Channelling (physics)

The grid guide or grid guide effect (English channeling ) is a physical phenomenon that occurs in the ion beam physics. It describes the almost undisturbed penetration of an ion in a single crystal due to linear areas without lattice atoms in certain crystal lattices.

Description

Are irradiated single crystals with focused ion at an angle close to a crystal axis, they penetrate deep into the crystal. In typical backscattering experiments ( Rutherford back scattering) to measure a lower backscatter rate, because it is precisely along the crystal axis of the collision with lattice atoms is unlikely. Had the atoms perfectly bayed behind the other, then the ions would even see a single plane; it is as if the particles would penetrate along the grating through channels, even by the repulsive effect of the lattice atoms in the channel held ( since both positively charged), which explains the English and German name.

If one detects the particles in two dimensions, one obtains typical backscatter patterns that depend on the crystal structure of the specific crystal axis and the lattice site of the backscattering atoms. This also speeds up the measurement compared to the one-dimensional detectors.

Even negative and neutral particles show the lattice guiding effect. The experiment can be carried out using radioactive ions. In this case, the ions are no longer emitted from them but particles are detected; then one speaks of emission channeling ( engl. emission channeling ).

History

This effect was seen advance even before the first experiments with X-ray diffraction on single crystals have shown effects at all. 1912 such a hypothesis was published (John Stark). More than 50 years later came the first experimental evidence ( Piercy et al., 1963) and the first computer simulations ( Robinson and Oen, 1963). Meanwhile, the studies to this effect learn a very good if not fully developed theoretical and experimental development.

Applications

Direct application of this technique is the determination of the crystalline structure and the distribution and position of defects or impurities of a sample ( for example, thin films ). An interesting use is, for example, the lattice position determination if the doping of a semiconductor, in the manufacture of microelectronic circuits, a radioactive isotope which is normally stable elements is used. Since the radiation after the doping proceeds directly from the doping authorities to allow the distribution and position of the doping points and thus the semiconductor properties over method of the external sources of radiation work ( eg X-ray structure analysis ) can be determined much more accurately. The lattice guiding effect has not only an impact on the doping, but also contributes to the directional dependence of the resulting radiation ( at α - and β - radiation) at. Other applications are in surface physics ( surface relaxation, surface contamination, structural analysis at the interface between two different layers, etc.).

Importance in the semiconductor industry

The grid guide is an undesirable effect in the production of semiconductor devices. In this case, the silicon wafers are intentionally doped with impurity atoms to achieve the desired electrical characteristics. One method of doping ( in addition to the diffusion) the ion implantation, wherein said wafer is bombarded with ions. In this case the effect occurs and alters the otherwise very well to simulate penetration of the ions.

Contrary taxes can be grid guide basically in two ways: on the one hand, by the wafer relative to the implantation direction is slightly tilted (about 7 °). Alternatively or additionally, one formed on the wafer prior to implantation a thin layer of silicon dioxide ( with a coating method or by thermal oxidation). At this amorphous layer, the ions are scattered by elastic and inelastic collisions and penetrate so no longer preferentially along the crystal planes a.