CA1088210A

CA1088210A - Substrate assembly for an electronic circuit having multilayered wirings, each comprising thin titanium and nickel base films for a wiring metal film

Info

Publication number: CA1088210A
Application number: CA276,908A
Authority: CA
Inventors: Nobuhiko Eguchi; Toshio Takaba; Toshimasa Kobayashi; Shoji Nakakita; Masashi Takahashi
Original assignee: Nippon Electric Co Ltd
Current assignee: NEC Corp
Priority date: 1976-04-26
Filing date: 1977-04-25
Publication date: 1980-10-21
Also published as: JPS52131167A; BR7702582A; FR2349962B1; FR2349962A1

Abstract

ABSTRACT OF THE DISCLOSURE

A substrate assembly for an electronic circuit has multilayered wirings each of which comprises a thin titanium film either on a substrate or on an insulator layer, a thin nickel film on the titanium film, and a wiring film on the nickel film with the insulator layer covering the wiring films with predetermined areas thereof uncovered. The wiring films may be of gold or rhodium and may comprise a thin palladium film between one each of the nickel and gold or rhodium films. The titanium and nickel films are formed according to thin-film techniques. The wiring films are formed by selective plating. The insulator layer or layers are formed according to thick-film techniques.

Description

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This invention relates to a substrate assembly for an electronic circuit and, more particularly, to one for a hybrid integrated circuit. ;
A substrate assembly for an electronic circuit generally comprises multilayered wirings. Conventional methods of manufacturing such substrate assemblies may be broadly classified into two, one resorting to thick-film techniques and the other resorting to thin-film techniques. In accordance with the thick-film techniques, the multilayered wirings are formed on a ceramic substrate by alternately screen-printing thereon electroconductive -~
paste including a noble metal, such as silver or palladium, as the chief ingredient and insulating paste including powder of an insulating material, such as glass or alumina, as the chief ingredient and firing the printed paste patterns each time. Although advantageous in readily forming the patterns, the screen printing is disadvantageous in that restriations are put on the configuration and accuracy of the patterns. Use of the noble metal in the paste raises the cost. Firing in each instance complicates the process and further raises the cost. According to thin-film techniques, the multilayered wirings are formed on a ceramic substrate by evaporating a metal for wiring onto the substrate to form a thin metal film, selectively etching away the metal film to leave wirings of a first level, evaporating $
an insulating material to cover the wirings with a thin insulator layer, selectively etching away the insulator layer to expose predetermined areas of the wirings, and repeating formation of the metal film and insulator layer of other level or levels. Although patterns of the wirings and the exposed ~
areas are readily formed with high accuracy, the thin-film techniques are defective in that the metal film is only slowly formed to adversely affect the productivity, that pinholes are unavoidable in the insulator layer to ~ -bring about liability to shorting the wirings of dlfferent levels and to provide considerably large capacitances therebetween, and that it is difficult to etch the insulator layer away.

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In Japanese Patent No. 698,229 (Patent P~blication No. 47,943 of 1972), a combination of the thick-film and thin-film techniques is proposed to improve the method of manufacturing such substrate assemblies. As will later be described with reference to one figure of the accompanying drawing, each wiring of a first level formed on an alumina substrate comprises a first thin film of an alloy of nickel and chromium (trade mark NICHROME) evaporated onto the substrate, a second thin film of gold evaporated onto the first thin film, and a relatively thick film of gold plated on the second thin film. Those portions of a thin gold film on which no gold film is plated are etched away to provide the second thin films. Such wirings of each level are selectively covered with an insulator layer formed of a fired print ~ -of glass paste including a vitreous materials as the chief ingredient. The proposal is still defective in that selective etching away of the thin gold film is uneconomical and raises the cost, that quick etching of gold is ~ -technically difficult, and that the etchant for the thin gold film is inevitably brought into direct contact with the plated gold film to harm the reliability. Furthermore, it is inevitable on firing the glass paste print that diffusion of the nickel-chromium aIloy and the gold takes place to undesirably raise the resistivity of the wirings and to adversely affect 2Q the adhesion of the first-level wirings to the substrate.
It is therefore an object of the present invention to provide an inexpensive and yet reliable substrate assembly for an electronic circuit.
It is another object of this invention to provide a substrate assembly of the type described, whereby it is possible to provide a highly dense and integrated hybrid integrated circuit.
It is still another object of this invention to provide a substrate assembly of the type described, wherein use is made of a base film of a inexpensive and readily etchable metal for selective plating thereon of a wiring metal.

