- Email: [email protected]
Growth of Niobium Thin Films on Si Substrates by Pulsed Nd:YAG Laser Deposition
Accepted Manuscript Growth of Niobium Thin Films on Si Substrates by Pulsed Nd:YAG Laser Deposition Gontad Francisco, Lorusso Antonella, Ph.D., Solombrino Luigi, Koutselas Ioannis, Vainos Nikos, Perrone Alessio PII:
S1005-0302(15)00101-2
DOI:
10.1016/j.jmst.2015.06.007
Reference:
JMST 519
To appear in:
Journal of Materials Science & Technology
Received Date: 19 December 2014 Revised Date:
13 February 2015
Accepted Date: 16 February 2015
Please cite this article as: G. Francisco, L. Antonella, S. Luigi, K. Ioannis, V. Nikos, P. Alessio, Growth of Niobium Thin Films on Si Substrates by Pulsed Nd:YAG Laser Deposition, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.06.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Growth of Niobium Thin Films on Si Substrates by Pulsed Nd:YAG Laser Deposition Gontad Francisco1, Lorusso Antonella1, *, Solombrino Luigi1, Koutselas Ioannis2, Vainos Nikos2, Perrone Alessio1 Department of Mathematics and Physics “E. De Giorgi”, and National Institute of Nuclear
RI PT
1
Physics, University of Salento, 73100 Lecce, Italy 2
Department of Materials Science, University of Patras, 26500 Patras, Greece
[Received 19 December 2014; Received in revised form 13 February 2015; Accepted 16 *
Corresponding author. Ph.D.; Tel.: +39 832 297501.
SC
February 2015]
M AN U
E-mail address: [email protected] (L. Antonella).
The growth of Nb thin films on Si(100) substrates by pulsed Nd:YAG laser deposition (PLD) under different laser fluences (4‒15 J/cm2) was reported. The influence of laser fluence on ablation rate and deposition rate was discussed. X-ray diffraction (XRD) investigations of the
TE D
deposited films showed an amorphous structure. The droplet density on the film surface observed by scanning electron microscopy (SEM) analyses was extremely low. It was experimentally proved that the droplets on the film surface originated from liquid phase on the target surface. Profilometric measurements of the deposited Nb films revealed a substantial asymmetry in the
EP
film thickness related to the plume deflection effect. The measured electrical resistivity of the Nb film was higher than that of high purity Nb bulk. The present investigations of ablation and
AC C
deposition process of Nb thin films are related to its potential application in superconducting radio-frequency (SRF) cavities.
Key words: Pulsed laser deposition; Nb thin films; Ablation and deposition rate 1. Introduction Pulsed laser deposition (PLD) technique has been widely used to grow thin films of any material[1] and to develop innovative nanostructured devices[2]. One of the reasons for its success is its flexibility and simplicity. Nevertheless, in spite of its many advantages, the presence of droplets on the film surface and the inhomogeneity in the film thickness due to the plume 1
ACCEPTED MANUSCRIPT deflection effect prevent PLD technique from emerging as reliable technology for the deposition of high quality films[3‒6]. We found out, in our recent research activity on the deposition of metallic thin films, that droplet density on the film surface can be reduced by detailed parametric studies[7]. Moreover, it has been found that the laser fluence is the most important laser parameter. With this in mind, we performed ablation and deposition investigations of Nb thin
RI PT
films at different laser fluences over the laser ablation threshold. Niobium is a metal of great interest because it is the leading superconductor to build superconducting radio-frequency (SRF) cavities often used in particles accelerators[8‒10]. There exists an extensive research activity and literature on metallic thin films[11‒13] but little has been done on Nb thin films, less than never
SC
with Nb thin films grown by PLD[14,15]. PLD technique is distinguished from the other physical and chemical deposition techniques by the high kinetic energy of ablated material that improves
M AN U
the adhesion of the deposited film on their substrate. Preliminary results on the Nb thin films grown on Si(100) by PLD are reported here, together with a discussion concerning its future application in the SRF cavities, where the photocathode can be formed by a bulk of Pb with a
2. Experimental
TE D
film of Nb in the configuration given in Ref.[16].
Depositions were performed at high vacuum (<10−5 Pa) and at room temperature using the
EP
deposition system shown in Fig. 1. The irradiating source was a frequency-quadrupled Q-switched Nd:YAG laser (Continuum Powerlite 8010) emitting 7 ns pulses at a wavelength of 266 nm. In order to eliminate any native contaminant on the target surface, a pre-irradiation
AC C
cleaning of 2000 pulses was applied. During the laser cleaning treatment, a retractable shield was introduced between the target and the substrate. A total of 20000 laser pulses (800 pulses/site) were applied to the Nb target for the deposition of one film. The laser beam was incident at an angle of 45° on the rotating Nb target. The ablated material was collected on a Si(100) substrate kept parallel to the target at a distance of 50 mm from it. The present target-substrate distance was a good compromise between respectable deposition rate (which generally decreases with the distance) and low density of droplets on the film surface (which decreases with the distance). The depositions were performed with a frequency of 10 Hz and at different laser fluences of 4, 6, 9 and 15 J/cm2. The lowest laser fluences were obtained by means of attenuators. During the
2
ACCEPTED MANUSCRIPT ablation process of Nb target, mass spectra were recorded by a quadrupole mass spectrometer (Hiden Analytical HALO 201 RC) to study the composition and the ionization states of ablated material. Scanning electron microscopy (JEOL-JSM-6480LV) and X-ray diffraction (Bruker AXS D8 Advance X-ray diffractometer using Cu Kα line) were used to study the morphology and the structure of the deposited films. The thickness of the films was deduced in different points by
RI PT
using a profilometer (Tencor Alphastep). The average ablation rate was obtained by weighing the target before and after the irradiation process. Finally, electrical resistivity measurements of the
SC
deposited films were carried out by four-point probe method.
