Development of Test-Setup for Secondary Electron ...

Development of Test-Setup for Secondary Electron Detector Assembly for SEM J. K. Mukherjee, S. S. Pany Electronics & Instrumentation Services Division Bhabha Atomic Research Centre, Mumbai Considering the fact that the overall performance of SEM instrument is greatly influenced by that of the SE detector assembly, finetuning its design and operating parameters should be an indispensible step before integrating it into a working system. Here, we discuss about the design and construction of a test facility for performance evaluation of SE detectors. We also describe the experiments being carried out with the setup and present some preliminary results. Currently all experiments are being performed with YAP:Ce scintillator. There are plans to use YAG:Ce and P47 scintillators to compare them on the basis of overall yield for a typical SEM. We also plan to carry out the experiments with higher scintillator voltage (about 10-15 kV) and by varying the attractor voltage as well. Although the current test facility is being used with SEM SE detectors, it has been made useful for various other experiments involving low energy electrons.

• Beam Energy: 5 to 100eV • Typical Energy Spread: ~0.25eV FWHM •X-Y Static Deflectors for beam positioning • Provision for External Kinetic Energy Sweep • Provision of control for Kinetic Energy, Filament Current, Anode and Grid (Wehnelt) Bias voltage, Beam Focus and Deflection 3D Computational electromagnetic simulations have been carried out to compare between efficiencies of different SE detector designs. It has been observed that the faraday cage not only attracts the SE, but also acts as electrostatic single potential gridded focusing lens. The spherical aberration coefficient of the gridded lens is a crucial parameter

1. PMT output v/s scintillator voltage: Energy of the beam was fixed at 5 eV, attractor voltage was fixed at 1kV and electron gun heating current at 1 A. Scintillator voltage was gradually varied from 0 to 4.5 kV and the corresponding DC and AC components of PMT output were recorded.

2. Detector output v/s Filament heating current: The purpose of the experiment was to determine PMT base-line signal value when scintillator and attractor supplies are off. For this, the PMT output voltage was recorded by varying the LAB6 filament heating current of the electron gun, with no zero voltage supply to attractor and scintillator.

References: 1. Čudek, Pavel. “SCINTILLATION SECONDARY ELECTRON DETECTOR FOR VP-SEM”. 2. Everhart, T. E. and Thornley, R. F. M. (1960), ”Wide-band detector for micro-microampere low-energy electron currents”. Journal of Scientific Instruments, 37 (7). pp. 246-248. ISSN 0950-7671. 3. Konvalina, I. I., Quantification of Detection Efficiency of the Detector of Secondary Electrons in SEM, (Phd dissertation) , ISSN: 1213-4198