EP1664933A1

EP1664933A1 - Euv projection lens with mirrors made from materials with differing signs for the rise in temperature dependence of the thermal expansion coefficient around the zero transition temperature

Info

Publication number: EP1664933A1
Application number: EP03755564A
Authority: EP
Inventors: Udo Dinger; Frank Eisert; Stefan Koehler; Andreas Ochse; Johannes Zellner; Martin Lowisch; Timo Laufer
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2003-09-27
Filing date: 2003-09-27
Publication date: 2006-06-07
Also published as: WO2005040925A1; US7557902B2; KR20060107523A; JP4817844B2; AU2003273409A1; US20070035814A1; JP2007524214A; KR101052386B1

Abstract

The invention relates to a projection lens (1) for short wavelengths, in particular, for wavelengths lambda < 157 nm, provided with several mirrors (M1, M2, M3, M4, M5 and M6), arranged in a precise position with relation to an optical axis (5). The mirrors (M1, M2, M3, M4, M5 and M6) comprise multi-layer coatings. At least two different mirror materials are provided which differ in the rise in thermal expansion coefficient as a function of temperature around the zero transition of the thermal expansion coefficient, in particular, in the sign of the value.

Description

EUV PROJECTION LENS WITH MIRRORS MADE OF MATERIALS WITH DIFFERENT SIGNS OF THE INCREASE IN THE TEMPERATURE DEPENDENCY OF THE THERMAL EXPANSION COEFFICIENCY I DUCK NEAR THE ZERO TRANSITION TEMPERATURE

The invention relates to a projection objective for short wavelengths, in particular for wavelengths λ <157 nm, with a plurality of mirrors, which are arranged in an exact position with respect to an optical axis, and the mirrors have multilayer layers. Furthermore, the invention relates to a project exposure device for EUV lithography and an X-ray optical subsystem for X-rays of wavelength λ _R.

Projection lenses, which are used in the extreme ultraviolet range, are irradiated with soft X-rays. The wavelength range here is 10 to 30 nm. For the extreme UV light used, the materials previously used for optics are opaque, whereby the imaging rays are no longer guided through lenses by refraction, but only mirrors can be used. The mirrors used should have the highest possible reflectivity in the EUV area. Such mirrors comprise a substrate which is provided with a multilayer system, a so-called multilayer. This allows the realization of mirrors with high reflectivity in the X-ray range in the case of non-grazing incidence, that is, normal incidence mirrors (vertical incidence). Layer systems of this type, for example with Mo / Si (molybdenum / silicon), Mo / Be (molybdenum / beryllium), Mo-Ru / Be layer stacks with 40 to 100 layer pairs, can be used for such multilayer systems, with λ = in the EUV range 10 to 20 nm peak reflectivities of 70 to 80% can be achieved NEN. Depending on the wavelength of the light to be reflected, different layer systems can be used.

A high reflectivity of the layer stack can be achieved by phase-appropriate superimposition and constructive interference of the partial wave fronts reflected on the individual layers. However, the layer thicknesses should typically be controlled in the sub-angstrom range (<0.1 nm).

Multilayer-coated X-ray mirrors are operated near vertical incidence and are always preferred to mirrors with grazing incidence, which are covered with simpler layers, when high imaging quality due to low aberrations, preferably in imaging systems, is required. However, the reflectivity of grazing incidence mirrors can still be increased by applying a multilayer.

For mirrors, in particular X-ray mirrors, of an EUVL projection objective or projection system, the properties described below should be fulfilled at the same time, which guarantee a mask-true transfer of the structures to the wafer and enable a high contrast of the image and a high reflectivity of the mirror layer.

The first property to be mentioned is a good fine pass (figure), i.e. errors in the low spatial frequency range. This generally means structure sizes between 1/10 of the bundle cross sections assigned by the individual pixels up to a free diameter of the mirror. This means that the defects have lateral dimensions on the order of one millimeter to several decimeters. Such errors lead to aberrations and thus reduce the image fidelity and limit the resolution limit of the overall system.

