Abstract:
Techniques for adjusting the surface acousic wave velocity of a packaged SAW device are described. A first technique involves depositing a film from a cover of the device onto a surface wave propagating surface thereby providing a localized region on said surface where the acoustic properties and, hence, the velocity characteristics of the surface wave propagating surface are altered. An alternate embodiment of a pair of beams are directed through a transparent cover and converge on the surface wave propagating surface, selectively removing a portion of said surface to provide a localized alteration in the acoustic properties of the surface wave propagating surface.

Description:
This application is a divisional of application Ser. No. 442,404 filed Nov. 28, 1989, now U.S. Pat. No. 5,010,270, which is a continuation of application Ser. No. 293,607, filed Jan. 5, 1989, ABN which is a continuation of application Ser. No. 153,186, filed Feb. 8, 1988, ABN which is a continuation of application Ser. No. 944,911, filed Dec. 22, 1986, ABN. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to surface acoustic wave devices and more particularly to adjusting the frequency characteristics of a surface acoustic wave device. 
     As it is known in the art, surface acoustic wave devices such as resonators, delay lines, filters and pressure transducers are used in a variety of applications. Generally, a SAW device comprises at least one transducer, having a set of conductive members, which is disposed on or recessed within an upper surface portion of a piezoelectric substrate. 
     A new type of package has been developed which hermeticaly seals the SAW device substrate. This new package includes a pair of piezoelectric substrates which are sealed together by a glass frit, such as described in conjunction with a paper entitled &#34;Saw Resonator Frit Bonded Pressure Transducer&#34; by D. Weirauch et al, Proceedings of the IEEE Ultrasonic Symposium, 1979, p. 874, and an article entitled &#34;Long Term Aging, A Mechanical Stability of 1.4 GHz SAW Oscillators&#34; by M. Gilden et al, Proceedings of the IEEE Ultrasonic Symposium, 1980, p. 184, a second type of package is shown in Parker et al U.S. Pat. No. 4,270,107, issued May 26, 1981 entitled &#34;Stabilized Surface Wave Device&#34; and assigned to the same assignee as the present invention. 
     Although these packages provide improved packaging and handling capabilities for SAW devices, one problem associated with these packages is that during the fabrication of the package, the frequency characteristics of the package SAW device are changed from that of the prepackaged SAW device. For example, for SAW resonators operating around 350 MHz, this frequency shift may average approximately 45 ppm and may have a standard deviation as high as ±15 ppm. For other types of devices and devices which are designed to operate at higher frequencies, both the magnitude and standard deviation of this shift increase making accurate setting of the frequency characteristics of the SAW device more difficult. In many applications, accurate frequency settability of ±1 ppm would be desirable. 
     One solution to this problem has been to use external electrical components to adjust the frequency characteristics of the circuits employing the SAW device. The problem with this approach is that this type of external frequency adjustment is very time consuming, relatively expensive since the number of components required in any particular SAW device application is increased, and furthermore, the components themselves may contribute to frequency changes in circuits employing such SAW devices. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method of changing the surface wave velocity characteristic of a surface which supports surface wave propagation is provided by directing energy through a first one of a base and cover, which enclose said surface, towards a first material disposed over said surface, to provide a layer of a second material on said surface to change the velocity characteristic of said surface. With this particular arrangement, the layer of second material on the base provides a small, localized change in the surface wave velocity characteristic of the surface which supports surface wave propagation, thus permitting adjustment in the frequency characteristics of packaged SAW devices. 
     In accordance with a further aspect of the present invention, a method of changing the surface wave velocity of a packaged surface acoustic wave device is provided by directing incident energy through a cover of the packaged device to remove a portion of a layer of material disposed over a surface wave propagation surface of the SAW device. With this particular arrangement, the removed material provides a small, localized change in the acoustic properties of the surface wave propagation surface, thus providing a concomitant change in the frequency characteristics of the SAW device. 
