CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 21154221.2 which was filed on 29 January 2021 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to an apparatus for removing a contaminant from an optical element of a lithography apparatus. The present invention has particular, but not exclusive, use in connection with EUV lithographic apparatus and EUV lithographic tools. The present invention also relates to methods for removing a contaminant from an optical element of a lithographic apparatus, lithographic tools, and the use of such apparatuses or methods in a lithographic apparatus or process.

BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A pattern may be imparted to a radiation beam in a lithographic apparatus using a patterning device (e.g. a mask or reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. Contamination on the surface of the patterning device can cause manufacturing defects on the substrate. A membrane assembly, also referred to as a pellicle, may be provided to protect the patterning device from airborne particles and other forms of contamination.

[0006] The use of pellicles in lithography is well-known and well-established. A pellicle in a lithographic apparatus is a membrane (also referred to as a pellicle membrane) which is located away from the patterning device and is out of the focal plane of a lithographic apparatus in use. As the pellicle is out of the focal plane of the lithographic apparatus, contamination particles which land on the pellicle are out of focus in the lithographic apparatus. Consequently, images of the contamination particles are not projected onto the substrate. If the pellicle were not present, then a contamination particle which landed on the patterning device would be projected onto the substrate and would introduce a defect into the projected pattern. [0007] Even though the pellicle protects the reticle from contaminants, the pellicle itself can be a source of contamination as material from the pellicle can be transferred to the reticle during operation. Similarly, other optical elements within a lithographic apparatus, such as mirrors, sensors, or pellicles, may need to be cleaned from time to time. Optical elements can be sensitive to damage caused by cleaning and selecting the wrong cleaning apparatus or method can results in either ineffective cleaning or damage to the optical element.

[0008] It is desirable to provide an apparatus which is able to remove contamination from optical elements of a lithography apparatus, particularly reticles, and to provide methods for removing contamination from optical elements of a lithography apparatus. Of course, non-optical elements of a lithography apparatus may also be cleaned using the apparatus and methods, although primarily it is desirable for the optical elements to be cleaned. The present invention has been devised in an attempt to address at least some of the problems with cleaning contaminated optical elements of a lithography apparatus.

SUMMARY

[0009] According to a first aspect there is provided an apparatus for removing a contaminant from an optical element of a lithography apparatus, said apparatus including a chamber for receiving the optical element, a gas supply configured to provide a gas, and a plasma generator or an ion/electron source to generate ions from the gas, wherein the gas comprises from about 0.01 vol% to about 10 vol% of at least one hydrocarbon, and/or about 0.01 vol% to about 50 vol% of at least one of He, Ne, or Ar. [00010] The optical element may be a reticle, a mirror, a sensor, a pellicle, or a collector of a lithography apparatus. Preferably, the optical element is a reticle.

[00011] Reticles of lithography apparatuses, particularly EUV lithography apparatuses, include a pattern of absorber material on top of a ruthenium coated multi-layer mirror. EUV radiation which is incident upon the reticle is absorbed by the absorber material and reflected by the reflective areas of the reticle. In this way, the radiation beam is patterned. There are lines of different shapes and width on the reticle in order to provide the desired pattern to the EUV radiation beam. The front side of the reticle, which is the side which is illuminated by the radiation beam, includes two different materials which are exposed to the radiation beam, and thereby form two different areas, namely a reflective area having a capping layer and an absorber area. The reflective area may include a multi-layer mirror. The capping layer, such as ruthenium, protects the multi-layer mirror and comprises the reflective portion of the front side of the reticle. The absorber area, which may comprise tantalum, is provided as a patterned layer, which may be provided on top of the capping layer. Additionally or alternatively, the absorber layer may be replaced by a shift rotating layer, which primarily alters the phase of the reflected radiation to cause interference with radiation reflected from the reflective area of the reticle, whether constructive or destructive, and to form the desired pattern in the resist. [00012] As mentioned, pellicles may be used to protect the surface of the reticle. The pellicles are located a few millimeters from the reticle so that particles smaller than around 10 microns landing on pellicle do not adversely affect imaging. Some pellicles include silicon, whether that is in the form of elemental silicon or compounds of silicon, such as silicon oxynitride or silicon oxide or a metal silicide. In operation, the pellicle is located within an atmosphere of hydrogen. Hydrogen plasma is able to react with the silicon in the pellicle to form volatile silicon compounds, such as silane. The silane is then able to diffuse away from the pellicle and towards the reticle. The silane then breaks down to deposit silicon on the reticle. The silicon can be preferentially deposited on certain areas of the reticle and this results in a loss of contrast between the reflective areas and the absorber areas of the reticle. In turn, there is an adverse effect on imaging. The silicon may become oxidized in situ or when the reticle is exposed to the atmosphere. If this continues, some of the features of the reticle will become out of specification and the reticle therefore needs to be cleaned or replaced. The contamination caused by the action of hydrogen plasma on materials within the lithography apparatus may be referred to as hydrogen plasma induced outgassing (HIO) depositions.

