Fraught with seeming insurmountable challenges even a year ago, 157-nm lithography seems to be nearing feasibility as a production process. The challenges that remain are considered engineering tasks, not showstoppers. Our guest authors from ASML, Canon Inc., and Nikon Corp. offer updates on their timelines and the status of their programs. projection optics The projection lens of ASML’s first 157-nm production tool is specified with a numerical aperture (NA) of at least 0.85 to be competitive with 193-nm systems and a field size of 26  33 mm to reach the imaging performance for the 65-nm node. Despite some improvements in narrowing the laser bandwidth, the lack of a glass material in addition to CaF 2 to compensate for chromatic aberrations makes a catadioptric design the only possible choice for the projection optics. Thanks to new annealing procedures, CaF 2 material is now available with levels of [111] stress birefringence better than 1 nm/cm, and a refractive index homogeneity that reaches the performance specifications set for both [111] and [100] orientations. Nevertheless, growth yield improvement in CaF 2 material and additional investments will be necessary to reduce the costs and have sufficient material quantity available for the volume production of 157-nm exposure systems. Intrinsic birefringence, which has been confirmed at 11 nm/cm for 157 nm, 2 can be efficiently compensated for in the lens design by combining and clocking lens elements of [111] and [100] crystal orientations, yielding a residual level of 0.3 nm/cm below the level of the stress birefringence. The lens design has been further optimized to minimize the total CaF 2 content as well as the amount of [100]-oriented material, due to its higher-stress birefringence and lower-yield growth, thereby improving both the high-NA imaging contrast loss performance and the cost of the lens. Antireflection coatings have been developed by Carl Zeiss (Oberkochen, Germany) with a transmission value larger than

99% in production. Lifetime tests showed no degradation for at least 5 gigapulses of exposure at 25 mJ/cm 2 energy density. The material degradation reported by the Massachusetts Institute of Technology (Cambridge, MA) was not observed, nor did coating stability require a lowwater- vapor environment. 3 Initial concerns of low system efficiency due to poor optical transmission are being resolved with better illumination systems, improved optical materials, and higherpower lasers. Tools for 157-nm tools should meet the throughput of previous 248-nm or 193-nm generation tools. purge and contamination control Purging of the reticle and wafer stages has been verified on prototypes. 4 The concept of local purge allows immediate access to the machine in case of service action or operator intervention. The relatively small purge compartments greatly minimize the system downtime during maintenance of the tool, with a nitrogen flow chosen to optimize both the optical performance and cost of operation of the tool. Because optical absorption for contaminants such as oxygen, carbon dioxide, and water vapor at 157 nm is three orders of magnitude higher than at 193 nm, the maximum allowable concentration of these contaminants must be in the sub-parts-per-billion range. A tool has been developed to measure the concentration of hydrocarbon compounds in a nitrogen-purge gas flow to the level of parts-per-thousand by using thermal desorption and gas chromatography. The unit’s portability and sub-30-minute response time make it ideally suited to monitor and ensure contaminationfree operation in a production environment. ASML will offer the Micrascan VII in 2003 as the industry’s first 157-nm full-field scanner. Its 0.75-NA projection optics and field size of 26 mm  34 mm will provide imaging capability for sub-100-nm structures. Early system illumination before final setup has better than 1.25% uniformity across the field, including purge. This tool’s development has been instrumental in driving the industry’s CaF 2 suppliers and will be used to enable resist and process development. Experience will be gained in purging and contamination monitoring, lifetime degradation of components, and optical coatings. Learning in catadioptric design, aspheric lenses manufacturing, and CaF 2 bulk and surface scattering will lead to better optical performance of the production systems.