Surface wave filter having photoresist damping material thereon and method for manufacturing the same

A surface wave filter with a wafer-shaped substrate formed of monocrystalline piezoelectric material having a surface, input and output transducers disposed on the surface of the substrate, a coupler for coupling the transducers being spaced apart from the transducers on the surface of the substrate defining zones therebetween, and electric conductor runs being connected to the transducers on the surface of the substrate and having ends, includes a damping layer formed of photoresist material with a plastic base covering substantially the entire surface of the substrate except for a region bounded by the transducers, the coupler, the ends of the electric conductor runs and the zones between the transducers and the coupler, and a method for manufacturing the filter.

The invention relates to a surface wave filter with a slab or wafer-shaped 
substrate of monocrystalline piezoelectric material, particularly lithium 
niobate; a damping compound and input and output transducers and possibly 
a coupler for coupling the transducers to each other, each being disposed 
on one surface of the substrate. 
To fabricate such filters, a metal layer is usually vapor-deposited on the 
substrate. The transducers, couplers and conductor runs together with 
their ends are developed from the metal layer by means of a photo-etching 
technique. The substrate patterned in this way is cemented on to a metal 
support sheet and the ends of the conductor leads are electrically 
connected by bonding wires to the contact legs of the filter. The system 
is then inserted into a plastic housing and filled with casting compound. 
If electrical signals are fed to the input transducer, it emits surface 
waves at the same frequency but in different directions. The coupler picks 
up a part of the surface waves and specifically the part directed toward 
the coupler and also passes on the part of the surface waves in the 
direction toward the output transducer which converts the surface waves 
back into electrical signals. Reflected waves, are especially generated at 
the severed edges of the substrates, and are also caused by crystal 
structural faults. The reflected waves are therefore time-delayed and 
partially suppressed by certain technical means. For this purpose, a 
damping compound consisting, for instance, of a heat sealing adhesive, is 
melted on the surface between the end faces of the substrate and the 
transducers. However, only a relatively small area parallel to the severed 
edges of the chip area can be damped by this process. Undesirable 
reflections of waves, which have an interfering effect on the reception of 
the surface waves by the transducers and the coupler which are all 
attached to the substrate, are therefore unavoidable in spite of the use 
of these damping compounds. 
It is accordingly an object of the invention to provide a surface wave 
filter and method of manufacturing the same, which overcomes the 
hereinbefore-mentioned disadvantages of the heretofore-known devices of 
this general type, and which permits nearly complete damping or 
suppression of the interfering reflected waves. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a surface wave filter with a wafer-shaped 
substrate formed of monocrystalline piezoelectric material having a 
surface, input and output transducers disposed on the surface of the 
substrate, and optionally a coupler for coupling the transducers, the 
couplers being spaced apart from the transducers on the surface of the 
substrate defining zones therebetween, and electric conductor leads being 
connected to the transducers on the surface of the substrate and having 
electrodes, comprising a damping layer formed of photoresist material with 
a plastic base covering substantially the entire surface of the substrate 
except for a region bounded by the transducers, the coupler, the 
electrodes of the electric conductor leads and the zones between the 
transducers and the coupler. 
This nearly complete covering of the free areas by means of a damping 
layer, ensures that not only can the reflections occurring at the severed 
edges of the substrate be suppressed but also that the reflections which 
are caused by the most varied crystal structure faults in the substrate 
can be suppressed as well. 
In accordance with the invention, the dampening layer is applied by means 
of a well known photolithographic method as a layer formed of photoresist 
material with a plastic base over substantially the entire surface of the 
substrate except for a region bounded by the transducers, the electrodes 
of the electric conductor leads and the zones between the transducers. By 
means of this method it is possible to deposit the damping layer with 
accuracy in regard to position, so that the area of the chip surface which 
is free of the damping layer can be limited to a minimum. If this method 
is already applied in the processing of the wafers, i.e., in the 
processing of the substrate wafer which is divided into numerous chips but 
not yet mechanically separated, a considerable cost-saving in production 
is attained. Resists with a plastic base i.e. negative and positive photo 
resists are applied to the substrate with a layer thickness of preferably 
more than 10 .mu.m. 
In accordance with an additional feature of the invention, the substrate is 
formed of lithium niobate. 
In accordance with a concomitant mode of the invention there is provided a 
method which comprises omitting the damping layer over the coupler and the 
zones between the transducers and the coupler. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
surface wave filter and method for manufacturing the same, it is 
nevertheless not intended to be limited to the details shown, since 
various modifications and structural changes may be made therein without 
departing from the spirit of the invention and within the scope and range 
of equivalents of the claims.

Referring now to the figures of the drawings in detail and first 
particularly to FIGS. 1 and 2 thereof, there is seen a substrate 1 on 
which transducers 2, 3, a strip-shaped coupler 4 and conductor leads 5 
through 8 having electrodes 12 through 15, are disposed. a damping 
compound which may be in the form of a heat sealing adhesive is melted on 
the surface of the substrate 1 in areas 9, 10 between the ends of the 
substrate 1 and the transducers 2,3. As can be seen, this process provides 
a relatively small area parallel to the edges of the substrate 1 which can 
be damped. This damping process avoids the occurrence of undesirable wave 
reflections which interface with the transducers 2, 3 and the coupler 4. 
FIGS. 3 and 4 illustrate a device and method according to the invention, 
for improving the damping. In FIGS. 3 and 4, the input and output 
transducers 2 and 3, the strip-shaped coupler 4 and the conductor leads 5, 
6, 7 and 8 with their electrodes 12, 13, 15, 14, respectively, are 
designated with the same reference symbols as in FIGS. 1 and 2. The 
transducers are formed of interdigitated electrodes, as is conventional. 
According to the invention, a photoresist layer 11 with a plastic base 
covers the entire substrate surface except for the outlined area which is 
bounded by the transducers 2 and 3, the coupler 4, the electrodes (12, 13, 
14, 15) of the electric conductor leads (5, 6, 7, 8) and the zones between 
the transducers (2, 3) and the coupler 4. The substrate may be a wafer or 
slab-shaped body of monocrystalline piezoelectric material, such as 
lithium niobate. The layer 11 may be formed of negative or positive 
photoresist material and be applied by photolithographic methods. 
The foregoing is a description corresponding in substance to German 
Application No. P 32 34 95.6, dated Sept. 21, 1982, the International 
priority of which is being claimed for the instant application, and which 
is hereby made part of this application. Any material discrepancies 
between the foregoing specification and the aforementioned corresponding 
German application are to be resolved in favor of the latter.