Abstract:
A transparent force sensor for use in touch panel displays (touch screens) and method for fabricating the same are disclosed. The transparent force sensor is capable of detecting touch by measuring local pressure applied by a touch input to a display area of the touch screen.

Description:
[0001]    CROSS REFERENCE TO RELATED APPLICATION(S) 
         [0002]    The present non-provisional application is related to co-pending U.S. application Ser. No. 12/052,365 filed Mar. 20, 2008 and entitled ‘Transparent Pressure Sensor and Method for Using,’ the contents of which are incorporated herein in their entirety. 
     
    
     TECHNICAL FIELD 
       [0003]    This disclosure relates to a computing device input arrangement, and more particularly to a transparent force sensor and method of fabrication for use in touch panel displays. 
       BACKGROUND 
       [0004]    Today, in many electronic devices, such as portable communication devices, touch panel displays (touch screens) present information to a user and also receive input from the user. A touch screen is especially useful in portable communication devices where other input devices, such as a keyboard and a mouse, are not easily available. 
         [0005]    There are many different types of touch sensing technologies in use today, including capacitive, resistive, infrared, and surface acoustic wave. These technologies sense the position of touches on a screen. However, they do not respond to the pressure that is applied against the touch screen. 
         [0006]    For example, it has been proposed in U.S. Pat. No. 6,492,979 to Kent et al. to use a combination of capacitive touch screen and force sensors to prevent false touch. This approach, however, can complicate the sensor interfaces and does not lend itself to sensing different touch forces at the same time. U.S. Pat. No. 7,196,694 to Roberts proposes using force sensors at the peripherals of the touch screen to determine the position of a touch. This however does not offer a capability of multi-touch. It has also been proposed in US Patent Publication No. 2007/0229464 to use a capacitive force sensor array, overlaying a display to form a touch screen. Although this approach offers multi-touch capability, a capacitive pressure sensor has limited spatial resolution and is subject to environmental interferences such as electromagnetic interference (EMI) and capacitive coupling of fingers and other input devices. 
         [0007]    Accordingly, there is a need for improved touch sensing technologies and devices. 
       SUMMARY 
       [0008]    A transparent force sensor for use in touch panel displays (touch screens) and method for fabricating the same are disclosed. The transparent force sensor is capable of detecting touch by measuring local pressure applied by a touch input to a display area of the touch screen. 
         [0009]    Various aspects of the touch screen relate to force sensing. For example, according to one aspect, a force sensing touch screen includes a first plurality of transparent conducting oxide (TCO) electrodes (traces) disposed on a first surface of a transparent polymer matrix and arranged in a first direction, a plurality of transparent conducting nanoparticles dispersed in the transparent polymer matrix, and a second plurality of TCO electrodes disposed on a second surface of the transparent polymer matrix opposing the first surface and arranged in a second direction overlaying the first direction. Upon a force being applied at a cross section of one of the first and second plurality of TCO electrodes, the transparent conducting nanoparticles at the cross section provide a conducting path through the transparent polymer matrix. 
         [0010]    In one embodiment, for example, the polymer matrix is disposed on a transparent substrate. The transparent conducting nanoparticles can include indium tin oxide (ITO), zinc oxide (ZnO), tin dioxide (SnO2), or combinations thereof. In one embodiment, a drive voltage is applied to the first plurality of TCO electrodes using a drive multiplexor and a measurement resistor is connected to each of the second plurality of TCO electrodes using a sense multiplexor. In yet another embodiment, the force sensing touch screen includes a plurality of operational amplifiers that are adapted to minimize cross talk among the plurality of TCO electrodes. 
         [0011]    In another aspect, a method of fabricating a force sensing touch screen having a transparent force sensor includes forming a transparent polymer matrix including a plurality of transparent conducting nanoparticles, disposing on a first surface of the transparent polymer matrix a first plurality of transparent conducting oxide (TCO) electrodes, the first plurality of TCO electrodes arranged in a first direction, and disposing on a second surface of the transparent polymer matrix opposing the first surface a second plurality of TCO electrodes, the second plurality of TCO electrodes arranged in a second direction overlaying the first direction. Upon a force being applied at a cross section of at least one of the first and second plurality of TCO electrodes, the transparent conducting nanoparticles at the cross section provide a conducting path from the at least one of the first plurality of TCO electrodes to the at least one of the second plurality of electrodes. 
