Abstract:
Sensor systems, methods of making sensor modules and circuit modules, and methods of making expanded sensor systems including the sensor modules and circuit modules. A sensor module can include a flexible substrate, internal conductor lines, edge conductor lines for module interconnection, and sensors integrated thereon. One sensor module includes an array of interconnected capacitive pressure tactile sensors (taxels), being row addressable from address lines and readable through data lines. The sensor modules can be bonded to each other to form a larger array of sensors. One bonding method utilizes anisotropic conductive paste (ACP). The sensor system provided can be flexible and easy to expand to cover large areas. By using various sensor modules, the sensor system can be used for various applications. Readout modules can be coupled to the exterior edges to read all the individual sensors. Applications include robotic skin and wearable sensor fabrics.

Description:
RELATED APPLICATIONS 
     The present invention is a non-provisional patent application of U.S. Provisional Patent Application No. 60/762,649, filed Jan. 28, 2006, and claims priority to Korean National Patent Applications KR 10-2005-0039802, filed May 12, 2005, and KR 10-2005-0044570, filed May 26, 2005. 
    
    
     FIELD OF INVENTION 
     The present invention is related generally to sensors. More specifically, the present invention is related to systems and methods for joining groups of multiple flexible sensors to cover larger areas. The invention can be used to form tactile sensitive robot skin and flexible, wearable electrically coupled tactile sensors. 
     BACKGROUND 
     Many researchers have been conducting research to integrate electronic device functions (and functionality) such as MP3 players, computers, and health monitoring sensor systems into flexible packaging, for example, into clothes. Research into flexible electronics generally, such as flexible displays, has also been pursued. Key challenging technologies include implementing flexible electronic modules and connecting them. In one example, Infineon Technology in Germany presented a work titled “Enabling Technologies for Disappearing Electronics in Smart Textiles” at ISSCC 2003. 
     Research into ubiquitous sensor systems which consist of a large a network of sensor modules that can sense, process, and transmit various signals has been conducted. Some commercial products are currently available. The concept was summarized and presented at the 2004 International Conference on Applications and the Internet, in a paper titled “Ubiquitous Services and Networking: Monitoring the Real World.” 
     These technologies have tremendous potential. According to these technology trends, implementing a flexible and independent system of various functions by connecting flexible functional modules may likely be a key technology in the near future. 
     What would be desirable are improved methods and systems for creating sensors which cover larger, expanded areas by joining smaller sensors into groups. 
     SUMMARY 
     The present invention provides a sensor system comprising: a circuit module comprised of a first flexible substrate, a plurality of routing conductor lines, a plurality of interconnection conductor lines, IC chips integrated on the first flexible substrate; and a sensor module comprised of a second flexible substrate on which routing conductor lines and a plurality of interconnection conductor lines are formed, having sensors integrated on the substrate, wherein the sensor module and circuit module are electrically coupled to each other. In some sensor systems, the sensor modules and circuit modules are directly connected to each other with anisotropic conductive paste (ACP), or other electrical bonding methods. The sensor modules and circuit modules may be indirectly connected to each other through ACP bonded to a flexible connection module. In other embodiments, at least two sensor modules and two circuit modules are connected to each other electrically. The sensor modules may be arranged as a matrix having 2 dimensions to form exterior edges, the circuit modules attached to at least one exterior edge along each dimension, and at least one circuit module attached at a corner of the matrix where the two exterior edges meet, so that all the circuit modules are electrically connected. The substrate may be flexible polymer. 
     The present invention also provides a sensor device comprising: a plurality of sensor modules with the sensor modules including a plurality of sensors on a flexible substrate, the sensors configured to measure a physical property and to output data corresponding to the measured data, the sensors being coupled to a plurality of address lines for selecting the sensors and coupled to a plurality of data lines for reading the sensor data. The sensor device also includes the sensor modules having edge address conductors and edge data conductors. The device has the sensor modules grouped adjacent to each other, the device having adjacent address edge conductors electrically coupled together, the device having adjacent data edge conductors electrically coupled together, such that at least some sensor module edges remain on an exterior of the module grouping which are not between adjacent sensor modules. There exists sensors within the sensor modules that can be addressed through adjacent interposed sensor module address lines and have data that can be read through adjacent interposed sensor module data lines. 
     The sensor device can further have addressing circuitry electrically coupled to at least some sensor module exterior edge address conductors. The device may also have readout circuitry electrically coupled to at least some sensor module exterior edge data conductors and measurement circuitry electrically coupled to at least some sensor module exterior edge data conductors. The device may also include control circuitry coupled to the addressing circuitry and to the measurement circuitry for controlling the addressing of the sensors and for reading the data from the selected sensors. In some devices, at least some of the sensors generate analog data signals having a range of possible values. At least some of the sensors may generate binary data signals and/or serial digital signals. At least some of the adjacent edge conductors may overlap and be joined using ACP. 
     In some sensor devices, at least some of the adjacent edge conductors are joined with a flexible joining module which overlaps the adjacent edge conductors and is electrically coupled to the adjacent edge conductors and which establishes electrical continuity between the adjacent edge conductors. The sensors may be selected for reading at least in part by reading data from the sensor. The sensor modules can be arranged in rows and/or columns, in some devices. At least some of the sensors are pressure sensors. 
