Transparent strain sensitive devices and method

A transparent strain sensitive surface is disclosed which is suitable for strain gages, video terminal touch panels, security panels (windows, display cases, etc.). The strain gage includes a transparent, strain sensitive conductor of indium tin oxide (ITO) or indium oxide (IO) on a transparent substrate. The strain sensitive panels include a crossed xy matrix of transparent IO or ITO strain sensitive conductors formed on the same side or on opposite sides of a transparent substrate.

1. CROSS-REFERENCE TO RELATED PATENT(S) 
This patent application is related to commonly assigned U.S. Pat. No. 
4,851,095, entitled MAGNETRON SPUTTERING APATUS AND PROCESS, issued 
Jul. 25, 1989, to inventors Scobey, Seddon, Seeser, Austin, LeFebvre, and 
Manley from application Ser. No. 154,177 filed Feb. 8, 1988. This patent 
application is also related to commonly assigned U.S. Pat. No. 4,361,114, 
entitled METHOD AND APATUS FOR FORMING THIN FILM OXIDE LAYERS USING 
REACTIVE EVAPORATION TECHNIQUES, issued Nov. 30, 1982, to inventor Gurev 
from application Ser. No. 260,047 filed May 4, 1981. The '114 and '095 
patents are incorporated by reference. 
2. BACKGROUND OF THE INVENTION 
a. Field of the Invention 
The present invention relates to discrete thin film strain gauge devices, 
to devices utilizing strain sensitive surfaces, and to methods of 
manufacturing such devices. 
b. Definition(s) 
As used here with reference to the present invention, "transparent" 
includes non-visual translucent. Also, "transparent" includes 
"translucent" and vice versa. 
c. Current State of the Relevant Technology 
To my knowledge, conventional strain measurement technology involves two 
relevant types of devices. 
First, conventional strain gages use a straight or curved conductor element 
(for example, a serpentine conductor) whose electrical resistance varies 
with strain or pressure. The opaque sensor element comprises metal wire, 
semiconductor material or a foil of metal such as copper. 
The second application is to strain sensitive surfaces, which may comprise 
an xy grid of conductors separated by a pressure sensitive, variable 
resistance film or membrane. The opaque membrane provides variable 
electrical output (resistance and current) in the circuit paths defined 
through the membrane at the crosspoints of the opaque x and y conductors. 
Preferably, the membrane has linearly variable, repeatable resistance as a 
function of pressure. 
U.S. Pat. No. 4,734,034, issued Mar. 29, 1988 to Maness et al, discloses 
the use of strain surfaces in an occlusal diagnostic device for displaying 
the points of contact between a patient's upper and lower teeth. The 
sensor input device comprises top and bottom layers, each containing an 
array of parallel conductors. The x and y conductors are formed by opaque 
metal lines or conductive inks and are separated by a conductive/resistive 
layer. When the flat sensor input device containing the layered composite 
is positioned in a patient's mouth between the teeth, biting the device 
compresses the separation layer and this decreases the resistance between 
an associated crosspoint of the xy conductor arrays. The resistance of the 
separation layer effects a switching action such that when the resistance 
is decreased below a threshold value, current flows through the upper and 
lower conductors at the associated crosspoint causing the associated 
crosspoint location to be displayed on the system monitor as one of the 
contact points within an outline of the patient's teeth. 
Another example of the application of the xy strain surface technology is 
in so-called digitizing pads which are used in CAD (computer assisted 
design) systems. The digitizing pads are used for inputing information 
such as lines or circles to a computer, which manipulates or otherwise 
operates on the manually-generated input. Here, as in the above systems, 
the xy conductors and the supporting resistive layer(s) are opaque. 
It is highly desirable to mount such xy-conductor digitizing pads directly 
on the CRT monitor. Obviously, such an approach requires a large (for 
example, 14 in. square) strain sensitive transparent device. In addition, 
it is highly desirable to mount strain or pressure gages on glass panels 
and windows, for example to provide security for homes, offices, display 
cases, etc. However, such uses are not practical with the existing strain 
measurement technology, which uses opaque materials. 
To my knowledge, the prior art strain sensitive surface technology is 
limited to the peripheral conductor approach disclosed in Ng et al U.S. 
