One of the key disciplines of industrial laser material processing (or more specifically laser beam machining) is laser drilling. This means the generation of holes, which can have the following characteristics:
- Typically, one uses laser drilling for holes with diameters up to a few millimeters, but frequently for much smaller diameters below 100 μm, in extreme cases even for sub-micrometer holes (micro-drilling).
- They either go to a limited (and hopefully well-defined) depth (blind holes) or through the full thickness of some metal plates, for example.
- In some cases, a rather high aspect ratio (length divided by diameter) of e.g. 50 is possible. For example, holes with 50 μm diameter can be made in hard metals to a depth of several millimeters.
- The holes are often perpendicular to the workpiece surface, but can also be made at variable angles.
A wide range of materials can be processed, including metals (even quite hard metals like stainless steel or titanium alloys), ceramics, glasses, semiconductors and other crystals. Frequently, no alternative drilling methods would be available for such materials.
Applications and Limitations of Laser Drilling
Applications of laser drilling are very diverse. Some examples:
- Very small diameter holes, sometimes with a high aspect ratio are required for certain machine parts, e.g. for injection nozzles as used for fuel injectors in combustion engines, and (with much smaller holes) for ink jet printers. This belongs to the area of laser micromachining.
- Large numbers of tiny holes are needed for some types of filter sieves.
- Larger holes are required for air cooling of turbine blades, for venting purposes, for instrumentation and various other purposes.
- In laser cutting processes, one often needs to start with the generation of an initial hole (piercing), from which the actual cutting process can continue.
- In electronic manufacturing, many small holes for contacting components on printed circuits boards need to be fabricated quickly. Microvias are realized in high density interconnect (HDI) substrates. Similarly, silicon solar cells are often contacted on the backside with such methods, with contacts lead through laser-made holes.
- For various purposes, one requires tiny holes, but often with a substantial depth, in glass, sapphire or ceramic materials. Such holes in hard and brittle materials would be difficult or impossible to achieve with conventional drilling methods.
- Polymer foils, metal foils and paper can be equipped with large numbers of perforation holes with a very high speed, often with on-the-fly methods while the material is moved relative to the drilling laser beam with high speed.
Laser drilling is particularly suitable when very thin holes with large aspect ratio (ratio of length to diameter) need to be generated, which is hard with conventional mechanical methods. Also, it is often the only choice for fragile materials, which would break when applying mechanical processes.
On the other hand, there are various limitations:
- Particularly for larger holes, the processing speed is often less than desirable because of the need to remove a lot of material.
- There can be problems with a non-constant hole diameter (conicity) or an elliptical hole cross section. That can be caused e.g. by non-ideal beam profiles, beam divergence or beam polarization; precisely cylindrical holes are needed for some applications. (Note, however, that some amount of conicity may even be desirable, for example for fuel injection nozzles.) Another common problem is the deposition of material around the hole. Laser drilling processes are often carefully optimized to avoid such imperfections to a large degree, which however may require more expensive laser sources (e.g. ultrafast lasers) and/or longer processing times.
Pulsed Processing with Short or Ultrashort Pulses
For longer pulses, the resulting process can be essentially of thermal nature, but particularly in the high-intensity regime with ultrashort pulses one realizes “cold ablation” in the sense that basic thermal effects play only a minor role, and the surrounding material is only weakly affected by heat – there is only a very thin heat-affected zone.
Single-pulse and Percussion Drilling
In some thin materials (e.g. metal or polymer foils), a hole can be generated with a single sufficiently intense laser pulse (on-the-fly drilling). If a laser with a high pulse repetition rate (e.g. 10 kHz or higher) is used and the beam position can be accurately controlled with a suitable laser scanner, a large number of holes can be generated in a short time.