, 1~)8~2~0 According to one aspect of the invention there is provided, in a substrate assembly for an electronic circuit comprising a substrate of an insulating material, first thin metal film portions thereon in a first pre-determined pattern, first relatively thick metal films on said metal film portions, respectively, an insulator layer on said relatively thick metal films with predetermined areas thereof uncovered, second thin metal film portions on said insulator layer in a second predetermined pattern, and second relatively thick metal films on said second thin metal film portions, respectively, the improvement wherein each of said first and second thin metal film portions comprises an underlying film of titanium and an overlying film of nickel. .
According to another aspect of the invention there is provided a method of manufacturing substrate assembly for an electronic circuit com-prising the steps of successively forming a first thin titanium and a first thin nickel film on a substrate of insulating material, selectively plating a wiring metal on said nickel film in a first predetermined pattern to form first relatively thick metal films and to leave exposed portions of said nickel film, etching away the exposed portions of sald nickel film to uncover the underlying portions of said titanium film, etching away said underl~ing titanium film portions to leave said first thin metal film portions on said ~; substrate, forming a thick-film insulator layer on said relatively thick metal films, successivoly forming a second thin titanium and a second thin ~;
r~ckel film on said insulator layer, selectively plating said wiring metal on said second nickel film in said second predetermined pattern to form said second relatively thick metal films and to leave exposed portions of said second nickel film, and successivel~ etching away the exposed portions of said second nickel film and the underlying portions of said second titanium film to leave said second thin metal film portions on said insulator layer.
The invention will now be further described in conjunction with ,,,,~. , ~ . ' -` 1088Z10 the accompanying drawings, in which:
Figure 1 schematically shows an exploded sectional view of several parts during manufacture of a conventional substrate assembly for an electronic circuit;
Figure 2 is a schematic exploded view of several parts during manufacture of a substrate assembly according to a first embodiment of the instant invention; and Figure 3 is a schematic sectional view of a substrate assembly according to a second embodiment of this invention.
Referring to Figure 1, a conventional substrate assembly according to the above-referenced Japanese Patent will be described at first for a better understanding of the present invention. At the outset of manufacture, an alwmina substrate 10 is provided, on which a thin film lla of a nickel-q~/C h~qrk~
.i chromium alloy known as Nichrome/and another thin film llb of gold are successively evaporated (Figure la). After being applied onto the gold film llb, a photoresist film llc is exposed to light by the use of a mask -~
(not shown) of a first predetermined pattern, developed, and selectively etched away to uncover the gold film llb of the predetermined pattern (Figure lb). Gold is plated on the exposed areas of the gold film llb to provide relatively thick gold films lld (Figure lc) with the remaining photoresist film portions llc sub9equently removed. With the plated gold films lld used as resists, uncovered portions of the thin gold and nickel-chromium films llb and lla are removed successively by etchant therefor to leave wirings lle of a first level (Figure ld). According to the thick-film techniques, paste of an insulating material including a vitreous material as the chief ingredi-ent is applied at least onto the plated gold film lld with predetermined areas thereof uncovered (Figure le) and subsequently fired to form an insula-tor layer llf for the first-level wirings lle. With s;~;lar processes, wirings 12 of a second level are formed on the insulator layer llf in another ''A~ ~ , 1~)88210 predetermined pattern which may provide electric contact of the second-level wirings 12 with the first-level wirings lle. It will readily be understood that the substrate assembly has the defects pointed out in the preamble of ;~
the instant specification.
Referring now to Fig. 2, a substrate assembly according to a first embodiment of the present invention comprises a substrate 10 of a refractory material, preferably alumina ceramics, a thin titanium film lla thereon, and a thin nickel film llb on the titanium film lla (Fig. 2a)~ It is suffi-cient that the substrate 10 should withstand about 900C. The titanium and nickel films lla-llb are successively formed in accordance with the thin-~,. ...
film techniques either by evaporation or sputtering without breaking meanwhilethe ~acuum. The titanium film lla shows excellent adhesivity to both the alumina ceramic substrate 10 and the nickel film llb and is readily etched.
The nickel film llb is covered with a photoresist film llc, which is exposed to light by the use of a mask (not shown) of a first predetermined pattern, developed, and selectively etched away to uncover the nickel film llb of the ; predetermined pattern (Fig. 2b). Although known photoresist materials are applicable, a preferred photoresist film is provided by a film known as KTFR
manufactured and sold by Eastman Kodak Company, Rochester, N.Y., U.S.A. Gold is selectively plated on the exposed areas of the nickel film llb to provide relatively thick or wiring films lld (Fig. 2c) of the predetermined pattern with the remaining photoresist film portions llo subsequently removed by a known remover. The wiring films lld tenaciously adhere to the nickel film llb, which is also readily etchable. Preferably, the photoresist film llc should be a little thicker than the wiring films lld so as to give clear-cut configuration to the predetermined pattern of the wiring films lld and thereby to raise accuracy of the pattern. Selective plating of gold is resorted to in order not to waste the precious metal and in order to expedite formation of the gold films lld of a desired thickness to a considerable extent as com-pared with the formation either by evaporation or by sputtering.

