Bongsang Kim1, Li-Wen Hung and Clark Nguyen
Defense Advanced Research Projects Agency
To date, micromechanical resonators have been implemented using MEMS-technology with resonance frequencies in the UHF band and with tremendous quality factor on the order of 11,000, making them potentially suitable as on-chip frequency reference elements for ultra stable oscillators needed in wireless communications. Indeed a UHF oscillator with long- and short-term stability on par with that of quartz crystal oscillators would be highly desirable. However, before such an oscillator can be achieved, a number of device issues must be overcome, such as high impedance, limited linearity and noise sources, all of which can get worse as device dimensions are scaled to achieve higher frequency. To combat such scaling-induced issues, their mechanism must first be modeled, which should then reveal the most effective strategies for nulling them, such as arraying and electrode-to-resonator gap scaling for high power handling and noise suppression.
ALD (Atomic Layer Deposition) of HfO2 was used to partially fill the electrode-to-resonator gap of a wine-glass disk resonator so as to reduce its effective gap spacing from 97 nm to 37 nm (Figure 1), and thereby greatly lower its motional resistance. This reduction of gap spacing additionally improves the maximum power handling capability of the resonator when emplaced in the oscillator circuit shown in Figure 2, and this in turn allows for better far-from-carrier phase noise. To verify this, the phase noise of a 60-MHz wine-glass disk oscillator circuit was compared using an HP E5500 phase noise measurement setup, before and after ALD partial-gap filling of the reference wine-glass disk resonator. As shown in Figure 3, after ALD-coating, and for the case where amplitude limiting is achieved via a voltage clamping circuit, the “far-from-carrier” phase noise performance of the oscillator was improved by more than 15 dB from -125 dBc to -110 dBc.
Figure 1: SEM and schematic of ALD partial gap-filled resonators. The effective gap size was reduced from 97 nm to 37 nm.
Figure 2: a) Schematic of the oscillator circuit, which consists of a MEMS resonator, a transimpedance amplifier, and a clamping amplifier; b) Picture of oscillator PCB. The MEMS resonator is mounted in the circuit board and connected to the circuit through wire-bonds; c) Oscillator circuit is installed in a custom-made vacuum test setup for phase noise measurement in high vacuum (~μTorr)
Figure 3: Phase noise comparison plot of 60 MHz wine-glass mode disk resonator oscillator before and after ALD partial-gap filling. As can be seen, its “far-from-carrier” phase noise has been improved by more than 15 dB while it has shown some degrade in “close-to-carrier” phase noise.