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DC sputter deposition of YBa2Cu3O7 thin films for two sided coating
PWSICA B
Physica C 209 (1993) 369-372 North-Holland
DC sputter deposition of YBa2Cu307 thin films for two sided coating G. Koren PhysicsDepartment. Technion - Israel Instituteof Technology,Halfa, 32000, Israel Received 6 January 1993 Revised manuscript received 2 March 1993
A simple vacuum system for the deposition of YBa2Cu30, (YBCO) thin films by DC sputtering is described. This system enables one to perform YBCO film deposition on both sides of any appropriate wafer such as MgO for applications in passive microwave devices. The films are characterized by measurements of their X-ray diffraction, thickness uniformity and transport properties. These measurements indicate that ( 1) the films are perfectly c-axis oriented normal to the wafer, (2) thickness uniformity of 10% over an area of 10 mm diameter is obtained in the center of the film (from a 24 mm diameter target), (3) the critical temperature is T,(R =0) = 90 K and (4) the critical current density at liquid nitrogen temperature J,( 77 K) is over 2 X lo6 A/cm’.
Wafers for use in passive microwave devices must be coated with YBCO films on both sides because of the need for a superconducting ground plane in the basic circuits of the stripline variety. To obtain wafers coated on both sides by laser ablation deposition, a method that was developed already in 1988 [ 1 ] and yields excellent films, one obviously has to avoid the common technique of gluing the substrates to the heated block by silver paste. The gluing is necessary for obtaining good thermal contact with the heater and deposition is carried out at typically 780°C block temperature. Without gluing, it would be necessary to raise the heater temperature by an additional 150200°C which poses severe material problems, or alternatively, to work with an oven heater which is also quite complicated [ 2 1. It is well known that active oxygen, such as that produced in sputtering, enables the deposition of very good films at temperatures about 50- 100 ’ C lower than those needed in the laser process. This is especially true in systems where the oxygen plasma is in very close proximity to the deposited YBCO film. In the planar, high pressure DC sputtering geometry this requirement is fulfilled, and we therefore employed this method in the present study. DC sputtering has already been used successfully 0921-4534/93/$06.00
for the deposition of single sided YBCO films [ 3,4]. To the best of our knowledge, this method has not been previously used for two sided coating. In the present study, we demonstrate the capabilities and problems involved in using this method for two sided coating. In addition, we show that a small and simple vacuum system pumped only by a roughing pump, is sufficient for the DC sputter deposition of high quality YBCO films without the need for high vacuum systems. A cylindrical stainless steel chamber of 15 cm diameter and 15 cm height was adapted for the deposition of the YBCO films. A schematic drawing of the sputtering setup is given in fig. 1. The heater block has a planar shape with an elevated rim for better radiative heating also from the sides. Facing it is the YBCO sputtering target, which is a standard ceramic YBCO pellet of 24 mm diameter, home made and glued with silver paste to a cooled copper vessel. Oxygen flow onto the ti!m is achieved by forcing the flow around the target as shown in fig. 1, similar to the flow design of ref. [ 3 1. A flow of 1 l/min at a background O2 pressure of typically 4 Torr is maintained in the cell by continuous pumping with a small roughing pump. The substrates used in the present study are rectangular ( 100) MgO wafers of up to 25 x 25 mm’ area, optically polished on both
0 1993 Elsevier Science Publishers B.V. All rights reserved.
G. Koren /DC sputter deposition of YBCO thin films
370
DC SPUTTERING
SETUP
SUBSTRATE
BLOCK HEATER
Fig. 1. A schematic DC sputtering.
