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Oxygen ion etching of YBa2Cu3O7−y
Nuclear Instruments North-Holland
and Methods
in Physics Research
B59/60
(1991) 1415-1417
1415
Oxygen ion etching of YBa,Cu,O,_, Yoshitake Department
Nishi, Yasuo Takahashi, of Materials
Seiichiro
Takagi,
Science, Tokal University, I II 7 Kitakaname.
Toshio
Hiratsukq
Shima and Noriyuki
Kanagawa
2XW
Inoue
Japan
The oxygen ion etching rate (R,) is investigated for the YBa@,O,_, system. A higher etching rate is observed for oxygen than for argon ions. An increase of T, with etching is found. Furthermore, excess etching does not decrease T, below the T, value before oxygen etching, although argon ion etching decreases T, below 4.2 K. Thus, we conclude that oxygen ion etching is useful.
1. Introduction Argon ion etching of brittle YBa,Cu,O,_, has been investigated and a high rate of etching was found [l]. Furthermore, the influence of the argon etching on T, has been studied for a short period of ion etching 121. We found that the argon etching slightly increased T, for short etching times. On the other hand, the oxygenenriched YBazCu30,_, shows a high T, [3]. If the etching and enrichment are performed simultaneously, the oxide may increase T,. Thus, we have started the present study to investigate the influence of oxygen etching on T,.
2. Experimental Samples with nominal composition YBa,Cu ,O, _y are prepared by high-purity CuO (99.9%), BaCO, (99.9%) and Y,O, (99.99%). The powders are mixed and reacted at 1210 K for 2 h and then air-cooled under pure oxygen. After crushing, a pelletized tablet, 0.8 mm thick and 13 mm diameter, is sintered at 1210 K for 8 h and furnace-cooled under pure oxygen. The cooling rate is 4.2 K/mm at 973 K. The ion etching is performed under 1.6 x 10e4 Torr oxgyen atmosphere just above the specimen by a Kaufman-type ion milling apparatus (ISM-S, Elionix, Tokyo), which is shown schematically in fig. 1. The ion beam etching system can be broken down into three major sections: a plasma source which generates the ions, the extraction grids which extract the ions from the plasma source and accelerate them towards the sample, and the sample holder. Kaufman sources of this type are operated with diameters of 70 mm for the use in ion etching stations [4]. The energy for the ion acceleration and the current density are 1.0 keV and 0.6 mA/cm*, respectively. The ion beam was perpendicular to the sample surface. The etching rate, R,, was measured with a 0168-583X/91/$03.50
0 1991 - Elsevier Science
Publishers
micrometer and a mass balance, whose resolutions are below 1 pm and 0.1 mg, respectively. T, is measured using a standard four-probe technique and a Keithley 181 nanovoltmeter. The temperature is measured by a Au* Fe-chrome1 thermocouple attached to the specimen in a cryostat. The structures of the samples are examined by means of X-ray diffraction (Cu Ka).
3. Results and discussions 3.1. Ion etching
rate (R,)
The solid, dotted and broken lines in fig. 2 show the depth of the etching (d) against the etching time (t) for the YBa2Cu,0,_ y system of the oxygen and the argon ion etching [l] and Si [5] etched by the argon, respectively. The etching increases d. R, is the slope of the d-t line in fig. 2. R, slightly depends on the oxgyen etching time (t). R, (0.93 rim/s)) of the ion etching by the oxygen is about 10% larger than that (0.8 rim/s)) by
SUMPLE HOLDER
Fig. 1. Schematic
B.V. (North-Holland)
diagram
of ion milling apparatus.
