**3. Proposed methods of deposition on rotating substrate**

Three relatively simple methods of laser deposition of large area thin films are proposed [3, 4]. The first method is based on the target tilt control relative to the laser beam and its focal spot while the substrate is maintained in the same position. Specifics of other two methods include the deposition of a compound on a substrate through a mask with consideration of various slits on it. In one of the options, the mask has a slit in the form of a sector symmetrical to the substrate radius and with variation of angular sizes at different values of the radius. Another option uses a mask with two bent sector‐shaped slits located symmetrically relative to the line of equal velocity of the compound mass transfer path.

Conditions of films deposition at experimental verification of the proposed methods were the following. The deposition was performed using the third harmonic of an Nd3+:YAG laser (*λ* = 355 nm) with the pulse characteristics: energy, 15 mJ; width, 10 ns; and repetition rate, 20 Hz. The target and substrate rotation speeds were 37 and 2 rpm, respectively. AMBIOS XP‐1 profiler was used to control the thickness of the obtained films.

**Figure 2.** Geometry of tilting target method.

**Figure 3.** Dependence of film thickness on substrate radius. (a) *α* = +18°: *d* = 0.8 mm (1), 0.65 mm (2), 0.45 mm (3), 0.3 mm (4); (b) *α* = −30°: *d* = 1.05 mm (1), *α* = −24°: *d* = 1.05 mm (2), *α* = −30°: *d* = 0.8 mm (3), *α* = −24°: *d* = 0.65 mm (4); and (c) *α* = −24°: *d* =1.05 mm and *α* = +18°: *d* =0.45 mm.

### **3.1. The method of tilting target**

The first method directing the plume flux to various substrate areas is illustrated in **Figure 2**. Here, the target with a center in point O is tilted around the axis passing through the center and perpendicular to the laser beam. This AOA' axis is parallel to the substrate plane rotating around O'E. In original substrate position, the plume axis coinciding orthogonal to the target plane is crossing the substrate at point B. With tilted substrate, this intersection point is traveling from point D to point D' as it is marked by the dashed lines.

Angular distribution of the material ablated by the laser beam has a number of parameters it depends on. We have narrowed the variability to the slant angle (*α*) and the laser beam diameter (*d*). **Figure 3** shows the dependence of deposited film thickness on mentioned parameters as a function of substrate radius. It is clear from the charts in **Figure 3** that the proposed target tilting method allows achievement of a thicker deposition on the center and edges of the substrate. So it is possible to obtain uniform film thickness by superposing two tilts as it is shown in **Figure 3c**. Variation of the thickness of the film deposited by such a technique did not exceed ±3.3%.

**Figure 4.** Geometry of mask method of deposition on a rotating substrate. 1, laser beam; 2, target; 3, substrate; 4, plas‐ ma torch; 5, substrate holder; 6, mask; 7, axis of the plasma plume; 8, center of rotation of the substrate; 9, point of intersection of the line connecting the focal spot and the center of rotation of the substrate with the mask plane.

### **3.2. Mask method of deposition**

Conditions of films deposition at experimental verification of the proposed methods were the following. The deposition was performed using the third harmonic of an Nd3+:YAG laser (*λ* = 355 nm) with the pulse characteristics: energy, 15 mJ; width, 10 ns; and repetition rate, 20 Hz. The target and substrate rotation speeds were 37 and 2 rpm, respectively. AMBIOS XP‐1

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

**Figure 3.** Dependence of film thickness on substrate radius. (a) *α* = +18°: *d* = 0.8 mm (1), 0.65 mm (2), 0.45 mm (3), 0.3 mm (4); (b) *α* = −30°: *d* = 1.05 mm (1), *α* = −24°: *d* = 1.05 mm (2), *α* = −30°: *d* = 0.8 mm (3), *α* = −24°: *d* = 0.65 mm (4); and (c)

The first method directing the plume flux to various substrate areas is illustrated in **Figure 2**. Here, the target with a center in point O is tilted around the axis passing through the center

profiler was used to control the thickness of the obtained films.

**Figure 2.** Geometry of tilting target method.

*α* = −24°: *d* =1.05 mm and *α* = +18°: *d* =0.45 mm.

