Patent Publication Number: US-9897627-B2

Title: Test probe substrate

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 14/495,606 filed Sep. 24, 2014, the complete disclosure of which is expressly incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD 
     The present disclosure relates to the physical sciences, and, more particularly, to test probe structures and methods of fabrication thereof. 
     BACKGROUND 
     Integrated circuits can be mass produced by forming arrays of chips on semiconductor wafers. The wafers can be subsequently diced in order to form dies, each die including a circuit. Test probes are employed for testing integrated circuits, either as parts of a wafer or as discrete chips. The electrical characteristics of an integrated circuit are tested to ensure they conform to specifications and/or will perform as intended. Integrated circuit chips include surface contacts comprised of contact pads, solder bumps (e.g. C4 bumps), or other types of electrical contacts. Conductors corresponding to the surface contact locations of the integrated circuit chips are provided on test probes. 
     SUMMARY 
     Principles of the present disclosure provide an exemplary fabrication method that includes obtaining a structure comprising a test probe substrate including a top surface, one or more contact locations, and electrical conductors within the substrate, the contact locations being electrically connected to one or more of the electrical conductors, and forming a first dielectric layer over the top surface of the substrate. A plurality of first openings is formed within the first dielectric layer, thereby exposing the contact locations. Lateral extensions of one or more of the openings are formed within the first dielectric layer. An electrically conductive material is deposited within the first openings and lateral extensions. The method further includes depositing a second dielectric layer over the first dielectric layer and forming a plurality of second openings within the second dielectric layer, one or more of the second openings communicating with the lateral extensions. An electrically conductive material is deposited within the second openings to form an array of electrical contact regions electrically connected to the contact locations. The second dielectric layer is planarized. 
     A further exemplary fabrication method includes obtaining a test probe substrate including a top surface and a plurality of contact locations electrically connected to one or more electrical conductors and determining whether the contact locations match predetermined coordinate positions. A first dielectric layer is formed over the top surface of the substrate and a plurality of first openings are formed within the first dielectric layer, thereby exposing a plurality of the contact locations. Lateral extensions of the first openings are formed in the directions of the predetermined coordinate positions for the contact locations that do not match the predetermined coordinate positions. An electrically conductive material is deposited within the first openings and lateral extensions such that the conductive material contacts the contact locations. A second dielectric layer is deposited over the first dielectric layer. A plurality of second openings are formed within the second dielectric layer matching the predetermined coordinate positions, one or more of the second openings communicating with the lateral extensions. An electrically conductive material is deposited within the second openings to form an array of electrical contact regions electrically connected to the contact locations and conforming to the predetermined coordinate positions. The second dielectric layer is planarized. 
     A test probe structure in accordance with an exemplary embodiment includes a substrate including a top surface, a plurality of rows of contact locations, and electrical conductors within the substrate, the contact locations being electrically connected to one or more of the electrical conductors, one or more of the contact locations being misaligned with respect to one or more of the rows. A first dielectric layer is on the top surface of the substrate. A plurality of first openings corresponding to the contact locations extend through the first dielectric layer, each of the first openings being aligned with a corresponding one of the contact locations. At least one of the first openings corresponds to one of the misaligned contact locations and includes a lateral extension. A second dielectric layer is positioned over the first dielectric layer. A plurality of second openings extends through the second dielectric layer, the second openings being arranged in rows corresponding to the rows of contact locations. The second openings are arranged in rows without misalignment. An electrically conductive material is within each of the first and second openings and the lateral extension, the electrically conductive material contacting the contact locations. The second dielectric layer includes a planar top surface. 
     As used herein, “facilitating” an action includes performing the action, making the action easier, helping to carry the action out, or causing the action to be performed. Thus, by way of example and not limitation, instructions executing on one processor might facilitate an action carried out by instructions executing on a remote processor, by sending appropriate data or commands to cause or aid the action to be performed. For the avoidance of doubt, where an actor facilitates an action by other than performing the action, the action is nevertheless performed by some entity or combination of entities. 
