REVIEW OF SCIENTIFIC INSTRUMENTS 86, 043901 (2015)
An ultra-compact, high-throughput molecular beam epitaxy growth system A. A. Baker,1,2 W. Braun,3,a),b) G. Gassler,4 S. Rembold,3 A. Fischer,3,a) and T. Hesjedal1,2
1
Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom Diamond Light Source, Didcot OX11 0DE, United Kingdom 3 CreaTec Fischer & Co. GmbH, Industriestr. 9, 74391 Erligheim, Germany 4 Dr. Gassler Electron Devices GmbH, List Str. 4, 89079 Ulm, Germany 2
(Received 9 January 2015; accepted 24 March 2015; published online 13 April 2015) We present a miniaturized molecular beam epitaxy (miniMBE) system with an outer diameter of 206 mm, optimized for flexible and high-throughput operation. The three-chamber system, used here for oxide growth, consists of a sample loading chamber, a storage chamber, and a growth chamber. The growth chamber is equipped with eight identical effusion cell ports with linear shutters, one larger port for either a multi-pocket electron beam evaporator or an oxygen plasma source, an integrated cryoshroud, retractable beam-flux monitor or quartz-crystal microbalance, reflection high energy electron diffraction, substrate manipulator, main shutter, and quadrupole mass spectrometer. The system can be combined with ultrahigh vacuum (UHV) end stations on synchrotron and neutron beamlines, or equivalently with other complex surface analysis systems, including low-temperature scanning probe microscopy systems. Substrate handling is compatible with most UHV surface characterization systems, as the miniMBE can accommodate standard surface science sample holders. We introduce the design of the system, and its specific capabilities and operational parameters, and we demonstrate the epitaxial thin film growth of magnetoelectric Cr2O3 on c-plane sapphire and ferrimagnetic Fe3O4 on MgO (001). C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4917009]
I. INTRODUCTION
Molecular beam epitaxy (MBE) is a widely used versatile growth technique. The concept is based on assembling a crystal through direct deposition of constituent atoms, achieving superior purity through the use of ultrahigh vacuum conditions.1–3 MBE has been extensively applied to the synthesis and investigation of a wide range of materials,4 including complex oxides,5 semiconductor devices,6 and magnetic tunnel junctions.7 The technique allows unmatched thickness and composition control, and it can be applied to thin-film structures both close to8 and far away9 from thermodynamic equilibrium. MBE is therefore an indispensable tool at the forefront of materials research and surface science. The interest in ever more complex material systems has required the combination of growth systems with surface analysis equipment modules to allow for in vacuo transfers.10 This is especially true for synchrotron experiments, where time is limited and growth optimization must be performed ondemand. It is therefore a great advantage to have a dedicated, versatile deposition system combined with the measurement chamber directly at the end station.11 Severe restrictions are placed upon such a growth system, particularly in terms of sample geometry and transfer procedures, in order to minimize time between sample fabrication and measurement. Growth must be reliable and reproducible, to ensure consistency
a)Authors to whom correspondence should be addressed. Electronic ad-
dresses: [email protected] and [email protected]
b)Present address: Max Planck Institute for Solid State Research, Heisen-
bergstr. 1, 70569 Stuttgart, Germany. 0034-6748/2015/86(4)/043901/7/$30.00
across measurements, and fast, to adapt to new demands as the experiments proceed. Above all, it needs to be able to grow the highest quality samples, which requires optimized sources and source geometries, pumping, cooling, and UHV motion components. Here, we present the design of the miniMBE, a new, miniaturized MBE system. It is ideally suited for applications where smaller samples are beneficial, such as surface science systems with small sample holders or for more fundamental research that does not include device processing. We will demonstrate that this system has a number of advantages over standard size deposition systems opening up new possibilities for economic, high-quality, and high-throughput MBE.
II. GENERAL DESIGN CONSIDERATIONS
The key concept of the miniMBE is to rigorously reduce the size of the system to increase versatility and throughput and ultimately to save on running costs. All components are light and small enough so that the entire system can be assembled and disassembled manually, without the need for a gantry crane. The lengths of all horizontal vacuum ports are minimized. Even the growth chamber (GC), being the largest vacuum part of the system, can be cleaned or etched by immersing it in a cylindrical vessel of comfortable size which fits into a standard fume or flow hood. This concept results in a system that can be disassembled and cleaned with a minimum of effort, allowing great flexibility when changing to new material systems or performing routine maintenance. Scaling the circular sample size of a standard MBE system of 2 in. (50.8 mm) in diameter down to 10 mm implies
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a reduction in sample area of a factor of 25. Even scaling to (10 × 10) mm2—or 15 mm in diameter—still reduces the sample area by more than a factor of 10. The crucible sizes of the effusion sources can therefore be scaled accordingly while maintaining typical time-to-refills. Starting from a crucible volume of 25 cm3 for a standard DN 40 CF 2.75 in. effusion cell, a crucible volume of 2 cm3 is therefore sufficient. In the present design, we have chosen cylindrical crucibles with an inner diameter of 7 mm and a length of 80 mm, providing a volume of 3 cm3. Together with the deposition area reduction by a factor of 10 to 50, an economic use of source materials is achieved, which is particularly advantageous when expensive source materials are required. As the volume of the chamber scales with the third power of the sample diameter, small chambers allow tremendous pumping speed improvements with compact pumps. The pump port diameter on the miniMBE growth chamber is half the chamber diameter and fitted with a 250 l/s turbomolecular pump. The vacuum volume of the chamber is on the order of 5 l. The ratio of the two is around 50/s. This compares to a pump port diameter of DN 200 CF, a 1100 l/s turbomolecular pump (Pfeiffer MAG W 1500CT), and a 500 l/s ion pump with titanium sublimation pump (Gamma), a main chamber diameter of 560 mm and a volume of ∼160 l for a traditional standard size MBE system (e.g., the Riber 32P system configured for oxides). The pumping speed to chamber volume ratio is below 10/s. The load lock (LL) chamber on the miniMBE is so small that the pumpdown time is practically the free acceleration time of the turbo pump rotor. The increased throughput mostly results from shorter vent, pump-down, cooling and heating cycles, and a compact transfer system with a minimum number of degrees of freedom. A schematic view of the stand-alone miniMBE system is shown in Fig. 1. The frame footprint is only (800 × 875) mm2. After removing the load lock transfer rod, the system can be rolled through any standard office door and fits in standard elevators.
