MEMS INFRARED RESONANT DETECTORS WITH

NEAR-ZERO POWER READOUT FOR MINIATURIZED LOW

POWER SYSTEMS

A Thesis Presented

by

Sila Deniz Calisgan

to

The Department of Electrical and Computer Engineering

to partial fulfillment of the requirements

for the degree of

Masters of Science

in the field of

Electrical and Computer Engineering

Northeastern University

Boston, Massachusetts

December 2020

Acknowledgments

This work wouldn’t have been accomplished without the support of my col-

leagues, friends and family. I would like to express my thanks to;

My advisor, Prof. Matteo Rinaldi for giving me the opportunity to con-

tinue my studies in graduate level. His precious advice guided me along my

research work. The creative lab environment he built motivates his students

for innovative thinking.

Prof. Zhenyun Qian, for being my mentor in every step of the research

process. His dedication to solve research problems has initiated and greatly

supported this work.

Prof. Marvin Onabajo, for his technical discussions and feedbacks. He’s

kindly agreed to join me in my committee.

Sungho Kang, Vageeswar Rajaram and Antea Risso for their immense

support. Together, we overcame all the challenges from fabrication to exper-

iments and finally became life-long friends by sharing many experiences.

My lab members; Bernard Herrera, Flavius Pop, Giuseppe Michetti,

Michele Pirro, Meruyert Assylbekova, Xuanyi Zhao, Yao Yu, Guofeng Chen,

Luca Colombo for the countless days that we spent in labs preparing for con-

ferences. They made this journey easier with friendship and happy memories.

I am grateful for their helpful discussions that advanced my work.

Sivasubramanian Somu and Scott McNamara, for their technical support

and assistance in the Kostas cleanroom.

i

Murat Onen, for being my dearest supporter. His incredible determina-

tion inspires me throughout my studies and in many other adventures. It

would be much harder to be here without him.

My family, for always making me feel loved and supported. They gave me

courage to live abroad and most importantly taught me to make a difference.

ii

Contents

1 Introduction 11.1 IR Spectroscopy of Molecules . . . . . . . . . . . . . . . . . . 31.2 Infrared Absorption with Metametarials . . . . . . . . . . . . 5

2 Zero Power Sensing 82.1 Near-Zero Power IR Switch . . . . . . . . . . . . . . . . . . . 82.2 Device Description . . . . . . . . . . . . . . . . . . . . . . . . 122.3 Device Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 13

3 Nondispersive IR Sensing with AlN Resonant Detectors 153.1 Thin-Film AlN Resonator . . . . . . . . . . . . . . . . . . . . 17

3.1.1 Piezoelectric Materials . . . . . . . . . . . . . . . . . . 173.1.2 Thin-film Acoustic Resonators . . . . . . . . . . . . . . 183.1.3 Modified Butterworth Van Dyke (MBVD) model . . . 19

3.2 MEMS/NEMS Resonant IR Detectors . . . . . . . . . . . . . 203.3 Device Fabrication . . . . . . . . . . . . . . . . . . . . . . . . 23

4 Hazardous Material Sensing with AlN Resonant IR Detector 254.1 Device Characterization . . . . . . . . . . . . . . . . . . . . . 274.2 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2.1 Sample Preparation and Verification . . . . . . . . . . 334.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

5 Near-Zero Power MEMS-CMOS Systems for IR Detectors 415.1 System Approach . . . . . . . . . . . . . . . . . . . . . . . . . 435.2 Device Characterization . . . . . . . . . . . . . . . . . . . . . 455.3 Near-Zero Standby Power Readout Circuitry . . . . . . . . . . 465.4 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . 505.5 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

6 Conclusion 54

iii

List of Figures

1 a) Schematic of transmission spectroscopy with broadbandlight source, liquid cell containing the sample and detectorto absorb the transmitted light. The thickness of the liq-uid cell defines the pathlenght (𝑥𝑐). b) The transmission ofthe sample is the ratio between the incident and transmittedlight intensity. The absorbance is the logarithm of incidentc)Absorbance depends on the chemical content in the liquidcell, the concentration and the pathlength, 𝑥𝑐. . . . . . . . . . 4

2 a) The measured absorbance of multiple plasmonic absorberswith various dimensions. b) The plasmonic absorber materialstack where 𝑤 is used for width and Γ is used for the periodicityof the square gold patches. . . . . . . . . . . . . . . . . . . . 6

3 The measured absorbance of two different absorbers with samemetal width (1.9𝜇𝑚 maximum top metal dimension). Thecross-type absorber has ultra-narrow absorption spectrum fileswfalse[21]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 a) Schematic drawing of micromechanical photoswitch. Theinner legs connected with thermal isolation links to minimizethe heat flow towards outer legs. The magnified image showsthe plasmonic nanostructures integrated on top of the absorb-ing head. The finite-element method simulated response ofthe MP to b) selectively absorbed infrared radiation, c) an in-crease of ambient temperature. The outer legs with symmetricdesign provide temperature and stress compensation [40]. . . 10

5 a) Scanning electron microscopy (SEM) image of fabricatedswitch. b) The thermal isolation link between the inner andouter legs. c) The close up image of compensating beamformed with 2𝜇𝑚 𝑆𝑖𝑂2 and 500𝑛𝑚 𝐴𝑙. d) The bowl-shapedplatinum contact tip and plasmonic nanostructures on absorb-ing head. e) The air gap between up and bottom contacts [40].. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6 Fabrication steps of the micromechanical photoswitch. Thecross sections along A-A’ and B-B’ are shown in the top figureon left. The image is taken from [32]. . . . . . . . . . . . . . 14

iv

7 a) The 3-Dimensional schematic of piezoelectric material stackedbetween two metal layers and released from silicon wafer. b)The cross section of the unit resonant structure and it’s firstmode of vibration when electric field applied across the piezo-electric material. c) The combined mode of vibrations in theresonator body when it’s excited from interdigitated electrodesin bottom layer. The top metal is left floating without con-nected to any voltage source. + and − show polarities ofapplied voltages. . . . . . . . . . . . . . . . . . . . . . . . . . 18

8 Basic resonator MBVD model [57] . . . . . . . . . . . . . . . . 209 a) The integration of plasmonic absorbers on top of thin film

AlN resonator. b) The resonance shift of admittance vs. fre-quency curve where the resonance frequency decreases withincreasing temperature due to impinging IR radiation. c) Theadmittance change at a single frequency around the resonancewhen IR power is toggled between ON and OFF . . . . . . . 21

10 The 5-mask layer fabrication process of the resonators. A-A’cross-section are taken along width and B-B’ cross sectionstake taken along length. . . . . . . . . . . . . . . . . . . . . . 23

11 Schematic illustration of transmission spectroscopy-based chem-ical sensing using spectrally-selective IR detectors and a broad-band source. The transmission spectrum of the chemicals inthe liquid solution shows specific spectral bands (𝜆1, 𝜆2, 𝜆3)where the transmission through the chemicals in the liqiudcell is minimal. Each band can be a signature of a specificfunctional group which can be used to indicate its concentra-tion. The absorption spectra of the detectors are designed tomatch these spectral bands. . . . . . . . . . . . . . . . . . . . 26

12 The IR spectrum of Benzonitrile and n-Hexane (normal Hex-ane chain). The wavenumber 2240𝑐𝑚−1 shows the IR trans-mission region of nitrile functional group. Taken from NISTChemistry Webbook [65] . . . . . . . . . . . . . . . . . . . . . 28

13 Plasmonic IR absorbers are integrated on top of the AlN nanoplate resonator with pitch size 𝑊0 = 20𝜇𝑚. Lithographicallydefined gold patches (with patch size 𝑎 = 1.2𝜇𝑚, and peri-odicity of Γ = 2.2𝜇𝑚) are designed to have the absorptionpeak at ∼ 4.5𝜇𝑚. Absorbers were designed to fully cover the60 × 144𝜇𝑚2 AlN resonator. . . . . . . . . . . . . . . . . . . 29

v

14 a) The absorption spectrum of the plasmonic absorbers mea-sured with FTIR spectrometer b) The photograph of AlN res-onator integrated with plasmonic absorbers taken through op-tical microscope. The darkest colored region shows the etchwindow for 𝑋𝑒𝐹2 etching and the Pt anchor (light coloredstrip connecting electrical pad to bottom electrodes) suspendson the air gap. . . . . . . . . . . . . . . . . . . . . . . . . . . 30

15 Measured admittance curve around resonance with modifiedButterworth-Van Dyke model fitting results [67]; operatingpoint for tests is marked in red. TCF of the device was mea-sured as 27.9 ppm/K . . . . . . . . . . . . . . . . . . . . . . . 31

16 Experimental setup with IR source, liquid cell with ZnSe win-dows, converging optical 𝐶𝑎𝐹2 lens. The MEMS resonator isplaced inside the vacuum chamber and aligned vertically withIR source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

