Wet etching of cured polyimide

Abstract: Polyimide is etched by contacting the polyimide with an aqueous solution of a metal hydroxide followed by contact with an acid followed by contact with an aqueous solution of a metal hydroxide. Etching of chemically cured polyimide can be enhanced by employing a presoaking in hot water. Also, partially etched chemically cured polyimide is removed with a concentrated acid solution. In preparing a metal coated polyimide structure for subsequent gold plating, two flash etching steps with the polyimide etch between are employed after developing the photoresist.

Description: The present invention is concerned with wet etching a fully, or at least substantially fully thermally or chemically cured polyimide layer. The selective etching of polyimide films to provide openings or vias therein is important for various uses of polyimide. For instance, in the packaging of semiconductor chips, substrates are often provided with polyimide layers. In a number of these situations, it is necessary to form vias in the polyimide layer to allow for electrical connections to be made between the different layers of metallurgy. In order that the interconnection be as accurate as possible, it is necessary that the polyimide films be fully cured to avoid distortion of the desired polyimide pattern and prevent attack from other wet processing chemicals. For instance, in the formation of multi-layer substrates for mounting chips, one configuration employs an insulating substrate of ceramic material onto which is deposited a pattern of metallic conductors. Usually, the conductors are three layers of metal being a layer of chromium or nickel, followed by a layer of copper, followed by an overlayer of chromium or nickel. On top of the metallized ceramic substrate is placed a film or layer of polyimide; on top of the polyimide a second layer of pattern conductors is provided. The conductors are generally three layers of metal being chromium or nickel, followed by copper, followed by chromium or nickel. It is necessary to electrically contact some of the conductors in the upper or second layer of metallization to some of the conductors on the lower or first layer of metallization. In order to do so, the polyimide must be selectively etched to form the desired vias therein to allow for metal connection between the upper and lower levels of metallization. Another use would be as a dielectric and/or circuit carrier for flexible circuits. This would involve, for example, spray coating or roller coating polyamic acid onto a sheet of metal (such as stainless steel or aluminum). The film is then thermally cured or imidized, resulting in a film which is fully or substantially fully thermally cured. The metal which the polyimide is on can be imaged, removed, or maintained. On top of the polyimide, three layers of metal are deposited such as by either evaporation or sputtering. The conductors are chromium or nickel, followed by a layer of copper, followed by a layer of chromium or nickel. By means of photolithographic operations, this metal is imaged into circuits. Depending on the use of the circuit, the polyimide may or may not be imaged either before or after the formation of the circuit. One technique for wet etching fully or substantially fully cured polyimide is using either hydrazine hydrate or ethylene diamine. Both are extremely dangerous chemicals and are avoided by industry wherever possible. Ethylene diamine is highly carcinogenic and hydrazine hydrate is highly unstable and extremely explosive. If used, both would have tools which are extremely expensive because of safety concerns. Moderate or slightly thermally cured polyimide (significant amounts of polyamic acid remain) can be etched by using a metallic hydroxide. Etch time is dependent on etchant temperature and concentration, the films degree of cure (percent imidization), and film thickness. In addition, chemically cured polyimide can be etched using relatively high concentrated hot potassium hydroxide such as about 8 to about 15 molar. Furthermore, the by-products formed by the etching are somewhat difficult to remove. Accordingly, methods to etch polyimides and especially fully or at least substantially fully cured polyimides have not been entirely satisfactory and could stand significant improvement. It is an object of the present invention to provide a process for etching of fully cured or substantially fully cured polyimide that is relatively fast without requiring dangerous or explosive chemicals. It is still a further object of the invention to provide processes for etching polyimide with improved uniformity. These objects are achieved by processes as disclosed in claim 1. A quick water rinse can be used if metal hydroxides and an acid are alternatingly applied to minimize the dragout of the chemicals from one bath to another which could cause the baths to neutralize each other and significantly decrease the bathlife. In addition, the etching processing does not adversely affect any metallic circuitry that might be present along with the photoresist material that might be employed to pattern the polyimide to provide the desired selective etching pattern for the polyimide. In particular, the present invention is especially suitable for forming vias in thermally or chemically cured polyimide layers that separate metallic layers from each other to allow for selective metal connection between the separated metallic layers. Moreover, the