Abstract:
An interface structure for use in a semiconductor integrated circuit tester for connecting a test head interface to a DUT interface includes a first frame member having first and second opposite main faces, a second frame member having first and second opposite main faces, and a spacer securing the first and second frame members together in spaced relationship. A first cable assembly header is received in an aperture of the first frame member and includes a conductive element and electrically conductive terminal members exposed at a main face of the first frame member and electrically insulated from the conductive element of the first header. A second cable assembly header is received in an aperture of the second frame member and includes a conductive element and electrically conductive terminal members exposed at a main face of the second frame member and electrically insulated from the conductive element of the second header. Coaxial cables connect each terminal member of the first header to a corresponding terminal member of the second header.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to an interface structure for semiconductor integrated circuit test equipment. 
   A semiconductor integrated circuit die has an array of contact pads distributed in a predetermined pattern over a major surface of the die. 
   Semiconductor integrated circuits may be tested at the wafer stage, prior to dicing the wafer and packaging the individual integrated circuit chips, and at the device stage, after dicing and packaging. In either case, the test equipment typically includes a test head for supplying stimulus signals to, and receiving response signals from, the device under test (DUT). 
   In wafer stage testing, a wafer prober positions the DUT at a test location for testing whereas for packaged device testing, a device handler is used to position the DUT for testing. For convenience in the following description it will be assumed that the DUT is in wafer form and that the test head is in the so-called DUT down orientation in which the test head is oriented to engage a DUT whose major surface is presented upwards. 
   The test head of a conventional general-purpose semiconductor integrated circuit tester includes a chassis, a docking plate attached to the chassis at the bottom of the test head, and multiple pin cards mounted in the chassis. Referring to  FIG. 1 , each pin card  4  is provided with a pogo block and switch module  8 . Alignment pins  12  project from the module  8  and are received in alignment bores  14  of a docking plate  18  for precise positioning of the module  8  relative to the docking plate. The module  8  includes twenty-six electrical spring probe pins or contact pins  22 . A suitable spring probe pin is commonly referred to as a pogo pin and includes a socket that is firmly secured in the body of the module  8 , a barrel that is press fit into the socket, a plunger that is a sliding fit inside the barrel, and a spring inside the barrel urging the plunger toward a projecting position. As shown in  FIG. 1 , the plungers of the spring probe pins  22  project downwards beyond the docking plate. 
   The twenty-six spring probe pins  22  are arranged in two row of thirteen pins, and only one of these rows can be seen in  FIG. 1 . The thirteen pins in each row include one ground pin and eight signal I/O pins connected to the tester channel circuitry of the pin card and five auxiliary pins used for utility connections, for example for relay control. The sixteen signal I/O pins support eight or sixteen tester channels depending on tester configuration. 
   The spring probe pins  22  of the test head are distributed over an area that is much greater than the area of the major surface of the DUT. A prober interface structure is interposed between the spring probe pins  22  and the DUT and includes a prober interface board that is attached to the docking plate  18  and has on its upper side (the test head side) an array of pads that are engaged by the spring probe pins  22  and on its lower side (the DUT side) an array of pads distributed over an area that is smaller than the area occupied by the pads on the upper side of the prober interface board. A probe card is disposed parallel to the prober interface board and has an array of contact pads at its upper side corresponding to the array of contact pads at the lower side of the prober interface board and has probe needles projecting from its lower side for engaging the contact pads of the DUT. For reasons relating to the configuration of the conventional wafer prober, the probe card must generally be spaced by several centimeters from the prober interface board. Conventionally, this spacing is provided by a so-called pogo tower between the prober interface board and the probe card. A pogo tower typically comprises a generally cylindrical support structure and an array of double-ended spring probe pins that connect each contact pad on the lower side of the prober interface board to the corresponding contact pad on the upper side of the probe card. 
   During set-up of the tester, the prober interface board is positioned so that plungers of the spring probe pins  22  engage the pads on the upper side of the prober interface board and the prober interface board is then displaced towards the test head and secured to the docking plate, establishing electrically conductive pressure contact between the tip of each plunger and the respective contact pad. 
