WO2017025662A1

WO2017025662A1 - Method for manufacturing thin films by laser ablation by applying laser pulses with a rotating target

Info

Publication number: WO2017025662A1
Application number: PCT/FI2016/050554
Authority: WO
Inventors: Ville KEKKONEN; Jarkko PIIRTO; Jari Liimatainen; Fergus CLARKE
Original assignee: Picodeon Ltd Oy
Priority date: 2015-08-10
Filing date: 2016-08-08
Publication date: 2017-02-16
Also published as: FI20155578A

Abstract

The present invention introduces a method for manufacturing thin films by pulsed laser deposition technology so that the target (15) has essentially an annular cross-section, and laser pulses (12', 12a', 12b') are directed to the inner or outer surface of the target from at least one laser source (11, 11a, 11b). In the invention the target (15) is brought to a rotational movement, and the latest optical element (13, 13a, 13b) directing the pulses to the target can be placed farther away from the target (15), which thus facilitates the protection of the optical elements from contamination.

Description

Method for manufacturing thin films by laser ablation by applying laser pulses with a rotating target

Field of the invention The invention relates to the manufacture of metallic and inorganic ceramic thin films onto the surface of different types of substrate materials without the use of intermediate layers.

Background of the invention

Different kinds of methods for manufacturing thin films are used in the manufac- ture of metallic and ceramic coatings, such as physical and chemical vapour phase coating, and as the final result it is possible to produce dense and porous coatings. In addition to the material to be coated, the suitability of different methods for different applications depends also on the substrate material, the properties required of the coating and acceptable manufacturing costs. With coating solutions based on laser ablation it is essential, in addition to the quality of the coating, to achieve sufficient productivity i.e. the transfer efficiency of the material from the target material to a functional coating. In principle this refers to the amount of transferring material in a time unit from the target to the substrate to be coated. By increasing the power of the laser source it is generally possible to raise the productivity almost linearly up to a certain point, if the so-called ablation process can otherwise be kept constant and the laser pulses do not cause unde- sired phase changes, cracking, melting or other impacts relating to warming-up in the target material. Often it is the objective to simplify the ablation process by trying to separate individual pulses from each other and to control the process on the basis of micro-level impacts generated by a single laser pulse. This can be achieved by scanning the laser beam, by combining the scanning with the moving of the target material and by adjusting these parameters together with the repetition frequency of laser pulsing and in certain cases with the spot size of the laser beam. These general principles have been described e.g. in the specification Haruta: US 5,760,366.

In the maximizing of the use of laser pulse distribution produced by linear scanning the productivity of a single coating station is limited by the speed of scanning and by the separation of laser pulses from each other carried out by scanning. By means of the scanning techniques known at the moment it is possible to achieve at best a scanning speed of approximately 500-1000 m/s when making use of so- called polygon scanning (polyhedron mirror), which sets a certain theoretical maximum for the productivity of the coating process. For example, the apparatus ac- cording to the patent Fl 124523 uses a polyhedron mirror in the scanning.

One way to intensify the detaching of materials from a target with laser pulses is to use a monogon mirror located in the middle of an annular target and to make the laser pulses hit the surface of the target material by means of a rotating monogon mirror. Such a solution has been disclosed in the Finnish patent application Fl 20146142. This technology includes certain limitations; for example, high precision requirements for the rotating mirror, vibrations at great speeds and the protection of the monogon mirror against contamination and even damage caused by the material detaching in laser ablation. Because the monogon mirror has to rotate around its axis, it is difficult to protect the mirror from contamination, if the protec- tion mechanism has to rotate with the monogon mirror.

Summary of the invention

The invention introduces a method for manufacturing thin films, the method comprising the following steps: a. an annular, tubular, cylindrical or conical target or a target in the shape of a truncated cone or a target with a sloping bevelled end is manufactured of at least one material;

b. the direction of the normal of the target surface to be ablated in the ablation point or ablation points is set to be divergent in relation to the laser pulse sequence arriving at the respective ablation point. Characteristic features of the method are, that further in it c. the target is set to rotate around its axis;

d. a laser pulse sequence is transmitted from at least one laser source and each transmitted laser pulse sequence is targeted at the target through at least one optical element, the laser pulse sequence hitting either the inner or outer surface or both the inner and outer surfaces of the laser pulse sequence, detaching material from the target; and

e. the substrate is placed to such a point, which the material detached from the target hits at least partly and adheres to, forming a thin film to the surface of the substrate. In an embodiment of the invention, the optical element directing the laser pulses to the inner surface of the target is a stationary mirror, which is placed to the centre of the target.

