Coaxial wire for bonding semiconductors and a method of its manufacure

Abstract: A miniature coaxial cable, suitable for bonding to semiconductor dies, is formed by forming an insulating layer (22) over a conductive core (20), and forming a conductive layer (24) over the insulating layer (22). The insulating layer (22) is preferably silicon dioxide. The conductive layer (24) is doped silicon or aluminum. The insulating layer (22) and the conductive layer (24) are formed by an electron cyclotron resonance plasma process or by a pulsed plasma process. Selected areas (18) of the core (20) may be masked, so that they are not overcoated with the insulating and conductive layers (22, 24).

Description: The invention relates to improved bonding wires, and is particularly concerned with bonding wires for use in microelectronic assemblies and similar applications. There are various ways of making connections from a chip carrier or package to a semiconductor (microelectronic) device (die), among which are wire bonding. The bond wires extend from the die to a lead frame, or to inner ends of lead fingers in a package, and carry power, ground and signals to and from the semiconductor die. Typically, there are many closely-spaced bond wires extending from the die to the lead-frame (or package). Each bond wire is "small", namely on the order of 0.025 mm (one thousandth of an inch), or less, in diameter. As semiconductor devices become faster and faster, the signals carried by the wires become higher and higher in frequency. In the connection of components in microelectronic assemblies and other similar applications, it is desirable to reduce the mutual inductance of the bond wires - in other words, the "cross-talk" between closely adjacent bond wires, particularly those bond wires carrying signals, especially higher frequency signals, to and from the semiconductor device. At the high operating speeds of modern semiconductor devices, this becomes a non-trivial problem. The same problem can manifest itself in other, similar applications. In US-A-4,581,291 a microminiature coaxial cable is disclosed having a ribbon inner conductor surrounded by a dielectric polystyrene or parylene and a circumferential conductor aluminium or copper. In EP-A-0 465 113, filed June 26, 1991, published January 8, 1992, a coaxial cable is disclosed which comprises an inner conductor consisting of a very fine metal wire having a diameter of 120 µm or below an insulating layer of an insulating synthetic resin material or teflon coating the inner conductor and an outer conductor Cu or Ag coating the insulating layer. In US-A-4 956 749 a semiconductor integrated device support structure having a transmission line interconnected structure in a metal block on which the devices are mounted is disclosed. A plurality of stacked layers contains the complete wireline interconnection network. Each wire has an inner conductor, surrounded by a dielectric material, for which silicone or teflon is proposed, and an outer conductor. It is therefore an object of the present invention to provide an improved bond wire which manifests less mutual inductance, especially with respect to neighboring bond wires. The improved bond should be formable by an improved method. According to the present invention, a small diameter conductive wire as in claim 1 is provided with a first coating of an electrically insulating material, and is further provided with a second coating of electrically conductive material superposed over the first insulating material. In this manner, a "small" coaxial wire is formed, having a conductive "core", an intermediate insulating layer, and an outer conductive layer. The first coating is formed of silicon dioxide and the second coating is formed of doped silicon to facilitate the process. The invention further describes a method of producing such a coaxial wire - namely, the techniques for treating the initial wire which serves as the core of the coaxial wire. The invention is especially well-suited to semiconductor applications, as a bond wire extending between a semiconductor die and a lead frame or package leads. By reducing the mutual inductance of the bond wires, signals, especially higher frequency signals, can be conveyed by the wires, with less interference from signals on neighboring wires. The application of the second, electrically conductive layer of doped silicon over the intermediate insulting layer formed of silicon dioxide effectively makes each bond wire a miniature coaxial conductor. DRAWINGS Figure 1 is a cross-sectional view of a bond wire, according to the prior art. Figure 2 is a cross-sectional view of a bond wire, such as that set forth in Figure 1, provided with a first overlying layer (coating) of SiO 2 , according to the present invention. Figure 3 is a cross-sectional view of the bond wire of Figure 2, further provided with a second overlying layer (coating) of doped silicon according to the present invention. Figure 4 is a perspective view of a bond wire, being formed according to an alternate embodiment of the present invention. Figure 5 is a perspective view of the completed bond wire of Figure 4, according to the present invention. DESCRIPTION Figure 1 shows a cross-section of a conductive bond wire 10, formed of a material such as gold, copper or the like. The bond wire has a diameter in the order of 0.025 mm (one thousandth of an inch), or less, and is suitable for bonding from the bond pads (not shown) of a semiconductor device (die) to a lead frame, or the like. The use of many such "small" bond wires is well known for connecting semiconductor chips (dies), and is known to provide some sort of insulation, such as a gel, over the bond wires to prevent their shorting to one another. According to the present invention, bond wires are made into miniature coaxial conductors. This can occur prior to being bonded to the die, or after, the latter (after) being somewhat preferable. (If overcoated before bonding to the die, the intermediate insulating layer and outermost conductive layer must be "stripped away" at the location of bonding.) Figure 2 shows a small conductive wire, such as the bond wire 10 of Figure 1, being treated. In this case, there is a "core" conductor 10. Arrows (↓↓↓) indicate a processing step, wherein the bond wires are covered with a layer of silicon dioxide as an electrically insulating material. The insulating material can be applied using any one of various methods, for example an electron cyclotron resonance plasma process, or a pulsed plasma process. Further, if this step is performed after the core 10 is bonded to the die, surrounding areas of the die can be covered with the insulating material in this same step. As will be seen, the insulating layer 12 is used as an "intermediate" layer, according to the present invention. Turning to Figure 3, after the insulating layer 12 has been applied, an electrically conductive outer layer (or coating) 14 is formed over the insulating layer. This electrically conductive coating (layer) 14 is of doped silicon. This layer 14 can be deposited using a similar process as was used in forming the intermediate layer 12 - namely an electron cyclotron resonance plasma process, or a pulsed plasma process. Hence, the completely formed wire, shown as 16 in Figure 3, is effectively a miniature coaxial conductor, suitable for making connections to semiconductor dies. Figures 4 and 5 show an alternate embodiment of the invention. In Figure 4, an area 18, such as an end of a conductor 20 (similar to conductor 10) is "masked", in other words covered with a masking material 21, prior to applying an insulating layer 22. The insulating layer 22, similar to the insulating layer 12, is applied over the wire 20 and over the mask 21, in a manner similar to that described with respect to Figure 2. The partially formed coaxial conductor (core 20 and insulating layer 22) is then further processed similar to Figure 3, to provide an outer conductive layer 24 overlying the intermediate insulating layer 22. The mask 21 may then be removed, exposing the core 20 in the masked area 18, as shown in Figure 5. This technique of masking is especially suited to forming a coaxial wire for bonding, prior to bonding, and without requiring stripping away outer insulating and conductive layers. Preferably, the outer conductive layer 14 (or 24) can be connected to a ground plane associated, for example, with a package containing the semiconductor die. This can be achieved by wire bonding (such as by using another wire) from the ground plane to the outer layer (14 or 24). The miniature coaxial conductor of the present invention provides a capability of carrying higher frequency signals to and from a semiconductor device, without cross-talk from adjacent (neighboring) signal-carrying wires. Preferably, all of the signal-carrying wires associated with a semiconductor die are formed as miniature coaxial conductors (cables). Additionally, shorts between adjacent bonding wires are alleviated, due to the insulating feature of the insulating layer. As noted above, the diameter of the core (10, 20) is on the order of 0.025 mm (one thousandth of an inch, 25 microns), or less. The thickness of the intermediate insulating layer (12,22) is on the order of a few µm (microns), or less. The thickness of the outer conductive layer (14,24) is also on the order of a few µm (microns), or less. Hence, the overall diameter of the coaxial conductor (16) is not significantly increased over that of a bare bonding wire.

