Patent Publication Number: US-8120375-B2

Title: Method for using probe card

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. utility patent application Ser. No. 10/902,188 filed Jul. 28, 2004 now U.S. Pat. No. 7,098,650, and claims the benefit of U.S. provisional patent application Ser. No. 60/490,621 filed Jul. 28, 2003, the entire content of which is incorporated herein by this reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to systems for testing integrated circuits in wafer form and, more particularly, to apparatus for planarizing probe cards for use therewith. 
     BACKGROUND OF THE INVENTION 
     Manufacturers in the electronics industry use automatic test systems or testers to test various electronic components, integrated circuits (ICs) and other devices under test (DUTs) to cull out defective devices. For example, data patterns are delivered to an integrated circuit with specific timing and voltage settings through timing generators and pin electronic channels to test the functionality of the integrated circuit. Data is then read from the integrated circuit to ensure that the device responds correctly. A variety of parametric timing tests may also be performed to validate correct operation of the integrated circuit as well as adherence to its specifications. Generally, it is desirable to test integrated circuits at several points during the manufacturing process, including while they are still part of a wafer or substrate. 
     Equipment used in wafer testing can include a wafer prober and an automated test system. A conventional wafer prober has a movable chuck which serves to transport the wafer to a position underlying a probe card mounted to the top deck of the wafer prober by being clamped at its periphery. A probe array is typically provided on the underside of the probe card for engaging the bonding pads of one or more die on the wafer. A plurality of contact elements, electrically coupled with the probe array, are usually provided at the periphery on the top of the probe card. The automated test system includes one or more testers which overlie the top deck of the wafer probe and have an electrical interface for electrically engaging the contact elements of the probe card. 
     Unfortunately, changes in temperature and mechanical forces experienced by the probe card during testing can result in distortion of the probe card, thus deflecting the generally unsupported central portion of the probe card where the probe array is located. The larger the probe card, the more deflection can occur in the probe card. Such distortion of the probe card can result in undesirable alignment errors between the probe array of the probe card and the chuck, thus compromising the accuracy or completeness of the testing of die carried by the chuck. 
     In view of the foregoing, it would be desirable to minimize distortion of the probe card, particularly in the vicinity of the probe array mounted to the underside of the probe card. 
     SUMMARY 
     An apparatus for use with a wafer prober and a probe card comprising a stiffening member having a feature defining a first plane is provided. The stiffening member is mountable atop the central portion of the probe card. A reference member is provided to mount to the wafer prober and has an underside with a feature defining a second plane. When the feature of the stiffening member defining the first plane is urged against the feature of the reference member defining a second plane the probe tips of the probe card are substantially planarized relative to the wafer prober. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view of an automatic test system and wafer prober using the apparatus for planarizing a probe card of the present invention. 
         FIG. 2  is an exploded, schematic side elevational view of a portion of the automatic test system and wafer prober of  FIG. 1  showing the apparatus for planarizing a probe card of the present invention. 
         FIG. 3  is a schematic, perspective view of the wafer prober of  FIG. 1  having a top deck with a reference plate thereon. 
         FIG. 4  is a schematic, top plan view of a probe card for use with the apparatus for planarizing a probe card of  FIG. 2 . 
         FIG. 5  is a schematic, side elevational view taken along the line  5 - 5  of  FIG. 4 , of the probe card of  FIG. 4 . 
         FIG. 6  is a perspective view of a latching plate of the apparatus for planarizing a probe card of  FIG. 2 . 
         FIG. 6A  is an enlarged view of a portion of the latching plate of  FIG. 6  indicated by the circle  6 A of  FIG. 6 . 
         FIG. 7  is a top plan view of the latching plate of  FIG. 6 . 
         FIG. 7A  is an enlarged view of a portion of the latching plate of  FIG. 6  indicated by the circle  7 A of  FIG. 7 . 
         FIG. 8  is a top plan view of the apparatus for planarizing a probe card of  FIG. 2  in a first position. 
         FIG. 9  is a cross-sectional view of the apparatus for planarizing a probe card of  FIG. 2  taken along the line  9 - 9  of  FIG. 8 . 
