# Direct-Write Dewetting of High Melting Temperature Metals on Flexible Substrates
## 1 Introduction  
Microelectromechanical systems (MEMS) and microfluidic devices encompass a great variety of physical designs. As such, the patterning processes utilized in MEMS fabrication must be equally as varied and versatile, expanding to frontiers beyond restrictive techniques such as traditional lithography. A great deal of research has been conducted regarding the dewetting of micro-scale thin films or nano-scale ultra-thin films because of the potential to create both periodic and free-form film structures without the need for complicated deposition, masking, and etching protocols. Moreover, dewetting may allow for a greater utilization of deposited material and can be used to impose periodic structures at small length scales at the expense of a precise control of geometry. Beyond the basic, randomized dewetting regimes that exhibit thickness-dependent periodicities [1; 2; 3; 4], a number of researchers have conducted studies on methods to impose user-defined length scales and spatial ordering to the dewetting events. Such imposed spatial ordering can be achieved via probe-assisted perturbations [5], stamp generated dewetting [6; 7; 8], pre-patterned surfaces [9; 10], and a number of other methods. Dewetting of a liquid film occurs when the spreading pressure of a film (as determined by the surface tensions in the system) is overcome by the long-range intermolecular forces. The point at which a thin liquid film may spontaneously dewet is dependent upon the critical height, as shown in Equation (1), where \(\lambda\) is the capillary length and \(\theta_{e}\) is the equilibrium contact angle of the liquid on the solid. In this regime, films may be metastable or unstable [11]. Dewetting then proceeds via two mechanisms: nucleation and the growth of holes, or spinodal dewetting. The former case may be triggered by defects in the substrate, while the latter case is characterized by the increased amplitudeof capillary waves within the film. Solid metallic thin films are metastable depending on the choice of substrate and will often require additional energetic input before dewetting is observed [12]. This can involve raising the system temperature past the melting point of the metal; however, dewetting has also been observed in films while they were in the solid phase and at room temperature (\(\sim\)20 \({}^{\circ}\)C) [13; 14; 15]. Past studies have shown that the dewetting of metallic films can be induced by the use of a laser input from both defocused and focused sources [16; 17; 18]. \[h_{c}=2\lambda\sin\frac{\theta_{e}}{2} \tag{1}\]  
The use of a laser opens the possibility of a rastered laser to develop dewetted structures that may be \"written\" on demand. [PERSON] et al. recently demonstrated that a focused 532 nm laser radiation can be used in conjunction with an X-Y movement stage to create dewetted polymer structures of polystyrene films deposited upon a silicon substrate [19]. In parallel, our group has demonstrated the direct-write patterning and structure formation of low melting point metal films on polymer substrates by using a high speed scanning laser [20]. Recently, [PERSON] et al. have detailed a similar technique for plasmonic color printing on glass [21]. More specifically to our initial work, low melting temperature bismuth and tin films deposited upon Parylene-C (Par-C) coated glass slides were patterned using a focused 1060 nm laser and high-speed galvanometer scan system. During the irradiation process, the films experienced a localized dewetting event that was initiated by melting and thermocapillary flow in the heated area. As the laser heated the target area, a thermal gradient formed beginning at the center of the focal point and radiating outward. A concurrent and parallel surface tension gradient forms in this area due to the variation of the surface tension with the temperature. This results in Marangoni flow away from the center and outward.