Measurement Considerations When Specifying Optical Coatings

ADVANCES IN OPTICAL MANUFACTURING

Measurement considerations when specifying optical coatings

PETE KUPINSKI and ANGUS MACLEOD

Design, specification, and procurement single layers of the con- of optical coatings all benefit when the stituent coating materials designer has a good understanding in the past and has determined, within the mea- of measurement techniques and surement uncertainty of uncertainties. their spectrophotometer, Accurately measuring the performance that there is little scatter or absorption of an optical thin-film coating can be as in any given layer of the filter. challenging as designing and manufac- Once the coatings are deposited, if the turing it. Understanding measurement performance is tested and reported only techniques and uncertainty when spec- in transmission, the broadband mirror ifying and procuring optical coatings is and passband regions of the filter will important for optical system designers. both appear to be within the designer’s This article is meant as a brief intro- specified tolerance. With broadband duction to optical coating measurement dielectric mirrors, however, it is possi- for optical system designers, quality en- ble to have very different performance gineers, and purchasing agents. In the when measured in transmission vs. re- course of the article, we hope to touch flection. Broadband dielectric mirrors on several of the more common mea- create resonant structures within the surement questions that are asked of multilayer stack, leading to field ampli- coating engineers when optical systems fication at certain wavelengths. Even a are being specified. very small amount of loss in individual coating layers results in large losses for Understanding how the filter at these resonant frequencies. A dielectric multilayer stack coating, measuring 0.5 µm in total thickness, that performance is measured When measured in transmission, this has been intentionally released from the The following is an example of a simple resonant loss has a positive influence optic surface before imaging. The image was measurement oversight with a potential- on blocking, making the mirror appear taken at 200x using a Nomarski microscope. ly large effect on system performance. better than it actually is (see Fig. 1a). This example is meant to highlight When measured in reflection, this same the need to ask questions about how resonance creates an obvious and sig- coating laboratories. They are more things are measured when purchasing nificant ripple in reflectivity across the versatile and less expensive than la- a coated optic. visible spectrum, and consequently the ser-based measurement systems and, In this example, a system designer is coating doesn’t meet the specification when used properly, can be very ef- interested in including an optical filter for the mirror (see Fig. 1b). fective. Most coating companies use in the system that reflects a minimum commercially available spectrophotom- of 99% intensity in the 350–450 nm Measuring specular eters; standard features include grat- wavelength band and transmits greater performance ing-based monochromators and a ref- than 99% at longer wavelengths. The Spectrophotometers are the prima- erence beam to deal with light source coating engineer has measured loss in ry measurement tool in most optical and detector drift. Reprinted with revisions to format, from the October 2015 edition of LASER FOCUS WORLD Copyright 2015 by PennWell Corporation a) 100-measured % transmission 100 95 90 85 80 75 70 320 370 420 470 nm

