GB2601498A

GB2601498A - Projection lens with Alvarez lens system

Info

Publication number: GB2601498A
Application number: GB2018925.4A
Authority: GB
Inventors: Bouman Wim; Fahrni Francis; Klinkhamer Friso; Thomas Ivo; Baselmans Jan
Original assignee: Carl Zeiss AG
Current assignee: Carl Zeiss AG
Priority date: 2020-11-27
Filing date: 2020-11-27
Publication date: 2022-06-08
Also published as: GB202018925D0; GB2601498A9

Abstract

A projection lens for a projection exposure apparatus for microlithography is for imaging a pattern arranged in an object field at an object plane 8 of the projection lens into an image field at an image plane 9 of the projection lens by means of electromagnetic radiation having an operating wavelength λ< 260 nm. Optical elements having optical surfaces are arranged in a projection beam path between the object plane 8 and the image plane 9 in such a way that a pattern arranged in the object field of the object plane can be imaged into the image field of the image plane by means of the optical elements. An Alvarez lens system 11 comprising at least two Alvarez lenses (11a, 11b, figure 2) for manipulating the filed curvature is arranged in the projection beam path outside a field plane in the projection beam path. The projection lens may have at least two manipulators 11, 12 for compensating for distortion, which field dependency can be described by a polynomial, wherein each of the manipulators is designed for compensating for lower order terms of the polynominal.

Description

PROJECTION LENS WITH ALVAREZ LENS SYSTEM

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention refers to a projection lens for a projection exposure apparatus for mi-crolithography for imaging a pattern arranged in an object field at an object plane of the projection lens into an image field at an image plane of the projection lens by means of electromagnetic radiation having an operating wavelength < 260 nm. Moreover, the present invention refers to a projection exposure apparatus and a method of operating a projection exposure apparatus.

PRIOR ART

A projection exposure apparatus is used for micro] ithographic processes for forming micro -structured and nano -structured components for micro -electronic or micro -mechanical de-vices. Due to the ongoing miniaturisation of the dimensions of the structures to be built with microlithographic processes decreasing into the range of several nanometres the demands with respect to resolution and accuracy in imaging of structures by projection exposure apparatuses are increasing.

In this respect problems arise for a precise and accurate imaging of structures provided at a reticle in the object plane of a projection lens of the projection exposure apparatus to a wafer in an image plane of the projection lens, if the reticle or the structures provided at the reticle or if the wafer in the image plane of the projection exposure apparatus do not lie exactly in a flat plane, since deviations of the reticle from the object plane and the wafer from the image plane may lead to imaging errors.

Accordingly, attempts have been made to solve this problem by influencing the geomeuical form of the reticle or the wafer with corresponding manipulators which deform the reticle or the wafer in order to compensate reticle or wafer curvature. In addition, US 2013/0162 964 A proposes to include an Alvarez lens system into a projection lens of a lithographic apparatus in order to provide adjustments of the field curvature in order to compensate for the optical effects of the curvature of the reticle or the wafer. However, according to US 2013/0162 964 A the Alvarez lens has to be positioned in the field plane or an optically conjugated plane of the projection lens. This leads however to design restrictions and to restrictions on the application of such an Alvarez lens system, since in a complex projection lens the space for the positioning of such correction means is limited.

Another problem associated with projection exposure apparatuses is the correction of imaging errors which are inevitably associated with any actual embodiment of a projection objective, like astigmatism or distortion. While distortion, which field dependency can be described by a polynomial, can be compensated for lower order terms of the polynomial, a correction of dis-tortion effects associated with the terms of higher order of the polynomial is still required.

DISCLOSURE OF THE INVENTION OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a projection objective for a projection exposure apparatus for microlithography in which the problems associated with the curvature of the reticle and / or the wafer as well as problems associated with the distortion are solved or at least reduced. Moreover, the proposed solution should allow a relatively simple design of the projection lens as well as a relatively simple and reliable operation of the projection exposure apparatus.

TECHNICAL SOLUTION

The above-mentioned object is solved by a projection lens having the features of claim 1 as well as a projection exposure apparatus according to claim 10. Moreover, a method for operating a projection exposure apparatus according to claim 11 is proposed. Further advanta-geous embodiments are subject matter of the dependent claims.

