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Meng Yee Ling
systems for sensing applicationsDesign and Fabrication of a near-infrared spectroscopy
Academiejaar 2007-2008Faculteit IngenieurswetenschappenVoorzitter: prof. dr. ir. Paul LagasseVakgroep Informatietechnologie
Scriptie ingediend tot het behalen van de academische graad van
Begeleider: Joost BrouckaertPromotor: prof. dr. ir. Dries Van Thourhout
i
Toelating tot bruikleen
De auteur geeft de toelating dit afstudeerwerk voor consultatie beschikbaar te stellen en de-len van het
afstudeerwerk te copieren voor persoonlijk gebruik. Elk ander gebruik valt onder de beperkingen van het
auteursrecht, in het bijzonder met betrekking tot de verplichting de bron uitdrukkelijk te vermelden bij het
aanhalen van resultaten uit dit afstudeerwerk.
Permission for usage (English version)
The author gives her permission to make this work available for consultation and to copy parts of the work
for personal use. Any other use is bound to the restrictions of copyright legislation, in particular regarding
the obligation to specify the source when using the results of this work.
Meng Yee Ling 6th
June 2008
ii
ACKNOWLEDGEMENTS
I would like to thank the opportunity given to me by the Erasmus Mundus Consortium (EMMP) Master of
photonics for studying in University of Ghent, Belgium. Special thanks to Roel Baets, Geert Morthier and
Dave Steyaert for the welcoming stay in Belgium. This has been an extraordinary learning experience that I
will cherish for life.
I would also like to thank my promoter Dries Van Thourhout and my supervisor Joost Brouckeart for being
so magnanimous throughout my learning experience. They have been extremely considerate, especially
generous in providing me advice and constant guidance during my project. I have gained invaluable
knowledge, and even more, patience and determination.
My gratitude also goes to all members of INTEC department for their hospitality and help. Special thanks
goes to Steven Verstuyft for helping me with the metallization and electroplating processing work in the
cleanroom, Peter Geerinck for the wirebonding of detectors, Jeroen Allaert for the electronics board design
and implementation, Iwan Moreels and Professor Dirk Poelman for near infrared absorption experiments. I
am greatly indebted with their expertise, guidance and kindness.
Last but not least, to my beloved family and friends, thank you for instilling faith, hope and joy in me. They
are my pillar of strength and inspiration. The journey could not have been better, without you.
iii
ABSTRACT
The project presents the design and fabrication of a spectrometer-on-a-chip based on a photonic integrated
chip (PIC) with a planar concave grating (PCG) and metal-semiconductor-metal (MSM) photodetectors
integrated on the silicon-on-insulator (SOI) waveguides. The planar concave grating (PCG) is the
wavelength demultiplexer of the spectrometer system which diffracts and focuses the incident light into 30
output waveguide channels with a spectral resolution of 3.2nm. The subsequent optical signals of each
output waveguide are detected by InAlAs/InGaAs MSM photodetectors integrated on top of these output
waveguides.
The spectrometer chip is glued onto a ceramic package with the MSM photodetectors wire bonded to
package. The package is further mounted onto an electronic circuit board. The board contains switches and
a transimpedance circuit to convert the photocurrent to a voltage output. Two biasing schemes have been
presented for MSM and PIN detectors respectively. The MSM detectors are tested and experiments showed
a 0.06 A/W responsivity at 1570nm with an internal responsivity is 0.71 A/W. Photocurrent read out from
the MSM detectors resembles the fibre coupler response. The transimpedance circuit introduces an error of
5% in voltage output for photocurrent in the range of 10nA to 10µA.
Design and fabrication of a near-infrared spectroscopy
system for sensing applications
Meng Yee Ling
Supervisor: Ir. Joost Brouckaert, Promotor: Prof. Dr. Ir. Dries Van Thourhout
Abstract- We present the design and fabrication of a
spectrometer-on-a-chip based on a photonic integrated
circuit (PIC). This PIC is fabricated on a silicon-on-insulator
(SOI) substrate and the main components are a planar
concave grating (PCG) and heterogeneously integrated
InGaAs metal-semiconductor-metal (MSM) photodetectors.
The PCG acts as a wavelength filter and is fabricated with
deep UV lithography. It diffracts and focuses the incident
light into 30 output waveguide channels with a spectral
resolution of 3.2nm. The optical signals of the 30 channels
are then detected by the 30 MSM detectors integrated on top
of the output waveguides. The detectors are wire bonded
onto a package and the corresponding photocurrents
generated are converted into electrical read out signals by a
custom designed printed circuit board.
Keywords – Near infrared spectroscopy, planar concave
grating, MSM photodetector
I. INTRODUCTION
Conventional Near infrared (NIR) spectroscopy systems are
big and bulky lab equipments which have to be contained in
a stationary location. They are also expensive and require
costly technical maintenance. They offer extensive range of
wavelength measurements and usually exceed the
requirements of industrial applications.
Here we introduce the spectrometer-on-a-chip which
can operate as a miniature NIR spectrometer at lower cost.
The spectrometer is based on planar concave grating (PCG)
which acts as wavelength diffraction component and metal-
semiconductor-metal (MSM) photodetectors.
The operation of the spectrometer is discussed in
section II, followed by the wire bonding and packaging in
section III and finally experiment results in section IV.
II. OPERATION OF THE DEVICE
The optical signal is injected from a single mode fibre onto
the spectrometer chip via a fibre coupler [1]. An silicon-on-
insulator (SOI) waveguide will then route the light to the
PCG.
The PCG used in this chip is a 30 channel wavelength
filter which acts as a demultiplexer grating defined by
completely etching through the 220nm thick silicon layer. It
yields a free spectral range (FSR) of 115nm and an
operational wavelength range spanning from 1500nm to
1600nm. The corresponding 30 output channel response is
spaced 3.2nm apart with a FWHM of 1nm [2].
There are 30 InAlAs-InGaAs MSM detectors integrated on
top of the 30 output waveguides [3]. These detectors each record
signals of different wavelength region of the dispersed spectrum.
The photocurrent generated will be proportional to the input
optical power.
III. WIRE BONDING AND PACKAGING
A. Wire bonding
A mask is designed for metallization of Ti/Au (20nm/200nm)
Schottky contacts on the detectors. There are 30 MSM
photodetectors on top of the 30 output waveguide channels.
Since the output waveguides are spaced 25µm apart, we need to
design a fan out from the detector contacts to a bigger wire bond
contacts. The big wire bond contacts are 250µm x 250µm to
accommodate aluminium wire (32µm) bonding.
The metallization is done with lithography and deposition of
Ti/Au aligned onto the underlying MSM detectors. The thickness
of Au (200nm) is not sufficient for wire bond and electroplating
(KAuCn2) is used to add the thickness of the wire bond contacts
(3-5µm) as shown in figure 2.
Figure 2: Added thickness on the wire bond contacts.
The spectrometer chip is the glued onto a PGA 68 ceramic
package. Aluminium wires are bonded onto the package and
then to the contacts on the chip via ultrasonic bonding. The
results can be seen in Figure 3.
Figure 3: Wire bonding process and final product.
B. Electrical read out circuit
A bias voltage is supplied to the MSM detectors to sweep
out the carriers generated. The photocurrent will then be
converted to a voltage reading by a transimpedance circuit.
The basic electrical read out configuration is as in figure 4.
Figure 4: Schematic for electrical read out circuit.
We use CMOS analogue switches to sequentially
switch the photocurrents from the detector to the
transimpedance circuit. The digital switching is done via
parallel port with Labview interface vi.
IV. EXPERIMENTS
A tunable laser is used as a light source. Under no
illumination, the dark currents of the 30 MSM detectors are
measured for a bias voltage from -6V to 6V. We would
expect a dark current of 4nA to 10nA [3]. With a
comparison, we found out that 15 detectors are functioning.
Among 15 faulty detectors, 8 are suspected to be short-
circuited showing extremely high dark currents while others
are disconnected (very low dark currents).
By tuning the laser to the peak transmission of each
detector, the I-V curve for different optical power of input
fibre is measured. The external responsivity of the detectors
measured at a bias voltage of 6V is calculated from the I-V
curve and plotted in figure 5.
External responsivity of detectors
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
R ext (A/W)
Figure 5: External responsivity of detector 9 to 23.
Since the detectors circuit from 1570nm onwards are
not functioning, it is difficult to compare the whole response
to the response of the fibre coupler and the transmission
grating. However, the responsivity of the detectors is
observed to increase from 1520nm to 1570nm which agrees
with the fibre coupler response.
On-chip loss (Planar concave grating) of the spectrometer is
approximately 4.6 dB at 1570nm and the coupling loss at
1570nm is 6dB. The insertion loss is thus approximately 10.6dB.
From experiment, the external responsivity, Rext of detector 23
(1570nm) is 0.062 A/W. Internal responsivity, Rint is calculated
as waveguideoutputP
fibreinputPRR extInt
__
__*= . For the transmission at
1570nm, the internal responsivity is 0.71 A/W.
Photocurrent versus wavelength plot
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
7.E-02
8.E-02
9.E-02
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
Photocurrent (m
A)
det 23
det 22
det 21
det 20
det 19
det 18
det 17
det 16
det 15
det 14
det 13
det 12
det 11
det 10
det 9
Figure 6: Optimised photocurrent versus wavelength plot.
Under a constant optical power of input fibre (1mW) and
tuning the laser to the peak transmission of each detector, the
photocurrent is plotted as in figure 6. The electrical board
photocurrent read out of the detectors showed that the peak
wavelength response of detectors agrees with the PCG response
with a resolution of approximately 3.2nm.
However, the crosstalk of the photocurrent read out from
the electronic circuit is approximately -11dB. This crosstalk has
increased from the crosstalk of -25dB before the packaging of
the spectrometer. The reason for this might be due to the losses
introduced during the packaging such as the metallization, wire-
bonding and electrical board circuit read out. All this processing
steps could introduce on chip losses.
V. CONCLUSION
A spectrometer on a chip connected to an electronic read
out board is presented. The responsivity of the detectors is
measured and the photocurrent of the detectors is measured with
the electrical circuit board.
VI. REFERENCES
1. D. Taillaert , F. V. Laere, M. Ayre, W. Bogaerts, , D. V.
Thourhout, P. Bienstman and R. Baets, “Grating couplers for
coupling between optical fibers and nanophotonic waveguides,”
Japanese Journal of Applied Physics, Vol. 45, No. 8A, pp. 6071-6077,
2006.
2. J. Brouckaert, W. Bogaerts, P. Dumon, D. V. Thourhout and R. Baets,
“Planar Concave Grating Demultiplexer Fabricated on a Nanophotonic Silicon on Insulator Platform”, J. Lightwave technology, Vol. 25, No. 5,
pp. 1269-1275, 2007.
3. J. Brouckaert, G. Roelkens, D. V Thourhout and R. Baets, “Compact InAlAs-InGaAs Metal Semiconductor Metal Photodetectors Integrated on
Silicon on Insulator Waveguides”, IEEE Photonics Technology Letters,
Vol. 19, No. 19, pp. 1484-1486, 2007.
iv
Contents
CHAPTER 1 Introduction 1
1.1 Near Infrared (NIR) spectroscopy background physics………………………………………. 1
1.1.1 The harmonic oscillator……………………………………………………………… 2
1.1.2 The anharmonic oscillator……………………………………………………………. 3
1.2 Beer Lambert’s Law…………………………………………………………………………… 4
1.3 Conclusion……………………………………………………………………………………… 5
CHAPTER 2 Aim of Project 6
CHAPTER 3 Miniaturized Near Infrared Spectrometers 8
3.1 Grating based spectrometer…………………………………………………………………….. 9
3.1.1 Aluminium grating spectrometer……………………………………………………… 10
3.1.2 Planar double grating spectrometer…………………………………………………… 11
3.1.3 Waveguide concave grating spectrometer…………………………………………….. 12
3.2 Design and operation of spectrometer chip of the project………………………………………12
3.2.1 Fibre coupler………………………………………………………………………… 12
3.2.2 Planar concave grating………………………………………………………………… 13
3.2.3 Metal-semiconducto-metal (MSM) photodetectors…………………………………… 14
3.3 Comparisons of miniature spectrometers………………………………………………………..15
CHAPTER 4 Absorption experiments and Calibration 16
4.1 Measurement of water content in ethanol……………………………………………………… 16
4.1.1 Preparation of samples and experiment setup………………………………………… 16
4.1.3 Spectra of water ethanol mixtures……………………………………………………... 17
4.2 Measuring blend levels of biodiesel with conventional fuels…………………………………... 18
4.2.1 Preparation of samples and experiment setup…………………………………………. 19
4.2.2 Spectra of blends levelsof biodiesel with conventional fuel…………………………... 19
4.3 Calibration of absorption spectrum…………………………………………………………….. 20
4.3.1 Univariate Analysis: Single Wavelength calibration…………………………………..20
4.3.1.1 Prediction in univariate analysis……………………………………………... 21
4.3.2 Multivariate Analysis: Principle Component Analysis (PCA)………………………... 21
4.3.2.1 Predictions in PCA…………………………………………………………… 22
4.3.3 Error analysis………………………………………………………………………….. 25
4.4 Analysis of Results …………………………………………………………………………….. 26
4.4.1 Univariate calibration:water ethanol mixtures………………………………………… 26
4.4.2 PCA calibration: water ethanol mixtures……………………………………………… 27
4.4.3 Univariate calibration: blend levels of biodiesel with conventional fuel……………… 30
4.4.4 PCA calibration: blend levels of biodiesel with conventional fuel……………………. 31
4.4.5 Comparisons of univariate analysis and PCA analysis……………………………….. 33
4.4.6 Advantages of Principle Components Analysis (PCA)……………………………….. 33
v
CHAPTER 5 Planar Concave Grating responses 34
5.1 Planar concave grating…………………………………………………………………………. 34
5.1.1 Crosstalk………………………………………………………………………………. 35
5.1.2 -10dB Crosstalk……………………………………………………………………….. 36
5.1.3 -20dB Crosstalk……………………………………………………………………….. 37
5.1.4 Grating response of the spectrometer…………………………………………………... 38
5.2 Limitation………………………………………………………………………………………. 39
CHAPTER 6 Wire Bonding and Electrical Read-Out 40
6.1 Layout of MSM detectors ……………………………………………………………………… 40
6.1.1 MSM photodetector metallization mask………………………………………………. 41
6.1.2 PIN photodetector metallization mask………………………………………………… 42
6.1.3 Metallisation processing steps………………………………………………………… 43
6.1.4 Wire bond test………………………………………………………………………… 46
6.2 Electrical read-out board………………………………………………………………………... 48
6.2.1 Schematics of voltage read out circuit………………………………………………… 48
6.2.1.1 MSM photodetector board…………………………………………………… 49
6.2.1.2 PIN photodetector board……………………………………………………... 50
6.2.2 Selection of electronic components…………………………………………………… 50
6.2.3 Error in output voltage reading………………………………………………………... 52
CHAPTER 7 Experiments on Spectrometer-on-a-chip and the electrical board 54
7.1 Measuring the dark current of the detectors……………………………………………………. 54
7.1.1 Experimental results…………………………………………………………………… 55
7.1.2 Defects of detectors…………………………………………………………………… 56
7.2 Measuring the responsivity of the detector……………………………………………………... 57
7.2.1 Experimental results…………………………………………………………………… 57
7.3 Measuring the output current and voltage from the printed circuit board……………………… 60
7.3.1 Crosstalk………………………………………………………………………………. 62
7.3.2 Voltage reading from printed circuit board………………………………………….... 63
Conclusion and perspective 65
Appendices 66
References 72
List of Figures 74
List of Tables 77
vi
List of Symbols
k force constant
m mass
h Plank constant
I1 transmitted light intensity
I0 incident light intensity
c concentration of the sample
l optical path length
ε molar extinction coefficient
NIR near infrared
PIC photonic integrated chip
SOI silicon on insulator
MSM metal-semiconductor-metal
PCA principle component analysis
λ wavelength
ηext external quantum efficiency
R responsivity
q coulomb charge
v frequency of incident light
1
CHAPTER 1
Introduction
1.1 Near Infrared (NIR) Spectroscopy background physics
Spectroscopy is a study of interaction of light waves with matter. When light radiates on the object of interest,
it will be absorbed, transmitted or scattered. Hence, by comparing the light properties before and after
penetrating through the sample, information about the sample could be obtained. Near infrared (NIR)
spectroscopy is a measurement technique based on absorption of matter using light in near infrared region,
780µm to 2500µm.
