Method of detecting ghost particles from patterning device inspection data

Abstract

A method of processing patterning device inspection data is provided. The method includes: receiving a map of points on a patterning device where contaminant is suspected to be present; detecting a geometric pattern formed by the points in the map; and based on the detected geometric pattern, selecting a subset of the points at which the patterning device is determined to be free from contaminant.

Description

METHOD OF DETECTING GHOST PARTICLES FROM PATTERNING DEVICE INSPECTION DATACROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 734,547 which was filed on 16 December 2024, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present technology relates to techniques of detecting ghost particles from patterning device inspection data.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiationsensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the "scanning "-direction) while synchronously scanning the substrate parallel or antiparallel to this direction.

[0004] The imaging of the pattern including small structures, possibly protected by a pellicle, is very sensitive to dust and other contamination of the patterning device and substrate. Therefore, before imaging, the patterning device (and / or the pellicle protecting the small structures thereof) and substrate are tested for contamination, in particular for particles. In conventional lithographic apparatus, a particle detection system directs a beam of radiation, in particular (but not necessarily) monochrome radiation, i.e., radiation having substantially one wavelength, on a surface of an object, for example, but not limited to, the patterning device or the substrate. The object and / or the beam move in order to scan the surface of the object. When the beam of radiation engages the surface of the object, the radiation is partially reflected according to physical laws of reflection (an exit angle is identical to an angle of incidence with respect to a fictitious line perpendicular to the surface (the normal)). Another part of the incident radiation may enter the object, such as the patterning device or substrate, and is refracted. In both cases,the beam is anisotropically redirected. When the beam of radiation engages a contaminating particle, the radiation is scattered, i.e., reflected isotropically.

[0005] A radiation detector is positioned with respect to the surface and the beam of radiation such that radiation reflected on the surface is not incident on the detector, but a part of the radiation scattered, i.e., being reflected in substantially every direction, by a particle or other contamination is incident on the detector. Thus, the detector receives radiation when the beam of radiation is scattered by a particle or other contamination.

[0006] A part of the radiation incident on the surface of the object enters the object and is refracted, as above mentioned. Inside the object, the beam may be refracted and / or diffracted by the chrome pattern and / or reflected one or more times. Depending on a number of parameters, such as the material, the size, the geometry, and the like, a part of the radiation that entered the object will leave the object again in the direction of the detector. For example, this may be caused by properties of the reticle chrome pattern, such as pitch, density, etc., and also sometimes due to the flaws in the optical modules. In that case, the detector detects radiation not being scattered by a particle. As a result, a detection circuit receiving a signal from the detector determines that a particle is present, although no particle is actually present. Such a detected, but not actually present, particle will hereinafter be referred to as a ghost particle.

[0007] In other conventional systems for detecting particles, a microscope may be used. Such systems use a microscope to scan the surface and may perform a detailed analysis of any detected particle. However, such systems are expensive and less suitable for mere in-line detection of particles.SUMMARY

[0008] An object of the present technology is to detect contaminant particles on patterning devices.

[0009] Another object of the present technology is to reduce false positive detections caused by ghost particles.

[0010] In accordance with an embodiment of the present technology, a method of processing patterning device inspection data is provided. The method includes: receiving a map of points on a patterning device where contaminant is suspected to be present; detecting a geometric pattern formed by the points in the map; and based on the detected geometric pattern, selecting a subset of the points at which the patterning device is determined to be free from contaminant.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic apparatus according to an embodiment of the present technology;Figure 2 schematically illustrates redirection of a beam of radiation on an object or on a particle; Figure 3 schematically illustrates a beam of radiation leaving an object being detected by a detector system;Figure 4 schematically illustrates how a beam of radiation may internally be refracted, diffracted, and / or reflected before leaving the object in the direction of a detector system;Figure 5 schematically illustrates how a beam of radiation may internally and externally be refracted, diffracted, and / or reflected before leaving the object in the direction of a detector system;Figure 6 schematically illustrates another way in which a beam of radiation may internally be refracted, diffracted, and / or reflected before leaving the object in the direction of a detector system;Figure 7 schematically illustrates a process of removing ghost particles; and Figure 8 schematically illustrates an example of patterning device inspection data.

