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Generation of High Frequency Acoustic Surface Waves on Piezoelectric Wafers

IP.com Disclosure Number: IPCOM000078881D
Original Publication Date: 1973-Mar-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 57K

Publishing Venue

IBM

Related People

Brady, MJ: AUTHOR [+2]

Abstract

Generation of acoustic surface waves (ASW) on piezoelectric substrates, using planar surface technologies can currently be obtained in only one way, by an interdigital electrode array. The interdigital or alternate phase electrode transducers, depicted in Figs. 1 and 2, consists of two sets of interwoven metallic electrodes, usually of the same width and separation "d". ASW are then generated at a frequency f = v/Lambda, where v is the ASW velocity in the wafer and Lambda = 4d is the wavelength. Conventional photolithographic techniques limit the spacing d to about 3 x 10/-4/ cm. Since the surface velocity is typically 3 x 10/5/ cm/sec, they have a typical maximum frequency of: f(max) = v over 4d = 3x10/5/ over 4x3x10/-4/ approx./= 2.5x10/8/ Hz or 250 MHz.

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Generation of High Frequency Acoustic Surface Waves on Piezoelectric Wafers

Generation of acoustic surface waves (ASW) on piezoelectric substrates, using planar surface technologies can currently be obtained in only one way, by an interdigital electrode array. The interdigital or alternate phase electrode transducers, depicted in Figs. 1 and 2, consists of two sets of interwoven metallic electrodes, usually of the same width and separation "d". ASW are then generated at a frequency f = v/Lambda, where v is the ASW velocity in the wafer and Lambda = 4d is the wavelength. Conventional photolithographic techniques limit the spacing d to about 3 x 10/-4/ cm. Since the surface velocity is typically 3 x 10/5/ cm/sec, they have a typical maximum frequency of: f(max) = v over 4d = 3x10/5/ over 4x3x10/-4/ approx./=

2.5x10/8/ Hz or 250 MHz.

Described is a scheme which, for the above-mentioned example, will allow generation of ASW at 500 MHz while employing the same conventional photolithographic fabrication techniques as discussed above. In general, the scheme will double the highest ASW frequency which is presently obtainable by conventional processes, using the same fabrication methods. The scheme employs two sets of electrode arrays (as in the interdigital case) operating in a single-phase mode, as shown in Fig. 2. Fabrication proceeds in accordance with the following steps:
Step 1 - A set of metal electrodes of width and separation "d" is fabricated on the piezoelectric wafer, using the

standard positive

photoetch techniques. At the end of this process,

the metallic electrodes are still covered by the baked

photoresist which protected them during the etching

step, Fig. 4.

Step 2 - An etching agent appropriate for etching the piezoelectric material

is used for controlled etching of the spacings

between the electrode array (which is still

protected by the previous

treatment). This results in a narrow cavity between

the electrodes, plus some underetching below the

electrodes, as shown in Fig. 5.

Step 3 - A second set of metallic electrodes is sputtered or evaporated on the device

surface through a mask, and then acetone is used

to expand the photoresist and remove the excess metal.

The result is

shown in Fig. 6. Due to the underetching of the

pr...