Machining with Long Pulse Lasers
Note that almost all the commercial lasers used in industrial settings today fall in the "long pulse" laser category. Let's first take a look at what happens when material is machined with these long pulse lasers.
The most fundamental feature of material interaction in the long pulse regime is that the heat deposited by the laser in the material diffuses away during the pulse duration, as shown in Figure 3.1. Technically speaking, the laser pulse duration is longer than the heat diffusion time. This may be desirable if you are doing laser welding, but for most micromachining jobs, heat diffusion into the surrounding material is undesirable. Why? There are several reasons why heat diffusion is detrimental to the quality of the machining.
Figure 3.1: Long-Pulse Laser Matter Interaction. Click here to view an animation of the process.
Heat diffusion reduces the efficiency of the micromachining process. Heat diffusion sucks energy away from the work spot - energy that would otherwise go into removing material. Think of it as trying to fill a bucket full of holes with water. You have to pour a lot more water into the bucket to compensate for the water that leaks away. The higher the heat conductivity of the material the bigger the size of the holes and the more water you need to be poured into the bucket to fill it.
Figure 3.1 highlights the numerous physical phenomena that are present when machining with a long laser pulse. These effects are best observed in the animated version of Figure 3.1. The absorption of the long laser pulse leads to melting and then sputter evaporation of the material which can contaminate the surrounding area, produce micro-cracks, and remove material over dimensions much larger than the spot. Other adverse effects are damage to adjacent structures, delamination, formation of recast material, and poor shot-to-shot reproducibility.
Heat-diffusion also reduces the temperature at the focal spot (the machining spot), clamping the working temperature not much above the melting point of the material. Material is removed by depositing a lot of energy into the melted material which boils. As shown in Figure 3.1, this boiling ejects globs of the molten material away from the work zone. The ejected globs form drops that fall back onto the surface and contaminate the sample. These droplets can be rather large. They retain a fair amount of residual heat and may bind strongly to the sample. Removal of these contaminants may be difficult or impossible without damaging the target.
Heat-diffusion also reduces the accuracy of the micromachining operation. Typically, heat diffuses away from the focal spot (and there is plenty of heat because the process is inefficient!) and melts an area that is much larger than the laser spot size. It is therefore difficult to do very fine machining. In other words, the boiling that results in material removal is not limited to the spot size of the beam itself. Thus, while the minimum laser spot size might be in the range of one micron or less, in many materials it is not possible to create features with dimensions much smaller than 10 microns diameter.
Heat-diffusion affects a large zone around the machining spot. This zone is referred to as the "heat-affected zone" or HAZ. The heating (and subsequent cooling) waves that propagate through the HAZ causes mechanical stress and can create microcracks (or in some cases macrocracks) in the surrounding material (see Figure 3.1). These defects are 'frozen' in the structure when the material cools. In subsequent routine use, these cracks may propagate deep into the bulk of the material and cause premature device failure. A closely associated phenomena is the formation of a recast layer of material around the hole. This resolidified material often has a physical and/or chemical structure that is very different from the unmelted material. This recast layer may be mechanically weaker and must often be removed. In some applications, for example arterial stent manufacturing, this recast layer (also called 'slag') is removed through extensive and expensive post-process cleaning before the device can be used inside the human body.
Heat-diffusion is sometimes associated with the formation of surface shock waves. These shock waves can damage nearby device structures or delaminate multilayer materials. While the amplitude of the shock waves varies with the material being processed, it is generally true that the more energy deposited in the micromachining process the stronger the associated shock waves.
Clearly, heat diffusion is associated with numerous phenomena that affect the micromachining process. Reducing, or better, eliminating, heat diffusion is therefore desirable. We will get back to this in Chapter 5.
There are other limitations associated with laser machining. For example traditional lasers cannot readily machine transparent materials. That is not too surprising!
But ultrafast lasers can! Yes, as surprising as this may sound, ultrafast lasers can machine transparent material. We will review this in Chapter 13.
To summarize, in the case of micromachining with conventional long pulse lasers (or more conventional machining tools), heat-diffusion dominates the micromachining process. This introduces numerous undesirable side effects that reduces the value of the machining.