ETCHANT COMPOSITION

Abstract: An etchant composition comprising (a) a cerium (IV) salt, (b) a nonionic or anionic fluorine-containing surfactant, (c) water, and (d) optionally, at least one compound selected from the group consisting of perchloric acid, acetic acid, sulfuric acid, nitric acid and hydrochloric acid and salts thereof, which can homo­ geneously etch a resist pattern having thin gaps as well as wide gaps on a chrome layer.

Description: The present invention relates to an etchant composition. More particularly, it relates to an etchant composition suitable for use in etching of a chromium or chromium oxide material (hereinafter referred to as "chrome" material). The chrome material is conventionally etched with an aqueous composition comprising a cerium (IV) salt such as ammonium cerium (IV) nitrate [(NH₄)₂Ce(NO₃)₆] and cerium (IV) sulfate [Ce(SO₄)₂] and optionally perchloric acid, acetic acid, nitric acid and the like. Ratios and concentrations of these components depend on properties of the chrome material such as the oxidation degree of chromium, contents of impurities and a thickness of the chrome material as well as an etching rate. However, such conventional etchant composition is not suitable for use in etching a pattern with very thin gaps. For example, a chrome mask, which is used for transferring a specific pattern onto a silicon wafer in the production of semiconductor integrated circuit elements, is produced by forming a chrome thin layer on a substrate made of a material which is transparent to visible light, ultraviolet light or far-infrared light to be used for transferring the pattern onto the wafer (e.g. glass and silica) followed by application of a photosensitive agent on the chrome thin layer to form a resist layer, then providing a latent image on the resist layer by irradiation of ultraviolet light, X-ray, an electron beam and the like, developing a designed pattern in the resist layer. Then, the chrome thin layer is etched through the pattern-formed resist layer as a mask so as to transfer the pattern to the chrome layer As the degree of integration of the semiconductor integrated circuit increases, line gaps in the image to be transferred become thinner and thinner. In such circumstance, the conventionally used etchant can hardly penetrate into the thinner gaps formed in the resist layer so that some parts of the chrome layer are not etched. To overcome such defect of the conventional etchant, there are proposed to add a pretreating step comprising dipping the resist in an organic solvent such as alcohol or an aqueous solution of a surfactant to improve the wettability of the resist (cf. Japanese Patent Kokai Publication (unexamined) No. 62854/1984) or to add an operation comprising vibrating the etchant with ultrasonic wave or agitating the etchant to facilitate the penetration of the etchant into the very thin gaps. These proposals, however, are not favorable since they increase the number of steps of the etching process, or the etching quality is influenced by the fluctuation of concentration of the etchant. Further, the precisely formed resist image may be damaged, the chrome layer may drop out, or flaws are formed on the chrome layer. FR-A-2101771 discloses an etching composition for a chrome mask which comprises cerium (IV) ions, an acid such as nitric acid and additives in minor quantities. GB-A-837703 and FR-A-1480356 disclose the use of fluorinated surfactants as a component of an etching bath solution. SUMMARY OF THE INVENTION One object of the present invention is to provide an etchant composition which can be improved in the wettability of the resist layer without deteriorating the properties of the conventional etchant composition comprising a cerium (IV) salt. Another object of the present invention is to provide an etchant composition which can homogeneously etch the resist pattern having thin gaps as well as wide gaps in the chrome layer. A further object of the present invention is to provide an etchant composition which does not form any precipitate and maintains its activity after storage. Accordingly, the present invention provides an etchant composition comprising: (a) a cerium (IV) salt, (b) at least one anionic