Mechanisms and Manipulation of Ion Beam Pattern Formation on Si(001)

Ion beam pattern formation is a versatile and cost-efficient tool for the fabrication of well-ordered nanostructures. Furthermore, silicon is known to be a prime material in microelectronics. The thesis at hand deals with pattern formation on Si(001) through 2keV Kr+ ion beam erosion under ultra high vacuum conditions investigated by in situ scanning tunneling microscopy, ex situ atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. Under highly pure conditions, at room temperature, and for fluences of F ≈ 1 × 10^22 ions/m^2, no ion beam induced patterns develop for ion incidence angles ϑ ≤ 55° with respect to the global surface normal. In fact, the ion beam induces a smoothing of preformed patterns. Only for grazing incidence angles 60° ≤ ϑ < 81° pronounced ripple and tiled roof patterns develop. Analysis of the fluence dependence of pattern formation was conducted at ϑ = 75° in the unstable ion incidence angular range. The initially flat surface develops small amplitude, regular ripple patterns which then evolve to large amplitude, irregular facet patterns. Experiments were conducted to rule out or determine the processes of relevance in ion beam pattern formation on Si(001) with impurities. Co-deposition of stainless steel during ion beam erosion results in well developed hole, dot and ripple patterns for even small ion fluences of F ≈ 5 × 10^21 ions/m^2. The key factor determining the type of pattern realized is the ion-to-impurity arrival ratio. While in a broad range from 140K to 440K pattern formation tends to be temperature independent, dramatic changes take place above a threshold temperature of about 600K, when structures of crystalline iron silicide are shaped upon the surface. For these high temperatures nanopillars and spongelike patterns with amplitudes in the order of 100nm and directed towards the ion beam evolve. Furthermore, variation of the angle between ion beam and impurity source has a significant effect on pattern formation. The larger this angle is, the more efficient the pattern formation. This observation highlights the relevance of shadowing. Our investigations on the phenomenology of metal assisted ion beam pattern formation identify height fluctuations, local flux variations, induced chemical inhomogeneities, silicide formation and ensuing composition-dependent sputtering to be of relevance

Is keV ion induced pattern formation on Si(001) caused by metal impurities?

We present ion beam erosion experiments performed in ultra high vacuum using a differentially pumped ion source and taking care that the ion beam hits the Si(001) sample only. Under these conditions no ion beam patterns form on Si for angles below 45 degrees with respect to the global surface normal using 2 keV Kr ions and fluences of 2 x 10^22 ions/m^2. In fact, the ion beam induces a smoothening of preformed patterns. Simultaneous sputter deposition of stainless steel in this angular range creates a variety of patterns, similar to those previously ascribed to clean ion beam induced destabilization of the surface profile. Only for grazing incidence with incident angles between 60 degrees and 83 degrees pronounced ion beam patterns form. It appears that the angular dependent stability of Si(001) against pattern formation under clean ion beam erosion conditions is related to the angular dependence of the sputtering yield, and not primarily to a curvature dependent yield as invoked frequently in continuum theory models.Comment: 15 pages, 7 figures. This is an author-created, un-copyedited version of an article published in Nanotechnology. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from i

Evolution of ion beam induced patterns on Si(001)

In the range of incidence angles between 58 degrees and 79 degrees, Si develops erosion patterns through room-temperature exposure to 2 keV Kr+, while for other incidence angles it remains flat. We investigate the formation of these patterns through all in situ sample preparation and investigation under ultrahigh vacuum conditions. The ion fluence is varied by a factor of 1000 for the incidence angles of 63 degrees and 75 degrees. The resulting morphologies are imaged by scanning tunneling microscopy and quantitatively analyzed in view of roughness, wavelength, disorder, and surface slopes. We find it necessary to distinguish between low-fluence and high-fluence regimes of pattern formation. While in both low-fluence regimes a similar parallel mode ripple pattern evolves, the high-fluence regimes are distinctly different and evidence either the evolution of disordered perpendicular ripples or a roof-tile structure for 63 degrees or 75 degrees, respectively. The data are compared to other experimental data for ion beam erosion of Si and Ge enabling us to assess the universality of our observations. Comparison to existing models for the surface evolution under ion exposure allows us to draw conclusions on the applicability of these models for pattern formation on Si(001)

Silicide induced ion beam patterning of Si(001)

Low energy ion beam pattern formation on Si with simultaneous co-deposition of Ag, Pd, Pb, Ir, Fe or C impurities was investigated by in situ scanning tunneling microscopy as well as ex situ atomic force microscopy, scanning electron microscopy, transmission electron microscopy and Rutherford backscattering spectrometry. The impurities were supplied by sputter deposition. Additional insight into the mechanism of pattern formation was obtained by more controlled supply through e-beam evaporation. For the situations investigated, the ability of the impurity to react with Si, i.e. to form a silicide, appears to be a necessary, but not a sufficient condition for pattern formation. Comparing the effects of impurities with similar mass and nuclear charge, the collision kinetics is shown to be not of primary importance for pattern formation. To understand the observed phenomena, it is necessary to assume a bi-directional coupling of composition and height fluctuations. This coupling gives rise to a sensitive dependence of the final morphology on the conditions of impurity supply. Because of this history dependence, the final morphology cannot be uniquely characterized by a steady state impurity concentration