TRIMMING OF PIEZOELECTRIC COMPONENTS

Abstract: A method of frequency trimming piezoelectric devices (1) such as quartz crystal resonators and surface acoustic wave filters. The device may be entirely enclosed in glass encapsulation (43) or be provided a housing (52) with a window (51) transparent to optical energy at a predetermined wavelength, for example, at 1.06 micrometer. A Q-switched, Nd-YAG laser (4) is directed by an optical x-y deflection system (5) so as to penetrate the housing (52) or window (51) and impinge on conductive patterns or electrodes (42) deposited on the piezoelectric substrate (41) of the device. The frequency characteristics of the device vary as the conductive material is caused to evaporate. The intensity and direction of the laser beam is controlled so as to bring the frequency characteristics, that is, resonant frequency, frequency response, etc., within specification. The device may be stored for a period of time between encapsulation and the final trimming procedures so as to substantially obviate significant short-term aging phenomena.

Description: TRIMMING OF PIEZOELECTRIC COMPONENTS TECHNICAL FIELD The subject invention relates to the fabrication of piezoelectric components and, more particularly, to the frequency trimming of same. BACKGROUND OF INVENTION As is well known, piezoelectric materials, that is materials characterized by an ability to transform elec¬ trical energy to mechanical energy, and vice versa, have widespread application in electronic equipment. To wit: such materials are extensively used, for example, as resonators and filters and as such are required to exhi¬ bit stringent frequency response (accuracy and stability) characteristics. As a result both bulk and surface wave filters and resonators typically demand some degree of "trimming" to compensate for finite tolerances attribut¬ able to material and production variances. With regard to bulk wave devices, trimming processes have heretofore been largely limited to ad hoc addition or removal of pieozelectric material in order to achieve the specified frequency characteristics. Surface wave devices, on the other hand, have been frequency trimmed by ablation or abscission of conductive coatings pre¬ viously deposited on the surface of peizoelectric mater- ial. For example, quartz crystal resonators are trimmed by the removal or addition of precisely controlled amounts of material from or to one or both of the elec¬ trodes deposited on the major faces of the blank. A typical quartz filter illustrating the quartz blank (11), frequency plating (12), and conductive electrodes (13), is shown in Figure 1. The modification in massloading is accompanied by a frequency change in the resonator and is monitored and continued until the specified performance is achieved. In practice quartz filters are typically comprised of two or more resonators arranged in various configurations. Here to the filters composite frequency response can be trimmed by adjusting the resonant frequency of the component resonators irrespective of whether the resonator are arranged as discrete blanks, stacked arrays or multi-resonator structures deposited on a single wafer. Historically trimming had been effected by vacuum deposition of controlled amounts of a precious metal onto the electrode or by exposure of the electrodes to a reac- tive atmosphere such as iodine (for silver electrodes). More recently a laserbea has been used to evaporate material deposited on the pieozelectric material. The resonant frequency of the resonator increases as the attendant massloading decreases. A plurality of methods for trimming surface wave components have also been disclosed. Here to lasers have been utilized to cut portions of predeposited conductive structures, thereby disconnecting those structures form the piezoelectric substrate. Figure 2 depicts a repre- sentative surface acoustic wave (SAW) filter including a number of interdigitated fingers (21), some of which (22), have been disconnected so as to achieve frequency trimming. Although generally effective, the above-mentioned trimming techniques suffer from a common disadvantage: each requires the trimming procedure to be performed prior to final encapsulation of the component, the grava¬ men being that the encapsulation process itself will likely have a significant effect on the frequency charac- teristics of the device. Such effects are posited to to have their origin in the therminal or mechanical stresses induced by a moisture, changes in air pressure, and stray capacitance introduced by the encapsulation process. Because such phenomena are effectively inamen- able to amelioration once the component has been sealed, it is necessary that they be anticipated and, to the extent predictable, accomodated during the trimming pro¬ cedure. That is, the resonant frequency of the device is trimmed to a frequency offset by a predetermined amount from the desired frequency with the expectation that the final frequency, after encapulation, will be accurate. Clearly a trimming scheme allows final trimming to be performed subsequent encapulation represents a welcome advance in the art of fabricating frequency-selective piezoelectric components. As will become evident, such an advance in the art is achieved by the subject inven¬ tion. DISCLOSURE OF THE INVENTION The above and other objects, advantages and capabi¬ lities are achieved in one aspect of this invention by a method of trimming frequency-selective devices of the type characterized by a piezoelectric substrate upon which is deposited a conductive material. The device is enclosed in a housing at least a portion of which is transparent to optical energy at a predetermined wave¬ length, for example, at 1.06 micrometer. Optical energy, typically derived from a laser, at the predetermined wavelength and appropriate intensity is directed at the device so that it impinges on the conductive material, thereby causing evaporation of that material. The rele¬ vant frequency-dependent characteristics of the device are monitored and the direction and intensity of the optical energy controlled in a manner that allows thse characteristics to be brought within desired tolerances. In a preferred application of the invention, a storage time is introduced between the encapsulation of the device and subsequent final frequency trimming, thereby obviating the effects of the short-term aging phenomena. