Effect of Plasma Treatment and TMCTS Vapor Annealing on the Reinforcement of Porous low-k Films
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B3.10.1
Effect of plasma treatment and TMCTS vapor annealing on the reinforcement of porous low-k films Kazuo Kohmura1, Hirofumi Tanaka1, Shunsuke Oike1, Masami Murakami1, Tetsuo Ono1, Yutaka Seino2, and Takamaro Kikkawa2, 3 1 MIRAI-ASET, Tsukuba, Japan 2 MIRAI-ASRC-AIST, Tsukuba, Japan 3 RCNS, Hiroshima Univ., Higashi-Hiroshima, Japan ABSTRACT A novel process of TMCTS vapor annealing combined with a plasma treatment has been developed for improving the mechanical strength of porous silica films having ultralow dielectric constant. When porous silica films annealed under 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor were treated with argon plasma and then re-treated with TMCTS vapor, the mechanical strength (i.e., elastic modulus, hardness) of the films increased significantly. Results of Fourier transform infrared spectroscopy (FT-IR) suggested an accelerative effect resulted from the plasma treatment on the conversion of Si-CH3 and Si-H groups to Si-OH groups. The latter group appears to react faster with TMCTS from the second annealing to form cross-linked polymer network on the porous silica wall surfaces. The resulting cross-linked network is thought to keep the low permittivity and enhance the mechanical strength of the low-k films. INTRODUCTION As design rules go to 45nm and beyond, improvements in the performance of the large scale integrated circuits (ULSIs) require ultra-low dielectric constant (k < 2.1) films for metal interconnects. Introducing pores to silica films reduces the k-value, while at the same time it is the cause of instability of the films in air because of water adsorption in the pores. Furthermore, it weakens the mechanical strength of the films. We found in the previous work that elastic modulus and hydrophobicity were improved when porous silica films were annealed with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor [1]. On the other hand, it is known that plasma treatment processes such as dry etching and photo-resist ashing induce the conversion of Si-CH3 bonds in porous silica films to Si-OH [2] , and they are also the cause of the deterioration of low-k films’ performance. Porous silica film damage induced with plasma processing would be recovered by their initial electrical properties when treated with TMCTS [3-4]. In this work, we studied a combination of plasma treatment with TMCTS vapor annealing aiming at accelerating TMCTS cross-linking polymerization in order to achieve both ultra-low dielectric constant and high mechanical strength in porous silica films.
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EXPERIMENTAL Porous silica films were prepared using a sol-gel method based on the self-organization of surfactant templates [5-7]. A precursor solution was prepared by mixing 4.8 mmol of tetraethoxysilane (TEOS), 5 mmol of H2O and 0.1 mmol of HCl in 10 ml ethanol at 50 ºC; it was kept for 90 min under agitation. Then 4.6 g of Brij78 (Aldrich Corp.) as a surfactant template and 40 ml of ethanol solution were added in 21 ml of the precursor solution at 30ºC which was left further for 2 hours. Next, 8 ml
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