     In accordance with a further aspect of the present invention, a method of changing the surface wave velocity of a packaged surface acoustic device includes the steps of: providing a cover over the surface wave device forming an enclosure over said surface wave device with at least one of said cover and base being comprised of a material which is substantially transparent to energy over a selective wavelength range. A layer of material is disposed on an inner surface portion of the cover. Energy is directed towards this layer of material through the transparent cover or base, to remove selected portions of the material from the layer, and have said selected removed portions redeposit onto the surface wave propagating surface. With this particular arrangement, the surface wave velocity characteristic and hence, the frequency characteristics of a packaged SAW device may be adjusted after the SAW device is packaged. The redeposited material produces a small, localized alteration in the acoustic properties of the piezoelectric surface which changes the surface wave velocity of propagating surface waves. Therefore, this change in surface wave velocity provides an accompanying change in frequency characteristics of the SAW device. 
     In accordance with a still further aspect of the present invention, the cover portion of the material is quartz, and the quartz is transparent to light energy from a laser having a wavelength in the range of about 0.185 μm to 4 μm. Disposed on the inner portion of the quartz cover is a pad, comprising a first layer of a thermally buffering material, a second layer of an energy conversion material, and a third layer of an ablatable or evaporatable material. The incident optical energy is directed towards this pad. The quartz cover and the first layer comprising the thermally buffering material are substantially transparent to the incident optical energy. The incident energy, however, heats up the intermediate energy conversion layer disposed between thermally buffering layer and the ablatable layer. The heat generated in the energy conversion layer causes the ablatable or evaporatable layer to ablate or to evaporate and redeposit onto underlying portions of the piezoelectric surface. The first dielectric layer is provided to buffer the quartz cover from the heat associated with the energy conversion layer. With this particular arrangement, the frequency characteristics of a SAW may be adjusted by the use of a laser, which selectively redeposits a predetermined portion of the ablatable or evaporatable layer over the surface wave propagating surface of the SAW device, adjusting the surface wave velocity characteristic and, hence, the frequency characteristics of the SAW device. 
     In accordance with a still further aspect of the present invention, a method of changing the surface wave velocity characteristic of a packaged SAW device includes the steps of providing a cover comprised of a material which is transparent to electromagnetic energy over a selected wavelength range, with the cover being disposed to enclose a surface wave propagating surface. A highly convergent beam or a pair of beams, each at a selected incident angle to converge on said surface, is directed towards said surface to produce a beam of sufficient intensity to selectively remove portions of said propagating surface. With this particular arrangement, by selectively removing portions of the piezoelectric propagating surface, a small, localized alteration in the acoustic properties and, hence, surface wave velocity characteristic is provided. Accordingly, the frequency characteristics of said SAW device also will be altered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description of the drawings, in which: 
     FIG. 1 is a plan view of a packaged SAW resonator prior to frequency trimming in accordance with the present invention; 
     FIG. 2 is a cross-sectional view taking along line 2--2 of FIG. 1; 
     FIG. 2A is a blown-up view taking along line 2--2 of FIG. 2; 
     FIG. 3 is a plan view of a packaged SAW resonator after frequency trimming in accordance with the present invention; 
     FIG. 4 is a cross-sectional view taking along lines 4--4 of FIG. 3; 
     FIG. 4A is a blown-up view of a portion of FIG. 4 taking along line 4A--4A of FIG. 4; 
     FIG. 5 is a cross-sectional view of an alternate embodiment of the present invention, using converging pairs of directed energy beams; 
     FIG. 5A is a blown up view taken along line 5A--5A of FIG. 5; 
     FIG. 6 is a cross-sectional view of a further embodiment of the present invention using converging pairs of directed energy beams to ablate a pad deposited on a surface wave propagating surface. 
     FIG. 6A is a blown up view taken along line 6A--6A of FIG. 6; 
     FIG. 7 is a cross-sectional view of a further embodiment having a gas mixture enclosed in the package with incident energy directed towards said mixture to provide a layer of material over the surface wave propagating surface; and 
     FIG. 7A is a blown up view taken along lines 7A--7A of FIG. 7. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1, 2, and 2A a packaged SAW device here a resonator 10 includes a base 12 having a surface 12a which supports surface wave propagation, a cover 14, and a glass frit seal 16 as shown. Base 12 is here comprised of ST-cut or rotated ST-cut quartz, having disposed on surface 12a thereof, a pair of interdigitated transducers 20 and 24 (IDT) and reflective gratings 27, 29. Interdigitated transducers 20 and 24 are coupled to busbars 18a and 18b, and 19a and 19b respectively, as shown. Disposed over the base 12 is the cover 14 here also comprised of ST-cut or rotated ST-cut quartz. Quartz is typically 85% to 95% transparent to energy having wavelengths in the range of about 0.185 μm to 4.0 μm, for samples 1-3 mm thick. Alternatively, base 12 and cover 14 may be comprised of other transparent materials such as lithium niobate. The base 12 and the cover 14 are sealed together using a suitable glass frit seal 16. The cover 14 has a width w c  less than the width w b  of the base 14 such that when the cover is sealed to the base, busbars 18a and 18b, and 19a and 19b are exposed on a peripheral portion of the base thereof, to provide electrical contact to external components. 
