Optical thin films consisting of nanoscale laminated layers

Article abstract of DOI:10.1063/1.1467622

The control of the refractive index of laminated coatings consisting of alternating stacks of nanoscale Al₂O₃ and TiO₂ sublayers grown by atomic layer deposition has been achieved. The refractive index of the coating linearly changed from 1.870 to 2.318 as the thickness of the single TiO₂ sublayer was varied from 2.0 to 39 A while that of the single Al₂O₃ sublayer was kept constant at 5.5 A. The refractive index could be varied by adjusting only the number of growth cycles of each material. This approach will have potential applications to optical multilayer coatings consisting of well-controlled extremely thin layers.

Full text of DOI:10.1063/1.1467622

Optical thin films consisting of nanoscale laminated layers
Shin-ichi Zaitsu, Takahisa Jitsuno, Masahiro Nakatsuka, Tatsuhiko Yamanaka, and Shinji Motokoshi
Citation: Applied Physics Letters 80, 2442 (2002); doi: 10.1063/1.1467622
View online: http://dx.doi.org/10.1063/1.1467622
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APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 14
8 APRIL 2002
Optical thin films consisting of nanoscale laminated layers
Shin-ichi Zaitsu,a) Takahisa Jitsuno, Masahiro Nakatsuka, and Tatsuhiko Yamanaka
Institute of Laser Engineering, Osaka University, 2-6 Yamada-oka, Suita, Osaka 565-0871, Japan
Shinji Motokoshi
Institute for Laser Technology, 1-8-4 Utubo-honmachi, Nishi-ku, Osaka 550-0004, Japan
͑
Received 12 November 2001; accepted for publication 14 February 2002͒
The control of the refractive index of laminated coatings consisting of alternating stacks of
nanoscale Al O and TiO sublayers grown by atomic layer deposition has been achieved. The
2
3
2
refractive index of the coating linearly changed from 1.870 to 2.318 as the thickness of the single
TiO sublayer was varied from 2.0 to 39 Å while that of the single Al O sublayer was kept constant
2
2
3
at 5.5 Å. The refractive index could be varied by adjusting only the number of growth cycles of each
material. This approach will have potential applications to optical multilayer coatings consisting of
well-controlled extremely thin layers. © 2002 American Institute of Physics.
͓
DOI: 10.1063/1.1467622͔
Composite films whose refractive index can be adjusted
by mixing two materials at different ratios are of consider-
able interest from the viewpoint of optical applications.
based on the alternating chemical reactions of reactant va-
pors with surface species. The self-limiting nature of the
chemical reactions on the surface results in the precise con-
trol of film thickness and large-area uniformity.
1
–9
Tailoring the refractive index of film materials provides
much benefit to optical coatings because thin films with the
refractive index which does not exist naturally enable the
flexible design of multilayer optical filters. In addition, the
control of the refractive index makes it possible to fabricate
coatings with the refractive index varying in the thickness
direction ͑gradient-index coatings͒, which will have attrac-
tive spectral performance exceeding that of regularly stacked
normal multilayer coatings.2
In this letter, we present the application of ALD to the
fabrication of optical coatings consisting of extremely thin
͑Ӎa few nm͒ laminated films. We attempted to control the
refractive index by alternately depositing sublayers whose
thickness is much less than the wavelength in the visible
region. Al O and TiO grown by ALD were used as the low
2
3
2
and high refractive index materials, respectively, for the ex-
tremely thin sublayers that constitute the laminated coating.
ALD has overcome the difficulties associated with the thick-
ness control of optical thin films deposited by conventional
methods, and has realized an alternate approach to form thin
films with variable refractive index.
Nanoscale laminated films were deposited on silica and
silicon substrates at the growth temperature of 200 °C. The
details of the equipment used and the basic procedure for
film growth were described elsewhere.15 In brief, alternate
The refractive index of composite films is determined by
adjusting the volume concentration of the constituents in the
materials of the films. Such composite films have been pre-
3
pared by a number of coating methods such as dip coating,
4
–6
or laser-aided7 evaporation, ion-beam
sputtering, and chemical vapor deposition, and used in
many fields of application such as optical waveguides, rug-
ate filter, narrow band filters, and broadband antireflection
electron-beam
5
,8
9
3
4
,5
6
7
͑AR͒ coatings.
chemisorption of vapors of reactant A and H O onto the
2
Another approach to obtain films with a variable or pro-
surface led to the formation of the oxide film on the sub-
filed refractive index is the approximation of the refractive
index by using very thin alternating layers of high and low
refractive index materials.10 When the thickness of the layers
is much less than the wavelength of interest, the structure
appears to be optically continuous. Adjusting the relative
thickness of high and low refractive index layers results in
the desired average refractive index. The main problem of
this approach is the difficulty in accurately controlling the
optical thickness of the very thin ͑Ӎa few nm͒ layers.
Atomic layer deposition ͑ALD͒ is a thin-film growth
method that is capable of forming one-dimensional structures
of the nanoscale order.11 This method is utilized in applica-
tions that require extremely thin films whose thickness is
controlled to nanometer-level accuracy, such as nanolami-
strate. Reactant A is trimethylaluminum ͓Al(CH ) ;(TMA)͔
3
3
for Al O3 film growth or tetrachlorotitanium (TiCl ) for
2
4
TiO film growth. The vapors of the reactants were intro-
2
duced into the growth chamber thorough timer-controlled
electric valves. For Al O growth, the TMA vapor was con-
2
3
Ϫ2
fined in the growth chamber at a pressure of 10 Torr by
closing the gate valve mounted between the vacuum pump
and the growth chamber. After an interval of 5 s, the vapor
Ϫ4
was evacuated to pressures below 10 Torr. During the in-
terval, TMA chemisorbed to the surface and reacted with
surface OH species. Subsequently, H O vapor was intro-
2
Ϫ1
duced into the chamber at a pressure of 10 Torr, allowed
to react with the surface during the 5 s interval, and finally
evacuated. The same process was conducted for TiO growth
2
1
2,13
nates for insulators with high permittivity
and mirrors for
Ϫ2
at a TiCl pressure of 5ϫ10 Torr. Thus, one cycle of the
4
the soft x-ray wavelength region.14 This coating method is
ALD process comprises these sequential operations: dosing
reactant A ͑0.1 s͒, keeping the gas in the growth chamber ͑5
a͒Electronic mail: zaitsu@ile.osaka-u.ac.jp
s͒, exhausting reactant A ͑10 s͒, dosing H O ͑0.2 s͒, keeping
2
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003-6951/2002/80(14)/2442/3/$19.00 2442 © 2002 American Institute of Physics
28.193.164.203 On: Sun, 21 Dec 2014 08:11:19
0
1