[00013] The reaction of silicon oxide in a pellicle with hydrogen plasma is:

SiO ₂ + {H* + H ⁺ => SiH ₄ + H ₂ O

Once the gas is transported to the reticle, it can deposit silicon through the following reactions:

SiH ₄ + Ru(s) + {hv _euv + H* + H ⁺ => Ru(s") + S’i(s)

SiH ₄ + Ta(s) + {hv _euv + H* + H ⁺ } => Ta(s) + Si(s)

[00014] Optical elements may also be contamination by other materials. For example, in operation, the pellicle may be heated to, for example, around 500°C or even higher. Materials with low evaporation enthalpy, such as molybdenum oxide, may evaporate from the pellicle and diffuse towards the reticle where they can be deposited.

[00015] It is very difficult to remove the silicon oxide or other contaminant materials from the reticle. In particular, it has been found that it is not possible to remove the silicon oxide from the reticle where the silicon oxide has been deposited on ruthenium using hydrogen plasma. Whilst halogens could be used to remove the silicon oxide using reactive ion etching, there is a high risk of damaging the reticle, particularly the layers of the multi-layer mirror, since molybdenum and silicon are more reactive than ruthenium and tantalum. The use of Cl, Br, or I to reactively ion etch silicon oxide demonstrates a lack of selectivity and reactivity towards the materials, such as Mo and Si, of the optical element, and is also damaging to Ru and Ta. Fluorine is also unsuitable due to its toxicity and the danger in handling it. [00016] It has been found that the addition of a small amount of hydrocarbon to the hydrogen plasma provides a plasma which is highly selective towards contaminants, particularly silicon oxide, and which is mild towards the materials of the surface of the reticle, in particular ruthenium and tantalum. Without wishing to be bound by scientific theory, it is believed that the addition of hydrocarbons to the gas creates a plasma comprising carbon species which are reactive towards silicon oxides to provide gaseous carbon species, such as carbon monoxide or carbon dioxide. The removal may occur by the chemical sputtering of CH3 radicals and/or selective physical sputtering by CHx ions. In addition, removal of silicon is faster than removal of silicon oxide and the silicon is believed to be removed, at least partially, via volatile silicon-carbon-hydrogen molecules. As such, the present invention allows for the highly selective removal of silicon oxide from a reticle, which has previously not been possible. A further advantage of the present invention is that if any carbon from the hydrocarbon is deposited on the reticle front side, it is easily removed by hydrogen plasma. As such, where it is desirable to remove carbon, the composition of the gas can be adjusted to reduce or remove the amount of hydrocarbons present so that the gas can remove any deposited carbon. The composition of the gas can then be adjusted again to include hydrocarbon useful for stripping contaminants.

[00017] Alternatively or additionally, a light noble gas, such as He, Ne, or Ar, may be added to the hydrogen or hydrocarbon/hydrogen mixture. The He, Ne, or Ar ions have been found to be able to remove contaminants, particularly molybdenum oxide, from the reticle front side. The selectivity is provided by the differences in the physical sputtering threshold of contaminants and the materials of the optical element. For example, the energy required to sputter lighter elements, such as Si, O, or P, is around 30 to 100 eV lower than the energy required to sputter heavier elements, such as Ru or Ta. In addition, certain contaminants, such as molybdenum oxide, are easier to remove in their oxidized form than in their elemental form. As such, certain oxides with melting points lower than that of the elemental form are easier to remove without being reduced first and so no hydrocarbons necessarily need to be included.

[00018] This being the case, there are three gas mixtures disclosed. Firstly, a mixture of at least one hydrocarbon in hydrogen, which is advantageous for removing silicon or silicon oxide contaminants. Secondly, a mixture of at least one hydrocarbon, at least one of He, Ne, and Ar in hydrogen, which is advantageous for removing silicon or silicon oxide contaminants. Thirdly, a mixture of at least one of He, Ne, and Ar, in hydrogen, which is advantageous for the removal of molybdenum oxide, or any oxide which has a lower enthalpy of evaporation than the corresponding elemental form.

