Process for metallized imaging

Abstract: Formation of well-adhered metal layers on aromatic polymeric substrates through a two-step process is based upon reversible charge storage in the electroactive center-containing polymeric substrate. The process comprises the steps of l) injecting a stored charge into a polymer surface having electroactive sites, and 2) reducing metal ions with said stored charge to form metal in or on said polymer surface and applying a masking means to the polymer surface either before step l) or between steps l) and 2). Articles particularly useful for electronic, imaging and solar applications are produced. The process may be a totally additive process such that articles can be produced in a continuous manner.

Description: This invention relates to the formation of metal images on polymeric surfaces and particularly to the formation of circuitry on polymer surfaces. Metallized organic polymers are utilized in numerous applications requiring conductive or reflective coatings. The primary methods for the metallization of the polymers have been vapor deposition (evaporation and sputtering) and standard or conventional electroless deposition techniques. Metallized films of polyimides (PIm) are particularly desirable in the fabrication of large-scale integrated circuits (the polyimide being primarily used as an insulating dielectric layer), flexible printed circuitry, and photovoltaic devices (primarily as a flexible substrate which can withstand the temperatures associated with the deposition of amorphous silicon). Circuit elements are generally formed by the formation of resist layers or masks over the metallized polymer surface, followed by the plating and/or etching of circuit elements. A major concern in metallizing polyimide films, particularly for electronic applications, is the adhesion of the metal film to the polymeric substrate. It is necessary that the metal film stay well-adhered to the polymer during and after processing, which often involves electroplating and selective etching of metal film off the substrate by strong acids. This processing can lead to undercutting of metal film and loss of adhesion. Perhaps the most popular method of achieving well-adhered copper films on polyimide today is done by sputtering techniques. In this process, chromium is sputtered in the presence of oxygen onto the polyimide substrate and then copper is sputtered onto this "primed" substrate. It has been claimed that this presputter with chrome in the presence of oxygen results in the covalent bonding of the chrome oxide layer to the substrate. This covalent bonding mechanism may be subject to a hydrolysis reaction and may generally be expected to show reduced persistence after exposure to ambient conditions. U.S. Patent No. 4,459,330 discloses an electroless plating process for plating at least one main group metal on a surface of an aromatic polyimide substrate comprising the steps of forming a nonaqueous solution containing a Zintl complex, a salt or alloy of an alkali metal in a positive valence state and at least one polyatomic association of a main group metal in a negative valence state, the polyatomic main group metal being selected from the group consisting of Ge, Sn, Pb, As, Sb, Bi, Si and Te. An aromatic polymeric substrate is chosen which is reducible by the solubilized salt and is resistant to degradation during the reaction. A redox reaction is effected between the salt in solution and the substrate by contacting the solution with the substrate for a sufficient time to simultaneously oxidize and deposit the main group metal in elemental form to produce a plated substrate. The alkali metal is retained in the plated substrate, and the substrate becomes negatively charged by electrons transferred from the main group metal during the redox reaction. Only polyatomic complexes of at least seven atoms are shown. Haushalter and Krause (Thin Solid Films, 102, 1983, 161-171 "Electroless Metallization of Organic Polymers Using the Polymer As a Redox Reagent: Reaction of Polyimide with Zintl Anions") extended the polyimide metallization discussed above to certain transition metals by using the PIm as a reducing agent toward an oxidized metal species in solution. Specifically, the treatment of PIm with methanol solutions of Zintl salts, e.g., salts of K₄SnTe₄ provides a reduced intercalated material, K x PIm, with no surface metallization. The reaction of K x PIm with solutions of transition metal cations with reduction potentials more positive than that of K x PIm results in metal deposition. The metal films deposited by this method show varied properties depending on the element and amounts deposited. For example, reaction of K x PIm with Pt²⁺ or 5 Pd²⁺ in dimethylformamide (hereinafter DMF) rapidly gives uniform highly reflective films with conductivities approaching that of the bulk metal. In contrast, Ag⁺ ions, noted for their high mobility in solids, give films with resistances several orders of magnitude higher than that of palladium films containing similar amounts of metal. Apparently, the Ag⁺ ions can diffuse into the solid at a rate roughly comparable with the diffusion rate that the K⁺ and electrons exhibit in moving to the surface of the polymer (the rate of charge propagation towards the surface). The polymer is therefore partially metallized throughout the bulk solid. European Patent Publication No 245016 forms part of the state of the art by virtue of Article 54(3) & (4) and discloses an improved process for the deposition of metal layers onto a polymeric surface having electroactive sites contained therein, which process comprises contacting the polymeric