GB2091634A - Transfer lamination of vapour deposited copper thin sheets and films - Google Patents

Abstract

A copper-clad laminate having special utility in the production of high resolution printed circuit patterns is made by a method which includes forming an initial copper film by a vapor depositing copper directly in contact with e.g. an as-rolled aluminum carrier sheet at a temperature between about 100 DEG and 250 DEG C so that the carrier release peel strength is between about 0.5 and 2.0 pounds per inch. The transferred copper layer has the advantage of being pin-hole free.

Description

SPECiFICATION Transfer lamination of copper thin sheets and films, method and product The present invention relates generally to laminated materials and is more particularly concerned with a novel copper-clad laminate having special utility in the production of highresolution printed circuit boards and with a new method for making those products.

This invention is related to that disclosed and claimed in our British Patent Application No. 8119496. Thus, the extremely smooth, virtually pinhole-free and fine-grained copper surfaces of the copper-clad laminates produced by the process of that invention are features of importance in the products of the preferred practice of the present invention.

The invention disclosed and claimed herein is also related to that disclosed and claimed in our British Patent Application No. 8125381.

As used herein, and in the appended claims, the term "carrier" includes aluminum sheet material which is of gauge thickness such that it can be run through a processing line and rolled for storage or shipment, and also includes such sheet material of other metals such as copper as well as of plastics, such as duPont commercial products known as MYLAR and KAPTON and other organic polymeric materials of similar flexibility. All these materials share the ability to withstand the processing temperatures involved in this invention and have the strength at the temperature of deposition of the copper film and the characteristics of inertness and bondability to copper films necessary to preserve the integrity of the film-carrier sheet combination through subsequent processing and substrate attachment and to permit mechanical stripping of the carrier sheet without damaging the copper film.

"Ultra-thin" designates thickness less than about 16 microns.

"Film" and "foil" in this same context mean, respectively, an ultra-thin coating and the combination of such coating and one or more ultra-thin coatings of another metal or other material.

"Vapor deposition" means and includes sputtering, physical evaporation (i.e., electron beam, inductive and/or resistive evaporation), chemical vapor deposition, and ion plating.

"Substrate", as that term is used in this specification and the appended claims, means and refers to that part of the copper-clad laminate product or other article of manufacture of this invention which serves as the physical support means for the metal film or foil being suitably a glass-epoxy body provided in the form of a prepreg for cure in contact with copper or other metal foil. Other materials useful for this purpose include, but are not limited to, that known in the trade as "phenolic paper resins" which are paper sheet products impregnated with a resin curable to provide an adhesive bond between the substrate and the metal foil of the laminate. Still other such materials are polyimides and polyester resins.

Foils for copper-clad laminates suitable for printed circuit board production have heretofore been made, for the most part, by electrodeposition. The many advantages of this method, including speed of production, economy and a very fully developed technology, are, however, offset to a substantial extent by important disadvantages. A very important disadvantage is the difficulty of producing pin hole- free foils of less than 16 microns thickness.

Another is the inherent environmental impact of electrodeposition practice. While the pinhole problem may be minimized to some degree by electroplating copper on an aluminum carrier surface specially prepared in accordance with the procedure described in U.S. Patent 3,969,199 to Berdan and Luce, it is at the expense of substantially increased processing complexity and costs.

These shortcomings of the prior art can be avoided through the use of the inventions disclosed and claimed in the above-referenced copending patent applications. They can now also be avoided in still another different way represented by the process of the present invention. Thus, instead of coating the carrier surface with a release agent prior to laying down the copper film, the copper is deposited directly on the bare carrier surface in such a way as to enable subsequent non-destructive release of the film from the carrier. In other words, this invention enables elimination of the release agent layer while retaining its function. This is accomplished through control of the film bonding action involved in depositing copper on the carrier.This invention, thus, is based upon our new concept of vapor depositing copper under conditions of carrier surface temperature, roughness and cleanliness, resulting in a virtually pinhole-free, high quality, copper film which is bonded to the carrier but releasable upon application of peeling or stripping force within an acceptable range. As a general rule, according to this invention, the carrier surface will be clean and free from adhering oil and dirt which would impair adequate bonding or contaminate the resulting copper film, and will be relatively smooth and free from gross physical irregularities. Further, the temperature of the carrier surface will, in accordance with this invention, be between about 1000 and about 25000 so that bonds which are strong enough but not too strong are formed between the copper film and the carrier.

