Resist sensitivity and thickness-based beam count optimization for parallel low energy E-beam exposure systems

Abstract

Proximity effects during electron beam exposure have been kept under control by using sophisticated correction algorithms and software, combined with a strategy which aims at increasing the electron beam energy to 50 keV and 100 keV. At these energies, the proximity effects are more uniform and provide a situation where they are easier to correct. However, as feature sizes shrink, and the pattern density increases, this task becomes extremely complex, since tolerances to pattern definition errors are becoming more restricted. An alternate approach is to move to lower electron energies where proximity effects become negligible. Several programs are underway to develop massively parallel electron beam (MPEB) writer systems that have greatly reduced energy in the ≤5keV regime. Selection of the electron beam energy becomes critical below 10 keV, since the tolerance window where proximity effects are indeed negligible is very small. A shot noise model has been elaborated providing minimum exposure doses required for resists at technology nodes of 45 nm and below. These doses increase rapidly with reducing linewidth and impose a minimum number of electron beams for MPEB systems in order to be able to pattern a surface corresponding to a standard full field 6 inch reticle in a reasonable time, and to directly pattern 300 mm wafers at rates of 5, 50 and 100 wafers per hour. An overall set of results is obtained indicating minimum number of electron beams and electron beam current. CONTEXTThe International Technology Roadmap for Semiconductors[1] places stringent requirements on the lithography required to address the next generation of technology nodes for integrated circuit fabrication. Figure 1 illustrates in a summary fashion the required linewidths in resist for the future technology nodes as well as the acceptable limit on linewidth roughness (LWR, which is restricted to less than 8% of the resist linewidth). It can be seen that for 10 nm linewidths in resist, corresponding to the 18 nm node, LWR must be less than 1 nm. This places severe constraints on the lithography process, requiring control of the process far exceeding what is achievable today.Electron beam lithography (EBL) has become a required technology for patterning high resolution photomasks and remains a candidate technology for next generation lithography through direct-write patterning as well as in the form two possibilities for projection and proximity lithography [1]. In addition to the continued development of high energy electron beam lithography systems (typically at the 75 to 100 keV energy range in order to obtain best overall performance), several massively parallel low energy electron beam lithography systems have been under study. This approach presents one potential solution to the long write times associated with EBL for large patterns: large numbers of electron beams are controlled independently and simultaneously in order to bring a major reduction in the overall write time of any given pattern on a wafer or mask blank.However, as the c...