Laser ablation means removing some material from a solid surface using intense laser light, usually in the form of a laser beam. This term is often not regarded as a laser application as such, but rather as a process which is utilized in the context of specific applications in subtractive laser material processing (laser machining), such as laser engraving, cutting or drilling.
Applications of Laser Ablation
Laser ablation processes are utilized in many areas of laser material processing:
- In laser machining processes like laser cutting, drilling and laser milling, the removal of some amount of material is required.
- The same holds for laser engraving for purposes of laser marking or others. Here, one often needs to remove material up to a well-defined depth. The achieved uniformity and a low roughness of the resulting lowered surface can be important. Usually, the ablation is done with a sequence of many laser pulses, each one slightly moved against the position of the previous one. Such processes are possible with a wide range of materials, such as metals, ceramics, glasses and polymers.
- Machine parts are processed to get microtextured surfaces, e.g. in order to reduce friction of lubricated parts – for example on cylinders and pistons of combustion engines.
- Some kinds of laser surface modification also involve ablation, normally on a microscopic scale.
- Laser cleaning means the removal of some unwanted type of material, which often absorbs the laser radiation better than the underlying substrate. That selectivity is in practice often very helpful for completely removing all unwanted material while preserving the substrate material.
- Thin-film photovoltaic panels need to be insulated at their boundaries, i.e., a metallic layer needs to be ablated.
- Pulsed laser deposition utilizes laser ablation of a material in order to deposit it elsewhere.
- An exotic application would be laser propulsion, utilizing the recoil of ablated material . For recoil velocities well above the exhaust gas velocities of rockets, laser propulsion may be more efficient in terms of required mass of propellant, while requiring more energy (e.g. from a nuclear reactor).
Non-technical applications of laser ablation are mainly in the area of medicine:
- Laser surgery allows one to very accurately remove fine structures (e.g. parts of malignant tumors) without significantly affecting their neighborhood; the processing speed, however, may be quite low.
- Laser ablation can also be used in dentistry for curing caries; when using lasers with suitably long wavelength, one can obtain selective removal of caries-affected tissue while preserving the unaffected parts of a tooth.
See the links to encyclopedia articles for further details.
The Physics of Laser Ablation
Absorption of Laser Light
Some of the incident laser light is absorbed, and its energy is converted into heat. Typically, the wavelength of the laser light should be chosen such that the absorption length is rather short. That way, the incident power is absorbed in a small volume, causing correspondingly strong heating. Reflection losses will then also normally be reasonably small.
Laser ablation is often done with near-infrared light, mainly because the best-performing and cheapest lasers are available in that spectral region. However, increased absorption may often be achieved by frequency doubling, e.g. from the 1-μm wavelength region to the region around 0.5 μm, effectively obtaining a green laser. For example, the absorption in copper is much better for green light than for the original infrared light, so that the ablation process can become more effective despite the substantial loss of pulse energy in the frequency doubling process (usually of the order of 50%). With frequency tripling or frequency quadrupling, one even gets into the ultraviolet region, where very strong absorption in many materials is obtained. Alternatively, one may directly start with in ultraviolet laser, e.g. an excimer laser.
A special case is the cutting of transparent materials such as optical glasses with laser light e.g. in the near infrared, which is normally not absorbed to any significant extent. Nevertheless, one may obtain substantial absorption by nonlinear processes when applying light with very high optical intensity; this is possible by applying amplified ultrashort pulses of light. Initially, the process may start with multiphoton absorption, and once the state of the material is changed substantially by the deposited energy, the absorption may rise further substantially (anomalous absorption).
Even if sufficiently strong absorption is obtained already at low intensity levels, significant anomalous absorption often occurs during laser ablation processes.
In some cases, selective absorption (material-dependent absorption) is utilized – for example, in laser cleaning (see below). Here, the proper choice of laser wavelength can be particularly important.
Ejection of Material
When applying only moderate optical intensities to a metal, for example, the deposited energy may be sufficient mostly only for melting. A little amount of generated metal vapor may be used to expel the liquid (melt) via its vapor pressure. Alternatively, one may use a gas jet for that purpose; gravity alone would normally not be sufficient. More frequently, however, one applies higher intensities, such that all the removed material is vaporized and leaves the site in the form of a plume. Often, the interaction with the laser light further ionizes the vapor, leading to a plasma.
