Abstract:
A chuck for a probe station that include a first chuck assembly element defining a substantially planar upper and lower surfaces, and another chuck assembly element defining a substantially planar surface. The chuck includes a spacing mechanism having exactly three independent supports interconnecting the first chuck assembly element and the another chuck assembly element defining the spacing between the first chuck assembly element and the another chuck assembly element in such a manner that the substantially planar lower surface of the first chuck assembly element and the substantially planar upper surface of the another chuck assembly element are in opposing relationship with respect to one another.

Description:
This application claims the benefit of U.S. patent application Ser. No. 60/230,212 filed Sep. 5, 2000. 

   BACKGROUND OF THE INVENTION 
   The present application relates to an improved chuck. 
   With reference to  FIGS. 1 ,  2  and  3 , a probe station comprises a base  10  (shown partially) which supports a platen  12  through a number of jacks  14   a ,  14   b ,  14   c ,  14   d  which selectively raise and lower the platen vertically relative to the base by a small increment (approximately one-tenth of an inch) for purposes to be described hereafter. Also supported by the base  10  of the probe station is a motorized positioner  16  having a rectangular plunger  18  which supports a movable chuck assembly  20  for supporting a wafer or other test device. The chuck assembly  20  passes freely through a large aperture  22  in the platen  12  which permits the chuck assembly to be moved independently of the platen by the positioner  16  along X, Y and Z axes, i.e., horizontally along two mutually-perpendicular axes X and Y, and vertically along the Z axis. Likewise, the platen  12 , when moved vertically by the jacks  14 , moves independently of the chuck assembly  20  and the positioner  16 . 
   Mounted atop the platen  12  are multiple individual probe positioners such as  24  (only one of which is shown), each having an extending member  26  to which is mounted a probe holder  28  which in turn supports a respective probe  30  for contacting wafers and other test devices mounted atop the chuck assembly  20 . The probe positioner  24  has micrometer adjustments  34 ,  36  and  38  for adjusting the position of the probe holder  28 , and thus the probe  30 , along the X, Y and Z axes, respectively, relative to the chuck assembly  20 . The Z axis is exemplary of what is referred to herein loosely as the “axis of approach” between the probe holder  28  and the chuck assembly  20 , although directions of approach which are neither vertical nor linear, along which the probe tip and wafer or other test device are brought into contact with each other, are also intended to be included within the meaning of the term “axis of approach.” A further micrometer adjustment  40  adjustably tilts the probe holder  28  to adjust planarity of the probe with respect to the wafer or other test device supported by the chuck assembly  20 . As many as twelve individual probe positioners  24 , each supporting a respective probe, may be arranged on the platen  12  around the chuck assembly  20  so as to converge radially toward the chuck assembly similarly to the spokes of a wheel. With such an arrangement, each individual positioner  24  can independently adjust its respective probe in the X, Y and Z directions, while the jacks  14  can be actuated to raise or lower the platen  12  and thus all of the positioners  24  and their respective probes in unison. 
   An environment control enclosure is composed of an upper box portion  42  rigidly attached to the platen  12 , and a lower box portion  44  rigidly attached to the base  10 . Both portions are made of steel or other suitable electrically conductive material to provide EMI shielding. To accommodate the small vertical movement between the two box portions  42  and  44  when the jacks  14  are actuated to raise or lower the platen  12 , an electrically conductive resilient foam gasket  46 , preferably composed of silver or carbon-impregnated silicone, is interposed peripherally at their mating juncture at the front of the enclosure and between the lower portion  44  and the platen  12  so that an EMI, substantially hermetic, and light seal are all maintained despite relative vertical movement between the two box portions  42  and  44 . Even though the upper box portion  42  is rigidly attached to the platen  12 , a similar gasket  47  is preferably interposed between the portion  42  and the top of the platen to maximize sealing. 
