An Investigation of Metal Thin Films Using X-ray Reflectivity and
Atomic Force Microscopy
D.M.SOLINA,
R.W.CHEARY,
Microstructural
F.A.LUPSCHA,
and P.D.SWIFT
Analysis Unit, Department of Applied Physics
University of Technology,
Sydney
P.O.Box 123, Broadway, 2007. Australia.
ABSTRACT
Magnetron
sputtered thin films of tungsten,
100 silicon were investigated
as part of a systematic
of the interfaces.
titanium and molybdenum
using X-ray Reflectivity
results are presented
atomic number, and gas pressure during fabrication.
the surface
film roughness
obtained
Microscopy
are noted. Roughness
on
and Atomic Force Microscopy
study on the effects of deposition
Preliminary
deposited
conditions
on the roughness
on the effects of film thickness,
Significant
using X-ray Reflectivity
differences
between
and Atomic
Force
was found to increase with increasing gas pressure,
and with lower atomic number materials. Little effect on roughness was observed with
thickness.
INTRODUCTION
Multilayers,
can be used
consisting
of alternating layers of material such as Tungsten and Carbon,
as mirrors
in x-ray astronomy,
soft x-ray lithography,
and x-ray
microscopy[‘l. In these devices one material acts as the diffracting plane for the x-rays
and the other a spacer. The requirements of a high reflectivity mirror are that the step
in the refractive index of the two bilayer materials be as large as possible, and that the
interface between the two materials be well defined and very smootht2]. X-ray mirrors
are used widely in the soft x-ray region (clkev), but their use in the hard x-ray region
(-SkeV) has only recently been exploited13]. Hard x-ray mirrors require ultra-smooth
surfaces and interfaces to maintain high reflectivity and ultra-thin layers to give
reflection at angles higher than grazing incidence (i.e. >0.5”8). For example a spacing
of approximately 15w is required to reflect at 28 = 6 ’ using 1.54A (8 keV) radiation.
In order to identify optimum conditions for fabricating smooth interfaces a systematic
study is being carried out to determine the relationship between the deposition
parameters and the smoothness of single layer metal films. In this paper, the variation
of roughness with film thickness, and gas pressure during magnetron sputtering has
been investigated using tungsten, titanium and molybdenum deposited on 100 silicon.
This is part of a larger study which encompasses other deposition techniques including
cathodic arc deposition and e-beam evaporation.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
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Centre for Diffraction Data 1997
This document was presented at the Denver X-ray
Conference (DXC) on Applications of X-ray Analysis.
Sponsored by the International Centre for Diffraction Data (ICDD).
This document is provided by ICDD in cooperation with
the authors and presenters of the DXC for the express
purpose of educating the scientific community.
All copyrights for the document are retained by ICDD.
Usage is restricted for the purposes of education and
scientific research.
DXC Website
– www.dxcicdd.com
ICDD Website
- www.icdd.com
EXPERIMENTAL CONDITIONS
All samples were deposited
cut from highly polished,
on polished
substrates, of dimensions
18mm by 30mm,
100 silicon supplied by Infra-Red Products. Substrates were
cleaned ultrasonically, first in ANALAR grade acetone and then PET ether, followed
by drying with dry nitrogen. Prior to deposition the samples were further dried with a
heat gun to remove any condensed water.
The films were produced by magnetron
Parameters investigated
COOLING
sputtering using the set-up shown in Figure 1.
are listed in Table 1.
w*TER?yy-
WORKIN
GAS
VACUUM
I
I
CHAMBER
I
’
VACUUM
PUMPS
I
Figure 1. Set-up for DC Magnetron Sputtering.
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Centre for Diffraction Data 1997
The X-ray Reflectivity
(XRR) measurements
for this work were carried out on a
SIEMENS D5000 8-28 X-ray diffractometer.
to measurement
the equipment
The set-up is shown in Figure 2. Prior
was carefully aligned to obtain accurate zero positions
for the detector angle and the specimen angle. The tilt and vertical displacement
of
the specimen were adjusted to an accuracy of +O.O05mm with three micrometers
fixed to the specimen
stage as shown in Figure 3. The XRR scans were done in two
parts. In the vicinity of the critical angle, an attenuator was placed in the beam owing
to high count rates. At higher angles the attenuator was removed. All scans were made
by collecting rocking curves at equal intervals in 28 of 0.02” along the specular ridge.
For a typical sample the rocking curves were recorded over the range 8+0.1” in steps
of 0.005’ with a 1 second dwell time at each point. For a range of 0.6-6.0” 28 the
collection
time is approximately
3.5 hours. The reflectivity
value at each 28 was
obtained by first subtracting the background
and diffuse scatter from the rocking curve
and then integrating
intensity.
the specular reflected
higher angle data sets were merged
reflectivity
and normalised
The attenuated
with I,,
low angle and
corresponding
= 1. An example of data obtained from a smooth high atomic number (W)
film of nominallv 2OOAcan be seen in Figure 4.
