Lead-on-chip semiconductor device

Abstract: A semiconductor device (10) has a lead-on-chip (LOC) configuration. Leads (24) of the device have on-chip portions (36, 38) which are electrically coupled to peripheral bond pads (14) by conductive wires (30) and off-chip portions (42). On-chip portions (38) of the leads extend from the central portions toward centerline A-A for improved adhesion. At least one on-chip portion (28) is forked and includes a branch which lacks a direct wire bond connect, also for improved adhesion. In one embodiment, tie bar (22) of leadframe (16) is used to distribute power across semiconductor chip (12). The leadframe may also include chip alignment features (50) and tape alignment features (52) to align chip (12) and insulating tape (18) to the leadframe, respectively.

Description: Field of the Invention The present invention relates to semiconductor devices in general, and more specifically to lead-on-chip semiconductor devices and methods for making the same. Background of the Invention Lead-on-chip (LOC) is a packaging technique employed by several semiconductor manufacturers, particularly those who make semiconductor memory devices, including DRAMs (dynamic random access memories) and SRAMs (static random access memories). Conventional LOC devices have a plurality of leads which are disposed on and attached to an active surface of a semiconductor chip, thus the name lead-on-chip. A primary advantage of LOC is that the ratio between the size of the semiconductor chip and the size of a package which encapsulates that chip is quite high. This advantage is achieved because a chip mounting area, also known as a flag or a die pad, is not required since the chip is instead attached to the leads. The chip size to package size ratio is an important factor to semiconductor manufacturers because customers continue to demand smaller and smaller devices. While LOC is an attractive packaging alternative in some regards, many proposed LOC schemes are not flexible enough to accommodate various chip designs. For instance, much of the existing LOC technology requires bond pads of a semiconductor chip to be located in a central row of the chip. Several semiconductor manufacturers, however, have existing chip designs which have peripheral bond pads. Other LOC techniques have been developed for chips with bond pads on two opposing ends of a chip. Nonetheless, these techniques are not applicable to chips having bond pads distributed around the entire periphery of the chip. In order to benefit from the advantages of LOC, semiconductor manufacturers must re-design the peripheral bond pad chips to have end-only bond pads or centrally located bond pads. Chip re-designs are costly and time-consuming. Furthermore, re-designing the chip may result in a chip which is larger than the originally designed chip. As another example, some semiconductor manufacturers design a chip so that the chip can be packaged using any one of several techniques rather than having to re-design the chip for each type of package. A common case is a chip which is designed to be packaged using either a metal leadframe or a TAB (tape automated bonding) leadframe. Much of the existing LOC technology is directed solely to the use of metal leadframes and cannot be implemented in TAB. EP-A-400 324 (IBM) discloses a semiconductor chip having bonding terminals deposited within an encapsulating material having a plurality of self-supporting, unitary, discrete, and continuous lead frame conductors positioned at various locations around the semiconductor chip and cantilevered out of the encapsulating material, so that discrete wires can be used to connect the conductors to the terminals. Excessively long bonding wires are avoided by connecting selected lead frame conductors to a parallel conductor by a jumper wire and then connecting the parallel conductor to a desired terminal with a short wire. EP-A-409 173 (NEC) discloses a semiconductor device that includes a semiconductor pellet, an insulating film, a conductive plate, and a lead frame. A plurality of electrodes and a plurality of active elements are formed on the semiconductor pellet. The insulating film is bonded to a surface of the semiconductor pellet on which the active elements are formed. The conductive plate is arranged on the insulating film. The lead frame includes a plurality of connecting terminals selectively arranged in predetermined regions on the conductive plate through another insulating film, and leads laterally extending from the connecting terminals. According to the present invention there is provided a lead-on-chip semiconductor device according to claim 1. Summary of the Invention The present invention overcomes many of the disadvantages associated with the prior art. In one form, a semiconductor device in accordance with the present invention has a semiconductor chip having a periphery, an active surface, and a centerline. A plurality of bond pads are formed on the active surface of the chip along at least two sides of the periphery. A plurality of leads overlies the active surface and the chip such that the leads are interspersed with the bond pads. Each lead has an on-chip portion which is wire bonded to a bond pad and an off-chip portion. The on-chip portion of at least one lead is configured to be forked with one branch of the forked lead lacking a direct wire bond connection to a bond pad. These and other features, and advantages, will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. It is important to point out that the illustrations may not necessarily be drawn to scale, and that there may be other embodiments of the present invention which are not specifically illustrated. Brief Description of the Drawings An embodiment of the present invention is now described, by way of example only, and with particular reference to Figure 3, Figures 1 and 2 being capable of having the inventor applied and are FIG. 1 is a plan view of a semiconductor device FIG. 2 is an exploded