1 CCQM Pilot Study P-140: Quantitative Surface Analysis of Multi-Element Alloy Films Kyung Joong KIM 1 , Jong Shik JANG 1 , An Soon KIM 1 , Jung Ki SUH 1 , Yong-Duck CHUNG 2 , Vasile-Dan HODOROABA 3 , Thomas WIRTH 3 , Wolfgang UNGER 3 , Hee Jae KANG 4 , Oleg POPOV 5 , Inna POPOV 5 , Ilya KUSELMAN 5 , Yeon Hee LEE 6 , David E. SYKES 7 , Meiling WANG 8 , Hai WANG 8 , Toshiya OGIWARA 9 , Mitsuaki NISHIO 9 , Shigeo TANUMA 9 , David SIMONS 10 , Christopher SZAKAL 10 , William OSBORN 10 , Shinya TERAUCHI 11 , Mika ITO 11 , Akira KUROKAWA 11 , Toshiyuki FUJIMOTO 11 , Werner JORDAAN 12 , Chil Seong JEONG 13 , Rasmus HAVELUND 14 , Steve SPENCER 14 , Alex SHARD 14 , Cornelia STREECK 15 , Burkhard BECKHOFF 15 , Axel EICKE 16 , Ralf TERBORG 17 1 Korea Research Institute of Standards and Science (KRISS), Republic of Korea 2 Electronics and Telecommunications Research Institute (ETRI), Republic of Korea 3 BAM Bundesanstalt für Materialforschung und – Prüfung, Germany 4 Chungbuk National University (CNU), Republic of Korea 5 National Physical Laboratory of Israel (INPL), Israel. 6 Korea Institute of Science and Technology (KIST), Republic of Korea 7 Loughborough Surface Analysis Ltd (LSA), UK 8 National Institute of Metrology (NIM), China. 9 National Institute for Materials Science (NIMS), JAPAN 10 National Institute of Standards and Technology (NIST), USA 11 National Metrology Institute of Japan (NMIJ), Japan. 12 National Metrology Institute of South Africa, (NMISA), South Africa. 13 National Nanofab Center (NNFC), Republic of Korea. 14 National Physical Laboratory (NPL), UK 15 Physikalisch-Technische Bundesanstalt (PTB), Germany 16 Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW), Germany. 17 Bruker Nano, Germany * Electronic mail: [email protected]
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1
CCQM Pilot Study P-140: Quantitative Surface Analysis
of Multi-Element Alloy Films
Kyung Joong KIM1, Jong Shik JANG1, An Soon KIM1, Jung Ki SUH1, Yong-Duck CHUNG2,
Vasile-Dan HODOROABA3, Thomas WIRTH3, Wolfgang UNGER3, Hee Jae KANG4, Oleg POPOV5,
Inna POPOV5, Ilya KUSELMAN5, Yeon Hee LEE6, David E. SYKES7, Meiling WANG8, Hai
WANG8, Toshiya OGIWARA9, Mitsuaki NISHIO9, Shigeo TANUMA9, David SIMONS10,
Christopher SZAKAL10, William OSBORN10, Shinya TERAUCHI11, Mika ITO11, Akira
KUROKAWA11, Toshiyuki FUJIMOTO11, Werner JORDAAN12, Chil Seong JEONG13,
Rasmus HAVELUND14, Steve SPENCER14, Alex SHARD14, Cornelia STREECK15,
Burkhard BECKHOFF15, Axel EICKE16, Ralf TERBORG17
1 Korea Research Institute of Standards and Science (KRISS), Republic of Korea 2 Electronics and Telecommunications Research Institute (ETRI), Republic of Korea 3 BAM Bundesanstalt für Materialforschung und – Prüfung, Germany 4 Chungbuk National University (CNU), Republic of Korea 5 National Physical Laboratory of Israel (INPL), Israel. 6 Korea Institute of Science and Technology (KIST), Republic of Korea 7 Loughborough Surface Analysis Ltd (LSA), UK 8 National Institute of Metrology (NIM), China. 9 National Institute for Materials Science (NIMS), JAPAN 10 National Institute of Standards and Technology (NIST), USA 11 National Metrology Institute of Japan (NMIJ), Japan. 12 National Metrology Institute of South Africa, (NMISA), South Africa. 13 National Nanofab Center (NNFC), Republic of Korea. 14 National Physical Laboratory (NPL), UK 15 Physikalisch-Technische Bundesanstalt (PTB), Germany 16 Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW), Germany. 17 Bruker Nano, Germany *Electronic mail: [email protected]
A pilot study for a quantitative surface analysis of multi-element alloy films has been
performed by the Surface Analysis Working Group (SAWG) of the Consultative Committee
for Amount of Substance (CCQM). The aim of this pilot study is to evaluate a protocol for a
key comparison to demonstrate the equivalence of measures by National Metrology Institutes
(NMIs) and Designated Institutes (DI) for the mole fractions of multi-element alloy films.