     In some sensor devices, the sensors include pressure sensors including a first, flexible plate conductor supported in spaced part relation to a second plate conductor and having a compressible dielectric therebetween, forming a capacitor between the two plates, such that increasing deflection of the first plate towards the second plate increases the capacitance of the capacitor. The compressible dielectric is a gas, which may be air. The gas may exit from between the two plates in response to the increased deflection, in some devices. The gas may exit into other capacitors, in some embodiments, and may be sealed or open to the outside atmosphere, depending on the embodiments. 
     In some sensor devices, the address lines serve to address a set including a plurality of sensors at the same time, and the data lines serve to read out data from the addressed sensors. The address lines may select a row of sensors, where the data lines select individual sensors from the selected row by reading the data from the individual sensors. The data read can include capacitance values from the sensors selected and read through the data lines. At least some of the plates may include electrically conductive material bonded to an electrically insulating polymer. At least some of the sensor modules may have sensors for measuring different physical properties and/or different ranges of those properties within the same sensor. Sensors may be selected from the group consisting of pressure, optical, temperature, continuity, pH, humidity, ion concentration, and combinations thereof, depending on the embodiment. 
     The present invention also provides methods for making the sensors, sensor modules, other modules, and systems made of these modules. One method includes: grouping a plurality of flexible sensor modules having edge conductors into adjacent locations to each other; and physically bonding adjacent electrical conductors into electrically continuity with each other, such that the grouped plurality of flexible sensor modules remain flexible. The method may have the bonding accomplished using Anistropic Conductive Paste (ACP). The grouping may include overlapping adjacent edge conductors with each other, and in which the bonding includes bonding the overlapping edge conductors. The grouping can include overlapping adjacent edge conductors with a flexible joining connector, where the flexible joining connector is configured to establish electrical continuity between adjacent edge conductors which the joining connector overlaps, and in which the bonding includes bonding the overlapping joining connector to the edge conductors. 
     The sensor system described in this invention can be applied where flexible and expandable sensor systems are required. Depending on the sensors used, the system can be applied in various fields. For example, if a tactile sensor is used, the system can be used for robot skin, for example, to provide touch, to sense being touched, and to be used on the feet to assist in balance and walking. If a pressure sensor array is used, measuring pressure distribution as applied to human feet when walking will be possible, for medical purposes, prosthetic purposes, and the like. Medical sensors can be used, for example, to make a health monitoring cloth having temperature and pressure sensors combined. 
     The present invention can includes a modular expandable sensor system which can be deployed on a curved surface in any size by stitching functional modules for artificial skin for robots, smart clothes which can monitor human body status, wearable computers, or any other applications where a large flexible sensor array may be deployed. These modules may be implemented using a flexible polymer platform, and the sensor system can be deployed on any curved surface such as robot or human bodies. A sensor can be integrated in this sensor system for itself or in combination with other kinds of sensors. 
     The present invention can provide an electrode layer for capacitors having high flexibility and a method of manufacturing the electrode layer. The present invention can also provide a unit sensor that can be easily manufactured while the unit sensor has high flexibility and resolution. The present invention can further provide a tactile sensor having high flexibility and easy extendability. 
     In accordance with one aspect of the present invention, some devices can be made by the provision of an electrode layer for capacitors whose capacitance is changed depending upon the variation in distance between two electrode layers, wherein the electrode layers comprise: a polymer substrate; an electrode formed on the polymer substrate; and a signal transmission line formed on the polymer substrate, such that the signal transmission line is connected to the electrode. 
     In accordance with another aspect of the present invention, there is provided a method of manufacturing an electrode layer for capacitors, wherein the method comprises the steps of: forming a sacrifice layer on a silicon substrate; forming an electrode and a signal transmission line at a predetermined area on the sacrifice layer; coating the sacrifice layer (on which the electrode and the signal transmission line are formed) with liquid-state polymer, and hardening the liquid-state polymer; and removing the silicon substrate and the sacrifice layer. 
     In accordance with another aspect of the present invention, there is provided a unit sensor comprising: an upper electrode layer including a polymer substrate, an upper electrode formed on the polymer substrate, and a signal transmission line formed on the polymer substrate, such that the signal transmission line extends in the side-to-side direction of the upper electrode. The sensor also includes a lower electrode layer including a polymer substrate, a lower electrode formed on the polymer substrate, and a signal transmission line formed on the polymer substrate such that the signal transmission line extends in the front-to-rear direction of the lower electrode. The sensor further includes a spacer layer, made of polymer and disposed between the upper electrode layer and the lower electrode layer, the spacer layer being provided at a predetermined area thereof with an opening, through which the upper electrode and the lower electrode face each other. 
     In some embodiments, the unit sensor also comprises an insulating layer, made of polymer, disposed between the upper electrode layer and the spacer layer. Alternatively, the insulating layer may be disposed between the lower electrode layer and the spacer layer. The unit sensor may further comprise a bump layer, made of polymer, disposed on the upper electrode layer. 