Pat. No. 4,476,463 issued Oct. 9, 1984. The Ng '463 patent uses a 
peripheral array of electrodes along the different edges of a faceplate 
(such as a CRT or monitor faceplate) to measure the impedances of the 
faceplate surface. The peripheral conductors do not interfere with the 
central viewing area of the faceplate. 
3. SUMMARY OF THE INVENTION 
The above and other objectives are accomplished in a strain sensitive 
transparent composite embodying my invention and comprising a transparent 
substrate having formed thereon at least one transparent electrical 
conductor, the electrical resistance of the conductor being proportional 
to the strain or load applied to the conductor. The substrate preferably 
is a sheet. 
In one preferred embodiment, the conductor forms a strain gage. In another 
preferred embodiment, the composite comprises arrays of conductors formed 
on one side, or on opposite sides of the substrate, defining an xy grid.

5. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The utility of materials such as indium tin oxide (ITO) and indium oxide 
(IO) in strain applications of the type described in the "Background of 
the Invention" was demonstrated and characterized by measuring the change 
in resistance of a thin strip of ITO coating due to an applied load as 
follows. Referring to FIG. 1, a commercially available 16 in. square, 
0.060 in. thick sheet of glass 12 having a coating 14 of 5 ohms per square 
ITO formed thereon was masked and sand-blasted to define an ITO strip 
0.050 in. wide and 14 in. long. Conductive leads 16--16 were attached to 
each end of the strip. Loads of about 58 oz. were applied to the composite 
10 in directions transverse to the sheet 12 to place the ITO coating 14 in 
compression or tension and an ohmmeter (not shown) was attached to the 
leads to measure the resistance of the conductor. The direction of 
application of the illustrated load in FIG. 1 placed the thin film coating 
14 in compression. The results are given below. 
______________________________________ 
Compression 
Tension 
______________________________________ 
No load 1.10964 k.OMEGA. 
1.10988 k.OMEGA. 
Full load 1.10989 k.OMEGA. 
1.10852 k.OMEGA. 
Difference +0.00025 k.OMEGA. 
-0.00036 k.OMEGA. 
______________________________________ 
Please note, the compressive and tensile strains unexpectedly resulted in 
increased and decreased resistance, respectively, the opposite of the 
behavior of the conventional materials discussed in the Background of the 
Invention. The measured resistance difference between no load and full 
compressive and tensile load conditions for the relatively crude 
0.050.times.14 in.&gt;5.OMEGA./.quadrature. gage are equivalent to a 
resistance change of about 300 ppm (parts per million). Changes of this 
magnitude are readily detected and thus useful in strain gages, 
touchpanels/digitizing pads, and other strain sensitive devices. 
Furthermore, the above tensile and compressive strains are approximately 
symmetric. Symmetry is a desirable but not necessary characteristic in 
strain materials because of the elimination or reduction of circuitry 
and/or software necessary to compensate for differences in the response to 
compressive and tensile strains. It also is expected that more precise 
devices and instrumentation would demonstrate even greater resistance 
changes and, perhaps, even greater symmetry. Because of the desirable 
properties of transparency, strain sensitivity and durability, the 
composites described here are useful for example as touch panels for 
inputting information to a computer, in strain sensitive windows such as 
architectural windows and display case windows in which the strain 
sensitive surface is used to monitor the pressure and/or strain and/or 
integrity of the window, and in pressure windows in airplanes, spacecraft, 
submarines, etc., where, again, the strain sensitive surface is used to 
monitor the pressure and/or strain and/or integrity of the window. 
FIG. 2 depicts one embodiment of a strain sensitive device which uses the 
discovery, discussed above relative to FIG. 1, that transparent ITO (and 
IO) and similar transparent materials such as alloys of indium oxide and 
tin oxide, particularly indium-doped tin oxide (ITO) and antimony-doped 
tin oxide (ATO), as well as aluminum doped zinc oxide (Al:ZnO), have the 
required load-varied resistance characteristics which make them useful in 
strain measurement devices. The illustrated device 20 comprises a 
transparent substrate 22 of glass or plastic such as glass substrates 
(float glass (preferred), borosilica glass, and silicon dioxide), and 
plastic substrates (polycarbonate (preferred; G. E. LEXAN) polyester 
(Dupont MYLAR D), and polyethylene), having a thin film coating of ITO 
formed to a serpentine strain gage pattern 24 thereon. The ends of the 
conductor 24 form enlarged pads 26--26 for attaching electrical leads 
28--28 to associated monitoring circuitry, FIG. 5. As alluded to above, 
preferably the transparent strain sensitive material used in my strain 
sensitive composites is indium tin oxide (ITO). Other materials including 
indium oxide (IO) are suitable. Preferably the strain sensitive material 
such as ITO is formed using the sputter deposition, sequential oxidation 
process described for example in the incorporated '095 patent. 