17 The power distribution emitted from an ideal blackbody at800∘𝐶 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

18 The methodology to collect data points . . . . . . . . . . . . 3419 a) The experimental setup b) Measured FTIR transmission

spectrum of the benzonitrile focused between 3.5𝜇𝑚 and 6𝜇𝑚.c) Fabricated resonator absorption curve between 3.5𝜇𝑚 and6𝜇𝑚. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

20 a) The transmittance curves of 100% hexane, 100% benzoni-trile and 𝐶𝑎𝐹2 lens and the absorbtance of the detector inIR range of the spectrum between 3 − 10𝜇𝑚. b) The trans-mission spectrum of the samples around 4.5𝜇𝑚. 0% samplerepresents pure hexane and 100% represents pure benzonitrile(analyte). c) Verification of the FTIR measurements of thesamples around 4.5𝜇𝑚. The absorbance (𝐴 = −𝑙𝑜𝑔(𝑇 )) showslinear relation with the concentration. . . . . . . . . . . . . . . 36

vi

21 a) The admittance vs. frequency curve of the resonator mea-sured around its resonance frequency when the IR power is ONand OFF. b) Admittance change measured from single obser-vation frequency while IR power is modulated. The admit-tance difference depends on the concentration of the sample,larger concentration of the targeted chemical results in lesspower transmission therefore 𝑐1 < 𝑐2 since 𝑐1 creates largerfrequency shift then 𝑐2. c)The photograph of the VNA screenshowing admittance vs. frequency curve of the IR detector (x-axis is shown in between 178.80−178.91𝑀ℎ𝑧). The frequencyshift is created due to IR radiation that is applied in squarewave form with chopper at 1 Hz. The red envelope shows theadmittance curve when IR radiation is present, whereas theblue envelope shows the admittance curve without any IR ra-diation. Depending on the liquid cell transmission the amountof the frequency shift (𝑓𝑂𝐹𝐹 − 𝑓𝑂𝑁) changes. . . . . . . . . . . 37

22 Measured frequency shift of the resonant IR detector on expo-sure to IR transmitted through increasing benzonitrile concen-trations. The relative power density represents the expectedtrend of frequency shift to increasing concentration. The mea-sured frequency shift data follows the expected trend well withresults showing a detection range from 0.01% to 61%. . . . . . 38

23 The liquid transmission spectrum of pure hexane, pure ben-zonitrile and the absorption of the IR detector. The hexaneabsorbtion around 3.4𝜇𝑚 contributes to the frequency shift inthe detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

24 a) A schematic illustration of the proposed sensor system con-sists of an oscillator based on an AlN resonant IR detector thatis powered through a micromechanical photoswitch controlledloadswitch. Both of the IR sensing elements have matchingabsorption wavelength to ensure near-zero stand by power con-sumption as well as the analog IR reading at the output. b)A schematic of the threshold-triggered output of the proposedsensor system. The oscillator is turned ON by the microme-chanical photoswitch when the power of incident IR radiationwas increased to a pre-determined threshold and frequencychange is shown with changing IR intensity. . . . . . . . . . . 42

vii

25 SEM images of a) The fabricated micromechanical photoswitch.b) The bowl-shaped switch contact with an air gap of∼ 500𝑛𝑚.c) The array of gold patches on the top surface of metal-insulator-metal plasmonic absorbers that are integrated in bothresonator and micromechanical photoswitch. d) The fabri-cated resonator with 5𝜇𝑚-wide platinum anchors for betterthermal isolation. . . . . . . . . . . . . . . . . . . . . . . . . . 44

26 The IR absorbance of MP and AlN resonant detector measuredusing a FTIR spectrometer. The transmission spectrum of the3.5𝜇𝑚 filter used in the experiment is also plotted. . . . . . . 46

27 The measured admittance curve (red line) of AlN IR detectorand its MBVD model fitting (blue line). . . . . . . . . . . . . 47

28 Circuit diagram for ADS simulations of the self sustaining os-cillator. The noise source is implemented to mimic the envi-ronmental/temperature noise in the circuitry. . . . . . . . . . 48

29 The harmonic balance simulations of the a) output voltageand b) current drawn from the supply. Data point m1 showsthe DC component of output voltage and m2 shows the am-plitude of the signal at resonance frequency of the resonator(∼ 182.67𝑀𝐻𝑧). After MP is triggered the oscillator circuitryuses ∼ 9𝑚𝐴 from 3.5𝑉 voltage supply. . . . . . . . . . . . . . 49

30 The measured Allan deviation versus measurement time of theoscillator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

31 a)A circuit diagram of the proposed sensor microsystem. IRsensitive components are shown with arrows. b)Photographsof PCBs and MEMS chips that are later on electrically con-nected inside a vacuum chamber. . . . . . . . . . . . . . . . . 51

32 a)Experimental setup with IR source, 𝐶𝑎𝐹2 window, chop-per wheel, and the vacuum chamber. IR sensitive devices areplaced inside the vacuum chamber. b) The components insidethe vacuum chamber. Both MP and IR detector are alignedto IR light source. . . . . . . . . . . . . . . . . . . . . . . . . . 52

33 Response of the sensor system when both the photoswitchand the resonator are exposed to an increasing IR radiation at3.5𝜇𝑚. a) The recorded current flow through the MP. b) Thefrequency shifts with respect to the first data point recordedafter MP is activated. . . . . . . . . . . . . . . . . . . . . . . . 53

viii

List of Acronyms

ADS Advanced Design Systems

AlN Aluminum Nitride

CMOS complementary metal oxide semiconductor

DC direct current

DNPD dinitropentano nitrile

FBAR thin-film bulk acoustic resonator

FOM figure of merit

FTIR Fourier transform infrared

FWHM full-width at half-maximum

HEM highly explosive material

ICP Inductively Coupled Plasma

IoT Internet of Things

IR infrared

LOR lift off resist

MBVD Modified Butterworth-Van Dyke

MEMS micro electromechanical systems

MIM metal-insulator-metal

N/MEMS nano/micro electromechanical systems

NEP noise equivelent power

NIR near infrared

NIST National Institute of Standards and Technology

ix

PCB printed circuit board

PECVD plasma-enhanced chemical vapor deposition

PR photoresist

PZT lead zirconate titanate

QCL Quantum Cascaded Laser

RF radio frequency

SEM scanning electron microscopy

SNR signal to noise ratio

TCF temperature coefficient of frequency

VNA vector network analyzer

x

Abstract

The demand for low-cost and low-power microsystems for spectrally-selective

IR sensing has been rising with the proliferation of Internet of Things (IoT)

for applications such as security surveillance and natural disaster monitoring.

As a result, there is a need for low-power, high sensitivity IR sensors with

minimum deployment and maintenance cost that can detect trace levels of

chemicals. This thesis reports on the first experimental demonstrations of

passive integrated microsystems based on transmission spectroscopy using

narrowband uncooled microelectromechanical resonant infrared (IR) detec-

tors. Moreover, the MEMS-CMOS (complementary metal oxide semicon-

ductor) integrated microsystem can turn itself ON to quantify the intensity

of infrared radiation when an above-threshold IR signature is present, but

otherwise remain dormant with near-zero standby power consumption. The

proposed sensor system combines the unique advantage of two recently de-

veloped technologies, namely, the zero-power nature of micromechanical pho-

toswitches (MPs) and the high resolution of aluminum nitride (AlN) MEMS

resonant infrared detectors, to achieve an unprecedented IR sensing capa-

bility. Thanks to the spectral selectivity enabled by the plasmonically en-

hanced thermo-mechanical transduction in MEMS structures, the proposed

sensor system is capable of discriminating the spectral content of incoming

IR radiation for the identification of events of interest. The prototype pre-

sented here is automatically powered up by the MP when the incoming IR

xi

radiation exceeds 440𝑛𝑊 showing a high IR detection resolution in active

state and a near-zero power consumption (∼ 3𝑛𝑊 ) in standby. The ul-

trathin plasmonic absorber with narrow bandwidth (FWHM < 17% ) and

near-perfect IR absorption (𝜂 > 92%) coupled with the high IR detection

capability (𝑁𝐸𝑃 ∼ 463𝑝𝑊/√𝐻𝑧) of the AlN resonator was exploited for a

filter-free spectroscopic chemical sensor based on uncooled AlN resonant IR

detectors with a minimum concentration detection limit of < 0.01% (Ben-

zonitrile in Hexane).

xii

1 Introduction

Microelectromechanical systems (MEMS) are emerging class of electronic de-

vices with unique electronic and mechanical functions and properties. The

development of portable devices has increased rapidly due to the demand on

Internet of the Things (IoT). Over the last decades to satisfy this demand,

immense progress has been made in the advancements of wireless technolo-

gies and in the deposition conditions for thin films [1]. These advancements

are utilized for the development of portable infrared (IR) spectrometers for

surveillance and environmental monitoring. Furthermore, IR spectroscopy

has been used for standoff chemical detection. Chemical analysis methods

based on molecular spectroscopy coupled with advanced multivariate statis-

tical methods to identify chemicals from its spectrum are a powerful tool to

detect chemicals at low concentrations [2, 3].

