MEMS LOHANI1

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ME ETC (ECI) MEMS

Introduction toIntroduction to

MEMS TechnologyMEMS Technology

Dr. R. B. Lohani

Professor & Head

ETC Dept.

Goa College of Engineering. Farmagudi


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Course Overview

MEMS applications in engineering, Properties of single crystal silicon. Anisotropy, Photolithography. Environment for microfabrication, Surface micromachining techniques:

Doping and the solution of the diffusion equation.

Thin film characterization Residual stresses, thermal stress calculation in thin films. Bulk silicon micromachining. Sensing principles: Piezoresistivity, sensitivity of anisotropic

piezoresistors. Capacitive sensing and actuation, piezoelectric

sensing and actuation. Elasticity effects: stiffness and equilibrium. Stability of

electrostatic actuators, dynamic responses of MEMSstructures. Microfluidics, viscous flow calculations


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Why MEMS?Why MEMS?

Micro Electro Mechanical Systems


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Abstract


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Relative ScalesRelative Scales



What are MEMS?What are MEMS?

Micro - Small size, microfabricated structures Electro - Electrical signal /control ( In / Out )

Mechanical - Mechanical functionality ( In / Out )

Systems - Structures, Devices, Systems Control



History of MEMS TechnologyHistory of MEMS Technology..

Richard Feynman "There's Plenty of Room at the Bottom- Presentation given December 26,1959 at California Institute of

Technology- Tries to spur innovative miniature fabrication techniques for

micromechanics- Fails to generate a fundamentally new fabrication technique

Westinghouse creates the "Resonant Gate FET" in 1969- Mechanical curiosity based on new microelectronics fabrication

techniques Bulk-etched silicon wafers used as pressure sensors in 1970s Kurt Petersen published -Silicon as a Structural Material in 1982

- Reference for material properties and etching data for silicon Early experiments in surface-micromachined polysilicon in 1980s

- First electrostatic comb drive actuators- micropositioning disc drive heads Micromachining leverages microelectronics industry in late 1980s

- Widespread experimentation and documentation increases publicinterest

Simplified slide follows



REVIEW:REVIEW: What is MEMS ?What is MEMS ?

Micro Electro Mechanical Systems micro scale dimensions (1mm = 1000 microns) electrical and mechanical features systems (features combined to perform a function)

MEMS fabrication techniques Originally used IC (computer chip) fabrication techniques

and materials

More MEMS-specific fabrication techniques and materialsare now in use

MEMS is an enablingtechnology

Smaller device size

Batch processing for low cost,uniform production Distributed device placement More precise sensing

4000 die per 6 inch wafer



A nm 0.1 m 10 m 1mm 100mm10 matom

DNA

virus bacteriadust

hair

MEMS

People

nanotechnology microsystems meso macrosystems

Size matters: scales, miniaturizationSize matters: scales, miniaturization

http

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1947~1999

1 transistor42 x 106 transistors

Courtes

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Scaling

Scaling Length 1/r (meters-1) 1 meter 1

1 mm 1,000

1 mm 1,000,000 1 nm 1,000,000,000

Surface to volume ratio varies as 1/r:



Why do we make things small?Why do we make things small?

Cost ReducedBatch Fabrication

Larger wafer in diameter

Speed IncreasedShorter distance between elements

Reduce RC delay

Rigidity EnhancedVery High Resonant Frequency

Mostly Single Crystal Silicon. No Fatigue!

Compatibility

Integration with IC/electronics

Capability of Arrays

Avoidable DrawbacksNoise Amplification

High Developing CostFundamental Limitations



Moores law (1964)Moores law (1964)

Number of transistors per chip doubles every 1.5-2 years

Information store on Si = 2^(Time-1962)

Case Study: Accelerometers

6.152J class note

Moore's Law, the empirical observation that the transistor

density of integrated circuits doubles every 24 months;



Moores law (1964)Moores law (1964)

Case Study: Accelerometers

6.152J class note

Processor Transistor count Date of introduction Manufacturer

Intel 4004 2,300 1971 Intel

Pentium II 7,500,000 1997 Intel

Core 2 Duo 291,000,000 2006 Intel

Quad-Core Itanium 2,000,000,000 2008 Intel
http://en.wikipedia.org/wiki/Intel_4004http://en.wikipedia.org/wiki/Intelhttp://en.wikipedia.org/wiki/Pentium_IIhttp://en.wikipedia.org/wiki/Core_2_Duohttp://en.wikipedia.org/wiki/Core_2_Duohttp://en.wikipedia.org/wiki/Pentium_IIhttp://en.wikipedia.org/wiki/Intelhttp://en.wikipedia.org/wiki/Intel_4004


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Integrated circuits (ICs)Integrated circuits (ICs)

This picture comes from an excellent introductory discussion about IC fabrication at: icknowledge.com

Start Finish


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Why is MEMS useful?Why is MEMS useful?

