October 10, 2011 - Imec says it has built an integrated poly-SiGe-based piezoresistive pressure sensor on top of 0.13μm copper backend CMOS, the first such device directly fabricated above its readout circuit and the first poly-SiGe MEMS device of any kind processed on top of Cu-backend CMOS.
The MEMS-last approach for CMOS-MEMS monolithic integration leads to smaller die areas and allows use of standard foundry CMOS processes. Poly-SiGe as a new MEMS structural material (vs. for example CMOS top interconnect layers to fabricate the MEMS device) offers more flexibility since the MEMS fabrication and CMOS fabrication can be completely decoupled, imec explains. Aggressive scaling has pulled the technology away from aluminum interconnects and toward copper metallization with its lower resistivity and improved reliability.
The new integrated sensor, fabricated at imec in 0.13μm standard CMOS, incorporates a surface-micromachined piezoresistive pressure sensor, with a poly-SiGe membrane and four poly-SiGe piezoresistors, and an instrumentation amplifier, with Cu- interconnects (two metal layers), oxide dielectric, and tungsten-filled vias. Maximum processing temperature of the complete sensor (including poly-SiGe piezoresistors) was kept below 455°C, and "an appropriate passivation layer" was added to protect against the aggressive MEMS etch and deposition processes. The CMOS circuit "showed no significant deterioration after the MEMS processing," and even with the lower processing temperature, the sensor by itself (250×250μm2) showed ~2.5mV/V/bar sensitivity; the integrated device on top of the Cu-based amplifier showed ~64× higher sensitivity (~158 mV/V/bar).
Cross-section SEM picture of the integrated sensor. At the bottom, the two Cu metal lines of the CMOS circuit can be observed. Above, the MEMS layers (poly-SiGe membrane and piezoresistors, oxide sealing layer, and metal interconnects) are visible. (Source: IMEC)