drawing
of the thin film deposition
system by
sides (Synergy Superconductive Technologies, Jerusalem ), and ( 100 ) SrTi03 wafers polished on one side (Lamprect, Neuhausen). They are simply placed on the heater and heated by radiation (mostly in the infrared), as well as by convection in the oxygen background pressure. The target is biased negatively at 230-260 V and a whitish plasma is formed whose light-emitting volume extends half-way from the cathode to the substrate. The fast ions are slowed down in the relatively high pressure of the discharge, the positive ions hit only the target and therefore no ion damage occurs to the film. The current density in the discharge is about 20-25 mA/cm’, the target to substrate distance is 13 mm and the block temperature during deposition is about 860-870°C. After deposition the cell is filled up with 0.7 atm of oxygen and the sample is cooled down with a half hour dwell time at 450°C for oxygen intake to assure a high T,. Under these conditions, typical deposition rates are 80-100 rim/h.. The resulting films are black and smooth but not free of particulates which are less than 500 nm in size. The morphology of the films and the parameters affecting it will be investigated in the future. Films of 300-600 nm thickness are generally prepared in 48 h runs. Because of the radiative heating, the film temperature changes continuously during the beginning of the deposition process even though the block
temperature is kept constant. This is due to the transparency of the MgO and SrTiO, substrates to infrared radiation of wavelengths shorter than N 67 urn [ 5 1. Once a black YBCO film has been grown on the substrate, the infrared absorption becomes constant, and the film continues to grow at a constant temperature. One can compensate for the lower temperature of the film during the first stage of the deposition process by driving the block temperature further up at this time. Since this was not done in the present study, the bottom layer of the films is inferior in quality to the top layer. This is reflected in the observation of broader and weaker X-ray diffraction peaks, and in lower T, and J, values of films thinner than about 150 nm. Figure 2 shows an X-ray diffraction pattern of a 500 nm thick film on ( 100) MgO substrate. Only the (OOn) peaks of the YBCO structure are observed, which indicates a perfect c-axis orientation normal to the wafer. The absence of the (00 1) peak is simply due to masking at low angles. The strongest (006 ) peak has a width of 0.25” which is close to the resolution of the X-ray machine, and this shows good crystallinity of the film. From the results in fig. 2 we cannot infer that the film is epitaxial, but this will follow from the observed high critical currents as described below. The thickness uniformity of the deposited films was tested with the aid of a mechanical stylus (Tencor) on patterned features across the film. The resulting thickness distribution versus distance from the center of the wafer is shown in fig. 3. One can see that a very good thickness uniformity is obtained in the center of the film (5% within a circle of 8 mm di-
28(deg) Fig. 2. X-ray diffraction of a 0.5 pm thick YBCO film on MgO substrate, after deposition of another film on the back side of the wafer.
371
G. Koren /DC sputter deposition of YBCO thin films
0
I
(unifwwtyl
300
i--., t
dimfty
{
' t
250 t
I
edge of wafer
150
loo’;“““““’
10
5
r(mm) Fig. 3. Thickness distribution vs. the distance from the center of an YBCO film deposited on a SrTiO, substrate.
ameter), whereas thinner areas are found further away from the center. This is quite reasonable, considering the small target size of 24 mm diameter. It is expected that with the use of a larger target, a larger area with uniform thickness will be obtained. Figure 4 shows the resistivity as a function of temperature of the film of fig. 2. For this measurement the film was patterned by deep UV lithography with PMMA resist and dilute phosphoric acid into a microbridge and measured in a four probe DC setup. The higher than expected resistivity at room temperature (by a factor of about 2) is a result of the inferior quality of the bottom layer of the film as explained before. The right value of the resistivity would have resulted if only the top layer of about half
SPUT503
the film thickness (0.25 pm) had been taken into account. Except for that, the film quality is excellent. The resistivity is metallic and extrapolates to zero at 0 K. T,(R=O)is obtained to be 90 K and the transition width is less than 1 K. Moreover, the critical current density in the film is over 2.5 X lo6 A/cm2 at 77 K when the overall film thickness of 0.5 pm is used. Keeping in mind that only the top layer of the film is of the best quality, one can deduce an expected critical current density of about 5 x lo6 A/cm2 for this layer. As is well known, this high J, value is obtained only in the highest quality epitaxial films. We can therefore assume that the present films are also epitaxial. Poppe et al. [ 3 ] reported on high quality thin films of only 30 nm thickness deposited by DC sputtering also at 860-870°C. Even though this is the same block temperature as used here, the contrast with our results seems to be due to a higher effective film temperature in their work, where a larger target of 35 mm diameter was used. In ref. [ 3 ] it was also shown by TEM measurements that the first 15 nm layer of their films has many defects in it but the film on top of this layer is defect-free. The bottom layer of inferior quality in our films is about an order of magnitude thicker. This may result not only from the lower effective temperature as described above, but also from contamination, since, as mentioned before, only a roughing pump was used in the present experiments. In the near future we plan to study the properties of our films as a function of their thickness to elucidate the growth mechanism, structure and epitaxy, especially during the first stage of the deposition pro-
5512
Jc(77K)=2.5x106
0.0
0
A/cm2
200
100
T(K) Fig. 4. Resistivity
vs. temperature
of the film of fig. 2.