     Test probe structures and fabrication methods as disclosed herein can provide substantial beneficial technical effects. For example, one or more embodiments may provide one or more of the following advantages:
         Effective use of probe substrate materials subject to distortion;   Corrections for non-planarity of probe substrates;   Corrections for positional inaccuracy of probe contact locations;   Enables use of LTCC (low temperature co-fire ceramic) substrate or glass or silicon substrates, which have a coefficient of thermal expansion near or matched to silicon;   Enables use of high modulus substrate base such as LTCC substrate to both facilitate uniformity of applied force to probes on substrate and to minimize non-coplanarity of probes with or without applied mechanical force and across temperature ranges of testing dies and/or wafers;   Effective design of wafer probe substrate to support high power delivery during test (current) and low voltage to support full power/pulsed power and at speed testing of circuits.   Power levels of &gt;200 watts per die and current of 0.1 amps per pin to over 1 amp per pin are possible with proper choice of base substrate, probe and low contact resistance;   Effective design of wafer probe substrate to support low frequency (MHz) to high frequency (GHz) test measurements such as with using short distance connections, appropriate pitch/spacing of input-output signals and shielded wiring;   Enables use of single die to multiple die or wafer level test probe substrates, providing flexibility to have high throughput to meet low cost and volume applications;   Testing of multiple products using the same test probe substrates.       

     These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic top plan illustration of a test probe substrate base including a plurality of vias or via pads, some of which are not in position to match test hardware; 
         FIG. 2  is a schematic top plan illustration of the substrate of  FIG. 1  including a dielectric layer formed thereon and processed to provide openings in the dielectric layer; 
         FIG. 3  is a schematic illustration showing a solder injection head and the substrate following injection of molten solder within the openings in the dielectric layer; 
         FIG. 4  is a schematic top plan illustration of the structure of  FIG. 3  showing a second dielectric layer deposited on the substrate, opened to form a desired grid, and following injection of molten solder by the solder injection head; 
         FIG. 5A  is a schematic side elevation view of a first embodiment of a test probe structure formed in accordance with a first exemplary method; 
         FIG. 5B  is an enlarged sectional view of a portion of the structure shown in  FIG. 5A ; 
         FIG. 6  is a schematic side elevation view of an alternative embodiment of a test probe structure formed in accordance with the steps shown in  FIGS. 1-4 , and 
         FIG. 7  is a schematic side elevation view of a further alternative embodiment of a test probe structure including an interposer layer. 
     
    
    
     DETAILED DESCRIPTION 
     Test probes used for testing integrated circuits include electrical contacts intended for contacting surface pads or other contact structures formed on IC chips. Such contact structures may be employed for power connections or I/O signals. Substrate materials employed for fabricating test probes may be subject to deformation, thereby causing misalignment of the electrical contacts on the test probe substrate. Ceramic substrate materials, for example, offer good strength and rigidity. Such substrates are typically fabricated using a sintering process. Wiring layers are formed within the substrate, which may comprise ceramic layers, for electrically connecting the probe contacts with testing equipment. The ceramic material undergoes shrinkage and possible distortion if the shrinkage is non-uniform. Such distortion can result in misalignment of the electrical contacts on the probe substrate with the corresponding contacts on an integrated circuit to be tested. It can further result in a loss of planarity of the surface that includes the electrical contacts. 
