Wire bonding for thin semiconductor package

[0002] The present invention is related in general to the field of semiconductor devices and more specifically to very thin wire-bonded semiconductor packages.

[0003] An ongoing trend in the semiconductor industry is the need for devices with a thinner profile. One of the major hurdles to achieve thinner devices is an inherent feature of the wire ball-bonding technology, which is the dominant assembly technique for the majority of integrated circuit chips. In the wire ball-bonding process, a free air ball, approximately spherical in shape, is created at the tip of the wire by using a flame or a spark technique. It is unavoidable that the heat necessary to melt a small volume of the wire for forming the ball also spreads along a certain distance of the wire, without actually melting the metal. The extent of the heat-affected zone depends, among other parameters, on the thermal conductivity of the wire metal. Most commonly, gold, copper, aluminum, or alloys of these metals are used as wire materials. When the ball cools and solidifies, the cooling of the heat-affected zone causes a re-crystallization of the wire; a result of which is that the originally microcrystalline metal tends to develop fewer and larger crystals. These crystals, in turn, are easier to separate under force and thus reduce the mechanical strength of the wire in the heat-affected zone.

[0004] In the conventional ball-bonding technology, the reduction of mechanical strength in the heat-affected zone is compensated by a procedure in which the ball is first attached to the contact pad (for example, of the integrated circuit), and the wire is then allowed to attain a vertical position over the ball for the length of the heat-affected zone. Only after this vertical stretch is the wire formed into the curvature and bending of the loop which bridges the distance to the contact pad of the stitch bond. The usually high loop necessitated by the heat-affected zone is a hallmark of wire ball bonding. Would the curvature and bending of the wire initiate directly over the ball, the wire would easily shear off and break.

[0005] A need has therefore arisen for a coherent, low-cost method of wire ball bonding without the need of high, arching wire loops. The innovative wire bonding method should use the installed equipment base so that no investment in new manufacturing machines is needed. The method should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability.

[0006] One embodiment of the invention is an assembly of a semiconductor chip having an integrated circuit (IC) including at least one contact pad on its surface, wherein the contact pad has a metallization suitable for wire bonding, and an interconnect bonded to said contact pad. This interconnect includes a wire attached to the pad by ball bonding, a loop in the wire closed by bonding the wire to itself near the ball, and a portion of the remainder of the wire extended approximately parallel to the surface of the chip. The interconnect can be confined to a space equal to or less than about three ball heights from the surface.

[0007] In another embodiment of the invention, an electrically insulating substrate with first and second surfaces and a plurality of electrically conductive routing strips integral with the substrate, has a first plurality of contact pads disposed on the first surface and a second plurality of contact pads disposed on the second surface. An IC chip with a plurality of bonding pads is attached to the first substrate surface. At least some of the bonding pads have a wire interconnect attached by ball bonding. The interconnect has a loop in the wire which is closed by bonding the wire to itself near the ball, and a portion of the remainder of the wire extends approximately parallel to the surface, spanning the chip surface to the edge of the chip. Each interconnect can thus be confined to a space equal to or less than about three ball heights from the active surface. Finally, each wire is bonded to its respective substrate contact pad on the first substrate surface, and chip and wires are encapsulated. Interconnection elements may be attached to each of the contact pads on the second substrate surface.

[0008] Another embodiment of the invention is a method for forming a low profile wire bond for a low profile package for a semiconductor chip having on its surface an IC and at least one contact pad with a metallization suitable for wire bonding. In this method, a wire is attached to the pad by ball bonding. A loop is then formed in the wire and the loop is closed by stitching the wire to itself near said ball. As a result, the mechanical strength of the recrystallized portion of the wire near the ball is improved. The loop may be formed at a distance less than about three ball heights from the chip surface.

[0009] Embodiments of the present invention are related to thin devices and packages, which are, for example, required in stacks of memory devices and miniaturized products. The ICs for these and similar products can be found in many semiconductor device families such as standard linear and logic products, digital signal processors, microprocessors, wireless devices, and digital and analog devices. The embodiments apply to wires of various metals, such as gold, copper, or aluminum.

