Photolithography method and apparatus

Abstract

A method of determining a sensor contribution to an apodization measurement. The method comprises directing a radiation beam through an aperture when the aperture is in a first configuration having a first aperture diameter, the first aperture diameter being smaller than a diameter of the radiation beam; receiving the radiation beam at a sensor; obtaining a first measurement of an amount of radiation detected by the sensor in a first region of the sensor, wherein the radiation beam is not incident on the first region; and determining a sensor contribution to the apodization measurement based on the first measurement.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 17189827.3 filed on September 7, 2017, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to photolithography methods and apparatus. Background Art

[0004] A lithographic apparatus is a machine constructed to apply a desired pattern to a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device (alternatively referred to as a mask or reticle) can be used to generate a circuit pattern corresponding to a separate layer of the IC, and the pattern can be imaged onto a target portion (e.g., a portion including one or more dies) having a layer of radiation-sensitive material (resist) on a substrate (e.g., a silicon wafer). Typically, a single substrate will contain a network of adjacent target portions that are continuously exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing the entire pattern to the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern in a given direction (the "scanning" direction) with a light beam while simultaneously scanning the substrate parallel or antiparallel to that direction.

[0005] The lithographic apparatus comprises an illumination system and a projection system. The immersion lithographic apparatus also comprises a liquid layer. Both the illumination system and the projection system have an inherent apodization property. Apodization describes the angular transmission of radiation passing through an optical system (such as an illumination system and a projection system). It is desirable to determine the apodization property of the projection system so that the lithographic errors caused by the apodization property of the projection system can be taken into account. A photodetector (or camera) can be used to measure the angular intensity distribution of radiation transmitted through the lithographic apparatus. However, the output of the photodetector contains the apodization contribution of the illumination system, the apodization contribution of the projection system, and the contribution of the photodetector itself. U.S. Patent No. US9261402, which is incorporated herein by reference, describes a technique for removing the apodization contribution of the illumination system from the measurement value. It is desirable to provide, for example, a method for determining the contribution of a photodetector to the apodization measurement value of the projection system of the lithographic apparatus, which avoids or mitigates one or more problems in the prior art, whether pointed out herein or elsewhere. Summary of the invention

[0006] Apodization may negatively impact the imaging performance of the projection system and, therefore, the performance of the lithographic apparatus. Apodization properties of the lithographic apparatus may be determined using a sensor within the lithographic apparatus. Measurements made using the sensor may include angle-dependent properties of the sensor itself, which negatively impact the accuracy of the apodization measurement.

[0007] Once determined, the sensor contribution to the apodization measurement can be removed from future apodization measurements of the projection system, thereby enabling a more accurate determination of the apodization property of the projection system. The determined apodization property of the projection system can then be taken into account when performing a lithographic exposure, thereby enabling a more accurate lithographic exposure.

[0008] According to a first aspect described herein, there is provided a method of determining a sensor contribution to an apodization measurement. The method comprises directing a radiation beam through an aperture when the aperture is in a first configuration having a first aperture diameter. The first aperture diameter may be smaller than the diameter of the radiation beam. The radiation beam is received at a sensor and a first measurement of the intensity of radiation detected by the sensor in a first region of the sensor is obtained, wherein the radiation beam is not incident on the first region. The sensor contribution to the apodization measurement may then be determined based on the first measurement. By measuring the intensity of radiation detected by the sensor in regions of the sensor where it is known that no radiation is incident, any radiation detected at those regions may be assumed to be caused by sensor artifacts and may therefore be used to determine the sensor contribution to the apodization measurement.

[0009] The method may further include: adjusting the aperture so that the aperture is in a second configuration having a second aperture diameter, wherein the second aperture diameter is different from the first aperture diameter and the second aperture diameter is smaller than a beam diameter of the radiation beam; and obtaining a second measurement of an amount of radiation detected by the sensor in a second region of the sensor, wherein the second region is different from the first region and the radiation beam is not incident on the second region. Determining a sensor contribution to the apodization measurement may be based on the first measurement and the second measurement. In this way, a radial component of the sensor contribution may be characterized.

