Catadioptric Projection Objective With Intermediate Images
A catadioptric projection objective has a first objective part, defining a first part of the optical axis and imaging an object field to form a first real intermediate image. It also has a second, catadioptric objective part forming a second real intermediate image using the radiation from the first objective part. The second objective part has a concave mirror and defines a second part of the optical axis. A third objective part images the second real intermediate image into the image plane and defines a third part of the optical axis. Folding mirrors deflect the radiation from the object plane towards the concave mirror; and deflect the radiation from the concave mirror towards the image plane. The first part of the optical axis defined by the first objective part is laterally offset from and aligned parallel with the third part of the optical axis.
This application is a Continuation of co-pending U.S. application Ser. No. 14/672,497, filed on Mar. 30, 2015, which is a Continuation of co-pending U.S. application Ser. No. 14/143,000, filed on Dec. 30, 2013, now U.S. Pat. No. 9,134,618, issued on Sep. 15, 2015, which is a Continuation of co-pending U.S. application Ser. No. 14/079,026, filed on Nov. 13, 2013, now U.S. Pat. No. 8,913,316, issued on Dec. 16, 2014, which is a Continuation of co-pending U.S. application Ser. No. 13/361,707, filed on Jan. 30, 2012, now U.S. Pat. No. 9,019,596, issued on Apr. 28, 2015, which is a Continuation of U.S. application Ser. No. 11/596,868, filed on Oct. 6, 2008, now U.S. Pat. No. 8,107,162, issued on Jan. 31, 2012, which was the National Stage of International Application No. PCT/EP2005/005250, filed on May 13, 2005, which claims the benefit of US Provisional Application No. 60/571,533 filed on May 17, 2004. All of these prior applications are hereby incorporated into the present application in their entireties by reference.
BACKGROUND OF THE INVENTION Field of the InventionThe invention relates to a catadioptric projection objective for imaging of a pattern, which is arranged on the object plane of the projection objective, on the image plane of the projection objective.
Description of the Related ArtProjection objectives such as these are used in microlithography projection exposure systems for the production of semiconductor components and other finely structured components. They are used to project patterns of photomasks or reticles which are referred to in the following text in a general form as masks or reticles, onto an object which is coated with a light-sensitive layer, with very high resolution and on a reduced scale.
In this case, in order to produce ever finer structures, it is necessary on the one hand to enlarge the image-side numerical aperture (NA) of the projection objective, and on the other hand to use ever shorter wavelengths, preferably ultraviolet light at wavelengths of less than about 260 nm, for example 248 nm, 193 nm or 157 nm.
In the past, purely refractive projection objectives have been predominantly used for optical lithography. These are distinguished by a mechanically relatively simple, centered design, which has only a single optical axis, that is not folded. Furthermore, it is possible to use object fields which are centered with respect to the optical axis, which minimize the light transmission level to be corrected, and simplify adjustment of the objective.
However, the form of the refractive design is primarily characterized by two elementary imaging errors: the chromatic correction and the correction for the Petzval sum (image field curvature).
Catadioptric designs, which have at least one catadioptric objective part and a hollow mirror or a concave mirror, are used to simplify the correction for the Petzval condition and to provide a capability for chromatic correction. In this case, the Petzval correction is achieved by the curvature of the concave mirror and negative lenses in its vicinity, while the chromatic correction is achieved by the refractive power of the negative lenses upstream of the concave mirror (for CHL) as well as the diaphragm position with respect to the concave mirror (CHV).
One disadvantage of catadioptric designs with beam splitting is, however, that it is necessary to work either with off-axis object fields, that is to say with an increased light conductance value (in systems using geometric beam splitting) or with physical beam splitter elements, which generally cause polarization problems. The term “light conductance value” as used here refers to the Lagrange optical invariant or Etendue, which is defined here as the product of the image field diameter and the image-side numerical aperture.
In the case of off-axis catadioptric systems, that is to say in the case of systems with geometric beam splitting, the requirements for the optical design can be formulated as follows: (1) reduce the light transmission level to the maximum extent, (2) design the geometry of the foldings (beam deflections) such that a mounting technology can be developed for this purpose, and (3) provide effective correction, in particular the capability to correct the Petzval sum and the chromatic aberrations jointly in the catadioptric mirror group.
In order to keep the geometric light conductance value (Etendue) low, the folding of the design should in principle take place in the region of low NA, that is to say for example close to the object, or in the vicinity of a real intermediate image.
However, as the numeric aperture increases, the object-side numerical aperture also increases, and thus the distance between the first folding mirror and the reticle, so that the light transmission level rises. Furthermore, the diameter of the hollow mirror and the size of the folding mirror increase. This can lead to physical installation space problems.
These can be overcome by first of all imaging the reticle by means of a first relay system onto an intermediate image, and by carrying out the first folding in the area of the intermediate image. A catadioptric system such as this is disclosed in EP 1 191 378 A1. This has a refractive relay system, followed by a catadioptric objective part with a concave mirror. The light falls from the object plane onto a folding mirror (deflection mirror) which is located in the vicinity of the first intermediate image, from there to the concave mirror and from there onto a refractive object part, with a second real intermediate image being generated in the vicinity of a second deflection mirror, and the refractive object part images the second intermediate image on the image plane (wafer). Concatenated systems having, in that sequence, a refractive (R), a catadioptric (C), and a refractive (R) imaging subsystem will be denoted “R-C-R” type systems in the following.
Systems of type R-C-R with a similar folding geometry are disclosed in WO 2004/019128 A, WO 03/036361 A1 and US 2002/019946 A1. Patent application US 2004/0233405 A1 discloses R-C-R type projection objectives with different folding geometries including objectives where the first folding mirror is arranged optically downstream of the concave mirror to deflect radiation coming from the concave mirror towards the image plane.
Other catadioptric systems with two real intermediate images are disclosed in JP 2002-372668 and in U.S. Pat. No. 5,636,066. WO 02/082159 A1 and WO 01/04682 disclose other catadioptric systems with more than one intermediate image.
SUMMARY OF THE INVENTIONIt is one object of the invention to provide a catadioptric projection objective which allows very high resolutions to be achieved, with a compact design with optimized dimensions. It is another object to allow correction of the Petzval sum and of the chromatic aberrations with good manufacturing conditions.
As a solution to these and other objects this invention, according to one formulation, provides a catadioptric projection objective for imaging of a pattern, which is arranged on the object plane of the projection objective, on the image plane of the projection objective, having: a first objective part for imaging of an object field to form a first real intermediate image; a second objective part for generating a second real intermediate image using the radiation coming from the first objective part; and a third objective part for imaging the second real intermediate image on the image plane; wherein the second objective part is a catadioptric objective part with a concave mirror; a first folding mirror for deflecting the radiation coming from the object plane in the direction of the concave mirror and a second folding mirror for deflecting the radiation coming from the concave mirror in the direction of the image plane are provided; and a field lens with a positive refractive power is arranged between the first intermediate image and the concave mirror, in a region close to the field of the first intermediate image.
According to another formulation, a field lens with a positive refractive power is arranged geometrically between the first folding mirror and the concave mirror in a region close to the field of the first intermediate image. This position is optically between the first intermediate image and the concave mirror if the first intermediate image is created optically upstream, i.e. before the field lens in light propagation direction. The first intermediate image may also be positioned optically downstream, i.e. behind the field lens, or may partly extend into the field lens.
