Abstract:
A resistive touch screen includes a transparent substrate defining a touch area; a first layer of conductive material formed on the transparent substrate and extending over the touch area; an electrical connection to the first layer of conductive material; a transparent flexible cover sheet; a second layer of conductive material formed on the transparent flexible cover sheet, the cover sheet being mounted in a spaced apart relationship from the substrate, whereby a touch in the touch area results in an a electrical contact between the first and second layers of conductive material at the point of touch; an electrical connection to the second layer of conductive material; and at least one of the first or second layers of conductive material having a variable conductivity.

Description:
FIELD OF THE INVENTION 
   This invention relates to resistive touch screens and more particularly, to the formation of a resistive layer in the resistive touch screen. 
   BACKGROUND OF THE INVENTION 
   Resistive touch screens are widely used in conventional CRTs and in flat-panel display devices in computers and in particular with portable computers.  FIGS. 2 and 3  show a portion of a prior art four-wire resistive touch screen  10 , which includes a transparent substrate  12 , having a first conductive layer  14  defining a touch area. This conductive layer typically comprises indium tin oxide (ITO) or conductive polymers such as polythiophene. A flexible transparent cover sheet  16  includes a second conductive layer  18  that is physically separated from the first conductive layer  14  by spacer dots  20 . Conductive patterns  30  having lower resistance than the conductive layer  14  defining an edge area are arranged over the conductive layer  14  at opposite edges of the conductive layer  14  on transparent substrate  12 . The conductive patterns  30  are provided by an additional layer of material that is in electrical contact with conductive layer  14 . Conductive patterns  30  are also provided in electrical contact with and at opposite edges of the conductive layer  18  on the flexible transparent cover sheet  16  (because of the four conductive patterns  30  this is commonly referred to as a four-wire design). The shape of these conductive patterns  30  can be adjusted to improve the linearity of the response of the touch screen. See for example U.S. Pat. No. 4,625,075 issued Nov. 25, 1986 to Jaeger. These conductive patterns  30  are used to provide electrical connection to the conductive layers  14  and  18 . 
   In an alternative design (commonly called a five-wire design) all four conductive strips  30  are located on the substrate  12  and the second conductive layer  18  is the so-called fifth wire. The five-wire design may also utilize specially chosen patterns for the four conductors  30  on substrate  12  to improve the linearity of the device response. 
   The flexible transparent cover sheet  16  is deformed, for example by finger pressure, to cause the first and second conductive layers  14  and  18  to come into electrical contact. A voltage is applied across the conductive layers  14  via electrical connections  33  and a resulting signal is measured on the electrical connections  31  connected to layer  18  to determine the location of the touch in one direction. The voltage is then applied across the conductive layer  18  and the signal is measured on the electrical connection  33  to determine the location of the touch in the orthogonal direction. The conductive layers  14  and  18  have a resistance selected to optimize power usage and position sensing accuracy. 
   In conventional prior-art manufacturing processes, the conductors  30  are made of silver inks screen printed onto the conductive layers  14  and  18 . In practice, this process has a number of disadvantages. First, the silver inks are costly and the screen printing process is expensive in that additional manufacturing steps and materials are needed. Second, unless they are carefully prepared and printed, the silver inks do not adhere well to the conductive layers. Moreover, the process of adhering the inks to the conductive layers may require high temperatures, creating problems for other materials in a touch screen or associated display system. Furthermore, the width of the edge area of the touch screen may need to be relatively large to accommodate the patterns used to linearize the response of the touch screen. 
   There is a need therefore for an improved means to provide conductive patterns for a resistive touch screen and a method of making the same that can reduce the width of the edge area, improve the robustness of the touch screen and reduce the cost of manufacture. 
   SUMMARY OF THE INVENTION 
   The need is met by providing a resistive touch screen that includes a transparent substrate defining a touch area; a first layer of conductive material formed on the transparent substrate and extending over the touch area; an electrical connection to the first layer of conductive material; a transparent flexible cover sheet; a second layer of conductive material formed on the transparent flexible cover sheet, the cover sheet being mounted in a spaced apart relationship from the substrate, whereby a touch in the touch area results in an a electrical contact between the first and second layers of conductive material at the point of touch; an electrical connection to the second layer of conductive material; and at least one of the first or second layers of conductive material having a variable conductivity. 
