Scanner influences on resolution capabilities

In the past, the primary methods to achieve ever-tightening resolution requirements were reducing exposure wavelength and increasing the projection lens NA. Today however, photo engineers are pushing optical lithography well beyond the realm of what was once considered practical. The exposure wavelength in leading-edge semiconductor manufacturing facilities has reached 193 nm and the lens numerical aperture (NA) has recently been pushed beyond 0.90, rapidly approaching the dry lithography limit of LO. The industry is working aggressively to adopt immersion lithography; however, the extension of cost-effective dry ArF technology- continues to be imperative. The remaining variable useful in the extension of dry 193 nm technology´ is through the optimization of the process dependent factors that impact resolution. A substantial portion of resolution enhancement techniques has included modifications to mask design, incorporating strategies such as phase shifting and pattern splitting on dual masks. While such methodologies are effective for enhancing resolution, they significantly reduce the throughput, while also increasing the cost of mask generation. When reviewing process influences, it is necessary to take into account all of the areas that impact resolution, and that includes many contributions from the lithography scanner itself. When considering the scanner´s effect on resolution, one must look at the earliest stages of the lens design and manufacturing process, as both are major contributors to the system imaging capabilities. CaF₂ usage, in addition to lens polishing, annealing and assembly technology can be key differentiators between scanner manufacturers. It is well known that DUV lithography systems are far more sensitive to the environmental molecular contamination present in fabs than the early generations of i-line steppers. However, there are system factors that can minimize the exposure to these contaminants and their negative effects. In- addition, with the aggressive imaging challenges for sub-90 nm applications, the scanner´s polarization control system will have a dramatic impact on image contrast and process latitude. Furthermore, due to the significant capital cost associated with lithography equipment, there are on-going efforts towards the continuous improvement of imaging performance of existing litho systems, thus enabling performance far beyond initially specified levels. With pattern-specific lens optimization, the scanner is able to provide significant imaging improvements yielding enhanced full-field resolution capabilities, and expanded deployment flexibility. Many factors throughout scanner design and manufacturing impact its resolution capabilities. In addition, various scanner functions can be used to further enhance imaging performance. This paper will discuss numerous ways in which the scanner can be manipulated to achieve today´s critical imaging requirements, in addition to how such factors are vital in the continued extension of dry ArF lithography