Handbook of Modern Coating Technologies

Physical basis of the scanning nuclear microprobe

A SNM represents a combination of several devices and systems based on different physical principles. Their traditional layout is illustrated in Fig. 5—4. As a rule, a plasma ion source produces a beam accelerated to the necessary energy of a few megaelectron-volts in an elec­trostatic accelerator. Ions of the desired type are selected by an analyzing magnet having an output slit.

The beam formation in SNMP is performed with controling electromagnetic fields, and the system transforming an ion beam into the probe is known as the probe-forming system (PFS). Here, the preliminarily formed beam is transformed into the probe by the object and angular collimators with the help of a focusing system (FS) composed of a set of active ion-optical elements, such as quadrupole lenses or a superconducting solenoid. The probe position at the sample surface is altered by the mechanical or electromagnetic systems that displace either the sample or the probe, respectively.

This traditional SNMP setup is a product of historical developments. Ion beam microanal­ysis became one of the applications of electrostatic accelerators after the main nuclear con­stants had been determined with the help of these facilities, and the physics of charged particle beams had centered on high-energy ranges. Local irradiation of biological objects with the analysis of nuclear reactions required focusing MeV-energy light ion beams for increasing beam density with the aim to reduce the irradiated area; hence, the name

"nuclear microprobe.” In fact, it was originally just an attachment now referred to as a chan­nel of an electrostatic accelerator. The early history of SNMP developments is described by one of the patriarchs of the field, Legge [28].

Comparison with such MIFs as a high-voltage electron microscope leads to the conclu­sion that the electrostatic accelerator with an ion source and the analyzing magnet of SNMP function in the aggregate as a high-voltage ion gun. The major requirement for the improve­ment of both ion and electron guns is the enhancement of brightness and current stability of the beam coupled to the reduction of energy spread of the charged particles. PFSs must show high acceptance scaled to the probe size directly related to the high reduction factor D (or scale factor, a characteristic similar to that of optical microscopes) at low aberrations. These requirements govern trends in the development of beam formation methods in SNMP, which are based on the traditional layout of its constituent elements. The main fac­tors determining spatial resolution of a microprobe (minimal probe size at a given current strength) include:

• brightness of the ion source,
• energy spread of the beam particles at the exit from the accelerator,
• quality of PFS elements,
• choice of optimal PFS parameters,
• effect of external parasitic electromagnetic fields,
• magnitudes of vacuum pressure, vibrations, etc.