Flip-Chip Packaging for a Two-Dimensional Thermal Flow Sensor Using a Copper Pillar Bump Technology

A flip-chip packaged two-dimensional (2-D) thermal flow sensor fabricated in CMOS technology is presented. The sensor consists of polysilicon resistor heaters, Al/polysilicon thermopiles, and a substrate bipolar transistor located in the center of the sensor chip. The thermopiles and the transistor were used to measure the change of the flow-induced temperature distribution on the flow-sensing surface. The sensor chip was flip-chip packaged on a thin ceramic substrate using a copper pillar bump technology. The polysilicon resistor provides the necessary overheat of the chips, and thermal interactions with the flow are achieved via the pillar bump and the thin ceramic substrate. The operating principle for the packaged sensor remains the same as before packaging. The backside of the ceramic substrate provides a smooth surface for the sensor to be exposed to the flow. Meanwhile, the ceramic substrate holds the sensor chip and protects it from being contaminated or even destroyed by the corrosive environment. The packaged flow sensor shows the good performances compared with the unpackaged sensors. It can detect airflow speed up to 30 m/s with accuracy of 0.5 m/s and airflow direction in a full range of 360 with an accuracy of 6 at room temperature.