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Depth-profile analysis of thermoelectric layers on Si wafers by pulsed r.f. glow discharge time-of-flight mass spectrometry
Spectrochimica Acta Part B 76 (2012) 175–180
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Depth-profile analysis of thermoelectric layers on Si wafers by pulsed r.f. glow discharge time-of-flight mass spectrometry☆ K.-G. Reinsberg a, C. Schumacher b, A. Tempez c, K. Nielsch b, J.A.C. Broekaert a,⁎ a b c
Institute for Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany Institute for Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany HORIBA Jobin Yvon, 16‐18 rue du Canal, F-91160 Longjumeau, France
a r t i c l e
i n f o
Article history: Received 3 April 2012 Accepted 15 June 2012 Available online 26 June 2012 Keywords: Thermoelectrics Pulsed r.f. glow discharge Time-of-flight mass spectrometry Depthprofiling
a b s t r a c t In this work the depth-profile analysis of thermoelectric layers deposited on Au and Cr covered Si wafers with the aid of pulsed radiofrequency glow discharge time-of-flight mass spectrometry (pulsed RF-GD-TOFMS also called plasma profiling TOFMS (PP-TOFMS™)) is described. For thermoelectric materials the depth resolutions obtained with both PP-TOFMS and secondary ion mass spectrometry (SIMS) are shown to be well comparable and in the order of the roughness of the corresponding layers (between 20 and 3700 nm). With both methods a direct solid analysis without any preparation steps is possible. In addition, the analysis of the samples with PP-TOFMS proved to be faster by a factor of 26 compared to SIMS, as sputtering rates were found to be 80 nm s−1 and 3 nm s−1, respectively. For the analyzed samples the results of PP-TOFMS and SIMS show that a homogeneous deposition was obtained. Quantitative results for all samples could also be obtained directly by PP-TOFMS when the stoichiometry of one sample was determined beforehand for instance by inductively coupled plasma optical emission spectrometry (ICP-OES) and scanning electron microscopy energy dispersive X-ray fluorescence spectrometry (SEM-EDX). For Bi2Te3 the standard deviation for the main component concentrations within one sample then is found to be between 1.1% and 1.9% and it is 3.6% from sample to sample. For Sb2Te3 the values within one sample are from 1.7% to 4.2% and from sample to sample 5.3%, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric materials have the unique property to convert heat into electrical power and therefore they can be used to recover energy, wherever it is lost by undesired heat production. For the use of thermoelectrics at room temperature, Bi2Te3, Sb2Te3 and Bi2−xSbxTe3 are the materials of choice [1,2]. The simplest and inexpensive method for the preparation of a system capable of gaining electrical power by thermoelectrics makes use of electrochemical deposition from an aqueous electrolyte solution [3–5]. As shown in our previous work [6,7], the synthesis conditions with respect to the composition of the layers have to be optimized in detail. It has been shown in the literature, that thermoelectric properties of materials such as electrical conductivity (σ), thermal conductivity (κ) and the Seebeck coefficient (S = ΔU/ΔT, the generated voltage for a given temperature difference) critically depend on the composition [8] and the homogeneity [9] of the materials. The electrochemical deposition in its turn depends on many parameters, which can affect the composition. Especially for non-stoichiometric synthesis products a non-homogeneous deposition has been reported [10]. ☆ This paper is dedicated to Gary M. Hieftje, on the occasion of his 70th birthday, in recognition of his boundless contributions to spectroscopy and analytical chemistry. ⁎ Corresponding author. E-mail address: [email protected] (J.A.C. Broekaert). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.06.005
Great efforts have therefore been undertaken to determine deposition conditions, which allow it to obtain homogeneous synthesis products [2] and complex Pourbaix-diagrams for such studies on the depostion can be found in the literature [11]. After all, however, the homogeneity of the layers ought to be controlled by depth-profiling methods such as pulsed RF-GD-TOFMS also called plasma profiling TOFMS (PP-TOFMS™) and secondary ion mass spectrometry (SIMS).
1.1. PP-TOFMS PP-TOFMS is a suitable technique to gain depth-profile information in the case of electrically conducting as well as of non-conducting samples. The so called “source profile” generated from the transient signals of the pulsed RF plasma can be divided into three different time windows, namely the prepeak, the plateau and the afterglow time windows. In the prepeak time window the plasma is formed by the action of an electric field. In this time window only plasma gas ions are detectable as sample sputtered species are not yet present in the plasma to be ionized [12]. In the plateau region, the discharge reaches a stationary state, where the production of electrons and ions is balanced by their loss through diffusion to the chamber walls. In this time window the ionization is known to be mainly the result of electron impact (aproximately 90%) and collision with metastable atoms [13]. After
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sputtering. Bi2Te3 and Sb2Te3 layers were deposited on the substrates under the application of a pulsed potential. The latter was found to be essential for obtaining relatively smooth surfaces, compared to those deposited under non-pulsed potential. Pulse cycles as well as the average potential were optimized, so as to achieve the correct stoichiometric composition [6]. For this optimization work elemental determinations of the main components in the layers were carried out with ICP-OES [7]. As the homogeneity of the layers cannot be measured with this bulk analysis procedure an additional depth-profile analysis was required. For this study a series of samples was produced under optimized conditions. The Si wafer was cut into 25 × 25 mm 2 pieces on which a spot of 15 mm diameter was deposited. The samples have then been investigated with the PP-TOFMS instrument. They were taken as they were produced and mounted onto the GD source. In Fig. 1 a drawing of a typical sample is shown. The same samples have also been analyzed with secondary ion mass spectrometry (SIMS) so as to acquire depth-profiles with a reference profiling method. The sputtering rates and the depth resolution obtained with both methods are compared.