[0010] It is a technical advantage of one or more embodiments of the invention that the embodiments can reach the goals of the invention with a low-cost manufacturing method without the cost of equipment changes and new capital investment, by using the installed fabrication equipment base, especially established automated wire bonding machines.

[0011] Another advantage which may flow from one or more embodiments of the invention is to produce thin outline devices with packages having interconnection elements including reflowable material, or just with pressure contacts. Other embodiments of thin outline devices include packages with leadframes. Embodiments of the invention generally apply to semiconductor package types such as PDIPs, SOICs, QFPs, SSOPs, TQFPs, TSSOPs, TVSOPs, and Ball Grid Array devices employing leadframes.

[0012] The technical advances represented by certain embodiments of the invention will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.

[0013] FIGS. 1A to 1D are schematic and simplified cross sections of a portion of a semiconductor chip and substrate on a hot stage in process steps of attaching a wire ball bond according to known technology.

[0014] FIG. 1E shows a schematic cross section of a portion of a semiconductor chip, which is wire ball bonded, attached to a substrate and encapsulated, according to known technology.

[0015] FIG. 2 illustrates a schematic cross section of a portion of a semiconductor chip, which is wire ball bonded according to the invention, attached to a substrate and- encapsulated.

[0016] FIG. 3 is a schematic cross section of the wire ball bond interconnection scheme according to the invention.

[0017] FIGS. 4A to 4F are schematic and simplified cross sections to describe process steps of the assembly method according to the invention.

[0018] FIG. 5 illustrates the schematic cross section of an embodiment of the invention, showing a completed thin-outline ball grid array product having the low-rise ball bond interconnection according to the invention.

[0019] FIG. 6 is a schematic cross section of an embodiment of the invention, showing a multi-chip product having chip interconnections according to the invention.

[0020] FIG. 7 is a schematic cross section of an embodiment of the invention, showing a stacked-chip product having chip interconnections according to the invention.

[0021] FIG. 8 is a schematic top view of an embodiment of the invention, showing bond pads of an IC interconnected according to the invention, as a substitute for an additional high power metallization level.

[0022] The technical advances of the present invention can best be appreciated by a comparison with the shortcomings of the known technology.

[0023] As FIG. 1A schematically shows for known technology, the typical wire bonding process begins by positioning the semiconductor chip 101, attached to a substrate 102, on a heated pedestal 103 to raise the temperature to between 150 and 300° C. The wire 104 is strung through a capillary 105 and extends a short distance 106 outside the tip of the capillary. As an example, for a 25 urn diameter gold wire 104, the distance 106 may be 50 to 150 μm.

[0024] As illustrated in FIG. 1B, at the tip of the wire, a free air ball 107 is created using either a flame or a spark technique 108. The ball 107 has a typical diameter from about 1.2 to 4.0 wire diameters, and should preferably be outside of the capillary. Dependent on the thermal conductivity of the wire metal or alloy chosen, on the time of exposure to the flame or spark, and on other parameters such as the proximity of the capillary, heat extends a certain distance 110 from the air ball along the wire, without actually melting the wire but with enough temperature rise to affect the micro-crystallinity of the wire. As a rule of thumb, the extent of this “heat-affected zone” 110 has a length of about 1 to 1.5 free air ball diameters with significant re-crystallization, and sometimes an additional length 111 of up to 10 air ball diameters with minor re-crystallization.

[0025] Due to this re-crystallization, the mechanical strength of the wire for bending or pulling is considerably weakened, at least for the length of the heat-affected zone 110. For example, wire bending within the heat-affected zone may lead to premature wire breakage due to easy slipping along the macro-crystalline grain boundaries.

[0026] The capillary is moved towards the bonding pad 120, and the ball is pressed by the capillary against the metallization of the pad, as depicted in FIG. 1C. In this process, the ball is deformed; a typical shape looks like a “nailhead”, but for the sake of convenience it is still referred to as the “ball” of the bond. After formation of the bond (often involving ultrasonic energy in addition to the compression force), the capillary has to be lifted straight upward in order to avoid wire bending at least for the length 110 of the heat-affected wire zone. Thereafter, the capillary moves in a pre-determined and computer-controlled manner through the air to create a wire arch of well defined shape, before the capillary attaches the wire by stitch bond 122 to the contact pad 121 of substrate 102. The height 112 of the wire arch is in principle required by the weak heat-affected zone 110.

