Diffusion reaction of oxygen in HfO2/SiO2/Si stacks

Area of Science:

• Materials Science
• Semiconductor Physics
• Surface Chemistry

Background:

• Understanding silicon oxidation is crucial for semiconductor device fabrication.
• High-k dielectrics like Hafnium dioxide (HfO2) are used in advanced transistors.
• The role of HfO2 in the underlying silicon oxidation process requires detailed investigation.

Purpose of the Study:

• To elucidate the oxidation mechanism of silicon beneath a thin HfO2 layer.
• To identify the species responsible for silicon dioxide (SiO2) growth.
• To determine the rate-limiting step in the oxidation process.

Main Methods:

• Annealing of HfO2/SiO2/Si stacks in an 18O2 atmosphere.
• Analysis of 18O distribution using time-of-flight secondary ion mass spectrometry (ToF-SIMS).
• Investigating the dependence of oxidation velocity on oxygen partial pressure and annealing temperature.

Main Results:

• Molecular oxygen (O2), not atomic oxygen, is responsible for SiO2 growth.
• The rate-determining step is the oxygen exchange at the HfO2/SiO2 interface.
• Chemisorbed moisture in HfO2 significantly accelerates silicon oxidation, mimicking wet oxidation kinetics.

Conclusions:

• The oxidation mechanism is primarily governed by molecular oxygen diffusion and interface exchange.
• The presence of moisture within the HfO2 layer drastically alters the oxidation rate.
• Controlling moisture in HfO2 films is critical for managing silicon oxidation during fabrication.