WO1996010324A1

WO1996010324A1 - Laser target for use in an apparatus for generating radiation and atomic particles

Info

Publication number: WO1996010324A1
Application number: PCT/NL1995/000323
Authority: WO
Inventors: Leonid Shmaenok
Original assignee: FOM-INSTITUUT VOOR PLASMAFYSICA RIJNHUIZEN
Current assignee: FOM-INSTITUUT VOOR PLASMAFYSICA RIJNHUIZEN
Priority date: 1994-09-26
Filing date: 1995-09-26
Publication date: 1996-04-04
Anticipated expiration: 1997-03-26
Also published as: AU3687895A; NL9401560A

Abstract

Method for generating radiation and particles, more specifically x-rays and atomic particles, by focusing a pulsed laser beam on a target. The target surface section onto which the pulsed laser beam is focused is moved along the focal spot of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated with such a velocity that, because of the vectorial addition of said velocity and the different initial velocities of the various radiation components, at least a partial spacial separation between the various radiation components will be obtained.

Description

Laser target for use in an apparatus for generating radiation and atomic particles

The invention relates to a laser target for use in an apparatus for generating radiation including particle radiation, more specifically x-rays and atomic particles, by focusing a pulsed laser beam on said target.

Targets of the above-mentioned type are used in apparatus which are either predominantly destined to generate X-ray radiation or predomi- nantly destined to generate atomic particles in the form of ions, atoms, and molecules.

One of the most important and prospective applications of radiation sources of this type is the so-called soft X-ray projection lithography. Prior art examples of this type of apparatus are known for instance from the article "Prototype high-speed tape target transport for a laser plasma soft X-ray projection lithography source" by Steven J. Haney, et al. in Applied Optics, Vol. 32, Nr. 34, 1 December 1993, pages 6934-6937.

An other wide field of utilization of the laser interaction with solids, as a source of atomic particles, is laser evaporation (ablation) of materials for deposition of thin films and multilayer structures in production of high temperature superconductors and X-ray optics. Embodiments thereof are also known from the prior art.

The problem encountered in all prior art methods and apparatus is that with each laser pulse various kinds of radiation and particles are generated, i.e. X-rays, atomic particles and inevitably also macroparticles, whereby all these various kinds of radiation and particles are generated in the same directions. However, in the various applications of these type of targets in general only radiation or particles of one type is needed and preferably all other types of generation have to be suppressed in one way or another. Although in some applications where particles are used the simultaneously generated radiation is not harmful, in applications where radiation is needed the particles are in general parasitic. Especially the appearing macroparticles are very disturbing in most applications and should be avoided.

Different types of proposals are already described to obtain a separation between the X-ray radiation and the atomic particles and macroparticles. According to one proposal a very thin tape target is used to reduce the amount of generated atomic particles. According to anothe proposal a rotating shutter is used synchronized with the pulsed lase beam source such that the X-rays are transmitted whereas the atomi particles, arriving at the shutter πust a small time fraction later, ar stopped. Further proposals include the use of cryogenic targets or th use of a supersonic ^et for splitting macroparticles into small fragment and removing them.

All these known matters are complicated and/or expensive and i any way not sufficiently effective. So, there is still a need for a method and apparatus for generating radiation and particles in such a manner, that at least th macroparticles will become separated from the atomic particles an radiation and preferably such that also a separation between x-rays an atomic particles is obtained. In agreement with this object the invention provides a target for use in an apparatus for generating radiation and particles, more specifically x-rays and atomic particles, by focusing a pulsed laser bea on said target, characterised in that the target surface section ont which the pulsed laser beam is focused moves along the focal spot of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated with such a velocity that, because of the vectorial addition of said velocity and the different initial velocities of the various radiation components, at least a partial spacial separation between the various radiation components will be obtained.

In general the generated macroparticles are rather slow in comparison with the relatively fast atomic particles. By moving the target surface section, from which both the macroparticles and the atomic particles are released, with a predetermined velocity perpendicular to the initial average direction in which both types of particles are released, the relatively slow macroparticles will be dragged to one side whereas the faster atomic particles will only show a rather small change in direction. So the result will be a partial spacial separation between a subspace in which macroparticles and atomic particles appear and another subspace in which only atomic particles are generated (apart from the x-rays which appear in both subspaces).