Furthermore, the X-ray mirrors should have a low roughness in the MSFR (mid spatial frequency roughness) range (middle spatial frequency range). Such local wavelengths typically occur in the range between approx. 1 μ and approx. 1 mm and lead to stray light within the image field and thereby to contrast losses in the imaging optics.

The necessary prerequisites for achieving high reflectivities are sufficiently low layer and substrate roughness in the so-called HSFR (high spatial frequency roughness) range. Depending on the point of view, the HSFR area leads to light losses due to scattering outside the field of view of the optics or due to disturbances in the microscopic superimposition of the partial wave trains. The relevant local wavelength range is limited at the top by the criterion of scatter outside the image field and, depending on the application, is in the range of a few μm for EUV wavelengths. In general, no limit is defined for the high-frequency limit. In this case, a useful guideline value in the range of half the wavelength of the incident light can be given, since even higher spatial frequencies can no longer be seen by the incident photons based on the knowledge known to date. The HSFR can be measured with the well-known Atomic Force microscopes (AFM), which have the necessary lateral and vertical resolution. In projection optics, both figure as well as the MSFR and the HSFR can be controlled within a few angstroms rms (root ean square - square mean).

Furthermore, materials should be used for the X-ray mirrors which have the lowest possible thermal expansion coefficient, such as ZERODUR ^® or ULE ^® . The surface shape of the mirror can thus be kept stable even during operation under thermal loads. Single-crystalline silicon could also be used as a carrier, since it allows very low roughness. Furthermore, the higher coefficient of thermal expansion in silicon can be partially compensated for by the significantly higher thermal conductivity and suitable cooling. However, silicon has mechanical anisotropy and can generally only be used for small mirror sizes due to the required single crystallinity. Another major disadvantage is the comparatively high price of the single-crystal material. Silicon will therefore only be used at very high thermal loads, for example in lighting systems.

It has been shown that only suitable glass-ceramic materials, e.g. Schott: ZERODUR ^® , Ohara: CLEARCERAM-Z ^® , or amorphous titanium silicate glasses, e.g. Corning: ULE, have been considered for such mirrors, since they have a thermal coefficient of thermal expansion ( CTE), which can be made to disappear at a certain temperature, which is also referred to as zero crossing temperature (ZCT). In the event of finite, local and global deviations in the operating temperature from the zero crossing temperature, the thermal expansion coefficient disappears. However, the surface is not completely deformed. The tolerances for these deformations are around 100 nm for global, homogeneous deformations of the mirror, and for local, locally varying deformations in the range from 50 pm to 200 pm. When simulations have been carried out, it has been shown that, in particular, distortion of the projection lens, the optical components it contains either consist only of ULE or only ZERODUR, are so sensitive to thermal loads that they are compensated for during operation by expensive manipulators while accepting dead times have to.

When using the materials currently used, the achievable system quality with regard to X-ray optics is severely impaired in different ways.

With regard to the projection optics for EUV lithography and the X-ray optical components used, reference is made to DE 100 37 870 AI and to US 6,353,470 B1, the statements of which are incorporated in full in the present application.

The titanium silicate glass, also known as ULE ^® , is specified specifically in WO 01/08163 AI for projection lenses in EUV lithography. In this document, a projection lithography method for the production of integrated circuits and generated patterns with extremely small object dimensions is described. An illumination subsystem illuminates a mask or a reticle with X-rays. A projection subsystem has reflective, multilayer-coated titanium silicate glasses which have a flawless surface. In the method according to the invention, the Titanium silicate glasses are heated to an operating temperature by means of the X-ray radiation, the titanium doping substance level preferably being regulated such that the glass has a thermal coefficient of thermal expansion which is centered around zero at the operating temperature. The titanium silicate glass specified here thus has a variation in the thermal coefficient of thermal expansion of <10 ppb.

It is an object of the invention to provide a projection lens of the type mentioned at the outset, avoiding the disadvantages of the prior art, in which the overall image quality is improved even when the temperature rises and the scattered light characteristic is optimized.

