Lateral Templating for Guided Self-Organization of Sputter Morphologies

Methods for the fabrication of large areas of nanoscale features with controlled period and intraperiod organization are of interest because of the potential for high-throughput mass production of nanoscale devices. Due to their potential in this regard, much recent attention has been devoted to self-organization processes, in which processing causes the spontaneous emergence of a nanoscale pattern. The short-range order can be quite high but some envisaged applications require long-range order, which is destroyed by uncontrolled topological defects arising spontaneously from the self-organization process. A potentially successful hierarchical fabrication strategy is the fabrication of controlled features at a small, but lithographically tractable, length scale by methods such as conventional mask or optical-standing-wave lithography, in order to guide a self-organization process at the finest length scale. Topographic patterning has been used for templating the local disorder in two-dimensional (2D) self-assembled monolayers and for templating defect organization or elimination in three-dimensional (3D) colloidal crystallization. Topography has also been used to manipulate semiconductor quantum-dot placement, composition, and strain, through its effect on stress, surface energy, and mobility. 3D short-range ordering of grown-in quantum-dot short-period superlattices can result from multilayer growth; nanoscale topographic templating of the first layer could dramatically accelerate the development of order and lead to true long-range order. Lithographically and focused ion beam (FIB)-patterned topographies have recently been used to template quantum-dot growth in linear chains, periodic 2D lattices, and in more complex configurations that are promising for novel nanoelectronic architectures, such as quantum cellular automata. The finest features have been templated by serial writing with a FIB, a prohibitively expensive process for mass production that might be circumvented by using a hierarchical fabrication strategy. Here we report the influence of patterned boundaries on the primary material of complementary metal oxide semiconductor (CMOS) technology, i.e., a Si(001) substrate, in guiding self-organized topographic ripples spontaneously appearing during uniform irradiation with low energy Ar ions. We show that the long-range order of the features can be greatly enhanced by this lateral-templating approach. The emerging pattern can be manipulated by changing the boundary spacing and misorientation with respect to the projected ion-beam direction. We develop a scalar figure of merit, a dimensionless topological defect density, to characterize the degree of order of the pattern. At small boundary separation, greatest order is observed when the separation is near an integer multiple of the spontaneously arising feature size. The defect density is exceedingly low up to a critical misorientation angle, beyond which topological defects develop in proportion to the incremental misorientation. These results suggest the potential utility of lateral templating ion-irradiation-induced topographic patterns for highthroughput fabrication at sublithographic length scales. Sputter patterning is the spontaneous formation of oneand two-dimensional arrays of topographical ripples and dots on the surfaces of solids eroded by a directed ion beam. This phenomenon has been observed for a wide range of ion species, energies, and incident angles on a variety of materials, including glass, semiconductors, metals, and insulators. Spontaneously emerging feature sizes, which are selected by the kinetics and, consequently, are continuously tunable via the operating conditions, have been reported as small as 15 nm. In general, controlling the behavior of such driven systems is important in many areas of science and engineering. The success of lateral templating raises the possibility of controlling the morphology at a fine length scale in mass production. Because ion irradiation is currently used for doping control in the mass production of semiconductor devices, there may be little impediment to its rapid uptake for morphology control. We investigate the control of the self-organized sputter pattern by using lithography to control only the boundaries of the field over which morphological features evolve. The system chosen for study was silicon with argon-ion irradiation under conditions known to create a topographically rippled surface with small ordered domains at a spontaneously arising “natural” wavelength of k= 140 nm. Details are reported in the Experimental section. The morphological development of templated and non-templated regions was compared after exposure under identical conditions. Figure 1a shows an atomic force microscopy (AFM) topograph of a templated substrate after ion bombardment. The initial configuration consists of a series of 200 nm mesas and 200 nm valleys with a periodicity of 400 nm, as shown in the top trace in Figure 1b. Figure 1a and the bottom trace of Figure 1b C O M M U N IC A IO N S