view of region 34 of FIG. 1; and FIG. 3 is a plan view of a semiconductor device in accordance with a preferred embodiment of the present invention. Detailed Description of a Preferred Embodiment The present invention provides chip design flexibility to semiconductor manufacturers. In one form of the invention, a semiconductor chip has peripheral bond pads and uses a lead-on-chip (LOC) concept. Leads extend across an active surface of a ,chip between the bond pads to accommodate the peripheral bond pads. In addition, leads extend well beyond the bond pads, toward a centerline of the package, in order to increase lead surface area. Increasing lead surface area has at least two advantages. One advantage is that a larger surface area improves adhesion between the leads and the chip, and between the leads and a molded resin package body. Another advantage is that a larger lead area also improves dissipation of heat from the active chip surface to the ambient through the thermally conductive leads. Although the leads of a semiconductor device in accordance with the present invention extend beyond the bond pads, wire bond lengths are kept short because wire bonds are made to a central portion of the leads as opposed to lead tips. Short wire bonds reduce the probability of wire sweep during a package molding procedure and also lower the probability of electrically short circuiting two wires or short circuiting a wire to an inappropriate lead. Each of these advantages and other desirable features of the present invention are described in more detail below. FIG. 1 illustrates in plan view a semiconductor device 10. Device 10 includes a semiconductor chip 12. An active surface of the chip is illustrated, and includes a plurality of bond pads 28 positioned along a periphery of the chip. The chip has four sides, two of which are much longer than the other two. This rectangular chip shape is common in semiconductor memory devices. It is to be understood, however, that the present invention may be practiced with other device types, such as gate arrays, microprocessors, analog devices, and the like. The active surface of semiconductor chip 12 is bonded to a leadframe 16 by an insulating tape 18 having an adhesive layer or adhesive coating on each major side. The double-sided adhesive insulating tape is used to bond leads 14 to chip 12. Insulating tape 18 may be any of the commercially available tapes used in conventional LOC packaging. Commonly used insulating tapes are often polymers, such as a polyimide material. Adhesive materials may also be a polyimide, or may be an epoxy, an acrylic, a variant of these materials, or the like. Only a portion of leadframe 16 is illustrated in FIG. 1. Those portions illustrated include rail portions 20, tie bar 22, and a plurality of leads 24. Leadframe 16 is in strip form so that multiple semiconductor chips can be attached to one leadframe. However, for purposes of understanding the present invention, it is necessary only to show the leadframe portions associated with one semiconductor chip. Leadframe 16 is formed of materials which are used in conventional leadframes, for instance copper, a copper alloy, an iron-nickel alloy, or the like. As illustrated in FIG. 1, tie bar 22 extends along a centerline A-A of chip 12 and joins opposing rail portions 20 of leadframe 16. Tie bars in a conventional leadframe are used to suspend a chip mounting area, also known as a flag or a die pad. LOC leadframes do not require a die pad since a chip is attached to leads. However, a tie bar serves two other purposes. First, tie bar 22 of leadframe 16 provides additional surface area to adhere to chip 12 and to adhere a molded resin material which will eventually form a package body (not shown). If necessary, tie bar 22 can be modified to include support members 26 for additional surface area and improved adhesion. Another purpose in having tie bar 22. The tie bar can also be used to distribute voltage in the device. Tie bar 22 is used for power distribution, however any voltage may be distributed throughout the device. As illustrated in FIG. 1, tie bar 22 is electrically coupled to bond pads 28 by conductive wires 30. A lead 32 is integral to tie bar 22. Thus, a voltage supplied to lead 32 is distributed across the chip by the tie bar. Although device 10 has two bond pads 28 at each of two sides of chip 12 which are electrically coupled to the tie bar, this is not a requirement of the invention. Voltage distribution can be achieved by having one or more bond pads and one or more leads electrically coupled to the tie bar . Furthermore, more than one tie bar may be used for voltage distribution purposes. Although FIG. 1 illustrates tie bar 22 as distributing voltage from one end of chip 12 to another, a tie bar may distribute voltage to and from any portion of a chip. For example, a voltage bond pad could be electrically coupled to one of the support members 26 of FIG. 1 by a wire bond to distribute voltage from the center of the chip to another portion of the device. Leads 24 of leadframe 16 each have three different portions. A lead of FIG. 1 highlighted by a region 34 is illustrated in an exploded view in FIG. 2 to more clearly point out each of the three lead portions. Lead 24 has a central portion 36 to which is bonded a conductive wire 30. Dimensions of central portion 36 should be sufficient to provide a reasonable bonding target area during bonding of the conductive wires. An inner portion 38 of lead 24 is a second lead portion and is also illustrated in FIG. 2. Inner portion 38 extends from central portion 36 toward centerline A-A of chip 12 as illustrated in FIG. 1. Conventional leadframes bond a conductive wire to a tip of a lead rather than to a central portion. By extending the lead beyond the bonded portion toward the center of the chip, more lead surface area is provided and adhesion of the lead to insulating tape 18, chip 12, and a molded resin package body (not shown) is improved. Also, chip 12 may