Method, device and system for sensor surface cleaning

Abstract

The invention provides a method for cleaning a sensor surface using a cleaning device which comprises a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space, and an extract arrangement, wherein the method comprises: placing the cleaning device in proximity to the sensor surface, delivering the cleaning gas and the cleaning liquid to the cleaning space, controlling the cleaning device in such a way that during use a cleaning mixture is created and confined in the cleaning space, the cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both, extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement, and removing the cleaning device from the sensor.

Description

METHOD. DEVICE AND SYSTEM FOR SENSOR SURFACE CLEANINGCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24182838.3 which was filed on 18 June 2024 which is incorporated herein in their entirety by reference.FIELD

[0002] The present invention relates to a method for cleaning a sensor surface using a cleaning device, to a cleaning device for use in the method, and to a cleaning system and an exposure apparatus comprising such a cleaning device.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 (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law.’ To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. 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. 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 are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 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] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on the substrate. A critical aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, a lithographic apparatus may comprise an optical alignment system which may be used to determine and improve an alignment between the patterning device and the substrate. The patterning device may include a marker that may be imaged by aprojection system of the lithographic apparatus. The marker may impart a radiation beam with a pattern or image, such as an aerial image of the marker, which may subsequently be measured in order to derive one or more properties of the lithographic apparatus. The optical alignment system may comprise a sensor apparatus, for example an image sensor module, configured to detect the image of the marker and thereby determine a position of the substrate relative to the patterning device, and / or determine a property of the projection system, for example image quality and optical aberrations.SUMMARY

[0006] It is an object of the invention to provide a method for cleaning a sensor surface using a cleaning device. It is a further object of the invention to provide a cleaning device and a cleaning system for use in the improved method for cleaning a sensor surface. According to a first aspect of the invention, there is provided a method for cleaning a sensor surface using a cleaning device, the cleaning device comprising: a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space between the cleaning device and the sensor surface during use, and an extract arrangement, wherein the method comprises the steps of: placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, delivering the cleaning gas and the cleaning liquid to the cleaning space, controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both, extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement, and removing the cleaning device from the sensor.

[0007] The method cleans the sensor surface by providing a two-phase, i.e. gas-liquid, cleaning mixture in contact with the sensor surface, wherein contact surfaces between the cleaning gas and the cleaning liquid are formed during use. By doing so, the surface tension of the cleaning liquid at the contact surfaces provides an increased force to remove contaminants from the sensor surface, such that an improved cleaning effect, and thereby an improved reliability and durability of the sensor, may be achieved.

[0008] The cavity in a bottom side of the cleaning device, as shown in the figures, may form part of cleaning space.

[0009] The cleaning gas may be air, preferably clean dry air, further preferably extra clean dry air (XCDA). The cleaning gas may also be oxygen, carbon dioxide, hydrogen, nitrogen, or a mixture of two or more of the above. The cleaning liquid may be water, preferably ultra-pure water. The cleaning gas and cleaning liquid may also be selected depending on the material and / or compositionof the contaminants on the sensor surface for an improved cleaning effect. For example acidic gas and / or liquid may be chosen to remove alkaline contaminants. To improve the cleaning effect, it is beneficial to choose a combination of the cleaning gas and the cleaning liquid such that the cleaning gas does not easily dissolve in the cleaning liquid under the working temperature and pressure, and gas-liquid contact surfaces, in a form like for example bubbles, may be formed and maintained for an improved cleaning effect.

[0010] The sensor may be a level sensor, an alignment sensor, an image sensor, a laser beam sensor, a position sensor, or any other sensor which is sensitive to contamination and need to be cleaned to maintain a desirable measurement accuracy. The method for cleaning a sensor surface may also be applied in other applications, for example cleaning a surface of an object like a substrate table, a substrate, or an optical element.

[0011] The cleaning device may comprise a gas channel, a gas inlet, and a liquid channel, wherein the gas inlet opens up into the liquid channel. The controlling the cleaning device may comprise controlling a flow rate of the cleaning liquid to create a reduced pressure in the liquid channel with respect to the gas channel, allowing the cleaning gas to enter the liquid channel via the gas inlet, thereby forming the cleaning mixture. The controlling the flow rate of the cleaning liquid may comprise measuring a flow rate of the cleaning liquid, and regulating using a control valve the flow rate regardless of pressure fluctuation. As such, changes in various pressure settings based on a command signal or setpoint may be compensated.

[0012] The controlling the cleaning device may further comprise controlling a parameter to form a confined flow of the cleaning mixture across the sensor surface, that substantially no cleaning mixture leaks to the surrounding environment. The parameter may be the reduced pressure at the extract arrangement relative to the pressure at the supply arrangement.

