Actinic Mask Inspection at the ALS Initial Design Review
This report is the first milestone report for the actinic mask blank inspection project conducted at the VNL, which forms sub-section 3 of the Q1 2003 mask blank technology transfer program at the VNL. Specifically this report addresses deliverable 3.1.1--design review and preliminary tool design. The goal of this project is to design an actinic mask inspection tool capable of operating in two modes: high-speed scanning for the detection of multilayer defects (inspection mode), and a high-resolution aerial image mode in which the image emulates the imaging illumination conditions of a stepper system (aerial image or AIM mode). The purpose and objective of these two modes is as follows: (1) Defect inspection mode--This imaging mode is designed to scan large areas of the mask for defects EUV multilayer coatings. The goal is to detect the presence of multilayer defects on a mask blank and to store the co-ordinates for subsequent review in AIM mode, thus it is not essential that the illumination and imaging conditions match that of a production stepper. Potential uses for this imaging mode include: (a) Correlating the results obtained using actinic inspection with results obtained using other non-EUV defect inspection systems to verify that the non-EUV scanning systems are detecting all critical defects; (b) Gaining sufficient information to associate defects with particular processes, such as various stages of the multilayer deposition or different modes of operation of the deposition tool; and (c) Assessing the density and EUV impact of surface and multilayer anomalies. Because of the low defect density achieved using current multilayer coating technology it is necessary to be able to efficiently scan large areas of the mask in order to obtain sufficient statistics for use in cross-correlation experiments. Speed of operation as well as sensitivity is therefore key to operation in defect inspection mode. (2) Aerial Image Microscope (AIM) mode--In AIM mode the tool is configured so that the collected data emulates the aerial image of a stepper system, thereby enabling rapid evaluation of mask defects and patterning without the need for a resist exposure step. The main uses of the microscope in this mode would be: (a) Review of multilayer and pattern defects to determine their printability; (b) Defect review following a repair process to assess the success of the operation; (c) Investigation of the effects of illumination and NA on the printed image; (d) Process window analysis of defects and other mask features; and (e) Characterizing defects on both patterned and unpatterned masks (i.e. blanks). An essential characteristic of operation in this mode is that the illumination and imaging conditions through focus should emulate as accurately as possible that of a production stepper system. This mode is designed for local review of defects over a small sub-field of the mask; therefore it is not necessary to have the same high-speed throughput required for defect inspection mode. We first describe some technical background relating to EUV masks and defect scanning, with the aim of defining the context of the tool and experiments to be performed. We then present an overview of several candidate optical system configurations for achieve the above objectives, and analyze the ability of each system to achieve the stated project goals. Of key importance is throughput and sensitivity in inspection mode, whilst in AIM mode key points are spatial resolution and the ability to perform stepper emulation imaging. We then down-select the best candidate from this set of solutions to one system, which is then investigated in further detail with a view to producing a preliminary tool design and estimates of overall system performance.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15003862
- Report Number(s):
- UCRL-CR-152460; TRN: US1005096
- Country of Publication:
- United States
- Language:
- English
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