METHOD AND TARGET-SUBSTRATE ARRANGEMENT FOR BUILDING A HOMOGENEOUS COATING OF UNIFORM THICKNESS ON THE SUBSTRATE FROM A PLASMA

PRODUCED BY A PULSED LASER

The invention relates to a method for building a homogeneous coating of uniform layer thickness on a substrate from a plasma produced by a pulsed laser from a target. The invention also relates to a target-substrate arrangement for building a homogeneous coating of uniform layer thickness on the substrate from a plasma produced by a pulsed laser from the target. The invention further relates to a homogeneous coating of uniform film thickness prepared on the' substrate from a plasma produced by a pulsed laser from the target.

To fabricate thin films of desired composition on a substrate area, thin film growth from a plasma generated by a pulsed laser (that is, the "pulsed laser deposition", PLD) is a widely applied technique nowadays. The essence of this technique lies in exposing the target generally arranged within a vacuum chamber to laser pulses of desired energy. A thin surface layer of the exposed target portion comprising the constituents of the layer to be fabricated are brought (ablated) into a plasma phase by means of the laser pulses. The species in the plasma phase (atoms, ions, and/or the clusters thereof) leave the target in the form of a rapidly expanding plasma plume, in which the largest components of the velocity vectors of the outgoing species are - in accordance with the experi- ments and theoretical models describing the phenomenon - perpendicular to the surface of the exposed target portion, independently of the angle of incidence of the laser pulses used for the ablation. The plasma plume generated within the vacuum chamber is optionally mixed with further (inert and/or reactive) gaseous substance(s) also fed into the vacuum chamber. The plasma species may interact with these substances through physical or chemical reactions. The desired layer on the surface to be coated (ie. the deposition surface), arranged in the path of the expanding plasma plume, is formed by the species re- condensing from the plasma phase. The laser pulses applied are capable of transforming all constituents of the target into the plasma phase, and hence involving in the film growing process. The chemical composition of the film being deposited can be fine-tuned and set in a relatively accurate manner: the composition of the layer to be deposited is

determined by the composition of the target, in combination with that of the gaseous substance optionally being also present within the vacuum chamber.

At the moment of the plasma plume being created by the laser pulses, the largest components of the velocity vectors of the target constituents being present in the plasma plume are perpendicular to the ablated target portion and point away therefrom. If the normal to the target is defined as a unit vector pointing from the bulk of the target towards the ablated portion and being perpendicular to the ablated portion, and if the terms "forward" and "backward" are introduced for the components of the velocity vectors of the plasma species pointing into the same and into an opposite direction, respectively, relative to the normal to the target, then the majority of the components of the velocity vectors of the plasma plume species generated initially are forward oriented. Simultaneously with the expansion of the plasma plume, the angular distribution of the velocity vectors of the plasma species changes in time: due to scattering, as well as plasmady- namic effects, e.g. changes of the pressure conditions prevailing within the plasma plume, the velocity vectors having backward oriented components also appear. As far as deposition is concerned, the velocity vectors' angular dependency results in an uneven film growth rate on the deposition surface arranged in the path of the plasma plume: the growth rate will be the greatest in a portion of the deposition surface that lies opposite to the source of the plasma plume, and decreases in any directions away from this portion. This leads to a layer of uneven thickness and of varying physical and chemical properties, that is, the coating fabricated (considering the full area thus coated) is not going to be of even thickness and homogeneous, which is one of the greatest drawbacks of thin films prepared by the PLD technique. To eliminate said drawback of PLD, a great number of solutions exists nowadays. U.S. Patent No. 5,468,930 discloses a PLD apparatus, wherein the ablated portion of the target and the deposition surface are arranged facing to each other, however not in a coaxial position, and the normal to the target and the rotation axis of the deposition surface are at a given angle relative to each other. The layer deposition onto the deposition surface being continuously rotated is accomplished by those species of the plasma plume produced by a laser which have velocity vectors pointing towards the deposition surface, ie. which are parallel with and oriented to the same direction as the normal to the target. The uniformity of the film being formed on a large surface is aimed to be achieved

through tilting of the plane containing the ablated portion of the target relative to the plane of the deposition surface.

