Patent Publication Number: US-6664130-B2

Title: Methods of fabricating carrier substrates and semiconductor devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 09/652,880, filed Aug. 31, 2000, now U.S. Pat. No. 6,462,399, issued Oct. 8, 2002, which is a continuation of application Ser. No. 09/008,678, filed Jan. 16, 1998, now U.S. Pat. No. 6,133,065, issued Oct. 17, 2000, which is a divisional of application Ser. No. 08/812,476, filed Mar. 6, 1997, now U.S. Pat. No. 6,040,618, issued Mar. 21, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the formation of a conductor trace-bearing carrier substrate from semiconductor material and the fabrication of a multi-chip module (“MCM”) from the substrate. More particularly, the present invention relates to forming the semiconductor carrier substrate as a segment of a micromachined silicon wafer and fabricating the MCM therefrom. 
     2. State of the Art 
     Chip-on-Board (“COB”) techniques are used to attach semiconductor dice to a printed circuit board, including flip-chip attachment, wirebonding, and tape automated bonding (“TAB”). Flip-chip attachment consists of attaching a “flip-chip” to a printed circuit board or other substrate. A flip-chip is a semiconductor chip that has a pattern or array of terminations spaced around an active surface of the flip-chip for face-down mounting of the flip-chip to a substrate. Generally, the flip-chip active surface has one of the following electrical connectors: Ball Grid Array (“BGA”)—wherein an array of minute solder balls or other conductive material elements is disposed on the electrical connection locations on the active surface of a flip-chip that attaches to the substrate, or Slightly Larger than Integrated Circuit Carrier (“SLICC”)—which is similar to a BGA, but having a smaller solder ball/conductive material element pitch (spacing) and diameter than a BGA. 
     Flip-chip attachment requires (in the case of solder ball connections) the formation of solder-joinable contact sites or terminals on the metal conductors of a carrier substrate such as a printed circuit board (“PCB”), which sites are a mirror-image of the solder ball arrangement on the flip-chip. The terminals on the substrate are usually surrounded by non-wettable barriers so that when the solder balls of the bond pads are placed in contact with the chip contact sites to melt and merge (“reflow”), surface tension holds the semiconductor chip by solder columns, suspending it above the substrate. After cooling, the chip is essentially brazed face-down to the carrier substrate by these very small, closely-spaced solder column interconnections. An insulative underfill encapsulant, such as an epoxy, is then generally disposed between the semiconductor die and the substrate for environmental protection and to enhance the attachment of the die to the substrate. 
     Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. To meet these challenges, attention has been directed to wafer level packaging. U.S. Pat. No. 4,670,770 issued Jun. 2, 1987 to Tai (“the &#39;770 patent”) illustrates wafer level integrated circuits formed by placing “flipped” semiconductor chips on a wafer substrate. The wafer substrate carries solder metal contacts for attaching to metallic contacts on the semiconductor chips. However, the &#39;770 patent requires specialized metallic contacts on the semiconductor chips to make contact with the solder metal contacts on the wafer substrate. These specialized contacts increase the cost of manufacturing the assembly because of the additional fabrication steps required. 
     Silicon wafers have also been used as carrier substrates for temporary electrical connection with an unpackaged semiconductor die for testing, such as disclosed in commonly owned U.S. Pat. Nos. 5,326,428, 5,478,779, 5,483,741, 5,559,444, and U.S. patent application Ser. No. 08/387,687, each hereby incorporated herein by reference. The patents and application generally disclose raised contact members with sloped walls formed on a silicon wafer by an anisotropic etch process. The raised contact members have one or more projections at their outer ends adapted to penetrate contact locations (bond pads) on the semiconductor die under test and to pierce any residual oxide or other insulating material on the surface of the semiconductor die bond pads to establish an ohmic connection therewith. However, as these projections penetrate the surface of the die bond pads on the semiconductor die under test to ensure good electrical connection, such penetration may, in some instances, degrade the physical integrity of the bond pad, or might pierce right through the bond pads making physical contact to the devices underneath, damaging and destroying them. 
