Patent Application
20140151582
The present invention relates to the technical field of generating droplets. It refers to a droplet dispensing device according to the preamble of claim 1. It further refers to a light source comprising such a droplet dispensing device.
In different technical fields it is necessary to generate a continuous sequence of droplets of a predetermined size and droplet frequency, which is ejected on a predetermined trajectory. The material of the droplets may for example be a liquid solution or a molten metal. One technical field, where such a droplet dispensing device is used, is a light source emitting extreme ultraviolet or soft x-ray light.
Extreme ultraviolet (EUV) light is electromagnetic radiation with wavelengths between 121 nm and 10 nm, while soft x-rays range from 10 nm to 1 nm. In EUV or soft x-ray sources, a radiation emitting plasma is produced by irradiating a target material. A regenerative solution for delivering target material to the production site comprises a target dispensing device based on liquid droplets. The radiation exciting the target material can be a pulsed laser beam, thus producing a laser produced plasma (LPP). The target delivery must be synchronized with the pulsed laser beam. The radiation is typically collected and directed to an intermediate region for utilization outside of the light source.
The formation and delivery of the target material to the focus of the light collector is closely linked to thermal and fluid management, as well as droplet generation issues of the dispensing device.
A method for creating continuous droplet streams with a vibrator coupled to an injection nozzle is disclosed in H. M. Hertz et al., “Debris free soft x-ray generation using a liquid droplet laser plasma target”, U.S., SPIE, Vol. 2523, pp. 88-93, with application in a soft x-ray light source. Another method, based on magnetic coil vibrators is disclosed in “A continuous droplet source for plasma production with pulse lasers”, U.K., Journal of Physics E: Scientific instruments, Vol. 7, 1974, pp. 715-718.
Other dispensing systems are used in soldering applications, such as the apparatus shown in document U.S. Pat. No. 5,171,360, which produces a stream of metal droplets at a temperature of 200° C. A similar droplet generator developed is disclosed in Rev. Sci. Instr. 58 (1987) 279. Another typical continuous or on-demand soldering jetting device is disclosed in document U.S. Pat. No. 6,224,180. A method and apparatus to generate on-demand droplets by acoustic impulses, including an impulse transmitting device inside liquid metal, is disclosed in document U.S. Pat. No. 5,598,200.
Document EP 0 186 491 discloses an apparatus for producing soft x-rays from generating a plasma source which comprises a low pressure vessel, energy beam means, such as a laser beam, associated with the low pressure vessel for generating and supplying a high energy beam to an impact area inside the low pressure vessel, a liquid target, such as mercury, capable of emitting x-rays when impacted by a high energy beam target supply means associated with the low pressure vessel for supplying the liquid target material to the impact area inside the low pressure vessel, and control means coupled to the energy beam means so that the high energy beam impacts the liquid material target in the impact area of the low pressure vessel. The mercury drops are formed at the end of a fine tube in a vessel by the action of surface tension and are caused to drop by vibration set up by a piezoelectric element, which is connected to the fine tube (see also US 2009/0230326 or US 2010/0090133).
Document WO 2006/093687 discloses an EUV light source plasma source material handling system and method, which may comprise a droplet generator having droplet generator plasma source material reservoir in fluid communication with a droplet formation capillary and maintained within a selected range of temperatures sufficient to keep the plasma source material in a liquid form; a plasma source material supply system having a supply reservoir in fluid communication with the droplet generator plasma source material reservoir and holding at least a replenishing amount of plasma source material in liquid form for transfer to the droplet generator plasma source material reservoir, while the droplet generator is on line; a transfer mechanism transferring liquid plasma source material from the supply reservoir to the droplet generator plasma source material reservoir, while the droplet generator is on line.
The droplet dispensing devices known so far have the disadvantage that the droplet size and trajectory are not precise enough to generate extreme ultra-violet or soft x-ray radiation in a laser-driven light source with improved efficiency.
It is an object of the invention, to create a droplet dispensing device with improved precision of the droplet size and trajectory even when the liquid used is a molten material from a heated reservoir.
It is another object of the invention to have a light source with an improved efficiency.
These and other objects are obtained by a droplet dispensing device according to claim 1, a light source according to claim 20.
The droplet dispensing device according to the invention comprises a reservoir to for receiving a liquid material, an outlet nozzle in fluid communication with said reservoir and a piezoelectric actuating means acting on said liquid material at said outlet nozzle to exit said outlet nozzle in a sequence of droplets. It is characterised by said piezoelectric actuating means comprising a piston, which is actuated by a piezoelectric actuator at one end and dips the other, free end into said liquid material just upstream of said outlet nozzle.
According to an embodiment of the invention said liquid material is a molten material and said reservoir is heated by a heating means to keep said molten material in the molten state.
According to another embodiment of the invention said reservoir is surrounded by an outer cylinder and said heating means comprises a resistive heater, which is wound around said outer cylinder and preferably said outlet nozzle and thermally and mechanically fixed to said outer cylinder, especially by means of laser welding.