We have found in the course of using this invention that electron beam evaporation is an especially satisfactory method of carrying out the copper deposition step, carrier surface temperature being readily controllable in various ways under such condition and a film of requisite thickness being quickly established uniformly over the carrrier surface as required. We also contemplate, however, the possibility of ion plating deposition of the copper film which would involve biasing the carrier and, if required, introducing an inert gas such as argon into copper vapor to establish the necessary ionization effect.

Again, in the case of ion plating, carrier surface temperature would be amenable to easy control by a variety of alternatives. Induction (RF) evaporation of the copper instead of electron beam evaporation is also contemplated as a means of producing the vapor phase copper required for physical vapor deposition and again, of course, carrier surface temperature would be readily controllable. If, however, sputtering is the method of deposition to be carried out in the practice of this invention, it will be necessary for special heat removal measures to be taken so as to maintain carrier surface temperature in the range necessary to limit the development of bonding strength to the range we have found to be critical to the new results and advantages of this invention.

It will be understood by those skilled in the art that in whatever manner the invention is carried out to provide the copper film on the carrier surface, one has the choice of proceeding to produce the laminated body by either of the methods disclosed and claimed in the aforesaid co-pending patent applications. The disclosures of each of those patent applications regarding the steps of producing the laminated products are incorporated herein by reference, each of the separate steps in each of those different processes being set forth in general and specific terms in those disclosures.

The invention provides a copper-clad laminate product comprising an uncoated metal carrier sheet and a fine-grain vapor-deposited film of copper in direct contact with the carrier sheet surface and adhering thereto with a release peel strength between about 0.5 and about 2.0 pounds per inch.

The invention also provides a method of making a copper-clad laminate which comprises the steps of forming a copper film on a carrier by vapor deposition of copper directly on the carrier surface at a temperature between about 1 000C and about 2500C, depositing a substrate bonding layer on the copper film, laminating the resulting body with a substrate so as to create a strong adhesion between the said body and the substrate and, finally, removing the carrier, leaving the said body adhered to the substrate. In accordance with the invention of British Patent Application No. 81 19496, the substrate bonding layer is an electrolytically deposited film which is preferably of copper and in any event is so produced as to have or provide a nodular or dendritic surface form promoting substrate bonding.Alternatively, the substrate bonding layer is an electrolytically deposited ultra-thin film of zinc, aluminum, tin or chromium overlaid with an ultra-thin film of silicon dioxide or aluminum oxide, as set forth in our British Patent Application No. 8125381.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which.~ Fig. 1 is a schematic diagram representing a cross-sectional view of a laminate product of this invention at an intermediate stage production.

Fig. 2 is a view like that of Fig. 1 of the product of this invention prior to removal of the aluminum carrier sheet.

Fig. 3 is a view like that of Fig. 1 illustrating the step of removing the carrier sheet by stripping by the application of peeling force of 0.5 to 2.0 pounds per inch.

As illustrated in Fig. 3, a product 10 of this invention as it may appear as an item of commerce for ultimate use in, for example, the production of printed circuit boards, comprises a substrate 12, having a vapor-deposited copper film 14 bonded to the substrate by an electrolytically deposited or otherwise established bonding layer 16. It also includes carrier sheet 18 preferably of aluminum or aluminum alloy on which the vapor-deposited copper film is established in the key novel step of the process of this invention.Thus, as also illustrated in Fig. 1, the step of the process to which is attributable the principal virtues and advantages of the present invention as detailed above, is the establishment of initial copper film 14 on the carrier sheet by vapor deposition under conditions which permit ultimate separation by peeling or stripping by the application of a force of only such small proportion as not to cause damage or destruction of the finished product laminate as by ripping or breaking the copper film on which a printed circuit is ultimately to be provided as by semi-additive or subtractive technique.