include bond pads which are located near the middle of the chip rather than along the chip periphery. In this case, a conductive wire can be bonded to inner lead portions 38 to accommodate the internal bond pads, as illustrated in region 40 of FIG. 1. In one example, inner portions 38 are made wider than other portions of the leads to further improve adhesion. Yet another benefit of inner portions 38 is that the inner portions of the leads can be used as a clamping area during wire bonding. In wire bonding conductive wires between leads of a leadframe and bond pads of a semiconductor chip, it is important that the leads be held still to ensure proper bonding. As part of a leadframe, leads are relatively long cantilevers which can easily be flexed. To prevent flexing of the leads during bonding, and to prevent movement of the leadframe in general, outer portions of the leads are typically held down by a clamping tool of a wire bonding apparatus. The clamping tool is designed to permit access to a semiconductor chip and to bonding portions, most often the tips, of the leads. The clamping area of a leadframe most often encircles, and is spaced apart from, a semiconductor chip. For example, the region beyond a package outline 44 might represent a conventional clamping region. With respect to LOC packaging techniques, however, conventional clamping methods often do not provide sufficient lead stability. Because LOC leadframes have leads which extend over a chip, the portion of the lead not clamped by a peripheral clamping tool is longer in comparison to other leadframe designs. The longer the unclamped portion of the lead, the easier it is for the lead to move during the wire bond operation. Inner lead portions 38 may be used as a clamping area. One possible clamping area is defined in FIG. 1 as clamping area 41. Clamping area 41 is in the shape of a cross; however, this is not a requirement of the present invention. Any amount or portion of the inner portions 38 of leads 24 can be used as a clamping area. In clamping leadframe 16 during a wire bond operation, it is important that all bond pads and all lead portions which are to be bonded are accessible. For example, if using a clamping tool having clamping area 41, one would not be able to include the internal bond pad of region 40 illustrated in FIG. 1 since this region would be clamped. By clamping the leadframe along clamping area 41, the cantilevered ends of leads 24 are securely stationed near the central bonding portions 36 of the leads. In clamping the leadframe in a region overlying the active chip surface, it is also important that pressure applied by the clamping tool does not damage the chip. A third portion of each lead 24 is an outer portion 42, illustrated in FIG. 2, which extends away from central portion 36 and chip 12. Upon forming a package body for device 10, outer portions 42 will also extend out of the package body into a predetermined lead configuration. While an actual package body is not illustrated in FIG. 1, a package outline 44 is included for reference. Outer portion 42 of lead 24 includes an opening 48. The opening is provided to improve adhesion between the lead and the package body material, but is not necessary in practising the invention. Device 10 illustrated in FIG. 1 also includes various alignment features. In particular, several leads are provided with chip alignment features 50. Chip alignment features 50 are protrusions which extend from the leads, and define the appropriate position of chip 12. In bonding chip 12 to leadframe 16, an automated tool or human operator can use alignment features 50 to properly position the chip relative to the leadframe. In device 10, eight leads have the alignment features, each being aligned to one side of the chip. It is not necessary that eight leads be equipped with alignment features to ensure proper alignment. For example, two or more features which are aligned to either a side or a corner of the chip may be used to determine position of the chip. Tape alignment features 52 are also included in areas of leadframe 16. Tape alignment features 52 are implemented in a similar manner as chip alignment features 50. An adhesively coated insulating tape is typically attached first to a leadframe. The leadframe, having the tape attached thereto, is then aligned and bonded to a semiconductor chip. The tape alignment features illustrated in FIG. 1 aid automated manufacturing equipment and human operators in determining the proper position of insulating tape 18 relative to leadframe 16. Tape alignment features are provided on both leads 24 and on support members 26. As mentioned with respect to the chip alignment features, the specific number of tape alignment features used can vary and the features can be aligned to either a side of insulating tape 18 or to a tape corner. Illustrated in FIG. 3 is a plan view of a semiconductor device 70, in accordance with the present invention. Elements of device 70 which are analogous to elements of device 10 are referenced by the same numerals as in FIG. 1. Some differences between device 10 and device 70 are that device 70 does not have a tie bar and that bond pads 14 of device 70 are peripheral, but only along two sides of chip 17. As discussed previously, tie bars are not necessary in LOC applications. However, in one embodiment of the present invention, tie bars are used to distribute power in a device and serve to improve the adhesion of the leadframe to other device elements. Semiconductor memory devices often have bond pads on only two sides of a chip. FIG. 3 illustrates one way in which the present invention can be used in conjunction with such a memory device. Other distinctive features of device 70 are the utilization of two insulating tapes 18 rather than one, and an internal, rectangular clamping area 41 rather than a cross-shaped clamping area. As previously mentioned leads 24 of device 70 are spaced between bond pads 14. Conductive wires 30 are used to electrically