**4. Effective utilization of the target material**

transfer rate of the target material to the substrate will be maximum and the deposition time

158 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

of a film of specified thickness will be minimum.

**Figure 9.** Schematic of the new technique for depositing thin films of arbitrary sizes.

**Figure 10.** Surface profile of a CuO film deposited on a substrate transferred at a velocity *V* = const/*r*.

The surface profile of a CuO film deposited on a substrate transferred with a velocity *V* = const/ *r* is shown in **Figure 10**. The average film thickness is 85 nm. The thickness deviation from the average value did not exceed ±3% over the entire substrate surface. The method proposed A commercial of PLD is hindered by its few drawbacks, main among those are a nonuniform thickness of the deposited film and a low coefficient of effective utilization of the target material. Consider the second drawback in more detail. In this method, the films are deposited due to ablation of the target material by a laser beam. Obviously, if both the target and the laser beam are immovable, then a crater arises on the target in a certain time interval. The crater affects the angular distribution of the evaporated material, that is, the thickness of the film deposited per unit time on a certain area of the substrate.

One more undesirable result is overheating of the target and distortion of its composition. The target is also overheated in the case when the laser beam is focused onto the entire surface of the target. In this geometry, no crater is formed in the target; however, the target undergoes overheating. Thus, in the case of PLD with both immovable target and the laser beam, the thickness and composition of the film deposited would vary in time. There is a simple solution to the problem—rotation of the target, which substantially facilitates the situation preventing the target from overheating and prolonging the duration of deposition without disturbing the film composition and angular distribution of the evaporated material.

However, such geometry does not solve the problem yet. A groove arises in the rotating target (**Figure 11**) and the angular distribution of the evaporated material changes with time. After several deposition cycles with a variation in the distance of the laser beam from the target center, concentric grooves arise on the surface of the target and the latter becomes unsuitable for further employment. In the conventional geometry, utilization efficiency of PLD (the fraction of its evaporated volume) is very poor (0.01−0.02). It is not important if the target material is cheap. However, if the films of rare metals and its alloys are deposited or the target is made of very high purity chemicals or isotopes, then such a low efficiency is inappropriate.

**Figure 11.** Target made of YBa2Cu3O7–δ of diameter 50 mm after five deposition cycles performed by the conventional PLD method at magnification (a) 10× and (b) 60×.
