Femtosecond Laser Pulses and Thin Metallic Films: Mixing in the Vapor Dome and the Solid State

Abstract

This thesis covers the different mechanisms for mixing thin films with ultrashort laser pulses and refining the techniques for use in morphologically- and heat-sensitive applications. The irradiating fluence accesses different mechanisms for material modification. Irradiation at high fluences can induce extreme states where the temperatures and pressures experienced by the film can reach several thousands of Kelvins and several GPas, respectively. Low fluence pulses can modify the material surface after repeated exposures due to point defect formation and diffusion. Both of these fluence regimes are explored as pathways for mixing Ni-W films. These films are sputter deposited as alternating layers on a Ni heatsink and irradiated on the film stack surface. The resulting material is characterized by a combination of scanning electron microscopy, high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS). The first section attempts to mix Ni-W films at high fluences. An EDS map shows that irradiation by a pulse of fluence of 0.20 J cm-2 is insufficient for mixing a Ni-W film stack. Films irradiated by a 0.40 J cm-2 pulse are mixed. Hydrodynamic simulations are performed to model the responses of Ni films to femtosecond laser irradiation at these fluences. Irradiation by a 0.40 J cm-2 pulse pushes the material into a region in its temperature-density phase diagram known as the vapor dome at a higher temperature than irradiation by a 0.20 J cm-2 pulse. The increased kinetic energy and subsequent prolonged cooling period are thought to contribute to the onset of mixing at 0.40 J cm-2. The next section demonstrates mixing in Ni-W films through the generation and diffusion of point defects at low fluences. Repeated irradiation with 1000 pulses of fluence 0.10 J cm-2 results in the formation of high spatial frequency laser induced periodic surface structures (HSFL). HAADF and EDS show that mixing is confined to a depth of approximately 20 ± 2 nm. Next, the ability to finely control the mixing depth is demonstrated by increasing the thickness of the middle Ni layer to 5 nm. The mixed region was further confined to 13.5 ± 2 nm and likely results due to the modified extinction depth. The third section attempts to suppress HSFL formation in order to refine mixing Ni-W films at low fluences. HSFL formation in semiconductors occurs due to the diffusion of point defects to the surface to alleviate a highly-stressed interstitial state. A barrier to stress relaxation is thus created by altering the air-W boundary to a rigid boundary—a sapphire-W boundary. Irradiation occurs through the transparent sapphire onto the Ni-W film stack. HAADF images and EDS maps show that the sapphire successfully suppresses HSFL formation in the mixed films. The final section develops a theory for suppressing Coulomb explosion (CE) in wide-band-gap dielectrics. Higher harmonics generated in the sapphire can cause photoemission of electrons from Ni. Depositing a Ni film on a sapphire substrate lowers its work function and allows photo-injection of free electrons into sapphire. A static electric field across the sapphire is theorized to allow the dispersion of free electrons through its depth and linearly absorb light incident on the sapphire. Distributing the deposited energy through the sapphire instead of allowing it to concentrate on the surface is postulate to suppress CE.