Lab-on-a-Chip Diagnostic Biosensor - Colorado State …projects-web.engr.colostate.edu/ece-sr-design/AY10/biosensors/... · 1 Lab-on-a-Chip Diagnostic Biosensor First Semester Report

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Lab-on-a-Chip Diagnostic Biosensor

First Semester Report

Fall Semester 2010

By

Justin Grantham

John Blatt

Daniel Higley

Andrew Armstrong

Kelli Luginbuhl

Kelly Nienburg

Prepared to partially fulfill the requirements for ECE401

Department of Electrical and Computer Engineering

Colorado State University

Fort Collins, Colorado 80523

Project advisors: Dr. Kevin Lear, Dr. Dave Kisker, Dr. Susan Hunter

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Abstract The inability to diagnose tuberculosis quickly and accurately in developing countries has been pointed to

as a possible source of the disparity of deaths due to tuberculosis and other diseases in developing

countries as opposed to developed countries. The local evanescent array coupled waveguide biosensor

chip is a potential combatant to this disparity due to its quick diagnosis time (2-5min). The LEAC chip

optically senses the presence of antigens or antibodies in a test sample of blood.

In helping the progress of the LEAC chip, the senior design team working on the project this year

decided to focus primarily on automating the current LEAC chip measurement process for the fall

semester. Before this semester, the standard method of probing photodetectors on the LEAC chip was

to individually guide single probes to the metal pads of the LEAC chip. The senior design team decided

to implement a probe card based system to quickly and more effectively collect data from the LEAC chip.

This system uses a probe card to contact many pads on the LEAC chip at the same time. It also uses

precision rotational and translational stages to guide the probe card, LEAC chip and a laser precisely

together. Finally, this system uses an amplifier and switching circuit to interface with the probe card

and LabVIEW software to automatically collect data from these components. At the end of the

semester some data was collected, though there are still some modifications to be made before it will

be working fully.

The senior design team also made significant progress towards implementing a polydimethylsiloxane

(PDMS) microfluidic network on top of the LEAC chip to carry a blood sample over the waveguides on

top of the LEAC chip. A PDMS microfluidic network was not actually implemented this semester, but

this task should be completed early next semester.

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Table of Contents Abstract ......................................................................................................................................................... 2

Table of Figures ............................................................................................................................................. 4

Chapter I: Introduction and Motivation ........................................................................................................ 5

Introduction .............................................................................................................................................. 5

Motivation................................................................................................................................................. 5

Chapter II: Background ................................................................................................................................. 6

Local Evanescent Array Coupled Waveguide Biosensor Basics ................................................................ 6

Summary of Previous Work ...................................................................................................................... 7

Chapter III: Project Goals and Requirements................................................................................................ 8

Chapter IV: Chip Fabrication ......................................................................................................................... 9

Fabrication Basics ...................................................................................................................................... 9

Photolithography .................................................................................................................................. 9

Etching................................................................................................................................................. 10

Deposition ........................................................................................................................................... 10

LEAC Chip Fabrication ............................................................................................................................. 11

Chapter V: Probe Card Requirements and Selection .................................................................................. 13

Chapter VI: Probe Card, Sample, and Fiber Launch Motion Control .......................................................... 16

Overview ................................................................................................................................................. 16

Requirements and Considerations .......................................................................................................... 16

The Sample Mount .............................................................................................................................. 16

The Fiber Launch Mount ..................................................................................................................... 17

The Probe Card Mount ........................................................................................................................ 17

Chapter VII: Addition of Micro-fluidic Channels using PDMS ..................................................................... 19

Chapter VIII: Test Biomolecule Selection .................................................................................................... 21

Chapter IX: Probe Card Electronic and Software Interface ......................................................................... 21

Multiplexer Circuit .................................................................................................................................. 21

LabVIEW Technical Considerations ......................................................................................................... 23

Chapter X: Conclusions and Future Work ................................................................................................... 24

References .................................................................................................................................................. 25

Acknowledgements ..................................................................................................................................... 25

Appendix A: Abbreviations ......................................................................................................................... 26

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Appendix B: Budget Summary .................................................................................................................... 26

Appendix C: Guide to Probe Cards .............................................................................................................. 26

Appendix D: SU-8 Molds Fabrication Process ............................................................................................. 28

Appendix E: LEAC Chip Fabrication Flow .................................................................................................... 31

Appendix F: LEAC Fabrication Diagrams ..................................................................................................... 38

Appendix G: Gantt Chart for Fall Semester ................................................................................................. 40

Table of Figures Figure 1: The LEAC waveguide biosensor (Courtesy of Rongjin Yan)............................................................ 7

Figure 2: 64 to 1 MUX Circuit with Amplifier (Courtesy of Matt Duwe) ....................................................... 8

Figure 3: Current LEAC Chip Mask Design (Top View) .................................................................................. 9

Figure 4: Positive and Negative Photoresists .............................................................................................. 10

Figure 6: Micro Reactive Ion Etching Machine ........................................................................................... 10

Figure 5: Electron Beam Evaporation Machine .......................................................................................... 10

Figure 7: PECVD Machine ............................................................................................................................ 11

Figure 8: Mask of LEAC chip ........................................................................................................................ 11

Figure 9: Probe card received from Alpha Probes Incorporated ................................................................ 13

Figure 10: Close up view of the probes and epoxy rings ............................................................................ 14

Figure 11: The LEAC chip topology. Black dots indicate ground pads and red ovals enclose sets of pads 14

Figure 12: Block diagram of probing apparatus .......................................................................................... 16

Figure 13: Sample XYZ and rotation stage and Fiber Launch...................................................................... 17

Figure 14: Probe station with addition of the probe card .......................................................................... 18

Figure 15: Microscope image of probe card in contact with metal pads (Each pad is 105um Wide) ........ 19

Figure 16: PDMS outline ............................................................................................................................. 20

Figure 17: Original microchannel mask design ........................................................................................... 20

Figure 18: New microchannel mask design ................................................................................................ 20

Figure 20: MUX to probe card interface ..................................................................................................... 22

Figure 19: Interface Card to connect switching circuit to the probe card .................................................. 22

Figure 21: Edge Sensor Circuit .................................................................................................................... 23

Figure 22: Probe Card with Edge Connector Attached ............................................................................... 27

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Chapter I: Introduction and Motivation

Introduction A biosensor is a biologically sensitive device that is used to detect the presence of some biologically

reactive material or analyte. A biosensor consists of three parts, a biologically sensitive material, a

detector element and peripheral electronics to display the data. These three parts work together to

sense the presence of the substance in question, the analyte. A common example is a blood glucose

meter used by people with diabetes to monitor their blood sugar. In the case of the blood glucose

meter, the analyte is the glucose concentration in the blood, an enzyme called glucose oxidase is the

biologically sensitive material that reacts with the glucose in the blood and an electrode that senses

currents resulting from this reaction, is the detector.

If the biosensor is used to detect the presence of an illness and thus diagnose the illness, then it is a

diagnostic biosensor. When detecting the presence of an illness, specific types of bio-molecules must be

sensed. When a disease is present in the body of a host organism, the disease releases various chemicals

into the body of the host organism. These chemicals resulting from the disease organism are called

antigens. When the host organism senses the presence of these antigens, the host organism produces

antibodies. These antibodies bind to the antigens neutralizing them and preparing them for transport

out of the body of the host. Thus, when attempting to detect diseases with biosensors, the analyte could

be an antigen generated by that disease organism or an antibody generated by the body of the host in

response to the presence of the disease organism. Detecting either the antigen or the antibody is a good

indication of the presence of the disease. Furthermore, if an antigen is the analyte then the

corresponding antibody is a logical choice as the biologically sensitive material used to sense the analyte

and vice versa.

