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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
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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
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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
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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
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Batch Processingand miniaturization
Cost Reduction
Low Power operation
Biomedical and
aerospace Applications
Reliability &
Reproducibility
Batch Processing
Miniaturization
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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
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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
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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
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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
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Miniaturization approaches
Conventional Micromachining
Silicon Micromachining
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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.
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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
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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.
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1 atmosphere=1bar= 105Pa.1Pascal=1N/m2
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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
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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.
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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.
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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
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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.
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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
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