`
`Trends in Optical Design of Projection Lenses for UV -and EUV(cid:173)Trends in Optical Design of Projection Lenses for UV -and EUV(cid:173)Trends in Optical Design of Projection Lenses for UV -and EUV(cid:173)
`
`
`Lithography Lithography Lithography
`
`
`
`Willi Ulrich*, Susanne Beiersdorfer, Hans-Jiirgen Mann Willi Ulrich*, Susanne Beiersdorfer, Hans-Jiirgen Mann Willi Ulrich*, Susanne Beiersdorfer, Hans-Jiirgen Mann
`
`
`Carl Zeiss, D-73446 Oberkochen, Germany Carl Zeiss, D-73446 Oberkochen, Germany Carl Zeiss, D-73446 Oberkochen, Germany
`
`
`
`
`
`ABSTRACT ABSTRACT ABSTRACT
`
`
`
`The continuing trend towards higher integration density of microelectronic circuits requires steadily decreasing feature The continuing trend towards higher integration density of microelectronic circuits requires steadily decreasing feature The continuing trend towards higher integration density of microelectronic circuits requires steadily decreasing feature
`
`
`sizes. The SIA roadmap defines the technologies needed to meet this challenge. One of the fundamental requirements for sizes. The SIA roadmap defines the technologies needed to meet this challenge. One of the fundamental requirements for sizes. The SIA roadmap defines the technologies needed to meet this challenge. One of the fundamental requirements for
`
`
`lithography with a resolution of 100 nm and below is the development of new high-performance optical designs for lithography with a resolution of 100 nm and below is the development of new high-performance optical designs for lithography with a resolution of 100 nm and below is the development of new high-performance optical designs for
`
`
`projection lenses. projection lenses. projection lenses.
`
`
`Many sophisticated new design concepts have been created and further ideas are in the process of development. Optical Many sophisticated new design concepts have been created and further ideas are in the process of development. Optical Many sophisticated new design concepts have been created and further ideas are in the process of development. Optical
`
`
`designers continue to search for new ideas and suitable optical means to reduce residual aberrations. The driving forces are designers continue to search for new ideas and suitable optical means to reduce residual aberrations. The driving forces are designers continue to search for new ideas and suitable optical means to reduce residual aberrations. The driving forces are
`
`
`decreasing wavelength and increasing numerical aperture, while the solution space is limited by several conflicting decreasing wavelength and increasing numerical aperture, while the solution space is limited by several conflicting decreasing wavelength and increasing numerical aperture, while the solution space is limited by several conflicting
`
`
`constraints such as diffraction limited performance, reasonable overall dimensions, minimum number of optical elements, constraints such as diffraction limited performance, reasonable overall dimensions, minimum number of optical elements, constraints such as diffraction limited performance, reasonable overall dimensions, minimum number of optical elements,
`
`
`availability of material, limits on the angles of incidence posed by coating properties and mechanical sensitivities, avoidance availability of material, limits on the angles of incidence posed by coating properties and mechanical sensitivities, avoidance availability of material, limits on the angles of incidence posed by coating properties and mechanical sensitivities, avoidance
`
`
`of vignetting, control of aspheric departure, etc. of vignetting, control of aspheric departure, etc. of vignetting, control of aspheric departure, etc.
`
`
`
`
`
`Keywords: lens design, optical systems, microlithography, UY, EUY Keywords: lens design, optical systems, microlithography, UY, EUY Keywords: lens design, optical systems, microlithography, UY, EUY
`
`
`
`
`
`1. OPTICAL LITHOGRAPHY AT THE EDGE OF RALEIGH'S LAW 1. OPTICAL LITHOGRAPHY AT THE EDGE OF RALEIGH'S LAW 1. OPTICAL LITHOGRAPHY AT THE EDGE OF RALEIGH'S LAW
`
`
`
`According to Moore's law, the leading-edge performance of integrated circuits doubles every 18 months without any According to Moore's law, the leading-edge performance of integrated circuits doubles every 18 months without any According to Moore's law, the leading-edge performance of integrated circuits doubles every 18 months without any
`
`
`increase in manufacturing cost. Recently the actual rate of development has even exceeded the prediction of this "Golden increase in manufacturing cost. Recently the actual rate of development has even exceeded the prediction of this "Golden increase in manufacturing cost. Recently the actual rate of development has even exceeded the prediction of this "Golden
`
`
`Rule".J Rule".J Rule".J
`
`
`How can optical lithography continue to be the enabling technology for this constant miniaturization of feature sizes? How can optical lithography continue to be the enabling technology for this constant miniaturization of feature sizes? How can optical lithography continue to be the enabling technology for this constant miniaturization of feature sizes?
