NL2020893A - Apparatus and methods for cleaning - Google Patents

Abstract

A method of cleaning a surface of an element of a lithographic apparatus comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the distance between the surface to be cleaned and the carbon dioxide nozzle is from between around 20 mm to around 100 mm, and/or the angle subtended by the surface to be cleaned and the carbon dioxide snow stream is around 45 to around 90', and/or wherein the angle subtended between the surface to be cleaned and the direction of the carbon dioxide snow stream is from around 45° to around 90, and/or wherein the direction of the streams of gas and carbon dioxide snow are controlled by a cooperative robot and/or wherein the gas stream is heated to from around 20'C to around 80'C and/or wherein the pressure of the gas stream is from around 1 bar to around 20 bar; and apparatus for the same.

Description

APPARATUS AND METHODS FOR CLEANING FIELD

[0001] The present invention relates to apparatus and methods for cleaning. In particular, the present invention relates to apparatus and methods for cleaning lithographic apparatus, more particularly optical elements, and even more particularly, collectors for EUV lithography apparatus.

BACKGROUND

[0002] 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), the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), Ihin-lïlm magnetic heads, etc. A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate. The present invention is not limited to optical lithography and may be used in other lithographic applications, for example imprint lithography.

[0003] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0004] In the lithographic apparatus, EUV radiation is generated by the ionisation of liquid tin droplets by a radiation source, such as a laser. The liquid tin droplets are passed in front of the radiation source and as the radiation hits the tin droplets, the tin droplets are ionised and release EUV radiation. The EUV radiation released by the tin droplets is released in all directions and it is therefore necessary for the EUV radiation to be focused by a collector before it can be used. One issue associated with generating EUV radiation in this way is that the tin can contaminate the collector. The tin may also contaminate the walls of the apparatus, and, although this is less of an immediate issue, it may be desirable to clean the walls from time to time. The tin may also contaminate other parts of the lithography apparatus, and it is also desirable to clean these too. Indeed, it is desirable to clean the surface of any optical element as well as sensors.

[0005] Contamination of the collector leads to a reduced reflectivity of the collector. A consequence of this is that less of the incident EUV radiation is reflected by the collector and focused, thereby reducing the power of the EUV source. Due to the reduced power of the EUV source, a longer exposure is required during lithography, which reduces the throughput of the lithographic apparatus. Similarly, if optical elements are contaminated, this will reduce their performance.

If sensors are contaminated, this may adversely affect the sensitivity and/or accuracy of the sensors and may even render them inoperable. Although the cleaning of collectors used in EU V lithography apparatus is primarily discussed, it will be appreciated that the apparatus and methods of the present invention are applicable to a wide range of surfaces to be cleaned and may be applied to optical elements, mirrors, sensors, and the like. Further, contaminants other than tin may also be cleaned from a surface using the apparatus and methods of the present invention.

[0006] During normal usage, a collector requires cleaning around every 13 weeks. The cost of new collectors is very high and there is limited manufacturing capacity of new collectors. The production capacity is currently less than the rate at which collectors are required, so it is necessary for existing collectors to be cleaned. The removal and cleaning of a collector is a long and difficult process that requires significant skill and expertise. Therefore, collectors currently have to be sent back to the manufacturer for cleaning. It can take more than five weeks for a single collector to be cleaned. In addition, transportation can also take around three weeks. Therefore, during cleaning, a collector may be out of service for around eight weeks. It is therefore necessary to have a pool of collectors available for use. Since collectors currently require cleaning around every three months and there is around a two-month cleaning turnaround time, it is necessary to have almost double the number of collectors as lithography machines to ensure that a clean collector is available to replace the collector being used in a lithography apparatus when the collector in use becomes contaminated.

[0007] There are a number of known methods for cleaning the collector, each with its own benefits and limitations. For example, hydrogen radicals can be reacted with the tin to form tin hydride. Tin hydride has a boiling point of -52° and is therefore a gas at temperatures at which cleaning is undertaken. Whilst this is able to clean the tin from the surface of the collector, the process is very slow and can take around a week or longer. In addition, tin hydride is a dangerous gas which can ignite on contact with air and therefore presents a significant safety hazard.

[0008] Other methods for cleaning tin from collectors include converting the deposited tin from the ductile, metallic beta allotrope of tin to the nonmetallic, alpha allotrope of tin. Alpha tin is very brittle and could be simply swept away from the surface of the collector. However, the conversion of beta to alpha tin requires a high activation energy and the process is slow. The conversion can be initiated by using low temperatures, such as minus 30=C, but such low temperatures may pose a risk to the collector itself and it is undesirable to cool the collector to such low temperatures. Using temperatures below zero could lead to the deposition of water ice on the surface of the collector. If there are any cracks on the surface of the collector, any water contained within such cracks may freeze and expand, thereby damaging the surface of the collector, which could lead to decreased performance of the collector. In addition, the collectors comprise cooling channels which may contain water for use in cooling. If temperatures below ÖC are used, this may lead to the formation of ice in the cooling channels, which could also damage the collector. Further, the use of very low temperatures may result in distortions of the surface due to the differential contraction and/or expansion of the surface.

