MANUFACTURING
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New benefits
Micro laser machining has undergone major advances in recent years with the progressive development of microsecond, nanosecond and femtosecond laser ablation systems (Table I). With each reduction in pulse length, increasingly fine machining capability has been gained. Each reduction in pulse length was 1000 times shorter than the previous one, and there was a jump between nano and femto of 1 million. Ultra-short pulse lengths have the disadvantage that the energy intensity becomes too intense at higher pulse energies and this led to a worse than expected surface finish. The solution to this was to reduce the energy per pulse. But the consequence was that material removal rates dropped significantly with each shortening of pulse length, thus calling into question the commercial viability of these machining systems. Recent developments in picosecond laser systems that fill the gap, that is, the missing 1000 times jump in pulse length, have allowed the industry to explore the benefits of a new laser ablation technology that combines fine ablation capability with an acceptable material removal rate.
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Picosecond laser ablation
Laser machining involves removing material using a pulsed, high-intensity laser beam of often extremely short pulse duration. The beam is generated by any number of different laser technologies, but this article discusses pulsed laser systems in the picosecond range, and compares these with other pulsed lasers such as femtosecond and microsecond lasers.
Short pulse (femtosecond, picosecond and to some extent nanosecond) laser ablation can be regarded as a direct phase change from solid to gas with no liquid phase. The material changes from solid to vapour transition (sublimation) with the advantage that the application (pulse) time is much shorter than the time it takes for the substrate material to absorb the energy. Therefore, there is negligible thermal conduction into the surrounding material and almost no heat-affected zone. The energy concentration is so intense that virtually any material can be ablated, including all metals, polymers, ceramics, glass and even diamond. Each pulse creates a plasma, that is, an extremely hot gas of ionised particles, which expands rapidly from the source, thus taking with it most of the heat energy. Because there is little substrate heating, short-pulsed lasers are ideal for machining low melt temperature materials (polymers and metals) and give excellent machined edges (Figure 1 and Figure 2). However, as described below, the plasma created by all laser drilling methods can have adverse secondary effects when laser drilling deep relief structures.
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Figure 2 [7]: Ultra-short pulse (picosecond and femtosecond) laser ablation.
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Longer pulse (microsecond and to some extent nanosecond) lasers suffer from heat-affected zones, because the pulse time is sufficiently long to allow heat dissipation into the work piece. Other problems are significant recast layers, that is, molten debris thrown out by the ablation process that has cooled (Figure 3 and Figure 4), and micro cracking around the machined area. This can often be removed by post processing, but the result is usually poor compared with ultra-short pulse lasers.
Another differentiating feature of the picosecond laser is that the material removal rate is much higher compared with other ultra-short pulse lasers such as femtosecond laser systems. This makes the picosecond laser more commercially attractive.
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Figure 3: Microsecond laser drilled 50-micron diameter hole showing recast layer debris around edge of feature and micro cracks (prior to cleaning).
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Mode of operation
“Direct write” pulsed laser systems manipulate a generated laser beam so that a focused spot of light is produced. Various optical systems, mirrors and lenses are used and these can be controlled so that the spot can be moved about the surface of the work piece in much the same way a milling tool is in conventional milling processes. However, the main difference in approach with laser systems relates to the interpretation of computer-aided design (CAD) data. The emphasis in these systems is on defining the volume of material to be removed, rather than the solid object left behind after machining. This is because laser ablation rates vary significantly depending on many variables, the material type being a major factor. Given constant laser process parameters, different materials ablate at different rates and it is common for machining trials to be run on representative materials to establish predictable and reproducible ablation characteristics.
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The laser beam is focused at the small spot and this creates a focus cone. The focus cone angle, typically 3° to 10°, is dependent on the focal length of the lenses involved. The effect is to produce an increasing intensity of light towards the apex of the cone with a maximum at the spot, and then a decreasing light intensity below the focus spot as the light is defocused. This cone prevents vertical wall machining unless there is provision in the machine’s control for tilting the light cone axis away from vertical, or tilting the work piece away from vertical so that the cone edge is parallel to the machined wall. This can be achieved when the aspect ratio (hole depth to width ratio) of the feature being machined is low (no more than about 2), but this is not possible for deep drilling of holes. Inevitably, a tapered hole or wall will result. This is acceptable for many applications especially when plastic mould tools are being machined because this effectively creates draft angle.
