We have compared the elemental composition, growth rates, and hardness of silicon dioxide layers grown from hexamethyldisiloxane (HMDSO), octamethylcyclotetrasiloxane (OMCTS), and tetramethylcyclotetrasiloxane (TMCTS) in a. high-density electron cyclotron resonance oxygen plasma. Hard colorless coatings could be grown from each precursor; coating composition, growth rates, and hardness depended strongly on the ratio of oxygen to precursor flow. At the highest O-2/precursor flow ratios (4:1 for HMDSO and 8:1 for OMCTS and TMCTS), the coatings were SiO2-like with oxygen-to-silicon ratios close to 2:1 and carbon percentages as low as 12% for HMDSO, 3.4% for OMCTS, and 1.4% for TMCTS. Growth rates were 0.7-1.1 mum/min, initially increasing with the O-2/precursor flow ratio; final coating thicknesses after 5 min were 3.5 mum to 5.5 mum. Coating hardness values generally increased with, increasing O-2/precursor flow ratio, attaining . 6 GPa, 7.4 GPa, and 8.7 GPa for HMDSO, OMCTS, and TMCTS, respectively. (C) 2003 American Vacuum Society.