`
`
`The answer to this question is given by the Rayleigh resolution formula2 The answer to this question is given by the Rayleigh resolution formula2 The answer to this question is given by the Rayleigh resolution formula2
`
`
`A. A. A.
`
`
`res=k . -res=k . -res=k . -
`
`
`J NA J NA J NA
`
`
`
`
`
`(1) (1) (1)
`
`
`
`Figure 1 shows the various applicable wavelengths, the numerical aperture (NA) of the exposure tool and the kJ-factor Figure 1 shows the various applicable wavelengths, the numerical aperture (NA) of the exposure tool and the kJ-factor Figure 1 shows the various applicable wavelengths, the numerical aperture (NA) of the exposure tool and the kJ-factor
`
`
`associated with a given resolution node. For each node a reasonable lens NA is given together with a resulting kJ-factor. associated with a given resolution node. For each node a reasonable lens NA is given together with a resulting kJ-factor. associated with a given resolution node. For each node a reasonable lens NA is given together with a resulting kJ-factor.
`
`
`Resolution below 100 nm simultaneously requires lenses of extremely high NA, wavelengths far in the deep ultraviolet and Resolution below 100 nm simultaneously requires lenses of extremely high NA, wavelengths far in the deep ultraviolet and Resolution below 100 nm simultaneously requires lenses of extremely high NA, wavelengths far in the deep ultraviolet and
`
`
`advanced low- kJ imaging techniques.3 advanced low- kJ imaging techniques.3 advanced low- kJ imaging techniques.3
`
`
`
`-.-.. -.. -------.----------.---.----------... ---.---.------ 0.9 -.-.. -.. -------.----------.---.----------... ---.---.------ 0.9 -.-.. -.. -------.----------.---.----------... ---.---.------ 0.9
`
`
`numerical aperture (N~A) • numerical aperture (N~A) • numerical aperture (N~A) •
`
`
`
`
`
`0.8 0.8 0.8
`
`
`
`0.6 L-------0.6 L-------0.6 L-------
`........ -k1.factor~
`~--k1-factor~
`........ -k1.factor~
`-+-12Snm I 0.1
`-+-126nm I 0.1
`-+-12Snm I 0.1
`
`
`
`
`
`0.7 0.7 0.7
`
`
`
`Fig. 1: Exposure tool NA and kJ for resolution nodes Fig. 1.- Exposure tool NA and kJ for resolution nodes Fig. 1: Exposure tool NA and kJ for resolution nodes
`from 250 nm down to 50 nm. The wavelength is
`from 250 nm down to 50 nm. The wavelength is
`from 250 nm down to 50 nm. The wavelength is
`indicated by the plotting symbols.3
`indicated by the plotting symbols.3
`indicated by the plotting symbois.3
`
`
`0.6 0.6
`0.6
`
`0.5
`0.5
`0.5
`
`0.4
`0.4
`0.4
`
`0.3 0.3
`
`0.3
`
`0.2
`0.2
`0.2
`
`0
`
`0 0
`50
`
`50 50
`
`resolution node [nm]
`
`resolution node [nmJ resolution node [nm]
`
`
`
`
`
`0.9 0.9 0.9
`
`
`
`O.B O.B O.B
`
`
`0.7 0.7 0.7
`
`0.5
`0.5
`0.5
`0.4
`0.4
`0.4
`0.3
`0.3
`0.3
`0.2
`0.2
`0.2
`
`
`0.1 0.1
`0.1
`o
`
`o o
`250
`
`250 250
`
`..... 24Bnm ...... 24Bnm
`
`..... 24Bnm
`
`*193nm +193nm
`
`*193nm
`
`"'157nm "'157nm
`
`"'157nm
`
`200
`
`200 200
`
`150
`
`150 150
`
`100
`
`100 100
`
`• Correspondence: Email ulrich@zeiss.de; Telephone +49-7364-20-2303; Fax +49-7364-20-2344 • Correspondence: Email ulrich@zeiss.de; Telephone +49-7364-20-2303; Fax +49-7364-20-2344
`
`• Correspondence: Email ulrich@zeiss.de; Telephone +49-7364-20-2303; Fax +49-7364-20-2344
`
`Downloaded from SPIE Digital Library on 05 Oct 2009 to 80.152.5.244. Terms of Use: http://spiedl.org/terms
`
`Trends in Optical Design of Projection Lenses for UV-and EUV..
