Frank Staals

Advanced wavefront engineering for improved imaging and overlay applications on a 1.35 NA immersion scanner

In this paper we describe the basic principle of FlexWave, a new high resolution wavefront manipulator, and discuss experimental data on imaging, focus and overlay. For this we integrated the FlexWave module in a 1.35 NA immersion scanner. With FlexWave we can perform both static and dynamic wavefront corrections. Wavefront control with FlexWave minimizes lens aberrations under high productivity usage of the scanner, hence maintaining overlay and focus performance, but moreover, the high resolution wavefront tuning can be used to compensate for litho related effects. Especially now mask 3D effects are becoming a major error component, additional tuning is required. Optimized wavefront can be achieved with computational lithography, by either co-optimizing source, mask, and Wavefront Target prior to tape-out, or by tuning Wavefront Targets for specific masks and scanners after the reticle is made.

Verification and application of multi-source focus quantification

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. This work will show that chip topography can be predicted from reticle density and perimeter density data, including experimental proof. Different pixel sizes are used to perform the correlation in-line with the minimum resolution, correlation length of CMP effects and the spot size of the scanner level sensor. Potential applications of the topography determination will be evaluated, including optimizing scanner leveling by ignoring non-critical parts of the field, and without the need for time-consuming offline topography measurements.

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

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.

Intra-field patterning control using high-speed and small-target optical metrology of CD and focus

The continuing trend of shrinking dimension and the related specifications requires tightening of control loops. To support the tighter control loops, the metrology sampling plans will require increasing sampling density and frequency. This study shows that tighter control of scanner focus and AEI CD and profile parameters requires sampling schemes with intra-field measurement points, and a frequent update of the corrections. This drives the need for high-speed smalltarget metrology. Experiments show that this can be achieved by the YieldStar angle resolved scatterometer, demonstrating measurements on areas of 16×16μm2 for focus metrology and 12×12μm2 for CD metrology, at a MAM time below 0.5s.

Product layout induced topography effects on intrafield levelling

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 2, 5] show that even though the intrafield component stays the same, it 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. In a previous paper, the feasibility of anticipating the scanner levelling measurements (Level Sensor, Agile and Topography) has been shown [1]. This model, built using a multiple variable analysis (PLS: Partial Least Square regression) and GDS densities at different layers showed prediction capabilities of the scanner topography readings up to 0.78 Q² (the equivalent of R² for expected prediction). Using this model, care areas can be defined as parts of the field that cannot be seen nor corrected by the scanner, which can lead to local DOF shrinkage and printing issues. This paper will investigate the link between the care areas and the intrafield focus that can be seen at the wafer level, using offline topography measurements as a reference. Some improvements made on the model are also presented.

Improving focus performance at litho using diffraction-based focus metrology, novel calibration methods, interface, and control loop

In advanced optical lithography the requirements of focus control continues to tighten. Usable depth of focus (DoF) is already quite low due to typical sources of focus errors, such as topography, wafer warpage and the thickness of photoresist. And now the usable DoF is further decreased by hotspots (design and imaging hotspots). All these have put extra challenges to improve focus metrology, scanner focus stability calibrations and on-product correction mechanisms. Asymmetric focus targets are developed to address robustness in focus measurements using diffraction-based focus (DBF and μDBF) metrology. A new layout specific calibration methodology is introduced for baseline focus setup and control in order to improve scanner focus uniformity and stability using the measurements of the above mentioned asymmetric targets. A similar metrology is also used for on product focus measurements. Moreover, a few novel alternative methods are also investigated for on-product focus measurements. Data shows good correlation between DBF and process on record (POR) method using traditional FEM. The new focus calibration demonstrated robustness, stability and speed. This technical publication will report the data from all the above activities including results from various product layers.