Sensors and Actuators -

two classes of MEMS

Sensors - measure the environment

SMALL => interfere less with the environment it is

measuring

SMALL => redundancy (more than required necessary

data), batch fabricated,

Actuators - perform work on the environment

SMALL => small motion but very precise

SMALL => can be placed in small spaces


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ApplicationsApplications

Etching

Deposition


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ApplicationsApplications

A device that converts some kind of power, such as hydraulic

or electric power, into linear motion.


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Mechanical ActuatorMechanical Actuator

Advantages

Cheap.

Repeatable. No power source required. Self contained.

Identical behaviour extending or retracting .

Disadvantages Manual operation only.

No automation.


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Electro-mechanical ActuatorElectro-mechanical Actuator

Advantages

Cheap. Repeatable. Operation can be automated. Self contained.

Identical behaviour extending or retracting. Disadvantages Many moving parts prone to wear. Linear motor


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Piezoelectric ActuatorPiezoelectric Actuator

Advantages Very small motions possible. Disadvantages Requires position feedback to be repeatable.

Short travel. Low speed. High voltages required. Expensive.


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Linear motor ActuatorLinear motor Actuator

Advantage

Simple design. Minimum of moving parts. High speeds possible. Self contained.

Identical behaviour extending or retracting. Disadvantages Relatively low force.


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Hydraulic ActuatorHydraulic Actuator

Advantages Very high forces possible. Disdvantages Can leak.

Requires position feedback for repeatability.External hydraulics pump required. Some designs good in compression only.


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MultidisciplinaryMultidisciplinary


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Other Examples.Other Examples.


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S


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MEMS ApplicationsMEMS Applications

Accelerometers Biotechnology Communications
http://upload.wikimedia.org/wikipedia/commons/a/a8/Airbag_system.jpg


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A l t


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AccelerometerAccelerometer

A mechanical orelectromechanicalinstrument that measuresacceleration. The two general types of accelerometers measure eitherthe components of translational acceleration or angular acceleration.

Built using MEMS technology, accelerometers detect impact and deployautomobile airbags as well as retract the hard disk's read/write headswhen a laptop is dropped. Digital cameras employ them in their imagestabilization circuits.

They are used in washing machines to detect excessive vibration

and in pedometers for more accurate distance measurement.

A l tA l t
http://www.answers.com/topic/electromechanicalhttp://www.answers.com/topic/electromechanical


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AccelerometersAccelerometers

MEMS accelerometers are quickly replacingconventional accelerometers for crash air-bagdeployment systems in automobiles. The conventionalapproach uses several bulky accelerometers made ofdiscrete components mounted in the front of the carwith separate electronics near the air-bag; thisapproach costs over $50 per automobile.

MEMS and Nanotechnology has made it possible tointegrate the accelerometer and electronics onto asingle silicon chip at a cost between $5 to $10. TheseMEMS accelerometers are much smaller, morefunctional, lighter, more reliable, and are produced for afraction of the cost of the conventional macroscaleaccelerometer elements.

Bi t h l A li tiBi t h l A li ti


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Biotechnology ApplicationsBiotechnology Applications

Invasive and noninvasive biomedical sensors Miniature biochemical analytical instruments Cardiac management systems (e.g., pacemakers,

catheters)

Drug delivery systems (e.g., insulin, analgesics) Neurological disorders (e.g., neurostimulation)


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C i ti A li tiC i ti A li ti


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Communications ApplicationsCommunications Applications

Telecommunication optical fiber components andswitches Mass data storage systems RF and wireless electronics

C i ti


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Communication

MEMS?MEMS?


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MEMS?MEMS?

REVIEW f S i d tREVIEW of Semiconductors


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REVIEW of SemiconductorsREVIEW of Semiconductors

The tremendous wealth of information accumulated onsilicon and its compounds over the last few decadeshas made it possible to innovate and explore newareas of application extending beyond themanufacturing of electronic integrated circuits.

It becomes evident that silicon is a suitable materialplatform on which electronic, mechanical, thermal,optical, and even fluid-flow functions can be integrated.

Ultra pure, electronic-grade silicon wafersavailable for the integrated circuit industry are common

today in MEMS.

Selecti e Part of Periodic TableSelective Part of Periodic Table


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Selective Part of Periodic TableSelective Part of Periodic Table

W l t th W ld f Sili


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Welcome to the World of Silicon.

About SiliconAbout Silicon


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About SiliconAbout Silicon


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Silicon Wafers


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Silicon Wafers

Structural properties compared to Steel / Aluminum:

Density u t E C

Ma te ria l g/cc Mpa GPa x

Silicon 2.33 4000 130 4.

Steel [1020] 7.87 420 205 12

Alum inum [6061 T6]2.7 310 69 24

Properties of Single crystal SiProperties of Single crystal Si


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Properties of Single crystal SiProperties of Single crystal Si

Silicon Silicon is one of very few materials that is

economically manufactured in single crystal substrates.This crystalline nature provides significant electricaland mechanical advantages.

The precise modulation of silicons electrical

conductivity using impurity doping lies at the very coreof the operation of electronic semiconductor devices.

Mechanically, silicon is an elastic and robust materialwhose characteristics have been very well studied anddocumented (see Table 2.1).

