EP1815294A2

EP1815294A2 - Protection of surfaces exposed to charged particles

Info

Publication number: EP1815294A2
Application number: EP05791381A
Authority: EP
Inventors: Robert Bruce Grant
Original assignee: BOC Group Ltd
Current assignee: Edwards Ltd
Priority date: 2004-11-26
Filing date: 2005-10-11
Publication date: 2007-08-08
Also published as: KR20070084558A; CN101061435A; WO2006056730A2; WO2006056730A3; JP2008522399A; GB0426036D0; TW200632571A

Abstract

A method is described of protecting a surface of a mirror (20) located in a chamber (10) exposed to extreme ultra violet (EUV) radiation (14) . The EUV radiation is generated from a plasma (12) , which emits both EUV radiation and electrically charged particles. Organic molecules are supplied to the chamber, which interact with the EUV radiation to form a coating of carbonaceous deposits on the mirror surface. The charged particles emitted from the plasma impact the deposits, causing the deposits to be sputtered from the mirror surface. By controlling at least one of the rate of deposition of deposits on the mirror surface and the rate of removal of the deposits from the mirror surface, the thickness of the coating can be actively controlled both to prevent impact of the charged particles directly on to the mirror surface and to minimise the loss of reflectivity of the mirror surface due to the formation of the coating. The method is also suitable for protecting the surface of a window used to transmit EUV radiation from the chamber.

Description

PROTECTION OF SURFACES EXPOSED TO CHARGED PARTICLES

This invention relates to the protection of surfaces exposed to highly charged particles. The invention finds particular use in the protection of multi-layer mirrors used in optical systems for reflecting extreme ultra violet (EUV) radiation, and in the protection of surfaces through which EUV radiation is transmitted into a photolithography 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.

The use of EUV radiation for lithography creates many new difficulties, both for the optics in the lithography tool, and also in the apparatus for supplying EUV radiation to the tool.

One problem is that EUV radiation has poor transmissibility through most gases at atmospheric pressures, and therefore much of the mechanical, electrical and optical equipment involved in the lithography process must be operated in a high- purity vacuum environment. A further problem is that lens materials used for projection and focussing of radiation in DUV lithography, such as calcium fluoride, are not suitable for transmission of EUV radiation, and it is usually necessary to use reflective optical devices (mirrors) in place of transmissive optical devices (lenses). These mirrors tend to be formed from alternate layers of molybdenum and silicon, each layer typically 5 to 10 nm thick, and generally terminate in a layer of silicon or a layer of ruthenium or other metallic species.

The source of EUV radiation is typically housed within a chamber located adjacent the lithography tool. In order to isolate the source from the ultra clean lithography tool, a spectral purity filter (SPF) is often used as a window through which EUV radiation is transmitted into the lithography tool. An SPF typically comprises a very thin foil, usually formed from zirconium, nickel or silicon.

The source of EUV radiation may be based on excitation of tin, lithium, or xenon. For example, when xenon is used in the EUV source, a xenon plasma is generated, either by stimulating xenon by an electrostatic discharge or by intense laser illumination. Electronic transitions of highly charged xenon species Xe⁺¹⁰ within the plasma to Xe⁺¹¹ generate EUV radiation. Consequently, the source of EUV radiation also acts a source of highly charged particles. These particles can impact on the surfaces of multi-layer mirrors and the SPF located within the chamber, and cause atoms to be sputtered from those surfaces. This can reduce the reflectivity of the mirrors, and thus reduce the intensity of the EUV radiation transmitted to the lithography tool. As the intensity of the EUV radiation output from the chamber decreases, this can lead to variations in the quality of the patterns formed on the wafers subject to lithography using the EUV radiation. Due to the high cost of these components, it is always undesirable to replace them, and in many cases it is completely impractical. Furthermore, the generation of "holes" in the SPF can result in contamination of the lithography tool.

In accordance with one aspect of the present invention, there is provided a method of protecting a surface from damage due to impact by charged particles, the method comprising the steps, during the exposure of the surface to the charged particles, of supplying to the surface a source of carbon for forming a coating of carbonaceous deposits on the surface, and controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to actively control the thickness of the coating.

