Flip-chip packaging for thermal CMOS anemometers

We developed a new packaging method based on flip-chip mounting, for integrated micro sensors. The technique is demonstrated with the fabrication, packaging, and characterization of CMOS anemometers. The sensors are produced using the commercial 2 μm CMOS process of EM Microelectronic-Marin SA, Switzerland, followed by bulk silicon micromachining. They consist of a membrane of the CMOS dielectrics heated by integrated polysilicon resistors. Integrated thermopiles detect wind-induced temperature differences on the membrane. The sensor chip is flip-chip mounted on a ceramic substrate. An opening in the substrate is aligned with the sensor, and enables free advection. The electrical interconnections between the chip bumps and the substrate contact pads are soldered with PbIn₅₀. Concurrently a bump frame on the chip is soldered to a corresponding structure on the substrate. This bump frame seals the rest of the chip from the medium flowing over the sensor membrane. The sensor responses were characterized in a wind-tunnel as a function of sensor orientation, air velocity, and packaging parameters. The output signals increase monotonically with the air velocity up to 36 mV at 38 ms^-1 (12 Beaufort) at a heating power of 4.7 mW