DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0041] FIG. 1 shows a typical configuration of a projection exposure system for microlithography in accordance with an embodiment of the invention. The reflective reticle 5 is imaged via a demagnifying imaging optics onto the wafer 6 at a typical imaging scale β of −0.25±0.15. The illuminated field on the wafer 6 has a diameter of at least 10 mm. Rectangular fields having an x-y aspect ratio of 1:1 to 1:4 are typical. The image end numerical aperture is greater than 0.5. The imaging takes place via the optical elements 71 and 72 . A beam splitter cube 3 is integrated into the imaging beam path 200 of the reduction objective between reflective reticle 5 and wafer 6 for illuminating the reflective reticle 5 . The beam splitter cube can, for example, be a polarization beam splitter cube wherein a layer system is located between the prism surfaces. This layer system almost completely reflects polarized light parallel to the beam splitter surface 30 ; however, the beam splitter surface 30 is light transmissive for polarized light perpendicular to the beam splitter surface 30 .

[0042] A condition precedent for the arrangement of FIG. 1 is therefore that the illuminating light is polarized in parallel to the incidence plane of the beam splitter surface 30 mounted at an angle of 45°. Polarized light of this kind is reflected at the beam splitter surface 30 and is deflected in the direction of reflective reticle 5 . A λ/4 plate 4 is mounted between the beam splitter cube 3 and the reflective reticle 5 and this plate 4 is run through a total of two times. The first time is in the illuminating beam path 100 so that the linearly polarized light is polarized circularly. After the reflection at the reticle 5 , the circularly polarized light in the imaging beam path 200 runs the second time through the λ/4 platelet 4 and is now again linearly polarized. The polarization direction now, however, is aligned perpendicular to the beam splitter surface 30 of the beam splitter cube 3 so that the beam splitter cube 3 is passed through without reflection. In this way, a separation of the illuminating light beam path 100 and of the imaging beam path 200 is provided in the combination of the following: polarization beam splitter cube 3 , two-time passthrough of the λ/4 platelet 4 and the reflective reticle 5 . A plane-parallel beam splitter plate would have the disadvantage compared to the polarization beam splitter cube 3 that rotationally-symmetrical imaging errors would not be introduced by the beam splitter plate of finite thickness positioned at an angle of 45°.

[0043] The polarization beam splitter cube 3 should be mounted within the imaging beam path 200 at a location at which the rays impinging on the beam splitter surface 30 exhibit a slight divergence. This is the case when the polarization beam splitter cube 3 is disposed at a location having an almost collimated beam path. For this reason, optical elements 71 having an overall positive refractive power are to be provided between reflective reticle 5 and the polarization beam splitter cube 3 . The optical elements 71 essentially collimate the diverging beam coming from the reticle. The optical elements 72 can, in accordance with the type of design, be configured differently but also likewise have a positive refractive power in order to achieve imaging on a possible intermediate image plane or on the wafer plane 6 .

[0044] One can view the optical elements 71 and 72 taken together as a refractive reduction objective having a typical imaging scale β of −0.25±0.15. In the design of the refractive objective, the λ/4 platelet 4 and the beam splitter cube 3 are to be provided between the optical elements 71 and the optical elements 72 .

[0045] The reflective reticle 5 is illuminated with the aid of the illuminating system 2 . In the design of the illuminating system 2 , the beam splitter cube 3 , the λ/4 platelet 4 and the optical elements 71 need be considered. The interface between the illuminating system 2 and the imaging optic is therefore not the reticle 5 as would be the case in a transmission reticle or when there is an inclined illumination of the reticle; instead, the interface is the input of the beam splitter cube 3 facing toward the illuminating system 2 .