A similar effect can be obtained with respect to x-rays and atomic particles. If the velocity of the moving target surface section is selected in the same order as the initial velocity with witch the atomic particles are released from the target surface, the majority of said atomic particles will be dragged to one side and will be confined within a predetermined subspace. The result thereof will be "clean" x-rays outside said subspace without atomic particles (and without macroparticles). Under these circumstances the macroparticles are confined to an even more restricted subspace so that the larger part of said cloud of atomic particles is also free of macroparticles.

Apparatus in which at least part of the target surface is moving are already known as such, for instance from the above-mentioned article by Steven J. Haney, et al. However, in all these prior art apparatus the target movement is only implemented to transport a virgin target surface section into the laser focus for each laser pulse. According to said article for instance a target tape velocity of 300 cm/s may be required. However, at such target tape velocities the direction, in which the particles leave the target surface, is not noticeably influenced and therefore no separation between the macroparticles and the other radiation or between the X-rays and the atomic particles is obtained. As indicated above to obtain a separating effect it will be necessary to select the velocity of the target surface section rather high, in the same order of the velocities with which the macroparticles or the atomic particles respectively are released from the target.

The invention provides an apparatus for generating radiation and particles, more specifically x-rays and atomic particles, comprising a target and a pulsed laser beam source producing during operation a pulsed laser beam wich is focused on a predetermined surface section of said target to emit particles and radiation therefrom, characterised in that the apparatus comprises furthermore drive means to move said predetermined surface section of the target onto which the laser beam is focussed along the focal spot of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated with such a velocity that, because of the vectorial addition of said velocity and the different initial velocities of the various radiation components, at least a partial spacial separation between the various radiation components will be obtained.

Although in principle various ways can be imagined to move the target surface section according to a very practical solution it is preferred that said target surface section is part of a rotatable target body and that the drive means are emboddied as a drive motor coupled to said target to rotate the target such that the surface section of th target onto which the pulsed laser beam is focussed moves with th required velocity along the laser beam focal spot.

Within the scope of the invention various target embodiment are concievable.

In a first embodiment the target body is a cylindrically shape target body which is rotated by said drive motor along its longitudina axis and that said target surface section is part of the cilindrica outer target surface. In a second embodiment the target body is a cylindricall shaped target body which is rotated by said drive motor along it longitudinal axis and that said target surface section is part of th cilindrical inner target surface.

In a further embodiment the target body is a disk shaped targe body which is rotated by said drive motor along its longitudinal axis an that said target surface section is restricted to a circular ring shape section on the front or back surface of the target.

In yet another embodiment the target body comprises a hub and number of bades or spokes extending radialy from said hub whereby th surface near the free end of each blade or spoke forms part of sai target surface section.

Figure 1 illustrates a first embodiment of an apparatu according to the invention.

Figures 2a, 2b, 2c and 2d illustrate schematically in two dimensional angular velocity diagrams the spatial separation betwee radiation and particles ejected from the target surface.

Figure 3 illustrates a second embodiment of an apparatu according to the invention.

Figure 4 illustrates a variant of the embodiment illustrated i figure 1.

Figure 5 illustrates another target to be used in the apparatu according to figure 3.

Figure 1 illustrates an apparatus comprising a cylindricall shaped target body 1 connected to the shaft 2 of a drive motor 3. Th actual target surface is formed by the cylindrical outer surface 4 of th target body 1. A source 5, generating a pulsed laser beam, is located i such a position in relation to the cylindrical target 1 , respectively th target surface 4, that the pulsed laser beam 6 will hit said targe surface 4 at a predetermined focal spot 7. Under the influence of eac laser beam pulse on the one hand X-ray radiation will be generated whereas on the other hand atomic particles and macroparticles will be released from the surface.

In agreement with the invention the drive motor 3 rotates the target body 1 with a rather high velocity. The effect thereof is that all particles released from the target surface 4 receive an additional vel¬ ocity vector in a direction tangential to the circumference of the target body 1. The result thereof is schematically illustrated in figures 2a, 2b and 2c. Figure 2a illustrates schematically the target body 1 rotating around the shaft 2 of the (not visible) drive motor 3 in the direction of the arrow 8. It is assumed that the pulsed laser beam hits the target surface of the target body 1 at the focal spot 7.

