01 Apr 2005
Successful UV optics buying requires an understanding of fabrication limitations and careful reading of manufacturers’ specifications, say David Collier and Rod Schuster.
Ultraviolet (UV) laser processing offers advantages over longer wavelengths for a number of applications. In materials processing, these benefits include the ability to produce smaller features, to precisely control maximum processing depth and to remove material without affecting surrounding areas of the workpiece. However, the limited number of materials that transmit in the UV and the destructive power of high-energy UV photons makes producing long-lived optics for this wavelength range challenging. That is why it is important that buyers and vendors understand each other's needs. Here, we offer some guidance on how to choose and specify the most economical UV optics that will perform properly within a given application.
Most UV optics are made from either fused silica or calcium fluoride (CaF2). Fused silica is a robust material that is relatively easy to fabricate into a wide range of different-shaped optical components, making it an economical choice of substrate material (see figure 1).
Fused silica comes in a variety of different grades, with the two major categories based on UV transmission. The "UV grade" is useful for optics that need to transmit light down to about 200 nm, while "standard-grade" fused silica starts to attenuate at significantly below 260 nm. It is difficult to obtain UV-grade fused silica in diameters much larger than 100 mm, so components of this size and over are quite expensive. Within the UV- and standard-grade classifications, a number of different grades are available, based on parameters such as bubble and inclusion content, the homogeneity of the refractive index and the amount of birefringence.
The second common material option, CaF2, is a hygroscopic crystalline material and typically transmits down to about 170 nm. However, the natural anisotropy and brittleness of CaF2 render it prone to chipping and fracturing during polishing, and make it necessary to control the orientation of the crystal's axis with respect to the polished face. This makes it difficult for the fabricator to simultaneously meet tight surface-quality (scratch-and-dig) and flatness specifications. It is therefore important not to overspecify CaF2 components, otherwise they will become more costly than necessary. Since CaF2 components are typically more expensive than the equivalent fused-silica optics, they are usually only used at the short wavelengths at which fused silica cannot transmit well, most notably at the 193 nm excimer wavelength.
Surfaces and coatings
Successfully obtaining UV optics of a given flatness or surface accuracy requires a careful understanding of the manufacturer's specifications and how they are generated. One cause of confusion is that most optics suppliers specify flatness at a red wavelength of 632.8 nm, rather than at a wavelength in the UV. This means that you have to be careful about interpreting datasheets. For example, a flatness specification of λ/10 at 632.8 nm translates into about λ/6 at 355 nm, and only λ/3 at 193 nm.
When specifying a custom optic, the clear aperture (the area over which the flatness specification applies) should be kept as small as the application will permit, since a larger clear aperture value rapidly drives up part cost. It is also critical to understand that the flatness specifications of most manufacturers only apply to the substrate prior to coating. For visible-wavelength optics there is usually not much difference between pre- and post-coating substrate flatness, but this is not true in the UV and deep-UV for two reasons.
First, most optical thin films are deposited at high temperature, which results in mechanical stress as the part cools. The stress in each coating layer results in either compression or tension, depending upon the materials used and the deposition conditions. Most longer-wavelength coatings deliberately use a mixture of different materials in the layers to give a net stress that is very small. However, the choice of coating materials is much more limited in the UV - at 193 nm and below, virtually all the materials generate tension. These stressed coatings can often pull a substrate out of its flatness specification.
The second part of the problem comes from water absorption by the coating materials which can change the coating stress and thus the component shape. The insidious aspect of this problem is that, at a given temperature and humidity, the part may exhibit a flatness that is within specification, but under different environmental conditions, changing stress levels may put it out of specification (see figure 2).
Although there are methods for producing dense thin films which possess reduced water absorption, such as ion-assisted deposition or sputtering, these films contain inherently more stress to begin with. As a result, they are more likely to warp a substrate out of its original shape. Furthermore, dense films often exhibit lower damage-threshold characteristics than porous coatings.
For high-flatness and high-damage-threshold optics, porous films are often the best choice. With porous coatings, it may be necessary for the supplier to control the environmental conditions of their testing, and state at what temperature and humidity values their specifications apply. For demanding applications, the customer and supplier must work together to agree on test conditions that accurately reflect the intended use.
Fortunately, there are techniques the manufacturer can apply to minimize the effects of coating stress on part shape. The most simple of these is to lower the aspect ratio of the part's diameter to its thickness. Part deformation increases with the square of aspect ratio, so just moving from the industry standard value of 6:1 down to 5:1 results in a nearly 30% improvement in distortion resistance. On a related note, it is typically easier to control surface accuracy on circular parts than rectangular components.
Another process, useful for front-surface reflectors, involves depositing a second coating on the back surface of the part, solely for the purpose of combating stress induced in the front surface. It is also possible to prefigure an optic substrate with a slight curvature in the opposite direction to the calculated coating stress. The coating stress then serves to deform the part towards higher flatness, rather than away from it.
The topic of laser damage to thin-film coatings and bulk materials at UV wavelengths has been widely investigated and there are many different approaches to the problem. Nevertheless, it is still possible to make some important generalizations about maximizing the laser damage-threshold of UV optics.
Of critical concern is subsurface damage (SSD), a phenomenon first identified by researchers at Lawrence Livermore National Laboratory in California. SSD refers to the fractures and scratches that occur during the grinding and polishing process, which then become partially or totally covered by the polishing redeposition layer - a thin layer of material that flows over the surface of the substrate while it is being processed. Trace amounts of residual polishing compound may become incorporated into the redeposition layer, or trapped in micro-fractures and surface defects. Producing optics with minimum SSD involves frequent and thorough cleaning between each fabrication step.
Surface roughness is also an important contributor to the damage-resistance of UV optics because it affects coating adhesion and causes scatter that ultimately leads to laser damage. Because scatter increases strongly with decreasing wavelength, achieving a given scatter specification at 193 nm requires a surface an order of magnitude smoother than that needed for visible wavelengths. This issue is now being addressed by the widespread use of superpolished substrates with surface roughness at the 1-2 Å level. By comparison, typical "laser-grade" optics have a surface roughness in the 10-15 Å range.
UV optics are very sensitive to contamination and the same level of care used to avoid problems during fabrication must extend all the way to the customer's door. Traditional optics packaging and wrapping, which utilizes lens tissue, plastic bags and foam packing material, may not be adequate.
Ideally, all packaging materials should be inert and not produce particulates or outgas. This is important because optics may be exposed to extremes in temperature and even pressure (for example, in an aeroplane hold) during shipping. Part containers should securely hold the optic by its corners and not touch the clear aperture (see figure 3).
The production of high-performance UV components requires techniques that differ substantially from those used for longer-wavelength optics. In addition, specifications and parameters not usually considered for other optics may play an important role in UV components. Successfully sourcing UV optics requires working with a vendor who thoroughly understands the relevant issues. It is advisable to engage in a dialogue so that specifications are clearly communicated and all of the factors affecting performance and cost are considered.