Anal. Chem. 2000, 72, 2814-2820
Feasibility Study on the Characterization of Thin Layers by Charged-Particle Activation Analysis K. De Neve, K. Strijckmans,* and R. Dams
Laboratory of Analytical Chemistry, Institute for Nuclear Sciences, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium
The purpose of this feasibility study was to investigate the possibilities and limitations of Charged-Particle Activation Analysis (CPAA) as a thin layer characterization method, i.e., the determination of the mass thickness or the composition of a thin layer. Therefore industrially important layers of sputtered Al, AlOx, TiOx (all three from the packaging industry), YBa2Cu3O6+δ, and Y2O3-stabilized ZrO2 (both superconducting industry) on different substrates were analyzed, and thereby the accuracy, the detection limits, and the precision of the method were studied. To test the accuracy, the same materials were also analyzed with neutron activation analysis (NAA) and inductively coupled plasma mass spectrometry (ICPMS). The results of CPAA compared with the results of NAA and ICPMS showed no significant difference at the 95% confidence level. The detection limits expressed as mass thickness were about 10-2 µg cm-2 or expressed as thickness 0.04 nm for a monatomic layer of Al. The experiments showed that the precision of the method depends only on the counting statistics. Generally we can conclude that CPAA is an absolute method for the characterization of “thin” layers, with respect to composition and mass thickness determinations. Several methods (electron spectroscopy for chemical analysis (ESCA), secondary ion mass spectrometry (SIMS)) are commonly used for the characterization of thin layers such as surface layers.1,2 However, the results obtained with some of these methods can differ significantly,3 owing to different lateral resolutions (important for inhomogeneous samples) and the use of different reference materials for the calibration. The latter fact suggests that new solid standards are required, which can be used for the quantitative analysis of thin layers. Charged-particle activation analysis (CPAA) is a quantitative analytical method for the determination of the elemental concentrations in the bulk of solid samples. CPAA is based on charged particle induced nuclear reactions leading to radionuclides. * Corresponding author: Karel Strijckmans, Laboratory of Analytical Chemistry, Institute for Nuclear Sciences, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium; (Tel) + 32 9 264 66 16; (Fax) + 32 9 264 66 99; (E-mail)
[email protected]. (1) Werner, H. W.; Torrisi, A. Fresenius’ Z. Anal. Chem. 1990, 337, 594-613. (2) Willich, P. Mikrochim. Acta 1992, 12, 1-17. (3) Dubourdieu, C.; Didier, N.; Thomas, O.; Se´nateur, J. P.; Valignat, N.; Rebane, Y.; Kouznetsova, T.; Gaskov, A.; Hartmann, J.; Stritzker, B. Fresenius’ J. Anal. Chem. 1997, 357, 1061-1065.
2814 Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
Identification of these radionuclides by the energy of the emitted radiation or their half-life yields qualitative information, while measurement of the number of photons or particles emitted yields quantitative information. Charged particles lose their energy when penetrating the sample and are stopped at a depth called the “range”. The surface layer analyzed is slightly less than the range. For the charged particles and samples used in this work, the range is 0.4-0.8 mm (p) and 0.2-0.4 mm (d). Up until now, CPAA has been successfully applied in a number of analytical problems for the determination of trace element concentrations (mass fraction, i.e., mass of the analyzed element per mass unit of the sample) in the bulk of samples. The experimental conditions are that (1) the thickness of the sample is at least equal to the range (≈millimeters) of the charged particles used in the sample and (2) the outer surface layer (≈micrometers) is removed after irradiation and before radioactivity measurement, if this outer surface layer may be subject to