A laminated capacitive touch-pad having a thin film touch-plate electrode deposited upon a first (exterior) surface of a first, relatively thin dielectric layer and having spaced transmitter and receiver electrodes deposited upon a second surface of the first layer within the outline of and opposite to the touch electrode, with a relatively thick backing layer of dielectric material laminated upon the second surface to provide a total thickness, as measured between the furthest opposed surfaces of the first and second layers, as required for high voltage insulation purposes and to provide additional impact strength. The touch, transmitter and receiver electrodes may be of thin film construction.

BACKGROUND OF THE INVENTION 
The present invention relates to capacitive touch-pads and, more 
particularly, to a novel laminated capacitive touch-pad providing greater 
dielectric coupling between touch plate and transmit and receive 
electrodes thus greatly enhancing the ratio of desired coupling 
capacitance to parasitic coupling between transmit and receive electrodes 
while maintaining required high voltage insulation parameters and 
facilitating a greater total dielectric thickness which is required in 
some applications. 
Touch-pads capable of changing capacitance, when approached or contacted, 
are well known. Capacitive touch-pads are often used in high voltage 
environments where relatively thick dielectrics are required for safety 
purposes; the value of a parasitic capacity between a pair of spaced 
transmitting and receiving electrodes, fabricated upon a first surface of 
a relatively thick dielectric substrate, often approaches the capacitance 
value between each of the electrodes and a touch-pad electrode fabricated 
upon the remaining surface of the substrate, whereby erroneous signals are 
frequently generated. The problem is compounded when arrays of discrete 
touch-pad devices are utilized; the separation distance between each 
device in the array and all other adjacent devices must be relatively 
great to prevent additional parasitic capacitance, and the associated 
signal leakage attributable thereto, from totally masking the desired 
change in signal when the touch electrode is approached and/or contacted. 
A capacitive touch-pad having a relatively large ratio of desired coupling 
capacity to parasitic capacity, yet retaining a relatively thick 
dielectric for safety reasons in high voltage environments, is desirable. 
BRIEF DESCRIPTION OF THE INVENTION 
A laminated capacitive touch-pad, in accordance with the invention, has a 
first, relatively thin dielectric layer having a conductive touch 
electrode fabricated upon an exterior surface thereof and has a 
transmitting electrode and a receiving electrode fabricated, within the 
outline of the touch-pad, upon the remaining substrate surface. A second 
layer of dielectric material, of relatively greater thickness with respect 
to the thickness of the first layer, is laminated against the surface of 
the first layer bearing the spatially separated transmitting and receiving 
electrodes to achieve a total dielectric thickness as required for safety 
reasons, while having a dielectric thickness between the touch electrodes 
and the transmitting and receiving electrodes of relatively small 
dimension to cause the touch-pad capacity to be relatively greater than 
the parasitic capacity between transmitting and receiving electrodes, 
thereby preventing erroneous signal generation, even when a plurality of 
the laminated capacitive touch-pads are arrayed with relatively small 
separation distances therebetween. 
In a preferred embodiment of the invention, the electrodes are fabricated 
by thin film deposition techniques; the receiving and transmitting 
electrode leads are of the thin film type and are integrally fabricated 
with the electrodes. Thick film leads and electrodes may also be utilized. 
Accordingly, it is one object of the present invention to provide a novel 
capacitive touch-pad having a relatively large dielectric thickness while 
minimizing parasitic capacitance effects. 
It is another object of the present invention to provide a novel capacitive 
touch-pad having laminated dielectric layers. 
It is a further object of the present invention to provide a laminated 
capacitive touch-pad having thin film electrodes. 
These and other objects of the present invention will become apparent upon 
consideration of the following detailed description and accompanying 
drawings.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Referring initially to FIGS. 1a and 1b, a prior art capacitive touch-pad 10 
comprises a dielectric substrate 11, upon a front, or outwardly-facing, 
surface 11a of which is fabricated a touch electrode 12 of conductive or 
semiconductive material and having a preselected boundary, herein 
illustrated as being of rectangular shape. A pair of electrodes 14 and 15, 
commonly referred to as a transmitting electrode and a receiving 
electrode, respectively, are fabricated of a conductive or semiconductive 
material upon the remaining, inwardly-facing surface 11b of the substrate. 
Both transmitting and receiving electrodes 14 and 15 are typically of 
substantially smaller area than, and are positioned substantially within 
the boundaries of, the area of touch electrode 12. The closest points 
between transmitting and receiving electrodes 14 and 15 are separated by a 
preselected distance D, while each of electrodes 14 and 15 are separated 
from touch electrode 12 by a preselected dielectric thickness T, derived 
in accordance with the insulation characteristics to be achieved. Each of 
transmitting and receiving electrodes 14 and 15 have an associated 
conductive lead wire 16 and 17, respectively, coupled to a point thereon 
for connection of the touch-pad to known circuitry (not shown for purposes 
of simplicity) configured to drive the transmitting electrode with a 
periodically-varying waveform and to recognize a change in the peak 
amplitude of that waveform at the receiving electrode as the effective 
series coupling impedance between transmitting and receiving electrodes 
varies when touch electrode 12 is approached and/or contacted. 
