WO2008007134A2

WO2008007134A2 - Method of controlling contamination of a surface

Info

Publication number: WO2008007134A2
Application number: PCT/GB2007/050350
Authority: WO
Inventors: Robert Bruce Grant
Original assignee: Edwards Limited
Priority date: 2006-07-14
Filing date: 2007-06-22
Publication date: 2008-01-17
Also published as: WO2008007134A3; TW200821762A; GB0614028D0

Abstract

Apparatus for generating extreme ultra violet (EUV) radialson comprises a tin EUV source, a chamber housing at least one optical element for directing EUV radiation generated by the EUV source towards a radiation outlet from the chamber, at least one vacuum pump for evacuating the chamber, and means for supplying to the chamber an organic halide for reacting with the deposited on said at least one optical element to form an organo-tin halide.

Description

METHOD OF CONTROLLING CONTAMINATION OF A SURFACE

The invention relates to a method of controlling contamination of a surface. The method finds particular use in removing tin from a surface located in an evacuated chamber.

Photolithography is an important process step in semiconductor device fabrication. In overview, in photolithography a circuit design is transferred to a wafer through a pattern imaged on to a photoresist layer deposited on the wafer surface. The wafer then undergoes various etch and deposition processes before a new design is transferred to the wafer surface. This cyclical process continues, building up the multiple layers of the semiconductor device.

In lithographic processes used in the manufacture of semiconductor devices, it is advantageous to use radiation of very short wavelength in order to improve optical resolution so that very small features in the device may be accurately reproduced. In the prior art, monochromatic visible light of various wavelengths have been used, and more recently radiation in the deep ultra violet (DUV) range has been used, including radiation at 248 nm, 193 nm and 157 nm. In order to further improve optical resolution, it has also been proposed to use radiation in the extreme ultra violet (EUV) range, including radiation at 13.5 nm.

EUV radiation has poor transmissibility through all materials and gases at atmospheric pressures, and therefore much of the mechanical, electrical and optical equipment located in the lithography tool must be operated in a high-purity vacuum environment. The source of EUV radiation is typically housed within a chamber located adjacent the lithography tool. In order to isolate the radiation source from the lithography tool, a thin foil, usually formed from zirconium, nickel or silicon, is often used as a window through which EUV radiation is transmitted into the lithography tool. In addition to separating the tool from the radiation source, the foil can act as a spectral purity filter (SPF) by restricting the bandwidth of frequencies of electromagnetic radiation entering the tool. The source of EUV radiation may be based on excitation of tin, lithium, or xenon. A plasma is generated from the target material either by stimulating the target material by an electrical discharge or by intense laser illumination. Initially, xenon was used as the target material, and many variants of xenon-based EUV sources were produced, but as xenon has an energy conversion efficiency of only around 1 %, extremely high powered lasers or high energy discharges were required to supply sufficient EUV radiation for the lithography tool.

In order to make EUV lithography systems more realisable, tin, having an energy conversion efficiency of at least double that of xenon, may be used as the target material of the source of EUV radiation. However, the use of tin as the target material results in the release of tin debris, such as high velocity tin ions, neutral tin atoms and clusters of tin particles, from the source. This debris can collect on the surfaces of EUV source optical elements, such as multi-layer mirrors, located within the chamber for directing the EUV radiation generated by the source towards the SPF. The formation of a tin layer over these optical elements can reduce the effectiveness of these elements, leading to reduced illumination and consequent loss of tool productivity. For example a layer of tin having a thickness of the order of 1 nm can reduce the reflectivity of a multi-layer mirror by around 10%. Due to the high cost of these optical elements, it is always undesirable to replace them, and in many cases it is completely impractical.

One known method of removing tin deposits from the surfaces of EUV source optical elements is described in WO2006/020080. In this method, tin deposits are removed by reaction with gaseous halides such as HBr, HCI, CI₂ or Br₂ to create volatile tin (IV) halides. These tin halides can then be pumped away from the optical elements by the vacuum pumping system used to evacuate the chamber. A problem associated with the use of gaseous species such as HCI and Br₂ is that these species are highly corrosive, and will attack metallic surfaces within the chamber and the pumping system, in particular the surfaces of aluminium components of the pumping system, resulting in premature failures. This problem is made significantly worse if water vapour is present, as this can convert metal halides into acidic mixtures which then further attack the metal components of the chamber and the pumping system.

