Influence of rapid thermal processing on carrier concentration in high resistivity silicon

Abstract

In this study, we report changes in the carrier concentration of high resistivity Si wafers after rapid thermal annealing (RTA) anneals measured by spreading resistance technique. Spreading resistance technique (SRP) profiles clearly show the generation of donor centers with concentrations and depth distributions comparable to those of vacancy-related centers reported in the literature. Changes of carrier concentrations as a function of RTA temperature, duration, ramp down rate, and subsequent annealing in the 800-1000 8C range are consistent with the earlier literature data. The influence of annealing ambient is also studied. Annealing in pure Ar atmosphere leads to profiles dominated by in-diffusion of vacancies generated at the surface, while annealing in oxidized ambient results in well-known profiles controlled by outdiffusion of vacancies generated in the wafer bulk. Studies of the wafers with different oxygen content show that the concentration of the generated donors is directly proportional to O i concentration.

Trap‐Limited Vacancy Diffusion in Silicon at High Temperatures

“…The prefactor in D includes ρ/ N , a very large ratio for a moderate trap concentration. By the data given by Capello et al (circles in Figure ), the activation energy is about 4.5 eV, whereas the prefactor is roughly 10 9 cm 2 s −1 ; these numbers strongly indicate to a trap‐limited vacancy diffusion.…”

“…Different kinds of vacancy (or a mixture of them) can be manifested depending on the experimental conditions, for different time scales of observation. Now, with an extended database (including vacancy-related donors in RTA wafers [15] ), it has become evident that there is a much simpler reason for a small D (and for a strong scatter in D): the wafers can be contaminated with some impurity (or impurities) that acts as efficient traps for vacancies even up to 1250 C. The apparent (trap-limited) vacancy diffusivity D can be much lower than the true diffusivity D V (of free vacancies) depending on a random amount of traps. This is a peculiarity of wafers; in growing crystals, the vacancies are not trapped at the void formation temperature (around 1100 C) judging by the observed void size and density.…”

“…The trap concentration, N = [X i ], can be roughly estimated by the concentration of vacancy‐related donors that are presumably the V*O (X s O) complexes of a concentration well smaller than [V*]. The highest observed donor concentration is 3 × 10 13 cm −3 , and [V*] is still well higher, which is about 10 14 cm −3 or more. The trap concentration N should be still higher, which is 3 × 10 14 cm −3 or more.…”

“…The donor profiles to the full wafer depth are available for two cooling rates: 45 and 5 K s −1 (Figure ). The fitting procedure is as follows.…”

“…Recently, it was reported that RTA (at 1100–1250 °C, in Ar) creates donors in high‐resistivity wafers. The donor depth profiles are of a complicated shape, sometimes resembling the Pt‐monitored vacancy profiles.…”