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When the first CMP systems appeared in chip fabs in the mid-1990s, many engineers were aghast at the possibility of subjecting their precious wafers to the perils of grinding and polishing with slurries. Since then, CMP has become a key enabling process of record in the relentless pursuit of the semiconductor roadmap.

In the MEMS world, however, CMP is far from being a POR. But as a paper presented at the recent Advanced Semiconductor Manufacturing Conference showed, CMP could soon enter the MEMS production mainstream.

A Finnish group from VTT Microelectronics and Okmetic teamed with 3M's German unit to investigate the use of a fixed abrasive pad to smooth and planarize bulk micromachined substrates. It turns out that many advanced MEMS structures and processes—such as thick-film SOI, buried metal lines, capping of bulk micromachined silicon or deposited films, backside polishing for double-side processing, and surface preparation for bonding of different substrates—might benefit from CMP.

But conventional chip-style CMP doesn't necessarily work for MEMS. Although both device lines require angstrom-level surface roughness, the IC polishing process is not designed for tall or big structures and is suited for removing a pattern's shape, not maintaining it. The research team's solution? A slurry-free, fixed-abrasive-pad approach.

Their test results bode well. The roughness was comparable to conventional CMP, the pattern geometries held up even better than the norm, and the abrasive-polished wafers were bonded successfully to the silicon substrate. Although further investigations are in order, the fabrication of MEMS should get smoother—on the surface anyway.

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