mineaturization concepts, benefits and materials for mems

7/25/2019 Mineaturization Concepts, Benefits and Materials for MEMS

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E3-222 Micromachining for

MEMS Technology

Module-1Miniaturization Concepts, Benefits and

Technology common for for MEMS and VLSI

Professor K.N.Bhat

CeNSE / ECE Department

Indian Institute of Science

Bangalore -560 012

Email :[email protected]


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Module-1Miniaturization Concepts, Benefits and

Materials for MEMSOutline:

Need for Miniaturizing mechanical sensors and actuators

Benefits of Micromachining and Scaling for MEMS

Materials for Micromachining and MEMS

Why and how Silicon is the best material for MEMS

Silicon Processes common to VLSI and MEMS

Prof K.N.Bhat


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Micro Electro Mechanical Systems (MEMS)

Deals with

1.Miniaturizationand batch processing of

Sensors , Actuators and microstructures

2. Integration of mechanical components with

electronics

This is a Revolution similar to

VLSI in Microelectronics

Prof K.N.Bhat


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Classification of MEMS

MEMS structures and devices can be classified intofour major groups:

Passive(nonmoving) structures

Sensors,which respond to the world, (eg), pressure

Actuators(reciprocal of the sensors), which useinformation to influence something in the world. (eg)pump, valve, Resonating structures, filters etc

Micro-Systems that integrate both sensors andactuators to provide some useful function

Prof K.N.Bhat


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Transportation Communications Analytical &

Medical

MEMS

Structures

Infrared

Imagers

Optical & RF

Signal Guides,

Field EmissionArrays

Micro Filters,

Micro

Channels& - Mixers

MEMS

Sensors

Pressure ,

Acceleration,

& AngularRate

Acoustic

sensors

Gas sensors

MEMSActuators

AerodynamicFlow Control

Displays, Opticalswitches, & RF

Switches &Filters

Micro-pumps& -Valves

MEMS Categoriesand Application areas

Categories

Application

Areas

Prof K.N.Bhat


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Batch Processingand miniaturization

Cost Reduction

Low Power operation

Biomedical and

aerospace Applications

Reliability &

Reproducibility

Batch Processing

Miniaturization

Prof K.N.Bhat


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(a) Intracranial Pressure (ICP)-15 to 30mmHg

(b) Blood Pressure (BP) 80/120mmHg

monitoring

.Mapping pressure on

the aerofoil of Aircraft

Oceanography- CTD(Conductivity, Temperature & Pressure)

for Marine Engg (NPOL , Kochin)

P=1 bar for D = 10mters of water

Need for Miniaturizationof Sensors

The size of pressure sensor

to be inserted into the

Ventricle in the brain shouldbe within 1mm diameter and

it should be biocompatible

LCA


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Miniaturized Pressure sensors in automobile

MAP sensor measures the absolute (with ref to vacuum)

pressure in the fluid intake to the manifold on a cars

engine. This enables the engine's electronic control unit(ECU) to determine the air density and determine the

engine's air mass flow rate.This determines the required fuel metering for optimum

combustion and influence the advance or retard of

ignition timing .

1 Manifold Absolute Pressure (MAP) sensor

MAP sensor is a piezoresistive

Absolute Pressure sensor

Maximum operation range = 1bar


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Prof K.N.Bhat 9

2. Tire Pressure Monitoring System (TPMS)

(compulsory in USA) for real time sensing exact

pressure inside tire

Remote sensing modulecomprises Pressure

sensor, Signal Processor, Temperature Sensor

and RF Transmitter.Pressure measurement information is displayed

in the cabin of the car

The Motorola TPM pressure

sensor is capacitive type

and requires C to V

conversion stage uses less

than 0.5!A in standbymode.


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Prof K.N.Bhat 10

Principle of Operation of Pressure Sensors

Most of the pressure sensors operate based on

monitoring the deflection of a diaphragmusingtransducers

Diaphragm: Metal foil anchored all around or any other

material such as Si, SiC, SiO2, SiN , Diamond .

Transducers: capacitance , piezoresistor, piezoelectric

Silicon Micromachined Piezo-

resistive Pressure sensor


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Acceleration sensor in automobile

Fast deceleration, during a collision, triggers the air

bag sensors ( accelerometer) which turns on a switchand heats the propellant, the chemical reactionproduce N2which inflates the cloth air bag. The air

bag fully inflates in less than 1/20 of a second, and then

it starts deflating, cushioning the impact.

