Glancing angle deposition: Fabrication, properties, and applications of micro- and nanostructured thin films
(a) Definition of the deposition angle and the substrate rotation angle . The deposition plane is defined by plane containing the incident flux direction and the substrate normal. (b) The GLAD apparatus as it is typically implemented in a standard PVD system. Substrate movement is accomplished by two independent motors. The motors are computer controlled, receiving feedback from thickness monitors.
Archetypal GLAD fabricated microstructures, (a) tilted columns, (b) chevronic or zig-zag columns, (c) helical columns, and (d) vertical columns. The particular microstructure is determined by the substrate movement during deposition, with substrate rotation being used to “shadow sculpt” the columns.
Structure zone model of Movchan and Demchishin is succinctly presented by plotting the observed microstructure over a range of temperatures (normalized to the melting point of the material). The transition temperatures between zones I and II and between zones II and III occur at 0.3 and 0.5, respectively. Image adapted from (Ref. 14 ).
Overview of the evolutionary model for columnar thin film growth, as proposed by Messier et al. A given columnar structure is composed of smaller columns. Reprinted with permission from (Ref. 18 ), Copyright 1984.
Nuclei distributed across the surface will lead to ballistic shadowing of the surrounding regions. This restricts further growth to the tops of the nuclei which develop into columnar structures.
Columns will grow oriented toward the source, forming an angle with the substrate normal .
Plot of four different analytic curves relating the column tilt angle and the deposition angle , the tangent rule (solid line) (Ref. 21 ), Tait’s rule (line with circles) (Ref. 22 ), Meakin’s expression (line with triangles) (Ref. 23 ), and Lichter and Chen’s expression (line with squares) (Ref. 24 ).
Features patterned onto the substrate can initiate the shadowing process in a controlled manner. On an unmodified substrate the nucleation process is random leading to a random distribution of columnar structures. On a patterned substrate, however, the columns will adopt a planar order based on the patterned features [tetragonal in (a) and hexagonal close packed in (b)]. Image (a) from Ref. 55 , Copyright 2005 IEEE. Image (b) reprinted from Ref. 58 , Copyright 2006.
Using properly designed seed layers, the columnar structures produced by the GLAD process are much more uniform than those produced by GLAD on unmodified substrates and exhibit planar ordering. This is a key requirement of certain applications, such as photonic band gap materials. Image from Ref. 55 , Copyright 2005 IEEE.
PhiSweep technique involves incremental growth from two different directions to avoid the broadening and competition effects seen in traditional GLAD processes. Image reprinted from Ref. 59 , with kind permission from Springer Science and Business Media.
Substrate swing method can be used to control the columnar cross section and avoid broadening, leading to more uniform structures. Image from Ref. 54 .
By depositing Al at oblique angles onto high temperature substrates, nanoscale rod and wire structures are formed. The structures appear to be single crystalline. Image from Ref. 79 , Copyright 2007.
columns fabricated using the GLAD technique self-assemble in an ordered manner. The columns themselves maintain their shape throughout the thickness of the film and exhibit a much greater uniformity from column to column. Images adapted from Ref. 81 , Copyright Wiley-VCH Verlag GmbH & Co. Reproduced with permission.
Periodicity of the helical columnar structure will lead to a circular analog of the Bragg effect. Circularly polarized light with handedness equal to the structure (right handed, in this case) will be reflected, whereas the orthogonal polarization is unaffected. Image from Ref. 153 .
GLAD fabricated graded-index antireflection coatings demonstrate excellent performance, with a maximum of 99.9% transmittance at normal incidence. Over 99% of incident light (both TE and TM polarizations) is transmitted through the sample over a broad wavelength range even at angles up to 30°. Image from Ref. 168 .
Square spiral structures, when arranged in a tetragonal lattice, are 3D photonic crystals. Image (a) from Ref. 181 .
Synopsis of experimentally reported values for the scaling exponents of various morphological parameters.
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