WO1991017840A1

WO1991017840A1 - Process for making fine conductor lines

Info

Publication number: WO1991017840A1
Application number: PCT/US1991/003257
Authority: WO
Inventors: Dietmar Dudek; Thomas Pfeiffer; Richard H. Jensen; Carola Hafner
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1990-05-12
Filing date: 1991-05-13
Publication date: 1991-11-28
Also published as: DE4113483C2; EP0457071A3; EP0457071A2; DE4113483A1; JPH05506965A

Abstract

A process for producing conductor lines for electrical circuits, which process enables high packing density of the conductor lines and is adaptable to a large number of different conductors and support materials. The process comprises producing a powder image of a conductor line pattern by means of a preferably photographically prepared, positive, tacky pattern on an intermediate support; removing the tacky pattern, and transferring the powder image onto a final support, the powder image being consolidated into a conductor line pattern either before or after the transfer by sintering.

Description

Process for Making Fine Conductor Lines DESCRIPTION The subject of the invention is a process for making fine conductor lines of metallic conductive materials, for example, for printed circuits and multilayer circuits on rigid and flexible supports with flat or curved surfaces.

With the progressive miniaturization of integrated switching circuits, the density of the connections between the switching circuit and the substrate also increases. Known technologies can handle this situation with smaller, more densely packed conductor lines only up to certain limits. A multilayer circuit cannot have as many circuit levels as might be desired to increase packing density, because the speed of signal processing gained by miniaturization is then lost.

The published patent application WO 86 03 930-A1 teaches the preparation of fine conductor lines for electrical switching circuits on a support by the use of an intermediate support with an auxiliary surface, comprising the steps :

- selecting the support and the intermediate support in a manner such that their characteristics permit them to be separated from each other after they are brought into contact,

- forming a pattern establishing the configuration of the conductor lines on the auxiliary surface,

- applying an electrically conductive material onto the auxiliary surface in the configuration defined by the pattern,

- removing the pattern from the auxiliary surface, leaving only the conductive material, - adhering the conductive material to the support with an adhesive strength greater than the adhesive strength of the conductive material to the auxiliary surface, by bringing the support and auxiliary surface into mutual contact, and

- separating the support from the intermediate surface, leaving the electrically conductive material adhered to the support in the configuration determined by the pattern. The embodiment described in the cited publication with this generally worded teaching uses a negative pattern formed from a liquid or dry film photoresist. Here, "negative" means that the pattern-forming material is absent from the areas corresponding to the conductor line pattern to be formed. Compact metal is deposited galvanically as the electrically conductive material in the open areas of this negative. However, this embodiment has an array of limitations.

First, the attainable fineness of the conductor lines depends on the resolving power of the photoresist. This is about 40 μm for dry film resist. Thus, the process is useless for conductor lines whose width has to be less than 40 μm.

Further, the support material and the conductive material cannot be selected freely. For example, if compact metal conductor lines are transferred onto green ceramic sheets, the lines become detached and destroyed by shrinkage in the subsequent sintering. This is substantiated, for example, by US 47 53 694, which proposes a solution to this special problem different from the present invention.

Finally, the auxiliary surface must be provided with a conductive overcoating for the known process, if the intermediate support is not conductive. If this overcoating is transferred in the separation, then it must be removed by an additional etching process.

The problem involved in the present invention is to develop the known process to overcome the cited limitations.

This problem is solved by a process in accordance with Claim 1.

Specifically, and surprisingly, it was found that not only finished conductor line patterns of compact metal, but also powder images can be transferred onto a support after removal of the tacky pattern from an auxiliary surface, without the lines being damaged or destroyed.

The practicability of the process of the invention, especially the separation, is facilitated if at least the support or the intermediate support has a certain flexibility so that one can be stripped away from the other. In practice, this occurs mostly if a sheet structure is used. However, fairly rigid structures can also be processed, if one is rolled onto the other or care is taken to have relatively low adhesive strength and contact area between the support and the auxiliary surface.

Examples of suitable intermediate supports are sheets of synthetic resins, such as polyethylene terephthalate and polyimide, glass plates, sheet metal, preferably of stainless steel, sintered ceramic structures, vitrified structures. In the selection of this material, it is obvious that attention must be paid to its ability to withstand handling for removal of the tacky pattern, for example, by heating or solvent, without dimensional change.

