WDM optics drive improvements in coatings technology
Coatings vendors have risen to the challenge of complex optics needs. Designers of components must understand the capabilities of the chamber and the coatings, plan for monitoring, and consider the costs involved in custom vs. off-the-shelf choices.
Market forces are pushing the performance of optics to their limits. Optical components must be developed to provide the best possible combination of manufacturability, performance, and price. One vital step to success in creating WDM optics lies in a discipline that is often overlooked or misunderstood—coating engineering.
Coating requirements for components such as wavelength-division multiplexers, which require complex and difficult-to-achieve wavelength accuracies and durability, are driving the development of coating technology.
HOW COATINGS WORK
Coatings use constructive and destructive interference in thin films to create a specific spectral response (which can be a mirror, partially reflecting mirror, an antireflection coating, or a filter) over the spectral region of interest. The coatings are thin dielectric films deposited on glass. Dielectric materials are nonabsorbing (in other words, exhibit very high transmission) from the UV through the visible and, of particular interest for WDM applications, well into the IR.
To understand interference, consider light as sine waves. When the lightwave encounters an interface, the reflected portion of this wave changes phase. The total phase change is a result of the combination of phase changes at interface reflections in combination with phase changes due to the optical path length the light travels. The phase change is related to the thickness of the interface layer. Typically, dielectric layers are deposited on the surface of the component in alternating high and low refractive indexes of quarter-wave optical thickness (QWOT).
The QWOT is prevalent throughout optical coating designs because it produces the maximum change in phase for any single dielectric layer. Layers of half-wave optical thickness, also known as absentee layers, do not alter the performance at the design wavelength but may be used to modify transmission away from the design wavelength. The resultant added wavefront from all the reflections presents either an additive or subtractive effect. An additive effect is that of a high reflector, and a subtractive effect result as in an antireflection coating (see Fig. 1.)
A COATING CHAMBER
It is easier to describe how a coating is made if one understands the parts of a coating chamber. A typical optical coating chamber is 24 to 40 in. in interior diameter and contains an array of components. The coating chamber includes several subsystems (see Fig. 2). In this article we focus on the process and limitations of the most common method for making WDM components—vapor deposition.
The first subsystem holds and rotates the components being coated. It is either a planetary dual rotation or calotte single rotation mechanical structure. Planetary tooling is preferred if precision and uniformity are critical; the calotte is used if tight tolerances are not specified, and provides more parts per coating run. The planetary spins the components. Each tool includes a set of standard diameter holes that hold custom inserts, which in turn hold the components being coated. These inserts are made, if not already available, for each type of component being coated.
Moving down in the chamber, the next subsystem is the element heaters. These are placed along the perimeter of the chamber to aid in heating the chamber and specifically the substrate or components being coated. The chamber is typically heated to between 250°C and 300°C.
Next is the focus point of the chamber: an electron-beam gun vaporizes a target, held in a crucible, to create the vapor that fills the chamber and deposits onto the components (as well as all the other surfaces in the chamber). A complex system of crucibles and shutters allows the correct material to be vaporized for the correct amount of time. These crucibles are loaded into a rotating wheel. The coating machine or the operator moves the correct material in front of the gun at the correct time to deposit the next layer. The shutter stops vaporization after the correct material thickness is deposited.
In some systems an ion gun is used to add energy to the material as it is vaporized for better control of the process. This ion-assisted deposition (IAD) method increases the density, or packing factor, of the coating. This in turn decreases the voids in the coating and opportunity for moisture to comingle with the layer. Moisture changes the effective index of a thin film and causes the coatings properties to shift. Moisture in the coating limits the accuracy possible in a coating.
The layers are required to be a specific thickness, on the order of 1/10 of a wavelength of light. Two primary measuring methods are quartz crystal frequency monitoring and optical monitoring.
Crystal monitoring is based on the film being deposited on the crystal the same as the components of interest. As the thickness builds up, the characteristics of the crystal change accordingly. This change can be monitored and directly related to the thickness of the film. The second method uses the same concept, with the exception that it uses an optical detection basis. All of these systems work in concert to deposit very accurate layers of dielectric films to produce the result of the coating design. The chambers can create complex coating structures in excess of 100 layers.
Coating a single surface takes nine separate steps, and a two-sided component takes 16 steps. Each step is labor- and time-intensive (see photo, p. 51). A typical broadband antireflective (BBAR) coating can take more than three hours of machine cycle time.
It takes the same coating time to coat an entire chamber full of parts as it does to coat a single part. The nine-step process involves:
- Prepare the tooling inserts for the coating run. If these inserts do not exist for the specific parts, they must be machined. The machining process can take up to several days depending on complexity of the components to be coated, and the number that can fit into an insert.
- Clean and load the components into the tooling. Depending on the size of the part, and the number of them to coat, this process can take from seconds per part to minutes.
- Prepare the coating chamber for the run. The chamber needs to put through a series of checks to make sure all systems are functioning and all necessary surfaces in the chamber are covered.
- Load the planetary tools into the coating chamber.
- Evacuate the chamber down to 2 x 10-5 Torr, and heat the chamber to between 250°C and 300°C. The vacuum removes airborne containments and moisture from the chamber as well as allowing more mobility to the material being vaporized.
