Integrated circuit substrate having laser-embedded conductive patterns and method therefor

[0001] The present application is related to U.S. patent application Ser. No. 09/931,144 filed Aug. 16^th, 2001 by the same inventors and assigned to the same assignee. The specification of the above-referenced patent is herein incorporated by reference.

[0002] The present invention relates generally to semiconductor packaging, and more specifically, to a substrate having laser-embedded conductive patterns for providing electrical inter-connection within an integrated circuit package.

[0003] Semiconductors and other electronic and opto-electronic assemblies are fabricated in groups on a wafer. Known as “dies”, the individual devices are cut from the wafer and are then bonded to a carrier. The dies must be mechanically mounted and electrically connected to a circuit. For this purpose, many types of packaging have been developed, including “flip-chip”, ball grid array and leaded grid array among other mounting configurations. These configurations typically use a planar printed circuit etched on the substrate with bonding pads and the connections to the die are made by either wire bonding or direct solder connection to the die.

[0004] The resolution of the printed circuit is often the limiting factor controlling interconnect density. Photo-etch and other processes for developing a printed circuit on a substrate have resolution limitations and associated cost limitations that set the level of interconnect density at a level that is less than desirable for interfacing to present integrated circuit dies that may have hundreds of external connections.

[0005] As the density of circuit traces interfacing an integrated circuit die are increased, the inter-conductor spacing must typically be decreased. However, reducing inter-conductor spacing has a disadvantage that migration and shorting may occur more frequently for lowered inter-conductor spacings, thus setting another practical limit on the interconnect density.

[0006] Therefore, it would be desirable to provide a method and substrate having improved interconnect density with a low associated manufacturing cost. It would further be desirable to provide a method and substrate having reduced susceptibility to shorting and migration between conductors.

[0007] A substrate having laser-embedded conductive patterns and a method for manufacturing generate a circuit pattern within a substrate having circuits embedded beneath the surface of the substrate. An outer dielectric layer is injection molded or laminated over a thin metal layer and channels outlining a desired circuit pattern are cut in the surface of the plastic layer using a laser. Conductive material is then plated or paste screened into the channels. The thin metal layer may be etched, mechanically drilled or punched to provide through holes for vias and to create separate power and ground paths within the metal layer. The process can be extended to multiple layers to create a sandwich structure for multilayer applications.

[0012] The invention, as well as a preferred mode of use and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals indicate like parts throughout.

[0013] The above-incorporated patent application discloses a process and structure for manufacturing a low-cost substrate having high conductor density and electrical integrity by embedding the conductive patterns beneath the surface of a substrate. The substrate is an embossed substrate requiring tooling to form channels for the conductive patterns. While embossing provides a low cost and high throughput manufacturing process for the substrate base, the tooling must be remanufactured when design changes are made, as it is unique to a particular design. For low volume applications such as prototyping, the cost to tool the embossing process may be prohibitive and in general, the techniques of the present invention will provide a lower cost alternative, except in designs or portions of designs that have large areas that are recessed such as wells for integrated circuit dies.

[0014] The present invention provides an alternative that does not require custom tooling for producing channels for conductors within a substrate and provides a manufacturing process, that in general, has fewer steps and lower overall cost in producing a substrate. For some embodiments of the present invention, a punching tool is required to make a metal frame, but for other embodiments of the present invention, the metal frame is etched or mechanically drilled and therefore no punching tool is required, reducing the cost of taking a particular design to the manufacturing process. As a result, the present invention provides a low cost alternative to the techniques of the above-incorporated patent application and an alternative with a greatly reduced startup or low-volume production cost. Also, the substrate material is not deformed to generate circuit channels in the techniques of the present invention, providing use of a wider range of materials for the dielectric layer and eliminates any reduction in the mechanical properties of the dielectric that are cause by deformation. A combination of the techniques described in the above-incorporated patent application and the techniques of the present invention may be used to emboss a large area, such as an integrated circuit die well, within the manufacturing process disclosed herein.

