Micromachining with Lasers
- Thursday, 01 January 2009
Laser micromachining involves using light to remove material. Lasers can also be used in many other applications such as welding, marking, additive manufacturing and surface alteration, but these fall outside the definition. As a general rule, laser micromachining involves working on substrates that are less than 1 mm in thickness — usually much less — and feature sizes that are also less than 1 mm, with the lower end on the order of a few microns.
Why use lasers for materials processing? There are several reasons. They are non-contact, minimizing risk of damage to parts or tools. They can be used to selectively remove one material and leave another behind by varying parameters such as laser wavelength and energy density on target. They can be extremely flexible, allowing rapid prototyping and development without significant tooling costs. Finally, they can do things that other complementary technologies like mechanical methods, chemical etch and EDM (electrical discharge machining) simply cannot do.
Commercially available lasers have wavelengths spanning from the far IR (CO2 lasers at about 10 microns) through the visible and down into the vacuum UV (157 nm F2 excimer laser). For micromachining purposes, the power output is a few watts to maybe a few hundred watts on the high side with pulse lengths ranging from milliseconds to nanoseconds on most lasers, but with some available down to pico- or even femtosecond. Almost all these lasers are “pulsed” in some manner so that the combination of small spot size (frequently less than 25 microns), short pulse length and high energy per pulse produces high peak power intensity on the part — this seems to be the key to clean and efficient material removal. Smaller spot sizes can be achieved using shorter wavelength lasers. In general, the material removal is also much cleaner, but it also appears that wavelength dependence, probably because of multi-photon processes, becomes less important as the pulse length gets very short — below 1 ns.
The most important thing in material removal applications is that there is strong absorption of the incident photons — at least 50% absorption is needed and the closer to 100% the better. Absorption depth is a function of the material, the incident energy density and also the laser wavelength — as a general rule, UV photons are absorbed within fractions of microns of the material surface whereas IR photons have a penetration depth on the order of 10s of microns or more per pulse.
Lasers commonly used in micromachining applications include excimer, solid state, DPSS (diode pumped solid state), CO2 and fiber lasers. In general, these lasers all have good beam quality (measured by a ‘focus-ability factor’- called M2, with 1.0 being a perfect beam and larger numbers deviating further from “perfect”). For a Gaussian beam laser, M2 close to 1 is ideal; for imaging lasers, a very high-order multi-mode beam is used and instead of being focused, an aperture is imaged on target. The laser itself is basically a light bulb and the beam must be conditioned and directed to the work area — this is the function of the beam delivery system (BDS). The BDS consists of a series of refractive/reflective optics, sometimes enclosed and purged, to get the photons to the work piece efficiently. Beam delivery paths can be simple or quite complicated depending on the tasks, but in general it is best to use the minimum number of optical elements needed in order to do the job.