In the past few decades, studies have been conducted to investigate photon assisted oxidation of silicon substrates. Most of these efforts have focused on understanding and modeling the oxide growth mechanism using photon energies spanning the visible (1.55 eV – 3.0 eV) to the UV range (3.0 eV – 6.5 eV). In this work, we study the influence of higher energy x-rays (10 keV) on silicon oxidation. We found that x-ray irradiation of silicon substrates, performed at ambient temperature and atmospheric pressure conditions, can significantly affect the formation of silicon oxide. The oxide formation is influenced by the dose rate and total dose of x-ray irradiation, as well as the initial silicon surface preparation.

 

Boron doped silicon samples (0.01 Ω-cm - 0.02 Ω-cm) were irradiated at dose rates ranging from 5.8 krad(SiO2)/min to 31.5 krad(SiO2)/min using a 10 keV x-ray source (ARACOR Model 4100). The samples were cleaned in dilute HF acid to remove the initial native oxide and obtain a clean surface prior to irradiation. The resulting silicon oxide thickness was estimated using spectroscopic ellipsometry (J. A. Woolam M-2000). Different models for the optical properties of the surface layer were evaluated. Oxide growth on the irradiated samples was compared to oxide growth on control samples, which were placed in ambient air at room temperature. The ellipsometry analyses suggest that the oxide growth on the irradiated samples was greater than that on the control samples. Initial XPS analysis showed that the oxide layers on the irradiated and control samples were chemically different, suggesting that differences observed in the ellipsometry analyses may be due, in part, to differences in the optical properties of the oxide layers and not purely due to a change in thickness.

 

We will present the results of the oxidation study in addition to XPS and AES characterization of the resulting oxide films. The impact of heating the silicon substrate up to 150°C during x-ray irradiation will also be discussed. A mechanism to explain the experimental observations is proposed based on ozone concentration measurements performed during irradiation. The possible generation of atomic oxygen, due to dissociation of molecular oxygen, by the high energy x-rays is suggested as a key factor in the observed x-ray irradiation induced silicon oxidation.

 

Acknowledgement: This work was supported in part by the DTRA Basic Research Program (Grant No. HDTRA1-10-0041).