Photoresist exposure control using light scatter

Abstract

Design geometries of microelectronics devices are shrinking. Today, CDs of 0.8 μm are common, and 0.35 μm and smaller are goals of the near future. These small geometries place increasingly stringent requirements on fabrication process control, such as photoresist exposure. This is especially significant when it is realized that up to 50% of the processing time for advanced devices can be in lithography. Changes in the optical properties of a surface influence the amount of light coupled into photoresist during exposure. This, in turn, causes a change in the CD of the developed photoresist pattern, which is undesirable in light of shrinking CDs. Changes in optical properties of a surface occur, for example, from variations in optical thickness of films (including photoresist) on a wafer and from variation in optical thickness of films (including photoresist) on a wafer and from variation to metal roughness that might occur from the process variations. There is presently no direct monitor of photoresist exposure. Calibration samples (send-ahead wafers) are used to determine optimal exposure time; subsequent processing is performed open loop using this established constant exposure time. Slight changes in optical properties of wafers are not accommodated in this arrangement. We have used light scattered from latent images in photoresist as a direct monitor of exposure dose. Using this technique, we have demonstrated a 3× improvement in CD control for variations of film thickness on wafers compared to using fixed exposure time. The technique can be incorporated for real-time control of photoresist exposure.