Abstract:
Disclosed herein are exemplary embodiments of a contact system (referred to as a “Z-block”) for interfacing a semiconductor wafer to an electrical tester, and methods for making the same. In a preferred embodiment, the Z-block comprises three stacked pieces or layers: an upper and lower piece which are similar in structure, and a unique middle piece. The pieces each contain corresponding locking holes and probe pin holes. The locking holes are strategically arranged on each of the pieces to allow the stacked piece structure to be locked together at various points during its manufacture. After alignment of the probe pin holes in the various pieces, probe pins are injected into these holes. The probe pins are then aligned and locked into place by moving the middle piece relative to the upper and lower pieces. Such locking of the probe pins is accomplished through interaction of the middle piece with the shape of the probe pins, which prevents the probe pins from slipping out of the probe pin holes.

Description:
FIELD OF THE INVENTION 
   This invention relates a contact system for testing semiconductor devices, and preferably to a contact system for interfacing a semiconductor wafer to an electrical tester. 
   BACKGROUND 
   As is known, semiconductor integrated circuit devices (“chips”) can be tested while they are still present on the semiconductor wafer on which they were formed. Such wafer level testing is traditionally accomplished on a per chip basis, in which probe tips are brought into contact with bond pads for a given chip, the chip is stimulated and tested through the probe tips via a tester, and then the wafer is indexed and moved to the next chip which is similarly tested, etc. 
   However, systems also exist which are capable of testing an entire semiconductor wafer, i.e., all chips on the wafer simultaneously and without the need to index from chip to chip. While such systems can be used to test the chips on the wafer for basic functionality, they are also particularly useful to stress the chips for a limited period of time for the purpose of weeding out early latent failures, what is known in the art as “burn in.” When burning in a wafer, and as shown in  FIG. 1A , signals are sent from a tester  10  through a cable conduit  12  and edge connector  14  to a burn in board or “fan out” board  20 . As one skilled in the art will appreciate, the burn in board  20 , which typically constitutes a Printed Circuit Board (PCB), contains contact points  22  on its underside (shown in phantom) which match the locations of the bond pads  42  on the wafer  40  being tested or burned in. (Only several corresponding contact point  22 /bond pad  42  pairs are shown in  FIG. 1A  for simplicity, although one skilled in the art will recognize that tens of thousands or more can be present). Such bond pads  42  may constitute redistribution bond pads routed to convenient locations and at a more relaxed pitch that is easier to probe than are traditional bond pads. 
   To electrically and physically mate the contact points  22  with the bond pads  42 , an intermediary structure is interposed therebetween. This intermediary structure is sometimes known as a “Z-block,” which is so named because it mates the contact point  22 /bond pad  42  pairs along the Z (or vertical) axis. The Z-block  30  contains holes  45  therethrough which similarly match the location of each contact point  22 /bond pad  42  pair. Interposed in each hole is a conductive probe pin  32 . When the burn in board  20 , the Z-block  30 , and the wafer  40  are sandwiched together during a testing or burn-in operation, and as shown in cross-section in  FIG. 1B , the ends of the probe pins  32  will contact the contact points  22  on one side of the Z-block  30  and the bond pads  42  on the wafer  40  on the other side. Because of the mechanical nature of the coupling of this stack, it is preferred that the probe pins  32  be deformable in a spring like manner. Many shapes are possible for the probe pins  32 , which may comprise helical spring, leaf springs, “pogo pins,” springs made from flat sheets of metal, etc.; the S-shaped probe pin shown in the Figures is merely exemplary, and other probe pin designs will be discussed in this disclosure. 
   It can be difficult to construct a Z-block  30 , as a number of parameters constrain its design. For example, the Z-block must capture the probe pins  32  so that the pins will not fall through the Z-block. This usually requires the use of a probe pin  32  having two effective diameters: an end diameter D 1  and a body diameter D 2 , as shown in  FIG. 2A . In this example, the probe pin  32  is essentially a helical spring with straight ends used to contact the contact points  22 /bond pads  42 , although other pin shapes are possible. To accommodate and capture such a probe pin  32 , the Z-block  30  must in turn also contain through holes  45  with two different diameters: one diameter D 3  sufficiently large for the effective diameter of the probe pin end (D 1 ) to fit through, but smaller than the effective diameter of the probe pin body diameter (D 2 ); and a second diameter D 4  sufficiently large to accompany the probe pin body diameter (D 2 ). 