At room temperature, the atomic bonds of matter (solid, liquid or gas) vibrate at lowest energy state or
otherwise at fundamental frequencies. When near infrared light illuminates on the matter, the atom-to-atom
bonds will feel this incident photon energy. If the energy or frequency of the photon matches the energy needed
to excite the molecules to higher energy state (overtones and combination vibrations), this photon will be
absorbed by the molecules.
This absorption further leads to a low transmission at the corresponding photon energy (frequencies). Thus, by
analysing the spectrum of light after penetration of a sample, we could identify the molecular bonds which
possess those particular vibration frequencies.
Different molecules have different bonding strength and hence different vibrational frequencies. This renders
each molecule a unique body with different identification. Therefore, near infrared spectroscopy is popular for
qualitative analysis and quantitative measurements of functional groups of OH, CH, NH and CO and has been
widely adapted as a sensor in many industrial applications, i.e. pharmaceutical, food and beverages etc.
In the coming section we will study in brief the physics that governs the vibrational frequencies of atomic
bonds, followed by Beer Lambert’s Law which is used to relate absorbance to concentration of samples.
Chapter 1 Introduction
2
1.1.1 The Harmonic Oscillator
At room temperature, molecules vibrate in the least energy state allowed. By assuming Hooke’s Law and
deriving from the vibration of a diatomic harmonic oscillator, we can calculate the lowest or fundamental
frequencies of any two atoms connected by a chemical bond.
Figure 1.1. Diatomic oscillator with masses m1 and m2.
Consider a diatomic oscillator model in figure 1.1 with vibrating masses m1 and m2, the vibrational frequency
vo is [1]: m
kvo π2
1= ------------Eq. (1)
where k is the force constant of the bond, m is reduced mass of the two atoms
21
21
mm
mmm
+= .
Although the reduced masses of molecules such a CH, OH and NH are quite similar, the fundamental
vibrational frequencies between them would differ due to different bond strength, k and length [1].
Instead of continuous energy levels as predicted by the classical model, quantum mechanics has shown
otherwise that there are only several discrete vibrational energies, En that are allowed.
+=
2
1nhvE on ; ...2,1,0=n ------------Eq. (2)
where h is the Plank’s constant.
For polyatomic molecules, we can treat them as a series of diatomic, independent, harmonic oscillators.
Equation 2 can then be generalized as [1]:
hvnnnnEN
i
i )2
1(,...),,(
63
1
321 += ∑−
=
; ...)2,1,0,...,,( 321 =nnn ------------Eq. (3)
Chapter 1 Introduction
3
A nonlinear molecule containing N atoms will have 3N − 6 vibrational degrees of freedom, while a linear
molecule has only 3N − 5. These vibrational degrees of freedom correspond to the number of fundamental
vibrational frequencies of the molecule.
Energy levels are equidistant for harmonic oscillator model and the selection rule only allows transitions
between neighbouring energy levels such that 1±=∆n [1]. At room temperature, most molecules populate the
ground level, n =0. Transitions from ground state to n=1 is termed as the fundamental transition.
1.1.2 The Anharmonic Oscillator
In molecules, electron clouds and the charges of nuclei of the two bound atoms will impose a limit during
compression step, hence creating an energy barrier. On the other hand, when the stretch of electron clouds
exceeds the bond restoring ability, the bond will break and thus the vibrational energy will reach the
dissociation energy (figure 1.2) [1].
It is also observed that the energy levels in anharmonic oscillator are not equal as in the harmonic oscillator
model. The energy levels become closer as the vibrational energy increases [1]. The energy levels are modified
to be:
2
2
1
2
1
+−
+== nvnvhc
EG oo
nn χ ------------Eq. (4)
where c is the speed of light and χ is the anharmonicity constant.
Figure 1.2. Energy diagram of an ideal diatomic oscillator and anharmonic diatomic oscillator.
Chapter 1 Introduction
4
In the anharmonic oscillator model, the selection rules loosen up to include transitions to more than one energy
level. Apart from the fundamental modes, vibrational transitions corresponding to ,...3,2 ±±=∆n are also
allowed. These additional transitions are called first, second, and so forth overtones [1].
Besides the overtones, combination bands are also observed where the vibrational transitions are the sum and
difference of fundamental and overtone bands. Near infrared region is filled with overlaps of overtone and
combination bands.
1.2 Beer Lambert’s Law
Beer Lambert’s Law relates the absorbance of light to the concentration of the samples. The law states that the
reflected or transmitted light through a substance is dependent on the chemical (molecular absorbance),
physical (scattering/reflective) properties of the sample and optical path length traversed.
Figure 1.3. Transmission through sample contained in a cuvette.
Transmission of light, T through a liquid sample can be defined as the ratio of transmitted light intensity to
incident light intensity: lcc
I
IT εα −− === 1010
0
1 ------------Eq. (5)
where I1 is the transmitted light intensity, I0 is the incident light intensity, c is the concentration of the sample, l
is the optical path length and ε is the molar extinction coefficient1.
Absorbance for liquids is then defined as:TI
IA
1loglog 10
0
110 =
−= ------------Eq. (6)
Overall, absorbance of a liquid sample can be stated as lcA ε= ------------Eq. (7)
1a measure of how strongly a chemical species absorbs light at a given wavelength.
Chapter 1 Introduction
5
1.3 Conclusion
Near infrared spectroscopy operates in the region of 780µm to 2500µm and is based on molecular overtone and
combination vibrations.
Generally, overtones and combination bands have a much lower intensity than fundamental modes in the mid
infrared region. Hence the molar extinction coefficient in the near infrared region is small. This results in a
lower sensitivity of the probing system. However, this also allows near infrared light to penetrate much further
into the sample and this is particularly useful for probing samples with little to no preparation as compared to
mid infrared spectroscopy.
We have also seen the Beer Lambert’s law which relates the absorption of a material to the concentration of the
absorptive element. This relation will be used to analyze concentration of liquids in subsequent near infrared
absorption measurement.
6
CHAPTER 2
Aim of Project
The goal of the project is to design and fabricate a miniaturized Near Infrared (NIR) spectroscopy system based
on a planar concave grating (PCG) on a silicon on insulator (SOI) photonic integrated chip (PIC). This PIC
consists of a fibre coupler, a planar concave grating and InGaAs metal-semiconductor-metal (MSM)
photodetectors heterogeneously integrated on top of the outgoing SOI waveguides.
Figure 2.1: Schematic view of spectrometer on a chip.
Figure 2.1 shows a schematic view of the spectrometer-on-a-chip design. Light is first coupled into the chip via
a fibre coupler. The light is the guided to the planar concave grating (PCG) demultiplexer which separates and
focuses the light into 30 different wavelength channels. 30 InGaAs MSM photodetectors are integrated on top
of the 30 output waveguides to measure the optical power in the 30 different wavelength channels respectively.
Fibre coupler
grating
Planar Concave
Grating
MSM detectors
Ti/Au wire bond
pads
Aluminium wire
bond
Single mode
fibre
Ceramic
package
SOI platform
Chapter 2 Aim of Project
7
Now, the project will focus on converting the light signals detected by the detectors into electrical output
signals. These electrical output signals (current or voltage) will be a proportional to the optical power detected
by the MSM photodetectors.
The proposed read out mechanism is to package the spectrometer on-a-chip by gluing it onto a ceramic
package. The 30 MSM photodetectors is wire bonded onto the ceramic package contact pads and the package is
mounted onto an electronic circuit board. This board contains switches and a current to voltage converter
(transimpedance circuit) to produce linear voltage read out that corresponds to the photocurrent of the detector.
The 30 detectors will be sequentially biased by controlling the switches and the corresponding voltage will be
read out using a multimeter. The control of switches is done from a computer and the output voltage of
transimpedance circuit is read out on a multimeter which is synchronised to the computer.
The final product of the project would be to produce a miniature spectrometer with electrical output readings
from the detectors.
The next chapter will start with information of present miniature spectrometers and comparing it with the
spectrometer-on-a-chip of this project. Chapter 4 will be on absorption measurements and calibration. Chapter
5 will be on crosstalk analysis of the planar concave grating responses. Then we will proceed to the design of
wire bonding and electronic board. Finally, the thesis will include the experiment on characterization of
photodetectors and the read out of photocurrent from electronic board.
8
CHAPTER 3
Miniaturized Near Infrared Spectrometer
Conventional Near infrared (NIR) spectroscopy systems are big and bulky lab equipments which have to be
contained in a stationary location. They are also expensive and require costly technical maintenance. These
equipments offer extensive range of wavelength measurements and the performances usually exceed the
requirements of industrial applications. Therefore a NIR spectrometer which is smaller, easier to maintain
while performs in the specific wavelength of interest is much desirable.
Having said so, miniaturized near infrared (NIR) spectrometers have been realized over the past few years to
better suit some of the industrial needs. New generations of miniaturized optical spectrometers involves
microfabrication of optical components and subsequent assembly on a micro-optical platform. The dimension
of spectrometers can be further reduced with nano-scale integration of photonics components on a silicon on
insulator (SOI) platform.
In the coming section we will discuss some of the work conducted by research groups to produce miniaturized
spectroscopy systems. Following the review, we shall introduce the design and operation of the spectrometer-
on-a-chip of this project.
Spectrometers are usually classified by their wavelength selection/filter method and among the popular
wavelength filter devices are gratings, interferometers and prisms. Here, we shall only discuss the grating based
spectrometers.
Chapter 3 Miniaturized Near Infrared Spectrometer
9
Grating based spectrometer
Figure 3.1: Basic grating configuration.
A grating based spectrometer uses a grating to filter light into different wavelengths. As can be seen in figure
3.1, the incident light passes though an opening slit and is collimated by a mirror or lens. The light is then
diffracted into different spectral components upon illuminating on the grating. The diffracted wavelength
spectrum will depend strictly on the grating resolution and design.
Different wavelengths will be focused and scanned successively onto the exit slit by rotating either the grating
or the output focusing mirror. The incident light can then be analysed by studying the wavelength components
of the distributed spectrum.
3.1.1 Aluminium grating spectrometer
Kong et al. reported an aluminium grating spectrometer integrated on a silicon wafer [4]. The infrared
spectrometer consists of two independently processed silicon wafers. The first wafer contains the aluminium
grating while the second wafer contains the thermopile based detector array. Two wafers are bonded using a Si-
Si low temperature fusion bonding technique.
Figure 3.2: Schematic structure of the aluminium grating spectrometer [4].
Chapter 3 Miniaturized Near Infrared Spectrometer
10
Figure 3.2 shows the incident light is diffracted by the aluminium grating. Then, it propagates through the
silicon substrate and it is detected by an array of polysilicon thermopiles. Silicon is transparent for wavelengths
exceeding 1µm and thus is suitable as a platform for the near infrared optical path.
Polysilicon thermal detectors are implemented in this design due to the inability of silicon to detect infrared
light. They also require no electrical biasing and are reported to be easier to fabricate [4]. However, the system
requires a chopper to modulate the light beam because the thermal detectors do not respond to continuous
radiation.
The device reported a detectable wavelength range of 13µm for a silicon optical path for a 4µm grating
constant [4].
3.1.2 Planar double grating spectrometer
Grabarnik et al. reported a miniature spectrometer built from two flat diffraction gratings. In this design, the
second grating provides compensation of aberrations introduced by the first grating.
The spectrometer works in reflection mode with two glass wafers aligned parallel facing each other. The light
is first reflected from a stripe mirror which also acts as a slit. Then the reflected light undergoes twice
diffraction upon impinging on grating 1 and 2. The light further passes through the glass and is recorded by the
detector.
Figure 3.3: Schematic view of compact planar spectrometer [5].
The device is mounted on the surface of a charge coupled device (CCD) sensor that records the light spectrum.
The operating wavelength range is from 450nm to 750nm giving a 300nm visible bandwidth. The spectral
resolution is reported to be 3 nm and the device is 3 x 3 x 11 mm3 in size [5].
Chapter 3 Miniaturized Near Infrared Spectrometer
11
Figure 3.4: The experimental set up of compact planar spectrometer [5].
3.1.3 Waveguide concave grating spectrometer
Figure 3.5: Waveguide concave grating spectrometer [6].
Mohr et al. reported a planar grating spectrograph based on 3 layer resist polymer waveguides. The 3 layer
resist consists of 50µm thick core of polymethylmethacrylate (PMMA) with n1 =1.49 and 17.5µm thick
cladding layers of copolymer composed of methlmethacrylate (MMA) and tetrafluorpeopylmethacrylate
(TFPMA) with n2 ranging from 1.49 to 1.425.