   ADVANTAGES 
   The touch screen of the present invention has the advantages that it is simple to manufacture, reduces costs, and provides a larger active area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing a cross sectional view of a four-wire touch screen according to the present invention; 
       FIG. 2  is a schematic diagram illustrating a prior art four-wire touch screen; 
       FIG. 3  is a schematic diagram showing a side view of the prior art four-wire touch screen; 
       FIG. 4  is a schematic diagram illustrating a side view of process for manufacturing a touch screen according to the present invention; 
       FIG. 5  is an end view of the manufacturing process; 
       FIG. 6  is a topographical representation of a variable conductive layer having continuous variation in the touch area according to the one embodiment of the present invention; 
       FIG. 7  is a schematic diagram of a variable conductive layer having width variation in the edge area according to the one embodiment of the present invention; 
       FIG. 8  is a schematic diagram of a variable conductive layer having thickness variation in the edge area according to the one embodiment of the present invention; and 
       FIG. 9  is a schematic diagram showing a display and driver for adjusting an image signal to compensate for variations in transparency or color in the touch screen. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , the problems of the prior art resistive touch screens are overcome through the use of a variably conducting layer  32  deposited on the substrate  12 . A flexible transparent cover sheet  16  having a second conductive layer  34  is separated from the variably conducting layer  32  by conventional means, for example spacer dots  20 . The conductive layer  32  deposited on the substrate  12  and/or the conductive layer  34  deposited on the flexible transparent cover sheet  16  are variably conducting. 
   The variation in conductivity of the variably conducting layers  32  and/or  34  may be continuous or discontinuous. If the conductivity of a layer is continuous, the sheet resistance of the layer varies continuously and gradually from location to location in the layer. If the conductivity of a layer is discontinuous, a specific location in the layer will have a conductivity that is substantially different from a nearby location. 
   In prior art touch screens, the sheet resistance of a conductive layer in the touch area of a resistive touch screen is a constant value typically in the range of 300 to 600 ohms per square. Resistance values outside of this range may be employed for different purposes, for example reduced power consumption or reduced errors. In one embodiment of the present invention, the sheet resistance may vary in the touch area within the range of 300 to 600 ohms or go well outside the range. According to another embodiment of the invention, the edge area, defined by the conductive patterns  30  of  FIG. 2 , are replaced by more highly conductive portions  36  of the variably conductive layer  32 . 
   Typical material used for transparent conductive coatings include indium tin oxide (ITO), indium zinc oxide (IZO), or conductive polymers such as polythiophene. As these materials are coated on a substrate, their sheet resistance will vary with the thickness of the deposition. By depositing the material with varying thickness, a variably conducting layer may be formed. If the material is deposited with twice the thickness, its sheet resistance may drop by half. Alternatively, the composition of the material may be varied in a single layer to vary the conductivity of the layer. The substrate on which the variable conductivity pattern is formed can be either rigid or flexible. 
   In accordance with one embodiment of the invention, the at least one variable conductive material layer may define a region of uniform conductivity covering the touch area and an edge area having a higher conductivity than the uniform conductivity, with the electrical connection to the at least one layer being made to the edge area. The edge area may have a variable conductivity effective to linearize electric fields in the touch area of the at least one layer of conductive material, or to compensate for the resistivity of the edge area. The edge area may have a variable conductivity provided by a pattern of variable width or thickness. In a further embodiment, the at least one variable conductive material layer may have a variable conductivity in the touch area effective to linearize electric fields in the layer of conductive material in the touch area. 
   In particular, discontinuous conductive edge patterns  30  such as those formed to improve the linearity of the touch screen response (for example as shown in U.S. Pat. No. 5,736,688 issued Apr. 7, 1998 to Barrett et al.) or to provide a connection to the resistive layer may be constructed to form the variably conductive layer  32 . For example the pattern shown in this patent can be formed in the variably conductive layer  32  according to the present invention. Moreover, the variably conductive layer  32  may be continuously varying in the touch area so as to also improve the linearity of the device response. Alternatively, variation in both the edge area and the touch area may be employed to improve the linearity of the touch screen response and to reduce the width of the edge area. 
