Deposition Characteristics

Angular Distribution. In general, the angular distribution of material from the laser-generated plasma is sharply peaked with respect to the target normal, with some tilting of the distribution toward the incident laser beam. Early studies on the PLD of multicomponent films indicated that the spatial distribution of deposited material was also accompanied by a spatial variation in film composition. In thermal evaporation, a small effusive source is described as having a cos(9) distribution (9 is the angle at which the material is ejected from the target relative to the surface normal of the target; see Fig. 1). This results in a cos4(9) thickness distribution of the film on the substrate. In a purely effusive source, there are no collisions from the evaporation region to the substrate surface. In PLD, a large number of collisions take place in the high-pressure plasma and are responsible for the forward peaking. It is difficult to generalize on the exact functional dependence of the angular distribution of material because it is sensitive to a number of the deposition parameters (e.g., laser wavelength, fluence, spot size and geometry, target-substrate distance, and background gas pressure). As an example, in vacuum the angular distributions, as determined from film thickness, for single and multicomponent materials have yielded values of x (cosx(9)) ranging from 2.3 to 30 (Ref 46).

The angular distribution of ejected material is also sensitive to the shape of the laser spot on the target. The rectangular shape of excimer laser beams typically results in a rectangular focused spot on the target. In some systems, it has been observed experimentally that the deposited film has a rectangular image, with the long and short axis of the film reversed from the image on the target. The rotation of the image is an indication that plume expansion proceeds fastest along the shorter axis (i.e., there are fewer collisions and the particles can achieve greater velocities).

Effects of Added Gas. The addition of a background gas has a strong effect on the angular distribution of the evaporated material. Where direct comparison measurements have been made, the distribution has been less sharply peaked at an elevated pressure. Measurements on multicomponent oxides (e.g., YBCO) are complicated by gas-phase chemistry. For example, a comparison study has shown a reduction from cos23(9) to cos17(9) associated with a change from vacuum to 100 mtorr of oxygen (Ref 46).

The angular distribution changes nonlinearly with the added gas pressure. For elemental systems such as copper (Ref 47) or platinum (Ref 48), the initial addition of either argon (for copper) or oxygen (for platinum) leads to a forward focusing effect; that is, ejection of material occurs at smaller angles. Forward focusing is manifest as a narrowing of the angular distribution function (increasing x) and an increase in the average film deposition rate. This behavior reaches a maximum effect at low pressures (~1 to 50 mtorr). Above this pressure, scattering dominates and the average deposition rate decreases as material is scattered over larger angles.

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