Abstract:
A planarization gauge assures probe card-to-wafer parallelism in semiconductor automatic test equipment (ATE) used for wafer test, and provides a standard system reference plane during the building and testing of ATE components. The planarization gauge has two planar and parallel surfaces that may serve as a system reference plane The planarization gauge has at least one access hole for a depth gauge, and at least one optical target recognizable by a prober&#39;s upward looking camera. The planarization gauge is mechanically interchangeable with a probe card; thus, it is compatible with different planarization methods and platforms used in building and testing ATE components. The planarization gauge is manufactured and inspected in a manner as to assure traceability to established standards such as NIST. When used by all ATE vendors, the planarization gauge ensures correlation between the vendors&#39; various planarization methods.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention is directed towards the field of semiconductor automatic test equipment, and more specifically, towards probe card-to-wafer parallelism (also described as planarity) in semiconductor automatic test equipment at wafer probe.  
         BACKGROUND OF THE INVENTION  
         [0002]    Automatic Test Equipment Configuration  
           [0003]    Within the semiconductor industry, an essential step in the manufacturing process is wafer test, also known as wafer probe or wafer sort. During wafer sort, each individual die on the wafer is electrically tested for functionality before packaging. FIG. 1A is a high-level sketch showing a sample configuration of the automated test equipment (ATE), also known as an ATE test cell or test cell, used in wafer sort. This configuration shall hereinafter be referred to as a direct-docking system. The equipment that controls and runs the tests on the wafer is called a tester  101 . The tester  101  has a moveable test head  103  that is positioned over a wafer  105  during test. A prober  107  loads and unloads each wafer  105  onto a prober stage  109 . The prober stage  109  (also known as a prober chuck) maneuvers each wafer  105  into position for testing, and is capable of movement in x-, y-, and z-directions. An arrow  121  points in the direction of the z-axis for the system. The x- and y-axis are in the plane of the wafer  105 . In this ATE test cell configuration, the test head  103  rests on docking supports  111 , which are adjustable in height. In other test cell configurations, the test head  103  may be suspended above the prober  107  using appropriate means other than docking supports.  
           [0004]    The test head  103  makes contact with the wafer  105  via probe card  113 , which can be attached to the tester interface  120  with a number of possible mechanisms, including but not limited to vacuum attachment, mechanical latching, or retention using electromechanical connectors. In some alternate test cell configurations, such as the one shown in FIG. 1B, the probe card  113  is mounted directly onto a prober head plate  114  of the prober  107 . The configuration of FIG. 1B shall hereinafter be referred to as a conventional docking system. The probe card  113  holds an array of probes  115  that have been manufactured to line up with contact pads on the wafer  105 . Ideally, all of the probes  115  are aligned in the same plane, parallel to the wafer surface, such that contact is made with all of the contact pads on the wafer  105  simultaneously, minimizing the required z-direction travel of the prober stage  109 . The probe depth  116  is defined to be the distance in the z-direction from the tester interface  120  to the tip of the probes  115  as illustrated in FIG. 1A. Each probe card  113  is custom-made for the specific circuitry of the wafer  105  that is to be tested, and has an interface that is electrically and mechanically matched to the tester specific interface on the test head  103 . The prober  107  typically has a prober vision system with an upward looking camera  117  that can optically measure distances in the z-direction.  
           [0005]    A fixed point, usually the center of x-y travel of the prober stage  109 , is designated as the probing center of the ATE. The test cell has a system reference plane  119 , which is typically a flat surface on a mechanical portion of the test cell. The system reference plane  119  is the surface against which the planes of other surfaces in the test cell are measured relative to. In the direct docking system of FIG. 1A, the system reference plane  119  is also the tester interface  120 . In other test cell configurations, such as a conventional docking system, the system reference plane  119  may be another surface such as the prober head plate  114  or other flat surface. Each probe card  113 , depending upon the probe technology employed and other application-specific factors, has a manufacturing planarity tolerance, which specifies the maximum distance that can be tolerated between the lowest and highest hanging probe  115  on the probe card  113  before the wafer  105  can no longer be accurately tested. The components that make up a test cell, consisting of tester  101 , prober  107 , and probe card  113 , are typically supplied and supported by different vendors. For example, it is very common for the tester  101  to be supplied by one vendor, the prober  107  supplied by another, and the probe card  113  supplied by a third vendor.  
