Another Puzzle - this time Thin Films Related

Glancing Angle Deposition (GLAD)

Colorized SEM collage with permission of Prof. Michael Brett University of Alberta
Description & pics below with permission of Prof. Kevin Robbie Queens University

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Glancing Angle Deposition (GLAD) is a technique for fabricating materials with controlled structure. It is based on thin film deposition, by evaporation or sputtering, and employs oblique angle deposition flux and substrate motion to allow nanometer scale control of structure in engineered materials.
The substrate is oriented at a large oblique angle relative to the incident vapour flux (a > 75). This leads to an effect called atomic shadowing and results in a porous structure with isolated columns of material growing toward the vapour source. The substrate is then rotated or tilted during deposition to engineer desired microstructures.

Atomic shadowing occurs when oblique vapour flux is shadowed by previously deposited film material from reaching areas on the substrate. These areas are shadowed. If the mobility of the atoms on the surface (adatoms) is limited, there is no growth in the shadowed regions. This results in the porous inclined microstructure observed in oblique deposition.

This figure shows the fundamental techniques used in GLAD with MgF2 films deposited on Si or glass. All 4 films were deposited with the polar flux angle a equal to about 85 degrees. In a), the substrate was held stationary and the flux arrived from the right for the entire deposition. A porous inclined microstructure resulted. In b), the direction of arrival of the flux was alternated from the left and right 12 times during deposition. Note that the polar angle a was kept fixed at 85 degrees for the entire deposition. In c), the substrate was rotated continuously during deposition. The film growth was always toward the vapour source so this resulted in a helical structure. A feedback control system was used to adjust rotation rate to accommodate for varying deposition rate and achieve constant pitch helices. In d), a combination of the techniques used in b) and c) was used. The substrate was rotated in 90 degree steps during deposition. A short pause for deposition followed each 90 degree rotation and the resulting structure is a 4 sided 'square helix'.

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