In an embodiment of the invention the optical element directing the laser pulses to the inner surface of the target is a stationary mirror, which is located on the rotational axis of the target on a different side of the target from the substrate.

In an embodiment of the invention the optical element directing the laser pulses to the inner surface of the target is a fixed mirror, which is located elsewhere than on the rotational axis of the target. For example, in the case of a cylindrical target, also the distance from the optical element to the ablation point can be selected bigger than the radius of the target cylinder so that the contamination stays sufficiently small for the optical element. In a second embodiment the distance from the optical element to the ablation point can be selected to be bigger than the diameter of a cylindrical target or the diameter of the base of a conical target so that the risk of contamination decreases further. In case of several different laser sources these conditions can be set to concern each laser pulse sequence arriving at the target and the location point of the respective optical element.

In an embodiment of the invention the end of a tubular or cylindrical target is convex or in the form of a truncated cone, and the at least one optical element direct- ing the laser pulse sequences to the outer surface of the target has been placed outside the volume defined by the target.

In an embodiment of the invention the said optical elements have been located so that the direction of the laser pulse sequences arriving at the ablation points is 90 degrees in relation to the rotational axis of the target. In an embodiment of the invention laser pulses are reflected to the inner or outer surface of the target from at least two laser sources, both of which can be controlled independently with own parameters.

In an embodiment of the invention the pulse sequence of a laser source travels via, reflected by or through at least one optical element, which is a monogon mir- ror, a polyhedron mirror, lens, partly permeable element or some other element reflecting, refracting, focusing or filtering light. In an embodiment of the invention the pulse sequences of at least two laser sources can be directed simultaneously to several points on the inner or outer surface of the target through one polyhedron mirror.

In an embodiment of the invention the pulse sequences of the laser sources hitting the target in contact points detaches simultaneously at least two different materials.

In an embodiment of the invention the target is manufactured either as a uniform piece, a ring or cylinder consisting of segments, a cylinder assembled of annular superimposed parts or nested cylinders. In an embodiment of the invention the target is manufactured of annular parts, one ring in the target having been made of one material and the materials of the rings varying with each other.

In an embodiment of the invention the location of at least one optical element is changeable in relation to the target so that as the target material wears during the process the focusing of the laser pulses to the surface of the target can be maintained as desired.

In an embodiment of the invention a thin film is produced as a controlled mixture. In this case at least two rings of the target are made of different materials and the laser pulses are directed to the target at least as two pulse sequences so that the first laser source produces a first pulse sequence hitting the first ring of the target and the second laser source produces a second pulse sequence hitting the second ring of the target so that different materials hit the substrate essentially on the same area.

In an embodiment of the invention a thin film is produced as a controlled multilayer coating so that at least two rings of the target are made of different materials and the laser pulses are directed to the target at least as two pulse sequences so that the first laser source produces a first pulse sequence hitting the first ring of the target and the second laser source produces a second pulse sequence hitting the second ring of the target, the different materials hitting the substrate in different points and when moving the substrate, a first coating is first formed on the substrate and after this a second coating s formed on the first coating.

In an embodiment of the invention the substrate is arranged to travel from the first roll to the second one during the coating process, in which process the coating with at least one material and/or mixture occurs on the coating area between the rolls.

In an embodiment of the invention at least one laser source with its optical elements is placed inside a protective casing, from where the laser pulse sequence transmitted to the target is directed through apertures in the protective casing.

In an embodiment of the invention the length of the laser pulses is at most 1000 ns.

In an embodiment of the invention the repetition frequency of the laser pulses is at least 100 kHz. In an embodiment of the invention the target material comprises one or several of the following materials: gold, copper, silver, platinum, palladium, iridium, rhenium, tungsten or molybdenum.

In an embodiment of the invention the target material comprises inorganic material. In an embodiment of the invention the target material comprises metal oxide.

In an embodiment of the invention the target material comprises at least 40 atomic percent of carbon.

In an embodiment of the invention the substrate material to be coated is a polymer. The inventive idea of the invention also comprises the apparatus carrying out the method for the manufacture of thin films. The said apparatus comprises the following parts: a. at least one laser source for transmitting at least one laser pulse sequence, and

b. at least one optical element for directing the laser pulse sequence.

