MEMS-based oscillators are an emerging class of highly miniaturized, batch manufacturable timing devices that can rival the electrical performance of well-established quartz-based oscillators. In this review, a description is given of the key properties of a MEMS resonator that determine the overall performance of a MEMS oscillator. Piezoelectric, capacitive and active resonator transduction methods are compared and their impact on oscillator noise and power dissipation is explained. An overview is given of the performance of MEMS resonators and MEMS-based oscillators that have been demonstrated to date. Mechanisms that affect the frequency stability of the resonator, such as temperature-induced frequency drift, are explained and an overview is given of methods that have been demonstrated to improve the frequency stability. The aforementioned performance indicators of MEMS-based oscillators are benchmarked against established quartz and CMOS technologies.

https://iopscience.iop.org/article/10.1088/0960-1317/22/1/013001/pdf

A review of MEMS oscillators for frequency reference and timing applications

In this paper, we review a recent technology development based on coupled MEMS resonators that has the potential of fundamentally transforming MEMS resonant sensors. Conventionally MEMS resonant sensors use only a single resonator as the sensing element, and the output of the sensor is typically a frequency shift caused by the external stimulus altering the mechanical properties, i.e. the mass or stiffness, of the resonator. Recently, transduction techniques utilizing additional coupled resonators have emerged. The mode-localized resonant sensor is one example of such a technique. If the mode localization effect is utilized, the vibrational amplitude pattern of the resonators changes as a function of the quantity to be measured. Compared to using frequency shift as an output signal, the sensitivity can be improved by several orders of magnitude. Another feature of the mode-localized sensors is the common mode rejection abilities due to the differential structure. These advantages have opened doors for new sensors with unprecedented sensitivity.

A review on coupled MEMS resonators for sensing applications utilizing mode localization

This paper demonstrates two variations of Temperature compensated (TempCo) FBAR resonators with high Kt2. One 1.5 GHz non-symmetric stack design TempCo FBAR resonator has a Kt2 of 4.28% and linear TCF of 0 ppm/°C. A second, quasi-symmetric stack design 1.5GHz TempCo FBAR resonator has Kt2 as high as 5.6% and linear TCF of −6 ppm/°C. Significant Kt2 improvement comes from optimal design of stack film, interposer electrode effect and novel process development of a sealant for the oxide to protect it from HF etching. This paper also discusses the trade-off between two parameters (linear TCF vs. Kt2). High Kt2 TempCo FBAR resonator is ideal for FBAR oscillator application with wide frequency pulling range.

High coupling coefficient Temperature compensated FBAR resonator for oscillator application with wide pulling range

High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO2-AlN-SiO2 sandwiched between electrodes have been developed. Acoustic isolation was achieved by means of deep silicon etching to obtain membrane type thin film bulk acoustic wave resonators (TFBARs). Three different device versions were investigated. The SiO2 film thicknesses were varied (0 nm, 70 nm, 310 nm, and 770 nm) while the piezoelectric AlN film had a constant thickness of 1.2 mum. The sputter-deposited AlN film grown on the amorphous, sputter-deposited SiO2 layer exhibited a d33,f of 4.0 pm/V. Experimental results of quality factors (Q) and coupling coefficients (kt 2) are in agreement with finite element calculations. A Q of 2000 is observed for the first harmonic of the 310 nm oxide devices. The most intense resonance of the 770 nm oxide device is the third harmonic reaching Q factors of 1450. The temperature drift reveals the impact of the SiO2 layers, which is more pronounced on the first harmonic, reducing the TCF to 4 ppm/K for the 3rd harmonic of the 310 nm oxide devices.