[0010] The second area of ​​the sensor may at least partially overlap with the first area of ​​the sensor. Alternatively, the second area of ​​the sensor may be completely different from the first area of ​​the sensor. In each case, adjusting the aperture diameter so as to vary the non-incident area of ​​the sensor allows determining the sensor contribution to the apodization measurement for different areas of the sensor.

[0011] The diameter of the hole may define a numerical aperture in the projection system of the projection system.

[0012] The sensor may take any suitable form and may include a photodetector or a camera. For example, the sensor may include an integrated lens interferometer at scanner (ILIAS) sensor.

[0013] The method may further comprise obtaining an apodization measurement. Obtaining the apodization measurement may comprise obtaining an initial apodization measurement and subtracting the determined sensor contribution from the initial apodization measurement. In this way, an apodization measurement may be obtained in the lithographic apparatus excluding the sensor contribution.

[0014] The method may further comprise determining a contribution of the illumination system to the apodization measurement. Obtaining the apodization measurement may further comprise: subtracting the determined contribution of the illumination system from the initial apodization measurement. In this way, an apodization measurement of a projection system of a lithographic apparatus may be obtained.

[0015] The method may further comprise causing an alarm to be output in response to the apodization measurement obtained. For example, the alarm may indicate that a cleaning or replacement operation is to be performed. Alternatively, the alarm may automatically schedule a cleaning or replacement operation.

[0016] The method may further comprise controlling the lithographic apparatus based on the obtained apodization measurement.

[0017] According to another aspect described herein, there is provided a computer program comprising computer readable instructions configured to cause a computer to perform the method according to the first aspect. The computer program may be stored on a computer readable medium.

[0018] According to another aspect described herein, there is provided a computer device for determining a sensor contribution to apodization measurement, the computer device comprising: a memory storing processor-readable instructions; and a processor arranged to read and execute the instructions stored in the memory; wherein the processor-readable instructions include instructions arranged to control a computer to perform the method according to the first aspect.

[0019] According to another aspect described herein, there is provided a lithographic apparatus comprising: an illumination system; a substrate table for holding a substrate, the substrate table comprising a sensor; a projection system for projecting a radiation beam onto the sensor, the projection system comprising a hole through which the radiation beam is guided; and an adjustable stop for controllably adjusting a diameter of the hole.

[0020] The adjustable stop may include a diaphragm.

[0021] The lithographic apparatus may further comprise a processor configured to perform the method according to the first aspect.

[0022] According to another aspect described herein, there is provided a projection system for a lithographic apparatus for projecting a radiation beam towards a substrate table, wherein the projection system comprises an aperture through which to guide the radiation beam and an adjustable stop for controllably adjusting a diameter of the aperture.

[0023] It will be appreciated that features described in conjunction with one aspect may be combined with features described in conjunction with other aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0025] Figure 1 schematically depicts an immersion lithography apparatus including a photodetector and a processor that can be used to measure apodization in an embodiment of the present invention;

[0026] Figure 2 The wavefront projected onto the sensor is schematically depicted;

[0027] Figure 3 The optics of a lithographic apparatus are schematically depicted;

[0028] Figure 4 is a flow chart illustrating an exemplary process that may be performed by a processor to determine a sensor contribution to an apodization measurement;

[0029] Figure 5 is a flow chart illustrating an alternative exemplary process that may be performed by a processor to determine a sensor contribution to an apodization measurement; and

[0030] Figure 6 is shown which can be performed to determine Figure 1 Flowchart of an exemplary process for apodizing at least a portion of a lithographic apparatus. DETAILED DESCRIPTION

[0031] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guides and detection patterns for magnetic domain memories, liquid crystal displays (LCDs), thin film magnetic heads, etc. Those skilled in the art will understand that in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed before or after exposure, for example in a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a measurement or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. In addition, the substrate may be processed multiple times, for example to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[0032] The terms "radiation" and "beam" as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 4 nm to 20 nm), as well as particle beams (such as ion beams or electron beams).