All of the dimensionally stable, as well as flexible support materials known for preparing printed and hybrid switching circuits can be used, for example, green ceramic sheets, sintered ceramic structures, optionally provided with printed dielectric pastes or with laminated dielectric films, whereby these pastes and films can also be light-sensitive, glass plates, enamelled metal plates, plates and sheets of synthetic resins, optional fiber-reinforced, such as polyimide, epoxy resin, and polyethylene terephthalate.

"Green ceramic sheet" here means a flexible sheet structure containing a sinterable ceramic powder and an organic binder. On firing, the binder is vaporized from the sheet by evaporation or burning off and a dimensionally stable ceramic structure is produced from the ceramic portion, with a certain shrinkage being observed. The ceramic powder can consist of, for example, aluminum oxide and small quantities of glass frit. The thickness of the green ceramic sheet is usually between 100 and 400 μm.

Although the smoothest possible auxiliary surface on the intermediate support is advantageous for the practice of the process of the invention, supports with a certain surface roughness can also be used, provided this roughness is less than the thickness of the conductor lines to be produced. Thus, equalization layers, for example, on rough, fired ceramic become superfluous .

The support can be a shaped structure with not only a flat, but also with a simple curved surface, for example, a cylindrical surface. The term "positive, tacky pattern" means that the auxiliary surface bears either a coating that is tacky in the areas corresponding to the conductor line pattern to be formed or that is tacky and is present only in the specified areas. The tacky pattern is formed preferably by means of a radiation- sensitive coating, which changes its tackiness on irradiation. These are coatings that become tacky on irradiation (negative working) and also those that lose their original tackiness on irradiation (positive working) . Patent Specification DE 12 10 321 gives an example of a positive-working coating. Particularly suitable are negative-working coatings with dihydropyridine compounds of Patent Specification DE 27 58 209-C3 and those with derivatives of nitrophenyl dihydropyridine of Patent Specification DE 34 29 615-C1. The tacky pattern can also be made by the imagewise application of an adhesive, for example, by a printing process, A further possibility is to use a pattern made from a polymer-containing mixture and formed by imagewise exposure and washoff, and making it tacky by treatment with a plasticizer or by heating.

As the powder image is not suitable, from the standpoint of its stability and also its conductivity, as a conductor line pattern without further processing, a processing step to consolidate the powder image is provided. This occurs preferably in a thermal sintering procedure that is conducted preferably after the support and auxiliary surface have been separated from each other. In another preferred embodiment of the process of the invention, the powder image is subjected to a sintering process after removal of the tacky pattern and before adhesion to the support, that is, while it is still on the auxiliary surface. This enables the use of supports that are not adequately heat- resistant to withstand a firing step. Thus, the process is useful for preparing conductor line patterns on flexible synthetic resin sheets. This processing step can be combined in an especially simple manner with removing the pattern by heat treatment. Obviously, the intermediate support must be heat- resistant in this instance and the powder should not contain additives that increase the adhesion of the sintered powder to the auxiliary surface. Glass plates or polished thin film ceramics are well suited as intermediate supports for this embodiment, because of their especially smooth surface.

The metal-containing powders can be primarily powders of pure metals or alloys with a chemical composition corresponding to the desired conductor line material, for example, powders of gold, silver, copper, palladium, platinum, molybdenum, and/or tungsten. In addition to metals, the powders can also contain additives that influence the sintering step or the adhesion of the conductor lines to the support and, for example, have been incorporated in accordance with the process in Patent Specification DE 38 06 515-C1. It is also possible to apply first the pure metal and incorporate the additives in a second coating step, as described in German Patent Application P 39 13 115.7.

The powder can be applied on the tacky pattern, for example, with a brush, a wad of cotton, or any other known method. It is particularly advantageous to work with a bed of powder consolidated by vibration in accordance with Patent Specification DE 37 36 391-C1.

After application of the powder, the tacky pattern is removed. To do this, various processes are available, the selection depending on the type of material in the pattern. For example, a suitable solvent can be condensed on the auxiliary surface, the runoff film of solvent dissolving the pattern, as described in EP 01 92 301-A2. If the tacky pattern consists of only low molecular weight volatile compounds, for example, nitrophenyl dihydropyridine derivatives, it can be volatilized thermally by simple heating. Another possibility is heating in the presence of oxygen, whereby the material of the tacky pattern is burned off.