   With reference to  FIGS. 5A and 5B , the top of the upper box portion  42  comprises an octagonal steel box  48  having eight side panels such as  49   a  and  49   b  through which the extending members  26  of the respective probe positioners  24  can penetrate movably. Each panel comprises a hollow housing in which a respective sheet  50  of resilient foam, which may be similar to the above-identified gasket material, is placed. Slits such as  52  are partially cut vertically in the foam in alignment with slots  54  formed in the inner and outer surfaces of each panel housing, through which a respective extending member  26  of a respective probe positioner  24  can pass movably. The slitted foam permits X, Y and Z movement of the extending members  26  of each probe positioner, while maintaining the EMI, substantially hermetic, and light seal provided by the enclosure. In four of the panels, to enable a greater range of X and Y movement, the foam sheet  50  is sandwiched between a pair of steel plates  55  having slots  54  therein, such plates being slidable transversely within the panel housing through a range of movement encompassed by larger slots  56  in the inner and outer surfaces of the panel housing. 
   Atop the octagonal box  48 , a circular viewing aperture  58  is provided, having a recessed circular transparent sealing window  60  therein. A bracket  62  holds an apertured sliding shutter  64  to selectively permit or prevent the passage of light through the window. A stereoscope (not shown) connected to a CRT monitor can be placed above the window to provide a magnified display of the wafer or other test device and the probe tip for proper probe placement during set-up or operation. Alternatively, the window  60  can be removed and a microscope lens (not shown) surrounded by a foam gasket can be inserted through the viewing aperture  58  with the foam providing EMI, hermetic and light sealing. The upper box portion  42  of the environment control enclosure also includes a hinged steel door  68  which pivots outwardly about the pivot axis of a hinge  70  as shown in  FIG. 2A . The hinge biases the door downwardly toward the top of the upper box portion  42  so that it forms a tight, overlapping, sliding peripheral seal  68   a  with the top of the upper box portion. When the door is open, and the chuck assembly  20  is moved by the positioner  16  beneath the door opening as shown in  FIG. 2A , the chuck assembly is accessible for loading and unloading. 
   With reference to  FIGS. 3 and 4 , the sealing integrity of the enclosure is likewise maintained throughout positioning movements by the motorized positioner  16  due to the provision of a series of four sealing plates  72 ,  74 ,  76  and  78  stacked slidably atop one another. The sizes of the plates progress increasingly from the top to the bottom one, as do the respective sizes of the central apertures  72   a ,  74   a ,  76   a  and  78   a  formed in the respective plates  72 ,  74 ,  76  and  78 , and the aperture  79   a  formed in the bottom  44   a  of the, lower box portion  44 . The central aperture  72   a  in the top plate  72  mates closely around the bearing housing  18   a  of the vertically-movable plunger  18 . The next plate in the downward progression, plate  74 , has an upwardly-projecting peripheral margin  74   b  which limits the extent to which the plate  72  can slide across the top of the plate  74 . The central aperture  74   a  in the plate  74  is of a size to permit the positioner  16  to move the plunger  18  and its bearing housing  18  a transversely along the X and Y axes until the edge of the top plate  72  abuts against the margin  74   b  of the plate  74 . The size of the aperture  74   a  is, however, too small to be uncovered by the top plate  72  when such abutment occurs, and therefore a seal is maintained between the plates  72  and  74  regardless of the movement of the plunger  18  and its bearing housing along the X and Y axes. Further movement of the plunger  18  and bearing housing in the direction of abutment of the plate  72  with the margin  74   b  results in the sliding of the plate  74  toward the peripheral margin  76   b  of the next underlying plate  76 . Again, the central aperture  76   a  in the plate  76  is large enough to permit abutment of the plate  74  with the margin  76   b , but small enough to prevent the plate  74  from uncovering the aperture  76   a , thereby likewise maintaining the seal between the plates  74  and  76 . Still further movement of the plunger  18  and bearing housing in the same direction causes similar sliding of the plates  76  and  78  relative to their underlying plates into abutment with the margin  78   b  and the side of the box portion  44 , respectively, without the apertures  78   a  and  79   a  becoming uncovered. This combination of sliding plates and central apertures of progressively increasing size permits a full range of movement of the plunger  18  along the X and Y axes by the positioner  16 , while maintaining the enclosure in a sealed condition despite such positioning movement. The EMI sealing provided by this structure is effective even with respect to the electric motors of the positioner  16 , since they are located below the sliding plates. 
