Factors Affecting Linewidth Control Including Multiple Wavelength Exposure And Chromatic Aberration

Photoresist linewidth variations are particularly difficult to control in the vicinity of steps where resist thickness changes occur. These variations are primarily due to the interference phenomena which affect the coupling of energy into the resist and vertical standing wave intensity contrast in the resist. In this paper the user oriented program SAMPLE is used to quantitatively explore linewidth control in the vicinity of steps as a function of exposure, resist, and substrate parameters. Nominally, projection printing of 1.0 um line and space patterns in 1.0 urn of AZ13.50J resist crossing 1.0 urn steps on silicon, oxide and aluminum substrates is considered. The linewidth variation, resist profile shape and linewidth control sensitivity are characterized by the behavior of the two extreme cases. These are the maximum linewidth which occurs at the largest resist thickness on the bottom of the step corresponding to a destructive interference node and the minimum linewidth which occurs at the smallest resist thickness on the top of the step corresponding to a constructive interference node. The significance of the aerial image contrast and substrate reflectivity are explored. The possibility of using increased optical absorption and post exposure bake procedures to improve process control are considered. The use of multiple wavelength exposures with 6 raction limited optics is shown to significantly improve both linewidth control and the resist profile shape. The degree to which a refractive optical system can realize this improvement can be limited by the extent to which the change in focal error with wavelength reduces the contrast. This limit is particularly important in assessing the potential advantages of multiple wavelength systems, because the performance of such lenses is expected to be limited by chromatic aberration effects.