Fig. 1. A typical sample consisting of a thermoelectric layer deposited on top of Au and Cr layers sputtered on a Si wafer (drawing is not to scale).
the electric field has been switched off the electrons are no longer accelerated to the anode. By collisions with atoms they quickly loose their initial energy and it comes to a thermal equilibrium with the surrounding gas. By this thermalization process the electron density is expected to sharply increase by several orders of magnitude, as described by Bogaerts [14]. Due to the thermalization a dissociative recombination process between electrons and Ar2+ ions is induced during the afterglow and the number of highly excited Ar species reaches a maximum in the afterglow. Therefore this ionization process explains qualitatively the intense Ar signals during the afterglow. Analyte signal increases during the afterglow may be explained in a similar way or by enhanced Penning ionization (ionization by Ar metastables) [14]. One can easily imagine that every element will behave differently in response to well-defined afterglow conditions depending on its energetic properties. Therefore, the source profiles for the isotopes 209Bi and 130Te are studied by PP-TOFMS in the case of thermoelectric layers of Bi2Te3 deposited on Si substrates. 2. Experimental 2.1. Samples Prior to electrochemical deposition, Si substrates are coated with a 20 nm thick Cr layer followed with a 20 nm thick Au layer by
2.2. PP-TOFMS parameters and analysis conditions The PP-TOFMS system (HORIBA Jobin Yvon, France) used consists of a GD plasma source, ablating the sample and delivering ions to an orthogonal extraction time-of-flight mass spectrometer. A similar system is described by Lobo [12,15] and Pisonero [16]. The GD is operated with high purity (6 N) argon at a pressure of approximately 6 mbar, leading to sample ablation and ionization of the sputtered species. The ions are then transferred via an interface to the time-of-flight mass-analyzer equiped with a 70 cm long flight tube and a double stage reflectron at a pressure of 5 × 10 −7 mbar. The sample is mounted against an O-ring to close the plasma chamber. A ceramic ring between the sample and a grounded 4 mm diameter anode tube defines the appropriate 0.15 μm spacing. A water cooled (10 °C) applicator is held by a piston on the back of the sample. It brings the RF power from an RF-generator with a maximum power of 150 W and operated at 13.6 MHz to the sample. The RF generator is operated in pulsed mode at a frequency of 250 Hz with 2 ms long pulse, so that the entire period of interest including the afterglow is 4 ms. Within this time frame full mass spectra are acquired at a frequency of 27.5 kHz. The interface includes a quadrupole filter, which allows to attenuate up to four ions of different masses in order to prevent overloading of the detector and to increase the dynamic range of the system. Due to the relatively weak adhesion of the thermoelectric layer to the Au substrate a low GD operation power of 30 W was choosen to ablate the samples. The detection was carried out in the positive ion mode.
Fig. 2. Source profiles obtained for Sb2Te3 (left) and Bi2Te3 (right). Source profile data shown here are acquired for both samples 100 s after the glow discharge was started so that variations seen cannot be attributed to surface effects. The ion counting rates for the most abundant isotopes of the elements of interest are shown.
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obtained by PP-TOFMS analysis. The thickness determined with SEM-EDX was used to convert sputtering time to depth for the thermoelectric layer. To do the time to depth conversion for the Au, Cr and Si parts of the sample the thickness of each of the Cr and Au layers was assumed to be 20 nm, which is known from the deposition rates for the Au and Cr layers during the production process of the samples. To do the time to depth conversion within the thermoelectric layer for the PP-TOFMS analysis the occurrence of the half value of the maximum intensity of the Au peak was considered to indicate the end of the sample film. To do the time to depth conversion within the sputtered Au and Cr layers and for the Si part of the sample the depth between the occurrence of the half of the maximum intensity for the 197Au peak and the occurrence of the half of the maximum intensity for 30Si is accepted to have a thickness of 40 nm. For the calculations in the case of SIMS analyses the crater depths were measured with the aid of a profilometer and all other calculations were done as in PP-TOFMS. A schematic representation for the time to depth conversion is given in Fig. 3. Fig. 3. Time to depth conversion for Sb2Te3. Two sputtering rates, one for the Sb2Te3 layer and one for the Au, Cr and Si part of the sample. SEM was also used to study the crater shape.
2.3. SIMS For the SIMS measurements a CAMECA IMS 4550 (CAMECA, Genevilliers, France) was used with Cs + primary ions at an energy of 5 keV. 3. Results and discussion 3.1. PP-TOFMS “Source Profiles” In Fig. 2 a typical source profile is shown. As expected in the afterglow region (once RF potential is turned off) ion signals strongly increase, due to the effects described before. Three integration time windows have been selected to generate depth-profiles: the full source profile, a period during the plateau and a period in the afterglow regime, in which peaks exhibit a maximum. The most “interesting” part of the glow discharge operation is certainly the afterglow region in which higher signal-to-noise ratios are observed. It was found that the behavior of Te greatly differs from the behavior of Bi in the afterglow region. While for Te one peak maximum occurs in the afterglow region two peaks are observed for Bi. 3.2. Time to depth conversion The exact thickness of the thermoelectric layer was determined by scanning electron microscopy (SEM) as a complementary method. SEM measurements were performed on cross sections of the craters
3.3. Qualitative analysis of the samples The peak maxima in the afterglow pulse periode for all isotopes measured, namely 121Sb, 209Bi, 130Te, 52Cr and 197Au were used for the depth profile measurements with PP-TOFMS. For the isotopic measurements a mass-resolving power R of 2700 (measured on m/ z = 197) under the experimental conditions used was found. From the comparison of the results in Figs. 4 and 5 it can be concluded that the layers were rather homogeneous. With both SIMS and PP-TOFMS, however, small deviations from perfectly flat profiles for the signals of 130Te and 121Sb or 130Te and 209Bi, are found. PP-TOFMS and SIMS profiles are not identical as SIMS is extremely sensitive to matrix changes, whereas PP-TOFMS is much less matrix dependent. For both techniques and both sample types the expected order of layers, as it can be seen in Fig. 1, was found. The thermoelectric layer, consisting of Sb2Te3 or Bi2Te3, is followed by the Au and the Cr layer and the Si wafer is located below all others. It can be seen that sputtering changes the surface morphology of the sample around the crater and produces rounded crater edges. Particles from the sample are deposited onto the surrounding sample surface. In Fig. 6 sample morphologies and crater shapes are shown. 3.4. Comparison of the sputtering performances in PP-TOFMS and SIMS Depth resolution is defined as the distance between 84% and 16% of the depth-profile and it relates to interface sharpness [17]. Depth resolutions were calculated accordingly making use of the depths, for which the calculation was explained above, and are given in Table 1. The sputtering rates, which were calculated from the measured depth and the sputtering time can be found in Table 2. The
Fig. 4. Depth-profiles obtained with pulsed r.f. PP-TOFMS for Sb2Te3 (left) and Bi2Te3 (right). For a better overview the ion counting rates have been normalized to their maxima.