As characteristic features the apparatus further comprises: c. an annular, tubular, cylindrical or conical target or a target with an end in the form of a truncated cone or a target with a sloping bevelled end, which consists of one or several materials and in which at least one laser pulse se- quence is directed to the target each from a different direction from that of the normal of the surface to be ablated in the respective ablation point, detaching material from the target;

d. a substrate, which the material detached from the target at least partly hits and adheres to, forming a thin film to the surface of the substrate; and e. rotation elements, which are arranged to bring the target to a rotating motion around its axis.

Other characteristic features described in connection with the method can naturally be realized with this apparatus so that the said additional parts, elements and/or functionalities can be added to the above apparatus comprising the basic ele- ments of the invention.

Brief description of the drawings

Figure 1 illustrates an ablation arrangement with a rotating annular exemplary target, in which the incoming laser beam arrives at the target in the direction of the radius of the ring, Figure 2 illustrates an ablation arrangement with a rotating annular exemplary target, in which the laser source with its monogon mirror is located entirely on the other side of the plane of the annular target,

Figure 3 illustrates an ablation arrangement with a rotating annular exemplary target, in which there are two separate laser sources, with which several simultane- ous plasma clouds can be generated for producing a multilayer coating or desired mixtures or for increasing the transfer efficiency of the material, and

Figure 4 illustrates an ablation arrangement with a rotating annular exemplary target, in which there are four separate laser sources, with which several simultaneous plasma clouds can be generated on the outer circumference, i.e. outer surface of the annular target.

Detailed description of the invention

The present invention introduces a method and apparatus for achieving a very efficient material flow produced by laser pulses from an annular target to an object to be coated, by making use of high-powered laser pulse technology. Generally, also the term laser ablation is used of this principle. An annular target rotating at a desired speed is used in the method, laser pulses being directed to the inner or outer surface of the target, which pulses remove material from the hitting points by using a desired laser pulse repetition frequency. In the method of the invention the target material is of an annular form and it is brought to rotate around its axis; in addition, at least one laser pulse sequence is directed to the target from the centre of the target material (from the inside of the target material ring) or, when desired, from a stationary mirror system from outside the plane of the ring, the laser pulse sequence removing material from the target, and the material flow is directed as a coating to the surface of the substrate material to be coated. The repetition frequency of the laser pulses is selected so that, combined with the rotation of the target material around its middle axis, it achieves a superimposed structure of la- ser pulses of a desired size or, when desired, the hitting points of the laser pulses remaining separate on the target. Other parameters for the laser pulses are selected so that the ablation process can be carried out in a high-quality manner concerning the functionality and properties of the target material and desired coating. The optical system guiding the laser beam to the target material can divide the beam into several separate beams, which are directed to the inner and outer surface of the rotating annular target. Thus it is possible to use as high-powered lasers as possible to increase productivity without too big local energy density, which might cause heating, melting or cracking of the target material and thus weaken the quality of the developing material flow and the coating produced of it.

Laser beam pulses can be brought to the optical element directing the target material also from several different laser sources so that the optical element has separate optical surfaces to guide the laser beams to the target material. This makes possible the simultaneous use of several maximum-powered laser sources or sev- eral different laser beams in a desired way on selected output levels. Further the radiation stress directed to the optical elements from one laser source decreases, when the laser beams are aimed at the target material through several optical surfaces. This increases the operating life of these elements and reduces the risk of damage. The use of several different laser sources makes also possible the simul- taneous use of different laser parameters.

During the coating it is essential to minimize the contamination of optical elements, caused by the part of material detaching from the target, which is not directed as a coating i.e. thin film to the substrate material. It is critical for the operational continuity of the ablation process and good productivity of industrial coating processes that the contamination of essential elements is as low as possible. Coating per area unit decreases as the function of distance so that the contamination of optical elements can be significantly reduced by placing the elements as far away as possible from the coating material source i.e. the surface of the target. In practice this is best managed by placing optics outside the target ring. When the optics are inside the target ring the distance of the material source from the optics can be raised by increasing the size of the target ring's diameter. In addition, migration of the material and its adhesion to optical surfaces can be prevented mechanically, magnetically, electromagnetically or by deflecting the material flow away from the optical surfaces e.g. by means of a gas flow. Depending on the size, speed, charge and formation point of the particles, slightly different types of means can be used. A method, in which a stationary optical element inside an annular rotating target distributes the laser beam to the inner or outer surface of the target, renders a possibility to best protect the optical element corresponding to each laser pulse sequence, because the output aperture of the beam in the middle of the casing protecting the optical element can be limited to be very narrow. The small diameter of the output aperture reduces the migration risk of contaminations to optical surfaces and also contributes to the use of above-mentioned gases preventing the migration of the target material. When needed, the outlet point of laser beams can be protected e.g. by heating it locally or by cleaning it at certain intervals or realtime e.g. with a second laser beam. In the illustration of the invention reference is first made to Figure 1 illustrating the physical arrangement. The invention is partly based on the technique known as laser ablation.

