Acoustic Surface-wave Filters And Methods Of Manufacture Therefor

Abstract

A surface-wave filter has a wave-propagating medium of ferroelectric material that exhibits a rate of change of surface-wave velocity which increases with increasing degree of poling more rapidly than its surface-wave coupling factor increases as the poling level is raised. The ultimate transducer interaction efficiency is a function of the coupling factor. Moreover, the material is poled in an amount that effects a desired surface-wave velocity while, at the same time, producing a surface-wave coupling factor that is significantly less than that corresponding to an optimum value with respect to transducer interaction efficiency. Completing the structure, an input transducer serves to launch acoustic surface waves and an output transducer responds to those waves by developing an output signal.

Description

BACKGROUND OF THE INVENTION

The present invention pertains to surface-wave filters. More particularly, it relates to surface-wave filters which may be mass produced with good control of ultimate characteristics and to methods for achieving such control.

It is known that an electrode array composed of a pair of interleaved combs of conducting teeth may be coupled to a piezoelectric medium to launch or respond to acoustic surface waves. Such a device, with its small size, is particularly useful in conjunction with solid-state functional integrated circuitry where signal selectivity is desired. A number of different versions of these devices, together with various modifications and adjustments thereof, are described and others are cross-referenced in United States Letters Patent 3,582,840 issued June 1, 1971 and assigned to the same assignee as the present invention.

A salient characteristic of surface-wave filters is the sharp selectivity that may be obtained. Moreover, the particular selectivity pattern and center frequency may be tailored more or less as desired by means of appropriate engineering design. Consequently, the filters represent an attractive component for use in such systems as the intermediate-frequency stages of radio and television receivers where a particular bandpass characteristic is necessary. These devices are capable of eliminating the need for the critical and usually much larger and more cumbersome components such as wound coils.

Of course, use in such apparatus as radio and television receivers necessitates adaptability to mass production. In turn this necessitates a high degree of reproducibility. Even in the case of laboratory production, however, surface-wave filters have been found to exhibit significant variations from one unit to the next.

It is, therefore, a general object of the present invention to provide a new and improved method of manufacturing surface-wave filters that at least reduces such variations during production.

A corresponding object of the present invention is to provide a new and improved surface-wave filter that enables mass production with a higher degree of reproducibility.

A specific object of the present invention is to produce surface-wave filters that exhibit a reduced variation in the center frequency as between successive filters turned out during manufacture.

The invention thus relates to methods of making surface-wave filters and to the filters themselves. Each filter has a poled wave-propagating medium, an input transducer which responds to input signals and launches acoustic surface waves along the medium together with an output transducer which is responsive to those acoustic waves for developing an output signal. Selected as the propagating medium is a ferroelectric material that exhibits a rate of change of surface-wave velocity which increases with increasing degree of poling more rapidly than its surface-wave coupling factor increases with the same increasing degree of poling. The transducers exhibit an ultimate efficiency that is a function of the surface-wave coupling factor. During poling, the material is subjected to a controlled degree of temperature. The poling itself is accomplished by applying across the material an electric poling field that has a strength and persists for a time interval which is selected to obtain a desired surface-wave velocity. Concomitantly, a surface-wave coupling factor is obtained which is significantly less than that which would correspond to optimum ultimate transducer efficiency.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which:

FIG. 1 is a partly schematic plan view of a now known acoustic-wave filter;

FIG. 2 is a plot which exhibits a desired bandpass characteristic of the intermediate frequency stages of a color television receiver;

FIG. 3 includes a pair of related plots that depict correlation between surface-wave velocity and surface-wave coupling factor as against degree of poling in a surface-wave filter;

FIG. 4 is a plot of the phase angle .phi. vs frequency obtained by measuring the driving point impedance of a surface-wave filter as a function of frequency;

FIGS. 5a, 5b and 5c are plots which represent a function of the degree of poling of a surface-wave filter as a result of time, temperature and field strength; and

FIGS. 6a and 6b are plots which represent the effect of temperature, to which the filter may be subjected after poling has been accomplished, on surface-wave velocity and surface-wave coupling factor.

As is now known, surface-wave filters may take a variety of forms. A simple and yet typical form is shown in FIG. 1. Thus, an input signal source 10 is connected across an electrode array 12 which is mechanically coupled to a piezoelectric acoustic-wave-propagating medium or substrate 13 to constitute therewith an input transducer. An output electrode array 14 also is mechanically coupled to substrate 13 to constitute therewith an output transducer. Electrode arrays 12 and 14 are each constructed of two interleaved comb-type electrodes of a conductive material, such as gold or aluminum, which may be vacuum deposited on the smoothly lapped and polished planar upper surface of substrate 13. The piezoelectric material is one, such as lead zirconate titanate (PZT), that propagates acoustic surface waves.

