WO2016113113A1

WO2016113113A1 - Arrangement for reducing contamination in a microlithographic projection exposure apparatus

Info

Publication number: WO2016113113A1
Application number: PCT/EP2015/081412
Authority: WO
Inventors: Joachim Hartjes; Toralf Gruner
Original assignee: Carl Zeiss SMT GmbH
Current assignee: Carl Zeiss SMT GmbH
Priority date: 2015-01-13
Filing date: 2015-12-30
Publication date: 2016-07-21
Anticipated expiration: 2017-07-13
Also published as: TW201629638A; DE102015200327A1

Abstract

The invention relates to an arrangement for reducing contamination in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus is designed for operation in the EUV and comprises an illumination device, wherein during the operation of the projection exposure apparatus EUV light generated by a plasma excitation of a target material brought about by a first laser light source enters the illumination device via an intermediate focus (IF). The arrangement comprises a second laser light source (140), which directs directional electromagnetic radiation into a region adjoining the intermediate focus (IF) within the illumination device, wherein said second laser light source (140) is designed in such a way that the directional electromagnetic radiation of the second laser light source (140) brings about an evaporation or plasma excitation for at least part of target material that has entered said region during the operation of the projection exposure apparatus.

Description

Arrangement for reducing contamination in a microlithographic projection exposure apparatus

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of German Patent Application DE 10 2015 200 327.2 filed on January 13, 2015. The content of this application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to an arrangement for reducing contamination in a microlithographic projection exposure apparatus.

Prior art

Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus comprising an illumination device and a projection lens. The image of a mask (= reticle) illuminated by means of the illumination device is in this case projected by means of the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate. In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process. The EUV light can be generated by means of an EUV light source based on a plasma excitation, with respect to which Fig. 3 shows one exemplary conventional construction.

Said EUV light source firstly comprises a CO₂ laser (not illustrated in Fig. 3) for generating infrared radiation 306 having a wavelength of λ« 10.6 μηι, which is focused via a focusing optical unit (not illustrated in Fig. 3), passes through an opening 321 present in a collector mirror 320 embodied as an ellipsoid and is directed onto a target material 310 (formed by tin droplets 332 in the example) generated by means of a target source 335 and fed to a plasma ignition position. The infrared radiation 306 heats the target material 310 situated in the plasma ignition position in such a way that said target material undergoes transition to a plasma state and emits EUV radiation. The spectral range used by the microlithographic projection exposure apparatus can be for example λ« 13.5 + 0.5 nm. Said EUV radiation is focused via the collector mirror 320 onto an intermediate focus IF (= "Intermediate Focus") and enters a downstream illumination device through said intermediate focus IF, the boundary 340 of said illumination device merely being indicated and said illumination device having a free opening 341 for the entrance of light. A light trap 330 serves for preventing the infrared radiation 306 from passing through directly (i.e. without prior reflection at the collector mirror 320) into the illumination device.

During the operation of a projection exposure apparatus equipped with such an EUV light source, the problem occurs that particles of the target material (e.g. tin) used for transfer to the plasma state can enter the illumination device. Such particles can deposit on the individual optical components and lead to a loss of reflection for the reflective optical elements, a reduction of the lifetime of the individual components and an impairment of the performance of the projection exposure apparatus.

With regard to the prior art, reference is made merely by way of example to DE 10 2008 041 827 A1 , DE 103 37 667 A1 and US 2013/0313423 A1 .

SUMMARY OF THE INVENTION It is an object of the present invention to provide an arrangement for reducing contamination in a microlithographic projection exposure apparatus which enables an effective reduction of contaminations proceeding from an EUV light source during the operation of the projection exposure apparatus. This object is achieved by means of the arrangement according to the features of the independent claims.

An arrangement according to the invention for reducing contamination in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus is designed for operation in the EUV and comprises an illumination device, wherein during the operation of the projection exposure apparatus EUV light generated by a plasma excitation of a target material brought about by a first laser light source enters the illumination device via an intermediate focus, comprises: - a second laser light source, which directs directional electromagnetic radiation into a region adjoining the intermediate focus within the illumination device;

- wherein said second laser light source is designed in such a way that the directional electromagnetic radiation brings about an evaporation or plasma excitation for at least part of target material that has entered said region during the operation of the projection exposure apparatus. The concept underlying the invention is, in particular, that particles of the target material which proceed from a plasma light source of a projection exposure apparatus, enter the illumination device or are situated directly before this entrance, said target material being used in the plasma light source for generating EUV light, are prevented from depositing on the sensitive optical elements of the illumination device by virtue of the fact that said target material evaporates or is transferred to the plasma state by the action of directional electromagnetic radiation of an additional laser light source. In this case, according to the invention, an initially highly localized particle contamination before or after entrance into the illumination device is converted into a "cloud" of significantly smaller or atomic or molecular particles that is comparatively widely distributed or extended spatially (owing to the evaporation or plasma excitation).