     None of the prior art uses of wafers in wafer level semiconductor die packaging and testing as described above teach a cost efficient method of forming a wafer level carrier substrate which can be used for packaging or testing of semiconductor dice, and that does not require any specialized processing steps or which will not damage the bond pads of the semiconductor chip. Furthermore, these prior art techniques address only temporary connection between the bond pads and the substrate. Therefore, it would be advantageous to develop a technique for forming a carrier substrate from a silicon wafer which would achieve these goals while utilizing known semiconductor device fabrication techniques. 
     SUMMARY OF THE INVENTION 
     The present invention enables semiconductor packaging at a wafer level by forming an MCM from a micromachined carrier substrate, preferably of silicon. The formation of the micromachined substrate begins with providing a substrate of semiconductor material such as monocrystalline silicon (traditional wafer), silicon-on-glass, or silicon-on-sapphire, germanium, or ceramic, which is coated on one surface with a masking material, such as a layer of silicon nitride (Si 3 N 4 ). The mask material is selectively etched to form strips across the surface of the substrate. The substrate is then etched except under the protective mask strips to form elongated mesas having sidewalls extending to a lower substrate surface. It is, of course, understood that groups of the strips may be placed in mutually transverse orientation to form “box canyons” of mesas adapted to receive a semiconductor chip with bond pads arrayed about the periphery of its active surface. 
     After mesa formation, the remaining mask material is removed, preferably using a wet etch. An insulating or dielectric layer is then formed on the substrate, including the elongated mesas and sidewalls. The insulating layer is preferably formed by oxidizing the substrate and may be accomplished by exposing the substrate to an oxidizing atmosphere in a reaction chamber. Other insulating techniques include deposition of silicon dioxide or silicon nitride by chemical vapor deposition (CVD), and injecting TEOS (tetraethyl-orthosilane) into the reaction chamber to grow silicon dioxide (SiO 2 ) at a temperature of about 400° C. Silicon dioxide is preferred due to its low dielectric constant, which results in reduced capacitance and increased signal speed on the substrate traces. Other dielectrics such as silicon nitride can also be employed. 
     A conductive material layer is then formed on the insulating layer. The conductive material layer can be any known low-resistivity material such as a metal, preferably copper. The conductive material layer is then patterned and etched to form or define conductive traces on the dielectric-covered substrate surface. The conductive traces can be patterned to route signals between semiconductor dice carried on the substrate and/or to circuitry external to the substrate. A stack of conductive materials such as copper-coated palladium can also be used. 
     It is understood that the conductive traces can be formed by a number of alternate conventional techniques other than patterning and etching a metal layer discussed above, such as: depositing a conductive paste on the substrate by silk screening the conductive traces directly thereon; directly extruding a conductive paste to form the conductive traces; or applying a second insulating layer on the first insulating layer, etching a trough in the second insulating layer, filling the trough with a conductive material, and removing the excess conductive material. The conductive traces are preferably formed in the vicinity of chip-mounting sites to include a portion which extends to and over a mesa defining a chip site, down the sidewall and onto a lower, previously-etched portion of the substrate where a chip is to be mounted. 
     The forming of the conductive traces on a semiconductor substrate devoid of any integrated circuitry enables the conductive traces to be made of copper or any other low-resistivity material, because such a substrate can be processed at extremely elevated temperatures (as high as 1000° C.) since the substrate does not contain any temperature sensitive integrated circuitry. Substrates containing such integrated circuitry are limited to upper temperatures ranging from about 500 to 700° C. 
     Once the conductive traces are formed on the substrate, they may be brought into input/output communication with external circuitry along a substrate edge with wirebonds, leads, clips, TAB tape attachment, or other connectors known in the art. 