In many cases, however, a sequence of many pulses needs to be applied, where each pulse removes only a tiny part of the volume; this is called percussion drilling. A higher pulse repetition rate, associated with a correspondingly high average power of the laser, is then desirable in order to reach a higher processing speed. Particularly for ultrafast laser sources, the average power is often limited, leading to less than desirable processing speeds. However, the performance of such sources is more and more improved.
Liquid vs. Vapor Expulsion; Processing Efficiency
Most efficient processing is generally achieved when using relatively long high-energy pulses. Here, material expulsion largely occurs in liquid form, driven by a high induced pressure gradient.
The processing quality is tentatively better when using vapor expulsion, which can require substantially higher overall laser energies. This is because vaporization, often also accompanied by ionization, is a very energy-intensive process.
Frequently, a processing gas is applied which assists the expulsion of material mostly through the exit hole. This may improve both the processing efficiency and the resulting quality of the holes. By using oxygen one may exploit the additional heat input from oxidation of the blown-out material.
A common problem is that some amount of recast is formed on the surface by deposition of ablated material. That can be a problem concerning the required quality of results. Recast formation can be minimized by optimizing the drilling process in various ways, e.g. concerning the laser parameters but also the application of a transverse gas flow for blowing away the ablated material. In some cases, some post-processing is required to remove recast.
Lasers often emit linearly polarized light. For drilling operations, this is actually not ideal, since the angle between polarization direction and the machined surface is then strongly dependent on the location around the hole.
A simple and common way to restore the symmetry is to use circularly polarized light, which can simply be obtained with a quarter waveplate from a linearly polarized beam.
With somewhat more sophisticated optical elements, one can obtain radially or azimuthally polarized light. The former leads to stronger light absorption at the hole walls. Particularly for drilling deep holes, it can be more advantageous to use azimuthal polarization, where one obtains more reflection and thus more like propagation down the hole to the bottom where it needs to be extended further.
The processing quality of the back side of the workpieces is often a concern, because some of the removed material can be deposited there. In some cases, the quality can be improved by applying a special coating (e.g. of silicon or some ceramics) to the back side before doing the drilling.
Laser Drilling in Hard and Brittle Transparent Materials
Laser drilling cannot only be done in metals or soft materials like polymers, but also in very hard and brittle materials such as various types of glasses and crystalline materials like sapphire, which also happen to be transparent for visible and near-infrared light. Here, the key challenges are to obtain sufficient absorption of the laser light and to avoid damage by fracturing.
Absorption of laser light can be achieved in two different ways. One of them is to use light with sufficiently long wavelength – normally around 10.6 μm from a CO2 laser. The other possibility is to use intense pulses from a femtosecond laser, where some initial absorption is given by multiphoton absorption, followed by laser-induced breakdown. The best results particularly in demanding applications are typically achieved with ultrashort pulse lasers, but one may prefer a CO2 laser where possible because of the substantially lower cost.
The brittle nature of such hard materials is much less of a problem for “cold ablation” with femtosecond pulses, because the remaining material is not subject to that much heat. In that respect, the much longer pulses from a CO2 laser are more problematic. Here, however, a solution can be to drill in glass materials which are strongly heated such that they get somewhat soft. Such processes have been well developed for drilling in fused silica (quartz glass) and in borosilicate glasses, for example. The drilling operation may be integrated with our manufacturing steps where the glass is already hot, so that one does not need to apply an additional heating process.
A helpful detail for drilling glass, for example, can be that the walls of the created hole reflect the laser radiation. The resulting channeling of the radiation allows one to obtain holes with large aspect ratio despite the beam divergence.
With femtosecond pulses, when can also realize laser drilling in backward direction. Here, the laser light is focused through a plate such that the beam focus is located at the back end (see Figure 3). Only there, the resulting optical intensity is high enough to create strong nonlinear absorption. Material is then ablated from that back side. Subsequently, the beam focus can be pulled back more and more, until a hole through the full depth of material is obtained.