b) % re ection 100 95 90 85 80 75 70 320 370 420 470 nm

ADVANCES IN OPTICAL MANUFACTURING

a) 100-measured % transmission b) % re ection 2. Detectors are often spatially vari- 100 100 able. The effect of a small change in beam 95 95 position from reference to sample mea- 90 90 surement is small when measured in re- 85 85 flection and large in transmission for the 80 80 reason outlined in point 1. As the position 75 75 70 70 of the beam changes between baseline and 320 370 420 470 320 370 420 470 sample measurements, the accuracy of the nm nm measurement suffers. Beam-path changes FIGURE 1. Spectral measurements are shown for a multilayer broadband dielectric mirror in become more significant as sample thick- transmission (a) and reflection (b). Ripple in the mirror as measured in reflection is caused by ness, index of refraction, and measurement resonance and amplified loss. angle increase. The degree of beam-position change is described by Snell’s law of It is important to note that the follow- tightly controlled index of refraction as the refraction, the effect of which can be am- ing discussion is of general considerations high-signal reference instead of an open plified by multiple Fresnel reflections from for commercially available instruments. beam. Schott N-BK7 glass is common- opposing optical surfaces. For thick wit- In some cases, optical coating companies ly used because of its stability in refrac- ness samples mounted at high angles, it will develop their own spectrophotome- tive index. can become nearly impossible to get a rea- ters that have measurement advantages Uncertainty in wavelength for commer- sonably accurate transmission measure- over more general instruments for their cial systems is typically less than ±0.3% ment (a witness sample is typically a small, particular application. for ultraviolet (UV) through near-infrared flat plate of glass that is coated along with A large range of accessories are available (NIR) wavelengths. It is relatively straight- the rest of the components, serving as a for commercial instruments. Each of these forward to check wavelength calibration testable example and record of the partic- accessories has advantages and disadvan- using lamp emission lines and/or trace- ular coating run). Measuring transmission tages for different measurement types. A able glass standards doped to give sharp through a lens using a spectrophotome- full review of these accessories and their absorption peaks in the wavelength range ter is challenging for the same reasons. strengths and weaknesses is beyond the of interest. The challenges outlined above can be ad- scope of this article; we will instead focus dressed by adding an integrating sphere on trying to provide an understanding of Spectrophotometer-based to the detector, using thin witness sam- some general considerations when mea- transmission measurements ples, and decreasing the distance from the suring with spectrophotometers. One of the fundamental challenges of op- sample to the detector. Larger integrat- To make a measurement with a spec- tical coating measurement is that it is dif- ing spheres typically achieve better results trophotometer, known reference measure- ficult to measure AR coatings and mir- because of averaging of light over many ments with intensities less than and great- ror coatings accurately and precisely in bounces within the integrating sphere be- er than that for the surface under test are transmission and reflection, respective- fore it strikes the detector. They typically made across the wavelength band of in- ly. The following points highlight two contain baffles to eliminate direct bounc- terest. A detector-specific equation is then of the larger sources of error when mea- es to the detector and a smaller port-win- used to correlate measured signals at each suring AR surfaces in transmission using dow fraction to reduce the proportion of wavelength to sample intensity within this spectrophotometers: light leaving the sphere.2 range. Uncertainty associated with this 1. For AR surfaces measured in re- Using larger integrating spheres can have calculation is often referred to as error flection, a small error in photometric drawbacks, however, when measuring with from detector nonlinearity.1 In the simplest accuracy equates to a typically insig- a small beam (required for small samples case, reference measurements are taken nificant measurement error of coating or high angles), in that less light reaches with a blocked beam (0% intensity) and performance. For example, a 1% error the detector, leading to potential signal-to- open beam (100% intensity). in measured reflected intensity (R) for noise issues. With a very good setup on a Photometric, or ordinate, uncertainty a surface that only reflects 0.1% of in- commercial instrument, a best-case mea- is partially a function of the range used cident light equates to an inaccuracy in surement uncertainty for a flat, thin AR- for the reference measurements. For ex- measurement of 0.001%R for the sur- coated witness sample at normal incidence ample, when measuring surfaces with face under test. The same error in trans- is typically in the range of ±0.1%T. antireflection (AR) coatings in reflection, mission (T) would be much more signif- Polarizers and variable-angle detector it is common practice to reduce uncer- icant; 99.9% transmission × 1% error modules can be purchased for commer- tainty in detector nonlinearity by using equates to an inaccuracy in transmis- cial spectrophotometers. Variable-angle a single-surface reflection of glass with sion of nearly 1%T. reflectance accessories do an adequate job ADVANCES IN OPTICAL MANUFACTURING