The enlargement of the numerical aperture which is required in order to achieve very high resolutions frequently leads in conventional systems to a major increase in the diameter of the optical components which are located in the area of preferred diaphragm positions. The invention counteracts this effect.
The expression “field lens” describes an individual lens or a lens group with at least two individual lenses. The expression takes account of the fact that the function of a lens can fundamentally also be carried out by two or more lenses (splitting of lenses). The refractive power of this field lens is arranged close to the field, that is to say in the optical vicinity of a field plane. This area close to the field for a field plane is distinguished in particular by the chief ray height of the imaging being large in comparison to the marginal ray height. In this case, the marginal ray height is the ray height of a marginal ray which leads from the innermost point of the object field, which is closest to the optical axis, to an edge of an aperture diaphragm, while the chief ray (principal ray) leads from the outermost field point of the object field parallel to, or at an acute angle to, the optical axis and intersects the optical axis in the area of the system diaphragm, that is to say at a diaphragm location which is suitable for the fitting of an aperture diaphragm. The ratio between the marginal ray height and the chief ray height is thus less than unity in the area close to the field.
The expression “intermediate image” describes the area between a paraxial intermediate image and an marginal ray intermediate image. Depending on the correction state of the intermediate image, this area may extend over a certain axial range in which case, by way of example, the paraxial intermediate image may be located in the light path upstream or downstream of the marginal ray intermediate image, depending on the spherical aberration (undercorrection or overcorrection). The paraxial intermediate image and the marginal ray intermediate image may also essentially coincide. For the purposes of this application, an optical element A, for example a field lens, is located “between” an intermediate image and another optical element B when at least a portion of the optical element A is located between the (generally axially extended) intermediate image and the optical element B. The intermediate image may thus also partially extend beyond an optical surface which, for example, may be advantageous for correction purposes. The intermediate image is frequently located completely outside optical elements. Since the radiation energy density in the intermediate image area is particularly high, this may be advantageous, for example, with respect to the radiation load on the optical elements.
Positive refractive power in the divergent beam path between an upstream intermediate image and the concave mirror contributes to the capability for the downstream lenses in the beam path and the concave mirror to have small diameters. This applies in particular to the at least one negative lens which is provided in preferred embodiments in the immediate vicinity upstream of the concave mirror and which, in conjunction with the concave mirror, makes a major contribution to the correction of the chromatic longitudinal aberration CHL. If the chromatic longitudinal aberration is corrected in some other way, there is no need for this negative lens.
The insertion of positive refractive power between a field plane upstream of the concave mirror and the concave mirror leads in its own right to a contribution to the image field curvature which is proportional to the strength of the positive refractive power. In order to at least partially compensate for this effect, the concave mirror should have greater curvature than in the absence of the positive refractive power. In order, on the other hand, to keep the aberrations which are introduced by the reflection on the concave mirror as small as possible, it is advantageous for the radiation which strikes the concave mirror to strike it essentially at right angles. When positive refractive power is inserted downstream from the intermediate image, this leads to an increase in the negative refractive power directly upstream of the concave mirror, in order to ensure largely perpendicular radiation incidence by virtue of the scattering effect. The increase in the negative refractive power upstream of the concave mirror can at least partially compensate for the reduction in the CHL correction by reducing the size of the lens diameter in this area, so that good CHL correlation is ensured even with a relatively small mirror diameter.
In preferred embodiments, the first intermediate image is located in the vicinity of a folding mirror, which makes it possible to keep the Etendue of the system small. The field lens can generally be fitted very close to the intermediate image without being adversely affected by the folding mirror, thus allowing effective correction of imaging errors. In particular, the objective parts can be suitably designed in order to ensure that at least the intermediate image which is close to the field lens is subject to severe aberration. This allows particularly effective correction of imaging errors. The effectiveness of the correction can be assisted by designing at least that lens surface of the field lens which faces the intermediate image as an aspherical surface.
In one embodiment, the field lens is arranged geometrically between the concave mirror and at least one folding mirror in a region through which the beam passes twice, in such a manner that a first lens area of the field lens is arranged in the beam path between the object plane and the concave mirror, and a second lens area of the field lens is arranged in the beam path between the concave mirror and the image plane.
The field lens can be arranged such that it is arranged not only in the optical vicinity of an intermediate image plane which is located in the beam path upstream of the concave mirror, but also in the optical vicinity of an intermediate image plane which is located in the beam path downstream from the concave mirror. This results in an arrangement close to the field with respect to two successive field planes, so that a powerful correction effect can be achieved at two points in the beam path.
At least one multiple area lens can be arranged in a region of the projection objective through which the beam passes twice and has a first lens area, through which the beam passes in a first direction, and a second lens area, through which the beam passes in a second direction, with the first lens area and the second lens area not overlapping one another, at least on one side of the lens. This multiple area lens may be used as a field lens. If the footprints of the beam paths do not overlap on at least one of the two lens faces, a multiple area lens such as this allows two lenses which act independently of one another to be geometrically moved to a common point. It is also possible to physically manufacture two lenses which act independently of one another as one lens, specifically as an integral multiple area lens, from one lens blank. A multiple area lens such as this can be clearly distinguished from a conventional lens through which the beam passes twice since, in the case of a multiple area lens of this type its optical effect on the beams which pass through it independently of one another can be influenced by suitable independent shaping of the refractive surfaces of the lens areas independently of one another. Alternatively, a lens arrangement with one or two half-lenses or lens elements can also be arranged at the location of an integral multiple area lens in order to influence the beams as they pass one another, independently of one another.
Projection objectives with geometric beam splitting, an intermediate image and a multiple area lens have been disclosed, for example, in WO 03/052462 A1 from the same applicant. The disclosure in this patent application is included by reference in the content of this description.
The projection objective preferably has an image-side numerical aperture of NA>0.85, and an image-side working distance of A≦10 mm. Projection objectives such as these may be used, if required, or immersion lithography with NA>1. The image-side working distance or the working distance in the image area is the (shortest) axial distance between the exit surface of the objective and the image plane. This working distance in the image area, which is filled with a gas during operation in dry systems, is filled with an immersion medium during operation in immersion systems, with the immersion medium having a refractive index which is relatively high in comparison to that of gas.
It is generally advantageous for the image-side working distance not to fall below a minimum value. In this case, it should be noted that scratches, dirt and inhomogeneities on or in the last optical element can lead to a disturbance of the image if the working distance is too short. A finite working distance of, for example, 1 mm or more can in contrast lead to relatively large subapertures (footprints of one specific field point) with the high image-side numerical apertures, so that an averaging effect can occur and any image disturbance is reduced or suppressed.
Particular criteria must be taken into account for the definition of the working distance in the image area in immersion systems. On the one hand, a long working distance results not only in greater radiation losses owing to the normally lower transmission of immersion liquids (in comparison to gases) but also to a greater amount of aberration of the surfaces which are located in the vicinity of the image plane, specifically for spherical aberration. On the other hand, the image-side working distance should be sufficiently large to allow laminar flow of an immersion fluid. It may also be necessary to provide space for measurement purposes and sensors. In preferred embodiments, the image-side working distance is between about 1 mm and about 8 mm, in particular between about 1.5 mm and about 5 mm. When using an immersion fluid between the exit surface and the image plane, preferred embodiments have an image-side numerical aperture of NA≧0.98, with the image-side numerical aperture preferably being at least NA=1.0, or at least NA=1.1. The projection objective is preferably matched to an immersion medium which has a refractive index of nI>1.3 at an operating wavelength.
