an introduction to polysilicon micromaching robert w. johnstone rjohnsto

An Introduction to Polysilicon Micromaching

Robert W. Johnstone

www.sfu.ca/~rjohnsto/

www.sfu.ca/immr/

An Introduction to Polysilicon Micromachining 2

Personal Information

Robert W. Johnstone Graduate Student at

Simon Fraser University School of Engineering

Science Simon Fraser University 8888 University Drive,

Burnaby, BC Canada V5A 1S6 Tel: (604) 291-4971 Fax (604) 291-4951


Outline

Introduction Fabrication MEMS Technology Sensors Actuators Packaging Issues and

Integration MUMPs Examples Design Issues Evaluations and

Questions


Introduction


Introduction: Terminology

Micromachining Microfabrication Microelectromechanical

Systems (MEMS) Microsystems Technology

(MST)


Introduction

Microfabrication

Micromachining

Microelectronics

BulkMicromachining

LIGA Process

SurfaceMicromachining

RaisedStructures


Introduction: Features of MEMS

Miniature mechanical systems

Batch fabrication approach

Utilizes microelectronic manufacturing base

Common technology for sensors, actuators and systems


Introduction: Why Miniaturize?

System Integration Avoid assembly of discrete components Better reliability Lower costs Better Performance

Better Response Smaller devices have less inertia, less thermal

mass, less capacitance, etc. Increased Reliability

Mass decreases faster than structural strength


Introduction: Systems-on-a-Chip

Traditional Hundreds of components Manual/semi automated

assembly Plenty of solder joints Sensitive to shock and vibration

Future Single chip No assembly Minimal solder joints Batch fabrication Insensitive to shock & vibration


Introduction: Growth Prediction

Hard disk drive heads Inkjet print heads Heart pacemakers In vitro diagnostics Hearing aids Pressure sensors Chemical sensors Infrared imagers

Accelerometers Gyroscopes Machine monitoring Micro fluidics Magnetoresistive sensors Microspectrometers Micro optical systems Military systems

Technologies experiencing growth.


Introduction: Growth Prediction

MEMS Device RevenuesSource: SEMI

MEMS use in existing systemsSource: MST News


Introduction: Applications

Telecommunication relies on routing optical signals

Present systems use large and centralized networks

A low cost optical switch can revolutionize telecommunications technology

MEMS enables practical, low cost micro-mirrors

Relevant Examples



MEMS enables low cost chips that can monitor motion and position

Enables integration of inertial measurement in systems not possible with traditional technology

Applications in air bags, skid control, machine tools, sports equipment etc

Inertial Measurement



Ink jet printing TAS – Micro Total Analysis

System (chemical analysis) Environmental monitoring:

Detection of pollutants and pathogens

Biomedical devices: heart/lung and kidney Dialysis machine, dosing systems etc

DNA analysis systems for diagnostic, therapeutic

And forensic studies

Micro Fluidics



Strategy #1 Build the best one possible to

meet the most stringent requirements

Strategy #2 Build them cheap and worry about

performance later

Development Strategy



New products in old fabs Seamless integration into existing

fabrication plants Minimal additional investment Risk is low Logical next step

Industry’s Interest in MEMS


Introduction: Major Challenge

Application specific technologies

Differently tuned technology for different devices/applications

Presently low synergy or cooperation in formulating a common technology

Technology Standards


Fabrication


Fabrication: Technology

Basic fabrications processes based on IC technology


Fabrication: Spectrum

IC technology Bipolar CMOS BiCMOS

MEMS related technology

Bulk micromachining Surface micromachining LIGA, LIGA-like Micro EDM 3D stereo lithography Laser micromachining Focused ion beam milling


Fabrication: Basic Processes

Lithography Oxidation Diffusion Thin film deposition

CVD process Thermal evaporation Sputtering

Silicon Processing


Fabrication: Lithography

Lithography is the process of transferring a pattern from a mask to a photoresist using a photographic tool (mask aligner), and to the silicon substrate using etching techniques.

