Catadioptric lens development for DUV and VUV projection optics
`
`Yasuhiro Ohmura, Masahiro Nakagawa, Tom Matsuyama, Yuichi Shibazaki
`IC Equipment Division, Precision Equipment Company, Nikon Corp.
`201-9, Miizugahara, Kumagaya, Saitama, 360-8559 Japan
`
`ABSTRACT
`According to the International Technology Roadmap for Semiconductors (ITRS), the 65nm technology node is forecast
`to appear in 2007. In this paper, we propose two specifications for the projection optics at 65nm nodes. The one is over
`1 .0 numerical aperture (NA) at 193nm lithography by liquid immersion'. The other is 0.85 NA at 157nm lithography2'
`3)• Since it is almost impossible for traditional dioptric optics to realize these specifications, catadioptric is supposedly
`the leading optics for an extreme optical lithography, like 65nm node. Described in the paper are feasibility study for
`catadioptric optics, and our assembly strategy. Emphasis is placed on our selection methodology among a variety of
`catadioptric configurations.
`
`Keywords: 193nm, 157nm, projection optics, catadioptric, dioptric, liquid immersion, assembly strategy
`
`1. INTRODUCTION
`The resolution required to projection optics for exposure tools has been rapidly increased. This can be represented in
`Rayleigh's equation, such that;
`
`(1)
`
`R=k1--—
`NA
`where R is the resolution, A is wavelength of exposure light, and k1 is process constant determined by resist
`performance, mask and illumination condition. In order for the optics to improve the resolution, it has been an orthodox
`approach to shorten the wavelength of exposure light and to increase the NA of projection optics. However, if the
`wavelength of the exposure light is shortened, the number of types of available glass materials is limited to a few due to
`the absorption of light. In fact, usable materials for 193nm are only fused silica and calcium fluoride, and the only one
`for 157nm is calcium fluoride. Moreover, if the NA is increased, many lens elements with large diameter are required.
`Because the situation is like mentioned above, it has become all the more important to select proper optics type and
`study minutely for its assembly strategy for the upcoming lithography optics.
`
`2. CATADIOPTRIC VS. DIOPTRIC
`
`a)
`
`c—
`
`b)
`
`Fig. 1: Sample designs of "Catadioptric" and "Dioptric" optics
`
`Optical Microlithography XVI, Anthony Yen, Editor, Proceedings of SPIE
`Vol. 5040 (2003) © 2003 SPIE · 0277-786X/03/$15.00
`
`781
`
`Downloaded from SPIE Digital Library on 30 Jul 2010 to 150.135.248.118. Terms of Use: http://spiedl.org/terms
`
`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 1
`
`

`

`The catadioptric optics, which is constructed with refractive elements and mirrors, has been utilized in a camera
`objective and an astronomical telescope for a long time. For the projection optics for microlithography, it has already
`been studied for 20 years4, an example of which is given in Fig. la. However, it has not become the leading optics for
`the lithography thus far. The dioptric optics (See Fig. ib) is constructed with only refractive elements. The optics, all
`the elements of which are rotational symmetric and aligned along one optical axis, can be assembled within one lens
`barrel. So, it can be concluded that the dioptric has an advantage in terms of lens producibility. The assembling
`procedure that we have established through many experiences makes the conclusion firm. This superiority is a main
`reason why the dioptric has been used in actual exposure equipment. Nevertheless, the catadioptric is advantageous in
`terms of the volume of glass material and correcting chromatic aberrations. The following section deals with the reason
`briefly.
`
`2.1 Satisfaction of Petzval condition
`The projection optics as a whole can be regarded as a positive lens. From very simple considerations, it is clear that a
`positive lens has an inward-curving field. The effects can readily be understood by examining Fig. 2.
`Ax
`
`Yi
`
`Fig. 2: Field curvature.
`
`The reticle and the wafer have a flat surface, so a flat field is required for the projection optics. When the optics is free
`of astigmatic image, a field curvature (See Fig. 2) is given by
`
`yj2m
`
`(2)
`
`where Pj is refractive power of each lens element, which is given by inverse focal length of the m thin lenses forming
`the system, zlX is the displacement of an image point at height Yl from the paraxial image plane. To have a flat field,
`the lens system needs to satisfy the following expression.
`
`1=o
`
`(3)
`
`This is the so-called "Petzval condition".
`For the dioptric, many lens elements with large diameter are needed to satisfy the Petzval condition. Fig.3 shows an
`example of power arrangement for dioptric system.
`
`object
`
`mage
`
`+P
`
`—P—P
`
`+P
`
`Fig. 3: Example of power arrangement for dioptnc system
`
`782 Proc. of SPIE Vol. 5040
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`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 2
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`

