We have developed a novel two-step baking process to achieve high-aspect-ratios in UV photolithography with a conventional positive thick photoresist. We newly report high-aspect-ratio (greater than 10:1) results in a single coated 91 micrometer- thick photoresist AZ9262, which was introduced Hoechst at SPIE in 1996. From extensive experiments, we improved the aspect ratio by minimum exposure, diluted development, reabsorption of sufficient water before exposure, and especially by extended and effective soft bake in two steps. In the optimum two-step baking, first the baking is performed at an intermediate temperature in a forced convection oven for hours to evaporate large amounts of solvent. Second, the photoresist is heat-treated at an elevated temperature on an air-gapped hotplate with cover for minutes to enhance aspect ratios. The reason for this improvement has been studied based on the photochemical process of the DNQ/novolac-type positive photoresist. Using this two-step baking, we have obtained lines of 4.4 micrometer-wide bottom in a single coated 91 micrometer-thick photoresist (aspect ratio: 20). The line width in the mask was 8.9 micrometer, and hence 2.2 micrometer undercut was observed.