Portland, Ore. -- To hear vendors tell it, the only right way to do microelectromechanical systems is "my way." So, what method of fabricating MEMS is really best?
Which is preferable: MEMS-first or MEMS-last? Are proprietary processes best, or should MEMS methods be able to run on any foundry line? Are different approaches better for different applications?
All vendors agree that the holy grail is seamless integration of MEMS structures onto the same CMOS chips as the circuitry to which they interface. But only one MEMS startup claims to be there already, though that vendor adds a step to the standard CMOS fabrication process.
"Ours is the only MEMS that is not just compatible with CMOS--we are CMOS," said Davin Yuknis, vice president of marketing and product management at Akustica Inc. (Pittsburgh). "The traditional way of doing MEMS is a two-chip approach: the MEMS transducer and an ASIC that holds its electronics. Then there is CMOS-compatible MEMS, where you first fabricate a CMOS wafer, then deposit MEMS structures on top of the wafer. Then there is our technology, which uses the metal and oxide layers in the CMOS itself to create the movable MEMS structures."
Akustica developed the first MEMS microphone to integrate electronics and digital output (search www.eetimes.com for article ID: 180207742). Knowles Electronics LLC (Itasca, Ill.) and Sonion MEMS A/S (Roskilde, Denmark) both have MEMS microphones, but only Akustica has a single-chip solution that users can run on any CMOS line, since it requires only one post-CMOS processing step to etch away sacrificial material.
The fabless vendor says it has successfully sampled parts from nine foundries using 11 design rules, from low-cost 0.6-micron processes using aluminum interconnect to high-performance 0.18-micron processes using copper interconnect. The company has one customer and two design wins so far using its portable CMOS fabrication technique.
On the other hand, old-timer Analog Devices Inc. has shipped more than 250 million devices over the past 15 years using its proprietary iMEMS approach to making single-chip motion sensors by integrating surface-micromachined mechanical elements and BiMOS circuitry for on-board signal conditioning. Likewise, Texas Instruments Inc. invented its MEMS digital micromirror device (DMD) in 1987 by adding a movable mirror atop a finished CMOS wafer. TI has shipped more than 10 million DMDs since 1995.
Besides ADI and TI, a third large MEMS chip maker--Bosch Sensortec (Reutlingen, Germany)--annually ships more than 100 million MEMS devices based on its deep-reaction ion-etching technique. Bosch has licensed the process to MEMS startup SiTime Inc. (Sunnyvale, Calif.), which trumpets it as MEMS-first. SiTime calls MEMS-first superior because it uses extremely high temperatures to perform very deep etching.
SiTime said the bane of MEMS oscillators is moisture, which can condense on the tiny resonator during operation, thereby changing its frequency in an uncontrollable way. The MEMS-first process solves that problem by sealing the MEMS cavity in silicon at extremely high temperatures, making it impossible for moisture or stray gases to be sealed in. MEMS-last waits until the very last step to put a glass cap on a wafer at relatively low temperatures.
"The difference between MEMS-first and MEMS-last is that we can take advantage of high-temperature processing to create an ultraclean environment where all of the contaminants are removed, after which a silicon seal is placed on it," said SiTime's strategic-alliances manager, Joe Brown. "A MEMS-last process is subject to variations in moisture and other contaminants that are sealed in with the vibrating resonator, because you have to use some sort of low-temperature adhesive agent--glass or epoxy--which introduces process variations."
Only Bosch and SiTime use these high-temperature techniques during MEMS fabrication, putting most other MEMS vendors in the MEMS-last camp. To compensate for any stray moisture or gases that get sealed inside the cavity, MEMS-last players add "getter" films inside the cavity that immobilize stray moisture.
But promoters of MEMS-last, such as Discera Inc., maintain that MEMS-first solves a problem with high temperatures that they do not have to deal with at all--namely, burning off the caustic acids required by Bosch's process to create the deep trenches characteristic of MEMS-first.
"We believe that MEMS-last is best, because the relief step, where you etch away sacrificial material, is the most important step," said Venkat Bahl, vice president of marketing at Discera. "By saving etching for last, we don't have to go to high temperatures to clean out the cavity, because we don't have to etch deep trenches with hydrofluoric acid and then heat it up to high temperatures to get rid of the etchant."
The proof is in the pudding, Discera insists, saying its MEMS-last achieves the industry's lowest measurable moisture sensitivity rating--MSL1--by using an industry-standard getter to immobilize moisture or gases trapped in the MEMS cavity.
The newest kid on the block, Silicon Clocks Inc. (Fremont, Calif.), argues that customers do not really care whether the process is MEMS-first, MEMS-last, MEMS CMOS or even MEMS at all. What they do care about, Silicon Clocks founder Andrew McCraith said, is high performance, flexibility and low cost.
Indeed, Silicon Clocks says its first oscillator chip, due out next year, will not even use MEMS but will instead increase the current limit on quartz crystal oscillators from 150 MHz today to as high as 650 MHz--a frequency now attainable only with expensive surface-acoustic-wave oscillators (see story, below). Silicon Clocks does plan to introduce MEMS oscillators, but not until it has squeezed every last drop of utility out of quartz crystals. The company's proprietary silicon germanium MEMS process runs at temperatures low enough for the oscillators to be added to standard CMOS chips after fabrication.
Advanced Diamond Technologies Inc. (ADT), a spin-off from the U.S. Energy Department's Argonne National Laboratory, also has a low-temperature process for adding diamond-film MEMS to standard CMOS chips after fabrication. ADT, working with MEMS pioneer Innovative Micro Technology (Santa Barbara, Calif.), plans to deliver its first integrated diamond-MEMS-on-CMOS parts to the Department of Defense in 2007.