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Patent Application Titled "Radiation Source and Method for Lithographic Apparatus and Device Manufacturing Method" Published Online

July 3, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- According to news reporting originating from Washington, D.C., by VerticalNews journalists, a patent application by the inventors Loopstra, Erik Roelof (Eindhoven, NL); Dijksman, Johan Frederik (Weert, NL), filed on July 4, 2012, was made available online on June 19, 2014.

The assignee for this patent application is ASML Netherlands B.V.

Reporters obtained the following quote from the background information supplied by the inventors: "A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

"Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

"A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):"

In addition to obtaining background information on this patent application, VerticalNews editors also obtained the inventors' summary information for this patent application: "Fuel droplet generators may comprise a nozzle through which molten fuel is driven under pressure to be injected from the nozzle as a stream of droplets. The natural break-up of a stream of liquid issuing from a nozzle is known as Rayleigh break-up. The Rayleigh frequency, which corresponds to the rate of droplet production of the nozzle is related to the mean velocity of the fuel at the nozzle and the diameter of the nozzle, as represented in equation (2):

"f Rayleigh = mean velocity 4.5 nozzle diameter ( 2 ) ##EQU00002##

"Although Rayleigh break-up of a stream of fuel may occur without excitation, a vibrator such as a piezoelectric actuator may be used to control the Rayleigh break-up by modulating or oscillating the pressure of molten fuel at the nozzle. Modulating the pressure inside the nozzle may modulate the exit velocity of the liquid fuel from the nozzle, and cause the stream of liquid fuel to break-up into droplets in a controlled manner directly after leaving the nozzle.

"If the frequency of oscillation applied by a vibrator is sufficiently close to the Rayleigh frequency of the nozzle, droplets of fuel are formed, the droplets being separated by a distance which is determined by the mean exit velocity from the fuel nozzle and by the oscillation frequency applied by the vibrator. If the oscillation frequency applied by the vibrator is substantially lower than the Rayleigh frequency, then instead of a periodic stream of fuel droplets being formed, clouds of fuel may be generated. A given cloud of fuel may include a group of droplets travelling at a relatively high speed and a group of droplets travelling at a relatively low speed (the speeds being relative to the average speed of the stream of fuel exiting the nozzle). These clouds may coalesce together to form a single fuel droplets. In this way a periodic stream of fuel droplets may be generated by applying an oscillation frequency to the vibrator which is significantly lower than the Rayleigh frequency. The spacing between the droplets is still governed by mean exit velocity and the oscillation frequency: the spacing between the droplets increases with decreasing oscillation frequency.

"Piezoelectric transducers may be used as vibrators to apply oscillation to a nozzle. For instance, where the nozzle is in the form of a capillary tube acting as a fuel feed chamber arranged to feed fuel, supplied from a fuel reservoir connected to its proximal end, to a nozzle formed as a restriction at its distal end, a piezoelectric vibrator may be in the form of a sleeve cemented or adhered to an outer face of the capillary. The use of molten fuel such as molten tin means that the cement or adhesive used to adhere the vibrator to the outer face of the capillary should be one which does not lose adhesion at the operating temperature of the fuel supply.

"In extended use, adhesion between a vibrator and the outer face of the feed chamber (such as the outer face of the capillary) may be lost, leading to loss of transmissivity (i.e. poor acoustic coupling) between the vibrator and the molten fuel in the feed chamber.

"Furthermore, piezoelectric vibrators may be brought to a temperature at or just below that of the molten fuel, because of direct contact with the outer wall of the feed chamber, and this may mean that the piezoelectric vibrators operate at a temperature above their Curie temperature, leading to low efficiency.

"Vibrators that are not of a piezoelectric nature may be unable to operate effectively at the high temperature required to maintain the fuel in a molten state.

"An aspect of embodiments of the invention, amongst others, is to provide apparatus and methods for generation of streams of fuel droplets for use in lithographic radiation sources and apparatus and methods for controlling fuel droplet size and separation in such streams which address or overcome at least some of the problems set out above. In particular, it is an aspect of embodiments of the invention to provide apparatus and methods which may be used to cause oscillatory stimulation of a fuel supply at a nozzle, wherein such apparatus and methods provide alternatives to prior art apparatus and methods, and enable efficient control of the breakup of a stream of molten fuel exiting the nozzle.