Very pure water for which n≈1.43 is suitable as an immersion medium for an operating wavelength of 193 nm. The article “Immersion Lithography at 157 nm by M. Switkes and M. Rothschild, J. Vac.Sci. Technol. B 19(6), November/December 2001, pages 1 et seq proposes immersion liquids based on perfluoropolyethers (PFPE) which are sufficiently transparent for an operating wavelength of 157 nm and are compatible with a number of photoresist materials that are currently used in microlithography. One tested immersion liquid has a refractive index of nI=1.37 at 157 nm.
The optical design also allows use for non-contacting near-field projection lithography. In this case, sufficient light energy can be injected into the substrate to be exposed via a gap which is filled with gas, provided that a sufficiently short image-side working distance is maintained, averaged over time. This should be less than four times the operating wavelength that is used, in particular less than the operating wavelength. It is particularly advantageous for the working distance to be less than half the operating wavelength, for example less than one third, one quarter or one fifth of the operating wavelength. These short working distances allow imaging in the optical near field in which evanescent fields, which exist in the immediate vicinity of the last optical surface of the imaging system, are used for imaging.
The invention thus also covers a non-contacting projection exposure method in which evanescent fields of the exposure light which are located in the immediate vicinity of the exit surface can be used for the lithographic process. In this case, if the working distances are sufficiently short (finite), a light component which can be used for lithography can be emitted from the exit surface of the objective, and can be injected into an entry surface, which is immediately adjacent at a distance, despite geometrical total internal reflection conditions on the last optical surface of the projection objective.
Embodiments for non-contacting near-field projection lithography preferably use typical working distances in the region of the operating wavelength or less, for example between about 3 nm and about 200 nm, in particular between about 5 nm and about 100 nm. The working distance should be matched to the other characteristics of the projection system (characteristics of the projection objective close to the exit surface, characteristics of the substrate close to the entry surface) so as to achieve an input efficiency of at least 10%, averaged over time.
A method for production of semiconductor components and the like is thus possible within the scope of the invention, in which a finite working distance is set between an exit surface for exposure light which is associated with the projection objective and an entry surface for exposure light which is associated with the substrate, with the working distance within an exposure time interval being set, at least at times, to a value which is less than a maximum extent of an optical near field of the light emerging from the exit surface.
Use as a dry objective is also possible, if required, with minor modifications. Dry objectives are designed such that a gap which is filled with gas is produced during operation between the exit surface of the projection objective and the entry surface of an object to be exposed, for example a wafer, with this gap width typically being considerably greater than the operating wavelength. The achievable numerical apertures with systems such as these are restricted to values NA<1, since total internal reflection conditions occur on the exit surface on approaching the value NA=1, preventing any exposure light from being emitted from the exit surface. In preferred embodiments of dry systems, the image-side numerical aperture is NA≧0.85 or NA≧0.9.
The third objective part immediately upstream of the image plane is preferably designed to be purely refractive, and can be optimized in order to produce high image-side numerical apertures (NA). It preferably has a first lens group, which follows the second intermediate image, and has a positive refractive power, a second lens group, which immediately follows the first lens group and has a negative refractive power, a third lens group which immediately follows the second lens group and has a positive refractive power, a fourth lens group which immediately follows the third lens group and has a positive refractive power, and a pupil surface which is arranged in a transitional region from the third lens group to the fourth lens group and in whose vicinity a system diaphragm can be arranged. The third lens group preferably has an entry surface which is located in the vicinity of a point of inflection of a marginal ray height between the second lens group and the third lens group, with no negative lens with any substantial refractive power being arranged between this entry surface and the system diaphragm. There are preferably only positive lenses between this entry surface and the image plane. This allows a material-saving design, with moderate lens diameters.
The last optical element in the projection objective immediately upstream of the image plane is preferably a plano-convex lens with a high spherical or aspherically curved entry surface and an exit surface which is essentially planar. This may be in the form of a plano-convex lens which is virtually hemispherical or is not hemispherical. The last optical element, in particular the plano-convex lens, may also be composed of calcium fluoride in order to avoid problems resulting from radiation-induced density changes (in particular compaction).
The first objective part may be used as a relay system in order to produce a first intermediate image, with a predetermined correction state at a suitable position, from the radiation coming from the object plane. The first objective part is generally purely refractive. In some embodiments, at least one folding mirror is provided in this first objective part, which images the object plane to form a first intermediate image, such that the optical axis is folded at least once, and preferably no more than once, within the objective part which is closest to the object.
In some embodiments, the first objective path is a catadioptric objective part with a concave mirror and an associated folding mirror, which may be used as the first folding mirror for the overall projection objective.
The provision of at least two catadioptric subsystems has major advantages. In order to identify significant disadvantages of systems with only one catadioptric subsystem, it is necessary to consider how the Petzval sum and the chromatic aberrations are corrected in a catadioptric part. The contribution of a lens to the chromatic longitudinal aberration CHL is proportional to the square of the marginal ray height h, to the refractive power φ of the lens, and to the dispersion v of the material. On the other hand, the contribution of a surface to the Petzval sum depends only on the surface curvature and of the sudden change in the refractive index (which is −2 in the case of a mirror in air).
In order to allow the contribution of the catadioptric group to the chromatic correction to become large, large marginal ray heights (that is to say large diameters) are thus required, and large curvatures are required in order to allow the contribution to the Petzval correction to become large (that is to say small radii, which are best achieved with small diameters). These two requirements are contradictory.
The contradictory requirements based on Petzval correction (that is to say correction of the image field curvature) and chromatic correction can be solved by introduction of (at least) one further catadioptric part into the system. Since the first catadioptric objective part can be designed such that both the image field curvature and the chromatic longitudinal aberration can be largely or completely corrected, the first intermediate image may have a defined correction state with respect to these aberrations, so that the subsequent objective parts may have an advantageous design.
In one embodiment, the first objective part is a catadioptric objective part with a physical beam splitter, which has a polarization-selective beam splitter surface which is used as a folding mirror and at the same time separates that radiation which leads to the concave mirror of the first objective part from that radiation which is reflected by this concave mirror.
In some embodiments, a concave mirror is provided which is designed as an active mirror, so that the shape of the concave mirror surface can be varied by a suitable drive. This can be used to compensate for various imaging errors.
Some embodiments of projection objectives according to the invention have a crossed beam path at at least one point. For this purpose, they are designed such that a first beam section which runs from the object plane to a concave mirror and a second beam section which runs from the concave mirror to the image plane can be produced, and one folding mirror is arranged with respect to the concave mirror in such a manner that one of the beam sections is folded on the folding mirror, and the other beam section passes through the folding mirror without any vignetting, and the first beam section and the second beam section cross over in a crossing region.
The crossed beam path in the region of a catadioptric objective part, allows projection objectives with a compact and mechanically robust arrangement of the optical components. In this case, a beam path without any vignetting can be achieved, so that no folding mirror intersects a beam which is either reflected on the folding mirror or is passed by the folding mirror without reflection. In this way, only the system diaphragm limits the angular distribution of the rays which contribute to imaging, in an axially symmetrical manner. At the same time, even with the largest numerical apertures, which are associated with large maximum beam diameters and, possibly, highly convergent or divergent beams in the region of the field planes, it is possible to achieve a moderate size for the overall field to be corrected. In this case, the expression “overall field” describes the field area which is enclosed by a minimum circle around the generally rectangular field. The size of the overall field to be corrected increases with the field size and the lateral offset of an axially asymmetric field with respect to the optical axis, and should be minimized in order to simplify the correction process.