The present invention provides a method of removing tin from a surface located within an evacuated chamber, the method comprising supplying to the surface an organic halide for reacting with tin to form an organo-tin halide.

The present invention thus avoids the introduction of corrosive halides or halogen compounds within the evacuated chamber or the generation of such compounds within the chamber or within a pumping system used to evacuate the chamber, by supplying to the surface an organic halide having the general formula RX, where R represents an organic, for example an alkyl, radical and X represents a halogen. The reaction between tin and the organic halide can be written as:-

2RX + Sn → R₂SnX₂

The reaction can also produce, depending on the conditions at the surface, tri-alkyl tin mono-halide and traces of alkyl tin tri-halide when an alkyl halide is supplied to the surface. All of these three reaction products are very volatile and can be readily removed from the chamber by the pumping system without corroding the metallic surfaces of the chamber or the metallic surfaces of the pumping system.

The invention is particularly suitable for removing tin from the surface of an extreme ultra violet (EUV) source optical element, such as a multi-layer mirror, a grazing incidence mirror or a spectral purity filter, located within an evacuated chamber of an apparatus for generating EUV radiation, and so the present invention also provides a method of in situ removal of tin from a surface of an extreme ultra violet (EUV) source optical element located within an evacuated chamber, the method comprising the step of supplying to the surface an organic halide for reacting with tin to form an organo-tin halide. The reaction between the organic halide and the tin deposits can proceed at temperatures in the range of 150 to 200⁰C. Such temperatures may be routinely generated within EUV source optical elements, and so this heating may be used to drive the reaction when the apparatus has been turned off. Consequently, the organic halide may be supplied to the surface subsequent to the generation of EUV radiation by the EUV source. As the EUV radiation generated by the EUV source may activate the reaction between the tin deposits and the organic halide, the organic halide may alternatively, or additionally, be supplied to the surface during the generation of EUV radiation by the EUV source.

The reaction may be photo-activated using an ultra violet (UV) light source, for example a mercury lamp, or from UV radiation generated by electrodes of the EUV source, or the reaction may be electro-activated by electrons from an electron gun. The use of an electron gun may be particularly, but not exclusively, suitable for activating the surface of a spectral purity filter (SPF) to promote the reaction of the organic halide with tin deposits located on the surface thereof.

The organic halide is preferably supplied to the evacuated chamber entrained within a carrier gas, such as nitrogen or argon. A gaseous organic halide is preferably supplied to a stream of carrier gas through a flow restricted orifice, whilst a vaporised liquid organic halide is preferably supplied to the carrier gas stream through a diffusion tube. Both of these supply techniques can enable a known amount of gaseous or vaporised organic halide to be mixed with a carrier gas without the problems associated with direct injection or control via conventional mass flow controllers.

A gas stream drawn from the evacuated chamber by the pumping system is preferably treated by an abatement system to remove any organo-tin halides therefrom.

The present invention further provides apparatus for generating extreme ultra violet (EUV) radiation, the apparatus comprising a tin EUV source, a chamber housing at least one optical element for directing EUV radiation generated by the tin EUV source towards a radiation outlet from the chamber, at least one vacuum pump for evacuating the chamber, and means for supplying to the chamber an organic halide for reacting with tin deposited on said at least one optical element to form an organo-tin halide.

Features described above in relation to method aspects of the invention are equally applicable to apparatus aspects, and vice versa.

By way of example, an embodiment of the invention will now be further described with reference to the accompanying figure, which illustrates schematically an example of an apparatus for generating extreme ultra violet (EUV) radiation. The apparatus comprises a chamber 10 containing or interfacing with a source 12 of EUV radiation. The EUV source 12 may be a discharge plasma source or a laser- produced plasma source. In a discharge plasma source, a discharge is created in a medium between two electrodes, and a plasma created from the discharge emits EUV radiation. In a laser-produced plasma source, a target is converted to a plasma by an intense laser beam focused on the target. In this apparatus, the medium used for a discharge plasma source or as a target for a laser-produced plasma source is tin, which radiates EUV radiation at a wavelength of around 13.5 nm and with a higher energy conversion efficiency than xenon.

