S4-2
Solid-Vapor multiphase plasma: Mesoplasma processing )#,
Makoto Kambara$"Department of Materials Engineering, The University of Tokyo
7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan(&&   %!
113-8656 ('*+7-3-1
Mesoplasma can be distinguishably defined as a plasma in the 0.1 – 10 Torr range which possesses thecharacteristics of both low pressure and thermal plasmas. Upon application of such a non-equilibrium
plasma flow to chemical vapor deposition, formation of controlled growth precursor similar to the lowpressure plasma and its fast transport toward the substrate at high flux are anticipated, which coincides withthe requirements of the technologies for large scale production of functional devices/materials. In the caseof Si deposition by CVD as one example, it is revealed that liquid like nanoclusters are formed as precursor
and their unique dynamics facilitate fast rate Si epitaxial film deposition. In such a way, mesoplasma
potentially offers a unique environment for simultaneous attainment of quality and production speed.

1. Introduction
Recent devices in many applied fields require
mostly simultaneous attainment of functionalities
and low cost, and thus seek for production routes
that combine high quality and high throughputs. In
respond to such a demand, especially related to film
deposition, a plasma in the 0.1 – 10 Torr range is
expected to offer a non-equilibrium plasma flow
environment in which selected species are formed
and transported toward substrate at high flux. Oneof the important characteristics is that, in contrast to
the 3D space associated with formation depositionprecursor for the conventional low pressure
processing, a space for precursor formation can beconfined in the 2D region in this pressure range
because of gas flow characteristics of mesoplasma.
As a result, despite the difference in the mean freepath of such species between these pressures,
Knudsen number under mesoplasma can becomparably as large as that under low pressure
environment. If active species for deposition form
through cooling of high temperature chemistries
within the 2D space that is present at the tail ofmesoplasma flow ahead of the substrate, thedeposition itself can be controlled as nicely as low
pressure processing, while maintaining rather fast
rates owing to its flow dynamics under high density
environment. In fact, we have demonstrated fast
rate deposition of silicon epitaxial films thatmaintain wafer-equivalent qualities under themesoplasma condition [1]. This in turn suggeststhat the deposition precursors that potentiallyformed within the 2D region could have a clue tothis unique characteristic of the mesoplasma
processing.
2. Formation of liquid–like nanoclusters as
deposition precursors
In the case of the thermal plasma chemical vapor
deposition, raw materials that are introduced into
the plasma in a form of either gas, liquid or powder
are decomposed completely to atomic state at hightemperature, and these vapors are expected tocondense within the 2D thermal boundary region
present ahead of substrate. However, unlike thenanoparticle formation for thermal plasma CVD,
less agglomerated species with several nanometerin size are expected under the mesoplasma
condition because of the condensation in a reducedpressure. To identify such species that arepotentially in several nanometer immediately after
nucleation, x-ray scattering was employed to detect
them in-situ within the 2D reaction space during
CVD. In brief, x-ray was introduced into thethermal boundary region with several 100 µm
parallel to the substrate surface such that potential
growth precursors scatter x-ray spatially andanalysis of the scattering intensities provides thesize and structure of the scatterers. Details of themeasurement system can be found elsewhere [2,3].
Although no significant scattering signal of theincident x-ray (Cu-Kα) was monitored undervacuum or Ar plasma condition, appreciable
scattering intensity was detected from the region of