According to the invention, the object is achieved in that at least two different mirror materials which differ in the slope of the thermal expansion coefficient as a function of the temperature in the region of the zero crossing of the thermal expansion coefficient, in particular in the sign of the slope, are provided, advantageously being used in an EUV Range with wavelengths λ <20 nm is provided.

At such wavelengths of λ <20 nm, the projection lens is constructed with mirrors that reflect the light radiation. The construction of the projection lens with at least two different mirror materials, the mirror materials having a very small thermal coefficient of thermal expansion, is advantageous in that the image errors of the projection lens are assigned by a suitable assignment of the materials to the individual mirrors can be balanced or compensated for local and global temperature increases so that the resulting effects are minimized. By suitably arranging the mirror materials in the projection lens, the projection lens can be operated with stronger light sources, which consequently guarantees a higher wafer throughput and thus increased productivity. On the other hand, with a fixed thermal load, the requirements for the thermal coefficient of thermal expansion (CTE) of the materials can be reduced, which consequently enables a higher yield and therefore more economical use of the materials.

An advantageous embodiment of the invention can provide that at least one mirror made of a glass-ceramic material and at least one mirror made of an amorphous titanium silicate glass is provided.

The coefficients of thermal expansion of a glass-ceramic material and an amorphous titanium silicate glass are so small that they can be made to disappear at a certain temperature. Using such materials as mirror substrates with correct assignment of the materials to the mirrors, image errors can be significantly minimized and the overall system quality can be improved.

Embodiments of the invention will be explained in more detail with reference to basic drawings. It shows :

Figure 1 is a schematic representation of a 6-mirror projection lens, as known from DE 100 378 70 AI;

FIG. 2 shows the dependence of the CTE (T) on the temperature in the region of the zero crossing temperature (ZCT).

FIG. 3 shows a graphical representation of the sensitivities of the imaging errors without manipulator correction;

FIG. 4 shows a graphical representation of the compensation of thermally induced image errors by means of manipulators; and

Figure 5 is a graphical representation of a material mix optimization for distortion (NCE).

FIG. 1 shows an exemplary 6-mirror projection lens 1 known from the prior art, for example according to DE 100 37 870 A1, when used in the EUV range with wavelengths λ <157 nm, in particular λ <20 nm, with a Object 0 is located in object level 2. In this case, the object 0 to be imaged represents a mask or a reticle in the lithography. The object 0 is generated via a first mirror Ml, a second mirror M2, a third mirror M3, a fourth mirror M4, a fifth mirror M5 and a sixth mirror M6 mapped in an image plane 3. For example, a wafer is arranged in the image plane 3 in the lithography. With the six mirrors Ml, M2, M3, M4, M5 and M6 are aspherical mirrors, the first mirror M1 being designed as a convex mirror.

A diaphragm B limits the rays 4 passing through the system 1. The diaphragm B is located directly on the second mirror M2 or in the immediate vicinity of the mirror M2. The overall system is arranged centered on an optical axis 5 and has a telecentric beam path in the image plane 3. Furthermore, as can be clearly seen in FIG. 1, an intermediate image Z is formed between the fourth mirror M4 and the fifth mirror M5. This intermediate image Z is in turn imaged into the image plane 3 via the mirrors M5 and M6.

The thermal expansion coefficients (CTE) of suitable glass-ceramic materials, such as ZERODUR ^® or ClearCeram-Z ^® , as well as of amorphous titanium silicate glasses, such as ULE ^® , can be set at a certain temperature that can be set in certain ranges, namely the zero crossing temperature - zero crossing temperature (ZCT) - can be made to disappear, as shown schematically in FIG. However, the two material classes differ, among other things, in the dependence of the coefficient of thermal expansion on the temperature in the area of zero crossing temperature. The special functional profile of the thermal coefficient of thermal expansion with respect to the temperature and the distribution of inhomogeneities is also different. The zero crossing temperature should be in the range between 0 and 100 ° C, advantageously between 10 and 50 ° C. In the case of ZERODUR ^®, the Term - CTE (T) negative, while in the case of ULE ^® the Ter dT

- CTE (T) is positive. A typical target value for the Abso dT d, this value is - CTE (T) = 1.6 ppb / K. Here, however, the exact values of these terms may differ from those mentioned here, although special attention is paid to the different sign of the two CTE slopes in the area of the zero crossing temperature (ZCT). Materials should advantageously be used which have an increase in the coefficient of thermal expansion of less than 100 ppb / K ² , in particular less than 10 ppb / K ² . However, materials which have a CTE slope of less than 2ppb / K ² , such as ZERODUR ^® or ULE ^® , are particularly preferred.