PATTERN TRANSFER



Coat the wafer with an adherent and etch-resistant photoresist

Selectively remove the resist to leave the desired pattern by exposure and development steps

Etch to transfer the mask pattern to the underlying material

Remove (strip) the photoresist and clean the wafer



Silicon substrate

Mask Pattern

Photoresist

Oxide

Photomask

UV Light

Transparentregion

Opaqueregion



Silicon substrate

Silicon substrate

Silicon substrateExpose and develop

Etch oxide

Strip resist

Positive Photoresist



Silicon substrate

Silicon substrate

Silicon substrateExpose and develop

Etch oxide

Strip resist

Negative Photoresist


Fabrication: LithographyFilm

Mask

Etch

After maskremoval

Mask

Afterlithography

Film

Subtractive vs. Additive Pattern Transfer


PR Spinner

Dispenseresist

Spin

Spin complete

Spin Coating of Photoresist




Contact Printers Mask and wafer in direct contactVery high resolution1X magnification

Proximity Printers Mask and wafer separated by a few micron gapModerate resolution

Projection Printers Accomplished via mirror and lenses

Step and Repeat Projection Printers

High resolution5X and 10X reduction possibleRelaxes reticle tolerances and defect requirements

Types of Lithographic Tools



Wafer and maskout-of-contact

during alignment

Wafer and maskin-contact

during exposure

Contact Printing


Fabrication: LithographyProjection Printing (using Wafer Stepper)


Fabrication: Oxidation

Thermal oxidation is a high temperature process used to grow a continuous layer of high-quality silicon dioxide on silicon substrate

Dry oxidation: oxidizing species is oxygen

Wet oxidation: oxidizing species is water vapour



Oxidation process

After oxidation



Ref: Fundamentals of Silicon Integrated Device Technology

Dry

Ox id

a ti o

n Ra

te



Ref: Fundamentals of Silicon Integrated Device Technology

We t

Oxi d

ati o

n Ra

te



Oxidation process

Oxidation complete

Oxide removed

Oxidation Through a Window in the Oxide



Silicon nitride deposition

Oxidation

Oxidation complete

Silicon nitride removed

Local Oxidation



Film Thickness (Microns)

Color and Comments Film Thickness (Microns)

Color and Comments

0.05 Tan 0.60 Carnation pink

0.07 Brown 0.58 Light orange or yellow to pink borderline

0.10 Dark violet to red violet 0.57 Yellow to "yellowish" (At times it appears to be light creamy gray or metallic)

0.12 Royal blue 0.56 Green yellow

0.15 Light blue to metallic blue 0.54 Yellow green

0.1 Metallic to very light yellow green 0.52 Green (broad)

0.20 Light gold or yellow - slightly metallic 0.50 Blue green

0.22 Gold with slight yellow orange 0.49 Blue

0.25 Orange to melon 0.48 Blue violet

0.27 Red violet 0.47 Violet

0.30 Blue to violet blue 0.46 Red violet

0.31 Blue 0.44 Violet red

0.32 Blue to blue green 0.42 Carnation pink

0.34 Light green 0.41 Light orange

0.35 Green to yellow green 0.39 Yellow

0.36 Yellow green 0.37 Green yellow

Silicon Processing for the VLSI Era: Volume 1- Process Technology

Oxide Layer Color Chart


Fabrication: Diffusion

Diffusion is a process by which atoms of impurities (eg., B, P, As, Sb) move into solid silicon as a result of the presence of a concentration gradient and high temperatures.


Fabrication: DiffusionDiffusion Through an Oxide Window



Diffusion fromunlimited source

Diffusion fromlimited source

Diffusion fromconcentration step

Diffusion Profiles



Irvines’sCurves

Resistivity of Diffused Layers in Silicon



Separate furnacesfor oxidation and

diffusion processes

Oxidation/Diffusion Furnace


Fabrication: Thin Film Deposition

Chemical Vapor Deposition (CVD) Processes Physical Vapor Deposition (PVD) Processes

Thermal evaporation Sputtering



CVD is the formation of a solid film on a substrate by the reaction of vapour phase chemicals which are decomposed or reacted on or near the substrate.