[0033] The term "patterning device" as used herein should be broadly interpreted as referring to a device that can be used to impart a radiation beam with a pattern in its cross-section to create a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate. Typically, the pattern imparted to the radiation beam will correspond to a specific functional layer in a device created in the target portion, such as an integrated circuit.

[0034] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography and include mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted to reflect an incoming radiation beam in different directions; in this way, the reflected beam is patterned.

[0035] The support structure holds the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions (e.g., whether the patterning device is held in a vacuum environment). The support may use mechanical clamping, vacuum, or other clamping techniques (e.g., electrostatic clamping under vacuum conditions). The support structure may be, for example, a frame or table that can be fixed or moved as needed and that can ensure that the patterning device is, for example, in a desired position relative to a projection system. Any use of the term "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

[0036] The term "projection system" as used herein should be broadly interpreted to cover, for example, various types of projection systems suitable for the exposure radiation used, including refractive optical systems, reflective optical systems and catadioptric optical systems. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system".

[0037] The illumination system may also encompass various types of optical components, including refractive, reflective and catadioptric optical components for directing, shaping or controlling the radiation beam, and such components may also be collectively or individually referred to as "lenses" hereinafter.

[0038] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more support structures). In such a "multi-stage" machine, the additional tables may be used in parallel, or preparatory steps may be performed on one or more tables while one or more other tables are being used for exposure.

[0039] The lithographic apparatus may be of a type in which the substrate is immersed in a liquid having a relatively high refractive index (e.g. water) to fill the space between the final element of the projection system and the substrate. Immersion techniques for increasing the numerical aperture of a projection system are well known in the art.

[0040] Figure 1 An immersion lithography apparatus including a photodetector and a processor that can be used to measure apodization in an embodiment of the present invention is schematically depicted. Of course, it will be appreciated that in embodiments of the present invention, a processor that can be used to measure apodization may be used. Figure 1 The depicted lithographic apparatus is different from a lithographic apparatus. For example, a non-immersion lithographic apparatus may be used in some embodiments of the present invention. An EUV lithographic apparatus may be used in some embodiments of the present invention.

[0041] Figure 1 Equipment depicted includes:

[0042] - an illumination system (illuminator) IL for conditioning a radiation beam PB (eg UV radiation);

[0043] a support structure MT for supporting a patterning device (eg a mask) MA and connected to a first positioning device PM for accurately positioning the patterning device relative to the item PL;

[0044] a substrate table (e.g. wafer stage) WT for holding a substrate (e.g. resist coated wafer) W and connected to a second positioning device PW for accurately positioning the substrate relative to the item PL; and

[0045] - a projection system (eg, a refractive projection lens) PL configured to project the pattern imparted to the radiation beam PB by the patterning device MA onto a target portion C of the substrate W (eg, comprising one or more dies).

[0046] As depicted here, the device is transmissive (eg, employing a transmissive mask). Alternatively, the device may be reflective (eg, employing a reflective mask or a programmable mirror array as described above).

[0047] The illuminator IL receives a radiation beam from a radiation source SO. For example, when the source is an excimer laser, the source and the lithographic apparatus may be separate entities. In such a case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is transferred from the source SO to the illuminator IL with the aid of a beam delivery system BD, which includes, for example, suitable directional mirrors and / or a beam expander. In other cases, for example, when the source is a mercury lamp, the source may be an integral part of the apparatus. The source SO and the illuminator IL together with the beam delivery system BD (as required) may be referred to as a radiation system.

[0048] The illuminator IL may include adjustment means AM for adjusting the angular intensity distribution of the beam. Typically, the intensity distribution of at least the outer and / or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL typically includes various other components such as an integrator IN and a condenser CO. The illuminator provides a conditioned radiation beam PB having a desired uniformity and intensity distribution in its cross-section.

