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Optimization results for YBaCuO film sputtering on polycrystalline zirconia substrates with various growth and oxygenation conditions
Physica C 235-240 (1994)669-670 North-Holland
PUIS[CA
Optimization results for YBaCuO film sputtering on polycrystalline zirconia substrates with various growth and oxygenation conditions* A. Degardin a, E. Caristan a, T. Pech a, Z. Ben Ayadi a, J-C. Martin a, A. Kreislera and C. Dolin b aLGEP, Universitrs Paris 6 et Paris 11, URA 127 CNRS, SUPELEC, Plateau de Moulon, 91192 Gif sur Yvette CEDEX, FRANCE bLPM, CNRS-Bellevue, 1 Place Aristide Briand, 92195 Meudon CEDEX, FRANCE In situ elaboration of YBaCuO films sputtered on polycrystalline YSZ was studied thanks to a model of reactive sputtering and the in situ measurements of the film resistance during various oxygenation processes.
1. I N T R O D U C T I O N One of the most successful methods for the elaboration of YBaCuO thin films is in situ rf magnetron sputtering which consists of two steps: - a deposition phase at Tmax = 600-800 °C substrate temperature; an oxygenation phase during cooling down. To identify the pertinent parameters governing this elaboration technique, an optimization study involving several aspects has been undertaken. The first aspect is related to reactive sputtering in an Ar+O2+H20 vapor atmosphere. To optimize this process, a refined model, taking into account the various mechanisms of oxygen exchange between the target and its environment has been established. The second aspect concerns the film resistance monitoring during the oxygenation phase which helps to understand the oxygen exchange between the film and the substrate and to optimize oxygenation parameters (02 pressure, temperature profile, etc.). M o r e o v e r as yttria stabilized polycrystalline zirconia substrates (YSZ) with 8 mol. % Y203 were used, this monitoring was rendered delicate by the ionic conductivity of the substrate due to a high oxygen vacancy generation rate. These two points are considered first; film morphology and superconducting characteristics are then briefly presented. 2. R E A C T I V E S P U T T E R I N G M O D E L [1] Numerous experiments were performed with varied choices of parameters (such as gas partial pressures, gas fluxes, rf power...).
Two different types of sputtering regimes have been established: - an instable one, in which there is a continuous oxygen depletion of the target; - a stable one, where the oxygen flux impinging onto the target surface is capable to maintain an outgoing flux preventing the oxygen depletion of the target. Moreover it was shown that the dc bias voltage is a pertinent parameter to describe the plasma/target interaction. Besides, the favorable effect of H 2 0 vapor (in the plasma mixture) on the film quality was confirmed. 3. I N SITU M E A S U R E M E N T S
This study is an extension of previous in situ electrical characterizations of low temperature sputtered YBaCuO thin films during a rapid thermal annealing (RTA) process [2]. 3.1.
Films
One example o f in situ e l e c t r i c a l characterization is shown in fig. 1. We observe a sharp resistance drop as oxygen is introduced into the reaction chamber (tetragonal to orthorhombic transformation) and a metallic behaviour during cooling down. We have to stress that extreme care should be taken while interpreting this plot. In fact, the high temperature resistance data are strongly influenced by the very large ionic conductivity values characterizing the YSZ substrates above 600 °C. Under these conditions the observed resistance values should result from a parallel combination of
* Study supported by the Indo-French Centre for the Promotion of Advanced Research (CEFIPRA), project # 808-2. 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00895-7
670
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0
___,
Ih, t~ardin cl a/
~
P/n,w~a (" 255 240 (t9!).1~ 6h9 67(t
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Sample
1"32
300
8()0
6()0~
.~ 2oo
,
8
~
4oo
",
2oo
', I o0
~
o
()
"<~/
~5o lO]l~
\
The superconducting properties of the fihns wcrc measured using the four point method. Values of Tc(R = 01 .-: 85 K and Jc 3000 A/cm 2 (at 77 K. H = () and 5 btV/cm) were obtained. This value for Jc is lhnited by the granular nature of the substrate.
-k
~
1
"0
0.25 0.5 0.75 I 1.25 1.5 1.75 Time (h)
Figure l. Sample resistance and temperature hehaviour during oxygenation (after sputtering at Tma x = 745 °C and p(Ar+O2+H20) = 0.3 torrL both the fihn and substrate resistances. 3.2. S u b s t r a t e s
I? 2 4 ¢= :-: ~,yn 1L-I Figure 2. Morphology ol sample # T33 (deposiled al Tma x = 745 ' C and p(Ar+O2+H20) = 0.3 torr).