In Figure 1 there is illustrated a first example of an apparatus, in which the laser beam arriving at the target 15 comes to the inner surface of the target in the direc- tion of the radius of the annular target. The apparatus includes a laser source 1 1 , which transmits pulse-shaped laser light 12. The parameters of the light transmitted by the laser source are adjustable by means of an internal controller or external control signal (not in the Figure). Laser light 12 can be processed optically by different means, which is illustrated by an optical element 14. This can be a lens, mirror, filter, semipermeable film or some other light directing or focusing element. After this the laser pulses 12 are directed to a monogon mirror 13, which is placed fixedly in place. The reflected laser pulse sequence 12' is directed accurately towards the inner surface of the target 15 in the shape of a ring, cylinder or truncated cone. The laser pulses meet the target in the contact point 15C. In this point there occurs detachment of material due to the short-term laser pulses hitting the material, but only relatively slight heating of the material. The material flow can comprise particles of different sizes, atoms, molecules, particles, plasma and in the worst case bigger material blocks, the latest one being more undesired material coming loose. The loosening material cloud 16 begins to separate from the target 15, and its direction can be controlled by forming the direction of the target's inner surface suitably inclined in relation to the direction of the laser pulses. It is desired that the material flow 16 extends towards the substrate 17 to be coated, which in this example has been placed between two rolls 18, 19. When the target 15 is set to rotate around its axis with a desired angular velocity, and the substrate is set to horizontal movement discharging from the first roll 8 and collecting to the second roll 19, an arrangement is achieved, in which the target 15 wears evenly around the whole circumference, and in addition, a coating of a desired thickness can be produced to the lower surface of the substrate 17 (in the example in the Figure). Seen from the outside, the contact point 15C thus remains seemingly in place, but examined in the own coordinate system of the target, the contact point 15C moves at desired speed around the inner circumference of the target. In one embodiment the rotational speed of the target is kept constant so that also the wearing down of the target is even, and the quality of the material coming loose can thus be kept good. The rotation of the target around its axis and the motion speed of the substrate of the roll-to-roll arrangement can be controlled by the same or different controller element as the laser. Because the radius of the target 15 can be selected relatively freely to be of different sizes in different applications, target material can be released very efficiently per time unit, when necessary. The productivity achieved by the apparatus is bigger compared to the previous solutions and makes possible many kinds of industrial coating applications. It has to be noted that the monogon mirror 13 has to stay as clean as possible during the process, and in the method it has to be ensured that the mirror 13 is protected properly from the material cloud 16 hitting the mirror itself. The protection can be executed by a protective bunker, i.e. protective ring (not in the Figure) surrounding the mirror 13, in which there are cylindrical narrow apertures for the laser beams 12 and 12'. Because the mirror 13 remains in place during the process, also the protective bunker can be installed to a fixed point in the apparatus. The narrower the aperture left for the laser beam 12', the better the protective bunker protects the monogon mirror 13 from contamination.

The final result is a coated substrate 17, in which the thin coating or thin film is illustrated by the striped section in Figure 1 . In Figure 2 there is illustrated a second exemplary arrangement of an efficient coating arrangement by laser ablation. The essential difference of this example compared to the previous one is that the laser pulse sequence arriving at the target 15 does not need to come from the centre of an annular or cylindrical target, but the monogon mirror 13 can also be placed to some other point on the rotational axis of the target. In this example the laser source 1 1 , optical element 14 and monogon mirror 13 are all placed on the same side of the plane formed by the annular target. The reflection angle of the mirror 13 can in principle be selected freely, and because also the inner surface of the target can be inclined, the output angle of the leaving material cloud 16 can be adjusted to be suitable in relation to the plane of the target ring 15.

In another embodiment it is possible to use a stationary mirror in form of a pyramid or polyhedron instead of the mirror 13, to which beams of several laser sources can be directed at the same time, on beam per one side of the polyhedron. When the shape of the polyhedron and the sizes of the angles have been selected in a suitable manner, more than one laser pulse sequence can be directed to the target 15 via one stationary polyhedron mirror. For example, a pyramid with a square base can be used to produce, for example, four reflecting laser pulse sequences, which can be directed to hit the inner surface of the target ring at intervals of 90 degrees. When such a target 15 is set to rotate with a sufficiently big angular velocity, very high productivity can be achieved, when needed, i.e. the amount of material coming loose within a time unit.

