Contour Modifiers for Optical Lithography

Abstract

The shrinking design rules of integrated circuits require precisely controlled patterned resist linewidths with minireal dimensional deviations. In the presence of steps (topography) on the substrate, dimensional deviations greater than 0.5 ~m are not uncommon. Several factors contribute to dimensional deviations in positive resist films patterned on topography with optical lithography. These include: resist thickness changes, standing wave effects within the resist film, sidewall reflections, and focus differences between the top and bottom of the .step. Standing wave induced dimensional deviations appear to present the most severe difficulties (1-3). These interferometric effects produce oscillating linewidth deviations as a function of resist thickness and are particularly prominent on highly reflective substrates (>50%). The necking of the patterned resist line near the edge of a step has been termed "reflective notching." This reflective notching near the top edge of a step and ballooning near the bottom edge of a step have been largely attributed to a standing wave effect. In this work we discuss another factor that also contributes to "reflective notching" and propose a new procedure for minimizing its effects. In some cases the latitude of the resist process can be improved and deviations in linewidth at critical regions of the device layout minimized with this procedure. ExperimentalAlthough standing wave effects certainly play an important role in resist dimensional deviations from optical lithography, our experiments suggest the origin of reflective notching and ballooning arise in part from a different and more complicated mechanism. Figure 1 shows a schematic of the contour of a spun-on resist film coating a step and the resulting light ray paths. The curvature of the resist contour changes the direction of incident light so that it no longer travels perpendicular to the substrate surface. Furthermore, near the top of the step several different paths through the resist film can expose the same point on the substrate surface or within the thickness of the resist film. The curvature of the contour acts as a concave lens near the top edge of a step to focus the light and as a convex lens to diffuse the light near the bottom edge of the step. The focused light tends to produce a "reflective notch" and the diffuse light a "ballooned" linewidth.To correct these exposure anomalies a spun-on, contour modifier overcoat is proposed to reduce the curvature of the contour at the air-polymer film interface. The overcoat layer is a water soluble polymer and is specifically designed to planarize the surface. Similar layers have been overcoated on resist films for other applications. Polyvinylalcohol, PVA, layers have been coated on x-ray resists to prevent oxygen from penetrating into the resist film (4). PVA or polyvinylpyrrolidone, PVP, layers have also been proposed as lubricants for resist layers to be used in contact printing (5). However, to obtain suitable planarization properties some special requirements a...

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