[0012] The Figures are schematic. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described.DETAILED DESCRIPTION

[0013] There is a trend in the semiconductor industry (often known as “Moore’s law”) to reduce the physical dimensions of structures representing circuit components on a substrate and / or to increase the packing density of such structures, in order to reduce the physical size of electronic devices and / or enhance the computing power of electronic devices. The physical dimensions of such structures may be reduced and / or the packing density of such structures may be increased by increasing lithographic resolution. Manufacturing processes of semiconductor IC chips can have hundreds of individual steps. An error in any step of the manufacturing process has the potential to adversely affect the functioning of the electronic device. It is desirable to improve the overall yield of the manufacturing process. For example, to obtain a 75% yield for a 50-step manufacturing process (where a step may indicate the number of layers formed on a substrate), each individual step must have a yield greater than 99.4%. If an individual step has a yield of 95%, the overall yield of the manufacturing process would be as low as 7- 8%. It is desirable to determine defects quickly so as to maintain a high substrate throughput, defined as the number of substrates processed per hour.

[0014] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patternsfor magnetic domain memories, liquid-crystal displays (LCDs), thin-fdm magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion," respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, 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.

[0015] The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 248, 193, 157, or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0016] The term "patterning device" used herein should be broadly interpreted as referring to a device that can be used to impart a beam of radiation with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the beam of radiation may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the beam of radiation will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0017] Patterning devices 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 so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. In each example of patterning device, the support structure may be a frame or table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example, with respect to the projection system. Any use of the term "mask" herein may be considered synonymous with the more general term "patterning device."

[0018] The term "projection system" used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate, for example, for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term "lens" herein may be considered as synonymous with the more general term "projection system."

[0019] The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a "lens."

[0020] The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and / or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.

[0021] The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g., water, so as to fdl a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0022] Figure 1 schematically depicts a lithographic apparatus according to an embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to provide a beam PB of radiation (e.g., UV radiation), and a first support structure (e.g., a mask table) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device with respect to the projection system, item PL ("lens"), apparatus also includes a substrate table (e.g., a wafer table) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate with respect to the projection system, item PL ("lens"); the projection system (e.g., a refractive projection lens) PL being configured to image a pattern imparted to the beam of radiation PB by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0023] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above).

[0024] The illuminator IL receives a beam of radiation from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example, when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source may be integral part of the apparatus, for example, when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0025] The illuminator IL may include an adjusting device AM configured to adjust the angular intensity distribution of the beam. Generally, at least the outer and / or inner radial extent (commonly referred to as o-outer and o-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL generally includes various other components, such as an integrator IN and a condenser CO. The illuminator provides a conditioned beam of radiation, referred to as the beam of radiation PB, having a desired uniformity and intensity distribution in its cross-section.

[0026] The beam of radiation PB is incident on the mask MA, which is held on the mask table MT. Having traversed the mask MA, the beam of radiation PB passes through the lens PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g., an interferometric device), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the beam PB. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the mask MA with respect to the path of the beam PB, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the object tables MT and WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the positioning device PM and PW. However, in the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks PI, P2.

[0027] The depicted apparatus can be used in the following modes:

[0028] Step mode: the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the beam of radiation is projected onto a target portion C at once (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.

[0029] Scan mode: the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the beam of radiation is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT is 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 (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

[0030] Another mode: the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the beamof radiation is projected onto a target portion C. In this mode, generally a pulsed radiation source is 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 that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

[0031] Combinations and / or variations on the above-described modes of use or entirely different modes of use may also be employed.

[0032] To show the principle of particle detection by incident radiation and how artifacts may occur, it is illustrated in Figures 2-5 how isotropic, e.g., by a particle or other contamination scattered, and non- isotropic, e.g., diffracted or reflected, radiation may be incident on a detector system.

