- Email: [email protected]
On the optimization of the PIXE technique for thickness uniformity control of ultra-thin chromium layers deposited onto large surface quartz substrate
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 249 (2006) 447–450 www.elsevier.com/locate/nimb
On the optimization of the PIXE technique for thickness uniformity control of ultra-thin chromium layers deposited onto large surface quartz substrate K. Zahraman a, B. Nsouli b
a,*
, M. Roumie´ a, J.P. Thomas b, S. Danel
c
a IBA Laboratory, Lebanese Atomic Energy Commission, National Council for Scientific Research, P.O. Box 11, 8281 Beirut, Lebanon Institut de Physique Nucle´aire de Lyon, Universite´ Claude Bernard Lyon 1, 43 Bd. 11 Novembre 1918, 69622 Villeurbanne Cedex, France c LETI, Commissariat a` l’Energie Atomique, Grenoble, France
Available online 3 May 2006
Abstract Chromium is a good candidate for obtaining conductive ultra-thin layers on insulator substrates such as quartz. The resistivity of such layers is highly related to the quality of the deposited chromium film. In order to optimize the deposition process, there is a need for rapid and accurate monitoring of such films (film thickness, thickness uniformity over a big surface, etc.). In this paper, we demonstrate the ability of the LE-PIXE technique, using proton energies <1 MeV, for the monitoring of the thickness and the thickness uniformity of ultra-thin (0.5 nm < t < 20 nm) chromium layers deposited onto quartz substrates. The acquisition time needed to obtain results with less than 3–4% precision was 5 min for the thinnest layers. The validation for the use of the LE-PIXE technique was checked by means of conventional RBS technique. 2006 Elsevier B.V. All rights reserved. PACS: 41.75.Ak; 29.30.Kv; 32.30.Rj Keywords: PIXE; LE-PIXE; RBS; Chromium; Thin layers
1. Introduction Chromium is a good candidate to obtain ultra-thin conductive layers on insulator substrates like quartz. Besides the interesting mechanical properties of the Cr/quartz interface which are due to the good wetability of Cr on quartz, the conductivity of such systems becomes measurable from thickness values as low as 0.2 nm [1]. The resistivity of such systems is highly connected to the quality of the deposited chromium film (thickness, thickness uniformity, interface properties, etc.). In order to optimize the deposition parameters of such films, a rapid, accurate and sensitive monitoring of their thickness and thickness
*
Corresponding author. Tel.: +961 1 450812; fax: +961 1 450810. E-mail address: [email protected] (B. Nsouli).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.028
uniformities over a relatively big surface (100 cm2) is needed. RBS is the mostly used technique for thin film thickness determination and for film/substrate interface characterization. A chromium thin film deposited onto a quartz substrate is a favorable case to perform RBS measurements with a high sensitivity. However, the accurate thickness determination of ultra-thin layers (<1 nm) needs relatively large acquisition time which is a limiting factor when the number of samples is big. In the same way, and in the case of no significant sample surface roughness, the low energy PIXE (LE-PIXE) is now recognized as a powerful tool, being a commonly used method for ultra-thin film and surface characterization [2–4]. In fact, when the proton energy decreases, the bremsstrahlung decreases much more than the cross-section for inner shell ionization. Consequently, the LE-PIXE, using protons with energy <1.5 MeV, could
K. Zahraman et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 447–450
enhance significantly the peak to background ratio for light and semi-light elements. This can permit the quantification of these elements with a low limit of detection (LOD) and with a high sensitivity [4,5]. In this paper, we report, using the PIXE technique, on the experimental optimum conditions for accurate and rapid thickness determination of ultra-thin chromium layers deposited onto a quartz substrate. The conventional RBS technique has been used to validate PIXE measurements.
10000
Si-K
1000
Cr-Kα
Counts
448
100
with C coating without C coating
Cr-Kβ
10
2. Experimental 2.1. Samples Chromium films with different thicknesses (0.5–20 nm) were deposited onto a quartz substrate under vacuum by using electron bombardment. For PIXE and RBS analysis, each sample was cut into 20 sub-samples (1 · 1 cm2). In order to ensure good electrical contact between the sample and the aluminum frame sample holder, the sample was glued by using a double side carbon tape. For the very thin chromium layers (<1.5 nm), a thin ultra-pure carbon layer was evaporated on the surface of the sample in order to ensure a good surface conductivity, which is required for in vacuum IBA techniques.
