WO2009077943A1

WO2009077943A1 - Method for laser-based plasma production and radiation source, in particular for euv radiation

Info

Publication number: WO2009077943A1
Application number: PCT/IB2008/055221
Authority: WO
Inventors: Jakob Willi Neff
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.; Fraunhofer Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 2007-12-14
Filing date: 2008-12-11
Publication date: 2009-06-25

Abstract

The present invention relates to a method for laser-based plasma production and a radiation source, in which this method of plasma production is used. In the method, a target material in a target region is evaporated with one or more laser pulses and heated to produce the laser-based plasma. The target material is moved in the method as a liquid film (9) on a carrier (8) through the target region. Increased wear of the components for the supply of the target material can be avoided and stable operation of the radiation production achieved by the method and the associated radiation source.

Description

METHOD FOR LASER-BASED PLASMA PRODUCTION AND RADIATION SOURCE, IN PARTICULAR FOR EUV RADIATION

Field of the invention

The present invention relates to a method for plasma production, in particular for radiation sources emitting extreme ultra-violet (EUV) radiation and/or soft X-ray radiation, in which a target material in a target region is evaporated with one or more laser pulses and heated to produce a laser-based plasma. The invention also relates to a radiation source, which uses a method of this type for plasma production. Background of the invention

Plasma sources are frequently used as light sources for short-wave radiation in the technical application. Basically, two different methods are used here to produce and heat plasmas in order to obtain virtually punctiform, short-wave light sources. In one method, the plasma is heated by a short current pulse and, in the other method, by a short laser pulse. The target material which supplies the material for the plasma, may be present or supplied in the two cases in a gaseous, liquid or solid state.

An electrode system of an anode, cathode and hollow cathode is described in EP 1036488 Bl, in which the target material, for example xenon, is present in a gaseous state at a pressure of about 100 Pa. The anode and cathode are low- inductively connected to a capacitive energy store. In the hollow cathode, the xenon gas is pre-ionized and electrically conductive at a certain electric voltage. It spreads into the discharge gap between the anode and cathode and causes a current pulse from the energy store, which heats it and excites it thereby to produce radiation. In order to produce high radiation outputs, the current pulses have to be repeated at a high frequency. This leads to heating and also by a sputtering action to the destruction of the electrodes in the long term. The output and service life of the electrode system are therefore severely limited in this type of light production.

EP 1665907 Bl shows a special structure of the electrodes, the current supply and the cooling and a special technique for providing the radiating medium. The electrode system consists of two rotatably mounted disc-shaped electrodes, which each partially dip into a temperature-controlled bath with liquefied metal, for example molten tin. A liquid metal film forms on the electrodes during rotation out of the melt owing to the wetting of the surface of the electrode wheels with the liquefied metal. By radiating in a laser pulse onto the surface wetted with the liquid metal at the narrowest point between the electrode wheels, a part of the metal film located on the electrodes evaporates and bridges the electrode spacing. An electric breakdown occurs at this point and also a very high current flow from the capacitor bank used as an energy store. This current heats the metal vapor to temperatures, at which the latter is ionized and emits the desired EUV radiation in a pinch plasma. By applying the liquid target material, the electrode surface loaded by the gas discharge is constantly regenerated, so no further wear occurs on the basic material of the electrode wheels. The rotation of the electrode wheels through the metal melt leads to good heat contact, by means of which the wheels heated by the gas discharge can efficiently emit their energy to the melt. As a very low electric resistance exists between the electrode wheels and the metal melt, it is easily possible to transmit very high currents via the melt. However, the area, which emits the radiation lies very close to the surface of the electrode wheel, onto which the laser beam producing the vapor is focused. As a result, depending on the forming of the two electrodes, the usable solid angle to output the radiation may be severely limited. Electrodes for supplying electric energy are completely dispensed with in DE

102005014433 B3. Here, individual droplets from a droplet jet of liquid target material are evaporated, ionized and heated exclusively with laser pulses. To maximize the efficiency, the evaporation of the droplet and the heating thereof may be separated, in that firstly one or more laser pulses are radiated in as pre-pulses with low energy and, at a short time interval, a last pulse with high energy follows. The pre-pulses condition the target, i.e. they adjust the optimal particle and electron density, so the last, energy-rich laser pulse is completely absorbed as far as possible and a plasma temperature suitable for the radiation production is reached.