         FIG. 10  is a top plan view the apparatus for planarizing a probe card of  FIG. 2  positioned relative to the reference plate of the wafer prober. 
         FIG. 11  is a top plan view, similar to  FIG. 8 , of the apparatus for planarizing a probe card of  FIG. 2  in a second position. 
         FIG. 12  is a cross-sectional view, similar to  FIG. 9 , of the apparatus for planarizing a probe card of  FIG. 2  taken along the line  12 - 12  of  FIG. 11 . 
         FIG. 13  is a top plan view, similar to  FIG. 8 , of the apparatus for planarizing a probe card of  FIG. 2  in a third position. 
         FIG. 14  is a cross-sectional view, similar to  FIG. 9 , of the apparatus for planarizing a probe card of  FIG. 2  taken along the line  14 - 14  of  FIG. 13 . 
         FIG. 15  is a top plan view, similar to  FIG. 8 , of the apparatus for planarizing a probe card of  FIG. 2  in a fourth position. 
         FIG. 16  is a cross-sectional view, similar to  FIG. 9 , of the apparatus for planarizing a probe card of  FIG. 2  taken along the line  16 - 16  of  FIG. 15 . 
         FIG. 17  is a schematic, side elevational view of the apparatus for planarizing a probe card of  FIG. 2  in an engaged position. 
         FIG. 18  is a schematic side elevational view, similar to  FIG. 2 , of a portion of the automatic test system and wafer prober of  FIG. 1  showing the apparatus for planarizing a probe card of  FIG. 2  in an engaged position. 
         FIG. 19  is a schematic top plan view of a wafer prober utilizing another embodiment of the apparatus for planarizing a probe card of the present invention. 
         FIG. 20  is a schematic, cross-sectional view of the wafer prober of  FIG. 19  taken along the line  20 - 20  of  FIG. 19 . 
         FIG. 21  is a schematic, top plan view of a circular probe card for use with the apparatus for planarizing a probe card of  FIG. 19 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates the main components of a wafer testing system  14 , namely a wafer prober  15  and an automated test system  20 . Although any suitable wafer prober can be utilized, a suitable wafer prober is of the type manufactured as model number UF3000 by Accretech located in Tokyo, Japan. Wafer prober  15  generally includes a wafer loader  17 , controls  19  and a housing  21  provided with a top deck  22  and an access door  24  (see  FIG. 1 ). 
     Automated test system  20  can be of any suitable type, such as of the type disclosed in copending U.S. application Ser. No. 10/170,916 filed Jun. 12, 2002, the entire content of which is incorporated herein by this reference. The automated test system  20  includes one or more testers and as shown a single tester  25  having a test head  26 . The tester  25  is supported above the wafer prober  15  by a support arm  27  that is vertically adjustable on a post  28  upstanding from a wheelable base  29 . The system  20  further includes controls  30 .  FIG. 2  shows a schematic and enlarged view of portions of the wafer prober  15  and test system  20  and specifically the interface between the test head  26  and the wafer prober  15  for performing automated wafer testing. The wafer prober  15  includes a movable chuck  30 , located behind the access door  24  of the wafer prober, having a top substantially planar surface  32  on which a wafer  33  is placed. Typically, top deck  22  of the wafer prober  15  is a reference plane to which probe manufacturers planarize chuck  30  so that the top deck  22  is therefore planar and parallel with respect to the top surface  32  of the chuck  30 . A plurality of die (not shown) have been formed on the wafer  33 , each have a plurality of bonding pads, contact pads or other contact interconnects formed on its top surface. 
     Each test head  26  includes an electrical interface  36 , for example input/output blocks, for outputting test signals to the die undergoing testing and receiving response signals from the die to be analyzed by the automate test system  20  (see  FIG. 2 ). The electrical interface  36  includes a plurality of test pins  38  for electrically connecting the test head  26  to the probe card  34  and hence to the die electrically connected to the probe card  34 . The test pins  38  are arranged to contact a corresponding plurality of contact pads  39  on the probe card  34 , which are illustrated in  FIG. 4 . 
