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1、1,Chapter 2,Microfabrication(Electronics Manufacturing),2,Learning Objectives,Be able to describe the basic processes of microfabricationBe able to explain the principles of photolithography.Be able to describe the basic mechanisms of the additive processes,including relative comparisons among them.
2、Physical Vapor Deposition(evaporation,sputtering)Chemical Vapor DepositionBe able to describe the basic mechanisms of the subtractive processes,including relative comparisons among them.Wet Etching(isotropic,anisotropic)Dry Etching(physical,chemical,physical-chemical)Be able to describe the process
3、of bonding and packaging,3,What is LIGA?,LIGA is a fabrication process for high aspect ratio microstructures consisting of three major process steps.X-ray lithography(LI=lithographie)to generate primary microstructures.(DXRL=deep X-ray lithography,UDXRL=ultra-deep X-ray lithography)Electroplating/El
4、ectrodeposition(G=Galvanik)to produce microstructures in metal.Molding(A=Abformung)to batch produce secondary microstructures in polymers,metals,ceramics,4,Brief History of LIGA,Electrodeposition and X-ray lithographyRomankiw et al at IBM,1975.Adding moldingEhrfeld et al at KfK,1982.In Germany,LIGA
5、was initially independent from semiconductor industry.Guckel(U.Wisc)integrated LIGA with existed semiconductor industry so that LIGA becomes one major micro fabrication process.Today,more and more prototypes have been fabricated.And some LIGA companies have been founded and provided professional ser
6、vices to users.,5,Why Is LIGA Interesting?,Very high aspect ratio structures can be achievedHeight(typical)20-500 m,this can not be achieved by state of art silicon surface micromachiningHave more material selectivity in final productsCould be metal,polymer,and even ceramics.Vertical and better surf
7、ace roughness in sidewall Vertical slope1m/mm,Surface roughness 0.03-0.05 m(max.peak to valley)It can be applied in both MEMS and traditional precision manufacturing.,6,LIGA ProcessX-ray lithography,7,LIGA ProcessElectroplating,8,LIGA ProcessMolding,9,LIGA Materials,PolymersUsed in X-ray lithography
8、 period.The materials must be able to allow photochemical reaction under exposure under X-rayThick PMMA is a popular choiceMetalsUsed in electrodeposition phase to form a mouldNickel is a popular choice.Polymer againUsed in molding periodThe final LIGA product is usually made of polymers,10,LIGA App
9、lications,high aspect ratio microstructures,Micro turbine rotor,KfK,11,LIGA Applications,Spinneret Capillary,IMM,12,Problem Associated with LIGA,You need to buy a big X-ray sourceexpansiveNeed long time to exposure and developTime consuming(several days)Other compete technology are rapidly catching
10、upLIGA-LikeSi DRIE technologyThe X-ray lithography process can be replaced to reduce the cost.,13,LIGA-Like Process Overview,14,DRIE System,Plasmalab System 100 Modular ICP-RIE Etching System Oxford Instruments,15,http:/,Overview of DRIE,Reactive ion etching(RIE)is a dry etching method which combine
11、s plasma etching and ion beam etching principles.Choice for most advanced product lineIts not well suitable for deep etching(10m)Inductively coupled plasma(ICP)reactors have been introduced for silicon RIE process leading to the deep reactive ion etching(DRIE)technique.Higher plasma densityHigher et
12、ching rate,either for anisotropic and isotropic etchingHigher aspect ratio(AR)Reduction of parasitic effects,16,http:/,DRIE Applications,Use ICP etcher to create deep etching in silicon substrate,17,DRIE Applications,The silicon structures were part of an innovative escape mechanism of the Ulysse Na
13、rdin Freak watch.(CSEM SA,IMT),Silicon parts:LighterLower friction coefficientInsensitive to magnetic fields,18,Silicon Review,In a perfect crystal,each of silicons four outer electrons form covalent bonds,resulting in poor electron mobility(i.e.insulating)Doping silicon with impurities alters elect
14、ron mobility(i.e.semiconducting)Extra electron(“N-type”,with phosphorous,for example)Missing electron(“P-type”,with boron,for example),19,Silicon Circuits,Silicon makes the transistorsTransistors can be made to very large scale integration(VLSI)and ultralarge scale integration(ULSI),A Pentium 4 proc
15、ess contains 42 million transistors,20,Silicon Micromachines,The other application is micromachines,also called the microelectricmechanical system(MEMS),which have the potential of making the computer obsoleteThe micromachines include:Fuel cellsDNA chips,21,Microfabrication,Silicon crystal structure
