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A multiple target system using laser ablation for multilayer deposition
Vacuum/volume 47Inumber l/pages 1 to 311996 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $9.50+.00
0042-207X(95)00212-X
A multiple target system using laser ablation for multilayer deposition RP Campion, D R Dye, P J King and R G Ormson, Department of Physics, University of Nottingham, Nottingham
NG7 2RD, England
received 26 July 7995
The laser ablation technique requires the laser spot to be scanned across the surface of a target, while the fabrication of multi-layer thin film devices by laser deposition requires the in-situ selection of a number of different targets. The design and construction of a multiple target system for use under vacuum or in oxygen is described. The system uses a single drive both to select targets and to scan across their complete surfaces, and so eliminates the need for beam rastering or two drive mechanisms.
1. Introduction
Pulsed laser deposition has been used successfully for preparing thin films of a wide range of materials including insulators, metals, magnetic layers, and the oxide superconductors. A pulsed laser is used to ablate material from a target, and the material removed is carried away in a plume, to be deposited as a thin film on a suitably positioned substrate. One of the strengths of the ablation method is that the stoichiometry of a multi-element target may be preserved in the film provided that certain conditions are met. For the high temperature superconductors these include positioning the substrate close to the plume axis, and ensuring that only a small number of consecutive laser pulses hit the same point on the target.‘,2 The laser spot must therefore be scanned across the target surface. Device applications of many materials including the high temperature superconductors frequently require the sequential deposition of several layers of different materials. This requires the ability to change target without breaking vacuum. The successful deposition system therefore requires a mechanism which enables the surface of a target to be scanned by a focused laser spot while maintaining the position of the spot with respect to the substrate, and which enables a number of targets to be selected in-situ. Partial or complete computer control of the process may well be desirable. An early published design by Rao and Moodera’ uses a motor to rotate the single target, while a motorised mirror external to the deposition chamber enables the laser spot to be scanned across the target. This method does not preserve the geometry of the substrate with respect to the plume. A later design uses two vacuum stepper motors and an arrangement of gears to select any one of six targets, and to scan the laser spot across the surface of that target4. The method is capable of using the entire surface of each target, and maintains spot-substrate geometry. It does however require two motors which are positioned within the vacuum system. In 1994. a somewhat similar commercial design
was described which drives the gears through two rotary seals using two motors positioned outside the deposition chamber (see Ref. 2, page 44). The design by Jackson et al.’ uses a slide mechanism to select one of a number of targets. The selected target is then rotated by a motor following engagement using a clutch mechanism. A movable lens may be used to raster the spot across the target. This design is simple, avoiding the use of large number of gears, but the utilisation of the full target surface, rather than an annulus, requires beam rastering and this does not maintain spot substrate geometry. The method uses a three distinct mechanical movements which does not make it ideal for automatic control. The system described here uses only one rotary drive and feedthrough and has no requirement for beam rastering, yet still allows target selection and scanning of the entire area of each target.
2. Target mechanism design The basis of the system is a turntable which supports four target mounts. As the turn-table rotates, the target mounts move radially towards or away from the centre of the turn-table depending on the turn-table rotation direction (Figure 1). This enables the focused laser spot to be scanned across a selected target by rotation of the turn-table. After each full rotation of the turntable, the target mount has moved radially by about one spot diameter and a different track across that target is addressed. Obviously a quarter revolution of the turn-table will allow use of a different target. In this way the single stepping motor which controls the rotation of the turn-table may be used to scan the full area of any or all of the four targets, at the expense of holding a “map” of the system in the controlling microcomputer. The mechanism which provides the radial movement of the four target mounts is shown in Figure 2. The main shaft which 1
R P Campion
et a/: Multi-layer
deposition
ryl :: :: :: :: :: :: :: :: :: :: :: :: ,I
II I,
::
C
:: II
::
:i
::
::
::
ii
::
::
::
::
::
::
::
:: ::
::
:: ::
B: Particle
A: Target
shield
::
::
,,
II
C: Turn-table
Figure 1. Overview of the turn-table
A: Fluorosint bush
of the targets as it rotates. target is shown inset.