`Lithography
`
`Willi Ulrich*, Susanne Beiersdörfer, Hans-JUrgen Mann
`Carl Zeiss, D-73446 Oberkochen, Germany
`
`ABSTRACT
`
`The continuing trend towards higher integration density of microelectronic circuits requires steadily decreasing feature
`sizes. The SIA roadmap defines the technologies needed to meet this challenge. One of the fundamental requirements for
`lithography with a resolution of 100 nm and below is the development of new high-performance optical designs for
`projection lenses.
`Many sophisticated new design concepts have been created and further ideas are in the process of development. Optical
`designers continue to search for new ideas and suitable optical means to reduce residual aberrations. The driving forces are
`decreasing wavelength and increasing numerical aperture, while the solution space is limited by several conflicting
`constraints such as diffraction limited performance, reasonable overall dimensions, minimum number of optical elements,
`availability of material, limits on the angles of incidence posed by coating properties and mechanical sensitivities, avoidance
`of vignetting, control of aspheric departure, etc.
`
`Keywords: lens design, optical systems, microlithography, UV, EUV
`
`1. OPTICAL LITHOGRAPHY AT THE EDGE OF RALEIGH'S LAW
`
`According to Moore's law, the leading-edge performance of integrated circuits doubles every 18 months without any
`increase in manufacturing cost. Recently the actual rate of development has even exceeded the prediction of this "Golden
`Rule".1
`How can optical lithography continue to be the enabling technology for this constant miniaturization of feature sizes?
`The answer to this question is given by the Rayleigh resolution formula2
`2
`res=k •—
`NA
`Figure 1 shows the various applicable wavelengths, the numerical aperture (NA) of the exposure tool and the k1-factor
`associated with a given resolution node. For each node a reasonable lens. NA is given together with a resulting k1-factor.
`Resolution below 100 nm simultaneously requires lenses of extremely high NA, wavelengths far in the deep ultraviolet and
`advanced low- k1 imaging techniques.3
`.—-
`numerical aperture (NA)
`
`(1)
`
`.