     As shown in particular in FIG. 2A, the inner surface 14a of cover 14 has disposed over a portion thereover, a layer here preferably a composite layer 26 comprised of a thermally buffering or shock absorbing layer 26a, a thermally absorbing or energy conversion layer 26b, and a thermally evaporating or ablating layer 26c. In this preferred arrangement, the thermally shock absorbing layer 26a comprises a dielectric material which is substantially transparent to the incident energy, which will be directed through the cover 14 of the SAW device in a manner to be described, but which provides sufficiently thermal resistance to thermally buffer the cover 14 from the heat generated in the thermally absorbing layer 26b as will be described. Energy conversion layer 26b is a material which substantially absorbs the incident energy directed through the cover 14 and the thermal shock layer 26a, and converts said incident directed energy into radiant energy or heat. The thermally ablating or evaporation layer 26c is disposed over the conversion layer 26b and absorbs some of the radiant energy generated in layer 26b, and if the energy therein is of sufficient intensity, the film 26c will be selectively removed, in particular, ablated or evaporated off the cover and will redeposit or condense onto the surface 12a, as shown in FIG. 4. The amount of this redeposited mass is selected to sufficiently alter the surface wave velocity characteristic of the SAW device, and thus, change the frequency characteristics of the SAW device, as required. 
     Referring now to FIGS. 3, 4, and 4A, incident energy 30 is shown directed through cover 14 towards the trim pad 26. The incident energy passes through transparent buffer layer 26a and is absorbed by selected underlying portions of energy conversion layer 26b. Layer 26b, converts the directed energy to heat. The heat in layer 26b causes the ablation or evaporation of portions of layer 26c as shown. In accordance with the pulse width, power level, wavelength, and repetition rate of said energy, the layer 26c is either evaporated off at low energy levels or ablated off at higher energy levels. Moreover, at sufficiently high levels, selected portions of each of the layers 26a-26c may be removed. The removed material whether ablated or evaporated condenses and redeposits as a layer 34 across the base portion 12 of the SAW device 10. The presence of layer 34 on surface 12a of base 12 reduces the surface wave velocity of the wave in this region due to mass loading effects, as the surface wave propagates. By increasing the number of these layers 34 and their width and total mass, the range of adjustment of the surface wave velocity characteristic is increased. 
     Specific examples of devices fabricated in accordance with the teachings of the present invention will now be described. As shown in FIGS. 3, 4, and 4A, incident energy here in the form of light beams, from laser sources as set forth in Table, are formed having lengths which substantially traverse the length of the acoustic wave propagating surface and have widths which are generally on the order of the SAW wavelength (typically in the range of 2.5 μm to 50 μm. That is, a long narrow line across the length of the acoustic propagating surface and centered within and parallel to the acoustic aperture of the interdigitated transducer elements is directed towards the quartz cover. This energy passes through the quartz cover 14 and is absorbed by the composite trim pad 26, disposed on the inner surface 14a of the cover 14. With this laser beam pattern, the aluminum layer 26b is selectively heated in regions thereof, impinged by the incident laser beam. In response to this localized heating of the aluminum layer, portions of the aluminum oxide layer 26c are evaporated or ablated off the aluminum layer 26b and are redeposited onto the underlying surface wave propagating surface 12a. In particular, since the amount of frequency adjustment is related to the mass of the redeposited material on the surface wave propagating surface, the amount of frequency adjustment per line of the laser may be controlled by controlling the width and energy density of the laser beam, as well as the amount of material provided on the inner portion of the cover 14. It is believed that the maximum amount of frequency shift could exceed 10,000 parts per million if the total area disposed between the interdigitated transducers was used. However, it is also believed that shifts of this order would degrade both the insertion loss and unloaded Q of the device. However, since only about 25 to 50 parts per million is generally required to account for the uncertainty in the frequency after sealing of the package, only a small fraction of the pad area is generally redeposited onto active area between the IDTs. 