M AN U
3. Results and Discussion
The technique of PLD was applied to study the ablation process of the target and the deposition process of the films at different laser fluences. The deposited films were extremely adherent to the Si substrates as the scotch tape and scratch tests established. The good adhesion of the films is due to the high kinetic energy of the ablated material[17,18]. This tribological
SRF cavities.
TE D
property is strongly demanding for application of photocathodes based on metallic thin film in
The precise ablation threshold of metals, Fthr, is somehow difficult to estimate due to the large energy loss by the heat diffusion into the bulk during the laser irradiation. The calculation of this value has been done according to the following equation[19]:
ρ LT cs ∆T + ∆Hf + ∆He
EP Fthr =
3
(
1− R
) (1)
AC C
where ρ is the material density (8.57 g/cm3), cs is the specific heat (0.265 J/(g K)), ∆T is the difference between the evaporation temperature (5017 K) and room temperature, ∆Hf fusion enthalpy (292.76 J/g),
is the
∆He is the evaporation enthalpy (7497.58 J/g), LT = (2k×τ)1/2 =
0.58 µm, which is the thermal diffusion length, k (0.24 cm2/s) is the thermal diffusivity and τ is the laser pulse duration. LT so defined indicates the distance at which the amplitude of the heat flux reduces e times from its value at sample’s surface[20]. The computed value of Fthr is about 2.9 J/cm2 taking into account the surface reflectance value of the Nb target, R, of about 0.48 given in Ref.[21]. The relatively high laser ablation threshold is caused by the high values of evaporation temperature and evaporation enthalpy of the present metal. Eq. (1) is always valid for nanosecond laser ablation of metals where the optical penetration depth of the laser is much less than LT. 3
ACCEPTED MANUSCRIPT The average ablation rate normalized to the energy (µg/(pulse J)) as a function of laser fluence, given in Fig. 2, was calculated by weighing the target before and after the laser-ablation process. The experimental data are well approximated by a logarithmic law, whose extrapolation gives the ablation threshold value of 3.8 J/cm2. Error bars in Fig. 2 indicate the experimental uncertainty in measuring the laser fluence and the ablation rate. Doeswijk et al. measured a
RI PT
threshold fluence value of 3.5 J/cm2 for ablation of Nb at 248 nm[21]. This discrepancy between theoretical and experimental values is probably due to the uncertainty on surface reflectance value of the Nb. The laser ablation threshold for the Nb ablation was also found empirically by observing the worsening of the vacuum from a background pressure of 6 × 10−6 Pa, before the
SC
laser irradiation, down to 5 × 10−5 Pa by increasing slowly the laser energy. The occurrence of mass peak of Nb at 93 amu, during mass spectrometry investigations, was observed at around 4
M AN U
J/cm2. At laser fluence values up to 6 J/cm2 a fast growing of ablation rate was observed. At higher fluences (>6 J/cm2) a self-regulating ablation rate regime, due to plasma absorption of laser beam, is encountered.
The thickness value of films measured on the center area and the average deposition rate are listed in Table 1. The average deposition rate of the film on the Si substrate increases almost
TE D
linearly with the laser fluence used in our experimental conditions (see Fig. 3). The behavior of the average deposition rate does not follow completely the trend of average ablation rate due to the lowering of plume deflection angle with the laser fluence (see Fig. 4), which improves the deposition in the center area of the substrate.
EP
The detailed studies of the plume deflection effects during the laser ablation of Nb also showed that during the first laser pulses/site, less than a dozen, the visible plume was still
AC C
perpendicular to the target surface, but as the number of pulses per site increased (~ 24 pulses/site), an evident deviation of the plume towards the laser beam direction was observed even at naked eyes. This confirms that the target surface modification produces variations in the plasma plume expansion[12]. A sequence of ablation plume at different laser fluences and at the end of laser ablation process (800 pulses/site) is shown in Fig. 5. The image of the plume taken from the top window of the deposition chamber is the result of a single laser pulse. The plume deflection angle decreases with increasing laser fluence as just shown in Fig. 4. This behaviour can be explained by observing the microstructures morphology of the ablating target surface after the laser ablation process[22, 23]. Fig. 6 illustrates the SEM images of the target surface morphology irradiated with different laser fluences. At the highest laser fluence (Fig. 6(a)), the microstructures 4
ACCEPTED MANUSCRIPT on the target surface, at the end of the laser ablation process, are smooth and less developed producing a weak plume deflection effect. At lower laser fluences (Fig. 6(c, d)), the microstructures on the target surface present more asperities and they are more resistant to the lower laser density and persist longer implying a more evident plume deflection[24,25]. The shortcoming of plume deflection is to worsen the thickness uniformity over the whole
RI PT
film surfaces (10 mm in diameter) for a maximum value of about 20% at the lowest laser fluence. In order to study the ions generated by the direct laser ablation of Nb target, mass spectrometry investigations were performed. It must be stressed that during the laser ablation process, the vacuum level is lowered down to 5×10−5 Pa. Fig. 7 shows the time integrated mass
SC
spectrum recorded during the ablation process of the target at the highest laser fluence of 15 J/cm2. The mass spectrum was purged by the contribution of the residual gases. The mass peaks at