This thesis mainly focuses on the applications with new generation IR

sensors combined with AlN resonant IR detection technology that do not con-

sume any electrical power in standby and also provide analog readings of the

IR intensity. These applications of miniaturized MEMS-CMOS systems are

enabled by the unique absorption characteristics of the metal-insulator-metal

abosrbers in mid-IR range. The peak wavelength of narrowband absorbing

structures are lithographically tuned. This provides having multiple such

sensors occupying a small chip area to target different chemicals, resulting in

overall improvement in sensor capabilities while enabling its miniaturization.

1

The envisioned sensor node has compact form factor and it is compatible

with the commercially available electronics. This thesis will be organized as

follows:

In sections 2 and 3, the background of the IR sensitive devices are ex-

plained in details. The basics of nondispersive infrared (NDIR) sensing based

on transmission spectroscopy is demonstrated. The fabrication of IR detector

based Aluminum Nitride (AlN) nanoplate resonators and micmomechanical

photoswitches (MPs) are presented. Compared to previous works, the detec-

tor used here features 2× improved sensitivity to IR owing to optimized an-

chor geometry.The narrowband absorption property of the plasmonic struc-

tures allow the use of a blackbody as infrared source to characterize the

transmission of chemicals in the spectral bands of interest without the need

of any lasers or filters.

In sections 4 and 5, the novel demonstrations are presented. In section

4 of this thesis, a narrowband resonant IR detector with an integrated plas-

monic absorber (absorptance ∼ 95% with a full width at half maximum

∼ 800𝑛𝑚) tuned to 4.7𝜇𝑚 wavelength is demonstrated for use in spectro-

scopic detection of nitriles (𝑅−𝐶 ≡ 𝑁). Experiments show the quantification

of the concentration of benzonitrile in a binary mixture with hexane base on

the 𝐶 ≡ 𝑁 stretching vibration band by transmission spectroscopy using a

narrow-band MEMS IR detector as a thermal detector. Furthermore, thanks

to the optimized thermal isolation of the device, a high resolution for ther-

mal detection (noise equivalent power ∼ 463𝑝𝑊/√𝐻𝑧) is achieved, which

2

translates to a minimum detectable concentration of ∼ 0.01% (Benzonitrile

in Hexane). In this section 5, we discuss the integration of zero-power IR

sensors and IR detectors for an advanced IR sensor node with zero standby

power consumption. Both of the plasmonically enhanced MEMS devices in

this demonstration are tuned to operate around ∼ 3.5𝜇𝑚 (corresponds toni-

trogen dioxide (𝑁𝑂2) ). The IR absorptivity of MP (Absorptance > 98%

and FWHM < 650𝑛𝑚 ) and AlN resonant detector (absorptance > 94% and

FWHM < 700𝑛𝑚) is reported. A self-sustaining Pierce oscillator is built

around the resonator and triggered by the MP when the incident IR power

is increased to 3.41𝑚𝑊/𝑐𝑚2 corresponding to an absorbed power of 440𝑛𝑊

in the MP (i.e. threshold of the MP) and absorbed power of 177𝑛𝑊 in the

resonator.

1.1 IR Spectroscopy of Molecules

Infrared region is a part of the light spectrum that covers the wavelengths

between 700 𝑛𝑚 and 16 𝜇𝑚. Molecular vibrations emit or absorb radiation

in IR region of the spectrum which can be used to determine the functional

groups in the material.

IR spectroscopy has emerged as one of the key methods to analyze chem-

ical content. Absorption of specific wavelengths of IR radiation by chemi-

cals depends on their constituent molecular bonds that have characteristic

resonant modes of vibration at different wavelengths. When IR radiation

matching specific wavelength passes through a sample containing targeted

3

Figure 1: a) Schematic of transmission spectroscopy with broadband lightsource, liquid cell containing the sample and detector to absorb the transmit-ted light. The thickness of the liquid cell defines the pathlenght (𝑥𝑐). b) Thetransmission of the sample is the ratio between the incident and transmittedlight intensity. The absorbance is the logarithm of incident c)Absorbancedepends on the chemical content in the liquid cell, the concentration and thepathlength, 𝑥𝑐.

chemical bonds (Fig.1a), the electromagnetic energy gets coupled to one of

the vibration resonant modes of the bond, resulting in reduced IR power

exiting the sample at that wavelength (Fig.1b).

The ratio of IR power entering and exiting the sample (i.e. transmission)

has dependence on the sample and it’s governed by Beer-Lambert’s Law;

4

𝐼(𝜆𝑐, 𝑥𝑐) = 𝐼0 × 𝑒𝑎𝜆𝑥𝑐𝑐 (1)

where, 𝐼(𝜆𝑐, 𝑥𝑐) is the IR power at wavelength 𝜆𝑐 exiting the sample of

path length 𝑥𝑐 (subscript c denotes for targeted chemical bond). 𝐼0 is the

incident IR power, 𝑎𝜆 is wavelength dependent absorption coefficient and 𝑐

is the concentration of the solution. Thus, for a certain chemical bond and

fixed path length, the concentration can be determined by measuring the

power transmitted through the sample (Fig.1c).

1.2 Infrared Absorption with Metametarials

Metamaterials (i.e. periodic structures with smaller dimensions in compari-

son to the interacted wavelength) have been used to engineer the optical prop-

erties of arrays of nanostructures and to create reflectors [4], absorbers [5],

emitters [6] and lenses [7–9] in all the electromagnetic spectrum from ra-

dio frequency (RF) to visible [10–20]. The matematerial absorbers in IR

region have shown near perfect absorption with small dependency on angle

and polarization of the incoming radiation [21–24]. Lithographically defined

plasmonic absorbers have enhanced the thermomechanical coupling of the

structure for selective light absorption [25–27].

One of the most common electromagnetic IR absorbers have been demon-

strated with a lossy dielectric material placed between two metal layers to

confine the absorbed power in the dielectric material then convert the elec-

5

tromagnetic energy to heat (Fig.2b). These metal-insulator-metal (MIM)

narrowband IR absorbers have been designed as nanostructured arrays which

had lithographically tunable peak absorption wavelength and the full-width

half maximum (FWHM) (Fig.2a). The design steps, equivelent circuit model

and detailed characteristics of the plasmonic absorbers are explained in [22,

23].

Figure 2: a) The measured absorbance of multiple plasmonic absorberswith various dimensions. b) The plasmonic absorber material stack where 𝑤is used for width and Γ is used for the periodicity of the square gold patches.

Advanced fabrication methods have enabled the compact MIM absorbers

while maintainning near perfect absorption. The high absorption efficiency

and substrate-independent spectral band tunability has made this technology

the ideal candidate for integration with various nano/micro electromechanical

systems (N/MEMS) structures [23,28–30].

The narrowband IR absorber array studied in this thesis consists of gold

- silicon dioxide - platinum material stack integrated on top of N/MEMS

6

Figure 3: The measured absorbance of two different absorbers with samemetal width (1.9𝜇𝑚 maximum top metal dimension). The cross-type ab-sorber has ultra-narrow absorption spectrum fileswfalse [21].

device where the top layer width (𝑤) and periodicity (Γ) of the square metal

patches is defined by electron-beam lithography (Fig.2b). Moreover, our

group has previously demonstrated ultra-narrow bandwidth and near-perfect

IR absorption based on cross-type MIM plasmonic absorbers (Fig.3).

7

2 Zero Power Sensing

Sensors are extensively exploited in the development of the IoT with new

requirements on cost and power consumption [31]. They play a key role since

applications such as security surveillance and natural disaster monitoring

However, a key technical challenge preventing the deployment of large-

scale sensor networks has not been addressed yet because of the continuous

power consumption of active electronics in the sensors built with existing

technologies. Recent studies thus have been focused on reducing the standby

power consumption of environmental sensors with a hope of decreasing or

even eliminating the maintenance cost associated with the frequent battery

replacements [32–35].In this context, zero-power IR sensors that utilize the

energy in the signal of interest to operate without consuming any electrical

power have been proposed and experimentally demonstrated recently [32].

2.1 Near-Zero Power IR Switch

Passive IR sensors largely depend on the pyroelectric materials which gener-

ate different voltage levels based on the detected temperature change such as

gallium nitride (𝐺𝑎𝑁) [36, 37]. They generally show broadband absorbtion,

therefore they are suitable for hot object detection for example human body

detection with occupancy sensors [38, 39]. In order to obtain spectral infor-

mation and identify the chemicals, these sensors require additional filters.

These additional components create bulky and power hungry sensors that

8

aren’t suitable for most deployment scenarios.

Our group has been working on smart IR detection technologies with

minimum deployment and maintenance costs. With this idea in mind, re-

cently zero-power micromechanical photoswithches (MPs) have been devel-

oped which can harvest energy from the incident radiation in specific mid-

infrared band of interest. MPs are combined with spectrally-selective MIM

plasmonic absorbers whose absorption characteristic can be lithographically

defined. The digital output of the plasmonically enhanced switch can be used

to generate a wake up bit (e.g. ON signal) in the presence of above-threshold

IR signal. The switch (with the entire system connected to MP) remains in

dormant state with zero power consumption until woken up by the targeted

IR source.

