JPH0247556B2 - DOETSUCHINGUNOHOHOOYOBIETSUCHINGUKOZOTAI - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates generally to copper etching, and more particularly to copper etching methods and etching structures for removing copper during the manufacturing process of printed wiring boards. In conventional printed wiring board manufacturing, copper foil is removed from a layered board using a liquid etchant. In this method, the copper is removed by immersing the substrate in an etchant solution until the copper is dissolved, or by spraying the etchant onto the substrate. In either case, the etchant is a reactive and corrosive liquid that is difficult to handle, and the reaction liquid and reaction products are usually difficult to dispose of properly. Furthermore, there is a limit to the minimum conductor width that can be formed with liquid etchants because liquid etchants create undesirable undercuts in conductors formed on the substrate. It is known that copper reacts with gaseous nitrogen dioxide in the relationship shown in the chemical formula below. Cu+4NO ₂ →Cu(NO ₃ ) ₂ +2NO It is also well known that the oxidation effect in this reaction, ie the removal of copper, can be increased or accelerated by the use of electrolytic organic catalysts. The mechanism of this chemical reaction is thought to be a reaction between the nitrosonium ion NO ⁺ and metallic copper, that is, Cu + NO ⁺ →Cu ⁺ + NO, and the reaction rate is highest when the number of NO ⁺ is maximum. Although nitrogen dioxide itself is gaseous, the catalysts used to promote the reaction between nitrogen dioxide and copper are highly volatile and flammable liquids, and methods using nitrogen dioxide and these catalysts are essentially is wet etching. To the applicant's knowledge, such methods have not been used in the manufacture of printed wiring boards;
It shares many of the same limitations and drawbacks as liquid etching. It is also known to react copper with chlorine in the presence of water, but this method is considered slow and unsuitable for copper removal in printed wiring board manufacturing. The etchants used to remove copper from conventional printed wiring boards are water-based processes that involve processing in alkaline solutions or in acidic solutions. In these etching systems, copper exists in one of three states: That is, metallic copper Cu
(O) or as cuprous Cu() or cupric Cu(). The most commonly used etching agents have different components that control the relative stability of copper oxides, Cu() and Cu(), which complex with and solvate the copper oxides. Conventional etching methods used to dissolve metallic copper on printed wiring boards are based on the oxidation of metallic copper with cupric Cu() as the basic chemical process of copper dissolution. In these methods,
Two types of cuprous Cu() are produced by the reaction. In these methods, the oxidant, cupric Cu(), and the product, cuprous Cu(), must be sufficiently soluble to facilitate their removal from the printed wiring board surface. . Cu() and Cu() in aqueous solution
Since it is difficult to maintain both in reasonable amounts, these methods require precise adjustment of parameters such as temperature, pH, concentration of constituents, and reaction time. However, this adjustment is often difficult, and a common problem with these methods is the deposition of insoluble surface slag containing Cu() species. Such surface slag is difficult to remove and may cause problems in subsequent processes or result in corrosion in the finished product. Another limitation of conventional copper removal etchants is that the reaction with metallic copper is isotropic, meaning that the etching action of the copper film around the resist pattern occurs equally in the desired vertical direction and in the undesired horizontal direction. It happens with speed. As a result, a small amount of copper is removed from the underside of the resist, causing distortion in circuit elements formed on the substrate. In extreme cases, such undercuts can lead to problems such as short circuits and raised traces, placing a practical limit on circuit element size and, therefore, circuit density. These limitations become critical when substrate density becomes much lower than circuit element (semiconductor) density. Conventional etchants used to remove copper use aggressively corrosive solutions, which impose material constraints on the construction of processing equipment and require the use of metal parts made of relatively expensive materials such as titanium. Furthermore, as mentioned above,