   With particular reference to  FIGS. 3 ,  6  and  7 , the chuck assembly  20  is a modular construction usable either with or without an environment control enclosure. The plunger  18  supports an adjustment plate  79  which in turn supports first, second and third chuck assembly elements  80 ,  81  and  83 , respectively, positioned at progressively greater distances from the probe(s) along the axis of approach. Element  83  is a conductive rectangular stage or shield  83  which detachably mounts conductive elements  80  and  81  of circular shape. The element  80  has a planar upwardly-facing wafer-supporting surface  82  having an array of vertical apertures  84  therein. These apertures communicate with respective chambers separated by O-rings  88 , the chambers in turn being connected separately to different vacuum lines  90   a ,  90   b ,  90   c  ( FIG. 6 ) communicating through separately-controlled vacuum valves (not shown) with a source of vacuum. The respective vacuum lines selectively connect the respective chambers and their apertures to the source of vacuum to hold the wafer, or alternatively isolate the apertures from the source of vacuum to release the wafer, in a conventional manner. The separate operability of the respective chambers and their corresponding apertures enables the chuck to hold wafers of different diameters. 
   In addition to the circular elements  80  and  81 , auxiliary chucks such as  92  and  94  are detachably mounted on the corners of the element  83  by screws (not shown) independently of the elements  80  and  81  for the purpose of supporting contact substrates and calibration substrates while a wafer or other test device is simultaneously supported by the element  80 . Each auxiliary chuck  92 ,  94  has its own separate upwardly-facing planar surface  100 ,  102  respectively, in parallel relationship to the surface  82  of the element  80 . Vacuum apertures  104  protrude through the surfaces  100  and  102  from communication with respective chambers within the body of each auxiliary chuck. Each of these chambers in turn communicates through a separate vacuum line and a separate independently-actuated vacuum valve (not shown) with a source of vacuum, each such valve selectively connecting or isolating the respective sets of apertures  104  with respect to the source of vacuum independently of the operation of the apertures  84  of the element  80 , so as to selectively hold or release a contact substrate or calibration substrate located on the respective surfaces  100  and  102  independently of the wafer or other test device. An optional metal shield  106  may protrude upwardly from the edges of the element  83  to surround the other elements  80 ,  81  and the auxiliary chucks  92 ,  94 . 
   All of the chuck assembly elements  80 ,  81  and  83 , as well as the additional chuck assembly element  79 , are electrically insulated from one another even though they are constructed of electrically conductive metal and interconnected detachably by metallic screws such as  96 . With reference to  FIGS. 3 and 3A , the electrical insulation results from the fact that, in addition to the resilient dielectric O-rings  88 , dielectric spacers  85  and dielectric washers  86  are provided. These, coupled with the fact that the screws  96  pass through oversized apertures in the lower one of the two elements which each screw joins together thereby preventing electrical contact between the shank of the screw and the lower element, provide the desired insulation. As is apparent in  FIG. 3 , the dielectric spacers  85  extend over only minor portions of the opposing surface areas of the interconnected chuck assembly elements, thereby leaving air gaps between the opposing surfaces over major portions of their respective areas. Such air gaps minimize the dielectric constant in the spaces between the respective chuck assembly elements, thereby correspondingly minimizing the capacitance between them and the ability for electrical current to leak from one element to another. Preferably the spacers and washers  85  and  86 , respectively, are constructed of a material having the lowest possible dielectric constant consistent with high dimensional stability and high volume resistivity. A suitable material for the spacers and washers is glass epoxy, or acetyl homopolymer marketed under the trademark Delrin by E. I. DuPont. 