couple the central portions of the leads, while inner portions of the leads extend inward toward centerline A-A. Both of these aspects enable a device in accordance with the present invention to have several advantages over conventional LOC devices. For instance, a device in accordance with the present invention has improved adhesion between a leadframe, a semiconductor chip, an adhesive tape, and a molded resin packaging material. A primary reason for this advantage is an overall increase is leadframe surface area. Aspects of the present invention which contribute to the increase in leadframe surface area are the extension of leads from the wire bonded portion toward the center of the chip, the use of a tie bar, and making inner portions of the leads wider than the minimum lead width. Using a tie bar also creates a convenient way of distributing power in the device. Another advantage of the present invention is that it is applicable to chips having peripheral bond pads. Thus, chips which are used in an LOC implementation may also be used in other packages without having to re-design or re-layout the chip. Yet another advantage of the present invention is the ability to clamp inner portions of leads which overlie a chip during wire bond to reduce lead movement, thereby improving bonding accuracy. Furthermore, accuracy of attaching a chip or an adhesive tape to a leadframe in accordance with the present invention is improved by the use of alignment features included on leads and other leadframe members. Thus it is apparent that there has been provided, in accordance with the invention, a lead-on-chip (LOC) semiconductor device and method for making the same that fully meets the need and advantages set forth previously. Although the invention has been described and illustrated with reference to specific embodiment thereof, it is not intended that the invention be limited to these illustrative embodiment. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. For example, the present invention is not limited to devices having a dual-in-line or SOJ (small outline J-lead) lead configuration. The invention may be practiced with semiconductor devices with leads on any number of sides of a package body. In addition, a device in accordance with the invention is not limited to a specific number of leads, bond pads, chips, etc. Nor is the invention limited to any specific types of materials or types of semiconductor chips. It is also important to note that bond pads along the "periphery" and "peripheral" bond pads as referenced herein do not exclude the use of non-peripheral bond pads, and does not imply that pads must lie along the entire periphery. Although a molded resin package has been specifically mentioned, the present invention is not limited to any specific package type. Furthermore, a centerline of a chip need not bisect two short sides of the chip as illustrated herein. A centerline of a semiconductor chip may bisect any two opposing sides of a chip in accordance with the present invention. Moreover, an electrically insulating adhesive material used in accordance with the present invention can be selected to provide alpha particle protection. Therefore, it is intended that this invention encompass all such variations and modifications as fall within the scope of the appended claims.

Claims: A lead-on-chip semiconductor device (70) comprising: a semiconductor chip (12) having a periphery and an active surface, the active surface having a centerline (A-A) which intersects two opposing sides of the chip; a plurality of bond pads (14) formed on the active surface of the chip along at least two sides of the periphery; and a plurality of leads (24) having portions which overlie the active surface of the chip and being interspersed with the plurality of bond pads, each lead of the plurality of leads comprising an on-chip portion which is electrically coupled to at least one of the plurality of bond pads and which extends from the periphery of the chip toward the centerline of the chip; and an off-chip portion which extends form the periphery of the chip away from the chip; wherein the on-chip portions of the plurality of leads are wire bonded to the plurality of bond pads, and characterized in that the on-chip portion of at least one lead of the plurality of leads is configured to be forked with one branch of the forked lead lacking a direct wire bond connection to a bond pad. The semiconductor device of claim 1 wherein the on-chip portion of each lead comprises an electrical connection portion (36) located adjacent to the periphery of the chip which receives a wire bond, and an extension portion (38) which extends from the electrical connection portion toward the centerline of the chip, wherein the extension portion lacks a wire bond and is provided to improve adhesion between the chip, the lead, and a molded package body. The semiconductor device of claim 1 or 2 wherein the chip periphery comprises four sides and wherein at least one bond pad is positioned along each of the four sides. The semiconductor device of claim 1, 2, or 3 further comprising a tie bar (22) which overlies the active surface of the chip. The semiconductor device of claim 4 wherein the tie bar (22) is electrically coupled to at least one bond pad (28). The semiconductor device of any preceding claim wherein the semiconductor chip has four comers and four sides and wherein at least one of the plurality of leads has a protrusion (50) aligned to either a side or a corner of the chip. The semiconductor device of any preceding claim wherein each of the plurality of leads has a minimal width and wherein the inner portions of each of the plurality of leads has a width which is larger than the minimal width.

Inventors: AFSHAR DAVID D, BIGLER CHARLES G, CASTO JAMES J, MCSHANE MICHAEL B, AFSHAR, DAVID D., BIGLER, CHARLES G., CASTO, JAMES J., MCSHANE, MICHAEL B.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1993
Main classification: H01L23
IPC codes: H01L23