In the case of the LEAC waveguide diagnostic biosensor, the analyte is a Tuberculosis related antigen,

the biologically sensitive material is the corresponding antibody, the waveguide and related features are

the detector and peripheral electronics are used to interact with those detectors.

Following in this report will be a discussion of the motivations for working on this project. Chapter 2

goes into further detail on the function and fabrication of the LEAC chip as well as prior work done with

it. Chapter 3 will then include analysis of the project goals and requirements for this semester. Chapters

4 through 9 discuss the work done by this senior design team this semester which flows into plans for

work next semester in Chapter 10 followed by several appendices that will be cited throughout the

report.

Motivation This project has the potential to lead to a final product with a very positive impact on society. The

technology utilized provides significant benefits over current diagnostic practices and, when finalized,

could lead to a product that would be used in developing nations to increase the rate of diagnosis and

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treatment of Tuberculosis. Increased rates of diagnosis and treatment will lead in the long term to a

decreased death rate due to Tuberculosis in the areas where the product is implemented.

In developing nations such as many in Africa, the death rate due to Tuberculosis is significantly higher

than that of developed nations. As a whole, the continent of Africa has over 11 times the deaths

(normalized to population) due to Tuberculosis than the Americas (Citation Here). This disparity is a

result of many factors including environment, lifestyle and access to healthcare facilities. The goal of this

project is to combat the problem by increasing the access to healthcare in these nations.

There are currently several groups working towards this goal in various ways, however their efforts are

often impeded by the unavailability of inexpensive, transportable means of diagnosing tuberculosis. One

highly transportable diagnostic tool is the Tuberculin skin test. In this test, a substance called Tuberculin

is injected under the skin of an individual by a trained healthcare professional. The individual must then

return 2-3 days later to have the healthcare professional examine the injection site to make a diagnosis.

The problem is that the individual often doesn’t return to the clinic to be diagnosed and thus never

receives treatment. Other diagnostic practices may suffer from this time delay between sample

collection and diagnosis. In other cases, diagnosis is achieved more quickly, but there is a need for a full

laboratory environment to perform diagnosis which is expensive. The LEAC diagnostic biosensor seeks to

solve this problem by providing diagnosis in a matter of minutes in a cost effective way. The decreased

cost compared to other systems and the increased speed of detection will increase the availability of

treatment and have a positive impact on quality of life in these regions.

Chapter II: Background The local evanescent array coupled (LEAC) waveguide biosensor is a research project here at CSU that

has been going on for quite a few years under Dr. Lear. It is currently headed by Phd student Rongjin

Yan. He has numerous publications regarding the LEAC theory and application which can be easily found

online[2][3]. In this section the physics of the LEAC chip will be discussed as well as a very brief

summary of some of Rongjin Yan’s previous results.

Local Evanescent Array Coupled Waveguide Biosensor Basics The LEAC chip operates on the principle of total internal reflection (TIR) to optically sense biomolecules.

A laser light is optically coupled into the silicon nitride waveguide structure at the edge of the chip.

Since the index of refraction of the silicon nitride (n=1.8) is larger than the refractive index of the

surrounding silicon dioxide (n=1.48) and air (n=1), there is a critical angle where the light entering will

become trapped inside and will propagate down the waveguide. This is the same principle that is used

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in fiber optic communication channels to send information with light.

Figure 1: The LEAC waveguide biosensor (Courtesy of Rongjin Yan)

The waveguide is seen in Figure 1 as the thin green layer on the top of the chip. Even though the light is

totally internally reflected, there is a component of the electric field that is present on the outside of the

waveguide. This component is known as the evanescent field and it is this field that we use to detect

the biomolecules. When a biomolecule is attached to the top of the silicon nitride waveguide the

increased index of refraction causes the evanescent field to shift upwards and couple into the

biomolecule. This shift in power will reduce the field beneath the waveguide that is in contact with the

polysilicon detectors. The evanescent field profile and the polysilicon detectors are shown in Figure 1.

The field reduction on the polysilicon detectors increases the resistance of the detectors and thus

decreases the current flowing through them for a given voltage bias. The voltage bias is applied to the

ends of the detectors through metal vias also pictured in Figure 1. This change in current is how the

biomolecule layer can be detected at any point along the waveguide where there are buried detectors

that can be biased.

Summary of Previous Work This will be the first semester that a senior design team will be working in the LEAC project. Previously it

has been worked on solely by graduate students and thus has a lot that can be expanded on by

undergraduates. There currently exist two different chips that are referred to as LEAC chips. The first

generation was a design that was on a corner of a chip that was manufactured by Avago. The most

recent version of the chip however is fabricated in the CSU cleanroom. This new generation of LEAC

chip allows for much more freedom of experimentation since new chips can be fabricated at will.

Currently Rongjin has manual probe arms that he guides under a microscope to each individual pad to

apply the bias. We wish to automate this system as described in Chapter III: Project Goals and

Requirements.

A switching circuit has also been designed already by graduate student Matt Duwe. This circuit consists

of 9 separate 3-8 MUX’s arrayed together, allowing for the selection of only one of 64 channels. This

selection is made by inputting 6 digital control lines into the multiplexers, choosing between the 26=64

channels. This chosen channel is then passed through a transimpedance amplifying stage with a gain of

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500 million V/A. Further information may be found in [1]. The top view of the circuit can be seen below

in Figure 2.

Figure 2: 64 to 1 MUX Circuit with Amplifier (Courtesy of Matt Duwe)

Chapter III: Project Goals and Requirements Throughout this semester and into next semester we as a group were hoping to achieve a real time

sensing capability with the LEAC chip. As mentioned previously, right now the only way to measure each

buried detector pad is to tediously move individual probes to each position. This makes it difficult and

time consuming to obtain the information we need to analyze each experiment. In order to expedite

this process we decided one of our main goals was to assemble and build a probe station that we could

use to maneuver a probe card to the chip. Once the probe card is in contact with the chip we would

then be able to easily toggle between all of the different detectors along the waveguide.

In addition to this automation of the LEAC chip, we also desired to better implement the microfluidics

that will be utilized on the chip. Right now antigens and antibodies are simply printed and rinsed off of

the surface of the waveguide. This process also required that the biomolecules dry on the surface since

the user had to individually probe each detector. Now with the automation process in operation, we

will be able to flow biomolecules down a microfluidic channel on top of the waveguide and take a

measurement instantly after the injection. We also wish to further explore different variations of the

chip through the fabrication process. A mask has been made with a large number of variations in widths

of features that we can test to find which dimensions and arrangements work the best.

Once we implement the microfluidics and the probe station set up, we wish to further characterize the

ability of the LEAC chip to sense the biomolecules. This includes achieving a given sensitivity (detect

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whether a molecule is present) and specificity (whether we can tell biomolecules apart). This

characterization portion of the project will be the main focus of the spring semester.

Chapter IV: Chip Fabrication Fabrication of the chip is done in the cleanroom located in the basement of the engineering building.

