Chrome migration or aging phenomenon is known for 193nm binary photomasks since a few years. 193nm irradiations and time generate an oxide growth on chrome sidewalls and then cause a non-uniform increase of critical dimensions (CD) , , [3, . If not prevented or detected early enough, wafer fabs are likely to face process drifts, defectivity issues and even lower yield on wafers in the worst cases. Fortunately, some solutions have been put in place in the industry. A standard cleaning and repel service at the maskshop has been demonstrated as efficient to remove the grown materials and get the mask CD back on target. Some detection methods have been already described in literature, such as wafer CD intrafield monitoring (ACLV) , giving reliable results but also consuming additional SEM time with less precision than direct reticle measurement. Another approach is to monitor the CD uniformity directly on the photomask, concurrently with defect inspection for regular requalification to production for wafer fabs . This enables ultimately to trigger the preventive cleanings rather than on predefined thresholds. However, may the 193nm Phase Shift Masks (PSM) be impacted too? In other words, should wafer fabs pay attention to this form of aging? Indeed, some publications , ,  report a growth of SiO2, leading to the development of a high duration MoSi (modification of MoSi composition). This study will characterize the aging behaviour on a 193nm PSM contact hole layer, 40nm logic technology node. During this study, the aging phenomenon has been accelerated with the use of a test bench, to reach a CD increase up to 11nm after a cumulated exposure dose of 10kJ/cm2 (equivalent to exposures of >32,000 wafers 300mm). Two dice were compared, one kept as reference without any exposure, whereas the other die was aged on the accelerated test bench. Exhaustive characterization has been performed, with CD measurements on the mask and on wafers, evaluation of lithography process windows for usual patterns and most critical features (Optical Proximity Correction hotspots). It appears that despite a consistent CD increase on the mask, the impact on wafer can be neglected, at least at this amount of exposures. Aerial CD were also analysed through a Zeiss WLCDTM to enable a prediction of wafer impact. An advanced inspection tool (KLA-Tencor X5.2 model) has been challenged as an inline monitoring method to detect the aging degradation on PSM. The Intensity Critical Dimension Uniformity option (iCDUTM) was firstly developed to provide feed-forward CDU maps for scanners intrafield corrections, from arrayed dense structures on memory masks. Due to layout complexity and differing feature types, CDU monitoring on logic masks used to pose unique challenges. CDU monitoring on logic masks is now available, the latest Delta-Die and Delta-Time options gives all the needed information, as shown in this paper. In this study, iCDU has demonstrated its ability to catch a slight degradation of CD uniformity. In the end, this study shows evidences that standard cleanings used in maskshops cannot recover the mask back to its original CD. Finally, Transmission Electron Microscopy (TEM) was used to confirm the chemical nature of the grown material on sidewalls. TEM cuts provide a comparison between a production mask (aging over many years in production) and the test mask (accelerated aging on a test bench).
As Moore's law drives the semiconductor industry to tighter specifications, challenges are becoming real for overlay metrology. A lot of work has been done on the metrology tool capability to improve single-tool precision, tool-to-tool matching and Tool-Induced Shift (TIS) variability. But nowadays these contribute just a small portion of the Overlay Metrology Error (approximately 10% for 90nm technology). Unmodeled systematic, scanner noise and process variation are becoming the major contributors. In order to reduce these effects, new target design was developed in the industry, showing improvements in performance. Precision, Residual analysis, DI/FI (Develop Inspection / Final Inspection) bias and Overlay Mark Fidelity (OMF) are common metrics for measurement quality. When we come to measurement accuracy, we do not have any direct metric to qualify targets.
In the current work we evaluated the accuracy of different AIM (developed by Kla-Tencor) and Frame-In-Frame (FIF) targets by comparing them to reference “SEM” targets. The experiment was conducted using a special designed 65nm D/R reticle, which included various overlay targets. Measurements were done on test wafers with resist on etched poly printed on 248nm scanner.
The results showed that, for this "straight-forward" application, the best accuracy performance was achieved by the Non Segmented (NS) AIM target and was estimated in the order of 1.5 nm site-to-site. This is slightly more accurate than hole-based target and far more than NS FIF target in this particular case. When using the non-accurate NS FIF target, correctable parameters and maximum overlay prediction error analysis, showed up to 24nm overlay error at the edge of the wafer. We also showed that part of this accuracy error can be attributed to the non-uniformity of BARC deposition.