     The system of the invention further includes a reference member or plate  42  mounted to or integral with the top deck plate  22  of the prober  15  (see  FIGS. 2 and 3 ). The reference plate  42 , more clearly illustrated in  FIG. 3  which shows a perspective top view of the wafer prober  15  without the test head  26 , is supported above the probe card  34  by the top deck  22 . Reference plate  42  preferably bridges over the central opening in top deck  22 , as shown in  FIG. 3 , and is more preferably centered over the top deck. The reference or support member  42 , which can be a thick, highly planar plate made from any suitable material such as metal or ceramic. Reference member  42  preferably has a feature defining a plane, which is preferably on the underside of the reference member  42 . Such feature is planar with the top deck  22 , and hence parallel to and planar with respect to the top surface  32  of the chuck  30 , and is preferably a planar bottom surface  44 . To achieve a high degree of planarity for reference surface  44 , the reference plate  42  is preferably made from a rigid material, for example a metal such as steel or aluminum. Access openings  46  are provided between the reference plate  42  and the top deck  22  to permit the electrical interface  36  of the test head  26  to access the underlying probe card  34 . 
     A probe card  34  is carried by the wafer prober and provides an electrical interface between the one or more test heads  26  of the test system  20  and a die of the wafer  33  undergoing testing (see  FIG. 2 ). Probe card  34  includes a substrate layer which can be made from any suitable dielectric material and is preferably a printed circuit board (PCB)  48  having a first or top surface  49  and an opposite second or bottom surface  50  (see  FIGS. 4 and 5 ). A plurality of contact pads  39  are formed on the top surface  49  of the PCB  48  and are preferably located in a peripheral portion  52  of the probe card  34 . The contact pads  39 , some of which are shown in  FIG. 4 , are arranged in one or more patterns and accessible from the top of the probe card  34 . The plurality of test pins  38  of the test head  26  are arranged in a corresponding one or more patterns as the pattern of contact pads  39  to permit contact and registration of the test pins  38  with the contact pads  39 . Contact pads  39 , including pluralities of distinct arrays thereof, can extend partially or completely around the peripheral portion  52  of the probe card  34 . Accordingly, it is appreciated that the invention is broad enough to cover cards  34  having contact pads  39  disposed only at portions of the periphery  52 , such as along two sides as shown in  FIG. 4 . The probe card  34  can further include a substrate member, for example block  54  made from ceramic or any other suitable material, secured or rigidly coupled to the bottom surface  50  of the PCB  48  and located within a central portion  56  of the probe card  34 . The central portion  56  of the card  34  is preferably that portion of the card  34  interior of contact pads  39  and more preferably that portion of the card  34  beneath the central portion of the reference plate  42 . 
     A probe array or assembly  58  descends or depends from the centrally-disposed block  54 , which supports the probe array  58  (see  FIG. 5 ). The probe elements or probes  59  of the probe array  58  are arranged in a pattern corresponding to the pattern of contact bonding pads or balls of the die undergoing testing. Each of the probes  59  has a probe end or tip which registers with and engages a contact pad of the die when the probe array  58  engages the wafer  33 . The probe tips of the probe array  58  are aligned in a plane, sometimes referred to herein as the plane of the probe array  58 . Electrical leads are provided in the probe card  34 , for example conductive traces (not shown) within the PCB  48 , for electrically coupling the contact pads  39  on the top surface  49  of the PCB  48  to the probes  59  of the probe array  58  descending from the central portion  56  of the underside of PCB  48 . 
     The probe card  34  preferably further includes a stiffening member or plate  60  attached to the top surface  49  of the PCB  48  within the central portion  56  of the probe card  34  (see  FIGS. 4 and 5 ). The stiffener plate  60  is preferably made from any suitable rigid material such as metal and may be attached to the PCB  48  by screws (not shown). The stiffener plate  60  adds rigidity to the central portion  56  of the probe card  34 . The probe card  34  further includes three or more planarizing adjustment screws  62  for adjusting the plane of the probe array  58 , and specifically the plane formed by the tips of the probes  59 , with respect to the plane of the stiffener plate  60 . More specifically, screws  62  permit the plane of the probe array  58  to be made substantially parallel to the plane of the stiffener plate  60 . 