1 0
3. Results and discussion For the chromium layers with thickness values <1.5 nm, a charging effect was observed in both the RBS and the PIXE spectra, which makes it impossible to perform any accurate quantitative analysis. The deposition of a very thin carbon layer (3 nm) at the surface of such samples was needed to overcome the problem. Fig. 1 presents the PIXE spectra of a 0.5 nm thick chromium deposited onto a quartz substrate, using 750 keV proton beam, before and after carbon deposition. Furthermore, the charge effect eliminated totally the chromium signal in the RBS spectra.
10
15
20
Energy (keV) Fig. 1. PIXE spectra of a 0.5 nm thick chromium layer deposited onto quartz substrate: (a) with and (b) without carbon coating using 0.75 MeV proton beam.
Conventional RBS using 2 MeV helium beam was used for the analysis. The chromium film thicknesses were determined with 2–10% precision, depending on the layer thickness. For the thinnest films (<1.5 nm thick), the acquisition time needed to obtain less than 3% statistical error, was
2.2. PIXE and RBS instrumental set-up and data analysis
1000 3 MeV proton 100
10
1 2 MeV proton
1000 100 10
Counts
Proton and helium beams were delivered by the 1.7 MV 5-SDH NEC tandem accelerator of the Lebanese Atomic Energy Commission. The beam dose varied between 1 and 75 lC. The spot diameter of the beam on the target was about 3 mm. X-ray emission from targets was detected using a Si(Li) detector situated at 135 referring to the beam direction. A 131 lm thick Kapton filter was used. For RBS measurements, a silicon PIPS detector situated at 165 referring to the beam direction, was used. A detailed description of our experimental set-up has been reported elsewhere [6]. The PIXE spectra were analyzed using the GUPIX computer code [7]. The RBS spectra were simulated using the SIMNRA code [8]. Finally, the count rate was <1000 Hz and the dead time, in all measurements, was less than 1%.
5
1 1 MeV proton
100
10
1
Cr-Kα
Si-K
0.75 MeV proton 100
Cr-Kβ
10
1 0
5
10
15
20
Energy (keV) Fig. 2. PIXE spectra of a 0.5 nm chromium layer deposited onto quartz substrate, bombarded with different proton energies.
K. Zahraman et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 447–450
Cr Signal/Background
10 0.03
Cr (S/B) Cr (LOD)
8
6
0.02
4 0.01 2
0
1000
1500
2000
2500
3000
Proton energy (keV) Fig. 4. Chromium signal on background (S/B) ratio and limit of detection (LOD), calculated for the 0.5 nm thick chromium layer versus proton energy.
chromium film, the acquisition time necessary to obtain less than a 2% statistical error was 6 min, by using a beam current of 60–70 nA. In this way, the determination of the chromium thickness for the different studied samples, was achieved with better than 3–4% precision. With the applied methodology the thickness uniformity study via the analysis of a large number of sub-samples, even for the very low chromium thicknesses (<1.5 nm) was practically possible and not time consuming. The LE-PIXE results showed that the samples were practically uniform in thickness over a big surface even for the edge areas. Finally, the validation of the LE-PIXE technique for the characterization of ultra-thin chromium layers can be appreciated via the excellent agreement between the thicknesses determined by this technique versus the ones determined by RBS (Fig. 5).