The conventional IR sensor systems that are continuously scanning the en-

vironment even in stand by (i.e. when the targetted IR source is not present)

cause constant drain on the battery and require frequent replacements. The

battery and the replacement cost clashes with the idea of densely-deployed

sensor networks and bottlenecks the scalibility. This technology uniquely

combines the zero-power consumption in dormant state and the spectral se-

lectivity for event-driven sensing capabilities that enable unattended sensor

deployment with long life time.

System-level implementation of the MP was demonstrated. The device is

vacuum-packaged by using a standard third-party packaging process and in-

terfacing with a commercial-off-the-shelf wireless transmitter. This resulted

9

Figure 4: a) Schematic drawing of micromechanical photoswitch. The in-ner legs connected with thermal isolation links to minimize the heat flowtowards outer legs. The magnified image shows the plasmonic nanostruc-tures integrated on top of the absorbing head. The finite-element methodsimulated response of the MP to b) selectively absorbed infrared radiation,c) an increase of ambient temperature. The outer legs with symmetric designprovide temperature and stress compensation [40].

10

Figure 5: a) Scanning electron microscopy (SEM) image of fabricated switch.b) The thermal isolation link between the inner and outer legs. c) The closeup image of compensating beam formed with 2𝜇𝑚 𝑆𝑖𝑂2 and 500𝑛𝑚 𝐴𝑙.d) The bowl-shaped platinum contact tip and plasmonic nanostructures onabsorbing head. e) The air gap between up and bottom contacts [40].

11

in the first near-zero standby power spectrally-selective wireless infrared sen-

sor node with orders of magnitude lower power consumption than the state-

of-the-art [30].

2.2 Device Description

The MP consists of two symmetrical sides with folded beams (legs) and center

regions (heads) (Fig.4a). The bottom and top contacts are placed on one of

the heads. These contacts are seperated with nanoscale air gap to ensure the

zero-stand by power consumption and define the threshold of the mechanical

switch. One of the heads is integrated with plasmonic nanostructures (i.e.

abosrbing head) which also carries the top contact in the tip region.

The lithographically defined plasmonic absorbers enhance the thermo-

mechanical coupling and IR absorption on the absorbing head, whereas the

other head is covered with gold reflector. When the absorbed IR radiation

exceeds the defined threshold, the absorbing head bends down enough to

close the air gap closing the bottom and top contacts which creates a con-

ductive path (Fig.4b). The inner bimaterial legs cause bending down motion

in the presence of a temperature increase in the head due to the existance of

targetted IR radiation (Fig.4b).

The thermal isolation link after the inner leg provides confinement of the

temperature increase only to the inner leg pair. Moreover, the folded beam

structure of the legs (i.e. inner and outer bimaterial beams connected via

thermal isolation link) create immunity of the contact gap to fluctuations in

12

the ambient temperature or stress in the bimaterial legs (Fig.4c).

2.3 Device Fabrication

The MP [40] was fabricated with standard microfabrication steps using an

8-mask process which starts with single side polished silicon (Si) wafer. 2𝜇𝑚

thick silicon dioxide (𝑆𝑖𝑂2) was deposited by plasma-enhanced chemical va-

por deposition (PECVD). Then 100𝑛𝑚 platinum (Pt) was deposited and

patterned via lift-off in both heads (Fig.6a). Another thin 𝑆𝑖𝑂2 was de-

posited by PECVD to define the dielectric layer for plasmonic absorbers.

The next metal layer was deposited for routing and bottom contact (Fig.6b).

The both of the previous metal layers were deposited with electron-beam

deposition and patterned with LOR-assisted lift-off to avoid sidewalls. Next,

1𝜇𝑚 aluminum (Al) was deposited and patterned with wet etch (Fig.6c).

Then, the reflector metal of 150𝑛𝑚 gold (Au) and the plasmonic absorbers

(50𝑛𝑚 of Au) were deposited by e-beam evaporation and patterned by lift-off

in two different lithography steps (Fig.6d). The contour of the device was

defined by the dry etching of Al and 𝑆𝑖𝑂2 in single patterning step to ensure

co-etching of the materials and perfectly align the bimaterial legs(Fig.6e).

The air gap between the contacts was defined by thickness of the sacrificial

amorphpus silicon (a-Si) layer (500𝑛𝑚) (Fig.6f). After deposition of a-Si, a

layer of photoresist (PR) was patterned to define basis of the tip. The PR

was hard baked and its thickness was reduced to create a planar PR plug

(Fig.6g). 500𝑛𝑚 Pt sputtering and lift-off were used to fabricate the bowl-

13

Figure 6: Fabrication steps of the micromechanical photoswitch. The crosssections along A-A’ and B-B’ are shown in the top figure on left. The imageis taken from [32].

shaped tip (the sidewalls of the thick metal is kept during the lift-off process)

(Fig.6h). In the last step, the device is released by etching of bulk silicon

and sacrificial a-Si by dry etching with xenon difluoride (𝑋𝑒𝐹2) (Fig.6i).

14

3 Nondispersive IR Sensing with AlN Reso-

nant Detectors

Nondispersive IR (NDIR) sensing technology is based on spectroscopy with-

out using any dispersive elements such as prism or light gradients. NDIR

sensors technology uses IR lamp and bandpass filters to create monochro-

matic IR radiation and a broadband IR detector to quantify the transmitted

light at each spectral band.

State-of-the-art IR spectroscopy technology for example Fourier Trans-

form IR (FTIR) spectrometers relies on sophisticated optical accessories,

delicate motorized moving parts, and cooled IR detectors to acquire high

resolution spectra. As a result, they are bulky, expensive, power hungry

and highly susceptible to movement, preventing their use in fast screening

of chemicals outside lab environments. Therefore, developing a portable and

low-cost chemical detection equipment based on IR spectroscopy requires a

system intrinsically insensitive to movement, which is uncooled, has a high-

resolution IR detector and uses a readily available source of radiation. NDIR

sensor technology is a suitable option since it uses a low-cost IR components

however, the use of external filters not only limits the spectral resolution, but

also limits device miniaturization [41, 42]. In this context, quantum cascade

lasers (QCLs) are an ideal candidate to replace broadband IR sources and

filters in terms of increasing the spectral resolution and sensitivity [43]. But

the use of QCLs comes with much higher cost and increased complexity of

15

the system due to the need for synchronization between the source and the

detector.

Transmittance spectroscopy measurements based on broadband light source

and uncooled narrowband NDIR sensors is a cost effective solution for detec-

tion of traces of chemical threats in gas or liquid forms /citepopa2019towards.

Recently, spectrally-selective MEMS resonant IR detectors [27, 28, 44, 45]

have been demonstrated showing ultralow noise equivalent power (NEP)

operating at room temperature with narrow IR absorption bandwidth and

lithographically-defined absorption wavelength. This emerging uncooled IR

detector technology effectively integrates the function of a bandpass filter on

the device itself while simultaneously achieving a higher performance com-

pared to conventional uncooled detectors, thus showing great potential to be

used for IR spectroscopy with broadband sources. Thanks to the elimination

of bulky filters, multiple detectors with different targeting wavelengths can

be placed at the focal plane of an IR beam transmitted through a chemical

sample to measure its absorption in the spectral bands of interest at the

same time. Such an approach is particularly useful for fast screening of a

wide range of explosive or toxic chemicals with known characteristic absorp-

tion peaks, which is a task typically requiring a compact and low-cost tool

to fulfill on site.

16

3.1 Thin-Film AlN Resonator

3.1.1 Piezoelectric Materials

Piezoelectric materials show a linear relationship between electrical displace-

ment (D-field) and stress tensors [46] which are used to build resonant struc-

tures. In these electromechanical devices, the energy is periodically converted

from one energy domain to another (e.g. electrical to mechanical, mechanical

to electrical) under applied signal. The losses associated with these energy

conversions (e.g Joule, dielectric, electromechanical, elastic losses) degrade

the performance of a resonator. There are two key parameters to define the

performance of the resonator over these losses; coupling coefficient, (𝑘2𝑡 ) and

the quality factor, 𝑄. 𝑘2𝑡 depends on piezoelectric constants of the material

and describes the effectiveness of energy stored in the device such as the ra-

tio of mechanical stored energy in the system to the input electrical energy.

𝑄, represents the ratio between the stored energy and the energy consumed

in every cycle [47, 48]. Figure of merit (FOM) for resonators is defined by

multiplication of these two factors such as 𝑘2𝑡 × 𝑄.

AlN stands out from other piezoelectric materials with fabrication of high

quality films at low temperatures and CMOS compatibility. Therefore, AlN

is suitable for sensors requiring high signal to noise ratio (SNR) for example

resonator applications at high frequencies. Moreover, AlN is an electrically

insulating material with a quite large bandgap of 6.2𝑒𝑉 at room temperature.