[0013] The supply arrangement of the cleaning device may have a central inlet that opens into a central location of the cleaning space. The cleaning device may have multiple outlets at an outer side of the cleaning device, preferably each at a radial distance from said inlet. The method may further comprise delivering the cleaning gas and the cleaning liquid to the cleaning space via the central inlet, and extracting the cleaning mixture and any cleaning residue via the multiple outlets, such that the confined flow is radially outwards. The multiple outlets may be arranged to cover more than 20% of an area at the outer side of the cleaning device for an effective cleaning of the sensor surface. The area of the outer side of the cleaning device may have for example a hollow shape like a hollow circle, oval, and rectangle, which encloses the multiple outlets.

[0014] The delivering the cleaning gas and cleaning liquid may also be from one side of the cleaning device, and the extracting the cleaning mixture and any cleaning residue being from an opposite side of the cleaning device, such that the confined flow is substantially one-directional.

[0015] The delivering the cleaning gas and the cleaning liquid may alternatively be substantially from multiple inlets at an outer side of the cleaning device, and extracting the cleaning mixture andany cleaning residue being substantially from a center of the cleaning device, such that the confined flow is radially inwards.

[0016] The choice of the inlets and outlets arrangement may be based on the shape of the sensor surface to be cleaned, and / or the source of the contaminants. It is beneficial to choose an arrangement that the cleaning mixture flow flushes across substantially the whole sensor surface, and the flow does not introduce additional contaminants to the sensor surface.

[0017] The method may further comprise measuring a parameter or a property indicative of the cleanliness of the sensor surface. The measuring the parameter or the property may comprise measuring and / or analysing a parameter or the property of the cleaning mixture and any cleaning residue, or examining the sensor surface using an optical sensor or a camera.

[0018] The method may further comprise controlling the cleaning device and / or other steps of the method at least partially based on the measured parameter or property.

[0019] According to a second aspect of the invention, there is provided a cleaning device for use in the method according to the invention for cleaning a sensor surface, wherein the cleaning device comprises, a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space between the cleaning device and the sensor surface, and an extract arrangement.

[0020] The cleaning device may further have any of the above described features of the cleaning device used in the method according to the first aspect of the invention.

[0021] The cleaning device may also be applied in other applications, for example cleaning a surface of an object like a substrate table, a substrate, or an optical element.

[0022] According to a third aspect of the invention, there is provided a cleaning system configured to clean a sensor surface, for use in the method according to the first aspect of the invention for cleaning a sensor surface, wherein the cleaning system comprises, the cleaning device of the second aspect, a controller for controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both.

[0023] The cleaning system may further comprise a measurement device for measuring a parameter or property indicative of cleanliness of the sensor surface, the measurement unit may be a chemical analyzer, a camera, or an optical sensor comprising a light source, a detector and / or optical elements, or any measurement device suitable for measuring a relevant parameter or property indicative of the cleanliness of the sensor surface.

[0024] The cleaning system may also be applied in other applications, for example cleaning a surface of an object like a substrate table, a substrate, or an optical element.

[0025] According to a fourth aspect of the invention, there is provided a data processing apparatus comprising means for carrying out the method of the first aspect.

[0026] According to a fifth aspect of the invention, there is provided a computer-readable storage comprising instructions which, when executed by a computer, cause the computer to carry out the method of the first aspect.

[0027] According to a sixth aspect of the invention, there is provided an exposure apparatus comprising a sensor and a surface thereof, and the cleaning device of the second aspect or the cleaning system of the third aspect, wherein the cleaning device or the cleaning system is configured to clean the sensor surface using the method of the first aspect. The exposure apparatus may be a lithographic apparatus, or more specifically a dry lithographic apparatus. The exposure apparatus may also be an optical metrology apparatus for measuring overlay, edge placement error, or critical dimension of exposed patterns on an integrated circuit substrate. The exposure apparatus may also be an imprint apparatus, a packaging apparatus or an optical or electron-beam inspection apparatus for defect inspection of a substrate. The exposure apparatus may also be a stand-alone measurement apparatus for measuring wafer alignment and / or leveling before the wafer being transferred to and exposed by another exposure apparatus like a lithographic apparatus.