EUV radiation generated in chamber 10 is supplied to a chamber (not shown) of a lithography tool optically linked or connected to chamber 10 via, for example, one or more windows 14 formed in the walls of the chamber 10. This chamber houses a lithography tool which projects a beam of EUV radiation beam on to a mask or reticle for the selective illumination of a photoresist on the surface of a substrate, such as a semiconductor wafer. The window 14 may be provided by a spectral purity filter (SPF) comprising a very thin foil, typically formed from zirconium, nickel or silicon, for transmitting EUV radiation into the lithography tool chamber whilst preventing contaminants from passing into the lithography tool chamber from the chamber 10. In order to direct EUV radiation generated by the EUV source 12 towards the SPF 14, the chamber 10 houses one or more EUV source optical elements 16, which in this embodiment are provided by at least one multi-layer mirror (MLM). Each MLM comprise a plurality of layers, each layer comprising, from the bottom a first layer of molybdenum and a second layer of silicon. A metallic layer, typically formed from ruthenium, may be formed on the upper surface of each MLM to improve the oxidation resistance of the MLM whilst reflecting substantially all of the EUV radiation incident thereon.

Due to the poor transmissibility of EUV radiation through most gases, a vacuum pumping system comprising vacuum pumps 18, 20 is provided for generating a vacuum within the chamber 10. In view of the complex variety of gases and contaminants which may be present in the chamber 10, in this embodiment the pumping system includes transfer pumps 18, 20, such as a turbomolecular pump, which may be backed by a roughing pump (not shown).

The tin EUV source 12 can also be a source of tin debris during the generation of EUV radiation. High velocity tin ions, neutral tin atoms and clusters of tin particles emitted from the EUV source 12 can collect on the surfaces of the SPF 14 and the EUV source optical elements 16 to form a layer of tin over these surfaces. These tin layers can reduce the effectiveness of such optical elements, leading to reduced illumination of a wafer located in the lithography tool chamber and consequent loss of tool productivity.

In view of this, the apparatus is provided with a system for supplying to the surfaces of the SPF 14 and the optical elements 16 an organic halide, having the general formula RX, where R is an organic radical and X is a halogen, for reacting with tin located on these surfaces to form an organo tin-halide. The organic radical may be an alkyl radical, such as a methyl, ethyl, propyl, butyl, or phenyl radical, or an isomer thereof, and the halogen may be one of chlorine, bromine and iodine. One example is C₂H₅I, which reacts with tin to form (C₂Hs)₂SnI₂. The reaction can also produce, depending on the conditions at the surfaces of the optical elements 14, 16, tri-alkyl tin mono-halide and traces of alkyl tin tri-halide. All of these reaction products are very volatile, and so can be readily pumped away by the vacuum pumps 18, 20. However, as these reaction products each have some degree of toxicity, an abatement device 22 is provided for receiving from conduit 24 a gas stream exhaust from the vacuum pumps 18, 20 and for removing organo tin-halides from the gas stream. An example of a suitable abatement device 22 is a gas reactor column (GRC) produced by BOC Edwards.

The organic halide enters the chamber 10 through chamber inlet 26. The organic halide is preferably supplied to the evacuated chamber entrained within a carrier gas, such as nitrogen or argon, and so in this example the system for supplying organic halide to the chamber 10 comprises a source 28 of carrier gas, in this case nitrogen, and a mass flow controller 30 for controlling the rate at which the carrier gas is supplied to the chamber 10. If a gaseous organic halide is used, the organic halide enters the stream of carrier gas output from the mass flow controller 30 through a flow restricted orifice, whereas if a low boiling point liquid organic halide is used, the organic halide is vaporised and enters the carrier gas stream through a diffusion tube. A second mass flow controller 32 may be provided for controlling the rate at which the organic halide is supplied to the carrier gas stream.

