Yoshiaki Oku

Theoretical Analysis of Elastic Modulus and Dielectric Constant for Low-k Two-Dimensional Periodic Porous Silica Films

To lower the dielectric constant k of interlayer-dielectric films with two-dimensional pore structures while maintaining their mechanical strength, the influences of pore arrangement on the elastic modulus E and k of the films were investigated. It was found that periodicity in pore structure enhances E with constant k. Periodic porous silica films having a hexagonal arrangement of circular cylindrical pores with k 3 GPa were demonstrated to be feasible at a porosity of 0.614 using a bulk material with a k of 4.0 and E>21 GPa.

Theoretical Investigation of Dielectric Constant and Elastic Modulus of Two-Dimensional Periodic Porous Silica Films with Elliptical Cylindrical Pores

The effects of film shrinkage during the thermal curing of two-dimensional periodic porous silica films on the relative dielectric constant k with respect to the vacuum value and the relative elastic modulus E with respect to the skeletal (pore-wall) value Ew were investigated by theoretical calculations. Two extreme models of the film thickness shrinkage were assumed in the calculation: the constant porosity model and the constant wall volume model. It was found that E/Ew decreased more markedly upon film thickness shrinkage in the constant porosity model than it did in the constant wall volume model. The calculated results were confirmed by the experimental data together with the elliptical cylindrical pore shape. It is shown that the film shrinkage of ultralow-k porous silica films must be suppressed for achieving the higher elastic modulus while keeping the k value constant.

Theoretical Investigation of Dielectric Constant and Elastic Modulus of Three-Dimensional Isotropic Porous Silica Films with Cubic and Disordered Pore Arrangements

The dielectric constant (k) and elastic modulus (E) of self-assembled three-dimensional porous silica films were investigated by analytical and numerical calculations to reveal the relationship between k and E. It was found that cubic pore arrangements have E values higher than those of random pore arrangements and two-dimensional periodic hexagonal pore arrangements for the same k. It was also found that disordered isotropic porous silica films having cylindrical pores with well-controlled pore size distributions exhibit an E vs k relationship similar to that of two-dimensional hexagonal periodic porous silica films. The elastic modulus of the skeletal silica was determined to be 40 GPa from the combination of the calculated results and experimental data on ultralow-k disordered porous silica film with a k value of 2.0 and a modulus of 8 GPa.

Improved thermal stability of mesoporous molecular sieves by vapor infiltration treatment

A mesoporous silica has been prepared by base-catalyzed hydrolysis and polymerization of silicate ions followed by a vapor infiltration (VI) treatment using tetraethoxysilane (TEOS). X-ray diffraction analyses suggested that the VI-treated mesostructured silica did not contract during calcination, showing high structural stability. Fourier-transform infrared spectroscopy studies confirmed that a high density of surfactant molecules remained inside the pores during the VI treatment. The pore wall density increased after the VI treatment. The change in weight of mesostructured silica under VI treatment, and nitrogen adsorption/desorption studies, demonstrated that TEOS molecules penetrated into originally deposited silicate but did not deposit on the outer surface of particles and inside the pores. The denser pore wall of the VI-treated mesostructured silica has a high structural stability and hardly contracts under calcination. This post-synthesis method provides good support for the preparation of mesoporous silica with high structural stability and can also be applied to film preparation.

Comparative Studies of Ultra Low-κ Porous Silica Films with 2-D Hexagonal and Disordered Pore Structures

Periodic 2-dimensional (2-D) hexagonal and the disordered pore structure silica films have been developed using nonionic surfactants as the templates. The pore structure was controlled by the static electrical interaction between the micelle of the surfactant and the silica oligomer. No X-ray diffraction peaks were observed for the disordered mesoporous silica films, while the pore diameters of 2.0-4.0 nm could be measured by small angle X-ray scattering spectroscopy. By comparing the properties of the 2-D hexagonal and the disordered porous silica films which have the same porosity, it is found that the disordered porous silica film has advantages in terms of the dielectric constant and Youngs modulus as well as the hardness. The disordered porous silica film is more suitable for the interlayer dielectrics for ULSI.