[0049] The radiation beam PB is incident on a patterning device (e.g. a mask) MA held on the support structure MT. After passing through the patterning device MA, the radiation beam PB passes through the projection system PL which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioning device PW and a position sensor IF (e.g. an interferometry device), the substrate table WT can be precisely moved, for example in order to position a different target portion C in the path of the radiation beam PB. Similarly, the first positioning device PM and a further position sensor (in the example of FIG. 1 ) can be used to precisely position the substrate table WT, for example after mechanical retrieval from a mask library or during scanning. Figure 1 The support structure MT may be connected to a short-stroke actuator only, or may be fixed, for example. The patterning device MA and the substrate W may be aligned using patterning device alignment marks M1, M2 and substrate alignment marks P1, P2.

[0050] The depicted device can be used in the following preferred modes:

[0051] 1. In step mode, the support structure MT and substrate table WT are held substantially stationary while an entire pattern imparted to the radiation beam PB is projected at once onto a target portion C (i.e. a single static exposure). The substrate table WT is then shifted in the X and / or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

[0052] 2. In scan mode, the support structure MT and substrate table WT are scanned synchronously while projecting a pattern imparted to the radiation beam PB onto a target portion C (i.e. a single dynamic exposure). The speed and direction of the substrate table WT relative to the support structure MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width of the target portion (in the non-scanning direction) in a single dynamic exposure, while the length of the scanning movement determines the height of the target portion (in the scanning direction).

[0053] 3. In another mode, the support structure MT holding the programmable patterning device is held substantially stationary, and the substrate table WT is moved or scanned whilst a pattern imparted to the beam PB is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed, and the programmable patterning device is updated as required after each movement of the substrate table WT, or between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography utilizing a programmable patterning device, such as a programmable mirror array of the type described above.

[0054] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0055] Apodization may be defined as the angular dependence of the transmission of an optical system. Apodization may negatively affect the imaging performance of the projection system PS and thus the performance of the lithographic apparatus. Apodization may lead to lithographic errors that limit the imaging performance of the lithographic apparatus. For example, variations in critical dimensions of the lithographic apparatus may be negatively affected by effects attributed to apodization.

[0056] When the optical components of the projection system PS are first manufactured, the apodization properties of the projection system PS may be determined. However, the apodization properties of the projection system PS may change over time. For example, contaminant particles present in the projection system PS may absorb some of the radiation passing through the projection system PS. This may change the intensity of the radiation incident on the substrate relative to the angle of incidence of the radiation, thereby changing the apodization properties of the projection system PS. It is desirable to have a method of determining the apodization properties of the projection system PS of a lithographic apparatus. Once determined, the apodization properties of the projection system PS may be partially or fully compensated when using the lithographic apparatus to perform lithographic exposures.

[0057] The apodization properties of the lithographic apparatus may be determined using sensors (including, for example, photodetectors) in the lithographic apparatus. The photodetector may be located at the image plane of the projection system PS (e.g., near the substrate W). The photodetector may be located, for example, in or on the substrate table WT. The photodetector may be configured to measure the intensity of radiation across the pupil plane of the projection system PS. For example, the photodetector may include an integrated lens interferometer at scanner (ILIAS) sensor. The ILIAS sensor is an interferometric wavefront measurement system that can perform static measurements of lens aberrations up to high orders. The ILIAS sensor may be implemented as an integrated measurement system for system initialization and calibration. Alternatively, the ILIAS sensor may be used for "on-demand" monitoring and recalibration. U.S. Patent No. US7282701B2, incorporated herein by reference, discloses an ILIAS sensor that may be used to determine the intensity profile of radiation across the pupil plane of the projection system PS. The intensity profile of radiation across the pupil plane of the projection system PS may be considered to be an angular distribution image of the radiation transmitted by the projection system PS. Points in the pupil plane of the projection system PS may correspond to angles of incidence in the field plane of the projection system, and vice versa.

