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Design and Fabrication of CMOS ISFET

for pH Measurement

MSc viva voce presentation by:

Chin Seng Fatt

Supervised by:

1. Prof. Dr. Uda bin Hashim

2. En. Mohd Khairuddin bin Md Arshad

School of Microelectronic Engineering

Universiti Malaysia Perlis


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Presentation Outline

Introduction TCAD Simulation

Mask Design

Device Fabrication Device Packaging

Device Characterization

Conclusion Acknowledgments


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What is pH?

The pH relation originated from Danish

Chemist Sorenson in1909. pH is the unit of measurement for

determining the acidity of alkalinity of a

solution. The math definition of pH is the negative

logarithm of the molar H+:

pH = - log ([H+])

Source: wwww.emersonprocess.com


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Importance of pH Measurement

Control a Chemical Reaction

Most inorganic reactions are pH neutralizations The rate of many reactions depend on the

availability of H+ or OH- ions.

Bacterial growth is pH dependent.

Corrosion Control

Water and Wastewater Treatment

Raw Material and Product Quality Control

Source: wwww.emersonprocess.com


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pH Measurement Methods

Litmus paper Simple Quick measurement

Color indication

Glass pH Electrode

Higher accuracy Better selectivity


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This research proposed an ISFET that can measuresthe ionic activity in a electrolyte solution and can befabricated using CMOS technology and materialswithout extra processing steps

The advantages of this proposed ISFET include: Fast and direct in-situ monitoring Robust and sturdier Small size

This research proposed an ISFET that can measuresthe ionic activity in a electrolyte solution and can befabricated using CMOS technology and materialswithout extra processing steps

The advantages of this proposed ISFET include: Fast and direct in-situ monitoring Robust and sturdier Small size

Litmus paper

Color indication2 pH value limitationPreliminary measurement

Glass pH electrode

Bulky, fragileHigh cost of Initial setupRoutine maintenance


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What is ISFET?

ISFET is Ion Sensitive Field Effect Transistor

Known as chemical or ion sensor

Sensing method based on potentiometric detection

First developed by Prof. Bergveld in 1970 by usingSiO2 as sensing layer

Advantages: small size, robust, fast response

Applications: medical, agriculture, food industry,environment monitoring


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MOSFET and ISFET

Basically the structure of the ISFET is

similar to MOSFET

The physical difference in the ISFETis the replacement of the gateelectrode of the MOSFET by the

series combination of referenceelectrode, electrolyte and ion sensinglayer

MOSFET operation was controlled bythe gate electrode while ISFEToperation was controlled by ionconcentration in the electrolyte


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Flowchart of How ISFET works

ISFET

ISFET

Gatevoltage

exceedsthreshold

Gatevoltage

exceedsthreshold

Inversionlayer

formed atSiO2/Si

Inversionlayer

formed atSiO2/Si

N+ sourcesupply

electrons

N+ sourcesupply

electrons

N+ drainmake

electronsflow

N+ drainmakeelectrons

flow

Positivevoltage

applied ton+ drain

Positivevoltageapplied ton+ drain

Electronsflow from S

to D

Electronsflow from S

to D

Gate voltagecontrols

electrons and Id

Gate voltagecontrols

electrons and Id


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Objective of Research

To characterize the

ISFET

To design the ISFET

To fabricate the ISFET

Research Goals


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Scope of Research

Reviewing and understanding the principles of ISFET

Design and simulate the ISFET with TCAD

Design and fabricate the ISFET masks

Fabrication of the ISFET Testing of the ISFET


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Synopsys TCAD is used to

perform process and devicesimulation on ISFET.

Process simulation models the

fabrication steps of the ISFET.

Simulation starts with definition

of structure and finishes with a

complete device

The process simulator used is

TSUPREM4Virtual ISFET simulated by

TSUPREM4

TCAD Process Simulation

N N

Si3N4 / SiO2 gate

Source metalcontact

Drain metalcontact

Gate metalcontact


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TCAD Device Simulation

I-V simulation by Medici

Device simulation in TCAD is the

simulation of the device electricalcharacteristics.

The TUSPREM4 ISFET is simulated

for its gate and drain characteristics.

The characteristics of the ISFET are

simulated by applying a set of

voltage biases and sweep the biases

from one point to another.

Device simulator used is Medici.


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Mask Design

Layout of ISFET similar to MOSFET: gate,source, drain, contacts.

