(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2004/0183031 A1
`Silverman et al.
`(43) Pub. Date:
`Sep. 23, 2004
`
`US 20040183031A1
`
`(54) DUAL HEMISPHERICAL COLLECTORS
`
`Publication Classification
`
`(75)
`
`Inventors: Peter J. Silverman, Palo Alto (CA);
`Michael Goldstein, Ridgefield, CT
`(US)
`
`Int. Cl.7 ................................................... .. G03F 7/207
`(51)
`(52) U.S. Cl.
`................................. .. 250/492.2; 250/504 R
`
`Correspondence Address:
`FISH & RICHARDSON, PC
`12390 EL CAMINO REAL
`SAN DIEGO, CA 92130_2081 (US)
`
`(73) Assignee;
`
`[me] Corporation
`
`(21) Appl, No;
`
`10/394,412
`
`(22)
`
`Filed:
`
`Mar. 20, 2003
`
`(57)
`
`ABSTRACT
`
`A system and method for collecting radiation, which may be
`used in a lithography illumination system. The system
`comprises a first surface shaped to reflect radiation in a first
`hemisphere of a source to illuminate in a second hemisphere
`of the source; and a second surface shaped to reflect radia-
`tion in the second hemisphere of the source to an output
`plane.
`
`102
`
`ASML 1415
`
`ASML 1415
`
`

`
`Patent Application Publication Sep. 23, 2004 Sheet 1 of 6
`
`US 2004/0183031 A1
`
`FIG. 1
`
`

`
`Patent Application Publication Sep. 23, 2004 Sheet 2 of 6
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`US 2004/0183031 A1
`
`FIG.
`
`

`
`Patent Application Publication Sep. 23, 2004 Sheet 3 of 6
`
`US 2004/0183031 A1
`
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`
`Patent Application Publication Sep. 23, 2004 Sheet 4 of 6
`
`US 2004/0183031 A1
`
`

`
`Patent Application Publication Sep. 23, 2004 Sheet 5 of 6
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`US 2004/0183031 A1
`
`
`
`

`
`Patent Application Publication Sep. 23, 2004 Sheet 6 of 6
`
`US 2004/0183031 A1
`
`PROVIDING A FIRST REFLECTIVE SURFACE IN A FIRST
`HEMISPHERE OF A RADIATION SOURCE AND A SECOND
`
`REFLECTIVE SURFACE IN A SECOND HEMISPHERE OF THE
`RADIATION SOURCE
`
`704
`
`REFLECTING RADIATION FROM THE SOURCE IN THE SECOND
`HEMISPHERE TO A PRE-DETERMINED PLANE.
`
`700
`
`02
`
`REFLECTING RADIATION FROM THE SOURCE IN THE FIRST
`
`HEMISPHERE TO THE SECOND HEMISPHERE
`
`FIG. 7
`
`