FIG. 1. The miniMBE standalone system. Dimensions are given in millimeters.
Rev. Sci. Instrum. 86, 043901 (2015)
III. SYSTEM DESCRIPTION
The miniMBE system is composed of three chambers: the LL, the storage chamber (SC), and the GC. Due to the small sample size, 40 mm outer diameter tubes are sufficient for the sample handling chambers as well as the tubes connecting the chambers, allowing the use of economic DN 40 CF valves for loading doors and chamber isolation. The miniMBE can be configured to use circular disc shaped sample holders that are transferred and stored face-down throughout in the system. With the tube diameter limit, such holders allow substrate sizes of up to 1 in. Transfer is achieved by linear horizontal movement—plus a limited z linear movement for handover of the disc holders—using magnetically coupled transfer rods. This z movement can be integrated either in the sample stations and manipulators or in the transfer rod, which minimizes of the number of mechanical degrees of freedom. The transfer is essentially friction-free, resulting in drastically extended lifetimes of the transfer components and minimized particulate generation. Some misalignment can be accommodated for during sample handover; it is therefore possible to fully automate the system to include batch processing of samples. A second sample handling and transfer option, realized in the present system, are the standard surface science sample holders with a handle that allow substrate sizes up to about (10 × 10) mm2. Sample handling and handover takes place via bayonet tools on the transfer rods and spring-loaded guides which hold the sample in place in the storage racks and manipulators. The present miniMBE system was installed at the highresolution angle-resolved photoemission spectroscopy (HRARPES) beamline I05-1 of the Diamond Light Source (Didcot, Oxfordshire, United Kingdom) (see Fig. 2). It is designed for the growth of oxide materials, with a heated
FIG. 2. Picture of the MBE system (center) at Diamond beamline I05-1, which is coupled to a surface science chamber (right) and a HR-ARPES system (not shown).
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gas inlet and an oxygen plasma source. It is fully automated with a PC-based control system (EMERALT) that controls all growth related parameters, including the source and substrate heaters, shutters, and chamber pressures. The EMERALT system is based on a real-time core controlled by user-defined programs. The programming language is a subset of the programming language C, with additional real-time control commands. As the language includes a complete mathematical library and a considerable variety of control structures, it allows the growth of very complex layer structures including modeling of source depletion and shutter transients. Modules and functions further enhance flexibility by encapsulating such requirements into discrete, reusable entities. Automation is not limited to a growth sequence; adaptable automated pumpdown, vent, and sample loading or bake-out cycles can also be controlled through EMERALT. Interlocks and safety relevant functions are handled independently by an autonomous system based on a programmable logic controller, ensuring maximum reliability of the system. The bake-out system accommodates temperatures up to 200 ◦C, consisting of self-supporting, extremely lightweight foam panels that are assembled to form a free-standing box around the MBE system. The panels are coated with Al foil, reflecting infrared radiation and limiting heat transfer through diffusion of hot air. The low weight of the panels avoids possible damage to the machine during assembly. Bake-out heaters are plugged vertically into the main frame of the system and a high temperature fan circulates heated air within the box to eliminate temperature gradients. A. Load lock chamber
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C. Growth chamber
The growth chamber (Fig. 3) has an outer diameter of 206 mm. The largest flange on the system is therefore a DN 200 CF, and all UHV seals are CF. The entire unused free volume inside the growth chamber is filled by a liquid nitrogen cooling shroud to reduce the pumped volume and to achieve the lowest possible base pressure. The openings in the shroud are minimized in number and size for best performance, except for the pumping port which has the full inner diameter of the shroud and directly faces the volume where deposition takes place. The growth chamber is equipped with eight equivalent effusion cell ports DN 40 CF for mini effusion sources and one DN 63 CF port for either a multi-pocket electron beam evaporator or an oxygen plasma source. This larger multifunctional port can also be fitted with high volume sources or a cluster of mini sources, should a larger variety of materials be needed. It allows the use of full-size sources for components that are not yet available in a suitable form for the mini-source ports. At the expense of a mini-source port, the multifunctional port may also be located in a tilted orientation beneath the pump port for applications such as nitride growth in which Ga dripping from the sample manipulator may damage the large diameter source. The source ports are designed with a small angle of 21◦ to the substrate normal, and the hot zones of the sources are completely surrounded by the liquid nitrogen shroud. This avoids crosstalk and allows the use of hightemperature sources beyond 2000 ◦C. Apart from the ports for the linear shutters, a number of other DN 40 CF ports are provided for a retractable beamflux monitor or quartz-crystal microbalance, a main shutter in
The load lock chamber is based on a DN 40 CF cross with minimum volume. Substrates are loaded directly onto the transfer rod through a viton sealed fast loading door (with view port). The chamber is pumped by a Pfeiffer HiPace 80 turbomolecular pump with a pumping speed of 73 l/s, backed by a dry diaphragm pump. A halogen lamp enables quick desorption of water from the substrate and holder, which can be rotated to face the lamp using the transfer rod. The small volume of the chamber allows 10−6 millibars to be reached in 5 min and 5 × 10−7 millibars after a short bake-out with the halogen lamp within 20 min, suitable for transfer to the sample storage chamber. The chamber pressure is monitored using a Vacom Atmion wide range pressure measurement unit with double sensor covering the range from 1000 millibars to 1 × 10−10 mbar. B. Sample storage chamber
The sample storage chamber consists of a 40 mm diameter vertical tube, with the possibility to fit an ion pump and an optional heating stage. In the present system, no pump is mounted because this chamber also connects the growth system to the analysis chamber and the HR-ARPES branch line. The rack can be rotated around the vertical axis to allow transfer to both transfer rods from the load lock and into the growth chamber. A third rod connects the chamber at a higher level to the analysis chamber.