`
`. 0.9
`
`0.9
`0.8
`0.7
`0.6
`0.5
`
`03
`
`0.1
`
`0.7
`
`0.6
`
`0.5
`
`0.4
`
`Fig. 1: Exposure tool NA and k1 for resolution nodes
`from 250 nm down to 50 nm. The wavelength is
`indicated by the plotting symbols.3
`
`Z k,-factor
`
`[48nm 193nm '157nm 126nm
`100
`150
`200
`
`250
`
`50 resolution node mm]
`
`*
`
`Correspondence: Email ulrich@zeiss.de; Telephone +49-7364-20-2303; Fax +49-7364-20-2344
`
`Soft X-Ray and EUV Imaging Systems, Winfried M. Kaiser, Richard H. Stulen, Editors,
`Proceedings of SPIE Vol. 4146 (2000) © 2000 SPIE. · 0277-786X/00/$15.00
`
`13
`
`ZEISS 1018
`
`

`

`This paper will describe how the quest for ever higher apertures and ever shorter wavelengths has influenced the
`This paper will describe how the quest for ever higher apertures and ever shorter wavelengths has influenced the
`This paper will describe how the quest for ever higher apertures and ever shorter wavelengths has influenced the
`development of optical designs for lithographic projection lenses. An overview of the progress in lens design for deep
`
`development of optical designs for lithographic projection lenses. An overview of the progress in lens design for deep development of optical designs for lithographic projection lenses. An overview of the progress in lens design for deep
`
`ultraviolet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) micro lithography will be given. Different ultraviolet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) micro lithography will be given. Different
`ultraviolet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) micro lithography will be given. Different
`
`design concepts for dioptric, catadioptric and catoptric projection lenses will be discussed against the background of design concepts for dioptric, catadioptric and catoptric projection lenses will be discussed against the background of
`design concepts for dioptric, catadioptric and catoptric projection lenses will be discussed against the background of
`technical and technological constraints. Relative performance, challenges, feasibility and limits of different optical designs
`
`technical and technological constraints. Relative performance, challenges, feasibility and limits of different optical designs technical and technological constraints. Relative performance, challenges, feasibility and limits of different optical designs
`will be compared.
`
`will be compared. will be compared.
`
`•
`
`• •
`
`• Section 2 shows the rapid increase of lens volume with increasing numerical apertures and the effects achieved through
`
`• Section 2 shows the rapid increase of lens volume with increasing numerical apertures and the effects achieved through • Section 2 shows the rapid increase of lens volume with increasing numerical apertures and the effects achieved through
`
`the use of aspheres. the use of aspheres.
`the use of aspheres.
`In Section 3 the question of how optical designs could enable the transition to shorter wavelength will be discussed.
`
`In Section 3 the question of how optical designs could enable the transition to shorter wavelength will be discussed. In Section 3 the question of how optical designs could enable the transition to shorter wavelength will be discussed.
`Various dioptric and catadioptric design examples for 193 nm and 157 nm will be described.
`
`Various dioptric and catadioptric design examples for 193 nm and 157 nm will be described. Various dioptric and catadioptric design examples for 193 nm and 157 nm will be described.
`• Section 4 focuses on the development of all-reflective lenses for EUV-lithography
`
`• Section 4 focuses on the development of all-reflective lenses for EUV-lithography • Section 4 focuses on the development of all-reflective lenses for EUV-lithography
`
`
`• •
`In Section 5 a conclusion is given. In Section 5 a conclusion is given.
`•
`In Section 5 a conclusion is given.
`
`2. ASPHERES FOR COMPACT HIGH-NA LENSES
`
`2. ASPHERES FOR COMPACT HIGH-NA LENSES 2. ASPHERES FOR COMPACT HIGH-NA LENSES
`
`Designing projection lenses with highest numerical apertures for micro lithography combines all the difficulties of lens
`
`Designing projection lenses with highest numerical apertures for micro lithography combines all the difficulties of lens Designing projection lenses with highest numerical apertures for micro lithography combines all the difficulties of lens
`
`design. Field sizes comparable to those of photographic lenses have to be achieved at numerical apertures close to those of design. Field sizes comparable to those of photographic lenses have to be achieved at numerical apertures close to those of
`design. Field sizes comparable to those of photographic lenses have to be achieved at numerical apertures close to those of
`high-aperture microscope objectives, while maintaining near-perfect wavefront correction. Not at least of all, such lenses
`
`high-aperture microscope objectives, while maintaining near-perfect wavefront correction. Not at least of all, such lenses high-aperture microscope objectives, while maintaining near-perfect wavefront correction. Not at least of all, such lenses
`
`actually have to be manufacturable in quantity. actually have to be manufacturable in quantity.
`actually have to be manufacturable in quantity.