In Figure 3 there is illustrated an arrangement, in which laser sources have been placed even more freely than in the previous examples. In principle, the arriving laser beams can originate from any place, from which there is a clear visual connection to a desired point on the inner surface of the target. Monogon mirrors 13 focusing to the contact point 15C of the beam can in principle also be placed farther away, i.e. the length of travel of the beam between the mirror 13 and the contact point 15C can be distinctly longer than the length of the diameter of the target ring 15. In the example in Figure 3 there is a situation, in which there are several laser sources and their laser pulse sequence is directed simultaneously to different points on the inner circumference of the target. The first laser beam 1 1 a transmits the laser pulse sequence 12a, and it is directed to the first mirror 13a with an accurately set direction through the optical element 14a. As the final result the laser pulses 12a' of the first laser source are directed accurately towards the target, where they meet the contact point 15C, a. If the target has been assembled, for example, of two superimposed material rings, the first pulse sequence 12a' can be directed, for example, to the lower one of these so that the material flow 16c consists expressly of material A of the lower ring.

Consistently with what has been disclosed above, the second laser source 1 1 b is placed to a desired second point, and as above, it transmits its own pulse sequence 12b, the pulse sequences travel via or through the optical element 14b and are then reflected from the stationary mirror 13b. In one embodiment the direction of the second laser pulse sequence 12b' can be chosen so that it hits the upper material of the two superimposed material rings in the contact point 15C, b. Because the mirrors 13a, 13b do not move, it is possible, when so desired, to arrange the above mentioned protective bunker or other protection around them, which prevents the material cloud from travelling back to the mirror 13a, 13b. For example, the protection can be spherical with narrow apertures provided at the place of the laser beams. When a target ring assembled of two superimposed material rings is set to a rotary movement, a laser ablation process wearing down both rings uniformly is achieved, in which two material clouds 16a and 16b are simultaneously generated. If the impact points of the clouds when hitting the substrate 17 are essentially on the same area, a coating can be produced, which is a mixture of a desired type of the first and second materials of the target ring. The mixture can naturally contain a desired number of components, and this is not limited to two different materials only. By using the roll-to-roll method, the substrate can be efficiently brought to the coating area from the roll 18, then subject the substrate to both material clouds 16a, 16b for producing a mixed coating and collect the coated substrate to the roll 19.

A second option is to coat the substrate according to the case illustrated in Figure 3 first only with the material cloud 16a and let it form the first layer on top of the substrate 17. This is illustrated by the simple striped section in Figure 3. When the place of the material cloud 16b is set farther away from the meeting area of the cloud 16a and substrate 17, a second separate material layer can be produced of the material cloud 16b on top of the first material layer. This is illustrated by the checkered section in the laminar substrate 17. The final result is thus a two-layer coating, in which there is no need for separate intermediate layers and the material layers are selected by assembling them suitably of rings or cylinders and by di- recting the pulse sequences of the same or different laser sources to appropriate points 15C, a and b on the inner surface of the target. The method is naturally not restricted to two-layer coatings only, but by using even more laser sources (or by optically distributing the light transmitted by the one and same laser source) it is in principle possible to produce a desired multi-layer coating with a desired material combination. The advantage caused expressly by the rotational movement of the target 15 is that such a multi-layer coating is faster to manufacture than before, because the productivity is distinctly better than before.

The farther away the laser sources 1 1 a, 1 1 b are from the contact points 15C, the easier it is in principle to protect the parts of the apparatus from contamination. On the other hand, aligning the beams accurately to the target and also requiring a compact structure for the apparatus means that there also has to be a reasonable maximum for the distance.

Instead of building the target of superimposed rings or cylinders, the target can also be assembled of partly superimposed parts so that a conical surface is generated. A second option is to build the rings in a nested manner so that the outer surface of the inner material ring sets against the inner surface of the outer material ring. In this case the inner material can first be worn out during the ablation process thus forming a first coating layer, and after this the contact point 15C shifts to "wear" the outer layer of a different material, without altering the ablation arrangement or parameters so that a second material layer is generated on top of the first coating.

