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Are mechanical laws different at small scales? YES!
If we scale quantities by a factor ‘S’
Area S2 Volume S3
Surface tension S Electrostatic forces S2
agnetic forces S3 Gravitational forcesS4
• Surface Area/Volume effects• Stiction: “Sticky friction”, due to molecular forces
- surface tension pulls things together
SCALING OF: Mechanical systems Fluidic systems Thermal systems Electrical and Magnetic systems Chemical and Biological systems
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Which dynamical variables are scaled? - depends on our choice
e.g. Mechanical systems Constant stress Scale independent elastic deformation, scale independent shape
Electromagnetic systems Constant electrostatic stresses/field strengths
Thermal systems Constant heat capacity & thermal conductivity
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Scaling Issues in Fluids Viscosity & Surface Tension
• Definition: A fluid cannot resist shear stresses
Re is the ratio of inertial and viscous forces,v: velocity, : density. l: linear dimension
Viscosity dominates at: Re < 1
Re for whale swimming at 10 m/second ~ 300,000,000Re for a mosquito larva , moving at 1mm/sec ~ 0.3 Re marks the transition betweenLaminar/Smooth flow & Turbulent Flow (mixing)
η
l ρ v (Re)number sReynold'
In MEMS: always laminar flow!
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Thermal Issues
• Thermal Mass (specific heat X Volume) scales as l3, but heat removal scales as l2 (proportional to area)
• Evaporation or Heat loss increases as Surface Area/Volume increases
Easier to remove heat from a smaller sample
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Electrophoresis
- Stirring vs. Diffusion, Diffusion is the dominant mixing process in MEMS
- Separation of bio-molecules, cells by the application of electric fields
Separation of different types of blood cells
E = 0 E > 0
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Micro-fabricated DNA capture chip(Cepheid, CA)
Fast, on-site, real time testing
Miniature Clinical Diagnostic Systems
• Polymerase Chain Reaction (PCR) for DNA amplification
Principle: High Isolation, Low Mass, Localized heating possible
Scaling of Minimal Analytic Sample Size
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Scaling in Electricity and Magnetism
• Potentiometric devices (measure voltage) are scale invariant
• Amperometric devices (measure current) are more sensitive when miniaturized
e.g., -array electrochemical detectors (Kel-F) for trace amounts of ions
Electroplating is faster in MEMS
Courtesy: M. Schoning
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Scaling in electromagnetic systems
Voltage Electrostatic field · length L
Resistance Length L-1
Ohmic current Voltage L2
Current density (I/A) is scale invariant
Constant electrostatic stresses/field strengths
Area
Resistance
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Scaling in Electricity and Magnetism
Electric:: dielectric permittivity (8.85 . 10-12 F/m)
E: electric field(Breakdown for air: 30 kV/cm)
Magnetic: : permeability (4) B: Magnetic field
2electric E ε
2
1U
μ
B
2
1U
2
magnetic
Rotor
Stator
Human Hair !
Sandia M
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S
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Judy, Smart Mater. Struc, 10, 1115, (2001)
Electrostatics is more commonly used in MEMS
Macroscopic machines: Magnetic basedMicroscopic machines: Electrostatics based
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Electrostatic force Area · (Electrostatic field)2 L2
Electrostatic energy Volume · (Electrostatic field)2 L3
Electrostatics vs. magnetostatics
Magnetic field Current L
distance
Magnetic force Area · (magnetic field)2 L4
Magnetic forces are much weaker compared to electrostatic forces
Magnetic energy Volume · (Magnetic field)2 L5
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Power and Power density scaling
Power Force · speed L2
Power density Power L-1
Volume
Small devices made through strong materials can have very large power densities
e.g. 10 nN force in a 1m3 volume ~ 103 J/mm2
c.f. a thin-film battery ~ 1J/mm2
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Power in MEMS
Compact power sources needed, but Power scales by mass
Energy stored in 1 mm3
Currently: Fuel cells, micro-combustors, Radio frequency/optical sources
Power capacitor 4 J/mm3 1 W for 4 s
Thick Film Battery 1 J/mm3 270 W for 1 hour
Thin Film Battery 2.5 J/mm3 0.7 mW for 1 hour
Solar Cell (1 X 1 X 0.1 mm3) 0.1 mW
Gasoline 300 J/mm3 3 mW for 1 day
178 Hf > 10 MJ/mm3
160 mW
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MEMS devices: How do we make them?
Sandia MEMS
Gear chain Hinge Gear within a gear
A mechanism
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Making MEMS
• How to make a MEMS device - deposit and etch out materials
• Introduction to Micro-machining - Wet and Dry etching - Bulk and surface micro-machining
• What kinds of materials are used in MEMS?-Semiconductors- Metals- Polymers
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Basic MEMS materials Silicon and its derivatives, mostly
• Micro-electronics heritageSi is a good semiconductor, properties can be tunedSi oxide is very robustSi nitride is a good electrical insulator
Substrate Cost Metallization Machinability
Silicon High Good Very good
Plastic Low Poor Fair
Ceramic Medium Fair Poor
Glass Low Good Poor
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Materials in MEMS
Dominant: SEMICONDUCTORS (Silicon centric)
MEMS technology borrows heavily from the Si micro-electronics industry
The fabrication of MEMS devices relies on the processing ofSilicon and silicon compounds (silicon oxide, nitride …)
METALS: used in electrical contacts (Al,Cu), magnetic elements (Ni, NiFe)
POLYMERS: used as sacrificial layers, for patterning (photoresist/polyimide)
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Making MEMS
• Planar technology,constructing components (MEMS & electronics) on initially flat wafers
> Wafer level processes> Pattern transfer
• Introduction to Micro-machining - Wet and Dry etching - Bulk and surface micro-machining
• What kinds of materials are used in MEMS?-Semiconductors- Metals- Polymers
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Photolithography
Photoresist
Silicon substrate
MASK
Light
DepositMetal
Silicon substrate
MASK
Light
Positive photoresist
Negative photoresist
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-Deposit and remove materials precisely to create desired patterns
The photo-lithography processJ.
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Negative
Remove deposit and etch
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Surface micromachining
How a cantilever is made:ht
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One can make devices as complex as one wishes using deposition and micromachining processes
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Any MEMS device is made from the processesof deposition and removal of material
e.g. a state-of-the art MEMS electric motor
www.cronos.com
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The History of MEMS
Y.C.Tai, Caltech