A Novel Organosiloxane Vapor Annealing Process for Improving Elastic Modulus of Porous Low-k Films
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A Novel Organosiloxane Vapor Annealing Process for Improving Elastic Modulus of Porous Low-k Films Kazuo Kohmura1, Shunsuke Oike1, Masami Murakami1, Hirofumi Tanaka1, Syozo Takada2, Yutaka Seino3, and Takamaro Kikkawa3,4 1 2
MIRAI-ASET, Tsukuba, Japan ASRC-AIST, Tsukuba, Japan
3
MIRAI-ASRC-AIST, Tsukuba, Japan
4
RCNS, Hiroshima Univ., Higashi-Hiroshima, Japan
ABSTRACT A novel organosiloxane-vapor-annealing method has been developed for improving the mechanical strength of porous silica films with a low dielectric constant. Treatment of a porous silica film with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) under atmospheric nitrogen above 350 °C significantly enhanced the mechanical strength (i.e., elastic modulus and hardness) of the film. Results of Fourier transform infrared spectroscopy (FT-IR) and thermal desorption spectroscopy (TDS) suggested the formation of cross-linked poly(TMCTS) network on the porous silica internal wall surfaces by the TMCTS treatment. Such TMCTS cross-linked network is thought to enhance the mechanical strength of the low-k film. INTRODUCTION Considerable efforts have been devoted to the development of ultralow dielectric constant (k < 2.0) films with a relevant mechanical strength because of the great demand for low-k/Cu interconnects in ultra large scale integrated circuits (ULSIs). Porous silica films possess considerable potential for ultralow-k values and high mechanical strengths. The drawbacks of these porous films include the presence of hydrophilic silanol groups on the internal wall surfaces and the lack of mechanical strength (e.g., elastic modulus, hardness). Although the silanol groups can be transformed to the hydrophobic moieties by treatment with hexamethyldisilazane (HMDS) [1-4], the mechanical strength of the resulting films is insufficient as ULSI interconnects. The objective of this work is to develop porous silica films that possess both ultralow dielectric constant and mechanical strength.
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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 4.8mmol of tetraethoxysilane (TEOS), 5mmol of H2O and 0.1mmol of HCl in 10 ml ethanol at 50 ºC, being kept for 90 min under agitation. Then 2.9g of Pluronic P123(BASF Corp.) as a surfactant template and 40ml of ethanol solution were added in 21ml of the precursor solution at 30ºC and further left for 2 hours. Next, 8 ml of H2O and 20ml of N,N-dimethylacetoamide were added to it at a molar ratio of TEOS:P123:H2O:HCl = 1:0.01:10:0.02 . The resulting solution was spin-coated onto Si substrates and calcined at 400ºC for 3h in air to form a porous silica film. The film was treated with TMCTS at temperature from 250 to 400 ºC for 30 sec – 180 min under atmospheric nitrogen. For comparison, the film was also treated with HMDS under identical conditions. The mechanical properties of the resulting films were evaluated by nanoindentation at the shallow indentation depth compared to the film t
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