Method of manufacturing a substrate for microwave integrated circuits

Abstract: A microwave integrated circuit formed on a sub­ strate (22) comprising a body of insulating material having upper and lower surfaces with one or more holes (21) formed in said body to extend between the upper and lower surface and a metal filling said holes and extending between and coplanar with the upper and lower surfaces.

Description: The present invention is directed to a method of forming a substrate for a microwave integrated circuit, the substrate having via holes filled with metal for either electrical grounding or heat dissipation or both. In the manufacturing of microwave integrated circuits formed on a substrate, it is often desirable to create direct connections between specific points of the circuit on the upper surface and the metallization on the lower surface. Such connections may be required for two different reasons: 1) to create a low inductance/low resistance path to ground; and 2) to provide an effective heat sinking, specifically for active devices. In the present state of the art when the substrate used is a ceramic substrate, such connections are typically obtained by cutting holes, referred to as "via holes" or "vias", through the substrate and by metallizing the wall of the holes. The holes may be cut either before or after processing the substrate. In the first case, the presence of holes significantly hinders the photolithography process, typically preventing the definition of fine details in the neighborhood of a hole. In the second case, the hole cutting process is severely limited by the need to preserve the integrity of the circuit already fabricated. This is especially true in the case of ceramic materials, such as alumina, where holes cannot be easily etched chemically and have to be drilled mechanically or by laser. Both processes are damaging to delicate circuit structures. In addition, the metallization of the wall alone may provide an adequate electrical connection but does not generally provide adequate heat conduction in the case of high power devices. In the present state-of-the-art, the vias or holes are often filled with a conductive paste ("ink") of the type used in connection with thick film circuits. However, this process is only partially succesful because during filling of the holes with the paste and subsequent curing, the volume of the paste is significantly reduced thereby creating voids in the filling. The use of metal filling by electroplating has been studied. For example from GB-A-2046514 is known a microwave integrated circuit substrate comprising a body of dielectric material wherein one or more holes extending between the upper and lower surface are formed and filled with an electroplated copper conductive material after metallization of the substrate itself. However the procedure requires the use of solder which leaves undesirable contamination in the substrate. D. Tomsa et al. in IBM Technical Disclosure Bulletin vol.21, September 78, pages 1396 - 1397 describe a multilayer integrated circuit chip carrier fabricated by successive laminations beginning from prepreg containing metal layers, and employing solid electroplated interconnections between layers. Such arrangements and methods of manufacture, as well as those indicated in US-A-4 396 467, however, do not guarantee that the substrate with filled holes is free from any residual material remaining from the fabrication process and specifically any metals except obviously the metal filling the holes. It is an object of the present invention to provide a process capable of forming and completely filling with metal by plating the holes or vias in an insulating substrate used in microwave integrated circuits, before the metallization of the substrate itself. The foregoing object of the invention is achieved by a method as defined in claim 1. Advantageous embodiments are specified in the dependent claims. Figure 1 shows a portion of a microwave integrated circuit with metallized via holes in accordance with the prior art. Figures 2A-2D show in cross-section, a portion of a substrate and the method of forming metal filled via holes in accordance with the present invention. Figure 3 shows a finished substrate with filled via holes ready for metallization and photolithography processing. A typical prior art microwave integrated circuit is shown in Figure 1. The substrate 11, typically alumina or beryllia, is coated on the lower surface with a metal coating or layer 12, typically a two-layer coating comprising a metal which adheres to the substrate such as chromium, and a good conductor, such as gold. A similar coating on the upper side is formed and shaped by photolithographic and etching techniques to yield the desired circuit geometry 13. Various components such as capacitors, resistors and, most important, active devices 14, may be attached at specific locations on the upper circuit. Selected points of the upper circuit are connected to the back side of the substrate by via holes 16 which have metallized walls 17. Active devices are typically located in the vicinity of the via holes both because one of their electrodes typically needs to be grounded to the backside metal layer 12 but also in order to help conduct heat generated by the device to the back surface where it is dissipated. However, unless the via holes are solidly filled, the device cannot be located on top of the hole, where it would be most desirable for heat dissipation. In the preferred embodiment of the present invention, via holes 21 are drilled by a laser beam in the ceramic substrate 22 at selected locations, Figure 2A. The substrate is then coated with a laminated layer such as a layer including a positive dry film resist 23 carried by a mylar film 24, Figure 2B. A suitable laminated layer including a dry film resist polymer is "Resolve" sold by Minnesota Mining and Manufacturing Company. The polymer film carried by the mylar preferably has good adherence to both ceramic and glass, and, to some extent, even to the mylar backing. The film supported by the mylar backing is preferably covered with a liner which is removed before applying the film to the substrate for lamination. The substrate with the applied film is then preferably heated, whereby the polymer film becomes brittle. The mylar backing is then removed. When the mylar backing is removed from the film, the film remains adhered to the substrate, however, the brittle portions over the holes are removed with the mylar whereby the substrate will be uniformly coated and the holes remain open. The laminating temperature is such as to achieve the desired brittleness and adhesion of the polymer film but avoid completely drying out the film so that by using a higher temperature, a second lamination can be carried out. As an example, a temperature of around 120 deg.C for the first lamination and 140 deg.C for the second lamination has been used in the case of the above mentioned laminated layered photoresist. In an alternative embodiment of this invention the desired coverage of the substrate with polymer is achieved by laminating the polymer at a lower temperature, for example 90 deg.C, thereby avoiding brittleness. The polymer can then be exposed either thrpugh the substrate itself or through a suitable mask and is dissolved away to leave the open holes with a polymer surface layer on the substrate. After the desired coverage of the substrate is achieved with one of the described processes, or any other alternate process which will form a substrate with an adhesive layer, the substrate undergoes a second lamination process during which the substrate, already covered with the polymer film, is attached to a smooth, metallized backing plate, so that one end of the holes is closed by a metal. In the preferred embodiment of this invention as shown in Figure 2C, a glass slide 26 is metallized with a thick layer of copper 27 (about 10 microns), followed by a thin layer of gold 28 (about .1 micron). The gold layer is used to avoid oxidation while the thick layer of copper provides a low electrical resistance of the metal film as well as ease of etching after the process is completed. Any other suitable metal conductive layer can be used. After the second lamination is completed, the substrate attached to the backing plate by the polymer layer 23 is ready for a plating operation. An electrical contact 31 is made to the metal backing layer which is slightly larger than the substrate, so that a metallized edge 32 will remain uncovered and provide easy electrical contact. The substrate, attached to the backing plate, is immersed in a plating bath such as a neutral gold bath suitable for low-stress fine grain deposits. Several suitable plating solutions are readily available and other metals besides gold might be considered, e.g. copper. Because the walls of the holes are not metallized, the lines of the electric field during plating will be approximately parallel to the hole walls and therefore the plating will take place approximately parallel to the bottom so as to fill the hole uniformly. After the holes are filled with metal, they start to develop the mushroom shape characteristic of plating not constrained by lateral walls, as shown in Figure 2D, which depicts the cross-section of a portion of the substrate after the plating operation. At this point the backing plate is separated from the substrate. In the specific embodiment described this is accomplished by etching the relatively thick copper layer deposited on the glass backing. In order to facilitate this operation, no adhesive layer is used on the glass so that the adherence is minimal and the etching solution (e.g. nitric acid) will readily penetrate. A variety of alternatives are clearly possible, including using a backing plate which is itself readily etchable, such as an aluminum sheet barely thick enough to provide minimal mechanical integrity. This could be easily etched away, e.g., with a sodium or potassium hydroxyde solution. After removing the backing plate, the residues of the gold film 28 and the polymer 23 are removed, e.g. by a combination of an ultrasonic treatment and ashing, so that the result of the described process is a substrate with holes filled by a solid column of gold 29. The side originally attached to the backing plate is typically fairly planar. The other, especially when some overplating has taken place, may require an additional planarization step. This is easily accomplished by a lapping operation where the surface of the ceramic substrate is hardly modified, since the gold is much softer and readily abraded. The substrate at this point appears as in Figure 3 and is ready for metallization and further processing to form a microwave integrated circuit having a metallized lower layer with circuits on the upper surface of the substrate.

