Micromachining By Ronald D. Schaeffer, PhD, CEO—PhotoMachining Inc., and Neil S. Ball, President—Directed Light Inc. Laser micromachining involves the machining of small features into various materials using lasers through material removal. By “small,” we defi ne the feature size as being less than 1 mm and the material thickness as being less than 1 mm, and both are usu- ally a lot less.
Lasers are used for a variety of reasons. First, the noncontact nature minimizes the risk of damage to the material and does not introduce tool wear. The feature resolution when using UV lasers is unmatched by any traditional machining technology, with the smallest attainable features on the order of a few microns, using UV lasers and high-quality optics. By choosing the correct wavelength and energy density on target, selective material removal can even be achieved. Finally, the use of lasers provides great fl ex- ibility especially in the prototyping and R&D stages. For micromachining applications the key to clean
and low taper processing is peak power intensity, which is energy density per unit area. In other words, the best results are obtained when using lasers with high-pulse energy and short-pulse length and where the laser spot is focused to a small size. This is one of the reasons that USP (ultra- short pulse, meaning picosecond and femtosecond lasers) are becoming very popular—the short-pulse length greatly increases peak power at the target, even with relatively low pulse energy. Figure No. 1 shows a graph of wavelength vs. pulse length where all of the microma- chining applications are on the lower half of the Y axis, where longer pulse lengths are used primarily for applications requir- ing a lot of heat on target. Armed with a variety of lasers with
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wavelengths from the IR through the UV, and pulse lengths from milliseconds to femtoseconds, numerous applications can be addressed in the fi elds of automo- tive manufacturing, semiconductors, microelectron- ics, medical devices, alternative energy, aerospace and defense. All materials can be addressed by using the right laser and optical setup with the stipulation that maximum material thickness is somewhat de- pendent on the output power of the laser, especially when speed/cost are an issue. Hard/brittle materi- als like ceramics and glass have a higher ablation threshold and therefore require higher energy density on target to achieve material removal, while soft materials like polymers generally require less energy density on target to affect clean removal. For the same laser and optical setup, much greater material thick- ness can be processed with soft materials than with hard. The factor of overriding importance, however, is absorption. The more photons a material absorbs at a particular wavelength and optical setup, the better the processing will be.
Figure No. 1: Courtesy of PhotoMachining Inc.
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