Catadioptric projection objectives with a crossed beam path are disclosed, for example, in the US provisional application with the serial number 60/511,673, which was filed on Oct. 17, 2003, or the US patent application with the Ser. No. 10/734,623, which was filed on Dec. 27, 2003, or the US provisional application with the Ser. No. 60/530,622, which was filed on Dec. 19, 2003, by the same applicant. The disclosure content of these patent applications is included by reference in the content of this description.
In preferred embodiments, an off-axis effective object field arranged in the object surface of the projection objective is imaged onto an off-axis effective image field arranged in the image surface of the projection objective. Here, the term “effective object field” relates to the object field which can be effectively imaged by the projection objective without vignetting at a given numerical aperture. The amount of a lateral offset between the effective object field and the first part of the optical axis defined by the first objective part may be characterized by a finite object center height h. Likewise, on the image side of the projection objective, the effective image field is laterally offset with respect to the image side part of the optical axis by a finite image center height h′ related to the object center height h by the magnification ratio 13 of the projection objective according to h′=|β·h|. In some conventional projection objectives having a refractive first objective part, a catadioptric second objective part, and a refractive third objective part (also denoted type R-C-R) efforts have been made to align the parts of the optical axis defined by the object side refractive objective part and the image side refractive objective part such that no lateral axis offset exists between these parts of the optical axis. However, under these conditions, a finite value of an object-image-shift (OIS) defined between an object field center and an image field center results. Where the object surface and the image surface of the projection objective are parallel to each other, the object-image-shift may be defined as a lateral offset between an object field center axis running parallel to the object side optical axis through the center of the effective object field and an image field center axis running parallel to the image side part of the optical axis through the center of the effective image field. It has been found that small values of object-image-shift may be desireable e.g. if the projection objective is to be incorporated into a projection exposure system designed for scanning operations. Also, measuring techniques used for the qualification of the projection objective may be simplyfied with respect to conventional measuring techniques if small amounts of object-image-shift are obtained. Therefore, in preferred embodiments, the following condition holds:
0≦OIS<|h·(1−|β|)|.
In embodiments obeying this condition, the object-image-shift OIS is smaller than the object-image-shift of designs where the object side part of the optical axis and the image side part of the optical axis are coaxial. In preferred embodiments, no object-image-shift is present such that the condition OIS=0 is fulfilled.
These conditions may be useful in embodiments of the invention having a field lens with positive refractive power arranged between the first intermediate image and the concave mirror in the optical vicinity of the first intermediate image. However, small values for OIS may also be useful for conventional designs having no field lens of this type, such as shown e.g. in WO 2004/019128 A.
Another aspect of the invention enables designing projection objectives having potential for very high image side numerical apertures NA>1.2 or NA>1.3 while at the same time the overall track length (axial distance between object surface and image surface) can be limited to values allowing incorporation of the projection objective in conventional projection exposure systems and, at the same time, allowing to limit the maximum size (diameter) of lenses in the refractive objective parts upstream and/or downstream of a folding mirror. To this end, in preferred embodiments, a refractive power and a position of the field lens is set such that for a first chief ray direction cosine CRA1 at the first intermediate image the following condition holds:
|CRA1|<|β1*(YOB)/(LHOA)|
where β1 denotes the magnification of the first objective part , YOB is the object height of the outermost field point for which the chief ray is considered and LHOA is the geometrical distance from the first intermediate image to the concave mirror (length of the horizontal axis). With other words, it may be desireable that the chief ray is telecentric or almost telecentric at a an intermediate image. A chief ray obeying the condition given above will be denoted as an “essentially telecentric chief ray” in the following. Providing an essentially telecentric chief ray at a folding mirror close to such interrmediate image allows to limit the size of lenses immediately upstream and/or downstream of the folding mirror. In addition, it has been found that installation space within the third objective part responsible for providing high image side numerical apertures is obtained.
Further, in some embodiments it has been found beneficial for obtaining very high image side numerical apertures if a first axial length AL1 of the first objective part is smaller than a third axial length AL3 of the third objective part, wherein the axial length AL1 is measured between the object plane and an intersection of the optical axis with the first folding mirror and the axial length AL3 is measured between the intersection of the optical axis with the second folding mirror and the image plane. In preferred embodiments, the condition AL1/AL3<0.9, more preferably AL1/AL3<0.8 holds
Systems according to the invention can preferably be used in the deep UV band, for example at 248 nm, 193 nm or 157 nm, or less.
These features and further features are evident not only from the claims but also from the description and the drawings, in which case the individual features can be implemented in their own right or together in the form of subcombinations of embodiments of the invention, and in other fields, and can represent advantageous embodiments, as well as embodiments which are worthy of protection in their own right.
In the following description of preferred embodiments, the expression “optical axis” means a straight line or a sequence of straight line sections through the centers of curvature of the optical components. The optical axis is folded on folding mirrors (deflection mirrors) or on other reflective surfaces. Directions and distances are described as being on the “image side” when they are directed in the direction of the image plane or of the substrate which is located there and is to be exposed, and as on the “object side” when they are directed toward the object plane or toward a reticle located there, with respect to the optical axis. The object in the examples is a mask (reticle) with the pattern of an integrated circuit, although it may also be a different pattern, for example a grating. The image in the examples is projected onto a wafer which is provided with a photoresist layer and is used as a substrate. Other substrates, for example elements for liquid crystal displays or substrates for optical gratings, are also possible.
A device 40 (reticle stage) for holding and manipulating a mask 6 is arranged behind the illumination system in such a way that it is located on the object plane 4 of the projection objective 5, and can be moved in a departure direction 7 (y direction) on this plane, for scanning purposes.
The plane 4, which is also referred to as the mask plane, is followed by the catadioptric reduction objective 5, which images an image of the mask on a reduced scale of 4:1 on a wafer 10 which is covered with a photoresist layer. Other reduction scales, for example 5:1, 10:1 or 100:1 or more, are likewise possible. The wafer 10 which is used as a light-sensitive substrate, is arranged such that the planar substrate surface 11 together with the photoresist layer essentially coincides with the image plane 12 of the projection objective 5. The wafer is held by a device 50 (wafer stage) which comprises a scanner drive in order to move the wafer synchronously with the mask 6 and parallel to it. The device 50 also has manipulators, in order to move the wafer both in the z direction parallel to the optical axis 13 of the projection objective and in the x and y directions at right angles to this axis. A tilting device is integrated, and has at least one tilting axis which runs at right angles to the optical axis 13.
The device 50, which is provided for holding the wafer 10, is designed for use for immersion lithography. It has a holding device 15, which can be moved by a scanner drive and whose base has a flat depression or recess for holding the wafer 10. A flat liquid-tight holder, which is open at the top, for a liquid immersion medium 20 is formed by a circumferential rim 16, and the immersion medium 20 can be introduced into the holder, and can be carried away from it, by devices that are not shown. The height of the rim is designed such that the filled immersion medium completely covers the surface 11 of the wafer 10, and the exit-side end area of the projection objective 5 can be immersed in the immersion liquid between the objective exit and the wafer surface while the working distance is set correctly. The entire system is controlled by a central computer unit 60.
