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.
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.
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
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.
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.