           [0006]    Probe Card Planarization  
           [0007]    Before wafer sort, it is imperative that the probe card  113  is leveled so that the tips of the probes  115  lie in a single plane, parallel to the wafer surface. This process, known as probe card planarization, ensures that the probes  115  all simultaneously contact the corresponding pads on the wafer  105 . FIG. 2 shows an ATE test cell (a direct docking system) in which the test head  103  and probe card  113  are slightly tilted (exaggerated for clarity in the figure) and therefore not parallel to the wafer  105 . As the test head  103  is brought down to rest upon the docking supports  111 , the first probe  115 A contacts the wafer  105  before any of the other probes  115 . The test head  103  cannot be positioned any lower without damaging the first probe  115 A and/or the wafer circuitry, but the remaining probes  115  have yet to make contact with the wafer  105 . The test head  103  and probe card  113  must be leveled and made substantially parallel to the wafer  105  (within the tolerance of each ATE test cell) by adjusting the height of the docking supports  111 , before the wafer  105  can be successfully tested. Probe cards  113  in conventional docking systems also require planarization before running wafer sort. In conventional docking systems, the probe card  113  is planarized by making adjustments to the prober head plate  114 .  
           [0008]    There are various ways to planarize a probe card  113 . One method, used in direct docking systems, requires using a custom-made leveling apparatus. The leveling apparatus is mounted onto the docking supports  111  of the prober  107 , and has three holes in its body that are positioned above the prober stage  109 . A mechanical depth gauge is inserted into each hole to measure the distance between the leveling apparatus (which is a planar reference surface) and the prober stage  109 . The height of each docking support  111  is adjusted until the measured distances are equal, indicating the docking supports  111  themselves are planar. If the docking supports  111  are planar, then it is presumed the test head  103  and its probe card  113  will also be planar when the test head  103  is set down upon the docking supports  111 .  
           [0009]    Unfortunately, the described leveling apparatus is flawed because it does not replicate the physical setup of the ATE test cell during wafer sort. Once the leveling apparatus is removed and the test head  103  is lowered onto the docking supports  111 , the weight of the test head  103  (which can exceed  1000  pounds in some systems) alters the height of the docking supports  111  so that the test head  103  is no longer planarized. Furthermore, the leveling apparatus cannot utilize the measurement capabilities of the upward looking camera  117 , nor can it be used in conventional docking systems  
           [0010]    Another probe card planarization method is described in U.S. Pat. No. 5,861,759 to Bialobrodski et al. U.S. Pat. No. 5,861,759 uses the prober&#39;s upward looking camera to gauge the distance between 3 selected probes on the probe card. The test head rests on one fixed support and 2 adjustable, motorized supports. The camera communicates to a central microprocessor any adjustments that need to be made to the tilt of the test head in order to planarize the probe points to the wafer surface. In response, the central microprocessor adjusts the height of the motorized supports accordingly. Unfortunately, this method and apparatus requires additional setup steps and a costly motion control system to control the motorized supports.  
           [0011]    Planarity Verification Methods  
           [0012]    To ensure probe card planarity, any component that interfaces with the wafer  105  and/or probe card  113  must also be planar and, in combination with the other components, meet the manufacturing planarity tolerance of the ATE. The prober stage  109 , the probe card  113  and probes  115 , and the system reference plane  119  should all be planar and parallel to each other in the final assembly. Vendors verify the planarity of their components by measuring the distance between a known, flat reference plane and the surface in question at multiple points. If the distances are equal, then the surface is verified to also be planar and parallel to the reference plane. Unfortunately, the vendors use completely different methods and tools to verify planarity. As a result, the different verification methods may not necessarily correlate to each other; that is, a surface that is determined to be planar using one method may not necessarily be found planar using another method.  
           [0013]    To verify the planarity of a prober stage  109 , one prober vendor attaches images of crosshair targets in three different locations onto a dummy probe card. The dummy probe card serves as the system reference plane  119 , and is installed into a tester interface emulator that is mechanically equivalent to the tester interface  120  in the ATE in which the prober  107  will be used. Then, the upward looking camera  117  is used to measure the distance between each crosshair target and the prober stage  109 . When the three distances are equal, the prober stage  109  is determined to be planar and parallel to the dummy probe card.  
