FIB milling offers excellent machining resolution, but is slow for ‘large’ structures. Laser micromachining offers an alternative fabrication route to produce micron scale structures. The technology is widely reported in recent years [2.40] [2.41] [2.42] [2.43]. Normally, laser micro- machining uses a focused optical beam to selectively remove material from a substrate creating a desired feature. Unlike mechanical machining techniques, laser machining induces low heat deposition to the work piece. The technology is capable of rapid replication of microstructures with a high degree of robustness, high throughput, wide range of compatible materials, and ‘low-cost’ manufacturing. These features make laser micro-machining an ideal candidate for fabrication of micro-cantilever structures.
Laser micro- machining can be divided into various regimes, nanosecond, picosecond and femtosecond, according to the pulse width of the laser. The mechanism of material removal during laser micro-machining includes different stages such as melting, vaporization, and chemical degradation (chemical bonds are broken which causes the materials to degrade) depending on how the laser energy interacts with the material. When a high energy density laser beam is focussed onto the work surface, energy is absorbed and the surface is heated. As a result, molten, vaporized or chemically
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changed states are formed. The comparison between long pulse and short pulse laser machining process is shown in Figure 2.9.
Figure 2.9 Laser machining effect on the same substrate by using ns pulses (left) and fs pulses (right) (adapted from [2.43]).
For a nanosecond laser pulse, material is removed by thermal ablation and local heating to the near boiling point. Figure 2.9 shows that as the laser pulses interaction time with material becomes shorter, the laser energy doesn’t transfer to lattice when interacting with electronic lattice thus no shock wave and micro cracks will be produced during the machining. Furthermore, the heat affected zone is quite different in each case: for longer ns laser pulses, heat is transferred to surrounding material, making the machined hole bigger, whilst for fs laser pulses, the heat affect zone is so small that the surface roughness of the machined surface is much better compared with ns laser pulses. The phenomenon observed shows that longer pulses can deliver more energy and a larger machining area at a high speed, however, the machined material can be easily melted, damaging the surrounding areas. In contrast, short-pulsed laser shows promising manufacturing advantages due to a small heat affected zone and better surface roughness.
Although femtosecond lasers offer new and promising ways to micro- machine almost any solid material, they do have a few drawbacks for industrial applications. First of
Surface damage M icro crack Shock wave ns pulse laser Debris fs pulse laser Plasma Plume No damage caused to adjacent surface
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all, fs- laser machining is a slow process. Usually, the interaction of laser beam with the sample is to modify the refractive index change rather than ablation. Secondly, the system contains many more optical components than longer pulse length lasers which might influence the stability of operation by environmental conditions like temperature, vibrations, etc. However, on the other hand, fs- lasers can be used for high quality applications which are not achievable by traditional laser micro- machining. Table 2.1 shows the basic property of lasers with different pulse width.
Table 2.1 Comparison of different laser micromachining techniques.
Damage to material
Precision Heat affect
zone
Processing speed
Cost
ns High Low Large Fast Low
ps Low High Small Fast Medium
fs Very low Very high Very small Slow High
To summarise the different pulse widths used for laser micromachining, the choice strongly depends on the application. In terms of machining big metal sheets, nanosecond pulse or longer pulse duration/Continuous Wave (CW) lasers remain the best choice, since they can deliver very high average power of up to tens of kilowatts while maintaining a high machining speed. In terms of small feature size, picosecond and femtosecond laser might be ideal candidates because of their low heat affected zone and small spot size.
In micro-cantilever machining, silica, polymer and silica oxide are the commonly used materials due to their optical and mechanical properties over other materials. With different ultrashort pulse, different feature sizes can be machined to meet the best application performance. For the purpose of micro-cantilever bending measurement, polymer materials would be a good choice because of its low Young’s modulus and mechanical characteristics. However, a trade-off between high sensitivity and laser machined surface quality must be considered before it can be used for practical application.
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One of the most important parameters used for evaluating the machined surface quality is called surface roughness which is the arithmetic mean roughness (Ra). It is assumed
that a roughness profile has been filtered from the raw profile data and the mean line has been calculated. The roughness profile contains n ordered, equally spaced points along the trace, and yi is the vertical distance from the mean line to the ith data point. Height is
assumed to be positive in the up direction, away from the bulk material. Then Ra can be
defined as:
n i i a y n R 1 1 (2.12) Generally speaking, laser machining is capable of generating a surface roughness ofm
Ra 0.3 in stainless steel,Ra 0.9min Alumina andRa 0.434min dielectrics [2.45]. When a laser process is used for micro polishing, the average surface roughness can be decreased from Ra 0.112m to Ra 0.015m [2.45] by optimizing the processing parameters such as pulse energy and scanning speed.
Surface roughness of machined materials is one of the most important factors that affect its application. By using ultrashort (up to 10 ps) laser pulses, many materials can be machined to very high precision. The ability to machine such a wide range of materials is very different from that of conventional longer pulse lasers. The interaction between laser and material is independent of the linear absorption properties of the material and is applicable to materials which would otherwise be transparent to the laser wavelength. Machining down to micron scale precision with small damage to the remaining material is achieved by ablating material faster than heat transferring time.
Petkov et al. [2.46] employed four different laser milling systems with different pulse durations by ablating a field with dimensions of 1 by 1mm in order to investigate the relationship between laser pulse widths and surface roughness. The characteristics of the laser sources employed in this experimental study are shown in Table 2.2.
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Table 2.2 laser source characteristic ([2.47]).
Laser source Laser process parameter Surface roughness
(Ra)(μm) fs-laser Power:20mW Scanning speed:100mm/min Fluence:0.25J/cm2 0.35 ps-laser Power:100mW Scanning speed:100mm/s Fluence:0.25J/cm2 0.29 ns-laser Power:10W Scanning speed:100mm/s Fluence:2J/cm2 0.86 ms-laser Power:5.2W Scanning speed:305mm/s Fluence:1.8J/cm2 2.18
From the above table, it is clearly that when applying ultra-short pulses, significant improvements of surface roughness can be achieved by using ps and fs pulse lasers, whereas a marginally better surface quality was achieved when performing laser milling with a ps- laser source compared to a fs- laser. This might due to the non- linear effects that usually happen for processing materials in the fs regimes, as well as the specific machining materials with the laser wavelength used for machining.
In order to investigate ps-laser machined surface roughness, an AFM was used to measure the surface quality of a machined optical fibre end facet. Figure 2.10 shows preliminary measurement results. Surface roughness of the selected area is approximate 200nm, which indicates that ps- laser is a good choice to improve the surface finish as also shown in Table 2.2 from the previous research (detailed discussion will be presented in Chapter 4).
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Figure 2.10 AFM scanned results of a ps-laser machined optical fibre end surface.
Existing studies show that a trade-off between the surface finish and the material removal rate is usually found. For high surface quality with small material removal area, ultrashort pulsed laser might be a good candidate. Therefore, the optimum laser parameters used for micromachining need to be investigated for each specific application.