Investigation of Laser-Induced Etching of Ti in Phosphoric Acid
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at 1.064 jim. Depths and widths of etched grooves were measured both with a mechanical profilometer (DEKTAK IIST) and SEM investigations of the cross section of etched grooves. During some investigations the electrical potential of the samples was determined with a high-impedance voltmeter (>1013 Ohm) against a standard calomel electrode (SCE), which was either directly in contact with the etchant or connected to the etchant by a Luggin capillary. The surface of the Ti sheets was polished in order to achieve equal optical properties of the sample surfaces. The reflectivity of the Ti surface is strongly dependent on the sample pretreatment. Published results suggest a reflectivity of about 0.5 to 0.55 at 514 nm and of 0.6 to 0.7 at 1.064 Vim [15]. It is assumed in this work that the laser radiation is not or only weakly absorbed by the etchant. RESULTS AND DISCUSSION Investigations on the dependence of etch rate on laser power were carried out with both lasers by measuring the time needed for through etching Ti foils of known thicknesses. Fig. I shows the dependence of etch rate on laser power obtained with a 25 pjm thick foil in 3M H 3 PO4 at a wavelength of 514 nm. In Fig. I two different etching regions are to be distinguished. At laser powers below 0.4 W an approximately exponential increase of the etch rate with increasing laser power was observed. Consequently, it is deduced that in this range etch rates are dominated by Arrhenius-type thermally activated chemical reactions. This region will be referred to as the pure chemical etching region. At laser powers of about 0.1 W boiling of the etchant in the irradiated area was observed. The measured etch rate in this case was comparable to the etch rate obtained in hot H3 PO 4 (1.3 nm/s at 95 oC at 5M concentration). Around 0.4 W a sharp rise of the etch rate by approximately one order of magnitude is achieved within a power interval of less than 50 mW. The strong enhancement of the etch rate is
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Fig. 1: Dependence of laser-induced etch rates on laser power obtained on Ti foils (25 jim) in 3M phosphoric acid at 514 nm 480
presumably due to melting of the laser irradiated surface area resulting in more effective reactions. Evidence for melting is gained by SEM inspections of holes etched near the transition region. Below threshold holes show a very smooth inner surface exhibiting an estimated surface roughness less than 0.5 jm. Holes prepared just above threshold were usually rough (estimated roughness of several microns). Additional indication of possible melting is the glowing of the sample surface inside the laser spot during etching at laser powers above threshold. Above the threshold the etch rate is nearly constant with increasing laser power. This saturation effect is most probably caused by two reasons: vivid boiling of the liquid and strong hydrogen evolution in etched blind holes. Both effects lead to scattering and deflection of
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