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24 February 2009 Optical characterization of the mask writing process in bimetallic grayscale photomasks
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Bimetallic thin films of Bi/In and Sn/In oxidize becoming transparent under laser exposure. By controlling the laser power, direct-write binary and grayscale photomasks can be produced with the mask's transparency, or optical density (OD), ranging between ~3.0 (unexposed) to <0.22 OD (fully exposed). An OD measurement system has been developed that provides real time OD and laser exposure power measurements while the masks are being written. Measurements are obtained for each combination of films, characterizing their response when patterned with a raster-scanned v-groove mask. The characterization is performed by writing v-groove step patterns and modifying the mask's writing parameters such as velocity, line spacing and step width. Stationary results demonstrate Sn/In takes longer to expose compared to Bi/In. With a moving beam, the oxidation of Sn/In also occurs over a wider power range suggesting film materials with delayed or slower oxidations may offer power ranges that are better suited for grayscale masks. A narrow power range is less desirable for grayscale as more control is required over the writing laser. The stationary exposures also demonstrate both films can produce >64 distinct OD levels provided there is sufficient control over the laser power and exposure duration. The physical characteristics of the films are also examined to determine a more accurate method of verifying each film's composition. Combining weight, area, and thickness measurements allows for better characterization of the films as the thickness for bi-layer films are found to differ significantly from the sum of the individual layers.
© (2009) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
James M. Dykes and Glenn H. Chapman "Optical characterization of the mask writing process in bimetallic grayscale photomasks", Proc. SPIE 7201, Laser Applications in Microelectronic and Optoelectronic Manufacturing VII, 72010S (24 February 2009);

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