- Email: [email protected]
Laser annealing of Nb coatings for superconducting RF accelerating cavities
& -_
Nuclear Instruments and Methods in Physics Research A 365 (1995) 28-35
m-I --
64!!3 ELSEVIER
NUCLEAR INSTRUMENTS &METHODS IN PHVSICS RESEARCH SectronA
Laser annealing of Nb coatings for superconducting RF accelerating cavities E. Radicioni
ap1,C. Benvenuti
ay* , M. Bianconi b, L. Correra b
a CERN 1211, Gendve 23, Switzerland b CNR-LAMEL, uia Gobetti 101, 40129 Bologna, Italy Received 15 March 1995
Abstract The effects of laser annealing on niobium films sputter-coated on OFHC copper are reported. It is shown that single shots (40 ns, 5.5 J/cm’) of a XeCl excimer laser result in surface melting and recrystallization of the Nb film, and in turn in the complete removal of surface defects and in a large increase of grain size and RRR values. This procedure may be of practical interest to improve the performance of Nb-coated RF accelerating superconducting cavities, such as those produced for the LEP energy upgrade from 5.5 to about 90 GeV per beam.
1. Introduction Nb sputter-coated copper accelerating cavities have been chosen for the energy upgrade of LEP (Large Electron Positron collider at CERN) from 55 to about 90 GeV per beam. They provide higher stability against quench, higher quality factor (Q,,) values at operating fields (6 MV/m), insensitivity to the earth’s magnetic field, at lower cost compared to Nb sheet cavities [l]. Nevertheless, they suffer from performance limitations due to both localised and global RF losses. Localised surface defects may be generated during the manufacturing process or during the cavity setup; they are detrimental to both RF performance and system reliability, due to enhanced local RF losses and/or electron field emission. Chemical surface finishing prior to Nb coating may cause the formation of holes and protrusions on the copper surface, which are subsequently replicated by the Nb film, leading to unwanted surface defects [2]. Other types of defects may consist of foreign particles embedded in the Nb coating or localised film peel-off. In the absence of localised defects, Nb-coated cavities show a faster Q, degradation with increasing accelerating field compared to those made of bulk Nb. Recent models suggest that film granularity may be responsible for Q, degradation [3-51.
* Corresponding author, tel. f41 22 767 37 18, fax +41 22 767 91 50, e-mail: cristoforo_ [email protected]. ’ ASP (Associazione Scientifica Piemonte) fellow.
According to these models, the Nb film behaves like a network of superconducting grains coupled by Josephson junctions (the grain boundaries). This gives rise to a RF dependent term in the surface resistance. The reduction of the number of grain boundaries per unit area and the improvement of their quality should result in the reduction of the anomalous increase of surface resistance observed when increasing the accelerating field [l]. We present here a study on the potential benefits of laser annealing as a final surface treatment of Nb-coated cavities, aimed at destroying localised defects and at recrystallizing the film to a more suitable granularity and morphology.
2. Experimental setup and procedures 2.1. Laser annealing system The experimental setup is sketched in Fig. 1. The beam is produced by a Lumonix HE-480-UB-A XeCl excimer laser, providing up to 1 J/pulse energy at 308 nm, 40 ns pulse duration and 50 pps repetition rate. This laser is particularly suitable for transient annealing processes, due to both the high spatial homogeneity (better than 5% rms over 15 X 15 mm2 diaphragmed beam) and the 3% pulseto-pulse reproducibility. A single quartz lens focuses the beam to a 2 X 2 mm2 spot on the sample, while a stack of beam splitters is used to give the required energy density (fluence). The sample is mounted in a specially designed stainless steel chamber with a quartz window that can be pumped down to lo-’ Pa and filled with inert or reactive
0168-9002/95/$09.50 0 1995 Elsevier Science B.V. AI1 rights reserved SSDI 0168-9002(95)00411-4
29
E. Radicioni et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
E = 5 J/cm2
Surface
I\
4000
2
Lens f = 270 mm
8
3000
s E k c” 2000
Quartz window 1000
X-Y moving system with step-by-step
motors
Fig. I. Laser beam focusing system and annealing chamber. The chamber is equipped with flanges (not shown in the figure) to accommodate connections to a diffusion pumping station and to a venting valve.
