MEMS On The Move: Motion Sensors For The Masses

The MEMS motion sensor is no longer the bastion of just the automotive and industrial markets. Thanks to the maturation of design and manufacturing methods, these accelerators and gyroscopes are meeting the price points for mass-market requirements. As a result, they've extended their reach into many consumer, computer, well-being, well-care, and security applications.

For instance, low-cost, small-form-factor, multi-axis MEMS sensors are being used in high-volume consumer and portable-computing applications. They protect these products from damage, improve wireless connectivity, or provide new user-input commands based on movement instead of keyboard entries or buttons.

Research company IC Insights believes inertial sensors (accelerometers and gyroscope devices) will become the largest product category in solid-state sensors in 2008. By next year, they'll overtake the pressure and magnetic-field sensors used in automotive, industrial, and other applications (Fig. 1).

ACCELEROMETERS DRIVE THE MARKET
The expansion of MEMS sensors in consumer electronics applications can be credited to MEMS accelerometers. Early single-axis models that sensed in only one direction along a line have given way to dual-axis units. Both Freescale Semiconductor and Analog Devices have pioneered the way toward the application of low-cost MEMS accelerometers in consumer applications. Another pioneer in 3-axis MEMS accelerometers is ST Microelectronics (see the sidebar).

Most MEMS accelerometers are manufactured on a bulk micromachining process, necessitating the use of an additional chip for signal processing. Here, the two chips are interconnected via wafer bonding or wafer-scale packaging. In bulk processing, a single crystal of silicon is anisotropically etched to form the 3D MEMS structure. This is a subtractive process in which the silicon on the wafer is selectively removed.

The other MEMS manufacturing technique is surface micromachining. Analog Devices was the first company to produce sensors on a surface-micromachined process by investing in and developing its own proprietary process. Its iMotion sensors are formed on top of the silicon using deposited thin-film materials. The deposited materials form the sensor and the sacrificial layer that define the gap between the structural layers. Signal-processing circuitry can be formed on the same wafer holding the MEMS sensor (Fig. 2).

The transduction mechanism for commercially available MEMS accelerometers employs either the piezoresistive or capacitive principles. In piezoresistive-implanted materials, the change in the stress experienced by a cantilever beam or a diaphragm causes a strain and a corresponding change in resistance. A Wheatstone bridge measures this change in resistance and then converts it to a voltage.

The capacitive approach uses interlocking fingers or elements, which alternate between a fixed position and a moving position caused by stress or strain of the inertial motion. The capacitance differences between these states, which are proportional to the acceleration changes, are then measured and used to produce potential (Fig. 3).

MEMSIC takes a different approach. The proof mass for its dual-axis accelerometers is based on heat transfer by natural convection using a gas (Fig. 4). A single heat source, centered on the silicon chip, is suspended across a cavity. Equally spaced aluminum/polysilicon thermopiles (groups of thermocouples) are located equidistantly on all four sides of the heat source.

Under zero acceleration, a temperature gradient is symmetrical about the heat source. That makes the temperature the same on all four thermopiles, causing them to produce the same voltage. Acceleration in any direction disturbs the temperature profile, due to free convection heat transfer, causing it to be asymmetrical. The temperature differential at the thermopiles is proportional to the acceleration.