Electronic Design

MEMS Digital Dual-Axis Gyro Improves Camera Stabilization

Addressing lower-cost opticalimage stabilization (OIS) as a major goal, InvenSense Inc. has unveiled the first digital dual-axis pitch and roll (X and Y) gyroscope for camera phones and digital still cameras. Its nextgeneration IDG-2000 family offers the smallest such devices on the market, housed in a 4- by 4- by 0.9-mm quad flat no-lead (QFN) package (Fig. 1). The chip is designed for OIS applications in digital still cameras and high-resolution camera phones of 8 Mpixels and above.

The IDG-2000 features a pair of integrated 16-bit analog-to-digital converters (ADCs), one per axis, as well as a programmable digital low-pass filter, anti-aliasing filters optimized for OIS applications, and an integrated sensor for temperature-compensated bias adjustment. The ADCs allow for direct communication with advanced image processors over an I2C or serial-peripheral interface (SPI) bus, eliminating costly discrete components.

“We went from using 6-in. wafers to 8-in. wafers to include more functions and to lower chip costs,” says Steve Nasiri, founder, president, and CEO of InvenSense. “We developed a grinding process that allows us to make wafers 20% thinner, going from 1.2 mm to 0.9 mm.”

Nasiri estimates that the OIS market needs a solution that’s closer to the $5 mark than the present $10 and higher level, and he says that the IDG-2000 will go a long way toward meeting this need. “As part of the OIS solution, the IDG-2000 will cost well below $1/axis,” he says.

“Optical image stabilization is quickly becoming a standard key feature consumers look for in new cameras,” says Chris Chute, research manager for research firm IDC’s Worldwide Digital Imaging Practice. “Even with millions of digital cameras still being sold worldwide, competition for product differentiation coupled with lower price points are forcing camera vendors to manage costs more closely than ever. New technologies like the IDG-2000 can help camera vendors remain competitive in the market.”

The gyroscope family features lower phase delay from the higher bandwidth of the low-pass filter. Other key features include automatic gain amplification control, anti-aliasing filters, and low cross-axis sensitivity of less than ±1%.

The IDG-2000 measures hand jitter across a wide frequency range of 0.1 Hz to 20 Hz with exceptionally low noise at 0.033°/s-rms. The operating frequency in the 30-kHz range is immune to ambient noise. These features are required when taking photos in low ambient light with high-resolution sensors or when using a zoom function at any resolution. Power consumption is reduced by 50% compared to analog gyros with a low 4-mA maximum current at 2.1 V in the active mode and 5 µA of maximum current in the sleep mode.

Key to the IDG-2000’s development is the patented Nasiri Fabrication platform. It employs bulk silicon processing, wafer-level packaging, and innovative integration techniques to achieve a lowcost single-chip microelectromechanical- systems (MEMS) gyroscope (Fig. 2). The process is also useful for producing accelerometers, rotation sensors, actuators, and potentially other MEMS functions.

The process suits MEMS capacitivesensing circuits. The CMOS wafer is bonded directly on top of the MEMS wafer, using off-the-shelf bonding equipment. Though it is proprietary, the patented bonding process allows eutectic bonding of MEMS wafers to the aluminum layer of the CMOS wafer holding the signal-conditioning circuitry, without the addition of any other layers on the aluminum. Hundreds of thousands of electrical interconnects are made simultaneously between the MEMS structure and the CMOS wafer’s aluminum layer. The MEMS gyroscope element employs patented out-of-plane resonating structures.

This approach forms the cornerstone of a vibrating dual-mass, tuning-fork design that InvenSense says surpasses other approaches used by competitors in terms of low cost and is needed to meet the cost requirements of high-volume consumer electronic products.

The Nasiri process reduces wafer-scale integration to a single step. It streamlines wafer-scale packaging and provides full hermetic sealing for the sensitive MEMS structures within the package, resulting in resistance to high temperature and humidity levels. Even though the Nasiri process is proprietary, it has been ported to several high-volume MEMS foundries and has been ramped up for large-volume production without the typical constraints faced by other manufacturers of MEMS gyroscopes.

The IDG-2000 will begin sampling this month to selected OEM partners.



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