M AN U
93 and at 46‒47 amu (atomic mass unit) can be easily associated with singly and doubly charged Nb ions, respectively. In these experimental conditions, no niobium oxides were observed even if its formation in gas phase cannot be ruled out. Ionic species were also detected with the ionizer switched off and under the same experimental conditions. The time integrated mass spectrum recorded during the laser ablation of a pure niobium target is shown in Fig. 7 with black columns. The appearance of signals localized 46‒47 and 93 amu, even with the ionizer switched off, clearly
TE D
demonstrates that the main ionic species are present in the ablation plume are Nb++ and Nb+. The mass spectra recorded at lower laser fluences are similar but with weaker peak intensity in particular for the doubly-charged ions. It has been also observed that after laser ablation process
EP
the quality of vacuum is improved, because the gas phase reactions between ablated material and residual gas reduce the partial pressure of oxygen-containing molecules (O2, H2O, CO2)[26]. The surface morphology of the film deposited at 15 J/cm2 is shown in Fig. 8. The film is
AC C
almost free of droplets and fragments, which slightly decrease with the laser fluence. The SEM images of the films deposited at lower laser fluences were quite similar and not reported here for sake of brevity. In the inset of Fig. 8 is highlighted one of the biggest droplets observed in the SEM image. The spherical shape of this particle and its chemical composition obtained by energy dispersive X-ray spectroscopy (EDX), shown in the right side of the inset, demonstrate its origin from liquid phase on the target surface. Two dimensional maps of Si, Nb and O distributions on the sample surface were obtained by EDX technique. The SEM-EDX maps of the thickest sample deposited with 15 J/cm2 are shown in Fig. 9. They were obtained with very low electron energy (5 keV) because the thickness of Nb film was not enough to reduce the contribution of the X-ray
5
ACCEPTED MANUSCRIPT coming from Si substrate. The left area of the image relative to EDX map of Si coloured in green shows the uncoated Si substrate. On the contrary, the right area, coated with the Nb film, appears almost black (dark greenish) because only a few X-ray comes from the underneath Si substrate. The uniform map of Nb does not present voids or cracks, which means that good quality film has been deposited. The oxygen map shows a higher concentration of this element in the Nb film area.
RI PT
The reason of this effect is probably due to gas phase reactions of Nb with the residual gas of the vacuum chamber and the higher chemical reactivity of Nb with respect to Si when the sample is exposed in open air.
XRD investigations were carried out to deduce the structure of the deposited film. All the
SC
XRD patterns show an amorphous structure of the films probably due to the low deposition temperature of the substrate and he lack of sufficient film thickness.
M AN U
Finally the sheet resistance of the Nb film with the highest thickness was around 5.5 Ω/sq and the corresponding electrical resistivity was (7.5±0.7)×10−5 Ω cm. This average value of the resistivity measured on different area of the deposited film by four-point probe technique is higher than that of high purity Nb bulk (1.5×10−5 Ω cm). At the moment, it seems to be the only weak point for its potential application in SRF cavities since the deposition of thicker films can be
pulses.
EP
4. Conclusion
TE D
obtained by lowering the target-substrate distance and by increasing the total number of laser
In the present work the ablation process of Nb target and the deposition process of Nb thin
AC C
film at different laser fluences were investigated. The fabricated metallic films were characterized by SEM, EDX, XRD, profilometry and electrical measurements. The amorphous nature of the deposited films was deduced by XRD analysis. Morphological studies show that no significant number of droplets is observed on all the deposited films. The data of average ablation rate as a function of laser fluence are well approximated by a logarithmic law, which takes into account of the laser-plasma absorption. The deposition rate behavior is, instead, quasi linear due to the lowering of the plume deflection angle with the laser fluence. The relatively low deposition rate of this metal could be enhanced by lowering the target-substrate distance down to 30 mm. The laser ablation threshold was computed and compared to that deduced by the experiments. Finally, the low droplets density and the high adhesion of the Nb films deposited by PLD are
6
ACCEPTED MANUSCRIPT promising properties for its potential application in SRF cavities. Acknowledgments We gratefully acknowledge Professor M. Di Giulio for profilometric measurements and Mr. L. Monteduro for his expert technical support. This work was supported partially by the Italian
RI PT
Ministry of Research in the framework of FIRB-Fondo per gli Investimenti della Ricerca di Base (Project no. RBFR12NK5K) and the Italian National Institute of Nuclear Physics (INFN).
SC
References
[1] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley-VCH, New York, 2003.
M AN U
[2] Y. Li, T. Sasaki, Y. Shimizu, N. Koshizaki, J. Am. Chem. Soc. 130 (2008) 14755‒14762. [3] L. Cultrera, M.I. Zeifman, A. Perrone, Phys. Rev. B 73 (2006) 075304. [4] E. van de Riet, C.J.C.M. Nillesen, J. Dieleman, J. Appl. Phys. 74 (1993) 2008‒2012. [5] N. Pryds, J. Schou, S. Linderoth, Appl. Surf. Sci. 253 (2007) 8231‒8234. [6] W. Liu, J.P. Fang, W.P. Cai, J.H. Liang, X.S. Zhou, X.G. Long, Chin. Phys. B 23 (2014)
TE D
098103.
