Hyun Wook Ro

Quantifying residual stress in nanoscale thin polymer films via surface wrinkling.

Residual stress, a pervasive consequence of solid materials processing, is stress that remains in a material after external forces have been removed. In polymeric materials, residual stress results from processes, such as film formation, that force and then trap polymer chains into nonequilibrium stressed conformations. In solvent-cast films, which are central to a wide range of technologies, residual stress can cause detrimental effects, including microscopic defect formation and macroscopic dimensional changes. Since residual stress is difficult to measure accurately, particularly in nanoscale thin polymer films, it remains a challenge to understand and control. We present here a quantitative method of assessing residual stress in polymer thin films by monitoring the onset of strain-induced wrinkling instabilities. Using this approach, we show that thin (>100 nm) polystyrene films prepared via spin-coating possess residual stresses of approximately 30 MPa, close to the crazing and yield stress. In contrast to conventional stress measurement techniques such as wafer curvature, our technique has the resolution to measure residual stress in films as thin as 25 nm. Furthermore, we measure the dissipation of residual stress through two relaxation mechanisms: thermal annealing and plasticizer addition. In quantifying the amount of residual stress in these films, we find that the residual stress gradually decreases with increasing annealing time and plasticizer amounts. Our robust and simple route to measure residual stress adds a key component to the understanding of polymer thin film behavior and will enable identification of more effective processing routes that mitigate the detrimental effects of residual stress.

Synthetic control of molecular weight and microstructure of processible poly(methylsilsesquioxane)s for low-dielectric thin film applications

Processible poly(methylsilsesquioxane)s (PMSSQs) were prepared in refluxing THF solutions under nitrogen atmosphere in the presence of HCl catalyst. It was found that various reaction parameters such as concentration, temperature, reaction time, relative amount of water, and relative amount of acid catalyst could affect the molecular weight, microstructure, and the amount of functional end-groups of synthesized PMSSQs. PMSSQ thin films prepared with high molecular weight PMSSQ samples synthesized in solutions exhibited a much improved crack resistance over commercially available samples, probably due to the effects of different microstructures of polymers. The dielectric constants of the fully cured thin films prepared in this study were found to be ca. 2.7, which is nearly the same as those for commercially available samples.

Anisotropic, hierarchical surface patterns via surface wrinkling of nanopatterned polymer films.

By combining surface wrinkling and nanopatterned polymer films, we create anisotropic, hierarchical surfaces whose larger length-scale (wrinkling wavelength) depends intimately on the geometry and orientation of the smaller length-scale (nanopattern). We systematically vary the pattern pitch, pattern height, and residual layer thickness to ascertain the dependence of the wrinkling wavelength on the nanopattern geometry. We apply a composite mechanics model to gain a quantitative understanding of the relationship between the geometric parameters and the anisotropy in wrinkling wavelength. Additionally, these results shed light on the effect of surface roughness, as represented by the nanopattern, on the metrology of thin films via surface wrinkling.

High performance airbrushed organic thin film transistors

Spray-deposited poly-3-hexylthiophene (P3HT) transistors were characterized using electrical and structural methods. Thin-film transistors with octyltrichlorosilane treated gate dielectrics and spray-deposited P3HT active layers exhibited a saturation regime mobility as high as 0.1 cm2 V−1 s−1, which is comparable to the best mobilities observed in high molecular mass P3HT transistors prepared using other methods. Optical and atomic force microscopy showed the presence of individual droplets with an average diameter of 20 μm and appreciable large-scale film inhomogeneities. Despite these inhomogeneities, near-edge x-ray absorption fine structure spectroscopy of the device-relevant channel interface indicated excellent orientation of the P3HT.

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.

Real-time X-ray scattering studies of film evolution in high performing small-molecule–fullerene organic solar cells

We have studied the influence of the formulation additive 1,8-diiodooctane (DIO) on the structural evolution of bulk heterojunction (BHJ) films based the small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-5-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) and phenyl-C71-butyric-acid-methyl ester ([70]PCBM). Real-time, in situ, grazing-incidence X-ray scattering experiments allow us to characterize the development of crystalline order via diffraction and phase separation via small angle scattering. The performance of p-DTS(FBTTh2)2 based solar cells exhibits a distinct optimum with respect to volume fraction of DIO in the coating solution, unlike many polymer–fullerene systems that exhibit plateaus in performance above a certain additive volume fraction. Increasing the DIO volume fraction increases the crystallinity of p-DTS(FBTTh2)2 and dramatically increases the phase separation length scale even at small DIO amounts. These results suggest that the existence of an optimal DIO amount is a consequence of the phase separation length scale and its relationship to the optimal length for exciton dissociation. The effects of DIO on the time evolution of the drying films indicates that it acts as both a solvent and a plasticizer for p-DTS(FBTTh2)2, controlling its nucleation density and promoting its crystal growth.