XI. HIGH-T
SUPERCONDUCTORS
Y. Nishi et al.
1416
0 ion etching of YBaZCu30,
0 0
1
2 etching
3 4 5 timex10m3(s) Fig. 2. Etching depth (d) vs etching time ( te); solid, dotted and broken lines are for oxygen etching, argon etching [l] of the YBa,Cu ,O, _ )’ system and Si [S], respectively.
the argon and about four times as large as that of Si by the argon. This shows that the oxygen ion etching is useful for the YBa@@_, system, because it is rapidly and easy to form without fracture. The behavior of ions, with acceleration energies ranging from 10 eV to 10 keV, on the metallic crystal was deduced by Ishitani and Shimizu [6]. Except for backscattering argon ions, most of the argon ions lose their kinetic energy and become implanted atoms. The kinetic energy gained by an atom is larger than that of the atom bond energy, and therefore, knockon cascades are formed [7]. If the knockon cascades happen at the sample surface and the kinetic energy of the ion (atom) is greater than the bonding energy. the surface atoms are ejected from the sample surface. This phenomenon is called sputtering. The mass and atomic radius of the argon are two or three times as large as that of the oxygen, so these values do not agree with the R, model. Thus, we suggest that the oxygen ions remove the metals (Y. Ba. Cu) from the YBa,Cu@_, surface more readily than the argon ions, because the attractive force between the oxygen and the metals (Y, Ba, Cu) is extremely larger than that between the argon and the metals. 3.2. Influence
of ion etching
_ I‘
90
92
94
96
98
T (K) Fig. 3. Changes in electrical resistivity (R) with temperature (T) of YBa,Cu a0, I’systems etching before and after etching (1, = 1000 s).
94.2 K. Namely, the sputtering increases the superconducting transition temperature. The solid and broken lines in fig. 4 show changes in T, differences (AT,) against etching time (t,) for the YBa,Cu,O, _y system of the oxygen and argon ion etching, respectively (at an ion acceleration energy of 1 keV). For oxygen ion etching, AT, increases with time at 1000 s. The maximum AT, is 1.6 K until 1000 s. It is smaller than that by the argon etching. However, the oxygen etching does not decrease the T, value below the original value without etching, while the argon etching at 1000 s decreases T, down to 4.2 K. This shows that the oxygen etching is useful. Fig. 5 shows X-ray diffraction patterns of the samples before and after the etching (t, = 0, 1000 and 5000 s). Most peaks can be identified as orthorhombic structures, which correspond to oxygen-deficient perovskite. The short period of etching increases the angle value of the peak (see peaks for t, = 0 and 1000 s in fig. 5). The higher the angle, the smaller the lattice constant be-
on T,.
Fig. 3 shows the temperature dependence of electrical resistivity of the YBa,Cu -,O,_ v system before and after etching (t, = 0 and 1000 s). The lower the temperature, the lower the electrical resistivity becomes. Onset corresponds to the deviation of the electrical resistivity. The deviated point is taken at dR/dT = O.O3R,, K in the present work, where d R/dT and R,, K are the slope of R-T and resistivity at 300 K, respectively. The midpoint of the transition is designated T,. T, is 92.6 K before the etching. After the etching for 1000 s, T, is
_I --+--Aretching I
102
I I I
I
b
3
I lo4
te A0 Fig. 4. Change in midpoint temperature difference (AT) against etching time (t,). Solid and broken lines are for oxygen and argon [2] etching, respectively
Y. Nishi et al. / 0 ion etching of YBa,Cu30,_
n
1417
4. Conclusion
after etching te =5ooos
after
,
In summary, the oxygen ion etching rate (R,) and the influence of ion etching on T, are studied for the YBazCu 307__V system. R, of the oxygen ion etching (0.93 rim/s)) is larger than that of the argon ion etching (0.8 rim/s)) for the YBa,Cu @_ y system. T, increases for short etching times and does. not decrease for long etching times down to the T, value without etching. The ion etching is performed by a tungsten filament cathode. The resistance to the oxidation is not so high. Therefore, the Takayama group in Tokai university in Japan has developed an ion source with a plasma cathode to attain a long lifetime for the oxygen ion production [ 111. If the ion source is applied, the oxygen ion etching may be a good tool to etch the high-T, YBa,Cu,O,_ ,, system commercially.
etchinq
before etching T, = 92.6K
References
I
1
47 40 28 (deg) Fig. 5. X-ray diffraction patterns of YBa $21 jO, _ ,, etching for 0,lCOO and 5000 s.
comes. The lattice constant depends on the compressive stress [8] and the oxygen concentration [3]. Since both increase T, [9,10], the T, increase in fig. 3 is explained. We conclude that the high T, which is induced by the oxygen etching, is caused by the formation of the oxygen concentrated phase with the compressive stress. After excess oxygen etching (t, = 5000 s) T, approaches the K value without etching. Excess oxygen etching decreases the angle (see 1, = 1000 and 5000 s in fig. 5). It approaches to the mean angle before etching (see fig. 5). The X-ray results agree with the T, value.
