Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published May 1996

PROCESSING

RONALD SCHAEFFER

Lasers provide unique processing capabilities for the manufacture of many products, including disposable medical devices. In some cases, the use of laser-micromachining technology allows more cost-effective solutions compared with other available methods, even though competing technologies may offer similar technical capabilities. In many other instances, laser-based manufacturing may provide the only technical solution to unique fabrication problems. It is the job of laser integrators to review all customer

requirements--including tangible elements such as processing speed, throughput, feature size, and budget, as well as intangibles such as "feature quality" and "user-friendliness"--and propose the most sensible, effective approach. Rigid attention to detail is necessary both in the actual production and in the quality control necessary to ensure product conformity to specifications. This article reviews the fundamental physics of carbon dioxide (CO₂), excimer, and solid-state lasers as related to material interaction and discusses considerations relevant to optimizing a laser-based system for a particular manufacturing environment. Finally, factors relevant to quality control in manufacturing are discussed.

COMMON INDUSTRIAL LASERS

Excimer, CO₂, and solid-state (primarily Nd:YAG and Nd:YLF) lasers represent the three most common types of lasers used in industrial micromachining applications. Each has its own unique characteristics, and together they provide very complementary capabilities (see Table I). (Tables and figures are not yet available on-line.)

Carbon dioxide lasers are the most familiar and widely used photon sources. Employed in industry since the 1960s, they are still quite popular because of their wide acceptability, ease of use, and simplicity. While some varieties of these lasers are available at multi-kilowatt power levels, most medical-related micromachining requires less than 150 W of average power. The wavelength of emitted photons with CO₂ lasers is about 10 µm, which means that they are infrared photon sources (see Figure 1).

The fundamental physics of material interaction with infrared photons is primarily between ro-vibrational energy levels of the molecules. In essence, absorption of 10-µm photons by most materials causes an increase in the vibrating frequencies of molecules, which leads to a temperature increase. Therefore, the primary interaction is thermal. In most materials, these thermal effects are quite pronounced on a microscopic scale (see Figure 2). If these effects can be tolerated, the CO₂ laser provides a high-speed method of material processing.

In addition, since the ultimate achievable feature resolution is related to the wavelength of the photons, the CO₂ laser, while theoretically capable of producing 10-µm feature resolution, is limited to 50-µm resolution in the majority of cases. Finally, in most (but not all) applications, the CO₂ laser is set up in a focal-point machining configuration (see Figure 3), which means that it is normally possible to machine only one feature at a time.

Solid-state lasers, first used industrially in the 1970s, are best represented by the more recognizable Nd:YAG (neodymium:yttrium, aluminum, and garnet) sources, although some different processing capabilities are currently being investigated using Nd:YLF (neodymium: yttrium, lithium, and fluoride) lasers.¹ Both of these lasers emit photons with about a 1-µm wavelength--which also places them in the infrared region of the spectrum--so the considerations discussed previously regarding thermal material interaction would again apply. Solid-state lasers can also be frequency converted using nonlinear crystals to produce photons at double, triple, and quadruple the fundamental frequency, which gives flexibility for material processing at visible and UV wavelengths. Most of the

current processing work is being done at fundamental frequencies, but it is probable--given the development of diode pumping of the rod at high efficiencies and high repetition rates--that more work will soon be done at frequency-converted wavelengths, especially in the UV region, and that solid-state UV lasers will take the place of traditional UV sources for many applications. Ultimate feature resolution depends on the wavelength used, but is approximately 2 µm for the fundamental frequencies, while the ultimate practical resolution is from 10 to 25 µm. Like CO₂ lasers, solid-state lasers are generally used in a focal-point configuration.