of measuring AR performance in reflec- associated with reflectance standards. skewed significantly when using poorly tion. A measure of the Brewster’s angle Ideally, a traceable calibrated mirror collimated test beams. Cropping the ap- can be used to gauge uncertainty in an- is used for the reference measurement. erture of the beam on noncollimated sys- gle and polarization; with a good setup, Degradation of these reference mirrors tems helps, the tradeoff being a reduction these sources of uncertainty are typically over time because of metal oxidation and/ in signal to noise. small compared to the photometric sourc- or mechanical degradation must be con- Also, for high-precision filters, the sig- es of error. A good summary of sources sidered. Total uncertainty in measuring nal bandwidth as dictated by the grating of uncertainty in transmission measure- high reflectors depends on all the terms and slit width can become limiting. The ments can be described using the follow- outlined in equation 1.1 for transmission. best measurements of high-precision fil- ing formula:3 However, for mirror measurement, the ters and polarizers are typically made with term for ordinate uncertainty (µ ) is in- custom (collimated) spectrophotometers µtotal = N fluenced to a greater degree by uncertain- and/or laser based measurement systems. √(µ )2+(µ )2+(µ )2+(µ )2+(µ )2+(µ )2 s A N p o w ty in the reflectivity of the reference. A Commercially available spectrophotom-

The combined uncertainty (µtotal) is a general measurement stage on a commer- eters typically have a half-cone angle of function of measurement repeatability (µs), cially available spectrometer is capable of 2–3º. An example of the effect cone an- beam alignment (µA), ordinate linearity measuring high reflectors with approxi- gle can have on coating performance can

(µN), polarization uncertainty (µp), angle mately ±0.5 %R uncertainty, while a care- be seen in Fig. 2. of incidence uncertainty (µo), and wave- fully aligned and calibrated commercial length uncertainty (µw). spectrophotometer with a precision reflec- Witness samples A suggested approach to gauging spec- tance accessory is capable of measuring All the considerations mentioned above tral performance of most AR coatings is to to approximately ±0.05 %R uncertainty. are important; however, even if every- perform measurements in reflection over thing is done perfectly, the data is of no the complete angle range of the specifica- Cone angle value if the witness sample is not repre- tion and a measurement in transmission Spectrophotometers generally have some sentative of optic performance. It is nec- to verify loss at normal incidence. For crit- cone angle associated with the test beam. essary to use witness samples because in ical applications, it is important for sys- The best spectrophotometers for optical % R tem designers to know whether they are applications are more-collimated, high 2.0 receiving measurements over the full an- f-number systems. Understanding the ef- gle range or theoretical data at high an- fects of measurement-system cone angle is 1.5 gles when they purchase a lens. especially important for coatings such as 1.0 polarizers and precision filters. The place- 0.5 Spectrophotometer-based ment of a filter or polarizer edge can be Optic Optic edge center reflection measurements 0.0 % Rp % Rs 900 950 1000 1050 1100 1150 1200 There is always some minimum angle (typ- nm ically less than 10º) at which a sample can 3 100.0 FIGURE 3. Performance of an uncorrected be measured in reflection because of the 99.8 2 1064 nm V-coat AR at 0° on a 150 mm 99.6 physical constraints of light source and de- convex-radius-of-curvature lens is plotted. 99.4 tector placement. For most optical coat- 1 At the center of the optic (blue line), the AR ings, the difference in performance from 99.2 is centered correctly at 1064 nm. At a 50 0 0 to 10º angle of incidence is insignificant. 99.0 mm radial distance from center (red line), 53 54 55 56 57 58 59 60 the coating is 12% thinner and reflectivity at An accurate measurement of highly re- Angle (°) the edge of the clear aperture climbs to just flective surfaces using a spectrophotom- FIGURE 2. Performance of a 1064 nm less than 1%. This plot is meant to represent eter is challenging for many of the same Brewster’s angle (56.6°) thin-film polarizer is typical uncorrected performance using a line reasons outlined above for AR transmis- shown as a function of incident angle. A 2–3° of sight coating process; actual uniformity will sion measurements. Small changes in mea- half-cone angle on the spectrophotometer vary depending on process and equipment. sured signal at the detector between the test beam can have a significant effect on reference and measurement lead to sig- measured performance of the p-polarized nificant errors in measured performance. (blue line) passband transmission. It is most cases it is very difficult to get an ac- Inserting a test sample into the measure- interesting to note that leakage of p-polarized curate measurement directly off the op- light can also be an application issue when ment path without changing the path used tic. For AR coatings, it is crucial that the using noncollimated light in an optical system. in baseline reference measurements is al- P-polarized light will leak through the polarizer index of refraction and dispersion of the ways tricky when measuring in reflection. because of skew rays in rough proportion to witness sample closely match (or exactly Additionally, there can be challenges the square of the illumination cone apex angle. match) that of the optic. ADVANCES IN OPTICAL MANUFACTURING