surface with first solution in which at least 10 molar percent of all negative charge intercalation ions within that solution are simple or complex negative charge intercalation ions, thereby reducing the polymer of the polymeric surface but without substantial plating of metal onto the polymeric surface and then contacting the surface of the reduced polymer with a second solution containing reducible metal cations thereby reducing the metal ions to metal and depositing a layer of metal onto and within the polymer surface. European Patent Publication No 245016 also discloses the image-wise deposition of metal onto the polymer surface by the application of water or methanol insoluble ink materials to the polymer surface before reduction to prevent charge transfer to that surface region. This provides an imaging process suitable for printing circuit manufacture which can be a totally additive one. To demonstrate this, arbitrary circuits are patterned onto polyimide by a high speed offset printing technique using an ink and the printed polyimide reduced in the manner described above and then oxidized in electroless copper or nickel. Only the polyimide film surface that has not been covered by the offset print is metallized with no etching necessary. For less detailed work it has now been found possible to inject the charge prior to application of the mask and then apply the masking means before reducing metal ions to form the initial metal image. However, there will be some horizontal movement of metal ions and therefore fringe images created by such a process. This is intolerable where resolution of less than one µm (micron) is important but more than acceptable in processes where detail less than two to five µm (microns) is unimportant. Thus, according to the present invention there is provided a process for image-wise deposition of metal onto at least a portion of a polymer surface comprising the steps of (1) injecting a stored charge into a polymer surface having electroactive sites, and (2) reducing metal ions with the stored charge to form metal in or on the polymer surface in which a masking means is applied to the polymer surface after injecting the stored charge but before reduction of the metal ions. A metallization process is utilized for imagewise diffusing metals into at least a portion of the surface of a polymeric substrate having electroactive centers and subsequently imagewise plating a metal to a desired thickness. A charge is first imagewise injected and reversibly stored in the polymer, which charge is subsequently used for the reduction and deposition of transition metal in elemental form. A mask or coating is then used to cause the imagewise deposition of metal. The metallized product may be used for electronic circuitry or photomasks. The process of the present invention can be used with any process that enables the storage of charge in polymers having electroactive sites therein. It is particularly useful wherein the injection of those charges is effected from a liquid solution of active ingredients. Such injection processes are shown in U.S. Patent No. 4,459,330 and EP-A-245016. The process of the invention basically requires the following steps: (1) injecting a charge into the surface of a polymer having electroactive sites (2) masking in an imagewise pattern the surface of the polymer, and (3) reducing metal ions with said charge to form an imagewise distribution of metal on or in said polymer. Once the charge in the polymer has been used to deposit metal, the deposited metal may be used as plating sites for the further deposition of metal by other means such as electroless plating. The masking material may comprise photoresist materials (either positive or negative acting) in either liquid or dry film formats, inks printed in the desired negative pattern, waxes, paints or closely adhering stencils. The preferred charge injecting process for use in the present invention is that disclosed in European Patent Publication No. 245016. The basic process comprises injecting electrons into a polymer containing electroactive centers without the coincident deposition of a metal film on or in the surface of the polymer and the subsequent reduction of metal ions in solution by the transfer of electrons from the polymer to the metal cations causing the formation of metal in or on the surface of the polymer. The deposited metal is then used as a site for further deposition of metal by reduction of metal cations. This process is preferred because it is not necessary to use exotic Zintl complexes and Zintl anions to inject the charge into the polymer. It is preferred that no Zintl complexes or anions be used in the practice of the present invention. It is preferred to use some amount of simple transition metal or tellurium or complexed transition metal ions which can be used to inject charges into the polymer as a partial or total replacement for the expensive and difficult to manufacture Zintl complexes and anions. For example, the injection solutions should comprise at least ten molar percent of monoatomic negative charge intercalation ions out of the total molar amount of negative charge intercalation ions. This percentage is preferably at least 25%, more preferably 60%, highly preferred as at least 90% and most preferred as at least 98% or 100% of monoatomic negative charge intercalation ions. The most preferred negative charge intercalation ions are Te²⁻, V(ethylenediaminetetra-acetate)²⁻, and Co(bipyridyl) + / 3 which are conveniently provided as M x Te²⁻ (wherein M is an alkali metal cation and x is the value of 2 divided by the valence state of the cation) or produced by the