[0027] In a later assembly step, indicated in FIG. 1E, chip 101 and wire 104 are protected by an encapsulation material 130. As FIG. 1E shows, a major portion 131 of the thickness 132 of the finished packaged device is determined by the height 112 of the wire arch inside the package. However, for the ongoing technology trend toward thinner semiconductor packages, height 132 is incompatible with the need of many electronic products, which require thinner packages.

[0028] The schematic FIG. 2 shows one embodiment of the invention, which solves the package thickness dilemma by eliminating the wire arch and the weakness of the heat-affected wire zone, which necessitated the arch in the first place. The thickness 232 of the finished packaged device is now only determined by the height portion 231, which is essentially a function of the encapsulation method (such as transfer molding).

[0029] Inside the package is a semiconductor chip 101, which has an integrated circuit including at least one contact pad 120 on its surface 101ahaving a metallization suitable for wire bonding. Examples for such metallization include aluminum, copper, gold, silver, palladium, or alloys thereof. Bonded to contact pad 120 is an interconnect which consists of a wire 204 attached to pad 120 by ball bonding 205. Examples for preferred wire materials include gold, copper, aluminum, and alloys thereof. Near “ball” 205 (deformed by pressing the capillary during the bonding process for instance into a nailhead shape) is a loop 206 in wire 204, which is closed by bonding the wire 204 to itself near ball 205. A portion 207 of the remainder of wire 204 is extended approximately parallel to chip surface 101a. The end of interconnect wire 204 is connected at bond 222 to contact pad 121 of substrate 102. For example, bond 222 may be a stitch bond. Preferred examples of contact pad materials are copper and gold-clad copper.

[0030] In another embodiment of the present invention, schematically illustrated in FIG. 3, the interconnect is confined to a space 308 equal to or less than about three ball heights 309 from the IC surface 301a. The semiconductor chip 301 is shown with its surface 301a, including the IC and one contact pad 320. The contact pad 320 has a metallization suitable for wire bonding, such as aluminum,. copper, gold, or alloys thereof. The wire 304 consists preferably of gold, copper, aluminum, or alloys thereof.

[0031] The ball bond 305 of the interconnect is depicted in FIG. 3 as resembling a nailhead, metallurgically welded to bond pad 320. At its neck 310 with ball 305, wire 304 is strongly curved, directly in the heat-affected zone where the wire material re-crystallization is most pronounced. After forming loop 306, the loop is closed along a wire length 307 so that this length 307 is pressed as a stitch bond by the capillary directly in the curved wire portion in the heat-affected zone. As a result, the mechanical strength of the recrystallized portion of the wire next to the ball is reinforced. It should be stressed that the shape of the loop depicted in FIG. 3 is only one example of several possible wire shapes which provide the goal of reinforcing the mechanical strength of the heat-affected zone of ball bonds in order to enable interconnects in close proximity to the IC surface. A portion 311 of the remainder of wire 304 is extended approximately parallel to surface 301a.

[0032] In another embodiment of the present invention, a process for forming a low-rise wire loop is illustrated in the sequence of the schematic and simplified FIGS. 4A to 4F. In this sequence, the capillary of an automated bonder machine is moved by a computer-controlled program. Examples of commercially available computerized bonders are bonder 8020 by Kulicke & Soffa, Willow Grove, Pa., U.S.A., or ABACUS SA by Texas Instruments, Dallas, Tex., USA or a bonder by Shinkawa, Japan. Moving the capillary in a predetermined and computer-controlled manner through the air will create a wire looping of well defined shape.

