Two processes are developed to fabricate electrostatic comb drives hermetically encapsualted in a silicon nitride or polysilicon micropackage. To fabricate the silicon nitride micropackage, high-doped phosphosilicate glass is deposited, after the electrostatic comb-drive process is complete except for the release etch, and reflowed to form a mould for the silicon nitride microshell. The mould is patterned with a 5:1 BHF wet etch, and then the etch channel layer of phosphosilicate glass is deposited and patterned with a wt etch. The silicon nitride microshell layer is depositied, and holes are plasma etched in the nitride to access the etch channels. The sacrifical PSG mould is removed with 10:1 HF to rlease the electrostatic comb drive within the silicon nitride micropackage. The etch holes in the silicon nitride microshell are sealed with another layer of silicon nitride.
To fabricate the polysilicon micro package, highly-doped phosphosilicate glass is deposited upon completion of the comb-drive process ave the release etch to provide a spacer between the second, microstructural polysilicon layer and the third, micro package polysilicon layer. The PSG spacer is patterned with a 5:1 BHF wet etch, and the etch channel layer of PSG is deposited and patterned. The third, micro shell polysilicon layer is deposited, and a high temperature anneal in N2 is performed to inflate the polysilicon and cause plastic deformation. Holes are plasma etched in the polysilicon at access the etch channels, and the sacrificial PSG is moved to release the comb drive within the polysilicon cavity. The etch holes can then be sealed with either silicon nitride or polysilicon, although this sealing process step was not demonstrated here.
These processes produced mechanically free electrostatic comb drive microstructures covered by silicon nitride or polysilicon shells. The highest point of the silicon nitride shell above the silicon nitride substrate is 10 um, while the highest point of the polysilicon shell is 20 um. The microstructures enclosed in the micro shells could be moved mechanically with a probe tip after breaking away part of the microshell, but they could not be electrostatistically actuated. The silicon nitride micro package was translucent enough to permit observing the mechanically driven motion of the comb drive, while the polysilicon microshell was opaque, so motion of the comb drive shuttle was confirmed by observing motion where part of the polysilicon shell had been removed. These results are important steps in the process development of a hermetically sealed resonating electrostatic comb drive.