Optical Coating Innovations Push Performance

HANK HOGAN, CONTRIBUTING EDITOR, hank.hogan@photonics.com

Optical coatings improve the performance of mirrors, lenses, and light-emitting or absorbing semiconductors by enhancing transmission and reducing reflection. They also harden surfaces, allowing components to withstand harsh environments or take pounding from high-power lasers. To continue to meet rising demands, however, coatings need advancements in control, measurements, and cost — areas vendors seek to address by rolling out innovations.

A case in point comes from photonic component and subsystem supplier II-VI Inc. of Saxonburg, Pa. The company recently posted record sales, in part due to emergence of extreme ultraviolet (EUV) lithography. This new semiconductor manufacturing technology illustrates an important trend toward increasing laser power levels.

Diamond overcoat (DOC) on zinc selenide (ZnSe). Coatings form a protective layer, allowing components to better withstand harsh environments and high-power lasers. Courtesy of II-VI.

In the past, the company would designate lasers 1 kW and above as high power, said Jim Bacon, IR thin film engineering manager. Now the trend is to apply this category to 4-kW lasers and beyond. Coatings have been forced to keep pace with these growing power levels, which in some situations are quite high.

“In the case of EUV systems used in the semiconductor market, the optics could be exposed to 20- to 40-kW power,” Bacon said.

In an EUV system, an IR laser bombards a tin droplet, which then produces the extreme UV light used in the lithography. That IR beam must be steered to the droplet, and all the optics along the way are subjected to the high-intensity beam. In the future, power levels may rise, as this is one way to increase the production of EUV and thereby boost throughput while cutting costs.

Automated IR thin films coating systems for laser optics. Courtesty of II-VI.

Another less extreme example of this trend shows up in OLEDs. Organic light-emitting diode manufacturing demands high-speed, high-performance scanning systems for cutting, marking, and drilling a variety of materials, which in turn places new requirements on beam delivery components.

Optical coatings typically are made by depositing thin films on a substrate, with the films forming a durable layer. The films can be anywhere from nanometers to microns thick, so processing capabilities must enable control on an even finer scale.

The deposition is done in a vacuum chamber, with an electron beam, plasma, or other means supplying the energy to transform a material into something that can be deposited. The materials are chosen, and the layer thickness and composition is set so the desired optical properties are achieved.

Interior view of the chamber of a fully automated electron beam (e-beam) deposition system capable of evaporating a selection of materials onto a substrate. These systems make constructing complex optical coatings possible. Courtesy of Thorlabs.

A few years ago, in response to the need to improve the durability of coatings, II-VI developed and began using what it calls a diamond-like carbon overcoating. This coating improves the lifetime of the optics, with minimal impact on laser absorption, Bacon said. It offers additional benefits because it allows operating parameters to be changed. Thanks to this coating, for instance, the transmission of windows on zinc selenide, silicon, and germanium components increases by several percent compared to traditional diamond-like carbon coatings, according to Bacon.

Using a proprietary process and equipment, II-VI can incorporate a protective outer layer in any part of the coating process. This is done in a single deposition chamber, which means the chamber does not have to be opened. Keeping it closed minimizes the exposure of the components within to contaminants that can impact performance, durability, or other parameters of the coating stack.

Medicine and the life sciences could see high growth in the future. For such applications, biocompatibility is a critical need. Some customers are using the diamond-like carbon overcoat for this purpose, said Stan Himelinski, senior thin film development engineer in II-VI’s IR division.

“Life sciences and biotechnology are advancing at a rapid pace, pushing the need for cleaner and safer solutions,” he said.

According to Mike Scobey, CEO of Santa Rosa, Calif.-based Alluxa, one of the largest markets for optical coatings is an everyday application: windows. The company designs and manufactures optical filters and coatings using a proprietary plasma deposition process. Prior to joining Alluxa, Scobey co-founded a smart glass company.

Smart glass and smart windows reveal future challenges for optical coatings. Smart glass is smart because it changes from virtually transparent to almost opaque on command. A variety of methods exist to accomplish this, with one being a response by electrochromic glass to the application of voltage. The glass has multiple thin layers of metal oxide inside it, with these forming a metallic ceramic coating. The voltage change causes the ions to move between layers, which alters the material’s structure and thereby its tint.

