There are two different ways of material removal by laser light; pyrolytic and photolytic.
If the process is predominantly pyrolytic, the material removal is due to heating of the substrate material, and it is therefore mainly associated with materials that melt and evaporate, i.e. metals. At present, generally, pulsed lasers producing light in the infrared or visible part of the spectrum (mainly Nd:YAG (fundamental mode and frequency multiplied) and CO2) are used for pyrolytic drilling.
For the pyrolytic process, the power density in the laser spot should be of the order of 106 W/cm2 or higher. The material melts and some material evaporates at the bottom of the drilling hole. Due to the rapid expansion of the vapour, a steep pressure gradient arises with the surrounding atmosphere, causing molten material and vapour to be rapidly ejected from the hole ('molten ejection'). This usually leaves a recast layer on the inside wall of the hole, and sometimes leads to dross build up at the entrance or exit to the hole. At higher power densities (over 108 W/cm2) material removal can become evaporation controlled, generally leading to less recast and build up, and therefore a better hole quality.
If the process is predominantly photolytic, material removal results from the breaking of chemical bonds. It is therefore also often referred to as laser ablation or 'cold machining'. This method of material removal is mainly associated with drilling using excimer lasers in organic materials (for example wood and plastics). In order to break molecular bonds, the energy level of each single photon needs to be higher than the bond energy, of the order of a few electron volts. Excimer lasers emit light of very short wavelength (i.e. in the UV part of the spectrum). The photon energy is then in the range of 4 to 8eV, powerful enough to break many of the molecular bonds of natural and synthesised materials. Frequency quadrupled solid state lasers (in which a crystal is used to shorten the wavelength of the laser light) are also beginning to be used for photolytic processing.
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