The process of this invention in its preferred form involves as a first step the cleaning of the aluminum carrier sheet. This is an as-rolled sheet product, the surface of which may be specially prepared as by cleaning or it may be used as produced in the course of normal commercial manufacture of the sheet metal. In any event, the aluminum or aluminum alloy sheet will typically have a naturally-formed oxide coating overall of something less than about 50-Angstroms thickness and in usual situations, from 10 to 30-Angstroms. The carrier sheet is preferably of thickness of 1 to 7 mils but can be somewhat thinner or much thicker, if desired, and it will have surface finish better than + eight microinches root mean square. Cleaning of the sheet as it is obtained on the market or freshly produced in a commercial rolling operation, is essential to the production of uniform laminate product as it is necessary that the copper film applied to the aluminum sheet as the first stage of the process of deposition be bonded, although relatively weakly, to the carrier sheet for integrity in subsequent processing. Fume-cleaning with Freon liquid or other suitable material can be useful but we prefer plasma etching or ozone cleaning which are techniques known and established in the art for general cleaning purposes where a consistently high degree of surface cleanliness is required.

With the surface of the carrier sheet thus prepared, the application of copper by vapor deposition technique, as described above, is conducted and a film of copper is established on the clean aluminum carrier sheet surface. In accordance with our preference, electron beam means are used and in any case, the step is carried out to provide a coating which may be ultra-thin or somewhat thicker, up to 25 microns, as desired.

As deposited in this manner, or by alternative method such as with induction heating or by ion plating, the copper film will be of grain size of the order of from a few hundred to a few thousand Angstroms in contrast to the much larger grain size of electroplated copper films running of the order of one to two microns as a minimum. To achieve this result, the aluminum carrier sheet surface on which the copper film is deposited, is maintained throughout the deposition operation at a temperature from about 100 to 25O0C.

Temperatures somewhat higher have been found to result in the establishment of a bond between the copper film and the aluminum carrier sheet which is strong enough to make mechanical stripping or peeling of the film from the sheet difficult or impossible without damage to the film.

On the other hand, somewhat lower temperatures have been found to result in inadequate bonding to maintain copper film-aluminum carrier integrity through subsequent processing steps.

With the establishment of the copper film by vapor deposition technique completed, the resulting assembly is ready for the next step of the process. This involves preparing the copper film for attachment to a substrate and this step may be carried out either in accordance with the process disclosed and claimed in above-referenced Patent Application No. 8119496 or by that disclosed and claimed in the other referenced Patent Application No.8125381. It will be understood, however, that it may be otherwise treated by present or future methods to accomplish the ultimate desired result in respect to the bonding of the copper film to the ultimate substrate body with which it is to be laminated and used in the production of printed circuit boards or for other purposes.Accordingly, that portion of the aforesaid Patent Application No. 8119496, concerning preparation of the copper film surface and the ensuring substrate bonding operation steps is incorporated herein by reference. Likewise, the corresponding part of the specification of Patent Application No. 8125381 is in its entirety incorporated herein by reference.

At this time, we have no preference for one of these procedures over the other in the practice of this invention because they are equally reliable and comparably economical in use.

Substrate bonding, then, by either referenced method, or some other method of choice, is carried out to the establishment of the finished product and it only remains for the ultimate user to remove the carrier sheet by mechanically stripping or peeling it away. This is the preferable method of removal of the carrier but it will be understood that chemical means, i.e., dissolution, may be an alternative of choice under some circumstances. Stripping or peeling action, however, is carried out in the simplest possible mechanical way and requires only the application of a limited peeling force which is non-destructive in respect to the integrity of the copper film which serves as the active portion of the laminate to be involved directly in the printed circuit board production or other use of similar type of the laminate.