     Optionally, a passivation layer may be applied over the substrate and conductive traces, which layer is then selectively etched to expose only discrete contact areas on the conductive traces. 
     When the completed semiconductor carrier substrate comprises an entire wafer, it can be used as a burn-in test substrate. Thus, an integrated circuitry-carrying (IC) wafer with electric contact points corresponding to the conductive traces on the completed semiconductor substrate allows for the testing of the entire circuitry-carrying wafer through bum-in without the need for preliminary dicing of the IC wafer into individual dice for testing. When performing such a bum-in test, it is preferable that the coefficient of thermal expansion (CTE) for the carrier substrate be the same or about the same as that of the IC wafer. Thus, a micromachined silicon carrier substrate is an ideal test bed for a silicon integrated circuit wafer. 
     A variety of semiconductor dice and/or circuitry-carrying wafer configurations (collectively, “semiconductor elements”) may be attached to the silicon carrier substrate. In general, a standard bond pad array on the semiconductor die, such as a central row or rows, parallel side rows, rectangular periphery arrangement, or combinations thereof, with a conventional bond pad pitch of about 2-3 mils or 75-150 microns, can be used in the present invention. However, if necessary to prevent potential shorting, the bond pad arrangement on the active surface of the die can be rerouted to achieve a sufficiently greater pitch for use with the present invention, although such rerouting is undesirable from a cost standpoint. Furthermore, if the carrier substrate and integrated circuitry-bearing wafer each comprise entire wafers, dicing of these wafers into individual packages for use in a computer system may not be necessary. 
     Electrical contact between the carrier substrate and semiconductor element is achieved with conductive connectors formed on either the semiconductor element, the carrier substrate, or both (although not preferred as incurring extra cost). The conductive connectors each preferably make contact with both the portion of the conductive trace extending down the mesa sidewall adjacent a chip mounting site and the portion of the conductive trace on the etched portion of the substrate at the site itself. This enhanced, even substantially doubled, contact surface area results in the following advantages: 
     1. The bond has a lower contact resistance. 
     2. The bond is more robust than single contact (conventional flat terminal) attachment to the carrier substrate. 
     3. The bond is stronger, and thus mechanically more reliable. 
     4. An underfill material may not be required to strengthen the chip-to-substrate attachment. 
     5. The mesa sidewall acts at least as a rough alignment guide in at least one X-Y direction. When the conductive connectors are solder balls, reflowing the solder achieves an automatic fine alignment due to wetting of the conductive traces carried on the mesa sidewalls. 
     If parallel, elongated mesas are closely spaced to accommodate the width of a single connector such as a solder ball, the conductive connectors may even make contact with a trace portion extending down a sidewall, a contiguous second portion lying flat on the etched portion of the substrate, and a contiguous third portion extending up a sidewall of an adjacent mesa. This “triple” surface area contact achieves all of the above advantages for the “double” surface area contact, as well as providing an even stronger bond. 
     The semiconductor element, such as a large, single semiconductor die or a larger circuitry-bearing substrate such as a wafer or partial wafer, may also straddle the mesa(s) to make electrical contact with various conductive traces placed “outside” of the mesas. Additionally, the conductive connectors on the semiconductor element may be angled to form a matched angular attachment to both the substrate and mesa sidewall portions of the conductive traces. This feature may be employed with non-solder conductive elements such as conductive epoxies or conductor-filled epoxies, as well as with solder conductive connectors wherein a single reflow is employed for attachment of a semiconductor element to substrate traces. 
     The present invention also includes a stacked configuration of semiconductor elements and carrier substrates. After attachment of semiconductor elements, the resulting semiconductor carrier substrate assemblies can be stacked to form a high density stacked configuration. 
