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A substrate heater design for high temperature deposition in an oxidizing plasma
Vacuum/volume 43lnumbers Printed in Great Britain
A substrate deposition E E Inameti, University
i/2/pages
121 to 12311992
0042-207x/92$5.00+.00 @ 1991 Pergamon Press plc
heater design in an oxidizing
M S Raven, Y M Wan and B G Murray,
of Nottingham,
Nottingham
NG7 2RD,
for high temperature plasma Department
of Electricaland
Electronic
Engineering,
UK
A novel contact heater which can operate reliably in air, vacuum and highly oxidizing atmospheres has been designed and constructed. It uses a fully enclosed element which makes good thermal contact with a stainless steel plate on which the substrates are clamped. Adequate radiation shields are used to reduce heat loss. Power consumption is less than 200 W for an operational temperature of 750°C.
1. Introduction The great desire to exploit the properties of high temperature superconducting (HTS) materials in a new technology has demanded the growth of high quality thin films. These can be produced mainly by in situ vacuum deposition techniques such as sputtering, evaporation, molecular beam epitaxy and laser ablation. Common to all in situ HTS vacuum deposition methods is the use of (a) oxygen in the growth chamber and (b) high substrate temperature (6OG8OO’C). Oxygen is required to oxidize the sputtered species as they arrive on the substrate and condense to form the required phase of the material. It is also used for post-deposition oxygenation to convert the insulating phase to the superconducting orthorhombic phase’. The partial pressure of oxygen can be used to control the lattice orientation of the films in sputtering, and the value of the lattice parameter as in laser ablation”‘. Various forms of oxygen : atomic (O-), molecular (O,, 0,) and compound (N,O) have been used in growing superconducting films. The best films with excellent superconducting properties are grown in situ at temperatures between 600 and 800°C. This allows the films to grow epitaxially or highly oriented as well as yielding smooth and shiny surfaces for device processing. The high temperature and high pressure oxygen requirements for HTS film growth present the need for reliable substrate heaters which can operate for long periods in an oxidizing environment. It has therefore been necessary to design an effective but simple heater which can be used reliably in such an environment. Several groups use radiation heating from one or more filament bulbs4. The problem here is the limited lifespan of the bulbs and the serious damage to the target and chamber in the event of explosion of the bulbs. Ceramic heaters are available commercially but cost considerations and adaptability demands may make these not so attractive. In this paper we report the design, construction and performance of a simple and reliable contact heater which can handle the severe conditions for HTS film deposition. The heater is designed for long periods of use in either vacuum, highly oxidizing atmospheres on in air with reduced
efficiency. The heater has been used on regular basis to produce high quality YBa$u,O, films by rf magnetron sputter deposition.
2. Design and construction The heating element used in this design was a Thermocoax heating element supplied by Thermocoax (Philips), France. This is a coaxial element with the element made of Ni/Cr wire around which is compacted MgO powder to isolate the wire from the outer sheath which is made of zirconium copper alloy. The element is supplied in wire form and was wound into a rectangular spiral making sure the bends were not too acute to damage the insulation and cause a short between the wire and the sheath. The coaxial connectors were first brazed onto the sheath and then to the exposed Ni/Cr wire using high temperature brazing material. All the other heater parts including nuts and screws were of stainless steel material. The element E was sandwiched between two flat and parallel plates P of dimensions 65 x 50 mm and thickness 1.2 mm, Figure I. On the upper plate of the sandwich a rectangular slot S 45 x 10.5 mm and depth of 0.4 mm was milled out to hold the substrates. Dimensions of this slot may be altered to suit different substrate sizes. This slot allowed easy loading of the substrates as well as good heat transfer to the substrates-the upper sandwich plate being thinnest along the slot. The thickness of the sandwich plates was chosen to be minimal to allow maximum heat transfer but also give rigidity and flatness after assembly. After assembly the plates were held in place by four long screws making sure that the entire length of the element made surface contacts with both top and bottom plates. To reduce heat loss the sandwich had two radiation shields on either side. The inner pair of shields had 90” flaps at the ends which almost enclosed the sandwich. Both shields over the top sandwich plate had openings to allow sputtered vapour onto the substrates. The support for the connectors of the element were attached to the inner lower shield. The leads from the power supply were screwed on to these terminals. The heater was attached within the chamber by only two thin screws 121
et a/: Substrate
f f lnamefi
heater
design Temperature
(Cl
10001
800
600
4oL1
/
200
/
0 0
2
4 Current
Figure 3. Plot of the power heater.
Lil 0
0
0
Figure 1. A schematic diagram of a heater showing the clement E and the sandwich plates P. the slot S where substrates are clamped.
10 reduce
loss by conduction. The heater could be mounLed or vertically without further adaptations. Tcmthe slot was monitored by a type K thermocouple next to the mounted substrates. Figure 2 is a photoassembled heater.
heat
horizontally perature on spot welded graph of the
3. Performance of the substrate heater The temperature variation along the slot was 5 C and although the slot may take up to 4 x IO mm square substrates it was easier to load not more than three substrates to achieve uniform temperature. It was necessary to place a thin silver foil between
input
6 ,nput
against
8
the temperature
TEMPERATURE
ICI
ij !