By controlling at least one of the rate of deposition of deposits and the rate of the subsequent removal of the deposits from the surface (through impact of charged particles on the deposits), the thickness of the coating can be actively controlled at or around a predetermined thickness which both prevents impact of the charged particles directly on to the surface and minimises the loss of reflectivity or transmissivity of the surface (due to the formation of the coating). Furthermore, deliberately supplying a carbon source can overwhelm the effects of background carbon containing impurities inevitably present in the mirror environment. An additional benefit of this approach is that it involves high rates of turnover of the carbonaceous species, thus maintaining the latter in a more reactive, easily removed chemical state. When this is not the case, aging of the carbonaceous deposits leads to its graphitisation, resulting in an optically deleterious and very stable surface coating that cannot be removed.

Controlling the partial pressure of the carbon source at the surface can provide one mechanism for controlling the deposition rate. By adjusting the partial pressure of the carbon source, the steady state coverage of carbonaceous deposits can be regulated at an acceptable level. The partial pressure can be conveniently controlled by controlling the rate of supply of the carbon source to the surface. By monitoring the thickness of the coating using a suitable sensor sensitive to the build-up of a thin film, such as a quartz crystal oscillator or a surface acoustic wave device, a signal can be supplied to a mass flow controller for adjusting the rate at which the carbon source is supplied to the surface.

The impact rate may be controlled by controlling the rate of supply of charged particles to the coating. For example, some of the charged particles may be selectively neutralised, for example, by passing the charged particles through a gas curtain prior to impact with the deposits. In such a case, a fixed partial pressure of carbon source may be provided in combination with a variable pressure gas curtain to enable the thickness of the coating to be controlled. The pressure of the gas curtain can be controlled in a similar manner to the partial pressure of the carbon source. Optionally, a buffer gas can be supplied to maintain a constant pressure in the vicinity of the surface. The maximum allowable total pressure of the mixture of buffer gas and carbon source depends on the absorption cross-section for EUV radiation of the gaseous species and will typically be less than 0.1 mbar.

The charged particles are preferably emitted from a source thereof located in a chamber housing the surface. In the preferred embodiment, the source of charged particles is a plasma generated within the chamber for also emitting electromagnetic radiation, preferably EUV radiation, which promotes the deposition of carbonaceous deposits on the surface by stimulating the emission of secondary electrons from the surface which interact with the carbon source to form the carbonaceous deposits. A number of different materials may be used as the source of the plasma, for example, one of lithium, tin and xenon.

The surface may be a surface of a window for emitting EUV radiation from the chamber, typically a foil formed from zirconium, nickel or silicon, for example. Alternatively, the surface may be a reflective surface, such as a surface of a multi¬ layer mirror. By maintaining a substantially constant thickness on the surface of such components, the transmissivity of the window and the reflectivity of the mirror can be maintained at substantially constant levels, thereby maintaining a substantially constant intensity of the EUV radiation emitted from the chamber and thus providing a stable EUV radiation source for a lithography tool.

The carbon source is preferably a source of organic molecules. The choice of the carbon source is determined by a number of criteria, including the probability and rate of dissociative chemisorption on the surface, adequate cross-section for activation by secondary electrons, stability against polymerisation, and gas phase adsorption cross-section to EUV radiation. Examples include carbon monoxide, alkanes, alkynes, alkenes, aryl oxygenates, aromatics, nitrogen-containing species and halogen-containing species.

In a second aspect, the present invention provides a method of protecting a surface located within a chamber within which extreme ultra violet (EUV) radiation and charged particles are generated, the method comprising the steps of supplying to the chamber a source of carbon for forming a coating of carbonaceous deposits on the surface in the presence of EUV radiation, impacting the coating with charged particles to remove deposits therefrom, and controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to maintain the thickness of the coating at or around a predetermined value.

In a third aspect, the present invention provides apparatus for protecting a surface from damage due to impact by charged particles, the apparatus comprising means for supplying to the surface a source of carbon for forming a coating of carbonaceous deposits on the surface during the exposure of the surface to the charged particles, and means for controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to actively control the thickness of the coating.

In a fourth aspect, the present invention provides apparatus for generating extreme ultra violet (EUV) radiation, the apparatus comprising a chamber having a window through which a EUV radiation is output from the chamber, a source of EUV radiation and charged particles located within the chamber, at least one reflective surface located within the chamber for focussing EUV radiation from the source towards the window, means for supplying to the chamber a source of carbon for forming a coating of carbonaceous deposits on said at least one reflective surface in the presence of EUV radiation, and means for controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to maintain the thickness of the coating at or around a predetermined value.

