WO2011116898A1

WO2011116898A1 - Steering device for controlling the direction and/or velocity of droplets of a target material and extreme euv source with such a steering device

Info

Publication number: WO2011116898A1
Application number: PCT/EP2011/001304
Authority: WO
Inventors: Reza Abhari; Andrea Giovannini
Original assignee: Eth Zurich
Priority date: 2010-03-25
Filing date: 2011-03-16
Publication date: 2011-09-29

Abstract

The invention relates to a steering device (24) for controlling the direction and/or velocity of droplets (19) of a target material, which are dispensed by a target delivery system (17) to move to a EUV production site (20) for being irradiated by a focused laser beam (34). Increased conversion efficiency and frequency of operation is achieved by said steering device (24) comprising deflecting and/or accelerating means (27) arranged along a droplet path from the target delivery system (17) to the EUV production site (20).

Description

DESCRIPTION

STEERING DEVICE FOR CONTROLLING THE DIRECTION AND/OR VELOCITY OF DROPLETS OF A TARGET MATERIAL AND EUV SOURCE WITH SUCH A

STEERING DEVICE

BACKGROUND OF THE INVENTION

The present invention relates to the generation of extreme ultraviolet radiation. It refers to a steering device for controlling the direction and/or velocity of droplets of a target material according to the preamble of claim 1. It further refers to a EUV source with such a steering device.

PRIOR ART

The next generation of semiconductor devices will be manufactured using extreme ultraviolet (EUV) lithography. The plasma is generated by droplets that are irradiated by a laser (Laser Produced Plasma LPP). Fig. 1 shows a simplified configuration of a EUV source. The EUV source 0 comprises a chamber 11 containing a collector or mirror 15 and a target delivery system 17, which is attached to the chamber 11 by means of a mechanical support 16 and emits a chain of droplets 19 of the target material. A high power and high repetition rate drive laser 12 ignites the target material at a EUV production site 20. The focused drive laser pulse 14 enters the chamber 11 through a flanged window 13. The spatial and temporal characteristics of the laser pulse match the target size and location in order to maximize conversion efficiency (CE), i.e. the ratio of EUV energy and laser energy. An optical system 23 may be used to detect and control the droplets 19 coming from the target delivery system 17.

The collector 15 collects the EUV light 18 generated at the EUV production site 20. The collector 15 has a first focus at the EUV production site 20, and a second focus 21 , called intermediate focus (IF), where the EUV light 18 is bundled for further use in a subsequent EUV lithography tool (not shown in Fig. 1 ). The collector 15 has an aperture 22 for the laser light to reach the EUV production site 20. Similar configurations are shown in documents WO 2006/091948(A1 ) or WO 2009/025557(A1 ) or WO 2010/017892(A1 ).

In order for the droplets to be a mass limited target, they should have a diameter smaller than 30pm. In addition, the laser beam, when it irradiates the droplets, should have a spot size smaller than 150pm in order to generate a plasma hot enough to produce EUV radiation. These two limitations lead to a challenge: how to hit the droplets fully with every laser shot. This is necessary if the EUV source is used for high volume manufacturing, since it has a direct impact on the conversion efficiency. In order to achieve the optimum conversion efficiency, the laser beam in the focus should be as close as possible to the droplet diameter. Then the positioning of the droplets becomes even more critical.

Document US 2009/0218522 discloses a EUV light source apparatus comprising a charging electrode for charging the droplets when the target material in the jet form injected from the injection nozzle is broken up into the droplets. The intensity of the radiated EUV light can thus be stabilized by homogenizing the intervals between the droplets with the repulsive force between the droplets due to the electric charge. As the droplets are charged, the repulsive force between the droplets results in an acceleration of the droplets. However, the trajectories of the droplets are not corrected or controlled.

Document US 2010/0025223 discloses an extreme ultraviolet light source device, wherein an offset in the ejection direction of target material droplets is corrected in order to stabilize the EUV output of the light source device. A trajectory correction device is provided with a block electrode, which forms a so-called einzel lens (a unipotential lens). As a result, the electric field generated by the block electrode acts on the previously charged droplets as an electrostatic lens, thus functioning similarly to a convex lens. The block electrode therefore can exert a focusing force in the X and Y dimensions without accelerating or decelerating the droplets in the Z direction. Thus, although the trajectories of the droplets are corrected, there is no

acceleration.

Finally, Document EP 1367441 (A2) discloses a target material delivery system in the form of a nozzle for a EUV radiation source. The nozzle includes a target material supply line having an orifice through which droplets of a liquid target material are emitted, where the droplets have a predetermined size, speed and spacing there between. The droplets are mixed with a carrier gas in a mixing chamber enclosing the target material chamber and the mixture of the droplets and the carrier gas enter a drift tube from the mixing chamber. The droplets are emitted into an accelerator chamber from the drift tube where the speed of the droplets is increased to control the spacing there between.

