- Email: [email protected]
High pressure d.c. sputtering of high Tc superconducting films: properties and deposition conditions
6#HYAC_
,GMTING8
ELSEVIER
Surfaceand CoatingsTechnology74-75 (1995) 1038-1042
ffoeHAIOLOM
High pressure d.c. sputtering of high superconducting films: properties and deposition conditions A. Geerkens, K. Fiilber, H.-J. Frenck Institute of Physics, Technische Universiti~t Cottbus, PO Box 101344, D-03013 Cottbus, Gel'many
Abstract We used high pressure d.c. sputtering to deposit superconducting thin films (YBa2Cu3OT-o (YBCO) and BiaSraCaCu2Os+x). With a.c. susceptibility measurements one can not only quickly characterize the superconducting properties of the sample, but also identify the layered growth of the films. Measurements of the gas phase composition of the sputter deposition show an unexpectedly high oxygen/metal ratio. The reason for this ratio is investigated. Measurements of the evaporation of YBCO target material yield spectra are presented, where the most important metal and metal-oxygen species are identified. Keywords: Thin films; Mass spectroscopy; Superconductors; YbaaCu3OT; Bi2Sr2CaCugOs
1. Introduction The availability of thin film technologies (deposition and structuring) is a key to commercial utilization of high T~ superconductors. While there has been a lot of effort and also a lot of success in preparing thin films with high onset temperatures and critical current densities using a variety of methods, comparatively little is known either about the exact mechanisms of film formation or the mechanism of superconductivity in thin films. We used high pressure d.c. sputtering to deposit YBa2Cu3OT-a and Bi2Sr2CaCuaOs+x as examples for a class of materials. While being a known method and simple to implement, d.c. sputtering has the advantage of providing the species impinging on the substrate and forming the film with sufficient energy to enable c-axis-oriented growth. This layered growth resulting in a film of more or less independent layers is known to be the basis of superconductivity in perovskite material. Yet it is not well known what causes the layered growth. We investigated the mechanism responsible for current transport. The behaviour of current flux lines and/or regions of weak or strong superconductivity are studied through the angular dependence of the magnetoresistance p and the critical current density jo. These can be measured using a.c. susceptibility with a special set-up allowing the tilting of the sample during ramping of the
investigated parameter. Among others, our results show that the magnetization of the sample in low magnetic fields depends not only on the external field, but also on the history of the sample (with optical excitation this is known as the memory effect [1]). As this is a contactless method it may also serve as a quick reference to overall film quality before the sample is processed further. A key to understanding of these processes will be to monitor the species in the gas phase and those impinging on the substrate surface. Based on the capability to monitor in situ the species that bombard and are eventually trapped in the film, one can assess directly a number of issues in thin film formation. Gas phase analysis will lead to more precise data on film formation and thus easier interpretation of the electrical data.
2. Experimental set-up The process termed high pressure d.c. sputtering is characterized by the d.c. discharge at about 30 W (200 V) and an oxygen atmosphere of typically 2-5 mbar. Typical substrate temperatures are between 750780 °C (Bi2Sr2CaCu2Os+.~ deposition) and 700-730 °C (YBa2Cu3OT_o deposition). No further annealing is done on the samples. This approach is thought of as advantageous [2,3] because it is simple to implement, inexpensive and scalable in the case of success. The mean free
A. GeerIcenset aL/Smface and Coatings Technology 74-75 (1995) 1038-7042
path of the species in the discharge in this pressure range is of the order of 10 tam. Thus species in the gas phase suffer a number of collisions, so that gas phase reactions will contribute to the production of species impinging on the surface. In turn, this will also lead to a reduction of resputtering by ionized oxygen atoms [2]. The high oxygen pressure counteracts possible loss of oxygen in the target resulting in changes of target stoichiometry. As it has been observed experimentally that the electrical quality of the deposited thin films strongly depends on the stoichiometry and the electrical properties of the target itself, there should either be some presputtering of the target with oxygen or O2 itself should be used as sputter gas. A schematic drawing of the inner part of the deposition reactor is given in Fig. 1. In principle it can be characterized as a conventional sputtering system. The target is bonded to a cooled copper plate approximately 2 cm above the substrate. The distance is chosen so that species impinging on the surface still have energy to contribute to alteration of the deposited layers. The sputtering gas is introduced sideways above the target and pumped directly below the substrate to ensure laminar flow between the electrodes and to minimize the amount of parasitic deposition. To the side of the reactor in Fig. 1 is the flange where the mass spectrometer for in situ investigations of the deposition process is fitted via a differentially pumped
1039
cell at the height the substrate is placed. The cell is just below the substrate and the quadrupole analyser is in line of sight with the extraction bore.
