A modern alternative to the positive-tone PMMA resist is the ZEP 520A (Nippon Zeon) brand co-polymer resist, which offers a higher sensitivity and etch durability for electron beam lithography. However, the molecular mechanisms are not entirely understood, and the relative performance of two resists for various process conditions of nanofabrication is not readily predictable. The authors report a thorough experimental comparison of the performance of PMMA 950k and ZEP 520A resists in MIBK:IPA, ZED, and IPA:water developers. Interestingly, ZEP resist performance was found to depend significantly on the developer. ZED developer increases the sensitivity, whereas IPA:water optimizes line edge roughness and conceivably the resolution at the expense of sensitivity. The authors also describe two alternative numerical models, one assuming an enhancement of the main chain scission in ZEP as a result of electronic excitations in side groups, and another without such enhancement. In the second case, the differences in ZEP and PMMA resists performance are attributed to their different interaction with the developers. Using both approaches, the authors parameterize the respective models of ZEP development by fitting numerical results to the experimental resist morphologies, and analyze the outcomes.

The authors report a numeric simulation tool that they developed for the modeling and analysis of electron beam lithography (EBL) of nanostructures employing a popular positive tone resist polymethylmethacrylate (PMMA). Modeling and process design for EBL fabrication of 5–50 nm PMMA structures on solid substrates is the target purpose of the simulator. The simulator is functional for exposure energies from 1 to 50 keV with arbitrary writing geometries. The authors employ a suite of kinetic models for the traveling of primary, secondary, and backscattered electrons in the resist, compute three-dimensional (3D) distributions of the yield of main-chain scission in PMMA, and convert these into the local volume fractions of fragments of various sizes. The kinetic process of development is described by the movement of the resist-developer interface with the rate derived from the mean-field theory of polymer diffusion. The EBL simulator allows the computation of detailed 3D distributions of the yield of main-chain scission in PMMA for various conditions of exposure, the corresponding volume fractions of small fragments, and the clearance profiles as functions of the development in time and temperature. This article describes the models employed to simulate the EBL exposure and development, reports examples of the computations, and presents comparisons of the predicted development profiles with experimental cross-sectional resist profiles in dense gratings.

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ZEP brand electron beam resists are well-known for their high sensitivity and etch durability. The various performance metrics such as sensitivity, contrast, and resolution of ZEP resist depend strongly on the development process. In this work, we investigate the development of ZEP-520 resist through contrast curves, dense gratings, and surface roughness measurements using three different classes of developer systems of varying solvation strength, ZED-N50, methyl isobutyl ketone (MIBK) : isopropyl alcohol (IPA) 1:3, and IPA : H2O 7:3, at the ambient temperature (22 °C) and cold (-15 °C) development conditions. In order to provide a deeper insight into the ZEP development process, we propose a novel kinetic model of dissolution for ZEP, and develop an efficient analytical method that allows determining the microscopic parameters of ZEP dissolution based on experimental contrast curves. We also observe experimentally and characterize the negative tone behavior of ZEP for dense grating patterning and compare its performance with positive tone behavior.