GB1596077A - Surface acoustic wave transducer fabrication to reduce reflections - Google Patents

Description

(54) SURFACE ACOUSTIC WAVE TRANSDUCER FABRICATION TO REDUCE REFLECTIONS (71) We, RAYTHEON COMPANY, a corporation organised and existing under the laws of the State of Delaware, United States of America, of Lexington, Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention pertains to surface acoustic wave transducers.

In the construction of surface acoustic wave devices, it is almost always desired to provide one or more interdigital transducers for launching and receiving surface waves upon a piezoelectric substrate or upon a non-piezoelectric substrate with an overlay film of piezoelectric material. Such transducers are ordinarily constructed as two sets of interleaved metal stripes or fingers positioned upon the surface of the substrate or piezoelectric film. The lengths of the fingers within each transducer can be constant or varied in accordance with a predetermined frequency characteristic as desired. Although such transducers function well to launch and receive surface waves, the presence of the transducer upon the substrate or piezoelectric film also causes unwanted reflection of surface waves propagating to or past the transducer or launched by the transducer itself.Such reflections cause a distortion in the transfer function of the surface wave device employing the transducer. According to the present invention, there is provided a surface accoustic wave transducer comprising an ST-cut quartz substrate havana a surface which, in operation supports surface accoustic waves, and transducer electrodes so recessed into the said surface as to render the transducer substantially reflectionless.

Reflections caused by transducers generally arise from any or all of three different mechanisms. First, if the metal of the transducer has different elastic properties than the substrate, a discontinuity in the acoustic impedance of the device in the region of the transducer results, producing a mismatch between the transducer and substrate. Secondly, the low electrical resistance of the metal of the transducer shorts out the piezoelectric field of the substrate and causes a change in velocity of surface waves and hence a reflection in the region of the transducer.

Thirdly, the presence of the metal causes a periodic topographical variation at the surface of the substrate.

The transducer electrodes are preferably of a metal which has similar elastic properties to those of the substrate. The preferred materials are aluminium for the electrodes and quartz for the substrate. The surface of the substrate may be substantially planar.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a surface acoustic wave oscillator embodying the invention having a partially cut away portion; and Figs. 2A-2E are a series of cross-section views showing steps in the construction of one of the transducers of the device shown in Fig. 1.

Referring first to Fig. 1, there is shown a surface acoustic wave oscillator. The oscillator includes two primary operating elements, a surface acoustic wave delay line 5 and a feedback amplifier 11.

The delay line 5 includes input and output.

transducers 14 and 12 respectively, constructed upon a piezoelectric substrate 10. Surface waves produced by the input transducer 14 propagate to the output transducer 12 with a delay time determined by the distance between the two transducers and the velocity of propagation of surface waves upon the substrate 10. The surface waxes received at the output transducer 12 are amplified by amplifier 11 and subsequently coupled back to the input transducer 14 and the output transducer 12.

Both the input transducer 14 and the output transducer 12 are fabricated as part of the surface acoustic wave delay line 5. In the preferred embodiment, the metal forming the fingers of both transducers is aluminium while the substrate 10 is fabricated from ST-cut quartz. These two materials are preferred as they have similar elastic properties so that aluminium is a relatively good acoustic match to quartz. Also ST-cut quartz has a relatively low piezoelectric coupling coefficient compared to other well-known piezoelectric commonly used for surface wave devices. This is advantageous in that the piezoelectric shorting effect caused by the presence of the metal is minimal.

The metal fingers of both input transducer 14 and output transducer 12 are recessed within the surface of substrate 10 so that the upper surface of the metal of both transducers is substantially flush with the upper surface of the substrate 10. In this manner, because of the good acoustic match between the metal of the fingers and the substrate, there is substantially no reflection of surface waves due to periodic topographical variations as the surface over the transducers is substantially smooth and of constant elastic properties. Hence, with the transducer and surface wave devices as shown in Fig. 1, substantially all of the reflection mechanisms at the transducers have been eliminated. Because the transducers are substantially reflectionless, the distortion of the passband characteristics of prior art devices is eliminated.

Referring next to the series of crosssectional views of Figs. 2A--2E, a method of fabricating transducers for a device such as shown in Fig. 1 will be described. First, as shown in Fig. 2A, a substrate 10 of ST-cut quartz is provided having at least one surface suitable for surface wave propagation thereupon. Substrate 10 is next covered with a layer of vanadium 16 using any of a number of well-known metal deposition techniques. A thickness of 300 A of vanadium has been found adequate. On top of the vanadium layer 16 is then deposited a layer of photoresist 18. Next, as shown in Fig. 2B, the photoresist layer 18 is masked, exposed to developing radiation, and removed chemically in the pattern shown. The portions of the vanadium laver 16 exposed through the openings in the photoresist layer 18 are then removed using a technique such as ion-beam etching.

Then, as shown in Fig. 2C, grooves 20 are etched into the substrate 10 to a predetermined depth. A depth of 1500 A has been found adequate. A layer of aluminium 22 is next deposited within the grooves 20 and on the photoresist layer 18 until the aluminium in the grooves reaches the level of the surface of the substrate 10.

