US 7,404,910 Bl  1  2  ETCHING SOLUTION, ETCHED ARTICLE  AND METHOD FOR ETCHED ARTICLE  TECHNICAL FIELD  BACKGROUND ART  DISCLOSURE OF INVENTION  Examples of the organic solvent having a hetero atom  include methanol (relative dielectric constant: 32.6 (25°  C.)),  ethanol (relative dielectric constant: 24.6 (25°  C.)), isopro› panol (IPA, relative dielectric constant: 19.9 (25°  C.)), 1-pro› panol (relative dielectric constant: 22.2 (25°  C.)), 1-butanol  (relative dielectric constant: 17.1 (25°  C.)), 2-butanol (rela- tive dielectric constant: 15.5 (19°  C.)), t-butanol (relative  Examples of the organic acid include acetic acid (relative  dielectric constant: 6.15 (20°  C.)), propionic acid (relative  dielectric constant: 3.4 ( 40°  C.)), butyric acid (relative dielec› tric constant: 2.97(20°  C.)), isobutyric acid (relative dielectric  constant: 2.73( 40°  C.)), valeric acid, caproic acid (relative  dielectric constant: 2.63(71 ° C.)), caprylic acid (relative  dielectric constant: 2.45(20° C.)), monochloroacetic acid  (relative dielectric constant: 21 (20°  C.)), dichloroacetic acid  (relative dielectric constant: 8.08(20°  C.)), trichloroacetic  acid (relative dielectric constant: 4.6 (60°  C.)), monofluoro› acetic acid, difluoroacetic acid, trifluoroacetic acid, a-chlo- robutyric acid, ~-chlorobutyric acid, y-chlorobutyric acid,  lactic acid (relative dielectric constant: 22(70°  C.)), glycolic  acid, pyruvic acid, glyoxalic acid, acrylic acid and like mono› carboxylic acids, methanesulfonic acid, toluenesulfonic acid  and like sulfonic acids, oxalic acid, succinic acid, adipic acid,  tartaric acid, citric acid and like polycarboxylic acids.  Preferable examples of the inorganic acid include inor› ganic acids having a pKa value at 25°  C. of 2 or lower, for  example, hydrochloric acid (pKa =-8), nitric acid (pKa=- 1.8), hydrobromic acid (pKa=-9), hydroiodic acid (pKa=- 10) and perchloric acid (a pKa-unmeasurably strong acid).  The water content is not higher than 70% by weight, pref› erably not higher than  30%  by weight, more preferably about  30-5% by weight. The relative dielectric constant of the etch› ing solution expresses an arithmetic mean of the relative  dielectric constants of the components of the etching solu› tions other than the HF and inorganic acid.  The present invention relates to the items 1-16 listed below.  The etch rate of the etching solution of the invention can be  calculated as the difference in thickness of a film (BSG;  BPSG; THOX; TEOS and like USG, etc.) before and after  etching divided by etch time.  An  object of the present invention is to provide an etching  solution and an etching method for selectively etching oxide  films doped with impurities relative to TEOS and THOX.  In case of using TEOS instead ofTHOX, the ratio ofBSG  etch rate/TEOS etch rate and/or the ratio of BPSG etch rate/  TEOS etch rate at 25°  C. is/are 5 or higher, preferably 10 or  higher, more preferably 50 or higher, particularly 100 or  higher.  According to the etching solution of the invention, the ratio  of BSG etch rate/TH OX etch rate and/or the ratio of BPSG  etch rate/THOX etch rate at 25°  C. is/are 10 or higher, pref› erably 20 or higher, more preferably 50 or higher, particularly  100 or higher.  However, the buffered hydrofluoric acids etch not only  doped oxide films such as BSG films, BPSG films, phospho› silicate glass (PSG) films, arsenic silicate glass (AsSG) films  and the like, but also undoped oxide films such as USG  including TEOS (oxide obtained by CVD method using tet› raethoxysilane gas) films, THOX and the like. Therefore, the  buffered hydrofluoric acids can not selectively etch the doped  oxide films.  Conventionally, as etchants for silicon wafers and the like  have been used buffered hydrofluoric acids comprising HF  (50% by weight) and NH$_{4}$ F (40% by weight) at such a ratio  that can achieve a desired etch rate.  The present invention relates to an etching solution, a  method for producing an etched article and an etched article  produced by the method, more specifically, an etching solu› tion and a method for producing an etched article for selec› tively etching a doped oxide film, particularly BSG or BPSG  relative to an undoped oxide film, particularly THOX, and an  etched article produced by the method.  Item 1:  An  etching solution comprising hydrofluoric acid,  wherein an a ratio of etch rate of a boron silicate glass film  (BSG) or boron phosphosilicate glass / an etch rate of a  thermal oxide film (THOX) at 25°  C. is 10 or higher.  Item 2: The etching solution according to item 1, wherein a  solvent in the etching solution has a relative dielectric  constant of 61 or lower.  Item 3: The etching solution according to item 1, the solution  containing at least one member selected from the group  consisting of an organic acid and an organic solvent having  a hetero atom.  Item 4: The etching solution according to item 1, the solution  containing (i) water and (ii) at least one member selected  from the group consisting of an organic acid and an organic  solvent having a hetero atom, the water being contained in  a concentration of 70% by weight or lower.  Item 5: The etching solution according to item 1, wherein the  weight ratio ofHF:isopropyl alcohol:water is 0.1-50% by  weight: 30-99% by weight: 0-70% by weight.  Item 6: The etching solution according to item 1, wherein the  weight ratio ofHF:acetic acid:water is 0.1-50% by weight:  30-99 .9% by weight: 0-70% by weight.  Item 7: The etching solution according to item 1, wherein the  weight ratio of HF:tetrahydrofuran:water is 0.1-50% by  weight: 30-99.9% by weight: 0-70% by weight.  Item 8: The etching solution according to item 1, wherein the  weight ratio of HF: acetone:water is 0.1-50% by weight:  30-99 .9% by weight: 0-70% by weight.  Item 9: The etching solution according to item 1, wherein the  weight ratio of HF: methanol: water is 0.1-50% by weight:  30-99 .9% by weight: 0-70% by weight.  Item 16:  An  etched article which is obtainable by the method  of item 15.  Item 15: A method for producing an etched article by etching  an article to be etched with the etching solution as defined  inanyofitems 1-14.  Item 14: The etching solution according to item 11, wherein  the weight ratio ofHF:HNO$_{3}$:wateris 0.01-50% by weight:  1-70% by weight: 0-99% by weight.  Item 13: The etching solution according to item 11, wherein  the weight ratio ofHF:HCl:water is 0.01-50% by weight:  1-36% by weight: 0-99% by weight.  Item: 12 The etching solution according to item 11, wherein  the inorganic acid has a pKa value at 25°  C. of 2 or lower.  Item 11: The etching solution according to item 1, the solu› tion comprising an inorganic acid.  Item 10: The etching solution according to item 1, wherein the  weight ratio of HF:ethanol:water is 0.1-50% by weight:  30-99 .9% by weight: 0-70% by weight. 