   This relationship can be accomplished in different ways. For example, and as shown in  FIG. 2A , the Z-block  30  can be formed of two separate pieces  50   a  and  b . Each piece has holes  45  drilled therein corresponding to the eventual location of the probe pins  32 . Through the use of an angled drill bit, holes  45  having a conical ends can be formed which meet the two-diameter requirement noted above for retaining the pins  32 . Such holes  45  must be drilled precisely along the Z-axis lest the diameters of the holes (e.g., D 3 ) become skewed or non-uniform, which can be very difficult or expensive to accomplish. Ultimately, the probe pins  32  are placed within the holes  45  in the lower of the two pieces  50   a , and thereafter the upper piece  50   b  is affixed (e.g., bolted, glued, etc.) to the lower piece  50   a  to capture the probe pins  32 , as shown in  FIG. 2B .  FIGS. 2C and 2D  disclose other Z-block geometries for capturing the probe pins  32  which again use multiple affixable pieces, although in these examples the pieces are drilled with perfectly cylindrical holes  45 . 
   Regardless of the scheme used to form the Z-block  32 , ultimately all of the schemes involve the same step of (1) placing the pins  32  into holes  45  in at least one lower piece of the Z-block, and (2) placing at least one other piece of the Z-block over the pins to capture them and affixing the pieces together. But this is difficult to do in practice. The probe pins  32  are very delicate, being on the order of 100 mils in length and made of wire which may have a thickness of approximately 3 mils for a 100 mil length coil spring pin. It is therefore difficult to insert the probe pins  32  into the lower piece  50   b  such that their ends protrude through the smaller diameter holes (D 3 ). Moreover, if the probe pins  32  lay askew in their holes  45  when the upper piece  50   b  is placed on top of the lower piece  50   a , the ends of the pins  32  may not pass through the holes  45  in the upper piece, and instead will become bent within the holes and unable to make contact with the points/pads  22  or  42 . This problem is exacerbated when it is recognized that a typical Z-block  30  may contain tens of thousands of probe pins  32 , thus requiring the ends of the pins to simultaneously pass through the upper piece  50   b  when it is mounted to the lower piece  50   a , a formidable challenge. Damage to any one of these pins  32  may require opening and re-working the Z-block  30  to replace affected or damages pins, which requires unaffixing (e.g., unbolting) the upper and lower pieces  50   a  and  50   b . This reopening procedure too can cause problems, as this operation can tend to drag the otherwise properly-aligned pins  32  out of their holes  45 , which can occur if the delicate pins bind to the smaller diameter portions (D 3 ) of the holes. This can be a problem even if no pins  32  are damaged through assembly of the Z-block, but instead become worn though use and need replacement. 
   Moreover, capturing the spring limits the sorts of probe pins that can be used with known prior art Z-block approaches. As noted above, the probe pins  32 , be they springs, pogo pins, etc., necessarily must contain end portions with smaller effective diameters (D 1 ) than the main body portion of the pins (D 2 ) so that they can be captured, but still protrude from, the holes  45  (D 3 , D 4 ). Such a limitation to the design of the probe tips is unfortunate. For example, consider the probe pin  32  of  FIG. 3 , which has a flat, long end  32   a . This end design for the pin may be used, for example, as the portion of the pin  32  to make contact with the bond pads  42 , which is beneficial as a flat end  32   a  is less likely to stab and damage the bond pads  42 . However, such a pin design cannot be used with prior art means for capturing the springs as shown in  FIGS. 2A–2D , because the effective diameter of end  32   a  (D 5 ) is greater than the effective diameter of the body of the spring (D 6 ). 
   Another problem that prior art Z-block designs do not adequately address is the issue of probe pin orientation within the holes of the Z-block. As shown by the probe pin design of  FIG. 3 , not all probe pins are rotatably symmetrical about their long axis  60 . But probe pins  32 , when placed in the holes  45  in prior art Z-blocks, will be able to rotate freely within the holes  45 . This is undesirable, especially for asymmetric probe pins. For example, as shown in  FIG. 4 , if the probe pin of  FIG. 3  was mounted within a prior art Z-block, it would be allowed to turn such that the various ends  32   a  of the probe pins  32  would become misaligned (in solid lines). Such non-uniformity is undesirable as it may make contact of such ends to bond pads  42 /contact points  22  unreliable or could result in shorting and limiting probe design. It would therefore be preferable for the probe pins  32  to be non-movably aligned in the holes, which would, for example, allow the ends  32   a  to have a uniform orientation (in dashed lines). 