     Four devices operating at 402 MHz were trimmed using the above described technique. A trilevel film, as shown in FIG. 4, comprising a 400 A thick layer of aluminum oxide Al 2  O 3  corresponding to layer 26a, a 100 A thick layer of aluminum corresponding to layer 26b, and a 250 A thick layer of aluminum oxide corresponding to layer 26c were deposited on the inner portion of the cover 14. The thicknesses of the deposit layers of 12a-12c may be selected to provide different amounts of mass loading per pulse of incident energy light. Therefore, the layer 26a may have a thickness typically in the range of 200 A to 1000 A, layer 26b will have a thickness in the range of 25 A to 200 A, and layer 26c will have a thickness in the range of 50 A to 500 A. 
     The power level, number of pulses, and the number of redeposited lines provided to four illustrative examples of the present invention, as well as, the initial frequency, final frequency, and the frequency shift of these devices are shown in the Table, examples 1-4. 
     Alternatively, the incident laser beam may be directed through the base 12 towards the trim pad 26. The incident laser energy then may be used to ablate regions off of the trim pad onto the cover, for example by turning the SAW device upside down to again change the mass loading in said region, and accordingly, the surface wave velocity characteristics of the SAW devices. 
     
                                           TABLE__________________________________________________________________________ DEVICE  TRIM         LENGTHWAVE-                     PER PULSEDENSITYENERGY                            LENGTHPULSE                                   RATEREP                                       WIDTHLINE                                            OF#                                                fo  f                                                        ##STR1##I.D.  TECHNIQUE         LASER              μm J/cm.sup.2                           ns     hz  μm                                           LINES                                               MHz MHz PPM__________________________________________________________________________1.    Trilevel         Excimer              0.193 5      15      1   7   5   401.831                                                   401.817                                                       -35 film on cover2.    Trilevel         &#34;    &#34;     5      15      1   7   5   401.840                                                   401.828                                                       -30 film on cover3.    Trilevel         Nd:YAG              1.06  0.0025 250    1024                                      50   5   402.178                                                   402.166                                                       -30 film on cover4.    Trilevel         Nd:YAG              1.06  0.00325                           250    1024                                      60   1   402.219                                                   402.203                                                       -40 film on cover5.    Removal Excimer              0.193 7      15     50  50   0.1 402.174                                                   402.170                                                       -10 of SAW Surface6.    Chemical         Excimer              0.193 5      15     10  100  5   401.962                                                   401.955                                                       -17 Conversion of Al--O Cermet__________________________________________________________________________ 
    
     Referring now to FIG. 5, an alternate embodiment of the present invention for selectively changing the frequency characteristics of a packaged SAW device is shown. Here the packaged SAW device 10&#39; includes the aforementioned base 12, and a cover 14 as generally described in conjunction with FIGS. 1 and 2. In accordance with this aspect of the present invention, directed energy here in the form of light energy from an excimer laser is directed through the quartz cover 14 towards the base 12. Here a pair of beams of excimer laser light are directed through the quartz cover 12 at a selected incident angle such that the beams converge on the quartz base 12 to selectively remove or ablate a portion of the quartz base 12 in a region thereof, disposed between the pair of interdigitated transducers, as shown in particular in FIG. 5A. Alternatively, three or more beams or a highly convergent beam may be used. The power density at the point where the beam or beams converge must be sufficient to remove selected amounts of material. Since the cover comprised of quartz typically 1-2 mm thick is about 85%-95 % transmissive, the power density at the point of convergence must be sufficient to remove the quartz at that region. With this particular arrangement, since the acoustic properties of the surface wave propagating surface between the pair of transducers 20 and 24 is altered by the selectively removal of the material, the surface wave velocity characteristics between said pairs of transducers is also altered. Thus, the frequency of the device incorporating this arrangement is selectively altered. Again, the number, depth, and width of these regions 12c disposed within the surface wave propagating surface portion of the base 12 may be selected to provide a selected shift in the acoustic and, hence, frequency characteristics of the surface acoustic wave device. Typically, the depth of the grooves will be approximately 0.5 microns, and the width of the grooves will be in the order of the surface wave propagating wavelength (2 to 50 microns). With this particular arrangement, the intensity of beams 30a and 30b directed through the quartz cover 14 is insufficient to disrupt or disturb the material of the cover 14. However, since at the point of convergence of the two beams, the intensity of the beams are added, then the intensity of the beam is sufficient to ablate a selected portion 12c of the surface wave propagating surface 12a. Since the mass and propagation path of the surface wave propagating surface is altered by this arrangement, the surface wave propagating velocity and thus, frequency of the device will also be altered. Example 5 in the Table shows the results of a device trimmed using this technique. 
