Identification and Traceability for Optical Elements
Manufacturers and users of glass optics would benefit from identifiable and traceable components but, until recently, marking glass efficiently, clearly and permanently was not a viable option.
In any manufacturing process, information about the creation and nature of the product is critical to optimizing the process. Permanent, unambiguous symbols such as part or serial numbers save time, facilitate in-process testing and reduce mistakes. Marking parts in the metal fabrication and electronics packaging industries, for example, has increased the productivity, traceability and quality of their manufacturing processes.
Despite explosive growth, the optics industry has been unable to implement similar practices. Laser marking is widely used to identify metals, plastics and other materials, but only recently has it been suggested for glass components. This is largely because marking optics efficiently and permanently continues to be a challenge. Even so, technologies exist that can mark glass components with a variety of symbols, including bar codes and two-dimensional portable data files.
Figure 1. Direct laser marking of optics continues to be difficult to accomplish without damaging the glass. However, with help from either a CO2 or an Nd:YAG laser, new coating materials can quickly and efficiently fuse to glass surfaces. Similar to a photomask, the material is applied (top), exposed and then washed off, leaving a permanent and durable mark (bottom).
In a typical manufacturing scenario, a glass optical element might receive an identifying mark via a sticker, pencil, grease marker or diamond-tip scribe or by a stress-free etching process, such as the Airgrit system from Matthews International Corp. in Pittsburgh. These identification methods provide varying levels of benefit, specific to certain applications.
Stickers can interfere with manufacturing processes. Pencils or grease markers, while inexpensive, can leave marks with questionable legibility and permanence. Also, the amount and type of information that can be encoded is limited, and solvents or subsequent processing steps can alter or remove the marks.
Diamond-tip scribes leave a robust mark, but they can cause scratches and illegible marks. One improperly read character can result in a useless component or, worse, a useless assembly that the customer rejects. Therefore, use of diamond-tip scribes is typically slow, labor-intensive and challenged by a greater degree of risk, damage or waste. And again, there is a limit to the type and quantity of information that can be encoded.
The use of an Airgrit glass etching system requires the placement of a rubber screen or mask over the area to be marked. Sand or other ultrafine particles are then blasted over the screen pattern using forced air at 80 to 100 psi. After the mask is removed, the image of the original mask pattern is left as a frosted or etched impression in the glass. This method works on both flat and round optical elements, but it requires the extra step and the expense of fabricating the mask and providing the processing environments and abrasives.
In addition, readable marks or characters produced by this method must measure more than 1.5 mm, which can limit the size and shape of optical elements. Serialization is not too practical because each component marked requires a different mask arrangement.
Although the up-front cost of laser marking may be higher, these other methods may incur costs associated with poor-quality marks, repeated steps or rejected parts.
Many industrial laser marking processes today use CO2 or Nd:YAG sources to mark an assortment of parts materials, including stainless steel, anodized aluminum, plastics and ceramics. Innovative processes have enabled these same lasers to succeed in a variety of glass-marking applications.1 See also, US Patent Nos. 6,075,223 and 6,238,847.
The well-established advantages of using computer-controlled lasers for marking parts — flexibility, speed, low cost and the ability to imprint machine-readable codes — are now available to the small optics shop.
Figure 2. Besides fluid applications, materials for laser marking glass can be applied as tape. The marking process is the same — applying it to the glass, exposing it to laser energy and removing the unexposed substance.
For example, Synrad Inc. of Mukilteo, Wash., a manufacturer of CO2 laser systems, documented significant process developments concerning localized thermal stress induced in glass components by absorption of CO2 laser radiation.2 Initially, this stress caused unacceptable degrees of fracturing and chipping and made marks difficult to read. Synrad claimed to have overcome these problems for certain glasses and reported processes in which CO2 lasers create fractures in the glass surface ranging from 50 to 75 μm in depth.
Producing readable marks with a CO2 laser while avoiding subsurface damage to the optical element depends on process parameters and on the thermal properties of the glass.
Nd:YAG-based laser marking systems also can mark glass materials directly. It is well-established that a solid-state laser can mark the surface or interior of certain glass substrates. In these cases, as in CO2 marking, the laser radiation creates microfractures in the glass, resulting in a visible mark.
When a mark is required but damage or fracturing is undesirable, both CO2 and Nd:YAG lasers can be used in conjunction with material solutions. One example is Cermark, a coating manufactured by Ferro Corp. of Cleveland. This product is composed of inorganic compounds: a combination of frit (amorphous glassy material) and metal oxides. Applied to a glass surface, the material reacts with the localized heat of an Nd:YAG laser and fuses to the glass. With appropriate process parameters for laser power and marking speed, residual fractures are reportedly less than 5 μm below the surface of the glass substrate, although this has not been verified. This process may offer better performance relative to the direct CO2 laser processing.
