Surface-Micromachined CMOS Process Enhances IC Tire-Pressure Sensor

Aug. 19, 2002

An innovative design by Motorola's Sensor Products Division in Tempe, Ariz., optimally integrates a surface-micromachined capacitive automotive tire-pressure sensor with a 0.8-µm CMOS double-metal, double-polysilicon process. Improving pressure linearity over conventional CMOS designs, this chip includes an optimum signal-processing algorithm and EEPROM to store pressure and temperature calibration data.

A careful examination of conventional CMOS designs showed that designers have to micromachine the sensor element before they perform CMOS' temperature-sensitive steps to maximize sensor performance. The source/drain implants were identified as the temperature-sensitive steps.

The chip has a full-scale output of 0 to 255 counts with a pressure linearity of −1.5% of full scale. The offset temperature coefficient and sensitivity are 0.01 counts/°C and −0.005%/full scale/°C, respectively. Its pressure range is 0 to 637.5 kPa, and its thermal hysteresis is just 0.04%/full scale.

"With most designs, adding a MEMS device to a CMOS process affects performance, either thermally or topographically, and the CMOS Spice design parameters involved," explains Bishnu Gogoi, principal staff scientist for technology development. "We also designed this chip so that we can reuse the CMOS process fabrication steps as much as possible."

In the new process, the pressure sensor's bottom is defined using floating-gate polysilicon, followed by an isolation nitride layer. Next, the spacer layer is defined. The pressure-sensitive diaphragm is then formed using a polysilicon layer. A hydrogen-fluoride solution is used to remove the spacer layer. The etch holes are scaled to complete the formation of the sensor. The CMOS process is resumed with interlayer dielectric layers and metal deposited. Finally, the chip is passivated with the sensor and bonding pads left exposed (see the figure). Although he refused to be more specific, Gogoi says the chip is in engineering evaluation by a number of customers.

For details, contact Gogoi at (480) 413-8836 or [email protected].

About the Author

Roger Allan

Roger Allan is an electronics journalism veteran, and served as Electronic Design's Executive Editor for 15 of those years. He has covered just about every technology beat from semiconductors, components, packaging and power devices, to communications, test and measurement, automotive electronics, robotics, medical electronics, military electronics, robotics, and industrial electronics. His specialties include MEMS and nanoelectronics technologies. He is a contributor to the McGraw Hill Annual Encyclopedia of Science and Technology. He is also a Life Senior Member of the IEEE and holds a BSEE from New York University's School of Engineering and Science. Roger has worked for major electronics magazines besides Electronic Design, including the IEEE Spectrum, Electronics, EDN, Electronic Products, and the British New Scientist. He also has working experience in the electronics industry as a design engineer in filters, power supplies and control systems.

After his retirement from Electronic Design Magazine, He has been extensively contributing articles for Penton’s Electronic Design, Power Electronics Technology, Energy Efficiency and Technology (EE&T) and Microwaves RF Magazine, covering all of the aforementioned electronics segments as well as energy efficiency, harvesting and related technologies. He has also contributed articles to other electronics technology magazines worldwide.

He is a “jack of all trades and a master in leading-edge technologies” like MEMS, nanolectronics, autonomous vehicles, artificial intelligence, military electronics, biometrics, implantable medical devices, and energy harvesting and related technologies.