These properties of the glass-ceramic materials, as well as the amorphous titanium-silicate glasses, can now be used to compensate for global and local temperature increases in such a way that the resulting effects or still existing imaging errors are minimized. This is done by appropriately assigning the two different mirror materials, which differ in the slope of the coefficient of thermal expansion as a function of temperature, in particular in the sign of the size, to the individual mirrors M1, M2, M3, M4, M5 and M6. For this purpose, the temperature distributions to be expected and, in turn, the resulting surface deformations are first determined using finite element analyzes. These are then superimposed on the ideal surfaces in an optical beam tracing program (eg code V) and the resulting imaging errors are determined. By selecting the mirror from the right materials, the most prominent aberrations such as distortion (NCE), field curvature (FPD), astigmatism (AST), coma (Zernike coefficient 7/8), spherical aberration (Zernike coefficient Z9) and influences the root mean square of the wavefront errors (RMS) and the resulting effects are minimized.

By balancing the temperature-related aberrations through the use of ZERODUR ^® and ULE ^® for the mirrors, the overall projection lens 1 can be operated with stronger light sources, which means that a higher wafer throughput and an increased productivity can be guaranteed. Thus, a composition of the projection objective 1 can be achieved from mirrors Ml, M2, M3, M4, M5 and M6, which are arranged to minimize thermal errors due to their mirror materials.

Due to the process, ULE ^® is a layered material. The use of ULE ^® may cause a low-frequency MSFR due to the streaks that occur, at least on curved surfaces, which in turn leads to small-angle scattering. Such scattering is particularly disruptive on mirrors close to the pupil, since it affects a field-dependent non-uniformity of the illuminance on the wafer or on the image plane 3. On mirrors close to the field, the small-angle scatter essentially leads to nonuniformities in the illuminance in the pupil, that is to say the angular distribution of the light rays in a field point. This effect can be classified as much less critical than the non-uniformity of the illuminance on the wafer. The crystallite structure of glass ceramic materials, in particular ZERODUR ^® , is prepared by certain manufacturing processes (see WO 03/16233 AI or DE 101 27 086 AI) and preferably contributes to high-frequency MSFR components and HSFR components, that is, wide-angle scattering is caused. It is therefore advantageous if the glass-ceramic material or ZERODUR ^{® is} preferably used in mirrors in which these spatial frequencies scatter in angular ranges that do not reach the wafer through vignetting or masking. In this case, the material should preferably be used in mirrors with large bundle cross sections, it also being possible for mirrors to be arranged in the front part, that is to say in the region remote from the wafer, of the lens 1 so that the scattered light is masked out by the aperture B or the other mirrors, so that it does not or minimized arrives in the wafer level 3. An optimization of the scattered light distribution in the wafer plane can thus be achieved by a suitable composition of the mirror materials.

FIG. 3 shows the sensitivities of the above-mentioned imaging errors in connection with the assignment of the materials to the individual mirrors Ml, M2, M3, M4, M5 and M6 for an exemplary objective relative to a suitably defined tolerance range, the sensitivities of the absolute errors in nm, arranged according to combinations of the materials ZERODUR ^® and ULE ^® . The combinations are symmetrical with regard to a common change of sign of the CTE (T) slope on all mirrors Ml, M2, M3, M4, M5 and M6. The plus sign stands for the material ULE ^® and the minus sign for the material ZERODUR ^® in the relevant arranged mirror. The analyzes were carried out with the following, exemplary, but realistic heat loads, which represent the absorbed power of the respective mirrors:

The CTE inhomogeneities (location variations) were not taken into account in these calculations.

