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Sept. 5, 2002 — MEMS devices such as accelerometers, sensors and wireless or optical components usually need to be tightly sealed against dust, water vapor, gases or other contaminants.
Yet packaging MEMS remains an expensive final step that is typically outsourced. For complex devices like micromirror arrays in digital projectors, packaging can run as high as 80 percent of the cost. Even in simpler sensors it can make up half the cost.
Ziptronix Inc. in Research Triangle Park, N.C., hopes to seal its own success with a simple, low-cost process for hermetically encapsulating entire MEMS wafers inside a foundry’s clean rooms.
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This “wafer-level” process entails fusing a second silicon wafer with cavities etched into it over the wafer of MEMS. That way, when the wafer is cut into individual devices, there is no risk of silicon dust or water from this “dicing” procedure ruining some of the MEMS devices.
Ziptronix’s process also produces a thin package that, according to CEO Doug Milner, enables MEMS devices to be more easily interconnected and integrated directly onto a computer chip.
Jim Walker, a principal analyst with Gartner Dataquest, said Ziptronix’s ability to package an entire wafer of MEMS is one of the keys to reducing their cost.
While other companies such as Amkor Technology Inc. in West Chester, Pa., and EV Group of Austria offer wafer-level packaging, Walker said Ziptronix’s approach is unique because it doesn’t use exotic chemicals or require conditions like high temperatures, pressure or vacuum.
The company recently revealed a room temperature bonding process that atomically locks together a wafer surface and silicon cover using chemicals and equipment that it says most MEMS facilities have on hand.
Milner said that the process should significantly reduce costs. Encapsulating a wafer’s worth of complex devices individually now costs as much as $50,000. Sealing them with Ziptronix’s method while they’re all on the wafer could cost as little as $1,500 to $2,000.
“We believe the low cost of the technology will open up new markets for MEMS devices, especially for price sensitive products such as mobile phones,” Milner said.
That the approach works at room temperature is also important because certain MEMS, such as those in radio frequency devices, contain metal layers that would melt in high temperature packaging processes.
According to Milner, Ziptronix’s ability to integrate MEMS with IC chips is one of the company’s key advantages over its packaging competitors. Formed in 2000 as a spinoff of the Research Triangle Institute, Ziptronix ultimately hopes its bonding technology will enable chipmakers to build 3-D semiconductor devices with stacks of components such as memory, logic, MEMS and processors sandwiched on top of each other.
Gartner’s Walker described this emerging world of stackable silicon as “system-in-a-package.” In contrast to a “system-on-a-chip” approach that would integrate components spread across a flat silicon surface, Walker said, companies could stack off-the-shelf chips to create customized computer cubes that would take up less space than their flat counterparts.
But it is in packaging MEMS that Walker said Ziptronix really shines today.
Andrew Swiecki, vice president of marketing and sales with MEMS maker Corning IntelliSense, only recently heard of Ziptronix but agreed there was a great need for low temperature, inexpensive packaging techniques for certain MEMS devices.
However, he said he’d want to know more about how the technology had been tested and who Ziptronix was working with.
While Milner declined to name specific customers, he said that the company was already at the prototype or engineering stage with many MEMS makers. “We’re in the process of evaluating and discovering how we can provide solutions for specific applications,” Milner said.
To date, the company has also produced prototypes of an encapsulated MEMS pressure sensor and a 3-D imaging chip.
With respect to testing, Milner said that independent labs had found that the company’s bonding technology had exceeded the industry standard for hermetic sealing. The company said that the bonds have also passed the Motorola pressure/Temperature cycle test, an automotive industry standard.
Milner noted further that the chemicals used to create the strong, covalent bonds have been tested to ensure that they don’t interfere with the tiny moving parts critical to how a MEMS device works.