         [0012]    In one embodiment, forming the transparent polymer matrix includes dispersing the plurality of transparent conducting nanoparticles in a first solvent system, dispersing a transparent polymer in a second solvent system, and combining the first and second solvent systems in a predetermined ratio determined by a percolation value for the combination. The method can also include degassing the combination, disposing the degassed combination on a substrate to form a dry film, and curing the dry film. 
         [0013]    Several benefits can be derived from the present invention. For example, the force sensor can simplify the input process by enabling different combinations of positions and forces on a touch screen. An additional advantage is that the force sensor is not limited to only detecting finger touch and can accept input from many other devices including, but not limited to, a stylus and glove. The force sensor is also more tolerant to environmental noises, such as Electromagnetic Interference (EMI). 
         [0014]    Additional features and advantages will be readily apparent from the following detailed description, the accompanying drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a schematic of a touch screen sensor according to an embodiment of the present invention. 
           [0016]      FIG. 2  is a flow chart of a method for fabricating the touch screen sensor shown in  FIG. 1 . 
           [0017]      FIG. 3  is an example of a force-resistance curve generated using the touch screen sensor of  FIG. 1 . 
           [0018]      FIG. 4  is an example of a transmission spectrum generated using the touch screen sensor of  FIG. 1 . 
           [0019]      FIG. 5  is a partial cross section of exemplary components included in the touch screen sensor of  FIG. 1 . 
           [0020]      FIG. 6  is a perspective view of an exemplary embodiment including the touch screen sensor of  FIG. 5 . 
           [0021]      FIGS. 7A-C  illustrate exemplary device configurations for the touch screen sensor shown in  FIG. 5 . 
           [0022]      FIG. 8  is a block diagram of a device incorporating the touch screen sensor of the present invention. 
           [0023]      FIGS. 9A-C  are schematics of drive schemes for interfacing with the touch screen sensor of  FIG. 1 . 
       
    
    
       [0024]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0025]    The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
         [0026]    Referring to  FIG. 1 , a transparent touch screen sensor  10  is disclosed. The sensor  10  can be implemented in various electronic devices having displays including, but not limited to, mobile computers, computer monitors, mobile phones, personal digital assistants (PDAs), and service terminals. The sensor  10  is configured to detect touch by measuring local pressure exerted on a display area, such as a touch screen. Advantageously, the sensor  10  allows a third dimension of input, namely force, to be analyzed in addition to time and location inputs, thereby enabling a whole new range of applications. 
         [0027]    In one embodiment, the sensor  10  comprises a force sensing transparent polymer-conductor composite (TPCC)  15 , which consists of transparent conducting oxide (TCO) nanoparticles  12  dispersed in a transparent polymer matrix  14 . The TPCC  15  can comprise, for example, phenoxy resin, polyester, silicone rubber, polimide, or combinations thereof. In one embodiment, for example, the TCO nanoparticles  12  are sized less than 100 nm. 
         [0028]    As shown in  FIG. 1 , one or more TCO electrodes (traces)  16 ,  18  are disposed on each opposite side of the TPCC  15 . Upon a pressure being applied to at least one of the TCO electrodes  16 ,  18  on one side of the TPCC  15 , the resistance across the TCO electrodes  16 ,  18  is decreased and is measured by the electrodes  16 ,  18 . The resistance of the TPCC  15  is highly sensitive to pressure near the composition of percolation threshold. 
         [0029]    The transparent conducting nanoparticles  12  of the transparent polymer matrix  14  provide a conducting path  19  through the TPCC  15 . For example, as shown in  FIG. 1 , current can flow through the TPCC  15  via the nanoparticles  12 , either directly when the nanoparticles  12  are in contact with each other, or by tunneling when the particles are separated by a very small distance. When pressure is applied to the TPCC  15 , the TPCC  15  deforms and increases the number of conductive paths, thereby lowering the resistance. 