3. Properties of deposited films 3.1. Overall fihn properties The d.c. sputtered films with a thickness between 70 and 250 nm are highly oriented with the c axis perpendicular to the surface of the SrTiQ single-crystal substrate. As-deposited YBazCu3OT-o films show onset of superconductivity between 85 K .~"~o~r°nset-.~88 K with transition widths between 1 and 4 K. This is shown in Fig. 2 where a typical non-contact a.c. susceptibility measurement is presented. Typical critical current densities are of the order of 3 x 106 Acm -2 at 77K. The corresponding Bi2Sr2CaCu2Os+x films yield Tc°nset of 75K with transition widths of 1 - 4 K and Jc of 1 x 106 A cm -2. The angular dependence of the critical current density and the magnetoresistance were measured with a pulsed current method to avoid warming up of the bonded gold wire contacts on the structured film. Conduction paths of 20 gm width and 2 mm length were prepared by wet chemical etching. The sample holder allows variation of the angle between the magnetic field and
Feedthrough for substrate heating
02
02
cathode
cooling water
seal
et
chuck
sputtering pre-sputtering
sample / manipulator
! therm couple feedthrough
cooling pipes
~auue =~____
position
_~
~
~
~, pump line
Fig. 1. Schematic drawing of the d.c. sputtering reactor concept. The mass spectrometer may be connected to the reactor via flange.
A. Geerkenset al./Swface and Coatings Technology 74-75 (1995) 1038-1042
I040 I
2,0 4
7 &
,3
g
1,0
Te"
2
d...
0,5 I
.
.
.
86
.
.
.
,........... i.........
i
i
88 Temperature [K]
0,0
90
Fig. 2. ImaNnary part ( - - - ) and reaI part ( susceptibility of a deposited YBa2Cu3Ov-a thin film,
) of the a.c.
the c axis of the film (angle O) as well as the angle between the current direction and the O rotation plane. The temperature was varied between 4.2 and 77 K and magnetic fields between 10 mT and 8 T were used.
3.2. Angular dependence offilm properties To study the pinning of the high temperature superconductor films they were investigated by measuring the critical current density and the magnetoresistance dependent on the temperature and the strength and the angle of the magnetic field. Furthermore current-voltage curves were taken. The results for YBa2Cu3OT_~ and Bi2Sr2CaCu2Os+~ films are compared due to the anisotropy of the chosen superconductor. Fig. 3 shows some typical results for an investigated
4
YBa2Cu307-~
2~
3 it: (cos20+a2sin2e)la
tk Y = 77 K : 2 p o=oo
Ot
I
0
i ~
°°
2 Tesla
>..o
I
30
r
r
t
60
~
T
I
90
r
T
I
120
o (deg) Fig. 3. Critical current density jo (O) in high magnetic field.
YBa2Cu3OT_ a film. These measurements of the angular dependence of the critical current density were made in high magnetic fields and at a temperature of 77 K. As criterion for the critical current density a voltage of lbtVcm -i was chosen. At O = 0 ° the e axis of the YBa2Cu3OT_a film is parallel to the magnetic field B and at O = 90 ° the c axis is perpendicular to the magnetic rid& Due to the layered structure of this superconductor the critical current density jo(O) reaches its maximum at O = 90 o, when the magnetic field is parallel to the CuO2 planes I-4]. This indicates the strong pinning at this angle, where the flux lines are located by extrinsic pinning centres. The minimum ofjo(O) appears for the magnetic field being parallel to the e axis of the film (O=0 °). The fits of the data were made using the function jo(O)cc(cos2 0 + e 2 sin 2 O) -~, For this fit the effective magnetic field is reduced to Br~a= B(0 °)(cos2 0 + e2 sin 2 O) * with e being the anisotropy parameter [5]. For ~2=0.008 at 2T and 52=0.006 at 6 T this function fits the data quite well. The small value for e, which decreases with increasing magnetic field, shows the very anisotropic behaviour of the YBa2Cu3OT-a film in high magnetic fields. Besides measurements on YBazCuaOT_a thin films, sputtered BizSr/CaCuaOs +.,, films were investigated too. Results show that the maxima (see also [6]) of the critical current density sharpen compared with the results for YBa2CuaOT-o films. This is related to the stronger anistropy of the layered structure of these films. For the nearly two-dimensional behaviour of this superconductor in high magnetic fields the function jo(O)oclcos20[ -~ is best fitting [7]. However, the function used for the YBa2CuaOT_~ results with a very small anisotropy parameter also fits the Bi2Sr2CaCu2Os+.,: measurements quite well.
