|Publication number||USRE40743 E1|
|Application number||US 10/916,650|
|Publication date||Jun 16, 2009|
|Filing date||Aug 11, 2004|
|Priority date||Feb 5, 2000|
|Also published as||DE10005189A1, DE50112921D1, EP1122608A2, EP1122608A3, EP1122608B1, US6590718, US20010022691|
|Publication number||10916650, 916650, US RE40743 E1, US RE40743E1, US-E1-RE40743, USRE40743 E1, USRE40743E1|
|Inventors||Gerhard Fuerter, Christian Wagner, Uwe Goedecke, Henriette Mueller|
|Original Assignee||Carl Zeiss Smt Ag|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (24), Non-Patent Citations (3), Referenced by (11), Classifications (14), Legal Events (3)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention relates to a projection exposure system having a reticle which operates in reflection.
Projection exposure systems having a reflective reticle have been used in the past, inter alia, together with 1:1 Dyson objectives. These projection exposure systems are described in the following publications:
The incoupling of the illumination takes place via a partially transmitting mirror as shown, for example, in U.S. Pat. No. 4,964,705 (FIGS. 3A and 3B). Beam splitter cubes or beam splitter plates are not provided in these designs.
Reflective reticles are used exclusively in the area of lithography utilizing soft X-rays (EUVL). The beam splitting of illuminating and imaging beam paths is realized by an inclined incidence of the illumination. Beam splitter cubes or beam splitter plates are not used. The objectives are pure mirror objectives having a non-axial symmetrical beam path. The inclined incidence of the illuminating light on the reflective reticle has the disadvantage that the raised mask struts lead to vignetting.
Japanese patent publication 9-017719 discloses a wafer projection exposure system having a reflex LCD as a special reticle. According to
U.S. Pat. No. 5,956,174 discloses a catadioptric microscope objective wherein the illuminating light is coupled in via a beam splitter cube between the microscope objective and the tube lens. This type of illumination is conventional in reflected light microscopes. The illuminating field sizes are only in the order of magnitude of 0.5 mm.
Catadioptric systems for wavelengths of 193 nm and 157 nm are known. Catadioptric projection objectives having beam splitter cubes without an intermediate image are shown, for example, in U.S. Pat. Nos. 5,742,436 and 5,880,891 incorporated herein by reference.
Catadioptric projection objectives having a beam splitter cube and an intermediate image are disclosed in U.S. Pat. No. 06/424,471 6,424,471.
Illuminating devices for microlithography are disclosed in U.S. Pat. No. 5,675,401 and U.S. Pat. No. 6,285,443. So-called REMA objectives for imaging a reticle masking device (REMA) into the plane of the reticle are disclosed in U.S. Pat. No. 5,982,558 and U.S. Pat. No. 6,366,410, also incorporated herein by reference. With these objectives, inter alia, the entry pupil of the downstream projection objective is illuminated.
The production of transmission reticles (that is, masks operated in transmission for microlithography) is difficult for deep ultraviolet wavelengths, especially 157 nm, inter alia, because of suitable transmitting carrier materials. The materials CaF2 or MgF2 can be considered. However, reticles made of CaF2 or MgF2 are difficult to process and are therefore very expensive. Furthermore, a reduction of the minimal structural size which can be applied to a semiconductor chip results because of absorption and the thermal expansion of the reticle resulting therefrom when there are multiple illuminations. When possible, materials such as MgF2 are avoided because they are also double refracting.
The alternative are reflective reticles. To reduce the requirements imposed on the reticle, it is advantageous when the projection objective is configured as a reduction objective and the reticle is imaged so as to be demagnified. The reticle can then be provided with larger structures.
In conventional reduction objectives, the use of reflective reticles is not easily possible. The typical entry intersection distance of, for example, 30 mm reduces the illumination at suitable angles of incidence.
It is an object of the invention to provide a projection exposure system having a reduction objective which functions without problem with reflective reticles.
The projection exposure system of the invention is for microlithography and includes: a light source; an illuminating system mounted downstream of the light source for transmitting light from the light source as an illuminating beam along an illuminating beam path; a reflective reticle; a reduction objective defining an imaging beam path and being configured for imaging the reticle onto an object; and, a beam splitter cube mounted in the imaging beam path for mutually superposing the illuminating beam path and the imaging beam path.
According to a feature of the invention, a beam splitter cube functions to superpose the illuminating and imaging beam paths. In this way, numerous objective design concepts for reflective reticles can be adapted as will be shown in the following examples. Erroneous entries by the beam splitter plate are avoided by utilizing a beam splitter cube in lieu of a planar parallel beam splitter plate. The beam splitter plate is operated in passthrough and mounted at 45°.
According to another feature of the invention, optical elements are provided between the beam splitter cube and the reticle. With these optic elements, it is possible to reduce the angle of incidence of the main beams of the reduction objective on the reticle in such a manner that the incident angle has values between −15 mrad and +15 mrad.
According to still another feature of the invention, the illuminating system is so configured that the illuminating beam path passes over into the imaging beam path with deviations of less than ±2.5 mrad. This deviation can be measured in that the angles with respect to the reticle plane are determined for the centroidal rays after the reflection and the deviation to the angles of the corresponding chief rays is computed. The angles of the centroidal rays are dependent upon the emission characteristics of the light source and the design of illuminating system and the angles of the chief ray are exclusively dependent upon the design of the reduction objective.
According to another feature of the invention, a polarization beam splitter cube is used in order to reduce transmission losses at the beam splitter cube and so that no scattering light is deflected onto the wafer. For an optimal operation, the illuminating light should be linearly polarized to more than 95%. The polarization direction is dependent upon whether the illuminating beam path is intended to be reflected or not at the beam splitter layer. In the case of a reflection, the illuminating light has to be polarized parallel to the beam splitter surface and, in the case of the transmission, the illuminating light has to be polarized perpendicularly to the beam splitter surface.
