A six-circle diffractometer system for synchrotron X-ray studies of surfaces and thin film growth by molecular beam epitaxy

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1 Nuclear Instruments and Methods in Physics Research A 572 (2007) A six-circle diffractometer system for synchrotron X-ray studies of surfaces and thin film growth by molecular beam epitaxy Hawoong Hong a,, T.-C. Chiang a,b a Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, 104 South Goodwin Avenue, Urbana, IL , USA b Department of Physics, University of Illinois at Urbana-Champaign, 1110 West Green Street, Urbana, IL , USA Received 6 September 2006; received in revised form 13 November 2006; accepted 14 November 2006 Available online 15 December 2006 Abstract A new ultrahigh vacuum (UHV) surface diffractometer system equipped with molecular beam epitaxy (MBE) capabilities has been developed. It has a versatile 6-circle configuration for defining the diffraction geometry, and a three-axis translation stage for controlling the sample position. Rugged mechanical components are employed in the design to allow accurate diffraction measurements. Sample cooling is facilitated by passing liquid nitrogen into a reservoir in the base of a sample mount. The sample can be heated to very high temperatures by either direct current heating or electron beam bombardment. During film growth and processing, the sample temperature, monitored by thermocouples, can be continuously and rapidly varied between 110 K to above room temperature. A charge coupled device (CCD) camera, attached to the m circle, allows rapid reciprocal space mapping for real time studies of sample growth and evolution during deposition and annealing. A beam stop and a baffle are implemented to minimize stray scattered radiation. r 2006 Elsevier B.V. All rights reserved. PACS: Kf; Jy; Qe; Mc Keywords: X-ray diffractometer; X-ray surface diffraction; Molecular beam epitaxy; Low temperatures 1. Introduction Corresponding author S. Cass Ave, Bldg 438D, Argonne, IL 60439, USA. Tel.: ; fax: address: h-hong@uiuc.edu (H. Hong). X-ray diffraction is a powerful technique for characterizing the atomic structure of surfaces and thin films. Unlike other surface probes such as scanning tunneling microscopy and low energy electron diffraction, which probe the top few atomic layers only, X-rays penetrate deeply and can provide detailed information for all atomic layers in thin films including the interfacial structure. Furthermore, the typical X-ray scattering geometry is well suited for realtime studies of film growth and surface processing. To utilize this technique, a UHV chamber with sample growth and manipulation capabilities must be coupled to an X-ray diffractometer. A number of such systems exist worldwide [1 7]. We have examined the existing designs and have constructed a new system with improved stability, accuracy, flexibility, and functionality. Specifically, a full 6-circle geometry is implemented in the new design that allows accurate reciprocal spacing mapping over wide ranges of momentum transfer. Ease of use, alignment, and maintenance is also a priority in the design. Here we present a description of the essential features of the new system. 2. Instruments 2.1. The diffractometer A 6-circle diffractometer configuration suitable for use at a synchrotron source is chosen for our design [6,8]. Compared to a 2+2 configuration (2 circles for the sample and 2 circles for the detector) [5], the 6-circle configuration allows much easier sample alignment and scanning of the incident and outgoing beam directions. The geometry also provides a larger sample mounting space. Fig. 1 shows a picture of our system with the major /$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi: /j.nima

$A CCD camera for recording diffraction patterns was removed in order to show the other components of the system. Fig. 2. A schematic diagram for the 6 circles of the diffractometer.$ components labeled. Fig. 2 is a schematic diagram illustrating the six circles of motion. Here, we adopt the 6-circle naming convention used in Ref. [8].

components labeled. Fig. 2 is a schematic diagram illustrating the six circles of motion. Here, we adopt the 6-circle naming convention used in Ref. [8].

