A Flip Chip Package for Thermal Mass Flowmeter

This paper reports on a new package of thermal mass flow meter. The new package is to etch a flow channel on the Al₂O₃ substrate, and bond the chip to the substrate using flip chip technology. The fluid flows along with the channel and across the sensor element which is made by platinum and this material's resistance changes with the temperature. When fluid flows, there will be a Δ T between upstream element and downstream element, which will result in a Δ R and Δ U, therefore we can measure Δ U to measure the fluid velocity. The velocity distribution in the channel is simulated by software Fluent. A conclusion that there is a fixed velocity ratio between the fluids inside and outside the channel is obtained. This sensor chip is fabricated using bulk micromachining technology on a silicon substrate. As the thermal layer platinum and the insulation layer silicon nitride are very thin and fragile, the nitrogen is charged in the sensor chip's cavity to form a preliminary pressure on the back of the membrane. The fabrication process is presented in the paper. Using flip chip bonding overcomes the drawbacks associated with presently available flow sensors which use wire bonding.