present invention is useful to image polyimide which is used as a circuit carrier. Other advantageous embodiments of the process of claim 1 are disclosed in the subclaims. The invention will become more apparent from the following detailed description. The etching process of the present invention is concerned with etching of fully or at least substantially fully cured polyimide. In particular, the fully or at least substantially fully cured polyimides etched pursuant to the present invention are those having a significant amount of ring closure from chemical and/or preferably from subsequent thermal treatment. In the preferred aspects of the present invention, the cured polyimide has about 100 % imidization content. The polyimides treated pursuant to the present invention can be the chemically cured or the thermally cured polyimides. The difference between the thermally cured polyimide and chemically cured polyimide is the structure of the film. The heat treatment of the polyimide "packs" the polymer chains very closely together. This packing gives the thermally cured polyimide very good mechanical properties. This film is quite impervious to most chemical attacks. To be wet etched, a very aggressive process is required. Chemically cured polyimides (such as Kaptons®) structure is different since the packing of the film is not nearly as dense as thermally cured polyimides. Therefore, the chemically cured polyimides can be etched with chemicals that are less aggressive than those needed or desired for the thermally cured polyimides. The polyimides that can be etched in accordance with the present invention include unmodified polyimides, as well as modified polyimides such as polyester imides, polyamide-imide-esters, polyamide-imides, polysiloxane-imides, as well as other mixed polyimides. Such are well-known in the prior art and need not be described in any great detail. Generally, the polyimides include the following recurring unit: where n is an integer representing the number of repeating units to provide a molecular weight usually about 10,000 to about 100,000. R is at least one tetravalent organic radical selected from the group consisting of: R₂ being selected from the group consisting of divalent aliphatic hydrocarbon radicals having from 1 to 4 carbon atoms and carbonyl, oxy, sulfo, hexafluoroisopropylidene and sulfonyl radicals and in which R₁ is at least one divalent radical selected from the group consisting of: in which R₃ is a divalent organic radical selected from the group consisting of R₂, silico, and amino radicals. Polymers containing two or more of the R and/or R₁ radicals, especially multiple series of R₁ containing amido radicals, can be used. Commercially available polyimide precursors (polyamic acid) and particularly various polyimide precursors from DuPont are available under the trade designation Pyralin®. This polyimide precursor comes in many grades, including those available Pyralin® polyimide precursors from DuPont under the further trade designations PI-2555, PI-2545, PI-2560, PI-5878, PIH-61454, and PI-2540. A number of these are Pyromelletic Dianhydride-Oxydianaline (PMDA-ODA) polyimide precursors. Commercially available chemically cured polyimides are various polyimides from DuPont and available under the trade designation Kapton®, including H-Kapton®, V-Kapton®, HN-Kapton®, and VN-Kapton®, which are all chemically cured polyimides. The chemically cured polyimides are generally cured with acetic anhydride and pyridine. In order to obtain the thermally cured polyimides etched pursuant to the present invention, the polyimide precursors are subjected to elevated temperatures of at least 250°C, preferably at least 300°C, and most preferably at least 350°C. The maximum temperature must not be greater than the breakdown temperature of the polyimide and depending upon the specific polyimide can be up to about 600°C. For many polyimides it is preferred that the temperature not be greater than 485°C. The thermal cure is usually for at least 15 minutes and preferably at least 30 minutes. It is understood that the present invention is concerned with etching fully or at least substantially fully cured polyimides as distinguished from partially cured polyimides that include only about 20% to 70% imidization. The first step of the process of the present invention is to contact the preselected areas of the polyimide to be etched with an aqueous solution of a metal hydroxide and particularly an aqueous solution of an alkali metal hydroxide such as potassium hydroxide or sodium hydroxide or mixtures thereof. The concentration of the hydroxide is usually at least 4 molar and preferably 4 to 15 molar. In addition, in the preferred aspects of the present invention, up to 15% by volume and preferably 3% to 15% by volume of N-methyl-2-pyrrolidone is employed along with the metal hydroxide. The N-methyl-2-pyrrolidone enhances the speed of the etching process and the sharpness of the etched image. The etching with the hydroxide bath is usually carried out from 1 to 5 minutes and preferably 2 to 3 minutes (depending upon the thickness of the film to be etched). The bath is generally at a temperature of 60°C to 100°C. Next, the polyimide is contacted with a strong, inorganic and/or organic acid such as hydrochloric acid, sulfuric acid, or acetic acid. When an aqueous solution of the acid is employed, usually the concentration of the acid is at least 1 molar, preferably at least 3 molar, and