[0015] Referring now to the figures and in particular to FIG. 1A, a side view of a metal layer 10 for use in preparing a substrate in accordance with an embodiment of the present invention is depicted. Metal layer 10 is used to form a substrate in a novel process that permits embedding circuits beneath the top and/or bottom surface of a substrate and isolating the circuits in channels. Metal layer 10 is generally a copper core that may be etched or die-cut, but other suitable metal layers may be used for form the core of the substrate of the present invention, such as a copper-INVAR-copper laminate. The ratio of copper to Invar can be varied to provide adjustment of the coefficient of thermal expansion (CTE) of the substrate. Holes 11 are generated in metal layer 10 to permit the passage of circuit paths through metal layer 10, while avoiding electrical contact with metal layer 10. Referring now to FIG. 1B, a top view of metal layer 10 is shown. A die aperture 12, for mounting an integrated circuit die is provided in the central area of metal layer 10. Isolating cuts 13 separate metal layer 10 into multiple conductive planes, such as power plane 15 and ground plane 14. A frame (not shown) can be provided around the periphery of metal layer 10 to hold the isolated planes in place until after the manufacture of the substrate.

[0016] Referring now to FIG. 2A, the first stage in the preparation of a substrate 20 in accordance with an embodiment of the present invention is depicted. A dielectric outer layer 21 has been added to the top and bottom surface of metal layer 10 and can be provided by injection molding a plastic material around metal layer 10 or by laminating a dielectric such as KAPTON film or PTFE on each side of metal layer 10.

[0017] Referring now to FIG. 2B, the next stage in the preparation of substrate 20 is depicted. Substrate 20 is laser-ablated to form substrate 20A having an outer dielectric layer 21A as shown. Substrate 20A includes channels on both surfaces of the dielectric layer defining channels 23 for conductive paths, blind vias 22 for connection to ground and power planes formed in metal layer 10 and through vias 11A having a diameter smaller than holes 11 in metal layer 10, providing an insulating layer around holes 11. Blind vias 22 show a conical shape, which is preferred for addition of conductive material and can be generated by varying the laser angle or beam diameter as the dielectric material 21 is ablated.

[0018] Next, referring to FIG. 2C, the next step in the preparation of substrate 20B providing a substrate 20C having conductive circuit paths. Conductive material is added within channels 23, blind vias 22 and through vias 11A to provide conductive paths 23A conductive blind vias 22A and conductive through vias 11B. The conductive material may be a silver or copper paste that is screen printed into channels 23, blind vias 22 and through vias 11A, and planarized to remove conductive material on the surface of outer dielectric layer 21A after printing. Alternatively, an electroplating process (generally copper electroplate) can be used to add conductive material within channels 23, blind vias 22 and through vias 11A and a planarization process or chemical etching process can be used to remove excess conductive material on the surface of dielectric layer 21A.

[0019] Multiple conductive layers may be generated by repeating the steps above, adding a second outer dielectric layer to the top and/or bottom surface of substrate 20B to form a multi-layer circuit on one or both sides of substrate 20B. Further, embossing steps in accordance with the above-incorporated patent application may be used to generate large area recesses in one or both sides of outer dielectric layer 21A, such as die mounting recesses.

[0020] Finally, top plating 24 is electroplated on the conductive surfaces deposited within the channels of substrate 20B to form plated substrate 20C. Nickel-Gold is generally used to provide a barrier migration layer and to provide electrical contact for wire or chip bonding in subsequent manufacturing steps. In general, silver-nickel is an appropriate electroplating material and if a silver paste was used to form conductive channels 23A, electroplating may not be needed to provide solderable conductive connections, but may be added to eliminate oxidation.

[0021] While the figures illustrate conductive circuit channels, the figures are depicting only a portion of the total substrate. Hundreds of circuit channels 23 will generally be used in an integrated circuit design and may be oriented in any direction within the surface of substrate 20C. The present invention provides a process for forming circuits within channels in a substrate that are below the top surface of the substrate. This an improvement over the present state of the art similar to that provided by above-incorporated patent application in that the prior art generally provides only surface conductors. The channels formed by laser ablation place the conductors below the surface and the conductors are thereby insulated from adjacent conductors by the substrate. The use of laser ablation techniques further provides improvement over the techniques of the above-incorporated patent application.

[0022] Referring now to FIG. 3, an integrated circuit 30 in accordance with an embodiment of the invention is depicted. A die 31 having electrical contacts is attached to substrate 20C and is electrically connected to conductive channels 23A by wires 35. Ball grid array (BGA) connections for the integrated circuit package are provided by solder balls 36 attached to the bottom channels 23A formed in substrate 20C.

[0023] The above description of embodiments of the invention is intended to be illustrative and not limiting. Other embodiments of this invention will be obvious to those skilled in the art in view of the above disclosure and fall within the scope of the present invention.