   With reference to  FIGS. 6 and 7 , the chuck assembly  20  also includes a pair of detachable electrical connector assemblies designated generally as  108  and  110 , each having at least two conductive connector elements  108   a ,  108   b  and  110   a ,  10   b , respectively, electrically insulated from each other, with the connector elements  108   b  and  110   b  preferably coaxially surrounding the connector elements  108   a  and  10   a  as guards therefor. If desired, the connector assemblies  108  and  110  can be triaxial in configuration so as to include respective outer shields  108   c ,  110   c  surrounding the respective connector elements  108   b  and  110   b , as shown in  FIG. 7 . The outer shields  108   c  and  110   c  may, if desired, be connected electrically through a shielding box  112  and a connector supporting bracket  113  to the chuck assembly element  83 , although such electrical connection is optional particularly in view of the surrounding EMI shielding enclosure  42 ,  44 . In any case, the respective connector elements  108   a  and  110   a  are electrically connected in parallel to a connector plate  114  matingly and detachably connected along a curved contact surface  114   a  by screws  114   b  and  114   c  to the curved edge of the chuck assembly element  80 . Conversely, the connector elements  108   b  and  110   b  are connected in parallel to a connector plate  116  similarly matingly connected detachably to element  81 . The connector elements pass freely through a rectangular opening  112   a  in the box  112 , being electrically insulated from the box  112  and therefore from the element  83 , as well as being electrically insulated from each other. Set screws such as  118  detachably fasten the-connector elements to the respective connector plates  114  and  116 . 
   Either coaxial or, as shown, triaxial cables  118  and  120  form portions of the respective detachable electrical connector assemblies  108  and  110 , as do their respective triaxial detachable connectors  122  and  124  which penetrate a wall of the lower portion  44  of the environment control enclosure so that the outer shields of the triaxial connectors  122 ,  124  are electrically connected to the enclosure. Further triaxial cables  122   a ,  124   a  are detachably connectable to the connectors  122  and  124  from suitable test equipment such as a Hewlett-Packard 4142B modular DC source/monitor or a Hewlett-Packard 5284A precision LCR meter, depending upon the test application. If the cables  118  and  120  are merely coaxial cables or other types of cables having only two conductors, one conductor interconnects the inner (signal) connector element of a respective connector  122  or  124  with a respective connector element  108   a  or  110   a , while the other conductor connects the intermediate (guard) connector element of a respective connector  122  or  124  with a respective connector element  108   b ,  110   b . U.S. Pat. No. 5,532,609 discloses a probe station and chuck and is hereby incorporated by reference. 
   The chuck assembly  20  with corresponding vertical apertures  84  and respective chambers separated by O-rings  88  permits selectively creating a vacuum within three different zones. Including the three O-rings  88  and the dielectric spacers  85  surrounding the metallic screws  96  permits securing adjacent first, second and third chuck assembly elements  80 ,  81  and  83  together. The concentric O-rings  88  are squeezed by the first and second chuck assembly elements and assist in distributing the force across the upper surface of the chuck assembly  20  to maintain a flat surface. However, the O-rings and dielectric spacers  85  have a greater dielectric constant than the-surrounding air resulting in leakage currents. Also, the additional material between adjoining chuck assembly elements  80 ,  81 , and  83  decreases the capacitance between the adjoining chuck assembly elements. Moreover, the dielectric material of the O-rings and dielectric spacers  85  builds up a charge therein during testing which increases the dielectric absorption. The O-rings and dielectric spacers  85  provides mechanical stability against warping the chuck when a wafer thereon is probed so that thinner chuck assembly elements  80 ,  81 , and  83  may be used. The height of the different O-rings and dielectric spacers  85  tend to be slightly different which introduces non-planarity in the upper surface when the first, second, and third chuck assembly elements  80 ,  81 , and  83  are secured together. 
   The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a partial front view of an exemplary embodiment of a wafer probe station constructed in accordance with the present invention. 
       FIG. 2  is a top view of the wafer probe station of  FIG. 1 . 
       FIG. 2A  is a partial top view of the wafer probe station of  FIG. 1  with the enclosure door shown partially open. 
       FIG. 3  is a partially sectional and partially schematic front view of the probe station of  FIG. 1 . 
       FIG. 3A  is an enlarged sectional view taken along line  3 A— 3 A of  FIG. 3 . 
       FIG. 4  is a top view of the sealing assembly where the motorized positioning mechanism extends through the bottom of the enclosure. 
       FIG. 5A  is an enlarged top detail view taken along line  5 A— 5 A of  FIG. 1 . 