16、 is regular,well-understood,and to a large extent controllable.It is all about control:the size of a transistor is 1 m,the doping must therefore less than have of that How to control?,22,Microfabrication Techniques,23,Process of Microfabrication,Single crystal growing,Waferslicing,Filmdeposition,Oxi
17、dation,Diffusion,Ionimplantation,Etching,Lithography,Metallization,Bonding,Packaging,Testing,24,Crystal Growing,Silicon occurs naturally in the forms of silicon dioxide and various silicates and hence,must be purified The process of purifying silicon:Heating to produce 95%98%pure polycrystalline sil
18、iconUsing Czochralski(CZ)process to grow single crystal silicon,1 rev/s,10 m/s,Liquid silicon,Illustration of CZ process,25,Crystal Growing,26,Wafer Slicing,This step includesSlice the ingot into slices using a diamond sawPolish the surface,andSort,27,Film Deposits,This step is used to add a special
19、 layer on the surface of the silicon for maskingMany types of films are used for insulating/conducting,including polysilicon,silicon nitride,silicon dioxide,tungsten,and titanium.Films may be deposited using various method,includingEvaporationSputteringChemical Vapor Deposition(CVD),28,Film Deposits
20、,The process of CVDContinuous,atmospheric-pressure CVDLow-pressure CVC,29,Photolithography,Photolithography is a process by which an image is optically transferred from one surface to another,most commonly by the projection of light through a mask onto a photosensitive material.Photoresist is a mate
21、rial that changes molecular structure when exposed to radiation(e.g.ultraviolet light).It typically consists of a polymer resin,a radiation sensitizer,and a carrier solvent.,30,Photolithography,Adding a photoresist layer on the waferA photomask is typically manifested as a glass plate with a thin me
22、tal layer,that is selectively patterned to define opaque and transparent regions.,31,Photolithography,A positive photoresist is weakened by radiation exposure,so the remaining pattern after being subject to a developer solution looks just like the opaque regions of the mask,A negative photoresist is
23、 strengthened by radiation exposure,so the remaining pattern after being subject to a developer solution appears as the inverse of the opaque regions of the mask.,32,Etching Mask for a Mask,The mask must withstand the chemical environment.A typical mask/substrate combination is oxide on silicon.Resi
24、lient masks are typically grown or deposited in whole films,and must therefore be patterned through a photosensitive mask,Reusable mask,Photoresist coating,Functional mask,33,Dry Etching Mechanisms,PhysicalRemoval based on impact&momentum transferPoor material selectivityGood directional controlHigh
25、 excitation energyLower pressure,100 mTorrPhysical/ChemicalGood directional controlIntermediate pressure,100 mTorr,34,Isotropic Wet Etching,Etch occurs in all crystallographic directions at the same rate.Most common formulation is mixture of hydrofluoric,nitric and acetic acids(“HNA”:HF+HNO3+CH3COOH
26、).Etch rate may be very fast,many microns per minute.Masks are undercut.High aspect ratio difficult because of diffusion limits.Stirring enhances isotropy.Isotropic wet etching is applicable to many materials besides silicon,35,Anisotropic Wet Etching,Etch occurs at different rates depending on expo
27、sed crystalUsually in alkaline solutions(KOH,TMAH).Heating typically required for rate control(e.g.80 oC).Etch rate typically 1 m/min,limited by reactions rather than diffusion.Maintains mask boundaries without undercut.Angles determined by crystal structure(e.g.54.7).Possible to get perfect orthogo
28、nal shapes outlines using 1-0-0 wafers.,36,Etching a Comparison,ISOTROPICWide variety of materialsNo crystal alignment requiredMay be very fastSometimes less demand for mask resilience,ANISOTROPICPredictable profileBetter depth controlNo mask undercuttingPossibility of close feature arrangement,Mult
29、iple layers are common,37,Etching a Comparison,38,Ion implantation,Ion implantation is used to alter the electrical characters of the silicon in specific regions.The process,39,Ion implantation,The main disadvantage is that it can only process a wafer at a timeDiffusion,40,Bonding and Packaging,Wires(25 m)are bonded to package leadsThe bond wires are attached using thermocompression,ultrasonic,or thermosonic techniques,41,Bonding and Packaging,Packaging is done by surface mount technology,