; ~ _5_ 10~3~Z10 Further referring to Figure ~, those portions of the nickel film llb which are uncovered by the removal of the photoresist film portions llc are selectively etched away by the use of a nickel etchant to expose the underlying portions of the titanium film lla. A preferred nickel etchant is a solution of 200 grams of ferric chloride in 750 cc of water with addi-tion of 50 cc of 35% hydrochloric acid. Thereafter, the exposed titanium film portions are selectively etched away by a titanium etchant to leave wirings lle (Figure 2d) of a first level on the substrate 10 in the clear-cut and accurate predetermined pattern. A preferred titanium etchant is a mixture of 1 part of 46% hydrofluoric acid, 1 part of 70% nitric acid, and 50 parts of water, the parts being by volume. The wiring films lld weIl withstand both the nickel and titanium etchants. No change occurs even in the color of the wiring films lld. It is therefore feasible to thoroughly -~
carry out the nickel and titanium etching, with the etching processes readily administered. It is now understood that the titanium and nickel films lla-llb of each wiring lle serve as a composite base film portions for the wiring metal film lld. Paste of an insulating materialm such as alumina, is screen-printed at least on the wirings lle leaving predetermined area or areas of the wirings lle uncovered and subsequently fired to form an insulator layer llf (Figure 2e) for the first-level wirings lle. The firing is carried out in a usual manner in an oxidizing atmosphere. Protected by the wiring films lld, the base film portions lla-llb are never oxidized. In marked contrast to the prior-art base film portions, it has now been confirmed that the firing even reduces the reslstivity of the wirings lle, rather than giving rise to diffusion of nickel to raise the resistivity, as exemplified in the following table for a good number of samples wherein the wirings lle are only 0.1 mm wide, the titanium and nickel films lla-llb are 0.1 and 0.2 micron thick, respectively, and the firing is carried out at 860 C, with the thickness of the gold film lld varied as listed. Furthermore, it has been 1~882~0 proven that th~ firing augments the adhesion between the films lla, llb, and lld. With like processes, wirings 12 of a second level are formed on the insulator layer llf (Figure 2f). It is now possible by repeating the above-mentioned processes to manufacture, if desired, a substrate assembly having wirings, such as lle and 12, of more than two levels.

