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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. References R. Falster, M, Pagani, D. Gambaro, M. Cornara, M. Olmo, G. Ferrero, P. Pichler, M. Jacob, Solid State Phenom., 57-58, 129 (1997). M. Pagani, R. Falster, G. R. Fisher, G. C. Ferrero, M. Olmo, Appl. Phys. Lett., 70 (12), 1572 (1997). V. Akhmetov, G. Kissinger, and W. von Ammon, Appl. Phys. Lett., 94, 092105 (2009). G. Kissinger, J. Dabrowski, A. Sattler, C. Seuring, T. Müller, H. Richter, W. von Ammon, J. Electrochem. Soc., 154, H454 (2007). D. Kot, G. Kissinger, and A. Sattler, Semicond. Sci. Technol. 32, 104006 (2017). L.I. Murin, J. L. Lindström, B. G. Svensson, V. P. Markevich, A. R. Peaker, C. A. Londos, Solid State Phenomena, Vols. 108-109, pp. 267-272 (2005).

With the introduction of the Czochralski method for growing of silicon single crystals oxygen in silicon became an issue with positive and negative impact on silicon devices. On the one hand the oxygen strengthens the wafers against slip dislocation formation, on the other hand precipitating oxygen in the device active area can degrade the device performance. However, oxygen precipitates in the bulk are beneficial because they act as gettering sinks for metallic impurities and thus can improve the device performance. Therefore, oxygen is a major issue in silicon defect engineering. Extensive research on oxygen precipitation started in the seventies of the 20th century and it is still ongoing because new tools become available which offer new possibilities of characterization, higher resolution and lower detection limits. In the field of modeling, the tremendous advances of computer technique and software during the last decades made it possible for a much broader community to run larger models and to make predictions being much more sophisticated. The challenges of defect engineering in silicon device technology changed over the years. In the eighties, two or three step heat treatments were applied to out-diffuse the interstitial oxygen from the device active area followed by nucleation and growth of oxygen precipitates. This procedure is very time consuming. In the nineties, a new RTA based method was discovered. Vacancy profiles, which controlled the spatial distribution of oxygen precipitation in the wafers, were generated by high temperature RTA treatment. During this method, the oxygen does not diffuse out. Because intrinsic point defects are very fast diffusors, a good adjustment to the following device process has to be made. Today, one of the major requirements of high end silicon device technology is void-free silicon because with ongoing decreasing feature size voids became a major yield decreasing problem. There are several solutions to this issue like the growth of void-free silicon crystals and void dissolution in silicon wafers by heat treatment at very high temperature. Both methods are strongly connected with oxygen precipitation. Growth of nearly void-free silicon often leads to radial inhomogeneities of the concentrations of intrinsic point defects [1, 2]. The challenge of defect engineering is to create a radially homogeneous concentration of oxygen precipitates. The ongoing increase of wafer diameter imposes additional problems to oxygen precipitation. If the density of oxide precipitates becomes too high, overlay problems in mask alignment occur [3]. This requires new adjustments of oxygen precipitation. The low feature size in addition to the large wafer diameter asks for low thermal budget processing. A new problem arises because the oxygen precipitates need a certain surface and density to be able to getter enough metallic impurities. Examples from our own recently published results about ab initio calculation for understanding of the initial stages of oxygen precipitation [4], EELS investigation of the stoichiometry of oxygen precipitates [5], the gettering mechanism of Cu at oxygen precipitates [6], and N-doping for the homogeneous oxygen precipitation during high temperature annealing in wafers optimized with respect to voids [1, 2] will be presented. In summary, it can be said that oxygen in silicon remains an ongoing issue. Control of oxygen precipitation becomes more and more demanding for high end device technologies. Oxygen precipitation is not yet fully understood but the current developments of diagnostic methods allow new insights. [1] G. Kissinger, G. Raming, R. Wahlich, T. Müller, 300 mm Czochralski silicon wafers optimized with respect to voids with laterally homogeneous oxygen precipitation, Physica B 407, 2993 (2012). [2] G. Kissinger, G. Raming, R. Wahlich, T. Müller, Nitrogen doped 300mm Czochralski silicon wafers optimized with respect to voids with laterally homogeneous internal getter capabilities, Mat. Sci. Forum 725, 221 (2012). [3] K. Izunome, Advanced silicon wafers for leading-edge semiconductor devices, Proceedings of the 6th Int. Symposium on Advanced Science and Technology of Silicon Materials (JSPS Symposium), Nov. 19-23, 2012, Kona, Hawaii, USA, pp. 9-13. [4] G. Kissinger, J. Dabrowski, T. Sinno, Y. Yang, D. Kot, A. Sattler, Ab initio calculations and rate equation simulations for vacancy and vacancy-oxygen clustering in silicon, J. Cryst. Growth 468, 424 (2017). [5] G. Kissinger, M. A. Schubert, D. Kot, T. Grabolla, Investigation of the composition of the Si/SiO2 interface in oxide precipitates and oxide layers on silicon by STEM/EELS, ECS J. of Solid State Sci. and Technol. 6, N54 (2017). [6] G. Kissinger, D. Kot, M. Klingsporn, M. A. Schubert, A. Sattler, T. Müller, Investigation of the copper gettering mechanism of oxide precipitates in silicon, ECS J. of Solid State Sci. and Technol. 4, N124 (2015).

In order to elucidate the influence of stress and in-diffused nitrogen on oxide precipitation after rapid thermal annealing (RTA), we carried out experiments with one-side and double-sided nitride layers accompanied by simulation models which help to understand the behavior of intrinsic point defects, nitrogen, and strain. We found that the presence of a nitride layer of any thickness, within the rage which we investigated, in direct contact with the silicon surface is sufficient to markedly change the precipitation behavior of interstitial oxygen after RTA at 1175 and 1250 °C. The presence of the nitride layer during the stabilization and growth of the oxide precipitates is not of any influence on the precipitation behavior. Therefore, the RTA of wafers covered with silicon nitride is the crucial step controlling the bulk microdefect (BMD) depth profiles. A 10 nm oxide between silicon substrate and nitride layer prevents any change of the BMD depth profile. Only in a direct contact with the nitride layers the vacancy supersaturation, which enhanced the oxide precipitation compared to wafers without nitride layers, was generated. Nitrogen peaks below the silicon surface generated by in-diffusion of nitrogen during RTA lead to an enhanced oxygen precipitation only for RTA at 1250 °C and not for RTA at 1175 °C. We propose a model based on very tiny coherent α-Si3N4 precipitates generated at nitrogen-vacancy (NV) complexes which can act as nucleation sites for oxygen precipitation. Because the stability of NV seems limited to temperatures above 1200 °C, it would not be effective for RTA at 1175 °C. RTA treatment of silicon wafers with one-sided nitride layers at 1250 °C leads to very sharp and small defect denuded zones in subsequent annealing and would be suitable for proximity gettering. The depth of the denuded zone is nearly independent of the thickness of the nitride layer.