Optimum Stepper Performance through Image Manipulation

It is the goal of this paper to investigate the potential advantages of using flexible steppers for "image manipulation," that is, varying the aerial image of a given mask pattern by varying the optical parameters of the stepper. Obviously, the purpose of image manipulation will be to improve the lithographic process in some way. Thus, the first step will be to define some metric or metrics of image quality that can be related directly to lithographic quality. Next, the available optical parameters will be varied and the conditions which give the best image (i.e., the maximum value of the image metric) will be found. The three variables that will be used to manipulate the image will be the objective lens numerical aperture (NA), the partial coherence of the illumination (0'), and the shape of the illumination source. The advent of flexible steppers, allowing variation in the numerical aperture, partial coherence, and possibly other optical parameters, allows new opportunities for optimization. This paper will discuss a method for picking the optimum numerical aperture and partial coherence for a given mask pattern. The intuitive solution, that the highest possible numerical aperture is the best, is true only when the image is in perfect focus. For moderate amounts of defocus, increasing the numerical aperture may actually decrease the quality of the aerial image resulting in degraded stepper performance. Thus, an optimum numerical aperture and partial coherence will be defined for a given amount of defocus. The use of annular sources of illumination will be shown to give some improvement in high resolution performance. AERIAL IMAGE BEHAVIOR INTRODUCTION In previous work4.5 the advantages and disadvantages of various metrics for image quality were discussed. It was shown that the slope of the log of the aerial image (called the log-slope) is directly proportional to the gradient of photoactive compound and the relative gradient of development rate. Through these relationships. the log-slope is the dominant feature of the aerial image which determines exposure latitude. In general. the value of the log-slope at the mask edge is sufficient to A trend is emerging in optical step-and-repeat equipment oward greater flexibility and user control. The latest offering from Ultratech Stepper incorporates a variable numerical aperture objective lens (ranging from 0.2 to 0.4) with variable partial coherence of the illumination.1 Scanning projection printers from Perkin-Elmer have long been able to pick among many possible wave-