A 100 keV heavy ion accelerator for implantation studies

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A 100 keV HEAVY I O N A C C E L E R A T O R F O R I M P L A N T A T I O N S T U D I E S R. BENOIT, G. BERTOLINI, F. CAPPELLANI and G. RESTELLI

Electronics Service, Technology Division, EURATOM-CCR, 21020-Ispra (Varese), Italy Received 8 October 1971 An ion implanter has been designed and constructed at the CCR-ISPRA. The equipment enables implants to be carried out at energies ranging from 0 to 100 keV with a wide range of ion species and choice of target temperature and orientation.

'The schematic layout of the 100 keV ion accelerator is shown in fig. 1. As can be seen from fig. 2 the acceleraling voltage has been split in two steps; the first one can be set from +25 to +75 kV in steps of 10 kV each while the second one can be continuously adjusted from -:25 to + 2 5 kV. The ion source potential with respect to the grounded target is given by the algebraic sum of the two voltages: therefore it is continuously variable from 0 to 100 kV. 'The main advantage of this configuration is the high extraction potential (at least 25 kV) even for the lowest ion energies thus assuring a good ion extraction yield. The ion source is the Danfysik 910 Model. 'The extraction and focusing system consists of a three electrodes Einzel lens. The extraction gap as well as the position of the ion source outlet in the plane normal to the beam axis can be varied without breaking the vacuum. The configuration of the electrodes has been designed according to the results of computer ca][culations for ion trajectoriesl'2). The analysing magnet is of sector type with an aperture angle of 127 °, radius 60 cm and a mean gap width of 32 m m , the mass x energy product is equal to 16 amu MeV. Double focalization is accomplished by means of diverging poles with hyperbolic shape3); a circular beam of parallel trajectories at the magnet input is focalized in one point at the end of the magnet (neglecting space charge effects)g). Some peaks of the Xe beam mass spectrum shown in fig. 3, as displayed by the beam scanner, placed at 25 cm from the magnet output, allow to estimate the separation of the peaks and their resolution. 'The post-acceleration is provided by a set of ten electrodes calculated to give a constant electric field di,;tribution on the beam axis. From the experimental results it appears that the beam dimension at the target is not influenced by the operation of the constant field tube (retardation or acceleration). Uniform irradiation of the target area is ensured by sweeping the beam 421

in X - Y directions. Beam sweeping over a square of 3 cm side is performed by two sets of electrostatic deflection plates. These are driven by 2000 V peak to peak triangulal waves of 20 and 2000 Hz frequency respectively for the X and Y sweep. The maximum deflection angle results _+ 1.5 °. A remote control order, given by a current integrator connected with the target, stops the beam sweeping positioning the beam out of the bombarded area when a preset charge has been implanted. The target chamber consists of a cylinder provided with a terminal flange which can be easily interchanged with other normalized flanges. The r o o m temperature facility allows implantations of up to eight specimens to be performed subsequently without breaking the vacuum. Using the other flanges, four samples to be bombarded can be cooled by liquid nitrogen down to 100 K or heated up to 800 K by an electron gun. Beam collimators and secondary electron suppressor are incorporated in the target chamber. A beam viewing facility is provided for the r o o m temperature implantation flange: KBr layers deposited on conductive glass behave satisfactorily even for the lowest ion energies and ensure long duration. A two-axis goniometer can be fitted to the target chamber for implantation in channeling conditions. The goniometer designed and constructed in the laboratory allows positioning of the specimen with an accuracy of 0.05 °. The sample on the goniometer can be cooled or heated during the implantation. The vacuum system has been designed to reduce the surface contamination of the target to a minimum. Therefore a turbo-molecular and an ion p u m p have been used in the second part of the accelerator after the magnet. Electrical and vacuum commends, controls, protection circuits and the beam control systems shown in the simplified block diagram of fig. 2 have been assembled in a control desk.

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1) C. Mongini-Tamagnini, to be submitted to Nucl. Instr. and Meth. '2) G. Gaggero, to be submitted to Nucl. Instr. and Meth. a) L. Stanchi, Nucl. Instr. and Meth. 73 (1969) 313. a) G. Gaggero, E U R - R e p o r t , to be published.