Strain relief technique for surface acoustic wave devices

A strain isolation technique for a surface acoustic wave (SAW) device having a piezoelectric SAW substrate is disclosed. A cut in the surface of the piezoelectric SAW substrate forms an isolated surface region where active SAW signal propagation occurs. The cut prevents undesirable surface strains from affecting SAW signal propagation in the isolated region.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to surface acoustic wave (SAW) devices and more 
particularly relates to SAW devices which are constructed to reduce or 
substantially eliminate the effects of certain undesirable strains which 
affect the operating characteristics thereof. 
2. Description of the Prior Art 
Devices incorporating SAW delay lines and resonators, are known and are 
useful as component parts of pressure sensors and high frequency 
oscillators, and the devices are useful in a number of other applications. 
Some of the outstanding features of SAW devices are their ability to 
provide real time delays of electromagnetic waves within relatively 
compact substrate materials, and their inherent rugged construction. These 
features permit the use of SAW devices in high vibration and G-force 
environments. 
Typically, a SAW device may be constructed to provide a SAW delay line or 
resonator with fixed operating characteristics, which do not depend on a 
strain-inducing phenomenon, for use as part of a SAW oscillator or other 
such SAW system. Alternatively, SAW devices may be constructed for use in 
measuring parameters which induce strain in the substrate of the devices 
and correspondingly affect the surface wave properties thereof. For 
example, the propagation velocity of a surface acoustic wave and the 
length of the propagation path of a SAW delay line, are both functions of 
strain in the substrate at and near the surface, whereby the operating 
frequency, of an oscillator dependent on a SAW delay line located on the 
surface, varies with the strain at the surface. In general, any type of 
strain-inducing parameter, such as pressure, temperature, force, 
acceleration and other similar mechanical parameters can be measured by a 
SAW device fabricated with a suitably deformable substrate. 
In SAW devices, since the surface acoustic waves exist near the surface of 
the SAW substrate with atomic particle motion confined to a depth of 
approximately one SAW wavelength (referred to herein as "at the surface"), 
the problem of surface contamination of the SAW substrate may be 
particularly acute. Therefore, there are many applications for SAW devices 
which require that the devices be vacuum encapsulated, such as in the 
manner shown in U.S. Pat. No. 4,213,104 to Cullen, et al. which relates to 
a vacuum encapsulation structure for a SAW device wherein the 
encapsulating structure is fabricated from the same kind of material as 
the SAW substrate and is attached to the substrate by a glass-frit seal. 
The vacuum encapsulation of SAW devices presents stability problems, such 
as strain which is induced in the SAW substrate from thermal expansion and 
contraction of the encapsulating structure and which may induce 
undesirable strains in the SAW propagation region. Also, the seal between 
the SAW substrate and a vacuum encapsulating structure, electrical 
connections, and clamps and other such holders for retaining a SAW 
substrate in a vacuum encapsulation structure as part of a SAW device, may 
all act as sources of stress which induce undesirable strains in the SAW 
propagation region. These undesirable strains distort the SAW substrate 
resulting in undesirable changes in the SAW propagation path and velocity 
for the device. 
Techniques have been developed for reducing or substantially eliminating 
undesirable strain effects due to expansion and contraction of the 
propagation region of a SAW device as a function of temperature. For 
example, U.S. Pat. No. 4,100,811 to Cullen, et al. relates to a SAW 
pressure transducer having two SAW oscillators whose outputs are 
subtractively combined to increase the pressure sensitivity of the SAW 
device while reducing the temperature sensitivity of the device. However, 
this particular SAW transducer structure is not designed to reduce or 
substantially eliminate the effects of undesirable strains which are due 
to bonds between a vacuum encapsulation structure, or such other similar 
structure, and the SAW substrate of the device. 
U.S. Pat. No. 4,085,620 to Tanaka does relate to a strain relief technique 
for isolating a piezoelectric element from strains originating at a bond 
between a supporting member for the element and a metal substrate. The 
supporting member comprises a connecting section and a neck section of a 
relatively smaller diameter which is constructed to absorb stresses 
originating at the bond between the supporting member and the substrate. 
However, this strain relief technique is not satisfactory since it does 
not isolate the active region from stress in the substrate per se. 