   The prober interface board must be manufactured with a high degree of precision to ensure that all the contact pads will remain in the correct positions, within the applicable tolerances, over the intended useful life of the board. The stringent requirements regarding the physical structure of the prober interface board result in the prober interface board being rather expensive to manufacture. 
   Although the test head of the conventional tester mentioned above can accommodate up to 64 pin cards, each of which may support sixteen I/O paths (for a total of 1024 I/O paths), some users of the conventional tester may require fewer than 1024 I/O paths and purchase a test head with fewer than 64 pin cards. 
   The conventional prober interface structure hitherto has been generally satisfactory, but as the frequencies of the signals that must be propagated between the pin cards and the probe card increases, the conventional prober interface structure approaches the limits of its performance. In particular, the I/O path should be able to propagate signals at frequencies in excess of 4 GHz with minimal cross talk and low return loss. Preferably, the signal paths should be matched in length to minimize need for deskew and to provide uniform I/O capacitance and performance. For example, the two paths that carry the two components of a differential signal should be matched in length to within about 2.5 mm. It is difficult to meet these demanding requirements in an interface structure that includes a printed circuit board of the size of a conventional probe interface board. 
   Another conventional prober interface structure comprises a prober interface board and a tower structure that is permanently attached to the prober interface board. The prober interface board has on its upper side an array of pads that are engaged by contact pins in the test head and the tower structure incorporates contact pins that project downwardly from the prober interface structure for engaging contact pads on the upper side of the probe card. The prober interface board has an array of pads distributed over its lower side, and each of these pads is connected to a corresponding contact pin of the tower structure by a cable that is attached at one end to the contact pad of the prober interface board and at its other end to the pogo pin of the tower structure. 
   SUMMARY OF THE INVENTION 
   In accordance with a first aspect of the invention there is provided an interface structure for use in a semiconductor integrated circuit tester for connecting a test head interface to a DUT interface, comprising a first frame member having first and second opposite main faces and defining an aperture that opens at the first and second main faces of the first frame member, a second frame member having first and second opposite main faces and defining an aperture that opens at the first and second main faces of the second frame member, a spacer securing the first and second frame members together with their second main faces in spaced confronting relationship, and a cable assembly comprising a first header received in the aperture of the first frame member and including a conductive element and a plurality of electrically conductive terminal members exposed at the first main face of the first frame member and electrically insulated from the conductive element of the first header, a second header received in the aperture of the second frame member and including a conductive element and a plurality of electrically conductive terminal members exposed at the first main face of the second frame member and electrically insulated from the conductive element of the second header, and a plurality of coaxial cables connecting each terminal member of the first header to a corresponding terminal member of the second header. 
   In accordance with a second aspect of the invention there is provided an interface structure for use in a semiconductor integrated circuit tester for connecting a test head interface to a DUT interface, comprising a first frame member having first and second opposite main faces, a second frame member having first and second opposite main faces, a spacer securing the first and second frame members together in substantially parallel relationship with their second main faces in spaced confronting relationship, and a plurality of flexible conductors each connected between a terminal exposed at the first main face of the first frame member and a corresponding terminal exposed at the first main face of the second frame member, and wherein the second frame member can be secured to the spacer in at least first and second different locations relative to the first frame member. 
   In accordance with a third aspect of the invention there is provided an interface structure for use in a semiconductor integrated circuit tester for connecting a test head interface to a DUT interface, comprising a first frame member having first and second opposite main faces, a second frame member having first and second opposite main faces, the second frame member being secured to the first frame member with the second main faces of the first and second frame members in confronting relationship, a plurality of flexible conductors each connected between a terminal exposed at the first main face of the first frame member and a corresponding terminal exposed at the first main face of the second frame member, and at least one energy storage device interposed between the first and second frame members and urging the frame members apart. 
   In accordance with a fourth aspect of the invention there is provided an interface structure for use in a semiconductor integrated circuit tester for connecting a test head interface to a DUT interface, comprising a first frame member having first and second opposite main faces, a second frame member having first and second opposite main faces, the second frame member being secured to the first frame member with the second main faces of the first and second frame members in confronting relationship, with the second frame member spaced from the first frame member along an axis, and in a manner allowing positively limited movement of the second frame member relative to the first frame member in directions perpendicular to said axis, and a plurality of flexible conductors each connected between a terminal exposed at the first main face of the first frame member and a corresponding terminal exposed at the first main face of the second frame member. 