A Cu(In,Ga)Se2 (CIGS) film with non-uniform depth distribution was chosen as a
representative multi-element alloy film. The mole fractions of the reference and the test CIGS
films were certified by isotope dilution - inductively coupled plasma/mass spectrometry. A
total number counting (TNC) method was used as a method to determine the signal intensities
of the constituent elements acquired in SIMS, XPS and AES depth profiling. TNC method is
comparable with the certification process because the certified mole fractions are the average
values of the films.
The mole fractions of the CIGS films were measured by Secondary Ion Mass
Spectrometry (SIMS), Auger Electron Spectroscopy (AES), X-ray Photoelectron
Spectroscopy (XPS), X-Ray Fluorescence (XRF) Analysis and Electron Probe Micro
Analysis (EPMA) with Energy Dispersive X-ray Spectrometry (EDX). Fifteen laboratories
from eight NMIs, one DI, and six non-NMIs participated in this pilot study.
The average mole fractions of the reported data showed relative standard deviations from
5.5 % to 6.8 % and average relative expanded uncertainties in the range from 4.52 % to 4.86 %
for the four test CIGS specimens. These values are smaller than those in the key comparison
K-67 for the measurement of mole fractions of Fe-Ni alloy films. As one result it can be
stated that SIMS, XPS and AES protocols relying on the quantification of CIGS films using
the TNC method are mature to be used in a CCQM key comparison.
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1. INTRODUCTION Several pilot studies and key comparisons have been performed by the Surface Analysis
Working Group (SAWG) of the Consultative Committee for Amount of Substance
(CCQM) since 2004.[1-4] The aim of the pilot studies and key comparisons is to ensure
equivalency of measurements made by National Metrology Institutes (NMIs) and Designated
Institutes (DIs). Up to now, two key comparisons have been performed by the SAWG for the
thickness measurement of nanometer SiO2 films and for the quantitative analysis of binary
alloy films. Cu(In,Ga)Se2 (CIGS) thin film solar cells are next-generation solar cells with high
conversion efficiency and low fabrication cost.[5,6] The conversion efficiency of CIGS thin
film solar cells depends on the chalcopyrite crystal structure of the CIGS film.[7] For the
formation of chalcopyrite CIGS films, the relative mole fractions and in-depth distributions
of the constituent elements should be regulated precisely to control the fabrication procedure.
