Mask for EUV Lithography, EUV Lithography Apparatus and Method for Determining a Contrast Proportion Caused by DUV Radiation

Abstract

A mask (M) for EUV lithography includes: a substrate (7), a first surface region (A.sub.1) formed by a surface (8a) of a multilayer coating (8) embodied to reflect EUV radiation (27), said surface (8a) facing away from the substrate (7), and a second surface region (A.sub.2) formed by a surface (18a) of a further coating (18) embodied to reflect DUV radiation (28) and to suppress the reflection of EUV radiation (27), said surface (18a) facing away from the substrate (7). The further coating is a multilayer coating (18). Also disclosed are an EUV lithography apparatus that includes such a mask (M) and a method for determining a contrast proportion caused by DUV radiation when imaging a mask (M) onto a light-sensitive layer.

Claims

1. A mask for extreme ultraviolet (EUV) lithography, comprising: a substrate having: a first surface region (A.sub.1) formed by a surface of a multilayer coating embodied to reflect EUV radiation, said multilayer coating surface facing away from the substrate, and a second surface region (A2) formed by a surface of a further coating embodied to reflect deep ultraviolet (DUV) radiation and to suppress the reflection of the EUV radiation, said further coating surface facing away from the substrate, wherein the further coating is a further multilayer coating, wherein the wavelength-dependent reflectivity of the further multilayer coating for the DUV radiation in the wavelength range between 140 nm and 400 nm does not deviate by more than +/−5% from the wavelength-dependent reflectivity of the multilayer coating.

2. The mask as claimed in claim 1, further comprising: a third surface region (A.sub.3) formed by a surface of a coating absorbing the EUV radiation, said EUV radiation absorbing surface facing away from the substrate.

3. The mask as claimed in claim 1, wherein the reflectivity of the further coating is less than 0.3% at a used wavelength (λ.sub.B) of the EUV radiation at which the reflectivity of the multilayer coating is at a maximum.

4. The mask as claimed in claim 1, wherein the multilayer coating comprises a plurality of alternating layers made respectively of a layer material with a high refractive index and a layer material with a low refractive index.

5. The mask as claimed in claim 4, wherein the layer materials of the alternating layers of the multilayer coating and of the further multilayer coating are identical.

6. The mask as claimed in claim 1, wherein the surface of the multilayer coating forms a contiguous first surface region (A.sub.1) of the mask, said first surface region covering 30% or more of the surface (A.sub.1+A.sub.2+A.sub.3) of the mask provided for imaging.

7. The mask as claimed in claim 1, wherein the surface of the further multilayer coating forms a contiguous second surface region (A.sub.2) of the mask, said second surface region covering 30% or more of the surface (A.sub.1+A.sub.2+A.sub.3) of the mask provided for imaging.

8. An EUV lithography apparatus comprising: a mask as claimed in claim 1.

9. A method for determining a contrast proportion (K.sub.Duv/K.sub.DUV+Euv) caused by DUV radiation when imaging a mask onto a light-sensitive layer, comprising: illuminating the mask with radiation for imaging the mask onto the light-sensitive layer, determining a radiation dose (D.sub.1) required for exposing a first region (B.sub.1) of the light-sensitive layer, wherein radiation which is reflected at a multilayer coating of the mask is incident on the light-sensitive layer in the first region, said multilayer coating being embodied both to reflect EUV radiation and to reflect DUV radiation, and determining a radiation dose (D.sub.2) required for exposing a second region (B.sub.2) of the light-sensitive layer, wherein radiation which is reflected by a further coating of the mask is incident on the light-sensitive layer in the second region (B.sub.2), said further coating being embodied to suppress EUV radiation and to reflect DUV radiation, and determining the contrast proportion (K.sub.DUV/K.sub.DUV+Euv) by comparing the radiation doses (D.sub.1, D.sub.2) required for exposing the first region (B.sub.1) and for exposing the second region (B.sub.2), wherein the wavelength-dependent reflectivity of the further coating, which is embodied as a multilayer coating, for DUV radiation in the wavelength range between 140 nm and 400 nm is selected to not deviate by more than +/−5% from the wavelength-dependent reflectivity of the multilayer coating.

