WO2006066816A1

WO2006066816A1 - Optical element

Info

Publication number: WO2006066816A1
Application number: PCT/EP2005/013572
Authority: WO
Inventors: Karl-Heinz Schuster
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2004-12-22
Filing date: 2005-12-16
Publication date: 2006-06-29
Anticipated expiration: 2007-06-22
Also published as: DE102004063091A1

Abstract

An optical element is suggested which is made of a mixed crystal with at least two metal oxides and a semiconductor oxide, like, for example Mg3Al2Si3O12, Ca3Al2Si3O12, Al2Mg2CaSi3O12, and Al2MgCa2Si3O12. An optical element of this kind can be used in a projection objective for microlithography, in particular for immersion lithography. In one embodiment of the invention the composition of the mixed crystal can be chosen in such a way that the overall intrinsic birefringence of the optical element is adjusted to a particular value, preferably minimized.

Description

Optical Element

The invention relates to an optical element , an obj ective with at least one optical element , a proj ection obj ective, as well as a microlithography projection exposure apparatus with a proj ection obj ective .

In the time ahead, there will be a need in the field of microlithography for materials of exceptionally high refractivity . This holds particularly for the last lens or several last lenses , which are located adjacent to the wafer . Such highly refractive materials take on a special importance in the areas of immersion lithography or optical near-field lithography .

The invention has the obj ect to propose an optical element which in addition to a high index of refraction also has good transmissivity for DUV wavelengths , in particular the wavelength of 193 nm.

This obj ect is attained through an optical element in accordance with the independent claims 1 and 7. Advantageous further developed embodiments are presented in the dependent claims 2 to 6 and 8 to 17.

It is a further obj ect of the invention to propose an obj ective, in particular a proj ection obj ective, whose transmissivity is as high as possible at the aforementioned wavelengths and which has refractive indices that are suitable for obtaining a high resolution and depth of focus . The latter obj ect is attained through the use of a material in accordance with claim 18. Advantageous further embodiments are presented in the dependent claims 19 to 23.

As a material for an optical element , in particular for use in an obj ective in a microlithography proj ection exposure apparatus, a crystal is proposed which comprises at least two metal oxides and a semiconductor oxide . For example, this could be a mixed crystal consisting of two metal oxides and a semiconductor oxide . In another example, it could be a mixed crystal consisting of three metal oxides and a semiconductor oxide .

It is particularly advantageous to use pyrope, which is a mixed crystal of MgO , AI₂O₃ and SiO₂. Its crystal structure is cubic . Its chemical formula is AI2O3 ^• 3MgO - 3SiO₂ or Mg₃Al₂Si₃Oi₂. The refractive index of pyrope at a wavelength of 193 nm is close to the refractive index of magnesium spinel (MgAl₂O₄) , which at 193 nm has a refractive index of 1.8.

It is likewise very advantageous to use grossularite, a mixed crystal which, instead of the MgO found in pyrope, contains CaO . Accordingly, the chemical formula of grossularite is Al₂O₃ - 3CaO - 3SiO₂ or Ca₃Al₂Si₃Oi₂. Like pyrope, it has a cubic/isotropic crystal structure . Its refractive index is notably higher than for pyrope .

At a wavelength of 193 nm the refractive index of pure MgO is around 2.0 , while the refractive index of CaO is around 2.7. Accordingly, as the component 3MgO in pyrope crystal is replaced by 3CaO, the refractive index of grossularite is notably higher in comparison to pyrope . Besides mixed crystals of two metal oxides and a semiconductor oxide, other materials that can be considered for optical elements , in particular for use in an obj ective of a microlithography projection exposure apparatus, include crystal materials that are formed of three different metal oxides and a semiconductor oxide .

