DE102015201581A1 - Method for producing a reflective optical element for EUV lithography - Google Patents

Abstract

For the gentle production of a reflective optical element for EUV lithography, a method is proposed comprising the steps of: providing (101) a substrate; - depositing (103) an extremely ultraviolet wavelength reflective coating; - applying (105) a protective layer to the surface of the reflective coating; - Further processing (107) of the reflective optical element; Removal of the protective layer, wherein non-selectively material is removed in a first removal step (109) and then, in a second removal step (111), protective layer material is selectively removed such that the original surface of the reflective coating is exposed.

Description

The present invention relates to a method for producing a reflective optical element for EUV lithography.

In the manufacture of a reflective optical element, a substrate is coated with a reflective coating. The surface of this reflective coating is very sensitive to chemical contaminants, e.g. As with hydrocarbons, metals or by oxidation, as well as mechanical damage or particles. These impurities can cause the properties of the reflective optical element to be adversely affected. However, it may be necessary to further process reflective optical elements after coating the substrate, for example in the fabrication of micromirror arrays, patterned EUV mirrors, pupil or field facet mirrors, and EUV masks. Subsequent processing may also be performed immediately after the reflective coating to correct for any aberrations, such as local partial removal of the reflective coating. The various further processing steps can lead to mechanical impairments of the reflective coating, such as scratches or particles, as well as to contamination by chemical contamination.

From the WO 2009/152885 A1 For example, a method for cleaning an optical element is known in which optical surfaces are cleaned in-situ before an operating vacuum is applied. For this purpose, own cleaning units can be provided. In particular, carbon layers that have been applied before a system has been opened for repair or maintenance purposes can subsequently be removed by means of atomic hydrogen.

It is an object of the present invention to specify a method for producing an optical element in which negative influences on the optical properties of the finished optical element caused by manufacturing steps are prevented or at least minimized.

• – Bereitstellen eines Substrats;
• – Aufbringen einer im extrem ultravioletten Wellenlängenbereich reflektierenden Beschichtung;
• – Aufbringen einer Schutzschicht auf die Oberfläche der reflektierenden Beschichtung;
• – Weiterbearbeiten des reflektiven optischen Elements;
• – Entfernen der Schutzschicht, wobei in einem ersten Entfernungsschritt nichtselektiv Material entfernt wird und anschließend in einem zweiten Entfernungsschritt Schutzschichtmaterial selektiv derart entfernt wird, dass die ursprüngliche Oberfläche der reflektierenden Beschichtung freigelegt wird.

This object is achieved by a method for producing a reflective optical element for EUV lithography with the following steps:

• - Providing a substrate;
• Applying a coating which reflects in the extreme ultraviolet wavelength range;
• Applying a protective layer to the surface of the reflective coating;
• - Further processing of the reflective optical element;
• Removal of the protective layer, wherein non-selectively material is removed in a first removal step and then in a second removal step protective layer material is selectively removed so that the original surface of the reflective coating is exposed.

An advantage of the present invention is that mechanical or chemical changes in the further processing of the reflective optical element affect only the protective layer on the surface of the reflective coating of the reflective optical element, which is removed before the start of the reflective optical element. By applying a selective removal in a second removal step, that is, selecting a procedure in which the removal process measurably acts more on the material of the protective layer than on the material of the surface of the underlying reflective coating, the removal can be controlled such that the protective layer is removed as thoroughly as possible without the reflective coating would be affected. In the nonselective removal step, protective layer material is first removed by a procedure that is not specifically optimized for the present materials, but is i.a. at high removal rates with little effort to perform the removal of the protective layer economically, yet efficiently.

In a preferred embodiment of the method, the protective layer is applied in the form of at least two partial layers of different material. This has the advantage that for each of the two partial layers, a material can be selected which is most suitable for the respective protective function performed. By way of example, a material can be selected for the sublayer closing off from the environment, which in particular offers protection of the reflective optical element during its further processing. For the other, directly adjacent to the reflective coating sub-layer, a material can be selected, which protects in particular the reflective coating in the removal of the outer sub-layer.

Advantageously, a material is selected for the sub-layer immediately adjacent to the surface of the reflective coating, to which the second removal step acts as differently as possible on the surface of the reflective coating. This allows the most complete possible removal of the directly adjacent to the surface of the reflective coating sub-layer without the reflective coating attacked and thus negative in their properties is affected, and simplifies the execution of the second, selective removal step.

