An introduction to laser micromachining
A dimple texture applied to a tool by laser micromachining firm Lightmotif: the curved flute surface of a drill is machined with a texture consisting of oblong shaped dimples with a non-symmetrical cross section
Laser micromachining involves lasers being used to machine fine structures – including holes, grooves and patterns, typically between a few microns and a few hundred microns in size – on the surfaces of materials or components.
Typical laser micromachining includes drilling, cutting, milling and structuring/texturing.
Advantages of laser micromachining
The use of laser-based methods to machine fine structures on parts overcomes many of the limitations associated with more traditional mechanical approaches.
Although they can deliver high throughputs at low cost, mechanical machining methods require contact to be made between the tool and the workpiece, and their efficacy is dependent on the quality and condition of the tool being used. Tools degrade with wear, affecting the consistency of the features they create and necessitating their frequent replacement. Laser-based processes, on the other hand, are non-contact by nature – and the number of consumables needed to carry them out is negligible.
The fine features and complex geometries that can be produced through laser-based techniques can be difficult or even impossible to realise using conventional techniques, depending on their dimension and shape. In addition, when a high-quality finish is required, secondary processes such as polishing may have to be carried out following conventional techniques. This is not requred as often with laser micromachining and is sometimes not necessary at all.
Furthermore, certain materials such as plastics that melt at low temperatures, and brittle or hard materials such as glass and ceramics, can also be challenging to machine using conventional methods. The precision and control guaranteed by laser-based methods makes processing these materials easier.
Finally, mechanical techniques tend to be noisy and can create waste, such as contaminated cooling water, which requires remediation and disposal. Such issues are dramatically reduced or even eliminated when laser micromachining is used.
Types of lasers used for micromachining
A wide variety of lasers can be used for micromachining and work continues to improve their performance by, for instance, increasing their pulse energies and optimising their pulse durations and repetition rates, reducing their cost and size, and improving their reliability.
In many cases, the spatial resolution that can be achieved through laser micromachining is determined by the radius of the beam used, which, in turn, is determined by the diffraction and aperture of the focusing optics employed. Depending on the circumstances, a spatial resolution of approximately 1μm or less can be generated while, in certain situations, a substantially higher resolution – creating features of around 100nm in size – can also be achieved.
The lasers used in micromachining must be able to deliver near-perfect beams to target areas so that the size of the heat-affected zone (HAZ) that they create is limited, thereby preserving the integrity of surrounding materials. This means that lasers with shorter wavelengths and narrower pulse-widths are often employed. For example, ultrashort pulse (USP) lasers, wielding picosecond and femtosecond pulse widths, are often used for laser micromachining. These lasers generate intense peak powers that result in the instantaneous vaporisation of materials and a negligible HAZ – so-called ‘cold ablation’ processing.
Being readily absorbed by a wide range of materials, visible and ultraviolet (UV) wavelength USP lasers are being increasingly used for laser micromachining. UV lasers in particular can be focused into very tight spots for even smaller, more precisely machined features.
Laser micromachining firm Lightmotif uses the technology to micro-mill stamp tools (Image: Lightmotif)
Higher ablation rates can be achieved by increasing the average output power of USP lasers, but a balance must be struck. Excess fluence (the energy delivered per unit area), for instance, is partly imparted as heat into the material, causing a reduction in throughput and quality, while insufficient fluence results in reduced ablation rates. This is particularly true for ultrashort pulses. High repetition rates with sufficiently high pulse energies are therefore necessary. One approach to improving both throughput and the precision of laser micromachining is to use tailored pulse bursts and pulse shapes. Using this approach, the temporal profile of the energy deposited can be optimised for a particular material so that incident energy is directed almost entirely to removal, rather than heating.
The laser beam profile and the performance of the beam-delivery optics are also of critical importance for laser micromachining applications. The beam-profile must be delivered in a single spatial mode to realise a tight, round focal spot on the sample. Each optical element between the laser and the target workpiece can distort the beam incrementally, so both the number and quality of the optical elements need to be optimised for each application. If this isn’t done, optics with insufficient surface quality might be used, which can distort the shape of the beam. Alternatively, optics with an insufficiently clear aperture might be used, which can crop the beam and introduce undesirable diffractive effects. Such distorted or cropped beams would reduce the overall quality and accuracy of the micromachining process.