We have recently just acquired all of the technology needed to manufacture the LEAC chip completely in

our laboratory. In this section, a brief introduction to the fabrication processes and technology used in

the manufacture of the LEAC chip will be introduced. A short walkthrough of the LEAC chip fabrication

in particular will then be discussed. The full Fabrication flow for our chip may be found in Appendix E:

LEAC Chip Fabrication Flow and Appendix F: LEAC Fabrication Diagrams.

Fabrication Basics The LEAC chip is fabricated using many common processes and techniques used in the semiconductor

manufacturing industry. Among these are photolithography, etching, and deposition. Pictured in Figure

3 is one of the mask designs that are used in the fabrication

of the chip. Using individual layers of this mask one at a

time, all of the features can slowly be developed from a

featureless silicon wafer.

Photolithography

The technology to actually use the individual mask layers of

the chip is called photolithography. Every sequence of the

photolithography process starts by applying a layer of

photoresist to the top surface of the chip. Photoresist is a

liquid substance that is spun on to the chip using a vacuum

chuck that holds the chip in place. Resist is placed on the

top surface, and spun for a specified amount of time

depending on how thick the user wishes the resist layer to

be. Upon applying the resist to the surface, the chip is then placed under the mask aligner machine,

where an individual mask layer is positioned over the chip. UV light is then exposed to the chip,

chemically reacting with the photoresist depending on whether it is positive or negative. Upon

development, positive photoresist will leave a mask of resist in the shape of the features that were

prevented from being exposed to the UV light. Negative photoresist behaves in the opposite way,

leaving a layer on the chip where the resist was exposed to the light, thus achieving a “negative”

replication of the mask. A cross section diagram is shown of the difference in Figure 4.

Figure 3: Current LEAC Chip Mask Design (Top View)

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Figure 4: Positive and Negative Photoresists

This mask of photoresist is now used to shield certain parts of the chip so that etching and deposition

can now be done on only the surfaces exposed.

Etching

Etching is the process of removing a specified amount of material from the surface of the chip. This is

typically done by either a wet etch or a dry etch. Wet etching refers to dipping the whole chip in acid

and removing material. Dry etching is performed by bombarding the surface with ions, which removes

material at a specified rate. Dry etching is the cleanroom is done by

using a micro reactive ion etching (micro-RIE) machine as pictured in

Figure 6. The micro-RIE machine uses a strong radio frequency

electric field to ionize a certain gas and create plasma, which reacts

with the material on the chip, removing it. Wet and dry etching each

has advantages and disadvantages. Both normally involve dangerous

gases or chemicals, so caution must be taken when performing both

processes. An advantage to dry etching is that its etching profile is

normally more anisotropic. This means that the etching primarily

acts in the vertical direction and will not etch sideways into material

that is covered by photoresist. Wet etching on the other hand is

normally very isotropic, meaning

it etches in all directions.

Figure 6: Micro Reactive Ion Etching Machine

Deposition

Deposition is the process of adding a layer of material on to the

surface of the chip. This can be a layer of metal, semiconductor, or

even an insulator. Metal is deposited on the surface of the chip

using a technology called electron beam evaporation physical vapor

deposition. The machine that performs this is pictured in Figure 5.

This involves placing the sample inside a vacuum chamber with a

crucible of the desired metal. The metal is then heated to very high

temperatures by shooting it with a beam of electrons, which then

evaporates it. The evaporated material flies through the vacuum

Figure 5: Electron Beam Evaporation Machine

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until it contacts the chip, leaving a layer of metal on top of the chip.

To deposit layers of semiconductor or insulator, a technology called plasma enhanced chemical vapor

deposition (PECVD) is used. In this process the sample is again placed inside of a vacuum. A specific

mixture of gases is then pumped into the chamber and inductively turned into plasma. A chemical

reaction then occurs on the surface of the chip which then builds up the layer of material. The PECVD

machine is pictured below in Figure 7.

Figure 7: PECVD Machine

By cycling through these basic steps we are able to create all of the features needed on the LEAC chip.

LEAC Chip Fabrication The LEAC chip is fabricated using multiple runs of

the processes describe above. The fabrication flow

described here will correspond to the cross

sectional view seen in Appendix F: LEAC Fabrication

Diagrams. The following is just one of the

fabrication flows possible that will produce a

variation of the LEAC chip. The specific process

depends on the available wafer substrate we have

available to start with, as well as if we are

experimenting with slightly different structures.

This, however, is the most common as of now and

chips using this flow have been successfully

produced. Shown below in Figure 8 is the newest

mask design that is used in the fabrication of the

LEAC chip. Figure 8: Mask of LEAC chip

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The four different quadrants show the four main mask layers: Metal, Waveguide, Metal Via, and Buried

Detectors. Within each quadrant are variations within each mask layer. These variations have different

dimensions and features that will be tested on the LEAC chip. Refer back to Figure 3 for a top view of the

chip with all 4 layers stacked on top of each other.

Fabrication begins with a silicon wafer with 100nm silicon dioxide (SiO2) and 255nm silicon nitride

(Si3N4) layers on top of it. The SiO2 layer is simply for insulating the polysilicon layer from the silicon

substrate. The first step in fabrication is etching the polysilicon layer into the buried detector structures.

This is done by using the buried detector mask (top right quadrant of Error! Reference source not

ound.) to place a mask of photoresist on the top of the chip and etching it down, as described in

Fabrication Basics. In this case the etching is performed by the micro-RIE machine, and thus is dry

etched. Recently we have begun experimenting with a wet etch of KOH to remove the polysilicon as the

dry etching is not very selective and at times has removed our insulating silicon dioxide layer. In this

case, however, a mask of Si3N4 must be used as the photoresist cannot stand up to the KOH.

Next the metal pads and wires are laid down. This is done by first applying a layer of negative

photoresist and performing what is known as lift off. After the photoresist is applied and has been

developed using the metal mask layer, aluminum is deposited using the e-beam evaporation machine.

After, the whole sample is submerged in acetone with dissolves the photoresist and thus “lifting off” the

metal lying on top of it. This leaves the aluminum only in the positions that we wish.

After this a layer of insulating silicon dioxide must be deposited to separate the buried detectors from

the waveguide. This is done by using the PECVD machine as described above, which will deposit silicon

dioxide all over the top surface of the chip. Since a lot of insulation is desired we normally deposit at

least 1um of SiO2 on the surface. Immediately following this insulator deposition a layer of Si3N4 is

then applied directly on top of the SiO2, again using the PECVD machine. This layer of Si3N4 is then

etched into the waveguide features by using positive photoresist (S1818) and the waveguide mask.

All that is left after these steps is to gain access to the metal pads that are buried beneath the SiO2

layer. To achieve this we use the “via” mask to cover the whole chip with photoresist except for the

locations with the metal pads. The SiO2 layer is so thick that dry etching would take way too long to

perform so we use wet etching once again. For this step hydrogen fluoride (HF) is used. It eats through

SiO2 very fast and thus only requires about an 8sec dip before the vias are opened all of the way

through.

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Chapter V: Probe Card Requirements and

Selection This section describes the requirements for the probe card ordered this semester, our reasoning for

ordering a probe card and decisions on parameters of the probe card. If further information is desired

on the general aspects of probe cards, Appendix C:

Guide to Probe Cards, serves as an introductory

guide to probe cards.