[7] A. Lorusso, V. Fasano, A. Perrone, K. Lovchinov, J. Vac. Sci. Technol. A 29 (2011) 031502. [8] J. Smedley, T. Rao, J. Warren, P. Kneisel, J. Sekutowicz, J. Iversen, D. Klinke, D. Kostin, W. Möller, A. Muhs, P. Strzyzewski, QE Measurements of a Nb‒Pb Photoinjector, in:
EP
Proceedings of the Energy Recovery Linacs, Daresbury, 2007, 95. [9] J.K. Sekutowicz, P. Kneisel, R. Nietubyc, T. Rao, J. Smedley, QE tests with Nb‒Pb SRF
AC C
Photoinjector and Arc Deposited Photocathodes, In: Proceedings of the International Particle Accelerator Conference, Kyoto, 2010, 4086. [10] C. Xu, C. Reece, M. Kelley, Appl. Surf. Sci. 274 (2013) 15‒21. [11] A. Lorusso, F. Gontad, A. Perrone, Thin Solid Films 520 (2011) 117‒120. [12] L. Cultrera, A. Lorusso, B. Maiolo, L. Cangueiro, R. Vilar, A. Perrone, J. Appl. Phys. 115 (2014) 093192. [13] M. Zheng, J. Shen, J. Barthel, P. Ohresser, Ch. V. Mohank, J. Kirschner, J. Phys.-Condens. Matter 12 (2000) 783‒794. [14] S. Haindl, M. Weisheit, T. Thersleff, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 21 (2008) 045017.
7
ACCEPTED MANUSCRIPT [15] V. Grosse, C. Pansow, A. Steppke, F. Schmidl, A. Undisz, M. Rettenmayr, A. Grib, P. Seidel, J. Phys.-Conference Series 234 (2010) 012015. [16] A. Lorusso, A. Cola, F. Gontad, I. Koutselas, M. Panareo, N.A. Vainos, A. Perrone, Nucl. Instrum. Methods Phys. Res. Sect. A 724 (2013) 72‒75. [17] L. Torrisi, F. Caridi, L. Giuffrida, Nucl. Instrum. Methods Phys. Res. Sect. B 268 (2010)
RI PT
2285‒2291.
[18] G. Baraldi, A. Perea, C.N. Afonso, Appl. Phys. A 105 (2011) 75‒79.
[19] F. Gontad, A. Lorusso, A. Perrone, Thin Solid Films 520 (2012) 3892‒3895.
[20] J.F. Ready, Effects of High Power Laser Radiation, Academic Press, New York, 1971.
SC
[21] L.M. Doeswijk, G. Rijnders, D.H.A. Blank, Appl. Phys. A 78 (2004) 263‒268. [22] N. Arnold, D. Bäuerle, Appl. Phys. A 68 (1999) 363‒367.
M AN U
[23] A.G.J. Mank, P.R.D. Mason, J. Anal. At. Spectrom. 14 (1999) 1143‒1153. [24] J.C. Conde, L. Lusquinos, P. Gonzalez, J. Serra, B. Leon, A. Dima, L. Cultrera, D. Guido, A. Zocco, A. Perrone, Thin Solid Films 453‒454 (2004) 323‒327.
[25] L. Cultrera, P. Miglietta, A. Perrone, J. Alloy. Compd. 504 (2010) S399‒S404.
AC C
EP
TE D
[26] F. Gontad, A. Lorusso, A. Perrone, Thin Solid Films 520 (2012) 5211‒5214.
8
ACCEPTED MANUSCRIPT Figure and table captions Table 1 Film thickness and average deposition rate at different laser fluences Fig. 1. Scheme of the typical experimental setup used for the PLD technique. M: Mirror; A:
RI PT
Attenuator; L: Lens; W: Laser Window; S: Substrate; T: Target; TR: Target Rotation feedthrough; SH: Shutter; F: x‒y‒z substrate motion system; MS: Mass Spectrometer.
Fig. 2. Average ablation rate of the target normalized to the energy (µg/(pulse J)) at different laser fluences. In this figure as in the next ones the bars represent the maximum errors obtained by
SC
reproducibility studies. Continuous line is the data-fitting curve taking into account a logarithmic law.
M AN U
Fig. 3. Average deposition rate normalized to the energy as a function of laser fluence. The curve line is added to guide the eye.
Fig. 4. Plume deflection angle vs the laser fluence. The curve line is added to guide the eye. Fig. 5. Sequence of ablation plume deviation at different laser fluences: (a) 15 J/cm2; (b) 9 J/cm2; (c) 6 J/cm2; (d) 4 J/cm2.
Fig. 6. Target surface morphology irradiated with different laser fluences: (a) 15 J/cm2; (b) 9
TE D
J/cm2; (c) 6 J/cm2; (d) 4 J/cm2.
Fig. 7. Time integrated mass spectrum obtained with electron impact ionizer turned on (red color) and off (black color) with laser fluence of 15 J/cm2. Fig. 8. SEM image of the film deposited with 15 J/cm2. The inset highlights one of the biggest
EP
droplets with the relatively EDX map.
Fig. 9. EDX maps of the sample deposited with 15 J/cm2 obtained with electron beam energy of 5
AC C
keV.