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 depicts a typical quartz crystal filter including the quartz blank, frequency plating, and conductive electrodes. Figure 2 depicts a representative SAW (Surface Acoustic Wave) filter including a number of interdigi- tated conductive fingers, some of which have been severed so as to achieve frequency trimming. Figure 3 illustrates an apparatus for effecting frequency trimming of an encapsualted piezoelectric component, for example, a quartz crystal resonator. A quartz resonator, including a quartz blank, con¬ ductive electrodes and a glass cover, especially amenable to laser trimming is illustrated in Figure 4. An alternative configuration in which the resonator is enclosed in a standard metal cover which has been pro¬ vided a transparent window is shown in Figure 5. A corresponding SAW configuration is shown in Figure 6. BEST MODE FOR CARRYING OUT THE INVENTION For a better understanding of the present invention, together with the objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in conjunction with the above- described drawings. In accordance with this invention, there is illus¬ trated in Figure 3 an apparatus for providing a frequency trimming of an encapulated piezoelectric component, be it a quartz crystal resonator, surface acoustic wave filter or similar device. The device to be trimmed, 1, is in¬ serted in a test circuit 2 in such a manner that it is directed toward an optical beam 3 generatedby Q-switched Nd-YAG laser 4. The laser is a pulsed Nd-YAG type cap¬ able of delivering a focused beam that will produce suf- ficient heat to evaporate metal from the electrodes of, for example, a quartz resonator. The laser beam is appropriately directed by an X-Y deflection system 5 equipped with the necessary optical devices including, by way of illustration, a pair of optical mirrors 6. The direction of the beam is controlled by a test system 7 that delivers control signals to the deflection system and to the laser power control 8. The test system, power control and x-y deflection system operate to control the intensity and direction of the laserbeam so as to si ul- taneously scan the surface of the device to be trimmed and to modulate the trimming rate as the resonant fre¬ quency (or some other specified characteristic frequency) approaches its final value. A quartz resonator especially amenable to trimming is illustrated in Figure 4. The resonator includes a quartz blank 41, electrodes 42 and a glass cover 43. A salient feature of the resonator is the glass cover 43. The cover is transparent to the optical energy generated by the laser so that the laserbeam is allowed to impinge on the electrodes of the resonator and thereby cause the evaporation of sufficient electrode mass to achieve trim¬ ming. Such a quartz resonator may be fabricated according to the following technique. First, the quartz crystal is conductively bonded to lead-in wires, that is to say, one electrode is electrically and physically connected to one of the lead-in wires and the other electrode is electri¬ cally and physically connected to the other lead-in wire. The conductive bond may be.made, for example, by electri- cally conductive bonding material, for example, silver- filled epoxy. Or it may be made by soldering, welding and the like. The quartz crystal is then inserted into an open-ended glass tube of suitable diameter and length. The extremities of the lead-in wires protrude outside the glass tube and are secured in a suitable external holder to properly position the quartz crystal within the glass tube. The end of the glass tube is then heated to its softening point and pressed together to seal the end by forming a press seal, the lead-in wires being embedded in the press seal. For this purpose, the lead-in wires are of a type readily sealable to glass, for example, Du et wire for sealing to soft glass. Dumet comprises a nickel-iron core within a copper sheath. Upon cooling, the press seal solidifies and rigidly holds the lead-in wires and quartz crystal. Next, a circumferential section of the glass tube near the other end thereof is heated to its softening point and the end is then drawn apart from the main body of the glass tube to form a necked-down portion in the glass tube. Said end is then cut off or otherwise removed, leaving the necked-down portion which is of smaller diameter than the glass tube and is suitable as an exhaust tubulation to exhaust, tipoff and seal the glass tube with the quartz crystal therewithin. After exhausting (by vacuum) the atmosphere within the glass tube, the exhaust tubulation may be tipped off under vacuum, to maintain a vacuum within the glass tube. Or an inert gas, for example, dry nitrogen, may be introduced into the glass tube prior to tip-off of the exhaust tube. If necessary, a cooling gas, e.g. -fυR E A r OMPl ^^rte-f nitrogen may be flowed into the glass tube during the sealing steps in order to cool the quartz crystal and prevent it from being heated above the curie point of the quartz material. The drawing shows one embodiment of an encapsulated quartz crystal in accordance with this invention. The quartz crystal comprises a flat circular disk about 8mm in diameter by 0.5mm thick. The metallized portion of each surface is about 6mm in diameter. Lead-in wires are made of Dumet, 0.35mm thick, and are fastened to the electrodes. A glass tube is 11mm outside diameter by 20mm long (internal length). A press seal about 9mm wide by about 7mm long and is about 2.5mm thick. After exhausting and filling with nitrogen, the glass tube is sealed at tip-off. If desired, the lead-in wires can be embedded in glass bead prior to press sealing in order to bend and hold the wires in the correct position for fast¬ ening the crystal. In some cases, it may be desirable that the glass tube be flattened into, say a flat sub- stantially rectangular, as opposed to circular shape, in order to reduce the size of the glass tube or in order to accommodate a rectangular quartz crystal. In such a case, the open-ended round glass tube would be heated and flattened prior to mounting of the quartz crystal/lead-in wire assembly therein. After embedment of said assembly in a press seal at one end of the flatened glass tube, the other end could also be sealed by a press seal with¬ out the need of an exhaust tubulation. In such a case, nitrogen, for example, could be introduced into the interior of the flattened glass tube by means of a small diameter hollow metal needle inserted therein while the glass was heated to its softened point. At the proper time, the needle would be removed and the press seal made immediately, thereby providing the desired nitrogen fill within the glass tube. JVJR-EATT OMPI ^ ?