Clark-MXR, Inc. Ultrafast lasers for science, industry and micromachining

While most of the unique characteristics associated with micromachining with ultrafast lasers can be explained by the lack of thermal diffusion, some important features, notably the extreme high shot-to-shot repeatability, the ability to create sub-micron features, and the capability to machine inside transparent materials, requires a more detailed look at the physics behind the interaction of light with matter. How does light interact with matter? Figures 9.1, and 9.2 will help us get a deeper understanding.

Figure 9.1 Avalanche Ionization - Example 1

Figure 9.1 shows a long laser pulse interacting with a typical sample. The sample is formed of atoms and electrons. For clarity Figure 9.1 shows only the electrons. The electrons are either “bound” or “free”. Bound electrons are tightly attached to the local atoms. In contrast free electrons are not tightly attached to the local atoms. The ratio of bound electrons to free electrons is a function of the material. Metals have mostly free electrons, while semiconductors and insulators have very few free electrons. Figure 9.1 shows very few free electrons. It is representative of a semiconductor.

Click on the above image to view a series of three animations of long-pulse avalanche ionization and a side-by-side comparison of the results.

NOTE:
You must have the Macromedia™ Flash Player to view the animations.

You may "right-click" on the animation to display a pop-up menu for controlling the animation or use the Pause and Play buttons in the animation itself.

Now lets look at the other key component present in the animations viewed through Figure 9.1: the laser pulse. So far we have represented laser pulses as a “solid chunk of energy." Now we need to think of the optical pulse as an electromagnetic wave packet. The wave frequency corresponds to the “color” of the laser pulse, the length of the wave packet corresponds to the duration of the laser pulse, and the amplitude of the wave corresponds to the peak power of the laser.

As the optical wave packet (i.e. laser pulse) enters the sample the electrons start to oscillate. The bound electrons are tightly localized and can only “wiggle” slightly. In contrast, the free electrons, which are unbound, can oscillate strongly once they are in the laser field. While oscillating they occasionally collide with the surrounding atoms. If the laser field is intense enough, a free electron colliding with a surrounding atom will knock off an additional electron. Now there are two free electrons that are being driven by the light field. They in turn can knock two more electrons off atoms in the surrounding material. These four electrons create four more free electrons through collisions, and so on.

This type of multiplication effect is called an avalanche effect, and because it creates electrons by ionizing atoms, it is called 'avalanche ionization.'

The full process is shown in the animations as seen through Figure 9.1. For this avalanche process to start, a free electron must be present initially in the electromagnetic field. The absence of free electrons prevents the avalanche process from significantly starting and ultimately hinders the material ablation.

In metal there are plenty of free electrons; the avalanche process starts immediately. This leads to reproducible machining (there may be other problems associated with heat diffusion).

In semiconductors or isolators there are naturally very few free electrons. The avalanche process may start right away or may not, depending on the presence or absence of a free electron in the beam path. If we have initially several free electrons in the electromagnetic field, then the process will be very “efficient”. If we have no free electron, then the process will not start. We are basically relying on luck to get the machining process going! This variability, which is inherent to the physical process, leads to unstable machining rates. Note that the laser may be perfectly stable, the beam spot size and amount of energy in the pulse may be precisely the same from shot-to-shot, yet the material ablation will vary significantly from shot to shot. This is a serious limitation when trying to do very fine machining.

Can we do anything about it? Can we outsmart the physics of the interaction?

Yes, that is possible. If we can somehow create, a priori, large quantities of free electrons then the presence or absence of naturally occurring free electrons will no longer be an important factor. What matters is the total number of free electrons, not their origin.

So, how do we produce a large quantity of free electrons?

There are at least two ways to do it. Both approaches rely on the same underlying principle: We always have a lot of electrons in our work piece. It is just that the vast majority of them may be bound and not useful to get the avalanche process going. If we could turn these bound electrons immediately into free electrons then we would have solved our problem.

We can achieve this goal using lasers working in the ultraviolet, or using ultrafast lasers. Figure 9.2 shows what happens with an ultrafast laser.

Figure 9.2 Avalanche Ionization - Example 2

Figure 9.1 shows a long laser pulse interacting with a typical sample. The sample is formed of atoms and electrons. For clarity Figure 9.1 shows only the electrons. The electrons are either “bound” or “free”. Bound electrons are tightly attached to the local atoms. In contrast free electrons are not tightly attached to the local atoms. The ratio of bound electrons to free electrons is a function of the material. Metals have mostly free electrons, while semiconductors and insulators have very few free electrons. Figure 9.1 shows very few free electrons. It is representative of a semiconductor.

Click on the above image to view an animated version of avalanche ionization initiated with an ultrafast laser.

NOTE:
You must have the Macromedia™ Flash Player to view the animations.

You may "right-click" on the animation to display a pop-up menu for controlling the animation or use the Pause and Play buttons in the animation itself.

The amplitude of the electromagnetic field corresponds to the peak power of the laser. Ultrafast lasers generate tremendous peak power as shown in Figure 9.2.

Remember what we said earlier: "…as the optical wave packet (i.e. laser pulse) enters the sample, the electrons start to oscillate. The bound electrons are tightly localized and can only “wiggle” slightly." This is correct under normal conditions. But we are not operating under “normal” conditions when the sample is illuminated with ultrafast pulses and their extremely intense electromagnetic fields.

The electromagnetic field is so high that the “bound” electrons are knocked free. Immediately we find ourselves with a large quantity of free electrons and the avalanche ionization process can start immediately, reliably, and reproducibly. This leads to high-quality machining, as shown clearly in Figure 9.3.

Figure 9.3 Avalanche Ionization - Example 3

Figure 9.3 show a comparison between long and short pulsed lasers.

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