Let's now use this high-reproducibility concept to create sub-micron features in materials.
If your objective is to create the smallest feature you can possibly make by machining with light, you must focus that light on the smallest spot you can possibly make. The size of the spot is determined by several factors, but for our purposes we limit this discussion to making the statement that the smallest spot that you can get is about the same as the wavelength of the light you are using to make it. Thus, if the wavelength of light is about 0.5 microns, then the smallest spot you can create is about 0.5 microns. However, as noted above, while both ultrafast pulse lasers and long pulse lasers can both operate at wavelengths of 0.5 microns, the long pulse laser is not capable of creating a machined feature that is much less than about 10 microns because of heat diffusion into the surrounding material.
Referring to Figure 10-1, below, we can see how an ultrafast laser pulse can create features substantially below that of the central wavelength of the laser pulse itself. First, we focus the ultrafast laser on a spot with a profile which has a peak intensity in the center of the beam and smoothly decreases radially outward from the center (a "Gaussian" spot). Remember we said earlier that machining with ultrafast laser pulses is a threshold process? If we adjust the intensity of the laser spot on the surface of the material (which is a very easy thing to do) so that just the peak of the beam is above threshold, then we will remove material only in that very limited area! That very limited area can be as little as one-tenth the size of the spot itself.
Figure 10.1: Creating Sub-Micron Features. Click here for an animation of the process.
Now imagine that we generate ultrafast laser pulses that have a central wavelength of 0.2 microns. Using ultrafast pulses of light we should be able to create features as small as 0.02 microns, or 20 nanometers.
It is important to note that it is the unique combination of multiphoton absorption and saturated avalanche ionization provided by ultrafast laser pulses that makes it possible to machine materials on dimensions much smaller than 1 micron. Without this unique characteristic of machining with ultrafast laser pulses and the highly deterministic nature of the process, it would not be possible to achieve these results without a heat-affected zone. More specifically, it is not possible to get comparable highly repeatable sub-micron machining with long pulse laser systems.