Make Si into a conductor or insulator by doping Boron, phosphorous, arsenic, antimony

Relative PropertiesRelative Properties


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Relative PropertiesRelative Properties

Conductors Semi-conductors Insulators

10-3 -cm 102 -cmResistance

SiliconSilicon


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SiliconSilicon

silicon is not an active optical materialsilicon-basedlasers do not exist. Because of the particularinteractions between the crystal atoms and theconduction electrons, silicon is effective only indetecting light; emission of light is very difficult toachieve.

At infrared wavelengths above 1.1 m,silicon istransparent, but at wavelengths shorter than 0.4 m (inthe blue and ultraviolet portions of the spectrum), itreflects over 60% of the incident light (see Figure 2.3).

The attenuation depth of light in silicon (the distance

light travels before the intensity drops to 36% of itsinitial value) is 2.7 m at 633 nm (red) and 0.2 m at436 nm (blue-violet).

Optical Reflectivity for Silicon & selectedOptical Reflectivity for Silicon & selected


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Optical Reflectivity for Silicon & selectedOptical Reflectivity for Silicon & selectedmetalsmetals

At infrared wavelengths above 1.1 m,silicon is transparent, but

at wavelengths shorter than 0.4 m (in the blue and ultraviolet

portions of the spectrum), it reflects over 60% of the incident light

(see Figure 2.3).


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Silicon ProductionSilicon Production


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Silicon ProductionSilicon Production

Wafer IdentificationWafer Identification


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Wafer IdentificationWafer Identification


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Wafer Flats - orientation for automatic equipmentand indicate type and orientation of crystal.

Primary flat The flat of longest length located inthe circumference of the wafer. The primary flathas a specific crystal orientation relative to the

wafer surface; major flat. Secondary flat Indicates the crystal orientation

and doping of the wafer.


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-I

-I I


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StructureStructure


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StructureStructure

Atomic Order of a Crystal StructureAtomic Order of a Crystal Structure


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Atomic Order of a Crystal StructureAtomic Order of a Crystal Structure

Amorphous Atomic StructureAmorphous Atomic Structure


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pp

Unit Cell in 3-D StructureUnit Cell in 3-D Structure


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Unit cell

Miller IndicesMiller Indices of Crystal Planesof Crystal Planes


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Miller IndicesMiller Indices of Crystal Planesof Crystal Planes

Z

X

Y

(100)

Z

X

Y

(110)

Z

X

Y

(111)

Figure 4.9

? ? ?

Miller Index of a Plane


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e de o a a e


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All lattice planes and lattice directions are described by a

mathematical description known as a Miller Index. In the cubic

lattice system, the direction [hkl] defines a vector direction normal

to surface of a particular plane or facet.

A crystal can always be divided into a fundamental shape with a

characteristic shape, volume, and contents.

Identification of a CutIdentification of a Cut


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de t cat o o a Cut

Cubic Arrangement


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ME ETC (ECI) MEMSSilicon Crystal Structure


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Planes and directions are defined using x, y, z coordinates. [111] direction is defined by a vector of 1 unit in x, y and z.

Planes defined by Miller indices Theirnormal direction(reciprocals of intercepts of plane with the x, y and z axes).

Crystals are characterized by a unit cell which repeats in the x, y, z directions.


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Silicon has the basic diamond crystal structure

two merged FCC cells offset by a/4 in x, y and z.

Faced-centered Cubic (FCC) Unit CellFaced-centered Cubic (FCC) Unit Cell


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( )( )

Figure 4.5

Silicon Unit Cell: FCC Diamond StructureSilicon Unit Cell: FCC Diamond Structure


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Faced-centered Cubic (FCC)Faced-centered Cubic (FCC)


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Basic FCC Cell Merged FCC Cells

Omitting atoms

outside Cell Bonding of Atoms

(Extra line of atoms)Defects


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Various types of defects can exist in a crystal (or can beVarious types of defects can exist in a crystal (or can beprocessing steps).processing steps).In general, these cause electrical leakage andIn general, these cause electrical leakage and

are result in poorer devices.are result in poorer devices.

Defects

DefinitionsDefinitions


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Point DefectsPoint Defects


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Vacancy defect

Interstitial defect Frenkel defect

DefectsDefects


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Dynamics of VacancyDynamics of Vacancy


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Point Defects in Ionic SolidsPoint Defects in Ionic Solids


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Frenkel DefectFrenkel Defect


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In simple words;

Line DefectsLine Defects


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DislocationsDislocations


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AnisotropyAnisotropy


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AnisotropyAnisotropy


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Typical Sizes of Semicoductor WafersTypical Sizes of Semicoductor Wafers


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(The Diameter of a wafer is measured through its center

and not through any flats):

1 inch or 25mm 2 inch or 50mm

3 inch or 75mm 4 inch or 100mm 5 inch or 125mm 6 inch or 150mm

8 inch or 200mm

12 inch or 300mm

CleavingCleaving


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Cleaves will run according to the crystalorientations.

If the crystal orientation of the Si is cleave at90 deg. angles.

If the crystal orientation of the Si is cleave at60 deg. angles.