Excimer lasers are comparatively newer photon sources, having been first used in industrial settings in the 1980s. The utility of these laser sources is that they directly emit photons at high average powers in the UV region without frequency conversion. Also, because of the incoherent and highly divergent nature of the beam, excimer lasers are used in an imaging configuration, allowing multiple, simultaneous feature machining and easy splitting of the main beam for multiple part processing. Discreet spectral lines are emitted, depending on the gas mixture employed (see Table II). Longer-wavelength, 308-nm excimer lasers are primarily used for marking purposes. This wavelength produces a photochemical color change in materials like plastics and ceramics, and also produces visible and indelible marks on some metals. The 193-nm lasers are used primarily for specialty applications in which materials do not absorb well at other wavelengths. These lasers are not routinely used because of the difficulties associated with working at UV wavelengths below 200 nm, which include atmospheric absorption of the photons, short laser gas lifetime, low laser output powers, high maintenance costs, and color-center formation in optics.

More than 90% of all applications involving excimer lasers use 248-nm photons because of the aggressive material interaction and highly reliable laser operation at this wavelength. Practical feature resolution of 1 µm is achievable, with most applications having feature resolutions greater than 10 µm. Below 10 µm, extreme attention to detail in equipment design is necessary to avoid mechanical, thermal, vibrational, and optical aberrations. Because of the halogen gases used in excimer lasers and the high cost and relative difficulty of operation, these laser sources are usually reserved for applications for which neither CO₂ nor solid-state sources are practical. Nevertheless, excimer lasers fulfill an important niche processing area and frequently are the best--or the only--technology capable of performing certain operations.

Several other comments apply to laser micromachining in general. First, the production of straight-walled features is usually not possible, since some degree of inherent taper is normally present and increases as the part aspect ratio (feature size to depth) increases. Second, debris from the lasing process may be a problem in some instances. In order to maximize cleanliness--and sometimes to enhance processing speed or to give higher quality results--assist gases are frequently used. It is important to optimize the gas type, pressure, nozzle shape, and direction of gas flow in order to achieve the best possible processing conditions. Finally, facility requirements or safety requirements may, in some cases, override technical considerations in the choice of laser sources. An example is the use of a 308-nm (XeCl) excimer laser chosen instead of a 248-nm (KrF) excimer laser primarily because chlorine sources are somewhat less toxic then fluorine sources, even though the shorter-wavelength light gives better processing results.

SYSTEM CONSIDERATIONS

Once the appropriate laser has been chosen, it must be integrated into a complete processing system. The first step is to identify the environment in which the system will be located. It is important that the system be built in accordance with available facilities, especially as concerns electrical and cleanroom requirements: most medical products require at least Class 100,000 clean conditions or postprocess cleaning. It is also necessary to decide whether the system will be used as a stand-alone or on-line operation. The integration of laser-based systems into automated serial production lines, most often using conveyor-belt carriers, requires a detailed knowledge and understanding of how the line as a whole functions. The production line should offer centralized control of all line stations and be designed so that the product will flow smoothly through each operation.

A laser-beam-delivery system performs three primary tasks. First, it propagates photons onto the workpiece. Second, optical elements within the system are used to shape and condition the beam in order to maximize the efficiency of photon use. This can include beam splitting, beam homogenization, or beam motion (such as a galvanometer-driven head). Finally, the system must protect operators from any dangerous light or surface reflections.

Many laser processing systems use a fixed beam--moving the parts rather than the beam. In addition to conveyor-belt motion, x-y-z-* stages can be used to position the parts properly during lasing. Systems can have many axes of motion--all controlled from a centralized host computer--including z-axis adjustment of the focus head and automated demagnification. In some instances, robotics are used for efficient part handling. For actual part holding, dedicated tooling integrated into the motion control is designed for quick and accurate part positioning. Vacuum chucks with solenoid control are commonly employed when the part being lased is a thin film, especially for roll-to-roll applications.

Even though some production systems do not require the use of cameras for visual inspection, some form of part viewing is usually necessary or recommended. Systems can be designed for viewing the part either while it is being lased or once it is off the lasing axis. By using both optical and electronic magnification (perhaps incorporating zoom) and video crosshairs for targeting, images of 500* or higher are easily achievable. For highly automated and extremely accurate alignment, more sophisticated vision systems can be selected that grab an image, digitize it, compare stored data to the target site, and align to predetermined fiducials.