There is a problem with the disposal of large amounts of aqueous melt containing copper. Even if the molten copper precipitates out to relatively low concentrations, it often contains other materials that require further treatment before disposal. A general object of the present invention is to provide a new and improved method and etching structure for etching copper in printed wiring board manufacturing and other applications. Another object of the present invention is to provide an etching method and an etched structure of the above character which eliminates the need for the use of reactive and corrosive etchants in wet etching. It is another object of the present invention to provide an etching method and etching structure of the above-mentioned character which does not produce dilute copper effluent that requires considerable treatment before disposal. Another object of the present invention is to provide an etching method and an etched structure having the above-mentioned characteristics in which the copper oxide is in a solid state which is convenient for recovery. These and other objects are achieved by the dry etching method of the present invention, in which copper is contacted with a gaseous oxidizing agent in the presence of a catalyst that promotes the reaction between the copper and the oxidizing agent. In one embodiment of the invention, the catalyst is supported in a non-liquid medium, which also acts as a receptor for the copper oxide produced by the reaction. In other embodiments, the non-liquid medium has a layered structure including a first layer carrying a catalyst and a second layer receiving copper oxide. In the dry etching according to the present invention, the copper to be removed is contacted with a gaseous oxidizing agent such as nitrogen dioxide in the presence of an organic catalyst, and the copper oxide produced by the reaction is substantially removed as a solid. The reaction can also be carried out at room temperature and atmospheric pressure. The main role of this catalyst is to promote the dissociation of the NO ₂ dimer to produce nitrosonium/ion NO ⁺ which rapidly reacts with the copper to be removed. The dissociation of NO ₂ dimer is shown by the following chemical formula. (NO ₂ ) ₂ →NO ⁺ + (NO ₃ ) ^-This reaction occurs at a faster rate in the presence of a medium of relatively high dielectric constant, since it involves the separation of species with opposite charges. However, it was discovered that a large amount of this dielectric medium was not required, and that the reaction proceeded at a reasonable rate simply by covering the surface of the copper with a thin film of an appropriate organic substance. In general, organic compounds useful as catalysts for promoting the reaction between copper and nitrogen dioxide either have electron donor properties that promote ionic dissociation of the NO ₂ dimer, or have high dielectric properties that sustain this ionic dissociation. Should have. Table 1 shows examples of such polar organic compounds.

[Table] Acetate

[Table] Among these compounds, adiponitrile, ethyl acetate, ethylene glycol diacetate, dimethyl succinate, dimethyl glutarate, dimethyl adipate, dioxane, acetic acid, acetic anhydride,
Particularly preferred are ethyl cyanoacetate, diethylene glycol monoethyl ether acetate, and mixtures thereof. Relatively high boiling points (e.g. 200
C or higher) and are easier and safer to handle than compounds with lower boiling points. Polymers with catalytically active functional groups can also be used in the dry etching. Polymers may be preferred because they allow easier control over the spatial distribution of the catalyst. Table 2 below gives examples of such polymers. Table 2 Polymer Catalyst Nitrile Polymer for Accelerating Copper Etching Reaction Poly(acrylonitrile) Cyanoethyl cellulose (nitrile and ether groups) Ester poly(vinyl acetate) Poly(ethyl acrylate) Poly(methyl methacrylate) Poly(ethylene terephthalate) Cellulose acetate (esters and ether groups) Ethers poly(ethylene oxide) poly(methyl vinyl ether) carboxylic acids poly(acrylic acid) carboxylic anhydrides maleic anhydride-butadiene copolymer amides poly(N-vinylpyrrolidone) polyacrylamide hydroxyl polymer poly( Ethylene glycol) Poly(vinyl alcohol) Cellulose When using a resist material where copper removal should be prevented, it is preferable that the copper foil can be etched without extending laterally or undercutting the resist. In embodiments, exposed copper and resist material surfaces are covered with an aqueous gel comprising a polymer such as polyacrylamide or poly(acrylic acid) and water and containing a surfactant. NO ₂
Anisotropic etching occurs when exposed to Suitable surfactants include Dupont's Zonyl FSC, Zonyl FSK, Zonyl FSN and 3M Fluorad FC-135.
Its effective concentration is 0.001-0.01% by weight, preferably
It is 0.002-0.003% by weight. A dry thin film type organic resist such as DuPont's Riston or a wet thin film type organic resist such as Kodak 752 may be used. Addition of a small amount of phosphoric acid and a trace amount of an organophosphate such as Zonyl ESP detergent to the above mixture will anisotropically etch copper protected by lead-tin solder without appreciably damaging the solder resist. becomes possible. A suitable amount of phosphoric acid is 5-25% by weight, preferably 8-12% by weight, and the amount of phosphate is similar to the concentration of the surfactant, i.e. 0.001-0.01% by weight, preferably 0.002-0.003%. Weight%. Polymer catalysts are used in a variety of ways. For example, "solid poly(ethylene oxide)" becomes a catalytically active material for the Cu-NO ₂ reaction when heated to its melting point.
