Faced with the semiconductor shortage of 2020-21, semiconductor manufacturers have had to maximize utilization of their existing fab assets, including 200mm fabs. As a result, according to SEMI, capital spending for 200mm fabs will reach $4 billion in 2021 – and, for the first time, some of these new 200mm fabs will be producing MEMS.
This is in stark contrast to past years when MEMS manufacturers relied almost exclusively on retired semiconductor fabs and equipment to enable MEMS manufacturing at low cost. This was convenient—not only because MEMS devices have larger design rules than semiconductors, but also because the tools could then be reconditioned and customized to best process the materials used. MEMS utilize a wider variety of substrates (silicon, glass, silicon-on-insulator, sapphire) and a more diverse range of thin films than semiconductors. Nonetheless, MEMS companies benefit from the mature characterization and know-how associated with semiconductor toolsets.
MEMS are, by nature, custom-designed devices typically developed by fabless MEMS companies, often in conjunction with universities and research institutes. When a product is designed and ready to ramp from prototype to production, the process is transferred to a MEMS foundry that has the process toolkit to fabricate the product at the required volume with high yield. This often requires dedicated yet somewhat flexible tools to ensure cost-effective manufacturing.
The basic processes that MEMS share with silicon fabs include thermal oxidation, low-pressure chemical vapour deposition (LPCVD) nitride, reactive-ion etching (RIE) and ion milling, plasma-enhanced chemical vapour deposition (PECVD), electroplating, lithography, etching, and wafer cleaning.
Some tools required for MEMS manufacturing did not originate with the silicon industry. These include vapour-phase etching (VPE) and self-assembled monolayer deposition, the latter of which coats the final MEMS device and protects it from environmental damage. Both processes compete with legacy batch wet-processing tools. Silicon deep reactive-ion etching (DRIE), the first process tool developed for MEMS, is widely used to create cavities for micromachines or to create through-silicon vias to electrically connect devices on different wafers. Many wafer-to-wafer bonding processes are used to marry ICs to MEMS or to create the MEMS package. Bonding of silicon or silicon-on-insulator (SOI) wafers uses glass frit, anodic, or alloy bonding methods.
The demand for these tools has created opportunities for companies whose specialisation is remanufacturing older products – either those still available from OEMs that require some revamping, or previously owned tools that may need repair or refurbishment, as well as hardware and/or software updates. For more on MEMS and the 200mm renaissance, download our latest white paper here.