Research On Laser Irradiation Characteristics Of DUV Optical Coatings

Surfaces of optical elements applied in fields of deep ultraviolet (DUV)wavelength and high energy laser systems are coated with reflection reducing filmsor high reflecting films in most of the situations. The stability of films’ performancesis very important. Impurities on film surfaces or base materials and defects such ascolor centers in them will generate single photon or two-photon absorption underlaser irradiation, and the absorbed energy will convert to heat energy, which willresult in thermal expansion or even light degradation and will decrease theperformances of optical elements and reduce their lifetimes, also restrict the range ofapplications. Film elements used in193nm wavelength ultraviolet lithographysystems need to last working for thousand million or even more pulses, the study ofabsorption characteristics of optical thin film irradiated by DUV laser is an essentialwork in the producing and application of optical elements with high quality and theresults can also be used to evaluate the coating process. The absorptioncharacteristics of fluoride film coated by different coating processes were studiedunder193nm laser irradiation, by using laser calorimetry, the main works wereintroduced as following.Development and application introductions of DUV optical films; themeasurement principles and characteristics of several main absorption methodsincluding laser calorimetry were then introduced; the absorption phenomenon of fluoride film and fused silica material under193nm laser irradiation were analyzedfrom influences of laser irradiation frequency, laser irradiation fluence and materialdefects on absorption.The principle of exponential method used to calculate the measurement resultsof laser calorimetry was analyzed, and then the absorptance of fused silica wasmeasured. The influence factors of measurement uncertainty were found out andtheir estimates and relative standard uncertainties were calculated. By using matlabprogram, the fitted parameters A and γ were corrected which could decrease theinfluence of temperature drift in the sample chamber on the uncertainty ofabsorptance result. Finally, the combined uncertainty and expanded uncertainty ofabsorptance were calculated.The dose effect, nonlinear absorption and unrecoverable absorptionphenomenon of base material and thin film in the measuring process of absorptionwere analyzed and then standard measurement process used to measure theabsorptance of fluoride film applied in193nm wavelength ultraviolet lithographysystems by laser calorimetry was established, also the relationships betweenabsorptance of fused silica and laser irradiation fluence and frequency were obtained.Then the fused silica was coated with LaF3and MgF2by thermal evaporationmachine, the relationships between absorptance of film sample and laser irradiationfluence before and after being coated by different methods were measured based onthe established process. And then actual absorptance of thin film in workingcondition was calculated, also the absorptance, roughness and waviness of LaF3thinfilms under different deposition temperatures were obtained. Finally, theabsorptances of fluoride films coated by thermal evaporation method and ion beamsputtering method were compared, also the absorptances and extinction coefficientsin working condition were figured out.