Method of forming crossover connections

#This invention relates to the fabrication of thin film and hybrid integrated circuits, and in particular to the formation of crossover connections on said circuits.

#Thin film and hybrid circuits presently enjoy extensive use in a wide variety of applications, such as filter circuits and memories switching and transmission systems. This extensive use has recently prompted the development of a new interconnect multilayer metallization system for such circuits comprising layers of titanium-copper-nickel-gold with an optional layer of palladium between the titanium and copper layers. See U.S. patent application of Lesh, Morabito and Thomas, Ser. No. 576,711 filed May 12, 1975, which is incorporated by reference herein. The basic advantage of such a system is the requirement of only a minimum amount of gold, resulting in great cost savings over the prior art systems.

#One of the problems associated with this new system, however, was that a new fabrication technique was needed for forming "crossover" connections necessary for electrically connecting certain metallization across any intervening elements or metallization in the circuit. In particular it was desirable to utilize at least a portion of said interconnect metallization as a base layer for forming the copper spacing layer commonly utilized for crossover fabrication. This created additional problems in that the removal of the spacing layer would undercut the copper layer of the interconnect metallization.

#These and other features of the invention will be delineated in detail in the description to follow. In the drawing:

#FIGS. 1A - 1H are cross-sectional views of a portion of a circuit illustrating various processing steps in accordance with one embodiment of the invention.

#It will be appreciated that these drawings are not made to scale.

#The method in accordance with one embodiment of the invention is described with reference to FIGS. 1A - 1H which illustrate various steps in the fabrication of a single crossover on a portion of a typical thin film or hybrid integrated circuit. It will be realized that several such crossovers may be batch processed on a single circuit and further that an actual circuit would typically include many more interconnect elements as well as various circuit elements such as resistors, capacitors and integrated circuit chips.

#FIG. 1A illustrates a portion of a typical circuit after formation of the interconnect metallization. The insulating substrate, 10, typically comprises a ceramic material. Formed over essentially the entire surface of the substrate were successive layers of titanium, 11, and copper, 12, by standard evaporation techniques. Additional copper layer, 13, was then selectively plated over the areas which were to form the interconnect pattern. (Layer 13 is shown as distinct from layer 12 for purposes of illustration.) This was followed by plating a layer of nickel, 14, on the plated copper layer, and plating a layer, 15, of gold on the nickel layer to complete the interconnect metallization scheme. (The process for forming such metallization is described in more detail in U.S. patent application of Lesh, Morabito and Thomas, Ser. No. 576,711, filed May 12, 1975 which has been incorporated by reference herein.)

#In the fabrication of the crossover connection, the evaporated copper layer, 12, utilized for forming the interconnect metallization, was also used as the base layer to build up the spacing layer to be described. Since this evaporated copper layer, as well as the plated copper layer, 13, was also part of the interconnect metal, it was desirable to protect the copper layer from subsequent etching operations. Therefore, as illustrated in FIG. 1B, a thin layer, 16, of nickel was plated over the evaporated copper layer and interconnect metal pattern. Preferably, the nickel layer is formed over essentially the entire circuit, although the layer can be selectively formed over the necessary portions of the circuit if desired. Plating was performed in a bath comprising nickel sulfamate. The particular bath used was that sold by Allied-Kelite Co. under the trade name Barrett type SN which basically comprises nickel sulfamate and boric acid. Other commercially available nickel plating baths may be utilized. It was found that a minimum nickel thickness of approximately 6,000 A was desirable to insure edge coverage and provide adequate protection of the copper in the interconnect metal. Although electro-plating is the preferred method of forming the nickel layer, other methods such as evaporation might be employed.