0 Time (ns)
Fig. 2. Computed temperature variation as a function of time from the beginning of the laser pulse (5 J/cm”). The curves refer to different depths (km). T,, indicates the melting temperatures of Nb and Cu.
gases. The sample holder can be heated up to 300°C. Annealing of large area substrates is obtained by an X-Y computer controlled moving system which holds the whole chamber. The surface status of the irradiated area is monitored on-line by a time resolved reflectivity system 161. 2.2. Modelling
of the annealing
1200_ sol
1000
process
The determination of the correct fluence is a key step. A programme to simulate the laser thermal effects has been applied to the actual sample structure (1 km Nb on bulk Cu). Due to the fast electron-phonon energy transfer a local and instantaneous conversion of the photon energy into heat can be assumed. Radiation losses at the surface are neglected due to time scale considerations. Finally, as the beam dimensions are larger than the UV light absorption length (= 10 nm), the lateral heat diffusion can be neglected and the process can be described by one-dimensional heat diffusion [7]. A critical aspect of the simulation is the knowledge of the thermal and optical parameters of the materials involved and their dependence on temperature. Table 1 reports values of these parameters used for the calculations. Most of data are from Ref. 181. For thermal conductivity and specific heat dependence on temperature, simple linear fits were assumed. The solid niobium reflectivity was measured by a spectrophotometer, the obtained value being in good agreement with the literature data. As the
liq
1 f
600
0 P
Time (ns)
Fig. 3. Computed phase evolution (sol = solid, liq = liquid, fg = Fine grains, Ig = large grains) J/cm2 laser irradiation.
of the Nb/Cu
system during
a ii
E. Radicioni et al. /Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
30 Table 1 Thermal and optical parameters s = solid, 1= liquid
Thermal conductivity [W/cm K] Specific heat [J/g K] Melting temperature [KI Melting enthalpy [J/g] Vaporization temperature [K] Vaporization enthalpy [J/g] Density [g/cm31 Reflectivity Absorption coefficient [cm-’
I
of Nb and Cu. The (* ) indicates
that the corresponding
values are estimated
as described
in the text;
S-Nb
I-Nb
s-cu
1-cu
0.476 + 2 X 10-4T 0.255 + 4 x 10-4T 2741 288
0.64 0.36
4.52 + 1.5 X 10-4T 0.356 + lo- 4T 1356 205
1.28 + 2.8 x 10-4T 0.459
_ 8.57 0.44 9.5 x 106
5015 7319 7.83 0.6 ( * 1 106(‘)
_
optical parameters of liquid niobium are not available, typical values of liquid metals were assumed. Fig. 2 shows the computed evolution of the temperature at different depths in the sample vs. time for a 5 J/cm* laser pulse. The surface approaches the Nb boiling temperature and about one half of the film is melted. The formation of a buried liquid layer of copper is also predicted by the simulation. However the diffusion of the Cu
Fig. 4. Surface defect consequent
_ 8.96
2840 4803 8.00
into the Nb is not expected because the two liquid layers are separated by a solid region. The computed phase evolution of the sample is reported in Fig. 3. The resolidification of the surface Nb liquid layer, as usually observed in laser annealing processes, is expected to produce an increase of the grain size. This effect is actually confirmed by the structural characterisation of the irradiated samples as shown in Section
to excessive laser power density (above 5.5 J/cm2J.
31
E. Radicioniet al./Nucl. Instr. and Meth. in Phys.Res. A 365 (1995) 28-35
Fig. 5. Niobium film evaporation
consequent
to poor Nb/Cu
3.5. The above results indicate that the suitable fluence to produce the deepest annealed region, without any surface damage and/or Nb/Cu intermixing, is about 5 J/cm’. 2.3. Sample preparation, laser treatments and evaluation procedures Niobium films (about 1 p,m thick) were coated on OFHC, 1 mm thick Cu substrates by magnetron sputtering. The sputtering configuration and parameters were the same as used for the coating of accelerating cavities [l]. The samples were laser-annealed in different conditions and at different energy densities. They have been analysed using X-ray diffractometry, Auger spectroscopy, SEM imaging and residual resistivity ratio (RRR) measurements. In view of annealing an entire cavity, it is of practical importance to establish whether the laser processing should be applied under vacuum or under controlled atmosphere. To clarify this point, several samples were annealed in air, argon and vacuum (1O-5 Pa); Auger analysis was employed to monitor surface composition and to ascertain if the annealing was detrimental for the surface purity. Samples used for this purpose had only half of their surface laser-annealed, to allow a direct comparison with the unprocessed surface.
bonding at energy densities of about 5 J/cm’.
