Rethinking MEMS sensor design for the masses

MEMS INERTIAL SENSoRS have been around for more than 25 years, from the first prototypes developed in universities to initial product offerings from the likes of Analog Devices, Bosch, Motorola (now Freescale) and STMicroelectronics. Driven by the Defense Advanced Research Projects Agency, with the promise of ubiquitous inertial navigation, MEMS sensors saw their first commercial success as collision detectors in vehicle airbag systems.

Fast-forward 20 years. Airbags that started on high-end cars are now available on every vehicle. The use of sensors in cars has expanded into all manner of vehicle dynamics. Improvements in the cost, size and power of current sensors, along with expansion of the vendor base, have allowed MEMS sensors to spill over into consumer electronics such as game controllers and smartphones.

MEMS inertial sensor employing variable capacitor transduction. Source: HP Labs

Their performance, however, remains largely unchanged. The systems are a bit more digital now, but factors such as noise floor and stability, which are key to navigation, have not seen the same improvements as cost, size and power usage.

It is time to examine why MEMS inertial sensors have failed to live up to their initial promise, and to propose a different approach that could jump-start those advancements. MEMS fabrication technologies, such as high-aspect-ratio etching, wafer bonding and packaging, have all seen dramatic improvements from the first university prototypes. A new, nano-based approach that could dramatically move the needle toward realizing a MEMS-based inertial measurement unit suitable for consumer electronics.

Hewlett-Packard's 25-plus years of nanofabrication experience was not in sensors, but instead focused on creating printheads with ever-increasing nozzle density, shrinking droplet size and improved power efficiency. Using that knowledge, HP took out a clean sheet of paper and designed a MEMS-based storage device. In the process, we discovered a technology platform with all the attributes needed to create a new generation of MEMS inertial sensors.

The storage device sought to miniaturize a CD-RW-like system onto a chip. The rotating disk and laser were replaced by an XY positioner, dubbed the micromover, and an array of field-emitting electron guns. Both devices use phase-change media to store data bits.

The program developed several unique MEMS technologies, but the micromover, created with high-aspect ratio etching and wafer bonding, was the clean sheet needed to transform MEMS inertial sensors. By taking the precision XY actuator and running it backward, the CD-RW-like system-on-chip became a high-performance sensor.