Particulates

One of the most often cited drawbacks to PLD is the rough surface morphology of the deposited films. The observed surface roughness can originate from a variety of mechanisms. For the same deposition conditions (i.e., laser parameters), the particulate density (number of particles/cm2 • A of film) varies significantly from material to material (Ref 49). No one model is able to predict the size and distribution of the particulates. One of the main causes of surface roughness is the ejection of micron-size particles from the target. These particles can be problematic for electronic materials, especially those involving multilayers of conductors and insulators, as well as optical materials in which particles can serve as scattering centers. There are at least three mechanisms associated with formation of surface particulates: subsurface boiling (Ref 50), shock wave recoil (Ref 51), and exfoliation (Ref 52). Subsurface boiling occurs when the required heat transfer time is less than the evaporation time. The subsurface reaches a molten temperature before the surface layer has evaporated. The effect of subsurface boiling can be minimized by reducing the laser power, which also reduces the deposition rate. Above the target surface, a shock wave is formed at the boundary of the high-pressure region and the vacuum. Recoil pressure from the shock wave into the target can also result in the ejection of micron-size particles. These particles can be reduced by reducing laser power. Continuous heating and cooling of the laser target leads not only to erosion but also to modification of the target surface. Micron-size cones that point back at the direction of the incident laser beam are observed on the surface of ceramic targets and can be broken off by the shock of the laser. The presence of these structures can be minimized by mechanical polishing of the target surface.

Mechanical Filters. Although the presence of particulates seems to be an inherent property of the PLD process, careful control of deposition parameters can minimize their density. Additional particulate reduction can be achieved mechanically through the use of a particle filter (Ref 53). A velocity filter operates on the principle that particle size can be correlated with particle velocity. A shutter can be used to allow fast-moving (106 cm/s) atomic vapor to pass while blocking the slower-moving (103 cm/s) particulates (Ref 34). In practice, a rotating chopper (3000 to 10,000 rpm) can be synchronized to the firing of the laser. Mechanical filters are effective at reducing the aerial density of particulates regardless of the mechanism of origin.

Off-Axis PLD. The most common PLD geometry has the substrate facing the target, separated by a few centimeters. This is designed to maximize the film deposition rate. Recently, there have been a few reports of off-axis PLD. Nonnormal geometries have been successfully used in the PLD of YBCO thin films, and two geometries have been reported. Mounting the substrate surface so that it is parallel to the target normal is essentially equivalent to an off-axis sputtering geometry (Ref 54). The majority of the ejected vapor (in addition to the particulates) travels across the substrate surface, without depositing. Although the deposition rate is decreased, there is substantial discrimination of the larger particulates in the depositing thin film. Additionally, substrates have been mounted facing away from the target surface (Ref 55). Films are deposited in high background pressures of reactive gases. Preferential slowing of the atomic vapor over the particulates allows deposition of smooth films and a substantial reduction in the average deposition rate. The required thermalization of the evaporated material and the absence of energetic ions in the depositing vapor does not seem to affect the properties of YBCO films.

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