Thermodynamic underpinnings of cell alignment on controlled topographies.

H /H o Surface topography is an important environmental cue for controlling cellular responses such as morphology, adhesion, alignment, migration, and gene expression. [ 1–7 ] Surface topographies with feature sizes covering the range of cell and cell components, i.e., from a few nanometers to tens of micrometers, have been broadly investigated with respect to effects on cell contact guidance (CG). [ 2 , 8 ] Despite the signifi cant work done to date, there has not been a satisfactory general explanation for the phenomenon, although many hypothesize that it is related to a biological response. In this paper, we fabricate a platform with precisely controlled surface topography, and use it to perform systematic cell studies that lead us to a new mechanistic understanding of CG under these conditions, which indicates that the response is rapid and largely physical rather than biological in nature. Below, we describe a two-step approach to fabricate submicrometer polymer gratings with continuous variations in grating height ( H ). First, large-area uniform gratings consisting of equally spaced lines were generated via nanoimprint lithography [ 9 , 10 ] on polystyrene (PS) and polymethylmethacrylate (PMMA). For each polymer, two sets of gratings were created with one-to-one line-to-space ratios, each with a pitch ( Λ ) of approximately 420 and 800 nm. Next, the uniformly patterned area was transformed to a continuous gradient in height by annealing on a thermal gradient stage for a fi xed time (see Supporting Information for details). A sketch of an annealed pattern with a height gradient is shown in the inset of Figure 1 . As indicated, the direction of the gradient is parallel to that of the polymer lines. Figure 1 shows position-dependent grating heights for two PS gratings ( Λ = 420 and 800 nm). The grating heights were characterized by atomic force microscopy (AFM) and are normalized in Figure 1 by the maximum height, H 0 , at x = 0.

Cubic Silsesquioxanes as a Green, High-Performance Mold Material for Nanoimprint Lithography

Optical lithography deep in the UV spectrum is the predominate route for high-resolution, high-volume nanoscale pattering. However, state-of-the-art optical lithography tools are exceedingly expensive and this places serious limitations on the applications, technical sectors, and markets where highresolution patterning can be implemented. To date the only substantial market for high-end optical lithography tools has been semiconductor fabrication. Nanoimprint lithography (NIL) has recently emerged as an alternative to optical lithography and combines the potential of sub-fi ve-nanometer patterning resolution with the low cost and simplicity of a stamping process. [ 1–4 ] This has led to signifi cant efforts to implement NIL methods, not only for semiconductor logic devices, but also in fi elds as diverse as the direct patterning of interlayer dielectrics (ILDs) for back-end-of-line (BEOL) interconnect structures, [ 5–7 ] bitpatterned magnetic media for data storage, [ 8 , 9 ] and high-brightness light-emitting diodes (LEDs). [ 10 ] Some of these are new areas where nanoscale patterning has previously not been considered, and are made possible here by the low cost and simplicity of the NIL stamping processes.

Scatterometry for in situ measurement of pattern reflow in nanoimprinted polymers

We use optical scatterometry to extract the time evolution of the profile of nanoimprinted lines in low and high molecular mass polymer gratings during reflow at the glass transition temperature. The data are obtained continuously during the anneal using a spectroscopic ellipsometer and analyzed using a rigorous-coupled-wave model. We show excellent agreement of scatterometry results with ex situ measurements of line height by atomic force microscopy and specular x-ray reflectivity. The in situ scatterometry results reveal differences in the shape evolution of the grating lines indiscernible by other methods.

Nanoimprint pattern transfer quality from specular x-ray reflectivity

Specular x-ray reflectivity is used for high precision measurements of the pattern height, residual layer thickness, and the line-to-space ratio for parallel line and space patterns fabricated with nanoimprint lithography. The line-to-space ratio is profiled vertically to reveal relative linewidth variations as a function of the feature height. These relative linewidth variations are quantified through an external measure of the average pitch to fully define the line shape profile or cross section. An excellent fidelity of the nanoimprint pattern transfer process is quantified by comparing the line shape profiles of the mold to the imprinted pattern.