surface contamination. For example, for the determination of the oxygen concentration in the bulk of an Al sample, the oxidized surface layer must be removed. However, CPAA has more, unexplored possibilities for surface characterization. The same principle can be applied to determine the partial mass thickness, i.e., mass of the analyzed element per surface unit (e.g. mg Ti per cm2 for an TiO2 surface layer). Therefore, the experimental conditions are (1) the surface layer containing the analyzed element is thinner (µm or less) than the range of the charged particles used in the sample, and (2) the substrate (i.e. the layer on which the thin layer is deposited) does not contain the element(s) to be analyzed. So, the elemental composition of a surface layer (e.g. fraction of Ti in TiO2) can be determined, or the total mass thickness of a surface layer (i.e. the mass thickness of the compound on the substrate; e.g. AlOx) with known elemental concentration (e.g. Al2O3) can be determined. This principle has been worked out theoretically.4,5 NUCLEAR DATA The nuclear reactions in Table 1 were used for the determination of Al, Ba, Cu, Ti, Y, and Zr. Important parameters for the reactions are the isotopic abundance θ (in %) of the target nuclide, (4) Strijckmans, K. Charged Particle Activation Analysis. In Surface Characterisation: A Practical Approach; Hellborg, R., Brune, D., Eds.; Scandinavian Scientific Press & VCH: Weinheim, Germany, 1997. (5) Strijckmans, K. Charged particle activation analysis. Article no. A6203 In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley: Chichester, UK, 2000. 10.1021/ac000187c CCC: $19.00
© 2000 American Chemical Society Published on Web 05/06/2000
Table 1. Nuclear Reactions Used for the Determination of Al, Ba, Cu, Ti, Y, and Zr by CPAA; for Beam Intensity Monitoring in CPAA; for the Determination of Al and Ti by NAA and Its Possible Interferences reaction
θ (%)
T1/2
27Al(d,p)28Al
100 0.10 0.10 69.17
2.2414 min 24.3 min 4.8 h 38.47 min
0/3.3 5.5/9.4 5.5/9.4 4.2/6.0
30.83 73.72
244.26 d 15.9735 d
2.2/6.0 4.9/4.9
100 51.45
78.41 h 14.60 h
3.7/7.4 7.0/7.5
26.22 68.08
3.333 h 17.53 h
0/5.7 1.4/6.0
100 5.18 99.75 99.75 73.72
2.2414 min 5.76 min 5.76 min 3.743 min 43.67 h
132Ba(p,n)132mLa 132Ba(p,n)132La 63Cu(p,n)63Zn 65Cu(p,n)65Zn 48Ti(p,n)48V 89Y(p,n)89Zr 90Zr(p,n)90Nb 60Ni(d,n)61Cu 58Ni(p,R)55Co 27Al(n,γ)28Al 50Ti(n,γ)51Ti 51V(n,p)51Ti 51V(n,γ)52V 48Ti(n,p)48Sc
Et/EC (MeV)
excitation function (EXFOR no.7)
Eγ (keV)/Iγ (%) 1778.9/100 464.6/22.3 464.6/76 669.6/8.2 962.1/6.5 1115.5/50.6 983.5/100 1312.1/97.5 909.2/99.0 141.2/66.8 1129.2/92.7 283.0/12.2 931.1/75 477.2/20.2 1778.9/100 320.1/93.1 320.1/93.1 1434.1/100 983.5/100 1037.5/97.6 1312.1/100
c A0510, B0017, B0054, B0057, B0058, a A0333, A0510, B0057, a, c A0510, B0043, D4060 A0403, A0510, B0001, B0018, a A0510, P0021, a
a Shubin, Yu. N.; Lunev, V. P.; Konobeyev, A. Yu.; Dityuk, A. I. Cross Section Data Library MENDL-2 to Study Activation and Transmutation of Materials Irradiated by Nucleons of Intermediate Energies, Report IAEA, INDC (CCP)-385, Wenen, 1995. b Dimitriev, P. P.; Konstantinov, I. O.; Krasnov, N. N. Soviet J. At. Energy 1968, 24, 346-351. c Flores, J. M. Phys. Rev. 1962, 127, 1246-1249.
the half-life, T1/2, of the induced radionuclide,6 the threshold energy, Et, (in MeV),7 the Coulomb barrier, EC, (in MeV), the γ energy, Eγ, (in keV), the absolute γ intensity, Iγ, (in %) of the emitted radiation,6 and the reference of the excitation function are given.8 The table also contains nuclear data for charged-particle beam intensity monitoring and for neutron activation analysis of Al and Ti. STANDARDIZATION For the standardization one can use “thick” standards (in which the charged particle beam is completely stopped, D g R) or “thin” standards (in which the energy of the charged-particle beam is slightly degraded, D , R). Depending on this choice, different equations must be used to calculate the partial mass thickness cxDx (g cm-2) of a thin layer4 for a given element.