     FIG. 6  shows a topographical representation of variable conductivity in the touch area  35  to improve the linearity of response for a five-wire device having electrical connections  33  at each corner of the variably conductive layer  32 . 
     FIG. 7  shows a top view of variable conductivity in the highly conductive edge area  36  having variable width to improve the linearity of response for a five-wire device having electrical connections  33  at the center of each edge area. 
     FIG. 8  shows a cross sectional view of a variably conductive edge area  36  shaped to improve the linearity of response wherein varying thickness to provide the variable conductivity. 
   A variably conductive layer  32  may be formed on the substrate  12  or the flexible transparent cover sheet  16  by a variety of means. U.S. Pat. No. 6,214,520 issued Apr. 10, 2001 to Wolk et al. describes the use of a thermal transfer element for forming a multi-layer device. Alternatively, inkjet devices can be configured to deposit liquid materials such as polythiophene in varying amounts and thickness to provide a variably conducting layer. Applicant has demonstrated the pixel-wise deposition of conductive materials using an inkjet device. Moreover, both approaches can be used to deposit varying types of materials, providing a multi-component layer with different materials as necessary to provide the preferred conductivity. These techniques are also readily used to provide discontinuous deposits as well as deposits that vary continuously over a surface. 
   Another useful technique may be sputtering. Techniques known in the art may be applied to continuous roll manufacturing processes to provide a variably conducting layer by passing a continuous substrate beneath a sputtering station with the necessary masking and aperture control devices. Referring to  FIG. 4 , a side view of a continuous substrate  40  passing above material deposition stations  42  and  44 . The deposition stations  42  and  44  heat material  47  that is evaporated and condensed on the surface of the continuous substrate  40 . By controlling the deposition of material, the time that a particular portion of the substrate is exposed to the material deposition can be controlled, for example with a shutter that opens and closes or a mask that restricts deposition to particular locations on the substrate. Referring to  FIG. 5 , an end view of the substrate  40  is shown with a deposition station  44  having a mask  46  to provide an area  48  of greater material deposition. 
   Another deposition method is liquid coating. By using a hopper containing liquid material, the material can be flowed in a controlled fashion onto a continuously moving substrate. By varying the thickness and location of the deposition, a variably conducting layer may be provided. Yet another technique of providing a variable conductive coating is the use of photo lithography by depositing a uniform layer of transparent conductive material and selectively removing the material to provide a variable conductive layer. 
   Once the variably conducting layer is provided on a substrate, the substrate may be combined with other elements to form a touch screen, as is known in the art. 
   The transparency of the deposited material is a critical factor for any touch screen. The variably conductive layer may have a correspondingly varying thickness and transparency or color. For locations that are not part of the display area (for example the conductive patterns  30 ), this is of no consequence. For locations that are a part of the display area, the image signal employed to drive the display may be adjusted to accommodate any variation in transparency or color of the touch screen. By using a transparency or color map wherein each pixel in a display is adjusted in brightness or color to compensate for the transparency of the touch screen, a display with corrected brightness and color may be obtained. As shown in  FIG. 9 , the means for modifying the image signal  54  to provide a modified image signal  56  that compensates for variations in color or transparency of the touch screen can be provided in a display controller  50  having a lookup table  52  that provides a brightness and/or color adjustment for each pixel element of a display with a touch screen  58 . 
   The present invention may be used in conjunction with any flat panel display, including but not limited to OLED and liquid crystal display devices. Moreover, a substrate or cover of an OLED display may be used as the substrate for a resistive touch screen. 
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, some of the advantages of the present invention may be obtained by combining a variable conductive layer with the conventional silver ink conductors in the edge areas, to reduce the amount of silver ink required and reduce the width of the edge area. 
   PARTS LIST 
   
       
         10  resistive-wire touch screen 
         12  substrate 
         14  first conductive layer 
         16  cover sheet 
         18  second conductive layer 
         20  spacer dots 
         30  conductive patterns 
         31  electrical connections 
         32  variably conducting layer 
         33  electrical connections 
         34  conductive layer 
         35  touch area 
         36  highly conductive portions 
         40  continuous substrate 
         42  deposition station 
         44  deposition station 
         46  mask 
         47  material 
         48  area of greater material deposition 
         50  display controller 
         52  look up table 
         54  image signal 
         56  modified image signal 
         58  display