Alternatively, other suitable processes include vacuum evaporation, 
electron beam evaporation and PAS PVD processes such as the PAS electron 
beam evaporation disclosed in the referenced '114 patent. 
FIG. 3 illustrates the use of the strain gage 20, FIG. 2, to monitor the 
stress in a depositing thin film. Here, the gage 20 is joined to the back 
of the substrate 33 on which material 32 is (being) deposited, forming 
thin film 35. The strain gage is connected via leads 28--28 to the 
monitoring system described below relative to FIG. 5. During deposition of 
the thin film 35, strain gage 20 monitors stress across the film in a 
manner well known in the art and which, thus, is not described in detail 
here. Please note, it is necessary that the strain gage composite 20 be 
securely joined to the substrate 33. Suitable joining techniques include 
screws (preferred), mechanical clamping, welding, brazing, and adhesive 
bonding. A transparent strain gage is more desirable in the in-situ film 
stress monitoring applications where it is desirable to also monitor layer 
thickness with an optical monitor. 
FIGS. 4A-4C depict one example of a transparent composite 40 comprising an 
xy grid of conductors 44X and 44Y formed on a substrate 42. The device 40 
differs from conventional devices in that preferably the conductor arrays 
44X and 44Y and the substrate 42 are all transparent.) Specifically, a 
transparent glass or plastic substrate 42 is coated on opposite sides with 
a transparent conductive coating 44 of material such as ITO. The coatings 
can be deposited by various processes, such as sputtering and oxidation, 
or electron beam evaporation (with or without PAS) and oxidation. For 
temperature sensitive substrate materials such as plastics, assignee's 
above-referenced '095 patent process process is preferred. To form the 
composite 40, the coatings are deposited, either sequentially or 
simultaneously, then one of the coatings is masked and etched, to form the 
thin conductor lines 44X or 44Y. The opposite coating is then masked and 
etched to form the perpendicular array 44Y or 44X. Alternatively, both 
coatings can be masked before they are etched. Other patterning techniques 
such as laser oblation, and different deposition, masking and removal 
sequences, can be used. 
Alternatively, both sets of conductors can be formed on the same side of 
the substrate 42 with an intervening transparent insulating layer. In this 
alternative, one of the coatings is deposited and patterned, a transparent 
insulating layer such as silicon oxide is deposited, then the second 
coating is deposited and patterned. 
The FIG. 5 block diagram generically depicts a system 50 comprising 
interface electronic circuit 52 and computer 54, for monitoring the 
magnitude and/or the magnitude and location of the pressure or strain in a 
strain sensitive surface such as the strain gage 20 and the xy grid device 
40. FIG. 6 is a more detailed schematic of a system 60 for monitoring the 
magnitude and/or the magnitude and location of the pressure or strain of 
an xy grid device. Specifically, the strain sensitive xy grid device, 
which is mounted on a window 58 or other surface to be monitored, 
comprises transparent substrate 62, active surface 64 surrounded by four 
sensors 66--66, and interconnect wires 68--68 connected to interface 
electronics circuit 52, which in turn is connected via cable 69 to 
computer 54. The computer 54 receives the amplified, digitized electrical 
signals from the interface circuit 52 and, as is well understood in the 
art, provides output information in electronic, visual, magnetic, paper, 
etc. media regarding the magnitude and/or location of the load applied to 
the strain sensitive surface. Alternatively, (for example, where the 
strain sensitive surface is a digitizer pad), the computer uses the 
electronic signals as input to an applications program such as a CAD 
program. 