First attention is payed to the macroparticles. Depending on various parameters such as the type of the target surface material and the energy of the laser beam pulse, the initial velocity of macroparticles will in general vary between 10² and 10⁴ cm/sec. If the target body 1 is not rotating then the macroparticles, which will be released from the target surface 4 of the target body 1 by each laser pulse will start moving in random directions. Each released particle can be represented by a vector 9a, 9b, ... , pointing in the direction in which the respective macroparticle will start moving after leaving the target surface 4 of the target body 1. The length of each vector represents the velocity of the respective macroparticle. All different vectors 9a, 9b, ..., are confined within a semi-spherical space, determined by the longest vectors, i.e the fastest macroparticles with a velocity of around 10⁴ cm/sec. The cross section of this semi-spherical space with the plane of the drawing is represented by the semi-circular line 10. In agreement with the invention the target body 1 is rotated at high speed in the direction of the arrow 8 around the shaft 2. Because of this rotation each particle will receive an additional vector indicated in figure 2a by reference number 12. For the sake of simplicity it is assumed in figure 2a that the vector 12 has the same length as one of the longest vectors, the vector 9a which points in the opposite direction. Because of this additional vector 12 a particle emitted from the target surface 4 of the body 1 with the vector 9a will be neutralized and will be halted. All other particles will become under the influence of a sum vector. If all vectors 9a, 9b, ..., are vectorially summed with the additional vector 12, then the result will be a new array of vectors 11a, 11b, ..., which are bounded by a space the cross section of which with the plane of the drawing being represented schematically by the boundary line 14. As appears clearly from figure 2a the whole cloud of macroparticles will move in general to the left in figure 2a and no macroparticles will move in the direction of the right hand lower corner anymore

On the other hand, however, the generated atomic particles will also be influenced by the additional vector 12 but to a far lesser extend as will be explained with reference to figure 2b.

Again depending on various parameters such as the type of the target surface material and the energy of the laser beam pulse, the initial velocity of the atomic particles will in general vary between 10⁴ and 10⁶ cm/sec. If the target body 1 is not rotating then the atomic particles, which will be released from the target surface 4 of the target body 1 by each laser pulse, will start moving in random directions. Each released particle can be represented by a vector 13a, 13b, ..., pointing in the direction in which the respective atomic particle will start moving after leaving the target surface 4 of the target body 1. The length of each vector represents the velocity of the respective atomic particle. All different vectors 13a, 13b, ..., are confined within a semi-spherical space, determined by the longest vectors, i.e the fastest atomic particles with a velocity of around 10⁶ cm/sec. The cross section of this semi-spherical space with the plane of the drawing is represented by the semi-circular line 15.

In agreement with the invention the target body 1 is rotated at high speed in the direction of the arrow 8 around the shaft 2. Because of this rotation each atomic particle will receive an additional vector indicated in figure 2b by reference number 12. This vector is equal to the vector 12 in figure 2a. Because of this additional vector 12 all atomic particles will become under the influence of a sum vector. If all vectors 13a, 13b, ..., are vectorially summed with the additional vector 12, then the result will be a new array of vectors 16a, 16b, ..., which are bounded by a space the cross section of which with the plane of the drawing being represented schematically by the boundary line 17. As appears clearly from figure 2b the atomic particles are only slightly influenced by the additional vector 12.

The combined effect of the additional vector 12 on both the macroparticles and the atomic particles is illustrated in fig 2c. The vectors representing the macroparticles are bounded by the boundary line 14 as explained with reference to figure 2a and the vectors representing the atomic particles are bounded by the boundary line 17 as explained with reference to figure 2b. It will be clear from figure 2c that a partial spacial separation is obtained such that at the left hand of an imaginary plane represented by its cross section 18 with the plane of the drawing macroparticles and atomic particles are generated whereas at the right hand side only atomic particles are generated. (Apart from the particles x-rays which are generated at both sides). In the above explanation it is assumed that the vector 12 has the same length as the vector 9a. It was furthermore assumed that the vector 9a represented one of the fastest macroparticles with a velocity of approximately 10⁴ cm/sec. To create the vector 12 the drive motor should have a rotational speed which can be calculated as follows: Assume that: D = diameter of the target body in centimetres ω = rotational speed of the target body in rev/min v = velocity caused by the rotation of the target body

then: v = — . π D cm/ sec

60

If for instance the diameter of the target body is 10 cm and the required velocity v = 10⁴ cm/sec then according to the above formula the rotational speed of the drive motor 2 can be calculated at 2. 10⁴ rev/min.