300
372
G. Koren /DC sputter deposition of YBCO thin films
cess. We also plan to investigate how well epitaxial growth is obtained on a highly defective and strained bottom layer, an effect that was already observed in ref. [ 3 1, but with a much thinner layer. The results in figs. 2 and 4 were actually measured on a film that was first deposited by a standard procedure (Sl film) and then flipped over for the deposition of YBCO on the second side of the MgO wafer (S2 film). We call such a film which is temperature cycled twice, the second time when it faces the hot stainless steel block, an S12 film. Hence the results in figs. 2 and 4 are even more impressive since they were recorded on an S12 film. We still have problems with the S2 films because of surface contamination of the back side of the MgO wafer when the Sl film is deposited (the uncoated S2 side faces the hot block first). The resulting S2 films still have T, values of 90 or 89 K, but the critical currents are lower by about an order of magnitude than those of the S 1 or S12 films. Clearly, noncontaminating hot blocks should be used to avoid this problem. To summarize, high quality thin films of YBCO were obtained by the high pressure DC sputtering method in a small, simple and quite “dirty” vacuum system pumped by a roughing pump only. The film coated on the first side (S 1) kept most of its good properties even after a temperature cycling for the deposition of YBCO on the back side of the wafer. Problems remain with the radiative heating during the first stage of the deposition process and with contamination of the back side of the MgO wafer when Sl is deposited. The suggested solutions to these problems are to increase the growth temperature at the beginning of the deposition process and to use a
heating block from a noncontaminating
material.
Acknowledgements The author wishes to thank E. Polturak, E. Aharoni and D. Cohen for useful discussions, M. Reisner for the X-ray measurements and S. Hoida and M. Ayalon for their skillful technical assistance. This research was carried out within the framework of the Israeli Consortium for Superconductivity supported by the Israeli Ministry of Industry and Commerce. It was also supported by the Technion V. P. R. Fund - Alexander Goldberg Memorial Research, and by the Fund for the Promotion of Research at the Technion.
References [ 1] G. Koren, E. Polturak, B. Fisher, D. Cohen and G. Kimel, Appl. Phys. Lett. 53 (1988) 2330; X.D. Wu, A. Inam, T. Venkatesan, C.C. Chang, E.W. Chase, P. Barboux, J.M. Tarascon and B. Wilkens, Appl. Phys. Lett. 52 (1988) 754. [2] J.A. Greer, J. Vat. Sci. Technol. A 10 (1992) 1821; B. Holzapfel, B. Roas, L. Schulz, P. Bauer and G. SaemannIschenko, Appl. Phys. Lett. 6 1 ( 1992) 3 178. [ 3 ] U. Poppe, N. Klein, Dlhne, H. Soltner, CL. Jia, B. Kabius, K. Urban, A. Lubig, K. Schmidt, S. Hensen, S. Orbach, G. Miiller and H. Piel, J. Appl. Phys. 71 (1992) 5572 and references therein. [4] G.K. Muralidhar, G. Mohan Rao, J. Raghunathan and S. Mohan, Physica C 192 ( 1992) 447. [ 51 Handbook of Infrared Optical Materials, ed. P. Klocek (Optical Engineering vol. 30) (Marcel Dekker, New York, 1992) p. 308 and p. 374.