FIG. 7 illustrates with greater detail one exemplary arrangement of an 
interface circuit 52 comprising an amplifier 71 and an A/D converter 73. 
FIG. 7 also depicts a strain sensitive surface in the form of a strain 
gage 66 which uses a very simple strain gage pattern. The pattern 
comprises a glass substrate with thin grid lines on the top, bottom, and 
sides. The grid is connected at either end to heavier, nearly strain 
insensitive, tabs which facilitate soldering to leadwires. This soldering 
has been successfully done using ultrasonic soldering techniques. For 
example, soldering copper wire or foil to ITO is accomplished using solder 
containing an indium alloy such as that available from Arconium Corp of 
America under the tradename/product identification OST 298-300 
(80%indium/15%lead/5%silver). The solder is fed into an ultrasonic 
soldering machine, one example being a Sunbonder USM III from Asahi Glass 
Ltd. 
Each gage can be connected into a Wheatstone bridge, as previously 
described, amplified at 71, converted to a digital signal at 73, and sent 
for processing to computer 54. Only one gage is shown with the electrical 
connections. 
To improve the resolution of the strain sensitive surface, more gauges 
could be constructed, as shown in FIG. 8. This process could be extended 
to as many gauges as desired. To increase the sensitivity of each 
individual gauge, a looping could be done as shown in FIG. 9. The 
sensitivity or "gage factor" can be increased by increasing the length and 
decreasing the width of each gauge element. 
FIG. 12 is a more-detailed schematic depiction of a suitable Wheatstone 
bridge interface circuit 52, one which incorporates or integrates the 
strain sensitive conductor/surface as one side of the bridge, for the 
purpose of monitoring the variation in pressure/strain in strain sensitive 
components such as the strain gage conductor 24, FIG. 4 and the strain 
sensitive conductors/conductor array/surfaces of FIGS. 4 and 7-9. 
Considering the FIG. 12 circuit 52 with specific reference to the FIG. 4 
xy array, a conductor 44X or 44Y is connected via bus bar 43 as the 
variable resistor/impedance with three known, equal-valued resistors R. 
Illustratively, the output of the bridge is connected to amplifier 45, and 
the resulting amplified output is applied as input to an analog-to-digital 
converter 47 which, in turn, is connected to computer 54. 
FIG. 13 depicts one application of the FIG. 12 circuit, to a 
multi-conductor strain sensitive surface such as 40. Each 44X or 44Y 
conductor is incorporated into a bridge circuit as described in FIG. 12. 
In order to accommodate the multiple bridge circuits, a multiplexer 49 is 
inserted between the output of the analog-to-digital converters 47 
associated with the different bridge circuits and the computer input. 
Strain Surface Operating Procedure 
Consider now an exemplary operating procedure, using the system of FIG. 7 
as an example. To calibrate the system, first, all gages are read, and 
these readings are stored as ZERO values (nulling the initial bridge 
unbalance). The designed full scale weight is then placed in the very 
center of the surface and all gauges are read and stored as FULL SCALE. If 
desired, one half of the full scale weight can be substituted for the full 
scale weight, readings taken and stored as HALF SCALE readings to correct 
for non-linearity. All subsequent readings can now be referred to the ZERO 
and FULLSCALE values. 
When the full scale weight is placed at various positions around the 
surface the following normalized reading (in millivolts at the input to 
the amplifier with a ten volt excitation) would be typical: 
______________________________________ 
Weight Top Gauge Bottom Left Right 
Position Reading Reading Reading 
Reading 
______________________________________ 
None 0 0 0 0 
Center 5 5 5 5 
Top 9 1 5 5 
Bottom 1 9 5 5 
Left 5 5 9 1 
Top Right 9 1 1 9 
______________________________________ 
When a half scale weight is placed at various positions around the surface 
the following normalized readings (in millivolts at the input to the 
amplifier with a ten volt excitation) would be typical: 
______________________________________ 
Weight Top Gauge Bottom Left Right 
Position Reading Reading Reading 
Reading 
______________________________________ 
None 0 0 0 0 
Center 2.5 2.5 2.5 2.5 
Top 4.5 0.5 2.5 2.5 
Bottom 0.5 4.5 2.5 2.5 
Left 2.5 2.5 4.5 0.5 
Top Right 4.5 0.5 0.5 4.5 
______________________________________ 
By reading all four gages, scaling, and performing the calculations; it is 
possible to accurately determine both the position and the magnitude of 
the applied force. 