     An advantage of the present invention is that the carrier substrates of the invention are particularly adapted to the use of “partial” memories. Partials are individual semiconductor memory dice which do not yield the design or expected number of bits, such as when a 64 MB memory die as a DRAM yields only, for example, 37 MB. Rather than scrapping such partials, conductive traces or bit lines can be connected to two or more such “partial” dice to form a larger array of usable memory. This process using the inventive substrate makes use of the lower-yielding, less desirable partials, thereby increasing the overall wafer yield. Furthermore, this bit line process for using partials can also be used for full-capacity memory dice. For example, 120 or more memory dice (or equivalent partial wafer segments) of 16 MB each could result in a multi-gigabit memory array usable in lieu of a hard disk drive in a computer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the advantages of this invention can be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a side cross-sectional view of a mask coated substrate; 
     FIG. 2 is a side cross-sectional view of the mask coated substrate of FIG. 1 having a portion of the coating removed to expose strips of the substrate; 
     FIG. 3 is an oblique view of the exposed substrate of FIG. 2; 
     FIG. 4 is a side cross-sectional view of the mask coated substrate of FIG. 2 after etching of the substrate; 
     FIG. 5 is a side cross-sectional view of the etched substrate of FIG. 4 after removal of the mask strips; 
     FIG. 6 is an oblique view of the etched substrate of FIG. 5; 
     FIG. 7 is a side cross-sectional view of the etched substrate of FIG. 5 coated with an insulating layer; 
     FIG. 8 is a side cross-sectional view of the insulated substrate of FIG. 6 with a conductive coating; 
     FIG. 9 is a side cross-sectional view of the insulated substrate of FIG. 6 with a multiple layer conductive coating; 
     FIG. 10 is a side cross-sectional view of the conductively coated substrate of FIG. 8 after etching of the conductive coating to form conductive traces; 
     FIG. 11 is a side cross-sectional view of the conductively coated substrate of FIG. 8 after etching of the conductive coating to form conductive traces, bond wires being depicted to connect the conductive traces to external circuitry; 
     FIG. 12 is an oblique view of the conductively traced substrate of FIG. 11; 
     FIG. 13 is a side cross-sectional view of the conductive trace-bearing carrier substrate of FIG. 9 having select portions passivated; 
     FIG. 14 is a side cross-sectional view of the passivated carrier substrate of FIG. 13 with a semiconductor element flip-chip attached; 
     FIG. 15 is a side cross-sectional view of the passivated carrier substrate of FIG. 13 with a straddling semiconductor element flip-chip attached; 
     FIG. 16 is a side cross-sectional view of the passivated carrier substrate of FIG. 13 with an integrated circuitry-carrying, bond padded substrate attached; 
     FIG. 17 is a side cross-sectional view of the passivated carrier substrate of FIG. 13 in a stacked configuration with other such semiconductor-element carrying substrates; 
     FIG. 18 is a side cross-sectional view of a passivated, closely spaced mesa configuration with a straddling semiconductor element attached; 
     FIG. 19 is a side cross-sectional view of a wafer-to-wafer semiconductor element substrate arrangement; 
     FIG. 20 is a top elevation of a box canyon mesa and conductive trace arrangement carrying a semiconductor element; and 
     FIG. 21 is a top elevation of a carrier substrate bearing alignment boxes thereon for alignment of a semiconductor element therewith through engagement with alignment bumps or protrusions on the element. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a mask coated carrier substrate  100 . The mask coated carrier substrate  100  comprises a carrier substrate  102  (such as monocrystalline silicon, silicon-on-glass, silicon-on-sapphire, germanium, or ceramic) coated with a mask layer  104 , such as silicon nitride (Si 3 N 4 ). While a semiconductor material substrate is currently preferred, other materials from which a substrate may be fabricated according to the present invention are equally suitable, subject to some limitations. For example, a carrier substrate  102  is preferably formed from a material having a coefficient of thermal expansion (CTE) that closely matches the CTE of the semiconductor die and/or other circuitry-bearing substrate (not shown) to be later attached to the completed carrier substrate. 
