Laser-induced shock wave cleaning of EUV photomasks

With the arrival of the next generation extreme ultraviolet (EUV) lithography tools, printed features on integrated circuits will continue to shrink in the future. In the absence of a protective pellicle, the requirements for contamination control are getting more and more demanding. In some cases the presence of a single particle as small as 30 nm on the surface of a photomask can already lead to costly production errors, which is why the semiconductor industry is searching for a quick, dry, and contact–free method to remove particles from contamination critical surfaces. In light of these developments, this thesis presents an investigation into the feasability of laser–induced shock wave cleaning as a novel method for removing small submicron particles from the surface of an EUV photomask. This particular cleaning method removes particles by exposing them to a fast moving shock wave, which is generated by the laser–induced breakdown of a gas above the surface. To study its properties and to validate the proposed shock theory, the shock wave produced by the laser–induced breakdown is visualized with specialized techniques such as shadowgraphy and Schlieren photography. The recorded images clearly reveal the transient behaviour of the shock wave, from which it can be concluded that the laser–induced breakdown process is consistent with the Taylor & Sedov solution of an intense point explosion. By combining this result with particle adhesion theory, a model has been developed that is able to predict the diameter of the smallest removable particle as a function of the main cleaning parameters. The results of this model were found to be in agreement with the experimental results. To assess the cleaning capabilities of laser–induced shock wave cleaning, a series of cleaning experiments were performed on flat unpatterned 300 mm silicon wafers. The wafers were purposely contaminated with polystyrene latex and silica spheres with sizes between 60 and 150 nm, after which they were exposed to various laser–induced shock wave cleaning treatments. The results of these cleaning experiments showed that in some cases it is possible to fully remove polystyrene latex spheres with a diameter below 60 nm. Furthermore, it is expected that the removal of particles from patterned surfaces is more problematic, because the particles that are located at the botom of a trench are able to escape the full force of the shock wave. Unfortunately, damage assessments on the fragile multilayer mirror of the EUV photomask have shown that the laser–induced shock wave cleaning method is too damaging for the surface of the EUV photomask. It is therefore concluded that laser–induced shock wave cleaning can be an effective method of particle removal from flat surfaces, but that it is not suitable for the cleaning of EUV photomasks.