Handbook of Modern Coating Technologies

High-voltage ion gun of the scanning nuclear microprobe

Today, charged particle beams are accelerated by various methods designed to build up an accelerating voltage. The main problem is to ensure that a charged particle accelerator pro­duces a high-brightness beam of the desired energy with its small spread; otherwise, chro­matic aberrations of the focusing elements (FEs) will not allow creating a probe of the required size.

Electrostatic accelerators are classified in terms of the type of conductor of the charging unit of the high-voltage terminal. Van de Graaf accelerators have a conveyer belt to transport charges [29]. The rubber or synthetic belt carries a positive charge toward the high-voltage terminal where the belt is discharged. A disadvantage of such a mechanism is that it is diffi­cult to dynamically control discharging, which leads to temporal instability of the conductor potential and restricts, in turn, the energy spread of the particles in the beam at the level of Д£/Б~10^_3. Current instability lies in the range of ±10%—30% [30,31] and does not appre­ciably change the resolving power. However, this parameter is of importance in charge nor­malization for quantitative analysis and obtaining a contrast image of the sample surface with secondary electrons. The nonuniform brightness of the image in this case is attributable to the impossibility of accurately controling the charge coming with beam ions.

The accelerators of the National Electrostatic Corporation, USA, have an inductive type P electron charging unit with the energy spread improved up to ДБ/Б~10^_4 [32,33]. One of the main drawbacks of both accelerator models is the mechanical charge transport fraught with vibrations, which hamper the achievement of high resolution in a nuclear microprobe.

Currently Cockroft— Walton type accelerators with a specific high-voltage terminal [34] are considered to be the most promising for microprobe applications. This approach is based on using electronic high-voltage-multiplication circuits containing no mechanical parts and is therefore free of vibrations. To date, the highly stable singletron electrostatic accelerator has been developed by High Voltage Engineering Europe (HVEE), the Netherlands with a maxi­mum conductor voltage of 3.5 MeV [35], which ensures an energy spread of the beam parti­cles up to Д£/£~10^-5 and practically eliminates chromatic aberrations.

A major characteristic of an ion source in accelerators is its brightness, defined as current density per unit solid angle. The total current at the surface of a sample or a target is the product of paraxial brightness of the ion beam and its transverse phase volume that the PFS is able to transport into the target plane with minimal spot size. Therefore the ion sources with the highest brightness are utilized to obtain the highest beam current at a maximally attainable transverse phase volume in accelerators with a SNM channel. High-frequency (HF) sources have found wide application in electrostatic accelerators. The search for new physical principles and technologies called to modernize HF sources with a view of increas­ing the brightness of HF sources is currently underway. For example, the standard HF source

installed at the Institute of Applied Physics (IAP) of the National Academy of Sciences of Ukraine (NASU) was modified by mounting a system of annular permanent Sa—Co magnets creating such a magnetic field configuration that increases the source brightness by a factor of three or more, up to 10 pA/pm² mrad² MeV. The use of a special configuration of the magnetic field and optimization of the beam extraction and transport system permitted enhancing the paraxial brightness of the HF source by a factor of >10 [36—38]. However, these data indicate that electron guns afford brightness exceeding that of the available HF sources at least by two orders of magnitude.

Essentially different ion sources for SNMPs, besides HF ones, were also considered with a view to radically increase the ion beam brightness up to that of electron sources. Liquid-metal ion sources (LMISs) with the brightness on the order of 10⁶ pA/pm² mrad² MeV proved inap­plicable for certain microanalytic techniques that require a significant rise in the ion beam energy to ensure similar conditions of interaction between hydrogen and helium ions and the atoms of the sample material. The application of LMISs with Ga¹ ions for creating a micro­probe with a beam energy of 500 keV to modify materials for microelectronics was considered in paper [39]. However, the low current of such a source and the large divergence angle of the primary beam impose strong aberration constraints on the entire optical system of the device. The failure to obviate them prevented a substantial improvement in resolution.

Results of numerous studies of gas field ion sources with a fantastic brightness of 10⁹ pA/ pm² mrad² MeV [40,41] indicate that their straightforward application in SNMP encounters dif­ficulties arising from certain technical constraints (temperature <77K and pressure <10^_1° Torr). It is practically impossible to create such conditions in the high-voltage part of an electrostatic accelerator under a conductor placed in an insulating gas medium at high pres­sure. The general requirements for high-brightness sources to be used in electrostatic accelera­tors for SNMPs are expounded in Refs. [42,43]. The authors emphasized the necessity of the most thorough analysis of ion beam-forming optics, bearing in mind that large divergence angles in these sources may enhance the effect of spherical aberrations on the beam transport conditions in the accelerating tract and the primary beam formation at the entrance to the PFS.

Also worthy of note is the use of cyclotrons as the accelerators for an SNMP ion gun. The first application of a cyclotron for this purpose was demonstrated in Amsterdam in 1979 [44], despite the low brightness of the beam generated in the ion sources based on electron- cyclotron resonance. Currently the most successful cyclotron in operation, the JAEA AVF in Takasaki, Japan, utilizes heavy (mostly single) ions for research in biology and microelectron­ics. The upgrade of the cyclotron allowed achieving an energy resolution of 0.02% for 90- MeV proton, and 260-MeV²⁰Ne⁷¹ ions [45,46].