In operation, direct piezoelectric surface-wave transduction is accomplished by input transducer 12. Periodic electric fields are produced across the comb array when a signal from source 10 is applied to the electrodes. These fields cause perturbations or deformations of the surface of substrate 13 by piezoelectric action. Efficient generation of surface waves occurs when the strain components produced by the electric fields in the piezoelectric substrate substantially match the strain components associated with the surface-wave mode. These mechanical perturbations travel along the surface of substrate 13 as generalized surface waves representative of the input signal.

Source 10 might represent the intermediate-frequency output signal from a television receiver tuner. That signal is converted by transducer 12 into acoustic waves. Those surface waves are then transmitted along the substrate to output transducer 14 where they are converted to an electric signal for transmission to a load 15 connected across the two interleaved combs in output transducer 14. In this example, load 15 represents a subsequent video or audio stage of the receiver. Utilizing PZT as the substrate material in the example, the teeth of both transducers 12 and 14 are each about twelve microns wide and are separated by a center-to-center spacing of 24 microns for the application of an intermediate-frequency signal in the standard 40 megahertz range. The spacing between transducer 12 and transducer 14 is on the order of 80 mils and the width of the wavefront is approximately 0.1 inch.

The potential developed between any given pair of successive teeth in electrode array 12 produces two waves traveling along the surface of substrate 13, in opposing directions, perpendicular to the teeth for the illustrative case of a ceramic which is poled perpendicular to the surface. When the center-to-center distance between the teeth is one-half of the acoustic wavelength of the wave at the desired input signal frequency (the so-called center frequency), relative maxima of the output waves are produced by piezoelectric transduction in transducer 12. For increased selectivity, additional electrode teeth are added to the comb patterns of transducers 12 and 14. Further modifications and adjustments are described and others are cross-referenced in the aforementioned Letters Patent for the purpose of particularly shaping the response presented by the filter to the transmitted signal. Techniques are also there mentioned for attenuating or advantageously making use of the one of the two surface waves that travels to the left from transducer 12 in FIG. 1. It will suffice for purposes of understanding the present invention to consider only the acoustic surface waves that travel to the right from transducer 12 in the direction toward transducer 14.

FIG. 2 depicts a typical bandpass characteristic for a television receiver intermediate frequency amplifier. It will be observed to include the need for well defined comparatively deep traps particularly at the frequencies of the associated and adjacent sound. These are for the purpose of precluding the appearance of sound caused interference in the reproduced picture. Additionally, proper picture or video fidelity requires accurate shaping of the main response lobe relative to the picture carrier and the color subcarrier. The aforementioned Letters Patent describes in detail various techniques in the practical design of surface-wave filters intended for use in such an application. For present purposes, it is sufficient to note that it is necessary to assign to each transducer a selected and fixed center frequency of maximum response. To that end, as previously indicated, the inter-tooth spacing of the interleaved combs is chosen to be one-half the wavelength in the propagating material of the acoustic surface waves. The acoustic wavelength in the medium is, in turn, a function of the surface-wave velocity.

Also necessarily of interest in the design of a surface-wave filter in such an application is the insertion loss encountered by its inclusion in a system. Of course, as indicated initial consideration is the desirability of minimizing the amount of insertion loss, since any such loss normally has to be overcome by the inclusion in the system of an equivalent amount of compensating amplification. Accordingly, it is customary to seek ultimate maximum efficiency of transducer interaction. That is, a maximum is sought in the surface-wave coupling factor k.sub.s. At least generally, the surface-wave coupling factor is a function of the degree to which the substrate is poled. Consequently, the situation indicates the use of as high a poling field strength as can be obtained without electrical breakdown across the material.

In practice, it is found that the rate of change of surface-wave velocity increases with an increased degree of poling. Similarly, the surface-wave coupling factor also increases with an increased degree of poling. However, the rate of change of surface-wave velocity with such an increase is more rapid than that of the surface-wave coupling factor. With a very high degree of poling, the change of surface-wave velocity with any change in degree of poling is substantial. Consequently, even small changes in poling level during manufacture can result in significant changes in surface-wave velocity. With the intertooth spacing of the transducers already fixed as a matter of design, this variation in surface-wave velocity leads to a difference in the actual center frequencies as between any one transducer and other supposedly identical transducers. The end result is errors in placement of the different carrier frequencies and trap positions on the overall frequency-response characteristic. Even with very careful control of the poling level, it has been found that a spread of .+-. 2 percent may be expected in the ultimate center frequencies of a succession of filters.

In accordance with the present invention, the applied electric poling field is caused to have a strength and to persist for a time interval selected in view of the temperature so as to obtain the desired surface-wave velocity while, at the same time, obtaining a surface-wave coupling factor that is significantly less than that corresponding to ultimate maximum transducer interaction efficiency. Because the surface-wave velocity increases with increased poling more rapidly than the concomitant increase in surface-wave coupling factor, the approach herein contemplated results in attaining a surface-wave velocity at a poling level where there is a significantly less change in surface-wave velocity for any deviation in poling degree. Consequently, enhanced reproducibility of result, as between successively produced filters, is achieved at the expense of some reduction in surface-wave coupling factor.