Reaction Types

Heterogeneous reactionChemical reaction takes placeVery close to the surface

Good quality films

Homogeneous reaction

Chemical reaction takes placeIn the gas phase

Poor quality films

Reaction EnergyThermalPhotonsElectrons

Processes

APCVD – Atmospheric pressure CVD

LPCVD – Low pressure CVD

PECVD – Plasma enhanced CVD



Mass Transport Limited Reaction Rate Limited

Temperature not criticalRegulation of reactant species on wafer surface is important

Temperature sensitiveReactant flux not critical

Deposition Conditions



Deposition condition and reaction chemistry determine the crystalline nature of the film

Crystallographic Forms



Atmospheric pressure chemical vapor deposition

Large volume of carrier gases needed

Poor step coverage Low throughput Primarily used for LTO

Process gases

APCVD



Low-pressure chemical vapor deposition

Reaction rate limited operation

Operates at 0.1 to 1Torr pressure

Good quality films Conformal coverage Typically used for HTO,

Poly-silicon, some metal films and nitride

LPCVD



Plasma enhanced chemical vapor deposition

Low temperature operation Good conformal step coverage Primarily used for passivation and

inter-level dielectrics

PECVD



Film Reactant Gases(Carrier)

Temp°C

Deposition Ratenm/min

APCVD

Epitaxial SiCold Wall (CW)

SiCl4H2 (H2)

SiHCl3 / H2 (H2)

SiH2Cl2 (H2)

SiH4 (H2)

1125 – 12001100 – 11501050 – 11001000 - 1075

500 – 1500500 – 1500500 – 1000100 - 300

Poly Silicon(CW)

SiH4 (H2) 850 - 1000 100

Si3N4(CW)

SiH4 / NH3 (H2) 900 - 1100 20

SiO2 SiH4 / O2 (N2) 200 - 500 100

CVD Chemistry


Film Reactant Gases(Carrier)

Temp°C

Deposition Ratenm/min

LPCVD

Epitaxial Silicon SiH2Cl2 (H2)

(30 – 80 Torr)

1000 - 1075 100

Poly Si 100% SiH4(0.2 Torr)

23% SiH4 (N2)

(1.0 Torr)

620

640

10

19

Si3N4 SiH2Cl2 / NH3

(0.3 Torr)

800 4

SiO2 SiH2Cl2 / N2O

(0.4 Torr)

900 8

SiO2 SiH4 / O2

SiH4 / PH3 / O2

(0.7 Torr)

450450

1012

PECVD

Si3N4 SiH4 / NH3 (N2) 300 10




Parallel plate PECVD(Low throughput)

High throughput PECVD

PECVD Systems



Physical vapor deposition is a process in which the material to be deposited is converted from a solid phase into vapor phase, then moved through a region of low pressure, with the vapor condensing on the substrate, to form a solid thin film.

Evaporation: Source material is converted into liquid phase and next into vapour phase usually by thermal process

Sputtering: Physical dislodging of atoms from a target

Primarily used for interconnect metal

deposition

Physical Vapor Deposition (PVD)



Evaporator Evaporation Sources

Thermal Evaporation



Provides very clean and high purity metal films

Electron Beam Evaporation



DC Sputtering DC voltage between target and substrate,

used for conductive targets (metal films)

RF Sputtering RF voltage between target and substrate,

used for insulators (dielectrics)

Magnetron Sputtering

Magnetic field confines electrons near the target,

increasing the number of electrons causing

ionization collisions and, thereby, deposition rates

Reactive Sputtering

Sputtering a target material in presence of a

reactive gas, thereby, depositing a compound

Sputtering Systems


Fabrication: Etching Thin Films

Wet Etch Liquid phase wet chemical etchUnder cut problemsNot useful for fine dimension control

Dry Etch Use of a gas plasma to abrasively etch the thin filmExcellent dimension control

Reactive Ion Etch

Fluorine, Chlorine

based chemistry

Use of a reactive gas species that reacts with the thin film and produce a gaseous by productExcellent dimension and sidewall control