[0033] Figure 2 shows an object 2 such as a lithographic mask or substrate. Referring to the left-hand side of Figure 2, a beam 4A hits the surface of the object 2. At the location where the beam 4A hits the surface, the normal 6, i.e., a line perpendicular to the surface, is indicated. A reflection beam 8 may be reflected according to physical laws known to the person skilled in the art (an exit angle is the same as the angle of incidence with respect to the normal 6). The incident beam 4A may partially be refracted, indicated by a refraction beam 10. Depending on a refraction index of the material of object 2 and on the refraction index of the medium through which the radiation beam 4A travels, the refracted beam 10 is bent towards or away from the normal 6. The amount of radiation being refracted and / or reflected depends on the material of object 2, a surface coating of the object 2, and / or on the angle of incidence, among others.

[0034] A detector system 12 detects radiation coming from the location of incidence of the radiation beam 4A, and being directed towards the detector system 12, indicated by a detection cone 14. As is seen from the left-hand side of Figure 2, an incident beam 4A is anisotropically reflected as a reflection beam 8 and / or anisotropically refracted as a refraction beam 10. Thus, in this case, no radiation is incident on the detector and the detector may output a signal having a noise and / or bias level, but not having a significant particle detection level.

[0035] Now referring to the right-hand side of Figure 2, a beam of radiation 4B is incident on a contaminating particle 16 present on the surface of object 2. A part of the incident radiation may be absorbed by the particle 16. Another part may be reflected. Due to the surface shape of the surface of the particle 16, the incident radiation is scattered, i.e., isotropically reflected. Isotropically reflected radiation, indicated by arrows 18 is directed in substantially every direction. Therefore, a part of the reflected radiation 18 lies within the detection cone 14 of the detector system 12. Thus, the detector system 12 detects radiation and outputs a signal corresponding to the detected radiation having a level above the particle detection level, i.e., a threshold level.

[0036] In Figure 3, an incident beam of radiation 4 is indicated to hit the surface of the object 2. From the location of incidence, a beam of radiation 19 lies within a detection cone 14 of detector system 12 and is incident on the detector system 12. The beam 19 may be radiation having been scattered by a contaminating particle, the detection circuit thus correctly detecting the particle.

[0037] However, a beam 20 coming from inside the object 2, as a result of diffraction, refraction, and / or reflection as will be explained hereinafter, may leave the object 2 and be refracted such that the beam 19 results. So, if a beam 20 comes from inside the object 2 having such an angle with respect to the normal that its refracted beam 19 lies within the detection cone 14, the detector system 12 detects radiation which was not scattered by a contaminating particle. A detection circuit receiving a signal from the detector system 12 however determines that the signal is above a predetermined threshold level and erroneously indicates that a particle is present. Such a detected, but not actually present, particle is herein referred to as a ghost particle.

[0038] As will be explained in detail below in relation to Figures 4 and 5, an important contributor to the detection of ghost particles is a diffraction pattern. When the object includes a pattern, for example a reflective chrome pattern at a surface, at which surface an entered radiation beam internally diffracts, a diffraction pattern may result. The diffraction pattern may internally reflect and refract and then exit the object such that at least a part of the diffraction pattern will be incident on the detector. Such a diffraction pattern is an anisotropic contribution to the radiation incident on the detector.

[0039] A diffraction pattern may include a number of orders, i.e., a zero order pattern, a first order pattern, and higher order patterns. A shape, orientation, and spacing of the orders of the diffraction pattern is dependent on the shape, orientation, and spatial frequency of the diffracting pattern. If a diffracting pattern has a two-dimensional periodic structure, the related diffraction pattern will also be two-dimensional. The energy (intensity) in the diffracted orders depends among others on a duty cycle of the pattern (i.e., a spatial characteristic of the pattern) and on height differences in the pattern. The direction of the diffraction pattern determines whether no, one, or more orders of the diffraction pattern may reach the detector system.

[0040] In Figure 4, it is illustrated how a beam may originate from inside the object 2. The illustrated object 2 is a mask. On one surface, the mask 2 includes a mask pattern 22, which is made, for example, of chromium. An opposite surface of the mask 2 is scanned for particles. There is no particle actually present in the case illustrated in Figure 4.