     The central portion of probe card  34 , and preferably stiffener plate  60 , includes a feature defining a plane. Such feature is preferably a plurality of at least three alignment elements  65  are attached to the stiffener plate  60  within the central portion  56  of the probe card  34  and extend outwardly, and preferably upwardly, from the probe card  34  (see  FIGS. 4 and 5 ). The alignment elements  65  have top surfaces  68  and are arranged on the stiffener plate  60  such that the top surfaces  68  substantially define a plane. As shown in  FIG. 4 , the alignment elements  65  are spaced apart so as to form the shape of a triangle, preferably an equilateral triangle, when viewed in plan. The alignment elements  65  are preferably made out of metal that can be machined with a high degree of accuracy. The plane formed by the top surfaces  68  of the alignment elements, that is the feature of the probe card defining a plane, is preferably substantially parallel to the plane of the plane of the probe array  58  and preferably the plane of the stiffener plate  60 . 
     A means or cooperating mechanism is included for urging the feature of the probe card  34  defining a plane against the reference member  42  and preferably the feature of the reference member  42  defining a plane. In the illustrated embodiment, such means or apparatus  69  includes alignment elements  65  and serves to make the plane formed by the tips of probes  59  planar or parallel with the bottom surface  44  of reference plate  42 . Apparatus  69  can part of a cooperating mechanism of the invention that is carried by the reference member  42  and the central portion of the probe card  34  for rigidly coupling the central portion of the probe card  34  to the reference member  42 . Such cooperating mechanism and apparatus  69  preferably include a latching mechanism  70  mounted to the reference plate  42  for latching the alignment elements  65  of the probe card  34  to the reference plate  42 . Preferably, the latching mechanism  70  lifts and urges the top surface  68  of the alignment elements  65  against the bottom, planar surface  44  of the reference plate  42 . Latching mechanism  70  is rigidly coupled to wafer prober  15 , and preferably to top deck  22  thereof. In one preferred embodiment, illustrated herein, the latching mechanism is bolted or otherwise rigidly secured to reference plate  42 . 
     Gripping or latching mechanism  70  according to one embodiment of the invention includes a latch member or plate  171  having a first or top surface  172  and an opposite second or bottom surface  173  (see  FIGS. 6 and 7 ). A plurality of holes or slots  176  adapted to engage or receive at least a portion of respective alignment elements  65  extend through surfaces  172  and  173  of the latching plate. The alignment elements or latch pins  65  are preferably in the form of mushroom latch pins with indentations such as annular groves  177  extending around the cylindrical body of the alignment element for facilitating latching of the element  65  by the plate  171  (see  FIGS. 2 and 9 ). Each keyhole slot  176  includes an enlarged opening  178  large enough for the head  179  of an alignment element  65  to pass therethrough and an inner ledge  180  that slides into the groove  177  of the alignment element  65  to engage the alignment element  65  beneath the head  179  of the element  65 . 
     The gripping or latching assembly or mechanism  170  further includes a lift plate  182  positioned above the latch plate  171 , as shown in  FIGS. 8 and 9 . Side portions  184  of the latch plate  171  extend beyond the lift plate  171 , as shown in  FIG. 8 , and underneath the reference plate  42  of the wafer prober  15 , shown in phantom lines in  FIG. 8  and solid line in  FIG. 9 . The keyhole slots  76  are located within the side portions  184  of the latch plate  171  and are therefore not covered by the overlying lift plate  182 . The lift plate  182  is mechanically coupled to the underlying latch plate  171  by six mechanical couplers or fasteners  185 . The latching mechanism  70  includes an actuator or mechanism  186 , shown schematically in  FIG. 8 , that is mechanically coupled to the lift plate  182  for moving the lift plate  182  and the latch plate  171  mechanically coupled to the lift plate  182  laterally and upwardly. The actuator may be of any suitable type, such as pneumatic, hydraulic, electric, mechanical or any combination of the foregoing. 
     Each mechanical coupler  185 , one of which is illustrated in some detail in  FIG. 9 , is preferably from an I-shaped body or pin  187  extending through the lift plate  182 . The pin  187  has a flanged top end  189  seated against a spring  191  and a flanged bottom end  194  seated in a key shaped slot  195 , similar to slot  176 , formed in the latch plate  171 . The spring  191  is preferably a suitable stack of bellevelle washers extending concentrically around the pin  187  and seated or disposed within a cavity or bore  196  formed in the lift plate  182 . The spring  191  biases the pin  187  by having a first or top end rest or push against the top end  189  of the pin  187  and the second or bottom end rest or push against an inner surface of a flange  196  forming the bottom of cavity  192 . 