     Referring to  FIG. 1 , a top plan view of a test probe substrate base following a sintering process is provided. In one or more embodiments, the substrate base comprises LTCC (low temperature co-fire ceramic) having a coefficient of thermal expansion near or matched to silicon so probes can support tests at die level up to wafer level across a range of temperatures from less than 0° C. to greater than 100° C. The use of a high modulus ceramic substrate base such as an LTCC substrate facilitates uniformity of the applied force when the probe is employed for testing and minimizes non-coplanarity of probes with or without applied mechanical force and across the temperature range(s) of the testing dies and/or wafers. As shown schematically, contact locations  22 ,  22 A forming rows of via conductors or via pads are formed in the substrate  20 . The contact locations are on the substrate surface in the exemplary embodiment. Electrical conductors within the substrate  20  are electrically coupled to the via conductors or via pads. The electrical conductors may be comprised, at least in part, by the via conductors. The array of such via conductors or pads exposed at the illustrated surface of the substrate base is intended to correspond in some embodiments to surface contacts of an integrated circuit or other electronic device to be tested. In this exemplary embodiment, the arrangement of the rows and columns of via conductors or via pads only partially conforms to the desired configuration wherein the rows and columns would be parallel and the via conductors or via pads would be equally spaced. In some test probe substrates, such as ceramic substrates, distortion tends to be greater near the periphery of the surface than near the middle portion thereof. As shown in  FIG. 1 , some of the contact locations  22 A at the corner portions of the array are misaligned. The rows of contact locations are accordingly not entirely linear nor are the rows entirely parallel to each other as desired. In addition to the positional inaccuracy of the contact locations  22 A, the illustrated surface is non-planar. The X, Y and Z coordinates of some of the contact locations accordingly are not suitable for matching the test hardware (not shown). The contact locations  22 ,  22 A are mapped and stored, preferably electronically in a memory device, for future reference. These locations are compared to a set of predetermined coordinate positions that, for example, match the coordinate positions of the electrical contact surfaces of test hardware. As discussed below, the positional inaccuracy of the contact locations and non-planarity of the base substrate are both addressed in order to provide probe compatibility with the electronic devices to be tested and/or intermediate structures. 
     Referring to  FIG. 2 , a first dielectric layer  24  is provided on the surface of the substrate  20  shown in  FIG. 1 . In one exemplary embodiment, the first dielectric layer comprises a polyimide adhesive that is laminated or spun on the base substrate  20  and cured. In some exemplary embodiments, a polyimide adhesive layer having a thickness between three and ten microns is formed. A thicker dielectric layer, for example between fifty and one hundred microns, may be employed in other embodiments. The dielectric layer  24  may comprise a single layer or a series of layers. A mask (not shown) is provided on the dielectric layer  24 . The mask facilitates patterning of the dielectric layer to open the contact locations  22 ,  22 A through use of the stored data relating to contact location positions. A laser is employed in some embodiments to cut through the dielectric layer  24 . As shown in  FIG. 2 , the openings  26  in the dielectric formed by laser ablation are round where the contact locations  22  are satisfactory. With respect to the contact locations  22 A that are not in true X, Y and/or Z coordinate positions, tails  28  are formed in the dielectric layer  24  that extend from the round openings in the appropriate directions. The round portions of the openings are in registration with the contact locations  22 ,  22 A while the tails  28  extend in the directions of the correct locations for matching to test hardware or other elements. It will be appreciated that the openings  26  may include laterally extending portions having configurations that differ from those of the tails  28  shown in the figures. 
     Referring to  FIG. 3 , wiring  30  is provided by the injection of molten solder into the openings  26  and tails  28  in one exemplary embodiment. This may be accomplished by passing an injection mold solder (IMS) head  32  over the surface of the patterned dielectric layer  24  formed on the base substrate  20 . The head  32  includes a solder slot  34  that provides molten solder to the surface of the dielectric layer, filling the openings  26  down to the contact locations  22 ,  22 A as well as filling the laterally extending tails  28 . The solder injection process is conducted in a low oxygen environment, preferably less than 10,000 ppm oxygen in a nitrogen or forming gas environment. The IMS head  32  may further include an optional blade (not shown) that contacts the surface of the dielectric layer to prevent solder leakage. U.S. Pat. Nos. 5,244,143 and 7,784,673, which are incorporated by reference, disclose techniques and apparatus for filling cavities with molten solder. The contact locations  22 ,  22 A may include a nickel/gold finish or other suitable finish to facilitate wetting. The wiring  30  may be formed by processes other than molten solder injection. Exemplary processes include sputter deposited metallurgy and plated metallurgy. The surface of the substrate  20 , which now includes the first dielectric layer  24 , is planarized in some embodiments following the deposition of the wiring layer to provide a planar surface and to remove any excess metallurgy. Planarizing dielectric layer(s) formed prior to deposition of a final dielectric layer is beneficial when planarity is far from specification, but is not required when close to target specifications. As discussed above, in one or more exemplary embodiments, the contact locations are compared to a set of predetermined coordinate positions that correspond to the target specifications. This information is used to determine whether planarization can be satisfactorily effected by planarizing only the final dielectric layer or whether one or more intermediate dielectric layers, such as the first layer  24 , requires planarization. In one exemplary embodiment, the dielectric layer has an initial thickness of about five microns. A grinding or chemical mechanical polishing (CMP) process is employed if appropriate to form a planar surface having an average thickness of about two microns. Typically, planarization is utilized for each deposition layer but is particularly important with respect to the final deposition layer if the targeted specification of the probe planarity has not been achieved (for example, 2 μm in some fine pitch probe testing applications). The thickness of the resulting dielectric layer  24  may or may not be uniform depending on the surface characteristics of the substrate  20 . 
