Array Compressor with a Compression Ratio of 5120, SURFACE ACOUSTIC WAVE DEVICES AND Ultrasonics Symposium Proceedings, Williamson et al.,
METHOD OF MANUFACTURE THEREOF IEEE> New York, 1973 pp. 490-493. Williamson et al.
rnvcBvucvT ... c contemPIate placing a metal film of variable width be
GOVERNMENT SUPPORT 5 tween the two grating structures to slow the wave and
The Government has rights to this invention pursuant advance its phase (p. 492). In practice, the photolitho
to Contract No. AF19628-C-0002 sponsored by the graphic processing required to define the film pattern
United States Air Force. perturbs both the amplitude and phase response of the TECHNICAL FIELD 10 wave and precludes independent phase adjustment be
T,. . t. • ■ r u r c _r * vond 2 degrees r.m.s. or less.
This invention is m the field of Surface Acoustic XT„ , _ ... ■ u *
... /c . , ,. , , —. . . No known amplitude-compensation technique, short
Wave (SAW) devices and, more particularly, relates to . . ^ . . •
alaser photo-chemical etching process for in situ trim- °f f mPlete fra'Ion 0!,dey_lce fabncat'fn w.th a modiming of thin film deposited on the surface of piezoelec- fied groove-depth profile, has previously been demontrie SAW structures. 15 strated.
BACKGROUND ART DISCLOSURE OF THE INVENTION
Surface acoustic waves containing congressional and In accordance with the invention, a compound SAW shear components in phase quadrature propagate non- compensating film comprising first and second layers is dispersively along solid surfaces. This phenomena was 20 deposited on the surface of a piezoelectric SAW subpredicted by Lord Rayleigh in the 1880's. More re- strate, preferably formed of LiNbOj, between the two cently, microminiature SAW devices have been fabri- conventional grating structures. The first layer is comcated in which the surface acoustic waves are generated prised of ^ ampiitude attenuating material intermediate by electrical signals which are converted to acoustic the resistivity of a metal and a dielectric. Preferably, in signals by transducers formed on piezoelectric maten- 25 the of 10+3_10+9ohms/sq. The precise resistivity a s. These devices are used for the analog processing of on gAW ... electrical signals. r . . .1,1. ,?
Significant SAW devices include bandpass filters, fi1lm laver ,s a metalllc phase compensating layer of resonators and oscillators and pulse compression filters. electrically conductive material, such as molybdenum. System applications for such devices are numerous and 30 The second layer is deposited over the cermet layer and include color television, radar, sonar, communication, extends laterally beyond the cermet layer onto the subnon-destructive testing and fast Fourier transform pro- strate surface between the grating structures, cessors. This compound film is initially patterned to the ap
The basic SAW structure comprises an InterDigi- proximate desired dimensions using conventional pho
tated metal film Transducer (IDT) deposited on a pla- 35 tolithography masking techniques,
nar optically polished surface of a piezoelectric sub- The second layer of metallic conducting material
strate, such as lithium niobate or quartz niobate. In effectively masks or short-circuits the cermet attenuat
Reflective Array Compressor (RAC) SAW devices the mg first iayer. Thus, by selectively removing portions
acoustic wave generated by the IDT is propagated of the ^ iayer overiying the first layer, the wave
along an array of suitably angled reflective slots etched 40 can ^ litude compensated. Subsequently, or con
into the substrate surface. These sk>ts form a dispersive ... fe Actively removing portions of the sec
delay line grating. The spacing of the slots determines _, , , • .1 ..f .i. u
the frequency selectivity of the grating and the depth of ond laver overlying the substrate, the wave can be
the slots determines the amplitude weighting applied to pnase compensated.
the input pulse. Two such gratings are arrayed side-by- 45 To do thls>the SAW devlce> including the compound
side on the substrate surface. The slots on the second film of the invention, is made operational by bonding
grating are matched to the first to reform and counter- input and output leads to the transducers and is loaded
propagate the SAW beam parallel to but laterally dis- into a laser etching chamber. The device is energized
placed from the incident SAW beam (McGraw-Hill and the phase and amplitude characteristics measured
Encyclopedia of Electronics and Computers, 1984, pp. 50 by well-known techniques, using a network analyzer.
793-796). To the extent these characteristics deviate from the
Slot depths are typically l/100th of the acoustic ideal; the second layer, or film, of molybdenum is laser
wavelength, and slot spacings are typically one wave- photo chemically etched to remove selected portions
length between centers. Needless to say, such stringent and to thereby compensate the SAW wave indepen
requirements necessitate extremely precise fabrication 55 dentiy in phase a„d/0r amplitude,
techniques. In practice, there is a significant amount of ^ phot0chemical etching process produces a fast
device-to-device vanation, due to the sensitivity of low t rature reaction without damaging the under
device performance tc.fabrication steps j m LiNbQ sub wWch ^ extremel susceptible
In addition, even if fabrication technology were per- / . r i- 1 i_ Jt 1
feet, it is difficult to exactly predict device performance 60 t0 dama*e fr0m nonlinear optical absorption and local
from device design because of substrate variability. ;zed heatln«- In thl* etchlnS Process low P°w,er laser
Due to these difficulties, viz., imperfect device de- hght 18 focused on the selected P0^11 of the fllm to
sign, imperfect device fabrication and substrate variabil- removed while a flowing vapor containing an etchant
ity it is important to develop an ability to correct device reactant, such as Cl2, is passed over the film. Light
performance after fabrication. 65 photons dissociate the vapor reactant producing free
Phase compensation of pulse compression SAW grat- atoms of CI which combines with the heated metal film
ings has been suggested as a means for improving de- to etch the film and produce volatile vapors which are
vice response by Williamson et al. in L-Band Reflective- evaporated and exhausted.