`
`US 2004/0183031 A1
`
`Sep. 23, 2004
`
`DUAL HEMISPHERICAL COLLECTORS
`
`BACKGROUND
`
`[0001] A microchip manufacturing process may deposit
`various material layers on a wafer and a photosensitive film
`or photoresist on the deposited layers. The process may use
`lithography to transmit light through transmissive optics or
`reflect light from reflective optics and a reticle or patterned
`mask onto the photoresist, which transfers a patterned image
`onto the photoresist. Aprocess may remove portions of the
`photoresist that are exposed to light. A process may etch
`portions of the wafer which are not protected by the remain-
`ing photorcsist. Some of these actions may be rcpcatcd.
`
`[0002] Extreme ultraviolet (EUV) is one form of lithog-
`raphy. AEUV lithography tool may be used to print a pattern
`on a photoresist with dimensions that are smaller than
`dimensions achieved by other lithography tools.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0003] FIG. 1 illustrates one embodiment of an Extreme
`Ultraviolet (EUV) lithography tool.
`
`[0004] FIG. 2 is a cross-sectional side view of an appa-
`ratus which may be used in a Dense Plasma Focus (DPF)
`electric discharge source.
`
`[0005] FIG. 3 is a cross-sectional view of a dual hemi-
`spherical collector system, which may be used in a lithog-
`raphy illumination system.
`FIG. 4 illustrates a table of values associated with
`
`[0006]
`FIG. 3.
`
`[0007] FIG. 5 is a cross-sectional view of another embodi-
`ment of a dual hemispherical collector system, which may
`be used in a lithography illumination system.
`
`[0008] FIG. 6 illustrates an example of a hyperbola and
`ellipse for a shape of the second collector in FIG. 5.
`
`[0009] FIG. 7 illustrates a method of using a dual hemi-
`spherical collector system.
`
`DETAILED DESCRIPTION
`
`[0010] Dual hemispherical collectors are described herein
`with lithography systems, but the dual hemispherical col-
`lectors may be used with other light sources for car head-
`lights, movie/video/slide projectors and other applications
`outside of lithography.
`
`[0014] The EUV lithography tool 100 may create plasma
`by using a laser-produced plasma (LPP) source 104 or an
`electric discharge source 200. A laser-produced plasma
`(LPP) source 104 produces plasma by focusing a laser 102
`onto a gas, liquid, or filament jet of a material or materials,
`such as Xenon.
`
`[0015] An clcctric dischargc sourcc produccs plasma by
`pulsing a current discharge
`(like a powerful arc
`welder) through a gas, such as Xenon. The plasma emits
`visible and EUV radiation. Excitation of the Xenon mol-
`ecules causes the electrons to transition between their shells
`
`to produce EUV photon radiation.
`
`[0016] The EUV source 104 may produce radiation (pho-
`tons) with a very short wavelength, such as about 13
`nanometers. In other embodiments, the photons may have
`other wavelengths.
`
`[0017] As an example, it may be desirable for the EUV
`lithography tool 100 to deliver about 50-120 watts of
`“clean” power to projection optics for a throughput of 80
`wafers per hour.
`
`[0018] Single Hemisphere Radiation Collection
`
`[0019] FIG. 2 illustrates an example of radiation collec-
`tors 232 in a single hemisphere. FIG. 2 is a cross-sectional
`side view of an apparatus 200 which may be used in a Dense
`Plasma Focus (DPF) electric discharge source. The appara-
`tus 200 includes an anode 208, cathode 202 and insulator
`212. The apparatus 200 may be used with a buffer gas 206,
`such as Helium, a source gas 214, such as Xenon, a foil trap
`230, a grazing incidence collector 232 and a pump 234. The
`anode 208 may be coupled to a high voltage source 216, and
`the cathode 202 may be grounded.
`
`[0020] Extreme ultraviolet (EUV) light sources, particu-
`larly laser produced plasma EUV sources, may have a finite
`power output that is radiated into a large solid angle of up
`to about 4H. Laser plasma sources tend to be very small,
`e.g., about 300 microns in diameter. Due to their small size,
`a laser plasma source may be modeled as a quasi-point
`source.
`
`[0021] The ability to collect the large solid angle light has
`been limited by several factors, including mirror fabrication,
`coating requirements, and the large solid angle itself. Cur-
`rently, near-normal incidence collectors or grazing incidence
`collectors try to resolve these challenges to collect light. But
`the collectors are only in a single hemisphere, as shown in
`FIGS. 1 and 2.
`
`[0011] EUV Lithography
`
`[0022] Dual Hemisphere Radiation Collection
`
`[0012] FIG. 1 illustrates one embodiment of an Extreme
`Ultraviolet (EUV) lithography tool 100, which may also be
`called a “lithographic exposure system” or a “EUV scanner.”
`The lithography tool 100 may include a laser 102, a laser
`produced plasma (LPP) source 104, a plurality of condenser
`optics 106, a reflective reticle 107 with a pattern, and a
`plurality of reflective reduction optics 108.
`
`[0023] The above challenges may be addressed by using
`collectors in both hemispheres (e.g., 320 and 330 in FIG. 3)
`around the radiation source/object (e.g., 306), as described
`with reference to FIGS. 3-7. Dual hemisphere radiation
`collectors may be implemented with a laser produced
`plasma (LPP) source 104 of FIG. 1 or any other light source
`that radiates light into a large solid angle up to about 411.
`
`[0013] Other embodiments of the EUV lithography tool
`100 may include other components instead of or in addition
`to the components shown in FIG. 1. For example, instead of
`a laser produced plasma source 104, the lithography tool 100
`may have an electric discharge EUV source 200, as shown
`in FIG. 2.
`
`[0024] FIG. 3 is a cross-sectional view of a dual hemi-
`spherical collector system 300, which may be used in a
`lithography illumination system, such as the system 100 of
`FIG. 1. A radiation source 306 in FIG. 3 emits forward
`radiation 312 in one hemisphere 320 and backward radiation
`310 in another hemisphere 330. The backward radiation 310
`
`

