Atomic layer deposition for high power laser applications: Al2O3 and HfO2

Abstract

Optical coatings are essential components reflecting or transmitting light in optical systems. Laser components demand more complex properties of optical coatings, such as precise thickness, uniformity over large area, weak absorption, high laser induced damage threshold (LIDT), low stress, high film density and so on. ALD process is investigated to deposit Al2O3 and HfO2 thin films. The film properties are studied and compared to Ion Beam Sputtering (IBS) Al2O3 and HfO2 single layer films. Anti-reflection coatings for 1ω-4ω of Nd: YAG laser are prepared with ALD HfO2/Al2O3 double layers. The anti-reflection coatings are characterized and compared to the coatings prepared by IBS HfO2/Al2O3. Both ALD HfO2 and Al2O3 have thickness with high resolution due to the self-terminating feature. Thin films can be controlled by counting the cycle number. Both ALD films have uniformity >99% in ϕ60 mm area. At 1064 nm, the ALD single layer films show absorption as weak as 3.7 ppm, and similar LIDT compared to the IBS films. Annealing at 300/400 °C promote the LIDT of ALD Al2O3 by 30%, but does not promote that of ALD HfO2. For 1ω-4ω of Nd: YAG laser, the ALD anti-reflection coatings have generally higher LIDT than IBS coatings. The LIDT of the ALD anti-reflection coating is limited by HfO2 film layer. The damage of both ALD single layers and anti-reflection coatings are induced by defects. ALD films, both single layers and multilayers, were found to have severe tensile stress. A model for the origin of the film stress is established. Taking advantage of the high LIDT and opposite stress of ALD and IBS films, a concept is proposed to develop high LIDT and stress compensated coatings with ALD Al2O3 and IBS SiO2. An anti-reflection coating consisting of 4 layers was prepared following this concept. The compensated stress is much smaller than either ALD Al2O3 or IBS SiO2 single layer. The LIDT of the multi-layer is lower than the single layers because of the defects introduced during shift between the ALD and IBS coating chambers. This concept has potential application in high power laser components if a proper coating plant is developed to reduce the defects. ALD films have high density, thus could be used as capping layer against vacuum-air-shift when the environment changes. The capping effect of ALD Al2O3 on evaporated Ta2O5/SiO2 multilayers was studied and discussed. The application of ALD films in nanolaminates and chirped mirror is discussed. The study in this thesis indicates the high versatility of ALD films for applications in high-power coatings.