Claims: Miniature coaxial cable, comprising: an inner conductive core (10, 20); an insulating layer (12, 22) deposited onto the core (10, 20); and an outer conductive layer (14, 24) formed over the insulating layer (12; 22) characterized in that the insulating layer (12, 22) is formed of silicon dioxide and that the outer layer (14, 24) is formed of doped silicon. Miniature coaxial cable according to claim 1, wherein said inner conductive core (10, 20) is on the order of 0.025 mm (one thousandth of an inch), or less, in diameter. Miniature coaxial cable according to claim 1 or 2, wherein the core (10, 20) is formed of gold. Miniature coaxial cable according claim 1 to 3, further comprising: an area (18) along the core (20) where the core is exposed; and the remaining core being covered with the insulating layer and the outer conductive layer. Method of forming a bond wire for bonding semiconductor dies said bond wire consisting of a miniature coaxial cable as set forth in claims 1 to 4, comprising the steps of: (a) providing the core conductor (10, 20); (b) forming the intermediate insulating layer (12, 22) over the core conductor (10, 20); and (c) forming the outer conductive layer (14, 24) over the intermediate insulating layer (12, 22), wherein the bond wires are in place and connected between the die bond pads and the leadframe before the step of forming said intermediate insulating layer (12, 22). Method according to claim 5, wherein the intermediate layer (12, 22) is formed using an electron cyclotron resonance plasma process. Method according to claim 5, wherein the intermediate layer (12, 22) is formed using pulsed plasma deposition. Method according to claim 5, wherein the outer layer (14, 24) is formed using an electron cyclotron resonance plasma process. Method according to claim 5, wherein the outer layer (14, 24) is formed using pulsed plasma deposition.

Inventors: MIAOULIS NIKO, MIAOULIS, NIKO
Applicants: LSI LOGIC CORP, LSI LOGIC CORPORATION
Application year: 1992
Main classification: H01B11
IPC codes: H01B11, H01L21, H01L23