Ion Implanter Cyclotron Apparatus System

Transcription

1 Ion Implanter Cyclotron Apparatus System A. Latuszyñski, K. Pyszniak, A. DroŸdziel, D. M¹czka Institute of Physics, Maria Curie-Sk³odowska University, Lublin, Poland Abstract In this paper the authors have described an apparatus system consisting of an ion implanter and a small cyclotron (24 cm ion radius in magnetic field), which is used in the Institute of Physics, Maria Curie Sk³odowska University (IF UMCS), Lublin. The implanter can generate ion beams, at an intensity up to 1 ma and with kev energy, of practically all elements occurring in nature, including isotopes of radioactive elements. The cyclotron allows us to obtain proton beams and alpha particles of energies in 0,8-5 MeV range at an intensity up to 0,05 ma and not exceeding 1% energy scatter. Introduction Studies of physical processes occurring due to interaction of ion beams with solid surface, and ion modification of the properties of surface layers find a wide interest in technological application. As yet the methods of improving the mechanical, anticorrosion properties of solids by their ion coating with a thin layer of material of appropriate properties have reached a stabilized application. The spectrum of physical phenomena occurring under ion bombarding on the surface and in the layer is especially varied. The most important of them are: 1. back scattering of the primary ion beam; 2. secondary emission of various particles knocked out from the solid surface; 3. penetration of ions getting into the solid subsurface area and therewith connected transfer of momentum and energy. The energy range of ion beams used both in technological application and research methods is very wide; its range is usually from 10 ev to several MeV. This necessitates construction of equipment forming ion beam of possibly a large number of elements with energies from the range mentioned above. An apparatus system set constructed in the Institute of Physics, UMCS, Lublin, consisting of an implanter allowing obtaining ion beams of energy up to 100 kev of most elements occurring in nature, as well as a small cyclotron generating proton beams and alpha particles of 0,8 5 MeV energy, has been described in this paper. A particular attention has been given to the description of ion sources cooperating with the implanter, because the quality of their work largely determines the utilization advantage of the whole research apparatus. The authors have described a plasma ion source with slit optics providing generations of appropriate ion beams for the cyclotron and source cooperating with the implanter, including the thermoemission source and a special plasma source with a screening grid destined for effective ionization of radioactive elements. The construction idea of the cyclotron implanter apparatus of ions was to obtain the possibility of using various kinds of ion beams in one place, i.e. in a collector chamber common for both devices. Such a possibility of obtaining simultaneously two different charged beams forms unique on-line study conditions of the properties of a solid surface modified by these beams. This particularly allows: a) studying the surface conditions of solids by the methods: RBS, ERD and resonance nuclear reaction, b) quantitative and qualitative analysis of microamounts of substances (PIXE and RBS method), c) ion implantation (low and high energetic) of stable and radioactive isotopes, d) studying radiation defects and other crystal defects by the RBS and channelling methods,

2 e) on-line studying secondary processes accompanying ion implantation, as well as following implantation kinetics ( in situ methods). Experimental apparatus A scheme of the apparatus system of IF, UMCS, in cross section vertical to the force lines of the magnetic field of the implanter and cyclotron is shown in Fig.1. The basic parameters characterising both devices are listed in Table1. Fig.1. A scheme of the cyclotron ion implanter system Tabela1 Basic working parameters of the implanter and cyclotron. implanter cyclotron accelerated masses (a.m.u.) 1 10 (a.m.u.) particle energy kev MeV ion current (max) 500 µa 50 µa energy broadening 1% 2% ion rad. in mag. field 160 cm 24 cm magnetic field 0.3 T 1.8 T vacuum 10-6 Torr 10-6 Torr The cyclotron chamber, inside which there is a pair of duants of 48 cm working diameter is placed in the magnet yoke, the coils of which are fed with direct current A. Variable electric field from generator HF of 20 kw is applied to the duants by a special system of resonators cooled with water. The generator, consisting of three subsequent lamp amplification degrees, allows obtaining sinusoidal impulse of regulated tension to 10 kv and frequency in 8-24 MHz interval. The resonance frequency necessary for proton or alpha particles of required energy is obtained through appropriate change of the resonator length. A