"Throughout this specification, the term 'comprising' or 'comprises' means including the component(s) specified but not to the exclusion the presence of others. The term 'consisting essentially of' or 'consists essentially of' means including the components specified but excludes other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention.

"Whenever appropriate, the use of the term 'comprises' or 'comprising' may also be taken to include the meaning 'consists essentially of' or 'consisting essentially of' and may also be taken to include the meaning of 'consists of' or 'consisting of'.

"Optional and/or preferred features as set out herein be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional and/or preferred features for each aspect of the invention set out herein are also applicable to any other aspects of the invention, where appropriate.

"An aspect of the invention provides a radiation source comprising a fuel droplet generator arranged to provide a stream of droplets of fuel and at least one laser configured to vaporize at least some of said droplets of fuel, whereby radiation is generated, wherein the fuel droplet generator comprises a nozzle, a feed chamber, a reservoir, and a pumping device arranged to supply a flow of fuel in molten state from the reservoir through the feed chamber and out of the nozzle as a stream of droplets, wherein the feed chamber has an outer face in contact with a drive cavity, wherein the drive cavity is filled with a liquid, and the liquid is arranged to be drivable to undergo oscillation by a vibrator operably connected to the drive cavity, and wherein said oscillation is transmissible to said molten fuel in the feed chamber from the outer face of the feed chamber through the liquid.

"An aspect of the invention provides a lithographic apparatus comprising a radiation source described above, arranged to generate a radiation beam, and further comprising an illumination system configured to condition the radiation beam, a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam, a substrate table constructed to hold a substrate, and a projection system configured to project the patterned radiation beam onto a target portion of said substrate.

"An aspect of the invention provides a method comprising emitting a stream of fuel droplets from a nozzle and using a laser to vaporize at least some of the droplets of fuel to generate radiation, wherein molten fuel is pumped from a reservoir, through a feed chamber and out through the nozzle as the stream of droplets, wherein the feed chamber has an outer face in contact with a first cavity, filled with a liquid, and wherein the first cavity is driven to undergo oscillation by a vibrator, and the oscillation is transmitted through the liquid and through the outer face of the feed chamber to the fuel in the feed chamber.

"The radiation source according to an aspect of the invention described above is particularly suitable for putting into effect the method of an aspect of the invention described above.

"The radiation source according to an aspect described above, and for use in the other aspects of the invention, comprises a fuel droplet generator arranged to provide a stream of droplets of fuel. At least one laser is configured to vaporize at least some of said droplets of fuel, whereby radiation is generated by the radiation source of the first aspect of the invention.

"It should be understood that the fuel droplet generator as described herein, which forms part of the radiation source of the first aspect of the invention, may be considered independently as an aspect of the invention in its own right. Hence, an aspect of the invention provides a fuel droplet generator for providing a stream of droplets of fuel for a radiation source, wherein the fuel droplet generator comprises a nozzle, a feed chamber, a reservoir, and a pumping device arranged to supply a flow of fuel in molten state from the reservoir through the feed chamber and out of the nozzle as a stream of droplets, wherein the feed chamber has an outer face in contact with a drive cavity, wherein the drive cavity is filled with a liquid, and the liquid is arranged to be drivable to undergo oscillation by a vibrator operably connected to the drive cavity, and wherein said oscillation is transmissible to said molten fuel in the feed chamber from the outer face of the feed chamber through the liquid.

"The radiation source of the invention will typically be configured to generate radiation such as EUV (extreme ultraviolet radiation). The EUV radiation may for example have a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.

"The fuel droplet generator may comprise a nozzle, a feed chamber, a reservoir, and a pumping device arranged to supply a flow of fuel in molten state from the reservoir through the feed chamber and out of the nozzle as a stream of droplets. The pumping device may simply be a pressure generator applied to the reservoir to force the fuel in its molten state from the reservoir through the feed chamber and out of the outlet orifice of the nozzle a stream of droplets.