           [0014]    Another prober vendor uses a dummy probe card with three holes as a system reference plane  119 . The dummy probe card is installed into a tester interface emulator. The tester interface emulator has a center opening large enough to expose the holes on the tester side of the dummy probe card. The holes are wide enough to let the plunger of a mechanical depth gauge pass through to make measurements. This allows the mechanical depth gauge to measure the distance between the system reference plane  119  of the dummy probe card and the surface of the prober stage  109 . When the three distances are equal, the prober stage  109  is determined to be planar and parallel to the dummy probe card.  
           [0015]    The probe card vendor uses an altogether different method to verify planarity of the probe points on a probe card  113 . Due to the complexity of the probe array, a special instrument known as a metrology tool is used to check that a probe card  113  is planar to its probes  115 . The metrology tool, which is provided by yet another vendor, also needs to have its planarity verified. Verification of the metrology tool can be performed in various ways, including the above-mentioned method of using a dummy probe card with holes.  
           [0016]    In all of these aforementioned examples, the vendors rely on their own measurement instruments, tools, emulations of the tester interface  120 , and/or emulations of the system reference plane  119  to verify planarity during the various stages of probe card manufacturing, measurement, and use. Unfortunately, with each vendor using a different method to determine planarity, and few (if any) of these methods providing for measurement traceability, often the disparate tools and methods do not correlate to each other. This lack of correlation causes probe card planarization difficulties during production, which means valuable time that could have been used in wafer sort must be wasted in planarizing the probe card  113  instead.  
           [0017]    The use of non-correlating verification methods may also result in the erroneous rejection of a good probe card during wafer sort. For example, the probe tips on probe cards  113  should lie in a plane parallel to the system reference plane  119  of the subject ATE test cell. The probe tips&#39; planarity is verified by the probe card vendor. However, if the methods of verifying planarity in the ATE test cell do not correlate with the probe card vendor&#39;s methods, then it may be impossible to planarize the probe card  113  in the ATE test cell. In most such cases, the probe card is assumed to be defective (even though the probe card vendor had already independently verified the probe tips&#39; planarity) and returned to the probe card vendor. These types of mistakes increase reject rates, probe card inventory requirements, and average setup time for an ATE test cell.  
           [0018]    Miscorrelation between verification methods of different vendors is not the only problem. There may also be lack of correlation between a vendor&#39;s own internal manufacturing and verification methods. For example, the probe card vendors have tip-planarizing tools (such as a sanding station or tip-etch system) that are used during the manufacturing process to sand, etch, or otherwise align the probe tips of a probe card within a plane. Then, the planarity of the probe tips is verified using a metrology tool. However, there can be miscorrelation between the tip-planarizing tools and the metrology tool. Miscorrelation between such internal tools is a problematic source of yield fallout in the probe card manufacturing environment.  
           [0019]    Better Planarization Tool and Better Correlation Needed  
           [0020]    Therefore, a need remains for an improved planarization tool, one that can more accurately replicate the physical setup of the ATE test cell during wafer sort and be used with the upward looking camera  117 . There is also a need for better correlation between the various planarity verification methods used by the vendors, as well as better correlation between manufacturing and planarity verification tools. These needs are especially urgent as wafers (and the probe cards to test them) grow larger in array size and manufacturing planarity tolerances become stricter. The solution should be compatible with direct-docking and conventional docking systems, as well as with the different methods and platforms used by vendors to fashion and verify the planarity in the components of an ATE.  
         SUMMARY OF THE INVENTION  
         [0021]    The present invention meets the above-mentioned needs. In accordance with an illustrated preferred embodiment of the present invention, a planarization gauge has a mechanical layout identical to that of a probe card  113 , so as to be mechanically interchangeable with a probe card  113 . The planarization gauge is installed in the tester  101  or tester interface emulator in the same manner as a probe card  113 . The planarization gauge is functionally and mechanically compatible with the ATE in which it is to be used, and is built within the manufacturing planarity tolerance of the ATE. The planarization gauge provides a front planar surface and a back planar surface, which are substantially parallel to each other. Either or both of the surfaces may be used as a system reference plane  119  when verifying planarity in the individual components of the ATE. During probe card planarization in the ATE test cell, the back planar surface is typically blocked by the test head  103 , but the front planar surface remains accessible as a system reference plane  119 . The planarization gauge is a single tool that provides depth gauge access holes for measurements using depth gauges, and optical targets for measurements using an upward looking camera  117 . An optical target is hereinafter defined as any image or object that can be recognized by an upward looking camera  117  and used as an endpoint in a measurement of distance.  
