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First and second etching solutions 23 and 24 are used for etching treatments of glass substrates 11 and 12 . The first etching solution 23 is faster in etching rate than the second one 24 . The first etching solution first etches surface 11 a of the glass substrate 11 and the surface 12 a of the glass substrate 12 . Even where there are the origins of pits 14 (concave defects) on the surfaces 11 a and 12 a, this etching treatment can remove them from the surfaces 11 a and 12 a because the etching runs faster than the occurrence of the pits. The second etching solution 24 which is slower in etching rare than the first one is then applied to etch the surfaces 11 a and 12 a. As a result, the etching treatments make the surfaces 11 a and 12 a as thin as desirable and sufficiently flat.