Note: Descriptions are shown in the official language in which they were submitted.
11~81383
Background of the Invention
_
1. _ eld of the Invention.
The invention pertains to surface acoustic wave devices
and particularly to the construction o-f transducers used for
launching and receiving surface waves upon such devices.
2. Description of the Prior Art.
-
In the constructlon of surface acoustic wave devices,itis 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 trans-
:
ducers are ordinarily constructed as two sets of interleaved
metal stripes or flngers positioned upon the surface of the
substrate or pi~ezoelectric film. ~he length of the fingers
within each transducer could be constant or varied in
accordance with a predetermined frequency characteristic as
~, desired.
Although such transducers functioned well to launch and
receiv~ surface waves, the physical presence of the transducer
upon the substrate or piezoelectric Eilm also caused unwanted
reflection of surface waves propagating to or past the transducer
or launched by the transducer itself. Such reElections caused
a distortion in the transfer function of the surEace wave
derice employing the transducer.
Re-Elections caused by transclucers generally arose from any
or all of three cliferent mechanisms. ~lrst, if the metal oE
the ~ransducer had diEferent elastic properties than the substrate,
a discontinuity in the acoustic impedance of the device in the
region of the transducer resulted producing a mismatch between
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the transducer and substrate. Secondly, the low electrical
resistance of the metal o.~ the transducer shorted out the
piezoel.ectric field of the substrate and causing a change
in velocity of surface waves and hence a reflection in the
region of the transducer. Thirdly, the physical presence of
the metal caused a periodicc topographical variation at the
surface of the substrate.
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~ummary of the Invention
Accordingly, it is an object of the present invention to
provide a surface acoustic wave device having one or more
transducers with substantially reduced reflection of sur~ace
waves at the transducers.
It is also an object of the present invention to provide
a surface acoustic wave device with minimal distortion in the
transfer function oE the device.
These as well as other objects of the invention, may be
met by providing the combination of a substrate having ~a
surface suitable for supporting sur-face wave propagation and
transducer means coupled to the surface wave supporting means
wherein the topographical surface wave propagation properties
are substantially constant in the region of the transducer.
The transducer preferably is a metal which has similar elastic
properties to those of the substrate. The preferred materials
~ are aluminum for the metal of the transducer and quartz for the
- substrate. l'he surface of the substrate may be substantially
planar.
The invention may also be practiced by providing the
combination of a substrate having a surface suitable -for
supporting surface wave propagation and at least one transducer
coupled to the substrate which has a plurality of interleaved
parallel conductive stripes which are at least partially
~mbedded within the substrate. The upper surface oE each of the
s~ripes shoulcl be substantially flush wlth the sur:Eace oE the
substrate. The metal stripes of the transducer are constructed
within a plurality of substantially parallel grooves extending
;Erom the surface into the substrate for predetermined distance.
As beEore, the substrate and conductive stripes preEerably have
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similar elastic pro[)erties. The substrate may be quartz t
particularly ST-cut qilartz, while the metal stripes are
fabricated of aluminum.
Purther objects of the invention may be met by providing
the combination of a substrate of pi.ezoelectric material having
at least one surface adapted for surface wave propagation and
having first and second sets of substantially parallel grooves
extending from the surface into the substrate for predetermined
distance and a plurality of stripes of metal filling the first
and second sets of grooves in the substrate to form first and
second transducers, one surface o~ the metal stripes being
: substantially flush with the surface of the substrate. A-t
least some of the me~al stripes within each transducer are
electrically coupled to other ones of the stripes wi~hin the
transducer such as in two groups. In a preferred embodiment
as a surface acoustic wave oscillator device, amplifying means
are coupled between the first and second transducers. Again,
the substrate may comprise quartz, such as ST-cut quartz, while
the metal stripes are aluminum.
The invention may also be practiced by a method comprising
the steps of providing a substrate of piezoelectric material
oovering at least a portion of the substrEIte with a first ~ayer
oE metal, removing portions of the first layer of metal in the
Eorm o:E parallel stripes in one or more regions of the first
layer oE metal, removing portions of the substrate underlying
the removed portions of the first layer o:f metal to a predeterm:ined
depth to form a plurality oE parallel grooves in the substrate,
Eilling the grooves through the removed portions of the first
layer of metal with a second layer of metal to substantially
the original level o~ the surface o~ the substrate, and removing
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the remaining portions of the first layer of metal, as well as unwanted
portions of the second layer of metal. The substrate is preferably
quartz while the second layer of metal is aluminum. The first layer of
metal may be vanadium.
In accordance with the invention, there is provided in combi-
nation: a pie~oelectric substrate comprising ST-cut quartz having an
uncoated surface supporting surface wave propagation; at least one sub-
stantially reflectionless transducer coupled to said surface waves on
said substrate; and said transducer comprising a plurality of conductors
positioned in grooves in said substrate whereby the mechanical energy of
surface waves outside saia transducer conductors is substantially
entirely within said substrate.