"The feed chamber has an outer face in contact with a drive cavity. The drive cavity is filled with a liquid, and the liquid is arranged to be drivable to undergo oscillation by a vibrator operably connected to the drive cavity. The oscillation is transmissible in use to the molten fuel in the feed chamber, with the oscillation transmitted as acoustic waves from the outer face of the feed chamber and through the liquid.

"The feed chamber may have a first resonant frequency and the drive cavity may have a second resonant frequency.

"The drive cavity may suitably comprise a tuning device whereby the second resonant frequency of the drive cavity is variable.

"The drive cavity may comprise a first cavity in direct contact with the outer face of the feed chamber, wherein the first cavity is in fluidic connection with a second cavity through a bore of a connection tube, and wherein the second cavity has the vibrator operably connected thereto, wherein the first cavity, second cavity and connection tube are filled with the liquid, and wherein the first cavity is drivable to undergo oscillation by acoustic transmission of said oscillation from the second cavity through the liquid, through the connection tube.

"The connection tube is suitably a tube with a rigid wall or walls whereby acoustic energy is transmissible from the second to the first cavity through the connection tube.

"The second cavity may comprise a tuning device whereby the second resonant frequency of the drive cavity is variable. The tuning device may be, for instance, a device for adjustment of the volume of the second cavity.

"The vibrator may suitably be arranged to oscillate an outer wall of the second cavity to drive the liquid to undergo oscillation in use. Hence, the liquid in the second cavity is oscillated and the oscillation may be transmitted as acoustic energy through the connection tube to the first cavity to drive the molten fuel in the feed chamber.

"The fuel droplet generator may comprise a cooling device arranged to maintain the vibrator, in use, at a temperature lower than the temperature required to maintain said fuel in a molten state.

"For instance, the vibrator, in use, may be maintained at a temperature of 100.degree. C. or less, such as about 50.degree. C. or less, for example about 30.degree. C. or less.

"The cooling device may include a cooling device arranged to maintain the second cavity and vibrator, in use, at a temperature lower than the temperature required to maintain the fuel in a molten state. The cooling device may comprise a cooling chamber enclosing the second cavity and vibrator.

"The vibrator may be a piezoelectric actuator and the cooling device may arranged to maintain the piezoelectric actuator in use at a temperature lower than the Curie temperature of the piezoelectric actuator. Although a piezoelectric actuator may still be effective to oscillate and act as a vibrator at temperatures above its Curie temperature, piezoelectric materials are considerably more efficient when operated at a temperature below their Curie temperature.

"The vibrator may suitably be a piezoelectric actuator driven in bending mode.

"The feed chamber may be in direct fluid connection with the nozzle. For instance, the feed chamber may be a capillary tube and the nozzle may be a narrowing at a distal end of the capillary tube.

"The feed chamber is desirably substantially acoustically decoupled from the feed reservoir. For instance, the fuel may enter the feed chamber through a restriction arranged to minimise transmission of acoustic energy from the feed chamber to the reservoir, such as a restriction having a cross sectional area of less than 5.times.10.sup.-6 m.sup.2.

"The liquid may be maintained at a pressure sufficiently in excess of atmospheric pressure to inhibit cavitation of the liquid in use. Suitably the liquid is degassed. For instance, the liquid may be maintained at a pressure from 0.1 to 5 MPa in excess of atmospheric pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

"Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

"FIG. 1 schematically depicts a lithographic apparatus according to an embodiment of the invention;

"FIG. 2 is a more detailed view of the apparatus of FIG. 1, including a laser produced plasma (LPP) source collector module;

"FIG. 3 schematically depicts a first embodiment of a fuel droplet generator forming part of a radiation source according to an embodiment of the invention, which may form part of the lithographic apparatus shown in FIGS. 1 and 2; and

"FIG. 4 a second embodiment of a fuel droplet generator forming part of a radiation source according to an embodiment of the invention, which may form part of the lithographic apparatus shown in FIGS. 1 and 2."

For more information, see this patent application: Loopstra, Erik Roelof; Dijksman, Johan Frederik. Radiation Source and Method for Lithographic Apparatus and Device Manufacturing Method. Filed July 4, 2012 and posted June 19, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=4800&p=96&f=G&l=50&d=PG01&S1=20140612.PD.&OS=PD/20140612&RS=PD/20140612

Keywords for this news article include: ASML Netherlands B.V.

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