Atomic layer deposition of metal fluorides through oxide chemistry
Transcript of Atomic layer deposition of metal fluorides through oxide chemistry
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Atomic layer deposition of metal fluorides through oxide chemistry
Matti Putkonen,*ab Adriana Szeghalmi,c Eckhard Pippelc and Mato Knezc
Received 26th April 2011, Accepted 15th July 2011
DOI: 10.1039/c1jm11825k
In this paper, we report on the atomic layer deposition of magnesium, calcium and lanthanum
fluorides utilising two different approaches with hexafluoroacetylacetonate as a fluorine source. The
first approach is based on the reaction of fluorinated metal precursors with a strong oxygen source. A
deposition rate of 0.3 �A per cycle was obtained when calcium hexafluoroacetylacetonate (Ca(hfac)2)
was used as a metal precursor and ozone as a second reactant at 300 �C. From Rutherford
backscattering spectroscopy (RBS) measurements, the film stoichiometry was determined to be
CaF2.17 with less than 5 atomic% of oxygen. The second and more feasible approach is based on
metal oxide formation from traditional nonfluorinated b-diketonate type metal precursors and an
oxygen source, followed by fluorination using hexafluoroacetylacetonate (Hhfac) and ozone. The
MgF2, CaF2 and LaF3 thin films prepared using this method showed refractive indices of 1.429, 1.472
and 1.687, respectively. The oxygen content of these films was below the detection limit of the RBS,
which is 2 at%.
Introduction
Traditionally, the atomic layer deposition (ALD) chemistry of
binary compounds relies on the chemisorption of the precursor
onto the surface and subsequent reaction with oxygen or
nitrogen containing compounds, such as H2O, O3 or NH3. This
concept has proved to be useful for metal oxides or nitrides,
because counter precursors are available. However, for metal
fluorides and phosphates the selection of reactive non-metal
precursors is very limited. In principle HF or PH3 would be
suitable to use as precursors, but due to their corrosive nature
and toxicity they are not widely used.1 Alternatively, ex situ
fluorination with application of CF4 plasma to ALD deposited
Al2O3 has been performed.2 We have previously shown that
phosphorus can be incorporated into carbonate films by
exchange reactions to produce hydroxyapatite-like thin films.3 In
this work we adopt a similar approach using exchange reactions
to convert metal oxides to metal fluorides during ALD
processing.
Typically, nonfluorinated metal b-diketonates are used as
ALD precursors although they are relatively inert and require
strong oxidizer such as ozone to produce metal oxide films. Their
use as precursors for nitride and sulfide films is not straightfor-
ward due to the already present metal–oxygen bond. Typical
aBeneq Oy, P.O. Box 262, FI-01511 Vantaa, Finland. E-mail: [email protected]; Fax: +358 9 7599 5310; Tel: +358 40 5203099bLaboratory of Inorganic Chemistry, Aalto University School of ChemicalTechnology, P.O. Box 16100, FI-00076 Aalto, Espoo, FinlandcMax-Planck-Institut f€ur Mikrostrukturphysik, Weinberg 2, D-06120Halle, Saale, Germany
This journal is ª The Royal Society of Chemistry 2011
examples are the formation of yttrium oxysulfide from the Y
(thd)3 + H2S,4 and indium sulfide from In(acac)3 + H2S.
5
In principle b-diketonates can also hydrolyze in the presence of
water at very high temperatures, but at the same time they
undergo thermal decomposition, thus limiting their use in H2O
based processes. Another problem associated with some b-
diketonates, such as Ca, Sr, and La, is the carbonate formation.
For example, films made by using Ca(thd)2 + O3 consist of
almost pure CaCO3.6 Fluorinated b-diketonate ligands are
known to be relatively high-vapor pressure precursors for
chemical vapor deposition (CVD) but their use for ALD
precursors is rather scarce.7 Although they are quite bulky, they
are reactive with strong oxidizers such as O3 or oxygen plasma.
Quite recently Pilvi et al. reported on the deposition of metal
fluorides (MgF2, CaF2, LaF3) with metal b-diketonates as metal
precursors and solid TiF4 or TaF5 as fluorine source.8,9 These
reactions proceed through a ligand exchange reaction between
the metal b-diketonate and the metal fluoride producing titanium
or tantalum b-diketonates as volatile byproducts. It is worth
noting that although oxygen coordinated b-diketonate ligands
are present, no significant amount of oxygen is present in the
films. However, small amounts of titanium or tantalum are
observed as impurities limiting the use of these films for deep UV
applications because of reduced transmittance.