`Projection lenses for lithography were derived from photo lenses, particularly from the double-Gauss type, about 25 years
`
`Projection lenses for lithography were derived from photo lenses, particularly from the double-Gauss type, about 25 years Projection lenses for lithography were derived from photo lenses, particularly from the double-Gauss type, about 25 years
`
`ago. With comparable field sizes and comparable numerical apertures, lens designers had to improve image quality ago. With comparable field sizes and comparable numerical apertures, lens designers had to improve image quality
`ago. With comparable field sizes and comparable numerical apertures, lens designers had to improve image quality
`
`significantly. First of all field curvature had to be reduced. E. Glatzel4 proposed the correction means of multiple "bulges" significantly. First of all field curvature had to be reduced. E. Glatzel4 proposed the correction means of multiple "bulges"
`significantly. First of all field curvature had to be reduced. E. Glatzel4 proposed the correction means of multiple "bulges"
`
`according to the recommendation of H. Slevogt.5 This method can be used several times in the same design. according to the recommendation of H. Slevogt.5 This method can be used several times in the same design.
`according to the recommendation of H. Slevogt.5 This method can be used several times in the same design.
`
`In time numerical aperture increased due to the need for improved resolution. As a consequence, the complexity of the In time numerical aperture increased due to the need for improved resolution. As a consequence, the complexity of the
`In time numerical aperture increased due to the need for improved resolution. As a consequence, the complexity of the
`optical systems rose, particularly in the last, aperture-dominated, bulge of the lens. This part of the lens became more and
`
`optical systems rose, particularly in the last, aperture-dominated, bulge of the lens. This part of the lens became more and optical systems rose, particularly in the last, aperture-dominated, bulge of the lens. This part of the lens became more and
`more similar to high-NA microscope lenses.
`
`more similar to high-NA microscope lenses. more similar to high-NA microscope lenses.
`
`Volume vs. NA
`
`Volume vs. NA Volume vs. NA
`
`O.7NA
`
`O.7NA O.7NA
`
`O.SNA
`
`O.BNA O.BNA
`
`a.SNA
`
`O.BNA O.BNA
`
`
`* aspherical * aspherical
`* aspherical
`
`
`surfaces surfaces
`surfaces
`
`yrtore,\
`
`/'rtoM\ /'rtoM\
`
`I max. dlame~
`
`I max. diame~ I max. diame~
`o
`
`\.0 \.0
`
`
`309 309
`309
`
`
`2:'4 mm 2:'4 mm
`234 mm
`
`Patent Appl. DE 19855108
`Patent Appl. DE 19855108
`Patent AppL DE 19655106
`
`Patent Appl. DE 19922209
`Patent Appl. DE 19922209
`Patent AppL DE 19922209
`
`Fig. 2: The development of compact high-NA lenses for 248 nm DUV-lithography. All of
`Fig. 2: The development of compact high-NA lenses for 248 nm DUV-lithography. All of
`Fig. 2: The development of compact high-NA lenses for 248 nm DUV-lithography. All of
`these lenses expose a slit height of26 mm. The track length of the 0.8 NA all-spherical
`
`these lenses expose a slit height of26 mm. The track length of the 0.8 NA all-spherical these lenses expose a slit height of26 mm. The track length of the 0.8 NA aU-spherical
`system is 1150 mm, the other two lenses have a track length of 1 000 mm.
`system is 1150 mm, the other two lenses have a track length of 1 000 mm.
`system is 1150 mm, the other two lenses have a track length of 1 000 mm.
`
`
`
`5.00 5.00
`
`
`
`·· .. ·· _____ .... __ ...... ··. __ ...... __ ....... ·· ........ H ....... _ •• -·· .. ·· _____ .... __ ...... ··. __ ...... __ ....... ·· ........ H ....... _ •• -
`
`
`
`••• _ •••• _ . __ ._ ._ . { _ ._ ._ . , ••• _ •••• _ . __ ._ ._ . { _ ._ ._ . ,
`
`.. ----.--.. -.-----_____ . __ .. /-___ J .. ----.--.. -.-----_____ . __ .. /-___ J
`
`
`
`
`
`4.50 4.50
`
`
`
`.. -.. -
`
`
`
`spherical designs spherical designs
`
`
`:::.=~~=~:e~:==~:~~~:~:==:::=~=.~=~::,~~:.~~j :::.=~~=~:e~:==~:~~~:~:==:::=~=.~=~::,~~:.~~j
`
`i~~ i~~
`
`
`~ 3.00 ~ 3.00
`
`.2 .2
`
`
`...... _ ..... _ ..... _ ....... _ ....... _ ....... -._. . ...... __ ........................... __ ... __ ...... . .... J.: ... --.... . .... /..~ ...... _ ..... _ ..... _ ....... _ ....... _ ....... -._. . ...... __ ........................... __ ... __ ...... . .... J.: ... --.... . .... /..~
`
`.' 200x?! .' 200x?!