**3.1. The method of tilting target**

We have proposed two essentially different solutions of the problem of obtaining large‐size films on a rotating substrate. The first method is based on the use of a mask with a slit in the form of a sector of different angular sizes at different distances from the substrate rotation axis. The slit configuration is determined on the basis of the angular dependence of the mass‐transfer rate so that the thickness uniformity of the deposited film is provided. The second method uses the slits in the form of a sector with a curved symmetry axis coinciding with the equal thickness line of the film deposited on a stationary substrate. **Figure 4** shows schematically the geometry of film deposition on a rotating substrate. It is seen that it differs from the conven‐ tional off‐axis deposition (**Figure 1a**), in that the deposition is performed through a mask placed in the immediate vicinity of the substrate.

### *3.2.1. Sectorial slit with varying angular sizes*

This technique supposes deposition through a mask with a slit in the form of a sector symmetric with respect to the substrate radius and having different angular sizes at different distances from the axis of substrate rotation. The sector vertex must be in the point of intersection of the straight line connecting the focal spot and the center of the rotation of the substrate with the mask plane. **Figure 5a** shows the variation of the thickness of films along the substrate radius in different deposition processes. Curve 1 presents the process with no mask and rotating substrate. Curve 2 corresponds to deposition on the rotating substrate through a mask with a slit in the form of a sector. As expected, in this case, we obtain films of nonuniform thickness. However, curve 2 indicates where the sector should be narrowed and where broadened. Performing several depositions with different masks, we succeeded to reveal a mask configu‐ ration (**Figure 5b**) providing the thickness uniformity of the films (curve 3).

**Figure 5.** Radial distribution of film thickness (a) slit configuration which provided film thickness uniformity (b).

### *3.2.2. Calculation of the configuration of slit in the mask*

In order to exclude experimental optimization of the mask‐slit configuration, a method has been designed to calculate the slit form based on the data from one deposition process onto a stationary substrate. With this purpose, the deposition is performed onto a non‐rotating substrate without a mask. **Figure 6** shows the relative thickness of a CuO film obtained under indicated conditions. After choosing the position of the plume axis with respect to the axis of rotation of the substrate and taking the center of rotation for the origin, we determine the variation of the film thickness along an arc of circumference for different values of *R* inside a sector with an angle *δ*.

*3.2.1. Sectorial slit with varying angular sizes*

This technique supposes deposition through a mask with a slit in the form of a sector symmetric with respect to the substrate radius and having different angular sizes at different distances from the axis of substrate rotation. The sector vertex must be in the point of intersection of the straight line connecting the focal spot and the center of the rotation of the substrate with the mask plane. **Figure 5a** shows the variation of the thickness of films along the substrate radius in different deposition processes. Curve 1 presents the process with no mask and rotating substrate. Curve 2 corresponds to deposition on the rotating substrate through a mask with a slit in the form of a sector. As expected, in this case, we obtain films of nonuniform thickness. However, curve 2 indicates where the sector should be narrowed and where broadened. Performing several depositions with different masks, we succeeded to reveal a mask configu‐

ration (**Figure 5b**) providing the thickness uniformity of the films (curve 3).

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

**Figure 5.** Radial distribution of film thickness (a) slit configuration which provided film thickness uniformity (b).

In order to exclude experimental optimization of the mask‐slit configuration, a method has been designed to calculate the slit form based on the data from one deposition process onto a stationary substrate. With this purpose, the deposition is performed onto a non‐rotating substrate without a mask. **Figure 6** shows the relative thickness of a CuO film obtained under indicated conditions. After choosing the position of the plume axis with respect to the axis of rotation of the substrate and taking the center of rotation for the origin, we determine the

*3.2.2. Calculation of the configuration of slit in the mask*

**Figure 6.** Relative thickness of the film deposited from CuO target on a resting substrate (1), ellipse of equal thickness (2), and the part of the plasma plume passing through the mask (3).

**Figure 7.** Distribution of film thickness along the arc at different radii *R*: 1, 10 mm; 2, 20 mm; 3, 30 mm; 4, 40 mm; and 5, 50 mm. The pattern is symmetric and the maximal thickness is associated with the origin of the *lR* coordinate.

