materials for MEMS - TAUhanein/teaching/materials for MEMS.pdf · 11 March 2004 Materials...

11 March 2004 Materials Applications Yael Hanein

Materials for MEMS

Dr. Yael Hanein


Materials for MEMS

• MEMS (introduction)• Materials used in MEMS• Material properties• Standard MEMS processes


MEMS

• The world’s smallest guitar is about 10 micrometers long—about the size of a single cell. It was built to illustrate how micro-electro-mechanical technology can be used to create structures. Cornell.


MEMS

Tiny MEMS pressure sensors


What are MEMS ?• MEMS (micro electro mechanical systems)

or MST (micro system technology)• The fabrication of devices with at least

some of their dimensions are in the micrometer range (Madou)

• A portfolio of techniques to design and create miniature systems. (Maluf)


1 micron scale

Visible

1 µmAtoms

Microwaves

-6 -1-10

Infrared

Cells

WaveLength (m)


MEMS applications

Mechanical Applications• Pressure sensors• Accelerometers (air bag

sensors)


Optical Application

• Optical switches• Digital Light Processing


Inkjet nozzles

Step4 - RefillChambers are filled again by the ink through microscopic channels

Step3 - Drop EjectionThe vaporized part of the ink is propelled towards the paper in a tiny droplet

Step2 - Bubble GrowthBy heating the liquid ink a bubble is generated

Step1 - NucleationInk-filled chambers are heated by tiny resistive heating elements


MEMS applicationsCommercial • Biomedical sensors• Drug delivery systems• Neurological

disorders• Telecom optical fiber

switching• Monitoring structural

health• Automotive safety

Military • Biochemical warfare

detection• Inertial systems for

guidance and navigation


U.S. MEMS markets

87213632476

4979133

438575995

129216444

255491879

1994199820022004

TotalMilitaryInformatiotech.&Inds**

MedicalAutomotive*Year

In $1,000,000* 80% air bags** 1998: inkjets 75%, displays 5.4Updates info: http://www.smalltimes.com


Why MEMS?A MEMS solution is attractive if:• A new function• Significant cost reduction• Both** Size reduction is seldom sufficient as the sole

reason MEMS is justified when:• Added value• Increased productivity• Cost competitiveness (batch process)• Revenue and profit


MEMS: Evolution from Si IC technology

• VLSI (FET as an example)


Evolution from Si IC technology

/ may01/mems.htmloemagazine.com/fromTheMagazine

http://puma.wellesley.edu/~cs110/lectures/history/history_computing.html

http://www.materials.ox.ac.uk/ocamac/ocamac/newsletters/Newsletter%209/April96_5of6.html

asci/mems.html/ www.evl.uic.edu/scharver


Photolithography


IC - Jack Kilby…After proving that integrated circuits were possible (1958), I headed teams that built the first military systems and the first computer incorporating integrated circuits. I also worked on teams that invented the handheld calculator and the thermal printer, which was used in portable data terminals.


Moore's Law

• Gordon Moore (co-founder of Intel) predicted in 1965 that the transistor density of semiconductor chips would double roughly every 18 months.


Main IC Benefits• High quality materials (SC silicon, GaAs)• CAD• Possessing infrastructure• Batch processes


MEMS History• Original technology was very similar; same materials

(silicon), similar processing (photolithography) but different applications: Pressure sensors, accelerometers

• Silicon for MEMS• SC (integration with VLSI)• Extensive experience• Piezo-resistive• High quality material available• Infrastructure available• mechanically strong


MEMS vs IC• Some Issues are relevant to both IC and MEMS

design: Stress, selective etching, Pattern transfer, cleanliness, structure release.

• Some are unique: wet environment, 3D, moving parts.

• Integration requires considering limitations of both technologies.

• MEMS evolved and now use various materials, processes for a wide range of applications:


MEMS Today

Nikolas Chronis and Luke P. Lee, MEMS2004

Gripper end – 8 µm


Standard MEMS sensors/actuators

• Piezo-resistors

• Comb drives

• Thermal expansion

• Surface chemistry


MEMS today• Materials: SC (Si,GaAs,SiGe), Glass,

Silicone.• Application: Bio, RF, Chemistry, Optical,

Mechanical.• Physics/engineering: Elasticity, piezo-

resistivity, surface properties, capacitive actuators

• Processes: Bulk machining, surface machining, DRIE, LIGA, Polymers


Functionality

• Transducer – Converts energy from one form to another

• Sensors (Transducers) – A device that detects or measures

• Actuator –


SiO2/Quartz/glass

• The stable oxide is one of the key elements for the success of silicon in IC

• Excellent thermal and electrical insulation• Sacrificial layers in surface

micromachining processes (selectively etched in HF)


Relevant Material Properties

• Electrical (SC, metals, insulators)• Mechanical (elasticity) • Thermal (Heat conductivity)• Chemical, electro-chemical • Biological (bio-compatibility)• Optical (roughness)• Processing


MEMS and materials

Processing

Electrical (SC, metals, insulators)

Mechanical (elasticity)

Thermal (Heat conductivity)

Chemical, electro-chemical

Biological (bio-compatibility)Optical (roughness)