     In accordance with yet another aspect of the present invention, there is provided a tactile sensor comprising: a unit sensor array including a plurality of unit sensors formed in a two-dimensional array, the upper electrodes of the unit sensors being electrically connected with each other by the sequential interconnection of the signal transmission lines for the upper electrodes, the lower electrodes of the unit sensors being electrically connected with each other by the sequential interconnection of the signal transmission lines for the lower electrodes. The sensor can also include connection lines disposed at the ends of the signal transmission lines for the upper electrodes and the signal transmission lines for the lower electrodes to connect the unit sensor array to the outside. 
     Some embodiments of the present invention provide a fabrication method of a circuit module which includes: preparing a silicon wafer as a carrier; coating a sacrificial layer on the silicon wafer; forming routing lines and interconnection lines on the sacrificial layer; and coating and curing a polymer layer on the sacrificial layer to make a flexible substrate. The method also includes removing the sacrificial layer to peel off the completed flexible substrate; forming an insulation layer on the routing lines except where any IC chips are to be bonded; and bonding IC chips on the flexible substrate. In some of these methods, a step forming a bump mold by etching part of sacrificial layer is included after forming the sacrificial layer, and bumps are formed during electroplating for interconnection lines. In some such methods, the IC chips include bumps for integration. 
     Some embodiment sensor module fabrication methods can include preparing a silicon wafer as a carrier; coating a sacrificial layer on the silicon wafer; forming a plurality of routing lines and interconnection lines on the sacrificial layer; and coating and curing a polymer layer on the sacrificial layer to make a flexible substrate. The method also includes removing the sacrificial layer to peel off the completed flexible substrate; forming an insulation layer on the routing lines except where the sensors are to be bonded; and bonding sensors on the flexible substrate. Such methods may also include a step forming a bump mold by etching part of the sacrificial layer after forming the sacrificial layer, where bumps are formed during electroplating for interconnection lines. In some methods, the sensors include bumps for integration. 
     A fabrication method of a sensor is provided in some embodiments, which includes: preparing a silicon wafer as a carrier; coating a sacrificial layer on the silicon wafer; forming routing lines and interconnection lines on the sacrificial layer; and coating and curing structure polymer to make a flexible substrate. The method also includes removing sacrificial layer to peel off the completed flexible substrate, and forming an insulation layer on the routing lines except where the IC chips and sensors are bonded, and bonding IC chips and sensors on the flexible substrate. The method may also include a step forming a bump mold by etching part of the sacrificial layer after forming the sacrificial layer, and bumps are formed during electroplating for interconnection lines. In some methods, IC chips and sensors include bumps for integration. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a high level diagram of an interface circuit module which can be used to interface to a sensor module. 
         FIG. 1B  is a high level diagram of a tactile sensor module having an array of taxels and edge row and column pad conductors for connecting to other tactile sensor modules and for connecting to interface circuits. 
         FIG. 2  is a high level diagram of a tactile system including four of the tactile sensor modules of  FIG. 1B  coupled together, two interface circuits of  FIG. 1A  coupled to the column edge conductor pads, two interface circuits of  FIG. 1A  coupled to the row edge conductor pads, and a control circuit coupled to two adjacent interface circuits. 
         FIGS. 3 and 4  are process flow diagrams of a fabrication process of a circuit module and a sensor module. 
         FIGS. 5A and 5B  are schematic views of methods for joining modules together. 
         FIG. 6  is a photograph of a fabricated circuit module according to the present invention. 
         FIG. 7  is a schematic view of one modular expandable sensor system 
         FIG. 8  is schematic diagram of a sensor module and other modules. 
         FIG. 9  is diagram of the integration of ICs or sensors on a flexible polymer platform. 
         FIG. 10  is a diagram of two module bonding methods. 
         FIG. 11  includes two photographs of a tactile sensor module. 
         FIG. 12  is a diagram of a modular expandable tactile sensor scheme. 
         FIG. 13  is a schematic diagram of a modular expandable readout circuit scheme and a timing diagram. 
         FIG. 14  is photograph of a fabricated universal readout circuit chip. 
         FIG. 15  is an illustrated process diagram of a fabrication process using a polymer substrate as a platform for readout circuitry modules. 
         FIG. 16  is an illustrated process diagram of a flip chip assembly process using ACP. 
         FIG. 17  is a group of four photographs of the fabricated PDMS substrate and the assembled chip on it. 
         FIG. 18  is a photograph of the flip chip bonded on polymer substrate and an image captured from a tactile sensor using the sensor array with readout circuit chips. 
         FIG. 19  is schematic view of a modular expandable tactile sensor. 
         FIG. 20  is cross-sectional view of a tactile cell with dimensions. 
         FIG. 21  is an illustrated process diagram of a fabrication process for a tactile sensor module. 
         FIG. 22  is a group of four photographs of the fabricated sensor module. 
         FIG. 23  is a photograph of a measurement setup for testing the sensor module. 
         FIG. 24  is a plot of capacitance ratio vs. applied force for a sensor module. 
         FIG. 25  is schematic of a readout circuit and a photograph of a sensor module test setup utilizing a computer display. 
         FIG. 26  is group of photographs of rubber stamp letters and the corresponding tactile sensor captured images. 
         FIG. 27  is a photograph and a diagram of the expanded tactile sensor module, one bonding process, and a captured image. 
         FIG. 28A  is a perspective view schematically illustrating an electrode layer for capacitors according to a preferred embodiment of the present invention. 
         FIG. 28B  is a sectional view schematically illustrating a method of manufacturing the electrode layer for capacitors shown in  FIG. 28A . 
         FIGS. 29A and 29B  are an exploded perspective view, and a sectional view, schematically illustrating a unit sensor using the electrode layer for capacitors shown in  FIG. 28A . 
         FIG. 30A  is a sectional view schematically illustrating a method of manufacturing a spacer layer used in the unit sensor shown in  FIGS. 29A and 29B . 
         FIG. 30B  is a sectional view schematically illustrating a method of manufacturing an insulating layer used in the unit sensor shown in  FIGS. 29A and 29B . 
         FIG. 30C  is a sectional view schematically illustrating a method of manufacturing a bump layer used in the unit sensor shown in  FIGS. 29A and 29B . 
