(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2013/0329283 A1
`(43) Pub. Date:
`Dec. 12, 2013
`Nakano et al.
`
`US 20130329283A1
`
`(54)
`
`(75)
`
`(73)
`
`(21)
`
`(22)
`
`CATADIOPTRIC OPTICAL SYSTEM WITH
`TOTAL INTERNAL REFLECTION FOR HIGH
`NUMERICAL APERTURE IMAGING
`
`Inventors: Masatsugu Nakano, Tucson, AZ (US);
`Jose Manuel Sasian-Alvarado, Tucson,
`AZ (US)
`Assignees:
`
`The Arizona Board of Regents on
`behalf of the University of Arizona,
`Tuscon, AZ (US); CANON
`KABUSHIKI KAISHA, Tokyo (JP)
`
`Appl. No.:
`
`13/492,078
`
`Filed:
`
`Jun. 8,2012
`
`Publication Classi?cation
`
`(51)
`
`Int. Cl.
`G02B 1 7/08
`
`(2006.01)
`
`(52) US. Cl.
`USPC ......................................... .. 359/366; 359/731
`
`(57)
`
`ABSTRACT
`
`A catadioptric optical system includes, in order from an
`object side to an image side and arranged along an optical
`axis, a ?rst catadioptric unit, a second catadioptric unit dis
`posed in axial alignment With the ?rst catadioptric unit and
`With a space therebetWeen; and a lens group disposed in axial
`alignment With the ?rst and second catadioptric optical units.
`Light rays arriving from an object plane undergo a ?rst re?ec
`tion at the image-side surface of the ?rst catadioptric optical
`unit, a second re?ection at the object-side surface of the ?rst
`catadioptric optical unit, a third re?ection at the image-side
`surface of the second catadioptric optical unit, and a fourth
`re?ection at the object-side surface of the second catadioptric
`optical unit. Advantageously, the sum the outWard PetZval
`curvatures is cancelled out by the sum of inWard PetZval
`curvatures.
`
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`IPR2013-00362
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`Patent Application Publication
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`US 2013/0329283 A1
`
`Dec. 12, 2013
`
`CATADIOPTRIC OPTICAL SYSTEM WITH
`TOTAL INTERNAL REFLECTION FOR HIGH
`NUMERICAL APERTURE IMAGING
`
`BACKGROUND OF THE INVENTION
`
`[0001] 1. Field of the Invention
`[0002] The present invention is generally related optical
`imaging, and more particularly it is related to a catadioptric
`optical system With total internal re?ection for high numeri
`cal aperture imaging; the catadioptric optical system may ?nd
`industrial application in microscope objective systems or
`lithographic projection systems, among others.
`[0003] 2. Description of the Related Art
`[0004] Imaging apparatuses, such as a microscope, a litho
`graphic projection system, or even a telescope, use purely
`re?ective (catoptric), purely refractive (dioptric), or a combi
`nation of re?ective and refractive (catadioptric) optical ele
`ments to form an image of an object. A microscope uses an
`objective optical system to observe a sample, such as a bio
`logical tissue, a defect on a semiconductor Wafer or a surface
`of material. A lithographic projection system uses a projec
`tion objective to project an image of a pattern on a reticle onto
`a planar image surface of a semiconductor substrate (Wafer).
`In a telescope, an objective lens, larger in diameter than the
`pupil of a human eye, permits the collection of enough light to
`make visible distant point sources such as stars that otherWise
`may not be observed. To produce a good image, these instru
`ments must collect enough light re?ected from (or transmit
`ted through) an object, separate the details in the image,
`magnify the image, and render the details visible to the human
`eye or resolvable by an optical detector.
`[0005] The ability to resolve ?ne object details at a ?xed
`object distance, regardless of Whether the details correspond
`to physically close features (as in a microscope) or to features
`separated by a small angle (as in a telescope), is determined
`by the instrument’s resolution. Resolution (R) of a micro
`scope is given by Equation (1).
`
`A
`R = 0.61>< —
`NA
`
`(1)
`
`[0006] Where 7» is the Wavelength of the light used, NA is
`the numerical aperture of the microscope’s object space, and
`0.61 is derived from the Rayleigh criterion.