After the tacky pattern is removed, the powder image has a certain stability, enabling defect-free transfer onto the support surface. This is surprising, because the most freely flowing powder possible is usually employed for producing the image, that is, powder with the lowest possible form stability in the deposited powder. The cause of this surprising behavior is unknown, but it appears possible that, as a result of the limited duration in the performance of this process step, traces of the material of the tacky pattern or its reaction products are occluded in the narrow crevices between the contiguous powder particles and cause a definite cohesion of the powder image.

It is advantageous for the surface layer of the support to contain substances that act as an adhesive, at temperatures between 0 and 200°C, for the conductive material deposited on the auxiliary surface. This facilitates the adhesion step. For example, this can be a thin layer of an adhesive or a melt adhesive. If the support is a green ceramic sheet, the organic polymer present in the sheet as binder can have this melt adhesive property, thus simplifying the process considerably. In this instance, pressing the auxiliary surface and support together at an appropriate temperature suffices for adhesion. It is also possible obviously to adhere the conductive material to a rough support surface if the auxiliary surface is so smooth that the adhesion to it is lower than to the support after the two elements are pressed together. A suitable adhesive layer can also be produced on the support surface by coating it with a material that becomes tacky on exposure and diffuse exposure of this coating.

Conductor line patterns can be prepared on ceramic substrates in a particularly simple manner by practicing the process of the invention on a green ceramic sheet support. The metal in the powder and the ceramic material can be selected advantageously so that the melting point of the metal is above the sintering temperature of the ceramic. For example, an appropriate combination is gold with Du Pont Green Tape®. In this embodiment, continuous and good, adhering conductor lines are obtained, surprisingly, independently of the degree of shrinkage in the ceramic on sintering. This also applies particularly whenever several green ceramic sheets are superimposed before sintering to produce a multilayer circuit.

In another preferred embodiment of the process of the invention, the steps of producing a powder image on the intermediate support, removing the tacky pattern, and transferring the powder image to the surface of the support are repeated with the same support one or more times before sintering. If the same pattern and the same conductive material are used, a thicker powder layer is attainedd on the support. Therefore, the conductor lines formed by sintering the powder are thicker and have lower electrical resistance.

This thickening of the conductor line pattern can obviously be limited to the individual areas in which the tacky pattern is formed. In this manner, defects in the transferred powder image can also be repaired.

If, in the process step repetition, the pattern used is only partially covered with the first pattern, the conductor lines of the second pattern adhere surprisingly well to the support in e overlapping areas after sintering and the overlying conductor lines do not break away.

In the process step repetition, another electrically conductive material can also be used. For example, conductor lines of alloys can be built up or conductor lines of different metals can also be bonded together. In this case, if necessary, experience from thick film technology from the standpoint of the conflicting behavior of components on sintering should be used in constructing the pattern.

If a positive-working, light sensitive material is used, powder images comprising different powders in different areas can be produced in a known manner on the intermediate support. In this instance, the process steps of exposure and powder application are repeated on the same intermediate support, but with different makes and powders. For this purpose, powders of dielectric materials can be used in addition to the metal powders used for making conductor lines.

Such combination powder images can be used advantageously in the invention's process. For example, a first powder image of conductive metal can be formed on a first intermediate support and transferred onto a green ceramic sheet. A second powder image contains metal powder only where a thickening of the finished conductor lines, perhaps for contact points, is desired and comprises on the entire remaining surface a dielectric powder. This second powder image is first transferred accurately onto the ceramic sheet . After firing, a conductor line pattern is obtained with thickened, exposed contact points and is otherwise covered with a dielectric protective layer. The second powder image can be formed from a material with low conductivity. Thus is obtained a switching circuit with integrated resistors. Suitable powders can be prepared in a known manner from commercial thick-film resistor pastes by drying and milling. In this case, it is important, as in thick- film technology, that the two powder images overlap about 50-100 μm where the metal joins the resistor material, that is, in the terminal area. Surprisingly, a defect-free connection of the metallic conductor lines to the resist can be thus attained in the terminal area, whereas resistor circuits prepared by a one-step powder process separate at the terminal, probably because of the different shrinkages of the metal powder and the resistor powder.