[0028] Embodiments of the invention which are described herein may have use in an EUV lithographic apparatus. Embodiments of the invention may have use in a deep ultraviolet (DUV) lithographic apparatus or another form of lithographic apparatus, such as, for example, nano-imprint systems and advanced packaging systems. Embodiments of the invention may have use in optical systems that form part of other substrate processing apparatus such as, for example, e-beam systems, metrology, or inspection systems.BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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 schematic overview of a lithographic apparatus;Figure 2(a-b) schematically depicts a perspective view and a top view, respectively, of a cleaning device for use in a method according to some embodiments;Figure 2c depicts cross-section A-A of Figure 2b;Figure 3 depicts a flow diagram of a method according to some embodiments;Figure 4 schematically depicts the cross-section of the cleaning device of Figure 2c, in use in the method according to Figure 3;Figure 5 depicts a flow diagram of a method according to some other embodiments;Figure 6 depicts the cross-section of the cleaning device of Figure 2c, in use when being controlled according to the flow diagram of Figure 5;Figures 7(a) and 7(b) schematically depict a perspective view and a bottom view of a cleaning device for use in a method according to some embodiments;Figure 8 depicts a flow diagram of a method according to some other embodiments;Figure 9(a-c) depict side views of three cleaning systems for use in the method of Figure 8, wherein the cleaning device is shown in cross-section in accordance with Figure 2c;Figure 10 depicts a flow diagram of a method according to some embodiments;Figure 11 schematically depicts a side view of a cleaning system for use in the method of Figure 10;DETAILED DESCRIPTION

[0030] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0031] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0032] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0033] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0034] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0035] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fdl a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0036] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0037] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid or a surface of the sensor. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0038] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0039] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis, and a z-axis. Each of the three axis is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y- axis is referred to as an Ry-rotation. A rotation around about the z-axis is referred to as an Rz -rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.

[0040] The lithographic apparatus LA may comprise an optical alignment system which may be used to determine and improve an alignment between the patterning device and the substrate. The patterning device MA may include a marker that may be imaged by a projection system of the lithographic apparatus. The marker may impart a radiation beam with a pattern or image, such as an aerial image of the marker, which may subsequently be measured in order to derive one or more properties of the lithographic apparatus. The optical alignment system may comprise a sensor apparatus, for example an image sensor module, configured to detect the image of the marker and thereby determine a position of the substrate relative to the patterning device, and / or determine a property of the projection system, for example image quality and optical aberrations. The sensor apparatus may comprise a sensor surface where patterns, for example optical gratings, are arranged to receive an image of the marker and to create diffraction lights to be measured by an image sensor module. The sensor surface may be contaminated due to outgassing of the photoresist, redeposited on the sensor surface, and catalysed by the exposure light. The contamination impacts the quality of the alignment and / or image quality measurements. The contaminants may not be easily removed by manual cleaning or ultrasound cleaning, because of the risk of damage to the delicate optical gratings on the sensor surface.

[0041] Figure 2(a) schematically depicts a perspective view of a cleaning device for cleaning a sensor surface according to some embodiments. Figure 2(b) is a top view of the cleaning device and Figure 2(c) is a cross section of the cleaning device seen from A-A of Figure 2(b). The cleaning device CD comprises a supply arrangement SUP for delivering a cleaning gas and a cleaning liquid to a cleaning space CSP and an extract arrangement EXT for extracting a cleaning mixture and any cleaning residue from the cleaning space CSP. The cavity CV in the bottom side of the cleaning device, forms part of cleaning space.

[0042] Figure 3 depicts a flow diagram of a method according to a first embodiment, wherein the method comprises steps of:- SI: placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, that means, the cleaning space is bounded by, or defined by, the cleaning device and the sensor surface,- S2: delivering the cleaning gas and the cleaning liquid to the cleaning space,- S3 : controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both,- S4: extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement, and- S5 : removing the cleaning device from the sensor.

[0043] The steps S1-S5 are not necessarily performed one after another according to the sequence shown in Figure 3. The steps may be performed in a different sequence, for example first controlling the cleaning device according to a pre-defined setting, then placing the cleaning device in proximity to a sensor, and then delivering the cleaning gas and cleaning liquid to the cleaning space. Another example may be performing S2, S3 and S4 in parallel. Other sequences of performing the steps may be chosen based on specific circumstances.

[0044] Figure 4 schematically depicts the cross-section of the cleaning device of Figure 2c, in use in the method according to Figure 3. The working principle of the method after the cleaning device being placed in proximity to a sensor SENS and before being removed from the sensor is further explained in more details using Figure 4. The cleaning gas GAS and the cleaning liquid LIQ are delivered via the supply arrangement SUP to the cleaning space CSP. The gap between the lower surface of the cleaning device and the sensor surface, i.e. the height of the cleaning space CSP is exaggerated in the figures to explain the invention. It is preferred to place the cleaning device as close to the sensor surface as possible for a better cleaning effect and a better confinement of the cleaning mixture. The gap may be determined based on mechanical tolerance of structures, for example support structure of the cleaning device.

