Permeable Polysilicon Etch-Access Windows for Microshell Fabrication

The use of permeable polysilicon etch access windows for the removal of underlying sacrificial PSG has been implemented in order to fabricate low-pressure, hermetically sealed micro encapsulation shells. Permeable polysilicon provides a dense array of etchant access passages (each less than 100 nm in diameter) that allow HF to penetrate and remove the underlying PSG. Subsequent to the etch process, the same permeable polysilicon window acts as a virtual substrate during the sealing process. Therefore, in contrast to previous microshell fabrication methods, etch-access is through the top of the shell. This eliminates the geometry constraint caused by perimeter located etch holes and vastly reduces etching times. Shells 3 pm high and as wide as 1,000 pm have been cleared of sacrificial PSG using concentrated HF in less than 120 sec. In addition, low-pressure hermetic sealing has been demonstrated with low-stress silicon nitride, with less than 100A of nitride depositing inside of the shell. By reducing exposure time to HF, minimizing internal sealing film deposition, and eliminating the geometry constraints due to etch hole placement, this process is attractive for applications including not only encapsulation but also the formation of surface micromachined fluid passages.