Table 1
Laser fluence
Thickness on the
(J/cm2)
film center (nm)
rate (nm/(pulse J))
4
10
0.010
6
24
0.017
9
59
0.027
15
137
0.038
9
Average
deposition
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
Figure List
AC C
Fig. 2
EP
TE D
Fig. 1
Fig. 3
10
RI PT
ACCEPTED MANUSCRIPT
Fig. 5
AC C
EP
TE D
M AN U
SC
Fig. 4
11
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Fig. 7
EP
TE D
Fig. 6
Fig. 8
12
EP AC C
Fig. 9
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
13
S1005-0302(15)00101-2
DOI:
10.1016/j.jmst.2015.06.007
Reference:
JMST 519
To appear in:
Journal of Materials Science & Technology
Received Date: 19 December 2014 Revised Date:
13 February 2015
Accepted Date: 16 February 2015
Please cite this article as: G. Francisco, L. Antonella, S. Luigi, K. Ioannis, V. Nikos, P. Alessio, Growth of Niobium Thin Films on Si Substrates by Pulsed Nd:YAG Laser Deposition, Journal of Materials Science & Technology (2015), doi: 10.1016/j.jmst.2015.06.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Growth of Niobium Thin Films on Si Substrates by Pulsed Nd:YAG Laser Deposition Gontad Francisco1, Lorusso Antonella1, *, Solombrino Luigi1, Koutselas Ioannis2, Vainos Nikos2, Perrone Alessio1 Department of Mathematics and Physics “E. De Giorgi”, and National Institute of Nuclear
RI PT
1
Physics, University of Salento, 73100 Lecce, Italy 2
Department of Materials Science, University of Patras, 26500 Patras, Greece
[Received 19 December 2014; Received in revised form 13 February 2015; Accepted 16 *
Corresponding author. Ph.D.; Tel.: +39 832 297501.
SC
February 2015]
M AN U
E-mail address: [email protected] (L. Antonella).
The growth of Nb thin films on Si(100) substrates by pulsed Nd:YAG laser deposition (PLD) under different laser fluences (4‒15 J/cm2) was reported. The influence of laser fluence on ablation rate and deposition rate was discussed. X-ray diffraction (XRD) investigations of the
TE D
deposited films showed an amorphous structure. The droplet density on the film surface observed by scanning electron microscopy (SEM) analyses was extremely low. It was experimentally proved that the droplets on the film surface originated from liquid phase on the target surface. Profilometric measurements of the deposited Nb films revealed a substantial asymmetry in the
EP
film thickness related to the plume deflection effect. The measured electrical resistivity of the Nb film was higher than that of high purity Nb bulk. The present investigations of ablation and
AC C
deposition process of Nb thin films are related to its potential application in superconducting radio-frequency (SRF) cavities.
Key words: Pulsed laser deposition; Nb thin films; Ablation and deposition rate 1. Introduction Pulsed laser deposition (PLD) technique has been widely used to grow thin films of any material[1] and to develop innovative nanostructured devices[2]. One of the reasons for its success is its flexibility and simplicity. Nevertheless, in spite of its many advantages, the presence of droplets on the film surface and the inhomogeneity in the film thickness due to the plume 1
ACCEPTED MANUSCRIPT deflection effect prevent PLD technique from emerging as reliable technology for the deposition of high quality films[3‒6]. We found out, in our recent research activity on the deposition of metallic thin films, that droplet density on the film surface can be reduced by detailed parametric studies[7]. Moreover, it has been found that the laser fluence is the most important laser parameter. With this in mind, we performed ablation and deposition investigations of Nb thin
RI PT
films at different laser fluences over the laser ablation threshold. Niobium is a metal of great interest because it is the leading superconductor to build superconducting radio-frequency (SRF) cavities often used in particles accelerators[8‒10]. There exists an extensive research activity and literature on metallic thin films[11‒13] but little has been done on Nb thin films, less than never
SC
with Nb thin films grown by PLD[14,15]. PLD technique is distinguished from the other physical and chemical deposition techniques by the high kinetic energy of ablated material that improves
M AN U
the adhesion of the deposited film on their substrate. Preliminary results on the Nb thin films grown on Si(100) by PLD are reported here, together with a discussion concerning its future application in the SRF cavities, where the photocathode can be formed by a bulk of Pb with a
2. Experimental
TE D
film of Nb in the configuration given in Ref.[16].
Depositions were performed at high vacuum (<10−5 Pa) and at room temperature using the
EP
deposition system shown in Fig. 1. The irradiating source was a frequency-quadrupled Q-switched Nd:YAG laser (Continuum Powerlite 8010) emitting 7 ns pulses at a wavelength of 266 nm. In order to eliminate any native contaminant on the target surface, a pre-irradiation
AC C
cleaning of 2000 pulses was applied. During the laser cleaning treatment, a retractable shield was introduced between the target and the substrate. A total of 20000 laser pulses (800 pulses/site) were applied to the Nb target for the deposition of one film. The laser beam was incident at an angle of 45° on the rotating Nb target. The ablated material was collected on a Si(100) substrate kept parallel to the target at a distance of 50 mm from it. The present target-substrate distance was a good compromise between respectable deposition rate (which generally decreases with the distance) and low density of droplets on the film surface (which decreases with the distance). The depositions were performed with a frequency of 10 Hz and at different laser fluences of 4, 6, 9 and 15 J/cm2. The lowest laser fluences were obtained by means of attenuators. During the
2
ACCEPTED MANUSCRIPT ablation process of Nb target, mass spectra were recorded by a quadrupole mass spectrometer (Hiden Analytical HALO 201 RC) to study the composition and the ionization states of ablated material. Scanning electron microscopy (JEOL-JSM-6480LV) and X-ray diffraction (Bruker AXS D8 Advance X-ray diffractometer using Cu Kα line) were used to study the morphology and the structure of the deposited films. The thickness of the films was deduced in different points by
RI PT
using a profilometer (Tencor Alphastep). The average ablation rate was obtained by weighing the target before and after the irradiation process. Finally, electrical resistivity measurements of the
SC
deposited films were carried out by four-point probe method.