20
18
PIXE thickness (nm)
16
[YCr/ σCr]/[YSi/ σSi]
0.04
12
LOD (nm)
50 min. Under the used evaporation conditions, chromium diffusion into the quartz substrate was not clearly evidenced. When the chromium thickness >3.5 nm, the thickness uniformity of the samples was properly checked (analysis of more than 15 sub-samples). This is due to the relative large acquisition time needed per sample for the thinnest chromium films. The RBS results demonstrate that the samples were practically uniform in thickness. The standard deviation was more or less within the accuracy of the measurements. Fig. 2 shows the PIXE spectra of a 0.5 nm thick chromium layer deposited onto a quartz substrate, obtained at different proton energies. It can be shown that the use of the conventional PIXE, using protons of 2.5–3 MeV energy, gives rise to a large bremsstrahlung radiation thus considerably limiting the chromium detection. At lower energies, the chromium signal starts to contribute more and more to the integral X-ray spectrum due simultaneously to the surface effect and to the decrease of the bremsstrahlung. Fig. 3 shows, for a 0.5 nm thick chromium layer, the variation of the ratio between the Cr-Ka yield and its X-ray production cross-section divided by the Si one for different beam energies. The significant increasing of this ratio for proton energy <1 MeV indicates a clear surface effect for low energy proton beam due to a significant smaller analyzed depth [2]. Fig. 4 shows the variation of the chromium signal to background ratio and the LOD of chromium versus proton energy for the 0.5 nm thick chromium layer. The sensitivity versus the detection of chromium increases around 25 times when the proton energy decreases from 3 to 0.75 MeV. In the same way, the LOD of Cr decreases by one order of magnitude when the proton energy decreases from 3 to 0.75 MeV. Accordingly, the LE-PIXE using 0.75 MeV proton beam was used to characterize the Cr/quartz samples. In fact, at this energy and for the sample with a 0.5 nm thick
449
14
12
10
8 1000
1500
2000
2500
3000
Proton energy (keV) Fig. 3. [YCr/rCr]/[YSi/rSi] versus proton energy. YCr and YSi are the number of counts/lC for Cr-Ka line and Si-Ka line respectively. rCr and rSi are the Cr-Ka and Si-Ka X-ray production cross-section, respectively.
15
10
5
0
0
5
10
15
20
RBS thickness (nm) Fig. 5. The experimental thickness found by using LE-PIXE (0.75 MeV proton energy) versus the one found by using conventional RBS for Cr/ SiO2 samples with different chromium thicknesses (r2 = 0.996).
450
K. Zahraman et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 447–450
4. Conclusion In this work, it has been demonstrated that LE-PIXE, using proton energies <1 MeV, is an accurate, rapid and powerful technique for the characterization of ultra-thin chromium layers deposited onto quartz substrates. The 0.75 MeV proton beam permitted the determination of chromium thicknesses with a very low LOD (<0.003 nm), high sensitivity and within few minutes of acquisition time. The precision of the measurement was 3–4%. This analytical methodology permitted the rapid and accurate monitoring of the thickness uniformity of the chromium layers over a big surface, by analyzing more than 15–20 sub-samples for each sample. The high accuracy is especially needed for the thinnest chromium layers (<1.5 nm). The special deposition geometry used in this work for chromium deposition was very satisfactory and yielded Cr layers with high thickness uniformity over a relatively big surface (100 cm2).
Finally, the conventional RBS technique has been used to validate PIXE measurements. References [1] J.A.J. Lourens, S. Arajis, H.F. Helbig, El-Sayed A. Mehanna, L. Cheriet, Phys. Rev. B 37 (1988) 5423. [2] B. Nsouli, M. Roumie´, K. Zahraman, J.P. Thomas, M. Nasreddine, Nucl. Instr. and Meth. B 192 (2002) 311. [3] J. Miranda, Nucl. Instr. and Meth. B 118 (1996) 346. [4] F.M. El-Ashry, M. Goclowski, L. Glowacka, M. Jaskola, J. Marczewski, A. Wolkenberg, Nucl. Instr. and Meth. B 22 (1987) 450. [5] K. Wittmaack, B. Hietel, Nucl. Instr. and Meth. B 161–163 (2000) 814. [6] M. Roumie´, B. Nsouli, K. Zahraman, A. Reslan, Nucl. Instr. and Meth. B 219–220 (2004) 389. [7] J.A. Maxwell, J.L. Campbell, W.J. Teesdale, Nucl. Instr. and Meth. B 43 (1989) 218. [8] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max-PlankInstitut fu¨r Plasmaphysik, Garching, Germany, 1997.
