Improved resolution of an i-line stepper using a phase-shifting mask

Improved resolution of an available i-line (365 nm) 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 reduce the sizes of isolated space images for printing fine single spaces 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 light passing through the main aperture and those through 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 surface is simulated by comparing the images obtained with a phase-shifting mask and those obtained with a conventional transmission mask. Printing fine patterns are performed using an i-line stepper with a numerical aperture (NA) of 0.42. 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. The process latitudes and the effects of variations in the optical phase of the additional apertures are also investigated. The image simulations and experimental results suggest that the phase-shifting mask improve not only resolution but also exposure latitude and focus latitude. Furthermore, it is found that it is possible to control the position of the best focal plane by changing the optical phases of the additional apertures.