Chapter Experimental Setup 38 _______________________________________________________________________________________________________ 3.4 HRBS Endstation Motorized slits Switching magnet 45°° beamline Spectrometer magnet MCP-FPD chamber Control cabinet Main chamber Fig. 3.6 Layout of the 45° beamline and the HRBS endstation. Overview The HRBS endstation was fabricated by the Machinery Company of Kobe Steel Ltd and installed at CIBA in 2003. The general setup (Fig 3.6) consists of: i. Main chamber with a load lock chamber ii. UHV Vacuum sytem (pumps, valves and interlocks) iii. 5-axis goniometer iv. Control cabinet v. Spectrometer magnet and MCP-FPD chamber (HRBS detection system) Both the main and the MCP-FPD chambers are constantly maintained under UHV with two turbo-molecular pumps, which are located beneath the main chamber and Chapter Experimental Setup 39 _______________________________________________________________________________________________________ the MCP-FPD chamber. Sample exchange is performed by a transfer rod which transfers a target holder onto the goniometer attachment in the main chamber from a load-lock chamber through a gate valve. A controller program at the control cabinet oversees the vacuum interlocks system and allows for programmed or manual control of all valves as well as the goniometer rotation axes. During measurements, the divergence of the ion beam is defined using the motorized slits located ~ m before the main chamber. The scattering target is suspended by a precision 5-axis goniometer and is applied with an electrical potential for secondary electron suppression. Backscattered ions subsequently enter the detection system, which consists of a 90° spectrometer magnet and a Micro-Channel Plate – Focal Plane Detector (MCP-FPD) stack. The system is mounted on a circular track which allows the scattering angle to be varied via detection ports. The signal output from the FPD is processed by a network of electronics on the control cabinet to extract the position of incidence of the ion along its length. The spectrum is sorted by an MCA before being output by a software on a computer. Chapter Experimental Setup 40 _______________________________________________________________________________________________________ 3.4.1 Main chamber, load lock and vacuum system During RBS measurements, the sample is placed in a specially designed sample holder which is held on the goniometer attachment in the main chamber. Insertion and removal of the sample holder is done with a transfer rod normally contained within the load lock chamber. (a) (b) Goniometer Gate Valve V1 V1 Main Chamber Load lock chamber (c) Main chamber Main viewport Fig. 3.7 (a) Main chamber, goniometer and load lock. (b) Load lock with a sample holder (inset picture) attached to the insertion rod (c) View of sample holder on the goniometer attachment within the main chamber through the main viewport. A UHV vacuum of < × 10−9 mbar is maintained by a Mitsubishi FT-800WH turbomolecular pump (TMP) in main chamber (TMP 1) and a Mitsubishi PT-50 TMP in the MCP-FPD chamber (TMP 2), as shown in Fig. 3.8. Both TMPs are being backed by a Mitsubishi DS-251L scroll pump with valve V2 perpetually open. Valve V3 allows the load-lock to be pumped (also by the scroll pump), V4 controls the venting with N2 during sample change and V1 isolates the load lock from the main chamber. The network is regulated by an interlocks system at the control cabinet, preventing Chapter Experimental Setup 41 _______________________________________________________________________________________________________ any sudden change in vacuum pressure by accidental activation of inappropriate valves. V1 Main Chamber MCP-FPD Chamber Load lock V4 TMP TMP N2 V3 V2 Scroll Pump Fig 3.8 Schematic of the vacuum pump and valve network. During sample change, programmed sequences can be initiated from the graphical user interface on the control cabinet which automatically activates the appropriate valves in the correct sequence. During the insertion of the sample into the main chamber, the sample holder containing the target is placed onto the transfer rod within the load lock with all valves except V2 closed. The insertion sequence is activated, where V3 is first opened to allow the load lock to be pumped to a pressure of ~ 10−2 mbar before opening V1 to allow the user to transfer the sample into the main chamber. Once that is done and the inserting rod is retracted back into the load lock, both V1 and V3 are closed, ending the sequence. The sample removal sequence is slightly different: again V3 is first opened to pump down the load lock before opening V1. Once the user removed the sample and the transfer rod is retracted into the load Chapter Experimental Setup 42 _______________________________________________________________________________________________________ lock, both V1 and V3 are closed. V4 is then opened to allow N2 to flood the load lock back to atmospheric pressure, after which the load lock chamber can be opened to remove the sample. 3.4.2 Goniometer A Kitano Seiki 5-axis goniometer controls φ Motor the sample orientation within the main chamber. It allows for translation in the x, y θ Motor and z axes, as well as rotation about the θ and φ axes (Fig. 3.9). The translation resolution is 0.01 mm with a repeatability of z Control y Control ≤ 0.05 mm along all directions, while rotations have resolution of 0.05° and are repeatable to within ± 0.05°. An electric potential of ∼ +480 V is applied to the x Control Goniometer attachment attachment which holds the sample holder during measurements, suppressing secondary φ electron emissions to allow for accurate z x beam current readings. θ y Fig. 3.9 (Right) Schematic of the HRBS goniometer. Source: [37] Chapter Experimental Setup 43 _______________________________________________________________________________________________________ 3.4.3 Control cabinet The goniometer and vacuum system controls are accessed from a touch-screen control panel that is found on the control cabinet, which also houses the vacuum gauge readouts, MCP electronics, as well as the MCA. Vacuum gauge readouts Emergency Stop and alarm MCP electronics MCA Control panel Electron suppression power supply (Concealed) Cabinet circuit breakers Fig. 3.10 Front view of the control cabinet. The power supply for the electron suppression voltage applied on the sample is also installed within the cabinet, comprising two 240V dry cells in