D.E. Oates

SAW/FET Programmable Transversal Filter with 100-MHz Bandwidth and Enhanced Programmability

The authors previously reported on a SAW/FET filter that uses air-gap coupling of a LiNbO/sub 3/ SAW delay line to a Si integrated circuit, thus implementing a programmable tapped delay line. The device incorporates edge-bonded transducers for wide bandwidth and short (15 ns) pedestal of delay. Modifications to the Si circuit are reported here, which both increase the programmable RF bandwidth to 100 MHz and provide means of increasing the programmable tap on/off ratio to 30 dB. This filter has 350 metal-oxide-silicon (MOS) analog sampling fingers spanning 1/5 microsec. of delay. The varactor inherent in each sampling finger provides programmable tap weighting; a digital shift register controls programming FETs that connect each MOS varactor tap to a common programming voltage line. The 100-MHz bandwidth results from doubling the number of taps by the addition of a 2-to-1 analog multiplexer to the FET array.

Holographic‐grating acoustic devices

We report observations of acoustic wave reflections by an optically produced holographic grating within iron‐doped LiNbO3. The stored hologram, through the electroacoustic effect, causes a spatially varying acoustic velocity change which can serve to reflect acoustic waves. We have demonstrated the reflection of bulk acoustic waves from a 4‐μm period grating in LiNbO3 with an iron doping of 0.015%. The acoustic frequency was 0.9 GHz. A strong interaction with a reflection coefficient of 4×10−4 per grating element was found. Memory retention exceeds the three months over which we have observed the effect. The reflections can be exploited to produce practical filters and resonators.

Surface photoacoustic wave spectroscopy of thin films

Time‐resolved detection of the surface acoustic waves (SAW) generated upon relaxation of energy optically absorbed in surface films has been used to measure surface absorption spectra and laser‐induced desorption cross sections. This new background‐free spectroscopic technique is shown to be highly sensitive to the details of the film/substrate bonding. Minimum detectable molecular surface densities of 2×1011 cm−2 over an area of 10−3 cm2 have been demonstrated for rhodamine‐590 dye on a crystal quartz substrate using a pulsed laser source and an edge‐bonded SAW detector.

SAW/FET programmable filter with varistor taps for improved performance

The SAW (surface acoustic wave)/FET (field-effect transistor) programmable transversal filter utilizes a LiNbO/sub 3/ SAW delay line that is coupled to a silicon integrated circuit across a 300 nm air gap. A SAW/FET with an integrated circuit that incorporates a varistor-controlled tap-weighting circuit has been built. The varistor is a MOSFET (metal-oxide-silicon FET) whose resistance determines the tap weight and is controlled by the voltage stored on the capacitance of the MOSFET gate. The integrated circuit contains the taps, tap weighting and RF summing circuitry to accomplish the transversal filter function. A digital shift register operates as a commutator that allows serial programming of the individual analog tap weights. There are 350 analog taps that yield a 1.5- mu s delay with a programmable bandwidth of 100 MHz at a center frequency of 175 MHz. The programmable tap on/off is greater than 30 dB with a +or-1.0-dB uniformity. When programmed as a narrowband filter, the out-of-band rejection is greater than 35 dB across the entire 100-MHz programmable bandwidth.<<ETX>>

Adaptive and Matched Filtering with a SAW/FET Programmable Transversal Filter

We describe the operation of a SAW/FET programmable transversal filter as both a matched filter and an adaptive filter. The SAW/FET device incorporates surface-acoustic-wave (SAW) signal propagation in conjunction with a field-effecttransistor(FET)-controlled sampling array. The SAW/FET device has a 1.5-us impulse response, 350 taps, and 100-MHz programmable bandwidth centered at 175 MHz. The on/off ratio of each individual tap is 30 dB. As a matched filter, the SAW/FET device is demonstrated using both 31- and 63-chip m-sequences, showing nearly ideal sldelobe performance. The adaptive processor uses a digital computer to calculate the tap-weight updates according to the least-mean-squares (LMS) algorithm and achieves up to 25 dB of CW cancellation.

High-sensitivity surface-photoacoustic spectroscopy

The sensitivity of surface-acoustic-wave detection is extended by several orders of magnitude to a surface-specific absorbance of alphal ~ 10(-9) for a power density of 1 GW/cm(2) using a narrow-bandwidth interdigitated surface-acousticwave detector and an optical irradiation pattern to provide a matched acoustic signal. Major advantages include narrow-bandwidth detection and a large irradiated area that permits more optical energy on the sample. A rapid, nondestructive, reproducible liquid-bonding technique, which permits the extension of these measurements to a wide variety of samples, is demonstrated. Results for AlN and ZrO(2) films (alphal ~ 10(-4)) and fused-silica substrates (alphal ~ 10(-6)-10(-7)) are reported.

