Abstract:
An interference device to communicate electrical signals from a probe card used to test electronic circuits. The interface device includes at least one interposer configured to electrically couple to the probe card and a plurality of mechanical springs mechanically coupled to the at least one interposer. Each of the plurality of mechanical springs is removably arranged such that one or more of the plurality of mechanical springs may be removed. A flexible circuit is electrically coupled to the plurality of mechanical springs. The flexible circuit is further configured to mechanically couple to the at least one interposer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 60/862,883 entitled “Low Cost, High Pin Count, Wafer Sort Automated Test Equipment (ATE) Device under Test (DUT) Interface for Testing Electronic Devices in High Parallelism” filed Oct. 25, 2006 which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention is related generally to electronic device testing. More specifically, the present invention is related to device-under-test (DUT) interface for mating to a probe card used in testing electronic devices. 
       BACKGROUND 
       [0003]    Complexity levels of electronic device testing vary tremendously, from simple manual low-volume/low-complexity testing performed with perhaps an oscilloscope and voltmeter, to personal computer-based medium-scale testing, to large-scale/high-complexity automated test equipment (ATE). Manual and personal computer-based testing are typically applied to testing discrete devices, specific components of an integrated circuit, or portions of a printed circuit board. In contrast, ATE testing is used to test functionality of a plurality of complex integrated circuits such as memory circuits or hundreds of dice on a wafer prior to sawing and packaging. 
         [0004]    When testing ICs on a wafer, it is cost effective to test as many devices as possible in parallel, thus reducing the test time per wafer. Test system controllers have evolved to increase the total number of channels and hence the number of devices that can be tested in parallel. However, a test system controller with increased test channels is typically a significant cost factor for a test system, as is a probe card with complex routing lines used to accommodate multiple parallel test channels. Thus, an overall probe card architecture that allows increased test parallelism without requiring increased test system controller channels and without increased probe card routing complexity and cost is desirable. 
         [0005]      FIG. 1  shows a block diagram of an automated test system  100 . The test system  100  includes a test system controller  101 , a test head  105 , and a test prober  107 . The test system controller  101  is frequently a microprocessor-based computer and is electrically connected to the test head  105  by a communication cable  103 . The test prober  107  includes a stage  109  on which a semiconductor wafer  111  may be mounted, and a probe card  113  for testing DUTs on the semiconductor wafer  111 . The stage  109  is movable to contact the wafer  111  with a plurality of test probes  115  on the probe card  113 . The probe card  113  communicates with the test head  105  through a plurality of channel communications cables  117 . 
         [0006]    In operation, the test system controller  101  generates test data which are transmitted through the communication cable  103  to the test head  105 . The test head in turn transmits the test data to the probe card  113  through the plurality of communication cables  117 . The probe card then uses these data to probe DUTs (not shown explicitly) on the wafer  111  through the plurality of test probes  115 . Test results are then provided from the DUTs on the wafer  111  back through the probe card  113  to the test head  105  for transmission back to the test system controller  101 . Once testing is completed and known-good dice are identified, the wafer is  111  diced. 
         [0007]    Test data provided from the test system controller  101  are divided into individual test channels provided through the communications cable  103  and separated in the test head  105  so that each channel is carried to a separate one of the plurality of test probes  115 . Channels from the test head  105  are linked by the channel communications cables  117  to the probe card  113 . The probe card  113  then links each channel to a separate one of the plurality of test probes  115 . 
         [0008]    With reference to  FIG. 2  and continued reference to  FIG. 1 , a cross-sectional view of components contained within the probe card  113  indicate routing of mechanical and electrical couplings between the wafer  111  and the test head  105 . The probe card  113  provides both electrical pathways and mechanical support for the plurality of test probes  115  that will directly contact the wafer  111 . Electrical pathways on the probe card  113  are provided through a printed circuit board (PCB)  201 , an interposer  203 , and a space transformer  205 . Test data from the test head  105  are provided through the channel communications cables  117  typically connected around the periphery of the PCB  201 . A plurality of channel transmission lines  207  distribute signals from a plurality of electrical interconnects  209  (only two electrical interconnects are shown for clarity) mounted on the PCB  201  to match the routing pitch of pads on the space transformer  205 . The interposer  203  includes a substrate  211  with a plurality of spring probe electrical contacts  213  disposed on both sides. The interposer  203  electrically connects individual pads on the PCB  201  to pads forming a land grid array (LGA, not shown explicitly) on the space transformer  205 . A plurality of electrical traces  215  in a substrate  217  of the space transformer  205  distribute or “space transform” connections from the LGA to the plurality of test probes  115 , configured in an array. The space transformer substrate  217  is typically constructed from either multi-layered ceramic or organic-based laminates. The space transformer substrate  217  with embedded circuitry, probes, and LGA is referred to as a probe head. 