For the final sub-layer to the environment may preferably be selected a material that is particularly resistant to influences during further processing. This reduces the probability that the reflective optical element and in particular the reflective coating will be adversely affected during further processing.

In a preferred embodiment, carbon, a carbide, a nitride or a fluoride is applied as the sub-layer adjoining the surface of the reflective coating. On the one hand, these materials are comparatively inert to many chemical and physical cleaning processes, but can be removed very selectively by means of adapted purification processes. The surface layer adjacent to the sub-layer can advantageously be applied with a thickness between 2 nm and 100 nm. These materials are also preferred when a one-piece protective layer of only one material is applied to the reflective coating of the reflective optical element.

As the sub-layer terminating in the environment, it is preferable to apply a lacquer, in particular a photoresist, an oxide or a metal. A partial protective layer of lacquer offers, in particular, protection against further processing steps which serve for structuring by means of wet chemical or dry chemical etching. Photoresist offers the particular advantage of protecting specific areas of the surface of the reflective coating by exposure and development. The use of oxides is advantageous in further processing steps that could cause oxidation of the surface of the reflective coating. A final sub-layer of metal to the environment is particularly inert against cleaning of the reflective optical element by means of plasma, UV light irradiation or wet-chemical cleaning. The surrounding sub-layer may be, for example, to a thickness of 500 nm to 20 μm in a paint or photoresist or a thickness of e.g. be applied in the range of about 50 nm for the oxide or the metal.

The first removal step is preferably carried out as sputtering, polishing, wet-chemical removal or dry-chemical removal. Such removal methods enable the thorough removal of the sub-layer or the contaminants thereon at the same time at high removal rates.

The second removal step is preferably carried out as cleaning with hydrogen radicals or as cleaning with oxygen radicals. Such ablation processes are highly dependent on the material to be removed removal rates, which are rather low in conventional surface materials of reflective coatings, so that you can remove with their help the surface adjacent to the sub-layer or the remaining part of the protective layer, without the reflective coating attack. They therefore allow a very selective removal of protective layer material.

In a preferred embodiment, the protective optical element provided with protective layer is further processed by irradiation with electrons, cleaning, annealing or mechanical processing. The cleaning can i.a. Plasma cleaning, wet chemical cleaning or UV light cleaning may include and may be, for example, removal of debris, adhesive residue or other debris on a fixture or substrate side of the reflective optical element. For mechanical processing, for example, separating, cutting, such as laser cutting or water jet cutting, milling, drilling or turning may include, for example, to bring the reflective optical element in its final form. Annealing can also influence the optical properties of the reflective optical element as well as its radiation and heat resistance.

As the reflective coating, it is preferable to apply a multilayer system of alternately arranged layers of a higher real part refractive index material at a wavelength in the extreme ultraviolet wavelength range and a material of lower refractive index material at the wavelength in the extreme ultraviolet wavelength range. Such a reflective coating is particularly suitable for almost normal incidence or angles of incidence, which do not differ too much, and can be well aligned with respect to its subsequent use, in particular on wavelength and angle of incidence range. Alternatively, it is also possible to provide a reflective coating based on a metallic main layer, which is more suitable for use in grazing applications.

Advantageously, a metal or an oxide is applied as cover layer of the multi-layer system or surface layer of the reflective coating. Metals and oxides have the advantage of being inert to a large number of removal processes, in particular by means of radicals, and also of being able to protect the multilayer system from external influences during operation without having to do so high reflectivity losses would have to be accepted.

The present invention will be explained in more detail with reference to a preferred embodiment. Show this

1 a flowchart of an embodiment of the method presented here for producing a reflective optical element;

2 a schematic diagram of a first reflective optical element with protective layer;

3 a schematic diagram of a second reflective optical element with protective layer; and

4 a schematic diagram of a third reflective optical element with protective layer.

1 schematically shows a flowchart as a method for producing a reflective optical element for EUV lithography. In a first step 101 a substrate is provided on which first a reflective coating (step 103 ) and then a protective layer is applied (step 105 ).

In the 2 to 4 the structure of various, provided with protective layer optical reflective elements is shown schematically.

2 schematically shows a first reflective optical element 10 in which a substrate 14 with a reflective coating 16 was provided and then with a protective layer 12 on the surface 18 the reflective coating. This in 3 schematically illustrated second reflective optical element 20 with protective layer 26 differs from the first reflective optical element 10 out 2 in that the protective layer 26 two partial layers 22 and 24 includes.

