22nm logic lithography in the presence of local interconnect

Abstract

The 22nm logic node is being approached from at least two different scaling paths. One approach "B" will use Gate and 1x Metal pitches of approximately 80nm, which, combined with the appropriate design style, may allow single exposure to be used. The other combination under consideration "A" will have a Gate pitch of ~90nm and a 1x Metal pitch of 70nm. Even with immersion scanners, the Rayleigh k1 factor is below 0.32 for 90nm pitch and below the single exposure resolution limits when the pitch is below 80nm. Although highly regular gridded patterns help [1,2,3], one of the critical issues for 22nm patterning is Contact and Via patterning. The lines / cuts approach works well for the poly and interconnect layers, but the "hole" layers have less benefit from gridded designs and remain a big challenge for patterning. One approach to reduce the lithography optimization problem is to reconsider the interconnection stack. The Contact layer is complex because it is connecting two layers on the bottom - Active and Gate - to one layer on the top. Other layers such as Via-1 only have one layer on the bottom. A potential solution is a Local Interconnect layer. This layer could be formed as part of the salicide process module, where a patterned etch would replace the blanket strip of un-reacted metal of the silicide layer. Local interconnect lines would run parallel to the Gate electrodes, eliminating "wrong-way" lines in the Active layer. Depending on the final pitch chosen, Local Interconnect could be single or double patterned, or could be done with a self-aligned process plus a cut mask. Example layouts of standard cells have shown a significant benefit with local interconnects. For example, the Contact count is reduced by ~25%, and in many cases Via-1 and Metal-2 usage was eliminated. The simplified Active pattern, along with reduced contact count and density, permit a different lithography optimization for the cells designed with Local Interconnect. Metal-1 complexity was also reduced. Details of lithography optimization results for critical layers, Active, Gate, Local Interconnect, Contact, and Metal-1 will be presented.