This bandgap prevents dielectric losses and enables high operating temper-

17

atures for various applications [49]. The state-of-the art laterally vibrating

AlN resonators have been demonstrated with 𝐹𝑂𝑀 > 30 [50–53]. However,

it has lower piezoelectric coupling coefficient compared to ferroelectric ma-

terials such as lead zirconate titanate (PZT). This lower value hinders the

application fields where high force and displacement is required like actuators

and transducers.

3.1.2 Thin-film Acoustic Resonators

Figure 7: a) The 3-Dimensional schematic of piezoelectric material stackedbetween two metal layers and released from silicon wafer. b) The cross sectionof the unit resonant structure and it’s first mode of vibration when electricfield applied across the piezoelectric material. c) The combined mode ofvibrations in the resonator body when it’s excited from interdigitated elec-trodes in bottom layer. The top metal is left floating without connected toany voltage source. + and − show polarities of applied voltages.

One of the latest resonant structures are formed by a piezoelectric thin-

film sandwiched between metallic electrodes (Fig.7a) to create lateral vibra-

tions. This type of resonators are called thin film bulk acoustic resonators

(FBARs) [48,54–56]. An important feature of laterally vibrating structures is

18

the ability to lithographically define the resonance frequency without chang-

ing other operating parameters. This enables the design of filters and multi-

plexers (that can operate with different frequencies) for RF front-ends on the

same substrate without any dependence on the material thickness [52,57].

For laterally vibrating structures, the first mode of resonance frequency

can be calculated as;

𝑓𝑟𝑒𝑠 =1

2𝑊0

√𝐸𝑒𝑞

𝜌𝑒𝑞(2)

where 𝑊0 is the width of the metal plate, 𝜌𝑒𝑞 and 𝐸𝑒𝑞 are the mass

density and the Young’s modulus of the resonant structure (including metal

electrodes), respectively. The cross-section of the overall structure is shown

in Fig.7c.

Here, we utilized the bottom metal layer for frequency scaling and lower

anchor loss. The patterned bottom electrode creates similar effect to com-

bination of the unit resonant structures (Fig.7b). Two consecutive interdigi-

tated electrodes (i.e. fingers) are excited with 180∘ phase difference to have

symmetric vibration pattern across the resonator body [55]. The resonance

frequency depends on the width of the interdigitated electrode 𝑊0 (Eqn. 2).

3.1.3 Modified Butterworth Van Dyke (MBVD) model

These resonant structure can be modeled with a series 𝑅𝐿𝐶 branch in par-

allel with another capacitor. 𝐶𝑚, 𝐿𝑚 and 𝑅𝑚 are motional parameters. This

19

simplified Modified Butterworth Van Dyke (MBVD) model is used to simu-

late the resonator in electrical circuits. The physical dimensions such as film

thickness and electrode width define the 𝑅𝐿𝐶 components in the derived

model. The series resonance mode (where admittance curve creates a sharp

peak) defines the operation frequency of the devices in this thesis. The paral-

lel capacitance 𝐶0 comes from the electrostatic capacitance between top and

bottom electrodes (much higher than series capacitance in high 𝑄 devices)

which defines the anti-resonance peak where the admittance curve creates a

valley.

Figure 8: Basic resonator MBVD model [57]

3.2 MEMS/NEMS Resonant IR Detectors

MEMS resonant IR detectors have shown unique advantages such as high

thermal sensitivity and low noise performance [44]. Our group has previously

demonstrated uncooled MEMS resonant IR detectors with narrow bandwidth

and near-perfect IR absorption based on AlN nano-plate resonators inte-

grated with plasmonic absorbers [23,28].

20

Figure 9: a) The integration of plasmonic absorbers on top of thin filmAlN resonator. b) The resonance shift of admittance vs. frequency curvewhere the resonance frequency decreases with increasing temperature dueto impinging IR radiation. c) The admittance change at a single frequencyaround the resonance when IR power is toggled between ON and OFF

The AlN resonators can be used as narrowband IR detectors by func-

tionalizing the top layer with plasmonic absorbers as explained in previous

chapter. The floating electrode of the thin film AlN resonator is purposely

designed to act as the bottom metal layer for the absorber material stack.

When IR radiation matching the absorption wavelength of the absorber

is incident on the detector, the absorber efficiently confines the incident IR

radiation in the sub-wavelength thickness of the structure [27, 58], convert-

ing the electromagnetic energy into heat. The latter induces a temperature

increase ∆𝑇 in the well thermally-isolated suspended nano plate. High ther-

mal isolation is enabled by the nanoscale metallic anchor. ∆𝑇 then causes a

frequency shift ∆𝑓 in the resonance frequency of the device due to the tem-

21

perature coefficient of frequency (TCF). The resonant frequency decreases

when the temperature of the resonator increases (Eqn. 2).

The ∆𝑇 for an incident IR power P is given by;

∆𝑇 ≈ 𝜂𝑃𝑅𝑡ℎ (3)

where, 𝜂 is the IR absorption efficiency and 𝑅𝑡ℎ is the resonator’s effective

thermal resistance [28]. Then, the device performance can be represented by

its responsivity;

𝑆 = 𝜂𝑅𝑡ℎ𝑇𝐶𝐹𝑓0 (4)

where 𝑓0 is the resonance frequency.

The minimum detectable absorbed IR power (i.e. sensitivity of the device)

can be calculated from the noise level of the detector (Eqn. 5). Based on the

extracted noise spectral density (𝑓𝑛), the NEP of the detector is calculated

by dividing 𝑓𝑛 to its responsivity (Eqn. 4). The NEP of the detector is;

𝑁𝐸𝑃 = 𝑓0/𝑆 (5)

which is used to calculate the figure of merit (Eqn. 6) of the detector

along with the thermal resistance.

𝐹𝑂𝑀 =1

𝑁𝐸𝑃 *𝑅𝑡ℎ

(6)

22

3.3 Device Fabrication

AlN resonant IR detectors were fabricated with integration of narrowband

plasmonic IR absorbers whose absorption wavelength is designed to match

the targeted chemical bond’s specific wavenumber.

Figure 10: The 5-mask layer fabrication process of the resonators. A-A’cross-section are taken along width and B-B’ cross sections take taken alonglength.

The device fabrication shown in Fig.10 was started with single side pol-

ished, high resistivity silicon (Si) wafer (> 20 × 103Ω) as a substrate. The

interdigitated electrodes were patterned by lift-off and 80𝑛𝑚 of platinum

(Pt) was deposited with electron beam (e-beam) deposition. 5𝑛𝑚 of tita-

nium (Ti) was used as adhesion layer between the substrate and electrodes

(Ti layer is not shown in the Fig.10). Then, AlN thin film of 500𝑛𝑚 was

23

sputter-deposited Fig.10a. The floating Pt electrode (serves as ground plate

for plasmonic absorbers) was patterned via e-beam evaporation and lift-off.

Then, 100𝑛𝑚 of silicon dioxide (𝑆𝑖𝑂2) was deposited as shown in Fig.10b.

𝑆𝑖𝑂2 and AlN were patterned by plasma etching. Dry etching with Induc-

tively Coupled Plasma (ICP) was used to define the resonator contour and

wet etching with phosphoric acid (𝐻3𝑃𝑂4) for AlN was used for vias to

reach the bottom metal layer (Fig.10c). Thick gold (𝐴𝑢) layer of 100𝑛𝑚 is

patterned for probing pads by lift-off and e-beam metal deposition (Fig.10d).

Next, sqaure-shaped plasmonic absorbers (50𝑛𝑚 of Au) were patterned via e-

beam lithography (Fig.10e). Finally, the resonators are isotropically released

by xenon difluoride (𝑋𝑒𝐹2). The final cross section is shown in Fig.10f.

24

4 Hazardous Material Sensing with AlN Reso-

nant IR Detector

Screening for hazardous materials in airports, public places, and government

facilities is important to homeland security. The development of technology

for fast standoff detection of highly explosive materials (HEMs) with high

accuracy is decisive in early identification of safety threats. Well-established

HEM detection methods range from explosive-sniffer dogs to spectroscopic

measurements [2,3,59–62]. Usually, taking the sample to the lab environment

is dangerous and time consuming because it requires physical handling of the

substance. Therefore, there is a need for portable chemical scanners that can

be operated without contact with the unknown sample.

HEM detection using transmission spectroscopy relies on the detection

of characteristic IR spectral absorption bands of specific functional groups

such as 𝑁𝑂2, peroxide, and −𝐶𝑁 that are indicative of explosive materi-

als. Reliable detection of HEMs is possible by irradiating a sample with IR

radiation and using a calibrated IR detector to quantify the transmitted IR

intensity in the spectral bands of interest corresponding to these functional

groups. This in turn depends on the concentration of the molecules: greater

the concentration, lower is the transmitted intensity in those spectral bands.