The following examples give representative parameters for processes and 
devices which embody my invention. 
EXAMPLE 1 
Transparent Strain Gage (FIG. 2) 
An 18 kA.degree. thick coating of indium tin oxide (ITO) is formed by 
depositing ITO by electron beam evaporation, using the process summarized 
in Table 1, on a float glass substrate 0.5 in. wide by 1.0 in. long by 
0.010 in. thick. The evaporation process of Table 1 does not use a plasma 
activated source (PAS), but PAS can be used. (Alternatively, as described 
below, the process described in the referenced '095 patent could be used) 
Using conventional masking and etching, the ITO layer is formed to a 
standard serpentine pattern conductor 0.05 in. wide (transverse width) 
using standard masking and etching steps. Then, silver paint bonding pads 
are coated on the ends of the conductor for external electrical 
connection. Alternatively, an insulating/passivation layer is formed over 
the assembly for the purpose of mechanical protection and/or humidity 
protection. For example, a silicon dioxide layer one micron (micrometer) 
thick deposited by the well-known electron beam PVD process serves the 
above described protective functions. The resulting strain gage is 
transparent to visible radiation. 
TABLE 1 
______________________________________ 
Electron Beam Evaporation 
______________________________________ 
Substrate: Float Glass 
Rotary Motion: Double 
Material co-evaporation 
Source 1 Sn (tin) 
Source 2 In (Indium) 
C.R. (Cathode rate): 
1-2 .ANG./s 
Gas: Oxygen 400 sccm 
Power: Indium Power 6.0 KV .3 Amps 
Tin Power 6.0 KV .3 Amps 
Evaporation Pressure: 
0.1 microns 
Process Temp: 300.degree. C. 
Post Operation Bake: 
None 
______________________________________ 
EXAMPLE 2 
Transparent Video Terminal Touch Panel (FIG. 4) Using Scobey et al '095 
Technology 
10 ohms per square ITO coatings .about.0.020 in. on a side and .about.0.7 
microns thick are formed on opposite sides of a 12 in. square, 0.060 in. 
thick substrate of float glass, using the process summarized in Table 2. 
Using conventional masking and etching, the ITO layer is formed into an 
array of close spaced parallel conductor lines interconnected at each end 
by a bus line. The arrays are oriented perpendicular to one another, that 
is, they form an xy grid. The conductor lines are .about.0.020 in. wide by 
.about.9.0 in. long, while the bus lines are .about.0.100 in. wide by 
.about.10.0 in. long. The resulting touch/pad can be mounted directly to a 
CRT or other monitor or display by 0.020 stand-offs at each corner or can 
be mounted in a plastic bezel added to a finished computer terminal 
similar to the GLAREGUARD.RTM. anti-glare panels. Touching the pad changes 
the resistance of the underlying ITO conductors, thereby altering the 
current through the conductors at the crosspoint(s), allowing an 
associated computer to define the xy position of contact, as well as the 
"z" axis or magnitude of force. The resulting strain sensitive touch panel 
is transparent to visible radiation and has the transmittance 
characteristics depicted in the FIG. 11 wave/length scan for 3.45 k.ANG. 
thick, 37.0 ohms per square ITO formed using the process described in the 
incorporated '095 patent. 
TABLE 2 
______________________________________ 
ITO PER PROCESS OF THE '095 PATENT 
______________________________________ 
Substrate: Polycarbonate 
Rotary Motion: Single 
Material: Indium tin alloy to form ITO 
C.R.: 3-6 .ANG./s 
Gas: Argon 600 sccm 
Power: 2.5 KW 
Argon Sputter Pressure: 
2.5 microns 
Ion Source Operation: 
3.0 amps; 280 sccm O.sub.2 
Post Operation Bake: 
None 
______________________________________ 
EXAMPLE 3 
Transparent Video Terminal Touch Panel (FIG. 4) Using PAS E-Beam 
Evaporation (Gurev '114) 
10 ohms per square ITO coatings .about.0.020 in. on a side and .about.0.7 
microns thick are formed on opposite sides of a 12 in. square, 0.060 in. 
