<title>Pulsed laser ablation mechanisms of thin metal films</title>

Abstract

The ablation of thin films by single laser pulses is a well known technique with widespread industrial applications. Ablation occurs in a well defmed power density region if a supported thin film is illuminated by a single laser pulse. In the literature there are a number of theoretical description of ablation, but a very few based on in-situ experiments. In our study we have directly visualised the ablation processes with fast photography based on application of dye laser probe pulses. The ablation of chromium and tungsten layers supported onto glass substrates with pulses of ArF excimer laser was investigated. The ablated area was illuminated by a delayed short pulse of a fluorescein dye laser or a Rhodamine6G dye laser. Snapshots of initial phase of ablation and the forthcoming material transport were recorded by an optical system and a video camera. Blowing-off mechanisms and thermo-mechanical mechanisms are considered to take place during ablation. Pressures formed during laser ablation were calculated and compared with experimental data. It was found that thermo-desorbtion of gas adsorbed on to the substrate surface, substrate material evaporation and film exfoliation by its longitudinal thermal enlargement may be acting during laser ablation of thin films.

Laser-induced forward transfer of pure metals

“…In particular, if the optical penetration length is smaller than the film thickness and ejection occurs prior to thermal diffusion over the film thickness, solid-state ejection is observed [27]. In these cases, the ejection threshold depends on the yield strength of the film or its adhesion to the carrier substrate [35,36]. In that case, equation (5.7) no longer applies.…”

“…Elastic stress release and partial vaporization of the film are well-established driving mechanisms for metal LIFT [35,36,[38][39][40]54]. However, the actual driving mechanisms are more complex.…”

“…Attempts to visualize LIFT-processing of Au [31], Ni [32], Al [33], and Cr [34] do not provide sufficient spatial resolution to track the process in detail. Therefore, theories describing the ejection mechanism have been proposed based on postprocess analysis of the craters left in the donor layer or deposited features on the receiver substrate [35][36][37]. In addition, numerical simulation has been performed [38][39][40].…”

“…This is because two challenging requirements have to be simultaneously fulfilled for consistent deposition: the impact location of a single droplet has to be limited to the previously deposited droplet's impact area, and good adhesion between the deposited droplets is required. For solid [35,51] material transfer, which is mainly pursued to transfer intact parts of the donor film to the receiver, the adhesion is expected to be limited. Therefore, controlled deposition is often achieved in liquid phase.…”

“…In addition, numerical simulation has been performed [38][39][40]. Two driving mechanisms of the ejection process are commonly proposed [35,36,41]. First, relaxation of thermally induced stresses [42] could drive the ejection.…”