Abstract:
A flexible probe card according to the present invention includes a compression layer; a transport layer coupled to the compression layer; and a contact layer coupled to the transport layer. The compression layer is formed of encapsulated closed cell polyurethane foam. The transport layer includes connectors for coupling the flexible probe card to a tester. The contact interface layer includes embedded conductive wires placed in a fixed grid pattern in a silicon rubber layer without a specific connector pattern associated either with the transport layer or a device under test.

Description:
RELATED APPLICATIONS 
       [0001]    The present application relates to and claims priority of U.S. provisional patent application (“Copending Provisional Application”), Ser. No. 62/297,701, filed on Feb. 19, 2016. The disclosure of the Copending Provisional Application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The present invention generally relates to testing of semiconductor devices, and, more particularly, to a method and test probe card for testing large format semiconductor devices, such as flat panel image sensor devices. 
         [0004]    Relevant Background 
         [0005]    Current state of the art methods of testing semiconductor devices require a means of providing temporary electrical contacts to the device under test without damaging the connection surfaces that will later be used in the device packaging after successful testing of device. 
         [0006]    Current Semiconductor Industry temporary test contact technologies consist of two basic methods including Probe Cards and Flexible Circuits. 
         [0007]    Probe Cards are now discussed. This test contact method utilizes a mechanical assembly that serves as an interface between an electronic test system and the Device Under Test, herein known as “DUT”. Typically the probe card is mechanically docked to a prober and electrically connected to a tester. Its purpose is to provide an electrical path between the test system and the circuits on the wafer, thereby permitting the testing and validation of the circuits at the wafer level, usually before they are diced and packaged. This probe head generally utilizes machined alloy housings combined with a rigid circuit board that will contain many individual mechanical pins that each serves as a contact point for a single device line to be tested. Each probe can contain as few as ten to twenty contact pins, or up to thousands of total pins depending on the specific application. Probe cards are broadly classified into three types: 
         [0008]    Needle type: These pins are made from wire with bends to provide upward resisting spring force when moved into contact with the DUT. 
         [0009]    Vertical type: These pins are also made from wire but are placed in a vertical orientation with the beam rigidity of the wire serving as the spring loading force for the pin contacts. These types of cards generally have a design where each pin will have its own spring loaded housing, also known as pogo pins. 
         [0010]    MEMS (Micro Electro-Mechanical System) type depending on shape and forms of contact elements. MEMS type is the most advanced technology currently available. This most advanced type of probe card currently can test an entire 12″ wafer with one touchdown. The micro-machined nature of this type of contact method makes this solution very expensive. 
         [0011]    Because each probe card is manufactured to very exact tolerances for pin placements in all axes, all three types of probe cards are very expensive to build and to maintain and are not scalable to different product sizes without all new tooling. Additionally, manufacturers will typically require a new probe card for each new device to be tested due to the unique footprint for each product layout; this can be a very significant cost factor when many different product layouts must be tested. 
         [0012]    Flexible Circuits are now discussed. This test contact method utilizes a Polyimide based flexible circuit with exposed trace lines that are pressed against the DUT contact lines by applying downward pressures using an air initiated or solid tube. This temporary contact then serves as an interface between the electronic test system and the DUT. Typically the flexible circuit is held in position by being glued to a mechanical block that has the ability to be correctly positioned using extraneous hardware incorporated with the block mounting design. Each flexible circuit can contain as few as 80 contact trace lines or up to 256 lines depending on the specific application. This design is very low cost per contact line when compared to probe cards. This design also offers advantages of having the ability to replace and align an individual flex assembly very quickly when compared to probe card based test heads. This design approach also can have numerous disadvantages: 
         [0013]    Poor Contact Performance related to surface area. Since electrical contact is being achieved by pressing together two basically flat surfaces, intermittent contact performance can be problematic. A general rule of thumb is that as contact surface area decreases, so does contact performance. 
         [0014]    Poor Contact Performance related to deterioration or damage of contact trace lines on product. The flexible circuits used for temporary test contact can become contaminated or damaged, or in some cases be very abrasive/rough, which in turn will damage the contact trace surfaces of DUT which greatly limits the number of head down contacts that result in adequate test contact performance needed to successfully complete testing. 
         [0015]    Poor Contact Performance related to alignment of flex assemblies. For each test head aligned there can be reduced contact performance due to positioning differences between flex assemblies. The more accurate the alignment of the entire group of flexes per test head, the greater the tool positioning error that can be tolerated and still achieve usable contact performance. A general rule of thumb is: as the pitch and line sizes are reduced, the greater the accuracy required for flex alignments. Most 100 to 120 micron pitch flex based test heads can function with tool accuracy at ±10 microns. Recent 75 micron pitch products require tool positioning accuracy less than or equal to 5 microns to achieve usable contact performance. 