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 following figure, which illustrates schematically an example of an apparatus for generating extreme ultra violet (EUV) radiation. The apparatus comprises a chamber 10 containing a source 12 of EUV radiation. The 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. A suitable medium for a discharge plasma source and for a target for a laser-produced plasma source is xenon, as xenon plasma radiates EUV radiation at a wavelength of 13.5 nm. However, other materials, such as lithium and tin, may be used as the target material, and so the present invention is not limited to the particular material or mechanism used to generate EUV radiation.

EUV radiation, indicated at 14, generated in chamber 10 is supplied to another chamber 16 optically linked or connected to chamber 10 via, for example, one or more windows 18 formed in the walls of the chambers 10, 16. The chamber 16 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. In order to direct EUV radiation generated by the source 12 towards the window 18, the chamber 10 houses a plurality of reflective surfaces provided by multi-layer mirrors (MLMs) 20. The MLMs 20 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 20 to improve the oxidation resistance of the MLMs 20 whilst transmitting substantially all of the EUV radiation incident thereon. The window 18 is 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 chamber 16 whilst preventing contaminants from entering the lithography tool chamber 16 from the chamber 10.

Due to the poor transmissibility of EUV radiation through most gases, a vacuum pumping system (not shown) is provided for generating a vacuum within the chambers 10, 16. In view of the complex variety of gases and contaminants which may be present in the chambers, the pumping system may include, for each chamber, both a cryogenic vacuum pump and a transfer pump, such as a turbomolecular pump, backed by a roughing pump.

The source 12 of EUV radiation can also be a source of charged particles. For example, when a xenon plasma is used as the EUV source, Xe⁺¹⁰ ions can be emitted from the source. These ions can impact on the surfaces of SPF 18 and MLMs 20 located within the chamber 10, and cause atoms to be sputtered from those surfaces. If sputtering is allowed to proceed, this can reduce the reflectivity of the MLMs 20, and thus reduce the intensity of the EUV radiationJxansmitted to the chamber 16, and can generate "holes" in the SPF 18, resulting in contamination of the chamber 16.

In the presence of EUV radiation, secondary electrons are released from within the surfaces of the SPF 18 and MLMs 20, which electrons can interact with species present on the surfaces. In particular, cracking of adsorbed hydrocarbon contaminants can form graphitic-type carbon layers adhering to the SPF 18 and MLMs 20. For example, a hydrocarbon having the general formula C_xH_y dissociates in the presence of EUV radiation as per equation (1) below:

C_xHy + e → C_xH_y-I + H(a) + e^" → C_xHy_-2 + H(a) + e → →xC + yH(a) (1 ) with deposition (adsorption) of x amount of C on the surface of the SPF 18 and MLMs 20.

A layer of carbon on the surfaces of the SPF 18 and MLMs 20 clearly would normally be undesirable; the presence of a carbon coating on the surface of the SPF 18 would reduce its transmissivity, whilst a carbon coating of the surfaces of the MLMs 20 would reduce their reflectivity. However, in the presence of highly charged ions such as Xe⁺¹⁰, the formation of a carbon coating on the surfaces of these components of the apparatus can serve to protect the surfaces from sputtering due to the impact of these ions with the surfaces. In view of this, a carbon source for the controlled deposition of carbonaceous deposits on the surfaces of the SPF 18 and MLMs 20 under EUV radiation is introduced into the chamber 10 from a supply 22. Deliberately supplying a carbon source can overwhelm the effects of background carbon-containing impurities inevitably present in the chamber 10.

The carbon source is preferably selected from the group comprising carbon monoxide, alkynes, alkenes, aryl oxygenates, aromatics, nitrogen-containing species and halogen-containing species. Examples of suitable oxygenates are alcohols, esters and ethers. Examples of suitable nitrogen-containing compounds are amines, pyrrole and its derivatives, and pyridine and its derivatives. Examples of suitable halogen-containing compounds are saturated aryl hydrides, unsaturated aryl hydrides, saturated alkyl hydrides, and unsaturated alkyl hydrides. In one preferred example, the carbon source is ethyne (C₂H₂).