By way of further explanation, the velocity of surface waves propagating in the basal plane of a ferroelectric material, such as those of the PZT-type, depends upon density, four elastic constants, three piezoelectric constants and two dielectric constants. All of these different constants are dependent upon the degree of poling of the material. As discussed in "Variation of Electroelastic Constants of Polycrystalline Lead Titanate Zirconate with Thoroughness of Poling," J.A.S.A., Vol. 36, No. 3, pp. 515-520, March 1964, by D. Berlincourt, the variation of the elastoelectric constants can be measured as a function of the invariant coupling factor k.sub.i3. This invariant coupling factor is the highest piezoelectric coupling factor obtainable for a given electric field and certain specified elastic stress conditions. Invariant coupling factor k.sub.i3 serves as a measure of the degree of poling.

Using PZT-5 as a material for example, the elastic constants decrease from 5 to 11 percent as between an unpoled and a fully poled condition. The dielectric constants decrease by 31 and 47 percent for the same change. Two of the piezoelectric constants gradually increase with degree of poling while the other decreases after an initial small increase. The end result of these different variations as the poling is changed is the establishment of a definite relationship between the surface-wave velocity V.sub.s and the degree of poling as shown in the upper trace of FIG. 3. Thus, the surface-wave velocity varies from 1662 meters/second for unpoled material to 2085 meters/second for a substantially fully poled material, a variation of approximately 25 percent. As already indicated, the rate of change of the surface-wave velocity increases as the degree of poling, represented by the invariant coupling factor k.sub.i3, is increased. For materials of a particular symmetry, the surface-wave velocity curves for both a free surface and a metallized surface may be calculated utilizing the set of equations derived by C--C. Tseng in his Doctoral Dissertation published at the University of California, Berkeley, in 1966. The solid line curve in the upper portion of FIG. 3 represents the result of such a calculation for a free surface. On the other hand, the dashed-line curve represents actual experimentally measured values of that surface-wave velocity.

The lower trace in FIG. 3 depicts the change in surface-wave coupling factor as the degree of poling is increased. In this case, the rate of change is almost constant, increasing to a maximum value for the surface-wave coupling factor k.sub.s of about 0.23. From the difference .DELTA. V.sub.s between the surface-wave velocity on a free surface and that on a metallized surface, the surface-wave coupling factor k.sub.s may be directly calculated. Again in the lower portion of FIG. 3, the solid line trace represents the calculated determination while the dashed line depicts the values as measured experimentally.

Intrinsic coupling factor k.sub.i3 may be calculated directly from measurements of the radial coupling factor, the thickness coupling factor and the low- and high-frequency dielectric constants. By measuring the driving point impedance of the resulting transducer as a function of frequency, the apparent surface-wave velocity and the surface-wave coupling factor may be calculated. FIG. 4 is a plot of the phase angle .phi. as determined by such measurement of the driving point impedance. It will be observed that the total phase angle is a function not only of the surface-wave development but also of the bulk mode, series and dielectric losses, .phi..sub.l, .phi..sub.s and .phi..sub.d, respectively. Thus a complete determination necessitates allowing for the existence of the latter losses. The surface-wave response curve represented by .phi..sub.A is typical, exhibiting a main lobe A between a pair of minor lobes separated by nulls B and C respectively. That is, the response is basically a sin x/x function.

As indicated, it is contemplated herein to trade a certain amount of surface-wave coupling factor in order to obtain improved reproducibility of surface-wave velocity which, in turn, yields improved reproducibility of center frequency. Returning to FIG. 3, it will be observed that .DELTA.V.sub.s /.DELTA.k.sub.i3 decreases as the value of the invariant coupling factor k.sub.i3, or the poling level, is decreased. Consequently, the degree to which the material is poled preferably is selected to have an ultimate value of approximately 0.5 as represented by the invariant coupling factor k.sub.i3. Accordingly, poling is accomplished to a level at which the velocity curve in the upper portion of FIG. 3 is much flatter than would be the case if maximum surface-wave coupling factor were to be sought.

