AP Interview: Ann Witvrouw, manager, MEMS integration research program, IMEC Advanced Packaging talked with IMEC’s Ann Witvrouw about recent developments in MEMS and CMOS processing, and how the technology could lead to advanced packaging techniques across the spectrum of MEMS structures. Witvrouw explained that micro-mirrors developed on a CMOS wafer, in conjunction with IC functions, are based on a process that has undergone three stages: proof-of-concept; a first product, which was a gyroscope; and application to a range of products within the scope of MEMS, proven by the micro-mirror development. IMEC’s process involves growing hydrogenated microcrystalline silicon germanium (μc-SiGe:H) micro-mirrors from a multi-layer process atop a silicon wafer, Witrvrouw explained. Hydrogen encourages crystallization of the SiGe particles on a CMOS wafer, post-processing. The IC is already formed when the MEMS is grown. “SiGe grows in very small particles, and this enables development of smaller mirrors and hinges,” Witvrouw said. The mirrors are flat and uniform, sized from 7.5 to 16 μm2, and have submicron torsional hinges. All processing temperatures are maintained at or below 420°C, which creates compatible environments for CMOS and MEMS. Witvrouw believes this technology is on the path to becoming a generic process, replacing wafer bonding in MEMS-IC applications and opening markets for smaller, less-expensive MEMS packages. Now that the poly-SiGe technology has been integrated into two different MEMS processes – a fully functional on-chip gyroscope and actuated micro-mirrors grown atop IC drivers – IMEC will consider the possible implications for processing CMOS and MEMS compatibly. Similar processing could generate MEMS-scale packaging, and multiple use of the SiGe MEMS module production process could lead to stacked 3D MEMS structures. The key is maintaining a reproducible process below the 420°C thermal budget.