Wei-Chang Li

Oscillator far-from-carrier phase noise reduction via nano-scale gap tuning of micromechanical resonators

Substantial improvements in the far-from-carrier phase noise of oscillators referenced to stand-alone (as opposed to arrayed) capacitively transduced microme-chanical disk resonators have been attained via the use of atomic layer deposition (ALD) to tune the electrode-to-resonator capacitive gaps. Specifically, ALD of about 30nm of hafnia (HfO2) onto the surface of a released 60-MHz micromechanical disk resonator to reduce its effective resonator-to electrode gap size from 92nm to 32nm provides an increase in power handling leading to more than 15–20dB reduction in the far-from-carrier phase noise of an oscillator referenced to this resonator. This ALD-enabled nano-scale gap tuning provides a simple and effective method to satisfy increasing demands for higher short-term stability in frequency references for electronic applications.

Quality factor enhancement in micromechanical resonators at cryogenic temperatures

Quality factors as high as 362,768 have been measured for 61-MHz polysilicon wine-glass disk micromechanical resonators operated at cryogenic temperatures down to 5K. The measured results not only represent a ~2.5× increase in Q over the room temperature value, equivalent to a nearly 10-dB improvement in phase noise; but also provide some limited insight into the intrinsic material damping and other loss mechanisms that dominate over certain temperature ranges. In particular, a measured Q versus temperature curve verifies a peaking point for phonon-phonon interaction losses at a temperature around 150K. In addition, measurement versus theory for resonators with different support designs suggests that anchor losses probably still dominate the Qs of the resonators over the entire measured temperature range, implying that improved anchor isolating designs are needed if the true intrinsic material Qs of polysilicon are to be measured at cryogenic temperatures.

Enhancement of micromechanical resonator manufacturing precision via mechanically-coupled arraying

A statistical comparison between the resonance frequency variations of stand-alone micromechanical disk resonators and mechanically-coupled array composites of them reveals that mechanically-coupled arraying of on-chip micromechanical resonators can very effectively enhance the manufacturing repeatability of resonance frequencies. In particular, twenty 3-disk resonator array-composites on a single die achieve a measured resonance frequency standard deviation as small as 165.7 ppm around a 61.25 MHz average, which is significantly smaller than the 316.4 ppm measured for twenty stand-alone disk resonators on the same die. This new standard deviation reduces the expected filter percent bandwidth achievable with a 90% confidence interval without the need for trimming from the 1.89% of previous work to now just 0.86%. Larger arrays should further reduce the frequency standard deviation, perhaps to the point of allowing trim-free RF channel-select bandwidths with reasonable manufacturing confidence interval.

Micromechanical Resonant Displacement Gain Stages

Micromechanical resonant displacement gain stages have been demonstrated that employ directionally engineered stiffnesses in resonant structures to effect displacement amplification from a driven input axis to an output axis. Specifically, the introduction of slots along the output axis of a 53-MHz wine-glass mode disk resonator structure realizes a single gain stage with a measured input-to-output displacement amplification of 3.08x. Multiple such mechanical displacement gain stages can then be cascaded in series via half-wavelength beam couplers to achieve multiplicative gain factors; e.g., two cascaded gain stages achieve a total measured gain of 7.94x. The devices have also been operated as resonant switches, where displacement gain allows impact switching via actuation voltages of only 400mV, which is 6x smaller than for previous resoswitches without displacement gain. The availability of such high frequency displacement gain strategies for resonant switches may soon allow purely mechanical periodic switching applications (such as power amplifiers and power converters) with much higher efficiencies than current transistor-based versions.

Digitally-specified micromechanical displacement amplifiers

A micromechanical displacement amplifier comprising two asymmetric resonator array composites coupled by a quarter-wavelength beam has been demonstrated that permits specification of gain factor by mere (digital) selection of an appropriate ratio of the number of resonators in an input array to that in an output array. Like the method of [1], this displacement gain circuit is a key enabler for resoswitch-based mechanical power amplifiers and power converters, because it can prevent unwanted drive electrode-to-resonator impact in such circuits. This design, however, differs from that of [1] in that 1) it can be applied to radial-contour mode disks that can achieve much higher frequency than the wine-glass disks of [1]; 2) it preserves the frequency and Q of its constituent resonators (whereas the method of [1] changed the frequency and lowered the Q); and 3) its digital method for gain specification is much more straightforward, accurate, and repeatable.

