NL9401560A - Method and device for generating radiation and atomic particles. - Google Patents

Description

Method and device for generating radiation and atomic particles

The invention relates to a method and apparatus for generating radiation and particles, more particularly X-rays and atomic particles, by focusing a pulsating laser beam on a target.

Devices of the above type are either primarily intended to generate X-rays or primarily to generate atomic particles in the form of ions, atoms and molecules.

One of the most important and promising applications of radiation sources of this type is the so-called lithography with soft X-ray projection. Examples of this type of prior art device are known, for example, 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 Options, Vol. . 32, no. 34, December 1, 1993, pages 6934-6937.

Another wide field of application of the laser interaction with solids as a source of atomic particles is laser evaporation (ablation) of thin film deposition materials and multilayer structures in the production of high temperature superconductors and X-ray optics. Embodiments thereof are also known in the art.

The problem that occurs with all known methods and devices is that with each laser pulse various types of radiation and particles are generated, i.e. X-rays, atomic particles and inevitably also macro particles, all these different types of radiation and particles being generated in the same directions. In the various applications of this type of targets, however, generally only radiation or only one type of particles is generally required and preferably the other products generated must be suppressed in some way. Although in some applications where particles are used the radiation generated at the same time is not harmful, in applications where radiation is required the particles are generally parasitic. In particular, the occurring macro particles are very disturbing in most applications and should be avoided.

Several proposals have already been described to obtain a separation between the X-rays and the atomic particles and the macro particles. According to one proposal, a very thin band-shaped target is used to reduce the amount of atomic particles generated. According to another proposal, a rotary shutter that is synchronized with the pulsating laser beam source is used such that X-rays are transmitted while the atomic particles reaching the shutter a fraction of a time unit later are retained. Other proposals include the use of cryogenic targets or the use of a supersonic beam, which splits and removes macro particles into small fragments.

All of these known proposals are complicated and / or expensive and generally not sufficiently effective.

There is therefore still a need for a method and apparatus for generating radiation and particles in such a way that at least the macro particles are separated from the atomic particles and the radiation and preferably such that also a separation between the X-rays and the atomic particles are obtained.

In accordance with this objective, the invention provides a method of generating radiation and particles, more particularly X-rays and atomic particles, by focusing a pulsating laser beam on a target, characterized in that the surface portion of the target, on which the pulsating laser beam is focused moves past the focal point of the laser beam in a direction perpendicular to the mean direction in which the various radiation components are initially generated at such a rate that, due to the vectorial addition of this speed and the different initial velocities of the various radiation components, at least a spatial separation is obtained between the various radiation components.

In general, the generated macro particles are rather slow compared to the relatively fast atomic particles. By moving the surface part of the target, from which both the macro particles and the atomic particles are detached, at a predetermined speed perpendicular to the initial mean direction in which both types of particles are released, the relatively slow macro particles will be pulled to one side while the faster atomic particles will only undergo a rather small change of direction. Thus, the result will be a partial spatial 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 that occur in both subspaces).

A similar effect can be obtained with regard to X-rays and atomic particles. If the velocity of the moving surface part of the target is chosen in the same order as the initial velocity at which the atomic particles are released from the target surface, the majority of the atomic particles will be drawn to one side and limited within a predetermined subspace . The result will be "clean" X-rays outside this subspace with no atomic particles (and no macro particles). Under these conditions, the macro-particles are confined to an even more limited subspace so that most of the said atomic particle cloud is also free from macro-particles.

Equipment in which at least part of the target surface moves is already known, for example, from the aforementioned article by Steven J. Haney, et al. In all these known equipment, the movement of the target is implemented only around a virgin surface portion of the target prior to each laser pulse. bring the focal point of the laser. According to the said article, a speed of a band-shaped target of 300 cm / s can for instance be requested. However, at such tape speeds, the direction in which the particles leave the target surface is not appreciably affected, and therefore no separation is obtained between the macro particles and other generated components or between the X-rays and the atomic particles. As indicated above, in order to obtain a separating effect, the velocity of the target surface portion will have to be selected rather high, in the same order as the rates at which the macroparticles or atomic particles are released from the target, respectively.

The invention provides an apparatus for generating radiation from particles, more particularly X-rays and atomic particles, comprising a target and a pulsating laser beam source, which source produces a pulsating laser beam during operation, which is focused on a predetermined surface portion of the target in order to emit radiation and particles therefrom, characterized in that the device further comprises driving means for moving the surface part of the target on which the pulsating laser beam is focused along the focal point of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated at such a speed that, due to the vectorial addition of this speed and the different initial speeds of the various radiation components, at least a partial spatial separation between the various radiation components will be received.

