Patent Publication Number: US-6711948-B2

Title: Method and apparatus for testing of sheet material

Description:
RELATED APPLICATIONS 
     This application is a continuation of copending U.S. patent application No. 09/812,741, filed Mar. 20, 2001, now U.S. Pat. No. 6,443,002, which is a continuation of U.S. patent application No. 09/316,677, filed May 21, 1999, now U.S. Pat. No. 6,202,482, which is a continuation-in-part of U.S. patent application No. 09/274,487, filed Mar. 23, 1999, now U.S. Pat. No. 6,205,852, which claims priority from U.S. Provisional Patent Application No. 60/079,058, filed Mar. 23, 1998, which application is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This application relates to the field of testing of sheet materials, and in particular testing of thin glass sheets, such as glass sheets for use in computer flat panel displays, or for semiconductor wafers. 
     BACKGROUND OF THE INVENTION 
     Glass sheets or panels are conventionally maintained, after fabrication of the glass sheets, and before assembly into products, such as flat panel displays, in cassettes. Similarly, semiconductor wafers are placed in cassettes. Cassettes are essentially boxes sized to accommodate sheets or cassettes of a selected size. In one existing design of cassettes, there are provided projecting inward from the sides of the cassette panel supports. A defined distance separates the panel supports. The defined distance is selected to permit an end effector of a robot to pass between the panel supports, so as to remove or insert the panel in the cassette. 
     After fabrication, and after various steps during processing, panels, semiconductor wafers, and other materials in sheet form, are tested for a variety of physical, electrical, mechanical and chemical properties. Typically, upon fabrication, the panels or wafers are placed in the cassette by a robot with an end effector that engages the panel or wafer in such a manner as to minimize damage. When it is desired to test the panel or wafer, an end effector of a robot is inserted into the cassette, engages the panel or wafer, and transports the panel or wafer from the cassette to a testing device. The robot then places the panel or wafer on suitable supports on the testing device. A handler in the testing device moves the panel or wafer relative to test heads that carry out various tests on the panel or wafer. Upon completion of testing, the robot again engages the panel or wafer and removes the panel or wafer from the test equipment and returns it to the cassette. 
     This presents several difficulties. Testing time includes time to remove the panel from the cassette and transport it to the test device, and the time required to remove the panel from the test device and return it to the cassette. Each time the panel or wafer is engaged or disengaged by handling equipment, such as the robot end effector or the handler of the test device, there is a risk of damage. The time required to position the sample or sheet before measurement or testing and replace the sample after measurement or testing is increased by the need to have several devices successively engage and release the wafer or panel. 
     OBJECTS AND ADVANTAGES OF THE INVENTION 
     It is an object of the invention to provide a method and apparatus for testing of physical, chemical, electrical and mechanical properties of material in sheet form, such as panels and wafers, that reduces the process time associated with testing the material. 
     It is a further object of the invention to provide a method and apparatus for testing of material in sheet form that reduces the risk of damage associated with testing of the material. 
     It is an advantage of the invention that the foregoing objects are achieved. 
     Additional objects and advantages of the invention will become evident from a review of the detailed description which follows. 
     SUMMARY OF THE INVENTION 
     An apparatus for testing of material in sheet form includes a cassette adapted to store one or more sheets of material and one or more sensors rigidly mounted with respect to the cassette. 
     The sensors may be mounted adjacent a test location exterior to the cassette. The cassette and the sensors may be so configured and positioned that a suitable end effector may move sheets of material between storage locations in the cassette and test locations adjacent the sensors. 
     A method for testing sheet material includes the steps of placing the sheet material in a cassette, and conducting tests employing one or more sensors rigidly mounted with respect to the cassette. The method may include employing an end effector to remove the sheet from the cassette, to position the sheet stepwise in several positions relative to the sensors, and to replace the sheet in the cassette upon completion of testing. 
     An apparatus for testing of material in sheet form includes sensors that can be positioned adjacent to a surface of material in sheet form located in a cassette and supports positioned to reduce sag of the material. 
     A method for testing sheet material includes the steps of placing the sheet material in a cassette, and testing the sheet material while in the cassette. 
     A cassette according to the invention includes shelves having defined therein test heads for testing properties of material in sheet form. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a somewhat schematic drawing of an apparatus of the invention positioned relative to an exemplary cassette with an exemplary panel in the cassette. 