[0058] The measurement made using the photodetector may contain contributions from the apodization properties of the projection system PS, the apodization properties of the illumination system IL, and the angle-dependent properties of the photodetector itself. The angle-dependent properties of the photodetector may, for example, include ghosting effects caused by unwanted internal reflections occurring within the photodetector, electronic crosstalk within the photodetector, and the like. As described above, U.S. Patent No. US9261402 discloses a technique for removing the contribution of the illumination system IL from the apodization measurements made by the photodetector. However, in the measurement made by the photodetector, there may still be a contribution from the angle-dependent properties of the photodetector. If the angle-dependent properties of the photodetector are known, they may be removed from the apodization measurements made by the photodetector. However, the angle-dependent properties of the photodetector are generally unknown and may change over the lifetime of the photodetector.

[0059] Figure 2A spherical wavefront 10 is schematically depicted, projected through an aperture 26 in a pupil plane onto a sensor 36 comprising a planar photodetector (hereinafter referred to as a planar sensor). A problem associated with projection onto a planar sensor is that the portion of the wavefront 10 incident on the edge of the planar sensor 36 is distributed over a larger area of ​​the planar sensor 36 than the portion of the wavefront 10 incident on the interior of the sensor 36, which results in a larger signal-to-noise ratio (SNR) at the edge of the sensor 36. It can be seen that the same solid angles 12 and 14 of the projected wavefront 10 are spread across areas of different sizes of the sensor 36 (as shown by brackets 12a and 14a). This means that the intensity of the radiation measured at area 12a may be different from the intensity measured at area 14a. In particular, the measured intensity will experience a significantly significant "roll-off" at the edge of the sensor 36. That is, the intensity measured at the edge of the sensor 36 will be much lower than the intensity measured at the center of the sensor 36. Additionally, sensor artifacts such as internal reflections and leakage can cause undesirable effects such as "ghosting", which degrades sensor accuracy at the pupil edges.

[0060] Figure 3 Schematically illustrates parts of a lithographic apparatus arranged to facilitate determination of a sensor contribution to apodization measurement. Figure 3 Various components of the projection system (such as lenses) are omitted from the schematic diagram of FIG. After exiting the illumination system (not shown), the radiation beam R passes in the propagation direction indicated by the depicted arrow, passing through a mask 22 which is generally located in the focal plane of the illumination system. The mask may be a so-called pinhole mask which can be used to provide a radiation beam of known size. The mask 22 is located in the focal plane of the illumination system. Figure 1 The mask MA will be in the plane where it will be located. After passing through the mask plate 22, the radiation beam R enters the projection system PL. Within the projection system PL, the radiation beam R passes through the hole 28. The diameter of the hole 28 can be adjusted using an adjustable stop 26. For example, the adjustable stop 26 can take the form of a diaphragm or iris that is obvious to the technician. However, the adjustable stop 26 can be implemented in any way that allows the diameter of the hole 28 to be adjusted. The adjustable stop 26 can be operable to adjust the diameter of the hole 28 between, for example, 1.35 and 0.85. The hole 28 defines the numerical aperture of the projection system PL. By using the adjustable stop 26 to adjust the diameter of the hole 28, the radiation beam R can be tailored. That is, the adjustable stop 26 can be used to make the diameter of the hole 28 smaller than the diameter of the radiation beam R. Since the radiation beam R may not be completely circular (for example, due to the cross-sectional shape imparted to the radiation beam R by the adjustable stop 26), it should be understood that, as used herein, the term diameter of the radiation beam R also includes the maximum range (maximum cross-sectional range) in a cross section of the radiation beam R taken in a plane perpendicular to the propagation direction of the radiation beam R.

[0061] After passing through aperture 28, radiation beam R is directed through aperture 31 at plane 32, which corresponds to the plane in which substrate W will be located during use. Aperture 31 may be sized larger than the projection of reticle 22 at plane 32 so that no diffraction occurs when radiation beam R passes through aperture 31.

[0062] After the aperture 31, the radiation beam is directed to a sensor 36, which may include, for example, a photodetector. As described above, the sensor 36 may, for example, include an ILIAS sensor or other suitable sensor. The sensor 36 may include other components in addition to the photodetector. For example, where the sensor 36 includes a photodetector, the photodetector may be placed in a far field plane without an imaging optical device, or may be located in a pupil plane with an imaging optical device. Although the sensor 36 may include other components, for convenience herein, the term sensor is generally used to refer to the photodetector (or other similar component) on which the radiation beam is ultimately incident.