`
`
`
`0.60 0.60
`
`·-··--¥.l9&-·----------·--·-······--i
`····--¥.l9&-··-------···-··-······--i
`
`
`o.oo+---~--~--~--~--' o.oo+---~--~--~--~--'
`D.OO+----~--~--~--~--'
`o.ao
`O.SO
`0.50
`
`
`0.50 0.50
`0.60 0.60
`
`0.70 0.70
`0.70
`0.80
`0.60
`0.90
`0.90
`0.90
`NA
`
`NA NA
`
`
`Fig. 3: Lens volume as afunction of numerical Fig. 3: Lens volume as afunction of numerical
`Fig. 3: Lens volume as afunction of numerical
`
`aperture. Compact high-NA lenses become aperture. Compact high-NA lenses become
`aperture. Compact high-NA lenses become
`feasible through to the use of asp heres
`
`feasible through to the use of asp heres feasible through to the use of aspheres
`
`Figure 2 visualizes how design modifications have enabled the increase in NA for DUV lenses from 0.7 to 0.8.
`
`Figure 2 visualizes how design modifications have enabled the increase in NA for DUV lenses from 0.7 to 0.8. Figure 2 visualizes how design modifications have enabled the increase in NA for DUV lenses from 0.7 to 0.8.
`Without new design ideas, lenses would have exploded in diameter and volume (cf. Figs. 2, 3), if the NA was increased up
`
`Without new design ideas, lenses would have exploded in diameter and volume (cf. Figs. 2, 3), if the NA was increased up Without new design ideas, lenses would have exploded in diameter and volume (cf. Figs. 2, 3), if the NA was increased up
`to 0.8 or more. Due to essentially three design measures a very compact 0.8 NA design7 could be provided:
`
`to 0.8 or more. Due to essentially three design measures a very compact 0.8 NA design7 could be provided: to 0.8 or more. Due to essentially three design measures a very compact 0.8 NA design7 could be provided:
`
`Downloaded from SPIE Digital Library on 05 Oct 2009 to 80.152.5.244. Terms of Use: http://spiedl.org/terms
`
`This paper will describe how the quest for ever higher apertures and ever shorter wavelengths has influenced the
`development of optical designs for lithographic projection lenses. An overview of the progress in lens design for deep
`ultraviolet (DUV), vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) microlithography will be given. Different
`design concepts for dioptric, catadioptric and catoptric projection lenses will be discussed against the background of
`technical and technological constraints. Relative performance, challenges, feasibility and limits of different optical designs
`will be compared.
`. Section 2 shows the rapid increase of lens volume with increasing numerical apertures and the effects achieved through
`the use of aspheres.
`.
`In Section 3 the question of how optical designs could enable the transition to shorter wavelength will be discussed.
`Various dioptric and catadioptric design examples for 193 nm and 157 nm will be described.
`I Section 4 focuses on the development of all-reflective lenses for EUV-lithography
`.
`In Section 5 a conclusion is given.
`
`2. ASPHERES FOR COMPACT HIGH-NA LENSES
`
`Designing projection lenses with highest numerical apertures for microlithography combines all the difficulties of lens
`design. Field sizes comparable to those of photographic lenses have to be achieved at numerical apertures close to those of
`high-aperture microscope objectives, while maintaining near-perfect wavefront correction. Not at least of all, such lenses
`actually have to be manufacturable in quantity.