[0009] It is also possible to clean surfaces such as collectors using various chemical methods. For example, it has been proposed to apply hydrochloric acid to the surface of the collector in order to react it with the tin contaminant. However, this is a time consuming process and there is risk to an operator in using strong acids and also a risk of inadvertent damage to the collector. Further, the deposits on the surface of the surface to be cleaned are not always solely comprised of tin and other metals or non-metals may be present. Therefore, in order to ensure that the surface is as clean as possible, multiple chemical cleaning methods may be required. However, this has the disadvantage of essentially selectively etching the surface of the collector. The chemical cleaning methods may not only remove the unwanted material, such as tin or other metals or non-metals, from the surface of the collector, but may also remove materials comprising the collector itself. The collectors are very complex multi-layered mirrors with finely tuned refractive indices on each layer. Thus, if the metals which are used to form the various layers of the mirror are selectively etched from the layers, this may alter the refractive index of the layer or otherwise damage the surface. Whilst this may be acceptable if only one chemical cleaning method were used, since a number of different chemical cleaning methods are required, this will result in an increased amount of damage to the collector in a short period of time.

[0010] Other methods for cleaning collectors include polymer film peeling and atmospheric plasma torch cleaning. However, these can be slow', expensive, and/or have safety concerns.

[0011] It is known to use carbon dioxide snow to clean a wide variety of surfaces. Carbon dioxide can be used to dean surfaces via three different methods, namely using macroscopic dry ice pellets, snow streams spraying the surface to be cleaned with either microscopic or macroscopic carbon dioxide snow1 particles, or by using supercritical carbon dioxide.

[0012] In systems which use macroscopic dry ice pellets, cleaning is achieved by abrasive action and momentum transfer. The pellets are able to physically dislodge surface contaminants and the sublimation of the solid carbon dioxide (commonly referred to as ‘dry ice’) into gaseous carbon dioxide is able to push contaminants from the surface to be cleaned.

[0013] In systems which use supercritical carbon dioxide, the low viscosity of the supercritical fluid means that it is able to enter tight spaces where it can dissolve contaminants. Supercritical carbon dioxide is able to dissolve organic contaminants readily and is therefore used in the extraction of caffeine from coffee beans or nicotine from tobacco.

[0014] In systems which rely on carbon dioxide snow, the snow is generated by passing either liquid or gaseous carbon dioxide through an orifice. The drop in pressure as the liquid or gaseous carbon dioxide passes though and out of the orifice causes at least some of the carbon dioxide to solidify to form carbon dioxide crystals in the form of “snow”.

[0015] The use of snow cleaning is standard in many industries, such as the food industry, automotive industry and the manufacturing industry. However, it has not been used to remove metallic deposits, in particular tin deposits, from lithography equipment, particularly collectors or mirrors. Furthermore, known methods of and conditions used in carbon dioxide snow cleaning have been surprisingly found to be sub-optimal. Further, it has also been surprisingly found that carbon dioxide snow cleaning is particularly suitable for cleaning metallic contamination from a mirror, whereas previously it has only been used to clear off materials which are only loosely adhered to the surface to be cleaned or which are not in themselves particularly robust, such as paint.

[0016] In known snow cleaning apparatus, the carbon dioxide is passed out of a central nozzle which causes the carbon dioxide snow to form. Surrounding tire central nozzle, there may be provided an annular flow of clean, dry air which assists in directing the flow of the carbon dioxide snow. In this way the flow of carbon dioxide snow is surrounded by an annular curtain of clean, dry air.

[0017] Due to the very low temperature of the carbon dioxide snow, there is a risk that if the carbon dioxide snow is directed to a portion of a surface for too long, this could result in overcooling, which could lead to damage due to water ice formation or possible distortion of the surface due to the differential contraction of one portion of the surface relative to another portion. Further, in the event of water ice formation, this may result in the carbon dioxide cleaning apparatus only cleaning off the water ice rather than the contaminants which were the reason for the cleaning. In carbon dioxide snow' cleaning, it is preferable to have the outlet of the nozzle as close to the surface to be cleaned as possible, typically from about 5 to about 10 mm from the surface as this has been found to provide the best cleaning results. Further, it is also desirable to direct the carbon dioxide snow stream at a shallow angle to the surface to be cleaned so that the flow of the carbon dioxide snow stream will tend to push away any loosened material from the surface. The surface of the collector is also very sensitive to damage and any inadvertent contact of the carbon dioxide snow' cleaning apparatus may result in damage to the collector.

[0018] Whilst the present application generally refers to EUV lithography apparatus throughout and particularly collectors, the invention is not limited to solely EUV lithography apparatus and it is appreciated that the subject matter of the present invention may be used to clean any surface of any lithography apparatus, in particular any optical element, mirror, reflective surface, wall, and sensor or similar in the lithography apparatus.

SUMMARY

[0019] The present invention has been made in consideration of the aforementioned problems with known methods of cleaning lithography apparatus, in particular EUV lithography apparatus and even more particularly, optical elements of EUV lithography apparatus, including mirrors, reflective surfaces, collectors, walls, and sensors used in EUV lithography apparatus.

