(Invited) Current Stage of the Investigation of the Composition of Oxygen Precipitates in Czochralski Silicon Wafers
Authors: Kot Dawid , Kissinger Gudrun , Schubert Markus Andreas , Sattler Andreas 
Published: February 2020

The oxygen precipitates formed in Czochralski (CZ) silicon wafers in consequence of the thermal treatment were investigated since many decades. The main reason why so much effort was directed towards these defects was the impact which they have on integrated circuits and properties of silicon wafer itself. Oxygen precipitates can increase the resistivity of CZ silicon, change the wafer strength and cause warpage of the wafers. It was also demonstrated that metal impurities can be effectively trapped at oxygen precipitates in the process of gettering, what is a huge advantage. All these features of the oxygen precipitates require the control of precipitation in the production process of silicon devices. However, this cannot be optimally executed if the features of oxygen precipitates like their composition are not fully known. In spite of the wide knowledge of oxygen precipitation, the composition of oxygen precipitates SiO
x
still remains under ongoing discussion. This is due to the different x values varying from x=1 to x=2 which can be found in the literature.
In this work, we look on the current stage of the investigation on the composition of oxygen precipitates obtained with the help of different techniques. Moreover, we present our recent investigation on the composition of oxygen precipitates carried out by means of energy dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS) and Fourier transform infrared spectroscopy (FTIR). The FTIR spectra measured at liquid helium temperature will be compared with the spectra simulated on the basis of experimental results obtained by scanning transmission electron microscopy (STEM).
The EDX gives direct information about the composition of oxygen precipitates. However, the analysis of the EDX spectrum requires a special calibration. This is due to the limitation of EDX for light elements like oxygen. The intensity of characteristic X-rays of oxygen depends on the thickness of TEM lamellae. Therefore, in this work we measured a thermally grown SiO2 layer at different thickness of TEM lamellae in order to determine the influence of the lamellae thickness on the intensity of oxygen characteristic X-rays. After doing this, we measured the composition of oxygen precipitates located at very thin TEM lamellae. The results shown that x is in the range between 1.8 and 1.9 (1).
To make sure that the EDX results are correct EELS measurements were carried out. EELS is a complementary method to EDX. It allows to avoid the limitation of EDX in the measurement of the composition of oxygen precipitate especially in the case if the precipitate does not go through the TEM lamellae. Deconvoluting the characteristic plasmons of Si, SiO2 and suboxides in the electron energy loss spectra we found that the oxygen precipitate consist of SiO2 and it is coated by thin 2-3nm SiO layer (2).
Finally, we investigated the composition of oxygen precipitates by FTIR. The absorption band of oxygen precipitates were observed in FTIR spectra measured at liquid helium temperature. The position and the shape of the bands fit very well to the simulated spectra. The simulation of the spectra was conducted on the basis of the effective medium theory (EMT) where we considered SiO2 spheroids coated by SiO layers embedded in a Si matrix.
In summary, the composition of oxygen precipitates was investigated by EDX, EELS and FTIR whereby the first two methods are direct methods and the latter one is an indirect method. All methods delivered similar results. Hence, we can conclude that the oxygen precipitates consist of a SiO2 core which is coated by an SiO layer.
References

1D. Kot, G. Kissinger, M. A. Schubert, M. Klingsporn, A. Huber, and A. Sattler, Phys. Status Solidi RRL 9, 405 (2015).

2G. Kissinger, D. Kot, M. Klingsporn, M.A. Schubert, A. Sattler, T. Müller, ECS J. Solid State Sci. Technol. 4 (9), N124-N129 (2015).