`Projection lenses for lithography were derived from photo lenses, particularly from the double-Gauss type, about 25 years
`ago. With comparable field sizes and comparable numerical apertures, lens designers had to improve image quality
`significantly. First of all field curvature had to be reduced. E. Glatzel4 proposed the correction means of multiple "bulges"
`according to the recommendation of H. Slevogt.5 This method can be used several times in the same design.
`In time numerical aperture increased due to the need for improved resolution. As a consequence, the complexity of the
`optical systems rose, particularly in the last, aperture-dominated, bulge of the lens. This part of the lens became more and
`more similar to high-NA microscope lenses.
`
`0.7 NA
`
`0.8 NA
`
`* aspherical
`surfaces
`
`0.8 NA
`
`*
`
`Volume vs. NA
`
`*:
`
`aperture stop
`
`\ I
`ri::: 0:309 mm \::
`
`".:
`
`I
`
`—.
`
`.
`max. diameter
`
`\
`\ :'
`
`/
`
`•'
`
`255 mm
`
`Patent Appi. DE 19855108
`
`Patent Appt. 02 19922209
`
`Fig. 2: The development ofcompact high-NA lensesfor 248 nm DUV-lithography. All of
`these lenses expose a slit height of26 mm. The track length ofthe 0.8 NA all-spherical
`system is 1150 mm, the other two lenses have a track length ofl000 mm.
`
`E
`
`0.50
`
`0.60
`
`0.70
`
`NA
`
`0.80
`
`090
`
`Fig. 3: Lens volume as afunction of numerical
`aperture. Compact high-NA lenses become
`feasible through to the use of aspheres
`
`Figure 2 visualizes how design modifications have enabled the increase in NA for DUV lenses from 0.7 to 0.8.
`Without new design ideas, lenses would have exploded in diameter and volume (cf. Figs. 2, 3), if the NA was increased up
`to 0.8 or more. Due to essentially three design measures a very compact 0.8 NA design7 could be provided:
`
`14
`
`Proc. SPIE Vol. 4146
`
`

`

`
`
`• Three aspheric surfaces in the front part of the lens to smooth the aberration distributions across the whole field • Three aspheric surfaces in the front part of the lens to smooth the aberration distributions across the whole field • Three aspheric surfaces in the front part of the lens to smooth the aberration distributions across the whole field
`
`
`
`
`• • •
`Shortening of the first bulge and shifting of positive power into the second bulge Shortening of the first bulge and shifting of positive power into the second bulge Shortening of the first bulge and shifting of positive power into the second bulge
`
`
`• • •
`
`
`Splitting of the strong negative lens element in the second waist into two elements to bend the marginal ray more Splitting of the strong negative lens element in the second waist into two elements to bend the marginal ray more Splitting of the strong negative lens element in the second waist into two elements to bend the marginal ray more
`
`
`gently4 gently4 gently4
`
`
`
`Within the same track length of 1000 mm the lens diameter rose only by 10% relative to the all-spherical 0.7 NA design. A Within the same track length of 1000 mm the lens diameter rose only by 10% relative to the all-spherical 0.7 NA design. A Within the same track length of 1000 mm the lens diameter rose only by 10% relative to the all-spherical 0.7 NA design. A
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`
`further positive refractive element between the second waist and the aperture stop is also advantageous for increasing the further positive refractive element between the second waist and the aperture stop is also advantageous for increasing the further positive refractive element between the second waist and the aperture stop is also advantageous for increasing the
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`NA. Of course all of these designs could be provided with flat elements on the wafer side (shown in the spherical designs of NA. Of course all of these designs could be provided with flat elements on the wafer side (shown in the spherical designs of NA. Of course all of these designs could be provided with flat elements on the wafer side (shown in the spherical designs of
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`Fig. 2) and/or the reticle side, e.g. for lens protection. Fig. 2) and/or the reticle side, e.g. for lens protection. Fig. 2) and/or the reticle side, e.g. for lens protection.