[00019] The at least one hydrocarbon may be a saturated, unsaturated, and/or partially oxidized hydrocarbon. The hydrocarbon may be methane, ethane, propane, or butane. The hydrocarbon is preferably methane. The at least one hydrocarbon may be a C1-C4 hydrocarbon. The gas may comprise a mixture of different hydrocarbons or may contain just one type of hydrocarbon, for example methane. The hydrocarbon may have the formula C _x H _y O _z where 1 < x < 4, y < 10, z < 3. Whilst longer chain hydrocarbons could be used, these are less volatile and there is a greater risk that they will produce particles, which could be contaminants in themselves. As such, hydrocarbons having 1 to 4 carbon atoms are preferred. Methane is the most preferred hydrocarbon. Whilst unsaturated hydrocarbons could be used, they are less preferable since they are able to polymerise to form heavier compounds and are more likely to produce particles. It will be appreciated that there may be unavoidable impurities within the gas.

[00020] The gas may comprise from about 0.1 vol% to about 10 vol% hydrocarbons, from about 0.2 vol% to about 7 vol% hydrocarbons, or from about 0.3 vol% to about 5 vol% hydrocarbons, or from about 0.3 vol% to 3 vol% hydrocarbons. Alternatively or additionally, the gas may comprise from about 0.1 vol% to about 50 vol% of at least one of He, Ne, and Ar, about 0.1 vol% to about 10 vol% of at least one of He, Ne, and Ar, from about 0.2 vol% to about 7 vol% of at least one of He, Ne, and Ar, from about 0.3 vol% to about 5 vol% of at least one of He, Ne, and Ar, or from about 0.3 vol% to 3 vol% of at least one of He, Ne, and Ar. The composition of the gas may alter during operation of the apparatus or method.

[00021] The exact composition of the gas can be adjusted depending on the cleaning requirements of the apparatus. For example, the proportion of hydrocarbons can be increased where a high degree of cleaning is required and then reduced if any build-up of carbon is observed. In other terms, the composition of the gas and/or cleaning time may be selected such that there is a particular number of hydrocarbon ions delivered to the surface of the reticle during the cleaning per each atom of Si or O to be removed, for example such number can be more the 10 or more than 100. Similarly, the concentration of noble gas or gases, when present, can be selected and adjusted depending on the amount of contaminant to be removed.

[00022] The balance of the gas may be hydrogen or a hydrogen/noble gas mix, particularly comprising one or more of He, Ne, or Ar. Hydrogen is used within lithography apparatuses and the materials used in the different component parts of the lithography apparatus are selected to tolerate an atmosphere of hydrogen EUV-induced plasma. The gas is preferably free from halogens. Whilst halogens could be used to clean optical elements, they are able to damage the optical elements and/or leave contamination on the optical elements, so it has been found to be undesirable to include halogens. As such the gas may comprises from about 0.01 vol% to about 10 vol% hydrocarbons and/or from about 0.01 vol% to about 50 vol% of one or more of He Ne, and Ar, with the balance hydrogen and no halogens. It will be appreciated that there may be unavoidable impurities including halogens, but no halogens are intentionally included.

[00023] The plasma may be generated by any suitable means and the invention is not particularly limited to the exact mean used. Particularly appropriate means for generating the required plasma include an electron cyclotron resonance source or an electron beam.

[00024] The plasma generator may be configured to generate a plasma having ions with energy of from around 1 eV to around 100 eV at the reticle front side or other optical element, preferably with energy of from around 5 eV to around 30 eV. If the energy of the ions is too high, it can begin to damage the optical element being cleaned. Typically, the energy of the ions is between around 1 and 50 eV, and preferably between around 10 and 30 eV.

[00025] The reticle front side may be biased with respect to grounded walls of the apparatus in order to increase the ion energy at the reticle front side up to between around 1 eV and around 30 eV. The magnitude of the bias may be from around 1 to around 30 V. It will be appreciated that, where possible, other optical elements which are to be cleaned may similarly be biased.

[00026] The apparatus may comprise one or more controllers to control the composition and/or the pressure of the gas within the chamber. As mentioned, the composition of the gas can be adjusted as required. Similarly, it may be desirable to increase or decrease the pressure within the chamber to achieve optimal cleaning.

[00027] The apparatus may include a conditioning unit. The conditioning unit may be configured to control the temperature of the optical element. The optical element may be heated during exposure to the plasma and it is therefore desirable to have a means by which the temperature can be controlled.

[00028] The apparatus may be configured to screen pre-selected areas of the optical element from the plasma. As such, the apparatus may include barriers or shields which prevent the plasma from reaching portions of the optical element which are susceptible to damage by exposure to plasma. For example, where the optical element is a reticle, the sides and rear portion of the reticle include ultralow expansion glass, which comprises silicon oxide. As such, since the plasma described herein is especially suitable for removing silicon oxide from surfaces, if the ultra-low expansion glass were to be exposed to the plasma derived from the hydrocarbon containing gas mixture, it would become damaged. Similarly, some reticles or other optical elements may include regions on their surface which need to be protected from the plasma.