FIG. 3. Cutaway view of the miniMBE growth chamber. The cooling shroud fills most of the chamber volume.
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front of the substrate, and a quadrupole mass spectrometer to characterize the chamber background pressure. Three DN 40 CF viewports with window shutters allow a view to the sample manipulator for transfer, a side view of the sample during growth, and a view onto the source orifices to check the crucibles. With the small chamber geometry and the correspondingly smaller working distances, port sizes of DN 40 CF for the reflection high-energy electron diffraction (RHEED) gun and DN 63 CF for the RHEED screen are sufficient. The sample manipulator is mounted on the DN 63 CF port of the chamber top flange. The chamber is pumped by a two-stage turbomolecular pumping (TMP) system, backed by a membrane pump. This allows for a base pressure of 5 × 10−10 millibars without cooling and a base pressure of better than 1×10−9 mbar during typical growth operations with multiple cells in excess of 1000 ◦C (with the aid of the cryoshroud). The primary turbo pump is a Pfeiffer HiPace 300 M with a pumping speed of 255 l/s (N2) and magnetic bearings, allowing it to be pumped out to UHV when not in operation as it has no bearings with liquid lubricants. The pump is therefore attached directly to the growth chamber, without a gate valve, to achieve the best pumping geometry with maximum solid angle as seen from the substrate of the pumping cross section. The secondary turbo pump, a Pfeiffer HiPace 80 with a hybrid bearing and a pumping speed of 67 l/s (N2), is connected to the primary pump through a DN 16 ISO-KF pneumatic right-angle valve. This valve serves as the vacuum isolation valve in case of a pump or power failure and is interlocked to the chamber pressure. The secondary turbo pump is equipped with a N2 gas ballast input to cope with the high oxygen partial pressure in the pumped residual gas. This combination of two turbo pumps is necessary to achieve a base pressure in the lower 10−10 millibars range, as the compression ratio of a single pump is insufficient to reach low UHV with currently available roughing pump pressures. 1. Effusion sources
The growth chamber provides 8 cell ports for miniature effusion cells with reduced diameter and one central full diameter DN 63 CF compatibility port normal to the substrate for a standard size evaporation source, a cluster of three effusion cells or four electron beam evaporators, or a plasma source. A maximum of up to 12 different source materials are therefore available in the system. The source ports make a small angle of 21◦ with the substrate normal, thereby reducing oscillatory growth rate variations with substrate rotation.12 This is an important advantage for the growth of layers that sensitively depend on material stoichiometry as it efficiently reduces short-period compositional variations that might trigger, e.g., the premature nucleation of dislocations in strained systems. The small inclination angle of the effusion sources allows a complete enclosure of the hot zones of the cells by the liquid nitrogen shroud and at the same time a small diameter of the growth chamber. This concept leads to long sources compared to the scale of the chamber, an advantage as the hot zones of the sources are far away from the mounting flanges, permitting
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efficient shielding and cooling. The design therefore enables very high crucible temperatures beyond 2000 ◦C using TUBO high temperature sources.13 In addition, the extended overlap of the sources with the cooling shroud enables the use of long and narrow crucibles that offer distinct advantages in terms of material utilization and therefore longer growth runs. Long crucibles require longer heating elements and larger heated surfaces, which at first sight appears to be a disadvantage. However, heat dissipation at higher temperatures is dominated by black-body radiation, which goes as T 4, over heat conduction through the body of the shroud, which is linear in T. Radiative losses are effectively minimized by surrounding the heater and crucible by layers of radiation shields, with the only unavoidable losses happening through the source orifice. Therefore, the effusion cell power dissipation at high temperatures scales directly with the square of the crucible diameter and thus source opening, hence depending only weakly on source length. Despite their reduced diameter around the heater and crucible, the sources are built on standard DN 40 CF flanges and have the standard in-vacuum length (284 mm) of typical DN 40 CF based sources. The mini sources can therefore be used in conventional MBE chambers as well, maintaining backward compatibility. Depending on the design of the source, crucible volumes from below 2 cm3 to more than 4 cm3 can be achieved. For high vapor pressure materials, even longer high capacity crucibles with significantly larger capacities are possible. The system described here is equipped with TUBO sources with cylindrical crucibles and a crucible volume of 3 cm3. Using Al2O3 crucibles, temperatures up to about 1600 ◦C can be achieved. The thermocouple is in direct contact with the crucible, resulting in a tight control loop that produces both high agility and good temperature stability. Oversized electrical feedthroughs for the high temperature mini sources with large cross-section connections to the hot region additionally cool the source by transporting heat out of the chamber into the power leads, hence avoiding the need for water cooling. The power lead diameters are chosen such that about 50 cm of their length outside the source act as radiative coolers through heat exchange with air. 2. Shutters
The source ports are equipped with individual, horizontally mounted linear shutters, except for the central port for which the main shutter in front of the sample can be used. The shutters have double blades for high thermal insulation of the substrate in the closed position. Their design is based on a new principle of motion in vacuum14,15 that eliminates both gliding friction and elastic deformation, resulting in drastically improved lifetimes of the mechanism. In traditional designs of linear shutters, the shutter rod is either suspended from a flange attached to the chamber by a flexible bellows or moved by a magnetic coupling, in which case the rod needs to be guided by sliding bearings or ball bearings on the vacuum side. In the first case, the bellows tend to fail after larger number of movements due to fatigue in the stainless steel of the bellows. In the second case, sliding friction usually leads to failure before the required number of moves, given by the shutter cycles per sample
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and the number of samples per growth campaign, is reached. Gliding friction exists not only in glide bearings but also in ball bearings, in which the balls slide against each other or the ball cage. In UHV, without any lubricants, small imperfections on the gliding surfaces, caused by manufacturing tolerances or particles that get into the mechanism, quickly lead to accelerated abrasion and failure. We experimentally found both types of traditional designs to be limited to around 1–2 × 106 (back and forth) movement cycles before they fail. The design used here consists of a horizontal metal bar rolling on two metal cylinders as shown in Fig. 13 of Ref. 15. The horizontal bar is moved back and forth by magnetic coupling through the vacuum case. The magnetic force at the same time presses the horizontal bar onto the cylinders, thereby eliminating any play in the mechanism. Compared to the spheres of a ball bearing, such cylindrical rolling elements do not make a point contact with the tread, but touch and roll on a line. This drastically improves their loading capacity and lifetime as well as their tolerance to surface coating and particles. The cylinders are guided by teeth which again touch in line contacts, eliminating the need for a cage or other support. This design avoids gliding friction completely, and hence the need for solid-state lubricants such as polyether ether ketone (PEEK), other organic substances, carbon or non-refractory coatings which are problematic in UHV, in particular in proximity to hot parts. The motion is coupled magnetically, using magnetic stainless steel instead of permanent magnets on the vacuum side. The entire shutter can be baked up to 350 ◦C and it can be operated at temperatures up to 250 ◦C. Apart from the magnetic coupling, the mechanism is held within its housing at only one point, allowing the mechanism to adapt to changing temperatures by expanding without building up significant strain. If required, the shutters can be operated at high speeds. In test runs, we have demonstrated continuous cycling at a rate of 5 Hz in UHV for more than 10 × 106 cycles without significant wear. We therefore estimate a lifetime of at least 100 × 106 cycles, which is several orders of magnitude larger than what conventional linear shutter designs are capable of. The shutters are driven by compact linear pneumatic actuators. The relatively weak magnetic coupling in the movement direction and essentially friction free in-vacuum movement result in a soft movement of the shutter blades.