Although in principle various ways can be devised to move the surface part of the target on which the pulsating laser beam is focused, according to a very practical solution it is preferred that said surface part of the target be part of a rotatable target body, and that said drive means are configured as a drive motor connected to the target in order to rotate this target so that the surface portion of the target on which the pulsed laser beam is focused moves at the required speed past the focal point of the laser beam.

In another practical embodiment, the target for use in a device according to the invention is characterized in that the surface part of the target forms part of a tape-shaped target body, which tape is wound from a supply reel and wound on a take-up reel, said drive means being designed as a drive motor connected to one reel or to both reels to move the tape between the two reels such that the surface portion of the tape on which the pulsating laser beam is focused moves at the required speed along the focal point of the laser beam.

The belt may also be in the form of an endless belt wound around two capstans, said drive means being in the form of a drive motor coupled to a capstans or both capstans, in order to move the belt around these two capstans such that the surface part of the band on which the pulsating laser beam is focused moves at the requested speed past the focal point of the laser beam.

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

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

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

Figure 4 illustrates a variant of the embodiment illustrated in Figure 1.

Figure 5 illustrates another target plate for use in the device of Figure 2.

Figure 1 illustrates an apparatus comprising a cylindrical shaped target body 1, which is connected to the shaft 2 of a drive motor 3. The actual target surface is formed by the outer cylindrical surface of the target body 1. A source 5, which generates a pulsating laser beam, is such positioned with respect to the cylindrical target 1 and the target body 4, respectively, that the pulsating laser beam 6 will hit the said target surface 4 at a predetermined focal point 7. Under the influence of each laser beam pulse, on the one hand X-rays are generated, while on the other hand atomic particles and macro particles of the surface will be released.

In accordance with the invention, the drive motor 3 rotates the target body 1 at a fairly high speed. The effect of this is that all particles released from the target surface 4 acquire an additional velocity 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 schematically illustrates the target body 1 rotating about the axis 2 of the (invisible) drive motor 3 in the direction of the arrow 8. It is assumed that the pulsating laser beam strikes the target surface of the target body 1 at the focal point 7.

First of all, attention is paid to the macro particles. 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 macro particles will generally vary between 102 and 10 cm / sec. If the target body 1 does not rotate, the macro particles released from the target surface 4 of the target body 1 will move through each laser pulse in different directions. Each loose particle can be represented by a vector 9a, 9b,. . pointing in the direction in which the respective macro-particle will start to move after leaving the target surface 4 of the target body 1. The length of each vector represents the speed of the respective macro particle. All the different vectors 9a, 9b, ..., are bounded within a hemispherical space defined by the longest vectors, i.e. the fastest macro particles at a rate of around 104 cm / sec. The cross section of this hemispherical space with the plane of the drawing is indicated by the semicircular line 10.

In accordance with the invention, the target body 1 rotates at a high speed in the direction of the arrow 8 around the axis 2. Because of this rotation, each particle will have an additional vector which is indicated by the reference numeral 12 in Figure 2a. For the sake of simplicity in Figure 2a it is assumed that the vector 12 has the same length as one of the longest vectors, the vector 9a, which points in opposite directions. Because of this additional vector 12, a particle released from the target surface 4 of the body 1 with the vector 9a will be neutralized and stopped. All other particles will come under the influence of a sum vector. If all vectors 9a, 9b, ..., are summed vectorially with the additional vector 12, the result will be a new array of vectors 11a, 11b, ..., which are bounded by a space whose cross section is with the plane of drawing is schematically represented by the boundary line 14. As is clear from Figure 2a, the whole cloud of macro particles will generally shift to the left in Figure 2a and no macro particles will move in the direction of the lower right corner.

On the other hand, however, the generated atomic particles will also be affected by the additional vector 12 but to a much lesser extent, 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 generally vary between 104 and 10 cm / sec. If the target body 1 does not rotate, the atomic particles released from the target surface 4 of the target body 1 will move in various directions with each laser pulse. Each released particle can be represented by a vector 13a, 13b, ..., pointing in the direction in which the respective atomic particle will move 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 the different vectors 13a, 13b, ... are bounded within a hemispherical space defined by the longest vectors, i.e. the fastest atomic particles at a rate of about 106 cm / sec. The cross section of this hemispherical space with the plane of drawing is represented by the semicircular line 15.