[00029] The apparatus may be configured to alter the composition of the gas in response to a predetermined cleaning stage being reached. For example, where there is a build-up of carbon, the composition of the gas may be altered to have a higher proportion of hydrogen in order to etch away the carbon deposits.

[00030] The contaminant may be silicon oxide. Whilst the description mainly refers to silicon oxide, it has also been found that the apparatus and method of the present invention are also able to remove low-sputtering threshold metals (such as Mg, Cu) or metal oxides, such as MoOs (that has a relatively low evaporation enthalpy) as well as phosphorus, in different oxidation states. As such, physical sputtering by ions derived from hydrocarbons and/or by noble gas ions is able to remove such contaminants, particularly MoOs.

[00031] According to a second aspect of the present invention, there is provided a method for removing a contaminant from an optical element of a lithographic apparatus, the method including providing a gas comprising comprises from about 0.01 vol% to 10 vol% hydrocarbons and/or about 0.01 vol % to about 50 vol% of at least one of He, Ne, and Ar, converting at least a portion of the gas into ions or a plasma, and contacting the contaminant with the ions or plasma to remove at least a portion of the contaminant.

[00032] As described in respect of the first aspect of the present invention, the addition of the hydrocarbon and/or He, Ne, Ar to the hydrogen gas and subsequently the presence of hydrocarbon or noble gas ions in the plasma provides for a plasma which is able to selectively remove contamination by reactive chemical or selective physical sputtering, particularly silicon oxide, from a surface. Previously, the plasma comprised only hydrogen and this was not able to remove silicon oxide from optical elements, including when deposited on ruthenium, silicon or silicon oxide and/or other hydrogen induced outgassing contamination elements, deposited on ruthenium resist etching by pure hydrogen plasma due to the high recombination properties of ruthenium towards atomic hydrogen. Addition of gases with physical or chemical selectivity for elements deposited on the ruthenium allows to overcome such effects. Note that any carbon deposited on ruthenium or tantalum or other elements on optical elements, that are robust to hydrogen plasma, can be completely removed by pure hydrogen plasma, and note that noble gas ions or atoms cannot accumulate in the topmost layers of the optical element and affect its reflectivity.

[00033] The gas may comprise saturated, unsaturated, or partially oxidized hydrocarbons. The hydrocarbon is preferably methane. The partially oxidized hydrocarbon may have the formula C _x H _y O _z where 1 < x < 4, y < 10, z < 3.

[00034] The gas may comprise from about 0.1 vol% to about 10 vol% hydrocarbons, from about 0.2 vol% to about 7 vol% hydrocarbons, from about 0.3 vol% to about 5 vol% hydrocarbons, or from about 0.3 vol% to 3 vol% hydrocarbons. Alternatively or additionally, the gas can comprise one or more of He, Ne, Ar with the concentration up to 50%, preferably 0.1-10%. The gas may comprise from about 0.1 vol% to about 10 vol% of at least one of He, Ne, and Ar, from about 0.2 vol% to about 7 vol% of at least one of He, Ne, and Ar, from about 0.3 vol% to about 5 vol% of at least one of He, Ne, and Ar, or from about 0.3 vol% to 3 vol% of at least one of He, Ne, and Ar.

[00035] The balance of the gas may be hydrogen. Preferably, the gas is free from halogens.

[00036] The ions at the optical element have an energy in the range of from around 1 eV to around 100 eV, preferably of from around 5 eV to around 30 eV. This can be achieved by control of the temperature of electrons in the plasma over the optical element surface optionally combined with a bias of the optical element surface.

[00037] One or more controllers may control the composition and/or the pressure of the gas.

[00038] The method may further comprise controlling the temperature of the optical element. As described in relation to the first aspect of the present invention, this may be achieved by way of a conditioning unit.

[00039] The composition of the gas may be altered in response to a predetermined cleaning stage being reached. For example, the relative proportion of hydrogen in the gas could be increased to remove any carbon build-up observed during the cleaning process or which may already be present. [00040] The optical element may be a reticle, a mirror, a sensor, a pellicle, or a collector. Although the method according to the present invention may be applied to other surfaces and optical elements, it is mainly directed to the cleaning of reticles. Reticles are more sensitive to damage that non-optical elements of a lithography apparatus and so a cleaning method and apparatus which is used to clean non- optical elements can not necessarily simply be applied to optical elements, including reticles or mirrors. [00041] Alternatively or additionally, it has been found that an etching process comprising hydrogen peroxide at above room temperature or piranha solution is sufficiently selective to remove certain contaminations, such as molybdenum oxide, without causing damage to any Ru or Ta present.

