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A phase-shifting mask can be used to print groove servo patterns for optical storage disks and provides improved contrast and resolution in printed photoresist images when used in a projection printer such as a Perkin-Elmer MicralignTM-500 mask aligner. Such a phase- shifting photomask structure can be fabricated in the following way. Onto a conventional photoresist coated chrome on fused quartz photomask blank is written a pattern of concentric lines and spaces or a spiral pattern. The photoresist is developed and the exposed chrome etched in the conventional way to yield a transmission mask pattern of twice the pitch desired in the final groove pattern. The quartz is then etched, using the photoresist/chrome and/or chrome pattern as the resist, by either a wet or dry process, to a uniform depth. The depth of quartz etch must be controlled as well as the width of the groove and land in the mask as well as the radius at the bottom of the etched groove walls. The chrome is then completely removed to form a 'chromeless' phase-shifting mask. Phase-shifting masks of this sort have been fabricated and used successfully to print photoresist images on glass disks yielding patterned photoresist structures of 1.5 micrometers pitch, 1.2 micrometers pitch, and 1.0 micrometers pitch. The resulting photoresist pattern could be transferred into the glass substrate by wet or dry (RIE) etch processes to generate the desired glass etch pattern after photoresist removal. Phase- shifting masks of this design require careful dimensional control in 3 dimensions. If the dimensions are not carefully controlled, the desired photoresist pattern is not obtained. For example, if the dimension of the mask groove is too large or if the bottom of the groove is rough, an alternating pattern of groove dimensions is obtained in the photoresist. Grooves etched from such a defective pattern would not be optimal for use as the servo pattern for an optical disk. One modification of the phase mask design described which simplifies the fabrication process is to alter the relative dimensions of the groove and land. If the land is made very small in relation to the groove, then the groove prints as though it were a clear feature in the transmission mask. Conversely, the land prints as though it were a chrome ('dark') feature. The contrast obtained in the image is much larger that can be obtained from a conventional transmission mask of similar dimensions. This 'darker than dark' phase- shifting mask phenomenon may prove valuable in the fabrication of complex patterns.
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