EECS 179 9/29 Lec

Macro to nano Macro - measured in meters

o Bridges, airplanes, cars Meso - measured in millimeters

o snails, rice, ants, sand Micro - measured in micrometers <--- main focus

o hair, dust, pollen, cells, bacteria Nano - measured in nanometers

o viruses, nuclei, transistors, cell apparatus, proteins

Scaling laws of small Laws of physics same, manifestation different Most important scaling law: volume Memorize first 3-4 okay

o Volume: V ~ L^3o Mass: M ~ L^3o Surface: SA ~ L^2o Strength: F ~ L^2o Acceleration: A ~ 1/Lo Frequency: f ~ 1/Lo Power: P ~ L^2o Power density: P ~ L^2o Voltage: V ~ constanto E Field: E ~ 1/Lo Resistance: R ~ 1/Lo Capacitance: C ~ Lo Current: I ~ Lo Magnetic wire: B ~ constanto Heat capacity: Cv ~ L^3o Heat flow: dT/dt ~ 1/L^2o Turbulence: Re ~ L

Typical small values Size=100um

o Typical volume: 1 nanolitero Typical mass: 1 microgramo Typical force: 10-100nN (1-10ug)o Typical E field (at 1V): 10,000 V/mo Typical frequency: 10-100kHzo Typical time constant: 10-100usec

Size=10umo Typical volume: 1 picolitero Typical mass: 1 nanogramo Typical force: 0.1-1nN (10-100ng)o Typical E field (at 1V): 100,000 V/mo Typical frequency: 0.1-1MHzo Typical time constant: 1-10usec

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Small scale phenomena Surface tension

o In microscale, forces hard to generateo Gravity almost DNE in microscaleo Surface tension force for 100um opening: 5.7uNo Typical force for 100um device is 10nNo Surface tension is #1 force in microscale, about 500x

stronger than any other forceo Pressure = constant/drop diameter

Laminar flowo Liquids at microscale behave viscouso Inertiao Viscosityo Reynolds number (Re)scales as lengtho Typical Reynolds Number for 100um device is Re ~ 0.1o Onset of turbulence is at Re ~ 2000o Characteristic flows in microchannels are smooth and viscous

with no mixing Rapid heat transfer

o Generate heat quickly, also get heat dissipation quicklyo surface area compared to volume is very large (at small

scales), so lots of surface area to lose heato heat capacity depends on masso heat transfer is very fast at small scaleso temperature of small filament in 75W light bulb is ~2500C,

ramp up time is ~20ms, ramp down time is ~60mso heat flows through small devices quicklyo hard to have temperature gradient

Chemical transfero Surface area to volume is large at small scaleso Mass flow saturates quickly in small volumeso Equilibrium can be reached very quickly

Diffusion happens quickly b/c of high surface areao Micro-scale systems must utilize physical barriers (cell

walls) to maintain concentration gradientso surface contamination is a serious issue at small scaleso Mass flows through small devices quicklyo hard to maintain concentration gradient

Loss of continuityo at sizes below ~50um, granularity of nature becomes relevanto many bulk scale physical laws no longer accurateo typical grain size is ~10um; affects physical, thermal,o electrical propertieso mean free path of N2 at atmosphere is 60nm. affects dynamics

in air ex Paschen effect

Atomic granularityo at sizes below ~100um, bulk properties are meaninglesso atomic level understanding required

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Noiseo at sizes below ~50um, thermal induced fluctuations are

noticeableo heat is not a continuous "fluid"o heat is not "fluid"; vibrations (due to heat) and Brownian

motion add statistical fluctuation and noiseo thermal fluctuations depend on size and temperatureo everything bouncing around -> noise

Residual forceso examples: electrostatic charging, hydrogen bonding, dipole

interactions, Van der Waals forceso Small scales allow very close interaction (eg Van der Waals)

and large surface interfaceo Dry state is favorable for electrostatic chargingo Wet/humid state mediates hydrogen bonding

fast time scaleso typical time constant is 10-100usec

Engineering small how small is small enough?

o from engineering perspective, need to solve real problemo after a point, integration more important than

miniaturizationo academic research: hubris and professional standing push for

smallero technology for technology sake?