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Fig. 5. Depth-profiles obtained with SIMS for Sb2Te3 (left) and Bi2Te3 (right). For a better overview the ion counting rates have been normalized to their maxima.
B
A
C
D
E
Fig. 6. Typical crater shapes and sample surfaces after GD sputtering. A) Needlelike structure of a sample area, which was unaffected by the GD. B) Crater rim: the unaffected area is on the left side and the crater is on the right side. C) Crater area. D) Cross section of the sample cut through a crater rim. E) Depth-profile as measured with a mechanical profilometer and confirming the crater shape, which is shown in D).
standard deviations for the sputtering rates and the depth resolutions in the case of the GD measurements are calculated from measurements at three samples. In the case of the SIMS measurements only one sample of each type was analyzed. The depth resolution can be calculated from the decrease of the signals for 209Bi, 121Sb or 130Te at the interface with the Au layer, from the increase of the signal for 197Au at this interface or from the decrease of the signal for 197Au at its interface with the Cr layer, from both the
Table 1 Depth resolution at the different interfaces. Depth resolution [nm] PP-TOFMS
SIMS
Sb2Te3
121
Increasing signal Decreasing signal
–
–
1700
1000 81
3400
3700
24
31
–
Bi2Te3
209
130
197
52
30
Increasing signal Decreasing signal
–
–
320
170
53
–
–
–
–
–
210
190
350
310
–
–
–
–
–
–
Sb
Bi
130
Te
Te
197
Au
Au
52
Cr
Cr
30
Si
Si
121
Sb
130
Te
197
Au
52
Cr
30
Si
–
–
250
110
61
67
67
41
55
–
52
30
209
Bi
130
Te
197
Au
Cr
increase of the signal for 52Cr and the decrease at the Au or the Si interface and from the increase of the signal for 30Si at the Cr interface (see Fig. 1). Since the sample morphology significantly changes at the interfaces mentioned the depth resolution also may vary dramatically, which is shown by calculated depth resolutions reported in Table 1. It can be seen clearly that the depth resolution in the case of PP-TOFMS strongly depends on the sample morphology. Also one has to consider that for the PP-TOFMS a large sample area is sputtered (4 mm diameter) so the depth resolution is clearly related to the layer roughness. For the porous thermoelectric layer of Sb2Te3 the resolution is therefore poor. At the Au, Cr and Si interfaces this seems to be no longer the case. In terms of analysis time and sample throughput, PP-TOFMS is clearly more advantegous, since the sputtering rate is higher by a factor of 26 than in SIMS. For both methods the adhesion of the sample to the Si wafer under the experimental conditions is crucial. For both sample types there were indications from unsucessfull test runs that parts of the sample may peel off from the surface of the Si
Table 2 Sputtering rates obtained with PP-TOFMS and SIMS. Si
Sample type
Sb2Te3 Bi2Te3
Sputtering rate [nm s−1] PP-TOFMS
SIMS
79 ± 0.6 82 ± 2.6
3.37 2.65
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Fig. 7. Determination of the composition of Sb2Te3 (left) and Bi2Te3 (right) layers with different methods. For PP-TOFMS the integration time zone (in the source profile) giving the best precision was selected, i.e. for Sb2Te3 the full profile and for Bi2Te3 the plateau region.
wafer. This clearly leads to a deterioration of the depth resolution. Therefore, the calculated depth resolutions measured in this work are significantly worse than those, which can be found in the literature for perfectly flat samples [18].
mentioned above the isotopic abundance is considered as being proportional to the elemental abundance and no further correction was needed. The standard deviations for measurements at different samples both for Bi2Te3 and Sb2Te3 are larger than those obtained at the same sample.
