Optical Coating Technology and Applications: Past and Present to Future

Optical coating technology and applications: past and present to future

Amber Czajkowski, Edmund Optics Inc., Pennsburg, PA, USA

Deposition technology has evolved tremendously since the ﬁ rst single layer optical interference coating was pre- sented more than 70 years ago. Today, methods involv- ing evaporation deposition, IBS, and APRS are amongst the preferred technologies for thin-ﬁ lm production. This article discusses the differences between various coating technologies and their applications.

Increasing demand for high-precision and deposition process. Fundamen- durable thin fi lms in biophotonic, defense, tal understanding of these key and laser-driven applications is perpetu- ingredients is what caused the ally propelling the development of cutting- industry to prosper as such. edge technologies and cost-effective man- Each of these ingredients ufacturing processes. The article explores requires profound research the evolution of optical coating technolo- and development. Advance- gies and the applications that they are best ments in software and compu- suited for. ter technology have rendered a number of very powerful 1 Coating fundamentals thin-fi lm design programs that can solve sophisticated thin- Figure 1: For IAD coating processes at 200–300°C, cham- The design and development process of fi lm equations and develop ber pressure is typically reduced to <10.7·10-6 mbar thin-fi lm coatings has matured signifi cantly theoretical solutions to com- since the fi rst single-layer antirefl ection plicated application problems. coating procedure was invented, real- Additionally, material sciences have sig- materials are transformed into a vapor ized, and patented in 1935 by Alexander nifi cantly contributed to the design and state by either resistance heating or elec- Smakula while working for Carl Zeiss in fabrication process. Material selection is tron-beam bombardment inside a vacuum Jena, Germany. Branded the fi rst practi- quite an arduous process because of the chamber, at pressures on the scale of cal method of thin-fi lm deposition, he limited number of optical materials suited ~2·10-6 to 60·10-6 mbar, depending on the employed vacuum technology to apply for coating applications; however, it largely type of coating. The evaporated materials a fl uoride compound to a glass lens. As determines a coating’s production viability. then condense and adhere to substrates a result, adverse refl ection effects intrin- A theoretical design is virtually useless if it that are loaded into planetary work-holders, sic to the glass were eliminated. Not relies upon materials that can’t be depos- which rotate within the chamber to achieve only did Smakula’s contribution to optical ited in an effective way. coating uniformity. The resulting fi lms are interference coatings signifi cantly improve Methods including evaporation deposition, generally porous with columnar microstruc- the performance of many optical devices, ion-beam sputtering (IBS), and advanced tures exhibiting spectral shift when exposed but it prompted a new interest amongst plasma reactive sputtering (APRS) are to varying thermal conditions or changes in researchers to further explore thin-fi lm amongst the preferred technologies com- humidity; unless the deposition process is technology. monly employed for thin-fi lm production. enhanced by ion assisted deposition (IAD). At that time, photographic lenses were In the following, each of these processes In this process variant (fi gure 1), additional the primary application for thin-fi lms; since is analyzed for technological differences, bombardment with energetic ions provide then, the optical coating industry has manufacturing effi ciency, product perform- for more densely packed layers with less grown to serve a versatile market, consist- ance, costs, and the application or market built-in strain and better adhesion to the ing of approximately $2.1 billion world- it best suits. substrate. wide. Single layer fl uoride deposits have morphed into complex multi-layer, multi- 2 Evaporation deposition 2.2 Capabilities functional designs. However, there have The evaporation deposition process infl icts consistently remained three requirements 2.1 Technology fairly low operational costs while being for quality optical coating production: (1) a The fi rst effective method used to apply compatible with an assortment of mate- good theoretical design, (2) reliable materi- thin-fi lm coatings was evaporation deposi- rials, ranging from dielectrics to metals. als, and, most importantly, (3) a practical tion, a thermal process by which source Additionally, the chamber geometry cre-