According to the invention a projection lens for a projection exposure apparatus for micro-lithography is proposed wherein an Alvarez lens system comprising at least two Alvarez lenses for manipulating the field curvature is arranged in the projection beam path outside the field plane. Contrary to the disclosure of US 2013/0162 964 A it has been surprisingly found that a. sufficient adjustment of field curvature is even possible for an arrangement of the Alva.-rez lens outside of the field plane or any plane optically conjugated to the field plane. Thus, a manipulator for field curvature adjustment can be placed at various positions of the beam path which increases design variability.

The manipulator of the field curvature comprises an Alvarez lens system having at least two elements wherein the Alvarez lens system is adjusted by moving at least one element in a direction perpendicular to an optical axis of the lens. The Alvarez lens system may comprise two such elements with each element comprising a planar surface and a curved surface, with the curved surfaces of the two elements being complementary in shape. Alternatively, the lens may comprise three elements, an outer pair of elements and a middle element located between the outer pair, each element of the outer pair comprising a planar surface and a curved surface facing the middle element, and the middle element comprising two curved surfaces, each curved surface of the middle element being complementary in shape to the facing curved sur-face of the outer pair.

The Alvarez lens system is know-n from US Pat. No. 3,305,294, incorporated herein by reference in its entirety. The Alvarez lens system is adjusted by introducing a relative lateral translational movement in a direction perpendicular to the optical axis of the projection lens. For small movements the extent of the optical correction is proportional to the amount of relative movement. The two parts of the lens pair may be configured so that the curved surfaces face each other or so that the planar surfaces face each other.

The Alvarez lens system is designed such that optical effects of a curvature of a reticle comprising the pattern to be imaged or optical effects of a curvature of a wafer onto which the pattern is to be imaged can be compensated at least partially.

According to the invention the Alvarez lens system is disposed at a position outside a field plane of the projection lens or an optically conjugated plane. The projection lens may comprise one or two or more intermediate images formed in the projection beam path between the object plane and the image plane. Therefore, the Alvarez lens system is arranged out of the planes of the intermediate images.

The position of the Alvarez lens system in the beam path of the projection lens may be defined by the subaperture ratio SAR which can be used for quantifying the position of an optical element or an optical surface in the beam path.

In accordance with an illustrative definition, the subaperture ratio SAR of an optical surface of an optical element in the projection bea.m path is defined as the quotient between the sub-aperture diameter SAD and the optically free diameter DCA in accordance with SAR: : SAD / DCA. The subaperture diameter SAD is given by the maximum diameter of a partial surface of the optical element that is illuminated with rays of a beam emerging from a given single field point. The optically free diameter DCA is the diameter of the smallest circle around an optical axis of the optical element, wherein the circle includes the region of the surface of the optical element which is illuminated by all rays coming from all field point of the object field.

In a field plane (object plane or image plane or intermediate image plane), SAR 0 accord- ingly holds true. In a pupil plane, SAR 1 holds true. "Near-field" surfaces thus have a sub-aperture ratio which is close to 0, while "near-pupil" surfaces have a subaperture ratio which is close to 1.

In this application, the optical proximity or the optical distance of an optical surface with re-spect to a reference plane, e.g. a field plane or a pupil plane, is desciibed by the so-called (signed) subaperture ratio SAR. The subaperture ratio SAR of an optical surface is defined for the purposes of this application as follows: SAR sign CRH (MRH/(ICRH MRHI)) where MRH denotes the marginal ray height, CRH denotes the chief ray height and the sig-num function sign x denotes the sign of x, where according to convention sign 0 = 1 holds true. Chief ray height is understood to mean the ray height of the chief ray of a field point of the object field with maximum field height in terms of absolute value. The ray height should be understood here to be signed. Marginal ray height is understood to mean the ray height of a ray with maximum aperture proceeding from the point of intersection of the optical axis with the object plane. This field point does not have to contribute to the transfer of the pattern ar-ranged in the object plane -particularly in the case of off-axis image fields.