Ax Is (1 - e-λti,s) cx Dx ) c s As Ix (1 - e-λti,x) cx Dx ) cs
∫
Ei
Et
σ(E)/σ0 Ss(E)
dE
Ax Is (1 - e-λti,s) [R (E ) - Rs(Et)] As Ix (1 - e-λti,x) s i
cx D x ) c s D s
Ax Is (1 - e-λti,s) As Ix (1 - e-λti,x)
(1)
(2)
(3)
Here, cx and cs are the element concentration (g g-1) in the sample and standard, respectively; Dx and Ds are the total mass thickness (g cm-2) of the sample and standard, respectively; Axand As are (6) National Nuclear Data Center. http://www.nndc.bnl.gov/nndc/nudat/ (accessed Jan 2000). (7) Lund Nuclear Data WWW Service. http://nucleardata.nuclear.lu.se/database/ masses (accessed Jan 2000). (8) National Nuclear Data Center. http://www.nndc.bnl.gov/nndc/exfor (accessed Jan 2000).
the activity at the end of the irradiation (counts s-1) for sample and standard, respectively; Ix and Is are the beam intensity (particles s-1) for sample and standard, respectively; ti,x and ti,s are the irradiation time (s) for sample and standard, respectively; λ is the decay constant of the induced radionuclide (s-1); Ei and Et are the incident energy of the charged particles and threshold energy of the nuclear reaction, respectively; Rs is the mass range of the charged particles used in the standard (g cm-2); Ss(E) is the mass stopping power of the standard for the charged particles used (MeV g-1 cm2); σ(E) is the cross section of the nuclear reaction (cm2); σ0 is the cross section at the incident energy (cm2). Equations 1 and 2 should be used for “thick” standards and eq 3 only for “thin” standards. For the present work, a preliminary study was performed in order to determine which manner of standardization should be used. For five nuclear reactions (on Al, Cu, Ti, and Zr in Table 1) one “thick” sample and at least two different “thin” samples (e.g., 5 and 50 µm) were irradiated and measured. These thin samples were metal foils (thickness between 5 and 50 µm) with mass thickness experimentally determined by measuring the surface and weighing the metal foil. For each thin sample, the mass thickness was calculated by eqs 1 and 2 using the thick sample as a standard. Also for a thin sample (e.g., 5 µm), the mass thickness was calculated by eq 3 using a thicker sample (e.g., 50 µm) as a standard. According to this study, the standardization via eqs 1 and 3 can be used.9 Equation 2 was rejected because of the oversimplified energy dependence, whereby σ ) 0 for Ei < Et and σ ) σ0 for Ei > Et. For eq 1, the results obtained via different σ data sets differ a lot. Unlike in the conventional CPAA where eq 2 is valid,10 the exact knowledge of the cross section σ at E ) Ei is essential in the present investigation. (9) De Neve, K. Nieuwe ontwikkelingen in geladen deeltjesactiveringsanalyse (GDAA): cyclische activering en karakterisatie van dunne lagen. Ph.D. Thesis (In Dutch with English summary), Universiteit Gent, Belgium, 1998.
Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
2815
In addition, some experimental excitation functions were determined via the “stacked foil” technique, wherein a stack of target foils containing the element of interest is irradiated. The total thickness of the foils must be g R, i.e., sufficient to stop the beam, and in principle, the energy loss in each foil must be small. The activity induced in each individual target foil is measured, and this activity is a measure for the cross section at the mean energy of each foil. For all reactions examined, the results (for the thick standardization) obtained using this excitation function were in good agreement with the experimentally determined mass thickness. Still, the standardization with thin standards was to be preferred to the standardization with thick standards, because for eq 1 a systematic study concerning the applicability of the different σ data sets should be made and/or an experimental excitation function should be determined. Conditions for Thin-Layer Analysis with Thin Standards. For the nuclear reactions on Al, Cu, Ti, Y, and Zr listed in Table 1, the excitation functions (σ as a function of E) were examined. For a few reactions the excitation function was also experimentally determined by the stacked-foil technique. To achieve a high sensitivity for the analyses, the incident energy Ei for the sample and standard was chosen in an interval E1 - E2, wherein the cross section is higher than 95% of σmax, the maximum cross section for the reaction examined. The induced activity for a thin sample is, in fact, proportional to the cross section. This is in contrast with the conventional CPAA where Ei > Emax () energy at the maximum cross section σmax). Since the thin standards are relatively thicker than the thin samples, the incident energy Ei of