Particularly advantageous among these materials is a mixed crystal of Al₂O₃ , MgO, CaO and SiO₂ with the composition Al₂O₃ - 2MgO - CaO - 3SiO₂ or Al₂Mg₂CaSi₃Oi₂. Likewise a possible choice is a mixed crystal with the same components , but with different proportions of CaO and MgO, such as for example Al₂O₃ -MgO - 2CaO - 3SiO₂ or Al₂MgCa₂Si₃Oi₂.

Calcium oxide CaO and magnesium oxide MgO exhibit intrinsic birefringence at DUV wavelengths , particularly at 193 run. At these wavelengths the sign of the intrinsic birefringence of CaO is opposite to the sign of the intrinsic birefringence of MgO : While MgO exhibits a positive intrinsic birefringence, the birefringence of CaO is negative . The inventor has found that in a mixed crystal of at least two, in particular three, metal oxides and a semiconductor oxide, these mutually countervailing effects can be put to use by specifically selecting the respective proportions of the positive- birefringent MgO and the negative-birefringent CaO so that the overall intrinsic birefringence of the mixed crystal becomes minimal , with particular preference zero . The value of the intrinsic birefringence is strongly dependent on the wavelength of the transmitted radiation . Accordingly, the wavelength has to be taken into account in optimizing the composition of the mixed crystal .

It is in this context particularly advantageous to use a mixed crystal of the composition A1₂O₃ - 3 [A MgO • B CaO] - 3SiO₂ , wherein A + B = 1.0 , with A and B being factors for the respective proportions of MgO and CaO . This mixed crystal , too , has a cubic crystal structure . The factors A and B are preferably selected in such a way that the intrinsic birefringence of the crystal at a given wavelength is minimized, for example if the optical element is used in an obj ective of a proj ection exposure apparatus . It is further advantageous if A and B are selected in such a way that the resultant mixture is eutectic .

In comparison to pure metal oxide crystals , the foregoing crystal composition with variable shares of MgO and CaO has the further advantage that its absorption edge is located farther away from 193 run. This is true in particular in the case of a eutectic mixture .

In a microlithography proj ection obj ective, particularly for applications in the areas of immersion lithography or near- field lithography, it is advantageous to select a material of the highest refractivity possible, such as for example MgO,

YAG (yttrium aluminum garnet, Y₃AI₅O1₂) or grossularite for the optical element that is located closest to the image plane . Adj acent to this element , as a further optical element , it is advantageous to provide an optical element of one of the aforementioned materials which is suitable to compensate the birefringence of the optical element that lies closest to the image plane . This adjacent optical element can have a lower refractive index. Consequently a larger selection of materials is available that can be used for compensating the intrinsic birefringence . Particularly suited for this is a mixed crystal of the composition A1₂O₃ - 3 [A MgO • B CaO] - 3SiO₂ , wherein A + B = 1.0 , because its intrinsic birefringence can be adjusted by varying the factors A and B . It is particularly advantageous , especially for applications in lithography, to select the composition of the oxide-based mixed crystal in such a way that the refractive index is above 1.7 and that at the same time an intrinsic birefringence is achieved with an absolute value of less than 100 nm/cm, preferably less than 50 nm/cm, and with special preference less than 30 nm/cm.

Each of the preceding types of crystals can also be compensated by itself through four crystal orientations , which requires four lens elements that are distributed in the obj ective, preferably also following each other in a direct sequence . This can be achieved, for instance, by providing four lens elements , for example made of pyrope, which are either positioned adjacent to each other or distributed in the objective and whose crystal orientations are shifted with respect to each other . This technique is frequently referred to as "clocking" .

As another advantageous means for minimizing the intrinsic birefringence, optical elements made of a mixed crystal with a positive overall intrinsic birefringence are compensated by other optical elements made of a mixed crystal with a negative overall intrinsic birefringence, and vice versa . When implementing this concept , it is particularly advantageous to orient the optical elements in such a way that the crystallographic axis with the highest symmetry runs parallel to the optical axis of the obj ective .