         [0030]      FIG. 2  illustrates an exemplary method of fabricating the sensor  10  shown in  FIG. 1 . As shown in  FIG. 2 , in one embodiment, for example, preparation of the TPCC  15  begins with obtaining transparent conducting nanoparticles  20 . In one embodiment, for example, the nanoparticles are indium tin oxide (ITO) alloys. In another embodiment, for example, the nanoparticles are zinc oxide (ZnO) alloys. In yet another embodiment, the nanoparticles are tin dioxide (SnO2) alloys. Next, the obtained nanoparticles are dispersed in a first solvent system  22 . The first solvent system can include methyl ethyl ketone (MEK), Toluene, Methonal, 1-Phenoxy-2-Propanol (DPPH), or any other suitable solvent having similar solvent qualities. 
         [0031]    Preparation of the sensor  10  also includes obtaining a transparent polymer  24 . Example transparent polymers that can be used with the present invention include, but are not limited to, phenoxy resin, polyethers, acrylic, silicone, lacquer, or other types of transparent elastomers, or combinations thereof. The obtained transparent polymer is then dissolved in a second solvent system  26  having qualities similar to the first solvent system. The two solvent systems  22 ,  26  are then measured in predetermined ratios and combined to form a matrix  28 . In one embodiment, for example, the measurements are determined by a percolation point associated with combining the two solvent systems  22 ,  26 . In one embodiment, for example, the combination matrix includes approximately twenty percent (20%) to thirty percent (30%) volume ratio of nanoparticles to matrix. Combining the two solvent systems  28  can be done utilizing a conventional high speed mixer, shaker, or the like. 
         [0032]    Next, the combination matrix is degassed  30 . In one embodiment, the degassing is performed in a vacuum chamber. In another embodiment, degassing of the matrix is done under ambient air pressure. The degassing step allows for any gas that may have been introduced into the combination matrix to be removed. 
         [0033]    Substrates with transparent conducting traces having a predetermined pitch are then processed using standard photolithography, etching, and/or screen printing process  32 . Once the substrates are processed, the mixture of the polymer composite is then disposed onto the substrates  34 . In one embodiment, for example, a spin coating technique is used to apply the mixture to the substrate. Dip coating, screen printing, or any of the suitable thick film deposition techniques may be used to apply the polymer composite onto the substrate. Typically, the dry film thickness achieved from deposition ranges between 1 and 10 um. For example, in one embodiment, for example, the dry film thickness ranges between 6 and 8 um. 
         [0034]    Once the film is deposited on the substrate, in one embodiment, for example, the substrate is cured in an oven  36 . Various types of ovens can be used for curing the film, including a vacuum oven, convection oven and hot plate. Several heating stages can be applied to achieve a uniformly cured film. The final curing temperature can range between 120 and 200 degrees Celsius, inclusive. For example, in one embodiment, the final curing temperature of the transparent polymer matrix is approximately one-hundred and seventy (170) degrees Celsius. 
         [0035]    The electrical response of combining the above materials in the matrix can be characterized by the force resistance curve and the transmission characteristics of the material. A typical resistance range can extend from over twenty (20) megohm at zero (0) pressure to approximately five (5) kilohm at less than one (1) kilogram of pressure.  FIG. 3  illustrates an exemplary logarithmic resistance versus force curve measured on matrices produced by the above-described method. 
         [0036]    Furthermore, using the TPCC described herein, over ninety percent (90%) of transmissions can be achieved with a film thickness of approximately 1 um. For example, in one embodiment, referring to  FIG. 4 , a transmission spectrum from three hundred nanometre (300 nm) to eight hundred nanometer (800 nm) for a one 1 um film thickness on a glass substrate can be achieved. 
         [0037]    Referring to  FIG. 5 , to form the structure of the sensor  10 , in one embodiment, two perpendicular sets (row and column) of transparent conducting oxide (TCO) electrodes  16 ,  18  are disposed on both sides of the TPCC  15 . A conducting path  19  is formed at each row and column intersection to form an array of addressable pixels. A force applied at each pixel location can be measured by probing the resistances at each row column intersection. A force sensing touch screen then can be formed by depositing the sensor  10  on a suitable transparent substrate, such as glass or plastic sheet. Scan and read signals are sent and received through the tab connectors attached to each set of TCO electrodes. An exploded view of such a structure is described and shown in connection with  FIG. 6 . 
