Aerogel Films Research Articles

Modification of GaAs and copper surface by the formation of SiO 2 aerogel film as an interlayer dielectric

For the application of SiO 2 aerogel film to GaAs based devices, the changes of interfacial chemical bonding state of HCl-cleaned (or S-passivated) GaAs surface were investigated using monochromatic X-ray photoelectron spectroscopy after immersing the GaAs in each constituent of SiO 2 sol. A large amount of oxide was formed on the HCl-cleaned GaAs after the treatment using tetraethoxysilane and de-ionized water due to hydroxyl group in SiO 2 sol, while the S-passivated GaAs was not oxidized. The interfacial surface of GaAs was also investigated after the removing SiO 2 aerogel film formed by supercritical drying. The aerogel film showed 80% of porosity and low dielectric constant of 1.8. Normally the oxidation of GaAs was successfully suppressed with S-passivation, however during the supercritical drying due to the high temperature and pressure, S-passivation layer was completely decomposed, and GaAs-oxides and elemental As were generated somewhat, but less than the case of HCl-treated GaAs. Furthermore, the formation of SiO 2 aerogel film on copper metal substrate was revealed to induce a modification of metal surface. The modified and oxidized state of copper surface formed during the formation of the aerogel film was found to be not greatly influenced on the leakage current behavior of SiO 2 aerogel/Cu system.

The role of vacuum ultraviolet in H 2 plasma treatment on SiO 2 aerogel film

SiO 2 aerogel film has been drawn an attention as excellent dielectric material due to its ultra low dielectric constant. SiO 2 aerogel film could exhibit the improved electrical property after plasma treatment due to the removal of organic group. However, the role of vacuum ultraviolet (VUV) was not investigated in spite of high energy of VUV radiated during plasma processing. In this work, the radiation effect of VUV on SiO 2 aerogel film was investigated during H 2 plasma treatment. H 2 gas was selected from its lightest atomic mass and strong photon emission in the VUV region. LiF single crystal was used as a VUV filter to permit the transmittance of the VUV radiated from H 2 plasma but not with chemical species and reactive ions. It was found that VUV radiation during H 2 plasma treatment plays a role of removal of organic group and rearrangement of siloxane bond.

Synthesis of window glazing coated with silica aerogel films via ambient drying

An ambient drying process (1 atm, 270 °C) has been developed in order to synthesize window glazing coated with silica aerogel films. The aerogel film could be manufactured by this process of wet gel films obtained via a dip-/spin-coating of the silica sol on a glass slide. Before drying, the isoproponol solvent in wet gels was exchanged with n-heptane to minimize the drying shrinkage. The thickness, refractive index, and porosity of silica films were 0.16–10 μm, 1.08–1.09, and 80–84%, respectively. The transmittance of window glazing was over 90% and we could predict that the optimal thermal conductivity (0.2 W/(m K)) of the window glazing would be obtained at the aerogel thickness of 100 μm (0.016 W/(m K)).

Control of surface residual OH polar bonds in SiO 2 aerogel film by silylation

SiO 2 aerogel film was effectively silylated with n-hexane solution containing 6 vol.% of trimethylchlorosilane (TMCS). Through the silylation, hydrophilic polar OH bond was exchanged with hydrophobic non-polar Si(CH 3) 3 bond. Approximately 12% of reduction in dielectric constant and almost 2 orders of improvement in leakage current were observed after the silylation. Silylation behavior was examined with various volume concentrations of TMCS. The silylation of SiO 2 aerogel film with 70% porosity was maximized when using 6 vol.% of TMCS. The fully silylated film showed an excellent moisture-resistant behavior. Even after 5 days of exposure to an air atmosphere with 45–50% of controlled humidity, an increase in dielectric constant due to the generation of polar OH bond through moisture absorption was not observed. It was clearly revealed that the improved film properties such as low dielectric constant, low leakage current and high resistance to moisture absorption could be obtained by exchanging terminal polar OH bond with non-polar Si(CH 3) 3 bond by the silylation with TMCS.

Ambient pressure dried SiO 2 aerogel film on GaAs for application to interlayer dielectrics

The interfacial layer between ambient pressure dried SiO 2 aerogel film and GaAs has been examined with emphasis on varying the concentration of aerogel modification agent, trimethylchlorosilane (TMCS) in n-hexane solution. Through surface modification of aerogel, HCl was formed from the reaction between TMCS and OH bonds of silica aerogel surface, and the GaAs surface could be influenced according to the TMCS concentration in surface modifying solution. When mol concentration of TMCS is 0.066%, the GaAs interface with aerogel was roughly etched from the reflection of surface microstructure of aerogel. However, when the aerogel was modified with 0.05 mol.% of TMCS, uniform shape of the GaAs interface could be obtained and the resultant electrical property, especially leakage current density, showed almost the same behavior when the aerogel was applied to Si system.

