Ultra-thin films of a few nanometers thick of the DNA bases adenine, guanine and cytosine were prepared on Si substrates by applying laser ablation at 157 nm to etch initially prepared crystal samples from solutions. This was possible because only photochemical dissociation is taking place on DNA bases following illumination with laser light at 157 run and localized damage. High-resolution AFM images reveal similar morphology between areas exposed or not to radiation, suggesting a limited chemical change on the surface of the remaining radiated substrate. For a laser fluence of 1 mJ/cm(2), an average film thickness of 0.5, 03 and 0.7 nm was removed from the initial adenine, cytosine and guanine crystal samples; this allows an accurate sub-nanometer control of the film thickness with the laser fluence. In addition, the surface roughness of laser-treated crystals was reduced down to nanometer-scale dimensions. (c) 2005 Elsevier B.V. All rights reserved.