[0063] When the radiation beam R is incident on the sensor 36, the cross-sectional area of ​​the radiation beam R (in a plane perpendicular to the direction of propagation) is smaller than the area of ​​the detection area of ​​the sensor 36. Figure 3 When the adjustable stopper 26 is operated to reduce the diameter of the hole 28, the radiation beam R is clipped (e.g., Figure 3 4. As a result, the non-incident area on sensor 36 becomes larger to include area 40. The first diameter may be the widest diameter of aperture 28, so that even at the maximum diameter of aperture 28, radiation beam R may fall completely within the detection area of ​​sensor 36. However, in other embodiments, aperture 28 may be adjustable so that the cross-sectional area of ​​radiation beam R fills the detection area of ​​sensor 36 (i.e., so that there is no non-incident area).

[0064] The sensor contribution to the apodization measurement obtained from the measurement made by the sensor 36 may be determined by measuring the intensity of radiation detected at non-incident regions 38 of the sensor 36. Since the radiation beam R is not incident on those regions 38, any radiation detected at the regions 38 may be attributed to sensor artifacts. The determined contribution of the sensor artifact may then be used to determine the contribution of the sensor 36 to the apodization of the overall measurement. The determined sensor contribution to the measured apodization may then be subtracted from future overall apodization measurements in order to remove the effect of the sensor artifact from the overall apodization measurement.

[0065] In some embodiments, the adjustable stopper 26 is used to change the diameter of the aperture and repeat the measurement of radiation at the (now changed) non-incident region. In this way, the radial component of the sensor artifact can be characterized. For example, a first measurement can be performed when the aperture 28 is at its maximum diameter and a measurement is made of how much radiation is detected in the current non-incident region of the sensor 36. The current non-incident region can be considered as the first region of the sensor 36. The adjustable stopper 26 can then be used to reduce the diameter of the aperture 28. When the aperture is smaller, the non-incident region(s) become larger because more of the radiation beam R is blocked from passing through the aperture 28.

[0066] A second measurement may then be made of how much radiation is received in a second area of ​​the sensor 36. The second area may be the portion of the (now larger) non-incident area that does not overlap with the first area (i.e., the portion of the non-incident area for which the first measurement was performed in the previous step). Alternatively, the second area may at least partially overlap with the first area. In some embodiments, the second area may completely overlap with the first area, such that the second area includes the entire larger non-incident area.

[0067] In one embodiment, the sensor contribution may be calculated using the following equation:

[0068]

[0069] in To measure the vector, having an element for each pixel of the sensor 26, is the ideal result (e.g., the intensity expected at each pixel of sensor 26), and is the transfer matrix. In an ideal case, the transfer matrix is the unit matrix. When the diameter of hole 28 is changed, and will change, allowing determination

[0070] During this process of determining the sensor contribution to the apodization measurement, it may be desirable to ensure that only the size of the aperture 28 is changed (e.g., without changing the illumination setting), since it may be observed that sensor artifacts may differ for different illumination modes. In other embodiments, the process of measuring the intensity distribution for different aperture sizes (as described above) may be performed for each of a plurality of illumination modes. In some embodiments, the above reference may be used Figure 3 The described techniques can be used to calibrate for differences in angular sensitivity between different sensors using measurements obtained using the described techniques. For example, if the angular distribution of radiation received at the sensor pupil 34 of the lithographic apparatus is known, the angular distribution can be varied to calibrate the sensor 36 by adjusting the aperture 28 using the adjustable stop 26 and recording the output from the sensor 36 to be calibrated.

[0071] In some embodiments, stopper 26 may include an iris including a plurality of blades such that in at least one position of stopper 26, the outer edge of aperture 28 is not completely circular. In the event that the outer edge of aperture 28 is not circular, this will affect the shape of radiation beam R. Any changes in the shape of radiation beam R may be taken into account when determining which areas of sensor 36 should be considered as non-incident areas.