Due to the high technological interest of CIGS films, elemental depth profiles have been
studied by several characterization methods, among them many conventional analytical
techniques, confirming the analytical challenges when appropriate calibration samples are not
available.[8-14]
Although Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy
(XPS) are generally used for the surface compositional analysis of multi-element alloy films,
accurate surface composition analysis is difficult due to matrix effects. Pure element relative
sensitivity factors (PERSFs) determined from pure elements are generally applied for the
quantification of alloy materials. However, matrix effects due to atomic density, attenuation
lengths of electrons and the electron backscattering factor in the matrix materials should be
taken into account. [15-17]
A calibration method using alloy reference materials is recommended for the
quantitative analysis of binary alloys to compensate for the matrix effects. The best method
for the quantification of binary alloys is to use an alloy reference with a similar composition
to the sample to be analyzed, and the next best approach is to use a calibration curve
measured using a series of alloy reference materials with different compositions spanning the
unknown composition. [18-21]
A CIGS film with a non-uniform depth distribution was chosen as a representative multi-
element alloy film. The mole fractions of the reference and test CIGS films were certified by
4
the method of inductively coupled plasma mass spectrometry with isotope dilution (ID-
ICP/MS). Total number counting (TNC) method was used as a method to determine the
signal intensities of the constituent elements in the CIGS films after SIMS, XPS and AES
sputter depth profiling by most participants. This method is comparable to the certification
process by ID-ICP/MS because the film material is fully dissolved within a solvent in ID-
ICP/MS and the measured mole fractions are the average values over the depth of the film.
The analyses of CIGS films were confirmed to be quantitative by Secondary Ion Mass
Spectrometry (SIMS) and AES depth profiling analyses using the TNC method. [22,23]
In this CCQM pilot study, the mole fractions of CIGS films were measured by various
surface analysis methods such as SIMS, AES, XPS, X-ray Fluorescence (XRF) and Electron
Probe Micro Analysis (EPMA) with Energy Dispersive X-ray Spectrometry (EDX). 18 data
sets were collected from 15 laboratories. The relative expanded uncertainties of this pilot
study are smaller than 5 %. The quantification of CIGS films was found to be a good subject
for a CCQM key comparison.
2. SPECIMENS Polycrystalline CIGS thin films of about 2 μm thickness were grown on 100 mm x 100
mm soda-lime glass (SLG) substrates by 3-step thermal evaporation.[24] Before the growth
of the CIGS layer, a Mo back contact layer of about 900 nm was deposited on a soda lime
glass substrate using a DC sputtering system. Four CIGS test films with different mole
fractions were produced by varying the mole fraction of Ga. The mole fractions of Cu, In, Ga
and Se are not homogeneous with depth, which is similar with those of the real solar cells.
The specimens were kept in vacuum packs to prevent a change of surface state due to
oxidation by exposure to the atmosphere. Figure 1 shows photographs of the specimen
packages delivered to the participants.
Figure 1. Photographs of the reference specimen package and test specimen package.
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The relative mole fractions of Cu, In, Ga and Se in the reference and test CIGS films
were certified by ID-ICP/MS. The isotopic ratios of n(63Cu)/n(65Cu), n(113In)/n(115In),
n(69Ga)/n(71Ga) and n(78Se)/n(82Se) were precisely measured for the quantification of metallic
elements.
The CIGS film wafers of 100 mm x 100 mm were cut to small specimens of 10 mm x 10
mm for the certification. The sample solution was prepared by dissolving the CIGS films in a
30% HNO3 solution. The mass difference of the CIGS films after dissolution was about 1.8
mg. The enriched spike isotopic standards were added quantitatively to an aliquot of the
sample solution to yield isotope ratios in the range 0.5 - 1. The isotope ratios in this range
minimize the error propagation factor and measurement uncertainty. The spiked sample
solutions were diluted with deionized water appropriately for ICP/MS measurement.
The enriched isotopes provided by Oak Ridge National Lab were 65Cu (Batch 165740), 113In (Batch 115840) and 82Se (Batch 195291). The isotope of 71Ga (99.60%) was purchased
from U.S Service Inc. All analytical measurements were performed by means of a quadrupole
ICP/MS.
Table 1. Uncertainty in the determination of the mass fraction of Cu in the CIGS thin film CRM by ID-ICP/MS.
No Analytical results (mg/kg)
Standard deviation due to systematic effects (mg/kg)
Table 6. Example of an uncertainty budget for the quantification of the test2 CIGS film by
SIMS with a 5 keV O2+ ion beam using the ARRSFs.