In other embodiments of the invention, the beam splitter cube functions exclusively for incoupling the illuminating beam path. To be able to more easily integrate the beam splitter cube into the design of the reduction objective, it is advantageous to subdivide the reduction objective into two component objectives with a first intermediate image having an imaging scale of −1.0±0.25 and a second image having an imaging scale of −0.25±0.15. The beam splitter cube is integrated into the first intermediate image. The second image can be configured to be strictly refractive or catadioptric.
The coupling in of the illuminating beam path with a beam splitter cube is especially advantageous when the beam splitter cube is already a part of the reduction objective. Then, the fourth unused face of the beam splitter cube can be used to couple in the illuminating beam path.
If the design of the catadioptric objective includes a deflecting mirror, then the deflecting mirror can be replaced by a beam splitter cube via which the illuminating light is coupled in.
The design of the catadioptric objective can be configured with or without an intermediate image.
In another embodiment of the invention, a special beam splitter plate is provided in the projection exposure system. This beam splitter plate is operated in pass through in the illuminating beam path and is operated reflectively in the imaging beam path. Here, reflection in air is provided, that is, in the optically thinner medium which can also be a vacuum or a special gas mixture or a gas such as nitrogen or helium. The beam splitter plate is so configured that astigmatic errors because of the plate mounted at an angle can be refractively corrected.
The common inventive concept is that the imaging beam path is held free of disturbances by the beam splitter arrangement and the illuminating beam path is corrected with less requirements directly via the beam splitter arrangement. For a beam splitter cube, only rotationally-symmetrical imaging errors are introduced which can be corrected within the illuminating system via rotationally-symmetrical optical elements such as spherical lenses. In the beam splitter plate according to a feature of the invention, the correction of the illuminating beam path is provided by the special configuration of the side of the beam splitter plate facing toward the illuminating system.
According to still another feature of the invention, the beam splitter plate is provided with a non-planar corrective surface. By mounting the beam splitter plate at an angle, the corrective surface exhibits no rotational symmetry, rather, a simple symmetry with respect to the meridian plane.
The beam splitter plate is configured to have a wedge shape in accordance with another embodiment of the invention for correcting the astigmatism of the lowest order. The use of a beam splitter plate is especially advantageous when it is used in lieu of a deflecting mirror provided in the design of the reduction objective.
The superposition of the illuminating optics and the projection optics make possible the use of reflective reticles especially at operating wavelengths in the range from 100 to 200 nm. In this way, the difficulties are avoided which occur in the manufacture of transmission reticles because of machining of the materials transparent at these wavelengths.
The invention will now be explained with reference to the drawings wherein:
A condition precedent for the arrangement of
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.
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.
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.
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.
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.
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 parallely 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.
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.
The operation of these elements is disclosed, for example, in U.S. Pat. No. 6,285,443, 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. Pat. No. 6,366,410 and from U.S. Pat. No. 5,982,558, both incorporated herein by reference.
As a light source, a DUV laser or VUV laser can be used, for example, an ArF laser at 193 nm, a F2 laser at 157 nm, an Ar2 laser at 126 nm and a NeF laser at 109 nm.
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.
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
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.
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.
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.
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.
In the configuration of the reduction objective 12 of
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
The deflection mirror 122 in
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
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.
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.
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.
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.
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.
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.
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.
Usually, the object is imaged onto the intermediate image with an imaging scale of β1=−1.0±0.25.
A refractive reduction imaging having an imaging scale of, for example, β2=0.25±0.15 follows the intermediate image 95. In
It is also possible to arrange the deflecting mirror 97 forward of the optical components 96.
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.
For the embodiment shown in
The embodiment of
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. SiO2 identifies quartz glass and CaF2 identifies calcium fluoride monocrystals.
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.
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.
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.
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. Pat. No. 6,366,410, 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.
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
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.
Object Height at Reticle
Chief Ray Angle at Reticle
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|US8730572||May 14, 2012||May 20, 2014||Carl Zeiss Smt Gmbh||Catadioptric projection objective|
|US8908269||Dec 30, 2013||Dec 9, 2014||Carl Zeiss Smt Gmbh||Immersion catadioptric projection objective having two intermediate images|
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|US9134618||Dec 30, 2013||Sep 15, 2015||Carl Zeiss Smt Gmbh||Catadioptric projection objective with intermediate images|
|US9639005||Feb 13, 2014||May 2, 2017||Carl Zeiss Smt Gmbh||Imaging catoptric EUV projection optical unit|
|US20090034061 *||May 13, 2005||Feb 5, 2009||Aurelian Dodoc||Catadioptric projection objective with intermediate images|
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|U.S. Classification||359/732, 359/726, 359/649|
|International Classification||G02B17/08, G03F7/22, H01L21/027, G03F7/20, G02B17/00|
|Cooperative Classification||G03F7/70225, G03F7/70283, G03F7/70066|
|European Classification||G03F7/70D2, G03F7/70F14, G03F7/70F2|
|Jan 4, 2011||FPAY||Fee payment|
Year of fee payment: 8
|Jan 18, 2011||AS||Assignment|
Owner name: CARL ZEISS SMT GMBH, GERMANY
Free format text: A MODIFYING CONVERSION;ASSIGNOR:CARL ZEISS SMT AG;REEL/FRAME:025763/0367
Effective date: 20101014
|Dec 31, 2014||FPAY||Fee payment|
Year of fee payment: 12