       FIG. 5B  is an enlarged top sectional view taken along line  5 B- 5 B of  FIG. 1 . 
       FIG. 6  is a partially schematic top detail view of the chuck assembly, taken along line  6 — 6  of  FIG. 3 . 
       FIG. 7  is a partially sectional front view of the chuck assembly of  FIG. 6 . 
       FIG. 8  is a perspective view of a chuck illustrating a set of spacers and vacuum interconnections. 
       FIG. 9  is a plan view of the bottom surface of the upper chuck assembly element. 
       FIG. 10  is a plan view of the upper surface of the upper chuck assembly element. 
       FIG. 11  is a cross sectional view of a multi-layer chuck. 
       FIG. 12  is an enlarged cross sectional view of the interconnection between a pair of chuck assembly elements of the chuck of  FIG. 11 . 
       FIG. 13  is an enlarged cross sectional view of the interconnection between a pair of chuck assembly elements of the chuck of  FIG. 11  illustrating a minimum air breakdown distance. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Traditionally chuck designers use thin chuck assembly elements and many radially arranged screws in order to permit the screws to be tightened tightly without significantly warping any of the chuck assembly elements, and in particular the upper chuck assembly element. Maintaining a flat planar upper chuck assembly element is important to permit accurate probing of the wafer and avoid breaking, or otherwise damaging, the wafer while probing. In a multi-layered chuck, the lower chuck assembly element is secured to the middle chuck assembly element, the middle-chuck assembly element in turn is secured to the upper chuck assembly element, which results in any non-uniformities of slightly different thicknesses of the chuck assembly elements and interposed dielectric elements creating a cumulative non-planarity. For example, non-uniformity in the planarity of the lower chuck assembly element and differences in the thickness of the dielectric spacers may result in the middle chuck assembly element being slightly warped when secured thereto. Non-uniformity in the planarity of the middle chuck assembly element, the slight warping of the middle chuck assembly element, and the differences in the thickness of the dielectric spacers and O-rings, may result significant warping of the upper chuck assembly element when secured to the middle chuck assembly element. Accordingly, the thicknesses and planarity of (1) each chuck assembly element, (2) dielectric spacers, and (3) O-rings, needs to be accurately controlled in order to achieve a planar upper surface of the upper chuck assembly element. 
   After consideration of the thin chuck assembly elements and the desire to minimize warping of the upper chuck assembly element, the present inventor came to the realization that a three point securement system, including for example three pins, permits defining the orientation of the upper chuck assembly element without inducing stress into the upper chuck assembly element  180 , as illustrated in  FIG. 8 . Preferably, the pins are substantially equal distant from one another. Changes in the spacing of the height of any of the pins  200 ,  202 ,  204  results in pivoting the upper chuck assembly element  180  about the remaining two pins in a manner free from introducing added stress and hence non-planarity of the upper surface  198  of the upper chuck assembly element. There are preferably no dielectric spacers which maintain, or otherwise define, the spacing between the upper and middle chuck assembly elements, other-than the pins  200 ,  202 ,  204 . The elimination of dielectric spacers, such as O-rings, avoids stressing the upper chuck assembly element when under pressing engagement with the middle chuck assembly element. Another benefit that may be achieved by using a three point system is that the orientation of the upper surface of the upper chuck assembly element may be defined with respect to the prober stage and probes with minimal, if any, planarization of the intervening layers. In other words, if the planarity of the middle and lower chuck assembly elements is not accurately controlled, the planarity of the upper chuck assembly element will not be affected. Normally the spacing between the upper/middle and middle/lower chuck assembly elements is relatively uniform to provide relatively uniform capacitance between the respective chuck assembly elements. It is to be understood that any suitable interconnection assembly involving three discrete points or regions of the chuck assembly elements may be employed. 
   Minimization of the spacers, such as O-rings, between the upper and middle chuck assembly elements reduces the capacitive coupling between the upper and middle chuck assembly elements to less than it would have been with additional dielectric layer material there between. The elimination of additional spacers likewise increases the resistance between adjacent chuck assembly elements. 