The emerging plasma plume may substantially interact with the laser radiation, in some cases shielding the workpiece from the laser light. Such an effect may substantially degrade the efficiency of the process. In some cases, however, the plasma even supports the ablation process by absorbing more of the laser light (i.e., reducing power losses by reflection) and transferring energy via thermal radiation.
In this context, the term sublimation is often used. What is meant is essentially that no substantial amounts of the liquid phase are present at any time, because the rapid supply of heat vaporizes any material rather quickly. This is sublimation in a practical sense, not exactly with the meaning as in thermodynamics.
Thermal Conduction; Using Short Laser Pulses
If continuous laser light or long pulses would be applied, much of the generated heat could diffuse into the material by thermal conduction. That is usually unwanted, because even if the heating is still strong enough to cause ablation, there can be a large heat affected zone (HAZ) remaining after the ablation process. That zone may exhibit oxidation, changes of geometrical shape or other kinds of degradation. Particularly in metals, exhibiting high thermal conductivity (because of additional heat conduction by electrons), thermal conduction can be quite detrimental. In insulators like ceramics, polymers and glasses, it is less of an issue.
A simple way to limit effects of thermal conduction is to apply the energy within a very short time. With laser light, this means applying short or even ultrashort pulses of light, as can be generated with various kinds of pulsed lasers:
- Q-switched lasers and some other laser types typically generate pulses with durations between a few nanoseconds and about 100 ns (→ nanosecond lasers). While such pulse durations do not completely suppress effects of thermal conduction, they already improve the situation substantially. The pulse energy which such lasers can provide (e.g. dozens of millijoules, often without relying on complicated technology) is directly suitable for many ablation processes, without employing additional optical amplifiers.
- Far shorter pulse durations are possible with picosecond lasers and femtosecond lasers, and with such extremely short pulse durations one gets into the regime where thermal conduction plays only a minor role. As a result, one can achieve “cold ablation” (athermal ablation) – not in the sense that high temperatures are avoided, but that the immediately neighbored non-ablated material does not experience much of the heat; the heat affected zone is then rather small, allowing for a very high processing quality even when making rather fine structures (→ laser micromachining). Note, however, that because the ultrashort pulses are usually obtained with rather small energies from mode-locked lasers (or sometimes from picosecond diode lasers), and sufficiently high energies for ablation (some microjoules for micromachining or millijoules for macro-ablation) can be achieved only with high-gain optical amplifier systems. Mainly for that reason, picosecond and femtosecond laser systems which are suitable for laser ablation are substantially more expensive than nanosecond lasers. Only in the low microjoule region, relatively simple fiber amplifier technology can be used.
See the article on lasers for material processing for more details on laser sources.
As mentioned above, particularly femtosecond pulses can at the same time provide beneficial nonlinear absorption, extending the range of materials which can be ablated.
Another interesting aspect is that the energy is initially transferred only to the electrons of the material. These rapidly thermalize, exchanging energy with each other, within the order of only 100 fs. However, it takes a much longer time, typically some tens of picoseconds, for that energy to be transferred to the lattice, i.e., to cause thermal vibrations. This means that with pulse durations of femtoseconds or at least no longer than a few picoseconds, one can deposit the energy mainly in the electron system, causing a very highly excited state of matter which subsequently leads to “explosion” of the originally solid structure.
Of course, substantial movement of material (including the plume) is not possible during the duration of a picosecond or femtosecond pulse. Even light could propagate only over 0.3 mm (in vacuum) within one picosecond, and the material velocities are far lower than the velocity of light.
Effective ablation of material often occurs only once a certain threshold value for the pulse energy is exceeded; below that threshold, there may be nearly no ablation at all. The value of the ablation threshold can depend on the type of ablated material, the surface roughness and impurities on the surface, the laser beam radius and the pulse duration. For shorter pulse durations, the threshold energy can be somewhat smaller, because thermal conduction losses are reduced.
When the applied pulse energy is only slightly above the ablation threshold, the amount of ablated material is rather small, since the somewhat lower intensity slightly inside the material (reduced by absorption) is already below the threshold. As a result, only a small part of the deposited energy may be usable for the ablation. The unused energy may even contribute to an unwanted heat affected zone (see below).
On the other hand, a much higher pulse energy leads to deeper penetration, i.e., to the ablation of more material, but it may inject far more energy into some of the material (also into the plume) than necessary for ablation. Therefore, the ablation efficiency is not necessarily much increased.