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

The combined effect of the additional vector 12 on both the macro particles and the atomic particles is illustrated in Figure 2c. The vectors representing the macroparticles are bounded by the boundary line 4 as explained above 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 apparent from Figure 2c that a partial spatial separation is obtained such that on the left side of an imaginary plane represented by its section 18 with the plane of drawing, macro-particles and atomic particles are generated, while on the right-hand side only atomic particles are generated. (Apart from the X-rays generated on both sides).

In the above explanation, it has been assumed that the vector 12 has the same length as the vector 9a. Furthermore, the vector 9a is believed to represent one of the fastest macro particles at a rate of about 104 cm / sec. To create the vector 12, the drive motor must have a rotational speed that can be calculated as follows:

Suppose that: D = diameter of the target body in centimeters t ω = revolution speed of the target body in rpm v = speed caused by the rotation of the target body then:

For example, if the diameter of the target body is 10 cm and the required speed v = 104 cm / sec, then according to the above formula, the rotation speed of the drive motor 2 can be calculated at 2.10 * rpm.

Referring back to Figure 2b, it can be explained that with increasing rotation speed of the target body such that the additional vector 12 becomes equal to the vector 13a, which represents one of the fastest atomic particles, a further spatial separation can be obtained. As illustrated in Figure 2d, in that case, all atomic particles will be generated in a subspace delimited by a semicircular line 21 on the left of an imaginary plane represented by its cross section 20 with the plane of drawing, while on the right only X-rays are generated. As further illustrated, the macro particles are confined to a relatively small subspace, denoted by 22, with the result that most of the generated atomic particles are to the left of the plane 20, but to the right below a further imaginary plane defined by cross section 23 with the plane of drawing are free from macro-particles.

A second embodiment of a target system according to the invention is illustrated in figure 3. The main difference between figures 1 and 3 is that in figure 3 the pulsating laser beam 6a emitted by the laser source 5a is not aimed at the side peripheral surface of a cylindrically shaped rotating target body 1a, but directed at the edge section of the front face of a disc-shaped target body 1a. It will be clear that with a sufficient rotation speed of the drive motor 2a, the edge section of the target disc 1a will have a sufficient speed to achieve the desired effect as described with reference to Figures 2a-2d.

Regarding the target lifetime, it is noted that materials such as Re, W and Ta, which are often used to generate X-rays, can be exposed to a large number of laser beam pulses, much larger than S / a, where S is the target surface and a the focal area. That is, it is possible to shoot at the same place many times because the deformation of the surface after each shot is relatively small.

In some cases, however, it is preferable to take measures to gradually shift the focal point across the target surface section. In that connection, it is conceivable to move the target as a whole in a direction perpendicular to the direction of the pulsating laser beam. For example, in Figure 1, it is preferable that the drive motor 3 is coupled to a mechanism (not illustrated) by means of which the drive motor 3 and thereby the target body 1 can be moved in the direction of the arrow 30 perpendicular to the direction of the pulsating laser beam 6. Each pulse of the pulsating laser beam 6 will affect a rather limited surface area of the target surface 4 of the body 1, the diameter of this surface area being, for example, 100-200 µm in practical conditions. If the drive motor 3 is mounted in a fixed place, then all these areas will be in a circular line around said surface of the target body 1, the circumference length of this line and the diameter of the areas together being the maximum number of laser pulses determine what the target surface section can be used for. However, if the drive motor 3 is gradually moved in the direction of the arrow 30 (up or down) during operation, then all areas will be located on a helix. Depending on the longitudinal dimension of the body 1, the life of the target surface section 4 can thereby be increased very significantly.

Likewise, in the embodiment illustrated in Figure 3, it is preferable to provide means that allow smooth movement of the drive motor 3a and thereby of the target disk 1a in a direction 30a, perpendicular to the direction of the laser beam 6a. It will be clear that the effect of such a movement is a spiral track along the edge portion of the disk 1a on which track the various central areas are located.

Instead of moving the target body, as described above, in some cases it is possible to take steps to gradually change the direction of the laser beam such that a helical track or a spiral track on the target surface obtained.