U.S. Patent No. 5,672,211 teaches a PLD apparatus, wherein the film deposition onto a deposition surface takes place from a forward moving plasma plume produced by a laser from the target. A uniform film growth on the substrate is achieved through a continuous movement of the target, an appropriate turning off of the deposition surface, as well as by guiding the ablating laser spot by means of a relatively complicated (additional) optical system. The deposition surface is optionally subjected to a rotation, whereby it is kept perpendicular to the plasma plume used for the film deposition in every single position thereof.

U.S. Patent Appl. No. 2003/0180462A1 discloses a coating apparatus and method, wherein the deposition surfaces to be coated are capable of rotating around two independent rotation axes and arranged at equal distances from the target in such a manner that the coating substance leaving the target (due to the action of an electron beam) reaches each of the deposition surfaces just at right angle. As a consequence of the peculiar, simultaneous rotation around the two axes, a uniform coating will grow on each deposition surface. The coatings are formed with the help of the forward moving substance stream, ie. the stream that moves away from the surface of the target.

European Pat. No. 702,416Bl describes a PLD method and apparatus for depos- iting a superconducting layer onto a deposition surface, whereby the target and the deposition surface lie in planes essentially perpendicular to each other and rotate around rotational axes perpendicular to said planes with given angular velocities. During the method, a plasma plume with an axis being essentially parallel to the deposition surface is created from the target through laser ablation and is used for film growing. In this arrangement, the deposition surface is always located above the plane of the ablated target portion, that is, to build a layer on the deposition surface, the forward oriented constituents of the plasma plume are exploited.

A great number of technical applications of PLD would require a film growth at relatively high pressures (typically at least a couple of tens of Pa's) and in a controlled atmosphere of a background gas comprised of reactive or inert gas(es). It is known, however, that an increase in the pressure induces a decrease in the mean free path of the plasma species, that is, the plasma plume is practically compressed to smaller and smaller

volumes. As far as film growth is concerned, this means that the plasma species can travel forwardly shorter and shorter distances. At the same time, when pressure increases more and more plasma species become scattered on the molecules of the background gas, and hence the amount of forward moving plasma species, that are suitable for film growth via the PLD technique, is getting smaller and smaller. As for accomplishing a film growth on the deposition surface the forward moving plasma species leaving the ablated target surface must reach the deposition surface, an increase in the pressure of the background gas encumbers industrial application of PLD. As far as feasibility of PLD is concerned, this can be compensated only by arranging the substrate closer to the plasma source. At background gas pressures of at most 1 Pa the target-substrate distance, which is typically about a few centimetres, can be decreased only within a certain limited range due to technical limitations. This means that the pressure of the background gas used in a PLT technique can be increased to a certain upper limit.

Former solutions have tried to eliminate the drawbacks of PLD emerging when large deposition surfaces are to be coated (uneven film thickness, lateral inhomogeneity, surface roughness) on the one hand by rotating the deposition surface at a suitable angular velocity and on the other hand by exploiting such a target-substrate geometry, whereby the deposition surface is located at every single instant of the film growing process in that half-space limited by a tangent plane laid through the ablated portion to the surface of the target, in which the plasma plume is generated by the laser pulses. A further common feature of the PLD processes used is that the film growth was realized basically by the plasma plume species having forward oriented velocity components. In order to reach film growth rates being suitable (ie. about rates of 0.1 to 0.5 μm/min) for the application of PLD feasibly and economically on the industrial scale, relatively complicated technical solutions are necessary. Moreover, in the case of PLD processes used nowadays the harmful effect on film growth of the increased background gas pressure failed to be eliminated, and hence said processes can be applied for an economical fabrication of coatings/layers on an industrial scale only at background gas pressures of at most a few Pa's. The aim of the present invention is to provide a deposition method for forming homogeneous coatings of uniform film thickness, by means of which the above detailed drawbacks of PLD can be eliminated or at least reduced to a great extent. A further aim

