Droplet dispensing device, method for providing droplets, and light source for providing UV or X-ray light

Abstract

The invention relates to a droplet dispensing device (4) comprising a reservoir (9) for containing a liquid medium (10), an outlet (11) for dispensing droplets of said liquid medium (10) from said reservoir (9), an actuation means (12) for generating and transmitting a mechanical oscillation at an excitation frequency, and a resonant structure comprising a piston (15) coupled to said actuation means (12) for transmitting said mechanical oscillation to the liquid medium (10) contained in said reservoir (9) such that droplets are formed from said liquid medium (10), wherein a resonance frequency of said resonant structure is sufficiently close to said excitation frequency, such that resonance occurs.

The invention further relates to a UV or X-ray light source, comprising a droplet dispensing device (4) according to the invention, and a method for providing a stream, in particular a monodisperse stream, of droplets by means of the droplet dispensing device (4).

Claims

1. Droplet dispensing device (4) comprising a reservoir (9) for containing a liquid medium (10), wherein the droplet dispensing device (4) comprises an outlet (11) for dispensing droplets of said liquid medium (10) from said reservoir (9), an actuation means (12) for generating and transmitting a mechanical oscillation at an excitation frequency, characterized in that the droplet dispensing device (4) comprises a resonant structure coupled to said actuation means (12), wherein said resonant structure has a resonance frequency which is sufficiently close to said excitation frequency, such that resonance occurs when the mechanical oscillation is transmitted from the actuation means (12) to the resonant structure at said excitation frequency, and wherein said actuation means and/or said resonant structure comprises a piston (15), wherein the piston (15) comprises a tip (16), which is immersed or immersible in said liquid medium (10), wherein said piston (15) is adapted to transmit said mechanical oscillation from said actuation means (12) to said liquid medium (10), such that droplets are formed from said liquid medium (10).

2. Droplet dispensing device (4) according to claim 1, characterized in that the excitation frequency is in the range of 1 kHz to 1000 kHz, particularly 1 kHz to 200 kHz.

3. Droplet dispensing device (4) according to claim 1, characterized in that said actuation means (12) comprises an electro-actuatable element, particularly a piezoelectric element, which is adapted to transmit said mechanical oscillation to said piston (15).

4. Droplet dispensing device (4) according to claim 1, characterized in that said piston (15) comprises at least a first section having a first cross-sectional area perpendicular to a longitudinal axis (L), along which said piston (15) extends, and a second section having a second cross-sectional area, wherein the second cross-sectional area is larger or smaller than the first cross-sectional area, and wherein the second section is adapted to contact the liquid medium (10), such that the displacement of the piston (15) can be amplified, wherein in particular the displacement of the mechanical oscillation of the piston tip immersed in the liquid medium is maximized, such that the induced pressure waves in the liquid medium are maximized.

5. Droplet dispensing device (4) according to claim 1, characterized in that said piston (15) is mechanically connected to a cover (19) of said reservoir (9) or to an inside wall of said reservoir (9), wherein the connection between said piston (15) and said cover (19) or said inside wall forms a region of smaller displacement than the displacement of the tip (16) of said piston (15).

6. Droplet dispensing device (4) according to claim 1, characterized in that said resonant structure comprises a backing mass attachment wherein said backing mass attachment is coupled to said actuation means (12), wherein particularly said backing mass attachment is positioned outside of said reservoir (9).

7. Droplet dispensing device (4) according to claim 1, characterized in that said actuation means (12) is positioned outside of said reservoir (9).

8. Droplet dispensing device (4) according to claim 1, characterized in that said droplet dispensing device (4) comprises a filter (17) for filtering said liquid, wherein said filter (17) is positioned upstream of said outlet, and wherein said filter (17) is coupled to said resonant structure.

9. Droplet dispensing device (4) according to claim 8, characterized in that said filter (17) is flexibly connected to said resonant structure.

10. Droplet dispensing device (4) according to claim 1, characterized in that said resonant structure comprises said reservoir (9).

11. Light source (1) for providing UV and/or X-ray light, comprising a droplet dispensing device (4) according to claim 1, which is adapted to provide droplets of a liquid medium (10), a laser source, wherein the laser source is adapted to provide a laser beam (6), and direct said laser beam (6) onto at least one of said droplets, wherein said laser beam (6) is adapted to excite atoms and/or molecules comprised in said droplets, such that UV and/or X-ray light is emitted by said atoms and/or molecules.