[0041] A beam 4 is directed at and is incident on the surface to be scanned. A part of the incident radiation may be reflected (not shown) and another part may be absorbed and refracted. An absorbed and refracted beam 10 travels through the mask 2. At the opposite surface, the refracted beam 10 hits the mask pattern 22. The mask pattern 22 is a periodic pattern. Due to the periodicity the radiation isdiffracted. A diffracted beam 24 travels through the mask 2 and is reflected at the surface of incidence. Subsequent internal reflections at other surfaces may occur, indicated by reflected beams 26. Eventually, an internally reflected beam 26 may approach the location of incidence of the beam 4 such that it leaves the mask 2 and is refracted towards the detector system 12, as indicated by the beam 19. Thus, a ghost particle is detected.

[0042] In Figure 5, again, a refracted beam 19 coming from inside the mask 2 is incident on the detector system 12, but due to another series of reflections, refractions, and / or diffractions compared to the case illustrated in Figure 4. In the case illustrated in Figure 5, an incident beam 4 enters the mask 2 and is refracted as refracted beam 10. Given the angle of incidence and the surface conditions, the refracted beam 10 is diffracted or refracted at the opposite surface and leaves the mask 2 as diffracted beam 28. If the diffracted beam 28 is reflected at a surface of another object, such as a pellicle, a reflected beam 26 may hit the surface of the mask 2 again.

[0043] The reflected beam 26 is diffracted by periodic mask pattern 22 and enters the mask 2 as a diffracted beam 30. The indicated diffracted beam 30 leaves the mask 2 such that it is refracted towards the detector system 12 and is detected. Similar to Figure 4, a ghost particle is detected by a detection circuit, although no particle is present.

[0044] Ghost particles can also be caused by refractions and reflections, without involving diffraction. For example, as shown in Figure 6, a beam 4 is directed at and is incident on the surface to be scanned. A part of the incident radiation may be reflected (not shown) and another part may be absorbed and refracted. An absorbed and refracted beam 10 travels through the mask 2. At the opposite surface, the refracted beam 10 hits the mask pattern 22, and reaches the surface to be scanned, and leaves the mask 2 and is refracted towards the detector system 12, as indicated by the beam 19. Thus, a ghost particle is detected.

[0045] In the above description in relation to Figures 3-6, it should be noted that reflection, refraction, and diffraction may be anisotropic. The resulting redirected radiation is included in one or more beams as opposed to scattered, isotropic radiation being redirected in substantially every direction. The multiple diffracted beams are known as (diffraction) orders.

[0046] The cross sections of the beams in a diffraction pattern are dependent on the shape of the crosssection of the incident beam of radiation. When, for instance, the incident beam is a single round beam of radiation, the diffraction pattern will be a series of single round beams.

[0047] Further, it is noted that Figures 2-6 are for illustrative purpose only and are non-exhaustive, since many other light trajectories are possible. For example, although not shown in Figures 2-6, radiation coming from other directions than from inside the illustrated cone 14 may be incident on the detector system 12, and thus resulting in the detection of a ghost particle. Other series of reflections, refractions,and / or diffractions may result in radiation being directed at the detector system such that a detection of a ghost particle could result.

[0048] In order to reduce the likelihood of patterning defects, it may be desirable to halt the exposure cycle when contaminant particles 16 are found to be present in the patterning device inspection data of the patterning device 2. The patterning device 2 may thus be cleaned or replaced before exposure cycles can resume. However, some of the detected contaminant particles 16 may in fact be ghost particles. In some cases, it is possible that the patterning device inspection data contains ghost particles exclusively, i.e., the patterning device 2 is in fact free of contaminant particles 16 but is nevertheless reported as contaminated. In such cases, the exposure cycle would be needlessly halted, and the patterning device 2 may be needlessly cleaned or replaced. This increases the down time of the lithographic apparatus, and reduces its manufacturing throughput.

[0049] It may thus be desirable to distinguish ghost particles from genuine contaminant particles 16. In particular, it may be desirable to analyze patterning device inspection data to determine which, if any, of the reported contaminant particles 16 are, or likely are, ghost particles that may be disregarded.