of the present invention is to provide a target-substrate geometry, by the usage of which homogeneous coatings of uniform film thickness can be prepared on the deposition surfaces to be coated, even in the case of optionally applied background pressures that are relatively high (that is, typically at least a couple of tens of Pa's) compared to the highest pressures of PLD processes in general.

The present invention is based on our experimental findings according to which the species of the plasma plume, generated by the laser pulses, having backward oriented (ie. pointing into the opposite direction relative to the normal to the target portion ablated by the laser pulses) velocity components can also be utilized successfully for a film deposition. Moreover, according to our experiences in such a film growth process every effect due to which the amount of plasma constituents with backward directed velocity components increases within the plasma plume produced by the laser pulses (e.g. scattering of the plasma plume species on each other, usage of a reactive atmosphere creating a background pressure) facilitates the film growth and improves the homogeneity of the film thus fabricated.

In accordance with this, in one aspect of the present invention a method for building a homogeneous coating is provided which comprises the steps of

— arranging the deposition surface in a closed half space located on that side of a tangent plane laid through the ablated part of the ablation portion of the target which is opposite to the plasma plume in such a position, wherein a normal to the deposition surface has a vector component being parallel and unidirectional with the normal to the ablated part of the ablation portion of the target;

- simultaneously with the ablation, continuously rotating the deposition surface around a rotational axis parallel with the normal to the deposition surface, while - utilizing those constituents of the plasma plume for building the coating on the deposition surface that also have velocity components pointing into the opposite direction relative to the normal to the ablated part of the ablation portion of the target.

Further embodiments of the method according to the present invention are revealed by claims 2 to 11. In a further aspect of the present invention such a target-substrate arrangement is provided, wherein the deposition surface is arranged in a closed half space located on the target side of a tangent plane laid through the ablated part of the target, is capable of be-

ing rotated around a rotational axis parallel with the normal to the deposition surface, and is located in the path of that portion of the plasma species, the constituents of which also have velocity components pointing into the opposite direction relative to the normal to the ablated part. Further preferred embodiments of the target-substrate arrangement of the present invention are covered by claims 13 to 19.

In a yet further aspect of the present invention such a coating is achieved, which is deposited by making use of a target-substrate arrangement in accordance with the present invention by any of the possible methods according to the present invention. The invention will now be explained in detail with reference to the accompanied drawing, wherein

- Figure IA is a schematic cross sectional view of a possible setup for effecting the method according to the present invention;

- Figure IB is a schematic cross sectional view of the setup shown in Figure IA with a modulating member arranged within the reaction volume;

- Figure 2 is a perspective view of a possible embodiment of the target-substrate arrangement according to the invention to be used within the setups shown in Figures IA and IB;

- Figures 3A and 3B are the perspective and cross sectional views, respectively, of a possible other embodiment of the target- substrate geometry according to the invention to be used within the setups illustrated in Figures IA and IB;

- Figures 4A and 4B are the cross sectional and top views, respectively, of a sample holder for a possible further embodiment of the target-substrate geometry according to the invention to be used within the setups illustrated in Figures IA and IB, when the target and the substrate are removed;

- Figures 5A and 5B are schematic views of some possible further embodiments of the target-substrate geometry according to the invention;

- Figure 6 illustrates a target-substrate geometry formed with a target provided specifically by a cylindrical rod; - Figures 7A and 7B are atomic force microscopic (AFM) images of carbon-nitride

(CN) films grown on a silicon substrate from a graphite target in the presence of a

nitrogen background gas by a traditional PLD method and by a film forming method according to the invention, respectively; and

- Figure 8 shows the normalized growth rates measured for the carbon-nitride films of Figures 7 A and 7B as a function of the nitrogen background gas pressure in a given point each of the deposition surfaces.

