Head - C. Mallik
SUPERCONDUCTING CYCLOTRON ECR SECTION
Electron Cyclotron Resonance Ion Source (ECRIS) is an ion source that produces high current high charge state ion beams of almost all elements in the periodic table. Plasma is created in a chamber with the aid of microwave power. Permanent magnets and electromagnets are used to confine the plasma where the electrons gain energy by absorbing microwave power resonantly. Magnets are arranged in such a way that electrons are reflected from the edges of volume plasma towards the centre where the magnetic field is week. Electrons and ions have long life time in the plasma and hence undergo large number successive collisions and form high charge state ions. Several processes for ionization takes place in the plasma chamber at the same time. Out of many the electron – neutral, electron – ion, ion-neutral, ion-ion collisions are the major ionization processes that take place in the ion source. Microwave power, gas pressure and magnetic field profiles in the plasma chamber are the control knobs to change the charge state distribution in the ECR ion source.
Energy of the ions after acceleration in cyclotron is proportional to square of the charge state and hence higher the charge state of injected ion beam, higher is the energy gain by ions in the cyclotron.
An ECR (ECR-2) ion source operating at 14.45 GHz is installed to produce ion beams and inject into the cyclotron. Figure 1 presents a view of the ECR ion source where microwave injection system, water cooling lines etc can be seen.
Figure 2. shows the layout of the ion source and the injection line. There is another ECR ion source (ECR-3) with similar specifications is going to be installed as shown in the layout. The entire system is assembled in the high-bay area, just above the superconducting cyclotron.
At present the (ECR-2) ion source is being used to produce ion beam and inject into the cyclotron.
Figure 3 shows the physical layout as it exists on high-bay area. Ion beam extracted from ECR ion source is mass analyzed and bent twice to inject into the cyclotron. There are several optical elements such as Glazer lenses, bending magnets and steerers. There are a host of beam diagnostic elements such as Faraday cups and collimators distributed through-out the beam line. Three gridded ion beam buncher is installed in the vertical injection line to enhance the efficiency of injection and acceleration.
Control system of ECR ion source follows a distributed architecture to reduce enormous cabling and to improve the reliability. All parameters are controlled and monitored from one single computer placed in ion source hall of high-bay. In addition, all the parameters can also be controlled from another terminal in the main control room of superconducting cyclotron. Fig 4 shows the view of control room.
Figure 5. is the typical control panel of the ion source and injection line parameters. The page has tab controls and each tab is meant for control and monitor of particular group of parameters, e.g, first tab is for ion source control, second tab is for injection line control, third tab is for vacuum system control. The control system is developed in such a way that the ion beam from faraday cup and collimators can be observed on any page. This feature is very important because the ion beam position and profile down the beam line are affected by any optical elements placed upstream of the Faraday cup.
Control software also has a feature to continuously log the data and save the latest parameters for quick restart of the system in case of the power failures.
Sevearl ion beams have been developed and few of them are listed below in graphs shown in figure 5. Ion beams of solid elements are produced by using micro-oven and by sputtering method depending on the vapor pressures of different elements.
Ion beam sputter deposition of thin films.
ICP ion source developed for producing focused ion beams has also been characterized with larger extraction apertures. With 3 mm extraction apertures, > 2 mA of Ar ion beam has been extracted. Heavy ion beams have high sputtering yields and hence are very efficient in sputtering the target materials on large areas. Sputter material has sufficient energies to form high quality thin films on practically any materials including insulators. By controlling ion beam density, ion species and energy, a very fine control on the growth of the thin films can be achieved easily. Thin films of Cu, Hf, ZnO2, Ni, Cr, W etc of thicknesses in the ranges of 10 – 100 nm have been developed on Al, glass, kapton, Si and Teflon substrates. Fig 1. Shows few examples of coating of tungsten on Ceramic, aluminum foil and carbon fiber.
Fig 1. A) Tungsten deposited on beam viewer to facilitate current reading as well as beam viewing. B) Tungsten coated on Aluminum foil, Tungsten coating on carbon fibre.