     In some embodiments, the deformation of the base substrate  20  may require the deposition of one or more additional dielectric and wiring layers. Each additional dielectric layer would be patterned and provided with corrective wiring to achieve positional accuracy using RDL (redistribution layer) processing. In one or more embodiments, openings and tails similar to those shown in  FIG. 2  are formed in subsequent dielectric layers. Further processing (e.g. CMP) is preferably, though not necessarily, performed following deposition of each dielectric layer to obtain a substantially planar surface on the resulting structure and, if applicable, to facilitate the deposition of molten solder. As discussed above, planarization of the final dielectric layer may be particularly important to meet product specifications. 
     Once the positional accuracy of the contact locations is acceptable via redistribution as described above, a final dielectric layer  38  may be deposited and patterned. In one exemplary embodiment, the final dielectric layer  38  is a second such layer deposited on the first layer  24 . Openings  40  are formed in the dielectric layer  38  as shown in  FIG. 4 . Referring to  FIG. 3  for exemplary purposes, some of the openings  40  are aligned with the contact locations  22  and openings  26  in the underlying dielectric layer  24 . Other openings  40  are aligned with the tails  28  formed in the underlying dielectric layer  24  as the underlying contact locations  22 A corresponding to some of the openings  40  are misaligned. As no further adjustment for contact location inaccuracy is necessary in this exemplary embodiment, none of the openings  40  requires an extension in any direction. The openings  40  within the final dielectric layer  38  which forms the top surface of the test probe substrate form parallel rows with no misaligned openings. In the exemplary embodiment of  FIG. 4 , the openings  40  are round. The openings can be formed by laser ablation or etching. The openings  40  are filled with an electrically conductive material such as solder. Exemplary materials for filling the openings  40  include Sn/Ag/Cu with Sn 96.5%,/Ag 2.5%/Cu 1%, SnCu with Sn 98.3%/Cu 1.7%, and Cu 6 /Sn 5  or other metallic compounds. The dielectric layer  38  is planarized by CMP or other suitable process in one or more exemplary embodiments. An injection molded solder process as described above may again be employed to extend the wiring  30  formed in previous step(s). In some applications, the perimeter of the substrate  20  is removed by grinding to permit the contact area of the structure to extend above the level of the remaining portion of the substrate. A step  42  is accordingly formed in one or more embodiments. 
     Finish metallurgy is deposited in some embodiments and is in electrical communication with the previously formed wiring  30 . Finish metallurgy may comprise copper having a finish layer of Ni/Au or alternative metallurgy that facilitates wetting. A test probe substrate  50  is shown in  FIG. 5A .  FIG. 5B  shows such a test probe substrate including finish metallurgy. The finish metallurgy  75 , such as Cu/Ni/Au, Cu/NiFe/Au, Ti/Cu/Ni/Au, TiW/Cu/Ni/Au, Cu/Ni/Pt or Cu/Ni/Rh or alternate composition are used to complete the structure for contact. The finish metallurgies may have a thin bonding metallurgy such as Ti or TiW of &lt;0.1 μm, Cu, Ni or NiFe metallurgies on the order of 0.1 to 5 μm, or Au, Pt, Rh or other finish metallurgy of &lt;0.5 μm. Finish metallurgy is usually deposited on top of underlying Cu or other metallurgical via or wire and is not used to fill openings such as described above with respect to  FIG. 4 . Wiring  77  within the substrate  20  and electrically contacting the contact locations  22 ,  22 A of the probe substrate  20  is further shown in the schematic illustration. 
