Thin polymer films on rough surfaces

X-ray diffraction measurements in the region of small incidence and exit angles on thin polystyrene (PS) films deposited on laterally structured surfaces are performed. From fits of the data we obtain how the Fourier-components of the substrates are damped by the adsorbed films. The results are compared with theoretical predictions for liquid films. It turns out that PS-films of low molecular weight (less than or equal to 300k) behave like frozen liquids. Within films of a molecular weight of 1000k viscoelastic forces dominate over the substrate-adsorbate van der Waals-interactions so that even a 680 Angstrom thick film is not totally flat on top of a grating with a height of 130 Angstrom.