Silicon Micromachined Capacitive sensor is used as crash sensor


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Principle of operation of Accelerometers

Mass supported by a spring anchored

to the frame at the other end

Prof K.N.Bhat

Monitor the displacement x by

capacitive, piezoresistive or

piezoelectric method .

How do we realize this spring-

mass system by silicon

micromachining ? What are thedesign considerations ?


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Prof K.N.Bhat 1313

Mcromachined Silicon

accelerometer

Beam

Mass

Electrodes

(Cr / Au)

Si or

PolySi

SiO2 (1 m)

Bulk Si


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Micropumps for l/minute pumping

(1) Drug delivery drug dosage control(2) Lubricating bearings of gyro motor space application.

Actuation

Mechanism

PumpDiaphragm

Inlet Valve

Pump ChamberOutlet Valve

Valve Threshold

Pressure "p (crit) Stroke Volume "VChamber Pressure "p

Chamber Volume Vo

Inlet Outlet

Miniaturization of Actuators

Prof K.N.Bhat


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Prof K.N.Bhat 1515

Inlet check valveOutlet check valve

Pyrex

Spacer

layer 4!m

Deformable diaphragm

4mmx4mmx25!m

Counter

electrode

MICRO PUMP

pumping rate is 70micro liters /min. at 25Hz


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Micropump Animation

Fluid

Pulse


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Micropump Actuation

Fluid

Pulse


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RF resonators and filters for

defense and communicationGHz frequency

DNA analysis

Miniaturization of cantilevers and beams

Prof K.N.Bhat


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Prof K.N.Bhat 19

SiO2 Cantilever beams:fabricated at CEN IISc Bangalore

by TMAH etching of Si using bulk micromachining: L= 65

m , W= 15 m thickness =0.52 m , Stiffness

k=0.134N/m


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Field emission tips for high frequency

Vacuum Electron Devices

MicrostructuresMicro gears

Micro turbine

Nanometer

size AFM Tip

Miniaturization of structures

Prof K.N.Bhat


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Biomedical : pressure sensors (Intra Cranial Pressure,

blood pressure) ), cantilever beams (DNA analysis),

micropump ( controlled micro dose of drug Delivery)

Aerospace and Automobile: Pressure sensors ,

accelerometers.

Space programs and missiles: Pressure sensors ,

accelerometers, gyro,micropump

Micro fluidicschannels and mixers: Chemical analysis

and synthesis and lab on Chip concept

Defense : Explosive detection, gas sensors, pressure

sensors , acceleration sensors and RF MEMS

Miniaturization of Devices is

important for many Applications

Prof K.N.Bhat


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Miniaturization approaches

Conventional Micromachining

Silicon Micromachining

Prof K.N.Bhat


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Conventional Micromachining Techniques

Each component must be made piece by piece. Lowprice for large production volumes are the result ofmechanization.

Ultrasonic machining, sandblasting, laser ablation andspark erosion have aided in miniaturization.

Finest details that can be machined are one to two

orders larger than what photolithography makes

possible.

Prof K.N.Bhat


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Silicon Micromachining

Suitable for batch processing similar tofabrication of ICs.

Production costs of whole production isindependent from number of componentsfabricated.

Miniaturization with finest details in the rangeof 0.1 to 10m possible based on photo-litho-graphy

Prof K.N.Bhat


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Requirements for MEMS

Microfabrication process used to create the device

must be scalable and suitable for batch processingto realize a low cost of production.

Material and process must enable integration

between electronic and non-electronic function.

High performance, high-strength and high

reliability materials for mechanical elements.

Materials for transducer elements which permitpower or signal conversion from one physical

domain to another.

Prof K.N.Bhat


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1 atmosphere=1bar= 105Pa.1Pascal=1N/m2

Prof K.N.Bhat


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Reference:Peterson K.E.

Silicon as a

mechanical material.

