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Figure 1. Coarse grained polysilicon. Columnar grains and a rough surface can be seen in this SEM of a 2 gim thick polysilicon film deposited at 635TC. The edge was formed by etching in a CF4/O 2 plasma, and removal of a portion of the supporting oxide in HF. 3 Mat. Res. Soc. Symp. Proc. Vol. 605 © 2000 Materials Research Society

Figure 2. Fine grained polysilicon. This 2 jim thick film shows no visible texturing and a very smooth conformal surface finish. important issue [1]. Polycrystalline silicon; poly; is typically produced from high purity silane, SiH 4, a gas which decomposes into silicon and hydrogen when heated above 550*C. Since silane oxidizes readily the decomposition must occur in a reducing ambient or in a vacuum. Low pressure chemical vapor depositions, LPCVD, of poly are typically performed between 570'C to 650'C at pressures which are set by the reactor and the desired silane

flow rate. The growth rate increases with temperature, however, the film morphology also changes and varies from amorphous to aligned crystal structure. This is illustrated by Fig. 1 and Fig. 2. Either film morphology may be acceptable for microelectronic applications. However, for micro mechanics the film in Fig. 2 is much preferred. There is a chance that the film in this figure may have direction independent; isotropic; mechanical behavior. If this can be verified and if polysilicon shares the excellent mechanical behavior of single crystal silicon the use of conformal depositions via LPCVD of polysilicon films would have a major influence on surface micro machined micro mechanical devices. POLYSILICON FOR MICROMECHANICS The possibility of using polysilicon in a surface micro machining sequence becomes feasible if certain conditions are met. Surface micro machining involves the construction material from which devices are fabricated and sacrificial material, which supports structures during fabrication and is eventually removed by lateral etching [2]. In a polysilicon-silicon dioxide system the polysilicon is the construction material and the oxide forms the sacrificial layer. This system becomes interesting if [3,4]: (1) the polysilicon is fine grained without preferred directions. This may be

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Figure 3. Buckled polysilicon plates. These plates are made from a 2.0 micron thick polysilicon film using a sacrificial silicon dioxide film. The plate dimensions range from 50 to 250 jim in length restated by requiring the texture function for the film to be close to unity. This function is used to compute mechanical properties of the film from single crystal data. And if (2) the etch rate of the construction material is zero in the etchant for the sacrificial layer. In the present case polysilicon should not be attacked by HFsolutions which are used to remove the Si0 2 . From the silicon perspective this would require a very clean film without non-silicon nucleation centers. This in turn requires careful reactor design. The etchant, HF, should not contain impurities; and additives, such as NH 4F, have to be tested for func