Ionen-geïnduceerde nano-patronen op metaaloppervlakken
Skeren, Tomás; S0207665
Irradiation with keV ions often leads to the formation of nanosized periodic undulations on surfaces (mounds, pits, ripples...). This phenomenon is attracting a lot of interest as a potential nanofabrication tool due to its relative simplicity and high throughput. Despite substantial effort towards a complete theoretical description of this phenomenon many aspects of ion-induced pattern formation are still not satisfactorily understood. We focus on the patterning of a specific group of materials i.e. metals. In contrast to other surfaces typically studied in this context (e.g. Si, SiO2), metals do not amorphize during the ion irradiation process and their persistent crystalline nature has a significant impact on the patterning behavior. The existing theory for the pattern formation, however, usually does not explicitly account for the crystalline structure of the surface and it is inconsistent with many experimental observations for metallic surfaces. A characteristic observation valid for a majority of metallic surfaces is that grazing incidence ion irradiation leads to the formation of ripples parallel to the ion beam direction. In the context of the conventional theory, ripples are seen as waves with a specific orientation and wavelength which are formed and amplified due to the presence of an anisotropic linear instability resulting from the erosion process. In contrast to this paradigm, we propose a different mechanism which explains the formation of the parallel ripples. Its action can be described as follows: if a localized protrusion or depression appears on the surface, the oblique irradiation leads to the elongation of this feature in the direction of the ion beam. In contrast to the linear instability mechanism, this elongating tendency is a purely non-linear phenomenon, stemming from the variation of the local irradiation conditions between up-stream and down-stream slopes of the surface protrusion/depression. This elongating tendency is a very general consequence of the oblique irradiation process and is present on any surface by grazing angle ion irradiation. In order to actuate the non-linear elongation, an additional, independent roughening mechanism needs to be active on the irradiated surface. We investigated the behavior of single-crystalline Ni(001) and polycrystalline Ni surfaces and concluded that substantially different processes dominate the roughness formation on the two surfaces. Roughening of the Ni(001) surface is caused by the diffusive Ehrlich-Schwoebel instability while the roughening of polycrystalline Ni surface is caused by the variation of the erosion rate with the crystalline orientation of the particular grain. We formulate mathematical models describing the surface morphology evolution in the two cases and by extensive computer simulations we show that the proposed mechanism excellently reproduces all experimental observations. This conclusion suggests that the formation of parallel ripples at grazing incidence irradiation is a distinct phenomenon which is only superficially similar to the formation of ripples perpendicular to the ion beam which are often observed on amorphous surfaces. We discuss the general aspects of this mechanism in the context of the existing theory and we review its applicability to a broader range of material systems. This study significantly improves the detailed understanding of the process of ion-induced pattern formation on metallic surfaces and it contributes to the general physical picture of this phenomenon as a whole.