different perspectives, different communitieso sub-miniature (~1mm)

conventional precision engineering, primarily metals, glass

o MEMS (~10-100um) sensors and useful devices, primarily in silicon

o NEMS (~100nm-1um) electronic devices, concept devices, primarily in

silicono Nanotechnology (<100nm)

mostly materials

engineered systems how is a system engineered today?

o materials: basic building stuff, often with special properties; processed in batch

o components: simple parts designed to be put togethero devices: assembly of components that can perform a simple

functiono sub-systems (modules): assembly of devices designed to

perform a complex functiono systems: assembly of sub-systems designed to perform desired

application examples: automobile, human body

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Engineered systems with nanotechnology the promise of nanotechnology: integration

o nano is an enabling technology; integration is the key

conventional manufacturing assembly

o materials are processed, formed into componentso components are assembled together to build more complex

moduleso materials are standardizedo interfaces are standardizedo manufacturing methods are standardizedo design method is matureo test methods are matureo primary method of manufacturing for engineered systems

batch fabricationo materials are processed in batcho new materials are layered and patterned over other materialso final devices are packaged and assembled on host systemo primary method of manufacturing for semiconductor systems

how to manufacture microsystems? assembly (top down)

o High precision actuators move atoms from place to placeo Micro tips emboss or imprint materialso Electron (or ion) beams are directly moved over a surface

Nano-scale assembly has been demonstrated using atomic force microscopes

Slow and not suitable for large scale production growth (bottom up)

o Chemical reactors create conditions for special growtho Biological agents sometimes used to help processo Materials are harvested for integration

Nano-scale structures are readily formed using bottom up approaches

Mostly materials; hard to directly integrate or build devices

growth and patterning (top down and bottom up)o Chemical reactors used to grow nanomaterialso Lithographic techniques used to selectively remove some

materialso Process is repeated multiple times o Nano-scale structures and micro-scale structures are readily

formed using top down and bottom up approaches. Best chance for integration.

Biological manufacturing of microsystems batch growth

o small creatures often farm their materials

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o many biostructures are made by successively layering of materials and letting them harden

mechanical assemblyo insects+other small creatures regularly assemble

microsystems molecular assembly

o complex molecules (nature's engines) are manufactured using molecular assembly

o RNA acts as coding template to attach specific amino acids to form a peptide chain

EECS 179 10/1 Lec

Semiconductor processing*on wafers

Clean rooms for micro and nano fabrication*need clean room processing facilities*originally developed by NASA and aerospace industry for satellite manufacturing*now used for all MEMS and semiconductor manufacturing*particle free walls, furniture, and accessories must be used*airflow through 0.3 microns (or better) filters*positive pressure inside clean room ensures removal of particles*classifications+applications

*main function of clean rooms is control of particlecontamination*requires control of air flow, water and chemical filtrations,human protocol, operational procedures*particle = something greater than 0.3um*Class 10,000: fewer than 10,000 particles (>0.5um) in cubic footof air PC boards, electronic packaging, medical devices*Class 1,000 - MEMS, electronic packaging, hard disk drives*Class 100 - MEMS, RF/Photonic ICs*Class 10 - integrated circuits

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basic idea behind lithographic processing*coat, protect, expose, etch, repeat*start with substrate*put SiO2 on top*then put polymer (essentially paint) to protect; AKA photoresist*shine light through mask (glass w/ dark areas), change chemical properties of polymer (makes soluble)*left w/ photoresist that matches mask*etch SiO2 w/ acid, only at places w/ no photoresist*strip off photoresist*end with wafer with SiO2 that has pattern *advantages: high precision, all patterns done at once (batch fabrication), relatively easy

basic idea behind lithographic processing*result: multiple patterned layers of different materials*wafers

*start w/ wafer (a clean, flat surface)*boules

*film growth/deposition*put in oven*grow thin film of desired material

*photoresist spinning*spin coat a protective polymer resist layer*polyer goes on wet, then is dried after spinning*gives perfectly flat and smooth surface

*masking+exposure*expose to UV light through a mask*mask is aligned to wafer before exposure

*developing the pattern*resist is removed from exposed areas*remaining resist faithfully reproduces mask pattern

*etch the material*resist protects selected regions during etch*pattern is transferred to substrate material