[0045] The cleaning gas may be for example air, preferably clean dry air, further preferably extra clean dry air (XCDA), Carbon dioxide, Hydrogen, Nitrogen, or a mixture of two or more of the above. The cleaning liquid may be for example water, preferably ultra-pure water, or any liquid suitable for the purpose. The cleaning gas and cleaning liquid may also be selected depending on the material and / or composition of the contaminants on the sensor surface for an improved cleaning effect. For example acidic substances may be chosen to remove alkaline contaminants. To improve the cleaning effect, it is beneficial to choose a combination of the cleaning gas and the cleaning liquid such that the cleaning gas does not easily dissolve in the cleaning liquid under the working temperature and air pressure, and gas-liquid contact surfaces, in a form like for example bubbles, may be formed and maintained.

[0046] The sensor may be for example a level sensor, an alignment sensor, an image sensor, a laser beam sensor, or a position sensor, which is sensitive to contamination and need to be cleaned to maintain a desirable measurement accuracy. The cleaning device is controlled during use to create acleaning mixture MIX comprising the cleaning gas and the cleaning liquid. The said cleaning mixture is in contact with the sensor surface SF. The cleaning gas and the cleaning liquid define contact surfaces CSF between both, in a form like for example bubbles.

[0047] The cleaning mixture with any cleaning residue from the cleaning space is then extracted via the extract arrangement EXT by applying a reduced pressure P_ext. A certain level of the reduced pressure relative to the pressure at the supply arrangement SUP is required to confine the cleaning mixture within the defined cleaning space, preventing the cleaning mixture from leaking and contaminating surrounding environments like wafer table and wafer stage. In the embodiment shown in Fig. 4, the flow of the cleaning liquid and the reduced pressure at the extract arrangement together create a flow of the cleaning mixture in the directions indicated by the arrows, flushing across the sensor surface and removing contaminants. Compared to a cleaning fluid comprising only a gas or a liquid, the said cleaning mixture comprises contact surfaces CSF between the cleaning gas and the cleaning liquid, thereby the surface tension of the cleaning liquid at the contact surfaces provides an increased cleaning force to remove contaminants from the sensor surface, thus, to improve the cleaning efficiency.

[0048] Figure 5 depicts a flow diagram of another embodiment according to the invention, wherein the method comprises steps of:- SI: Placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, that means, the cleaning space is bounded by, or defined by, the cleaning device and the sensor surface;- S2: delivering the cleaning gas and the cleaning liquid to the cleaning space;- S3 : controlling a flow rate of the cleaning liquid to create a reduced pressure in a liquid channel with respect to a gas channel, allowing the cleaning gas to enter the liquid channel via a gas inlet, thereby forming a cleaning mixture, wherein the cleaning gas and the cleaning liquid define contact surfaces between both;- S4: controlling the cleaning device in such a way that during use the cleaning mixture being confined in the cleaning space, and in contact with the sensor surface;- S5: extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement;- S6: removing the cleaning device from the sensor.

[0049] The skilled person should understand that the steps S1-S6 are not necessarily performed one after another according to the sequence shown in Figure 5. The steps may be performed in a different sequence, or may be performing in parallel. Other sequences of performing the steps may be chosen based on specific circumstances. Embodiments of Figure 5 being used are further described using Figure 6.

[0050] Figure 6 schematically depicts a side view of the cleaning device being controlled by a controller CTR using the method in Figure 5. The cleaning device comprises a gas channel GASCH, a gas inlet GASINL, one or more outlets OUTL and a liquid channel LIQCH, wherein the gas inlet GASINL opens up into the liquid channel LIQCH. The controlling the cleaning device comprises controlling a flow rate of the cleaning liquid. According to Coanda effect, a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surrounding tends to develop a region of lower pressure. The flow of the cleaning liquid in the liquid channel LIQCH at a certain flow rate creates a reduced pressure in the liquid channel with respect to the gas channel GASCH, allowing the cleaning gas to enter the liquid channel via the gas inlet GASINL, thereby mixing with the cleaning liquid and forming the cleaning mixture.

[0051] The flow rate of the cleaning liquid also partially influences the flow rate of the cleaning mixture MIX across the sensor surface SF. The flow rate of the cleaning liquid can be controlled by the controller CTR. A higher flow rate creates a more reduced pressure, allowing more cleaning gas to enter the liquid channel and mix with the cleaning liquid, and increases the flow rate of the cleaning mixture. A lower flow rate creates a less reduced pressure, allowing less cleaning gas to enter the liquid channel, and decreases the flow rate of the cleaning mixture. The flow rate may be controlled for a desired bubble and thus the mentioned contact surfaces formation and sufficient confinement of the cleaning mixture without leakage. As such, the amount of cleaning gas in the cleaning mixture and the flow rate of the cleaning mixture can be controlled to achieve a desired cleaning efficiency. The controller may comprise a flow rate sensor for measuring the flow rate of the cleaning liquid, and a control valve to regulate the flow rate regardless of pressure fluctuation. The controller may be configured to compensate for changes in various settings based on a command signal or setpoint.