Thus, the liquid droplets are prevented from immediately flash boiling and disintegrating as they are emitted from the nozzle into the source vacuum. Also, the source parameters are tightly controlled to insure the resulting size and consistency of the droplets at the target location is correct. Additionally, the speed, spacing and frequency of production of the droplets are controlled. SUMMARY OF THE INVENTION

The primary object of the present invention is the precise control of the position of the droplets at the irradiation point, where the laser is focused.

This object is obtained by a steering device for controlling the direction and/or velocity of droplets of a target material, which are dispensed by a target delivery system to move to a EUV production site for being irradiated by a focused laser beam, whereby said steering device comprises deflecting and/or accelerating means arranged along a droplet path from the target delivery system to the EUV production site.

Other electrodes may be placed elsewhere within the EUV source, especially in a droplet catcher that is located on the other side of the plasma production site.

Also the interaction between plasma and droplets is taken into account, which leads to the need to accelerate the droplets also axially increasing the distance between plasma and next droplet.

According to an embodiment of the invention a plurality of deflecting and/or accelerating means is arranged one after the other along said droplet path.

According to another embodiment of the invention said plurality of deflecting and/or accelerating means comprises at least one of an electric, a magnetic and a fluidic means for influencing the path and/or velocity of said droplets.

According to another embodiment of the invention the steering device is of a tubular design with an entrance and an exit for said droplets, whereby a droplet charging means is arranged at the entrance of the steering device, and at least one of said deflecting and/or accelerating means comprises an electrode. According to another embodiment of the invention a plurality of deflecting and/or accelerating means comprises an electrode, whereby said electrodes are each connected to a control to be held each on a predetermined electric potential. According to another embodiment of the invention said electrodes are arranged in electrode rings being concentric with respect to a central axis, such that the electric field of said electrodes has a concentric component toward the central axis. According to another embodiment of the invention the deflecting and/or accelerating means comprises at least one nozzle for deflecting and/or

accelerating said droplets by means of a gas flow.

According to another embodiment of the invention the gas flow contains argon or helium or nitrogen or hydrogen, or a mixture of these gases.

According to another embodiment of the invention the gas flow within the nozzle has a swirl. According to another embodiment of the invention the gas within the nozzle is charged.

According to another embodiment of the invention the steering device is shielded from the heat coming from the EUV production site by a cooled shielding means

According to another embodiment of the invention the shielding means is cooled by a liquid or gaseous cooling medium, especially water or oil, or one of argon, nitrogen and helium. According to another embodiment of the invention a droplet catcher is provided to catch droplets that have passed the EUV production site, whereby the droplet catcher comprises an electrode. Other electrodes and/or magnetic coils and/or natural magnet and/or fluid inlet and/or outlet may be placed between the EUV production site and the droplet catcher.

The EUV source according to the invention comprises a target delivery system in the form of a droplet dispenser and a EUV production site, where said droplets of said droplet dispenser are ignited by means of a focused laser beam, whereby an inventive steering device is placed between the droplet dispenser and the EUV production site to deflect and/or accelerate said droplets on their way from the droplet dispenser to the EUV production site.

According to an embodiment of the inventive EUV source at least one droplet sensing means is arranged at the path of the droplets sensing the position and/or velocity of the droplets for controlling and/or triggering the operation of the EUV source.

According to another embodiment of the inventive EUV source the droplet dispenser ejects a jet of target material, which breaks up into droplets in a droplet break-up region, that the steering device comprises a droplet charging means in form of a charging electrode, and that the charging electrode is placed in the droplet break-up region.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

Fig. 1 shows a simplified configuration of a EUV source; Fig. 2 schematically shows the steering and accelerating device

coupled with the droplet generator and aligned with the laser spot size; Fig. 3 shows a possible configuration for one type of

steering/accelerating means comprising

electrodes;

Fig. 4 shows a possible configuration for another type

of steering/accelerating means comprising an

electrode; Fig. 5 shows an exemplary module for a steering tube that uses gas to induce the concentric and accelerating force; shows a longitudinal section of an actual embodiment of a steering/accelerating tube with accelerating/deflecting electrodes according to the invention; and

Fig. 7 shows the results of a simulation of droplet trajectories in a

steering/accelerating tube according to Fig. 6.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

The aim of the steering device of the present invention is to achieve a position accuracy of the droplets of less than ±10% of the droplet diameter. Hence, the smallest portion of laser energy is lost.

Fig. 2 shows an embodiment of such a steering device 24. A jet 25 of target material comes out from the droplet dispenser 17 and enters the first part of a steering/accelerating tube 28 at an entrance 24a of the steering device 24 in a transition region, where Rayleigh break-up of the jet 25 into separate droplets 19 occurring. In this first section of the steering/accelerating tube 28 a charging electrode 26 is placed that charges the passing droplets 19. The charging electrode 26 has a cylindrical shape, and the droplets pass through in the middle. The target material thus enters as a jet 25 and exits as a train of charged droplets 19.

