The standard inspection flow typically consists of transmitted light pattern inspection (die-to-die or die-to-database) and STARlightTM (Simultaneous Transmitted And Reflective Light) contamination inspection. The initial introduction of TeraScan (DUV) inspection system was limited to transmitted pattern inspection modes. Hence, complete inspections of critical mask layers required utilizing TeraScan for maximized pattern defect sensitivity and the previous generation TeraStar (UV) for STARlightTM contamination inspection.
Recently, the reflective light die-to-database (dbR) inspection mode was introduced on the DUV tool to compliment transmitted light die-to-database (dbT) inspection. The dbR inspection mode provides not only pattern inspection but also contamination inspection capabilities.
The intent of this evaluation is to characterize the dbR inspection capability on pattern defects and contaminations. A series of standard programmed defect test plates will be used to evaluate pattern inspection capability and a PSL test plate will be used to determine the contamination performance. Inspection results will be compared to the current inspection process of record (dbT + STARlightTM).
Lastly, the learning will be used to develop and implement an optimal dbR inspection flow for selected critical layers of the 65-nm node to meet the inspection criteria and minimize the cycle time.
Progressive mask defect problem is an industry wide mask reliability issue. During the start of this problem when the defects on masks are just forming and are still non-critical, it is possible to continue to run such a problem mask in production with relatively low risk of yield impact. But when the defects approach more critical state, a decision needs to be made whether to pull the mask out of production to send for clean (repair). As this problem increases on the high-end masks running DUV lithography where masks are expensive, it is in the interest of the fab to sustain these problem masks in production as long as possible and take these out of production only when absolutely necessary; i.e., when the defects have reached such a critical condition on these masks that it will impact the process window. During the course of this technical work, investigation has been done towards understanding the impact of such small progressive defects on process window. It was seen that a small growing defect may not print at the best focus exposure condition, but it can still influence the process window and can shrink it significantly. With the help of a high-resolution direct reticle inspection, early detection of these defects is possible, but fabs are still searching for a way to disposition (make a go / no-go decision) on these defective masks. But it is not an easy task as the impact of these defects will depend on not only their size, but also on their transmission and MEEF. A lithographic detector has been evaluated to see if this can predict the criticality of such progressive mask defects.
As design rule continues to shrink towards ITRS roadmap requirements, reticle defect capture criteria are becoming ever more challenging. Pattern fidelity and reticle defects that were once perceived as insignificant or nuisance are now becoming a significant considerable yield impacting factor. More defects are also detectable and presented with increase in implementation of new generation reticle inspection systems. Therefore, how to review and characterize defects accurately and efficiently is becoming more significant. In particular, defect classification time often corresponds directly to the cost and the cycle time of mask manufacturing or new technology development.
In this study we introduce a new mask defect review tool called ReviewSmart, which retrieves and processes defect images reported from KLA-Tencor's high sensitivity TeraScan inspection tool. Compared to the traditional defect review method, ReviewSmart provides a much better method to manage defects efficiently by utilizing the concept of defect grouping disposition.
Through the application and qualification results with respectable reticle production cases, the implementation of ReviewSmart has been proven to be effective for reducing defect classification loading and improving defect characterizing efficiency. Moreover, the new review tool is helpful to categorically identify tool or process variations thus allowing users to expedite the learning process for developing production worthy leading node processes.
DUV lithography has introduced a progressive mask defect growth problem widely known as crystal growth or haze. Even if the incoming mask quality is good, there is no guarantee that the mask will remain clean during its production usage in the wafer fab. These progressive defects must be caught in advance during production in the fabs. The ideal reticle quality control goal should be to detect any nascent progressive defects before they become yield limiting. So a high- resolution mask inspection is absolutely needed, but then the big question is: "how often the fabs need to re-inspect their masks"?