     The mask layer  104  is etched to form mask layer strips  106 , as shown in FIGS. 2 and 3. The carrier substrate  102  is then etched around the mask layer strips  106  to form elongated mesas  108  with sidewalls  110 , as shown in FIG.  4 . Typical etching techniques include wet anisotropic etching with a solution of KOH:H 2 O. A 40% KOH solution applied at 70° C. can achieve etch rates of 30 μm/hr. This type of etching is also known in the art as bulk micromachining. With such an anisotropic etch, the sidewalls  110  are formed with the elongated mesas  108  which are sloped at an angle of about 54° with the horizontal. 
     As shown in FIGS. 5 and 6, the mask layer strips  106  are removed, preferably using a wet etch. For mask layer strips  106  formed of silicon nitride, an etchant such as H 3 PO 4  that is selective to the carrier substrate, can be used. An insulating or dielectric layer  112  is then formed on the carrier substrate  102 , including the elongated mesas  108  and the sidewalls  110 , as shown in FIG.  7 . The insulating layer  112  is preferably formed by oxidating the carrier substrate  102  and may be accomplished by exposing the carrier substrate  102  to an oxidizing atmosphere in a reaction chamber. Other techniques include deposition of silicon dioxide or silicon nitride by chemical vapor deposition (CVD), and injecting TEOS (tetraethylorthosilane) into the reaction chamber to grow silicon dioxide (SiO 2 ) at a temperature of about 400° C. Silicon dioxide is a preferred insulating layer  112  for its lower dielectric constant and consequently reduced capacitance, which results in increased speed of signal transmission in the overlying, subsequently-formed traces. 
     As shown in FIG. 8, a conductive material layer  114  is then formed on the insulating layer  112 . The conductive material layer  114  can be any known, low-resistivity material such as a metal, preferably palladium, tungsten, copper or aluminum. It can also be a stack of two or more metals, as noted below. When the semiconductor dice (not shown) to be attached are attached by lead/tin solder balls, the conductive material layer  114  is preferably comprised of multiple layers. Most preferably, as shown in FIG. 9, the multiple layers are a chrome layer  118  applied over the insulating layer  112 , a middle layer of copper/chrome alloy  120 , followed by an uppermost layer of copper  122 . This multiple layer configuration provides both good adherence to the material of carrier substrate  102  and an excellent wetting surface for the later attachment of the solder balls of semiconductor dice. Additionally, a fourth metal layer (not shown) of palladium or gold is occasionally placed atop the copper layer  122  to prevent oxidation of the copper. Furthermore, titanium or titanium/tungsten alloys may be used as alternatives to chrome, and nickel may be used as an alternative to copper. 
     The conductive material layer  114  is then masked, patterned and etched to form conductive traces  116 , as shown in FIG.  10 . It is also understood that the conductive traces  116  can be formed by a number of alternate conventional techniques, such as: depositing a conductive paste on the carrier substrate by silk screening the conductive traces  116  directly thereon; directly extruding a conductive paste to form the conductive traces  116 ; or applying a second insulating layer on the insulating layer  112 , etching a trough in the second insulating layer, filling the trough with a conductive material, and removing the excess conductive material. The conductive traces  116  are preferably formed to include portions which extend over the elongated mesas  108 , up one sidewall  110 , over the mesa top  111  and down the opposing mesa sidewall  110  onto the flat, etched portion  124  of the carrier substrate  102  at chip mounting sites to form sidewall conductive trace terminal portions  126  and carrier substrate conductive trace terminal portions  128 , respectively. 
