James A. Greer

Extremely low-phase-noise SAW resonators and oscillators: design and performance

The authors describe prototype low-noise SAW (surface acoustic wave) resonator oscillators which have demonstrated state-of-the-art phase-noise performance not only at their fundamental operating frequencies in the 400- to 600-MHz range but also after 16* frequency multiplication to X-band as well. SAW resonator designs with overmoded cavities, very wide apertures, and dual apertures, as well as modified fabrication techniques, have been used to realize an overall reduction in an oscillators phase-noise spectrum, i.e. white phi M, flicker FM, and random-walk FM. The S resonators can typically handle incident RF power in excess of +20 dBm, a key requirement to achieving an extremely low oscillator-phase-noise floor. A novel burn-in procedure at relatively high incident-RF-power levels (>27 dBm) was used to reduce both the flicker FM and random-walk FM phase-noise levels. Using these various techniques, a 5- to 15-dB improvement in the overall phase-noise spectrum for several prototype oscillators was demonstrated.<<ETX>>

1/f noise in etched groove surface acoustic wave (SAW) resonators

Measurements of 1/f (or flicker) frequency fluctuations in SAW resonators fabricated with etched groove reflectors on single crystal quartz have shown that the observed noise levels vary inversely with device size. These measurements were made on sixteen 450 MHz resonators of four different sizes. The 1/f noise levels were also evaluated on twenty-eight other SAW resonators ranging in frequency from 401 to 915 MHz. This additional data provides valuable information on the dependence of the flicker noise levels on resonator frequency. A model based an localized, independent velocity fluctuations in the quartz is proposed which correctly fits the observed size and frequency dependence of the measured 1/f noise levels. This model suggests that the velocity fluctuations originate in small regions (much less than /spl sim/5 /spl mu/m in diameter) randomly distributed throughout the quartz with an average separation of about 5 /spl mu/m between independent (incoherent) sources. The magnitude of the localized fractional velocity fluctuations, /spl Delta/v/v, averaged over a 5 micron cube is on the order of 1/spl times/10/sup -9/.<<ETX>>

SAW oscillators with low vibration sensitivity

Surface acoustic wave (SAW) oscillators with vibration sensitivities well below 1*10/sup -9/ fractional change in frequency per g have been demonstrated. Data are presented which indicate the need for a rigid support structure under an inherently low vibration sensitivity all quartz package (AQP) SAW device to achieve low vibration sensitivity at the oscillator level. An alumina stiffener supporting a hybrid circuit SAW oscillator provides this essential rigidity. Vibration sensitivity data for 26 oscillators in a 1.22-inch-square hybrid circuit package mounted on a 0.30-inch-thick alumina stiffener yielded an average value for the magnitude of the vibration sensitivity vector of 3.3*10/sup -10//g at a vibration frequency of 500 Hz. It also appears that some degree of vibration isolation at the AQP SAW device level may be achieved with the proper choice of material used to mount the AQP SAW device into the hybrid circuit package.<<ETX>>

Laser-Induced Forward Transfer Of Metal Oxides To Trim The Frequency Of Surface Acoustic Wave Resonator Devices

The frequencies of surface acoustic wave resonators (SAWRs) sealed in novel all quartz packages have been accurately set using laser-induced forward transfer (LIFT) of aluminum-oxide thin films. This technique allows accurate frequency adjustment of SAWRs over -40 parts per million (PPM) with a resolution of better than ±1 PPM. This technique does not significantly degrade relevant electrical SAWR device characteristics and provides the user with substantial cost and time savings when setting a SAWR oscillator to frequency. However, some degradation in the long-term stability of oscillators driven by laser-trimmed SAWR devices has been measured. The quality of the LIFT-deposited oxide film plays an important role in both the frequency sensitivity and long-term stability of laser-trimmed SAWR devices.

Applications of laser-trimming for all-quartz package, surface acoustic wave devices

A laser-based technique that allows the frequency of sealed, all-quartz package (AQP) surface acoustic wave (SAW) resonators to be accurately set to within +or-1 p.p.m. of a prescribed frequency has previously been reported (J.A. Greer et al., 1987). SAW resonator devices in the range of 300 MHz to 1 GHz have been laser-trimmed by as much as 100 p.p.m. The laser-trimming process is extended to a variety of new applications, including accurately adjusting the insertion phase shift of AQP SAW delay lines by as much as 90 degrees ; effectively suppressing the distortion caused by the third-order transverse mode in resonators with nonapodized transducers, which usually results in improved insertion phase shift and insertion-loss responses for the device; and frequency trimming, used either to form accurately matched pairs of AQP SAW resonators for high-power applications by effectively doubling the acoustic aperture using two SAW resonators fabricated on the same substrate surface, or to form matched pairs of devices fabricated on opposite surfaces of the same substrate. The latter application is for high-vibration environments, reducing the combined vibration sensitivity due to the cancellation of strain-induced frequency shifts. The influence of laser-trimming on the residual flicker noise and long-term frequency stability of AQP SAW devices is discussed.<<ETX>>