`[0007] From Equation (1), therefore, the resolution R can
`be improved by decreasing the Wavelength 7», or increasing
`the NA. In terms of decreasing the Wavelength 7», the use of
`ultraviolet (UV), deep ultraviolet (DUV), X-ray, and electron
`beam radiation has been investigated extensively for high
`resolution applications in microscopy and lithography. HoW
`ever, these applications are prohibitively expensive, and
`accordingly there is greater need for imaging using the visible
`spectrum (Wavelengths betWeen 400-700 nanometers
`approximately), as in the case of optical microscopes.
`[0008] Therefore, the vast majority of optical microscopes
`have objectives designed to ful?ll certain NA requirements.
`NA is determined by the instruments’ ability to gather enough
`light to resolve ?ne object details. In terms of its ability to
`gather enough light, the NA of a microscope is de?ned by
`Equation (2).
`NA :No sin 0m
`
`(2)
`
`[0009] Where No is the refractive index of the medium in
`object space, GM is the angle formed betWeen the marginal ray
`
`that comes from the object and the normal to the surface
`Where the marginal ray impinges (hereinafter GM is referred to
`as the “marginal angle”).
`[0010] From the perspective of Equation (2), therefore, in
`order to obtain a high NA value, either the angle GM of the
`marginal ray or the refractive index No of the medium in
`object space need to be large. As it is generally knoWn to
`persons having ordinary skill in the art, the medium in the
`object space of a microscope can be air or an immersion ?uid.
`When air (N51) is used in the object space, the maximum
`value of NA cannot be greater than unity, but When the object
`space is ?lled With a ?uid of index larger than 1 (N 0>1) a NA
`larger than 1 can be achieved. Incidentally, mo st conventional
`optical microscopes use objectives With NA values in the
`approximate range of 0.08 to 1.30, With the proviso that NA
`values greater than 0.95 can typically be achieved only by
`using an immersion ?uid in the object space. Accordingly, to
`further increase the NA value, regardless of the medium in the
`object space, the angle GM of the marginal ray needs to be
`increased. HoWever, this requires signi?cantly complicated
`optical arrangements for correcting aberrations.
`[0011] Speci?cally, many conventional optical designs for
`high NA values use catadioptric optical elements to minimiZe
`optical aberrations. See, for example, US. Pat. No. 5,650,
`877, international publication number WO2008/ 101676
`(herein “WO2008/101676”), and the article “A NeW Series of
`Microscope Objective: I. Catadioptric NeWtonian Systems,”
`JOSA 39, No. 9, 719-723 (1949), by Grey et al. (herein
`“Grey”).
`[0012] US. Pat. No. 5,650,877 discloses a lithographic
`reduction system in Which a catadioptric optical element hav
`ing specially con?gured front and back faces projects a
`reduced image of a reticle onto a substrate. The back face of
`the optical element has a central aperture surrounded by a
`concave re?ective surface. The front face has a partially
`re?ective surface that transmits a portion of the light beam
`toWard the concave re?ecting surface and re?ects a portion of
`the remaining light beam returned by the concave re?ective
`surface on a converging path through the central aperture. The
`substrate is aligned With the aperture, and is therefore
`exposed With high-resolution.
`[0013] WO2008/101676 discloses a lithographic projec
`tion system in Which a catadioptric optical element made of a
`high-index transparent material has a ?rst surface on an
`object-side of the element and a second surface opposite to
`the ?rst surface. The second surface has a transmissive por
`tion in a central region around the optical axis and a concave
`re?ective portion in a Zone around the transmissive portion.
`The ?rst surface has a transmissive Zone to transmit radiation
`coming from the object surface toWards the second surface
`and oriented relative to the second surface such that at least a
`portion of radiation re?ected by the re?ective portion of the
`second surface is totally re?ected by the transmissive portion
`of the ?rst surface toWards the transmissive portion of the
`second surface.
`[0014] Grey discloses a microscope objective With a last
`solid lens made of ?uorite or quartz-?uorite, Where both
`obj ect-side and image-side surfaces of the lens contain re?ec
`tive coating on certain regions thereof to achieve NA values
`greater than 0.95 at 220 to 540 nanometer Wavelengths pur
`portedly With negligible aberrations.