         [0009]    Mechanical support for the electrical components is provided by a back plate  219 , a probe head bracket  221 , a probe head stiffener frame  223 , a plurality of leaf springs  225 , and leveling pins  227 . The frame  223  surrounds the probe head and maintains a close tolerance to the bracket  221  such that lateral motion is limited. 
         [0010]    The leveling pins  227  complete the mechanical support for the electrical elements and provide for leveling of the space transformer  205 . The leveling pins  227  are adjusted so that brass spheres  229  provide a point contact with the space transformer  205 . The spheres  229  contact outside the periphery of the LGA of the space transformer  205  to maintain electrical isolation from electrical components. Motion of the leveling pins  227  is opposed by the plurality of leaf springs  225  so that the spheres  229  are kept in contact with the space transformer  205 . 
         [0011]    The complexity of the automated test system  100  an the probe card  113  demonstrates an inherent potential problem in contemporary ATE systems. For example, a critical component of the probe card  113  is the plurality of electrical interconnects  209 . All generated test data and resulting DUT data are funneled through the plurality of electrical interconnects  209 . In contemporary ATE systems, the plurality of electrical interconnects  209  are designed using pogo pins, coaxial cables, zero insertion force clamp assemblies, or other expensive interconnect technologies. Pogo pins suffer from reliability problems associated with repeatable contact resistance. Coaxial cables are large in diameter and can not be contained in a small volume of space. Zero insertion force clamp assemblies are mechanically complex and mechanics associated with operations of the assembly occupy valuable real estate which could otherwise be used for more interconnects. 
         [0012]    Therefore, what is needed is a simple, economical, and robust means of interacting bidirectional electrical signals between the test head and probe card. Such an interface should have individually replaceable contact points with a sufficient z-dimension deformational stroke to allow for slight misalignment errors or irregularities in the surface of the probe head. Further, the interface should reduce a deflection in the probe card by minimizing an applied load to compress the contact points. The reduced deflection allows a large contactor array to be mounted on the probe card, further increasing parallelism. 
       SUMMARY OF THE INVENTION 
       [0013]    In an exemplary embodiment, the present invention is an interface device to communicate electrical signals from a probe card used to test electronic circuits. The interface device includes at least one interposer configured to electrically couple to the probe card and a plurality of mechanical springs mechanically coupled to the at least one interposer. Each of the plurality of mechanical springs is removably arranged such that one or more of the plurality of mechanical springs may be removed. A flexible circuit is electrically coupled to the plurality of mechanical springs. The flexible circuit is further configured to mechanically couple to the at least one interposer. 
         [0014]    In another exemplary embodiment, the present invention is an interface device to communicate electrical signals from a probe card used to test electronic circuits. The interface device includes a plurality of interposers configured to be electrically coupled to the probe card and a plurality of mechanical springs mechanically coupled to each of the plurality of interposers. Each of the plurality of mechanical springs is at least partially formed from an electrically conductive material. Each of the plurality of mechanical springs further has a stroke of at least 100 μm and is removably arranged such that one or more of the plurality of mechanical springs may be removed for replacement. A plurality of flexible circuits is configured to be mechanically coupled to select ones of the plurality of interposers and electrically couple to select ones of the plurality of mechanical springs. 
         [0015]    In another exemplary embodiment, the present invention is an interposer for communicating electrical signals from a probe card used to test electronic circuits. The interposer includes a plurality of mechanical springs mechanically coupled to the interposer and arranged in a matrix. Each of the plurality of mechanical springs is at least partially formed from an electrically conductive material. Each of the plurality of mechanical springs further has a stroke of at least 100 μm and is removably arranged such that one or more of the plurality of mechanical springs may be removed for replacement. At least one flexible circuit is configured to be mechanically coupled to the interposer and electrically coupled to select ones of the plurality of mechanical springs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a block diagram of an ATE system of the prior art. 
