THIN SILICON-NITRIDE FILMS FOR REDUCTION OF LINEWIDTH AND PROXIMITY EFFECTS IN ELECTRON-BEAM LITHOGRAPHY

A thin silicon nitride layer (50-300 nm), deposited on a semicoductor substrate, prior to resist deposition, greatly enhances the resist resolution during electron beam lithography. The resolution enhancement was manifested by smaller feature sizes, for a given dose and full resolution of individual array elements at higher doses than on a bare semiconductor substrate. The effect has been observed in 50-100 nm of both SAL-601 and PMMA spun onto silicon nitride coated Si and GaAs substrates. The samples were patterned with a 50 keV, 15 nm diam probe generated by a JEOL JBX-5DII system. Improved resolution was found on two types of silicon nitride film grown by chemical vapor deposition: one deposited at 800-degrees-C on Si and the other deposited at 200-degrees-C on GaAs. Linewidth reductions in SAL-601 of 40% at low doses and an order of magnitude at high doses were observed on silicon nitride coatings of 50-300 nm thickness. In PMMA, the resolution enhancement was less than in SAL-601, with only a 15% linewidth reduction observed at high doses (greater-than-or-equal-to 10 nC/cm). A reduction in proximity effects due to the presence of the nitride layer, is apparent in SAL-601, for array periods of 1 mum and less. Analysis of the results, combined with Monte Carlo simulations and electron microscopy suggests that fast secondary electrons, generated by the substrate are prevented by the silicon nitride layer from exposing the resist.