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Ref. 16, Fig. 4. It consists of one single moving part inside the vacuum, a hollow magnetic stainless steel tube rolling on the inside of the non-magnetic stainless steel outer vacuum tube. A comb of magnets rotating around the tube on the air side causes the magnetic tube rotor to describe a cycloidal motion with an offset of 2.5 mm of the magnetic tube axis to the non-magnetic tube axis. The substrate therefore describes a planetary motion on a 5 mm diameter orbit. The vertical position of the inner rotor is defined by tangential grooves in its outer surface that provide a vertical restoring force when facing the comb of alternatingly poled permanent magnets of the air rotor (Ref. 16, Fig. 8). Analogous to the design of the linear shutters, this design avoids point contacts and sliding friction, and therefore is extremely robust. In addition, it is insensitive to thermal expansion as it may expand and contract without hard mechanical constraints: the magnitude of any force on the rotor cannot be larger than its magnetic coupling force to the outside rotor. Sample holders are placed in a support plate at the lower end of the inner rotor. As the torsional stiffness of a cylinder with radius r increases with r 4, the tube can be made very thin towards the lower end, allowing good thermal insulation. Further, there are a number of radiation heat shields both on the rotating tube and the substrate heater. The top flange of the substrate manipulator is entirely independent of the motion, supporting the free standing heater. This is fixed in space and reaches down through the manipulator to the back side of the sample holder. Due to the free-standing design, ceramic insulators either can be moved away from the hot zone of the heater or can be eliminated altogether allowing for high temperature operation. Using a foil heater similar to the ones used in the TUBO mini sources, substrate temperatures of up to 1150 ◦C can be maintained. As the heater is separated from the rotational assembly, it can be easily replaced or modified, without having to disassemble or remove the rotation drive. This allows further upgrade to new heater technologies. For example, after replacing the heater flange by a transmission window, the sample may be laser-heated from the back side, allowing almost arbitrary power densities and therefore substrate temperatures, independent of the partial pressures inside the growth chamber. 4. Flux calibration and quadrupole mass spectrometry
3. Substrate manipulator
The substrate manipulator is directly attached to the top flange of the growth chamber without any x- y-z adjustment stage. This results in a very compact design, the rotation around the vertical axis is the only mechanical degree of freedom of the manipulator. The necessary alignments, required to adjust the sample transfer position, are included in the transfer rod and thereby outside the growth chamber. This follows the design principle to move complex components outside of the vacuum and away from the lowest base pressure chambers whenever possible to increase robustness and uptime of the system. The substrate manipulator uses a hollow core design in which the motion and heating subsystems are completely separated. Sample rotation is realized by a concept described in
Growth rate calibrations can be performed with a Bayard-Alpert (BA) gauge operating as a beam-flux monitor or, alternatively, with a piezoelectric quartz crystal microbalance. The BA gauge is located underneath the main shutter and can be moved independently, allowing flux calibrations directly before deposition. Since its diameter is significantly larger than the sample size and it is practically transparent to the molecular beams, depositions may even be done through the flux gauge as a reference. Whenever layer-by-layer growth is present, the RHEED system (see below) is available for growth rate calibrations as well. A separate port at the side of the growth chamber allows the permanent installation of a quadrupole mass spectrometer for residual gas analysis. One of the cell ports is configured for line-of-sight mass spectrometry. The port is shorter, allowing the attachment
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of a double walled, water-cooled DN 40 CF tube instead of an effusion source with direct line of sight to the sample surface. A QMS mounted on this port directly monitors any species leaving the sample surface, by reflection or by desorption. Having the same geometry as the adjacent source ports inside the vacuum, the background is reduced due to the liquid nitrogen shroud forming the front aperture of the detector geometry. The shutter in front of the shroud allows the alternating acquisition of spectra with and without the line-of-sight contribution from the sample for background subtraction. Line-of-sight mass spectrometry provides direct access to surface kinetic processes during growth.17 5. RHEED system
The chamber is equipped with a newly developed RHEED gun which combines compact size with high beam quality [Fig. 4(a)]. The RHEED gun has a length of 242 mm, a DN 40 CF mounting flange, and can be baked up to 200 ◦C. It is designed for an energy range of 5–15 keV with a maximum beam current of 1 mA and a beam spot size of
An ultra-compact, high-throughput molecular beam epitaxy growth system A. A. Baker,1,2 W. Braun,3,a),b) G. Gassler,4 S. Rembold,3 A. Fischer,3,a) and T. Hesjedal1,2
1
Clarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, United Kingdom Diamond Light Source, Didcot OX11 0DE, United Kingdom 3 CreaTec Fischer & Co. GmbH, Industriestr. 9, 74391 Erligheim, Germany 4 Dr. Gassler Electron Devices GmbH, List Str. 4, 89079 Ulm, Germany 2