The power of the laser sources, wavelengths, pulse lengths and pauses between the pulses as well as the beam spot size can naturally be adjusted so that material clouds 16a, 16b of a desired type are obtained, and the properties of the clouds 16a, 16b can deviate even considerably from each other. In Figure 4 there is illustrated an exemplary arrangement of an efficient coating arrangement using laser ablation, the essential difference from the arrangements disclosed above being that at least one laser source is targeted at the outer surface of the annular target instead of the inner surface. The shape of the target can be a truncated cone or an erect cylinder rounded in a convex manner from its up- per or lower end. There may also be several incident laser beams and separate laser sources; in this example there are four incident laser pulse sequences so that also contact lines 15C to the target are formed as four separate lines. Beam- reflecting mirrors 13 illustrate the focusing optics in the respective way as above. In the arrangement in the Figure, the mirrors 13 can also be located up or oblique- ly up in relation to the target in addition to the horizontal direction. Here it is im- portant that a visual straight line exists from the mirror 13 to the desired target line 15C as the target 15 rotates about around its axis. This renders more freedom for arranging the optics. In other words, laser beams are brought to the surface of the target 15 in different planes so that each one of them creates an own separate ablation groove i.e. contact line 15C to the surface of the target. In one embodiment the place of the ablation point can be the closest point of the ablation groove on the target in relation to the respective mirror 13. In another embodiment the ablation point is some other point on the visual straight line on the inclined end surface of the target, and it does not need to be the point closest to the respective mirror of the generating ablation groove. In this latter embodiment contamination could be reduced for each mirror 13, as far as the mirrors are placed sensibly (e.g. far enough) in relation to the ablation points.

The direction of the inner or outer surface of an annular or cylindrical target can be set inclined in relation to the direction of incident laser pulses so that the target material coming loose is made to go to the direction of the substrate to be coated. In other words, the direction of the normal of the surface to be ablated is made to deviate from the directions of the incident laser pulses.

In an embodiment of the invention the angle between the laser pulse sequence arriving at the target and the normal of the surface to be ablated is 60 degrees or approximately 60 degrees in the ablation point. In another embodiment of the invention the said angle of arrival of the laser pulses to the target is selected from the range of 40 - 60 degrees.

The target material can be prepared as a uniform piece, a ring consisting of superimposed rings (or nested cylinders, the upper surfaces of which form a uniform conical surface), in which the rings can be made of different materials. The use of several laser sources makes possible the use of different laser parameters (wavelengths, pulse lengths, energies... ) between the laser sources. Target rings or cylinders made of different materials make possible controlled mixtures. Especially two different laser pulse sequences can be made to hit the targets, i.e. parts of a target of different materials close to each other.

Transferring the mirror/mirrors and lens/lenses in the direction of the rotation axis during the process can be carried out so that the focusing of the pulse-shaped laser beam to the conical inner or outer surface of the target stays as desired and the wearing down of the target can be controlled. The object is to make the target wear evenly in different places so that sharper "craters" are not generated to the surface of the target, which could affect the quality of the material ablating i.e. loosening from the target.

For the even wear of the target, the compensation of the wear and the general control of the ablation process, the target can be moved freely and controllably in the direction of the rotation axis.

The location of at least one optical element (for example a lens) is changeable in relation to the target so that as the target wears during the process, the focusing of the laser pulses to the inner or outer surface of the target can be set as desired. In addition to that the exact focusing of laser pulses can be executed to the surface of the target, this way it is also possible to choose a desired point of the target to be ablated by changing the locations of the optics and the target in relation to each other. In one example the target can be moved, for example, in the direction of the rotation axis. In a second example the places of the optical elements can be moved physically change the position (for example the angle of orientation) can be changed in a controlled manner or the parameters of the optical elements, such as focusing or damping can be adjusted.

When using several laser beams to the outer and/or inner surfaces of an annular target, the laser beams can be aligned so that they hit the target at different levels. In this case it is possible to execute the ablation over a larger area on the surface of the target ring during the rotational movement. If the superposition of the ablation grooves formed by different laser beams further is optimal and there is a sufficient number of laser beams to cover the entire cross-sectional area of the cylindrical target, the target can be made to wear down evenly through its entire cross- sectional area. Then as the process proceeds, the target material can be fed as a cylinder in the direction of the rotation axis and thus make possible a longer uninterrupted coating process.