In this report we have studied the deposition of metal fluorides
by two novel methods. The first approach is based on the
traditional metal oxide ALD chemistry with fluorinated metal
chelates, whereas fluorinated hydrocarbons were used as the
fluorine source in the second approach. Both of these concepts
are based on the exchange reactions of oxygen containing
precursors and adsorbed fluorine species at the surface.
J. Mater. Chem., 2011, 21, 14461–14465 | 14461
Fig. 1 Deposition rate of the Ca(hfac)2/O3 process as a function of
Ca(hfac)2 pulse time at 300 �C.
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Experimental
In the first approach Ca(hfac)2 (obtained from Volatec Oy)
and O3 were used as precursors ((hfac ¼ 1,1,1,5,5,5-hexa-
fluoroacetylacetonate). In the second approach Mg(thd)2, Ca
(thd)2 and La(thd)3 (thd¼ 2,2,6,6-tetramethyl-3,5-heptanedione)
were used as metal precursors and O3 as the oxygen source.
Hhfac (1,1,1,5,5,5-hexafluoroacetylacetonate) (Sigma-Aldrich)
was used as a fluorine source. The films were deposited on
a commercial TFS 200 or TFS 500 cross-flow ALD reactor
manufactured by Beneq Oy. The film depositions were carried
out onto Si(100) wafers measuring 100–200 mm in diameter. For
measuring the optical properties of the ALD films, also quartz
substrates were used. Ozone was used as the oxygen source and it
was generated from O2 (99.999%) in an ozone generator
(Wedeco). Nitrogen (>99.999%) was used as the carrier gas
without any additional purification.
The film thicknesses were measured by ellipsometry using
either a PLASMOS SD 2300 or a synchrotron beam at the
BESSY Helmholtz-Zentrum Berlin. Details of the experimental
setup at BESSY can be found in ref. 10. The measurements were
performed in the 9.5–3.5 eV energy region at an angle of
incidence of ca. 67.5� using a MgF2 Rochon polarizer. X-Ray
diffraction using Cu Ka radiation (Philips MPD 1880) was used
to characterize crystalline phases. TEM images of the represen-
tative samples were taken by using a Titan 80–300 TEM
from FEI.
The compositions of the selected thin film samples were
determined using Rutherford backscattering spectrometry (RBS)
with a 2 MeV 4He+ ion beam backscattered at 160�. The 2 mm
diameter beam was incident at 7� to the sample surface normal.
The SIMNRA simulation program11 was used to obtain the
elemental ratios by comparison of simulated and experimental
RBS spectra.
Fig. 2 Deposition rates of Ca(thd)2/O3 and Ca(thd)2/O3 + Hhfac/O3
processes.
Results and discussion
In the first approach we used fluorinated metal b-diketonates as
a metal source and ozone as the second precursor.
Initial experiments were carried out with Ca(hfac)2 and water
as precursors, but no film was obtained up to 300 �C, which is
probably due to the already expected low reactivity. By changing
the oxygen source to O3, a deposition rate of 0.3 �A per cycle was
obtained which is reasonable in view of the bulkiness of the
fluorinated b-diketonate precursor. The pulsing time of the
precursors did not affect the deposition rate if kept over 200 ms,
indicating an ALD type of growth (Fig. 1). With optimized
deposition parameters a max–min uniformity of 1.5% was
obtained over a 100 mm silicon wafer. The refractive index was
approximately 1.46 which is much lower than that expected for
CaO or CaCO3 thin films and closer to the refractive index of
1.43, reported for CaF2 thin films.
According to the RBS measurements, the film composition
was Ca 30 at%, F 65 at% and O <5 at%, indicating a stoichi-
ometry close to CaF2, with less than 5% of oxygen as impurity.
However, the oxygen impurity is the most probable reason for
the increased refractive index compared to the bulk values.
Typically O3 has been used as an oxygen source for b-diket-
onate precursors to deposit oxide films. Therefore it is rather
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surprising that with fluorinated metal precursors the reaction
yields metal fluorides. Previously, fluorinated metal b-diketo-
nates have been utilized for the deposition of palladium12 and
copper,13,14 and recently Goldstein and George reported that
during the Pd(hfac)2 + formalin process the released Hhfac
ligand is chemisorbed to the surface, blocking the reactive sites.15
This is the main reason for the low deposition rate and rough-
ening of the Pd film since hfac cannot be easily removed from the
surface.