According to another embodiment of the invention said reservoir and preferably said outlet nozzle are enclosed by a band-heater. According to another embodiment of the invention a filter is provided upstream of the outlet nozzle.
According to another embodiment of the invention said outlet nozzle, said actuated piston and said filter, respectively, are detachably coupled to said reservoir.
According to another embodiment of the invention said outlet nozzle is configured to deliver a continuous stream of droplets of said liquid material at a desired frequency when said actuated piston is periodically excited by means of said piezoelectric actuator.
According to another embodiment of the invention said outlet nozzle comprises a separate nozzle disk, which is retained in a nozzle casing and is detachably coupled to said reservoir by a clamping device.
According to another alternative embodiment of the invention said outlet nozzle comprises a one-piece nozzle unit, which is detachably coupled to said reservoir by a clamping device.
According to another embodiment of the invention said nozzle disk or nozzle unit, respectively, are made of a metal or ceramic and comprise a micromachined nozzle orifice.
According to another embodiment of the invention means for applying a pressure to the liquid material in the reservoir is provided, so that said liquid material is urged from the reservoir into said outlet nozzle.
Preferably, said pressure applying means comprises a connector tube leading to the interior of said reservoir.
According to just another embodiment of the invention said reservoir comprises a cover at a side opposite to said outlet nozzle, said actuated piston is mounted on said cover, and said piezoelectric actuator is arranged between said cover and said actuated piston to generate displacements of said actuated piston in to order to induce pressure waves in the liquid material at the outlet nozzle to impose a desired frequency to said sequence of droplets.
According to another embodiment of the invention said reservoir is suspended at a side opposite to said outlet nozzle in a casing by means of a thermally insulating flange and said piezoelectric actuator is arranged between the flange and the casing.
According to another embodiment of the invention said reservoir is provided with a cooling system for cooling the piezoelectric actuator to maintain the temperature of the piezoelectric actuator below detrimental temperatures.
Preferably, said cooling system is arranged outside said heated reservoir with a defined heat path to said piezoelectric actuator.
According to another embodiment of the invention a mechanical preload mechanism is provided for said piezoelectric actuator.
According to another embodiment of the invention a heat shield is provided for at least part of said piston, which reduces the heat flux from the liquid material to said piezoelectric actuator.
According to just another embodiment of the invention said reservoir and said outlet nozzle are part of replaceable cartridge, which is arranged in a casing and which is thermally insulated from said casing by insulating means, preferably by insulating flanges.
The light source for producing extreme ultraviolet or soft x-ray light according to the invention comprises a chamber that contains a production site for producing extreme ultraviolet or soft x-ray light, a droplet dispensing device for dispensing droplets of a liquid material into said production site, the liquid material being capable of radiating extreme ultraviolet or soft x-ray light when excited into a higher energy state, and irradiating means for irradiating the droplets at said production site. It is characterized by said droplet dispensing device being a is droplet dispensing device according to the invention.
The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.
The invention is directed to a droplet dispensing device which can be used for various purposes. The invention also encompasses a light source, comprising a production chamber that contains a light production site, a droplet dispensing device for dispensing droplets of a material into the production site, the material being capable of radiating in the target wavelength window when excited into a higher energy state, and irradiating means for irradiating the droplets in the production site. Said droplet dispensing device is a droplet dispensing device acs cording to the invention.
In one embodiment the material reservoir and nozzle may be heated using an electrical heating resistance. Droplet generation is achieved by an actuated piston inside the material reservoir. The excitation source of said piston is a piezoelectric actuator. The piston may be provided with a cooling system and a heat shield for said actuator. The material reservoir may be a replaceable and refillable cartridge with a backpressure connector. A filter may be placed upstream of the outlet nozzle in order to avoid clogging of the outlet nozzle. The outlet nozzle may have a micro-machined orifice. The dispensing unit may be supported by thermally insulating flanges in order to reduce the outer dispenser casing temperature. Instead of actuating the piston directly, the desired droplet train frequency may be imposed by displacing the dispensing unit relative to the casing via piezoelectric actuators.
The collector 42 may be an elliptical reflector, e.g., an elliptical EUV or soft x-ray (e.g., Mo/Si) mirror, with a first focus at the production site 48 and a second focus, called intermediate focus (IF), at an intermediate focus location 49, where the light is bundled for further use in an outside tool (not shown). The collector 42 has an aperture for the laser light to reach the light production site 48. The collector 42 may be replaced by a set of collection optics, which also bundles the light for further use. A catcher 47 catches the droplets or remains passing the production site 48.
The target or droplet dispensing device 43, which is mounted to the chamber 41 by means of a fixture 44, delivers the plasma source material in form of a sequence or stream of droplets to the ignition or production site 48. The liquid source material can be chosen by the skilled artisan in accordance with the requirements. It may either be liquid droplets of metals, e.g., Sn, Li, In, Ga, Na, K, Mg, Ca, Hg, Cd, Se, Gd, Tb, and alloys of these metals, e.g. SnPb, SnIn, SnZnIn, SnAg, liquid non-metals, e.g. Br, liquefied gases, e.g. Xe, N2 and Ar, or a suspension of a target material in a solution, e.g., in water or alcohol. In terms of requirements for the droplet dispensing device 43, the delivery of the source material may be at constant repetition rate (frequency) and target (droplet) size. Target sizes are in the range of 5 to 100 μm in order to minimize the amount of neutral particles after plasma formation, as well as the amount of residual source material.