   In accordance with a fifth aspect of the invention there is provided an interface structure for use in a semiconductor integrated circuit tester for connecting a test head interface to a DUT interface, comprising a first frame member having first and second opposite main faces and defining an aperture that opens at the first and second main faces of the first frame member, a second frame member having first and second opposite main faces and defining an aperture that opens at the first and second main faces of the second frame member, a spacer securing the first and second frame members together with their second main faces in spaced confronting relationship, and a cable assembly comprising a first header received in the aperture of the first frame member, a second header received in the aperture of the second frame member, and a plurality of compliant, elongated signal propagating elements each having a first end fitted in the first header and a second end fitted in the second header, for propagating respective signals between the first main faces of the first and second frame members. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which 
       FIG. 1  is a partial schematic sectional view of a test head in accordance with the prior art, 
       FIG. 2  is a side elevation of a test head equipped with a prober interface structure embodying the invention, the test head being shown in DUT down orientation, 
       FIG. 3  is a perspective view of the test head in DUT up configuration, 
       FIG. 4  is an enlarged view of the prober interface structure shown in  FIGS. 2 and 3 , 
       FIG. 5  is an exploded view of the prober interface structure, 
       FIG. 6  is a perspective view of the prober interface structure in inverted orientation relative to  FIG. 4 , 
       FIG. 7  is a perspective view of a cable assembly that is included in the prober interface structure, 
       FIG. 8  is a side elevation of the cable assembly, 
       FIG. 9  is a partial enlarged view of one part of the cable assembly, 
       FIG. 10  is a partial enlarged view of another part of the cable assembly, 
       FIG. 11  is a partial sectional view of a second prober interface structure embodying the invention, and 
       FIG. 12  is a plan view of tart of a cable assembly that may be used in another embodiment of the invention. 
   

   Words of orientation that are used in the specification in connection with a structure or element are relative to the orientation of the structure or element when the test head is in the DUT down orientation, as shown in  FIG. 2  of the drawings. It will be appreciated, however, that this is merely for convenience in description and is not intended to limit the claims. 
   At least one embodiment of the invention is described in detail below with reference to the drawings. For the sake of clarity and definiteness of the detailed description, the detailed description may refer to specific values or ranges of values, but it should be understood that unless the context indicates otherwise, the values are given by way of example and it is not intended that these values or ranges should limit the scope of the claims. 
   DETAILED DESCRIPTION 
     FIG. 2  shows the test head  40  of a semiconductor integrated circuit tester in DUT down orientation. The test head is mounted in a yoke  44  that is attached to a test head manipulator (not shown). The manipulator and yoke allow the test head to be moved in three translational degrees of freedom and three rotational degrees of freedom. The test head includes a chassis (not shown) in which up to 64 pin cards, similar to the pin card  4  shown in  FIG. 1 , are mounted so that they radiate from a vertical axis. The test head  40  further includes a housing  48  surrounding the chassis and the pin cards, and a docking plate  52  attached to the chassis and provided with a docking mechanism  56  for docking the test head to a wafer prober (not shown). A prober interface structure  60 , which is shown partially in  FIG. 3 , is attached to the test head chassis. The switch modules of the pin cards are precisely aligned relative to the docking plate  18  by alignment pins that are received in alignment bores in the docking plate (similarly to the arrangement shown in  FIG. 1 ) and the prober interface structure is precisely aligned relative to the docking plate by alignment pins that project from the docking plate and are received in alignment bores in the interface structure. In this manner, the switch modules are precisely aligned relative to the prober interface structure  60 . 
   Referring to  FIGS. 4–6 , the prober interface structure  60  comprises a multi-part body  62 . An upper frame  64  that is generally annular in configuration and has eight sectors  68 , each of which has eight rectangular slots  72  ( FIG. 6 ), is attached to the body  62 . 
   The prober interface structure includes a lower frame  76  which is generally annular in configuration but is smaller in diameter than the upper frame  64 . The lower frame  76  has eight sectors  80  ( FIG. 5 ) and each sector has eight substantially rectangular slots  84  ( FIG. 4 ). 
