THIN SILICON-NITRIDE FILMS TO INCREASE RESOLUTION IN E-BEAM LITHOGRAPHY

A physical method of reducing feature size and proximity effects in su b-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 reso lution. The samples were patterned with a 50-keV, 15-nm-diam probe gen erated by a JEOL JBX-5DII e-beam lithography system. Point spread func tion 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 fil ms, 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 ea ch half of the sample. On examination of the samples after postexposur e processing and development, reduced feature sizes and proximity effe cts 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 co uld only be successfully resolved when the intermediary nitride layer was present. Monte Carlo simulations were performed on a CM-200 connec tion machine. The results show a large number of fast secondary electr ons 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 electro ns entering the resist is reduced by the silicon nitride layer. Simula tions compare the thin-layer technique to a bilayer resist technique, used to improve resolution at larger dimensions.