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1、Chapter 2, Microfabrication (Electronics Manufacturing) Learning Objectives Be able to describe the basic processes of microfabrication Be able to explain the principles of photolithography. Be able to describe the basic mechanisms of the additive processes, including relative parisons among them. P
2、hysical Vapor Deposition (evaporation, sputtering) Chemical Vapor Deposition Be able to describe the basic mechanisms of the subtractive processes, including relative parisons among them. Wet Etching (isotropic, anisotropic) Dry Etching (physical, chemical, physical-chemical) Be able to describe the
3、 process of bonding and packaging What is LIG A? 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) Ele
4、ctroplating/Electrodeposition (G=Galvanik) to produce microstructures in metal. Molding (A=Abformung) to batch produce secondary microstructures in polymers, metals, ceramics Brief H istory of LIG A Electrodeposition and X-ray lithography Romankiw et al at IBM, 1975. Adding molding Ehrfeld et al at
5、KfK, 1982. In Germany, LIGA was initially independent from semiconductor industry. Guckel (U. Wisc) integrated LIGA with existed semiconductor industry so that LIGA bees one major micro fabrication process. Today, more and more prototypes have been fabricated. And some LIGA panies have been founded
6、and provided professional services to users. Why Is LIG A Interesting? Very high aspect ratio structures can be achieved Height(typical) 20-500 m, this can not be achieved by state of art silicon surface micromachining Have more material selectivity in final products Could be metal, polymer, and eve
7、n ceramics. Vertical and better surface roughness in sidewall Vertical slope10m) Inductively coupled plasma (ICP) reactors have been introduced for silicon RIE process leading to the deep reactive ion etching (DRIE) technique. Higher plasma density Higher etching rate, either for anisotropic and iso
8、tropic etching Higher aspect ratio (AR) Reduction of parasitic effects ipe.cuhk.edu.hk/ DRIE ApplicationsUse ICP etcher to create deep etching in silicon substrate DRIE Applications The silicon structures were part of an innovative escape mechanism of the Ulysse Nardin Freak watch. ( CSEM SA, IMT) S
9、ilicon parts: Lighter Lower friction coefficient Insensitive to magnetic fields 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 electron mobility (i.e. semicond
10、ucting) Extra electron (“N-type”, with phosphorous, for example) Missing electron (“P-type”, with boron, for example) Silicon Circuits Silicon makes the transistors Transistors can be made to very large scale integration (VLSI) and ultralarge scale integration (ULSI) A Pentium 4 process contains 42
11、million transistors Silicon Micromachines The other application is micromachines, also called the microelectricmechanical system (MEMS), which have the potential of making the puter obsolete The micromachines include: Fuel cells DNA chips Microfabrication Silicon crystal structure is regular, well-u
12、nderstood, 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? Microfabrication Techniques Process of MicrofabricationSingle crystal growing Waferslicing Filmdeposition OxidationDiffusion Ionim
13、plantation Etching Lithography Metallization Bonding Packaging Testing 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 silicon Using Czochralsk
14、i (CZ) process to grow single crystal silicon 1 rev/s10 m/sLiquid silicon Illustration of CZ process Crystal Growing Wafer Slicing This step includes Slice the ingot into slices using a diamond saw Polish the surface, and Sort Film Deposits This step is used to add a special layer on the surface of
15、the silicon for masking Many types of films are used for insulating / conducting, including polysilicon, silicon nitride, silicon dioxide, tungsten, and titanium. Films may be deposited using various method, including Evaporation Sputtering Chemical Vapor Deposition (CVD) Film Deposits The process o
16、f CVD Continuous, atmospheric-pressure CVD(a) Low-pressure CVC Photolithography Photolithography is a process by which an image is optically transferred from one surface to another, most monly by the projection of light through a mask onto a photosensitive material. Photoresist is a material that ch
17、anges molecular structure when exposed to radiation (e.g. ultraviolet light). It typically consists of a polymer resin, a radiation sensitizer, and a carrier solvent. Photolithography Adding a photoresist layer on the wafer A photomask is typically manifested as a glass plate with a thin metal layer
18、, that is selectively patterned to define opaque and transparent regions. PhotolithographyA 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 strengthen
19、ed 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. Etching Mask for a Mask The mask must withstand the chemical environment. A typical mask/substrate bination is oxide on silicon. Resilient masks ar
20、e typically grown or deposited in whole films, and must therefore be patterned through a photosensitive maskReusable maskPhotoresist coatingFunctional mask Dry Etching Mechanisms Physical Removal based on impact & momentum transfer Poor material selectivity Good directional control High excitation e
21、nergy Lower pressure, 100 mTorr Physical/Chemical Good directional control Intermediate pressure, 100 mTorr Isotropic Wet Etching Etch occurs in all crystallographic directions at the same rate. Most mon formulation is mixture of hydrofluoric, nitric and acetic acids (“HNA”: HF + HNO3 + CH3COOH). Et
22、ch 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 Anisotropic Wet Etching Etch occurs at different rates depending on expose
23、d crystal Usually 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 perf
24、ect orthogonal shapes outlines using 1-0-0 wafers. Etching a ComparisonISOTROPIC Wide variety of materials No crystal alignment required May be very fast Sometimes less demand for mask resilience ANISOTROPIC Predictable profile Better depth control No mask undercutting Possibility of close feature a
25、rrangementMultiple layers are mon Etching a Comparison Ion implantation Ion implantation is used to alter the electrical characters of the silicon in specific regions. The process Ion implantation The main disadvantage is that it can only process a wafer at a time Diffusion Bonding and Packaging Wires (25 m) are bonded to package leads The bond wires are attached using thermopression, ultrasonic, or thermosonic techniques Bonding and Packaging Packaging is done by surface mount technology
硅片電子制造