SEM imaging was carried out without applying the chemical etching normally used to increase the contrast on grain boundaries, the contrast obtained being judged adequate. Sputter deposition of a thin gold layer was unavoidable to eliminate charging during scanning with the electron probe and to improve stigmator response. Four magni-
--a--
0 --
Oxygen,
,.~","",~'~.,~~~'I"~',".' 0 10 5 Time
laser-processedsample
:
Oxygen, standard sample
15
20
25
30
Iminl
Fig. 6. Evolution of the oxygen concentration as a function of the ion etching time monitored by Auger spectroscopy. The sample laser annealed in argon atmosphere (energy density 5.5 J/cm’) presents an oxyde layer about 30% thinner than the unprocessed sample.
E. Radicioni et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
32
fications (i.e. 1.5 K, 5 K, 15 K, 50 K) were standardised to examine both the global surface aspect and the microscopic morphology. Special care is required in preparing samples for RRR measurements because the Cu substrate must be dissolved by an appropriate chemical treatment. It was verified that HNO, etching has, to a first approximation, no influence on the RRR value of the film. To do so, the RRR value was measured on Nb films coated on quartz, before and after dipping them in HNO, while dissolving a piece of copper. The same RRR values were found in both cases. Sample preparation proceeds as follows. The copper is dissolved chemically, and the film is then rinsed and dried before being glued on a Kapton self-adhesive ribbon. The ribbon is finally glued onto a copper substrate. Copper has been chosen because its large thermal conductivity ensures uniform temperature of the sample in the cryostat probe.
lower than 4.5 J/cm2 did not produce any effect, while densities higher than about 6 J/cm2 may cause catastrophic defects, i.e. craters consequent to surface explosion (Fig. 4). Elemental analysis reveals only copper inside the resulting holes showing that the Nb coating has been completely vaporised. It should be noted that another possible cause of film evaporation is poor bonding of the Nb film to the substrate: if the thermal contact is not perfect, all laser energy is accumulated in the film which immediately evaporates (Fig. 5). It should also be noted that the adhesion of the film to the substrate is preserved by the annealing. If the first annealing deteriorates film bonding, a subsequent laser shot would vaporise it. This phenomenon does not take place because superposition of laser shots is always feasible without any such detrimental effect.
3. Results
3.2. Crystal lattice
3.1 I Laser jluence
As a first check of the effects of laser processing, an X-ray analysis has been carried out on treated samples. The X-ray diffraction measurement revealed that the vertical lattice spacing changes from 2.350 _&(0.280 A
Different annealing energy densities around the calculated value have been applied. It was found that densities
Fig. 7. Nh unprocessed
surface observed by electron microscopy
with 5 K magnification.
Tilt angle: 45’.
E. Radicioni et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
FWHM) to 2.326 A (0.269 A FWHM), i.e. that laser annealing relaxed the crystal structure. The lattice parameter of the standard Nb is 2.336 A,. This result indicates that our Nb films are usually compressed parallel to the surface and confirms that film melting and recrystallization occurred.
3.3. Surface composition Auger data show no influence of the annealing atmosphere on surface composition. Small differences are noticeable in the thickness of the oxide layer which is naturally formed on the surface when exposed to air. Samples annealed in argon atmosphere show a thinner oxide layer with respect to unprocessed samples (Fig. 6). Auger spectra are taken, in this case, during ion etching to measure the oxide thickness. Tal$ng into account that the applied etching speed is 5.85 A/min, ihe unprocessed sample presents an oxide layer about 80 A thick, while in the case of the laser-annealedOsamples oxide layer is only about 50 A.
the thickness of the
33
3.4 Localised defects The surface of a standard Nb film obtained by sputtering is shown in Fig. 7. Pits and irregularities are evident. Holes are generated on the copper substrate by the chemical etching [2]. Laser annealed surfaces look very smooth and without holes (Fig. 8). Taking into account that the surface temperature approaches that of the Nb boiling point during the laser irradiation, laser annealing is not only able to cure surface morphology defects, but also to vaporise nearly any contaminant material present on the surface of the cavity. Tungsten contamination, which is often produced by the electrode of a TIG welding machine, would survive to such a treatment, but the cavity half-cells are usually welded by electron beam, therefore there is no reason why this contamination should occur. 3.5. Film structure and RRR values Standard film grains are irregular in shape. Their dimension ranges from 80 to 110 nm. They have a tendency
Fig. 8. Nb surface processed at 5.5 J/cm* observed by electron microscopy with 5 K magnification. Tilt angle: 45”.