The preparation of fine conductor lines is significantly simplified by the process of the invention. For example, special process steps, necessary in the current state of the art for making negative patterns by washoff or for adjusting the electrical conductivity of the auxiliary surface, are eliminated.

The quality of conductor lines produced by the process of the invention is outstanding. In particular, conductor lines can be made finer, that is below 40 μm wide, than in the current state of the art. It is also possible to apply conductive material on green ceramic so that the conductive pattern is not destroyed when the ceramic is fired. Thus is made possible a particularly economical process for preparing ceramic multilayer circuits.

The process of the invention can be adapted very flexibly in many ways to various requirements and material combinations. This gives, for example, economical advantages such that supports with low surface quality or with voids or pits arising in manufacture are still useful.

The invention can be employed for preparing fine conductor lines on all types of substrates, such as, for example, those used for printed circuits and hybrid circuits in microelectronics.

Examples of Embodiments

Example 1

Borosilicate glass plates, 2 mm thick and 5 x 5 cm, were coated with a solution of 1 g dimethyl ester of 2, 6- dimethyl-4- (2 '-nitrophenyl)-1, 4-dihydropyridine- 3,5-dicarboxylic acid and 1 g diethyl ester of 2,6- dimethyl-4- (2 '-nitrophenyl)- 1, 4-dihydropyridine-3, 5- dicarboxylic acid in 10 ml methyl ethyl ketone and dried. The dry, light-sensitive layer had a coating weight of 0.2 mg/cm². The coating was exposed with a test pattern of 25 μm wide lines and spaces . Pure gold powder with a 2 μm average particle size was applied onto the resulting tacky pattern with the aid of the vibration process of DE 37 36 391-C1 to a coating weight of 8 mg/cm² of surface to be exposed. After a three minute wait, pure bismuth powder was applied by the same process at a coating weight of 0.8 mg/cm².

The glass plates with the powder images were heated 10 minutes at 400°C, volatilizing the tacky pattern formed by the light-sensitive material. After the plate was cooled to 70°C, a green ceramic sheet (Du Pont Green Tape®) was pressed on the powder image one minute at 20 MPa pressure. After being completely cooled, the ceramic sheet was carefully separated from the glass plate. The powder image had now been completely transferred onto the ceramic sheet. The sheet was fired 10 minutes at 850°C; firmly adhering, continuous conductor lines, 25 μm wide, with good conductivity on a dimensionally stable ceramic substrate were obtained.

Example 2

Example 1 was repeated, but with plates of thin film ceramic (99.6% AI2O3; Rubalit® 710 from Hoechst's CeramTec) as an intermediate support. After the gold powder was applied, the plates were fired immediately for 10 minutes at 850°C. There resulted a satisfactory conductor line pattern, which, however, adhered only slightly to the intermediate support surface. For a demonstration of transferability, a Cromalin® sheet with the polypropylene cover sheet removed was pressed on the tacky coating side of the pattern with 0.2 MPa pressure. After careful separation of the sheets, the conductor lines were completely transferred.

Example 3

An intermediate support of a 5 x 5 cm thin film ceramic plate was given a light-sensitive coating as described in Example 1 and exposed with a test pattern. Then, a metal powder was applied by the vibration process and the light-sensitive coating was removed by burn-off. The intermediate support was now brought into contact with a green ceramic sheet and the powder image was transferred onto this sheet at 74°C under pressure applied for five minutes. After removal of the ceramic sheet from the intermediate support and firing in a continuous throughput oven at 850°C, a sintered ceramic structure was obtained with high quality, continuous conductor lines on its surface. Other process parameters for different conductive metals are listed in the following Table 1.

