Depth of focus in high-NA EUV lithography: a simulation study

Abstract

High-NA EUV lithography tools are projected to be deployed in HVM by 2025/2026, with the introduction of the N1.4 node. This should allow single-exposure EUVL at 30 nm pitch and below and reduce cost and complexity. One of the main challenges in process control will be the budgeting of depth of focus. With Rayleigh’s equations in mind and considering a relatively thick photoresist needed for etching purposes, an overall focus window of less than 40 nm is expected for high-NA EUV exposures. This is concerning, considering that after almost two decades at roughly constant depth of focus (~100 nm for 1.35 NA ArF immersion and 0.33 NA EUV), we may potentially be dealing with 2-3X lower usable focus range. On top of that - and differently from the DUV case - increasing NA in EUVL exacerbates the impact of complex 3D mask effects which, in a non-telecentric tool with an oblique incidence at the mask, negatively impact best focus shifts across mask pitch and feature CD. In this paper, we look at how to mitigate lines/spaces depth of focus loss across pitch by exploring source, mask pattern and mask stack optimization. Moreover, we compare the best strategy between bright/dark exposures, mask transmissivity, and phase-shifting combined with assist features, resist thickness, and feature orientation. Finally, we compare the optical performance with defect-aware process windows at 0.33 vs. 0.55 NA based upon stochastic resist calibration for generic positive-tone organic CAR, and negative-tone inorganic platforms.

Depth of focus in high-NA EUV lithography: the role of assist features and their variability budget

“…It should be mentioned that bright field maks show a more generous process window with tighter PPE control across focus, as previously reported for 22nm minimum pitch [2]. However, such masks are still difficult to apply in manufacturing due to mask blank defectivity concerns.…”

“…Moreover, stochastic effects are expected to be more prominent at the dimensions that high NA EUV is targeting, which could cause a further reduction of the focus window. In an earlier report [2], we demonstrated how the illuminator, mask stack, mask tonality, and assist feature placement can be used for maintaining a reasonable DoF for process control when targeting a minimum pitch of 22nm. We demonstrated how pattern placement is especially sensitive to subtle changes in illuminator and mask design (Figure 1).…”

EUV APSM mask prospects and challenges

“…The simulation in Figure 2 demonstrates the impact of depth of focus (DOF) variation on the exposure latitude (EL) of both APSM and binary masks 9,10 . Compared to the binary mask, APSM exhibits a larger EL across all DOF values.…”

Pattern deformation mitigation for EUV photoresists using wafer backside cleaning techniques

“…This implies that, for instance, if the DOF at NA 0.33 is 100 nm, at NA 0.55, the DOF would be ∼30-40 nm. [21][22][23][24][25] Under these exposure conditions, concerns arise about wafer surface distortions caused by unintended particles or objects on the wafer backside, a consideration absent in previous generations of lithography, such as ArF immersion. 26,27)…”