`

`For easy understanding, in Fig. 3, the system has 1X magnification and very simple configuration. Assume the
`refractive power of each element is P. Then, the sum of each absolute refractive power is 4P, while the sum of the
`power is 0 to satisfy the Petzval condition. By contrast, the concave mirror has positive refractive power like a positive
`lens elements but it works for a negative Petzval sum. This leads an optical system to consist of low refractive power.
`Fig. 4 shows an example of power arrangement for catadioptric system.
`
`Concave mirror
`
`termed :ate
`rase
`
`N
`
`--------
`
`beamsp! iter
`
`---
`
`----
`
`---
`-- --- -
`
`4P
`
`mage
`
`—P
`
`-
`
`-
`
`'e±
`
`-
`
`Fig. 4: Example of power arrangement for catadioptric system
`
`The sum of each absolute refractive power is 2P in this conceptual configuration. From the intuition of lens designer,
`the total refractive power would be proportional to difficulty of correcting aberrations. Therefore, it is possible for the
`catadioptric to be scaled down, or to reduce all the linear constructional dimensions. For example, when the whole
`system of Fig. 4 is scaled down by half, the total refractive power becomes 4P, that is same as total of the dioptric
`system in Fig. 3. That is why the catadioptric has an advantage about glass material volume.
`
`2.2 Correcting chromatic aberrations
`The refractive index of glass changes with wavelength. Normally, the refractive power is larger at shorter wavelengths.
`This effect can readily be understood by examining Fig. 5. For a positive lens, the focal point for red light is farther
`from the lens than that for blue light. Therefore at least two kinds of material that have different dispersive powers are
`needed for dioptric, otherwise, ultra-line-narrowed laser which has a narrow spectral bandwidth would be required.
`
`I ow
`
`Fig. 5: Axial chromatic aberration for positive lens
`
`Fig. 6: Axial chromatic aberration for a combination of a
`Concave mirror and a negative lens
`
`Nevertheless, the combination of a concave mirror and a negative lens (See Fig. 6) also has positive refractive power,
`but the mirror and the negative lens work oppositely for correcting chromatic aberrations. The negative lens is the
`so-called "Schupmann achromat.5' 6) For the catadioptric system, chromatic aberrations can be corrected with single
`material as these are combined. That is why the catadioptric has an advantage about correcting chromatic aberrations.
`
`2.3 Motivation for 193nm microlithographic lens
`Designing projection optics with higher NA will require larger diameter for lens elements. Fig. 7 shows a graph of the
`relation between NA and lens diameter at 193nm projection optics. All spherical lenses had constituted the projection
`
`Proc. of SPIE Vol. 5040 783
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`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 3
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`