Figure 1 illustrates a laser coating apparatus 10 for effectuating the method according to the invention. The coating apparatus 10 has a source 20 of laser light and a reaction chamber 12. The reaction chamber 12 encloses a reaction volume 8. The reaction chamber 12 has an optical window 14 for transmitting a laser beam 22 emitted by the Ia- ser source 20 and an outlet 16 and an inlet 18 provided with means, preferably in the form of valves for maintaining a controlled atmosphere within the chamber 12. The outlet 16 and the inlet 18 serve for the creation of a pressure (vacuum or a background pressure differing from vacuum, also including a reactive atmosphere) prevailing within the reaction volume 8 of the reaction chamber 12. Therefore, the outlet 16 can be connected to a pump not shown in the drawing, while the inlet 18 can be connected to the reservoir of a background gas, which is not shown in the drawing either.

Within the reaction volume 8, a sample holder 30 firmly connected to a shaft 36 and rotatable together with it is arranged in the path of a laser beam 22 entering through the optical window 14. The sample holder 30 is provided with a support surface 31 which faces to the reaction volume 8. The sample holder 30 can be of any shape. The support surface 31 of the sample holder 30 serves for the target 32 representing a source of substance to be promoted to the plasma state, as well as for the substrate 34 to be coated to be placed and mounted within the reaction chamber 12. The support surface 31 is preferentially formed as a planar surface. In a possible further embodiment of the sample holder 30, the support surface 31 can be provided with a recess/recesses receiving at least parts of the target 32 and of the substrate 34. To ensure rotatability of the target 32 and the substrate 34, in the embodiment of the sample holder 30 illustrated on Figure IA, the shaft 36 is connected to a driving means, preferably to a motor (not shown in the drawing) that can be equally arranged outside or inside the reaction volume 8. Simultaneous rotation of the target 32 and the substrate 34 is most simply achieved by means of the setup outlined in Figure IA, however, other constructions are also possible. An axial rotation of the target 32, which prolongs the lifetime of the target 32, is preferred, but not

required. Accordingly, an embodiment of a sample holder 30 ensuring the axial rotation of only the substrate 34 can also be used in a preferred manner in the coating apparatus 10.

The source 20 of laser light is a pulsed, preferentially an excimer or a YAG laser. Generally, any kind of laser source can be used as the laser source 20 that emits laser pulses between 10 ^'15 s and 10 ^"6 s in length and of an energy density sufficient to ablate the material of the target 32 when reaching its surface. In the path of the laser beam 22 between the laser source 20 and the optical window 14 an optical unit (not shown in the drawing) can also be arranged to accomplish focusing of the laser beam 22 to a required extent and to effect the displacement of the laser beam 22 relative to the ablated surface of the target 32. As the construction of such an optical unit is known by the person skilled in the relevant field, its details are not discussed here.

When deposition is to be carried on, following the arrangement of the target 32 and the substrate 34 on the sample holder 30, the latter is arranged within the reaction volume 8 and the reaction chamber 12 is closed in an air-tight manner, at a closed position of the inlet 18 the reaction volume 8 is subjected to a vacuum by means of a vacuum pump connected to the outlet 16. When the required amount of vacuum is attained, the outlet 16 is closed or in case of a further, so-called flow-through type variant of the method a constant pressure is maintained within the reaction volume 8 via opening the inlet 18 and applying a suction through the outlet 16 with a simultaneous continuous gas feed through the inlet 18. If deposition should be carried out in a controlled atmosphere, the required amount of a reactive or inert background gas is fed from the appropriate reservoir connected to the inlet 18 into the reaction volume 8 at the open position of the inlet 18. To. arrange the target and the substrate within the coating apparatus, any of the target- substrate geometries shown in Figure IA and IB, as well as to be discussed in detail below with reference to Figures 2 to 6 can be used. Having set the required atmosphere within the reaction volume 8, the substrate 34 (and also the target 32 if e.g. the target- substrate geometry of Figure IA is used) is brought into rotation around an axis 36. Simultaneously, the laser source 20 is also activated and the ablation of the surface of the target 32 is commenced by the laser pulses through the optical window 14 of the reaction chamber 12. As a result, the substance in a thin layer of the target 32 is promoted to the plasma state and leaves the target 32 in the form of a plasma plume 24. The maximal