Analysis of Results
Particularly when holes with rather small diameter and high aspect ratio are generated, it is not easy to precisely assess the obtain quality. Therefore, destructive methods are sometimes needed, in particular the grinding right through the hole, which allows the inspection of the hole cross-section throughout the length. Unfortunately, that is a quite time-consuming method, which is not practical for regular use in a fabrication process, but mainly for the initial development.
Making Large Holes
Larger holes are efficiently generated with trepanning, i.e., with cutting out the contour of the hole.
For moderate hole diameters, one use specialized laser trepanning heads, which can move the laser beam around a circle. A modified method is helical drilling, where the whole laser beam including its focus is moved, such that the focus describes a kind of helical path. That path can be cylindrical but also positively or negatively conical, depending on the desired hole geometry. An interesting aspect is that the reduced beam diameter (often much smaller than the hole diameter) supports a much improved surface quality.
For larger holes, laser scanners are more suitable and most flexible.
Laser processing heads for drilling operations may contain fixed optics or some kind of laser scanner for maximum flexibility. Some of them are specially equipped with suitable means for trepanning or helical drilling. Often, one injects some kind of process gas, either in the direction of the laser beam or perpendicular to it (forming a cross-jet). There can also be components for process control.
Laser drilling processes can also be optimized for very small hole diameters, even in conjunction with substantial aspect ratios. This is explained in the article on laser micromachining.
Lasers Used for Drilling
Laser drilling is in most cases done with pulsed lasers – typically with nanosecond lasers. At the workpiece, one may e.g. apply a peak intensity of 109 W/cm2 over a time of 20 ns. If the laser beam radius is 100 μm, for example, this leads to a pulse energy around 6 mJ. Therefore, millijoule pulses are usually required, except for very small holes.
Pulsed solid-state nanosecond lasers (Q-switched lasers) are most frequently employed for drilling. For finest processing results (tiniest holes, highest processing quality), ultrafast lasers (laser–amplifier systems) are more and more used. The continuing performance advances allow the realization of quite impressive results, for example multi-beam drilling of thousands of sub-micrometer holes per second.
However, laser drilling is in some areas – e.g. for drilling in dielectrics including ceramics – still done with CO2 lasers or excimer lasers.
For drilling in materials with low thermal conductivity, substantially longer laser pulses can be used, e.g. from free-running lamp-pumped lasers.
Many lasers emit linearly polarized light. This is not ideal for drilling, however, since the angle between electric field direction and the created hole surface then varies strongly around the circumference, and that can lead to an elliptical hole cross-section. Better results are often achieved with radially polarized beams, which may be generated by transforming a TEM00 beam with a specialized kind of beam shaper. Note that radial polarization is associated with zero intensity on the beam axis, i.e., a kind of doughnut beam shape.
Laser Drilling Machines
A typical industrial laser drilling machine contains the following:
- The core part is the laser source, e.g. a Q-switched solid-state laser system with an average output power of several hundred watts or several kilowatts. It usually requires some auxiliary units, particularly for cooling the laser.
- A beam delivery system transports the laser light to the application area. For bridging large distances, one often uses high-power fiber cables.
- The laser processing head needs to be moved relative to the workpiece. That may be done by moving the workpieces only, by moving the laser head only (flying optics), or with hybrid solutions, assigning certain degrees of freedom to one or the other side. The processing head is often equipped with a supply of processing gas.
- There may be additional facilities for automatically loading and removing workpieces, removing debris and fumes, etc.
- Due to the substantial laser powers, laser safety is an important issue. It may be provided with very different means, e.g. by doing the drilling only within a closed housing with an interlock, or by not having any personnel in the whole area during automated production.
Advanced laser cutting machines can often control movements with five or even six axes, allowing not only to drill holes into a fixed plane, but also 3D parts, e.g. accessed by a laser head on a robot arm from different directions.
While some drilling machines are entirely optimized for one specific application, there are also multi-purpose machines with substantial flexibility to adapt to different scenarios.
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