Advantageously, for the sub-layer 24 immediately adjacent to the surface 18 the reflective coating 16 a material to which the second removal step to be explained later for removing the protective layer is selected 26 acts as different as possible on the surface of the reflective coating. This property is also beneficial if only a one-piece protective coating 12 is applied. For example, carbon, carbides, nitrides and fluorides have proven to be useful.

For the sub-layer that completes the environment 22 is advantageously chosen a material that is particularly resistant to influences in the further processing, which will be discussed in more detail below. This property is also advantageous if only a one-piece protective layer 12 is applied. Coatings, in particular photoresists, oxides or metals, have proven to be suitable as coating material.

It should be noted that the protective layer 12 respectively. 26 in further modifications may also have three, four, five or more sub-layers.

The reflective coating 16 may be designed for more grazing or more normal incidence of the radiation to be reflected. For more normal incidence in the EUV wavelength range, as a reflective coating, multilayer systems of alternating layers of a higher refractive index real material at a wavelength in the EUV wavelength range and a lower refractive index real material at that wavelength are particularly suitable.

In 4 is schematically the structure of a reflective optical element 30 with two-part protective layer 26 on a as a multi-day system 32 trained reflective coating on a substrate 14 shown. In the multi-day system 32 These are essentially alternating layers of a material with a higher real part of the refractive index at the operating wavelength at which, for example, the lithographic exposure is carried out (also spacer 36 called) and a material with a lower real part of the refractive index at the operating wavelength (also absorber 35 called), wherein in the example shown here, an absorber-spacer pair a stack 34 forms, which corresponds in periodic multilayer systems of a period. This somehow simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection occurs. The thicknesses of the individual layers 35 . 36 as well as the repeating stack 34 can over the entire multi-day system 32 be constant or vary, depending on which spectral or angle-dependent reflection profile is to be achieved. The reflection profile can also be selectively influenced by the basic structure of absorber 35 and spacers 36 is supplemented by more more and less absorbing materials to increase the maximum possible reflectivity at the respective operating wavelength. For this purpose, in some stacks absorber and / or spacer materials can be exchanged for each other or additional layers of other materials can be provided. The absorber and spacer materials can have constant or varying thicknesses over all stacks in order to optimize the reflectivity. Furthermore, in individual or all stacks, additional layers can also be used, for example, as diffusion barriers between spacer and absorber layers 36 . 35 be provided. At the first to the substrate 14 adjacent location may be an absorber, a spacer or an additional layer. For the EUV wavelength range, for example, silicon as spacer material and molybdenum as absorber material have proven successful. But other suitable materials are well known to those skilled in the art.

In the in 4 represented reflective optical element is as the uppermost layer 38 of the multi-day system 32 a metal or an oxide applied. Coatings of metals or oxides protect the underlying absorber and Spacerlagen before various influences during use of the finished reflective optical element in lithographic processes. They can protect against oxidic and carbon contamination during irradiation with EUV radiation at the working wavelength and, if necessary, against contamination by metal that can escape from plasma radiation sources.

The protective layer provided with protective optical element is now in one step, depending on the requirement 107 further edited. These may include, for example, irradiating the optical surface of the optical reflective element with electrons, ions or photons to correct its optical properties, annealing to increase long-term stability, removal of adhesive residues on holders or substrate or other residues, mechanical Processing steps for producing micromirrors for micromirror arrays or facet or pupil mirrors. Further processing can also be used to structure EUV mirrors or EUV masks. During further processing, the protective layer, whether one-piece or multi-part, protects the reflective coating from scratches, contamination by particles or contamination that could render the reflective optical element useless.

For further chemical processing, the protective layer should, inter alia, have sufficient thickness and chemical resistance. Typically, for chemical processes, a thickness of the protective layer of about 10 nm to 30 nm is sufficient. To mechanical loads during further processing, z. For example, when fishing, polishing, milling, drilling, turning, etc., must withstand the protective layer 12 have a certain hardness and resistance. For example, silicon nitride and titanium nitride are very hard and durable materials that are useful as a protective coating for the reflective coating. Depending on the hardness of the protective layer, its thickness can be varied; in general, such a layer is applied with a layer thickness of less than 100 nm.