`
`US 2004/0183031 A1
`
`Sep. 23, 2004
`
`may be reflected by a first collector (C1) 309 (also called a
`“surface,”“mirror” or “condenser”) at near normal
`inci-
`dence, i.e., almost perpendicular, e.g., 75-89 degrees. The
`first collector 309 may have a spherical or aspherical shape,
`i.e., elliptic or other conics.
`
`[0025] A second collector 302 may have a grazing inci-
`dence, e.g., less then about 60 degrees, and may reflect
`forward directed radiation 312 from the source 306. Side
`
`302B is a continuous bottom part of the second collector
`302A. The second collector 302 may have an aspherical
`shape, e.g., elliptic.
`
`[0026] Light from both hemispheres 320 and 330 around
`the source 306 may be focused with a single reflection by the
`collectors C1, C2309, 302 towards an output point or x-z
`plane 308 with a “z” axis into the page and an “x” horizontal
`axis. The collectors 302, 309 may be shaped, sized and
`positioned to reflect light from the source 306 to the output
`point or plane 308. Examples of values for FIG. 3 may be
`expressed as follows. For the first collector (C1) 309:
`
`[0027] NA1
`mm,
`
`(Numerical Aperture)=0.75, L1=100
`
`[0028]
`
`theta1=sin‘1 (NA1)eH1=113.389 mm.
`
`[0029]
`
`1/L1+1/(a+c)=2/rl
`
`[0030] where “a” and “c” are variables to solve for an
`elliptic collector in FIGS. 3 and 4. “a” is a major
`axis of the ellipse 400 in FIG. 4. “b” is a minor axis
`of the ellipse 400. “c” is a focal point of the ellipse
`400 from the origin. rl is the radius of curvature of
`the aspherical collector C1309. r2 is the radius of
`curvature of the ellipse 400 at point “a” in FIG. 4. If
`L1, “a” and “c” are known, then the equation above
`may be solved for rl.
`
`[0031] For the second collector (C2) 302:
`
`[0032]
`
`(2c—L4)tan [sinl (NA2)]=
`
`[0033]
`L4))
`
`(b/a)(sqr rt of (a—c+L4))(sqr rt of (2a—a+c—
`
`[0034]
`
`c2=a2—b2 and
`
`[0035]
`
`2(sqr rt of (c2+b2))=
`
`[0036]
`L.»
`
`(sqr rt of (H42+(2c—L4)2))+(sqr rt of (H42+
`
`[0037] with L4=400, NA2=0.25/4=0.0625,
`
`[0038]
`
`theta4+H4
`
`[0039] FIG. 4 illustrates a table of values for theta4 and a,
`b, c, c/a, r2, f1=a—c and r1. FIG. 4 also illustrates equations
`for an ellipse 400:
`z=a(1—5qr rt of (1—x2/b2))
`x=(b/a)(5qr rt of z)(5qr rt of (2a—z))
`z=(x2/r2)/(1+5qr rt of (1—(1+k)(x/r2) 2))
`
`[0040] FIG. 5 is a cross-sectional side view of another
`embodiment of a dual hemispherical collector system 500,
`which may be used in a lithography illumination system. In
`FIG. 5, a light source 506 generates light 510 backward in
`a first hemisphere 530 to a first collector (C1) 509 (also
`called “surface,”“mirror” or “condenser”) and generates
`light 512 forward to a set of second collectors (C2) 502, 504,
`506.
`
`[0041] The first collector 509 may have a spherical or
`aspherical shape, i.e., elliptic or other conics as well. The
`first collector 509 focuses light 510 back through a region
`around the source 506, and the light undergoes reflections
`from the set of second collectors (C2) 502, 504, 506. Thus,
