Plasma-Enhanced Atomic Layer Deposition of HfO₂ with Substrate Biasing: Thin Films for High-Reflective Mirrors

Area of Science:

• Materials Science
• Thin Film Deposition
• Plasma Physics

Background:

• Controlling material properties of functional coatings is crucial for advanced applications.
• Plasma-enhanced atomic layer deposition (PEALD) offers precise control over thin film growth.
• Hafnium dioxide (HfO2) is a key material for optical coatings, but its deposition presents challenges.

Purpose of the Study:

• To systematically analyze the influence of substrate biasing on HfO2 thin film properties.
• To investigate the impact of ion energy tuning on structural, mechanical, and optical characteristics.
• To demonstrate the fabrication of high-performance high-reflective (HR) mirrors using biased HfO2 deposition.

Main Methods:

• Deposition of HfO2 thin films using tetrakis(dimethylamino)hafnium (TDMAH) and oxygen plasma at 100 °C.
• Systematic variation of substrate bias voltage during PEALD.
• Characterization of film properties including microstructure, density, refractive index, OH content, mechanical stress, and optical performance.

Main Results:

• Unbiased HfO2 films exhibited amorphous structure, low density, low refractive index, high OH content, and tensile stress.
• Low substrate bias improved film density, refractive index, and reduced OH incorporation, with stress becoming compressive.
• High bias led to decreased density, refractive index, increased OH, and crystalline inclusions; biased deposition enabled crack-free HR mirrors.

Conclusions:

• Substrate biasing is a versatile technique for precisely tuning HfO2 thin film properties in PEALD.
• Optimized ion energy control via substrate biasing is essential for fabricating high-quality optical coatings.
• The developed method allows for stress-compensated, crack-free HR mirrors with excellent optical and environmental stability.