           [0022]    Since the planarization gauge is mechanically interchangeable with a probe card  113 , the planarization gauge can be used in any ATE configuration, such as direct-docking or conventional docking systems. In a direct-docking system, the planarization gauge can be used while latched to the test head  103 . The test head  103  can be set down on the docking supports  111  during planarization, thus replicating the physical setup of the ATE during wafer sort. Also, due to its interchangeable nature, the planarization gauge is compatible with conventional docking systems, as well as with the different methods and platforms used by vendors while building and verifying individual ATE test cell components. It can be used by the aforementioned prober vendors in place of the dummy probe cards. The probe card vendor and metrology tool vendor can use it to verify a metrology tool. Each vendor can also use it to calibrate and correlate internal manufacturing and planarization processes by verifying their own tools with the same planarization gauge. When used by all the ATE vendors, the planarization gauge provides a uniform standard for building and verifying all ATE components, ensuring correlation between the various methods of verifying planarity. Furthermore, the planarization gauge can be manufactured and inspected in a manner as to provide traceability to a standard, such as National Institute of Standards and Technology (NIST). Additionally, it serves as an excellent debugging tool for determining which components are at fault when probe card planarization is a problem.  
           [0023]    One embodiment of the planarization gauge consists of a front plate fastened to a back plate. One surface of the back plate is the back planar surface of the planarization gauge; one surface of the front plate is the front planar surface of the planarization gauge. The back plate is adapted to attach to a test head  103  in the same manner as a probe card  113 . The front plate, made of glass, has three optical targets etched onto the front planar surface. In addition to the three optical targets, three depth gauge access holes run through the back and front plates, large enough to allow room for the plunger of a mechanical depth gauge to fit through.  
           [0024]    Vendors using a mechanical depth gauge to measure planarity insert its plunger through the depth gauge access holes, using the back planar surface as the system reference plane  119 . Vendors using an upward looking camera  117  optically measure the distance to the optical targets, using the front planar surface as the system reference plane  119 . Either method of verifying planarity is valid and will correlate to the other method, since the back and front planar surfaces are parallel and planar within a strict tolerance. The depth gauges used with the planarization gauge are not limited to just mechanical depth gauges, either. Any other instrument that is capable of measurement in the z-direction with a precision sufficient for the ATE in question can also be used. Laser measurement equipment is one such alternative instrument.  
           [0025]    The optical targets in this embodiment are singular dots that are recognizable by a prober vision system utilizing, for example, the prober&#39;s upward looking camera  117 . Directional lines, also etched onto the glass, trace two paths from the center of the front plate to each optical target. The first path is a direct path from the front plate center to each target. The second path is broken down into x- and y-vectors. The directional lines in this embodiment allow an ATE operator to locate the tiny optical targets with ease, especially when the front planar surface is viewed at magnification through the upward looking camera  117 . Additional directional lines to the optical targets may be added. Alternatively, the routine for locating and focusing on the dots of the planarization gauge can be automated and run by the prober  107 .  
           [0026]    The distance from the probing center to an optical target is hereinafter defined as an optical target radius, and the distance from the probing center to a docking support  111  in a direct-docking system is hereinafter defined as a docking support radius. Each docking support  111  has a corresponding optical target. The optical targets can be positioned at locations that duplicate, on a smaller and proportional scale, the locations of the docking supports  111  on the prober  107 . The optical targets are located within the radial axis of the docking supports  111 , and are positioned such that the ratio of each optical target radius to its respective docking support radius is the same.  
           [0027]    Planarizing the test head  103  to the prober stage  109  becomes a straightforward process in a direct-docking system when the optical target radii are proportional to the docking support radii. First, the planarization gauge is latched onto the test head  103 . Then, the distance between the prober stage  109  and each of the three optical targets is measured. The difference between the three measurements is proportional to the height adjustments that need to be made to the docking supports  111 . This method of positioning the optical targets is optional and is not needed in a conventional docking system.  
           [0028]    In an alternate embodiment, the planarization gauge is a single plate with parallel back and front planar surfaces. The front planar surface has three optical targets. These optical targets can be probes  115 , probe-like protrusions, light-colored dots against a dark background, or any other image or object recognizable by the upward looking camera  117 . The single plate also has three holes that allow access for a depth gauge. The single plate is adapted to attach to a test head  103  in the same manner as a probe card  113 .  
           [0029]    Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1A is a high-level sketch showing the typical automated test equipment (ATE) used in wafer sort.  