After the further processing step 107 the protective layer is removed, wherein in a first removal step 109 nonselective material is removed and then in a second removal step 111 Protective layer material is selectively removed so that the original surface of the reflective coating is exposed in order to bring the reflective optical element as possible without loss of quality can be used. The first removal step 109 can be carried out, for example, as sputtering, polishing, wet-chemical removal or dry-chemical removal in order to be able to remove thicker protective layers or partial layers as quickly as possible. The second removal step 111 can be carried out, for example, as cleaning with hydrogen radicals or oxygen radicals. Cleaning processes are advantageous, which have very different removal rates depending on the material to be removed. Advantageously, the one or more materials of the protective layer or the partial layers and the removal methods in the non-selective and especially in the selective removal step are coordinated.

The method proposed here will be further explained with reference to some embodiments.

Example 1:

Within the production of a reflective optical element for EUV lithography, irradiation with high-energy electrons should take place in order to increase the radiation resistance or to relax the stresses built up by the individual layers of a multilayer system as a reflective coating.

First, a multi-layer system is applied as a reflective coating to a substrate and sputtered thereon a 50 nm thick carbon layer as a protective layer. Subsequently, the electron irradiation is carried out. In this case, the reflective coating is effectively protected by the carbon protective layer against a variety of impurities occurring during electron irradiation. In a first removal step, these impurities are non-selectively removed on the carbon layer and part of the carbon layer is removed by argon sputtering. In a modification, this can also be done by polishing. Subsequently, in a selective removal step, the remaining part of the carbon protective layer is removed by means of atomic hydrogen purification. Although the removal rate is lower than in the first removal step, but higher than the usual absorber materials of multi-layer systems or even of conventional cover layer materials such as that against atomic Hydrogen very inert ruthenium. This makes it possible to control the removal process very accurately so that it is terminated before the surface of the reflective coating is attacked more than tolerable.

In a modification, the multi-layer system has a covering layer of titanium oxide. In this case, the carbon protective layer is selectively removed by means of an oxygen plasma, forming volatile carbon monoxide and carbon dioxide. The reaction rate with titanium dioxide is orders of magnitude smaller, so that a timely termination of the second removal step is possible and the surface of the reflective coating remains intact.

Example 2

The optical properties and the long-term stability of the reflective coating should be influenced by annealing prior to the completion of a reflective optical element for EUV lithography.

On a substrate, a multilayer system is first applied as a reflective coating. Then, a first partial protective layer is applied, which can be removed particularly well from the surface of a multi-layer system, in particular a cover layer on the absorber and Spacerlagen. For this purpose, a 50 nm thick carbon layer is sputtered in the present example. In addition, a second partial protective layer is applied, which is particularly resistant to the oxygen-containing residual gas atmosphere in a bake chamber, here in the form of a 50 nm thick titanium dioxide layer. The thus prepared element is tempered in the bake chamber. The reflective coating is effectively protected from oxidation by the oxygen contained in the residual gas atmosphere of the bake chamber. After completion of the annealing process, first the titanium dioxide layer and possibly also a part of the underlying carbon layer is removed by argon sputtering. Subsequently, the remaining carbon layer is converted by means of atomic hydrogen into volatile hydrides, which can be pumped out. This process can be very well stopped because of the very different reaction rates on the one hand with carbon and on the other hand with, for example, ruthenium, before measurably the top layer of the multi-layer system reacts with the atomic hydrogen.

In a modification, the multi-layer system has a covering layer of titanium dioxide and the second removal step is carried out with an oxygen plasma.

Example 3

At the reflective optical element processing steps must be performed, leading to residues, adhesive residue or contamination on the holder or on the sides of the substrate. These must be removed by cleaning.

On a substrate, a reflective coating in the form of a multi-layer system with a covering layer of ruthenium is applied. Preferably, before the aforementioned processing steps, a first partial protective layer of 50 nm carbon is applied directly to the multilayer system and above a second partial protective layer of sputtered aluminum of a thickness which leads to a dense, closed, in particular light-tight layer, for example of 50 nm Cleaning adapted to the dirt to be removed optionally by a plasma cleaning, a wet-chemical cleaning or a cleaning by irradiation with UV radiation to be cleaned. Due to the protective layer of the two mentioned sub-layers, the reflective coating remains unaffected. After cleaning, the aluminum layer and possibly part of the carbon layer is removed by sputtering or polishing. Subsequently, the remaining carbon layer is removed with atomic hydrogen, so that the surface of the reflective coating is intact available.

In a modification, the multilayer system of the reflective coating has a covering layer of titanium dioxide and the remaining carbon protective layer is removed with the aid of an oxygen plasma.

