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 homogeneously etch a resist pattern having thin gaps as well as wide gaps on a chrome layer.

Description:
This application is a continuation-in-part of application Ser. No. 798,407 filed on Nov. 15, 1985, abandoned. 
    
    
     FIELD OF THE INVENTION 
     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 &#34;chrome&#34; material). 
     BACKGROUND OF THE INVENTION 
     The chrome material is conventionally etched with an aqueous composition comprising a cerium (IV) salt such as ammonium cerium (IV) nitrate [(NH 4 ) 2  Ce(NO 3 ) 6  ] and cerium (IV) sulfate [Ce(SO 4 ) 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. 
     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. 
     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) 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 (e.g. alkali metal salts such as sodium salt and potassium salt, and ammonium salt) (hereinafter referred to as &#34;acid component&#34;). 
     A preferred etchant composition according to the present invention comprises 
     (a) a cerium (IV) salt, 
     (b) an anionic fluorine-containing surfactant of the formula: 
     
         Rf&#39;COOX or Rf&#39;SO.sub.3 X 
    
      wherein Rf&#39; is a C 4  -C 10  perfluoroalkyl or ω-hydroperfluoroalkyl group, and X is a hydrogen atom, an alkali metal (e.g. sodium and potassium) atom or an ammonium group, 
     (c) water, and 
     (d) optionally, at least one acid component. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The etchant compositions of the present invention may be roughly classified into two kinds: 
     (A) ones which form precipitate after kept standing for 1 to 7 days but once filtered, do not re-form any precipitate, and 
     (B) ones which form precipitate after kept standing for 1 to 20 days and even filtered, re-forms precipitate repeatedly. 
     Those belonging to the kind A are ones containing a surfactant of the formula: 
     
         Rf&#39;COOX and Rf&#39;SO.sub.3 X 
    
     wherein Rf&#39; is the same as defined above. 
     Among the composition belonging to the kind A, 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(CF.sub.2 CF.sub.2).sub.3 COOH, 
    
     
         H(CF.sub.2 CF.sub.2).sub.3 COONH.sub.4, 
    
     
         C.sub.8 F.sub.17 SO.sub.3 H, and 
    
     
         C.sub.8 F.sub.17 SO.sub.3 NH.sub.4. 
    
     Those belonging to the kind B can be used before the formation of the precipitate. 
     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 be any one of conventional nonionic and anionic fluorine-containing surfactants. 
     Preferred are those having at least one of functional groups of the formulas: --O--, --OH, --COO--, --COOH, --SO 4 , --SO 3  --, and --PO 4  &lt;. 
     Specific examples of the surfactant preferably used according to the present invention are: 
     
         ______________________________________Nonionic onesRfAOH,RfACH(OR.sup.1)CH.sub.2 OQ.sub.n R.sup.2,RfBN(R.sup.3)(C.sub.2 H.sub.4 O).sub.n H,RfACH(OR.sup.1)CH.sub.2 OQ.sub.n CH.sub.2 CH(OR.sup.1)ARf, ##STR1##Anionic onesRfCOOX,RfBN(R.sup.3)ACOOX,RfBN(R.sup.3)AOSO.sub.3 X,RfSO.sub.3 X,RfCH.sub.2 O(CH.sub.2).sub.m SO.sub.3 X,RfBN(R.sup.3)APO.sub.4 X,RfACH(OR.sup.1)CH.sub.2 N(R.sup.3)ACOOX______________________________________ 
    
     wherein X is the same as defined above; Rf is a C 3  -C 21  fluoro-aliphatic group a carbon-carbon bonding of which may be interrupted by an oxygen atom; A is a group of the formula: --(CH 2 ) p  --, --[CH 2  --CH(CH 3 )] p  -- or --[CH 2  --CH(OY)--CH 2  ]-- in which Y is a hydrogen atom or a C 1  -C 3  acyl group and p is an integer of 1 to 10; B is --CO-- or --SO 2  --; R 1  is a hydrogen atom or a C 1  -C 5  acyl group; R 2  is a hydrogen atom or a substituted (with, for example, a C 1  -C 4  alkyl group) or unsubstituted aromatic group (e.g. a phenyl group) or alkyl group having, for example, 1 to 4 carbon atoms; R 3  is a hydrogen atom or a C 1  -C 4  alkyl group; R 4  is a C 1  -C 4  alkyl group; Q is a group of the formula: --C 2  H 4  O-- or --CH(CH 3 )CH 2  O--; n is an integer of 6 to 22; 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: 
     
         C.sub.3 F.sub.7 OCF(CF.sub.3)CF.sub.2 O(CH.sub.2 CF.sub.2 CF.sub.2 O).sub.u CH.sub.2 CF.sub.2 -- 
    
     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: 
     
         ______________________________________Base Composition______________________________________Ammonium cerium (IV) nitrate                165       gPerchloric acid (70%)                40        mlWater                1         l______________________________________ (Surface tension, 72-75 dyne/cm at 25° C.). 
    
     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. 
     