Silicon etchingSilicon etching


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The classic example of this is the crystal plane sidewalls that appear

when etching a hole in a silicon wafer in a chemical such as

potassium hydroxide (KOH). The result is a pyramid shaped hole instead of

a hole with rounded sidewalls with a isotropic etchant.

Plane hkl etching speed:Vhkl =dhkl/ t

where: dhkl - etching deep, t - etching time

Silicon etchingSilicon etching


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W = a0 + 21/2

x (d-m) - 2uwhere:

u = (V111 x t)/sin,

= 54.74

Vhkl =dhkl/ t

where: dhkl - etching deep, t - etching time

Plane hkl etching speed:

Environment for microfabrication


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Microfabrication TechniquesMicrofabrication Techniques


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Surface Micromachining -additive process (2-D)

Create device by adding materials on top of a wafer Easily mixed with IC fabrication techniques

Limited to film thickness less than 4 microns(human hair 50-75 microns, IC gate ~ 0.15 microns)

Typically used for high volume, low precision, lowcost devices

Bulk Micromachining -subtractive process (3-D)

Create device by etching material out of a wafer

Distinct processes and materials from IC fabrication

Flexible for design of diverse device features sizes

Advantageous for high precision sensors/actuators

Basic Process Steps for Wafer PreparationBasic Process Steps for Wafer Preparation


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Crystal GrowthCrystal Growth

ShapingShaping

Wafer SlicingWafer Slicing

Wafer Lapping

and Edge Grind

Wafer Lapping

and Edge Grind

EtchingEtching

PolishingPolishing

CleaningCleaning

InspectionInspection

PackagingPackaging

Figure 4.19

Fabrication StepsFabrication Steps


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OverviewOverview


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MicroengineeringMicroengineering


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Basic Process flow in MicromachiningBasic Process flow in Micromachining


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Micro-Fabrication Overview


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DepositionOxidation orDeposition

LithographyAdd resistTransfer patternRemove resist

EtchWet isotropic orWet anisotropic orRIE

Sacrificial Etch

Wafers2D

Devices

Repeat as Necessary

Needed for previousexample

Review: Process FlowReview: Process Flow


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LithographyLithography


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PHOTOLITHOGRAPHYPHOTOLITHOGRAPHY


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PHOTOLITHOGRAPHYPHOTOLITHOGRAPHY


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OVERVIEWOVERVIEW


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WAFER CLEANINGWAFER CLEANING


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SPIN COATINGSPIN COATING


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SPIN COATINGSPIN COATING


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PREBAKINGPREBAKING


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PREBAKINGPREBAKING


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PREBAKINGPREBAKING


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DEVELOPMENT STAGESDEVELOPMENT STAGES


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WAFER ALLIGNMENTWAFER ALLIGNMENT


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POSTBAKEPOSTBAKE


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PHOTORESIST STRIPPINGPHOTORESIST STRIPPING


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EXPOSURE CHOICESEXPOSURE CHOICES


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BASIC MASK STRUCTUREBASIC MASK STRUCTURE


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MASK ALLIGNER TECHNOLOGYMASK ALLIGNER TECHNOLOGY


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ReviewReview:: PhotoPhotolithography (lithography (opticaloptical Lithography)Lithography)

Expose PR to UV light thru mask


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Properties change in exposed regions Masks: laser etched or print-transferred chrome

Positive resist destroy bonds, soluble Negative resist crosslinking, less soluble

PositiveMask

PRSubstrate

UV light

SubstrateExpose

PR Reaction

Develop

NegativeWe use:

PhotolithographyPhotolithography


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Photolithography

Photoresist (PR) used to transfer patterns


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Viscous PR spun onto wafer Dispense, then spin

Speed controls thickness Thickness: 1 10 m Requires baking (~100C)

DispenserPR

Wafer

Chuck

Spin Xrpm

7 m Positive ThickResist

1. Dispense, Spin 3500rpm2. Pre-bake 90C 60min

3. UV mask expose 15 sec

4. Develop in aqueous

solution

5. Post-bake 90C 30min

Photolithography

Expose PR to UV light thru mask


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Properties change in exposed regions Masks: laser etched or print-transferred chrome

Positive resist destroy bonds, soluble Negative resist crosslinking, less soluble

PositiveMask

PRSubstrate

UV light

SubstrateExpose

PR Reaction

Develop

NegativeWe use:

PhotolithographyPhotolithography


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What Is Micromachining?What Is Micromachining?


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Micromachining is the set of design and fabrication

tools that precisely machine and form structures andelements at a scale well below the limits of our humanperceptive facultiesthe microscale.

Micromachining is the underlying foundation of MEMSfabrication; it is the toolbox of MEMS.