`[0015] A feature common to each of the above-discussed
`background references is the last optical element Which a
`catadioptric optical element (COE) in Which a central obscu
`
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`US 2013/0329283 A1
`
`Dec. 12, 2013
`
`ration blocks a portion of the light from passing through the
`central region thereof. Generally, the obscuration ratioi
`Which characterizes the fraction of blocked illuminationiis
`de?ned by the folloWing equation (3):
`
`,
`sinO,
`obscuration : , sinOm
`
`(3)
`
`[0016] Where 61 is the loWest angle to achieve the required
`obscuration ratio (hereafter 61 Will be referred to as the “loW
`est obscuration angle”), and GM is the marginal angle, as
`de?ned in Equation (2). Accordingly, a central obscuration
`larger than a certain threshold (e.g., 25%) can cause signi?
`cant degradation in image contrast and loss of light intensity,
`Which in turn degrades the quality of a resultant image.
`[0017] According to US. Pat. No. 5,650,877, the central
`obscuration may be limited by controlling the siZe of the
`illumination beam to block no more than 15 percent of the
`projected image. HoWever, although relatively loW obscura
`tion may be obtained by controlling the siZe of the illumina
`tion beam, substantial energy loss is caused by this technique.
`[0018] On the other hand, in the catadioptric optical ele
`ment disclosed by WO2008/ 101676 total internal re?ection
`(TIR) is used to minimize obscuration While achieving a
`desired level of NA. FIG. 1 illustrates a concept of the cata
`dioptric optical element disclosed by WO2008/ 101676.
`[0019] The left side of FIG. 1 illustrates a side vieW of a
`catadioptric lens 10, Which has a ?rst surface 11 and a second
`surface 12 opposite to each other. The ?rst surface 11 is
`generally concave When seen from the side of the second
`surface 12, and the second surface 12 is substantially planar
`(?at). A plane vieW of the substantially planar second surface
`12 is illustrated on the right side of FIG. 1. The ?rst surface 11
`has a transmissive portion in a central region around the
`optical axis AX and a concave re?ective portion in a region
`around the transmissive portion. That is, the transmissive
`portion and the concave re?ective portion are concentric to
`each other. The second surface 12 is generally transparent and
`has a total internal re?ection (TIR) region 16 and transmissive
`region 17, Which are concentric to each other and also cen
`tered on the optical axis AX. Light rays illuminating an object
`O passes through the transmissive portion of the ?rst surface
`11 and impinges ?rst on the second surface 12. More speci?
`cally, light rays R2 and R3 having angles of incidence
`betWeen the critical angle 66 and the marginal angle 6m
`undergo total internal re?ection on the TIR region 16 of the
`second surface 12, and are therefore re?ected toWards the
`re?ective portion of the ?rst surface 11. In turn, the re?ective
`portion of the ?rst surface 11 re?ects these rays forWard
`toWards the second surface 12 as light rays R2‘ and R3‘. This
`time, since the incident angles of rays R2‘ and R3‘ are less than
`the critical angle 66, the rays R2‘ and R3‘ are transmitted
`through the TIR region 16 of the second surface 12.
`[0020] On the other hand, light rays R1 propagating
`through the transmissive region of the ?rst surface 11 and
`impinging on the transmissive region 17 of the second surface
`12, at an incident angle less than the critical angle 66 (e.g.,
`incident at the minimum obscuration angle 61), cannot be
`re?ected by the second surface 12, but instead these rays are
`refracted as a light rays R1‘. The refracted rays R1‘ may be
`scattered or blocked by a central obscuration or ?eld stop
`aperture; thus, the light rays R1 With an incident angle 61 less
`than the critical angle 66 do not contribute to image formation.
`
`Moreover, the transmissive region 17 immediately around the
`optical axis AX is obscured because the object O itself blocks
`light incident normal to the object. Therefore, light rays
`impinging on the transmissive region 17 at incident angles
`small than the critical angle 66 may degrade image contrast
`and cause loss of light intensity.
`[0021] Furthermore, since a catadioptric optical element
`includes the above-described curved re?ective surfaces, other
`problems in terms of chromatic aberration, PetZval curvature
`and alignment arise.
`[0022] Correcting chromatic aberration, in particular,
`across the visible spectrum of Wavelengths is particularly
`challenging. As it is knoWn to persons having ordinary skill in
`the art, a microscope can be thought of as a positive lens. In
`that sense, the poWer of the positive lens produces What is
`knoWn as “undercorrected” axial chromatic aberration. To
`compensate for it, overcorrected axial chromatic aberration is
`intentionally generated by adding specially designed optical
`elements Within the microscope’s optical system.