The extended source drain regions separatesthe metal contacts from gate region duringimmersion and for straightforwardencapsulation.

Mask making process is straightforward: CADdesign and mask printing.

CAD design of individual dies replicated on awafer to create the wafer layout and then

transferred to actual mask. A total of 6 masks created. Material used as

the actual mask is transparency.Schematic design of the ISFET


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Well Mask

(b)

(c)

(a)

(a) Schematic design of well

(b) AutoCAD design of the Well mask(c) Photograph of the actual mask


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Gate Mask

(b)

(c)(a)

(a) Schematic design of Gate (b) AutoCAD design of the Gate mask

(b) Photograph of the actual Gate mask


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Contact Mask

(b)

(c)

(a) Schematic design of Contact (b) AutoCAD design of Contact mask

(b) Photograph of actual Contact mask

(a)


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Metal Mask

(b)

(c)

(a)

(a) Schematic design of metal contact

(b) AutoCAD design of the Metal mask(c) Photograph of the actual Metal mask


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Fabrication of ISFET

ISFET is fabricated using CMOS technology without any post

processing steps. All fabrication steps are performed in-house in Microfabrication Lab.

The starting material is a 4 inch p-type Silicon wafer.

The gate material of the ISFET is made of SiO2 and Si3N4, both

CMOS compatible materials.

Six masking steps: creation of n-well, n and p source drains, gate,

contact and metal.

The etching of Si3N4 and SiO2 is done using buffered oxide etch

(BOE) solution.


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Equipment modules

PECVD system PVD system Wet/dry oxidation furnace N/P diffusion furnace Mask aligner/exposure system Wet etch module Wafer spinner

Hot plate

Consumables

Silicon wafer 4 inch Buffered oxide etch(BOE)

Acetone

Positive photoresist DI water Aluminum foil Aluminum etchant

SiH4 gas Purified oxygen gas Purified nitrogen gas


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Process Flow of ISFET Fabrication

1. Starting material

Si, p-type,

1. Starting material

Si, p-type,

2. Field oxidation

Wet oxidation, 1000C95 min 5598 Wet oxidation furnace

2. Field oxidation

Wet oxidation, 1000C95 min 5598 Wet oxidation furnace

3. Well creationWell Mask, positive photoresistResist development: 30sOxide etch: 30 min

3. Well creationWell Mask, positive photoresistResist development: 30sOxide etch: 30 min


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4. Well phosphorus diffusion

Spin-on dopant phosphorusDiffusion drive-in: 6 hours

N-diffusion furnace

4. Well phosphorus diffusion

Spin-on dopant phosphorusDiffusion drive-in: 6 hoursN-diffusion furnace

6. Boron source drain formationSource Drain Mask, positive photoresistSpin on dopant - boronP-Diffusion furnace: 900C, 30 min

6. Boron source drain formationSource Drain Mask, positive photoresistSpin on dopant - boronP-Diffusion furnace: 900C, 30 min

5. Phosphorus source drain formation

Source Drain Mask, Positive photoresistSpin on dopant - phosphorusN-Diffusion furnace: 850C, 25 min

5. Phosphorus source drain formation

Source Drain Mask, Positive photoresist

Spin on dopant - phosphorusN-Diffusion furnace: 850C, 25 min


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7. Gate oxide formation

Gate Mask photolithographyGate oxidationDry oxidation furnace

1000C 60 min 556

7. Gate oxide formation

Gate Mask photolithographyGate oxidationDry oxidation furnace

1000C 60 min 556

8. Silicon nitride PECVD deposition

Deposition rate: 24.34nm/minDeposited thickness: 486.7

8. Silicon nitride PECVD deposition

Deposition rate: 24.34nm/minDeposited thickness: 486.7

9. Contact BOE etch formation

Oxide & nitride etch with BOEEtch time:30 min

9. Contact BOE etch formation

Oxide & nitride etch with BOEEtch time:30 min


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10. PVD contact metallization

PVD moduleAluminum thickness: 1541 Annealing 450 C, 45 min, N2 gas

Metal Mask, positive photoresistEtch rate Al: 308.2 /minEtch time: 5 min approx.

10. PVD contact metallization

PVD moduleAluminum thickness: 1541 Annealing 450 C, 45 min, N2 gas

Metal Mask, positive photoresistEtch rate Al: 308.2 /minEtch time: 5 min approx.