Typically photoresist is used as a masking layer


Fabrication: Planarization

Chemical mechanical polishing (CMP) Planarization process used in IC technology

Non planarized surface micromachining produces stringers and non-flat surfaces

Yield and reliability problems Not suitable for micro-optics


Fabrication: PlanarizationNon CMP

CMP

Stringers Non uniform staple Non planar link Non planar hinge

Uniform and flat staple Planar link Planar hinge

Sandia National labs


MEMS Technology


MEMS Technology

Major MEMS technologies Bulk micromachining Surface micromachining LIGA …


MEMS Technology

Historically: Silicon Micromachining3-D Sculpting of silicon and silicon compoundsOffshoot of IC fabrication technologyUses lithography & mass production

Modern: Non silicon MEMSElectroformingMolding


Silicon wafer

Oxidize

Lithography

Diffuse impurity

IC Technology Micromachining

Silicon wafer

Oxidize

Lithography

Etch the substrate

MEMS Technology: Roots



Isotropic Etching

Anisotropic Etching

Etch cavity bound by the crystal planes

Basic Etching Processes



Bulk Micromachining Surface MicromachiningDeposit thin films on substratePattern thin films lithographicallySelectively etch away a portion of the substrate to form a free standing 3D microstructure bound by a cavity

Deposit thin films on substratePattern thin films lithographicallySelectively etch away one (or more) of the intermediate thin films to form a free standing 3D structure standing on top of the substrate surface

Micromachining Classification


MEMS Technology: Bulk

Relies mostly on anisotropic etching(wet as well as dry etch)


MEMS Technology: Bulk

Etchant Mask Etch Stop Etch Rate

m/hr

Etch Ratio

(100):(111)

Potassium Hydroxide (KOH)

SiO2, SiN Boron > 1020 cm-3 reduce etch rate by 20

~85 ~400

Ethylene Diamine Pyrocatechol (EDP)

SiO2, SiN, Au Boron > 5x1019 cm-3 reduces etch rate by 50

~70 ~35

Tetramethyl Ammonium Hydroxide (TMAH)

SiO2, SiN Boron > 1020 cm-3 reduce etch rate by 40

~60 ~10

Silicon Anisotropic Etchants


MEMS Technology

Etch Stop Techniques Heavily boron doped

silicon can act as an etch stop

For more precise thickness control use electrochemical techniques

Technology developed for silicon pressure sensors and single crystal silicon resonators

MEMS Specific Etching


MEMS Technology

BOSCH Patent STS, Alcatel, Trion, Oxford Instruments …

Uses high density plasma to alternatively etch silicon and deposit a etch-resistant polymer on side walls

Polymer deposition Silicon etch using SF6 chemistry

Polymer

Unconstrained geometry90o side wallsHigh aspect ratio 1:30Easily masked (PR, SiO2)

Process recipe depends ongeometry

Deep Reactive Ion Etching (DRIE)


MEMS Technology: LIGAX-Rays

X-Ray maskHigh aspect ratio

Metallic microstructures

Suitable for magneticactuation and sensing

Enables micro assembly

Liga-like techniqueuses thick photoresist and

UV lithography

Thick Photoresist(PMMA)

Electroplatemetal

Dissolve resist


MEMS Technology: Surface

Structural Layer: Must have good mechanical and electrical properties

Sacrificial Layer: Must be stable during deposition and processing should etch quickly during release step

Both layers should be IC process compatible and should have excellent etch selectivity

Sacrificial and Structural Layers



Sacrificial Materials Silicon dioxide Doped silicon oxides Photoresist Polyimides Carbon and few metals

Structural Materials Polysilicon Aluminium Silicon nitride Silicon carbide Nickel

Sacrificial and Structural Layers



Prototypical surface micromachining process

Three structural layers Polycrystalline silicon

(polysilicon) First layer is not

moveable Often called zero layer



Substrate

Silicon Nitride

Poly-silicon

Silicon Dioxide

Metal



Substrate

Silicon Nitride

Poly-silicon

Silicon Dioxide



Substrate

Silicon Nitride

Poly-silicon



Micro bridge Rotating part

Sacrificial LayerIsotropic Etching

Structural Layer (Poly)

