Systems and Methods for Cooling an Optic

Alexander L Ershov

Robert L. Morse

William N. Partlo

Christopher P. Pate

CROSS-REFERENCE TO RELATED APPIJCATIONS

This application claims die benefit of United States Provisional Patent Application Serial Number 61/324,702, riled on April 15, 2010, and United States Utility Application Serial Number 12/900,762, filed on October 8, 2010, the contents of which are hereby incorporated by reference herein.

FIELD

The present disclosure relates to extreme ultraviolet CEUV) light sources mat provide EUV light from a plasma that is created from a target material and collected and directed to an intermediate region for utilization outside of die EUV light source chamber, e.g., by a lithography scanner/stepper.

BACKGROUND

Extreme ultraviolet ("EUV") light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.

Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include heat, high energy ions and scattered debris from die plasma formation, e.g., atoms and/or clumps/microdroplets of source material that is not fully ionized in die plasma formation process.

These plasma formation by-products can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and / or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and die laser input window. The heat, high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission or interact with light to produce undesirable heat, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into mem.

For an LPP system, optics such as the laser beam focusing lens and/or laser beam input window may experience undesirable heat loading due to exposure to the high fluence, e.g. 150-325 W/cm ² from the relatively high powered drive laser, which may be, for example, a pulsed, focused lO-12kW CO ₂ drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000 - 200,000 tin droplets per second.

In some cases, the laser focusing lens may also function as a laser input window, forming a par of the vacuum chamber, and may be placed under considerable stress due to a pressure differential between the relatively high vacuum in the plasma chamber and the pressure, e.g. atmospheric pressure, outside the plasma chamber. For these windows, deposits and heat can combine to fracture (i.e., crack) the element resulting in a loss of vacuum and requiring a costly repair. To accommodate this stress and prevent fracture, laser input windows have generally been rather thick, and, as a consequence, are subject to thermal lensing. This thermal lensing, in turn, can reduce the ability to properly steer and focus a laser beam to a desired location within the plasma chamber. For example, for use in some LPP EUV light sources, it is contemplated that a lens or mirror be used to accurately focus a high power laser beam to a focal spot sized to irradiate droplets having a diameter between about 10- 100μm.

With the above in mind, Applicants disclose systems and methods for cooling an optic.

SUMMARY

In a first aspect, a device is described herein which may comprise an optic having a non-planar surface, the non-planar surface having an optically active portion; and a flow guide directing gas upon the non-planar surface to produce turbulent flow on at least a portion of the optically active portion of the non-planar surface to cool the optic.

In one embodiment, turbulent flow may be produced on at least seventy-five percent of the optically active portion of the non-planar surface.

In a particular embodiment, the flow guide may comprise a ring having a plurality of nozzles each nozzle directing gas toward the optic. In some arrangements, each nozzle may be a diverging nozzle and in one implementation, the flow guide may have at least six uniformly spaced nozzles. In a particular arrangement, the ring may be formed with a single gas flow inlet and an internal passageway connecting the inlet to each nozzle.

In a particular implementation, the nozzles may be positioned and arranged to flow gas along a spiral path from a peripheral region of the optically active portion to a central region of the optically active portion.

In one embodiment, the flow guide may be made of a thermally conductive material and a thermally conductive spacer may be positioned between the optic and ring to conduct heat from the optic to the ring.

In one setup, the optic may be a lens and the optically active surface portion may be convex. In one arrangement, a first nozzle may produce an output jet oriented to disturb an output jet from a second nozzle, and in a particular arrangement, a first nozzle may produce an output jet oriented to disturb an output jet from a second nozzle at a peripheral region of an optically active portion of the optic.

In another aspect, a device is described herein which may comprise an optic; a laser irradiating the optic; and a flow guide having a plurality of nozzles arranged to direct gas on the optic during irradiation of the optic to produce turbulent flow on at least a portion of the optic.

In one embodiment of this aspect, the flow guide may comprise a ring, In one arrangement of this aspect, each nozzle may be a diverging nozzle.

In a particular embodiment, the flow guide may have at least six uniformly spaced nozzles.

In one embodiment, the nozzles may be positioned and arranged to flow gas along a spiral path from a peripheral region of an optically active portion of the optic to a central region of the optically active portion.