The invention differs in particular from conventional approaches in which a chemical conversion of the contamination is brought about (e.g. by the use of an additional gas). Instead, the invention pursues the concept, rather, of converting the relevant contamination into smaller particles by mechanically "smashing" or "blowing up" larger particles, said smaller particles then moving in comparatively non-critical directions until they either deposit on a wall of the illumination device, for example, or are caught by a particle trap provided at a suitable location. In other words, the concept according to the invention is not based on a chemical conversion, but rather on a mechanical reduction in size or "smashing" of the contamination particles.

In this case, the invention makes use of the circumstance, in particular, that the target material forming the undesired contamination particles enters the illumination device via the intermediate focus on a narrowly delimited path and, accordingly, can be effectively smashed or converted in terms of its state of matter in the manner described above by the use of directional electromagnetic radiation in a region directly adjoining the intermediate focus.

To a certain extent, in the concept according to the invention, in the process used within the plasma light source for generating the desired EUV radiation, namely the plasma excitation of the target material, is repeated directly before or after the undesired entrance of particles of said target material into the illumination device with a lower laser power; in other words, the principle of plasma excitation used firstly for generating the EUV radiation is therefore now used for reducing or eliminating contamination.

In accordance with one embodiment, the directional electromagnetic radiation of the second laser light source has a direction of propagation which runs at an angle of 90°±20°, in particular 90°±10°, more particularly 90°±5°, with respect to the direction of propagation of the EUV radiation that entered the illumination device via the intermediate focus.

In accordance with one embodiment, the region adjoining the intermediate focus is situated downstream of the intermediate focus or in the illumination device relative to the light propagation direction.

In accordance with one embodiment, the region adjoining the intermediate focus is situated upstream of the intermediate focus relative to the light propagation direction.

In accordance with one embodiment, the arrangement furthermore comprises a particle trap for catching target material that has entered the region adjoining the intermediate focus.

In accordance with a further aspect, the invention also relates to an arrangement for reducing contamination in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus is designed for operation in the EUV and comprises an illumination device, wherein during the operation of the projection exposure apparatus EUV light generated by a plasma excitation of a target material brought about by a first laser light source enters the illumination device via an intermediate focus, comprising:

- a second laser light source, which directs directional electromagnetic radiation into a region adjoining the intermediate focus; and

- a particle trap for at least partly catching target material that has entered said region.

In accordance with one embodiment, said particle trap is configured for at least partly catching the target material after the evaporation or plasma excitation thereof by the electromagnetic radiation of the second laser light source.

In accordance with one embodiment, the invention furthermore comprises a detector for detecting target material that has entered the illumination device. This makes it possible for the contamination particles that have entered the illumination device via the intermediate focus or are situated directly before this entrance to be detected on their movement path and, accordingly, for the laser light source used for the evaporation and/or plasma excitation of said contamination particles to be correspondingly aligned with the relevant contamination particles. This is particularly advantageous insofar as the laser power required for the evaporation or plasma excitation rises proportionally to the volume into which the directional electromagnetic radiation is focused.

In accordance with one embodiment, the target material is a metallic target material, in particular tin (Sn) or lithium (Li).

In accordance with one embodiment, the second laser light source has a laser power of at least 300 watts (W).

The invention furthermore also relates to a microlithographic projection exposure apparatus comprising an EUV light source, an illumination device and a projection lens, wherein the projection exposure apparatus comprises an arrangement having the features described above.

Further configurations of the invention can be gathered from the description and the dependent claims.

The invention is explained in greater detail below on the basis of exemplary embodiments illustrated in the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures: Figure 1 shows a schematic illustration for elucidating one exemplary construction of an arrangement according to the invention for reducing contamination in one embodiment;

Figure 2 shows a schematic illustration for elucidating one possible conventional construction of a microlithographic projection exposure apparatus which is designed for operation in the EUV and in which the invention can be realized; and

Figure 3 shows a schematic illustration for elucidating one possible conventional construction of an EUV light source in a microlithographic projection exposure apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Firstly, one exemplary construction of a microlithographic projection exposure apparatus designed for operation in the EUV is described below with reference to Fig. 2. The projection exposure apparatus comprises an illumination device and a projection lens. In this case, the illumination device comprises in particular a field facet mirror 203 and a pupil facet mirror 204. The light from a light source unit comprising a plasma light source 201 and a collector mirror 202 is directed onto the field facet mirror 203. A first telescope mirror 205 and a second telescope mirror 206 are arranged in the light path downstream of the pupil facet mirror 204. A deflection mirror 207 is arranged downstream in the light path and directs the radiation impinging on it onto an object field in the object plane of the projection lens, which comprises six mirrors 221 -226 in the example. At the location of the object field, a reflective structure-bearing mask 231 is arranged on a mask stage 230, said mask being imaged with the aid of the projection lens into an image plane, in which a substrate 241 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 240.