Nuclear Instruments and Methods in Physics Research B 249 (2006) 447–450 www.elsevier.com/locate/nimb
On the optimization of the PIXE technique for thickness uniformity control of ultra-thin chromium layers deposited onto large surface quartz substrate K. Zahraman a, B. Nsouli b
a,*
, M. Roumie´ a, J.P. Thomas b, S. Danel
c
a IBA Laboratory, Lebanese Atomic Energy Commission, National Council for Scientific Research, P.O. Box 11, 8281 Beirut, Lebanon Institut de Physique Nucle´aire de Lyon, Universite´ Claude Bernard Lyon 1, 43 Bd. 11 Novembre 1918, 69622 Villeurbanne Cedex, France c LETI, Commissariat a` l’Energie Atomique, Grenoble, France
Available online 3 May 2006
Abstract Chromium is a good candidate for obtaining conductive ultra-thin layers on insulator substrates such as quartz. The resistivity of such layers is highly related to the quality of the deposited chromium film. In order to optimize the deposition process, there is a need for rapid and accurate monitoring of such films (film thickness, thickness uniformity over a big surface, etc.). In this paper, we demonstrate the ability of the LE-PIXE technique, using proton energies <1 MeV, for the monitoring of the thickness and the thickness uniformity of ultra-thin (0.5 nm < t < 20 nm) chromium layers deposited onto quartz substrates. The acquisition time needed to obtain results with less than 3–4% precision was 5 min for the thinnest layers. The validation for the use of the LE-PIXE technique was checked by means of conventional RBS technique. 2006 Elsevier B.V. All rights reserved. PACS: 41.75.Ak; 29.30.Kv; 32.30.Rj Keywords: PIXE; LE-PIXE; RBS; Chromium; Thin layers
1. Introduction Chromium is a good candidate to obtain ultra-thin conductive layers on insulator substrates like quartz. Besides the interesting mechanical properties of the Cr/quartz interface which are due to the good wetability of Cr on quartz, the conductivity of such systems becomes measurable from thickness values as low as 0.2 nm [1]. The resistivity of such systems is highly connected to the quality of the deposited chromium film (thickness, thickness uniformity, interface properties, etc.). In order to optimize the deposition parameters of such films, a rapid, accurate and sensitive monitoring of their thickness and thickness
*
Corresponding author. Tel.: +961 1 450812; fax: +961 1 450810. E-mail address: [email protected] (B. Nsouli).
0168-583X/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.028
uniformities over a relatively big surface (100 cm2) is needed. RBS is the mostly used technique for thin film thickness determination and for film/substrate interface characterization. A chromium thin film deposited onto a quartz substrate is a favorable case to perform RBS measurements with a high sensitivity. However, the accurate thickness determination of ultra-thin layers (<1 nm) needs relatively large acquisition time which is a limiting factor when the number of samples is big. In the same way, and in the case of no significant sample surface roughness, the low energy PIXE (LE-PIXE) is now recognized as a powerful tool, being a commonly used method for ultra-thin film and surface characterization [2–4]. In fact, when the proton energy decreases, the bremsstrahlung decreases much more than the cross-section for inner shell ionization. Consequently, the LE-PIXE, using protons with energy <1.5 MeV, could
K. Zahraman et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 447–450
enhance significantly the peak to background ratio for light and semi-light elements. This can permit the quantification of these elements with a low limit of detection (LOD) and with a high sensitivity [4,5]. In this paper, we report, using the PIXE technique, on the experimental optimum conditions for accurate and rapid thickness determination of ultra-thin chromium layers deposited onto a quartz substrate. The conventional RBS technique has been used to validate PIXE measurements.
10000
Si-K
1000
Cr-Kα
Counts
448
100
with C coating without C coating
Cr-Kβ
10
2. Experimental 2.1. Samples Chromium films with different thicknesses (0.5–20 nm) were deposited onto a quartz substrate under vacuum by using electron bombardment. For PIXE and RBS analysis, each sample was cut into 20 sub-samples (1 · 1 cm2). In order to ensure good electrical contact between the sample and the aluminum frame sample holder, the sample was glued by using a double side carbon tape. For the very thin chromium layers (<1.5 nm), a thin ultra-pure carbon layer was evaporated on the surface of the sample in order to ensure a good surface conductivity, which is required for in vacuum IBA techniques.