the beam for the standard will be degraded to the outgoing energy Eo. The outgoing energy was also chosen within the interval E1 - E2, implying that the cross section is more or less constant in the interval Ei - Eo, so that eq 3 is applicable. EXPERIMENTAL SECTION Samples and Standards. The thin layers analyzed in this work were made by sputtering deposition. In sputtering deposition a gas discharge is created between a cathode and anode in order to deposit material from the cathode (the sputtering target) onto a substrate. This gas discharge can be created by means of a direct current (dc) or a alternating current (ac) between both electrodes. All metal foils used as standard foils (Al, Cu, Ti, Y, and Zr), beam intensity monitor foils (Ni), substrates (Al, Ta) and beamdegradation foils (Al, Ni) were obtained from Goodfellow, Inc. with a minimum purity of 99.5%. The thin layers of Al and AlOx were made by Innovative Sputtering Technology N.V. (Zulte, Belgium). For AlOx, reactive sputtering was used: a cylindrical Al target was the cathode and the sputtering gas was a mixture of Ar and O2. Both layers were deposited on a 12-µm-thick polymer substrate, called Melinex, which is PET (poly(ethylene terephthalate)). As the thin standard, an Al foil of 5-µm thickness was used. TiOx was also made by Innovative Sputtering Technology N.V. via reactive sputtering from a cylindrical Ti target and is also deposited on a Melinex substrate. Ti foils with thicknesses of 12.5 µm were used as standards.
These thin layers (Al, AlOx, and TiOx) were deposited on the Melinex substrate to increase its waterproof and airproof properties. YBa2Cu3O6+δ (YBCO) belongs to the high-temperature superconductors. The YBCOs were sputtered at the Department of Solid State Physics (Ghent University) on Ta substrates (50 µm). The target cathode was a thick YBCO bar which is commercially available. As standards, a Cu foil of 8-µm thickness and a Y foil of 100-µm thickness were used. Since no thin standards of Ba or any Ba compound are commercially available, a thin Ba standard was created by means of two thick BaCO3 standards: one was irradiated with 12 MeV protons and the other with 11.3 MeV protons (by inserting an extra Al foil of 100 µm). The difference between both standard activities can be seen as the activity induced in a thin BaCO3 standard, which is responsible for the energy degradation of 12 to 11.3 MeV. For this energy degradation a corresponding mass thickness of BaCO3 can be calculated. The Y2O3-stabilized ZrO2 layers (YSZ) were also made by sputtering at the same department. These materials are used as substrates for the YBCOs. The Zr/Y ratio is very important since it determines the crystal structure of the YSZ and also the crystal structure of the YBCO which is deposited on it. These layers were not deposited on Ta substrates, but on Al substrates because the Mo impurities in the Ta substrate induce 96Tc, 99mTc, and 99Mo, which interfere spectrally with 90Nb (see measurements). A Zr foil of 5-µm thickness and a Y foil of 100 µm were used as standards. Irradiation. The samples and standards were placed in a water-cooled target holder and irradiated under a He atmosphere11 with proton and deuteron beams from the CGR-MeV 520 cyclotron of Ghent University. The He atmosphere was separated from the cyclotron vacuum by a Ti foil (Goodfellow, Inc.; 22.94 mg cm-2). For all layers analyzed the irradiation conditions are given in Table 2. For each layer or element the energy of the charged particles used (i.e., the cyclotron energy Ecyclo), the different foils inserted in front of the samples in order to monitor the beam intensity (Ni 12.5 µm) and to decrease the incident Ecyclo to the effective incident energy Eeff, the beam intensity I, and the irradiation time ti are given. For the analysis of the YBCO layers, Cu and Y were determined first. For the determination of Ba in YBCO, the irradiated samples were cooled for two weeks. Measurements. Gamma spectrometry was applied by means of a HP Ge detector (20.8% efficiency; resolution, 1.75 keV at 1332.5 keV; peak-to-Compton ratio, 56.9 at 1332.5 keV). As the beam intensity monitor Ni foils of 12.5 µm were used. For the deuteron-induced reactions the 283.0 keV photopeak of 61Cu (see Table 1) was measured for the beam intensity (by the determination of Al). For the proton-induced reactions, the 931.1 keV photopeak of 55Co (see Table 1) was measured for the beam intensity (by the determination of Ba, Cu, Ti, Y, and Zr). In Table 2 the experimental conditions are given for the analysis of the different layers. For each layer or element the cooling time, tc, the measuring time, tm, and the geometry are mentioned.