fluorine-containing surfactant selected from Rf'COOX and Rf'SO₃X wherein Rf' is a C₄-C₁₀ perfluoroalkyl or ω-hydroperfluoro-alkyl group and X is a hydrogen atom, an alkali metal atom or an ammonium group, (c) water, and (d) optionally, at least one compound selected from the group consisting of perchloric acid, acetic acid, sulfuric acid, nitric acid and hydrochloric acid and salts thereof (e.g. alkali metal salts such as sodium salt and potassium salt, and ammonium salt) (hereinafter referred to as "acid component"). DETAILED DESCRIPTION OF THE INVENTION The etchant compositions of the present invention form a precipitate after being kept standing for 1 to 7 days but once filtered, do not re-form any precipitate. Among the composition of the invention, preferred are those having substantially the same surface tension before and after filtration and sufficiently lower surface tension than one containing no surfactant. Preferred surfactants for such compositions are of the formulas: H(CF₂CF₂)₃COOH, H(CF₂CF₂)₃COONH₄, C₈F₁₇SO₃H, and C₈F₁₇SO₃NH₄. For the etchant composition of the invention, the preferred cerium (IV) salt is a cerium (IV) salt of a strong acid or a complex salt of said cerium (IV) salt and an ammonium salt of the strong acid. Specific examples of the cerium (IV) salt are cerium (IV) sulfate, cerium (IV) nitrate, ammonium cerium (IV) sulfate, ammonium cerium (IV) nitrate and mixtures thereof. The amount of the cerium (IV) salt to be added in the etchant composition of the invention is generally from 10 to 300 g per liter of water. The acid component is optionally added to the composition. When it is used, its amount to be added is up to 120 g per liter of water. The fluorine-containing surfactant to be used according to the present invention may include at least one further functional group selected from -O-, -OH, -COO-, -COOH, -SO₄, -SO₃-, and -PO4<. Specific examples of the surfactant preferably used according to the present invention are: RfCOOX, RfBN(R³)ACOOX, RfBN(R³)AOSO₃X, RfSO₃X, RfCH₂O(CH₂) m SO₃X, RfACH(OR¹)CH₂N(R³)ACOOX wherein X is the same as defined above; Rf is a C₃-C₂₁ fluoro-aliphatic group a carbon-carbon bonding of which may be interrupted by an oxygen atom; A is a group of the formula: -(CH₂) p -, -[CH₂-CH(CH₃)] p - or -[CH₂-CH(OY)-CH₂]- in which Y is a hydrogen atom or a C₁-C₃ acyl group and p is an integer of 1 to 10; B is -CO- or -SO₂-; -R³ is a hydrogen atom or a C₁-C₄ alkyl group; and m is an integer of 1 to 10. In the above formulas, the fluoro-aliphatic group may be a saturated or unsaturated, straight or branched one, preferably a perfluoroalkyl group, ω-hydroperfluoroalkyl group and a group of the formula: C₃F₇OCF(CF₃)CF₂O(CH₂CF₂CF₂O) u CH₂CF₂- wherein u is an integer of 1 to 4. The fluorine-containing surfactants may be used alone or as a mixture thereof. The amount of the surfactant to be added is from 0.00001 to 10 % by weight, preferably from 0.001 to 1.0 % by weight, especially not more than 0.1 % by weight based on the whole weight of the etchant composition. When the amount of the surfactant is less than the above lower limit, the effect of the addition of the surfactant is not achieved. When it is larger than the above upper limit, the surfactant does not exert its effect in proportion to the increased amount of the surfactant, this being uneconomical. The etchant composition of the invention may be prepared by only mixing all the components, and the order of the addition of the components is not critical. For example, an aqueous solution of the surfactant is added to an aqueous solution of the the cerium (IV) salt and the optional acid component. When the precipitate is formed in the composition after kept standing for 1 to 7 days, it is removed by filtering it off. The precipitate does not materially influence the composition, etching activity and surface tension of the etchant composition since its amount is very slight. In addition to the above components, the etchant composition may contain any component that does not adversely affect the activity of the cerium (IV) salt. Examples of such additional component are water-soluble organic solvents such as alcohols (e.g. methanol, ethanol, butanol and the like) and ketones (e.g. acetone, methyl