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Physical sensors drive MEMS consumerization wave (Part 2)

Posted: 08 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:accelerometers? MEMS? pressure sensors? micromachining? SiP?

For accelerometers and gyroscopes, ST pursues the SiP approach with a dual chip solution (THELMA Sensing Element and CMOS controller) in a single package. The THELMA micromachined mechanical element, sensitive to inertia or to Coriolis' force, and the analog or digital controller chip can be assembled in two equivalent configurations: side-by-side or stacked (Figure 6).

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Figure 6

In the SiP approach, the micromachined sensor chip translates the acceleration into a differential capacitive change and an interface IC chip converts these small capacitance changes (in the range of few atto-farad) into an output signal, in an analog or digital format. Micro-machined structures are similar to those shown in Figure 1.

Moreover, the SiP approach enables a faster development of new motion sensors like multi-axis gyroscopes. In fact, mechanical and electrical designers can re-use some of the blocks already designed and qualified for multi-axis accelerometers. Gyro-specific mechanical part and electrical IC (Figure 7) can use the same technology platforms already in production for other multi-axis accelerometers. Companies pursuing the SiP approach can really play the LEGO game offering the customer the fastest and most inexpensive solution.

ST can pick any couple of two elements from a wide collection of chips and realize fastly the final product, even adapting the pinout thanks to the LGA package configuration.

Figure 7: LEGO Game rule to make a Motion Sensor: pick one mechanical element on the left side and one electrical chip on the right side, combine them in a single package you want and you get a complete accelerometer or a gyroscope.

Tapping pressure sensors
Pressure sensors have long been in service measuring pressure and flow in a variety of industrial, automotive and medical applications. Micromachined pressure sensors are fabricated in Silicon with the physical sensing mechanism being either a variable resistance or a variable capacitance. The fabrication is either "Bulk" or "Surface" micromachining or a combination of the two. Standard silicon substrates or more expensive Silicon-On-Insulator substrates are equally used as a starting point.

The variable resistance type uses the piezo-resistive nature of silicon to convert stress in a small diaphragm into a very small resistance change. And bulk micro-machining technology is the preferred technique. The variable capacitance type uses two parallel plates. One is fixed and the other is a thin diaphragm moving in direction orthogonal to the plane of the chip. A very small change in capacitance then occurs between these two plates. Surface micromachining methods are preferred for this class of pressure sensors. The interface circuitry that converts resistance to voltage or capacitance to voltage can be integrated at chip level or at package level, as for motion sensors. But also in this case the SiP solution guarantees faster commercialization time and higher flexibility for the customer.

XSA-0720S

Figure 8

No ideal manufacturing process exists, but each company follows the micromaching approach that exploits better its experience and its manufacturing assets. Nonetheless the plethora of processes running in different semiconductor fabs the high-volume consumer market players had to accept the compromise related to price, size and performances of the pressure sensor. And this compromise limited strongly the wide adoption of this sensor in the consumer market.

Only recently some companies found a viable solution for the consumer market. In particular, ST VENSENS technology enables the manufacturing of a "Full-Silicon" small (0.8mm x 0.8mm), thin (0.3mm) and inexpensive pressure sensor. The performances of the "Full-Silicon" sensor are independent on the specific package, that for classical bulk-micromachined solutions represents the biggest part of the sensor cost. And this opens the door of the consumer market definitely.


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