Optimisation of HSQ e-beam lithography for the patterning of FinFET transistors

FinFET transistors are presented as promising candidates to extend CMOS lifetime. One major challenging technological step in the process fabrication of such devices is the patterning of narrow, sharp, densely packed silicon fins. As an attempt to face this challenge, hydrogen silsesquioxane (HSQ) is used as a negative tone e-beam resist. This paper presents a study on the pre- and post- e-beam exposure process of the HSQ resist to limit the impact of proximity effects. The post application bake (PAB) and the concentration of tetramethyl ammonium hydroxide (TMAH) in the developer solution have been investigated. It is found that contrast can be improved by 7× using a lower pre-bake temperature (80 °C) and a more concentrated TMAH solution (25%). The impact of contrast improvement on the control of the critical dimension (CD) is also studied. Cross-sectional scanning electron microscope (SEM) views show fin networks with a pitch ranging from 40 to 200 nm. The HSQ line profiles are steep with top-angles measured near 90° and an excellent uniformity across the whole network is obtained. The HSQ lines have also been successfully transferred to a silicon layer by dry etching using a SF6/N2/O2 based chemistry. The anisotropy of the etching process is demonstrated by measuring the top-angle of the lines. The process proposed in this paper enables the fabrication of very dense patterns such as silicon fins as thin as 15 nm with 25 nm spaces between lines.