`

`optics under O.7NA. If it went over 0.7, the larger lens diameter required for the optics would increase rapidly, and
`exceed a practical limit. At that time "Aspheres" brought about the breakthrough7. They can work for the reduction of
`lens diameter and the realization of over O.8NA optics. When NA goes over 1 .0, we should choose liquid immersion
`optics, of course. However, as long as we persist in the dioptric, its lens diameter will go over a practical limit
`immediately. I would say, " The next breakthrough will be brought by Catadioptric". It will make possible to realize
`over 1 .2NA optics.
`
`>
`-
`
`0)
`
`>
`
`--S-'
`
`r'J'_#
`::
`
`¼)
`
`1i:
`
`::2
`
`ii
`
`Fig. 7: Lens Dia. vs NA
`
`2.4 Motivation for 157nm microlithographic lens
`In case of 157nm projection optics, the motivation for the catadioptric system is obvious. We know that development of
`the second material, or line narrowed laser and sufficient supply of glass material are necessary conditions for the
`dioptric. However, for the 157nm optics, usable material is only calcium fluoride, and it is difficult to make
`achromatized dioptric optics with a single lens material. Ultra-line-narrowed F2 laser with the spectrum bandwidth of
`0. l5pm FWHM or less makes it possible to realize an achromatized imaging by dioptric optics with only one kind of
`the material, but around 1 .Opm FWHM is current bandwidth of practical F2 lasers with adequate output power and
`stability. What is worse, supply of calcium fluoride lens is not enough. So, we would take a pessimistic view of all
`these items. Then our decision is made on catadioptric optics for 157nm projection optics3.
`
`3. CATADIOPTRIC OPTICS TYPE
`As I have mentioned, the concave mirror and the Schupmann achromat are very important elements for the catadioptric
`system. They work to compensate chromatic aberrations and the Petzval sum. However, incident and reflected rays
`interfere with each other above the mirror, and the imaging ray or the reflected ray must be separated from the incident
`ray to make the system practical. The catadioptric optics can be classified into three types according to the method how
`to separate these rays changing the propagation direction.
`The first one is the beamsplitter type called type-A, hereafter.8' 9),10), 11), 12) Fig. 8 shows a conceptual configuration of
`type-A, which has Quarter Wave Plate (QWP) and Polarized Beam Splitter (PBS) prism close to the concave mirror
`and the Shupmann achromat. They work to separate beam of returning from the concave reflecting mirror and beam of
`going to the concave mirror without the loss of energy. The location of prism has a little more allowance than other
`catadioptric optics. So it allows non-intermediate image in optical system can consist of only the elements with low
`refractive power. Nevertheless, PBS-coating, large calcium fluoride prism, and large QWP could be difficulty in
`realizing the optics.
`The second one is the central obscuration type called type-B, hereafter (See Fig. 9)13)14), 15), 16) which can be
`considered the Schwarzschild objective'7 plus refracting relay. This type-B produces not passing area in the pupil of
`optics to separate the reflected ray from the incident ray. Each of two mirrors has a central hole, and they are arranged
`close to image plane to keep the obscuration small. The type-B is uni-axial system like dioptric optics, and has an
`advantage of lens producibility. Nonetheless, we have to remember that the type-B lacks the capability of resolving
`
`784 Proc. of SPIE Vol. 5040
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`Downloaded from SPIE Digital Library on 30 Jul 2010 to 150.135.248.118. Terms of Use: http://spiedl.org/terms
`
`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 4
`
`

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`Proc. of SPIE Vol. 5040 785
`
`Downloaded from SPIE Digital Library on 30 Jul 2010 to 150.135.248.118. Terms of Use: http://spiedl.org/terms
`
`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 5
`
`

