Ion beam sputtering (IBS) technology has strongly advanced UV laser optics over the past 25 years because of its ability to deposit high-quality optical films for ultraviolet (UV) laser applications (see Figure 1). High-quality optics manipulate and direct the laser beam and are critical to laser performance and lifetime. Biomedical, semiconductor processing, micromachining and other UV laser applications continue to grow thanks to IBS technology.
While UV optical films face many challenges, the advent of IBS technology has made it possible to deposit high-quality UV optical films.
Meeting the Challenges of UV Laser Optics
The challenges faced by UV laser optics are twofold: absorption enhancement, which reduces the laser power, and scattering enhancement, which diminishes the intensity of the laser. Optical films can be further damaged if film stress, stoichiometry or film density are not optimized. Coating is the weakest link, and optical coatings will be optimized if improvements are made to key processing steps such as optical coating design, substrate cleaning, deposition and post-deposition processing.
Our research focuses on various production aspects of optical coating, including target selection, oxygen pressure, sputtering energy and annealing time, with the goal of improving the quality of optical coatings for IBS systems (see Figure 2) [1]. The effects of different process conditions and post-deposition annealing on HfO2 and SiO2 optical thin films have been investigated in recent years. The parameters analyzed include:
-Effect of metallic and dielectric sputtering targets on UV properties;
-Effect of oxygen (O2) partial pressure on stoichiometry and film properties;
-The effect of ion-assisted sources and beam energy on film and deposition properties;
-The effect of annealing on stoichiometry, stress and film properties.
This project at Veeco focuses on optical coatings in Nd:YAG lasers. Oxide films can be sputtered from oxide or metal targets. [2] Metal targets have lower absorption but higher sputtering rates. A higher sputtering rate will improve the coating efficiency as long as other film parameters are satisfied. When depositing HfO2 thin films, the partial pressure of O2 is a crucial process parameter, and if the O2 partial pressure does not meet the requirements, the films are prone to structural defects. In particular, oxygen deficiency produces sub-bandgap electronic states that induce laser damage to optical components. Unequilibrium HfO2 films with low oxygen content are highly absorbing and opaque, and cannot meet the requirements of UV laser optics.
Ion beam energy and annealing
Ion beam energy is another critical process element. When plating SiO2 films, lowering the ion beam energy reduces the absorption of the SiO2 film, thereby improving the performance of the laser system. However, this comes at a cost. While lowering the ion beam energy improves film quality, it also reduces the deposition rate, which affects productivity. In the SiO2 process, the use of an auxiliary beam helps to increase the transfer rate.
For HfO2 films, the resistance to laser damage decreases with increasing auxiliary beam energy. Obviously, the optimization of sputtering energy plays a crucial role in the quality of the film.
Annealing also plays a key role in the quality of the film, as it is a critical step to obtain the lowest loss and the highest resistance to laser damage. During the sputtering process, the deposited films are subject to compressive stresses and defects due to high-energy deposition. Annealing helps to release the tension and eliminate dangling bonds created during the sputtering process.
The annealing process also slightly alters the stoichiometric ratio of the film. Ideally, annealing should reduce film stress and result in optimal optical properties.Veeco's research has shown that annealing can improve the properties of deposited films, but annealing for too long or at too high a temperature can also be detrimental. When the annealing conditions are not proper, the interface roughness increases and the film can crystallize. The number of layers and composition of optical films can vary for different UV laser applications, so the annealing time should be optimized for each layer.