In this section, we discuss a way to improve the detector’s IR sensitivity

and show that it is possible to use such a spectrally-selective IR detector for

liquid phase chemical sensing by demonstrating the detection of the nitrile

25

Figure 11: Schematic illustration of transmission spectroscopy-based chem-ical sensing using spectrally-selective IR detectors and a broadband source.The transmission spectrum of the chemicals in the liquid solution shows spe-cific spectral bands (𝜆1, 𝜆2, 𝜆3) where the transmission through the chemicalsin the liqiud cell is minimal. Each band can be a signature of a specific func-tional group which can be used to indicate its concentration. The absorptionspectra of the detectors are designed to match these spectral bands.

functional group (−𝐶 ≡ 𝑁) in a binary mixture of liquids through transmis-

sion spectroscopy (Fig.11).

In particular, as a proof of concept, we demonstrate our AlN resonant IR

detector by measuring the IR detector’s response to varying concentrations

of solutions containing the nitrile functional group (−𝐶 ≡ 𝑁) which has its

characteristic stretching mode around 4.5𝜇𝑚(∼ 2200𝑐𝑚−1) [63]. This bond

is targeted since it is found in explosive materials such as dinitropentano

nitrile (DNPD) [64] and can be tested in a lab environment using safer and

relatively non-toxic chemicals like benzonitrile. For the experiments, different

26

concentrations (by volume) of benzonitrile in a binary solution with hexane

as the solvent are prepared. Hexane is chosen for its near-unity transmittance

around the operation wavelength of 4.5𝜇𝑚 and its easy accessibility as shown

in Fig.12.

4.1 Device Characterization

The IR detector used in this section comprises an AlN nano plate resonator

with a narrowband plasmonic IR absorber integrated on top of the resonator

(Fig.13). The absorber comprises a metal-insulator-metal stack [28] featuring

polarization- and angle-independent absorption, narrow FWHM (< 17%),

and high absorption efficiency (> 92%) at the center wavelength (4.7𝜇𝑚):

all of which are critical parameters for obtaining high performance of the

proposed sensor.

The absorber efficiently confines the incident IR radiation in the resonant

structure which results in a temperature increase ∆𝑇 . The ∆𝑇 depends on

the absorption efficiency 𝜂 and thermal resistance of the detector 𝑅𝑡ℎ for a

constant input power as shown in Eqn. 3.In this demonstration, 𝑅𝑡ℎ is im-

proved by scaling the anchor width to maximize ∆𝑇 . When operating in

vacuum, the primary path of heat loss to the substrate is through the ultra-

thin (80𝑛𝑚) platinum (Pt) anchors (Fig.13) which supply the electrical signal

to the interdigitated bottom electrodes and support the free-standing AlN

nano plate. By decreasing the width of Pt anchor to only 5𝜇𝑚 while main-

taining the mechanical integrity of the suspended structure, 𝑅𝑡ℎ is doubled

27

Figure 12: The IR spectrum of Benzonitrile and n-Hexane (normal Hexanechain). The wavenumber 2240𝑐𝑚−1 shows the IR transmission region ofnitrile functional group. Taken from NIST Chemistry Webbook [65]

28

Figure 13: Plasmonic IR absorbers are integrated on top of the AlN nanoplate resonator with pitch size 𝑊0 = 20𝜇𝑚. Lithographically defined goldpatches (with patch size 𝑎 = 1.2𝜇𝑚, and periodicity of Γ = 2.2𝜇𝑚) aredesigned to have the absorption peak at ∼ 4.5𝜇𝑚. Absorbers were designedto fully cover the 60 × 144𝜇𝑚2 AlN resonator.

compared to the previous work [28].

The absorption characteristics of the plasmonic absorbers (integrated on

top of the resonant structure) are measured with FTIR spectroscopy. Nar-

row FWHM ∼ 795𝑛𝑚 (∼ 17%) and high absorption efficiency of > 92% at

the center wavelength are achieved (Fig.14). All of these contribute to the

demonstration of a narrowband high-resolution uncooled IR detector. The

center absorption peak is measured at 4.7𝜇𝑚 which offsets from the target

wavelength of 4.5𝜇𝑚 due to fabrication imperfections. It is worth noting that

the FWHM of the plasmonic absorber can be further improved by employing

cross-type absorbers as described in [23] for better rejection of noise from

other wavelengths [66].

The electromechanical characteristics of the resonant IR detector mea-

sured with a Vector Network Analyzer (VNA) show that despite the integra-

29

Figure 14: a) The absorption spectrum of the plasmonic absorbers measuredwith FTIR spectrometer b) The photograph of AlN resonator integrated withplasmonic absorbers taken through optical microscope. The darkest coloredregion shows the etch window for 𝑋𝑒𝐹2 etching and the Pt anchor (lightcolored strip connecting electrical pad to bottom electrodes) suspends on theair gap.

tion of the plasmonic absorber on top of the AlN nanoplate resonator. A high

electromechanical performance is achieved with high quality factor 𝑄 ∼ 1600

and electromechanical coupling coefficient 𝑘2𝑡 ∼ 1.37% which is extracted us-

ing MBVD model fitting (Fig. 15). Furthermore, improved thermal isolation

(simulated 𝑅𝑡ℎ ∼ 4.8 * 105𝐾/𝑊 in vacuum) of the resonant thermal detec-

tor is achieved by optimizing anchor geometry. The frequency noise spectral

density, 𝑓𝑛, of the device is extracted by monitoring the short-term (1 ms)

frequency instability and found to be ∼ 1.02𝐻𝑧/√𝐻𝑧 with −20𝑑𝐵𝑚 input

RF power. The NEP is calculated to be ∼ 463𝑝𝑊/√𝐻𝑧 by dividing 𝑓𝑛 by

the responsivity (Eqn. 4).

30

Figure 15: Measured admittance curve around resonance with modifiedButterworth-Van Dyke model fitting results [67]; operating point for tests ismarked in red. TCF of the device was measured as 27.9 ppm/K

4.2 Experimental Setup

The transmission spectroscopy setup is used to analyze the content of liquid

cell. The fabricated AlN nanoplate resonant IR detector is placed inside a

vacuum chamber under an IR transparent calcium fluoride (𝐶𝑎𝐹2) window

with a calibrated broadband blackbody source positioned above (Fig.16).

The IR power source used in the experiments can be treated as ideal

blackbody (previously calibrated with IR power meter) and the temperature

is set to 800∘ C. The emitted power of the ideal blackbody radiation is shown

in Fig.17.

31

Figure 16: Experimental setup with IR source, liquid cell with ZnSe windows,converging optical 𝐶𝑎𝐹2 lens. The MEMS resonator is placed inside thevacuum chamber and aligned vertically with IR source.

An IR liquid transmission cell containing the solution of benzonitrile and

hexane with known concentration is centered on the window above the detec-

tor. Liquid cell used here has zinc selenide (𝑍𝑛𝑆𝑒) window and its transmis-

sion characteristics are measured with the chemicals. The path length of the

liquid cell is measured as 0.108𝑚𝑚 from the FTIR measurements. The pre-

pared sample concentrations are verified by FTIR measurements. The 𝐶𝑎𝐹2

window, 𝐶𝑎𝐹2 lens and 𝑍𝑛𝑆𝑒 liquid cell windows have high transmission in

mid-IR region (> 95%). The delivered power levels are calculated by the

combination of the window transmissions and the content of chemical sam-

ple. Chopped IR radiation (at 1 Hz) from the blackbody is used to irradiate

the liquid sample and the transmitted radiation is then detected by the IR

detector as an induced ∆𝑓 . The IR beam is aligned using a converging 𝐶𝑎𝐹2

32

Figure 17: The power distribution emitted from an ideal blackbody at 800∘𝐶

lens to maximize the delivered power to the detector through the liquid cell.

The steps for each data point is shown in Fig.18 (calibration of RF probes is

done once for all the data points).

4.2.1 Sample Preparation and Verification

The measured FTIR transmission of the targeted analyte and the absorption

of the detector is shown in Fig.19. The absorption peak of the plasmonic

absorber shifts due to imperfections in release process of the MEMS device,

thus creating a mismatch between the absorbtion peaks.

A set of samples with concentrations from 0% to 100% of benzonitrile in

hexane are used for testing the proposed chemical sensor. The samples are

prepared with milli-syringes and collected in 2𝑚𝐿 sample containers. The

liquid cell has been filled and cleaned twice for each measurement. The trans-

mission spectrum of the pure chemicals and the absorptance of the detector

33

Figure 18: The methodology to collect data points

are measured in IR range between 3−10𝜇𝑚 (Fig.20a). The prepared sample

concentrations are verified by FTIR measurements around 4.5𝜇𝑚 as shown

in Fig.20b which shows matching results to Beer-Lambert Law (Eqn. 1) ex-

pectations. Absorbance is linearly changing with the concentration when the

pathlength is kept constant (Fig.20c).