*repeat process*strip resist, and do process again *eventually, 3D structure built up

*final release*etch away one of the sacrificial materials to release the part

microdevice integration: packaging*for microdevice to be useful, it must be packaged*packaged device can be inserted into system

commercial micro devices: sensors and actuators*pressure sensors, gas sensors, accelerometers, gyros, specialty structures, mirror arrays, micro array plates, microfluids*pressure sensors, ink jet nozzles, accelerometers are biggest markets

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future direction (industry)*Develop robust, standardized manufacturing Every MEMS device is a one-of-a-kind manufacturing operation*Solve the packaging/interface problem 80% of the cost is in packaging; MEMS are so delicate*Understand reliability, QC Need to understand reliability of MEMS better*Evolve robust design tools Need comprehensive software, design libraries*Killer applications Need economic incentive to justify investment in MEMS development

future direction (research)*Novel, integrated manufacturing Creative new ways to build things, more materials, more 3D*Push smaller, discover new materials Exciting opportunities to develop new materials with unique properties*High performance masterpieces Demonstrate the potential of miniaturization and nanotechnology*Bio/Nano Bring biological sciences and engineering together; Molecular engineering?*Killer applications Need public benefit to justify investment in research

First microengineered device: integrated circuit*1959 Jack Kilby (TI) invented the monolithic integrated circuit*Design improved by Robert Noyce (Fairchild semiconductor) to produce planar technology

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BULK MICROMACHINING

*additive process*subtractive process <- bulk micromachining

substrate: silicon*single crystal silicon *up to 99.999999999% pure (11 nines)*single crystal silicon has a diamond cubic structure: two FCC lattices displaced by .25,.25,.25*silicon is amazing material b/c strong and metallic, acts like resistor but changes resistance by orders of magnitude if impurities find their way into silicon*boules; slice boule into wafers

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single crystal silicon growth*silicon is grown from a single seed*typically use Czochralski (CZ) method

*melt silicon in a quartz crucible in an inert atmosphere*Over the liquid pool of silicon, a specially prepared seed crystal is mounted on a rotating rod*This seed crystal is lowered until it just comes in contact with the surface of a molten pool of silicon*When the seed crystal comes in contact with the liquid silicon, a combination of liquid surface tension and capillary action act to form a shoulder where solidification occurs*The seed crystal's structure is replicated in the solid*Initially solidification is both radial and parallel to the normal*By precise control of extraction rate, the diameter of the solidifying crystal is established*After the desired diameter is obtained, unidirectional solidification occurs*The solidified ingot is called a boule*Once the liquid reservoir is exhausted, the length of the boule is decided

single crystal silicon wafer*silicon is cut into wafers using a diamond saw*sliced perpendicular to the axis of extraction*resulting discs are then ground on both sides and then the width+length of the tile are cut from diameter*To increase yield, the diagonal of the tile should closely approach the diameter of the boule*Ideally, a square with diagonals approaching the diameter of the boule would be the least expensive to produce*Alternatively, tiles can be cut longitudinally or parallel to the direction of extraction

*somewhat more difficult to accomplish as it poses problems in fixturing, sawing, blade cooling and dimensional control along the length of the cut

*The yield of full width tiles is correspondingly low, thus increasing costs significantly

silicon wafer cuts*Miller indices indicate by gound edges called "flats"*primary flat: which way atoms lined up*n type and p type refer to doping

*N - negative (phosphorus)*P - positive (boron)

silicon is a semiconductor*silicon electrical properties change w/ addition of impurities

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main steps in bulk micromachining vs semiconductor manufacturing*semiconductor manufacturing

*oxidation*film deposition*metalization*lithography*etch*ion implantation*diffusion*strip

*bulk micromachining: no ion implantaton+diffusion, don’t need impurities

*oxidation of silicon*not adding onto surface, changing the surface*stick wafer into oven w/ oxygen*oxygen in presence of silicon causes surface to oxidize to its natural oxide SiO2

