Step And Flash Imprint Lithography Process Research Articles

Pinning at template feature edges for step and flash imprint lithography

We present a theory of the pinning of an air-liquid interface at the feature edge of a template during the imprint step of step and flash imprint lithography (SFIL). Pinning occurs when the pressure at the air-liquid interface reaches the pressure of the bulk liquid. At this condition, there is no pressure gradient or driving force to move the liquid and fill the feature. We examined several parameters, including residual layer thickness, wettability of the feature edge, applied force on the template, and the radius of curvature of the feature edge, to study their effect on pinning. An optimal SFIL process window and template modifications are proposed that minimize the pinning at the feature edge while still preventing any extrusion along the mesa (pattern containing area on the template) edge.

Simulation of fluid flow in the step and flash imprint lithography process

Step and flash imprint lithography (SFIL) is a photolithography process in which the photoresist is dispensed onto the wafer in its liquid monomer form and then imprinted and cured into a desired pattern instead of using traditional optical systems. The mask used in the SFIL process is a template of the desired features that is made using electron beam writing. Several variable sized drops of monomer are dispensed onto the wafer for imprinting. The base layer thickness at the end of the imprinting process is typically about 50 nm, with an approximate imprint area of 1 in 2. This disparate length scale allows simulation of the fluid movement between the template and wafer by solving governing equations of fluid mechanics simplified by lubrication theory. Capillary forces are also an important factor governing fluid movement; a dimensionless capillary number describes the relative importance of these forces to the viscous forces in the fluid. This paper presents a simulation to model the flow and coalescence of the multiple fluid drops and the effect the number of drops dispensed has on imprint time. The imprint time is shown to decrease with increasing numbers of drops or with an applied force on the template. Appropriate filling of features on the template is an important issue in SFIL, which is presented in this study by simulating the interface movement into and around the feature by a modified boundary condition on the governing equations. It is found that above a critical aspect ratio, features do not fill and fluid does not spread outside the mask edge. The simulation provides a predictive tool for understanding and optimizing fluid management in SFIL.

Nanofabrication with step and flash imprint lithography

Step and flash imprint lithography (SFIL) has made tremendous progress since its initial development at The University of Texas at Austin in the late 1990s. The SFIL process went from laboratory to commercialization in under five years, and the number of technical hurdles that must be cleared before it is recognized as fully competitive with optical or EUV lithography for sub-50-nm patterning is dwindling. Patterning resolution has been demonstrated down to 20 nm, with the limit so far being only the template fabrication process. The SFIL method was developed from the beginning with the precision overlay/alignment requirements of multilevel device fabrication in mind. It was recognized that it would be inherently easier to achieve overlay and alignment accuracy with a constant temperature and low pressure imprinting process, and already tool designers have built on SFIL's advantages to produce tools that are viable for multilayer device fabrication. Early tools have demonstrated better than 10-nm alignment resolution, and no insurmountable fundamental issues have been identified that would prevent alignment resolution from reaching the tight tolerances required for integrated circuit manufacturing. With any contact printing method, process-generated defects are a concern, but the SFIL process has proven to be surprisingly robust with an inherent self-cleaning mechanism for removing particle contamination. Furthermore, new template surface treatments have been developed that improve mold lifetime and minimize defect generation. SFIL shows promise as a low cost manufacturing tool for a wide variety of semiconductor, microelectromechanical, optoelectronic, microfluidic, and other devices. This work summarizes the state of development of step and flash imprint lithography and discusses its potential as a general nanofabrication tool.

Simulating fabrication and imprinting distortions in step-and-flash imprint lithography templates

Step-and-flash imprint lithography (SFIL) is a novel approach for producing chips with sub-45-nm features. The technology uses a quartz template to imprint a dispensed polymer solution on the device wafer in a 1 × pattern-transfer process. The accuracy of the imprint fabrication technology requires minimization of the errors associated with the template fabrication, appropriate dispensing of the polymer, and reduction of template distortion during the interaction with the polymer and the device wafer as a result of the imprint. In this paper, two key issues, template fabrication and polymer dispensing, are investigated with numerical models in order to characterize the response of system components during the SFIL process. The template distortion during fabrication is simulated using finite-element models and the polymer solution dispensing is modeled using computational fluid dynamics simulations.

Vinyl ethers in ultraviolet curable formulations for step and flash imprint lithography

Until now, acrylates have been the monomers of choice for use for step and flash imprint lithography (SFIL) etch barrier formulations, in part because of the commercial availability of silicon-containing acrylates (necessary for etch resistance), together with their low viscosities and capability for rapid photopolymerization. However, despite many desirable properties, the polymerization of acrylates via radical chain propagation causes some potential issues in the SFIL process as a result of the inhibition of these processes by oxygen. Vinyl ethers are prime candidates to replace acrylates. Their curing proceeds by a cationic mechanism, which is insensitive to oxygen and very rapid, while the vinyl ether group contribution to viscosity is significantly lower than that of an acrylate, silicon-containing vinyl ethers are not widely commercially available, and so were synthesized for this study. As expected, formulations based on these vinyl ethers were lower viscosity and faster curing than the acrylate etch barrier formulations presently employed, while the tensile strength of cured vinyl ether formulations were found to be higher than their acrylate counterparts. In spite of about twice higher template separation force compared with acrylates, the tensile strength of vinyl ether etch barrier formulations is about five times higher than that of acrylate formulations. Therefore, these vinyl ether-based formulations were used to pattern down to 50 nm features using the SFIL process.

Step and Flash Imprint Lithography Modeling and Process Development

Step and Flash Imprint Lithography (SFIL) is a low pressure, room temperature process capable of faithfully reproducing nanometer scale features. By eliminating expensive imaging optics, SFIL is able to realize this pattern transfer at low cost. Previous work has described the SFIL process in detail including a general process description, etch transfer processes, initial defect studies, polymerization kinetics, and device demonstrations. This paper documents photopolymerization induced shrinkage of the etch barrier and its effect on pattern fidelity and line profile. Results from finite element and mesoscale models are compared to imprinted etch barriers of varying shrinkage.

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 (<20 nm) 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, scanning electron microscopy (SEM) and defect inspection are compromised. By incorporating a conductive and transparent layer of indium tin oxide 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.

Patterning nonflat substrates with a low pressure, room temperature, imprint lithography process

Step and flash imprint lithography (SFIL) is a technique that has the potential to replace photolithography for patterning resist with sub-100 nm features. SFIL is a low cost, high throughput alternative to conventional photolithography for high-resolution patterning. It is a molding process in which the topography of a template defines the patterns created on a substrate. The ultimate resolution of replication by imprint lithography is unknown but, to date, it has only been limited by the size of the structures that can be created on the template. It is entirely possible to faithfully replicate structures with minimum features of a few hundred angströms. SFIL utilizes a low-viscosity, photosensitive silylated solution that exhibits high etch contrast with respect to organic films in O2 reactive ion etching. In this article we describe the SFIL process, the development of a multilayer etch scheme that produces 6:1 aspect ratio features with 60 nm linewidths, a method for patterning high-aspect-ratio features over topography, and a metal lift-off process. A micropolarizer array consisting of orthogonal 100 nm titanium lines and spaces fabricated using this metal lift-off technique is reported.

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.