         FIG. 30D  is a sectional view schematically illustrating a method of manufacturing a unit sensor using the spacer layer, the insulating layer, and the bump layer shown in  FIGS. 30A ,  30 B, and  30 C, respectively. 
         FIG. 31  is a plan view schematically illustrating a tactile sensor using the unit sensor shown in  FIGS. 29A and 29B . 
         FIG. 32  is a photograph of the tactile sensor shown in  FIG. 31 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  illustrates a circuit module  100  which is comprised of a substrate  102  on which routing lines and interconnection lines  106  are formed and IC chips  104  that are integrated on the substrate  102 .  FIG. 1A  illustrates the circuit module with integrated IC chips. Substrate  102  can be made of rubbers, polyimides, or other polymers for flexibility. The routing lines, and interconnection lines  106  can be formed by electroplating and connected electrically. IC chips or integrated circuits  104  can be integrated on the substrate using anisotropic conductive paste (ACP). In order to integrate chips  104 , the substrate or IC chips  104  can have bumps. Multiple routing lines may be formed on substrate  102 . 
       FIG. 1B  illustrates a sensor module  108  which is comprised of a substrate  110  on which routing lines and interconnection lines  112  are formed, and sensors  112  that are integrated on substrate  110 . Like circuit module  100 , the substrate  110  can be made of polymers, and the routing lines and interconnection lines  114  can be formed by electroplating. Sensors  112  can be integrated on substrate  110  using anisotropic conductive paste, in some embodiments. In order to integrate sensors  112 , the substrate  110  or sensors  112  can have bumps. 
     According to a first embodiment, a sensor system is formed by connecting circuit modules  100  and sensor modules  108  through interconnection lines  114 . The connection can be done by attaching modules directly or indirectly using an anisotropic conductive paste or by attaching modules using other flexible connection modules that contain interconnection lines and using an anisotropic conductive paste. 
       FIG. 2  illustrates a second embodiment which includes expansion of the sensor system by attaching multiple modules. A sensor system can be formed by connecting multiple sensor modules and circuit modules electrically. Modules can be connected directly to each other, or connected using flexible connection modules that contain interconnection lines. The arrangement of modules is not limited to the arrangement shown in the  FIG. 2  and can be changed as required. 
     Referring again to  FIG. 2 , sensor modules  116  are arranged to form a matrix. Circuit modules are placed on the edge of the sensor matrix. Circuit module  118  can select or address a specific row of the sensor matrix. Circuit module  120  can select a specific column and read out the sensor response. Circuit module  122  can control the other modules and transmit the measured signals to the outside. In some embodiments, each circuit module can include each function separately. In other embodiments, each module can include all functions, so that a proper function can be selected automatically by specific location or by a signal as required. 
     A third embodiment relates to implementing a sensor system on a single platform. All the functions of the second embodiment can be integrated in a single system of the third embodiment, if a sensor system of specific size and shape is required. The fabrication method and materials can be the same as with embodiment 2, except that the system is fabricated on a single substrate. 
       FIG. 3  illustrates a fourth embodiment, a fabrication process of the circuit modules. According to  FIG. 3 , a sacrificial layer  132  is formed on a silicon wafer  130  first. Bump molds  131  are formed by etching part of sacrificial layer  132 . Next, routing lines  140 , interconnection lines  136 , and bumps  138  are formed on the sacrificial layer by electroplating. Then, a polymer is spin coated and cured to form a substrate  134 . By removing sacrificial layer  132 , the substrate is detached from the silicon wafer  130 . Next, an insulation polymer  137  is applied on the substrate  134  except on bumps and interconnection lines to prevent short circuits between routing lines and IC chips  135  that will be integrated. Finally, anisotropic conductive paste is applied where the IC chips  135  will be located and IC chips  135  are placed on bumps. Then, IC chips  135  are bonded to the substrate by applying pressure and heat. 
       FIG. 4  illustrates a fifth embodiment, in which, during the fabrication of circuit modules, bumps can be placed on IC chips  135  instead of substrate  134 . According to  FIG. 4 , a sacrificial layer  132  is formed on a silicon wafer  130  first. Next, routing lines  140  and interconnection lines  136  are formed on the sacrificial layer by electroplating. Then, a polymer is spin coated and cured to form a substrate  134 . By removing sacrificial layer  132 , the substrate is detached from the silicon wafer  130 . Next, an insulation polymer  137  is applied on the substrate  134  except where bumps will be bonded and on interconnection lines to prevent short circuits between routing lines and IC chips  135  which will be integrated. Finally, anisotropic conductive paste is applied where the IC chips  135  will be located and IC chips  135  with bumps are placed. Then, IC chips  135  are bonded to the substrate by applying pressure and heat. 
     In another aspect, a sixth embodiment is related to fabricating sensor modules. All the processes can be the same as in the fourth and fifth embodiments, except that sensors are bonded to the substrate instead of IC chips. 
       FIG. 5A  illustrates a seventh embodiment, related to connecting circuit modules  170  and sensor modules  172 . Circuit modules and sensor modules can be connected to each other through interconnection lines using anisotropic conductive paste (ACP) as shown in  FIG. 5A . Extra connection modules  176  can be used to connect modules as shown in  FIG. 5B . 
     Another aspect, an eighth embodiment, is related to fabricating a sensor system mentioned in the third embodiment. The fabrication process can be the same as that of the circuit modules except that both IC chips and sensors are bonded together on a single substrate. 
       FIG. 6  is a photograph of a fabricated circuit module according to the present invention. 
       FIG. 7  shows a schematic view of one expandable sensor system  180 . In this figure, there are four basic modules: sensor modules  182 , readout modules  184  and  186 , a control module  188 , and a wireless I/O module  190 . In addition, other modules with different functions can be included if required. Information sensed by a sensor array in a sensor module is selected and can be converted to an electrical signal by a readout module and then transferred to the wireless I/O module. A wireless I/O module can transmit sensor data to an external base station and may receive control signals from it. A control module may manage these operations. As shown in  FIG. 7 , the sensor system can be expanded to a larger area by stitching more sensor modules and readout modules. 
