Low-Temperature SiGe Processing Advances MEMS Integration

It’s possible to integrate MEMS devices above a silicon-based process using silicon germanium (SiGe). Silicon Clocks employs a SiGe low-temperature liquid-phase chemical vapor deposition (LPCVD) process to produce a resonator circuit. The work is based on research done at the University of California at Berkeley.

As a back-of-the line (BOL) process, it enables the vertical stacked integration of MEMS and supporting circuitry. BOL refers to the post-processing of MEMS circuitry on top of other electronics, which is sometimes called a MEMS-last process.

The resulting product features a significant reduction in parasitic effects and an increase in timing circuit performance over a conventional two-chip (i.e., MEMS and signal-conditioning) approach.

The poly-SiGe can be etched very selectively to silicon (Si), SiGe, silicon-dioxide (SiO₂), and silicon-nitride (Si₃N₄) materials in a heated hydrogen-peroxide solution. Consequently, the poly-SiGe can be used as a sacrificial material, eliminating the need to protect the CMOS electronics during the MEMS-release etching step.

This development is useful not only for producing MEMS timing circuits, but also for developing inertial, optical, and RF MEMS ICs. It allows for true 3D IC integration. The use of poly-SiGe is envisioned for many more above-IC applications, such as accelerometers and resonators. Also, wafer-level, thin-film packaging can be done by using SiGe or nitrogen, allowing for truly highly integrated systems.

Working with other industrial partners, IMEC is developing an integrated poly-SiGe MEMS gyroscope with dedicated MEMS and ASIC designs. The gyroscope targets low-noise, high-resolution applications (0.15°/s for a 50-Hz bandwidth). The high-voltage (20-V), 0.35-µm, commercial double-polysilicon CMOS process has five wiring levels and 500-nm silicon-oxide (SiO)/ Si₃N₄ passivation.

After the deposition and patterning of an etch-stop layer, a thick, undoped sacrificial SiO layer is deposited and panelized. Contacts to the top metal level are etched through the sacrificial layer, the etch-stop layer, and the CMOS passivation.

The poly-SiGe structural layer, which is at least 10 µm thick for increased sensitivity, is deposited by using an advanced multilayer technology. This results in high-quality films at low temperature (≤450°C). The release is done using wet processing and carbon-dioxide (CO₂) super-critical drying to eliminate stiction. Thus, free-moving gyroscopes were obtained above fully functioning CMOS.

Though processing of the integrated gyroscopes is finished, packaging and evaluation are still ongoing at the project partner’s site.