Physica C 209 (1993) 369-372 North-Holland
DC sputter deposition of YBa2Cu307 thin films for two sided coating G. Koren PhysicsDepartment. Technion - Israel Instituteof Technology,Halfa, 32000, Israel Received 6 January 1993 Revised manuscript received 2 March 1993
A simple vacuum system for the deposition of YBa2Cu30, (YBCO) thin films by DC sputtering is described. This system enables one to perform YBCO film deposition on both sides of any appropriate wafer such as MgO for applications in passive microwave devices. The films are characterized by measurements of their X-ray diffraction, thickness uniformity and transport properties. These measurements indicate that ( 1) the films are perfectly c-axis oriented normal to the wafer, (2) thickness uniformity of 10% over an area of 10 mm diameter is obtained in the center of the film (from a 24 mm diameter target), (3) the critical temperature is T,(R =0) = 90 K and (4) the critical current density at liquid nitrogen temperature J,( 77 K) is over 2 X lo6 A/cm’.
Wafers for use in passive microwave devices must be coated with YBCO films on both sides because of the need for a superconducting ground plane in the basic circuits of the stripline variety. To obtain wafers coated on both sides by laser ablation deposition, a method that was developed already in 1988 [ 1 ] and yields excellent films, one obviously has to avoid the common technique of gluing the substrates to the heated block by silver paste. The gluing is necessary for obtaining good thermal contact with the heater and deposition is carried out at typically 780°C block temperature. Without gluing, it would be necessary to raise the heater temperature by an additional 150200°C which poses severe material problems, or alternatively, to work with an oven heater which is also quite complicated [ 2 1. It is well known that active oxygen, such as that produced in sputtering, enables the deposition of very good films at temperatures about 50- 100 ’ C lower than those needed in the laser process. This is especially true in systems where the oxygen plasma is in very close proximity to the deposited YBCO film. In the planar, high pressure DC sputtering geometry this requirement is fulfilled, and we therefore employed this method in the present study. DC sputtering has already been used successfully 0921-4534/93/$06.00
for the deposition of single sided YBCO films [ 3,4]. To the best of our knowledge, this method has not been previously used for two sided coating. In the present study, we demonstrate the capabilities and problems involved in using this method for two sided coating. In addition, we show that a small and simple vacuum system pumped only by a roughing pump, is sufficient for the DC sputter deposition of high quality YBCO films without the need for high vacuum systems. A cylindrical stainless steel chamber of 15 cm diameter and 15 cm height was adapted for the deposition of the YBCO films. A schematic drawing of the sputtering setup is given in fig. 1. The heater block has a planar shape with an elevated rim for better radiative heating also from the sides. Facing it is the YBCO sputtering target, which is a standard ceramic YBCO pellet of 24 mm diameter, home made and glued with silver paste to a cooled copper vessel. Oxygen flow onto the ti!m is achieved by forcing the flow around the target as shown in fig. 1, similar to the flow design of ref. [ 3 1. A flow of 1 l/min at a background O2 pressure of typically 4 Torr is maintained in the cell by continuous pumping with a small roughing pump. The substrates used in the present study are rectangular ( 100) MgO wafers of up to 25 x 25 mm’ area, optically polished on both
0 1993 Elsevier Science Publishers B.V. All rights reserved.
G. Koren /DC sputter deposition of YBCO thin films
370
DC SPUTTERING
SETUP
SUBSTRATE
BLOCK HEATER
Fig. 1. A schematic DC sputtering.
