Abstract
The method of atomic layer deposition (ALD) is considered one of
the primary candidates for the uniform and conformal deposition of ultrathin
films vital for the continuous miniaturization in the semiconductor
industry and related high-technology markets. By the virtue of two selflimiting
surface reactions, the ALD technique yields an ultimate control
of film growth in the sense that a submonolayer of material is deposited
per so-called ALD cycle. With established materials being at the verge
of industrial implementation, efforts are continuously undertaken to optimize
and develop new ALD configurations and processes. So far, the main
emphasis within the field of ALD has been on the materials characterization
of the films by means of ex situ analysis. The research described
in this thesis aims at the development of the relatively new configuration
of plasma-assisted ALD and at in situ diagnostics studies of the (plasmaassisted)
ALD processes.
In plasma-assisted ALD, a plasma is used to activate the reactants in
the gas phase in order to supply additional reactivity to the ALD chemistry.
Plasma-assisted ALD is researched to provide benefits in the development
of new ALD processes and materials. In particular, the opportunities to
improve and tune the film properties, and to deposit films at reduced substrate
temperatures have been addressed in this thesis. This work has
been accompanied by studies using various in situ diagnostics, from which
fundamental insight into the reaction mechanisms governing the ALD processes
can be obtained. Moreover, in situ techniques provide the opportunity
to monitor, optimize, and control the ALD process. In this work
the use of in situ spectroscopic ellipsometry, transmission infrared spectroscopy,
mass spectrometry, and optical emission spectroscopy has been
demonstrated in studies of the plasma-assisted ALD processes of metal nitrides
and metal oxides. The results of the film characterization obtained
by these techniques have been corroborated and complemented by extensive
ex situ analysis. In particular, the combination of in situ spectroscopic
ellipsometry and the layer-by-layer ALD growth has been explored comprehensively.
The merits of this in situ technique during ALD have been
demonstrated by addressing various aspects relevant to ALD processes and
materials.
A large part of the work has concentrated on the plasma-assisted ALD
process of the metal nitrides TiN and TaN. The merits of plasma-assisted
ALD were observed in the deposition TiN films with excellent conductivity
and low impurity content, even at low deposition temperatures. Furthermore,
it was shown that by variation of the plasma condition in the ALD
process of TaN, the film properties could be tailored from conductive, cubic
TaNx;x??1 to semiconductive, amorphous Ta3N5. These aspects were clearly
demonstrated by in situ spectroscopic ellipsometry, where the transition
in TaNx phase could be distinguished by monitoring the energy dispersion
in the optical constants. For the conductive films, the light absorption by
free conduction electrons could be probed and that enabled extraction of
the electrical film properties from the ellipsometry data. The latter was
valuable to demonstrate electron-impurity scattering and finite size effects
in TiN films. Furthermore, fundamental insight into the reaction mechanisms
of plasma-assisted ALD process of TaN was obtained by detection of
the volatile reaction by-products by mass spectrometry and optical emission
spectroscopy.
The possibilities for plasma-assisted ALD to improve the material properties
and to deposit at reduced temperatures have been demonstrated for
the process of Al2O3. The Al2O3 films were deposited at substrate temperatures
down to room temperature and these films yielded good moisture
permeation barrier properties as relevant for encapsulation purposes.
The fundamental reaction mechanisms of this plasma-assisted ALD process
were elucidated by transmission infrared spectroscopy in order to understand
and further improve the film properties obtained at these reduced
deposition temperatures. It was established that the surface chemistry is
ruled by –CH3 and –OH surface groups created by the Al(CH3)3 precursor
adsorption and the combustionlike reactions during the O2 plasma step,
respectively. Moreover, infrared spectroscopy provided insight into the influence
of deposition temperature on the material properties. It was shown
that by prolonging the plasma exposure, i.e., by supplying more plasma
reactivity to the ALD process, the surface chemistry at low temperatures
was enhanced and the impurity content in the Al2O3 was reduced.
In conclusion, the knowledge gained through the in situ diagnostic
studies in this work is relevant to further develop the ALD technique. The
insight obtained into the reaction mechanisms and the material properties
of the ALD films in this work are particularly useful to further exploit
the possibilities and opportunities of the plasma-assisted ALD technique
in the synthesis of novel (complex) materials.
Original language | English |
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Qualification | Doctor of Philosophy |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 6 Oct 2008 |
Place of Publication | Eindhoven |
Publisher | |
Print ISBNs | 978-90-386-1368-0 |
DOIs | |
Publication status | Published - 2008 |