thick substrate of float glass, using electron beam evaporation with PAS, 
the evaporation process of TABLE 1, with the PAS assist described for 
example in the incorporated Gurev '114 patent. Without PAS, the process 
involves flowing oxygen into the chamber and allowing it to react with the 
evaporated tin and indium to form ITO. Using the PAS process involves 
flowing oxygen into the chamber through the PAS unit as described in the 
Gurev '114 patent. RF energy is coupled into the oxygen stream exciting 
the oxygen to higher energy states. The excited oxygen is significantly 
more reactive and results in better oxidation of the indium, resulting in 
ITO having lower sheet resistance than is formed using non-PAS electron 
beam evaporation. 
Using conventional masking and etching, the ITO layer is formed into an 
array of close spaced parallel conductor lines interconnected at each end 
by a bus line. The arrays are oriented perpendicular to one another, that 
is, they form an xy grid. The conductor lines are .about.0.020 in. wide by 
.about.9.0 in. long, while the bus lines are .about.0.100 in. wide by 
.about.10.0 in. long. The resulting touch/pad can be mounted directly to a 
CRT or other monitor or display by 0.020 stand-offs at each corner or can 
be mounted in a plastic bezel added to a finished computer terminal 
similar to the GLAREGUARD.RTM. anti-glare panels. Touching the pad changes 
the resistance of the underlying ITO conductors, thereby altering the 
current through the conductors at the crosspoint(s), allowing an 
associated computer to define the xy position of contact, as well as the 
"z" axis or magnitude of force. The resulting strain sensitive touch panel 
is transparent to visible radiation and has the transmittance 
characteristics depicted in the FIG. 11 wave/length scan for 28.8 k.ANG. 
thick, 0.91 ohms per square plasma activated source (PAS) assisted 
electron beam evaporation formed ITO. 
TABLE 3 
______________________________________ 
Electron Beam Evaporation with PAS 
______________________________________ 
Substrate: Float Glass 
Rotary Motion: Double 
Material co-evaporation 
Source 1 Sn (tin) 
Source 2 In (Indium) 
C.R. (Cathode rate): 
1-2 .ANG./s 
Gas: Oxygen 400 sccm 
Power: Indium Power 6.0 KV .3 Amps 
Tin Power 6.0 KV .3 Amps 
Evaporation Pressure: 
0.1 microns 
Process Temp: 300.degree. C. 
Post Operation Bake: 
None 
Plasma-Activated Source (PAS): 
Frequency: 13.56 MHz 
Incident Power: 1.8 KW 
______________________________________ 
EXAMPLE 4 
Transparent Video Terminal Touch Panel 
This touch panel is the same as that of Example 2, except that the crossed 
conductor coatings are formed on the same side of the substrate, separated 
by an insulating later. One ITO conductor coating about 0.7 microns thick 
is deposited and patterned, including forming bonding pads on the 
conductor ends; then a 1.0 micron thick insulating coating of silicon 
dioxide is deposited; and the second ITO conductor coating, about 0.7 
micron thick, is deposited and patterned and bonding pads are formed. A 
passivation/insulating coating of SiO.sub.2 material, 1-2 micron thick is 
deposited over the assembly. 
EXAMPLE 5 
Transparent Video Terminal Touch Panel on Flexible Plastic Base 
This touch panel is the same as that of Example 2 except that the low 
temperature process described in the incorporated '095 patent is used to 
form the coatings on a plastic substrate. Here, the substrate is a 
flexible plastic material such as polycarbonate. The touch panel is formed 
as described in Example 2 using the process described in the referenced 
Scobey, Seddon et al. patent, U.S. Pat. No. 4,851,095, to form the ITO 
coatings and, preferably, the silicon dioxide passivation or protective 
coatings. The advantages of the process described in the incorporated '095 
patent include low temperature processing, which permits the use of 
materials such as plastics having low melting and/or softening 
temperatures, in turn permitting the use of a touch panel which is 
flexible as well as translucent/transparent and, thus, suitable for 
mounting on curved surfaces such as curved monitor viewing screens. 
Based upon the above description and examples, those of usual skill in the 
art will readily practice the invention set out in the claims defined 
below.