Frequency stability in thin-film piezoelectric-on-substrate oscillators

Abstract

For many years, crystal oscillators have been used as the de facto frequency reference in almost all electronic platforms because they offer excellent stability and superior phase noise. This is mainly due to the high quality factor (Q) and exceptional temperature stability of quartz crystals. However, the size of quartz resonators is relatively large, and they cannot be readily integrated with microelectronics. This ultimately impedes the complete integration of the high-performance oscillators with the electronics. Achieving such integration will enable frequency control devices with a smaller form factor, lower cost, greater flexibility, and potentially higher reliability. Microelectromechanical systems (MEMS) resonator technology is gradually gaining popularity as a solution for the integration barrier and high-performance micro-machined oscillators have been presented by researchers and companies recently. However, one of the most important drawbacks of MEMS resonators has been their relatively large and linear temperature coefficient of frequency (TCF) (e.g., around -30 ppm/C for Si-based).

The subject of this presentation is on the frequency stability in thin-film piezoelectric-on-substrate oscillators (TPoS). In this regard, jitter and temperature dependency of the oscillation frequency are studied. The dependency of jitter of TPoS on the resonator characteristics (i.e. quality factor and motional impedance) is studied where the results provide experimental validation for the suppression of overall oscillator circuit noise through the operation of the resonator beyond the bifurcation.

A novel temperature compensation technique for silicon-based lateral-extensional MEMS oscillators is introduced, which is based on the properly orienting an extensional-mode resonator on a highly doped n-type silicon substrate. The existence of a local zero temperature coefficient of frequency (i.e., turnover point) in extensional-mode silicon microresonators, fabricated on highly n-type-doped substrates and aligned to the [100] crystalline orientation is demonstrated. It is shown that the turnover point in TPoS resonators is a function of doping concentration and orientation. Moreover, the turnover point can be adjusted by changing the thickness ratio of Si and the piezoelectric film (e.g., AlN) in the resonant structure. MEMS oscillators with controlled temperature coefficient of frequency (TCF), assembled through mixing the frequencies of two oscillators that are made of silicon micro-resonators with known and dissimilar TCF, are also introduced. Based on this method, a TPoS MEMS oscillator is assembled in which the first-order TCF is virtually cancelled resulting in a parabolic TCF curve (second-order TCF).

The frequency tuning in TPoS resonators is also reported which results show a great potential application in temperature compensated oscillators. Tuning is demonstrated through varying the termination load connected to an isolated tuning port. The dependency of frequency tuning on the design features of the resonator is studied as well.