velocity of the expanding plasma is essentially perpendicular to the ablated portion of the target 32 and the initial velocity vectors of the plasma species mainly point forwards, ie. into the same direction as the normal n (see Figure 3B) to the ablated portion of the target 32, or at least possess components pointing into the same direction as the normal ή . As a result of plasmadynamical processes taking place within the plasma during its buildup and the collisions among the plasma species (self-scattering), plasma species will also appear in the plasma plume 24, the initial velocities of which have backward directed components, ie. components that point into the opposite direction relative to the normal n to the ablated surface of the target 32. Along with a continuous rotation of the sub- strate 34, just those plasma species are used to fabricate the coating in the method according to the invention that approach the target 32 and are not used for film fabrication in traditional PLD methods. The uniform thickness and homogeneity of the coating/film being grown is assured by the continuous, but not necessarily steady rotation of the substrate 34, while varying the distance between the substrate 34 and the plasma source in a lateral direction, it is possible to grow layers of different properties.

When film fabrication is carried out in the presence of an inert or reactive gas, the constituents of the background gas are also incorporated into the layer formed on the substrate 34, and hence it is possible to prepare coatings with properties different from the properties of the target 32 and with compositions much more diversified than the composition of the target 32. A further function of the background gas fed into the reaction volume 8 is to serve as a scattering medium for the constituents of the plasma plume 24. The higher the pressure of the background gas within the reaction volume 8 is, the stronger the scattering effect of the background gas is.

As it is shown in Figure IB, the growth rate distribution of the film formation can be modulated by arranging a mechanical barrier (which optionally transmits the laser beam 22), e.g. a modulating member 38 formed as a surface, opposite to the substrate 34 within a certain distance limited by the size of the reaction volume 8. The modulating member 38 hampers the plasma plume's forward traveling species of high energy, thus these species cannot be backscattered from the atmosphere. At the same time a lateral backscattering is not impeded, whereby the plasma plume 24 broadens and the thickness distribution of the layer being formed on the surface of the substrate 34 becomes much more homogeneous. The modulating member 38 can be mounted opposite to the sub-

strate 34 by any suitable manner; in the embodiment illustrated in Figure IB it is affixed onto the sample holder 30 by means of e.g. spacer elements 37 of adjustable length.

Figure 2 shows a target-substrate geometry according to the invention in an enlarged view arranged on the sample holder 30, wherein the substrate is formed by the tar- get 32 itself. In the shown arrangement, deposition takes place directly onto the target 32; an ablation portion 33 of the target 32 arranged on the sample holder 30 is ablated on a laser spot 23 by the laser beam 22 reaching the target 32. The film to be grown forms both within an annular zone (in a portion of the target 32 denoted by a dashed line in Figure 2) swept over by the laser spot 23 due to the rotation of the sample holder 30 around the axis 36 and outside thereof. This latter deposition, if desired, can be removed from the target 32 through reablation effected by displacing the laser beam 22 radially outwards. Figures 3 A and 3B illustrate a further embodiment of the sample holder suitable for use in the coating apparatus 10. As it can be seen in Figure 3B, the target 32 and the substrate 34 are arranged on and affixed to the support surface 31 in such a manner that the ablation portion 33 of the target 32 and the deposition surface 35 of the substrate

34 define a single plane. Figure 3 A and 3B show a target-substrate geometry of cylindrical symmetry comprising a disc shaped substrate 34 having a circular deposition surface

35 and a target 32 having an ablation portion 33 encircling the deposition surface 35 in the form of a ring. Naturally, the solution according to the present invention can also be accomplished with target-substrate geometries comprising a target 32 and a substrate 34 having shapes differing from what was described above. In what follows, some of these geometries will be discussed in detail. It should also be noted here that to form a film, the target 32 can be encircled by the substrate from the outside, too.

