Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
MEASUREMENT OF THE SILICON DIOXIDE CONCENTRATION IN
HAFNIUM SILICATE GATE DIELECTRICS WITH A TXRF
Chris M. Sparks1, Patrick Lysaght2, and Todd Rhoad1
ATDF, Inc.1 a wholly owned subsidiary of SEMATECH2
Austin, Texas
ABSTRACT
As complimentary metal oxide semiconductor (CMOS) devices continue to scale along the rapid
pace of Moore’s Law, gate dielectric materials with significantly higher dielectric constant (k =
10 – 25) are being evaluated as replacements for conventional silicon dioxide, SiO2 (k = 3.9), and
silicon oxynitride. This allows for the introduction of a physically thicker film with lower
leakage current and with capacitance equivalent to a thinner (1.0 nm and below) SiO2 layer [1-3].
Although binary metal oxide films such as HfO2 and ZrO2 exhibit higher permittivity than their
corresponding silicates and aluminates, alloyed with various molecular percents of SiO2 or
Al2O3, respectively, they are compromised by lower onset of crystallization temperature which
contributes a higher degree of interfacial microroughness and increased gate leakage current due
to dislocations and oxygen vacancies generated along grain boundaries. Accordingly,
development of hafnium silicate has been the subject of intense investigation as an advanced gate
dielectric thin film designed to meet the device manufacturing requirements of thermal stability
in direct contact with substrate silicon and metal gate electrode materials. In this paper, we
present results corresponding to the utilization of total reflection X-ray fluorescence
spectroscopy (TXRF) as a quick, accurate, non-destructive technique for hafnium silicate
composition determination based on detection of the Hf: Si ratio of (HfO2)x(SiO2)1-x, where x
varies over the range 0.2 – 1.0.
INTRODUCTION
The effort to introduce a high-k gate dielectric thin film with equivalent electrical silicon oxide
thickness (EOT) < 1 nm that meets required performance characteristics including low leakage
current and high electron mobility, while demonstrating the capability of continued scaling, has
evolved with the realization that the combination of many specific chemical treatments in
conjunction with variations in Hf silicate (HfSiO) composition may be necessary. In addition,
electrical performance has been further compromised by an unacceptably high density of
interface states at the Si / dielectric interface and a high density of electron trap centers in the
bulk film, giving rise to explorations of the process parameters during HfSiO deposition in order
to optimize the resultant film quality. The frequency of experiments designed to improve the
HfSiO film quality and the number of variable process parameters that may influence the
resultant silicate composition is appreciable. To this end, it has been deemed essential to
establish a means, other than the relatively slow, destructive technique of Rutherford
Backscattering (RBS), to identify the HfSiO composition. Since TXRF is extremely sensitive to
surface elemental composition, it was investigated as a method of measuring the percent SiO2 of
the HfSiO high-k films under development and as an means of monitoring the statistical process
control of the deposition tool for consistent recipe reproduction over time.
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
EXPERIMENTAL
The TXRF used in this study was a Technos model 630T with a rotating tungsten anode (W-Lß1
at 9.67 keV) and a silver tube anode (Ag-Kα at 22.1 keV). The Hf-Lα (7.898 keV) and the SiKα (1.74 keV) were the main lines monitored. Each wafer was analyzed at three spots per wafer
at angles ranging from 0.01°- 0.30° for the tungsten anode and 0.01°-0.20° for the silver anode.
To reduce excessive dead time on the Si(Li) detector at higher incident angles, the tungsten
anode was operated at 20 kV and 20 mA and the silver anode was operated at 40 kV and 15 mA.
Counting times for all spectra in this paper were 100 seconds although recent experiments
suggest this could be reduced to 10 seconds.
Hafnium silicate films were deposited by atomic layer deposition (ALD) on 200 mm Si (100)
wafers. The films were deposited with a wafer temperature of 330°C and a pressure of 1 Torr
Three sets of five film compositions have been characterized; a five wafer set of 15 nm thick
films deposited by the ALD tool supplier, and two complimentary five wafer sets of 15 nm and 3
nm thick films deposited in ATDF’s fab.