Industries served by micromachining
The use of lasers to micromachine fine features on parts has become an essential part of high-volume manufacturing in industries such as consumer electronics, clean energy, medical device manufacturing and automotive. Generally speaking, these industries are increasingly demanding parts that are smaller and have finer, more densely packed features.
Smartphones, for instance, contain thousands of components featuring huge numbers of holes and precision cuts, and well over a billion of them are sold each year. The need to add more functionality to smartphones has forced manufacturers to fabricate printed circuit boards (PCBs) with smaller, more densely packed features. As such, laser micromachining is used for this purpose, and in the fabrication of semiconductor chips and their packaging. The manufacture of high-resolution touchscreen displays also relies on many laser micromachining processes.
Lightmotif can also apply pillar textures to mould inserts (Image: Lightmotif)
In the clean energy industry laser micromachining is used, for example, to rapidly create fine features on photovoltaic panels to increase their efficiency, or in the production of lithium-ion batteries.
In the automotive and aerospace industries, lasers are used to machine parts made from lightweight materials such as carbon fibre-reinforced plastics (CFRPs), which can be difficult, or even impossible, to machine using conventional processes.
In the medical industry, laser micromachining is being used to produce the increasingly complex geometries of implantable devices such as stents, intraocular lenses, prosthetics and catheters, which now have smaller features to facilitate new treatments and improve patient outcomes. In the case of metal stents, including drug-eluting metal stents, reducing their dimensions allows for introduction into smaller coronary, peripheral and neurovascular blood vessels. There may be a correlation between positive clinical outcomes and the amount of metal deployed within the vessels, so makers of stents have had to find the means to create thinner-walled and smaller-diameter tubes with more complicated features. The addition of precise surface textures to stents and prosthetics can improve biocompatibility, for example, to reduce the risk of restenosis (the recurrence of arterial narrowing after treatment).
The main benefit of laser drilling is that it can be used to create very small holes quickly, precisely and repeatedly. One of the earliest industrial uses of laser micromachining was for creating small (30μm or less) holes in the nozzles of inkjet printers. Micrometre-scale holes can be drilled in thin foils, for the production of sieves and filters, for instance, using lasers with pulse durations in the nanosecond, picosecond or even femtosecond ranges. Using pulse repetition rates in the kilohertz range, thousands of holes can potentially be drilled every second.
For drilling thicker plates, particularly in metals, small hole diameters imply large aspect ratios, and therefore the beam divergence angle of the laser being used becomes a significantly more important consideration than for drilling holes in foils.
Oxford Lasers can drill 5μm holes in a tube with a 1.5mm outer diameter
The best results for drilling holes with large aspect ratios are usually achieved using picosecond or even femtosecond lasers.
Pulsed lasers can also be used to create microvias – interconnects between different layers of microelectronic circuits. Some microvias cross over multiple layers of high-density interconnect (HDI) substrates and can be created by drilling sub-millimetre holes and filling them with electrically conductive metals. Here, lasers are a faster, cheaper and more reliable alternative to tungsten-carbide drills.
USP laser sources can also be used to cost-effectively drill precise holes in the high-pressure fuel-injection nozzles employed in diesel engines.
Laser cutting and milling
Laser cutting and milling (through which a material is ablated layer-by-layer) are being used to micromachine many metals, semiconductors, ceramics, glasses, polymers and composites. Consequently, a wide range of different pulsed lasers are used for these processes, including diode-pumped solid-state lasers, CO2 lasers and excimer lasers.
For the processing of translucent materials, such as glasses, diamond or sapphire, ultrafast lasers in the UV and infrared wavelengths are proving to be particularly effective.
Laser micromachining can also be used to create micrometre-scale structures on large parts. Laser honing, for instance, is often applied to pistons and cylinders for combustion engines to improve their durability and reduce friction. Here, intense pulses of UV light from an excimer laser are directed at the surfaces of the parts in a nitrogen atmosphere to yield surface textures of around 2μm in depth that, in use, are filled with lubricant.
USP lasers are also being increasingly deployed to produce nanostructures that imbue surfaces with a range of functional properties. Surfaces can be made to be hydrophobic/hydrophilic, antibacterial, produce less friction, and reflect/absorb more/less light.
Other applications of laser micromachining include the trimming of electrical resistors. Here, small amounts of a conducting material are ablated until the desired electrical resistance is achieved.
Laser-based micromachining can also be used to write waveguide structures into certain glasses, and fibre Bragg gratings can also be written point-by-point with a tightly focused laser beam.
Lead Image: Lightmotif