Channeling Contrast Microscopy - spatially resolved ion channeling at Lipsion

Dipl.-Phys. Niklas Liebing

Ion channeling is an established technique since 1950 [1]. The channeling effect relies on the regular arrangement of atoms in a crystal. The atomic rows or planes can be considered as strings or sheets of charge. The average of the individual atomic potentials can be described as a continuum potential. The channeled ions are steered by the rows or planes and suffer only small angular scattering. Therefore, the alignment of a major direction of a crystal (rows or planes) leads to a large reduction in the rate of energy loss and scattering probability. This steering ability gives rise to a number of phenomena useful for the investigation of the crystal structure. Anything that disturbs this regular arrangement can be studied using this technique.

The traditional channeling technique with a broad, unfocused beam is limited to distin- guish between different types of defects and delivers information about their depth distri- bution. However, the exact nature of the defect cannot be determined unambiguously. Spatially resolved information cannot be produced using conventional broad beam channeling analysis. The nuclear microprobe offers the capability to raster a sample with a focused beam. This allows to study crystalline materials using the traditional channeling technique in addition with spatial information from the sample (Fig. 1). The spatially resolved channeling techniques are called Channeling Contrast Microscopy (CCM) [2] and Channeling Scanning Trans- mission Ion Microscopy (CSTIM) [3] CCM has now been established at the ion nanoprobe LIPSION. In general for CCM a very high accuracy in sample motion is required. Therefore, a eucentric two-axis goniometer was installed and a new software was developed for an automated alignment procedure which links the data acquisition with the goniometer control.

Figure 1: CCM image106 x 106 µm2) of Si (along {100}) irradiated on a  54 x 54 µm2 area with 2 MeV 4He+ ions. The dechanneling at the outer edges of this area which was swollen shows up in light grey.

A first application of CCM was the investigation of the ion beam induced swelling of a silicon single crystal
[4]. For this purpose the beam was aligned with the {100} axis and the normalized RBS yield (c_aligned/c_random) was monitored. As can be seen the ion beam induced swelling leads to a misalignment of the crystallographic axis at the outer edge of the irradiated area with respect to the ion beam (Fig.1). This results in an increased back-scattering yield.

Furthermore, we studied the dependency of the dechanneling due to swelling on scan area and ion fluence (Fig.2). Whereas the minimum yield c_min of the total scan area increases significantly with ion fluence due to the contribution of the misalignment in the outer edge, good channeling conditions remain in the center of the scan..

Figure 2: Left: CCM image of a Si crystal irradiated with 2 MeV ⁴He⁺ ions. Right: Minimum yield c_min extracted from the regions indicated in the CCM image as a function of ion fluence. Whereas the dechanneling in the total scan area increases remarkably with ion fluence due to the contribution of the misalignment in the outer edge, good channeling conditions remain in the center of the scan.

Crystal defects influence the electrical properties of materials in different ways. Thus, the imaging of the crystal quality is an important application for ion microbeams in the field of materials analysis of semiconductors. Channeling Contrast Microscopy offers the capability to perform these analyses even for micron-sized samples. Further developments focus on the introduction of CSTIM (high resolution single ion technique) which allows a better spatial resolution without having ion beam induced swelling as a limiting factor, but requires thin samples.

Literature:
[1] L.C. Feldman et al.: Materials analysis by ion channeling: Submicron crystallography (Academic Press, New York 1982)
[2] C.J. McCallum et al.: Appl. Phys. Lett.42(9), 827 (1983)
[3] M. Cholewa et al.: Nucl. Instr. Meth. 54(1-3), 397 (1991)
[4] N. Liebing: Diplomarbeit, Fakultät für Physik und Geowissenschaften, Universität Leipzig (2009)