Isotropic Etching

A combination of sacrificial and structural layers


Sacrificial Layer

Isotropic Etching

Micro bridge Rotating part

Structural Layer (Poly)

Isotropic Etching




Silicon NitrideWafer (Silicon)



Silicon DioxideSilicon NitrideWafer (Silicon)



Poly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



PhotolithographyPoly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



Silicon DioxidePhotolithographyPoly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



PhotolithographySilicon DioxidePhotolithographyPoly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



PhotolithographyPoly-siliconPhotolithographySilicon DioxidePhotolithographyPoly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



PhotolithographyMetalPhotolithographyPoly-siliconPhotolithographySilicon DioxidePhotolithographyPoly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



ReleasePhotolithographyMetalPhotolithographyPoly-siliconPhotolithographySilicon DioxidePhotolithographyPoly-siliconSilicon DioxideSilicon NitrideWafer (Silicon)



2-Level

Simple sensors &actuator

3-Level

Gearsgear train

4-Level

Pin-joints,gear train

5-Level

Multilevelgears and

advanced MEMS

Actuator Gear Hub

Drive link

Movable plate

Actuator

Gear Hub

Actuator Drive link

Gear Hub Actuator

CRONOS and variousuniversity technology

Sandia’s SUMMiTtechnology

State-of-the-Art Surface MicromachiningNumber of structural layers determine the

complexity/advancement



2-Level

Simple sensors &actuator

3-Level

Gearsgear train

4-Level

Pin-joints,gear train

5-Level

Multilevelgears and

advanced MEMS

CRONOS and variousuniversity technologies

Sandia’s SUMMiTtechnology

40m20m

State-of-the-Art Surface Micromachining


Sensors


Sensors: Transduction Principles

Physical Sensors Accelerometer Gyroscope Pressure sensor Mass flow sensor Temperature sensor Proximity sensor Magnetic sensor

Chemical Sensors Gas detector pH Detector

Micro-fluidics

Bio-analysis


Sensors: AccelerationTransduction:PiezoresistiveCapacitiveResonance based

Analog Devices ADXL-50integrated Accelerometerwith on board electronics

(BiCMOS)

Based on comb structureand capacitive pick-up

Translation direction

180° out-of-phase signalsfed to this pair of

stationary electrodes


Sensors: Pressure

Piezoresistive Bulk micromachining Electrochemical etching Anodic bonding to a

PYREX base


Sensors: Pressure

Cross-Sectional and Side Views of a commercial Bulk Micromachined Pressure Sensor


Sensors: Gas

Adsorption Foreign chemical species

enter interstitial or bonding sites at or near the surface

Changes the interface properties

Absorption Foreign chemical species

enter interstitial or bonding sites within the bulk material

Changes the materials bulk properties


Sensors: Gas Concentration

Metal Oxide Gas sensor FET Gas Sensor

Adsorbed gas moleculealters the conductivity

Adsorbed gas moleculealters the threshold voltage

Gas Sensor Principles


Sensors: Sandia Developments


Sensors: 3-axis Acceleration SensorSandia National Labs


Sensors: Surface Micromachined Pressure Sensor

Sandia National Labs


Sensors: Combustible Gas SensorSandia National Labs


Actuators


Actuators

Microactuators are the special contribution of MEMS technology

Actuation Mechanisms Electrostatic Thermal Magnetic Piezoelectric

Can be easily implementedusing most of the surface micromachining technology

Silicon is neither piezoelectricnor magnetostrictive, therefore,additional thin films have to be added to the microstructures


Actuators: ElectrostaticIn surface micromachining, often popularly known as comb drives.

