RF MEMS Switches and Their Packaging Challenge

Though still in its infancy, MEMS (microelectromechanical systems) technology is more than a vision — it is a reality. Reed Business Information's In-Stat Group reported a total market volume of more than $4B in 2001 for MEMS devices, predominantly for sensors and display devices. It estimates that this market will grow to $12B by 2005. One untapped area of potential promise for this technology is in the fast-growing arena of rf applications. Although the total current market for rf MEMS devices is less than $1M, In-Stat estimates that it will grow to more than $350M in the next five years.

The use of micromachining — etching macro structures in silicon — has already been applied to fabricating integrated passives. By etching around the sides of spiral inductor elements, for example, potentially lossy dielectric can be removed and the Q of filters significantly increased. Integrated passives fabricated with micromachining will play an important role in increasing the integration level of rf devices, moving toward the goal of the single-chip rf system-on-a-chip.

But integrated passive devices alone are not leveraging the real value of MEMS technology — mechanical motion. A new type of device is just beginning to realize true potential from the marriage of rf technology and MEMS: rf switches.

Cantilevered rf MEMS switch, color-coded for mechanical displacement. (Source: Coventor)

Rf switches are currently implemented with PIN diodes or MESFETs, and are found in phase shifters, switchable filters, and transmitters and receivers for radar systems — ranging from large installations to anti-collision radar in cars and communication systems, and from base stations to cell phones. Two important terms describe the performance of an rf switch: the isolation in the open state, and the insertion loss in the closed state.

In MEMS-based rf switches, the isolation of the open state is only limited by the 2-4 fF of capacitance between the top and bottom electrodes. This results in an isolation of >50 dB at 1 GHz and 15 dB at 100 GHz. The closed state is set by the contact resistance, typically <1 Ω. This offers an insertion loss of <0.1 dB. No solid-state device can compete with these specifications, especially while consuming virtually zero power.

With their performance advantage, lower power consumption, obvious small size and potential for integration, it looks like a bright future for rf MEMS switches. But there is a large barrier to volume applications: the high cost of packaging.

An important constraint on the packaging for rf MEMS with moving parts is the requirement for hermetic or near-hermetic packaging. Any residual moisture or organic contamination will cause charging of the suspended beams, shift switching thresholds or impair reliability.

Their high surface-area-to-volume ratio makes the hermetic environment critical. Current prototype discrete units are packaged in hermetic ceramic packages. These packages increase the size and the cost, and degrade the performance of the bare MEMS switch.

MEMS-based rf switching will only realize its potential if a high-volume, low-cost packaging technology can be developed. It is generally believed this will be a wafer-scale packaging (WSP) technology.

"Everyone recognizes the performance advantages and smaller size of MEMS switches," said Arthur Morris, director of technology development at Coventor (Cary, N.C.), a leading developer of MEMS technology. "But the only solution for high-volume applications must use a cost-effective, robust packaging approach where the MEMS chip can be handled like any other chip."

Evolutionary advances in existing WSP approaches can be applied to MEMS wafers "with the addition of a few tricks," Morris said. "To integrate rf MEMS switch elements with other rf passive or active components, a near-hermetic WSP technology will be essential." He and his team expect to have WSP-assembled rf MEMS switches available for sampling in early 2003.

Early applications might be in the test equipment market, where an n × n switching matrix might be used to apply test signals for telecom, rf and radar products. As packaging costs drop, real volume opportunities in the automotive and consumer products markets may open.

For additional information on assembly and packaging, go to www.semiconductor.net/assembly