Resolution enhancement techniques for optical lithography

Optical lithography has been the key enabling technology for scaling down dimensions of devices on VLSI chips. While electron beam and X ray lithography techniques promise higher resolution, optical lithography remains the most economical technique for defining fine patterns on a chip. The critical dimensions in current VLSIs are typically smaller than the wavelength of commonly available optical sources. Therefore, special techniques are required to achieve such high resolution. Commonly used resolution enhancement techniques are: optical proximity correction, phase shift masking and off-axis illumination. Often, a combination of these techniques can be used to good effect. In this paper, we describe the development of simulation programs which help us determine the effectiveness of these techniques in achieving a given resolution at a specified wavelength. An aerial image simulator permits us to quantify the quality of the mask image formed using resolution enhancement techniques. A resist development simulator then determines how successfully the image can be transferred to the photoresist on wafer