         [0031]    [0031]FIG. 1B is a high-level sketch showing an alternate configuration of ATE used in wafer sort.  
         [0032]    [0032]FIG. 2 shows a system in which the test head and probe card are not planarized to the wafer surface.  
         [0033]    [0033]FIG. 3 shows a high-level sketch of an ATE with a planarization gauge installed.  
         [0034]    [0034]FIG. 4A is a side view of a preferred embodiment of a planarization gauge, made in accordance with the teachings of the present invention.  
         [0035]    [0035]FIG. 4B shows a top view of the back plate of a planarization gauge.  
         [0036]    [0036]FIG. 4C shows a top view of the front plate of a planarization gauge.  
         [0037]    [0037]FIG. 4D shows a close-up view of the area enclosed in dashed circle D′ shown in FIG. 4C.  
         [0038]    [0038]FIG. 5A shows a top view of an alternate embodiment of a planarization gauge.  
         [0039]    [0039]FIG. 5B shows a side view of the alternate embodiment of a planarization gauge shown in FIG. 5A.  
         [0040]    [0040]FIG. 6 shows a top view of another alternate embodiment of a planarization gauge.  
     
    
     DETAILED DESCRIPTION  
       [0041]    [0041]FIG. 3 shows a high-level sketch of an ATE test cell with a planarization gauge  301  installed. The planarization gauge  301  has a back planar surface  303  and a front planar surface  305 , both of which are substantially planar and substantially parallel to each other. Either or both the back planar surface  303  and the front planar surface  305  may be used as a system reference plane  119  in verifying the planarity in the individual components of the ATE test cell. When attached to the test head  103  during probe card planarization, the back planar surface  303  is typically blocked by the test head  103 , but the front planar surface  305  remains accessible as a system reference plane  119 . The planarization gauge  301  has depth gauge access holes running through its structure, and optical targets on the front planar surface  305  for measurements using an upward looking camera  117 . An optical target is hereinafter defined as any image or object that can be recognized by an upward looking camera  117  and used as an endpoint in a measurement of distance. Because FIG. 3 shows a side view of the planarization gauge  301 , neither the depth gauge access holes nor the optical targets can be seen.  
         [0042]    The planarization gauge  301  is mechanically interchangeable with a probe card  113 , so it can be used while latched to the test head  103 . For example, in direct-docking systems during planarization of the probe card  113 , the test head  103  can be directly set down upon the docking supports  111 , thus replicating the physical setup of the ATE test cell during wafer sort. Because the planarization gauge  301  is interchangeable with a probe card  113 , it is compatible with conventional docking systems, too. It is also compatible with the different methods and platforms used by vendors in building the individual components in a test cell, and in verifying their planarity.  
         [0043]    [0043]FIG. 4A is a side view of a preferred embodiment of the planarization gauge  301 , made in accordance with the teachings of the present invention. A front plate  401  is centered and secured to a back plate  403  using screws, adhesives, latches, or any other well-known means of attachment. When the planarization gauge  301  is installed into an ATE test cell, the back planar surface  303  faces the test head  103 , and the front planar surface  305  faces the prober  107 .  
         [0044]    [0044]FIG. 4B shows a top view of the back plate  403 , as seen from the prober side of the ATE test cell. The back plate  403  is adaptable to attach to a test head  103 , in the same manner as a probe card  113 . The embodiment shown in FIG. 4B has outer edge holes  405  that fasten to a frame (not shown) that, in turn, mates to the test head  103 . Other methods of adapting the back plate  403  to fit a test head  103  are possible. The back plate  403  has three depth gauge access holes  407  that are large enough for the plunger of a mechanical depth gauge to fit through. The depth gauge access holes  407  are usually blocked when the planarization gauge  301  is latched into an actual test head  103 , but are accessible when the planarization gauge  301  is latched into a tester interface emulator. Most tester interface emulators have a center opening through which the depth gauge access holes  407  can be reached. However, the center opening is limited in size, so the depth gauge access holes  407  must be positioned to fall within the center opening when the planarization gauge  301  is installed into the tester interface emulator. The size of the center opening may vary among the different tester interface emulators. For illustrative purposes only, in an actual working embodiment, the depth gauge access holes  407  are approximately 6.5 mm in diameter, and located within a 6.3-inch radius of the center of the back plate  403 .  
