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 CMOS.
"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