Developing defect-tolerant demolding process in nanoimprint lithography

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Demolding, the process to separate stamp from molded resist, is most critical to the success of ultraviolet nanoimprint lithography (UV-NIL). In the present study we first investigated adhesion and demolding force in UV-NIL for different compositions of a model UV-curable resist containing a base (either tripropyleneglycol diacrylate with shorter chain length or polypropyleneglycol diacrylate with longer chain length), a cross-linking agent (trimethylolpropane triacrylate) and a photoinitiator (Irgacure 651). The demolding force was measured using a tensile test machine after imprinting the UV resist on a silicon stamp. In general, the shorter monomer shows a larger demolding force. Decreasing the cross-linking agent content from 49 to 0 wt% results in a decreased adhesion force at the resist/stamp interface thereby facilitating the demolding. Demolding stress in general is mainly generated due to shrinkage of the resist in the UV curing step and also adhesion and friction at the stamp/resist interface in the subsequent demolding step. In the second part of this study the effect of resist compositions on the stress generation was studied by numerical simulation of the curing and demolding steps in UV-NIL. Input parameters required for the simulation were determined experimentally. As the cross-linking agent content increases the fracture strength of the resist also increases. At the same time, shrinkage stress due to cure and also adhesion at the stamp/resist interface both increase. By normalizing the overall maximum local stress by the fracture stress of the resist, we found that there is an optimum for the cross-linking agent content that leads to the most successful imprinting. In the third part of our study a simple method was developed to obtain the polymerization shrinkage stress exerted on the sidewalls of resist/stamp interface in UV NIL. This method is based on the measurements of demolding force which is the sum of adhesion and friction forces. The mean polymerization shrinkage stress on the sidewalls can readily be decoupled from overall demolding force by independently measuring the friction coefficient, adhesion force, and geometries of stamp structures. The polymerization shrinkage stress on the sidewalls is overall larger than adhesion and increases by adding more cross-linking agent to the resist composition.