Atomic Layer Processing of Molybdenum Disulfide Thin Films

Author

Jake Alan Soares, Boise State UniversityFollow

Publication Date

12-2022

Date of Final Oral Examination (Defense)

8-17-2022

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

Supervisory Committee Chair

Elton Graugnard, Ph.D.

Supervisory Committee Member

Kris Campbell, Ph.D.

Supervisory Committee Member

David Estrada, Ph.D.

Supervisory Committee Member

Steven Hues, Ph.D.

Abstract

As feature sizes in semiconductor devices continue to shrink, it is of upmost importance to synthesize materials that can accommodate the drastic degree of scaling. One such material receiving great attention is molybdenum disulfide (MoS₂), which is a semiconducting two-dimensional (2D) material in its most favorable few-layer form. The distinctive electrical properties make few to single-layer MoS₂ a potential candidate to replace silicon in many microelectronic devices. MoS₂ research is commonly conducted on mechanically exfoliated films due to the high quality, low defect layers that can be prepared. However, exfoliation is not a scalable method due to the lack of dimensional control and poor layer reproducibility. Currently, there is a lack of suitable methods for integrating MoS₂ films into manufacturing. Thus, there is a need for scalable industry-compatible processing methods to enable integration of MoS₂ in modern electronics manufacturing.

One processing technique that can be used for MoS₂ integration is atomic layer deposition (ALD). This technique is suitable because of its self-limiting, vapor-phase surface reactions used for thin film deposition. This process offers low temperature deposition of thin and conformal films with angstrom level control. This method is commonly used in high volume manufacturing, making it a clear choice as the processing technique that can be used for MoS₂ integration. One drawback, however, is the lack of in-depth knowledge of ALD MoS₂ thin films. By investigating the nucleation and growth of MoS₂ films, key insights can be established to allow for greater control over the deposition process and resulting material quality. This understanding of the ALD nucleation process can also help identify new processing methods, such as area-selective ALD (ASALD). ASALD can further support the efforts towards MoS₂ integration. This process can help solve the issue of placement errors found in standard lithography patterning. It can additionally provide another tool for creating complex device structures. Lastly, other processing techniques such as atomic layer etching (ALE) are also critical in manufacturing. Similar to ALD, ALE is the complementary vapor phase technique for layer-by-layer etching of uniform thin films. Combined, ALD and ALE provide scalable approaches for precise atomic layer processing and advanced manufacturing. To realize these useful processing methods, efforts need to be made to better understand the film substrate interactions and subsequent film growth or removal.

In this work, we present the study of the early stages of growth and nucleation of MoS₂ films on common metal oxide surfaces used in semiconductor manufacturing. We show the temperature dependence of nucleation over the range of 150-250 °C. This work identifies that hydroxyl concentrations on metal oxide surfaces are directly related to the disassociation of MoF₆ precursor on the substrate surface. This precursor disassociation leads to metal fluoride bonding, revealing the interface layer formation between deposited MoS₂ films and the substrate. Film morphology was additionally studied, revealing the critical role that temperature has on growth mechanisms during MoS₂ ALD. At increased growth temperatures, MoS₂ films exhibited higher degrees of MoS₂ bonding and crystalline grains oriented perpendicular to the growth surface. This study of the nucleation and growth process provides a greater understanding of 2D film-substrate interactions and offers more control over processing. Additionally, this work explores ASALD of MoS₂ films on common semiconductor surfaces by exploiting inherent differences in surface chemistry between substrate materials. Selective ALD was established between various materials including alumina and thermal oxide substrates. To our knowledge, this is the first ASALD process for a 2D material that achieves selectivity without the use of inhibitors. Lastly, we established a new thermal ALE process for the removal of MoS₂ films. This work identifies the removal of the MoS₂ films by means of fluorination and oxidation to create volatile moly-oxyfluoride byproducts. This method was shown to etch both amorphous and crystalline ALD films, where the etch rates were highly dependent on crystallinity and temperature.

This work provides insights and processing required for MoS₂ integration into nanoscale electronics, as well as many other applications. By the study of both the deposition and etching of MoS₂ films, we provide a greater depth of knowledge that will be required for MoS₂ integration into nanoscale manufacturing.

DOI

https://doi.org/10.18122/td.2037.boisestate

Recommended Citation

Soares, Jake Alan, "Atomic Layer Processing of Molybdenum Disulfide Thin Films" (2022). Boise State University Theses and Dissertations. 2037.
https://doi.org/10.18122/td.2037.boisestate