   The prober interface structure  60  further includes a cable assembly for each of the pin cards. Since there may be up to 64 pin cards, the prober interface structure may include up to 64 cable assemblies. The cable assemblies are not shown in  FIG. 3  or in  FIGS. 4–6  but one cable assembly, designated  88 , is illustrated in  FIGS. 7–10 . Referring to  FIGS. 7 and 8 , the cable assembly  88  comprises an upper header  90 , a lower header  92 , and multiple coaxial cables  94  each attached at one end to the upper header  90  and at its opposite end to the lower header  92 . 
   The upper header is a composite structure and comprises a dielectric body  98  and a conductive insert  98 A fitted in a recess in the body  98 . The upper header is dimensioned to fit in one of the slots  72  of the upper frame  64 . The body  98  has two ears  100  that are formed with holes for attachment screws for attaching the upper header to the upper frame  64 . Each ear is provided with an alignment pin  102  for entering an alignment bore in the frame  64  for positioning the header relative to the frame. The alignment pins  102  are asymmetrically positioned so that the header will fit in the selected slot of the upper frame  64  in only one orientation. The header  90  is also formed with multiple bores  106  ( FIG. 9 ) for receiving the upper ends of the coaxial cables  94 . The upper end of each bore is countersunk as shown at  110  to accommodate a washer  114  made of insulating dielectric material such as PTFE. A contact element  118  is placed over the washer  114  and covers the center hole of the washer. 
   The coaxial cables are of conventional structure and each includes a center conductor  122 , an insulating sleeve (not shown), typically made of PTFE, an outer shield conductor  126 , and a protective outer jacket  130 . The upper end of the cable is prepared by stripping the protective jacket  130  over a length slightly less than the depth of the hole  106 , exposing the shield conductor  126 . The shield conductor  126  and the insulating sleeve are stripped from the center conductor  122  over a small length thereof so as to expose a short stub of the center conductor, as shown in  FIG. 9 . The upper end of the cable is force-fit into the bore  106 , or is soldered into the bore  106 , in order to provide a good electrically conductive connection between the conductive insert  98 A and the shield conductor  126 , and the center conductor  122  is electrically connected at its upper end to the contact element  118 , for example by soldering the upper end into a hole or recess at the underside of the contact element  118 . 
   Referring to  FIG. 10 , the lower header  92  is dimensioned to fit in one of the slots  84  of the lower frame  76  and is a composite structure that comprises a dielectric body  134  and a conductive insert  134 A fitted in a recess in the body  124 . The body  134  has ears  136  and also has alignment pins  138 , similar to the alignment pins  102  of the upper header  90 . The lower header  92  is also formed with bores  142  for receiving the lower ends of the cables  94  respectively. The lower length segment of each bore  142  is occupied by a filler sleeve  146  of dielectric insulating material such as PTFE. The filler sleeve has an axial passage that is of substantially uniform diameter over most of its length from the lower end of the filler sleeve towards the upper end thereof but is provided with a narrow throat at its upper end. A cup-like socket  150  is a tight press fit in the wider part of the axial passage. A conventional single-ended spring probe pin  154  is also press fit into the axial passage of the sleeve  146 . The barrel of the spring probe pin  154  fits firmly into the socket  150 . 
   The lower end of the cable  94  is prepared in similar fashion to the upper end and the portion from which the protective jacket has been stripped is soldered or force-fit into the bore  142 . The protruding end of the center conductor  122  is electrically connected to the socket  150 , for example by soldering or crimping. 
   The conductive insert  134 A provides a common signal ground for all the cables  94 . However, should a common signal ground not be desired, the lower header  92  may be made entirely of electrically insulating material. 
   The cable assembly  88  can be made using conventional mass production techniques so that the length of the conductive path from the top of the contact element  118  to the tip of the spring probe pin  154  (when flush with the lower surface of the lower header) differs by an insignificant amount from a specified nominal path length. In addition, the electrical characteristics of the I/O paths employing coaxial cables are more favorable for propagation of high frequency signals than those of the conductive traces of a conventional printed circuit board. 
   Each cable can be tested, for example by use of time domain reflectometry, before fitting in the upper and lower headers and rejected if its electrical behavior is out of tolerance. Quality control of this nature is not possible when the conductive path is a trace on a printed circuit board. Moreover, the cables can be tested after installation in the upper and lower headers and replaced in the event of deterioration or damage. 