M AN U
3. Results and Discussion
The technique of PLD was applied to study the ablation process of the target and the deposition process of the films at different laser fluences. The deposited films were extremely adherent to the Si substrates as the scotch tape and scratch tests established. The good adhesion of the films is due to the high kinetic energy of the ablated material[17,18]. This tribological
SRF cavities.
TE D
property is strongly demanding for application of photocathodes based on metallic thin film in
The precise ablation threshold of metals, Fthr, is somehow difficult to estimate due to the large energy loss by the heat diffusion into the bulk during the laser irradiation. The calculation of this value has been done according to the following equation[19]:
ρ LT cs ∆T + ∆Hf + ∆He
EP Fthr =
3
(
1− R
) (1)
AC C
where ρ is the material density (8.57 g/cm3), cs is the specific heat (0.265 J/(g K)), ∆T is the difference between the evaporation temperature (5017 K) and room temperature, ∆Hf fusion enthalpy (292.76 J/g),
is the
∆He is the evaporation enthalpy (7497.58 J/g), LT = (2k×τ)1/2 =
0.58 µm, which is the thermal diffusion length, k (0.24 cm2/s) is the thermal diffusivity and τ is the laser pulse duration. LT so defined indicates the distance at which the amplitude of the heat flux reduces e times from its value at sample’s surface[20]. The computed value of Fthr is about 2.9 J/cm2 taking into account the surface reflectance value of the Nb target, R, of about 0.48 given in Ref.[21]. The relatively high laser ablation threshold is caused by the high values of evaporation temperature and evaporation enthalpy of the present metal. Eq. (1) is always valid for nanosecond laser ablation of metals where the optical penetration depth of the laser is much less than LT. 3
ACCEPTED MANUSCRIPT The average ablation rate normalized to the energy (µg/(pulse J)) as a function of laser fluence, given in Fig. 2, was calculated by weighing the target before and after the laser-ablation process. The experimental data are well approximated by a logarithmic law, whose extrapolation gives the ablation threshold value of 3.8 J/cm2. Error bars in Fig. 2 indicate the experimental uncertainty in measuring the laser fluence and the ablation rate. Doeswijk et al. measured a
RI PT
threshold fluence value of 3.5 J/cm2 for ablation of Nb at 248 nm[21]. This discrepancy between theoretical and experimental values is probably due to the uncertainty on surface reflectance value of the Nb. The laser ablation threshold for the Nb ablation was also found empirically by observing the worsening of the vacuum from a background pressure of 6 × 10−6 Pa, before the
SC
laser irradiation, down to 5 × 10−5 Pa by increasing slowly the laser energy. The occurrence of mass peak of Nb at 93 amu, during mass spectrometry investigations, was observed at around 4
M AN U
J/cm2. At laser fluence values up to 6 J/cm2 a fast growing of ablation rate was observed. At higher fluences (>6 J/cm2) a self-regulating ablation rate regime, due to plasma absorption of laser beam, is encountered.
The thickness value of films measured on the center area and the average deposition rate are listed in Table 1. The average deposition rate of the film on the Si substrate increases almost
TE D
linearly with the laser fluence used in our experimental conditions (see Fig. 3). The behavior of the average deposition rate does not follow completely the trend of average ablation rate due to the lowering of plume deflection angle with the laser fluence (see Fig. 4), which improves the deposition in the center area of the substrate.
EP
The detailed studies of the plume deflection effects during the laser ablation of Nb also showed that during the first laser pulses/site, less than a dozen, the visible plume was still
AC C
perpendicular to the target surface, but as the number of pulses per site increased (~ 24 pulses/site), an evident deviation of the plume towards the laser beam direction was observed even at naked eyes. This confirms that the target surface modification produces variations in the plasma plume expansion[12]. A sequence of ablation plume at different laser fluences and at the end of laser ablation process (800 pulses/site) is shown in Fig. 5. The image of the plume taken from the top window of the deposition chamber is the result of a single laser pulse. The plume deflection angle decreases with increasing laser fluence as just shown in Fig. 4. This behaviour can be explained by observing the microstructures morphology of the ablating target surface after the laser ablation process[22, 23]. Fig. 6 illustrates the SEM images of the target surface morphology irradiated with different laser fluences. At the highest laser fluence (Fig. 6(a)), the microstructures 4
ACCEPTED MANUSCRIPT on the target surface, at the end of the laser ablation process, are smooth and less developed producing a weak plume deflection effect. At lower laser fluences (Fig. 6(c, d)), the microstructures on the target surface present more asperities and they are more resistant to the lower laser density and persist longer implying a more evident plume deflection[24,25]. The shortcoming of plume deflection is to worsen the thickness uniformity over the whole
RI PT
film surfaces (10 mm in diameter) for a maximum value of about 20% at the lowest laser fluence. In order to study the ions generated by the direct laser ablation of Nb target, mass spectrometry investigations were performed. It must be stressed that during the laser ablation process, the vacuum level is lowered down to 5×10−5 Pa. Fig. 7 shows the time integrated mass
SC
spectrum recorded during the ablation process of the target at the highest laser fluence of 15 J/cm2. The mass spectrum was purged by the contribution of the residual gases. The mass peaks at
