METHODS FOR PULSED LASER DEPOSITION OF LARGE-AREA FILMS USING MORE THAN ONE TARGET

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International Journal of Modern Physics: Conference Series Vol. 15 (2012) 170 –178

 World Scientific Publishing Company DOI: 10.1142/S2010194512007118

170

METHODS FOR PULSED LASER DEPOSITION OF LARGE-AREA FILMS USING MORE THAN ONE TARGET

A.A. KUZANYAN

Institute for Physical Research, National Academy of Sciences, Ashtarak, 0203, Armenia [email protected]

V.A. PETROSYAN

A.S. KUZANYAN

Several new methods of pulsed laser deposition for fabricating large-area thin films of uniform thickness and composition on a rotating substrate and onto a moving ribbon are proposed. The peculiarities of the methods are the laser deposition of a compound upon a substrate through a diaphragm or the mask placed in immediate proximity of the substrate together with a use of more than one target. The proposed method makes it possible to obtain thin films of uniform thickness on substrates with sizes limited only by the deposition chamber size. Some of the methods are experimentally verified by depositing CuO thin films and the deviation of the film thickness from the average value does not exceed ±3%. Given the advantage of laser deposition, the offered methods should find practical use, in particular, in micro-electronics, optical industry, development of superconducting coated conductors, deposition of thin films of functional materials and other modern technologies.

Keywords: Pulsed laser deposition; large-area films; target.

1. Introduction

For tackling the new problems arising in modern applied science and hi-tech fields it becomes more and more important to synthesize new materials in different forms, particularly, in the form of large-area films and coatings. The pulsed laser deposition method (PLD) allows one to obtain high-quality films of superconductors, semiconductors, dielectrics, metals and alloys1,2. The advantages of this method are the high deposition rate, the good correspondence between the target and film compositions, the possibility of changing pressure in the deposition chamber in a wide range; moreover, it allows one to fabricate high-quality films from small targets (in contrast to the requirement of big targets in sputtering)3. On the other hand, for overcoming one of the

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disadvantages of the PLD method, namely the difficulty of obtaining large-area films4,5, the most advanced proposal currently is the rasterizing deposition from a large target6. Thus the method loses one of its major advantages. The solution of this contradiction could help to move out the PLD method from research laboratories into industrial application.

We have developed special techniques for depositing large-area thin films on rotating disks and translationally-moving ribbons7–11. The method of obtaining thin films of uniform thickness on substrates with sizes limited only by the deposition chamber size is proposed in Refs. 12 and 13.

In this paper we propose new solutions, which employ more than one target for deposition, allowing in case of a rotating disk substrate to increase the deposition area and in case of a ribbon substrate to increase the ribbon width. For both substrate types one can achieve a speed increase in depositing a film of the same thickness. Apart from PLD, the proposed methods are applicable to all methods of deposition from a point source.

2. PLD of Large-Area Films onto a Substrate Undergoing Translational Motion

A Nd:YAG laser with 355 nm wavelength was used with a pulse duration of 20 ns and various energy. The pulse repetition rate was 20 Hz. The geometry of the method of laser deposition of large-area uniform-thickness films onto translational-motioned ribbon is shown in Fig. 1. The laser beam (2) intercepts the target (1) with an incidence angle of 45°. The mask and the moving ribbon (4, 4΄) are placed perpendicularly to the plasma plume axis (3). The arrow (5) represents the ribbon’s motion direction. In the immediate vicinity of the ribbon a mask with slit(s) of particular configuration is placed through which the laser-evaporated target material passes onto the ribbon. We did not show it in Figure 1 so as not to complicate the figure. The experiments were conducted at two different positions of the ribbon with respect to the plasma plume axis. In one case the plume axis intercepts the ribbon on its half-width (position I); in the other case it falls on the ribbon edge (position II).