By controlling the partial pressure of the carbon source within the chamber, and thus the rate of carbonaceous deposition on the surfaces of the SPF 18 and MLMs 20, an equilibrium can be reached between carbonaceous deposition and the removal of the carbonaceous deposits due to impact with the ions emitted from the source 12, after which the coating thickness remains substantially constant. The thickness of the coating can be monitored using a sensor 24 sensitive to the build¬ up of a coating on one of its surfaces, such as a quartz crystal oscillator or a surface acoustic wave device, and strategically placed so as to have a similar exposure to the carbon source, EUV radiation and ions as the MLMs 20. The sensor 24 outputs a signal indicative of the thickness of the coating formed thereon to a controller 26, which, in response to the received signal, outputs a control signal to a mass flow controller 28 for controlling the rate of supply of the carbon source from the supply 22 to the chamber 10 through inlet 30. By varying the rate of supply of carbon source to the chamber 10, the partial pressure of the carbon source within the chamber 10 can be carefully controlled, thereby allowing the rate of formation of the coating on the MLMs 20 and SPF 18 to be controlled so as to maintain the thickness of the coating at or around a predetermined value.

As an alternative to, or in addition to, controlling the rate at which the carbonaceous deposits are formed on the surfaces of the SPF 18 and the MLMs 20, the apparatus can be configured to control the rate at which the deposits are removed from these surfaces by the ions emitted from the source 12. For example, a gas can be introduced from a source thereof into the chamber 10 to form a gas curtain between the source 12 and the MLMs 20. For example, the gas supply 22 may be replaced by a source of gas for forming the gas curtain. Some of the ions emitted from the source 12 collide with the gas in the gas curtain and are neutralised, thereby reducing the rate at which ions impact the coating. Whilst a relatively dense gas curtain would be undesirable, due to the absorption by the gas of the EUV radiation emitted from the source, nonetheless a variable, relatively low density gas curtain could be beneficial to maintaining a relatively constant coating thickness. Similar to the control of the rate of supply of the carbon source, the rate of supply of the gas for forming the gas curtain may be controlled by mass flow controller 28 using signals output from the controller 26 in response to the thickness of the coating formed on sensor 24. As opposed to replacing the supply 22 with a source of gas for forming the gas curtain, a separate source of gas for the gas curtain may be provided opposite or adjacent the supply 22 for supplying the gas into the chamber through a separate inlet, with the controller 26 supplying control signals to a separate mass flow controller to control the rate of supply of gas for forming the gas curtain. In summary, a method is described of protecting a surface of a mirror located in a chamber exposed to extreme ultra violet (EUV) radiation. The EUV radiation is generated from a plasma, which emits both EUV radiation and electrically charged particles. Organic molecules are supplied to the chamber, which interact with the EUV radiation to form a coating of carbonaceous deposits on the mirror surface. The charged particles emitted from the plasma impact the deposits, causing the deposits to be sputtered from the mirror surface. By controlling at least one of the rate of deposition of deposits on the mirror surface and the rate of removal of the deposits from the mirror surface, the thickness of the coating can be actively controlled both to prevent impact of the charged particles directly on to the mirror surface and to minimise the loss of reflectivity of the mirror surface due to the formation of the coating. The method is also suitable for protecting the surface of a window used to transmit EUV radiation from the chamber.

Claims

1. A method of protecting a surface from damage due to impact by charged particles, the method comprising the steps, during the exposure of the surface to the charged particles, of supplying to the surface a source of carbon for forming a coating of carbonaceous deposits on the surface, and controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to actively control the thickness of the coating.

2. A method according to Claim 1 , wherein the deposition rate is controlled by controlling the partial pressure of the carbon source at the surface.

3. A method according to Claim 2, wherein the partial pressure of the carbon source is controlled by controlling the rate of supply of the carbon source to the surface. „,,

4. A method according to Claim 2 or Claim 3, wherein the thickness of the coating is monitored and the partial pressure of the carbon source is varied in dependence on the thickness of the coating.

5. A method according to any preceding claim, wherein the carbon source is a source of organic molecules.

6. A method according to Claim 5, wherein the carbon source is selected from the group comprising carbon monoxide, alkanes, alkynes, alkenes, aryl oxygenates, aromatics, nitrogen-containing species and halogen-containing species.

7. A method according to Claim 6, wherein the oxygenates comprise alcohols, esters and ethers.

8. A method according to Claim 6 or Claim 7, wherein the nitrogen- containing compounds comprise amines, pyrrole and its derivatives, and pyridine and its derivatives.

9. A method according to any of Claims 6 to 8, wherein the halogen- containing compounds comprise saturated aryl hydrides, unsaturated aryl hydrides, saturated alkyl hydrides, and unsaturated alkyl hydrides.

10. A method according to any preceding claim, wherein the impact rate is controlled by controlling the rate of supply of charged particles to the coating.

11. A method according to Claim 10, wherein the supply rate of the charged particles is controlled by selectively neutralising some of the charged particles prior to impact with the deposits.