[l] Y. Nishi, S. Moriya, N. Inoue, S. Tokunaga and T. Shima. J. Mater. Sci. Lett. 7 (1988) 281. (21 Y. Nishi. S. Moriya, N. Inoue, S. Tokunaga and T. Shima, J. Mater. Sci. Lett. 7 (1988) 997. [3] A. Ono and Y. Ishizawa, Jpn. J. Appl. Phys. 26 (1987) 1043. [4] P.G. Gloerson, J. Vat. Sci. Technol. 12 (1975) 28. [5] S. Hosaka, I. Kawamata and A. Hashimoto. Shinku 18 (1975) 384. [6] T. Ishitani and R. Shimizu, Phys. Lett. 46 (1974) 487. [7] Y. Nishi, M. Tachi. T. Kai and T. Ono. Script. Metall. 19 (1985) 1367. [S] S. Yomo. C. Murayama, W. Utsumi, H. Takahashi, T. Yagi. N. Mori, T. Tamegai, A. Watanabe and Y. lye, Jpn. J. Appl. Phys. 26 (1987) 1107. [9] Y. Nishi, N. Ninomiya and S. Tokunaga. J. Mater. Sci. Lett. 7 (1988) 361. [lo] E. Takayama-Muramachi. Y. Uchida, M. Ishii. T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) 1156. [ll] T. Shiono, T. Shibuya. Y. Harano, E. Yabe and K. Takayama, Nucl. Instr. and Meth. B37/38 (1989) 166.
Xl. HIGH-T
SUPERCONDUCTORS
and Methods
in Physics Research
B59/60
(1991) 1415-1417
1415
Oxygen ion etching of YBa,Cu,O,_, Yoshitake Department
Nishi, Yasuo Takahashi, of Materials
Seiichiro
Takagi,
Science, Tokal University, I II 7 Kitakaname.
Toshio
Hiratsukq
Shima and Noriyuki
Kanagawa
2XW
Inoue
Japan
The oxygen ion etching rate (R,) is investigated for the YBa@,O,_, system. A higher etching rate is observed for oxygen than for argon ions. An increase of T, with etching is found. Furthermore, excess etching does not decrease T, below the T, value before oxygen etching, although argon ion etching decreases T, below 4.2 K. Thus, we conclude that oxygen ion etching is useful.
1. Introduction Argon ion etching of brittle YBa,Cu,O,_, has been investigated and a high rate of etching was found [l]. Furthermore, the influence of the argon etching on T, has been studied for a short period of ion etching 121. We found that the argon etching slightly increased T, for short etching times. On the other hand, the oxygenenriched YBazCu30,_, shows a high T, [3]. If the etching and enrichment are performed simultaneously, the oxide may increase T,. Thus, we have started the present study to investigate the influence of oxygen etching on T,.