Source to Divergence
slit : 1lOmm
Sample to Receiving
Figure 2. Schematic diagram of X-ray Reflectivity
Slit : 215mm
set-up.
I
Three Pin Micrometer
Height
djustment
4
Three Pin Micrometer
Height Adjustment
I
Side View
I
to a
Sample Holder
I
I
I
-lF
Spring loaded
sample support
Figure 3. Schematic diagram of sample holder.
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Centre for Diffraction Data 1997
Tar, View
le.6
le-7
I
0
1
Figure 4. Measured
2
3
reflected
silicon.
Atomic Force Microscopy
intensity
4
5
6
for a smooth nominal
7
8
28
9
2OOA W film on 100
(AFM) was also used to measure the rms roughness
of the
top surface and the film thickness. AFM scans were made with an AutoProbe Atomic
Force Microscope from Park Scientific. The tip was cone shaped and crafted from
silicon with a radius of curvature lOO& Scans were made over areas of 20ym by
20um to lum by lym. Six or more different areas on the sample were scanned to give
an average value for the rms roughness
of the top surface. The rms roughness
was
calculated with the manufacturer’s software, Proscan. The AFM was also used to
measure the thickness of the film. To do this, steps were produced on the film with a
mark of ink on a of silicon substrate which was sputtered at the same time as the XRR
samples. These marks were then dissolved with high purity acetone and the thickness
subsequently
determined
by scanning across the revealed step.
MODELING THE XRR DATA
The C++ software written to analyse and fit the reflectivity data implements Rouard’s
treatment [41 for multilay ers and incorporates a Debye-Waller factort5’ to represent
interface roughness (o,.&. The parameters obtained from fitting were the interfacial
roughness ((T,,), film thickness (t) and density (p) of each layer of the film. All the
XRR data in this study were fitted by synthesising XRR curves over a range of
conditions and comparing the log values to obtain the best fit using the following
expression to represent the goodness of fit :
Goodness of Fit = i
“2
C
i=n
(lnRpbS - lnRfa1c)2
1
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Centre for Diffraction Data 1997
N
where ni and n2 define the angular region being fitted and N the number of points
fitted.
Various models were examined to represent the structure of the film. The first model
examined
(Model Ai) consisted
densitiest6-‘I
of a layer of tungsten
but allowing the thickness and roughness
on silicon using the bulk
of the tungsten and silicon to
refine. This model was unable to fit the data (see Figure S).The fit in Al can be
improved by allowing the tungsten density to refine also (Model AZ) but the value
obtained for the density of the tungsten film was unrealistically
low and the thickness
obtained was much less than that measured by AFM. Various other layers were added
to this model to obtain a better fit. In the model that we have adopted, an oxide layer
has been added on both the silicon
parameters
and tungsten
surfaces
(Model
B). The film
obtained for models Ai, AZ, and model B are drawn up in Table 2. It is
worth noting that in all cases the thickness of the tungsten layer was constant to within
f 1A for each model. Errors with such fittings are hard to define but from the above
we can give an estimation of the uncertainties to be c 2% for thickness, ~5% for
density and c 10% for rms roughness. The results for the oxide film model are
consistent in a number of ways:
1.
The inclusion of an oxide layer on both the tungsten and silicon
surfaces gives a density for the tungsten layer only 6% less than that for
the bulk material,
2.
X-ray Photoelectron Spectroscopy on the films has shown the presence
of high concentrations of oxygen on the surface of the tungsten film
and at the tungsten-silicon interface,
3.
AFM measurements to determine the film thickness have been found to
be consistently greater than the measured XRR thickness for the
tungsten layer (Table 3). The AFM thickness is comparable to the
combined thickness of the tungsten oxide and pure tungsten layers
found by XRR,
4.
Ellipsometry measurements have also shown the presence of a native
layer of SiO2 on the surface of the silicon with a thickness in the
vicinity of 10-20,&t91,
5.
The densities obtained for the amorphous WO, and SiO2 layers from
the XRR results are comparable to the densities of the bulk materials
WO3 (6.3-7.2 gcme3) and amorphous silica (-2.1 gem-3).
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le+O
____** -B
.
Measured
le- 1
h
le-2
P
;?
s
$
le-3
0
B
’
le-4
le-5
le-6
/
0
Figure 5.
I
I
1
2
3
4
Fits obtained using models Ai and B for a nominally
Table 2. XRR parameters
ANALYSIS
29
6
lOOA W film on silicon.
obtained for the W films on 100 Si using models Ai, AZ,
OF RESULTS
The AFM results and the fitted XRR fitting parameters
groups listed in Table 1 are given in Tables 3,4, and 5.
The general conclusions
follows.
1.
5
obtained
for the different
drawn from the three groups of results are summarised
Increasing the thickness of the W layer has little or no effect on the
roughness or densities of the layers.