Optical fibres are adjusted to couple light into the polymer waveguide in horizontal direction by structuring
fixed fibre grooves. The light is then diffracted by the self focusing reflecting grating fabricated on the polymer
by deep-etch X-ray lithography. The diffracted spectrum is then focused and projected onto 10 optical fibres
that further channel the spectral components to a photodetector array.
The device area is 18 x 6.4mm2 and the operation wavelength range is from 720nm to 900nm with a spectral
resolution of 20nm [6].
Chapter 3 Miniaturized Near Infrared Spectrometer
12
3.2 Design and operation of the spectrometer chip of the project
The spectrometer-on-a-chip of this project has similar operating principles as the waveguide concave grating
spectrometer mentioned in section 3.13. However, we have utilise the high refractive index difference of SOI
waveguides (nsilicon=3.47, nsilicon_oxide = 1.44) and also integrated on-chip photodetectors instead of output fibres
fixed on the grooves connected to detectors. This greatly reduces the size of the spectrometer.
The spectrometer chip of the project consists of fibre coupler, planar concave grating and InGaAs
photodetectors integrated on silicon on insulator (SOI) platform. Silicon on insulator (SOI) platform consists of
a layer of 220nm thick silicon on top of a 1µm thick oxide layer on a silicon substrate. The large refractive
index difference between the silicon and silicon oxide layer enables very compact integration of optical
waveguides and components.
In this project, light is coupled onto the spectrometer chip from free space via a fibre coupler. The light
propagates through photonic wire waveguides and diffracted by the planar concave grating onto 30 output
waveguide channels. The optical signals of 30 channels are detected by 30 InGaAs metal-semiconductor-metal
photodetectors respectively. The following section will briefly explain the workings of each optical component
on the spectrometer chip.
3.2.1 Fibre coupler
Due to the large mismatch of the mode size of a typical single mode fibre with a diameter of 9um and a single
mode waveguide with a cross-section of 0.1um2, there will be huge losses of optical power when trying to
couple light directly from one to the other [10]. Therefore, a fibre grating coupler is used to efficiently couple
light onto the chip.
Figure 3.6: Diffraction grating layout for fibre to waveguide coupling [10].
Chapter 3 Miniaturized Near Infrared Spectrometer
13
The coupler grating operates on the Bragg diffraction principle whereby the incident light waves interfere with
the scattered light wave to form constructive interference, θλ sin2dm = , where m is the diffraction order, λ is
the incident wavelength, d is the grating pitch and θ is the incident angle.
The input single mode fibre is titled at 10 degrees incidence angle to operate in first order Bragg diffraction
from the grating. The first order diffracted light is then coupled into the 12µm broad ridge waveguide. This
light further propagates through the tapered waveguide into a narrow photonic wire with a width of 500nm
which is formed by etching completely through the 220nm thick silicon layer. The peak coupling efficiency of
the fibre coupler is 30% (-5.2dB) at 1550nm and the 1dB bandwidth is approximately 40nm [10].
Figure 3.7: Coupling efficiency for fibre coupler with air interface and 630nm grating pitch[10].
3.2.2 Planar concave grating (PCG)
This light from the photonic wire propagates in a slab mode when the wire opens up to the unetched free
propagation region (FPR) of the planar concave grating (PCG). The PCG is designed based on conventional
Rowland geometry and is able to diffract light and to focus the light into a series of output waveguides [12].
Figure 3.8: Planar Concave Grating based on Rowland configuration [12].
Chapter 3 Miniaturized Near Infrared Spectrometer
14
The Planar concave grating used in this chip is a 1x 30 demultiplexer defined on a SOI wafer with a silicon top
layer of 220nm and a buried oxide layer of 1um [12]. The grating is defined by completely etching through the
220nm thick silicon layer. The input diffracted light by the grating can be focused onto the circumference of
the Rowland circle with a reflection angle,di θθθ −= , where θi is the incident angle with respect to the centre
of the circle, θd is the diffracted angle with respect to the centre of the circle This is further determined by using
the grating equation ( )eff
din
mdλ
θθ =+ sinsin , where d is the grating pitch, m is the order of diffraction, λ is the
free space wavelength and neff is the effective refractive index of the slab mode.
The design of the grating on the chip yields a free spectral range (FSR) of 115nm and the 30 output channels
range from 1500nm to 1600nm. The corresponding 30 output channel response is spaced 3.2nm apart with a
FWHM of 1nm.
Following the diffraction, different wavelengths are re-focused onto the 30 output waveguides and are detected
by photodetectors integrated on top of the waveguides. III-V photodetectors are integrated on SOI waveguides
due to the transparency of silicon to infrared lights of >1µm.
3.2.3 Metal-semiconductor-metal (MSM) photodetectors
Figure 3.9: Schematic view of waveguide integrated MSM detector [13].
Due to its high absorption, InGaAs are good semiconductor material to be used as near infrared light detectors
especially for the telecommunication wavelength region, 1330nm to 1550nm. The composition of InxGa1-xAs
can be tailored to yield a bandgap absorption wavelength from 800nm to 2600nm.
Metal-Semiconductor-Metal (MSM) detectors are defined by bonding unprocessed InAlAs-InGaAs dies onto
processed SOI waveguide wafer using low temperature divinyl-disiloxane benxocyclobutane (DVS-BCB)
bonding process [13]. The InP substrate is then removed and the photodetectors can be further defined on a
wafer-scale and lithographically aligned with respect to the underlying SOI waveguides [13].
Chapter 3 Miniaturized Near Infrared Spectrometer
15
No fibre couplers are needed as in the case for PIN photodiodes and directional coupling of light from
waveguide to the detector is achieved.
In0.53Ga0.47As is used as the active absorption layer in the detector. A thin layer of InAlAs is implemented
between the Ti/Au schottly contacts and InGaAs active layer to raise the Schottky barrier to prevent leakage
current (dark current). There is a 20nm InAlAs-InGaAs digital graded superlattice layer to decrease the
bandgap discontinuities between InAlAs and InGaAs absorption layer [13]. Two Schottky electrodes (Ti/Au)
are further deposited on top of the thin film detector to provide biasing contacts while at the meantime create a
lateral confinement for underlying waveguide [13].
The reported detector has a responsivity of 1.0 A/W at a wavelength of 1.55um and low dark current of 4.5nA
[13].
3.3 Comparisons of miniature spectrometers
Brief comparisons are made between the miniaturized spectrometers mentioned previously:
Device
Dimension
Operating
spectral range
Spectral
resolution
Remarks
Aluminium grating
spectrometer
(Year 2001)
Not reported
grating pitch of
4um, wavelength
range < 4um (air)
Not
reported
Grating pitch limits the
spectral resolutions
Planar double grating
microspectrometer
3 x 3 x 11 mm3
450nm – 750nm
3nm
Non coplanar optical path.
Stray light problems
Waveguide concave
grating spectrometer
(Year 1991)
18mm x 6.4mm
720nm – 900nm
20nm
No integrated detectors on
chip
Planar Concave
grating spectrometer
(SOI chip used in this
experiment)
6x3mm2
1500nm-1600nm
3.2nm
Planar structure and no
moving parts
Table 3.1: Comparison of grating spectrometers.
16
CHAPTER 4
Experiments and calibration
We are interested to measure absorbance of material in the near infrared region which could subsequently be
measured by the grating spectrometer on a chip of this project. In this chapter, we will conduct two absorption
measurements: measuring the water content in ethanol and blend levels of biodiesel with conventional diesel.
These two measurements yield distinguishing absorbance bands in the near infrared range from 1400nm to
1700nm.
Once the absorbance spectrum is obtained, there are several calibration techniques that can be used for spectral
analysis. We will look at the shortcomings of univariate analysis and then proceed to the multivariate analysis
method, Principle Components Analysis (PCA) for generating calibrations for future predictions of analyte
concentrations.
4.1 Measuring the spectrum of water ethanol mixtures
4.1.1 Preparation of samples and experiment setup
We prepare a calibration set of water ethanol mixtures by mixing pure compounds. 10 samples of de-ionized
(DI) water with concentration from 0% to 20% with increase step of 2% in ethanol were prepared.
Figure 4.1: Cuvette holder used in experiment.
Chapter 4 Absorption experiments and Calibration
17
The liquid samples were contained in a plastic cuvette holder with an optical path length of 1cm. SMA
multimode fibres are used as transmission fibres. The transmission spectra of the samples were then measured
with an Optical Spectrum Analyser (OSA) with resolution setting of 2nm. A superluminescence broadband
Light Emitting Diode (SLED) with centre wavelength at 1500nm is used as the light source. The SLED
spectrum is recorded as in figure below.
Superluminescence LED spectrum
-45
-35
-25
-15
1450 1500 1550 1600 1650
wavelength
dB
m
Figure 4.2: SLED output spectrum.
The transmission spectra of the liquid samples were recorded in a wavelength region from 1450nm to 1600nm
and further converted into absorbance spectra using the formula, onTransmissi
Absorbance1
log=
4.1.2 Spectra of water ethanol mixtures
The transmission spectra of samples are collected and normalized with respect to an empty cuvette
transmission spectrum. The spectra are further normalized with the pure ethanol transmission spectrum and
converted to absorbance spectra.
1460 1480 1500 1520 1540 1560 1580 1600
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Absorbance spectra of water mix ethanol ethanol100
water2
water4
water6
water8
water10
water12
water14
water16
water18
water20
Ab
so
rba
nce
wavelength (nm)
Figure 4.3: Transmission through different water ethanol mixtures contained in a cuvette.
Chapter 4 Absorption experiments and Calibration
18
Figure 4.3 shows the recorded absorbance of samples in the wavelength range of 1450nm to 1590 nm. The
water absorption at 1450nm increases with increasing concentration of water in ethanol. This absorption is
caused by the first overtone band of the OH stretching mode in the infrared region (3450 cm-1
x2 =6900 cm-1
=
1450nm) [14].
Another absorption band at 1580 is also observed. However, this OH overtone peak is attributed from the OH
bond from ethanol as well as from water. Thus, we will only analyse the water concentration by taking into
account the peak at 1450nm.
1460 1480 1500 1520 1540 1560 1580 1600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Relative Absorbance of water mix ethanol
water2
water4
water6
water8
water10
water12
water14
water16
water18
water20
Re
lative
Ab
so
rba
nce
wavelength (nm)
1460 1480 1500 1520 1540 1560 1580 1600
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Gaussian Fit Relative Absorbance
water2
water4
water6
water8
water10
water12
water14
water16
water18
water20
Re
lative
Absorb
ance
wavelength (nm)
Figure 4.4:.Normalised absorbance curve of water ethanol mixtures (left) and gaussian fit curve (right).
Figure 4.4 shows the normalised absorbance of different samples with respect to pure ethanol. The noisy
spectra are due to the use of multimode (SMA) fibres in the experiment to couple the light in and out of the
cuvette. Thus before generating the calibration, the spectra are smoothed by a Gaussian fit and the
corresponding absorbance curve is shown on the right graph.
We can be concluded that the absorbance increases as the water content in ethanol increases. The peak
absorbance is noticed to be at 1450nm.
4.2 Measuring blend levels of biodiesel with conventional fuels
It is becoming more crucial to develop a method that can accurately, rapidly monitor the blend levels of
biodiesel with conventional fuel at a reduced cost. Near infrared spectroscopy has potential to fill these
requirements due to ease of sample handling [15].
Chapter 4 Absorption experiments and Calibration
19
4.2.1 Preparation of the samples and experiment setup
We prepared a calibration set of different blend levels of biodiesel with conventional fuels by mixing pure
compounds. 10 samples of biodiesels with concentration from 0% to 100% with increase step of 10% in
conventional fuel were prepared.
The transmission spectra of the samples were recorded with a CARY 5000 UV-VIS-NIR photospectrometer by
Varian. Using the Cary WinUV software, we recorded transmission spectra of the liquid samples from 1400nm
to 2400nm with grating resolution set at 1nm.
4.2.2 Spectra of blend levels of biodiesel with conventional fuel
Normalised Absorbance of Biodiesels blend levels with
conventional fuel
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
1651 1653 1655 1657 1659 1661 1663 1665 1667 1669
wavelength (nm)
Ab
so
rba
nc
e
Biodiesel 10%
Biodiesel 20%
Biodiesel 40%
Biodiesel 60%
Biodiesel 80%
Biodiesel 90%
biodiesel 100%
Biodiesel 30%
Biodiesel 50%
Biodiesel 70%
Figure 4.5: Normalised absorbance spectrum of different blends of biodiesel in conventional fuel.
Figure 4.5 shows the normalised absorbance spectra of biodiesels with respect to conventional diesel fuel in the
wavelength range from 1651nm to 1670 nm. This region demonstrated an increase in absorbance with respect
to increase of biodiesel blend levels in conventional diesel.
This region will be included in the calibration range to predict the blend levels of biodiesel, in this case the
methyl soyate with the petroleum derived diesel fuel.
Chapter 4 Absorption experiments and Calibration
20
Blend levels of biodiesel in conventional fuel
0.1
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.2
1900 1910 1920 1930 1940 1950 1960
wavelength (nm)
Transmission %
diesel_100%
DF_9_BIO_1
DF_8_BIO_2
DF_7_BIO_3
DF_6_BIO_4
DF_5_BIO_5
DF_4_BIO_6
DF_3_BIO_7
DF_2_BIO_8
DF_1_BIO_9
biodiesel_100%
Figure 4.6: Transmission spectrum of different blends of biodiesel and conventional fuel.
Another region to distinguish the blend levels is observed around 1930nm. This region can be included in the
calibration technique for the prediction. However, as can be seen in figure 4.6, the transmitted signal is very
low and this poses a limit to the sensitivity of the calibration system.
4.3 Calibration of absorption spectrum
It is important to generate a calibration equation that relates the concentration of analyte to the optical data we
recorded. This is to provide an interpretation and also a means to predict future samples.
4.3.1 Univariate Analysis: Single Wavelength calibration
The analyte concentration versus wavelength relationship could be deduced at the peak of the absorbance band.
Figure 4.7: Error free single absorbance band [1].
Chapter 4 Absorption experiments and Calibration
21
In a simplified and idealized model, the height of the absorbance peak is strictly proportional to the
concentration of the analyte. Taking unity path length of 1cm and substituting in Beer’s law, we have:
CA ε=
where A is the absorbance, ε is the molar extinction coefficient and C is the concentration of analyte. We will
rearrange the equation above in terms of absorbance because we are interested in concentration. The inverse
Beer’s Law is thus AC 1−= ε or rather bAC = , where the inverse molar extinction coefficient 1−ε is a
constant and now replaced by b . The graph for the univariate analysis at wavelength of choice can then be
plotted as in figure 4.8.
Figure 4.8: Calibration line in univariate analysis [1].