(Received 9 January 2015; accepted 24 March 2015; published online 13 April 2015) We present a miniaturized molecular beam epitaxy (miniMBE) system with an outer diameter of 206 mm, optimized for flexible and high-throughput operation. The three-chamber system, used here for oxide growth, consists of a sample loading chamber, a storage chamber, and a growth chamber. The growth chamber is equipped with eight identical effusion cell ports with linear shutters, one larger port for either a multi-pocket electron beam evaporator or an oxygen plasma source, an integrated cryoshroud, retractable beam-flux monitor or quartz-crystal microbalance, reflection high energy electron diffraction, substrate manipulator, main shutter, and quadrupole mass spectrometer. The system can be combined with ultrahigh vacuum (UHV) end stations on synchrotron and neutron beamlines, or equivalently with other complex surface analysis systems, including low-temperature scanning probe microscopy systems. Substrate handling is compatible with most UHV surface characterization systems, as the miniMBE can accommodate standard surface science sample holders. We introduce the design of the system, and its specific capabilities and operational parameters, and we demonstrate the epitaxial thin film growth of magnetoelectric Cr2O3 on c-plane sapphire and ferrimagnetic Fe3O4 on MgO (001). C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4917009]
I. INTRODUCTION
Molecular beam epitaxy (MBE) is a widely used versatile growth technique. The concept is based on assembling a crystal through direct deposition of constituent atoms, achieving superior purity through the use of ultrahigh vacuum conditions.1–3 MBE has been extensively applied to the synthesis and investigation of a wide range of materials,4 including complex oxides,5 semiconductor devices,6 and magnetic tunnel junctions.7 The technique allows unmatched thickness and composition control, and it can be applied to thin-film structures both close to8 and far away9 from thermodynamic equilibrium. MBE is therefore an indispensable tool at the forefront of materials research and surface science. The interest in ever more complex material systems has required the combination of growth systems with surface analysis equipment modules to allow for in vacuo transfers.10 This is especially true for synchrotron experiments, where time is limited and growth optimization must be performed ondemand. It is therefore a great advantage to have a dedicated, versatile deposition system combined with the measurement chamber directly at the end station.11 Severe restrictions are placed upon such a growth system, particularly in terms of sample geometry and transfer procedures, in order to minimize time between sample fabrication and measurement. Growth must be reliable and reproducible, to ensure consistency
a)Authors to whom correspondence should be addressed. Electronic ad-
dresses: [email protected] and [email protected]
b)Present address: Max Planck Institute for Solid State Research, Heisen-
bergstr. 1, 70569 Stuttgart, Germany. 0034-6748/2015/86(4)/043901/7/$30.00
across measurements, and fast, to adapt to new demands as the experiments proceed. Above all, it needs to be able to grow the highest quality samples, which requires optimized sources and source geometries, pumping, cooling, and UHV motion components. Here, we present the design of the miniMBE, a new, miniaturized MBE system. It is ideally suited for applications where smaller samples are beneficial, such as surface science systems with small sample holders or for more fundamental research that does not include device processing. We will demonstrate that this system has a number of advantages over standard size deposition systems opening up new possibilities for economic, high-quality, and high-throughput MBE.
II. GENERAL DESIGN CONSIDERATIONS
The key concept of the miniMBE is to rigorously reduce the size of the system to increase versatility and throughput and ultimately to save on running costs. All components are light and small enough so that the entire system can be assembled and disassembled manually, without the need for a gantry crane. The lengths of all horizontal vacuum ports are minimized. Even the growth chamber (GC), being the largest vacuum part of the system, can be cleaned or etched by immersing it in a cylindrical vessel of comfortable size which fits into a standard fume or flow hood. This concept results in a system that can be disassembled and cleaned with a minimum of effort, allowing great flexibility when changing to new material systems or performing routine maintenance. Scaling the circular sample size of a standard MBE system of 2 in. (50.8 mm) in diameter down to 10 mm implies
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a reduction in sample area of a factor of 25. Even scaling to (10 × 10) mm2—or 15 mm in diameter—still reduces the sample area by more than a factor of 10. The crucible sizes of the effusion sources can therefore be scaled accordingly while maintaining typical time-to-refills. Starting from a crucible volume of 25 cm3 for a standard DN 40 CF 2.75 in. effusion cell, a crucible volume of 2 cm3 is therefore sufficient. In the present design, we have chosen cylindrical crucibles with an inner diameter of 7 mm and a length of 80 mm, providing a volume of 3 cm3. Together with the deposition area reduction by a factor of 10 to 50, an economic use of source materials is achieved, which is particularly advantageous when expensive source materials are required. As the volume of the chamber scales with the third power of the sample diameter, small chambers allow tremendous pumping speed improvements with compact pumps. The pump port diameter on the miniMBE growth chamber is half the chamber diameter and fitted with a 250 l/s turbomolecular pump. The vacuum volume of the chamber is on the order of 5 l. The ratio of the two is around 50/s. This compares to a pump port diameter of DN 200 CF, a 1100 l/s turbomolecular pump (Pfeiffer MAG W 1500CT), and a 500 l/s ion pump with titanium sublimation pump (Gamma), a main chamber diameter of 560 mm and a volume of ∼160 l for a traditional standard size MBE system (e.g., the Riber 32P system configured for oxides). The pumping speed to chamber volume ratio is below 10/s. The load lock (LL) chamber on the miniMBE is so small that the pumpdown time is practically the free acceleration time of the turbo pump rotor. The increased throughput mostly results from shorter vent, pump-down, cooling and heating cycles, and a compact transfer system with a minimum number of degrees of freedom. A schematic view of the stand-alone miniMBE system is shown in Fig. 1. The frame footprint is only (800 × 875) mm2. After removing the load lock transfer rod, the system can be rolled through any standard office door and fits in standard elevators.