2 H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) Fig. 1. A picture of the system with major components labeled. A CCD camera for recording diffraction patterns was removed in order to show the other components of the system. Fig. 2. A schematic diagram for the 6 circles of the diffractometer. components labeled. Fig. 2 is a schematic diagram illustrating the six circles of motion. Here, we adopt the 6-circle naming convention used in Ref. [8]. The sample orientation is controlled by the usual 3 circles j, w, and y. The w movement is restricted to 751 out of the possible 7201 range because of space limitations caused by mechanical interference between the sample XYZ translational stages and the bore holes of the y and d stages. The detector orientation is controlled by two circles, a d circle that is parallel to the y circle, and an orthogonal g circle. The five circles described above are mounted on a base plate, which also supports the UHV chamber. The entire assembly sits on a large m circle which rotates about a vertical axis. With appropriate software for synchronous scanning of the 6 circles, both the sample and the detector angular orientations can be independently scanned. The use of a CCD area detector provides additional flexibility for reciprocal space mapping. Two model 440 goniometers manufactured by Huber Diffraktionstechnik Gmbh, Rimstring, Germany, are used for the y and d circles. A larger goniometer, Huber model 480, is used for the m circle, which supports the rest of the diffractometer system and the vacuum chamber. With our chamber and diffractometer placement, the offcenter load for the m circle is well within the specifications of the Huber 480. As a result, no counter weight is necessary for this circle. By contrast, the off-center load on the d circle is rather large due to the attached g circle. A counter weight is necessary and is implemented for this circle. The w motion is provided by two parallel circular segments mounted on the y goniometer. These segments are PWM series goniometers made by Franke Gmbh, Aalen, Germany. Ball bearings, instead of dovetails, are used in these goniometers, resulting in a smooth, low friction motion. One of the two segments, with a motor drive, serves as the master, and the other segment serves as a slave. This is different from some of the earlier designs in which the two segments are driven simultaneously either by a mechanical linkage or by two synchronously driven motors. Those designs, however, could suffer from excessive stress or even jamming because of mechanical errors. The f circle is a PDM series goniometer made by Franke Gmbh. It is mounted on the two w segments. The sample manipulator, to be described below, is mounted on this circle. Previous designs of the g motion generally used a composite mechanism involving a translational motion and a synchronized rotational motion. The distance between the detector and the sample varies during scanning, which must be taken into account for proper intensity normalization. The synchronization of the two motions adds complexity to the system, and the assembly can be relatively bulky. Instead, we employ a single motion guided by a curved rail purchased from THK Co. Ltd., Tokyo, Japan. A schematic drawing of the subsystem is shown in Fig. 3. The detector assembly rides on the rail by re-circulating ball bearings, maintaining its aiming at the sample, and its position is varied by a ball screw drive. The motor end of the ball screw drive is attached to a ball bearing, which allows the ball screw to pivot as the detector assembly travels along the curved rail. The ball screw drives a traveling block which is attached to the detector assembly by another ball bearing to provide a second pivot point. The frames and mounting brackets of the assembly are made of aluminum for its light weight, and dowel pins are used for precision positioning. The system, with a dynamic load rating over 30 kn, has excellent stability and rigidity. The ball screw drives the traveling block along a sliding chord of the curved rail. The g angle is related to the linear