[0050] As surprisingly found by the present inventors, whereas genuine contaminant particles 16 have a tendency to be randomly distributed, ghost particles are more likely to be appear with a certain regularity or with some resemblance of a geometric pattern. Sometimes, the distribution of ghost particles may appear to be random to the human eye, but may in fact have hidden regularities that can be recognized through data analysis.

[0051] Therefore, in accordance with an embodiment, ghost particles are detected by a method of processing patterning device inspection data. As shown in Figure 7, the method includes, in some embodiments, receiving a map of points 71 on a patterning device 2 where a contaminant is suspected to be present, detecting a geometric pattern formed by the points in the map, and, based on the detected geometric pattern, selecting a subset of the points 721 at which the patterning device 2 is determined to be free from contaminant. In other words, the subset of points 721 may be deemed as ghost particles.

[0052] The map of points 71 may contain a list of suspected contaminant particles. Each suspected contaminant particle may be associated with coordinates of its position on the patterning device 2. Furthermore, each suspected contaminant particle may be associated with a parameter representing the size of the suspected contaminant particle. For example, the parameter may be a brightness measurement, which may be correlated with the size of the suspected contaminant particle. The map of points 71 may be obtained by scanning a surface of the patterning device 2 with an illumination beam 4. Each point in the map where contaminant is suspected to be present is a point on the patterning device 2 where a reflection of the illumination beam 18, 19 indicative of the presence of contaminant is detected.

[0053] A second map 73 may be generated by subtracting the subset of points 721 from the first map 71.That is, the second map 73 may be generated by making a copy of the list of suspected contaminant particles that make up the first map 71, and removing from the list any suspected contaminant particles having matching coordinates with the any one of the subset of points 721. As a result, the second map 73 may contain contaminant particles 731 that are assumed to be genuine.

[0054] The patterning device 2 may be determined to be contaminated if the second map 73 contains at least one point 731. Conversely, the patterning device may be determined as not contaminated if the second map 731 is empty (i.e., contains no points 731).

[0055] Different methods may be used to detect one or more geometric patterns that may be present in the points of the first map 71.

[0056] For example, in some embodiments, collinear points are detected from the first map of points 71. Specifically, detecting the geometric pattern includes identifying unique groups of three or more points in the first map 71 that are substantially collinear. It should be understood that the three or more points need not be exactly collinear for them to be identified as collinear. A predetermined margin of error may be allowed, so that three or more points that are close to collinear may nevertheless be identified as collinear. This may accommodate errors that are inherent in measurements, and / or manufacturing tolerances of features of the patterning device 2 that are responsible for generating ghost particles.

[0057] The detection of a group of three or more collinear points need not necessarily immediately result in a determination of these points as ghost particles. Instead, it may be recognized that a detection of collinearity increases the probability that the points are ghost particles. The probability may be represented in various ways. For example, for each of the unique groups of three or more points that are substantially collinear, a score associated with each of the points forming the group may be incremented. It should be recognized that a given point may be a member of several groups of three or more points that are substantially collinear. Therefore, the score associated with a point may increase each time the point is found to be in a unique group of three or more points that are substantially collinear. Furthermore, as surprisingly found by the present inventors, if a given point is a member of many unique groups of three or more points that are substantially collinear, the likelihood that the point is a ghost particle is correspondingly high. Thus, the higher the score, the more likely it is that the associated point is a ghost particle.

[0058] In some embodiments, a stricter test for a geometric pattern is used. For example, in some embodiments, in addition to collinearity, the method includes looking for points that repeat at regular distances, such as points forming a grid. More specifically, detecting the geometric pattern further includes, in some embodiments, determining whether the points in each unique group of three or more points that are substantially collinear are also substantially equidistant. It should be understood that, as with the collinearity test, the three or more points need not be exactly equidistant for them to be identifiedas equidistant. A predetermined margin of error may be allowed, so that three or more points that are close to equidistant may nevertheless be identified as equidistant.

[0059] With this stricter test, which requires both collinearity and equidistance, the score associated with a given point may be incremented only if it is in a collinear and equidistant relationship with two or more other points. That is, for each of the unique groups of three or more points that are substantially collinear and equidistant, the score associated with each of the points forming the group may be incremented.