[0020] According to a first aspect of the present invention, there is provided a carbon dioxide snow cleaning apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source: a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the distance between the carbon dioxide snow nozzle and a surface to be cleaned is variable from between around 20 mm to around 100 mm.

[0021] Preferably, the carbon dioxide snow nozzle is separated from the surface by from around 40mm to around 80 mm, preferably around 55 mm to around 65 mm, and most preferably around 60 mm. The size of the opening of the carbon dioxide snow' nozzle may vary depending on the required operating conditions.

[0022] According to a second aspect of the present invention, there is provided a carbon dioxide snow' cleaning apparatus comprising; a carbon dioxide source; a carbon dioxide snow' nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the angle subtended between the surface to be cleaned and the direction of the carbon dioxide snow' stream is from around 45’ to around 9(1.

[0023] Preferably, the angle subtended between the surface to be cleaned and the direction of the carbon dioxide snow stream is greater than around 6Ö, preferably greater than around 75; and most preferably substantially around 90.

[0024] It will be appreciated that the carbon dioxide snow stream will have a particular w'idth. The skilled person will appreciate that the angle will be measured from the central line of the stream and that there is a degree of uncertainty in the measurements, such that slight deviations from the angles disclosed herein are also within the scope of the invention.

[0025] According to a third aspect of the present invention, there is provided a dioxide snow' cleaning apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the apparatus comprises a cooperative robot.

[0026] In some embodiments, the cooperative robot is a multiple degrees of freedom robot. Preferably, the cooperative robot is a six degrees of freedom robot.

[0027] A cooperative robot is a robot which is specifically designed to be used in combination with other robots or one or more humans. As such, a cooperative robot has additionally safety measures compared to non-cooperative robots in order to avoid any collisions between the cooperative robot and other robots and/or humans. In addition, cooperative robots have the same or similar capabilities as humans and are therefore able to manipulate the nozzle in a similar way to a human. Whilst prior art carbon dioxide snow cleaning apparatus is only able to be used in 2D, the use of a cooperative robot with six degrees of freedom allows the apparatus of the present invention to be used to clean much more complex shapes, such as parabolic or otherwise curved surfaces.

[0028] According to a fourth aspect of the present invention, there is provided a dioxide snow cleaning apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the apparatus comprises a heater to heat the gas stream which surrounds the carbon dioxide nozzle. Preferably the heater is used for heating of compressed, clean air.

[0029] The heater may be selected to allow it to provide a gas stream at temperatures of from around 20C to around 8ÖC, preferably from around 30C to around 7(JC, more preferably from around 40C to around 65C, and even more preferably to around 5ÖC.

[0030] According to a tilth aspect of the present invention, there is provided a dioxide snow cleaning apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the pressure of the gas stream is from around 1 bar to around 20 bar, more preferably from about 5 bar to about 15 bar, and most preferably substantially about 10 bar.

[0031] The subject matter of the first to fifth aspects of the present invention may be combined together in any combination. As such, the parameters defined in the first to fifth aspects of the present invention may be covered by a single embodiment of the invention and combined with any aspect of the present invention. For example, the subject matter of the first aspect of the present invention may be combined with that of tire second and/or third and/or fourth and/or fifth aspect of the present invention. The same applies to the second, third, fourth, and fifth aspects of the present invention, which may be combined with any of the other aspects of the present invention. As such, disclosed herein is a carbon dioxide snow cleaning apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the distance between the carbon dioxide snow nozzle and a surface to be cleaned is variable from between around 20 mm to around 100 mm and/or wherein the angle subtended by the surface to be cleaned and the carbon dioxide snow stream is around 45 to around 90, and/or wherein the apparatus further comprises a cooperative robot and/or wherein the apparatus further comprises a heater to heat the gas stream and/or wherein the pressure of the gas stream is from around 1 bar to around 20 bar.

[0032] The apparatus according to any aspect of the present invention may comprise a gas outlet through which the stream of gas is provided. The outlet may be of any suitable shape to provide a gas stream which substantially surrounds the carbon dioxide snow stream. The gas stream may be provided through a single outlet which substantially surrounds the carbon dioxide nozzle.

However, the gas stream may be provided by more than one outlet. The gas stream outlet and the carbon dioxide nozzle may be concentric. The gas comprising the gas stream may be any suitable gas. Preferably, the gas is air, although it will be appreciated that any other suitable gas, such as nitrogen, may be used. Preferably the gas is substantially dry. Preferably the gas is clean and substantially free of contaminants, such as dust, particles, organic and inorganic compounds, and the like.

[0033] Preferably, the carbon dioxide nozzle is concentric with the gas stream. The gas stream is provided from any suitable source.

[0034] Preferably, the carbon dioxide source comprises liquid carbon dioxide, although it is appreciated that any suitable carbon dioxide source may be used. Where the carbon dioxide source comprises liquid carbon dioxide, the carbon dioxide may be held at a pressure of around 50 to 70 bar, preferably around 55 to 65 bar, such as 60 bar. However, it will be appreciated that the exact pressure of the carbon dioxide source can be selected depending on the design of the nozzle and the required pressure of the carbon dioxide snow. When the carbon dioxide leaves the nozzle, the pressure will drop to substantially atmospheric pressure. Due to the rapid expansion of the carbon dioxide, the carbon dioxide will form carbon dioxide snow, which can then be used for cleaning.