One exemplary construction of an arrangement according to the invention for reducing contamination will now be described below with reference to the schematic depiction in Fig. 1 .

In Fig. 1 , the target material situated in the plasma ignition position on the part of the EUV light source and undergoing transition to a plasma state with emission of EUV radiation is designated by "101 ". In accordance with Fig. 1 , the EUV radiation emitted by said target material in the plasma state is directed via a collector mirror 102 onto the intermediate focus IF, from where the EUV radiation enters the illumination device. The high-energy process leading to the generation of the EUV radiation also has the consequence, however, that particles 103 of the target material are blasted away in an undesired manner and, as indicated in Fig. 1 , can likewise penetrate into the illumination device via the intermediate focus IF. According to the invention, a further laser light source 140 (i.e. a second laser light source in addition to the first laser light source or the CO2 laser (not illustrated) used within the EUV light source for heating the target material) now serves to direct directional electromagnetic radiation onto said particles 103 in a region directly adjoining the intermediate focus IF with the consequence that the particle contamination that is still highly localized in said region is converted mechanically or by a change in the state of matter into a spatially comparatively greatly distributed particle cloud consisting of particles having a significantly smaller particle size.

Even though the region which adjoins the intermediate focus IF and into which the directional electromagnetic radiation of the second laser light source 140 is directed is situated within the illumination device (that is to say that the relevant particles have already penetrated into the illumination device) in the exemplary embodiment in Fig. 1 , the invention is not restricted thereto. In this regard, in further embodiments, the relevant region which adjoins the intermediate focus IF and into which the directional electromagnetic radiation of the second laser light source 140 is directed can also still be situated upstream of the intermediate focus IF relative to the light propagation direction or within the EUV light source.

In accordance with the exemplary embodiment (but without the invention being restricted thereto), the laser light source 140 is an argon ion laser, wherein the electromagnetic radiation generated by the laser light source 140 is likewise preferably oriented substantially perpendicularly (preferably at an angle of 90° ± 20°) to the direction of propagation of the EUV radiation that entered the illumination device via the intermediate focus IF. In Fig. 1 , "160" denotes a detector via which the particles 103 that entered the illumination device (in further embodiments also the particles situated directly before this entrance) can be detected in their movement path, such that the electromagnetic radiation of the laser light source 140 can be correspondingly aligned with or focused onto the particles 103 in a targeted manner.

In embodiments, the region which adjoins the intermediate focus IF and into which the second laser light source 140 directs the directional electromagnetic radiation can comprise a region which is at a distance of less than 25 mm, preferably less than 10 mm, more preferably less than 5 mm, from the physical separation point or the mechanical constriction between source region or EUV light source, on the one hand, and illumination system, on the other hand, or from the intermediate focus IF. If monitoring of the resulting "debris" or particles of the target material and alignment of the radiation on the basis thereof are carried out, a larger distance can also be complied with, wherein it is possible to reduce the risk of a significant proportion of the evaporation cloud penetrating through the opening or the intermediate focus IF. The focusing takes place in a non-targeted manner, by contrast, preferably onto a region as close as possible to the opening or the intermediate focus IF, thereby taking account of the circumstance that only a comparatively small volume can be heated with realistic laser powers.

Furthermore, a particle trap 150 is indicated in Fig. 1 , said particle trap serving to catch part of the "contamination cloud" generated by evaporation or plasma excitation of the particles 103. The particle trap 150 can be produced for example from steel suitable for operation in ultra-high vacuum and can comprise lamellae or a grooved structure. However, the invention is not restricted to the presence of a particle trap 150 since, in embodiments of the invention, if appropriate, deposition of the abovementioned contamination cloud e.g. on a housing wall of the illumination device can also be accepted (wherein in this case, too, at least a direct impairment of the sensitive optical components or mirrors of the illumination device is prevented). In further embodiments, the particle trap can also be configured or arranged for catching particles situated upstream of the intermediate focus IF relative to the light propagation direction or within the EUV light source.

Of the optical components of the illumination device, for the sake of simplicity Fig. 1 merely indicates a field facet mirror 1 10 and a pupil facet mirror 120

(having pupil facets 120a, 120b, 120c, ... ), from where the EUV light impinges on the mask or reticle, designated by "130". The following quantitative consideration serves for estimating the laser powers or photon energies suitable in particular for the evaporation of the contamination particles by the directional electromagnetic radiation of the laser light source 140.