PrincipleConsider parallel plate 1 & 2

Force of attraction (along y direction)

Fp = ½ eA(V2/d2)

Plate-1

Plate-2

Plate-3

V (volts)

x

yd

Consider plate 2 inserted between plate 1 and 3Force of attraction (along x direction)

Fc = e (t/d) V2

Constant with x-directional translation


Actuators: Electrostatic

Sandia cascaded comb drive(High force)

Close-up viewof the shuttle

CRONOS comb drive

Comb Drives


Actuators: ElectrostaticSqueeze Film: Texas Instruments DMD


Actuators: Thermal

Uses thermal expansion for actuation

Small thermal expansion is mechanical amplified

Very effective and high force output per unit area


Actuators: Thermal

Current output

terminal

Current output pad

Hot arm

Cold arm Direction of actuation

Ground plane


Acknowledging ZYVEX (www.zyvex.com)

Actuators: Thermal


Actuators: Motors

Electrostatic Micromotor Wobble Micromotor(also electrostatic)

Stator

HubRotor

Stator

Hub

Rotor


Actuators: Motors

Sandia’s wedge stepping motor

LIGA – electro-magnetic microactuator


Actuators: MotorsAcknowledging Sandia National Labs (www.sandia.gov)


Actuators: MotorsAcknowledging Rotary Stepper Motor (www.zyvex.com)


Actuators: MotorsLinear Stepper Motor


Actuators: MotorsVibromotor


Actuators: Steam Engine

Structure immersed in working fluid (DI water) Vapor bubble formed at right end Vapor condenses at the piston end Expansion of vapor bubble moves the piston


Pvapour

CylinderPiston Liquid

Vapour

Heater Element


Actuators: Steam Engine

Single piston Multi piston



Actuators: Fluidics

Glass micromachined DNA purification system

DRIE etched fluidic handling system


2 cm

Actuators: Fluidics

Muscle-cell analysis

Plant pathogen detector

Dr. Paul Li, department of chemistry, Simon Fraser university


Actuators: FluidicsPhase Transformation Fluid Pump


Actuators: Photonics

UC Berkeley micromirror

Sandia micromirror Clip-on and virtual retinal displays

Fresnel Zone Plate and Laser Diode (UCLA)


Acknowledging Sandia National Labs (www.sandia.gov)

Actuators: Movies


Packaging


Packaging: Process

Wafer dicing (diamond saw) Release and dry Die attach Wire bonding Multi-chip modules and flip-chip bonding Hermetic sealing (for physical sensor) Potting to protect from shock Orientation of sensors (inertial sensors)


Packaging: Release

Normal drying after deionized (DI) water rinse creates a meniscus between the substrate and microstructure

This process collapses the freestanding microstructure

In general, want to avoid surface coming into contact to avoid adhesion


Packaging: Release

Solutions Make structures stiffer in Z-

direction (high aspect ratio) Add dimples to reduce surface

contact area Treat surfaces to make them

hydrophobic Avoid creating the meniscus by

drying in supercritical CO2 Freezing and sublimating

the solvent

Supercritical CO2 Drying


Packaging: Electronics

TechniqueUse an existing industrial CMOS technology as a baseIntroduce special layout design techniquesPerform micromachining step as a post-process

AdvantagesNo need for a in-house fabCan integrate microstructure and

electronics on the same chip

DisadvantagesLimited assortment of microstructures A CMOS Micromachined

integrated IR emitter pixel

CMOS Compatible Micromachining


Packaging: Why Integrate?

Cost: Batch fabrication

Performance: Reduced parasitics

Manufacturability: Integrated contacts have higher yield than wire bonds and flip-chip bonds

Reliability: Integrated systems are more reliable than hybrids

Size: Offers the ultimate level of miniaturization


MUMPs Examples


MUMPs Examples

Layout Tool L-Edit (Tanner Tools) Cadence AutoCAD

Output File Format CIF GDS II DXF

DesignLayout Generation Masks for fabrication


MUMPs Examples

The Multi-User Micromachining Process (MUMPs) is a polysilicon surface-micromachining process

Provided to public by Cronos Uses three structural layers Uses two sacrificial layers


MUMPs Examples

Process Layers Nitride Polysilicon-0 1st Oxide Polysilicon-1 2nd Oxide Polysilicon-2 Metal

