Optical EngineeringStress, reflectance, and temporal stability of sputter-deposited Mo/Si and Mo/Be multilayer films for extreme ultraviolet lithography
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Precisely figured optics coated with high-reflectance multilayer films are an integral part of an extreme ultraviolet (EUV) projection lithography system. Since multilayer film stress deforms these precisely figured optics, it is important that this stress be characterized and that suitable methods are developed to negate the effects of the stress. However, these techniques must reduce or compensate for the effects of film stress without significantly reducing the EUV reflectivity, since the reflectivity has a strong impact on the throughput of an EUV lithography system. Different techniques for reducing multilayer stress are examined. The technique of varying the base pressure (impurity level) yielded a 10% decrease in stress with a 2% decrease in reflectance for our multilayers. A study of annealing during Mo/Si deposition is performed; a stress reduction of 70% is observed at 200°C, similar to what has been found for postdeposition annealing. However, the reflectance loss was 3.9% versus 1.3% for postdeposition annealing, indicating that if annealing is performed on our films it should be done after multilayer deposition. A decrease of approximately 9% in the bilayer period thickness was observed for annealing during deposition at temperatures near 120 to 140°C, much larger than the thickness changes observed during postdeposition annealing. A new athermal buffer-layer technique is developed to compensate for the effects of stress. Using this technique with amorphous silicon and Mo/Be buffer layers it is possible to obtain Mo/Be and Mo/Si multilayer films with a near zero net film stress and less than a 1% degradation in reflectivity. It is important that the multilayer coated optics are stable with time and that the stress reduction and compensation techniques do not degrade the stability. The temporal stability of 100-nm-thick Mo, Si, and Be single-layer films, Mo/Si and Mo/Be multilayer films, and films treated with various stress reduction/compensation techniques are investigated. Results are reported showing the reflectance peak wavelength to be stable to within 0.15% for both the multilayers and the stress-compensated multilayers. Therefore, stability is maintained when these buffer-layer stress compensation techniques are applied. Mo/Si multilayer films exhibit a ~10% short-term decrease in the stress over the first few months after deposition, but the stress stabilizes thereafter. The Mo/Si reflectance is observed to be stable to within 0.4% over a period of over 400 days. The Mo/Be multilayer film stress was stable to within about 2% over 300 days, and the reflectance was observed to decrease by about 1.5% over the same period. For Mo/Si on a Mo/Be buffer layer the stress changed from -28 to +3 MPa, and the reflectance decreased by approximately 0.4% for a period of over 300 days. For Mo/Be on an a-Si buffer layer, the stress changed from -23 to -3 MPa, and the reflectance decreased by â€”1.8% for a period of over 300 days; this drop in reflectance is believed to be due primarily to the Mo/Be and not the addition of the buffer layer.