Figures 4A and 4B illustrate in a cross sectional view and in a top view, respec- tively, a further embodiment of the sample holder 130 to be used in the coating apparatus 10, which is suitable for adjusting the relative position of the ablation portion of the target and the deposition surface of the substrate both arranged on the sample holder. The sample holder 130 consists of two pieces: a substrate holder 140 provided with an upper substrate holder surface 143 and a base surface 144, and a target holder 120 having a support surface 131 and a base surface 121. The upper substrate holder surface 143 faces into the same direction as the support surface 131, while the base surface 144 faces into the same direction as the base surface 121. A shaft 142 is connected perpendicular to the

base surface 144 of the substrate holder 140. A hollow tubular shaft 136 perpendicular to the base surface 121 is connected to the target holder 120 on that side thereof which is located opposite to the support surface 131 (ie. from the direction of the 121 base surface). The connection is provided by bracing members 137 attached to the base surface 121 at one end thereof and to the tubular shaft 136 at the other end thereof. At its end located closer to the base surface 121, the tubular shaft 136 is provided with a flange 139. The flange 139 is located below the plane containing the base surface 121.

The target holder 120 is provided with a through hole 141 extending from the support surface 131 to the base surface 121 perpendicular to the latter and in the full height of the target holder 120. A section of the hole 141 parallel to the base surface 121 conforms to the shape of the substrate holder 140. When the sample holder 130 is assembled, on the one hand the substrate holder 140 preferably fits into the hole 141 of the target holder 120 with no clearance and can be shifted parallel to the shaft 142 within the hole 141, on the other hand its shaft 142 is coaxial with and extends within the tubular shaft 136 of the target holder 120. The substrate holder 140 can be shifted between two extremal positions in the hole 141. The substrate holder 140 reaches one of said extremal positions when its base surface 144 bears against the flange 139 of the tubular shaft 136. The other extremal position of the substrate holder 140 corresponds to the position where the portion ablated by the laser pulses of the target arranged on the target holder 120 and the deposition surface of the substrate placed onto the substrate holder surface 143 lie in the same plane. The substrate holder 140 can be fixed and/or shifted by any suitable means, e.g. mechanically in any position between said extremal positions thereof. The hole 141 formed in the target holder 120 and the substrate holder 140 that fits into the hole 141 preferably have shapes of cylindrical symmetry, however they can be formed with any other shapes preferably fitting complementary to each other.

To rotate, if necessary, the target holder 120 and thereby the target being arranged on the support surface 131, the tubular shaft 136 is coupled to a drive unit (not shown in the drawings), preferably to a motor. To effect required rotation of the substrate holder 140, the free end of the shaft 142, which is opposite to the end connected with the base surface 144, is coupled to a drive unit (not shown in the drawings either), preferably to a motor.

The sample holder 130 detailed above enables rotating (ie. at different angular velocities and/or in different rotation directions) the target and the substrate around a common axis independently of each other, applying targets and substrates with various thicknesses as well as making use of an ablation portion and a deposition surface lying in different heights.