The following illustrative, but not limiting, examples are offered in further exposition of the special new features and advantages of the present invention: EXAMPLE I Three 3-mil aluminum carrier sheets in asrolled condition were plasma-etch cleaned and clamped to a 4 by 4-inch glass plate (1/4 inch thick) and a thermocouple was attached to one edge of the sheet in each instance. By means of two six-inch quartz lamps disposed on either side of the assembly in a vacuum chamber, the temperature of the carrier sheet in each instance was brought to 3000C and maintained while pressure in the chamber was brought down to 8 x 10-7 torr. Then, through the use of an electron beam device, a 5-micron film of copper was vapor deposited on the exposed surface of the aluminum sheet. During the 10 minute period that the copper was being melted, vacuum chamber pressure was 3.6 x 10-6 torr.Then, during the evaporation stage, the pressure was 6 x 10-6 torr.

The temperature of the aluminum sheet was maintained at 3000 + 50C throughout the 45-minute deposition stage of the process. On test, the peel strength of one of the copper film specimens was found to be 3.04 pounds per inch.

The second specimen was nodularized in a copper sulfate bath at one ampere per square inch for 30 seconds and pressed onto an FR4 epoxy glass board at 160 pounds per square inch pressure and 1 660C for 35 minutes as disclosed in the aforesaid application Serial No. 1 89,003. The peel test performed on the carrier sheet by cutting a one-eighth inch strip from the edge resulted in the copper film being pulled away from the board and thus destroying the composite body for its intended printed circuit board production use.

The third specimen was assembled with the substrate which was then heated at 3000C for 45 minutes with the consequence that the copper annealed with the aluminum forming a composite which could not be cleanly separated through the application of peeling or stripping forces.

EXAMPLE II In still another experiment like that of Example I, except that the carrier sheet was Freon fume-cleaned, the temperature of the aluminum copper sheet was maintained at about I 200C throughout the copper deposition period. On test, the peel strength of the copper film was found to be 1.6 pounds per inch. This test was performed as described in the Example I by assembling the copper film coated carrier sheet with the bonding layer and substrate, as described in Example I. The test then, for peeling strength, being made on the one-eighth inch strip cut from the specimen and a peel testing device being used as in the Example I for the measurement of the peel strength.

EXAMPLE Ill In still another test like that of Example II, using a Freon fume-cleaned aluminum carrier sheet of 3-mil thickness, and with the other conditions of deposition of the copper film and the production of the substrate laminate product being the same, it was found on test carried out in the manner described above that the peel strength of the laminate product was zero. A preliminary peel strength test carried out on the film prior to assembly with the substrate also gave a zero reading.

EXAMPLE IV In another experiment like that of Example II, except that the temperature of the carrier sheet was maintained at 2500C throughout the copper film deposition period, it was found on test of the ultimate substrate laminate body that the peel strength was between 0.88 and 1.00 pound per inch.

EXAMPLE V Another experiment like that of Example II resulted in peel strengths of 1.4 pound per inch and 1.76 pound per inch when the temperature of the aluminum carrier sheet was maintained at 2600C throughout the copper film deposition period and other conditions were unchanged.

EXAMPLE Vl In another experiment like that of Example V, except that the carrier temperature was maintained at 2400C throughout the deposition period, the peel strength in one instance measured 0.24 pound per inch, while in another, it was 0.48 pound per inch and in still another, it was 0.80 pound per inch.

In another recent experiment, we prepared a 0.3 micron vapor-deposited film of copper on an uncleaned, as-rolled aluminum sheet at room temperature. The peel strength of that film proved to be zero.

