Wen-Li Wu

Properties of nanoporous silica thin films determined by high-resolution x-ray reflectivity and small-angle neutron scattering

A new methodology based on a novel combination of a high-resolution specular x-ray reflectivity and small-angle neutron scattering has been developed to evaluate the structural properties of low-dielectric-constant porous silica thin films about one micrometer thick supported on silicon wafer substrates. To complement these results, film composition was determined by high-energy ion scattering techniques. For the example thin film presented here, the overall film density was found to be (0.55±0.01) g/cm3 with a pore wall density of (1.16±0.05) g/cm3 and a porosity of (53±1)%. The characteristic average dimension for the pores was found to be (65±1) A. It was determined that (22.1±0.5)% of the pores had connective paths to the free surface. The mass fraction of water absorption was (3.0±0.5)% and the coefficient of thermal expansion was (60±20)×10−6/°C from room temperature to 175 °C. Lastly, model fitting of the specular x-ray reflectivity data indicated the presence of a thin surface layer with an increa...

Characterization of internal fracture process of double network hydrogels under uniaxial elongation

Previously we revealed that the high toughness of double network hydrogels (DN gels) derives from the internal fracture of the brittle network during deformation, which dissipates energy as sacrificial bonds. In this study, we intend to elucidate the detailed internal fracture process of DN gels. We quantitatively analysed the tensile hysteresis and re-swelling behaviour of a DN gel that shows a well-defined necking and strain hardening, and obtained the following new findings: (1) fracture of the 1st network PAMPS starts far below the yielding strain, and 90% of the initially load-bearing PAMPS chains already break at the necking point. (2) The dominant internal fracture process occurs in the necking and hardening region, although the softening mainly occurs before necking. (3) The internal fracture efficiency is very high, 85% of the work is used for the internal fracture and 9% of all PAMPS chains break at sample failure. (4) The internal fracture is anisotropic, fracture occurs perpendicular to the tensile direction, in preference to the other two directions, but the fracture anisotropy decreases in the hardening region. Results (1) and (2) are in agreement with a hierarchical structural model of the PAMPS network. Based on these findings, we present a revised description of the fracture process of DN gels.

Small angle x-ray scattering for sub-100 nm pattern characterization.

Characterization of sub-100 nm photolithographic patterns with nanometer scale resolution is demonstrated using small angle x-ray scattering. The transmission scattering geometry employed potentially enables high throughput measurements for future technology nodes of the semiconductor industry, organic and inorganic nanoscale devices, and three-dimensional structures. The method is demonstrated through the characterization of a series of polymer photoresist gratings using a synchrotron x-ray source. Quantities, such as periodicity and line width, are extracted using minimal modeling. Additional quantities and the potential of a laboratory-based x-ray system are briefly discussed.

Structural characterization of porous low-k thin films prepared by different techniques using x-ray porosimetry

Three different types of porous low-k dielectric films, with similar dielectric constants, are characterized using x-ray porosimetry (XRP). XRP is used to extract critical structural information, such as the average density, wall density, porosity, and pore size distribution. The materials include a plasma-enhanced-chemical-vapor-deposited carbon-doped oxide film composed of Si, C, O, and H (SiCOH) and two spin cast silsesquioxane type films—methylsilsesquioxane with a polymeric porogen (porous MSQ) and hydrogensilsesquioxane with a high boiling point solvent (porous HSQ). The porous SiCOH film displays the smallest pore sizes, while porous HSQ film has both the highest density wall material and porosity. The porous MSQ film exhibits a broad range of pores with the largest average pore size. We demonstrate that the average pore size obtained by the well-established method of neutron scattering and x-ray reflectivity is in good agreement with the XRP results.

Thermodynamic Interactions in Double-Network Hydrogels

Double-network hydrogels (DN-gels) prepared from the combination of a moderately cross-linked anionic polyelectrolyte (PE) and an uncross-linked linear polymer solution (NP) exhibit mechanical properties such as fracture toughness that are intriguingly superior to that of their individual constituents. The scheme of double-network preparation, however, is not equally successful for all polyelectrolyte/neutral polymer pairs. A successful example is the combination of poly(2-acrylamido-2-methyl-1-propane sulfonic acid) (PAMPS) cross-linked network and linear polyacrylamide (PAAm), which results in DN-gels with fracture strength under compression approaching that of articular cartilage ( approximately 20 MPa). Small-angle neutron scattering was used to determine the thermodynamic interaction parameters for PAMPS and PAAm in water as a first step to elucidate the molecular origin responsible for this superior property. Measurements on PAMPS/PAAm DN-gels and their solution blend counterparts indicate that the two polymers interact favorably with each other while in water. This favorable PAMPS/PAAm interaction given by the condition chi(PE-NP) < chi(PE-water) <chi(NP-water), where chi is the Flory-Huggins interaction parameter, is consistent with some of the salient features of the DN structure revealed by SANS, and it may also contribute to the ultimate mechanical properties of DN-gels.

