NanoImprint Lithography (NIL) is one of the most promising low-cost, high-throughput technologies for manufacturing nanostructures. In the imprint, a stamp with nanostructures on its surface is used to deform a thin resist film deposited on a substrate. In this work, mathematical models and simulation results for two variants of NIL are presented: Thermal NIL (TNIL) and UltraViolet NIL (UVNIL).

In TNIL, a thermal plastic resist is heated above its glass transition temperature during imprint, where the resist becomes viscous liquid and the stamp is pressed into the resist. Then the resist is cooled before being separated from the stamp. The design of "an optimum" stamp geometry is an essential issue in TNIL. The optimization must exclude stamp zones under which the resist has to flow laterally over large distances. In these zones locally increased values of pressure are achieved resulting in a non-uniform deformation of stamp/substrate and consequently in inhomogeneities of the residual layer thickness. If these inhomogeneities fall outside of specific tolerances then pattern transfer will be difficult. The stamp optimization produces the greatest benefit if its implementation is performed before expensive stamp manufacturing starts. To do this requires an effective simulation tool for prediction of the residual layer thickness from a given stamp structure. Here the mathematical model for simultaneous calculation of the resist viscous flow in TNIL and the stamp/substrate deformation is presented. The model specifies the 2D temporal distributions of the pressure and the normal displacement of the stamp/substrate surface for a given resist thickness and a stamp velocity. By the numerical approximation of the model, special coarse-grain approach is applied. This finite difference approach provides high precision of simulation results by using a reasonably coarse grid. The model has been realized in an effective simulation tool for the verification of stamp structures on standard personal computers. From the presented comparison of calculated and experimental results, it can be concluded that the coarse-grain modeling allows predicting the residual layer thickness with accuracy better than 10%.

UVNIL is a photolithography process in which a liquid monomer (resist) is dispensed onto the substrate in the form of several variable sized droplets and then imprinted by UV-transparent stamp. The stamp is "flashed" with UV light to photopolymerize the monomer into a desired pattern.

A computer optimization of the droplets distribution is required to attain defectless pattern transfer in UVNIL. In this work, a coarse-grain method for the simulation of the viscous flow of the resist in UVNIL is presented. The method is realized in software for the quantitative prediction of resist spreading at given the stamp geometry (the distribution of cavities and protrusions, the cavities depth) and the process parameters (the resist properties, the number of droplets, the droplet size, the stamp loading regime). The software is applied to evaluate resist optimal dispensing, which provides perfect pattern transfer for tested structures. The obtained results indicate the inadequacy of the approach whereby droplet sizes are chosen only depending on local filling factor of imprinted stamps. Analysis of the results provides strong evidence that even in relatively simple cases the optimal dispensing should be designed using the resist flow simulation.

Phase-change materials based on chalcogenide alloys are of great technological importance due to their ability to undergo reversible and fast transitions between the amorphous and crystalline phases upon heating. This property, together with the strong optical and electronic contrast between the two phases, is exploited in rewritable optical discs (CDs, DVDs, Blu-Rays discs) and prototype non-volatile memories. Up to now, very few works have addressed the properties of phase change materials doped with magnetic impurities. Recently, it was shown experimentally [1,2] that Ge2Sb2Te5 doped with Fe atoms exhibits phase-change behavior for low concentrations of Fe and that both the amorphous and the crystalline phases are ferromagnetic at low enough temperatures. Moreover, the two phases were found to have different saturation magnetization. This finding opens up the possibility of exploiting the phase-change behavior for fast magnetic switching in e.g. spintronic devices.
We have investigated the structural, electronic and magnetic properties of Fe-doped Ge₂Sb₂Te₅ by first-principles simulations based on Density Functional Theory [3]. Both amorphous and crystalline (hexagonal and cubic rocksalt) phases of Ge₂Sb₂Te₅ were considered. Our results show that a) in the crystalline phases, the most stable substitutional sites for Fe impurities are Ge and Sb, whereas interstitial sites are energetically costly, b) for small Fe concentrations, the structural properties of amorphous Ge₂Sb₂Te₅ are not significantly affected by the presence of Fe and c) in the amorphous phase, the magnitude of the magnetic moments of the Fe impurities is reduced with respect to the crystalline phases, due to the different local geometries and chemical environments, which explains the experimentally observed magnetic contrast between the two phases.

[1] W.-D. Song, L.-P. Shi, X.-S. Miao, and C.-T. Chong, Adv. Mater. 20, 2394 (2008).
[2] W.-D. Song, L.-P. Shi, and T.-C. Chong, J. Nano. Nanotechnol. 11, 2648 (2011).
[3] Y. Li and R. Mazzarello, submitted.