Yoonkee Kim

Noise in microelectromechanical system resonators

Microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) based resonators and filters, ranging in frequencies from kHz to GHz, have been proposed. The question of how the stabilities of such resonators scale with dimensions is examined in this paper, with emphasis on the noise characteristics. When the dimensions of a resonator become small, instabilities that are negligible in macro-scale devices become prominent. The effects of fluctuations in temperature, adsorbing/desorbing molecules, outgassing, Brownian motion, Johnson noise, drive power and self-heating, and random vibration are explored. When the device is small, the effects of fluctuations in the numbers of photons, phonons, electrons and adsorbed molecules can all affect the noise characteristics. For all but the random vibration-induced noise, reducing the dimensions increases the noise. At submicron dimensions, especially, the frequency noise due to temperature fluctuations, Johnson noise, and adsorption/desorption are likely to limit the applications of ultra-small resonators.

Uncooled IR imaging array based on quartz microresonators

A quartz crystal resonators resonance frequency is sensitive to temperature. This sensitivity has been exploited in the past in thermometers made of single, macroscopic quartz resonators that can accurately detect temperature changes of /spl mu/K. Using semiconductor microfabrication techniques, it is now possible to fabricate a large number of microresonators from a single quartz wafer. It is shown that combining the small thermal mass and high thermal isolation capability of such microresonators, the steep frequency versus temperature characteristics of resonators made of certain cuts of quartz and the low-noise characteristics of quartz crystal oscillators can result in high-performance infrared (IR) sensors and sensor arrays. In a microresonator sensor, the temperature change produced by the absorption of IR energy results in a frequency change that can be measured with a resolution that corresponds to a change in the resonators temperature of less than a /spl mu/K. Calculation shows that an array of microresonators in the 200 MHz-1 GHz range can be the basis of an uncooled IR imaging system with a noise equivalent temperature difference, NETD, of <0.01 K. The design and fabrication problems to be overcome before such microresonator arrays can be realized are discussed.

ZnO films on {001}-cut -propagating GaAs substrates for surface acoustic wave device applications

A potential application for piezoelectric films on GaAs substrates is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Knowledge of the SAW properties of the layered structure is critical for the optimum and accurate design of such devices. The acoustic properties of ZnO films sputtered on {001}-cut <110>-propagating GaAs substrates are investigated in this article, including SAW velocity, effective piezoelectric coupling constant, propagation loss, diffraction, velocity surface, and reflectivity of shorted and open metallic gratings. The measurements of these essential SAW properties for the frequency range between 180 and 360 MHz have been performed using a knife-edge laser probe for film thicknesses over the range of 1.6-4 /spl mu/m and with films of different grain sizes. The high quality of dc triode sputtered films was observed as evidenced by high K/sup 2/ and low attenuation. The measurements of the velocity surface, which directly affects the SAW diffraction, on the bare and metalized ZnO on SiO/sub 2/ or Si/sub 3/N/sub 4/ on {001}-cut GaAs samples are reported using two different techniques: 1) knife-edge laser probe, 2) line-focus-beam scanning acoustic microscope. It was found that near the <110> propagation direction, the focusing SAW property of the bare GaAs changes into a nonfocusing one for the layered structure, but a reversed phenomenon exists near the <100> direction. Furthermore, to some extent the diffraction of the substrate can be controlled with the film thickness. The reflectivity of shorted and open gratings are also analyzed and measured. Zero reflectivity is observed for a shorted grating. There is good agreement between the measured data and theoretical values.<<ETX>>

Sensing the properties of liquids with doubly rotated resonators

Immersion of a resonator into a liquid results in changes in the resonators frequency and impedance. These changes have been used to characterize liquid properties. Frequency can be measured with far higher accuracy than impedance or any other quantity. The goal of the project described in this paper was to investigate whether or not it is possible to measure a liquids properties by means of frequency measurements alone. When a doubly rotated resonator (/spl theta//spl ap/35/spl deg/ and /spl phi/>0/spl deg/) is operated in a liquid, the displacement of the surface is partly out of the plane of the plate. By controlling the /spl phi/ angle, one may control the ratio of in-plane to out-of-plane displacements. The out-of-plane component of the displacement propagates a damped compressional wave into the liquid, while the in-plane component propagates a damped shear wave. The frequency changes of doubly rotated resonators have been measured in glycerol solutions of a variety of concentrations. At each concentration, the frequency change was found to increase with increasing /spl phi/ angle. A method is proposed for the determination of a fluids density, viscosity and acoustic wave velocity.