         [0038]    Turning now to  FIG. 6 , in one embodiment, for example, the transparent pressure sensor  10  includes a transparent substrate  64  that can be a rigid material, for example, glass or a polymer, but may be a flexible material. A patterned layer  60  of transparent conductive electrodes  61  is deposited on the substrate  64  using any of the deposition techniques referenced earlier. In one embodiment, for example, the electrodes  61  are aligned in a first direction and are formed to have a pitch of 0.05-10 mm, (preferably 1.0 mm), a width less than the pitch but larger than 0.001 mm, and a thickness of 1.0-1000 nm, (preferably 40 nm). The transparent electrodes  61  may be a transparent conductive oxide, for example, indium tin oxide, zinc oxide, and tin&#39;oxide, as described previously. A tab  62  is electrically coupled to the electrodes  61  for providing connection to other circuitry. 
         [0039]    The TPCC  15  is disposed on the electrodes  61  as a layer or in a predetermined pattern. As described previously, the TPCC  15  can be a transparent elastomeric matrix such as polyester, phenoxy resin, or silicone rubber. Transparent conductive or semiconductive particles such as indium tin oxide, zinc oxide, tin oxide, or combinations thereof are dispersed within the composite matrix as discussed above. 
         [0040]    As shown in  FIG. 6 , a patterned layer  56  of transparent conductive electrodes  57  is disposed over the TPCC  15 . The placement of the transparent conductive electrodes  57  creates a plurality of intersections, each including one of the transparent conductive electrodes  61 . A tab  54  is electrically coupled to electrodes  57  for providing a connection to other circuitry. A substrate layer  52  of a transparent protective material, such as glass or a polymer, is disposed over the patterned layer  56 . 
         [0041]    Turning now to  FIGS. 7A-C , several different device configurations for the sensor  10  are shown. In one exemplary embodiment, as shown in  FIG. 7A , bottom TCO electrodes  60  are deposited on a transparent substrate (bottom substrate)  64 . The TPCC  15  is deposited in a blanket pattern over a top surface of bottom TCO electrodes  60 . Spacers  70  are deposited in between electrodes of top TCO electrodes  56  to separate the top TCO electrodes  56  from the TPCC  15 . Spacers  70  can be formed from polymers or polymer-like materials, as is known in the art. 
         [0042]    In another exemplary embodiment, as shown in  FIG. 7B , the TPCC  15  is patterned to form a plurality of separate islands, with each island formed at an intersect of top TCO electrodes  56 . Spacers  70  are deposited in between electrodes of the top TCO electrodes  56  to separate the top TCO electrodes  56  from the bottom TCO electrodes  60  and TPCC  15 . In yet another exemplary embodiment, as shown in  FIG. 7C , the spacers are eliminated and the TPCC  15  is patterned to form separate islands that also operate as spacers. 
         [0043]    While the transparent pressure sensor device described herein can be used in electronic devices in general, a block diagram of a force imaging system  100  as an example using the above-described pressure sensor is depicted in  FIG. 8 . As shown in  FIG. 8 , in one embodiment, a touch screen controller  106  provides drive signals  110  to a force sensing touch screen  102 , and a sense signal  104  is provided from the force sensing touch screen  102  to the touch screen controller  106 , which periodically provides a signal  108  of the distribution of pressure received to a processor  112 . The processor  112  interprets the controller signal  108 , determines a function in response thereto, and provides a display signal  114  to a display device  116 . 
         [0044]    Various interface electronics for driving the force sensing touch screen  102  are disclosed. For example, referring now to  FIG. 9A , in one example embodiment, a drive voltage  91  is applied to each row of TCO electrodes  93  via a control circuit  90  sequentially through a multiplexer  92 . Within each row  93 , a resistor value of each column of TCO electrodes  95  is read through a sense multiplexer  94 . The resistor value is read out using a measurement resistor  96  as a voltage divider. A digital read out is then extracted through an A/D convertor  98  which is connected to the measurement resistor  96 . 
         [0045]    In another exemplary embodiment, referring to  FIG. 9B , an operational amplifier  97  is used in addition to the measurement resistor  96  for a more linear interpretation of the voltage-force curve. In yet another exemplary embodiment, as shown in  FIG. 11 , an operational amplifier  97  is used for each column of TCO electrodes  95 . Advantageously, by implementing an operational amplifier at each column of TCO electrodes  95 , cross talk between columns of TCO electrodes  95  can be reduced for a more precise read out and determination of pressure applied. 
         [0046]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.