(10) Vandecasteele, C.; Strijckmans, K. J. Radioanal. Chem. 1980, 57, 121136.
(11) Wauters, G.; Vandecasteele, C.; Hoste, J. J. Radioanal. Nucl. Chem. 1986, 98, 345-351.
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Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
Table 2. Irradiation and Measurement Conditions for the Analysis of Al, AlOx, TiOx, YBCO, and YSZ Layers sample
Ecyclo (MeV)
Al/AlOx
d 10
TiOx
p 15
YBCO: Cu and Y YBCO: Ba YSZ
p 15 p 15 p 15
foils Ni(12.5 µm) Ni (50 µm) Ni(12.5 µm) Al(3 × 100 µm) Ni (12.5 µm) Al(2 × 100 + 5 µm) Ni (12.5 µm) Al(3 × 100 + 5 µm) Ni (12.5 µm) Al (100 + 50 + 20 µm)
Eeff (MeV)
I (nA)
ti
tc
tm
geometry (cm)
5.7
400-500
30 s
4-5 min
5 min
1
12.0
300
40 min
3d
24 h
0
12.7
1000
5 min
12.0
1000
30 min
Cu: 20 min Y: 1 d 4h
20-30 min 30 min 2h
6 2 0
13.0
1000
10 min
5h
20 min
6
For the Al and AlOx samples a lead absorber of 1-cm thickness was used to diminish the annihilation radiation of 511 keV. For the determination of Ba via 132La, the 464.6 keV photopeak is spectrally interfered by the 464.6 keV photopeak of 132mLa (see Table 1). After 4 h of cooling the 132mLa activity was reduced to less than 0.3% of the 132La activity. This minimum cooling period was determined by decay-curve analysis (DCA).12 To correct for the difference in geometry between the YBCOs and the BaCO3 standards, a geometry factor had to be taken into consideration. This factor was determined by means of the 477.2 keV photopeak of 55Co (see Table 1), which is produced from Ni by a (p,R) reaction on 58Ni. First, the irradiated Ni foil was measured in the YBCO geometry, then it was dissolved in a mixture of 3 mL of 6 M HNO3 and 200 µL of 20% H2O2. This solution was diluted to 25 mL, of which 1 mL was measured in the BaCO3 geometry. The ratio of both activities measured yields the geometry factor. For these experiments, self absorption by the BaCO3 matrix was negligible. Calculations. To determine the thickness of a thin layer, the composition and the density of the sputtered layer should be exactly known. The partial mass thickness cxDx must be divided by cx, i.e., concentration of the analyte in the thin layer, to determine the total mass thickness Dx. Division of Dx by the density of the thin layer gives the thickness of the thin layer expressed in units of length. On the other hand, to determine the composition or the atomic ratios, i.e., stoichiometry, of a sputtered compound, the ratio of the partial mass thicknesses cxDx for two elements must be divided by the ratio of the relative atomic masses of these elements. Neutron Activation Analysis for Al, AlOx, and TiOx. The Al, AlOx, and TiOx samples were also analyzed via neutron activation analysis (NAA) (see Table 1). To determine the mass thickness of the samples analyzed via NAA, the mass of the analyte was determined as well as the surface of the sample analyzed from the weight of the sample (almost 100% PET) and the experimentally determined mass thickness of PET. For the determination of Al in Al and AlOx layers, the 27Al(n,γ)28Al reaction was used (see Table 1). Sample and standard were irradiated together in a channel of Thetis reactor (INW, Ghent University), at a thermal neutron fluency rate of 1.9 × 1012 cm-2 s-1 for 2 min. The samples were measured for 3 min, 2 min after irradiation. The standards were measured for 2 min after a cooling time of 20 min. The influence of the interfering reaction, 30Si(n,p)28Al, was less than 1% as experimentally determined. (12) Cumming, J. B. CLSQ, the Brookhaven Decay Curve Analysis Program; Report BNL (6470); Brookhaven National Laboratory, 1962 (unpublished).