Cost


Silicon*• One of very few materials that can be

economically manufactured in single crystal substrates

• Diamond lattice• * Not to be confused with silicone


Silicon


Czochralski Crystal Growth Process


Float zone pulling


Silicon boules


Silicon1. Crystal Growth

2. Single Crystal Ingot

3. Crystal Trimming and Diameter Grind

4. Flat Grinding

5. Wafer Slicing

6. Edge Rounding

7. Lapping

8. Wafer Etching

9. Polishing

10. Wafer Inspection

Slurry

Polishing table

Polishing head

Polysilicon Seed crystal

Heater

Crucible


Silicon SummaryProperties:• Extensive studies and documentation• Suitable for electronic, mechanical, thermal, and optical integration• Can sustain harsh (mechanical) handling conditions• Crystalline: mechanical properties are uniform across wafer lotsStructure: • Crystalline, • Polycrystalline-polysilicon• amorphousConductivity: • Semiconductor


SiliconMechanical: • Hard and brittle material, deforms elastically, robust• Tensile yield strength – 7 GPa• Maintain mechanical integrity up to 500 °C. >500 °C

plastic deformation.• Properties independent of doping (stress when impurities

reach 1020 cm-3)• Polycrystalline and amorphous: properties vary with

deposition conditions, but similar to crystalline silicon.• Polycrystalline and amorphous: high levels of intrinsic

stress, requires annealing (>900 °C).• Polycrystalline and amorphous: unstable, >250 °C.


SiliconFabrication: • Crystalline: Wafers • Polysilicon: thin film deposition• Amorpous : thin film depositionOptics:• Not an active optical material (indirect band gap)• Transparent at IR• <0.4 µm reflects 60% of incedent light

Chemistry/biology• Stable and resistant (brake fluid, biological medium)• Suitable for high purity gases • Benign in the body, does not release toxic substances.Cost:• Low – ultra pure electronic grade silicon wafers are available for IC


SiO2

SiO2 – Silica• Fused silica is a purer version of Fused quartz that is made

synthetically from various Silicon gasses. 17 crystalline phasesQuartz – single crystal material, low impurity concentration• Fused quartz is the amorphous form of quartz. • Fused quartz is made from natural crystalline quartz, usually quartz

sand that has been mined. Glass - amorphous solid, impurities, low melting temperature • Borosilicate glass is an "Engineered" glass developed specifically

for use in environments such as laboratories and heating applications where Thermal, mechanical and chemical conditions are too much for standard, household type glass. Some common names are Pyrex™ by Corning, and Duran™ by Schott Glass. Like most glasses, the dominant component of Borosilicate glass is SiO2 with boron and various other elements added to give it its excellent qualities.


SiO2

Properties• Color table (Sze)Fabrication• Thermally grown by oxidizing silicon at temperature >

800 °C.• Spin on glass• BondingMechanical• High stress (difficult to control or anneal) – limited use as

beams or membranes• Uses• Cost


MetalsAluminum• Basic electrical interconnections (common and easy to

deposit)• Non-corrosive environment only• T < 300 °C (melting temperature = ? )• Good light reflector (visible light)Gold/ titanium/tungsten• Better for higher temperature• Harsher environments• Gold is good light reflector in the IRPlatinum and palladium• Stable for electrochemistry


Common Metals in MEMS

High T elect interconnects5.5W

ElectrochemistryBio-potential sensors

5.1, 10.6Ir, PtTransparent interconnects300-3000ITO

Elect interconnects1.7CuIntermediate adhesion layer12.9, 42, 75-200Cr, Ti, TiW

High T elect interconnectOptical refl IR

electrochemistry

2.4Au

Elect interconnectsOptical reflection

2.7AlElectrochemistry1.58Ag

Applicationsρ (10-6 Ω×cm)Metal

Maluf table 2.3, 24


Polymers Properties• Spin coated with varying thickness; few nm – hundreds of microns• Used in sensing of chemical gases and humidity• Used as Photoresists, • SU8: Epoxy based photoresist can form layers up to 100 µm• Polyimide

Fabrication• Spin-on,molding

Cost• Low


Thermal conductivity

0.5-250.028Air300.6Water

300Au

236Al

178W

1.4SiO2

1.9Si3N4

156Si (SCS)

Convection W/m2K

At 300K (W/m K)Material


Thermal Expansion

25Al4.5W

0.4-0.55SiO2

2.8Si3N4

2.616Si (SCS)At 300K (10-6/°C)Material


Processing

• Bulk machining vs surface machining• Etching, selective etching, sacrificial

layers, isotropic vs anisotropic• Release processes• Deposition processes


Si Etching

highhazard

NoNoYesYesYesNoP++ etch stop

1010<0.10.21010-30SiO2 etch (nm/min)

200200<0.10.1<1LowNitride etch

NoneNone50:135:1100:1None111/100selectivity

1-30.1-0.50.5-1.50.750.5-21-20Rate(µm/min)

YesvariesYesYesYesNoAnisotropic

PlasmaPlasmaWetWetWetWetType

SF6/C4F8(DRIE)

SF6(CH3)4NOH(TMAH)

Ethylene diaminepyrochatechol(EDP)

KOHHF:HNO3:CH3COOH


MEMS ProcessingProcesses (geometry/design) are

material specific


Standard MEMS sensors/actuators

• Piezo-resistors• Comb drives• Thermal expansion


Standard MEMS Processes

• Piezo-resistive Pressure sensor


Standard MEMS Processes

• Comb driver


Bio-potential electrodes


MEMS applications

Mechanical Applications: Actuators


Micro-fabricated cilia

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