   Connecting each vacuum line(s) directly to the center of the upper chuck assembly element  180  normally requires at least one corresponding hole drilled radially into the upper chuck assembly element from which vertically extending vacuum chambers provide a vacuum to the upper surface  198  of the upper chuck assembly element. Machining the combination of radial and vertical holes requires highly accurate machining which is difficult, time consuming, and expensive. Machining such holes becomes increasingly more difficult as the size of the chucks increases. 
   After consideration of the difficulty of machining accurate holes into the side of the upper chuck assembly element  180 , the present inventor determined that machining a set of airways  210   a – 210   e  in the lower surface  208  of the upper chuck assembly element is easier and tends to be more accurate, as shown in  FIG. 9 . In addition, the airways  210   a – 210   e  in the lower surface  208  of the chuck may be readily cleaned of dust and debris. The lower surface  208  of the upper chuck assembly element is covered with a cover plate  212  (see  FIG. 11 ), which is preferably thin. The cover plate  212  is preferably secured to the upper chuck assembly with glue (not shown) and a thin layer of vacuum grease to provide a seal there between. Preferably, the cover plate  212  is conductive material electrically connected to the upper chuck assembly element. It is to be understood that the cover plate may be made of any material having any thickness, as desired. Referring to  FIG. 10 , a plurality of “zones” defined by vacuum holes  214   a – 214   e  to the upper surface  198  may be achieved, each of which is preferably concentric in nature, so that each “zone” may be individually controlled and provided a vacuum, if desired. This provides accurate pressure control for different sizes of wafers. For example, the diameters of the concentric rings may be, 2½″, 5½″, 7½″, and 11½″ to accommodate wafers having sizes of 3″, 6″, 8″, and 12″. This permits the system to be selectively controlled to accommodate the size of the wafer being tested so that uncovered vacuum holes are not attempting to provide a vacuum, which may reduce the vacuum pressure available and pull contaminated air through the system. Dust and other debris in contaminated air may result in a thin layer of dust within the vacuum interconnections, described later, resulting in a decrease in electrical isolation between the upper and middle chuck assembly elements. It is to be understood that any suitable structure may be used to define a series of airways between adjacent layers of material, such materials preferably being conductive and in face-to-face engagement. The definition of airways may even be used with chucks where the vacuum lines are interconnected to the upper chuck assembly element, together with the definition of airway. 
   The elimination of the O-rings between the adjacent upper and middle chuck assembly elements creates a dilemma as to of how to provide a vacuum to the top surface of the upper chuck assembly element, if desired. The present inventor determined that it is normally undesirable to attach a vacuum tube directly to the upper chuck assembly element because the exterior conductive surface of the vacuum tube is normally connected to shield potential. The shield potential of the exterior of the vacuum tube directly adjoining the upper chuck assembly element would result in an unguarded leakage current between the upper chuck assembly and the vacuum tube. 
   To provide a vacuum path between the middle chuck assembly element and the upper chuck assembly element a vacuum pin  206  interconnects respective vacuum lines and particular vacuum holes (e.g., “zones”) on the upper surface of the upper chuck assembly element, as illustrated in  FIG. 11 . Normally, one vacuum line and one vacuum pin is provided for each “zone.” The vacuum pins are preferably recessed into respective openings  220   a  and  220   b  in the facing surfaces  208  and  224  of the upper and middle chuck assembly elements. Each vacuum pin includes a pair of O-rings  222   a  and  222   b  which provides a seal within respective openings  220   a  and  220   b  and likewise permits the vacuum pins  206  to move within the openings. The spacing between the facing surfaces  208  and  224 , depth of the openings  220   a  and  220   b , and length of the vacuum pins  206  are preferably selected such that changes in the spacing between the surfaces still permit the vacuum pins  206  some movement within the openings  220   a  and  220   b . Accordingly, the vacuum pins “float” within the openings and do not determine, or otherwise limit, the spacing between the upper and middle chuck assembly elements. Further, the vacuum pins are not rigidly connected to both the upper and middle chuck assembly elements. Alternatively, the vacuum pins may be rigidly connected to one of the upper and middle chuck assembly elements, if desired. The vacuum pins are preferably constructed from a good dielectric material, such as Teflon or PCTFE. Preferably, the vacuum pin(s) are positioned at locations exterior to the pins  200 ,  201 ,  204  (e.g., the distance from the center of the chuck to the pins is less than the distance from the center of the chuck to the vacuum pins) to minimize noise. It is to be understood that any non-rigidly interconnected set (one or more) of vacuum paths that do not define the spacing may be provided between a pair of chuck assembly elements. 