[0046] In order to simplify the optical configuration of the illuminating optics 2 , it is advantageous when the chief ray angles are less than ±15 mrad with reference to the reticle plane, that is, the reticle 5 is virtually telecentrically illuminated. The chief rays are so defined in the reduction objective that they intersect the optical axis at the location of the system diaphragm. For larger chief ray angles, the design of the illuminating optics 2 is thereby made more difficult because the centroidal rays of the illuminating beam path 100 have to pass in the reticle plane 5 into the chief rays of the imaging beam path 200 . Because of the reflection at the reticle, the incident angles of the centroidal rays have to exhibit the reverse sign from the incident angles of the chief rays. In this way, the illuminating beam path 100 is different from the imaging beam path 200 within the optical components 71 . The distribution of the chief ray angles over the illuminated field has to be overcompensated by the illuminating system 2 . The chief ray angle distribution at the reticle 5 is determined primarily by the optical elements 71 and these optical elements 71 are fixedly pregiven for the design of the illuminating system 2 . For these reasons, optical components have to be provided in the illuminating system 2 , such as a sequence of converging and diverging lenses, which operate on the centroidal ray angle on the reticle 5 .

[0047] The optical components in the illuminating system 2 are so configured that the centroidal rays of the illuminating beam path 100 , after the reflection at the reflective reticle 5 , are coincident with the chief rays up to a maximum angle deviation of ±2.5 mrad depending upon field height. The chief rays are pregiven by the design of the reduction objective. Otherwise, the usually required telecentricity in the wafer plane 6 is deteriorated.

[0048] The illuminating system 2 has to have a unit for changing the polarization state of the illuminating light. In linearly polarized light of the source 1 , the polarization direction has to be rotated, as required, for example, via double refracting crystals or double refracting foils. For unpolarized light of the source 1 , polarizers are used for generating light which is polarized perpendicularly or parallelly to the beam splitter surface 30 . Preferably, these components for influencing the state of polarization are introduced directly forward of the polarization beam splitter cube 3 . The polarization direction is dependent upon whether or not the illuminating beam path 100 should be reflected at the beam splitter layer 30 . In the case of a reflection, for example, the illuminating light has to be polarized parallel to the beam splitter surface 30 .

[0049] Conventionally, the illuminating system 2 includes integrators for homogeneously illuminating the reticle plane 5 . The integrators are, for example, honeycomb condensers, hollow conductors or glass rods. For varying the illumination mode, the illuminating system can include: two zoom optics, axicon elements, filter plates in the pupillary planes and/or masking devices in the pupillary field planes or in the intermediate field planes.

[0050] The operation of these elements is disclosed, for example, in U.S. patent application Ser. No. 09/315,267, filed May 20, 1999, and incorporated herein by reference. Objectives within the illuminating system 2 for adapting the centroidal ray angles of the illuminating beam path 100 to the chief ray angles of the reduction objective are known as REMA objectives for the correct illumination of the entry pupil of the reduction objective from U.S. patent application Ser. No. 09/125,621, filed Dec. 3, 1997 and from U.S. Pat. No. 5,982,558, both incorporated herein by reference.

[0051] As a light source, a DUV laser or VUV laser can be used, for example, an ArF laser at 193 nm, a F 2 laser at 157 nm, an Ar 2 laser at 126 nm and a NeF laser at 109 nm.

[0052] FIG. 2 shows a further embodiment of the projection exposure system of the invention for microlithography. Components in FIG. 2 which correspond to those in FIG. 1 are identified with the same reference numerals. The imaging system ( 7 , 8 ) in FIG. 2 includes an intermediate image plane 103 . The intermediate imaging system 7 includes the optical elements 101 , the λ/4 platelet 4 , the polarization beam splitter cube 3 and the optical elements 102 . The intermediate imaging system 7 then provides an intermediate imaging of the reflective reticle 5 onto the intermediate image plane 103 . The imaging scale β 1 of this intermediate imaging can, for example, be β 1 =−1.0±0.2. Also possible is a reduction imaging at an imaging scale β 1 =−0.5±0.2 if thereby the design of the downstream optical system 8 is simplified. In this case, the incoupling of the illuminating light takes place via the polarization beam splitter cube 3 with the downstream λ/4 platelet 4 within the intermediate imaging optics 7 . The optical elements 101 and 102 each have a positive refractive power. The polarization beam splitter cube 3 is disposed in a region having an almost collimated beam path. Optical elements 104 follow the intermediate image plane 103 and image the intermediate image plane 103 onto the wafer plane 6 at an imaging scale of β 2 =−0.25±0.15 or β 2 =0.5±0.15. In this embodiment, the reduction objective is subdivided into the intermediate imaging system 7 and the reduction system 8 . This affords the advantage that, in the intermediate imaging system 7 , adequate space is provided for the polarization beam splitter cube 3 . Also in this configuration, the optical elements 101 , the λ/4 platelet 4 and the beam splitter cube 3 are included in the design of the illuminating system 2 . It is advantageous when the intermediate imaging optics 7 are so configured that the reflective reticle 5 is almost entirely telecentrically illuminated. The angles of incidence of the chief rays on the reflective reticle 5 should then be less than 15 mrad.