Force-frequency effect of Y-cut langanite and Y-cut langatate

Most recently, langasite and its isomorphs (LGX) have been advanced as potential substitutes for quartz, owing to their extremely high-quality (Q) factors. At least twice higher Q value of LGX than that of quartz has been reported. High Q translates into potentially greater stability. In order to make such materials practical, the environmental sensitivities must be addressed. One of such sensitivities is the force-frequency effect, which relates the sensitiveness of a resonator to shock and vibration via the third-order (non-Hookean) elastic constants. In this paper, we report measured force-frequency coefficients of a Y-cut langanite (LGN) resonator and a Y-cut langatate (LGT) resonator as a function of the azimuthal angle, which is the angle between the crystalline X-axis of a resonator plate and the direction of in-plane diametric force applied to the periphery of the resonator. It was found that the LGN and the LGT behave like AT-cut quartz in the polarity of the frequency changes and the existence of zero-coefficient angle. The maximum magnitudes of the coefficients of the LGN and the LGT are five and seven times smaller than that of stress-compensated cut (SC-cut) quartz, respectively (or, 7 and 10 times smaller comparing to AT-cut quartz). The coefficients of planar-stress, which represent the superposition of a continuous distribution of periphery stresses, also were obtained as 0.52/spl times/10/sup -15/ m/spl middot/s/N and 0.38/spl times/10/sup -15/ m/spl middot/s/N for the LGN and the LGT, respectively.

A temperature insensitive quartz microbalance

Mass deposition onto a microbalance is generally accompanied by a temperature change. By measuring a single frequency only, it is not possible to separate the frequency change due to mass change from that due to temperature change. In the temperature insensitive microbalance technique, measurements of two frequencies, the fundamental mode and third overtone frequencies of an SC-cut resonator, yield two equations with two unknowns. This allows the separation of mass change effects from temperature change effects. Dual mode excitation can be used for highly accurate resonator self-temperature sensing over wide temperature ranges. SC-cut resonators are also thermal transient compensated. These unique properties allowed the development of a temperature compensated microbalance that is highly sensitive to mass changes, which can be used in rapidly changing thermal environments, over wide temperature ranges, and which requires neither temperature control nor a thermometer other than the resonator. To demonstrate the performance of this microbalance, SC-cut resonators were coated with thin polymethylmethacrylate (PMMA) photoresist films then placed into a UV-ozone cleaning chamber that initially was at about 20/spl deg/C. When the UV lamp was turned on, the UV-ozone removed PMMA from the surfaces while the chamber temperature rose to about 60/spl deg/C. The frequency changes due to mass changes could be accurately determined, independently of the frequency changes due to temperature changes.

Aging of dual mode resonator for microcomputer compensated crystal oscillator

A microcomputer compensated crystal oscillator (MCXO) utilizes the dual c-mode excitation (fundamental mode and 3rd overtone (OT)) of an SC-cut resonator for self-temperature sensing and compensation. The long-term stability of the MCXO depends primarily on the aging of the dual mode resonator. When two modes age differently in time, the aging MCXOs output frequency curve would shift with a tilt over its operating temperature range. In this paper, we report the aging of the dual modes of the 20 MHz 3rd OT SC-cut MCXO resonators. The resonators were measured over the -55degC ~ +85degC temperature range with the self-temperature sensing technique utilized for the MCXO. The measured aging of the two modes ranges from 0.083 ppb to 0.386 ppb per day. The fundamental mode shows more aging than the OT. Observing the different aging of the two modes is not surprising since the mode shape effect and stress relief would be different between the two modes. The different aging rates exacerbate the long-term aging in the MCXO by increasing the offset more than the worse aging rate of the two

