Partial Restriction. Restriction lift date: 2025-12-31
Modelling of thin film oxide growth and etching
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Date
2023
Authors
Mullins, Rita
Journal Title
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Publisher
University College Cork
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Abstract
As integrated circuit technology follows Moore's Law and continues to shrink, conventional methods for depositing and etching thin films encounter numerous challenges. Moreover, the challenges intensify due to the expected widespread adoption of 'more-than-Moore' devices designed for non-conventional computing applications. Traditional deposition methods will struggle to deliver continuous films at increasingly thinner levels and on complex 3D structures. Moreover, traditional continuous-wave plasma etching faces problems in achieving precise critical dimension control, enhanced selectivity, and with minimal plasma damage, especially below the 10 nm scale. In response to these limitations, atomic-scale processing techniques have emerged; producing thin films tailored to meet the demands of smaller and more intricate structures crucial for future semiconductor devices.
Atomic layer deposition (ALD) and atomic layer etching (ALE) offer uniform and conformal processing with precise thickness control and can be achieved using sequential, self-limiting thermal surface reactions. These are used in various applications, such as employing ALE for etching high dielectric metal oxides necessary for gate dielectrics in complex transistor structures such as Gate All-Around or Complementary FET, and utilizing ALD to deposit barrier/liner thin films within interconnects. Presently, ALD is widely used in the semiconductor industry whereas thermal ALE is in the early stage of development and is an emerging and promising frontier in thin film processing.
It is difficult to investigate ALD and ALE reactions directly using experimental techniques. First principles density functional theory (DFT) can give deep insights into precursor chemistry and reaction mechanisms of ALD and ALE processes. In my thesis I studied the hydrogen fluoride (HF) pulse as the first step in thermal ALE of high dielectric metal oxides. A thermodynamic analysis is used to predict the temperature at which the targeted self-limiting (SL) reactions are favoured over continuous spontaneous etching (SE) in an ALE cycle. Furthermore, calculations of HF adsorption are performed on the oxide surfaces to understand the mechanistic details of the HF pulse and calculate theoretical etch rates. The results are compared between the metal oxides studied: monoclinic HfO2 and ZrO2, orthorhombic HfO2, amorphous HfO2 and ZnO. HCl is examined as an alternative to HF for crystalline HfO2, ZrO2 and ZnO. The second step in thermal ALE, the ligand exchange reaction is examined for crystalline HfO2 using HF and SiCl4 allowing us to determine how the target Hf species can be chlorinated before being eliminated as volatile Cl-containing species.
A combined barrier and liner material incorporating Ru or Co into TaN has been proposed to replace the tri-layer stack of TaN/Ta/Cu for advanced interconnect technology. This will accommodate high aspect ratio trench structures with the continued miniaturization of devices and extend the use of Cu in interconnects for the next generation of electronic devices. Ruthenium and cobalt are also potential replacements for Cu in next-generation interconnects. In this thesis, DFT calculations are also used to explore the nature of N2/H2 terminated TaN surfaces that are produced after a plasma pulse (with H2 or N2/H2 plasma) in Plasma Enhanced ALD to incorporate Ru or Co into TaN. The reactivity of Ru and Co precursors is studied on stable NHx-terminated TaN surfaces. This work will help guide experimental PEALD for the incorporation of ruthenium or cobalt into TaN as a combined barrier and liner material. Finally the mechanism of Ru ALD using the novel precursor RuO4 and molecular H2 was investigated to elucidate the role of both reactants.
Description
Partial Restriction
Keywords
Atomic layer etching , Atomic layer deposition , High dielectric materials , Interconnects , Semiconductor devices , Materials modelling , Computational chemistry , Density Functional Theory
Citation
Mullins, R. H. 2023. Modelling of thin film oxide growth and etching. PhD Thesis, University College Cork.