S. V. Sreenivasan

Step and flash imprint lithography: Template surface treatment and defect analysis

We have finished the construction of an automated tool for step and flash imprint lithography. The tool was constructed to allow defect studies by making multiple imprints on a 200 mm wafer. The imprint templates for this study were treated with a low surface energy, self-assembled monolayer to ensure selective release at the template-etch barrier interface. This surface treatment is very durable and survives repeated imprints and multiple aggressive physical and chemical cleanings. The imprint and release forces were measured for a number of successive imprints, and did not change significantly. The process appears to be “self-cleaning.” Contamination on the template is entrained in the polymerizing liquid, and the number of defects is reduced with repeated imprints.

Patterning curved surfaces: Template generation by ion beam proximity lithography and relief transfer by step and flash imprint lithography

Submicron patterning of 1 in. diameter curved surfaces with a 46 mm radius of curvature has been demonstrated with step and flash imprint lithography (SFIL) using templates patterned by ion beam proximity printing (IBP). Concave and convex spherical quartz templates were coated with 700-nm-thick poly(methylmethacrylate) (PMMA) and patterned by step-and-repeat IBP. The developed resist features were etched into the quartz template and the remaining PMMA stripped. During SFIL, a low viscosity, photopolymerizable formulation containing organosilicon precursors was introduced into the gap between the etched template and a substrate coated with an organic transfer layer and exposed to ultraviolet illumination. The smallest features on the templates were faithfully replicated in the silylated layer.

Mammalian cells preferentially internalize hydrogel nanodiscs over nanorods and use shape-specific uptake mechanisms

Significance Nanoparticles are widely investigated for intracellular drug delivery and molecular imaging and should be designed to maximize cell uptake. Here the effects of particle geometry to maximize nanoparticle uptake by mammalian cells are evaluated. The findings show that uptake is governed by a combination of cell–particle adhesion, strain energy for membrane wrapping around the particle, and local particle concentration at the cell membrane, all of which are particle-shape–dependent. Under typical culture conditions, disc-shaped hydrophilic nanoparticles were internalized more efficiently than nanorods. Interestingly, larger nanodiscs and rods had higher uptake compared with the smallest particles tested. Mechanisms of uptake were also shape- and cell type-specific. These results provide important insights for rational design of nanocarriers to maximize intracellular delivery efficacy. Size, surface charge, and material compositions are known to influence cell uptake of nanoparticles. However, the effect of particle geometry, i.e., the interplay between nanoscale shape and size, is less understood. Here we show that when shape is decoupled from volume, charge, and material composition, under typical in vitro conditions, mammalian epithelial and immune cells preferentially internalize disc-shaped, negatively charged hydrophilic nanoparticles of high aspect ratios compared with nanorods and lower aspect-ratio nanodiscs. Endothelial cells also prefer nanodiscs, however those of intermediate aspect ratio. Interestingly, unlike nanospheres, larger-sized hydrogel nanodiscs and nanorods are internalized more efficiently than their smallest counterparts. Kinetics, efficiency, and mechanisms of uptake are all shape-dependent and cell type-specific. Although macropinocytosis is used by both epithelial and endothelial cells, epithelial cells uniquely internalize these nanoparticles using the caveolae-mediated pathway. Human umbilical vein endothelial cells, on the other hand, use clathrin-mediated uptake for all shapes and show significantly higher uptake efficiency compared with epithelial cells. Using results from both upright and inverted cultures, we propose that nanoparticle internalization is a complex manifestation of three shape- and size-dependent parameters: particle surface-to-cell membrane contact area, i.e., particle–cell adhesion, strain energy for membrane deformation, and sedimentation or local particle concentration at the cell membrane. These studies provide a fundamental understanding on how nanoparticle uptake in different mammalian cells is influenced by the nanoscale geometry and is critical for designing improved nanocarriers and predicting nanomaterial toxicity.

Characterization and modeling of volumetric and mechanical properties for step and flash imprint lithography photopolymers

Step and flash imprint lithography (SFIL) is an alternative approach to high-resolution patterning based on a bilayer imprint scheme. SFIL utilizes the in situ photopolymerization of an oxygen etch resistant monomer solution in the topography of a template to replicate the template pattern on a substrate. The SFIL replication process can be affected significantly by the densification associated with polymerization and by the mechanical properties of the cured film. The densities of cured photopolymers were determined as a function of pendant group volume. The elastic moduli of several photopolymer samples were calculated based on a Hertzian fit to force–distance data generated by atomic force microscopy. The current SFIL photopolymer formulation undergoes a 9.3% (v/v) densification. The elastic modulus of the SFIL photopolymer is 4 MPa. The densification and the elastic modulus of the photopolymer layer can be tailored from 4% to 16%, and from 2 to 30 MPa, respectively, by changing the structure of the ph...

Step and flash imprint lithography: Defect analysis

Step and flash imprint lithography (SFIL) is a promising, low cost alternative to projection printing. This technique has demonstrated very high resolution and overlay alignment capabilities, but it is a contact printing technique so there is concern about defect generation and propagation. A series of experiments has been carried out with the goal of quantifying the effect of defect propagation. To that end, each unit process in SFIL was studied independently. The number of particles added during handling and transportation and due to SFIL machinery was deemed acceptable, and the added particles should not complicate the inspection of process defects. The concept of a “self-cleaning” process in which the imprint template becomes cleaner by imprinting was revisited. Inspection of an imprint template before and after imprinting revealed that the template actually becomes cleaner with imprinting. Visual inspection of multiple imprints did not reveal any systematic generation or propagation of defects. The inspection area used in this study was limited, however, since the inspection was both manual and visual. Imprinting for this defect study was performed at the University of Texas in a Class 10 cleanroom, and inspection was performed at International SEMATECH.