*Si (solid) + O2 (gas) -> SiO2 (dry oxidation)*after first layer of oxide (25 Angstrom) is formed, protects silicon underneath from oxidation how to get oxygen under the skin of the first layer? diffusion – heat driven*to get thickness of SiO2, drive temp up for diffusion*Si (solid) + H2O (gas) -> SiO2 (wet oxidation) less dense than try oxide, but faster diffusion times*0.5 microns = ~ 6 hrs at 1200C (~1 hr wet oxide)

photoresist*photoresist is polymer (plastic) that has light sensitve chemicals*positive resist composition: DQN

*main components: polymer, solvent, __, (like paint)*photoactive compound (PAC) -> diazonaphthoquinone (DQ)*matrix or resin -> novolac (N)*solvents

photoresist spinning*spinning is easiest and most accurate way to coat a wafer's surface*typical speed is 3500rpm*liquid material sheets out to layer that is only a few microns thick*spin curve: film thickness vs spin speed; be on flat part of spin curve so less error in thickness due to spin speed being a bit faster/slower

photoresist exposure*exposing resist to light causes it to change its chemistry*expose to UV light*develop in hydroxide solution (TMAH, NaOH)*exposed areas are soluble in hydroxide*rinse

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photoresist contrast*high contrast resist required for high resolution

photoresist properties*many properties of relevance when selecting photoresist*photospeed, viscosity, adhesion, thermal stability, etch resistance, contamination, shelf life, pinhole density, charging, ease of processing

proximity printing/contact printing: put mask on top*easiest method*in industry: project shadows

optical lithography*use photographic mask to shine light and shadows on a photoresist covered wafer

lithographic masks*binary masks: light field masks and dark field masks

*good for ~1 micron work*transfer pattern to substrate*made of glass, quartz, with photographic emulsion or metal such as chrome of iron oxide*defects, dust, pinholes all affect patterning quality*masks made by mask writer (laser or e-beam)

*problems with masks: x-y, theta, pincushion, barrel, magnification error, random*phase shift masks: to compensate light bending at edges (diffraction)

*good for submicron work (<.3 micron)*not used for MEMS applications

lithographic light source*mercury bulbs used for most lithographic work*makes good, cheap UV source*UV light has short wavelength, so less diffraction, also good for high resolution*most popular wavelength: i-line (365nm)*high numerical aperature (NA) -> low depth of focus*MEMS: 3D structure not flat

dependences on wavelength*UV light has small wavelength, so good for high resolution

wet isotropic etching*isotropic: all directions*some chemicals can etch silicon; photoresist not strong enough

*nitric acid*almost all etch techniques in bulk micromachining require the prepatterning of a hard mask*strong material (like silicon oxide or silicon nitride) must be grown over the wafer, then patterned to make a stencil*photoresist, in general, isn't strong enough (selectivity is poor)

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wet anisotropic etching*some chemicals (example KOH) can etch silicon, but have preferential etch along an axis*allows one to design an anisotropic etch*crystal lattice plays a very important role for some wet wtchants*etchant is very much faster in 111 direction*typically 500x (100) and 600x (110)*angle: 54.74 degrees*anisotropic etch follows crystal orientation, not mask*mask misalignment with respect to the crystal lattice will result in uneven undercutting since the etch will proceed along crystal planes*for long, narrow patterns the effect is to make the sidewalls look "rough"*cantilever

dry etch*Xenon difluorie etch: a vapor

*2XeF2 + Si -> 2Xe + SiF4*rough etch, rough texture (10mm)*generates heat locally*may form silicon fluoride polymer on surface if not dehydrated*must operate in pulsed mode

*Plasma etch*most common is reactive ion etch (RIE)*use plasma (glow discharge) to create energetic ions*etch rate depends on plasma power, pressure, etc; typically 0.1um/min

*parallel plate plasma*plasma in RIE system contains reactive gases that react w/ materials to etch them*process: ionize gas, stick wafer in, ionized gas reacts with Si*precise control*regular plasma uses physical bombardment (erosion)*regular RIE only good for thin films (<1 micron)

*deep RIE (DRIE)*magnetic fields (inductive) plasma, faster than parallel plate plasma

*use high power, high density plasma*alternate: ETCH and DEPOSIT*create high aspect ratio etches, up to 1mm*etch rate is 203 mm/min or more