FIG. 1. The miniMBE standalone system. Dimensions are given in millimeters.
Rev. Sci. Instrum. 86, 043901 (2015)
III. SYSTEM DESCRIPTION
The miniMBE system is composed of three chambers: the LL, the storage chamber (SC), and the GC. Due to the small sample size, 40 mm outer diameter tubes are sufficient for the sample handling chambers as well as the tubes connecting the chambers, allowing the use of economic DN 40 CF valves for loading doors and chamber isolation. The miniMBE can be configured to use circular disc shaped sample holders that are transferred and stored face-down throughout in the system. With the tube diameter limit, such holders allow substrate sizes of up to 1 in. Transfer is achieved by linear horizontal movement—plus a limited z linear movement for handover of the disc holders—using magnetically coupled transfer rods. This z movement can be integrated either in the sample stations and manipulators or in the transfer rod, which minimizes of the number of mechanical degrees of freedom. The transfer is essentially friction-free, resulting in drastically extended lifetimes of the transfer components and minimized particulate generation. Some misalignment can be accommodated for during sample handover; it is therefore possible to fully automate the system to include batch processing of samples. A second sample handling and transfer option, realized in the present system, are the standard surface science sample holders with a handle that allow substrate sizes up to about (10 × 10) mm2. Sample handling and handover takes place via bayonet tools on the transfer rods and spring-loaded guides which hold the sample in place in the storage racks and manipulators. The present miniMBE system was installed at the highresolution angle-resolved photoemission spectroscopy (HRARPES) beamline I05-1 of the Diamond Light Source (Didcot, Oxfordshire, United Kingdom) (see Fig. 2). It is designed for the growth of oxide materials, with a heated
FIG. 2. Picture of the MBE system (center) at Diamond beamline I05-1, which is coupled to a surface science chamber (right) and a HR-ARPES system (not shown).
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gas inlet and an oxygen plasma source. It is fully automated with a PC-based control system (EMERALT) that controls all growth related parameters, including the source and substrate heaters, shutters, and chamber pressures. The EMERALT system is based on a real-time core controlled by user-defined programs. The programming language is a subset of the programming language C, with additional real-time control commands. As the language includes a complete mathematical library and a considerable variety of control structures, it allows the growth of very complex layer structures including modeling of source depletion and shutter transients. Modules and functions further enhance flexibility by encapsulating such requirements into discrete, reusable entities. Automation is not limited to a growth sequence; adaptable automated pumpdown, vent, and sample loading or bake-out cycles can also be controlled through EMERALT. Interlocks and safety relevant functions are handled independently by an autonomous system based on a programmable logic controller, ensuring maximum reliability of the system. The bake-out system accommodates temperatures up to 200 ◦C, consisting of self-supporting, extremely lightweight foam panels that are assembled to form a free-standing box around the MBE system. The panels are coated with Al foil, reflecting infrared radiation and limiting heat transfer through diffusion of hot air. The low weight of the panels avoids possible damage to the machine during assembly. Bake-out heaters are plugged vertically into the main frame of the system and a high temperature fan circulates heated air within the box to eliminate temperature gradients. A. Load lock chamber
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C. Growth chamber
The growth chamber (Fig. 3) has an outer diameter of 206 mm. The largest flange on the system is therefore a DN 200 CF, and all UHV seals are CF. The entire unused free volume inside the growth chamber is filled by a liquid nitrogen cooling shroud to reduce the pumped volume and to achieve the lowest possible base pressure. The openings in the shroud are minimized in number and size for best performance, except for the pumping port which has the full inner diameter of the shroud and directly faces the volume where deposition takes place. The growth chamber is equipped with eight equivalent effusion cell ports DN 40 CF for mini effusion sources and one DN 63 CF port for either a multi-pocket electron beam evaporator or an oxygen plasma source. This larger multifunctional port can also be fitted with high volume sources or a cluster of mini sources, should a larger variety of materials be needed. It allows the use of full-size sources for components that are not yet available in a suitable form for the mini-source ports. At the expense of a mini-source port, the multifunctional port may also be located in a tilted orientation beneath the pump port for applications such as nitride growth in which Ga dripping from the sample manipulator may damage the large diameter source. The source ports are designed with a small angle of 21◦ to the substrate normal, and the hot zones of the sources are completely surrounded by the liquid nitrogen shroud. This avoids crosstalk and allows the use of hightemperature sources beyond 2000 ◦C. Apart from the ports for the linear shutters, a number of other DN 40 CF ports are provided for a retractable beamflux monitor or quartz-crystal microbalance, a main shutter in
The load lock chamber is based on a DN 40 CF cross with minimum volume. Substrates are loaded directly onto the transfer rod through a viton sealed fast loading door (with view port). The chamber is pumped by a Pfeiffer HiPace 80 turbomolecular pump with a pumping speed of 73 l/s, backed by a dry diaphragm pump. A halogen lamp enables quick desorption of water from the substrate and holder, which can be rotated to face the lamp using the transfer rod. The small volume of the chamber allows 10−6 millibars to be reached in 5 min and 5 × 10−7 millibars after a short bake-out with the halogen lamp within 20 min, suitable for transfer to the sample storage chamber. The chamber pressure is monitored using a Vacom Atmion wide range pressure measurement unit with double sensor covering the range from 1000 millibars to 1 × 10−10 mbar. B. Sample storage chamber
The sample storage chamber consists of a 40 mm diameter vertical tube, with the possibility to fit an ion pump and an optional heating stage. In the present system, no pump is mounted because this chamber also connects the growth system to the analysis chamber and the HR-ARPES branch line. The rack can be rotated around the vertical axis to allow transfer to both transfer rods from the load lock and into the growth chamber. A third rod connects the chamber at a higher level to the analysis chamber.