[0072] Figure 4 A flow chart illustrating the steps of an exemplary method of determining a sensor contribution to an apodization measurement is shown. Figure 4 The processing of may be performed, for example, by a controller MP operable to receive a signal from the sensor 36 and determine an apodization contribution of the sensor 36. The method comprises, at a first step S1, directing a radiation beam R through an aperture 28, wherein the aperture 28 has a first diameter, the first diameter being smaller than a beam diameter of the radiation beam R and smaller than a width of a detection region of the sensor 36. At a second step S2, the radiation beam R is received at the sensor 36. At a third step S3, a first measurement of the amount of radiation detected by the sensor 36 in a first region of the sensor 36 (e.g., a first non-incident region) is obtained. At a fourth step S4, the controller determines a contribution of the sensor 36 to the apodization measurement (i.e., the sensor contribution) using the first measurement.

[0073] Figure 5 A flow chart illustrating another exemplary method of determining a sensor contribution to apodization measurement is shown. Steps S1 to S3 are similar to those described above with respect to Figure 4 The steps in the method described are the same. However, Figure 5 In the method of , at step S5, the adjustable stop 26 is used to adjust the diameter of the aperture 28 to a second diameter, the second diameter being different from the first diameter. Similar to the first diameter, the second diameter is smaller than the beam diameter of the radiation beam R and smaller than the width of the detection area of ​​the sensor 36. At step S6, a second measurement of the amount of radiation detected by the sensor 36 is performed in a second area of ​​the sensor 36 (i.e., the now changed non-incident area). At step S7, a sensor contribution to an apodization measurement of the projection system of the lithographic apparatus is determined based on the first measurement and the second measurement. As described above, the first measurement and the second measurement can be used to determine the sensor contribution at different radial portions of the sensor 36.

[0074] Figure 6 is a flow chart showing a process that may be performed to determine the apodization of an optical column (comprising illumination system IL and projection system PL) or projection system PL. At step S10, an initial apodization measurement is obtained using sensor 36. At step S12, the contribution of sensor 36 to the apodization measurement is subtracted from the initial apodization measurement (using, for example, Figure 4 or Figure 5The processing of step S12 provides a "no sensor contribution" apodization measurement. Processing optionally proceeds to step S14 (shown in dashed lines) where the contribution of the illumination system IL is also subtracted from the no sensor contribution apodization measurement obtained at step S12. As described above, the determination of the contribution of the illumination system IL may be performed using the techniques described in U.S. Pat. No. US9261402, which is incorporated herein by reference. The processing of step S14 provides a measure of the projection system contribution to the apodization.

[0075] At step S16, the controller MP may be further operable to perform an action in response to determining the apodization contribution of the sensor 36 or in response to determining the apodization of the optical column (e.g., including the illumination system IL and the projection system PL) or in response to determining the apodization of the projection system. For example, in one embodiment, the controller MP may be operable to output an alarm. For example, the controller MP may output an alarm to indicate that a maintenance operation is required, such as a cleaning or lens replacement operation. In another embodiment, the projection system PL may include one or more actuators that are operable to adjust the apodization of the projection system PL in response to the determination made by the controller MP. In another embodiment, the controller MP is operable to control other components of the lithographic apparatus LA in response to the apodization measurements obtained at steps S12 or S14.

[0076] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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 a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and such actions are actually caused by a computing device, processor, controller, or other device that executes firmware, software, routines, instructions, etc.

[0077] In one embodiment, the present invention may form part of a mask inspection device. The mask inspection device may use EUV radiation to illuminate the mask and may use an imaging sensor to monitor the radiation reflected from the mask. The image received by the imaging sensor is used to determine whether there are defects in the mask. The mask inspection device may include an optical device (e.g., a reflector) configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam guided at the mask. The mask inspection device may further include an optical device (e.g., a reflector) configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection device may include a processor configured to analyze the image of the mask at the imaging sensor and determine whether there are any defects on the mask based on the analysis. The processor may be further configured to determine whether the detected mask defects cause unacceptable defects in the image projected onto the substrate when the lithographic device uses the mask.