To make the process of testing the LEAC chip much

quicker the team decided to implement a probe

card to be able to probe many of the LEAC chip's

photodetectors simultaneously. This probe card

was purchased and implemented this semester.

Before the implementation of this probe card,

making measurements with the LEAC chip entailed

probing individual pads with individual probes and

measuring the current across a photodetector for a

specific voltage with each iteration. After the

implementation of the probe card, probing the first

32 photodetectors connected to any one waveguide

on the LEAC chip simply required setting down the

probe card once and using a system of multiplexers,

amplifiers and LabVIEW software to systematically

measure the currents running through all individual

photodetectors for a given applied voltage. The

LabVIEW probe card interface is described in

Chapter IX: Probe Card Electronic and Software

Interface. The motion control system necessary to

precisely align the probe card with the LEAC chip is

described in Chapter VI: Probe Card, Sample, and

Fiber Launch Motion Control.

The requirements for the probe card were fairly

standard since we do not have a high-frequency,

high-power, high-temperature, or low-temperature system. Also, the LEAC chip has aluminum pads to

be contacted (as opposed to gold pads) since high contact resistance is not a large problem. Therefore,

Figure 9: Probe card received from Alpha Probes Incorporated

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we chose to use standard tungsten probes. Since the LEAC chip requires a moderately high probe

density (probe pitch, or distance between adjacent probes, of 105 mils) and does not need any of the

benefits blade cards provide, we chose to use an Epoxy-ring probe card topology.

We chose to order our probe card from Alpha Probes Incorporated, which is based in Colorado Springs,

Colorado because they provided us with the cheapest quote ($235 for Alpha Probes versus $500 for

Accuprobe) out of the vendors we contacted and they are relatively local. Figure 9 is an image of the

probe card we received. Figure 10 is a

zoomed in view of the probes and epoxy-ring

configuration on the probe card.

Essentially, our probe card consists of a large

PCB (4.5” width, 9 3/8” length, 0.062”

thickness) which has a hole to accommodate

probes on one side and metallic fingers to

accommodate connection of the probes to

external electronic circuits on the other. The

probes are very precisely aligned on the

bottom of an aluminum ring using epoxy. The

metallic fingers are accessed via a standard

edge card connector. Our probe card has a

total of 34 probes and 2 edge sensors. We chose to use this many probes because, it allows us to probe

all of the probes for a given waveguide

simultaneously well avoiding the significant

cost of added probes. Thirty-two of these

probes are used to contact the biasing side of photodetectors and the other two are used to contact

ground pads for the photodetectors. All of the photodetectors for a single waveguide share a common

ground. The reason for having two ground probes was because the ground pad is on a different side of

the pads, depending on which section of

the LEAC chip is being probed. Figure 11

shows this. The edge sensors were placed

in line with the rest of the probes but at a

strategic distance away to avoid damaging

any features on the chip. The edge sensors

are used to determine when contact is

made. Our utilization of them is further

described in Chapter IX: Probe Card

Electronic and Software Interface. We had

the choice between insulating and non-

insulating edge sensors when we ordered

the card. We went with the non-insulating

Figure 10: Close up view of the probes and epoxy rings

Figure 11: The LEAC chip topology. Black dots indicate ground pads and red ovals enclose sets of pads

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for two reasons; they were cheaper ($15 versus $35)[1] and our chip has an insulating layer on the top

surface and would not be effected by the live circuit.

An application specific problem we had during the design process was that we will have a ~4mm tall

obstacle in the center of our chip which had to be avoided. This obstacle was the

Polydimethylsiloxane (PDMS) which is further described in Chapter VII: Addition of Micro-fluidic

Channels using PDMS. The approximate location of PDMS is the orange rectangle in Figure 11. In order

to avoid the possibility of any part of the probe card contacting the PDMS, we increased the height of

the epoxy-ring holding our probes from the standard 80 mils to 250 mils. We also left out the half of the

epoxy-ring (white substance in Figure 10) that would be interior to the LEAC chip when probing.

Another possibility we considered was cutting the PCB of the probe card completely off halfway through

the hole accommodating probes. This option was discarded when our vendor informed as that this

might damage the structural integrity of the card and possibly warp it. We also considered using probes

with longer probe tip lengths, but our vendor informed us that this would cause the probes to be much

too fragile and that we would easily break them[2].

Because our application for this probe card is necessarily very low noise, we thought it would be good to

place the amplifier circuit for signals exiting photodetectors on the probe card itself. Unfortunately, our

vendor informed us that this would add thousands of dollars to the cost and we could not afford such an

increase so we were forced to keep the circuit external and make sure we had effective shielding for the

probe card electronic interface.

Ordering and receiving the probe card took much longer than we originally anticipated and was a major

factor for us not making our originally stated goals. We had planned on receiving the probe card by the

20th of October but did not actually receive it until the 22nd of November. The major contributing factors

to this delay were difficulty in determining exactly how we wanted the probe card configured (due to a

lack of readily available information) and difficulty in communicating with the vendor exactly what we

needed for our application.

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Chapter VI: Probe Card, Sample, and Fiber

Launch Motion Control

Overview Figure 12 above shows a block diagram of the Probing Apparatus used in our project. To take a

measurement from a sample of the LEAC chip, one must first bring probes into contact with metal pads

on the chip. To achieve the goal of automated measurement, a row of 32 probes must be

simultaneously aligned to 32 corresponding metal pads on the sample. Once the probes are in contact,

laser light is then coupled into a waveguide on the LEAC chip. This process is very sensitive and thus

requires the use of an extremely precise piezo-electrically controlled XYZ stage called a fiber launch.

Once everything is in place, 32 measurements are taken very quickly through the switching circuit with

the help of the LabVIEW interface. This set up is fairly complex, especially compared to a single probe

measuring system. This results in a longer set up time per sample. However, even if this set-up requires

four times longer getting ready, the number of measurements that can be taken without modifying the

setup (unlimited) more than compensates for this disadvantage.

Requirements and Considerations There are a number of things that needed to be taken into consideration throughout the design of the

probe station. With so many movable stages we had to thoroughly plan how every piece would

translate together and in the order they would do so.

The Sample Mount

The pads on the sample must be precisely aligned to the probes on the probe card. This is achieved

through the use of an XYZ stage combined with a rotation stage. With these degrees of freedom, the

probe card can be arbitrarily fixed in position and then the sample moved precisely into a position just

below it. Planarity must also be considered so that some probes do not contact the sample before the

Figure 12: Block diagram of probing apparatus

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others. To guarantee planarity, the metal bracket that connects the XYZ and rotation stages must be

fabricated very precisely. For this reason we had the metal bracket fabricated to very strict tolerances

using a digitally controlled mill in a machine shop. It was also designed to accommodate the previously

mentioned rotation stage and a second stage that allows a planarity adjustment if we think we need it.

The Fiber Launch Mount

The control of the fiber must be very precise to allow light coupling into the waveguide. To achieve this

we took advantage of a piezo-controlled x-y-z translation stage that we found in the lab. This allows us

to precisely align the fiber to within micrometers of where we need it to acheive the most optical power

into the waveguide. Considerations must be made to eliminate any tall features on this stage to allow

clearance for the probe card to move down to the pads. We were able to extend the fiber only in the

direction toward the sample by utilizing glass slides as a sort of “plank” to support it as seen in Figure 13.