Different techniques are known for providing the target material in purely laser-based radiation sources of this type. Gaseous substances may be forced, for example, under high pressure through nozzles into a vacuum chamber, in which the electrode system is arranged. The laser pulse, however, then has to be focused in a region very close to the nozzle, in which the gas is still adequately dense or ice clusters have formed owing to the cooling during the pressure relief. As a result, the service life of the nozzles is low because of the erosion by the plasma and the output of sources of this type is low because of the lack of cooling possibilities. An improvement is achieved in that the gas is cooled and is introduced as a liquid jet through the nozzle into the vacuum chamber. It is now possible to focus the laser at a greater spacing from the nozzle and to thus reduce the reaction on the nozzle. However, it has been shown that the laser pulses disturb the jet in such a way that it changes its direction or even disintegrates completely, so radiation sources of this type run very unstably.

As a remedy, in DE 102004005242 B4, the jet of target material is chopped with a laser pulse with low energy in terms of time and also spatially before the main laser pulse with as little disruption as possible, so the following disruption can only go back to this point and the remaining jet remains uninfluenced up to the nozzle. In other cases, it is achieved by the action of ultrasound on the nozzle that the jet is already periodically disrupted when leaving the nozzle in such a way that it disintegrates into a pulse train of individual drops. However, a complex optical monitor system is then required in order to be able to synchronize the laser pulses with regard to place and time with the individual droplets.

If the materials used as the target are present in solid form, the target has to be moved relative to the site of the laser focus, as the surface is destroyed by the plasma formation. For this purpose, different embodiments are known. For example, the target may have the form of a cylinder, which is rotatably and displaceably mounted. Furthermore, the solid target may be configured as a band, which is pulled through the laser focus. In each case, the target is a wear part here, which gives rise to frequent maintenance intervals of the radiation source.

All the techniques described hitherto for preparing the target material with purely laser-based radiation sources have certain advantages and disadvantages with regard to their service life and the tolerable average output of the laser used.

The object of the present invention is to provide a method for plasma production for a radiation source and a radiation source working with the method, which do not have the disadvantages of the described techniques and can be used in particular to produce EUV radiation and/or soft X-ray radiation Summary of the invention

The object is achieved by the method and the radiation source according to claims 1 and 12. Advantageous configurations of the method and the radiation source are the subject matter of the sub-claims or can be inferred from the following description and the embodiments. In the proposed method for plasma production, in a known manner, a target material is evaporated in a target region with one or more laser pulses and heated to produce a laser-based plasma. This is a purely laser-based method. The method is distinguished by the fact that the target material is moved as a liquid layer on a carrier through the target region.

The associated radiation source correspondingly comprises a device for providing target material in a target region and at least one laser, which emits laser pulses for the evaporation of the target material in the target region and to heat the evaporated target material to produce a laser-based plasma. The device for providing target material in this case has a carrier and means for applying the target material as a liquid film onto the carrier and is configured to move the carrier with the liquid film through the target region.

An erosion of the supplying components, in the present case the carrier, can be avoided by the proposed supply of the target material as a liquid film on a carrier to the target region, i.e. to the region at which the target material is evaporated, so that no wear caused thereby with short maintenance intervals connected therewith, occurs.

In comparison to gas discharge sources with electrode wheels, as presented in the introduction to the description, the optical accessibility of the radiation-emitting area is not restricted by electrodes. Moreover, the method can also be operated at the high pressure of a buffer gas in comparison to gas discharge sources and this has an advantageous effect on the service life of a downstream optical system. Undesired deposits of evaporated target material on optical surfaces can be better suppressed by a high pressure of a buffer gas (debris suppression). In comparison to laser-based radiation sources, in which the target material is supplied in droplet form, the proposed method can be operated significantly more stably. The fluctuation of the radiation emission from pulse to pulse is also very low in the proposed method. Thus, this fluctuation depends to a high degree on the reproducibility of the production of the target. In the case of a droplet jet target, this strongly depends on how precisely a droplet can be spatially synchronized with the laser pulses. Even the smallest deviations lead to substantially different intensities on the droplet surface. In the target proposed here, only the spatial and time synchronization of the laser pulses with one another is required and can be adjusted very precisely.

In a particularly advantageous configuration of the method and of the radiation source, the thermal loading of the carrier is further reduced. In this configuration, electric and/or magnetic fields are used to counteract a lateral spreading of the evaporated material and to thereby keep the material together better spatially. Thus, a strong magnetic field oriented, for example, in the target region perpendicularly to the surface of the liquid film influences the spreading of the ionized material vapor in such a way that the latter preferably moves away parallel to the field lines from the surface. Thus, the vapor cloud is kept together laterally thereby, so a second high-power laser pulse can be focused at a greater spacing from the surface into the vapor cloud. Thus a lower thermal loading of the carrier occurs. The liquid target material is preferably evaporated with at least a first laser pulse and the evaporated target material is heated with one or more second laser pulses. The at least one first laser pulse may in this case have lower energy than the one or more second laser pulses.

