OBTAINING HIGH FLATNESS OPTICS

Understanding coating, substrate and environmental factors is essential when purchasing high flatness optics.

By David Kemp, Ph.D. and Wayne Pantley, Alpine Research Optics

David Kemp, Ph.D. is Eastern Regional Sales Manager and Wayne Pantley is Sales Manager and Marketing Manager at Alpine Research Optics, 3180 Sterling Circle, Boulder, CO 80301, 303-444-3420, FAX 303-444-1686, E-mail bouldersales@saint-gobain.com, www.arocorp.com

Many vendors currently offer optics with a flatness of λ/10 or better. However, flatness specifications often only apply to the substrate prior to coating, and mechanical stress in a thin film coating can warp a component substantially out of shape. Environmental factors, such as temperature and humidity, also affect part shape. Because of these reasons, performance specifications for many off-the-shelf optics fail to tell the whole story.

This article is intended to educate buyers of high performance optics about the factors that affect part flatness and provide some guidance for understanding manufacturers' specifications and qualifying vendors. It also gives an overview of fabrication techniques for achieving high flatness optics on a production basis.

Stress and Flatness

Most optical thin films are deposited at high temperature, and mechanical stress is introduced into a coating when the part subsequently cools; this is demonstrated in figure 1. The stress in each coating layer can be either compressive or tensive, depending upon the materials used and the exact deposition process parameters. In addition, water absorption due to atmospheric humidity can later cause additional, time varying changes in internal thin film stress. The net result of all these effects is that an optic originally specified at λ/20 might end up with a surface accuracy of only λ/4 in actual use. The magnitude of temperature and humidity effects is shown in figure 2.

Water absorption effects are most pronounced in porous thin films. Various coating densification methods, such as ion assisted deposition (IAD) or sputtering, deliver more environmentally stable films. Unfortunately, these dense films contain inherently more stress to begin with. Thus, they're more likely to warp a substrate out of its original shape. Furthermore, dense films often exhibit lower damage threshold characteristics than porous coatings. Thus, for high flatness and high damage threshold optics, porous films remain the predominant choice. As a result, the issue of humidity related stress must be addressed, and conditions in the final use environment must be known in order to accurately predict component performance.

The total amount that a part can be distorted by coating stress is primarily determined by its aspect ratio: the ratio of the part's longest dimension to its thickness. It has been fairly standard in the optics industry to use a substrate aspect ratio of 6:1 or less when producing λ/10 or better optics. But based on our experience at ARO, we have found that it's safer to use an aspect ratio of 5:1 or less in order to consistently produce high flatness optics in volume. This is because part deformation increases with the square of the aspect ratio; thus, going from just 6:1 to 5:1 results in a nearly 30% improvement in distortion resistance. Substrate material itself also plays a role, because various materials have different stiffness characteristics.

The substrate fabrication process also can introduce stress into the bulk material, even before coating. This is because operations such as milling, grinding and polishing apply pressure to the substrate and cause sub-surface damage. It's most effective if any internal stress is relieved before final polishing, so that the part doesn't "spring," or continue to change shape, during final figuring. Stress relief can be accomplished by process steps such as polishing part edges or polishing the second surface of an optic, instead of just producing a find grind (the latter only applies to optics that operate in a first surface mode, such as high reflectors). Parts can also be temperature cycled in order to pre-anneal them before final processing.

Understanding Specifications

Part flatness can be specified in a variety of ways. The first step in obtaining flat optics is to fully understand how a manufacturer defines and specifies flatness. Any discussions with that vendor must be conducted in a common language to avoid misunderstandings, and customer produced drawings and specifications should clearly state what definitions are being used and how they are applied.

One simple way of specifying flatness is peak-to-valley. This is the height difference between the highest and lowest parts on the surface of the piece. Another definition of flatness divides the specification into two separate components: power and irregularity. Power is the deviation in overall part shape from the desired surface - a perfect flat plane in the case of a flat optic. Irregularity refers to small scale surface imperfections.

