The Atom Scatterer
The purpose of a vacuum chamber is to provide a clean environment, free from extraneous atmospheric influences, where
controlled experiments may be performed. For surface analysis, evacuation of atmospheric gases from a sample's vicinity
allow that sample to be cleaned atomically and investigated on a reasonable timescale before recontamination from
residual gas occurs. For HAS, a vacuum is also required to ensure the unhindered passage of the helium beam. All
atom-beam machines have a similar structure and can be divided into three main stages: the beam source and differential
pumping stages; the experimental chamber(s); the detector chamber and pumping stages. Those machines designed for
surface structural investigations are designed to probe as large a portion of reciprocal space as possible, within
physical limitations. It is difficult to rotate a helium beam source about a fixed crystal, so most atom beam machines
hold the source fixed and rotate the crystal instead. In general, the detector itself is also fixed, restricting the
portion of reciprocal space that can be mapped.
Figure 2: A side-view of the Cambridge atom scatterer with cut-away to reveal the
rotatable detector. For clarity, the chambers are printed in black, UHV pumps (backing pumps are omitted) in red and
frames and supports in blue. The purple dotted line indicates a typical path for the helium beam when using the
Figure 2: A side-view of the Cambridge atom scatterer with cut-away to reveal the rotatable detector. For clarity, the chambers are printed in black, UHV pumps (backing pumps are omitted) in red and frames and supports in blue. The purple dotted line indicates a typical path for the helium beam when using the rotatable detector.
All three of the main stages of the Cambridge atom scatterer are sketched in figure 3. The source and differential stages are indicated to the right of the figure and the main scattering chamber is to the centre. Unusually, the Cambridge atom scatterer has two distinct detectors: one to the top of the scattering chamber, offering a fixed angle detector with low background signal and high angular resolution; and one inside the scattering chamber itself, which can be rotated about the sample position and thereby probe a far greater range of scattering geometries. Indeed, the combination of a rotatable detector and a sample with three additional degrees of rotational freedom allow the same flexibility in scattering geometries that the 'ideal' set-up of a fixed sample and positionable source and detector would have achieved.
The first stage is the helium beam source. A nearly-free supersonic expansion source is used to produce an intense, monochromatic helium beam (right hand side of figure). A compressor (not shown) is used to pressurise bottled helium, which is then regulated to a working pressure of approximately 70 bar. The helium gas is passed through a liquid nitrogen-cooled filter, to remove condensible contaminants, before being expanded through a 10 micron diameter nozzle into the evacuated source chamber. A helium refrigerator and resistive heater can be used to control the nozzle (and hence beam) temperature between 50 K and 350 K, and the whole nozzle assembly is mounted on a three-axis manipulator. The source is pumped by a 1500 l/s turbo-molecular pump, backed by a Roots pump and rotary vane pump during beam operation, or by a smaller rotary pump when the beam is off. For a nozzle pressure of 70 bar, the typical operating pressure in the source chamber is 5E-4 mbar , whilst the chamber is under high vacuum conditions (~5E-8 mbar) when the beam is off. To select a fine helium beam, a skimmer (see Chapter 1) is placed down-stream from the nozzle, typically 10 mm from the nozzle position (optimised by moving the nozzle) and with an orifice diameter of 0.5 mm. The beam then passes through two differential pumping stages to reduce background gas levels, both pumped by 1000 l/s diffusion pumps and backed by the same rotary pump. Finally, the beam passes through a limiting aperture, 1.0 mm diameter and approximately 525 mm downstream from the nozzle, into the main scattering chamber.
The main scattering chamber (figure 3, centre) is a 1.0m diameter, 0.55m deep stainless steel cylindrical UHV chamber, pumped by two 1000 l/s turbo-molecular pumps and a liquid nitrogen-cooled titanium sublimation pump. The compression ratio of the main pumps is increased by backing them with a 100 l/s diffusion pump before the rotary pump. After baking for four days at 130 degrees C and degassing, the main chamber's base pressure is typically 2E-10 mbar, the residual gas composing primarily of hydrogen.
All sample manipulation and processing is conducted within the main chamber. The sample is mounted on a six-axis in-vacuo manipulator, inserted horizontally through the uppermost flange of the scattering chamber, as seen in figure 3. Three translational and three rotational independent degrees of freedom allows the crystal face and its rotation axis to be placed coincident with the rotation axis of the detector. Computer-controlled stepper motors mounted outside the chamber control each motion through rotary drives, with a rotational precision of 0.01 degrees and a positional precision of 0.05 mm. A liquid nitrogen cooled cold-block and rear filament allow the crystal temperature to be controlled and monitored via a thermocouple whilst water-cooled metal deposition sources mounted on the manipulator and infront of the sample allow in-situ measurements of metal deposition.
To clean the crystal in-vacuo, an electron-bombardment argon ion gun is mounted on the main chamber. The ion gun is housed within a 40 mm-bore stainless steel tube, terminated by a ~10 mm aperture approximately 300 mm from the crystal surface. Gas is admitted into the ionisation region by a leak valve. The design allows the argon pressure in the gun's ionisation region to remain ~2 orders of magnitude higher than in the main chamber. Typically, the gun is operated at an ion energy of 800 V, with a sample current of 7 microA. Also mounted on the main chamber is a leak valve for gas dosing, an ion vacuum gauge and a combined Auger and Rear-view LEED system for elemental and structural analysis.
Mounted on the opposite flange to the manipulator is a rotatable, differentially pumped detector. Detector rotation is concentric with that of the sample. The detector is positioned at the end of a stainless steel tube and consists of an ionisation source, quadrupole mass spectrometer (tuned to helium) and an off-axis secondary electron multiplier. Crystal and ionisation source are separated by 470 mm, with the line of sight between restricted by a movable 2 mm-diameter aperture 280 mm from the crystal. After alignment, the angular resolution of this detector is 0.53 degrees. To reduce background gas levels, the detector and rotating arm are pumped by a 340 l/s turbo-molecular pump, backed by a 100 l/s diffusion pump and a rotary pump. Combined with the six degrees of freedom afforded by the manipulator, this rotatable detector offers a highly flexible system capable of probing a large fraction of reciprocal space, as required for accurate structural analysis. High resolution time-of-flight studies can be made by a second, fixed detector situated at the top of the main scattering chamber (see figure), which is used in conjunction with a pseudo-random mechanical chopper.