4. Mass spectroscopic measurements To our knowledge so far there have been no reports about in situ mass spectroscopic identification of metal species in d.c. sputtering processes. Attempts to investigate the gas phase during laser ablation [8-10] or evaporation [11,12] can be useful, however, to compare any results. During laser ablation singly charged metal ions (Cu, Y, Ba) and the respective oxygen compounds are detected. With evaporation processes only the oxygen content is measured mostly in order to control the deposition rate and the oxygen content of the gas phase. In order to investigate directly the influence of deposition parameters on film properties, first attempts were made to monitor the gas phase composition in situ. Those measurements showed that the ratio of oxygen to metal atoms is larger than 106 : 1. As the ratio of oxygen to metal atoms in the target as well as in the deposited
A. Geerkens et al./Smfaee and Coatings Technology 74-75 (1995) 1038-1042
layers is approximately equal, this would either indicate that the sticking coefticient of the oxygen is lower by an appropriate amount or that, in spite of the assumption made earlier, there is still significant contribution of resputtering of the film by impinging oxygen atoms or radicals. In spite of the measurements mentioned above, however, we did not manage to measure any signal relating to the emission of any of the metals or metal oxides. Therefore and to verify the measurement during sputtering concerning the oxygen/metal ratio we evaporated YBa=Cu3OT_~ target material. The measurements in Fig. 4 show that upon heating the material a number of peaks emerge. As the background spectrum in Fig. 4 has been measured with the boat upon which the material rests already at the required temperature for about 60 min, it is ensured that in the spectrum measured when evaporating the material there is no contribution from any contamination or evaporation of material other than YBa=Cu30~_o. The material is then continuously fed onto the heated boat. It is evident from Fig. 4 that there are a number of compounds measured. Emissions from the metallic species and the related oxygen compounds are identified. This is in agreement with the literature. Nevertheless there are also a number of peaks that are still unidentified. As can also be seen from Fig. 4, there is no emission at mass 134-138, which is the range of mass of the barium isotopes. Presumably barium should be easy to measure as the vapour pressure of the material
14
Spectrum#2
t.,O tO) *-,
6
We have shown in our contribution that high pressure d.c. sputtering is a method to deposit superconducting thin films. With a.c. susceptibility measurements one can identify the layered growth of the films. Measurements of the gas phase composition of the sputter deposition show an unexpectedly high oxygen/metal ratio. Measurements of the evaporation of YBa=Cu3OT_~ target material yield unexpectedly rich spectra, where the most important metal and metal-oxygen species are identified. More experiments are needed in order to be able to interpret measurements of the gas phase composition of the sputter process.
Spectrum #1
CuO
E
tt
-
References
~"-.~ i
50
5. Concluding remarks
Some of this work has been conducted in close cooperation with the Institute of Physics, RWTH Aachen. We therefore are thankful for the help of F. Stellmach and M. Meven. This work has in part been financed by the German Ministry of Research and Technology under Contracts 2.I3A.6.A and 13N5487.
8
-2
is comparatively high. Also this is in contrast with laser ablation experiments where there is emission noted at the mass of barium. Several conclusions can be drawn from these experiments. First, the ratio of oxygen/metal species measured during sputtering represents the gas phase composition. The impact of this finding is that one has to reconsider the role of oxygen in this process. Experiments are currently under way to investigate this effect. Second, the number of unidentified peaks hints to the fact that one may have to look for additional compounds other than the metal or metal oxides in the gas phase. We have indications of at least one such species, and will report separately on the appearance of this.