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Deep drilling of small holes or slots that are of high aspect ratio (greater than 2 and up to 10) creates its own challenges. It becomes increasingly difficult to maintain the cone parallel to the walls and ultimately the cone interferes on the other side of the hole. When this happens, further drilling or milling can take place, but the walls start to taper. The taper can be small, of no more than 1° or 2° from the vertical. However, as the hole becomes deeper, the plasma that is created causes damage to the side walls and actually erodes away the material, thus exaggerating the taper in the near-surface part of the hole. Drilling still deeper leads to increasing absorption and plasma disruption of the beam until, ultimately, the ablation process stops. The possible depth of drilling varies with material type and aspect ratio, but in certain materials the outer part of the light cone can be reflected off the walls of the hole and back towards the main beam, thus maintaining the energy intensity for longer. This can lead to deeper drilling and is a common phenomenon with most types of focused beam laser.
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Figure 5 [10]: Picosecond laser machining centre showing
355-nm and 532-nm beam paths and five axes work piece stage movement. |
Picosecond laser ablation in practice
Figure 5 shows a multi-axis picosecond laser milling machine (Oxford Lasers Ltd, www.oxfordlasers.com [11]). It has been designed to have great flexibility in optical characteristics and control, and work piece manipulation and control. The available wavelengths are 355 nm and 532 nm and under optimum conditions a 1-micron spot is achievable, although for most applications a 6-micron spot is sufficient. This allows complex three-dimensional (3D) structures in the order of a few tens of microns to be produced and simple structures such as shallow holes and trenches of less than 10 microns width in all materials including glass and quartz (fused silica), the latter being optically transparent to longer wavelength lasers. The controllable optics allow for trepanning of larger diameter holes, whereas fixed beam operation allows deep drilling of small diameter holes and features from only a few microns in dimension and up to several hundred microns are possible.
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Figure 6: Medical micro-filter components (essentially arrays of laser drilled holes).
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The work piece is mounted onto a multi-axis stage that allows movement accuracy of one micron in the x, y and z axes, but also in the x and y rotation. Using these functions concurrently allows vertical wall milling. By using a telecentric scanner, the laser beam can be scanned across the surface of the work piece while at the same time the work piece is moved and tilted so that the edge of the laser cone is parallel to the desired wall. Using various milling paths, complex vertical all 3D objects can be machined. The resulting surface roughness from the milling process depends on the type of material being machined and also on the speed of material removal. Fast removal rates require higher energy pulses and this can lead to a rougher surface. This can be compensated for by performing many rough cuts on the bulk of the material to be removed and then slowing down the material removal by reducing the energy in each pulse, or by deliberately defocusing the beam slightly for the final finishing cuts. Fast removal rates can be up to 3 or 4 microns depth per pass depending on the material and this can be reduced to 1 micron or less for finer control.
The picosecond laser system has an inbuilt feedback system for controlling the milling process. There is a vertical depth sensor that accurately measures the relative vertical position of the milled surface. This measurement process can be selected to measure after every milling pass or after groups of passes. However, the point is to provide real-time feedback on the material removal rate, which can then be used to update the pulse energy. This is a powerful tool because the laser power output naturally varies by a small amount over time. This is the result of a number of different factors such as temperature changes in the components or in the environment or other factors that are difficult to eliminate completely. The structures that can be made are numerous and these can be developed using a 3D CAD system. Sophisticated computer-aided manufacturing software converts the 3D model into a cutting strategy and this can be further optimised by manual input to suit any material type.
Applications
There are many applications for the picosecond milling machine. These are all based around machining discrete micro-featured parts or mould tools for replication in polymer by suitable processes.
Minimally invasive surgery components, microfluidic components, micro filters (Figure 6), biosensor components, implants and drug delivery systems, and micro-optic and fibre optic components are just a few possible applications of picosecond laser machining.
Dr. Robert Hoyle is MicroBridge Operations Manager at The Manufacturing Engineering Centre MEC, Cardiff University, The Parade, Newport Road, Cardiff CF24 3AA, UK, tel. +44 29 2087 0018, e-mail: manufacturing@cardiff.ac.uk [12], www.microbridge.cf.ac.uk [13]
The MicroBridge project is funded by the UK Department of Trade and Industry and the Welsh Assembly Government.