DE102022202072A1 - Method of handling reflective optical elements - Google Patents

Abstract

To protect against contamination by deposits and particles, a method for handling reflective optical elements, in particular for the extreme ultraviolet wavelength range, having a reflective surface, is proposed with the steps: - applying a protective coating to the reflective surface; - handling the reflective optical element;- removing the protective coating.

Description

The present invention relates to a method for handling reflective optical elements, in particular for the extreme-ultraviolet wavelength range, having a reflective surface.

In order to be able to produce ever finer structures in the production of semiconductor components using lithographic methods, work is being carried out with ever shorter wavelength light. If one works in the extreme ultraviolet (EUV) wavelength range, especially at wavelengths between approx. 5 nm and 30 nm, it is no longer possible to work with lens-like elements in transmission, but instead illumination and projection lenses or masks made of reflective ones are used in EUV lithography devices optical elements used that have a reflective coating. Typical EUV lithography devices have eight or more reflective optical elements. In order to nevertheless achieve a sufficient overall intensity of the EUV radiation used, the reflective optical elements must have reflectivities that are as high as possible. This is because the total intensity is proportional to the product of the reflectivities of the individual reflective optical elements.

High cleanliness requirements apply not only, but especially for small reflective optical elements or for reflective optical elements that are arranged in the vicinity of the field, in order to avoid undesirable imaging errors. In EUV lithography, for example, a loss of contrast during structuring can lead to increased rejects. If impairments of the reflective surface occur, for example due to deposits, particles, mechanical damage to reflective optical elements that are arranged particularly close to the field in the beam path, they are particularly difficult to compensate for.

From the DE 10 2013 226 678 A1 It is known to transport reflective optical elements for EUV lithography, in particular masks, in a special transport device which provides a gas curtain flowing flatly along the reflective optical element in order to prevent the deposit of contaminating substances or particles on the surface behind the gas curtain area to be avoided.

It is an object of the present invention to show a further possibility of protecting reflective optical elements from the deposit of contaminating substances or particles.

• - Aufbringen einer Schutzbeschichtung auf die reflektierende Oberfläche;
• - Handhaben des reflektiven optischen Elements;
• - Entfernen der Schutzbeschichtung.

This object is achieved by a method for handling reflective optical elements, in particular for the extreme-ultraviolet wavelength range, having a reflective surface, with the steps:

• - applying a protective coating to the reflective surface;
• - handling of the reflective optical element;
• - Removal of the protective coating.

It has been found that this procedure can provide good protection against contamination of the reflective surface in the form of deposits and particles. During handling of the reflective optical element, the protective coating is compromised in place of the delicate reflective surface. After the handling is completed, when the protective coating is removed again, any contamination that may be present is also removed with the protective coating and the reflective optical element with the reflective surface exposed can be used.

The reflective optical element is preferably handled in the form of storage, transport and/or assembly. It is true that storage and transport can be carried out with greater effort under a protective atmosphere or clean room conditions. But at the beginning or at the end of such handling, it is difficult to avoid that the reflective optical element and its reflective surface are not exposed to normal air. The situation is particularly serious when the reflective optical element is installed in, for example, an optical system of an EUV lithography device or a measuring device for checking semiconductor wafers or other reflective optical elements for the EUV wavelength range. Because assembly usually has to take place in the open air. In particular, when the reflective optical element comes into contact with air, its reflective surface can be protected particularly well against contamination by deposits or oxidation or impairment by particles by the proposed procedure. Furthermore, a protective coating can also be applied before a maintenance operation and then removed again.

In particularly preferred embodiments, the reflective optical element is located at a different location when the protective coating is removed than when the protective coating was applied. This is the case, for example, when the protective coating is applied as soon as possible after the reflective optical element has been manufactured is brought and is only removed again after final assembly. As a result, the reflective optical element can be protected particularly comprehensively from damage to its reflective surface during handling.

Advantageously, the reflective optical element is placed in a coating chamber to apply a protective coating to its reflective surface. In the case of reflective optical elements in particular, the reflective surface of which is formed by a reflective coating, this allows the protective coating to be applied to the reflective surface immediately after the reflective coating has been applied, before there is an opportunity for it to become contaminated. Any chemical and/or physical deposition process can be used to apply the protective coating.

In preferred embodiments, a carbonaceous layer is applied as a protective coating. A layer of carbon is particularly preferably applied as a protective coating. Protective coatings that contain carbon can not only be grown on with comparatively little effort, but can also be removed particularly well, especially if they are essentially made of carbon. Among other things, when stored in air with a certain concentration of hydrocarbons in the atmosphere, a self-terminating protective carbon-containing or carbon coating can grow that has a few atomic or molecular layers and that can also protect against contamination. As a rule, it can be a few molecular or atomic layers thick.