[0053] FIG. 3 shows an additional embodiment of the projection exposure system of the invention for microlithography. The imaging between reflective reticle 5 and wafer plane 6 takes place with two intermediate image planes 113 and 118 . The intermediate imaging system 9 of reflecting reticle 5 to intermediate image plane 113 is configured similar to the intermediate imaging system 7 of FIG. 2 . The imaging of intermediate image plane 113 on the wafer 6 takes place first with the aid of the catadioptric intermediate imaging system 10 and a downstream refractive reduction system 11 . The catadioptric intermediate imaging system 10 comprises the optical elements 114 , a deflecting mirror 115 , the optical elements 116 and the concave mirror 117 . The object field of the intermediate imaging system 10 is not centered with respect to the optical axis because of the reflective deflecting mirror 115 ; instead, the object field is outside of the optical axis. This means in this case that the component systems 10 and 11 must be arranged offset to the component system 9 . For these projection objectives, the image end numerical aperture can have values in the range from 0.65 to 0.8 or more. Field sizes in the wafer plane 6 in the range from 20 mm×7 mm to 30 mm×10 mm are possible. Objectives of this kind are disclosed in U.S. patent application Ser. No. 09/364,382, filed Jul. 29, 1999, and incorporated herein by reference.

[0054] The incoupling of the illuminating beam path 100 into the imaging beam path 200 can be done in an especially advantageous manner when a beam splitter cube is already provided in the imaging beam path 200 as is the case in some catadioptric objective types. Catadioptric objective types having beam splitter cubes are known in various configurations.

[0055] FIG. 4 shows a possible catadioptric projection objective having a beam splitter cube 31 which is assembled without an intermediate image. Objectives of this kind comprise, starting with the reticle 5 : a first lens group 121 , a deflecting mirror 122 , a second lens group 123 , the beam splitter cube 31 , a third lens group 124 , a concave mirror 125 , a fourth lens group 126 and a diaphragm which is arranged between the elements 123 and 126 . For these objectives, the following can be considered: an imaging scale β of −0.25±0.15; an image end numeric aperture of >0.5; and, an image field diameter >10 mm, preferably >20 mm.

[0056] The first lens group 121 and the second lens group 123 can be so arranged that the divergence of the rays on the beam splitter surface 310 of the polarization beam splitter cube 31 is minimized. If one views a peripheral ray which originates from an object point on the optical axis, then the sine of the angle of this ray with respect to the optical axis can be reduced up to 40% by the first and second lens groups 121 and 123 . The lens group 124 must have a negative refractive power in order to obtain an adequate color correction together with the concave mirror 125 . The lens group 126 generates the image in the wafer plane 6 and therefore exhibits a positive refractive power. The reduction objective 12 , which is shown in FIG. 4 , comprises the optical elements 121 , 122 , 123 , 124 , 125 , 126 and the beam splitter cube 31 . This reduction objective 12 is taken from U.S. Pat. No. 5,880,891 incorporated herein by reference.