     In an alternative embodiment, the finish metallurgy may comprise copper pillars having solder tips or solder interconnection bumps  62 . The structure  60  shown in  FIG. 6  is obtained in such a manner. The copper pillars may be formed by electroplating or other suitable process. The IMS head  32  may again be employed to fill the openings  40 . In some embodiments, the solder may remain solder after fill. In other embodiments, a heat treatment causes conversion of the solder to an intermetallic compound such as Cu 6 Sn 5  or alternate composition(s). The surface pads can have additional copper pillars (not shown) plated with subsequent surface finishing metallurgy. Alternatively, the pads may include added solder by means of the IMS head  32  that injects molten solder to add height above the finished pads or finished pillar metallurgical structures. Bumps  62  extending above the surface of the dielectric layer  38  are formed upon solidification of the solder. 
     In one or more embodiments, copper pillars are formed in the openings  40  in the dielectric layer  38  or on top of finished metallurgy, for example copper pillars extending 5 to 40 μm above the filled openings. Alternatively, a solder deposited by IMS can fill the openings  40  or be deposited on top of the filled openings  40 . Solder above the surface of the structure can be deposited through a mask (not shown) using IMS or can be otherwise filled into a resist (not shown) that has openings above the dielectric layer  38  and then resist stripped after filling. The bumps  62 , like the openings  40  from which they extend, are arranged in parallel rows that are correctly aligned for interfacing with the surface contacts of the test substrate or an intermediate structure such as an interposer layer. The bumps are also substantially coplanar even though the surface of the underlying substrate  20  may not be. 
     A further alternative embodiment is shown in  FIG. 7 . Fine pitch test probes are required for some applications where the surface contacts of electronic devices to be tested have a pitch of about one hundred fifty microns or less. A test probe substrate  70  as shown in  FIG. 7  is fabricated once a structure such as shown in  FIG. 6  is obtained. A silicon interposer layer  72  (or layers) is attached to the structure using flip chip attachment (controlled collapse chip attachment) techniques, thermal compression bonding (with or without integrated adhesive and interconnections) or alternative attachment techniques. An adhesive underfill  73  is applied in one or more embodiments. The interposer layer  72  is configured (wired) to function as a redistribution layer wherein the pitch of the wiring layer  30  formed on the substrate  20  is transposed to the pitch required for the wafer to be tested. The silicon interposer layer  72  includes probe tips  74  in one or more embodiments conforming to the layout of surface contacts of the integrated circuit or other device to be tested. In some embodiments, the probe tips comprise shaped copper pads with contact metal deposition such as Ni/Au or Pt or alternative metallurgy. The interposer layer is likely to be used for transposing a relatively large pitch layout (e.g. two hundred microns), such as the layout of bumps  62  shown in the exemplary embodiment of  FIG. 6 , to a layout of probe tips  74  having a finer layout (e.g. one hundred microns) for wafer testing. The interposer layer  72  is planar and the contacts on the interposer for interfacing with the contact regions (e.g. bumps  62 ) of the underlying structure (e.g. structure  60 ) are also coplanar. The planarity of the contact surface formed by the final dielectric layer  38  and correct alignment of the contact regions associated with the contact surface accordingly facilitate proper attachment to the interposer layer  72 . 
