WO2007107783A1

WO2007107783A1 - Spectral filter repair

Info

Publication number: WO2007107783A1
Application number: PCT/GB2007/050108
Authority: WO
Inventors: Robert Bruce Grant
Original assignee: Edwards Limited
Priority date: 2006-03-23
Filing date: 2007-03-07
Publication date: 2007-09-27
Also published as: TW200741375A; GB0605725D0

Abstract

A method is described for repairing a spectral filter (18) through which electromagnetic radiation (14) passes from a chamber (10) housing a radiation source (12) to a lithography tool, in which a source of carbon for forming carbonaceous material on the filter is periodically supplied to the chamber to at least reduce the size of holes grown in the filter during use of the tool. In a preferred embodiment, the method comprises the steps of detecting the leakage of gas from the chamber through the holes, and, depending on the amount of gas passing through the filter, supplying the carbon source to the chamber.

Description

SPECTRAL FILTER REPAIR

This invention relates to the repair of a spectral filter through which EUV or other electromagnetic radiation is transmitted from a chamber housing the radiation source.

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 ultra clean 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. 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 as a source of high energy particles. These particles can impact on the surface of the SPF, and cause atoms to be sputtered from the SPF. SPFs tend to have a thickness in the range from 200 to 1000 nm, and so the impact of the charged particles on the surface of the SPF can lead to the growth of holes in the SPF, both by the enlargement of existing holes in the SPF and by the formation of new holes. This can result in contamination of the tool by the passage of gas from the chamber through the SPF. Once the holes have grown beyond a critical value, the SPF will need to be replaced, resulting in costly process downtime and potential re-qualification after the replacement of the SPF.

In a first aspect, the present invention provides a method of repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the method comprising the step of periodically supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of holes grown in the filter during generation of the radiation.

Electromagnetic radiation, such as EUV radiation or X-rays, can promote the deposition of carbonaceous deposits on the filter by stimulating the emission of secondary electrons from the surface of the filter, which interact with the carbon source to form the carbonaceous deposits. This carbonaceous material can block, or at least reduce the size of, holes grown in the filter during generation of the radiation through impact with high energy particles, such as photons and ions. As mentioned above, the growth of the holes in the filter may be caused by the generation of holes in the filter, and/or by the enlargement of holes already present in the filter. By blocking the holes in this manner whilst the filter is in situ, contamination of a lithography tool or other device that receives the radiation passing through the filter can be reduced.

The supply of the carbon source to the chamber may be initiated on a time basis. Alternatively, in a preferred embodiment the leakage of gas from the chamber through the holes is detected, and the carbon source is supplied to the chamber depending on the amount of gas passing through the filter. Therefore, in a second aspect the present invention provides a method of repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the method comprising the steps of detecting the leakage of gas from the chamber through holes grown in the filter during generation of the radiation, and, depending on the amount of gas passing through the filter, supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of the holes.

The supply of carbon source to the chamber is preferably initiated when the amount of gas passing through the filter is at or above a first value, and is preferably discontinued when the amount of gas passing through the filter is at or below a second value lower than the first, that is, when contamination of a tool that receives the radiation from the chamber is at or below an acceptable level. The leakage of gas through the filter can be detected by a sensor, for example a mass spectrometer or ionisation gauge, for detecting the presence of gas within the tool.

Controlling the partial pressure of the carbon source within the chamber 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.

In the preferred embodiment, the radiation source is a plasma generated within the chamber. A number of different materials may be used as the source of the plasma, for example, one of lithium, tin and xenon. Alternatively, the source may generate radiation through impact of electrons on a metal or semiconductor surface. For example, X-rays may be generated through the impact of electrons on an aluminium or magnesium surface, or EUV may be generated by impact of electrons on a silicon of beryllium surface.

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 third aspect, the present invention provides apparatus for repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the apparatus comprising means for supplying to the chamber a source of carbon for forming carbonaceous material on the filter, and means for controlling the duration of the carbon source supply to the chamber so that the carbonaceous material blocks holes grown in the filter during generation of the radiation.