10. The method as claimed in claim 9, wherein the contrast proportion K.sub.DUV/K.sub.DUV+EUV is determined from the radiation dose D.sub.1 required for exposing the first region (B.sub.1) and the radiation dose D.sub.2 required for exposing the second region (B.sub.2) in accordance with the following formula:
K.sub.DUV/K.sub.DUV+EUV=D.sub.1/D.sub.2.

11. The method as claimed in claim 9, further comprising: determining a radiation dose (D.sub.3) required for exposing a third region (B.sub.3) of the light-sensitive layer, wherein radiation which is reflected by a coating which absorbs EUV radiation is incident on the light-sensitive layer in the third region (B.sub.3), and determining the contrast proportion (K.sub.DUV/K.sub.DUV+EUV) taking into account the radiation dose (D.sub.3) required for exposing the third region (B.sub.3).

12. The method as claimed in claim 11, wherein the contrast proportion K.sub.DUV/K.sub.DUV+EUV is determined from the radiation dose D.sub.1 required for exposing the first region (B.sub.1), the radiation dose D.sub.2 required for exposing the second region (B.sub.2) and the radiation dose D.sub.3 required for exposing the third region (B.sub.3) in accordance with the following formula:
K.sub.DUV/K.sub.DUV+EUV=(A.sub.3/D.sub.3+(A.sub.1+A.sub.2)/D.sub.2)/(A.sub.3/D.sub.3+(A.sub.1+A.sub.2)/D.sub.1), where A.sub.1, A.sub.2, A.sub.3 denote areas of the surfaces of the multilayer coating, the further coating and the coating which absorbs EUV radiation.

13. The mask as claimed in claim 1, wherein the wavelength-dependent reflectivity of the further multilayer coating for the DUV radiation in the wavelength range between 140 nm and 400 nm does not deviate by more than +/−1% from the wavelength-dependent reflectivity of the multilayer coating.

14. The mask as claimed in claim 3, wherein the reflectivity of the further coating is less than 0.1%, at a used wavelength (λ.sub.B) of the EUV radiation at which the reflectivity of the multilayer coating is at a maximum.

15. The method as claimed in claim 9, wherein the further coating is embodied as a multilayer coating.

16. The method as claimed in claim 9, wherein the wavelength-dependent reflectivity of the further coating, which is embodied as a multilayer coating, for DUV radiation in the wavelength range between 140 nm and 400 nm is selected to not deviate by more than +/−1% from the wavelength-dependent reflectivity of the multilayer coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Exemplary embodiments are illustrated in the schematic drawing and are explained in the following description. In the figures:

[0045] FIG. 1 shows a schematic illustration of an EUV lithography apparatus comprising an illumination system for illuminating a mask and comprising a projection system for imaging the mask onto a light-sensitive layer,

[0046] FIG. 2A,B show schematic illustrations of a mask for the EUV lithography apparatus of FIG. 1 for the purposes of determining a contrast proportion caused by DUV radiation when exposing the light-sensitive layer,

[0047] FIG. 3A,B show schematic illustrations of a mask analogous to FIG. 2A,B, which comprises a multilayer coating, an absorbing coating and a further multilayer coating,

[0048] FIG. 4A,B show illustrations of the reflectivity of the multilayer coating and of the further multilayer coating of the mask of FIG. 3A,B in the EUV wavelength range, and

[0049] FIG. 5 shows an illustration of the reflectivity of the multilayer coating and of the further multilayer coating of the mask of FIG. 3A,B in the DUV wavelength range.

[0050] Identical reference signs are used in the following description of the drawings for components that are the same or functionally the same.