[0033] In FIG. 4A, the capillary 401 bonds the free air ball 402 to the bond pad of the IC on the surface 403aof the semiconductor chip 403. In FIG. 4B, capillary 401 rises and thus exposes a length 404 of wire 405, which contains the heat-affected zone beginning at neck 404a. In FIG. 4C, capillary 401 moves sidewise in one direction, bending wire 405 in the heat-affected zone near the ball 402 and extends the wire in the direction of movement. In FIG. 4D, capillary 401 slightly rises, bending the wire at position 407, and then reverses the sidewise movement into the opposite direction, extending the wire at position 408. In FIG. 4E, capillary 401 moves sidewise in the opposite direction until it is positioned vertically above neck 404a. Capillary 401 then moves toward ball 402, thus bonding the wire 405 to itself near the ball (as shown in detail in FIG. 3). This stitch bond closes the loop 409 and reinforces the heat-affected wire zone at wire neck 404a. In FIG. 4F, capillary 401 continues to move sidewise in the opposite direction, approximately parallel to the chip surface 403aand preferably at a distance equal to or less than about three ball heights from the surface 403a. This continued sidewise motion of the capillary spans at least the chip surface 403auntil the edge 403bof the chip. Thereafter, the wire 405 may be attached to a contact pad on an external part such as a substrate.

[0034] An example of a complete device assembly, including the assembly process flow in FIGS. 4A to 4F, is shown by another embodiment of the present invention, illustrated schematically in FIG. 5. A semiconductor chip 501 has an active surface 501aand a passive surface 501b. The active surface includes an IC and a plurality of contact pads with a metallization suitable for wire bonding. A ball bond interconnection 502, produced by the process flow described in FIGS. 4A to 4F, has been attached to each contact pad. Due to the loop 503 in the wire near each ball bond, the mechanical strength of the interconnection has been reinforced. A portion 504 of the interconnection 502 is extended approximately parallel to the surface 501aat a distance equal to or smaller than about three times the ball height.

[0035] Furthermore, the embodiment illustrated in FIG. 5 includes an electrically insulating substrate 510 having a first surface 510aand a second surface 510b. Substrate 510 includes a plurality of integral electrically conductive routing strips. A first plurality of contact pads 511 is disposed on the first surface 510a, and a second plurality of contact pads 512 is disposed on the second surface 510b. The passive chip surface 501bis attached to the first substrate surface 510a, using a chip attach material in layer 520.

[0036] The wire interconnections 502 are completed by extending the balance of the wires not used in portions 504 to stitch bonds onto substrate contact pads 511. Furthermore, chip 501 and the plurality of completed interconnections 502 are encapsulated by an encapsulation compound 530. A preferred material is an epoxy molding compound, applied in a transfer molding process. Due to the close proximity of the interconnections 502 to the chip surface 501a, the thickness 530aof the encapsulation compound can be kept at a profile as thin as the encapsulation technology allows. In the embodiment depicted in FIG. 5, an interconnection element 540 is attached to each contact pad 512 on the second surface 510bof the substrate 510. Examples for interconnection elements are bodies made of reflowable materials such as tin, indium, tin alloys including tin/indium, tin/silver, tin/bismuth, and tin/lead, conductive adhesives, and z-axis conductive materials.

[0037] In another embodiment of the present invention, the contact pads 512 on the second surface 510bof the substrate 510 are employed as pressure contacts to external parts.

[0038] While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the material of the semiconductor chip may comprise silicon, silicon germanium, gallium arsenide, or any other semiconductor or compound material used in IC manufacturing.

[0039] As another example, the low-rise interconnection scheme including the wire loop reinforcing the strength of the recrystallized zone of the ball-bonded wire may be employed for interconnecting IC chips in a multi-chip product, as illustrated in the schematic cross section of FIG. 6, or in a stacked-chip product, as illustrated in the schematic cross section of FIG. 7.

[0040] As another example, the low-rise interconnection scheme including the wire loop reinforcing the strength of the recrystallized zone of the ball-bonded wire may be used on the surface of a chip as a substitute for an additional metallization level in ICs, where regions requiring high power have to be interconnected. An example is shown in the schematic top view of FIG. 8.

[0041] It is therefore intended that the appended claims encompass any such modifications or embodiments.