Simultaneous source-mask-laser spectrum optimization

Tuning the spectrum of a scanner's excimer laser light source is a well-known technique to achieve improved depth of focus (DOF) for an exposure. Previous studies have focused on the imaging improvement capabilities of readily available spectral shapes such as Gaussian E95 single peak width, as well as dual-peak spectral shapes where the spacing of the peaks can be varied. It is commonly known that adjustments in the laser spectrum must be carefully considered since exposure latitude (EL) can also be reduced as DOF is increased. By carefully engineering the laser's spectrum, DOF can be maximized with very little impact to exposure latitude. Undesirable speckle contrast can also be reduced as laser bandwidth is increased. Traditionally, these approaches have been used to improve DOF of thick resist applications such as CMOS image sensors. Other areas such as NAND Flash have introduced laser spectrum engineering (SE) to compensate for topography effects between periphery and array regions. Recently, advances in laser hardware have enabled new and unique spectral shapes to be used for imaging. In this paper, we explore the various spectral profiles possible on the latest Gigaphoton GT66A ArFi light sources. New applications of these spectrum shapes are considered for foundry logic-type levels. First, a representative 14nm-node logic layer is considered. Source Mask Optimization (SMO) is performed while various widths of spectral profiles are used as input, such as traditional E95 Gaussian profiles, dual-peak, and flat top. By accounting for the laser spectrum during SMO, common DOF can be improved by ≥ 20% without a noticeable decrease in exposure latitude. The SMO sources and optimized masks are also different from each other and the nominal cases, depending on the spectral distribution used. Next, we explore simultaneously optimizing the laser spectrum, source, and mask in an experimental SMO+SE. Spectra can be constrained to keep symmetric profiles about the 193nm center point. Improved lithographic performance can result from the application of programmable laser spectra combined with pixelated sources and inverse lithography (ILT).