electrochemical reduction of tellurium and as further shown in the Examples. The term negative charge intercalation ion indicates that an electron is injected into the polymer from the ion. The intercalation ion does not itself necessarily pass into the polymer composition. Rather, an electron is injected into the polymer. After injection of the charge into the electroactive centers of the polymer substrate without substantial surface metallization (that is, less than 5 nm (50Å) of deposition, preferably less than 2.5 nm (25Å) of deposition, most preferably zero deposition by weight and volume), the activated polymer may now be contacted with solutions of metal salts, particularly transition metal salts, to cause deposition of the metal. As subsequently pointed out, the choice of the metal can determine the depth of the initial deposition, with highly migratory metal cations being capable of reduction at depths of up to about 4 µm. As the charge is exhausted, the depth of penetration will be reduced until substantially only surface deposition will occur, but at that point, conventional electroless (or other) deposition may be used to further thicken the metal layer. The preferred electroless process for plating transition metals on a polymeric substrate having electroactive centers (e.g., polymeric films containing imide groups in the polymer network, e.g., polyimide [PIm], pyromellitimide) is accomplished in a two-step process. First, by taking advantage of the oxidation states of the simple ions or transition metal complexes and the alkali metal cations and small quaternary ammonium cations (e.g., tetramethyl ammonium) permeability of PIm, it is possible to reduce the polymer without surface metal deposition. Second, the reduced solid is then used to deposit transition metal cations reductively from solution. For example, the reduced, deeply green-colored alkali metal or quaternary ammonium cation diffused mono-anion polyimide is prepared by immersion of the film in aqueous or methanolic reductants. The time of immersion varies from a few seconds up to several hours depending upon the extent of reaction desired. Along with the reduction of the polyimide film is the concomitant diffusion of the counter cation into the film. The size of the counter cation appears to be very important. Alkali metals freely diffuse into the film as reduction proceeds. Intermediate sized quaternary ammonium cations such as tetramethylammonium and tetraethylammonium do diffuse into the polyimide film and reduction of the film to produce a deeply colored radical anion film occurs. However, the ammonium cation appears unstable as the counter cation. This is indicated by a gradual fading of the film color to lighter shades of green. Polyimide reduction with Te²⁻ is best accomplished in methanol, although the halfwave potentials (E1/2) for the oxidation of Te²⁻ in water or methanol are essentially the same. The differences in reducing polyimide in water, as opposed to methanol, appear to arise from the inability of the water solutions to adequately wet the polymer and facilitate rapid electron transfer. Reduction of polyimide using aqueous Te²⁻ is generally a much slower reduction step and can produce inhomogeneous results. Surfactants added to these aqueous Te²⁻ solutions have reduced the inhomogeneity. Alternately, it has been discovered that polyimide film reduction without the formation of metal surface layers can be accomplished using complexed vanadium (V[II]) or complexed cobalt (Co[I]) species in aqueous or methanolic based systems as shown in Table I below. The reductants are easily regenerated electrochemically by applying a suitable potential to the solution. This makes possible the use of a closed loop system for the reduction of polyimide. Only electrolyte need be added to the system to make the film reduction continuous. Additionally, no special environmental problems are encountered in the use of this system. Films of copper, cobalt, cobalt/phosphorous alloy, gold, nickel-boride alloy, nickel-phosphorous alloy and nickel were deposited on polyimide film when any of the vanadium reductants in Table I were used. It is believed that polymers having electroactive sites of the appropriate standard reduction potentials such as aromatic polyimides, polysulfones and copolymers of styrene and vinyl pyridine would provide favorable results. In view of the increased rates of the redox reaction and platings produced on the aromatic polyimides and polysulfones, the presence of electron-withdrawing groups are preferred adjacent to the aromatic ring either in the polymeric backbone or as substituents. Accordingly, suitable polymers include aromatic polyimides, polyamides, polysulfones, styrene polymers with vinyl pyridine, substituted styrene polymers with electron-withdrawing groups and other polymers with the above characteristics. The preferred polymers include the polyimides and polysulfones. Advantageously, the polymers include electron-withdrawing groups in the backbone or as substituents on the aromatic groups. Illustrative of those in the backbone are carbonyl and sulfonyl groups while the groups substituted on the aromatic groups may include nitrile, thiocyanide, cyanide, ester, amide, carbonyl, halogen and similar groups. As is known, aromatic polyimides may be illustrated by the following where R₁ and R₂ are single or multiple aromatic groups. Polysulfones may be illustrated by the following: where R₁ and R₂ represent single and multiple aromatic groups as in In the copolymer of styrene and vinyl pyridine, the general repeating units are Polyimide films, particularly those containing pyromellitimide centers