Although in the embodiment illustrated in Figure 1, the laser beam is aimed at the outer surface of a cylindrical target, it is also possible to use a hollow cylinder and direct the laser beam at the inner surface of this cylinder. Such an embodiment is illustrated in Figure 5. The illustrated target body comprises a cylindrical body portion 4c on the inner surface of which the laser beam 6c generated by the source 5c is directed. To reduce the weight of the entire target structure, the cylindrical body portion 4c is connected to the shaft 2c of the drive motor 3c through a plurality of spokes, one of which is designated by the reference numeral 32.

Instead of a contiguous disc-shaped target as illustrated in Figure 3, a target consisting of a number of radially projecting spokes or blades as illustrated in Figure 6 may also be used. By means of a number of cuts, the original disc-shaped target has been reformed such that a number of blades, one of which is designated 33, protrudes from a hub 34. Through this hub 34, the target can be mounted on 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 can stretch under the influence of the tangential forces thereby reducing the risk of damage caused by these tangential forces . It will be understood that the number of blades or spokes and their actual shape is not limited to the number and shape illustrated in Figure 6.

Optionally, a number of these (thin) target plates can be placed one behind the other with small mutual angular shift, so that a (more or less) closed target is nevertheless formed.

Claims (14)

A method of generating radiation and particles, in particular X-rays and atomic particles, by focusing a pulsating laser beam on a target, characterized in that the surface portion of the target on which the pulsating laser beam is focused, moves along the focal point of the laser beam in a direction perpendicular to the mean direction in which the various radiation components are initially generated at such a speed that, because of the vectorial addition of this speed and the different initial speeds of the various radiation components, at least a spatial separation is obtained between the various radiation components. A method according to claim 1, characterized in that, depending on the target material, the velocity of said target surface portion is selected such that partial spatial separation is obtained between the majority of the inevitably generated relatively slow macro particles and at least one part of the generated relatively faster atomic particles. Method according to claim 1, characterized in that, depending on the target material, the velocity of said target surface portion is selected such that at least a partial spatial separation is obtained between the majority of the generated atomic particles and at least one part of the generated radiation. Device for generating radiation of particles, more particularly X-rays and atomic particles, comprising a target and a pulsating laser beam source, which source produces a pulsating laser beam during operation, which is focused on a predetermined surface portion of the target in order therefrom to emit radiation and particles, characterized in that the device further comprises driving means for moving the surface part of the target on which the pulsating laser beam is focused along the focal point of the laser beam in a direction perpendicular to the average direction in which the various radiation components are initially generated at such a rate that, due to the vectorial addition of this speed and the different initial speeds of the various radiation components, at least a partial spatial separation between the various radiation components will be obtained. Target for use in an apparatus according to claim 4, characterized in that said target surface part is part of a rotatable target body, and said drive means are designed as a drive motor connected to the target in order to achieve this purpose rotate that the surface portion of the target on which the pulsed laser beam is focused moves at the required speed past the focal point of the laser beam. Target according to claim 5, characterized in that the target body is a cylindrically shaped target body rotated by the drive motor about its longitudinal axis and that said target surface portion forms part of the outer cylindrical surface of the target body. Target according to claim 5, characterized in that the target body is a cylindrically shaped target body, which is rotated by the drive motor about its longitudinal axis and that said surface part of the target forms part of the inner cylindrical surface of the target body. Target according to claim 5 or 6, characterized in that the longitudinal dimension of the surface portion of said target cylindrical surface is sufficient to guide the pulsating laser beam along said surface portion along a helical track with a predetermined number of turns . Target according to claim 6, 7 or 8, characterized in that the diameter of the surface portion of the cylindrical surface of the target is sufficient to obtain the required linear velocity at a predetermined number of revolutions per unit time of the target body. Target according to claim 5, characterized in that said target body is a disc-shaped target body rotated by the drive motor about its longitudinal axis and that the surface portion of the target is limited to an annular section on the front surface or the rear surface of the target body. A target according to claim 10, characterized in that the inner diameter and the outer diameter of said annular surface section are selected such that the pulsating laser beam can be guided along said surface section in a spiral track with a predetermined number of turns. A target according to claim 10 or 11, characterized in that the inner diameter of said target surface section is sufficient to achieve the required linear velocity at a predetermined number of revolutions with time unit of the target body. A target according to claim 5, characterized in that said target body comprises a hub and a plurality of blades or spokes projecting radially from said hub, the surface near the free end of each blade or spoke being part of said surface portion of the target. Target according to claim 13, characterized in that a number of the defined mutually angular shifted bodies serve as targets.