FIG. 3. Cutaway view of the miniMBE growth chamber. The cooling shroud fills most of the chamber volume.
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front of the substrate, and a quadrupole mass spectrometer to characterize the chamber background pressure. Three DN 40 CF viewports with window shutters allow a view to the sample manipulator for transfer, a side view of the sample during growth, and a view onto the source orifices to check the crucibles. With the small chamber geometry and the correspondingly smaller working distances, port sizes of DN 40 CF for the reflection high-energy electron diffraction (RHEED) gun and DN 63 CF for the RHEED screen are sufficient. The sample manipulator is mounted on the DN 63 CF port of the chamber top flange. The chamber is pumped by a two-stage turbomolecular pumping (TMP) system, backed by a membrane pump. This allows for a base pressure of 5 × 10−10 millibars without cooling and a base pressure of better than 1×10−9 mbar during typical growth operations with multiple cells in excess of 1000 ◦C (with the aid of the cryoshroud). The primary turbo pump is a Pfeiffer HiPace 300 M with a pumping speed of 255 l/s (N2) and magnetic bearings, allowing it to be pumped out to UHV when not in operation as it has no bearings with liquid lubricants. The pump is therefore attached directly to the growth chamber, without a gate valve, to achieve the best pumping geometry with maximum solid angle as seen from the substrate of the pumping cross section. The secondary turbo pump, a Pfeiffer HiPace 80 with a hybrid bearing and a pumping speed of 67 l/s (N2), is connected to the primary pump through a DN 16 ISO-KF pneumatic right-angle valve. This valve serves as the vacuum isolation valve in case of a pump or power failure and is interlocked to the chamber pressure. The secondary turbo pump is equipped with a N2 gas ballast input to cope with the high oxygen partial pressure in the pumped residual gas. This combination of two turbo pumps is necessary to achieve a base pressure in the lower 10−10 millibars range, as the compression ratio of a single pump is insufficient to reach low UHV with currently available roughing pump pressures. 1. Effusion sources
The growth chamber provides 8 cell ports for miniature effusion cells with reduced diameter and one central full diameter DN 63 CF compatibility port normal to the substrate for a standard size evaporation source, a cluster of three effusion cells or four electron beam evaporators, or a plasma source. A maximum of up to 12 different source materials are therefore available in the system. The source ports make a small angle of 21◦ with the substrate normal, thereby reducing oscillatory growth rate variations with substrate rotation.12 This is an important advantage for the growth of layers that sensitively depend on material stoichiometry as it efficiently reduces short-period compositional variations that might trigger, e.g., the premature nucleation of dislocations in strained systems. The small inclination angle of the effusion sources allows a complete enclosure of the hot zones of the cells by the liquid nitrogen shroud and at the same time a small diameter of the growth chamber. This concept leads to long sources compared to the scale of the chamber, an advantage as the hot zones of the sources are far away from the mounting flanges, permitting
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efficient shielding and cooling. The design therefore enables very high crucible temperatures beyond 2000 ◦C using TUBO high temperature sources.13 In addition, the extended overlap of the sources with the cooling shroud enables the use of long and narrow crucibles that offer distinct advantages in terms of material utilization and therefore longer growth runs. Long crucibles require longer heating elements and larger heated surfaces, which at first sight appears to be a disadvantage. However, heat dissipation at higher temperatures is dominated by black-body radiation, which goes as T 4, over heat conduction through the body of the shroud, which is linear in T. Radiative losses are effectively minimized by surrounding the heater and crucible by layers of radiation shields, with the only unavoidable losses happening through the source orifice. Therefore, the effusion cell power dissipation at high temperatures scales directly with the square of the crucible diameter and thus source opening, hence depending only weakly on source length. Despite their reduced diameter around the heater and crucible, the sources are built on standard DN 40 CF flanges and have the standard in-vacuum length (284 mm) of typical DN 40 CF based sources. The mini sources can therefore be used in conventional MBE chambers as well, maintaining backward compatibility. Depending on the design of the source, crucible volumes from below 2 cm3 to more than 4 cm3 can be achieved. For high vapor pressure materials, even longer high capacity crucibles with significantly larger capacities are possible. The system described here is equipped with TUBO sources with cylindrical crucibles and a crucible volume of 3 cm3. Using Al2O3 crucibles, temperatures up to about 1600 ◦C can be achieved. The thermocouple is in direct contact with the crucible, resulting in a tight control loop that produces both high agility and good temperature stability. Oversized electrical feedthroughs for the high temperature mini sources with large cross-section connections to the hot region additionally cool the source by transporting heat out of the chamber into the power leads, hence avoiding the need for water cooling. The power lead diameters are chosen such that about 50 cm of their length outside the source act as radiative coolers through heat exchange with air. 2. Shutters
The source ports are equipped with individual, horizontally mounted linear shutters, except for the central port for which the main shutter in front of the sample can be used. The shutters have double blades for high thermal insulation of the substrate in the closed position. Their design is based on a new principle of motion in vacuum14,15 that eliminates both gliding friction and elastic deformation, resulting in drastically improved lifetimes of the mechanism. In traditional designs of linear shutters, the shutter rod is either suspended from a flange attached to the chamber by a flexible bellows or moved by a magnetic coupling, in which case the rod needs to be guided by sliding bearings or ball bearings on the vacuum side. In the first case, the bellows tend to fail after larger number of movements due to fatigue in the stainless steel of the bellows. In the second case, sliding friction usually leads to failure before the required number of moves, given by the shutter cycles per sample
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and the number of samples per growth campaign, is reached. Gliding friction exists not only in glide bearings but also in ball bearings, in which the balls slide against each other or the ball cage. In UHV, without any lubricants, small imperfections on the gliding surfaces, caused by manufacturing tolerances or particles that get into the mechanism, quickly lead to accelerated abrasion and failure. We experimentally found both types of traditional designs to be limited to around 1–2 × 106 (back and forth) movement cycles before they fail. The design used here consists of a horizontal metal bar rolling on two metal cylinders as shown in Fig. 13 of Ref. 15. The horizontal bar is moved back and forth by magnetic coupling through the vacuum case. The magnetic force at the same time presses the horizontal bar onto the cylinders, thereby eliminating any play in the mechanism. Compared to the spheres of a ball bearing, such cylindrical rolling elements do not make a point contact with the tread, but touch and roll on a line. This drastically improves their loading capacity and lifetime as well as their tolerance to surface coating and particles. The cylinders are guided by teeth which again touch in line contacts, eliminating the need for a cage or other support. This design avoids gliding friction completely, and hence the need for solid-state lubricants such as polyether ether ketone (PEEK), other organic substances, carbon or non-refractory coatings which are problematic in UHV, in particular in proximity to hot parts. The motion is coupled magnetically, using magnetic stainless steel instead of permanent magnets on the vacuum side. The entire shutter can be baked up to 350 ◦C and it can be operated at temperatures up to 250 ◦C. Apart from the magnetic coupling, the mechanism is held within its housing at only one point, allowing the mechanism to adapt to changing temperatures by expanding without building up significant strain. If required, the shutters can be operated at high speeds. In test runs, we have demonstrated continuous cycling at a rate of 5 Hz in UHV for more than 10 × 106 cycles without significant wear. We therefore estimate a lifetime of at least 100 × 106 cycles, which is several orders of magnitude larger than what conventional linear shutter designs are capable of. The shutters are driven by compact linear pneumatic actuators. The relatively weak magnetic coupling in the movement direction and essentially friction free in-vacuum movement result in a soft movement of the shutter blades.