       FIG. 8  shows a high level diagram of each module. Sensor modules  182  can include a sensor array and interconnection lines  196  on a flexible polymer platform. All of the sensors  192  may be connected to each other and to interconnection lines electrically through routing metal lines  194 . Actually, interconnection lines  196  can be the edge part of the routing metal lines, and are also referred to as edge pads or edge conductors in some views of the invention. The edge conductors need not (but may) extend all the way to the edge, as the electrical bonding may be accomplished using overlap of adjacent modules or overlap of interconnection modules, in some embodiments. Routing lines  194  may be viewed as address and/or data lines in some views of some embodiments of the invention, and my serve both functions in some devices. All other modules including a readout, control, and wireless I/O module may have similar structures at a high level. They can include an IC (Integrated Circuit)  199  mounted on flexible polymer platform and interconnection lines. The integrated ICs can determine the basic functions of the module such as readout, control, wireless I/O or others. There can be routing metal lines on the platform which connect ICs and interconnection lines. Each module may be connected through interconnection lines electrically and mechanically by using Anisotropic Conductive Paste (ACP). 
       FIG. 9  displays one way to integrate an IC or a sensor, both referenced as  200 , on the flexible polymer platform  202 . Basically, a polymer platform can include a polymer substrate  202 , patterned routing metal lines  206 , and an insulating polymer layer  208  for preventing a short circuit. Multi-level metal lines can be employed if required. ACP  207  can be used to integrate ICs or sensors on the platform. ACP is a kind of commercial adhesive which includes tiny electrically-conducting metal balls. Initially, it is insulating because the concentration of metal balls is too low for conduction. 
     However, once it is squeezed between metal pads (or bumps) and cured, ACP provides fairly good conduction through squeezed metal balls in a vertical direction as well as providing good mechanical bonding strength. ACP is cured with adequate pressure and heat for a short period of time. Bumps are often important structures for integration. Bumps can be formed either on the polymer platform or pads of an IC or a sensor. If a sensor is composed of polymer structure, it can be directly implemented on the platform during the platform process instead of being bonded by using ACP. 
       FIG. 10  illustrates some module bonding methods. ACP can also be used for module bonding. Modules may be attached directly or by using a connection module as shown. If we attach modules directly, sensors  212  and ICs  210  will see the opposite direction. If we use a connection module  216  as in  FIG. 10(   b ), sensors and ICs can be placed on the polymer platform facing the same direction. 
     This invention can provide a flexible sensor system to which a modular concept is introduced to make the system expandable. This invention can provide a common flexible platform for sensor systems for artificial tactile skin for robots, and flexible bio-monitoring systems in smart clothes, and wearable computers. This can provide a modular concept to make a sensor system expandable to any size. This invention can also provide a simple method to attach modules and integrate ICs or sensors by using commercial ACP on a flexible polymer platform such as a silicone rubber. 
       FIG. 11  shows a prototype of a capacitive tactile sensor. The sensor is composed of a 16×16 cell array, with a spatial resolution of 1 mm and having a contact resistance less than 0.1 ohms using ACP. A flip chip assembly process using ACP was used to accommodate all the readout circuits on a soft polymer substrate. 
       FIG. 12  illustrates a diagram of a modular expandable tactile sensing scheme. The proposed tactile sensor system consists of four components: 16×16 sensor array modules, row and column readout circuits, and a control block. Each array sensor module and readout circuit module can be separately fabricated. To form the expanded integrated tactile sensing system, each module can be assembled and electrically connected through metal interconnections on soft polymer substrate. As an example, an expanded 32×32 array sensor system is shown in  FIG. 12 . 
       FIG. 13  shows a schematic diagram of a modular expandable readout circuit scheme. For simplicity, a 4×4 array sensor is shown with row/column circuit chips and a control block chip. Capacitance of each taxel (tactile cell) is measured by a simple charge amplifier. The initial capacitance of a taxel has been measured as 180 fF. The readout procedure can be as follows. First, each taxel is selected by row and column shift registers and a reset signal is applied in order to reset the amplifier. Then, a step-function signal is applied to the target capacitor and the stored charge C pix  during the reset phase is transferred to the feedback capacitance C f  which generates output voltage by a charge amplifier in the control block. The output voltage variation, ΔV out , becomes
 Δ V   out   =−ΔV   step ×( C   pix   /C   f )[ V]   
     Where C pix  is the cell capacitance in the array, C f  is the feedback capacitance of the charge amplifier, and ΔV step  is the applied step voltage to C pix . V out  is determined by the capacitance of C pix  which is changed by the applied force onto a taxel. When a strong force is applied, capacitance C pix  becomes larger. All this operation can be controlled by the timing generator in the control block. In order to extend the sensor array, simply more chips are added to the end of the column and/or row of the existing sensor array. However, it is cumbersome and expensive to fabricate three different types of ID chips for each different function such as the row/column decoders and the control block with a timing generator. 
     To overcome this obstacle, we have devised a universal chip in which all three circuit blocks are realized. A specific block function in the universal generic chip can be automatically self-configured by its position and neighboring ships in the system. The micrograph and characteristics of the finishes readout circuit are show in  FIG. 14  and Table 1, respectively. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Characteristics of the chip 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Technology 
                 0.35 micrometer 2 poly 3 metal 
               