The Probe Card Mount

The probe card must be affixed to something that allows very precise Z-motion in order to bring the

probes into contact with the sample. To achieve such precise tolerances for something as large as the

probe card appeared to be a difficult task until we came upon the mounting station as seen in Figure 14.

These metal slats proved to have a very precise z-motion capability (2mil per full revolution of the

handle) which was very much in line with our 1.5mil over travel specification on the probe card. The

probe card however has exposed wires beneath it so it could not simply be setting on top of the metal

slats. In order to achieve isolation between the items we had glass custom cut to sit beneath the probe

card. This also proved to be beneficial in allowing clearance for the edge connector to interface with

the probe card.

Figure 13: Sample XYZ and rotation stage and Fiber Launch

Fiber extension

18

Notice above in Figure 13 the glass slides extending the fiber to the center of the rotation stage. This is

desirable for the alignment of the fiber to the LEAC chip. In addition, the sample is the tallest feature in

this setup, guaranteeing clearance for the probe card.

Figure 14 below shows the probe station setup with the addition of the probe card. Notice how the

entire sample and fiber apparatus has been rotated 90 degrees to accommodate the probe card.

Figure 14: Probe station with addition of the probe card

Probe Card

Metal slat of probe station

19

Figure 15: Microscope image of probe card in contact with metal pads (Each pad is 105um Wide)

Figure 15 above shows the contact of the probe card tips with the metal pads on the surface of the LEAC

chip. The blurry probe structures on the right are the edge sensors which are further discussed in

Chapter V: Probe Card Requirements and Selection and Chapter IX: Probe Card Electronic and Software

Interface.

Chapter VII: Addition of Micro-fluidic Channels

using PDMS One of the primary goals of this design project is to integrate microfluidic channels onto the chip. The

integration of these channels will enable the implementation of a real-time detection system using

LabVIEW. The initial setbacks with this portion of the design were limitations in fabrication needs. The

lab did not possess a vacuum oven or the polymer media needed to fabricate the channels. The

polymer used to create microchannels is polydimethylsiloxane (PDMS).

Having also acquired training in the clean room for fabricating the SU-8 mold, from which the PDMS is

cast, fabrication of the microfluidics is on track. The clean room process for making the SU-8 molds can

be found in Appendix D: SU-8 Molds Fabrication Process. Next semester we hope to optimize the

channels and to test surface treatments and new designs that promote binding and mixing of the fluid

to bring the biomolecules to the surface. We also intend to optimize the positioning of the PDMS piece

for integration and test leakage, different PDMS to surfactant ratios for varied flexibility of the polymers,

and different curing techniques that alter the level of reversible binding for chip reuse.

20

In addition to learning the methods needed for fabrication this

semester, it was found that the PDMS needs cut to within 500

microns to a specific size so as not to cover the sensors on the chip.

However, we also want the PDMS to be big enough to cover the

first few sensors, which are the most accurate and have the

strongest signal, are still utilized. These requirements yield a PDMS

piece about 0.9 mm2. The group came up with several ideas

including the assembly of a metal dish of the exact size, which the

PDMS would be poured into, or redesigning the mask with cutting

guides.

The mold is typically placed in the bottom of a Petri dish and the

PDMS then fills the dish and is cut out around the features with a razor blade. By designing and creating

a dish of the exact dimensions needed for overlay on the chip, the final PDMS piece would be the exact

size and less material would be wasted in excess pouring. The PDMS can be as thin as 0.5 mm and as

thick at 5 mm with a pouring precision of about 0.3 mm. This thickness will be affirmed next semester

when the size of the pumping reservoir is determined.

The team decided to try a mask redesign with cutting guides first.

Unfortunately, when opening the old mask designs to add cutting

features on the LASI program, it was seen that the original design

is too big for the size guidelines and the microfluidic features

themselves are already outside the bounds needed to keep the

pads exposed. The design needs to be completely revamped to

reduce the channels and reservoirs and enable implementation of

razor blade guides outlining the desired size for the PDMS square.

Figure 16 shows an approximate PDMS perimeter, positioned

correctly over the chip.

In order to cut out the entire design, the PDMS square would cover

the entire chip. After reducing the size of the reservoirs (circles) and adjusting the channel positioning,

the design of Figure 18 was created, with a cutting guide border.

This design still needs to be run by the advisors and we hope to

schedule a meeting with Dr. Lynn to discuss whether the new

parameters, particularly channel thickness and reservoir size, will be

appropriate for fabrication. Once all of those issues have been

addressed, the new mask will be ordered, a new mold processed in

the clean room, and the properly-sized PDMS pieces fabricated and

ready for integration studies.

Figure 16: PDMS outline

Figure 17: Original microchannel mask design

Figure 18: New microchannel mask design

21

Chapter VIII: Test Biomolecule Selection In order to make a diagnosis our system is dependent on the occurrence of antibody/antigen

bonding. Antibodies are a part of the immune system produced by a person in order to bind to and

thus, inactivate foreign objects. These foreign particles can be a virus or bacteria and are the antigens.

Their structure is basically universal, consisting of a heavy chain and a light chain, however there is a

section on every antibody called the variably region. This region is very specific to the type of antigen

the antibody will bind to. This specificity is what allows us to test for a disease in someone’s blood.

Once a probe molecule, an antigen or antibody, is printed onto the chip or on the microfluidic channel, if

the sample has the corresponding disease, then the preprinted biomolecule will bind to the target in the

sample.

For initial testing of the system, bovine serum albumin (BSA) will be used. BSA is an accessible and

relatively cheap biomolecule. Using fluorescently labeled BSA will allow us to verify that the

biomolecules are patterned correctly. Eventually the use of a tuberculosis strain will be used but

currently it is more cost effective to use a cheaper biomolecule for the testing out the system.

One of the further off goals for this project is to have a lab-on-a-chip with various different channels to

test multiple biomolecules. These different biomolecules could be different forms of tuberculosis that

would specify the test more or could test for other diseases effecting a similar population. Size is also an

important factor in determining which biomolecules to test for in this system. Both malaria and hepatitis

B are diseases that could show interest to look into testing in the future as both affect similar

populations as tuberculosis and are two of the most common and deadly diseases. They both also have

similar sizes to tuberculosis biomolecules thus would be likely candidates to work within our system.

Chapter IX: Probe Card Electronic and Software

Interface

Multiplexer Circuit In order to efficiently measure all 32 of the pads connected to the buried detectors a multiplexing circuit

was designed by Matt Duwe (See Summary of Previous Work). This circuit was capable of using six

binary input channels from a computer to select and let pass through only one of the sixty-four separate

inputs. The circuit is then followed by a transimpedance amplifier and an additional voltage amplifier.

These two amplifying stages provide a gain of 500 million times. This is needed since the buried

detector resistances are so high and thus for a reasonable bias we would only be seeing currents in the

range of 10-100 nA.

22

While this circuit was a great start, it needed a lot of peripheral connections made with it. This including

all of the input channels going from the

circuit board and interfacing with the

probe card. Deciding how to achieve this

connection proved to be quite a bit more

complex than was originally thought. The

probe card size we ordered require a 70

conductor edge connector, which itself is

not a very common item. Due to this an

edge connector could not be found that

would interface directly with a ribbon

cable, thus an interface card had to be

made that would allow the conduction

paths to successfully interface with the

two connectors (Figure 19). The fully

assembled circuit can be seen in Figure 20 .