ethyl ketone, methyl isobutyl ketone and the like). The present invention will be hereinafter explained further in detail by following examples. Example 1 An etchant composition was prepared by mixing the following base composition with a 0.5 % aqueous solution of a surfactant listed in Table 1 to its final concentration of 0.01 % by weight: A surface tension at 25°C of the etchant composition and a period of time till the start of the precipitation (storage time) are shown in Table 1. Example 2 An etchant composition was prepared by mixing the following base composition with a 0.5 % aqueous solution of a fluorine-containing surfactant listed in Table 2 to its final concentration of 0.01 % by weight: A surface tension at 25°C of the etchant composition and a period of time till the start of the precipitation (storage time) are shown in Table 2. Example 3 An etchant composition having a following composition was prepared and examined for its surface tension (at 25°C) and storage time. Example 4 An etchant composition was prepared by mixing the following base composition with a 0.5 % aqueous solution of a surfactant listed in Table 3 to a predetermined final concentration: The prepared etchant composition was kept standing for three days and filtered out the precipitate to give an etchant composition. A surface tension at 25°C of the filtered etchant composition and a period of time till the start of the precipitation (storage time) after preparation and after filtration are shown in Table 3. Examples 5-7 and Comparative Examples 1-2 A mask substrate having a chromium film and a chromium oxide film formed on the chromium film (AMFR 2509 (P) LR manufactured by Ulvac Coating Corp.) was coated with Fluororesist FBM-120 (a solution of a copolymer of 99 % by weight of 2,2,3,4,4,4-hexafluorobutyl methacrylate and 1 % by weight of glycidyl methacrylate manufactured by Daikin Industries Ltd.) by means of a spinner with a thickness of 5,000 Å, in which a pattern consisting of 400 through-holes of each 2 µm square and a pattern consisting of 400 through-holes of each 5 µm square were drawn by an electron beam radiating equipment. After pretreatment with isopropanol, the patterns were developed with a developing agent comprising ethanol and isobutanol to form resist patterns. Then, it was rinsed with isopropanol and post-baked at 80°C for 15 minutes and descumed by a reactive ion etching equipment at 100 W of high frequency output for 20 seconds followed by etching with an etchant composition having the composition shown in Table 4. Then, the number of the through-holes which were not etched was counted by means of an optical microscope. The results are listed in Table 4.

Claims: An etchant composition comprising (a) a cerium (IV) salt, (b) at least one anionic fluorine-containing surfactant selected from Rf'COOX and Rf'SO₃X wherein Rf' is a C₄-C₁₀ perfluoroalkyl or ω-hydroperfluoro-alkyl group and X is a hydrogen atom, an alkali metal atom or an ammonium group , (c) water, and (d) optionally, at least one compound selected from the group consisting of perchloric acid, acetic acid, sulfuric acid, nitric acid and hydrochloric acid and salts thereof. An etchant composition according to claim 1, wherein the anionic surfactant is one selected from the group consisting of H(CF₂CF₂)₃COOH, H(CF₂CF₂)₃COONH₄, C₈F₁₇SO₃H, and C₈F₁₇SO₃NH₄. An etchant composition according to claim 1, wherein the amount of the surfactant is from 0.00001 to 10 % by weight based on the weight of the etchant composition. An etchant composition according to claim 1, wherein the cerium (IV) salt is at least one selected from the group consisting of cerium (IV) sulfate, cerium (IV) nitrate, ammonium cerium (IV) sulfate and ammonium cerium (IV) nitrate. An etchant composition according to claim 1, wherein the amount of the cerium (IV) salt to be added in the etchant composition is from 10 to 300 g per liter of water. An etchant composition according to claim 1, wherein the amount of the acid component to be added is up to 120 g per liter of water.

Inventors: DEGUCHI, TAKAYUKI, FUJII, TSUNEO, TAMARU, SHINJI
Applicants: DAIKIN INDUSTRIES, LIMITED
Application year: 1985
Main classification: C23F1
IPC codes: C09K13, C23F1