>o ' The technique described above results in a resonator entirely enclosed by glass. As an alternative the reso¬ nator may be enclosed by a standard metal cover which has been provided with a glass window as shown in Figure 5. The window 51 may be comprised of any otherwise suitable material transparent to energy at the laserbeam wave¬ length. (In a particular embodiment this wavelength was 1.06 micrometers.) The window may be preferrably disc¬ shaped and susceptible to attachment to the metal cover 52 by, for example, glue or a glass-to-metal seal. The diameter of the disc-shaped window should be large enough so that the laserbeam is allowed to impinge on substan¬ tially the entire electrode surface. In practice a diameter equal to or somewhat less than that of the elec- trode is sufficient since the extremities of the elec¬ trode surface have been found less sensitive to frequency trimming by evaporation. A SAW structure such as the one illustrated in Figure 6 is amenable to the laser trimming technique described herein. The trimming technique and implementing apparatus have been found to offer numerous significant advantages in the area fabrication and trimming of piezoelectric components. To wit: The post-encapsulation trimming of those devices permits less stringent handling procedures resulting in fewer rejected parts. Avoidance of the pre- encapsulation offset trimming technique provides more precise trimming and a closer approach to the desired ultimate frequency characteristics of the device. And, probably most significantly, the post-encapsulation trim- ming technique allows the encapsulated device to be stored for a period of time before the final trimming procedure is performed. This is decidedly an advantage because of the "aging" effect characteristics of such device. That is to say, a large portion of the total frequency shift of the device is found to occur within a relative short period after fabrication. By postponing the final trimming procedure the effects of this "short term" aging can be accordingly circumvented. As an example the total long-term frequency drift (i.e., the total drift after a period on the order of one year) can be expeced to be reduced from approximate 10 ppm (parts per million) when trimmed before encapsulation to about 3 ppm when trimmed subsequent encapsulation. Accordingly, while there has been shown and desribed what at present are considered to be the preferred embod¬ iments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the appended claims. INDUSTRIAL APPLICABILITY The subject invention is useful in the fabrication of frequency selective piezoelectric devices.

Claims: WHAT IS CLAIMED IS: 1. A method of trimming a frequency-selective device characterized by a piezoelectric substrate upon which is deposited a conductive material, said method comprising the steps of: enclosing the device in a housing, a portion of which is substantially transparent to optical energy at a predetermined wavelength, providing a source of optical energy at the prede- termined wavelength with sufficient intensity to cause evaporation of the conductive material, directing the source of optical energy at the trans¬ parent portions of the housing is such a fashion such that the optical energy impinges on and thereby causes evaporation of the conductive material, monitoring desired frequency-dependent characteris¬ tics of the device, and controlling the direction and intensity of the opti¬ cal energy so as to achieve the desired frequency-depen- dent characteristics of the device by selective evapora¬ tion of the conductive material. 2. A method as defined in Claim 1 wherein the housing comprises a glass window. 3. A method as defined in Claim 1 wherein the housing comprises a quartz window. 4. A method as defined in Claim 1 wherein the housing is substantially comprised of glass. 5. A method as defined in Claim 1 wherein the source of optical energy is a laser. 6. A method as defined in Claim 5 wherein the laser is a Q-switch Nd-YAG laser. 7. A method as defined in Claim 6 wherein the laser generates an optical beam at a wavelength of approxi¬ mately 1.06 micrometer. 8. A method as defined in Claim 1 wherein the source of optical energy is directed by an x-y deflection system. 9. A method as defined in Claim 9 wherein the x-y deflection system comprises a pair of optical mirrors. 10. A method as defined in Claim 1 wherein the device is a crystal resonator comprising a quartz blank upon which are depsoited a pair of electrically conductive elec¬ trodes. 11. A method as defined in Claim 10 wherein at least one of the electrodes is substantially disc-shaped. 12. A method as defined in Claim 11 wherein the housing comprises a substantially disc-shaped window. 13. A method as defined in Claim 12 wherein the disc¬ shaped window is slightly smaller than the disc-shaped electrode. 14. A method as defined in Claim 1 wherein the device is a Surface Acoustic Wave filter. 15. A method as defined in Claim 1 wherein a storage period is introduced between the time the device is enclosed in the housing and subsequent exposure of the device to the optical energy, the storage period being of a duration sufficient to substantially obviate short- term aging phenomena. ' υREAT O PI

Inventors: CLAES ROGER, VANNOPPEN JEAN, CLAES, ROGER, VANNOPPEN, JEAN
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1982
Main classification: H01L41
IPC codes: H03H3