M AN U
93 and at 46‒47 amu (atomic mass unit) can be easily associated with singly and doubly charged Nb ions, respectively. In these experimental conditions, no niobium oxides were observed even if its formation in gas phase cannot be ruled out. Ionic species were also detected with the ionizer switched off and under the same experimental conditions. The time integrated mass spectrum recorded during the laser ablation of a pure niobium target is shown in Fig. 7 with black columns. The appearance of signals localized 46‒47 and 93 amu, even with the ionizer switched off, clearly
TE D
demonstrates that the main ionic species are present in the ablation plume are Nb++ and Nb+. The mass spectra recorded at lower laser fluences are similar but with weaker peak intensity in particular for the doubly-charged ions. It has been also observed that after laser ablation process
EP
the quality of vacuum is improved, because the gas phase reactions between ablated material and residual gas reduce the partial pressure of oxygen-containing molecules (O2, H2O, CO2)[26]. The surface morphology of the film deposited at 15 J/cm2 is shown in Fig. 8. The film is
AC C
almost free of droplets and fragments, which slightly decrease with the laser fluence. The SEM images of the films deposited at lower laser fluences were quite similar and not reported here for sake of brevity. In the inset of Fig. 8 is highlighted one of the biggest droplets observed in the SEM image. The spherical shape of this particle and its chemical composition obtained by energy dispersive X-ray spectroscopy (EDX), shown in the right side of the inset, demonstrate its origin from liquid phase on the target surface. Two dimensional maps of Si, Nb and O distributions on the sample surface were obtained by EDX technique. The SEM-EDX maps of the thickest sample deposited with 15 J/cm2 are shown in Fig. 9. They were obtained with very low electron energy (5 keV) because the thickness of Nb film was not enough to reduce the contribution of the X-ray
5
ACCEPTED MANUSCRIPT coming from Si substrate. The left area of the image relative to EDX map of Si coloured in green shows the uncoated Si substrate. On the contrary, the right area, coated with the Nb film, appears almost black (dark greenish) because only a few X-ray comes from the underneath Si substrate. The uniform map of Nb does not present voids or cracks, which means that good quality film has been deposited. The oxygen map shows a higher concentration of this element in the Nb film area.
RI PT
The reason of this effect is probably due to gas phase reactions of Nb with the residual gas of the vacuum chamber and the higher chemical reactivity of Nb with respect to Si when the sample is exposed in open air.
XRD investigations were carried out to deduce the structure of the deposited film. All the
SC
XRD patterns show an amorphous structure of the films probably due to the low deposition temperature of the substrate and he lack of sufficient film thickness.
M AN U
Finally the sheet resistance of the Nb film with the highest thickness was around 5.5 Ω/sq and the corresponding electrical resistivity was (7.5±0.7)×10−5 Ω cm. This average value of the resistivity measured on different area of the deposited film by four-point probe technique is higher than that of high purity Nb bulk (1.5×10−5 Ω cm). At the moment, it seems to be the only weak point for its potential application in SRF cavities since the deposition of thicker films can be
pulses.
EP
4. Conclusion
TE D
obtained by lowering the target-substrate distance and by increasing the total number of laser
In the present work the ablation process of Nb target and the deposition process of Nb thin
AC C
film at different laser fluences were investigated. The fabricated metallic films were characterized by SEM, EDX, XRD, profilometry and electrical measurements. The amorphous nature of the deposited films was deduced by XRD analysis. Morphological studies show that no significant number of droplets is observed on all the deposited films. The data of average ablation rate as a function of laser fluence are well approximated by a logarithmic law, which takes into account of the laser-plasma absorption. The deposition rate behavior is, instead, quasi linear due to the lowering of the plume deflection angle with the laser fluence. The relatively low deposition rate of this metal could be enhanced by lowering the target-substrate distance down to 30 mm. The laser ablation threshold was computed and compared to that deduced by the experiments. Finally, the low droplets density and the high adhesion of the Nb films deposited by PLD are
6
ACCEPTED MANUSCRIPT promising properties for its potential application in SRF cavities. Acknowledgments We gratefully acknowledge Professor M. Di Giulio for profilometric measurements and Mr. L. Monteduro for his expert technical support. This work was supported partially by the Italian
RI PT
Ministry of Research in the framework of FIRB-Fondo per gli Investimenti della Ricerca di Base (Project no. RBFR12NK5K) and the Italian National Institute of Nuclear Physics (INFN).
SC
References
[1] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley-VCH, New York, 2003.
M AN U
[2] Y. Li, T. Sasaki, Y. Shimizu, N. Koshizaki, J. Am. Chem. Soc. 130 (2008) 14755‒14762. [3] L. Cultrera, M.I. Zeifman, A. Perrone, Phys. Rev. B 73 (2006) 075304. [4] E. van de Riet, C.J.C.M. Nillesen, J. Dieleman, J. Appl. Phys. 74 (1993) 2008‒2012. [5] N. Pryds, J. Schou, S. Linderoth, Appl. Surf. Sci. 253 (2007) 8231‒8234. [6] W. Liu, J.P. Fang, W.P. Cai, J.H. Liang, X.S. Zhou, X.G. Long, Chin. Phys. B 23 (2014)
TE D
098103.