     FIG. 2 is a partial sectional view along line  2 — 2  of an apparatus of FIG.  1 . 
     FIG. 3 is a somewhat schematic representation of an alternative embodiment of an apparatus of the invention. 
     FIG. 4 is a partial plan view of the apparatus of FIG.  3 . 
     FIG. 5 is a somewhat schematic representation of an alternative embodiment of an apparatus of the invention. 
     FIG. 6 is a top plan view of a substrate of the invention of FIG.  5 . 
     FIG. 7 is a sectional view of the substrate of FIG.  6 . 
     FIG. 8 is a top view of an alternative embodiment of the substrate of FIG.  6 . 
     FIG. 9 is a sectional view taken along line  9 — 9  of FIG.  8 . 
     FIG. 10 is a bottom plan view of the substrate of FIG.  8 . 
     FIG. 11 is a plan view of an alternative design of the upper surface of a substrate of the invention. 
     FIG. 12 is a somewhat schematic representation of an alternative embodiment of the invention. 
     FIG. 13 is a top plan view of an alternative embodiment of a cassette shelf arrangement according to the invention. 
     FIG. 14 is a partial front cross-sectional view of the embodiment shown in FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to FIG. 1, there is shown an apparatus  10  according to the invention. Apparatus  10  is an end effector of a robot arm. More broadly, apparatus  10  is a movable unit. Apparatus  10  is generally a flattened end effector, relatively thin in height compared to length and width. Apparatus  10  is relatively thin in height in this embodiment to permit apparatus  10  to fit beneath a panel or wafer to be tested and over a lower panel or wafer supported in the cassette with adequate clearance. The height of apparatus  10  is dictated principally by the need to clear the next lower panel or wafer in the cassette. 
     Apparatus  10  has three projecting fingers  15 . Each finger  15  has a sensor or test head  20 . The number of fingers  15  may be varied as desired depending on the test pattern to be achieved. Each sensor  20  is suitable for positioning adjacent to a panel or wafer positioned in a cassette. Sensor  20  may be any one of numerous types of known compact test heads for testing properties of sheet materials. A wide variety of physical, chemical, mechanical and electrical properties of materials may be tested by use of suitable test heads. For example, the test heads may provide for eddy current sheet resistance testing. Photoreflectance testing may be provided. If photoreflectance testing is provided, then optical fibers are provided to the test heads. Suitable circuitry and wiring are provided in the body of apparatus  10  to permit communication between sensor  20  and controllers and data storage and read out devices. Communication may include control signals sent to sensor  20 , and data signals received from sensor  20 . Three sensors  20  are shown merely as examples. The number of sensors may be selected as desired, depending on the number of locations to be tested at any one time. 
     As an alternative, pairs of coplanar sensors and detectors for eddy current detection may be provided in place of each individual sensor  20 . 
     Disposed on opposite sides of sensor  20  are devices  25  for engaging a sheet of material. Devices  25  may be small vacuum heads, or vacuum hold downs, as are well known in the art. Devices  25  serve to prevent relative movement between the panel and sensor  20  while testing is carried out. Air lines (not shown) are provided between vacuum heads  25  and electrically controlled valves (not shown). Device  25  may also support the panel. As there may be sag in the panel, which is otherwise supported only by the panel supports, devices  25  may serve to reduce sag. This may also serve to reduce stresses in the panel that result from sag. 
     FIG. 1 also shows a cassette  30  having panel or sheet  35  therein. Cassette  30  is generally a box, having a planar horizontal bottom, planar vertical parallel sides, a planar horizontal top, and a planar back wall. A front opening is provided opposite the back wall. The term cassette as used herein includes any container for retaining and protecting multiple sheets of material, such as panels or wafers. Panel supports  40  project inward from the two side walls. FIG. 1 shows only one set of panel supports  40 . For convenience of viewing panel supports  40  are shown through panel  35 . Cassette  30  has numerous such supports. Numerous panels are supported in a horizontal position in cassette  30 . 