The power and irregularity definition is particularly useful when addressing post-coating flatness issues. This is because the coating process and subsequent environmentally induced changes most commonly distort an optic in a way that creates either spherical or cylindrical power. Conversely, small scale irregularities in an optical surface are there from the outset and generally don't vary due to coating and post-coating factors. Thus, the irregularity specification provides a clear measure of the best a part will ever perform under any circumstances, while the power number gives an indication of the magnitude of post-coating related effects. In contrast, a peak-to-valley flatness measurement only provides a broad description of what is happening to component shape without clarifying which factors of imperfection are dominating the result. Furthermore, the absolute value of the power added to irregularity always produces a number that is greater than or equal to the peak-to-valley result, making it a more conservative measure of part flatness.

When specifying parts its important to remember that the actual effect on wavefront depends on whether a part is reflective or transmissive. In the case of a reflector, the wavefront is distorted by twice the value of the surface specification at normal incidence. On the other hand, a transmissive optic only distorts the wavefront by roughly half the surface specification; the actual value depends on the refractive index and angle of incidence.

These factors underline the importance of carefully dialoging with the manufacturer. Indeed, a number of questions should be asked of any prospective vendor of high flatness optics. First, it should be established whether flatness or transmitted wavefront specifications are meant to apply just to the substrate, or to the finished coated part. Also, it's more useful to specify optics in terms of actual wavefront distortion, rather than flatness. This takes into account all the use dependent factors.

It is also essential to ask how specifications are measured. Ideally, flatness and wavefront distortion should be measured with an interferometer. The buyer of high performance optics should feel entitled to an interferogram of that very part, not a representative sample; however, one should expect to pay for this service.

It is also necessary to ascertain the conditions under which the testing is performed. Are parts measured under the intended temperature and humidity use conditions? If not, can the vendor reliably calculate the performance shift between the measured and end use conditions? In some cases, the substrates themselves have been fabricated and measured by another manufacturer - the vendor thus loses control over the precise testing conditions.

For this article, ARO purchased optics from several major catalog suppliers. The table lists the results of interferometric testing of a few of these samples, and highlights the need for obtaining proof of performance. In the worst case, an optic specified as λ/10 had an actual surface figure of only λ/5.

However, some of the burden for achieving the required part performance rests with the user. Specifically, it's necessary to accurately specify the nominal operating temperature and humidity, as well as the total possible variation in both these parameters. Understand, however, that the broader the operating range, the more expensive the part.

Achieving High Post-Coating Flatness

There are a number of fabrication techniques targeted at achieving high post-coating flatness. One approach involves prefiguring. This is purposefully polishing a slight curvature on a substrate with the intention of having the coating stress distort the optic back into flatness. Unfortunately, this is a somewhat empirical process and it's also difficult to reliably produce the very slight curvatures required. The result is that production yields using this approach are low.

Another technique, useful for first surface reflectors, is backside coating. Specifically, a single layer coating, typically SiO2, is placed on the second surface of the optic. The purpose is to balance and null out the overall mechanical stress on the part. This approach is particularly popular with scanner mirrors, which often have a poor aspect ratio. Once again, the major drawback of this method is its unpredictability, which makes it problematic for use in volume manufacturing.

ARO has developed a new approach for reliably achieving high post-coating flatness, based on our experience in producing demanding optics for the Lawrence Livermore National Ignition Facility project. This process begins with the production of a substrate possessing high flatness in terms of both power and irregularity. This is accomplished by careful application of relatively traditional optical fabrication techniques. A multilayer dielectric thin film is then applied which is specifically optimized to introduce minimal distortion to the part. This is done by use of a combination of coating materials with both tensive and compressive characteristics. In addition to minimizing overall stress, the design is also optimized to produce the required damage threshold, mechanical durability and spectral performance. Most important, ARO has successfully characterized the relationship between temperature/humidity and coating stress. Understanding how coating stress changes between production conditions and the final operating environment has enabled the design and consistent production of optics that meet specification in actual use.

In conclusion, successfully obtaining high flatness optics requires an understanding of how manufacturers make, measure and specify parts. System builders who require very low wavefront distortion should develop a relationship with an optics vendor who understands all the issues involved and can consistently produce optics that meet specification in the final use environment.

Figures

Figure 1: In these interferograms of reflected wavefront, a three inch diameter component prior to coating shows a peak-to-valley wavefront distortion of better than λ/20; after coating, the reflected wavefront distortion has dropped to approximately λ/14.

Figure 2: The graph shows change in surface flatness (power) as humidity varies and temperature is held constant, and as temperature varies and humidity is held constant, for a three inch diameter, 0.6 (thick, fused silica substrate.)