By: Ralph Delmdahl and Rainer Pätzel
Coherent GmbH, Germany
UV wavelengths are particularly advantageous in laser microprocessing because the high energy photons can remove material by direct bond breaking in most materials, including plastics and glasses. This photoablation process generates virtually no heat and hence only marginal peripheral thermal damage as compared to longer wavelength lasers.
Excimer lasers emitting in the UV region deliver pulse energies scalable up to 1000 mJ and output powers up to many hundred watts. Thus, they are the key to fast and effective large area ablation and to machining structures as small as one micron. The excimer laser with appropriate beam delivery concepts is a key enabler in industrial large-area micro-processing. This laser technology is unparalleled in selectively patterning or annealing thin and functional layers.
There is growing demand for low unit cost, miniaturized electrical circuits and large-area, reel-to-reel UV processing lends the required precision and productivity for antennae circuits, disposable medical sensors and the like. Laser direct patterning enables repetitive production of complex patterns such as sensor circuits on flexible substrates. In this technique, the homogenized excimer laser beam passes a mask containing the pattern for one or even several complex circuits and is imaged onto a flexible polymer substrate.
Single-pulse direct patterning can generate as many as 18,000 circuits per minute with micron size resolution at substrate feed rates of tens of meters per second.
The maximum penetration depth up to which a material is influenced by the laser radiation correlates with the optical and thermal properties of the materials. When considering transparent substrates such as diamonds, eye glass blanks or dielectrics, highest marking and engraving precision is obtained using 193nm excimer lasers. Whereas long wavelengths penetrate into the material as much as many microns, the short-wavelength 193nm radiation is effectively absorbed already in the first nanometers.
Therefore, efficient large-area laser processing in a depth ranging from sub-micron (engraving of eye glasses and diamonds) to about 10µm to 20µm (photochemical marking of polymers) is achieved in a controlled and crack-free manner leading to highest laser machining precision and process reproducibility.
Precision and productivity are indispensable factors in industrial microprocessing. Sophisticated UV beam delivery systems fuelled by the enabling UV output power of today`s mature excimer laser technology meet these requirements in a unique manner.