Accurately measuring coating perfor- The accuracy of loss measurements us- compared to the substrate that mechanical mance often requires that coating hous- ing spectrophotometers is typically lim- properties of the film can be ignored. The es maintain a large inventory of witness ited by the accuracy of transmission stiffness of the film itself, or resistance to samples covering the full range of sub- measurements, as discussed previously. bending, is ignored—as seen by the lack strate optical properties. Placement of the High-accuracy measurements of mirror of a film elastic modulus term in the equa- witness sample(s) is also of critical impor- reflectivity can also be accomplished us- tion. The accuracy of this approximation tance. Witness samples should be placed ing cavity ring-down by placing the sample starts to break down as film thickness and in such a way as to match both the height under test inside the cavity. This technique film stiffness increase. Freund and Suresh and angle of the surface inside the coating is often used to measure the reflectivity of outline the limitations of the Stoney ap- chamber. For large and/or steeply curved laser mirrors when greater than roughly proximation in their book on the subject.7 surfaces, a second witness sample at the 99.95% reflectivity is required. Using their approach, we estimate that outside edge of the clear aperture can be careful selection of wafer size allows for a required. The uniformity of an optical Coating-induced figure change calculated uncertainty in measured stress coating changes inside of a chamber as Optical coatings, even though very thin, of less than 5% for most optical coat- a function of radial distance, height, and have enough internal stress to impart sig- ings. The following are some important surface angle. Figure 3 shows an example nificant force to the surface of the optics points of note as related to post-coat sur- of the effect of uncorrected nonuniformi- on which they are deposited. Too much face deformation: ty on coating performance. coating stress can distort the figure of an 1. For optics that are round and have flat optic and/or lead to mechanical failure of and parallel surfaces, the coating-stress-in- Laser-based measurement the coating itself. duced bending is almost all in the form of techniques For lenses and windows that have AR power (radius change). For optics of this For the highest-precision measurements coatings on both sides, the net effect that geometry, it is straightforward to predict of reflectivity, transmission, and loss, la- the coatings have on surface figure is very the post-coating figure change using the ser-based measurement techniques can be small, as the forces exerted by these coat- Stoney formula. More complicated geom- required. A dual-beam laser-based reflec- ings have equal and opposite effects. Figure etries can be successfully modeled using tometer (LR) is capable of measuring with distortion can be a significant challenge, finite-element analysis. great accuracy and precision across wide however, for relatively thick coatings such 2. It is important to note that bend- angle ranges in reflection and transmission. as dielectric mirrors, polarizers, and filters. ing scales linearly with film thickness (tf) When designed well, these instruments An effective way to measure stress is and with the square of substrate thick- have significant advantages in beam sta- to deposit the coating onto a thin, round ness (ts). Thin optics with large diame- bility and test beam geometry and all the wafer, measure bending of the wafer af- ters and thick coatings have more coat- advantages of a collimated light source. A ter coating, and then back-calculate the ing-stress-induced bending. well-built LR can have measurement un- stress. A significant contributor to coat- 3. Low-strain-point optical glasses have certainties of ±0.01 to ±0.05% for mirrors ing stress is thermomechanical in nature, been observed to occasionally change and highly transmitting surfaces as mea- driven by elevated process temperatures shape plastically when exposed to ele- sured on flat witness samples.4–6 and the thermal-expansion mismatch be- vated coating temperatures. This change Laser-based cavity ring-down measure- tween the optic and materials in the coat- ment systems can be used to measure op- ing. To get a good measurement of coating Re ected phase (°) tical loss (absorption + scatter) or reflec- stress, it is often necessary to use wafers 350 tivity with very high sensitivity. These that have a thermal-expansion coefficient b) systems measure the change in intensity similar to that of the optic. The Stoney 300 -0.5% change in coating thickness of light leaking out of an optical cavity equation can be used to calculate stress formed by two highly reflective mirrors. in the coating from the change in radius 250 a)