[0060] In some embodiments, a still stricter test is used that takes into account the (apparent) particle size of a suspected contaminant particle, in addition to collinearity and equidistance. As noted above, the particle size may be measured by a parameter that correlates with particle size, for example, brightness. That is, detecting the geometric pattern further includes, in some embodiments, determining whether the points in each unique group of three or more points that are substantially collinear and substantially equidistant are also within a predetermined particle size range. Accordingly, the score associated with a given point may be incremented only if the point is in a collinear and equidistant relationship with two or more other points, and only if it is within a predetermined particle size range. That is, for each of the unique groups of three or more points that are substantially collinear and equidistant and within the predetermined particle size range, the score associated with each of the points forming the group may be incremented.

[0061] In some embodiments, in addition to or as an alternative to collinearity and / or equidistance and / or particle size, a geometric pattern is determined based on the angle of lines formed by pairs of suspected contaminant particles, represented by points in the first map 71. As is typical in lithographic design, the mask pattern 22 often contains rectilinear features and periodic features. Therefore, as surprisingly found by the present inventors, it can be expected that ghost particles will have a tendency to be aligned along lines in certain directions, notably at 0° and 90° (these angles are, of course, dependent on the choice of frame of reference of the patterning device 2). Ghost particles may also be aligned along diagonals (e.g., 45° if the mask pattern 22 has features that repeat in a square grid), or indeed any angled lines that coincide with the periodicity of the features of the mask pattern 22. Therefore, in some embodiments, the angular distribution of the points in the first map 71 (i.e., the distribution of angles formed by pairs of points in the first map 71) provide useful information for detecting ghost particles.

[0062] In some embodiments, detecting the geometric pattern therefore includes calculating an angular distribution. For each unique pair of points from the points in the first map 71, the angle of a straight line joining the pair of points may be determined. The angle may be associated with the unique pair of points. This process may be repeated for every possible unique pair of two points in the first map 71. The unique pairs of points may be classified into bins of angular ranges according to the respective associated angles. As a result, the angular distribution of the points in the first map 71 may be represented as a histogram.

[0063] An example of such a histogram is shown in Figure 8(b), which is calculated from the example of a first map 71 shown in Figure 8(a). The histogram ranges from -90° to just under 90°, with 0° being at the center. By arbitrary convention, 0° refers to the horizontal direction in the first map 71 shown in Figure 8(a). As shown in the histogram of Figure 8(b), there is a clear spike at 0°. This is as expected because the first map 71 in Figure 8(a) visually contains many points that appear to be vertically aligned. A smaller, but still clear, spike appears at -90° in the histogram. This reflects the horizontal alignment of many of the points in the first map 71 shown in Figure 8(a). Visually, it can be seen that the vertical alignment is more predominant than the horizontal alignment. Other smaller spikes at diagonal angles can also be observed in the histogram of Figure 8(b). Generally, as surprisingly found by the inventors, the taller the spike, the more likely it is that the suspected contaminant particles contributing to that spike are ghost particles.

[0064] In some embodiments, a score-based technique similar to that described above in relation to the test based on collinearity and / or equidistance and / or particle size is used. For example, it may be determined whether the number of unique pairs of points classified into each of the bins of angular ranges exceeds a predetermined threshold. For each bin in which the predetermined threshold is exceeded, the score associated with each of the points forming each of the unique pairs of points classified into the bin may be incremented.

[0065] It should be understood that a given point in the first map 71 may have its score incremented each time it is found to form a straight line with another point at an angle that falls within one of the bins in which the predetermined threshold is exceeded. A given point may thus have its score incremented several times by virtue of it forming straight lines with several other points at several angles that fall within bins in which the predetermined threshold is exceeded. Such a point may thus accumulate a high score, which may be indicative of a high likelihood of being a ghost particle.

[0066] In some situations, the design of the patterning features 22 of the patterning device 2 may not be accessible. However, in situations where it is accessible, it may be used additionally or alternatively for detecting ghost particles. Specifically, detecting the geometric pattern may include receiving patterning features 22 of the patterning device 2, identifying points in the first map 71 that coincide with the patterning features 22, and incrementing the score associated with each of the points that coincides with the patterning features 22.