Ti was determined via the 50Ti(n,γ)51Ti reaction (for nuclear data see Table 1). The nuclear interference from the 51V(n,p)51Ti reaction was negligible, because (1) the cross section is low (σj n,p ) 0.9 mb) and (2) no V was detected by the 51V(n,γ)52V reaction, as the 1434.1 keV photopeak of 52V was not observed. In contrast with the determination of Al, the TiOx samples were first analyzed via NAA. Applying CPAA prior to NAA, a high residual background (Compton continuum) would be induced in the γ spectrum by 48V. Applying NAA prior to CPAA could introduce spectral interference in CPAA due to the neutron-induced 48Ti(n,p)48Sc reaction. However, this interference was negligible because (1) the cross section of this reaction is low (σj n,p ) 0.3 mb) and (2) the 1037.5 keV photopeak of 48Sc was not observed. The sample and standard were irradiated together in the Thetis reactor for 6 min again at the same thermal neutron fluency rate. The samples were measured for 6 min, 2 min after irradiation, and the standards for 4 min after a cooling time of 40 min. Inductively Coupled Plasma Mass Spectrometry of YBCO and Y2O3-Stabilized ZrO2. After the analysis via CPAA, the YBCOs were dissolved in 10 mL of 0.14 M HNO3 and the solution was diluted to 100 mL with 0.14 M HNO3. These solutions were analyzed with a Perkin-Elmer Sciex Elan 5000 ICP mass spectrometer in standard configuration. This means that the instrument is equipped with a multichannel peristaltic pump (Minipuls-3), a GemTip cross-flow nebulizer, a Perkin-Elmer Type II spray chamber made of Ryton, drained by the same peristaltic pump, and a Perkin-Elmer corrosion-resistant torch with standard alumina injector. External standardization was used with a 20 µg L-1 solution of Ba, Cu, and Y. To correct for possible instability of the apparatus, In (20 µg L-1) was added to all solutions as an internal standard. For all solutions, 137Ba+, 138Ba+, 63Cu+, 65Cu+, and 89Y+ were monitored. The Y2O3-stabilized ZrO2 was analyzed via ICPMS too. The samples were dissolved in 5 mL of 95% H2SO4 and diluted with 0.14 M HNO3 to 100 mL. As the standard, a solution of 20 µg L-1 Y and Zr was used. 89Y, 90Zr+, 91Zr+, and 92Zr+ were monitored. RESULTS AND DISCUSSION Al and AlOx. In Table 3 the results for the Al and AlOx layers via CPAA and NAA are given. Also, the Melinex substrate appeared to contain Al. All results in Table 3 are already corrected for this blank. The results obtained via CPAA and NAA are not significantly different at the 95% confidence limit. The AlOx layers are assumed to be Al2O3 with a density of F ) 3.97 g cm-3 for the thickness calculations. From Table 3 it is clear that a large spread of the AlOx values is Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
2817
Table 3. Results of the Sputtered Al and AlOx Layers via CPAA and NAAa
Al AlOx
Melinex 800 a
CPAA
NAA
8.83 (0.64) nm 2.38 (0.17) µg cm-2 (n ) 13) 45 (14) nm Al: 9.5 (2.9) µg cm-2 Al2O3: 17.9 (5.5) µg cm-2 (n ) 12) 0.55 (0.12) µg cm-2
9.10 (0.59) nm 2.46 (0.16) µg cm-2 (n ) 6) 43 (16) nm Al: 9.0 (3.5) µg cm-2 Al2O3: 16.9 (6.6) µg cm-2 (n ) 12) 0.547 (0.012) µg cm-2
(n ) 10)
(n ) 6)
Standard deviations in parentheses; n ) number of samples.
