Characterization of Porous Silicate Low- k Films by Ellipsometric Porosimetry and Variable-energy Positron Annihilation
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L8.19.1
Characterization of Porous Silicate Low-k Films by Ellipsometric Porosimetry and Variable-energy Positron Annihilation Spectroscopy Kenji Ito, Yoshinori Kobayashi, Ryoichi Suzuki, Toshiyuki Ohdaira, Runsheng Yu, Kiminori Sato, Kouichi Hirata, Hisashi Togashi, Miki Egami,1 Hiroki Arao,1 Akira Nakashima1 and Michio 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 We applied ellipsometric porosimetry and variable-energy positron annihilation spectroscopy to the pore characterization of spin-on-glass silicon-oxide-backboned porous thin films with different relative dielectric constants between 2.3 and 3.2. It was found that the relative dielectric constant decreases linearly with increasing open porosity deduced by ellipsometric porosimetry. Comparison of the open porosity with the average pore size deduced by positron annihilation lifetime spectroscopy suggested that mesopores less contribute to open porosity and are not so effective in decreasing film relative dielectric constant in comparison with micropores. INTRODUCTION Low dielectric constant (low-k) materials have attracted much attention in recent years because of the need of the semiconductor industry to reduce the interconnect signal delay in ultra large-scale intergraded circuits (ULSI) [1]. Low-k dielectrics can be engineered by incorporation of nanoporosity into dielectrics such as silicon oxide, methyl-silsesquioxane (MSSQ) and hydrogen-silsesquioxane (HSSQ). One promising class of low-k dielectrics is spin-on-glass films with nanoporosity introduced by decomposition of thermally degradable pore generating material (porogen). A key issue for the development of the low-k films is the optimization of pore characteristics such as porosity, pore size, shape, and distribution. Small closed pores are desirable because large open pores could lead to low mechanical strength and breakdown voltage. Recently, several advanced techniques such as ellipsometric porosimetry [2], variable-energy positron annihilation [3], small angle X-ray scattering [4] and X-ray porosimetry [5] have been implemented for the pore characterization of low-k films to overcome the low sensitivity of conventional nitrogen adsorption. Ellipsometric porosimetry measures the variation of film refractive index upon successive adsorption/desorption of gas molecules such as heptane under different pressures. Based on the Lorentz–Lorenz equation adsorption/desorption isotherms are obtained from the measured refractive indices. This technique can give specific information on open pores to which gas molecules are accessible. Positron annihilation measures the 3γ annihilation probability and lifetime of ortho-positronium (o-Ps), the spin-parallel bound state between a positron and an electron. Positronium (Ps) is formed when energetic positrons are injected into dielectrics such
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