Poling the material to the desired value of the invariant coupling factor is a function of time, field strength and temperature as illustrated in FIGS. 5a-5c. The particular values there represented were determined experimentally utilizing Honeywell S* ceramic ferroelectric material. However, analogous characteristics will be exhibited by other piezoelectric ceramics. Moreover, the ordinate in each of these three figures as measured in this case was the thickness coupling factor k.sub.t. However, the changes in the surface-wave coupling factor and surface-wave velocity in the material are a function of the thickness coupling factor, which, in turn, is a function of the invariant coupling factor k.sub.i3. Particularly with reference to FIG. 5a it will be seen that, for a field strength of 100 volts/mil and at a temperature of 100.degree.C., the poling level achieved remains relatively constant after the first few minutes. With reference to FIG. 5b, wherein the same field strength is applied and the time interval is 30 minutes, it will be observed that the degree of poling, as represented again by the thickness coupling factor k.sub.t, increases fairly consistently as the temperature is elevated. Finally, FIG. 5c depicts the degree of poling obtained with increase in field strength and at a time interval of thirty minutes. In this case, two results are shown, one at 26.degree.C. and another at 100.degree.C. Thus, at the much lower temperature, the degree of poling continues to increase as the field strength is raised. At a very high temperature, however, an actually decreased degree of poling is obtained as the field strength is increased.

The curves of FIGS. 5a-5c thus reveal that considerable flexibility is offered in choosing the actual poling conditions to be employed in any given manufacturing operation. Whatever the actual time, temperature and field strength characteristics for a particular material, the combination of those variables is selected so as to achieve an ultimate degree of poling level that corresponds approximately to an invariant coupling factor of 0.5 or a flatter part of the curve which relates the characteristic of surface-wave velocity as against degree of poling.

Whatever the precise degree of poling attained, it is also necessary to insure that subsequent manufacturing operations do not serve to change the desired degree of poling. Usually, the actual transducer electrodes and all connecting leads are deposited subsequent to the poling operation. Finally, the entire device is encapsulated in a hermetically sealed package. These subsequent operations may involve subjecting the poled ferroelectric material to a temperature of the order of 200.degree.C. It is important to see that such thermal shock does not serve to depole the material and thus lower the surface-wave velocity. Accordingly, the material selected should be one which does not experience any significant degree of depoling at whatever temperature levels are established during the subsequent manufacturing steps.

FIGS. 6a and 6b depict the effect of temperature in terms of change in surface-wave velocity V.sub.s and surface-wave coupling factor k.sub.s for four different typical present-day ferroelectric ceramic materials. It is clear from an examination of FIGS. 6a and FIG. 6b that PZT-5 or PZT-6 constitute the better choices for use in any case where the manufacturing techniques require that the poled substrate be subjected to any significant temperature level during subsequent manufacturing steps.

It is necessary to recognize that the different curves presented in the drawings and the particular values discussed in the above description are but examples. Substantial quantitative change in the different characteristics may be expected as between a variety of ferroelectric materials. In a particular case, the surface-wave velocity and surface-wave coupling factors are determined as a function of the degree of poling, and a specified poling level is then selected so as to correspond with a portion of the velocity curve which is reasonably flat. Controlling the temperature during poling, the poling time and the field strength are then selected so as to achieve the desired degree of poling. As a result, the variation from one unit to another in the surface-wave velocity during mass production is reduced. In turn, this results in improved consistency of transducer center frequency as between successively produced units.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

I claim:

1. In the method of making a surface-wave filter having a poled wave propagating medium, exhibiting surface-wave velocity and interaction efficiency characteristics varying as functions of its surface-wave coupling factor, and an invariant coupling factor varying as a function of the poling of said medium, an input transducer responsive to input signals for launching acoustic surface waves along said medium and an output transducer responsive to said acoustic waves for developing an output signal, the steps comprising:

selecting as said medium a ferroelectric material exhibiting a rate of change of said surface-wave velocity that increases with increasing degree of poling more rapidly than said surface-wave coupling factor increases with said increasing degree of poling;

subjecting said material to a controlled degree of temperature;

and applying across said temperature-subjected material an electric poling field having a strength and persisting for a time interval sufficient to obtain a desired stabilized surface-wave velocity at a value of surface-wave coupling factor which is significantly less than that corresponding to maximum transducer interaction efficiency, and an invariant coupling factor of approximately 0.5.

2. A method as defined in claim 1 in which said controlled degree of temperature is higher than any temperature to which said material subsequently is subjected during completion of said filter.

3. A surface wave filter comprising:

a wave-propagating medium of ferroelectric material exhibiting a rate of change of surface-wave velocity that increases with increasing degree of poling more rapidly than its surface-wave coupling factor increases with said increasing degree of poling, the transducer interaction efficiency of said material being proportional to said surface-wave coupling factor, the degree of poling of said material being represented by its invariant coupling factor, said material being poled in an amount effecting a desired surface-wave coupling factor significantly less than that corresponding to the maximum value of said transducer interaction efficiency and at an invariant coupling factor of less than 0.5;

an input transducer disposed on a surface of said medium near one end thereof and responsive to input signals for launching acoustic surface-waves along said surface;

and an output transducer disposed on said surface near the other end thereof and responsive to said acoustic waves for developing an output signal.