Linear Regression is used to find the best fit line across all the points. This line is called the calibration line and
has the equation 110ˆ AbbC += , where C is the concentration of analyte, 0b is the intercept from the
regression, ib is the regression coefficient at wavelength i and iA is the corresponding absorbance.
4.3.1.1 Prediction in univariate analysis
The calibration line we obtain at the chosen wavelength is used for future prediction of the analyte
concentrations. For samples with an unknown analyte concentration, the absorbance at the chosen wavelength
is substituted on the calibration equation and the corresponding water concentration can be calculated.
4.3.2 Multivariate analysis: Principle Components Analysis (PCA)
In multivariate analysis, the spectrum that relates to the change in analyte concentration is analysed at more
than one wavelength.
Chapter 4 Absorption experiments and Calibration
22
Figure 4.9: Absorbance band of multivariate case (more than one wavelength) [1].
Often there are absorbance band overlaps at the spectral peak as is shown in figure 4.9. In this situation, the
peak absorbance accounts for absorbance from different chemical bonds. In this case, there is a need to include
more wavelengths in the calibration.
Other reason on why univariate analysis is insufficient is due to the peak absorbance shifting caused by
hydrogen bonding, refractive index shifts or other physical parameters. This renders the chosen peak
absorbance wavelength unsuitable to generate a linear calibration line as per Beer’s Law. Therefore, more than
one wavelength has to be included for calibration to yield a better result.
There are several popular multivariate analysis method such as Multiple Linear Regression (MLR), Partial
Least Squares (PLS) and Principle component analysis (PCA). However, only PCA will be use to analyse the
results.
4.3.2.1 Predictions in PCA
Principle Component Analysis (PCA) is a mathematical technique which constructs an approximation to the
absorbance spectrum to be predicted. PCA generates new axes to define the data. These axes are generated with
the intention of plotting the data in terms of maximum variance. In other words, when plotted against the new
axis, the data has a maximum variance from the axis [1].
The steps to PCA analysis are as below. More detailed understanding of PCA can be found in literature [1].
a. Obtain optical data
In the experiment, n spectra are obtained for a range of wavelengths. For example in the figure 4.10, 4 samples
of different analyte concentration from 1450nm to 1600nm are measured. This yields n = 4 number of spectra,
m number of wavelength points.
Chapter 4 Absorption experiments and Calibration
23
Figure 4.10: Example of 4 absorbance spectra for a m wavelength points.
b. Subtract the mean
A mean spectrum for the 4 spectra is calculated, mXXXX ,...,, 321
. Then for each of the 4 spectrums, we
subtract the mean spectrum. This results in a new mean subtracted optical spectrum for the 4 samples.
iii XXXmean −=_ ; for i = 1,2,3….m.
c. Calculate the cross product matrix and the sum of cross product matrix
A m x m cross product matrix is created from each spectrum (n spectra = n matrices). The i,j th term in the
matrix corresponds to the product of mean subtracted absorbance at i th wavelength, and the mean subtracted
absorbance at j th wavelength. )_)(_(),(_ ji XmeanXmeanjiprodX = ; for i, ,j = 1,2,3….m.
The sum of cross product matrix is thus just the summation of all cross product matrices.
∑=
=n
k
kprodXprodXsum1
___
d. Calculate the principle components
The principle components, PC are the eigenvectors of the sum_X_prod matrix. This gives:
[ ][ ] [ ]prodXsumkprodXsumPC ____ = , where X = sum_X_prod matrix, PC is the eigenvector and k is the
eigenvalue.
The eigenvectors with corresponding large eigenvalues accounts for maximum variance in data points due to
effects of real physical phenomena on the spectra. Since we are interested in the difference of spectra with
relation to changes in analyte concentration, we shall choose the eigenvectors that with large eigenvalues.
These eigenvectors are then arranged in descending order. For mathematical reason, the number of
eigenvectors, p included has to be equal or smaller than the lesser of n and m. eigenvectors with very low
Chapter 4 Absorption experiments and Calibration
24
eigenvalues can be discarded without compromising the prediction analysis. Hence, data compression is
achieved here.
This set of newly selected and arranged eigenvectors (p x m matrix) are called the principle components (PC).
The eigenvectors are new axes that characterize the data. Now we have to transform the data such that they are
expressed in terms of the eigenvector axis.
e. Calculate the scores of Principle Components
The scores (S) are calculated as, ( ) dataAbsorptionOpticalPCSi __∗= . The scores are related to the concentration
as ...ˆ22110 +++= SbSbbC , where ib are the calibration coefficients. We can then obtain the calibration
coefficients with mulitlinear regression fitting [1].
f. Calculate coefficients of optical data
We retain the calibration coefficients, ib and the principle components, PC used to create the calibration. We
will now relate the scores and the calibration coefficients to the optical data.
Since score are defined as ...2211 ++= APAPS iii , where Aj is the optical data for the jth wavelength and Pji
is the value of the ith principle component at the jth wavelength. Substituting Si into concentration equation, we
obtain ( ) ( ) .........ˆ232322212113132121110 +++++++++= APbPbPbAPbPbPbbC .
This can be further simplified as ...ˆ22110 +++= AkAkbC --------------------------Eq (4.1)
where km is the coefficients of the optical data Am.
g. Perform prediction calculations
After obtaining all the optical data coefficients, b0 and km, the absorbance spectrum, Am (m wavelengths data
points) can be directly substituted into the equation 4.1 to give a predicted concentration. Figure 4.10 below
shows the flow of PCA analysis.
( )
•
•
•
•
•
•=
mp
m
m
m
pp PC
PC
PC
PC
PC
PC
PC
PC
PC
PC
PC
PC
PC
.
,3
,2
,1
2,
2,3
2,2
2,1
1,
1,3
1,2
1,1
..... Descending order of eigenvalues
Chapter 4 Absorption experiments and Calibration
25
Figure 4.11: Principle Component Flow Chart [1].
4.3.3 Error analysis
The error, e is defined as the difference between the predicted concentration, C and the reference concentration,
C . CCe ˆ−= . The root mean square error (RMSE) will be used to compare the performances of calibration
models: ( )
∑=
−=
n
i
ii
n
ccRMSE
1
2ˆ
, where n is the number of samples[1].
Chapter 4 Absorption experiments and Calibration
26
4.4 Analysis of Results
4.4.1 Univariate calibration: water ethanol mixtures
Univariate analysis: water ethanol mixtures at
1450nm
y = 0.0905x - 0.0274
0%
5%
10%
15%
20%
25%
0 0.5 1 1.5 2 2.5 3
Absorbance
% w
ate
r concentr
atio
n
Figure 4.12: Univariate calibration line calculated for water ethanol mixtures at 1450nm.
Figure above is a plot of percentage of water concentration in ethanol versus the absorbance at 1450nm for
each sample. The calibration set consists of 10 ethanol samples with 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%,
18% and 20% of water concentration. After fitting with partial least regression in excel, the calibration line
obtained gives 0b = -0.0274 and 1b =0.0905.
Referring to the inverse Beer’s Law equation, 110
ˆ AbbC += , we will plot the predicted water concentration
versus the known reference water concentration in ethanol by using the newly obtained calibration equation,
10905.00274.0ˆ AC +−= . Table 4.1 shows the reference value versus the prediction value calculated.
Reference
water concentration
Predicted
water concentration
2 % 1.52 %
4 % 4.69 %
6 % 4.84 %
8 % 8.80 %
10 % 10.47 %
12 % 12.44 %
14 % 14.59 %
16 % 16.73 %
18 % 16.27 %
20 % 19.61 %
Table 4.1: The univariate prediction table of water ethanol mixtures.
Chapter 4 Absorption experiments and Calibration
27
Univariate analysis:
Prediction concentration versus known
concentration for water ethanol mixtures
R2 = 0.9780
5
10
15
20
25
0 5 10 15 20 25
known water concentration %
pre
dic
ted %
Figure 4.13: Prediction versus known water concentration in ethanol mixtures at 1450nm.
The root mean square error, RMSE (see 4.3.3 Error Analysis) is 0.85% and the correlation coefficient is 0.978.
Correlation coefficient, R2 is a measure of how well the data fits the regression line with R
2=1 being the best
fit. The error calculated assumed the test samples are prepared with no error. However, the samples are
prepared with an uncertainly of ± 0.1ml yielding an error of 1% in 10 ml sample.
4.4.2 PCA calibration: water ethanol mixtures
Now we will proceed with multivariate analysis of water ethanol mixture. The generation of principle
components (PCs) are done with Matlab and the generation of calibration coefficients from the scores of PCs
are done with Excel linear regression fit.
Principle components are calculated and the corresponding eigenvalues are plotted as in figure 4.13. PCs with
large eigenvalue are retain for further calibration. This greatly reduces the data dimension and barely
compromises the calibration accuracy. Eigenvalues that has low values and plateau to a constant do not
contribute to the efficiency of the calibration and are not included.
Eigenvalues generated from the sum of cross product
matrix for water ethanol mixtures
-10
40
90
140
0 50 100 150
eig
env
alu
es
Figure 4.14: Eigenvalues of calculated from the sum of cross product matrix.
Chapter 4 Absorption experiments and Calibration
28
It is observed here that there is one single eigenvalue which has a very high value, nearly 140 times higher than
other eigenvalues. Including PCs with such low magnitude eigenvalues <1, would not contribute to the
performance of the prediciton. In this case, we retained only one set of principle component knowing that it
does not compromise the prediction and this greatly reduces the data dimension. The scores for each sample are
then calculated as: ( ) ( )m
m
i AAAAA
PC
PC
PC
PC
PC
S ••∗
•
•
•= 4321
4
3
2
1
where Si is the scores for ith sample and PCm is the set of principle components and Am is the absorbance of
optical data for whole range of wavelength, m. Figure 4.15 plots the Scores for each sample calculated based on
the principle component retained.
Water Ethanol Mixtures: Scores of
principle components plot
y = 0.0149x - 0.03
0%
5%
10%
15%
20%
25%
0 5 10 15 20
Scores
%w
ate
r concentr
ation
Figure 4.15: Plot of concentration of water in ethanol versus Scores.
The linear regression fit gives a calibration coefficient 03.00 −=b and 0149.01 =b . These calibration
coefficients will be retained together with the principle component to generate optical data coefficients that
cover the whole range of the spectrum (m wavelengths).
•
•∗+=
•
•
mm PC
PC
PC
bb
k
k
k
2
1
10
2
1
Chapter 4 Absorption experiments and Calibration
29
This is very convenient towards future prediction of sample concentrations because the optical spectrum
obtained can be directly substituted with the optical data coefficients to calculate the concentration. There will
be no wavelength selection as in univariate case to generate a prediction.
( ) ( )mi AAAAAtscoefficiendataopticalC ••∗= 4321__
We can now predict the water concentration in ethanol by multiplying the spectrum (A1, A2, A3,… Am) for each
sample with the newly obtained optical data coefficients (b0,, k1, k2, … km) as shown by the relation above.
PCA Reference
water concentration
Predicted
water concentration
2 % 1.34 %
4 % 4.83 %
6 % 4.78 %
8 % 8.88 %
10 % 10.60 %
12 % 12.73 %
14 % 14.42 %
16 % 16.92 %
18 % 16.16 %
20 % 19.29 %
Table 4.2: The PCA prediction table of water ethanol mixtures.
PCA analysis:
Prediction concentration versus known
concentration for water ethanol mixtures
R2 = 0.9715
0%
5%
10%
15%
20%
25%
0% 5% 10% 15% 20% 25%
known water concentration %
pre
dic
ted
%
Figure 4.16: Prediction versus known water concentration in ethanol mixtures using PCA.
The Root mean square error (RMSE) is 0.96% and the correlation coefficient is 0.9715.
Chapter 4 Absorption experiments and Calibration
30
4.4.3 Univariate calibration: blend levels of biodiesel with conventional fuel
The same steps as in previous section are repeated for biodiesels blend levels analysis.
Univariate analysis: Biodiesel blend levels at 1664nm
y = 347.58x - 2.1105
0
20
40
60
80
100
120
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
Normalised Absorbance
% B
iodie
sel
Figure 4.17: Calibration line calculated for biodiesel conventional fuel mixtures at 1664nm.
Figure 4.17 is a plot of the percentage of biodiesel in conventional diesel versus the absorbance at 1664nm for
each sample. The calibration set consists of 10 conventional fuel samples mixed with 10%, 20%, 30%, 40%,
50%, 60%, 70%, 80%, 90% and 100% of biodiesel. After fitting with partial least regression in excel, the
calibration line obtained gives 0b = -2.1105and 1b =347.58.
From the calibration line, predictions of samples are carried out:
Univariate prediction of
Biodiesel %
Reference %
10 7.76
20 20.93
30 30.16
40 40.11
50 51.07
60 60.88
70 71.84
80 79.41
90 89.01
100 98.80
Table 4.3: The univariate prediction table of biodiesel blend levels.
Chapter 4 Absorption experiments and Calibration
31
Univariate analysis:
Prediction concentration versus known
concentration for Biodiesel blend levels
R2 = 0.9983
0
20
40
60
80
100
120
0 50 100
known water concentration %
pre
dic
ted
%
Figure 4.18: Prediction for biodiesel conventional fuel mixtures at 1664nm.
The root mean square error (RMSE) is 1.18% and the correlation coefficient is 0.9983.
4.4.4 PCA calibration: blend levels of biodiesel with conventional fuel
In the multivariate analysis for biodiesel blend levels, the principle component with the largest eigenvalue is
retained for further calibration.
Eigenvalues (Principle components) for
biodiesel blend levels
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
-4 1 6 11 16 21
eig
en
va
lue
s
Biodiesel blend levels:
Scores of principle components plot
y = 121.67x - 0.7222
0
20
40
60
80
100
120
0 0.5 1
Scores
%b
iod
iese
l
co
nce
ntr
atio
n
Figure 4.19: Plot of eigenvalues of PC (left) and plot of scores (right) for biodiesel blend levels.
This will give a plot of one dimensional Scores plot as seen in figure 4.20 (right).
Chapter 4 Absorption experiments and Calibration
32
Biodiesel blend levels:
Scores of principle components plot
y = 121.67x - 0.7222
0
20
40
60
80
100
120
0 0.5 1
Scores
%b
iod
iese
l
co
nce
ntr
atio
n
Figure 4.20: Plot of biodiesel concentration in conventional fuel versus Scores.
The linear regression fit gives a calibration coefficient 7222.00 −=b and 67.1211 =b . This coefficient is
retained together with the principle component to generate the full spectrum optical data coefficients using the
same way as before.Prediction is carried out by multiplying the spectrum with the optical data coefficients.