The resistance behaviour of a bare subslralc undergoing a temperature cycle silnilar to that used during an YBaCuO fihn elaboration has been fitted according to the relationship R = R ~ exp(T0/T). In lhe heating up phase, we find T 0 = 7 K in fair agreement with O 2- ion diffusion. For the cooling d o w n phase, we find T 0 = 2 . 7 K for high temperatures, and T0=0.4 K for lower temperatures. in line with hole conductivity resulting from oxygen vacancies created by the vacuum treallnent.
mill
60 nm
1
30 nln
(} 1111~
i
O()
2
q(
75
I()() tim
Figure 3. AFM morphology of a polycrystallinc YSZ substrate doped with 8 tool. 9~ Y203 .
4. R E S U L T S 5. C O N C L U S I O N
The surface morphology was investigated t, sing AFM. For a fihn elaborated under non optimized conditions we observe a rapid growth (2500 i / h ) in the shapoe o f pyramids, hence a rough film (= 2500 A rms). On the contrary, a fihn obtained during the stable regime of sputtering (fig. 2) has a slower growth (1750 A/h) and is much smoother (200-250 A rms). We notice too that its surface has reproduced the granular state of the subslrate (fig. 3). SIMS analysis shows a composition Y:Ba:Cu which is stable (to within 2 %) in the YBaCuO layer. We note also a diffusion of the Zr element in the Y B a C u O film, which is due to the high temperature deposition of long duration. The X-ray diffraction technique shows that our films have highly predominant c-axis orientation. The value of the c lattice parameter (11.68 A) indicates a good oxygenation rate of the film.
We have shown the validity of the sputtering model and the resistance monitoring technique for optimization of the elaboration parameters, even if the resistance variation is influenced by the ionic conduction processes occuring in the substrate at high temperature. In order to obtain high Jc polycrystalline fihns. an in-plane texturing of the substrate is under investigation. REFERENCES
1 2
T. Pech, E. Caristan and A. Kreisler, sublnittcd lo J. Vac. Sci, Technol. t 7. Hosseini Teherani, E. Caristan, J-M. Depond, T. Pech, F. Carri~ and A. Kreisler, J. of Alloys and Compounds 19S, 21 I, (1993),
PUIS[CA
Optimization results for YBaCuO film sputtering on polycrystalline zirconia substrates with various growth and oxygenation conditions* A. Degardin a, E. Caristan a, T. Pech a, Z. Ben Ayadi a, J-C. Martin a, A. Kreislera and C. Dolin b aLGEP, Universitrs Paris 6 et Paris 11, URA 127 CNRS, SUPELEC, Plateau de Moulon, 91192 Gif sur Yvette CEDEX, FRANCE bLPM, CNRS-Bellevue, 1 Place Aristide Briand, 92195 Meudon CEDEX, FRANCE In situ elaboration of YBaCuO films sputtered on polycrystalline YSZ was studied thanks to a model of reactive sputtering and the in situ measurements of the film resistance during various oxygenation processes.
1. I N T R O D U C T I O N One of the most successful methods for the elaboration of YBaCuO thin films is in situ rf magnetron sputtering which consists of two steps: - a deposition phase at Tmax = 600-800 °C substrate temperature; an oxygenation phase during cooling down. To identify the pertinent parameters governing this elaboration technique, an optimization study involving several aspects has been undertaken. The first aspect is related to reactive sputtering in an Ar+O2+H20 vapor atmosphere. To optimize this process, a refined model, taking into account the various mechanisms of oxygen exchange between the target and its environment has been established. The second aspect concerns the film resistance monitoring during the oxygenation phase which helps to understand the oxygen exchange between the film and the substrate and to optimize oxygenation parameters (02 pressure, temperature profile, etc.). M o r e o v e r as yttria stabilized polycrystalline zirconia substrates (YSZ) with 8 mol. % Y203 were used, this monitoring was rendered delicate by the ionic conductivity of the substrate due to a high oxygen vacancy generation rate. These two points are considered first; film morphology and superconducting characteristics are then briefly presented. 2. R E A C T I V E S P U T T E R I N G M O D E L [1] Numerous experiments were performed with varied choices of parameters (such as gas partial pressures, gas fluxes, rf power...).