     Once the conductive traces  116  are formed, the conductive traces  116  may be brought into input/output communication with external circuitry (not shown) along a carrier substrate edge  130 , either before or after semiconductor elements are attached thereto. FIG. 10 schematically illustrates an external connector  132 , such as a lead, wire bonded or TAB tape attachment, in electrical communication with the conductive trace  116  abutting the carrier substrate edge  130 . Preferably, the external connectors  132  comprise bond wires  134 , as shown in FIG.  1 . FIG. 12 illustrates an oblique view of the configuration shown in FIG.  11 . It is understood that a different metal or uppermost metal layer (not shown) can be disposed on the conductive traces  116  near the carrier substrate edge  130  to achieve a more effective site for bonding the external connector  132 . The topmost metal can be also optimized for direct electrical connection to electrical pins or other contacts, such as are used for spring-biased or plug-in connections to higher-level packaging. A passivation layer  136  is optionally applied over the insulating layer  112  and conductive traces  116 , and etched to expose contact areas  138  on the conductive traces  116 , as shown in FIG. 13, to form a completed carrier substrate  140 . If desired, the carrier substrate  140  may be edge-connected into a mating aperture in a carrier of a higher-level package, the aperture including conductive clips arranged to connect to contact areas  138 . 
     The completed carrier substrate  140  may accept numerous semiconductor dice and/or circuitry-carrying wafer or partial wafer (as noted previously, “semiconductor element”) configurations. FIG. 14 illustrates a conventional flip-chip semiconductor element  142  attached to the carrier substrate  140 . Typically, and in this instance, flip-chip semiconductor element  142  is only one of a plurality of flip-chip semiconductor elements  142  carried by carrier substrate  140  in a multi-chip module (MCM) assembly such as a SIMM, DIMM, or mother board. A plurality of bond pads  144  on the flip-chip semiconductor element  142  achieves electrical contact with the conductive traces  116  via a plurality of conductive connectors  146 , preferably lead/tin solder balls. The solder may be sputtered, stenciled, screen-printed, evaporated or plated on bond pads  144 , and then reflowed to produce a surface tension-constrained ball shape. When lead/tin solder balls are used as conductive connectors  146 , the solder balls are reflowed at about 230° C. Additionally, connections to the substrate may be enhanced by pre-placing a lead/tin solder paste at  117  on the exposed portions of conductive traces  116 , which paste is formulated to reflow at a lower temperature than the solder balls. For example, a lead/tin eutectic paste which will liquify at a temperature of about 160-180° C. may be sputtered, evaporated, screen-printed, stenciled or plated on terminal areas of conductive traces  116 . By heating to this temperature after placing flip-chip semiconductor elements  142  on carrier substrate  140 , both permanent and temporary electrical contact therebetween can be made. The lead/tin eutectic paste, which liquifies at a lower temperature than the solder balls, may be employed to make a secure, temporary connection by heating the assembly to the lower temperature and then cooling to resolidify the paste, bonding the conductive traces  116  to the solder balls. If a permanent connection is desired, a higher temperature may be employed to reflow the solder balls, as well as the paste on the traces. 
     The conductive connectors  146  may be formed on either the flip-chip semiconductor element  142  or the carrier substrate  140 , as desired, the other element being configured with cooperative contacts. It is preferred, however, that conductive connectors be formed flip-chip semiconductor element  142 . The conductive connectors  146  each preferably make contact with both the sidewall conductive trace portion  126  and the carrier substrate conductive trace portion  128 . 
     FIG. 15 illustrates a straddling semiconductor chip or other circuitry-bearing semiconductor element  148  which spans one or more elongated mesas  108  and makes electrical contact with the conductive traces  116  in a manner discussed above for FIG.  14 . As shown in broken lines, elongated mesas  108  may be formed a single connector-width apart to provide the previously-referenced triple surface contact area. 