Figure 20: MUX to probe card interface

As mentioned in the previous sections, the probe card contains edge sensor pins that we could interface

with to tell whether we have made contact with the chip. The Accuprobe website describes the edge

sensors as being normally closed (in contact with each other) and when contact is made by the probes

the circuit is broken. In order to take advantage of this system a very simple circuit was designed that

would indicate whether the edges sensors have come in contact. The edge sensor circuit design is

shown in Figure 21.

Figure 19: Interface Card to connect switching circuit to the probe card

23

Figure 21: Edge Sensor Circuit

It can be seen from the circuit diagram that if either of the edge sensors come in contact with the

surface of the chip it will break the connection. This break will stop current flow which will shut off the

corresponding LED. Thus, when the LED turns off we can tell whether that edge of the probe card has

come in contact.

LabVIEW Technical Considerations One of the main goals for this semester was to increase the speed of taking data by writing a LabVIEW

program that would automatically switch between the channels, take multiple data points, and figure

out the standard deviation of the data points. This would hopefully reduce the time to take data from

over an hour to just minutes.

The first iteration of this program had the ability for multiple channels to be selected, although they had

to be in series (For example, testing channels 5 through 12 is possible, but only testing channels 3, 7 and

24 is not.)

A further addition gave the ability to look at a reference channel after every other channel, comparing a

known value every time to make sure there is no drift in the system that would skew later results. There

was also a residence time feature added, which is a waiting period between when the Hytek iUSBDAQ

would switch channels and when it would start to record data points.

The limitations of this version of the program only allow it to take one point on each channel, and

output the data and channel number into one column to excel. Further testing still needs to take place,

as well as an update to the graphical user interface to make it more user friendly so that anyone from

the team can easily run test.

24

Chapter X: Conclusions and Future Work This fall semester has proven to be very successful. We have successfully worked together as a team to

fabricate the LEAC chips, design the PDMS channels, and partially automate the measurements. The

LEAC chip has been successfully probed and measurement s taken through a computer interface. We

are prepared to fabricate and begin tests using the new PDMS channel designs. Throughout this

semester we have also learned a lot about working in research laboratories with a large variety of

equipment and how to manage our time throughout the semester (See Appendix G: Gantt Chart for Fall

Semester).

Future work for the spring semester will consist of a few parts. The LabVIEW switching circuit still needs

to be finalized and tested thoroughly on old LEAC samples to be sure we are getting reliable data. We

also are ready to fabricate the newly designed PDMS microfluidic channels that we will use to locally

apply biomolecules to the surface of the chip. Lastly, we will characterize the ability of the LEAC chip to

sense different types of biomolecules and obtain sensitivity and selectivity baseline and try to improve

upon it if needed.

25

References [1] A. Nott, private communication, October-November 2010

[2+ Accuprobe, “Probe Card Tutorial,” 2003,

http://www.accuprobe.com/Downloads/Probe%20Card%20Tutorial.pdf

[3]Compiled by A. D. McNaught and A. Wilkinson. (1997) “IUPAC. Compendium of Chemical

Terminology, (the “Gold Book”). (2nd ed.) . *On-Line]. Available: http://goldbook.iupac.org [12/6/10]

[4] M. Duwe, “Low Noise Selecting Circuit for Local Evanescent Array Coupled Biosensor” (Senior Honors

Thesis), unpublished.

[5] R. Yan (2010, January 10). Review of label free optical biosensor.

[6] R. Yan, Mestas, S., Safaisini, R., & Lear, K. (2009, August 7). Label-free silicon photonic biosensor

system with integrated detector array. Lab on a Chip, 9(15), 2163-2168.

Acknowledgements We would like to acknowledge a few individuals that have considerably helped us throughout this

semester. It is with their guidance that we have been able to get so far on this project this semester.

Our advisors Dr. Kevin Lear, Dr. Dave Kisker, and Dr. Susan Hunter have been invaluable guides with so

much combined experience in research. PhD student Rongjin Yan has also been a great guide and has

been very patient in recommending goals and helping us get adjusted to the research lab environment.

Graduate students Tim Erickson and Iris Yi have also been a great help in the fabrication of the LEAC chip

and without them we could not have done it. Andy Nott at Alpha Probes has also been very patient and

has been willing to guide us for quite some time in the design of our probe card. Lastly we would like to

thank two professors on campus, Dr. Lynn in the Chemical Engineering Department and Dr. Henry in the

Chemistry Department, both were very generous with both their equipment and materials. Also a

postdoctoral researcher in Dr. Henry’s lab, Jason Emory, was generous enough to provide training on the

PDMS casting process and also expressed the potential that our group would be able to use the lab’s

fluorescence microscope for future imaging needs as well.

http://www.accuprobe.com/Downloads/Probe%20Card%20Tutorial.pdf

http://goldbook.iupac.org/

26

Appendix A: Abbreviations E-Beam PVD: Electron Beam Physical Vapor Deposition

LEAC: Local evanescent array coupled

LED: Light Emitting Diode

MUX: Multiplexer

PDMS: Polydimethylsiloxane

PECVD: Plasma enhanced chemical vapor deposition

S1818: Shipley 1818 positive photoresist

Via: A vertical “passage” connecting different layers of a silicon chip

Appendix B: Budget Summary Item Cost Spent From

Probe Card $305.00 Rongjin Yan's Research Budget

Switching Circuit Supplies $20.00 Personal

Probe Station $50.00 Rongjin Yan's Research Budget

$20.00 Personal

Total $395.00

Personal expenses may be reimbursed through the $50 per semester per student ($300 for the whole

year) at the end of the academic year. Rongjin Yan’s research budget may be further used at his or Dr.

Lear’s discretion. There are no guarantees of future funding through the research budget.

Appendix C: Guide to Probe Cards This appendix is intended to serve as an introduction to probe cards and the terminology

associated with using them. Chapter V: Probe Card Requirements and Selection outlines the

requirements and selection of the probe card used with the LEAC chip.

Probe cards are commonly used to manually and automatically probe test wafers in

industry. The probe card is essentially a printed circuit board (PCB) with a circular hole which

accommodates probes. The PCB part of the probe card generally has traces from these probes to

metallic fingers on one edge of the card. If making a custom probe card, the PCB can be layed

out using PCB layout software such that discrete components or entire circuits can be soldered

directly to the probe card. The metallic fingers which are electrically connected to the probes of

the probe card can be accessed through a card edge connector. Figure 22 shows the probe card

received to be used in conjunction with the LEAC chip with a card edge connector attached.

Figure 9 shows a more zoomed in view of this probe card.

27

There are two main types of probe cards. The traditional blade probe card has superior

mechanical stability and a higher integrity signal path than the more recently developed epoxy-

ring probe card. To make a blade probe card, ceramic blades are inserted into lands on the probe

card and soldered down. These blades have probes sticking off them which are bent into the

required shape for the component to be probed. Because of their superior mechanical stability,

blade probe cards are less likely to need re-planarization adjustments. Blade probe cards are

inferior to epoxy-ring probe cards when it comes to probe densities and probe counts (blade

probe cards generally have a probe count less than 88 where as epoxy-ring probe cards can have

probe counts greater than 2000) [2].

