Analysis of the effect of laser bandwidth on imaging of memory patterns

Abstract

Tighter CD control requirements of the smaller devices in modern semiconductor products demand control of all potential sources of change in imaging characteristics. Bandwidth of ArF lasers is known to be one of the important parameters to be controlled to improve CD control of wafers. CD changes of Device Critical Patterns for memory products, for example spacing of DRAM isolation patterns, due to laser bandwidth changes were investigated through simulations. The purpose of the simulation study was to find out if there are optimum combinations of layout and illumination setting, if variations can be compensated by illumination adjustments and if the bandwidth performance of the laser meets requirements. The simulations were carried out using Cymer proprietary methods for high accuracy using improved laser spectrum sampling techniques[1]. Different CD behavior was observed for different combinations of pattern layout, illumination and bandwidth. Preferred illumination settings were found which suppress CD changes caused by bandwidth variation, especially for diffusion layer of DRAM layouts. Adjustment of illumination settings was demonstrated to cancel out CD shifts due to bandwidth change for the diffusion layer case. For all example cases, which demonstrated typical DRAM product conditions, simulation verified that the amount of CD shift can be controlled within allowed tolerances if Cymer's ABS technology was used for bandwidth control.