Besides the aforementioned mixed crystals of at least two metal oxides and a semiconductor oxide, there are further oxide-based mixed crystals that could be chosen, in particular for objectives in microlithography proj ection exposure apparatus . These include in particular : calcium spinel (CaAl₂θ₄) , gahnite (ZnAl₂O₄ ) , scandium spinel ( ScAl₂O₄) , strontium spinel ( SrAl₂O₄ ) , excess-magnesium spinel (MgO - BAl₂O₃ ) and excess calcium spinel (CaO - SAl₂O₃ ) , which in regard to their structure are related to magnesium spinel (MgAl₂O₄) . Furthermore, scandium oxide (Sc₂O₃) , magnesium gallium oxide

(MgGaO₄) , scandium aluminum garnet (Sc₃Al₅Oi₂) and hydro- grossularite (Ca₃Al₂SiOs ( SiO₄) _I^_1n (OH) _41n) are also suitable choices .

The invention will be explained in more detail based on the drawing, wherein:

Figure 1 shows a schematic representation of a projection exposure apparatus for applications in immersion lithography, with a proj ection obj ective, and

Figure 2 schematically illustrates the two lens elements of in the last positions on the image side of a projection objective .

Figure 1 schematically illustrates a microlithography proj ection exposure apparatus 1 designed for manufacturing highly integrated semiconductor elements by means of immersion lithography. As a light source, the projection exposure apparatus 1 includes an excimer laser 3 with an operating wavelength of 193 run. Alternatively, it is also possible to use light sources with different operating wavelengths , for example 248 run or 157 run. An illumination system 5 following next in the light path produces in its exit plane or obj ect plane 7 a large illumination field which is sharply delimited, very homogeneously illuminated, and matched to the telecentricity requirements of the projection objective 11 which follows in the light path. The illumination system 5 includes devices for the control of the pupil illumination and for setting a predetermined state of polarization of the illumination light . In particular, a device is provided which polarizes the illumination light in such a way that the plane of oscillation of the electrical field vector is oriented parallel to the structures of the mask 13.

Downstream in the light path after the illumination system, a device (called reticle stage) for holding and moving a mask 13 is arranged in such a way that the mask 13 is positioned in the obj ect plane 7 of the proj ection obj ective 11 and is movable in this plane in a travel direction 15 to work in a scanning mode .

After the obj ect plane 7 which is also referred to as mask plane, the proj ection objective 11 follows next, projecting an image of the mask in a reduced scale onto a substrate 19 , for example a silicon wafer, which carries a light-sensitive lacquer, also called resist 21. The substrate 19 is arranged in such a way that the planar surface of the substrate carrying the resist 21 substantially coincides with the image plane 23 , which is one of the field planes of the projection obj ective 11. The substrate is held by a mechanism 17 which includes a drive source to move the substrate 19 synchronously with the mask 13. The mechanism 17 also includes manipulators which serve to move the substrate 19 in the z-direction parallel to the optical axis 25 of the projection obj ective 11 as well as in the x- and y-directions perpendicular to the optical axis 25. A tilting device is integrated in the mechanism with at least one tilt axis running perpendicular to the optical axis 25.

The mechanism 17 (wafer stage) which serves to hold the substrate 19 is designed for use in immersion lithography applications . The mechanism 17 includes a receiving device 27 that is movable by a scanner drive and whose bottom has a shallow recess to receive the substrate 19. With a surrounding border 29 , a shallow, liquid-tight receiving cavity that is open at the top is formed for an immersion liquid 31. The height of the border is dimensioned so that when the immersion liquid 31 is in place, it can completely cover the substrate surface with the resist 21 , and so that the end portion of the proj ection obj ective 11 on the exit side of the latter can penetrate into the immersion liquid, if the operating distance between the exit of the obj ective and the substrate surface has been correctly adjusted.