Downstream of the charging electrode 25 a succession of modular deflecting and accelerating means 27, each comprising one or more electrodes (see 37 and 39 in Fig. 3 and 4), is present. The deflecting electrodes 37, 39 of the deflecting and accelerating means 27 are seen at first by the charged droplets, because at small velocity it is easier to deflect the droplets 19 radially. The deflecting electrodes are of two kinds (Fig. 3 and 4): Preferentially an assembly of three electrodes 37 in the form of short cylinders placed at 120° from each other within an electrode ring 35 (Fig. 3), and/or a ring-like electrode 39 concentrically placed within an electrode ring 38 (Fig. 4). The two electrodes 37, 39 are different in form, in order to achieve a concentric electric field, capable to deflect the droplets 19 in the centre of the tube 28.

Instead of the three electrodes 37 shown in Fig. 3, there may be different numbers of electrodes between 2 and 12, whereby the electrodes are arranged around the central axis equidistant to each other.

The modules 35, 38 and/or 40 are placed either with always the same orientation along the steering tube 28, or changing the angular orientation continuously moving from the top to the bottom of the steering tube 28.

Downstream of the first deflecting stages, where a small voltage difference between electrodes 37, 39 is enough, the same kind of electrodes 37, 39 are used to accelerate the droplets 19. The same two types of electrodes 37, 39 are used in order to maintain a radial field component. Overall, the steering device 24 is modular; in fact just two types of electrodes are present and repeated for all the length of the tube 28 several times. The electrodes 37, 39 of the deflecting and accelerating means 27 are connected to a control 30, which cooperates with the droplet dispenser 17 and an external droplet sensing means 33 and/or 33', and which provides the electrodes 37, 39 with the various voltages or electric potentials, which are necessary to fulfil the tasks of deflecting and accelerating the droplets 19. However, instead of or in addition to the electrodes 37, 39, magnetic means may be used to interact with the charged droplets 19 according to the principles of the Lorentz force. The interaction between the charged droplets 19 and the modules of the steering tube 28 may be used to localize the droplets 19 and feeding this information into the control 30. According to the invention, the electrodes may be replaced by or combined with fluidic deflecting/accelerating means (see Fig. 5), i.e. with modules, where a gas is flowing to influence the direction and/or velocity of the passing droplets 19. Fig. 5 shows an exemplary fluidic module in form of a nozzle 40. The gas, which acts on the droplets 19, comes from the entrance 24a, where the droplets 19 enter the tube 28. The nozzles 40 each comprise a convergent section 41 , a throat 43 and a divergent section 42. In the convergent section 41 the gas flow is constrained to go towards the centre, and it applies thus a concentric force on the droplets 19.

Furthermore, exits or holes 44 are present in the throat 43, which connect the fluid in the throat 43 with the outside of the nozzle 40. Because of this a part of the flow is evacuated taking advantage of the vacuum outside of the steering tube 28. Part of the gas is evacuated in order to have a smaller centrifugal force on the droplet 19 from the gas expanding in the diverging section 42. The end of the steering tube 28 (at exit 24b) is close to the plasma generated the EUV production site 20. Therefore, it has to be protected and cooled. This is achieved by attaching the end of the tube to a preferably water-cooled shielding means 29. Fig. 6 shows a longitudinal section of an actual embodiment of a

steering/accelerating tube 28 with accelerating/deflecting electrodes 37, 39 according to the invention. The electrodes 37, 39 are each incorporated into a respective electrode ring 35, 38. The (modular) electrode rings 35, 38 are alternating arranged within tube 28. At the entrance of the tube 28 the cylindrical charging electrode 26 is placed and held on a potential VO. The charging electrode 26 is separated from the remaining column of the electrode rings 35, 38 by means of an electrically insulating spacer 45. The distance between the electrodes 37, 39 of the column is always the same. The electrodes 37, 39 of the column are held on potential differences (or voltages) V1 V8 with respect to the charging electrode

26. As an example, these voltages may have values of 0, -0.5kV, 0.15kV, -0.8kV, - 1 kV, -1.2kV, -1.5kV, -1.8kV, respectively. The electrode potential is not

continuously increasing, because it is optimized to shrink the droplet flux.

The charging electrode voltage V0 can be up to 2kV. Important is always the voltage difference between the electrodes. In the simulation where the listed voltages are used, the charge to mass ratio is chosen using the Rayleigh limit for a droplet of 20 microns diameter, which gives the maximum charge that can be stored in a droplet. A combination of voltages on the electrodes of the column is suited just for one deviation angle of the droplet trajectory (46 in Fig. 7). Therefore a controller has to be integrated with respective trajectory sensors in order to bring into focus droplets, even when there are changes in the initial velocity. The half angle of the droplet initial velocity in the present case is 3° (worst case); the axial velocity of the droplets 19 increases from 10m/s to 72m/s, and can go even higher.