12. Method for providing droplets, comprising the steps of: providing a droplet dispensing device (4) according to claim 1, providing a liquid medium (10) in said reservoir (9) of said droplet dispensing device (4), generating a mechanical oscillation at an excitation frequency by means of said actuation means (12), wherein the excitation frequency is sufficiently close to a resonance frequency of said resonant structure, such that resonance occurs, transmitting said mechanical oscillation to said liquid medium (10) at said excitation frequency by means of said resonant structure, forming said droplets from said liquid medium by means of said transmitted mechanical oscillation.

13. Method for providing droplets according to claim 12, wherein the excitation frequency is in the range of 1 kHz to 1000 kHz, particularly 1 kHz to 200 kHz.

14. Method for providing droplets according to claim 12, wherein said resonant structure comprises said liquid medium (10) in said reservoir (9) and/or said outlet (11).

15. Method for providing droplets according to claim 12, wherein said resonance frequency is determined, and wherein said excitation frequency is changed according to the determined resonance frequency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0104] FIG. 1 shows a simplified sectional view of a light source including an embodiment of the droplet dispensing device according to the invention;

[0105] FIG. 2 shows a schematic sectional view of an embodiment of the droplet dispensing device according to the invention;

[0106] FIG. 3 shows a sectional view of a part of the droplet dispensing device of the present invention comprising a piston according to a first embodiment;

[0107] FIG. 4 shows a sectional view of a part of the droplet dispensing device of the invention comprising a piston according to a second embodiment with a diameter step change of the piston at the lower nodal point and a higher longitudinal displacement of the piston tip;

[0108] FIG. 5 shows the formation of a jet emanating from the nozzle orifice and the break-up of the jet into a regular stream of droplets;

[0109] FIG. 6: shows the effect of increasing the excitation voltage applied to the electro-actuatable element from left to right (peak-to-peak voltages: 0.5V, 1V, 2V, 4V, 6V, 8V, 10V) on the droplet break-up resulting in a stable droplet stream towards higher excitation voltages;

[0110] FIG. 7: shows resonance curves illustrating the characteristic shapes of the electric impedance and phase between voltage and current signal applied to the electro-actuatable element exhibited as the dispenser is frequency swept through resonance when the piston is immersed in molten tin.

[0111] The invention is directed to a droplet dispensing device capable of producing a monodisperse micrometre sized droplet stream even for high temperature liquids such as molten metals and for operation conditions at low non-dimensional wavenumbers including values smaller than 0.3 which can be used for various purposes and in various applications. One particular application of the invention is the generation of micrometre-sized droplets as target material for EUV light sources such as the one shown in FIG. 1, the material being capable of radiating in the target wavelength window in the EUV region when irradiated by a high power laser and excited into a higher energy state. Further applications are related to the 3D printing of metals (including those with high melting points), where the generation of uniformly sized droplets is important.

[0112] FIG. 1 shows a light source 1 comprising a vacuum chamber 2 containing collector optics 3 for extreme-ultraviolet or soft X-ray light and a droplet dispensing device 4 according to the present invention, a possible embodiment of which is shown in FIG. 2 for the continuous delivery of target material 5 to the irradiation site 6. The target material 5 gets irradiated by a high power laser beam 7 at the irradiation site 6, the focus point of the laser, and forms an EUV light emitting plasma. The laser beam 7 is brought into the vacuum chamber 2 through a flanged window 8 and its temporal and spatial characteristics should be such that the conversion efficiency (ratio of emitted EUV light energy to laser energy) is maximized with respect to the size and location of the target material.

[0113] The droplet dispensing device 4, a possible embodiment of which is shown in FIG. 2 delivers the target material 5 in form of a continuous droplet stream to the irradiation site 6. The droplets may be of any material suitable for the generation of radiation upon irradiation by a high power laser, including metals such as Sn, Li, In, Ga, Na, K, Mg, Ca, Hg, Cd, Se, Gd, Tb, alloys of these materials such as SnPb, SnIn, SnZnIn, SnAg, liquid non-metals such as Br or liquefied gases such as Xe, N.sub.2, and Ar as well as suspensions of a target material in a solution, e.g. in water or alcohol. Droplet sizes can be in the range of 5 μm to 100 μm in order to reduce the amount of detrimental debris as a side product of the irradiation of the droplet and the plasma formation. The delivery of the target material 5 may be at a constant frequency and uniform droplet target size.