A previous work towards finding a cost effective mask re-qualification frequency was done by Vince Samek et al. of IBM and Dadi Gudmundsson et al. of KLA-Tencor in 1999 [1], but this work was prior to the above mentioned progressive defect problem that industry started to see at a much higher rate during just the last few years.
In this present paper a realistic mask re-qualification frequency model has been developed based on the data from an advanced DRAM fab environment that is using low k1 lithography. Statistical methods are used to analyze mask inspection and product data, which are combined in a stochastic model.
For the 90nm-lithography node, understanding the impact of various reticle pinhole defects on wafer printability is essential to optimize wafer yield and to create the best quality reticle defect specification. In this study, a new programmed pinhole test reticle was designed by UMC, TCE and KLA-Tencor based on UMC's process requirements for its 193nm lithography. The reticle was manufactured and inspected on KLA-Tencor's high-resolution reticle inspection system in die to database mode by TCE. The reticle was then printed on a wafer by UMC to characterize the printability impact of programmed pinhole defects. The programmed pinhole test reticle, "193PTM", consists of two IC background patterns - poly gate and contact with programmed pinholes at various locations. The pinhole size ranges from 20nm to 75nm in 5nm increments on the wafer. By comparing the high-resolution pattern inspection results to the wafer print data, we have established the correlation and the appropriate mask specifications based on wafer application guidelines.
High-resolution contamination inspection for advanced reticles remains crucial in light of the increasing trend of progressive defects such as crystal growth, haze, fungus, precipitate etc., introduced with DUV lithography, especially for low k1 processes. In most fab environments, routine incoming and re-qualification inspections for photomasks have been implemented. But although this high-resolution inspection provides necessary high-sensitivity, on advanced photomasks it often introduces inspection challenges. Aggressive OPCs and dense primary and secondary geometries are some of the many factors that can result in false-defect problems for the inspection systems. Thus, inspection needs to be desensitized. As an effort to identify a methodology to provide the inspectability while maintaining the necessary high-sensitivity, a characterization has been performed to evaluate a new combination-mode inspection. This technical paper will list the details of this special contamination inspection technique that will allow users to maintain the same high inspection throughput while providing similar or higher resolution inspection for these advanced reticles with superior inspectability.
High-resolution contamination inspection for advanced reticles remains crucial in light of the increasing trend of progressive defects such as crystal growth, etc., introduced with DUV lithography, especially for low k1 processes. In most fab environments, routine incoming and re-qualification inspections for photomasks have been implemented. But although this high-resolution inspection provides necessary high-sensitivity, on advanced photomasks it often introduces inspection challenges. Aggressive OPCs and dense primary and secondary geometries are some of the many factors that can result in false-defect problems for the inspection systems. Thus, inspection needs to be desensitized. As an effort to identify a methodology to provide the inspectability while maintaining the necessary high-sensitivity, a characterization has been performed to evaluate a new combination- mode inspection. This technical paper will list the details of this special contamination inspection technique that will allow users to maintain the same high inspection throughput while providing similar or higher resolution inspection for these advanced reticles with superior inspectability.
The Verimask inspection standard is widely used to qualify inspection systems due to its simplicity, ease of use in a production environment, and readily understandable defect sensitivity table. The Verimask's major drawback is that it does not characterize the runability of an inspection system. Runability refers to the system's ability to inspect various pattern types, a critical characteristic of inspection systems used for mask manufacturing. Comprehensive inspection system capability evaluation should include both sensitivity and runability tests. Other inspection test masks suffer the same shortcoming of Verimask, providing simple sensitivity analysis without runability evaluation. The Universal Inspection Standard was developed to expand the Verimask's sensitivity test and to provide a runability test. The UIS runability module contains several typical industrial feature types at multiple technology nodes. We have used UIS to evaluate and benchmark inspection system and algorithms. Future UIS versions will be available with different feature and defect types to keep pace with inspection system development. In short, UIS provides a means to quantify an inspection system's runability in addition to the traditional sensitivity evaluation.
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