A protective coating of carbon with a thickness of at least 15 nm, preferably at least 25 nm, is advantageously applied. Such a thick protective carbon coating protects the underlying reflective surface of the reflective optical element from deposits and particles, but also provides mechanical protection, such as from light scratches or impacts. However, it can still be easily removed together with the contaminants when special protection is no longer necessary.

In preferred embodiments, the protective coating is removed by photolysis and/or heat treatment. The protective coating can be converted into volatile compounds or directly converted into the gas phase by the energy input by means of irradiation and/or heat treatment. This also flakes off any deposits on the protective coating or removes any particles along with the protective coating from the reflective surface.

Advantageously, the protective coating is removed by photolysis using radiation in the extreme ultraviolet wavelength range. This is particularly advantageous when the protective coating is to be removed from a reflective optical element that has already been finally assembled, since the EUV radiation can be used for this purpose, with which the optical system in which the reflective optical element is installed is operated and which is the arrangement of the reflective optical element in the beam path impacts the reflective surface of the reflective optical element or the protective coating anyway, as long as it has not yet been removed. In addition, protective coatings containing carbon or hydrocarbons can be converted particularly efficiently into volatile compounds by EUV photolysis. If necessary, these can then be pumped out by the vacuum system of the optical system.

The protective coating is preferably removed in a hydrogen-containing or an oxidizing residual gas atmosphere. Both hydrogen, especially in atomic or ionized form, and oxygen and other oxidizing substances can react with the protective coating to form volatile compounds, so that the protective coating and any deposits and particles on it can be removed from the reflective surface of the reflective optical element . In particular, protective coatings containing carbon or hydrocarbons can be quickly converted into volatile compounds in this way. Particular preference is given to maintaining low partial pressures of hydrogen or oxidizing gases in the residual gas atmosphere combined with heat treatment and/or photolysis, in particular with EUV radiation, in order to enhance the effect of converting the protective coating into one or more volatile compounds.

In preferred embodiments, a stream of gas is passed over the surface of the protective coating prior to its removal. This is particularly advantageous for removing any particles or deposits that may be present that have such poor adhesion to the protective coating that they can be blown off the surface of the protective coating by the gas stream. This can prevent these contaminants from being deposited again on the reflective surface of the reflective optical element after the protective coating has been removed and impairing its optical properties. By performing this blow-off prior to removing the protective coating, there is a risk that the reflective surface is affected by blowing off is reduced, in particular that particles are blown off leaving scratches in the reflective surface.

• 1 eine Prinzipskizze eines reflektiven optischen Elements für den EUV-Wellenlängenbereich;
• 2 ein reflektives optisches Element, auf das eine Schutzbeschichtung aufgebracht wird;
• 3 ein reflektives optisches Element, von dem eine Schutzbeschichtung entfernt wird; und
• 4 ein reflektives optisches Element, nachdem eine Schutzbeschichtung entfernt wurde.

The present invention will be explained in more detail with reference to preferred exemplary embodiments. to show

• 1 a schematic diagram of a reflective optical element for the EUV wavelength range;
• 2 a reflective optical element to which a protective coating is applied;
• 3 a reflective optical element from which a protective coating is removed; and
• 4 a reflective optical element after a protective coating has been removed.

In 1 The structure of a reflective optical element 50 is shown schematically, which has a reflective coating 51 on a substrate 52, which in the present example is designed as a multi-layer system comprising layers of at least two different materials with different real parts of the refractive index at a wavelength in the extreme ultraviolet wavelength range has, which are arranged alternately. In this example, layers of a material with a higher real part of the refractive index at the working wavelength, for example a lithographic exposure is carried out or a wafer or a mask is checked, (also called spacer 54) and a material are applied alternately to a substrate 52 with a lower real part of the refractive index at the working wavelength (also called absorber 55), with an absorber-spacer pair forming a stack 53. In a way, this simulates a crystal whose lattice planes correspond to the absorber layers at which Bragg reflection takes place. Typically, reflective optical elements for an EUV lithography device or an optical system are designed in such a way that the respective wavelength of maximum reflectivity essentially corresponds to the working wavelength of the lithography process or other applications of the optical system.

