Showing papers by "Clark T.-C. Nguyen published in 2011"

Capacitive-piezoelectric AlN resonators with Q>12,000

Abstract: A “capacitive-piezo” transducer that separates a piezoelectric resonator from its electrodes via small gaps to eliminate resonator-to-electrode loss while maintaining strong electromechanical coupling [1], has enabled demonstration of a 50-MHz wine-glass disk (WGD) resonator array with Q=12,748. This is higher than exhibited by any sputtered thin-film AlN resonator measured to date at any frequency and more than 2.2× larger than previously achieved by similar devices using conventional (i.e., contacting) electrodes [2]. By attaining such high Q, this work erases a common belief that material losses in sputtered AlN dominate the Q's of resonators constructed in this material and confirms that sputtered AlN is a high-Q material. It further suggests that energy loss associated with contacting electrodes is primarily responsible for the low Q's of previous AlN resonators. With Q's over 10,000, this work identifies AlN as a viable material for use in channel-selecting RF front ends targeted for future software-defined cognitive radio [3].

Hot filament CVD conductive microcrystalline diamond for high Q, high acoustic velocity micromechanical resonators

Abstract: A capacitively transduced micromechanical resonator constructed in hot filament CVD boron-doped microcrystalline diamond (MCD) structural material has posted a measured Q of 146,580 at 232.441 kHz, which is 3× higher than the previous high for conductive polydiamond. Moreover, radial-contour mode disk resonators fabricated in the same MCD film and using material mismatched stems, cf., Figure 1, exhibit a Q of 71,400 at 299.86 MHz, which is the highest series-resonant Q yet measured for any on-chip resonator at this frequency. The material used here further exhibits an acoustic velocity of 18,516 m/s, which is now the highest to date among available surface micromachinable materials. For many potential applications, the hot filament CVD method demonstrated in this work is quite enabling, since it provides a much less expensive method than microwave CVD based alternatives for depositing doped CVD diamond over large wafers (e.g., 8”) for batch fabrication.

Voltage-controlled tuning to optimize MEMS resonator array-composite output power

Abstract: A voltage controlled electrical stiffness tuning method has been demonstrated to correct phase and amplitude mismatches between the constituent resonators in a half-wavelength (λ/2) mechanically coupled array-composite towards maximizing its output power. Via tuning, a nine-disk array-composite using 3 output resonators achieves an output current 2.91× larger than that of a single one of its constituent resonators, and only a bit short of the 3× theoretical maximum. Without tuning, the array-composite achieves only 2.78× the current of a single device, and the deviation from ideal is expected to increase with the number of resonators in the array. The amount of tuning available can be tailored in numerous ways, from sizing of electrode-to-disk gap spacing, to specifying the number of devices in the array involved with tuning, to simple variation of voltages across selected electrode-to-resonator gaps. By raising the power output of a high-Q micromechanical disk-array composite resonator, the method and design of this work stand to greatly lower the phase noise of oscillators referenced to such devices.

Hollow stems for higher micromechanical disk resonator quality factor

Abstract: 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 Q's attainable by all-polysilicon capacitively-transduced MEMS resonators.

A metal micromechanical resonant switch for on-chip power applications

Abstract: 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.

Etchant-free methods of producing a gap between two layers, and devices produced thereby

Abstract: Etchant-free methods of producing a gap between two materials are provided. Aspects of the methods include providing a structure comprising a first material and a second material, and subjecting the structure to conditions sufficient to cause a decrease in the volume of at least a portion of at least one of the first material and the second material to produce a gap between the first material and the second material. Also provided are devices produced by the methods (e.g., MEMS and NEMS devices), structures used in the methods and methods of making such structures.

Polysilicon-filled carbon nanotube grass structural material for micromechanical resonators

Abstract: Folded-beam capacitive-comb-driven micromechanical resonators constructed of polysilicon-filled carbon nanotube (CNT) grass structural material have been characterized via electrical measurement to extract numbers for mechanical properties. The process used here is based on that first demonstrated in [1], but improved by introduction of a guiding oxide mold and post-fabrication in situ localized annealing. Specifically, the measured frequency response for a 28.658-kHz in situ localized annealed version yields a Q of 3,230 twice as high as that of an un-annealed one; and an acoustic velocity of 9,042m/s slightly higher than the 8,024m/s of polysilicon, and inching towards the ∼12,000m/s typical of SiC. This material not only facilitates fabrication of high aspect-ratio microstructures, but also shows potential (via its silicon-carbon makeup) for approximating SiC, for which theory predicts frequency and Q advantages over silicon for resonant devices.