           [0017]      FIG. 2  is a cross-sectional view of a prior art probe card contained in the system of  FIG. 1 . 
           [0018]      FIG. 3  is a simplified block diagram of an exemplary embodiment of the present invention. 
           [0019]      FIG. 4  is a perspective view of an exemplary probe card of the present invention. 
           [0020]      FIG. 5  is a plan view of an exemplary interposer of the present invention used to interface electrical signals to the probe card of  FIG. 4  through a plurality of mechanical springs. 
           [0021]      FIG. 6  is an elevation view showing a specific embodiment with optional daughter cards mounting on the probe card of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    With reference to  FIG. 3 , a portion of a DUT interface  300  includes a probe card  301 , a probe card interposer  303 , and a mechanical backing plate  305  electrically and mechanically connected to a flex circuit  309 . Electrical communication is provide between the mechanical backing plate  305  and the probe card interposer  303  through a plurality of mechanical springs  307 . Ideally, each of the plurality of mechanical springs  307  should be designed to be individually field-replaceable with a sufficient stroke (i.e., greater than 100 μm) to allow for any misalignment error or surface irregularities between the probe card interposer  303  and the mechanical backing plate  305 . Additionally, the plurality of mechanical springs  307  may be mounted on either the probe card interposer  303 , the mechanical backing plate  305 , or both (i.e., a spring-to-spring contact). No connectors are required to be mounted directly on the probe card  301 . Each of the plurality of mechanical springs may take various forms known in the art and include various compressional spring types such as volute, helical, coil, cantilever, or leaf springs. Both macro-mechanical and micro-mechanical methods for producing various forms of spring elements are also known in the art. The probe card interposer is described in more detail with reference to  FIG. 5 , below. 
         [0023]    The flex circuit  309  may either be a simple flat cable interconnect or it may be a flexible electronic interconnect containing active and passive device circuitry. Flex circuits of the latter type involve fabricating various device types on plastic, such as a polyethyleneterephthalate (PET) substrate. PET substrates are commonly employed in lightweight circuit applications, such as a cellular telephone or personal data assistant (PDA). Such circuits are known in the art and electronic devices are formed on, for example, a PET substrate deposited with silicon dioxide and polysilicon followed by an excimer laser annealing (ELA) anneal step. In a simple case, flexible electronics can be made using similar components used on rigid printed circuit boards. 
         [0024]    The flex circuit  309  is electrically and mechanically connected to a pin electronic board  313  through a pin board interposer  311 . The pin board interposer  311  is fastened to the pin electronic board  313  by, for example, mechanical fasteners  315 . The mechanical fasteners  315  may be screws, rivets, wire bails, or other fastening means known in the art. 
         [0025]    The flex circuit  309  may be routed to the probe card  301  and bent by, for example, 90 degrees to lay substantially horizontal to a plane of the probe card  301 . The probe card interposer  303  may be placed on top of the flex circuit  309 . The probe card interposer  303  may be floating on top of mechanical springs to allow greater compliance along the vertical axis. The probe card  301  is clamped against or is otherwise attached to the mechanical backing plate  305 , which applies a load required to compress the probe card interposer  303 . The probe card interposer  303  may have its own set of springs to allow compression over an interface area of the probe card due to any surface irregularities or warpage of the probe card  301  caused by load and temperature. Hence, mechanical springs may be used to allow each of a plurality of the probe card interposers  303  to float individually. 