[0052] Alternatively or additionally, a reduced pressure P ext at the extract arrangement can also be controlled by the controller CTR. A more reduced pressure P ext increases the extraction force applied at the extract arrangement and thus at the outlets, thereby increases the flow rate of the cleaning mixture across the sensor surface. The level of the reduced pressure P_ext can be controlled by the controller to achieve a desired cleaning efficiency, and a sufficient confinement of the cleaning mixture in the cleaning space. P ext may also be adjusted depending on one or more parameters, for example the dimension of the cleaning space defined by the cleaning device design and placement, distribution, and dimensions of the outlets OUTL, properties of the cleaning mixture, for example viscosity, and flow rate of the cleaning liquid.

[0053] Figure 7(a) schematically depicts a perspective view of a cleaning device for use in a method according to some embodiments. Figure 7(b) depicts a bottom view of the cleaning device of Figure 7(a). The cleaning device of Figure 7 is designed to have a supply arrangement SUP at a center of the cleaning device, a plurality of outlets OUTL arranged at an outer side of the cleaning device, and an extract arrangement EXT connected with all the outlets, thereby the delivering the cleaning gas and the cleaning liquid is substantially from the center of the cleaning device, and the extracting thecleaning mixture and any cleaning residue is from the outer side of the cleaning device, such that the confined flow is radially outwards, as indicated by the arrows in Figure 7(b). The cross-section of the outlets may be circular as shown in Figure 7(b), or other shapes for example oval, rectangular, or square. The coverage of the outlets is preferable larger than 20% of an area at the outer side of the cleaning device to sufficiently confine the cleaning mixture within the cleaning space and extract the cleaning mixture and any cleaning residue. In the embodiment of Figure 7, the area is a hollow circle HC enclosing the outlets.

[0054] The skilled person should understand that the arrangement of the supply arrangement, the extract arrangement and the outlets may be designed differently, as long as a confined flow of the cleaning mixture substantially flushes across the sensor surface covered by the cleaning space. For example in another embodiment, the delivering the cleaning gas and cleaning liquid may be from one side of the cleaning device, and the extracting the cleaning mixture and any cleaning residue is from an opposite side of the cleaning device, such that the confined flow is substantially one-directional. In yet another embodiment, the delivering the cleaning gas and the cleaning liquid is substantially from multiple inlets at an outer side of the cleaning device, and extracting the cleaning mixture and any cleaning residue is substantially from a center of the cleaning device, such that the confined flow is radially inwards.

[0055] Figure 8 depicts a flow diagram of a further embodiment according to the invention, wherein the method comprises steps of:- SI: placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, that means, the cleaning space is bounded by, or defined by, the cleaning device and the sensor surface;- S2: delivering the cleaning gas and the cleaning liquid to the cleaning space;- S3 : controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both;- S4: extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement;- S5: removing the cleaning device from the sensor; and- S6: Measuring a parameter or a property indicative of the cleanliness of the sensor surface.

[0056] Compared to the method of Figure 3, the method of Figure 8 further comprises measuring a parameter or a property indicative of the cleanliness of the sensor surface. The measuring may be conducted at any stage, for example before, during or after cleaning the sensor surface using the method according to previous embodiments. The steps S1-S5 are not necessarily performed one after another according to the sequence shown in Figure 8. The steps may be performed in a different sequence, or may be performing in parallel. Other sequences of performing the steps may be chosenbased on specific circumstances. Embodiments of Figure 8 being used are described below using Figure 9.

[0057] Figure 9(a-c) schematically depict side views of the cleaning systems for use in the method of Figure 8 according to some embodiments. The cleaning system of Figure 9(a) comprises a cleaning device of Figure 4, and a sensor SENS comprising a sensor surface SF to be cleaned. The sensor surface SF may comprise patterned structures, for example a diffraction grating DG. Such diffraction grating may be used for measurements like for example wafer alignment, reticle alignment and leveling.

[0058] The cleaning system further comprises an optical sensor for examining the sensor surface. The optical sensor comprises a light source, preferably a laser for illuminating the sensor surface SF and a detector for detecting the reflected and / or diffracted light from the sensor surface. The optical sensor may comprise an analyzer for analysing the detected light, and deriving one or more parameters or properties of the detected light indicative of the cleanliness of the sensor surface and / or the patterned structures. The parameters or properties may be one or more of for example an average intensity, peak intensity, intensity distribution, contrast, signal-to-noise ratio (SNR) of the detected light or diffraction orders, a ratio of intensity between a clean reference patterned structure and the patterned structure being analyzed. For example, the cleaner a flat sensor surface is, the higher the contrast and the light intensity of the reflected light is; the cleaner a diffraction grating DG on the sensor surface is, the better the contrast and / or signal-to-noise ratio (SNR) of the diffraction orders is.