2. Experimental Samples with nominal composition YBa,Cu ,O, _y are prepared by high-purity CuO (99.9%), BaCO, (99.9%) and Y,O, (99.99%). The powders are mixed and reacted at 1210 K for 2 h and then air-cooled under pure oxygen. After crushing, a pelletized tablet, 0.8 mm thick and 13 mm diameter, is sintered at 1210 K for 8 h and furnace-cooled under pure oxygen. The cooling rate is 4.2 K/mm at 973 K. The ion etching is performed under 1.6 x 10e4 Torr oxgyen atmosphere just above the specimen by a Kaufman-type ion milling apparatus (ISM-S, Elionix, Tokyo), which is shown schematically in fig. 1. The ion beam etching system can be broken down into three major sections: a plasma source which generates the ions, the extraction grids which extract the ions from the plasma source and accelerate them towards the sample, and the sample holder. Kaufman sources of this type are operated with diameters of 70 mm for the use in ion etching stations [4]. The energy for the ion acceleration and the current density are 1.0 keV and 0.6 mA/cm*, respectively. The ion beam was perpendicular to the sample surface. The etching rate, R,, was measured with a 0168-583X/91/$03.50
0 1991 - Elsevier Science
Publishers
micrometer and a mass balance, whose resolutions are below 1 pm and 0.1 mg, respectively. T, is measured using a standard four-probe technique and a Keithley 181 nanovoltmeter. The temperature is measured by a Au* Fe-chrome1 thermocouple attached to the specimen in a cryostat. The structures of the samples are examined by means of X-ray diffraction (Cu Ka).
3. Results and discussions 3.1. Ion etching
rate (R,)
The solid, dotted and broken lines in fig. 2 show the depth of the etching (d) against the etching time (t) for the YBa2Cu,0,_ y system of the oxygen and the argon ion etching [l] and Si [5] etched by the argon, respectively. The etching increases d. R, is the slope of the d-t line in fig. 2. R, slightly depends on the oxgyen etching time (t). R, (0.93 rim/s)) of the ion etching by the oxygen is about 10% larger than that (0.8 rim/s)) by
SUMPLE HOLDER
Fig. 1. Schematic
B.V. (North-Holland)
diagram
of ion milling apparatus.
XI. HIGH-T
SUPERCONDUCTORS
Y. Nishi et al.
1416
0 ion etching of YBaZCu30,
0 0
1
2 etching
3 4 5 timex10m3(s) Fig. 2. Etching depth (d) vs etching time ( te); solid, dotted and broken lines are for oxygen etching, argon etching [l] of the YBa,Cu ,O, _ )’ system and Si [S], respectively.
the argon and about four times as large as that of Si by the argon. This shows that the oxygen ion etching is useful for the YBa@@_, system, because it is rapidly and easy to form without fracture. The behavior of ions, with acceleration energies ranging from 10 eV to 10 keV, on the metallic crystal was deduced by Ishitani and Shimizu [6]. Except for backscattering argon ions, most of the argon ions lose their kinetic energy and become implanted atoms. The kinetic energy gained by an atom is larger than that of the atom bond energy, and therefore, knockon cascades are formed [7]. If the knockon cascades happen at the sample surface and the kinetic energy of the ion (atom) is greater than the bonding energy. the surface atoms are ejected from the sample surface. This phenomenon is called sputtering. The mass and atomic radius of the argon are two or three times as large as that of the oxygen, so these values do not agree with the R, model. Thus, we suggest that the oxygen ions remove the metals (Y. Ba. Cu) from the YBa,Cu@_, surface more readily than the argon ions, because the attractive force between the oxygen and the metals (Y, Ba, Cu) is extremely larger than that between the argon and the metals. 3.2. Influence
of ion etching
_ I‘
90
92
94
96
98
T (K) Fig. 3. Changes in electrical resistivity (R) with temperature (T) of YBa,Cu a0, I’systems etching before and after etching (1, = 1000 s).
94.2 K. Namely, the sputtering increases the superconducting transition temperature. The solid and broken lines in fig. 4 show changes in T, differences (AT,) against etching time (t,) for the YBa,Cu,O, _y system of the oxygen and argon ion etching, respectively (at an ion acceleration energy of 1 keV). For oxygen ion etching, AT, increases with time at 1000 s. The maximum AT, is 1.6 K until 1000 s. It is smaller than that by the argon etching. However, the oxygen etching does not decrease the T, value below the original value without etching, while the argon etching at 1000 s decreases T, down to 4.2 K. This shows that the oxygen etching is useful. Fig. 5 shows X-ray diffraction patterns of the samples before and after the etching (t, = 0, 1000 and 5000 s). Most peaks can be identified as orthorhombic structures, which correspond to oxygen-deficient perovskite. The short period of etching increases the angle value of the peak (see peaks for t, = 0 and 1000 s in fig. 5). The higher the angle, the smaller the lattice constant be-
on T,.