DETAILED DESCRIPTION

[0051] FIG. 1 shows, very schematically, an optical arrangement in the form of an EUV lithography apparatus 1, which comprises an EUV light source 2 for generating EUV radiation having a high energy density in an EUV wavelength range between approximately 5 nm and approximately 20 nm. The EUV light source 2 may for example take the form of a plasma light source for generating a laser-induced plasma or be formed as a synchrotron radiation source. In the former case, in particular, a collector mirror 3 may be used, as shown in FIG. 1, in order to focus the EUV radiation of the EUV light source 2 into an illumination beam 4 and in this way increase the energy density further. The illumination beam 4 serves for the illumination of a reflective mask M using an illumination system 10, which comprises five reflecting optical elements 12 to 16 (mirrors) in the present example.

[0052] The wavelength spectrum emitted by the EUV light source 2 is not restricted to EUV radiation between approximately 5 nm and approximately 20 nm; rather, the EUV light source 2 also generates radiation at longer wavelengths, in particular in the DUV wavelength range between approximately 100 nm and approximately 400 nm and, possibly, radiation at even longer wavelengths in the VIS range or in the IR range.

[0053] The reflective mask M may comprise reflecting and non-reflecting or at least less strongly reflecting, or absorbing, regions, which form a structure to be imaged. In the shown example, a special mask M is used for the imaging, said mask being described in more detail below.

[0054] The mask M reflects part of the illumination beam 4 and forms a projection beam 5, which is radiated into a projection system 20, which generates an image of the mask M or of a respective portion thereof (see below) on a wafer W. The wafer W comprises a semiconductor material, for example silicon, and is arranged on a holder, which is also referred to as a wafer stage WS. A light-sensitive layer 6 (resist or photoresist), which is exposed by the projection beam 5, is applied onto the wafer W.

[0055] In the present example, the projection system 20 comprises six reflective optical elements 21 to 26 (mirrors) in order to generate an image of the mask M on the wafer W. The number of mirrors in a projection system 20 typically lies between four and eight; however, only two mirrors may also possibly be used.

[0056] In order to achieve a high imaging quality when imaging a respective object point OP of the mask M onto a respective image point IP on the wafer W or on the light-sensitive layer 6, highest requirements are to be imposed on the surface form of the mirrors 21 to 26; and the position or the alignment of the mirrors 21 to 26 in relation to one another and in relation to the mask M and the wafer W also requires precision in the nanometer range.

[0057] FIG. 2A,B show an example of a mask M for the EUV lithography apparatus 1 of FIG. 1, which comprises a substrate 7 made of a material with a low coefficient of thermal expansion, e.g. ULE®, Zerodur® or Clearceram®, in a plan view and in a section. The mask M typically has a rectangular basic form, with the sectional illustration shown in FIG. 2B being representative for the entire mask M; i.e., the structure of the mask M does not change in a direction perpendicular to the plane of the drawing.

[0058] A multilayer coating 8 comprising a plurality of alternating layers 9a, 9b made of a layer material with a high refractive index and a layer material with a low refractive index is applied to the substrate 7 of the mask M. The number of layers with a high refractive index and a low refractive index 9a, 9b depicted in FIG. 2 merely serves for illustrative purposes. Moreover, depicting a capping layer and depicting possibly present barrier layers for avoiding diffusion were dispensed with in the illustration of the multilayer coating 8 in FIG. 2B.

[0059] The typically periodic design of the reflective multilayer coating 8 (generally with pairs of layers 9a, 9b with an identical thickness) facilitates reflection of short-wavelength λ.sub.B EUV radiation with a wavelength λ.sub.B in the nm range (e.g. at a used wavelength λ.sub.B λ.sub.B of 13.5 nm). As a rule, the layers 9a made of the material with a high refractive index are made of silicon and the layers 9b made of the material with a low refractive index are made of molybdenum in the case of a used wavelength λ.sub.B λ.sub.B of 13.5 nm. Depending on the used wavelength λ.sub.B in the EUV wavelength λ.sub.B range, other material combinations such as e.g. molybdenum and beryllium, ruthenium and beryllium, or lanthanum and B.sub.4C are likewise possible.