It is evident from the data developed in the experiments described above that the new results and advantages of this invention can be obtained under certain circumstances which are critical in substantial degree. It is thus clear that the temperature at which the carrier sheet surface is maintained during the copper film deposition period is vitally important to the strength of the bond formed between the copper film and the aluminum carrier. Further, it is apparent that the nature of the surface of the aluminum carrier sheet on which the copper film is deposited has an important bearing upon the strength of that bond.

Finally, it is clear from these experiments that the cleanliness of the surface of the carrier sheet on which the copper film is established has an important bearing upon peel strength of the bond and, of course, also can be important in respect to the contamination of the resulting copper film and the value and utility of the ultimate laminate product. The cleaning process we favor is the plasma-etch cleaning technique known in the art.

The Freon evaporation and otherfume-cleaning processes produce variable result as is evident from the data reported above, and is consequently not favored in the best practice of this invention.

Claims (19)

1. A copper-clad laminate product comprising an uncoated metal carrier sheet and a fine-grain vapor-deposited film of copper in direct contact with the carrier sheet surface and adhering thereto with a release peel strength between about 0.5 and about 2.0 pounds per inch.

2. A laminated product as claimed in Claim 1 in which the carrier sheet is aluminum in as-rolled unanodized condition and consequently has a natural oxide coating overall of thickness less than about 50-Angstroms.

3. A laminated product as claimed in Claim 1 in which the carrier sheet is of organic polymeric material.

4. A laminated product as claimed in Claim 1 in which the carrier sheet is rolled aluminum, the surface of which has a finish better than 8.0 micro inches root mean square.

5. A laminated product as claimed in Claim 4 in which the carrier sheet has been cleaned.

6. A laminated product as claimed in any preceding Claim laminated with a substrate which is firmly bonded to the copper film.

7. A method of making a copper-clad laminate which comprises the steps of: forming a copper film on a carrier sheet by vapor-depositing copper directly on the carrier surface while maintaining the carrier surface in the temperature range from about 1000 to about 2500 C; depositing a substrate bonding layer on the copper film; laminating the resulting metal foil body with a substrate so as to create strong adhesion between the said body and the substrate; and removing the carrier sheet leaving the said body adhered to the substrate.

8. A method as claimed in Claim 7 in which the carrier sheet removal step is accomplished by physically stripping the metal foil body from the carrier sheet by applying a peeling force between about 0.5 and 2.0 pounds per inch to said metal foil body.

9. A method as claimed in Claim 7 or Claim 8 in which the copper film is produced by electron beam evaporation.

10. A method as claimed in Claim 7 or Claim 8 in which the copper film is formed by ion plating.

11. A method as claimed in'any of Claims 7 to 10 in which the copper sheet is as-rolled aluminum foil having an oxide film of thickness between 10 and 20-Angstroms, the copper film is formed by electron beam evaporation, and includes the step of electrolytically treating the copper film to produce a foil for increased adhesion to the substrate.

12. A method as claimed in Claim 9, including the steps of vapor depositing a film of zinc on the copper film and vapor depositing a film of silica on the zinc film.

13. A method as claimed in Claim 7 in which the substrate bonding layer deposition step involves electro-deposition of copper as a highly irregular coating on the copper film.

14. A method as claimed in Claim 7 in which the substrate bonding layer deposition involves vapor depositing an ultra-thin film of a metal selected from the group consisting of fine, aluminum, tin and chromium on the copper film and vapor depositing an ultra-thin film of an oxide selected from the group consisting of silicon dioxide and aluminum oxide on the resulting metal foil.

15. A product when made by a method as claimed in any of Claims 7 to 14.

16. A copper-clad laminate product, substantially as hereinbefore described with reference to, and as illustrated in, Figure 1 or Figure 3 of the accompanying drawings.

17. A copper-clad laminate product substantially as hereinbefore described in any of Examples I to Vl.

18. A method of making a copper-clad laminate product, substantially as hereinbefore described with reference to the accompanying drawings.

19. A method of making a copper-clad laminate product substantially as hereinbefore described in any of Examples 1 to Vl.