Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 08/736,586, filed Oct. 24, 1996, now U.S. Pat. No. 6,133,638, issued Oct. 17, 2000, which is a divisional of application Ser. No. 08/578,493, filed Dec. 22, 1995, now U.S. Pat. No. 5,686,318, issued Nov. 11, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to die-to-insert interconnections and, more specifically, to a method of forming a permanent die-to-insert electrical connection for a semiconductor die assembly by diffusing gold bumps on the insert into the bond pads of the die using relatively low elevated temperatures and low levels of constant force during the extended time of a burn-in process. 
     2. State of the Art 
     Currently, there are three primary chip-level interconnection technologies in practice. They include wirebonding (WB), Tape Automated Bonding (TAB), and Controlled Collapse Chip Connection (C 4 ). The method used to bond the interconnections is dependent upon the number and spacing of input/output (I/O) connections on the chip and the insert (i.e., substrate) as well as permissible cost. 
     WB is the most common chip-bonding technology because the required number of chip connections in many products can be accomplished in addition to providing the lowest cost per connection. WB is generally employed to electrically connect chips to the inner ends of the leads of a lead frame, the assembly subsequently being packaged as by transfer molding of a plastic package. For chips requiring more than 257 but less than 600 connections, TAB may be used. TAB employs lead frames of a finer pitch mounted on an insulative carrier tape which is integrated into the chip package. The C 4  process, however, is capable of creating up to 16,000 connections per chip (or partial wafer), potentially meeting the demand for any number of connections that the die or partial wafer design dictates. 
     When C 4  bonding is employed, the entire surface of the chip is normally covered with bond pads for the highest possible I/O count. Solder bumps are deposited on wettable metal terminals (bond pads) on the chip, and a matching footprint of solder-wettable terminals is located on the substrate. Both the bond pads and the terminals must be treated with solder flux. Moreover, the solder bumps must be constrained from completely collapsing (or flowing out onto the substrate bonding site) by using thick-film glass dams, or stops. The tendency for the solder to flow on the chip is contained by a special bonding pad metallurgy that consists of a circular pad of evaporated chromium, copper, and gold. The bond pad metallurgy is then coated by evaporation with, for example, 5Sn-95Pb or 2Sn-98Pb, to a thickness of 100 to 125 μm. Finally, the upside-down chip or die (flip-chip) is aligned to the substrate, and all chip-to-substrate conductive paths are made simultaneously by reflowing the solder. 
     The numerous process steps and extensive prebond preparation associated with C 4  makes it an expensive bonding method. Moreover, because of the expense added by the C 4  process, bumping the chip has been avoided. In the Very Large Scale Integration (VLSI) era, however, the expense has been necessary to obtain the required number of connections. 
     As disclosed in U.S. Pat. No. 5,435,734 to Chow, pressure contact interconnect methods are also known in the art. Pressure contacts are not actually bonded but rather form a continuous contact using a material deformation concept such as a metal spring or an elastic retainer. For example, two gold bumps (on chip and substrate) may be joined by a conductive rubber contact embedded in a polyamide carrier. However, this is a mechanically created connection and is, therefore, not as desirable as metallurgical bonding techniques for economic- as well as reliability-associated reasons. 
     Furthermore, all of the previously mentioned methods of forming chip-to-substrate interconnections are typically effected after a burn-in operation is performed on the chip to determine if the chip is defective. For burn-in, a chip is typically placed in a multi-chip carrier in resiliently biased or other temporary connection to a burn-in die or substrate (also called an insert) having circuit traces and contacts for electrical testing of the chip. During the burn-in process, the chips are generally subjected to electrical impulses and elevated temperatures (on the order of 125-150° C.) for extended periods of time, usually 24-48 hours, depending upon the chip and the characterization protocol. Low-temperature cycling to as low as −50° C. may also be employed on occasion, particularly for chips being qualified to military specifications. However, this is not common for chips destined for use in commercial applications. 
     If not proven defective, the chip is removed from its test fixture after burn-in and is then permanently attached to a substrate by means known in the art, such as those previously mentioned. Alternatively, the chip may be wirebonded to a lead frame or TAB-bonded to a taped lead frame, as known in the art, depending upon the ultimate application for the chip and preferred packaging for that application. In any case, burn-in connections and permanent operational connections are effected in the prior art in two distinct and different operations. While it would be possible to form permanent die-to-insert connections before burn-in, this would increase processing time and cost. It is known to package single die before burn-in, such as with wire- or TAB-bonded lead frame-mounted, plastic-packaged dice (e.g., DIP, ZIP), but such arrangements are not suitable for multi-chip modules (MCM&#39;s) such as single in-line memory modules (SIMM&#39;s) where failure of a single die will result in scrapping of the module. 