     In a method according to the invention, apparatus  10  is mounted on a robot arm. By robot arm, any device capable of supporting and precisely moving and locating apparatus  10  is meant. Apparatus  10  is positioned relative to a sheet  35  in cassette  30 . Initially, apparatus  10  is positioned so that testing may be conducted on sheet  35  at the set of locations closest to the opening of the cassette  30 , i.e., at locations  50 . Preferably, apparatus  10  is moved into position beneath sheet  35 , and then moved upward to contact sheet  35 , and then moved upward slightly further a distance sufficient to significantly reduce sag in sheet  35 . This last step may involve movement upward of about 5 mils, although the precise distance may vary depending on the distance between sheets and the susceptibility of the particular material and thickness to sag. Apparatus  10  remains with sensors  20  at locations  50 , and devices  25  holding sheet  35 , to prevent relative movement of sensors  20  to sheet  35 . After the test is completed, devices  25  release sheet  35 . Apparatus  10  is moved to the next selected set of locations. The process is repeated. Devices  25  engage sheet  35 . Sensors  20  carry out testing on sheet  35  at locations  55 . Devices  25  disengage sheet  35 , and the process is repeated until all desired locations are tested. Of course, the order of testing of various locations on the sheet may be varied. Apparatus  10  must be sufficiently long to permit sensors  20  to contact the desired test location closest to the back wall. 
     Fingers  15  are shown to provide a certain amount of independent vertical positioning for the three sensors  20 . Apparatus  10  may be constructed without fingers, and all sensors  20  supported on a single surface. 
     The number and relative location of sensors  20  may be selected as desired. 
     In summary, in the embodiment of FIGS. 1 and 2, there is provided a robot end effector  10  having one or more sensors  20  or test devices thereon. The end effector  10  is dimensioned to fit between sheets of material  35  in a cassette  30  for holding numerous flat sheets of material. The end effector&#39;s length is sufficient to provide testing over all or a substantial portion of a sheet fully inserted in the cassette without moving the sheet. Multiple sensors may be provided on the end effector, and devices may be provided to engage the sheet to prevent relative movement of sensors and sheet during testing. The end effector is dimensioned to fit between panel supports projecting from opposite sidewalls of the cassette. 
     In a method of the invention in accordance with the foregoing, an end effector with one or more sensors mounted thereon is inserted in a cassette holding sheets of material, and is moved relative to one of the sheets so that the sensor can test the material at selected locations on many points on the surface of the sheet. This process may be repeated for all sheets in the cassette. 
     Such an effector may be provided, for example, on a Gencobot 7 or 8 GPR robot, available from Genmark. 
     Referring now to FIGS. 3 and 4, there is shown an alternative end effector  110  according to the invention. End effector  110  is shown in FIG. 3 in proximity to cassette  130  which has an exemplary flat panel  135  to be tested located therein and supported on supports  140 . End effector  110  has an array of sensors  150 . Sensors  150  may be any suitable sensor or test head, as discussed above in connection with FIG.  1 . Sensors  150  are arranged linearly on each finger  115 . Vacuum hold downs  120  are provided adjacent to sensors  150 . Sensors are multiplexed to external control electronics and electronics for detecting and storing readings from sensors  150 . 
     In operation, end effector  110  is brought into contact with the sheet material by movement of a robot arm (not shown). End effector  110  is so positioned relative to the sheet material that sensors  150  are positioned adjacent to a substantial portion of the surface of the sheet material. As a result, in a single positioning, sensors  150  may conduct appropriate tests on the sheet material. Preferably, end effector  110  is moved vertically to engage the lower surface of the sheet material and move the sheet material slightly upward to reduce, and preferably to eliminate, sag in the sheet material. For example, end effector  110  may be moved upward about 5 mils after engaging the lower surface of the sheet material. Sag in the sheet material during testing is undesirable as such sag results in anomalous results in various tests. After the vertical movement, the vacuum hold downs  120  are engaged to maintain each sensor  150  stationary relative to the surface of the sheet material. The sensors  150  are then maintained in such proximity to test sites  155  to permit testing. The tests are then carried out. With multiplexing of the sensors, the tests are not necessarily carried out simultaneously, but in series along each finger of end effector  110 . Upon completion of the tests, the vacuum is released. End effector  110  is moved vertically downward until it is no longer in contact with the sheet material. End effector  110  is then moved outward from the opening of cassette  130 . End effector  110  is then positioned appropriately relative to the next sheet to be tested in cassette  130 , and the tests are conducted. The absence of a need for movement of the end effector during testing of a sheet improves throughput and reduces the risk of damage to the surface of the sheet material. 