         [0045]    The dashed circle  409  represents the location of the front plate  401  when it is fastened to the back plate  403 . The front plate  401  can be fastened to the back plate  403  using screws into screw holes  411  drilled into the back plate  403 . The back planar surface  303  of back plate  403  should be very flat, so as to provide a good system reference plane  119 . The planarity of the back planar surface  303  should meet the manufacturing planarity tolerance of the ATE test cell with which it will be used. For illustrative purposes only, in an actual working embodiment, the back plate  403  is approximately 355.6 mm in diameter, 6.35 mm thick, and the back planar surface  303  is planar within 5 um. The back plate  403  can be made of any rigid material such as stainless steel, aluminum, or titanium, although lightweight materials are preferred so that the planarization gauge  301  remains portable and easily hand-carried.  
         [0046]    [0046]FIG. 4C shows the front planar surface  305  of the front plate  401 , as seen from the prober side of the ATE test cell. The front plate  401  has thru-holes  413  around its outer edge that align with the screw holes  411  on the back plate  403  so that the front plate  401  can be fastened to the back plate  403  using screws. Depth gauge access holes  407  on the front plate  401  align with the depth gauge access holes  407  on the back plate  403 , and are wide enough to allow the plunger of a mechanical depth gauge to fit through. Three optical targets  417  are drawn on the surface of the front plate  401 , facing the prober  107 . The optical targets  417  can take any form or shape that can be detected as singular points by an upward looking camera  117 . Typically, the upward looking camera  117  is designed to detect a small, light-colored area against a darker background, which is characteristic of probe tips. In the present embodiment, the surface of the front plate  401  is a dark color, and the optical target  417  is a proportionally small and visually contrasting dot. An additional optical target  418  is located at the center of the front planar surface  305 . The center optical target  418  ideally lines up with the probing center of the ATE test cell. It is typically used by the upward looking camera  117  during initialization of the prober  107   
         [0047]    Directional lines  419  drawn on the front plate  401  trace two paths from the center of the front plate  401  to each optical target  417 . The first path  419 A is a direct path from the center of the front plate  401  to each optical target  417 . The second path  419 B is broken down into x- and y-vectors. The directional lines  419  are optional, but make it much easier for an ATE operator to locate the tiny optical targets  417 , especially when the front plate  401  is viewed at magnification through the viewfinder of a prober vision system  117 . To use the directional lines  419 , the ATE operator positions the prober vision system  117  at the probing center, and then traces along the directional lines  419  on the front plate  401  towards the optical target  417  until the optical target  417  is reached.  
         [0048]    The front plate  401  is preferably made of glass, so that the optical targets  417 ,  418  and directional lines  419  can be cleanly and precisely etched onto the front plate  401  using a photomask creation process. The smaller and sharper the optical targets  417 ,  418  are, the more accurate the measurements. Of course, the optical targets  417 ,  418  must not be so small as to be undetectable by the upward looking camera  117 . The front plate  401  can also be made of metal, plastics, and even paper. In an actual working embodiment, the front plate  401  was fashioned out of a sheet of paper, with small white dots against a darker background to represent the optical targets  417 ,  418 . However, using such materials may degrade the precision of the optical targets  417 ,  418  and affect the ability of the upward looking camera  117  to detect the optical targets  417 ,  418 . These alternative materials may also be difficult to manufacture consistently in repeatedly similar fashion, and keep stable over time. The materials that are acceptable for use will vary depending on the ATE test cell and its manufacturing planarity tolerance. In order to be a good reference plane, the front planar surface  305  should be very flat. The planarity of the front planar surface  305  should fall within the manufacturing planarity tolerance of the ATE test cell with which it will be used. For illustrative purpose only, in an actual working embodiment, the front plate  401  is 177.8 mm in diameter and 3.81 mm thick, the front planar surface  305  is planar within 5 um, and the front planar surface  305  is parallel to the back planar surface  303  within 5 um. The total thickness of the planarization gauge  301  (the thickness of the front plate  401  and back plate  403  added together) should not exceed the maximum probe depth  116  found in probe cards  113 .  
         [0049]    A close-up of an optical target  417  and surrounding directional lines can be seen in FIG. 4D, which is an enlargement of the area surrounded by dashed circle D′ in FIG. 4C. Arrows  423  along the length of the directional lines point towards the optical target  417 . An identifying label  420  may be placed next to the optical target  417  to help the ATE operator know which optical target  417  is being viewed. Additional directional lines  421  may also be placed near the optical target  417  to help pinpoint its location. For illustrative purposes only, in an exemplary working embodiment, the optical targets  417  have a diameter of 25 um, and the directional lines are 200 um thick.  