1 0
3. Results and discussion For the chromium layers with thickness values <1.5 nm, a charging effect was observed in both the RBS and the PIXE spectra, which makes it impossible to perform any accurate quantitative analysis. The deposition of a very thin carbon layer (3 nm) at the surface of such samples was needed to overcome the problem. Fig. 1 presents the PIXE spectra of a 0.5 nm thick chromium deposited onto a quartz substrate, using 750 keV proton beam, before and after carbon deposition. Furthermore, the charge effect eliminated totally the chromium signal in the RBS spectra.
10
15
20
Energy (keV) Fig. 1. PIXE spectra of a 0.5 nm thick chromium layer deposited onto quartz substrate: (a) with and (b) without carbon coating using 0.75 MeV proton beam.
Conventional RBS using 2 MeV helium beam was used for the analysis. The chromium film thicknesses were determined with 2–10% precision, depending on the layer thickness. For the thinnest films (<1.5 nm thick), the acquisition time needed to obtain less than 3% statistical error, was
2.2. PIXE and RBS instrumental set-up and data analysis
1000 3 MeV proton 100
10
1 2 MeV proton
1000 100 10
Counts
Proton and helium beams were delivered by the 1.7 MV 5-SDH NEC tandem accelerator of the Lebanese Atomic Energy Commission. The beam dose varied between 1 and 75 lC. The spot diameter of the beam on the target was about 3 mm. X-ray emission from targets was detected using a Si(Li) detector situated at 135 referring to the beam direction. A 131 lm thick Kapton filter was used. For RBS measurements, a silicon PIPS detector situated at 165 referring to the beam direction, was used. A detailed description of our experimental set-up has been reported elsewhere [6]. The PIXE spectra were analyzed using the GUPIX computer code [7]. The RBS spectra were simulated using the SIMNRA code [8]. Finally, the count rate was <1000 Hz and the dead time, in all measurements, was less than 1%.
5
1 1 MeV proton
100
10
1
Cr-Kα
Si-K
0.75 MeV proton 100
Cr-Kβ
10
1 0
5
10
15
20
Energy (keV) Fig. 2. PIXE spectra of a 0.5 nm chromium layer deposited onto quartz substrate, bombarded with different proton energies.
K. Zahraman et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 447–450
Cr Signal/Background
10 0.03
Cr (S/B) Cr (LOD)
8
6
0.02
4 0.01 2
0
1000
1500
2000
2500
3000
Proton energy (keV) Fig. 4. Chromium signal on background (S/B) ratio and limit of detection (LOD), calculated for the 0.5 nm thick chromium layer versus proton energy.
chromium film, the acquisition time necessary to obtain less than a 2% statistical error was 6 min, by using a beam current of 60–70 nA. In this way, the determination of the chromium thickness for the different studied samples, was achieved with better than 3–4% precision. With the applied methodology the thickness uniformity study via the analysis of a large number of sub-samples, even for the very low chromium thicknesses (<1.5 nm) was practically possible and not time consuming. The LE-PIXE results showed that the samples were practically uniform in thickness over a big surface even for the edge areas. Finally, the validation of the LE-PIXE technique for the characterization of ultra-thin chromium layers can be appreciated via the excellent agreement between the thicknesses determined by this technique versus the ones determined by RBS (Fig. 5).