3.5. Stoichiometry 4. Conclusions The Sb/Te and Bi/Te concentration ratios were determined with a high precision by ICP-OES after wet chemical digestion [7] and with scanning electron microscopy energy dispersive X-ray fluorescence spectrometry (SEM-EDX). Both methods give rather constant values over the whole sample series. The relationship between the ion counting rates in PP-TOFMS and the concentration was obtained by asigning the average of the ICP-OES results to the average ion counting rates. The isotopic ratios obtained for 121Sb/ 130Te and 209Bi/ 130 Te with PP-TOFMS were constant and, after the calibration to the average, in good agreement with ICP-OES and SEM-EDX data. Since the isotopic distribution is constant the isotopic abundance is proportional to the elemental abundance and no further correction is necessary for the calibration. The results obtained with the three methods are compared in Fig. 7. The precision obtained with PP-TOFMS was found to be not as good as in ICP-OES and worse than the results cited in the literature, where relative standard deviations between 0.1% and 1% were found for bulk concentrations larger than 1% [19]. However, having the source profiles in mind it is easy to imagine that the standard deviations for the elemental concentration ratios for Sb and Te as well as for Bi and Te, respectively, depend on the choice of the integration time window within the source profile measured, as it is shown in Fig. 2. It must be mentioned, however, that the precision could not be increased significantly by the choice of one of the indicated measurement time windows. For Bi2Te3 the plateau time window seems to be the best choice so as to obtain low standard deviations, whereas for Sb2Te3 a data acquisition over the full pulse duration gives a lower standard deviation. Together with the relative standard deviations for measurements of different samples the relative standard deviations for the 121Sb/ 130Te and 209Bi/ 130Te isotopic signal ratios obtained with one sample only are given in Table 3. As
Table 3 Precision obtained in PP-TOFMS for Bi2Te3 and Sb2Te3. No. sample
130
Te/121Sb RSD (full pulse)
31.3 31.8 31.11 Sb2Te3
1.7 2.5 4.2 5.3
PP-TOFMS is shown to be useful to determine the homogeneity of the deposition of thermoelectric materials based on Bi, Sb and Te deposited on Au and Cr coated Si wafers. The results were validated by a comparison with the results obtained with SIMS measurements at the same samples. With both methods it could be shown that the deposition was homogeneous. For the PP-TOFMS and SIMS analyses depth resolutions have been determined and were found to strongly depend on the substrate and the measurement mode. While for the Sb2Te3 layers, depth resolutions of 3 μm were found with PP-TOFMS, the depth resolution within the Au, Cr and Si layers for the same sample was found to be below 80 nm. For the same sample the depth resolutions determined with SIMS were found to be between 250 nm and 40 nm. This may be attributed to the respective measured areas in the case of both methods, which is an essential parameter for relatively rough samples. For a Bi2Te3 sample all calculated depth resolutions obtained with PP-TOFMS were between 530 nm and 200 nm. The large deviations are attributed to the relatively fragile character of the samples. For PP-TOFMS and SIMS the sputtering rates have been found to be approximately 80 nm s −1 and 3 nm s −1, which makes the use of PP-TOFMS clearly more advantageous as compared to SIMS. The precision of PP-TOFMS has been investigated and found to be within 1.1% and 1.9% for measurements at one sample of Bi2Te3, which is lower than the standard deviation from measuring at three samples of the same material, which was found to be 3.6%. The same trend was found for the Sb2Te3 samples, for which the standard deviations in the case of one sample were found to be between 1.7% and 4.2%. The standard deviation for measurements at three Sb2Te3 samples was found to be 5.3%. From this result it can be concluded that the homogeneity within one sample is superior to the variability from one sample to another.
Acknowledgments
No. sample
130
Te/209Bi RSD (plateau)
35.2 35.4 35.7 Bi2Te3
1.7 1.9 1.1 3.6
The authors thank the “Bundesministerium für Bildung und Forschung” (BMBF) (Germany) for funding the investigations through the EchemTE project. The staff at the SIMS laboratory of the “Fraunhofer Institut für Angewandte Festkörperphysik” (Fraunhofer
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IAF, Freiburg, Germany) is gratefully thanked for carrying out the SIMS measurements.
References [1] W.M. Yim, A. Smith, Bi–Sb alloys for magneto-thermoelectric and thermomagnetic cooling, Solid State Electron. 15 (1972) 1141–1165. [2] In: D.M. Rowe (Ed.), Handbook of Thermoelectrics, CRC Press, Boca Raton, 1995. [3] M. Takahashi, Y. Oda, T. Ogino, S.J. Furuta, Deposition of Bi–Te alloy films, J. Electrochem. Soc. 140 (1993) 2550–2553. [4] M.S. Martín-González, A.L. Prieto, R. Gronsky, T. Sands, A.M. Stacy, Insights into the electrodeposition of Bi2Te3, J. Electrochem. Soc. 149 (2002) C546–C554. [5] P. Magri, C. Boulanger, J.-M. Lecuire, Synthesis, properties and performances of electrodeposited bismuth telluride films, J. Mater. Chem. 6 (1996) 773–779. [6] C. Schumacher, K.-G. Reinsberg, L. Akinside, S. Zastrow, S. Heiderich, W. Toelnner, G. Rampelberg, C. Detavernier, J.A.C. Broekaert, K. Nielsch, J. Bachmann, Optimization of electrodeposited p-doped Sb2Te3 thermoelectric films by millisecond potentiostatic pulse, Adv. Energy Mater. 2 (2012) 345–352. [7] K.-G. Reinsberg, C. Schumacher, K. Nielsch, J.A.C. Broekaert, Precision improvements by the use of principal component regression and pooled regression applied to main component determinations with ICP-OES for thermoelectric films, J. Anal. At. Spectrom. 26 (2011) 2447–2482. [8] J.P. Fleurial, L. Gailliard, R. Triboulet, H. Scherrer, S. Scherrer, Thermal properties of high quality single crystals of bismuth telluride—part I: experimental characterization, J. Phys. Chem. Solids 49 (1988) 1237–1247. [9] J. Kuleshova, E. Koukharenko, X. Li, N. Frety, L.S. Nandhakumar, J. Tudor, S.P. Beeby, N.M. White, Optimization of the electrodeposition process of high-performance bismuth antimony telluride compounds for thermoelectric applications, Langmuir 26 (2010) 16980–16985.