[7] A. Lorusso, V. Fasano, A. Perrone, K. Lovchinov, J. Vac. Sci. Technol. A 29 (2011) 031502. [8] J. Smedley, T. Rao, J. Warren, P. Kneisel, J. Sekutowicz, J. Iversen, D. Klinke, D. Kostin, W. Möller, A. Muhs, P. Strzyzewski, QE Measurements of a Nb‒Pb Photoinjector, in:
EP
Proceedings of the Energy Recovery Linacs, Daresbury, 2007, 95. [9] J.K. Sekutowicz, P. Kneisel, R. Nietubyc, T. Rao, J. Smedley, QE tests with Nb‒Pb SRF
AC C
Photoinjector and Arc Deposited Photocathodes, In: Proceedings of the International Particle Accelerator Conference, Kyoto, 2010, 4086. [10] C. Xu, C. Reece, M. Kelley, Appl. Surf. Sci. 274 (2013) 15‒21. [11] A. Lorusso, F. Gontad, A. Perrone, Thin Solid Films 520 (2011) 117‒120. [12] L. Cultrera, A. Lorusso, B. Maiolo, L. Cangueiro, R. Vilar, A. Perrone, J. Appl. Phys. 115 (2014) 093192. [13] M. Zheng, J. Shen, J. Barthel, P. Ohresser, Ch. V. Mohank, J. Kirschner, J. Phys.-Condens. Matter 12 (2000) 783‒794. [14] S. Haindl, M. Weisheit, T. Thersleff, L. Schultz, B. Holzapfel, Supercond. Sci. Technol. 21 (2008) 045017.
7
ACCEPTED MANUSCRIPT [15] V. Grosse, C. Pansow, A. Steppke, F. Schmidl, A. Undisz, M. Rettenmayr, A. Grib, P. Seidel, J. Phys.-Conference Series 234 (2010) 012015. [16] A. Lorusso, A. Cola, F. Gontad, I. Koutselas, M. Panareo, N.A. Vainos, A. Perrone, Nucl. Instrum. Methods Phys. Res. Sect. A 724 (2013) 72‒75. [17] L. Torrisi, F. Caridi, L. Giuffrida, Nucl. Instrum. Methods Phys. Res. Sect. B 268 (2010)
RI PT
2285‒2291.
[18] G. Baraldi, A. Perea, C.N. Afonso, Appl. Phys. A 105 (2011) 75‒79.
[19] F. Gontad, A. Lorusso, A. Perrone, Thin Solid Films 520 (2012) 3892‒3895.
[20] J.F. Ready, Effects of High Power Laser Radiation, Academic Press, New York, 1971.
SC
[21] L.M. Doeswijk, G. Rijnders, D.H.A. Blank, Appl. Phys. A 78 (2004) 263‒268. [22] N. Arnold, D. Bäuerle, Appl. Phys. A 68 (1999) 363‒367.
M AN U
[23] A.G.J. Mank, P.R.D. Mason, J. Anal. At. Spectrom. 14 (1999) 1143‒1153. [24] J.C. Conde, L. Lusquinos, P. Gonzalez, J. Serra, B. Leon, A. Dima, L. Cultrera, D. Guido, A. Zocco, A. Perrone, Thin Solid Films 453‒454 (2004) 323‒327.
[25] L. Cultrera, P. Miglietta, A. Perrone, J. Alloy. Compd. 504 (2010) S399‒S404.
AC C
EP
TE D
[26] F. Gontad, A. Lorusso, A. Perrone, Thin Solid Films 520 (2012) 5211‒5214.
8
ACCEPTED MANUSCRIPT Figure and table captions Table 1 Film thickness and average deposition rate at different laser fluences Fig. 1. Scheme of the typical experimental setup used for the PLD technique. M: Mirror; A:
RI PT
Attenuator; L: Lens; W: Laser Window; S: Substrate; T: Target; TR: Target Rotation feedthrough; SH: Shutter; F: x‒y‒z substrate motion system; MS: Mass Spectrometer.
Fig. 2. Average ablation rate of the target normalized to the energy (µg/(pulse J)) at different laser fluences. In this figure as in the next ones the bars represent the maximum errors obtained by
SC
reproducibility studies. Continuous line is the data-fitting curve taking into account a logarithmic law.
M AN U
Fig. 3. Average deposition rate normalized to the energy as a function of laser fluence. The curve line is added to guide the eye.
Fig. 4. Plume deflection angle vs the laser fluence. The curve line is added to guide the eye. Fig. 5. Sequence of ablation plume deviation at different laser fluences: (a) 15 J/cm2; (b) 9 J/cm2; (c) 6 J/cm2; (d) 4 J/cm2.
Fig. 6. Target surface morphology irradiated with different laser fluences: (a) 15 J/cm2; (b) 9
TE D
J/cm2; (c) 6 J/cm2; (d) 4 J/cm2.
Fig. 7. Time integrated mass spectrum obtained with electron impact ionizer turned on (red color) and off (black color) with laser fluence of 15 J/cm2. Fig. 8. SEM image of the film deposited with 15 J/cm2. The inset highlights one of the biggest
EP
droplets with the relatively EDX map.
Fig. 9. EDX maps of the sample deposited with 15 J/cm2 obtained with electron beam energy of 5
AC C
keV.
Table 1
Laser fluence
Thickness on the
(J/cm2)
film center (nm)
rate (nm/(pulse J))
4
10
0.010
6
24
0.017
9
59
0.027
15
137
0.038
9
Average
deposition
ACCEPTED MANUSCRIPT
M AN U
SC
RI PT
Figure List
AC C
Fig. 2
EP
TE D
Fig. 1
Fig. 3
10
RI PT
ACCEPTED MANUSCRIPT
Fig. 5
AC C
EP
TE D
M AN U
SC
Fig. 4
11
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
Fig. 7
EP
TE D
Fig. 6
Fig. 8
12
EP AC C
Fig. 9
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
13