The proj ection obj ective 11 has an image-side numerical aperture of at least NA = 0.8 , but preferably more than 1.0 , and with the highest preference more than 1.3. Thus , the proj ection obj ective 11 is specifically adapted for the use of immersion liquids with a high refractive index.

The proj ection obj ective 11 has as its last optical element closest to the image plane 11 a hemispherical planar-convex lens 33 whose exit surface 35 is the last optical surface of the projection objective 11. When the projection exposure apparatus is operating, the exit surface of the last optical element is completely immersed in the immersion liquid 31 and is wetted by the latter . An additional adjacent optical element 34 is arranged above the planar-convex lens 33.

Figure 2 schematically illustrates the last lens elements on the image side of a proj ection obj ective for immersion lithography, serving to proj ect an image onto a wafer 219. An immersion liquid 231 is arranged between the last optical element 233 on the image side and the wafer 219 , for example water, cyclohexane, or a perfluorated ether compound . On the last lens element 233 on the image side, a protective layer system P is deposited which can consist for example of an anti-reflection layer system in combination with an SiO₂ layer forming an enclosure against the immersion liquid 231 and thereby protecting the optical element 233 from chemical attack by the immersion liquid 231. A further lens element 234 is arranged adjacent to the last lens element 233 on the image side .

Example 1 : In a first exemplary embodiment , the last element 233 on the image side consists of MgO with a refractive index of 2.0 at a wavelength of 193 nm and with a positive intrinsic birefringence . The adjacent lens element 234 , on the other hand, consists of Al₂O₃ -MgO • 2CaO - 3SiO₂. The refractive index of this material is somewhat lower than the refractive index of MgO . As a result of the contribution of CaO, which has a negative birefringence to the overall properties of the mixed crystal , the crystal overall has a negative intrinsic birefringence , so that by combining the lens elements 233 and 234 , the overall intrinsic birefringence of the lens group consisting of the last lens element 233 and the adjacent lens element 234 is reduced.

Example 2 : The last lens element 233 on the image side in a second exemplary embodiment consists of YAG. The adjacent lens element 234 consists of Al₂O₃ -MgO ^• 2CaO - 3Siθ2 - As in the first example above, combining these materials minimizes the overall intrinsic birefringence .

Example 3 :

The last lens element 233 on the image side in a third exemplary embodiment consists of pyrope (Al₂O₃ - SMgO - SSiO₂) - The adj acent lens element 234 consists of grossularite AI2O₃ ^• 3CaO - 3SiO₂. As the respective intrinsic birefringence values of MgO and CaO have opposite signs , the overall intrinsic birefringence is minimized by combining the two lens elements . In this example , the last lens element 233 on the image side has a lower refractive index than the adjacent lens element 234.

Example 4 : In a fourth example of an embodiment , both the last lens element 233 on the image side and the adjacent lens element 234 consist of a eutectic mixture of the composition Al₂θ₃ - 3 [A MgO - B CaO] - 3SiOa , wherein A and B are selected so that the intrinsic birefringence is minimized at a operating wavelength of 193 run.

Example 5 :

In a fifth example of an embodiment, the last lens element 233 on the image side consists of grossularite (AI2O3 ^• 3CaO - 3Siθ2) . The adjacent lens element 234 consists of pyrope

(Al₂O₃ - 3MgO - 3Siθ2) - As in the third example, the overall intrinsic birefringence is minimized due to the opposite signs of the respective birefringence properties of pyrope and grossularite . In contrast to the third example, the optical element 233 closest to the image has in this case the higher refractive index.

Even though the invention has been described with reference to specific embodiments , those skilled in the art will also see numerous variations and alternative embodiments , for example by combination and/or replacement of features of individual embodiments . A person skilled in the art will accordingly understand that such variations and alternative embodiments of the present invention are also covered, and the scope of the invention is restricted only in the sense of the attached patent claims and their equivalents .

Claims

Patent Claims :

1. Optical element of a mixed crystal with at least two metal oxides and a semiconductor oxide .

2. Optical element according to claim 1 , characterized in that the mixed crystal is formed of at least three metal oxides and a semiconductor oxide .