                       TABLE 1______________________________________                  Surface   Storage                  tension   timeSurfactant             (dyne/cm) (days)______________________________________Unidain DS-401.sup.( *.sup.1)                  27.8      4Unidain DS-401-A.sup.( *.sup.1)                  20.7      2Unidain DS-402.sup.( *.sup.1)                  35.0      20Unidain DS-101.sup.( *.sup.2)                  50.0      2C.sub.3 F.sub.7 OCF(CF.sub.3)CF.sub.2 O--(CH.sub.2 CF.sub.2 CF.sub.2O).sub.n --            45.0      14CH.sub.2 CF.sub.2 COOKUnidain DS-102.sup.( *.sup.2)                  35.0      2Unidain DS-403.sup.( *.sup.1)                  20.0      2Unidain DS-501.sup.( *.sup.3)                  40.0      3Unidain DS-502.sup.( *.sup.4)                  40.0      3______________________________________ Note: .sup.(*.sup.1) Perfluoroalkylethyleneoxide adduct (manufactured by Daikin Industries Ltd.). .sup.(*.sup.2) Perfluoroalkylcarboxylate salt (manufactured by Daikin Industries Ltd.). .sup.(*.sup.3) Perfluoroalkyl ether (manufactured by Daikin Industries Ltd.). .sup.(*.sup.4) Perfluoroalkyl acetal (manufactured by Daikin Industries Ltd.). 
    
     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: 
     
         ______________________________________Base Composition______________________________________Ammonium cerium (IV) nitrate                165       gPerchloric acid (70%)                40        mlWater                1         l______________________________________ (Surface tension, 72-75 dyne/cm at 25° C.). 
    
     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. 
     
                       TABLE 2______________________________________               Surface     Storage               tension     timeSurfactant          (dynes/cm)  (days)______________________________________C.sub.2 F.sub.5 (CF.sub.2 CF.sub.2).sub.s CH.sub.2 (OH)               37.5        14 ##STR2##(s = av. 3.5, t = av. 10)(CF.sub.3).sub.2 CF(C.sub.2 F.sub.4).sub.s CH.sub.2 CH(OH)               25.0        2CH.sub.2 O(C.sub.2 H.sub.4 O).sub.t CH.sub.3(s = av. 3.5, t = av. 8)(CF.sub.3).sub.2 CF(C.sub.2 F.sub.4).sub.s CH.sub.2 CH(OH)               22.5        2CH.sub.2 O(C.sub.2 H.sub.4 O).sub.t H(s = av. 3.5, t = av. 15)(CF.sub.3).sub.2 CF(C.sub.2 F.sub.4).sub.s CH.sub.2 CH(OH)               40.0        2CH.sub.2 O[CH(CH.sub.3)CH.sub.2 O].sub.2 CH.sub.2CH(OH)CH.sub.2 (C.sub.2 H.sub.4 O).sub.s CF(CF.sub.3).sub.2(s = av. 3.5) ##STR3##           40.0        2(s = av. 3.5)C.sub.2 F.sub.5 (C.sub.2 F.sub.4).sub.s CH.sub.2 CH(OH)CH.sub.2               35.0        2N(CH.sub.3)CH.sub.2 COOK(s = av. 3.5)______________________________________ 
    
     EXAMPLE 3 
     An etchant composition having a following composition was prepared and examined for its surface tension (at 25° C.) and storage time. 
     
         ______________________________________Composition IAmmonium cerium (IV) nitrate                  20       gCerium (IV) nitrate.hydrate                  40       gNitric acid (60%)      120      mlWater                  1        lC.sub.3 F.sub.7 OCF(CF.sub.3)CF.sub.2 O(CH.sub.2 CF.sub.2 CF.sub.2O).sub.n               0.01     wt %CH.sub.2 CF.sub.2 COOK (n = av. 3.5)Surface tension        45       dyne/cmStorage time           &gt;20      days.Composition IICerium (IV) nitrate.hydrate                  20       gNitric acid (60%)      40       mlWater                  1        lUnidain DS-402         0.01     wt %Surface tension        35       dyne/cmStorage time           &gt;20      days.Composition IIIAmmonium cerium (IV) nitrate                  165      gPerchloric acid (70%)  40       mlNitric acid (60%)      80       mlWater                  1        lUnidain DS-402         100      ppmSurface tension        50-60    dyne/cmStorage time           15       days.Composition IVAmmonium cerium (IV) nitrate                  165      gPerchloric acid (70%)  40       mlNitric acid (60%)      40       mlWater                  1        lC.sub.3 F.sub.7 OCF(CF.sub.3)CF.sub.2 O(CH.sub.2 CF.sub.2 O).sub.n                  100      ppmCH.sub.2 CF.sub.2 COOK (n = av. 3.5)Surface tension        50-60    dyne/cmStorage time           15       days.Composition VCerium (IV) nitrate.hydrate                  20       gNitric acid (60%)      40       mlWater                  1        lC.sub.2 F.sub.5 (C.sub.2 F.sub.4).sub.s CH.sub.2 CH(OH)                  100      ppm ##STR4##(s = av. 3.5, t = av. 10)Surface tension        35       dyne/cmStorage time           &gt;20      days.Composition VIAmmonium cerium (IV) nitrate                  165      gPerchloric acid (70%)  40       mlNitric acid (60%)      80       mlWater                  1        lC.sub.2 F.sub.5 (C.sub.2 F.sub.4).sub.s CH.sub.2 CH(OH)                  100      ppm ##STR5##(s = av. 3.5, t = av. 10)Surface tension        50-60    dyne/cmStorage time           15       days.______________________________________ 
    
     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: 
     
         ______________________________________Base Composition______________________________________Ammonium cerium (IV) nitrate                165       gPerchloric acid (70%)                40        mlWater                1         l______________________________________ (Surface tension, 72-75 dyne/cm at 25° C.). 
    