The one or more second laser pulses are preferably not directed onto the surface of the carrier or the liquid film, but focused at a spacing from the surface of the carrier into the evaporated target material, so that they do not impinge on the carrier. The spacing can still be advantageously increased by the above-described use of electric and/or magnetic fields to reduce the thermal loading. In an advantageous development, second laser pulses are focused from a plurality of directions, in particular symmetrically to one another, into the evaporated target material. Thus, an improvement in the symmetry of the plasma produced and therefore of the radiation-emitting region can be achieved.

In a configuration of the proposed method and the associated radiation source, a rotatably mounted disc-shaped element is used as the carrier. The target material is applied to the radial periphery of this disc-shaped element and supplied by the rotation of this element to the target region as a liquid film. The disc-shaped element may be a so lid- surface disc or a disc penetrated by one or more openings, which has a radial outer face configured suitably to receive the liquid film. The application of the liquid layer onto the outer face of the disc-shaped element advantageously takes place in that this element dips with a radial part portion of its outer face into a container with the liquid target material. By wetting the outer face during the rotation of the element through the liquid target material, a liquid layer of the target material then forms on this face. In addition, suitable skimming elements may be provided to maintain a certain thickness of the liquid layer or the liquid film on the outer face.

In a further advantageous configuration, instead of the disc-shaped element, a flexible band, preferably a continuous band is used that is guided by means of corresponding deflection elements. The target material is applied to the surface of this band and supplied by the revolution of the band to the target region. The liquid film can also be advantageously applied here by dipping a portion of this band into a container with the liquid target material, the surface of the band being wetted by the target material. However, the target material can obviously also be applied in a different manner to the band or the disc-shaped element, for example by spraying on.

The method and the radiation source can be used particularly advantageously to produce EUV radiation or soft X-ray radiation in the range of about 1 nm to 20 nm wavelength. Radiation sources of this type are required above all for EUV lithography or measuring. To produce EUV radiation, metallic target materials are advantageously used, which are heated to a temperature above melting point to produce the liquid film. Thus, when using tin as the target material, which has a melting point of 230⁰C, a suitable operating temperature is, for example, 300⁰C. Gas lasers (for example CO₂ lasers) or solid state lasers (for example Nd: YAG lasers) can be used, for example, as lasers to produce the laser pulses. Brief description of the drawings

The proposed method and the associated radiation source are again elucidated in more detail hereinafter with reference to embodiments in conjunction with the drawings.

In the drawings:

Fig. 1 - schematically shows an example of a configuration of the proposed radiation source; and Fig. 2 - shows an example of the production of a magnetic field for the spatial influencing of the evaporated material.

Description of preferred embodiments

The radiation source shown schematically in Fig. 1 is used to produce EUV radiation. Liquid tin is used here as the target material and is evaporated by laser pulses and heated to produce the radiation-emitting plasma 1. The radiation source in the present embodiment has two lasers. A laser pulse which evaporates target material is produced with the first laser 2. With the second laser 3, laser pulses with higher energy are radiated in and heat the evaporated target material to produce the plasma 1 emitting EUV radiation.

The device for supplying the target material has a metal block 4, which contains a tin reservoir 5 with liquified tin. The liquefied tin is held in the reservoir 5 over a cooling device 7 at a temperature slightly above the melting point of about 230⁰C. A disc 8 rotatably mounted about a pivot pin 10 is partially let into the metal block 4 and may consist of molybdenum, for example. The metal block 4 forms a counter-form to the disc 8, so that between the metal block 4 and the disc 8 there exists a narrow gap 13, to which liquid tin from the reservoir can be supplied by means of a supply channel 14 in the metal block 4. The liquid tin may in this case be conveyed by the pump 6 through this gap 13, which is formed between the disc 8 and the metal block 4. As a result, the periphery of the disc is continuously wetted with tin and efficiently kept at the temperature of the liquid tin. A device which cannot be seen here made of skimmers and smoothers, which ensure that a defined thickness of the tin film 9 here is produced on the two side faces and the outer face of the disc 8, is located at a point before the disc 8 rotates out of the metal block 4.

At least one laser beam, which evaporates material locally from the tin film 9 and heats it until EUV radiation is emitted, is focused at the uppermost point onto the radial outer face of the disc 8. In the present example, this is achieved with the two laser beams of the first 2 and second laser 3. Obviously, evaporation and heating may, however, also be implemented by an individual laser.