Design Layers POLY0 ANCHOR1 POLY1 ANCHOR2 P1P2VIA POLY2 METAL


MUMPs Examples

Poly-0 Defines the geometry of Polysilicon-0

Anchor-1 Attaches Poly-1 to Poly-0 or attaches Poly-1 to Nitride

Poly-1 Defines the geometry of Poly-1

Anchor-2 Attaches Poly-2 to Poly-0 or attaches Poly-2 to Nitride

Poly1-Poly2-Via Attaches Poly-2 to Poly-1

Poly-2 Defines geometry of Poly-2

Metal Defines geometry of metal.Preferably on top of Poly-2

Design layers’ functions


Second-Oxide

First-Oxide

Anchor-1

Poly-0

Poly-1

Poly1-Poly2-Via

Anchor-2

Design layers’ functions

MUMPs Examples

Silicon Substrate

Silicon Nitride

Poly-2

Metal


MUMPs Examples

Hinge

Poly-1

Anchor-2

Poly-2

Metal

Sample Design: Hinge


MUMPs Examples

When flat structures are released the surface contact will glue parts together and prevent movement

Very small indentations (bumps) created on Poly-1 and Poly-2 so that when structures are released they rest on the bumps

Rotor without dimples

Rotor with dimples

Dimples


MUMPs Examples

Poly-0

Anchor-1

P1-P2 Via

Poly-1

Dimple

Poly-2

Metal

Sample Design: Thermal Actuator


Anchor-1 P1-P2 ViaPoly-1Dimple Poly-2

Sample Design: Gear Train

MUMPs Examples


Sample Design: Gear Train

MUMPs Examples


Sample Design: Why double thickness structures

MUMPs Examples

Pawl can slip underneath gear teeth


Sample Design: Why double thickness structures

MUMPs Examples

Teeth between gear and motor

can slip


Poly-0

Anchor-1

Dimple

Poly-1

P1P2V

Anchor-2

Poly-2

Metal

Sample Design: Tower

MUMPs Examples


ANSYS

MUMPs Examples

At device level, simulation requires building 3D model.

Layout 3D model www.sfu.ca/immr/

Cif-input to 3D output. Ansys VRML

MEMS Pro Intellisuite

Simulation


MUMPs ExamplesMirrors


Bistable Mechanism

MUMPs Examples


Design Issues


Design Issues: Topography

In the MUMPs process, all growth is conformal

Processing steps depend heavily on the preceding steps

Managing topology is an important



Thin films conform closely to the topology of the previously deposited and patterned layers

Topology can trap a structure that was intended to move freely

Unless the preceding layers are designed to ensure the upper structural layers are flat where needed, the induced topology can have detrimental effects on device operation



Topography can create structural weaknesses

Topography provides stress concentration points and beam thinning may occur due to variation in film thickness across steps

Problems are compounded since lithography is more difficult along height changes



Both actuators are composed of a wide arm and narrow arm

Differential heating due to an applied current causes differential thermal expansion

This was supposed to cause the arm to curve upwards

However, the wide arm has conformed and is no longer flat

The wide arm’s bending stiffness is thus significantly higher, reducing any motion


Design Issues: Residual Stresses

Uniform stress is the average stress through the thickness of the film For singly supported structures, one should expect a dimensional

change as the structure relaxes to a non-stressed state Doubly supported structures can be more reliable, in that their length

will remain fixed Over a critical compressive stress they will buckle


Design Issues: Residual Stresses

A non-uniform stress is a residual stress with a gradient

Non-uniform stresses are both more difficult to handle theoretically and more difficult to measure.


Design Issues: Ground Planes

Ground planes are necessary to electrically shield devices from the wafer

Conducting bodies at different potentials will experience an attractive force; this includes surface micro-machined devices and the substrate

If the attractive force is strong enough, the device will be pulled down to the substrate surface

Even if the device does not adhere, significant friction will be present


Design Issues: Double-thickness

Contact surfaces should be avoided in surface micro-machined devices .

Where contact surface are needed, double-thickness parts will often be needed Because they are so thin,

surface micromachined devices will have significant vertical flexibility

There may be bowing.