[0035] According to a sixth aspect of the present invention, there is provided a method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas w'hich substantially surrounds the carbon dioxide snow stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the distance between the surface to be cleaned and the carbon dioxide nozzle is from between around 20 mm to around 100 mm.

[0036] It is known in the art that the nozzle from which the carbon dioxide snow is released should be kept as close to the surface to be cleaned as possible, and is usually held at a distance of from about 5 mm to about 10 mm from the surface. However, it has been surprisingly realized that having the nozzle further away from the surface results in improved cleaning performance. It is also known that carbon dioxide snow' cleaning can be used to clean organic materials, such as oil or grease, from a surface and to also remove particles w'hich are loosely attached to the surface. However, prior to the present invention, it was not appreciated that carbon dioxide snow cleaning could be used to remove metallic contaminants from the surface of a mirror. In an embodiment of the present invention, the nozzle is held at a distance of around 40 mm to around 80 mm from the surface to be cleaned, more preferably around 50 mm to around 70 nun from the surface to be cleaned, and most preferably around 60 mm from the surface to be cleaned. Without wishing to be bound by scientific theory, it is believed that the greater distance between the nozzle and the surface to be cleaned allows the carbon dioxide snow to be accelerated to a higher velocity and thereby strike the surface with a higher energy. Alternatively or additionally, the morphology of the carbon dioxide snow may change over greater distances, which also results in improved cleaning performance. An additional benefit of using a greater distance between the carbon dioxide snow nozzle and the surface to be cleaned is that there is a greater margin of error which reduces the likelihood of the nozzle coming into direct contact with the surface, thereby damaging the surface. Furthermore, again without wishing to be bound by scientific theory, it is believed that the carbon dioxide snow stream may reach its optimal shape at the distances disclosed herein. As the carbon dioxide snow stream is accelerated by the gas stream, it is also shaped and focused to provide the most effective cleaning result.

[0037] According to a seventh aspect of the present invention, there is provided a method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the angle subtended by the surface to be cleaned and the carbon dioxide snow stream is around 45 to around 901 [0038] it is also known in the art that the angle at which the carbon dioxide is directed against the surface to be cleaned should be a low or shallow angle. In particular, the angle subtended between the surface and the direction at which the carbon dioxide snow is directed to the surface is generally around 30' or less. The lowest angle is 0 since this represents the carbon dioxide snow being directed parallel to the surface to be cleaned. It is considered that a low or shallow angle is preferable as any contaminants which are dislodged from the surface are pushed away by the direction of flow of the stream of carbon dioxide snow. However, against what is known in the art, it has been surprisingly discovered that using a higher angle results in improved cleaning of the surface, in particular when metallic contaminants are to be removed from the surface of a mirror. As such, in one embodiment of the present invention, the angle subtended between the surface and the direction at which the carbon dioxide snow is directed to the surface is preferably around 45or more. In this context, the greatest angle will be 9Cf, which represents the case where the stream of carbon dioxide snow is directed at the normal to the surface, i.e. perpendicular to the surface. In other embodiments, the angle is preferably greater than around 6ff, more preferably more than around 75. and most preferably around 90.

[0039] According to an eighth aspect of the present invention, there is provided a method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the direction of the streams of carbon dioxide snow and gas are controlled by a cooperative robot.

[0040] In an embodiment, the cooperative robot is a multiple degrees of freedom robot. Preferably, the cooperative robot is a six degrees of freedom robot.

[0041] Having a cooperative robot control the direction of the streams of carbon dioxide snow and gas, allows the cleaning of the surface to be carefully controlled. Where the carbon dioxide snow cleaning is operated manually, there is a high risk of overcooling portions of the surface. This may be due to an area of the surface having an increased level of contaminant present which a human operator may try to clean by holding the carbon dioxide outlet on that particular area for a longer period of time, which will result in overcooling of the surface. In addition, unlike traditional surfaces which are cleaned by carbon dioxide snow cleaning, the apparatus of the present invention is able to clean much more complex shapes, such as curved surfaces. In manual carbon dioxide cleaning, since it is generally preferable to have the nozzle as close to the surface as possible, there is a risk that the nozzle will come into contact with the surface to be cleaned thereby damaging the surface. There is also a hazard to the human operator due to the use of very cold materials as well as the risk of possible suffocation due to a build-up in carbon dioxide gas in the local atmosphere.

[0042] According to a ninth aspect of the present invention, there is provided a method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow’ stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the stream of gas which substantially surrounds the carbon dioxide snow stream is heated.

[0043] In an embodiment of the ninth aspect of the present invention, the stream of gas which substantially surrounds the carbon dioxide snow stream is heated to from around 20C to around 8ÖC, preferably from around 30’C to around 7ÖC, more preferably from around 4GC to around 65C, and even more preferably to around 5ÖC.

