A.G. Ponomarev

Optimum collimator shape and maximum emittance for submicron focusing of ion beams. Determination of the probe-forming system resolution limit

Abstract Improvements in the spatial resolution of instrumentation using focused ion beams are connected with further reduction of the beam spot size, while the beam intensity is kept sufficiently high. In search for probe-forming systems (PFS) of new generation it is important to determine the lower theoretical resolution limit for each system. In this case the lower resolution limit can be taken to be quality-factor of the system. As a general approach permitting the resolution limit to be found the authors propose the method of the maximum beam emittance. By proper selection of the object and angular slit dimensions and their positions with respect to the axis of the PFS, one attains the maximum beam emittance for a prescribed beam spot size. Calculations involving only demagnification and aberration coefficients allow a comparison to be made between different PFS irrespective of their dimensions and focusing elements employed. Investigations into conventional magnetic quadrupole PFS show that parasitic sextupole field components present in the lenses can lead to the asymmetric position of the angular slit and thus, to asymmetric position of the beam along the optical line.

Numerical optimization of magnetic nonlinear quadrupole systems in an ion microprobe with given spot size on the target

Abstract The beam emittance is maximized for a given beam spot size. Five magnetic quadrupole systems (one triplet and four Russian quadruplets) are numerically investigated taking into account all geometrical aberrations of the third order. The parameters to be selected to optimize the systems are the positions of lenses and the excitations (or magnetic gradients) of lenses. The optimization is fulfilled for different spot sizes and the results are presented. The reduced parasitic sextupole and octupole aberration coefficients are also calculated.

Optimization of magnetic quadrupole probe-forming systems by the method of synthesis

Abstract The paper presents a general approach to the problem of synthesis of a focusing system (FS) from several magnetic quadrupole lenses. Two-parameter sets of focusing systems are constructed in which the system length and the object distance are varied. The subclass of orthomorphic FSs with two separate power supplies based on the triplet (three versions) and the Russian quadruplet is discussed in detail, including the ion-optic properties. Traditional and nontraditional designs are compared.

Precision magnetic quadrupole lens for a nuclear scanning microprobe based on an ÉGP-10 electrostatic tandem accelerator

Basic ideas underlying the design of a precision magnetic quadrupole lens for a nuclear scanning microprobe with a maximal accelerating voltage of 14 MV are set forth. Four magnetic quadrupoles are combined into doublets. The doublets are placed on adjusting gears, which bring the local coordinate system of each lens into coincidence with the laboratory system related to the axis of the beam. Each lens provides a maximal gradient of the field of 0.68 T/cm, which makes it possible to perform stigmatic focusing of the beam with a working distance of 22 cm. All lenses are nonseparable and made of one piece of high-quality electrical steel. A special lens-feeding unit is designed that provides manual and remote control of pole tip excitation.

Optimization of the probe-forming system for a scanning nuclear microprobe based on the ÉGP-10 electrostatic tandem accelerator

The probe-forming system of a nuclear scanning microprobe based on the parametric multiplets of quadrupole lenses is optimized. The optimization is aimed at creating an ion probe with energy of several MeV that produces a micrometer spot on the target at a current of ∼100 pA. The influence of different geometric and physical parameters on the ion-optical properties of the probe-forming systems considered is determined. The optimization is carried out by varying the parameters specifying a given parametric multiplet, and its efficiency is found from a quality criterion that takes into account the beam current for given sizes of the spot and target. The beam parameters at the entrance to and at the exit from the ÉGP-10 electrostatic tandem accelerator (produced by the VNIIÉF) are involved in the optimizing calculations. These are the maximal energy, normalized brightness, transport conditions, and chromatic inhomogeneity of the beam (i.e., the energy straggling of beam particles). Allowance is also made for the parasitic components of the magnetic quadrupole lens field, which arise because of quadrupole symmetry breaking by technological and physical reasons.

Proton beam scanning modular unit for a nuclear microprobe

General ideas underlying the design of a proton beam scanning modular unit intended for a nuclear microprobe consisting of a ferromagnetic x-y scanning system and a dynamic power supply are described. For an H+ ion energy of 14 MeV, the unit provides a rectangular scanning raster with a linear size of ±300 μm on the target and a minimal point-to-point switch time of 200 μs. The positioning time does not exceed 40 μs.