Note also that Gaussian beams are often not ideal, despite their highest possible beam quality. The problem is that the outer parts of the spatial profile do not reach the ablation threshold and therefore deposit non-useful energy. One may thus want to use a kind of beam shaper to get closer to a flat-top beam profile.
Effect of Multiple Pulses
While in some cases laser ablation on a small spot is done with a single laser pulse, one frequently applies a large number of pulses per plate to a higher depth and/or on a larger area. It is common to scan the laser beam along a line while applying a regular sequence of pulses, such that each hit the spot has a certain distance to the last one. With a suitable scanning pattern, one can also cover whole areas.
Typically, the temporal spacing of subsequent laser pulses is so large that the material can be assumed to be largely relaxed to its original state when the next pulse comes – even if that pulse hits the same spot again. For example, one may use a Q-switched laser system with a 1-kHz pulse repetition rate, i.e., with 1 ms between subsequent pulses; there may then only be some heating effect remaining from previous pulses. Essentially, each pulse ablates some volume of material without being much influenced by the effects of previous pulses. The pulse repetition rate can only influence the heat affected zone, as explained further below.
However, a burst mode laser may produce bursts (bunches, groups) of pulses where the temporal pulse spacing within each burst is rather small – e.g. only 1 ns. In that case, each pulse can be substantially influenced by effects caused by previous pulses. Some remaining heat near the top surface may allow for ablation even below the single-pulse ablation threshold. The process can thus become potentially more efficient. Still, an excessive accumulation of heat may be avoided by allowing for sufficiently long cooling times between the bunches. On the other hand, there may be problems with the generated plume, which cannot disappear within such short times and may to some extent shield the workpiece against the laser radiation.
The same laser average power, when applied with bursts instead of a regular pulse train, may ablate more material per second without depositing more heat in the remaining material. For such reasons, burst mode lasers are developed. Technically, this is not trivial, for example due to problems with gain saturation during a burst.
The processing speed is often limited by the achievable removal rate, e.g. in units of mm3 per minute. That removal rate can be roughly estimated by considering the energy needed to transform the originally solid material into the state of a vapor plasma, and also various kinds of power losses e.g. via incomplete absorption of laser radiation, thermal conduction, thermal radiation and plasma shielding. Even if the mentioned types of losses are relatively small, a substantial volume-specific amount of optical energy is needed – far more than for alternative ablation methods, for example of mechanical type. An additional factor is the limited wall-plug efficiency of laser systems.
The considerations in the previous section are also relevant for the ablation efficiency. As explained there, part of the removed material typically obtains well more energy than actually required for ablation, while some of the energy is lost in parts of the material where one remains below the ablation threshold.
Substantially higher removal efficiencies may be achieved if much of the material is expelled in liquid form. That approach, however, often has disadvantages in terms of the quality of results; for example, droplets may be deposited elsewhere, spoiling material. Also, the complicated flow dynamics may lead to unstable process conditions.
The limiting factor, after optimizing various efficiency-related aspects, is often the available average power of the used laser system. Particularly for ultrafast lasers, a challenge is to provide more output power without degrading pulse quality and increasing the cost too much.
Fiber laser systems have a particularly high potential to provide ultrashort pulses with high average power, as long as the pulse energy remains reasonably small. They can also be combined with a bulk amplifier as a pulse energy booster with moderate gain. The ongoing development of such laser systems can be expected to more and more provide economically viable solutions for high processing speeds.
Heat Affected Zone and Other Detrimental Effects
It has already been mentioned that the heat affected zone is often observed after the ablation. One generally needs to consider the heat affected zone not only for a single pulse, but for the application of a large number of pulses on a sample, e.g. in an engraving operation. It can happen that although a single pulse causes hardly any heat effects, a large number of pulses can create a substantial heat affected zone when applied within a relatively short time, i.e., with a high pulse repetition rate.
Note that even if heat conduction effects are negligible, some heat is deposited in parts of the material exposed to sub-threshold intensity levels. Therefore, using ultrashort laser pulses is not a complete solution to that problem, at least if high pulse repetition rates are needed for a high processing speed. However, one can sometimes develop strategies to get a better trade-off between speed and heat effects – for example, by applying a helpful scanning strategy.
Ablation processes can also have other unwanted side effects. For example, ablative material may be deposited somewhere else, and may have to be removed in some post-processing operation. In some brittle materials such as glasses, cracks may form as a result of thermally induced mechanical stress. Some amount of strain may be “frozen in” and course effects like stress-induced birefringence.
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