[0044] According to a tenth aspect of the present invention, there is provided a method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow' stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the pressure of the gas stream is from around 1 bar to around 20 bar, more preferably from about 5 bar to about 15 bar, and most preferably substantially about 10 bar.

[0045] According to any of the sixth to tenth aspects of the present invention, the gas stream is preferably provided by passing a stream of gas out of an outlet. The outlet may be of any suitable shape to provide a gas stream which substantially surrounds the carbon dioxide snow stream. The gas stream may be provided through a single outlet which substantially surrounds the carbon dioxide nozzle. However, the gas stream may be provided by more than one outlet. The gas stream outlet and the carbon dioxide nozzle may be concentric.

[0046] The gas comprising the gas stream may be any suitable gas. Preferably, the gas is air, although it will be appreciated that any other suitable gas, such as nitrogen, may be used. Preferably the gas is substantially dry. Preferably the gas is clean and substantially free of contaminants, such as dust, particles, organic and inorganic compounds, and the like.

[0047] The surface to be cleaned may be the surface of a component of a lithographic apparatus. The surface may be the surface of a collector or other reflective surface in a lithographic apparatus. Preferably, the lithographic apparatus is an EUV lithography machine, although in some embodiments, it may be any other type of lithographic machine. The surface to be cleaned may be any other surface of an EU V lithography machine or other lithography machine, for example any optical element, mirror, collector, sensor, or the like.

[0048] According to any of the sixth to tenth aspects of the present invention, the nozzle may be moved across the surface to be cleaned at any suitable speed. Preferably, the nozzle is moved at a speed of from around 10 mm/s to around 1.00 mm/s, more preferably from around 25 mm/s to around 75 mm/s, and even more preferably around 50 mm/s. At these speeds, it has been found that there is optimal cleaning performance, but without the risk of overheating or overcooling the surface.

[0049] According to the method of each of the sixth to tenth aspects of the present invention, as the gas stream forms a curtain substantially surrounding the carbon dioxide snow stream, as the nozzle passes across the surface of the surface being cleaned, the surface and any contaminants are first heated by the gas stream. Due to the different coefficients of expansion between the surface and the contaminant, the strength of the bond between the surface and the contaminant is reduced. As the apparatus continues to pass across the surface, the much colder carbon dioxide snow stream is directed onto the area which has just been heated by the gas stream. The contaminant has a lower thermal mass and therefore contracts more quickly than the surface, which further serves to loosen the contaminant from the surface. As the apparatus further continues to move across the surface, the gas stream then passes across the area which has been cooled by the carbon dioxide snow stream. This heats the area up and stops the temperature of the area from dropping too far and also ensures that no water condenses on the surface. A further benefit of the heated gas stream is that is provides a protective gas layer which is essentially free from water. As such, any moisture which is present on the surface is removed by the heated gas stream before the surface is cooled by the carbon dioxide snow stream. This reduces the possibility of any liquid water or water ice forming on the surface and either damaging the surface or reducing the effectiveness of the cleaning procedure. The gas stream may comprise air, or may comprise any other suitable gas, such as nitrogen. Preferably, the gas stream is substantially dry. Preferably the gas is clean and substantially free of contaminants, such as dust, particles, organic and inorganic compounds, and the like.

[0050] The methods according to the sixth to tenth aspects of the present invention may be combined together in any combination. As such, the parameters defined in the sixth to tenth aspects of the present invention may be covered by a single embodiment of the invention and may be combined with any aspect of the present invention. For example, the subject matter of the sixth aspect of the present invention may be combined with that of the seventh and/or eight and/or ninth and/or tenth aspect of the present invention. The same applies to the seventh, eighth, ninth, and tenth aspects of the present invention, which may be combined with any of the other aspects of the present invention, in this way, disclosed herein is a method of cleaning a surface comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of carbon dioxide snow and gas onto the surface to be cleaned, wherein the distance between the surface to be cleaned and the carbon dioxide nozzle is from between around 20 mm to around 100 mm, and/or the angle subtended by the surface to be cleaned and the carbon dioxide snow stream is around 45 to around 90;, and/or wherein the angle subtended between the surface to be cleaned and the direction of the carbon dioxide snow stream is from around 45° to around 90, and/or wherein the direction of the streams of gas and carbon dioxide snow are controlled by a cooperative robot and/or wherein the gas stream is heated to from around 2ÖC to around 8GC and/or wherein the pressure of the gas stream is from around 1 bar to around 20 bar.

[0051] In an alternative embodiment wliich does not utilize carbon dioxide snow cleaning, the method of cleaning a surface may comprise the steps of: passing a stream of hydrochloric acid from a nozzle onto the surface to be cleaned, wherein the hydrochloric acid stream is in communication with an acoustic transducer.

[0052] The acoustic transducer causes pressure fluctuations to be generated at the surface to be cleaned. It has been surprising found that the combination of hydrochloric acid and the pressure fluctuations results in an improved performance over what would be expected from the combination of the two.

[0053] In an embodiment, the direction of the stream of hydrochloric acid is controlled by a robot. The stream of hydrochloric acid may be passed from an ultrasonic-activated liquid gun, as sold under the name StarstreamTM by Ultrawave Precision Ultrasonic Cleaning Equipment Limited.

