Polycrystalline Silicon Films for Microelectromechanical Devices
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Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
EXPERIMENT The micromechanical devices investigated in this study have been fabricated by standard surface micromachining techniques [7], using a single-mask process. The fabrication sequence is summarized as follows: 100 mm diameter (100) silicon wafers were thermally oxidized (wet) to form a 2.5 gm thick silicon dioxide release layer; four different thicknesses of heavily in situ B-doped (_1020 cm-3 ) polysilicon films were chemical vapor deposited using Sil- 4 and B2 H6 gases at 1100"C; chemo-mechanical polishing was used to reduce the roughness of the polysilicon films [8]; the polysilicon films were thermally oxidized (wet) at 1000"C to form a silicon dioxide masking layer for the later polysilicon etch - the three thinner films were oxidized for 107 minutes to create a 0.5 gm oxide layer, while the thickest film was oxidized for 300 minutes to yield a 1.0 gm oxide; the wafers were subjected to a standard optical photolithography sequence; the masking oxide layers were dry etched in a CHF 3 :C2 F6 :He gas ambient; the polysilicon layers were dry etched in a C12:He gas mixture (the etch rate at 200 W was about 0.5 prn per minute, and the selectivity to silicon dioxide was about 24:1); and finally, the devices were etched in liquid hydrofluoric acid for 10 minutes to remove the release oxide. This last step was timed so that some oxide remained beneath the structures' larger features, leaving them anchored to the substrate, while the smaller features were fully released and free to move. The device chosen to investigate the elastic modulus of the films was the lateral resonant structure developed by Tang, et al. [4] A scanning electron micrograph of one fabricated structure is shown in Fig. 1. The folded beam supports are 2 gm wide and 100 pm long, and the comb fingers are also 2 gm wide. It is apparent that the polysilicon etching procedure has achieved nearly vertical, smooth sidewalls for this thick film and high aspect ratios. The device used for studying residual stress is shown in Fig. 2. The design for this pointer-type strain gauge was developed at the Charles Draper Laboratory [9]. Again, the success of the etch is obvious. RESULTS AND DISCUSSION Elastic Modulus The theory of operation of the lateral resonant structure shown in Fig. 1 has been described in detail in ref. 10. In brief, an ac drive voltage is applied between one of the side pads 120 um
Figure 1. Scanning electron micrograph of a lateral resonant structure fabricated from a 10 gm thick B-doped polysilicon film. Note the nearly vertical sidewalls of the 2 pm wide beams. 322
Figure 2. Scanning electron micrograph of a pointer-type strain gauge fabricated from a 10 gm thick B-doped polysilicon film. and the central structure. Because of the electrostatic attraction between the interdigitated comb fingers, at the correct ac frequency, the central structure will be driven into resonance. This resonant frequency, co, is then defined by Co2 =
23.261EI (1) L 3(M+0.531Ms
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