Morphology of Oxygen Precipitates in RTA Pre-Treated Czochralski Silicon Wafers Investigated by FTIR Spectroscopy and STEM

Rapid thermal annealing (RTA) is a commonly used technique to control precipitation of oxygen in Czochralski silicon wafers (1,2). RTA pre-treatment leads to the formation of intrinsic point defects, mainly vacancies, which are responsible for the enhanced precipitation of oxygen. Oxygen precipitates are widely known as getter sites for metal impurities in the silicon microelectronic technology (3). The getter efficiency of oxygen precipitates depends on their density and morphology (4,5). The density of the precipitates can be controlled by the temperature of RTA and it can be easily determined by preferential etching. The morphology of oxygen precipitates depends on the type and the concentration of dopants, the thermal history, and the annealing temperature and time (6-8). Recently, it was shown that the supersaturation of vacancies impacts the morphology of oxygen precipitates (9). Since RTA pre-treatments introduce vacancy supersaturation in silicon wafers its usage before nucleation annealing could modify the morphology of precipitates compared to wafers without RTA. The morphology of oxygen precipitates can be determined by transmission electron microscopy, however the preparation of samples is demanding. An alternative is the Fourier transform infrared (FTIR) spectroscopy at liquid helium temperature, which also allows distinguishing different types of oxygen precipitates. The oxygen precipitates give rise to absorption bands in the range between 1000 and 1300 cm^-1. There are two characteristic broad bands at 1095 cm^-1 and at 1225 cm^-1 which belong to spherical and plate-like precipitates, respectively (10-12). The sharp bands (1136 cm^-1 and satellites and 1205 cm^-1) are attributed to interstitial oxygen.

In this work, we investigate the morphology of oxygen precipitates by FTIR spectroscopy at liquid helium temperature and by scanning transmission electron microscopy (STEM) in silicon samples  pre-treated by RTA at 1250°C for 30s in O₂ containing Ar atmosphere subsequently  annealed in the temperature range between 800°C and 1000°C in N₂. The results show that the morphology of the oxygen precipitates in RTA pre-treated samples depends on the nucleation temperature. Fig. 1 presents absorption spectra of silicon samples pre-treated by RTA at 1250°C and annealed at 800°C (a), 900°C (b) and 1000°C (c) for 64h. As can be seen from the deconvolution of the spectra, the number and position of the bands is different for each annealing temperature. Based on the effective medium theory (EMT) (13) and STEM investigations the absorption bands can be matched to relevant morphologies.