REDUCTION AND ELIMINATION OF PROXIMITY EFFECTS

The physical processes limiting resolution and causing proximity effects in electron-beam lithography (EBL) are outlined, with the major emphasis on sub-250 nm lithography. A reduction of proximity effects and the enhancement of resolution in the resist can be achieved with a thin intermediate layer (such as 50-300 nm of silicon nitride or silicon dioxide) between the resist and the substrate. This has been observed on a number of substrate's (Si, GaAs, W, and InSb) and with two high-resolution e-beam resists. Monte Carlo simulations and experimental results show that secondary electrons play a key role in the understanding of this phenomenon. Furthermore, a large number of fast secondaries (500 eV-3 keV), emitted from substrate-resist interface, enter the resist within a approximately 100 nm radius of the primary beam. The thin intermediate layers reduce the number of secondary electrons entering the resist. In contrast, the elimination of proximity effects was observed with very low energy electron beams (< 100 eV) as can be formed by a scanning tunneling microscope (STM), operated in the field emission mode. STM lithography gives a highly localized exposure, which results in very high resist resolution over a wide process latitude. The elements of an implementation of very low voltage EBL are outlined.