E. Radicioni et al. /Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
34
Fig. 9. Morphology
Fig. 10. Morphology
of a Nb unprocessed
of a Nb surface processed
surface observed by electron microscopy.
at 5.5 J/cm*
Tilt angle: O”, magnification:
observed by electron microscopy.
15 K.
Tilt angle: 0”, magnification:
15 K.
E. Radicioni et al. / Nucl. Ins&. and Meth. in Phys. Rex A 365 (1995) 28-35
NblCu
26
.
annealed samples
24
0
!
0” v
18
0 Nb/quartz 0 reference
lb6
012345678
Laser energy density
u/cm21
Fig. Il. Comparison of RRR values for reference and laser-annealed samples at different energy densities (5 and 5.5 J/cm’).
to align radially around holes, and seem loosely bound to each other with sharp edges resulting in a noticeable surface roughness (Fig. 9). The laser treatment recrystallizes the grains to a regular shape, without sharp edges, thus reducing the total surface in contact with atmosphere (Fig. 10). The grain size of laser annealed films ranges from 300 to 400 nm, with individual grains up to 500 nm. The number of grain boundaries is therefore strongly reduced. The better contact between grains might imply cleaner grain boundaries and, consequently, smaller power dissipation. This improvement is directly indicated by RRR measurements (Fig. 11). Although a strong RRR increase with increasing laser fluence is observed, the latter has been limited to 5.5 J/cm* to avoid film damage. The measured mean RRR values are 22.7 f 0.9 at 5 J/cm’ and 25.2 + 0.8 at 5.5 J/cm’, while the corresponding value of unprocessed samples is about 18.
4. Conclusions The results presented here indicate that: - grain size is increased by the annealing as shown by surface electron microscopy and RRR measurements; - laser annealing also produces a better surface from the point of view of uniformity and absence of localised defects; - the annealing atmosphere does not play an important role; Auger spectroscopy indicates that the Nb film may be laser annealed in argon atmosphere without detrimental effects on surface composition.
35
These features suggest that laser annealing should improve cavity performance both in term of local heating and/or electron field emission consequent to localised defects, and with respect to Q, deterioration (increase of surface resistance when increasing the accelerating voltage). A possible configuration to process an entire cavity consists of a robot arm inserted in the cavity and equipped with a tilting mirror and a focusing telescope. The required spot size, of the order of some square millimetres, does not impose severe design constraints. As shown by Auger electron spectroscopy, an ultrahigh vacuum system is not required. Depending on laser power (determined by pulse energy and repetition rate), an entire 4-cell 352 MHz accelerating cavity, of LEP type, could be laser-annealed in a reasonable time, from a few hours to one day. At the best of our present knowledge, the fabrication of a cavity laser processing facility does not present insurmountable technological difficulties.
Acknowledgements We would like to thank M. Servidori and S. Nicoletti (LAMEL) for X-ray diffractometry and for part of the RRR measurements, R. Cosso and D. Latorre (CERN) for Auger spectroscopy, F. Scalambrin (CERN) for sample coating and J.M. Dalin (CERN) for electron microscopy. The authors are indebted to S. Calatroni and G. Orlandi for fruitful discussions.
References [l] C. Benvenuti, S. Calatroni, G. Orlandi, Physica B 197 (1994) 72. El S. Calatroni et al., Proc. 6th Workshop on RF Superconductivity, Newport News, Virginia, USA (1993) p. 687. [31 C. Attanasio, L. Maritato and R. Vaglio, Phys. Rev. B 43 (1991) 6128. [41 J. Halbritter, J. Supercond. 5 (1992) 331. El B. Bonin and H. Safa, Supercond. Sci. Technol. 4 (1991) 257. [61 C. Summonte, M. Bianconi and D. Govoni, Mater. Res. Sot. Symp. Prof. 297 (1993) 539. [71 G. Bentini, M. Bianconi and C. Summonte, Appl. Phys. A 45 (1988) 317. 181 E.A. Brandes and G.B. Brook (eds.), Smithells Metals Reference Book, 7th edition, (Butterworth-Heinemann, 1992).