It seems that the ozone reacts with the chemisorbed metal
precursor and the released fluoride-containing hydrocarbons
further react with the metal oxide surface forming CaF2. In
addition, although O3 is a strong oxidizer, it seems that O3 is not
reactive enough to convert the metal fluoride to the corre-
sponding metal oxide. Bulk CaF2 is reported to be stable in an
oxygen atmosphere at these temperatures,16,17 although it is
reported that slight surface oxidation of CaF2 occurs even at
room temperature under normal pressure.18
Metal fluoride deposition by using fluorinated b-diketonates
together with ozone is a quite limited strategy to produce a large
variety of metal fluorides, since the commercial availability of
suitable fluorinated metal chelates is rather restricted. Therefore,
we tried a more practical approach to synthesize fluoride films,
which is based on traditional non-fluorinated metal precursors.
This journal is ª The Royal Society of Chemistry 2011
Fig. 3 Schematic representation of ALD processing of metal fluorides using Hhfac as a fluorine source during the (a) Ca(thd)2/O3 and (b) Hhfac/O3
cycle.
Fig. 4 RBS depth profile of the CaFx thin film.Fig. 5 Crystallinity of CaFx (a) and LaFx (b).
Fig. 6 CaF2.03 thin film surface morphology on the silicon substrate
measured by AFM from a 2 � 2 mm area, height axis 10 nm.
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In our second approach, we used nonfluorinated metal precur-
sors as metal sources and provided fluorine as a separate
precursor. Since hfac is known to strongly adsorb to surfaces,15
we studied the possibility to fluorinate the oxides during an ALD
cycle.
CaF2 films were deposited by first using Ca(thd)2 + O3 sub-
cycle for incorporating Ca species to the surface and subse-
quently applying a Hhfac + O3 cycle for fluorination. The
deposition rate of Ca(thd)2 + O3 + Hhfac + O3 was 0.4 �A per
cycle (Fig. 2). It seems that the Hhfac + O3 pulses do not affect
the deposition rate, since the films grown from Ca(thd)2 and O3
had a similar deposition rate of 0.41 �A per cycle at 250–350 �C.3,6
Similarly using Mg(thd)2 and La(thd)2 resulted in uniform thin
films with deposition rates of 0.38 and 0.49 �A per cycle, respec-
tively. The deposition rate of lanthanum fluoride is quite close to
the deposition rate of the La(thd)3/O3 process.19
Although the Hhfac ligand gets adsorbed to the surface, it
seems that it is not able to get desorbed in the form of volatile Ca
(hfac)2, since no decrease in the deposition rate was observed,
even if the Hhfac pulse time was increased. Typical Hhfac doses
of 0.35–0.4 mg per cycle resulted in uniform films with constant
refractive index over the entire substrate. Even if the Hhfac dose
was increased up to 15 mg per cycle, the deposition rate and
refractive index still remained constant, provided the purge times
This journal is ª The Royal Society of Chemistry 2011
were kept sufficiently long. On the other hand, if the ozone pulse
was omitted after the Hhfac, the deposition rate was significantly
reduced. Based on these observations, the following mechanism
can be suggested (Fig. 3). First, the ALD reaction proceeds by
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Fig. 7 CaF2.03 film deposited by using the Ca(thd)2 + O3 + Hhfac + O3
process onto silicon wafer with native oxide.
Fig. 8 Refractive indices and k-values of metal fluorides on quartz
substrates.
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producing the metal oxide or carbonate from M(thd)x/O3. In the
following phase, Hhfac is adsorbed to the surface. This adsorp-
tion seems to be surface controlled, since the film growth rate or
refractive index did not change with different Hhfac doses.
However, ozone must be introduced in order to decompose the
adsorbed Hhfac at the surface. Although this step is analogous to
the oxidation step in the Ca(hfac)2 + O3 process, the process with
separated calcium and fluorine sources resulted in a better stoi-
chiometric material and lower oxygen content. One possible
reason might be that in the one step process the amount of
fluorine is fixed with the calcium precursor, leading to a lower
fluoride content. The Hhfac pulse ensures the presence of excess
fluorine at the surface. According to the RBS measurements, the
compositions were 33 : 67 for Ca–F films and 29 : 71 for La–F
films, giving stoichiometries of CaF2.03 and LaF2.56. The oxygen
content was below the detection limit of the RBS (<2 at%)
(Fig. 4).