Table 6. Thickness of the TiOx Layer G (in nm) Obtained via CPAA and NAA (Standard Deviations in Parentheses) sample
CPAA mean
NAA
CPAA/NAA
1 2 3 4 5
27.89 (0.50) 28.19 (0.21) 28.16 (0.53) 28.96 (0.01) 27.77 (0.05)
27.50 (0.36) 27.64 (0.36) 28.06 (0.50) 28.02 (0.28) 27.81 (0.47)
1.014 (0.023) 1.020 (0.015) 1.004 (0.026) 1.034 (0.010) 0.999 (0.017) mean: 1.014 ( 0.017
Table 7. Results of the YBCO’s on Ta-substrate via CPAA and ICPMS (Standard Deviations in Parentheses) CPAA
Table 4. Results of the AlOx Layers Obtained via CPAA and NAA (Standard Deviations in Parentheses) sample
CPAA (µg cm-2)
NAA (µg cm-2)
1 2 3 4 5
3.182 (0.064) 8.89 (0.17) 6.19 (0.14) 4.85 (0.11) 7.83 (0.16)
3.016 (0.045) 9.08 (0.14) 6.442 (0.074) 5.068 (0.086) 8.18 (0.11)
CPAA/NAA 1.055 (0.026) 0.979 (0.024) 0.961 (0.025) 0.957 (0.027) 0.957 (0.023) mean: 0.982 ( 0.052
Table 5. Results of the TiOx Layers Obtained via CPAA (Standard Deviations in Parentheses) sample
mean (µg cm-2)
mean (nm)
A B C D E F G
1.0721 (0.0072) 1.1565 (0.0092) 1.1520 (0.0085) 1.125 (0.018) 1.062 (0.011) 7.425 (0.042) 11.835 (0.021)
2.517 (0.017) 2.715 (0.022) 2.704 (0.020) 2.641 (0.043) 2.493 (0.027) 17.430 (0.010) 27.782 (0.050)
observed, because of the inhomogeneous thickness of the sputtered layers. The precision of the method of analysis only depends on the counting statistics. This was proven by analyzing five AlOx samples first via NAA (three times) and afterward via CPAA (three times). A paired t-test on the results in Table 4 shows that there is not a significant difference at the 95% confidence level between NAA and CPAA. The mean ratio of the CPAA versus NAA results is 0.982 with a confidence limit of 0.052. The detection limit is 0.04 µg cm-2 for Al (0.15 nm) and 0.13 µg cm-2 for Al2O3 (0.32 nm) under the same experimental conditions. TiOx. In Table 5 the results for the TiOx layers are given. Each sample was analyzed three times. A paired t-test shows that the results obtained for the 983.1 and 1311.6 keV photopeaks are not significantly different. The results prove that TiOx layers with a thickness of a few nanometers can be determined with a precision of 2% or better. The TiOx layers are assumed to be TiO2 with a density F ) 4.26 g cm-3 for the thickness calculations. Five samples of the G series were first analyzed via NAA (four times) and afterward via CPAA (once). The results, expressed as thickness in nanometers, are given in Table 6. A paired t-test shows that the results obtained via CPAA and NAA are not 2818 Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
sample
Cu/Y
1 2a 2b 2c 3a 3b 3c 4 5a 5b 5c 6a 6b 6c 7a 7b 7c 8 9 10
2.894 (0.041) 2.964 (0.044) 3.130 (0.041) 2.873 (0.037) 2.911 (0.047) 3.162 (0.051) 3.043 (0.049) 3.074 (0.058) 2.722 (0.044) 2.820 (0.051) 2.881 (0.052) 2.883 (0.055) 3.012 (0.063) 2.984 (0.063) 3.090 (0.068) 3.084 (0.062) 3.143 (0.057) 3.140 (0.079) 3.301 (0.079) 3.100 (0.081)
ICPMS Ba/Y
2.02 (0.17) 1.86 (0.22) 2.27 (0.82) 2.42 (0.41) 2.00 (0.20) 1.57 (0.69) 2.22 (0.22) 2.47 (0.42) 2.10 (0.21) 2.20 (0.40) 2.32 (0.22) 2.47 (0.24) 2.53 (0.25)
Cu/Y
Ba/Y
3.001 (0.081) 2.97 (0.10) 3.01 (0.11) 3.01 (0.13) 2.864 (0.074) 2.903 (0.093) 2.86 (0.14) 3.241 (0.029) 3.012 (0.048) 3.014 (0.027) 3.013 (0.051) 3.180 (0.057) 3.163 (0.028) 3.152 (0.050) 3.163 (0.051) 3.190 (0.022) 3.193 (0.057) 3.061 (0.080) 3.292 (0.089) 2.954 (0.074)
2.463 (0.057) 2.392 (0.069) 2.374 (0.059) 2.38 (0.10) 2.251 (0.047) 2.252 (0.072) 2.261 (0.090) 2.491 (0.035) 2.210 (0.024) 2.244 (0.027) 2.213 (0.040) 2.481 (0.035) 2.510 (0.030) 2.482 (0.037) 2.403 (0.034) 2.434 (0.032) 2.390 (0.036) 2.381 (0.060) 2.092 (0.054) 2.414 (0.065)
