Thin silicon nitride films to increase resolution in e-beam lithography

Abstract

A physical method of reducing feature size and proximity effects in sub-quarter-micrometer e-beam lithography is described. A thin layer (50 to 300 nm) of silicon nitride deposited on a semiconductor substrate, prior to resist deposition, has been found to enhance the resist resolution. The samples were patterned with a 50-keV, 15-nm-diam probe generated by a JEOL JBX-5Dll e-beam lithography system. Point spread function measurements in 60-nm-thick SAL-601 on Si are shown to illustrate the resolution enhancement in the nanolithographic regime (sub-100 nm). The technique has been applied to lithography on 400-nm-thick W films, such as would be used in x-ray mask fabrication. The 200 nm of SAL-601 was spun onto W film samples, which were half-coated with 200 nm of silicon nitride. Identical lithographic patterns were written on each half of the sample. On examination of the samples after postexposure processing and development, reduced feature sizes and proximity effects were seen on the sample half with the silicon nitride intermediary layer. For example, in a field effect transistor (FET) type pattern, with a coded 500-nm gap between the source and drain pads, the gate could only be successfully resolved when the intermediary nitride layer was present. Monte Carlo simulations were performed on a CM-200 connection machine. The results show a large number of fast secondary electrons are generated within a 100-nm radius of the incident electron beam. The implications of fast secondary electrons on resolution in e-beam lithography are discussed. The total number of fast secondary electrons entering the resist is reduced by the silicon nitride layer. Simulations compare the thin-layer technique to a bilayer resist technique, used to improve resolution at larger dimensions.