Acknowledgements
10 v
1041
~ T I
60
70
~ I
80
i
Background [
~ 1 T I
r
spectrum ~ T ~ i
[
T I
T I
T
90 100 110 120 130 140 150 160 I70
Mass (amu) Fig. 4. Mass spectra measured during evaporation of YBazCu3Ov-~ target material: ..., measured background spectrum; - - - , the first spectrum measured during evaporation; - - . , measured at a later point in time. The selected mass range is 60-i60 masses.
[17 P.P. Vysheslavtsev, G.M. Genkin, Y.N. Nozdrin and A.V. Okomelkov, Nonequilibrium resistive response and memory during optical-excitation of YBazCu307-o high T~superconducting film, JETP Left., 52 (1990) 658. [21 J. Schubert, U. Poppe and W. Sybertz, Direct production and properties of sputtered epitaxiaI YBa2Cu307 thin and ultrathin films on (100) and (110) SrTiQ, J. Less-Common Met., 151 (1989) 277. [3"1 U. Poppe, J. Schubert and W. Evers, Preparation of thin YBazCu307 films at low temperatures, Physica C, 153 (1990) 776. [4] P.H. Kes, J. Aarts, V.M. Vinokur and C.J. van der Beek, Dissipation in highly anisotropic superconductors, Phys. Rev. Lett., 64 (1990) 1063.
1042
A. Geerkens et al./Swface and Coatings Technology 74-75 (1995) 1038-1042
[51 G. Blatter, V.B. Geshkenbein and A.I. Larkin, Susceptibility in superconducting material, Phys. Rev. Lett., 68 (1992) 1626. [6] A. Schmidt, F. Stellmach, M. Boekholt, S. Ewert, K.N.R. Taylor and D. Elefant, Thermal expansion and magnetostriction of high temperature superconductors, AppI. Supercond., I (1993) 1033. [71 M. Tachiki and S. Takahashi, Anisotropy of critical current in layered superconductors, Solid State Commun., 72 (1989) 1083. [8"1 C.H. Chen and R.C. Phillips, Desorption spectra of laser ablation of Bi2Sr2CaCuzOs+x, J. Appl. Phys., 70 (1991) 4643. [91 P.O. Artamonov and A.A. Lisachenko, The effect of the superconducting transition on the time-of-flight spectrum of oxygen laser-desorbed from single-crystal BieSrzCaCuaOs+x, Soy. Phys. Tech. Lett., 18 (1992) 671.
[i0] C. Champeaux, D. Damiani, J. Aubreton and A. Catherinot, Mass spectrometric investigation of the KrF laser-induced plasma plume created above an YBa2Cu307 superconducting target: correlation with thickness distribution of deposited thin films, Appl. S,erf. Sci., 69 (1993) 169. [11] J. Hudner, H. Ohlsen and M. Osfling, Coevaporated thin films of Y-Ba-Cu-O utilizing quadrupole mass-spectrometer rate control, J. Less-Common Met., I64 (1990) 42. [12] H. Ohlsen, M. Ottosson, J. Hudner, M. Ostling, L. Stolt, P. Nordblad, J.C. Villegier, H. Moriceau, F. Weiss and O. Thomas, In-situ preparation of Y-Ba-Cn-O thin films using mass-spectrometer rate control and atomic oxygen, in Layered Superconductors: Fabrication, Properties and Applications, April 1992, p. 299.