significantly different at the 95% confidence limit. The mean ratio of the CPAA versus NAA results is 1.014 ( 0.017. The detection limit is 7.73 ng cm-2 Ti or 1.29 10-2 µg cm-2 TiO2 (0.03 nm) under the same experimental conditions. YBa2Cu3O6+δ. Owing to its high Coulomb barrier (EC ) 11.3 MeV for protons), the activity induced in the Ta substrate is relatively low. In Table 7 the Cu/Y and Ba/Y ratios obtained via CPAA and ICPMS are given. The Cu results obtained via both isotopes 63Zn (669.6 and 962.1 keV) and 65Zn show no significant difference. Also, the Cu/Y ratios (20 samples) obtained via CPAA and ICPMS are not significantly different at the 95% confidence limit. Cu/Y ratios of 3.011 ( 0.066 and 3.062 ( 0.059 are obtained for CPAA and ICPMS, respectively. For all YBCO samples analyzed, the Cu/Y ratio required for superconductivity, namely, three, was obtained. In Figure 1 the ratios of the CPAA versus ICPMS results are shown with a mean ratio of 0.984 ( 0.024. Because of the high standard deviation, the Ba/Y results for the samples 3C and 5B were rejected for the statistical treatment. For the other eleven samples a paired t-test shows that the results via CPAA and ICPMS are not significantly different at the 95% confidence limit. In Figure 2 the ratios of the CPAA versus ICPMS results are shown with a mean ratio of 0.951 ( 0.066. Mean Ba/Y ratios of 2.24 ( 0.15 and 2.36 ( 0.09 were obtained for CPAA and ICPMS, respectively. The Ba/Y ratio for all analyzed YBCO
Figure 1. Ratio of the CPAA results to the ICPMS results for the Cu/Y ratios in YBCO layers.
Figure 2. Ratio of the CPAA results to the ICPMS results for the Ba/Y ratios in YBCO layers.
samples was higher than the Ba/Y ratio expected for superconductivity (i.e., two). Y2O3-Stabilized ZrO2. In Table 8 the Zr/Y ratios obtained via CPAA (two photopeaks of 90Nb) and ICPMS (three Zr isotopes) are given. Again, a paired t-test could not detect any significant difference between the results obtained via CPAA and ICPMS. Thickness Determinations. For the sputtered layers of Al, AlOx, TiOx, and Y2O3-stabilized ZrO2, their thickness was compared with the thickness results obtained by commonly used methods such as X-ray Reflectometry (XRR) and Electron Spectroscopy for Chemical Analysis (ESCA). These results differ significantly (up to 50%), however. As mentioned before, the accuracy of the method to determine the mass thickness of thin layers has been experimentally proven. To calculate the thickness of thin layers via CPAA, the composition and the density of the sputtered layers must be known (see calculations). Both factors can vary a lot during the sputtering process. The composition depends on the sputtering yield of the different elements and compounds. The
Table 8. Zr/Y Ratios of the Y2O3-Stabilized ZrO2 via CPAA and ICPMS (Standard Deviations in Parentheses) sample
CPAA
1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b
1.063 (0.060) 0.998 (0.058) 1.072 (0.035) 1.064 (0.036) 1.139 (0.027) 1.132 (0.025) 1.116 (0.034) 1.110 (0.033) 1.116 (0.034) 1.117 (0.034) 1.161 (0.031) 1.153 (0.035)
ICPMS 1.031 (0.011) 1.031 (0.011) 1.113 (0.019) 1.113 (0.019) 1.0843 (0.0087) 1.0544 (0.0095) 1.023 (0.012) 1.0573 (0.0095) 1.0384 (0.0093) 1.120 (0.010) 1.109 (0.010)
density depends on the pressure inside the system and the substrate temperature. The composition can, however, be controlled by using optical plasma emission,13 whereby the intensity Analytical Chemistry, Vol. 72, No. 13, July 1, 2000
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of the light emission of the excited atoms in the plasma is monitored. To control the density during the sputtering process is much more difficult. That is why the density of thin layers (