are the preferred substrates in the present invention because of their excellent thermal and dielectric properties as well as their chemical resistance and dimensional stability. Also films containing pyromellitimide units therein to act as electroactive centers are useful. The polymers may contain pyromellitimide units through copolymerization, block copolymerization, graft copolymerization or any of the other known methods of combining polymer units. Other polymer units which provide electroactive centers (variously known as redox centers and charge transfer centers) may also be used as the polymeric substrate. An important factor in the diffusion and formation of the metal layer is a favorable free energy for the reaction of radical anion polyimide film and a particular metal cation. This is clearly the case for most copper salts. The reduction potential of the copper salts is dramatically affected by a change in the solvent or coordination sphere of the ion. Variation of the reduction potential provides a means of controlling the reaction between the metal ion and the polyimide film. In general, the more negative the free energy of the reaction between a metal cation and reduced polyimide film, the faster the metal film is formed. The rate at which a metal is deposited has considerable affect upon the properties of the deposit. Also important for the oxidation of the polyimide film is the size of the oxidizing species which does not necessarily have to be cationic. Thus oxidation of PIm⁻¹ by Ag⁺ can result in finely dispersed polycrystalline silver metal deep within the polymer (3-4µm). The presence of dispersed metal particles at depths in excess of 1 µm immediately after deposition of the metal tends to be a unique characteristic of the process of the present invention. In this case, the very small aquated Ag⁺ diffuses into the film at a rate much greater than the rate of charge propagation out to the film surface. Similarly, the oxidation of PIm⁻¹ by Au(CN) - / 2 results in the formation of gold metal within the polymer. This oxidation is quite slow as the reduction potential of Au(CN) - / 2 is -600mV. Alternatively, the oxidation of PIm⁻¹ by AuBr - / 4 is very rapid and results in primarily highly conductive surface layers of gold metal. A third important consideration in the metallization of polyimide is the pH of the oxidizing solution. At a pH below 7, protonation of the radical anion will occur and inhibit the charge propagation out of the polyimide film. This effect increases at lower pH and can completely inhibit metal layer formation. Although the green color characteristic of the radical anion polyimide film persists at low pH values, surface protonation can be sufficient to totally disable charge transfer to the polymer surface and to inhibit metallization. For the metallization of polyimide with copper in a copper oxidant, this effect is seen at a pH below 5 and a reaction time of the film and the solution at pH 5 of 1 minute. The polyimide film remains green but copper will not form on the surface of the film. Metallization can occur, to some extent, even at low pH if the rate of metal reduction is sufficiently fast. Under basic conditions, hydroxide mediated electron transfer reduction of PIm¹⁻ to PIm²⁻ can also occur, having an effect on the oxidizing specie's ability to diffuse into the film surface. The half-wave potential of the negative charge intercalation ions should be negative with respect to the half-wave potential of the polymer. By being negative with respect to the half-wave potential of the polymer, it is meant that the negative charge intercalation ion is capable of reducing the polymer. It is preferred that the negative charge intercalation ion be capable of injecting only one electron per charge transfer center, although ions injecting two electrons have been used. When the oxidant is Cu(OCOCH₃)₂·H₂O in methanol, 1 mg/ml, a brilliant mirror-like copper layer is formed which is electrically conductive. Likewise, when the oxidant is a saturated methanolic solution of CuI with KI (1 g/25 ml), a bright opaque copper film is formed which has conductivity approaching that of the bulk metal. The formation of copper layers through the oxidation of PIm¹⁻ is very surprising in view of the fact that when PIm¹⁻ is oxidized with CuCl₂·2H₂O in methanol, the characteristic green color of the polyimide film disappears as oxidation proceeds but no copper film is formed. Similarly, when the oxidant is CuCl₂·2H₂O in DMF, no copper metal film is formed. The copper films formed in the above examples are all quite thin films being generally much less than 1µm in thickness (e.g., 10µ-40 nm). For many current carrying applications it is necessary to have thicker coatings of copper metal. This can be accomplished through the redox chemistry of polyimide by using electroless copper solutions described in the examples as the oxidant. The oxidizing copper complex may be Cu(II)EDTA as in Example 3. The reduction of this complex by the polyimide leads to the thin copper film formation and then the catalytic properties of the electroless solution continue to build copper thickness. Whenever electroless copper oxidants are used, the polyimide reductions may be accomplished by Te²⁻. However, polyimide reduction by the vanadium or cobalt complexes will lead to particularly good quality copper films and is preferred. The formation of nickel films from electroless nickel oxidants has also been accomplished. The composition of the electroless nickel oxidants are given in the examples. The adhesion of both copper and nickel films is quite good. Tape peel tests with an aggressive tape did not