Figures 5A and 5B illustrate schematically two further target-substrate geometries without showing the respective sample holders. In said figures the target(s) and the substrate(s) are simply represented by disc shaped member(s) and rectangular block member(s), respectively, although it is obvious for a person skilled in the relevant art that the substrate(s) and the target(s) of the arrangements illustrated in the figures can have any shape. In the figures the rotation of the individual elements that constitutes a core feature of the present invention, and is not only essential for depositing an even and homogeneous film but also required for the prolongation of the lifetime of the target(s) is represented by clockwise or anticlockwise arrows around rotation axes drawn by dashed- dotted lines for each element. It should be emphasized here that it is only the rotation of the substrate(s) that is essential for growing even and homogeneous films. It should also be noted here that the directions and/or velocities of rotation of the elements shown in the figures can be chosen arbitrarily and independently of each other. Moreover, at the target-substrate geometries illustrated in Figures 5 A and 5B the rotation axes of the individ- ual elements can form angles with each other. Furthermore, in view of the sample holders discussed in detail in relation to Figures 1 to 4, possible further embodiments of the sample holder suitable for realizing a given target-substrate geometry are also apparent for a person skilled in the relevant art. It is also noted here that the target-substrate geometries shown in Figures 5A and 5B can be freely combined with each other if suitably formed/chosen sample holders are applied.

Figure 5A shows a "multiple substrates, single target" type arrangement that comprises a target 232 having an ablation portion 233 ablated on a laser spot 223 by a laser beam 222 and a plurality of (in this special case particularly two) substrates 234a, 234b with their own deposition surfaces 235a, 235b, arranged in such a manner that said deposition surfaces 235 a, 235b lie within or below a tangent plane (which is identical with the plane of the ablation portion 233 in this special case) laid through the ablated zone of the target 232. If the substrates 234a, 234b are arranged symmetrically, the pres-

ent arrangement is especially useful for forming identical films (ie. of identical composition, layer thickness, and/or change in layer thickness) on a plurality of separate substrates simultaneously, and hence for series production.

Figure 5B shows a "single substrate, multiple targets" type arrangement that comprises a substrate 334 having a deposition surface 335 and a plurality of (in this special case particularly two) targets 332a, 332b having ablation portions 333a, 333b ablated on respective laser spots 323a, 323b by laser beams 322a, 322b, arranged in such a manner that the deposition surface 335 lies within or below one of the tangent planes (which form a single common plane containing all the ablation portions 333a, 333b in this spe- cial case) laid through each ablated zone of the targets 332a, 332b separately. Furthermore, the substrate 334 is preferably arranged between the laser spots 323a, 323b at given distances therefrom. The laser beams 322a, 322 that ablate are produced by the same or by different laser light sources. If different laser light sources are used, the laser beams 322a, 322b can be of various properties (e.g. pulse length, wavelength, fluence, etc.). If targets 332a, 332b of different compositions are ablated one after the other for different periods of time, the present arrangement is particularly useful for growing a multilayer structure comprised of layers of different thicknesses on a given substrate. With simultaneously ablating the targets 332a, 332b it is also possible to tune the composition of the layer under fabrication (e.g. by varying the distances of the substrate 334 from the laser spots 323a, 323b).

Figure 6 illustrates a target-substrate geometry comprising a target 432 provided, in particular, as a rod and a substrate 434 of arbitrary shape (for the sake of simplicity illustrated as a rectangular block) provided with a deposition surface 435. To prolong the lifetime of the target 432, the target 432 and the laser beam 422 ablating the target 432 on a laser spot 423 are kept in motion relative to each other. The relative motion can be realized for example by way of rotating the target 432 around its longitudinal axis (drawn by a dashed-dotted line), via translating the target 432 along its longitudinal axis, by way of directing the laser beam 422 over the target 432 along one of its generatrices by means of a suitable optical means (not shown in the drawing) or by any combination of said ways. As a result of the relative motion applied, ablation portions 433 of various shapes are obtained. However, the substrate 434 and the target 432 are arranged relative to each other in such a way that the deposition surface 435 (or in case of a non-planar deposition