Computer control of the motion system is accomplished using a PC486 or equivalent with appropriate drivers, boards, and software. Integrated CAD/CAM capability is common. State-of-the-art manufacturing systems include keyboard, mouse, and touch-screen control. The latter feature is especially useful, allowing full programmability for senior engineers using the keyboard and mouse but limiting others' access to the software through the use of a touch screen programmed only with operator-level commands.

Finally, the entire system--including areas for beam propagation, part location, and laser output--must be fully interlocked and enclosed to ensure operator safety. Excimer lasers require further attention because of their use of high-pressure, halogen-containing gas mixtures. Additional accessories such as gas processors, gas cabinets, water chillers, and air-purification units may also be desirable.

PROCESSING CONSIDERATIONS

While disposable medical devices can be manufactured out of ceramics, glass, and metals, by far the most common material is some form of plastic. Catheters and injection-molded plastic parts make up the majority of disposable devices that require laser micromachining. The first step in processing any component is a detailed analysis of the material to determine its interactions with various laser sources, keeping in mind the requirements for feature size, processing area, and desired finish quality and expected production volume. Table III gives information on etch rates for different materials. There are many cases in which the technical objectives can be met by laser processing, but financial considerations do not allow the use of lasers because of limited throughput or unacceptably high amortized piece cost.

A very important decision for the customer is whether to incorporate laser systems into existing manufacturing environments or to rely on the use of laser contract manufacturing services or "job shops." This choice is usually based solely on the portability of the product and the relative costs involved. For instance, very-high-volume products could justify capital-equipment purchases (assuming that operating costs are also acceptable), whereas lower-volume jobs, perhaps requiring only a fraction of the available time, would not. Unless there are good reasons for capital purchases--such as a proprietary process or simply to retain full control of manufacturing--consideration should be given to contract manufacturing. There are a large number of CO₂ and Nd:YAG contract manufacturers, but only a few shops capable of using excimer lasers in any reasonable volume. For medical projects, a critical consideration is the ability of the laser shop to provide levels of product cleanliness and traceability that are acceptable to both the customer and FDA. The best partnerships are long-term relationships based on personal contact, audits, and historical evidence such as quality and on-time delivery, which assure the customer that "ship-to-stock" processing is possible. The importance of selecting a reputable contract manufacturer with a long history of involvement in medical device manufacturing cannot be emphasized enough.

DOCUMENT CONTROL AND TRACKING

Another important aspect of medical product manufacturing is documentation control, which starts with accepted engineering drawings. Most laser processing is done on raw material or molded parts, and is delivered in bulk. In general, there is a lot number associated with incoming material. If possible, it is a good idea to preserve this number, perhaps with additional qualifying characters, in order to retain traceability to the material supplier.

A typical lot-tracking document of a laser-services provider is shown in Figure 4. This document is attached to the incoming product by the receiving department and accompanies material through every stage of processing. In this case, the product must be manufactured in a Class 100,000 cleanroom environment, and associated information and instructions also appear. Additionally, this particular job requires using a split-beam approach, so lot-tracking numbers differentiate the beamlets. A six-digit designator is attached to the original lot number and identifies the month (first two digits), day (third and fourth digits), and beamlet number (fifth and sixth digits). All personnel handling the parts during any phase of the operation log onto this tracking document, and tallies are kept on both incoming and outgoing products. Note also that the outgoing part number carries a numerical suffix ("-1") indicating that the part has in fact been laser processed, since this is not always obvious to the naked eye because of the small size of the holes in typical molded parts. Copies of the tracking document ship with the product and are also kept in the job files.