     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. 
     
                                           TABLE 3__________________________________________________________________________Surfactant     After preparation                         After filtration      conc.          Surface tension                  Storage time                         Surface tension                                 Storage timeCompound   (wt %)          (dyne/cm)                  (days) (dyne/cm)                                 (days)__________________________________________________________________________H(CF.sub.2 CF.sub.2).sub.2 COOH      0.005          73      &gt;7     73      40      0.010          71      &gt;7     72      40      0.015          70      &gt;7     74      40H(CF.sub.2 CF.sub.2).sub.3 COOH      0.005          65      3      65      &gt;60      0.010          58      1      59      &gt;60      0.015          50      1      49      &gt;60H(CF.sub.2 CF.sub.2).sub.4 COOH      0.005          49      3      56      40      0.010          48      1      60      40      0.015          55      1      63      40C.sub.7 F.sub.15 COOH      0.005          47      3      55      30      0.010          45      1      65      30      0.015          46      1      57      30C.sub.10 F.sub.21 COOH      0.005          59      1      65      30      0.010          58      1      61      30      0.015          58      1      60      30C.sub.8 F.sub.17 SO.sub.3 H      0.005          31      3      31      &gt;60      0.010          30      1      30      &gt;60      0.015          28      1      27      &gt; 60H(CF.sub.2 CF.sub.2).sub.2 COONH.sub.4      0.005          54      7      58      &gt;60      0.010          46      7      51      &gt;60      0.015          44      3      48      &gt;60C.sub.7 F.sub.15 COONH.sub.4      0.005          43      7      58      30      0.010          46      3      51      30      0.015          45      3      48      30C.sub.8 F.sub.17 SO.sub.3 NH.sub.4      0.005          34      7      36      &gt;60      0.010          31      3      33      &gt;60      0.015          29      3      30      &gt;60__________________________________________________________________________ 
    
     EXAMPLE 5 AND COMPARATIVE EXAMPLE 1 
     A chromium film of 500 Å in thickness and a chromium oxide film of 500 Å in thickness successively formed on a mask substrate were 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 fine pattern having gaps of 0.5 μm and 1.0 μm, respectively were drawn up 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 followed by etching with an etchant composition having the following composition: 
     
         ______________________________________Composition______________________________________Ammonium cerium (IV) nitrate               165       gPerchloric acid (70%)               40        mlWater               1         lUnidain DS-402      0.01      wt %______________________________________ (Surface tension, 40 dyne/cm at 25° C.). 
    
     For comparison, the same procedures as above were repeated but using an etchant composition having the following composition: 
     
         ______________________________________Composition______________________________________Ammonium cerium (IV) nitrate               165       gPerchloric acid (70%)               40        mlWater               1         l______________________________________ (Surface tension, 75 dyne/cm at 25° C.). 
    
     The produced etching patterns were observed. 
     In Example 5, since the etchant composition contained the fluorine-containing surfactant, the surface tension of the composition was reduced so that its wettability with and penetration ability into the gaps in the resist film were improved. Therefore, the fine patterns having the gaps of 0.5 μm and 1.0 μm were precisely and homogeneously etched so that all the portions of which were etched. The etching could be carried out with good reproducibility. 
     On the contrary, in case of the comparative experiment, the etchant composition had large surface tension so that its wettability and penetrating ability were very poor. Thus, the nonuniformity was observed, and the number of portions not etched 30 portions/cm 2 . 
     EXAMPLES 6-8 AND COMPARATIVE EXAMPLES 2-3 
     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 the same resist as used in Example 5 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. 
     
                                           TABLE 4__________________________________________________________________________Example No.  6         7     C.2                           8         C.3__________________________________________________________________________Ammonium cerium (V)        165       165   165                           165       165nitratePerchloric acid (70%)        40        40    40 0          0Surfactant   H(CF.sub.2 CF.sub.2).sub.3 COOH                  C.sub.8 F.sub.17 SO.sub.3 H                        -- H(CF.sub.2 CF.sub.2).sub.3 COOH                                     --(g)          (0.01)    (0.005)  (0.02)Water (liter)         1         1     1 1          1Surface tension        58        43    75 34        74(dyne/cm)Etching time (sec.)        65        50    65 39        65Number of unetched     2 μm         0         0    10 0         13through-holes     5 μm         0         0     5 0          7__________________________________________________________________________