WO1988005990A1

WO1988005990A1 - Cladding of substrates with thick metal circuit patterns

Info

Publication number: WO1988005990A1
Application number: PCT/US1988/000116
Authority: WO
Inventors: Robert Lawrence; Richard N. Leyden
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1987-02-05
Filing date: 1988-01-19
Publication date: 1988-08-11
Anticipated expiration: 1989-08-05
Also published as: IL85197A0; ES2006071A6; JPH01502228A

Abstract

A method for forming a circuit on a surface of an insulating substrate (10) comprises the steps of adhering Angstroms thick metallization (16) to the surface of the insulating substrate, placing conductive material (24) on the metallization through a photoresist pattern (20) of the circuit, stripping the resist pattern from the substrate, and removing the metallization not covered by the circuit of the conductive material by use of the conductive material as a resist.

Description

CLADDING OF SUBSTRATES WITH THICK METAL CIRCUIT PATT BACKGROUND OF THE INVENTION

The present invention relates to a method for forming circuits on insulative substrates and, in particular, circuit lines of 2 mil and lesser widths without electrical shorting between the lines.

Size reduction of interconnections in electronic devices, in particular with respect to high density electronic packaging, has been an on-going effort to provide narrower leads and closer center-to-center spacing between the leads, thus to reduce the size, weight, cost and number of layers of multilayer printed wiring boards. It has also been an aim to provide greater shock and vibration resistance to such electronic devices which is possible if size reduction and consequent shorter spans between supports can be properly effected.

Conventional printed wiring boards are produced from one or more rolled or electroplated copper sheets which are laminated to "B-staged" substrates of polymer- glass, such as of a polyimide, an epoxy, and combinations thereof. The thinnest copper sheet is commonly 1/2 oz. or 0.7 mils thick. The result from presently available technology, using resist, patterning and etching techniques, are circuits whose narrowest lines and spaces measure 5 mils, respectively, over line lengths of less than 5 inches, and at a premium cost. Under laboratory conditions, shorter lines with spaces and line widths of 2 to 3 mils are obtainable. However, if 1/2 oz. copper, for example, is used on the outer faces of a multilayer board, when the vias are plated through with 1 mil thick copper, the original 1/2 oz. copper on the outer faces becomes 1.7 to 2 mil copper. Etching 5 mil wide lines is accomplished only by trial by error, and obtaining smaller widths of 2 and 3 mils is even more difficult. O 88/05990

In general, as the thickness of the copper increases, the problems with etch factor, dry film resist turbidity, that is, exposure light diffracting in the film, and the physical separation of artwork lines likewise increase.

SUMMARY OF THE INVENTION These problems and drawbacks are successfully addressed and avoided in the present invention by a method in which a circuit is formed on a surface by adhering Angstroms thick metallization to the surface of an insulating substrate, placing conductive material on the metallization through a photoresist pattern of the circuit, stripping the resist pattern from the substrate, and removing the metallization not covered by the circuit of the conductive material.

Several advantages are derived from this method. Narrow lines and close center-to-center spacing between the lines result in higher density in electronic packaging because of decreased size of printed wiring boards and a decrease in the number of layers in multilayer printed wiring boards, thus lowering the weight and cost of such boards. Because of shorter spans between supports in and for the boards, resistance to shock and vibration is enhanced.

Other aims and advantages, as well as a more complete understanding of the present invention, will appear from the following explanation of exemplary embodiments and the accompanying drawings thereof.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view of a section of a substrate for a printed wiring board, one surface thereof in process of being etched; FIG. 2 illustrates the placement of an Angstroms thick layer of metallization on the etched surface; FIG. 3 shows a photoresist pattern of a fine line portion of a circuit configuration;

FIG. 4 depicts metal deposited into the resist pattern openings to define the circuit fine lines;

FIG. 5 is a view of the printed wiring board following stripping of the resist pattern; and

FIG. 6 illustrates the completed wiring board, following removal of the metallization, using the fine line circuitry as a resist.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a substrate 10, such as of polymer material, polyimide-glass or epoxy-glass, is illustrated for having one surface 12 thereof etched. Etching is preferably accomplished by an oxygen plasma etch, such as is represented by arrows 14, in a conventional manner, to prepare the surface for subsequent adhesion to it by metallization.

