Patent GB-1445268-A (GB)
Title: Multilayer electric circuit structures and methods of  fabricating the same
IPC: H05K3/42
Filed: 1974-06-19
Abstract: 1445268 Printed circuits URAL POLITEKHNICHE INST IM SM KIROVA 19 June 1974 27166/74 Heading H1R A multilayer electric circuit structure (Fig. 1 not shown) comprises plural metallic layers separated by dielectric layers; wherein an aperture extending therethrough is coated with a conductive connecting layer of a solid solution in Hg of a metal more electropositive than the layer metal; overlain by an electrolytically deposited metallic film. The exposed surface of the interleaved dielectric layers 2 (Fig. 2) covered with the connecting layer 3 and metallic film 4 may be divided by additional metal layers 11 parallel to the plural metallic layers into portions of thickness equivalent to the thinnest dielectric layer 2<SP>1</SP>. The metallic layers may be Cu, Ag, Zn, Al, Pb, Sn, Ti, Fe or Ni. The interlayer connections 3 may be prepared by preforming on an exposed end surface of the structure a continuous conducting film 3<SP>1</SP> of metal having higher positive potential than that of the metal layers 1, and application of a mercury film 311 thereto. The composite film is selectively removed from the exposed end surfaces of the metal layers and an additional film 5 formed to cover the exposed end surfaces of the dielectric 2, on which a metal film 4 is electrolytically deposited to cover the ends of the dielectric and metal layers, whereby a mercury based solid solution is formed between the exposed ends of the metal layers and the electolytically deposited film (Figs. 3a to g). The interlayer connectors may alternatively be formed by applying a mercury film or amalgam to the exposed end surfaces of the metal layers of the multilayer structure, underlying the continuous conducting film. In practice the multilayer structure (Fig. 3a) is immersed in a solution of ions of a metal more electropositive than the layer metal, which enter into a reaction therewith; in which electron exchanges alter the electropotential of the metals ; the solution being, e.g. of AgNO 3 , + NH 4 OH, or PdCl 2  2H 2 O, + CuCl 2  2H 2 O, + H 2 SO 4 , or PdCl 2  2H 2 O + HCl ; and continuous conductive film 3<SP>1</SP> being formed from Ag or Pd (Fig. 3b) on the exposed end surfaces of the metal and dielectric layers in porous and monolithic form respectively. The structure may be previously sensitised with Sn or T<SP>1</SP> ions in solution, and during formation an alternating voltage may be applied between the external electrodes 1 of the structure or between autonomous electrodes adjacent thereto in the solution; whereby deposition is accelerated. An ultrasonic field may be superimposed in the solution and the immersed structure. Thereafter the structure is immersed in a solution of ionic Hg, e.g. Hg(NO 3 ) 2  + H 2 SO 4  to form an amalgamated film 3<SP>11</SP> of Hg which dissolves the metal of film 3<SP>1</SP> and metallic layers 1 to form an intermetallic interlayer connection 3. Metal film 4 may be electrolytically deposited on connection 3 (Fig. 3g) and accelerates formation of the amalgam. Alternatively the multilayer structure after formation of film 3<SP>11</SP> is dipped into a solution, interacting only with metal of layer 1, e.g. Cu, which may be H 2 O 2 + HCl or H 2 O 2  + H 2 SO 4  or HCl + CuCl 2  or H 2 O 2  + H 2 SO 4 + disodium ethylene diamine tetra acetate or H 2 SO 4  + H 2 O 2  + (NH 4 ) 2 S 2 O 8 . Thereby, porous areas of film 3 are removed from the exposed end of layers 1 while the monolithic areas on the exposed ends of layers 2 remain (Fig. 3e). An ultrasonic field may be superimposed. Thereafter the structure is washed and dipped in dilute H 2 SO 4 , and is then electrolytically deposited on the ends with metal film 4. In specific examples the multilayer circuit structure may comprise 4 copper layers interleaved with 3 epoxy glass layers and the solid solution comprises Hg, Pd, and Cu; Ag+Hg+ Cu; Au+Hg+Cu; while the electroplated layer is Cu. Prior to deposition of the conductive film the surface may be sensitised with SCl 2 . 2H 2 O+HCl. The circuit structure may also comprise interleaved Cu and polyimide layers, while surfaces may be protected against deposition by cellulose nitrite varnish.