In a particular arrangement, a first nozzle may produce an output jet oriented to disturb an output jet from a second nozzle at a peripheral region of an optically active portion of the optic.

In another aspect, a device is described herein which may comprise the steps of: providing an optic; irradiating the optic with a laser beam; and directing a plurality of gas jets on the optic during the irradiating step to produce turbulent flow on at least a portion of the optic.

In one implementation of the method, the optic may be a lens.

In a particular implementation, the method may include the steps of: producing a first output jet at a peripheral region of an optically active portion of the optic; and producing a first output jet oriented to disturb the first output jet BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a simplified, schematic view of an embodiment of an EUV light source;

Fig. 2 shows a cross-section through an example of a focusing unit;

Fig. 3 shows a perspective view of an example of a flow guide;

Fig. 4 shows a top view of an example of a flow guide;

Fig. 5 shows a top view of an example of a flow guide and illustrates a spiral flow pattern; and

Fig. 6 shows a cross-section through an example of a flow guide.

DETAILED DESCRIPTION

With initial reference to Fig. 1, there is shown a simplified, schematic view of an embodiment of an EUV light source, e.g., a laser-produced-plasma EUV light source 20. As shown in Fig. 1, the LPP light source 20 may include a system 22 for generating a train of light pulses and delivering the light pulses into a chamber 26. For the source 20, each light pulse may travel along a beam path from the system 22 and into the chamber 26 to illuminate a respective target droplet at an irradiation region 28.

Suitable lasers for use in the system 22 shown in Fig. 1, may include a pulsed laser device, e.g., a pulsed gas discharge CO ₂ laser device producing radiation at 9.3μηι or 10.6μιη, e.g., with DC or RF excitation, operating at relatively high power, e.g., lOkW or higher and high pulse repetition rate, e.g., 50kHz or more. In one particular implementation, the laser may be an axial-flow RF-pumped CO ₂ laser having an oscillator-amplifier configuration (e.g. master oscillator / power amplifier (MOPA) or power oscillator / power amplifier (POP A)) with multiple stages of amplification and having a seed pulse mat is initiated by a Q-switched oscillator with relatively low energy and high repetition rate, e.g., capable of 100 kHz operation. From the oscillator, the laser pulse may then be amplified, shaped and / or focused before reaching the irradiation region 28, Continuously pumped CO2 amplifiers may be used for the system 22. For example, a suitable CO2 laser device having an oscillator and three amplifiers (O-PAl -PA2-PA3 configuration) is disclosed in U.S. Patent Application Serial Number 11/174,299 filed on June 29, 2005, entitled, LPP EUV LIGHT SOURCE DRIVE LASER SYSTEM, Attorney Docket Number 2005-0044-01, now U.S. Patent No. 7,439,530, issued on October 21 , 2008, the entire contents of which are hereby incorporated by reference herein. Alternatively, the laser may be configured as a so-called "self- targeting" laser system in which the droplet serves as one mirror of the optical cavity. In some "self-targeting" arrangements, an oscillator may not be required. Self-targeting laser systems are disclosed and claimed in U.S. Patent Application Serial Number 11/580,414 tiled on October 13, 2006, entitled, DRIVE LASER DELIVERY SYSTEMS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0025-01, now U.S. Patent No. 7,491,954, issued on February 17, 2009, the entire contents of which are hereby incorporated by reference herein.

Depending on the application, other types of lasers may also be suitable, e.g., an excfmer or molecular fluorine laser operating at high power and high pulse repetition rate. Other examples include, a solid state laser, e.g., having a fiber, rod, slab or disk-shaped active media, other laser architectures having one or more chambers, e.g., an oscillator chamber and one or more amplifying chambers (with the amplifying chambers in parallel or in series), a master oscillator/power oscillator (MOPO) arrangement, a maste oscillator/power ring amplifier (MOPRA) arrangement, or a solid state laser that seeds one or more excimer, molecular fluorine or CO ₂ amplifier or oscillator chambers, may be suitable. Other designs may be suitable.