D: Turn-table
design, showing the radial movement The arced rastering of the whole surface of a
B: Mounting screw
C: Bearing halves
Figure 3. Cross section of one of the eight bearing assemblies.
is used to rotate the turn-table passes through a fixed bevel gear. Four bevel gears, one for each target, engage the fixed gear and rotate as the turntable is turned. Each bevel gear turns a threaded stainless steel shaft supported by two light bearings which are attached to the back of the turn-table as shown. Four phosphor bronze target mounts are constrained to move radially with respect to the turn-table by slots cut in the stainless steel turntable disc. A threaded hole in each mount engages with one of the threaded shafts, and the rotation of a shaft causes the radial movement of the corresponding target mount. Though not explicitly shown in the diagrams, for reasons of clarity, all potential trapped volumes have been provided with drilled vents. Figure 1 also shows that the targets are separated on the front of the turntable by 2cm high particle shields designed to prevent any possible cross contamination of target materials. The turntable is driven, via a single rotary vacuum seal (Edwards type 8RK25), by a belt reduction drive and a stepper motor (Radio-Spares type RS-344-631) mounted outside rhe ablation chamber. The design shown here is for four standard one inch diameter ablation targets, although the principal can easily be used to design a system for other sizes and numbers of targets. A slight variation on the design was considered in which the fixed bevel gear of Figure 2 is replaced by an independent driven gear. This would have the advantage of faster access to all points of any one target, but the disadvantage of requiring a second rotary seal and motor drive, thus complicating the mechanism.
Only two problems were encountered during the development of the system; vibration and sticking of the mechanism. The operation of a stepper motor may be smoothed by feeding the windings with suitably phased sine wave rather than phased square-wave current waveforms. We have used a compromise in which each cycle of a sine wave is approximated by 256 segments in order to ensure adequate smoothness of operation. This is easy to achieve with any computer and some simple interfacing electronics; in our system the segment data is stored on an e-prom. The sticking of the mechanism was traced to the bearings supporting the threaded stainless steel shafts. Since the use of lubricated bearings may lead to contamination the bearings were changed from metal on metal to metal on fluorosint, a material which contains mica particles embedded in polytetraflorethylene. This design is shown in Figure 3; no sticking has occurred since. Note that the bevel gear driving the threaded rod also bears on the fluorosint bush rather than the metal bearing support (Figure 2). Neither the radiative heating of the turntable caused by the proximity of the substrate heater used in the ablation process nor the oxidising atmosphere present during deposition have been a problem. The system in use in Nottingham is computer controlled by the same computer that controls the firing of the laser. This gives the added advantage of very precise control of the number of laser shots per target site. An optical detector is combined with an alignment hole in the gear wheel of the belt reduction mechanism mentioned earlier, to provide information to the computer on the position in the rotation cycle. A microswitch at either end of the target travel indicates the limits of motion. These inputs allow very accurate alignment of the laser beam with any position on any target.
3. Conclusions
A: Fixing screw:
B: Target mounts
C: Target
D: Target support
E: Bearing mounts
F: Threaded shaft
G: Fixed bevel gear
H: Main turn-table shaft
Figure 2. Section of the rear assembly, radial movement of the target mounts. 2
showing
the mechanism
driving
A system has been constructed which uses a single vacuum feedthrough and a single motor external to the vacuum system to enable the entire surface of up to four targets to be scanned in the ablation process. The mechanism described here is easily produced from commonly used vacuum materials, is inexpensive and has been found in use to be very reliable. It is very suitable for incorporation into a system in which target selection, target
R P Campion et al: Multi-layer
scanning, computer.
and
laser
deposition
fire control
are controlled
by a single
Acknowledgements We would like to thank Professor Saburo Iwama of the Daido Institute of Technology, Nagoya, Japan for invaluable assistance with early designs and the Science and Engineering research council (now EPSRC) for a maintenance grant.