`[0023] Image ?eld curvature is another imaging aspect to
`be considered. Speci?cally, since an image of a sample is
`generally captured by a sensor, such as CCD (charged
`coupled device) or CMOS (complementary metal oxide
`semiconductor) sensor, Which has a ?at surface, a ?at image
`is required at the plane Where the sensor is located. Generally,
`hoWever, since a microscope can be regarded as a positive
`lens, the poWer of the positive lens generates an image having
`an inWard-curving ?eld. The curvature of the resulting image
`is knoWn as the PetZval curvature. To compensate for inWard
`PetZval curvature, an outWard-curving ?eld is intentionally
`generated by adding specially designed optical elements
`Within the microscope’s optical system. Speci?cally, using a
`concave mirror has been knoWn to be an effective method for
`compensating the inWard PetZval curvature. It is clear, there
`fore, that correction of aberrations can considerably increase
`the number of lens elements that ultimately form the objective
`optical system of a microscope. This signi?cant increase in
`the number of optical elements often results in a tight-?t,
`di?icult to align, and oversiZed objective system.
`[0024] Accordingly, there is a need for objective optical
`systems that can provide minimum obscuration, correction of
`chromatic aberration and PetZval curvature, and alloW for
`appropriate alignment Without undue dif?culty.
`
`SUMMARY OF THE INVENTION
`
`[0025] According to an aspect of the present invention, a
`catadioptric optical system includes, in order from an object
`side to an image side and arranged along an optical axis, a ?rst
`catadioptric unit, a second catadioptric unit disposed in axial
`alignment With the ?rst catadioptric unit and With a space
`therebetWeen; and a lens group disposed in axial alignment
`With the ?rst and second catadioptric optical units. Light rays
`arriving from an object plane undergo a ?rst re?ection at the
`image-side surface of the ?rst catadioptric optical unit, a
`second re?ection at the object-side surface of the ?rst cata
`dioptric optical unit, a third re?ection at the image-side sur
`face of the second catadioptric optical unit, and a fourth
`re?ection at the object-side surface of the second catadioptric
`optical unit. The sum of outWard PetZval curvatures is can
`celled by the sum of inWard PetZval curvatures.
`[0026] Advantageously, embodiments of the present inven
`tion address the issues related to obscuration, correction of
`chromatic aberration and PetZval curvature, and alloW for
`appropriate alignment Without undue dif?culty.
`
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`Dec. 12, 2013
`
`[0027] Further features of the present invention Will
`become apparent from the following description of exem
`plary embodiments With reference to the attached drawings.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0028] FIG. 1 illustrates relevant portions of a conventional
`catadioptric optical element.
`[0029] FIG. 2 illustrates a catadioptric optical system
`including a plurality of catadioptric units With four times
`re?ection and one intermediate image, in accordance With an
`embodiment of the present invention.
`[0030] FIG. 3 illustrates side and plane vieWs of a ?rst
`catadioptric optical unit, in accordance With the present
`invention.
`[0031] FIG. 4 illustrates side and plane vieWs of a second
`catadioptric optical unit, in accordance With the present
`invention.
`[0032] FIG. 5 illustrates an exemplary ray tracing Within
`the ?rst and second catadioptric optical units, in accordance
`With the present invention.
`[0033] FIG. 6 illustrates a catadioptric optical system
`including a plurality of catadioptric units With six times
`re?ection and one intermediate image, in accordance With an
`embodiment of the present invention.
`[0034] FIG. 7 illustrates a catadioptric optical system
`including a plurality of catadioptric units With six times
`re?ection and tWo intermediate images, in accordance With an
`embodiment of the present invention.
`[0035] FIG. 8 illustrates a catadioptric optical system
`including a plurality of catadioptric units With eight times
`re?ection and tWo intermediate images, in accordance With an
`embodiment of the present invention.
