Obtaining flatness in optical MEMS devices

In addition to deformations due to stress and stress-gradients, the surface roughness of the thin films used to fabricate optical M EM S components can also have an impact, as deposited polysilicon can have significant roughness. The root mean square surface roughness of polysilicon in the MUMPS process has been measured and found to be approximately 12nm. 

Petersen, Silicon as a mechanical material, Proc. IEEE 70, pp. 420-457 (1982). 

Petersen, Dynamic micromechanics on silicon techniques and devices, IEEE Trans. Electron Devices ED-25(10), pp. 1241-1250 (1978). 

Kovacs, Micromachined Transducers Sourcebook, McGraw-Hill (1998). 

Mukherjee, Design, modeling and verification of MEMS silicon torsion mirror, Proc. SPIE 3226, p. 114 (1997). 

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Another source of topography on the mirror surface resulted from residual stress-gradients in the polysilicon film after it was released, as described previously in Section 4.5 on obtaining flatness in optical MEMS devices. Here it was found that a post-fabrication ion bombardment step could be used to increase the radius of curvature due to residual stress gradients, as shown in Figures 8.8 and 8.9 [19]. 

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