         [0016]    A final disadvantage is that Intensive Manual Alignments are required. With this current contact method it is required that manual mechanical alignments be performed for each contact trace tab group that exists on the product layout. This can range in tab group counts from as few as 16 to as many 104 individual alignments required for each product specific test head assembly. These alignments are generally repeatable after performing replacement of a test head for product swap, but at the least requires extra maintenance intervention time needed to verify these individual alignments. 
         [0017]    What is desired is a new test method and test probe card that remedies the disadvantages found in the prior art methods and test probe cards as discussed above. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention as described below provides temporary test contacts on high density contact features across large substrates with contact along on one or more of the perimeter edges. The test methods described below have the precision and performance of current probe card contact methods, but with much easier scalability and at a much lower cost than current wafer centric probe card technologies. 
         [0019]    A Flexible Probe Card according to the present invention comprises of three main components that are laminated together with adhesive to function as one assembly. The Flexible Probe Cards according to the present invention have contact features patterned on a flexible circuit to exactly reproduce the contact features that are patterned on the product layouts being tested. Contact interface is performed by Z-axis conductors that are laminated to the flexible circuit board and allow signals to be passed from DUT to the patterned features on the flexible circuit board. Contact force is applied by a polyurethane backing layer that also corrects for flatness errors during probe card contacts. 
         [0020]    A flexible probe card according to the present invention comprises a compression layer; a transport layer coupled to the compression layer; and a contact layer coupled to the transport layer. The contact layer is in contact with a device under test, which can comprise, for example, an image sensor device formed on a glass substrate. The compression layer can comprise encapsulated closed cell polyurethane foam. The transport layer comprises a plurality of connectors for coupling the flexible probe card to a tester. A spacing of a plurality of connectors on the transport layer mirrors a spacing associated with a plurality of connectors on the DUT. The contact interface layer comprises a plurality of conductive wires placed in a fixed grid pattern in a silicon rubber layer. The fixed pattern dimensions of the contact layer are of a sufficiently smaller pitch than the features present on the contact layer and the DUT. This pattern pitch reduction at the contact layer is what allows the signals to be passed only between the desired contact traces according to the present invention. The conductive wires can comprise gold, copper, or CuZn. 
         [0021]    The flexible probe card of the present invention can be part of a testing system according to the present invention comprising a flexible probe card comprising a compression layer, a transport layer, and a contact layer; a device interface board (“DIB”) in electrical communication with the flexible probe card; and a test control system in electrical communication with the device interface board. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
           [0023]      FIG. 1  illustrates a first cross-sectional view of a flexible probe card in contact with the DUT according to the present invention; 
           [0024]      FIG. 2  illustrates a second cross-sectional view of a flexible probe card in contact with the DUT according to the present invention; 
           [0025]      FIG. 3  further illustrates the compression layer according to the present invention; 
           [0026]      FIGS. 4A and 4B  further illustrate the transport layer according to the present invention; 
           [0027]      FIG. 5  further illustrates the contact layer according to the present invention; 
           [0028]      FIGS. 6A and 6B  illustrates a final assembly overview of the flexible probe card according to the present invention; 
           [0029]      FIG. 7  illustrates in an exploded view a final assembly shown to perform temporary electrical contact on the DUT and to pass signals to the DIB according to the present invention: 
           [0030]      FIG. 8  illustrates a block diagram of a testing system according to the present invention; and 
           [0031]      FIGS. 9-12  illustrate a test environment for implementing the test method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    The flexible probe card according to the present invention comprises three main components that are laminated together with adhesive to function as one assembly: 
         [0033]    Backing or Compression Layer: This is the base layer for the assembly, and is custom manufactured for dimensions using an encapsulated closed cell polyurethane foam or equivalent that has all cut edges sealed for uniformity and reduced shedding. One material that can be used for this application is Poron microcellular urethane foam available from Rogers Corporation. This base layer is intended to provide two different solutions in the overall design of the Flexible Probe Card: first, it will provide the ability for the Flexible Probe Card to conform to the shape and flatness errors that exist along the length of the contact tab sets of the image sensor product glass substrate as it rests on a quartz test stage. Secondly, it will provide the compression needed to press conductors against the DUT contact tab sets. The compression factor of this material is very light when used with smaller overdrive (amount of travel after first physical contact of probe card is achieved) dimensions but this hardness, or compression factor, increases exponentially as the over travel range increases. 
         [0034]    Transport Layer: This is the middle layer of the assembly and contains the connectors needed to interface probe cards to a Tester, and will also contain the identically patterned features that exist as the contact tab patterns present on product layouts. This material is best described and marketed as flexible printed circuit boards and is currently available, but currently not used or exploited for the application and method describe herein. There are multiple vendors providing this material in the marketplace currently. One such material is, for example, Dupont Pyralux flexible circuit material. 
