Contamination on Ultraviolet (UV) Optics
CONTAMINATION CAN NOT ALWAYS BE SEEN
Contamination can find its way onto all types of optics. Even if it is hard to see the contamination, it may be easy to see the damage it causes to optics that have been subjected to intense light. UV light is generally more likely to break molecular bonds and cause damage than visible or infrared light. Contamination can make a good optic appear to have a low threshold for laser induced damage. Here we’ll explain a few types of contamination, and we’ll show what can be done to prevent contamination on UV optics. Most of the ideas here pertain to optics for ultrashort pulses too. Contamination comes in many forms, and particles and condensation of volatile materials (sometimes called molecular contamination) are the two major concerns.
Another type of contamination can occur just BELOW the surface of an optical material, which is important for transmissive optics. Subsurface damage can be minimized through skillful optical fabrication processes. This topic is explained in a Gamdan Blog on Subsurface Damage. Here we will focus on contamination that can be on optical surfaces.
CONTAMINATION FROM PARTICLES:
Large particles such as metal chips are created in many mechanical processes, for example when screws are used. Gravity pulls large particles onto upward-facing surfaces. A key to eliminating large particles is to (preferably) avoid generating them in the first place or (if avoidance fails) to remove them before they cause damage of any optical surfaces by blowing, wiping, vacuuming, or washing them away.
Tiny particles, which may be micron-sized or smaller, are ubiquitous. These particles can attach to optical surfaces by electro-static forces, Van der Waals forces, or gravity. The density of small particles can be minimized by carefully cleaning the optics and all other parts that are near the optics.
Particle accumulation can also be prevented by working in a clean room and by filtering all gases that are used near the optics. Purging an optical system with a clean gas can also help prevent particles from settling on optics. Upward facing optics are much more susceptible to particulate contamination that horizontally facing optics, and both are far more susceptible than downward facing optics. The best practice for avoiding contamination by particles is to prevent the particles from ever settling upon optics. Only if necessary, particles might be cleaned from surfaces.
SHUTTING OUT CONTAMINATION
Protective optics can isolate valuable optics from particles or from other types of contamination. These protective optics can be sacrificial, meaning that they can be replaced if they get dirty or damaged. This is like the use of protective filters on camera lenses, something that has been done for about a century. The filters on cameras often also block unwanted wavelengths of light, but that light-blocking function is not relevant here. An important example of particle prevention with isolating protective optics is the use of thin UV-transmissive films called pellicles to protect high value photolithography masks from contamination, especially particle contamination. Pellicles or filters should be positioned so that any particles that settle on them are not in focus.
CONTAMINATION FROM VOLATILE MATERIALS
Volatile materials in the gas around optics will often condense and coat optics with contamination, causing damage where UV beams are incident. Problematic volatile molecules can come out of materials like hydrocarbons (lubricants), water, adhesives, and plastics (including insulation on wires), to name a few. Though not all volatile materials are close to equally bad for optics, it is critical that the presence of all volatile materials is minimized in the design and construction of an optical system. This can be done by avoiding materials that might outgas, by cleaning all parts before they are put in the optical system, and by baking out parts that go into the optical system. In some cases it is useful to purge an optical system with clean gas or to filter the gas inside an optical system. Often more than one of these approaches is needed and is used.
GETTERS—A NEW AND INTERESTING EXAMPLE
Materials called getters can make a huge difference in mitigating certain types of contamination in optical systems. A recent example of this is in extreme UV (EUV) photolithography using 13.5 nm exposure light. EUV photolithography is the production tool for the world’s most advanced patterning of semiconductor chips, currently made only by Dutch equipment manufacturer ASML. In these machines a powerful CO2 laser superheats droplets of tin so they emit the 13.5 nm light, and this is done about 50,000 times a second. Fifty thousand drops a second is a significant amount of vaporized tin, which obviously presents a contamination challenge for many of the optics. ASML disclosed that they remove the ablated tin from their system by letting it react with hydrogen gas at low pressure. The tin and hydrogen combine to form a gaseous compound that can be pumped out of the machine before it can contaminate the optics. This technique of using an additive (in this case hydrogen) to capture an unwanted material is called a gettering process.
GETTERS—FOR USE WITH UV OPTICS:
For many UV optics, not just EUV optics, gettering is a valuable tool for avoiding contamination from a wide range of volatile materials. Solid desiccants or getters can remove volatile materials from the gas in many types of sealed optical systems. Popular desiccants and getters include silica gel, activated carbon, or molecular sieve. They may need to be periodically replaced if they absorb too much contaminant, and they must be packaged so that the getters themselves do not introduce particulate contamination into an optical system. There are certain limitations—for example, getters will not work and will surrender the molecules they absorbed if the getter becomes too hot. However, getters can be extremely effective in the right applications.
SUMMARY:
Many materials, such as nonlinear optical crystals, can appear to be unduly susceptible to optical damage when their lifetime is actually limited by the combination of contamination and UV light. Even the best of optical materials, like the nonlinear material, Lithium Triborate (LBO), will never have a superior threshold for laser induced optical damage if the LBO is contaminated or in a contaminated environment. A careful strategy for avoiding contaminants is key to maximizing the lifetime of UV optics. The methods for doing this are avoiding contaminants, keeping the contaminants that remain from reaching critical optics, and in some cases removing the contaminants off the optical surfaces.
Are you looking for crystals that perform well for deep UV applications? Contact us today for a free consultation with experts that can help you determine the best material for your application.
DR. WILLIAM GROSSMAN, AUTHOR
Will Grossman is a consultant retained by GAMDAN, and his role is to help our customers be more successful with nonlinear optics. His technical expertise includes laser design, nonlinear optics, and laser reliability. Dr. Grossman’s laser designs are used around the world in commercial products. More on the author can be found here.