Improving Thickness Uniformity of Mo/Si Multilayers on Curved Spherical Substrates by a Masking Technique

Abstract

In this work, a masking technique was used to improve the thickness uniformity of a Mo/Si multilayer deposited on a curved spherical mirror by direct current (DC) magnetron sputtering with planetary rotation stages. The clear aperture of the mirror was 125 mm with a radius of curvature equal to 143.82 mm. Two different shadow masks were prepared; one was flat and the other was oblique. When using the flat mask, the non-uniformity considerably increased owing to the relatively large gap between the mask and substrate. The deviation between the designed and measured layer thickness and non-uniformity gradually reduced with a smaller gap. The second mask was designed with an oblique profile. Using the oblique mask, the deviation from multilayer thickness uniformity was substantially reduced to a magnitude below 0.8% on the curved spherical substrate over the clear aperture of 125 mm. Multilayers still achieved a smooth growth when deposited with obliquely incident particles. The facile masking technique proposed in this study can be used for depositing uniform coatings on curved spherical substrates with large numerical apertures for high-resolution microscopes, telescopes, and other related optical systems.

Preparation and Structural Investigation of Ultra-Uniform Mo Films on a Si/SiO2 Wafer by the Direct-Current Magnetron Sputtering Method

“…The nonuniformity of film thickness is a significant criterion for assessing film quality, as it can directly influence the usable size, productive yield, and overall production cost. − In industrial production, film thickness nonuniformity is often required to be lower than 3∼5%, and some processes even have stricter requirements. For example, in order to achieve constant optical properties on the optical element surfaces of high-resolution microscopes, telescopes, and other related optical systems, depositing uniform coatings with better than 0.8% nonuniformity was normally demanded . In addition, film thickness nonuniformity can impact the film’s performance, such as the photovoltaic property and optical constants .…”

“…For example, in order to achieve constant optical properties on the optical element surfaces of high-resolution microscopes, telescopes, and other related optical systems, depositing uniform coatings with better than 0.8% nonuniformity was normally demanded. 15 In addition, film thickness nonuniformity can impact the film's performance, such as the photovoltaic property 16 and optical constants. 17 Badgujar et al reported that to achieve high photoelectric conversion efficiency in copper indium gallium selenide (CIGS) thin-film solar cells, uniform and crystalline Mo(110) thin films were desired as a back contact.…”

Theoretical and Experimental Study of Particle Distribution from Magnetron Sputtering with Masks for Accurate Thickness Profile Control

“…IBF provides both high accuracy and high efficiency, and a figure error of 0.5 µrad (root mean square (RMS)) has been widely realized [8]. The addition methods include differential deposition [9][10][11][12][13] and profile coating [14][15][16][17][18][19] based on modern physical deposition techniques. These are also high-precision and high-efficiency correction techniques, and expensive substrates can be reused to create different profiles since the added correction layer can be removed with different methods [20][21][22].…”

“…The smallest RMS deviation between the deposited profile and the designed ellipse was below 0.41 nm [15]. Coating methods are easily implemented using standard coating machines, so there are gradually more people working on these methods [9][10][11][12][13][14][15][16][17][18].…”

“…A mask is placed between the target and substrate, and the opening width of the mask at different positions perpendicular to the substrate motion direction determines the coating thickness at the corresponding positions [19], as shown in Figure 1c. A one-dimensional correction profile is obtained perpendicular to the direction of substrate movement because of different opening widths at different positions on the mask, and the coating thickness along the substrate motion direction is uniform due to the constant-speed motion of the substrate [15][16][17][18]. In both differential deposition and profile coating, a dedicated mask is used to modify the spatial distribution of the deposited particles and generate a growing function for figuring.…”