A review of typical PLD arrangements: Challenges, awareness, and solutions

Pulsed laser deposition (PLD) is a unique technique to synthesize thin films, in which a target is ablated by scanning a laser beam throughout its surface. Over other methods, PLD has the main advantage of achieving a stochiometric transfer of the target material onto the substrate, even in multi-component systems such as the increasingly attractive high-entropy alloys. But this can only be guaranteed if the beam fluence remains constant -a requisite that is often overlooked. Furthermore, a change in fluence causes a change in the ablation rate, which may lead to an inefficient use of the target. We present an overview of the most common laser beam scanning techniques for conventional on-axis PLD set-ups, with the target surface either parallel, oblique, or perpendicular to the laser. These techniques synchronize the target rotation together with the beam movement, which is controlled through the optical assembly either by mirror displacement, mirror tilting, mirror rotation, or lens displacement. Different combinations of laser beam scanning techniques and PLD set-ups result in different arrangements. We review these typical PLD arrangements, for which we examine the changes in the laser spot size and shape along the scan. To this end, we provide a complete set of equations of the area and eccentricity of the spot as functions of the PLD set-up parameters. After reporting the maximum ratios of both spot area and eccentricity for typical set-up parameters, we discuss their effects on the fluence at the spot. We point out the problems encountered by some of the arrangements and explore strategies to overcome them. Also, we examine which of the typical arrangements can accomplish bi-directional ablation with either opposing or angled incidence; a technique proposed to avoid cones forming on the target and achieve particulate-free thin films. The analyses addressed in this study are aimed to help researchers optimize the use of the targets as well as the quality of the thin films fabricated by PLD.