Are mechanical laws different at small scales? YES!

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

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

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

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In MEMS: always laminar flow!

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

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

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

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

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

Scaling in Electricity and Magnetism

Electric:: dielectric permittivity (8.85 . 10-12 F/m)

E: electric field(Breakdown for air: 30 kV/cm)

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Electrostatics is more commonly used in MEMS

Macroscopic machines: Magnetic basedMicroscopic machines: Electrostatics based

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

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

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

MEMS devices: How do we make them?

Sandia MEMS

Gear chain Hinge Gear within a gear

A mechanism

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

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

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)

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

Photolithography

Photoresist

Silicon substrate

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Negative photoresist

-Deposit and remove materials precisely to create desired patterns

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Surface micromachining

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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

The History of MEMS

Y.C.Tai, Caltech