Self-Contained Frequency Trimming of Micromachined Silicon Resonators via Localized Thermal Oxidation

Abstract

An electronically controllable frequency trimming technique based on localized thermal oxidation of single crystalline silicon resonators is demonstrated. Thin layers of silicon dioxide can be thermally grown on silicon surfaces of resonant structures via extreme joule heating of such when biased with relatively large bias currents in an oxygen-rich environment. Changes in structural stiffness or oxidation induced internal stress causes a shift in the resonant frequency of the structures that can be used for post-fabrication frequency trimming of silicon resonators. As an added advantage, the positive temperature coefficient of Young's modulus for the added silicon dioxide layer can partially compensate the large negative temperature coefficient of frequency (TCF) for the silicon resonators. Frequency trimming as high as ∼3.7% and TCF as low as 0.2 ppm/°C has been demonstrated for a 54 MHz I-shaped resonator using this technique. Furthermore, it has been demonstrated that coupling of mechanical resonance to electrical resistance and consequently joule heating of the structure can lead to a cooling effect at mechanical resonant frequency of the structure that allows the localized oxidation to stop automatically as soon as the resonator frequency reaches a targeted actuation frequency applied to the structure. The viability of this concept is demonstrated by application of both off-resonance and at-resonance actuation signals to fabricated resonators showing that as opposed to the off-resonance signals, the at-resonance signals with the same intensity do not lead to further frequency shift.[2012-0337]

Concepts and Key Technologies of Microelectromechanical Systems Resonators

“…Using the cooling effect of the resonator during resonance, automatic trimming can be achieved. The result shows that a frequency trimming effect of approximately 3.7% can be obtained [77]. A similar method of forming silicon-metal bonds by heating the deposited metal on the MEMS resonator to diffuse can also achieve trimming in the range of approximately 4000 ppm [78].…”

“…An electronically controlled frequency trimming technique for the local thermal oxidation of a single crystal silicon resonator was illustrated [77]. When the MEMS resonator is biased with a relatively large current in an oxygen-rich environment, a thin layer of silicon dioxide can thermally grow on the silicon surface of the resonant structure.…”

“…The effective mass and stiffness determine the resonant frequency, so the resonant frequency can be trimmed by adding, removing, or changing the material of the resonator. An electronically controlled frequency trimming technique for the local thermal oxidation of a single crystal silicon resonator was illustrated [77]. When the MEMS resonator is biased with a relatively large current in an oxygen-rich environment, a thin layer of silicon dioxide can thermally grow on the silicon surface of the resonant structure.…”