The copper surface may be covered with a dry (pure) thin film of . The reaction occurs only in the molten polymer, and using this method, it is possible to form a pattern on copper using only an etching process without the need for a resist process. Other useful applications use polymer gels as supports for more fluid catalyst compounds. For example, a gel of just the right viscosity can be made from a solution of cyanoethyl cellulose (7% by weight) in ethyl cyanoacetate. In this case, the polymer gel serves to spatially immobilize the highly fluid catalyst, essentially behaving like a container without walls. The low polymer content of the gel increases the rate of diffusion of the reactants through the catalytic medium while maintaining a fairly strong structure. The reaction between nitrogen dioxide and copper can also be carried out using a polymer solution in an active catalyst solution. For example, adiponitrile with poly(N-vinyl pyrrolidone)
The copper surface may be coated with a viscous thin film formed from a combination of melted nitrogen dioxide and copper, which film is sufficiently permeable at high temperatures to allow sufficient reaction between nitrogen dioxide and the copper. Ethyl vinyl acetate (20% by weight)
Acetate solutions have a similar effect, but the higher proportion of polymer results in some suppression of the diffusion of reactants. It has been found that the reaction method can be simplified by introducing a predetermined amount of liquid oxidizing agent into the polymer-containing medium before coating it on the copper substrate. A suitable oxidizing agent for this purpose is N ₂ O ₄ , a liquid dimer of nitrogen dioxide. In one example of this method using a liquid oxidizer, approximately 5
% cellulose, 21.6% _N2O4 , 30.7% _acetonitrile , and 42.8% Dupont DBE (wt%). This mixture is applied as a thin film onto the copper surface and maintained in contact with the copper until it melts. After the reaction occurs, the thin film and the molten copper are removed from the substrate. Using 14 grams of this mixture, it takes approximately 5 minutes at room temperature to etch a 3 x 4 inch (7.62 x 10.16 cm) printed wiring board containing 1/2 oz (14.17 g) of copper coating. When the copper surface was partially covered with Kodatsu 752 micro-resist, almost all of the exposed copper was removed in about 5 minutes, with almost no undercuts in the copper circuitry beneath the resist. . DuPont DBE is a mixture of dimethyl esters of adipic acid, glutaric acid and succinic acid. In another example, 9.19 grams of a 10% cellulose acetate solution in acetonitrile, 0.36 grams of Henkel Aliquat 336
(surfactant) and 2.90 grams _{of N2O4} were mixed. This mixture was applied in a thin film onto a copper substrate provided with a photoresist. When the solution was removed after about 7 1/2 minutes of contact, substantially all of the copper not covered by the resist was removed. Although only two specific examples using liquid oxidizers are mentioned, it should be understood that liquid oxidizers can also be applied to other methods using gaseous oxidizers described above. It has been found that the catalytic amount of solvent required for the Cu- _NO2 reaction can also be supported on materials such as non-woven fabrics or certain woven fabrics. These materials also serve as catalyst supports and receptors for copper nitride and other reaction products. The catalytic solvent may be impregnated into such a material, or it may be supported on the surface of the material in the form of a thin film. The solvent may exist as a solid at room temperature;
The temperature may be raised to its melting point before starting the Cu- _NO2 reaction. Supports such as textiles can also carry patterns that define circuits formed in the etching process. Additionally, catalytically active compounds can be supported on solid supports such as the high surface area networks provided by nonwovens. In this method, the catalyst is supported on a carrier that immobilizes the catalyst on the copper surface, just like the polymer system described above. However, due to its porous structure, this nonwoven fabric has little resistance to the passage of gaseous reactants and reaction products. Examples of nonwoven fabrics suitable for use in this method are shown in Table 3 below.

