Optimization of MgF₂-deposition temperature for far UV Al mirrors

Area of Science:

• Materials Science
• Optical Engineering
• Space Science

Background:

• Far ultraviolet (FUV) coatings are critical for space missions like LUVOIR.
• Enhancing reflectance of MgF2-protected Al mirrors is an ongoing challenge.

Purpose of the Study:

• To determine the optimal MgF2 deposition temperature for maximizing Al mirror reflectance at 121.6 nm.
• To investigate the effect of deposition temperature on the stability and properties of FUV coatings.

Main Methods:

• Depositing Al films at room temperature and protecting them with MgF2 films at various temperatures (room temperature to 350°C).
• Analyzing mirror reflectance at 121.6 nm.
• Characterizing film roughness and density using x-ray grazing incidence reflectometry.
• Investigating the impact of post-deposition annealing.

Main Results:

• Mirror reflectance increased with MgF2 deposition temperature up to 250°C, reaching ~90% at 121.6 nm.
• Coatings deposited at 250°C showed less than 1% reflectance decay over time.
• High-temperature deposition reduced Al and MgF2 roughness and increased MgF2 density.
• Annealing MgF2-protected Al mirrors also enhanced reflectance.

Conclusions:

• Optimal MgF2 deposition temperature for enhanced Al mirror reflectance in the FUV is around 250°C.
• High-temperature deposition or annealing improves FUV coating stability and optical properties.
• Controlling deposition temperature is key to preventing pinholes in Al films for better FUV performance.