[0067] As noted above, the detection of ghost particles may employ one or several tests, including collinearity, equidistance, particle size, angular distribution, and coincidence with patterning features 22. Each of these tests may result in increments to the score associated with each point in the first map 71. It should be understood that the score associated with a given point in the first map 71 may be incremented by one or several of these tests. Furthermore, the sizes of the increment of these tests may be weighted,so that some tests may have a greater influence on the score than other tests.

[0068] As noted above, a higher score is generally correlated with a higher likelihood that a given point in the first map 71 is a ghost particle, and may be removed to form the second map 73. For example, it may be determined whether the score associated with each of the points in the first map 71 exceeds a second predetermined threshold. For each score that exceeds the second predetermined threshold, the point associated therewith may be added to the subset of points 721 at which the patterning device 2 is determined to be free from contaminants.

[0069] In some embodiments, further improvements to the detection of geometric patterns may be achieved by using machine learning in addition or as an alternative. Specifically, the steps of detecting the geometric pattern and selecting the subset of the points 721 may be performed using a trained machine learning model. The machine learning model may take one or more of the following as input:• the unique groups of three or more points in the first map 71 that are identified to be substantially collinear;• the unique groups of three or more points in the first map 71 that are identified to be substantially collinear and equidistant;• the angular distribution of the unique pairs of points in the first map 71 ;• the patterning features 22 of the patterning device 2;• an image of the intensity of reflected light indicative of the presence of contaminants obtained by scanning a surface of the patterning device 2 with an illumination beam 4; and• a particle size distribution extracted from the image.

[0070] The machine learning model may be trained using supervised learning. The machine learning model may include one or more neural networks, including one or more convolutional neural networks (CNNs).

[0071] Although Figures 2-6 depict a patterning device 2 of the transmissive type, it should be understood that the present technology can be applied equally to patterning devices 2 of the reflective type. Of course, the mechanisms by which ghost particles are generated may be different for different types of patterning device 2. However, since the present technology is based on pattern recognition, it may also have the advantage of being agnostic to the mechanisms by which ghost particles are generated, and thus agnostic to the type of patterning device 2.

[0072] In addition to a method of processing patterning device inspection data, there is also disclosed a method of inspecting a patterning device 2. In some embodiments, the method of inspecting a patterning device 2 includes scanning a surface of the patterning device 2 with an illumination beam 4, generating a map of points 71 on a patterning device 2 where contaminant is suspected to be present, each point of the map 71 being a point where a reflection of the illumination beam 4 indicative of the presence ofcontaminant is detected, and the method of processing patterning device inspection data as disclosed above.

[0073] Based on the result of the method of inspecting the patterning device 2, the patterning device 2 may be rejected if it is determined to be contaminated.

[0074] There is also disclosed a method of manufacturing devices including the above method of inspecting a patterning device 2, or the above method of processing patterning device inspection data.

[0075] There is also disclosed a computer program product including instructions which, when executed by a processor, cause the processor to perform the above method of processing patterning device inspection data.

[0076] There is also disclosed a patterning device handler assembly configured to perform the above method of processing patterning device inspection data.

[0077] There is also disclosed a lithographic apparatus including the patterning device handler assembly.

[0078] Where the context allows, embodiments of the present technology may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present technology may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc., and in doing that may cause actuators or other devices to interact with the physical world.

[0079] While specific embodiments of the present technology have been described above, it will be appreciated that the present technology 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 present technology as described without departing from the scope of the claims set out below.