result in failure of the metal/polymer adhesion. Importantly, the adhesion appears to be good even immediately after the film's formation in the electroless oxidants. This promotes processing in a continuous manner when copper thickness is to be increased to 2.54 x 10⁻⁵m (1 mil) or greater by electroplating. Peel tests on thick electroplated copper formed by the methods of the prior art have generally resulted in cohesive failure of underlying polyimide. It is commonly observed that the adhesion of metal to polymer increases with time as metal establishes a mechanical anchorage. Copper films deposited upon polyimide through the technique of the present invention were investigated by transmission electron microscopy (TEM) in order to characterize the polymer/metal interface. These investigations show that the adhesion of the film to the polymer is due to a mechanical anchorage of the metal caused by immediate diffusion of the metal complex just within the polymer surface where reduction occurs. Metal builds on top of this diffused region forming the thick, conductive, copper film. Many of the metallized films of the present invention have a distinct and unique physical appearance upon inspection by photomicrographic techniques. Metallized films laid down by conventional techniques such as electroplating, vapor deposition and sputtering have the metal deposited at the surface of the film with only some of the metal actually penetrating into the body of the film itself. The process of the present invention, on the other hand, forms the metal particulate within the body of the polymer with lesser amounts being on the surface of the polymer. For example, with gold deposition according to the process of the present invention, 75% and more of the gold is deposited below the surface, with some distinct particles at depths of 1 µm and more. It tends to be a characteristic of the present invention that at least 40% of the metal is below the surface of the polymer and that at least some of the particulate metal is present at a depth of at least 0.25 µm. Preferably at least 50% of the metal is present below the surface of the polymer and the particulate metal exists (even in very small percentages, e.g., between 0.01 and 1%) at a depth of at least 0.3 µm. Other metallization methods are not believed to be capable of producing such distributions of metal within the polymer surface. It has been theorized that one general suggestion made by Haushalter and Krause (supra) and the use of both K₄SnTe₄ and silver might produce a distribution of particulates similar to those of the present invention, but would have tin present as a residue of the zintl complex breakdown and would have silver as the only major metal particulate. Any film having the described characteristics without the presence of analyzable tin and with particulate metal other than silver would not be produced by that teaching. By the nature of this process, less than 2% of the metal could be at a depth of greater than 2µm (microns) as would occur with a gross coating of particles in a binder. Usually less than 1% is present at a depth of at least 1µm (micron). Additional utility realized through this unique metallization process is the ability to deposit metal only where it is desired on the polyimide substrate, e.g., by the application of water or methanol insoluble ink materials to the polyimide surface to provide an imaging process for printed circuit manufacture which can be a totally additive one. Conventional photoresists could be utilized as well for imaging. The preferred final product of the present invention comprises an article having a transition metal present as finely dispersed particles within the surface of a polymer having electroactive sites and having adhered to said polymer and to some of said particles a highly conductive metal film, at least 10% by weight of said metal particles penetrating at least 2nm (20 Angstroms) into said polymer and no more than 25% of said particles penetrating more than 400nm (4000 Angstroms) into said polymer. Certain metals will tend to have greater penetration than others, specifically silver and gold. Silver in particular penetrates to depths as much as 4µm (40,000 Angstroms), but is not preferred in certain electronic devices because of its migratory properties. It is preferred that no more than 25% of said particles penetrate more than 40nm (400 Angstroms) into polymer as is the case with copper. One surprising aspect of the present invention has been found to be the relative importance of the sequence of steps in producing the best bond strengths. Examples have been performed where the film is first reduced, then either oxidized/plated contemporaneously or oxidized approximately stoichiometrically then plated. The bond strengths in the second alternative were often multiples (e.g., two or three times) of the bond strengths of processes with simultaneous oxidation and electroless plating. The best results are obtained when the charged polymer film is oxidized stoichiometrically, that is, all of the charge is used in the oxidation of the film, prior to any deposition of metal by other means. This effect is observable to proportionately lesser degrees as the amount of oxidation prior to further metallization is less than full stoichiometry. However, the effect is believed to be observable when at least 25% of the oxidation is effected by utilization of the stored charge prior to any other type of metallization. Preferably at least 50% of the charge is utilized in the oxidation process prior to any other type of metallization. More preferably 75% of the charge is so used, and still more preferably 