`light 510 from the backward direction undergoes reflections
`from a near normal incidence collector 509 and then grazing
`incidence collectors 502, 504, 506.
`
`[0042] For the first collector (C1) 509:
`1/L1+1/L2=2/r1—>r=L1
`
`[0043] FIG. 5 also shows the cross-sectional tops 502A,
`504A, 506A and bottoms 502B, 504B, 506B of the set of
`mirrors or grazing incidence collectors 502, 504, 506. The
`collectors 502, 504, 506 may form a rotation about the optic
`axis 516. From point 508 looking back at the source 506, the
`collectors 502, 504, 506 may appear to be three concentric
`rings (having the same center). The collectors 502, 504, 506
`may be symmetric. The collectors 502, 504, 506 may be held
`together by one or more spokes 514.
`
`[0044] Each collector 502, 504, 506 may be a “Wolter”
`collector, which may be known to those of ordinary skill in
`the art. For example, the collector 502 may have a first part
`503B closer to the source 506 that is generally hyperbolic in
`shape and a second part 503A farther from the source 506
`that is generally elliptic in shape. The collectors 502, 504,
`506 collect and reflect forward directed radiation 512 to a
`
`point or plane 508.
`
`[0045] A Wolter collector includes two conic mirror seg-
`ments 503A, 503B used to form a grazing incidence imaging
`optic. The total reflectance loss for small and similar angles
`of incidence on the two mirrors may be approximately
`equivalent to the loss from a single larger reflection angle
`design.
`
`[0046] FIG. 6 illustrates an example of a hyperbola 602
`and ellipse 600 for modeling a shape of the Wolter collector
`502. The hyperbola 602 may have a real focus (focal point)
`at fl and a virtual focus at f2. The source 506 in FIG. 5 may
`be positioned at the real focus fl of the hyperbola 602. The
`ellipse 600 may have a first focus at B and a second focus
`at f3. Thus, the first focus of the ellipse 600 is at the virtual
`or second focus B of the hyperbola 602. The output point or
`image plane 508 may be positioned at f3, which is the
`second focus f3 of the ellipse 600.
`
`[0047] A light beam 604 from the source 506 reflects off
`the collector 502A to the point/plane 508. Another light
`beam 606 reflects off the hyperbolic part 503B and then the
`elliptic part 503A of the collector 502A to the point/plane
`508.
`
`[0048] The collector systems 300, 500 collect light in a
`large solid angle from quasi point sources 306, 506, such as
`a laser produced plasma source used in extreme ultraviolet
`(EUV) lithography. Common key aspects of the systems
`300, 500 may include:
`[0049]
`1. Acollector system 300, 500 composed of reflec-
`tive surfaces 302, 304, 309 and 502, 504, 509 in both
`forward and backward hemispheres 320, 330 and 520, 530
`around an object or light source 306 and 506.
`
`2. Reflective surfaces 302, 304, 309 and 502, 504,
`[0050]
`509 of the systems 300, 500 may be arranged to project light
`from the sources 306, 506 onto a point or a plane 308, 508
`along the x-z axes.
`
`