the 2D at a wide angle only when the source gas
was introduced. If atomic Si is present in the 2D
region, scattering decreases and transmission
increase because of its smaller cross section than Ar.
Therefore, this is an evidence that objects are notatomic species that scatter the x-ray. Furthermore,
small angle X-ray scattering revealed that theobjects are several nm in size and have a rather
loosely bound structure. Interestingly, the scattering
signals for the condition that the polycrystalline
films are deposited has suggested the formation ofrelatively large scatterers with liquid-like structure
similarly observed for the precursor for epitaxial
growth. It is therefore evidently identified thatnanosized clusters with liquid like atomic structure
plays determinant role in the film structural control
during fast rate CVD.
3. Unique dynamics of nanoclusters during
impingement on substrate
Molecular dynamics (MD) simulation wasemployed to reproduce formation of nanoclusters
through rapid condensation of high temperature Si
vapors and so to elucidate their roles in fast rate
epitaxy. Detailed computational conditions can befound elsewhere [4]. It is confirmed that a several
nm sized cluster having a loosely bound structure
forms as a transitional quasi-stable condensed phaseat a certain degree of undercooling duringcontinuous cooling before the constituent atoms areordered to form a crystal at low temperature. Such aliquid like structure was observed for relatively
large clusters and also for the binary system of Si-H.
Furthermore, during impingement of these clusters
on a substrate, globular clusters are found to exhibit
instantaneous deformation, allowing the constituent
atoms at least near the cluster / substrate interface torearrange their positions to follow the underlying
crystal structure. As a result, epitaxial film
deposition itself was reproduced after multiple
impingement of these liquid like clusters. For alarge cluster having 5 to 8 nm in size, in contrast,
although the structure was confirmed to be similarlyloosely bound, the constituent atoms far away fromthe interface was not properly ordered, resulting in
the amorphous / polycrystalline structural formation.
This proves therefore that the cluster determines thefilm structure and thus the quality. Also, since highdegree of sticking of clusters is observed in thepresent simulation, cluster could also be a key toattain fast rate deposition. Several MD simulations
have reported that epitaxial growth was attained bydeposition of nanosized clusters on the condition
that they are given enough high kinetic energies topromote surface diffusion. In contrast, the cluster

created during rapid condensation does notnecessarily require such a large kinetic energies forepitaxy. This is additional uniqueness of theclusters formed in the mesoplasma processing.
4. Summary and prospects
Liquid like cluster formed in the mesoplasma
environment can be an unique deposition precursors
that offer potentially both fast rate and reasonablehigh quality. Under such a non-equilibrium
condensation, we can also use the chemical
non-equilibrium, in a way that high temperaturestable chemistries are frozen-in. In the case of Si
production from SiHCl3 (TCS), SiCl4 (STC) that is
stable at low temperature limits the conversion ofTCS to be only around 30% at the maximum if theprocess based on the equilibrium chemistries isemployed. Under mesoplasma, in contrast, hightemperature gas vapors at temperature where STC
is no longer stable can be directly utilized fordeposition precursor formation, resulting in an
improved conversion yield of ~60 % [6].
Similar to Si nanoclusters for Si epitaxy, one canexpect multi-component clusters for alloy phasesingle crystal. For example, SiC is expected in avariety of devices ranging from high temperature
semiconductor to next generation jet turbine, bothof which by nature require high quality and large
quantities. Unlike Si, SiC sublimates and no
congruent melt exist under equilibrium, which is
why this material is synthesized primarily by Si2C
and SiC2 stable molecules. It is therefore expectedthat liquid-like SiC cluster forms as a transitional
precursor and facilitates fast rate SiC epitaxy imilar
to Si epitaxy. In such a way, mesoplasma
processing may open up a novel production path ofquality products of multicomponent.
Acknowledgments
This work is partly supported by Grant-in-Aid forExploratory Research 26600125.
References[1] M. Kambara, H. Yagi, M. Sawayanagi, and T.
Yoshida: J. Appl. Phys. 99 (2006) 074901.
[2] M. Kambara, Y. Hamai, H. Yagi, and T. Yoshida,Surf. Coat. Technol. 201 (2007) 5529.
[3] J.M.A. Diaz, M. Kambara and T. Yoshida; J. Appl.
Phys. 104 (2008) 013536-5.
[4] L.W. Chen, Y. Shibuta, M. Kambara, and T.
Yoshida: Chem. Phys. Lett. 564, (2013) 47.
[5] I. Kwon, R. Biswas, G. S. Grest, C.M. Soukoulis,
Phys. Rev. B 41 (1990) 3678.
[6] S. Wu, M. Kambara, T. Yoshida: Plasma Chem.
Plasma Process, 33 (2013) 433.