Preparation of Mesoporous Low-k Films from Crystalline Pure Silica Zeolite (PSZ) Nanocrystals and from Noncrystalline Silica Nanoparticles
Date Issued
2011
Date
2011
Author(s)
Lu, Hsin-Yan
Abstract
In this research, mesoporous low dielectric constant (low-k) films were prepared from coating solutions composed of surfactant Tween 80 and nanoparticles of different crystallinity synthesized through hydrothermal processes. The propose is to prepare mesoporous low-k films with dielectric constants of < 2 and high mechanical strength, hardness of > 1 GPa and elastic modulus of > 10 GPa. According to the results obtained in this research, for the films prepared from coating solutions containing PSZ nanocrystal only (no Tween 80 involved), the films showed serious cracking or were very rough. As Tween 80 was added into the solutions containing the PSZ nanocrystals prepared through a two-stage hydrothermal process, films with uniform surface morphology can be obtained. A film spin-coated from coating solution prepared under optimal experimental conditions possessed an ultra low k value of 1.83, a hardness of 1.39 GPa, an elastic module of 12.3 GPa, and a leakage current density of 1.35 × 10-7 A/cm2, all of which met the needs of the integrated circuits (IC) industry.
However, the two-stage hydrothermal process took 156 to 168 h to synthesize the PSZ nanocrystals that was too long and took too much energy. In order to address the concern, a faster one-stage hydrothermal processes which took only 27 to 51 h to prepare the coating solutions were developed in this research. Coating solutions containing noncrystalline silica particles with an average size of 4.6 nm were synthesized through the one-stage hydrothermal process that took only 24 h to synthesize the noncrystalline silica particles. As the hydrothermal time was increased to 36 h or higher, MFI-type structure of zeolite nanocrystals could be easily observed. The amount of silanol groups on the particle surface, measured through solid-state NMR spectra, was decreased with the increase of hydrothermal time, indicating that the amount of the silanol groups in the noncrystalline silica particles was the most of all samples. The mechanical strength of the films was also decreased with the increase of hydrothermal time, and the film prepared with the noncrystalline silica particles possessed the strongest mechanical strength. Due to that there were many silanol groups on the surface of the noncrystalline silica particles, the modification of the silanol groups to become hydrophobic might not be complete enough when the film was too thick. Therefore, the film thickness was reduced, and then a mesoporous low-k film possessed k values of smaller than 2, low leakage current densities (of order of 10-8 A/cm2) and high mechanical strength (hardness of 1.73 GPa and elastic modulus of 17.0 GPa) was obtained. It was found that for preparing mesoporous low-k films with high mechanical strength and low k values, the ratio of optical density of < 10 % (ratio of optical density, measured from FTIR spectra, which was the ratio of intensity for band 550cm-1 to band 450cm-1) of the nanoparticles is necessary to prepare the mesoporous low-k films.When the hydrothermal time was higher than 42 h in the one-stage hydrothermal process, the ratio of optical density of the so-obtained nanoparticles was higher than 10 % (between 15 % and 42 %); therefore, a centrifugation step should be applied to remove big nanoparticles, resulting in that the ratio of optical density of the nanocrystal in the resulting centrifuged coating solution could be smaller than 10 %. As a result, mesoporous low-k films which possessed k values of < 2, low leakage current densities of order of 10-8 A/cm2, and high mechanical strength (hardness of > 1 GPa and elastic modulus of > 10 GPa) were successfully prepared from the centrifuged coating solutions.
However, the two-stage hydrothermal process took 156 to 168 h to synthesize the PSZ nanocrystals that was too long and took too much energy. In order to address the concern, a faster one-stage hydrothermal processes which took only 27 to 51 h to prepare the coating solutions were developed in this research. Coating solutions containing noncrystalline silica particles with an average size of 4.6 nm were synthesized through the one-stage hydrothermal process that took only 24 h to synthesize the noncrystalline silica particles. As the hydrothermal time was increased to 36 h or higher, MFI-type structure of zeolite nanocrystals could be easily observed. The amount of silanol groups on the particle surface, measured through solid-state NMR spectra, was decreased with the increase of hydrothermal time, indicating that the amount of the silanol groups in the noncrystalline silica particles was the most of all samples. The mechanical strength of the films was also decreased with the increase of hydrothermal time, and the film prepared with the noncrystalline silica particles possessed the strongest mechanical strength. Due to that there were many silanol groups on the surface of the noncrystalline silica particles, the modification of the silanol groups to become hydrophobic might not be complete enough when the film was too thick. Therefore, the film thickness was reduced, and then a mesoporous low-k film possessed k values of smaller than 2, low leakage current densities (of order of 10-8 A/cm2) and high mechanical strength (hardness of 1.73 GPa and elastic modulus of 17.0 GPa) was obtained. It was found that for preparing mesoporous low-k films with high mechanical strength and low k values, the ratio of optical density of < 10 % (ratio of optical density, measured from FTIR spectra, which was the ratio of intensity for band 550cm-1 to band 450cm-1) of the nanoparticles is necessary to prepare the mesoporous low-k films.When the hydrothermal time was higher than 42 h in the one-stage hydrothermal process, the ratio of optical density of the so-obtained nanoparticles was higher than 10 % (between 15 % and 42 %); therefore, a centrifugation step should be applied to remove big nanoparticles, resulting in that the ratio of optical density of the nanocrystal in the resulting centrifuged coating solution could be smaller than 10 %. As a result, mesoporous low-k films which possessed k values of < 2, low leakage current densities of order of 10-8 A/cm2, and high mechanical strength (hardness of > 1 GPa and elastic modulus of > 10 GPa) were successfully prepared from the centrifuged coating solutions.
Subjects
mesoporous low-k films
Tween 80
PSZ MFI nanoparticles
noncrystalline silica particles
two-stage/one-stage hydrothermal process
ratio of optical density (particle crystallinity)
mechanical strength
Type
thesis
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