Lec6p1.ProcessModulesII

Lec6p1.ProcessModulesII - EE 245: Introduction to MEMS...

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EE 245: Introduction to MEMS Lecture 6: Process Modules II CTN 9/15/09 Copyright © 2009 Regents of the University of California EE C245 : Introduction to MEMS Design LecM 3 C. Nguyen 8/20/09 45 Epitaxy Use CVD to deposit Si on the surface of a Si wafer ª Si wafer acts as a seed crystal ª Can grow a single-crystal Si film (as opposed to poly-Si) Epitaxy : Modeling –similar to CVD in fact, the model discussed so far for CVD is more relevant to epitaxy than CVD! get similar curve: Log (growth rate) T 1 Mass transport limited Reaction rate limited Reactions – can use SiCl 4 , SiH 4 , SiH 2 Cl 2 for vapor phase epitaxy. SiCl 4 : Silicon tetrachloride SiH 4 : silane SiH 4 Cl 2 : dichlorosilane EE C245 : Introduction to MEMS Design LecM 3 C. Nguyen 8/20/09 46 Epitaxy (cont.) Most popular: SiCl 4 (gas) + 2H 2 (gas) Si (solid) + 4HCl (gas) (Note that this is reversible!) Reverse reaction (i.e., etching) if have excessive HCl sometimes used before deposition to clean the Si wafer surface. SiCl 4 (gas) + Si (solid) 2SiCl 2 (gas) Also get a competing reaction. Important that the right conc. of SiCl 4 is used! See Figure 4.2 ª Too much SiCl 4 etching rather than growth takes place! ª Growth rate too fast get polysilicon instead of Si. (> 2 μ m/min.) 1200°C EE C245 : Introduction to MEMS Design LecM 3 C. Nguyen 8/20/09 47 Epitaxial Growth Rate Dependencies Figure 4.2 EE C245 : Introduction to MEMS Design LecM 3 C. Nguyen 8/20/09 48 Epitaxy (cont.) Alternative reaction: pyrolytic decomposition of silane: SiH 4 Si + 2H 2 650°C not reversible, low T, no HCl formation ª however, requires careful control of the reaction to prevent formation of poly-Si ª also, the presence of an oxidizing species causes silica formation 1. Just add impurities during growth: Arsine, diborane, Phosphine ª Control resistivity by varying partial pressure of dopant species i. Arsine, Phosphine slow down the growth rate ii. Diborane enhances growth rate Doping of Epitaxial Layers :
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EE 245: Introduction to MEMS Lecture 6: Process Modules II CTN 9/15/09 Copyright © 2009 Regents of the University of California EE C245 : Introduction to MEMS Design LecM 3 C. Nguyen 8/20/09 49 Doping of Epitaxial Layers 2. Use “autodoping” when growing own heavily-doped substrates ª Impurity evaporates from wafer (or liberated by Cl etching of surface during dep.) ª Incorporates into gas stream
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Lec6p1.ProcessModulesII - EE 245: Introduction to MEMS...

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