The angular distribution of ablated material depends on many factors. For variable parameters we chose the laser spot dimensions and the laser fluence. At different values of these parameters the deposition on a fixed substrate was conducted and the angular distribution of ablated material was determined. For both positions of the ribbon, the mask configurations, which should provide homogeneity of film thickness over the width of the ribbon, and amount of the matter deposited on the ribbon through a slit in the mask in a unit of time were calculated. The data allowed selection of optimal conditions to achieve the largest thickness of the films.

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Fig. 1. The geometry of deposition (a) and calculated mask configurations: (b) position I; (c) position II.

Fig. 2. The dependence of CuO film thickness on ribbon width: (a) position I; (b) position II.

A deposition on 60-mm-wide transparent ribbons moving with a 2.5 mm/min velocity was conducted. For determining the influence of the mask on homogeneity of the CuO film thickness, the depositions were conducted utilizing a mask and without it. The films obtained were tested by microphotometer. We measured the optical density of the films and determined the variation of their relative thickness over the ribbon. Variation of the film thickness over the width of the ribbon for the case when the plasma plume axis intercepts the ribbon on its half width is shown in Fig. 2 (a). It follows that in the case of absence of the mask the film thickness changes from ribbon center to the edge by 60%, whereas the deposition through mask decreases non-uniformity of the film thickness to

1 2 3 ₅ 4 4′ 0 1 2 3 4 5 6 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 (c) W id th o f th e s lit , c m

Width of the ribbon, cm

(b) (a) 0 1 2 3 4 5 6 0 20 40 60 80 100

F ilm T h ic k n e s s , % without mask with mask (b) -3 -2 -1 0 1 2 3 40 50 60 70 80 90 100 (a)

F ilm T h ic k n e s s , % without mask with mask

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3%. The variation of the relative thickness of obtained films over the width of the ribbon for the case when the plasma plume axis intercepts ribbon edge is shown in Fig. 3. In this case the thickness non-uniformity of the film deposited without a mask reaches 90%. If we use a mask, the thickness non-uniformity does not exceed 3% in this geometry as well.

From these data it follows, that the proposed method can provide for deposition of films homogeneous by thickness on ribbons of width exceeding 100 mm. Therefore we hope for practical usage of the proposed method, in particular, in manufacturing of superconducting wide tapes, or several narrow tapes in one deposition process.

Two geometries of deposition onto a moving ribbon with multiple targets located along the length (a) and width (b) of ribbon are presented in Fig. 3. The geometry (a) allows by using N targets to speed up the deposition process of a film of same thickness N times, and geometry (b) allows in the same time span to obtain N times wider films.

Fig. 3. The geometry of deposition onto a moving ribbon with multiple targets. 1 - moving ribbon/substrate, 2 - plasma plume, 3 - target.

3. PLD Setup Including Diaphragm and a Moving Target

The geometry for PLD of uniform thin films, whose sizes are limited only by the deposition chamber dimensions, is shown in Figure 4. A laser beam is incident on a rotating target placed in a vacuum chamber. A diaphragm with a slit is located in the path of evaporated material between the target and substrate to select the plasma plume region where the mass-transfer rate is constant and maximum. Specifically this part of the plasma plume falls on the uniformly rotating substrate. It can be seen in Fig. 4 that the

(a) 2

3 1

(b)

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target undergoes translational motion with respect to the substrate. Obviously, in this geometry the substrate motion can be replaced by the joint motion of the target and diaphragm. To grow a film of uniform thickness over the entire substrate surface, it is necessary to determine the motion law for the target. The experiments have shown that films of uniform thickness are obtained when the target moves with a V = const/R velocity.

Fig. 4. The schematic of the technique for depositing thin films of arbitrary sizes.

In more detail this method is described in Ref. 13; now let us move on to improving the method in which we propose the use of several targets. The difficulty in this case is the requirement of a specific disposition of targets at the beginning of the deposition process, their independent movement with a certain velocity during the deposition process and the simultaneous finalization of depositing the entire area of the substrate from all targets. The geometry of such a type of the method using three targets is shown in Fig. 5. If we use n targets, the distance of target numbered l from the perpendicular to the substrate plane passing through its center at the initial time is given by Xl(t0)=R(l-1)1/2(n)-1/2, where

R is the substrate radius14. Next, each target with its diaphragm is moving parallel to the substrate radius at a velocity of V(t)=const/R(t), where R(t) is the distance from the substrate center to the point of intersection of the perpendicular passing through the laser focal spot on the target with the substrate at a given time t.