`

`large enough the Schupmann achromat sufficiently large diameter without the obscuration or the PBS prism. The
`type-D absolutely does have a competitive advantage when compared with dioptric or other catadioptric. What is more,
`the type-D can realize super-high specifications, such as a super high NA with wide achromatization. Accordingly, we
`conclude that we select this multi-barrel off-axis type for the future projection optics.
`
`4. ASSEMBLY STRATEGY FOR MULTI-BARRELS
`If we must find a drawback about the type-D, it is assembly for the multi-barrel construction. However, we have
`already established assembly strategy for multi-barrels.24 Our experiences and simulations reached an conclusion
`that: high performance optics with less than 0.0 10 waves RMS of wavefront aberration could be realized with the
`application of the assembly with the accuracy less than 1 micron
`
`4.1 Angle adjustment
`The angle between one lens barrel and the other can be easily adjusted by using well-known auto-collimator. At first,
`we prepare a joint-barrel with a folding mirror (See Fig. 12) and investigate the accuracy of the flange, to which
`horizontal barrel is to be connected, by placing plane parallel on it. In the same manner, the flange accuracy for vertical
`barrel is also investigated, and the tilting folding mirror would complete the alignment procedure.
`
`.......
`
`b
`
`I
`
`Fig. 12: Setting for angle adjustment
`
`4.2 Positional adjustment
`Positional adjustment with high accuracy is a tough work for us, but we have established strategy by using our
`conventional technique. At first, we prepare two tool barrels (See Fig. 13); one tool barrel has a transmission sphere,
`and the other has a concave mirror. The focal length of lens system and the curvature of spherical mirror can be
`measured accurately. The length "A" and "C" are known. The distance from edge of barrel (reference surface) to lens or
`mirror surface can be measured correctly. The length "B" and "D" are known, and also the length "L" and "M" will be
`known. We prepare two tool barrels on condition that "L" plus "M" equal "distance between two lens barrels".
`
`iresuiision s
`
`Fig. 13: Tool barrels
`Then an interferometer system is set with the two tool barrels and the joint barrel. The setting can readily be
`appreciated by examining Fig. 14. By shifting the tool barrel and by inserting a ring washer between the joint barrel and
`the tool barrel for positioning adjustment, the output of interferometer with one color can be obtained.
`
`786 Proc. of SPIE Vol. 5040
`
`Downloaded from SPIE Digital Library on 30 Jul 2010 to 150.135.248.118. Terms of Use: http://spiedl.org/terms
`
`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 6
`
`

`

`fl t c fe orn( ir
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`Fig. 14: Setting of interferometer system
`
`The two tool barrels are finally replaced with real lens barrels. We are now assembling multi-barrel construction with
`such high accuracy as less than imicron, based upon above-mentioned strategy.
`
`4.3 Fine adjustment
`We have already developed "adjustable" and "kinematic" lens mounting. (See Fig. 15) In this mounting, six degrees of
`moving freedom can be controlled. This mechanism can control lens element position perfectly without any lens
`deformation. Our adjustment procedure will never require disassembling multi-barrel construction or breaking gas
`sealing by using this mounting.
`Now we have strong confidence in achieving high performance optics with Multi-barrel Off-axis type.
`
`Fig. 15 : Concept of new lens positioning mechanism
`
`5. CONCLUSIONS
`In this paper, the right optical system for the future projection optics is discussed. Our feasibility study reaches the
`conclusion that the catadioptric is better than the dioptric. The reason is that developments of second material and
`line-narrowed laser are not needed, and that volume of glass material can be reduced. This reduction alleviates the
`supply problem of calcium fluoride and large lens material with high quality.
`Moreover, some types of catadioptric optics are compared with each other from the viewpoint of feasibility. Our
`selection is multi-barrel off-axis type, because this type realizes super-high specifications, and it has no drawbacks but
`lens producibility. We have already designed over 1 .2NA optics with this type.As to the solution to the drawback, we
`have already established assembly strategy for multi-barrel construction by using interferometer system. In addition,
`our new lens mounting works effectively at fine adjustment, and the projection optics with high performance will
`definitely be realized.
`
`ACKNOWLEDGEMENTS
`The authors would like to thank N. Shiraishi, Y. Ichihara, Y. Suenaga, T. Takahashi, M. Hatasawa, J. Nishikawa, I.
`Tanaka, T. Mon and K. Matsumoto of Nikon and D. M. Williamson of NRCA for their discussion about optical design
`and assembly strategy.
`
`Proc. of SPIE Vol. 5040 787
`
`Downloaded from SPIE Digital Library on 30 Jul 2010 to 150.135.248.118. Terms of Use: http://spiedl.org/terms
`
`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 7
`
`

`

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`788 Proc. of SPIE Vol. 5040
`
`Downloaded from SPIE Digital Library on 30 Jul 2010 to 150.135.248.118. Terms of Use: http://spiedl.org/terms
`
`CARL ZEISS V. NIKON
`IPR2013-00362
`Ex. 2006, p. 8
`
`