References ’ T J Jackson and S B Palmer, J Phys D: Appl Phys, 27, 1581 (1994). ‘D B Chrisey and G K Hubler (Eds), Pulsed Laser Deposition of Thin Films. John Wiley and Sons, New York (1994). 3A M Rao and J S Moodera, Review of Scientific Instruments, 62(4), 1107 (1991). 4 0 Auciello, J Emerlck, J Duarte and A Illingworth, Journal of Vacuum Science and Technology A, 11(l), 267 (1993). ’ T J Jackson, NJ Appleyard, M J Cooper, D H Richards and S B Palmer, Measurement Science and Technology, 6(l), 128 (1995)
3
0042-207X(95)00212-X
A multiple target system using laser ablation for multilayer deposition RP Campion, D R Dye, P J King and R G Ormson, Department of Physics, University of Nottingham, Nottingham
NG7 2RD, England
received 26 July 7995
The laser ablation technique requires the laser spot to be scanned across the surface of a target, while the fabrication of multi-layer thin film devices by laser deposition requires the in-situ selection of a number of different targets. The design and construction of a multiple target system for use under vacuum or in oxygen is described. The system uses a single drive both to select targets and to scan across their complete surfaces, and so eliminates the need for beam rastering or two drive mechanisms.
1. Introduction
Pulsed laser deposition has been used successfully for preparing thin films of a wide range of materials including insulators, metals, magnetic layers, and the oxide superconductors. A pulsed laser is used to ablate material from a target, and the material removed is carried away in a plume, to be deposited as a thin film on a suitably positioned substrate. One of the strengths of the ablation method is that the stoichiometry of a multi-element target may be preserved in the film provided that certain conditions are met. For the high temperature superconductors these include positioning the substrate close to the plume axis, and ensuring that only a small number of consecutive laser pulses hit the same point on the target.‘,2 The laser spot must therefore be scanned across the target surface. Device applications of many materials including the high temperature superconductors frequently require the sequential deposition of several layers of different materials. This requires the ability to change target without breaking vacuum. The successful deposition system therefore requires a mechanism which enables the surface of a target to be scanned by a focused laser spot while maintaining the position of the spot with respect to the substrate, and which enables a number of targets to be selected in-situ. Partial or complete computer control of the process may well be desirable. An early published design by Rao and Moodera’ uses a motor to rotate the single target, while a motorised mirror external to the deposition chamber enables the laser spot to be scanned across the target. This method does not preserve the geometry of the substrate with respect to the plume. A later design uses two vacuum stepper motors and an arrangement of gears to select any one of six targets, and to scan the laser spot across the surface of that target4. The method is capable of using the entire surface of each target, and maintains spot-substrate geometry. It does however require two motors which are positioned within the vacuum system. In 1994. a somewhat similar commercial design
was described which drives the gears through two rotary seals using two motors positioned outside the deposition chamber (see Ref. 2, page 44). The design by Jackson et al.’ uses a slide mechanism to select one of a number of targets. The selected target is then rotated by a motor following engagement using a clutch mechanism. A movable lens may be used to raster the spot across the target. This design is simple, avoiding the use of large number of gears, but the utilisation of the full target surface, rather than an annulus, requires beam rastering and this does not maintain spot substrate geometry. The method uses a three distinct mechanical movements which does not make it ideal for automatic control. The system described here uses only one rotary drive and feedthrough and has no requirement for beam rastering, yet still allows target selection and scanning of the entire area of each target.
2. Target mechanism design The basis of the system is a turntable which supports four target mounts. As the turn-table rotates, the target mounts move radially towards or away from the centre of the turn-table depending on the turn-table rotation direction (Figure 1). This enables the focused laser spot to be scanned across a selected target by rotation of the turn-table. After each full rotation of the turntable, the target mount has moved radially by about one spot diameter and a different track across that target is addressed. Obviously a quarter revolution of the turn-table will allow use of a different target. In this way the single stepping motor which controls the rotation of the turn-table may be used to scan the full area of any or all of the four targets, at the expense of holding a “map” of the system in the controlling microcomputer. The mechanism which provides the radial movement of the four target mounts is shown in Figure 2. The main shaft which 1
R P Campion
et a/: Multi-layer
deposition
ryl :: :: :: :: :: :: :: :: :: :: :: :: ,I
II I,
::
C
:: II
::
:i
::
::
::
ii
::
::
::
::
::
::
::
:: ::
::
:: ::
B: Particle
A: Target
shield
::
::
,,
II
C: Turn-table
Figure 1. Overview of the turn-table
A: Fluorosint bush
of the targets as it rotates. target is shown inset.