A resonance dynamical approach to faster, more reliable micromechanical switches

The resonance and nonlinear dynamical properties of micromechanical structures have been harnessed to demonstrate an impacting micromechanical switch with substantially higher switching speed, better reliability (even under hot switching), and lower actuation voltage, all by substantial factors, over conventional RF MEMS switches. The particular resoswitch implementation demonstrated in this work comprises a wine-glass mode disk resonator, driven hard via a 2.5V amplitude ac voltage at its 61-MHz resonance frequency so that it impacts electrodes along an orthogonal switch axis, thereby closing a switch connecting a signal through switch axis electrodes. The 61-MHz operating frequency corresponds to a switching period of 16ns with an effective rise time of ~4ns, which is more than 50 times faster than the mus-range switching speeds of the fastest conventional (non-resonant) RF MEMS switches. Furthermore, with the signal on during switching, a capacitive version of the switch has hot switched for more than 16.5 trillion cycles without failure, which is substantially more than the 100 billion cycles normally posted by conventional RF MEMS switches. The reliability of the present resoswitch benefits from the high stiffness of its actuating disk resonator, which provides a large restoring force with which to overcome sticking forces; and from the energy stored via resonance vibration that provides a momentum that further increases the effective restoring force. Resonance operation in turn allows the actuation voltage amplitude to be a mere 2.5V, despite the large spring restoring force. Such mechanically resonant switches (dubbed ldquoresoswitchesrdquo) used in place of the switching transistors in switched-mode power converters and power amplifiers stand to greatly enhance efficiencies by allowing the use of much higher power supply voltages than allowable by transistors.

Acceleration sensitivity of small-gap capacitive micromechanical resonator oscillators

The vector components of acceleration sensitivity Γ for a closed-loop oscillator referenced to a wine-glass disk array-composite resonator employing tiny (∼92nm) electrode-to-resonator capacitive transducer gaps were measured along axes perpendicular and parallel to the substrate to be Γvertical∼13.6ppb/g and Γlateral∼4.92ppb/g, respectively, which are on par with commercial quartz-based oscillator products. Interestingly, the measured acceleration sensitivity greatly exceeds the prediction of theory. In particular, models for frequency shifts due to variations in electrical stiffness and mechanical stress predict acceleration sensitivities orders of magnitude lower than measured here. Consideration of other microphonic contributors reveals that the measurements of this work were probably limited by the bond wires and package stresses of the board-level realization of the oscillator, so are very likely not representative of the performance actually achievable by a fully-integrated micromechanical resonator oscillator, where MEMS and transistors share a single chip. Still, the measured microphonic performance on par with mid-grade quartz oscillators at least provides some reassurance that the tiny electrode-to-resonator gaps used in high frequency capacitively transduced micromechanical resonators will not compromise the stabilities of oscillators referenced to them in conventional applications that currently accept mid-grade quartz resonators.

A metal micromechanical resonant switch for on-chip power applications

A micromechanical resonant switch, or “resoswitch” (c.f., Fig. 1), constructed in nickel metal rather than previously used polysilicon attains a switch FOM >50 THz, which is several times higher than so far attained by power FET devices and pin diodes. Here, the use of metal reduces the “on” resistance of the resoswitch to less than 1Ω, allowing it to generate 17.7dB of sustained electrical power gain at 25MHz when embedded in a simple switched-mode power amplifier circuit, marking the first successful demonstration of RF power gain using a micromechanical resonant switching device. The high FOM of this device may soon permit the near 100% efficiency predicted for Class-E switched-mode power amplifiers that has eluded transistor-based versions for decades. This in turn would greatly extend battery lifetimes for portable wireless communications and other applications.

Hollow stems for higher micromechanical disk resonator quality factor

The use of hollow support stems to reduce energy loss to the substrate while supporting all-polysilicon UHF micromechanical disk resonators has enabled quality factors as high as 56,061 at 329 MHz and 93,231 at 178 MHz - values now in the same range as previous disk resonators employing multiple materials with more complex fabrication processes. With a substantially smaller cross-sectional area compared with the full stems used by predecessors, the hollow stem of this work effectively squeezes the energy conduit between the disk structure and the substrate, thereby suppressing energy loss and maximizing Q for devices operating in radial-contour and whispering gallery modes. Measurements confirm Q enhancements of 2.6× for contour modes at 154 MHz and 2.9× for wine glass modes around 112 MHz over values previously achieved by full stem all-polysilicon disk resonators with identical dimensions. The measured results not only demonstrate an effective Q-enhancement method with minimal increase in fabrication complexity, but also provide insights into anchor loss mechanisms that have been largely responsible for limiting the Qs attainable by all-polysilicon capacitively-transduced MEMS resonators.

Soft-impacting micromechanical resoswitch zero-quiescent power AM receiver

A micromechanical resonant switch-based envelope detector employing a soft-impact cantilever output electrode to affect a linear input amplitude-to-output level gain has successfully received, filtered, amplified, and demodulated an input AM signal over a 63-kHz carrier, effectively demonstrating a zero-quiescent power AM receiver to complement a previous such FSK receiver. The RF front-end achieves a power gain of 30dB and demodulates without needing a power hungry local oscillator. Key to operation as an AM receiver is the linear analog amplification (as opposed to binary amplification in the work of Liu et al. (2015)) enabled by the flexible soft-impact output electrode. The higher Q of 1200 of the polysilicon device with Ru-silicide contact further increases the receiver sensitivity to -68dBm, which is 8dB better than that in the work of Liu et al. (2015). The ability of this device to AM demodulate while consuming zero power during standby periods makes it ideal for use in ultra-low power long-distance receiver applications, such as the clocks that wirelessly receive atomic clock time over thousands of miles via AM over a 60-kHz carrier.