PCA prediction of Biodiesel % Reference %
10 7.76
20 20.93
30 30.16
40 40.11
50 51.07
60 60.88
70 71.84
80 79.41
90 89.01
100 98.80
Table 4.4: The PCA prediction table of biodiesel blend levels.
PCA analysis:
Prediction concentration versus known
concentration for Biodiesel blend fuel
R2 = 0.9988
0
20
40
60
80
100
120
0 50 100
known biodiessel concentration %
pre
dic
ted
%
Figure 4.21: Plot of prediction value versus reference value of biodiesel concentration in diesel.
The Root mean square error (RMSE) is 1.00% and the correlation coefficient is 0.9988.
Chapter 4 Absorption experiments and Calibration
33
4.4.5 Comparisons of Univariate analysis and PCA analysis
An overall listing of the Root mean squared Error (RMSE) of both calibration methods:
Water ethanol mixtures Biodiesel mix fuel
Univariate analysis 0.85% 1.18%
PCA analysis 0.96% 1.00%
Table 4.5: Comparisons of analysis method.
Overall, biodiesel blend level measurements exhibit higher error as compared to water ethanol measurement
due to the large blend level range 0-100% tested. If the blend level is restricted to smaller range as in ethanol,
0-20%, the corresponding error would also decrease.
Principle component analysis (PCA) and univariate analysis of data show similar accuracy for the
measurements conducted above. However, univariate analysis is usually not preferred due to its wavelength
selection limitations. The error of the univariate analysis would also increase when analysing complex
absorbance band due to shifts of absorbance peak.
4.4.6 Advantages of Principle Components Analysis (PCA)
There is no need for a wavelength selection in calibration as is required in univariate analysis and multilinear
regression technique (MLR) [1]. It is relatively easier to obtain more precise results with the help of computer
calculations of principle components.
Since the coefficients of optical data calculated have the same dimension as the number of wavelengths in the
spectrum, it is easy to just relate the entire optical spectrum obtained and predict the analyte concentration [1].
34
CHAPTER 5
Planar concave grating responses
The planar concave grating (PCG) is the wavelength demultiplexer of the spectrometer-on-a chip. In this
chapter, we will study the effects of the resolution and the crosstalk of the PCG demultiplexer. Several grating
transmission spectrums with different crosstalks are compared.
5.1 Planar concave grating (PCG)
The transmission spectrum of the 30 channel Planar Concave Grating demultiplexer is plotted in figure 5.1. By
replacing each of the grating facet with a distributed Bragg reflector (DBR), the on-chip loss is decreased from
6.3dB down to approximately 3dB.
Figure 5.1: Grating transmission spectrum of 30 channels PCG demultiplexer.
The spectral peaks of each output channel are spaced approximately 3.2nm apart spanning from 1502nm to
1594.8nm. The loss of the peak transmission is approximately -3dB with full width half maximum (FWHM) of
the channels is around 1nm. The crosstalk between channels is better than -20dB.
Chapter 5 Planar Concave Grating responses
35
The transmission spectrum of the central wavelength (1550nm) channel is used as a model transmission
spectrum for all 30 channels spaced at 3.2nm apart. This newly generated transmission spectrum for all 30
channels will be used as a grating transmission reference in subsequent crosstalk simulations.
Figure 5.2: Newly generated grating transmission spectrum of the 30 channel PCG demultiplexer.
5.1.1 Crosstalk
Crosstalk is the signal that is undesirable and ideally does not belong to the channel. Crosstalk signals appear as
side lobes to the transmission spectrum and are reproducible from channel to channel. Comparing to the
transmission of optical slab modes without the grating, it is certain that the side lobe structure is due to the
errors in the grating[18]. Thus, crosstalk is highly dependent on the accuracy of the grating definition. In
reality, grating facets deviates from the ideal position and produces phase errors between each facet and its
neighbours. These phase errors further introduce sidemodes and deteriorate the channel spectra [18].
Figure 5.3 (pink curve) shows the ideal response which does not record any crosstalk signals whereas the blue
curve recorded crosstalks of signals outside the ideal spectral range.
grating response of single channel
-50
-40
-30
-20
-10
0
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
real
ideal
Figure 5.3: Qualitative spectrum view of the ideal channel response and the real channel response.
Chapter 5 Planar Concave Grating responses
36
Different crosstalk values are compared to have an idea on the influence on the output waveguide signals.
These output waveguide signals will be a summation of the total signals within the response range of the
channel. An incoming light source transmission spectrum with a narrow dip with FWHM of 5nm as shown in
figure will be superimposed with the grating transmission responses of the 30 channels.
5.1.2 -10 dB crosstalk
Grating transmission with -10dB crosstalk
-14
-12
-10
-8
-6
-4
-2
0
2
1500 1520 1540 1560 1580 1600
w avelength (nm)
optical signal (dBm)
Figure 5.4: Incoming optical signal (blue) superimpose on the grating transmission profile with -10db crosstalk.
Figure 5.4 shows the simulation of a 5nm absorbance band (blue curve) at 1550nm with -10dB loss and the 30
grating responses with -10dB crosstalk. We will simulate the corresponding signal strength of 30 output
waveguides for this -10dB crosstalk grating response.
Signal output for -10dB crosstalk
-12-10
-8-6-4-202
1500 1550 1600
wavelength (nm)
no
rmalise
d o
utp
ut
optical sig
na
l (d
Bm
)
output optical signal
input optical signal
Figure 5.5: Simulated output signals (blue) compared with input signal spectrum (red).
Figure 5.5 shows the normalised output signal for 30 waveguides (blue dots) as compared with simulated input
signal. As we can see, the low transmission (-10dBm) in the region of 1550nm can not be distinguished from
the output signals of the 30 channels (blue dots). This is due to the crosstalk at -10dB that introduces large
amount of noises from other channels. These noises have overwhelmed the true signal and corrupted the
information carrying signal.
Chapter 5 Planar Concave Grating responses
37
5.1.3 -20dB crosstalk
Signal output for grating transmissision with -20dB
crosstalk
-25
-20
-15
-10
-5
0
5
1500 1520 1540 1560 1580 1600
w avelength (nm)
sig
nal (
dB
m)
Figure 5.6: Incoming light profile (blue) superimpose on the grating transmission with -20dB crosstalk.
The same profile of transmission/absorption band of FWHM 5nm with an extinction ratio of 10dB is imposed
with grating responses of crosstalk -20dB as shown in figure 5.6.
signal output for grating transmission with -20dB
-12
-10
-8
-6
-4
-2
0
2
1500 1520 1540 1560 1580 1600
wavelength (nm)
norm
alis
ed o
utp
ut
sig
nal
(dB
m)
output signal
intput signal
Figure 5.7: Simulated output signals (blue dots) compared with input signal spectrum (red).
Figure 5.7 shows the output signals (blue dots) detected from the 30 output waveguides. The low transmission
at 1550nm is detected but with an extinction ratio of approximately -3.2dB. The noise caused by the high
crosstalk has decreased the extinction ratio of 6.8dB. This noise value is too high and has corrupted more than
half of the input signal.
Thus, crosstalk at -20dB is inadequate for transmission analysis of input signal with FWHM of 5nm or less.
Chapter 5 Planar Concave Grating responses
38
5.1.4 Grating response of the spectrometer
Signal output for spectrometer grating transmission
-50
-40
-30
-20
-10
0
10
1500 1520 1540 1560 1580 1600
wavelength (nm)norm
alised s
ignal (d
Bm
)
Figure 5.8: Incoming light profile (blue) superimpose on the spectrometer grating transmission with better than
-20dB crosstalk.
Now we will have a comparison with the grating transmission of the spectrometer chip. The crosstalk of the
grating is better than -20dB. Simulating the response of the incoming signal, we obtained the following signal
output for 30 channels.
Figure 5.9: Simulated output signals (blue dots) compared with input low transmission (red) spectral
information falling on the centre of channel response (left) and at the edge of channel response (right).
Figure 5.9 shows the output signals at 30channels can adequately reproduce the incoming light spectrum. The
input low transmission with an extinction ratio of 10dB at 1550nm is detected as a transmission with decreased
extinction ratio of 8.5dB at the output of the grating. 1.5dB deterioration is present on the signal.
This shows that the crosstalk value of the grating response of the design is sufficiently good to produce an
approximation to the light signal (FWHM = 5nm) with low transmission of -10dB falling on the centre or edge
of channel response.
signal output for spectrometer grating transmission, lower
transmission falling at the centre of channel response
-12
-10
-8
-6
-4
-2
0
2
1500 1520 1540 1560 1580 1600
wavelength (nm)
norm
alis
ed o
utp
ut
sig
nal
(dB
m)
output signal
intput signal
Low transmission (-10dB) of 5nm FWHM falling at the
edge of peak transmission
-12
-10
-8
-6
-4
-2
0
2
1500 1520 1540 1560 1580 1600
wavelength (nm)
no
rma
lis
ed
ou
tpu
t s
ign
al
(dB
m)
output signal
intput signal
Chapter 5 Planar Concave Grating responses
39
5.2 Limitation
The previous simulations were carried out on signal with an extinction ratio of 10dB at 1550nm with FWHM of
5nm. Most absorbance bands in the liquid are broader than 5nm and thus the grating resolution of 3.2nm is
sufficient for detecting liquid absorbance bands. This is true for spectral information (low transmission)
occurring at the centre of the channel transmission.
The worse case scenario is when optical signals contain spectral information less than the grating resolution of
3.2nm, i.e. , a very narrow dip in transmission of FWHM 1nm falling on the edge of the channel response as
shown in figure 5.10 (red curve). The grating would not be able to recover the spectral information. We would
not be able to observe any spectral changes by looking at the output signals of the channels.
Figure 5.10: Simulated low transmission falling on the edge of channel response (left) and corresponding
output signal of the 30 channels (right).
After some straightforward simulations, the spectrometer grating resolution of 3.2nm with a crosstalk better
than -25dB enables sufficiently good reproduction of optical signals containing spectral information more than
3.2nm. This is adequate for spectrometry measurements whereby the absorbance band of analytes are much
broader than the grating resolution.
To be able to better resolve spectral information, the grating channel responses should resemble a flat-top
response.
Signal (FWHM=1nm) falling on the edge of channel
response
-20
-15
-10
-5
0
5
1530 1540 1550 1560 1570
wavelength (nm)
no
rma
lis
ed
sig
na
l (d
Bm
)
Low transmission (-10dB) of 1nm FWHM falling at
the edge of peak transmission
-12
-10
-8
-6
-4
-2
0
2
1500 1520 1540 1560 1580 1600
wavelength (nm)
no
rma
lise
d o
utp
ut sig
nal
(dB
m)
output signal
intput signal
40
CHAPTER 6
Wire Bonding and Electronic read-out board
In this chapter we will describe the wire bonding of the on-chip MSM photodetectors to the ceramic package.
Processing steps to define gold contact pads for the photodetectors and the electronic board will be discussed.
We also designed a back-up bonding mask and electronic board for PIN photodetectors.
6.1 Layout of MSM detectors
There are 30 MSM photodetectors on top of the 30 output waveguide channels. The output waveguides are
spaced 25µm apart. Therefore, this limited space is not sufficient for electrical wire bond pads (wire bond
diameter 32µm). We need to define a fan out from the detector contacts to a bigger wire bond contacts.
Figure 6.1: 30 MSM detectors integrated (vertical straight line) and the Ti/Au contacts on the detectors [13].
The 30 MSM detectors are defined on top of the waveguides in a same line distanced 25µm apart as shown in
figure 6.1. The titanium/gold (thickness of 20nm/200nm) Schottky contacts with a dimension of 22µm x 40µm
are deposited on top of the MSM detectors.
25 µm
Detector 1
Detector 2
Detector 3
Ti/Au contact 1
Ti/Au contact 2
Ti/Au contact 3
40µ
m
Chapter 6 Wire Bonding and Electronic read out board
41
There will be only 31 Ti/Au contacts defined on the same plane for the 30 detectors as opposed to 60 contacts
(2 contacts for each detector) needed. Referring to figure 6.1, the Ti/Au contact 2 will be shared among
detector 1 and detector 2 while Ti/Au contact 3 will be shared by detector 2 and detector 3 and the list goes on
for the rest of the detectors.
6.1.1 MSM photodetector metallization mask
Figure 6.2 below shows the wire bond mask designed for 30 MSM photodetectors. Since the Schottky contacts
are coplanae, there is only one bonding mask needed for metallization of 30 detectors. The Schottkty contacts
of the detectors are connected to bigger contact pads for wire bonding. The wire bond pads are designed to be
250µm x 250µm spaced 50 µm apart.
Figure 6.2: The wire bond mask on MSM detectors.
The dimension of the spectrometer SOI chip is approximately 6mm x 3mm. Therefore, we have chosen Pin
Grid Array (PGA) 68 ceramic package with due to its big die cavity dimension, 1.16cm x 1.16cm that provides
sufficient space to glue the spectrometer chip.
Figure 6.3: The bonding mask for defining wire bond pads (blue) and PGA ceramic package top view.
250µm
250µm 30 MSM detectors
50µm
The layout of wire bonding mask is designed such that it is
compatible with the bonding pad layout in the chosen ceramic
package (See Appendix A for PGA 68 layout). The PGA 68
package has 17 contact pads on each side which gives a total
of 68 pins. The contact pads on the SOI chip are thus designed
to have 7 contacts on the top, 17 contacts on the right and 7
contacts on the bottom.
Chapter 6 Wire Bonding and Electronic read out board
42
6.1.2 PIN photodetector metallization mask
A wire bonding mask is designed for PIN photodiodes for back-up purposes. An InP/InGaAsP PIN
photodetector integrated on top of a SOI waveguide is shown below in figure 6.4.
Figure 6.4: III-V photodetectors bonded on SOI waveguide circuit [14].
The ohmic contacts of the PIN photodetectors have to be defined separately in the processing due to the fact
that these contacts are not in the same plane. Thus, two masks will be needed to define the n-ohmic contact and
p- ohmic contact separately. Figure 6.5 below shows the bonding mask for PIN photodetector contacts.
Figure 6.5: The wire bond mask on PIN detectors.
Two bonding masks are needed where the green mask (figure 6.5) is the common contact pad for p-ohmic
contact and the blue mask is the n- ohmic contacts. Due to the extra processing steps needed define the PIN
detector structure and to define the p-contacts and then the n-contacts, MSM detectors are preferred to PIN
photodiodes to be integrated on the chip.