     Thus, it would be advantageous to provide an economical method of chip-to-substrate interconnection that is capable of keeping up with the ever-increasing requirements for more I/O connections per chip, does not require all of the preparation and process steps associated with C 4  chip interconnections such as application of flux and the use of thick-film glass substrate dams, and removes at least one major step from the manufacturing process through use of a one-step chip-to-substrate electrical connection technique suitable for both burn-in and ultimate first-level packaging of a chip. 
     Additional non-C 4  ball- or bump-type chip-to-substrate electrical interconnect systems exist in the art, as disclosed in U.S. Pat. Nos. 5,451,274; 5,426,266; 5,369,545; 5,346,857; and 5,341,979. Such systems achieve electrical connections through use of relatively complex and sophisticated apparatus and process methodology, and thus are not suitable for use during chip burn-in a carrier or other fixture. 
     Temporary chip-to-burn-in die or insert connections are also known in the art and exemplified by the disclosures of U.S. Pat. Nos. 5,440,241; 5,397,997; and 5,249,450. None of the foregoing patents, however, discloses a methodology for forming suitably permanent die-to-substrate electrical connections during burn-in. 
     It is known in the electronics art to employ diffusion bonding to effect electrical connections between two or more substrates or circuit boards; U.S. Pat. No. 5,276,955 discloses such a process. However, diffusion bonding as known in the art is generally effected at relatively high temperatures just below the eutectic or peritectic temperatures of the bonding alloy, and for relatively short periods of time, such as one or two hours. Thus, state-of-the-art diffusion bonding as known to the inventors has no legitimate application to making chip-to-insert connections. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the invention, a method for forming a permanent chip-to-insert interconnection is herein disclosed. Gold bumps are attached to ends of conductive circuit traces on one side or the other of a nonelectrically conductive substrate, or even the exposed ends of internal conductors, by which electrical testing of a chip during burn-in is effected. As used herein, it should be understood that the term “gold” includes not only elemental gold, but gold with other trace metals and in various alloyed combinations with other metals as known in the semiconductor art. Typically, the die has bond pads on one surface (commonly termed the “front” or “active” surface) formed of aluminum or an aluminum alloy. The bond pads are arranged as a mirror image of the gold bumps located on the surface of the substrate. Thus, when the bond pads are placed on top of the gold bumps, they are in substantial alignment with each other. 
     The substrate material is selected such that the coefficient of thermal expansion (CTE) is similar to that of the die or semiconductor chip. This assures that both the substrate and the die expand and contract in a similar manner when subjected to elevated temperatures during a burn-in process so that the bond pads on the die will stay in relatively precise alignment with the gold bumps on the substrate, producing little or no shear force between any bond pad and its corresponding bump. By way of example only, the substrate may be comprised of Mullite, a ceramic material such as 203 aluminum oxide, or any other material known in the art that has a CTE similar to that of the die. 
     By applying a force to the die as it is located above and parallel to the plane of the substrate, the bond pads and gold bumps are pressed together. The die/substrate assembly is then heated to effect a bond between the conductive paths of the two components of the assembly. The heat applied, however, is not sufficient to melt the gold bumps or even to approach the eutectic or peritectic threshold of the gold, but only to the extent necessary to diffuse the gold to form a permanent aluminum/gold bond between the gold bump and the aluminum bond pad. Thus, the gold from the gold bump diffuses into the aluminum bond pads of the die. 
     The method herein disclosed is preferably performed during the burn-in process. During the heating cycle, the temperature can be set or cycled to provide the necessary diffusion energy to form the aluminum/gold bond. Moreover, the chips may be placed in chip carriers which utilize a spring or other biasing member to press the bond pads of the semiconductor die and the gold bumps of the substrate (burn-in die, insert) together. The assemblies are then subjected to selected temperatures for a selected period of time, the combination of temperature and time promoting diffusion of the gold into the aluminum bond pads of the die. Contrary to prior art diffusion bonding methods, the diffusion temperature of the present invention is markedly lower, and the diffusion time markedly longer. Of course, were this not the case, the semiconductor die circuitry, if not the die itself, would be damaged and its performance characteristics altered. 
     Since there is an initial biased electrical contact as soon as the die under test (DUT) is secured against the gold bumps of the substrate in the carrier, electrical testing with elevated potentials as well as thermal testing of the die may commence immediately and continue while the permanent, bump-to-pad diffusion bond is created. Each die that fails during the burn-in process may then simply be discarded at the termination of burn-in along with its attached substrate for recovery of the precious metals. Alternatively, the die may be mechanically removed and a new die attached to the die location on the substrate. It has been found to be, in terms of processing time versus ultimate yield, less expensive to form a permanent chip-to-substrate attachment during burn-in than to perform burn-in followed by a permanent chip attachment to a second substrate, even if some dice have to be pulled as defective or substandard and replaced. 