Nuclear Instruments and Methods in Physics Research A 365 (1995) 28-35
m-I --
64!!3 ELSEVIER
NUCLEAR INSTRUMENTS &METHODS IN PHVSICS RESEARCH SectronA
Laser annealing of Nb coatings for superconducting RF accelerating cavities E. Radicioni
ap1,C. Benvenuti
ay* , M. Bianconi b, L. Correra b
a CERN 1211, Gendve 23, Switzerland b CNR-LAMEL, uia Gobetti 101, 40129 Bologna, Italy Received 15 March 1995
Abstract The effects of laser annealing on niobium films sputter-coated on OFHC copper are reported. It is shown that single shots (40 ns, 5.5 J/cm’) of a XeCl excimer laser result in surface melting and recrystallization of the Nb film, and in turn in the complete removal of surface defects and in a large increase of grain size and RRR values. This procedure may be of practical interest to improve the performance of Nb-coated RF accelerating superconducting cavities, such as those produced for the LEP energy upgrade from 5.5 to about 90 GeV per beam.
1. Introduction Nb sputter-coated copper accelerating cavities have been chosen for the energy upgrade of LEP (Large Electron Positron collider at CERN) from 55 to about 90 GeV per beam. They provide higher stability against quench, higher quality factor (Q,,) values at operating fields (6 MV/m), insensitivity to the earth’s magnetic field, at lower cost compared to Nb sheet cavities [l]. Nevertheless, they suffer from performance limitations due to both localised and global RF losses. Localised surface defects may be generated during the manufacturing process or during the cavity setup; they are detrimental to both RF performance and system reliability, due to enhanced local RF losses and/or electron field emission. Chemical surface finishing prior to Nb coating may cause the formation of holes and protrusions on the copper surface, which are subsequently replicated by the Nb film, leading to unwanted surface defects [2]. Other types of defects may consist of foreign particles embedded in the Nb coating or localised film peel-off. In the absence of localised defects, Nb-coated cavities show a faster Q, degradation with increasing accelerating field compared to those made of bulk Nb. Recent models suggest that film granularity may be responsible for Q, degradation [3-51.
* Corresponding author, tel. f41 22 767 37 18, fax +41 22 767 91 50, e-mail: cristoforo_ [email protected]. ’ ASP (Associazione Scientifica Piemonte) fellow.
According to these models, the Nb film behaves like a network of superconducting grains coupled by Josephson junctions (the grain boundaries). This gives rise to a RF dependent term in the surface resistance. The reduction of the number of grain boundaries per unit area and the improvement of their quality should result in the reduction of the anomalous increase of surface resistance observed when increasing the accelerating field [l]. We present here a study on the potential benefits of laser annealing as a final surface treatment of Nb-coated cavities, aimed at destroying localised defects and at recrystallizing the film to a more suitable granularity and morphology.
2. Experimental setup and procedures 2.1. Laser annealing system The experimental setup is sketched in Fig. 1. The beam is produced by a Lumonix HE-480-UB-A XeCl excimer laser, providing up to 1 J/pulse energy at 308 nm, 40 ns pulse duration and 50 pps repetition rate. This laser is particularly suitable for transient annealing processes, due to both the high spatial homogeneity (better than 5% rms over 15 X 15 mm2 diaphragmed beam) and the 3% pulseto-pulse reproducibility. A single quartz lens focuses the beam to a 2 X 2 mm2 spot on the sample, while a stack of beam splitters is used to give the required energy density (fluence). The sample is mounted in a specially designed stainless steel chamber with a quartz window that can be pumped down to lo-’ Pa and filled with inert or reactive
0168-9002/95/$09.50 0 1995 Elsevier Science B.V. AI1 rights reserved SSDI 0168-9002(95)00411-4
29
E. Radicioni et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
E = 5 J/cm2
Surface
I\
4000
2
Lens f = 270 mm
8
3000
s E k c” 2000
Quartz window 1000
X-Y moving system with step-by-step
motors
Fig. I. Laser beam focusing system and annealing chamber. The chamber is equipped with flanges (not shown in the figure) to accommodate connections to a diffusion pumping station and to a venting valve.