[0060] The multilayer coating 8 comprises a surface 8a, at which the multilayer coating 8 is exposed, facing away from the substrate 7. The multilayer coating 8 comprises a portion 8b, onto which an absorbing coating 17 has been applied in the example shown in FIG. 2A,B. In the shown example, the absorbing coating 17 consists of a single layer having a metallic material, for example chromium, chromium oxide, titanium, titanium nitride, tantalum, tantalum nitride, etc. Where necessary, a barrier layer not depicted here may be applied between the absorbing coating 17 and the multilayer coating 8. In the mask M shown in FIG. 2A,B, the absorbing layer 17 has been applied onto the multilayer coating 8 over the entire area thereof, i.e. the absorbing coating 17 completely covers the portion 8b.

[0061] A further coating 18, which consists of a single layer of aluminum in the mask M shown in FIG. 2A,B, has been applied onto a portion 17b of the absorbing coating 17. Aluminum has a reflectivity of virtually 0% for incident EUV radiation 27 in a wavelength range between approximately 5 nm and approximately 20 nm while DUV radiation 28, i.e. radiation in a wavelength range between approximately 100 nm and approximately 400 nm, and consequently also between 140 nm and 400 nm or 300 nm, has a reflectivity of virtually 100%. Both the EUV radiation 27 incident on the mask M and the DUV radiation 28 incident on the mask M are part of the illumination beam 4 shown in FIG. 1.

[0062] As may be identified in FIG. 2a, the exposed surface 8a of the multilayer coating 8 forms a first surface region A.sub.1 of the mask M, the exposed surface 18a of the further coating forms a second surface region A.sub.2 of the mask M and the exposed surface 17a of the absorbing coating 17 forms a third surface region A.sub.3 of the mask M.

[0063] FIG. 3A,B show a further embodiment of the mask M which inter alia differs from the mask M depicted in FIG. 2 in that the further coating is embodied as a further multilayer coating 18. Moreover, the absorbing coating 17 is applied directly onto the substrate 7 and over the whole area thereof in the mask M depicted in FIGS. 3A,B. The multilayer coating 8 and the further multilayer coating 18 are applied onto the absorbing coating 17 at two different portions 17b, 17c and the surfaces 8a, 18a of said multilayer coating and further multilayer coating form a first surface region A.sub.l and a second surface region A.sub.2 of the mask M, said surface regions being surrounded by a third surface region A.sub.3 which is formed by the exposed surface 17a of the absorbing coating 17. It is understood that the mask M provided with the further multilayer coating 18 may alternatively have an embodiment as depicted in FIG. 2A,B.

[0064] The further multilayer coating 18 comprises a multiplicity of alternating layers 29a, 29b made of a layer material with a high refractive index and a layer material with a low refractive index, with the terms “high refractive index” and “low refractive index” relating to the refractive indices of the two layer materials 29a, 29b relative to one another, i.e. the layer material 29a with a high refractive index has a higher refractive index than the layer material 29b with a low refractive index.

[0065] In the shown example, the material of the layers 29a with a high refractive index is silicon and the material of the layers 29b with a low refractive index is molybdenum, i.e. the layer materials of the further multilayer coating 18 correspond to the layer materials of the multilayer coating 8. However, the layer thicknesses of the layers 9a, 9b of the multilayer coating 8 and the layer thicknesses of the layers 29a, 29b of the further multilayer coating 18 differ from one another, to be precise in such a way that the multilayer coating 8 has a maximum of the reflectivity R at a used wavelength λ.sub.B λ.sub.B of approximately 13.5 nm while the further multilayer coating 18 has a reflectivity of less than 0.3% in a wavelength range of +/−0.5 nm around the used wavelength λ.sub.B λ.sub.B, as may be identified on the basis of the two reflectivity curves, depicted in FIG. 4A,B, for the reflectivity R.sub.1 of the multilayer coating 8 and the reflectivity R.sub.2 of the further multilayer coating 18.