`
`US 2004/0183031 A1
`
`Sep. 23, 2004
`
`[0051] The systems 300, 500 may have light collection
`subtending a solid angle greater then 2H in lithography
`illumination. Each mirror may be configured and coated
`separately.
`
`[0052] Using both hemispheres 320, 330, 520, 530 may
`achieve a larger collection angle and more collected radia-
`tion.
`
`[0053] FIG. 7 illustrates a method of using a dual hemi-
`spherical collector system. The method includes providing a
`first reflective surface in a first hemisphere of a radiation
`source and a second reflective surface in a second hemi-
`
`sphere of the radiation source at 700; reflecting radiation
`from the source in the first hemisphere to the second
`hemisphere at 702; and reflecting radiation from the source
`in the second hemisphere to a pre-determined plane at 704.
`
`[0054] A number of embodiments have been described.
`Nevertheless, it will be understood that various modifica-
`tions may be made without departing from the spirit and
`scope of the application. Accordingly, other embodiments
`are within the scope of the following claims.
`
`1. An apparatus comprising:
`
`a first surface in a first hemisphere of a radiation source,
`the first surface reflecting radiation from the radiation
`source to a second hemisphere of the radiation source;
`and
`
`a second surface in the second hemisphere of the source,
`the second surface reflecting radiation in the second
`hemisphere to an output plane.
`2. The apparatus of claim 1, wherein the source comprises
`an extreme ultraviolet (EUV) radiation source.
`3. The apparatus of claim 1, wherein the source comprises
`a laser produced plasma (LPP) source.
`4. The apparatus of claim 1, wherein the first surface
`comprises one or more mirrors.
`5. The apparatus of claim 1, wherein the first surface is
`shaped to reflect backward radiation from the source.
`6. The apparatus of claim 1, wherein the first surface is
`shaped as a portion of a sphere.
`7. The apparatus of claim 1, wherein the first surface has
`an aspheric shape.
`8. The apparatus of claim 1, wherein the first surface has
`an elliptic shape.
`9. The apparatus of claim 1, wherein the first surface has
`a conic shape.
`10. The apparatus of claim 1, wherein the first surface is
`positioned to receive radiation from the source at an inci-
`dence of about 75 to 89 degrees.
`11. The apparatus of claim 1, wherein the first surface is
`configured to reflect radiation from the source through a
`region near the source to the second surface.
`12. The apparatus of claim 1, wherein the second surface
`comprises at least one mirror.
`13. The apparatus of claim 1, wherein the second surface
`has an elliptic shape.
`14. The apparatus of claim 1, wherein the second surface
`is shaped to reflect forward radiation from the source.
`15. The apparatus of claim 1, wherein the second surface
`is adapted to reflect radiation from the first surface to the
`output plane.
`
`16. The apparatus of claim 1, wherein the second surface
`is configured to receive radiation from the source at an
`incidence of less then about 60 degrees.
`17. The apparatus of claim 1, wherein the second surface
`comprises a first part closer to the source and a second part
`farther from the source,
`the first part being shaped as a
`portion of a hyperbola, the second part being shaped as a
`portion of an ellipse.
`18. The apparatus of claim 17, wherein the hyperbola has
`a real focus near the radiation source and a virtual focus near
`
`a first focus of the ellipse, the ellipse having a second focus
`near the output plane.
`19. The apparatus of claim 1, wherein the second surface
`comprises a set of concentric collectors, each collector
`having a first part closer to the source a second part farther
`from the source, the first part being shaped as a portion of
`a hyperbola, the second part being shaped as a portion of an
`ellipse.
`20. The apparatus of claim 1, wherein the second surface
`comprises a set of grazing incidence collectors.
`21. The apparatus of claim 1, wherein the first and second
`surfaces are both configured to reflect radiation to the output
`plane.
`22. The apparatus of claim 1, wherein the first and second
`surfaces are configured to collect radiation from the source
`at a solid angle of up to about 411.
`23. The apparatus of claim 1, wherein the source com-
`prises a light bulb.
`24. The apparatus of claim 1, wherein the apparatus is a
`vehicle headlight.
`25. The apparatus of claim 1, wherein the apparatus is a
`movie projector.
`26. The apparatus of claim 1, wherein the apparatus is a
`video projector.
`27. The apparatus of claim 1, wherein the apparatus is a
`slide projector.
`28. The apparatus of claim 1, wherein the output plane
`comprises an output point.
`29. A lithography system comprising:
`
`a radiation source;
`
`a first surface shaped to reflect radiation in a first hemi-
`sphere of the radiation source to illuminate in a second
`hemisphere; and
`
`a second surface shaped to reflect radiation in the second
`hemisphere of the radiation source to an output plane in
`the second hemisphere.
`30. The system of claim 29, wherein the source comprises
`an Extreme Ultraviolet (EUV) source.
`31. The system of claim 29, wherein the source comprises
`an Extreme Ultraviolet
`(EUV)
`laser produced plasma
`source.
`
`32. The system of claim 29, further comprising optical
`elements positioned to direct the radiation to a pre-deter-
`mined pattern.
`33. A method comprising:
`
`providing a first reflective surface in a first hemisphere of
`a radiation source and a second reflective surface in a
`
`second hemisphere of the radiation source;
`
`reflecting radiation from the source in the first hemisphere
`to the second hemisphere; and
`
`

`
`US 2004/0183031 A1
`
`Sep. 23, 2004
`
`reflecting radiation from the source in the second hemi-
`sphere to an output plane.
`34. The method of claim 33, further comprising generat-
`ing the radiation.
`35. The method of claim 33, further comprising collecting
`the radiation.
`
`36. The method of claim 33, wherein the source comprises
`an electric discharge extreme ultraviolet source.
`
`37. The method of claim 33, wherein the source comprises
`a dense plasma focus (DPF) electric discharge extreme
`ultraviolet source.
`
`38. The method of claim 33, wherein the source comprises
`a laser produced plasma (LPP) source.