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First eleven years of the phase-shifting mask

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Abstract

These days, all mass-produced integrated circuits are fabricated using techniques where the image of a mask is projected with light onto a photosensitive polymer. The size of the smallest features formed by such optical lithography techniques is limited by the laws of optics, to ~0.4 µm, but the next generation of integrated circuits will require smaller features. While shorter wavelength light or larger aperture lenses might allow smaller minimum features, those proposed developments would also reduce the depth of focus to unfeasibly small values. The solution to this impasse was actually discovered 11 yr ago. If the mask used to form the image has information about both the phase and the amplitude of the light (rather than just the amplitude), the minimum feature size can be reduced by as much as a factor of 2. In such a phase-shifting mask, destructive interference cancels some of the effects of diffraction and ensures that individual bright features are separated by the required dark lines. The phase-shifting mask idea languished in obscurity before being developed beginning about three years ago by various Japanese semiconductor manufacturers. In the last year or so, remarkable accomplishments have been made public. Working electronic circuits with minimum feature sizes down to 0.17 µm have been fabricated using light of wavelength 0.365 mm. Unhappily the design of phase-shifting masks is an engineering exercise which requires knowledge of optics.

© 1991 Optical Society of America

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