# Direct-Write Dewetting of High Melting Temperature Metals on Flexible Substrates
## 3 Results
### Directed-Dewetting of Ni and Ag on Par-C Substrates
#### 3.1.2 Silver
The beam was scanned in the direction of the arrow. Dewetted silver ridges exhibit an instability and degrade into discrete beads. The cross sections of the dewetted Ag digits are presented in Figure 7. Unlike the previously described Ni film, the dewetted ridges show no signs of porosity within the beads formed by the displaced material or film delamination. The absence of the pores may be a result of the lower melting point and higher thermal conductivity of the Ag relative to Ni, thus reducing the heat load on the underlying substrate and reducing vaporization at the metal-polymer interface. Furthermore, the increased thermal conductivity quickly dissipates the high heat concentration at the exposure point, leading to less substrate degradation. Like the pure Ni films, there is no visible destruction of the Par-C substrate--even in the face of a much higher laser intensity. To avoid the high intensity operation needed to overcome the reflectivity of the Ag, Ag films were coated with a 45 nm absorption layer of Bi or Ni to lower their reflectivity. The adsorption layers were kept to 10% of the thickness of the base silver film. Line arrays with a 100 \(\upmu\)m pitch were dewetted into both types of layered films. Initial dewetting trials were performed using a 50 kHz, 5 \(\upmu\)s processing regime and scanned at various speeds. Figures S2 and S3 in the Supplementary Material provide examples of the different speeds used in the resulting edge qualities. By doubling the frequency and halving the pulse duration, the energy per meter values were kept the same, while also giving slightly better edge qualities. The cross-sectional FESEM images can be seen in Figure 8; the edge quality is featured in Figure 9. Similarities can be seen between the two types of layered films. Ridge formations appear to flatten and widen to a greater degree relative to the uncoated Ag films of Figure 7, but still exhibit larger cross-sectional heights compared to the original film height. Figure 6: 3D profilometry mapping of an Ag linear array formed by directed dewetting with a spacing of 100 \(\upmu\)m. “Negative” and “Positive” relate to the surface of the original Ni film. The accounting of the displaced volume reveals that 7.9% of the excess positive space remains unaccounted for after the dewetting process. Figure 7: Red, color-enhanced, cross-sectional images of dewetted ridges of Ag for (**a**) 100 \(\upmu\)m and (**b**) 40 \(\upmu\)m pitched arrays. The right and left sides of two adjacent digits are shown. The metal films were characterized using energy dispersive x-ray spectroscopy (EDS) elemental maps to identify the distribution of the metals after dewetting. Figure 9 displays surface maps (top-down view) of the post-processed multi-layered films. For both films, there is evidence of a relatively uniform distribution of the absorption metal (Bi or Ni) among the Ag. Because of the penetration depth of approximately 1 \(\upmu\)m of the EDS analysis, it is not possible to determine the surface vs. bulk distribution of the elements. To verify the bulk distribution of the metals after the dewetting and subsequent displacement, similar EDS maps were performed on the cross sections of the metal displaced in the form of the \"ridge\". In the case of the Bi-coated sample, the Bi top film is incorporated within the bulk of the displaced metal, as clearly seen in Figure 10c. The low T\({}_{\text{m}}\) Bi film will melt first, and according to the phase diagram the Bi-Ag system exhibits a small degree of Bi solubility in Ag above 175 \({}^{\circ}\)C [43]. As the processing induces very fast cooling rates, it could be that a limited degree of Bi solubility in Ag occurs through super-cooling. In sharp contrast to the Bi absorption layer scenario, when using the Ni absorption layer, the underlying Ag will tend to melt prior to any thermocapillary action of the Ni. Thus, we expect a movement of the Ag film without mixing of the Ni film.