All deposited fluorine films were polycrystalline with
a preferred (111) orientation as determined by XRD (Fig. 5). The
CaFx films showed a strong (111) peak with a smaller (220) peak
only. From LaFx films other reflections such as (110) and (300)
were also observed. The crystallinity of the CaF2 films did not
change considerably as a function of deposition temperature
between 250 and 300 �C.According to AFM measurements, the roughness was depen-
dent on the film crystallinity. CaF2 and LaF3 films with a similar
thickness around 50 nm had roughness values around 5 nm
(Fig. 6). The increased roughness of ALD deposited fluorides has
also been observed earlier, when other precursor chemistries
have been applied.8,9
Increased roughness was also verified from the TEM micro-
graph (Fig. 7). Film crystallinity was also seen as there were
relatively large grains with a columnar structure on the deposited
films.
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Fluorides are well known transparent materials in the ultra-
violet spectral region. Magnesium8 and calcium9 fluorides have
some of the lowest refractive indices with reported values around
1.4, and are therefore widely used in various optical structures. In
multilayer interference optics, a high refractive index counterpart
material is necessary. While numerous adequate oxides can be
found for the visible spectral range, only few materials have both
low absorption and high refractive index values in the ultraviolet
spectral range.20 Al2O3 thin films have a relatively high refractive
index of ca. 1.7, but their extinction coefficient rapidly increases
below a wavelength of 200 nm. The band gap of Al2O3 films
deposited by physical vapor deposition is ca. 6.41 eV corre-
sponding to the first interband transition. Hence, their use in
vacuum UV is limited. Alternatively, LaF3 thin films are highly
promising high refractive index coatings with application
potential down to a wavelength of ca. 150 nm.21
Two aspects are highly important to achieve deep and vacuum
UV optics for analytical chemistry or lithography applications,
for spectrometers or laser instruments. High quality coating films
must be produced and excellent thickness control for ultra-thin
layers must be guaranteed. The film quality of the multilayer
structure is determined by its purity, homogeneity, low rough-
ness, uniformity, etc. Small amounts of impurities in the films
could drastically increase the UV absorption of the optics. The
RBS data presented here demonstrate high purity fluorides and
the AFM measurements indicate relatively smooth films, which
are very promising for optical coatings. Concerning the thickness
control, the individual layer thicknesses get downscaled with
decreasing wavelengths.22 Atomic layer deposition is a promising
coating technology achieving excellent thickness control for
ultra-thin layers of only a few nanometre thicknesses.23,24
The optical properties of the fluoride thin films were investi-
gated by vacuum ultraviolet ellipsometry using synchrotron
radiation. The ellipsometry parameters J and D have been
modeled by the Cauchy model to determine the refractive index
and film thickness of the ALD coating. An EMA (effective
medium approximation) layer to account for film roughness was
included in the fitting procedure. The EMA layer thicknesses
were in the range of 2 to 8 nm corresponding to the roughness
measurements performed by AFM. The spectral range was
limited to achieve a good fit of the ellipsometry data taking into
account the low absorption wavelengths. The optical properties
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are summarized in Fig. 8. The refractive indices measured at 320
nm for MgFx, CaFx and LaFx films were 1.429, 1.472 and 1.687,
respectively.
Transmittance of CaFx films deposited on various substrates
was evaluated by recording transmittance spectra (Fig. 8). The
films deposited on quartz substrates were highly transparent
down to ca. 140 nm wavelength, with the transmittance being
limited by the substrate material.
Conclusions
We have demonstrated a new ALD concept for producing metal
fluoride films using either fluorinated metal precursors with
ozone or conventional metal precursors with fluorine compounds
and ozone. Ozone proved to be essential for the activation of the
hexafluoroacetylacetonate adsorbed to the surface. Using Hhfac
as a fluorine source increased the purity of the films by reducing
the oxygen content below the detection limit of the RBS. The
deposited films are highly uniform and polycrystalline with (111)
as the preferred orientation. The refractive indices of the mate-
rials have been determined by vacuum ellipsometryand the
transmittance measurements of the films deposited on quartz
substrates indicate their application for UV optics.
Acknowledgements
The authors are grateful to C. Cobet for support with the
ellipsometer equipment. Dr Timo Sajavaara is acknowledged for
performing the RBS measurements at University of Jyv€askyl€a.
Financial support within the German Federal Ministry of
Education and Research BMBF project FKZ 03X5507 and by
the Helmholtz Zentrum Berlin is highly acknowledged.
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