Referring back to figure 2b it can be explained that by increasing the rotational speed of the target body such that the additional vector 12 will become equal to vector 13a representing one of the fastest atomic particles a further spacial separation can be obtained. As illustrated in figure 2d in that case all atomic particles will be generated in a subspace bounded by the semicircular line 21 at the left hand of an imaginary plane represented by its cross section 20 with the plane of the drawing whereas at the right hand side only x-rays are generated. As further illustrated the macroparticles are confined to a relatively small subspace indicated by 22 with the result that the major part of the generated atomic particles left of the plane 20 but right under a further imaginary plane defined by its cross section 23 with the plane of the drawing are free of macroparticles.

A second embodiment of a target system according to the invention is illustrated in figure 3. The basic difference betwee figures 1 and 3 is that in figure 3 the pulsed laser beam 6a, emitted by the laser source 5a, is not directed to the circumferential side surface of a cylindrically shaped rotating target body 1a, but is directed to the edge section of the front surface of a disk shaped target body 1a. It will be clear that with a sufficient rotational speed of the drive motor 2a the edge section of the target disk 1a will have a sufficient velocity to obtain the desired effect as is described with reference to figures 2a-2d.

With respect to the lifetime of the target it is remarked that materials like Re, W and Ta, which are often used for X-ray generation, can be exposed to a large number of laser beam shots, much larger than S/a, wherein S is the target surface and a is the focal spot area. I.e. it is possible to shoot many times on the same spot because the deformation of the surface after each shot is relatively small.

However in some cases it is preferred that provisions are taken to gradually move the focal spot across the target surface section. In that respect it is conceivable to move the target as a whole in a direc¬ tion perpendicular to the direction of the pulsed laser beam. In figure 1 for instance it is preferred that the drive motor 3 is connected to a mechanism (not illustrated) by means of which the drive motor 3 and therewith the target body 1 can be moved in the direction of the arrow 30 perpendicular to the direction of the pulsed laser beam 6. Each pulse of the pulsed laser beam 6 will influence a rather restricted surface section of the target surface 4 of the body 1 , the central spot diameter of said surface section being in practical circumstances for instance 100-200 μm. If the drive motor 3 is mounted in a fixed position then all these central spots will be arranged on a circular line around the said surface of the target body 1 , whereby the circumferential length of this line and the central spot diameter together determine the maximum number of laser pulses for which the target surface section can be used. However, if during operation the drive motor 3 is gradually moved in the direction of the arrow 30 (upwards or downwards) then all the central spots will become located onto a helical line. Depending on the longitudinal dimension of the body 1 the lifetime of the target surface section 4 is extended therewith very significantly.

In a similar manner it is preferred in the embodiment illustrated in figure 3 to provide means enabling a gradual movement of the drive motor 3a and therewith of the target disk 1a in the direction 30a perpendicular to the direction of the laser beam 6a. As will be clear, the effect of such a movement will be a spirally shaped track along the edge section of the disk 1a onto which track the various central spots are located.

In stead of moving the target body as explained above it is of course also possible to take measures for gradually changing the direc¬ tion of the laser beam such that a helical track or a spirally shaped track on the target surface will be obtained.

Although in the embodiment illustrated in figure 1 the laser beam is directed to the outer surface of a cylindrical target it is also possible to use a hollow cylinder and to direct the laser beam onto the inner surface of said cylinder. Such an embodiment is illustrated in figure 5. The illustrated target body comprises a cylindrical body part 4c onto the inner surface of which the laser beam 6c, emitted by the source 5c, is directed. To decrease the weight of the whole target construction the cylindrical body part 4c is connected to the shaft 2c of the drive motor 3c through a number of spokes, one of which is indicated by reference number 32.