Appl. Phys. Lett., Vol. 80, No. 14, 8 April 2002
Zaitsu et al.
2443
FIG. 1. Film thickness as a function of the number of cycles, and corre-
sponding structure of the laminated film constructed of alternating
Al O –TiO sublayers.
2
3
2
FIG. 2. Refractive index of Al O –TiO nanoscale laminated films as a
2 3 2
function of the volume concentration of Al O (C ) defined by Eq. ͑2͒.
Al O
2 3
2
3
H O in the growth chamber ͑5 s͒, and exhausting H O
2
2
͑10 s͒.
is varied from 5 to 10, 20, 50, or 100. It was found that the
The major characteristic of ALD is that the number of
refractive index varied from 2.39 for pure TiO to 1.61 for
2
molecules that attach to the surface in one cycle is definite
due to the saturation of the surface chemical reactions. As a
consequence, the growth rate per cycle is kept constant
throughout the process, showing that the film thickness lin-
early increases as a function of the number of cycles. At the
pure Al O . The theoretical results obtained using the linear
and Lorentz–Lorenz formulas are also shown in Fig. 2.
These formulas are expressed in the following forms:
2
3
1
6
nϭn
C
ϩnTiO2͑1ϪC
͒,
Al O
2 3
͑3͒
Al O
Al O
2
3
2
3
growth temperature of 200 °C, the growth rates of Al O3
2
2
2
n2Ϫ1
n2ϩ2
n
n
Ϫ1
ϩ2
n
n
Ϫ1
ϩ2
Al O
TiO
(
g
) and TiO (g ) are 1.1 Å/cycle and 0.39 Å/cycle,
TiO
2
2
3
3
2
Al O
2
2
3
ϭ
C
ϩ
͑1ϪC ͒, ͑4͒
Al O
2 3
2
Al O
2
TiO
2
3
respectively.
Al O
2
2
Figure 1 shows a schematic of the alternately deposited
nanoscale sublayers and the relationship between layer thick-
where n is the average refractive index of composite films,
and n
Al O
and nTiO2 are the refractive indices of pure Al O3
2 3
2
ness (d
and dTiO2͒ and the number of growth cycles
Al O
2
3
and TiO , respectively. The solid line in Fig. 2 is the linear
(
N
and NTiO2͒. The thickness of a single sublayer of
3
2
Al O
2
model assuming that the laser electrical field is parallel to the
internal boundaries between different material layers. The
dashed curve is the Lorentz–Lorenz model assuming that the
film consists of the two randomly assigned materials with
different polarizabilities. The experimental results agreed
well with the linear model ͑the solid line͒. This may be ac-
counted for by the highly dense and clearly discrete layered
structure based on the principle of ALD.17
Al O (d
) and TiO (d
) can be expressed as fol-
TiO
2
2
3
Al O
2
2
3
lows: d
ϭg
N
Al O
2
and dTiO2ϭgTiO2NTiO2, respec-
Al O
Al O
2
3
2
3
3
tively.
Al O –TiO bilayers were regularly stacked from the
2
3
2
first bilayer to the last one. Therefore, the volume concentra-
tion of Al O of the coating is given by
2
3
d
Al O
2
3
When the refractive index of the substrate is n , the
C
ϭ
3
.
͑1͒
s
Al O
2
d
ϩdTiO2
Al O
2
3
refractive index of an ideal single-layer AR coating (nAR) is
given by nAR
substrate with a high refractive index, we determined
NTiO2 /N so that the refractive index of the nanoscale
ͱ
ϭ ns. In order to obtain an AR coating for a
By using the growth rates and the number of cycles, C
Al O
2
3
is expressed as
Al O
2
3
laminated coating would give zero reflectance to a silicon
substrate. Figure 3 shows the reflection spectra of silicon
substrate with and without the AR coating. The AR coating
consisted of 60 pairs of Al O –TiO layers. An Al O single
1
C
ϭ
3
.
͑2͒
Al O
2
gTiO2 NTiO2
1
ϩ
g
N
Al O
2
Al O
2
3
3
2
3
2
2
3
layer was obtained after five cycles and a TiO single layer
2
Since g
and g
are constant in ALD, the volume
Al O
2 3
Al O
2
3
was obtained after ten cycles. The estimated thicknesses of
the Al O and TiO single layers were 5.5 Å and 3.9 Å,
concentration can be controlled by the ratio of the number of
).
2
3
2
growth cycles of the two materials (NTiO2 /N
Al O
respectively. We succeeded in growing the Al O –TiO
2
3
2 3 2
Figure 2 shows the dependence of the refractive index of
the coatings on the volume concentration of Al O . The re-
laminated coating with nARϭ1.91 for n ϭ3.8. The reflec-
tance was 0.3% at the wavelength of 560 nm.
s
2
3
fractive indices were measured with an ellipsometer at the
wavelength of 632.8 nm. The indices were averaged because
of the nanoscale structure of the coating that had a much
smaller thickness than the wavelength. In Fig. 2, closed
The conventional co-evaporation and co-sputtering
method frequently used in the deposition of a composite film
needs independent control of the relative deposition rates of
the two evaporation sources or sputtering targets to control
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circles represent results when N
is fixed at 5 and N
the composition of the film. Therefore, there has been much
Al O
TiO
128.193.164.203 On:2Sun, 21 Dec 2014 08:11:19
2
3