3 beam of protons or alpha particles is extracted from the area of the duants and directed to the ion duet I by a special deflector system (at kv potential) inside the cyclotron chamber. A slit ion source is utilised in the cyclotron (Fig.2), which is destined for ionization of hydrogen and helium atoms. Its basic elements are a directly annealed cathode K at a potential of several hundred volts and a graphite or molybdenum anode A with an rectangular extraction opening (2x10mm). The intensity of ion current extracted from the source depends above all on the anode voltage Ua and the arc current Ia and the amount of the dosed gas. The working dependences of ion currents of hydrogen and helium on Ua and Ia are presented in Fig.3a and 3b. The current intensity of helium ions (because of a high ionization potential) emitted from the source is definitely lower than that of hydrogen ions. As a result, far smaller is also the current intensity of helium ion beams emitted from the cyclotron. The relationships shown in Fig. 3 were obtained by measuring ion currents drawn through an extraction lens (at 10 kv potential) fastened to one of the duants. So they are maximal currents emitted from the source, and their values are in 1-5 ma interval. During acceleration the intensity of the ion beam decreases considerably, the result of which is that the intensity of the beams leaving the cyclotron is up to 50µA. Fig.2 A scheme of a slit ion source and the extraction system in the cyclotron chamber. A anode, K cathode, P plasma, S extraction lens, D duants a) b) Fig.3 Dependence of ion intensity currents of hydrogen I(H) and helium I(He) extracted from the slit ion source on anode voltage (Fig.3a) and arcing current (Fig.3b)

4 The second part of our apparatus system is an implanter of Scandinavian type (a symmetric system) with a 90 0 magnetic lens and a 160 cm ion radius, which provides effective separation of isotopes in the range a.m.n. with resolving power above 1000 (Fig.5b) [1]. The implanter is equipped with an appropriate set of ion sources, which enables generation of ion beams of an intensity up to 1mA and keV energy of practically all elements occurring in nature, including radioactive isotopes. The summary ion beam obtained from the source is respectively formed and diagnosed both in the area of the magnetic lens and after its resolution into individual isobaric lines. The collector chamber common for beams from the implanter and cyclotron (Fig.1) is equipped with a proper research accesory. The basic types of the ion sources used by us are schematically presented in Figs Classical sources of plasma type [2,3] shown in Fig.4.1 and 4.2. guarantee ionization of gases and readily volatile substances without problems. 1) 2) 3) Fig.4 Plasma ion source of the implanter. 1 Nielsen type source, 2 Sidenius type source, 3 source for ionization of slow volatile radioactive elements. K annealed cathode, A screening grid (anode), I insulator, Ex extraction opening of the source The source in Fig.4.3 [4] was constructed especially for effective ionization slow volatile elements of high ionization potentials (over 7eV). Effective ionization of such elements as: Be, Ti, V, Zr, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, Pt is particularly difficult in the case when we want to obtain ions of their short-lived isotopes with life times of the order of minutes or less. Our source (Fig4.3) consists of a cylindrical (d=15mm) molybdenum, tantalum or wolfram cathode K indirectly annealed to K by an external electron beam emitted from an annealed wolfram spiral K. Such a high temperature of the cylinder guarantees a sufficient density of electron current of thermoemission in the cathode volume and simultaneously shortens the time of the presence of the ionized elements on the source walls.

5 Inside the cathode cylinder, near its wall, there is a coaxially mounted tantalic grid. A, being at the anode potential ( V). The whole source is at a high potential (10-20 kv) in relation to the extraction opening (d=0,1mm) drilled in the cathode and anode cylinder. ACTIVITY [a.u.] Fig.5 A scheme of a thermoemission ion source. A mass spectrum of radioactive isotopes 165 Tm, 166 Yb, 167 Tm, obtained in the thermoemission ion source. K 1,K 2 annealing external cathode, J ionizer, P evaporator, Ex source extraction opening. The source of thermoemission type (Fig.5) [5,6] is characterized especially by a high working temperature. The source ampoule consisting of two parts ioniser I (r w ~ 5mm in inner diameter) and an evaporator P annealed to K by an external electron beam emitted from cathodes K. A stream of neutral particles diffused from the evaporator undergoes, in subsequent collisions with ioniser walls, ionisation with a probability determined by the Saha-Langmuir equation. Inside the ioniser volume, under definite conditions, isothermal plasma of a high density is formed, which allows a very effective (to 50%) ionisation of elements of an ionisation potential lower than 7 ev. This work was partly supported by the grant of JINR Dubna ( /2000). References: 1. W.Zuk, D.M¹czka, J.Pomorski. Nucl. Instr. and Meth, 37, 249, G.Sidenius. Radiat. Effects 44, 145, A.Drozdziel, A.Latuszyñski, D.M¹czka, K.Pyszniak. Nucl. Instr. and Meth. in Phys. Res. B 126, 58, J.M.Nitachke. Nucl. Instr. and Meth. in Phys. Res. A 236, A.Latuszyñski, V.I.Rajko. Nucl. Instr. and Meth, 125, 61, A.Latuszyñski, D.M¹czka. Nucl. Instr. and Meth. in Phys. Res. B 85, 798, 1994.