ST plots wireless MEMS roadmap
ST has been struggling on the digital side of its business in recent years but its MEMS (microelectromechanical systems) business has been a resounding success, propelling the company to a leadership position with annual sales of about $900 million in 2011 and an emphasis on ninertial and membrane-based MEMS.
The system-in-package (SIP) approach adopted, whereby the company uses different manufacturing processes for sub-elements, is one reason for that success, Benedetto Vigna, executive vice president and general manager of the analog, MEMS and sensors group, said. It already allows ST to make smart MEMS that include a mechanical die, an analog signal conditioning and calibration die and, on occasion, a digital interface die with that die sometimes including a microcontroller. The next step is to add an RF transceiver.
"That’s where we are aiming to and exactly where we are going," said Vigna. "We are looking at applications around the human body and in the home. There are clear use cases and we are working intensively on that program."
When asked if such wireless sensors nodes would be for medical applications, Vigna said it would be more in fitness and wellness applications, which do not need years of clinical trials to be licensed for use. He added that tire pressure monitoring systems would be another key application for MEMS components that could communicate wirelessly, although ST is not in this market at present.
ST’s high volume MEMS making is for accelerometers and gyroscopes based on capacitive motion sensing and environmental sensors based primarily on moving membranes, such as pressure and microphones.
Adding RF
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 CMOS.
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.
The fifth element
The fifth element is a little further out. That is the integration of gas sensors which should come in 2014.
The gas and humidity sensors are a natural fit with ST’s family of membrane-based environmentally connected sensors. Gas sensors, like pressure sensors and microphones, need a physical connection to the atmosphere explained Vigna.
Vigna expects ST to have a pressure, temperatureand humidity sensor in one package in 2013 with various types of gas sensor to be added later. "The gas sensor will be added as another die or potentially the sensing layer can be integrated with the humidity sensor. The gases that customers are looking to detect include CH4, CO, methane, NO."
But overall it is the system-in-package approach that allows this incremental progress at minimum cost, maximum flexibility and best time to market, Vigna concluded.
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