*Bosch process (AKA DRIE)*combination of DRIE and passivation step*electric field on bottom plate*etch a bit, then change gas for deposition to protect*change gas again to etch, etch only bottom*repeat process*scalloped walls*can get deep vertical trenches*high aspect ratio: 20:1

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10-6 Lec Surface Micromachining

last time:*bulk micromachining: subtractive process

*photolithograph pattern*etch *plasma process

surface micromachining is additive process

layering films to generate 3-D structure*layer by layer a 3D structure can be built up*Surface micromachining refers to building a MEMS device by adding layers of material to the substrate, then patterning each layer*Usually surface micromachining is done on silicon*Surface micromachining is becoming the most popular method for making MEMS

thin film strategies*MEMS similar to microelectronics fabrication*layers of films

***max 10um thick****layers deposited, patterned, etched*sacrificial material removed after completion*similar to standard semiconductor processes like CMOS

growing thin films*chemical vapor deposition: chemical reactions and condensation from vapor

*vapor deposition: gas state to liquid state*hope molecule sticks to surface

*if sticks, slides around on surface and may bump into other molecules + react, then condense and nucleate

*sticking*gas energy and substrate temperature play a large role in sticking*low gas energy -> high sticking coefficient*low substrate temperature -> high sticking coefficient

*nucleation*molecular species migrate on surface*agglomeration occurs at condensation points*heterogeneous nucleation occurs due to reaction in surface energy caused by surface/nucleus interface*hydrophilic*small contact angle -> low surface energy*adhesion layer

*island formation*condensation site density increases*sites merge together forming islands*mass transfer (diffusion) typically occurs*surface energy is reduced in the process

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*steady state growth*fine crystal islands merge*high mobility -> large grains*high treatment (anneal) can increase grain size

*heat up (to cause diffusion), cool down slowly

CVD processes (know types, don’t need to memorize)*Atmospheric pressure CVD*Low pressure CVD <--- most popular*plasma enhanced CVD

physical vapor deposition***condensation****materials in vapor phase are physically introduced to the substrate*no chemical reaction occurs*change of state occurs (condensation)*can be deposited on variety of different surfaces*good for thin films which cannot be easily grown with CVD such as metals*most common material: metals*metallization

*vacuum evaporation*heat metal so hot that it melts -> atoms leave as vapor*stick wafer in vapor*suck air out of chamber, so molecules in vapor can travel (in straight line)*mean free path = 10cm*wafer at room temp, atoms hot, so will cool down and stick*most common way to heat up metal: e-beam evaporation

*electron beam, magnet bends electrons to shoot them to surface

*wafer has to be in line of sight of metal*shadow b/c metal atoms coming in at angle

*solution: rotate wafer*lift off

*can pattern metal w/o etching*put photoresist first before metal (opposite of etching)*evaporate metal through openings of photoresist, then strip away photoresist

*sputtering*use Argon plasma (nonreactive)*plasma so energetic that it knocks metal off target, metal lands on wafer

coverage of vapor methods*CVD gives better step coverage than sputtering/evaporation

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annealing of films*good reflow allows easier planarization

adhesion layer, barrier layer??

using sacrificial materials*repeated deposition, pattern, etch process*sacrificial material (PSG) included*PSG removed by HF etch (a selective etch); etch away sacrificial material for free standing structure*stiction and water damage are problematic

sacrificial release: stiction*removal of sacrificial material usually in solution*need to dry out after dropping in etch*water goes away slowly*surface tension problem: it wants to pull structure down*hydrogen bonding bonds structures*one solution: put in methanol b/c less surface tension*another solution: coat surface in polymer that repels water*freeze drying: use sublimation (most common is liquid CO2)*using dry etch eliminates surface tension problem

released films: stress*thin films usually have tensile+compressive stress

*buckling/breaking can occur*wafer curvature measurements*on board sensors

*thermal: thermal mismatch between substrate and film*these are all heat processes, so different thermal expansion coefficients (thermal mismatch)

*intrinsic: dislocations, grain boundaries mismatched, etc*external: externally applied stress

10-8 Lec Non-silicon microfabrication/MEMS packaging

non-silicon, non-planar processes*non-traditional ways for building MEMS devices*efforts include approaches to build devices in ceramics, metals, polymers; like to build things with polymers*research is active into building high aspect ratio (2.5D) structures and true 3D micro-machining techniques

