412 Surface and Coatings Technology, 62 (1993) 412—416 Evolution of surface topography in pulsed-laser-deposited thin films of MoS 2 S. D. Waick, J. S. Zabinski and M. S. Donley WL/MLBT, Materials Directorate, Wright—Patterson Air Force Base, OH 45433-6533 (USA) J. E. Buitman Research Institute, University of Dayton, Dayton, OH 45469-0168 (USA) Abstract Films of MoS2 were grown at various thicknesses onto silicon substrates at room temperature using pulsed laser deposition. Planar and cleaved cross-sectional samples examined in a scanning electron microscope showed that very thick films (thicker than about 450 nm) transitioned from being fully dense to having extensive porosity. The porosity is directly correlated with the accumulated incorporation of spherical particles within the film. Transmission electron microscopy (TEM) results from samples of differential thicknesses, acquired from early and later stages of deposition, showed that the volume fraction of the particles increased with the thickness. Decreasing the energy density of the laser beam by underfocusing relative to the target was shown to decrease dramatically the number of particles. Simulation of the deposition process by adding digitized, binary TEM images of the differential thickness samples, to an equivalent thickness of 450 nm, indicated that the porosity begins at a total particle projected area fraction of about 35%. 1. Introduction 2. Experimental details A significant feature of films deposited by pulsed The films were deposited in a PLD system described laser deposition (PLD) is the incorporation of spherical previously [5, 6]. A Lambda Physik model 1 lOi excimer particles within the films. These particles originate from laser, operated at a wavelength of 248 nm, a repetition liquid droplets which are expelled from the target as a rate of 10 Hz and an energy of 100 mJ per pulse, was result of laser-induced recoil pressure. The number and used to ablate cold-pressed MoS2 targets to grow the size distribution of these particles depend on both the films at room temperature. To enhance the uniformity target material and the laser deposition parameters of the film, the target and substrate were rotated and [1, 2]. In most applications, they are undesirable, but the beam was rastered across the target. in the MoS2 system they do not appear to affect the Several room temperature PLD MoS2 thin films were tribological properties [3]. Because the PLD of MoS2 grown on silicon at different thicknesses, for the purpose at room temperature is by line-of-sight, the accumula- of calibrating the deposition process. These samples were tion of these particles in the growing films produces an used in this study to examine both the surface and cross- increasing surface topography with thickness, which sectional microstructures of the films by scanning eventually leads to porosity. It has been shown in a electron microscopy (SEM). The thicknesses that were previous cross-sectional transmission electron micro- scopy (TEM) study that the particles project their prepared were 129, 390, 838 and 1900 nm. Two other presence to the surface in the form of “caps” above the deposition runs of 800 and 1200 nm were performed to particles [4]. examine the change in particle size distribution during This paper investigates the relationship between the the PLD process. Before and after the deposition of presence of the particles in the room temperature PLD these films, DT samples of nominal thickness 40 nm films and the evolution of porosity in relatively thick were deposited on NaC1 single crystals for examination films. A technique was developed which samples a by TEM. To increase their strength, these films were differential thickness (DT) of growing material and can carbon coated, and were prepared for TEM by dissolving be used to relate the projected area of the particles to the substrates in distilled water, floating them on the the surface topography. surface and collecting them on a 400-mesh copper TEM Elsevier Sequoia