As a mask blank for a digging Levenson type phase shift mask, there is known a mask blank having a laminated structure wherein, from a side of a transparent substrate of a glass material, a light shielding film made of a chromium-based compound is laminated. In a phase shift mask formed using such mask blank, a transfer pattern is formed of a light shielding pattern, which is formed by patterning the light shielding film by dry etching using a mixture gas of a chlorine-based gas and oxygen, and a phase shift pattern, which is obtained by digging the transparent substrate by dry etching using a fluorine-based gas.
Moreover, when the light shielding film forms a part of the transfer pattern as in the digging Levenson type phase shift mask described above, both a front surface side and a back surface side of the light shielding film should have low reflectances with respect to exposure light used for exposure and transfer using the phase shift mask. Therefore, to reduce the reflectances of the light shielding film and to secure a light shielding property, a proposal has been made about the configuration in which a film of a low chromium composition, a chrome metal film, and a film of a low chromium composition are successively laminated in the stated order (see JP-A-2007-33470 (Patent Document 1)).
Meanwhile, for the purpose of increasing an etching rate in dry etching while maintaining optical characteristics of the light shielding film made of the chromium-based compound, there has been proposed the structure in which the light shielding film is made of a chromium-based material containing tin. In this case, there have been exemplified a configuration wherein a film having a content of tin that is 0.01 times a content of chromium or more is formed only about a layer which should mainly have an antireflection function or to the contrary, a configuration in which a film having a content of tin that is 0.01 times a content of chromium or more is formed only about a layer which should have mainly a light shielding function and any other configurations (see JP-A-2013-238777 (Patent Document 2)).
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