摘要
Differences between the developed photoresist profile and the optical image in the resist is a function of exposure and development conditions. Because of absorption of light in the photoresist and lateral etching by the developer, an overcut profile in the developed image of positive resists is inevitable, even if the distribution of the exposing light is perfectly verical. Intentional overexposure followed with quick development, or the use of highly reflective substrates only minimizes the overcut. This paper describes an approach using a rotating plane wave making a predetermined angle with the optical axis to expose a mask-wafer assembly. This type of illumination produces an undercut optical image which compensates for the absorption and lateral etching effects. It also reduces the coherence in the illumination to give better images. The angle of inclination as well as the exposure-development parameters can be adjusted to manipulate the angle of cutting. Analytical intensity distribution based on geometric optics is given to provide an insight to the dependence of the cutting angle to the beam angle and the exposure-development parameters. PMMA in the deep-uv wavelength region and AZ1350J in the near-uv region are used to demonstrate the variation of the cutting angle and the liftoff process.