     Referring to FIGS. 5,  6  and  7 , there is shown an alternative embodiment of an apparatus of the invention for in-cassette testing of sheet material. There is shown a cassette  230  according to the invention. Cassette  230  is essentially a rectangular box with an open front and walls on its other five sides. The walls may be made from conventionally used materials, for cassettes for use in storage of flat panels and wafers, such as metal or ceramics. Cassette  230  has a plurality of sheet shelves  240  for testing of sheet material. Each shelf  240  is a planar sheet of a rigid, non-contaminating material. For example, shelves  240  may be made of one of various ceramics. Shelves  240  may be of metal, such as aluminum; suitable insulation may be provided between the aluminum of the shelf and the conductors. It may be desirable to use heavier conductors than laminated copper for better shielding; if insulated wires are used, the conductive nature of the aluminum is less of a disadvantage. Each shelf  240  is rigidly supported on side walls of cassette  230 . Shelves  240  may be bonded to the side walls with an adhesive, be supported on projecting wires attached to the side walls, or otherwise be securely and rigidly supported. Each shelf  240  has a plurality of test heads or sensors  250  formed therein in an array. The array is preferably selected to be of a size relative to the surface of material to be tested to permit conducting of tests on test points including a substantial portion of the surface of the material. Sensors  250  are preferably recessed at the level of the upper, planar surface of shelf  240 . Sensors  250  are arranged in several lines, although the patterns of sensor locations may be varied. Vacuum hold downs  260  are provided in pairs adjacent each sensor  250 . Vacuum hold downs may be provided in smaller or larger numbers or in different locations as desired. Vacuum hold downs may be in the form of recesses or wells in the body of shelf  240 , which wells are in physical communication with a tube. Each shelf  240  preferably has defined therein one or more cutouts or recesses  270  intermediate rows of sensors  250 . Recesses  270  are defined to reduce the area of contact between shelf  240  and the sheet material. Recesses  270  also reduce the weight of shelf  240 , particularly intermediate the walls of cassette  230 . This reduction in weight tends to reduce sag of shelf  240 . 
     Exemplary wiring  280  is shown on the surface of shelf  240 . Wiring  280  may be placed on the lower surface of shelf  240 , or interior to shelf  240 , as desired. In fact, each sensor  250  may have control and readout lines associated therewith. Exemplary circuit boards  290 , on which appropriate control and memory electronics may be mounted, are shown immediately below each shelf  240 . The appropriate functionality to provide control and memory for test data for the sensors may be physically located elsewhere on the cassette, such as on an outer surface of a cassette wall. Alternatively, wiring  280  of shelves  240  may be electrically connected during testing to electronics mounted externally to cassette  230 . Suitable connectors may be provided on each shelf  240  for rapid connection and removal of connecting wiring. By employing externally mounted electronics, additional space is provided within cassette  230 . Rather than providing a single board  290  corresponding to each shelf  240 , two or more shelves  240  in cassette  230  may be multiplexed to a single board. 
     As many of the test devices or sensors provide analog data, the boards may include analog-to-digital converters to facilitate the exchange of information with digital devices. Alternatively, the boards may have solely analog electronics. The use of analog devices will reduce the complexity of the boards, although analog-to-digital converters will be required remotely. 
     If photoreflectance testing is to be provided, in place of wiring to the sensors, optical fibers may be provided. A pair, one for emission of radiation and one for detection of reflected radiation, may be provided at each test head. The optical fibers are preferably placed on the lower surface of the shelf or in cavities defined interior to the shelf. Optical emitters and detectors may be provided on the shelf itself, on an associated board, or remotely. 
     Vacuum hold downs have tubes or pipes leading thereto from electrically operated valves, which in turn are connected to a pump, vacuum manifold or the like. All hold downs on the surface of a shelf are preferably in physical communication with a single valve, so that all vacuum hold downs on a surface engage and disengage the sheet material simultaneously. The valves may be mounted on the cassette, or may be external. Appropriate couplings are provided for tubes or pipes leading from the vacuum hold downs. Couplings for vacuum lines may be provided to permit quick connection and release. 
     In an apparatus as shown in FIGS. 5-7, the sheet of material is placed in cassette  230  by a suitable robot end effector. The sheet is placed on the surface of the ceramic shelf  240  to provide contact with the vacuum hold downs  260 . If external electronics must be connected with the sheet, the connections are made. The vacuum hold downs  260 , as a result of a valve opening in response to a suitable signal from control board or from external electronics, engage the sheet. The test is then carried out During the testing, the test heads  250  are caused to emit suitable signals by control board  290  or external electronics, and to sense resulting fields, in accordance with well-known techniques Upon completion of the testing, any electrical connections can be removed. The sheets remain in the cassette, and the cassette can be transported to the location for the next processing step. 
