An error source in ellipsometry

When measured by ellipsometry: the thickness of a polymer (PolyDiMethylSilo xane: PDMS) microscopic film deposited on a solid substrate (silicon wafer) is shown to depend upon the exposure time and the size of the light spot. We measured experimentally the thinning of such films. This effect can be a non-negligible source of error in the determination of microscopic liquid film thickness. To explain our results, we propose the existence of a Maran goni effect which is due to a local overheating produced by light absorptio n in the substrate. (C) Academie des sciences/Elsevier, Paris.