InP undercladding
(n-doped)
InP uppercladding
(p-doped)
InGaAs absorbing layer
n- ohmic contact p- ohmic contact
30 PIN
detectors
250µm
250µm
Chapter 6 Wire Bonding and Electronic read out board
43
6.1.3 Metallization processing steps
The metallisation process of the photodetectors is carried out in the cleanroom in Zwijnaarde. The processing
include twice lithography, deposition of Ti/Au and electroplating.
Electroplating is needed to increase the thickness of the gold contact pads as thickness of the initial evaporation
of Ti/Au (20/200nm) is not sufficient for wire bonding. Adding the gold thickness by evaporation is expensive
and thus we have opted for gold electroplating.
1st step: Lithography for metallization
The SOI chip with processed III-V photodetectors is first spin coated with positive photo resist (AZ5214) and
put on the hot plate for soft bake and to remove solvents. The wire bonding mask is aligned onto the underlying
photodetectors on the chip with the mask aligner. The chip is then exposed with 300nm light in vacuum contact
mode for 16 seconds. The polymer chains of the exposed photoresist are broken render it vulnerable.
After exposure, the chip is heated on the hotplate at 120° for 2 minutes. This baking process will cross link the
broken polymer chains of the exposed photoresist. The bonding mask is now removed and the whole chip is
subjected to a flood exposure for 40 seconds. This cross linked part of the photoresist is now insoluble with
developer.
The chip is then developed by immersing in developer AZ 400 and water at 1:3 ratio for approximately 22
seconds. The cross-linked photoresist on the chip will remain while the others are washed away. The chip is
rinsed thoroughly in de-ionised (DI) water to stop further development.
The idea of using a positive photoresist and image reversal is to obtain a negative slope profile for the
photoresist. This is advantageous for further processing steps such as lift off of metal layer. After blow drying
the chip, the chip is observed under the microscope to make sure the photoresist pattern is fully developed.
2nd
step: Evaporation of Titanium/Gold
With the photoresist clearly defined, titanium (20nm) is evaporated onto the chip followed by 200nm of gold.
3rd
step: Lift-off Gold
After evaporation, the chip is immersed in acetone to lift off the remaining photoresist and the metal on top of
it. Now the schottky contacts on the MSM detectors, the connectors and big contact pads are defined (refer to
figure 6.2).
Chapter 6 Wire Bonding and Electronic read out board
44
4th
step: Cover the photodetectors with BCB
The chip is spin coated with BCB and cured. This is to protect the photodetectors and metal contacts on top of
photodetectors from oxidation. The BCB that covers the square wire bond pads is further etched away by
reactive ion beam etching.
5th
step: Deposition of Titanium on chip
Titanium is evaporated on the whole chip. This is to provide a conducting platform for further electroplating
process.
6th
step: Lithography + deposition of Ti/Au + Lift off to define big bonding contacts
The first 3 steps are repeated. However, the mask with only the square bonding contacts is used to open the
gold bonding contacts (250µm x 250µm) for added thickness. After lift off, Ti/Au big contact pads are define
for electroplating. This is done because for electroplating, a top later of gold is necessary.
7th
step: Lithography to open squares for gold electroplating
This lithography step using the mask of just square bonding pads is repeated. This is necessary to ensure the
gold is plated within the square bonding pads during the electroplating process.
8th
step: Electroplating of Gold
The chip is now ready for electroplating. The chip is clipped at the cathode of the electrode and immersed in a
mixture of gold salt, potassium gold cyanide (KAuCn2) and water. The solution is heated to 65°C and stirred
with a magnetic rod. Current (3mA) is the applied across the electrodes.
The plating thickness varies proportional with the immersed time and temperature of the solution. After about 2
minutes of electroplating, the chip is removed from the process and rinsed with water. Thickness of more than
800nm is required for good wire bonding. The measured gold thickness achieved from electroplating is
approximately 3 to 5um.
The chip is then dipped in mild hydrofluoric (HF) acid to remove the titanium conducting platform.
Chapter 6 Wire Bonding and Electronic read out board
45
Figure 6.6: The flow of defining bonding pad contacts.
Figure 6.7: Added thickness on the wire bonding pads (brown).
1st lithography step
Evaporation of Titanium (20nm) followed by
Gold (200nm)
Lift off of Ti/Au
2nd
lithography step + evaporationg of Ti/Au (20nm/200nm)
+ lift off
Electroplating
Dip in diluted HF to remove titanium
Coat with BCB + etching +
deposition of Ti (conductive platform)
Chapter 6 Wire Bonding and Electronic read out board
46
6.1.4 Wire bond test
The SOI chip with electroplated contact pads is glued onto the ceramic package with epoxy glue and cured for
robustness.
Figure 6.8: Top view of the spectrometer chip glued onto the ceramic package.
Aluminium wires of 32µm are used for wire bonding from SOI chip to package. An ultrasonic welding process
is used to make a bond in which a combination of vibration and force to effectively scrub the interface between
wire and substrate is used. This cause a very localised temperature rise, promoting the diffusion of molecules
across the boundary. Figure 6.9 shows the wedge being lowered to make a wire bond onto the contact pads.
Figure 6.9: Lowering the bonding wedge to make the bond.
The dimension of the bonding pads on the SOI detectors are 250um x 250um. The aluminium wire is first
bonded onto the gold pads of the ceramic package and then to the gold bonding pads on the chip.
The ceramic package provides a robust wire bond platform and thus the bonding starts from the package bond
pads. The bonded wire is then pulled from the bond pad on the package to the bond pad on the chip and with
Chapter 6 Wire Bonding and Electronic read out board
47
the right force, the bond wedge is lowered, pressed against the bond pads and terminate the wire bond
connection. This procedure is to prevent the gold bonding pads on the SOI chip to be lift off during the pulling
of the wires.
Figure 6.10: Successful bonding (left) and lift off of bonding pad (right).
Figure 6.10 shows the wire bonds on the SOI chip. One of the bond pads was lifted off together with the wire
after the termination of the bond. This is due to bad adhesion of the Ti/Au contacts with the underlying SOI
substrate.
Figure 6.11: Complete wire bond chip.
Figure 6.11 shows a complete bonded SOI chip on a package. Two of the contact pads on the chip were lifted
off during the blow drying process of the chip. Again, this is caused by the bad adhesion of the Ti/Au contact
pads with underlying BCB layer.
Adhesion could be improved by ensuring cleanliness of the chip before defining the contacts. Subjecting the
chip to 1 minute oxygen plasma to roughen the surface of BCB before defining the Ti/Au contacts can further
improve the adhesion of Ti/Au on BCB.
Chapter 6 Wire Bonding and Electronic read out board
48
6.2 Electrical read-out board
A photodetector generates a photocurrent which is proportional to the incident light intensity. For this
spectrometer chip, we will supply a bias voltage to the detectors to sweep out the carriers. And since we are
accustomed to measure electronic signals in terms of voltage, we will use a current to voltage converter o
produce a voltage reading proportional to the photocurrent generated by the detector.
There are 30 photodetectors integrated on the 30 output waveguides of the grating respectively. Each detector
measures a part of the diffracted light spectrum from the grating spanning from 1500nm to 1600nm. We design
2 boards for MSM photodetectors and PIN photodetectors respectively. The circuit have the same switches and
transimpedance circuit except with the different biasing path for the detectors.
6.2.1 Schematic of voltage read-out circuit
The idea is to read out the voltage signals of 30 photodetector. This can be done by sequentially switching
photocurrent generated from the 30 detectors to the current to voltage converter. The ceramic package carrying
the chip is plugged into a socket instead of soldering directly onto the board. This is done for the ease of testing
different samples.
The basic schematic for electronic read out circuit is shown in figure 6.12 (See Appendix B for complete
electronic schematic layout). The schematics and PCB board is drawn with Eagles 4.12r2 Light software.
Figure 6.12: Schematic of electronic read out circuit with switch and current to voltage converter.
Chapter 6 Wire Bonding and Electronic read out board
49
The photocurrent from the detectors can be selected by inserting analogue switches between them and the
current to voltage converter. The selection bits of the switch are controlled by the parallel port data out pins of
the computer. The photocurrent from each detector is then selected by sending the right selection bits to the
switch. A labview vi is used to interface and control the parallel ports pins (See Appendix C for selection bits
and Labview vi).
A bias voltage is supplied to the detector to sweep out the carriers generated by and the photocurrent is
converted into an output voltage by the OpAmp transimpedance circuit. A multimeter is used here to record the
output readings.
Two biasing schemes are needed for each MSM and PIN photodetectors due to the different layout of the
contacts for both the detectors (full schematics can be found in Appendix B).
6.2.1.1 MSM photodetector board
MSM detectors can be seen as back to back connected Schottky diodes. As these detectors are symmetric, the
current-voltage (I-V) characteristics should also be symmetric. So the polarity of the bias voltage is not
important. The bias scheme proposed has alternate biasing polarity for alternate MSM detectors.
Figure 6.13: Schematic of MSM detector biasing circuit.
Figure 6.13 shows the biasing scheme for the MSM detectors. The first MSM detector is positively biased
through the upper switch and the photocurrent will flow through the lower switch to the transimpedance circuit.
The second MSM detector will be negatively biased due to sharing of a contact with the first detector. The
Chapter 6 Wire Bonding and Electronic read out board
50
photocurrent of the second MSM detector will have opposite sign to the first MSM detector. This is designed
considering the responsivity of MSM detectors is symmetrical for positive ad negative bias.
6.2.1.2 PIN photodetector board
Figure 6.14: Schematic of PIN detector biasing circuit.
Figure 6.14 shows the biasing scheme for PIN photodetectors. The PIN photodetectors have a common ohmic
contact that can be biased (V_bias) at the same time. The photocurrent of each detector is switched to the
transimpedance circuit respectively. For these detectors, the polarity is important and the photodiodes should be
inversely biased.
6.2.2 Selection of electronic components
For the board of MSM photodetectors, four CMOS 4052 8x1 analogue switches and one CMOS 4053 4x1
analogue switch are used to sequentially switch the photocurrent from the photodetector. The CMOS family of
analogue switches are chosen for their good performance with low “OFF” leakage currents (10pA) and low
“ON” resistance (80Ω). They provide logic level conversion for digital addressing signals and also dissipate
extremely low power over full supply voltage range, up to 15V [19].
As we know, electronic components are not ideal and the operational amplifier (OpAmp) used in the
transimpedance circuit will introduce some offset voltage to the output readings. In reality, the input resistance
of the opamp is not infinite. This allows a small amount of current to flow into the opamp. This current will be
further amplified by the negative feedback gain.
Chapter 6 Wire Bonding and Electronic read out board
51
Furthermore, the dark current of the photodetector can impose error on the I-V gain of the circuit and produce
DC offset error in the converter’s output. This dark current offset error will be negligible for larger currents but
can cause considerable error in smaller currents. Since we are operating in a very low current region, from 5nA
to 10µA, this offset error could cause considerable error in the output voltage.
The simplest way to minimize the offset error would be to choose an OpAmp with low DC offset voltage and
low input bias current. By setting an error budget for the voltage read out from the transimpedance circuit, we
can choose a suitable OpAmp.
We will set our error budget to 1% of the output voltage. That means that for maximum output voltage of 1V,
we will have ± 0.01 V error. First we will determine the necessary open loop gain for this error budget.
OpAmp gain error is defined to be
CL
OL
A
Ae
+=
1
1; where AOL is the open loop gain and ACL is the closed loop
gain. Thus, the open loop gain is ( )11 −=e
AA CLOL .
For our design, the close loop gain, ACL is 100k (feedback resistance 100k) and error is 0.01. Hence, the
calculated minimum open loop gain, AOL has to be more than 70dB.
We have chosen OPA 132 as the OpAmp for the transimpedance circuit as it has an open loop gain of 120dB,
very low input bias current of 50pA and low input offset voltage of 0.5mV. Error will be calculated for this
model. The sources of offset voltage are as follow:
a) Dark current of photodetector
A dark current of 10nA will impose an offset voltage of 1mV.
RIV darkDoff ×−=_
b) Input Bias Current OPA 132
The low input Bias current of the OPA 132 of 50pA will induce an offset of 5µV which will be negligible.
c) Input offset voltage of OPA 132
Input offset voltage of the OPA 132 is 0.5mV and thus is within the 1% error region.
Chapter 6 Wire Bonding and Electronic read out board
52
The theoretical offset voltage is within tolerance range and the OPA 132 is suitable to be used in the
transimpedance circuit.
Figure 6.15 shows the PCB board layout of the circuit.
Figure 6.15: PCB board layout.
6.2.3 Error in output voltage reading
A voltage to current converter (transimpedance) circuit with a feedback resistance, R of 100kΩ is chosen here.
Figure 6.16: Schematic of transimpedance circuit with feedback resistance of 100kΩ.
The output voltage with respect to the incoming photocurrent is calculated as:
RIV pdout ×−=
Below is the expected photocurrent from the detectors and the converted output voltage:
Socket for
SOI chip
Analogue
switches
Supply
pins
Current to voltage converter
Parallel port connected to PC to control switches (labview .vi) interface
Chapter 6 Wire Bonding and Electronic read out board
53
Photodetector State Photocurrent I-V converter output voltage
dark 5nA 0V
bright 10µA 1V
Table 6.1: Expected photocurrent versus output voltage.
The dark current of the detector is expected to be in range of 5nA to 10 nA. When light is illuminating on the
detectors, 10µA is the expected average generated photocurrent. With a feedback resistance of 100kΩ, the
corresponding output voltage expected would be 1V. After obtaining the electrical board, the I-V gain curve is
tested on the transimpedance circuit by injecting current from 10nA to 50µA. Figure 6.17 shows the I-V gain
curve from 10nA to 10 µA.
Voltage-Current gain curve
y = 100.01x - 0.4776
-10
190
390
590
790
990
0 2 4 6 8 10
Current (uA)
Volta
ge (
mV
)
Voltage-Current gain curve
y = 100.01x - 0.4773
0.01
0.1
1
10
100
1000
10000
0.001 0.01 0.1 1 10 100
Logscale Current (uA)
Logscale Voltage (mV)
Figure 6.17: Plot of I-V gain curve of transimpedance circuit of the board, linear (left), logscale (right).
The transimpedance circuit exhibits good linear current to voltage conversion starting from current as low as
0.1µA. In the region of operation from 0.1µA to 10 µA, the linear regression fit gives Voltage (mV) =
100.01*current (µA) -0.4773. The offset voltage is approximately 4.7mV.
Thus for 1µA photocurrent generated, the conversion voltage by the transimpedance circuit yield 99.53mV.
When compared with ideal conversion voltage of 0.1V (gain=100000), an error of 0.5% is obtained.