As in the present example, some tin is advantageously firstly evaporated from the surface with a first laser pulse from the first laser 2, and spreads away from the radial outer face of the disc 8. The tin- vapor cloud produced is then heated with a second laser pulse from the second laser 3. The first laser pulse may in this case have significantly lower energy than the second laser pulse. This second laser pulse is advantageously radiated in at a spacing from the radial outer face of the disc 8 in such a way that it does not impinge on the disc 8. This may, for example, take place by radiation parallel to the pivot pin 10 of the disc 8 slightly above the radial outer face. As a result, there is no risk of too much tin evaporating from the tin film 9 or the disc surface being destroyed by the laser pulse. To increase the power, a plurality of high-power laser beams may also be simultaneously focused from slightly tilted angles into the vapor cloud. As a result, a more uniform (more symmetrical) heating effect and therefore also a radiation of the EUV radiation with a higher degree of isotropy is achieved.

The peripheral speed of the disc 8 should be selected such that a fresh point of the tin film 9 is always impinged upon with the respective following pulse. It may also be advantageous if the tin film is not produced on a metallic disc, but on a closed, peripheral metal band and transported into the region of the laser foci. With a higher rotational speed of the disc there is the risk that the tin film will be hurled away by the centrifugal force in the form of droplets. This effect can be completely avoided in this region with a band running straight in the target region. A further advantage of a band is the complete freedom of orientation of the band surface in relation to the direction of the gravitational force. It is therefore easily possible to deflect the band and orientate the laser radiation in such a way that the EUV radiation can radiate downward.

Owing to the operation of a radiation source of this type, tin is constantly being removed from the disc or the band. To screen sensitive optical elements possibly connected downstream, for example a collector optical system, against this removed material, also called debris, the same measures can be used as in other known source concepts, for example the use of so-called debris traps or gas curtains.

Fig. 2 shows a further configuration possibility of the proposed radiation source, in which the spreading of the evaporated material for the following heating is suitably restricted by a magnetic field. The figure shows the view perpendicular to the pivot pin 10 of the disc 8. In this example, the first laser 2, which evaporates the target material in the target region, radiates perpendicularly onto the radial outer face of the disc 8. The second laser 3, which heats the vapor cloud produced, is radiated tangentially via the radial outer face of the disc 8 into the vapor cloud. This means that the surface of the disc 8 is not damaged by the high-energy laser pulses of the second laser 3.

In the present example, two magnet coils 11 are arranged on the disc 8 and produce a directed magnetic field in the target region. The magnetic field lines 12 in the target region are indicated in the figure. This magnetic field prevents the flowing away of the charged particles of the evaporated material and the plasma produced therefrom perpendicular to the field lines. As a result, the evaporated material is laterally substantially held together compared to a configuration without a magnetic field of this type. The laser pulses of the high-power laser responsible for the heating, i.e. the second laser 3, can therefore be radiated in at a greater spacing from the radial outer face of the disc 8, so the thermal loading of the disc is reduced. It is also possible to focus a plurality of second lasers 3 at different angles into the tin- vapor cloud produced by the first laser 2. This leads to a symmetrical heating of the plasma and also prevents the plasma flowing away laterally. For this purpose, when using three high-power lasers, a symmetrical radiation in of the laser pulses at an angle spacing of 120° may take place, for example.

The site, at which the radiation is produced, should be adjustable in a range of at least 1 mm² precisely down to a few μm, and also in a manner which is stable in the long term, with respect to the optical elements. Both in the case of a rotatable disc and also with a band of a few mm in width, this can be achieved without problems by displacing the foci of the lasers over the surface. While in a droplet jet target the time of the radiation emission is determined by the individual droplets, this restriction does not exist with the present radiation source. The tin- vapor cloud can be generated at any desired time with a precision of better than 100 ns with the first laser pulse.

LIST OF REFERENCE NUMERALS:

1 plasma

2 first laser

3 second laser

4 metal block

5 tin reservoir

6 tin pump

7 cooling device

8 disc

9 tin film

10 pivot pin

11 magnet coils

12 magnetic field lines

13 gap

14 supply channel

Claims

CLAIMS:

1. A method for plasma production, in particular for radiation sources emitting extreme ultraviolet radiation and/or soft X-ray radiation, in which, with one or more laser pulses, a target material in a target region is evaporated and heated to produce a laser-based plasma (1), characterized in that the target material is moved as a liquid film (9) on a carrier through the target region.

2. A method as claimed in claim 1, characterized in that the target material is evaporated with at least one first laser pulse and is heated with one or more second laser pulses.

