Yuichi Shibazaki, Nikon Fellow and Litho Business Development Sector Manager, previewed the next-generation Nikon immersion scanner and the inline Alignment Station (iAS) at LithoVision 2017. Shibazaki-san opened his presentation by commenting that major Nikon scanner innovations have been implemented roughly every five to six years in order to boost tool performance and drive down chip costs/process effort. However, this often came with trade-offs in tool cost and process complexity. Meanwhile, as that process complexity has increased, it has brought with it complications that impact overlay and edge placement error (EPE) performance, requirements for which are being tightened. To better meet customer needs, Nikon has developed strategic products specifically focused on optimizing alignment to enable cost-effective lithography scaling.
Shibazaki-san introduced the next-generation Nikon immersion scanner, which integrates the inline Alignment Station (iAS) (Figure 1). The new system offers better alignment and overlay while still increasing throughput to maintain cost effectiveness. He contrasted the current-generation NSR-S631E scanner, which enables 250 wafers per hour (WPH) throughput using 16 point alignment measurement and global linear correction, with the advanced capabilities of the next-generation system. Using iAS, the next-generation immersion scanner can process 270 WPH while performing all shot alignment with four sophisticated shot shape modes, and providing global high order correction and shot linear correction (Figure 2A).
The iAS is a high speed, extremely accurate wafer pre-measurement module that provides feed forward alignment results for all shots on every wafer. Pre-measurement, dense sampling, and high order grid correction is performed by iAS, then traditional enhanced global alignment (EGA) is performed on the scanner wafer stage using sparse sampling and final linear correction. Since the method requires removal and re-chucking of the wafer from the iAS to the scanner, one immediate question is whether that step affects results. Nikon tests show that chucking repeatability is extremely well controlled. Minimal differences were observed between chuck/de-chuck and fiducial vs. product wafer alignment reproducibility measurements with non-linear 3σ consistently ≤ 0.4 nm (Figure 2B). In addition, the difference (3σ nm) between the wafer grid measured by the iAS and the grid measured by the scanner showed a constant offset that was independent of the wafers (Figure 3).
iAS has demonstrated excellent capabilities on distorted grid wafers, using high order correction capabilities and dense sampling to dramatically reduce overlay error. One evaluation using ten wafers having five different error types showed overlay results improved from 4.5 nm 3σ using 16 point EGA with linear correction only, to below 2 nm using 84 point EGA and 7th order correction with iAS (Figure 4A). Marked performance improvements have also been achieved using the advanced iAS die-by-die corrections. Testing showed 10 nm overlay 3σ results with 16 point EGA performance and linear correction, compared to 1.6 nm using 636 point EGA and die-by-die correction (Figure 4B).
The next-generation Nikon immersion scanner with integrated iAS makes comprehensive alignment data collection and high order adjustment viable for manufacturing. This successful pairing optimizes on-product overlay using all-shot alignment, without any impact to scanner throughput. iAS can also be retrofit to a number of existing scanner models, with minimal effect on the fab footprint. Shibazaki-san stated that these strategic alignment solutions “…expand the lithography horizon.”