ST uses two silicon die to implement the MEMS function: one for the MEMS element and a second for analog amplification, calibration and
conditioning of the signal. There is a third piece of silicon which provides the cap to the MEMS element.
ST prefers to use what Vigna calls a "stupid" cap with little or no functionality and therefore little or no processing. The caps seals the moving parts of the MEMS using a glass frit or metal-to-metal bonding. It may not be as elegant as using the front side of a CMOS wafer to cap the MEMS but it provides lower costs and better yields, said Vigna.
Vigna admits that ST has been thinking about the possibility of using the backside of a CMOS wafer to cap the MEMS element. "We've done some experiments. We haven't gone ahead so far."
The system-in-package allows the same MEMS transducer element to be produced in volume and then matched with different analog
ICs to create different MEMS products and even to perform customer specific variants, said Vigna.
Today there are already a lot of MEMS components with digital content added but again Vigna stresses it makes sense to choose the best process for each element. So the mechanical MEMS device is made in a MEMS-specialized 1-micron process, the analog conditioning IC is likely to be in 130-nm CMOS while digital ICs containing a microcontroller and memory would probably be in 90-nm
Click on image to enlarge.
A MEMS system sandwich contains a full low-power microcontroller on the bottom, a MEMS motion-detector in the middle and an ASIC signal conditioning companion chip on the top in a 3-mm by 3-mm assembly. Source: STMicroelectronics
"The fourth element could be, say the SPIRIT-1 die," said
Vigna. SPIRIT-1 is a 90-nm low power radio IC recently introduced by ST.
The radio is intended for sub-1-GHz applications with a programmable
data rate from 1 to 500 kbps. Vigna adds that ST will likely integrate
digital and RF in the same CMOS process keeping the assembly at 3 active
die plus a "dumb" silicon cap.
Vigna says ST will have such
wireless MEMS in the market before the end of 2013 as the company is
already selling MEMS plus RF transceivers at the board level and is
working with customers on domestic and body-worn applications.