Enhanced Control of Porous Thin Film Morphology via Ion Bombardment
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0960-N01-03
Enhanced Control of Porous Thin Film Morphology via Ion Bombardment Michael D. Fleischauer, Jason B. Sorge, Robert A. Joseph, and Michael J. Brett Electrical and Computer Engineering, University of Alberta, 9107-116 St. NW, Edmonton, Alberta, T5K 2Y3, Canada
ABSTRACT Ion bombardment was used during and after the deposition of controlled-porosity thin films produced using the Glancing Angle Deposition (GLAD) technique. Post-deposition ion bombardment was used to mill vertical-post morphology GLAD films, whose constituent columns varied in size due to competition effects. Smaller columns, although in the minority, can have a significant impact on the function of these films in a variety of optics applications, and can only be avoided by pre-selecting column location (i.e. seeding). Uniform milling eliminated smaller columns and left only remnants of the largest columns, which then acted as seeds during subsequent depositions. This simple, non-lithographic technique leads to more uniform column morphology. Column tilt angle can also be adjusted using ion bombardment. Methods to tune column tilt angle as a function of ion current using ion-assisted deposition are discussed, with an emphasis on improved square spiral photonic band gap crystal fabrication. Ion bombardment was found to increase the tilt angle of each spiral arm, moving the architecture closer to the theoretical optimum structure. INTRODUCTION Control over thin film morphology can be realized by combining complex substrate motion with limited adatom mobility and self-shadowing in the highly oblique deposition regime. This process, known as Glancing Angle Deposition (GLAD),1 has been used to fabricate films with a variety of morphologies including slanted posts, chevrons, square spirals, helices, vertical posts, and combinations thereof, on the tens to hundreds of nanometres scale. Morphology control is achieved via precise rotation of the substrate relative to the incoming flux. Rotation in the plane of substrate allows for control over film density, and in the case of slanted posts, column tilt angle. The angle between the incident flux and substrate normal, α, is typically greater than 80°. Rotation about the substrate normal (known as rotation in ϕ) controls the chiral nature of the film (no rotation – slanted column, slow rotation – helix, fast rotation – vertical post). The search for new and improved GLAD morphologies has lead researchers to investigate complex substrate motion routines,2 substrate seeding,3 and post-deposition annealing.4 Ion bombardment is commonly used to modify the structure and morphology of thin films fabricated using physical vapour deposition methods. Thus, initial studies of the effect of ion-assisted deposition and post-deposition milling have also been performed.5,6 The purpose of this paper is to further investigate the role of post-deposition milling and ion-assisted deposition. Focus is divided in to two sections: using post-deposition milling to develop a non-lithographic seeding technique; and control ove
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