Handbook of Modern Coating Technologies

Probe-forming systems of the scanning nuclear microprobe

PFSs of charged particle beams are concentrating ion-optical systems unintended for obtain­ing correct images of the object of interest. The main requirement for the PFS is an ade­quately sized probe with a beam current sufficient for experimental studies. The goal of minimizing the probe size that determines resolving power is in conflict with that of
maximizing the probe beam current directly related to the sensitivity of the microanalytic techniques being used. Indeed, the probe can be made smaller by decreasing the beam cur­rent, while an increase in the current results in a greater probe size.

The PFS can be categorized into passive and active based on the methods of beam forma­tion. The simplest and most primitive PFS scheme comprises two diaphragms that collimate the beam (Fig. 5—5A). The first diaphragm is called the object collimator, since its aperture sets the object size. The second diaphragm limits the angles of beam divergence and is referred to, in analogy with light optics, as the aperture or angular collimator. Such collima­tor systems were the first passive PFSs of SNMPs used for microanalysis based on resonance nuclear reaction method [48,49], with a probe size of ~100 pm.

Current density in the probe is naturally increased by utilizing active ion-optical elements focusing the beam (Fig. 5—5B). In such a setup, the PFS of a nuclear microprobe consists of object and angular collimators forming the initial phase space of the beam and a set of lenses making up the FS. The probe-forming process includes transformation of beam parti­cle phase coordinates from the plane of the object collimator into that of the sample image in accordance with the approximate solution of the nonlinear differential trajectory equations describing beam dynamics in electric and/or magnetic fields of different symmetry types. In the aberration theory for charged particle optics, this solution has the form

where (x_ob, y_ob, X_ob, and y_ob) are the phase coordinates of beam particles in the plane of the object collimator;(x_t, y_t) are the coordinates of particle deflection from the axis in the sample (target) plane;6 is the momentum spread of the particles;D_x(_y) are the PFS reduction factors; (x/26) and hy/y6) are chromatic aberrations;and (x/X^ly^J) and (y/X^ly^J) are intrinsic and par­asitic spherical aberrations of the PFS, respectively.

Relation (1) holds for stigmatic beam focusing and in the case when the contribution of other geometric and parasitic aberrations is smaller compared with chromatic, intrinsic, and parasitic spherical aberrations like those in PFSs of SNMPs. It follows from (1) that PFSs with large D are capable of ensuring small probe sizes with a sufficiently large object collimator. However, high-D systems have greater aberrations, which necessitate reducing the size of the angular collimator. This discrepancy peculiar to PFSs of SNMPs requires seeking solutions to increase reduction factors without a significant increase in aberrations. Therefore there are var­ious active PFSs in which probes are formed using various combinations of active elements to modulate the ion-optical properties of PFSs and influence the resolving power of SNMPs.

• Probe-forming system with a superconducting solenoid

A comparative analysis of the focusing properties of magnetic axially symmetric lenses used in SEMs revealed limitations on their application for light ion beams with an energy of sev­eral megaelectron-volts [8]. The estimation was made based on the value of magnetic induc­tion, B² втТ/q², necessary to focus a charged particle beam with energy T, mass m, and charge q. To reach parameters of the same order of magnitude as electron probe characteris­tics when using a proton beam with an energy of 1 MeV, magnetic induction must be an order of magnitude greater than the saturation limit ~2.5 T), even for special magnetic mate­rials. Insufficient value of the magnetic induction does not allow the working length g to be smaller than 20 cm for solenoids containing no superconducting elements;it restricts, in turn, the reduction factor to ~10 [50].

The use of superconducting solenoids somewhat improves the characteristics of axially symmetric ion-optical elements for PFSs of SNMPs. Today, a superconducting solenoid is used as the active FE of SNMP in Bochum, Germany [51,52]. The resolving power of this unit is 0.6 X 0.7 mm² with the proton beam current I & 100 pA and the ion energy 3 MeV. The length of the PFS measures ~6 m, maximum reduction factor D = 80 at the minimal working distance g = 7.5 cm and the maximum magnetic induction B = 8 T. A tandem electrostatic accelerator with a Dynamitron type charging unit (maximum voltage 4 MV, proton beam bright­ness b = 1.5 pA/pm² mrad² MeV, and energy spread of the beam particles AE/E = 5 X 10^_4) serves here as the ion gun. Nonetheless, despite the sufficient parameters of SNMP, supercon­ducting solenoid-based PFSs have limited application for some analytic techniques due to large scattered magnetic fields.