As further shown in Fig. 1, the EUV light source 20 may also include a target material delivery system 24 _» e.g., delivering droplets of a target material into th interior of a chamber 26 to the irradiation region 28, where the droplets will interact with one or more light pulses, e.g., zero, one or more pre-pulses and thereafter one or more main pulses, to ultimately produce plasma and generate an EUV emission. More details regarding various droplet dispenser configurations and their relative advantages may be found in United States Serial Number 12 214,736, filed on June 19, 2008, entitled SYSTEMS AND METHODS FOR TARGET MATERIAL DELIVERY IN A LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket No. 2006-0067-02; U.S. Patent Application Serial Number 11/827,803, filed on July 13, 2007, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE HAVING A DROPLET STREAM PRODUCED USING A MODULATED DISTURBANCE WAVE, Attorney Docket Number 2007-0030-01; U.S. Patent Application Serial Number 11/358,988, filed on February 21, 2006, entitled LASER PRODUCED PLASMA EUV LIGHT SOURCE WITH PRE-PULSE, Attorney Docket Number 200S-008S-01, and published on November 16, 2006 as US200670255298A-1; U.S. Patent Application Serial Number 11/067,124, filed on February 25, 2005, entitled METHOD AND APPARATUS FOR EUV PLASMA SOURCE TARGET DELIVERY, Attorney Docket Number 2004-0008-01 ; now U.S. Patent 7,405,416, issued on July 29, 2008; and U.S. Patent Application Serial Number 11/174,443, filed on June 29, 2005, entitled LPP EUV PLASMA SOURCE MATERIAL TARGET DELIVERY SYSTEM, Attorney Docket Number 2005-0003-01, now U.S. Patent 7,372,056, issued on May 13, 2008; the contents of each of which are hereby incorporated by reference.

The target materia! may include, but is not necessarily limited to, a material that includes tin, lithium, xenon or combinations thereof. The EUV emitting element, e.g., tin, lithium, xenon, etc., may be in the form of liquid droplets and/Or solid particles contained within liquid droplets. For example, the element tin may be used as pure tin, as a tin compound, e.g., SnBr ₄ , SnB ₂ ¾, SnH ₄ , as a tin alloy, e.g., tin- gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or a combination thereof. Depending on the material used, the target material may be presented to the irradiation region 28 at various temperatures including room temperature or near room temperature (e.g., tin alloys, SnBr ₄ ), at an elevated temperature, (e.g., pure tin) or at temperatures below room temperature, (e.g., Sn¾), and in some cases, can be relatively volatile, e.g., SnBR ₄ . More details concerning the use of these materials in an LPP EUV light source is provided in U.S. Patent Application Serial Number 11/406,216, filed on April 17, 2006* entitled ALTERNATIVE FUELS FOR EUV LIGHT SOURCE, Attorney Docket Number 2006-0003-01, now U.S. Patent 7,465,946, issued on December 16, 2008, the contents of which are hereby incorporated by reference herein. Continuing with Fig. I, the EUV light source 20 may also include an optic 30, e.g., a near-normal incidence collector mirror having a reflective surface in the form of a prolate spheroid (i.e., an ellipse rotated about its major axis) having, e.g., a graded multi-layer coating with alternating layers of Molybdenum and Silicon, and in some cases, one or more high temperature diffusion barrier layers, smoothing layers, capping layers and/or etch stop layers. Fig. 1 shows that the optic 30 may be formed with an aperture to allow the light pulses generated by the system 22 to pass through and reach the irradiation region 28. As shown, the optic 30 may be, e.g., a prolate spheroid mirror that has a first focus within or near the irradiation region 28 and a second focus at a so-called intermediate region 40, where the EUV light may be output from the EUV light source 20 and input to a device utilizing EUV light, e.g., an integrated circuit lithography tool (not shown), It is to be appreciated that other optics may be used in place of the prolate spheroid mirror for collecting and directing light to an intermediate location for subsequent delivery to a device utilizing EUV light, for example, the optic may be a parabola rotated about its major axis or may be configured to deliver a beam having a ring-shaped cross-section to an intermediate location, see e.g., U.S. Patent Application Serial Number 11/505,177, filed on August 16, 2006, published on February 21, 2008, as U.S. 2008/0043321 At, entitled EUV OPTICS, Attorney Docket Number 2006-0027-01, the contents of which are hereby incorporated by reference.

