Multilayer antireflection coatings for Ge optical window

Two multilayer antireflection coatings have been designed, prepared and characterized for a 1 mm thick Ge optical window in 3.25-5.25 mu band for optical and electro-optical application. Ta₂O₅ and TiO₂ have been used as high index materials along with SiO₂ as low index material. The prepared multilayer structures have been characterized optically and structurally using spectrophotometry, atomic force microscopy (AFM), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Post deposition annealing of both multilayer structures have been carried out in the temperature range of 150-450degC for 10 hours, to study the effects of annealing on structure and transmission characteristics of the two configurations. Optical analysis shows that multilayer structure exhibits high absorption for the as-deposited samples which improves with subsequent annealing for both configurations. Structural realignment has been observed as soon as post deposition annealing is started and the film structure fails at annealing temperature of 450degC for both configurations, before the realignment process could complete. Configuration 1 has shown predominantly amorphous like structure, where as configuration 2 exhibited pronounced peaks with crystalline structure. A considerable improvement in the transmission profiles for the two multilayer configurations has been observed with highest transmission observed at 350degC with peak and average transmission for the both configurations has been found exceeding 90%. However, at 450degC, delamination and cracking of the two multilayer structures have been witnessed and its reasons have been studied.