WO2022084317A1

WO2022084317A1 - Binary intensity mask for the euv spectral range

Info

Publication number: WO2022084317A1
Application number: PCT/EP2021/078952
Authority: WO
Inventors: Oliver Paul; Derick CHONG
Original assignee: Carl Zeiss Smt Gmbh; Asml Netherlands B.V.
Priority date: 2020-10-21
Filing date: 2021-10-19
Publication date: 2022-04-28
Also published as: JP2023546667A; DE102020213307A1; CN116569104A; KR20230088459A

Abstract

The invention relates to a binary intensity mask (100) for use in an EUV system working with EUV radiation, comprising a substrate (110) and a mask structure (140), which is applied to the substrate and contains absorber material. The mask structure (140) has a structured layer assembly, which comprises a first layer (151) made of a first layer material and a second layer (152) made of a second layer material. At the wavelength λ of the EUV radiation, the first layer material has a real part of the index of refraction, n1, of greater than 1 and the second layer material has a real part of the index of refraction, n2, of less than 1.

Description

Binary intensity mask for the EUV spectral range

This application is based on the German patent application with file number 10 2020 213 307.7, which was filed on October 21, 2020. The disclosure content of this German application is incorporated into the content of the present application by reference.

FIELD OF APPLICATION AND PRIOR ART

The invention relates to a binary intensity mask for use in an EUV system working with extreme ultraviolet radiation (EUV) and to a method for producing the binary intensity mask. Different possible uses of such a binary mask are described.

To produce semiconductor components and other finely structured components, such as masks for microlithography, photolithographic processes and projection exposure systems are used, among other things, in which the structure pattern to be generated is applied to a functional layer coated with a light-sensitive layer using a mask (also called a lithography mask or reticle). projected on a reduced scale and, after developing the photosensitive layer, transferred to the functional layer by means of an etching process.

In order to be able to produce finer and finer structures, optical systems have been developed in recent years that work with moderate numerical apertures and achieve high resolving power essentially through the short wavelength of the electromagnetic radiation used from the extreme ultraviolet range (EUV), especially with working wavelengths in the range between 5 nm and 30 nm, for example at working wavelengths around 13.5 nm.

Radiation from the extreme ultraviolet range (EUV radiation) cannot be focused or guided using refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at longer wavelengths. Therefore, mirror systems are used for EUV lithography.

A binary intensity mask of the type considered in this application has a laterally structured mask structure made up of structural elements and having absorber material. The mask structure should reflect the EUV radiation impinging on a structure element of the mask structure Radiation absorb as much as possible, while portions of the EUV radiation that fall on structural element-free areas next to structural elements of the mask structure on the mask are not absorbed by the mask structure. The areas covered by the mask structure should therefore be relatively opaque to the EUV radiation.

The known EUV lithography systems work with reflective masks. Reflective masks for productive operation (i.e. lithography masks, e.g. for the production of structured semiconductor components) have a mask structure that essentially corresponds to an enlarged structure of the structure of the desired functional layer to be produced in an exposure step.

A reflective binary intensity mask for use in an EUV system that works with EUV radiation comprises a substrate, a multi-layer arrangement applied to the substrate and having a reflective effect for the EUV radiation, and a mask structure applied to the multi-layer arrangement, which has at least one contains absorber material. Such binary intensity masks are also referred to as "binary intensity mask" (BIM) in the English-language literature. The substrate usually consists of a material with a very low coefficient of thermal expansion. The reflective multilayer arrangement (multilayer) can, for example, have a large number of alternating layers made of silicon (Si) or molybdenum (Mo), which have a highly reflective effect on the EUV radiation of the working wavelength. Tantalum (Ta) or tantalum nitride (TaN) is often used as the absorber material.

WO 2011/157643 A1 deals, among other things, with the impairment of imaging quality that can result from shadowing effects when using reflective EUV masks. Among other things, it describes how the thickness of the absorber layer and the absorber material can be suitably selected depending on the numerical aperture and other general conditions in order to obtain optimal imaging quality. It is recommended to keep the thickness of the mask structure relatively small. Numerous absorber materials and their absorption coefficients are included in the consideration.

US Pat. No. 6,610,447 B2 describes a method for producing a reflective binary intensity mask. In this case, an improved absorption layer (improved absorber layer) is produced on a reflective coating carried by a substrate. This absorption layer contains a first element which is doped with a second element, the ratio of the elements to one another changing in the direction of thickness. After the absorption layer has been applied, it is structured. US Pat. No. 9,709,844 B2 also describes a reflective binary intensity mask. The intensity mask includes a substrate containing a low thermal expansion material. A mirror structure is arranged on the substrate. A cap layer is arranged on the mirror structure. An absorber layer is arranged on the cap layer. The absorber layer contains a material that has a refractive index in a range from about 0.95 to about 1.01 and an extinction coefficient of greater than about 0.03. This is intended to reduce unwanted displacements of the aerial image (aerial image shift) during projection exposure with dipole lighting.

The technical article "Ni-Al Alloys as Alternative EUV Mask Absorber" by Vu Luong et al. in: Appl. May be. 2018, 8, 521; doi:10.3390/app8040521 describes systematic methods for evaluating potential absorber materials for binary intensity masks for EUV lithography masks with the aim of minimizing so-called mask 3D effects (M3D effects) that result from the three-dimensional structure of the EUV masks. One approach consists in using materials with a relatively large extinction coefficient, in which at the same time the real part of the refractive index is close to the value 1. The properties of Ni-Al alloys with a suitable property profile are presented in detail.

Investigations by the inventors have shown that even if all recommendations known from the prior art for optimizing the design of binary EUV mask intensity mask for the EUV range are observed, wavefront deformations can occur in the reflected EUV radiation compared to the irradiated EUV radiation, which can lead to imaging errors.