4.3 Results

The frequency shift of the admittance curve is observed by applying IR ra-

diation with a chopper wheel. The chopper wheel creates ON/OFF cycles

34

Figure 19: a) The experimental setup b) Measured FTIR transmission spec-trum of the benzonitrile focused between 3.5𝜇𝑚 and 6𝜇𝑚. c) Fabricatedresonator absorption curve between 3.5𝜇𝑚 and 6𝜇𝑚.

for IR radiation where ON due to the temperature change in the resonator

body. The resonance frequency shifts to a lower value (Fig. 21a) in the

presence of the IR radiation and this can be measured from one observation

frequency (Fig. 21b) as explained in the section 13. For an arbitrary sample,

the chopper ON/OFF cycles between two admittance curves is shown in Fig.

21c.

Fig. 22 shows the measured ∆𝑓 of the detector in response to different

concentrations of benzonitrile in hexane. Here, the VNA is set up to mon-

itor the resonator’s admittance at 179.04𝑀𝐻𝑧 where the high slope near

resonance (at 178.95𝑀𝐻𝑧) as explained in [68] (Fig. 9) ensures maximum

35

Figure 20: a) The transmittance curves of 100% hexane, 100% benzonitrileand 𝐶𝑎𝐹2 lens and the absorbtance of the detector in IR range of the spec-trum between 3−10𝜇𝑚. b) The transmission spectrum of the samples around4.5𝜇𝑚. 0% sample represents pure hexane and 100% represents pure ben-zonitrile (analyte). c) Verification of the FTIR measurements of the samplesaround 4.5𝜇𝑚. The absorbance (𝐴 = −𝑙𝑜𝑔(𝑇 )) shows linear relation withthe concentration.

36

Figure 21: a) The admittance vs. frequency curve of the resonator measuredaround its resonance frequency when the IR power is ON and OFF. b) Ad-mittance change measured from single observation frequency while IR poweris modulated. The admittance difference depends on the concentration of thesample, larger concentration of the targeted chemical results in less powertransmission therefore 𝑐1 < 𝑐2 since 𝑐1 creates larger frequency shift then𝑐2. c)The photograph of the VNA screen showing admittance vs. frequencycurve of the IR detector (x-axis is shown in between 178.80 − 178.91𝑀ℎ𝑧).The frequency shift is created due to IR radiation that is applied in squarewave form with chopper at 1 Hz. The red envelope shows the admittancecurve when IR radiation is present, whereas the blue envelope shows theadmittance curve without any IR radiation. Depending on the liquid celltransmission the amount of the frequency shift (𝑓𝑂𝐹𝐹 − 𝑓𝑂𝑁) changes.

37

admittance shift when the curve shifts left on exposure to IR; this shift is

due to the induced reduction in the resonance frequency (∆𝑓). Using the

value of slope around this frequency, the corresponding ∆𝑓 is calculated. As

shown in (Fig. 22), there is a clear reduction in ∆𝑓 as the concentration

of benzonitrile is increased. This closely follows the predicted trend in rela-

tive IR power density (Fig. 22) which was calculated for each concentration

by integration of the blackbody’s, sample’s and absorber’s measured FTIR

spectra (Fig. 20a).

Figure 22: Measured frequency shift of the resonant IR detector on expo-sure to IR transmitted through increasing benzonitrile concentrations. Therelative power density represents the expected trend of frequency shift toincreasing concentration. The measured frequency shift data follows the ex-pected trend well with results showing a detection range from 0.01% to 61%.

As expected from (Eqn. 1), due to the exponential relation, concentra-

tion changes at low concentrations (< 4%) show a larger ∆𝑓 . Using the slope

at low concentrations (< 4%) divided by the frequency noise spectral den-

38

sity, the minimum detectable concentration is calculated to be ∼ 0.01%. ∆𝑓

monotonically decreases up to 75% concentration, since beyond this, benzoni-

trile’s peak at 4.5𝜇𝑚 saturates (Fig. 23) and IR transmittance is determined

by hexane.

Figure 23: The liquid transmission spectrum of pure hexane, pure benzoni-trile and the absorption of the IR detector. The hexane absorbtion around3.4𝜇𝑚 contributes to the frequency shift in the detector.

At high benzonitrile concentrations absorbed power density increases.

This behavior is result of the hexane absorbtion peak in a different wave-

39

length. The increase in transmitted power (and ∆𝑓) above 75% as predicted

by the power density curve, is due to the decrease in wide hexane absorption

around 3.4𝜇𝑚 which depends on the amount of 𝐶 − 𝐻 bonds (Fig. 23).

The maximum detectable concentration is limited to 61% since, ∆𝑓 at 100%

is the same as that at 61%, reliable measurements above 61% cannot be

guaranteed.

40

5 Near-Zero Power MEMS-CMOS Systems for

IR Detectors

The zero-power IR sensor relies on a micromechanical photoswitch to gen-

erate a digitized wake-up signal for triggering an alarm when an above-

threshold signal of interest is present, otherwise remain completely dor-

mant [30, 32, 69]. However, unlike conventional IR sensors, they lack the

ability to measure the exact value of an above-threshold IR power. There-

fore, using MP(s) as the only IR sensing element is not sufficient for tracking

the progress and determine the severity of an IR radiation related event. For

example, first responders (e.g. fire fighters) could understand the situation

of a fire accident and get better prepared if the size of the fire and growth

rate are reported by pre-deployed sensors. The integration of an additional

IR sensor with MP is thus desirable for having an analog output while main-

taining zero power consumption in standby when the input is lower than a

predetermined IR threshold.

On the other hand, MEMS resonant IR detectors have relatively low

power consumption and miniaturized form factor which makes them ideal

candidates for the integration with MP. Among different resonant IR sensor

technologies AlN piezoelectric nano-plate IR detectors stand out as the most

promising technology for fast detection and CMOS integration thanks to the

mature thin film deposition process for AlN at room temperature [58]. As

explained in section 3, plasmonic absorbers [23] can be seamlessly integrated

41

Figure 24: a) A schematic illustration of the proposed sensor system consistsof an oscillator based on an AlN resonant IR detector that is powered througha micromechanical photoswitch controlled loadswitch. Both of the IR sensingelements have matching absorption wavelength to ensure near-zero stand bypower consumption as well as the analog IR reading at the output. b) Aschematic of the threshold-triggered output of the proposed sensor system.The oscillator is turned ON by the micromechanical photoswitch when thepower of incident IR radiation was increased to a pre-determined thresholdand frequency change is shown with changing IR intensity.

42

with AlN nano-plate resonant IR detector for spectrally-selective sensing in

the mid-IR range [27,28], which is an essential functionality for non-dispersive

IR spectroscopy and the discrimination between various IR sources [70,71].

5.1 System Approach

The proposed microsystem is composed of a single transistor oscillator based

on a narrowband uncooled MEMS AlN resonant detector, a MP with match-

ing IR absorption band, and an ultra-low leakage CMOS load-switch (Fig.

24 and Fig.25).

The first core element is a threshold-triggered MP which selectively har-

vests the energy from impinging IR radiation in the specific band of interest

to mechanically close the switch contacts (see previous sections). Any IR ra-

diation with an intensity lower than the predefined threshold will not trigger

the microsystem. Thanks to the physical gap in between the two contacts in

the MP (Fig. 25b), standby power consumption is eliminated.

The second core element is a high performance MEMS resonant detector

based on a AlN nano-plate resonator integrated with a plasmonic absorber.

As explained in the previous sections when the targeted IR radiation is in-

cident on the detector, the plasmonic absorber converts the electromagnetic

energy to heat. The confined heat results in a temperature increase and fre-

quency shift in the nano-plate resonator due to the high thermal isolation and

the TCF of the AlN nano-plate resonator. A CMOS single-transistor Pierce

oscillator circuity built around the resonator is used to sustain the oscillation

43

Figure 25: SEM images of a) The fabricated micromechanical photoswitch.b) The bowl-shaped switch contact with an air gap of ∼ 500𝑛𝑚. c) Thearray of gold patches on the top surface of metal-insulator-metal plasmonicabsorbers that are integrated in both resonator and micromechanical pho-toswitch. d) The fabricated resonator with 5𝜇𝑚-wide platinum anchors forbetter thermal isolation.

44

and monitor the resonance frequency (Fig. 31). The thermo-mechanical cou-

pling of both MEMS devices is enhanced by the integration of near-perfect

metal-insulator-metal plasmonic absorbers. The lithographically-defined size

and periodicity of the patches on the absorber set the wavelength and band-

width of the absorption peak. In this demonstration, the absorbers are de-

signed to operate around ∼ 3.5𝜇𝑚, which corresponds to the IR emission of

hot nitrogen dioxide (𝑁𝑂2) released from burning wood. The absorbers are

designed to have an active area of 150×150𝜇𝑚2 in the MP and 60×144𝜇𝑚2

in the resonant detector (Fig. 25a and d). Same material stack is inte-

grated with MP and nano-plate AlN resonator to achieve similar absorption

characteristics as shown in Fig. 25c.