In a fourth aspect the present invention provides apparatus for repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the apparatus comprising means for detecting the leakage of gas from the chamber through holes grown in the filter during generation of the radiation, means for supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of the holes, and means for controlling the supply of the source of carbon to the chamber depending on the amount of gas passing through the filter. In a fifth aspect, the present invention provides apparatus for generating extreme ultra violet (EUV) radiation, the apparatus comprising a chamber having a spectral filter through which EUV radiation is output from the chamber, the chamber housing a source of EUV radiation and means for focussing EUV radiation generated by the source towards the filter, and means for periodically supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of holes grown in the filter by the impact of particles generated by the source.

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. The invention is also applicable to apparatus generating other forms of electromagnetic radiation. For example, the source 12 may be a source of X-rays, in which X-rays are produced by the impact of electrons on a metal surface, for example Al or Mg.

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 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.

The chamber 16 houses a lithography tool which projects a beam of EUV radiation 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 SPF 18, the chamber 10 houses a plurality of reflective surfaces 20. For a laser-produced plasma, these reflective surfaces may be provided by multi-layer mirrors (MLMs), providing both collection and focussing optics for the radiation. The MLMs comprise a plurality of layers, each layer comprising, from the bottom a first layer of molybdenum and a second layer of silicon. An interface barrier layer, formed for example from boron carbide, may be provided between each molybdenum and silicon layer. A metallic layer, typically formed from ruthenium, may be formed on the upper surface of each MLM to improve the oxidation resistance of the MLMs whilst transmitting substantially all of the EUV radiation incident thereon. For a discharge-produced plasma, collection optics for the EUV radiation are provided by grazing incidence mirrors instead of MLMs.

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 high energy particles, such as ions and photons. 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 surface of SPF 18 located within the chamber 10, and cause atoms to be sputtered from those surfaces. The impact of the charged particles on the surface of the SPF 18 can lead to the growth of holes in the SPF 18, both by the enlargement of existing holes in the SPF 18 and by the formation of new holes. If allowed to continue unabated, the growth of holes in the SPF 18 can result in contamination of the chamber 16 by the passage of gas from the chamber 10 through the SPF 18.

In the presence of EUV or X-ray 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 the reflective surfaces 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_xHy-! + H(a) + e^" → C_xH_y-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 reflective surfaces 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 reflective surfaces 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. Furthermore, the carbon layer formed on the SPF 18 can at least partially block the holes grown in the SPF 18 during the generation of the EUV radiation.

In view of this, a carbon source for the controlled deposition of carbonaceous deposits on the surface of the SPF 18 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 adding a pre-defined partial pressure of the carbon source to the chamber 10, a controlled amount of carbonaceous material can be deposited on the surface of the SPF 18 such that the holes formed in the SPF 18 are at least partially blocked without significant reduction in the transmissivity of the SPF 18 for the EUV radiation.

The addition of the carbon source to the chamber 10 may be conducted on a time basis, for example after a chosen number of hours of generation of EUV radiation by the source 12. Alternatively, and as illustrated in the drawing, a sensor 24 may be provided for monitoring the amount of gas entering the chamber 16 from the chamber 10. The sensor 24 outputs a signal indicative of the amount of gas passing through the SPF 18 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. For example, when the signal received from the sensor 24 indicates that the amount of gas passing through the SPF 18 is at or above a first level, the controller 26 may control the mass flow controller 28 to supply the carbon source to the chamber 10 until the signal received from the sensor 24 indicates that the amount of gas passing through the SPF 18 has fallen to, or below, a second level lower than the first. A second sensor 32 may be provided in the chamber 10 for monitoring the partial pressure of the carbon source, with the controller 26 preferably being configured to control the supply rate of the carbon source to the chamber 10 in dependence on the output from this second sensor 32.

Claims

1 . A method of repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the method comprising the step of periodically supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of holes grown in the filter during generation of the radiation.

2. A method according to Claim 1 , wherein the leakage of gas from the chamber through the holes is detected, and the carbon source is supplied to the chamber depending on the amount of gas passing through the filter.

3. A method of repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the method comprising the steps of detecting the leakage of gas from the chamber through holes grown in the filter during generation of the radiation, and, depending on the amount of gas passing through the filter, supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of the holes.