drawing
of the thin film deposition
system by
sides (Synergy Superconductive Technologies, Jerusalem ), and ( 100 ) SrTi03 wafers polished on one side (Lamprect, Neuhausen). They are simply placed on the heater and heated by radiation (mostly in the infrared), as well as by convection in the oxygen background pressure. The target is biased negatively at 230-260 V and a whitish plasma is formed whose light-emitting volume extends half-way from the cathode to the substrate. The fast ions are slowed down in the relatively high pressure of the discharge, the positive ions hit only the target and therefore no ion damage occurs to the film. The current density in the discharge is about 20-25 mA/cm’, the target to substrate distance is 13 mm and the block temperature during deposition is about 860-870°C. After deposition the cell is filled up with 0.7 atm of oxygen and the sample is cooled down with a half hour dwell time at 450°C for oxygen intake to assure a high T,. Under these conditions, typical deposition rates are 80-100 rim/h.. The resulting films are black and smooth but not free of particulates which are less than 500 nm in size. The morphology of the films and the parameters affecting it will be investigated in the future. Films of 300-600 nm thickness are generally prepared in 48 h runs. Because of the radiative heating, the film temperature changes continuously during the beginning of the deposition process even though the block
temperature is kept constant. This is due to the transparency of the MgO and SrTiO, substrates to infrared radiation of wavelengths shorter than N 67 urn [ 5 1. Once a black YBCO film has been grown on the substrate, the infrared absorption becomes constant, and the film continues to grow at a constant temperature. One can compensate for the lower temperature of the film during the first stage of the deposition process by driving the block temperature further up at this time. Since this was not done in the present study, the bottom layer of the films is inferior in quality to the top layer. This is reflected in the observation of broader and weaker X-ray diffraction peaks, and in lower T, and J, values of films thinner than about 150 nm. Figure 2 shows an X-ray diffraction pattern of a 500 nm thick film on ( 100) MgO substrate. Only the (OOn) peaks of the YBCO structure are observed, which indicates a perfect c-axis orientation normal to the wafer. The absence of the (00 1) peak is simply due to masking at low angles. The strongest (006 ) peak has a width of 0.25” which is close to the resolution of the X-ray machine, and this shows good crystallinity of the film. From the results in fig. 2 we cannot infer that the film is epitaxial, but this will follow from the observed high critical currents as described below. The thickness uniformity of the deposited films was tested with the aid of a mechanical stylus (Tencor) on patterned features across the film. The resulting thickness distribution versus distance from the center of the wafer is shown in fig. 3. One can see that a very good thickness uniformity is obtained in the center of the film (5% within a circle of 8 mm di-
28(deg) Fig. 2. X-ray diffraction of a 0.5 pm thick YBCO film on MgO substrate, after deposition of another film on the back side of the wafer.
371
G. Koren /DC sputter deposition of YBCO thin films
0
I
(unifwwtyl
300
i--., t
dimfty
{
' t
250 t
I
edge of wafer
150
loo’;“““““’
10
5
r(mm) Fig. 3. Thickness distribution vs. the distance from the center of an YBCO film deposited on a SrTiO, substrate.
ameter), whereas thinner areas are found further away from the center. This is quite reasonable, considering the small target size of 24 mm diameter. It is expected that with the use of a larger target, a larger area with uniform thickness will be obtained. Figure 4 shows the resistivity as a function of temperature of the film of fig. 2. For this measurement the film was patterned by deep UV lithography with PMMA resist and dilute phosphoric acid into a microbridge and measured in a four probe DC setup. The higher than expected resistivity at room temperature (by a factor of about 2) is a result of the inferior quality of the bottom layer of the film as explained before. The right value of the resistivity would have resulted if only the top layer of about half
SPUT503
the film thickness (0.25 pm) had been taken into account. Except for that, the film quality is excellent. The resistivity is metallic and extrapolates to zero at 0 K. T,(R=O)is obtained to be 90 K and the transition width is less than 1 K. Moreover, the critical current density in the film is over 2.5 X lo6 A/cm2 at 77 K when the overall film thickness of 0.5 pm is used. Keeping in mind that only the top layer of the film is of the best quality, one can deduce an expected critical current density of about 5 x lo6 A/cm2 for this layer. As is well known, this high J, value is obtained only in the highest quality epitaxial films. We can therefore assume that the present films are also epitaxial. Poppe et al. [ 3 ] reported on high quality thin films of only 30 nm thickness deposited by DC sputtering also at 860-870°C. Even though this is the same block temperature as used here, the contrast with our results seems to be due to a higher effective film temperature in their work, where a larger target of 35 mm diameter was used. In ref. [ 3 ] it was also shown by TEM measurements that the first 15 nm layer of their films has many defects in it but the film on top of this layer is defect-free. The bottom layer of inferior quality in our films is about an order of magnitude thicker. This may result not only from the lower effective temperature as described above, but also from contamination, since, as mentioned before, only a roughing pump was used in the present experiments. In the near future we plan to study the properties of our films as a function of their thickness to elucidate the growth mechanism, structure and epitaxy, especially during the first stage of the deposition pro-
5512
Jc(77K)=2.5x106
0.0
0
A/cm2
200
100
T(K) Fig. 4. Resistivity
vs. temperature
of the film of fig. 2.