EnvironmentEnvironment


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Review:Review:


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In Brief: Silicon Micromachining TechniquesIn Brief: Silicon Micromachining Techniques


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Silicon Micromachining techniques

Bulk micromachining

Surface micromachining

Bulk micromachiningis based on the

etching of the silicon wafer (almostthe entire wafer)

Surface micromachiningis based on

additional thin layer deposition and

by selective etching of this layers

MicromachiningMicromachining


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MicromachiningMicromachining


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Surface MicromachiningSurface Micromachining


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Micromachining MaterialsMicromachining Materials


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Bulk MicromachiningBulk Micromachining


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Review: Micro-manufacturing

Surface/bulk micro-machiningMaterials: Silicon


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Materials: Silicon

Oxidation

Thin film deposition Photolithography Etching Lift-off Wafer Bonding Dispenser

PR

Wafer

Chuck

Spin

Silicon WafersSilicon Wafers


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Czochrolski method Molten Si bath Seed pulled at 2-5 cm/hr

Silicon-On-Insulator (SOI) Wafers

Multi-layered Si / Oxide


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1. Start with Si wafer

2. Grow oxide

3. Bond 2nd Si wafer to oxide

4. Grind top wafer to size

5. Repeat if necessary

Expensive~ Rs20000 vs. Rs1500 Reduces process time

Si Base Wafer

Si Base Wafer

Si Base Wafer

2nd Si Wafer

Si Base Wafer

Oxide

SOI

Material Addition / Removal

Additi Subtractive


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Additive

Casting Molding Oxidation Deposition

Physical

Evaporation

SputteringChemical

CVD

Spin-on

Machining

Grinding Etching

Wet

Dry

MEMSwww.memspi.com

DepositionDeposition


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Deposition TechnologyDeposition Technology

MEMS deposition technology can be classified in two groups:


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MEMS deposition technology can be classified in two groups:

Depositions that happen because of a chemical reaction: Chemical Vapor Deposition (CVD) Electrodeposition Epitaxy Thermal oxidation

These processes exploit the creation of solid materials

directly from chemical reactions in gas and/or liquidcompositions or with the substrate material. The solidmaterial is usually not the only product formed by thereaction. Byproducts can include gases, liquids and evenother solids.

Depositions that happen because of a physical reaction: Physical Vapor Deposition (PVD) Casting

Chemical Vapor DepositionChemical Vapor Deposition

Gases react to deposit film on surfaces


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Gases react to deposit film on surfaces

Example: Polysilicon at 580-650C

242HSiHSi +

ICL, MIT

Typical CVD Furnace:

Parameters DomainParameters Domain

Thermal (temperature heat and heat flow)


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Thermal (temperature, heat and heat flow)

Mechanical (force, pressure, velocity, acceleration,position)

Chemical (concentration, pH, reaction rate)

Magnetic (field intensity, flux density, magnetization)

Radiant (intensity, wavelength, polarization, phase) Electrical (voltage, charge, current)


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OxidationOxidation

Forming Silicon Dioxide by oxidation Uses: insulation, masking, sacrificial layer


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Easy to grow and etch (HF)

Good insulator/ diffusion barrier (Ex. B, P, As) High dielecytic breakdown field (500 V/ m); = 1016 ohm-cm Good Ge/ GaAs => higher mobility/direct bandgap, but no stable oxide => Cant

make MOS device

Si Wafer

O2 chamber

Si Wafer

O2 chamber

Oxide

3hr, 1000 C 1m

EvaporationEvaporation

Metal evaporated w/ electron beam


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Metal evaporated w/ electron beam

Vacuum environment prevent oxidation, directionalvapor travel

Can get shadowing:

S. G. Kim, MIT

SputteringSputtering

Materials: Metals and dielectrics


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Materials: Metals and dielectrics

Requires high vaccuum Argon ions bombard material target

Ejected material - ballistic path to wafers

S. G. Kim, MIT


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Deposition IssuesDeposition Issues

Thermal compatibility Topographic compatibility


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Residual stresses

Deposition over features Conformality:

Conformal Non-conformal

Non-conformal

Review


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Microscopic EtchMicroscopic Etch


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EtchantsEtchants


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REVIEW: Material Removal - Etching

Bulk micromachining remove Si substrate Surface micromachining add / remove films Masks for etch s rface


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Masks for etch surface:

Photoresist oxide, nitride

Isotropic vs. anisotropic Wet vs. dry etch Deep Reactive Ion Etching

Wet vs. Dry Etching

Wet etch Chemical reaction

Lo res

Dry etch Reactive ion etching (RIE) High res


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Low res

Low cost Hard to control direction Limited 2D geometries 1 10s of microns/min

High cost Very directional Unlimited 2D geometries 0.1 10 microns/min

Isotropic vs. Anisotropic Etching

Isotropic same etch rate in all directions Etch depth / surface uniformity problems Diffusion dependent


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Etch silicon wafer and deposited films

Anisotropic crystal plane dependent Etch ratios more than 100:1 silicon wafers only Crystal planes:

54.7 Si

Adapted from: S.M. Sze, Semiconductor Devices S. G. Kim, MIT

Deep Reactive Ion Etching

Alternating RIE - polymer deposition Protection of side walls during material removal

CHF /Ar forms Teflon like layer


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CHF3/Ar forms Teflon-like layer

Issues: Scalloping and tapering

EtchingEtching


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EtchingEtching


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EtchingEtching


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DopingDoping


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Band ModelsBand Models


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DopantsDopants


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Carrier distributionCarrier distribution


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Mass Action LawMass Action Law


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DiffusionDiffusion

Diffusion in general is motion of species in solidsin the direction of concentration gradient;


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in the direction of concentration gradient;

(i)diffusion of free charge carriers insemiconductor;

(ii) diffusion of dopant atoms in semiconductor atelevated temperature;

(iii) diffusion - common name of the hightemperature process during which dopant atomsare introduced into semiconductor by diffusion.

