MASS DENSITY OF POLYSTYRENE THIN-FILMS MEASURED BY TWIN NEUTRON REFLECTIVITY

Neutron reflectivity measurements on polystyrene thin films (6.5-79.0 nm thick) supported on silicon substrates indicate that the mass densi ty is near the bulk value regardless of film thickness. To account for possible inaccuracies arising from sample misalignment, reflectivity measurements were made both from the free-surface and silicon-substrat e sides of the thin film, a method termed twin reflectivity. For films spin coated on the hydrogen-terminated silicon surface the relative u ncertainty in the density measurement was on the order of 1%, but for films spin coated onto the silicon native-oxide surface the analysis w as more difficult because of subtleties in data fitting due to the oxi de layer. Nevertheless, within the limits of greater uncertainty, thes e films also showed no systematic change in density with thickness. (C ) 1998 American Institute of Physics.