1、CHAPTER 2.Micro fabrication technology1.Semi-conductor industry and MEMS materials.1)Introduction to modern Semiconductor industrySemiconductor industry is playing moreand more important role in modern society.We can say,no semiconductor industry,nomodern civilization!2)Commonly used MEMS materials
2、CategoryMaterialProperty or applicationMetalAu,Al,Cu,Ni,CrConductorSemiconductorSilicon Structure,semiconductorPoly-crystalline siliconStructure,semiconductorSingle-crystalline siliconStructure,Anisotropic etch propertiesGaAs Structure,optical propertiesQuartzCrystalline SiO2Substrate,structuresPoly
3、merParylene-CNafion Polymeric materialsNon-metalsDiamond(C)structure3)Crystal structure of Silicon 4)Crystal planes 5)Types of silicon wafer Geometric representation of crystal Si Definition:“()”specific plane “”equivalent plane“”specific direction “”equivalent direction Wafers are classified as how
4、 they are doped(n-type,p-type),and how the Si crystals are oriented as seen from the top of the shape of the wafers are cut differently.6)Why do we choose silicon as a MEMS structural material?Semiconductors are a class of materials which have the unique property that their electrical conductivity c
5、an be controlled over a very wide range by the introduction of dopants.While this property can easily be observed in crystalline,polycrystalline,or amorphous semiconductor materials,crystalline materials provide the most reproducible properties and the highest performance devices and are almost alwa
6、ys used in integrated circuits.Dopants are atoms that generally contain either one more or one fewer electrons in their outermost shell than the host semiconductor.They provide one extra electron or one missing electron(a hole)compared to the host atoms.These excess electrons and holes are the carri
7、ers,which carry current in semiconductor devices.The key to building semiconductor devices and integrated circuits lies in the ability to control the local doping and hence the local electronic properties of a semiconductor crystal.7)Semiconductor Devices Understanding of how the basic devices used
8、ICs operate is useful,because it provides some understanding of the objectives we have for IC technology.The most commonly used IC devices are:1)PN Diodes 2)Bipolar Junction Transistors 3)MOS Transistors2.Introduction to the Lithography Process1)Positive and Negative ResistKOH、Acetone、TMAHHF Basic s
9、tep of lithography:1）The maximum resolution of a device:where is the wave length of light used to makeimage transfer.e.g.UV=500nm2）Steps:substrate preparation thin film addition cast PR pre-designed mask expose in UV light develop PR in developing solution Etch the thin film covered with PR in a sol
10、ution that will attack the thin film but not the PR remove PR 2)Photoresist Spin Coating3)Clean-Rooms,Wafer Cleaning-A very clean environment is needed for micro fabrication to prevent dirt or other particles from contamination the micro devices.-Air purification:humidity(45%),temperature(70F)-If we
11、 need to maintain a“clean”environment,we need to pay a lot of cost.Definition of clean-room class:number of particles of size of air.100class,10class3.X-Ray lithography A type of light lithography techniquesusing short wavelength X-RaysProsFast process High aspect ratio Solves depth of focus problem
12、 High resolutions of .5 m Reduction in diffraction,reflection,and scattering effects Not affected by organic defects in mask Cons Shadow printing Lateral magnification error Brighter x-ray sources needed More sensitive resists needed Difficult fabrication of x-ray mask 4.Electron Beam LithographyAs
13、scanning electronmicroscopes,an electronbeam scans across thesubstrate surface andexposes electron sensitivecoating.Pros Computer-controlled beam No mask is needed Can produce sub-1 m features Diffraction effects are minimized Electron beam can detect surface features for very accurate registrar Con
14、sSwelling occurs when developing negative electron beam resists,limiting resolution Expensive as compared to light lithography systems Slower as compared to light lithography systems Forward scattering in the resist and back scattering in the substrate limit resolution 5.Diffusion and Implantation o
15、f Dopants6.Thin film additionPR Spin onThermal oxidationThermal evaporationE-beam evaporationSputtering processCVD(LPCVD,PECVD)Electroplating1)Thermal oxidation-growing on SiDry oxidation:(1150C,4hr)Wet oxidation:(800C1200C)o.44dSidWhere t:time;A、B:constant;when td B t/AA and B can be obtained from
16、handbook2)Deposition(no chemical reaction on surface)-Vacuum chambers are needed+thin film deposition-Al,Au,Cr3)Sputtering-A momentum-exchange process(not heating)-Low temperature process-We can sputter anything onto substrate theoretically4)CVD(Chemical Vapor Deposition)(1)(300500C)Typically CVD fi
17、lms are 0.12um thickness in MEMS use.(2)(700C900C)(3)(600C700C)5um50um poly-SiLPCVD（低（低压CVD）P 100um7.Micro Fabrication Technologies（一）（一）Surface micromachining（二）（二）Bulk micromachining1.Basic concept for Bulk Micromachining1)SelectivityA process is selectivity if it etches the desired direction at a
18、 higher rate and all other surface at a lower rate.2)Isotropic-Same etching rate at all directions.3)Anisotropic direction selected4)Wet etchingThis is the simplest etching technology.All it requires is a container with a liquid solution that will dissolve the material in question.Isotropic enchants
19、HNA):HF,HNO3,CH3COOHAnisotropic enchants:KOH,EDP,TMAH54.74(100)Note:the advantage of wet etching is that you will get good selectivity,but hard to control etch rate.5)Dry etchinga.Plasma etchpolySi:Cl,F based gases,such as Cl2,CF4Oxides:CHF3,C2F6 etc.b.RIE(reactive iron etch)c.DRIE（三）（三）Surface vs.
20、Bulk micromachingSurface micromachingBulk micromachingSurface MicromachiningBulk MicromachiningTrenchBridgeCantileversWafer SurfaceCavityNozzleMembrane（四）（四）LIGA(Lithographic,Galvanoformung,Abformung)(Deep x-ray lithography,electroplating,molding)1)Development and Electroplating2)Molding3)LIGA vs.Si micro-lithography（五）（五）CMOS process
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