Nitridated Pd/B₄C multilayer mirrors for soft X-ray region: internal structure and aging effects

Area of Science:

• Materials Science
• Thin Film Technology
• Optics

Background:

• High reflectance multilayers are crucial for soft X-ray optics.
• Palladium/Boron Carbide (Pd/B4C) multilayers are promising for soft X-ray applications.
• Understanding the impact of nitrogen incorporation and aging is essential for device stability.

Purpose of the Study:

• To investigate the effect of nitrogen incorporation during reactive sputtering on Pd/B4C multilayer properties.
• To evaluate the high reflectance performance of nitridated Pd/B4C multilayers in the soft X-ray region.
• To analyze the aging behavior of these multilayers when exposed to air.

Main Methods:

• Reactive sputtering using Argon and Nitrogen gas mixtures (4%, 8%, 15% N2 partial pressure).
• Characterization using reflectance measurements, transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS).
• Analysis of structural changes, stress relaxation, and chemical composition before and after aging.

Main Results:

• Nitrogen incorporation reduced compressive stress and slightly increased interface roughness.
• A maximum reflectance of 32% at 9.5 nm was achieved with 15% N2.
• Nitridated multilayers exhibited aging effects, including layer deterioration, interdiffusion, and reduced d-spacing, with higher nitrogen content showing less degradation.
• EDS confirmed reduced N and B concentrations in degraded areas, suggesting upward diffusion.

Conclusions:

• Reactive sputtering with nitrogen is effective in fabricating high-performance Pd/B4C multilayers for soft X-rays.
• Nitrogen and boron diffusion is identified as the primary mechanism for the observed aging effects.
• Optimizing nitrogen content can mitigate aging and enhance the long-term stability of these multilayers.