In a semiconductor manufacturing process, silicon nitride is utilized for a hard mask in forming of a silicon oxide film, a side wall for protecting a fine pattern, a stopper of CMP polishing, and the like. Moreover, in accordance with an increase in degree of semiconductor integration, miniaturizing of a structure and thinning of a film structure have advanced, and therefore the film thickness of a silicon oxide film has also been increasingly thinner. In order to etch silicon nitride for patterning, etching is conducted, and a phosphoric acid solution at about 160° C. has conventionally been used for such etching of a silicon nitride film (for example, Patent Literature 1). The phosphoric acid solution at about 160° C., however, not only removes a silicon nitride film, but also partially removes a silicon oxide film due to etching by the phosphoric acid solution.
In order to enhance productivity for preforming high precision patterning, it is necessary to promote etching of silicon nitride while selectively suppressing etching of silicon oxide, and an improved method has been conventionally proposed in which SiN is selectively etched while etching of SiO2 is suppressed in cleaning of a substrate on which SiO2 and SiN are exposed, thereby resulting in an increase in selection ratio that means the ratio of the etching rate of SiO2 to the etching rate of SiN (see, for example, Patent Literatures 2 and 3).
In Patent Literature 2, an etching solution containing sulfuric acid and fluoride as main components and containing water in an amount of 5% by mass or less has been proposed. Moreover, in Patent Literature 3, an etching liquid containing phosphoric acid, sulfuric acid, and an oxidant including no metal element has been proposed, and the oxidant has been, for example, ammonium peroxodisulfate.
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