|Laser Basics: Interactions with Materials|
The devices and tools used in medicine are becoming smaller. They have greater functionality and are expected to last longer than earlier devices. Design, manufacturing, and packaging the devices and components pose new challenges for manufacturers. Of the technologies available, the use of laser technology for such processes as welding, drilling, ablation, cutting, and marking can provide options for manufacturing small medical devices.
A laser beam is a source of energy that can be focused to a small spot. It interacts with materials without direct contact. Laser capabilities have increased over time. They now offer different wavelengths, power levels, and pulse durations that provide combinations that are ideal for specific processing challenges. To better understand the potential provided by lasers, engineers must be familiar with the technology and its nuances. Before deciding which type of laser to use, engineers should understand how lasers work, laser-material interaction, laser parameters, and opportunities for using lasers in medical materials processing. Such knowledge will enable them to make informed decisions when designing medical devices.
Device Opportunities for Lasers
Lasers are used in device manufacturing for a variety of processes. Laser cutting, for example, is a common application and is often employed for manufacturing small devices such as stents. Lasers can also be used for drilling either through-cut or blind holes. This process can be adapted for drilling microfluidic channels in medical diagnostic equipment and for holes in microsyringes used for drug delivery. Silicon-based micromachines for microsensors and actuators are being developed for lab-on-a-chip devices using laser processing. Laser welding and marking are often used for implantable and surgical tools. In addition, lasers are routinely used for surface texturing, such as surface modification of orthopedic implants, to improve surface adhesion.
How Lasers Work
Lasers work relatively simply. A photon encourages other photons to be emitted and travel with it to form a large number of photons traveling together in a beam of light. The beam, which may not be visible, comes out of the laser cavity and is then diverted toward the material process station. Based on the laser wavelength, the beam travels either via optical fibers or directly through optics.
Most of the lasers used today were demonstrated in the 1960s, including Nd:YAG, CO2, and semiconductor lasers. It took several more years to integrate lasers into machines that could be used in industrial settings. Although the technologies are mature, there have been advances in lasers, including the development of systems that produce short pulse widths such as picosecond and femtosecond lasers. In addition, unique arrangement of lasing materials in fiber lasers, disk lasers, and green welding lasers are opening up new frontiers in material processing.
Table I. (click to enlarge) Laser wavelengths that are commonly used in materials processing.
Laser wavelengths used in material processing extend from ultraviolet to infrared and include the visible spectra. Typical laser types and their wavelengths are listed in Table I. In addition to laser type, there are many important aspects of laser selection, including laser cavity design, delivery optics, and laser-material interaction. Most critically, medical device designers should understand how a laser beam will interact with various potential device materials and how it can be used for material processing.
As a beam of laser light impinges on a material's surface, energy is partially reflected, partially absorbed, and partially transmitted depending on the material type and laser wavelength. Of the light energy impinging on the surface, the portion that is absorbed is of interest in material processing.1,2 Light is absorbed in the form of electronic and vibration excitation of the atoms, and energy converts into heat, which dissipates to adjacent atoms. As more and more photons are absorbed, the material temperature increases, thereby increasing the fraction of light absorbed. The process sets off a chain reaction resulting in a rapid rise in temperature in a very short time—typically within a millisecond for welding applications. The rate of temperature rise depends on a balance between energy absorption and energy dissipated by the material.
The optical absorption length is the length over which photon energy is absorbed so that beam intensity drops to 1/e (37%) of its original value. The energy absorbed over this volume produces thermal energy that is diffused to a distance of
L = [4Dt]1/2,
where L equals diffusion length, D is the thermal diffusivity, and t is the pulse width of the laser.
If the thermal diffusion length is much longer than the absorption length, temperature rise at the laser spot will be limited. By contrast, if the diffusion length is shorter than the absorption length, there will be a very rapid rise in temperature, leading to melting and possible evaporation. To produce the desired result, whether for heating, soldering, welding, drilling, marking, cutting, or micromachining, engineers must choose a suitable wavelength and pulse width. By equating the optical absorption length to thermal diffusion distance, a threshold value can be obtained and used as a guide to selection of pulse-width duration for a particular frequency.
Table II. (click to enlarge) Varied pulse widths are required to equalize thermal diffusion distance-to-absorption length at excimer wavelengths for micromachining. Pulse durations in picoseconds are for comparison only; actual experimentation should be conducted for a complete evaluation.
Table II calculates the pulse width needed to contain heat diffusion for a 248-nm wavelength. Given that absorption distances are similar for metals, the difference in time scales results from the differences in diffusion distances. For example, stainless steel has poor heat conductivity compared with nickel and therefore can be micromachined by much longer pulse widths. On the other hand, silicon is quite conductive and requires shorter pulse durations, compared with nickel, to produce ablation.
With femtosecond pulses, the interaction between the laser and materials is thought to occur in a nonlinear multiphoton process because of the high power density and short time frames. The process is so fast that one can think of the beam practically plucking atoms from the surface without disturbing adjacent atoms. Femtosecond lasers are suitable for micromachining because they do not leave a disturbed layer on the exposed surfaces.
Table III. (click to enlarge) The time scale and energy density values for typical material processing applications.