               
                   
                   
                 Standard CMOS 
               
               
                   
                 Chip Size 
                 2 mm × 2 mm 
               
               
                   
                 Supply Voltage 
                 3.3 V 
               
               
                   
                 Frame Rate 
                 24 frames/s 
               
               
                   
                   
                 (@16 × 16 Array Size) 
               
               
                   
                 Power 
                 551.1 microW 9@control Block) 
               
               
                   
                   
                 13.3 microW 9@Column/Row) 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 15  illustrates a fabrication process of a polymer substrate platform for readout circuitry modules with embedded interconnection. The structure and material of the platform can be similar to those of the modular array sensor, except for small metal bumps on the embedded interconnection lines for assembly with IC chips. 
     Fabrication processes can be as follows. First LOR (Lift-Off-Resists) from Microchem is spin coated by 24 micrometers on a bare silicon wafer as a sacrificial layer. Then it is partially etched by 17 micrometers in AZ400K developer to form a mold for metal bumps by a plasma etcher. Copper lines and bumps for flip-chip bonding are simultaneously formed using electroplating. Bump size is 100 micrometers by 100 micrometers. Next, titanium is sputtered as an adhesion layer and PDMS is spin-coated at 300 micrometers. This PDMS layer should be cured at room temperature to prevent the layers from being deformed after release due to thermal expansion difference between the copper and PDMS layers. Also, they should be cured on a custom designed planarization stage to get uniform thickness. Thickness variation can be controlled under 20 micrometers over a 4 inch wafer. After vulcanizing the PDMS, the platform layer is cut and released from the carrier wafer. 
       FIG. 16  illustrates the hybrid integration of readout circuit chips on soft polymer substrate platform demonstrated by the flip-chip assembly process shown, using anisotropic conductive paste (ACP) which is widely used in PDP or LCD packaging. ACP is a kind of thermally curable epoxy adhesive including conducting balls. Once the ACP is squeezed between two electrodes and cured, the electrodes are connected electrically through the squeezed balls. To cure ACP, pressure of about 0.2 N per each bump has been applied at 140 degrees C. for 5 minutes as shown in  FIG. 16(   a ). All the bumps on the platform and all the pads in the readout circuit chips were electrically connected without failure, with contact resistance below 100 mOhms. Mechanical bonding strength is so strong that the assembled chip could not be detached from the platform without tearing. 
       FIG. 17  shows the fabricated PDMS substrate and the assembled chip on it. Photographs of the fabricated polymer substrate with metal lines and bumps, as well as a photograph of a flip-bonded chip (the backside) are illustrated along with a magnified view of ACP and the conducting balls. 
       FIG. 18  shows how the assembled chip was attached to the PCB and tested. The chip was flip bonded on polymer substrate in  FIG. 18(   a ). All the pads were successfully connected to the substrate.  FIG. 18(   b ) shows a captured 8×8 image of the letters ‘IML’ from the fabricated tactile sensor array assembled with readout circuit chips on a test board. 
     EXAMPLES OF SENSOR MODULES 
       FIG. 19  illustrates one modular expandable tactile sensor according to the present invention. Each sensor module can be connected through interconnection lines to form a larger sensor skin. One sensor module consists of a 16×16 cell array and interconnection lines. The spatial resolution is 1 mm which is similar to that of human skin. The cell is composed of five PDMS layers and copper electrodes sandwiched between PDMS layers. 
       FIG. 20  shows the cross-section of the proposed cell and its dimensions. Two electrodes form a capacitor separated by 6 micrometers via a spacer. The cell and electrode size are 600×600 micrometers and 400×400 micrometers, respectively. Initial capacitance of one cell has been estimated as 171 fF assuming the relative permittivity of PDMS as 2.75 from the product manual (Sylgard 184, Dow Corning). When pressure is applied to a bump, the upper PDMS deforms and the capacitance increases until the gap is closed. The thickness of the upper electrode layer and bump layer determines the sensitivity. Three cells with different PDMS thicknesses, 470, 720, and 900 micrometers, have been tested. 
       FIG. 21  illustrates a fabrication process of a sensor module. Each layer can be processed separately and bonded together after oxygen plasma treatment. For the electrode layers, LOR from Micro-chem is spin coated about 10 micrometers on a bare silicon wafer. LOR is used as a sacrificial layer. The copper electrode (20 micrometers) is formed using electroplating. Next, titanium is sputtered as an adhesion layer and PDMS is spin coated about 320 micrometers. PDMS should be cured at room temperature to prevent the layers from being deformed after release due to thermal expansion difference between the copper and PDMS layers. Also, they should be cured on a custom designed planarization stage to get uniform thickness. Thickness variation can be controlled under 20 micrometers over a 4 inch wafer. After vulcanization the PDMS at room temperature, the electrode layer is cut and peeled off. The insulation and spacer layers are formed by spin coating PDMS diluted with hexane on bare silicon wafers with sputtered platinum. Platinum has been used to weaken PDMS adhesion to the substrate. The thickness of both layers is 6 micrometers. Then, PDMS is patterned and etched in a RIE with 3:1 SF 6 /O 2  gas to form a spacer layer. The bump is formed using a silicon wafer etched in KOH as a mold. Five layers are aligned and bonded together using a conventional contact aligner with slight modification. The total thickness of the cells has been controlled by bump layer thickness. 
