RETICLE MODIFICATION FOR THE RESOLUTION ENHANCEMENT OF THE DAMASCENE LITHOGRAPHY

Abstract

In an attempt to pattern damascene for the sub-120nm node by using the 248nm KrF lithography system, this work has demonstrated a new mask design that shows an excellent resolution enhancement over the existing 8% attenuated phase shift mask (attPSM) which is capable of printing up to 140nm trench width. Studies review the resolution could be improved beyond 140nm and the minimum resolvable dimension could reach as low as 100nm, which is closed to the theoretical limit (90nm) and well below the coherent imaging (180nm) at 0.68 numerical aperture for the 248nm lithography. The primary design factor for the mask was built on the concept of chromeless phase shift technique. Alternating the adjacent regions for the phase and maintaining the phase shifter width at the same size was the key to the new design. With the conventional illumination mode at low partial coherence factor, the improvement could be enhanced further. Aerial image simulation of dense trench feature using the simulator Prolith 3D version 6.1 has proven a tremendous gain in the modulation transfer factor (MTF) and normalized image log-slope (NILS) of new design as compared with the attPSM. Both gains are about ×3 at 0.68 NA and partial coherence factor 0.31. Finally, the implementation of the dense feature into practical use has been investigated for the ability to extend this 248nm lithography system to print feature ?120nm by using this chromeless phase shift mask (CLM), in particular, patterning of the “comb” structure with CD 120nm in the IC designs. Specifically, process windows (depth of focus and exposure latitude) and mask manufacturing difficulty have been examined for the feasibility of mask design for this structure. It is desired that feature CD printed on target is to be insensitive to defocus and exposure changed, and the allowable minimum chrome width is to be kept large for quartz undercut.