Using a duration that is shorter than the threshold criteria calculated in Table II can be a necessary, but not entirely sufficient, condition for ablation. Additionally, there must be sufficient energy contained in the pulse to heat up a useful volume of the material to be processed in each pulse. For a given pulse energy, as the pulse time is reduced, the heat is increasingly confined to the vicinity of the laser spot. This progressively leads to heating, melting, ablation, and, ultimately, vaporization. Once a suitable wavelength is selected, it is the combination of pulse energy and pulse duration that defines the type of material processing obtained. Typical values of pulse duration and energy density for processing applications are listed in Table III.
Even though the basic laser-material interaction is similar, there are certain unique aspects among types of materials such as metals, ceramics, glasses, and plastics. Figure 1 shows a graph of absorption versus wavelength for metals, plastics, and ceramics and glasses. The profiles are presented in schematic form for discussion purposes only. The profiles shown also are valid only at room temperature. Specific absorption characteristics are available in Industrial Applications of Lasers and the LIA Handbook of Laser Materials Processing.1,2
Figure 1. (click to enlarge) A schematic shows the absorption characteristics of selected metals, glasses, and plastics.
Metals are not transparent to laser radiation and will partially absorb and reflect laser energy. Metals are poor absorbers of CO2 lasers, and absorption percentage increases at lower wavelengths, thereby increasing the efficiency of energy transfer. Even with low absorption at CO2 wavelengths, CO2 lasers can be effectively used for welding and cutting of metals if the energy density is very high.
In contrast to metals, ceramics and glasses absorb well at both ends of the spectrum. However, because of their poor thermal shock characteristics and high melting points, ceramics are harder to process than metals. Glasses absorb only a small fraction of the incident energy from YAG wavelengths, but are easily melted because they have poor thermal conductivity.
Table IV. (click to enlarge) Laser wavelengths in the UV range capable of breaking bonds of polymeric materials during processing.
Plastics are even better absorbers of laser energy, especially in the UV and CO2 regions. Plastics provide another level of opportunity because some laser wavelengths in the UV range have the energy to break certain bonds in the plastic molecule (see Table IV). These wavelengths could be used to selectively alter a material's surface properties. Furthermore, if the plastic is transparent enough, engineers can even alter properties under the surface.
The ability of a laser to produce desired results greatly depends on several laser parameters and often relies on their interdependence. It is important for engineers to fully understand these relationships before choosing a particular laser wavelength or machine.
Pulse Energy. Energy delivered in a single pulse is used as a starting point for most calculations. With the latest generation of power supplies, energy in each pulse can be programmed for a desired delivery profile, which allows ramp-up at the beginning and gradual cool down the end of the pulse. Pulse shaping helps to improve process control.
Power Density. Power density is essentially a measure of the number of laser photons impinging on the material. Power density is measured in terms of watts per centimeters squared and is calculated as pulse energy divided by spot size. Even in a single spot, the power density can vary significantly depending on the beam quality of the laser.
M2. M2 is a measure of energy distribution in the beam. A perfect beam with an M2 of 1 essentially has a sharp peak at the center with a Gaussian distribution away from the center and toward the edge. A low M2, close to 1, is preferred for micromachining, while a high M2 (of the order of 30–100) is preferred for heat-treating and welding applications.
Pulse Duration and Repetition Rate. The length of time that the laser energy pulse is on is defined as the pulse duration. The majority of laser processing applications are operated in pulse mode. In pulse mode, the laser emits energy in pulses of specified duration at a defined repetition rate. Applications where the laser is on continuously (referred to as continuous wave or CW mode) include welding, soldering, and heat-treating applications.
Peak Power. Even though a laser may have a fairly low average power rating, the peak power in each pulse can be very high. For example, a typical 10-W laser can have peak power of 5 kW. This is possible because the laser energy is delivered in a very short pulse. Peak power can be calculated as pulse energy divided by pulse width. A pulse that has 1 J of energy delivered in 1 millisecond will have a peak power of 1 kW. However, because the calculated value is the average for the pulse duration, the actual peak power could be even greater because energy is not uniformly delivered over the duration of a pulse.
Spot Diameter. Spot diameter depends on focal length, wavelength, M2, and beam diameter by the following relation:
Spot diameter = 2fλ M2 /D,
where f is focal length of the focusing lens, λ is the wavelength, M2 is the beam quality metric, and D is the beam diameter.
Note that excimer lasers have poor beam quality and are not focused to a spot. They are instead passed through a mask to produce a desired pattern and are commonly used for etching silicon chips.
Except for wavelength, which is set once a type of laser is selected, practically all other parameters are interdependent and must be set with care. For example, changing spot size by using a shorter-focal-length lens increases power density unless overall power is reduced in proper proportion.
Understanding the laser parameters and laser-material interaction opens up many opportunities to use lasers in medical device manufacturing. Engineers should review a laser's wavelength, power level, and pulse duration. But more importantly, they should understand how these qualities work with each other. As devices become smaller and more sophisticated, engineers must carefully consider the characteristics of laser systems and how they affect material processing.
Girish Kelkar is founder and owner of WJM Technologies (Cerritos, CA) and can be contacted through www.welding-consultant.com.
1. John F Ready, Industrial Applications of Lasers (New York City: Academic Press, 1997).
2. LIA Handbook of Laser Materials Processing, ed. John F Ready (Orlando, FL: Laser Institute of America, Magnolia Publishing, 2001).