   In short, there is room to improve to Z-block designs. Such improvement would preferably make assembly of the Z-block easier, installing and servicing of the probe pins easier, would accommodate a wide variety of probe pin designs, and would be cheaper to manufacture. This disclosure presents such solutions. 
   SUMMARY OF THE INVENTION 
   Disclosed herein are exemplary embodiments of a contact system (referred to as a “Z-block”) for interfacing a semiconductor wafer to an electrical tester, and methods for making the same. In a preferred embodiment, the Z-block comprises three stacked pieces or layers: an upper and lower piece which are similar in structure, and a unique middle piece. The pieces each contain corresponding locking holes and probe pin holes. The locking holes are strategically arranged on each of the pieces to allow the stacked piece structure to be locked together at various points during its manufacture. After alignment of the probe pin holes in the various pieces, probe pins are injected into these holes. The probe pins are then aligned and locked into place by moving the middle piece relative to the upper and lower pieces. Such locking of the probe pins is accomplished through interaction of the middle piece with the shape of the probe pins, which prevents the probe pins from slipping out of the probe pin holes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which: 
       FIGS. 1A and 1B  show the basic components of a prior art system for testing or burning in a wafer using an intermediary Z-block between the burn in board and the wafer. 
       FIGS. 2A–2D  shows prior art Z-block approaches in cross section, and shows capture of the probe pins within pieces of the Z-block. 
       FIG. 3  shows a spring design which can be used with the disclosed Z-block design, but which is not suitable for use with the Z-block designs of  FIGS. 2A–2D . 
       FIG. 4  shows rotational misalignment of the probe pins of  FIG. 3 . 
       FIGS. 5A–5B  show the various locking holes (b, c, d) and probe pins holes (a) in the upper, middle, and lower portions of an embodiment of the disclosed Z-block, and the relative orientation of those holes. 
       FIGS. 6–15  show sequential steps in assembling a Z-block using the pieces of  FIGS. 5A–5B , including initial locking of the stack, alignment of probe pin holes and temporary locking of the same, insertion of the probe pins, alignment of the probe pins, and locking of the probe pins in their holes. 
       FIGS. 16A–16D  show alternative designs for the probe pins and the pieces usable in the context of the disclosed Z-block. 
       FIG. 17  shows an alternative probe pin locking scheme. 
       FIGS. 18A–18C  show alternative Z-block designs using two, four, and five pieces. 
   

   DETAILED DESCRIPTION 
   Disclosed herein is an improved Z-block design, and methods for its manufacture. The disclosed design is simple to manufacture, easy to install the probe pins into, easy to service, and can accommodate a number of probe pin designs. Moreover, the design allows for rotation of the probe pins within the Z-block so that they can be aligned, and locking of those aligned pins into place. 
     FIGS. 5A and 5B  illustrate a simple illustrative embodiment of the Z-block  100 &#39;s design, and illustrate the various pieces of the Z-block in a plan view and perspective view respectively. In this embodiment, the Z-block  100  comprises three different pieces (e.g., plates): an upper piece  110 , a middle piece  120 , and a lower piece  130 . When the Z-block is assembled, each of these pieces is stacked and locked together to capture the probe pins  150 , as will be seen in subsequent drawings. 
   Each of the pieces  110 ,  120 ,  130  contains a number of holes (a, b, c, d) which perform various functions in the Z-block. Holes  110   a ,  120   a ,  130   a  are the holes that ultimately will contain the probe pins  150 ; only a few representative holes are shown in  FIGS. 5A and 5B  for clarity, but tens of thousand can be present. The b, c, and d holes allow the pieces of the Z-block  100  to be locked together at various points during the Z-block&#39;s assembly, or upon final assembly of the Z-block. Specifically, holes  110   b ,  120   b ,  130   b  allow the Z-block to be locked prior to probe pin insertion; holes  110   c ,  120   c ,  130   c  allow the Z-block to be locked during pin insertion; and holes  110   d ,  120   d ,  130   d , allow the Z-block to be locked after probe pin insertion and capture, which completes assembly of the Z-block. These various locking procedures will be described in further detail in conjunction with  FIGS. 6–15 . 
