High depth resolution rutherford backscattering spectrometry with a magnet spectrometer implementation and application to thin film analysis 2

Chapter 3 Experimental Setup 32
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Chapter 3
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Experimental Setup



3.1 CIBA Facilities
CIBA consists of two laboratories, each housing an accelerator. A single-ended 2.5
MV AN-2500 Van de Graaff accelerator from High Voltage Engineering (HVE) is
housed in the auxiliary laboratory, providing ion beams to a single beamline leading
to a NEC RC43 analytical endstation which allows for broad-beam conventional RBS
and PIXE research. In the main laboratory (Fig. 3.1), three beamlines are attached to a
single-ended 3.5 MV Singletron
TM
accelerator from High Voltage Engineering Europa
(HVEE) [33] that provides ion beams which are brighter and more stable than those
from the belt-driven Van De Graaff accelerators. At the 10° beamline, a state-of-the-
art proton beam writer [34] is present, which allows for direct-write micro-machining
while a nuclear microscope [35] is attached to the 30° beamline which is used for

advanced materials analysis and elemental mapping of biological samples. The 45°
beamline leads to the HRBS endstation, which is the focus of this thesis.
32

Chapter 3 Experimental Setup 33
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Fig. 3.1 Layout of the accelerator and beamlines

3.2 Singletron
TM
accelerator

Fig. 3.2 Schematics of the accelerator system. Source: [33]

Proton
Beam Writer
Nuclear
Microscope
HRBS
Endstation
Accelerator
Steerer table
Aperture
Analyzer
table
Switching
magnet
45
°
°°
°

90°
°°
° Analyzer
magnet

Chapter 3 Experimental Setup 34
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3.2.1 RF Ion source








Fig. 3.3 The RF Ion source. Source: [36]

The RF ion source model 173 (Fig. 3.3) is used within the Singletron as a heavy duty
profile source for proton, alpha and oxygen beams. The neutral gas is released into the
ion source, where an RF voltage is applied. Electrons within the gas atoms are excited
into oscillation by the RF field, quickly gaining enough kinetic energy to cause
ionization and forming a plasma. Positive ions from the plasma are pushed out of the
source by an applied electric field from the probe, and the resultant ion beam is
focused by applied voltage at the extraction electrode. A set of permanent magnets
apply an axial magnet field to create a toroidal motion of electrons that concentrates
the plasma near the extraction electrode, increasing the ion beam current.

3.2.2 High voltage power supply
The high voltage power supply (Fig 3.4) provides the terminal voltage which
accelerates the ions extracted from the ion source along the accelerator tube. The

Chapter 3 Experimental Setup 35
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driver electrodes are connected to RF oscillator coils which generate an AC voltage

which is transferred to the rectifier stack by means of capacitative coupling.








Fig. 3.4 The high voltage power supply. Source: [33]

The RC time constant of the stack is much larger than the period of the RF generated
by the voltage driver, so that an equilibrium high positive voltage is generated at the
top of the stack. Alternate capacitors along the stack are charged in the same electrical
orientation during each half-cycle of the AC, with the rectifiers ensuring a net flow of
current towards the positive accelerator terminal over the AC cycles. Potential
differences across each charged capacitor along the stack eventually add up to the
terminal voltage. A generating voltmeter (GVM) measures the potential of the
terminal relative to the laboratory ground, while a capacitative pickup unit (CPU)
measures the ripple of the terminal voltage and feeds the signal back to the voltage
regulator controlling the voltage driver. The voltage driver then adjusts the terminal
voltage accordingly to reduce the ripple, improving the energy stability of the beam.
In the event of a tank spark, the corona/spark interlock will trigger a warning on the
control computer and the voltage regulator will switch off the terminal voltage.

Chapter 3 Experimental Setup 36
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3.2.3 High voltage insulation and Electron suppression
The accelerator tank is filled with a heavy non-toxic insulating gas SF

6
at a pressure
of 8 bar. SF
6
has a dielectric strength approximately 2.5 to 3 times that of air, and
serves to prevent corona buildup and tank sparks. Other forms of electrical insulation
are the vacuum and glass insulation within the accelerator tube, the plastic support
members in the power supply and a variety of insulating cable covers. Also, electron
suppression is employed via small permanent magnets within the accelerator to
reduce the level of emitted radiation.

3.3 Beam steering and stablization
The beam output from the accelerator is adjusted at the steerer table (Fig. 3.5(a)) by
electrostatic steerers along x and y axes along the x-y plane normal to the beam (z)
direction and subsequently passes through a beam defining slit. A circular aperture
checks the exact beam position and defines the maximum beam diameter just before
the 90° analyzing magnet. The desired ion species is then selected by the magnet to
pass into the stabilizer slit at the analyzer table (Fig. 3.5(b)). The currents created by
the beam hitting on the left and the right of the stabilizer slit are monitored, and the
slit assembly is connected via a feedback loop to the voltage regulator in the
accelerator high voltage power supply. Any instability of the ion beam will cause
changes in the slit currents, which in turn triggers a compensation response from the
terminal voltage regulator in the accelerator, creating a stabilizing effect on the beam.




Chapter 3 Experimental Setup 37
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Fig. 3.5 (a) The steerer table and (b) the 90° analyzing magnet on the analyzer table. Source:
[33]

On both steerer and analyzer tables, a beam profile monitor (BPM) and a faraday cup
measure the beam profile along the x-y plane and the total beam current respectively.
This facilitates the proper steering and focusing of the beam before it enters the
switching magnet to be steered into one of the beamlines.
(a)
(b)