Atomic Layer Processing of Molybdenum Disulfide Thin Films
Authors: Soares Jake Alan 
Published: March 2023

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.