Packaging Challenges for Micro-Optoelectromechanical Systems

What Feynman introduced was the industry that now is termed microelectromechanical systems (MEMS). This involves any microscale structure that is designed to have a moving part. Cahners In-Stat Group estimates the total market for MEMS devices in 2000 at $200M, expected to grow to $1.5B by 2005. Popular MEMS applications may be dwarfed by the growing opportunities for micro-optoelectromechanical systems (MOEMS).

The first MOEMS device, introduced by Texas Instruments (TI, Dallas) in 1992 with 840 micromirrors, has grown today into 1280 × 1024 mirror arrays built on top of CMOS SRAM chips. Larry Hornbeck, a TI Fellow and inventor of the micromirrors, describes the quality of the images they produce as "far beyond what HDTV can do."

	Silicon Light Machine's 6-inch GLV wafer with precision glass windows soldered directly to the wafer. (Source: Silicon Light Machines)

"The SLM image was the best I had ever seen — better than my home theater and better than the best art theater," said T.J. Rogers, CEO of Cypress and a home theater enthusiast.

With volume applications of these MOEMS chips growing, robust manufacturing is a must. While the fabrication of the actual MOEMS wafer leverages standard silicon processes, the packaging infrastructure for MOEMS must be reinvented. There are three new packaging steps required for MOEMS devices that challenge the existing packaging infrastructure:

• Release: removing an inner layer to allow mechanical motion of the upper layers just prior to sealing.
• Hermetic sealing: eliminating any residual moisture to prevent stiction of moving parts.
• Optical system integration: precision chip alignment with a high-quality glass window seal.

In the case of the TI DMD chips, photoresist is the release layer. This is kept on the wafer through sawing. The chip is die bonded in the ceramic package using precision spacer balls in the adhesive. The photoresist is removed in an oxygen plasma ashing step. A film of fluorinated polymer, with one quarter the stickiness of Teflon, is vapor deposited on the exposed DMD to prevent stiction. A precision ground glass window with getter strips along the edges is fused to a kovar seal ring and welded to the package.

In the case of the SLM GLV chips, the release layer is polysilicon, and it is removed in a XeF₂ vapor phase etch process. Once released, precision windows with a metalized seal ring are brazed onto the GLV wafers in a controlled atmosphere furnace. "The atmosphere determines the way the ribbons move," said Josef Berger, vice president of engineering at SLM. "The best package is no package at all. A wafer with sealed chips is the ultimate in minimum packaging." After the window seal, the wafer is diced and assembled conventionally.

These early devices have pioneered the packaging technologies that will be essential for the widespread use of MOEMS technology. There are now more than 20 companies that have announced their intent to produce MOEMS-based optical switches for all-optical switching networks. This promises to be an even bigger opportunity than the display market.

However, limitations remain because of packaging challenges in cost, reliability and available infrastructure. As Berger explained, "Packaging technology lags behind the optical MEMS technology, yet it determines the success and cost of the devices."