     FIG. 16 illustrates a semiconductor chip or integrated circuitry bearing element  150  with conductive connectors  152  designed to be angled to form a matched attachment to both the substrate conductive trace portion  128  and the sidewall conductive trace portion  126 . The conductive connectors  152  are preferably lead/tin connectors of square wedge or frustoconical configuration. These conductive connectors  152  can be formed by sputtering or evaporating the conductive material, using conventional tin/lead solder compositions which liquify at about 230° C., or eutectic compositions which liquify at relatively low temperatures (about 160-180° C.), comprising about 63% by weight Pb and 37% by weight Sn, or by depositing the conductive material by stenciling, screen-printing, or plating the conductive material on top of conductive traces  116  on the carrier substrate  140  or on the bond pads  144  of the semiconductor chip  150 . When the lead/tin connectors are formed by the above method, a single reflow during attachment of the semiconductor chip  150  to the carrier substrate  140  results in a simultaneous attachment and formation of a solder ball. As a further option, a dielectric layer of relatively great thickness may be formed on the active surface of the die or carrier substrate to define the bond pad and solder ball locations to eliminate the need for an underfill. Alternatively, the connectors may be printed or otherwise formed of a conductive or conductor-filled epoxy with sidewalls angled to match those of the elongated mesas. Such connectors may comprise a so-called “B stage” epoxy which is partially curable after deposition on the semiconductor element to stabilize its structure, and subsequently completely curable to effect an adhesive connection to the substrate traces. As a further alternative, the conductive connectors may comprise conductive or conductor-filled thermoplastics. Non-conductive underfills between the carrier substrate edge  130  and semiconductor chip  150  can also be employed to enhance the mechanical bond between the components and provide environmental protection in the electrical connections. 
     After attachment of one or more semiconductor chips  150 , the carrier substrate  140  may be stacked with and attached to other semiconductor chip-carrying substrates  140  with an adhesive  156 , as shown in FIG. 17, to form a high density stacked configuration  158 . 
     FIG. 18 illustrates a closely spaced, elongated mesa configuration with a straddling semiconductor  160  attached. The elongated mesas  108  are spaced close enough together to make contact with three surfaces (i.e., a substrate etched portion  162 , a first mesa sidewall surface  164 , and a second mesa sidewall surface  166 ) of the conductive traces  116 . However, such an arrangement may not be possible if the bond pad pitch (spacing) is too small. 
     FIG. 19 illustrates a wafer-to-wafer arrangement  168 . The wafer to wafer arrangement  168  comprises a carrier substrate edge  170  formed from an entire silicon wafer attached, in a manner discussed above, to a semiconductor element  172  also formed from an entire wafer. 
     FIG. 20 illustrates the previously-noted “box canyon” arrangement of elongated mesas  108  with conductive traces  116  extending thereover. Such an arrangement, if used with solder ball connectors  146 , will result in automatic alignment of a semiconductor element  154  with the carrier substrate  140  in both X and Y directions upon solder reflow or, if conductive adhesive elements of precise placement, form and size are employed during the assembly of the semiconductor element  154  with the carrier substrate  140 , continued alignment is ensured by cure of the adhesive by heat, radiation, etc. as known in the art. 
     FIG. 21 illustrates the use of two alignment boxes  200  formed on a carrier substrate  140 , which may be mated with like-spaced alignment bumps or protrusions  202  on a semiconductor element  154  to be attached to carrier substrate  140 , in order to effect precise X-Y alignment during initial placement of the semiconductor element  154 . Of course, only a single alignment box  200  may be employed to ensure alignment in one of the X-Y directions, with elongated mesas  108  flanking the alignment box or boxes  200  ensuring alignment in the other direction by their contact with solder ball connectors  146 . Such alignment boxes are formed at the same time as the elongated mesas, using the same substractive techniques. However, there are no traces formed on or over the alignment boxes. Further, the alignment boxes may also comprise C-shaped structures, or even two parallel “tabs” extending from the carrier substrate, if alignment in only one direction is required, or two alignment boxes comprising an “L” or chevron shape may be employed to cooperate with two spaced alignment bumps on the semiconductor element. 
     Having thus described in detail preferred embodiments of the present invention, it is to be understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description as many apparent variations thereof are possible without departing from the spirit or scope thereof.