The subaperture ratio is a signed variable that is a measure of the field or pupil proximity of a plane in the beam path. The subaperture ratio is normalized by definition to values of between -1 and +1, the subaperture ratio being zero in each field plane and the subaperture ratio jumping from -1 to +1, or vice versa, in a pupil plane. A subaperture ratio of 1 in terms of absolute value thus determines a pupil plane. Notably the two above definitions of SAR are equivalent in terms of their absolute values.

Near-field planes therefore have subaperture ratios which are close to 0, while near-pupil planes have subaperture ratios which are close to 1 in terms of absolute value. The sign of the subaperture ratio indicates the position of the plane upstream or downstream of a reference plane.

Thus, at least one optical surface of an Alvarez lens of the Alvarez lens system, preferably all optical surfaces of an Alvarez lens system may have an absolute value of a subaperture ratio of equal to or more than 0.1, preferably equal to or more than 0.2 and especially equal to or more than 0.3.

Moreover, at least one optical surface of an Alvarez lens, preferably all optical surfaces of an Alvarez lens system may have an absolute value of a subaperture ratio of equal to or less than 0.9, preferably equal to or less than 0.8 and especially equal to or less than 0.7.

In addition to the Alvarez lens system at least a further manipulator may be disposed in the projection lens for cooperating with the Alvarez lens system in order to improve correction of the field curvature and / or to compensate aberrations other than the field curvature, especially parasitic aberrations introduced by the Alvarez lens system.

According to another aspect of the present invention, for which in combination with each and every feature of the invention described above and hereinafter protection is sought, a projec-Hon lens for a projection exposure apparatus for rnicrolithography is proposed wherein the projection lens comprises at least two manipulators for compensating for distortion, which field dependency can be described by a polynomial, wherein each of the manipulators is designed for compensating for lower order terms of the polynomial, while the combination of the manipulators provides a correction of distortion effects associated with the terms of higher order of the polynomial.

One of the manipulators for compensating for the lower order terms of the polynomial is an Alvarez lens system as described above.

Especially, the projection lenses described above may comprise a Z -manipulator, especially as a second manipulator for compensating for distortion effects associated with the terms of lower order of the polynomial, which is designed to enable movement of an optical element, in particular a lens of the projection lens in the z-direction, i.e. in direction along the optical axis in order to compensate imaging errors caused by distortion. By a combination of an adjustment of the position of a lens in 7-direction and of an Alvarez lens system especially distortion errors associated with terms of higher order of the corresponding polynomial describ-ing the field dependency of the distortion may be compensated. This can be achieved by using the difference between the compensation of the Z -manipulator and the compensation of the Alvarez lens system with respect to terms of lower order of the corresponding polynomials.

Accordingly, a term of higher order of a polynomial describing the field dependency of distortion of the projection lens can be compensated by shifting at least one Alvarez lens of an Alvarez lens system transverse to the optical axis relative to it's opposing Alvarez lens of the Alvarez lens system and by shifting an optical element, especially a lens of the projection lens in the z-direetion along the optical axis of the beam path.

Moreover, a method of operating a projection exposure apparatus may comprise an adjustment of the field curvature of the projection lens to a curvature of the reticle or the wafer or both by shifting at least one Alvarez lens of an Alvarez lens system transverse to the optical axis relative to it's opposing Alvarez lens of the Alvarez lens system.

SHORT DESCRIPTION OF THE FIGURES

The attached drawings show in a purely schematic view in Figure 1 a schematic illustration of a microlithography projection exposure apparatus in accordance with one embodiment of the invention, Figure 2 schematic views illustrating a two-part Alvarez lens for use in an embodiment of the present invention, Figure 3 schematically illustrates a three-pail Alvarez lens for use in an embodiment of the present invention.

EMBODIMENTS

Further advantages, characteristics and features of the present invention will become apparent from the following detailed description of the embodiments. However, the invention is not limited to these embodiments.

Figure 1 shows an example of a microlithographic projection exposure apparatus 100 which can be used in the production of semiconductor components and other finely structured corn- patients and operates for example with light or electromagnetic radiation from the deep ultra-violet range (DUV) in order to achieve resolutions down to fractions of micrometers. An ArF excimer laser having an operating wavelength A. of approximately 193 nm serves as primary radiation source or light source I. Other UV laser light sources, for example F2 lasers having an operating wavelength of 157 nm or KrF excimer lasers having an operating wavelength of 248 nm, arc likewise possible.