The epoxy ring probe card technology used to be more expensive, but with recent advances in

techniques and technology, has become around the same price as the blade probe card

technology. The epoxy-ring probe card topology has now overtaken the blade probe card

topology in terms of popularity. An epoxy-ring probe card can be used for most applications

except those which deal with high-frequencies (> 3 GHz) or very low leakage currents (< 1

pA/V) [2].

The most commonly used materials to make probes out of are tungsten, tungsten-rhenium and

beryllium-copper. Tungsten is the most commonly used and cheapest of these materials. It

provides good probe stability and probe life for most applications. Oxide crystals can become

trapped in Tungsten probes periodically due to their fibrous nature [2]. This means tungsten

probes will require occasional cleaning to maintain acceptable contact resistance. Tungsten-

rhenium probes are very similar to tungsten probes except they do not have the fibrous nature of

tungsten and thus, trap oxide crystals within them less readily. Beryllium-copper probes are used

when low contact resistance or high current is required. These probes wear faster because

beryllium-copper is softer than tungsten or tungsten-rhenium.

Typical epoxy-ring probes have a beam and taper which is where the probe comes out of the

epoxy-ring and begins to taper towards its tip size. At the junction of the taper of the probe and

Figure 22: Probe Card with Edge Connector Attached

28

the probe tip, the probe is bent so it points downward at the point to be contacted. The part of the

probe which is pointed downward is called the probe tip. An example probe beam and taper

lengths is a few hundred mils and a probe tip length of around ten mils. Probe tips can be

increased in length with the consequence that they become more fragile [1]. Probes should be

sized to be around 30-40% of the size of the bond pad to which they are to connect [2].

Edge sensors are sets of two or three probes which form a closed circuit between two probes

when they are not in contact with the device being probed and form an open circuit when they

are in contact with the device to be probed. Edge sensors are commonly used in industry to

automatically probe test wafers (the machine doing the probing can tell when contact is made).

For people that are experienced with using probe cards, edge sensors will likely not be necessary,

but we found that having edge sensors helped us tremendously in determining when contact was

made and determining if there were any planarity errors between the probes. Edge sensors can

be designed to have an isolated tip (3 probe configuration) to avoid interfering with the device

under test in case it contacts a conductive part of the device under tests [2].

Appendix D: SU-8 Molds Fabrication Process

Fabrication Process for SU-8 Molds

1. Clean a Si wafer (approximately 2”×2.5”) with Acetone, Methanol and DI water; blow dry with

N2, bake on the hot plate @120 °C for at least 5 min;

2. Pour an even layer of PR 2035 on the cleaned Si wafer (make sure the first attempt covers the

center area);

3. Make the PR reach the edge of the chip by tilting it slightly (note that wrinkles are BAD!);

4. Spin on PR with the speed shown in Fig 4.3 (ramp_1 = 100 rpm/sec determines how even the

layer would be, ramp_2 = 300 rpm/sec determines the final layer thickness);

29

Fig. 4.3 Spin speed to achieve an even SU-8 layer

5. Soft bake @60-65 °C for 2-5 min (it’ll even defects our, if any); then @90-95 °C for 15 min (until

the PR is hard); let sample cool down to room temperature;

6. Expose with UV-34 filter for 190 mJ/cm2;

7. Post bake @60-65 °C for 1 min, then @90-95 °C for 10 min, then ramp down to 60-65 °C for 2-3

min; let sample cool down to room temperature; if the exposure is adequate, the outline of the

features will show up after 1min of 90-95 °C baking;

8. Use PGMEA(Propylene Glycol Methyl Ether Acetate) as developer, develop until unexposed area

is clear (usually take 5-8 min);

9. PGMEA goes to solvent waste bottle immediately after use;

10. Before use, the mold can be cleaned by rinsing it with Methanol, then blow dry with N2.

And the PDMS channel fabrication process is:

1. Take a plastic container (with lid) and put it on the scale, then turn it on (this would zero the

scale reading);

2. Measure PDMS and its curing agent to a weight ratio of 10:1;

3. Stir slowly with a disposal stirrer to mix well;

4. Degas the mixture carefully until clear;

5. Pour adequate amount of the mixture into a mold loaded tray, and make the mold as parallel to

the fluid (gel) level as possible;

6. Degas again;

7. Put the tray in the oven and bake @85 °C for 2 hours, or @65 °C for 4 hours to reduce its

rigidity;

8. Use a Methanol cleaned razor blade to cut the cured PDMS and gently peel the desired area off;

(for short-time storage, put the PDMS on a piece of cleaned Si wafer);

0

600 600

4000 4000

0

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 20 40 60 80 100

Spin

Sp

eed

/rp

m

Spin Time /sec

Ramp_1

Ramp_2

30

9. Always throw away the first piece peeled off from a mold, it could only offer rough channel

edges;

10. If done, seal the container and put it in the fridge (it could only last a few days, even in the

fridge); if not, go to step 5 to make more pieces of PDMS.

31

Appendix E: LEAC Chip Fabrication Flow

LEAC sensor fabrication flow

1a) Buried detector fabrication for dry etching

a) etch the whole sample down to the desired thickness, poly SI etching rate is ~1.3nm/sec, not very

accurate, varies everytime, need filmmatrix to confirm

MicroRIE:

O2 Plasma, 40sccm, 40 W, 30secs

Place chip into the chamber with glass slips around it

CF4+8%O2, 40sccm, 50 W, starting pressure 108mT, time = d/1.3nm

b) BD PR mask

Clean the sample with acetone, methanol and water

Prebake, 120C, 2mins, let it cool to room temperature

[apply PMDS, 3000rpm, 30secs]

Spin S1818 (positive), 6000rpm, 30secs

Softbake: 100C, 2mins

Remove edge bead with razor

Exposure: total 191.4mJ, Right: 13.2mj, 14.5secs. Left: 10mj 19secs.

Develop: AZ400K, 17secs or more till all red color is gone. Avoid over-etching. Do not hesitate to rinse

sample with DI water and inspect sample under microscope during development process.

Postbake (reflow): 120C, 1min

c) Etching

MicroRIE:

[O2 Plasma, 40sccm, 40 W, 30secs]


CF4+8%O2, 40sccm, 50 W, starting pressure 108mT, time = d/1.3nm

d) Wash off PR with acetone, methanol and water

1.5) SiO2 layer deposition and planarization

a) Deposit 300-350nm thick SiO2

b) Planarization with BD features

Take the AZ2070 PR out from refrigerator >1 hr before fabrication (never use AZ2070 older than 1

day outside)




Spin AZ2070 (negative), 500-(10secs)-4000rpm, total 50secs


Remove edge bead with razor, critical step, make sure the edge bead is gone

32

Soak in water, 2mins

Exposure: Left: 10mj, 30secs?? (underdevelop it to get “enlarged” feature)

Postbake (harden): 110C, 100secs

Wet etching: 10-15 secs??

O2 plasma clean, 40sccm, 40W, 1min

c) Measured it using Alpha step to make sure the feature is as expected

1b) Buried detector fabrication using KOH etching

a) Deposit 10-15 nm of SiNx on sample surface using PECVE. Total deposition time is one minute. Follow

PECVD instructions carefully.

b) After deposition, acetone, methanol, water rinse sample.

c) Make BD PR mask






Exposure: total 191.4mJ, Right: 13.2mj, 14.5secs. Left: 10mj 19secs.

