Optically induced mask critical dimension error magnification in 248 nm lithography

One form of optical proximity effect that further complicates lithography i s the unexpected response of the printed image to small perturbations [crit ical dimension (CD) errors] in the reticle. In this way mask CD errors are actually magnified (they are reduced by less than the reduction factor of t he optics) during the optical transfer to the wafer. This effect will requi re even tighter specifications for mask CD control when the error magnifica tion factor is significantly above unity. The effect is particularly pronou nced for tight pitches of small features, but can also impact the printing of small isolated lines. Both resist and optical nonideal responses are inv olved in this mask error factor (MEF). This article discussed the optical e ffects that produce the MEF. This article will show where the MEF due to op tical effects can be ignored and where they cannot when using 248 nm lithog raphy with a high numerical aperture (NA) tool. We will demonstrate how the NA, partial coherence, and variations in focus can effect the mask error m agnification factor. We will also show that resolution enhancement techniqu es can be used to reduce the mask error magnification factor. In particular , Levenson phase shift masks show particularly low mask error magnification factors for small lines. Far some applications it should be possible to de sign the mask so that the mask error magnification factor of the smallest f eatures is significantly below unity. This would allow loosened reticle CD specifications and/or better CD control of the lithography process. (C) 199 8 American Vacuum Society. [S0734-211X(98)06906-6].