[10] X. Li, E. Koukharenko, I.S. Nandhakumar, J. Tudor, S.P. Beeby, N.M. White, High density p-type Bi0.5Sb1.5Te3 nanowires by electrochemical templating through ion-track lithography, Phys. Chem. Chem. Phys. 11 (2009) 3584–3590. [11] F. Xiao, C. Hangarter, B. Yoo, Y. Gheem, K.-H. Lee, N.V. Myung, Recent progress in electrodeposition of thermoelectric thin films and nanostructures, Electrochim. Acta 53 (2008) 8103–8117. [12] L. Lobo, J. Písonero, N. Bordel, R. Pereiro, A. Tempez, P. Chapon, J. Michler, M. Hohl, A. Sanz-Medel, A comparison of non-pulsed radiofrequency and pulsed radiofrequency glow discharge orthogonal time-of-flight mass spectrometry for analytical purposes, J. Anal. At. Spectrom. 24 (2009) 1373–1381. [13] M.A. Biondi, Diffusion, de-excitation, and ionization cross sections for metastable atoms. I, Phys. Rev. 88 (1952) 660–665. [14] A. Bogaerts, The afterglow mystery of pulsed glow discharges and the role of dissociative electron–ion recombination, J. Anal. At. Spectrom. 22 (2007) 502–512. [15] L. Lobo, N. Tuccitto, N. Bordel, R. Pereiro, J. Pisonero, A. Licciardello, A. Tempez, P. Chapon, A. Sanz-Medel, Polymer screening by radiofrequency glow discharge time-of-flight mass spectrometry, Anal. Bioanal. Chem. 396 (2010) 2863–2869. [16] J. Pisonero, L. Lobo, N. Bordel, A. Tempez, A. Bensaoula, N. Badi, A. Sanz-Medel, Quantitative depth-profile analysis of boron implanted silicon by pulsed radiofrequency glow discharge time-of-flight mass spectrometry, Sol. Energy Mater. Sol. Cells 94 (2010) 1352–1357. [17] G.H. Morrison, K.L. Cheng, M. Grasserbauer, General aspects of trace analytical methods—IV. Recommendations for nomencalture, standard procedures and reporting of experimental data for surface analysis techniques, Pure Appl. Chem. 51 (1979) 2243–2250. [18] J. Pisonero, Glow discharge spectroscopy for depth-profile analysis: from micrometer to sub-nanometer layers, Anal. Bioanal. Chem. 384 (2006) 47–49. [19] M. McClenathan, G.M. Hiefje, Absolute methods of quantitation in glow discharge mass spectrometry with a time-of-flight mass analyzer, J. Anal. At. Spectrom. 20 (2005) 1326–1331.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab
Depth-profile analysis of thermoelectric layers on Si wafers by pulsed r.f. glow discharge time-of-flight mass spectrometry☆ K.-G. Reinsberg a, C. Schumacher b, A. Tempez c, K. Nielsch b, J.A.C. Broekaert a,⁎ a b c
Institute for Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany Institute for Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany HORIBA Jobin Yvon, 16‐18 rue du Canal, F-91160 Longjumeau, France
a r t i c l e
i n f o
Article history: Received 3 April 2012 Accepted 15 June 2012 Available online 26 June 2012 Keywords: Thermoelectrics Pulsed r.f. glow discharge Time-of-flight mass spectrometry Depthprofiling
a b s t r a c t In this work the depth-profile analysis of thermoelectric layers deposited on Au and Cr covered Si wafers with the aid of pulsed radiofrequency glow discharge time-of-flight mass spectrometry (pulsed RF-GD-TOFMS also called plasma profiling TOFMS (PP-TOFMS™)) is described. For thermoelectric materials the depth resolutions obtained with both PP-TOFMS and secondary ion mass spectrometry (SIMS) are shown to be well comparable and in the order of the roughness of the corresponding layers (between 20 and 3700 nm). With both methods a direct solid analysis without any preparation steps is possible. In addition, the analysis of the samples with PP-TOFMS proved to be faster by a factor of 26 compared to SIMS, as sputtering rates were found to be 80 nm s−1 and 3 nm s−1, respectively. For the analyzed samples the results of PP-TOFMS and SIMS show that a homogeneous deposition was obtained. Quantitative results for all samples could also be obtained directly by PP-TOFMS when the stoichiometry of one sample was determined beforehand for instance by inductively coupled plasma optical emission spectrometry (ICP-OES) and scanning electron microscopy energy dispersive X-ray fluorescence spectrometry (SEM-EDX). For Bi2Te3 the standard deviation for the main component concentrations within one sample then is found to be between 1.1% and 1.9% and it is 3.6% from sample to sample. For Sb2Te3 the values within one sample are from 1.7% to 4.2% and from sample to sample 5.3%, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thermoelectric materials have the unique property to convert heat into electrical power and therefore they can be used to recover energy, wherever it is lost by undesired heat production. For the use of thermoelectrics at room temperature, Bi2Te3, Sb2Te3 and Bi2−xSbxTe3 are the materials of choice [1,2]. The simplest and inexpensive method for the preparation of a system capable of gaining electrical power by thermoelectrics makes use of electrochemical deposition from an aqueous electrolyte solution [3–5]. As shown in our previous work [6,7], the synthesis conditions with respect to the composition of the layers have to be optimized in detail. It has been shown in the literature, that thermoelectric properties of materials such as electrical conductivity (σ), thermal conductivity (κ) and the Seebeck coefficient (S = ΔU/ΔT, the generated voltage for a given temperature difference) critically depend on the composition [8] and the homogeneity [9] of the materials. The electrochemical deposition in its turn depends on many parameters, which can affect the composition. Especially for non-stoichiometric synthesis products a non-homogeneous deposition has been reported [10]. ☆ This paper is dedicated to Gary M. Hieftje, on the occasion of his 70th birthday, in recognition of his boundless contributions to spectroscopy and analytical chemistry. ⁎ Corresponding author. E-mail address: [email protected] (J.A.C. Broekaert). 0584-8547/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2012.06.005
Great efforts have therefore been undertaken to determine deposition conditions, which allow it to obtain homogeneous synthesis products [2] and complex Pourbaix-diagrams for such studies on the depostion can be found in the literature [11]. After all, however, the homogeneity of the layers ought to be controlled by depth-profiling methods such as pulsed RF-GD-TOFMS also called plasma profiling TOFMS (PP-TOFMS™) and secondary ion mass spectrometry (SIMS).