[00042] According to a third aspect of the present invention, there is provided a lithographic tool comprising a controlled environment with a holder to receive an optical element, a gas supply, and a plasma generator or an ion/electron source configured to generate ions from the gas.

[00043] The optical element may be a reticle, a mirror, a sensor, a pellicle, or a collector.

[00044] The gas supply may be configured to provide a gas having a composition as described in respect of the first or second aspects of the present invention.

[00045] According to a fourth aspect of the present invention, there is provided the use of an apparatus according to the first aspect of the present invention or a method according to the second aspect of the present invention in a lithographic apparatus or process.

[00046] It will be appreciated that features described in respect of one aspect of the present invention are equally applicable to any other aspect of the present invention. In addition, the features described in respect of any of the aspects may be combined with the features described in respect of any of the other aspects of the present invention.

[00047] The present invention will now be described with reference to the cleaning of a reticle of an EUV lithography apparatus. However, it will be appreciated that the present invention may also be applied to the cleaning of other optical elements of lithography apparatuses, including mirrors, such as multi-layer mirrors.

BRIEF DESCRIPTION OF THE DRAWINGS

[00048] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

Figure 2 depicts a cross section of a reticle assembly;

Figure 3 depicts a top view of an exemplary reticle;

Figure 4 depicts a schematic view of an apparatus according to the present invention; and Figures 5a and 5b depict examples of masking units or shields configured to protect the reticle.

DETAILED DESCRIPTION [00049] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[00050] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[00051] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated.

[00052] A pellicle 15 is depicted in the path of the radiation to protect the patterning device MA. The pellicle 15 comprises a thin film that is substantially transparent to EUV radiation (although it will absorb a small amount of EUV radiation) and acts to protect the patterning device MA from particle contamination.

[00053] Whilst efforts may be made to maintain a clean environment inside the lithographic apparatus LA, particles may still be present inside the lithographic apparatus LA. In the absence of a pellicle 15, particles may be deposited onto the patterning device MA. Particles on the patterning device MA may disadvantageous^ affect the pattern that is imparted to the radiation beam B and therefore the pattern that is transferred to the substrate W. The pellicle 15 provides a barrier between the patterning device MA and the environment in the lithographic apparatus LA in order to prevent particles from being deposited on the patterning device MA.

[00054] In use, the pellicle 15 is positioned at a distance from the patterning device MA that is sufficient that any particles that are incident upon the surface of the pellicle 15 are not in the focal plane of the radiation beam B. This separation between the pellicle 15 and the patterning device MA acts to reduce the extent to which any particles on the surface of the pellicle 15 impart a pattern to the radiation beam B. It will be appreciated that where a particle is present in the beam of radiation B, but at a position that is not in a focal plane of the beam of radiation B (i.e., not at the surface of the patterning device MA), then any image of the particle will not be in focus at the surface of the substrate W. In some cases, the separation between the pellicle 15 and the patterning device MA may, for example, be between 2 mm and 3mm (e.g. around 2.5 mm). In some cases, a separation between the pellicle 15 and the patterning device may be adjustable. The pellicle membrane 15 often comprises silicon, whether in elemental form or in a compound, such as silicon oxide or silicon oxynitride, alternatively, pellicle can comprise a metal silicide or a metal oxide. In use, the pellicle membrane 15 is exposed to hydrogen plasma, which is able to react with the silicon and form volatile silicon compounds, including silane. In use, pellicle is heated due to absorbed EUV radiation to temperatures in excess of 500 C, which can induce evaporation of materials with low evaporation temperature towards reticle, for example a metal oxide or metal hydroxide, for example MoCh.Thc volatile silicon compounds are able to travel to the front side of the patterning device MA, also referred to as a reticle, where they are able to break down and deposit silicon on the patterning device MA. The deposition of silicon on the patterning device MA ultimately results in a decrease in imaging performance and it is therefore necessary to clean the patterning device MA. The silicon generally oxidises on the surface of the patterning device MA, possibly either in situ or when the reticle is exposed to the air for cleaning. It has not previously been possible to remove the silicon oxide layer in a manner which is suitable to avoid damage to the patterning device MA itself. It has not previously been possible to remove the metal oxide and/or metal hydroxide layer, especially mixed with silicon oxide, in a manner which is suitable to avoid damage to the patterning device MA itself. It will be appreciated that other optical elements may similarly require cleaning.

