Abstract:
A system ( 200 ) for transmitting and focusing surface acoustic waves ( 208 ) along a substrate ( 206 ) is disclosed. The system ( 200 ) comprising a curved transmission element ( 202 ) that is disposed upon the substrate ( 206 ). The curved transmission element ( 202 ) is adapted to propagate an acoustic wave ( 212 ). The system ( 200 ) also comprises a curved receiving element ( 206 ) that is disposed upon the substrate in relation to the curved transmission element ( 202 ). The curved receiving element ( 206 ) is adapted to receive the acoustic wave ( 212 ) and match the acoustic wave&#39;s size and shape.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates, in general, to the field of surface acoustic wave devices, and in particular, to a system for providing precision filters and oscillators that uses a curved transceiving element. 
     BACKGROUND OF THE INVENTION 
     Surface acoustic wave (SAW) devices are used extensively in modern electronic systems, especially in those involving communications or signal processing applications. A SAW device is formed starting with a piezoelectric substrate, onto the surface of which electrodes (also referred to as antennas) or other transceiving elements are patterned using known photolithographic processes. Typically, a transmitting set and a receiving set of multiple, inter-digited electrodes are formed into a lattice configuration. 
     The inter-digited electrodes now disposed upon the piezoelectric substrate operate to convert a voltage into a surface acoustic wave, or a surface acoustic wave into voltage. Specifically, as voltage is applied to the transmitting electrodes, an acoustic wave is formed in the substrate, due to the piezoelectric effect. The acoustic wave propagates in the substrate at a given velocity that differs depending upon the direction of propagation. Many devices operate in the surface mode, in which the relevant velocity is the surface acoustic velocity. When the acoustic wave reaches the receiving electrodes, the acoustic energy is reconverted to electric energy. 
     Designers may thus employ SAW devices to provide filter and oscillator functions in a signal processing or communication application. In such applications, a technical measure of SAW efficacy may be referred to as Quality factor (Q). Quality factor may be calculated by the formula Q=ω/Δω; where ω represents the frequency of operation and Δω represents the variance of the frequency. Variance of the frequency may be affected by a number of factors including transmission losses, phase jitter, and other distortions which may be intentionally and unintentionally introduced. 
     For a given frequency, the quality factor Q may be increased providing more precise filters and oscillators. This may be accomplished by increasing frequency or by decreasing the variance (or a combination of the two). Variance increases with losses in transmission of the acoustic wave, including diffractive losses. It is critical, therefore, to curtail diffractive losses in high precision SAW applications. 
     It has been found that in conventional SAW devices employing linear electrodes, the diffractive losses are unnecessarily high which causes a high variance and a low Q value. This phenomenon is illustrated in FIG. 1 wherein a conventional SAW device  100  uses linear electrodes. Transmitting electrode  102  propagates waves towards receiving electrode  104 , as shown by wave fronts  106 . As waves  106  are propagated from electrode  102 , they are diffractively altered in size and shape over the course of transmission. As they arrive at electrode  104 , waves  106  exceed the receiving area of electrode  104 , resulting in portions  108  of the wave being lost. 
     A further problem with straight electrodes pertains to phase delay errors caused by a variance in arrival time of the acoustic signal across the receiving electrode. In the standard SAW device configuration of parallel, straight transmitting and receiving electrodes, the leading phase front of an acoustic signal arrives at the center of the receiving electrode before arriving at the tips of the receiving electrodes. This spread, or dispersion, in arrival time lowers performance of the device by increasing Δω and lowering Q. 
     Some conventional systems have attempted to address this by significantly increasing the size of receiving electrodes. This approach, however, has unacceptable impacts on system efficiency and costs. Other conventional systems have used shaping and positioning of electrodes in an attempt to reflect acoustic waves. Mere redirection, however, fails to address diffractive losses, resulting in a low Q value. While other conventional systems have attempted to reduce variance emanating from the coupling of electrodes to a substrate, reduction of diffractive losses remains unaddressed. 
     A need has, therefore, arisen for a surface acoustic wave system that curtails diffractive losses. A need has also arisen for such a surface acoustic wave system that provides for optimal Q value. A need has further arisen for such a surface acoustic wave system that has increased precision. 
     SUMMARY OF THE INVENTION 
     In the present invention, a surface acoustic wave system curtails diffractive losses and phase delay errors by shaping an acoustic wave for propagation. The surface acoustic wave system of the present invention provides for a high Q value. The surface acoustic wave system of the present invention has increased precision which improves its performance, particularly when used as a filter or oscillator. 
     In the surface acoustic wave system of the present invention, curved transmission elements are provided in order to shape a wave for propagation. Curved receiving elements are provided in order to match the shape and size of the propagating wave front, thereby fully receiving the wave at a definite arrival time and eliminating diffractive losses. 
     In one embodiment of the present invention, both the transmission and receiving elements are semi-circular in shape. The elements are curved concavely with respect to one another and satisfy a defined size and positional relationship. Alternatively, both transceiving elements of the present invention may be semi-elliptical or substantially parabolic in shape, providing necessary wave front matching. 