   Although the locking holes (b, c, d) are preferably disposed around the periphery of the pieces  110 ,  120 ,  130  and away from the working area of the pieces where the probe pins are located, this is not strictly necessary. Moreover, although only four of each of these locking holes are shown, more or less may be present (three, six, etc.). 
   The various holes (a, b, c, d) are preferably the same in the upper and lower pieces  110  and  130 , and in this regard the same piece can be used for both. Holes  110   a ,  130   a , which ultimately hold the springs, have a diameter of approximately 15 mils, while holes  10   b–d ,  130   b–d  have a larger diameter of roughly 1 mm. Of course, other diameters can be used. The middle piece  120 , however, is different; its probe pin holes  120   a  are the same as that of the upper and lower pieces  110 ,  130 , as are its locking holes  120   c . However, locking holes  120   b  are larger in diameter than corresponding holes  110   b ,  130   b , measuring approximately 2 mm in diameter. Furthermore, locking holes  120   d  are linearly shifted with respect to corresponding holes  110   d ,  130   d , although they are equal in size (again, approximately 1 mm). (They may also be shifted is other ways, such an angularly along a common radius as shown in  FIG. 17 ). These differences in the location and sizes of the various holes between the middle piece  120  and the upper/lower pieces  110 ,  130  are illustrated in  FIG. 5A , in which corresponding holes for the upper/lower pieces are shown in dotted lines on the top view of the middle piece  120 . Of course, the sizes of the holes are greatly exaggerated to illustrate their differences and functions more clearly. 
   The pieces  110 ,  120 , and  130  of the Z-block  100  are preferably formed from Printed Circuit Board (PCB) materials, such as Arlon 55 NT or FR4, although other materials can be used as well. It is preferred to use insulating materials for the pieces (or materials that have been coated or treated to make then insulating) so that the probe pins  150  in the Z-block  100  will not short to each other. As the Z-block  100  may be used in high-temperature applications, such as during burn in, it is also preferred that materials with relatively low Coefficients of Thermal Expansion (CTEs) be used which have a similar CTE to the wafer being tested. If necessary, non-functional holes can be drilled in the pieces to reduce their bulk to effectively lower their CTEs and allow for constrainment. The pieces  110 ,  120 , and  130  can also be coated to reduce sliding friction and to prevent them from bonding or sticking together. (As will be seen herein, the ability to slide the pieces with respect to one other is an important aspect of some embodiments of the invention). Teflon or Parylene works well for such coatings. 
   As will be seen later, the thicknesses for the pieces  110 ,  120 ,  130  can vary depending on the probe pin design that is to be used with the Z-block  100 . However, in a preferred embodiment, the upper and lower pieces  110 ,  130  are approximately 60–70 mils thick, while the middle piece  120  is approximately 15–20 mils thick. While it is illustrated that the pieces  110 ,  120 ,  130  forming the Z-block  100  are circular (to match the wafer  40  with which it interfaces), the pieces may take on other shapes, such as a rectangular or square shapes, as shown in dotted lines. 
   Referring now to  FIG. 6 , a cross-sectional view of the beginning of the assembly of the Z-block  100  is illustrated. In this illustration, each of the locking holes (b, c, d) are shown so that their various functions can be better understood. However, as  FIGS. 5A and 5B  reveal, such holes need not necessarily occur along a given cross section. Also, for convenience, the diameter of the probe pin holes (“a”) are shown as being of the same diameter as some of the locking holes, although in reality they would be different, and typically smaller. 
     FIG. 6  shows the three pieces  110 ,  120 ,  130  mounted on an assembly plate  155  and spaced therefrom by spacers  157 . The spacers  157  are approximately 15 mils thick and as will be made clear later provide for clearance of the ends of the probe pins once they are inserted into the pieces. The assembly plate  155  is preferably formed of a smooth material which will not bind to the probe pins, such as Pyrex, but can comprise any number of materials. The entire assembly is preferably mounted under a low-resolution optical microscope to  158  allow an operators to more easily perform the various assembly steps discussed herein. 