An illumination system 2 disposed downstream of the light source 1 generates in its exit surface 13 a large, sharply delimited and homogeneously illuminated illumination field adapted to the requirements of the projection lens 3 arranged downstream thereof in the light path. In addition, the illumination system is in position to alternatively illuminate the exit surface by predefined illumination settings which are given by predefined intensity distributions of the illumination system's exit pupil. The construction of suitable illumination systems is known per se and will therefore not be explained in greater detail here.

The illumination radiation formed by the illumination system 2 is directed onto the reticle 5 arranged in the projection exposure apparatus comprising the pattern to be imaged on the wafer 6 on the wafer stage 7. The pattern arranged on the reticle lies in the object plane 8 of the projection lens 3, which plane coincides with the exit surface 13 of the illumination system and is also designated here as reticle plane. The mask is movable in this plane for scanner operation in a scanning direction (y-direction) perpendicular to the optical axis 4 (z-direction) of the projection lens with the aid of a scan drive.

Downstream of the reticle plane 8 there follows the projection lens 3, which acts as a reducing lens and images an image of the pattern arranged on the mask 5 on a reduced scale, for exam-ple on the scale 1:4 ( I = 0.25) or 1:5 ( f3 = 0.20), onto a substrate or wafer 6 coated with a photoresist layer, the light-sensitive substrate surface 9 of said substrate lying in the region of the image plane 9 of the projection lens 3.

The substrate to be exposed, which is a semiconductor wafer 6 in the case of the example, is held by the wafer stage 7 comprising a scanner drive in order to move the wafer synchronous-ly with the reticle 5 perpendicular to the optical axis 4 in a scanning direction (y-direction).

The wafer stage 7 and the scan drive, which is also designated as "reticle stage", are part of a scanner device controlled by means of a scanning control device, which in the embodiment is integrated into the central control device of the projection exposure apparatus.

The illumination field generated by the illumination system 2 defines the effective object field used during the projection exposure. Said object field is rectangular in the case of the example and has a height measured parallel to the scanning direction (y-direction), and a width measured perpendicularly thereto (in the x-direction). The effective image field in the image surface 9, said image field being optically conjugate with respect to the effective object field, has the same shape and the same aspect ratio between height and width as the effective object field, but the absolute field size is reduced by the imaging scale p of the projection lens.

According to the present invention, the shape of the focal plane of the projection lens is adjusted by means of field curvature correction. To provide the field curvature adjustment a ma- nipulator comprising a two-element or a three-element lens 11 of the type known as an Alva-rez lens II is provided in the projection lens 3. Moreover, the manipulator comprising the Alvarez lens 11 may be used together with another manipulator which may adapt the Z -position of a lens 12 of the projection objective 3 (Z -manipulator) to correct the distortion according to a higher order of the corresponding polynomial.

The Alvarez lens 11 is known from prior art (see US Pat. No. 3,3(15,294) and an example of a two-clement Alvarez lens 11 is shown in Fig. 2. This Alvarez lens 11 consists of two optical lenses lla, 11b. Each part of the lens pair 11a, 1lb has a planar surface and a curved surface with the curved surfaces of the two parts of the lens pairs 11 a. llb being complementary. The curved surfaces have aspheric shapes. The Alvarez lens 11 is adjusted by introducing a relative lateral translational movement in a direction perpendicular to the optical axis 4 as shown by the arrows in Fig. 2. The extent of the optical correction is proportional to the amount of relative movement up to a maximal amount. It should be noted that the extent of the curvature of the two curved surfaces is exaggerated in the figure for clarity. The two parts of the lens pair 11 a, I lb may be configured so that the curved surfaces face each other or so that the planar surfaces face each other.

Fig. 3 shows a three-element Alvarez lens II in which a middle third part 1 1 c is introduced located between the first and second parts 11a, 1 lb and which has two curved surfaces complementary to the curved surfaces of the first and second parts II a, llb of the lens 11. Again, the extent of the curvature is exaggerated for clarity. Again, adjustment of the lens 11 is achieved by introducing relative lateral movement in a direction perpendicular to the optical axis 4. Either the two outer first and second parts 11 a, 11 b may move relative to the middle third part 11c, or the outer parts 11 a, 11b may be fixed and the middle part 11c may move or a combination of both.