The folding mirrors 213, 223 are each located in the optical vicinity of the intermediate images, so that the light conductance value can be kept low. The intermediate images, that is the entire region between the paraxial intermediate image and the marginal ray intermediate image, are preferably not located on the mirror surfaces, thus resulting in a finite minimum distance between the intermediate image and the mirror surface, so that any faults in the mirror surface, for example scratches or impurities, are not imaged sharply on the image plane. The minimum distance should be set such that sub-apertures of the radiation, that is to say footprints of beams which originate from a specific field point or converge on it do not have a diameter of less than 5 mm, or 10 mm, on the mirror surface. Embodiments exist in which both the first intermediate image 211, that is to say the second intermediate image 221 as well, are located in the geometric space between the folding mirrors and the concave mirror 225 (solid arrows). This side arm is also referred to as the horizontal arm (HOA). In other embodiments, the first intermediate image 211′ may be located in the beam path upstream of the first folding mirror 213, and the second intermediate image 221′ may be located in the beam path downstream from the second folding mirror (arrows represented by dashed lines).
The folding angles in this exemplary embodiment are exactly 90°. This is advantageous for the performance of the mirror layers of the folding mirrors. Deflections by more or less than 90° are also possible, thus resulting in an obliquely positioned horizontal arm.
All of the objective parts 210, 220, 230 have a positive refractive power. In the schematic illustration, lenses or lens groups with a positive refractive power are represented by double-headed arrows with points pointing outwards, while lenses or lens groups with a negative refractive power are, in contrast, represented by double-headed arrows with heads pointing inwards.
The first objective part 210 comprises two lens groups 215, 216 with a positive refractive power, between which a possible diaphragm position exists where the chief ray 203, which is shown by a solid line, intersects the optical axis 204, which is shown by a dashed-dotted line. The optical axis is folded through 90° at the first folding mirror 213. The first intermediate image 211 is produced in the light path immediately downstream from the first folding mirror 213.
The first intermediate image 211 acts as an object for the subsequent catadioptric objective part 220. This has a positive lens group 226 close to the field, a negative lens group 227 close to the diaphragm, and the concave mirror 225 which is arranged immediately downstream from this and images the first intermediate image to form the second intermediate image 221. The lens group 226, which has a positive effect overall, is used as a “field lens” and is formed by a single positive lens, whose effect can also be produced, however, by two or more individual lenses with a positive refractive power overall. The negative lens group 227 comprises one or more lenses with a negative effect.
The second intermediate image 221, which is located optically immediately in front of the second folding mirror 223, is imaged by the third refractive objective part 230 on the image plane 202. The refractive objective part 230 has a first positive lens group 235, a second negative lens group 236, a third positive lens group 237 and a fourth positive lens group 238. There is a possible diaphragm position between the positive lens groups 237, 238, where the chief ray intercepts the optical axis.
One special feature of the system is that a biconvex positive lens 326, through which the beam passes in two opposite directions, is provided geometrically between the folding mirrors 313, 323 and the concave mirror 325 in a region of the projection objective through which the beam passes twice, with the beam passing through it both in the light path between the first intermediate image 311 and the concave mirror 325 and in the light path between the concave mirror and the second intermediate image 321, or the image plane 302, in mutually offset lens areas. The positive lens 326 is arranged closer to the folding mirrors 313, 323 than to the concave mirror 325, in particular in the first third of the axial distance between the folding mirrors and the concave mirror. In the region of the positive lens 326, the marginal ray height is small in comparison to the chief ray height, with the ratio of the marginal ray height to the chief ray height being approximately 0.3. The positive lens 326 is thus arranged close to the field both with respect to the first intermediate image 311 and with respect to the second intermediate image 321, and thus acts as a field lens for both intermediate images. The positive refractive power in the light path between the first intermediate image 311 and the concave mirror 325 ensures, inter alia, that the diameters of the subsequent lenses 327 and of the concave mirror 325 can be kept small. The positive refractive power in the light path from the concave mirror to the second intermediate image 321 and to the image plane results in a reduction in the incidence angle bandwidth of the radiation which also arrives at the second folding mirror 323 and can thus be coated with advantageous reflection layers, as well as for limiting the lens diameters in the refractive objective part 330 which is closest to the image field and is essentially responsible for production of the high image-side numerical aperture (NA=1.20) of the immersion projection objective.
The positive lens can be moved very close to the two intermediate images when required, without being impeded by the folding mirrors, so that a strong correction effect is possible. The positive refractive power which is arranged close to the field allows the horizontal arm to be longer. Because of the large aperture in the first intermediate image 311, the length of the horizontal arm would normally be shortened, so that the diameter of the concave mirror 325 and of the negative meniscus lenses in the negative group 327 which are arranged immediately upstream of it is linked to the color correction and should therefore not be indefinitely large. The inclusion of a positive lens group 326 close to the field also increases the refractive power of the negative lenses 327, owing to the compensation for the Petzval curvature (in comparison to the concave mirror), and thus increases the correction of the color longitudinal error for relatively small diameters in the area of the concave mirror. The catadioptric objective part can thus be designed to be compact and with relatively small lens dimensions, with adequate color correction.
The field lens 326 which is arranged in the immediate vicinity of two intermediate images 311, 321 also has major advantages with respect to optical correction, as will be explained in more detail in the following text. In principle, it is advantageous for the correction of imaging errors to have optical surfaces in the vicinity of intermediate images which are subject to major aberrations. The reason for this is as follows: at a long distance from the intermediate image, for example in the vicinity of the system diaphragm or its conjugate planes, all of the opening rays in a light beam have a finite and monotonally rising height with the pupil coordinate, that is to say an optical surface acts on all the opening rays. Opening beams which are located further outwards at the pupil edge also have an increasingly greater height on this surface (or more correctly: an increasing distance from the chief ray).
However, this is no longer the case in the vicinity of an intermediate image which is subject to severe aberrations. If one is, in fact, located within the caustic of the intermediate image, then it is possible for the surface to be located approximately in or close to the marginal ray image, that is to say effectively it does not act on the marginal rays, but has a considerable optical effect on the zone rays. It is thus possible, for example, to correct a zone error in the optical aberrations. This principle may be used, for example, in order to deliberately influence the spherical zone error.
The convex lens surface of the positive lens 326 which faces the intermediate images 311, 321 and is arranged in their immediate proximity is aspherically curved. In conjunction with the arrangement close to the field, this allows a very major corrective effect to be achieved.
At least the two to three lenses closest to the image can be manufactured from calcium fluoride, in order to avoid compaction problems. In order to compensate for intrinsic birefringence, the crystallographic major axes of the lenses can be rotated with respect to one another. The concave mirror 325 may also be in the form of an active mirror, in which the shape of the mirror surface can be varied by means of suitable manipulators. This can be used to compensate for various imaging errors. The beam path in the vicinity of at least one of the intermediate images is virtually telecentric.