0 Time (ns)
Fig. 2. Computed temperature variation as a function of time from the beginning of the laser pulse (5 J/cm”). The curves refer to different depths (km). T,, indicates the melting temperatures of Nb and Cu.
gases. The sample holder can be heated up to 300°C. Annealing of large area substrates is obtained by an X-Y computer controlled moving system which holds the whole chamber. The surface status of the irradiated area is monitored on-line by a time resolved reflectivity system 161. 2.2. Modelling
of the annealing
1200_ sol
1000
process
The determination of the correct fluence is a key step. A programme to simulate the laser thermal effects has been applied to the actual sample structure (1 km Nb on bulk Cu). Due to the fast electron-phonon energy transfer a local and instantaneous conversion of the photon energy into heat can be assumed. Radiation losses at the surface are neglected due to time scale considerations. Finally, as the beam dimensions are larger than the UV light absorption length (= 10 nm), the lateral heat diffusion can be neglected and the process can be described by one-dimensional heat diffusion [7]. A critical aspect of the simulation is the knowledge of the thermal and optical parameters of the materials involved and their dependence on temperature. Table 1 reports values of these parameters used for the calculations. Most of data are from Ref. 181. For thermal conductivity and specific heat dependence on temperature, simple linear fits were assumed. The solid niobium reflectivity was measured by a spectrophotometer, the obtained value being in good agreement with the literature data. As the
liq
1 f
600
0 P
Time (ns)
Fig. 3. Computed phase evolution (sol = solid, liq = liquid, fg = Fine grains, Ig = large grains) J/cm2 laser irradiation.
of the Nb/Cu
system during
a ii
E. Radicioni et al. /Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
30 Table 1 Thermal and optical parameters s = solid, 1= liquid
Thermal conductivity [W/cm K] Specific heat [J/g K] Melting temperature [KI Melting enthalpy [J/g] Vaporization temperature [K] Vaporization enthalpy [J/g] Density [g/cm31 Reflectivity Absorption coefficient [cm-’
I
of Nb and Cu. The (* ) indicates
that the corresponding
values are estimated
as described
in the text;
S-Nb
I-Nb
s-cu
1-cu
0.476 + 2 X 10-4T 0.255 + 4 x 10-4T 2741 288
0.64 0.36
4.52 + 1.5 X 10-4T 0.356 + lo- 4T 1356 205
1.28 + 2.8 x 10-4T 0.459
_ 8.57 0.44 9.5 x 106
5015 7319 7.83 0.6 ( * 1 106(‘)
_
optical parameters of liquid niobium are not available, typical values of liquid metals were assumed. Fig. 2 shows the computed evolution of the temperature at different depths in the sample vs. time for a 5 J/cm* laser pulse. The surface approaches the Nb boiling temperature and about one half of the film is melted. The formation of a buried liquid layer of copper is also predicted by the simulation. However the diffusion of the Cu
Fig. 4. Surface defect consequent
_ 8.96
2840 4803 8.00
into the Nb is not expected because the two liquid layers are separated by a solid region. The computed phase evolution of the sample is reported in Fig. 3. The resolidification of the surface Nb liquid layer, as usually observed in laser annealing processes, is expected to produce an increase of the grain size. This effect is actually confirmed by the structural characterisation of the irradiated samples as shown in Section
to excessive laser power density (above 5.5 J/cm2J.
31
E. Radicioniet al./Nucl. Instr. and Meth. in Phys.Res. A 365 (1995) 28-35
Fig. 5. Niobium film evaporation
consequent
to poor Nb/Cu
3.5. The above results indicate that the suitable fluence to produce the deepest annealed region, without any surface damage and/or Nb/Cu intermixing, is about 5 J/cm’. 2.3. Sample preparation, laser treatments and evaluation procedures Niobium films (about 1 p,m thick) were coated on OFHC, 1 mm thick Cu substrates by magnetron sputtering. The sputtering configuration and parameters were the same as used for the coating of accelerating cavities [l]. The samples were laser-annealed in different conditions and at different energy densities. They have been analysed using X-ray diffractometry, Auger spectroscopy, SEM imaging and residual resistivity ratio (RRR) measurements. In view of annealing an entire cavity, it is of practical importance to establish whether the laser processing should be applied under vacuum or under controlled atmosphere. To clarify this point, several samples were annealed in air, argon and vacuum (1O-5 Pa); Auger analysis was employed to monitor surface composition and to ascertain if the annealing was detrimental for the surface purity. Samples used for this purpose had only half of their surface laser-annealed, to allow a direct comparison with the unprocessed surface.
bonding at energy densities of about 5 J/cm’.