Holographic-Grating Acoustic Devices

We have previously proposed wideband signal processing devices using the interaction between holographically produced reflection gratings and bulk acoustic waves (BAW). The devices are promising because of wide bandwidth, long interaction time and simple fabrication methods. We report here the first observations of acoustic wave reflections by a holographically produced grating w ithin iron-doped LiNb03. The interference of two plane waves within the LiNb03 induces charge migration and thereby stores a spatially varying electric field, as is well known from the photorefractive effect. This stored field, through the electroacoustic effect, causes a spatially varying acoustic velocity change which can serve to reflect acoustic waves. We have successfully demonstrated the reflection of a BAW from a holographically created gratinq usinq a Z-propagating wave in a LiNb03 sample which has an iron doping of 0.015%. The grating was created by the interference of two argon-ion laser beams. The BAW frequency was 889 MHz and the angle of interference of the laser beams was chosen to create a hologram of period 0.8 vm. A strong interaction producing a reflection coefficient of 4 x per grating element was found. Memory retention exceeds the one month over which we have observed the effect. Write and erase time constants a re about 10 min. We discuss s everal possible devices using this effect.

RF Magnetron Sputtering of ZnO for SAW: Effects of Magnetic Field Strength and Configuration

Magnetron sputtering of ZnO has p roven to be a useful method of depositing piezoelectric films for surface-acoust ic-wave applications. Until now, however, little consideration has been given to the strength or configuration of the magnetic field employed in the magnetron target of the sputtering system. The parameters of the magnetic field affect the deposition rate and particle bombardment of the sample. It is believed that damage to the film caused by particle bombardment is one of the important factors which limits the acoustic and piezoelectric qualities of the deposited ZnO films. We present the results of sputter deposition of ZnO films under a variety of magnetic field conditions. The deposited films are evaluated using x-ray diffraction, SAW piezoelectric coupling constant and SAW propagation loss. The amount of charged particle bombardment has been confirmed in each case by separate measurements of substrate heating. We find that for ZnO films of 1.5-pm thickness (h/X = 1/20 at 170 MHz) propagation loss can vary from -1 dB/us to 4 dB/us and x-ray diffraction intensity can vary nearly an order of magnitude. Better films can he related to decreased particle bombardment .

Effects of Temperature-Dependent Anisotropy in RAC Devices and a Cut of Quartz for a Temperature-Compensated RAC

The analysis of temperature-dependent effects in RAC devices shows that not only uniform changes in delay, but also t he effects of temperaturedependent velocity anisotropy lead to temperaturesensitive behavior. S ince ST quartz is temperature compensated in only one direction of SAW propagation and because the anisotropy is temperature dependent, a RAC fabricated on ST quartz is not temperature stable. quartz is proposed for use in temperature-compensated RAC devices. Experimental evidence is presented for a 35.1O-rotated Y cut of quartz which possesses two temperature-compensated SAW propagation directions located at right angles to each other. A RAC fabricated on this cut is fully temperature compensated. The measured second-order temperature coefficient of delay of this new cut is smaller than that for ST-X quartz.

LiNbO 3 Surface-Acoustic-Wave Edge-Bonded Transducers on St Quartz and l001g-Cut GaAs

The ability to generate .wideband surface-acoustic waves (SAW) on a variety of substrates is important to develop a wider selection of devices, both acoustic and acoustoelectric, with performance characteristics appropriate to the signal-processin g requirements of real systems. While edge-bonded transducers (EBT) have been used before, we report the extension of t his technology to GaAs. In addition, a new model has been developed which allows, for the first time, an accurate description of EBT behavior. We have fabricated LiNb03 EBTs on both ST quartz and -cut GaAs, with efficient transduction and fractional bandwidths of 50% and 91%, respectively, at center frequencies of approximately 100 MHz. Single-transduce r c onversion loss as low as 4 dB has been obtained on ST quartz. These experimental results for both substrates are in good agreement with the model, which uses the Mason method to describe the behavior of the LiNb03 bulk-shear-wave transducer and uses normal modes to treat the conversion of shear-wave energy to SAW energy at the transducer-subst rate interface. A description of this model, as well as fabrication and experimental details will be presented.