The thicknesses of the individual layers 54, 55 as well as the repeating stack 53 can be constant over the entire multi-layer system 51 or also vary over the area or the total thickness of the multi-layer system 51, depending on which spectral or angle-dependent reflection profile or which maximum reflectivity of the working wavelength is to be achieved. If the layer thicknesses are essentially constant over the entire multi-layer system 51, one also speaks of a period 53 instead of a stack 53. The reflection profile can also be influenced in a targeted manner by adding more and less absorbing materials to the basic structure of absorber 55 and spacer 54 is supplemented in order to increase the possible maximum reflectivity at the respective working wavelength. For this purpose, absorber and/or spacer materials can be exchanged for one another in some stacks, or the stacks can be constructed from more than one absorber and/or spacer material. Furthermore, additional layers can also be provided as diffusion barriers between spacer and absorber layers 54, 55. A material combination that is common, for example, for a working wavelength of 13.4 nm is molybdenum as the absorber material and silicon as the spacer material. A period 53 often has a thickness of approximately 6.7 nm, with the spacer layer 54 usually being thicker than the absorber layer 55. Other common material combinations include silicon-ruthenium or molybdenum-beryllium. In addition, a protective layer 56 can be provided on the multi-layer system 51 to protect it during operation, which can also be designed in multiple layers.

In variants that are not shown, the reflective surface of the reflective optical element can be formed from a polished metal surface or a reflective coating from just a few layers. Such a reflective optical element is primarily suitable for operation with grazing incidence. an as in 1 In contrast, the reflective optical element shown, in which the reflective surface is based on a multilayer system, has a particularly high maximum reflectivity at normal incidence.

Typical substrate materials for reflective optical elements for EUV lithography are silicon, silicon carbide, silicon-infiltrated silicon carbide, quartz glass, titanium-doped quartz glass, glass and glass ceramics. In particular with such substrate materials, a layer can additionally be provided between the reflective coating 51 and the substrate 52, which is made of a material that has a high absorption of radiation in the EUV wavelength range, which is used during operation of the reflective optical element 50 to protect the substrate 52 to protect against radiation damage, such as unwanted compaction. Furthermore, the substrate can also be made of copper, aluminum, a copper alloy, an aluminum alloy or a copper-aluminum alloy.

In order to protect the sensitive reflective surface of the in 1 To protect the reflective optical element 50 shown before it is used from contamination by deposits or particles, it is proposed that the reflective Apply a protective coating to the surface and remove it again only after handling the reflective optical element. The application can, as for example in 2 shown, are carried out in a coating chamber 21 . It is particularly advantageous to apply the protective coating 1 immediately after the application of a multilayer system or another coating that leads to an increase in the reflectivity of a surface of the reflective optical element 50 in a wavelength range in which the reflective optical element 50 is to be used, in particular in the EUV wavelength range, eg for EUV lithography or in a measuring apparatus for checking semiconductor elements or EUV masks.

The protective coating particularly preferably has materials which can be converted into volatile compounds by means of energy input through exposure to high-energy radiation such as radiation in the EUV wavelength range and/or through heating. Most preferably, the protective coating consists of one or more such materials. Known chemical and/or physical deposition methods can be used for application, and PVD (physical vapor deposition) or CVD (chemical vapor deposition) methods in all their variants can also be used.

Protective coatings that have hydrocarbon compounds and/or carbon have proven particularly useful. Protective coatings made of carbon have proven particularly effective. They have the advantage that they can also be applied particularly easily as thicker layers using known coating processes, such as are also used for applying multi-layer systems, for example. For example, carbon atoms or particles 30 can be applied to the reflective surface of the reflective optical element 50 to be coated, for example by sputtering or electron beam evaporation, so that a protective coating 1 made of carbon grows in this way. Depending on how the reflective surface of the reflective optical element 50 was formed, the coating chamber 21 has little or no conversion after the usual coating process for the application of the protective coating and can be further coated without breaking the vacuum, so that the protective coating 1 is applied can be used before contamination can even occur. A protective coating of carbon with a thickness of at least 15 nm, particularly preferably 25 nm or more, is preferably applied. The thicker the protective coating 1 is, the better it can serve not only as protection against contamination, but also as mechanical protection, for example against scratches or impacts. Protection against contamination by deposits and/or particles can be achieved from as little as one or a few atomic or molecular layers.

In a variant that is not shown, the reflective optical element can also be stored in an atmosphere containing hydrocarbons or carbon. Over time, a self-terminating hydrocarbon or carbonaceous protective coating will grow. However, protective coatings that are not as controlled and thick as with targeted coating cannot be applied in this way.

The protective coating 1 protects the reflective surface of the reflective optical element 50 from contamination, in particular from deposits and/or particles, during various types of handling such as storage, transport and/or assembly. While the reflective optical element can be placed in a housing with an applied vacuum or protective atmosphere for storage or transport, and a certain degree of protection against deposits or particles can therefore already exist, it is often necessary for the assembly of the reflective element in an optical system, e.g For example, an EUV lithography device needs to be performed at least partially in air, and assembly often involves a number of mechanical operations that can release particles or other contamination. For this type of handling, the provision of a protective coating on the reflective surface of the reflective optical element is particularly preferred.