Table 1

Transfer Metal Powder, Burn-off Pressure

Particle Size Temperature Time (MPa)

(°C) (min)

* with adhesive agents of DE 38 06 515

Example 4 Several 10 x 10 cm intermediate support glass plates were coated with the light-sensitive layer described in Example 1. In each case, a plate was exposed with the specular, negative mask of the conductor line pattern of one of the first three layers of a multilayer circuit (Du Pont Green Tape Test

Pattern) . Pure gold powder (2 μm) was applied onto the exposed layer. The intermediate support was then heated for six minutes at 350°C to volatilize the light- sensitive material. Four 7.5 x 7.5 cm coupons were punched out of green ceramic sheeting (Du Pont Green

Tape® AT) . Holes (vias) were punched in the coupons to connect the conductor line patterns of the different levels and were filled with gold paste by the screen printing process. Intermediate supports with the matching powder images were placed in register on three of the sheets thus prepared and pressed together five minutes at 74°C with 4.5 MPa pressure. Two untreated sheet coupons, the three coupons with powder images and the fourth coupon, on which the conductor image of the fourth layer had been previously applied by the screen printing process, were superimposed. This laminate was pressed together in the process recommended for the ceramic sheet, preheated, and sintered. The electrical test showed complete functional capability in the resulting multilayer circuit .

Example 5

A glass plate was coated with a light-sensitive layer as described in Example 1, exposed, and treated with gold powder (2 μm) . Then, the light-sensitive material was removed by heating for eight minutes at 600°C. A borosilicate glass cylinder was coated with a tacky surface by immersion in a diluted commercial adhesive (UHU® plus dichloromethane at about 1:10) and rolled on the powder image. The transferred powder image was sintered by heating at 700°C for 10 minutes to yield a conductor line image with satisfactory adhesion on the glass surface.

Example 6

A glass plate was coated with a light-sensitive layer as described in Example 1, exposed, and treated with gold powder (2 μm) . The light-sensitive material was removed by heating for six minutes at 350°C. A cylindrical barium titanate structure was dipped in a solution described in Example 1 of Patent Application P 39 13 116.5 (containing light-sensitive dihydropyridine compounds of Example 1 and bismuth nitrate as an adhesive agent) , dried, exposed to diffuse light, and thus given a tacky surface. The tacky cylinder surface was rolled under pressure on the powder image. The transferred powder image could be sintered by 10 minutes firing at 900°C to yield conductor lines with good adhesion on the barium titanate.

Example 7

A tacky photopolymerizable dry film (Cromalin® 4 from the Du Pont Company) was laminated onto a 5 x 5 cm thin film ceramic plate and exposed imagewise. After application of a gold powder, the plate was heated in an IR continuous throughput oven at 600°C until the light- sensitive material was burned off. The plate with the powder image was brought into contact with a green ceramic sheet and the combination was pressed together five minutes at 74°C with 10 MPa pressure. The sheet with the transferred powder image was separated from the intermediate support and fired in a continuous throughput oven at 850°C. A sintered ceramic structure with high quality, continuous, fine conductor lines was obtained.

Example 8 A light sensitive layer was prepared on a 7.5 x 7.5 cm glass plate as in Example 1, exposed with a resolution pattern, and treated by the vibration process with a spherical gold powder of 2 μ particle size. The plate with the resulting powder image was immersed in a beaker of cyclohexane at room temperature. The solvent was heated to 50°C within 10 minutes and cooled. The light-sensitive material was thus removed by extraction. The plate was removed from the solvent and dried. Then, the powder image was transferred onto Green Tape® by pressure for five minutes at 74°C and 4.5 MPa.

Example 9 As described in Example 1, borosilicate glass plates (10 x 10 cm², 2 mm thick) were given a light- sensitive coating, exposed with a test pattern, and treated with gold powder. The light-sensitive material was removed by heating for 6 minutes at 350°C, so that the gold powder image remained lightly adhered to the glass plates.

A green ceramic sheet was placed on one of these glass plate sand the two pressed together 5 minutes at 70°C with 4.4 MPa. The still warm ceramic sheet was immediately peeled away from the glass plate. After cooling took place, another glass plate with a gold powder image was placed in register on the ceramic sheet and the two pressed together again. The second powder image was also completely transferred. After firing, defect-free conductor line networks were obtained with increased conductivity compared to Example 1.

Example 10 The process of Example 9 was repeated, except that the second powder image was not placed in register but was rotated 90°. After firing, conductor lines were obtained adhering completely to the ceramic structure and uninterrupted at crossover points. Example 11

Examples 9 and 10 were repeated, except that the second powder image was made of silver or platinum powder (see Example 3) . In additional experiments, a third powder image of platinum was transferred onto the ceramic sheet after the transfer of a gold powder image and a silver powder image and before firing. In all cases, the powder images were completely transferred without damage.

