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48 The PCB Magazine • August 2017 energies; at higher pulse energies, the feature size increases. The transferred copper is strong- ly anchored (it passes a typical tape test), and conductive patterns can be made directly from this technique using multiple passes and/or the proper pitch, although these multiple depos- its are not structurally strong on smooth glass surfaces. Figure 1 also shows that there is un- bound copper dust between the anchored fea- tures. This can easily be removed by gently wip- ing the surface. A company micromachining platform was utilized for copper forward transfer in instanc- es that required precise alignment of the donor substrate. The system utilized a third-harmon- ic Nd:YAG laser (355 nm) with pulse repetition frequencies up to 90 kHz, pulse duration ~10 ns, ~12 μm focused beam diameter, and maxi- mum average power of around 11 W. The same donor substrate described above can be utilized with proper laser dosing conditions, i.e., using sufficiently large bite sizes to minimize dam- age to the receiving substrate and sufficiently low pulse energies to maintain good resolution of the deposited copper. Other forward transfer processes that employ different lasers, process parameters (including laser wavelength, pulse duration, energy, pulse repetition rate, as well as offset of the substrates), and forward transfer substrates have been successfully implemented toward this approach and can offer seeding res- olution below 10 μm. These methods are the subject of a future paper. The copper deposits made using this tech- nique act as seeds for the electroless plating of copper. A mismatch in resolution between the ablated features and that of the copper seeds may require polishing of the surface after for- ward transfer such that copper seeds only re- main within the features. A second polishing step can be applied after copper plating to elim- inate any unwanted connections or growth of the copper outside of the laser ablated bound- aries. The entire process is shown schematical- ly in Figure 2. Electroless copper plating was carried out after seeding using standard recipes [16] . A typi- cal recipe utilizes distilled water as the solvent, copper(II) sulfate pentahydrate as the copper source, potassium sodium tartrate as a chelator, and formaldehyde as a reductant. The pH of the aqueous solution is raised with sodium hydrox- ide to tune the reduction potentials to drive the plating reaction. The plating was carried out at room temperature in a 200 ml borosilicate glass beaker with magnetic stirring at 200 rpm. All solvents and plating chemicals used in this work were reagent grade. After a thin copper lay- er is deposited from the electroless plating pro- cess, copper electroplating, which offers much faster plating rates than electroless plating, can be carried out to build up thicker copper lay- ers. After plating, polishing can be carried out to prepare a smooth surface with recessed con- ductive features, suitable for further layer build up. The process can be repeated, drilling blind vias instead of through-holes, to build up layers to prepare all-glass or mixed-material multilay- er structures. Modified methods can be used for making structures with embedded components in all-glass structures. The methods described above (laser ablation followed by laser forward transfer of a thin me- tallic foil and then plating) can also be applied to traditional and high-performance dielectric materials, as well as to the plating of various metals. Details of this work will be shared in an upcoming paper. Resistivity measurements of the copper de- posits after electroless plating were done using 4-point probe measurements. A simple design of two 1000 × 400 μm pads connected by a 5 mm or 10 mm long wire 25 μm wide was used (Figure 3, A-C). Prior to plating, the areas of the cross sections of the wires were determined us- ing a scanning laser microscope. The resistivity is calculated according to equation 1. ρ = (V*σ)/(I*L) (eq 1) Where V is the measured voltage across the wire, σ is the cross-sectional area of the wire, I is the applied current, and L is the length of the feature. Profile measurements were carried out on a production scanning laser microscope. Cross- sections of the engraved features were analyzed using the production analysis application. For the resistivity measurements, cross-sectional LASER PATTERNING AND METALLIZATION TO REDUCE PROCESS STEPS FOR PCB MANUFACTURING