|
According to ITRS roadmap, low-k1 imaging requires extremely tight control on Critical Dimension Uniformity for multiple features that have to be printed simultaneously across exposure field and within the wafer. Besides tight control, CDU imaging performance needs to be maintained constant within wafer, wafer to wafer and lot to lot, as well as from one exposure tool to another. Maintaining the same performance from one exposure tool to another translates into new imaging requirements, specific for low-k1 regime, the matching of multiple-feature CD response to tool and process disturbances having different time and spatial characteristics. Here we defined the tool processing errors as disturbance and their CD effect as feature response. The tool disturbances considered in this work are represented by variations in dose and focus setup as well as dose - focus drifts and by reticle mean CD variation, short-range flare, offset in illumination setting (sigma center) and variation in spherical aberration. We defined the multiple-feature CD response as the 'through-pitch' CD variation, i.e., all responses calculated here are for the common range of pitches starting from 1:1 is isolated. The CD-proximity concept has been introduced earlier (1,2) but the sensitivity of proximity curve to litho tool disturbances and their effect on CD matching are less accounted for. In our study we explored two types of multiple-feature CD responses, the total CDU (3Sigma) and the actual CD values calculated from tool disturbances mentioned above. The later is used to characterize tool to tool CD-matching. The dependencies of multiple-feature CD response to primary tool-induced disturbances was done on lithography patterns typical to real device layout for both line-spaces (1-D) and 2-D patterns (line-ends) in clearfield and darkfield polarity. Although some of the effects can be small (in the single nm range), the sum of all relevant contributors add up to significant values when compared to single feature CDU requirement. As a relevant example to characterize CDU imaging performance of ArF low k1 lithography we will study multi-feature CD response to disturbances that are less accounted for, such as short-range straylight and laser bandwidth. In the future, disturbances induced by immersion and polarization will make the multi-feature CD response even more complex. Certain actuators on the exposure tool, such as dose, focus or coherence, can be actively used to control multi-feature CD matching, very similar to the way single feature CDU can be controlled by applying dose offsets. In the study, we will also compare the approach for a 'single-tool' CDU control to the approach to perform 'multiple-feature' CD matching on multiple exposure tools and will present cases where feature response are opposite in sign and magnitude for two different disturbance, which opens the door for compensating effects from one disturbance by selectively applying an offset to its orthogonal disturbance. For both 1-D and 2-D structures, our results show that 3nm reticle MTT (Mean-To-Target) disturbance has one of the most significant effect on multiple-feature CDU response which places an even tighter requirement to reticle CD’s.
|