2222quantRSFCRMc uuuu ++=
∑=
=N
i i
iceff
uu1
44
νν
Uncertainty component Element
Cu In Ga Se Standard uncertainty uCRM (%) 1.09 1.47 1.94 1.29 Standard uncertainty uRSF (%) 0 0.82 0.80 0.93 Standard uncertainty uquant (%) 0.94 0.49 0.7 0.23
Combined standard uncertainty uc (%) 1.44 1.75 2.21 1.60 Degree of freedom,νCRM 11 11 6 10 Degree of freedom,νRSF 4 4 4 4 Degree of freedom,νquant 4 4 4 4
Effective degree of freedom, νeff 13 17 9 14 Coverage factor, k 2.16 2.11 2.26 2.14
U = kuc (%) 3.11 3.70 5.00 3.43
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4. RESULTS
The mole fractions reported by 15 laboratories including 8 NMIs, 1 DI and 6 non-NMI
laboratories are shown in Table 7. Average relative standard deviations of the Test 1, Test 2,
Test 3 and Test 4 specimens are 5.53 %, 6.16 %, 6.77 % and 6.07 %, respectively. These large
relative standard deviations are dominated by several results which show large discrepancies
from the certified values. In order to investigate the quality of the data sets, the relative
differences (Di) of the measured mole fractions (Mi) from the certified values (Ci) were
determined from the following equation,
(%)100×−
=i
iii C
CMD
--------------------------------------------- (8)
Table 8 shows the relative differences of the measured mole fractions from the certified
mole fractions for all data sets. The average differences of some laboratories (5, 6, 9, 10 and
18) are larger than 5 %.
The expanded uncertainties of the measured mole fractions of the four test CIGS films
were evaluated by Eqn 6. Table 9 shows the relative expanded uncertainties with 95 %
confidence level.
Table 7. Measured mole fractions of the four test CIGS films.
Average 4.32 3.80 4.16 - 4.16 3.59 4.00 3.84 9.86 3.53 3.79 5.58 3.51 6.84 3.35 3.34 6.35 * Uncertainty is difficult to determine due to the small number of measurement (one time). ** The number of independent measurements per sample varied between one and two; the respective
uncertainties were derived as described in section D.
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4.1. Secondary Ion Mass Spectrometry (SIMS)
In this P-140 pilot study, six laboratories (1 to 6) participated in the quantification of
CIGS films by SIMS as shown in Tables 7, 8 and 9. The details of the instruments and the
experimental conditions of the participating laboratories are described in Table 10. Five
magnetic sector SIMS and one TOF-SIMS were used.
In the case of NIST, the mole fractions and depth distributions of the CIGS thin films
were analyzed by a magnetic sector SIMS (IMS-1270, Cameca, France). An oxygen ion beam
(O2+) with impact energy of 8 keV at an incidence angle of 43o from the sample normal is
achieved by combination of a primary ion energy of 15 keV and a sample bias voltage of 7
keV was rastered over an area of 250 μm x 250 μm on the film surface. Secondary ions of 63Cu+, 71Ga+, 80Se+ and 113In+ were collected from an area of 40 μm x 40 μm in the center of
the rastered area and were detected by an electron multiplier. The raw SIMS depth profiles of
the test CIGS films were converted to compositional depth profiles using the TNC method.
KRISS used a magnetic sector SIMS (IMS-7f, Cameca, France). The CIGS films were
sputtered by an O2+ ion beam with an impact energy of 5 keV at an incidence angle of 46o.
63Cu+, 113In+, 71Ga+ and 80Se+ ions ejected from an area of 56 x 56 μm2 in the center of the
rastered area were detected by an electron multiplier. The experimental conditions of the
Cameca IMS-3f of LSA were very similar with those of KRISS.
Table 10. Details of the SIMS instruments and the experimental parameters of the
participating laboratories.*
*Certain commercial instruments are identified in this report to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology and the other NMIs nor is it intended to imply that the instruments identified are necessarily the best available for the purpose.
Laboratory NIST KRISS NPL LSA NNFC KIST
Maker Cameca Cameca ION-TOF Cameca Cameca Cameca
Model IMS-1270 IMS-7f TOF-SIMS IV IMS-3f IMS-7f IMS4FE7