With respect to FIG. 3, a combination of “+++++ -" d means a - CTE (T) = +1.6 ppb / K ² on the mirrors Ml, M2, M3, M4 dT

and M5 and a - CTE (T) = -1.6 ppb / K ² on the M6 mirror. dT

If one and the same material is assumed for all mirrors Ml to M6, then from the standpoint of non-correctable distortion (non-correctable error) it represents a poor starting position in order to correct this image error as best as possible. The combinations "++++++" and

"" Are almost 2.5 times outside the tolerance range.

With a suitable material mix, however, an almost 5 times better system quality can be achieved.

The material mix can be further optimized by arranging ULE ^® mirrors in areas close to the field and not near the pupil. In such a 6-mirror projection lens ULE ^® 1 may be preferably for the mirrors Ml be used M3 and M4. FIG. 4 shows the compensation of the thermally induced image errors by using manipulators which allow whole-body movement of the individual mirrors M1, M2, M3, M4, M5 and M6 during operation. FIG. 4 shows that for any material combinations, by changing the distances, the decentration and the tilting of the mirrors Ml, M2, M3, M4, M5 and M6, all image defects can be checked within their specified tolerance ranges.

Disadvantages of this compensation method are on the one hand the high price of the manipulators, which generally have to be operated remotely in a vacuum, and the dead time of the wafer exposure system, which is caused by the measurement and compensation of the image errors during operation. The dead time leads to production losses and thus to considerable economic losses.

A material mix optimization for distortion (NCE) is shown graphically in FIG. Here, the sensitivities without manipulator correction from FIG. 4 are shown arranged according to NCE residual errors. In FIG. 5, the material mix that produces the lowest NCE is shown in the left area of the graphic. For example, a mix of "+++ - + -" or "+ - +", as can be seen, provides the lowest NCE. With such a ULE ^® -ZERODUR ^® combination, it can be seen particularly clearly that all image defects, NCE, FPD, AST, Z7 / 8, Z9 and RMS can be found within the tolerance range without costly manipulator correction, as can be seen in FIG. 5 , It is now clearly shown that a combination of materials with ULE ^® / ZERODUR ^® can influence distortion (NCE) in particular. Other image errors can also be influenced and minimized by such a combination of materials.

The specified material distribution was determined for an exemplary system with an exemplary temperature distribution, without being restricted to this. For other optical systems or temperature distributions, different optimal material combinations can occur in the inventive sense.

It is to be expected that spatial inhomogeneities in the CTE distribution will lead to surface deformations when the temperature changes. These will more or less emulate the frequency response of the CTE variations. When carrying out simulations, however, it has been shown that essentially low and medium frequency errors are relevant (> 1 mm, MSFR or the shape error). Such relevant errors can be caused, for example, by the thermal deformation, which is proportional to the domain size. On the other hand, the cause can be the elastomechanical properties of the solid itself, which leads to increased damping at higher spatial frequencies. The frequency distribution of these inhomogeneities depends on the material, so that an optimization can also be carried out here.

It goes without saying that other materials for optimizing such image errors would also be conceivable. Likewise, the invention should not be limited to EUVL components. Depending on thermal and litter light specification, it can be advantageous to optimize reflective components, for example of a 157 nm lithography system, from these points of view.

Claims

Claims:

1. Projection lens for short wavelengths, in particular for wavelengths λ <157nm, with a plurality of mirrors which are arranged in an exact position with respect to an optical axis, and the mirrors have multilayer layers, characterized in that at least two different mirror materials, which differ in the gradient of the coefficient of thermal expansion as a function of the temperature in the region of the zero crossing of the coefficients of thermal expansion, in particular in the sign of the slope, for which mirrors are provided.

2. Projection lens according to claim 1, characterized in that an increase in the coefficient of thermal expansion in terms of amount is provided below 100 ppb / K ² , in particular below 10 ppb / K ² .

3. Projection objective according to claim 1 or 2, characterized in that the zero crossing temperature is in a range between 0 to 100 ° C, in particular between 10 to 50 ° C.