[0114] FIG. 2 depicts a possible embodiment of the droplet dispensing device 4 according to the invention in a cross-sectional view. The device 4 comprises a reservoir 9 for receiving a liquid medium 10, an outlet nozzle assembly 11 in fluid and acoustic communication with the reservoir 9 and an oscillating actuation means 12 for producing pressure waves in the liquid medium 10, in particular at the outlet nozzle assembly 11, such that a part of the liquid medium 10 exits the outlet nozzle assembly 11 in a sequence of droplets. Both the reservoir 9 and the outlet nozzle assembly 11 may be heated using a heater 23 (electrical, inductive infrared or other).

[0115] The actuation means 12 comprises a backing mass attachment and electro-actuatable element 14, and a piston 15, the tip 16 of which is immersed in the liquid medium 10. The electro-actuatable element may be actively cooled. A filter 17 may be placed upstream of or in the outlet nozzle assembly 11 in order to avoid clogging of the outlet nozzle assembly 11. The outlet nozzle assembly 11 may have a micromachined nozzle orifice 18. The droplet dispensing device 4 (which may comprise a casing or cartridge of the reservoir 9) may be replaceable (i.e. removable from and reinsertable into the light source 1) and refillable and connected to a backpressure of an inert gas in order to form a jet emanating from the nozzle orifice 18. A typical gas may be gas inert to any chemical reactions with the target material such as Ar, N, Kr or He.

[0116] FIG. 3 shows a part of the droplet dispensing device 4 comprising the actuation means 12 consisting of an electro-actuatable element, being attached to the cover of the reservoir 9, a backing mass attachment as well as a piston 15. The displacement along the longitudinal axis L is shown for operation at the resonant frequency.

[0117] FIG. 3 further shows a piston 15, which is actuated by an electro-actuatable element being mounted in-between the backing mass attachment and the piston 15. The backing mass attachment and electro-actuatable element 14 and the piston 15 are all positioned on the same longitudinal axis L. The backing mass attachment and electro-actuatable element 14 can either be positioned on the outside or inside of the reservoir 9. The free piston tip 16 is immersed in the liquid medium 10 just upstream of the outlet nozzle assembly 11. The actuation means 12 is actuated by applying an electric signal to the electro-actuatable element and generates acoustic waves inside the piston 15 and causing the piston 15 and its tip 16 to vibrate. This in turn induces pressure waves in the liquid medium 10 and can also lead to periodic displacements of the liquid medium 10 both of which can propagate to the outlet of the nozzle orifice 18. Both of these effects combined or alone can lead to the generation of a stable breakup of the jet emanating from the outlet nozzle assembly 11 resulting in a monodisperse droplet stream as shown in FIG. 5.

[0118] The length scales and material choices of all components of the actuation means 12 (backing mass attachment and electro-actuatable element 14, piston 15) have to be carefully chosen and calculated such that at the design frequency a standing wave is formed in the actuation means 12 making it behave as a resonant structure and the end of the backing mass attachment as well as the piston tip 16 form a region of maximum displacement as shown in FIG. 3 and in FIG. 4. At the same time the region where the piston 15 and the electro-actuatable element is connected to the cover 19 forms a region of minimum displacement. The piston 15 is attached to the cover 19 ideally by means of laser welding in order to achieve high precision of the combined unit. Alternatively the piston 15 and cover 19 can also be machined out of one uniform piece. The connection of the actuation means 12 to the cover 19 and the remaining structure of the droplet dispensing device 4 at a region of minimum longitudinal displacement ensures a minimum amount of vibrations being transferred into the remaining structure of the droplet dispensing device 4 which might also create acoustic pressure waves in the liquid medium 10. This ensures that as high a proportion as possible of the pressure waves induced in the liquid medium 10 is created through the piston tip 16 only such that a highly clear and uniform excitation signal is generated for the break-up of the jet. This is crucial for achieving a stable break-up and reducing undesired interference of noise signals.

[0119] As shown in FIG. 4, the piston 15 can exhibit a diameter step change 20 at the lower nodal region associated with minimum longitudinal displacement and maximum stress. This diameter step change 20 of the piston 15 leads to an amplification of the vibrations by a factor proportionate to the change in cross sectional area of the piston 15. As the stress is transferred onto a smaller cross section the associated force and velocity and thus displacement is amplified by the factor. This gain ratio of the setup (here the change of the cross-sectional areas at the step) can be used to amplify the acoustic vibrations in the piston 15 induced by the electro-actuatable element. Thereby the piston tip 16 velocity and displacement can be increased and accordingly the generated pressure strength. The gain ratio has to be calculated in accordance with the acoustic load imposed by the liquid medium 10 in order to prevent suppression of the resonance modes for too high gain ratios.