A switching valve 34 may be located in a conduit 36 for supplying the stream of organic halide and carrier gas to the chamber inlet 26. This can enable the stream to be selectively diverted along a chamber by-pass conduit 38 to the abatement device 22 when the supply of organic halide to the chamber 10 is not required, for example when no tin debris is being generated from the EUV source 12, and/or when the conditions in the chamber 10 are not favourable for a reaction between tin and the organic halide. The reaction between the organic halide and the tin deposits can proceed at temperatures in the range of 150 to 200⁰C, and such temperatures may be routinely generated within optical elements 16 during use of the apparatus. Consequently, the valve 34 may be switched to supply the organic halide to the chamber 10 following the generation of EUV radiation by the EUV source 12. Furthermore, as the EUV radiation generated by the EUV source 12 may activate the reaction between the tin deposits and the organic halide, the valve 34 may be switched to supply the organic halide to the chamber 10 during the generation of EUV radiation by the EUV source 12. Temperatures in the range of 150 to 200⁰C may also be generated at the surfaces of the SPF 14 and optical elements 16 photolytically by providing an ultra violet (UV) light source, for example a mercury lamp, within the chamber 10, or from UV radiation generated by electrodes of the EUV source, and from an Such temperatures may also be generated at these surfaces using an electron gun or through use of one or more heating elements.

Claims

1. A method of removing tin from a surface located within an evacuated chamber, the method comprising supplying to the surface an organic halide for reacting with tin to form an organo-tin halide.

2. A method according to Claim 1 , wherein the surface is a surface of an extreme ultra violet (EUV) source optical element located within a chamber of an apparatus for generating EUV radiation.

3. A method of in situ removal of tin from a surface of an extreme ultra violet (EUV) source optical element located within an evacuated chamber, the method comprising the step of supplying to the surface an organic halide for reacting with tin to form an organo-tin halide.

4. A method according to Claim 2 or Claim 3, wherein the optical element comprises one of a multi-layer mirror, a grazing incidence mirror and a spectral purity filter.

5. A method according to any of Claims 2 to 4, wherein the organic halide is supplied to the surface during the generation of EUV radiation.

6. A method according to any of Claims 2 to 5, wherein the organic halide is supplied to the surface subsequent to the generation of EUV radiation.

7. A method according to any preceding claim, wherein the surface is heated to a temperature of at least 150°C.

8. A method according to Claim 7, wherein the surface is heated using a heating element.

9. A method according to any of Claims 1 to 7, wherein the reaction between the tin and the organic halide is photo-activated.

10. A method according to any of Claims 1 to 7, wherein the reaction between the tin and the organic halide is electro-activated.

11. A method according to any preceding claim, wherein the organic halide is supplied to the evacuated chamber entrained within a carrier gas.

12. A method according to Claim 11 , wherein gaseous organic halide is supplied to the carrier gas through a flow restricted orifice.

13. A method according to Claim 11 , wherein vaporised liquid organic halide is supplied to the carrier gas through a diffusion tube.

14. A method according to any preceding claim, wherein a gas stream drawn from the chamber is treated to remove the organo-tin halide therefrom.

15. Apparatus for generating extreme ultra violet (EUV) radiation, the apparatus comprising a tin EUV source, a chamber housing at least one optical element for directing EUV radiation generated by the tin EUV source towards a radiation outlet from the chamber, a vacuum pump for evacuating the chamber, and means for supplying to the chamber an organic halide for reacting with tin deposited on said at least one optical element to form an organo-tin halide.

16. Apparatus according to Claim 15, wherein the optical element comprises one of a multi-layer mirror, a grazing incidence mirror and a spectral purity filter.

17. Apparatus according to Claim 15 or Claim 16, wherein the supply means is configured to supply organic halide to the surface during the generation of EUV radiation.

18. Apparatus according to any of Claims 15 to 17, wherein the supply means is configured to supply organic halide to the surface subsequent to the generation of EUV radiation.

19. Apparatus according to any of Claims 15 to 18, comprising means for heating the surface to a temperature of at least 150⁰C.

20. Apparatus according to any of Claims 15 to 19, comprising means for photo-activating the reaction between the tin and the organic halide.

21. Apparatus according to any of Claims 15 to 19, comprising means for electro-activating the reaction between the tin and the organic halide.

22. Apparatus according to any of Claims 15 to 21 , wherein the supply means is configured to supply organic halide to the evacuated chamber entrained within a carrier gas.

23. Apparatus according to Claim 22, wherein the supply means comprises a flow restricted orifice through which gaseous organic halide is supplied to the carrier gas.

24. Apparatus according to Claim 22, wherein the supply means comprises a diffusion tube through which vaporised liquid organic halide is supplied to the carrier gas.

25. Apparatus according to any of Claims 15 to 24, comprising an abatement device from removing organo-tin halide from a gas stream drawn from the chamber by the vacuum pump.