         [0035]    Contact Interface Layer: This layer provides the temporary contact surface which contacts the DUT trace lines. This technology is marketed as a Z-axis conductor, and is a series of conductive (ex. Au, Cu, CuZn, etc.) wires that are placed in a fixed grid pattern in a block of silicon; this block is then sliced into very thin layers (0.25 to 2.5 mm for average listed dimensional ranges). The conductive wires suspended in silicon film will then transfer electrical contacts in a Z-axis as long as there are conductive surfaces present both above and below this layer. An example of the material identified as is ShinEtsu Z-axis conductor material. One example of the conductor material includes 20 to 50 micron wire diameters placed at a 50 to 100 micron pitch in a square pattern. The suspension material is ideally Silicon rubber. The density of wires for a 30 micron wire on a 75 micron pitch, as an example, would be approximately 145,000 contacts per square inch. Those skilled in the art will realize that other dimensions and materials can be used in the contact interface layer. 
         [0036]    Referring now to  FIG. 1 , a first cross-sectional view of a flexible probe card  100  in contact with a DUT according to the present invention is shown. A compression layer  102  is shown laminated to transport layer  104 , including a transport layer trace pattern  110 . A contact layer  106  is laminated to transport layer  104 , including a plurality of Z-axis conductors  112  as previously described in a silicon rubber layer  114  as was also previously described. The Z-axis conductors contact the DUT  108 , which includes a DUT trace pattern  116  as shown. It will be apparent to those skilled in the art that the Z-axis conductors as shown are not necessarily to scale and the density of these Z-axis conductors can be different than as depicted in  FIG. 1 . 
         [0037]    Referring now to  FIG. 2 , a second cross-sectional view of a flexible probe card  200  in contact with a DUT according to the present invention is shown. The compression layer is omitted for sake of clarity in this view. Transport layer  204 . however, is shown including a transport layer trace pattern  210 . A contact layer  206  is laminated to transport layer  204 , including a plurality of Z-axis conductors  212  as previously described in a silicon rubber layer as was also previously described. The Z-axis conductors contact the DUT  208 , which includes a DUT trace pattern  216  as shown. It will be apparent to those skilled in the art that the Z-axis conductors as shown are not necessarily to scale and the density of these Z-axis conductors can be different than as depicted in  FIG. 2 . 
         [0038]    Referring now to  FIG. 3 , the un-laminated compression layer  302  is shown in greater detail. In an embodiment. typical dimensions for compression layer  302  include a width of 75 to 100 mm, a length of 130 to 430 mm, and a thickness of 1.0 to 3.5 mm. 
         [0039]    Referring now to  FIGS. 4A and 4B , the un-laminated transport layer shown includes a base layer  402  made of the flexible material previously discussed, as well as top and bottom views of a trace pattern  404  that mirrors the trace pattern of the DUT. In an embodiment, typical dimensions of the base layer  402  include a width of 75 to 100 mm, a length of 130 to 430 mm, and a thickness of 1.0 to 3.5 mm. The height of the portion of the trace pattern  404  extending above the plane of the base layer  402  is about 0.000 to 0.025 mm, and the spacing between individual traces in trace pattern  404  is about 0.030 to 0.075 mm. 
         [0040]    Referring now to  FIG. 5 , the contact layer  500  is shown as including a silicon rubber layer  502  about 25 mm thick as previously discussed, including Z-axis conductors  504  with a density of about 145,000 contacts per square inch as previously discussed. In  FIG. 5 , both a cross-sectional view of the contact layer  500  is shown, as well as plan view of a portion of the contact layer is shown in greater detail. 
         [0041]    Referring now to  FIGS. 6A and 6B , a final assembly overview  600  of the flexible probe card of the present invention is shown. The two main components shown are the compression layer  602 , the transport layer  604 , and the contact layer  606 . Final assembly is accomplished using adhesive lamination. 
         [0042]    Referring now to  FIG. 7 , a final assembly is shown to perform temporary electrical contact on the DUT and to pass the signals to the DIB. Depicted separately in  FIG. 7  are the DIB  702 . the flexible test card  704 , and the DUT  706  according to the present invention. 
         [0043]    Referring now to  FIG. 8 , a flexible probe card  806  according to the present invention is shown in a test system  800  including a test control system  802  in communication with a device interface board  804 , which is in turn in communication with the flexible probe card  806 . The flexible probe card is used to test the DUT  808  as was previously discussed. The test control system  802  can be, for example, a Windows-based PC with imaging software, such as VIVA software from Varian Medical Systems, and is responsible for image acquisition, control, and post processing in test system  800 . The device interface board  804  can be, for example, a Varian Paxscan 4336W Digital Image Receptor, and is responsible for timing and control of. as well as interpreting signal levels on individual read-out lines in test system  800 . 
