Sergey I. Zaitsev

A systematic experimental study of the positive-tone resist ELP-20 was conducted, covering its structural properties, film-formation behavior, and response to electron-beam exposure. Raman spectroscopy demonstrated the methacrylate nature of the resist and its spectral correspondence to poly(methyl methacrylate) PMMA, which enabled direct comparison both with PMMA itself and with existing methacrylate-based resists. Spin-coated films prepared from 3–11 wt.% solutions exhibited a robust power-law dependence of thickness on spin speed, h_∞ ∝ ω^{−0.48 ± 0.01}, and showed high thickness uniformity. The concentration dependence of the film thickness at a fixed spin speed allowed identification of the polymer–coil overlap region and enabled estimation of the effective molecular weight of the polymer base, M_eff = (25 ± 7) kg/mol. Lithographic characterization indicated a decrease in sensitivity with increasing electron energy, with a sensitivity of approximately 40 μC/cm² at 25 keV. A depth-dependent dose-distribution model provided an energy-independent average contrast value of γ ≈ 1.67. The results present a coherent and systematic description of ELP-20 behavior under electron-beam exposure and establish a basis for its further use in lithographic processing.

The paper presents an analytical model for determining the resist contrast in electron lithography with nonuniform deposited energy over the depth, which is typical for low-energy electron exposure. In the classical approach, the contrast is determined from the logarithmic dependence of the residual resist thickness on the exposure dose and assumes the homogeneity of the deposited energy over the layer depth, which leads to an overestimation of the contrast value in the presence of a gradient of the deposited energy. The proposed model takes into account the linear change in the energy profile in the resist, which is neglected in existing generally accepted model, thus allowing us to extract the "true" contrast value reflecting the resist properties under given development conditions. To validate the model, experiments were carried out with an ELP-20 resist 200 nm thick on silicon substrates at an electron beam energy of 5, 15 and 25 keV. Dose wedges were exposed for each energy, followed by development and topography analysis by atomic force microscopy. By fitting the model curves to the experimental dependence of the residual resist thickness on the exposure dose for each electron energy, the values of the contrast and the parameter characterizing the gradient of the deposited energy by depth were calculated. In this case, the contrast remains almost constant when varying the energy of incident electrons and has an average value of γ = 1.67. Thus, the increase in contrast with a decrease in the electron energy observed within the classical approach should be considered as an artifact of the model used. The proposed model is applicable for precision calibration of the processes of forming three-dimensional resist structures using the grayscale lithography.

In this study, the key lithographic performance of PMMA 950K resist was evaluated by exposure to a 30 keV focused gallium (Ga⁺) ion beam. The sensitivity and contrast of PMMA 950K were directly compared with those of electron exposure under identical development conditions. It was found that the sensitivity of PMMA 950K to Ga⁺ ions for 50 nm films reaches a value of about 0.4 μC/cm², which is more than 250 times higher than its sensitivity to electron exposure. A method for evaluating the resist contrast during ion exposure is proposed in this work, taking into account the highly non-uniform dose distribution across the resist depth; it yielded a contrast value of γ = 2.6, which is consistent with the result obtained with electron exposure (γ = 2.8). In addition, a pronounced dependence of the resist sensitivity on the resist thickness was found: with an increase in thickness from 10 nm to 60 nm the sensitivity decreases by an order of magnitude. The obtained results form a reliable methodological basis for characterizing the behavior of polymer resists under ion irradiation and provide valuable recommendations for optimizing lithography with a focused beam of Ga⁺ ions when creating nanostructures for microelectronics, photonics, and quantum technologies.

The paper presents a program for simulating electron scattering in layered materials ProxyFn. Calculations show that the absorbed energy density is three-dimensional, while the contribution of the forward-scattered electrons is better described by a power function rather than the commonly used Gaussian. It is shown that for the practical correction of the proximity effect, it is possible, nevertheless, to use the classical two-dimensional proximity function containing three parameters: α, β, η. A method for determining the parameters α, β, η from three-dimensional calculations based on MC simulation and development consideration is proposed. A good agreement of the obtained parameters and experimental data for various substrates and electron energies is shown. Thus, a method for calculating the parameters of the classical proximity function for arbitrary layered substrates based on the Monte Carlo simulation has been developed.

The feature of the promising tool of lithographic nanostructuring based on selective exposure of polymer resist by ion beam is very compact (of about tens of nanometers) beam interaction volume. Herewith the main part of beam energy is deposited in the resist and is spent to its modification. It causes the set of advantages specific for this method: sub-10 nanometer resolution achievable, very high energy efficiency and almost complete absence of proximity effect. But also due to this feature absorbed doze essentially inhomogeneous in resist and the dissolution rate is strongly dependent on depth. So the common procedure of resist contrast determination cannot be applied anymore. In the present work a new method for resist contrast determination considering the relation between dissolution rate and deposited energy density is suggested and realized. By using it for PMMA resist irradiated by 30 keV Ga⁺ ion beam the value of contrast was determined to be 3.1 and ions energy length was estimated to be 42 nm.

The use of a special device that limits the field of view of a detector, i.e., a confocal collimator, is one of the approaches to obtaining information about the spatial distribution of the detected material in the methods for radionuclide diagnostics (in medicine) and X-ray fluorescence analysis (in material science). This device is a monolithic construction with a large number of straight channels whose axes are directed towards a single point that is the focus of the confocal collimator. The possibility of fabricating confocal collimators using 3D printing technology is demonstrated. The advantages of 3D printing are simplicity, reliability, and wide availability. It is shown that the use of a confocal collimator in radionuclide diagnostics instead of a single-channel collimator is advantageous, since it substantially increases the sensitivity (by 2−7 times) with the simultaneous improvement of the resolution (approximately by 9 times). The applicability of the confocal collimator for determining the depth of occurrence of a radiation source, i.e., for measuring the three-dimensional distribution of the emitting (fluorescent) substance, is also demonstrated.

The OECD/NEA PSB-VVER project provided unique and useful experimental data for code validation from PSB-VVER test facility. This facility represents the scaled-down layout of the Russian-designed pressurized water reactor, namely, VVER-1000. Five experiments were executed, dealing with loss of coolant scenarios (small, intermediate, and large break loss of coolant accidents), a primary-to-secondary leak, and a parametric study (natural circulation test) aimed at characterizing the VVER system at reduced mass inventory conditions. The comparative analysis, presented in the paper, regards the large break loss of coolant accident experiment. Four participants from three different institutions were involved in the benchmark and applied their own models and set up for four different thermal-hydraulic system codes. The benchmark demonstrated the performances of such codes in predicting phenomena relevant for safety on the basis of fixed criteria.