Heavily boron‐doped silicon membranes which show substantial improvement in mechanical properties have been fabricated for x‐ray mask substrate by eliminating the misfit dislocation from the membrane. The measured surface roughness, fracture strength, and residual tensile stress of the membrane are 20 Å peak to peak, 1.39×1010 and 2.7×109 dyn/cm2, while those of the conventional heavily boron‐doped silicon membrane with high density of misfit dislocations are 500 Å peak to peak, 8.27×109 and 9.3×108 dyn/cm2, respectively. The differences between the two membranes are due to misfit dislocation. Young’s modulus has been extracted as 1.45×1012 dyn/cm2 for both membranes. Also, the lattice constant of strain‐free membrane, the in‐plane lattice constant of the conventional membrane, and the density of extra‐half plane contained in the conventional membrane have been extracted as 5.424 Å, 5.426 Å, and 2.3×104/cm, respectively.