[0120] The outlet nozzle assembly 11 and in particular the liquid medium 10 inside it can also form part of the resonant structure in order to further amplify the strength of the pressure excitation at the nozzle orifice 18. Effectively, the distance between the piston tip 16 and the nozzle orifice 18 has to be carefully calculated in consideration of the acoustic impedance of the characteristics of the liquid medium 10 in order to achieve the formation of a standing wave in the outlet nozzle assembly 11.

[0121] With the presented invention high excitation pressure waves in the order of several bars can be achieved. The generation of high amplitude pressure waves can be beneficial for the stable generation of droplets at low non-dimensional wavenumbers well below the value associated with the maximum growth rate. FIG. 6 shows high-resolution images of the break up region of the jet emanating the nozzle orifice 18 with increasing levels of the excitation strength (peak-to-peak voltages: 0.5V, 1V, 2V, 4V, 6V, 8V, 10V, increasing from left to right) obtained with the invention presented in this patent application. The importance of the increased excitation amplitude for the generation of a micrometer sized droplet stream with high temporal stability can be clearly seen.

[0122] The operation of the invention is, however, not only limited to the operation of the system at resonance frequency or limited to the operation of the system in which the cover 19 forms a point of zero displacement.

[0123] When the excitation frequency of the droplet dispensing device 4 is changed, the electric circuit exhibits a characteristic change in both electric impedance and phase as shown in FIG. 7, depicting the frequency response of the actuation means according to the present invention with the piston tip immersed in molten tin. Therein, a first graph 25 (thin line) illustrates the relative impedance values in Ohm at the respective excitation frequencies and a second graph 26 (thick line) illustrates the corresponding phase shift values in degrees (°). This characteristic shape can be used in a feedback control system to adapt the excitation frequency to account for any changes of the resonant frequency. These might occur due to increased self-heating effects of the electro-actuatable element over time or a changing liquid medium 10 level changing the heat load the electro-actuatable element is exposed to and thus also its temperature, which changes the electro-actuatable element impedance (in case of a piezoelectric actuator) and thus the resonant frequency of the entire system.

[0124] The electro-actuatable element can be placed outside of the high-temperature and high-pressure reservoir 9. This is particularly made possible through the resonance structure, in which the acoustic vibrations generated by the electro-actuatable element are effectively transmitted to the piston tip 16 while the region where the resonant structure is attached to the cover does not exhibit significant longitudinal displacement. Various cooling means can be employed on the outside of the high pressure and high temperature vessel to actively and directly cool the electro-actuatable element. One method is for example based on impingement cooling. For impingement cooling the cooling means comprises a high pressure zone separated from the electro-actuatable element by a plate equipped with holes through which the air flows from the high pressure zone into the low pressure zone and impinges directly onto the electro-actuatable element. This allows highly effective cooling of the actuator and thus ensures its effective operation (other means of cooling can also be based on a liquid cooling fluid, e.g. an electrically non-conductive cooling fluid). For piezoelectric actuators this is particularly important as a depolarization of the piezoelectric material can occur when temperatures above half the Curie temperature are exceeded (for common piezoelectric materials this corresponds to approximately 150° C.)

[0125] As shown in FIG. 5, the outlet nozzle assembly 11 includes the nozzle casing 21 with a micro-machined nozzle orifice 18 as well as a porous filter 17 placed upstream of the nozzle casing 21 preventing the clogging of the micrometer sized nozzle orifice 18 due to dirt particles. The porous filter 17 and nozzle casing 21 may be made out of or coated with materials inert to chemical reactions with the target material such as, tungsten, silicon nitride, diamond, sapphire, aluminium oxide, silica or stainless steel. The micro-machining process for the creation of the micrometre-sized nozzle orifice 18 should give low geometric tolerances on the quality of the nozzle orifice 18 in particular with respect to the surface roughness of the inner surface of the orifice channel. The micro-machining process can include but is not restricted to laser-drilling, electrical discharge machining and etching. A smooth surface on the inside of the micrometer-sized nozzle orifice 18 is important as surface defects can induce turbulences acting as undesired excitation sources on the jet break-up process. The nozzle channel may have various geometric forms including a tapered channel, a straight channel or a streamlined channel. Both filter 17 and nozzle can be easily replaced and exchanged.