series. The cabinet circuitry is electrically protected by a network of circuit breakers, while the vacuum and the spectrometer magnet cooling systems are monitored by an interlocks program, which is accessible from the control panel. An alarm will sound whenever the interlocks is triggered, and an emergency stop button allows for automatic emergency shutdown of the HRBS endstation. Chapter Experimental Setup 44 _______________________________________________________________________________________________________ 3.5 HRBS Detection system Superior energy resolution of HRBS is achieved by replacing the solid-state PIPs detector in normal RBS with a detection system that consists of a spectrometer magnet and a 100 mm long by 15 mm width Micro-Channel Plate (MCP) – Focal Plane Detector (FPD) stack. Backscattered ions of energy E enter the spectrometer magnet at a specific scattering angle where they follow circular trajectories with radii r that are proportional to E 0.5 (Fig 3.11). These ions then incident onto the MCP-FPD stack at the focal plane of the spectrometer. (a) (b) Spectrometer magnet Incident Beam MCP-FPD Target Fig. 3.11 (a) The actual and (b) schematic layout of the HRBS detection system. Ions backscattered from the target at a fixed scattering angle enters the spectrometer magnet. Ions with higher momentum will be bent with a larger radii (red) while lower momentum ions will have a smaller radii (green). Chapter Experimental Setup 45 _______________________________________________________________________________________________________ The incidence of a backscattered ion onto the MCP triggers an electron cascade, which deposits a charge/voltage pulse onto the FPD. The FPD, being of a resistive strip type, determines the position of the pulse and hence the ion incidence along its length using a simple charge division method. The spectrum obtained directly from the system is therefore a “position spectrum”, which is a position distribution of backscattered ions at fixed intervals of length along the MCP-FPD. The energy spectrum is subsequently obtained by processing the position spectrum, using a forward-binning process, which will be described in Chapter 4. 3.5.1 Micro-Channel Plate The MCP consists of a stack of channel plates with rows of micro-channels as shown in Fig. 3.12. Channel plates Incident ion 0V +1 kV MCP +2 kV Focal Plane Detector Fig. 3.12 Structure of the MCP. Ion incidence triggers an electron cascade down the channels. The channels of the two halves of the plate are oriented in different directions. The function of the 1-dimensional FPD is to detect the position of incidence of energy-analyzed backscattered ions along its length. However, the charge carried by each ion is insufficient to generate an electrical signal large enough for the subsequent electronic equipment to pick up. Therefore, an MCP acts as an amplifier of the Chapter Experimental Setup 46 _______________________________________________________________________________________________________ electrical signal using an electron multiplication process initiated by the incidence of a single back-scattered ion. The walls of the micro-channels have low electron-emission work function, and a bias of about kV is applied across each channel plate using an ORTEC 660 High Voltage Bias so that an incident backscattered ion will initiate an electron cascade down the channel. The electron multiplication process will saturate, depositing a charge pulse onto the FPD. Residual gas molecules within the channels may be ionized during the cascade process, which will accelerate back up the channels. To prevent such gas ions from gaining enough kinetic energy to initiate another cascade, the orientation of the channels within each plate stack are angled relative to each other. This is to ensure that the acceleration of gas ions produced within the bottom plate will be either stopped or interrupted at the angled junction between two plates, preventing them from gaining kinetic energy continuously through the entire kV bias. 3.5.2 Focal Plane Detector and HRBS electronics The FPD is a 100 mm long resistive strip of uniform resistance per unit length. Once the electron cascade deposits a charge pulse onto the FPD, the system performs a simple charge division calculation electronically to determine the position of the pulse, and hence the ion incidence, along the MCP-FPD stack as shown in Fig. 3.13. A voltage pulse V0 is deposited on the FPD at point X, which in turn causes currents IL and IR to flow towards the left and right ends of the FPD respectively. The charges collected QL and QR are measured at the left and right ends of the FPD respectively over the same time interval ∆t and are compared to the total charge collected to calculate the distance of X from the ends of the FPD. Chapter Experimental Setup 47 _______________________________________________________________________________________________________ Electron cascade from the MCP IL IR X Focal Plane Detector (FPD) x L-x V0 QL QR Pre-Amp Pre-Amp Amplifier Amplifier b a SUM b a+b PSDA b /(a+b) a+b ADC (Position) Fig. 3.13 ADC (Energy) Schematic of the HRBS electronics setup Let the length of the FPD be L. Since it has uniform resistance per unit length, IL L−x Q ∆t = = L IR x QR ∆t ⇒ x = QR L QL + QR The charge division is performed electronically by a network of analog processors. The charge pulses QL and QR are each processed first by ORTEC 113 Pre-amplifier and then by ORTEC 571 Amplifier. The resultant pulses were then added using ORTEC 533 Dual Sum and Invert card. A Seiko EG&G PSDA card is used to divide an amplified signal with the summed signal to obtain the position output, while a summed signal output (Energy) is also available. Thereafter, the output is fed into a Canberra 8706 ADC and subsequently sorted using a Labo NT2400 Multichannel Analyzer, before the spectrum is obtained, on the PC. . Goniometer A Kitano Seiki 5-axis goniometer controls the sample orientation within the main chamber. It allows for translation in the x, y and z axes, as well as rotation about the θ and φ axes. (Fig. 3. 9). The translation resolution is 0.01 mm with a repeatability of 0 05 . ≤ mm along all directions, while rotations have resolution of 0.05 ° and are repeatable to within ± 0.05°. An. the channels. To prevent such gas ions from gaining enough kinetic energy to initiate another cascade, the orientation of the channels within each plate stack are angled relative to each other.