Effect of solvent on the preparation of ambient pressure-dried SiO 2 aerogel films

SiO 2 aerogel film has a promising property as intermetal dielectrics (IMD) for its low dielectric constant. However, a stable and porous SiO 2 aerogel film was not properly synthesized due to the rapid evaporation of solvent during spin coating even in a solvent saturated atmosphere. So less evaporative solvent, 2-methoxyethanol (2MeEtOH), was introduced and the properties of film were compared with films using conventional solvent, ethanol (EtOH). It was found that gelation was faster with 2MeEtOH than EtOH. Highly porous and three-dimensionally well microstructured SiO 2 aerogel film could be fabricated with 2MeEtOH. The maximum porosity of the films using 2MeEtOH and EtOH was 85 and 71%, respectively. The amount of residual –OR, and –OH groups was smaller in the former. The compositional ratios of Si:O:C in the films were 1:2.1:0.2 for the former and 1:2.4:1.1 for the latter. And corresponded dielectric constants were 1.6 and 2.2, respectively.

Investigation on the interface formation of ambient-pressure-dried SiO 2 aerogel film deposited on GaAs

The interfacial state between ambient-pressure-dried aerogel and GaAs has been examined with specific emphasis on the solvent of modification. Trimethylchlorosilane (TMCS), being widely used to modify aerogel, was shown to be inadaptable due to its constitutional chlorine, which formed HCl and then etched GaAs oxides on GaAs surface. The interface of GaAs with aerogel was roughly etched from the reflection of surface microstructure of aerogel. Therefore, surface modification of aerogel with TMCS induced a great damage to interfacial GaAs surface, resulted in the change of surface characteristics of GaAs, and finally induced cracks and a collapse of aerogel fractal structure. On the contrary, hexamethyldisilazane, another modifying agent for aerogel surface, represented a relatively good interfacial state, while aerogel could be successfully formed on GaAs substrate.

Study of porosity and stiffness measurement of nanoporous aerogel films with wideband ultrasonic surface waves

Nanoporous aerogel films are of great scientific and commercial interest because of their outstanding potential for application to microelectronic interconnect, associated with low dielectric constant. Of the parameters which describe such aerogel films, density/porosity and stiffness are two of the most critical, but are difficult to measure. This paper shows how measurement of the dispersion of laser-generated surface acoustic wavepackets travelling on submicron-thick aerogel films on silicon substrates allows the density and Young's modulus to be extracted reliably. Particular attention is paid to accuracy of measurements and sensitivity to input data; and a method for extracting measurements on very thin films (<300 nm) is presented.

Integration of carbon aerogels in PEM fuel cells

We prepared resorcinol–formaldehyde (RF) aerogel films with a high molar ratio of resorcinol/catalyst (R/C) of about 1500 and a low mass ratio of 30%. At the onset of the gelation process, organic fibers or carbon fiber fleece were added to the sol in order to increase the mechanical stability. Upon pyrolysis the RF-aerogel as well as the organic fibers were transformed into porous nanostructured carbon. The thickness of the films was about 500 μm and below. As previously shown, the addition of fibers modifies the gelation and thus the structure of the carbon network as well as the particle and pore sizes in characteristic ways. To investigate this influence, samples with different fiber types and contents were prepared. Scanning electron microscopy was used to determine the particle and pore sizes. The largest pore sizes obtained were in the range of several microns. The highest electrical conductivity reached was about 28 S/cm for a sample with almost 80% porosity. Tests of single cells prepared with carbon aerogel sheets show that films with a rather coarse structure in the interior and a micron thin fine structured top and bottom layer are most suitable as gas diffusion electrodes in polymer electrolyte membrane (PEM) fuel cells. The top and bottom layer promote the formation of low resistance electrical contacts between the gas diffusion electrodes and the polymeric membrane as well as the current collector.