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Ref. 16, Fig. 4. It consists of one single moving part inside the vacuum, a hollow magnetic stainless steel tube rolling on the inside of the non-magnetic stainless steel outer vacuum tube. A comb of magnets rotating around the tube on the air side causes the magnetic tube rotor to describe a cycloidal motion with an offset of 2.5 mm of the magnetic tube axis to the non-magnetic tube axis. The substrate therefore describes a planetary motion on a 5 mm diameter orbit. The vertical position of the inner rotor is defined by tangential grooves in its outer surface that provide a vertical restoring force when facing the comb of alternatingly poled permanent magnets of the air rotor (Ref. 16, Fig. 8). Analogous to the design of the linear shutters, this design avoids point contacts and sliding friction, and therefore is extremely robust. In addition, it is insensitive to thermal expansion as it may expand and contract without hard mechanical constraints: the magnitude of any force on the rotor cannot be larger than its magnetic coupling force to the outside rotor. Sample holders are placed in a support plate at the lower end of the inner rotor. As the torsional stiffness of a cylinder with radius r increases with r 4, the tube can be made very thin towards the lower end, allowing good thermal insulation. Further, there are a number of radiation heat shields both on the rotating tube and the substrate heater. The top flange of the substrate manipulator is entirely independent of the motion, supporting the free standing heater. This is fixed in space and reaches down through the manipulator to the back side of the sample holder. Due to the free-standing design, ceramic insulators either can be moved away from the hot zone of the heater or can be eliminated altogether allowing for high temperature operation. Using a foil heater similar to the ones used in the TUBO mini sources, substrate temperatures of up to 1150 ◦C can be maintained. As the heater is separated from the rotational assembly, it can be easily replaced or modified, without having to disassemble or remove the rotation drive. This allows further upgrade to new heater technologies. For example, after replacing the heater flange by a transmission window, the sample may be laser-heated from the back side, allowing almost arbitrary power densities and therefore substrate temperatures, independent of the partial pressures inside the growth chamber. 4. Flux calibration and quadrupole mass spectrometry
3. Substrate manipulator
The substrate manipulator is directly attached to the top flange of the growth chamber without any x- y-z adjustment stage. This results in a very compact design, the rotation around the vertical axis is the only mechanical degree of freedom of the manipulator. The necessary alignments, required to adjust the sample transfer position, are included in the transfer rod and thereby outside the growth chamber. This follows the design principle to move complex components outside of the vacuum and away from the lowest base pressure chambers whenever possible to increase robustness and uptime of the system. The substrate manipulator uses a hollow core design in which the motion and heating subsystems are completely separated. Sample rotation is realized by a concept described in
Growth rate calibrations can be performed with a Bayard-Alpert (BA) gauge operating as a beam-flux monitor or, alternatively, with a piezoelectric quartz crystal microbalance. The BA gauge is located underneath the main shutter and can be moved independently, allowing flux calibrations directly before deposition. Since its diameter is significantly larger than the sample size and it is practically transparent to the molecular beams, depositions may even be done through the flux gauge as a reference. Whenever layer-by-layer growth is present, the RHEED system (see below) is available for growth rate calibrations as well. A separate port at the side of the growth chamber allows the permanent installation of a quadrupole mass spectrometer for residual gas analysis. One of the cell ports is configured for line-of-sight mass spectrometry. The port is shorter, allowing the attachment
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of a double walled, water-cooled DN 40 CF tube instead of an effusion source with direct line of sight to the sample surface. A QMS mounted on this port directly monitors any species leaving the sample surface, by reflection or by desorption. Having the same geometry as the adjacent source ports inside the vacuum, the background is reduced due to the liquid nitrogen shroud forming the front aperture of the detector geometry. The shutter in front of the shroud allows the alternating acquisition of spectra with and without the line-of-sight contribution from the sample for background subtraction. Line-of-sight mass spectrometry provides direct access to surface kinetic processes during growth.17 5. RHEED system
The chamber is equipped with a newly developed RHEED gun which combines compact size with high beam quality [Fig. 4(a)]. The RHEED gun has a length of 242 mm, a DN 40 CF mounting flange, and can be baked up to 200 ◦C. It is designed for an energy range of 5–15 keV with a maximum beam current of 1 mA and a beam spot size of
