In order to improve the properties, such as stress and crystallinity, of diamond films for use as x-ray masks, we have studied the effectiveness of the addition of O-2 to a CH4/H-2 gaseous mixture in magnetically enhanced microwave plasma chemical vapor deposition. As the O-2 concentration was increased, diamond grain growth became more dominant over the nondiamond component and grain growth continued. As a result, stress shifted toward the tensile regime. We explain the mechanism of this diamond stress shift by using the stress generation model proposed by Chaudhari. Under the optimum deposition conditions, i.e., CH4/O-2 /H-2 = 2.0/0.5/97.5 vol % at 920 degrees C, we obtained (Ill)-textured polycrystalline diamond, which had high crystallinity and improved stress properties. The average stress was 147 MPa (tensile) and the uniformity was less than or equal to +21.1%; - 12.2% was achieved within a 50-mm-diam are,for a 2.3-mu m-thick film. The biaxial Young＇s modulus was evaluated by the bulge test, and was found to be 1000+/-51 GPa. This diamond membrane caused little in-plane distortion-induced backetching, i.e., max (x,y) = (27,31) nm, due to its high biaxial Young＇s modulus.