In the method the transmission parameters for the laser sources are preferably selected so that the temperature of the target, measured at 0.5 mm from the surface which the laser pulses hit, remains below 200 °C during the ablation process. The porosity of the thin film or coating obtained as the final result can be achieved to be over 20 volumetric percentages by suitable selection of ablation parameters. In another example the porosity can be set to under 10 volumetric percentages, respectively. The method of the invention has the following advantages: i. The loosening speed of the material produced by the use of laser beam pulses and its transfer as a coating to a substrate material can be increased without the undesired excessive superposition of the laser beam pulses by using a rotating target material.

ii. The total power of a laser unit used in one coating station can be increased without excessive heating, melting or cracking of the target material.

iii. Laser beam pulses of several high-powered lasers can be focused to the optical element in the middle of the target material so that the total productivity of one coating station can be maximized.

iv. By distributing the power of the high-powered lasers through the mirror surface of several optical elements, the radiation and heat stress directed to the optical elements can be restricted and their operating life can be increased. v. The optical elements can be more easily protected from contamination caused by the material coming loose during the process.

vi. Technical problems relating to a fast rotating optical monogon (to the surface directing laser beam pulses to the target material) can be avoided, such as imbalance and vibration, and thus the optics used can be simplified and its optimization can be facilitated.

vii. A simultaneous use of several different laser parameters in the coating process can be executed linearly.

As the final result in the method there is achieved a significant increase in the productivity of a coating manufactured by laser beam pulses. This way, for example, the manufacturing time of larger surfaces can be shortened considerably.

In the invention it is possible to combine individual features of the invention mentioned above and in dependent claims into new combinations, in which two or several individual features can have been included in the same embodiment.

The present invention is not restricted to the shown examples only, but many variations are possible within the scope of protection defined by the enclosed claims.

Claims

Claims 1 . Method for manufacturing thin films, the method comprising the following steps:

1. manufacturing an annular, tubular, cylindrical or conical target (15) or a target (15) in the shape of a truncated cone or of a sloping bevelled cylinder of at least one material

ii. setting the direction of the normal of the surface to be ablated of the target (15) in the ablation point or ablation points (15C) to be divergent from the direction of the laser pulse sequence (12') arriving at the respective ablation point characterized in that further in the method: iii. the target (15) is brought into a rotational movement about its axis

iv. a laser pulse sequence is transmitted from at least one laser source (1 1 , 1 1 a, 1 1 b) each and each transmitted laser pulse sequence is directed through at least one optical element (13, 13a, 13b) to the target (15), the laser pulse sequence (12') hitting at least one of the following surfaces: the part of the target surface pointing towards the mass centre of the target, a straight upper or lower end of the cylindrical target, the outwards showing surface of a truncated cone, or the sloping surface of the upper or lower end of an erect cylindrical target with a conically rounded upper or lower end, detaching material (16, 16a, 16b) from the target, and v. a substrate (17) is placed to a point, which the material (16, 16a, 16b) detached from the target hits at least partly and adheres to, thus forming a thin film to the surface of the substrate (17).

2. Method according to claim 1 , characterized in that the optical element (13, 13a, 13b) directing the laser pulses to the inner surface of the target is a stationary mirror, which is placed to the centre of the target (15).

3. Method according to claim 1 , characterized in that the optical element (13, 13a, 13b) directing the laser pulses to the inner surface of the target is a stationary mirror, which is placed on the rotational axis of the target (15) on a different side of the target as the substrate (17).

4. Method according to claim 1 , characterized in that the optical element (13, 13a, 13b) directing the laser pulses to the inner surface of the target is a stationary mirror, which is placed elsewhere from the rotational axis of the target (15).

5. Method according to claim 1 , characterized in that the tubular or cylindrical target (15) has a convex end or is in the shape of a truncated cone, and that the at least one optical element (13, 13a, 13b) directing the laser pulse sequences (12') to the outer surface of the target is placed outside the volume defined by the tar- get.

6. Method according to claim 5, characterized in that the said optical elements (13, 13a, 13b) are placed so that the direction of the laser pulse sequences (12') arriving at the ablation points (15C) is 90 degrees in relation to the rotational axis of the target (15).

7. Method according to any of the preceding claims 1 - 6, characterized in that laser pulses are reflected to the inner or outer surface of the target (15) from at least two laser sources (1 1 a, 1 1 b), both of which can be controlled independently with own parameters.

8. Method according to any of the preceding claims 1 - 7, characterized in that the pulse sequence (12, 12a, 12b) of the laser source travels via, reflected by or through at least one optical element (13, 13a, 13b), the optical element (13, 13a, 13b) being a monogon mirror, polyhedron mirror, lens, partly permeable element or some other light reflecting, refracting, focusing or filtering element.

9. Method according to claim 7, characterized in that the pulse sequences of at least two laser sources (1 1 a, 1 1 b) can be directed simultaneously through one polyhedron mirror to several places on the inner or outer surface of the target (15).