TASK AND SOLUTION

It is an object of the invention to provide a binary intensity mask according to the preamble of claim 1, which can be produced in high quality using established manufacturing processes and does not cause any substantial wavefront deformations when it is used. A further object is to provide a method for producing such an intensity mask and to indicate possible uses.

To solve this problem, the invention provides an EUV mask with the features of claim 1. Furthermore, a method for producing the binary mask with the features of claim 17 is provided. Furthermore, use of such an intensity mask as a measurement mask in a measurement method and a corresponding measurement method and a measurement device are provided. Advantageous developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference.

According to one formulation, the invention provides a binary mask for use in an EUV system using EUV radiation. The binary mask, which can also be referred to as a binary EUV mask, includes a substrate that is preferably made of a material with a very low coefficient of thermal expansion. Furthermore, the mask has a mask structure which is applied to the substrate and contains absorber material. The mask structure can be applied to the substrate directly or with the interposition of at least one intermediate layer. The mask structure should have an absorbing effect on the EUV radiation, so that the mask is designed as a binary mask in which those parts of the EUV radiation that fall on the mask structure should be absorbed as well as possible, while those parts that fall between the structural elements of the mask structure falling on uncovered areas without absorber material are not absorbed by the mask structure, or are absorbed as little as possible. Such masks are also referred to as binary intensity masks in this application.

The mask structure has a structured layer arrangement that includes (at least) a first layer made of a first layer material and (at least) a second layer made of a second layer material. The first layer material has a real part of the index of refraction, n1, that is greater than 1 at the wavelength of the EUV radiation, while the second layer material has a real part of the index of refraction, n2, that is less than 1.

With a suitable design of the individual layers, the multilayer construction of the absorbing mask structure can optimize the phase delay of the waves when passing through the mask structure with respect to a reference wave that travels the same distance through a vacuum. Due to the multi-layer structure, it is not necessary to find a single material for optimizing the phase retardation, which at the same time has good extinction properties and sufficiently low phase retardation. Rather, it is possible, due to the layered structure of the mask structure, to combine individual layers that are to be produced in a relatively reliable process in such a way that their effects on the phase delay of the EUV radiation passing through are at least partially compensated. If necessary, the ratios can be set such that there is no phase delay (zero phase delay) between the radiation that has passed through the mask structure and the radiation that has not passed through the mask structure. However, this is usually not absolutely necessary, as long as the phase lags remain sufficiently small that even small non-zero phase lags can be beneficial.

A basic idea on which the invention is based is to coordinate the multiple layers of the mask structure with respect to their layer thicknesses and the refractive indices of their layer materials in such a way that targeted optimization of the phase delay is possible. Preferably, the layer thicknesses are designed taking into account the working wavelength X such that the first layer has a first layer thickness d1 and the second layer has a second layer thickness d2, the following condition applying: d1 * n1 + d2 * n2 = ( d1 + d2 ) ± 0.1X

Here n1 is the real part of the refractive index of the first layer material, while n2 is the real part of the refractive index of the second layer material.

The product of the layer thickness d and the corresponding refractive index n of the layer material determines the optical path length of the radiation through the layer. The condition therefore states that the optical path lengths through the individual layers of the mask structure should behave overall in such a way that they roughly correspond to the optical path length of the same EU radiation through a vacuum. Deviations of ± 10% of the working wavelength X can often be tolerated. So if this upper limit of the deviation is not significantly exceeded, any residual phase delays for the process are usually tolerable. If necessary, the deviation can also be smaller, for example a maximum of ±5% or ±2%. Then the residual phase delays are also smaller.

It can also be seen from the above condition that both a layer material with a refractive index n>1 and a layer material with a refractive index n<1 must be used so that the different phase delays in the individual layers can be at least partially compensated for.

In some embodiments, the mask structure has exactly one first layer made of a first layer material and exactly one second layer made of a second layer material, so that the mask structure comprises exactly two layers. As a result, production can be particularly simple. However, it is also possible for the mask structure to have two or more first layers (ie layers made from a first layer material with n1>1) and/or two or more second layers (ie layers made from a second layer material with n2<1). Such a mask structure has three or more individual layers, for example four, five or six. The layer thicknesses of the individual layers are then to be matched to one another, taking into account the real part of the refractive index of the layer materials, in such a way that the desired phase compensation is effected overall. Layer arrangements with more than two individual layers can, for example, be favorable in order to at least partially compensate for the negative effects of layer stresses.

For more than two layers, the general formula for zero phase delay is:

where the sum runs over the number of layers.

Embodiments are particularly favorable in which the first layer material has a real part of the refractive index, n1, of more than 1.002 at the wavelength of the EUV radiation. In the case of relatively large upward deviations from the value 1, the layer thicknesses required to compensate for the other layer acting in the opposite direction can be kept relatively small.

In order to ensure at the same time that the mask structure absorbs sufficiently strongly, it is provided in preferred embodiments that the first layer material and the second layer material each have an extinction coefficient k of more than 0.02 at the wavelength of the EUV radiation. The required layer thicknesses can thus be kept so small that any shadowing effects that could be caused by excessive layer thicknesses of the mask structure can be limited.

In order to achieve sufficiently strong compensation for the opposite phase delays, it is usually advantageous if the first layer has a first layer thickness and the second layer has a second layer thickness that is smaller than the first layer thickness. Many different second layer materials can thus be used to construct the second layer, the real part of the refractive index (n2) of which is significantly less than 1, for example less than 0.99 or less than 0.98.

In preferred embodiments, a first layer material that essentially consists of aluminum (Al) is used to produce the first layer. At a Layer material, which essentially consists of aluminum, the element aluminum is the element that determines the real part of the refractive index. The first layer material can consist predominantly (ie with a proportion of 90 at % or more) or almost exclusively of aluminum, so that apart from aluminum only residual impurities and/or stabilizing alloy components can possibly be contained. Pure aluminum can be used to form the first layer.