Proceedings of

IEEE, Vol. 70, 420-457, 1982.Prof K.N.Bhat


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Mechanical Properties of Silicon

1

2

Yield

StrengthThermal

Conductivity Young

sModulus

Density Hysteresis

Ratio of Silicon to Steel

1.66 1.61

0.90

0.29

Si Crystal same type as Diamond and is

harder than most metals and has higherelastic limits than steel in both tension and

compressionProf K.N.Bhat


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Silicon is the best material

Silicon is used as an electronic material in an

already advanced microfabrication technology

Hardness, Young's modulus and yield strength are

comparable to or better than steel

Free from hysteresis, creep and fatigue

Lighter than Aluminum and harder than steel

Miniaturized mechanical devices can be realizedon silicon with high precision and they can be

integrated with electronics

Prof K.N.Bhat


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Silicon is the best material(contd)

Even though brittle, it exhibits higher strength when

miniaturized. Silicon is 100 percent elastic up to its

breaking point. This is what makes silicon the ideal material

to use as the sensing diaphragm. ( other ductile material

suffer from thermally activated deformation processessuch as creep).

Micro-fabrication process ( etching/ Deposition)and

single crystal substrates with low defect density allows

the creation of structures with very fine surfaces and

therefore very high strengths.

Prof K.N.Bhat


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Summary

Miniaturization is required in most of the applications

Miniaturization gives additional benefits

Miniaturization is possible using silicon as the

mechanical element

Silicon allows the use of already existing maturemcroelectronics technology for miniaturization:

Photolithography to select the regions, change the

conductivity or etch in selected regions of the material

to realize the required mechanical structures, depositmechanical structures in the selected regions.

Prof K.N.Bhat


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Basis for Silicon Micromachining

MEMS devices such as pressure sensors,

accelerometers,gyroscopesand Micro pumps can

be realized by micromachining Silicon .

Miniaturization is possible with photolithographyand etching process.

Photolithography defines regions on the material (eg Si)

where machining is done.

Machining includes etching, doping, deposition of thinfilmsin the patterned regions

Prof K.N.Bhat


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Prof K.N.Bhat

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Basic Processes for Si MEMS Technology

Clean room conditions for Process

Silicon Wafer cleaning

Photolithography to define specified regions in the Oxide

Doping the selected regions to realize Electrical elements

Micromachining to realize mechanical components


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Clean Room requirement

Federal standard 209E of the USA classifies the clean room by

the maximum number of particles higher than 0.5m in each

cubic foot of air.

(e.g.) A class 100 clean room has less than 100particles of size 0.5m and larger per cubic foot.

This is about 3500 particles /m3

This is about FOUR orders of magnitude lowerthan that of ordinary room air

IC fabrication areas require class 100 Clean room areas .

Photolithography areas need Class 10 or lower.

Prof K.N.Bhat


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Prof K.N.Bhat

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Silicon Wafer Cleaning

Treatment with RCA-1 solution NH4OH : H2O2: H2O

(1:1:5) at 80C for 15-20 minutes to remove anyorganic contaminants, DI water rinse and dry innitrogen Jet.

Treatment with RCA-2 solution Hcl: H2O2: H2O (1:1:5)

at 80C for 15-20 minutes to remove any metalliccontaminants, DI water rinse and dry in nitrogen Jet .

Dilute HF (1:50 in DI water) dip to remove any native

oxide layer formed during the chemical treatment

followed by rinse in DI water and dry in Nitrogen Jet.


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Prof K.N.Bhat

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Thermal Oxidation of Silicon

Si +O2! SiO

2

Si+ 2

H2O! SiO2+ 2

H2

O2for Dry.

H2O vapor for

wet by

bubbling N2

through water

bubbler keptat 95C

tox

= 2.2tSi

Oxidation takes place by consuming Si. Due to the difference in

density and molecular weight, and oxide of thickness 1nm grows

by consuming 044nm thickness of Si


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Dry Oxidation

of Si


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Prof K.N.Bhat 38

Wet Oxidation

of Si


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Prof K.N.Bhat 39

High Pressure Oxidation enables the oxidationtemperature to be dropped by 30C for each

1atmosphere increase in pressure .

(eg ) Wet oxidation at 1200C for 5 hours gives oxide

layer thickness of 2 microns.By carrying out high pressure wet oxidation at

10 atmosphere pressure, the same thickness ofoxide can be grown at 900C in 5hours.