3. Optical element according to claim 1 or 2 , characterized in that the mixed crystal is selected from the group that consists of Mg₃Al₂Si₃O^, Ca₃Al₂Si₃Oi₂, Al₂Mg₂CaSi₃θi2/ and Al₂MgCa₂Si₃O₁₂.

4. Optical element according to claim 1 or 2 , characterized in that the proportions of the metal oxides in the composition of the mixed crystal are adjusted so that the intrinsic birefringence of the mixed crystal is minimized .

5. Optical element according to claim 4 , characterized in that the mixed crystal is composed of

A1₂O₃ - 3 [A MgO • B CaO] - 3SiO₂ , wherein A + B = 1.

6. Optical element according to claim 5 , characterized in that the mixed crystal is a nearly eutectic mixture .

7. Optical element of a mixed crystal , characterized in that the mixed crystal is selected from the group that consists of calcium spinel CaAl₂O₄, gahnite ZnAl₂O₄ , scandium spinel ScAl₂O₄, strontium spinel SrAl₂O₄ , excess-magnesium spinel

MgO - 3Al₂O₃ , excess calcium spinel CaO - 3Al₂O₃ , scandium oxide Sc₂O₃ , magnesium gallium oxide MgGaO₄, scandium aluminum garnet SCsAl₅Oi₂ , and hydro-grossularite Ca₃Al₂SiO₈ (SiO₄) i-_m (0H) _4in.

8. Obj ective with at least a first optical element according to at least one of the claims 1 to 7.

9. Obj ective according to claim 8 with at least a second optical element according to one of the claims 1 to 7.

10. Objective according to claim 9 , characterized in that the first optical element and the second optical element are arranged adjacent to each other .

11. Obj ective according to claim 10 , characterized in that the first optical element and the second optical element consist of different materials .

12. Obj ective according to claim 11 , characterized in that the material of the first optical element and the material of the second optical element exhibit intrinsic birefringence with, respectively, opposite signs .

13. Obj ective according to one of the claims 8 to 12 , having a field plane, characterized in that the first optical element is the optical element which is located closest to the field plane .

14. Proj ection obj ective for projecting an image of an object plane into an image plane, consisting of an objective according to claim 13 , characterized in that the field plane is the image plane .

15. Proj ection obj ective according to claim 14 , characterized in that an immersion liquid is arranged between the first optical element and the image plane .

16. Microlithography proj ection exposure apparatus with a projection objective according to one of the claims 14 or 15.

17. Proj ection exposure apparatus according to claim 16 , characterized in that the proj ection exposure apparatus is operated at a wavelength of less than 250 run, in particular less than 200 nm.

18. Use of a mixed crystal of at least two metal oxides and a semiconductor oxide as material for an optical element in a microlithography proj ection obj ective .

19. Use in accordance with claim 18 , characterized in that the mixed crystal is formed of at least three metal oxides and a semiconductor oxide .

20. Use in accordance with claim 19 , characterized in that the mixed crystal is selected from the group consisting of Mg₃Al₂Si₃Oi₂ , Ca₃Al₂Si₃Oi₂ , Al₂Mg₂CaSi₃Oi₂ , and Al₂MgCa₂Si₃Oi₂.

21. Use in accordance with one of the claims 18 to 20 , characterized in that the proj ection obj ective is designed for light with an operating wavelength of less than 250 nm, in particular less than 200 nm.

22. Use in accordance with claim 21 , characterized in that the respective proportions of the metal oxides in the composition of the mixed crystal are adjusted so that the intrinsic birefringence of the mixed crystal at the operating wavelength is minimized. 3. Use in accordance with claim 22, characterized in that the mixed crystal is composed of Al₂O₃ ^■ 3 [A MgO ^• B CaO] -3SiO₂, wherein A + B = 1.