Aging mechanism of the native oxide on silicon (100) following atmospheric oxygen plasma cleaning

Silicon native oxide surfaces were cleaned with a radio frequency, atmospheric pressure helium and oxygen plasma and with an RCA standard clean-1. Both processes create a hydrophilic state with water contact angles of <5 degrees and 16.2 degrees +/- 1.7 degrees, respectively. During subsequent storage in a chamber purged with boil off from a liquid nitrogen tank, the water contact angle increased over time at a rate dependent on the cleaning method used. Internal reflection infrared spectroscopy revealed that the change in water contact angle was due to the adsorption of organic molecules with an average hydrocarbon chain length of 10 +/- 2. The rate of the adsorption process decreased with the fraction of hydrogen-bonded hydroxyl groups on the surface relative to those groups that were isolated. On Si (100) surfaces that were cleaned by RCA standard clean-1 and then the plasma, 95.8% of the silanol groups were hydrogen bonded. The first-order rate constant for adsorption of the organic contaminant on this surface was 0.182 +/- 0.008 h(-1). (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3597436]