Design of orientation stages for step and flash imprint lithography

This paper presents the design of orientation stages for high-resolution imprint lithography machines. These machines implement a new lithography process known as Step and Flash Imprint Lithography (SFIL) and are intended for 1) sub 100 nm imprint demonstrations on flat substrates and 2) investigation of potential defect propagation during step and repeat imprinting. SFIL is an imprint lithography process that is a combination of chemical and mechanical steps and its implementation at room temperature and low pressure makes it an attractive process as compared to other imprint techniques. A critical component of an imprint machine is the orientation stage that is required to provide uniform intimate contact between the template and substrate surfaces. The orientation stage requirements are distinct from those used in photolithography since the depth of focus of projection optics allows for larger errors in the orientation alignment. Also, due to contact between the template and substrate surfaces in imprint lithography, the separation kinematics must be carefully controlled in the SFIL process. Two different orientation stages are designed for single- and multi-imprint machines. In order to eliminate the particle contamination due to frictional contacts, all joints are made with flexure joints. Imprint experiments have been performed to demonstrate sub 100 nm imprints.

Template fabrication schemes for step and flash imprint lithography

Abstract Step and flash imprint lithography (SFIL) is an attractive method for printing sub-100 nm geometries. Relative to other imprinting processes, SFIL has the advantage that the template is transparent, thereby facilitating conventional overlay techniques. The purpose of this work is to investigate alternative processes for defining features on an SFIL template. The first method considered using a much thinner (

High-resolution templates for step and flash imprint lithography

Step and Flash Imprint Lithography (SFIL) is an attractive method for printing sub-100 nm geometries. Relative to other imprinting processes SFIL has the advantage that the template is transparent, thereby facilitating conventional overlay techniques. In addition, the imprint process is performed at low pressures and room temperature, minimizing magnification and distortion errors. The purpose of this work was to investigate alternative methods for defining high resolution SFIL templates and study the limits of the SFIL process. Two methods for fabricating templates were considered. The first method used a very thin layer of Cr as a hard mask. The second fabrication scheme attempts to address some of the weaknesses associated with a solid glass substrate. Because there is no conductive layer on the final template, SEM and defect inspection are compromised. By incorporating a conductive and transparent layer of indium tin oxide (ITO) on the glass substrate, charging is suppressed during SEM inspection, and the transparent nature of the final template is not affected. Using ZEP-520 as the electron beam imaging resist, features as small as 20 nm were resolved on the templates. Features were also successfully imprinted using both types of templates.

Swelling behavior of nanoscale, shape- and size-specific, hydrogel particles fabricated using imprint lithography

Recently a number of hydrogel-based micro- and nanoscale drug carriers have been reported including top down fabricated, highly monodisperse nanoparticles of specific sizes and shapes. One critical question on such approaches is whether in vivo swelling of the nanoparticles could considerably alter their geometry to a point where the potential benefit of controlling size or shape could not be realized. Little has been reported on experimental characterization of the swelling behavior of nanoscale hydrogel structures, and current theoretical understanding is largely based on bulk hydrogel systems. Using atomic force microscopy (AFM) and environmental scanning electron microscopy (ESEM) capsules, we have characterized the swelling behavior of nano-imprinted hydrogel particles of different sizes and aspect ratios. Our results indicate a size-dependent swelling which can be attributed to the effect of substrate constraint of as-fabricated particles, when the particles are still attached to the imprinting substrate. Numerical simulations based on a recently developed field theory and a nonlinear finite element method were conducted to illustrate the constraint effect on swelling and drying behavior of substrate-supported hydrogel particles of specific geometries, and compared closely with experimental measurements. Further, we present a theoretical model that predicts the size-dependent swelling behavior for unconstrained sub-micron hydrogel particles due to the effect of surface tension. Both experimental and theoretical results suggest that hydrogel swelling does not significantly alter the shape and size of highly crosslinked nanoscale hydrogel particles used in the present study.

Initial study of the fabrication of step and flash imprint lithography templates for the printing of contact holes

Step and flash imprint lithography (S-FIL) is an attractive method for printing sub-100-nm geometries. Relative to other imprinting processes, S-FIL has the advantage of the template being transparent, thereby facilitating conventional overlay techniques. In addition, the imprint process is performed at low pressures and room temperature, minimizing magnification and distortion errors. As a result, it may be possible to use S-FIL to build integrated circuits. The purpose of this work is to investigate the fabrication methods needed to form templates capable of printing sub-100-nm contact holes. A positive resist process is used to image both holes and pillars on the template. After fabrication, the templates are used to print both contacts and pillars. The dense 80-nm imprinted contacts measure 65 nm, a consequence of undersizing on the template. For relaxed pitches, contacts smaller than 30 nm are observed. Pillars as small as 50 nm are also cleanly printed. At 40 nm, pillar size is inconsistent, and missing pillars are evident. Modifications to the template fabrication process will be necessary to study the feasibility of printing even smaller contacts and pillars.