     Given the discussion thus far, an exemplary fabrication method includes obtaining a structure comprising a probe substrate  20  including a top surface, one or more contact locations  22 ,  22 A, and electrical conductors  77  within the substrate, the contact locations being electrically connected to one or more of the electrical conductors and forming a first dielectric layer  24  over the top surface of the substrate. A plurality of first openings  26  is formed within the first dielectric layer  24 , thereby exposing a plurality of the contact locations. Lateral extensions, for example tails  28  as shown in  FIG. 2 , of one or more of the openings  26  are formed within the first dielectric layer. An electrically conductive material  30  is deposited within the first openings  26  and lateral extensions  28 . The method further includes depositing a second dielectric layer  38  over the first dielectric layer and forming a plurality of second openings  40  within the second dielectric layer, one or more of the second openings communicating with the lateral extensions. An electrically conductive material  30  is deposited within the second openings to form an array of electrical contact regions electrically connected to the contact locations. The second dielectric layer is planarized. In one or more embodiments, the step of depositing an electrically conductive material within the first openings  26  and lateral extensions  28  includes injecting molten solder, such as illustrated in  FIG. 3 . In some embodiments, the substrate is comprised of ceramic material (e.g. LTCC) and the first dielectric layer  24  comprises a polyimide adhesive. Substrates formed from glass, silicon, and organic materials are employed in other embodiments. Some embodiments of the exemplary method further include the steps of attaching an interposer layer  72  having a bottom surface and a top surface to the substrate over the second dielectric layer, the interposer layer including electrically conductive probe tips  74  extending from the top surface of the interposer layer, and causing the electrical connection (for example by solder reflow) of the probe tips to the contact locations of the substrate.  FIG. 7  shows a structure  70  including such an interposer layer. In some embodiments, the second dielectric layer  38  is deposited directly on the first dielectric layer  24 . In other embodiments, intermediate dielectric layers are formed between the first and second dielectric layers. Finish metallurgy  75  is formed on the contact regions in some embodiments, as shown for example in  FIG. 5B . Both the first and second dielectric layers are planarized in one or more embodiments. The method may further include the steps of mapping three dimensional (x, y and z) coordinates of the contact locations  22 ,  22 A, storing the mapped coordinates electronically in a memory device, comparing the stored coordinates to a set of predetermined coordinate positions, and determining a set of stored coordinates that do not match the set of predetermined coordinate positions. At least the step of forming the lateral extensions is performed using the set of stored coordinates that do not match the set of predetermined coordinate positions. As discussed above, the lateral extensions address the misalignment of contact locations. The stored coordinates may further be employed for controlling the planarizing of one or more dielectric layers. 
     A further exemplary fabrication method includes obtaining a test probe substrate  20  including a top surface and a plurality of contact locations  22 ,  22 A electrically connected to one or more of the electrical conductors. The method further includes determining whether the contact locations  22 ,  22 A match predetermined coordinate positions. A first dielectric layer  24  is formed over the top surface of the substrate and a plurality of first openings  26  are formed within the first dielectric layer, thereby exposing a plurality of the contact locations. Lateral extensions of the first openings are formed in the directions of the predetermined coordinate positions for the contact locations that do not match the predetermined coordinate positions. An electrically conductive material  30  is deposited within the first openings and lateral extensions such that the conductive material contacts the contact locations  22 ,  22 A. A second dielectric layer  38  is deposited over the first dielectric layer. A plurality of second openings  40  are formed within the second dielectric layer matching the predetermined coordinate positions, one or more of the second openings communicating with the lateral extensions  28 . An electrically conductive material  30  is deposited within the second openings  40  to form an array of electrical contact regions electrically connected to the contact locations and conforming to the predetermined coordinate positions. The second dielectric layer is planarized. In some embodiments, the second dielectric layer  38  is deposited on the first dielectric layer and the first dielectric layer  24  is deposited on the top surface of the substrate. As discussed above, the first dielectric layer  24  is planarized in some embodiments of the exemplary method. 
     A test probe structure in accordance with an exemplary embodiment includes a substrate  20  including a top surface, a plurality of rows of contact locations  22 ,  22 A, and electrical conductors  77  within the substrate, the contact locations being electrically connected to one or more of the electrical conductors, one or more of the contact locations  22 A being misaligned with respect to one or more of the rows. A first dielectric layer  24  is on the top surface of the substrate. A plurality of first openings  26  corresponding to the contact locations extend through the first dielectric layer  24 , each of the first openings being aligned with a corresponding one of the contact locations. At least one of the first openings corresponds to one of the misaligned contact locations  22 A and includes a lateral extension  28 . A second dielectric layer  38  is positioned over the first dielectric layer. A plurality of second openings  40  extends through the second dielectric layer, the second openings being arranged in rows corresponding to the rows of contact locations  22 ,  22 A. The second openings are arranged in rows without misalignment. An electrically conductive material  30  is within each of the first and second openings and the lateral extension, the electrically conductive material contacting the contact locations. The second dielectric layer includes a planar top surface. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components, and/or groups thereof. Terms such as “above” and “below” are used to indicate relative positioning of elements or structures to each other as opposed to relative elevation. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the various embodiments has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the forms disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the various embodiments with various modifications as are suited to the particular use contemplated.