10. Method according to claim 7, characterized in that the pulse sequences (12', 12a', 12b') of the laser sources hitting the target (15) in the contact points (15C) detaches simultaneously at least two different materials.

1 1 . Method according to any of the preceding claims 1 - 10, characterized in that the target (15) is manufactured either as a uniform piece, a ring or cylinder consisting of segments, a cylinder consisting of superimposed annular parts or nested cylinders.

12. Method according to claim 1 1 , characterized in that the target (15) is manu- factured of annular parts, in which target one ring is manufactured of one material and the materials of the rings vary with each other.

13. Method according to any of the preceding claims 1 - 12, characterized in that the location of at least one optical element (13, 13a, 13b) is changeable in relation to the target (15) so that as the material of the target wears during the process, the focusing of the laser pulses to the surface of the target (15) can be main- tained as desired.

14. Method according to claim 7, characterized in that a thin film is produced as a controlled mixture, in which at least two rings of the target (15) are made of different materials and the laser pulses are directed at least as two pulse sequences (12', 12a', 12b') to the target so that the first laser source (1 1 a) produces the first pulse sequence hitting the first ring of the target (15) and the second laser source (1 1 b) produces the second pulse sequence hitting the second ring of the target (15) so that the different materials hit the substrate (17) essentially on the same area.

15. Method according to claim 7, characterized in that a thin film is produced as a controlled multilayer coating, in which at least two rings of the target (15) are made of different materials and the laser pulses are directed at least as two pulse sequences (12', 12a', 12b') to the target so that the first laser source (1 1 a) produces the first pulse sequence hitting the first ring of the target (15) and the second laser source (1 1 b) produces the second pulse sequence hitting the second ring of the target (15), in which the materials hit the substrate (17) at different places and in which, upon moving the substrate, a first coating is first formed on the substrate (17) and after this a second coating is formed on top of the first coating.

16. Method according to any of the preceding claims 1 - 15, characterized in that the substrate (17) is arranged to travel during the coating process from the first roll (18) to the second roll (19), in which process the coating with at least one material and/or mixture occurs on the coating area between the rolls (18, 19).

17. Method according to any of the preceding claims 1 - 16, characterized in that at least one laser source (1 1 , 1 1 a, 1 1 b) with the optical elements (13, 13a, 13b) is arranged inside a protective casing, from which the laser pulse sequence (12', 12a', 12b') going to the target (15) is directed through the apertures in the protective casing.

18. Method according to any of the preceding claims 1 - 17, characterized in that the length of the laser pulses is at most 1000 ns.

19. Method according to any of the preceding claims 1 - 18, characterized in that the repetition frequency of the laser pulses (12, 12a, 12b) is at least 100 kHz.

20. Method according to any of the preceding claims 1 - 19, characterized in that the material of the target (15) comprises one or several of the following mate- rials: gold, copper, silver, platinum, palladium, iridium, rhenium, tungsten or molybdenum.

21 . Method according to any of the preceding claims 1 - 20, characterized in that the material of the target (15) comprises inorganic material.

22. Method according to claim 21 , characterized in that the material of the tar- get (15) comprises metal oxide.

23. Method according to any of the preceding claims 1 - 22, characterized in that the material of the target (15) comprises at least 40 atomic percentages of carbon.

24. Method according to any of the preceding claims 1 - 23, characterized in that the substrate material (17) to be coated is a polymer.

25. Apparatus for manufacturing thin films, the apparatus comprising the following parts: a. at least one laser source (1 1 , 1 1 a, 1 1 b) for transmitting at least one laser pulse sequence b. at least one optical element (13, 13a, 13b) for controlling the laser pulse sequence characterized in that the apparatus further comprises: c. an annular, tubular, cylindrical or conical target (15) or a target (15) in the shape of a truncated cone or of a cylinder with a sloping bevelled end, consisting of one or several materials, and at least one laser pulse sequence is directed to the target (15) each from a direction different from the direction of the normal of the surface to be ablated in the respective ablation point (15C) and the laser pulse sequence (12') hits at least one of the following surfaces: section of the target surface pointing towards the mass centre of the target, the straight upper or lower end of a cylindrical target, outwards showing surface of a truncated cone, or the sloping surface of the upper or lower end of the target in the shape of an erect cylinder rounded in a convex manner from its upper or lower end, detaching material (16, 16a, 16b) from the target (15), d. a substrate (17), which the material (16, 16a, 16b) detached from the target hits at least partly and adheres to, thus forming a thin film to the surface of the substrate (17), and e. rotation means, which are arranged to bring the target (15) to a rotational movement around its axis.