,
122
12
rix
ol’ the
substrate and plate to improve the thermal contact. No additional water cooling was used in the system. although it was found useful to direct a small electric fan at the bottom of deposition chamber to keep the temperature low enough to preserve the o-rings. Figure 3 shows the temperature of the slot vx power input to the heater, for (a) heating in air; (b) vacuum of IO ’ torr, and (c) 300 mtorr of Ar/O, mixture. The power input to reach a temperature of 750 C was less than 200 W. Figure 4 shows the heating and cooling curves of the heater. The time taken by the heater to reach 95% of the steady-state temperature was 5 min. The cooling cut-kc gives a temperature dccreasc from 750 to 1OO’C in less than 500 s. The heater was used for about I95 h before it required maintenance. This was due to a break at the connector end of the NiiCr wire caused by strain exerted on the connector by the power supply leads. An improved terminal design has taken away much of the strain off the connector. There
800
Figure 2. Photograph of the assembled heater.
10
IA)
Figure 4. Heating and coohng curves
01‘ the heater.
E E lnameti
Resistance
et al: Substrate
heater
design
4. Summary
(Ohms)
We have designed and built a reliable substrate heater with satisfactory heating output and rapid warming and cooling characteristics. It has maintenance free parts and does not need additional cooling. It has been used routinely and successfully to provide substrate temperature of up to 750°C with less than a 200 W power input in a highly oxidizing environment.
Acknowledgements
- 1 --0
----~
I
50
100
150
Temperature Figure 5. Resistance-temperature
plot
200
250
300
The authors wish to thank Mr K White for machining the stainless steel parts and Mr D Oakland for the high temperature brazing of the coaxial connectors to the heating element.
(K) of YBCO
film deposited on
References
SrTiO,(llO) at 735°C using the heater.
has been no significant parts. We have used this YBa,Cu,O, films, the these is shown in Figure
oxidation problems of any of the heater heater to produce good superconducting resistance-temperature curve of one of 5.
‘H S Kwok and Q Y Ying, Physica C, 177, 122 (1991). * .I Q Zheng, M C Shih, S Williams, S J Lee, H Kajiyama, X K Wang, Z Zhao, K Viani, S Jacobson, P Dutta. R P Chang and J B Ketterson, Appl Phys Left, 59,231 (1991). 3A Gupta, B W Hussey, A Kussmaul and A Segmuller, Appl Phys Left. 57,2365 (1990). 4 R C Estler, N S Noger, R E Muenchansen, X D Wu, S Foltyne and A R Garcia, Rev Scient Instrum, 62,437 (1991).
123
A substrate deposition E E Inameti, University
i/2/pages
121 to 12311992
0042-207x/92$5.00+.00 @ 1991 Pergamon Press plc
heater design in an oxidizing
M S Raven, Y M Wan and B G Murray,
of Nottingham,
Nottingham
NG7 2RD,
for high temperature plasma Department
of Electricaland
Electronic
Engineering,
UK
A novel contact heater which can operate reliably in air, vacuum and highly oxidizing atmospheres has been designed and constructed. It uses a fully enclosed element which makes good thermal contact with a stainless steel plate on which the substrates are clamped. Adequate radiation shields are used to reduce heat loss. Power consumption is less than 200 W for an operational temperature of 750°C.
1. Introduction The great desire to exploit the properties of high temperature superconducting (HTS) materials in a new technology has demanded the growth of high quality thin films. These can be produced mainly by in situ vacuum deposition techniques such as sputtering, evaporation, molecular beam epitaxy and laser ablation. Common to all in situ HTS vacuum deposition methods is the use of (a) oxygen in the growth chamber and (b) high substrate temperature (6OG8OO’C). Oxygen is required to oxidize the sputtered species as they arrive on the substrate and condense to form the required phase of the material. It is also used for post-deposition oxygenation to convert the insulating phase to the superconducting orthorhombic phase’. The partial pressure of oxygen can be used to control the lattice orientation of the films in sputtering, and the value of the lattice parameter as in laser ablation”‘. Various forms of oxygen : atomic (O-), molecular (O,, 0,) and compound (N,O) have been used in growing superconducting films. The best films with excellent superconducting properties are grown in situ at temperatures between 600 and 800°C. This allows the films to grow epitaxially or highly oriented as well as yielding smooth and shiny surfaces for device processing. The high temperature and high pressure oxygen requirements for HTS film growth present the need for reliable substrate heaters which can operate for long periods in an oxidizing environment. It has therefore been necessary to design an effective but simple heater which can be used reliably in such an environment. Several groups use radiation heating from one or more filament bulbs4. The problem here is the limited lifespan of the bulbs and the serious damage to the target and chamber in the event of explosion of the bulbs. Ceramic heaters are available commercially but cost considerations and adaptability demands may make these not so attractive. In this paper we report the design, construction and performance of a simple and reliable contact heater which can handle the severe conditions for HTS film deposition. The heater is designed for long periods of use in either vacuum, highly oxidizing atmospheres on in air with reduced
efficiency. The heater has been used on regular basis to produce high quality YBa$u,O, films by rf magnetron sputter deposition.
