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1、1,CHAPTER 2. Micro fabrication technology,Semi-conductor industry and MEMS materials. Introduction to modern Semiconductor industry Semiconductor industry is playing more and more important role in modern society. We can say, no semiconductor industry, no modern civilization!,2,2) Commonly used MEMS
2、 materials,3,3) Crystal structure of Silicon 4) Crystal planes 5) Types of silicon wafer,4, Geometric representation of crystal Si Definition: “( )” specific plane “ ” equivalent plane “ ” specific direction “” equivalent direction Wafers are classified as how they are doped (n-type, p-type), and ho
3、w the Si crystals are oriented as seen from the top of the shape of the wafers are cut differently.,5,6) 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 can be controlled over a very wid
4、e 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 always used,6,in integrated circ
5、uits. 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 carriers, which carry curr
6、ent 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.,7,7) Semiconductor Devices Understanding of how the basic devices used ICs operate is
7、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 Transistors,8,2. Introduction to the Lithography Process,1) Positive and Negative Resist,KOH、Acetone、TMAH,HF,9,10, Ba
8、sic step of lithography: 1）The maximum resolution of a device: where is the wave length of light used to make image transfer. e.g. UV =500nm 2）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 wit
9、h PR in a solution that will attack the thin film but not the PR remove PR,11,2) Photoresist Spin Coating,12,3) 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(
10、45%), temperature(70F) -If we need to maintain a “clean” environment, we need to pay a lot of cost.,13,Definition of clean-room class: number of particles of size of air. 100class, 10class,14,3. X-Ray lithography,A type of light lithography techniques using short wavelength X-Rays Pros Fast process
11、High aspect ratio Solves depth of focus problem High resolutions of .5 m Reduction in diffraction, reflection, and scattering effects Not affected by organic defects in mask,15,Cons Shadow printing Lateral magnification error Brighter x-ray sources needed More sensitive resists needed Difficult fabr
12、ication of x-ray mask,16,4. Electron Beam Lithography,As scanning electron microscopes, an electron beam scans across the substrate surface and exposes electron sensitive coating.,17,Pros Computer-controlled beam No mask is needed Can produce sub-1 m features Diffraction effects are minimized Electr
13、on beam can detect surface features for very accurate registrar,18,Cons Swelling 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 sca
14、ttering in the substrate limit resolution,19,5. Diffusion and Implantation of Dopants,20,6.Thin film addition,PR Spin on Thermal oxidation Thermal evaporation E-beam evaporation Sputtering process CVD (LPCVD,PECVD) Electroplating,21,Thermal oxidation -growing on Si Dry oxidation: (1150C, 4hr) Wet ox
15、idation: ( 800C1200C ),Si,d,22,Where t: time; A、B: constant; when td B t/AA and B can be obtained from handbook,23,2) Deposition (no chemical reaction on surface) -Vacuum chambers are needed + thin film deposition -Al, Au, Cr,24,3) Sputtering -A momentum-exchange process (not heating) -Low temperatu
16、re process -We can sputter anything onto substrate theoretically,25,4)CVD (Chemical Vapor Deposition) (1) ( 300500C ) Typically CVD films are 0.12um thickness in MEMS use. (2) (700C900C) (3) ( 600C700C ) 5um50um poly-Si,26,LPCVD（低压CVD）P102 Pa PECVD (等离子增强CVD),27,5)Electroplating -Another way to add
17、thin film on surface. Coating of an object with a thin layer of metal by use of electricity. -A classical method to “grow” metal onto surface -Typical materials used for plating: Cu, Au, Ag, Cr -Thickness 100um,28,7. Micro Fabrication Technologies,（一）Surface micromachining,29,（二） Bulk micromachining
18、 1.Basic concept for Bulk Micromachining SelectivityA process is selectivity if it etches the desired direction at a higher rate and all other surface at a lower rate. Isotropic- Same etching rate at all directions. Anisotropic direction selected,30,31,4) Wet etching This is the simplest etching tec
19、hnology. All it requires is a container with a liquid solution that will dissolve the material in question. Isotropic enchants (HNA):HF,HNO3,CH3COOH Anisotropic enchants: KOH, EDP, TMAH,54.74,(100),32,Note: the advantage of wet etching is that you will get good selectivity, but hard to control etch
20、rate. 5) Dry etching Plasma etch polySi :Cl, F based gases, such as Cl2, CF4 Oxides: CHF3, C2F6 etc. b. RIE( reactive iron etch ) c. DRIE,33,34,（三）Surface vs. Bulk micromaching,Surface micromaching,Bulk micromaching,35,36,（四）LIGA (Lithographic, Galvanoformung, Abformung) (Deep x-ray lithography, electroplating, molding),37,Development and Electroplating,38,2) Molding,39,3) LIGA vs. Si micro-lithography,40,（五）CMOS process,41,42,43,44,45,46,47,48,49,50,