A method for manufacturing a semiconductor substrate having a patterned group-III nitride compound layer without collapsing a formed mask pattern due to reflow or decomposition even when an etching method at a high temperature of 300° C.-700° C. is used, including the steps: forming a patterned mask layer on the substrate's group-III nitride compound layer, and etching the group-III nitride compound layer by dry etching at 300° C. or higher and 700° C. or lower using the mask pattern, to form patterned group-III nitride compound layer, wherein the patterned mask layer contains a polymer containing a unit structure of the following Formula (1): a polymer containing a unit structure of the following Formula (2): O—Ar1  Formula (2) a polymer containing a structural unit of the following Formula (3): O—Ar2—O—Ar3-T-Ar4  Formula (3) a polymer containing a combination of unit structure of Formula (2) and unit structure of Formula (3), or a crosslinked structure of the polymers.