         [0050]    The optical targets  417  are positioned at locations that duplicate, on a smaller and proportional scale, the locations of the docking supports  111  on the prober  107  in a direct-docking system. When the planarization gauge  301  is latched to the test head  103 , the optical targets  417  should be located within the radial axis of the docking supports  111  such that the ratio of each optical target radius to its respective docking support radius is the same. This method of positioning the optical targets  417  is optional, and does not have to follow this formula, especially if the planarization gauge  301  is used in a conventional docking system. However, calculating the height adjustment needed of the docking supports  111  in a direct-docking system during probe card planarization is simpler and more straightforward when the formula is followed. The depth gauge access holes  407  can also be positioned following this formula.  
         [0051]    After the planarization gauge  301  is built, it may be calibrated to a standard such as NIST to provide measurement traceability. In an actual working embodiment, the front planar surface  305  and the back planar surface  303  were each calibrated to a NIST standard and verified to be planar within 5 um. The front planar surface  305  was also verified to be parallel to the back planar surface  303  within 5 um.  
         [0052]    [0052]FIG. 5A shows a top view of an alternate embodiment of the planarization gauge  301 . FIG. 5B shows a side view of the same planarization gauge  301  that is shown in FIG. 5A. The optical targets in this embodiment are three probes  501  or probe-like protrusions that are attached at three different locations on the front planar surface  305  of a single plate  503 . The points of the probes  501  lie in the same plane and are recognizable by an upward looking camera  117 . Structures other than probes  501  may be used in this embodiment. For example, any protrusions from the single plate  503  that have an endpoint recognizable by the upward looking camera  117  can also be used. Three depth gauge access holes  407  are also provided, each one wide enough for the plunger of a mechanical depth gauge to fit through. The single plate  503  can be adapted to a fixture that loads onto a test head  103 , in the same manner as a probe card  113 . Although not shown in the embodiment of FIGS. 5A and 5B, directional lines  419  and a center optical target can also be included. The probes  501  can also be positioned such that the ratio of each optical target radius to a respective docking support radius is the same for all probes  501 .  
         [0053]    [0053]FIG. 6 shows a top view of another alternate embodiment of the planarization gauge  301 . A single plate  601  can be adapted to a fixture that loads onto a test head  103 . The single plate  601  has three optical targets in the form of x-shaped optical targets  603  on its front planar surface  305  facing the prober  107 . Each x-shaped optical target  603  is an image of a dark-colored cross with a light-colored dot in the middle. The dark-colored cross serves a function similar to the directional lines  419  shown in FIG. 4C; it helps the operator of the upward looking camera  117  to find the small, light-colored dot when looking through a magnified viewfinder. Three depth gauge access holes  407  are also provided. Although not shown in the embodiment of FIG. 6, directional lines  419  and a center optical target can also be included. The x-shaped optical targets  603  can also be positioned such that the ratio of each optical target radius to a respective docking support radius is the same for all x-shaped optical targets  603 .  
         [0054]    Additional features can be easily added to any of these illustrated embodiments. For example, each optical target can be labeled with its relative height in the z-direction (z-height). This information allows an ATE operator to pre-set the docking supports  111  in a direct-docking system near their final height when the actual probe card  113  is loaded, thus minimizing setup time. The z-height of each optical target is especially useful information when multiple ATE test cells use the same probe card  113 —it enables a common z-height across all probers  107 . The center optical target can also be labeled with the distance it is offset from the probing center of the ATE when the planarization gauge  301  is loaded. This information is helpful in initializing the prober  107 .  
         [0055]    Although the present invention has been described in detail with reference to particular preferred embodiments, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims that follow. For example, the plates are shown as circular in the figures, but they can be any shape so long as they can be adapted to attach to a test head  103 . Also, the figures show the depth gauge access holes  407  and optical targets  417  in sets of three. However, any number and combination of depth gauge access holes  407  or optical targets  417  can be used on the planarization gauge  301 . If the number of depth gauge access holes  407  is less than three, then the planarization gauge  301  must be able to be rotated in the test head  103  so as to provide at least 3 different measurements with the available depth access holes  407 . Similarly, if the number of optical targets  417  is less than three, then the planarization gauge  301  must be able to be rotated in the test head  103  so as to provide at least 3 different measurements with the available optical targets  417 .