Additional required documentation includes cleanroom operational and maintenance procedures, cleanroom certification (usually done once per year), job-related setup and operational procedures, and certificates of compliance. Cleanroom operational and maintenance procedures must be written and kept on file, and should be required reading for the restricted number of employees authorized to enter the area: signed statements from each involved employee verify that they have read and understood the procedures. Each long-run job should have an associated manual comprising a complete set of operational procedures, setup instructions, explanations of any required tooling, and specifications (including tolerances). When incoming QC at the level following laser processing is minimal or nonexistent, certificates of compliance are sometimes used to ensure "lot-to-stock" production.

QUALITY CONTROL AND ASSURANCE

Whenever possible, laser systems should be set up with fail-safe mechanisms, automated monitoring and recording of important process parameters, and periodic quality checks. Just as there are many different types of laser processing, so will the quality system depend on the material, the type of job being undertaken, and the particular circumstances of the project. For example, with flow devices, the actual flow of gas or liquid through the orifice under constant conditions of viscosity and pressure is the desired result. In situ, one might measure drilled hole size and correlate that size to a flow empirically. But in the case of using gas flow as a calibrant for liquid flow (liquid flow tests are usually destructive), care must be taken to avoid confusion induced by different geographic and atmospheric conditions.

If parts are being shipped to various locations for additional processing, it is best to have accepted, duplicate standards at each location. Actual measurements of physical feature sizes and the positions of the features (relative to part fiducials) can be performed using microscopes, optical comparators, and a whole host of other analytical equipment. It is sometimes more difficult to judge feature "quality" or cleanliness: the laser process will naturally leave debris, and processing conditions must be optimized to give the best quality while retaining manufacturability. However, in some cases--whether for technical or fiscal reasons--a secondary cleaning process may be necessary.

Process monitoring must be done on all conditions vital to the success of the run. For example, most laser systems monitor and record output energy as an important parameter affecting feature size and quality as well as processing speed. In some instances, however, the process depends less on total output energy than on some other factor perhaps considered secondary in nature, such as beam homogeneity.

All production jobs should have accepted quality programs documented in the job manual, offering explicit instructions on how to check the work both in progress and after processing. The final authority to ship product or move it to the next processing step should rest with someone other than the operators, and the entire job should be reviewed by the person with shipping authority to ensure final product conformation.

CONCLUSION

The growing use of laser micromachining in disposable medical device fabrication--whether conducted in-house or contracted through service bureaus--is providing opportunities for unique and cost-effective manufacturing solutions. Continued developments in laser systems have made the technology a viable commercial processing tool, with applications in R&D, prototyping, short-run or low-volume production, and high-volume, high-throughput manufacturing. Through familiarization with the technology and interaction with skilled and reputable subcontractors, medical device manufacturers can cross the comfort threshold and use laser-based processing as a valuable enhancement to traditional manufacturing techniques.

REFERENCE

1. Scheffer R, and Angell J, "Novel High-Power Nd:YLF Laser for CVD-Diamond Micromachining," presented to the SPIE Micromachining and Microfabrication '95 Conference, Austin, TX, October 1995.

BIBLIOGRAPHY

Pippert K, and Zaal G, "Excimer Lasers Carve Out Industrial Market Niches," Indust Las Rev, April, pp 13­16, 1995.

Kincade K, "Industrial-Laser Job Shops: The Road to Success," Indust Las Rev, April, pp 13­18, 1993.

Scaggs M, Sowada U, and Andrellos J, "Excimer Laser Micromachining/Marking," in SAE Technical Paper Series, Warrendale, PA, Society of Automotive Engineers, pp 7­13, 1989.

Ogura G, Andrew R, and Schaeffer R, "Practical Consequences of Matching Real Laser Sources to Target Illumination Requirements." To be published in Vol 2703 of the SPIE Photonics West '96 Conference Proceedings, Bellingham, WA, Society for Photooptical Instrumentation Engineers, 1996.

Ronald Schaeffer, PhD, has been with Resonetics Inc. (Nashua, NH)--a provider of laser micromachining services and systems--for the past four years, holding positions in both corporate and technical management. He currently serves as the company's director of corporate development. Schaeffer holds a doctorate in physical chemistry from Lehigh University.