# Direct-Write Dewetting of High Melting Temperature Metals on Flexible Substrates
## 4 Discussion  
The results have shown that it is possible to extend the thermocapillary directed dewetting technique beyond low-melting point metals to metallic films with very high melting points and a high reflectivity. Despite the high melting temperature of Ni and high reflectivity of Ag, dewetted features on polymer substrates are possible under scanned, near-infrared laser heating. Use of a powerful, single-mode laser with a high-beam quality remains essential for successful patterning, as the Gaussian spatial distribution of intensity drives the flow.  
During the laser exposure of Ni and Ag films deposited on a Par-C substrate, a very thin polymer-vaporization layer likely forms at the metal-polymer interface. Although evidence was only observable in the extreme case of Ni, this may be a prevalent feature of many of the dewetting events and may indeed be a critical factor by providing thermal insulation at the metal-polymer interface during the process. While the specific heat of Par-C is approximately double that of Ni [35], the thermal conductivity of Par-C is three orders of magnitude lower than Ni (see Table 1), which may explain why the Par-C does not incur more damage while in contact with the dewetting metal. The thermoplastic effects that are seen, such as the deformation of the Par-C, indicate that there is a significant temperature rise at the interface between the plastic and metal layers; however, this is likely a function of the speed of dewetting. The laser can, in certain instances, impart more damage to the Par-C than the dewetting metal. This may occur if the processing speed is slowed down, thus allowing the metal layer to dewet and exposing the Par-C directly to the laser energy for an increasing amount of time. The processing settings must consider the temperature increase of the metal, the temperature increase of the Par-C, and the absorptivity of the metal layers. As such, this deserves further characterization.  
In this study, thin absorption layers are shown to decrease the need for an exceptionally high average laser power output for highly reflective films such as Ag; however, the use of visible or ultraviolet laser radiation may offer another avenue for approach. Ag experiences an increase in theoretical absorptivity for wavelengths at the near-UV range to nearly 24%--a stark difference from the \(<5\%\) observed in the near-IR range. Wavelengths in the mid-UV range, such as those from a frequency quadrupled Nd:YAG laser (266 nm), are absorbed at a theoretical 75% and therefore would not require the high average power exerted by the current laser system. Shorter wavelength lasers would also allow for smaller ultimate spot sizes and thus higher resolution features with a more effective dewetting on thinner films. One drawback to using a UV laser source may manifest in the absorption properties of the polymer substrate and the subsequent damage. This technique is in its infancy; as such, the full range of processing parameters and material combinations is yet to be explored. The limits in the feature size and edge quality will have to be explored using various  
Figure 12: A color-enhanced example of a push structure using 640 nm nickel films on Par-C substrates, as characterized by **(a)** profilometry and **(b)** FESEM. Successive, overlapping laser scans induce a progressive and directional dewetting front in the nickel film that produces thicker, smoother ridges without the need for additional depositions. Yellow remarks the Par-C, while grey demarks the original surface of the nickel. The dotted square highlights the pushed ridge.  
geometries, substrate roughness values, and materials choices. Still, the technique offers the possibility of creating structures of varied thicknesses without additional deposition coatings, masks, or etching steps. In some ways, this approach represents a micro-additive fabrication technique. Eventually, the technique may be used in conjunction with traditional lithographic processes, or perhaps in place of some.