TABLE

Thickness of the gold Resistivity of the wiring (in ohms/cm) film (in microns) Before firin~ After firin~

2 1.53 + 0.06 1.31 + 0.03 2.5 1.25 + 0.02 1.02 + 0.01 4 0.76 + 0.02 0.73 + 0.02 Finally referring to Figure 3, a substrate assembly according to a second embodiment of this invention comprises similar parts designated by like reference numerals as in Figure 2. It is to be noted, however, that thin palladium films 13 are selectively plated on the respective exposed areas of the nickel film llb before the selective plating of gold. In other words, each wiring film comprises a thin palladium film 13 and a main wiring film lld. The palladium films 13 serre to prevent diffusion of nickel and gold into the gold films lld and the nickel film portions llb, respectively.
While this invention has thus far been described in conjunction with a few preferred embodiments thereof, it is now understood that this inrention is characterized by the use of a composite base film portion of a thin titanium and a thin nickel film, such as lla-llb, for each wiring film, such as lld or lld and 13, of each level, such as lle or 12, and is featured by ~he readily feasible processes of manufacture, the low cost, the high 11~88210 reliability, and the capability of providing a highly dense electronic cir-cuit. Instead of gold, use is pos~ible as the wiring metal of rhodium and other readily plated noble metals for selective plating of the main wiring films~ It is also possible to use photosensitive insulating material on ' forming the insulator layer or layers.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a substrate assembly for an electronic circuit comprising a sub-strate of an insulating material, first thin metal film portions thereon in a first predetermined pattern, first relatively thick metal films on said metal film portions, respectively, an insulator layer on said relatively thick metal films with predetermined areas thereof uncovered, second thin metal film portions on said insulator layer in a second predetermined pattern, and second relatively thick metal films on said second thin metal film portions, respectively, the improvement wherein each of said first and second thin metal film portions comprises an underlying film of titanium and an overlying film of nickel.

2. A substrate assembly as claimed in claim 1, wherein said first and second relatively thick metal films are of a member selected from the group consisting of gold and rhodium.

3. A substrate assembly as claimed in claim 1, wherein each of said first and second relatively thick metal films comprises an underlying thin palladium film and an overlying relatively thick film of a member selected from the group consisting of gold and rhodium.

4. A method of manufacturing substrate assembly for an electronic cir-cuit comprising the steps of successively forming a first thin titanium and a first thin nickel film on a substrate of insulating material, selectively plating a wiring metal on said nickel film in a first predetermined pattern to form first relatively thick metal films and to leave exposed portions of said nickel film, etching away the exposed portions of said nickel film to uncover the underlying portions of said titanium film, etching away said underlying titanium film portions to leave said first thin metal film portions on said substrate, forming a thick-film insulator layer on said relatively thick metal films, successively forming a second thin titanium and a second thin nickel film on said insulator layer, selectively plating said wiring metal on said second nickel film in said second predetermined pattern to form said second relatively thick metal films and to leave exposed portions of said second nickel film, and successively etching away the exposed portions of said second nickel film and the underlying portions of said second titani-um film to leave said second thin metal film portions on said insulator layer.

5. A method as claimed in claim 4, wherein said wiring metal is gold or rhodium.

6. A method as claimed in claim 4, wherein the step of selectively plating said wiring metal to form the relatively thick metal films on rele-vant ones of said first and second thin metal film portions comprises the steps of selectively plating palladium in a pertinent one of said first and second predetermined patterns to form thin palladium films on said relevant ones of first and second thin metal film portions, respectively, and selectively plating gold or rhodium on said palladium films to form in combination with said palladium films the relatively thick metal films on said relevant ones of first and second thin metal film portions, respectively.