Ion beam sputtering (IBS) technology has driven the development of ultraviolet (UV) laser optics for the past 25 years by enabling the deposition of high-quality optical coatings for UV lasers (see Fig. 1). High-quality optics manipulate and guide laser beams and are critical for optimal laser performance and lifetime. Biomedical, semiconductor processing, micromachining, and other UV laser applications continue to grow, thanks to IBS.
While critical challenges remain for UV optical coatings, IBS can be harnessed to deposit high-quality optical coatings.
Addressing UV laser optics challenges
The critical challenges in UV laser optics are increased absorption, which reduces the laser’s power, and increased scattering, which lessens its intensity. Optical films may be damaged further if the film stress, stoichiometry, or film density are not optimized. The coating is the weakest link; however, improving the critical processing steps, such as optical coating design, substrate cleaning, deposition, and post-deposition treatment, will optimize optical coatings.
Our latest research focuses on a variety of production aspects for optical coatings, including target selection, oxygen pressure, sputter energy, and annealing duration. Improving the quality of the optical coatings deposited by IBS systems is a key goal (see Fig. 2).1 Recent studies investigated the impact of different process conditions and post-deposition annealing on optical films, hafnium oxide (HfO2) and silicon oxide (SiO2). The parameters analyzed included:
- Effects of metal and dielectric sputter targets on UV performance;
- Impact of oxygen (O2) partial pressure on stoichiometry and film performance;
- Impact of the ion assist source and beam energy on film and deposition characteristics;
- Effect of the anneal on stoichiometry, stress, and the resulting film properties.
This Veeco project focused on optical coatings used in Nd:YAG lasers. Oxide coatings can be sputtered from either oxide or metal targets.2 Metal targets give a lower absorption but higher sputter rate. Higher sputter rates can deliver a yield and throughput advantage, provided the other film parameters meet the critical requirements. The partial pressure of O2 is a vital process parameter when depositing HfO2, because these thin films are prone to structural defects induced by nonstoichiometry. In particular, oxygen deficiencies, which create sub-bandgap electronic states, can be directly correlated to damage to the optics by the laser. Nonstoichiometric HfO2 films with low oxygen content are highly absorbing, opaque, and unsatisfactory for UV laser optics.
Running experiments with O2 partial pressure revealed that higher partial pressure in SiO2 and aluminum oxide (Al2O3) films results in a higher energy transfer of sputtered material into the growing film. This, in turn, leads to defect formation, illustrating how increased absorption in the film becomes problematic. The deposition process requires careful control and optimization of O2 partial pressure.
Ion beam energy and annealing
Ion beam energy is another critical process component. In depositing SiO2 films, experiments reducing the ion beam energy lowered the absorption of the SiO2 films, which improves the laser system’s performance. There is a tradeoff, however. While reducing the energy of the ion beam improves the film quality, it also reduces the deposition rate—which impacts the system throughput. In the SiO2 process, using the assist beam helps to improve the transmission rate.
For HfO2, the laser damage resistance decreases with increased assist beam energy. Clearly, optimizing the sputtering energy plays a critical role in the quality of the films.
Annealing also plays a vital role in the quality of the films because annealing is a critical processing step in attaining the lowest losses and highest laser damage resistance. During the sputtering process, the films deposited develop compressive stress and defects due to the highly energetic deposition process. Annealing helps to relax the tension and satisfy dangling bonds created by the sputtering method.
The annealing process can also slightly alter the stoichiometry of films. Annealing should ideally result in low-stress films and optimum optical properties. Veeco studies show that annealing improves the deposited film characteristics, but annealing for too long or at a temperature that is too high can also be detrimental. Increased interface roughness and crystallization of the films are two examples of undesirable outcomes when improper annealing conditions are used. The optical coatings for different UV laser applications can differ in layering and composition, so annealing time should be optimized for each film stack.
Harnessing IBS to deposit optical coatings
Veeco has been using IBS to deposit optical films for more than 25 years and remains active in producing technology for applications such as UV lasers. Our SPECTOR ion beam sputtering optical coating system was developed to facilitate the deposition of high-quality films and is a dual-IBS system with a 16-cm radio frequency (RF) deposition ion source using noble gases and a 12-cm RF ion assist source (see Fig. 3). This combination enables the deposition of optical films with near-bulk properties. Three target materials can be loaded simultaneously to provide optimum deposition flexibility in layering films, while the deposition source’s UV grids help reduce the coating’s molybdenum (Mo) content—which results in higher-quality films.
Veeco is committed to growing its optical coatings knowledge base and passing this expertise on to our customers. Our company’s engineers collaborate to enhance insights across the company, because cross-pollination from other deposition technologies within Veeco’s portfolio enables a better understanding of the laser optics space.
REFERENCES
1. M. Falmbigl, K. Godin, J. George, C. Mühlig, and B. Rubin, Opt. Express, 30, 12326–12336 (2022).
2. D. Howe, M. Falmbigl, J. George, K. Godin, and B. Rubin, Optica Technical Digest Series (Optica Publishing Group), paper WC.2 (2022).