   As initially shown in  FIG. 6 , the three pieces  110 ,  120 ,  130  are roughly aligned with one another to form a stack. However, to prepare for eventual insertion of the probe pins, the stack is preferably stabilized using locking pins or dowels  170 , as shown in  FIG. 7 . The dowels  170  are preferably stainless steel and 1 mm in diameter, and are placed in the “b” locking holes in the various pieces. Holes  110   b ,  130   b  are slightly undersized (approximately 0.97 mm) to allow for a snug fit when the dowels  170  are pressed into the “b” holes by an operator, which can occur by hand or by using a tool (e.g., tweezers). Once the dowels  170  are in place, the stack is basically mechanically stable. Dowels  170  are preferably of approximately the same thickness as the stacked pieces, but can also protrude from the top or bottom of the stack so long as they will not affect performance of the Z-block  100  in operation. Dowels  170  could also extend from both sides for the purpose of keying to PCB test structures and/or assembly structures, etc. 
   At this point, the upper  110  and lower  130  pieces of the Z-block are rigidly affixed to one another, but the middle piece  120  is slidable between the two by virtue of its larger hole  120   b . As this hole is preferably 2 mm in diameter, the middle piece  120  can be shifted approximately +/−0.5 mm with respect to the dowel  170  in the X or Y (i.e., horizontal) direction. For example, as shown in  FIG. 8 , the middle piece  120  has been shifted approximately 0.5 mm to the right. Because middle piece  120  is moveable, an operator can push or pull the middle piece  120  to align the probe pin holes  110   a ,  120   a ,  130   a  to allow for smooth insertion of the probe pins  150 . This is facilitated by making the middle piece  120  slightly larger in size (e.g., diameter) than the upper/lower pieces  110 ,  130  so that the edges of the piece  120  can be grasped by the operator. (This size difference is not shown in the Figures for clarity). To align the middle piece  120 , the operator looks through the microscope  158  at a certain sample of probe pins holes (“a”) across the working surface to ensure that perfect, non-eclipsed circles can be seen within the holes. A light mounted on the underside of the Pyrex assembly plate  155  (not shown) can facilitate this procedure. Optionally, alignment of the middle piece  120  can be automated, using a motorized X–Y stage and optical alignment routines similar to those traditionally used for wafer probing. Alignment can also be accomplished, or can be more easily implemented, by pushing dowels with tapered tips into the probe pin holes (“a”), or into other locking holes (not shown) having a diameter just under those of the probe pin holes (“a”). 
   Once the middle piece  120  has been aligned, the stack is mechanically fixed to lock the middle piece  120  into place so that it can longer be horizontally shifted. This is accomplished by placing a second set of dowels  180  into the “c” holes within the pieces  110 ,  120 ,  130 , as shown in  FIG. 9 . Dowel  180  is again preferably stainless steel and 1 mm in diameter. However, dowels  180  are substantially longer than dowels  170  and protrude from the top of the stack (perhaps 0.25 to 0.5-inch), which facilitates their eventual removal. In this regard, note that the “c” holes are all uniform in diameter and are drilled such that their alignment correspond to alignment of the probe pin holes (“a”). Thus, once the probe pin holes “a” have been manually aligned, the “c” holes are aligned also. The “c” holes are preferably slightly larger (e.g., 1.03 mm) than the dowel  180  to allow it to be easily inserted and retrieved. As a result, dowel  180  can come to rest on the surface of the assembly plate  155 . 
   Once alignment of the middle piece  120  has been accomplished and has been locked into place using dowels  180 , the probe pins  150  can be inserted into the probe pin holes (“a”) as shown in  FIG. 10 . The probe pins  150  can be inserted in any number of ways, such as by hand, through the use of a pneumatic “gun” to “shoot” the pins into the holes, etc. In the example shown, the probe pins  150  of  FIG. 3  are used, but of course many other type of probe pins designs (helical, leaf springs, etc.) can be used. 
   Two points are worthy of note at this stage. First, because the probe pin holes (“a”) have been aligned, the probe pins  150  are easily inserted into the Z-block  100 . This is much easier when compared to the prior art approaches discussed above in which the pins are first placed into non-uniform diameter holes and later sandwiched between two pieces of the Z-block. As a result of the disclosed approach, the probe pins  150  are much less susceptible to damage or binding during Z-block assembly. Second, notice that one of the probe pins  150  (second from the left) is rotationally misaligned with respect to the other pins. Specifically, this pin is rotated at 90-degrees with respect to the other pins, as best shown in the top view of the stack in  FIG. 11 . This is important to note now, because in subsequent steps this rotational misalignment within the holes will be fixed, thus illustrating another important advantage to the disclosed approach. 
