Composition, coverage and band gap analysis of ALD-grown ultra thin films

Importance of the topic

Thin high-k dielectric films such as HfO₂ are critical in advanced semiconductor devices, offering superior gate insulation and enabling further miniaturization. Precise control of composition, thickness, surface coverage and electronic properties (band gap) is essential for reliable device performance and process optimization.

Objectives and study overview

This study explores a multi-technique approach to characterize ALD-deposited HfO₂ films of varying thickness. The goals are to quantify deposited material, determine film thickness, assess surface coverage uniformity, and measure the band gap of ultra-thin HfO₂ layers as a function of ALD cycle number.

Methodology and instrumentation

A Thermo Scientific Nexsa XPS system was employed, combining:

• XPS for elemental composition and overlayer thickness measurement.
• ISS to probe the topmost atomic layer and evaluate film coverage.
• REELS for rapid band gap determination via electron energy loss onset.

The Avantage data analysis software applied Beer–Lambert–derived attenuation models to extract thickness values from XPS signal intensities. ISS spectra of He⁺ scattered ions provided qualitative coverage data by monitoring the Si signal attenuation. REELS spectra were collected with the system’s dual-source flood gun, enabling direct band gap calculations.

Key results and discussion

• Composition and thickness: XPS survey spectra showed a progressive increase in Hf signal with ALD cycle number. Calculated thicknesses ranged from sub-nanometer to ~10 nm after 100 cycles.
• Surface coverage: ISS measurements revealed diminishing Si peaks as HfO₂ deposition progressed. Full surface coverage was achieved between 20 and 50 ALD cycles.
• Band gap: REELS spectra recorded after 100 cycles exhibited a clear onset of inelastic losses. Automated analysis determined the band gap of the HfO₂ films, confirming expected high-k dielectric behavior.

Benefits and practical applications

The integrated multi-technique workflow enables:

• Quantitative verification of ALD film growth and uniformity.
• Rapid identification of incomplete coverage to guide process adjustments.
• Accurate band gap measurement without additional instrumentation.

Such comprehensive characterization supports quality control in semiconductor manufacturing, LED and photovoltaic material development.

Future trends and potential applications

Advances may include extension to in situ ALD monitoring, integration with other surface-sensitive methods (e.g., UV-vis spectroscopy), and application to emerging 2D materials or complex multilayer stacks. Automated feedback loops could further enhance process repeatability and device yield.

Conclusion

The Thermo Scientific Nexsa XPS platform, augmented with ISS and REELS capabilities, offers a robust solution for the detailed analysis of ultra-thin ALD films. By combining compositional, coverage and electronic property measurements in a single instrument, researchers and engineers gain actionable insights into high-k dielectric materials essential for next-generation semiconductor devices.

Instrumentation used

• Thermo Scientific Nexsa XPS System
• Avantage Data Acquisition and Analysis Software

Reference

• Paul Mack, Robin Simpson. Composition, coverage and band gap analysis of ALD-grown ultra thin films. Thermo Scientific Application Note AN52344, 2018.