Fig 2. shows the ion beam sputter deposition of steel on the inner surface of the Teflon. This shows the capability of deposition of metal films on complex inner surfaces of even insulators. Ion beam can be focused on to the target mm size so that the film from small size expensive targets also can be synthesized without loosing the material.
Fig 2. Steel coating on the inner surface of the Teflon ring. Image on right shows the Halfnium deposition on glass, cellophane, kapton and silicon.
Thin films of various meterials are routinely developed. High current beams are also being used for cleaning the surfaces by ion beam milling.
Focused ion beam system
Focused ion beam, also known as FIB, is a technique used particularly in Semiconductor and materials science fields for site specific analysis, deposition and milling. This set up resembles scanning electron microscope (SEM). In SEM, electron beam is focused on to the sample to image the surface, whereas in FIB, ion beam is focused on to the target. Liquid metal ion source are used in conventional and commercially available systems as the source of ions. These ion sources can only produce metallic ions which contaminate the target materials with metallic ions forming complex alloys leading to several changes in the target material properties. To overcome the limitations of the LMIS based FIB systems, a plasma based ion source is developed to produce ion beams of gaseous elements. Besides avoiding the contamination of the target material, carbides, nitrides and oxides of materials can be produces selectively in micron and submicron scales on the target. This feature of plasma ion source based FIB system eliminates several stages of fabrication using conventional surface micromachining techniques commonly used in semiconductor technologies.
A plasma based ion source is designed for the above said application. Here, inductive coupled plasma is chosen for an ion source as it has distinct advantages over other mechanisms of production of plasma. Advantages are: Ion source has no filaments and hence the life time of ion source is considerably high (> 1000 hours). As there is no filament or any types of electrodes, contaminations in the plasma are minimum and ions of reactive gases can be easily produced. Plasma potential in the ICP plasma are much lower than other sources. Plasma density is high and ion energy spread is low. The ion source developed for this application has a novel plasma chamber having two compartments to facilitate easy initiation of plasma at low RF power.
A two lens focusing column is designed to focus the ion beam. Both the lenses are einzel lenses that are carefully designed to minimize the aberrations. Ion column also has electrostatic deflectors to align the ion beam to the optical axis. Fig 1 shows the schematic of the setup. On the left of the schematic the potential distribution along the length of the source is indicated. Fig 2. Shows the the ion source and the vacuum chamber assembled on the active vibration isolation system. The vacuum pumps are mounted on to the vacuum chamber through vibration damping bellow. With use of active vibration isolation system and vibration damping bellows, vibrations from the surrounding and the pumps are eliminated by a factor of 99%.
Fig 1. Schematic of Plasma Ion Source Based FIB system. Image on right shows the focusing column and einzel lenses.
Fig 2. Assembly of Plasma ion source based FIB on vibration isolation platform. Image on the right shows the xenon plasma with 180W or RF power. Ion source uses no magnets and is air-cooled.
Characteristics of plasma ion source based FIB
Ion source and the focused spot are characterized. The above table summarizes the characteristics of the FIB system. Ion source is under operation over more than 500 hours and find no deterioration in the performance.
Figure 3 shows the ion beam extraction characteristics of Ar, Kr and Xe ion beams extracted with 1 mm anode aperture. Current density of more than 70mA/cm2 has been achieved for 8KeV Xe ions when the plasma is operated at 180W of RF power. Figure 4 shows the typical focused ion beam spot profile as obtained by knife edge measurement technique for Ar ion beam with currents of 400nA and 1300nA.
Several micropattering experiments were carried out. Few of the examples are shown in Figure 5. Experiments were carried out to measure the milling rates of steel with Ar ion beam of 6 KeV. Milling rates of more than 135µm/S has been observed. With Xe ion beam the milling rate is expected to be > 500µm/S.
Fig 5. A) Arrays of line of 500 µm long and 100µm and 200 µm gap on Si wafer. B) arrays of dots 30 µm diameter with 100 µm gap. C) VECC Logo milled on 200 µm x 200 µm area on Si. D) 5 µm dia with gap of 5µm on Si. E) milling on steel blade F) 18µm diameter through hole milled in steel.
|Nabhiraj P Y