Optimizing AQP SAW resonators for reduced vibration sensitivity

All Quartz Package (AQP) SAW oscillators with low vibration sensitivity are achievable by minimizing the external stresses that might otherwise be transferred to the AQP SAW device. If these external stresses are at their lowest possible level, then frit geometry becomes a factor in limiting just how low the vibration sensitivity may be. Optimizing the frit geometry often involves trade-offs with the overall AQP SAW devices size requirements. Through the use of finite element modeling, it was determined that the AQPs cover and substrate thicknesses could also be used to improve vibration sensitivity. The model predicted a decrease in vibration sensitivity, with increasing cover thickness, for the vibration sensitivity component, /spl gamma//sub 1/, in a direction normal to the SAW substrate. The model also predicted a degradation in vibration sensitivity with increasing substrate thickness. These results were confirmed experimentally. The other benefit of increased cover thickness was a sizable decrease in the variation of performance among the devices tested. The increased cover thickness decreases the sensitivity of the AQP SAW to variations in the mounting material. The influence of AQP cover thickness on an insufficiently stiffened oscillator was also examined. The results were similar, except that the variation in cover thickness had a more dramatic affect than in the sufficiently stiffened case.

Pulsed-laser deposition of oxides over large areas

Due to its short wavelength and high peak power the excimer laser has become the de facto choice for Pulsed Laser Deposition (PLD) of ceramic superconductors as well as other complex chemical compounds. This paper will describe a unique excimer-based laser deposition system which is capable of producing thin films of a variety of oxide compounds or other materials over large areas (up to 46 cm2). Well over a dozen chemical compounds have been deposited with this system for a wide variety of electronic applications. Also the PLD technique has been adapted to grow thin films on the internal surface of cored cylindrical substrates in order to form resonant microwave cavity structures. The uniformity morphology and electrical properties of films grown on both planar and cylindrical substrates will be discussed.

Metallizations for surface acoustic wave resonators. Film properties and device characteristics

All-quartz packaged surface acoustic wave (SAW) resonator devices with interdigital transducers fabricated using electron-beam evaporated aluminium or copper-doped aluminium films which incorporate small amounts of either silicon or titanium are evaluated. The electrical characteristics discussed include: sealing-induced frequency shifts and residual flicker noise, as well as random and systematic long-term frequency stabilities. The data obtained thus far indicate that doping the interdigital transducers with a combination of both copper and titanium can result in SAW resonator devices with both excellent residual flicker noise and long-term frequency stabilities, even when the devices operate under conditions resulting in extremely high acoustic stress levels. In contrast, resonator devices fabricated using aluminium films doped with only silicon or titanium display both poor flicker noise and long-term frequency stabilities, even when operated at relatively modest stress levels.<<ETX>>

On - and Off -AXIS Large-Area Pulsed Laser Deposition

Over the past few years Pulsed Laser Deposition (PLD) has become a popular technique for the deposition of a wide variety of thin films, and PLD systems are currently found in numerous industrial, government, university, and military laboratories. At present, it is estimated that well over 200 different materials have been deposited by PLD and the list keeps growing. However, even with all the interest in laser deposition the technique has not yet emerged as an industrial process. At the moment, industry still prefers standard thin film growth techniques such as magnetron and ion beam sputtering, chemical vapor deposition, and electron beam evaporation for production applications. These processes have been in use for decades and have demonstrated the ability to deposit films of most materials over large areas with excellent uniformity at reasonable cost and deposition rates. Furthermore, an entire infrastructure has been built up to support these processes including standardization of deposition rate monitors, power sources, target and crucible sizes, etc. On the other hand, laser-deposition is still an emerging technology, and relatively little infrastructure exists to adequately support either research or industrial applications. Since there are several materials which are difficult if not impossible to grow in thin-film form by more conventional techniques, it is expected that as pulsed laser-deposition matures this unique process will take its rightful place on the manufacturing line.

Stress In Molybdenum Films Used For Fea Display Technology

Flat-panel displays based on cold-cathode field emission are actively being developed in a number of laboratories world-wide. Once fully developed, Field Emitter Displays (FEDs) will compete directly with Liquid Crystal Displays (LCDs). FEDs offer several significant advantages over LCDs including higher screen brightness, wider viewing angle, lower power consumption, and operation over a broader temperature range, making FEDs desirable for applications such as lap-top computers. Presently, several materials are being evaluated for use as coldcathode field emitters including molybdenum 1 , silicon 2 , and DLC 3 . At this time it is unclear which material (or materials) will ultimately be incorporated into commercial or military flatpanel display products. Each potential cathode material has its own set of advantages and disadvantages, and the ultimate choice will depend on the particular display requirements and architecture.