In stead of a continuous disc-shaped target as illustrated in figure 3 the target may comprise a number of radially extending spokes or blades as is illustrated in figure 5. By means of a number of incisions the original disc-shaped target is reshaped such that a number of blades, one of which is indicated by 33, extends from a hub 34. Through said hub 34 the target can be installed onto the shaft of a drive motor. The advantage of this type of target is that during operation, when the target rotates very fast, each of the spokes or blades 33 is able to stretch under the influence of the tangential forces reducing thereby the risk of damaging caused by said tangential forces. It will be clear that the number of spokes or blades and the actual shape thereof is not restricted to the number and shape illustrated in figure 5.

Eventually a number of this type of (thin) target plates can be installed one behind the other each with a small mutual angular displacement so that as a whole a (more or less) continuous target is formed.

Claims

1) Method for generating radiation and particles, more specifically x-rays and atomic particles, by focusing a pulsed laser beam on a target, characterised in that the target surface section onto which the pulsed laser beam is focused is moved along the focal spot of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated with such a velocity that, because of the vectorial addition of said velocity and the different initial velocities of the various radiation components, at least a partial spacial separation between the various radiation components will be obtained.

2) Method according to claim 1, characterised in that, depending on the target material the velocity of said target surface section is selected such that a separation is obtained in the generated particle radiation between the majority of the inevitably generated relatively slow macroparticles and at least a part of the generated relatively faster atomic particles.

3) Method according to claim 1, characterised in that, depending on the target material the velocity of said target surface section is selected such that at least a partial spacial separation is obtained between the majority of the generated atomic particles and at least part of the generated radiation.

4) Apparatus for generating radiation and particles, more specifically x-rays and atomic particles, comprising a target and a pulsed laser beam source producing during operation a pulsed laser beam wich is focused on a predetermined surface section of said target to emit particles and radiation therefrom, characterised in that the apparatus comprises furthermore drive means to move said predetermined surface section of the target onto which the laser beam is focussed along the focal spot of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated with such a velocity that, because of the vectorial addition of said velocity and the different initial velocities of the various radiation components, at least a partial spacial separation between the various radiation components will be obtained. 5) Target for use m an apparatus according to claims 4, characterised in that said target surface section is part of a rotatable target body and that the drive means are emboddied as a drive motor coupled to said target to rotate the target such that the surface section of the target onto which the pulsed laser beam is focussed moves with the required velocity along the laser beam focal spot.

6) Target according to claim 5, characterised in that said target body is a cylindrically shaped target body which is rotated by said drive motor along its longitudinal axis and that said target surface section is part of the cilindrical outer target surface.

7) Target according to claim 5, characterised in that said target body is a cylindrically shaped target body which is rotated by said drive motor along its longitudinal axis and that said target surface section is part of the cilindrical inner target surface.

8) Target according to claim 6 or 7, characterised in that the longitudinal dimension of the target surface section of said cylindrical surface is sufficient to guide the pulsed laser beam along said surface section according to a helical track with a predetermined number of turns.

9) Target according to claim 6, 7 or 8, characterised in that the diameter of the target surface section of said cylindrical surface is sufficient to obtain the required linear velocity at a predetermined number of revolutions per time unit of the target body.

10) Target according to claim 5, characterised in that said target body is a disk shaped target body which is rotated by said drive motor along its longitudinal axis and that said target surface section is restricted to a circular ring shaped section on the front or back surface of the target.

11) Target according to claim 10, characterised in that the inner diameter and the outer diameter of said circular ring-shaped surface section are selected to enable guiding of the pulsed laser beam along said surface section according to a spiral track with a predetermined number of turns. 12) Target according to claim 10 or 11, characterised in that the inner diameter of said target surface section is sufficient to obtain the required linear velocity at a predetermined number of revolutions per time unit of the target body.

5

13) Target according to claim 5, characterised in that the target body comprises a hub and a number of bades or spokes extending radialy from said hub whereby the surface near the free end of each blade or spoke forms part of said target surface section.

10

14) Target according to claim 13, characterised in that a number of said defined bodies having a mutual angular displacement are used as target.

]5 *****