After completion of the handling of the reflective optical element 50, such as in 3 shown after completion of the assembly in the vacuum chamber 23 of an optical system, the protective coating 1 can be removed again. Advantageously, the protective coating 1 is exposed to heat (indicated by dashed arrows 43) or high-energy radiation 41 in order to convert the protective coating 1 into one or more volatile compounds or to convert it into the gas phase, which is present in the in 3 illustrated example can be removed by applying a vacuum in the vacuum chamber 23. Particularly when removing the protective coating 1 by photolysis when the reflective optical element 50 is in the vacuum chamber 23 of an optical system for its end use, the photolysis is preferably carried out with the radiation with which the optical system is operated, in particular with EUV Radiation that is already reflected by the intended beam path on the located under the protective coating 1 reflektie rende surface of the reflective optical element 50 impinges.

Photolysis can be combined with heat treatment to speed up the removal process, especially for thicker protective coatings 1 .

Advantageously, before the protective coating 1 is removed, a gas stream is passed over its surface in order to virtually blow off deposits and in particular particles with less strong adhesion to the surface of the protective coating 1 and, if possible, to remove them from the immediate vicinity of the reflective optical element 50 so that they can be removed after removal of the protective coating 1 does not again lead to contamination of the reflective surface of the reflective optical element 50 this time. Thanks to the protective coating 1, there is no risk of the reflective surface being scratched during blowing off. On the contrary, any scratches are embossed in the protective coating 1 and disappear when the protective coating 1 is removed.

In particular in the case of protective coatings containing hydrocarbons or carbon, work can be carried out in a residual gas atmosphere which contains hydrogen or oxidizing substances, in particular molecules containing oxygen. In particular under the influence of EUV radiation, hydrogen or oxygen or other oxygen-containing molecules can be split into radicals which react with the protective coating to form volatile compounds.

The dissolving of the protective coating also removes any deposits or particles on the protective coating and the reflective surface 57 of the reflective optical element is exposed and can be put to its intended use, for example as in 4 shown as a reflective optical element 50 in the vacuum chamber 23 of an optical element, which in turn can be part of an EUV lithography device, a device for measuring or checking semiconductor elements or EUV masks or other optical elements or part of other optical applications.

While in the preferred embodiment shown here the reflective optical element is in a different place when the protective coating is removed than when the protective coating was applied, in a variant that is not illustrated here, for example for maintenance purposes, a reflective optical element can first be provided with a protective coating in the assembled state to protect it as well as possible from contamination during maintenance work, during which the vacuum often has to be broken, and to remove the protective coating from the mounted reflective optical element after maintenance has been completed. For this purpose, for example, the partial pressure of one or more substances containing carbon or hydrocarbons in the residual gas atmosphere can be increased and split by means of the EUV radiation for operating the reflective optical element, so that a layer containing carbon or a hydrocarbon can grow as a protective coating. After the maintenance work has been completed, this protective coating can be removed again by increasing the partial pressure of hydrogen, oxygen or substances containing oxygen and breaking them down using EUV radiation.

Reference List

1

protective coating

21

coating chamber

23

vacuum chamber

30

coating particles

41

EUV radiation

43

thermal radiation

50

reflective optical element

51

multilayer system

52

substrate

53

stack

54

spacers

55

absorber

56

protective layer

57

reflective surface

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was 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.

Patent Literature Cited

• DE 102013226678 A1 [0004]

Claims (10)

Method for handling reflective optical elements, in particular for the extreme-ultraviolet wavelength range, having a reflective surface, with the steps: - applying a protective coating to the reflective surface; - handling of the reflective optical element; - Removal of the protective coating. procedure after claim 1 , characterized in that the handling of the reflective optical element is carried out in the form of storage, transport and/or assembly. procedure after claim 1 or 2 , characterized in that the reflective optical element is at a different location when the protective coating is removed than when the protective coating is applied. Procedure according to one of Claims 1 until 3 , characterized in that the reflective optical element is placed in a coating chamber to apply a protective coating to its reflective surface. Procedure according to one of Claims 1 until 4 , characterized in that a carbon-containing layer is applied as a protective coating. Procedure according to one of Claims 1 until 5 , characterized in that a protective coating of carbon with a thickness of at least 15 nm is applied. Procedure according to one of Claims 1 until 6 , characterized in that the protective coating is removed by photolysis and/or heat treatment. Procedure according to one of Claims 1 until 7 , characterized in that the protective coating is removed by photolysis using radiation from the extreme ultraviolet wavelength range. Procedure according to one of Claims 1 until 8th , characterized in that the protective coating is removed in a hydrogen-containing or an oxidizing residual gas atmosphere. Procedure according to one of Claims 1 until 9 , characterized in that prior to its removal, a stream of gas is passed over the surface of the protective coating.

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