VOn Complexes in RTA Treated Czochralski Silicon Wafers Investigated by FTIR Spectroscopy
Authors: Kot Dawid , Kissinger Gudrun , Dabrowski Jaroslaw , Sattler Andreas 
Published: February 2020

Rapid thermal annealing (RTA) in silicon wafer processing is a technique used to control point defects, vacancies and silicon interstitials (1,2). The presence of these defects influences the nucleation and precipitation of supersaturated oxygen in Czochralski (CZ) silicon wafers. The precipitation of oxygen, which occurs during annealing of silicon wafers, is a very important phenomenon. This is because the oxygen precipitates provide a natural getter sink for unwanted metallic impurities which can get into the devices active region in the fabrication process. Vacancies are of high importance because they enhance oxygen precipitation. In CZ silicon vacancies are mainly bound with oxygen atoms forming so called VOn complexes where n is the number of oxygen atoms. The investigation of VOn complexes in silicon wafers is extremely difficult because of their low concentration, which is in the range of 1013 cm-3. So far, irradiation experiments are the main source of information about VOn complexes. The irradiation allows increasing their concentration to a much higher level compared to the level, which can be found in processed wafers.
Akhmetov et al. (3) published the first detection of VOn complexes in non-irradiated but RTA pre-treated wafers. In order to be able to measure defects with very low concentration they used highly sensitive FTIR measurements at room temperature (RT). They found the absorption band at 985 cm-1 which they attributed to VO4. The estimated concentration of VO4 complexes was very close to calculated values from an ab initio model (4).
In this work, we used a slightly modified method in order to investigate VOn complexes in RTA pre-treated wafers. The measurements were carried out at 6 K what allowed us to enhance the sensitivity. Moreover, the FWHM of the interstitial oxygen band is much smaller at 6 K compared to RT. This means that the range of wave numbers next to the Oi band typically covered by this band at RT can be investigated now. This range is very important because some absorption bands of VOn complexes with n > 4 are located there.
In a previous study, in a wafer pre-treated by RTA at 1250 °C for 30 s in an Ar containing O2 atmosphere we found the absorption bands of VO4 at 985 cm-1 and 991 cm-1 in the measurements carried out at room temperature and at 6 K, respectively (5). In this wafer, we also detected an unknown band at 1030 cm-1 and the bands at 1096 cm-1 and 1099 cm-1, according to Murin et al. both assigned to VO5,6 (6).
In order to better understand these complexes we performed additional experiments. The wafers were pre-treated by RTA in the range between 1250 °C and 1375 °C and the behavior of the detected bands was analyzed and correlated with simulations. The process of vacancy-oxygen complex formation after RTA was simulated using a rate equation model. It was found out that the most stable VOn complexes are the ones with n = 6, 5 and 4.
In summary, the VOn complexes were investigated by FTIR spectroscopy at 6K in RTA pre-treated wafers. The process of vacancy-oxygen complex formation after RTA was modeled using a rate equation model.
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