Diffusion of oxygen molecules in fluorine-doped amorphous SiO2

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Abstract

Effects of fluorine doping on the diffusion of interstitial oxygen molecules (O2) in amorphous SiO2 (a-SiO2) were compared to those obtained from a-SiO2 containing SiOH groups. Incorporation of moderate concentration (1019 cm−3) of SiF groups gives rise to minor changes in diffusion parameters between 800 and 1100 °C: only a slight decrease in solubility and an increase in the activation energy for diffusion can be detected. Incorporation of SiOH groups has similar weak effects on the solubility and activation energy for diffusion. These minor changes are most likely due to the enhancement of the flexibility of local Si–O network as a result of the dissociation of the network by SiOH and SiF groups. However, in contrast to the SiF doping, SiOH doping leads to a notable decrease in the diffusion coefficient. The heat of solution changes by 0.1–0.2 eV at 1000 ° C and it is attributed to the glass transition of a-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 hν8 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 1273 nm, attributed to the transition from the lower excited singlet state (a1Δg) to the ground state (X3Σg) [19]. It is possible to detect as few as 1014 cm−3 interstitial O2 when the upper excited singlet state (b1Σg+) 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 1020 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 1.4×1019 cm−3 of SiF groups and 1–2×1018 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 (1.5 W at the sample position) and was measured using the detector part

Results

Fig. 1 shows the variation of Ca with annealing time t at 800, 900, 1000, or 1100 °C. Ca was proportional to t1/2 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 Ca 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 ΔEa, ΔH, D0, and S0 values are listed in Table 1, along with the measurement method, sample type, and abbreviated name. Agreements among data are good for D, ΔEa, and ΔH. However, our S data are 2.5 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

References (37)

  • K. Kajihara et al.

    Mater. Sci. Eng. B

    (2009)
  • M.A. Lamkin et al.

    J. Eur. Ceram. Soc.

    (1992)
  • K. Kajihara et al.

    J. Non-Cryst. Solids

    (2008)
  • K. Kajihara et al.

    J. Non-Cryst. Solids

    (2004)
  • R. Brückner

    J. Non-Cryst. Solids

    (1970)
  • P. Richet et al.

    Geochim. Cosmochim. Acta

    (1984)
  • K. Awazu et al.

    J. Appl. Phys.

    (1991)
  • K. Arai et al.

    Phys. Rev. B

    (1992)
  • M. Kyoto et al.

    J. Mater. Sci.

    (1993)
  • H. Hosono et al.

    Appl. Phys. Lett.

    (1999)
  • Y. Ikuta et al.

    J. Vac. Sci. Technol. B

    (2000)
  • K. Saito et al.

    J. Appl. Phys.

    (2002)
  • R.A.B. Devine et al.

    Phys. Rev. B

    (1989)
  • R.A.B. Devine et al.

    Phys. Rev. B

    (1990)
  • H. Imai et al.

    Phys. Rev. B

    (1993)
  • H. Hosono et al.

    Phys. Rev. Lett.

    (2001)
  • K. Awazu et al.

    J. Appl. Phys.

    (2003)
  • K. Kajihara et al.

    Phys. Rev. B

    (2008)
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