Fig. 1 also shows that the source 20 may include a beam conditioning unit 42 having one or more optics for expanding, steering, pulse shaping and / or shaping the beam between the system 22 and a focusing unit 44. Further details regarding beam conditioning are provided in, see e.g., U.S. Patent Application Serial Number 10/803,526, filed on March 17, 2004, entitled A HIGH REPETITION RATE LASER PRODUCED PLASMA EUV LIGHT SOURCE, Attorney Docket No. 2003-0125-01, now U.S. Patent 7,087,914, issued on August 8, 2006; and/or United States Serial Number 10 900,839 filed on July 27, 2004, entitled EUV LIGHT SOURCE, Attorney Docket No. 2004-0044-01, now U.S. Patent 7,164,144, issued on January 16, 2007, and U.S. Patent Application Serial No. 12 638,092, filed on December 15, 2009, entitled BEAM TRANSPORT SYSTEM FOR EXTREME ULTRAVIOLET LIGHT SOURCE, Attorney Docket No. 2009-0029-01, the contents of each of which are hereby incorporated by reference.

For the source 22, the focusing unit 44 may include one or more optics for focusing a beam to a focal spot at the irradiation site.

As used herein, the term "optic" and its derivatives includes, but is not necessarily limited to, one or more components which reflect and/or transmit and/or operate on incident light and includes, but is not limited to, one or more lenses, windows, filters, wedges, prisms, grisms, gradings, transmission fibers, etaJons, difrusers, homogenizers, detectors and other instrument components, input apertures, axicons and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence minors, specular reflectors, diffuse reflectors and combinations thereof. Moreover, unless otherwise specified, neither the term "optic" nor its derivatives, as used herein, are meant to be limited to components which operate solely or to advantage within one or more specific wavelength range(s) such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or some other wavelength.

As further shown in Fig. 1, a suitable focusing arrangement of unit 44 may include a convex lens 46 and a system 48 for cooling the lens 46. As shown, for this arrangement, the lens 46 serves to focus the beam to a focal spot at the irradiation region 28 and as a laser input window for the chamber 26. The system 48 may also advantageously cool structural components around the lens, e.g. housings, mounting hardware, etc., for example, by turbulent forced convection.

Fig. 2 is a cross-section through an example of a focusing unit 44, As shown, the focusing unit 44 may include an optic, which for the setup s own may be a 177mm lens 46 made of optical grade ZnSe and having an optically active portion on the cooled surface, e.g. clear aperture ISO, of about 135mm. As further shown, the lens 46 may be formed with a non-planar surface 50, which for the lens 46 shown, is convex. With this arrangement, a beam having a diameter of about 120mm and travelling in the direction of arrow 51 can be comfortably focused.

As best seen with cross-reference to Fig. 2 and Fig, 3, the focusing unit 44 may also include a system 48 for cooling the lens 46 having a gas source 52, mass flow controller 54 and flow guide 56. For this arrangement, the gas source may be a compressed gas cylinder containing, for example, Nitrogen or Argon or another suitable gas or gas mixture.

For the system 48, flow guide 52 may be provided to direct a flow of gas onto the surface 50 to produce turbulent flow on a substantial portion of the optically active portion of the surface 50 to cool the optic 46. Having uniform cooling on the lens minimizes focal shift due to temperature gradients, thermal expansion and change of optical index. Moreover, heat transfer is maximized by inducing a turbulent region of moving gas at the surface of the lens.

In more detail, the flow guide 52 may be made of a conductive material, e.g., metal such as 6061 Aluminum, and may include a ring portion 58 having an annular, inside facing, surface 60 formed with a plurality of nozzles 62. A conductive spacer 75 may be provided (separate from or integral with the ring) to conduct heat from the optic 46 to the ring portion 58. For the arrangement shown, the flow guide 52 includes ten nozzles 62, however, it is to be appreciated mat more than ten, and as few as one nozzle, may be used to produce turbulent flow on a substantial portion of the optically active portion of the surface 50.