`
`DESCRIPTION OF THE EMBODIMENTS
`
`[0036] Embodiments according to the present invention
`Will be described beloW With reference to the attached draW
`ings. In the various draWings discussed beloW, the left side of
`each ?gure Will be referred to as the front or object-side,
`Whereas the opposite side thereof (right side) Will be referred
`to as the back or image side. Therefore, as used herein, the
`side of an optical element (lens or mirror) Where the object to
`be imaged is placed is interchangeably referred to as the
`“obj ect-plane side”, “object side”, or “front side” of the opti
`cal element; and the side of optical element Where the image
`is formed is interchangeably referred to as the “image-plane
`side”, “image side” or “back side” of the optical element.
`[0037] FIG. 2 illustrates an exemplary embodiment of a
`catadioptric optical system 10. The optical system 10 gener
`ally includes, in order from the object side to the image side
`and aligned along an optical axis AX (in axial alignment), a
`?rst catadioptric optical unit CG1, a second catadioptric opti
`cal unit CG2 and a lens group LG. The ?rst catadioptric
`optical unit CG1 consists of a single catadioptric optical
`element (COE) 100; and the second catadioptric optical unit
`CG2 includes a catadioptric optical element (COE) 200. Pref
`erably, the second catadioptric unit CG2 is disposed at a small
`distance of (spaced apart from), and in axial alignment With,
`the ?rst catadioptric unit CG1. The separation betWeen the
`?rst catadioptric unit CG1 and the second catadioptric unit
`CG2 can be determined by a block BK, Which may be imple
`mented by either leaving an empty space (air gap) or intro
`ducing a substantially parallel piece of optically transparent
`material (for example glass) betWeen CG1 and CG2. Alter
`
`natively, the block BK may correspond to an optical ?lter, a
`phase plate, or the like. Notably, the separation betWeen CG1
`and CG2 serves the purpose of easy alignment. Speci?cally,
`When the image-side surface of CG1 and the object-side
`surface of CG2 are substantially ?at, the placement of a
`transparent block BK (or air gap) therebetWeen can make
`alignment thereof easier by avoiding direct contact betWeen
`these tWo delicate optical surfaces. In order for light coming
`from object to be re?ected by total internal re?ection inside
`CG1, space betWeen CG1 and CG2 needs to be ?lled With a
`medium (material) Whose refractive index is loWer than that
`of CG1. Especially, When the space is air, critical angle for
`total internal re?ection in the TIR region of CG1 can be
`reduced; this means that obscuration ratio can be also
`decreased.
`[0038] In operation, the catadioptric optical system 10 is
`con?gured to form, at an image plane IP, an image IM of an
`object O located at an object plane OP. The image plane IP
`may correspond to an image surface of a solid-state image
`sensor S, such as a CCD sensor or a CMOS sensor. The
`catadioptric optical system 10 can also operate in reverse
`Where the object plane OP and image plane IP are inter
`changed, as in the case of a lithographic projection system.
`[0039] FIG. 3 illustrates a detailed vieW of the ?rst cata
`dioptric optical unit CG1; and FIG. 4 illustrates a detailed
`vieW of the second catadioptric optical unit CG2. As illus
`trated in FIG. 3, a plane vieW of the curved object-side surface
`101 (front vieW) of COE 100 is shoWn on the left-most section
`of the ?gure; and a plane vieW of the ?at image-side surface
`102 (back vieW) of COE 100 is shoWn on the right-most
`section of the ?gure. A side vieW of COE 100 is shoWn in the
`central section of the ?gure. Speci?cally, FIG. 3 illustrates the
`COE 100 having an object-side surface 101 and an image
`side surface 102 opposite to each other. The image-side sur
`face 102 is a non-curved, substantially ?at (planar) surface.
`The object-side surface 101 is a curved surface and may have
`either a spherical or aspherical axis symmetrical (axisymmet
`ric) shape. In FIG. 3, as seen from the side of the image-side
`surface 102, the object-side surface 101 is concave.
`[0040] As shoWn in FIG. 3, the object-side surface 101
`includes a circular transmissive portion 130 (object-side
`transmissive portion) centered the optical axis AX and a
`re?ective portion 120 (object-side re?ective portion) in a
`rotationally symmetric Zone around the circular transmissive
`portion 130. The circular transmissive portion 130 centered
`on the optical axis AX is an optically transparent (transmis
`sive) region and serves to transmit therethrough light origi
`nated at an object O disposed on the optical axis AX and
`located at an object plane OP. At least the re?ective portion
`120 of the ?rst surface 101 has a curved shape concave
`toWards the image side thereof. The re?ective portion 120 of
`the obj ect-side surface 101 is preferably coated With a ?lm of
`highly re?ective materials to form What can be considered an
`inner (internal) concave mirror. That is, the re?ective portion
`120 is an area of COE 100 in an outer region thereof that
`serves as a ?rst mirror M1, and the circular transmissive
`portion 130 is an area concentric to the optical axis AX not
`coated With the re?ective coating ?lm. Optionally, the trans
`missive region 130 may be coated With an anti-re?ection
`coating (?lm) to increase transmission of light rays from the
`object O to the second surface 102.