In addition to Ferro’s black glassmarking material, the company has similar products available in blue and bronze. Originally developed for automotive glass applications, RD-6044 is a water-based product formulated for laser marking of glass, ceramic and similar materials, such as porcelain. The material appears as a dark black paste. It has a density of about 1.64 g/cc, and the suggested thickness for the (initially wet) coating is approximately 100 μm, although optimum coating thickness is a process parameter.
The marking process for this product consists of five steps: cleaning the substrate, applying the product, drying, laser marking and final cleaning of the substrate.
Cleaning and drying the glass surface rids it of any oils, dirt or residue from other manufacturing processes. The coating material may then be applied using a spray gun, paint brush, foam brush, roller, pad printer or screen-printing device. Ferro is also developing a tape that would provide a uniform coating layer on the glass substrate.
Obviously, the coating and subsequent marks should be placed on an area of the optical element outside of the clear aperture. Although the process tolerates some nonuniformities in the coating thickness, matching constant laser parameters with a uniform coating thickness enhances the quality of marks across a production run.
Figure 3. Surface-height profiles obtained with noncontact optical metrology have demonstrated that laser marks measuring about 1.75 mm have corresponding surface heights between 2 and 10 μm, depending on the substrate material, laser parameters and initial coating thickness.
Once the wet coating layer is applied, it must be allowed to dry completely; usually within two minutes in shoplike conditions, although drying equipment can shorten this time. When dry, the RD-6044 appears as a gray powderlike substance.
At this stage, the coated optical element is ready for laser marking. For marking flat glass parts such as slides, filters, diffraction gratings and cylindrical lenses, placing a suitable fixture under a galvo-scanned beam is sufficient to hold the part. For marking round lenses around the outside diameter, a fixture placed in an indexing head is required to rotate the glass part. In all cases, care is taken throughout the entire process to preserve the integrity of the optical element. The laser system patterns the coating layer according to the desired program, which may include a part number, serial number, logo, 2-D bar code or other laser-printable image.
After the marking process is completed, water or a wet towel can remove the remaining residue — essentially unsintered regions of the coating layer — leaving high-contrast features to identify and track the optical element. It is important to note also that the marks can be removed using a grinder.
In our experience with marking glass with RD-6044, the material has produced durable, high-contrast marks characterized by linewidths less than 0.25 mm. The dimensions of the identifying marks are important for micro-optical elements (e.g., small filters and lenses) and glass parts that have a very thin edge.
Ferro’s original application for the material was for automotive glass and required resistance to severe conditions. In this context, the company tested its RD series of materials for significant exposure to chemicals such as ethanol, gasoline, NaCl solutions and hydrogen fluoride. The marks reportedly survived all tests except the HF. This suggested that Ferro’s series would perform well in some parts of the optics manufacturing process.
Advantages of this marking process include processing speed, the ability to create small characters and resistance to subsurface damage and chipping. In terms of expense, the material costs approximately $200 a pound; the amount needed to coat 1 sq cm costs about a penny. The identification mark illustrated in Figure 1 was written at a rate of 1 in./s using about 10 W from an Nd:YAG at 1.064 μm.
The serialized mark took approximately 10 seconds to pattern, including the time needed to index the part in angle, but excluding time to load and unload it. Careful coating, cleaning, handling and perhaps fixturing of the components obviously requires additional time. It is possible to mark the material an order of magnitude faster than this in some cases. Process optimization and smaller characters can further reduce marking time and cost.
This indicates that robust human- and machine-readable identification marks can assist in improving the optics manufacturing process and life-cycle traceability, and a variety of cost-effective solutions are available. Laser marking processes can either be integrated into the manufacturing process or outsourced to contractors who understand the required optics handling procedures. As technologies and applications evolve, coupled with the integration of global optoelectronic manufacturing strategies, users of glass and glasslike components will demand excellent identification markings.
1. G.J. Shannon and J. Law (March 2001). Glass marking with CO2 lasers. INDUSTRIAL LASER SOLUTIONS FOR MANUFACTURING.
2. G.J. Shannon (July 2000). Glass marking — tips to minimizing fracturing. SYNRAD APPLICATIONS NEWSLETTER.
3. B. Smandek et al (October 2001). Diode-pumped laser for single-shot micromaterials processing. PHOTONICS SHOWCASE.
4. D.E. Smith (September 2000). GMT-lasers: the light of the future. CERAMIC INDUSTRY.
5. E.A. Axtell III (Nov. 13, 2001). Durability testing of laser marks on glass. Test Report, Ferro Corp.
Special thanks to Brian McMaster for technical discussions and calculations regarding topography and fracture analysis.
Meet the author
Mark Shaw is vice president at aScribe Inc. in Fairport, N.Y.
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