Carbonaceous contamination growth induced by resist outgassing under e-beam exposure

Abstract

The emerging lithography tools with large numbers of parallel electron beams and direct-write capabilities provide an alternative solution to achieve high-resolution and high-throughput lithography for advanced CMOS nodes [Pain et al., C. R. Acad. Sci., Ser. IIB 7, 910 (2006); de Boer et al., Proc. SPIE 8680, 86800O (2013)]. However, resist outgassing during exposure induces carbonaceous contamination of the optics projection systems. This contamination is a major issue, much more than in classical electron-beam lithography, as the outgassing level is much higher. Thus, it is crucial to study both the outgassing phenomena and the associated contamination growth mechanisms, to assess resist platforms and define realistic specifications in order to limit their impact on the exposure systems. A specific experimental setup has been designed at Leti to perform 5-keV electron bombardment of e-beam resist-coated 100-mm silicon wafers. The setup allows two different configurations: an outgassing measurement mode and a contamination mode. In the outgassing measurement mode, the induced outgassing is monitored with a quadrupole mass spectrometer. In the contamination mode, the wafer is exposed through a silicon micromachined membrane (called a mimic) that simulates the projection optics system of the multi-ebeam exposure tool. In this case, the wafer stage allows suitable displacements of wafers to expose the resists to the targeted dose, thus inducing carbonaceous contamination growth on mimic edges. Outgassing rates of resists under electronic exposure have been studied in a previous work [Mebiene-Engohang et al., Proc. SPIE 9049, 90492Q (2014)]. This paper will focus, rather, on the kinetics of the contamination growth on the mimic, as a function of current density during exposure, contaminant partial pressures surrounding the mimic, and potential presence of a top-coat designed to prevent outgassing. The contamination of the mimic induced by outgassed compounds during exposure was characterized using techniques such as scanning electron microscopy and x-ray photoelectron spectrometry.