95% or 100% of the stored charge is so used prior to any other form of metallization. The following Examples are not in accordance with the present invention, inasmuch as they do nor disclose the application of masking means to the polymer surface prior to metallization. However they do serve to illustrate the general principles of the preferred metallization process for use in the present invention. Example 1 Generally, all reductions and some oxidations were performed in an oxygen-free inert atmosphere such as nitrogen or argon. Most of the operations were conducted in a glove box under an argon atmosphere. In this example 1 g of K₂Te, obtained from Cerac Pure, Inc., was dissolved in 100 ml of methanol. Approximately 30 minutes was allowed for the dissolution of the salt. A 75 µm (micron) thick strip of an aromatic polyimide (available under the Kapton trademark) was immersed into the solution for about 30 seconds, removed, rinsed in methanol and wiped clean. The resultant deeply green colored polyimide film strip was then ready for metallization. An oxidizing solution of Cu(OCOCH₃)₂·H₂O in methanol (500 mg/500 ml) was prepared. The above prepared reduced green colored polyimide film strip was then immersed for 60 seconds in this oxidizing solution. A brilliant mirror-like reflective copper film was obtained. The copper film was thin (partially transparent when held up to the light) and electrically conductive. Example 2 A reduced green radical anion polyimide strip was prepared as in Example 1. An oxidizing solution of KI in methanol (1 g/25 ml) saturated with CuI was prepared. Again, approximately 30 minutes was allowed for the dissolution of the salts. The reduced polyimide strip was immersed for three minutes in this oxidizing solution. A bright opaque copper film was obtained with an electrical conductivity approaching that of the bulk metal. Example 3 A reduced polyimide strip was prepared as in Example 1. An electroless copper oxidizing solution was prepared using 28.5 g/l CuSO₄·5H₂O plus 12.0 g/l 37% HCHO plus 50 g/l Na₂EDTA plus 20 g/l NaOH in 175 ml/l methanol/water. The reduced polyimide strip was immersed for five minutes in this oxidizing solution in air. A bright copper deposit approximately 0.5 µm (micron) thick with near bulk electrical conductivity was obtained. Example 4 A reduced polyimide strip was prepared as in Example 1. A commercially available (CP-78 Electroless copper, Shipley Co., Newton, MA) electroless copper solution held at a temperature of 43°C was utilized. The reduced polyimide strip was immersed for 5 minutes in this oxidizing solution in air. A well-adhered bright copper layer with bulk electrical conductivity was obtained. Example 5 A copper metallized polyimide strip prepared as in Example 4 was electroplated to a thickness of approximately 25 µm (microns) in a standard acid copper plating bath. Three parallel strips of plater's tape (3 mm wide) were attached spaced at 6 mm intervals on one side of the electroplated strip to protect the underlying copper from a subsequent acid etch. The entire strip was then immersed into a 30% nitric acid solution and the unprotected copper regions were etched away. The plater's tape strips were then removed leaving three well-adhered copper lines on the polyimide strip. Example 6 A reduced polyimide strip was prepared as in Example 1. An electroless nickel solution was prepared using 21 g/l NiCl₂·6H₂O plus 24 g/l NaH₂PO₂·H₂O and 12 g/l NH₂CH₂COONa. The pH of this solution was adjusted to 6.0 with hydrochloric acid. The reduced polyimide was immersed in this oxidizing solution for five minutes at 85°C. A bright nickel deposit with near bulk electrical conductivity was obtained. Example 7 An aqueous solution was prepared using 0.8 g of VOSO₄·2H₂O and 6.1 g of ethylenediaminetetraacetic acid dipotassium salt dihydrate in 150 ml deionized water. Sufficient KOH was added to dissolve the K₂EDTA salt, the final pH being approximately 8-9. This solution was electrolyzed at a mercury pool cathode at -1.4 V versus a Ag/AgCl reference electrode until most of the vanadium had been reduced to the V²⁺ oxidation state as evidenced by a reduction in the amount of current flowing to approximately less than ten percent of the beginning current level. A platinum helix contained in a separate fritted compartment containing aqueous KI solution was used as the counter electrode. A 75 µm (micron) thick strip of an aromatic polyimide (available under the Kapton trademark) was immersed into the solution prepared above for about 30 seconds, removed and wiped dry. The resultant deeply green colored polyimide strip was metallized as in Example 1. Example 8 An aqueous solution was prepared using 1.2g of VOSO₄ and 8.76g of ethylenediaminetetraacetic acid in 300ml of deionized water. Solid tetramethyl ammonium hydroxide was added to solubilize the ingredients and adjust the final pH to between 7 and 10. This solution was electrolyzed at a mercury cathode pool at -1.4 V versus a Ag/AgCl reference electrode to accomplish the reduction of V(IV) to V(II). A platinum helix contained in a separate fritted compartment containing aqueous tetramethyl ammonium ethylenediamine-tetraacetate (0.1M) was used as the counter electrode. A 75 µm (micron) thick strip of an aromatic polyimide (available under the Kapton trademark) was immersed into the solution prepared above for about 60 seconds, removed and rinsed in water. The resultant deeply green colored polyimide strip was metallized as in Example 1. Example 9 An aqueous solution was prepared using 0.4 g VOSO₄·2H₂O plus 1.66 g K₂C₂O₄·H₂O in 100 ml of deionized water. Sufficient KOH was added to adjust the pH to