In accordance with another aspect of the invention, there is
provided the method of ~anufacturing a surface acoustic wave transducer
comprising the steps of: providing a piezoelectric substrate of ST-cut
quartz; covering at least a portion of said substrate with a first layer
of metal; removing portions of said fi.rst layer oP metal in the form of
; parallel stripes in one or more regions of said first layer of metal;
; re-mo~ing portions o~ said substrate underlying the removed portions of
~0 said first layer of metal to a predetermined depth to form a plurality
of parallel grooves in said substrate; filling said grooves through said
remoyed portions of said first layer of metal with a second layer of
metal to substantially the original leYel of said surface of said s~lbstrate;
and rffmoving the remaining portions of said first layer of m~tal.
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Brief Description of the Drawings
FIG. 1 is a perspective view of a surface acoustic wave
oscillator device in accordance with the invention having a
partially cut away portion; and
FIGS. 2A-2E are a series of cross-sectional views showing
steps in the constructi.on of one of the transducers of the
device shown in FIG. 1.
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~)escrip-tion of the l~re:ferred ~bodiments
Referring flrst to ~rG. 1 ~ there is shown a surface
acoustic wave osci.lla-tor device in which the present invention
is used to advantage. The oscilla~or device includes two
primary operating elementsg surface acoustic wave delay line 5
and feedback amplifier 11.
Delay line 5 includes input a.nd output transducers 14 and
12 respectively, constructed upon piezoelectric substrate 10.
Surface waves produced by input transducer 14 propagate to
output transducer 1~ with a delay time determined by the distance
between the two transducers and the velocity of propagation of
surface waves upon substrate 10. The surface waves received
at output transducer 12 are amplified by amplifier 11 and
subsequently coupled back to input transducer 1~. Amplifier 11
is connected so that the phase of the sur:face waves produced by
input transducer 1~ from the fed-back signal is such as to
continually reinforce the surface waves then propagating
between input transducer 14 and ou~put transducer 12.
Both input transducer 14 and output transducer 12 are
fabricated as part of surface acoustic wave delay line 5
in accordance with the teachings of the invention. In the
preferred embodiment, the metal forming the fingers of both
transducers is aluminum whil.e the substrate 10 is flbricated
o ST-cut quartz. These two materials are preferred as they
have similar elastic properties so that aluminum is a relatively
good acoustic match to quartæ. ~lso, S~ cut quartz has a
relatively low piezoelectric coupl:ing coefficient compared to
other well-known piezoelectric materials commonly used for
surface wave devices. This is advantageous :in that the
piezoelectric shorting effect caused by the presence of the metal
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is minimal.
In accordance with the inventioll, the metal ingers of both
input transducer 14 and output transducer 12 are recessed within
the surface o~ substrate lO such that the upper sur~ace of the
metal of both transducers is substantially flush with the upper
surface of substrate lO. In this manner, because o-f the good
acoustic match between the metal of the fingers and the substrat~,
there is substantially no reflection oE 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. l, 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 oE cross-sectional views of
FI~S. 2A-2E, a method of fabricating transducers for a device
~uch as shown in FIG. l ~ill be described. ~irst, as shown in
FIG. 2A, a substrate lO of ST-cut quartz is provided having at
least one surface suitable for surface wave propagation thereupon.
Substrate lO is next covered with a layer of vanadium 16 using
any of a number or well-known metal deposltion technlques. A
thickness o 300 A oE vanadium layer 16 has been Eound adequate.
Atop vanadium layer :l6 is then deposited a layer oE photoresist
L8. Next, as shown in FIG. 2B, photoresist layer 18 is masked
exposed to developing radiation, and removed chemically in the
pattern shown. The portions of vanadium layer 16 exposed through
the openings in photoresist layer 18 are then removed using a
technlque such as ion-beam etching.
Then, as sho~n in FIG. 2C, grooves 20 are etched into
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substratc 10 to a predetermined clepth. A depth of 1500 A has
been found adequate. A layer of aluminum 22 is next deposited
within grooves 20 and atop photoresist la~er 18 until the
aluminum in the grooves reaches -the level of the surface of
substrate 10. An evaporation deposition technique is preferred.
Finally, the remaining portions of vanadium layer 16 and
photoresist layer 18, as well as that portion of the aluminum
layer 22 lying atop photoresist layer 18, are chemically
stripped away leaving the completed transducer as shown in the
view of FIG. 2E.
This completes the description of the preferred embodiments
of the invention. Although preferred embodiments have been
described, it is believed that numerous modifications and
alterations thereto would be apparent to one having ordinary
skill in the art without departing from the spirit and scope
of the invention.
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