Mechanical Property and Network Structure of Porous Silica Films

We investigated the relationship between Youngs elastic modulus of a series of spin-on sol-gel silica films and the shift of the longitudinal-optical component of the asymmetric stretching vibration mode of Si–O–Si (LO4), by nanoindentation measurements and Fourier transform infrared reflection spectroscopy. With the increase of the annealing temperature from 673–1073 K in air, the position of the LO4 mode reflection peak shifted from 1203 to 1253 cm-1, in parallel with the increase of Youngs elastic modulus. The correlation between the LO4 peak position and Youngs elastic modulus was also confirmed among a set of silica films in which the preparation conditions and/or the postpreparation treatment conditions were varied. It was concluded that the shift of the LO4 mode peak position is a good measure of the skeletal silica mechanical property of porous silica films.

Effects of Surfactants on the Properties of Ordered Periodic Porous Silica Films

We report the effects of surfactants on the properties of ordered periodic porous silica films. Silica films with a periodically ordered structure were synthesized on crystalline silicon substrates by spin-coating an alkoxysilane solution mixed with a self-assembling template and a cationic alkyltrimethylammonium chloride (ATMACl, CH3(CH2)n-1N(CH3)3 Cl) surfactant, where n was 12, 14, or 16. The effects of surfactants on the properties of porous silica films with a periodic pore structure were investigated. Both dielectric constant and refractive index decreased with the length of alkylchain in the template molecule ATMACl. The pore structure was successfully controlled by the alkylchain length of the template molecule when the tetraethyl-orthosilicate (TEOS) vapor treatment was applied.

Structural and Electrical Properties of Ultralow-k, Disordered Mesoporous Silica Films Synthesized Using Nonionic Templates

We report the dielectric constant, Youngs elastic modulus, and hardness of disordered mesoporous silica films prepared by spin-coating a silica precursor and nonionic triblock copolymer surfactant templates. The effects of selection of surfactant molecule on the properties of mesoporous silica films with disordered pore structures were investigated. One of the disordered mesoporous silica films studied had a low dielectric constant k = 2.0 and a high mechanical strength of elastic modulus = 4.14 GPa and hardness = 0.4 GPa. It has been shown that the modulus and hardness of the disordered mesoporous films can be made as high as those of two-dimensional hexagonal ordered mesoporous films.

Recovery from Plasma-Process-Induced Damage in Porous Silica Low-k Films by Organosiloxane Vapor Annealing

It was demonstrated that recovery from dry etching and ashing damage in porous silica low-k films occurred by 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor annealing. The increase in k-value after Ar/C5F8/O2 plasma etching was reduced from 35 to 6.5% of the initial value (k=2.25) by TMCTS vapor annealing. Leakage current also returned to the initial level. Hydrofluoric acid wet etching revealed the sidewall damaged region in a porous silica trench due to plasma processes. The TMCTS vapor annealing was found to be effective for recovery from the sidewall damage. Fourier transformed infrared absorption spectroscopy indicated that the replacement of Si–CH3 bonds in low-k films by Si–O and Si–OH bonds occurred during plasma processes. The recovery mechanism involves hydrophobic bond (–CH3) reintroduction into the film followed by stable cross-linked poly(TMCTS) network formation on pore wall surfaces by TMCTS vapor annealing.

A New Approach of Thin-Film X-Ray Diffraction / Scattering Analysis for Ultra-Low-k Dielectrics with Periodic Pore Structures

We propose and demonstrate experimentally a structural characterization technique for ultra-low-dielectric-constant thin films with periodic porous structures [1-3] by employing X-ray diffraction / scattering measurements. The analytical approach that we propose here takes into account specular reflection, incoherent scattering from random distribution of electron density, and coherent scattering from periodically modulated distribution of electron density. From the analysis, inter-pore distances and pore diameters in the directions perpendicular and parallel to the film surface are determined with which the film porosity is calculated. Thus obtained porosity is then used to discuss the film density and dielectric constant in comparison to those of non-porous reference sample.