Wednesday, January 28th, 2009
Silicon Clocks, the developer of custom semiconductor timing solutions that has reinvented itself as a “custom product development house” with a new CEO, has announced a new product based on its CMOS-MEMS (CMEMS) embedded approach. CMEMS-ZeroThermal passive temperature compensation resonators exhibit comparable temperature stability to quartz crystals, while drastically simplifying oscillator design, and reportedly reducing power and circuit size. Company CEO Didier Lacroix told BetaSights to expect more CMEMS technology announcements in the near future.
Silicon Clocks has developed a <400°C surface-micromachining flow to create SiGe MEMS, so that these structures may be embedded on chip directly over CMOS metallization (see Figure). The flow starts by making connection to the last level of CMOS metal, much like a redistribution layer (RDL) module:
MEMS resonator over CMOS (source: Silicon Clocks)
1) Connection to CMOS,
2) Deposition of sacrificial Ge,
3) Deposition of 2µm SiGe,
4) MEMS lithography,
5) Sacrificial material removal, and
6) Capping process.
“When you add it all up it’s seven new masks in total; five for the MEMS and two for the cap,” explained Lacroix. “Really what’s different is depositing the SiGe instead of poly.”
Customers could do these additional steps at their existing fab line, or could send wafers post-CMOS to a different line depending upon the volume requirements. Other non-quartz-crystal timing companies—such as SiTime and MobiusMicrosystems—offer stand alone single-chip but not embedded solutions. This technology has reportedly been demonstrated across a wide range of resonator designs from the low kilohertz to several hundred megahertz. Silicon Clocks did their alpha and beta development at SVTC. “SVTC has the capabilities for production, so we consider them to have been our beta,” said Lacroix. SVTC announced major investment last year in 200mm wafer tools to be able to provide a MEMS pilot line, including many tools from SUSS and a Lam etcher.
First-generation MEMS oscillators typically showed 15-30ppm/°C variation with temperature, compared to quartz crystals in the 0.25-1.0ppm/°C range. Matt Hopcroft did excellent Ph.D. work on MEMS resonators, and his 2007 thesis is very worth reading. “Until now, MEMS technology could not compete for ultra-low-power sleep clock applications,” said Lacroix. The company’s completely passive (zero power; no dedicated electronic circuitry) mechanical temperature compensation method is what allows this MEMS technology to match quartz-crystal performance. –E.K.
Tags: clock, CMOS, crystal, integration, IP, MEMS, resonator, Si, SiGe