5.2 Device Characterization

Both MEMS devices are fabricated and characterized with the same meth-

ods that explained in the previous chapters. The absorption spectrum and

FWHM of the fabricated plasmonic absorbers were measured in FTIR spec-

trometer (Fig 26). The IR absorptivity of MP (Absorptance > 98% and

FWHM < 650𝑛𝑚 ) and AlN resonant detector (absorptance > 94% and

FWHM < 700𝑛𝑚) is plotted along with the calibrated blackbody radiation

with transmitted through the transmission spectrum of a 3.5𝜇𝑚 filter placed

in front of a calibrated blackbody to simulate a targeted source (Fig. 26).

The electromechanical performance of the fabricated resonator (Fig. 27)

is characterized with a VNA as explained in previous section. The quality

45

Figure 26: The IR absorbance of MP and AlN resonant detector measuredusing a FTIR spectrometer. The transmission spectrum of the 3.5𝜇𝑚 filterused in the experiment is also plotted.

factor and electromechanical coupling coefficient is extracted to be 𝑄 ∼ 1000

and 𝑘2𝑡 ∼ 1.57% using MBVD model fitting. Based on the extracted noise

spectral density 𝑓𝑛, a NEP ∼ 746.4𝑝𝑊/√𝐻𝑧 is calculated by 𝑁𝐸𝑃 = 𝑓𝑛/𝑆,

where 𝑆 is the responsivity of the resonant detector (Eqn. 4).

5.3 Near-Zero Standby Power Readout Circuitry

The self sustaining oscillator with high gain inverting amplifier is designed in

Advanced Design Systems (ADS) around the IR detector (Fig. 28). Pierce

oscillator is widely used because of its stable resonance frequency [72–74].

The feedback resistor (𝑅𝐹 in Fig. 31) charges the input capacitance with

output voltage. The resonator with parallel capacitors provides 180∘ phase

46

Figure 27: The measured admittance curve (red line) of AlN IR detector andits MBVD model fitting (blue line).

shift additional to the inverter. The resulting positive loop gain creates

unstable circuit which is utilized to design oscillators that is known as the

Barkhausen stability criterion.

The resonator is wire-bonded to a printed circuit board (PCB) containing

the oscillator circuit with an ATF-551M4 E-pHEMT GaAs transistor and

several discrete electrical components (𝑅𝐹 = 100𝑘Ω, 𝑅𝐵 = 100Ω, 𝐶1 = 1𝑝𝐹 ,

𝐶2 = 15𝑝𝐹 , 𝐶3 = 10𝑝𝐹 in Fig 31). The harmonic balance simulations are

shown in (Fig. 29). The Allan deviation of the oscillator was measured with

a frequency counter under various gate times (Fig. 30).

The oscillator circuit is activated by the MP with the help of an ultra-

low leakage CMOS load-switch [75]. The load-switch is used to handle the

47

Figure 28: Circuit diagram for ADS simulations of the self sustainingoscillator. The noise source is implemented to mimic the environmen-tal/temperature noise in the circuitry.

relatively high current required by the oscillator (The current handling of

MP is limited to ∼ 200𝜇𝐴 [32]). The MP is connected to the control pin of

the load switch while a sourcemeter is set to supply 1.5𝑉 and measure the

current across the port.

The sharp sub-threshold slope of the MP ensures the gate voltage of the

load-switch to be well below the threshold voltage of transistor in order to

have a near-zero leakage current. A pull-down resistor is used to hold the logic

signal at zero volts when the MP is open and also limit the current through

the MP when it is triggered ON. Once the MP is triggered to close, the 3.5𝑉

is supplied to the Pierce oscillator circuitry [76] to which the resonator is

48

Figure 29: The harmonic balance simulations of the a) output voltage andb) current drawn from the supply. Data point m1 shows the DC componentof output voltage and m2 shows the amplitude of the signal at resonance fre-quency of the resonator (∼ 182.67𝑀𝐻𝑧). After MP is triggered the oscillatorcircuitry uses ∼ 9𝑚𝐴 from 3.5𝑉 voltage supply.

49

Figure 30: The measured Allan deviation versus measurement time of theoscillator.

directly wire-bonded to and the frequency is recorded from voltage output

with a frequency counter.

5.4 Experimental Setup

The fabrication steps of the MP and the AlN resonant detector are covered

in previous sections. The fabricated AlN resonant detector and MP were

connected together with the load-switch in a vacuum chamber (Fig. 32). A

blackbody IR source with a narrow-band filter (around 3.5𝜇𝑚 with 0.5𝜇𝑚

bandwidth) were used to emulate the emission from gases in the mid-IR

range. The devices under test were exposed to filtered IR radiation through

50

Figure 31: a)A circuit diagram of the proposed sensor microsystem. IR sensi-tive components are shown with arrows. b)Photographs of PCBs and MEMSchips that are later on electrically connected inside a vacuum chamber.

an IR-transparent window made of 𝐶𝑎𝐹2 in the lid of the vacuum chamber.

Additional 𝐶𝑎𝐹2 lens was placed between the window and the blackbody

to increase the IR power density while maintaining the radiation area large

enough to cover both IR sensitive devices. The temperature of the blackbody

(i.e. delivered power) was increased over time to simulate an evolving IR

source with characteristic emission at 3.5𝜇𝑚 (Fig. 26).

5.5 Results

The transistor circuitry requires ∼ 9𝑚𝐴 to self-sustain the oscillation, which

corresponds to a power consumption of ∼ 3.15𝜇𝑊 . The load-switch ensures

51

Figure 32: a)Experimental setup with IR source, 𝐶𝑎𝐹2 window, chopperwheel, and the vacuum chamber. IR sensitive devices are placed inside thevacuum chamber. b) The components inside the vacuum chamber. Both MPand IR detector are aligned to IR light source.

proper handling of the relatively high current without creating any damage in

the MP. The inclusion of the ultra-low leakage CMOS load-switch introduces

a near-zero (3.01𝑛𝑊 ) power consumption in standby (due to the 0.86𝑛𝐴

leakage in the load-switch).

The oscillator was triggered by the MP when the IR power was increased

to 3.41𝑚𝑊/𝑐𝑚2 corresponding to an absorbed power of 440𝑛𝑊 in the MP.

The same input IR power (i.e. filtered blackbody radiation at 3.5𝜇𝑚) results

in an absorption of 177𝑛𝑊 in the resonator (calculated based on the ab-

sorption spectrum and area of the resonator). Fig. 33a shows the measured

MP current variation in time where the switch turned ON when the MP

threshold was reached (i.e 440𝑛𝑊 ) and kept conducting current in response

to further increased input power. The resonant frequency of the resonant

52

Figure 33: Response of the sensor system when both the photoswitch andthe resonator are exposed to an increasing IR radiation at 3.5𝜇𝑚. a) Therecorded current flow through the MP. b) The frequency shifts with respectto the first data point recorded after MP is activated.

detector was monitored by a frequency counter connected to the output of

the oscillator (Fig. 33b). The IR-induced frequency shift after the triggering

of MP was recorded showing the dormant time period with zero output and

active time period with analog read-out capability. The linear frequency shift

is expected due to the linearly increased IR intensity along time. (A stabi-

lization time for the blackbody to change its temperature was given between

the measurements and each data point was averaged over 5 seconds.)

53

6 Conclusion

In this thesis, plasmonically enhanced MEMS devices such as aluminum ni-

tride (AlN) infrared (IR) detector and micromechanical photoswitch (MP)

were designed, fabricated and tested. The results on the first experimen-

tal demonstration of transmission spectroscopy-based chemical sensing us-

ing narrowband uncooled microelectromechanical resonant infrared detec-

tors. An aluminum nitride nano-plate resonator integrated with a spectrally-

selective plasmonic absorber was used to measure the concentration of ben-

zonitrile by targeting its characteristic absorption wavelength (4.5𝜇𝑚).

It is demonstrated that a microsystem for zero-power IR sensing based on

the integration of a MP with an AlN MEMS resonant IR detector. The result-

ing MEMS-CMOS IR detector is capable of measuring the above-threshold

intensity of an IR signature with zero standby power. The MP in the com-

posed microsystem can selectively harvest the energy from impinging IR

radiation to power up the resonant detector part: When the absorbed IR

power exceeds 440𝑛𝑊 the MP automatically connects the resonant detector

circuit to a power source showing a high-resolution IR detection capability

(𝑁𝐸𝑃 ∼ 746.4𝑝𝑊/√𝐻𝑧) in the active state. When the IR power is lower

than the threshold, the microsystem stays dormant with near-zero power

consumption (∼ 3.01𝑛𝑊 ).

Such a threshold-triggered IR detector is expected to extend the capabil-

ities of the emerging dormant, yet always-alert sensor networks for various

54

demanding IoT applications. Future work is aimed at fabricating arrays of

such detectors targeting different wavelengths to realize a highly sensitive

and compact NDIR chemical sensor that can perform fast measurement and

identification of a variety of chemicals.

55

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