[0059] Before, during or after cleaning the sensor surface using the method according to some embodiments, the light source, preferably a laser, emits a light beam with a predetermined intensity, wavelength, and bandwidth to the sensor surface with an angle. The light beam is reflected and / or diffracted by the sensor surface, received and analyzed by the detector. As such, a parameter or property indicative of the cleanliness of the sensor surface can be determined.

[0060] Compared to the cleaning system of Figure 9(a) which comprises an optical sensor, the cleaning system of Figure 9(b) alternatively or additionally comprises a camera for examining the sensor surface. Before, during or after cleaning the sensor surface using the method according to some embodiments, the camera acquires one or more images of the sensor surface, and evaluates a parameter or property indicative of the cleanliness of the sensor surface and / or the patterned structures. The parameter or property may be for example number of particles, number of contamination clusters, density of contamination, percentage of functional area covered by contaminants, or colour changes of the sensor surface being examined.. The analysing and evaluating the parameter or property may be conducted by executing an algorithm or a software in the camera or in an analyzer separate from the camera. Visual checks may also be involved to inspect the images and evaluate the cleanliness of the sensor surface.

[0061] Alternatively or additionally, a chemical analyzer as shown in Figure 9(c) can be arranged in the cleaning system at a location after the cleaning mixture and the cleaning residue being extractedvia the extract arrangement EXT. The chemical analyzer for example a spectrometry measures and / or analyzes a parameter or a property of the extracted mixture comprising the cleaning mixture and the cleaning residue, thereby determining the cleanliness of the sensor surface and / or the patterned structures. The parameter or the property may be the chemical composition of the extracted mixture showing presence or absence of a contamination, pH, viscosity, surface tension of the extracted mixture indicating presence or absence of a contamination. The chemical analyzer can also be used to examine the materials and composition of the contaminants on the sensor surface, for diagnostics and troubleshooting purposes.

[0062] The chemical analyzer in Figure 9(c) may also be for example an optical tensiometer or a force tensiometer for conducting a contact angle measurement. Contact angle, 0 (theta), is a quantitative measure of wetting of a solid by a liquid, i.e. the contact angle indicates how well the wetting of the liquid is on a specific surface. The contact angle is geometrically defined as the angle formed by a liquid at the three-phase boundary where a liquid, gas, and solid intersect. The contact angle can change based on the properties of the liquid and may therefore be used as a measure for the liquid properties of the extracted mixture after cleaning the sensor surface.

[0063] The skilled person should understand that the measuring and the cleaning may be conducted in an iterative manner. For example, the method may comprise a first measurement before a first cleaning, i.e. before placing the cleaning device in proximity to the sensor, a second measurement after the first cleaning, i.e. after cleaning and removing the cleaning device from the sensor and a second cleaning after the second measurement, placing again the cleaning device in proximity to the sensor to clean the sensor surface. The iteration may continue until a satisfactory cleanliness of the sensor surface is achieved.

[0064] The measuring may also be conducted during a cleaning process, i.e. after placing and before removing the cleaning device from the sensor, while delivering the cleaning gas and the cleaning liquid, and extracting the cleaning mixture and the cleaning residue. The measuring may be conducted after placing the cleaning device in proximity to the sensor, while the delivering the cleaning gas and the cleaning liquid is not started yet or paused. The measuring may also be conducted after extracting the cleaning mixture and any cleaning residue and before removing the cleaning device from the sensor. Single or multiple measuring may be conducted during one cleaning process.

[0065] Figure 10 depicts a flow diagram of another embodiment according to the invention, wherein the method comprises steps of:- SI: placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, that means, the cleaning space is bounded by, or defined by, the cleaning device and the sensor surface;- S2: delivering the cleaning gas and the cleaning liquid to the cleaning space;- S3: controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both;- S4: extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement;- S5 : removing the cleaning device from the sensor;- S6: Measuring a parameter or a property indicative of the cleanliness of the sensor surface; and- S7 : Controlling the cleaning device and / or other steps of the method at least partially based on the measured parameter or property.

[0066] The skilled person should understand that the steps S 1-S5 are not necessarily performed one after another according to the sequence shown in Figure 10. The steps may be performed in a different sequence, or may be performing in parallel. Other sequences of performing the steps may be chosen based on specific circumstances. Also, the measuring the parameter or property in S6 and the controlling the in S7 may be performed at any stage, for example before SI, between or in parallel with any of S 1 -S5, or after S5. Further details of the embodiment being used are described below using Figure 11.

