When thinking of a high powered laser, the image that comes to mind is likely to have some connection to a sci-fi “death ray.” Some planet, some ship, some foe is being instantly reduced to a sub-atomic pile of junk. The truth is there are extremely powerful lasers available in a variety of wavelengths, but they are not used for destructive purposes.
High powered lasers, such as those in the ultra-violet (UV) wavelength range (100 to 400 nm) are now used in medical applications, micro-machining, semi-conductor chip manufacturing, automotive, and defense, and aerospace technologies. UV lasers have distinct advantages over visible light and infrared lasers. One key advantage of UV lasers is they do not generate as much heat as other types. Without excessive heat to worry about, there is less likelihood of damage to the surfaces being worked on. Shorter wavelengths also enable finer (sub-micron level) processing of components, but there is a trade-off.
Laser Induced Damage Threshold (LIDT)
UV laser systems are built with a variety of optical components usually requiring anti-reflective (AR) or high-reflectivity (HR) coatings. These optics may have a variety of dimensions and have different surface geometries including plano, spherical or aspherical. The chemical makeup of the coatings themselves are somewhat limited for UV performance. Common materials used include magnesium fluoride, silicon dioxide, hafnium oxide or aluminum oxide.
The substrates can have features that are not perfectly smooth. This may be as part of the design or a defect in manufacturing. Compared to visible or infrared laser light, UV light has higher energy and higher scatter, which means it is absorbed by the substrates. It also means there is the potential for the laser to interact with the materials it meets. Add to this a variety of methods for applying coatings to substrates, the wavelength of the laser and whether it is pulsed or continuous wave, and it quickly becomes possible — and does frequently happen — that the coatings used for UV light applications sustain some damage.
How much damage is acceptable before performance is impacted? This is where LIDT requirements come in. Threshold evaluation is a specialized type of testing often involving the use of microscopes and spectrophotometers. The goal is to determine the probability of coating damage based on the amount of laser radiation that is incident upon the surface. Incident radiation is defined as the amount of fluence for pulsed lasers or the maximum intensity for continuous wave lasers. In essence, LIDT testing tells the system designer how long the coating will perform when subjected to UV laser light.
Coatings Play an Important Role in Achieving High Laser Damage Threshold for Optical Components
A recent ZYGO study looked at the optical coating requirements for advanced deep ultra-violet (DUV) tools used in the lithography industry. This industry continues to pursue the resolution of smaller and smaller features. A krypton fluoride laser operating at 248 nm peak emission was used. The optics required high-performance anti-reflection coatings (Al2O3/MgF2) which were applied to UV-grade fused silica substrates with both plano and steeply curved geometries.
This study required the expertise of various groups within ZYGO’s vertically integrated optical fabrication group. Design and deposition of the coatings resulted in reduced reflectance, improved transmission, prevention of ghost imaging and damage from back reflectance in addition to improved durability. The laser damage threshold for the ZYGO AR coatings was 4 to7 J/cm2.When the study was completed, it showed that values between both curved and plano surfaces remained consistent. So, with the proper design and correct application of coatings, many of the issues with UV coatings can be minimized or eliminated — resulting in a more acceptable LIDT and better performance overall.
Coatings that Match the Application
As applications for UV lasers continue to evolve and grow, the challenges and opportunities for creating laser damage resistant AR and HR coatings will also continue to grow. With 50 years of experience in manufacturing custom optical components, ZYGO’s optical fabrication group can take the mystery out of optical coatings, optics fabrication and much, much more.