The high-reflectance phase-shifting mask (HR-PSM) is studied for patterning 36nm-pitch logic contact holes and compared with other mask absorbers in terms of imaging performance (ILS, LCDU, MEEF, etc.) and exposure dose. To this end, wafer-data-calibrated EUV resist models for CAR and MOR are used. Our simulation results show that a HRPSM produces dark-field images at large mask CD. However, as mask CD decreases, the tone of the images is reversed and bright-field images of good contrast can be generated. Based on this observation, a HR-PSM plus MOR patterning approach is proposed for through-pitch logic contact hole applications with a minimum pitch equal to 36 nm. We show that this approach demonstrates multiple enhancements in terms of through-pitch performance and enables us to extend the practical resolution of logic contact holes below the pitch of 40 nm using the 0.33NA EUV scanner.
We explore in this study imaging performance of horizontally oriented logic metal layer designs computationally, focusing on metal patterns with a pitch of 20nm (P20) and a tip-to-tip (T2T) range of 8- 35nm using 0.55 NA EUV single exposure. Simulations are performed by the method of source-mask optimization (SMO) using the TachyonTM software. The key assessed imaging parameters include the (normalized) image log-slope (NILS/ILS), overlapping depth of focus (DOF), and best focus (BF) shift. Simulation results show that traditional TaBN masks offer a reasonable overall contrast and DOF of both dense lines and through-pitch line patterns. Low-n masks offer benefits in contrast and dose. Low-n masks also show improvement in BF shift range, but such benefits vary with the extinction coefficient k as well as the pitch range and pattern selections. Other resolution enhancement techniques such as subresolution assistant features (SRAF) and bright-field imaging associated with metal-oxide-resists (MOR) also provide help to improve patterning performance. Metal designs with thick power rails have an impact on the contrast of neighboring patterns. Inner SRAF OPC and narrow power rails can effectively eliminate this impact. Patternable T2T size is type-dependent. Our study finds a flat ILS when T2T size is ≥15nm (for P20 lines) and a quick ILS drop when T2T size is < 15nm. It also reveals that complex T2T designs (e.g. staggered T2Ts) have lower contrasts. Additional care is needed in OPC other than avoiding such patterns in design. Our study confirms the potential replacement of 0.33 NA 3- to 4-time multipatterning by 0.55 NA single patterning.
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