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LIGA: lithographie, galvanoformung, abformung*shine xrays from synchrotron on a metal mask/stencil; xrays that get through irradiates PMMA (plastic) to expose pattern*PMMA is chemically developed to create a high aspect ratio, parallel wall mold*electroforming: metal/alloy is electroplated in the PMMA mold to create a metal micropart*put in solvent so PMMA is dissolved, leaving a 3D metal micropart; micropart can be separated from the base plate is desired

LIGA: process1. irradiation: polymer material (PMMA) is exposed to x-rays through a shadow mask; exposed regions are weakened and made soluble in suitable developer2. development: exposed materials are developed away leaving high aspect ratio structures; structures can be 20:1 aspect ratio, with thickness of several mm3. electroforming: metal is electroplated between the polymers (mold); processed called electroforming4. mold insert: metal part is used as part of a larger die for molding operations5. mold filling: standard molding operations, such as injection molding, are carried out6. mold release: final part is released from mold to produce low cost microfabricated device in polymer

LIGA: maskmaking*LIGA mask made using micromachining techniques since the mask must be opaque to xrays*put metal on surface, etch metal photographically

LIGA: advantages/disadvantages*pros

*high aspect ratio micro-structures built cheaply (b/c out of plastic)*allows fabrication in polymers and other materials

*cons*requires synchrotron radiation for xrays; masks expensive*mostly only single mask structures; complex 3D is difficult*integration difficult*good for small parts, but most useful devices require assembly

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SU-8: poor man's LIGA*SU-8 is photosensistive epoxy resist

*low cost negative resist for MEMS*capable of yielding high aspect ratio structures*processing uses standard lithography equipment: UV lightsource, standard masks*structures up to several mm thick have been demonstrated w/ aspect ratios of 15:1

SU-8: advantages/disadvantages*pros

*high aspect ratio micro-structures can be built cheaply*allows fabrication in polymers*uses standard lithography equipment in standard clean room

*cons*processing is tricky and difficult to get consistent results*resist almost impossible to remove

*sticky, like molasses*mostly only single mask structures*good for small parts, but most useful devices require assembly

micromachining polymers*thermoplastics <-- most familiar with

*soft plastic behavior at low temp, high viscosity at high temp*melts; melting point 170C

*easy for injection molding*usually soluble in common solvents*typically linear polymer chains, minimal cross linking, high crystallization

*thermosets*strong bonds and therefore

*very strong, rigid material*high melting points (they don't melt)

*typically two part "cure" (polymerization), some UV cure*typically 3D polymers w/ high degree of cross-linking

*elastomers (rubbers)*soft, elastic behavior at moderate temps*cured or "vulcanized" to induce cross-linking or polymerization*typically high molecular weight, non crystalline, sparse cross linking

polymerization+crosslinking*how a polymer becomes a solid*start with monomers -> polymerization+crosslinking -> long chain polymers*change in viscosity from liquid to solid

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photo-sensitive polymers*photoresist often used as a structural or functional material*may be used to make molds for soft casting applications*fabrication of gray-tone mask and pattern transfer in thick photoresists

reflow techniques for polymers*by special lithographic tricks or by inducing reflow, specialshapes can be created*reflow: heat up, surface tension makes it round and smooth

rapid prototyping with precision optics*rapid prototyping techniques have been applied to build MEMS devices; true 3D devices can be built one at a time*just like 3D printing*slow process, can't build a lot at a time

etching polymers using oxygen plasma*polyimide often used in micromachining applications since it can be spun-on like photoresist, then patterned using oxygen plasma (to etch polymer)*very slow

etching polymers using high energy lasers*laser ablation patterning of polymers using excimer lasers

*laser has short wavelength*ablation vaporizes polymers by extremely rapid heating*ablation: laser shines on polymer for very short amount oftime, polymer absorbs the high energy but has no time to heat up, so gets removed

injection molding polymers*Special inserts must be created to fit within standard dies*Pre-evacuation of mold may be needed*Use of plasticisers may be necessary*problem: viscous so hard to get into channels

injection molding, CD technology*use of compact disk technology can be applied to build low cost MEMS devices if aspect ratio is not too high