     An added advantage of the method of chip-to-substrate interconnection of the present invention is its capability of keeping up with the requirements for ever-increasing numbers of I/O connections, the reduction of process and preparation steps in comparison to C 4  bonding and other flip-chip bonding systems known in the art, and the deletion of at least one major step from the fabrication, testing and packaging sequence. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The present invention will be more fully understood and appreciated by those of ordinary skill of the art by a review of this specification, taken in conjunction with the appended drawings, wherein: 
     FIG. 1 is a side view of a semiconductor die contained against a gold-bumped substrate in a burn-in fixture in accordance with the method of the present invention; 
     FIG. 2 is a partial top perspective view of a gold-bumped substrate showing circuit traces thereon; 
     FIG. 3 is a schematic view of the active surface of a high bond pad density semiconductor die suitable for use in accordance with the present invention; 
     FIG. 4 is a schematic top view of a gold-bumped burn-in substrate suitable for use in accordance with the present invention; 
     FIG. 5 is an exploded side view of the semiconductor die, burn-in substrate and burn-in fixture of the embodiment shown in FIG. 1; and 
     FIG. 6 is an end view of the semiconductor device and burn-in fixture shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a side view of a semiconductor die assembly  10  positioned in a burn-in fixture  12  is shown. The term “die” as used herein may denote a single die (chip) from a wafer or a plurality of dies, up to an entire wafer if wafer-scale integration is employed for the unit under test. 
     The semiconductor die assembly is comprised of a nonelectrically conductive substrate  14  (also commonly termed an insert or burn-in die in the prior art) on which a plurality of gold bumps  16  is formed by means known in the art. The bumps  16  are located at the ends of circuit traces  17  extending to the periphery of the substrate  14  for electrical testing during burn-in (see FIG.  2 ). One suitable means of forming gold bumps on substrate  14  is through use of a thermosonic gold wire bonding apparatus as known in the art and commercially available from Kulicke and Soffa Industries of Willow Grove, Pa. The bumps may be coined to a desired configuration after deposition, as known in the art. (See U.S. Pat. Nos. 5,397,997 and 5,249,450 for a discussion of various bump-forming techniques.) The preferred compositions of the gold bumps employed in the present invention may comprise 99.99% pure gold (Au) bond wire, as well as Be- or Cu-doped Au, or other Au-based alloys as known in the art. Aluminum (Al) wire may also be used to form the bumps, using ultrasonic apparatus as known in the art. 
     A semiconductor die  18  with active and optionally passive components, as well as circuit traces, vias and other conductive paths as known in the art, is positioned on top of the gold bumps  16 . The substrate  14  is placed in the base  20  of the burn-in fixture  12  with the gold bumps  16  facing upwardly, away from the base  20 . The die  18  is aligned with the substrate  14  (the die and substrate planes being mutually parallel and die and substrate electrical contacts being coincident) and a lid or cover  22  is placed on top of the die  18 . 
     As better seen in FIGS. 3 and 4, the die  18  has a plurality of bond pads  13  in the same configuration as the gold bumps  16  on the substrate  14 . Thus, when the die  18  is placed on the substrate  14 , the bond pads  13  and the gold bumps  16  match. Moreover, for alignment purposes, one gold bump  15  may be offset from the rest, leaving a space  19  on the substrate surface, and one bond pad  13 ′ offset from the rest of the bond pads  13 , leaving a space  21  corresponding to the space  19 . Thus, correct rotational orientation of the die  18  relative to the substrate  14  can be easily ascertained. Spaces  19  and  21  may, of course, be eliminated and a bump  16  and bond pad  13  merely offset in alignment. Of course, other alignment methods known in the art, such as marking the components for alignment or creating a die/substrate interconnect pattern which can only be mated in one orientation, may also be employed. 
     The bridge clamp  24  of the burn-in fixture  12  comprises an upper plate  26  having a first end  28  and a second end  30  to which perpendicularly extending legs  32  and  34  are attached about their proximal ends  36  and  38 , respectively. The legs  32  and  34  have anchors  40  and  42  resiliently disposed at the distal ends  44  and  46  of the legs  32  and  34 , respectively. Spaced upwardly from the anchors  40  and  42  are stop members  48  and  50  extending outwardly from legs  32  and  34 . 
     Attached to the underside  52  of the bridge clamp  24  is a biasing member  54 . The biasing member  54  may be comprised of spring steel and configured as a leaf spring, coil spring or belleville spring, or be formed of some other resilient material known in the art and capable of withstanding the elevated burn-in temperatures, such as a silicone-based elastomers. The biasing member  54  should also be designed to apply a selected amount of force to the back side of die  18  when the burn-in fixture  12  is closed, within a broad range capable of providing sufficient force for bonding contact but not excessive, damaging force to the die  18 , the bumps  16 , or the substrate  14 . The biasing member  54  as shown is held in position by projections  51  and  53  extending from the underside  52  of the bridge clamp  24 . The projections  51  and  53  are angled inwardly toward one another and provide for an abutment of the biasing member  54 . Other connection means are possible and contemplated, including a tab or extension of biasing member  54  sliding into slots in bridge clamp  24 . 