     Referring now to FIGS. 8 and 9, there is shown an alternative embodiment of the shelf shown in FIGS. 5,  6  and  7 . Shelf  340  is configured for conducting tests that require sensor or emitter devices on opposing sides of a panel or wafer. Shelf  340 , as with shelf  240 , includes sensors or test heads  350 , and vacuum hold downs  360  located in recesses in the upper surface of the shelf, and recesses  370 . As shown in FIGS. 9 and 10, the lower surface of shelf  340  includes test heads or sensors  355  disposed in recesses formed in the lower surface. Test heads  355  are aligned with test heads  350 . Suitably designed test heads  350  on a first shelf and test heads  355  on a second shelf may be inductively coupled through a sheet or wafer to test various properties, in accordance with well-known techniques. Test heads  350  and test heads  355  may be designed in other manners to cooperate to achieve testing of various properties. For example, test head  350  may emit radiation, and test head  355  may detect radiation emitted by test head  350 , or radiation emitted by the material as a result of exposure to radiation emitted by test head  350 . Existing equipment designs need simply be miniaturized appropriately to fit in the recesses defined in the surface of shelf  340 . The design of FIGS. 8-10 may be readily incorporated in a cassette. 
     Also shown in FIGS. 9 and 10 are lines or tubes  375  running to vacuum devices or hold downs. These lines  375  all physically communicate to a single source. Lines  375  do not block sensors  355 . As noted above, a single valve may control communication with a vacuum manifold or pump. 
     Referring now to FIG. 11, there is shown in plan view an alternative embodiment of a shelf according to the invention. In this alternative embodiment, each test point  420  on shelf  440  includes two sensors  422 ,  424  that are positioned in sufficiently close physical proximity to one another to cooperate in testing materials. For example, both test devices  422 ,  424  may be coils that may be inductively coupled to each other. Such inductive coupling permits measurement of various properties. Exemplary wiring  480  is shown. Each test point  420  also includes pairs of vacuum hold downs or vacuum devices  450 . 
     Referring now to FIG. 12, there is shown in a somewhat schematic isometric view a combined cassette and test location unit  500 . Cassette  505  is a conventional cassette, in the form of a rectangular box with side walls  510  having sheet supports  515  projecting horizontally inward therefrom. Front opening  520  is defined in cassette  505 . Sheet supports  515  are so positioned to permit a suitable end effector to fit between vertically adjacent sheets and between the sheet supports  515 . Rigidly attached to the lower wall of cassette  505  is testing location  525 . Testing location  525  is also in the form of a rectangular box with a front opening. Testing location  525  includes test heads  530 , which are represented schematically. Test heads  530  are preferably contactless sensors for testing physical, mechanical, chemical or electrical properties of sheets placed in test location  525 . Such test heads are known in the art. For example, test heads  530  may be sheet resistance sensors, which are positioned in vertically aligned pairs at the front opening of test location  525 . Suitable electrical connections to controllers and to data recording software and hardware are provided in accordance with well-known techniques. 
     Also along the front opening of test location  525 , intermediate test heads  530 , there are provided vacuum chucks or vacuum hold downs  535 . Vacuum chucks  535  are positioned to engage sheets and hold the sheets motionless relative to test heads  530  during each testing step. Vacuum chucks  535  also minimize sag of the sheets during testing. Suitable vacuum lines are provided in accordance with well-known techniques. Inward of front opening of test location  525  there are located substrate supports  540 . The substrate supports  540  are elevated to support sheets when those sheets are placed in contact with vacuum chucks  535 . The substrate supports  540  are also sufficiently elevated above the floor of test location  525  to provide clearance for a suitable end effector. 
     Representative end effector  545  is schematically shown adjacent test location  525 . End effector  545  has two fingers  550 . Fingers  550  are so positioned and dimensioned, and test heads  530 , vacuum chucks  535 , and substrate supports  540  are all so positioned and dimensioned to permit fingers  550  to position a sheet on substrate supports  540  and vacuum chucks  535  in a position for testing by test heads  530 . 