     In other embodiments, a transmission element may be shaped differently than, and curved either concavely or convexly with respect to, a receiving element; where the receiving element provides necessary wave front matching. 
     In one embodiment of the present invention, a transmission element may be linearly shaped and combined with a semi-circular receiving element to satisfy a defined size and positional relationship. 
     In yet another embodiment, a transmission element is curved either concavely or convexly with respect to, and combined with, an array of receiving elements, where each receiving element provides necessary wave front matching. This system provides a phase sensitive processing capability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is an illustration of a prior art SAW device having linear electrodes; 
     FIG. 2 is a schematic illustration of a SAW device, having curved electrodes, according to the present invention; 
     FIG. 3 is a schematic illustration of a SAW device having curved electrodes, according to the present invention; 
     FIG. 4A is a schematic illustration of curved electrodes for use in a SAW device, according to the present invention; 
     FIG. 4B is a schematic illustration of curved electrodes for use in a SAW device, according to the present invention; 
     FIG. 5 is a schematic illustration of a linear transmitting electrode and a curved receiving electrode for use in a SAW device, according to the present invention; 
     FIG. 6A is a schematic illustration of curved electrodes, according to the present invention; 
     FIG. 6B is a schematic illustration of curved electrodes for use in a SAW device, according to the present invention; 
     FIG. 7 is a schematic illustration of curved electrodes for use with a SAW device, according to the present invention; and 
     FIG. 8 is a schematic illustration of a linear transmitting electrode and an array of curved receiving electrodes for use with a SAW device, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention. 
     The SAW design of the present invention provides greater precision in filtering and oscillator applications. The curvature of the electrodes of the present invention provides a precise focusing of an acoustic wave upon propagation and a matching of wave front size and shape, resulting in reception without diffraction loss. 
     The principles of a SAW device  200  according to the present invention are illustrated in FIG.  2 . Device  200  incorporates transmission element  202  and receiving element  204 , both transmissively coupled to a substrate  206 . Substrate  206  is typically a piezoelectric material, such as lithium tantalate or quartz. Other materials may be employed as desired to realize the benefits of the present invention. Similarly, elements  202  and  204  may be realized in a number of ways. Elements  202  and  204  are typically electrodes, though other contrivances may be employed based on desired operational characteristics. It should be apparent to those skilled in the art that elements  202  and  204  may be initially formed and then coupled to substrate  206 , or they may be formed directly on substrate  206 , by known methods such as deposition. All such possibilities are comprehended by the present invention. 
     Element  202  is engaged and begins propagating wave fronts  208  across the substrate  206 . The curvature of element  202  constrains the shape of the wave front path and focuses that path into progressively smaller and straighter wave fronts, through a convergence point  210  at which the wave front is substantially linear and tightly focused. Passing point  210 , the wave front path diffracts into progressively larger and more curvaceous wave fronts. As wave front  212  approaches element  204 , it comprises a certain size and curvature. Element  204  is designed to be of optimal size and curvature, and is positioned in relation to element  202  such that element  204  fully receives wave  212  without diffractive loss. It will be apparent to one of skill in the art that the size and position of transmitter  202  and/or receiver  204  may be designed to coincide with any of the wave fronts  208  along the wave front path. In addition, it should be apparent to one of skill in the art that SAW device  200  of the present invention reduces frequency variance, thereby providing a SAW system with high precision. 
     Referring now to FIG. 3, a SAW device of the present invention is depicted and generally designated  300 . A transmitting element  302  and receiving element  304  are disposed upon a substrate  306 . In the illustrated embodiment, elements  302  and  304  are depicted as interdigited electrodes, though, as previously noted, other implementations of the present invention may be employed. Within this embodiment, elements  302  and  304  are disposed in relation to one another in accordance with a constraint. 
     This constraint may be understood with respect to certain characteristics of elements  302  and  304 . Element  302  has a radius of curvature  308 . Similarly, element  304  has a radius Is of curvature  310 . In this embodiment, elements  302  and  304  are concave with respect to one another. As such, radii  308  and  310  would be considered to be positive values. Elements  302  and  304  are spaced apart at distance  312 . Provided that:        0   ≤       [     1   -         distance        312         radius        308         ]          [     1   -         distance        312         radius        310         ]       ≤   1                          
     the SAW system will propagate and receive without diffraction loss. 
     It should be appreciated by those skilled in the art that, within the context of this constraint, some special cases give rise to a symmetry of the condition. One such case is where radii  308  and  310  are both equivalent to distance  312 . Another such case is when radius  308  equals infinity (indicating element  302  is linear)and radius  310  equals one half of distance  312 . In both such cases, due to the symmetry of the condition, the curvature of the mirrors may be interchanged. 
     It should also be appreciated by those skilled in the art that, as illustrated above and more generally, the radii of curvature and the distance between elements can be varied to achieve a wide variety of functionality and to suit a great number of design requirements. 