Mass Transfer Equations: Fick's LawMass Transfer Equations: Fick's Law

Fick describe diffusion in solids;


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1st Fick's law describes motion by diffusion of anelement in the solid in the direction of theconcentration gradient;

2nd Fick's law determines changes ofconcentration gradient in the course of diffusion


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concentration gradient in the course of diffusion(function of time and diffusion coefficient).

Analogy with Air FreshnerAnalogy with Air Freshner

We said that mass transfer is the movement of matter from ahigh concentration to a low concentration. This means that


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g

the matter wants to "get away" from the other similar matterand go to a place where there is open space.

Let's relate this to an example. Everyone has sprayed air freshener before. When you spray

the air freshener, it does not just smell where you sprayed it,

but rather the smell spreads throughout the room. The airfreshener (matter) moves from an area of high concentration(where you sprayed it) to an area of low concentration that isfar from the place that it was sprayed. This movement of thematerial is called diffusion.

Diffusion can be represented by a basic equation.

The equation that we are about to study is often referred toas Fick's Law.

Relation between Mass Flux &Relation between Mass Flux &ConcentrationConcentration

J= -D * C/x

J The Mass Flux Flux is the movement of objects from one


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j

point to another in a given time. Units: the units of moles / (time * area)

D Diffusivity The diffusivity is the constant that describeshow fast or slow an object diffuses.

Units: the units of area / time

- Delta - Delta is the Greek letter that represents

the words "change in." When the symbol is used,

it refers to subtracting the beginning value from the

final value of the symbol following the .

For example: C = C2 C1 = the last C the first C,

where C represents concentration
http://blowers.chee.arizona.edu/cooking/units.htmlhttp://blowers.chee.arizona.edu/cooking/units.htmlhttp://blowers.chee.arizona.edu/cooking/units.htmlhttp://blowers.chee.arizona.edu/cooking/units.html


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The symbol C refers to the change in concentration

from when the object had not diffused at all, to the final

concentration when the object was done diffusing.

x Distance This refers to the distance that the object is diffusing

Therefore, returning to the equation, J = -D * C/ x It was described that the flux was equal to the negative

diffusivity times the change in concentration divided by thechange in distance.

A typical solution of the diffusion problem maylook like this:


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look like this:


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State Ficks 1st and 2nd Law.


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THIN FILM CHARACTERISATIONTHIN FILM CHARACTERISATION

Thin Film Characterization

Thin films can be characterized many different ways, for anyi thi fil ll l i t t d i th f


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given thin film we are normally only interested in three or fourcrucial properties.

These important properties will vary from application toapplication.

The most common thin film properties and their associatedmeasurement techniques are listed below :-

Thickness (spectrophotometer, ellipsometer, step height,Electron microscope, RBS, quartz crystal monitor) Optical properties : refractive index, extinction coefficient,

transmission, reflection, absorbtion(spectrophotometer,ellipsometer)

Crystal structure Microstructure

Chemical/Elemental Composition

IN DETECT Technique

high energy electrons (30 keV) backscattered and secondary electronsScanning Electron Microscopy
http://www.alacritas-consulting.com/thin_film_thickness_calculation.htmlhttp://www.alacritas-consulting.com/thin_film_thickness_calculation.html


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moderate energy electrons (5 keV) secondary electrons Auger Electron Spectroscopy

light (X-rays - 1 keV) secondary electrons X-ray Photoelectron Spectroscopy

low energy electrons (100 eV) diffracted electrons Low Energy Electron Diffraction

moderate energy electrons (5 keV) diffracted electrons Reflection High Energy Electron Diffraction

moderate-high energy ions (2-30 keV) secondary ions Secondary Ion Mass Spectrometry

polarized light (2 eV) polarized light ellipsometry


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Chemical Vapor Deposition (CVD)Chemical Vapor Deposition (CVD)


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Typical hot-wall LPCVD reactor.

CVD TechniquesCVD Techniques

In this process, the substrate is placed inside areactor to which a number of gases are supplied.Th f d t l i i l f th i th t


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The fundamental principle of the process is that achemical reaction takes place between the sourcegases. The product of that reaction is a solidmaterial with condenses on all surfaces inside thereactor.

The two most important CVD technologies inMEMS are the Low Pressure CVD (LPCVD) andPl E h d CVD (PECVD) Th LPCVD


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Plasma Enhanced CVD (PECVD). The LPCVDprocess produces layers with excellent uniformityof thickness and material characteristics. The mainproblems with the process are the high depositiontemperature (higher than 600C) and the relativelyslow deposition rate. The PECVD process canoperate at lower temperatures (down to 300 C)thanks to the extra energy supplied to the gasmolecules by the plasma in the reactor.