       FIG. 10  shows a top view of the latching mechanism  70  relative to the reference plate  42  of the wafer prober  15 . The lift plate  182  is positioned within an opening  194  of the reference plate  142 . The side portions  184  of the latch plate  171  are positioned beneath the reference plate  42 , and therefore not visible in  FIG. 10 , so as to position the keyhole slots  176  located within the side portions  184  beneath the lower planar surface  44  of the reference plate  42 .  FIG. 9  shows one of the keyhole slots  176  beneath the lower planar surface  44  of the reference plate  42 . 
     In the method of operating and using apparatus  69 , latching mechanism  70  engages and lifts the alignment elements  65  upwardly to urge the top surfaces  68  of the alignment elements  65  against the bottom planar surface  44  of the reference plate  42 . A latching sequence utilizing apparatus  69  is described with reference to  FIGS. 8-17 . The operator initially loads probe card  34  into wafer prober  15  by means of access door  24 . The probe card  34  is placed onto an internal mechanism of the wafer prober (not shown) which delivers the probe card to a position beneath latching mechanism  70 , which position is similar to the position illustrated in  FIG. 9  where only a portion of the probe card  34  is shown. Such internal mechanism aligns elements  65  with the large openings  178  of the keyholes slots  176 , as illustrated in  FIGS. 8 and 9 , and then raises the alignment elements  65  to align grooves  177  of the elements  65  with the inner ledges  180  of the keyhole slots  176 , as illustrated in  FIGS. 11 and 12 . The lift plate  171  is then slid laterally by the actuator  186  referred to above, thereby sliding the latch plate  171  coupled to the lift plate  182  laterally. This causes the inner ledges  80  of the keyhole slots  176  to engage the annular grooves  177  of the alignment elements  65  (see  FIGS. 13 and 14 ). The lift plate  182  is then raised upwardly by the actuator, thereby lifting the latch plate  147  upwardly until all of the top surfaces  68  on the heads  179  of the alignment elements  65  engage the bottom planar surface  44  of the reference plate  42  (see  FIGS. 15 and 16 ). The probe card  34  is electrically connected to the die by having the chuck  30  mechanically position the wafer  33  so that bonding pads of the die contact respective probe elements or probes  59  of the probe card  34 . 
     The nonrigid attachment of the latch plate  171  to the lift plate  182 , by means of couplers  185 , permits the latch plate to separate from the lift plate as necessary to accommodate any misalignment between the plates  171  and  182 , such as the top surface of the latch plate  171  not being parallel to the bottom surface  44  of the reference plate  42  when all of the top surfaces  68  of the alignment elements  65  have engaged the reference plate  42 . More specifically, each of the couplers  185  can float within its respective cavity  192  to permit such movement between plates  171  and  182 . Springs  191  serve to urge or push against the flanged top ends  189  of couplers  185  so as to continually urge top surfaces  168  of the alignment elements  65  against the planar surface  44  of the reference plate  42  regardless of any such movement of latch plate  171  relative to lift plate  182 . The amount of each such spring force can be predetermined by adjustment of the amount that the spring  191  is initially compressed within the lift plate  182 . Preferably, the alignment elements  65  are urged against the planar surface  44  with sufficient force to prevent translation and rotation of the alignment elements  65 , and the probe card  34  attached thereto, relative to the reference plate  22  during the operation of system  14 . 
       FIG. 17  shows the probe card  34  attached to the reference plate  22  and thus wafer prober  15  by means of apparatus  69  and latching mechanism  70  thereof. As shown therein, as well as in  FIG. 16 , a gap  196  exists between the bottom surface of latch plate  171  and the top surface of stiffener plate  60  when the probe card  34  has been referenced and affixed to the reference plate as described above. Gap  196  desirably accommodates any irregularities, misalignments or lack of planarity between the stiffener plate  60  and the reference plate  22  which may exist prior to the operation of system  14  or which may come about as a result of changes in temperature or mechanical forces experienced by the probe card  34 , during the operation of the system  14 . 