300
372
G. Koren /DC sputter deposition of YBCO thin films
cess. We also plan to investigate how well epitaxial growth is obtained on a highly defective and strained bottom layer, an effect that was already observed in ref. [ 3 1, but with a much thinner layer. The results in figs. 2 and 4 were actually measured on a film that was first deposited by a standard procedure (Sl film) and then flipped over for the deposition of YBCO on the second side of the MgO wafer (S2 film). We call such a film which is temperature cycled twice, the second time when it faces the hot stainless steel block, an S12 film. Hence the results in figs. 2 and 4 are even more impressive since they were recorded on an S12 film. We still have problems with the S2 films because of surface contamination of the back side of the MgO wafer when the Sl film is deposited (the uncoated S2 side faces the hot block first). The resulting S2 films still have T, values of 90 or 89 K, but the critical currents are lower by about an order of magnitude than those of the S 1 or S12 films. Clearly, noncontaminating hot blocks should be used to avoid this problem. To summarize, high quality thin films of YBCO were obtained by the high pressure DC sputtering method in a small, simple and quite “dirty” vacuum system pumped by a roughing pump only. The film coated on the first side (S 1) kept most of its good properties even after a temperature cycling for the deposition of YBCO on the back side of the wafer. Problems remain with the radiative heating during the first stage of the deposition process and with contamination of the back side of the MgO wafer when Sl is deposited. The suggested solutions to these problems are to increase the growth temperature at the beginning of the deposition process and to use a
heating block from a noncontaminating
material.
Acknowledgements The author wishes to thank E. Polturak, E. Aharoni and D. Cohen for useful discussions, M. Reisner for the X-ray measurements and S. Hoida and M. Ayalon for their skillful technical assistance. This research was carried out within the framework of the Israeli Consortium for Superconductivity supported by the Israeli Ministry of Industry and Commerce. It was also supported by the Technion V. P. R. Fund - Alexander Goldberg Memorial Research, and by the Fund for the Promotion of Research at the Technion.
References [ 1] G. Koren, E. Polturak, B. Fisher, D. Cohen and G. Kimel, Appl. Phys. Lett. 53 (1988) 2330; X.D. Wu, A. Inam, T. Venkatesan, C.C. Chang, E.W. Chase, P. Barboux, J.M. Tarascon and B. Wilkens, Appl. Phys. Lett. 52 (1988) 754. [2] J.A. Greer, J. Vat. Sci. Technol. A 10 (1992) 1821; B. Holzapfel, B. Roas, L. Schulz, P. Bauer and G. SaemannIschenko, Appl. Phys. Lett. 6 1 ( 1992) 3 178. [ 3 ] U. Poppe, N. Klein, Dlhne, H. Soltner, CL. Jia, B. Kabius, K. Urban, A. Lubig, K. Schmidt, S. Hensen, S. Orbach, G. Miiller and H. Piel, J. Appl. Phys. 71 (1992) 5572 and references therein. [4] G.K. Muralidhar, G. Mohan Rao, J. Raghunathan and S. Mohan, Physica C 192 ( 1992) 447. [ 51 Handbook of Infrared Optical Materials, ed. P. Klocek (Optical Engineering vol. 30) (Marcel Dekker, New York, 1992) p. 308 and p. 374.