This is important for imaging at the edges of an image field when fields are printed very close to each other on the wafer (so-called butted fields, with zero field to field spacing). DUV light is reflected from the reticle black border (BB) into a neighboring exposure field on the wafer. This results in a CD change at the edges and in the corners of the fields and therefore has an impact on CD uniformity. Experimental CDU results are shown for 16 nm dense lines (DL) and 20 nm isolated spaces (IS) (N7 logic design features) in the fields exposed at 0 mm and 0.5mm distance on the wafer. Areas close to the edge of the image field are important for customer applications as they often contain qualification and monitoring structures; in addition, limited imaging capabilities in this area may result in loss of usable wafer space.
In order to understand and control OOB DUV light, it must be measured in the scanner. DUV measurements are performed in resist using a special OOB reticle coated with Aluminum (Al) having low EUV reflectance and high DUV reflectance. A model for DUV light impact on the imaging is proposed and verified. For this, DUV reflectance data is collected in the wavelengths range 100-400 nm for Al and BB and the ratio of reflectances of these materials is determined for assumed scanner and resist OOB spectra. Also direct BB OOB test is performed on the wafer and compared to Al OOB results. The sensitivity of 16 nm DL and 20 nm IS to OOB light is experimentally determined by means of double exposure test: a wafer with exposed imaging structures undergoes a second flood exposure from a DUV reflective material (Al or BB).
Finally, several OOB mitigation strategies are discussed, in particular, suppression of DUV light in the scanner (~3x improvement), recent successes of DUV suppression for 16 nm imaging resist (~1.8x improvement) and DUV reflectance mitigation in the reticle black border (~3.8x). An overview of OOB test results for multiple NXE systems will be shown including systems with new NXE:3350 optics with improved OOB suppression.