As shown in FIG. 2, metallization 16 is then sputtered or vacuum deposited or otherwise suitably deposited onto etched surface 12 to ensure optimum adhesion to the substrate. In examples of reductions to practice, titanium and chromium were deposited onto surface 12 in thicknesses of about 250 to 500 Angstroms, and gold and copper were deposited onto the chromium and titanium in respective thicknesses of 1,000 and up to 20,000 Angstroms. The thickness of this composite is selected to withstand deleterious effects thereon of subsequent processing. The sputtered-metal surface is then cleaned so that it is "water-break-free," that is, there are no deposits on metallization 16 to impede flow of water thereover. The work in process is then dried and warmed in an oven, for example at 150 to 200 degrees Fahrenheit. A dry film resist 18, e.g., "Riston" (trademark of E.I. DuPont de Nemours and Company) and "Laminar" O 88/05990 '

(trademark of Dynache Corporation) is placed over the metallization. If desired, a liquid resist may be used; however, a dry film is preferred as providing the greater thicknesses desired, such thicknesses typically being 1/2 to 1-1/2 mils. The type of resist used will depend upon the circuit line thickness desired. A negative defining the desired circuitry is placed atop the resist and exposed to light. Portions of the resist are removed to form the resist into a pattern 20 (see FIG. 3) defining the desired electronic circuit, openings 22 comprising a portion of the pattern. All the foregoing resist treating procedures are in accordance with conventional techniques.

^"As illustrated in FIG. 4, metal 24, which forms the desired circuitry, is then deposited, preferably by conventional electroplating techniques, in the thickness desired, e.g., 1/2 to 1-1/2 mils, onto metallization 16 exposed by openings 22. Resist pattern 20 is then stripped from the metallization, see FIG. 5. Using metal 24 as a resist, unprotected portions of metallization 16 are removed to provide free-standing electrical lines or traces on substrate 10, with no electrical shorting between the lines, as depicted in FIG. 6. If desired, a liquid or dry film resist may also be used to protect the deposited metal lines, especially when the metal lines and the below metallization comprise the same metal. It is essential that the deposited metal 24 not exceed to height of the resist, if "blooming" or spreading of the metal is to be avoided. Although the invention has been described with respect to a particular embodiment thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims

Claims :

1. A method for forming a circuit on a surface of an insulating substrate comprising the steps of: adhering conductive material to the surface through a photoresist pattern of the circuit; stripping the resist pattern from the substrate; and removing the metallization not covered by the circuit of the conductive material.

2. A method according to claim 1 in which said adhering step comprises the step of adhering Angstroms thick metallization to the surface of the insulating substrate.

3. A method for forming a circuit on a surface of an insulating substrate comprising the steps of: etching the substrate surface for enabling adhesion of metallization to the surface in a subsequent step; adhering a metallization layer to the surface; applying a resist on the metallization layer; placing a pattern of the circuit on the resist; exposing portions of the resist through the pattern; developing the resist so exposed; removing portions of the resist for defining openings therein conforming to the pattern; placing conductive material in the openings for forming the circuit; stripping the resist from the substrate; and removing portions of the metallization layer not covered by the circuit of the conductive material. O 88/05990

4. A method according to claim 3 in which said etching step comprises the step of removing a layer of the substrate.

5. A method according to claim 3 in which said etching step comprises the step of plasma etching the surface with oxygen.

6. A method according to claim 3 in which said adhering step comprises the step of sputtering the metallization onto the etched surface..

7. A method according to claim 6 in which said sputtering step comprises the steps of sputter depositing titanium or chromium on the etched surface to a thickness of about 250 to 500 Angstroms and sputter depositing gold or copper on the titanium or chromium to a thickness of about 1000 to 2000 Angstroms.

8. A method according to claim 7 in which said adhering step comprises the step of vacuum depositing the metallization onto the etched surface.

9. A method according to claim 8 in which said vacuum depositing step comprises the steps of depositing titanium or chromium on the etched surface to a thickness of about 250 to 500 Angstroms and depositing gold or copper on the titanium or chromium to a thickness of about 1,000 and up to 20,000 Angstroms, respectively.

10. A method according to claim 3 in which the circuit pattern has lines whose widths are 3 mils or less to provides the openings and the conductive material thereon with similar widths. O 88/05

11. A method according to claim 10 in which said resist depositing step comprises the step of forming the resist with a thickness of at least 1 mil to provide the conductive material with a similar height.

12. A method according to claim 3 in which the metallization layer and the conductive material comprise different metals, and in which said metallization layer removing step comprises the step of using the conductive material as a resist.