10 Photonik international · 2010 Originally published in German in Photonik 1/2010

010-013-Czajkowski.indd 10 17.11.2010 12:53:08 Uhr Coatings

ates a substantial separation between the 3 Ion-beam sputtering signifi cantly reduced cost. One example material source and the optical parts, lead- is in advanced plasma reactive sputtering ing to superior coating uniformity even 3.1 Technology (APRS); more specifi cally, the commercial with complexly-shaped and steeply curved Ion-beam-sputtering (IBS) technology, APRS system “Helios” by Leybold Optics optical components. emergent in the 1970s, allows fabrication GmbH, Alzenau, Germany, has nearly Nevertheless, it is often an unreliable of high-precision theoretical designs with revolutionized the coating industry. APRS manufacturing process, which is heavily impressive accuracy. This sputter process enables complex coating designs consist- dependent on operator input and, when employs an ion gun to produce energetic ing of alternating high and low index compared to alternative methods, sub- ions which are accelerated up to tens of target materials. Deposition can exceed ject to a greater number of random and eV by an electric fi eld. Once they reach the 200 layers in a single run, with short pro- systematic errors. This is mostly attributed material target there is transfer of kinetic duction cycles. to changes in process variables such as energy to create a cascade of collisions that The high-precision results demonstrated vapor distribution, rate of deposition, ejects, or “sputters,” material from the tar- by APRS-coaters are attributed to recent vacuum pressure, temperature, and so get source. The sputtered particles can bal- developments in plasma sputtering, using forth. Layer-to-layer errors are therefore listically fl y from the target to energetically two dual-magnetron sources (fi gure 3), greater, resulting in inconsistent spectral impact the substrate, resulting in deposi- operating at mid-frequency. Material oxi- performance. Furthermore, it takes longer tion. Alternatively, at higher gas pressures dation is completed when the substrates to complete a full coating run in an evap- (typically inert argon gas), the particles can pass through an oxygen plasma, located oration chamber, and often at reduced collide with the gas atoms in the chamber along the rotational path of the turntable capacity. Depending on the design, it can and follow a random path to thermally that carries the substrates. Because the sometimes take up to two times longer condense on the substrate, which creates sputtering and the fi nal oxidation process than alternative APRS processes. Since a low-energy deposition alternative. Either are separated, the targets are not prone capacity, yield, and production-cycle time way, a hard dense fi lm growth process is to charge build-up effects, which can be are all signifi cant cost drivers, the end- accomplished. problematic in the sputter deposition of cost of an evaporation produced part is metal oxide fi lms. The deposition rate often less than ideal. 3.2 Capabilities (~0.5nm/sec), which is close to that of IBS yields the highest quality deposition, evaporation, is well-controlled, resulting 2.3 Applications resulting in very low-loss, stable, and shift- in highly predictable fi lms with increased The evaporation method lends itself well free optical coatings (fi gure 2). Drawbacks repeatability. Also, since the energy of the to coating mirrors and most antirefl ec- are associated with high equipment costs: A APRS deposition method is much higher tion designs, which have fairly unde- capital investment of more than 500 000 € than evaporation (20–30 eV compared to manding requirements. It can also be per machine is required for start-up, and a 1 eV), the outcome is denser, more stable, employed on various edge fi lter, beam large amount of maintenance is necessary shift-free fi lms. splitter, and notch fi lter designs where to keep the ion source operating properly. the number of layers and design com- Other disadvantages include low deposi- 4.2 Capabilities plexity is kept at a minimum; however, tion rates, thus, long production cycles, APRS technology produces higher through- once the specifi cations get too rigorous, and relatively small coating run capacities, put and more environmentally robust coats accuracy is greatly diminished. For this also resulting in high cost-per-part. than evaporation deposition, with faster reason, evaporation deposition is unfavo- lead times and improved reproducibility of rable for high-precision fi lters, which can 3.3 Applications complex designs. The cost per coated part contain > 100 layers. Moderate complex- Because of its desirable characteristics, is similar to evaporation, but signifi cantly ity fi lters can be fabricated, but this often IBS technology was at the forefront of reduced in comparison with IBS. takes several preliminary tests, resulting narrowband fi lter development for dense in longer development times. For exceed- wavelength-division multiplexing (DWDM) ingly demanding fi lter specs, an alternate in the 1990’s. Here, bandpass fi lters fabrication method is necessary. transmit narrow segments of light, while attenuating the surrounding wavelengths. Similar manipulation techniques are used for current-day applications in biomedical, laser, and defense fi elds, where stringently toleranced bandpass and notch fi lters are employed to transmit and/or block specifi c spectral ranges. IBS fulfi lls these high-precision needs, nevertheless, it’s often ruled out as a contender in terms of cost.

