Handbook of Modern Coating Technologies

Scanning nuclear microprobe

The scanning nuclear microprobe (SNMP) is a relatively new MIF designed for studying the structure and elemental composition of samples and direct proton beam writing (PBW) [7,8]. Used in this MIF is a focused beam of light (hydrogen and helium) ions with an energy of several megaelectron-volts. Such a beam passing through a test subject undergoes only slight
expansion in its transverse dimension and penetrates as deep as a few dozen micrometers, with bremsstrahlung background being very low. Due to this, the spatial resolution of SNMP depends on the probe size at the sample surface, while the sensitivity of the microanalysis of certain nuclear-physical methods is at the level of 1 ppm. It makes possible investigations of near-surface layers of thick samples without detriment to spatial resolution and sensitivity. During its 40-year history, SNMP has been widely applied in such different fields (Fig. 5—2) as materials science [9—11], microelectronics [12], geology [13], botany [14], biophysics and medicine [15—17], archeology and the arts [18], the environmental sciences [19,20], microim­plantation [21,22], and the manufacturing technology of 3D micro- and nanosized structures [23,24].

To better illustrate the peculiarities and the potential of SNMP, it is worth considering the processes associated with the passage of high-energy (MeV) ions through matter.

• Physical mechanisms of interaction between high-energy light ions and solid-state matter

Light ions with an energy of several megaelectron-volts are known to interact with both atomic electrons and nuclei of sample material. However, the probability of ion—electron interaction in the first half of the ion path is a few orders of magnitude larger than that of scattering by atomic nuclei [25].

Microelectronic;

3 MeV protons ion current 100 p. resolution 1 pm

Due to the great mass difference between interacting ions and electrons, their interplay does not significantly change the incident ion trajectory, which is virtually a straight line (Fig. 5—3A). The energy lost by the ion in such collisions being small (to conserve momen­tum), thousands of interactions with atomic electrons may occur before the ion loses all its kinetic energy. The steady loss of energy by a moving ion accounts for the practically uni­form depth distribution of the introduced dose. As the ion loses energy, and therefore slows down, the probability of its interaction with atomic nuclei increases and the trajectory appre­ciably bends, causing the ion to follow a curved path (Fig. 5—3A). A distinctive feature of mid-energy light ion beams compared with electron beams is the practically total absence of secondary electrons with an energy that could markedly affect the material fluence (proxim­ity effect). The ion penetration depth in a given material depends on the ion energy and is strictly determined; this important property permits creating 3D multilevel objects in

Scattering (pm)

high-resistance monolayer materials. Calculations using the SRIM numerical code [27] indi­cate that protons with an energy of 2 MeV traveling in silicon penetrate as deep as « 50 mm and are deflected from the axis within ~3 pm at the end of their path.

Traveling beam ions with sufficiently high energy can displace atoms in the Si crystal lat­tice and thereby alter the local electric properties along their trajectories. Fig. 5—3B shows that the ions lose most of their energy at the end of their path and create a large number of vacancies. Such defects in the Si bulk can be used in two ways. First, to create small-sized 3D structures during irradiation of a definite region by a focused beam of light ions with MeV energies and a fluence sufficient to produce porous silicon by electrochemical etching only in the nonirradiated region. After the completion of etching, the sample is placed in a KOH solution where the porous silicon is removed and only the irradiated region remains. See paper [26] for a more detailed description of this process. The other way of using the irradiated Si region has its origin in the formation of a deeper region in the last part of the ion path with an increased refractive index due to high vacancy density (Fig. 5—3B).

Atomic interactions with beam ions result in the formation of some secondary products, besides internal structural changes in bulk specimens of resistive materials; detection of these products provides information on the local elemental composition in the scanned region. Fig. 5—3C demonstrates secondary products, such as particle-induced X-ray emission (PIXE) brought about by beam ions, nuclear reaction products (n, p, y- and a-particles), Rutherford backscattering (RBS) ions impinged on atomic nuclei, secondary and Auger elec­trons, and visible light.