Optical reflectometry of radiation damage in ion-implanted silicon

Reflectivity vs dose experiments were performed by implanting ³¹P ⁺ into polished single crystal Si substrates at a constant ion energy of 50 keV and a dose range of 1 × 10 ¹⁴ to 5 × 10 ¹⁵ cm ^-2. The measured reflectivity of the wafers at λ 632.8 nm changed from the pre-implanted value of 33.6% to a maximum of 42.5% at a dose of 1 × 10 ¹⁵ cm ^-2. Three major regions were observed on the R vs N curve. In the low dose region ( N < 3 × 10 ¹⁴ cm ^-2) only small changes in the reflectivity are noticeable. In the intermediate region (4 × 10 ¹⁴ to 8 × 10 ¹⁴ cm ^-2) the reflectivity increases rapidly and monotonically with dose. In the third section of the curve ( N > 8 × 10 ¹⁴ cm ^-2) the reflectivity remains at its high value, decreasing only slightly with increasing dose. Three sets of experiments were also performed of reflectivity vs implantation energy. For each set, the implantation energies were varied in steps between 50 keV and 200 keV, keeping the dose constant. The three regions selected, namely 1.2 × 10 ¹⁴, 5.5 × 10 ¹⁴ and 2 × 10 ¹⁵ cm ^-2, are representative of the three regions identified in the R vs N curve. Three distinctly different curves are obtained for the three dose regions. The low and the high dose curves show only small variations as a function of energy, and oscillate around the 50 keV values. Above 180 keV the high dose curve approaches the low dose curve. The reflectivity at the intermediate dose exhibits rapid changes in the region 50-100 keV, and also approaches the low dose curve at high energies. The experimental results are modeled by means of a multilayer optical model, assuming the existence of a critical ion concentration for complete amorphization of a continuous layer in the host crystal. A non-linear least-squares procedure is employed in order to fit the theoretical reflectivity curve to the experimental observations. Excellent agreement between experiment and theory is obtained, with the optical constants and the profile parameters as output.