surface, the highest point of thereof) lies within or below a tangent plane of the target 432 laid through the laser spot 423. Moreover, the rotation axis (drawn by a dashed-dotted line) of the substrate 434 is oriented so as to be parallel or form an angle with the largest velocity vector of the expanding plasma that leaves the area ablated on the laser spot 423. It should be noted that the expression "within or below a tangent plane" used throughout the exposition of the target-substrate geometries of Figure 5 and 6 refers to such an arrangement, wherein after arranging the target(s) and the substrate(s) within suitable sample holder(s), the deposition surface(s) is (are) located in every instant during the film growing process in a closed half-space that is defined by the tangent plane(s) laid through the just ablated portion(s) and contains no plasma plume(s) generated for the purpose of film forming; here, by definition, the closed half-space also contains the tangent plane(s) delimiting it.

EXAMPLE In what follows, properties of thin films grown from the same plasma plume, ie. under fully identical deposition conditions, by a traditional PLD process (that is, from forward directed constituents of the plume on a deposition surface arranged just opposite to the ablation portion; from now on "PLD configuration") and by a deposition method according to the present invention (that is, from backward directed constituents of the plume on a deposition surface located within the plane of the ablation portion; from now on "inventive configuration") will be illustrated through an example.

To prepare the films to be studied a graphite target and two silicon substrates were used in a coating apparatus shown in Figure IA, with the only difference that one of the substrates was arranged opposite to the target at a distance of 53 mm apart from it (PLD configuration). The film fabrication was performed by using nitrogen as the back- ground gas at a background pressure of 5 Pa. The ablation of the target was performed by laser pulses having fluences of 7 J/cm ² and lengths of 20 ns, the pulses were generated by a Lambda Physics EMGl 5 OTMS C type KrF excimer laser operated at the wavelength of 248 nm. As a result of the film growth, a carbon-nitride film was formed on each substrate. Atomic force microscopic images of a 10 x 10 μm slice of each of the surfaces of the substrates are shown in Figure 7A (PLD configuration) and in Figure 7B (inventive configuration). Comparing the two figures it is apparent that while the film fabricated in the PLD configuration contains a relatively high amount of melt drops and is rather un-

even, the surface of the film formed in the inventive configuration is significantly evener. Furthermore, when the lateral variation in the thickness of the carbon-nitride film on a sample of a macroscopic size (of about 5 cm in diameter) grown in the inventive configuration was measured, the relative difference was obtained to be only a few percentage that means a film with an extreme evenness.

Using the above discussed arrangement, the pressure dependence of the growth rate of the films being fabricated on the substrates was studied in a chosen point of each of the substrates. For the inventive configuration, those points of the substrate were considered as the chosen points which are located within the range of 5 to 7 mm apart from the elliptical laser spot (measured along its major axis), while for the PLD configuration that point of the substrate was considered which is located opposite the laser spot. The pressure of the nitrogen background gas was varied within the range of 0.1 to 100 Pa, and the thicknesses of the films expressed in units of nm were measured. The growth rate values defined as a product of the film thicknesses normalized to a single laser pulse and (to compensate for the change in the porosity) the densities of the films deposited are shown in Figure 8, wherein the symbols * represent the values obtained for the substrate arranged in accordance with the PLD configuration, while the symbols □ stand for the values measured on the substrate arranged according to the inventive configuration. The effect of the background pressure on the film growth is apparent from Figure 8: when the inventive configuration is used an increase of the background pressure results in faster and faster film growth, and above about 10 Pa the film fabrication performed in the inventive configuration becomes much more advantageous and is uniquely faster while the PLD configuration cannot be used for film deposition in a feasible way.

The present example unambiguously shows that the drawbacks of traditional PLD-based film deposition (uneven layer thickness, lateral inhomogeneity, surface roughness) can be eliminated to a high extent by the method according to the present invention, moreover, the target-substrate geometries according to the present invention are suitable for film deposition from a plasma produced by laser pulses even at relatively high pressures.