Furthermore, the supporting member has a relatively complex structure 
which may increase the sensitivity of the piezoelectric element to 
vibrations. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a SAW substrate 
for a SAW device which is substantially isolated from sources of 
undesirable strains, especially strains at the surface (surface strains) 
which are due to stresses such as stresses resulting from a seal between a 
vacuum encapsulation structure and the SAW substrate or from a clamp or 
other such holder for the SAW substrate. 
Another object of the present invention is to provide a SAW substrate for a 
SAW device which is substantially isolated from sources of undesirable 
surface strains, which is relatively easy to construct, and which is 
relatively uncomplicated in structure. 
These and other objects of the present invention are attained by a SAW 
device having a piezoelectric SAW substrate with a cut in the surface of 
the substrate surrounding a SAW propagation region to isolate this region 
from undesirable surface strains. The SAW propagation region may have one 
or more acousto-electric couplers forming one or more SAW delay lines 
which are isolated from the remaining surface area of the substrate by the 
cut. Electrical connections to external circuitry are made across the cut 
by flexible connectors, such as wire bonds or ribbon leads, which are 
constructed so that they do not transmit stresses from the substrate 
surface outside the cut to the surface region isolated by the cut. 
A cover assembly may be attached to the SAW substrate to form a vacuum 
encapsulating structure enclosing the surface region isolated by the cut 
in the SAW substrate. The cut is designed to prevent undesirable surface 
strains, especially strains due to such factors as a seal between the 
vacuum encapsulating structure and the SAW substrate, from affecting the 
operation of the electro-acoustic couplers in the isolated SAW propagation 
region.

DETAILED DESCRIPTION 
Referring to FIG. 1, a vacuum encapsulated device 10 incorporating a SAW 
delay line is constructed according to the principles of the present 
invention. The SAW device 10 comprises a SAW base portion 12 and a vacuum 
encapsulation cover assembly including a top portion or cap 16 and a 
spacer 14. The base portion 12 includes acousto-electric couplers 29 and 
39 (coupling means) disposed on a SAW active signal surface region 41 of a 
major surface 20 of a piezoelectric substrate 18. According to the present 
invention, the region 41 is isolated from the rest of the substrate 
surface 20 by a cut 42 in the surface 20. Referring to FIG. 2, it can be 
seen that the couplers 29 and 39 are enclosed in a chamber 30. 
The spacer 14 and top portion 16 are made of the same material as the 
substrate 18 or are made of a material having the same thermal expansion 
properties as the substrate 18. Also, the spacer 14 and top portion 16 are 
bonded together, with the spacer 14 bonded to the major surface 20 of the 
piezoelectric SAW substrate 18, as described in U.S. Pat. No. 4,213,104 to 
Cullen, et al. 
As shown in FIG. 1, the couplers 29 and 39 each comprise an interdigital 
transducer, which launches and receives surface acoustic waves, each 
including a pair of opposite phase electrodes 27, 28 and 37, 38, 
respectively. Each electrode 27, 28, 37, 38 includes a plurality of 
fingers interleaved with those of the opposite phase electrode. There are 
a number of different acousto-electric coupler configurations, including 
variations of the interdigital pattern itself, and although the 
illustrated couplers 29 and 39 represent a common configuration for a SAW 
delay line which may be used in high frequency SAW oscillators or SAW 
pressure sensing devices, the exact configuration of the SAW couplers 29 
and 39 is not critical with respect to the present invention. 
The substrate 18 is preferably a piezoelectric material such as quartz, 
lithium niobate, lithium tantalate, or any other such material which 
exhibits an acousto-electric coupling. Additionally, non-piezoelectric 
elastic substrates such as silicon, having a suitable thin film coating of 
piezoelectric material, such as zinc oxide, in the regions where 
acousto-electric coupling is required, may be used as the substrate 18. A 
surface acoustic wave can propagate in non-piezoelectric material, so only 
the surface below the electrodes 27, 28, 37, 38 need be piezoelectric, to 
provide the required acousto-electric coupling. Thus, the piezoelectric 
substrate may either be intrinsically piezoelectric or it may be formed of 
a non-piezoelectric material with suitable piezoelectric coating, either 
partly or totally covering the surface of interest. 