2444
Appl. Phys. Lett., Vol. 80, No. 14, 8 April 2002
Zaitsu et al.
thickness, can meet requests from coating designs for which
extremely thin layers are needed. Moreover, a chirped mirror
with layers that gradually become thinner toward the air-
exposed surface, which is desired in recent femtosecond la-
ser systems,21 may be one of the possible applications of this
coating method.
In summary, we employed ALD to prepare coatings con-
sisting of nanoscale Al O –TiO laminated layers. The re-
2
3
2
fractive index of the nanoscale laminated coatings varied lin-
early from 1.61 to 2.39 with the volume concentration C
Al O
2
3
defined by Eq. ͑2͒. We tailored the refractive index to suit a
single-layer AR coating (nARϭ1.91) for a silicon substrate
(
nsϭ3.8) by controlling the ratio of the number of growth
cycles of the two materials (NTiO2 /N ). Finally, we
Al O
2
3
pointed out that this method will be applicable to optical
filters that require extremely thin layers for realizing ad-
vanced spectral performance.
FIG. 3. Reflection spectra of silicon substrate with and without the single-
layer nanoscale laminated coating consisting of 60 Al O –TiO bilayers.
This work was partly supported by the Research Fellow-
ships of the Japan Society for the Promotion of Science for
Young Scientists.
2
3
2
difficulty in the accurate control of the parameters. In our
coating method, however, the ratio of the number of growth
cycles of the two materials is the only parameter that effects
the refractive index of the coating. This feature is realized by
the accurate thickness controllability due to the self-limiting
nature of the surface chemical reactions which is unique to
ALD.
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