Are mechanical laws different at small scales? YES!

Are mechanical laws different at small scales? YES! • If we scale quantities by a factor ‘S’ • Areaa S2 Volume a S3 • Surface tensiona S Electrostatic forcesa S2 • Magnetic forcesa S3 Gravitational forces a S4 • 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, r: density. l: linear dimension • Viscosity dominates at: Re < 1 • Re for whale swimming at 10 m/second ~ 300,000,000 • Re for a mosquito larva , moving at 1mm/sec ~ 0.3 • Re marks the transition between • Laminar/Smooth flow & Turbulent Flow (mixing) In MEMS: always laminar flow!

Thermal Issues Easier to remove heat from a smaller sample • 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

Electrophoresis • Stirring vs. Diffusion, Diffusion is the dominant mixing process in MEMS • Separation of bio-molecules, cells by the application of electric fields E = 0 E > 0 Separation of different types of blood cells

Miniature Clinical Diagnostic Systems Fast, on-site, real time testing Principle: High Isolation, Low Mass, Localized heating possible Scaling of Minimal Analytic Sample Size • Polymerase Chain Reaction (PCR) • for DNA amplification Micro-fabricated DNA capture chip (Cepheid, CA)

Scaling in Electricity and Magnetism • Potentiometric devices (measure voltage) are scale invariant • Amperometric devices (measure current) are more sensitive when miniaturized • e.g., m-array electrochemical detectors (Kel-F) for trace amounts of ions • Electroplating is faster in MEMS Courtesy: M. Schoning

Scaling in electromagnetic systems Constant electrostatic stresses/field strengths Voltage  Electrostatic field · length  L Resistance  Length  L-1 Ohmic current  Voltage  L2 Current density (I/A) is scale invariant Area Resistance

Scaling in Electricity and Magnetism Human Hair ! Sandia MEMS Rotor Stator Electric: e: dielectric permittivity (8.85 . 10-12 F/m) E: electric field (Breakdown for air: 30 kV/cm) Magnetic: m: permeability (4p . 10-7 N/A2) B: Magnetic field

Electrostatics is more commonly used in MEMS Macroscopic machines: Magnetic based Microscopic machines: Electrostatics based Judy, Smart Mater. Struc, 10, 1115, (2001)

Electrostatics vs. magnetostatics Electrostatic force Area · (Electrostatic field)2  L2 Electrostatic energy  Volume · (Electrostatic field)2  L3 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 1mm3 volume ~ 103 J/mm2 c.f. a thin-film battery  ~ 1J/mm2

Power in MEMS Compact power sources needed, but Power scales by mass Currently: Fuel cells, micro-combustors, Radio frequency/optical sources Energy stored in 1 mm3

MEMS devices: How do we make them? A mechanism Gear chain Hinge Gear within a gear Sandia MEMS

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 heritage • Si is a good semiconductor, properties can be tuned • Si oxide is very robust • Si nitride is a good electrical insulator

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 of Silicon 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 Light Light MASK MASK Deposit Metal Photoresist Silicon substrate Silicon substrate Positive photoresist Negative photoresist

Deposit and remove materials precisely to • create desired patterns The photo-lithography process Positive Remove deposit and etch J. Judy, Smart Materials & structures, 10, 1115, 2001 Negative

Surface micromachining How a cantilever is made: http://www.darpa.mil/mto/mems

One can make devices as complex as one wishes using deposition and micromachining processes http://mems.sandia.gov/

Any MEMS device is made from the processes of 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

Are mechanical laws different at small scales? YES!