Precise design and control of the manufacturing process of optical coatings enable the deposition of layers for spectral balancing filters. The output of a xenon lamp (a), which has high spikes, can then resemble the output of the sun (b). This can be useful for testing measurement equipment and other applications. AM: air mass coefficient. Courtesy of Alluxa.

Despite years of development, smart glass is still not the standard, in part because users do not want to pay a price premium. Consequently, manufacturing needs to be as low-cost and as high-volume as possible, which presents a challenge for the optical coating manufacturer. Less than half a dozen layers are involved in an electrochromic window. However, the layers must be nearly perfect over a large area because anything less, without fixing a defect, can lead to unacceptable performance. A killer defect can even render smart glass dumb and inoperable.

“The defects can create shorts and color spots,” Scobey said. “So, you’ve got these 60- × 80-in. sheets of glass that really can’t have meaningful defects on them.”

Manufacturers could meet the challenge, however, as somewhat similar requirements are found in the manufacture of LCD panels. Manufacturers of LCDs also need to make large panels with few, if any, defects, and they’ve developed successful methods to do so.

For smart glass and, in general, all applications, optimizing optical coatings involves a manufacturing balancing act, Scobey said. The process involves depositing multiple layers of materials. Consequently, the ability to accurately control the thickness and composition of the window layers is critical to achieving transmissivity at specified wavelengths, reflectivity at the others, overall durability, and other performance parameters of the optical coating. This need for fine control points toward a requirement for deposition to take place as slowly as possible.

Automated precision cleaning systems for laser optics. Courtesy of II-VI.

At the same time, a competing need exists to put down the layers rapidly and at a low cost. Increasing the throughput will mean more components will be produced in an hour, which will mean the depreciation cost of the equipment will be spread out over more product. Depreciation can take up a significant chunk of the overall cost of high-tech manufacturing equipment. This expense occurs continuously every day, making it imperative that the manufacturing equipment churn out as much product as possible.

One way to address these competing needs is through more accurate in-process measurements, Scobey said. Alluxa does this via in situ monitoring of the deposition parameters. Having a more accurate picture enables optical coating manufacturers to optimize this constant balancing act.

“You can play that trade-off into any of the buckets,” Scobey said, “including faster or more accurate, or things of that nature.”

If improved instrumentation is important to the future of optical coatings, so too is expanding the selection of materials that make up those coatings, said Chuan Ni, a thin-film coating scientist with Thorlabs Inc., the Newton, N.J.-based company that is a photonics products manufacturer.

Current monitoring techniques that work while the deposition process is underway primarily focus on the thickness of the films, Ni said. This characterization is vitally important, but other critical parameters, such as refractive index and film composition, are typically measured post-process.

Having more in situ characterization options would be useful, according to Ni. For one thing, they could enable real-time optimization. Coatings are often made of multiple layers, and for cleanliness and throughput reasons, coating manufacturers prefer not to open the deposition chamber. So they will deposit multiple layers in a single run. Since this is performed under an automated control algorithm, it is possible to know the exact condition of a previously deposited layer, which makes it possible to take action if anything is amiss.

“As a result, it would be [possible] to correct the errors of previous layers to achieve a better overall coating performance,” Ni said.

He added that performance involves multiple aspects. Reflection and transmission for specific wavelengths, along with attributes such as film hardness, temperature and humidity resistance, threshold for laser damage, and more can play a part. Depending on the application, the most most important aspect may vary.

Regarding materials, the selection of those with the right spectral characteristics in UV and IR is limited, Ni said. Having more materials available would mean that spectral performance could be improved, by absorbing less of the wavelengths of interest, for example. Because coatings consist of stacks of materials, it is also necessary for materials to have a high contrast to one another, another area for possible improvement.

More contrast means coatings could be thinner, while offering the same or better overall spectral performance. Putting down a thinner coating would increase throughput and reduce material needs, both of which would cut costs.

More and better coating materials, as well as improved film characterization methods, are under development, Ni said. When rolled out, the combination should help the industry meet future demands for increased performance, improved robustness, and lower cost.

Discussing such improvements, Ni said, “Better coating design would help reduce the cost, like choosing less expensive materials in the film stack, making film stack [layer] thicknesses thinner, [while] being able to achieve performance, and making the coating less sensitive to process errors.”