Design Issues: Tethers

Free moving structures will flap around during release

This can damage the device itself as well as nearby devices

The device should be tethered to the wafer surface

These tethers are broken after release


Design Issues: Dimples

Dimples are small bumps on the underside of the first structural layer

A short wet etch is used to isotropically etch small cavities the first sacrificial layer

The first structural layer will then have bumps, as it will conformally fill the holes

Dimple Dimple Dimple


Design Issues: Raised Structures

Hinges allow parts to rotate

Properly design parts can rotate off the wafer surface


Please fill out the course evaluation

Thank-you


Selected References S. Wolf and R. N. Tauber, Silicon Processing for the VLSI Era, Volume 1 – Process Technology, California, Lattice Press 1986. S. Ghandi, VLSI Fabrication Principles, Silicon and Gallium Arsenide, Second Edition, John Wiley & Sons, Inc., New York, 1994. S. M. Sze, VLSI Technology, McGraw-Hill, 1983. Madou M., Fundamentals of Microfabrication, CRC Press, New York, 1997. M. Parameswaran, M. Paranjape, Layout Design Rules for Microstructure Fabrication Using Commercialy Available CMOS Technology, Sensors and Materials, 5, 2, 1993, pp. 113-123. How Semiconductors are Made, Harris Semiconductor Ernest Garcia, Jeff Sniegowski, Surface Micromachined Microengine, Sensors and Actuators A 48 (1995), pp2O3-214. Kurt E. Peterson , Silicon as a Mechanical Material, Proc. of IEEE, vol 70 no 5, May 1982. Joseph Shigley , Mechanical Engineering Design, 1989, ISBN 0-07-056899-5 S.M. Sze , Semiconductor Sensors, 1994, John Wiley & Sons, ISBN 0-471-54609-7 J. J. Sniegowski and E. J. Garcia, Surface Micromachined Gear Trains Driven by an On-Chip Electrostatic Microengine, IEEE Electron Device Letters, Vol. 17, No. 7, 366, July 1996. J. J. Sniegowski, S. M. Miller, G. F. LaVigne, M.S. Rodgers and P.J. McWhorter, Monolithic Geared- Mechanisms Driven by a Polysilicon Surface-micromachined On-Chip Electrostatic Microengine, Solid-State Sensor and Actuator Workshop, Hilton Head Is., South Carolina, June 2-6, 19969 pp. 178- 182. J. J. Sniegowski, Moving the World with Surface Micromachining, , Solid State Technology, Feb. 1996, pp. 83-90.


WWW MEMS References

http://www.mems.sandia.gov http://www.onixmicrosystems.com/ http://www.siliconsense.com/ http://www.intellisense.com/index.html http://www.siliconlight.com/ http://www.css.sfu.ca/sites/immr/ http://www.memsrus.com/ http://www.zyvex.com/Research/MEMS/

MEMS.html http://mems.engr.wisc.edu/liga.html http://www.biomems.net/ http://bsac.eecs.berkeley.edu/ http://www.cmc.ca/beams.html http://www.ece.cmu.edu/~mems/ http://www.mech.kuleuven.ac.be/ http://mishkin.jpl.nasa.gov/CSMT_PAGE http://muresh.et.tudelft.nl/dimes/

index.html http://www-bsac.eecs.berkeley.edu/

~ptjones/database.html

http://dolphin.eng.uc.edu/index.html http://mems.eeap.cwru.edu http://www.ida.org/MEMS/ http://www-mtl.mit.edu/home.html http://synergy.icsl.ucla.edu/index.html http://www.rgraceassoc.com http://cdr.stanford.edu/ http://www.shef.ac.uk/uni/projects/mesu/ http://www.mems.ecs.soton.ac.uk/title.htm http://www.trimmer.net/ http://www-mtl.mit.edu/semisubway.html http://www2.ncsu.edu/eos/project/

erl_html/erl_damemi.html http://www.laas.fr/mc2_Europractice/ http://www.tanner.com/ http://www.omminc.com/ http://www.microsensors.com/

an introduction to polysilicon micromaching robert w. johnstone rjohnsto

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