The Zen of Sensor Design

Contributed by Mike Stanley

Originally posted on Freescale’s Smart Mobile Devices Embedded Beat Blog

About two years ago, I joined the Freescale sensors team, which focuses on accelerometers, pressure sensors, and touch sensors.

Prior to that, I spent a number of years in the Freescale’s microcontroller solutions group, where I was an architect for several digital signal controller and microcontroller product families. One of the first things I learned when I moved into the sensors group was that certain “rules of the game” that relate to microcontroller design needed to be adapted when dealing with sensors. An example is package selection. With most microcontrollers, package selection is based upon number of functional and power pins required, PCB assembly processes targeted and (sometimes) thermal characteristics. Performance considerations are often secondary, if they exist at all. Sensors interact with the real world. Mechanical stresses introduced during both package assembly and PCB mounting can affect electrical performance of the device; often showing up as additional offset or variation of performance with temperature. Even the compound used for die attach has a demonstrable effect on sensor performance, and must be considered early in the design process. Continue reading

Evolving Intelligence with Sensors

Contributed by Michael Stanley, Freescale Semiconductor

Originally posted on Freescale’s Smart Mobile Devices Embedded Beat blog

I’ve always been fascinated by electronic sensors. The idea of being able to measure and interact with the physical world appeals to the ten-year-old inside me. Not so long ago, if you needed to measure some physical quantity as an input to your system, you bought an analog sensor, hooked up your own signal conditioning circuitry, and fed the result into a dedicated analog-to-digital converter. Over time, engineers demanded, and got, self-contained products which handled those signal conditioning and conversion tasks for them. Continue reading

Chipworks: Inside Analog Devices’ New MEMS Strategy

St. J Dixon-Warren, Manager, Process Analysis, Technical Intelligence, Chipworks Inc.

Analog Devices has recently changed their MEMS strategy. Since the introduction of the ADXL50 in 1991, Analog has built their MEMS inertial sensors using their iMEMS technology, which integrated the micromechanical structures and ASIC circuitry on a single die. The development of this technology culminated, in some respects, with the ADXL330, which was launched in 2006. The ADXL330 provided three sense axes in a 4 mm × 4 mm × 1.45 mm LFCSP package. An X-ray of the ADXL330 package, which contains a single integrated chip, is presented in Figure 1.

Figure 1 ADXL330 Package X-Ray

Figure 2 shows that the ADX330 MEMS sensor was fabricated as a single chip, with the MEMS structure in the centre of the die, beneath a hermetic cap, and the ASIC circuitry around the outside edge. The ASIC circuitry uses a single metal, single poly 3 µm BiCMOS process, while the MEMS is fabricated using three layers of polysilicon, with the top 4 µm thick poly 3 being used to form the MEMS structures, as shown in Figure 3.

Figure 2 Decapsulated ADXL330 Chip

Figure 3 ADXL330 MEMS Structures

The integration of BiCMOS and MEMS process technology onto a single die represented a significant technological achievement; however, the price of this integration was significant limitation on the complexity of the circuitry available to device designers, and limitations on the range of MEMS processing possible.

After more than twenty years, Analog has decided to abandon their integrated iMEMS technology, and adopt the more common strategy of using a separate MEMS die and ASIC die wire bonded together in a single package. The ADXL345 represents one of the first examples of Analog’s new strategy.

Figure 4 shows an X-ray of the ADXL345 3 mm × 5 mm × 1 mm LGA package. The X-ray clearly shows the presence of a separate ASIC die and MEMS die, with a hermetic cap. The internal structure of the device is more clearly seen in the SEM micrograph of the decapsulated device, Figure 5.

Figure 4 ADXL345 Package X-Ray

Figure 5 Decapsulated ADXL345 MEMS and ASIC Die

Abandoning their integrated iMEMS technology brings a number of benefits for Analog Devices. It allows the use of more advanced process technology in the ASIC, thus enabling greater functionality, including SPI and I2C digital outputs. It also allows them to more easily adopt a foundry strategy, with the possibility of separate foundries for the MEMS and ASIC die. Apparently, Analog has already started using TSMC to provide some MEMS foundry services.

Chipworks completed a full analysis of the ADXL330 in 2006. We are presently undertaking a detailed analysis of the ADXL345, which will include information on the process used to fabricate the MEMS and ASIC dies, plus a discussion of the MEMS architecture. For further information, please contact the author.

References

  • Analog Devices ADXL330 Three-Axis ±2 g MEMS Accelerometer Process Review (PPR-0602-801)
  • Analog Devices ADXL330 3-axis Accelerometer ICWorks Surveyor (MEMS) (ICS-0807-802)
  • Analog Devices ADXL345 Digital Accelerometer MEMS Process Review (MPR-0907-802)
  • Analog Devices ADXL345 3-Axis Accelerometer ICWorks Surveyor Report (MEMS) (ICS-0905-801)