Fig. 4 shows mat the nozzles 62 may be uniformly spaced around the periphery of the ring 58. Figs. 4 and 6 also illustrate mat each nozzle 62 may be a diverging nozzle, increasing in effective diameter from nozzle entrance to nozzle exit. Fig. 6 shows the divergence half angle, Θ, which may be about 15 degrees, for the example flow guide shown, and also shows that each nozzle may expand from an initial diameter of about 1.0mm to a final diameter of about 3.4mm. In other arrangements, a divergence half angle, Θ, in the range of about 0-20 degrees may be suitable. Figs. 4 and 5 also show that the nozzles 62 may be positioned and arranged to produce a gas flow wherein gas flows along a spiral path (as indicated by arrows 68a-e from a peripheral region of said optically active portion, to a central region 70 of said optically active portion. Figs. 5 and 6 show a suitable orientation of a nozzle axis relative to the plane of the ring, a, and the ring axis 80, β. For the arrangement shown, a is about 17 degrees and the β is about 38 degrees. In other arrangements, an orientation of a nozzle axis relative to the plane of the ring, a, may b in the range of 0-90 degrees and an orientation of a nozzle axis relative to the ring axis 80, β may be in the range of 0-90 degrees. It can further be seen that the ring 58 may be formed with a gas flow inlet 100 and an internal passageway 102, formed in the ring 58 that extends around the ring 58 connecting inlet 100 to each nozzle 62. For the ring 58, the passageway diameter may be sized larger, eg, two times larger or more, than each nozzle's effective minimum diameter to ensure mat each nozzle receives approximately the same nozzle input gas pressure. With this arrangement, all nozzles may be adequately supplied with gas from a single input line with each nozzle producing a similar output jet

With the arrangement that is described above, Fig.4 shows that the output jet from nozzle 62f meets and disturbs the output jet from adjacent nozzle 62g at point 152 which is near the periphery of the clear aperture 150. More specifically, the jet from nozzle 62p meets the jet from adjacent nozzle 62g near the output of nozzle 62g, e.g., within 1/3, or in some cases _* within ¼ of a lens/mirror diameter D from the periphery of the lens/mirror. Thus, with this arrangement, turbulent flow may be obtained near the periphery of the clear aperture 150. The interactions between the gas jets may advantageously cause an early transition to turbulent flow. The size, position, angle and spacing of the jets may be adjusted to enhance the transition to turbulent flow and may provide uniform cooling at the same time.

Note also, as shown in Fig. 5, the gas flows generally from the periphery to a center region in a generally spiral pattern, and this flow in the spiral pattern is also turbulent since it is downstream of the ten jet disturbance points 152 (of which only one is shown). In another arrangement (not shown) the nozzles may be arranged such that adjacent jet collisions are located either on or outside of the clear aperture of the lens or mirror.

In practice, the number, size and orientation of the nozzles may depend on the size and shape of the optic to be cooled and the heat load (e.g., fluence). Typically, a computation fluid dynamics (CFD) program such as Ansys CSX may be employed to verify and / or optimize a specific nozzle design. EXAMPLE

The setup described above was exposed to a 10,6um light at a fluence of about 177 W / cm ² and 100% duty cycle for a 4 second burs length with a mass flow rate of Nitrogen of 20 liters min entering the ring, During exposure, the lens was sufficiently cooled to substantially focus the beam at a focal spot without distortion as compared to the focusing of a single pulse.

While the particular embodiments) described and illustrated in this patent application in the detail required to satisfy 35 U.S.C. §112, are fully capable of attaining one or more of the above-described purposes for, problems to be solved by, or any other reasons for, or objects of the embodiments) described above, it is to be understood by those skilled in the art that the above-described embodiments) are merely exemplary, illustrative and representative of the subject matter which is broadly contemplated by the present application. Reference to an element in the following Claims in the singular, is not intended to mean, nor shall it mean in interpreting such Claim element "one and only one" unless explicitly so stated, but rather "one or more". All structural and functional equivalents to any of the elements Of the above-described embodiments) that are known, or later come to be known to those of ordinary skill in the art, are expressly incorporated herein by reference and are intended to be encompassed by the present Claims. Any term used in the Specification and/or in the Claims, and expressly given a meaning in the Specification and/or Claims in the present Application, shall have that meaning, regardless of any dictionary or other commonly used meaning for such a term. It is not intended or necessary for a device or method discussed in the Specification as a embodiment, to address or solve each and every problem discussed in this Application, for it to be encompassed by the present Claims. No element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the Claims. No claim element in the appended Claims is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase "means for" or, in the case of a method claim, the element is recited as a "step" instead of an "act".