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A national facility for high current isotope separation and ion implantation

A national facility for high current isotope separation and ion implantation
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   77 Solid State Physics (India) 43  (2000) S.L. Chaplot, T. Sakuntala and S.M. Yusuf (Eds) Copyright ©  2001, Narosa Publishing House, New Delhi, India   A National Facility For High Current Isotope Separation and Ion Implantation T. K. Chini, S. R. Bhattacharyya, S. Hazra, A. Datta and M. K. Sanyal Surface Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Calcutta 700 064, India  Abstract  A national facility for high current isotope separation and ion implantation has been set up at the Surface Physics Division of the Saha Institute of Nuclear Physics. The combination of the isotope separator and the ion implanter makes it a first high current low energy Accelerator facility in India. The basic configuration and present status of this facility will be reported in the talk.   THE FACILITY A state of the art High Current Isotope Separator followed by a 200 kV Ion Implanter has been set up at the Surface Physics Division of the Saha Institute of Nuclear Physics as a national facility to promote advanced research in surface science, materials science, nuclear science and atomic physics. Equipped with the very powerful ion source (that can operate in gas, sputter and oven mode), capable of producing several mA beam currents (which is much higher than the sources employed in the conventional isotope separators or 100–400 kV ion implanters) and implementing a 1250 mm radius spectrometer class analysing magnet, the accelerator performs high resolution isotope separation of the heaviest masses in the collection chamber. With the separated isotope passing through a slit the accelerator can deliver beam of maximum energy of 200 keV to the ion implantation beam line featuring electromagnetic beam focussing and scanning of the transported beam up to the target chamber. The unique feature of the combination of the isotope separator and the ion implanter makes it a first high current low energy accelerator facility in India. Besides, providing valuable enriched isotopes required for basic research in nuclear science community, isotopically pure and low energy (30–200 keV) high current beam of almost all the elements in the periodic table, obtainable from this facility, will be an important source to tailor the near surface properties of inorganic as well organic substances and to study the various kinds of ion-atom collision phenomena serving the need of solid state physics, chemistry and atomic physics community of our country. The accelerator (Fig. 1), is designed on the basis of the well-proven Danfysik high current ion implanter (Danfysik model 1090) and has been supplied to the Saha Institute by Danfysik, Denmark. COLLECTOR CHAMBERQUADRUPOLE TRIPLETBEAM SCANNINGMAGNETSCRYOPUMPION IMPLANTATION CHAMBERDIPOLE ANALYZING MAGNETWITH ROTATEABLE POLE TIPSVERTICAL BEAM STEERERACCELERATOR COLUMNION SOURCE SUPPLIESMOTOR GENERATOR150 kV ACCELERATION SUPPLYION SOURCE MODEL 921 WITH 50 kV EXTRACTION1 m   ION SOURCE MODEL 921 WITH 50 kV EXTRACTION   Fig. 1.  Schematic layout of the isotope separator and ion implanter machine. MASS DISPERSION AND RESOLUTION Dispersion and resolution are the primary parameters that determine the performance of a separator. Mass dispersion,  D , is expressed in terms of the displacement, d  , in the image plane, taken perpendicular to the beam axis, between focussed ion beams of mass  M   and  M   + ∆  M  . So,  D  = d  (  M   /  ∆  M  ). Mass resolution,  R  = d     M   /  b 1/2 ( ∆ M = 1) ≈  400–500 for this separator. Here b 1/2  = image width at half of the peak height (FWHM). We obtained separation ( d  ) of 26 mm between the peaks 98 and 97 of Mo (Fig. 2) and R = 425 with b 1/2  = 6 mm. This implies the efficiency of this separator for the complete separation of the isotopes of heavy elements.   78 3353403453503550246810 Mo 100 Mo 98 Mo 97 Mo 96 Mo 95 Mo 94 Mo 92      B  e  a  m   c  u  r  r  e  n   t   (     µ     Α    ) Analysing magnet current (A)   Fig. 2.  Mass spectrum of 150 keV Mo + . OBTAINED TEST CURRENT VALUES Both the isotope separator (collector) chamber and implantation chamber are equipped with water cooled Faraday cups for measuring the beam current. Moreover, for testing and optimisation purpose the ion beam can be exposed for long time on a water-cooled beam-stop located in the collector chamber. Some typical isotopic currents obtained at different test runs are given below: •   40 Ar +   →  3 mA at 200 keV •   98 Mo +   →  8.5 µ A at 150 keV •   98 Mo +   →  3.5 µ A at 55 keV •   208 Pb +   →  1 µ A at 110 keV First one is measured at beam stop and the second and third one are measured in the Faraday cup of the isotope separator (collector) chamber. The fourth one is monitored in the Farady cup of the implantation chamber. PREPARATION OF ENRICHED TARGET As a first step to realise the preparation of enriched target we tried to deposit 98 Mo isotope (at 50 keV) in Si(100) wafer (~450 µ m thick) in the isotope collection chamber of our accelerator. The following results show that a selected isotope can be put in a host matrix making it enriched with an isotope of a particular element which, can subsequently be used for nuclear science research. If  I   = beam current in µ A, t   = time of collection in hrs,  M   = mass number of the separated isotope, then, weight of the collected isotope m  = 0.0373 tIM   in µ g, assuming 100% collection. Here,  I   = 0.25 µ A over an area of 0.13 cm 2  (for 98 Mo) and t   = 13 min amounts to 1.6 µ g/cm 2  of collected material (equivalent dose of 10 16  ions/cm 2 ). Profile of the 98 Mo isotope in Si(100) matrix as monitored by secondary ion mass spectrometry (SIMS) confirms the isotopic deposition in a matrix (Fig. 3). 01020304050050100150200    I  n   t  e  n  s   i   t  y Sputtering time (min) 0100200300400500  Depth (nm)   Fig. 3 . SIMS profile of the 98 Mo +  isotope in Si(100). SURFACE AND MATERIAL PHYSICS USING LOW ENERGY ION BEAM Nanoscale ripple pattern or sand-dune like morphology on solid surface has potential application [1,2] in optical device fabrication and nanolithography. We have been able to produce such pattern on Si surface by 50 keV 40 Ar +  bombardment (with a dose of 10 18  ions/cm 2 ) at 60º ion incidence angle using this implanter facility. The morphology (Fig. 4) is pictured by atomic force microscopy (AFM). Other studies that can be pursued using the low energy ion beam of this facility are atomic scale topography of ion sputtered surface, implantation and other ion beam induced surface modifications including tribology of ion implanted materials, modifications of optical properties and ion beam mixing, to name a few. Fig. 4.  AFM image of 50 keV 40 Ar bombarded Si surface. REFERENCES 1. J. Erlebacher et al., Phys. Rev. Lett. 82  (1999) 2330 2. S. Rusponi et al., Appl. Phys. Lett. 75  (1999) 3318
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