It is possible to combine a first layer consisting essentially of aluminum with a second layer of a second layer material which does not contain aluminum and is selected, for example, with a view to the highest possible extinction coefficient, e.g. tantalum (Ta), nickel (Ni ), tellurium (Te), copper (Cu) or cobalt (Co).

In preferred embodiments, on the other hand, it is provided that both the first layer material and the second layer material contain aluminum. This can result in a particularly good layer adhesion between the first layer and the second layer at the interface between the first layer and the second layer due to similar chemical and/or structural properties.

In some embodiments, the first layer consists essentially of aluminum and the second layer consists essentially of aluminum nitride (AIN) or aluminum oxide (AI2O3).

With regard to the desired compensation of phase delays, the sequence of the first and second layers in the layer arrangement of the mask structure can be selected as desired. For example, it is possible to arrange the second layer between the first layer and the substrate. In contrast, in many embodiments the first layer is arranged between the substrate and the second layer. With this arrangement, the second layer can serve as a protective layer for the first layer.

Binary EUV intensity masks with a multilayer, phase-optimized mask structure of the type described in this application can be advantageous for different applications in different configurations.

According to one development, the binary intensity mask is designed as a reflective binary intensity mask, ie as an intensity mask that is used in reflection (binary reflection mask). A reflective binary intensity mask for use in an EUV system working with EUV radiation comprises a substrate, a multi-layer layer arrangement applied to the substrate and acting reflectively for the EUV radiation, and a layer arrangement on the Multi-layer layer arrangement applied mask structure containing absorber material. In this variant, a reflective multi-layer arrangement is therefore arranged between the substrate and the (phase-optimized) mask structure.

With regard to the long-term stability of the optical properties, it can be advantageous if the multi-layer arrangement that acts reflectively for the EUV radiation has a cover layer made of an oxidation-resistant layer material, with the mask structure being applied to the cover layer. The multi-layer arrangement can therefore be closed at the top by a thin protective layer (capping layer). The cover layer can consist, for example, of ruthenium (Ru) or other layer materials with comparable properties. The cover layer can then serve as a base for the first layer or the second layer of the mask structure.

One area of application for a reflective binary intensity mask is its use as a lithography mask in an EUV projection exposure system. In this case, the mask structure essentially corresponds to an enlarged structure of the structure to be produced in an exposure step of a functional layer of a semiconductor or the like to be structured.

The inventors have recognized that binary EU intensity masks of the type described in this application can also bring considerable advantages in the field of measurement technology.

For measuring operations for measuring the imaging quality of EUV projection systems using EUV radiation, reflective binary intensity masks are often used, which are then usually referred to as measuring masks or measuring reticles. In this case, the mask structure is designed as a measurement structure with which the EUV radiation used for measurement can be spatially structured (cf. e.g. WO 2018/007211 A1). The measurement structure can, for example, form a periodic grating (e.g. line grating or pinhole array). In the case of a functional layer mask (lithography mask), the mask structure can represent the conductor structure of a chip to be produced, and is therefore generally much more complex with regard to the lateral structure.

Measuring masks can be used alternately with lithography masks in an EUV projection exposure system (eg scanner) or in measuring machines set up only for measuring purposes. To measure the imaging quality of EUV projection systems, reflective binary intensity masks (measuring masks) are used in some measuring methods in the object plane of the optical imaging system to be measured, whereas partially transparent binary intensity masks (also referred to as binary transmission masks) are used in the image plane. In many measurement methods, eg in shearing interferometry, such a binary transmission mask is arranged in the beam path in front of a sensor, which is why a binary transmission mask provided for measurement purposes is also referred to as a sensor mask in this application. The mask structure can be embodied, for example, as a diffraction grating that acts to diffract EU radiation. The substrate should be sufficiently transparent for the EUV radiation used, which can be achieved by a suitable choice of material (eg SiN _x ) and/or by a small thickness. The substrate can be a thin membrane, for example, the thickness of which can preferably be less than 1 μm and/or less than 500 nm and/or less than 200 nm in order to allow sufficient permeability for EUV radiation.

In comparison to a reflective binary intensity mask, the multi-layer arrangement that acts reflectively for the EUV radiation is missing. The mask structure can be applied directly to the substrate surface. If necessary, an EUV-transparent intermediate layer can be arranged between the substrate and the mask structure, which can have properties that reduce reflection (anti-reflection property) and/or properties that improve layer adhesion, for example.

The invention also relates to a method for producing a binary mask for use in an EUV system working with EUV radiation. The method comprises the step of providing a substrate and the step of producing a mask structure containing absorber material on the substrate.

When producing the mask structure, a layer arrangement is produced with a first layer made of a first layer material and a second layer made of a second layer material. This layer arrangement is then structured using a suitable structuring method in order to uncover the areas between the structural elements of the desired mask structure that are as non-absorbent as possible. The method is characterized in that a layer material is used as the first layer material which has a real part of the refractive index, n1, greater than 1 at the wavelength of the EUV radiation, while a second layer material is used for the second layer which has a real part of the Index of refraction, n2, less than 1. If a reflective binary intensity mask is to be produced, the substrate is coated with a multi-layer arrangement having a reflective effect for the EUV radiation before the mask structure is produced on the multi-layer arrangement having a reflective effect. The mask structure is then arranged on the reflective multi-layer arrangement.

Numerous conventional coating methods can be used individually or in combination to produce the individual layers and the layer sequences, for example an evaporator method (physical vapor deposition, PVD), a method of chemical vapor deposition (chemical vapor deposition, CVD) or a sputtering method.

A first layer material, which consists essentially of aluminum, is preferably used to produce the first layer.

The second layer can be applied by any suitable coating method, either before the first layer is applied or after the first layer is applied.