Example 4

The reflective coating of the reflective optical element for the EUV lithography device is to be patterned.

On a substrate, a multi-layer system is applied as a reflective coating. Immediately following this, an additional layer of carbon is applied as the first partial protective layer. It should be at least a monolayer. For example, the carbon layer has a thickness of between 2 nm and 20 nm. The reflective coating is now sufficiently protected so that the reflective optical element can be removed from the coating system and, if necessary, optically measured for control purposes. Subsequently, a lacquer layer of a thickness between 500 nm and 20 μm is applied as the second partial protective layer. The paint will matched to the following structuring steps. It can be sensitive to light and be developed and removed at the sites to be structured. In a first variant, the multi-layer system has absorber layers of molybdenum and this is locally and partially removed by wet-chemical etching with, for example, aqua regia. In further variants, the structuring is carried out with the aid of a plasma, wherein the structuring can be carried out purely physically or as a dry chemical etching with the aid of process gases such as halides, eg chlorine with a chemical component, or the structuring is mechanical, such as by laser or water jet cutting , Milling, drilling, turning or similar carried out. Meanwhile, the protective coating of carbon and lacquer effectively protects the reflective coating from particles and residues from the patterning process. These are deposited as non-volatile reaction products, particles or auxiliaries such as lubricating oil on the lacquer layer. These impurities are wet-chemically removed together with the lacquer in an ultrasonic bath with, for example, acetone or tetrahydrofuran or another solvent. The solvent as well as the paint are tailored to give a high selectivity of the paint removal process over the carbon layer. If, for example, the carbon layer has a thickness of 20 nm and the lacquer layer has a thickness of 20 μm, it is particularly advantageous to select the lacquer and solvent such that the removal rate for carbon is at most one thousandth of the removal rate of the lacquer, which means a selectivity of at least 1000 corresponds. In a second removal step, the partial protective layer of carbon is also removed by means of atomic hydrogen for the full functionality of the reflective optical element.

LIST OF REFERENCE NUMBERS

10

 reflective optical element

12

 protective layer

14

 substratum

16

 reflective coating

18

 Surface of the reflective coating

20

 reflective optical element

22

 upper part layer

24

 lower part layer

26

 protective layer

30

 reflective optical element

32

 Multilayer system

34

 stack

35

 absorber

36

 spacer

38

 topcoat

101-111

 steps

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 2009/152885 A1 [0003]

Claims (11)

 Method for producing a reflective optical element for EUV lithography with the steps: - Providing a substrate; Applying a coating which reflects in the extreme ultraviolet wavelength range; Applying a protective layer to the surface of the reflective coating; - Further processing of the reflective optical element; Removal of the protective layer, wherein non-selectively material is removed in a first removal step and then in a second removal step protective layer material is selectively removed so that the original surface of the reflective coating is exposed. A method according to claim 1, characterized in that the protective layer is applied in the form of at least two partial layers of different material. A method according to claim 2, characterized in that for the sub-layer immediately adjacent to the surface of the reflective coating, a material is selected, to which the second removal step acts as different as possible on the surface of the reflective coating. A method according to claim 2 or 3, characterized in that a material is selected for the final to the surrounding sub-layer, which is particularly resistant to influences during further processing. Method according to one of Claims 2 to 4, characterized in that carbon, a nitride or a fluoride is applied as the sub-layer adjoining the surface of the reflective coating. Method according to one of claims 2 to 5, characterized in that is applied as the environment final part of a coating layer, in particular a photoresist, an oxide or a metal. Method according to one of claims 1 to 6, characterized in that the first removal step is carried out as sputtering, polishing, wet-chemical removal or dry chemical removal. Method according to one of claims 1 to 7, characterized in that the second removal step is carried out as cleaning with hydrogen radicals or oxygen radicals. Method according to one of claims 1 to 8, characterized in that the provided with protective layer reflective optical element is further processed by irradiation with electrons, cleaning, annealing or mechanical processing. Method according to one of claims 1 to 9, characterized in that as reflective coating a multilayer system of alternately arranged layers of a material having a higher real part of the refractive index at a wavelength in the extreme ultraviolet wavelength range and a material of lower real part of the refractive index at the wavelength in the extreme ultraviolet Wavelength range is applied. A method according to claim 10, characterized in that as the uppermost layer of the multi-layer system, a metal or an oxide is applied.

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