8 March 2016 Simulation of shotnoise induced side-wall roughness in electron lithography
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We have developed a fast three dimensional Monte-Carlo framework for the investigation of shotnoise induced side-wall roughness (SWR) formation. The calculation outline is demonstrated by an example for an exposure of a 100nm thick layer of negative tone resist (NTR) resist on top of an infinitely thick silicon substrate. We use our home built Monte-Carlo simulator for electron-matter interaction for the purpose of lithography. A pattern of an isolated line is written into the resist layer by scanning a beam with 20 keV electrons over an area of 32nm×1μm (width and length). During the exposure, we use a spot size of 20 nm, beam step size of 4nm and a Poisson distributed exposure dose of 80 μC/cm2, 60 μC/cm2 and 40 μC/cm2. During the exposure of the sample, we record the locations of the inelastic events within the resist layer. The distribution of released acids is determined under the simplified assumption that every inelastic event corresponds to a release. We now construct a three dimensional image of the (in)solubility of the resist layer within a cuboid of 128 nm(256px) wide, 800 nm(1024px) in length and 100 nm(128px) in height. It is obtained by summing the contribution of all acids to every voxel in the three dimensional image. We have used a three dimensional Gaussian with σx,y,z = rd =5nm for the diffusion of the acid. The boundary between exposed and unexposed resist is determined by a threshold. The resulting image of the (in)solubility is analyzed in different ways by considering slices and three dimensional views of the border. The average line edge roughness (LER) is obtained by calculating the standard deviation (one-sigma) of the left and right border from yz-slices. By considering all slices, ranging from the top of the resist layer to the bottom of the substrate, the average LER as a function of the depth from the top surface of the resist layer is obtained. Shotnoise effects are observed as we decrease the exposure dose. An increased effect of shotnoise is observed near the vacuum and substrate interface. One contribution relates to the actual number of acids, which due to the scattering is less near the interface than away from the interface. Another contribution stems from the fact that no acids are found on the vacuum side nor on the substrate side.
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T. Verduin, T. Verduin, S. R. Lokhorst, S. R. Lokhorst, C. W. Hagen, C. W. Hagen, P. Kruit, P. Kruit, "Simulation of shotnoise induced side-wall roughness in electron lithography", Proc. SPIE 9778, Metrology, Inspection, and Process Control for Microlithography XXX, 97781Z (8 March 2016); doi: 10.1117/12.2219295; https://doi.org/10.1117/12.2219295

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