11 March 2004 Materials Applications Yael Hanein
Materials for MEMS
Dr. Yael Hanein
11 March 2004 Materials Applications Yael Hanein
Materials for MEMS
• MEMS (introduction)• Materials used in MEMS• Material properties• Standard MEMS processes
11 March 2004 Materials Applications Yael Hanein
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.
11 March 2004 Materials Applications Yael Hanein
MEMS
Tiny MEMS pressure sensors
11 March 2004 Materials Applications Yael Hanein
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)
11 March 2004 Materials Applications Yael Hanein
1 micron scale
Visible
1 µmAtoms
Microwaves
-6 -1-10
Infrared
Cells
WaveLength (m)
11 March 2004 Materials Applications Yael Hanein
MEMS applications
Mechanical Applications• Pressure sensors• Accelerometers (air bag
sensors)
11 March 2004 Materials Applications Yael Hanein
Optical Application
• Optical switches• Digital Light Processing
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
MEMS: Evolution from Si IC technology
• VLSI (FET as an example)
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
Photolithography
11 March 2004 Materials Applications Yael Hanein
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.
11 March 2004 Materials Applications Yael Hanein
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.
11 March 2004 Materials Applications Yael Hanein
Main IC Benefits• High quality materials (SC silicon, GaAs)• CAD• Possessing infrastructure• Batch processes
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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:
11 March 2004 Materials Applications Yael Hanein
MEMS Today
Nikolas Chronis and Luke P. Lee, MEMS2004
Gripper end – 8 µm
11 March 2004 Materials Applications Yael Hanein
Standard MEMS sensors/actuators
• Piezo-resistors
• Comb drives
• Thermal expansion
• Surface chemistry
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
Functionality
• Transducer – Converts energy from one form to another
• Sensors (Transducers) – A device that detects or measures
• Actuator –
11 March 2004 Materials Applications Yael Hanein
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)
11 March 2004 Materials Applications Yael Hanein
Relevant Material Properties
• Electrical (SC, metals, insulators)• Mechanical (elasticity) • Thermal (Heat conductivity)• Chemical, electro-chemical • Biological (bio-compatibility)• Optical (roughness)• Processing
11 March 2004 Materials Applications Yael Hanein
MEMS and materials
Processing
Electrical (SC, metals, insulators)
Mechanical (elasticity)
Thermal (Heat conductivity)
Chemical, electro-chemical
Biological (bio-compatibility)Optical (roughness)
Cost
11 March 2004 Materials Applications Yael Hanein
Silicon*• One of very few materials that can be
economically manufactured in single crystal substrates
• Diamond lattice• * Not to be confused with silicone
11 March 2004 Materials Applications Yael Hanein
Silicon
11 March 2004 Materials Applications Yael Hanein
Czochralski Crystal Growth Process
11 March 2004 Materials Applications Yael Hanein
Float zone pulling
11 March 2004 Materials Applications Yael Hanein
Silicon boules
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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.
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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.
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
Thermal Expansion
25Al4.5W
0.4-0.55SiO2
2.8Si3N4
2.616Si (SCS)At 300K (10-6/°C)Material
11 March 2004 Materials Applications Yael Hanein
Processing
• Bulk machining vs surface machining• Etching, selective etching, sacrificial
layers, isotropic vs anisotropic• Release processes• Deposition processes
11 March 2004 Materials Applications Yael Hanein
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
11 March 2004 Materials Applications Yael Hanein
MEMS ProcessingProcesses (geometry/design) are
material specific
11 March 2004 Materials Applications Yael Hanein
Standard MEMS sensors/actuators
• Piezo-resistors• Comb drives• Thermal expansion
11 March 2004 Materials Applications Yael Hanein
Standard MEMS Processes
• Piezo-resistive Pressure sensor
11 March 2004 Materials Applications Yael Hanein
Standard MEMS Processes
• Comb driver
11 March 2004 Materials Applications Yael Hanein
Bio-potential electrodes
11 March 2004 Materials Applications Yael Hanein
MEMS applications
Mechanical Applications: Actuators
11 March 2004 Materials Applications Yael Hanein
Micro-fabricated cilia