     The burn-in fixture  12  is designed to apply pressure to the interfaces  56  between the gold bumps  16  and the die  18  transversely to the planes of the die  18  and substrate  14 . As shown in FIG. 5, the anchors  40  and  42  are deflected and inserted through slots  58  and  60 . The stop members  48  and  50  prevent the legs  32  and  34  from being inserted too far into the slots  58  and  60  and thus prevent excessive force from being applied by the biasing member  54  on the lid or cover  22 . 
     When the anchors  40  and  42  are properly secured to the bottom  62  of the base  20 , a predetermined amount of force is applied by the biasing member  54  to the surface of the lid or cover  22 . Because the lid or cover  22  substantially covers the die  18  and is of sufficient strength to resist bending or other deflection (FIGS.  1  and  6 ), the lid or cover  22  provides uniform pressure across the surface  64  of the die  18 . Moreover, the pressure may be sufficient to ensure that all of the gold bumps  16  are held in contact with the bond pads  13  on the surface  64  of the die  18 . 
     FIG. 6 shows the right side end view of the embodiment of FIG.  5 . As shown, the biasing member  54  extends over a substantial portion of the lid or cover  22  so that pressure is evenly applied to the top  57  of the lid or cover  22 . Because of the even pressure applied to the lid or cover  22  and subsequently between the gold bumps  16  and the bond pads  13 , diffusion between all gold bumps  16  and all bond pads  13  under burn-in temperatures can occur substantially simultaneously. 
     Because of the potential for inherent variation in the height of each gold bump  16 , some gold bumps  16  may not initially be in contact with the bond pads  13  of the substrate  14 . While solvable through a coining operation as previous mentioned, such an additional process step (if not performed during bump application) may desirably be omitted. Dimensional variation of the substrate-to-die electrical contacts presented substantial problems with the use of prior art burn-in substrates or inserts employing hard, electroplated contact bumps of nonporous nickel. However, as the gold bumps  16  that are in initial contact with the bond pads  13  relax in height slightly as they are compressed during assembly, the distance between the die  18  and the substrate  14  will decrease under the force applied by biasing member  54  until those gold bumps  16  not initially in contact with the bond pads  13  do, in fact, contact and diffuse into the bond pads  13 . 
     The semiconductor die assembly  10  that is contained in the burn-in fixture  12  is subjected to heat during a burn-in process to elevate the assembly  10  to a predetermined temperature above ambient, typically 125-150° C. as previously noted. The burn-in temperature, in combination with the relatively slight temperature elevation of the die due to electrical testing during burn-in, is sufficient to cause the gold of the gold bumps  16  to diffuse into the bond pads  13  of the die  18 , but is not high enough to cause the gold in the gold bumps  16  to liquify or to cause damage (beyond the normal purpose of a burn-in to identify defective DUT&#39;s) to the DUT. The elevated temperature is maintained for a selected period of time, until burn-in is completed and diffusion of the gold into the bond pads  13  has formed a permanent bond. It should be noted that certain semiconductor devices have recently been developed for operation at elevated temperatures, such as 180° C. or slightly higher in applications such as aerospace or oil and gas exploration. Burn-in for such chips would naturally be conducted at temperatures higher than 150° C., but still far short of the melting point of gold or most gold alloys. Thus, diffusion bonding according to the present invention would have equal utility for such chips. 
     The time required for sufficient diffusion bonding of the gold bumps of elemental gold or a given alloy to the bond pads can readily be determined, both mathematically and empirically, based on the bump metal or alloy employed and the temperature selected during which the die is biased against the adjacent, parallel substrate. The higher the temperature, the faster the diffusion rate. Thus, for a higher temperature, less time is required for the desired diffusion to occur for any given bump metal. 
     While the present invention has been described in terms of certain preferred embodiments, it is not so limited, and those of ordinary skill in the art will readily recognize and appreciate that many additions, deletions and modifications to the embodiments described herein may be made without departing from the scope of the invention as hereinafter claimed. For example, a plurality of dice may be simultaneously bonded to a like plurality of substrates in a carrier during burn-in; while the term gold “bumps” has been employed, that term may encompass gold balls, cylinders, cuboids, pyramids or cones (including truncated such structures); the term “bond pad” is intended to include and encompass all suitable terminal structures to which a diffusion bond may be made, including both elevated and recessed bond pads as well as flat, concave or convex bond pads and other terminal structures; and bond pads may be formed of gold-compatible materials other than aluminum.

Technology Category: h