MEMS: Beyond 'Borrowed' Packaging

During the past 10 years, microelectromechanical systems (MEMS) have largely “borrowed” packaging technology originally developed for military high-reliability applications. While this packaging has worked well for MEMS, the associated high cost — as much as 70% of the price of a single MEMS device — has not. Now, MEMS packaging is evolving along with advances in wafer-level packaging (WLP) and microfluidics.

The MEMS packaging market is a highly fragmented one that includes devices such as accelerometers, automobile airbag systems, gyroscopes, inkjet cartridges, digital projector chips, micromirrors and other kinds of sensors. And although the cost of custom packaging for MEMS is often proprietary information, Jim Walker, vice president of semiconductor manufacturing for market research firm Gartner Dataquest (Stamford, Conn.), estimates the global MEMS packaging market reached a little more than $2B in 2006. “I expect to see WLP grow substantially within the next few years, and believe that will lower the overall cost of MEMS devices,” he said.

One of the greatest challenges associated with MEMS is that device-specific packaging is required. “The package is at the heart of high tech,” explained Ken Gilleo, president of ET-Trends (Warwick, R.I.), a consulting and IP firm focused on emerging technologies and device packaging. “It is influenced by both the device it is packaging and the application.”

Ceramic is still the most common approach for MEMS packaging, although it's relatively expensive. “Advancements in low-temperature co-fired ceramics and in manipulating ceramics to come up with better compositions have reduced full-hermetic ceramic cavity MEMS package prices below 50 cents,” Gilleo said. “Although today's preferred package for MEMS is ceramic, I think that preference will change significantly during the next 10 years, driven by fluidic MEMS.”

Near-hermetic packages are another approach to MEMS packaging. These high-performance plastic cavity packages lack full hermeticity, however, which is usually desirable for MEMS packaging.

WLP enables creation of a package while MEMS devices are still in wafer form. “One of the most challenging issues associated with MEMS packaging is self-contamination,” Gilleo said. “WLP allows capping and sealing of the MEMS device in an ultraclean fab environment before sawing it. Analog Devices has led the way in wafer-level capping, and Motorola, Texas Instruments and STMicroelectronics also cap MEMS inertial sensors. Having a cap in place allows traditional plastic packaging and overmolding the capped device for a package that costs only 5 or 7 cents. This is the quad QFN, the polymer version of the old ceramic leadless chip carrier.”

Gilleo expects to see today's capping process evolve toward caps with through-vias and pads on the outside, so that with wafer-level you can saw both the cap material and MEMS chip simultaneously, yielding a chip with little wire bond pads on it. Another method, Gilleo suggests, is to create microvias and bring the MEMS pads to the bottom, put a regular cap on it with a little cavity, and bond it. “Companies doing 3-D stacking are developing nice through-via processes to create microvias in silicon, then plating them to allow passive stacking of the chip,” Gilleo said. “I expect companies will be using this MEMS via process to cap MEMS within the next two years. This will be done at the wafer level, hermetically, without any secondary packaging.”

The entire MEMS packaging process can be done at the wafer level. (Source: Ken Gilleo)

For pressure sensors, a popular MEMS product, Gilleo points out a couple of evolving packaging choices. You can let the measuring gas or fluid contact the MEMS device and build a protective membrane over it to expose only that part of the package. Or you can build a package with a flexible surface of metal, plastic or ceramic, so the device is well sealed, but is still able to transfer pressure through its package.

Many companies are also exposing MEMS devices directly to the environment. “You can build a MEMS sensor that can survive exposure to its environment or to motor oil or to Freon and still survive, so we're starting to see this area of MEMS packaging evolve too,” Gilleo noted.

Microfluidic MEMS advancements, such as biosensors and lab-on-a-chip, may bring the most dramatic change yet — requiring sophisticated packaging that is not available off-the-shelf today. “When we take an entire laboratory full of equipment and gadgets and reduce it to a chip using fluidic MEMS, that will be a real big deal,” Gilleo said.

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