Example 12

This experiment used- a conventional resistor test pattern comprising two masks. The first mask was the metallic conductor leads and terminals, whereas the second mask defined the resistor areas. Both marks overlapped in the terminal area by about 50 μm.

Two glass plates were given a light-sensitive coating as in Example 1. After exposure through the first mask, gold powder was applied to the first plate. After exposure through the second mask, a powder of a dried, commercial thick-film resistor paste based on glass frit, bismuth oxide, and ruthenium oxide was applied onto the second plate. After removal of the light-sensitive coating by heating, the powder image was transferred first from the first glass plate and then from the second glass plate onto the same green ceramic sheet. After firing, a ceramic structure was obtained with resistor areas in

Claims

Claims 1. Process for preparing fine conductor lines for electrical switching circuits on a support by the use of an auxiliary surface of an intermediate support, comprising the steps a) selecting the support and the intermediate support so that they can be separated from each other after being brought into contact, b) forming on the auxiliary surface a pattern defining the conductor lines, c) applying on the auxiliary surface an electrically conductive material in the shape defined by the pattern, d) removing the pattern from the auxiliary surface leaving only the conductive material, e) adhering the conductive material onto the support with an adhesive force greater than the adhesive force of the conductive material on the auxiliary surface, by bringing the support and auxiliary surface into contact, f) separating the support and the auxiliary surface leaving adhered on the support the electrically conductive material in the shape defined by the pattern characterized in that i. the pattern is a positive, tacky pattern, ii. the electrically conductive material is a powder comprising principally of one or more metals and/or alloys, iii. the powder present in the form of the positive, tacky pattern is compacted to a conductor line pattern.

2. Process in accordance with Claim 1, characterized in that the positive, tacky pattern is produced by imagewise irradiation of a positive-working or negative-working radiation-sensitive coating.

3. Process in accordance with Claim 2, characterized in that the radiation-sensitive coating comprises a tacky photopolymerizable mixture that loses its tackiness on irradiation.

4. Process in accordance with Claim 2, characterized in that the radiation-sensitive coating comprises a material or a materials mixture that becomes tacky on irradiation.

5. Process in accordance with Claim 4, characterized in that the radiation-sensitive coating contains dihydropyridine compounds .

6. Process in accordance with Claim 5, characterized in that the radiation-sensitive coating contains derivatives of nitrophenyl dihydropyridine.

7. Process in accordance with any of Claims 1 to

6, characterized in that the powder contains, in addition to metal components, materials to influence adhesion of the conductor lines on the support and/or the sintering of the powder.

8. Process if accordance with any of Claims 1 to

7, characterized in that at least the support or intermediate support is flexible enough that it can be pulled away from the other for separation.

9. Process in accordance with any of Claims 1 to

8, characterized in that the powder image is subjected to a sintering procedure for compaction.

10. Process in accordance with any of Claims 4 to 9, characterized in that steps b) to f) are repeated one or more time using other intermediate supports with the same or another pattern and with the same or another electrically conductive material, and are followed by compaction..

11. Process in accordance with Claim 10, characterized in that another pattern and a material with low electrical conductivity are use din repeating the steps.

12. Process in accordance with Claim 10, characterized in that the powder image transferred in repeating the steps contains areas of dielectric material in addition to areas of conductive material.

13. Process in accordance with any of Claims 1 to 12, characterized in that the support contains, in at least in its surface coating, materials that act as an adhesive for the conductive material at least at a temperature between 0 and 200°C.

14. Process in accordance with any of Claims 1 to 13, characterized in that the intermediate support and/or the support comprises a green ceramic sheet and the melting point of the metal in the powder is above the sintering temperature of the ceramic.

15. Process in accordance with any of Claims 1 to 9, characterized in that the intermediate support is temperature-stable and the powder applied between steps d) and e) is subjected to a sintering process.

16. Process in accordance with any of Claims 1 to

15, characterized in that the pattern is removed by thermal volatilization, firing, or extraction.

17. Process in accordance with any of Claims 1 to

16, characterized in that the powder is applied from a powder bed compacted by vibration.

18. Monolayer or multilayer switching circuit prepared the process in accordance with any of claims 1 to 17.