U. Stehr

Probing anchor losses in AlN-on-Si contour mode MEMS resonators through laser Doppler vibrometry

We present the novel application of high frequency laser Doppler vibrometry to measure the underlying mechanism leading to anchor loss that now limits the quality factor of low impedance contour mode aluminum nitride (AlN) on silicon resonators. We applied the technique to two resonators designed to have largely different quality factors (2100 vs. 12000) in order to correlate the findings from laser vibrometer measurements to anchor loss. We show that the vibrometer measurements are in close harmony with findings from finite element analysis. These results allow us to move beyond pure simulations in the course of analyzing anchor losses in AlN-on-Si contour mode resonators.

Hybrid-integrated RF MEMS-based reference oscillator using a silicon-ceramic composite substrate

In this paper, the design of a RF MEMS oscillator on a silicon-ceramic composite substrate using a high-Q Lamb-wave resonator as frequency-selective device is described. The MEMS resonator is designed on a 1.8 μm thick piezoelectric AlN layer, deposited on silicon using thin-film processes. The finite-element simulation results of the resonator structure are presented, and the derivation of the electrical equivalent-circuit is described. The active part of the MEMS oscillator, which was laid out in a Pierce topology, has been integrated in an application-specific integrated circuit fabricated in CMOS technology. Both, amplifying and frequency-selective parts are hybrid-integrated on a unique silicon-ceramic composite substrate, which enables a very compact high-quality module design with minimal parasitics. The MEMS oscillator serves as a technology demonstrator combining the advantages of microelectronic and microelectromechanical components towards a compact and power-efficient hybrid technology, e.g. for mobile communications or wireless sensors.

An electronically tuneable inductance with extended frequency range

The extension of the usable frequency range of an electronically tuneable inductance circuit is investigated. By means of numerical and analytical studies, the transistor-internal reverse transfer capacitance can be identified as a frequency-limiting element, which must be minimised. This is achieved by circuitry-wise compensation of the capacitive effect by introducing an additional inductance. The concept is verified through measurements of the practical implementation and shows an extension of the frequency range by two octaves throughout the VHF range, which proves beneficial for applications such as broadband matched and tuneable transmission lines, amplifier circuits, oscillators, or filters.

An Analytical Temperature-Dependent Design Model for Contour-Mode MEMS Resonators and Oscillators Verified by Measurements

The importance of micro-electromechanical systems (MEMS) for radio-frequency (RF) applications is rapidly growing. In RF mobile-communication systems, MEMS-based circuits enable a compact implementation, low power consumption and high RF performance, e.g., bulk-acoustic wave filters with low insertion loss and low noise or fast and reliable MEMS switches. However, the cross-hierarchical modelling of micro-electronic and micro-electromechanical constituents together in one multi-physical design process is still not as established as the design of integrated micro-electronic circuits, such as operational amplifiers. To close the gap between micro-electronics and micro-electromechanics, this paper presents an analytical approach towards the linear top-down design of MEMS resonators, based on their electrical specification, by the solution of the mechanical wave equation. In view of the central importance of thermal effects for the performance and stability of MEMS-based RF circuits, the temperature dependence was included in the model; the aim was to study the variations of the RF parameters of the resonators and to enable a temperature dependent MEMS oscillator simulation. The variations of the resonator parameters with respect to the ambient temperature were then verified by RF measurements in a vacuum chamber at temperatures between −35 ∘C and 85 ∘C. The systematic body of data revealed temperature coefficients of the resonant frequency between −26 ppm/K and −20 ppm/K, which are in good agreement with other data from the literature. Based on the MEMS resonator model derived, a MEMS oscillator was designed, simulated, and measured in a vacuum chamber yielding a measured temperature coefficient of the oscillation frequency of −26.3 ppm/K. The difference of the temperature coefficients of frequency of oscillator and resonator turned out to be mainly influenced by the limited Q-factor of the MEMS device. In both studies, the analytical model and the measurement showed very good agreement in terms of temperature dependence and the prediction of fabrication results of the resonators designed. This analytical modelling approach serves therefore as an important step towards the design and simulation of micro-electronics and micro-electromechanics in one uniform design process. Furthermore, temperature dependences of MEMS oscillators can now be studied by simulations instead of time-consuming and complex measurements.

Very-low phase noise RF-MEMS reference oscillator using AlN-on-Si resonators achieved by accurate co-simulation

Reference oscillators are crucial hardware components of radio-frequency receiver circuits, as their performance directly affects the system performance. Especially in GHz applications, such as 4G/5G mobile communications, a low error-vector magnitude is required, which is strongly influenced by the phase noise of the reference oscillator. This paper reports the results of the design, simulation, and measurement of a MEMS oscillator with very low phase noise. Therefore, it is suitable for use as reference oscillator operating at high frequencies in RF receiver systems. While the MEMS device is a plate-shaped contour-mode resonator in an aluminium-nitride-on-silicon technology, the active part of the oscillator is designed and fabricated in a 180 nm CMOS technology. By adding the parasitic effects of the assembly, taken from measurements of the submodules, the results from system simulation and measurement show good agreement, i.e. only 3 dB deviation in the noise floor of −142 dBc/Hz. The phase-noise level of the oscillator at an offset of 1kHz from the operating frequency of 256 MHz is −112 dBc/Hz, among the lowest values reported for MEMS-based oscillators at this high frequency.