In a variant, the first layer is first applied, consisting essentially of aluminum, and the second layer is produced on the first layer by surface reaction of the aluminum of the first layer with oxygen or nitrogen, a second layer adhering to the first layer Layer forms as a reaction layer of aluminum oxide or aluminum nitride. A second layer produced by oxidizing an aluminum layer or by nitriding an aluminum layer with ionic bonding between aluminum ions and oxygen or nitrogen ions creates a particularly good adhesive bond between the first layer and the second layer, with both layers containing aluminum as an important property-determining component.

The invention also relates to the use of a binary intensity mask of the type described in this application in a method for measuring an optical imaging system which is provided for imaging a pattern arranged in an object plane of the imaging system into an image plane of the imaging system. A binary intensity mask can be designed as a reflection mask (reflective measuring mask) which is arranged in the area of the object plane and irradiated with EUV radiation in order to carry out a measuring operation. Alternatively or additionally, a binary intensity mask in the form of a binary transmission mask can be used, which is arranged in the region of the image plane and irradiated with EUV radiation to carry out a measurement operation, which, after interacting with a reflection mask, passes through the imaging system to the transmission mask.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and aspects of the invention result from the claims and from the following description of preferred exemplary embodiments of the invention, which are explained below with reference to the figures.

1 shows a schematic section through a reflective binary intensity mask according to an embodiment;

2 shows a diagram that represents the course of the optical path length difference in comparison to the optical path length difference through a vacuum when EUV radiation is propagated through an AIN/AI mask structure;

3 shows a simulation result for a comparison of a conventional reference mask and a phase-optimized AI/AIN mask of the embodiment in the form of Zernike spectra of the wavefront;

4 shows a schematic representation of a microlithography projection exposure system in which a reflective lithography mask according to an exemplary embodiment is arranged in the object plane;

5 shows a schematic plan view of the mask structure of the reflective lithography mask;

6 schematically shows components of a measuring system equipped with measuring masks, which is used to measure the imaging quality of an EUV projection objective;

7 shows a plan view of the mask structure of a reflective measurement mask to be arranged on the object side;

FIG. 8 shows, in plan view, the mask structure of a binary transmission mask to be arranged on the image side; FIG. FIG. 9 shows a schematic section through part of the image-side measurement mask of FIG.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Various aspects of binary EUV intensity masks that can be used in EUV systems, e.g. as a lithography mask or as a measurement mask, are described below. An EUV system is a system that works with EUV radiation or with a working wavelength from the EUV range. The design and structure of the masks, as well as their production and possible uses, are explained using exemplary embodiments. The exemplary embodiments are designed for a wavelength of λ<<13.5 nm.

To describe the optical properties of a material when interacting with light, the complex index of refraction n is generally used, which can be described according to n = n + ik as the sum of the real part of the index of refraction, n, and the extinction coefficient k multiplied by i, where the product ik forms the imaginary part of the refractive index. The real part of the index of refraction describes the phase velocity v of light as it travels through a material compared to the speed of light c in vacuum, according to v = c/n. Light slows down when entering a material with higher real part of the index of refraction. Since the frequency of the light waves remains constant, the wavelength X is shortened. The extinction coefficient k describes the loss of wave energy to the material, i.e. the attenuation. The relationship with the absorption coefficient a is given at wavelength λ by the relationship a = 47tk/λ. Light loses intensity in an absorbing material according to the Beer-Lambert law l(x) = Io e ^-i “ ^x , where x is the path length in the material and Io is the original intensity. Thus, the extinction coefficient k refers to how quickly light disappears in a material or how strongly it is absorbed.

In the literature in the visible light range, the real and imaginary parts of the complex refractive index of a material are mainly denoted in the n-k notation. In the EUV and X-ray range, the δ-β notation is preferred due to the small deviation of the real parts of the refractive indices from the value 1. The relations n = 1 - ö and k = ß apply.

First, an example of a binary EUV intensity mask that acts reflectively for EUV radiation is described, that is to say a binary reflection mask for EUV. For the sake of simplicity, the binary EUV intensity mask is occasionally referred to simply as a “mask” below. 1 shows a schematic section through a reflective binary intensity mask 100 according to an embodiment. The intensity mask 100 includes a rigid, warp-resistant substrate 110 that serves as the supporting component of the mask. The substrate consists of a material with a very low coefficient of thermal expansion. For example, glasses that are commercially available under the names ULE® or Zerodur® can be used.

The substrate 110 has a planar substrate surface 112 that has been machined smooth with optical quality. An optically functional layer system with many layers of different layer materials is applied to this.

The layer system comprises a multi-layer arrangement 120 that acts reflectively for the EUV radiation and is applied to the substrate 110 directly or with one or more further layers interposed (e.g. to promote adhesion). The multilayer arrangement (multilayer) 120 has many pairs of layers with alternating low-index and high-index layer material. The pairs of layers can be constructed, for example, with the layer material combinations molybdenum/silicon (Mo/Si) or ruthenium/silicon (Ru/Si). A pair of layers in each case comprises a layer made of a layer material with a relatively high refractive index and a layer made of a layer material with a relatively low refractive index. Such pairs of layers are also referred to as "double layer" or "bilayer". In addition to the two layers of relatively high refractive index or relatively low refractive index layer material, a layer pair can also have one or more further layers, for example an interposed barrier layer to reduce interdiffusion between adjacent layers. A multi-layer arrangement with many pairs of layers acts in the manner of a "Distributed Bragg Reflector". The layer arrangement simulates a crystal whose lattice planes leading to Bragg reflection are formed by the layers of the material with the lower real part of the refractive index. The optimal period thickness of the layer pairs is determined for a given wavelength and for a given angle of incidence or angle of incidence range by the Bragg equation and is between 1 nm and 10 nm in the example.

