High piezoelectric coupling X-cuts of lead potassium niobate, Pb.sub.2 Knb.sub.5 O.sub.15 , for surface acoustic wave applications

#This invention relates to surface acoustic wave devices and to substrates of lead potassium niobate, Pb.sub.2 KNb.sub.5 O.sub.15, PKN) for use therewith.

#ST-cut quartz is often utilized as a piezoelectric substrate material for a wide variety of surface acoustic wave devices (SAW) such as filters, delay time encoders, decoders, correlators and other signal processing devices. Unfortunately, ST-cut quartz possesses a low piezoelectric coupling constant and, therefore, is not suitable for use in SAW devices designed to have low insertion losses and broad bandwidths. As a consequence, a considerable research effort has evolved in an attempt to find other materials for use as SAW substrates that are temperature compensated and possess a requisite high piezoelectric coupling constant. In attempting to find desirable materials, it was determined that, in order to be temperature compensated, the material often possesses either a positive temperature coefficient of an elastic constant or a negative coefficient of thermal expansion. That such a concept is valid has been demonstrated by the results of recent calculations of the SAW properties of berlinite (which has a positive temperature coefficient of an elastic constant) and .beta.-eucryptite (which has a negative coefficient of thermal expansion). Those calculations showed that both materials are indeed temperature compensated and have larger piezoelectric coupling constants than ST-cut quartz.

#However, berlinite and .beta.-eucryptite still fail to possess a piezoelectric constant as large as is desired for certain SAW applications and, in addition, lack the low temperature coefficient of time delay and small electromechanical power flow angle parameters also desired for SAW applications. In further attempts at finding materials which might prove useful and desirable for SAW applications, it was discovered that lead potassium niobate (PKN), which occurs in the tungsten bronze structure and belongs to the orthorhombic crystal class mm.sub.2 (C.sub.2v) is attractive for SAW applications. The most significant feature of this material is that its piezoelectric coupling constant is up to four times as large as that of berlinite. In addition, the diffraction spreading of PKN is less than that of an isotropic material, an attractive feature not shared by either quartz or berlinite. Calculations undertaken during the research effort have shown that a continuous series of propagation directions in the plane of the X-cut of lead potassium niobate provide substrates with piezoelectric coupling constants 14.5 to 17.1 times as large as that of ST-cut quartz. The particular crystallographic orientations of this invention are defined by the Euler angles: Lambda=90.0.degree.; Mu=90.0.degree.; and a range of from -10.6.degree. to +10.6.degree. Theta.

#In the drawings

#FIG. 1 is an isometric view illustrating the substrate material contemplated by this invention; and

#FIG. 2 is a graphical illustration showing various properties for the x-cut substrate of FIG. 1.

#Pursuant to the above-defined objects, it has been found that lead potassium niobate crystalline substrates with crystallographic orientations defined by the Euler angle Lambda=90.0.degree., Mu=90.0.degree. and a range of from -10.6.degree. to +10.6.degree. Theta are especially suitable for surface acoustic wave applications and exhibit piezoelectric coupling constants from 14 to 17 times as large as that of the previously known ST-cut quartz substrate.

#Initial interest in PKN as a possible SAW substrate material was stimulated by measurements of its bulk wave properties which showed it to have electromechanical coupling factors as large as 0.73 and opposite signs for the temperature coefficients of the fundamental resonant frequencies for various crystal-cut plates. This latter result suggested that temperature-compensated cuts could be found for intermediate orientations. Further impetus was provided by the results of measurements of the temperature coefficients of the elastic constants of PKN, some of which are positive. To investigate the SAW properties of PKN, a theoretical computer model was used to calculate the SAW velocity, the electromechanical power flow angle, an estimate of the piezoelectric coupling, and the first-order temperature coefficient of time delay for singly rotated standard crystallographic orientations. As input, this model required experimental values for the elastic, piezoelectric, and dielectric constants, their respective temperature coefficients, the density, and the coefficients of thermal expansion. Values for all but one of these necessary constants were obtained from the data of Regnault, Ph.D. dissertation, (The Pennsylvania State University, 1977) (unpublished). His measurements were made on small (4-5 mm) single-crystal samples of PKN which were obtained from larger cracked boules grown from a melt by the conventional Czochralski method. The samples were homogeneous and free of internal strains but contained 180.degree. (electrical) domains which adversely affected only the measurement of the dielectric constant; hence, for the SAW property calculations, a value for that quantity was obtained from the data of Yamada, J. Appl. Phys. 46, 2894 (1975). Note that the effect of the dielectric constants on SAW calculations is only second order in comparison to that of the elastic or piezoelectric constants.

#Crystalline orientations considered attractive for SAW applications were found for single rotations of the X-cut. Plots of the SAW properties of this cut are shown in FIG. 2. While FIG. 1 illustrates a lead potassium niobate substrate 10 with the Euler angles of this invention. The graphs of FIG. 2 show that there are a series of attractive independent SAW orientations for the X cut. As example, orientations having the Euler angle .theta. equal to -10.6.degree., 0.degree. and +10.6.degree. are listed in Table I with respective values for temperature coefficient of time delay (TCD), electromechanical power flow angle and its slope, piezoelectric coupling constant (.DELTA.V/V.infin.), and SAW velocity. For comparison, the ST cut of quartz, two orientations of berlinite, AlPO.sub.4, and two of thallium vanadium sulfide, Tl.sub.3 VS.sub.4, are included also.