   The pin securing the middle chuck assembly element  182  to the upper chuck assembly element  180  includes a portion thereunder that is open to the lower chuck assembly element, normally connected to shield. More specifically, the pin  204  electrically connected to the upper chuck assembly element  180  provides an unguarded leakage path through the middle chuck assembly element  182  to the lower chuck assembly element  184 . In existing designs, a small plate is secured over the opening to provide guarding. A more convenient guarding structure is a lower cover plate  230  over the pin openings, preferably covering a major portion of the middle chuck assembly element  182 . The lower cover plate  230  is electrically isolated from the pins. In addition, the plate  230  together with the middle chuck assembly element  182  defines vacuum paths. 
   Referring to  FIG. 12 , the pin structure provides both mechanical stability and electrical isolation. A threaded screw- 240  is inserted through the middle chuck assembly element  182  and threaded into a threaded opening  242  in the lower surface of the upper chuck assembly element  180 . A conductive circular generally U-shaped member  244  separates the upper and middle chuck assembly elements and is in pressing engagement with the upper chuck assembly element. The conductive U-shaped member  244  is electrically connected to the screw  240  and extends radially outward from the screw  240 . The conductive U-shaped member provides lateral stability of the chuck assembly. An insulating circular generally U-shaped member  246 , preferably made from PCTFE, opposes the conductive U-shaped member  244  and is in pressing engagement with the middle chuck assembly element. The insulating circular U-shaped member  246  self-centers to the conductive U-shaped member  244  within the upwardly extending portions thereof. A circular insulating insert  248  surrounds the threaded screw  240  within the opening  250  in the middle chuck assembly element and supports the inclined head portion  252  of the threaded screw  240 . In the case that the screw  240  does not have an inclined portion the insulating insert may support the head portion of the screw  240 . An insulating cover  254  is preferably placed over the end of the threaded screw  240  and preferably spaced apart therefrom. Over the end of the screw is the cover plate  230 , preferably connected to a guard potential. The pin structure may likewise be used, if desired, between other adjacent plates of the chuck assembly. 
   While making high voltage measurements the air between two conductors will break down, e.g., arc, if the conductors are sufficiently close together. For example, when testing at 5000 volts the spacing between conductors should be in excess of about 0.2 inches. Referring to  FIG. 13  (same as  FIG. 12 ), it may be observed that all of the paths through the air from the screw and conductive circular U-shaped member (signal potential) to another conductor at guard potential is greater than 0.2 inches, as indicated by the “- - - ” lines. For example, the fins of the U-shaped insulating member  246  may increases the creepage distance greater than about 0.2 inches. 
   After further consideration another factor impacting rigidly is the interconnecting materials themselves. Preferably, the conductive member is at least three times as thick as the insulating material between the adjacent chuck assembly elements, and more preferably at least six times as thick. In this manner, a major portion of the spacing material is rigid conductive material which is significantly less prone to compression than the insulating material under pressure. 
   After extensive testing the present inventor came to the further realization that the dielectric absorption of the dielectric material tends to drain faster when both sides of the dielectric material are in face-to-face contact with electrical conductors. In contrast, when only one side of the dielectric material is in face-to-face contact with an electrical conductor then the dielectric absorption drains slowly with changes in electrical potential and hence degrades the electrical performance. Accordingly, referring to  FIG. 12 , it may be observed that substantially all (or at least a major portion) of the insulating material in contact with a conductor has an opposing conductor. For example, the upper portion of the center insulating portion is not in contact with the conductive screw because it would be difficult to provide an opposing conductor, and be further complicated if a requisite spacing is necessary. 
   The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of-the invention is defined and limited only by the claims which follow.