In situations where relatively large values of dielectric thickness T are 
required, to protect personnel contacting touch electrode 12 from high 
voltages present in apparatus adjacent substrate rear surface 11b, the 
relative magnitude of a parasitic capacity C.sub.P (between the facing 
surfaces of the transmitting and receiving electrodes 14 and 15 separated 
by the relatively small distance D), approaches the magnitude of the 
desired series capacity C (C=CaCb/(Ca+Cb) between electrodes 14 and 15 via 
touch electrode 12. Similarly, when arrays of a plurality of such 
touch-pads are positioned with relatively close spacing, the transmitting 
electrode of one touch-pad is positioned relatively close to the receiving 
electrode of an adjacent touch-pad, whereby the magnitude of the 
pad-to-pad parasitic capacities approach the magnitude of electrode 
capacity through the substrate. These and other parasitic capacities cause 
relatively large coupling between transmitting and receiving electrodes 
leading to the generation of erroneous signals which are not always easily 
identified or rejected. The coupling problem is even more severe where the 
value of the touch-pad capacitor C is small due to increasingly greater 
dielectric thickness T. In many circumstances, dielectric thickness and 
the distance separating individual touch-pads of an array are configured, 
in high density arrays, such that device and parasitic capacitances are 
substantially equal and allow a signal, introduced into the transmitting 
electrode 14, to have substantial equal coupling to receiving electrode 15 
through either the parasitic or the intended capacitance path. 
In accordance with the invention, a capacitive touch-pad 20 (FIGS. 2a and 
2b) essentially overcoming the above problems of parasitic capacitance, 
while maintaining the required total thickness T of a dielectric material, 
comprises a first dielectric substrate 21 of a relatively small thickness 
T.sub.1, upon a front, or outwardly-facing, surface 21a of which is 
fabricated touch electrode 12. The spatially separated transmitting and 
receiving electrodes 14 and 15, respectively, are fabricated upon the 
remaining, or inwardly-facing, surface 21b of first dielectric layer 21 
and may be formed, in one preferred embodiment, by thin film fabrication 
techniques. It should be understood that touch electrode 12 may also be 
fabricated by thin film techniques. The electrodes and leads may also be 
fabricated as thick films, if desired, in a particular application. 
Lead means 23 and 24 each respectively associated with respective 
transmitting and receiving electrodes 14 and 15, are advantageously 
fabricated by thin film techniques, at the same time as the transmitting 
and receiving electrodes are themselves fabricated, whereby the members 
forming lead means 23 and 24 are integrally joined to their respective 
electrodes. 
A second layer 26 of dielectric material is fabricated to a thickness 
T.sub.2, which may be equal to, or different from, the thickness T.sub.1 
of first layer 21. Layer 26 is laminated against the inwardly-facing 
surface 21b of the first dielectric layer, and the transmitting and 
receiving electrodes fabricated thereon. Thus, the total thickness 
(T.sub.2 +T.sub.1) of the dielectric insulation of our novel laminated 
touch-pad is easily established to be equal to the total dielectric 
insulation thickness T of the prior art touch-pad 10, yet the total 
separation distance T.sub.1 between each of transmitting and receiving 
electrodes 14 and 15, respectively, and overlying touch electrode 12 is 
substantially reduced over the prior art embodiment, whereby the value of 
each series capacitance (between transmitting electrode 14 and touch 
electrode 12 and between touch electrode 12 and receiving electrode 15) is 
substantially increased and is relatively greater than the parasitic 
capacity between transmitting and receiving electrodes of a single 
touch-pad or the parasitic capacitance between electrodes of adjacent 
touch-pads in high density arrays. The relatively greater magnitude of 
signal current enabled between transmitting and receiving electrodes via 
the series path (through the touch-plate), as opposed to the parasitic 
path, enables touch-pad arrays of greater density than hitherto possible. 
Use of spacing similar to the array spacing achieved in prior art displays 
allows relatively lower amplitudes of the waveform driving the 
transmitting electrode to be used, due to the increased magnitude of the 
desired signal coupled between electrodes 14 and 15, via Ca, Cb and the 
touch plate electrode. Thus, lower implementation cost of associated 
driving and sensing circuitry is facilitated by reducing driving signal 
magnitude and the difficulty of sensing false signals. 
While the present invention has been described with reference to one 
preferred embodiment thereof, many variations and modifications will now 
become apparent to those skilled in the art. It is our intent, therefore, 
to be limited solely by the scope of the appending claims.