11 October 2011

Imec demos CMOS-integrated poly-SiGe piezoresistive pressure sensor

Imec of Leuven, Belgium has realized an integrated poly-SiGe-based piezoresistive pressure sensor fabricated directly above 0.13µm copper (Cu)-backend CMOS silicon technology. The research institute claims that this represents not only the first integrated poly-SiGe pressure sensor fabricated directly above its readout circuit, but also the first time that a poly-SiGe MEMS device has been processed on top of Cu-backend CMOS.

Polycrystalline silicon germanium (SiGe) has emerged as a promising micro-electro-mechanical system (MEMS) structural material since it provides the desired mechanical properties at lower temperatures compared to poly-silicon, allowing the post-processing on top of CMOS. The MEMS-last approach is the most interesting approach for CMOS-MEMS monolithic integration, belives Imec, as it leads to smaller die areas and enables integration of the MEMS without introducing any changes in standard foundry CMOS processes. Compared with alternative technologies, for example using the CMOS top interconnect layers to fabricate the MEMS device, poly-SiGe offers a more generic and flexible technology for above-CMOS integration, due to the fact that the MEMS fabrication can be completely decoupled from the CMOS fabrication, says Imec.

Imec says that it has previously proved the potential of poly-SiGe for MEMS above-aluminum-backend CMOS integration. However, aggressive interconnect scaling has led to the replacement of the traditional aluminum metallization by copper metallization, due to its lower resistivity and improved reliability. Imec’s latest results now broaden the applications of poly-SiGe to the integration of MEMS with the advanced CMOS technology nodes.

Graphic: Cross-sectional SEM of the integrated sensor, with two Cu metal lines of the CMOS circuit at the bottom, and the MEMS layers (the poly-SiGe membrane and piezoresistors, the oxide sealing layer and the metal interconnects) at the top.

The integrated sensor (fully fabricated at Imec) includes a surface-micromachined piezoresistive pressure sensor, with a poly-SiGe membrane and four poly-SiGe piezoresistors, and an instrumentation amplifier fabricated using Imec’s 0.13mm standard CMOS technology, with copper interconnects (two-metal layers), oxide dielectric and tungsten-filled vias.

To enable above-CMOS integration, the maximum processing temperature of the complete sensor, including the poly-SiGe piezoresistors, is kept below 455ºC. Moreover, an appropriate passivation layer was included to protect the electronic circuit from the aggressive etch and deposition steps needed to fabricate the MEMS devices.

Imec says that the CMOS circuit showed no significant deterioration after the MEMS processing. Despite the low processing temperature, the poly-SiGe piezoresistive sensor alone (250µm x 250µm membrane) showed a sensitivity of about 2.5mV/V/bar. The integrated sensor (same sensor + Cu-based CMOS amplifier underneath) showed a sensitivity of about 158mV/V/bar, which is about 64 times higher than the standalone sensor.

Tags: Imec CMOS SiGe

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