When a transmitting sample is placed in- of curvature of the wafer: 200 side the cavity, the change in leak-down 631 632 633 634 635 E • t 2 1 Wavelength (nm) time as light is absorbed or scattered by σ = s s f 6(1–v )t ( R) the sample can be used to calculate loss s f FIGURE 4. A comparison shows phase in reflection for a visible broadband dielectric in the sample with resolution on the or- where Es = modulus of elasticity of sub- der of parts per million. strate; v = Poisson’s ratio of substrate; t mirror coating (a) and the same coating s s shifted 0.5% thinner (b). The change in Loss can also be determined with low- = substrate thickness; R = radius of cur- thickness of the coating has no significant er accuracy using spectrophotometers or vature; and σf = stress in film. effect on optic performance, but appears laser reflectometers by subtracting mea- The Stoney approximation assumes that as a 70° change in reflected phase when sured %T from (100%–measured %R). the thickness of the film is small enough measured with a 632.8 nm interferometer. ADVANCES IN OPTICAL MANUFACTURING

a) 0.26 λ P-V b) 0.33 λ P-V c) 0.26 λ P-V figure. When done well, this approach can be effective. 2. Flash-coat a thin metal film on top of the dielectric multilayer coating to eliminate phase effects on reflection. The metal coating can then be stripped off after testing. This technique is not ideal for some applications (high-energy laser or UV, for example). FIGURE 5. Form (peak-to-valley) of a fused silica mirror is measured with a 632.8 nm 3. Measure the uncoated back surface interferometer: pre-coat (a), after applying a multilayer broadband dielectric mirror (b), and after of the optic before and after coating to flash-coating aluminum over the broadband dielectric mirror (c). All images are on the same test figure change. This approach is ef- scale. No significant form change was expected after coating based on mirror geometry and fective but adds cost in back-surface pol- measured coating stress. Form change in (b) is the result of a phase change from the center to edge of the mirror because of coating thickness change and optical interference effects in the ishing and testing. dielectric mirror. 4. Characterize post-coat surface roughness with an atomic-force micro- in shape is not coating-stress-induced, or index of refraction. Coating nonunifor- scope (AFM) instead of an interferome- but related to changes in the glass itself. mity occurs to some degree in all coating ter. Per best practices in measuring sur- For these optical glasses, low-tempera- processes and, as mentioned previously, face roughness, careful attention must be ture coating processes can be required to can be especially challenging on steeply given to spatial period filtering whenever achieve tight figure control. curved surfaces. changing measurement techniques. Also The example listed in Figs. 4 and 5 be aware that root-mean-squared (RMS) Measuring post-coat shows the effect of a broadband dielec- surface roughness can measure significant- figure change tric mirror (BBHR) on post-coat wave- ly lower using a small spot AFM (10–100 Accurately measuring the post-coat fig- front as measured with a helium-neon la- µm field of view) as opposed to a more ure and roughness of precision optics ser interferometer (632.8 nm wavelength). standard 20x interferometric test (350 × is challenging. In most optics shops, The optic was a large fused-silica convex 450 µm field of view ) as a consequence form error and roughness are measured mirror. Coating uniformity across the sur- of the RMS calculation.10, 11 with phase-shifting interferometers. A face had been corrected to <0.5%. Coating In extremely high-quality imaging and phase-shifting interferometer collects in- stress was tuned to a near-neutral level and interferometric systems, the change in formation on reflected phase while mod- modeling predicted no significant physi- phase—especially from a reflecting coat- ulating the distance between a reference cal distortion of the optic because of the ing—can become a performance issue, as and test surface. The phase information coating. Fused silica is known to have high the wavefront at any given wavelength is collected across the optical surface un- dimensional stability at coating process a function of both surface figure and the der test is used to calculate changes in temperatures. After coating, the custom- phase change induced by the coating. It surface height. er required a surface form measurement is important for such applications to con- Optical coatings use constructive and over the BBHR. A significant change in re- sider minimizing coating nonuniformity. destructive interference of light from a flected wavefront was measured post-coat- In coating design, a very rapid change in series of alternating high- and low-in- ing with a 632.8 nm interferometer. The phase with wavelength is a danger sign. As dex-of-refraction films on the surface of apparent change in measured post-coat mentioned previously, any measurement an optic to achieve the desired spectral figure was attributed to reflected phase of wavefront from a coated surface oth- performance. Testing a surface with an effects, which was verified by flash-coat- er than at the wavelength of use can give interference coating using an interferom- ing the optic with a thin aluminum coat- nonrepresentative results. When the ap- eter can give a result that is erroneous and ing and remeasuring with the same in- plication requires precise control of wave- not representative of actual surface form terferometer. In fact, the surface figure front, it is ideal (although not always fi- or roughness.8, 9 The exception to this is of the optic hadn’t changed significantly nancially possible) to test the coated optic when one is measuring wavefront of a during coating. in precisely the same spectral conditions coated surface to be used at a single wave- To quantify the degree to which an in- as those intended in use. length using a laser interferometer work- terference coating changes surface figure ing at the same wavelength. The reflect- or roughness, the following approaches Conclusion ed phase from an interference coating can have been used successfully: When procuring optics, it is important to change quickly with otherwise spectrally 1. Measure coating stress and model the ask how the spectral performance of the insignificant changes in coating thickness effect that the coating has on post-coat coating or coatings is being measured. Ask ADVANCES IN OPTICAL MANUFACTURING