Deviations in the location of the target imply a drop in conversion efficiency, as the target material is ignited either not at all or incompletely. Therefore, conversion efficiency is a function of the target delivery accuracy.
Turning now to
The cartridge with its reservoir 11 may be heated using an electrical heating resistance or resistive heater 14. The resistive heater 14 may be wound around an outer cylinder 16 containing the replaceable cartridge. The heater winding should extend over the full length of the cylinder 16 in order to ensure uniform heat transfer to the source material within the reservoir 11. The heating system may be extended to include the outlet nozzle 17. As shown in
Alternatively the heating system may be part of the cartridge. The heating system may be based on a band heater enclosing the source material reservoir 11 and the outlet nozzle 17. The heating power of the resistive heater 14 may be adjusted by a microprocessor or microcontroller (not shown). Temperature sensors (not shown) for monitoring the temperature may be installed on the dispensing device 10. Their temperature signals can be used for a heating control system and/or an emergency shutdown system.
Turning now to
Alternatively, the nozzle casing and disk may form one single nozzle unit 24, as shown in the outlet nozzle 17b of
The material of the nozzle disk 21 or nozzle unit 24 may be a micro-machinable ceramic, e.g. aluminium oxide, diamond, Macor, sapphire, Shapal M, silicon nitride, metal, e.g. aluminium, brass, stainless steel, tungsten. The nozzle material should give low geometric tolerances on quality of the nozzle orifice 23 and should have low wetting by the source or droplet material. The nozzle channel within the nozzle orifice 23 may be tapered, staged or streamlined for manufacturing reasons or improved inflow conditions.
The nozzle casing 20 may be attached to the material reservoir 11 via a standard pipe fitting. The gasket for the fitting may include a filter 19, e.g. sintered stainless steel, with pore sizes from 0.1 to 20 μm. Alternatively, the filter 19 may be placed further upstream of the nozzle. The outlet nozzle 17, 17a,b may be attached by a clamping device 18, e.g. a nut known in the art.
For continuous droplet generation, when the fluid is pressurized by means of a gas, a periodic signal is applied to the actuator 29. A Rayleigh type instability results in the break-up of the fluid column into droplets with the imposed frequency. Droplet volume is hereby a function of the characteristics of the fluid, backpressure, orifice diameter, and actuator driving parameters (voltage and timings).
The thermal management of the piezoelectric actuator 29 is crucial for long-term use of the droplet dispensing device. The maximum operating temperature of the piezoelectric material of the actuator 29 should not exceed 50% of the Curie temperature of the piezoceramic. In order to limit the heat flux from the molten source material 31, a heat shield 33 may be used surrounding the immersed section of the actuated piston 32. Heat, which flows along the axis of the piston 32 from the hot source material 31 at the free end of the piston 32, is preferably removed by a cooling system 26.
At the attachment point of the piston 32 and the removable cover 12, the heat flux from the piezoelectric actuator 29 may be confined, such that the cooling system 26 mainly removes heat from the piezoelectric actuator 29 and not from the removable cover 12. The heat path confinement may be realized by inserting a transition piece 28 made of thermal insulating and damping material, e.g. Polyether ether ketone (PEEK®), between a base plate 25 of the piezoelectric actuator 29 and the removable cover 12. The transition piece 28 and base plate 25 may be glued or flanged into the removable cover 12. The cooling system 26 may be based on convective cooling with a fluid, e.g. air, Ar, N2, water. A conventional cooler provided for power electronics cooling may be used. The cooling system 26 may be placed inside or outside the material reservoir 11. In case, the cooling system 26 is placed outside the material reservoir 11, the heat path may include a membrane 27 in the removable cover 12 (see
The actuated piston 32 may include a mechanical preload capability for the piezoelectric actuator 29. The base plate 25 of the piezoelectric actuator 29 may include a threaded end. By tightening a housing 30 of the piezoelectric actuator 29 to the base plate 25 with a predetermined torque, the preload of the actuator 29 may be set.
As shown in
Instead of the actuated piston 32, a piezoelectric actuator 39 may be integrated between the flange 38 and the casing 35, as shown in the droplet dispensing device 34b of
The embodiments of the present invention disclosed above are only preferred embodiments and do not limit the presented invention in any way and particularly not to a specific embodiment alone. Many changes and modifications can be made to the disclosed aspects of the embodiments described above. The following claims should cover not only the disclosed aspects of the embodiments described above but also apparent changes and modifications.
| Number | Date | Country | Kind |
|---|---|---|---|
| 11002817.2 | Apr 2011 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2012/001453 | 4/2/2012 | WO | 00 | 12/13/2013 |