Force-frequency effects of Y-cut langanite and Y-cut langatate

Most recently, langasite and its isomorphs (LGX) have been advanced as potential substitutes for quartz, owing to their extremely high-quality (Q) factors. At least twice higher Q value of LGX than that of quartz has been reported. High Q translates into potentially greater stability. In order to make such materials practical, the environmental sensitivities must be addressed. One of such sensitivities is the force-frequency effect, which relates the sensitiveness of a resonator to shock and vibration via the third-order (non-Hookean) elastic constants. In this paper, we report measured force-frequency coefficients of a Y-cut langanite (LGN) resonator and a Y-cut langatate (LGT) resonator as a function of the azimuthal angle, which is the angle between the crystalline X-axis of a resonator plate and the direction of in-plane diametric force applied to the periphery of the resonator. It was found that the LGN and the LGT behave like AT-cut quartz in the polarity of the frequency changes and the existence of zero-coefficient angle. The maximum magnitudes of the coefficients of the LGN and the LGT are five and seven times smaller than that of stress-compensated cut (SC-cut) quartz, respectively (or, 7 and 10 times smaller comparing to AT-cut quartz). The coefficients of planar-stress, which represent the superposition of a continuous distribution of periphery stresses, also were obtained as 0.52/spl times/10/sup -15/ m/spl middot/s/N and 0.38/spl times/10/sup -15/ m/spl middot/s/N for the LGN and the LGT, respectively.Most recently, langasite and its isomorphs (LGX) have been advanced as potential substitutes for quartz, owing to their extremely high quality (Q) factors. At least 3 times higher Q value of LGX than that of quartz has been reported. High Q translates into potentially greater stability. In order to make such materials practical, the environmental sensitivities must be addressed. One such sensitivity is the force-frequency effect, which relates the sensitivity of a resonator to shock and vibration via the third-order (non-Hookean) elastic constants. In this paper, we report measured force-frequency coefficients of a Y-cut LGN resonator and a Y-cut LGT resonator as a function of the azimuthal angle, which is the angle between the crystalline x-axis of a resonator plate and the direction of in-plane diametric force applied to the periphery of the resonator. It was found that the LGN and the LGT behave like AT-cut quartz in the polarity of the frequency changes and the existence of zero-coefficient angle. The maximum magnitudes of the coefficients of the LGN and the LGT are 5 and 7 times smaller than that of SC-cut quartz, respectively (or, 7 and 10 times smaller compared to AT-cut quartz). The coefficients of planar-stress, which represent the superposition of a continuous distribution of periphery stresses, were also obtained as 0.52/spl times/10/sup -15/ m/spl middot/s/N and 0.38/spl times/10/sup -15/ m/spl middot/s/N for the LGN and the LGT, respectively.

Hysteresis measurements of 20 MHz third overtone SC-cut MCXO resonators

The microcomputer compensated crystal oscillator (MCXO) utilizes dual c-mode excitation of an SC-cut resonator for self-temperature sensing and compensation. The dual mode operation requires well-behaved fundamental and 3/sup rd/ overtone c-modes over the operating temperature range (e.g. -55/spl deg/C to +85/spl deg/C). The stability of MCXO depends primarily on the frequency vs. temperature hysteresis of the resonator. The hysteresis of the 10 MHz 3/sup rd/ overtone MCXO resonator has been a few parts in 10/sup 8/. This resonator is packaged in an HC-40 metal can, which makes the resonator the largest single component in the MCXO. The size of this resonator has limited efforts to reduce the size of the MCXO. In this paper, results of hysteresis measurements of a 20 MHz 3/sup rd/ overtone SC-cut resonator packaged in a TO-5 metal can (/spl sim/13% the volume and /spl sim/57% the height of the HC-40 package) is reported. Hysteresis over the -55/spl deg/C to +85/spl deg/C temperature range was found to be a few parts in 10/sup 8/, comparable to that of the 10 MHz 3/sup rd/ overtone resonator.

Sensing of organic vapor adsorption on gold using a temperature insensitive microbalance

The adsorption of organic vapor molecules onto the gold electrode surface of a dual mode 10 MHz 3rd overtone SC-cut quartz crystal microbalance (QCM) was investigated. Using this dual mode QCM, the separation of mass and temperature change can be made in real time, with resolutions of 56 pg and better than 0.01 K, respectively. The QCM experiments were performed in a UV/ozone chamber so that the gold electrodes could be maintained in a clean state. Organic vapors or liquids (e.g., benzene, hexane, methanol, ethanol, isopropanol, or butanol) were injected into the chamber while the two modes of the dual mode QCM were monitored and the mass-frequency and temperature changes were extracted. This paper demonstrates the ability of the sensor to determine in a matter of seconds, sub-ppm (ppm/spl equiv/parts per million) gas concentrations of benzene and hexane while operating in an elevated temperature environment without any temperature stabilization. In addition, the mass changes for a series of adsorbed methanol, ethanol, isopropanol, and butanol were found to be proportional to molecular weights.