   Once the probe pins  150  are in place, the dowels  180  can be removed as shown in  FIG. 12 , which re-exposes the “c” holes. This once again allows the middle piece  120  to move relative to the upper/lower pieces  110 ,  130 . 
   As will be seen later, one aspect of the disclosed invention is ability to move the middle piece  120  to lock the probe pins into place. However, prior to this step in the assembly process, it may be desirable to rotationally align the pins in the probe pins holes (“a”). This is especially desirable if the pins are not rotationally symmetrical about their long axis, or if for some reason the probe pins have special ends that are advantageously oriented in a predictable format. Moreover, rotational alignment ensures that the probe pins can be subsequently locked without damage. Rotational alignment can also be critical for achieving consistent probe tip pointing accuracy necessary to interfacing with small pads or contacts, such as traditional wafer bond pads. 
   Such alignment of the probe pins can be affected by moving the middle piece  120 . In a preferred embodiment, and referring to  FIGS. 13A and 13B , the middle piece  120  is moved along a path  190 . Several routes for path  190  are possible to perform the alignment function, but in a preferred embodiment, the path  190  first contemplates moving the middle piece  120  to a position that ideally would lock the probe pins  150  into place, which in the illustrated example would be to the right relative to the upper/lower pieces  110 ,  130 . Of course, because not all of the probe pins are aligned at this point, free movement to the right (i.e., as far as dowels  170  would allow) might not be possible initially. Thereafter, the path of the middle piece  120  is moved in a circular fashion with respect to the upper/lower pieces  110 ,  130 . As best shown in  FIG. 13B , which shows a single hole  120   a  in the middle piece  120 , this path  190  tends to catch the probe pins  150  at some point along their cross section, and eventually coaxes the probe pins into the proper rotational alignment. 
   It is preferred to rotate the path  190  through at least one full circle and back to the eventual locking position, although more than one full circle could be transgressed to ensure that all of the probe pins have been coaxed and are rotating within their probe pin holes (“a”). Movement of the middle piece  120  can be performed by hand, although movement of the middle piece  120  can also be automated as discussed earlier. Regardless of the method used to move the middle piece  120 , care should be taken not to force the middle piece  120  to follow a strict set path  190 , as this could damage probe pins that are not yet properly rotationally aligned. Experience teaches that an operator moving the piece  120  by hand can tell when the middle piece  120  is binding and should not be forced further. When using automated alignment processes, feedback should be gauged and limits set to ensure that the middle piece  120  is not over-forced, and/or the path should be appropriately modified to ensure that alignment will be achieved. One possible alternative path  190  that might be suitable for such an automated system is a spiral, in which the displacement of the middle piece  120  from the upper/lower pieces  110 ,  130  is gradually radially increased as the middle piece is rotated around. 
   In any event, the end of the probe pin alignment procedure discussed above preferably results in bringing the middle piece  120  to a position which locks the aligned pins. As noted earlier, in the illustrated example, this occurs by moving the middle piece  120  to the right, as shown in  FIG. 14 . With the pins aligned, it can be seen that the middle piece  120  comes to rest within a bend in the probe pin  150 . Because the middle piece  120  is bounded by this bend, the probe pin  150  will be restrained in its vertical movement, and thus becomes captured in the stack of the Z-block  100 . With the pins aligned and locked into place, the assembly can be finally locked. Such locking is illustrated in  FIG. 15 , in which a third set of dowels  200  are press-fit into the “d” holes in the assembly. In this regard, note that the “d” holes in the middle piece  120  have been offset relative to the probe pins holes (“a”) such that bringing the piece  120  into the locked position renders the “d” holes wide open (see  FIG. 14 ) and ready to receive dowels  200 . This third set of dowels  200  are similar in construction to the first set of dowels ( 170 ), and are press fit into the slightly smaller diameter “d” holes, which like the “b” holes are approximately 0.97 mm in diameter. 
