Laser micromachining is widely used for patterning thin (thickness less than 0.1um) metal coating deposited on a surface of thicker polymer substrate. Removal of metal coating is based on a photo-thermal absorption of the laser energy and takes place when laser induced tensile pressure wave reaches the metal to polymer interface and tensile force exceeds forces of adhesion. This laser induced tensile spallation is also capable of volume material removal at rates higher than predicted by photo-thermal model; it also can be used in applications when the film has to be removed from the opposite to incident laser beam side. But most exciting for laser micromachining applications is the fact that a thin metal coating can be patterned to form electrodes arrays for wide range of applications. This direct write process has advantages over standard photolithography processes. Not only does it eliminate all intermediate steps of applying, exposing, developing and finally washing off photoresist; it also eliminates the step of chemical etching that in most cases involves strong etchant chemicals with all associated costs for safety measures and disposal. Screen printing is also widely used to print metal electrodes on polymer substrate, but on a small scale laser patterning allow forming of electrode buses down to 10um wide and 25um apart
Lasers with pulse duration in low nanometer range show efficient laser induced spallation. Excimer laser system allows efficient removal of the following metal coating: copper, nickel, gold, molybdenum and their combination. Excimer laser (wavelength 248nm) system setup used energy density 0.2-0.4 J/cm2, which is significantly less than ablation threshold (2.5-4J/cm 2) required for direct micromachining of these metals. For multiple metal/ polymer combinations, single pulse removal of material provided adequate electrical insulation between electrodes; other combinations require additional cleaning pulse; some combinations did not show visible material damage or removal after first pulse, but consecutive pulse removes material due to induced losses during first pulse. Similar results were achieved using galvo based doubled-frequency YAG laser. Most metals show significant reflection at the fundamental frequency of YAG laser -1.06um, in addition to that high energy density can provide damage and charring to the polymer substrate , but at a wavelength of 0.53um most of metals absorb most of the laser energy, and at energy density 1.5-2J/cm 2 depth of machining into polymer substrate is below 1um and does not alter dielectric properties of polymer.Fig.1 and 2 present fragments of glucose sensor electrodes machined in gold coating 0.1 um thick in 0.25mm thick PET substrate. Electrical insulation between electrodes after laser machining is more than 500MOhms.
For some applications, electrical current load requires thickness metal electrodes more than 1um thick. In this case, direct writing with a laser presents a significant challenge due to thermal damage to polymer base. In this case combination of laser patterning of thin seed layer less than 0.1um thick with added electrical connection to the bus electrodes allows electroplating to be used to grow enough thickness of metal for