[0054] The robot used in the method utilizing a stream of hydrochloric acid may be the same as the robot used in the method utilizing a carbon dioxide snow stream.

[0055] In an alternative embodiment, there may be provided a hydrochloric acid cleaning apparatus comprising; a hydrochloric acid stream; a hydrochloric acid nozzle in fluid communication with the hydrochloric acid stream; wherein the apparatus further comprises an ultrasonic transducer in communication with the hydrochloric acid stream.

[0056] By having an ultrasonic transducer in communication with the hydrochloric acid stream, when the hydrochloric acid stream is directed onto a surface to be cleaned, the ultrasonic transducer produces pressure fluctuations at the surface, which serve to enhance the cleaning ability of the hydrochloric acid. The combination of hydrochloric add and the pressure fluctuations result in improved cleaning performance over what would be expected.

[0057] The hydrochloric acid stream may be passed out of a nozzle. The hydrochloric acid is preferably an aqueous solution of hydrogen chloride. The molarity of the solution can be varied depending on the particular cleaning requirements and the required concentration may be determined routinely.

[0058] The apparatus may further comprise a robot which is coupled to the nozzle and is able to direct the nozzle, and therefore the direction of the hydrochloric acid stream. The robot may be a robot as described in respect of any aspect of the present invention. The apparatus may comprise a camera. The camera may be used to monitor the progress of the cleaning.

[0059] The apparatus may also comprise a vessel to house the object to be cleaned, such as a collector. The vessel is substantially sealed in order to stop the hydrochloric acid entering the surroundings and to protect users. The vessel may also comprise a drain to allow any waste liquid, tin fragments and other materials to exit the vessel. The vessel may also be provided with an exhaust to allow any gaseous products to exit the vessel.

[0060] It will be appreciated that hydrochloric acid is the preferred acid, although any other suitable acid may be used. A suitable acid is one which is able to react with the contaminant to be removed, such as tin. but which is substantially inert to the surface to be cleaned and therefore does not damage the surface when used or which reacts with the surface considerably more slowly than it reacts with the contaminant.

[0061] According to an eleventh aspect of the present invention, there is provided the use of carbon dioxide snow cleaning for cleaning an EUV lithography apparatus. It will also be appreciated that carbon dioxide snow cleaning may be used to clean any surface of the lithography apparatus, including any optical element, mirror, collector, sensor, wall, or the like. The use may comprise the use of a carbon dioxide cleaning apparatus in combination with a cooperative robot, which preferably has multiple degrees of freedom, more preferably six degrees of freedom. Preferably, there is provided the use of the method according to any of the sixth to the tenth aspects of the present invention to clean an EUV lithography apparatus. In another embodiment, the apparatus according to any of the first to fifth aspects of the present invention is used to clean an EUV lithography apparatus. Again, carbon dioxide snow cleaning may be used to clean any surface of the lithography apparatus, including any optical element, mirror, collector, sensor, wall, or the like.

[0062] In a further aspect of the present invention, there is provided the use of ultrasonic hydrochloric acid for cleaning an EUV lithography apparatus. It will also be appreciated that ultrasonic hydrochloric acid cleaning may be used to clean any surface of the lithography apparatus, including any optical element, mirror, collector, sensor, wall, or the like. Ultrasonic hydrochloric acid describes a stream of hydrogen chloride solution in communication with an ultrasonic transducer. The use may comprise the use of an ultrasonic hydrochloric acid cleaning apparatus in combination with a cooperative robot, w'hich preferably has multiple degrees of freedom, more preferably six degrees of freedom.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] 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 which may be cleaned using the apparatus and/or methods of the present invention;

Figure 2 depicts a schematic depiction of the carbon dioxide snow cleaning apparatus according to the first to fifth aspects of the present invention; and

Figure 3 depicts a schematic depiction of the hydrochloric acid cleaning apparatus according to a further aspect of the present invention.

DETAILED DESCRIPTION

[0064] Figure 1 shows a lithographic system including a collector 5 which may become contaminated with tin and thereby require cleaning using the apparatus or methods of the present invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. 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. A layer of the resist composition is provided on the substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[0065] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e. g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system 1L and/or the projection system FS.

[0066] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source. A laser 1, which may for example be a C02 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3. 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 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.

[0067] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below. Due to the way in which the EUV radiation is generated, the collector 5 may be contaminated with tin, which will reduce its reflectivity and thereby lower the power output of the apparatus.

[0068] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser I 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 1 and the radiation source SO may together be considered to be a radiation system.

[0069] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system 1L. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[0070] The radiation beam B passes from the radiation source SO into the illumination system 1L, which is configured to condition the radiation beam. The illumination system 1L 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 radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. 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.

[0071] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The projection system comprises a plurality of mirrors which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors in Figure 1, the projection system may include any number of mirrors (e.g. six mirrors).