     Referring now to FIG. 6, a further embodiment of the invention is shown to include a trim pad 26 disposed on the base within the acoustic aperture of the IDT&#39;s. Here a beam or preferable a pair of converging laser beams may be used to selectively irradiate a portion of the trim pad 26 to ablate a portion of the trim pad 26 providing pad 26&#39;, as shown in FIG. 6a. Alternatively the beams may be used to change the molecular bonding of that portion of the trim pad. Again, since the mass loading in the region where the composition of the material is changed, the acoustic properties and, hence, surface velocity and frequency characteristics of the device are also changed. 
     Several approaches may be used to change the molecular bonding of the material of the trim pad. For example, an aluminum-oxygen cermet such as described in conjunction with a paper entitled &#34;Submicron Patterning by Projecting Excimer-Laser-Beam Induced Chemistry&#34; by Ehrlich et al. J. Vac. Sci. Tech. B 3 (1) January/February 1985 may be used. In this case, a cermet layer is deposited as the trim pad 26. The excimer energy is directed towards the trim pad and in response Al--O bonds are broken and localized growth of Al 2  O 3  within the trim pad occurs. Again, the spacing and number of localized area of growth of Al 2  O 3  is selected in accordance with desired amount of change in surface wave velocity. An example of this trimming technique is shown as Example 6 in the Table. 
     Alternatively, layers of Al 2  O 3 , Al may be provided as the trim pad 26 and the trim pad 26 again is subjected in selective regions to excimer light and Al--Al and Al--O bonds are broken and a new phase Al 2  O x  would be provided where x would represent a non-stoichiometric composition. 
     A further technique would have a layer of Al deposited as the trim pad 26. The cavity of the package 10 is filled with a reactant gas O 2  or NO 2  for example, to a pressure typically in the range of 2×10 -6  torr to 1×10 -4  torr. Incident energy is again directed towards the trim pad 26 where Al--Al bonds are broken, causing Al 2  O 3  to form pyrolytically. By controlling the amount of area irradiated, the selected amount of surface wave velocity change is provided. 
     Referring now to FIGS. 7, 7A a further embodiment of the invention is shown. Here the packaged SAW device comprised of the cover 14 and base 12 has sealed within it reactant gasses 32. A laser beam such as excimer light, Nd-YAG etc. is directed through said device to photolytically break the bonds of these reactant gasses and have deposited out from said photolytic reaction a material onto the surface wave propagation surface. Here selected patterns of excimer light would be provided to deposit selected patterns of said material onto the surface. Reactant gasses may include oxygen and a source of a suitable metal such as aluminum, silicon, etc. For example, TMAl (Trimethylaluminum), SiH 4  silane etc. may be used. The particular gas chosen for the metal source for example, would have to be stable during the sealing operation of the glass frit in those types of packages employing a glass-frit seal. 
     The excimer laser light may be directed towards these selected gasses and photolytically break the bonds of the metal source gas. This free metal would then react with the oxygen and a metal oxide layer 34&#39; would be deposited out of the vapor over the substrate. Thus, again a localized region of Al 2  O 3 , for example, may be grown on the surface wave propagation surface. 
     Having described preferred embodiments in the invention, it will now become apparent to one of the skill in the art that other embodiments incorporating their concepts may be used. Further, it would now become obvious to one of the skill in the art that a beam or a pair of converging beams may be used to selectively remove a pad disposed between the pair of interdigitated transducers on the base 12, as shown in FIG. 6, or to remove and redeposit portions of a trim pad disposed on an inner portion of the cover 14, for example, by directing the beams through the base 12 or the cover 14. It is felt, therefore, that these embodiments should not be limited to disclosed embodiments, but rather should be limited only to by the spirit and scope of the appended claims.