Table 1 shows the specification of the design in tabular form. In this case, column 1 shows the number of the surface which is refractive, reflective or is distinguished in some other way, column 2 shows the radius r of the surface (in mm), column 3 shows the distance d between the surface and the subsequent surface (in mm), column 4 shows the material of a component, and column 5 shows the optically useable free diameters of the optical components (in mm). Reflective surfaces are annotated by “R” in column 1. Table 2 shows the corresponding aspherical data, with the arrow heights of the aspherical surfaces being calculated using the following rule:
p(h)=[((1/r)h2/(1+SQRT(1-(1+K)(1/r)2h2))]+C1*h4+C2*h6+
In this case, the reciprocal (1/r) of the radius indicates the surface curvature at the surface apex, and h indicates the distance between the surface point and the optical axis. The arrow height is thus p(h), that is to say the distance between the surface point and the surface apex in the z direction, that is to say in the direction of the optical axis. The constants K, C1, C2, etc. are shown in Table 2.
The immersion objective 300 is designed for an operating wavelength of about 157 nm, at which the calcium fluoride which is used for all of the lenses has a refractive index of n=1.5593. This is matched to a perfluoropolyether (Fomblin®) which is used in vacuum technology, as an immersion medium for which n=1.37 at 157 nm, and has an image-side working distance of about 1.5 mm. The image-side numerical aperture NA is 1.2, and the imaging scale reduction factor is 4:1. The system is designed for an image field whose size is 26×5.0 mm2, and it is double telecentric.
In this embodiment as well, a biconvex positive lens 426 which is used as a field lens is arranged in the horizontal arm in the immediate optical vicinity of the intermediate images 411, 421 which are arranged between the folding mirrors 413, 423 and the concave mirror 425, thus resulting in the horizontal arm having small dimensions and on the other hand in a major corrective effect to the intermediate images.
A further special feature of this embodiment is the design of the third, refractive objective part 430, which has a particularly compact configuration, with small dimensions and a small maximum diameter. The basic design with an initial positive group 435, followed by the negative group 436 and two subsequent positive groups 437, 438 with an aperture diaphragm (aperture stop) A in between corresponds to the design shown in
If there are no negative lenses with a significant refractive power in the region in which the beam diameter is relatively large then this allows the maximum diameters of the lenses to be limited to practicable sizes in this region. “Relatively large beam diameter” for the purposes of this application occur in particular when the marginal ray height on a lens is at least as large as half the marginal ray height at a potential diaphragm position, for example at the system diaphragm. This measure takes account of the fact that the scattering effect of a negative lens may admittedly be desirable for correction reasons, but that any scattering effect downstream from the negative lens has a tendency to lead to larger lens diameters than will be necessary in the absence of a negative lens. Furthermore, the rays of the beam are joined together in the direction of the downstream image plane, and positive refractive power is required for this purpose. The positive lenses which are required for this purpose may overall be designed relatively moderately provided that there is also no need to compensate for the scattering effect of negative lenses in the combination of the beams. Furthermore, the number of lenses may be limited. The invention thus allows compact projection objectives with minimal lens dimensions.
A comparison between the beam profiles in the systems shown in
In this embodiment as well, the positive field lens group 526 is located in the optical vicinity of both intermediate images, geometrically between the folding mirrors and the concave mirror, although the second folding mirror and the second intermediate image are somewhat further away from the positive lens 526.
One embodiment of a projection objective 600 will be explained with reference to
One major difference from the embodiments described so far is that the first objective part 610 is a compact catadioptric subsystem. The catadioptric objective part 610 has a concave mirror 615 whose optical axis is at right angles to the object plane, and a polarization-selective beam splitter 660 (Beamsplitter Cube, BSC) which is arranged between the object plane and the concave mirror and has a planar beam splitter surface 613 which is inclined at 45° to the optical axis 604 and is used as a first folding mirror for the projection objective 610. A λ/4 plate 661, a first positive group 662, a second positive group 663, the beam splitter 660, a further λ/4 plate 664 and a negative group 665 arranged immediately in front of the concave mirror are arranged in this sequence between the object plane and the concave mirror. This is followed by a further λ/4 plate 666 and a positive group 667 in the beam path downstream from the folding mirror 613. The basic configuration of the second, catadioptric objective part 620 with a positive group 626 close to the field corresponds essentially to the basic design shown in
In this exemplary embodiment, folding thus takes place within the first, catadioptric objective part, with positive refractive power in the form of at least one positive lens 667 being arranged between the folding mirror 613, which is responsible for this, and the first intermediate image 611, which is produced by the first subsystem. The overall system is operated with circularly polarized input light, which is converted by the first X/4 plate to linear-polarized radiation, which is p-polarized with respect to the obliquely positioned beam splitter layer 613 and thus essentially completely passes through it to the concave mirror 650. The λ/4 plate which is arranged between the beam splitter layer and the concave mirror is passed through twice by the linear-polarized radiation and in the process rotates the polarization preferred direction through 90° such that the radiation arriving from the concave mirror at the polarization splitter layer 613 is s-polarized with respect to this, and is reflected in the direction of the subsequent objective parts. The third λ/4 plate 666 converts the radiation to circularly polarized radiation, which then passes through the subsequent subsystems.
Since the first, catadioptric objective part 610 can be designed such that, in conjunction with the mirror curvature and the negative refractive power upstream of the mirror, it can largely or completely correct both the image field curvature and the chromatic longitudinal aberration, the subsequent partial objectives are not loaded, or are only slightly loaded, by these imaging errors. Furthermore, this arrangement allows the physical space between the object plane and the horizontally aligned, catadioptric objective part 620 to be enlarged, which can be used in order to reduce the light connductance value.
The aperture diaphragm A is preferably arranged in the third objective part 630, which is closest to the image, where the chief ray intersects the optical axis. Two further possible diaphragm positions are shown in the first and second objective part, in each case close to the concave mirrors 615, 625.
The first objective part may be physically compact.
In
The lenses of the first objective part 910 define a first part OA1 of the optical axis, which is the axis of rotational symmetry of the lenses and is perpendicular to the object surface 901. The axis of rotational symmetry of the concave mirror 925 and the lenses of the second objective part define a second part 0A2 of the optical axis which, in this embodiment, is aligned perpendicular to the object side first part OA1 of the optical axis. With other words, the optical axis is folded by the first folding mirror 913 at 90°. The lenses of the third objective part 930 define a third part 0A3 of the optical axis, which is parallel to the first part OA1 of the optical axis and perpendicular to the image surface 902. In this embodiment, the object-side first part OA1 of the optical axis and the image-side third part 0A3 of the optical axis are coaxial such that no lateral axis offset exists between these parts of the optical axis. This construction may be desireable with regard to mounting of the lenses of the refractive objective parts. A similar construction with coaxial first and third parts OA1, 0A3 of the optical axis is shown as projection objective 1000 in
In the projection objective 900 the lens surface ASP immediately upstream of the first folding mirror 913 is an aspheric surface, which is optically close to the first intermediate image. Efficient correction of field related imaging errors are obtained. In the projection objective 1000 the field lens 1026 has an aspheric lens surface ASP facing the concave mirror. This aspheric surface is the lens surface closest to both the first and second intermediate image 1011, 1021 and therefore very effective for correction at two positions along the beam path. The wave front aberration of this design is about 3 mλ rms.
The embodiment of a projection objective 1100 shown in
From an optical point of view, an off-axis effective object field OF is imaged by the first objective part 1110 into a first intermediate image 1111 arranged between a first folding mirror 1113 and a positive field lens 1126 of the second objective part 1120. The second objective part includes the concave mirror 1125 and is designed as an imaging subsystem to create a second intermediate image 1121 positioned between positive lens 1126 and a second folding mirror 1123. The third objective part 1130 serves as a focussing group to generate the off-axis effective image field IF at a very high image-side numerical aperture NA, wherein here NA=1.30.