[0072] The radiation sources SO shown in Figure 1 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral lilter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[0073] The term “EUV radiation” may be considered to encompass electromagnetic radiation having a wavelength within the range of 4-20 nm, for example within the range of 13-14 nm. EUV radiation may have awavelength of less than 10 nm, for example within the range of 4-10 nm such as 6.7 nm or 6.8 nm.

[0074] Although Figure 1 depicts the radiation source SO as a laser produced plasma LPP source, any suitable source may be used to generate EUV radiation. For example, EUV emitting plasma may be produced by using an electrical discharge to convert fuel (e.g. tin) to a plasma state. A radiation source of this type may be referred to as a discharge produced plasma (DPP) source. The electrical discharge may be generated by a power supply w'hich may form part of the radiation source or may be a separate entity that is connected via an electrical connection to the radiation source SO.

[0075] Figure 2 show s a schematic illustration of a carbon dioxide cleaning apparatus in accordance with the first to fifth aspects of the present invention in cross-section. A carbon dioxide source (not shown), which may be a cylinder of liquid carbon dioxide, is attached to a carbon dioxide snow nozzle 12. As the carbon dioxide stream 15 passes out of the nozzle 12 and into zone 14, the liquid carbon dioxide expands and at least partially freezes forming a carbon dioxide snow. A gas stream 13 is provided substantially surrounding the carbon dioxide snow stream 15. The gas stream 13a, b assists in accelerating the carbon dioxide snow stream 15. Both streams are directed onto surface 5 by a six-degrees of freedom cooperative robot (not shown). The gas stream 13 is heated to around 50C and the distance between the carbon dioxide snow nozzle 12 and the collector 5 is substantially around 60 mm. The pressure of the gas stream 13 is substantially around 10 bar. The angle Θ between the direction of the carbon dioxide snow stream 15 and the surface of the collector 5 is preferably substantially around 90°.

[0076] In use, the apparatus comprising the carbon dioxide snow nozzle 12 is passed across the surface of the collector 5 at a rate of around 50 mm/s. Taking the case where the apparatus is passed from the right to the left of the figure, a first portion of the gas stream 13a passes over the contaminant D and heats the contaminant D. This causes the contaminant D to expand as it heats up and the contaminant D will heat up more quickly than the underlying collector 5 meaning that they will expand at different rates, thus loosening the bond between the contaminant D and the surface of the collector 5. As the apparatus continues to pass across the surface of the collector 5, as the carbon dioxide snow comes into contact with contaminant D, this will rapidly cool the contaminant D and further weaken the bond between the contaminant D and the surface of the collector 5. The carbon dioxide snow' will also physically dislodge the contaminant D and push it away from the surface of the collector 5. As the apparatus continues to pass across the surface of the collector 5, a second portion of the gas stream 13b will pass across the area where the contaminant D was previously located and will warm the surface of the collector 5 up in order to avoid overcooling of the collector 5 and possible water ice or condensation formation.

[0077] Figures 3 is a schematic illustration of an apparatus utilising ultrasonic hydrochloric acid cleaning in cross-section. The apparatus comprises a vessel 19 which houses a hydrochloric acid nozzle 20. The hydrochloric acid stream 18 which is passed out of the nozzle 20 is in communication with an ultrasonic transducer (not shown). The ultrasonic transducer induces ultrasonic pressure waves in the hydrochloric acid stream w’hich serve to improve the cleaning performance of the hydrochloric acid. The vessel 19 comprises an outlet 17 w'hich allows liquids and solids, such as tin fragments or hydrochloric acid solution, to leave the vessel 19. The vessel 19 also comprises an exhaust 16 to allow any gaseous components to leave the vessel 19.

[0078] In use, the hydrochloric acid stream 18 is directed onto the surface of the collector 5 w'hich is being cleaned. The pressure fluctuations in the hydrochloric acid stream 18 caused by the ultrasonic transducer assist the hydrochloric acid in removing contaminants, such as tin, from the surface of the collector 5. The hydrochloric acid stream 18 is passed across the surface to be cleaned 5 and is preferably controlled by a cooperative robot. In this way, the robot can be programmed to ensure optimal cleaning of the surface.

[0079] The present invention utilises operating conditions which go against the common general knowledge in the field with respect to the distance between the carbon dioxide nozzle and the surface to be cleaned, and the angle at which the carbon dioxide snow stream is incident on the surface to be cleaned. Each of these parameters results in improved cleaning performance when compared to the parameters of the prior art. Further, the combination of a cooperative robot with a carbon dioxide snow cleaning apparatus surprisingly allows improved cleaning performance when compared to existing systems. Furthermore, this combination allow's collectors of EUV lithographic apparatus to be cleaned without the risk of damage. In addition, the combination of a hydrochloric acid stream and an ultrasonic transducer improves the cleaning performance of just hydrochloric acid alone.

[0080] 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 used in lithographic applications. An element of a lithographic apparatus may be for example a mirror or a sensor, . Embodiments of the invention may form a part (or element) of an EUV radiation source, 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.