On the radiation entry side facing away from the substrate 110, the multi-layer arrangement 120 has a cover layer 125 (capping layer) made of an oxidation-resistant layer material, in the example ruthenium (Ru) is used. The cover layer 125 can fulfill many functions, for example as protection against oxidation, as protection against degeneration and/or simply because particles adhere less and the surface can be cleaned more easily as a result. A laterally structured mask structure 140 containing absorber material is applied to the multi-layer arrangement 120, more precisely to the cover layer 125. The term "absorber material" refers to material whose extinction coefficient k for the EUV wavelength is sufficiently high to absorb a substantial part of the incident EUV radiation with a layer that is not so thick. As a result, the mask acts as a binary intensity mask in which those parts of the EUV radiation that fall on the mask structure 140 are absorbed to a significant extent, while those portions that fall apart from structural elements of the mask structure on the uncovered areas of the reflective multilayer layer arrangement are absorbed as little as possible and are predominantly reflected.

1 shows a structure element 145 of the mask structure 140 in section. The structural element can be, for example, a straight line of defined width that runs on the otherwise reflective multilayer (multilayer layer arrangement 120). The mask structure 140 consists of a plurality of layers, ie it is constructed as a structured layer arrangement. In the example, the mask structure has exactly two superimposed layers, namely a first layer 151, which can be applied directly to the free surface of the cover layer 125, and a second layer 152, which is applied directly to the first layer 151 on the radiation entry side of the mask .

In the exemplary embodiment, the first layer 151 essentially consists of aluminum (Al) and has a first layer thickness d1 of approximately 66.1 nm (nanometers). The second layer 152 applied immediately thereon essentially consists of aluminum nitrite (AlN) and has a second layer thickness d2 of approximately 10 nm.

In the manufacture of the intensity mask, the substrate 110 is first coated with the reflective multi-layer arrangement 120 including the cover layer 125 . After that, the mask structure containing the absorber material is produced on the multi-layer arrangement. For this purpose, an extensive first layer (made of aluminum) is first applied, for example by a PVD process or by sputtering. Thereafter, the second layer lying thereon is produced, for example, by a PVD process or by sputtering.

The layer arrangement is then structured by removing those areas that are not intended to belong to the mask structure 140 using a suitable material removal technique (e.g. by means of electron beam lithography), so that the surface of the reflective multilayer layer arrangement 120 is exposed between the structural elements of the mask structure and the structural elements with flanks that are as sharply defined as possible. The first layer material (in this case aluminum) and the second layer material (in this case aluminum nitride) are selected and their layer thicknesses are designed in such a way that the effects of the two layers when EUV radiation passes through are at least partially compensated in terms of the phase delay caused thereby, so that the mask structure as a whole has a relatively small phase-shifting influence on the EUV radiation passing through.

The real part of the refractive index of the first layer material (parameter n1) is greater than 1 at the design wavelength λ and, according to literature, is in the order of approximately n1=1.003. In contrast, the real part of the refractive index of the second layer material applied thereon (parameter n2) is less than 1 and, according to literature data, is approximately n2=0.981.

In the example, the layer thicknesses d1 and d2 of the first layer 151 and the second layer 152 and the real parts of the refractive indices of the two layer materials are matched to one another such that the condition d1*n1+d2*n2=(d1+d2)±0.1X is met is, which means that the optical path length of the EUV radiation through the two-layer mask structure essentially corresponds to the optical path length of the same EUV radiation through a vacuum and any path length differences should not deviate from the ideal value of zero by more than 10% of the working wavelength. Smaller deviations are cheaper, e.g. a maximum of 0.5 nm at a working wavelength of 13.5 nm.

2 shows a diagram in which the optical path length difference OPD (optical path difference) in nanometers (on the y-axis) is shown as a function of the depth position POS of the mask structure (in nanometers) on the x-axis. The curve shown is calculated according to the following relationship, where parameter z represents the depth position:

The position 0 corresponds to the radiation entry side, ie the free surface of the second layer 152. The two-layer mask structure ends at the position at approximately 76 nanometers where the cover layer begins. The diagram shows the course of the optical path length difference compared to the optical path length difference through a vacuum during propagation through the AlN/Al absorber from top to bottom. It can be seen that the EUV radiation penetrating the second layer builds up an increasingly negative optical path length difference OPD, i.e. a phase delay, compared to the reference wave passing through the vacuum, which reaches its extreme value at the transition to the underlying aluminum layer (first layer). (approx. - 0.19 nanometers). This is because the real part of the refractive index of aluminum nitride (AIN) is slightly less than 1. During the subsequent penetration of the aluminum layer (first layer), this optical path length difference is then reduced further and further until it is compensated for again on the underside of the first layer 151 . If the EUV radiation that has penetrated is then reflected on the underlying multi-layer structure 120, the same thing happens on the way back from bottom to top through the absorber, with a positive optical path length difference initially building up, which then decreases when passing through the thinner AIN layer is balanced or compensated again.

Some advantages of such reflective masks are explained below. The inventors carried out rigorous simulation calculations in which the physical processes in the interaction of EUV radiation with an EUV mask and in particular with mask structures were examined. The simulation calculations showed that a significant contribution of the reflective mask to the wavefront deformation arises from the fact that the proportion W1 of the incoming EUV wave that runs through the absorbing mask structure (also known as absorber structure) is not completely absorbed and after back reflection and through it again the absorber material has a phase offset (phase shift, phase shift) when exiting again compared to that part W2 of the EUV wave that ran past the absorber structure through the vacuum to the reflective multilayer coating and was reflected by it. This is illustrated in more detail using the diagram below.