54
CHAPTER 7
Experiments on Spectrometer-on-a-chip and the electrical
board
Experiments are carried out to characterise the 30 metal-semiconductor-metal (MSM) photodetectors integrated
on the output waveguides from the concave grating and also to read out the photocurrent and voltage output
from the electrical board. It is essential to check the functionality of all the 30 detectors and characterise their
dark currents and responsivities. We will then measure the corresponding electrical output from the detectors
and analyse the performance of the board.
There are 30 MSM detectors integrated on top of the 30 output waveguides from the grating. This filter splits
the incoming light into 30 spectral components that is subsequently detected by the 30 detectors. Thus, each
photocurrent detected by the MSM detector is proportional to the optical power of different spectral component
of the light spectrum.
7.1 Measuring the dark current of the detectors
Under no external photon injection, there should be ideally no electron-hole pair generation in the
photodetector and hence no current flow under a constant voltage bias. However, when the detector is subjected
to a voltage bias, there is a measure of small current in the absence of light. This small current is termed as the
dark current.
Dark current is due to thermal electron-hole pair generation in the material. It is also viewed as one of the
contributing noise factor in a detector. Dark current imposes an offset signal in light detection and the value of
dark current increases with temperature. Hence, a low dark current is desirable in a detector.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
55
We will measure the dark currents of the 30 detectors by biasing the MSM detectors with increasing voltage.
The dark current value can also indicate whether the detectors and its connecting paths are functioning.
7.1.1 Experimental result
The 30 photodetectors with defined Ti/Au schottky contacts and the connecting wire bond contact pads are
subjected to a voltage bias. This is achieved by probing two needles onto the two gold contacts on the ceramic
which are bonded to the detector and supplying a voltage bias from the voltage source.
Dark current versus biasing voltage
0
1
2
3
4
5
6
7
0 2 4 6 8
biasing voltage (V)
dark current (nA)
detector 16
Figure 7.1: The measured dark current versus biasing voltage for the detector 16 (1550nm).
Figure 7.1 shows a plot of dark current of detector 16 with biasing voltage from 0V to 6V. The dark current
increases as bias voltage increases. The plot of dark currents of all 30 MSM detectors for a bias voltage at 6V is
shown below in figure 7.2.
Dark current of 30 detectors versus biasing voltage of
6V
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 10 20 30
detectors
dark
curr
ent lo
g s
cale
(mA
)
Figure 7.2: The measured dark current at 6V bias for 30 detectors.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
56
From figure 7.2, we notice that some detectors exhibit extremely high or low dark current relative to others. As
the dark current expected at 6V bias for a functioning MSM detector is around 6nA to 10nA range, we would
render the detectors with abnormal dark current readings as malfunctioning.
The detectors with extremely high dark current and saturates to 0.1mA are suspected to be short circuited. This
could be due to the shorted contact pads between the detectors. Some detectors also showed nearly no
connection and the recorded dark current is very low at less than 0.6nA. This situation resembles an open
circuit where the bonding contacts might be lifted off and were not connected to the detectors.
From the dark current measurement we concluded that the 9 detectors are short circuited and 5 detectors are
disconnected by the bonding pads. Hence only 16 out of 30 detectors on the chip are functioning and subjected
to measurements.
7.2.2. Defect of detectors
The figure 7.3 below shows the detectors with the corresponding connecting pads.
Figure 7.3: Bonding pads of 30 detectors on a package.
Referring to the figure above, the first detector is connected from the top right first bond pad to the top right
second bond bad. By rotating from top right clockwise to the top left square wire bond pad, we would get 30
detectors.
Detector 3, 4 and detector 29, 30 are not connected to the package due to lift off of the contact pads during
processing. Detectors 5,6,7,8 and detectors 24, 25,26,26,28 are short-circuited.
Detector 2
Detector 1 Detector
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
57
7.2 Measuring the responsivity of detector
Responsivity, R is one of selecting criteria for a detector where it measures the ratio of detector output to the
input light signal. The external responsivity can be defined as:
)(___
)(
)(
)( detdet
WfibreinputpowerOptical
AI
WP
AIR ecter
fibre
ector
ext ==
where Idetector is the output current from the detector and Pfibre is the optical power of the input fibre.
Responsivity can also be written as 24.1
][ m
hv
qR extextext
µληη == , where ηext is the external quantum efficiency, q
is the coulomb charge, h is the Plank’s constant and v is the frequency of incident light. From the relation we
can see that responsivity is dependent on the incoming light wavelengths and also the external quantum
efficiency of the detector. External quantum efficiency is defined as
fibreinputinphotonsincident
ntphotocurretocontributepairsheext
____
____−=η . ηext of 1 assumes all incident photons contribute to the
photocurrent .
7.2.1 Experiment result
Tunics tunable laser is used as the light source in this experiment. Light is injected onto the grating chip via a
fibre grating coupler from a single mode fibre held at 10° incidence angle. A voltage bias from -6V to 6V is
then applied across the detector bonding pads and the corresponding photocurrent is measured.
Figure 7.4: Experiment set up with spectrometer chip and electrical board.
The current voltage (I-V) curve of detector 23 which corresponds to a wavelength of 1570nm is measured and
shown in figure 7.5.
Parallel port cable
Analogue switches Voltage supply pins and output pins
Input fibre
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
58
I-V curve of detector 23
-4.E-02
-2.E-02
0.E+00
2.E-02
4.E-02
6.E-02
8.E-02
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
bias voltage (V)
measure
d c
urr
ent
(mA
)
0.2625 mW
0.525 mW
0.7875 mW
1.05 mW
Figure 7.5: The measured photocurrent for detector 16 from -6V to 6V bias.
Figure 7.5 shows I-V curve for detector 23 for voltage bias of -6V to 6V for different input optical power of
fibre ranging from 0.26mW to 1mW. Considering the positive bias region, the photocurrent is observed to
increase with increasing voltage bias less that 3V but reaches constant as the bias increases more than 4V. The
external responsivity of detector 23 (1570nm) at bias voltage 6V is measured to be 0.0627 A/W.
The asymmetrical responsivity for detector 23 (1570nm) is observed where the responsivity in the positive bias
region differs from the negative bias region. We do not expect this as MSM detector exhibits a symmetrical
responsivity. A possible explanation to this could be due to processing error when defining the two schottky
contacts. It is possible that the area of two Ti/Au metal contacts deposited on the detector is not identical and
may be shifted to the left or right. This could produce a preference biasing direction for the detector.
By tuning the laser to the peak response wavelength of each detector, i.e.1570nm for detector 23 and 1566.9nm
for detector 22, the I-V curve (same as figure 7.5) for detectors 9 to 23 with different optical power of input
fibre is measured. The corresponding external responsivity for the photodetectors (detector 9 to detector 23) at
6V bias are measured and plotted in figure 7.6.
External responsivity of detectors
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
detectors
R ext (A/W)
Figure 7.6: The external responsivity of 15 functioning photodetectors (9 to 23) at bias voltage 6V.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
59
As each detector records optical power of different wavelength channels, figure 7.6 can be plotted against the
spectrum axis as below in figure 7.7.
External responsivity of detectors
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
R ext (A/W)
Figure 7.7: The external responsivity of 15 functioning photodetectors (9 to 23) plotted against wavelength.
The fibre coupler on-chip has a peak response around 1550nm. However, the peak response will be shifted to
longer wavelengths of around 1580nm taking into account the BCB layer deposited on the SOI chip which
increases the effective refractive index. Figure 7.8 shows the transmission response for fibre coupler (brown),
fibre coupler with BCB top layer (blue), PCG transmission (blue dots) and total response of fibre coupler
(BCB) and PCG (red curve).
Transmission versus wavelength
-22
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
1500 1520 1540 1560 1580
wavelength (nm)
Tra
nsm
issio
n (
dB
)
PCG
fibre coupler
fibre coupler+BCB
f_coupler+BCB+PCG
Internal responsivity of MSM detector
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1520 1530 1540 1550 1560 1570 1580
w avelength (nm)
Inte
rnal r
esponsiv
ity (
A/W
)
Figure 7.8: The fibre coupler response curve (brown), possible shifted response with BCB top layer (blue) [10],
the PCG transmission response (blue dots) and the shifted fibre coupler + PCG response (red). The left figure
shows the internal responsivity of MSM detector from 1520nm to 1580nm [13].
Since the detectors circuit from 1570nm onwards are not functioning, it is difficult to compare the experiment
transmission response to the total transmission response of the system (fibre coupler, PCG and MSM detector).
However, by observing the transmission response trend of fibre coupler and PCG and also taking into account
the internal responsivity of the MSM detectors reported in [13], we can conclude that the signal transmission
could be increasing towards the region of 1580nm. The responsivity (figure 7.7) from 1520nm to 1570nm does
resemble part of the coupling and PCG response with increasing transmission towards 1580nm region.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
60
External Responsivity can be expressed as fibreinputP
AntPhotocurreRext
__
)(= whereas Internal Responsivity (internal
quantum efficiency =1) is expressed as waveguideoutputP
AntPhotocurreR
__
)(int = .Hence, Rint is calculated as
waveguideoutputP
fibreinputPRR extInt
__
__*= .
We will calculate the internal responsivity of the MSM detector by doing a calculation at 1570nm and
assuming this is the peak transmission wavelength for the fibre coupler. On chip loss of PCG is approximately
4.6 dB at 1570nm and the fibre coupling loss at 1570nm is taken to be 6dB (peak response). The insertion loss
is thus approximately 10.6dB. For input fibre optical power of 1mW, the output waveguide optical power
would be 0.087mW. The external responsivity measure from the experiment at 1570nm is 0.062 A/W.
Hence, the internal responsivity of MSM detector is 0.71 A/W. This is less than the expected internal
responsivity of 1 A/W [13]. This might be due to the underestimation of fibre coupling losses at 1570nm which
could be higher than 6dB. Another possibility is the layer of BCB deposited on top of the spectrometer chip.
This decreases the refractive index contrast of the etched grating facets. This can result in a higher facet
reflection loss and hence, a higher on-chip loss.
7.3 Measuring the output current and voltage from the printed circuit board
Now we tune the laser to the peak transmission of each detector and supply with a constant input optical power
of 1mW. The electronic board then supply bias voltage of 5V to the detectors and the selection of detectors is
done through the parallel port control pins. The photocurrent from each detector is measured from 1500nm to
1600nm and plotted as in figure 7.9.
Photocurrent versus wavelength plot at 6V
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
7.E-02
8.E-02
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
current (m
A)
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
Figure 7.9: The photocurrent versus wavelength plot for 15 detector 9 to detector 23.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
61
It is observed from figure 7.9 that the photocurrent measured by the detectors shows alternate peaks whereby
detector 23 has high responsivity while detector 22 has lower responsivity and followed by detector 21 with
high responsivity. This again shows that there might be processing error that renders the MSM detectors with
asymmetrical responsivities for different biasing voltage polarity. The read out circuit is designed such that
detectors are biased at alternate voltage polarity, i.e. 5 V for detector 23 and -5V for detector 22 etc.
Photocurrent versus wavelength plot
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
7.E-02
8.E-02
9.E-02
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
Photocurrent (m
A)
det 23
det 22
det 21
det 20
det 19
det 18
det 17
det 16
det 15
det 14
det 13
det 12
det 11
det 10
det 9
Figure 7.10: Optimised photocurrent versus wavelength plot for detector 9 to detector 23.
Figure 7.10 shows the optimised plot where the bias voltage is reversed to give a higher responsivity and
smooth photocurrent spectrum. The photocurrent recorded by detector 21 (1564.8nm) exhibits very high offset
current (5µA). This detector registered very high dark current in previous measurement (section 7.1.1) of
approximately 1µA. Thus we can conclude this detector has very high offset current due to leakage current and
is not operating in a good condition.
From figure 7.10, we can also conclude that the MSM detectors are measuring the peak transmission at
wavelength which agrees with the PCG diffracted wavelength channels. The peak transmissions recorded by
the detectors are spaced approximately 3 nm apart. The photocurrent trend also agrees well with the fibre
coupler and PCG transmission response where the peak coupling efficiency is approximated to occur around
1580nm and roll off towards 1520nm.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
62
7.3.1 Crosstalk
From figure 7.11, we can notice the presence of crosstalk with similar crosstalk profile compared with the PCG
transmission (refer figure 5.3). The photocurrents are plotted in log scale in the figure 7.11 below.
Photocurrent versus wavelength plot
0.0001
0.001
0.01
0.1
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
Logscale Photocurrent (mA)
det 23
det 22
det 21
det 20
det 19
det 18
det 17
det 16
det 15
det 14
det 13
det 12
det 11
det 10
det 9
Figure 7.11: Photocurrent (log scale) versus wavelength plot for detector 9 to detector 23.
From figure 7.11, it is noticed that the crosstalk for detector 23 of 1570nm is -11.23dB. As compared with
grating response before packaging in figure 7.12 below, the crosstalk has increased from -20dB to -11.23dB.
This increase in crosstalk can be introduced during the packaging of the spectrometer chip. The optical signal
losses could increase during integration of MSM detectors and metallization of wire bond contacts onto the
chip. The electronic board might also introduce losses to the optical signal and measure lower photocurrent
readings.
Grating transmission before packaging
-30
-25
-20
-15
-10
-5
0
1520 1530 1540 1550 1560 1570 1580
waveength (nm)
dB
Figure 7.12: PCG transmission before packaging.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
63
7.3.2 Voltage reading from the printed circuit board
We tuned the laser to the peak transmission wavelength of the channel/ detector 23, and supply with a constant
input optical power of 1mW. We then read out the voltage reading from detector 9 to detector 23 (15 data
points). The procedure is then repeated for each of the detector. The output voltage from the transimpedance
circuit is plotted in figure 7.13.
transimpedance output voltage
-50
0
50
100
150
200
1520 1530 1540 1550 1560 1570 1580
w avelength (nm)
Vo
ut
(mV
)
23
22
21
20
19
18
17
16
15
14
13
12
11
10
Transimpedance Voltage output
0
50
100
150
200
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
Voltage (mV)
Figure 7.13: Transimpedance output voltage reading, peak voltage output for detector (right).
Figure 7.13 records the voltage output proportional to the photocurrent of the detector with a gain of 100000
(feedback resistor = 100kΩ).
Photocurrent comparison
0.00001
0.0001
0.001
0.01
0.1
1
1520 1530 1540 1550 1560 1570 1580
wavelength (nm)
logscale Current (m
A)
Ipd
Ipd_Vout_conversion
Figure 7.14: Comparison between photocurrent converted from transimpedance voltage output (pink dots) with
photocurrent measured previously (blue dots) with same supply optical power.