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`Refractive designs have greatly Refractive designs have greatly Refractive designs have greatly
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`terms of NA and residual aberrations over terms of NA and residual aberrations over terms of NA and residual aberrations over
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`Figure 4 shows the monochromatic wavefront of the aspheric 0.8 NA design. Figure 4 shows the monochromatic wavefront of the aspheric 0.8 NA design. Figure 4 shows the monochromatic wavefront of the aspheric 0.8 NA design.
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`Fig. 4: Aspheric lens design for 248 nm DUV lithopraphy (0.8 Fig. 4: Aspheric lens design for 248 nm DUV lithopraphy (0.8 Fig. 4: Aspheric lens design for 248 nm DUV lithopraphy (0.8
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`NA, slit height 26 mm) Patent Appl. DE 19922209 : NA, slit height 26 mm) Patent Appl. DE 19922209 : NA, slit height 26 mm) Patent Appl. DE 19922209 :
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`3. TRANSITION TO SHORTER WAVELENGTHS 3. TRANSITION TO SHORTER WAVELENGTHS 3. TRANSITION TO SHORTER WAVELENGTHS
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`Apart from the numerical aperture, the other key to improved resolution is the transition to shorter wavelengths. Typically, Apart from the numerical aperture, the other key to improved resolution is the transition to shorter wavelengths. Typically, Apart from the numerical aperture, the other key to improved resolution is the transition to shorter wavelengths. Typically,
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`two materials with different dispersion are necessary to achromatize a dioptric lens. This issue will be discussed in sec. 3.1. two materials with different dispersion are necessary to achromatize a dioptric lens. This issue will be discussed in sec. 3.1. two materials with different dispersion are necessary to achromatize a dioptric lens. This issue will be discussed in sec. 3.1.
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`Alternatively, several design approaches can be considered to meet the short wavelength challenge: Alternatively, several design approaches can be considered to meet the short wavelength challenge: Alternatively, several design approaches can be considered to meet the short wavelength challenge:
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`• Downscaling the whole system, including field size, to reduce aberrations. This option will not be discussed here, • Downscaling the whole system, including field size, to reduce aberrations. This option will not be discussed here, • Downscaling the whole system, including field size, to reduce aberrations. This option will not be discussed here,
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`because it is somehow "external" to the actual design problem. Nevertheless, this option should be considered carefully because it is somehow "external" to the actual design problem. Nevertheless, this option should be considered carefully because it is somehow "external" to the actual design problem. Nevertheless, this option should be considered carefully
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`as a means of reducing the amount of material needed. This is the reason why all catadioptric designs for 157 nm as a means of reducing the amount of material needed. This is the reason why all catadioptric designs for 157 nm as a means of reducing the amount of material needed. This is the reason why all catadioptric designs for 157 nm
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`discussed in sec. 3.3. are designed for a scanner slit height of 22 mm instead of 26 mm. discussed in sec. 3.3. are designed for a scanner slit height of 22 mm instead of 26 mm. discussed in sec. 3.3. are designed for a scanner slit height of 22 mm instead of 26 mm.
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`• Catadioptric systems: Mirrors have no chromatic aberration and a concave mirror acts as a field flattener. Catadioptric • Catadioptric systems: Mirrors have no chromatic aberration and a concave mirror acts as a field flattener. Catadioptric • Catadioptric systems: Mirrors have no chromatic aberration and a concave mirror acts as a field flattener. Catadioptric
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`designs with a beamsplitter will be shown in sec. 3.2. The consequence of catadioptric designs without a beamsplitter is designs with a beamsplitter will be shown in sec. 3.2. The consequence of catadioptric designs without a beamsplitter is designs with a beamsplitter will be shown in sec. 3.2. The consequence of catadioptric designs without a beamsplitter is
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`either a central pupil obscuration or an off-axis field. 3 Such designs will be presented in sec. 3.3. either a central pupil obscuration or an off-axis field. 3 Such designs will be presented in sec. 3.3. either a central pupil obscuration or an off-axis field. 3 Such designs will be presented in sec. 3.3.
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`• All-reflective designs have no chromatic aberrations, but are NA-limited. Design examples will be shown in sec. 4, in • All-reflective designs have no chromatic aberrations,