Basic mechanisms and main types of reliefs in laser direct nanostructuring technological materials

We experimentally demonstrated a possibility of direct laser surface nanostructuring germanium, nickel, platinum silicide, silicon nitride, stainless steel, titanium and zirconia by nanosecond ArF laser multipulse irradiation with the wavelength 193 nm. Similar results were obtained also for titanium and platinum when using nanosecond KrF laser irradiation with wavelength 248 nm. It was found that 5 main types of laser-induced profiles are possible. They correspond to five zones of laser spot when laser intensity decreases with an increase of the distance from the spot centre, i.e. to zones (1) of intense ablative material removal (in the crater in the central part of the spot irradiated with a high intensity), (2) of “deep” laser melt without significant ablative material removal, when melt layer thickness is of the order of 1 μm or few hundreds of nanometers, (3) of “shallow” melt, i.e., at laser intensity around or just above the melting threshold, when melt thickness is small compared to the case (2), (4) of spot periphery outside melting zone, and (5) of original non-irradiated surface. Besides of this, mentioned 5 main reliefs can also be exhibited in the form of corresponding superpositions in various transient regions between mentioned zones. It can give rise to a co-existence in some parts of the spot, for example, of micron and submicron reliefs, or of two different submicron reliefs. The proposed classification of reliefs for the first time unites in one scheme the results of previous papers, where many authors have observed the formation of micron structures under nanosecond laser irradiation, with recent results on studies of laser-induced submicron reliefs (or nanostructures). The physical mechanisms for formation of mentioned reliefs are proposed. The experimental manifestations of these reliefs in standing and moving laser spots are demonstrated and discussed.