Fig. 3 shows the temperature dependence of electrical resistivity of the YBa,Cu -,O,_ v system before and after etching (t, = 0 and 1000 s). The lower the temperature, the lower the electrical resistivity becomes. Onset corresponds to the deviation of the electrical resistivity. The deviated point is taken at dR/dT = O.O3R,, K in the present work, where d R/dT and R,, K are the slope of R-T and resistivity at 300 K, respectively. The midpoint of the transition is designated T,. T, is 92.6 K before the etching. After the etching for 1000 s, T, is
_I --+--Aretching I
102
I I I
I
b
3
I lo4
te A0 Fig. 4. Change in midpoint temperature difference (AT) against etching time (t,). Solid and broken lines are for oxygen and argon [2] etching, respectively
Y. Nishi et al. / 0 ion etching of YBa,Cu30,_
n
1417
4. Conclusion
after etching te =5ooos
after
,
In summary, the oxygen ion etching rate (R,) and the influence of ion etching on T, are studied for the YBazCu 307__V system. R, of the oxygen ion etching (0.93 rim/s)) is larger than that of the argon ion etching (0.8 rim/s)) for the YBa,Cu @_ y system. T, increases for short etching times and does. not decrease for long etching times down to the T, value without etching. The ion etching is performed by a tungsten filament cathode. The resistance to the oxidation is not so high. Therefore, the Takayama group in Tokai university in Japan has developed an ion source with a plasma cathode to attain a long lifetime for the oxygen ion production [ 111. If the ion source is applied, the oxygen ion etching may be a good tool to etch the high-T, YBa,Cu,O,_ ,, system commercially.
etchinq
before etching T, = 92.6K
References
I
1
47 40 28 (deg) Fig. 5. X-ray diffraction patterns of YBa $21 jO, _ ,, etching for 0,lCOO and 5000 s.
comes. The lattice constant depends on the compressive stress [8] and the oxygen concentration [3]. Since both increase T, [9,10], the T, increase in fig. 3 is explained. We conclude that the high T, which is induced by the oxygen etching, is caused by the formation of the oxygen concentrated phase with the compressive stress. After excess oxygen etching (t, = 5000 s) T, approaches the K value without etching. Excess oxygen etching decreases the angle (see 1, = 1000 and 5000 s in fig. 5). It approaches to the mean angle before etching (see fig. 5). The X-ray results agree with the T, value.
[l] Y. Nishi, S. Moriya, N. Inoue, S. Tokunaga and T. Shima. J. Mater. Sci. Lett. 7 (1988) 281. (21 Y. Nishi. S. Moriya, N. Inoue, S. Tokunaga and T. Shima, J. Mater. Sci. Lett. 7 (1988) 997. [3] A. Ono and Y. Ishizawa, Jpn. J. Appl. Phys. 26 (1987) 1043. [4] P.G. Gloerson, J. Vat. Sci. Technol. 12 (1975) 28. [5] S. Hosaka, I. Kawamata and A. Hashimoto. Shinku 18 (1975) 384. [6] T. Ishitani and R. Shimizu, Phys. Lett. 46 (1974) 487. [7] Y. Nishi, M. Tachi. T. Kai and T. Ono. Script. Metall. 19 (1985) 1367. [S] S. Yomo. C. Murayama, W. Utsumi, H. Takahashi, T. Yagi. N. Mori, T. Tamegai, A. Watanabe and Y. lye, Jpn. J. Appl. Phys. 26 (1987) 1107. [9] Y. Nishi, N. Ninomiya and S. Tokunaga. J. Mater. Sci. Lett. 7 (1988) 361. [lo] E. Takayama-Muramachi. Y. Uchida, M. Ishii. T. Tanaka and K. Kato, Jpn. J. Appl. Phys. 26 (1987) 1156. [ll] T. Shiono, T. Shibuya. Y. Harano, E. Yabe and K. Takayama, Nucl. Instr. and Meth. B37/38 (1989) 166.
Xl. HIGH-T
SUPERCONDUCTORS