RESULTS AND DISCUSSION
Figure 1 shows a plot of the ratio of Hf to Si counts taken from the TXRF for the series of 15 nm
films deposited at the ALD tool supplier’s site. The films ranged from a 0% SiO2 film (a HfO2
film) to an 80% SiO2 film as determined by RBS. The data points plotted are an average of three
sites across each wafer for a series of the incident radiation angles below the critical angle for
this type of film and for both the tungsten and silver anode source X-rays (0.01°-0.05°). The
relative standard deviations (RSD’s) for each point was < 5%. It was expected that this approach
would indicate a linear relationship between Hf and Si in the films, and that the response could
10
Hf/Si (cps)
9
8
W anode
Ag anode
7
6
5
0
20
40
60
80
100
% SiO2
Figure 1. Ratio of Hf to Si counts against the mol. % SiO2 composition for
15 nm films measured below the critical angle
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
be used as a calibration curve for determining the mol. % SiO2 in unknown Hf silicate films. The
tungsten anode does show a linear response from 20 to 80 mol. % SiO2 in the films. The loss of
linearity below mol. 20% may indicate a limit of this approach. The Si signal component of the
Hf to Si ratio of the HfO2 film is most likely due to excitation of Si from the substrate below the
HfO2 film, an overlap from the Hf-M lines, or a contribution from Si in the Si(Li) detector.
Based on the decrease in slope, the silver anode shows a decrease in sensitivity to composition
relative to the tungsten anode. This may be due to the higher incident energy radiation either
penetrating deeper into the sample exciting the Si substrate or having less efficient excitation of
the lines of interest. Under total reflection, the estimated minimum penetration depth of the
hafnium-based films is about 2 nm [4].
The results of the 3 nm HfSiO film measurements are shown in Figure 2. The films are too thin
for RBS verification of the composition but they were deposited using the same process recipes
used to produce the 15 nm thick film set (only the deposition time was adjusted to achieve the
desired thickness) and it is expected that the compositions match. The loss of detection response
linearity with composition is more pronounced for the Hf rich films and the Si substrate may be
influencing the measurements.
Hf/Si (cps)
7
6
W anode
Ag anode
5
4
0
10
20
30
40
50
60
70
% SiO2
Figure 2. Ratio of Hf to Si counts against the mol. % SiO2 composition for
3 nm films measured below the critical angle
Since an angle scan is a function of physical properties of a sample (for example, density) [5],
we collected data from the HfSiO films by varying the incident radiation angle of the TXRF. An
example of a Ag anode angle scan of the Hf to Si ratio from the 15 nm HfSiO films deposited at
the tool supplier’s site is shown in Figure 3. We can see that there is a distinct difference in the
lines based on the mol. % SiO2 composition of the films. Taking the area under the curve for
each of these lines, we get a plot as seen in Figure 4. The best-fit line that is shown has an R2
correlation value of 0.968. These points were averages of three measurements across a wafer
and three repetitions over the course of two months. This best-fit line can be used as a calibration
curve to quantify other films. Taking the area under the Hf to Si ratio angle scans for the 15 nm
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
films deposited in the ATDF’s fab and plotting them on the line from Figure 4 gives the results
seen in Table 1. The values calculated by plotting TXRF data on the line in Figure 4 agree
within a few percent to the values determined by RBS except for the 33% SiO2 where the TXRF
calculated value is a higher 43%. ATDF’s analytical characterization labs have also
experimented with making this measurement by secondary ion mass spectrometry (SIMS) [6],
and have reported on artifacts from analysis of high-k films with sputter-based techniques [7].
The SIMS data is closer in agreement to the target value for the low end of % SiO2, and similar
in variation to the target with the other concentrations calculated by TXRF.