20
18
PIXE thickness (nm)
16
[YCr/ σCr]/[YSi/ σSi]
0.04
12
LOD (nm)
50 min. Under the used evaporation conditions, chromium diffusion into the quartz substrate was not clearly evidenced. When the chromium thickness >3.5 nm, the thickness uniformity of the samples was properly checked (analysis of more than 15 sub-samples). This is due to the relative large acquisition time needed per sample for the thinnest chromium films. The RBS results demonstrate that the samples were practically uniform in thickness. The standard deviation was more or less within the accuracy of the measurements. Fig. 2 shows the PIXE spectra of a 0.5 nm thick chromium layer deposited onto a quartz substrate, obtained at different proton energies. It can be shown that the use of the conventional PIXE, using protons of 2.5–3 MeV energy, gives rise to a large bremsstrahlung radiation thus considerably limiting the chromium detection. At lower energies, the chromium signal starts to contribute more and more to the integral X-ray spectrum due simultaneously to the surface effect and to the decrease of the bremsstrahlung. Fig. 3 shows, for a 0.5 nm thick chromium layer, the variation of the ratio between the Cr-Ka yield and its X-ray production cross-section divided by the Si one for different beam energies. The significant increasing of this ratio for proton energy <1 MeV indicates a clear surface effect for low energy proton beam due to a significant smaller analyzed depth [2]. Fig. 4 shows the variation of the chromium signal to background ratio and the LOD of chromium versus proton energy for the 0.5 nm thick chromium layer. The sensitivity versus the detection of chromium increases around 25 times when the proton energy decreases from 3 to 0.75 MeV. In the same way, the LOD of Cr decreases by one order of magnitude when the proton energy decreases from 3 to 0.75 MeV. Accordingly, the LE-PIXE using 0.75 MeV proton beam was used to characterize the Cr/quartz samples. In fact, at this energy and for the sample with a 0.5 nm thick
449
14
12
10
8 1000
1500
2000
2500
3000
Proton energy (keV) Fig. 3. [YCr/rCr]/[YSi/rSi] versus proton energy. YCr and YSi are the number of counts/lC for Cr-Ka line and Si-Ka line respectively. rCr and rSi are the Cr-Ka and Si-Ka X-ray production cross-section, respectively.
15
10
5
0
0
5
10
15
20
RBS thickness (nm) Fig. 5. The experimental thickness found by using LE-PIXE (0.75 MeV proton energy) versus the one found by using conventional RBS for Cr/ SiO2 samples with different chromium thicknesses (r2 = 0.996).
450
K. Zahraman et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 447–450
4. Conclusion In this work, it has been demonstrated that LE-PIXE, using proton energies <1 MeV, is an accurate, rapid and powerful technique for the characterization of ultra-thin chromium layers deposited onto quartz substrates. The 0.75 MeV proton beam permitted the determination of chromium thicknesses with a very low LOD (<0.003 nm), high sensitivity and within few minutes of acquisition time. The precision of the measurement was 3–4%. This analytical methodology permitted the rapid and accurate monitoring of the thickness uniformity of the chromium layers over a big surface, by analyzing more than 15–20 sub-samples for each sample. The high accuracy is especially needed for the thinnest chromium layers (<1.5 nm). The special deposition geometry used in this work for chromium deposition was very satisfactory and yielded Cr layers with high thickness uniformity over a relatively big surface (100 cm2).
Finally, the conventional RBS technique has been used to validate PIXE measurements. References [1] J.A.J. Lourens, S. Arajis, H.F. Helbig, El-Sayed A. Mehanna, L. Cheriet, Phys. Rev. B 37 (1988) 5423. [2] B. Nsouli, M. Roumie´, K. Zahraman, J.P. Thomas, M. Nasreddine, Nucl. Instr. and Meth. B 192 (2002) 311. [3] J. Miranda, Nucl. Instr. and Meth. B 118 (1996) 346. [4] F.M. El-Ashry, M. Goclowski, L. Glowacka, M. Jaskola, J. Marczewski, A. Wolkenberg, Nucl. Instr. and Meth. B 22 (1987) 450. [5] K. Wittmaack, B. Hietel, Nucl. Instr. and Meth. B 161–163 (2000) 814. [6] M. Roumie´, B. Nsouli, K. Zahraman, A. Reslan, Nucl. Instr. and Meth. B 219–220 (2004) 389. [7] J.A. Maxwell, J.L. Campbell, W.J. Teesdale, Nucl. Instr. and Meth. B 43 (1989) 218. [8] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max-PlankInstitut fu¨r Plasmaphysik, Garching, Germany, 1997.