     FIGS. 4A and 4B illustrate embodiments wherein the size and position of the elements have been selected to coincide with the wave front on the wave front path. Although not depicted, it should be understood that, as in the previous embodiments, transmitter and receiver elements are disposed upon a substrate. In FIG. 4A, transmitter  400  is of a smaller size and greater radius of curvature than receiver  402 . As such, transmitter  400  focuses waves more immediately, and a convergence point is formed closer to transmitter  400 . Receiver  402  is still formed of a size and shape suitable to fully receive the wave fronts as they arrive. Conversely, but in similar fashion, the transmitter may be of greater size and smaller radius of curvature than the receiver, as depicted in FIG.  4 B. Transmitter  404  focuses waves through a convergence point which is closer to receiver  406 . Receiver  406  is still designed to be of a shape and size suitable to fully receive propagated wave fronts. 
     FIG. 5 depicts another embodiment of the present invention in which a linear transmitting element is employed in conjunction with a curved receiving element. SAW device  500  comprises transmitter  502  and receiver  504  disposed upon a substrate  506 . Transmitter  502  has an infinite radius of curvature (indicating linearity) and is designed to be of a size coinciding with a wave front at a convergence point. Receiver  504  is designed to be of optimal size and curvature, and is positioned in relation to element  502  such that element  504  fully receives wave  508  without diffractive loss. 
     An alternative embodiment, not shown, wherein the size and shape of elements  502  and  504  are interchanged, is possible if those elements are designed in accordance with the constraints previously described in relation to FIG.  3 . 
     FIGS. 6A and 6B illustrate other embodiments of the present invention. Again, it should be understood that although not depicted, transmitter and receiver elements are disposed upon a substrate. In FIG. 6A, transmitter  600  is of a smaller size than receiver  602 , and has a greater magnitude for its curvature radius. However, transmitter  600  is curved convexly with respect to receiver  602 , effectively rendering its radius of curvature negative. As such, transmitter  600  is positioned such that a convergence point is not formed and wave fronts are focused directly into receiver  602 . Receiver  602  is still formed of a size and shape suitable to fully receive the wave fronts as they arrive. Conversely, but in similar fashion, the transmitter may be of greater size and opposing radius of curvature than the receiver, as depicted in FIG.  6 B. Transmitter  604  focuses waves directly onto receiver  606 . Receiver  606  is designed to be of a shape and size suitable to fully receive the focused wave fronts. 
     Referring now to FIG. 7, an alternative embodiment of a SAW device according to the present invention is depicted. Transmission element  700  and receiving element  702  are both of an elliptical shape. It should be understood that, alternatively, the principles of the present invention may be practiced with both elements of a substantially parabolic shape, or any other curved or arced shape. The elliptical curvature of element  700  constrains the shape of the wave front path and focuses that path into progressively smaller and straighter wave fronts, through a convergence point. Passing that convergence point, the wave front path diffracts into progressively larger and more elliptical wave fronts. Element  702  is designed to be of optimal size and curvature, and is positioned in relation to element  700  such that element  702  fully receives the propagated wave without diffractive loss. 
     Finally, a SAW system  800  according to the present invention is shown in FIG.  8 . This embodiment is especially useful in communications applications requiring phase sensitive processing of signals (e.g. Phase Shift Keying), signal processing applications requiring an increase in common mode rejection ratio and other various differential mode based filtering applications. A transmitter  802  and an array of receivers  804 ,  806 ,  808  are disposed upon a substrate  810 . For purposes of illustration, transmitter  802  is depicted as linear, though it may be shaped in accordance with any of the embodiments previously disclosed. Elements  804 ,  806 ,  808  of the array are positioned to receive predetermined segments of the propagating wave fronts  812 , allowing the remainder to pass. System  800  may thus provide phase sensitive or differential SAW functionality. 
     The elements  804 ,  806 ,  808  of the array are here depicted as concavely curved with respect to transmitter  802  and with differing curvature radii. These elements may also be shaped in accordance with any of the embodiments previously described. 
     A SAW device with curved electrodes described herein may be fabricated using conventional photolithographic techniques, widely practiced in the SAW and semiconductor industries in general. A mask or set of masks may be designed and produced using laser scanner or e-beam methods. These masks may be either the electrode image itself or the negative of the electrode pattern; but it is preferable to use the negative pattern, and with it a pre-metallized piezoelectric substrate, because the adhesion quality and electrical properties of the metallization is higher in this case. The piezoelectric substrate, quartz or lithium tantalate, for example, are coated with a photoresist using a spinner. Photolithographic exposure transfers the mask pattern onto the photoresist, and the substrate is then developed chemically to remove the exposed photoresist. In the case of the negative mask and pre-metallized substrate, the metallization is now exposed while the electrode pattern is protected under the photoresist. Acid or, preferably, plasma etching may now be used to erode the metallization in places where it is not desired. 
     While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.