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Critical level photolithography optimization for 0.35 μm design rules faces new challenges threatening the process control and jeopardizing manufacturing yields. To address these challenges we have adopted a methodology combining photolithography simulation and limited proof-of-principle DOE. It allowed us to analyze the process control requirements and to establish photolithography strategies resulting in optimum manufacturing performance. As manufacturing CD's continue to shrink towards and beyond the wave length at which photolithography is performed, the lithography process control becomes limited by optical proximity effects. The proximity effects modify the geometry of the reticle features printed in the resist and thus they impact both CD control and overlay performance. The proximity effects can be highly non-linear and thus difficult to quantify unless sophisticated analytical methods are adopted. Another challenge faced by IC manufacturing rose with the advent of illumination options [l, 2]. In particular, off-axis illumination options offered with commercial exposure tools expand the exposure capabilities of steppers. Inevitably, these new illuminators complicate optimization of photolithography process. If the optimization task is to be executed in a purely empirical fashion, the optimization resources requirements become prohibitive. To simplify the photolithography optimization cycle, we have developed methodology combining process modeling [3] and limited proof-of-principle testing; the modeling was done with Nikon Corporation's proprietary simulator. Our methodology was employed in photolithography process optimization of a set of critical levels used in manufacture of a DRAM chip. Photolithography modeling in support of the process optimization resulted in a set of illumination and projection optics alternatives. The optimization of illumination strategies involved conventional and off-axis exposure options. The modeling identified illumination tradeoffs in terms of process depth of focus, DOF, exposure latitude EL, and stepper throughput. The results of the analysis suggested off-axis illumination as a basis for a robust exposure process. The results of the modeling served as a basis for a reduced design-of-experiment, DOE, quantifying the CD characteristics of the levels. We present the results of photolithography optimization of a chosen critical level exposed with an i-line stepper, and discuss the merits of the methodology. The discussion presents examples of optimization solutions. This report reviews the results of the modeling, including aerial image analysis and resist simulation. The report also reviews the results of the proof-ofprinciple metrology. We compare the modeling and the metrology and draw conclusions on the quality of the models' predictions. We interpret the model results in terms of CD characteristics of the critical level features exposed and developed in the resist. The aerial image analysis and resist simulation allowed us to reduce the optimization space. This, in tum, reduced the resource required to conduct the limited, proof-of-principle verification. We stress the advantage of using the methodology combining process modeling together with limited, proof-of-principle metrology to simplify and to shorten the exposure optimization process.
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