       FIG. 22  shows the fabricated tactile sensor module. The size of one sensor module is 22×22 mm including interconnection lines. The fabricated sensor shows flexibility, a seen in the photograph.  FIG. 22(   b ) shows the magnified view of four cells and  FIG. 22(   c ) shows the embedded electrode with a bonded spacer. Air channels are formed to prevent the squeezed air from affecting the cell response. These air channels connect all the cell cavities to atmosphere and maintain the pressure of each tactile cells cavity to the pressure of atmosphere. In other embodiments, the tactile cells cavities are in fluid communication with the atmosphere. 
       FIG. 23  shows a setup for cell measurement. A micro-force gauge with 1 mN resolution has been used with a precision motorized translation stage with 100 nm resolution. 
       FIG. 24  shows the measured response of the fabricated cells for various thicknesses of the upper PDMS layer. The Y axis represents the ratio of measured capacitance to initial capacitance as a function of applied force. The initial capacitance of a cell has been measured as 180 fF. Every cell shoes saturation after 40 nM (250 kPa), which means both upper and lower electrodes are in contact with an insulation layer in between them. The cell becomes more sensitive as the upper PDMS layer thickness reduces. A sensitivity of 3%/mN for a 470 micrometer thick membrane for a small deflection has been measured. 
       FIG. 25(   a ) shows the schematic of readout circuits used to capture tactile images.  FIG. 25(   b ) shows the system setup for image capture. Each tactile cell is selected by a row decoder and reset first. Then it is charged to V step . When it is selected by the column decoder the stored charge is transferred to feedback capacitance (C f ) and generates output voltage as described in the figure. 
       FIG. 26  shows the captured images. Pressure has been applied using rubber stamps with an alphabet letter on them, and the corresponding images have been captured clearly. 
       FIG. 27(   a ) shows the expandability of the sensor array in four sensor modules ‘stitched’ together. Anistropic Conductive Paste (ACP) was used, which is also used in PDP and LCD packaging. To cure ACP, pressure of about 0.4 MPa has been applied at 120 degrees C. for 15 minutes, as shown in  FIG. 27(   b ). All interconnection lines were electrically connected without failure with contact resistance below 100 mOhms.  FIG. 27(   c ) shows the tactile image of the letter ‘O’ captured by the expanded 2×2 sensor module array (a total 32×32 tactile cell array). 
       FIG. 28A  shows an electrode layer  300  according to one embodiment of the present invention, comprising a substrate  303 , an electrode  302 , and a signal transmission line  302 . The substrate  302  is made of flexible polymer, such as silicon-based rubber or polyimide. 
     The electrode  301  and the signal transmission line  302  are formed on the substrate  303 . The signal transmission line  302  is connected to the electrode  301 . Consequently, a signal generated by the change in capacitance of a capacitor, in which the electrode layer  300  according to this embodiment is used, is transmitted to the outside though the signal transmission line  302 , which may be referred to as a data transmission line. The ‘data”, as the term is used herein, may be any signal indicative of a physical property, including variations in voltage, current, charge, continuously variable changes, discrete (on/off) changes, digital, binary, and the like. 
       FIG. 28B  illustrates a method of manufacturing the electrode layer  300 . A sacrifice layer  370  is formed on a silicon substrate  360 . Next, a predetermined area on the sacrifice layer  370  is electroplated with a conductive material, such as copper or gold, to form an electrode  301  and a signal transmission line. Subsequently, the sacrifice layer  370 , on which the electrode  301  and the signal transmission line are formed, is coated with liquid-state polymer, and then the liquid-state polymer is hardened, to form a substrate  303 . Finally, the silicon substrate  360  and the sacrifice layer  370  are removed to manufacture an electrode layer  300 , which comprises the substrate  303 , the electrode  301 , and the signal transmission line  302 . 
     Using the electrode layer according to the first embodiment of the present invention allows a very flexible capacitor array to be manufactured. 
     Another embodiment of the present invention relates to a unit sensor using the electrode layer according to the first embodiment of the present invention as described in detail above. 
       FIGS. 29A and 29B  show a unit sensor according to another embodiment of the present invention comprises a lower electrode layer  320 , a spacer layer  402  stacked on the lower electrode layer  320 , an insulating layer  410  stacked on the spacer layer  402 , an upper electrode layer  310  stacked on the insulating layer  410 , and a bump layer  400  stacked on the upper electrode layer  310 . 
     The upper electrode layer  310  comprises an upper electrode  311 , a signal transmission line  312 , and a polymer substrate  313 . The lower electrode layer  320  comprises a lower electrode  321 , a signal transmission line  322 , and a polymer substrate  323 . The upper electrode layer  310  and the lower electrode layer  320  are identical to the electrode layer  300  according to the first embodiment of the present invention as described in detail, and therefore, a detailed description of the upper electrode layer  310  and the lower electrode layer  320  will not be given. However, the signal transmission line  312  of the upper electrode layer  310  extends in the side-to-side direction of the upper electrode  311 , and the signal transmission line  322  of the lower electrode layer  320  extends in the front-to-rear direction of the lower electrode  321 . Consequently, the signal transmission line  312  of the upper electrode layer  310  is perpendicular to the signal transmission line  322  of the lower electrode layer  320 , when the unit sensor is shown in a plan view. 
     The spacer layer  402  is disposed between the lower electrode layer  320  and the upper electrode layer  310 . The spacer layer  402  is provided at a predetermined area thereof with an opening  404 , through which the upper electrode  311  and the lower electrode  321  face each other. Consequently, the upper electrode  311  of the upper electrode layer  310  and the lower electrode  321  of the lower electrode layer  320  face each other through the opening  404  formed at the spacer layer  402 , and the capacitance is changed depending upon the increase or decrease in the distance between the upper electrode  311  and the lower electrode  321 . 