[0067] Figure 11 schematically depicts a side view of a cleaning system for use in the method of Figure 10. The cleaning system comprises a cleaning device of Figure 4, a sensor SENS to be cleaned which comprises a surface. The surface SF may comprise patterned structures, for example a diffraction grating DG. Such grating may be used for measurements like for example wafer alignment, reticle alignment and leveling. The cleaning system further comprises one or more of a camera, an optical sensor, and a chemical analyzer for measuring and / or analysing a parameter or a property of the extracted cleaning mixture and any cleaning residue, and / or examining the sensor surface as described in the previous embodiments of Figure 9. The cleaning system of Figure 11 further comprises a controller for controlling one or more parameters of the cleaning device, for example the flow rate of the cleaning liquid or the reduced pressure at the extract arrangement as described in the embodiment of Figure 6. Compared to the embodiment of Figure 6, before, during or after cleaning the sensor surface using the method according to some embodiments, the controller further receives one or more measured parameters or properties from one or more of the optical sensor, camera, and chemical analyzer, and controls the cleaning device at least partially based on the measured parameters or properties.

[0068] For example, the controller may receive a contrast or a signal-to-noise ratio (SNR) of the measured light from an optical sensor, based on which adjust one or more parameters of the cleaning device, for example viscosity of the cleaning liquid, bubble or contact surface ratio, or flushing speed of the cleaning mixture. The bubble or contact surface ratio may be adjusted by for example varyingthe flow rate of the cleaning liquid, adjusting the composition of the cleaning gas and / or cleaning liquid, or varying the working temperature. The flushing speed of the cleaning mixture may be adjusted by for example varying the reduced pressure at the extract arrangement. Similarly the material type and composition of the cleaning residue may be measured and provided by chemical analyzer to the controller, based on which a parameter of the cleaning device, like pH of the cleaning liquid, composition of the cleaning liquid and / or cleaning gas may be controlled.

[0069] An exposure apparatus for example the lithographic apparatus of Figure 1 may comprise a sensor and a surface thereof, and a cleaning device or a cleaning system according to the invention. The method according to the invention may be applied to clean the sensor surface using the cleaning device or the cleaning system.

[0070] Although specific reference may be made in this text to the use of a 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, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0071] 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). This apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0072] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography, and may be used in other applications, for example imprint lithography.

[0073] 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 otherdevices to interact with the physical world. Other aspects of the invention are set-out as in the following numbered clauses.1. A method for cleaning a sensor surface using a cleaning device, the cleaning device comprising: a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space between the cleaning device and the sensor surface during use, and an extract arrangement, wherein the method comprises the steps of: placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, delivering the cleaning gas and the cleaning liquid to the cleaning space, controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both, extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement, and removing the cleaning device from the sensor.2. The method of clause 1, wherein the cleaning gas is air, preferably clean dry air, further preferably extra clean dry air (XCDA) and / or the cleaning liquid is water, preferably ultra-pure water.3. The method of clause 1 or 2, the sensor being a level sensor, an alignment sensor, an image sensor, a laser beam sensor, or a position sensor.4. The method of clause 1, 2 or 3, wherein the cleaning device comprises a gas channel, a gas inlet, and a liquid channel, wherein the gas inlet opens up into the liquid channel, and the controlling the cleaning device comprises controlling a flow rate of the cleaning liquid to create a reduced pressure in the liquid channel with respect to the gas channel, allowing the cleaning gas to enter the liquid channel via the gas inlet, thereby forming the cleaning mixture.5. The method of any preceding clause, wherein the controlling the cleaning device further comprises controlling a parameter to form a confined flow of the cleaning mixture across the sensor surface.6. The method of clause 5, wherein the parameter is the reduced pressure applied at the extract arrangement.7. The method of clause 5 or 6, wherein the delivering the cleaning gas and the cleaning liquid is substantially from a center of the cleaning device, and the extracting the cleaning mixture and any cleaning residue is via multiple outlets arranged at an outer side of the cleaning device, such that the confined flow is radially outwards.The method of clause 7, wherein the multiple outlets are arranged to cover more than 20% of an area at the outer side of the cleaning device. The method of clause 5 or 6, wherein the delivering the cleaning gas and cleaning liquid is from one side of the cleaning device, and the extracting the cleaning mixture and any cleaning residue is from an opposite side of the cleaning device, such that the confined flow is substantially onedirectional. The method of clause 5 or 6, wherein the delivering the cleaning gas and the cleaning liquid is substantially from multiple inlets at an outer side of the cleaning device, and extracting the cleaning mixture and any cleaning residue is substantially from a center of the cleaning device, such that the confined flow is radially inwards. The method of any preceding clause, further comprising measuring a parameter or a property indicative of the cleanliness of the sensor surface. The method of clause 11, wherein the measuring comprises measuring and / or analyzing a parameter or a property of the extracted cleaning mixture and any cleaning residue, and / or examining the sensor surface. The method of clause 12, wherein the measuring and / or analyzing is using a chemical analyzer. The method of clause 12, wherein the examining the sensor surface is using a camera or an optical sensor comprising a light source, a detector and / or optical elements. The method of any of the clauses 11 -14, further comprising controlling the cleaning device and / or other steps of the method at least partially based on the measured parameter or property. A cleaning device for use in the method of any preceding clause for cleaning a sensor surface, wherein the cleaning device comprises a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space between the cleaning device and the sensor surface during use, and an extract arrangement. A cleaning system configured to clean a sensor surface for use in the method of any of the clauses 1-10 for cleaning a sensor surface, wherein the cleaning system comprises the cleaning device of clause 16, a controller for controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both. The cleaning system of clause 17, further comprising a measurement device for measuring a parameter or property indicative of cleanliness of the sensor surface, the measurement unit being a chemical analyzer, a camera, or an optical sensor comprising a light source, a detector and / or optical elements.19. A data processing apparatus comprising means for carrying out the method of any of the clauses 1-15.20. A computer-readable storage comprising instructions which, when executed by a computer, cause the computer to carry out the method of any of the clauses 1-15. 21. An exposure apparatus comprising a sensor and a surface thereof, and the cleaning device of clause 16 or the cleaning system of clause 17 or 18, wherein the cleaning device or the cleaning system is configured to clean the sensor surface using the method of any of the clauses 1-15.22. The exposure apparatus of clause 21, being a lithographic apparatus, or more specifically a dry lithographic apparatus.