,GMTING8
ELSEVIER
Surfaceand CoatingsTechnology74-75 (1995) 1038-1042
ffoeHAIOLOM
High pressure d.c. sputtering of high superconducting films: properties and deposition conditions A. Geerkens, K. Fiilber, H.-J. Frenck Institute of Physics, Technische Universiti~t Cottbus, PO Box 101344, D-03013 Cottbus, Gel'many
Abstract We used high pressure d.c. sputtering to deposit superconducting thin films (YBa2Cu3OT-o (YBCO) and BiaSraCaCu2Os+x). With a.c. susceptibility measurements one can not only quickly characterize the superconducting properties of the sample, but also identify the layered growth of the films. Measurements of the gas phase composition of the sputter deposition show an unexpectedly high oxygen/metal ratio. The reason for this ratio is investigated. Measurements of the evaporation of YBCO target material yield spectra are presented, where the most important metal and metal-oxygen species are identified. Keywords: Thin films; Mass spectroscopy; Superconductors; YbaaCu3OT; Bi2Sr2CaCugOs
1. Introduction The availability of thin film technologies (deposition and structuring) is a key to commercial utilization of high T~ superconductors. While there has been a lot of effort and also a lot of success in preparing thin films with high onset temperatures and critical current densities using a variety of methods, comparatively little is known either about the exact mechanisms of film formation or the mechanism of superconductivity in thin films. We used high pressure d.c. sputtering to deposit YBa2Cu3OT-a and Bi2Sr2CaCuaOs+x as examples for a class of materials. While being a known method and simple to implement, d.c. sputtering has the advantage of providing the species impinging on the substrate and forming the film with sufficient energy to enable c-axis-oriented growth. This layered growth resulting in a film of more or less independent layers is known to be the basis of superconductivity in perovskite material. Yet it is not well known what causes the layered growth. We investigated the mechanism responsible for current transport. The behaviour of current flux lines and/or regions of weak or strong superconductivity are studied through the angular dependence of the magnetoresistance p and the critical current density jo. These can be measured using a.c. susceptibility with a special set-up allowing the tilting of the sample during ramping of the
investigated parameter. Among others, our results show that the magnetization of the sample in low magnetic fields depends not only on the external field, but also on the history of the sample (with optical excitation this is known as the memory effect [1]). As this is a contactless method it may also serve as a quick reference to overall film quality before the sample is processed further. A key to understanding of these processes will be to monitor the species in the gas phase and those impinging on the substrate surface. Based on the capability to monitor in situ the species that bombard and are eventually trapped in the film, one can assess directly a number of issues in thin film formation. Gas phase analysis will lead to more precise data on film formation and thus easier interpretation of the electrical data.
2. Experimental set-up The process termed high pressure d.c. sputtering is characterized by the d.c. discharge at about 30 W (200 V) and an oxygen atmosphere of typically 2-5 mbar. Typical substrate temperatures are between 750780 °C (Bi2Sr2CaCu2Os+.~ deposition) and 700-730 °C (YBa2Cu3OT_o deposition). No further annealing is done on the samples. This approach is thought of as advantageous [2,3] because it is simple to implement, inexpensive and scalable in the case of success. The mean free
A. GeerIcenset aL/Smface and Coatings Technology 74-75 (1995) 1038-7042
path of the species in the discharge in this pressure range is of the order of 10 tam. Thus species in the gas phase suffer a number of collisions, so that gas phase reactions will contribute to the production of species impinging on the surface. In turn, this will also lead to a reduction of resputtering by ionized oxygen atoms [2]. The high oxygen pressure counteracts possible loss of oxygen in the target resulting in changes of target stoichiometry. As it has been observed experimentally that the electrical quality of the deposited thin films strongly depends on the stoichiometry and the electrical properties of the target itself, there should either be some presputtering of the target with oxygen or O2 itself should be used as sputter gas. A schematic drawing of the inner part of the deposition reactor is given in Fig. 1. In principle it can be characterized as a conventional sputtering system. The target is bonded to a cooled copper plate approximately 2 cm above the substrate. The distance is chosen so that species impinging on the surface still have energy to contribute to alteration of the deposited layers. The sputtering gas is introduced sideways above the target and pumped directly below the substrate to ensure laminar flow between the electrodes and to minimize the amount of parasitic deposition. To the side of the reactor in Fig. 1 is the flange where the mass spectrometer for in situ investigations of the deposition process is fitted via a differentially pumped
1039
cell at the height the substrate is placed. The cell is just below the substrate and the quadrupole analyser is in line of sight with the extraction bore.
3. Properties of deposited films 3.1. Overall fihn properties The d.c. sputtered films with a thickness between 70 and 250 nm are highly oriented with the c axis perpendicular to the surface of the SrTiQ single-crystal substrate. As-deposited YBazCu3OT-o films show onset of superconductivity between 85 K .~"~o~r°nset-.~88 K with transition widths between 1 and 4 K. This is shown in Fig. 2 where a typical non-contact a.c. susceptibility measurement is presented. Typical critical current densities are of the order of 3 x 106 Acm -2 at 77K. The corresponding Bi2Sr2CaCu2Os+x films yield Tc°nset of 75K with transition widths of 1 - 4 K and Jc of 1 x 106 A cm -2. The angular dependence of the critical current density and the magnetoresistance were measured with a pulsed current method to avoid warming up of the bonded gold wire contacts on the structured film. Conduction paths of 20 gm width and 2 mm length were prepared by wet chemical etching. The sample holder allows variation of the angle between the magnetic field and
Feedthrough for substrate heating
02
02
cathode
cooling water
seal
et
chuck
sputtering pre-sputtering
sample / manipulator
! therm couple feedthrough
cooling pipes
~auue =~____
position
_~
~
~
~, pump line
Fig. 1. Schematic drawing of the d.c. sputtering reactor concept. The mass spectrometer may be connected to the reactor via flange.