3 shows a simulation result for a comparison of a conventional reference mask REF and the phase-optimized AI/AIN mask of the exemplary embodiment. The dark bars stand for the conventional mask, the light bars for the exemplary embodiment according to the invention. A so-called EUV shearing interferometer was simulated, in which diffraction on a mask with a line grating produces copies of the wavefront that has passed through an imaging system and these copies are then superimposed on themselves. The wavefront can thus be reconstructed with the aid of a phase shift method (cf. eg DE 10 2016 212 477 A1 or the corresponding WO 2018/007211 A1). In the diagram of FIG. 3, some Zernike coefficients ZK are shown on the x-axis and the amplitudes AMP of the wavefront deviations (in picometers, pm) are shown on the y-axis. The diagram shows so-called Zernike spectra of the wavefront. The wavefront is broken down into Zernike polynomials and the amplitudes of the respective polynomials are plotted as a spectrum. For reasons of clarity, only Zernike coefficients with amplitudes greater than 0.5 pm are shown in the diagram below, and only Zernike coefficients greater than Z5.

In the simulation, it was assumed that the measured optical imaging system (projection lens for EUV lithography) is free of aberrations, so that even if the measurement technology were perfect, all amplitudes in the diagram would have to be 0. The upward and downward deflections represented by the bars thus directly show the measurement errors, which are mainly caused by the 3D mask effects mentioned at the beginning. It is easy to see that significant improvements result from the use of a phase-optimized reflected mask. This is especially true in the Zernikes with the largest amplitudes, for example Z9, Z16 and Z17.

An application example of a reflective intensity mask 500 designed as a lithography mask according to an exemplary embodiment is described with reference to FIGS. 4 and 5. FIG. FIG. 4 shows a schematic representation of a microlithography projection exposure system 400 for the production of finely structured semiconductor components by means of EUV radiation. The system has a radiation source 410, an illumination system 420 and a projection lens 430. The radiation source 410 generates primary radiation in an EUV wavelength range around a main wavelength, this radiation being guided into the illumination system 420 as a beam bundle 411 . The illumination system 420 changes the primary radiation by expansion, homogenization, changing the beam angle distribution, etc. and thereby generates an illumination beam 412 at its output, which hits the reflective intensity mask 500 at an angle, which carries a pattern to be imaged (see FIG. 5).

The projection lens 430 is an optical imaging system that is designed to image the pattern arranged in its object plane 431 into an image plane 432 that is optically conjugate to the object plane. After passing through the projection objective in the region of the image plane 432, the radiation impinges on the surface of a substrate 450 in the form of a semiconductor wafer, which is carried by a substrate holder 460.

The projection lens 130 defines a reference axis 433. The object field 435 is centered in the Y direction on this reference axis. The optical elements of the imaging system can be decentered to this reference axis. In the example, the radiation source 410 is an EUV radiation source that generates radiation in a wavelength range between approximately 5 nm and approximately 30 nm, in particular between approximately 10 nm and approximately 20 nm. In particular, the radiation source can be designed in such a way that the main wavelength is in the range of approx. 13.5 nm. Other wavelengths from the EUV range (for example in the range of approx. 6.9 nm) are also possible.

The illumination system 420 includes optical components that are designed and arranged in such a way that illumination radiation is generated with an intensity profile that is as homogeneous as possible and a defined beam angle distribution. In the example, all optical components of the illumination system provided for beam guidance and/or beam shaping are purely reflective components (mirror components).

The illumination radiation is reflected by the reflective mask 500 in the direction of the projection lens 430 and is modified with regard to angular distribution and/or intensity distribution. The radiation that reaches the substrate through the projection lens forms the imaging beam path, from which two beams 441 are shown schematically on the object side of the projection lens 430 (between the mask and the projection lens) and two beams converging onto a pixel on the image side (between the projection lens and the substrate). 442 are shown. The angle formed by the rays 442 running towards one another on the image side of the projection objective is related to the image-side numerical aperture NA of the projection objective. This can be, for example, 0.1 or more, or 0.2 or more, or 0.3 or more, or 0.4 or more.

The projection objective is designed to transfer the pattern from the region of the object field 435 of the projection objective to the image field 438 of the projection objective on a reduced scale. The projection lens 430 reduces by a factor of 4, also other reduction scales, for example 5-fold reduction, 6-fold reduction or 8-fold reduction or also less strong reductions, for example. 2-fold reduction are possible.

Embodiments of projection lenses for EUV microlithography typically have at least three or at least four mirrors. Exactly six mirrors are often advantageous (cf. FIG. 6). With an even number of mirrors, all of the mirrors can be arranged between the object plane and the image plane, and these planes can be aligned parallel to one another, which simplifies the integration of the projection objective in a projection exposure system. A Cartesian x, y, z coordinate system is shown in FIG. 1 to simplify the description of the projection exposure system. The z-direction is parallel to the reference axis 133, the xy-plane perpendicular to it, ie parallel to the object plane and to the image plane, with the y-direction lying in the plane of the drawing in the illustration.

The projection exposure apparatus 400 is of the scanner type. During operation of the projection exposure system, the mask 500 and the substrate 450 are moved parallel to the y-direction in opposite directions, so that different areas of the binary reflective mask 500 are transferred to the moving wafer one after the other. Stepper-type embodiments are also possible.

5 shows a schematic plan view of the mask structure of the reflective lithography mask 500. The mask structure corresponds to a functional layer of a semiconductor to be structured and includes an arrangement of differently shaped, straight, angled, U-shaped, T-shaped and differently designed structural elements (light). This absorbing mask structure has a multilayer and phase-optimized structure, e.g. according to Fig. 1, and is supported by a reflective multilayer arrangement.

Due to the fact that the reflective mask 500 is constructed according to an exemplary embodiment of the invention, the undesired wavefront deformations mentioned at the outset can be kept at a low level.