High-Q AlN/SiO 2 symmetric composite thin film bulk acoustic wave resonators

This paper reports on the demonstration of series tuning for lateral micromechanical oscillators and its application for electronic temperature compensation of piezoelectric lateral bulk acoustic resonator (LBAR) micromechanical oscillators. Two aluminum nitride-on-silicon (AlN-on-Si) piezoelectric LBARs, one operating at 427 MHz (Rm ≈180 Ω, Qunloaded ≈ 1400) and the other operating at 541 MHz (Rm ≈ 55 Ω, Qunloaded ≈ 3000) are interfaced with a 13 mW three-stage tunable TIA implemented in 0.18 μm 1P6M CMOS process to sustain the oscillation. Recognizing the impact on the frequency tuning range due to the body capacitances appearing in parallel with the ports of the resonator, the TIA uses parasitic cancellation techniques to neutralize this effect and boost the tuning range of 427 MHz and 541 MHz oscillators, by as much as 12× to 810 ppm and 1,530 ppm, respectively, with negligible impact on the phase noise performance. The shunt parasitic capacitor is either resonated out with an active inductor or is cancelled out by using a single-terminal negative capacitor of equal value. However, the oscillator that uses negative capacitance parasitic cancellation yields larger tuning. This extended tuning range is used for temperature compensation. A 2 mW bandgap-based temperature compensation circuit which uses second-order parabolic approximation is fabricated on the same chip. Using this temperature compensation circuit has lowered the overall frequency drift of a 427 MHz tunable oscillator using negative capacitance cancellation from ±390 ppm to ±35 ppm in the -10°C to 70°C temperature range. The phase noise of this oscillator reaches -82 dBc/Hz at 1 kHz offset. The total phase noise variation for offset frequencies below 10 kHz is under 5 dB within the specified tuning range, and the best phase noise floor is under -147 dBc/Hz . Due to the higher Q and lower insertion loss of the resonating tank, the 541 MHz oscillator achieves -86 dBc/Hz at 1 kHz offset, and lower phase noise floor of -158 dBc/Hz.

Electronic Temperature Compensation of Lateral Bulk Acoustic Resonator Reference Oscillators Using Enhanced Series Tuning Technique

A reference oscillator operating at 2 GHz and based on a temperature compensated bulk acoustic wave resonator is presented. Temperature compensation is realized by addition of an extra SiO2 layer in the resonator. The temperature drift of the resonance frequency of the resonator is -5 ppm/K. The oscillator circuit has the Butler topology and uses the BAW resonator at series resonance. The demonstrator incorporating a high impedance resonator shows a low phase noise: -99 dBc/Hz and -120 dBc/Hz at 10 kHz and 100 kHz offset, respectively. The drift of the oscillator frequency with temperature is almost linear and almost equal to that of the stand-alone resonator. The supply voltage dependence is 100 ppm/V in this oscillator

4D-3 A 2 GHz Reference Oscillator incorporating a Temperature Compensated BAW Resonator

For a three-dimensional description of rectangular BAW resonators, a simple vibrating membrane model is revisited. Displacement profiles from this model are compared to measured profiles. Furthermore, a method for extraction of dispersion characteristics from electrical measurements is proposed and verified against interferometry measurements.

11E-4 Application of a Vibrating Membrane Model to Bulk-Acoustic-Wave Resonators

A two dimensional model for Bulk Acoustic Wave (BAW) resonators and filters is applied to predict electrical behaviour for various configurations. The results are explained in terms of lateral mode excitation at the electrode edge. It is found that both the vertical and lateral particle displacement should be considered to achieve realistic results, especially for the case of thick top electrodes. Furthermore the industrialization of the solidly mounted bulk acoustic wave filters within Philips Semiconductors is discussed. The industrial flow towards a wafer-level chip-scale packaged product for the USPCS standard is described. The characteristics of the first samples are close to the product specifications.

F. Vanhelmont

Papers

4D-3 A 2 GHz Reference Oscillator incorporating a Temperature Compensated BAW Resonator

11E-4 Application of a Vibrating Membrane Model to Bulk-Acoustic-Wave Resonators

Bulk acoustic wave resonators and filters - 2d modelling and industrialization aspects