     As can be seen, apparatus  69  and the method of the present invention serve to planarize the top surfaces  68  of the alignment elements  65  with the bottom planar surface  44  of the reference plate  42  and hence the top surface  32  of the chuck  30 . Because the plane of the probe array  58  is planar with the plane of the top surfaces  68  of the alignment elements  65  and the plane of such top surfaces  68  is planar with the plane of the chuck  30  when urged against the planar surface  44  of the reference plate  42 , the plane of the probe array  58  is planar with the plane of the chuck  30  so as to facilitate proper engagement of the probe array  58  with die on the wafer  33  and thus accurate testing of such die. In the latched state, the latching mechanism  70  thus firmly holds the central portion  56  of the probe card  58  planar with the chuck  30 . 
     The system of the invention offers advantages over the prior art. One advantage is that distortions of the probe card  34  outside the central portion  56  of the probe card  34  have minimal effect on planarization of the probe array  58  because the central portion  56  is held planar by the reference plate  42 .  FIG. 18  illustrates how distortions of the PCB  48  of the probe card  34  do not affect the planarization of the probe array  58 . Apparatus  69  can thus reduce the need to provide a stiffener plate extending beyond the central portion  56  of the probe card  34  to maintain planarity of the probe array  58 , thereby reducing the weight of the probe card  34 . 
     Another advantage is that the reference plate  42  adds rigidity to the central portion  56  of the probe card  34  by holding the central portion  56  planar to the reference plate  42 . In addition, the reference plate  42  does not increase the thermal mass of the probe card  34  because the reference plate  42  is thermally isolated from the probe card  34  by a gap  96 , assuming negligible thermal conduction through the alignment elements  65 . As a result, the reference plate  42  is subject to significantly less thermal distortion during temperature testing of the wafer. Low thermal mass is important because it reduces the time required for the temperature of the probe card  34  to stabilize during temperature testing of the wafer. 
     Apparatus  69 , and the method of the present invention, serve to rigidly couple or secure the central portion  56  of the probe card  34  to the bridging support member  42  and thus to wafer prober  15 . In the preferred embodiment, the peripheral portion  52  of the probe card  34  is free of the reference plate  42 , that is not supported by the reference plate  42  or the wafer prober  15 . Notwithstanding the foregoing, the invention is broad enough to cover methods and apparatus where some small portion or percentage of the peripheral portion  52  of the probe card  34  is contacted by the wafer prober, for example for purposes other than supporting the probe card  34 . The invention is also broad enough to cover methods and apparatus where some small portion or percentage of the peripheral portion  52  of the prober card  34  is supported by the wafer prober, such as by the reference plate  42  or top probe deck  22 , so long as the main or significant proportion or majority of the weight of the probe card  34  is supported by the central portion  56  of the card  34 . 
     Supporting probe card  34  substantially or preferably solely by the central portion  56  advantageously serves to free the peripheral portion  52  of the probe card  34  for other purposes. For example, freeing the peripheral portion  52  of the probe card from support pins or other support elements can expand the methods and means by which test head  36  can engage the probe card  34 . 
     The invention can be used to planarize the probe arrays of various types of probe cards, including circular and other nonrectangular-shaped probe cards. In another embodiment of the apparatus for planarizing a probe card of the present invention, a reference plate  242  for use with a circular probe card  234  is provided. The reference plate  242 , shown on a circular top deck  222  of a wafer prober  215  in  FIGS. 19 and 20 , has a central portion  245  with spokes  247  extending therefrom and attaching to or integral with the circular top deck  222 . The spokes  247  rigidly support the reference plate  242  above the circular probe card  234 , a peripheral portion  252  of which is shown in  FIG. 19  underlying the reference plates  242 . Contact pads  239  are provided on the upper surface of the substrate layer or printed circuit board  248  of the card  234 . For simplicity, only a single array of the contact pads  239  are shown in  FIG. 19 . Spokes  247  are arranged to provide access openings  249  therebetween so that an appropriately configured test head  36  can access the underlying probe card  234 , and specifically contact pads  239  thereon. The latching mechanism and reference planar surface (not shown), similar to latching mechanism  70  and reference planar surface  44  described above, are located in the central portion  245  of the reference plate  142 . 
