Contamination monitoring and control on ASML MS-VII 157-nm exposure tool

This paper presents results of monitoring and control of contaminants in an ASML MS-VII 157nm full-field exposure tool at IMEC, as verified lithographically in terms of field uniformity, lens transmission, CD uniformity, and scattered light. The daily contamination monitoring system utilizes in-line photo-ionization detector, oxygen and moisture analyzers, as well as chemiluminescent detector, and gas chromatograph that is coupled to a mass spectrometer. On a monthly basis, contamination monitoring was performed with thermal desorption-gas chromatographi/mass spectrometric techniques. The following four locations within the optical path of the MS-VII are monitored: source optic assembly, condenser lens optic, 1X relay station, and projection optics box. Contamination control is realized in the system with an on-board purge control unit, which is equipped with gas purifiers that remove contaminants such as H 2 O, O 2 , CO, CO 2 , hydrocarbons, H 2 , and sulfur compounds. All the observed contaminants have been trending within expected values and no contamination-related tool performance degradation has been observed. The excursions observed in the contaminant concentrations are coincident with tool downtime/maintenance events. Siloxane levels appear to be consistently below 50 ppt in all the monitored locations within the optical path of the tool, except on one occasion when it reached 90 ppt in the projection optics. Volatile organic compounds (VOCs) concentration within the MS-VII enclosure show a stable background level of around 10-25 ppb during weekends and levels of 45-60 ppb (during working days). VOCs concentration variations inside the MS-VII enclosure during the working days correlate well with activities inside the clean room. Air recirculation and low intake of fresh air inside the MS-VII tend to slow down the speed with which the VOCs levels decreases to stable background level, whenever there was a major upward excursion in their concentration. Average light intensity through the projection optics correlates well with oxygen concentration. The average light intensity transmission through the PO lens has shown a steady increase over time due to in-situ laser cleaning with oxygen.