Two different types of sputtering regimes have been established: - an instable one, in which there is a continuous oxygen depletion of the target; - a stable one, where the oxygen flux impinging onto the target surface is capable to maintain an outgoing flux preventing the oxygen depletion of the target. Moreover it was shown that the dc bias voltage is a pertinent parameter to describe the plasma/target interaction. Besides, the favorable effect of H 2 0 vapor (in the plasma mixture) on the film quality was confirmed. 3. I N SITU M E A S U R E M E N T S
This study is an extension of previous in situ electrical characterizations of low temperature sputtered YBaCuO thin films during a rapid thermal annealing (RTA) process [2]. 3.1.
Films
One example o f in situ e l e c t r i c a l characterization is shown in fig. 1. We observe a sharp resistance drop as oxygen is introduced into the reaction chamber (tetragonal to orthorhombic transformation) and a metallic behaviour during cooling down. We have to stress that extreme care should be taken while interpreting this plot. In fact, the high temperature resistance data are strongly influenced by the very large ionic conductivity values characterizing the YSZ substrates above 600 °C. Under these conditions the observed resistance values should result from a parallel combination of
* Study supported by the Indo-French Centre for the Promotion of Advanced Research (CEFIPRA), project # 808-2. 0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00895-7
670
1
5
0
L 4()()~.
0
___,
Ih, t~ardin cl a/
~
P/n,w~a (" 255 240 (t9!).1~ 6h9 67(t
10Or)
Sample
1"32
300
8()0
6()0~
.~ 2oo
,
8
~
4oo
",
2oo
', I o0
~
o
()
"<~/
~5o lO]l~
\
The superconducting properties of the fihns wcrc measured using the four point method. Values of Tc(R = 01 .-: 85 K and Jc 3000 A/cm 2 (at 77 K. H = () and 5 btV/cm) were obtained. This value for Jc is lhnited by the granular nature of the substrate.
-k
~
1
"0
0.25 0.5 0.75 I 1.25 1.5 1.75 Time (h)
Figure l. Sample resistance and temperature hehaviour during oxygenation (after sputtering at Tma x = 745 °C and p(Ar+O2+H20) = 0.3 torrL both the fihn and substrate resistances. 3.2. S u b s t r a t e s
I? 2 4 ¢= :-: ~,yn 1L-I Figure 2. Morphology ol sample # T33 (deposiled al Tma x = 745 ' C and p(Ar+O2+H20) = 0.3 torr).
The resistance behaviour of a bare subslralc undergoing a temperature cycle silnilar to that used during an YBaCuO fihn elaboration has been fitted according to the relationship R = R ~ exp(T0/T). In lhe heating up phase, we find T 0 = 7 K in fair agreement with O 2- ion diffusion. For the cooling d o w n phase, we find T 0 = 2 . 7 K for high temperatures, and T0=0.4 K for lower temperatures. in line with hole conductivity resulting from oxygen vacancies created by the vacuum treallnent.
mill
60 nm
1
30 nln
(} 1111~
i
O()
2
q(
75
I()() tim
Figure 3. AFM morphology of a polycrystallinc YSZ substrate doped with 8 tool. 9~ Y203 .
4. R E S U L T S 5. C O N C L U S I O N
The surface morphology was investigated t, sing AFM. For a fihn elaborated under non optimized conditions we observe a rapid growth (2500 i / h ) in the shapoe o f pyramids, hence a rough film (= 2500 A rms). On the contrary, a fihn obtained during the stable regime of sputtering (fig. 2) has a slower growth (1750 A/h) and is much smoother (200-250 A rms). We notice too that its surface has reproduced the granular state of the subslrate (fig. 3). SIMS analysis shows a composition Y:Ba:Cu which is stable (to within 2 %) in the YBaCuO layer. We note also a diffusion of the Zr element in the Y B a C u O film, which is due to the high temperature deposition of long duration. The X-ray diffraction technique shows that our films have highly predominant c-axis orientation. The value of the c lattice parameter (11.68 A) indicates a good oxygenation rate of the film.
We have shown the validity of the sputtering model and the resistance monitoring technique for optimization of the elaboration parameters, even if the resistance variation is influenced by the ionic conduction processes occuring in the substrate at high temperature. In order to obtain high Jc polycrystalline fihns. an in-plane texturing of the substrate is under investigation. REFERENCES
1 2
T. Pech, E. Caristan and A. Kreisler, sublnittcd lo J. Vac. Sci, Technol. t 7. Hosseini Teherani, E. Caristan, J-M. Depond, T. Pech, F. Carri~ and A. Kreisler, J. of Alloys and Compounds 19S, 21 I, (1993),