[0081] 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses: 1. A carbon dioxide snow cleaning apparatus for cleaning a surface of an element of a lithographic apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle; wherein the distance between the carbon dioxide snow nozzle and a surface to be cleaned is variable from between around 20 mm to around 100 mm. 2. Apparatus according to Clause 1, wherein the carbon dioxide snow nozzle is separated from the surface by from around 40mm to around 80 mm, preferably around 55 mm to around 65 mm, and most preferably around 60 mm. 3. A carbon dioxide snow cleaning apparatus for cleaning a surface of an element of a lithographic apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzlewherein the angle subtended between the surface to be cleaned and the direction of the carbon dioxide snow stream is from around 45° to around 90°, and is preferably greater than around 6Cf, more preferably greater than around 15, and most preferably substantially around 9ff. 4. A carbon dioxide snow cleaning apparatus for cleaning a surface of an element of a lithographic apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle, wherein the apparatus further comprises a cooperative robot. 5. Apparatus according to Clause 4, wherein the cooperative robot is a multiple degrees of freedom robot, preferably a six degrees of freedom robot. 6. A carbon dioxide snow cleaning apparatus for cleaning a surface of an element of a lithographic apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle, wherein the apparatus further comprises a heater to heat the gas stream.

7. Apparatus according to Clause 6, wherein the heater heats the gas stream to temperatures of from around 2ÜC to around 80C, preferably from around 30C to around 70C, more preferably from around 4ÖC to around 65C, and even more preferably to around 50C 8. A carbon dioxide snow cleaning apparatus for cleaning a surface of an element of a lithographic apparatus comprising; a carbon dioxide source; a carbon dioxide snow nozzle in fluid communication with the carbon dioxide source; a gas stream substantially surrounding the carbon dioxide snow nozzle, wherein the pressure of the gas stream is from around 1 bar to around 20 bar, more preferably from about 5 bar to about 15 bar, and most preferably substantially about 10 bar. 9. Apparatus according to any preceding clause, wherein the gas stream is substantially dry and/or clean. 10. Apparatus according to any preceding clause, wherein the gas stream comprises air, nitrogen, or any other suitable gas. 11. A method of cleaning a surface of an element of a lithographic apparatus, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of gas and carbon dioxide snow onto the surface to be cleaned, wherein the distance between the surface to be cleaned and the carbon dioxide nozzle is from between around 20 mm to around 100 mm. 12. A method according to Clause 11, wherein the distance between the surface to be cleaned and the carbon dioxide nozzle is from between around 40 mm to around 80 mm. 13. A method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of gas and carbon dioxide snow' onto the surface to be cleaned, wherein the angle subtended by the surface to be cleaned and the carbon dioxide snow' stream is greater than around 60:, preferably greater than around 75‘, and more preferably around 9ff. 14. A method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow str eam; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of gas and carbon dioxide snow' onto the surface to be cleaned, wherein the direction of the streams of gas and carbon dioxide snow' are controlled by a cooperative robot. 15. A method according to Clause 11, wherein the cooperative robot is a multiple degrees of freedom robot, and preferably wherein the robot is a six degrees of freedom robot. 16. A method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snow stream; directing the streams of gas and carbon dioxide snow onto the surface to be cleaned, wherein the gas stream is heated to from around 20C to around 80C, preferably from around 30C to around 7ÖC, more preferably from around 40C to around 65C, and even more preferably to around 50C. 17. A method of cleaning a surface, the method comprising the steps of: passing a stream of carbon dioxide out of a carbon dioxide nozzle to form a carbon dioxide snow stream; providing a stream of gas which substantially surrounds the carbon dioxide snowr stream; directing the streams of gas and carbon dioxide snow' onto the surface to be cleaned, wherein the pressure of the gas stream is from around 1 bar to around 20 bat', more preferably from about 5 bar to about 15 bar, and most preferably substantially about 10 bar 18. A method according to any of clauses 11 to 17, wherein the carbon dioxide nozzle is moved across the surface to be cleaned at a rate of from around 10 mm/s to around 100 mm/s, preferably from around 25 mm/s to around 75 mm/s, and more preferably around 50 mm/s. 19. A method according to any of clauses 11 to 18, wherein the gas stream is provided by an outlet wliich substantially surrounds the carbon dioxide nozzle. 20. A method according to Clause 19, wherein the gas stream outlet is concentric with the carbon dioxide nozzle. 21. A method according to any of clauses 11 to 20, wherein the surface to be cleaned is a surface of a lithographic apparatus, which may be an EUV lithography apparatus. 22. A method according to any of clauses 11 to 20 where the surface to be cleaned is the surface of an optical element of an EUV lithography apparatus. 23. The use of a method according to any of Clauses 11 to 22 or the apparatus according to any of Clauses 1 to 10 to clean a surface of a lithographic apparatus. 24. The use according to Clause 23, wherein the surface is an optical element of an EUV lithographic apparatus.

Claims (1)

A lithography device comprising: an illumination device adapted to provide a radiation beam; a carrier constructed to support a patterning device, the patterning device being able to apply a pattern in a section of the radiation beam to form a patterned radiation beam; a sub-slate table constructed to support a substrate; and a projection device adapted to project the patterned radiation beam onto a target area of the substrate, characterized in that the substrate table is adapted to position the target area of the substrate in a focal plane of the projection device.