The Alvarez lens 11 can be disposed in the beam path of the projection objective 3 at a position outside of the field plane or any optically conjugated plane thereto. For example, the pro-jection objective 3 may have an intermediate image plane 10 in which an intermediate image of the pattern of the reticle 5 is formed so that the intermediate image plane 10 is optically conjugated to the object plane 8 and the image plane 9 of the projection objective 3. Accordingly, the Alvarez lens 11 is disposed at a position intermediate between the object plane 8 and the intermediate image plane 10 or between the intermediate image plane 10 and the iii-age plane 9 of the projection lens 3.

According to the position of the Alvarez lens system in the projection objective 3 the optical surfaces of an Alvarez lens system may have an absolute value of a subaperture ratio of above 0.2, especially equal to or more than 0.3.

In embodiments of the present invention the manipulator comprising the Alvarez lens I 1 can be designed to provide a range of field curvature adjustments to match anticipated variations in the surface topography of the reticle 5, the wafer 6 or the wafer stage 7, or a combination of such, respectively. Doing this would, however, create other Zernike errors in the optical system that would have a negative effect on imaging, focus and overlay. By the term "Zernike crrors" any optical aberrations arc meant that may be described by Zernike polynomials. For compensating these errors introduced by the Alvarez lens 11, other lens manipulators may be used in order to compensate such parasitic effects.

Moreover, the Alvarez lens 11 may be used together with another manipulator for especially compensating distortion effects. For this purpose, the Alvarez lens 11 may be used together with a manipulator which allows shifting of a lens 12 of the projection objective 3 in the di- rection of the optical axis which is the z direction. By such a combination the distortion ac-cording to higher orders of the corresponding polynomial describing the field dependency of the distortion may be compensated.

For example. the Alvarez lens 11 generates a 3'd order field dependency of distortion as well as the manipulator shifting lens 12 does. On condition that that 3rd orders slightly deviates from each other in terms of their absolute field position the combination of both, the Alvarez lens and the lens 12 manipulation, give cause of a higher order field dependency of the distor-

tion, say a 5th order field dependency.

A corresponding lens 12 is shown in figure 1. Lens 12 is designed such that the three-point support of the lens 12 allows for motion of the lens 12 in the z direction, as indicated by the double arrow in Fig. 1.

The Alvarez lens 11 can be designed to produce the desired correction to the field curvature (the Z4 Zernike error), and at the same time can be configured to provide a correction for distortion and/or other residual Zernike errors.

Although the present invention has been described in detail with reference to the embodi-ments, it will be understood by those skilled in the art that the invention is not limited to these embodiments, but rather modifications are possible in the manner that individual features can be omitted or other combinations of features can be realized without departing from the scope of the appended claims. In particular, the present disclosure includes all combinations of the individual features shown in the various embodiments, so that individual features that are de-scribed only in connection with an embodiment can also be used in other embodiments or in combinations of individual features not explicitly shown.

LIST OF REFERENCE NUMERALS

1 light source 2 illumination system 3 projection objective 4 optical axis reticle 6 wafer 7 wafer stage 8 object plane or reticle plane 9 image plane intermediate image plane 11 Alvarez lens 1 la part or optical lens of Alvarez lens 1 lb part or optical lens of Alvarez lens 11c part or optical lens of Alvarez lens 12 lens 13 exit surface projection exposure apparatus

