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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?

CMEMS technology also offers significant performance gains over complex, multi-chip designs where thermal slew across the assembly becomes a dominant factor in the compensation loop. In a typical two-chip stacked die assembly, the CMOS chip, wire bonds, and die attach epoxy, the MEMS chip and the package itself all affect heat transfer between the resonator and the temperature sensor used to compensate for thermal excursions. CMEMS-based solutions overcome these issues with mechanically-compensated devices, ultra-short thermal paths, and small thermal time constants which make them orders of magnitude more resistant to thermal slew than any other existing solution.

Finally, mechanical temperature compensation also plays a critical role in minimising the impact of environmental effects, for example, thermal strain induced offset of the temperature sensor which directly impacts solder reflow shift, ageing, and overall accuracy. Details of experiments that compare the thermal stability of CMEMS, two-chip MEMS, and quartz technologies are well-documented in the technical paper from which this article was derived [1]. The same white paper also includes detailed information on how CMEMS systems can be designed to minimise sensitivity to thermal, frequency, and voltage reference drift, which can occur due to environmental factors or the ageing processes that occur in the package and the product integrated devices themselves.

Based on these demonstrated results, CMEMS technology's outstanding control and robustness under adverse conditions allows CMEMS solutions to specify frequency initial accuracy and stability inclusive of all effects over the lifetime of the device. This key characteristic, also known as total accuracy, is not available from quartz or other MEMS-based oscillators.

A paradigm shift in the frequency controland beyond
CMEMS technology is poised to create a positive disruption in the frequency-control industry by combining all the advantages of MEMS-based solutions while retaining and even improving many of the best characteristics of quartz solutions. These beneficial characteristics include:
???Streamlined wafer level manufacturing in an advanced CMOS line
???Streamlined standard packaging
???Programmability and short lead times
???Reliability meeting IC industry standards
???Low ageing characteristics
???Low strain sensitivity (initial accuracy post solder reflow)
???Good temperature stability (mechanical compensation)
???Immunity to fast thermal transients
???Extended frequency range for low-noise references
???Immunity to EMI

CMEMS technology's ability to support wafer-level, chip-scale packaging opens the possibility of extending the packaging and form-factor roadmaps beyond standard oscillator footprints. If needed, finished CMEMS products can also be delivered in wafer form, enabling the sale of wafer-calibrated, guaranteed frequency references for system-in-package integration. Such a business model would allow these timing devices to be integrated into each single SoC as a companion chip or, in some cases, on the SoC itself using direct post-processing techniques.

Reference
1. E. P. Quevy, "CMEMS Technology: Leveraging High-Volume CMOS Manufacturing for MEMS-Based Frequency Control," a white paper published by Silicon Labs

About the author
Emmanuel Quevy is director of MEMS engineering for timing products at Silicon Labs, (www.silabs.com) overseeing design and integration of timing solutions using Silicon Labs' proprietary MEMS technologies. Prior to Silicon Labs, he was co-founder, director and CTO of Silicon Clocks (acquired by Silicon Labs in 2010), where he led technology and MEMS-based product developments. He has co-authored more than 40 technical publications in the field of MEMS, is co-inventor on more than 25 issued US patents, and regularly serves as reviewer and committee member of various journals and conferences. He received an Engineering Degree from ISEN Lille, France, and the M.Sc. degree in electrical engineering and computer science from the University of Science and Technology of Lille (USTL), France, both in 1999. He then received the Ph.D. degree in electrical engineering from USTL in 2002.

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