However, the quality of the films tend to be inferiorto processes running at higher temperatures.S dl t PECVD d iti t


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Secondly, most PECVD deposition systems canonly deposit the material on one side of the waferson 1 to 4 wafers at a time. LPCVD systems depositfilms on both sides of at least 25 wafers at a time. Aschematic diagram of a typical LPCVD reactor isshown in the figure below.

When do I want to use CVD?When do I want to use CVD?

CVD processes are ideal to use when you want athin film with good step coverage. A variety of

t i l b d it d ith thi t h l


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materials can be deposited with this technology,however, some of them are less popular with fabsbecause of hazardous byproducts formed duringprocessing. The quality of the material varies fromprocess to process, however a good rule of thumbis that higher process temperature yields a materialwith higher quality and less defects

Chemical Vapor Deposition Gases react to deposit film on surfaces Example: Polysilicon at 580-650C


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242HSiHSi +

Typical CVD Furnace:

CVDCVD


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ElectrodepositionElectrodeposition

This process is also known as "electroplating" and is typicallyrestricted to electrically conductive materials. There are basicallytwo technologies for plating: Electroplating and Electroless plating.

In the electroplating process the substrate is placed in a liquidl i ( l l ) Wh l i l i l i li d


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In the electroplating process the substrate is placed in a liquidsolution (electrolyte). When an electrical potential is appliedbetween a conducting area on the substrate and a counterelectrode (usually platinum) in the liquid, a chemical redoxprocess takes place resulting in the formation of a layer of materialon the substrate and usually some gas generation at the counterelectrode.

In the electroless plating process a more complex chemicalsolution is used, in which deposition happens spontaneously onany surface which forms a sufficiently high electrochemicalpotential with the solution. This process is desirable since it doesnot require any external electrical potential and contact to thesubstrate during processing. Unfortunately, it is also more difficultto control with regards to film thickness and uniformity. Aschematic diagram of a typical setup for electroplating is shown inthe figure below.

When do I want to use electrodeposition?When do I want to use electrodeposition?

The electrodeposition process is well suited tomake films of metals such as copper, gold and

nickel The films can be made in any thickness


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nickel. The films can be made in any thicknessfrom ~1m to >100m. The deposition is bestcontrolled when used with an external electricalpotential, however, it requires electrical contact tothe substrate when immersed in the liquid bath. Inany process, the surface of the substrate musthave an electrically conducting coating before thedeposition can be done.

Typical setup for electro-deposition


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EpitaxyEpitaxy

This technology is quite similar to what happens in CVD processes,however, if the substrate is an ordered semiconductor crystal (i.e.silicon, gallium arsenide), it is possible with this process to continue

building on the substrate with the same crystallographic orientation


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building on the substrate with the same crystallographic orientationwith the substrate acting as a seed for the deposition. If anamorphous/polycrystalline substrate surface is used, the film willalso be amorphous or polycrystalline.

There are several technologies for creating the conditions inside areactor needed to support epitaxial growth, of which the mostimportant is Vapor Phase Epitaxy (VPE). In this process, a number of

gases are introduced in an induction heated reactor where only thesubstrate is heated. The temperature of the substrate typically mustbe at least 50% of the melting point of the material to be deposited.

An advantage of epitaxy is the high growth rate of material, whichallows the formation of films with considerable thickness (>100m).Epitaxy is a widely used technology for producing silicon oninsulator (SOI) substrates. The technology is primarily used for

deposition of silicon. A schematic diagram of a typical vapor phaseepitaxial reactor is shown in the figure below.

Epitaxial reactor


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Typical cold-wall vapor phase epitaxial reactor.

When do I want to use epitaxy?When do I want to use epitaxy?

This has been and continues to be an emergingt h l i MEMS Th b


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This has been and continues to be an emergingprocess technology in MEMS. The process can beused to form films of silicon with thicknesses of~1m to >100m. Some processes require hightemperature exposure of the substrate, whereas

others do not require significant heating of thesubstrate. Some processes can even be used toperform selective deposition, depending on thesurface of the substrate


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Thermal oxidationThermal oxidation

This is one of the most basic deposition technologies. It is simplyoxidation of the substrate surface in an oxygen rich atmosphere.The temperature is raised to 800 C-1100 C to speed up the process.

This is also the only deposition technology which actuallyconsumes some of the substrate as it proceeds. The growth of the


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This is also the only deposition technology which actuallyconsumes some of the substrate as it proceeds. The growth of thefilm is spurned by diffusion of oxygen into the substrate, whichmeans the film growth is actually downwards into the substrate. Asthe thickness of the oxidized layer increases, the diffusion of oxygento the substrate becomes more difficult leading to a parabolicrelationship between film thickness and oxidation time for filmsthicker than ~100nm. This process is naturally limited to materialsthat can be oxidized, and it can only form films that are oxides of

that material. This is the classical process used to form silicondioxide on a silicon substrate. A schematic diagram of a typicalwafer oxidation furnace is shown in the figure below.