Claims

CLAIMSI. Projection lens for a projection exposure apparatus for rnicrolithography for imaging a pattern arranged in an object field at an object plane (8) of the projection lens into an image field at an image plane (9) of the projection lens by means of electromagnetic radiation having an operating wavelength X < 260 nm, comprising a multiplicity of optical elements having optical surfaces which are arranged in a projection beam path between the object plane (8) and the image plane (9) in such a way that a pattern arranged in the object field of the object plane can be imaged into the image field of the image plane by means of the optical elements, wherein an Alvarez lens system (11) comprising at least two Alvarez lenses (11a,11 b) for manipulating the field curvature is arranged in the projection beam path outside a field plane in the projection beam path.
2. Projection lens according to claim 1, characterised in that the Alvarez lens system (11) comprises two or three Alvarez lenses (11a,11b,11c), at least one of which is movable transverse to a beam propagation direction of the projection beam path.
3. Projection lens according to claim 1 or 2, characterised in that the Alvarez lens system (11) is designed such that the optical effects of a curvature of a reticle (5) comprising the pattern to be imaged or a curvature of a wafer (6) onto which the pattern is to be imaged can be compensated at least partially.
4. Projection lens according to any one of the preceding claims, characterised in that optical surfaces of two adjacent Alvarez lenses (1 I a,11b,1 1 c) facing each other are shaped complementarily to each other so that shifting of the Alvarez lenses to each other in a direction transverse to a beam propagation direction of the projection beam path lead to a manipulation of the field curvature of the projection lens.
5. Projection lens according to any one of the preceding claims, characterised in that at least one intermediate image is formed in a field plane between the object plane and the image plane, wherein especially two intermediate images are formed in the projec-tion beam path between the object plane (8) and the image plane (9).
6. Projection lens according to any one of the preceding claims, characterised in that at least one optical surface of an Alvarez lens (11 a,11b, I lc), preferably all optical surfaces of an Alvarez lens system (11) have an absolute value of a subaperture ratio of equal to or more than 0.1, preferably equal to or more than 0.2 and especially equal to or more than 0.3.
7. Projection lens according to any one of the preceding claims, characterised in that at least one optical surface of an Alvarez lens (11a,11b,11c), preferably all optical sur-faces of an Alvarez lens system (11) have an absolute value of a subaperture ratio of equal to or less than 0.9, preferably equal to or less than 0.8 and especially equal to or less than 0.7.
8. Projection lens according to any one of the preceding claims, characterised in that in addition to the Alvarez lens system (11) at least a further manipulator is disposed in the projection lens for cooperating with the Alvarez lens system (11) in order to compensate aberrations other than the field curvature, especially parasitic aberrations introduced by the Alvarez lens system (11).
9 Projection lens for a projection exposure apparatus for microlithography for imaging a pattern arranged in an object field at an object plane (8) of the projection lens into an image field at an image plane (9) of the projection lens by means of electromagnetic radiation having an operating wavelength X < 260 nm, comprising a multiplicity of optical elements having optical surfaces which arc arranged in a projection beam path between the object plane (8) and the image plane (9) in such a way that a pattern ar-ranged in the object field of the object plane can be imaged into the image field of the image plane by means of the optical elements, wherein the projection lens comprises at least two manipulators (11,12) for compensating for distortion, which field dependency can be described by a polynomial, wherein each of the manipulators is designed for compensating for lower order terms of the polynomial, while the combination of the manipulators provides a correction of distortion effects associated with the terms of higher order of the polynomial.
10. Projection lens according to claim 9, characterised in that one of the at least two manipulators is an Alvarez lens system (11).
11. Projection lens according to any one of the preceding claims, 1 0 characterised in that the projection lens comprises a Z -manipulator which is designed to enable movement of a lens (12) of the projection lens in the z-direction along an optical axis (4) of the projection lens.
12. Projection exposure apparatus for microlithography having a projection lens according to any one of the preceding claims.
13. Method of operating a projection exposure apparatus according to claim 12, wherein the field curvature of the projection lens is adapted to a curvature of the reticle (5) or the wafer (6) by shifting at least one Alvarez lens (11a.11b, I lc) transverse to the optical axis (4) relative to another Alvarez lens (11a,11b.11c) of the Alvarez lens system (11) and! or wherein a term of higher order of a polynomial describing distortion of the projection lens is compensated by manipulating at least two manipulators for compensating for distortion, wherein each of the manipulators is designed for compensating for lower order terms of the polynomial, while the combination of the manipulators provides a correction of distortion effects associated with the terms of higher order of the polynomial.
14. Method according to claim 13, characterised in that manipulating at least two manipulators comprises shifting at least one Alvarez lens (11a,11b,11c) transverse to the optical axis (4) relative to another Alvarez lens (11a,11b,11c) of the Alvarez lens system (11) and by shifting a lens (12) of the projection lens in the z-direction along the optical axis (4) of the beam path.