[0057] If one now uses this objective type with a reflective reticle 5 , then the illuminating light can be coupled in via the polarization beam splitter cube 31 . Advantageously, the fourth unused face of the polarization beam splitter cube 31 is used for this purpose. It is absolutely necessary that the illuminating light is polarized more than 95% perpendicularly to the beam splitter surface 310 so that no illuminating light is reflected at the beam splitter surface 310 in the direction of wafer 6 so that thereby contrast and resolution are not reduced. For this reason, it is advantageous to build in a polarization filter between illuminating system 2 and polarization beam splitter cube 31 . The polarization filter has a transmissive polarization direction which is orientated perpendicular to the beam splitter surface 310 .

[0058] A first λ/4 platelet 41 follows the polarization beam splitter cube 31 . The light beams of the illuminating beam path 100 are circularly polarized with the aid of this first λ/4 platelet 41 . The light beams of the imaging beam path 200 run from the reflective reticle 5 to the wafer 6 and are, in turn, linearly polarized by the λ/4 platelet 41 but parallel to the beam splitter surface 310 and are reflected at the beam splitter surface 310 to the concave mirror 125 . Before the light beams impinge on the concave mirror 125 , the beams are circularly polarized by a second λ/4 platelet 42 and, after the reflection at the concave mirror 125 with the second passthrough, are linearly polarized by the second λ/4 platelet 42 again parallel to the beam splitter layer 310 so that the light beams pass through the polarization beam splitter cube 31 in the direction of wafer 6 .

[0059] Except for the first λ/4 platelet 41 between polarization beam splitter cube 31 and reticle 5 , a conventional catadioptric reduction objective 12 having a polarization beam splitter cube 31 can be used unchanged with the reflective reticle 5 . What is decisive is that, in the design of the illuminating system 2 , the optical elements of the projection objective, which are likewise passed through by the illuminating light, also have to be considered.

[0060] The light of the light source 1 is so configured in the illuminating unit 2 that it illuminates the reflective reticle 5 in correspondence to the lithographic requirements after passing through the following: the polarization beam splitter cube 31 , the first λ/4 platelet 41 , the second lens group 123 , the deflecting mirror 122 and the first lens group 121 . The homogeneity of the illumination and the illuminating mode is made available by corresponding components in the illuminating system 2 . The illuminating mode includes coherent, incoherent, annular or quadrupole illumination. In order to correctly illuminate the entry pupil of the reduction objective 12 , the polarization beam splitter cube 31 and the optical elements 121 to 123 are considered as fixed components of the illuminating beam path 100 and their effect is to be considered in the design of the illuminating system 2 .

[0061] In the configuration of the reduction objective 12 of FIG. 4 , it is also possible to couple in the illumination light 100 via the deflection mirror 122 as shown in FIGS. 5 and 6 .

[0062] In FIG. 5 , the deflecting mirror 122 of FIG. 4 is replaced by a polarization beam splitter plate 32 . The illuminating light 100 should be so polarized that it passes through the polarization beam splitter plate 32 . A λ/4 platelet 43 is disposed between polarization beam splitter plate 32 and the reticle 5 and leads to the circular polarization of the illuminating light 100 . After the reflection at reticle 5 , the light beams of the imaging beam path 200 are polarized when passing through the λ/4 platelet 43 parallel to the beam splitter surface 321 so that the beam is reflected in the direction of polarization beam splitter cube 33 . The use of a known plane parallel beam splitter plate, which is positioned in the beam path 200 at an angle of 45°, would lead within the illuminating beam path 100 to non-rotationally symmetrical imaging errors such as astigmatism and coma in the axis. For this reason, the beam splitter plate 32 of the invention is utilized. This plate is configured as a wedge plate such that the astigmatism of lowest order can be completely eliminated by an optimized wedge angle. The wedge angle is so configured that the thicker end of the wedge is directed toward the illuminating system 2 and the thinner end is directed toward the reticle 5 .