Possibilities are described with reference to FIGS. 6, 7 and 8 of using binary intensity masks according to exemplary embodiments of the invention in a measuring system or in a measuring method for interferometric measurement of the imaging quality of an optical imaging system. For this purpose, FIG. 6 schematically shows components of a measuring system 600 which is used to measure the imaging quality of an optical imaging system in the form of an EUV projection objective 630 . In the example, this has a total of six mirrors M1 to M6, which are arranged and designed to image a pattern arranged in the object field in the object plane 631 of the projection lens into the image field arranged in the image plane 632 of the projection lens on a reduced scale. A reflective measurement mask 700, which is a binary intensity mask according to an exemplary embodiment, is arranged in the object plane. FIG. 7 shows the mask structure in plan view, which is arranged on a reflective multi-layer arrangement (cf. FIG. 1). A binary transmission mask 800 according to an exemplary embodiment is arranged in the image plane 632 . 8 shows the mask structure of the binary transmission mask in plan view.

The mask structure of the reflective measurement mask 700 to be arranged on the object side and the mask structure of the transmission mask 800 to be arranged on the image side are adapted to one another in such a way that an interference pattern is created when the mask structure of the object-side measurement mask 700 is imaged onto the image-side measurement mask 800 using the projection lens 630. This can be detected by a detector 650 for location-resolved detection of the interference pattern. In the example, the detector is arranged below the transmission mask 800, so that only measurement radiation which is transmitted through the transmission mask 800 and is influenced with the aid of its mask structure reaches the detector.

In the example, the mask structure of the reflective measurement mask 700 to be arranged on the object side is a simple line grating with a periodicity length P1 that corresponds to a multiple of the measurement wavelength of the EUV radiation. The periodicity length can be, for example, of the order of 1 pm or more, for example 2 pm or more. The absorbing, rectilinear structure elements of the mask structure each have a two-layer structure, the layer material of one layer having a real part of the refractive index less than 1 and the other layer having a real part of the refractive index greater than 1. The layer structure of the reflective measurement mask 700 is attached to an EUV-reflecting multilayer arrangement on the side of a relatively thick, torsion-resistant substrate that faces the projection objective.

The measurement mask 800 to be arranged on the image side is designed as a binary transmission mask. 9 shows a schematic section through part of the image-side measurement mask 800. The measurement mask has a stable base support or frame 805, which has at least one continuous cutout 806. The carrier can be made of silicon, for example, and have a thickness of several hundred micrometers in order to ensure sufficient stability. The substrate 810 of the binary transmission mask is attached to an upper side of the carrier. The substrate is in the form of a thin membrane that spans the recess 806 like a thin plane-parallel plate. The membrane or the substrate 810 should have the highest possible transmission for EUV radiation and is correspondingly thin. The thickness can be, for example, in the range from 50 nm to 200 nm, preferably in the order of approx. 80 nm to 120 nm, eg 100 nm. Silicon nitride (Sis1^U) or another silicon ceramic can be used as the substrate material, for example be used. The mask structure 840 is applied to the substrate surface opposite the carrier 805, which has exactly two superimposed layers, namely a first layer 851, which is applied directly to the free surface of the substrate 810, and a second layer 852, which is on the radiation entry side of the Measurement mask is applied directly to the first layer 851. In the exemplary embodiment, the first layer 851 consists essentially of aluminum (Al), while the second layer 852 consists essentially of aluminum nitride. This layer arrangement is structured laterally in such a way that a periodic pattern of circular holes 855 is formed, in the area of which the layer structure was removed, so that the substrate 810 underneath is exposed. The two-layer arrangement of the mask structure is retained between the holes. The hole pattern has a periodicity length P2 which is less than the periodicity length P2 and which can be less than 1 micron, for example between 300 nm and 700 nm.

In the measurement mode, after passing through the projection objective, EUV radiation will impinge on the binary transmission mask 800 from the radiation entry side. Those portions W2 that strike the membrane through the holes in the mask structure pass through it with low absorption in the direction of the detector. In the ideal case, those portions W1 which fall on the structural elements of the mask structure which have an absorbing action would be completely absorbed. According to the observations of the inventors, however, a certain proportion of the radiation intensity will generally pass through the absorbing two-layer structure and the substrate 810 in the direction of the detector through the transmission mask. Due to the multilayer structure of the structural elements, however, the phase compensation already described above occurs, which has the effect that the phase offset that is generated when passing through one layer is compensated again when passing through the other layer, so that those parts W1 of the EUV Radiation that passes through the transmission mask after absorption have essentially the same phase as those portions W2 that were absorbed only when passing through the substrate 810 without interacting with the mask structure. As a result, impairments in measurement accuracy that could otherwise be caused by possible phase differences can be avoided or kept at an acceptably low level.

Claims

- 22 - Claims

Claims 1. Binary intensity mask (100, 700, 800) for use in an EUV system operating with EUV radiation, comprising: a substrate (110, 810); and a mask structure (140, 840) applied to the substrate and containing absorber material, characterized in that the mask structure (140, 840) has a structured layer arrangement having a first layer (151, 851) made of a first layer material and a second layer (152, 852) of a second layer material, wherein the first layer material has a real part of the refractive index, n1, greater than 1 and the second layer material has a real part of the refractive index, n2, less than 1 at the wavelength X of the EUV radiation, wherein an optical path length of the EUV radiation through the mask structure essentially corresponds to the optical path length of the EUV radiation through a vacuum such that path length differences between the optical path lengths do not deviate from zero by more than 10% of the wavelength λ.

2. Intensity mask according to claim 1, wherein the first layer (151, 851) has a first layer thickness d1 and the second layer (152, 852) has a second layer thickness d2, wherein at least one of the following conditions applies:

A: (d1 + d2) - 0.1 X < (d1 * n1 + d2 * n2) < (d1 + d2) + 0.1 X

B: d1 * n1 + d2 * n2 = (d1 + d2) ± 0.1 X

C: the wavelength λ is 13.5 nm and the optical path length of the EUV radiation through the mask structure deviates by less than 0.5 nm from the optical path length of the EUV radiation through vacuum.

3. Intensity mask according to claim 1 or 2, wherein the mask structure (140, 840) has exactly one first layer (151, 851) and exactly one second layer (152, 852), so that the mask structure comprises exactly two layers.