[0066] Accordingly, the multilayer coating 8 is a coating which is highly reflective for EUV radiation 27 at the used wavelength λ.sub.B of approximately 13.5 nm, while the further multilayer coating 18 is embodied to suppress the reflection of EUV radiation 27 in a wavelength range lying around the used wavelength λ.sub.B. The further multilayer coating 18, more precisely the layer thicknesses of the layer materials 29a , 29b, is/are selected in such a way that the further multilayer coating 18 reproduces the reflectivity R of the multilayer coating 8 in the DUV wavelength range, i.e. with wavelengths between 100 nm and 400 nm, preferably between 140 nm and 300 nm, as accurately as possible. This may likewise be achieved by virtue of the layer thicknesses of the layers 29a, 29b and the number of layers of the further multilayer coating 18 being selected in a suitable manner, with the optimization typically being carried out with the aid of numerical calculations.

[0067] FIG. 5 shows the reflectivity R.sub.1 of the multilayer coating 8 and the reflectivity R.sub.2 of the further multilayer coating 18 in the wavelength range between approximately 140 nm and approximately 400 nm. Practically no difference can be identified in FIG. 5 between the reflectivity R.sub.1 of the multilayer coating 8 and the reflectivity R.sub.2 of the further multilayer coating 18 in this wavelength range. In general, what may be achieved by the optimization is that the wavelength-dependent reflectivity R.sub.2 of the further multilayer coating 18 for DUV radiation 28 in the wavelength range between 140 nm and 400 nm does not deviate by more than +/−5%, preferably by no more than +/−1%, from the wavelength-dependent reflectivity R of the multilayer coating 8.

[0068] A layer design for the further multilayer coating, which generates the wavelength-dependent reflectivity R.sub.1 or R.sub.2 shown in FIG. 4A,B and FIG. 5, is described below. The following periodic design was used for the multilayer coating 8: Vacuum/60× (3 nm Mo/4 nm a-Si)/substrate. The aperiodic design of the further multilayer coating 18 may be gathered from the following table:

TABLE-US-00001 TABLE 1 Layer thickness (nm) Material Vacuum 0.695 Mo 7.74 a-Si 7.896 Mo 2.083 a-Si 0.663 Mo 4.216 a-Si 14.972 Mo Absorber Substrate

[0069] The masks M shown in FIG. 2A,B and FIG. 3A,B serve to determine a contrast proportion K.sub.DUV/K.sub.DUV+EUV caused by the DUV radiation 28 when imaging the mask M onto the light-sensitive layer 6 in the EUV lithography apparatus 1 of FIG. 1 using a method described below.

[0070] For exposure or imaging purposes, the mask M of FIGS. 3a,b is positioned in the EUV lithography apparatus 1 and the light source 2 is activated such that the illumination beam 4, which contains both EUV radiation 27 and DUV radiation 28, is incident on the mask M. During the imaging, the first surface region A.sub.1 with the multilayer coating 8 is imaged onto a first region B.sub.1 of the light-sensitive layer 6 (cf. FIG. 1), the second surface region A.sub.2 with the further multilayer coating 18 is imaged onto a second region B.sub.2 of the light-sensitive layer 6 and the third surface region 17a, in which the absorbing coating 17 is exposed, is imaged onto a third region B.sub.3 of the light-sensitive layer 6.

[0071] Hence, EUV radiation 27 and DUV radiation 28, which was reflected at the multilayer coating 8, are incident on the first region B.sub.1, while only DUV radiation 28 is incident on the second region B.sub.2 since the further multilayer coating 18 is embodied to suppress the reflection of EUV radiation 27. The EUV light source 2 is switched off after a predetermined period of time. The procedure described above is repeated with an increasing length of time during which the EUV light source 2 is activated until a first radiation dose D.sub.1 (“dose to clear”), at which the light-sensitive layer 6 has been exposed through in the first region B.sub.1, and a second radiation dose D.sub.2 at which the light-sensitive layer 6 has been exposed through in the second region B.sub.2 may be determined.