In the example of a SAW device 10 which is shown in FIG. 1, each electrode 
27, 28, 37, 38 is connected to a related inner conductor 25, 26, 35, 36 
which extends from the corresponding electrode to the inside edge of the 
cut 42. A resilient wire 23, 24, 33, 34 connects each inner conductor 25, 
26, 35, 36 to a corresponding outer conductor 21, 22, 31, 32 which extends 
from the outer edge of the cut 42 to provide a means for connecting the 
device 10 to electronic circuitry (not shown). As shown in FIG. 1, the 
outer conductors 21, 22, 31, 32 extend between the spacer 14 and the 
substrate surface 20 and reach beyond the outer edge of the spacer 14. 
The conductors 21, 22, 25, 26, and 31, 32, 35, 36 may be deposited in 
recesses, as shown in the Figures, or on the surface 20, by any 
conventional process and may comprise thin-film aluminum, or aluminum 
lightly doped with copper. Also, the conductors 21, 22, 25, 26 and 31, 32, 
35, 36 may include other layers of metals, such as titanium or chromium 
for improved adhesion. The wires 23, 24, 33, and 34 are flexible to 
prevent stresses from reaching the isolated surface region 41 from the 
remainder of the surface 20. Ribbon leads or other flexible electrical 
connection means may be used instead of the wires 23, 24, 33, 34, if 
desired. 
The cut 42, as best shown in FIG. 2, should be generally perpendicular to 
the surface 20 of the substrate 18 and, for the device 10, is selected to 
be about one-half the thickness of the substrate 18. For the exemplary 
delay line device 10 shown in FIGS. 1 and 2 having a glass-frit seal for 
attaching the cover assembly to a 0.5 square inch surface 20 of a 
substrate 18 having a thickness of 100 mils, good results are achieved by 
a cut 42 having a depth between 25 and 75 mils (25% to 75% of the 
thickness of the substrate 18) which is centered about an axis through the 
center of the substrate 18 perpendicular to the surface 20. The cut may be 
as narrow as it can be made in practice, to save space. The key feature is 
that strains generated externally of the cut, such as those generated by 
stresses at the seal between the spacer 14 and the substrate 18, are 
prevented from inducing undesirable surface strains in the isolated 
surface region 41. The best depth, width and location for the cut 42 are 
dependent on the origin and magnitude of the undesirable surface strains, 
which are to be isolated from the region 41. Therefore, the exact location 
and configuration for a strain relief cut in a particular kind of SAW 
device may be determined best by testing prototypes of the particular SAW 
device which have different types of cuts. 
Preferably, the cut 42 may be made by relative motion between the surface 
20 and a diamond tipped hollow cylindrical cutter. Alternatively, the cut 
42 may be made by ultrasonic milling using a cutter made from a 
thin-walled piece of stainless steel tubing of an appropriate diameter and 
wall thickness. The stainless steel tubing is vibrated against the surface 
20 of the substrate 18 by an ultrasonic generator to bore into the 
substrate 18. 
A highly accurate oscillator may be formed by connecting the SAW device 10 
to electronic circuitry (not shown). As is known in the art, the frequency 
of operation for such a SAW oscillator is attained by constructing the SAW 
couplers 29 and 39 to have a particular periodicity. Oscillator 
frequencies in the range of 50 to 2000 megahertz are typical. A high 
frequency, stable SAW oscillator according to the present invention is 
suitable in many applications, such as frequency control, which require 
stability on the order of two parts in 10.sup.9 per week. The cut 42 in 
the surface 20 prevents undesirable surface strains from affecting the 
operation of the oscillator, thereby meeting the high stability 
requirements for such an oscillator. Similarly, the stability of other SAW 
devices is enhanced by means of the present invention. 
The present invention has been described in conjunction with a device 10, 
incorporating a SAW delay line for use in a SAW oscillator. However, the 
principles of the present invention are applicable to many other SAW 
devices such as SAW pressure, temperature and force sensors, SAW 
resonators, SAW filters, and other such systems. Therefore, while the 
present invention has been described in conjunction with a particular 
embodiment it is to be understood that various modifications and other 
embodiments of the present invention may be made without departing from 
the scope of the invention as described herein and as claimed in the 
appended claims.