【table】

[Table] The use of the fabric carrier described above also has the advantage that it is essentially a dry process. In one example, an adiponitrile catalyst is supported on Perron 954 nonwoven polyester;
This catalyst-supported cloth carrier is placed in close contact with the copper foil,
The entire structure was exposed to nitrogen dioxide gas. The reaction between nitrogen dioxide and copper began immediately, and during the reaction the copper nitride product was adsorbed onto the fabric carrier. The area where the copper was removed matched the contour of the polyester carrier. When a nonwoven fabric is used as a carrier for a solvent with catalytic action,
It was found that 1 oz/ft square (305 g/m ² ) of copper was removed in 20 minutes at room temperature in gaseous nitrogen dioxide. Almost all of the copper oxide was removed by the cloth carrier. The accompanying drawings illustrate a layered structure particularly suitable for applying a catalyst to a copper surface and removing copper oxide.
The structure consists of a carrier layer 11 and a receiving layer 12 as its backing structure. In one example, the carrier layer may consist of an ultra-thin hydrophobic layer of non-woven polypropylene and the receiving layer may consist of a thick hydrophilic porous material such as a mat of non-woven cellulose. For example, when nitrogen dioxide is the oxidizing agent and the support layer contains a polar organic catalyst, the cellulose mat acts as a receptor for the copper nitride produced in the reaction. Experiments have shown that none of these layers impedes the permeation of nitrogen dioxide or the reaction product copper nitride. If desired, a suitable phase transfer catalyst may be used to facilitate passage of the copper nitride through the hydrophobic film. In the actual process, the illustrated layered structure is placed on the copper foil 13 of the printed wiring board 14, and the support layer 11 is brought into close contact with the surface of the copper foil. The entire structure is then exposed to nitrogen dioxide or other gaseous oxidizing agent, and after a suitable reaction time (e.g., about 15 minutes or less) the layer is stripped from the copper surface, leaving behind the desired copper pattern and the copper oxide becomes a dry solid. removed in the state. The pattern to be etched may be defined by forming a suitable organic coating on the copper foil or on the surface of the support layer in contact with the copper. Since copper and nitrogen dioxide do not react in the absence of a catalyst, conventional photoresists may be used in this method. It should be noted that the reaction will only occur in the presence of a catalyst. Therefore, there is almost no undercut in the remaining copper. Examples of other gaseous oxidants suitable for the process of the invention are listed in Table 4 below. Such gases include nitrogen oxides, nitrogen oxyhalides, halogens, and mixtures thereof. Table 4 Gaseous oxidants Nitrogen oxides Nitrogen tetroxide (nitrogen dioxide dimer) Nitrogen sesquioxide (+oxygen) Nitrogen dioxide Nitrogen oxide (+oxygen) Nitrogen oxyhalides Nitrosylhalogen chloride Chlorine Silium Mixture of the above substances It was previously thought that the direct reaction between chlorine and copper was so slow that it was not practical for making printed wiring boards, but in the presence of an appropriate receptor and catalyst, the reaction proceeds at a practical rate. It has been found. Nitrogen oxide is suitable as a catalyst when gaseous chlorine is used, as are some of the other gaseous oxidants in Table 4. Water or other compounds of Tables 1 and 2 may be used as co-catalysts, and both the catalyst and co-catalysts may be supported in an essentially dry state on a cellulosic material or a receptor of a Table 3 material. The reaction between the gaseous chlorine and the copper occurs only where the receptor is in contact with the copper, providing a convenient method for forming patterns on the copper. Among the catalysts in Tables 1 and 2, hydroxyl compounds are particularly suitable as auxiliary catalysts when chlorine is used as an oxidizing agent. It has been found that etching processes using gaseous chlorine as the oxidizing agent proceed much faster in the presence of a cocatalyst than in the absence of a cocatalyst. For example, in one example, 10 milligrams of copper was removed from a copper circuit board using a fiberglass receptor containing only chlorine and water as catalysts.
Blowing nitrogen oxide into the gaseous oxidant stream removed 239 milligrams of copper from a similar substrate in 30 minutes. In the above example, the nitrogen oxide as a catalyst is gaseous NO
However, it may also be used in the form of nitrogen-containing compounds such as nitrogen dioxide or nitrosyl chloride listed in Table 4. Alternatively, it may be supported on the receptor as a salt such as sodium nitrite, in which case water or other liquid may be used as a phase transfer agent for the catalyst. A gaseous chlorine oxidizing agent can also be used in the illustrated laminated structure of an absorbent layer and a receptor layer. In this embodiment, the hydrophobic polymer layer 11 comprises a membrane of a hydrophobic phase transfer catalyst, such as Henkel Alicotto 336;
A fired layer 12 of cellulose was generated by the reaction.
