Large Area NanoImprint Patterning by High-Resolution Epoxysilsesquioxane Resists

Ideally, nanomanufacturing processes that incorporate imprint patterning techniques would not need thermal cycling. To address this challenge, significant focus has been directed towards photocurable resist materials for NIL. One materials system that has received a good deal of attention are silsesquioxane (SSQ) polymers that have been developed as imprinting materials which can also be incorporated as functional patterns.The hurdles for these materials systems to achieve high-rate, large area patterning, include oxygen inhibition effects for methacrylate-based POSS and mold adhesion issues for POSS synthesized incorporating high content of epoxide groups. Pina-Hernandez and colleagues report a versatile method to control the materials properties of SSQ resins for high-rate, large area NIL patterning through straightforward chemical modifications.

Reviewed by Jeff Morse, PhD., National Nanomanufacturing Network

• Pina-Hernandez C, Guo LJ, and Fu PF. 2010. High-Resolution Functional Epoxysilsesquioxane-Based Patterning Layers for Large-Area Nanoimprinting. ACS Nano 4(8): 4776–4784. DOI: 10.1021/nn100478a.

Nanoimprint lithography (NIL) and patterning is an technique which has shown significant promise for the high-throughput nanopatterning of a range of device applications.  NIL’s potential has garnered sufficient attention to place it as a future contender on the semiconductor industry manufacturing roadmap and as a viable patterning tool for extremely high-rate production platforms such as roll-to-roll processing. Yet challenges remain for NIL in achieving both high-rate and large-area patterning due to the resist materials used for nanoimprint patterning. Widely-used thermoplastic polymers have throughput limitations due to high viscosity, even at elevated temperatures, resulting in the need for high pressures and long imprint times, with the added cost of replacing damaged molds.

Ideally, nanomanufacturing processes that incorporate imprint patterning techniques would not need thermal cycling. To address this, significant focus has been directed towards photocurable resist materials for NIL. Several materials approaches have been introduced—including acrylate, thiol-ene, and vinyl-ene based materials—yet  various issues remain including high dimensional shrinkage during curing, high toxicity, poor coating coverage due to low viscosity, sensitivity to oxygen inhibition resulting in increased defect density, low tensile strength, poor demolding properties, and insufficient etch selectivity for subsequent pattern transfer process steps.

To address these issues, a resist material is needed that does not require thermal cycling, provides high tensile strength, can be modified to provide good substrate adhesion with a low surface energy for effective demolding, requires low pressure during imprinting, is resistant to plasma etch processes, and can be coated over large area substrates. Additionally, the development of patterned resist materials that not only function as sacrificial layers but also provide the option of remaining in the device as an integral component has attracted great interest. One materials system that has received a good deal of attention are silsesquioxane (SSQ) polymers that have been developed as imprinting materials which can also be incorporated as functional patterns. As an example, photocurable polyhedral oligomeric silsesquioxanes (POSS) have been used as patterning layers for step-and-flash imprint lithography. The challenges for these materials systems to achieve high-rate, large area patterning, include oxygen inhibition effects for methacrylate-based POSS and mold adhesion issues for POSS synthesized incorporating high content of epoxide groups.

Recently, Pina-Hernandez et.al. reported on their approach in synthesizing novel photocurable SSQ molecular constructs to address these issues. The authors approach was to specifically tailor the resist properties through incorporation of functional groups onto the same SSQ backbone. For instance, low surface energy groups such as methyl or perflouro groups provide good mold release, while high energy groups such as silanol impart good substrate adhesion. The addition of photocurable epoxy groups reduces the imprint process step to seconds at room temperature, while the presence of phenyl groups enhances the material toughness for high fidelity patterning.

The authors synthesized a number of SSQ resins with different chemical functionalities through a hydrolytic condensation of the corresponding trialkoxysilanes. In each case the relative molar ratio of the structure unit was varied in order to establish the structure-property relationship of each resin for NIL processes. A major advantage of the silsesquioxane system developed in this work is the possibility of an easy modification of their chemical structures via chemical synthesis to produce the desired imprinting material. In general, imprinted nanoscale structures are typically subjected to high mechanical stresses during the demolding step, which often leads to the breakage or the deformation of replicated structures if the materials do not have the appropriate mechanical strength. SSQ’s composition thereby can be tuned to obtain a material with outstanding mechanical properties to withstand the otherwise detrimental stress present during the mold release step. The authors were able to optimize the concentration of epoxy functionality incorporated into the SSQ resin in order to achieve the desired mechanical properties.

A facile approach to modifying a photocurable NIL resist has been reported wherein liquid resists have been solidified within seconds under UV exposure at room temperature, replicating structures as small as 20 nm with low-pressure nanoimprinting tools. In the presence of the fluoroalkyl groups in the SSQ resins, a mold release was provided after the imprint process, and the high silicon content of the SSQ material subsequently provides high resistance to plasma dry etching. Thus, a versatile method to control the materials properties of SSQ resins for high-rate, large area NIL patterning has been demonstrated through straightforward chemical modifications.

Image reproduced with permission from Pina-Hernandez C, Guo LJ, and Fu PF. 2010. High-Resolution Functional Epoxysilsesquioxane-Based Patterning Layers for Large-Area Nanoimprinting. ACS Nano 4(8): 4776–4784. DOI: 10.1021/nn100478a. Copyright 2010 American Chemical Society.