Hot embossing*nickel mold inserts created using standard micromachining techniques*polymer is stamped on moldplate using special equipment

Hot rolling*using heated, patterned rollers, low cost (laboratory) prototypes can be made*no vacuum needed

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casting: popular method for prototyping*casting: use gravity to pour liquid*thermoplastics

*thermal processing is bad*high viscosity in liquid state is bad*must use solvent casting

*thermosets*chemical cure (low viscosity in initial state) is good*low temp processing is good*high adhesion is bad

*elastomers*chemical cure (low viscosity in initial state) is good*low temp processing is good*air cure (eg latex) may be good

casting issues*micromachined master

*SU-8: processing problems*silicon etch: limited etch depth with wet etchants*need low cost high aspect ratio etch/material

*cast mold material*silicone: good, but expensive; slow, bubbles, etc*latex: not as good as silicone, but cheaper, faster; thin coatings only*teflon: good but expensive

*cast replica material*two part materials are good *solvent based materials are good*seek out coatings technology

*special equipment*vacuum mixers, process ovens, mild jigs, etc

casting for microfluidics*for polymer microfluidic device research, casting is probably the most common method

Microbonding polymers*To bond polymer layers together, several technologies are being pursued

*laser welding, micro-adhesives, plasma surface treatments, ultrasonic welding

Soft lithography*Molded elastomers can be used as rubber stamps to produce nonoptical lithography*patterning technique is known as “soft lithography”*Capable of very high resolution

Thick CVD processes*diamond material can be structures using molding techniques*patterned silicon forms the mold

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UV sensitive glass: Foturan*special glass (Foturan) can be made to etch faster after being exposed to UV light*UV exposure, crystallization (annealing step), anisotropic etching (etching step)

UV sensitive glass as mold for electroplating*patterned glass can be used as electroplating mold

metal injection molding*patterned polymer can be used as a mold for metal injectionmolding*process

*grind metal (not too small because metal oxidizes)*mix metal dust+polymer*sintering: heat up, polymer goes away, metals bond together

ceramic injection molding*ceramics can be cast molded on to micropatterned substrates*powder based molding processes have limitations, but may still be useful for ceramic MEMS

Ion milling assisted fabrication*ion beams can be used with conventional bulk micromachining to produce novel etch profiles*atomic beams can be used with shadow masks and turned objects to produce novel, 3D structures

electrochemical fabrication*laminated electrodeposition techniques can give rise to 3D structures in metal

PCB fabrication*PCBs have been used to fabricate microchannels

precision milling*conventional machining may still provide many solutions for MEMS fabrication*machining, EDM, jet milling, electrochemical machining, etc

microassembly*someday may see birth of microfactory

packaging*Micro device is not useful until it is placed in a “package” that allows it to interface with the real world*MEMS packaging is extremely difficult and expensive*80% of cost of MEMS is in package*package allows electrical+mechanical connections + protects device

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packaging process for microelectronics*wafer level test*cut into pieces (singulation); use high speed saw (need to flow water)*each die mounted on substrate*wire bonding (to make electrical connections)*injection mold plastic on top

problems with MEMS packaging*MEMS device cannot be tested at the wafer level, must be released in order to test*Released MEMS cannot be put through dicing process b/c too fragile*MEMS release must be performed after dicing*MEMS part cannot be “pick and placed” by automated machinery, must be treated carefully*MEMS part must be handled in clean room environment*MEMS part typically tested after dicing, cleaning, release, and packaging —after much cost has already been incurred*MEMS part cannot be overmolded*MEMS part often requires hermetic or vacuum sealing

wafer level packaging*best hope to reduce cost of packaging*Build package over MEMS part during cleanroom fabrication.*Perform release while in package*Treat similarly as semiconductor device*Apply temporary package array to wafer prior to testing*Perform wafer-level release*Perform testing, dicing, etc*Polymer package becomes part of final package

advanced packaging*packaging technology is approaching MEMS scales*Cell-phone and PDA markets drive size requirements down forpackaged chips*Packaged chips available at below 1mm footprint*Pick and place machines can perform component assembly towithin 2µm*Wire bonding can now achieve 35µm pitch*Cost for packaging ~ 5 cents/chip