How Small Can It Get?

We've been living in the world of "micro" electronics for more than three decades, where the size of features on a chip is measured in micrometers. Two decades ago, we entered the "nano"-second regime, where clock periods are measured in billionths of a second. Recently we entered the "pico"-second regime, where the bit period of an OC-768 signal is measured with just 25 psec. To describe what is sensed by the highest-volume microelectromechanical system (MEMS) component currently in production, we need to jump nine orders of magnitude smaller to units of zeptofarads. (For those interested, in decreasing order, it is micro, nano, pico, femto, atto, zepto and yocto.)

Every single new car manufactured today has at least one MEMS sensor used, for example, as a manifold absolute pressure sensor, in antilock brake systems, and in accelerometers for active suspension or airbag control. MEMS motion sensors are appearing in video games, industrial machine monitoring and navigation products.

According to the MEMS Industry Group, "It is estimated that there are 1.6 MEMS devices per person today in the U.S. and this number is expected to grow to nearly five devices per person by 2004." Analog Devices Inc. (ADI, Norwood, Mass.) just announced it has shipped 100 million iMEMS (integrated MEMS) sensors. Most of these have been designed as high-g accelerometers in automotive applications.

iMEMS gyro in a hermetic ceramic ball grid array (BGA) package. (Source: Analog Devices Inc.)

ADI recently applied its iMEMS accelerometer sensor technology to sense rotation using the Coriolis force. A suspended mass is set to vibrate in the plane of the surface, at ~15 kHz. When the chip is rotated about the vertical axis, there is a lateral force on the suspended, vibrating mass. This force causes a displacement against the spring force of the supports. Measuring the displacement with a differential capacitor structure allows a measurement of the rotation rate.

One of the first iMEMS gyros, the ADXRS150, has a full-scale sensitivity of 150°/sec rotation rate. The noise floor, according to David Krakauer, ADI's program manager for gyro products, is set by the smallest change in capacitance that can be sensed, 12 zF. This corresponds to 12 millionths of a femtofarad. This tiny amount of capacitance is a measure of the displacement of the suspended mass against the spring force of the support beams, a net displacement of about 16 nuclear radii!

The success of this high-volume sensor technology is due to the combination of design, fabrication, assembly and packaging decisions made when this technology was developed more than 15 years ago. The goal was to be able to measure very small effects, be highly reliable, and meet volume cost targets of <$10 per packaged sensor. The approach taken was to integrate the MEMS sensor right on the chip with the electronics, and to use conventional IC processing and off-the-shelf assembly and packaging technology wherever possible to keep the costs down.

The iMEMS wafers start with the sensor electronics and interface circuitry, using 3 µm features. A sacrificial layer of oxide and then polysilicon are deposited. The MEMS sensor is machined in the poly, tightly coupling the sensor and electronics, enabling the measurement of capacitance changes as small as 12 zF.

There are two MEMS-specific processes added. After the sacrificial oxide layer is removed — allowing the inertial mass to be suspended freely over the substrate — a proprietary coating is deposited onto the wafer to prevent stiction, a well-known MEMS reliability concern.

The dicing process uses a patented technique to saw the wafer while protecting the MEMS elements from water and particles. This technique — an upside-down saw method — uses a protective tape to cover the MEMS elements while the wafer is diced from the backside. The singulated die are then picked off the tape and assembled in small ceramic, hermetic packages ready for conventional board assembly.

ADI's bet that integrating MEMS and active silicon could be done at high yield, rather than optimizing the two processes independently and reintegrating in the package, has paid off and is key to being able to measure effects as low as 12 zF.

For additional information on semiconductor packaging, go to www.semiconductor.net/assembly.