4. A method according to Claim 2 or Claim 3, wherein the supply of carbon source to the chamber is initiated when the amount of gas passing through the filter is at or above a first value, and is discontinued when the amount of gas passing through the filter is at or below a second value lower than the first.

5. A method according to any of Claims 2 to 4, wherein the leakage of gas through the filter is detected by a sensor for detecting the presence of gas within a tool for receiving the electromagnetic radiation passing through the filter.

6. A method according to any preceding claim, wherein the rate of formation of carbonaceous material on the filter is controlled by controlling the partial pressure of the carbon source within the chamber.

7. A method according to Claim 6, wherein the partial pressure of the carbon source is controlled by controlling the rate of supply of the carbon source to the chamber.

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

9. A method according to Claim 8, 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.

10. A method according to Claim 9, wherein the oxygenates comprise alcohols, esters and ethers.

11. A method according to Claim 9 or Claim 10, wherein the nitrogen- containing compounds comprise amines, pyrrole and its derivatives, and pyridine and its derivatives.

12. A method according to any of Claims 9 to 11 , wherein the halogen- containing compounds comprise saturated aryl hydrides, unsaturated aryl hydrides, saturated alkyl hydrides, and unsaturated alkyl hydrides.

13. A method according to any preceding claim, wherein deposition of carbonaceous material on the filter is promoted by the electromagnetic radiation generated by the source.

14. A method according to any preceding claim, wherein the electromagnetic radiation is one of X-rays and extreme ultra violet radiation.

15. Apparatus for repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the apparatus comprising means for supplying to the chamber a source of carbon for forming carbonaceous material on the filter, and means for controlling the duration of the carbon source supply to the chamber so that the carbonaceous material blocks holes grown in the filter during generation of the radiation.

16. Apparatus according to Claim 15, comprising means for detecting leakage of gas from the chamber through the holes, and wherein the control means is configured to initiate the carbon source supply to the chamber depending on the amount of gas passing through the filter.

17. Apparatus for repairing a spectral filter through which electromagnetic radiation passes from a chamber housing a radiation source, the apparatus comprising means for detecting the leakage of gas from the chamber through holes grown in the filter during generation of the radiation, means for supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of the holes, and means for controlling the supply of the source of carbon to the chamber depending on the amount of gas passing through the filter.

18. Apparatus according to Claim 16 or Claim 17, wherein the control means is configured to initiate supply of carbon source to the chamber when the amount of gas passing through the filter is at or above a first value, and to discontinue supply of carbon source to the chamber when the amount of gas passing through the filter is at or below a second value lower than the first.

19. Apparatus according to any of Claims 16 to 18, wherein the detecting means is configured to detect the presence of gas within a tool for receiving the electromagnetic radiation passing through the filter.

20. Apparatus according to any of Claims 15 to 19, wherein the control means is configured to control the partial pressure of the carbon source within the chamber.

21. Apparatus according to Claim 20, wherein the control means is configured to control the rate of supply of the carbon source to the chamber.

22. Apparatus according to any of Claims 15 to 21 , wherein the carbon source is a source of organic molecules.

23. Apparatus according to Claim 22, 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.

24. Apparatus according to Claim 23, wherein the oxygenates comprise alcohols, esters and ethers.

25. Apparatus according to Claim 23 or Claim 24, wherein the nitrogen- containing compounds comprise amines, pyrrole and its derivatives, and pyridine and its derivatives.

26. Apparatus according to any of Claims 23 to 25, wherein the halogen- containing compounds comprise saturated aryl hydrides, unsaturated aryl hydrides, saturated alkyl hydrides, and unsaturated alkyl hydrides.

27. Apparatus according to any of Claims 15 to 26, wherein the electromagnetic radiation is one of X-rays and extreme ultra violet radiation.

28. Apparatus for generating extreme ultra violet (EUV) radiation, the apparatus comprising a chamber having a spectral filter through which EUV radiation is output from the chamber, the chamber housing a source of EUV radiation and means for focussing EUV radiation generated by the source towards the filter, and means for periodically supplying to the chamber a source of carbon for forming carbonaceous material on the filter to at least reduce the size of holes grown in the filter by the impact of particles generated by the source.