how the vendor verifies loss in AR coatings tradeoffs with coating engineers before 8. D. Savage and J. Watson, “The effect of phase and accuracy when measuring high reflec- quoting is recommended to get the best distortion on interferometric measurements of thin film coated optical surfaces,” Optimax tors. Make sure the coating vendor is us- overall system performance. Systems Inc. white paper (2010); http://bit. ing witness samples that have an equivalent ly/1Uvj16K. REFERENCES index of refraction and that witness-sam- 9. P. Baumeister, Optical coating technology, 1. A. Reule, Appl. Opt., 7, 6, 1023–1028 (1968). SPIE—The International Society for Optical ple placement is representative of the sur- 2. J. Taylor, “Integrating sphere functionality: Engineering, Bellingham, WA (2004). face being coated. For applications where The scatter transmission measurement,” 10. D. Aikens, “Meaningful surface roughness form (not phase) must be measured, avoid PerkinElmer technical note (2013); see http:// and quality tolerances,” Proc. SPIE, 7652, bit.ly/1IN8xsS. asking for post-coat interferometric mea- International Optical Design Conference (2010). 3. T. Germer et al., Spectrophotometry: Accurate 11. J. Bennett and L. Mattson, Introduction to surements unless they are being made at measurement of optical properties of materials, surface roughness and scattering, Optical the wavelength of use. For antireflection Academic Press, Amsterdam, Netherlands (2014). Society of America, Washington, DC (1989). coatings, the net effect of coating stress 4. A. Voss, Appl. Opt., 33, 36, 8370–8374 (1994). 5. S. Holler, Fordham University, correspondence Pete Kupinski is optical coating group lead- on surface figure is very small. The effect (Jun. 11, 2015). er at Optimax Systems, Ontario, NY; e-mail: of dielectric mirror and filter coatings on 6. C. Smith, University of Rochester, [email protected]; www.optimaxsi. form can be significant. In these cases, un- correspondence (May 26, 2015). com. Angus Macleod is president of Thin Film derstanding the coating process and result- 7. L. Freund and S. Suresh, Thin film materials: Center Inc, Tucson AZ, and professor emeritus Stress, defect formation and surface evolution, of optical sciences at the University of Arizona; ing coating stress is important. Discussing Cambridge University Press, Cambridge, e-mail: [email protected]. system requirements and manufacturing England (2003).