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Utilising BAW gyroscopes for inertial sensing

Posted: 18 Jul 2014     Print Version  Bookmark and Share

Keywords:MEMS  gyroscopes  bulk acoustic wave  BAW  HARPSS 

Operating at such elevated frequency allows for high-Q mode-matched operation which enables superior rotation sensitivity without the need for large drive displacement amplitudes, high-vacuum levels and force-feedback architecture. Figure 1a shows an SEM image of a 600-micron diameter BAW disc gyroscope implemented on 35-micron thick SOI substrate [1]. The device utilises a degenerate pair of in-plane "n=3" 10MHz BAW modes to sense rotation signal perpendicular to the plane of the disc (figure 1b).

The HARPSS fabrication platform
One of the most important aspects of the evolution of MEMS-based products is the symbiotic relationship between product design and production design. In the case of BAW MEMS, the performance advantages of BAW sensor designs are being realised using the versatility and scalability of a High Aspect-Ratio combined Poly and Single-crystalline Silicon (HARPSS) fabrication process. Implementing the BAW disc gyroscope design requires a fabrication platform that allows the capacitive air-gaps in both lateral and vertical directions to scale down to sub-micron range without requiring expensive nanolithography techniques.

Figure 2: An SEM close-up of the capacitive gap in a BAW gyroscope defined by the HARPSS process.

The HARPSS process is capable of producing poly- and monocrystalline silicon microstructures tens of microns thick, that are electrically isolated with self-aligned capacitive air gaps. Such high-aspect-ratio capacitive gaps, as seen in figure 2, increase the efficiency of capacitive transduction substantially, and enable an effective high-frequency interface for vibrating silicon microstructures. This structure produces the highest signal to noise ratios in capacitive MEMS devices and superior noise density enabling better resolution.

Dynamic range, bias stability and vibration immunity
In motion sensing, there are many applications that have varying requirements for the upper and lower detection range. A golf simulator is such an application, where the sensors have to be able to detect intense motions like a tee-off swing while at the same time be able to pick up delicate motion of putting or a wedge shot. The term dynamic range refers the ratio of the largest detectable signal to the smallest detectable signal.

Figure 3: Measured linearity of 0.05% of a BAW gyroscope across an input rotation range of ±1500 deg./sec (limited by measurement setup).

The operating frequency and construction of the BAW gyroscope enables the widest dynamic range of ±5000 deg/s with excellent linearity (figure 3) allowing designers to create a broader array of applications from a single sensor design. This attribute is particularly appealing for gaming platforms that include special purpose controllers such as the Wii remote or multipurpose consumer gaming platforms, like mobile handsets and tablets that frequently update their designs.

Figure 4: Measured root Allan deviation plot of a BAW gyroscope indicating a bias instability value of 25 deg./hr.

BAW gyroscopes operate at frequency range outside the range of flicker noise of standard CMOS interface circuits enabling a smaller detection limit. This enhances the overall noise in the system that leads to superior bias drift performance as seen in figure 4, which shows a measured root Allan deviation plot of a typical BAW gyroscope.

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