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Understanding sensor fusion, MEMS and 10-DoF solutions

Posted: 27 Sep 2012     Print Version  Bookmark and Share

Keywords:MEMS  sensors  3D-accelerometer  sensor fusion  10-DoF 

The examples shown here are representatives of the current inertial sensing products on the market. If one starts looking into the next generation products things look even brighter. As already mentioned, a further integration is already taking place and the form factor is shifting from discrete accelerometer and gyro devices to 6-DoF combo products on a single MEMS chip. This is possible because of the advancements in MEMS technology and packaging. One excellent example of this is STM's announcement of the use of TSV (through silicon via) technology in MEMS packaging. Using TSV through the CMOS IC chip enables active capping and elimination of the bond wires at the same time (figure 4). That way the new 6-DoF combo solution (accelerometer + gyro) is scaled down in all three dimensions (true 3D-scaling down), reducing the cost and improving performance. Very impressive indeed!

Figure 4: Active Capping and use of TSV by STM.

Barometric sensing
Let us turn now to barometric sensing. The key part of a barometer is a pressure sensor. The crucial part of every pressure sensor is a diaphragm. There are several variations of MEMS technology on how to make a silicon diaphragm. In part, this is related to underlying principle of operations of pressure sensor. Generally, two basic principles are exploit: piezoresistive effect and capacitive method. In the case of the piezoresistive effect, there is a piezoresitor embedded into a diaphragm. When the diaphragm moves it creates a change in the piezoresistor, which in turn is correlated to pressure. In the case of capacitive sensing, the diaphragm itself represents one of the electrodes of the capacitor, and the change in capacitance is correlated to the change in pressure. Here, we will discuss several examples of pressure sensors that represent the main stream in barometric sensing.

Figure 5: Piezoresitive pressure sensor made by using wet anisotropic etching.

Figure 5 shows one of the pressure sensors and its diaphragm. The diaphragm is made of silicon crystal by using wet anisotropic etching of the silicon wafer with the (100) orientation. This approach is known as a bulk-micromachining. The etch rate of the silicon crystallographic planes with (111) orientation is much slower than the surface with (100) orientation which leads to the unique sloped truncated, and pyramidal shape beneath the diaphragm. The piezoresistive resistors are placed at the edge of diaphragm. The MEMS die is completed by bonding the MEMS wafer to the holder wafer whereby a sealed cavity (reference chamber) for absolute pressure sensor is created. This approach is used by Honeywell and Freescale.

Figure 6shows another pressure sensor with a different diaphragm. In this case, a combination of dry and wet etching with monocrystaline silicon growth, CMP process, and sacrificial etching is used. It leads to the formation of a silicon diaphragm with a sealed cavity at the top of the wafer without the need for wafer-to-wafer bonding. This technique leads to a smaller MEMS die compared to the die made by using wet anisotropic etching. The piezoresitive elements are again placed at the edge of the diaphragm where the stress is the largest. This approach is used by STM – they call the process VENSENS.

It should be pointed out that the final barometer product includes two dies in a single package. One is the pressure sensor MEMS die and the other is the CMOS IC die. The two dies are typically packaged side-by-side. For example, STM and Freescale use the LGA package for their barometers. Naturally, there is a hole in the LGA package to allow ambient pressure to reach the pressure sensor. The CMOS IC is an ASIC that performs signal processing and also provides a digital output that is compatible with I2C and PSI protocols.

Figure 6: Piezoresistive pressure sensor made with a grown-monocrystalline Si-layer.

Finally, an example of a pressure sensor based on capacitive sensing is shown in figure 7. In this case, a surface micromachining is used in combination with a thin polysilicon layer (about 2 um) and sacrificial etching. The sealed cavity and the thin diaphragm that also serves as one of the plates of the capacitor are seated at the top of the MEMS die. This technology is essential for high pressure sensors. The capacitive pressure sensor is at the core of sensing solutions for TPMS (tyre pressure monitoring system). An excellent example of this type of product is a Freescale's TPMS solution that includes a capacitive pressure sensor and CMOS ASIC on a single chip, an RF-transmitter chip for wireless data transmission, and an MCU chip, all in a single package (all together three chips). This product is the darling of the automotive industry and is used in both cars and trucks for monitoring tyre pressures.

Figure 7: Capacitive MEMS pressure sensor.

The basic aspects of sensor fusion have been described in this paper including the 10-DoF solution. Sensor fusion is a powerful concept that demonstrates how the whole is greater than the sum of its parts. Then, the focus turned to motion, inertial, and barometric sensing since they are the key components of 10-DoF solutions. The select MEMS sensor examples shown here are representatives of real products. They illustrate well that there is no lack of inventiveness when it comes to MEMS technology. Some MEMS solutions are cautious, some are more radical. Either way, they deserve our full attention because the products made by MEMS technology are reliable, and cheap, and they fuel new applications that were unthinkable only a few years ago. One can state with certainty that MEMS technology and MEMS sensors are essential to sensor fusion and the 10-DoF solutions as well as to the next generation of m-DoF solutions.

About the author
Lj. Ristic is Managing Director at Petrov Group LLC.

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