Claims: A method of forming a substrate for microwave integrated circuits comprising the following sequence of steps: drilling by a laser beam holes or vias (21) at selected locations in a ceramic substrate (22), the holes or vias extending through the substrate from an upper surface to a lower surface thereof; applying an adhesive polymer film (23) on one of said surfaces by lamination; removing those portions of said polymer film which are located over the holes or vias whereby said one surface of the substrate will be uniformly coated and the holes or vias remain open; attaching by lamination to said one surface of the substrate by means of said adhesive polymer film a metallized plate (27) comprising a gold film (28) on a copper layer (27), so that one end of the holes or vias is closed by metal; electroplating metal through the holes or vias using said metallized plate as one of the electrodes in a plating bath in which is immersed the substrate; removing said metallized plate from the substrate; removing any residues of metal originating from the metallized plate and the polymer film to leave the substrate with vias or holes filled with a solid column (29) of metal and ready for metallization of the surfaces to form a microwave integrated circuit. The method of claim 1 wherein said polymer film is a positive dry film resist supported by a mylar backing (24) and the film portions above the holes are removed together with the mylar backing after heating the substrate with the applied film at a temperature which is such that the polymer film becomes brittle over the holes or vias but still adheres to the substrate and does not completely dry out. The method of claim 1 wherein the polymer film is photosensitive and its lamination to the substrate is performed at a low temperature to avoid reaching a brittle state and then the polymer is exposed either through the substrate itself or through a suitable mask in such a way that after development it is dissolved away over the holes or vias and maintained on the substrate. The method of claim 1 wherein the metallized plate is removed from the substrate after the electroplating step by etching the copper layer. The method of claim 1 further comprising the step of lapping the other surface of the ceramic substrate to fully planarize the metal column before the further step of metallization.

Inventors: BUJATTI MARINA, SECHI FRANCO NICOLA, BUJATTI, MARINA, SECHI, FRANCO NICOLA
Applicants: MICROWAVE POWER INC, MICROWAVE POWER, INC.
Application year: 1989
Main classification: H01L21
IPC codes: H05K1, H05K3, H01L23