[00055] After the generation of the patterned EUV radiation beam B’, the projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’ , thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[00056] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[00057] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and/or in the projection system PS.

[00058] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region. The laser beam is incident upon the tin at the plasma formation region. The deposition of laser energy into the tin creates a tin plasma at the plasma formation region. Radiation, including EUV radiation, is emitted from the plasma during de-excitation and recombination of electrons with ions of the plasma.

[00059] The EUV radiation from the plasma is collected and focused by a collector. Collector comprises, for example, a near-normal incidence radiation collector (sometimes referred to more generally as a normal-incidence radiation collector). The collector may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region, and a second one of the focal points may be at an intermediate focus, as discussed below.

[00060] The laser system may be spatially separated from the radiation source SO. Where this is the case, the laser beam may be passed from the laser system to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser system, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[00061] Radiation that is reflected by the collector forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus to form an image at the intermediate focus of the plasma present at the plasma formation region. The image at the intermediate focus acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus is located at or near to an opening in an enclosing structure of the radiation source SO.

[00062] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source or a free electron laser (FEL) may be used to generate EUV radiation.

[00063] Figure 2 depicts a cross-section of a patterning device MA, also referred to as a reticle 100. The reticle 100 includes a conductive back side 101 and a core comprising an ultra-low expansion substrate 102. The ultra-low expansion substrate 102 is generally ultra-low expansion glass comprising a mixture of silicon oxide and titanium oxide. A multi-layer mirror 103 is provided which comprises alternating layers of silicon and molybdenum, and the multi-layer mirror 103 is capped with a protective layer of ruthenium 104. A patterned absorber layer 105 is provided on the ruthenium layer 104. The patterned absorber layer 105 may comprise tantalum, but other suitable materials may be used. A so- called quality area 140 is provided on the front side of the reticle 100. The quality area 140 includes the pattern which is to be imprinted into the radiation beam. The quality area 140 may be disposed within a so-called black border region 130. In some reticles, the black border region 130 is etched down to the ultra-low expansion substrate 102. It will be appreciated that the black border region 130 and the ultra-low expansion substrate 102 need to be shielded from plasma which is configured to etch silicon oxide. [00064] Figure 3 depicts a top view of a reticle 100. As can be clearly seen, there is a quality area 140 which is surrounded by a black border 130. In turn, the black border 130 is surrounded by an absorber layer 120.

[00065] Figure 4 is a schematic depiction of an apparatus according to an embodiment of the first aspect of the present invention. The apparatus includes a chamber 200. The chamber 200 is configured to receive the optical element which is being cleaned, such as a reticle 100. The chamber 200 is also configured to provide a controlled environment therein. The controlled environment can be controlled to alter the composition of the gas therein, the rate of ionization of the gas, the amount of plasma contained in the chamber 200, the energy of the ions in the chamber 200, and/or the pressure within the chamber 200. The chamber 200 also comprises a gas supply 201 which is configured to provide a gas. The chamber 200 may optionally also comprise an exhaust system such as a gas outlet 202 to remove any gases as required. The gas supply 201 and optional gas outlet 202 can be controlled by a controller (not shown) to alter the composition of gas within the chamber 200 and to alter the pressure of the gas within the chamber 200. The gas supply 201 according to any aspect of the present invention is preferably disposed close to the optical element being cleaned. For example, where the optical element being cleaned is a reticle or pellicle, the gas supply may be provided via the so-called Y-nozzle as they are known in the art or via one or more other nozzles in the vicinity of the reticle or pellicle. For other optical elements, there may be a separate gas supply which is close to the surface to be cleaned. A plasma generator 203 is provided to convert at least a portion of the gas within the chamber into a plasma. The plasma generator 203 may be located at any appropriate location which allows the plasma generated to interact with the optical element being cleaned. The invention is not particularly limited by the exact plasma generator 203 used and electron beam ionization may be a preferred mode of operation of the plasma generator 203. The plasma generator may instead be an ion/electron source. As depicted in Figure 4, the reticle 100 may be supported on optional studs 221 which support the reticle 100 in the desired position within the chamber 200. The studs are configured to avoid damage to the rear side of the reticle 100 and to also avoid damage to the sides of the reticle 100. The apparatus further comprises a shield 220 which is configured to screen the sides of the reticle 100 from the plasma. The shield 220 may also be configured to screen preselected areas of the optical element from the plasma. The shield 220 can be applied parallel to the front side where a gap of from around 50 to around 500 pm is adequate. For example, the reticle sides and/or black border at the front side of the reticle should be shielded from the plasma chemistry that etches the silicon oxide or other contaminant. The shield 220 is also configured to screen preselected portions of the frontside of the reticle 100 from the plasma. For example, the shield 220 may be configured to screen the black border 130 and/or the absorber layer 120 which surrounds the black border 130 from the plasma. It will be appreciated that the shield 220 may be a single piece or may comprise multiple individual sections. In order to avoid damaging the frontside of the reticle 100, the shield 220 is spaced apart from the shield by distance H. The distance H may be any suitable distance, such as for example, 0.1 to 1 mm. The shield 220 overlaps the black border 130 by distance G. The distance G may be from around 0.1 to 1 mm. In order to reduce plasma depletion by recombination, the thickness of the shield W is preferably less than 1 cm, and W may be between around 0.1 mm to 5 mm. The walls of the shield 210 and the walls of the chamber 211 are made of or provided with coating which is resistant to sputtering, such as Mo, W, Ru, or Ta. The apparatus is preferably free of silicon, since this would be attacked and etched by the plasma.