Acts as a receptor for CuCl ₂ . This phase transfer catalyst contains a Cl ^-carrier that passes the CuCl ₂ product as a (CuCl ₃ ) ^-complex through a hydrophobic layer, thereby separating water from copper in the cellulose receptor and providing a favorable zone for reaction. The etching area can be well controlled by limiting the Other types of solid materials useful as catalyst supports and as receptors for reaction products are relatively high surface area micronized solids and such single solids. An essentially inert, high surface area, finely divided solid material can also be coated with adsorbent catalyst promoters and is an effective medium for carrying out a substantially dry copper etching process. Such suitable solid particulates include silica, alumina, powdered cellulose,
These are silicic acid (SiO ₂ ^* nH ₂ O), celite (diatomaceous earth, neutral silica) and aluminum oxide. Solid fine particles have a high ability to support organic liquids, and even when they finally support approximately 50% by weight of liquid, they appear to be dry, fluid powders. The mixture of particulate material and organic liquid (which is the solvent and catalyst for the product) has an open pore structure that allows gas to freely permeate. These compositions can directly act as catalysts for the Cu-NO ₂ reaction while simultaneously reducing the production of Cu.
It has been found that adsorption and removal of (NO ₃ ) ₂ can be carried out. In the etching operation, a dry powder adsorbed with a catalyst is applied to the surface of the copper to be etched prior to contact with the oxidizing agent gas. The reaction begins immediately upon contact with the oxidant gas, and the powder is dyed with the characteristic color of the reaction product. For example, when NO ₂ is used as the oxidizing agent, the powder is stained with the characteristic blue color of the reaction product Cu(NO ₃ ) ₂ . When using gaseous chlorine and water as a catalyst and solvent, the color of the powder is the bright green of the reaction product CuCl ₂ ^* H ₂ O. It is particularly advantageous that the powdered carrier is freely permeable to the NO by-product of the Cu--NO ₂ reaction. The use of particulate receptors is particularly effective in preventing the problem of "tenting", ie, the formation of foam that separates the copper and catalyst and interferes with the oxidation reaction. If a dry powder carrier is used, a fluidized bed reactant can be used to etch the copper. A fluidized bed is made of loosely packed solid particles in which an upward airflow is maintained to allow free convection of the solid particles. In such systems, particles have liquid-like properties,
For example, it flows under the influence of gravity, is freely displaced around the inserted solid material, and has properties such as rapid heat exchange between the solid material and the fluid particles. In the etching method of the present invention, the fluidized particles contain a catalyst promoter and act as receptors for the reaction products. For example, when NO ₂ is used as an oxidizing agent,
A mixture of NO ₂ and O ₂ can be used as the fluidizing gas, and this mixture regenerates NO ₂ from the NO by-product of the reaction. After the reaction, the particles that have received the reaction product can also be heated to recover the organic promoter by slow heating, and then heated at a high temperature (200~200℃).