most preferably 3 to 8 molar. The treatment with the acid is usually carried out in 1 to 5 minutes at temperatures of about room temperature or above. The polyimide is then contacted with an aqueous hydroxide solution like the first hydroxide bath for another 1 to 5 minutes and preferably 2 minutes. This is done to complete the etching process. In between the base and acid baths, it is desirable to have a quick water rinse to minimize bath neutralization due to chemical dragout. In addition, if desired, the polyimide after the etching can then be rinsed with hot water such as water at 95°C for 1 to 5 minutes and preferably for 2 minutes. This is then followed by a dilute acid bath to neutralize any residual base on the surfaces of the structure. This is then followed by another water rinse. By following the etching procedure of the present invention, about 10 µm of fully and substantially fully thermally cured polyimide can be etched in the total of 5 to 6 minutes. The above process is most advantageous for etching the thermally cured polyimides as contrasted to chemically cured polyimides which do not require the very aggressive etching characteristics achieved by the above process. In some instances, with chemically cured polyimides, this etching is too strong and will not allow proper formation of the interlevel connection or the carrier's image in chemically cured polyimides. In order to facilitate a further understanding of the present invention, the following non-limiting examples are presented: EXAMPLE 1 A 10 µm thick film of polyimide which is roller coated in the form of polyamic acid onto a stainless steel sheet of metal and cured at 350°C for 30 minutes is immersed in an aqueous solution of 9 molar potassium hydroxide containing 5% by volume of N-Methyl-2-Pyrrolidone at 95°C for 4 minutes. The polyimide is removed from the potassium hydroxide bath and then immersed in 3 molar aqueous hydrochloric acid bath at room temperature for 1 minute. The polyimide is then removed from the acid bath and is again immersed in the 9 molar aqueous potassium hydroxide bath containing 5% by volume N-Methyl-2-Pyrrolidone for an additional 2 minutes. The 10 µm of polyimide are removed by this etching. EXAMPLE 2 A 10 µm thick film of an H-Kapton® film that is fully cured by heating to about 350°C for 30 minutes is immersed in an aqueous solution of 9 molar potassium hydroxide containing 5% by volume of N-methyl-2-pyrrolidone at 90.56°C (195°F) for 3 minutes. The polyimide is removed from the hydroxide bath and then immersed in an aqueous 3 molar HCl solution at room temperature for 1 minute. The polyimide is then removed from the acid solution and again immersed in the first hydroxide bath for an additional 2 minutes. The etching removes the exposed 10 µm of polyimide. EXAMPLE 3 A 10 µm thick film of polyimide of the same type employed in Example 1 is immersed in an aqueous solution of 9 molar potassium hydroxide at 90.56°C (195°F) for 4 minutes. The polyimide is removed from the potassium hydroxide bath and then immersed in a 3 molar aqueous HCl bath for 1 minute at room temperature. The polyimide is then removed from the acid bath and again immersed in the 9 molar aqueous potassium hydroxide bath for an additional 2 minutes. The results obtained are similar to those of Example 1, except that the process takes somewhat longer because of the absence of the N-methyl-2-pyrrolidone in the potassium hydroxide bath.

Claims: A process for etching a fully cured or substantially fully cured polyimide layer which comprises contacting said polyimide layer with an aqueous solution of a metal hydroxide, followed by contact with an acid, followed by contact with an aqueous solution of metal hydroxide. Process according to claim 1, wherein said hydroxide is an alkali metal hydroxide and preferably sodium hydroxide, potassium hydroxide, or mixtures thereof. Process according to claim 1 or 2, wherein the concentration of said hydroxide is at least 4 molar and preferably 4 to 15 molar. Process according to any one of claims 1 to 3, wherein the solution of metal hydroxide contains up to 15% by volume and preferably 3% to 15% by volume of N-Methyl-2-Pyrrolidone. Process according to any one of claims 1 to 4, wherein the solutions of metal hydroxide are employed at temperatures of 60°C to 100°C. Process according to any one of claims 1 to 5, wherein said acid is an inorganic and/or organic acid with said acid being preferably selected from the group hydrochloric acid, acetic acid, sulfuric acid or a mixture thereof. Process according to any one of claims 1 to 6, wherein at least a 1 molar aqueous solution of said acid, preferably at least a 3 molar aqueous solution of said acid and most preferred a 3 to 8 molar aqueous solution of said acid is employed. Process according to any one of claims 1 to 7, wherein said polyimide is heat cured polyimide that was heated to temperatures of at least 350°C, but not greater than 485°C for at least 30 minutes.

Inventors: BARD STEVEN LYNDZ, FEGER CLAUDIUS, GLENNING JOHN JOSEPH, HOUGHAM GARETH GEOFFREY, MOLIS STEVEN EARLE, PAWLOWSKI WALTER PAUL, RITSKO JOHN JAMES, SLOTA PETER JR, SNYDER RANDY WILLIAM, BARD, STEVEN LYNDZ, FEGER, CLAUDIUS, GLENNING, JOHN JOSEPH, HOUGHAM, GARETH GEOFFREY, MOLIS, STEVEN EARLE, PAWLOWSKI, WALTER PAUL, RITSKO, JOHN JAMES, SLOTA, PETER, JR., SNYDER, RANDY WILLIAM
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: C08J7
IPC codes: H05K1, H05K3, C08J7, G03F7, H01L21