A. Geerkenset al./Swface and Coatings Technology 74-75 (1995) 1038-1042
I040 I
2,0 4
7 &
,3
g
1,0
Te"
2
d...
0,5 I
.
.
.
86
.
.
.
,........... i.........
i
i
88 Temperature [K]
0,0
90
Fig. 2. ImaNnary part ( - - - ) and reaI part ( susceptibility of a deposited YBa2Cu3Ov-a thin film,
) of the a.c.
the c axis of the film (angle O) as well as the angle between the current direction and the O rotation plane. The temperature was varied between 4.2 and 77 K and magnetic fields between 10 mT and 8 T were used.
3.2. Angular dependence offilm properties To study the pinning of the high temperature superconductor films they were investigated by measuring the critical current density and the magnetoresistance dependent on the temperature and the strength and the angle of the magnetic field. Furthermore current-voltage curves were taken. The results for YBa2Cu3OT_~ and Bi2Sr2CaCu2Os+~ films are compared due to the anisotropy of the chosen superconductor. Fig. 3 shows some typical results for an investigated
4
YBa2Cu307-~
2~
3 it: (cos20+a2sin2e)la
tk Y = 77 K : 2 p o=oo
Ot
I
0
i ~
°°
2 Tesla
>..o
I
30
r
r
t
60
~
T
I
90
r
T
I
120
o (deg) Fig. 3. Critical current density jo (O) in high magnetic field.
YBa2Cu3OT_ a film. These measurements of the angular dependence of the critical current density were made in high magnetic fields and at a temperature of 77 K. As criterion for the critical current density a voltage of lbtVcm -i was chosen. At O = 0 ° the e axis of the YBa2Cu3OT_a film is parallel to the magnetic field B and at O = 90 ° the c axis is perpendicular to the magnetic rid& Due to the layered structure of this superconductor the critical current density jo(O) reaches its maximum at O = 90 o, when the magnetic field is parallel to the CuO2 planes I-4]. This indicates the strong pinning at this angle, where the flux lines are located by extrinsic pinning centres. The minimum ofjo(O) appears for the magnetic field being parallel to the e axis of the film (O=0 °). The fits of the data were made using the function jo(O)cc(cos2 0 + e 2 sin 2 O) -~, For this fit the effective magnetic field is reduced to Br~a= B(0 °)(cos2 0 + e2 sin 2 O) * with e being the anisotropy parameter [5]. For ~2=0.008 at 2T and 52=0.006 at 6 T this function fits the data quite well. The small value for e, which decreases with increasing magnetic field, shows the very anisotropic behaviour of the YBa2Cu3OT-a film in high magnetic fields. Besides measurements on YBazCuaOT_a thin films, sputtered BizSr/CaCuaOs +.,, films were investigated too. Results show that the maxima (see also [6]) of the critical current density sharpen compared with the results for YBa2CuaOT-o films. This is related to the stronger anistropy of the layered structure of these films. For the nearly two-dimensional behaviour of this superconductor in high magnetic fields the function jo(O)oclcos20[ -~ is best fitting [7]. However, the function used for the YBa2CuaOT_~ results with a very small anisotropy parameter also fits the Bi2Sr2CaCu2Os+.,: measurements quite well.
