Predictability and impact of product layout induced topology on across-field focus control

Abstract

With continuing dimension shrinkage using the TWINSCAN NXT:1950i scanner on the 28nm node and beyond, the imaging depth of focus (DOF) becomes more critical. Focus budget breakdown studies [Ref 1, 5] show that even though the intrafield component stays the same this becomes a larger relative percentage of the overall DOF. Process induced topography along with reduced Process Window can lead to yield limitations and defectivity issues on the wafer. To improve focus margin, a study has been started to determine if some correlations between scanner levelling performance, product layout and topography can be observed. Both topography and levelling intrafield fingerprints show a large systematic component that seems to be product related. In particular, scanner levelling measurement maps present a lot of similarities with the layout of the product. The present paper investigates the possibility to model the level sensor's measured height as a function of layer design densities or perimeter data of the product. As one component of the systematics from the level sensor measurements is process induced topography due to previous deposition, etching and CMP, several layer density parameters were extracted from the GDS's. These were combined through a multiple variable analysis (PLS: Partial Least Square regression) to determine the weighting of each layer and each parameter. Current work shows very promising results using this methodology, with description quality up to 0.8 R² and expected prediction quality up to 0.78 Q². Since product layout drives some intrafield focus component it is also important to be able to assess intrafield focus uniformity from post processing. This has been done through a hyper dense focus map experiment which is presented in this paper.

Verification and application of multi-source focus quantification

“…These two parts can be processed separately by the leveling algorithm, and then algebracally summed to generate the wafer stage positioning setpoints. Additionally, in our previous communication [1], and sections above, we showed that layout design density modelled topography map reasonably correlate with the level sensor measured local intrafield topography or even high-resolution topography measurement with external tools (e.g. Wyko).…”

“…In this paper, the same methodology as the one described above (in part II) and in the SPIE 2015 paper [1] was applied in order to link the GDS to offline topography measurements. These were done on a Veeco WYKO NT9300 tool in LETI without the photolithography process stack [8].…”

“…Major contributors to interfield focus are scanner fingerprints [1], edge effects [7] and leveling non-correctable of the wafer topography [2, 3, 4, and 5].…”

“…The concept of the multi-source focus correlation method was presented in 2015 [1,2]. A more accurate understanding of real on-product focus can be obtained by gathering information from different sectors: design, scanner short loop monitoring, scanner leveling, on-product focus and topography.…”

“…A more accurate understanding of real on-product focus can be obtained by gathering information from different sectors: design, scanner short loop monitoring, scanner leveling, on-product focus and topography. In reference [1], the link between scanner monitoring and on-product focus was established as well as the correlation between design and the scanner level sensor measured intra field non-correctable errors after leveling. In [2], the on-product focus to topography correlation has been studied and the concept of smart leveling was proposed.…”