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Optimise CMOS for MEMS-based frequency control

Posted: 23 Jan 2014 ?? ?Print Version ?Bookmark and Share

Keywords:MEMs? micro-electromechanical systems? sensors? wafers? oscillator?

Over the last five years, frequency control and timing products based on micro-electromechanical systems (MEMS) technologies have progressively diminished the 100-year-old monopoly held by quartz crystal solutions. Manufactured using standard semiconductor fabrication techniques, MEMs-based products offer significant improvements in size, ruggedness, and cost.

However, the full potential of MEMS has been realised only recently with the advent of CMEMS (this acronym is derived from the contraction of CMOS + MEMS) fabrication technology. This unique process can be used to fabricate both micro-electromechanical and CMOS electronic devices on a single die, allowing designers to leverage the economies of scale afforded by standard semiconductor processes to create single-chip frequency-control products that enjoy smaller form factors, better performance, lower cost, and greater scalability.

MEMS technology uses standard semiconductor processes to fabricate micro-electromechanical elements such as pressure sensors, accelerometers, and resonators on silicon wafers or other commercially-available substrates. While the compact form factors afforded by MEMS technologies were the original motivation for their adoption, timing product manufacturers quickly discovered that they also offer several other significant advantages. These include improved lead times, supply stability, product reliability, and the ability to make finely-tuned price/performance trade-offs.

MEMS devices don't suffer from some of the inherent limitations of quartz-based technologies that required ceramic-based packaging (figure 1), off-chip matching capacitors, and complex, highly specialised manufacturing flows to achieve acceptable levels of reliability and performance. The resulting devices were able to achieve acceptable accuracy, but they still possessed an inherent sensitivity to environmental factors, such as thermal stress and, in particular, shock and vibration, which increased their potential for field failures.

Figure 1: Caption.

Figure 2: Micrograph of a multi-chip MEMS-based oscillator.

Until recently, MEMS elements were integrated into frequency-control and clock products using the same methods developed for quartz-based products and other MEMS products. While combining a MEMS resonator chip with a separate IC in a multi-chip module (figure 2) allows the use of more standard packaging techniques, it still relies on integrating separate devices from boutique (i.e. expensive) MEMS foundries and high-volume CMOS foundries, resulting in sub-optimal system performance and cost.

CMEMS: Monolithic integration of CMOS and MEMS
These shortcomings have been overcome largely by CMEMS, which enables the modular post-processing of MEMS devices directly on top of CMOS circuitry. This unique approach to MEMS integration is the first technology of its kind to allow direct post-processing of high-quality MEMS layers on top of advanced RF/mixed-signal CMOS technology (0.18?m and below). Manufacturers can now add MEMS elements to their sensors' signal processing and interface electronics using the same advanced CMOS manufacturing line which fabricates as a scalable, state-of-the-art, modular back-end-of-line option (figure 3).

Figure 3: Basic sequence of the CMEMS manufacturing flow. (a) Starting material in the form of a passivated and planar CMOS wafer on top of which (b) polycrystalline SiGe is surface-micromachined into integrated MEMS devices, which are (c) encapsulated in a vacuum using wafer-level bonding. The fully finished wafer continues to probing, (d) die singulation and standard small-size packaging assembly, just like a standard CMOS product.

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