Laser Ablation

Ablation, in the broadest sense, is removal of material because of the incident light. In most metals and glasses/crystals the removal is by vaporization of the material due to heat. In polymers the removal can be by photochemical changes which include a chemical dissolution of the polymer, akin to photolithography.

If the removal is by vaporization, special attention must be given to the plume. The plume will be a plasma-like substance consisting of molecular fragments, neutral particles, free electrons and ions, and chemical reaction products. The plume will be responsible for optical absorption and scattering of the incident beam and can condense on the surrounding work material and/or the beam delivery optics. Normally, the ablation site is cleared by a pressurized inert gas, such as nitrogen or argon.

If the material to be ablated has a poor absorption, such as diamond, but a thermally converted form of the material has relatively good absorption, such as graphite, then it is normal to cover the diamond surface with a thin coating of graphite. The laser will ablate the graphite and in doing so the surface of the underlying diamond will be converted to graphite allowing efficient absorption. Sequentially, the graphite is ablated and a new layer of diamond is converted.

The ability of the material to absorb laser energy limits the depth to which that energy can perform useful ablation. Ablation depth is determined by the absorption depth of the material and the heat of vaporization of the work material. The depth is also a function of beam energy density, the laser pulse duration, and the laser wavelength. Laser energy per unit area on the work material is measured in terms of the energy fluence. Some typical fluence values are shown below.

Lasers used for micromachining are normally pulsed excimer lasers which have a relatively low duty cycle, or less commonly they may be a continuous laser which is shuttered. That is, the pulse width (time) is very short compared to the time between pulses. This is shown below. Therefore, even though excimer lasers have a low average power compared to other larger lasers, the peak power of the excimers can be quite large. The peak intensity and fluence of the laser is given by

while the peak power is

There are several key parameters to consider for laser ablation. The first is selection of a wavelength with a minimum absorption depth. This will help ensure a high energy deposition in a small volume for rapid and complete ablation. The second parameter is a short pulse duration to maximize peak power and to minimize thermal conduction to the surrounding work material. This is analogous to a vibrating system where the mass is large and the forcing function is of high frequency. This combination will reduce the amplitude of the response. The third parameter is the pulse repetition rate. If the rate is too low, all of the energy which was not used for ablation will leave the ablation zone allowing cooling. If the residual heat can be retained, thus limiting the time for conduction, by a rapid pulse repetition rate, the ablation will be more efficient. More of the incident energy will go toward ablation and less will be lost to the surrounding work material and the environment. The fourth parameter is the beam quality. Beam quality is measured by the brightness (energy), the focusability, and the homogeneity. The beam energy is of no use if it can not be properly and efficiently delivered to the ablation region. Further, if the beam is not of a controlled size, the ablation region may be larger than desired with excessive slope in the sidewalls.

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Copyright Craig Friedrich 1998