[0126] The porous filter 17 may be made of a sintered material and have pore sizes in the range of 0.05 μm to 20 μm. The porous filter 17 may be equipped with a sealing ring 22 as shown in FIG. 2, which provides a high pressure suitable sealing solution with respect to both the nozzle casing 21 as well as to the material reservoir 9. Also, the porous filter 17 and the sealing ring 22 might be connected in a flexible manner such as through the use of a corrugated sheet.

[0127] The filter 17 can be mechanically coupled to the piston tip 16 in order to reduce the damping effect of the porous filter 17 on the induced pressure waves and increase the transmission thereof into the liquid medium 10 below the filter 17. Further, the fitter 17 may be flexibly attached to the structure of the reservoir 9 and outlet nozzle assembly 11, e.g. by the use of corrugated flexible connection between the sealing ring 22 of the filter 17 and the outlet nozzle assembly 11 and reservoir 9.

[0128] The piston 15 and the reservoir 9 can be manufactured out of or coated with materials inert to chemical reactions with the target material such as, tungsten, silicon nitride, diamond, sapphire, aluminium oxide, silica or stainless steel.

[0129] The droplet dispensing device 4 according to the invention can be operated with various liquid mediums including high temperature molten metals (e.g. aluminium, chromium, copper, nickel-chromium based alloys (such as alloys commercially available under the name “Inconel”), iron, magnesium, molybdenum, nickel, platinum, steel, tin, titanium and many more including alloys thereof) that have a melting point below the one of the material out of which the droplet dispensing device 4 is made (e.g tungsten). The droplet dispensing device according to the invention is particularly suited for such high temperature metals, as the electro-actuatable element is positioned outside of the high temperature and high pressure reservoir 9, where it can be effectively cooled.

[0130] In the presented invention a pre-stress can be applied to the electro-actuatable element via a bolt that goes through the hollow cylindrical electro-actuatable element and is threaded into the piston 15 structure. Other forms and shapes of an electro-actuatable element can be used, such as a cuboid, including other means to apply the pre-stress and connect the backing mass rigidly to the electro-actuatable element. In case of a hollow cylindrical electro-actuatable element a piece between the backing mass attachment with the integrated bolt and the electro-actuatable element itself can be prevented from rotary motion, and thereby any detrimental exertion of torsion on the electro-actuatable element can be prevented.

[0131] The cover 19 of the reservoir 9 is detachable from the container of the reservoir 9 and the cover 19 and the reservoir 9 are assembled employing a seal suitable to withstand high pressures and temperatures. Such a sealing ring can for example be made of a softer metal than the cover 19 and the reservoir 9 container and a mechanical seal can be achieved by mechanically deforming the sealing ring with knive edges manufactured in both the cover 19 and the reservoir 9 container. Many other methods can be employed, however, to achieve such a high temperature and high pressure resistant sealing between the cover 19 and the reservoir 9.

[0132] The electric voltage signal can exhibit various waveforms, including a sinusoidal waveform, a square waveform, a rectangular waveform, a sawtooth waveform or a peaked-nonsinusoidal waveform.

[0133] The applied electric signal can also be amplitude or frequency modulated in order to achieve ‘droplet merging’. Droplet merging refers to the phenomenon, in which multiple droplets are generated per modulation period, which exhibit relative velocity components towards each other such that after a certain time of flight and distance these droplets merge together and thus form one droplet per modulation period. Droplet merging, next to methods to increase the excitation amplitude, further allows reducing the lower limit of stable droplet formation In terms of the non-dimensional wavenumber. This is due to the initial droplet formation occurring at a higher non-dimensional wavenumber, while the eventual merging of the droplets generated per modulation period, leads to a droplet stream corresponding to a lower non-dimensional wavenumber.

LIST OF REFERENCE SIGNS

[0134] 1 Light source [0135] 2 Vacuum chamber [0136] 3 Collector optics [0137] 4 Droplet dispensing device [0138] 5 Target material [0139] 6 Irradiation site [0140] 7 Laser beam [0141] 8 Flanged window [0142] 9 Reservoir [0143] 10 Liquid medium [0144] 11 Outlet nozzle assembly [0145] 12 Actuation means [0146] 14 Backing mass attachment and electro-actuatable element [0147] 15 Piston [0148] 16 Tip [0149] 17 Filter [0150] 18 Nozzle orifice [0151] 19 Cover [0152] 20 Diameter step change [0153] 21 Nozzle casing [0154] 22 Sealing ring [0155] 23 Heater [0156] 24 Droplet stream [0157] 25 First graph [0158] 26 Second graph [0159] L Longitudinal axis