1.1. PP-TOFMS PP-TOFMS is a suitable technique to gain depth-profile information in the case of electrically conducting as well as of non-conducting samples. The so called “source profile” generated from the transient signals of the pulsed RF plasma can be divided into three different time windows, namely the prepeak, the plateau and the afterglow time windows. In the prepeak time window the plasma is formed by the action of an electric field. In this time window only plasma gas ions are detectable as sample sputtered species are not yet present in the plasma to be ionized [12]. In the plateau region, the discharge reaches a stationary state, where the production of electrons and ions is balanced by their loss through diffusion to the chamber walls. In this time window the ionization is known to be mainly the result of electron impact (aproximately 90%) and collision with metastable atoms [13]. After
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sputtering. Bi2Te3 and Sb2Te3 layers were deposited on the substrates under the application of a pulsed potential. The latter was found to be essential for obtaining relatively smooth surfaces, compared to those deposited under non-pulsed potential. Pulse cycles as well as the average potential were optimized, so as to achieve the correct stoichiometric composition [6]. For this optimization work elemental determinations of the main components in the layers were carried out with ICP-OES [7]. As the homogeneity of the layers cannot be measured with this bulk analysis procedure an additional depth-profile analysis was required. For this study a series of samples was produced under optimized conditions. The Si wafer was cut into 25 × 25 mm 2 pieces on which a spot of 15 mm diameter was deposited. The samples have then been investigated with the PP-TOFMS instrument. They were taken as they were produced and mounted onto the GD source. In Fig. 1 a drawing of a typical sample is shown. The same samples have also been analyzed with secondary ion mass spectrometry (SIMS) so as to acquire depth-profiles with a reference profiling method. The sputtering rates and the depth resolution obtained with both methods are compared.
Fig. 1. A typical sample consisting of a thermoelectric layer deposited on top of Au and Cr layers sputtered on a Si wafer (drawing is not to scale).
the electric field has been switched off the electrons are no longer accelerated to the anode. By collisions with atoms they quickly loose their initial energy and it comes to a thermal equilibrium with the surrounding gas. By this thermalization process the electron density is expected to sharply increase by several orders of magnitude, as described by Bogaerts [14]. Due to the thermalization a dissociative recombination process between electrons and Ar2+ ions is induced during the afterglow and the number of highly excited Ar species reaches a maximum in the afterglow. Therefore this ionization process explains qualitatively the intense Ar signals during the afterglow. Analyte signal increases during the afterglow may be explained in a similar way or by enhanced Penning ionization (ionization by Ar metastables) [14]. One can easily imagine that every element will behave differently in response to well-defined afterglow conditions depending on its energetic properties. Therefore, the source profiles for the isotopes 209Bi and 130Te are studied by PP-TOFMS in the case of thermoelectric layers of Bi2Te3 deposited on Si substrates. 2. Experimental 2.1. Samples Prior to electrochemical deposition, Si substrates are coated with a 20 nm thick Cr layer followed with a 20 nm thick Au layer by
2.2. PP-TOFMS parameters and analysis conditions The PP-TOFMS system (HORIBA Jobin Yvon, France) used consists of a GD plasma source, ablating the sample and delivering ions to an orthogonal extraction time-of-flight mass spectrometer. A similar system is described by Lobo [12,15] and Pisonero [16]. The GD is operated with high purity (6 N) argon at a pressure of approximately 6 mbar, leading to sample ablation and ionization of the sputtered species. The ions are then transferred via an interface to the time-of-flight mass-analyzer equiped with a 70 cm long flight tube and a double stage reflectron at a pressure of 5 × 10 −7 mbar. The sample is mounted against an O-ring to close the plasma chamber. A ceramic ring between the sample and a grounded 4 mm diameter anode tube defines the appropriate 0.15 μm spacing. A water cooled (10 °C) applicator is held by a piston on the back of the sample. It brings the RF power from an RF-generator with a maximum power of 150 W and operated at 13.6 MHz to the sample. The RF generator is operated in pulsed mode at a frequency of 250 Hz with 2 ms long pulse, so that the entire period of interest including the afterglow is 4 ms. Within this time frame full mass spectra are acquired at a frequency of 27.5 kHz. The interface includes a quadrupole filter, which allows to attenuate up to four ions of different masses in order to prevent overloading of the detector and to increase the dynamic range of the system. Due to the relatively weak adhesion of the thermoelectric layer to the Au substrate a low GD operation power of 30 W was choosen to ablate the samples. The detection was carried out in the positive ion mode.
Fig. 2. Source profiles obtained for Sb2Te3 (left) and Bi2Te3 (right). Source profile data shown here are acquired for both samples 100 s after the glow discharge was started so that variations seen cannot be attributed to surface effects. The ion counting rates for the most abundant isotopes of the elements of interest are shown.