1 - substrate,

2 - substrate rotation axis, 3 - laser beam, 4 - target, 5 - diaphragm, 6 - plasma plume V=const/R 1 2 4 R 5 6 3

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Fig. 5. The schematic of the technique for depositing thin films of arbitrary sizes.

The positions and velocities of targets at the beginning and the end of the deposition for the case of four targets and a substrate 150 mm in diameter are shown in Table 1. The table shows the relative values of velocity of moving targets, according to which for each concrete case of film material and the duration of the whole process the required velocities can be predicted.

Table 1. Calculation Xl(t) and V(t) for n=4 and R=150 mm.

l Xl(t0),mm Vl(t0), a.u. Xl(tend), mm Vl(tend), a.u.

1 5 120 75 8

2 75 8 106.1 5.66

3 106.1 5.66 129.9 4.62

4 129.9 4.62 150 4

4. PLD of Large-area Films by the Mask Method

Of course, the above method can be used for depositing very large films, but it is clear that the independent movement of several targets inside the vacuum chamber entails a significant complication of the deposition set-up. The mask method does not have this disadvantage. Figure 6 shows the geometry of the mask method using a single target. A peculiarity of this method is the deposition of matter through a mask on a rotating substrate with the mask being placed in the close vicinity of the substrate10. Various configurations of slits in the mask providing the thickness uniformity of deposited films are considered. Precise sizes of slits in the mask are calculated based upon the data of angular distribution of mass transfer in the plasma plume. Fig. 6 shows also one possible configuration of the slit in the mask providing uniformity of thickness and composition of deposited films8. This method allowed to obtain films with uniform thickness on

1 - substrate,

2 - substrate rotation axis, 3 - laser beam,

4 - target, 5 - diaphragm,

6 - plasma plume V=const/R

l=1 1 2 4 R X3 X2 l=3 l=2

X

l

(t

0

)

= R(l-1)

1/2

(n)

-1/2 5 6 3

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substrates with a diameter of about 100 mm. To obtain films on substrates with larger diameters we use several targets and a mask of special configuration shown in Fig. 7. As in the case of one target, the uniformity of film deposition is achieved by the sum of the widths of the slits for each radius being equal to the width of the sector AOB and the slits being cut so that their center line coincides with the equal thickness line of a film deposited under the same conditions on a motionless substrate15. This line is shown dotted in Fig. 6.

Fig. 6. Left – the geometry of the mask method of deposition on a rotating target. 1 – laser beam; 2 – target; 3 – substrate; 4 – plasma plume; 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. Right – one of the possible configuration of the slit in the mask.

Fig.7. Left – the mask for seven targets. Right – the geometry of the mask method using more than one target.

O A B 7 3 5 6 4 1 2 8 9 _α_α_α_α αααα

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The use of seven targets can increase the radius of the deposited films 3-fold, i.e. allow to obtain films with a diameter of 300 mm.

5. Conclusions

We believe that the main result of the present work is the proposal and the realization of a new, simple and reliable method of obtaining thickness- and composition-uniform thin films on large areas, with use of laser deposition and more than one target. The mask method of deposition on a rotating substrate can be used at any standard arrangement. The size of the obtained films and the achieved degree of the thickness uniformity are not limiting. In any specific case where the permissible degree of the thickness non-uniformity is given, one may calculate the slit configuration providing the maximal rate of the deposition. Taking into account the advantages of laser deposition as compared to other methods of the manufacture of thin films, we hope that the proposed methods will be used in elaboration of new technologies in microelectronics and optical industry. Acknowledgments

The authors acknowledge the CRDF/Government of Armenia/EIF Business Partnership Grant “Sensor for a thermoelectric Single-photon Detector”.

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