`[0041] The image-side surface 102 of COE 100 includes a
`central transmissive region 170 (image- side transmissive por
`tion) centered on the optical axis AX, a ring-shaped re?ective
`
`11
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`US 2013/0329283 A1
`
`Dec. 12, 2013
`
`region 150 (image-side re?ective portion) in a rotationally
`symmetric Zone around the central transmissive region 170,
`and a total internal re?ection (TIR) region 110 (image-side
`TIR portion) in a rotationally symmetric region around the
`ring-shaped re?ective region 150. At least the central trans
`missive region 170 and the TIR region 110 are transparent
`surfaces devoid of any re?ective coatings, so as to transmit
`light incident thereupon at predetermined incidence angles.
`The ring-shaped re?ective region 150 is rendered specularly
`re?ective preferably by coating a region of the image-side
`surface 102 With highly re?ective materials, or by any con
`venient and knoWn process. More speci?cally, as illustrated
`in FIG. 3, the ring-shaped re?ective region 150 (hatched
`region) includes an area of the image-side surface 102 cov
`ered With a specularly re?ective ?lm, in a rotationally sym
`metric Zone around the central transmissive region 170 and
`Within the TIR region 110. The ring-shaped re?ective region
`150 namely serves to extend (enlarge) the re?ective function
`of the TIR region 110 toWards the optical axis AX.
`[0042] The specularly re?ective ?lm that forms the ring
`shaped re?ective region 150 can be selected, for example,
`from a metal ?lm such as aluminum and silver or a multilay
`ered ?lm made of different materials. The thickness of the
`re?ective ?lm may be selected, for example, betWeen tens of
`nanometers and a feW hundreds of micrometers. More spe
`ci?cally, the thickness and material of the re?ective ?lm may
`be chosen in accordance With the Wavelength of light to be
`used. A material of the COE 100 can be selected, for example,
`from croWn glass, ?int glass, abnormal dispersion glass,
`fused silica, ?uorite, etc., including equivalents and combi
`nations thereof. Therefore, the COE 100 can be considered as
`a solid lens made of transparent material (for example glass)
`shaped as a plano-convex lens, and having an internal specu
`larly re?ective surface on at least one side thereof.
`[0043] Further details concerning the structure and func
`tion of the COE 100, and in particular the ring-shaped re?ec
`tive region 150 can be found in US. patent application
`entitled “CATADIOPTRIC OPTICAL ELEMENT AND
`OPTICAL SYSTEM INCLUDING SAME”, Attorney
`Docket No.: 2000-1 l32l-CINC, ?led concurrently With the
`present application by an assignee of the present application,
`Which is hereby incorporate by reference for all purposes.
`[0044] Referring noW to FIG. 4, the second catadioptric
`optical unit CG2 is discussed. In the present embodiment, the
`second catadioptric optical unit CG2 includes the catadioptric
`optical element (COE) 200, Which may also be implemented
`as a solid optical element made of one or more parts (e.g.,
`cemented catoptric and dioptric elements). As seen from the
`object side to the image side (left to right in FIG. 4), the COE
`200 has an obj ect-side surface 201 and an image-side surface
`202 opposite to each other.
`[0045] The image-side surface 202 of the COE 200
`includes a circular transmissive portion 270 (image-side
`transmissive portion) centered on the optical axis AX, and a
`re?ective portion 210 (image-side re?ective portion) in a
`rotationally symmetric Zone around the circular transmissive
`portion 270. The re?ective portion 210 is silvered or made
`specularly re?ective by any knoWn process. Accordingly, the
`re?ective portion 210 can be considered as a mirror M2.