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obtained by PP-TOFMS analysis. The thickness determined with SEM-EDX was used to convert sputtering time to depth for the thermoelectric layer. To do the time to depth conversion for the Au, Cr and Si parts of the sample the thickness of each of the Cr and Au layers was assumed to be 20 nm, which is known from the deposition rates for the Au and Cr layers during the production process of the samples. To do the time to depth conversion within the thermoelectric layer for the PP-TOFMS analysis the occurrence of the half value of the maximum intensity of the Au peak was considered to indicate the end of the sample film. To do the time to depth conversion within the sputtered Au and Cr layers and for the Si part of the sample the depth between the occurrence of the half of the maximum intensity for the 197Au peak and the occurrence of the half of the maximum intensity for 30Si is accepted to have a thickness of 40 nm. For the calculations in the case of SIMS analyses the crater depths were measured with the aid of a profilometer and all other calculations were done as in PP-TOFMS. A schematic representation for the time to depth conversion is given in Fig. 3. Fig. 3. Time to depth conversion for Sb2Te3. Two sputtering rates, one for the Sb2Te3 layer and one for the Au, Cr and Si part of the sample. SEM was also used to study the crater shape.
2.3. SIMS For the SIMS measurements a CAMECA IMS 4550 (CAMECA, Genevilliers, France) was used with Cs + primary ions at an energy of 5 keV. 3. Results and discussion 3.1. PP-TOFMS “Source Profiles” In Fig. 2 a typical source profile is shown. As expected in the afterglow region (once RF potential is turned off) ion signals strongly increase, due to the effects described before. Three integration time windows have been selected to generate depth-profiles: the full source profile, a period during the plateau and a period in the afterglow regime, in which peaks exhibit a maximum. The most “interesting” part of the glow discharge operation is certainly the afterglow region in which higher signal-to-noise ratios are observed. It was found that the behavior of Te greatly differs from the behavior of Bi in the afterglow region. While for Te one peak maximum occurs in the afterglow region two peaks are observed for Bi. 3.2. Time to depth conversion The exact thickness of the thermoelectric layer was determined by scanning electron microscopy (SEM) as a complementary method. SEM measurements were performed on cross sections of the craters
3.3. Qualitative analysis of the samples The peak maxima in the afterglow pulse periode for all isotopes measured, namely 121Sb, 209Bi, 130Te, 52Cr and 197Au were used for the depth profile measurements with PP-TOFMS. For the isotopic measurements a mass-resolving power R of 2700 (measured on m/ z = 197) under the experimental conditions used was found. From the comparison of the results in Figs. 4 and 5 it can be concluded that the layers were rather homogeneous. With both SIMS and PP-TOFMS, however, small deviations from perfectly flat profiles for the signals of 130Te and 121Sb or 130Te and 209Bi, are found. PP-TOFMS and SIMS profiles are not identical as SIMS is extremely sensitive to matrix changes, whereas PP-TOFMS is much less matrix dependent. For both techniques and both sample types the expected order of layers, as it can be seen in Fig. 1, was found. The thermoelectric layer, consisting of Sb2Te3 or Bi2Te3, is followed by the Au and the Cr layer and the Si wafer is located below all others. It can be seen that sputtering changes the surface morphology of the sample around the crater and produces rounded crater edges. Particles from the sample are deposited onto the surrounding sample surface. In Fig. 6 sample morphologies and crater shapes are shown. 3.4. Comparison of the sputtering performances in PP-TOFMS and SIMS Depth resolution is defined as the distance between 84% and 16% of the depth-profile and it relates to interface sharpness [17]. Depth resolutions were calculated accordingly making use of the depths, for which the calculation was explained above, and are given in Table 1. The sputtering rates, which were calculated from the measured depth and the sputtering time can be found in Table 2. The
Fig. 4. Depth-profiles obtained with pulsed r.f. PP-TOFMS for Sb2Te3 (left) and Bi2Te3 (right). For a better overview the ion counting rates have been normalized to their maxima.
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Fig. 5. Depth-profiles obtained with SIMS for Sb2Te3 (left) and Bi2Te3 (right). For a better overview the ion counting rates have been normalized to their maxima.
B
A
C
D
E
Fig. 6. Typical crater shapes and sample surfaces after GD sputtering. A) Needlelike structure of a sample area, which was unaffected by the GD. B) Crater rim: the unaffected area is on the left side and the crater is on the right side. C) Crater area. D) Cross section of the sample cut through a crater rim. E) Depth-profile as measured with a mechanical profilometer and confirming the crater shape, which is shown in D).
standard deviations for the sputtering rates and the depth resolutions in the case of the GD measurements are calculated from measurements at three samples. In the case of the SIMS measurements only one sample of each type was analyzed. The depth resolution can be calculated from the decrease of the signals for 209Bi, 121Sb or 130Te at the interface with the Au layer, from the increase of the signal for 197Au at this interface or from the decrease of the signal for 197Au at its interface with the Cr layer, from both the
Table 1 Depth resolution at the different interfaces. Depth resolution [nm] PP-TOFMS
SIMS
Sb2Te3
121
Increasing signal Decreasing signal
–
–
1700
1000 81
3400
3700
24
31
–
Bi2Te3
209
130
197
52
30
Increasing signal Decreasing signal
–
–
320
170
53
–
–
–
–
–
210
190
350
310
–
–
–
–
–
–
Sb
Bi
130
Te
Te
197
Au
Au
52
Cr
Cr
30
Si
Si
121
Sb
130
Te
197
Au
52
Cr
30
Si
–
–
250
110
61
67
67
41
55
–
52
30
209
Bi
130
Te
197
Au
Cr
increase of the signal for 52Cr and the decrease at the Au or the Si interface and from the increase of the signal for 30Si at the Cr interface (see Fig. 1). Since the sample morphology significantly changes at the interfaces mentioned the depth resolution also may vary dramatically, which is shown by calculated depth resolutions reported in Table 1. It can be seen clearly that the depth resolution in the case of PP-TOFMS strongly depends on the sample morphology. Also one has to consider that for the PP-TOFMS a large sample area is sputtered (4 mm diameter) so the depth resolution is clearly related to the layer roughness. For the porous thermoelectric layer of Sb2Te3 the resolution is therefore poor. At the Au, Cr and Si interfaces this seems to be no longer the case. In terms of analysis time and sample throughput, PP-TOFMS is clearly more advantegous, since the sputtering rate is higher by a factor of 26 than in SIMS. For both methods the adhesion of the sample to the Si wafer under the experimental conditions is crucial. For both sample types there were indications from unsucessfull test runs that parts of the sample may peel off from the surface of the Si
Table 2 Sputtering rates obtained with PP-TOFMS and SIMS. Si
Sample type
Sb2Te3 Bi2Te3
Sputtering rate [nm s−1] PP-TOFMS
SIMS
79 ± 0.6 82 ± 2.6
3.37 2.65
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Fig. 7. Determination of the composition of Sb2Te3 (left) and Bi2Te3 (right) layers with different methods. For PP-TOFMS the integration time zone (in the source profile) giving the best precision was selected, i.e. for Sb2Te3 the full profile and for Bi2Te3 the plateau region.