         [0044]    The method of the present invention is now explained, and also particularly with reference to drawings  FIGS. 9-12  in further detail below. In general, the flexible probe card of the present invention is mounted to the bottom of a test fixture frame, and cable to the DIB. The DUT is positioned on a quartz stage. The quartz stage is located so that the DUT is located directly underneath the test fixture frame. The test fixture framework is lowered to a point just before contacting the DUT, which allows a bottom-up view from cameras located underneath the quartz stage. The bottom-up view includes both the contact features of the DUT and the contact features of the flexible probe card of the present invention. This test intermediate test position is used for alignment of the flexible probe card of the present invention and also for fine positioning of the DUT before testing. After proper alignment has been completed. the test fixture frame is moved into final test position. A typical range downward travel after initial contact with the DUT is about 0.020 to 0.150 mm. The position of the test fixture after contact combined with the compression factors of the polyurethane compression layer provide all of the downward force required for successful test contact performance. 
         [0045]    Referring now to  FIG. 9 , a bottom-up view of a test fixture  900  according to the present invention includes a test fixture frame  902 , a flexible probe card  904 , interconnects  906 , and a device interface board  908 . The flexible probe card  904  is mounted in front of the DIB  908 . Flex connectors  906  that had been used for providing contacts to the DUT are now used as interconnects between the flexible probe card  904  and the DIB  908 . Exposed traces on the DUT (not shown in  FIG. 9 ) are contacted by the front edge of the flexible probe card assembly. Compression against the DUT for temporary electrical contacts is achieved by vertical travel of the test fixture and the compression factors of the polyurethane compression layer of the flexible probe card  904 . 
         [0046]    The flexible probe card according to the present invention can be mounted along one perimeter edge, or on all four edges if desired. The flexible probe card can be made modular or scalable to accommodate a wide range of testing environments and applications. Also, the flexible probe card according to the present invention in application is not limited to testing glass panel devices. The flexible probe card could be used to provide temporary test contacts on wide range of manufactured devices such as flexible devices or any device with a circuit card face to contact. Referring to  FIG. 9 , the bottom up view shows one flexible probe card for contacting the DUT, which can be expanded to incorporate a flexible probe card along any or all edges of the DUT. Ideally, there is only one flexible probe card per side used for reduced build costs and reduced setup and maintenance times. 
         [0047]      FIGS. 10-12  show cross-sectional views of the test fixture of the present invention.  FIG. 10  shows the test fixture  1000 A in an intermediate position wherein the flexible probe card of the present invention is not in contact with the DUT.  FIGS. 11 and 12  show the test fixture  1000 B and  1000 C in a final contact position wherein the flexible probe card of the present invention is in electrical contact with the DUT. 
         [0048]    Referring now to  FIG. 10 , a test fixture frame  1002 , a flexible probe card  1004 , a DIB  1008 , a DUT  1010 , a quartz stage  1012 , and a video alignment camera  1014  are shown in the intermediate position. 
         [0049]    Referring now to  FIG. 11 , the same test fixture elements are shown in the final electrical contact test position described above. 
         [0050]    Referring now to  FIG. 12 , the same test fixture elements are shown in the same final test position, however, the electrical signal path between the flexible probe card  1004 , the DIB  1008 , and a test control system  1016  are also shown. 
         [0051]    It is an advantage of the present invention that there is, with respect to prior art test methods, low cost per contact line, more margin in chuck and substrate height variation, better alignment capability on tighter pitch, easily scalable to large area (greater than 300 mm) substrates, and is well suited for testing devices with interconnects on more than one side. 
         [0052]    The flexible probe card according to the present invention serves as a low cost alternative to traditional mechanical probe cards with improved contact performance compared to current low cost flexible circuit temporary contact solutions. 
         [0053]    In summary, the Flexible Probe Card of the present invention comprises multiple layers that work together to compensate for non-flatness errors of DUT stage systems. An example system embodiment according to the present invention includes a Base layer made from closed cell Polyurethane, a Transport layer made from flexible circuit board material, and a contact layer that uses a Z-axis interconnect o provide electrical contact between a DUT and the transport layer circuit board. 
         [0054]    The Flexible Probe Card design according to the present invention realizes significant cost savings potential from using patterned circuit board for mechanical accuracy rather than complicated mechanical build and alignment of traditional probe cards. 
         [0055]    Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed. As but one example, many of the dimensions described herein can be changed to conform to a particular application. As another example, the materials described herein can be substituted to conform to a particular application. Finally, the exact test fixture configurations described herein can also be changed to accommodate various DUTs as required.