   At this point, the Z-block  100  assembly is finished, and advantages of the disclosed method can be appreciated. First, it is easy to mount the probe pins into the Z-block, as the holes into which the probe pins are put are straight cylindrical holes (see  FIG. 12 ). There is no need to sandwich the probe pins between two pieces of the Z-block, which as noted earlier has the propensity to damage them. The probe pins can all be aligned to a common orientation. Moreover, pins of varying geometries can be used when compared to the prior art methods disclosed earlier; for example, the pins of  FIG. 3  can be captured and used with the disclosed Z-block approach even though they have larger effective diameter probe ends ( FIG. 3 ,  32   a ). Additionally, the Z-block is relatively easy to service. If necessary, the pieces can be gently pried apart—for example, using a razor blade—to loosen the dowels  170 ,  200  present in the final assembly. After the dowels are loose, and specifically dowel  200 , the middle piece  120  can be moved to unlock the probe pins  150  for easy retrieval. 
   It should be noted that to lock the probe pins  150  in place, it is not necessary for the middle piece  120  to contact the pin in the horizontal direction, as is shown in  FIG. 15 . Instead, it is sufficient that the pin movement be impeded in the vertical direction so that the pins won&#39;t slip out of the probe pin holes (“a”). Such a means of capturing the probe pins is satisfactory even if the pins are allowed some vertical play within the probe pin holes (“a”). However, in a given design, it may be desired that the pins be more firmly held in the horizontal and/or vertical directions, and if so the middle piece can be made to contact the horizontal or vertical surfaces of the probe pins  150  by choosing appropriate hole spacing and piece thicknesses given the probe pin design at issue. 
   The disclosed alignment method (see  FIG. 13 ) may cause the probe pins to come to rest at different heights after they are locked. Consider for example helical probe pins. If one probe pin is rotated three-quarters of a turn during alignment, and another is only rotated half a turn, when locked they may come to rest at different heights given their helical nature. Should this cause concerns, the protruding probe tips in the Z-block  100  can be planarized after their manufacture and capture, for example, by using Chemical-Mechanical Planarization (CMP) techniques. 
   Many useful modifications can be made to the disclosed Z-block design. For example, and as shown in  FIGS. 16A–16D , several different types of probe pins designs can be used. The illustration of these different pin types shows that the design of the pin can dictate a logical design for the Z-block, and in particular the design of the thicknesses used for the various pieces so that the middle pieces will most logically line up with and capture the probe pin at issue. For example, in  FIGS. 16A–16B  the middle piece  120  is thicker to accommodate a thicker recess in the probe pin used in that Z-block. In  FIG. 16C , the probe pin holes (“a”) in the middle piece have beveled edges designed to mate with the spring (or helix) used for the probe pin.  FIG. 16D  shows a simplified design for a “pogo pin,” which has a spring inside of a cartridge. As seen, the cartridge has a niche cut out of it to mate with the middle piece  120  during locking. 
   Moreover, different locking schemes can be used that don&#39;t require linear shifting of the middle piece  120  to lock the pins. For example, as shown in  FIG. 17 , the “d” locking holes in the middle piece  120 —i.e., those used to finally lock the probe pins into position—can be offset set at an angle relative to the corresponding locking holes ( 110   d ,  130   d ) in the upper/lower pieces  110 ,  130 . Accordingly, to lock the probe pins, the middle piece  120  would simply be rotated with respect to the upper/lower pieces  110 ,  130 . 
   While the use of three pieces is particularly useful, other number of pieces could also be used, as illustrated in  FIG. 18 , which illustrates the use of Z-blocks with two pieces ( FIG. 18A ), four pieces ( FIG. 18B ), or five pieces ( FIG. 18C ) in which some or all of the pieces stand in slidable relation to one another.  FIG. 18A  is particularly interesting as it contains countersunk probe pin holes in the lower piece  130  to allow a portion of the probe pin to pass through to the upper piece  110  and ultimately out of the Z-block. By moving the upper piece  110  relative to the lower piece  130  (much as the middle piece  120  was moved in earlier embodiments), the probe pins can be aligned and locked. 
   While disclosed in the context of testing wafers, it should be understood that the disclosed Z-block can be used to test other sorts of planar electrical structures, such as PCBs. Moreover, while disclosed as being beneficial to the testing of entire wafers, the disclosed Z-block can be used to test individual chips on a wafer as well, and as such can be used in a more traditional wafer probe fashion. 
   Although it is preferred that the various pieces touch one another to form a stack of pieces, this is not strictly necessary, as the pieces can have spaces between them, which may ease their slidability. Referring to “stacked” pieces should thus be so understood. 
   Thus, it should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.