STMicroelectronics introduces MEMS-based rotational accelerometer

Aug. 20, 2001 – Geneva, Switzerland – STMicroelectronics has introduced a new rotational acceleration sensor device based on MEMS technology that addresses applications in hard disk drives and consumer products.

Housed in a standard SO-24 plastic package, the new L6671 device includes a capacitive rotational accelerometer sensor produced using MEMS technology plus a sensitive interface chip integrating the analog-to-digital conversion, filtering and interface functions.

The L6671 provides a digital output through a 3.3V (5V tolerant) three-wire serial bus and achieves a signal bandwidth of 800Hz, sensitivity of 2.5rad/s2, a full-scale sensitivity of 200rad/s2 and a signal-to-noise ratio of 37dB over 30-800Hz. Through the serial bus it is also possible to program device characteristics including gain, offset, filter performance and phase delay.

Inside the device, movement about the rotational causes a displacement of a structure inside the MEMS sensor thus producing a capacitive change. These changes in capacitance are converted by the interface chip into a digital output for direct connection to a microprocessor. The device can measure capacitance changes as small as 0.05fF (0.05×10-15F).

Sensitivity of the system is enhanced by the design of the interface chip, fabricated by ST in BiCMOS technology, according to the company. It is also enhanced by the use of ST’s in-house developed MEMS technology that has a deep layer of polysilicon used to make the moving parts of the sensor. This additional thickness translates into greater capacitance changes for a given acceleration, ST said. The ST MEMS process also includes a technique for etching the sacrificial silicon dioxide layer beneath the moving parts, minimizing stiction effects.

In hard disk drive applications, the L6671 sensor device allows drive designers to reduce the effects of mechanical vibration on head tracking and thus increase the number of tracks per inch and hence drive capacity, according to ST. With the L6671 it is possible to detect and measure rotational vibrations so that the firmware can generate a feedforward correction signal for the voice coil drive circuit to keep the head in the correct position, ST said. This approach means that the track-to-track distance can be reduced, increasing the density per unit area.


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