If a spherical geometry of the particles 103 having a radius of r= 10 m and a flight velocity of v= 100 m/s are taken as a basis merely by way of example, the primary interaction volume with the electromagnetic radiation of the laser light source 140 results as V= Ax*Ay*Az= 10μηΊ*10μΓη*100 m.

The energy required for evaporating the particles 103 (composed of tin in the example) results from the heat of fusion Wf_USion= 0.06 kJ/g and the heat of vaporization W_vaporization= 2.49 kJ/g as E= (Wfusion + Wvaporization) * P *(4ττ/3 * Γ³) = 7.8*10^"5 J (1 )

If an absorption coefficient for tin of oc= 20% is taken as a basis, taking account of the interaction time t=Az/v=1 *10^~6s the laser power required in the primary interaction volume results as

P = E/(a*t) = 390 W (2) With lower absorption, correspondingly higher laser powers are required. In the above example, the momentum of the particles results as p= m*v = ρ*(4π/3*Γ³)*ν= 3*10^"9 kg m/s. From this, the momentum required for a deflection of a (tin) particle by an angle of β=10° results as Δρ = p * tan( )= 5.4*10^"10 kg m/s. The photon energy required for this momentum results as Ephoton = Ap*c= 0.162 J. It is evident that for a significant change in direction of (tin) particles 103 on account of radiation pressure such high radiation powers are required that the particles 103 are evaporated beforehand with high probability. Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments are apparent to the person skilled in the art, e.g. by combination and/or exchange of features of individual embodiments. Accordingly, it goes without saying for the person skilled in the art that such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the accompanying patent claims and the equivalents thereof.

Claims

1 . Arrangement for reducing contamination in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus is designed for operation in the EUV and comprises an illumination device, wherein during the operation of the projection exposure apparatus EUV light generated by a plasma excitation of a target material brought about by a first laser light source enters the illumination device via an intermediate focus (IF), comprising:

• a second laser light source (140), which directs directional electromagnetic radiation into a region adjoining the intermediate focus (IF);

• wherein said second laser light source (140) is designed in such a way that the directional electromagnetic radiation of the second laser light source (140) brings about an evaporation or plasma excitation for at least part of target material that has entered said region during the operation of the projection exposure apparatus.

2. Arrangement according to Claim 1 , characterized in that the directional electromagnetic radiation of the second laser light source (140) has a direction of propagation which runs at an angle of 90°±20°, in particular 90°±10°, more particularly 90°±5°, with respect to the direction of propagation of the EUV radiation that entered the illumination device via the intermediate focus (IF).

3. Arrangement according to Claim 1 or 2, characterized in that the region adjoining the intermediate focus (IF) is situated downstream of the intermediate focus (IF) relative to the light propagation direction.

4. Arrangement according to Claim 1 or 2, characterized in that the region adjoining the intermediate focus (IF) is situated upstream of the intermediate focus (IF) relative to the light propagation direction.

5. Arrangement according to any of the preceding claims, characterized in that it furthermore comprises a particle trap (150).

6. Arrangement for reducing contamination in a microlithographic projection exposure apparatus, wherein the projection exposure apparatus is designed for operation in the EUV and comprises an illumination device, wherein during the operation of the projection exposure apparatus EUV light generated by a plasma excitation of a target material brought about by a first laser light source enters the illumination device via an intermediate focus (IF), comprising:

• a second laser light source (140), which directs directional electromagnetic radiation into a region adjoining the intermediate focus (IF); and

• a particle trap (150) for at least partly catching target material (103) that has entered said region.

7. Arrangement according to Claim 5 or 6, characterized in that said particle trap (150) is configured for at least partly catching the target material (103) after the evaporation or plasma excitation thereof by the electromagnetic radiation of the second laser light source (140).

8. Arrangement according to any of the preceding claims, characterized in that it furthermore comprises a detector (160) for detecting target material (103) that has entered the region adjoining the intermediate focus (IF).

9. Arrangement according to any of the preceding claims, characterized in that the target material (103) is a metallic target material, in particular tin (Sn) or lithium (Li).

10. Arrangement according to any of the preceding claims, characterized in that the second laser light source (140) has a laser power of at least 300 watts (W).

1 1 . Arrangement according to any of the preceding claims, characterized in that the second laser light source (140) is an argon ion laser.

12. Microlithographic projection exposure apparatus comprising an EUV light source, an illumination device and a projection lens, characterized in that the projection exposure apparatus comprises an arrangement according to any of the preceding claims.