When do I want to use thermal oxidation? Whenever you can! This is a simple process, which unfortunately

produces films with somewhat limited use in MEMS components. Itis typically used to form films that are used for electrical insulationor that are used for other process purposes later in a processsequence.

wafer oxidation furnace


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Typical wafer oxidation furnace


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Oxidation Forming Silicon Dioxide by oxidation

O2 chamber @ 800-1200 oC (steam optional)

Silicon reacts with O2 to grow oxide

2 micron max film thickness


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2 micron max film thickness

Si Wafer

O2 chamber

Si Wafer

O2 chamber

Oxide

3hr, 1000 C 1m

Oxidation Forming Silicon Dioxide by oxidation

Uses: insulation, masking, sacrificial layer Easy to grow and etch (HF)

Good insulator/ diffusion barrier (Ex. B, P, As)

High dielecytic breakdown field (500 V/ m); = 1016 ohm-cm


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High dielecytic breakdown field (500 V/ m); = 10 ohm-cm Good Ge/ GaAs => higher mobility/direct bandgap, but no stable oxide => Cant

make MOS device

Si Wafer

O2

chamber

Si Wafer

O2

chamber

Oxide3hr, 1000 C 1m

OxidationOxidation


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PVDPVD

Physical Vapor Deposition (PVD) PVD covers a number of deposition technologies in which

material is released from a source and transferred to thesubstrate. The two most important technologies are


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p gevaporation and sputtering.

When do I want to use PVD? PVD comprises the standard technologies for deposition of

metals. It is far more common than CVD for metals since it

can be performed at lower process risk and cheaper inregards to materials cost. The quality of the films are inferiorto CVD, which for metals means higher resistivity and forinsulators more defects and traps. The step coverage is alsonot as good as CVD.

The choice of deposition method (i.e. evaporation vs.sputtering) may in many cases be arbitrary, and may depend

more on what technology is available for the specific materialat the time.


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In evaporation the substrate is placed inside avacuum chamber, in which a block (source) of the

material to be deposited is also located. The


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psource material is then heated to the point where itstarts to boil and evaporate. The vacuum isrequired to allow the molecules to evaporate freelyin the chamber, and they subsequently condense

on all surfaces. This principle is the same for allevaporation technologies, only the method used tothe heat (evaporate) the source material differs.There are two popular evaporation technologies,which are e-beam evaporation and resistiveevaporation each referring to the heating method.


In e-beam evaporation, an electron beam is aimedat the source material causing local heating and

evaporation. In resistive evaporation, a tungsten


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p p , gboat, containing the source material, is heatedelectrically with a high current to make the materialevaporate. Many materials are restrictive in termsof what evaporation method can be used (i.e.

aluminum is quite difficult to evaporate usingresistive heating), which typically relates to thephase transition properties of that material. Aschematic diagram of a typical system for e-beamevaporation is shown in the figure below.


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Typical system for e-beam evaporation of materials.

Evaporation Metal evaporated w/ electron beam Vacuum environment prevent oxidation, directional vapor travel


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Can get shadowing:

S. G. Kim, MIT

Evaporation Metal evaporated w/ electron beam Vacuum environment prevent oxidation, directional vapor travel


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Can get shadowing:

S. G. Kim, MIT


Sputtering is a technology in which the material isreleased from the source at much lower

temperature than evaporation. The substrate isl d i h b ith th


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p pplaced in a vacuum chamber with the sourcematerial, named a target, and an inert gas (such asargon) is introduced at low pressure. A gas plasmais struck using an RF power source, causing the

gas to become ionized. The ions are acceleratedtowards the surface of the target, causing atoms ofthe source material to break off from the target invapor form and condense on all surfaces includingthe substrate.


As for evaporation, the basic principle ofsputtering is the same for all sputtering

technologies. The differences typically relate to thei hi h th i b b d t f th t t


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g yp ymanor in which the ion bombardment of the targetis realized. A schematic diagram of a typical RFsputtering system is shown in the figure below.



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Typical RF sputtering system.



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CastingCasting

In this process the material to be deposited is dissolved inliquid form in a solvent. The material can be applied to the

substrate by spraying or spinning. Once the solvent isevaporated, a thin film of the material remains on theb t t Thi i ti l l f l f l t i l


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substrate. This is particularly useful for polymer materials,which may be easily dissolved in organic solvents, and it isthe common method used to apply photoresist to substrates(in photolithography). The thicknesses that can be cast on a

substrate range all the way from a single monolayer ofmolecules (adhesion promotion) to tens of micrometers. Inrecent years, the casting technology has also been applied toform films of glass materials on substrates. The spin castingprocess is illustrated in the figure below.

When do I want to use casting?When do I want to use casting?

Casting is a simple technology which can be usedfor a variety of materials (mostly polymers). The

control on film thickness depends on exactconditions but can be sustained within +/-10% in a


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conditions, but can be sustained within +/-10% in awide range. If you are planning to usephotolithography you will be using casting, whichis an integral part of that technology. There are

also other interesting materials such as polyimideand spin-on glass which can be applied by casting.


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The spin casting process as used for photoresist in photolithography.


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