SEM imaging was carried out without applying the chemical etching normally used to increase the contrast on grain boundaries, the contrast obtained being judged adequate. Sputter deposition of a thin gold layer was unavoidable to eliminate charging during scanning with the electron probe and to improve stigmator response. Four magni-
--a--
0 --
Oxygen,
,.~","",~'~.,~~~'I"~',".' 0 10 5 Time
laser-processedsample
:
Oxygen, standard sample
15
20
25
30
Iminl
Fig. 6. Evolution of the oxygen concentration as a function of the ion etching time monitored by Auger spectroscopy. The sample laser annealed in argon atmosphere (energy density 5.5 J/cm’) presents an oxyde layer about 30% thinner than the unprocessed sample.
E. Radicioni et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
32
fications (i.e. 1.5 K, 5 K, 15 K, 50 K) were standardised to examine both the global surface aspect and the microscopic morphology. Special care is required in preparing samples for RRR measurements because the Cu substrate must be dissolved by an appropriate chemical treatment. It was verified that HNO, etching has, to a first approximation, no influence on the RRR value of the film. To do so, the RRR value was measured on Nb films coated on quartz, before and after dipping them in HNO, while dissolving a piece of copper. The same RRR values were found in both cases. Sample preparation proceeds as follows. The copper is dissolved chemically, and the film is then rinsed and dried before being glued on a Kapton self-adhesive ribbon. The ribbon is finally glued onto a copper substrate. Copper has been chosen because its large thermal conductivity ensures uniform temperature of the sample in the cryostat probe.
lower than 4.5 J/cm2 did not produce any effect, while densities higher than about 6 J/cm2 may cause catastrophic defects, i.e. craters consequent to surface explosion (Fig. 4). Elemental analysis reveals only copper inside the resulting holes showing that the Nb coating has been completely vaporised. It should be noted that another possible cause of film evaporation is poor bonding of the Nb film to the substrate: if the thermal contact is not perfect, all laser energy is accumulated in the film which immediately evaporates (Fig. 5). It should also be noted that the adhesion of the film to the substrate is preserved by the annealing. If the first annealing deteriorates film bonding, a subsequent laser shot would vaporise it. This phenomenon does not take place because superposition of laser shots is always feasible without any such detrimental effect.
3. Results
3.2. Crystal lattice
3.1 I Laser jluence
As a first check of the effects of laser processing, an X-ray analysis has been carried out on treated samples. The X-ray diffraction measurement revealed that the vertical lattice spacing changes from 2.350 _&(0.280 A
Different annealing energy densities around the calculated value have been applied. It was found that densities
Fig. 7. Nh unprocessed
surface observed by electron microscopy
with 5 K magnification.
Tilt angle: 45’.
E. Radicioni et al. / Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
FWHM) to 2.326 A (0.269 A FWHM), i.e. that laser annealing relaxed the crystal structure. The lattice parameter of the standard Nb is 2.336 A,. This result indicates that our Nb films are usually compressed parallel to the surface and confirms that film melting and recrystallization occurred.
3.3. Surface composition Auger data show no influence of the annealing atmosphere on surface composition. Small differences are noticeable in the thickness of the oxide layer which is naturally formed on the surface when exposed to air. Samples annealed in argon atmosphere show a thinner oxide layer with respect to unprocessed samples (Fig. 6). Auger spectra are taken, in this case, during ion etching to measure the oxide thickness. Tal$ng into account that the applied etching speed is 5.85 A/min, ihe unprocessed sample presents an oxide layer about 80 A thick, while in the case of the laser-annealedOsamples oxide layer is only about 50 A.
the thickness of the
33
3.4 Localised defects The surface of a standard Nb film obtained by sputtering is shown in Fig. 7. Pits and irregularities are evident. Holes are generated on the copper substrate by the chemical etching [2]. Laser annealed surfaces look very smooth and without holes (Fig. 8). Taking into account that the surface temperature approaches that of the Nb boiling point during the laser irradiation, laser annealing is not only able to cure surface morphology defects, but also to vaporise nearly any contaminant material present on the surface of the cavity. Tungsten contamination, which is often produced by the electrode of a TIG welding machine, would survive to such a treatment, but the cavity half-cells are usually welded by electron beam, therefore there is no reason why this contamination should occur. 3.5. Film structure and RRR values Standard film grains are irregular in shape. Their dimension ranges from 80 to 110 nm. They have a tendency
Fig. 8. Nb surface processed at 5.5 J/cm* observed by electron microscopy with 5 K magnification. Tilt angle: 45”.