       FIG. 30A , in connection with  FIGS. 29A and 29B , shows that the spacer layer  402  is prepared by applying liquid-state polymer to a silicon substrate  310 , hardening the liquid-state polymer, forming a pattern using photolithography, forming the opening  404  through the polymer using dry etching, and removing the silicon substrate  360 . When the photolithography is used to form the spacer layer  402  as described above, high resolution of below 1 mm is accomplished. 
     Referring back to  FIGS. 29A and 29B , the insulating layer  410  is disposed between the upper electrode layer  310  and the spacer layer  402  to prevent the upper electrode  311  and the lower electrode  321  from contacting each other. The insulating layer  410  may be disposed between the lower electrode layer  320  and the spacer layer  402 . 
       FIG. 30B , in connection with  FIGS. 29A and 29B , illustrates that the insulating layer  410  is prepared by applying liquid-state polymer to a silicon substrate  360 , hardening the liquid-state polymer, and removing the silicon substrate  360 . 
     Referring back to  FIGS. 29A and 29B , the bump layer  400  is disposed on the upper electrode layer  310  such that pressure applied by a user can be reliably transmitted to the upper electrode layer  310 . 
       FIG. 30C , in connection with  FIGS. 29A and 29B , shows that the bump layer  400  is prepared by etching the silicon substrate  360  to form a bump mold  361 , applying liquid-state polymer to a silicon substrate  360 , on which the bump mold  361  is formed, hardening the liquid-state polymer, and removing the silicon substrate  360 . 
     Not only the substrates  312  and  323 , which are used for the upper electrode layer  310  and the lower electrode layer  320 , but also the spacer layer  402 , the insulating layer  410 , and the bump layer  400  may be made of polymer, such as silicon-based rubber or polyimide. 
     Spacer layer  402 , insulating layer  410 , upper electrode layer  310 , bump layer  400 , and lower electrode layer  320 , which are prepared as described above, are attached to each other, such that spacer layer  402 , insulating layer  410 , upper electrode layer  310 , and bump layer  400  are sequentially stacked on the lower electrode layer  320 , to manufacture the unit sensor as shown in  FIG. 30D . Attachment of spacer layer  402 , insulating layer  410 , upper electrode layer  310 , bump layer  400 , and lower electrode layer  320  is not particularly restricted. For example, the surfaces of spacer layer  402 , insulating layer  410 , upper electrode layer  310 , bump layer  400 , and lower electrode layer  320  to be attached are treated using oxygen plasma, and are then aligned with each other. Subsequently, the surfaces are attached to each other, and then the attached surfaces are heated to a temperature of 60 degrees C. for approximately 50 minutes. 
     The operation of the unit sensor of  FIGS. 29A and 29B  can be as follows. When a user pushes the bump layer  400 , the distance between the upper electrode  311  and the lower electrode  321  is decreased, and therefore, the capacitance between the upper electrode  311  and the lower electrode  321  is increased. As a result, a signal generated by the change of the capacitance is transmitted to an external circuit though the signal transmission line  322  connected between the external circuit and the lower electrode  321 . When the pressure applied to the bump layer  400  is released, the distance between the upper electrode  311  and the lower electrode  321  is increased, and therefore, the capacitance between the upper electrode  311  and the lower electrode  321  is decreased. As a result, a signal generated by the change of the capacitance is transmitted to an external circuit though the signal transmission line  312  connected between the external circuit and the upper electrode  311 . 
       FIG. 31  shows a tactile sensor according to yet another embodiment of the present invention, comprising a unit sensor array including a plurality of unit sensors with the above-stated construction, which are arrayed in two dimensions, with edge conductors or connection lines  406 . 
     The upper electrodes  311  of the unit sensors constituting the unit sensor array are electrically connected with each other by the sequential interconnection of the signal transmission lines  312  for the upper electrodes. The lower electrodes of the unit sensors constituting the unit sensor array are electrically connected with each other by the sequential interconnection of the signal transmission lines  322  for the lower electrodes. The connection lines  406  are disposed at the ends of the sequentially interconnected signal transmission lines  312  for the upper electrodes and the sequentially interconnected signal transmission lines  322  for the lower electrodes. 
       FIG. 32  shows that the tactile sensor can have high flexibility. 
     As described above, the tactile sensor can be manufactured using the flexible unit sensors, and therefore, the tactile sensor can be very flexible. Furthermore, an extension to the tactile sensor can be easily accomplished using the connection lines. 
     Embodiments of the present invention can have the following aspects. In the electrode layer and the unit sensor, the substrate can be formed using liquid-state polymer. Consequently, the electrode layer and the unit sensor are very flexible and easily manufactured. In addition, photolithography is used to form the spacer layer of the unit sensor, and therefore, high resolution of below 1 mm is accomplished. 
     Consequently, the tactile sensor according to the present invention can be applied to the soles of shoes as well as robots. In the case that the tactile sensor according to the present invention is applied to the soles of the shoes, the distribution of the pressure applied to the feet of a person when the person walks may be measured by the tactile sensor, and the measured data may be utilized medically. Also, the tactile sensor has soft tactile sensation. Consequently, the tactile sensor can provide various interfaces between a human and a computer as an extension to an input device, such as a mouse. 
     The above text has described several examples of various embodiments of the present invention. These examples and embodiments are meant to be illustrative, not restrictive, as all possible examples of the invention would be too numerous and cannot be included in this finite document. The scope of the invention is found in the claims which follow.