Optical EngineeringReduction of residual stress in extreme ultraviolet Mo/Si multilayer mirrors with post deposition thermal treatments
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The as-deposited stress of typical high-reflectance extreme ultraviolet (EUV) Mo/Si multilayer mirrors is measured to be approximately ?410±10 MPa (compressive). These multilayers are deposited with dc magnetron sputtering and have near-normal incidence (5 deg) reflectances of 67.2±0.1% around 13.2 nm. The effect of both slow and rapid thermal treatments on multilayer stress and reflectance is measured. For both approaches, the stress of these multilayers is reduced quasi-linearly with the annealing temperature. Using the slow thermal anneal approach, it is possible to reduce the stress from ?410 MPa to zero by heating the sample to ~275°C with a corresponding reflectance loss of ~3.5% (absolute). If preserving the reflectance is critical, we show that heating the sample to ~220°C reduces the stress by 85% from ?407 to -63 MPa with a reflectance loss of only ~1.5% (absolute). It therefore appears possible to ‘‘tune’’ the stress of EUV Mo/Si multilayer mirrors with this postdeposition annealing technique and predict the loss in reflectance for any given desired final stress. Moreover, it seems that the relationship between the reflectance loss and the stress change does not depend on the type of thermal treatment, i.e., it is the same for both the slow and rapid thermal annealing techniques.