E. Radicioni et al. /Nucl. Instr. and Meth. in Phys. Res. A 365 (1995) 28-35
34
Fig. 9. Morphology
Fig. 10. Morphology
of a Nb unprocessed
of a Nb surface processed
surface observed by electron microscopy.
at 5.5 J/cm*
Tilt angle: O”, magnification:
observed by electron microscopy.
15 K.
Tilt angle: 0”, magnification:
15 K.
E. Radicioni et al. / Nucl. Ins&. and Meth. in Phys. Rex A 365 (1995) 28-35
NblCu
26
.
annealed samples
24
0
!
0” v
18
0 Nb/quartz 0 reference
lb6
012345678
Laser energy density
u/cm21
Fig. Il. Comparison of RRR values for reference and laser-annealed samples at different energy densities (5 and 5.5 J/cm’).
to align radially around holes, and seem loosely bound to each other with sharp edges resulting in a noticeable surface roughness (Fig. 9). The laser treatment recrystallizes the grains to a regular shape, without sharp edges, thus reducing the total surface in contact with atmosphere (Fig. 10). The grain size of laser annealed films ranges from 300 to 400 nm, with individual grains up to 500 nm. The number of grain boundaries is therefore strongly reduced. The better contact between grains might imply cleaner grain boundaries and, consequently, smaller power dissipation. This improvement is directly indicated by RRR measurements (Fig. 11). Although a strong RRR increase with increasing laser fluence is observed, the latter has been limited to 5.5 J/cm* to avoid film damage. The measured mean RRR values are 22.7 f 0.9 at 5 J/cm’ and 25.2 + 0.8 at 5.5 J/cm’, while the corresponding value of unprocessed samples is about 18.
4. Conclusions The results presented here indicate that: - grain size is increased by the annealing as shown by surface electron microscopy and RRR measurements; - laser annealing also produces a better surface from the point of view of uniformity and absence of localised defects; - the annealing atmosphere does not play an important role; Auger spectroscopy indicates that the Nb film may be laser annealed in argon atmosphere without detrimental effects on surface composition.
35
These features suggest that laser annealing should improve cavity performance both in term of local heating and/or electron field emission consequent to localised defects, and with respect to Q, deterioration (increase of surface resistance when increasing the accelerating voltage). A possible configuration to process an entire cavity consists of a robot arm inserted in the cavity and equipped with a tilting mirror and a focusing telescope. The required spot size, of the order of some square millimetres, does not impose severe design constraints. As shown by Auger electron spectroscopy, an ultrahigh vacuum system is not required. Depending on laser power (determined by pulse energy and repetition rate), an entire 4-cell 352 MHz accelerating cavity, of LEP type, could be laser-annealed in a reasonable time, from a few hours to one day. At the best of our present knowledge, the fabrication of a cavity laser processing facility does not present insurmountable technological difficulties.
Acknowledgements We would like to thank M. Servidori and S. Nicoletti (LAMEL) for X-ray diffractometry and for part of the RRR measurements, R. Cosso and D. Latorre (CERN) for Auger spectroscopy, F. Scalambrin (CERN) for sample coating and J.M. Dalin (CERN) for electron microscopy. The authors are indebted to S. Calatroni and G. Orlandi for fruitful discussions.
References [l] C. Benvenuti, S. Calatroni, G. Orlandi, Physica B 197 (1994) 72. El S. Calatroni et al., Proc. 6th Workshop on RF Superconductivity, Newport News, Virginia, USA (1993) p. 687. [31 C. Attanasio, L. Maritato and R. Vaglio, Phys. Rev. B 43 (1991) 6128. [41 J. Halbritter, J. Supercond. 5 (1992) 331. El B. Bonin and H. Safa, Supercond. Sci. Technol. 4 (1991) 257. [61 C. Summonte, M. Bianconi and D. Govoni, Mater. Res. Sot. Symp. Prof. 297 (1993) 539. [71 G. Bentini, M. Bianconi and C. Summonte, Appl. Phys. A 45 (1988) 317. 181 E.A. Brandes and G.B. Brook (eds.), Smithells Metals Reference Book, 7th edition, (Butterworth-Heinemann, 1992).