November- December 1998
eMail the Editor
Tracking Down MEMS at Surface Mount International
By Harvey S. Miller
—Contributing Editor
At the recent Surface Mount International show (SMI '98), Tink Dempsey Young, vice president of marketing and sales for Multichip Assembly Inc., showed me some of that company's repertory of packaged assemblies prototyped in its San Jose, Calif. facility.One example was a MEMS sensor packaged with an RF—signaling die in an 8—lead SOIC—now used in GM Europe automobiles to transmit low tire pressure information to a dashboard indicator.
Two days later at another meeting, an IEEE CPMT (Components, Packaging, and Manufacturing Technology Society) Technical Strategy Workshop in San Diego, chaired by Dr. Rao Tummala, I became really intrigued about MEMS.
Karen Markus, director of the MEMS Technology Center at MCNC (and soon to be a MEMS entrepreneur) addressed the need for more technology transfer from the packaging/interconnect community to our MEMS counterparts. Now, subsequent digging into present important and future fabulous applications has turned me into an even bigger MEMS enthusiast than Tink and Karen—if that's possible.
| MEMS and Application: Examples of Current Products | |||
| Company | Year | Device | Application |
| ADI | 1991 | Accelerometer | Now used in air bags. Future: active suspensions, gyroscopes, tuning forks, microphones and filters |
| Texas Instruments | 1996 | Digital Micromirror Devices | Used for presentation projection. Future: TV and other displays replacing CRTs |
| Form Factor | 1995 | Micromachined Wire Probes | Vertical wafer probing at 150µm pitch |
| Xerox Palo Alto Research Center | 1998 | Micro—imaged Wire Probes | Wafer probing at 6µm pitch |
| Affymetrix | 1996 | Gene Chip | DNA probes to identify disease mutations |
MEMS are Micro — Electro — Mechanical Systems. The term serves as a generic definition. But there are two alternative/complementary approaches to fabrication, which define them more precisely:
Generally, in the U.S., photolithography, sputtering, ion etch, and other semiconductor fabrication technologies are applied to a substrate, such as silicon, glass, stainless steel, etc., to create structures whose dimensions are usually in the 1 micrometer to 1 millimeter range. This compares to semiconductors in the nanometer range, an order of magnitude finer—and to chemical milling in the 3 to 10 millimeter range—one to three orders of magnitude more gross.
The other fabrication technique is by micro machining.
Photolithography, however, can produce far finer dimensions than micro machining which are shown in the table, compare Form Factor's 150µm to Xerox PARC's 6µm probes.Future applications stretch as far as the imagination. Nature, including the human body, provides models of tiny sensors and transducers by the thousands. Tiny MEMS will emulate and perhaps improve on them.
MEMS fabrication tools use equipment that is obsolete for semiconductor fabrication; essentially these tools are free. MEMS offer easy business entry opportunities to world and U.S. regions that may be too late for semiconductor fab investment. For example, Ohio is a major center of activity, and MEMS are a natural replacement for mature electromechanical device industries. MEMS are a natural for ingenious entrepreneurs with little capital, wherever they are.
Packaging MEMS must be done very carefully; these are tiny, fragile devices. Close coupling with semiconductor input—output devices is important. They are embedded in larger systems, another argument for package miniaturization. CSP packages that don't expose MEMS to underfill would be appropriate. Conductive epoxy bumps, which avoid both solder and wire bonding temperatures, are a natural.
Harvey Miller is president of KirkMiller Associates, Palo Alto, Calif. Contact him at 650.327.2029, fax 650.327.2360.