## 2 Materials and Methods
### Sensitivity Curve for Negative Epoxy-Based Photoresist  
To determine the proper exposure parameters, we used sensitivity (contrast) curve procedure, which is used to assess the lithographic performance of photoresist [18]. A set of samples was prepared to carry out sensitivity curve procedure to determine the proper exposure parameters. A thin (few nm) layer of hexamethyldisilazane was deposited on silicon wafers with a thickness of 525 \(\upmu\)m during substrate pretreating with heating up to 150 \({}^{\circ}\)C for 30 min to increase the adhesion of the photoresist to the wafer. Using different regimes to achieve layer thicknesses of 10 \(\upmu\)m, 15 \(\upmu\)m, and 20 \(\upmu\)m, the epoxy-based negative mr-X 10 photoresist (micro resist technology GmbH, Berlin, Germany) was spin-coated on the wafers. Thicknesses higher than 20 \(\upmu\)m were not used for the sensitivity curve procedure, as in this case the dose is not uniformly distributed along the photoresist thickness due to absorption; the ratio of the bottom and top dose is not close to unity, which violates the basic assumption of the approach [18]. The soft bake was performed using a hot plate at the temperature of 95 \({}^{\circ}\)C for 10 min. Exposure was carried out with a mercury UV lamp. The stability and distribution of the total incident radiation power were controlled by [PERSON] UV intensity meter model 1000. The intensity value was 15 mW/cm\({}^{2}\) with 1 mW/cm\({}^{2}\) deviation from the center to the edge, the measurements were performed for the i-line of the mercury spectrum (\(\lambda\) = 365 nm). The exposure was done using double-mask geometry: the lower mask was a Cr-quartz mask with alternating dark and light tone regions with a pattern of structure sizes from two up to 50 \(\upmu\)m; the upper mask was a slit that allowed dividing the exposure area into 15 sectors that received a different dose with a constant step. The dose was tuned within a determined range by changing the exposure time. The exposure was carried out using vacuum contact alignment to eliminate diffraction effects: the wafer and the mask were placed in the vacuum chamber to ensure the absence of a gap between the mask and the photoresist layer. The exposed samples were subjected to post exposure bake (PEB) in the oven for four hours with temperature changing from 70 \({}^{\circ}\)C to 90 \({}^{\circ}\)C and subsequent cooling to room temperature. A wet development of the exposed photoresist layers was carried out in propylene glycol monomethyl ether acetate in order to dissolve the unexposed area of mr-X 10 photoresist layers, and then the samples were rinsed in isopropanol alcohol. After development, the samples were air dried in a convection oven at 30 \({}^{\circ}\)C and then the height of the formed micropattern was measured with a Tencor P-2 long scan profiler with an instrumental accuracy of 25 A. The results of the sensitivity curve measurements, which describe the remaining photoresist fraction as a logarithm function of the applied exposure dose, are shown in Figure 1. To approximate the data, we used a generalized logistic function fit according to the approach reported previously [18]. As one can see, the sensitivity curves for the 15 and 20 \(\upmu\)m photoresist layers are shifted towards higher exposure doses. This is due to the absorption introduced by the photoresist layer, which decreases the delivered bottom dose. All curves exhibited a similar slope, which can be associated with photoresist contrast. The contrast value is defined by:  
\[\gamma=\left(\log_{10}\left(\frac{D_{2}}{D_{1}}\right)\right)^{-1} \tag{2}\]  
where \(D_{1}\) is the dose for which the remaining photoresist height is 10% of the original value, and \(D_{2}\) is the dose for which the remaining photoresist height is 90% of the original value. The value of the contrast generally represents the response of the photoresist to radiation [18]. From the fabrication point of view, the most valuable information for a given patterning photoresist material is the optimal dose.