10.2 Plasma-Enhanced Atomic Layer Deposition (PEALD)

Atomic Layer Deposition (ALD) is a powerful thin film deposition technique. It uses self-limiting surface reactions to grow films one atomic layer at a time, offering precise thickness control and conformal coverage on complex structures.

Plasma-Enhanced ALD (PEALD) takes this process further by incorporating plasma. It enables lower deposition temperatures, improved film properties, and a wider range of materials. However, PEALD also brings challenges like potential substrate damage and increased process complexity.

Fundamentals of Atomic Layer Deposition (ALD)

Principles of atomic layer deposition

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• Self-limiting surface reactions ensure precursors react with the surface until all available sites are occupied (chemisorption), and excess precursor is purged to prevent further growth (physisorption)
• Alternating exposure of precursors involves exposing the substrate to precursor A, purging, then exposing to precursor B, and purging again, with the cycle repeating until the desired film thickness is achieved (binary reaction sequence)
• Layer-by-layer growth deposits a single atomic layer per cycle, allowing precise control of film thickness by adjusting the number of cycles (digital control)
• Conformal coverage enables uniform deposition on high aspect ratio structures (trenches, vias) due to the self-limiting surface reactions that prevent excess growth

Plasma-Enhanced Atomic Layer Deposition (PEALD)

Role of plasma in ALD

• Plasma serves as a reactive species source by generating highly reactive radicals, ions, and excited species that enhance surface reactions and reduce process temperature (lower activation energy)
• Improved film properties result from using plasma, including higher film density, lower impurity levels, and better electrical (conductivity) and optical properties (refractive index) compared to thermal ALD
• Expanded material selection is possible with PEALD, enabling deposition of materials difficult to achieve with thermal ALD, such as nitrides (AlN, TiN), metals (Cu, Ta), and doped materials (Al-doped ZnO)

Key parameters for PEALD growth

• Plasma power and duration influence film properties, with higher power and longer plasma exposure improving properties, but excessive power or duration potentially causing plasma damage (ion bombardment)
• Precursor and plasma gas chemistry affect film composition and properties, with common examples including O2, N2, H2, and NH3 plasmas for depositing oxides (Al2O3, TiO2), nitrides (SiNx), and metals (Ru, Pt)
• Substrate temperature is typically lower in PEALD compared to thermal ALD, but still affects growth rate (Arrhenius behavior) and film properties (crystallinity, stress)
• Pressure and gas flow rates influence precursor and plasma species transport to the substrate, affecting film uniformity (thickness variation) and conformality (step coverage)

Benefits vs challenges of PEALD

• Benefits of PEALD include:
  1. Lower deposition temperatures that enable coating of temperature-sensitive substrates (polymers, organics)
  2. Improved film properties and expanded material selection compared to thermal ALD
  3. Conformal coverage on high aspect ratio structures (nanopores, MEMS devices)
  4. Precise thickness control at the atomic level (sub-nanometer resolution)
• Challenges of PEALD include:
  1. Plasma-induced damage to sensitive substrates or devices (charging, UV radiation)
  2. Complexity of integrating plasma sources with ALD reactors (remote vs direct plasma)
  3. Potential for increased particle generation and contamination (plasma instabilities)
  4. Optimization of plasma conditions for each material system (power, pressure, gas ratios)