300℃), copper nitride is thermally decomposed and the reactant NO ₂
can be recovered. Cu (NO ₃ ) ₂ + heat → CuO + (1/2) O ₂ + _2NO Particulate matter that has accepted copper oxide can be used repeatedly until it is completely covered with CuO, after which it can be sold for its copper content. It has the value of Alternatively, CuO may be used as the particulate material, in which case subsequent solvent recovery and nitrate decomposition leaves fairly pure CuO. If desired, solvent recovery and nitrate decomposition can be carried out continuously by circulating the fluidized particles through another reactant. Support/receivers using dry particulate carriers can also be used in vertical spray etching systems that traditionally employ high temperature, corrosive, reactive liquid etchants. Even in this case, the dry etching method offers significant advantages. A serious problem with vertical liquid spray etching methods is that the etching is not uniform because the substrate being etched is held vertically. The sprayed etching agent flows along the surface of the plate after it hits the plate, so that the lower part of the plate is etched more rapidly than the upper part. Furthermore, before the etching ends at the upper part, undercutting becomes significant at the lower part. Similarly, the undercut is greater above the horizontal line than below the horizontal line due to the accumulation of etchant along the top edge of the foil. The use of solid particles instead of liquid etchant in spray etching also effectively eliminates the problem of etchant flowing over the surface of the board. Furthermore, the particles bounce off the surface of the plate rather than sticking to it or flowing over its surface, allowing for more precise and controllable anisotropic etching of the copper. As in the case of fluidized bed reactants
Favorable results are obtained using CuO as a solid particle. The present invention greatly simplifies copper etching and provides a system with more accurate circuit elements, simplified process control, easy by-product disposal, and extremely low cost. The method of the present invention differs from the prior art in the essential chemical process involved in dissolving copper. When _NO2 or _Cl2 is used as the oxidizing agent, the actual active species participating in Cu(0) oxidation is the nitrosonium ion NO ⁺ . The reaction between this active species and Cu(0) is an extremely fast electron transfer from Cu(0) to NO ⁺ and generates copper oxide and nitrogen oxide NO. The latter is repeatedly used to react with O ₂ during the NO ₂ reaction process or Cl ₂ during the chlorine reaction process. It should be noted that the initially generated Cu() is unstable under the reaction conditions according to the process of the invention and is very rapidly converted to the final product Cu(). The method therefore facilitates cleaning of the circuit board for subsequent processing. An important feature of the present invention is that neither _NO2 nor _Cl2 readily reacts directly with metallic copper. Catalysts are used to obtain proper reaction rates and the reaction process can be easily controlled. Spatial control of the chemistry of the copper etch process is achieved either chemically or physically. As can be seen from the foregoing description, new and improved copper etching methods and structures in the manufacture of printed wiring boards are provided. Although only certain preferred embodiments have been described in detail, modifications can be made within the technical scope of the present invention, as will be apparent to those skilled in the art.

[Brief explanation of drawings]

The drawing is an isometric diagram showing a specific example of a structure comprising a catalyst support and a copper oxide receptor according to the present invention, with a portion thereof cut away. 11...Supporting layer, 12...Receiving layer, 13...Copper foil, 14...Printed wiring board.

Claims (1)

[Claims] 1. A method of dry copper etching, comprising a step of exposing copper to a gaseous oxidizing agent in the presence of a catalyst that promotes the reaction between the copper and the oxidizing agent, and using the reaction product copper oxide as a receptor. and removing the copper oxide from the copper by the acceptor. 2. The method of claim 1, wherein the gaseous oxidizing agent is selected from the group consisting of nitrogen oxides, nitrogen oxyhalides, halogens, and mixtures thereof. 3. The method of claim 1, wherein the catalyst comprises a surfactant-containing polymer. 4. The method of claim 3, wherein said polymer is selected from the group consisting of polyacrylamide and poly(acrylic acid). 5. The method of claim 1, wherein the oxidizing agent is nitrogen dioxide and the catalyst is a polar organic solvent. 6. Claim 1, wherein the catalyst is supported in a non-liquid medium in close proximity to the copper, and the copper oxide species produced by the reaction are received by and removed from the copper by the non-liquid medium. Method described. 7. The medium includes a solid support having a first layer and a second layer of a member that is permeable to a gaseous oxidizing agent, a catalyst is supported on the first layer, and the first layer is in close contact with the surface of the copper to be etched. 7. The method of claim 6, wherein the second layer is spaced apart from the surface of the copper, and the copper oxide species passes through the first layer and is received by the second layer. . 8. A structure used for copper etching using a gaseous oxidizing agent, comprising a first layer of a member that allows the gaseous oxidizing agent to pass through, and a catalyst supported on the first layer that promotes the reaction between copper and the oxidizing agent. , a second layer of a member permeable to the gaseous oxidant, which is placed in close proximity to the first layer and collects and retains the copper oxide produced by the reaction;
A structure comprising layers. 9. The structure according to claim 8, wherein the first layer includes a hydrophobic member and the second layer includes a hydrophilic member. 10. The structure of claim 8, wherein the first layer comprises a polymeric fabric. 11. The structure of claim 8, wherein the second layer comprises a cellulose mat.