An evaporation deposition technique is preferred. Finally, the remaining portions of vanadium layer 16 and the photoresist layer 18, as well as that portion of the aluminium layer 22, lying on the photoresist layer 18, are chemically stripped away leaving the completed transducer as shown in the view of Fig. 2E.

WHAT WE CLAIM IS: 1. A surface acoustic wave transducer comprising an ST-cut quartz substrate having a surface which, in operation, supports surface acoustic waves, and transducer electrodes so recessed into the said surface as to render the transducer substantially reflectionless.

2. A transducer according to claim 1, wherein the electrodes and the substrate have similar elastic properties.

3. A transducer according to claim 2, wherein the electrodes are aluminium.

4. A transducer according to any of claims 1 to 3, wherein the said surface is substantially planar.

5. A transducer according to any of claims 1 to 4, wherein the electrodes are flush with the said surface.

6. A method of making a transducer according to claim 1, wherein the substrate has a layer of a first metal deposited on the said surface, the metal layer is selectively removed to expose parallel strips of the said surface which are etched to form grooves, the grooves are filled with a second metal to form the said electrodes and the remaining portions of the first metal are removed.

7. A method according to claim 6, wherein the first metal is vanadium.

8. A method according to claim 6 or 7, wherein the second metal is aluminium.

Reference has been directed in pursuance of section 9, subsection (1) of the Patents Act 1949, to patent No. 1,372,067.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. are amplified by amplifier 11 and subsequently coupled back to the input transducer 14 and the output transducer 12. Both the input transducer 14 and the output transducer 12 are fabricated as part of the surface acoustic wave delay line 5. In the preferred embodiment, the metal forming the fingers of both transducers is aluminium while the substrate 10 is fabricated from ST-cut quartz. These two materials are preferred as they have similar elastic properties so that aluminium is a relatively good acoustic match to quartz. Also ST-cut quartz has a relatively low piezoelectric coupling coefficient compared to other well-known piezoelectric commonly used for surface wave devices. This is advantageous in that the piezoelectric shorting effect caused by the presence of the metal is minimal. The metal fingers of both input transducer 14 and output transducer 12 are recessed within the surface of substrate 10 so that the upper surface of the metal of both transducers is substantially flush with the upper surface of the substrate 10. In this manner, because of the good acoustic match between the metal of the fingers and the substrate, there is substantially no reflection of surface waves due to periodic topographical variations as the surface over the transducers is substantially smooth and of constant elastic properties. Hence, with the transducer and surface wave devices as shown in Fig. 1, substantially all of the reflection mechanisms at the transducers have been eliminated. Because the transducers are substantially reflectionless, the distortion of the passband characteristics of prior art devices is eliminated. Referring next to the series of crosssectional views of Figs. 2A--2E, a method of fabricating transducers for a device such as shown in Fig. 1 will be described. First, as shown in Fig. 2A, a substrate 10 of ST-cut quartz is provided having at least one surface suitable for surface wave propagation thereupon. Substrate 10 is next covered with a layer of vanadium 16 using any of a number of well-known metal deposition techniques. A thickness of 300 A of vanadium has been found adequate. On top of the vanadium layer 16 is then deposited a layer of photoresist 18. Next, as shown in Fig. 2B, the photoresist layer 18 is masked, exposed to developing radiation, and removed chemically in the pattern shown. The portions of the vanadium laver 16 exposed through the openings in the photoresist layer 18 are then removed using a technique such as ion-beam etching. Then, as shown in Fig. 2C, grooves 20 are etched into the substrate 10 to a predetermined depth. A depth of 1500 A has been found adequate. A layer of aluminium 22 is next deposited within the grooves 20 and on the photoresist layer 18 until the aluminium in the grooves reaches the level of the surface of the substrate 10. An evaporation deposition technique is preferred. Finally, the remaining portions of vanadium layer 16 and the photoresist layer 18, as well as that portion of the aluminium layer 22, lying on the photoresist layer 18, are chemically stripped away leaving the completed transducer as shown in the view of Fig. 2E. WHAT WE CLAIM IS:

1. A surface acoustic wave transducer comprising an ST-cut quartz substrate having a surface which, in operation, supports surface acoustic waves, and transducer electrodes so recessed into the said surface as to render the transducer substantially reflectionless.

2. A transducer according to claim 1, wherein the electrodes and the substrate have similar elastic properties.

3. A transducer according to claim 2, wherein the electrodes are aluminium.

4. A transducer according to any of claims 1 to 3, wherein the said surface is substantially planar.

5. A transducer according to any of claims 1 to 4, wherein the electrodes are flush with the said surface.

6. A method of making a transducer according to claim 1, wherein the substrate has a layer of a first metal deposited on the said surface, the metal layer is selectively removed to expose parallel strips of the said surface which are etched to form grooves, the grooves are filled with a second metal to form the said electrodes and the remaining portions of the first metal are removed.

7. A method according to claim 6, wherein the first metal is vanadium.

8. A method according to claim 6 or 7, wherein the second metal is aluminium.

Reference has been directed in pursuance of section 9, subsection (1) of the Patents Act 1949, to patent No. 1,372,067.