In contradistinction to the embodiments of
|AO|=h*(1+|β|).
Another beneficial aspect of preferred embodiments of the invention relates to an appropriate selection of positive refractive power for the field lens. As will be demonstrated exemplarily in the following, a proper selection of refractive power allows to manufacture projection objectives with very high image side numerical apertures, such as NA=1.3 or NA=1.35, while maintaining a maximum size of lenses upstream and/or downstream of the folding mirrors and the overall track length of the projection objective moderate. For demonstration purposes,
For comparison,
Next, some characteristic features of prior art systems related to the embodiment of
The first objective part is subdivided into a first lens group LG1 and a second lens group LG2 (each positive refractive power), a pupil surface being positioned between these lens groups where the chief ray CR intersects the optical axis OA. The third objective part includes, in that sequence, a third lens group LG3 with positive refractive power, a fourth lens group LG4 with negativ refractive power, and a fifth lens group LG5 with positive refractive power. An image side pupil surface is positioned in the third objective part where the chief ray crosses the optical axis. An aperture stop AS is usually positioned at this position. A pupil surface optically between the first and second intermediate image is positioned close to or at the concave mirror CM.
Alternatively an aperture stop may also be positioned in one of the other pupil surfaces, namely in the refractive relay group L1 or in the catadioptric group, close to the concave mirror.
The chief ray CR is convergent at the first intermediate image IMI1 and the first folding mirror optically close to that interemediate image. Here, a convergent chief ray is a chief ray where the chief ray height CRH, i.e. the radial distance between the chief ray and the optical axis, is decreasing in light propergation direction. On the other hand, the chief ray is divergent (i.e. chief ray height increasing in light propergation direction) at the second intermediate image IMI2 and the second folding mirror.
Due to the folding geometry having the intermediate images between the folding mirrors and the concave mirror, the lenses of the second lens group LG2 and the third lens group LG3 closest to the first intermediate image and the second intermediate image, respectively, are optically relatively far away from the intermediate images since the folding mirror is placed between these lenses and the intermediate images. As a consequence of the convergence/divergence of the chief ray these lenses closest to the folding mirrors have a tendency to become large (large lens diameter). Note that this effect may be weaker if a larger distance is set between the concave mirror and the folding mirrors, thereby forming a longer horizontal arm (HOA) of the objective.
Given these conditions, there is a tendency for the horizontal optical axis to become shorter if the image side numerical aperture NA is to be increased. This can be understood as follows. The primary purpose of the concave mirror is to correct the Petzvalsum (image field curvature) of the projection objective. The contribution of the concave mirror for Petzval sum correction is directly proportional to the curvature of the concave mirror. If the numerical aperture of the system is to be increased and, at the same time, the length of the horizontal arm HOA would remain constant, the diameter of the catadioptric group including the concave mirror would be increased. One potential consequence is that the curvature of the concave mirror would become smaller, whereby the effect of the concave mirror on Petzval sum correction would decrease. This is considered less desireable since the Petzval sum correction must then be provided in other parts of the projection objective, thereby making the design more complicated.
On the other hand, if it desired to maintain the correcting effect of the catadioptric group on Petzval sum correction, the diameter of the catadioptric group including the concave mirror should be maintained essentially constant. This, however, corresponds to a decreasing length of the horizontal arm which, in turn, leads to relatively large chief ray angles at the intermediate images, as shown schematically in
It is evident from
However, if it is desired to increase the numerical aperture, sufficient space for lenses must be provided in the third objective part, mainly in the vincinity of the closest pupil position next to the wafer. If it is further desired to limit the track length of the objective to reasonable values, it appears that it is desireable to design the first objective part (relay group L1) axially shorter and to decrease the diameters of the lenses immediately upstream of the first folding mirror.
These objects can be obtained by introducing a field lens having sufficient positive refractive power geometrically between the folding mirrors and the concave mirror optically close to the intermediate images, as shown schematically in
In the embodiment of
The embodiments of the following
An image side numerical aperture NA=1.30 is obtained for the projection objective 1500 in
In the projection objectives 1300 and 1500, the positive field lens 1326, 1526 in the horizontal arm is arranged very close to the folding mirrors such that intermediate images follow within a space free of optical material between that field lens and the concave mirror. However, as evident from the intersecting lens symbols, one or more truncated lenses must be used close to the folding mirrors, which makes lens mounting more complicated.
Such mounting problem is avoided for the projection objective 1600 in
The design type has potential for even higher numerical apertures, which is evident from projection objective 1700 shown in
As mentioned earlier, the invention allows to built catadioptric projection objectives with high numerical aperture, particularly allowing immersion lithography at numerical apertures NA>1, that can be built with relatively small amounts of optical material. The potential for small material consumption is demonstrated in the following considering parameters describing the fact that particularly compact projection objectives can be manufactured.
Generally, the dimensions of projection objectives tend to increase dramatically as the image side numerical aperture NA is increased. Empirically it has been found that the maximum lens diameter Dmax tends to increase stronger than linear with increase of NA according to Dmax˜NAk, where k>1. A value k=2 is an approximation used for the purpose of this application. Further, it has been found that the maximum lens diameter Dmax increases in proportion to the image field size (represented by the image field height Y′, where Y′ is the maximum distance between an image field point and the optical axis). A linear dependency is assumed for the purpose of the application. Based on these considerations a first compactness parameter COMP1 is defined as:
COMP1=Dmax/(Y′−NA2).
It is evident that, for given values of image field height and numerical aperture, the first compaction parameter COMP1 should be as small as possible if a compact design is desired.
Considering the overall material consumption necessary for providing a projection objective, the absolute number of lenses, NL is also relevant. Typically, systems with a smaller number of lenses are preferred to systems with larger numbers of lenses. Therefore, a second compactness parameter COMP2 is defined as follows:
COMP2=COMP1−NL.
Again, small values for COMP2 are indicative of compact optical systems.
Further, projection objectives according to preferred embodiments of the invention have at least three objective parts for imaging an entry side field surface into an optically conjugate exit side field surface, where the imaging objective parts are concatenated at intermediate images. Typically, the number of lenses and the overall material necessary to build an projection objective will increase the higher the number NOP of imaging objective parts of the optical system is. It is desirable to keep the average number of lenses per objective part, NL/NOP, as small as possible. Therefore, a third compactness parameter COMP3 is defined as follows:
COMP3=COMP1−NL/NOP.
Again, projection objectives with low optical material consumption will be characterized by small values of COMP3.
Table 18 summarizes the values necessary to calculate the compactness parameters COMP1, COMP2, COMP3 and the respective values for these parameters for each of the systems presented with a specification table (the table number (corresponding to the same number of a figure) is given in column 1 of table 18). Therefore, in order to obtain a compact catadioptric projection objective having at least one concave mirror and at least two imaging objective parts (i.e. at least one intermediate image) at least one of the following conditions (1) to (3) should be observed:
COMP1<11 (1)
Preferably COMP1<10,7 should be observed.
COMP2<340 (2)
Preferably COMP2<320, more preferably COMP2<300 should be observed.
COMP3<110 (3)
Preferably COMP3<100 should be observed.
In some embodiments COMP1<11 and, at the same time, COMP2<340, which allows particularly compact designs.