3. A method as claimed in claim 2, characterized in that a lateral spreading of the target material evaporated with the first laser pulse is counteracted at least until the arrival of the one or more second laser pulses, with the aid of at least one electric and/or magnetic field (12).

4. A method as claimed in claim 2 or 3, characterized in that the one or more second laser pulses are directed onto the evaporated target material in such a way that they do not impinge on the carrier.

5. A method as claimed in claim 4, characterized in that a plurality of second laser pulses are directed onto the evaporated target material from different directions.

6. A method as claimed in any one of claims 2 to 5, characterized in that the first laser pulse in the target region is directed perpendicularly to a surface of the carrier, on which the liquid film (9) is applied, onto the liquid film (9) and the one or more second laser pulses are directed tangent ially to this surface onto the evaporated target material.

7. A method as claimed in any one of claims 1 to 6, characterized in that a rotatably mounted disc-shaped element (8) with a radial outer face is used as the carrier, onto which the liquid film is applied, the liquid film (9) being moved by rotation of the discshaped element (8) through the target region.

8. A method as claimed in claim 7, characterized in that the disc-shaped element (8) dips with a radial part portion of its radial outer face in a container with liquid target material in order to receive the liquid film (9) by the rotation.

9. A method as claimed in any one of claims 1 to 6, characterized in that a peripheral band, on the surface of which the liquid film is applied, is used as the carrier.

10. A method as claimed in claim 9, characterized in that the band dips with a part portion into a container with liquid target material in order to receive the liquid film (9) while revolving.

11. A method as claimed in any one of claims 1 to 10, characterized in that a liquefied metal is used as the target material.

12. A radiation source, in particular for extreme ultraviolet radiation and/or soft X-ray radiation, comprising - a device (4 - 8) for providing target material in a target region and

- at least one laser (2, 3), which emits laser pulses for an evaporation of the target material in the target region and to heat the evaporated target material to produce a laser-based plasma, characterized in that the device (4 - 8) for providing target material has a carrier and means for applying target material as a liquid film (9) onto a surface of the carrier and is configured to move the surface of the carrier with the liquid film through the target region.

13. A radiation source as claimed in claim 12, characterized in that it comprises at least a first laser (2) and at least a second laser (3), which are configured in such a way that the target material can be evaporated with a first laser pulse of the first laser (2) and the evaporated target material can be heated with one or more second laser pulses of the second laser (3).

14. A radiation source as claimed in claim 13, characterized in that it comprises a device (11) for producing at least one electric and/or magnetic field (12) in the target region which counteracts a lateral spreading of the target material evaporated with the first laser pulse at least until the arrival of the one or more second laser pulses.

15. A radiation source as claimed in claim 13 or 14, characterized in that the at least one second laser (3) is arranged and/or combined with a beam guide optical system in such a way that the one or more second laser pulses are directed onto the evaporated target material in such a way that they do not impinge on the carrier.

16. A radiation source as claimed in claim 15, characterized in that it comprises a plurality of second lasers (3), which are arranged and/or combined with one or more beam guide optical systems in such a way that the second laser pulses impinge on the evaporated target material from different directions.

17. A radiation source as claimed in any one of claims 13 to 16, characterized in that the first laser pulse in the target region is directed perpendicularly to a surface of the carrier, on which the liquid film (9) is applied, onto the liquid film (9) and the one or more second laser pulses are directed tangentially to this surface onto the evaporated target material.

18. A radiation source as claimed in any one of claims 12 to 17, characterized in that the carrier is a rotatably mounted disc-shaped element (8) with a radial outer face, onto which the liquid film is applied.

19. A radiation source as claimed in claim 18, characterized in that the disc- shaped element (8) dips with a radial part portion of its radial outer face into a container with liquid target material.

20. A radiation source as claimed in claim 18, characterized in that the discshaped element (8) dips with a radial part portion of its radial outer face into a counter- form formed by a material block (4), by means of which a gap (13) is formed by the radial part portion between the material block (4) and the radial outer face of the disc-shaped element (8), into which liquid target material can be introduced.

21. A radiation source as claimed in claim 20, characterized in that a supply channel (14) for the supply of the liquid target material to the gap (14) is formed in the material block (4).

22. A radiation source as claimed in claim 21, characterized in that a reservoir (5), which is connected by means of the supply channel (14) to the gap (14), for the liquid target material (14) is formed in the material block (4).

23. A radiation source as claimed in any one of claims 12 to 17, characterized in that the carrier is configured as a revolving band, on the surface of which the liquid film is applied.

24. A radiation source as claimed in claim 23, characterized in that the band dips with a part portion into a container with liquid target material.