[0063] The remaining imaging errors of higher order can be compensated by a targeted aspherization of the surface 322 facing toward the illuminating system 2 . The aspherization can, for example, be undertaken by an ion beam or a robotic refinement. The aspheric shape is then, as a rule, not rotationally symmetric; instead, the aspheric form has a simple symmetry. The symmetry plane is the meridian plane. A correction of this kind via the wedge plate and the aspherized surface 322 is adequate within the illuminating beam path 100 in order to achieve the required specification for the correct illumination of the reticle 5 . In contrast, within the imaging beam path 200 , the use of a polarization beam splitter plate 32 in transmission would not be possible because of the introduced imaging errors. In a configuration of FIG. 5 , no adverse effect on the imaging beam path 200 occurs because, in the imaging beam path 200 , only the planar surface 321 of the beam splitter plate 32 is used in reflection so that the light rays of the imaging beam path 200 are reflected by air. With air, a medium having a refractive index of almost 1.0 is understood. In this connection, consideration can be given also to gas fillings, for example, with nitrogen, helium or partially evacuated air spaces.

[0064] The deflection mirror 122 in FIG. 4 or the beam splitter plate 31 in FIG. 5 can also be replaced by a polarization beam splitter cube 34 as shown in FIG. 6 . A polarization beam splitter cube 34 has the advantage compared to a beam splitter plate 32 that only rotationally symmetrical imaging errors are introduced which can be easily corrected. In comparison to the beam splitter plate 32 , a beam splitter cube 34 has the advantage that the additional glass path through the glass prisms leads to transmission losses which are disturbing especially at low wavelengths.

[0065] Coupling in the illuminating light via a polarization beam splitter cube 36 can also be done in another class of objective designs as shown in FIG. 7 . The reduction objective includes the following: a catadioptric component objective 15 having a polarization beam splitter cube 36 , an intermediate image 95 and a refractive reduction objective 16 . The catadioptric component objective 15 can be disposed after the reticle 5 as shown in FIG. 7 as well as forward of the wafer 6 . In the catadioptric component objective 15 , a polarization beam splitter cube 36 is already provided having a fourth and still unused face. Via this face, the illuminating light 100 can be coupled in.

[0066] The light coming from the illuminating unit 2 has to be very well polarized, advantageously to more than 95%, perpendicularly to the beam splitter surface 360 . In this way, one avoids an unwanted reflection in the direction of wafer 6 whereby contrast and resolution of the projection objective would have been reduced.

[0067] A first λ/4 platelet 47 has to be mounted between polarization beam splitter cube 36 and reticle 5 so that the light rays of the imaging beam path 200 are polarized after passing through the λ/4 platelet 47 so that they are reflected at the polarization beam splitter cube 36 in the direction of concave mirror 93 .

[0068] Optical elements 91 , which overall have a positive refractive power, are disposed between reticle 5 and polarization beam splitter cube 36 so that the beam splitter surface 360 is passed through in the almost entirely collimated beam path.

[0069] A second λ/4 platelet 48 has to be introduced between polarization beam splitter cube 36 and concave mirror 93 so that the light rays of the imaging beam path 200 can, after the deflection at concave mirror 93 , pass through the polarization beam splitter cube 36 undeflected in the direction of the intermediate image 95 .

[0070] The optical elements 92 having an overall negative refractive power are disposed between the polarization beam splitter cube 36 and the concave mirror 93 . The elements 92 are passed through by the light beam in two passthroughs and lead to a chromatic overcorrection. The concave mirror 93 affords the advantage that it introduces no chromatic aberrations and has an adequately positive refractive power so that the catadioptric component objective 15 overall has a positive refractive power.

[0071] If the polarization beam splitter cube 3 is passed through in the almost collimated beam path, then further optical elements 94 having overall positive refractive power are required ahead of the intermediate image 95 in order to generate the intermediate image.

[0072] One can omit optical elements 94 if the intermediate image 95 is already generated by the action of the concave mirror 93 and the optical elements 92 and if the collimated beam path within the polarization beam splitter cube 93 is omitted.