4. An intensity mask according to any one of the preceding claims, wherein the first layer material has a real part of the refractive index greater than 1.002 at the wavelength of the EUV radiation.

5. Intensity mask according to one of the preceding claims, wherein the first layer material and the second layer material each have an extinction coefficient k of more than 0.02 at the wavelength of the EUV radiation.

6. Intensity mask according to one of the preceding claims, wherein the first layer (151, 851) has a first layer thickness and the second layer (152, 852) has a second layer thickness which is smaller than the first layer thickness.

7. The intensity mask as claimed in one of the preceding claims, in which the first layer material essentially consists of aluminium, in particular in a proportion of more than 90 at %.

8. An intensity mask according to any one of the preceding claims, wherein both the first layer material and the second layer material contain aluminium.

9. Intensity mask according to one of the preceding claims, wherein the first layer (151, 851) consists essentially of aluminum and the second layer (152, 852) consists essentially of aluminum nitride (AlN) or aluminum oxide (Al2O3).

10. Intensity mask according to any of the preceding claims, wherein the first layer (151, 851) is arranged between the substrate (110, 810) and the second layer (151, 851).

11. Intensity mask according to one of the preceding claims, wherein the binary intensity mask is designed as a reflective binary intensity mask (100, 700), wherein between the substrate (110) and the mask structure (140) there is a multi-layer arrangement ( 120) is arranged.

12. Intensity mask according to claim 11, wherein the multi-layer arrangement (120) that acts reflectively for the EUV radiation has a cover layer (125) made of an oxidation-resistant layer material, in particular ruthenium, the mask structure (140) being applied to the cover layer (125). is.

13. Intensity mask according to one of claims 11 or 12, wherein the reflective binary intensity mask is designed as a lithography mask (100, 500), the mask structure (140) essentially corresponding to an enlarged structure of the structure of a desired functional layer to be produced in an exposure step.

14. Intensity mask according to one of claims 1 to 12, wherein the binary intensity mask is designed as a measurement mask (700, 800), the mask structure (840) having a measurement structure, in particular in the form of a periodic grating, which is preferably a line grating or a pinhole array forms.

15. Intensity mask according to one of claims 1 to 10 or 14, wherein the binary intensity mask is designed as a binary transmission mask (800).

16. Intensity mask according to claim 15, wherein the binary transmission mask (800) has a substrate in the form of a membrane (810) permeable to EUV radiation, the thickness of the membrane preferably being less than 1 μm and/or less than 500 nm and/or is less than 200 nm.

17. Method for producing a binary intensity mask for use in an EUV system working with EUV radiation, comprising:

providing a substrate;

Creating a mask structure containing absorber material on the substrate, wherein to create the mask structure a layer arrangement with a first layer (151, 851) made of a first layer material and a second layer (152, 852) made of a second layer material is created and the layer arrangement is then structured, wherein the first layer material has a real part of the refractive index, n1, greater than 1 at the wavelength X of the EUV radiation and the second layer material has a real part of the refractive index, n2, less than 1, with an optical path length of the EUV radiation through the mask structure such essentially corresponds to the optical path length of the EUV radiation through vacuum, that path length differences between the optical path lengths do not deviate from the value zero by more than 10% of the wavelength λ.

18. The method as claimed in claim 17, wherein a first layer material which consists essentially of aluminum (Al) is used to produce the first layer (151, 851). - 25 -

The method of claim 18 wherein the second layer (152, 852) is formed on the first layer (151, 851) by surface reaction of the aluminum of the first layer with oxygen or nitrogen to form a second layer of alumina adhered to the first layer (AL2O3) or aluminum nitride (AIN) is produced.

20. The method as claimed in claim 17, 18 or 19, characterized by coating the substrate with a multi-layer arrangement which acts reflectively for the EUV radiation and producing the mask structure containing absorber material on the multi-layer arrangement.

21. Use of a binary intensity mask according to any one of claims 1 to 12 or 14 to 16 as a measurement mask (700, 800) in a method for measuring an optical imaging system, which is provided for imaging a pattern arranged in an object plane of the imaging system in an image plane of the imaging system is, the measurement mask being arranged in the area of the object plane or the image plane and irradiated with EUV radiation in order to carry out a measurement operation.

22. Measuring system (600) for interferometric measurement of an optical imaging system (630), which is provided for imaging a pattern arranged in an object plane (631) of the imaging system in an image plane (632) of the imaging system, with: a first structural support to be arranged on the object side of the imaging system (700) with a first measurement structure, a second structure carrier (800) to be arranged on the image side of the imaging system and having a second measurement structure, wherein the first measurement structure and the second measurement structure are adapted to one another in such a way that when the first measurement structure is imaged onto the second measurement structure using the imaging system creates an interference pattern; and a detector (650) for spatially resolving detection of the interference pattern; characterized in that a reflective binary intensity mask according to any one of claims 1 to 12 or 14 is used as the first structural support and/or that a binary transmission mask (800) according to any one of claims 1 to 12 or 14 to 16 is used as the second structural support.

23. Measuring method for interferometric measurement of the imaging quality of an optical imaging system (630) for imaging in an object plane of the - 26 -

Imaging system arranged pattern in an image plane of the imaging system with the following steps:

Arranging a first structure carrier (700) with a first measurement structure in the area of the object plane (631) of the imaging system,

Arranging a second structure support 800) with a second measurement structure in the area

image plane (632) of the imaging system,

illuminating the first measurement structure with EUV radiation;

mapping the first measurement structure onto the second measurement structure to generate an interference pattern;

Spatially resolving detection of the interference pattern;

Determination of at least one imaging parameter describing the imaging quality of the imaging system from the interference pattern, characterized in that a reflective binary intensity mask according to one of Claims 1 to 12 or 14 is used as the first structure carrier and/or that a binary transmission mask (800) according to one of Claims 1 to 12 or 14 to 16 is used as the second structural support.