The output voltage measured after the transimpedance circuit with an input optical power of 1mW is converted
into photocurrent (pink dots) R
VI = , where R=100000 is the gain of the transimpedance circuit. The value is
compared with photocurrent measure in the previous section (blue dots) and can be seen in figure 7.14.
Chapter 7 Experiments on Spectrometer-on-a-chip and the electrical board
64
The photocurrent from both measurements is supplied with the same input optical power of 1mW. However,
the photocurrent measurement on the second day differs from the measurement value before by a factor of 100.
This is not expected as same input optical power of the fibre is supplied to the spectrometer chip.
Due to accidental damage done to the spectrometer chip, the transimpedance output voltage cannot be
reproduced and verified. However, possible explanation to this disparity can be due to not optimised alignment
and measurement. The tunable laser might not be tuned to the optimum wavelength of the detector before
conducting the voltage measurement. Furthermore, voltage readings are collected from each of the detector
which has a spectral separation of 3.2nm. Thus the voltage reading points are separated by 3.2nm. Information
is lost in between this separation and possible higher voltage readings might be obtained in this range.
Overall, we have measured the external responsivity of the MSM detectors and also measured the photocurrent
from the electrical board. The transimpedance circuit is measured to have an offset voltage of 4.7mV.
The photocurrents measured from the electrical board have transmission response that agrees with the
transmission response of the fibre coupler and PCG. However the crosstalk after packaging has increased from
-20dB to -11.23dB. Crosstalk of -11dB can corrupt the optical signal information containing spectral
information less than 5nm.
65
CONCLUSION AND PERSPECTIVE
The project has presented a packaged spectrometer on a chip with electrical read out signals. Metallization of
Ti/Au schottky contacts on the detectors and electroplating for added bonding pad thickness is carried out for
wire bonding the detectors to electrical board. The dimension of bonding pads of more than 150µm x 150 µm
and thickness of bond pads of more than 800nm is needed to ensure successful wire bonding. Electrical board
has been designed for the electrical read out of photocurrent with switches and transimpedance circuit.
Two schemes have been designed for MSM and PIN photodetectors respectively. However, only the MSM
detectors integrated on the spectrometer chip has been tested. The experiments measure the external
responsivity of the peak transmission (1570nm) to be 0.062 A/W and calculated internal responsivity is 0.71
A/W.
The electrical board is able to record the photocurrents from the MSM detectors. The spectral resolution of the
detector is spaced approximately 3 nm apart which agrees with the transmission response of the PCG before
packaging. Thus, the spectrometer-on-a chip can be operated with the electrical board. However, the crosstalk
after packaging has increased from -20dB (before packaging) to -11.23dB. This crosstalk value can corrupt
optical signal with spectral information less then 5nm. The increase in crosstalk could be due to losses
introduced during packaging. This required further verification. Overall, the transimpedance circuit shows
reliable current to voltage conversion whereby the offset voltage is 0.47mV for input photocurrent of 1µA to
10µA.
Near infrared absorption experiments have been carried out to study the absorbance bands of water in ethanol
and also blend levels of biodiesel in conventional fuel. Straightforward calibration techniques have been
studied for predictions of analyte concentration. These are among the applications of spectrometer on a chip of
this project. However, we have not tested the absorption measurements with the spectrometer-on-a-chip.
Perspectives
• The increase in crosstalk of the spectrometer-on-a-chip after packaging should be investigated.
• Electrical read out board integrated with amplifiers would be advantageous to measure photocurrents
of detectors. An automated voltage read out with respect to the detectors can be implemented.
• The input single mode fibre can be glued to the chip to present a more robust package.
• It would be advantagoues to measure absorption of liquids with the packaged spectrometer.
66
Appendix A
Ceramic package Pin Grid Array (PGA) layout and connecting pins
Figure A.1: Bottom view of the package pins. Figure A.2: Top view of the PGA 68 package.
Figure A.3: Table of connection pins of the PGA 68 package by Globalchipmaterials.
67
Appendix B
Schematics of electrical read out circuit.
B.1: MSM detectors circuit
Figure B.1: Schematic circuit for MSM detectors.
68
B.2: PIN detectors circuit
Figure B.2: Schematic circuit for PIN detectors.
69
Appendix C
C.1: Analogue switching bits of detectors.
MSM detectors PIN detectors
Figure C.1: Tables of analogue switching pins for MSM detectors (left) and PIN detectors (right).
70
C.2: Labview vi. for switching the MSM detectors
Figure C.2.1: Front panel of Labview vi. for switching MSM detectors.
Figure C.2.2: Subroutine vi. for out port and in port (Sending bits to parallel port).
Figure C.2.3: MSM detector switching vi.
71
C.3: Labview vi. for switching the PIN detectors
Figure C.2.4: Front panel of Labview vi. for switching PIN detectors.
Figure C.2.5: PIN detector switching vi.
72
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14. G. Roelkens, P. Dumon, W. Bogaerts, D. V. Thourhout and R. Baets, “Efficient silicon-on-insulator
fiber coupler fabricated using 248nm deep UV lithography”, Photonics Technology Letters, Vol. 17,
Issue 12, pp. 2613-1615, 2005.
15. J. Brouckaert, W. Bogaerts, P. Dumon, D. V. Thourhout and R. Baets, “Planar Concave Grating
Demultiplexer Fabricated on a Nanophotonic Silicon on Insulator Platform”, J. Lightwave technology,
Vol. 25, No. 5, pp. 1269-1275, 2007.
16. J. Brouckaert, G. Roelkens, D. V Thourhout and R. Baets, “Compact InAlAs-InGaAs Metal
Semiconductor Metal Photodetectors Integrated on Silicon on Insulator Waveguides”, IEEE Photonics
Technology Letters, Vol. 19, No. 19, pp. 1484-1486, 2007.
17. J. Brouckaert, G. Roelkens, D. V. Thourhout and R. Baets, “Thin-Film III-V Photodetectors Integrated
on Silicon-on Insulator Photonic ICs”, J. Lightwave Tech., Vol. 25, No. 4, pp. 1053-1060, 2007.
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18. S. Cho, H. Chung, Y. A. Woo and H. J. Kim, “Determination of Water Content in Ethanol by
Miniaturized Near-Infrared (NIR) system”, Bull. Korean Chem. Soc., Vol. 26, No. 1, pp. 115-118,
2005.
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23. J. Brouckaert, W. Bogaerts, S. Selvaraja, P. Dumon, R. Baets, and D. V. Thourhout, “Planar Concave
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List of Figures
1.1: Diatomic oscillator with masses m1 and m2…………………………………………………… 2
1.2: Energy diagram of an ideal diatomic oscillator and anharmonic diatomic oscillator………… 3
1.3: Transmission through sample contained in a cuvette………………………………………….. 4
2.1: Schematic view of spectrometer on a chip…………………………………………………….. 6
3.1: Basic grating configuration…………………………………………………………………….. 9
3.2: Schematic structure of the aluminium grating spectrometer…………………………………… 9
3.3: Schematic view of compact planar spectrometer………………………………………………. 10
3.4: The experimental set up of compact planar spectrometer……………………………………… 11
3.5: Waveguide concave grating spectrometer……………………………………………………… 11
3.6: Diffraction grating layout for fibre to waveguide coupling……………………………………. 12
3.7: Coupling efficiency for fibre coupler with air interface and 630nm grating pitch……………... 13
3.8: Planar Concave Grating based on Rowland configuration…………………………………….. 13
3.9: Schematic view of waveguide integrated MSM detector………………………………………. 14
4.1: Cuvette holder used in experiment……………………………………………………………... 16
4.2: SLED output spectrum…………………………………………………………………………. 17
4.3: Transmission through different water ethanol mixtures contained in a cuvette………………... 17
4.4: Normalised absorbance curve of water ethanol mixtures (left) and gaussian fit curve (right)…. 18
4.5: Normalised absorbance spectrum of different blends of biodiesel in conventional fuel……….. 19
4.6: Transmission spectrum of different blends of biodiesel and conventional fuel…………………20
4.7: Error free single absorbance band [1]………………………………………………………… 20
4.8: Calibration line in univariate analysis [1]………………………………………………………. 21
4.9: Absorbance band of multivariate case (more than one wavelength) [1]……………………….. 22
4.10: Example of 4 absorbance spectra for a m wavelength points…………………………………... 23
4.11: Principle Component Flow Chart [1]……………………………………………………………25
4.12: Univariate calibration line calculated for water ethanol mixtures at 1450nm………………….. 26
4.13: Prediction versus known water concentration in ethanol mixtures at 1450nm………………… 27
4.14: Eigenvalues of calculated from the sum of cross product matrix………………………………. 27
4.15: Plot of concentration of water in ethanol versus Scores………………………………………... 28
4.16: Prediction versus known water concentration in ethanol mixtures using PCA………………… 29
4.17: Calibration line calculated for biodiesel conventional fuel mixtures at 1664nm………………. 30
4.18: Prediction for biodiesel conventional fuel mixtures at 1664nm……………………………….. 31
4.19: Plot of eigenvalues of PC (left) and plot of scores (right) for biodiesel blend levels………….. 31
4.20: Plot of biodiesel concentration in conventional fuel versus Scores……………………………. 32
75
4.21: Plot of prediction value versus reference value of biodiesel concentration in diesel………….. 32
5.1: Grating transmission spectrum of 30 channels PCG demultiplexer……………………………. 34
5.2: Newly generated grating transmission spectrum of 30 channels PCG demultiplexer…………. 35
5.3: Qualitative spectrum view of the ideal channel response and the real channel response……… 35
5.4: Incoming optical signal (blue) superimpose on the grating transmission profile with -10db
crosstalk………………………………………………………………………………………….36
5.5: Simulated output signals (blue) compared with input signal spectrum (red)……………………36
5.6: Incoming light profile (blue) superimpose on the grating transmission with -20dB crosstalk… 37
5.7: Simulated output signals (blue dots) compared with input signal spectrum (red)………………37
5.8: Incoming light profile (blue) superimpose on the spectrometer grating transmission with better
than -20dB crosstalk……………………………………………………………………………. 38
5.9: Simulated output signals (blue dots) compared with input low transmission (red) spectral information
falling on the centre of channel response (left) and at the edge of channel response (right)……38
5.10: Simulated low transmission falling on the edge of channel response (left) and corresponding output
signal of the 30 channels (right)…………………………………………………………………39
6.1: 30 MSM detectors integrated (vertical straight line) and the Ti/Au contacts on the
detectors [13]…………………………………………………………………………………….40
6.2: The wire bond mask on MSM detectors………………………………………………………... 41
6.3: The bonding mask for defining wire bond pads (blue) and PGA ceramic package top view….. 41
6.4: III-V photodetectors bonded on SOI waveguide circuit [14]…………………………………... 42
6.5: The wire bond mask on PIN detectors………………………………………………………….. 42
6.6: The flow of defining bonding pad contacts…………………………………………………….. 45
6.7: Added thickness on the wire bonding pads (brown)…………………………………………… 45
6.8: Top view of the spectrometer chip glued onto the ceramic package……………………………46
6.9: Lowering the bonding wedge to make the bond……………………………………………….. 46
6.10: Successful bonding (left) and lift off of bonding pad (right)…………………………………... 47
6.11: Complete wire bond chip……………………………………………………………………….. 47
6.12: Schematic of electronic read out circuit with switch and current to voltage read out circuit….. 48
6.13: Schematic of MSM detector biasing circuit……………………………………………………. 49
6.14: Schematic of PIN detector biasing circuit……………………………………………………… 50
6.15: PCB board layout……………………………………………………………………………….. 52
6.16: Schematic of transimpedance circuit with feedback resistance of 100kΩ………………………52
6.17: Plot of I-V gain curve of transimpedance circuit of the board…………………………………. 53
7.1: The measured dark current versus biasing voltage for the detector 16 (1550nm)……………… 55
7.2: The measured dark current at 6V bias for 30 detectors………………………………………… 55
7.3: Bonding pads of 30 detectors on a package……………………………………………………. 56
76
7.4: Experiment set up with spectrometer chip and electrical board……………………………….. 57
7.5: The measured photocurrent for detector 16 from -6V to 6V bias……………………………… 58
7.6: The external responsivity of 15 functioning photodetectors (9 to 23) at bias voltage 6V……… 58
7.7: The external responsivity of 15 functioning photodetectors (9 to 23) plotted against
wavelength……………………………………………………………………………………… 59
7.8: The fibre coupler response curve (brown), possible shifted response with BCB top layer (blue)
[10], the PCG transmission response (blue dots) and the shifted fibre coupler + PCG response
(red). The left figure shows the internal responsivity of MSM detector from 1520nm to
1580nm [13]……………………………………………………………………………………. 59
7.9: The photocurrent versus wavelength plot for 15 detector 9 to detector 23…………………….. 60
7.10: Optimised photocurrent versus wavelength plot for detector 9 to detector 23…………………. 61
7.11: Photocurrent (log scale) versus wavelength plot for detector 9 to detector 23………………… 62
7.12: PCG transmission before packaging……………………………………………………………. 62
7.13: Transimpedance output voltage reading, peak voltage output for detector (right)…………….. 63
7.14: Comparison between photocurrent converted from transimpedance voltage output (pink dots) with
photocurrent measured previously (blue dots) with same supply optical power……………….. 63
A.1: Bottom view of the package pins………………………………………………………………. 66
A.2: Top view of the PGA 68 package……………………………………………………………… 66
A.3: Table of connection pins of the PGA 68 package by Globalchipmaterials…………………….. 66
B.1: Schematic circuit for MSM detectors………………………………………………………… 67
B.2: Schematic circuit for PIN detectors…………………………………………………………….. 68
C.1: Tables of analogue switching pins for MSM detectors (left) and PIN detectors (right)……….. 69
C.2.1: Front panel of Labview vi. for switching MSM detectors…………………………………….. 70
C.2.2: Subroutine vi. for out port and in port (Sending bits to parallel port)…………………………. 70
C.2.3: MSM detector switching vi…………………………………………………………………….. 70
C.2.4: Front panel of Labview vi. for switching PIN detectors………………………………………. 71
C.2.5: PIN detector switching vi………………………………………………………………………. 71
77
List of Tables
3.1: Comparison of grating spectrometers. ……………………………………………………………… 15
4.1: The univariate prediction table of water ethanol mixtures………………………………………...... 26
4.2: The PCA prediction table of water ethanol mixtures…………………………………………………. 29
4.3: The univariate prediction table of biodiesel blend levels…………………………………………… 30
4.4: The PCA prediction table of biodiesel blend levels………………………………………………….. 32
6.1: Expected photocurrent versus output voltage………………………………………………………… 53