4. Projection lens according to claim 1, characterized by use in the EUV range with wavelengths λ <20nm.

5. Projection lens according to claim 1, characterized in that at least one mirror (M1, M2, M3, 4, M5, M6) made of a glass-ceramic material and at least one mirror (M1, M2, M3, M4, M5, M6) made of one Amorphous titanium silicate glass is provided.

6. Projection lens according to claim 5, characterized in that the glass-ceramic material is ZERODUR ^® .

7. Projection lens according to claim 5, characterized in that the amorphous titanium silicate glass is ULE ^® .

8. Projection lens according to claim 5, characterized in that the glass-ceramic material is provided for mirrors with large bundle cross sections.

9. Projection objective according to claim 5, characterized in that the glass-ceramic material is provided for mirrors in the objective region remote from the water.

10. Projection objective according to claim 1, characterized by a composition of mirrors (M1, M2, M3, M4, M5, M6), which are arranged minimizing thermal image errors with regard to their mirror materials.

11. Projection objective according to claim 1, characterized by a composition of mirrors (M1, M2, M3, M4, M5, M6) which are arranged with respect to their mirror materials in such a way that an optimization of a scattered light distribution in a wafer plane (3) is provided.

12. Projection lens according to claim 1, characterized by a composition of mirrors (M1, M2, M3, M4, M5, M6) which are arranged with respect to their mirror materials in such a way that a minimization of wavefront errors caused by CTE inhomogeneities is provided.

13. Projection exposure device for EUV lithography with optical components, in particular mirrors, reticles or beam splitters, characterized in that for the optical components (M1, M2, M3, M4, M5, M6) at least two different substrate materials, which are in the slope of the thermal expansion coefficient as a function of the temperature in the region of the zero crossing of the thermal expansion coefficients, in particular in the sign of the slope.

14. Projection exposure device according to claim 13, characterized in that an increase in the coefficient of thermal expansion is provided in terms of amount below 100 ppb / K ² , in particular below 10 ppb / K ² .

15. Projection exposure device according to claim 13 or 14, characterized in that the zero crossing temperature has a range between 0 to 100 ° C, in particular between 10 to 50 ° C.

16. Projection exposure device according to claim 13, characterized in that at least one optical component (M1, M2, M3, M4, M5, M6) made of a glass-ceramic material and at least one optical component (M1, M2, M3, M4, M5, M6) is provided from an amorphous titanium silicate glass.

17. Projection exposure device according to claim 13, characterized by a composition of optical components (M1, M2, M3, M4, M5, M6), which with regard to their rer substrate materials thermally induced image defects are arranged.

18. Projection exposure device according to claim 13, characterized by a composition of optical components (M1, M2, M3, M4, M5, M6) which are arranged with respect to their substrate materials in such a way that an optimization of a scattered light distribution in a wafer plane (3) is provided is.

19. Projection exposure device according to claim 13, characterized by a composition of optical components (M1, M2, M3, M4, M5, M6) which are arranged with respect to their substrate materials in such a way that a minimization of wavefront errors caused by CTE inhomogeneities is provided.

20. X-ray optical subsystem, in particular a mirror, reticle or beam splitter, for X-rays of wavelength λ _R , characterized by at least two different substrate materials, which differ in the slope of the coefficient of thermal expansion as a function of the temperature in the region of the zero crossing of the coefficient of thermal expansion, in particular in the sign of the slope, differentiate.

21. X-ray optical subsystem according to claim 20, characterized in that the wavelength λ _R <200 nm, in particular λ _R <157 nm.

22. X-ray optical subsystem according to claim 20, characterized in that the substrate material is a glass-ceramic material.

23. X-ray optical subsystem according to claim 20, characterized ^¬ indicates that the substrate material is a titanium silicate glass.

24. X-ray optical subsystem for a projection lens according to one of claims 8 to 12.

25. Use of X-ray optical subsystems according to one of claims 20 to 23 in X-ray microscopy, X-ray astronomy or X-ray spectroscopy.