Develop: AZ400K, 17secs or more till all red color is gone. Avoid over-etching. Do not hesitate to rinse

sample with DI water and inspect sample under microscope during development process.


d) Remove SiNx layer not covering buried detector. Micro RIE etch for 30 seconds using CF4 Plasma at

25 sccm and 25 W.

e) Selectively remove SiNx covering buried detector. Heat 85% H2PO4 to 110 degrees C, by setting

hot plate to 180 degrees C. Place sample in acid bath for 2 minutes. Wear safety goggles/mask and

let others know that hot acid is on hot plate.

f) Remove sample from acid bath and rinse with DI water for 30 seconds to remove any remaining

acid.

33

1c) Buried detector fabrication for Si substrate / SOI planar detector

a) Deposit 250 nm of SiO2 layer using PECVD

b) BD PR mask, negative tone



Spin 2070 PR 4000rpm, 50secs



Exposure: Left: 10mj, 45 seconds


Develop: AZ300MIF, ~30secs and then examine under microscope and then further develop till all PR

is gone


c) Etch in buffered HF (1:6) for 10 seconds. Examine under microscope to make sure Si02 removed. If

possible, use filmetrics.


2) Metal pads fabrication

a) metal pads PR mask


day outside)




Spin AZ2070 (negative), 4000rpm, total 50secs



Exposure: Left: 10mj, 45 seconds (overdevelop it to get “shrink” feature)



is gone


34

b) E-Beam Deposition : follow E-Beam instruction. 300 nm aluminum.

c) Wash off PR with Acetone (airbrush), methanol and water

3) Deposit SiO2 and SiNx (See Operating Procedure). Deposition rate is roughly 15 nm/minute

for both SiO2 and SiNx. We need 1.5 microns. 100-200 nm of SiNx deposition depending on

desired waveguide modes.

PECVD Operating Procedure Version 1.0

This document will provide detailed instruction for depositing thin films of silicon Nitride

(SiNx) and silicon dioxide (SiO2) with the (Plasma-enhanced chemical vapor deposition)

PECVD located in the white room.

Deposition

1. Turn on the main circuit breaker (located on the bottom left of the equipment), press the

“electric safety test” button and if the system passes the safety test, the red light will lit;

2. Flip on remaining switches: pump, module supply, RF generation and heater, as well as

the power strip on the left side of the PECVD;

3. Turn on the chiller from the main switch on the back and press “On/OFF” on the front

panel until temperature is display on the front;

4. Turn on the “MAIN ON” switch and “HEAT ON” switch on the temperature controller

to increase the plate temperature to 250°C;

5. On the CPU-500 unite, press “ROTARY PUMP” then “PUMP DOWN”, make sure the

yellow LEDs are lit;

6. Go to the service corridor and manually turn on the pump by holding the “CYCLE”

switch to the left for 5 seconds, pump will be noisy when it starts;

7. Make sure the toggle switch on the top of the equipment is on “CVD” side;

8. Press “Left”, “MAN” to start a manual run;

9. At this time, turn on the vacuum gauge controller and monitor the pressure indicator (#1)

on the vacuum gauge controller, the pressure should be ~150mTorr within 1 minute;

10. After pumping down, by pressing the numbers and “ENTER”, input “min=0, sec=0” for

process time and “min=20, sec=0” for purge time, choose “P2” for purge gas to start a

35

dummy run to let the system enter an idle mode, in which “VENT or RESTART” is

prompt on the screen;

11. Press “VENT” and wait 20 seconds until the gas flow can be heard near the CVD

chamber, then press “VENT” again to turn it off;

12. Simultaneously press both green “UP” buttons (located in the middle of the two

chambers) and lift the CVD chamber lid;

13. Load the sample(s) with metal tweezers, make sure the sleeves of your suit or the gloves

are not touching the 250°C plate;

14. Close the lid by simultaneously pressing both red “DOWN” buttons;

15. Press “RESTART”;

16. Press “Left”, “MAN” to start another manual run;

17. After pumping down, input “min=20, sec=0” for process time (long enough to adjust the

gas flow and chamber pressure, can be changed during the process) and “min=5, sec=0”

for purge time, choose process gases (G1, G2, etc) and “P2” for purge gas;

18. Make sure all gas controller had the flow “off”;

19. Press “RUN” and make sure the countdown timer is running;

20. Turn on the process gas flow one by one, adjust the flow rate to desired values;

21. Adjust the dial that controls the opening of the valve between the chamber and the pump,

until desired chamber pressure is achieved;

22. Turn on the RF matching network (ATX-600) and the RF generator (RFX-600), check

the RF power set point and adjust it if necessary;

23. Press “RF ON” on the control unit CPU-500;

24. Press “TIMERESET” and input the new process time calculated based on the “total film

thickness / deposition rate”;

25. Turn on the RF power on the RF generator by pressing “RF ON” on RFX-600;

26. Check the color of the gas plasma through the windows on the chamber;

27. Fill out process log sheet;

28. The timer will turn off the RF and start the purge process once the timer goes to 0;

29. After the process is down, repeat step #11-15 to take out the sample(s);

30. If this is not the end of the process go back to step #16;

31. If this is the end of the process, press “PUMP DOWN” to turn off the pump;

32. Then turn off all switches, power strip and the chiller.

Cleaning routine

A. Electrode cleaning

If flake particles are seen in the chamber or on the shower head, and this is very common

when microns or more of film has been deposited in the chamber:

1. Make sure the plate temperature is below 80°C;

2. Spray methanol on a clean wipe and wipe the electrode plate and the shower head;

3. Wet another clean wipe with DI water and wipe again to remove the residual methanol.

36

B. Flush

A flush run should be done at the end of the day when all process is done, to remove the

gases in the pipes and flow sensors:

1. When system is under idle mode, press “RESTART”;

2. Press “Left”, “FLUSH” (instead of “MAN”);

3. Use “min=20, sec=0” for process time;

4. Turn off the system when the flush is done.

C. O2+CF4 clean

An O2+CF4 clean should be done after 5-6 process:

1. Use “O2+CF4” (pre-mixed gas) as process gas and “min=20, sec=0” for process time;

2. Use 200W RF power;

3. Tick on the “clean” column in the logbook to indicate a cleaning process is done.

4) Waveguide fabrication

b) Waceguide PR mask







Exposure: Left: 10mj 19secs??.

Develop: AZ400K, 17secs or more till red color is gone


c) Etching

MicroRIE:



CF4+8%O2, 40sccm, 40 W, etching rate 75nm/min, 1.25nm/sec, 85secs for 100nm


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5) Open via

a) Via PR mask


day outside)




Spin AZ2070 (negative), 500-(10secs)-4000rpm, total 50secs



Soak in water, 2mins

Exposure: Right: 14mj, 30secs (not overdeveloped)



is gone


b) Wet etching

Make HF(48%):H2O = 1:1 (24% HF)

Dip the sample into HF for 8secs (plastic tweezers)

c) MircoRIE



CF4+8%O2, 40sccm, 40 W, 5mins

d) Wash off PR with Acetone (airbrush), methanol and water

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Appendix F: LEAC Fabrication Diagrams

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Appendix G: Gantt Chart for Fall Semester

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Documents

Lab-on-a-Chip Diagnostic Biosensor - Colorado State …projects-web.engr.colostate.edu/ece-sr-design/AY10/biosensors/... · 1 Lab-on-a-Chip Diagnostic Biosensor First Semester Report