[0078] In one embodiment, the present invention may form part of a metrology apparatus. The metrology apparatus may be used to measure the alignment of a projected pattern formed in a resist on a substrate relative to an existing pattern on the substrate. Such measurements of relative alignment may be referred to as overlay. The metrology apparatus may, for example, be located adjacent to a lithographic apparatus and may be used to measure overlay prior to processing a substrate (and resist).

[0079] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection device, a metrology device, or any device for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0080] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0081] Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present 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 a read-only memory (ROM); a random access memory (RAM); a disk storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and such actions are actually caused by a computing device, processor, controller, or other device that executes firmware, software, routines, instructions, etc.

[0082] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method of determining a sensor contribution to an apodization measurement, the method comprising: directing a radiation beam through the aperture when the aperture is in a first configuration having a first aperture diameter, the first aperture diameter being smaller than a diameter of the radiation beam; receiving the radiation beam at the sensor; obtaining a first measurement of radiation intensity detected by the sensor in a first region of the sensor, wherein the radiation beam is not incident on the first region; A sensor contribution to the apodization measurement is determined based on the first measurement.

2. The method according to claim 1, further comprising: adjusting the aperture so that the aperture is in a second configuration having a second aperture diameter, wherein the second aperture diameter is different from the first aperture diameter and the second aperture diameter is smaller than a beam diameter of the radiation beam, and directing a radiation beam through the aperture when the aperture is in the second configuration; obtaining a second measurement of an amount of radiation detected by the sensor in a second region of the sensor, wherein the second region is different from the first region and the radiation beam is not incident on the second region; Wherein determining the sensor contribution to the apodization measurement is based on the first measurement and the second measurement. The method of claim 2 , wherein the second area of ​​the sensor at least partially overlaps the first area of ​​the sensor.

4. A method according to any one of the preceding claims, wherein the diameter of the hole defines a projection system numerical aperture of a projection system.

5. A method according to any preceding claim, wherein the sensor comprises a photodetector or a camera.

6. The method according to any one of the preceding claims, further comprising: Get apodization measurements, Wherein obtaining the apodization measurement comprises: obtaining an initial apodization measurement, and subtracting the determined sensor contribution from the initial apodization measurement.

7. The method according to claim 6, further comprising: Determine the contribution of the illumination system to the apodization measurement; and Wherein obtaining the apodization measurement further comprises: subtracting the determined contribution of the illumination system from the initial apodization measurement.

8. The method according to claim 6 or 7, further comprising: The lithographic apparatus is controlled based on the obtained apodization measurements.

9. A computer program comprising computer-readable instructions, the computer-readable instructions being configured to cause a computer to execute the method according to any one of claims 1 to 8.

10. A non-transitory computer readable medium carrying the computer program according to claim 9.

11. A computer device for determining a sensor contribution to an apodization measurement, the computer device comprising: a memory storing processor-readable instructions; as well as a processor arranged to read and execute instructions stored in said memory; Wherein the processor readable instructions comprise instructions arranged to control the computer to perform the method according to any one of claims 1 to 8.

12. A lithographic apparatus comprising: Irradiation system; a substrate stage for holding a substrate, the substrate stage comprising a sensor; a projection system for projecting a radiation beam onto the sensor, the projection system comprising an aperture through which the radiation beam is directed; an adjustable stopper for controllably adjusting the diameter of the hole; as well as A processor configured to execute the method according to any one of claims 1 to 8.

13. The lithographic apparatus of claim 12, wherein the adjustable stop comprises a diaphragm.

14. A projection system for a lithographic apparatus, the projection system for projecting a radiation beam towards a substrate table comprising a sensor, wherein the projection system comprises an aperture through which the radiation beam is directed and an adjustable stop for controllably adjusting a diameter of the aperture, wherein a radiation intensity detected by the sensor in a first region of the sensor when the aperture is in a first configuration with a first aperture diameter smaller than a diameter of the radiation beam is used as a first measurement, wherein the radiation beam is not incident on the first region, and based on the first measurement a sensor contribution to the apodization measurement can be determined.

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