     In a method of testing sheet materials using unit  500 , end effector  545 , or another robot end effector, is employed to load sheets into cassette  505 . End effector  545  then is inserted into cassette  505 , removes a sheet, and moves the sheet into test location  525 . When a desired position of the sheet in the horizontal plane relative to test heads  530  is reached, end effector  545  moves downward to cause the sheet to be supported on vacuum chucks  535  and substrate supports  540 . If desired, the sheet can be supported on end effector  545  as well. Vacuum chucks  535  then engage the sheet, and testing is carried out by test heads  530 . When testing is complete, vacuum chucks  535  disengage the sheet, and end effector  545  engages the sheet again. If measurement at additional locations is desired, end effector  545  moves in a suitable direction until the sheet is positioned properly relative to test heads  530 , and the above process is repeated This process is repeated at as many locations as desired. When the testing is complete, end effector  545  removes the sheet from test location  525  and returns the sheet to a storage location in cassette  505 . This process may be repeated for each sheet in cassette  505 . 
     The upper set of sensors may be mounted on a vertically movable mount. The mount may be guided on a vertical track. The mount in this configuration is moved vertically by a stepper motor. This configuration is desirable if the amount of sag in the sheet before engagement by the vacuum chuck is greater than the desirable separation between upper and lower sensors. If this configuration is employed, the upper set of sensors is in an upper position when insertion of the sheet by the end effector commences. After insertion of the sheet and engagement by the vacuum chucks, the upper set of sensors is lowered to a lower position. In the lower position, the separation between upper and lower sensors is selected such that any effects of the separation are minimized. 
     In the embodiment of FIG. 12, it is also possible to mount sensors so that they are movable in a horizontal direction. As it is not necessarily possible to move the sheet laterally within the test area, lateral movement of sensors will permit additional portions of the sheet surface to be tested. 
     It will be appreciated that the location of the sensors mounted relative to the cassette reduces the distance that sheets must be moved. Also, the use of the end effector, rather than a separate handler, at the test location, reduces the number of pieces of equipment involved and the number of items of equipment that must contact the sheet. 
     In the embodiment of FIG. 12, rather than a robot end effector, a handler positioned on an elevator may be employed to provide the required vertical and horizontal movement of the sheet into and out of the cassette, between the cassette and the test area, and to various positions within the test area. 
     Referring now to FIG. 13, there is provided a top view of an alternative shelf arrangement within a cassette. In this arrangement, there is provided a partial shelf  605  at only the opening of the cassette. The positions of sensors  610  and vacuum hold downs  615  are represented schematically. Suitable electrical connections for sensors  610  and air lines for vacuum hold downs  615  may be provided. Exemplary sheet supports  620  are also shown in the form of ledges protruding inward from the side and rear walls of the cassette. FIG. 14 is a partial front cross-sectional view along line  14 — 14  of FIG.  13 . FIG. 14 shows corresponding upper sensors  625 . In this embodiment, an end effector, engaging a sheet, moves the sheet to a desired location relative to sensors  610  and  625 . Vacuum hold downs  615  engage the sheet, and appropriate testing is conducted. Vacuum hold downs  615  then release the sheet, and the end effector moves the sheet to a next desired location. The process is repeated until a desired number of measurements have been made. As the sensors and hold downs are located only at the front opening of the cassette in this embodiment, repair and maintenance are simplified. In this embodiment, single sensors may also be employed if desired. 
     In the embodiments with the sensors in the substrate and in the end effector, either a coil cup, generally of metal foil, such as aluminum, may be used, or individual components may be separately fastened. A coil cup which supports all of the components is useful because it can be constructed so that each sensor may readily be mounted and removed for maintenance. 
     In any of the embodiments, it will be understood that heads which use microwave radiation to excite the carriers of electric current, induce localized heating, and utilize a thermographic imaging system to measure the physical, chemical, electrical and/or mechanical properties of the sample may be used. 
     It will be understood that testing is not confined to testing of wafers and panels merely after initial manufacture, but after any suitable stage in the process of manufacture of products incorporating wafers or panels. 
     It will be appreciated that the foregoing method and apparatus may be used in connection with testing any material in sheet form, and is not limited to use with the items, such as flat panels and semiconductor wafers, named as examples. 
     While an apparatus and method of the invention have been described with reference to particular embodiments, it will be understood that additional variations are within the scope of the invention.