High Pressure Oxidation


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Positive PR Negative PR

( d) Etch SiO2

(e) Remove ResistFinal image

(b) UV Light (collimated) exposure

through the Photomask

Photolithography and Pattern Transfer process illustrating the use of

Positive Photo Resist (PPR) and Negative Photo Resist (NPR)

(a) Spin coat PPR or NPR and bake

( c) Develop

Photolithography


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Doping by Diffusion

An oxide of the desired dopant atoms is deposited on the silicon

wafer surface kept at a high temperature (800C to 1200C) inside a

quartz tube furnace.

Examples of such reaction for diffusion of N-type dopants such as

phosphorous and P-type dopants such as boron from their oxides

are respectively given below.

2P2O5+5Si! 4P +5SiO

2

2B

2O3+ 3Si! 4B+ 3SiO

2

The dopant source can be solid, liquid or gas, the choice depends

upon the ease with which it can be incorporated.

Liquid sources: phosphorus oxy chloride (POCl3) for phosphorousand Boron tribromide (BBr3) for boron diffusion

Gas Sources: 1% Phosphine(PH3)in H2 or argon for Phosoporus

1% Diborane(B2H6) in H2or argon


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Halide bearing liquid source such as phosphorus oxy chloride (POCl3) is

used for phosphorous diffusion .Ph3is used in gas source systems

Phosphorus /Boron Diffusion Using Liquid/Gas source

4POCl3+ 3O

2! 2P

2O5+ 6Cl

2

2P

2O5+5Si! 4P +5SiO

2

POCl3for Phosphorus. BBr3for Boron

2B2O3+ 3Si! 4B+ 3SiO

2

4BBr

3+ 3O

2! 2B

2O3+ 6Br

2

Replace the Bubbler with gas cylinder for

gas source option- Phosphine (

B2H6+ 3O

2! B

2O3+ 3H

2O

2PH

3+ 4O

2! P

2O5+ 3H

2O


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Prof K.N.Bhat 43

Solid Solubility of

impurities in SiliconThe surface

concentration (No)

during the diffusion issolid solubility limited

when the supply ofdopants is present all

the time


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Prof K.N.Bhat

!"#$%$&'()*

!+

!,-./ 12344526 78

98:( ;87


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Prof K.N.Bhat 45

Diffusion from a limited Source gives a

Gaussian doping profile N(x,t)

N(x,t)=N

T

!Dte"z

2

z =x

2 Dt

NTis the total number of dopants /cm2initially

deposited on the surface of the silicon substrate


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Ion Implantation of dopantsIn the Ion implantation process dopants are introduced into

semiconductor at room temperature by means of an energetic ion

beam of the dopants. Ion energies in the 50-200KeV range are used

for implantation into silicon.

The implanted dopant profile is Gaussian with its peak located inside

the semiconductor at a distance Rpfrom the surface, referred to as

the Projected Range, with a standard deviation called as the

Straggle"Rp. Implantation is done at 7degrees angle to the normal.ND(x) =

NT

2! .("Rp)exp[#

1

2(x # Rp

"Rp)2]

.0

G24H

I5?7@ 56 AB9 )24H J

AB873CB @B5(B ! AK;9

?7;26A4 289 5);=26A9?

DE45=5(76

!L .F

xNT = ND (x)dx!

NTis the total dose /cm2

$%

$&!'

.FRp


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Prof K.N.Bhat

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Sheet Resistance, RS

Rs =!

xj=

1

qpNAxj=

1

qpNT

NT = N

A(x)dx! = Dose

Four Probe measurement

I

N-Si

Diffused or implanted P-layer

Rs = (C.F.)

V

I

When the sample size is 50 times the spacing

between the probes, C.F. =4.53

C.F. is the correction factor

RS= Ohms/square


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Summary

Miniaturization is important for biomedical , aerospace

and military Applications

silicon is best suited for batch processing ,

Miniaturization and Integration with Electronics

Photolithography is the basis for MEMS deviceprocessing using Silicon and several other materials

such as Nitride, quartz, glass, polymers such as SU-8

and compound semiconductors such as SiC, GaN , GaAs

Oxidation, metallization, Dopant Diffusion and

Implantation play very important role in micro and smart

systemsP f K N Bh

mineaturization concepts, benefits and materials for mems

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