0.3-micron Optical Lithography Using A Phase-Shifting Mask

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Abstract
Improved resolution of an available i-line (365nm) stepper using a phase-shifting mask is discussed. The resolution investigated here is not only for periodic lines but also for isolated spaces and hole patterns. To obtain a narrow bright line for printing a fine isolated space on a wafer, two additional line apertures with widths smaller than the critical dimension of the stepper lens are placed on each side of the main aperture of the mask. The optical phase of the main aperture and those of additional apertures are opposite. The additional apertures play a role in reducing the bright feature size to less than the line spread function of the lens. Similarly, printing a fine hole is accomplished by using a main aperture surrounded by four additional apertures. The intensity distribution on the wafer is calculated by comparing the results obtained with a phase-shifting mask and those obtained with a conventional transmission mask. Patterns are also printed on the wafer using an i-line stepper with a nominal 0.55 μm resolution. A pattern of 0.3-μm lines and spaces, 0.3-μm isolated spaces and 0.4-μm hole patterns are resolved using the phase-shifting mask. This resolution is impossible with a conventional transmission mask. The effects of variations in the optical phase of the additional apertures are also investigated. The intensity calculations and experimental results suggest that it is possible to control the position of the best focal plane by changing the optical phases of the additional apertures.

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