The cleaning requirements for EUV masks are more complex than optical masks due to the absence of available EUV-compatible pellicles. EUV masks must therefore be capable of undergoing more than 100 cleaning cycles with minimum impact to lithographic performance. EUV masks are created on substrates with 40 multilayers of silicon and molybdenum to form a Bragg reflector, capped with a 2.5nm-thick ruthenium layer and a tantalum-based absorber; during usage, both ruthenium and absorber are exposed to the cleaning process. The CrN layer on the backside is used to enable electrostatic clamping. This clamp side must also be free of particles that could impact printing and overlay, and particles could also potentially migrate to the frontside and create defects. Thus, the cleaning process must provide decent particle removal efficiencies on both front-and backside while maintaining reflectivity with minimal surface roughness change. In this paper, we report progress developing a concurrent patterned-side and clamped-side cleaning process that achieves minimal reflectivity change over 120 cleaning cycles, with XPS and EDS indicating the presence of ruthenium after 125 cleaning cycles. The change in surface roughness over 100 cleaning cycles is within the noise (0.0086nm) on a mask blank, and SEM inspection of 100nm and 200nm features on patterned masks after undergoing 100 cleaning cycles show no indications of ruthenium pitting or significant surface damage. This process was used on test masks to remove particles from both sides that would otherwise inhibit these masks from being used in the scanner.

Defect monitoring is increasingly required for advanced line maintenance. The resulting information from wafer inspection can be used in two ways, (1) excursion detection, which requires an appropriately measured response, and (2) pro-active process control. For the first, a critical decision is how to proceed with lot deposition if an excursion is detected. A methodology based on post-lithographic defect inspection and defective die count analysis was employed which provided effective process monitoring and yield maintenance. The methodology allows rapid decision-making with a minimum of information for post-litho lot disposition. For the second application of proactive control, the wafer inspection was utilized for dose forecasting with a feedforward mode used to appropriately modify the dose for optimization. Good line control and yield maintenance were observed as a result of the both the lot-disposition and dose forecasting. The purpose of the lot disposition is to separate significant excursions from temporary fluctuations in order to appropriately focus defect reduction resources. Wafers are systematically inspected post-litho with a patterned wafer inspection system, the WF736, and the number of die with killer defects is counted and then monitored with time. Every lot is inspected, full wafer inspection is performed, and all defect types are captured. By determining the killer defect progression after re-work, it is possible to establish lot disposition. If the count is still high, defect reduction analysis is then applied. Various defects were flagged and addressed, arising both from litho and from prior steps

Defect monitoring is increasingly required for advanced line maintenance. A critical decision is how to proceed with lot deposition if an excursion is detected. A methodology based on defect inspection and defective die count analysis was employed which provided effective process monitoring and yield maintenance. The methodology allows rapid decision-making with a minimum of information for lot disposition. The purpose is to separate significant excursions from temporary fluctuations in order to appropriately focus defect reduction resources. Wafers are systematically inspected post-litho with a patterned wafer inspection system, the WF736, and the number of die with killer defects is counted and then monitored with time. Every lot is inspected, full wafer inspection is performed, and all defect types are captured. By determining the killer defect progression after re-work, it is possible to establish lot disposition. If the count is still high, defect reduction analysis is then applied. Various defects were flagged and addressed, arising both from lithe and from prior steps. In addition, the wafer inspection was utilized for dose forecasting with a feed-forward to appropriately modify the optimal dose. Good line control and yield maintenance were observed.

Wafer inspection providing classified defect density with “on-the-fly” automatic defect classification (OTF-ADC) is increasingly being utilized to obtain the detailed defect information for rapid resolution of defectivity issues. The OTF-ADC defect categories allow the critical defect types to be segregated and tracked during the actual wafer inspection without any throughput reduction. Inspection with OTF-ADC can be utilized at various interconnect steps such as post-develop, post-etch, and post-metal-CMP step for advanced process development and line monitoring. Significant benefits are realized with early defect excursion detection by tracking the classified defect counts instead of simply the total defect counts. An associated requirement is the ability to capture all of the relevant defect types at key inspection points. Utilizing the Applied Materials WF736 with a combined bright-field and dark-field architecture, the full range of yield limiting defects was captured at two key interconnect stages: post litho, and post CMP. After-development inspection (ADI) with the WF found both macro (>15 um) and micro (<15 u) defects. In some instances, the micro-defects could only be imaged with a SEM. The OTF-ADC provided good accuracy for both ADI and CMP. Using an example from post metal CMP, OTF-ADC is shown to identify hidden excursions which would have had significant yield impact and which would have gone undetected with standard “total count” inspection methods. The use of OTF-ADC provides improved time-to-information and enhances the yield potential by permitting defect sourcing which facilitates corrective action