3 944 ARTICLE IN PRESS H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) The MBE chamber Fig. 3. A schematic diagram for the g circle drive at (A) g ¼ 01 and (B) g ¼ 451. The components are: (a) pivot point for the ball screw with the center fixed to the frame, (b) ball screw, (c) curved rail, (d) pivot point on the traveling block, and (e) detector. travel x of the ball screw by cosðg þ g 0 Þ¼ a2 þ b 2 ðxþx 0 Þ 2, 2ab where x 0 ¼ mm is the distance between the ball screw pivot point and the traveling block pivot point when g ¼ 01, b ¼ 600 mm is the radius of the curved rail, and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a ¼ b 2 þ x 2 0. This conversion relationship is built into the motor drive software. The six circles, after assembly, are aligned to within a sphere of confusion less than 25 mm. The parallelism and orthogonality among the various circles, except the g circle, are aligned to within 10 arcsec. The alignment follows standard procedures using a precision ball with an error less than , a precision cube with an error less than over 6 00, and an electronic dial gauge with a sensitivity of The whole assembly of the diffractometer and the UHV chamber is mounted on a large table with wheels and jack screw stands. The wheels permit easy transportation of the system. The table is motorized in two directions for adjustment of the diffractometer center in both the vertical and horizontal directions relative to the incoming X-ray beam. The MBE chamber has a large beryllium window in the form of a cylindrical sector, which allows the incoming and exiting X-ray angles to vary from 31 to 451 relative to the sample surface. The detector angle d ranges from 101 to The chamber has two OD flanged ports for evaporation sources aiming at the sample at the X-ray diffraction position. A remotely controlled pneumatic shutter is available to block the deposition. The chamber sits on a sliding table. It can be translated relative to the diffractometer and the sample manipulator by stretching a connecting bellows. When it is moved away from the X-ray diffraction position, a crystal thickness monitor, mounted on a bellows translation stage, can be moved into the deposition target position for calibrating the deposition rate. A reflection high energy electron diffraction (RHEED) system is mounted 9 00 away from the X-ray diffraction position, which can be used to characterize the sample surface. Also mounted on the chamber are a sputter gun and an argon gas line with a leak valve; these are used for sample cleaning. The chamber pressure is typically maintained at below Torr using an ion pump and a titanium sublimation pump. The sample manipulator is mounted on the f circle of the diffractometer. The coupling to the chamber is via a two-stage differentially pumped rotary feedthrough followed by a 6 00 OD bellows. The feedthrough, purchased from Thermionics, has a 6 00 OD flange on one side and a OD flange on the other. The two differential stages are pumped by a turbo pump and a small ion pump, respectively. This feedthrough permits the sample to execute the y and f motions in vacuum. The 6 00 OD connecting bellows has sufficient travel for the chamber to be translated 1 00 away from the X-ray diffraction position as discussed above. It also permits the w motion The variable temperature sample holder assembly and manipulator The sample holder assembly in our design allows heating, cooling, and temperature sensing to facilitate sample processing and MBE growth. It is mounted on an XYZ triple axis translation stage for sample alignment and for moving the sample from the X-ray diffraction position to the RHEED position. The translation stage must be stable and repeatable for diffractometer motions. Fig. 4 shows a schematic diagram for the XYZ manipulator assembly. On the left end of the manipulator is a OD end flange, as shown in Fig. 4(B). Welded onto this flange, on the atmospheric side, are several smaller flanges for electrical, thermocouple, and liquid nitrogen feedthroughs. Also welded onto this flange, but on the vacuum side, is a 2 00 OD stainless steel tube. This tube, with venting holes, functions as the main shaft for the Z motion. It is also the conduit for the electrical, thermocouple, and liquid nitrogen connections to the sample. A bellows section as

H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) 942 947 945 Fig. 4.

4 H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) Fig. 4. A diagram for the sample manipulator: (A) cooling line assembly (only one of the two cooling lines is shown), (B) linear translation stage, and (C) XYZ sample manipulator assembly with (A) and (B) included. The components are: (a) liquid nitrogen lines with coiled sections, (b) liquid nitrogen reservoir, (c) screw base for sample holders, (d) bellows for the Z motion, (e) 2 00 ID re-circulating ball bushing mounted inside a flanged nipple, (f) reservoir clamp, (g) stainless steel tube shaft, (h) ACME screw, (i) compact XY slide, and (j) tandem linear bushings. seen in the figure allows the Z motion of the shaft and the end flange. Attached to the right side of the bellows section is a linear bushing assembly for guiding the Z motion of the tube shaft. The internal details are not shown in Fig. 4. It consists of a precision in-vacuum re-circulating ball linear bearing with a 2 00 ID, and the two ends of the bearing are brazed directly to vacuum flanges. A picture of the inside of the ball bearing is shown in Fig. 5. The re-circulating balls are made of stainless steel and are coated with Dicoronite s dry lubricant. The body of the bearing and the ball races are made of steel. With proper cleaning, they do not affect the vacuum. To minimize the possibility of seizing of the tube shaft by the linear bearing caused by particle contamination, the OD of the tube was turned down slightly except for a portion corresponding to the position for X-ray diffraction, where positioning repeatability is critical. As shown in Fig. 4(C), the Z motion is driven by an ACME screw and is additionally supported by three hardened shafts, each with two tandem linear bearings. All of these components are outside the UHV. The external shafts and the large internal shaft can be over constraining if not properly aligned. To avoid this problem, the OD end flange of the manipulator is initially not rigidly attached to the Z motion mechanism. After the Z stage is assembled and evacuated to vacuum, the end flange is fixed to the Z motion by a set of push-pull screws. The result is a very rigid structure. Fig. 5. A picture of the internal bore of the UHV linear bearing for the manipulator Z motion. The shaft for the Z motion is removed for the picture. Also seen is the inside of the bellows of the manipulator. The Z stage is attached to an XY translation stage, which is in turn bolted onto the f circle. Crossed roller bearings with a high load capacity are used on the XY stage. The OD end flange of the linear bushing assembly is coupled to the differentially pumped rotary feedthrough mentioned above. The 6 00 OD bellows that connects the differentially pumped rotatry feedthrough and the MBE chamber provides the freedom needed for the XY motions. The liquid nitrogen feedthrough is mounted on a OD flange, as shown in Fig. 4(A). The lines inside the UHV have a coiled structure to absorb thermal expansion and contraction. The lines, electrically isolated by a ceramic cryogenic isolator, lead to a reservoir clamped to the end of the 2 00 tube shaft in the XYZ stage. The cooling lines and the reservoir can be pulled out through the OD mounting flange for interchange or repair. The sample side of the reservoir has a molybdenum cap with a protruding threaded stud for attaching a sample mount. Fig. 6 shows the sample mount. The base of the sample mount is made of molybdenum. One sample post is made of sapphire, which has a high thermal conductivity at low temperatures but a low thermal conductivity at high temperatures. This is a desirable property because it provides efficient sample cooling at low temperatures and some thermal isolation to the liquid nitrogen reservoir when the sample is being heated to high temperatures. The top surface of the sapphire post has a step. Each sample to be mounted is pushed against this step for alignment and clamped down with a tungsten sheet spring and a tantalum top cover. The other end of the sample is held by a tantalum clip constructed of a thin foil to minimize thermal conduction and mechanical constraint. Thermal expansion

