Study of Pore Architecture in Silicon Oxide Thin Films by Variable-energy Positron Annihilation Spectroscopy
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Study of Pore Architecture in Silicon Oxide Thin Films by Variableenergy Positron Annihilation Spectroscopy Kenji Ito,
Yoshinori Kobayashi,
Runsheng Yu,
Kouichi Hirata,
Hisashi Togashi,
Ryoichi Suzuki, Toshiyuki Ohdaira, M. Egami,1 H. Arao,1 C. Sakurai,1 A. Nakashima1 and M. Komatsu1 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305–8565, Japan 1
Fine Chemicals Multi Media Research Center, Catalysts & Chemicals Industries Co.,
Ltd. (CCIC), Wakamatsu–ku, Fukuoka 808–0027, Japan
ABSTRACT Application of porous silicon oxide thin films to nanotechnology is under intensive investigation. Introducing a large amount of nano pores into a silicon oxide matrix is important to develop low-k dielectrics for future ultra-large-scale integrated circuits (ULSI). In this work, we applied variable-energy positron annihilation to the characterization of porous silicon oxide thin films fabricated on silicon wafers by sputtering and spincoating. It was found that the sputtered film has higher open pore connectivity than that of the spincoated low-k film.
INTRODUCTION Control of nano scale pores is important in many applications of porous thin silicon oxide films. Some examples are high performance gas sensors [1] and low dielectric constant (lowk) insulators for next-generation ultra-large-scale integrated circuits (ULSI) [2]. In this work, we performed variable-energy positron annihilation γ-ray and lifetime measurements to study the pore connectivity and pore size of porous silicon oxide thin films prepared by different techniques. Some positrons implanted into silicon oxide form ortho-positronium (o-Ps), the spin parallel bound state between a positron and an electron. If o-Ps is formed in an interconnected, open pore of porous silicon oxide film, it may diffuse out to vacuum, and then self-annihilates into 3γ-rays with an intrinsic lifetime of 142 ns. The positron 3γ decay probability (I3γ ) can be determined from the positron annihilation γ-ray spectrum [3], and provides useful information on the open porosity [4]. Meanwhile if it is confined in an isolated, closed pore, o-Ps annihilates into 2γ-rays
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of 511 keV each. The lifetime of such confined o-Ps lifetime gives information on the pore size because the larger the pore size, the longer the lifetime [5].
EXPERIMENTAL SECTION Two kinds of silicon oxide backboned porous films were formed on silicon substrates. One is a silicon oxide film deposited by magnetron sputtering at 13.56 MHz under a condition of 1.5 Pa argon pressure and 450 W discharge power [6]. The other is a spincoated low-k film prepared from the nano-clustering silica (NCS), which was synthesized at CCIC, Japan. Film thickness was determined by spectroscopic ellipsometry to be 782 nm for the sputter-deposited film and 482 nm for the low-k film. Capped samples were prepared by deposition of 100 nm thick nonporous silicon oxide on top of each uncapped sample by sputtering. It was confirmed by scanning electron microscopy (SEM) that the capping l
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