[0072] In order to check whether the radiation dose D.sub.1, D.sub.2 has been achieved, the light-sensitive layer 6 or the wafer W is removed from the EUV lithography apparatus 1 and developed using a photochemical method. Typically, a number of exposures with different time durations are undertaken on adjacent surface regions of the same light-sensitive layer 6; i.e., it is not necessary to replace the light-sensitive layer 6 after each exposure.

[0073] The radiation dose D.sub.1 required for exposing the first region B.sub.1 is less than the radiation dose D.sub.2 required for exposing the second region B.sub.2 since both EUV radiation 27 and DUV radiation 28 are incident in the first region B.sub.1, i.e. D.sub.1<D.sub.2 applies. The contrast ratio K.sub.DUV/K.sub.DUV+EUV corresponds to the ratio of the two radiation doses D1, D2 i.e. the following applies:

D.sub.1/D.sub.2=K.sub.DUV/K.sub.DUV+EUV.

[0074] Additionally, a third radiation dose D.sub.3 may also be determined in the mask M shown in FIG. 2A,B and FIG. 3A,B, said third radiation dose being required for exposing the third region B.sub.3 of the light-sensitive layer 6. Radiation, in particular DUV radiation 28, which is reflected by the coating 17 which absorbs EUV radiation 27 is incident on the light-sensitive layer 6 in the third region B.sub.3, provided said absorbing coating does not facilitate a sufficient suppression of the reflection of radiation in this wavelength range. Since the masks employed for producing semiconductors in the EUV lithography apparatus 1 have a structured absorbing coating 17, in which coated regions of the multilayer coating 8 alternate with uncoated regions, the determination of the DUV contrast proportion may be refined by measuring the third radiation dose D.sub.3 required for exposing the third region B.sub.3 if the surface area A.sub.3 of the absorbing coating 17 is known or if the proportion of the absorbing coating 17 of the entire surface A.sub.1+A.sub.2+A.sub.3 of the mask M to be imaged is known.

[0075] In this case, the following formula may be used for determining the contrast proportion K.sub.DUV/K.sub.DUV+EUV:

K.sub.DUV/K.sub.DUV+EUV=(A.sub.3/D.sub.3+(A.sub.1+A.sub.2)/D.sub.2)/(A.sub.3/D.sub.3+(A.sub.1+A.sub.2)/D.sub.1),

where A.sub.1, A.sub.2, A.sub.3 denote areas of the surfaces 8.sub.a, 18a, 17a of the multilayer coating 8, the further coating 18 and the coating 17 which absorbs EUV radiation 27.

[0076] In order to simplify the determination of the radiation doses D.sub.1, D.sub.2, D.sub.3, it is advantageous if the absorbing coating 17 forms a contiguous surface region A3 of no more than approximately 30-40% of the entire surface A.sub.1+A.sub.2+A.sub.3 of the mask M, as is the case in the masks M shown in FIG. 2A,B and FIG. 3A,B. It is likewise advantageous if the multilayer coating 8 and the further multilayer coating 18 each form a contiguous surface region A.sub.1, A.sub.2, which respectively covers 30% or more of the entire surface A.sub.1+A.sub.2+A.sub.3 of the mask M. What this may achieve is that the first region B.sub.1, the second region B.sub.2 and the third region B.sub.3 of the light-sensitive layer 6 have approximately the same size, i.e. each one of the three regions B.sub.1 to B.sub.3 provides approximately a third of the exposed area of the light-sensitive layer 6 in each case.

[0077] In the manner described further above, it is possible to precisely determine the contrast proportion of the DUV radiation 28 which, in addition to the EUV radiation 27, contributes to the exposure of the light-sensitive layer 6. Typically, the light-sensitive layer 6 is not sensitive to radiation at longer wavelengths, i.e. in the VIS or IR wavelength range, and so radiation at these wavelengths does not contribute, or only contributes to a negligible proportion, to the contrast.