[00066] The gas within the chamber 200 comprises a mixture of hydrogen and methane at a concentration of from 0.1 vol% to 10 vol% of methane. Other Cl to C4 hydrocarbons could be used, but methane is preferred. Alternatively or additionally, the hydrogen gas may contain at least on of He, Ne, Ar with a concentration of from 0.1 vol % to 50 vol%. In use, the gas within the chamber 200 is converted at least partially into a plasma by the plasma generator 203. The plasma contains ionized hydrocarbon molecules or noble gas ions and is able to remove silicon oxide or other HIO elements contamination from the quality area 140 of the reticle 100.

[00067] During cleaning, the reticle 100 may be subject to a thermal load of, for example between 1 and 100 W. As such, the apparatus may optionally further include a conditioning system (not shown) which conditions the reticle 100. For example, the conditioning system may include a cooling plate, and there may be a flow of hydrogen gas over the backside of the reticle 100 which is able to extract heat from the reticle 100. The flow of hydrogen gas may be provided between the backside of the reticle and the cooling plate. The pressure of such a flow of hydrogen may be between around 0.1 to around 10 mBar.

[00068] Figures 5a and 5b depict two possible embodiments of shield 220. In Figure 5a, the shield 220 is provided as a single piece. The shield 220 frames the reticle quality area 140 and is spaced apart by distances F and G. The distances F and G may be selected independently and may be selected that the opening formed in the shield 220 is larger or smaller than the quality area 140 of the reticle 100. The shield 220 of Figure 5b is the same as that of Figure 5a, except that it is formed of more than one piece.

[00069] The time for which the optical element is cleaned may be any suitable time. For example, the cleaning time may range from 1 minute to 6 hours. The cleaning time may be less than 3 hours, less than 2 hours, less than 1 hour, or less than 30 minutes.

[00070] The present invention provides for the first known apparatus and method of etching a few nm of hydrogen plasma induced outgassing (HIO) depositions on an optical element of a lithography apparatus, particularly silicon depositions deposited on ruthenium. The present invention provides a way to remove such HIO depositions without the use of halogens and does not rely on wet etching, meaning that there is less or no damage to the optical element being cleaned and no contamination caused by the cleaning method. Using plasma or an ion beam comprising ions derived from methane or light noble gases, such as He, Ne, and Ar, in the range of 5 eV to 30eV results in zero physical sputtering of ruthenium or tantalum. Without the addition of the methane, silicon oxide is not removed and it has been surprisingly found that the addition of methane allows for the ready removal of silicon oxide.

[00071] It should be understood that the features of the above embodiments and aspects may be combined.

[00072] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[00073] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[00074] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[00075] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims and clauses set out below.

1. An apparatus for removing a contaminant from an optical element of a lithography apparatus, said apparatus including a chamber for receiving the optical element, a gas supply configured to provide a gas, and a plasma generator or an ion/electron source to generate ions from the gas, wherein the gas comprises from about 0.01 vol% to about 10 vol% of at least one hydrocarbon and/or about 0.01 vol % to about 50 vol% of at least one of He, Ne, and Ar.

2. The apparatus of clause 1, wherein the optical element is a reticle, a mirror, a sensor, a pellicle, or a collector of a lithographic apparatus.

3. The apparatus of clause 1 or clause 2, wherein the at least one hydrocarbon comprises one or more saturated, unsaturated, or partially oxidised hydrocarbons.

4. The apparatus of any preceding clause, wherein the at least one hydrocarbon is a C1-C4 hydrocarbon, is methane, or has the formula C _x H _y O _z where 1 < x < 4, y < 10, z < 3.