[0074] 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 set out below.

Claims

CLAIMS1. A method for cleaning a sensor surface using a cleaning device, the cleaning device comprising: a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space between the cleaning device and the sensor surface during use, and an extract arrangement, wherein the method comprises the steps of: placing the cleaning device in proximity to a sensor having the sensor surface such that the cleaning space is between the cleaning device and the sensor surface, delivering the cleaning gas and the cleaning liquid to the cleaning space, controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both, extracting the cleaning mixture and any cleaning residue from the cleaning space by applying a reduced pressure at the extract arrangement, and removing the cleaning device from the sensor.

2. The method of claim 1, wherein the cleaning gas is air, preferably clean dry air, further preferably extra clean dry air (XCDA) and / or the cleaning liquid is water, preferably ultra- pure water.

3. The method of claim 1 or 2, the sensor being a level sensor, an alignment sensor, an image sensor, a laser beam sensor, or a position sensor.

4. The method of claim 1, 2 or 3, wherein the cleaning device comprises a gas channel, a gas inlet, and a liquid channel, wherein the gas inlet opens up into the liquid channel, and the controlling the cleaning device comprises controlling a flow rate of the cleaning liquid to create a reduced pressure in the liquid channel with respect to the gas channel, allowing the cleaning gas to enter the liquid channel via the gas inlet, thereby forming the cleaning mixture.

5. The method of any preceding claim, wherein the controlling the cleaning device further comprises controlling a parameter to form a confined flow of the cleaning mixture across the sensor surface.

6. The method of claim 5, wherein the parameter is the reduced pressure applied at the extract arrangement.

7. The method of any preceding claim, further comprising measuring a parameter or a property indicative of the cleanliness of the sensor surface.

8. The method of claim 7, further comprising controlling the cleaning device and / or other steps of the method at least partially based on the measured parameter or property.

9. A cleaning device for use in the method of any preceding claim for cleaning a sensor surface, wherein the cleaning device comprises a supply arrangement for delivering a cleaning gas and a cleaning liquid to a cleaning space between the cleaning device and the sensor surface during use, and an extract arrangement.

10. A cleaning system configured to clean a sensor surface for use in the method of any of the claims 1-8 for cleaning a sensor surface, wherein the cleaning system comprises the cleaning device of claim 9, a controller for controlling the cleaning device in such a way that during use a cleaning mixture comprising the cleaning gas and the cleaning liquid is created and confined in the cleaning space, the said cleaning mixture being in contact with the sensor surface, and wherein the cleaning gas and the cleaning liquid define contact surfaces between both.

11. The cleaning system of claim 10, further comprising a measurement device for measuring a parameter or property indicative of cleanliness of the sensor surface, the measurement unit being a chemical analyzer, a camera, or an optical sensor comprising a light source, a detector and / or optical elements.

12. A data processing apparatus comprising means for carrying out the method of any of the claims 1-8.

13. A computer-readable storage comprising instructions which, when executed by a computer, cause the computer to carry out the method of any of the claims 1-8.

14. An exposure apparatus comprising a sensor and a surface thereof, and the cleaning device of claim 9 or the cleaning system of claim 10 or 11, wherein the cleaning device or the cleaning system is configured to clean the sensor surface using the method of any of the claims 1-8.

15. The exposure apparatus of claim 14, being a lithographic apparatus, or more specifically a dry lithographic apparatus.

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