Another aspect concerns the size of the concave mirror, which is particularly small in relation to the largest lenses in some embodiments, thereby facilitating manufacturing and mounting. In some embodiments the concave mirror has a mirror diameter DM, the projection objective has a maximum lens diameter Dmax, and the condition DM<0.75* Dmax holds. Preferably DM<0.70* Dmax may be fulfilled.
Table 18 shows that preferred embodiments according to the invention generally observe at least one of these conditions indicating that compact designs with moderate material consumption and/or small concave mirror are obtained according to the design rules laid out in this specification.
The invention has been described in detail using examples of R-C-R type catadioptric projection objectives having a first folding mirror for deflecting the radiation coming from the object plane in the direction of the concave mirror and a second folding mirror for deflecting the radiation coming from the concave mirror in the direction of the image plane. The invention can also be implemented in designs having different folding geometry, for example those where radiation coming from the object plane is directly directed at the concave mirror prior to a reflection on a first folding mirror arranged for deflecting the radiation coming from the concave mirror in the direction of the image plane. In those embodiments, a second folding mirror is usually provided downstream of the first folding mirror to allow a parallel arrangement of object plane and image plane.
It is self-evident that all of the systems described above may be complete systems, that is to say systems for forming a real image (for example on a wafer) of a real object (for example a photolithography mask). The systems may, however, also be used as subsystems for larger systems. For example, the “object” of one of the systems described above may thus be an image which is produced by an imaging system (for example a relay system) positioned upstream of the object plane. An image which is formed by one of the systems described above may likewise be used as an object for a system (for example a relay system) downstream from the image plane. The enumeration of the objective parts with the expressions “first objective part” and “second objective part” etc. relates to the sequence in which the beam passes through them when they are used as a reduction objective. The expressions “first” and “second” etc. should be understood as being relative to one another. The “first” objective part is arranged upstream of the “second” objective part in the direction in which the beam passes through them. This need not necessarily be the first objective part in the overall system, that is to say the objective part which immediately follows the image plane in the system. However, this is the case in the illustrated exemplary embodiments.
Claims
1. (canceled)
2. A lithographic process for making an integrated circuit, comprising:
- projecting, using ultraviolet light, a pattern of a mask onto a semiconductor wafer supporting a layer sensitive to the ultraviolet light, the projecting comprising: imaging the pattern to a first intermediate image using a first objective part of a catadioptric projection objective; imaging the first intermediate image to a second intermediate image using a second objective part of the catadioptric projection objective, the second objective part comprising at least two optical elements, the at least two optical elements comprising an active concave mirror; imaging the second intermediate image to a final image at the semiconductor wafer using a third objective part of the catadioptric projection objective, the third objective part comprising: a first lens group between the second intermediate image and the semiconductor wafer having a positive refractive power, a second lens group between the first lens group and the semiconductor wafer having negative refractive power, a third lens group between the second lens group and the semiconductor wafer having positive refractive power, a fourth lens group between the third lens group and the semiconductor wafer having positive refractive power, and an aperture stop arranged between the third lens group and the fourth lens group; and manipulating the active concave mirror to compensate for imaging errors in the final image at the semiconductor wafer,
- wherein the imaging defines a marginal ray having a marginal ray height, and wherein the marginal ray height has a point of inflection in the third objective part between the second intermediate image and the aperture stop, and
- the catadioptric projection objective has a numerical aperture of greater than 1.25 up to and including 1.35 at the semiconductor wafer.
3. The process of claim 2, wherein the numerical aperture at the semiconductor wafer is 1.35 and the ultraviolet light has a wavelength of 193 nm.
4. The process of claim 2, wherein the pattern is imaged to the first intermediate image by focusing the ultraviolet light solely by refraction.
5. The process of claim 4, wherein the first intermediate image is imaged to the second intermediate image by focusing the ultraviolet light by both refraction and reflection.
6. The process of claim 2, wherein the projecting further comprises providing liquid water in a path of the ultraviolet light between the catadioptric projection objective and the semiconductor wafer.
7. The process of claim 2, wherein the projecting further comprises illuminating the mask with ultraviolet light from an excimer laser.
8. The process of claim 7, wherein the illuminating comprises selecting an illumination mode selected from the group consisting of: annular field illumination, dipole illumination, and quadrupole illumination.
9. The process of claim 2, wherein the projecting further comprises synchronously scanning the mask and the semiconductor wafer relative to the catadioptric projection objective while imaging the pattern on the mask to the semiconductor wafer.
10. The process of claim 2, wherein the catadioptric projection objective further comprises at least one mirror in addition to the concave mirror of the second objective part.
11. The process of claim 10, wherein the at least one additional mirror reflects the ultraviolet light focused by a lens in the first objective part closest to the first intermediate image towards the concave mirror.
12. The process of claim 10, wherein the at least one additional mirror folds an optical axis defined by the catadioptric projection objective.
13. The process of claim 2, wherein manipulating the active concave mirror comprising varying a shape of the mirror to compensate for the imaging errors.
14. The process of claim 2, wherein the second lens group comprises two negative lenses.
15. The process of claim 14, wherein the two negative lenses are consecutive lenses in a path of the ultraviolet light.
16. The process of claim 14, wherein the point of inflection of the marginal ray height is located at a surface of one of the two negative lenses.
17. The process of claim 16, wherein the surface at which the marginal ray height is a faces the other negative lens of the two negative lenses.
18. The process of claim 2, wherein the third lens group comprises two meniscus lenses.
19. The process of claim 18, wherein the two meniscus lenses are adjacent one another in a path of the ultraviolet light.
20. The process of claim 2, wherein the third lens group comprises three aspheric lenses.
21. The process of claim 2, wherein the third lens group comprises at least two aspheric lenses having an aspheric surface facing the mask.
22. The process of claim 2, wherein there are fewer than five lenses in the fourth lens group.
23. The process of claim 22, wherein the fourth lens group comprises a meniscus lens.
24. The process of claim 23, wherein the meniscus lens has an aspheric concave surface.
25. The process of claim 2, wherein the catadioptric projection objective comprises at least one lens having a diameter of greater than 300 mm.
26. The process of claim 2, wherein the pattern is projected to a field at the semiconductor wafer useful for microlithography having a dimension of at least 5 mm.
27. The process of claim 26, wherein the field at the wafer useful for microlithography has a dimension of 26 mm.
28. The process of claim 2, wherein the concave mirror has a diameter, DM, that is smaller than a diameter, Dmax, of a largest lens of the projection objective.
29. The process of claim 2, wherein DM<0.75 Dmax.
30. The process of claim 2, wherein the first objective part has a first axial length, AL1, measured between the object plane and an intersection of the optical axis with a first folding mirror, the third objective part has a third axial length, AL3, measured between the intersection of the optical axis with a second folding mirror downstream in a path of the ultraviolet light from the first folding mirror and the image plane, and AL1/AL3<0.9.
31. The process of claim 2, wherein a parameter COMP1<11, where COMP1=Dmax/(Y′·NA2) in which Dmax is a diameter of a largest lens of the projection objective, Y′ is a maximum distance between an image field point and the optical axis, and NA is the numerical aperture at the semiconductor wafer.
Type: Application
Filed: Jun 30, 2017
Publication Date: Dec 21, 2017
Inventors: Aurelian Dodoc (Heidenheim), Wilhelm Ulrich (Aalen), Alexander Epple (Aalen)
Application Number: 15/638,567