Molecular Model for Toughening in Double-Network Hydrogels

A molecular mechanism is proposed for the toughness enhancement observed in double-network (DN) hydrogels prepared from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) polyelectrolyte network and poly(acrylamide) (PAAm) linear polymer. It is an extension of the phenomenological model set forth recently by Gong et al. ( Macromolecules 2007, 40, 6658- 6664 ). This mechanism rationalizes the changes in molecular structure of the DN gel constituents observed via in situ neutron scattering measurements, the composition dependence of the solution viscosity, and the thermodynamic interaction parameters of PAMPS and PAAm molecules obtained previously from neutron scattering studies. More specifically, this proposed mechanism provides an explanation for the observed periodic compositional fluctuations in the micrometer range induced by large strain deformation.

A SANS study of the plastic deformation mechanism in polyethylene

Abstract Small angle neutron scattering was used to investigate the role of partial melting and recrystallization in the solid state deformation of polyethylene. Blends containing 4 vol% deuteropolyethylene in protonated polyethylene were deformed in a pure shear mode. The scattering cross section of the undeformed blends was well in excess of that expected for randomly (statistically) dispersed molecules, due to isotopic segregation effects between the deuterated and the protonated species. A significant reduction in the excess scattering was observed as the blends were deformed beyond the yield point. Upon further deformation, the cross section underwent a further modest decrease. Such a reduction is known to occur on melting. The neutron scattering results, hence, support the notion of local melting during the yielding and the subsequent deformation processes.

Small-angle X-ray study of particulate reinforced composites

Abstract Small-angle X-ray scattering technique can be used to quantify the microvoids structure within a particulate reinforced composite. An expression for the correlation function of three-phase systems has been derived in terms of the correlation function of the individual phases. By using this expression and the scattered intensities from the damaged and the undamaged composites; it has been show that the volume fraction and the chord length of the microvoids can be obtained, provided no damage occurs to the reinforcement particles. In cases where the microvoids are preferentially oriented within the composites, an approximation scheme based on a linear transformation method has also been developed to measure the aspect ratio of the microvoids provided the volume fraction of these microvoids is much smaller than the other two phases.

Determination of the Internal Morphology of Nanostructures Patterned by Directed Self Assembly

The directed self-assembly (DSA) of block copolymers (BCP) is an emerging resolution enhancement tool that can multiply or subdivide the pitch of a lithographically defined chemical or topological pattern and is a resolution enhancement candidate to augment conventional lithography for patterning sub-20 nm features. Continuing the development of this technology will require an improved understanding of the polymer physics involved as well as experimental confirmation of the simulations used to guide the design process. Both of these endeavors would be greatly facilitated by a metrology, which is capable of probing the internal morphology of a DSA film. We have developed a new measurement technique, resonant critical-dimension small-angle X-ray scattering (res-CDSAXS), to evaluate the 3D buried features inside the film. This is an X-ray scattering measurement where the sample angle is varied to probe the 3D structure of the film, while resonant soft X-rays are used to enhance the scattering contrast. By measuring the same sample with both res-CDSAXS and traditional CDSAXS (with hard X-rays), we are able to demonstrate the dramatic improvement in scattering obtained through the use of resonant soft X-rays. Analysis of the reciprocal space map constructed from the res-CDSAXS measurements allowed us to reconstruct the complex buried features in DSA BCP films. We studied a series of DSA BCP films with varying template widths, and the internal morphologies for these samples were compared to the results of single chain in mean-field simulations. The measurements revealed a range of morphologies that occur with changing template width, including results that suggest the presence of mixed morphologies composed of both whole and necking lamella. The development of res-CDSAXS will enable a better understanding of the fundamental physics behind the formation of buried features in DSA BCP films.

Effect of initial resist thickness on residual layer thickness of nanoimprinted structures

Quantification and control of the residual layer thickness is a critical challenge facing nanoimprint lithography. This thickness must be known to within a few nanometers, yet there are very few nondestructive measurement techniques capable of extracting such information. Here we describe a specular x-ray reflectivity technique that can be used to not only quantify the thickness of the residual layer with sub-nm resolution, but also to extract the pattern height, the line-to-space ratio, and relative linewidth variations as a function of the pattern height. This is illustrated through a series of imprints where the initial film thickness is varied. For films with sufficient resist material to fill the mold, complete pattern filling is observed and the residual layer thickness is directly proportional to the initial film thickness. When there is insufficient resist material in the film to completely fill the patterns in the mold, a finite residual layer thickness of approximately 50–100A is still observed.