approximately 7. This solution was electrolyzed at a mercury pool cathode at -1.4 V versus a Ag/AgCl reference electrode as described in Example 7. A 75 µm (micron) thick strip of an aromatic polyimide (Kapton®) was immersed into the solution prepared above for about 30 seconds, removed and wiped dry. The resultant deeply green colored polyimide strip was metallized as in Example 4, except that it was performed in the absence of oxygen. Example 10 An aqueous solution was prepared using a 0.4 g VOSO₄·2H₂O plus 1.9 g of nitrilotriacetic acid in 100 ml deionized water. Sufficient KOH was added to dissolve the nitrilotriacetic acid and to raise the pH to approximately 8. This solution was electrolyzed at a starting voltage of -1.4 V and a finishing voltage of -1.9 V versus a Ag/AgCl reference electrode as described in Example 7. The final pH was 8.6. A 75 µm (micron) strip of an aromatic polyimide (Kapton®) was immersed into the solution prepared above for about 30 seconds, removed and wiped dry. The resultant deeply green colored polyimide strip was metallized as in Example 1. Example 11 A reduced polyimide strip was prepared as in Example 1. An oxidizing solution of CoCl₂·6H₂O in N,N-dimethylformamide (1.30 g/100 ml) was prepared. The reduced green colored polyimide was immersed for several minutes in this oxidizing solution. A brilliant, mirror-like reflection cobalt film was obtained. The cobalt film was thin (partially transparent when held up to the light) and electrically conductive. Example 12 An electroless cobalt bath was prepared as described in U.S. Patent No. 3,138,479. The solution was prepared using 25 g/l CoCl₂·6H₂O, 25 g/l NH₄Cl, 50 g/l Na₃C₆H₅O₇·2H₂O, and 10 g/l NaH₂PO₂·H₂O. Ammonium hydroxide was used to adjust the pH to approximately 8.5. The bath was heated to 60°C and the thin cobalt clad polyimide film from Example 11 was immersed in it for several minutes. A cobalt/phosphorous alloy was deposited which has a magnetic coercivity of 450 oersteds. Example 13 An aqueous solution was prepared using 0.4 g VOSO₄·2H₂O plus 3.7 g ethylenediaminetetraacetic acid dihydrate in 100 ml deionized water. Tetramethylammonium hydroxide was added in sufficient quantity to dissolve the Na₂EDTA salt and to raise the initial pH to between 8 and 9. This solution was electrolyzed at -1.4 V versus a Ag/AgCl reference electrode as described in Example 7. The final pH was about 9. A 75 µm (micron) thick strip of an aromatic polyimide (Kapton®) was immersed into the solution prepared above for about 30 seconds, removed and wiped dry. The resultant deeply green colored polyimide strip was metallized as in Example 4, except that it was performed in the absence of oxygen. Example 14 An aqueous solution was prepared using 2 g VOSO₄·2H₂O plus 21 g ethylenediaminetetraacetiC acid dipotassium salt dihydrate in 400 ml deionized water. KOH was added until the final pH was approximately 9 or greater. At least 1000 ml methanol was added to the blue solution, resulting in the formation of a white precipitate. This solution was filtered and the white precipitate discarded. The filtrate was stripped off by vacuum evaporation, leaving a blue solid. The solid was dissolved in a minimum of methanol, filtered and the solvent removed. One gram of the dry blue solid was dissolved in 100 ml methanol which was also 0.1 M in a supporting electrolyte, KI. This solution was electrolyzed at a mercury pool cathode at -1.4 V versus a Ag/AgCl reference electrode as described in Example 7. The final solution was orange-brown and was used to reduce a strip of Kapton® polyimide film by about a 30 second immersion of the film in the solution. A solution of AUBr 3- / 4 was prepared by dissolving 10 mg of AuBr in 20 ml of 0.1 M aqueous KBr. The above reduced polyimide strip was immersed in this solution for a few seconds resulting in the formation of a conductive gold film on the polymer surface. Higher Au¹⁺ concentrations and neutral pH conditions favor and enhance the rate and depth of gold film formation. Example 15 A methanolic solution was prepared using 0.95 g of CoCl₂·6H₂O plus 1.87 g of 2,2'-dipyridyl plus 3.0 g NaI in 200 ml of methanol. This solution was electrolyzed at a mercury pool cathode at -1.3 V versus a Ag/AgCl reference electrode until most of the cobalt had been reduced to the Co⁺ oxidation state as evidenced by a reduction in the amount of current flowing to approximately less than ten percent of the beginning current level. A platinum helix contained in a separate fritted compartment containing methanolic NaI solution was used as the counter electrode. Seventy-five µm (micron) thick strips of an aromatic polyimide (Kapton®) were immersed in the solution prepared above for about 60 seconds, removed and rinsed in methanol and wiped dry. The resultant deeply green colored polyimide strips were metallized as in Example 1 and as in Example 4, except that it was performed in the absence of oxygen. Example 16 Example 15 was repeated except substituting an equivalent concentration of tetramethyl ammonium bromide for the sodium iodide. Example 17 Preferred example (method) for the deposition of copper with good adhesion. A solution of 20 millimolar Co(bpy)₃(NO₃)₂ in methanol was prepared as in Example 15. The solution was made 0.1 molar in tetramethyl ammonium bromide and then in the absence of oxygen reduced to Co(bpy)⁺₃NO₃. Kapton® film was reduced in this solution for 60 seconds and then rinsed in methanol. The reduced film was then immersed in methanolic copper acetate with a concentration of 0.5 mg/ml. The film was allowed to oxidize for 3 minutes and then rinsed in methanol. The film, now containing a thin copper