Fabrication of Periodic Arrays of Nanoscale Square Helices
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Fabrication of Periodic Arrays of Nanoscale Square Helices Martin O. Jensen, Scott R. Kennedy and Michael J. Brett Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB, Canada T6G 2V4 ABSTRACT We demonstrate fabrication of periodic arrays of nanometre scale square helices, with potential applications in three-dimensional photonic bandgap (PBG) materials. Processing is performed using a thin film deposition method known as Glancing Angle Deposition (GLAD). Through advanced substrate motion, this technique allows for controlled growth of square helices in a variety of inorganic materials. Organization of the helices into periodic twodimensional geometries is achieved by prepatterning the substrate surface using electron beam lithography. The regular turns of the helices yield periodicity in the third dimension, perpendicular to the substrate. We present studies of tetragonal and trigonal arrays of silicon helices, with lattice constants as low as 300 nm. By deliberately adding or leaving out seeds in the substrate pattern, we have succeeded in engineering line defects. Our periodic nanoscale structure closely matches an ideal photonic band gap architecture, as recently proposed by Toader and John. While our fabrication technique is simpler than most suggested PBG schemes, it is highly versatile. A wide range of materials can be used for GLAD, manipulation of lattice constant and helix pitch ensures optical tunability, and the GLAD films are robust to micromachining. INTRODUCTION Glancing Angle Deposition is a physical vapour deposition (PVD) technique, in which the substrate is held at a highly oblique angle relative to the vapour source [1-4]. This produces thin films with high degrees of porosity (> 50 %), and GLAD films consequently have significantly different properties than conventional thin films deposited at near-normal flux incidence angles. Rather than being continuous, dense films, GLAD films consist of individual nanometre-scale structures, whose size and spacing depend on deposition parameters such as the flux incidence angle, temperature and material. The shape of the structures can be precisely controlled by advanced three-dimensional motion of the substrate, producing vertical and slanted posts, chevron structures, and circular and polygonal helices. The unique topography of GLAD films is the result of pronounced self-shadowing among the growing structures at high flux incidence angles, as well as limited adatom mobility and diffusion (see figure 1b below) [5-6]. When using smooth substrates, self-shadowing is initially enforced by clusters of atoms nucleating on the substrate, creating a stochastic array of structures. However, if the substrate is pre-patterned with an array of high aspect ratio seeds, film growth conforms to this pattern to yield a periodic film [7]. As with conventional PVD techniques, GLAD can be performed with a wide selection of inorganic materials such as metals, semiconductors, oxides, and nitrides. Numerous applications are being explored for
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