     The circular probe card  234 , shown in plan in  FIG. 21 , includes a stiffener plate  260  within a central portion  256  of the probe card  234  and exemplary arrays  240  of contact pads  239  arranged in a ring along a periphery  252  of the probe card  234 . A probe array, not shown but similar to probe array  58  described above, descends from a bottom of the probe card  234  within the central portion  256 . Three or more alignment elements  265 , which can be substantially similar to alignment elements  65  described above, are attached to the stiffener plate  270  within central portion  156  and extend upwardly from the probe card  234 . The alignment elements  256  cooperate with reference plate  234  in the manner described above to planarize or make parallel the probe array with respect to the top surface of the chuck (not shown) of the wafer prober  215 . More specifically, the top surfaces of the alignment elements  265  are urged against the bottom planar surface of the reference plate  242 . 
     It should be appreciated from the foregoing that the reference member or plate of the present invention can be configured and shaped to accommodate any sized and shaped probe card. As discussed above, for example, the reference plate can be rectangular or circular in plan. Openings or apertures can be provided in the reference plate, in any suitable configuration, for permitting access to the contact pads of a probe card positioned beneath the reference plate. For example, round reference plate  242  is provided with spokes which define openings therebetween for accessing arrays of contact pads  239  provided on a circular probe card  234 . 
     It is appreciated that the feature of the reference member defining a plane referenced above is not limited to a planar surface but can be of any suitable configuration. For example, such feature can be a plurality of spaced-apart, outwardly-extending alignment elements like elements  65  above. Similarly, the feature of the probe card defining a plane is not limited to the spaced-apart outwardly-extending alignment elements  65  discussed above, but can be of any suitable configuration. Thus, for example, outwardly-extending alignment elements, such as alignment elements  65  above, can be provided on reference member or plate  42  or elsewhere on the wafer prober, instead of on stiffening plate  60  as discussed above. Where, for example, the alignment elements depend from the reference plate  42  and have end surfaces defining a plane that is parallel to the upper surface  32  of the chuck  30 , a planar reference surface is provided on the probe card, for example as the upper surface of stiffening plate  60 , which is parallel to the plane of the probe array  58 . A latching assembly or mechanism, for example similar to latching mechanism  70 , can be mounted on the top of the central portion of the probe card for gripping the alignment elements of the reference member in the manner described above with respect to mechanism  70 . Alternatively, when the alignment elements are provided on the reference member or elsewhere on the wafer prober the latching assembly or mechanism can be rigidly coupled to the wafer prober, such as to reference member  42  as described above, for gripping a set of coupling elements extending upwardly from the stiffening plate  60  or elsewhere on the central portion of the probe card. Such an embodiment could thus have alignment elements depending from the reference member and coupling elements, for example similar to alignment elements  65 , upstanding from the stiffening plate  60 . It is further appreciated that any reference plane of the invention can be formed from a single surface or a set of distinct surfaces that extend in a single plane. 
     Although a latching mechanism was employed in the preferred embodiment, other means may be employed to urge the top surfaces of the alignment elements of the apparatus of the present invention against the bottom planar surface of the reference plate. For example, the entire probe card may be raised toward the reference plate so that the top surfaces of the alignment element abut against the bottom planar surface of the reference plate. This may be done, for example, by securing the probe card to a movable plate capable of raising and lowering the probe card. The probe card may be grasped, clamped or otherwise moved at its peripheral portion or any other portion. 
     As can be seen from the foregoing, an apparatus for planarizing a probe card has been provided which minimizes distortion of the probe card, particularly in the vicinity of the probe array mounted to the underside of the probe card. The apparatus can include a plurality, preferably a plurality of at least three, alignment elements disposed in the central portion of the probe card for aligning the planarity of the probe array provided on the underside of the probe card with a reference plate of a wafer prober. The alignment elements can be urged against a reference plane of the reference plate by being grasped by a latching mechanism or by engaging any other portion of the probe card so as to urge the alignments elements secured thereto against the reference plate.