   In order to install the cable assembly  98  in the prober interface structure  60 , the upper header  90  is positioned so that the main body of the header extends into the selected slot  72  and the ears  100  lie against the frame  64 , and correspondingly the lower header  92  is inserted from above through the corresponding slot  84  in the lower frame  76  and is manipulated to position the ears  136  against the lower frame with the main body of the lower header extending upwards into the slot  84 . The upper and lower headers are attached to the upper and lower frames respectively by screws extending through the holes in the ears of the respective headers and engaging the respective frames. The frames  64  and  76  and the headers  90  and  92  can readily be manufactured so that the upper surfaces of the headers  90  are essentially coplanar when installed in the frames  64 , and similarly the lower surfaces of the headers  92  are essentially coplanar when installed in the lower frame  76 . 
   When the prober interface structure  60  is attached to the test head and the test head is docked to the wafer prober, the lower surfaces of the headers  92  are in confronting relationship with the upper surface of the probe card. The headers  92  should be spaced from the probe card in order to reduce the capacitance between the conductive inserts  134 A and the conductive traces on the upper surface of the probe card, but the spacing should be limited in order that the pogo pins should not project from the conductive inserts, and therefore be unshielded, to an excessive extent. 
   The switch module of the pin card also includes additional spring probe pins (not shown) that are grounded by the pin card and engage the conductive insert  98 A. Since the shield conductors  126  of the coaxial cables are all electrically connected to the insert  98 A, a common ground is established at the upper header for all the coaxial cables at the lower header  92 . 
     FIGS. 7 and 8  also show several pins  158  that are connected to contact elements  160  on the upper side of the conductive insert  98 A of the upper header  90  and pins  164  that are connected to spring probe pins  166  having plungers that project from the lower face of the lower header  92 . Each of the pins  158  is connected to a corresponding pin  164  by a suitable flexible conductor (not shown), which may be a coaxial cable or an unshielded wire. The additional conductive paths that are provided in this manner may be used to provide utility connections between the probe card and the pin card. 
   The cable assemblies can be installed and removed as needed. For example, a user who initially requires a prober interface structure to support  32  pin cards can buy a prober interface structure that is populated with only 32 cable assemblies and thereby minimize the initial cost of the prober interface structure. If the user subsequently installs additional pin cards in the test head, additional cable assemblies can be installed in the prober interface structure and the investment in the original prober interface structure is preserved. If the user wishes to upgrade any or all of the I/O paths, the existing cable assemblies can be removed and replaced with assemblies manufactured to higher tolerances or with superior components without its being necessary to replace the housing. Further, if a cable assembly should be damaged, it can be replaced without requiring that any other cable assemblies be removed from the prober interface structure. 
   In some tester configurations, it is desirable that the lower annular frame  76  and the upper annular frame  64  should lie on a common vertical axis whereas in other configurations, it might be desirable for the central axis of the lower annular frame  76  to be offset horizontally from the central axis of the upper annular frame  64 . In particular, this may be necessary to avoid interference between the test head and the wafer prober. Referring to  FIGS. 3–5 , the multi-part body  62  includes a main housing  168 , to which the upper frame  64  is attached, a secondary housing  170 , to which the lower frame  76  is attached, and an adaptor plate  172  to which the secondary housing  170  is attached. The main housing  168  includes a bottom plate  176  that defines an oblong opening  180 . The adaptor plate  172  is oblong and is fitted in the oblong opening  180 , the longitudinal dimension of which is somewhat greater than the corresponding dimension of the plate  172 . The main housing is provided with screw holes (not shown) around the periphery of the opening  180  and the adaptor plate is provided with two sets of holes for receiving screws for attaching the adaptor plate to the main housing  168 . In order to attach the adaptor plate to the housing  168 , the adaptor plate  172  is placed in the oblong opening  180  and is positioned so that one of the two sets of holes in the plate  172  are aligned with the screw holes in the main housing  168 , and the plate  172  is then secured to the housing  168  by screws that pass through the selected set of holes in the plate  172  and engage the screw holes in the housing  168 . In this manner, the adaptor plate  172  can be attached to the main housing  168  either in the position shown in  FIGS. 3 and 4  or in a position in which the plate  172  and the secondary housing  170  attached thereto are displaced relative to the main housing  168  by a distance D in the direction of the arrow A. In either case, the gap between the adaptor plate  172  and the plate  176  can be filled by a crescent-shaped filler plate  184 . The different positions of the plate  172  relative to the main housing  168  are accommodated by the compliant nature of the coaxial cables. A corresponding capability is not available in the conventional tester that employs a prober interface board and a pogo tower. 