12. A method according to Claim 11 , wherein the charged particles are selectively neutralised by passing the charged particles through a gas prior to impact with the deposits.

13. A method according to any preceding claim, wherein the charged particles are emitted from a source thereof located in a chamber housing the surface.

14. A method according to Claim 13, wherein the source of charged particles is a plasma generated within the chamber.

15. A method according to Claim 13 or Claim 14, wherein electromagnetic radiation is also generated by the source of charged particles for promoting the deposition of carbonaceous deposits on the surface.

16. A method according to Claim 15, wherein the electromagnetic radiation is extreme ultra violet radiation.

17. A method according to any of Claims 13 to 16, wherein the surface is a surface of a window for emitting EUV radiation from the chamber.

18. A method according to any of Claims 1 to 16, wherein the surface is a reflective surface.

19. A method according to Claim 18, wherein the reflective surface is a surface of a multi-layer mirror.

20. A method according to Claim 19, wherein the mirror comprises a plurality of layers, each layer, comprising a first layer of molybdenum and a second layer of silicon.

21. A method of protecting a surface located within a chamber within which extreme ultra violet (EUV) radiation and charged particles are generated, the method comprising the steps of supplying to the chamber a source of carbon for forming a coating of carbonaceous deposits on the surface in the presence of EUV radiation, impacting the coating with charged particles to remove deposits therefrom, and controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to maintain the thickness of the coating at or around a predetermined value.

22. Apparatus for protecting a surface from damage due to impact by charged particles, the apparatus comprising means for supplying to the surface a source of carbon for forming a coating of carbonaceous deposits on the surface during the exposure of the surface to the charged particles, and means for controlling at least one of the rate of deposition of deposits on the surface and the rate of impact of charged particles on the deposits to actively control the thickness of the coating.

23. Apparatus according to Claim 22, wherein the control means comprises means for controlling the partial pressure of the carbon source at the surface.

24. Apparatus according to Claim 23, wherein the partial pressure control means comprises means for controlling the rate of supply of the carbon source to the reflective surface.

25. Apparatus according to Claim 23 or Claim 24, comprising means for monitoring the thickness. of the coating and for outputting a signal to the partial pressure control means indicative of the monitored thickness, the partial pressure control means being configured to adjust the partial pressure of the carbon source at the surface in dependence on the monitored thickness.

26. Apparatus according to any of Claims 22 to 25, wherein the carbon source is a source of organic molecules.

27. Apparatus according to Claim 26, wherein the carbon source is selected from the group comprising carbon monoxide, alkanes, alkynes, alkenes, aryl oxygenates, aromatics, nitrogen-containing species and halogen-containing species.

28. Apparatus according to Claim 27, wherein the oxygenates comprise alcohols, esters and ethers.

29. Apparatus according to Claim 27 or Claim 28, wherein the nitrogen- containing compounds comprise amines, pyrrole and its derivatives, and pyridine and its derivatives.

30. Apparatus according to any of Claims 27 to 29, wherein the halogen- containing compounds comprise saturated aryl hydrides, unsaturated aryl hydrides, saturated alkyl hydrides, and unsaturated alkyl hydrides.

31. Apparatus according to any of Claims 22 to 30, wherein the surface is located in a chamber housing a source of the charged particles.

32. Apparatus according to Claim 31 , wherein the source of charged particles is a plasma.

33. Apparatus according to Claim 31 or Claim 32, wherein the source of charged particles is also a source of electromagnetic radiation for promoting the deposition of carbonaceous deposits on the surface.

34. Apparatus according to Claim 33, wherein the electromagnetic radiation is extreme ultra violet radiation.

35. Apparatus according to any of Claims 22 to 34, wherein the surface is a surface of a multi-layer mirror.

36. Apparatus according to Claim 35, wherein the mirror comprises a plurality of layers, each layer comprising a first layer of molybdenum and a second layer of silicon.

37. Apparatus for generating extreme ultra violet (EUV) radiation, the apparatus comprising a chamber having a window through which a EUV radiation is output from the chamber, a source of EUV radiation and charged particles located within the chamber, at least one reflective surface located within the chamber for focussing EUV radiation from the source towards the window, means for supplying to the chamber a source of carbon for forming a coating of carbonaceous deposits on said at least one reflective surface in the presence of EUV radiation, and means for controlling at least one of the rate of deposition of deposits on the reflective surface and the rate of impact of charged particles on the deposits to maintain the thickness of the coating at or around a predetermined value.