Skin formation on RF aerogel sheets

Aerogel sheets a few hundred μm thick were made from fiber-reinforced resorcinol–formaldehyde (RF) gels with a high molar resorcinol/catalyst (R/C) ratio and a target density of 300 kg/ m 3 . The obtained gels were ambient pressure dried without noticeable shrinkage. The bulk of these samples show a coarse structure with micron-sized particles and pores while on the sheets' surface a nanostructured skin with a thickness of a few microns formed. The surface skin was peeled off the aerogel sheet. In addition, a non-reinforced aerogel film with the same thickness as the peeled-off skin was made. Both were investigated via small angle X-ray scattering (SAXS) and atomic force microscopy (AFM). It was found, that the surface skin on a Novoloid-fiber-reinforced RF aerogel possesses a higher density and smaller entities than the non-reinforced RF aerogel film. This is explained by a local superposition of the catalytic influence of the fibers and the sol–mold interface. If pyrolyzed, the skin covered aerogel sheets are most suitable e.g., as gas diffusion electrodes in PEM fuel cells.

Carbon cloth-reinforced and activated aerogel films for supercapacitors

The incorporation of carbon cloth into resorcinol formaldehyde (RF) aerogel precursors allows the manufacture of flexible, mechanically stable monolithic carbon aerogel electrodes with thicknesses in the 100 μm range. These were used to build button cell supercapacitors (diameter: 22 mm, thickness: 6 mm) with a capacitance of about 11 F and a serial resistance of about 170 m Ω. These cells were characterized by cyclic voltammetry (CV) and impedance spectroscopy (IS). A fit function for CV data was developed and confirmed with IS. Self discharge properties and long-term stability of these cells were investigated and compared to cells made with Maxsorb carbon black binder electrodes and activated Kynol fiber cloth electrodes. In these tests the carbon aerogel showed significantly better results than the commercial materials. Activation tests on carbon aerogel electrodes were performed with promising results.

Electrochemically induced surface modification of titanols in a `nanoglued' titania aerogel–silica aerogel composite film

Titania aerogel, thermally processed into its anatase form, has been bound as submicrometer particles to an electrode with silica sol. The resulting composite gel is supercritically dried to create a titania–silica composite aerogel film on the conducting substrate. Titania aerogel particles present in the mesoporous film are electrochemically addressable. In organic electrolyte containing isopropanol and added water, the electrochemical response negative of the flatband potential of TiO 2 indicates that electrogenerated reactants (hydroxides and isopropoxides) induce condenzation reactions with surface titanols (Ti–OH). The data suggest that titania aerogels can be electrochemically modified in a controllable way with a wide variety of compounds that have reducible –OH moieties.

Characterization of thin-film aerogel porosity and stiffness with laser-generated surface acoustic waves

Density/porosity is the most important parameter determining properties of porous films; however, it is difficult to measure. This letter reports characterization of density/porosity and Young's modulus values of a range of nanoporous silica aerogel films via dispersion of laser-generated, wideband, surface acoustic waves. The technique shows a clear relationship between dielectric constant, porosity and Young's modulus. Low Young's modulus values show that film stiffness is drastically reduced with increasing porosity. The technique is rapid, non-destructive and relatively inexpensive, and yields absolute values of aerogel elastic properties, which are useful for process control; such properties are difficult to access using other techniques.

The evolution of microstructure and surface bonding in SiO 2 aerogel film after plasma treatment using O 2, N 2, and H 2 gases

In this work, we investigated the effects of various gases (O 2, N 2, and H 2) plasma treatment on SiO 2 aerogel films in order to strengthen the film and improve the surface chemical bonding nature of the film. The plasma treatments could reduce the density of silanol (SiOH) and ethoxy (SiOR) groups. The physical, chemical, and electrical properties of SiO 2 aerogel film through curing with various plasma gases were evaluated. The modification of SiO 2 aerogel film by plasma gas treatment was related to the physical impingement effect of ions, chemical reaction, and irradiated vacuum UV.

Synthesis of Low‐Dielectric Silica Aerogel Films by Ambient Drying

A new ambient drying process that is simple, effective, and reproducible has been developed to synthesize low‐k SiO2 aerogel thin films for intermetal dielectric (IMD) materials. The SiO2 aerogel films having a thickness of 9500 Å, a high porosity of 79.5%, and a low dielectric constant of 2.0 were obtained by a new ambient drying process using n‐heptane as a drying solvent.