   It is possible that when the test head is docked to the wafer prober, the upper surface of the probe card will not be precisely parallel to the lower surface of the frame  76 . In this case, a spring probe pin in one region of the frame  76  might be fully depressed into its header  92  while the plunger of a spring probe pin in another region of the frame  76  still protrudes slightly below the lower surface of its header. It is, however, desirable with respect to the electrical characteristics of the I/O path that the spring probe pin be fully depressed, and that the plungers should not protrude below the lower surface of their headers. Referring to  FIG. 11 , the possibility of the lower surface of the frame  76  not being parallel with the upper surface of the probe card can be accommodated by interposing energy storage devices between the frame  76  and the housing  170 . Specifically, the screws  186  that attach the frame  76  to the secondary housing  170  pass through springs  188 , which for convenience are depicted as helical springs but may in fact be so-called Belville springs. The springs  188  are compressible along the common axis of the upper and lower frames, and the total force required to compress the springs  188  exceeds the total force required to compress all the spring probe pins such that the tips of the spring probe pins are flush with the lower surface of the frame  76 . Accordingly, when the test head is brought into docking relationship with the prober, and actuation of the docking mechanism draws the test head towards the wafer prober, urging the probe card upwards relative to the lower frame  76 , the docking force will act to compress all the spring probe pins and bring the frame  76  into parallel relationship with the probe card before the axial spring set is fully compressed. 
     FIG. 11  also illustrates a probe card support  190  that is attached to the wafer prober and a probe card  192  that is seated in the probe card support. Alignment pins  194  project upwardly from the probe card support and pass through bores in the probe card  192  and are received in alignment bores in the frame  76 . Clearance between the frame  76  and the screws  186  allows a limited range of horizontal movement of the frame  76  relative to the secondary housing  170 , which movement is accommodated by the compliant nature of the coaxial cables. By allowing limited movement of the frame  76  relative to the housing  170 , it is possible to provide a high degree of precision in aligning the contact pins  154  to the pads on the probe card without requiring that the multi-part body  62  and the test head secured thereto be positioned with the same degree of precision. 
   In the embodiment that has been described with reference to  FIGS. 2–11 , there is one cable assembly  88  for each pin card. Thus, each cable assembly serves a single pin card. In another embodiment of the invention, one or more of the cable assemblies serves multiple pin cards.  FIG. 12  is a plan view of the upper header  90 ′ of a cable assembly that is designed to serve four pin cards, in lieu of four of the cable assemblies  88 , and it will be seen that the header  90 ′ includes a dielectric body  234  and a conductive insert  234 A fitted in a recess in the body  234 . The body  23  has inner and outer flanges  238  that lie against the frame  64  when the cable assembly is installed in the prober interface structure, and the conductive insert  234 A has four pairs of rows of countersunk bores accommodating respective dielectric washers  114  that contain contact elements (not shown). 
   It has been proposed that at least some of the signals emitted and received by an integrated circuit should be optical signals, and accordingly it is desirable to be able to test such an integrated circuit using optical stimulus and response signals. Therefore, although the invention has been described in connection with compliant propagating elements in the form of coaxial cables, which provide electrical signal paths, in its broader aspects the invention is also applicable to compliant propagating elements in the form of optical fibers, which provide optical signal paths. Even though some or all of the propagating elements for stimulus and response signals may be optical fibers, in general it will be necessary to provide compliant electrical paths between the test head interface and the DUT interface in order to supply operating power to the DUT. 
   It will be appreciated that the invention is not restricted to the particular embodiments that have been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof. For example, although the illustrated embodiments of the invention have been described with reference to a prober interface structure that is attached to a test head for use in conjunction with a wafer prober for wafer stage testing, the invention is also applicable to an interface structure that is used in device stage testing. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated.