Fabrication of Helically Perforated Thin Films
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Fabrication of Helically Perforated Thin Films .'+DUULV./:HVWUD0-%UHWW
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, AB, Canada, T6G-2G7
ABSTRACT Using glancing angle deposition and templating techniques, we have fabricated a number of unique thin film microstructures. Engineered columnar thin films having the inverse of the desired structure (i.e., arrays of helices or chevrons) were first deposited by glancing angle deposition. These films were then filled with a solution of the desired material, and allowed to cure. The template material was then removed by chemical etching, leaving a perforated thin film. Such films have been produced of photoresist and spin-on-glass, on both silicon and glass substrates. The perforations have taken the form of chevrons and helices of large and small pitch, and have been arranged in both random and periodic (1µm spacing) arrays. INTRODUCTION In the literature, a number of methods have been described for producing thin films perforated by organized networks of pores. Photoanodic etching, for example, has been used to introduce high aspect ratio pores into silicon wafers.1 The pores, arranged with 3.5µm periodicity, were about 2µm in diameter and extended up to 400µm into the wafer. Other groups have employed different methods of achieving similar results. In high purity aluminum foils, for example, self-organized, hexagonally arranged pores up to 100µm deep were anodically etched with phosphoric acid.2 To date, perforated films have been generally limited to arrays of pores either circular or rectangular in crosssection, extending through the film in a direction perpendicular to the substrate. However, applications for films of a slightly different nature also exist, and have been proposed in earlier work.3 For instance, helical monoliths were recently found to be capable of rotating the polarization plane of linearly polarized incident light.4 It is likely that a helically perforated film could perform the same operation in the form of a more robust, less porous film. Furthermore, pores arranged into organized two-dimensional arrays are of interest to scientists studying photonic band gap crystals, and even more desirable structures would result if periodicity could be designed into the third spatial dimension through the fabrication of helical structures.5 Such opportunities provide the impetus for investigations into controlling the perforation shape in thin films.
FABRICATION Using glancing angle deposition (GLAD)6 in conjunction with standard photolithography processes, we have produced a variety of perforated thin films by a four step process. Using the production of a perforated photoresist film as an example, each fabrication step will be explained. D9.37.1
Step One – Deposition of the Template: In GLAD depositions, flux is incident upon a substrate from highly oblique angles, typically greater than 80°. This condition is sufficient to create a state of significantly enhanced surface shadowing (Figure 1). Any rando
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