2. Design and construction The heating element used in this design was a Thermocoax heating element supplied by Thermocoax (Philips), France. This is a coaxial element with the element made of Ni/Cr wire around which is compacted MgO powder to isolate the wire from the outer sheath which is made of zirconium copper alloy. The element is supplied in wire form and was wound into a rectangular spiral making sure the bends were not too acute to damage the insulation and cause a short between the wire and the sheath. The coaxial connectors were first brazed onto the sheath and then to the exposed Ni/Cr wire using high temperature brazing material. All the other heater parts including nuts and screws were of stainless steel material. The element E was sandwiched between two flat and parallel plates P of dimensions 65 x 50 mm and thickness 1.2 mm, Figure I. On the upper plate of the sandwich a rectangular slot S 45 x 10.5 mm and depth of 0.4 mm was milled out to hold the substrates. Dimensions of this slot may be altered to suit different substrate sizes. This slot allowed easy loading of the substrates as well as good heat transfer to the substrates-the upper sandwich plate being thinnest along the slot. The thickness of the sandwich plates was chosen to be minimal to allow maximum heat transfer but also give rigidity and flatness after assembly. After assembly the plates were held in place by four long screws making sure that the entire length of the element made surface contacts with both top and bottom plates. To reduce heat loss the sandwich had two radiation shields on either side. The inner pair of shields had 90” flaps at the ends which almost enclosed the sandwich. Both shields over the top sandwich plate had openings to allow sputtered vapour onto the substrates. The support for the connectors of the element were attached to the inner lower shield. The leads from the power supply were screwed on to these terminals. The heater was attached within the chamber by only two thin screws 121
et a/: Substrate
f f lnamefi
heater
design Temperature
(Cl
10001
800
600
4oL1
/
200
/
0 0
2
4 Current
Figure 3. Plot of the power heater.
Lil 0
0
0
Figure 1. A schematic diagram of a heater showing the clement E and the sandwich plates P. the slot S where substrates are clamped.
10 reduce
loss by conduction. The heater could be mounLed or vertically without further adaptations. Tcmthe slot was monitored by a type K thermocouple next to the mounted substrates. Figure 2 is a photoassembled heater.
heat
horizontally perature on spot welded graph of the
3. Performance of the substrate heater The temperature variation along the slot was 5 C and although the slot may take up to 4 x IO mm square substrates it was easier to load not more than three substrates to achieve uniform temperature. It was necessary to place a thin silver foil between
input
6 ,nput
against
8
the temperature
TEMPERATURE
ICI
ij !
,
122
12
rix
ol’ the
substrate and plate to improve the thermal contact. No additional water cooling was used in the system. although it was found useful to direct a small electric fan at the bottom of deposition chamber to keep the temperature low enough to preserve the o-rings. Figure 3 shows the temperature of the slot vx power input to the heater, for (a) heating in air; (b) vacuum of IO ’ torr, and (c) 300 mtorr of Ar/O, mixture. The power input to reach a temperature of 750 C was less than 200 W. Figure 4 shows the heating and cooling curves of the heater. The time taken by the heater to reach 95% of the steady-state temperature was 5 min. The cooling cut-kc gives a temperature dccreasc from 750 to 1OO’C in less than 500 s. The heater was used for about I95 h before it required maintenance. This was due to a break at the connector end of the NiiCr wire caused by strain exerted on the connector by the power supply leads. An improved terminal design has taken away much of the strain off the connector. There
800
Figure 2. Photograph of the assembled heater.
10
IA)
Figure 4. Heating and coohng curves
01‘ the heater.
E E lnameti
Resistance
et al: Substrate
heater
design
4. Summary
(Ohms)
We have designed and built a reliable substrate heater with satisfactory heating output and rapid warming and cooling characteristics. It has maintenance free parts and does not need additional cooling. It has been used routinely and successfully to provide substrate temperature of up to 750°C with less than a 200 W power input in a highly oxidizing environment.
Acknowledgements
- 1 --0
----~
I
50
100
150
Temperature Figure 5. Resistance-temperature
plot
200
250
300
The authors wish to thank Mr K White for machining the stainless steel parts and Mr D Oakland for the high temperature brazing of the coaxial connectors to the heating element.
(K) of YBCO
film deposited on
References
SrTiO,(llO) at 735°C using the heater.
has been no significant parts. We have used this YBa,Cu,O, films, the these is shown in Figure
oxidation problems of any of the heater heater to produce good superconducting resistance-temperature curve of one of 5.
‘H S Kwok and Q Y Ying, Physica C, 177, 122 (1991). * .I Q Zheng, M C Shih, S Williams, S J Lee, H Kajiyama, X K Wang, Z Zhao, K Viani, S Jacobson, P Dutta. R P Chang and J B Ketterson, Appl Phys Left, 59,231 (1991). 3A Gupta, B W Hussey, A Kussmaul and A Segmuller, Appl Phys Left. 57,2365 (1990). 4 R C Estler, N S Noger, R E Muenchansen, X D Wu, S Foltyne and A R Garcia, Rev Scient Instrum, 62,437 (1991).
123