Fig. 7 shows the results of a simulation of droplet trajectories 46 and the focussing effect in a steering/accelerating tube 28 according to Fig. 6, whereby only some of the alternating electrodes 37, 39 are taken into account.

LIST OF REFERENCE NUMERALS

10 EUV source

11 chamber

12 drive laser

13 window 14 drive laser pulse

15 collector (elliptic)

16 mechanical support

17 target delivery system (droplet dispenser) 18 EUV light

19 droplet (of target material)

20 EUV production site

21 second focus

22 aperture

23 droplet catcher

24 steering device

24a entrance (steering device)

24b exit (steering device)

25 jet

26 charging electrode

27 deflecting/accelerating means

28 steering/accelerating tube

29 shielding means

30 control

31 ,32 control line

33,33' droplet sensing means (e.g. optical)

34 laser beam (focused)

35,38 electrode ring

36 holder

37,39,47 electrode

40 nozzle

41 convergent section

42 divergent section

43 throat

44 exit

45 spacer

46 trajectory voltage (electrodes)

Claims

1. A steering device (24) for controlling the direction and/or velocity of droplets (19) of a target material, which are dispensed by a target delivery system (17) to move to a EUV production site (20) for being irradiated by a focused laser beam (34), characterized in that said steering device (24) comprises deflecting and/or accelerating means (27) arranged along a droplet path from the target delivery system (17) to the EUV production site (20).

2. A steering device according to claim 1 , characterized in that a plurality of deflecting and/or accelerating means (27) is arranged one after the other along said droplet path.

3. A steering device according to claim 2, characterized in that said plurality of deflecting and/or accelerating means (27) comprises at least one of an electric, a magnetic, and a fluidic means for influencing the path and/or velocity of said droplets (19).

4. A steering device according to claim 3, characterized in that the steering device (24) is of a tubular design with an entrance (24a) and an exit (24b) for said droplets (19), that a droplet charging means (26) is arranged at the entrance (24a) of the steering device (24), and that at least one of said deflecting and/or accelerating means (27) comprises an electrode (37, 39).

5. A steering device according to claim 4, characterized in that a plurality of deflecting and/or accelerating means (27) comprises an electrode (37, 39), and that said electrodes (37, 39) are each connected to a control (30) to be held each on a predetermined electric potential.

6. A steering device according to claim 3 or 4, characterized in that said electrodes (37, 39) are arranged in electrode rings (35, 38) being concentric with respect to a central axis, such that the electric field of said electrodes (37, 39) has a concentric component toward the central axis.

7. A steering device according to claim 3, characterized in that the deflecting and/or accelerating means (27) comprises at least one nozzle (40) for deflecting and/or accelerating said droplets (19) by means of a gas flow.

8. A steering device according to claim 7, characterized in that the gas flow contains argon or helium or nitrogen or hydrogen, or a mixture of these gases.

9. A steering device according to claim 7 or 8, characterized in that the gas flow within the nozzle (40) has a swirl.

10. A steering device according to one of the claims 7 to 9, characterized in that the gas within the nozzle (40) is charged.

11. A steering device according to one of the claims 1 to 10, characterized in that the steering device is shielded from the heat coming from the EUV production site (20) by a cooled shielding means (29).

12. A steering device according to claim 11 , characterized in that the shielding means (29) is cooled by a liquid or gaseous cooling medium, especially water or oil, or one of argon, nitrogen and helium.

13. A steering device according to one of the claims 1 to 11 , characterized in that a droplet catcher (23) is provided to catch droplets that have passed the EUV production site (20), and that the droplet catcher (23) comprises an electrode (47).

14. A EUV source comprising a target delivery system (17) in the form of a droplet dispenser and a EUV production site (20), where said droplets (19) of said droplet dispenser (17) are ignited by means of a focused laser beam (34), characterized in that a steering device (24) according to one of the claims 1 to 12 is placed between the droplet dispenser (17) and the EUV production site (20) to deflect and/or accelerate said droplets (19) on their way from the droplet dispenser (17) to the EUV production site (20).

15. A EUV source according to claim 13, characterized in that at least one droplet sensing means (33, 33') is arranged at the path of the droplets (19) sensing the dimension and/or the position and/or velocity of the droplets for controlling and/or triggering the operation of the EUV source.

16. A EUV source according to claim 13 or 14, characterized in that the droplet dispenser (17) ejects a jet (25) of target material, which breaks up into droplets in a droplet break-up region, that the steering device (24) comprises a droplet charging means in form of a charging electrode (26), and that the charging electrode (26) is placed in the droplet break-up region.