4 Advanced plasma Figure 2: Subsequent to deposi- reactive sputtering tion – here: 800 nm high performance shortpass fi lters – coatings can 4.1 Technology Figure 3: The APRS platform is capable exhibit adverse spectral shifting when Recent advances in sputtering technology of producing high precision coatings exposed to varying thermal conditions have shown promise of coating equip- quickly and effi ciently, making cus- or changes in humidity. More densely ment which provides much of the control tom fi lters more affordable for a wide packed fi lms greatly reduce this effect and characteristics of IBS fi lms, but with range of applications

Originally published in German in Photonik 1/2010 Photonik international · 2010 11

010-013-Czajkowski.indd 11 17.11.2010 12:53:10 Uhr Coatings

• High-performance edge fi lters are used ments. This process uses a vacuum tech- in applications ranging anywhere from nique based on the principle of sputtering, astronomy to Raman spectroscopy, and except with much higher power densities designed to isolate specifi c wavelengths (kW/cm2 range) delivered over very short, in the long-wave or short-wave regions. 50–200 µs, pulse lengths to heat the Optical densities >6 with transmission target. Another promising technology is >95% in the passband are readily achiev- that for Directed Vapor Deposition (DVD), able for fi lters of this sort. The spectral which precisely controls the transport of edge tolerance maintains <1% devia- vapor atoms from source to substrate. This tion, and can be enhanced to <0.2% for process generates higher deposition rates Figure 4: For today’s most common special cases. and reduced process times, compared to fluorophores, tailor-made optical • Notch fi lters are narrowband refl ectors classic evaporation techniques. However, bandpass fi lters provide OD>6 block- used to refl ect a specifi c and poten- further development is necessary to apply ing in the cut-off region and high tially destructive laser wavelength, while this technique to optical coating applica- transmission in the passband, here retaining a high level of out-of-band tions of interest. around 531 nm transmission, e.g. visible light. Coatings of this type are practical for demanding 6 Conclusion display applications or night-vision sys- APRS is suited for effi ciently realizing tems that need to reject harmful laser As the number of optical applications in diffi cult-to-manufacture coating designs, radiation. APRS is able to produce thin- the life sciences, military, and laser optic where high precision, high optical density, layer notch designs with <5% ripple, industries continues to grow, the demands high transmission, and steep slopes are key and OD>6 at the center wavelength, on optical coatings extend to designs of features. These capabilities are critical for comparable to rugate fi lter perform- increasing complexity with tighter toler- capturing emerging markets. ance. ances. Each of the discussed coating tech- • Enhanced polarization-controlled mul- nologies presents its own distinct advan- 4.3 Applications tilayers can be manufactured on the tages and disadvantages (cf. table 1). It Capabilities for this technology extend into fi rst attempt, such that measured per- was shown that evaporation deposition the entire range of fi lter designs including formance is almost identical to that of is well-suited for antirefl ection and mirror shortpass, longpass, laser-line bandpass, the theoretical design. These typically coatings, but when the complexity of the fl uorescence bandpass, dichroic, and notch include beam splitters, whose P- and design increases as does cost. One advan- fi lters. Some of these highly demanded S-polarization states are manipulated to tage that evaporation deposition holds precision fi lters are discussed below, with transmit or refl ect depending on the over APRS is in its ability to coat complex- reference to some common applications in application, or to produce non-polarizing shaped optics; however, the inaccuracies of growth markets of interest. results (fi gure 5). For the latter, the dif- evaporation due to random and systematic • Precision bandpass fi lters can be manufac- ference between P- and S-polarization errors make it unreliable for high-precision tured for applications in biomedical, laser, can be carefully controlled to well within tasks. IBS technology is very accurate and and defense fi elds. In biophotonics, there 5% for a given wavelength and angle of favorable to more stringent specs, but the is an increasing demand for high-precision incidence. high capital and maintenance costs rule it fl uorescence fi lters with discrete excitation/ out for several mainstream applications. emission wavelength separation. These fi l- 5 Technology forecast From a cost and precision standpoint, APRS ters are typically composed of 100–200 deposition technology often outweighs the layers in a multi-cavity design for a steep In the long-term, it’s anticipated that alternative methods discussed. This is criti- spectral transition from high transmission APRS will remain a preferred fabrication cal in such a competitive industry – where in the passband to optical-density (OD) method for high-precision fi lters, with it is essential to maintain high-performance attenuation levels >6. This creates signal/ cost potential to extend into lower-end of the optic at a reduced cost. noise and transmission/blocking ratios of coating markets. Researches continue to 6 greater than 10 :1 (fi gure 4). A speci- explore a High-Power Impulse Magnetron Literature: fi cation for a fl uorescence fi lter might Sputtering technique, known as HIPIMS [1] A. Thelen, Milestones in Optical Coating Technol- be OD>6, transmission >90%, and edge or HPPMS, due to its excellent adhe- ogy, Proc. of SPIE Vol.5963, 2005 steepness <0.5% over a less than 5 nm to sion capabilities and extreme durability in [2] P.W. Baumeister, Optical Coating Technology, SPIE greater than 50 nm FWHM bandpass. aggressive and high temperature environ- Press, Bellingham, Washington, 2004 [3] I. MacMillan, Helios Advanced Capability, Power- Point presentation, Apr. 2009 [4] H.A. Macleod, Optical Thin Films, from Optical Sci- ences (OPTI575) class notes, Thin Film Center Inc., Spring 2008 [5] I. MacMillan, Optical Fabrication: Advances in sputtering benefi t coating costs, Laser Focus World, www.laserfocusworld.com/articles/358143, Apr. 2009 [6] R. Jaeger, Film Deposition, Introduction to Micro- electronic Fabrication, Upper Saddle River, Prentice Hall (2002) [7] A. Jaunzens, Versatile coatings all fresh optical Figure 5: The accuracy of the APRS system becomes evident e.g. in the compari- properties, OLE, Feb. 2009 [8] Directed Vapor Technologies, Inc., Coating Tech- son of the theoretical design (left) vs. measured spectral data (right) for a 532 nm nology for Future, Charlottesville, VA, www. non-polarizing plate beamsplitter directvapor.com