Photography ofthe completed ISFET wafer

Photography ofthe completed ISFET wafer

ISFET die with metalgate for functionalityevaluation

ISFET die with metalgate for functionalityevaluation

ISFET die with Si3N4gate will be packagedand tested in pHsolutions

ISFET die with Si3N4

gate will be packagedand tested in pHsolutions


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Packaging of ISFET

The packaging process of ISFET: wafer dicing, die mounting,

wire bonding and encapsulation.

The ISFET die is separated from the wafer and mounted on a

PCB as a platform and contacts are wired from die to the PCB.

Since the ISFET will work in electrolyte solution, an epoxy is

used to encapsulate the edge of the ISFET die, the wire bonding

and the PCB.

The sensing gate is the only area which is exposed to the

solution will not be covered.

The type of epoxy used is silicone rubber.


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1. Diced ISFET from wafer1. Diced ISFET from wafer

4. ISFET encapsulation4. ISFET encapsulation

2. Mounting ISFET on PCB3. Wire bonding

2. Mounting ISFET on PCB3. Wire bonding

Packaging Flow of ISFET


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Characterization of ISFET

The operation of ISFET is analyzed from IdVd and IdVg curves.

IdVd and IdVg measurements are done using Keithley 4200

Semiconductor Parameter Analyzer.

Two tests are performed on ISFET: functionality test at wafer level

and pH test.

In functionality test, the ISFET with metal gate is under probes

connected to the analyzer.

In pH test, the ISFET is immersed in acidic, neutral and base

solutions (pH 4, pH 7, pH 10). All solutions obtained from Orion. All measurements were done using Ag/AgCl reference electrode

from Hanna Instruments.


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Functionality Test Setup

Schematic setupSchematic setup

(a)

(b)(a) Dark shielded box wafer probestation(b) Keithley 4200 Semiconductor

Parameter Analyzer


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pH Test Setup

(a) Keithley 4200 Semiconductor Parameter Analyzer(b) ISFET(c) Reference Electrode(d) pH buffer solutions

(a)

(b)(c)

(d)


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Output characteristics of ISFET

I-V measurements of ISFET performed using Keithley 4200 SPA through wafer probe stationI-V measurements of ISFET performed using Keithley 4200 SPA through wafer probe station


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IdVd curves at Vg=5.0V for n-ISFET whenmeasured in three levels of pH buffer

solution

IdVd curves at Vg=5.0V for n-ISFET when

measured in three levels of pH buffer

solution

Sensitivity, S = Vth / pH= Vg / pH

= 40.34 mV/pH

Sensitivity, S = Vth / pH

= Vg / pH

= 40.34 mV/pH

pH Response of n-ISFET


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IdVd curves at Vg=-5.0V for p-ISFET whenmeasured in three levels of pH buffer

solution

IdVd curves at Vg=-5.0V for p-ISFET when

measured in three levels of pH buffer

solution

Sensitivity, S = Vth / pH= Vg / pH

= 34.83 mV/pH

Sensitivity, S = Vth / pH

= Vg / pH

= 34.83 mV/pH

pH Response of p-ISFET


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Conclusions A CMOS ISFET pH sensor has been successfully designed,

fabricated and characterized.

Simulation on ISFET is successfully achieved using TCAD.

Mask layout successfully designed using AutoCAD and

fabricated on transparency masks.

Fabrication of ISFET using all in-house CMOS technologyrequired no extra masking or post processing step.

The ISFET successfully detected buffer solutions of different

pH. Based on the results obtained, the CMOS ISFET showed a

fairly good response as a pH sensor and has potential for

commercialization.


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Research Achievements

2

3

1

Bronze Medal

Silver Medal

Gold Medal

5Local Conference

3Regional Conference

4International Conference

1International Journal

Research Publications Award Medals


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Recommendation SPICE simulation

Simulation of pH response of ISFET

Miniaturization of ISFET

Sub-micron size device, chrome masks

Lower cost of fabrication, mass production

Packaging of the ISFET

Precise wafer dicing by automation

Proper encapsulation material and technique Sampling Experiments

Larger samples of pH on both acidic and basic tests


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Acknowledgement

The financial support from UniMAP and Malaysian Ministry of

Science, Technology and Innovation (MOSTI).

Guidance and advices from supervisors

Motivational supports from families, researchers, friends.

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