Methylsilsesquioxane (MSSQ) films were chemically modified by (i) the bulk incorporation of a short-chain polymer or (ii) surface treatment by an NH3 plasma, and the effects of these modifications on elastic, plastic, and fracture properties were investigated. In (i), a copolymer of 10 mol % 1,2-bis(trimethoxysilyl) ethane (BTMSE) and methyltrimethoxysilane was synthesized and cured to form an MSSQ: BTMSE copolymer film. In ( ii), an NH3 plasma treatment was applied to cured MSSQ films for 5, 15, 30, or 60 min. Films were characterized using scanning electron microscopy (SEM), IR spectroscopy, and ellipsometry. Depth-sensing indentation was used to determine plane strain modulus, E', and hardness, H. Indentation traces and SEM images of residual indentation damage were used to determine the threshold load for cracking, Pc. The addition of BTMSE to MSSQ doubled E', H, and P-c relative to MSSQ with a slight increase in relative dielectric constant, k, from 2.70 to 2.86. Apparent E' and H at the surface of plasma-treated films increased with plasma treatment time. IR spectroscopy indicated that improved properties resulted in (i) from increased organic and inorganic networking and in (ii) from the formation of a dense, largely inorganic, surface layer. Such chemical modifications may be used to improve the performance of organosilicates as low-k dielectrics in microelectronic interconnection structures. (C) 2004 The Electrochemical Society.