5. The apparatus of any preceding clause, wherein the gas comprises from about 0.1 vol% to about 10 vol% hydrocarbons, from about 0.2 vol% to about 7 vol% hydrocarbons, from about 0.3 vol% to about 5 vol% hydrocarbons, or from about 0.3 vol% to 3 vol% hydrocarbons, and/or wherein the gas comprises about 0.1 vol% to about 50 vol% of at least one of He, Ne, and Ar, from about 0.1 vol% to about 10 vol% of at least one of He, Ne, and Ar, from about 0.2 vol% to about 7 vol% of at least one of He, Ne, and Ar, from about 0.3 vol% to about 5 vol% of at least one of He, Ne, and Ar, or from about 0.3 vol% to 3 vol% of at least one of He, Ne, and Ar.

6. The apparatus of any preceding clause, wherein the balance of the gas is hydrogen.

7. The apparatus of any preceding clause, wherein the plasma generator comprises an electron cyclotron resonance source, an electron beam, or an ion beam, optionally wherein the plasma generator is a reactive ion etcher operable in capacitively coupled plasma mode

8. The apparatus of any preceding clause, wherein the plasma generator or the ion/electron source are configured to generate ions with energy of from around 1 eV to around 100 eV, preferably with energy of from around 5 eV to around 30 eV.

9. The apparatus of any preceding clause, wherein the apparatus comprises one or more controllers to control the composition and/or the pressure of the gas within the chamber and/or the ionization rate of the gas.

10. The apparatus of any preceding clause, wherein the apparatus further includes a conditioning unit configured to control the temperature of the optical element.

11. The apparatus of any preceding clause, wherein the apparatus is configured to screen preselected areas of the optical element from the plasma or ions.

12. The apparatus of any preceding clause, wherein the apparatus is configured to alter the composition of the gas in response to a predetermined cleaning stage being reached.

13. The apparatus according to any preceding clause, wherein the contaminant is silicon, silicon oxide, molybdenum oxide, a metal or metal oxide, or phosphorus, or phosphorus oxide.

14. A method for removing a contaminant from an optical element of a lithographic apparatus, the method including providing a gas comprising comprises from about 0.01 vol% to about 10 vol% of one or more hydrocarbons and/or about 0.01 vol % to about 50 vol% of at least one of He, Ne, and Ar, converting at least a portion of the gas into a plasma, and contacting the contaminant with the ions or plasma to remove at least a portion of the contaminant.

15. The method of clause 14, wherein the at least one hydrocarbon comprises saturated, unsaturated, or partially oxidised hydrocarbons.

16. The method of clause 14 or 15, wherein the hydrocarbon is a C1-C4 hydrocarbon, methane or has the formula C _x H _y O _z where 1 < x < 4, y < 10, z < 3.

17. The method of clauses 14 to 16, wherein the gas comprises from about 0.1 vol% to about 10 vol% hydrocarbons, from about 0.2 vol% to about 7 vol% hydrocarbons, from about 0.3 vol% to about 5 vol% hydrocarbons, or from about 0.3 vol% to 3 vol% hydrocarbons and/or wherein the gas comprises from about 0.1 vol% to about 50 vol% of at least one of He, Ne, and Ar, from about 0.1 vol% to about 10 vol% of at least one of He, Ne, and Ar, from about 0.2 vol% to about 7 vol% of at least one of He, Ne, and Ar, from about 0.3 vol% to about 5 vol% of at least one of He, Ne, and Ar, or from about 0.3 vol% to 3 vol% of at least one of He, Ne, and Ar.

18. The method of any of clauses 14 to 17, wherein the balance of the gas is hydrogen.

19. The method of any of clauses 14 to 18, wherein the ions have an energy in the range of from around 1 eV to around 100 eV, preferably of from around 5 eV to around 30 eV.

20. The method of any of clauses 14 to 19, wherein one or more controllers control the composition and/or the pressure of the gas and/or ionization rate.

21. The method of any of clauses 14 to 20, wherein the method controlling the temperature of the optical element.

22. The method of any of clauses 14 to 21, wherein composition of the gas is altered in response to a predetermined cleaning stage being reached.

23. The method of any of clauses 14 to 22, wherein the optical element is a reticle, a mirror, a sensor, or a pellicle.

24. A lithographic tool comprising a controlled environment with a holder to receive an optical element, a gas supply, and a plasma generator or an ion/electron source configured to generate ions from the gas.

25. The lithographic tool according to clause 24, wherein the optical element is a reticle, a mirror, a sensor, or a pellicle.

26. The use of an apparatus according to any of clauses 1 to 13 or method according to any of clauses 14 to 23 in a lithographic apparatus or process.