12 Photonik international · 2010 Originally published in German in Photonik 1/2010

010-013-Czajkowski.indd 12 17.11.2010 12:53:10 Uhr Coatings

Coating Technologies Evaporation IBS APRS Author contact:

Capabilities Amber Czajkowski Capital Investment & Operational Cost + + - + Thin-ﬁ lms Engineer Accomodation (materials, optic shapes) + + (-) - Edmund Optics Inc. Manufacturing Depart- Production Capacity (-) - + ment Process Reliability & Design Repeatability (-) + + + 601 Montgomery Ave. Spectral Precision / Tolerance Capability - - + (+) + + Pennsburg, PA 18073, USA Stability (spectral & adherence) - + + + + Tel. +1/215/679-6272 Prozess Time - - - + Fax +1/856/573-6295 Applications eMail: [email protected] Internet: www.edmundoptics.com Antireﬂ ection Coatings + + + + Mirror Coatings + + - Martin Weinacht High-Precision Bandpass & Notch Filters - - + + + Sales Director Edge Filters - + (+) + + Edmund Optics GmbH Zur Giesserei 19-27 Polarization Controlled Multilayers - + (+) + + 76227 Karlsruhe Germany Tight Tolerance Specs and >100 Layer Designs - - + (+) + + Tel. +49/721/6273730 + Favourable, - unfavourable, () neutral / variabel with process parameters Fax +49/721/6273750 eMail: [email protected] Table 1: A comparison of coating technologies and applications shows that no Internet: www.edmundoptics.de single technique provides a solution for all optical coating challenges. For opti- mum results, a deposition method needs to be chosen based on its capabilities and intended application

Originally published in German in Photonik 1/2010 Photonik international · 2010 13

010-013-Czajkowski.indd 13 17.11.2010 12:53:10 Uhr