`[0046] The object-side surface 201 includes a circular
`internally re?ective portion 230 (object-side internal re?ec
`tive portion) centered on the optical axis AX, and a transmis
`sive portion 220 (image-side transmissive portion) in a rota
`tionally symmetric Zone around the circular re?ective portion
`
`230. At least the re?ective portion 230 of the obj ect-side
`surface 201 has a curved shape convex toWards the image side
`thereof (convex toWards the image-side surface 202). That is,
`the re?ective portion 230 is in effect an internal convex mirror
`M3 centered on the optical axis AX and facing the image-side
`surface 202. The transmissive portion 220 is preferably ?at
`and may be devoid of any coatings. Alternatively, in certain
`embodiments the transmissive portion 220 may include a
`aspheric or spheric curved surface (Where the radius of cur
`vature is much greater than the radius of curvature of the
`re?ective region 230). In addition, the transmissive portion
`220 may be optionally coated With antire?ection materials to
`improve light transmittance therethrough. That is, as illus
`trated in FIG. 4, in the present embodiment, the second cata
`dioptric optical unit CG2 consists essentially of a single cata
`dioptric optical element (COE 200), Which has an internal
`specularly re?ective region 230 on the object-side surface
`201, and an internal specularly re?ective region 210 on the
`image-side surface 202. The image-side re?ective region 210
`and obj ect-side re?ective region 230 are respectively referred
`to as concave mirror M2 and convex mirror M3, as illustrated
`in FIG. 5.
`[0047] Considering noW a optical path from the object
`plane OP to the image plane IP (see FIG. 2), the image IM of
`an object O disposed at the object plane OP along the optical
`axis AX can obtained at the image plane IP, When light com
`ing from the object O undergoes a plurality of re?ections to
`travel from the object plane OP to image plane IP. As it is
`knoWn to persons having ordinary skill in the art, multiple
`re?ections may minimiZe the total length of the optical sys
`tem While signi?cantly increasing the total focal length,
`numerical aperture and magni?cation of the optical system.
`HoWever, this leads to an optical system design With large
`obscuration ratios and augmented aberrations. In the present
`application, hoWever, When properly constructed, the appro
`priate combination of catadioptric optical elements enables
`negative chromatic aberration to offset (cancel out) positive
`chromatic aberration, While as the same time positive PetZval
`curvature cancels out negative PetZval curvature. More spe
`ci?cally, as used herein the terms “offset” or “cancel out” are
`intended to denote an action to make something ineffective, to
`counteract, to nullify, to compensate to counterbalance, to
`offset an error or defect or undesired effect. Accordingly, in
`the speci?cation and claims, a ?rst value can be considered
`“canceled out” by a second value, When the ?rst value is made
`substantially ineffective by the second value. In addition, as
`disclosed herein, the appropriate combination of refractive
`and re?ective surfaces advantageously alloW for reducing the
`obscuration ratio, increasing NA, and providing easy align
`ment of the optical system. Accordingly, in the remainder of
`the present application, the various examples of embodiments
`are described based on the number of re?ections included in
`the optical path. To that end, each specularly re?ective surface
`Will be referred to as mirror Mi (Where “i” is a positive integer
`corresponding to a specularly re?ective surface counted in the
`direction and order in Which light travels from the object plain
`OP to the image-plain IP.
`
`<Four-Re?ection Catadioptric System>
`
`[0048] As illustrated in FIGS. 2-5, light rays originated at
`the object O enter the ?rst catadioptric optical unit CG1
`through the obj ect-side transmissive region 130 and impinge
`on the image-side surface 102 of COE 100. The incoming
`rays undergo a ?rst re?ection at the image-side surface 102
`
`12
`
`

`
`US 2013/0329283 A1
`
`Dec. 12, 2013
`
`namely due to total internal re?ection, but can also undergo
`specular re?ection, When the ring-shaped re?ective region is
`provided on the image-side surface 102. In any case, from the
`image-side surface 102, the incoming rays are re?ected
`toWard the object-side re?ective portion 120 (also referred to
`as mirror M1) Whence they are re?ected forWard toWard the
`image side, and exit through the image-side surface 102.
`Depending on the shape design of the obj ect-side surface 101,
`the light rays can exit through the image-side surface 102
`substantially parallel to the optical axis AX.
`[0049] Continuing to refer to FIG. 3, it should also be noted
`that the TIR region 110 on the image-side surface 10