$Two survey targets, when mounted on the diffractometer, provide a plumb plane going through the diffractometer center of rotation.$

5 946 ARTICLE IN PRESS H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) to within 50 mm. Two survey targets, when mounted on the diffractometer, provide a plumb plane going through the diffractometer center of rotation. This setup facilitates the alignment of the sample and the X-ray beam relative to the diffractometer center. 3. Auxiliary components Fig. 6. A picture of the sample mount with the major components labeled. and contraction of the sample during heating and cooling are absorbed by this weak connection. A filament heater is located underneath the sample. Two sets of thermocouples are available; each can be attached to the tantalum end clip, clamped down with the sample on the sapphire post, or pressed against the sample using a coiled section as a spring. A high voltage can be applied between the filament and the sample for electron beam bombardment heating. The substrate can also be heated by a direct current up to 15 A flowing through the sample. The external liquid nitrogen lines have a high pressure check valve for vapor bypassing. When liquid nitrogen is first turned on, the high pressure vapor from evaporation is vented through this valve, allowing the liquid to move forward. When the lines become cold, the pressure drops and the valve closes. The result is a fast cool down. Starting with the system at room temperature, the sample temperature drops to 110 K in 20 minutes. The sample can be heated to above room temperature with the liquid nitrogen running by using the filament heater or by passing a current through the sample Sample alignment A large 8 00 OD view port facing the sample surface (see Fig. 1) facilitates the centering of the sample in the XY plane. This is done by using external fiducial marks on the diffractometer and an auto-leveling telescope. The tilt angles of the sample surface can be adjusted relative to the y rotation plane by using the w and f rotations while bouncing a laser beam off the sample surface through this window. This alignment can be further checked with X-ray reflection. Alignment of the Z position of the sample is facilitated by a OD view port located on the side of the chamber downstream from the X-ray beam and oriented at 451 below the horizontal plane. A mirror and a jig transit are employed to position the sample Z position The 6-circle geometry described above is well suited for measurements involving rocking curves, line scans, and reciprocal space mapping. Another useful mode of measurement is reflection surface X-ray diffraction (RSXD). This is the X-ray equivalent of RHEED [9] and yields RHEED-like diffraction patterns from surfaces on an imaging device such as a CCD camera or an image plate. While RSXD images may appear similar to RHEED patterns, the information content is very different. X-rays have a much greater penetration depth, and the scattering is generally well described by a kinematic model. These properties permit detailed quantitative structural analyses to include the subsurface and interface regions. Stray X-ray scattering from chamber components, however, can be problematic. Surface diffraction is very weak compared to Bragg diffraction. It is easy for stray powder patterns from chamber components to fall on the detector and contaminate the signal from surface diffraction. To avoid this problem, the chamber is equipped with a beam stop and a scattering baffle in the chamber. The rectangular baffle, mounted on a motorized rotary feedthrough, has a circular notch along one edge for the incoming X-ray beam to pass through. The powder diffraction pattern from the X-ray entrance point on the Be window is thus blocked over the portion of reciprocal space of interest. The internal beam stop blocks and absorbs the main X-ray beam after it passes through the sample. Its position is adjustable in the vertical direction by a lever and a cam mechanism. A picture of these components is shown in Fig. 7. These components are found to be critical for the acquisition of high quality data suitable for quantitative analyses [10]. For most of our RSXD work, a CCD camera is used. This camera is mounted on a two dimensional linear positioner, which is in turn mounted on the m circle of the diffractometer. Fig. 1 shows the positioner, but the camera was removed in order to show the other components. 4. Concluding remarks The diffractometer described above has been successfully operated on the undulator beamline of Sector-33, UNI- CAT, at the Advanced Photon Source, Argonne National Laboratory. The high brilliance of this third generation synchrotron radiation facility makes it possible to take rocking curves, line scans, reciprocal space mapping, and diffraction images rapidly. The rapid data acquisition rate in turn makes it possible to study film growth and surface evolution during annealing in real time. Some of the recent