D: Turn-table
design, showing the radial movement The arced rastering of the whole surface of a
B: Mounting screw
C: Bearing halves
Figure 3. Cross section of one of the eight bearing assemblies.
is used to rotate the turn-table passes through a fixed bevel gear. Four bevel gears, one for each target, engage the fixed gear and rotate as the turntable is turned. Each bevel gear turns a threaded stainless steel shaft supported by two light bearings which are attached to the back of the turn-table as shown. Four phosphor bronze target mounts are constrained to move radially with respect to the turn-table by slots cut in the stainless steel turntable disc. A threaded hole in each mount engages with one of the threaded shafts, and the rotation of a shaft causes the radial movement of the corresponding target mount. Though not explicitly shown in the diagrams, for reasons of clarity, all potential trapped volumes have been provided with drilled vents. Figure 1 also shows that the targets are separated on the front of the turntable by 2cm high particle shields designed to prevent any possible cross contamination of target materials. The turntable is driven, via a single rotary vacuum seal (Edwards type 8RK25), by a belt reduction drive and a stepper motor (Radio-Spares type RS-344-631) mounted outside rhe ablation chamber. The design shown here is for four standard one inch diameter ablation targets, although the principal can easily be used to design a system for other sizes and numbers of targets. A slight variation on the design was considered in which the fixed bevel gear of Figure 2 is replaced by an independent driven gear. This would have the advantage of faster access to all points of any one target, but the disadvantage of requiring a second rotary seal and motor drive, thus complicating the mechanism.
Only two problems were encountered during the development of the system; vibration and sticking of the mechanism. The operation of a stepper motor may be smoothed by feeding the windings with suitably phased sine wave rather than phased square-wave current waveforms. We have used a compromise in which each cycle of a sine wave is approximated by 256 segments in order to ensure adequate smoothness of operation. This is easy to achieve with any computer and some simple interfacing electronics; in our system the segment data is stored on an e-prom. The sticking of the mechanism was traced to the bearings supporting the threaded stainless steel shafts. Since the use of lubricated bearings may lead to contamination the bearings were changed from metal on metal to metal on fluorosint, a material which contains mica particles embedded in polytetraflorethylene. This design is shown in Figure 3; no sticking has occurred since. Note that the bevel gear driving the threaded rod also bears on the fluorosint bush rather than the metal bearing support (Figure 2). Neither the radiative heating of the turntable caused by the proximity of the substrate heater used in the ablation process nor the oxidising atmosphere present during deposition have been a problem. The system in use in Nottingham is computer controlled by the same computer that controls the firing of the laser. This gives the added advantage of very precise control of the number of laser shots per target site. An optical detector is combined with an alignment hole in the gear wheel of the belt reduction mechanism mentioned earlier, to provide information to the computer on the position in the rotation cycle. A microswitch at either end of the target travel indicates the limits of motion. These inputs allow very accurate alignment of the laser beam with any position on any target.
3. Conclusions
A: Fixing screw:
B: Target mounts
C: Target
D: Target support
E: Bearing mounts
F: Threaded shaft
G: Fixed bevel gear
H: Main turn-table shaft
Figure 2. Section of the rear assembly, radial movement of the target mounts. 2
showing
the mechanism
driving
A system has been constructed which uses a single vacuum feedthrough and a single motor external to the vacuum system to enable the entire surface of up to four targets to be scanned in the ablation process. The mechanism described here is easily produced from commonly used vacuum materials, is inexpensive and has been found in use to be very reliable. It is very suitable for incorporation into a system in which target selection, target
R P Campion et al: Multi-layer
scanning, computer.
and
laser
deposition
fire control
are controlled
by a single
Acknowledgements We would like to thank Professor Saburo Iwama of the Daido Institute of Technology, Nagoya, Japan for invaluable assistance with early designs and the Science and Engineering research council (now EPSRC) for a maintenance grant.
References ’ T J Jackson and S B Palmer, J Phys D: Appl Phys, 27, 1581 (1994). ‘D B Chrisey and G K Hubler (Eds), Pulsed Laser Deposition of Thin Films. John Wiley and Sons, New York (1994). 3A M Rao and J S Moodera, Review of Scientific Instruments, 62(4), 1107 (1991). 4 0 Auciello, J Emerlck, J Duarte and A Illingworth, Journal of Vacuum Science and Technology A, 11(l), 267 (1993). ’ T J Jackson, NJ Appleyard, M J Cooper, D H Richards and S B Palmer, Measurement Science and Technology, 6(l), 128 (1995)
3