         [0026]    In  FIG. 4 , a top perspective view of the probe card  301  showing an exemplary arrangement includes a plurality of probe card interposers  303  and a plurality of optional daughter board edge connectors  401 . Mounting of the daughter board edge connectors  401  is described in more detail with reference to  FIG. 6 , below. A probe tip  403  is located on the bottom side of the probe card  301 . Due to the relatively small size of each of the plurality of probe card interposers  303 , a larger probe tip  403  array may be used to contact more devices on a substrate (e.g., a wafer) in parallel. In a specific exemplary embodiment, there are 64 probe card interposers  303  DUT interface and an equal number of daughter board edge connectors 401 interspersed with the probe card interposers  303 . Other arrangements and numbers could readily be envisioned by a skilled artisan based on layouts disclosed herein. Continuing with the specific exemplary embodiment, a diameter, D pc , of the probe card  301  is 510 mm and a diameter, D pt , of the probe tip is 400 mm. Each of these dimensions may be changed based on relative sizes needed for substrates probed (e.g., probing a next generation silicon wafer may require a 450 mm probe tip  403 ). 
         [0027]    In  FIG. 5 , a detailed plan view of the spring contact side of the probe card interposer  303  includes a plurality of mechanical springs  307 . In a specific exemplary embodiment, the plurality of mechanical springs is laid out in an 8 column by 52 row matrix (thus, there are 416 mechanical springs  307 ). The matrix has a 1 mm by 1.25 mm pitch respectively for the columns and rows. Consequently, an overall size of the probe card interposer  303  is 9 mm×66.25 mm. 
         [0028]    Depending upon spring type chosen, each of the plurality of mechanical springs  307  requires a force of only about 30 N (Newtons) or a total force of 12,480 N per each of the probe card interposers  303 . Thus, the total force on a probe card is significantly less than required under the prior art, allowing more interconnects to be used per probe card with less overall deflection. One field-replaceable spring type that may be used with the present invention is employed in the InterCon cLGA® land grid array socket system (manufactured by Amphenol InterCon Systems, Inc., Harrisburg, Pa.). The Amphenol spring has a beryllium copper base with a gold over nickel-plated overcoat. 
         [0029]    Significantly, the probe card interposer  303  allows for a much smaller footprint than the prior art since ZIF connectors or similar large and expensive connectors are not required. Thus, more interposers may be used, allowing a higher number of DUTs to be tested in parallel. The probe card interposer  303  may be mounted and remain permanently on the probe card  301  ( FIG. 3 ). In the specific exemplary embodiment described above with 64 probe card interposers  303  per probe card  301  and 416 mechanical springs  307  per interposer, a total of 26,624 signal I/O, ground, power, and sense locations are available per DUT interface. 
         [0030]    In  FIG. 6 , an elevation view of a portion of the probe card  301  indicates how each optional daughter card  601  may be mounted orthogonally through the use of the edge connector  401 . Contacts on the edge connector  401  (not shown) electrically connect to a plurality of edge fingers  603  mounted on an edge of the daughter card  601 . In a specific exemplary embodiment, the optional daughter card  601  is 6 mm thick with an area of 25 mm by 45 mm. Electronic devices may be mounted on one or both sides of the daughter card  601 . 
         [0031]    The probe card interposers  303  of the present invention provide significant advantages over the prior art. For example, due to the relatively small size of the probe card interposers  303 , a large printed circuit board may be employed for improved routing of high frequency traces, more space is available for mechanically clamping the probe card to a DUT interface, and sufficient volume is present to mount a large number of orthogonal daughter boards on the tester side of the probe card. Daughter boards may be used for additional circuitry to aid in increasing the number of DUTs which can be tested in parallel. Also, a larger footprint is available for mounting electronic circuitry on the probe card and a high ratio of power supply contacts to signal contacts may be used. This high ratio is especially advantageous for testing low pin count devices (e.g., memory devices). Further, no mechanical tooling holes need be drilled in the area of the probe card where traces need to be routed thus both reducing a layer count of the probe card and increasing the number of the DUTs that can be tested in parallel. 
         [0032]    In the foregoing specification, the present invention has been described with reference to specific embodiments thereof. It will, however, be evident to a skilled artisan that various modifications and changes can be made thereto without departing from the broader spirit and scope of the present invention as set forth in the appended claims. For example, various types of conducting materials may be used for the spring contacts. Alternatively, non-conductive spring materials may be employed which have a conductive outer layer, such as gold plating. Also, various fabrication technologies, such as micro-electromechanical systems (MEMS), may be employed in future generations of probe card interposers to manufacture spring contacts. These and various other embodiments and techniques are all within a scope of the present invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.