H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) 942 947 947 Chemical Society, and the US National Science Foundation (Grant no.

6 H. Hong, T.-C. Chiang / Nuclear Instruments and Methods in Physics Research A 572 (2007) Chemical Society, and the US National Science Foundation (Grant no. DMR ), for partial support of the personnel and beam line operations. The Advanced Photon Source at Argonne National Laboratory is supported by the US Department of Energy, Office of Basic Energy Sciences (Contract no. W ENG-38). References Fig. 7. A picture of the inside of the chamber showing the baffle and beam stop. The beam stop can be barely seen on the left edge of a bracket. results taken from this instrument can be found in the literature [9 16]. Acknowledgments The authors wish to thank Prof. Ian Robinson and Dr. D. Gibbs for invaluable discussions regarding the system design. Thanks are also due to Mr. David Robison for the mechanical design of the g circle and Mr. Sung Won Choi for the system alignment. This work is supported by the US Department of Energy, Division of Materials Sciences (Grant No. DEFG02-91ER45439). We acknowledge the Petroleum Research Fund, administered by the American [1] S. Brennan, P. Eisenberger, Nucl. Instr. and Meth. 222 (1984) 164. [2] P.H. Fuoss, I.K. Robinson, Nucl. Instr. and Meth. 222 (1984) 171. [3] E. Vlieg, A. van t Ent, A.P. De Jongh, H. Neerings, J.F. van der Veen, Nucl. Instr. and Meth. A 262 (1987) 522. [4] D. Gibbs, B.M. Ocko, D.M. Zehner, S.G.J. Mochrie, Phys. Rev. B 42 (1990) [5] K.W. Evans-Lutterodt, M.-T. Tang, J. Appl. Cryst. 28 (1995) 318. [6] S. Ferrer, F. Comin, Rev. Sci. Instrum. 66 (1995) [7] J.S.G. Taylor, C. Norris, E. Vlieg, M. Lohmeier, T.S. Turner, Rev. Sci. Instrum. 67 (1996) [8] M. Lohmeier, E. Vlieg, J. Appl. Cryst. 26 (1993) 706. [9] H. Hong, Z. Wu, T.-C. Chiang, P. Zschack, P. Jemian, H. Chen, R.D. Aurano, Rev. Sci. Instrum. 71 (2000) 3132; H. Hong, Z. Wu, T.-C. Chiang, P. Zschack, H. Chen, Rev. Sci. Instrum. 73 (2002) [10] H. Hong, C.-M. Wei, M.Y. Chou, L. Basile, H. Chen, M. Holt, T.-C. Chiang, Phys. Rev. Lett. 90 (2003) [11] P. Czoschke, H. Hong, L. Basile, T.-C. Chiang, Phys. Rev. B 72 (2005) [12] P. Czoschke, H. Hong, L. Basile, T.-C. Chiang, Phys. Rev. B 72 (2005) [13] P. Czoschke, H. Hong, L. Basile, T.-C. Chiang, Phys. Rev. Lett. 93 (2004) [14] L. Basile, H. Hong, P. Czoschke, T.-C. Chiang, Appl. Phys. Lett. 84 (2004) [15] P. Czoschke, H. Hong, L. Basile, T.-C. Chiang, Phys. Rev. Lett. 91 (2003) [16] H. Hong, L. Basile, P. Czoschke, A. Gray, and T.-C. Chiang, submitted for publication.

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