- Deposit the coating onto the component. (See "Monitoring coating deposition ensures correct wavelength positioning," this page) Depending on the complexity of the coating, this process can take from half an hour to days. In complex filtering technologies, such as those in telecommunications, multiple hours to days is the standard.
- Cool and vent the chamber back to room temperature and pressure.
- Remove the components from the chamber and test the witness sample. The witness sample is a window that is coated along with the components. This window is the piece that will go into the spectrometer to determine the spectral response of the coating. This window is necessary because the spectrometer cannot test a part with a curved surface. In addition to spectral testing, most coatings are checked for adhesion and abrasion resistance. Depending on the application, coatings may also be required to pass other environmental tests such as high humidity, high/low temperature cycling, salt spray and resistance to various solvents.
- Inspect and package the components.
CHALLENGES TO REPEATABILITY
Designing and making a coating is not an exact science. The design of a coating is highly dependent on the deposition chamber in which it will be made. The designer and operator must know and understand the nature of the calibration of the machine, as well as any issues with the performance of the individual subsystems being used. All the factors contribute to the accuracy and repeatability of the coating from run to run. In coatings that require many multiple layers, the risk goes up for effective monitoring of the process.
Monitoring is particularly problematic for the telecommunications industry. The complexity of these coatings drives the performance of typical optical coating chambers and special machines have been made to accommodate the production of wavelength-division multiplexers.
Other telecommunications coatings, expected to function for more than 20 years, can be made in typical coating chambers equipped for ion-assisted deposition. This is particularly helpful for coatings that require tight position accuracy of the center wavelength. Notch or edge filter coatings also benefit greatly from this enhanced technology.
In many cases, tolerances are the key to the simplicity or complexity of manufacture. The engineer who specifies the coating can reduce his company's costs and improve the coating yield by asking for realistic performance. If the standard offerings from coating vendors will not meet the customer's need, the customer will do well to keep his requirements as close to the standard versions as his application will allow. Better yet, call the coating company and discuss your requirement with a coating designer. Working with the coating vendor during the design stage can save money, time, and headaches during production.
As with any other product, however, off-the-shelf coatings are less expensive than custom coatings. Any standard coating eliminates costly development and should be available at a shorter lead time. Using tried and tested processes also reduces the probability of failure in the coating chamber.
Coating failures do happen, and no one wants to see several weeks — or months —worth of precision-manufactured glass tossed out because it has a bad coating. Designs made on the computer always claim that a coating is manufacturable, but the execution in the coating chamber can be a different story.
Wallace Latimer is director of the custom product group at Edmund Industrial Optics, 101 East Gloucester Pike, Barrington, NJ 08007. He can be reached at email@example.com.
Monitoring coating deposition ensures correct wavelength positioning
WDM applications demand precision wavelength positioning. Picking out one channel and rejecting the others requires that the center wavelength of coating pass or stop band be highly accurate. The accuracy of wavelength positioning depends on how carefully the deposition is monitored.
For typical edge filters, the accuracy can be ±1.0%, and deposition is monitored with a quartz crystal. For DWDM filters, however, accuracy can be as fine as ±0.002%, and deposition is optically monitored with state-of-the-art equipment.
Conventional optical monitoring charts the reflectance (or transmittance) at a preselected wavelength during the deposition of the layer. As the layer approaches quarter-wave optical thickness (QWOT), the percentage reflection or transmission reaches a turning point on the chart (because the QWOT produces the maximum change in R or T). The turning point may be used as a layer termination trigger or as a calibration level from which to calculate the eventual termination value.
Optical monitoring can be highly effective in producing coatings of tight tolerance provided the core design is a regular quarter wave stack. This is a result of a highly effective error compensation feature inherent in the turning point detection and layer termination method: if, during the coating process, the turning point is over—or under—shot, it is compensated by terminating the deposition at the turning point of the next layer. This compensating effect minimizes the cumulative error in the multilayer stack and can result in very accurate filter wavelength positioning, as demonstrated by "successful" production of DWDM narrowband transmission filters.
Quartz crystal monitoring measures the physical thickness of the depositing material, and, therefore, does not involve any turning point methodology. While this technique does not provide any error compensation, it is a very useful when monitoring layers that are significantly thinner than one quarter wave (layers less than QWOT have no turning point and therefore present difficulties for optical monitoring). This quartz crystal methodology is favored in the production of designs such as broadband antireflection coatings, where layers as thin as 10 nm are common. Precision multilayer coatings such as edge filters can be produced using quartz crystal monitoring; however, without the compensatory effects of optical monitoring, accurate material characterization and very tight process control are necessary to achieve specification.
Most coatings do not exhibit any significant polarization effects at angles of incidence less than 20°. At higher angles, the S and P states behave quite differently. This is a consequence of their effective angular refractive index, given by: Ns = Ncos(q) and Np = N/cos(q). At 45°, the variation in S and P performance can be very significant. A 50:50 beamsplitter (random polarization) may transmit 75% P and only 25% S. Polarization insensitive coatings can be produced at high angles for single-wavelength operation. Achieving nonpolarization over a broad waveband, however, presents a difficult challenge to both thin-film designers and engineers. In telecommunications, polarization control is critical, and many coatings include a minimum polarization-dependant loss (PDL) specification. In some telecommunication applications, PDL is significant at angles as small as 15°.