The effects of surface terminal bonds and microstructure of SiO 2 aerogel films on dry etching

The effects of microstructure and surface terminal bonds of SiO 2 aerogel films on dry etching were investigated using Ar, SF 6, and C 2F 6 plasma gases. With Ar plasma etching, physical effect of ion bombardment on porous film was found. In residue-free SF 6 plasma etching, reactive etchant transport and high-mass ion bombardment were observed. With C 2F 6 plasma etching, fluorocarbon residue layer was revealed to maintain surface morphology as acting a barrier to radical transport and ion bombardment. An etching of 450°C-annealed SiO 2 aerogel showed that a dense surface induced the decrease in reaction area, inhibition of etchant transport, and then uniform etching.

Carbon Cloth Reinforced Carbon Aerogel Films Derived from Resorcinol Formaldehyde

Carbon cloth reinforced RF (Resorcinol Formaldehyde) aerogel films have been produced with extremely high RC ratio (molar ratio of resorcinol to catalyst) or with no catalyst at all. The gels were subcritically dried. Carbon aerogel films were obtained by pyrolysis of the RF aerogel films. The structure of the composite porous films was investigated using infrared spectrometer, optical and electron scanning microscope. The IR measurements indicated that the bottom surface reflectance of the films is higher than the top surface. The microscopical pictures revealed that the bottom surface of the films has a denser structure than the top surface, and the presence of carbon cloth in the sol-gel process strongly influences the structure of the films. The carbon cloth fibers act as catalytic sites. The particles are more likely to form around the fibers instead of within the sol. This leads to a somewhat reduced specific surface area as was shown via N2 sorption and BET analysis.Using the 4-probe method the electrical conductivity of the carbon cloth reinforced carbon films was found to be higher than that of pure carbon aerogel films.

Thin-Film Aerogel Porosity and Stiffness Characterized by Surface Acoustic Wave Spectroscopy

ABSTRACTDensity/porosity and stiffness are the most critical parameters determining properties of porous films, however they are difficult to measure. Here we report characterization of density/porosity and Young's modulus values of a range of nanoporous silica aerogel films via dispersion of laser-generated wideband surface acoustic waves. Results are compared to dielectric constant, Rutherford Backscattering Spectrometry (RBS), Ellipsometric Porosimetry (EP) and Nanoindentation (NI). RBS density correlates well to SAW. EP and NI also show a correlation, however the absolute values do not match. Low Young's modulus values show that the film stiffness is drastically reduced with increasing porosity. The technique is rapid, nondestructive and relatively inexpensive, and yields absolute values of nanoporous aerogel elastic properties which are useful for process control and are difficult to access with other techniques.

The effects of plasma treatment on SiO 2 aerogel film using various reactive (O 2, H 2, N 2) and non-reactive (He, Ar) gases

In order to reduce the R–C time delay of ULSI circuits, interconnection materials with low resistance and/or interlayer films with low dielectric constant should be applied. A SiO 2 aerogel film processed by spin-coating and supercritical drying has high porosity and large integral surface area. Therefore, this material can offer a low dielectric constant. In this work, we demonstrated that various gas plasma treatments can control the internal surface chemical species of SiO 2 aerogel film, such as organic groups, hydroxyl groups, and adsorbed water. Through O 2, N 2, He, and Ar plasma treatments, condensation reaction between –OR and –OH groups happened and this induced the reduction of film thickness. After the treatments, –OH related bonds were formed due to adsorbed moisture. Therefore, the dielectric constant and the leakage current increased. However, the amelioration of electrical properties could be obtained after subsequent thermal treatment. On the contrary, a H 2 plasma treated SiO 2 aerogel film showed better leakage current behavior than that of the thermally treated one. This was due to the hydrophobic character of the H 2 plasma treated sample. The H 2 plasma was seemed to passivate porous SiO 2 aerogel film with hydrogen. Then H 2 plasma treatment was revealed as one possible post-treatment of the SiO 2 aerogel film for applying to an intermetal dielectric in a multilevel interconnection structure.

Effects of H2 Addition in Magnetized Inductively Coupled C2F6 Plasma Etching of Silica Aerogel Film

Effect of H2 addition to C2F6 plasma etching of SiO2 aerogel film was examined for low-k dielectric application. In this experiment, H2 plasma in itself was responsible for pore blocking and bond breaking of the SiO2 aerogel. With increasing hydrogen from 0 to 50%, etch rate of SiO2 aerogel was severely dropped at 20% of H2 addition. According to the increase in H2 addition, transition from fluorine-rich residue to carbon-rich one was gradually happened in SiO2 aerogel. Surface microstructure of SiO2 aerogel was so influenced with the increasing H2 addition that they transformed to be planar by the interaction between residue/network and ion bombardment and the condensation reaction of surface chemicals with H2 plasma.