Ellipsometry on ion implantation induced damage

The optical properties of semiconductors largely depend on the disorder in the crystal structure, especially in the photon energy range near the direct interband transition energies. The E1 and E2 critical point (CP) energies in silicon are about 3.4 eV (∼365 nm) and 4.2 eV (∼295 nm), respectively. These transitions are located in a photon energy range that is available in most commercial spectroscopic ellipsometers, which makes ellipsometry a powerful technique for the characterization of ion implantation-caused damage. Due to the absorption peaks at the CP energies the optical penetration depth is small. For example, in silicon it is about 10 nm and 5 nm at photon energies corresponding to the E1 and E2 CP energies, respectively. It means that current trends towards shallower junctions and lower ion implantation energies make ellipsometry even more sensitive to the near-surface crystal structure, and the sensitivity of depth profiles can further be increased preparing special samples for the measurements using wedge masks. Ellipsometry measures the complex reflectance ratio of the sample in form of a pair of ellipsometric angles (ψ,Δ) that can accurately be measured using commercial ellipsometers. It is more and more important to use proper optical models to evaluate the measured spectra. There are two key points when evaluating ellipsometric spectra measured on ion implanted semiconductors: (i) the parameterization of the dielectric function of disordered material and (ii) the parameterization of the damage depth profile. The dielectric function can be characterized using numerous methods including the generalized critical point model, the standard critical point model, and the model dielectric function. The depth profile can be described using coupled half-Gaussian profiles or error functions. Because ellipsometry is a non-invasive and non-destructive method, it is capable of the measurement of decreasing disorder in situ, during annealing in a vacuum chamber or a furnace. It has also been demonstrated that ellipsometry is a powerful tool for a quick and non-destructive mapping of large surfaces using special optical arrangements and proper optical models. Using this tool, it is possible to map the lateral homogeneity of the dose, to map the thickness of thin surface layers and any other near-surface properties that can be described by proper optical models.