film was immersed in electroless copper for 1 minute as in Example 4. The film was then rinsed in water. Films prepared in this manner, and subsequently electroplated to 2.54 x 10⁻⁵m (1 mil)thickness, yield, through an Institute of Printed Circuitry T peel test, a value of between 0.895 and 1.611 kg/cm (5 and 9 lbs/lineal inch). Example 18 Preferred example for the deposition of well adhered copper films. An aqueous solution 0.02 molar in VOSO₄ and 0.1M in ethylenediamine tetracetic acid was prepared and neutralized by the addition of tetramethylammonium hydroxide. The vanadium complex was then electrochemically reduced to V(II)EDTA²⁻ as in Example 8. The pH of the final reduced solution was adjusted with either tetramethylammonium hydroxide or concentrated H₂SO₄ to 9. Kapton® film was reduced in this solution for 60 seconds and then rinsed in deionized water. The reduced film was then oxidized in dilute aqueous cupric oxalate (0.004M - 0.005M) for 120 seconds until the film was discharged. The copper coated film was then immersed in electroless copper (Example 4) for 1 minute at 49°C (120°F). Films prepared in this manner were electroplated to 2.54 x 10⁻⁵m (1 mil) copper thickness. The films were then etched as in Example 5 and tested for adhesion by a standard IPC T peel test. Adhesion values in excess of 1.074 kg/cm (6 lbs/linear inch) were obtained. Example 19 Seventy-five µm (micron) thick aromatic polyimide (Kapton®) was coated with Dynachem® DCR 3118 negative photoresist and imaged with a phototool to provide a circuitry pattern composed of 2.5 x 10⁻⁵ meter resist lines and 10.2 x 10⁻⁵ meter spaces. The imaged film was reduced and metallized as in Example 4. The resulting 10.2 x 10⁻⁵ meter copper lines with 2.5 x 10⁻⁵ meter spacings were clear and well resolved. Example 20 Seventy-five µm (micron) thick aromatic polyimide (Kapton®) was coated with DuPont Chromacheck® Negative Working Color Overlay Proofing Film and imaged with a phototool to provide a circuitry pattern composed of 12.7 x 10⁻⁵ meter lines and 12.7 x 10⁻⁵ meter spaces. The imaged film was reduced as in Example 15, rinsed in methanol and then immersed for 30 seconds in methanolic copper acetate with a concentration of 0.5 mg/ml. The film, now containing a thin copper film in the exposed areas was immersed in electroless copper for 2 minutes as in Example 4. The resulting 12.7 x 10⁻⁵ meter copper lines with 12.7 x 10⁻⁵ meter spacings were clear and well resolved.

Claims: A process for image-wise deposition of metal onto at least a portion of a polymer surface comprising the steps of: (1) injecting a stored charge into a polymer surface having electroactive sites, and (2) reducing metal ions with the stored charge to form metal in or on the polymer surface in which a masking means is applied to the polymer surface after injecting the stored charge but before reduction of the metal ions. A process as claimed in Claim 1 including the steps of: (1) contacting the polymer surface with a first solution in which at least 10 molar percent of all negative charge intercalation ions within that solution are simple or complex negative charge intercalation ions, thereby reducing the polymer of the polymer surface without substantial plating of metal onto the polymer surface and (2) contacting the reduced polymer surface with a second solution having reducible metal cations therein so that the reduced polymer reduces the metallic ion to form a metal film on the polymer surface or metal particles within the polymer surface. A process as claimed in Claim 2 in which the simple or complex negative charge intercalation ions comprise at least 60 molar percent of all negative charge intercalation ions in the first solution. A process as claimed in Claim 3 in which the simple or complex negative charge intercalation ions comprise at least 90 molar percent of all negative charge intercalation ions in the first solution. A process as claimed in Claim 4 in which the simple or complex negative charge intercalation ions comprise 100 molar percent of the negative charge intercalation ions in the first solution. A process as claimed in any preceding claim in which the ions are selected from Te²⁻ ions, and Co(I) and V(II) complexes. A process as claimed in any preceding claim in which the polymer contains pyromellitimide electroactive sites. A process as claimed in Claim 2 in which the metal is present as finely dispersed particles within the surface of the polymer layer and at least 10% by weight of the metal particles penetrate at least 2nm into the polymer and no more than 25% by weight of the metal particles penetrate more than 400nm into the polymer. A process as claimed in Claim 8 in which 25% by weight of the metal particles do not penetrate more than 40nm into the polymer. A process as claimed in Claim 8 in which at least 40% of the metal is present below the polymer surface and at least some of the metal extends to a depth of at least 0.25µm below the polymer surface with less than 2% of the metal extending to a depth greater than 2µm. A process as claimed in Claim 10 in which at least 50% of the metal is below the polymer surface and at least some of the metal is present at a depth of 0.3µm. A process as claimed in any one of Claims 8 to 11 in which the metal is a transition metal. A process as claimed in any preceding claim in which after at least 50% of the reduced polymer is oxidized by the metallic ion to form metal, a second metallization process is begun.

Inventors: KRAUSE LARRY J, RIDER JACK A, KRAUSE, LARRY J., RIDER, JACK A.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1987
Main classification: C23C18
IPC codes: C23C14, C23C18