Diffusion of oxygen molecules in fluorine-doped amorphous SiO2
Introduction
Amorphous SiO2 (a-SiO2) is widely used as gate dielectric films for silicon microelectronic circuits, optical fibers for telecommunication, and optical components in excimer laser photolithography. Fluorine is one of the most important dopant for a-SiO2 used as such devices, because moderate fluorine doping increases the radiation hardness of a-SiO2[1], [2], [3], [4], suppresses the electrical breakdown of the gate dielectric films, and improves the optical transmittance near the absorption edge of a-SiO2 located at 8 eV [5]. These improvements are mainly due to the breaking up of Si–O network by Si–F bonds. It decreases the viscosity of a-SiO2[3] and enhances the structural relaxation [6], [7], facilitating the removal of “strained” Si–O–Si bonds, which are considered to be a major source of point defects in a-SiO2[8], [9], [10], [11], [12], [13]. Furthermore, Si–F bonds themselves are stronger than Si–O bonds that build the a-SiO2 network and are hardly decomposed. Thus, radiation hardness of fluorine-doped a-SiO2 is better than that of a-SiO2 containing other network modifiers, such as SiOH and SiCl groups. Similarly to SiF groups they enhance the structural relaxation, however, they can be converted to point defects under radiation or electrical stress.
Oxygen molecules dissolved in interstices of Si–O network (interstitial O2) are the main mobile oxygen species in a-SiO2[14], [15], [16]. They play a key role in thermal oxidation of silicon [17] and radiation induced defect processes in a-SiO2[18]. Interstitial O2 in a-SiO2 are sensitively detected by their characteristic infrared photoluminescence at nm, attributed to the transition from the lower excited singlet state () to the ground state () [19]. It is possible to detect as few as cm−3 interstitial O2 when the upper excited singlet state () is populated using a continuous-wave laser light at a wavelength of 765 nm [20]. The sensitivity is sufficient to detect interstitial O2 incorporated during thermal annealing in air [21], offering an easy and straightforward way to quantitatively study the thermal diffusion of interstitial O2 in a-SiO2[22]. Furthermore, this PL method is precise enough to evaluate the variations of the solubility and diffusion coefficient of interstitial O2 with the incorporation of cm−3 SiOH groups [23], [18], which are the most common network modifiers in synthetic a-SiO2.
The purpose of the present study is to examine the influence of the incorporation of SiF groups on the diffusion of interstitial O2 in a-SiO2 and to compare it with that of SiOH groups.
Section snippets
Experimental procedure
Fluorine-doped synthetic SiO2 glass containing cm−3 of SiF groups and 1–2 cm−3 of SiOH groups was cut into specimens in the form of 7 mm 10 mm 1 mm, and the two largest faces were polished to an optical finish. They were thermally annealed in air at 800, 900, 1000, or 1100 C to incorporate interstitial O2. The PL band of interstitial O2 in the O2-loaded samples was excited at 765 nm using an AlGaAs laser diode ( W at the sample position) and was measured using the detector part
Results
Fig. 1 shows the variation of with annealing time t at 800, 900, 1000, or 1100 C. was proportional to at small t, and saturated at a constant value at large t. This observation indicates that the dissolution of O2 from air is much faster than the following O2 diffusion in a-SiO2[22] and is consistent with previous results [22], [23]. Thus, the observed variation of with t was simulated well by an equation describing the simplest one-dimensional diffusion in a parallel sheet of a
Discussion
Fig. 2 also shows D and S values of interstitial O2 reported to date. The obtained , , , and values are listed in Table 1, along with the measurement method, sample type, and abbreviated name. Agreements among data are good for D, , and . However, our S data are times smaller than those reported in Ref. [15].
The LowOH sample is fluorine-free and contains SiOH groups in concentration comparable with that in the F-doped sample. In these two samples the behavior of diffusion of
Conclusions
Diffusion of interstitial oxygen molecules (O2) in fluorine-doped synthetic amorphous SiO2 (a-SiO2) was examined. The results are compared with the data taken from a-SiO2 containing SiOH groups and are analyzed in terms of the concentration of network modifiers (SiOH and SiF groups). The observations indicate that solubility decreases and the activation energy for diffusion increases with an increase in the concentration of network modifiers. A comparison of the results with those reported for
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