For given (parallel) position of substrate and target rotation axes, the film thickness on various *R* from the center of substrate inside the sector with *δ* angle is determined. **Figure 7** shows the case corresponding to *δ* = 50° (the distance between two rotation axes is 2 cm). Using these data, the average film thickness ℎ on various *R* is calculated in the first approximation. Then, the optimal length of the circle arc *lR* is determined (i.e., the slit width on various *R*). This provides obtaining films with the same thickness on various *R*, equal to the average thickness ℎ on the maximal radius *Rm*, according to the formula = ℎ ()/ℎ. Here, =/180° is the length of the arc of the radius *R* in the sector with an angle *δ*. More accurate values, 2, may be obtained if we repeat the calculation using the average values of the film thickness ℎ 2 for the slit corrected in the first approximation. After this step, a test of the obtained results is per‐ formed.

Both the calculation data and the experiments have shown that the described computation methodology provides the necessary precision for obtaining films with excellent thickness uniformity.

**Figure 8.** Distortion of sectorial slit. The mask slit curved along the uniform thickness ellipse (dashed line) provides both thickness and composition uniformity of deposited films.

### *3.2.3. Slit in the form of a sector with the curved symmetry axis*

The method of the mask with a slit in the form of a sector with the curved symmetry axis is realized also with the use of geometry presented in **Figure 4**. This possibility to obtain thickness‐uniform large‐area films is due to the following circumstances. For obtaining a thickness‐uniform film on rotating substrate, the slit in the mask must be a sector with the vertex coinciding with the rotation axis of the substrate. This is valid for the case of uniform flow of deposited substrate. Consider now the realistic pattern shown in **Figure 6**. If we intersect the three‐dimensional surface outlining the thickness of the film, by a plane parallel to the *xy*‐plane, we obtain the line of equal thickness of the film. This line is usually an ellipse. Intersecting the surface above by planes parallel to the *xy*‐plane at different heights, we obtain a set of concentric ellipses. **Figure 8** shows the ellipse passing through point 9 in **Figure 4**. In every point of this ellipse, we have the same thickness of the film. The reason for this is the spatial uniformity of the flow of deposited material along this line. Hence, the slit in the mask in the form of a curved sector having the middle line coinciding with equal thickness ellipse (**Figure 8**) should provide the uniformity of the film thickness. The choice of the ellipse is caused by striving to obtain a uniform film over the entire surface of the substrate. Symmetry of the pattern with respect to the substrate radius causes the possibility to use two slits which doubles the deposition rate. The deposition rate depends as well on the value of angle *α*.

**Figure 6** shows the domain corresponding to the part of the plasma torch passing through the mask. It is obvious that if in this domain the three‐dimensional surface outlining the film thickness has no abrupt changes in curvature, inside this domain the average thickness of the film along the radius of substrate rotation will be the same for different radii and equal to the thickness corresponding to the chosen ellipse. The smaller the angle *α*, the better this condition is met. On the other hand, the smaller is the angle *α*, the lower is the rate of film deposition. Hence, it is necessary to choose *α* in each case depending on the specific problem.

Implementation of both mask methods of deposition onto a rotating substrate has revealed a possibility of providing for thickness uniformity for both CuO and YBa2Cu3O7–*<sup>δ</sup>* films within ±3.3% on substrates with the diameter of 100 mm. The YBa2Cu3O7–*<sup>δ</sup>* films had a high value of temperature of a superconducting transition (*Tc*= 90 K) independent of the distance between the chosen domain and the center of substrate rotation which indicates radial uniformity of the composition of deposited films. This refers more to especially the method of mask in the form of a sector with curved symmetry axis, since the rate of mass transfer for components of complex compound may vary depending on the solid angle inside the plasma torch. Another advantage of such a mask is that the central part of the plasma torch is cut off. Thereby, the density of micron‐size particles inherent to laser deposition decreases.

We also point out the possibility to combine the principles underlying the two different types of slit configurations, when requirements to the film quality are especially rigid. This may be done if we employ a mask with slits having curved symmetry axis and different angular sizes at different distances from the axis of rotation of the substrate.
