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MEMSIC Case Study

How does a designer make analog ICs and MEMS devices with the same tool, and get the resulting chip into the opening ceremonies of the 2008 Olympic Games?

It’s hard enough to design mixed-signal processing onto the same chip as a MEMS device, but MEMSIC has managed to integrate these technologies on the same silicon and sell hundreds of thousands of accelerometers in a variety of industries.

The company has also overcome two other hurdles: keeping production costs low by sticking to a standard CMOS IC process, and standardizing development on a single, lean set of EDA tools.

Detecting Acceleration and Motion

Most accelerometers depend on moving mass to determine motion, but MEMSIC differentiates itself from its competitors through its use of a thermo-mechanical sensor in silicon.

In the center of the 1mm-square sensor is a heater operating at 100°C above ambient temperature. Around the heater are symmetrically placed thermopiles for reporting temperature in different locations. (A thermopile is a series of thermocouples, or temperature-sensing elements, connected in a series to boost voltage.) The entire sensor is hermetically sealed in an air/gas cavity, outside of which is analog circuitry for amplification, control, analog-to-digital conversion and, in the 3-axis models, digital compensation/calibration circuitry.

In the absence of motion, the thermal profile is balanced among the thermopiles, but any motion or acceleration modifies the convection pattern around the heater, such that the thermopiles in the direction of the acceleration become hotter than the others. The analog circuitry interprets the resulting signal changes from the thermopiles as motion and acceleration.

With no moving parts, MEMSIC’s accelerometers are longer-lasting, more reliable, and as much as 25 times more shock-resistant (up to 100,000 g) than their mechanical counterparts for measuring tilt, inclination, shock, or vibration. The chips appear in such products as car alarms, mobile electronics, global positioning systems, elevator controls, patient monitoring devices and head-mounted displays for gaming.

One Toolset for Analog and MEMS

"Back in the 1990's our design center used Tanner tools, so we adopted them and have used them ever since," explains MEMSIC's Director of Technology Partnership and Development, Yongyao Cai. "Our accelerometers combine MEMS IP and analog circuitry IP, and the Tanner tools are flexible enough for both our circuitry and the sensor.

"We model the sensor as a resistor, and we can also model it as a polarized resistor because the thermopile has a polarity. Tanner tools have been 100% reliable for us ever since we started using them in 1999. We've never had a tapeout error due to verification."

For the current generation of accelerometers, MEMSIC engineers use MEMS Pro, a tool from SoftMEMS that sits on top of L-Edit for designing and analyzing MEMS. In fact, early MEMSIC products were even simpler and did not require full mechanical simulation, so the MEMS designers worked directly in L-Edit.

The engineers at MEMSIC use MEMS Pro for 3D mechanical model extraction for finite element analysis. They use L-Edit to modify the details of the sensor, and to do layout and pattern list. After layout they use Tanner L-Edit LVS and L-Edit Standard DRC. Finally, they export from L-Edit to a GDS layout file and send the result to tapeout for TSMC.

MEMS Designs, CMOS Fabrication

To take advantage of lower fabrication costs, MEMSIC designs its sensors almost exclusively with standard CMOS layers: for example, the heater is gate polysilicon and the first layer of the thermopile is metal and polysilicon.

"We have a tremendous advantage over our competitors," continues Yongyao. "Our process is almost independent of the fabrication foundry because our design is 95-99% CMOS. We can easily change process and foundry to take advantage of better production pricing. Our competitors, on the other hand, use proprietary MEMS processes, fabricating either by themselves or through a specialized foundry, and that is always more expensive than working with a traditional CMOS foundry."

MEMSIC also enjoys an advantage when changing geometry. Most of its competitors are still producing at 1-2 micron, and a change to .25 micron in MEMS would result in a completely different process and a costly conversion. MEMSIC has produced in .6 and .25 micron - with .18 micron on the roadmap - and its standard CMOS IC process allows it to ramp up volume and production quickly after a change in geometry.

92,000 Accelerometers in Beijing

The marquee application of MEMSIC's technology was in the electronic "Waving Torch" distributed to all attendees of the opening ceremonies for the 2008 Olympics in Beijing.

The torch resembles a 20-30cm wand, with a linear array of LEDs. Shaken from side to side, the torch tricks the human eye into seeing iconic Olympic images - symbols for major sports, the Olympic logo, Chinese greetings, and the five Olympic mascots - displayed in mid-air as the LEDs switch on and off. The core technology in the torch includes a MEMSIC algorithm and accelerometer (designed with Tanner tools) to detect the user's back-and-forth hand movement and to fire each LED as needed for the image.

"We worked on this project for half a year as an Olympic promotional tool," says Yongyao. "The user waves the torch through the air, and the LEDs display the pattern according to the motion. It's a good example of how much information an accelerometer can provide on position, orientation and speed."

About MEMSIC

MEMSIC, Inc. designs, manufactures and markets CMOS Micro-Electro-Mechanical Systems (MEMS) IC products that have on-chip mixed signal processing. MEMSIC is the first and the only company that integrates a MEMS inertial sensor with mixed signal processing circuitry onto a single chip using a standard CMOS IC process. This combination of technology has successfully yielded products at substantially lower cost and higher system performance and functionality than competitive products in the market for sophisticated accelerometers. In addition, this technological approach allows the Company to easily integrate additional functions, or create new sensors, using a standard CMOS IC process to expand into other MEMS application areas beyond accelerometers.

The Company's accelerometers, sometimes called inertial sensors are used to measure tilt or inclination, shock or vibration, or inertial acceleration. Any application that requires the control or measurement of motion is a potential application for accelerometers.

About MEMS Pro 

MEMS Pro is a tool suite from SoftMEMS LLC for designing and analyzing MEMS. Its integration with Tanner and other tool suites shortens development time and provides designers reliable analysis for manufacture.

The MEMS Pro suite offers mixed MEMS/IC schematic capture and simulation, full custom mask layout capability and verification, 3D model generation and visualization, behavioral model creation, and links to 3D analysis packages.

SoftMEMS was founded in 2004 by Dr. Mary Ann Maher. The company's products are based on the MEMS Pro software developed by Dr. Maher's team at Tanner Research in 1997 and the MEMS Xplorer software developed by Dr. Jean Michel Karam's teams at TIMA and MEMSCAP.

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