7
6
Hf/Si (cps)
5
0 % S iO 2
2 0% S iO 2
4 0% S iO 2
6 0% S iO 2
8 0% S iO 2
4
3
2
1
0
0
0 .0 5
0 .1
0 .1 5
0 .2
angle
Figure 3. Angle scans of the Hf to Si ratio of
15 nm HfSiO films using the Ag anode
1
0 .9
Hf/Si area
0 .8
0 .7
0 .6
0 .5
0 .4
0 .3
0
20
40
60
80
% S iO 2
Figure 4. Area under Hf to Si angle scans versus mol. % SiO2
in 15 nm HfSiO films using the Ag anode
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
mol. % SiO2
RBS values
33
45
55
67
mol. % SiO2
TXRF calculated values
43
48
54
71
Table 1. Comparison of calculated mol. % SiO2 concentrations to
the RBS values for 15 nm films
However, applying this method to measure the 3 nm HfSiO films is not as practical. We see
plots similar to Figure 2 where there are poor line fits. Also, the repeatability of the area under
the Hf to Si ratio angle scan is worse with RSD’s around 20% for the 3 nm films. On the other
hand, if we do not consider the ratio of Hf to Si and just plot the area under the angle scan of Hf
using the Ag anode against the mol. % SiO2, we get the plot in Figure 5, remembering that the
mol. % SiO2 values for the 3 nm HfSiO films are assumed from RBS results of the 15 nm films
that were grown under similar conditions. For most points, the reproducibility is about 2% RSD
90
80
Hf area
70
60
50
40
30
20
0
20
40
60
80
% SiO 2
Figure 5. Area under Hf signal angle scans versus mol. % SiO2
in 3 nm HfSiO films using Ag anode
over three points per wafer and four measurements over the span of three months. The best-fit
correlation of this line (R2) is 0.89. From Figure 5, the linear response of the area under the
curve to mol. % SiO2 decreases at the highest value of mol. % SiO2 (67%). If we drop the 67%
SiO2 point, that would lead to an R2 of 0.972 for a line fit. It would be interesting to have HfSiO
films in the 10-15% SiO2 range to verify linearity of this method at the low SiO2 percentage
range.
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Copyright ©JCPDS - International Centre for Diffraction Data 2005, Advances in X-ray Analysis, Volume 48.
CONCLUSIONS
TXRF has shown promise in quantifying the amount of SiO2 in HfSiO films. This may lead to
an in-fab and non-destructive metrology procedure. For thicker (15 nm) films a linear TXRF
response to mol. % SiO2 can be obtained from either measuring the ratio of Hf to Si below the
critical angle or from the area below the curve of the angle scan of the Hf to Si ratio. For most of
the concentrations of SiO2, these results agree with results obtained from RBS and SIMS. We
also show a fairly linear TXRF response on the thinner (3 nm) HfSiO films by plotting the area
under the Hf signal angle scans against the concentration of SiO2.
REFERENCES
[1] D. G. Schlom and J. H. Haeni, MRS Bulletin Vol 27, p. 198, 2002.
[2] G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, p. 5243, 2001.
[3] A. I. Kingon, J-P. Maria, and S. K. Streiffer, Nature Vol 406, p. 1032, 2001.
[4] R. Klockenkämper, Total-Reflection X-Ray Fluorescence Analysis, (edited by J. D.
Winefordner, John Wiley & Sons, New York) p. 38, 1997.
[5] Ibid., p 30.
[6] J. Bennett, “Determination of Si Content in HfSiO Films”, 17th Annual SIMS Workshop,
Westminster, Colorado, May 17-24, 2004.
[7] C.F.H. Gondran, J. Bennett, M.R. Beebe “Sputter Artifacts in Depth Profile Analysis of HfO2
and HfSixOy”, AVS 49th International symposium, paper # AS-WeA2, Denver, Colorado,
November 3-8, 2002.
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