[0073] Usually, the object is imaged onto the intermediate image with an imaging scale of β 1 =−1.0±0.25.

[0074] A refractive reduction imaging having an imaging scale of, for example, β 2 =0.25±0.15 follows the intermediate image 95 . In FIG. 7 , the component objective between intermediate image 95 and wafer 6 comprises the optical elements ( 96 , 98 ) and the deflecting mirror 97 .

[0075] It is also possible to arrange the deflecting mirror 97 forward of the optical components 96 .

[0076] The diameter of the illuminated field in the wafer plane 6 is, in this class of objectives, greater than 10 mm for an image end numerical aperture greater than 0.5.

[0077] For the embodiment shown in FIG. 7 with beam splitter cube and intermediate image, FIG. 8 shows a specific embodiment for an imaging scale β=−0.25, for an image field having a diameter of 14.3 mm and for a image end numerical aperture of 0.7. The reference numerals in FIG. 8 correspond to the reference numerals in FIG. 7 . The optical data are set forth in Table 1.

[0078] The embodiment of FIG. 8 is taken from U.S. patent application Ser. No. ______ , filed ______ , incorporated herein by reference and claiming priority from German patent application no. 199 54 727.0 (Docket No. 99067 P).

[0079] In Table 1, the surface 7 is assigned to the beam splitter surface 360 for the first contact and the surface 19 is assigned to the concave mirror 93 . The surface 31 is assigned to the beam splitter surface 360 for the second contact and the surface 36 is assigned to the deflecting mirror 97 . The surface 38 is assigned to the intermediate image 95 . SiO 2 identifies quartz glass and CaF 2 identifies calcium fluoride monocrystals.

[0080] The optical elements 91 in this case comprise two converging lenses 131 and 132 . The converging lens 132 is mounted close to the polarization beam splitter cube 3 and reduces the divergence of the peripheral rays. In this way, the substantially collimated beam path is produced within the polarization beam splitter cube 3 . An almost telecentric chief ray trace is achieved at the reticle 5 with the converging lens 131 close to the reticle 5 .

[0081] Table 2 provides the chief ray angles with respect to the surface normal in mrad for seven object heights in the reticle plane 5 . The chief ray angles are positive when the chief rays run convergent to the optical axis after reflection at reticle 5 . The maximum chief ray angle in this embodiment is only 0.5 mrad. The entry at the reticle is thereby almost perfectly telecentric.

[0082] The adaptation of the centroid ray angles of the illuminating beam path 100 in the illuminating system 2 to the chief ray angle of the projection objective is, in this case, especially simple because the illuminating beam path 100 and the imaging beam path 200 substantially overlap within the common components 91 between beam splitter cube 3 and reticle 5 .

[0083] The polarization beam splitter cube 3 and the two converging lenses 131 and 132 are to be included in the design of the illuminating system 2 . If the last component of the illuminating system 2 forward of the reticle is a REMA objective as disclosed in U.S. Pat. No. 5,982,558 or in U.S. patent application Ser. No. 09/125,621, filed Dec. 3, 1997, then the REMA objective can be so modified without great difficulty that a refractive cube is integrated for the polarization beam splitter cube 3 and a refractive planar plate is integrated for the λ/4 platelet 47 and the two converging lenses 131 and 132 are integrated into the field lens of the REMA objective.

[0084] An incoupling of the illuminating light 100 via the deflecting mirror 97 is in this case not possible. The incoupling of illuminating light via a polarization beam splitter cube or a polarization beam splitter plate is only possible when the light does not impinge on a further beam splitter surface after passing through this first beam splitter surface but is reflected after passthrough by possibly further optical elements. However, in the configuration of FIG. 8 , if the illuminating light would be coupled in via the deflecting mirror 97 , no reflecting surface would follow but a further polarization beam splitter surface 360 of the beam splitter cube 3 . An incoupling would, in this case, be conceivable only via a geometric beam splitter plate or a geometric beam splitter cube. This, however, would lead to high transmission losses.

[0085] It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims. 1

[0086] 2