wafer. This clearly leads to a deterioration of the depth resolution. Therefore, the calculated depth resolutions measured in this work are significantly worse than those, which can be found in the literature for perfectly flat samples [18].
mentioned above the isotopic abundance is considered as being proportional to the elemental abundance and no further correction was needed. The standard deviations for measurements at different samples both for Bi2Te3 and Sb2Te3 are larger than those obtained at the same sample.
3.5. Stoichiometry 4. Conclusions The Sb/Te and Bi/Te concentration ratios were determined with a high precision by ICP-OES after wet chemical digestion [7] and with scanning electron microscopy energy dispersive X-ray fluorescence spectrometry (SEM-EDX). Both methods give rather constant values over the whole sample series. The relationship between the ion counting rates in PP-TOFMS and the concentration was obtained by asigning the average of the ICP-OES results to the average ion counting rates. The isotopic ratios obtained for 121Sb/ 130Te and 209Bi/ 130 Te with PP-TOFMS were constant and, after the calibration to the average, in good agreement with ICP-OES and SEM-EDX data. Since the isotopic distribution is constant the isotopic abundance is proportional to the elemental abundance and no further correction is necessary for the calibration. The results obtained with the three methods are compared in Fig. 7. The precision obtained with PP-TOFMS was found to be not as good as in ICP-OES and worse than the results cited in the literature, where relative standard deviations between 0.1% and 1% were found for bulk concentrations larger than 1% [19]. However, having the source profiles in mind it is easy to imagine that the standard deviations for the elemental concentration ratios for Sb and Te as well as for Bi and Te, respectively, depend on the choice of the integration time window within the source profile measured, as it is shown in Fig. 2. It must be mentioned, however, that the precision could not be increased significantly by the choice of one of the indicated measurement time windows. For Bi2Te3 the plateau time window seems to be the best choice so as to obtain low standard deviations, whereas for Sb2Te3 a data acquisition over the full pulse duration gives a lower standard deviation. Together with the relative standard deviations for measurements of different samples the relative standard deviations for the 121Sb/ 130Te and 209Bi/ 130Te isotopic signal ratios obtained with one sample only are given in Table 3. As
Table 3 Precision obtained in PP-TOFMS for Bi2Te3 and Sb2Te3. No. sample
130
Te/121Sb RSD (full pulse)
31.3 31.8 31.11 Sb2Te3
1.7 2.5 4.2 5.3
PP-TOFMS is shown to be useful to determine the homogeneity of the deposition of thermoelectric materials based on Bi, Sb and Te deposited on Au and Cr coated Si wafers. The results were validated by a comparison with the results obtained with SIMS measurements at the same samples. With both methods it could be shown that the deposition was homogeneous. For the PP-TOFMS and SIMS analyses depth resolutions have been determined and were found to strongly depend on the substrate and the measurement mode. While for the Sb2Te3 layers, depth resolutions of 3 μm were found with PP-TOFMS, the depth resolution within the Au, Cr and Si layers for the same sample was found to be below 80 nm. For the same sample the depth resolutions determined with SIMS were found to be between 250 nm and 40 nm. This may be attributed to the respective measured areas in the case of both methods, which is an essential parameter for relatively rough samples. For a Bi2Te3 sample all calculated depth resolutions obtained with PP-TOFMS were between 530 nm and 200 nm. The large deviations are attributed to the relatively fragile character of the samples. For PP-TOFMS and SIMS the sputtering rates have been found to be approximately 80 nm s −1 and 3 nm s −1, which makes the use of PP-TOFMS clearly more advantageous as compared to SIMS. The precision of PP-TOFMS has been investigated and found to be within 1.1% and 1.9% for measurements at one sample of Bi2Te3, which is lower than the standard deviation from measuring at three samples of the same material, which was found to be 3.6%. The same trend was found for the Sb2Te3 samples, for which the standard deviations in the case of one sample were found to be between 1.7% and 4.2%. The standard deviation for measurements at three Sb2Te3 samples was found to be 5.3%. From this result it can be concluded that the homogeneity within one sample is superior to the variability from one sample to another.
Acknowledgments
No. sample
130
Te/209Bi RSD (plateau)
35.2 35.4 35.7 Bi2Te3
1.7 1.9 1.1 3.6
The authors thank the “Bundesministerium für Bildung und Forschung” (BMBF) (Germany) for funding the investigations through the EchemTE project. The staff at the SIMS laboratory of the “Fraunhofer Institut für Angewandte Festkörperphysik” (Fraunhofer
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IAF, Freiburg, Germany) is gratefully thanked for carrying out the SIMS measurements.
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