4. Mass spectroscopic measurements To our knowledge so far there have been no reports about in situ mass spectroscopic identification of metal species in d.c. sputtering processes. Attempts to investigate the gas phase during laser ablation [8-10] or evaporation [11,12] can be useful, however, to compare any results. During laser ablation singly charged metal ions (Cu, Y, Ba) and the respective oxygen compounds are detected. With evaporation processes only the oxygen content is measured mostly in order to control the deposition rate and the oxygen content of the gas phase. In order to investigate directly the influence of deposition parameters on film properties, first attempts were made to monitor the gas phase composition in situ. Those measurements showed that the ratio of oxygen to metal atoms is larger than 106 : 1. As the ratio of oxygen to metal atoms in the target as well as in the deposited
A. Geerkens et al./Smfaee and Coatings Technology 74-75 (1995) 1038-1042
layers is approximately equal, this would either indicate that the sticking coefticient of the oxygen is lower by an appropriate amount or that, in spite of the assumption made earlier, there is still significant contribution of resputtering of the film by impinging oxygen atoms or radicals. In spite of the measurements mentioned above, however, we did not manage to measure any signal relating to the emission of any of the metals or metal oxides. Therefore and to verify the measurement during sputtering concerning the oxygen/metal ratio we evaporated YBa=Cu3OT_~ target material. The measurements in Fig. 4 show that upon heating the material a number of peaks emerge. As the background spectrum in Fig. 4 has been measured with the boat upon which the material rests already at the required temperature for about 60 min, it is ensured that in the spectrum measured when evaporating the material there is no contribution from any contamination or evaporation of material other than YBa=Cu30~_o. The material is then continuously fed onto the heated boat. It is evident from Fig. 4 that there are a number of compounds measured. Emissions from the metallic species and the related oxygen compounds are identified. This is in agreement with the literature. Nevertheless there are also a number of peaks that are still unidentified. As can also be seen from Fig. 4, there is no emission at mass 134-138, which is the range of mass of the barium isotopes. Presumably barium should be easy to measure as the vapour pressure of the material
14
Spectrum#2
t.,O tO) *-,
6
We have shown in our contribution that high pressure d.c. sputtering is a method to deposit superconducting thin films. With a.c. susceptibility measurements one can identify the layered growth of the films. Measurements of the gas phase composition of the sputter deposition show an unexpectedly high oxygen/metal ratio. Measurements of the evaporation of YBa=Cu3OT_~ target material yield unexpectedly rich spectra, where the most important metal and metal-oxygen species are identified. More experiments are needed in order to be able to interpret measurements of the gas phase composition of the sputter process.
Spectrum #1
CuO
E
tt
-
References
~"-.~ i
50
5. Concluding remarks
Some of this work has been conducted in close cooperation with the Institute of Physics, RWTH Aachen. We therefore are thankful for the help of F. Stellmach and M. Meven. This work has in part been financed by the German Ministry of Research and Technology under Contracts 2.I3A.6.A and 13N5487.
8
-2
is comparatively high. Also this is in contrast with laser ablation experiments where there is emission noted at the mass of barium. Several conclusions can be drawn from these experiments. First, the ratio of oxygen/metal species measured during sputtering represents the gas phase composition. The impact of this finding is that one has to reconsider the role of oxygen in this process. Experiments are currently under way to investigate this effect. Second, the number of unidentified peaks hints to the fact that one may have to look for additional compounds other than the metal or metal oxides in the gas phase. We have indications of at least one such species, and will report separately on the appearance of this.
Acknowledgements
10 v
1041
~ T I
60
70
~ I
80
i
Background [
~ 1 T I
r
spectrum ~ T ~ i
[
T I
T I
T
90 100 110 120 130 140 150 160 I70
Mass (amu) Fig. 4. Mass spectra measured during evaporation of YBazCu3Ov-~ target material: ..., measured background spectrum; - - - , the first spectrum measured during evaporation; - - . , measured at a later point in time. The selected mass range is 60-i60 masses.
[17 P.P. Vysheslavtsev, G.M. Genkin, Y.N. Nozdrin and A.V. Okomelkov, Nonequilibrium resistive response and memory during optical-excitation of YBazCu307-o high T~superconducting film, JETP Left., 52 (1990) 658. [21 J. Schubert, U. Poppe and W. Sybertz, Direct production and properties of sputtered epitaxiaI YBa2Cu307 thin and ultrathin films on (100) and (110) SrTiQ, J. Less-Common Met., 151 (1989) 277. [3"1 U. Poppe, J. Schubert and W. Evers, Preparation of thin YBazCu307 films at low temperatures, Physica C, 153 (1990) 776. [4] P.H. Kes, J. Aarts, V.M. Vinokur and C.J. van der Beek, Dissipation in highly anisotropic superconductors, Phys. Rev. Lett., 64 (1990) 1063.
1042
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