Copyright (C) JCPDS-International Centre for Diffraction Data 1997
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as
Table 3. Values obtained for fitting parameters
whilst varying thickness
of tungsten
deposited on silicon 100 at 3mTorr.
Nominal
Thickness
-1oots
-2OOA
-300w
AFM
t
(A)
(TInIS (8)
113+4
1.8kO.4
218f6
1.8fO.l
3Olk1.5
1.6kO.2
wo,
t
(fi)
%ls (8)
p (gem”)
16.5
2.1
5.2
16.6
2.8
4.4
15.2
2.7
4.8
w
t
(A)
d Ems (A)
p (gem”)
97.2
1.1
18.1
189.8
1.5
18.1
280.0
1.1
18.1
Si02
t
(-Q
%I, (A)
p (gcme3)
24.6
2.3
1.7
24.6
1.5
1.7
24.6
2.3
1.7
Si
%Is (A)
p (gem”)
1.4
2.33
1.6
2.33
1.4
2.33
XRR
Table 4. Values obtained for fitting parameters
at varying gas pressures for tungsten
deposited on silicon 100.
Table 5. Values obtained for fitting parameters
titanium deposited on silicon 100 at 3mTorr.
AFM
1 Metal-
for tungsten,
molybdenum
Metal
W
MO
Ti
0 mls (A)
1.820.4
1.9kO.6
4.2k1.9
16.6
15.0
16.0
t
(A)
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and
2.
With increasing gas pressure during fabrication there is marked
increase in the roughness at the W/WO, and the WOJAir
interfaces.
The XRR fitting results also indicate that the density of the tungsten
layer increases with increasing gas pressure.
3.
Roughness
at the metal/oxide
interface does not appear to display any
systematic changes with atomic number. The oxide on the titanium
film was found to be the roughest with a thickness approximating
that
of the measured roughness. Further analysis using a FEGSEM will be
carried out examining the nature of these surfaces.
Table 3 also shows that the film thickness obtained using AFM is invariably greater
than the W thickness
thickness
measured
by XRR, but are comparable
of the tungsten and oxide films. In this table the roughness
interface values given by AFM and XRR are comparable.
to the combined
of the WOJAir
In other films the agreement
is not so good.
I
le-6 i
0
1
2
3
4
5
ze
6
Figure 6. XRR data for 8 mTorr tungsten on silicon.
Figure 6 shows the rapid damping of the interference fringes observed in the XRR
data which arises when the rms roughness becomes large (-Sfi). Although the XRR
data and XRR fitting parameters (Table 4 and 5) show high roughness, it is not
reflected in the AFM results. The low levels of roughness measured using AFM are
probably due to the large size of the tip used to probe the surface. The tip is
approximately lOOA in diameter, therefore cannot probe into the deep, narrow crevices
on the surface, As a consequence the AFM will always
lateral correlation distance is small [lo, ‘ll.
The results given in Table 4 and 5 also show that with the
roughness of the metal-oxide surface is comparable
metal/metal-oxide
interface. This indicates that the upper
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give a low roughness
when
exception of the Ti film, the
with the roughness of the
oxide/air interface conforms
with the metal/metal-oxide
roughness
interface.
In the case of the Ti film, the very large
suggests some breaking up of the oxide which will need to be investigated
further for confirmation.
FUTURE WORK
Further investigations
are needed to confirm and quantify the preliminary
conclusions
in the present work. The work will be extended to include other film materials and
deposition
Diffuse
procedures
including
electron beam evaporation
X-ray and X-ray Photoelectron
characterise
the roughness
concentrations
and conformity
Spectroscopy
and filtered cathodic arc.
will also be investigated
of the interfaces
to
as well as the elemental
with depth.
ACKNOWLEDGMENTS
I would like to thank the Australian Research Council for providing
funding to carry
out this work. Also Maree Anast, Richard Wuhrer and Geoff McCredie for helping to
make this work possible.
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Schuster, M.and Gobel, H. (1995) J. Phys. D : Appl. Phys., 28. A270-A275
Heavens, “Optical Properties of Thin Solid Films” Butterworth Scientific
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Chason, E. Falco, C.M. Ourmazd, A. Schubert, E.F. Slaughter, J.M.and
Williams, R.S. (1993) MRS Symp. Proc., 280.203-238
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11.
Cullity, B.D.,“Elements
of X-ray Diffraction - 2nd Ed.” Addison Wesley
Publishing Company, Inc. (1978)
Lide, D.R, Editor in Chief.“Handbook of Chemistry and Physics- 74th Ed”
CRC Press. (1993-1994)
Ghez, R. “A Primer of Diffusion Problems” John Wiley & Sons. (1988)
Westra K.L., and Thomson D.J., (1995) J.Vac. Sci. Technol. B 13 (2). 344-349
Lengeler B., and Huppauff M., (1993) Mat. Res. Sot. Symp. Proc. 280.245250
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