Optical lithography techniques for 0.25 μm and below: CD control issues

It is generally accepted that optical lithography will be the technology of choice for the fabrication of devices for several generations to come. In this paper some of the current challenges to print 0.25 μm dimensions are addressed. Although the mainstream technologies for 0.35 μm and 0.25 μm processes are more or less settled, the main challenge will most likely be CD control. Optical proximity effects and CD variations resulting from substrate reflections start to play a major role in the CD budget. Several methods to reduce these contributions (such as the use of optical proximity correction techniques and the use of anti-reflection coatings and surface imaging processes) are presented. Moreover, it will be indicated that considerable improvements in CD control can be obtained by optimization of stepper parameters. Finally, in an attempt to indicate the future of optical lithography, a simulation study is presented, indicating the available process latitudes which can be obtained by combining several of the above listed techniques