[0080] Example embodiments of the present technology are described in the following numbered clauses.1. A method of processing patterning device inspection data, the method comprising: receiving a map of points on a patterning device where contaminant is suspected to be present;detecting a geometric pattern formed by the points in the map; and based on the detected geometric pattern, selecting a subset of the points at which the patterning device is determined to be free from contaminant.2. The method of clause 1, wherein the map of points is obtained by scanning a surface of the patterning device with an illumination beam, and each point in the map where contaminant is suspected to be present is a point on the patterning device where a reflection of the illumination beam indicative of the presence of contaminant is detected.3. The method of clause 1 or clause 2, further comprising generating a second map by subtracting the subset of points from the first map.4. The method of clause 3, further comprising determining that the patterning device is contaminated if the second map contains at least one point.5. The method of clause 3 or clause 4, further comprising determining that the patterning device is not contaminated if the second map is empty.6. The method of any one of the preceding clauses, wherein the detecting the geometric pattern comprises identifying unique groups of three or more points in the first map that are substantially collinear.7. The method of clause 6, further comprising, for each of the unique groups of three or more points that are substantially collinear, incrementing a score associated with each of the points forming the group.8. The method of clause 6 or clause 7, wherein the detecting the geometric pattern further comprises determining whether the points in each unique group of three or more points that are substantially collinear are also substantially equidistant.9. The method of clause 8, further comprising, for each of the unique groups of three or more points that are substantially collinear and equidistant, incrementing the score associated with each of the points forming the group.10. The method of clause 8, wherein the detecting the geometric pattern further comprises determining whether the points in each unique group of three or more points that are substantially collinear and substantially equidistant are also within a predetermined particle size range.11. The method of clause 10, further comprising, for each of the unique groups of three or more points that are substantially collinear and equidistant and within the predetermined particle size range, incrementing the score associated with each of the points forming the group.12. The method of any one of the preceding clauses, wherein the detecting the geometric pattern comprises calculating an angular distribution by: for each unique pair of points from the points in the first map, determining the angle of a straight line joining the pair of points and associating the angle therewith; andclassifying the unique pairs of points into bins of angular ranges according to the respective associated angles.13. The method of clause 12, further comprising: determining whether the number of unique pairs of points classified into each of the bins of angular ranges exceeds a predetermined threshold; and for each bin in which the predetermined threshold is exceeded, incrementing the score associated with each of the points forming each of the unique pairs of points classified into the bin.14. The method of any one of the preceding clauses, wherein the detecting the geometric pattern comprises: receiving patterning features of the patterning device; identifying points in the first map that coincide with the patterning features, and incrementing the score associated with each of the points that coincides with the patterning features.15. The method of any one of clauses 7, 9, 11, 13 or 14, further comprising: determining whether the score associated with each of the points in the first map exceeds a second predetermined threshold; and for each score that exceeds the second predetermined threshold, adding the point associated therewith to the subset of points at which the patterning device is determined to be free from contaminant.16. The method of any one of the preceding clauses, wherein the steps of detecting the geometric pattern and selecting the subset of the points are performed using a trained machine learning model.17. The method of clause 16, wherein the trained machine learning model takes one or more of the following as input: the unique groups of three or more points in the first map that are identified to be substantially collinear; the unique groups of three or more points in the first map that are identified to be substantially collinear and equidistant; the angular distribution of the unique pairs of points in the first map; the patterning features of the patterning device; an image of the intensity of reflected light indicative of the presence of contaminant obtained by scanning a surface of the patterning device with an illumination beam; and a particle size distribution extracted from the image.18. The method of any one of the preceding clauses, wherein the patterning device is of the transmissive type.19. The method of any one of the preceding clauses, wherein the patterning device is of the reflectivetype.20. A method of inspecting a patterning device comprising: scanning a surface of the patterning device with an illumination beam; generating a map of points on a patterning device where contaminant is suspected to be present, each point of the map being a point where a reflection of the illumination beam indicative of the presence of contaminant is detected; and the method of any one of the preceding clauses.21. The method of clause 20, further comprising rejecting the patterning device if it is determined to be contaminated.22. A method of manufacturing devices comprising the method of any one of the preceding clauses.23. A computer program product comprising instructions which, when executed by a processor, cause the processor to perform the method of any one of clauses 1 to 21.24. A patterning device handler assembly configured to perform the method of any clauses 1 to 21.25. A lithographic apparatus comprising the patterning device handler assembly of clause 24.

Claims

CLAIMS1. A method of processing patterning device inspection data, the method comprising: receiving a map of points on a patterning device where contaminant is suspected to be present; detecting a geometric pattern formed by the points in the map; and based on the detected geometric pattern, selecting a subset of the points at which the patterning device is determined to be free from contaminant.

2. The method of claim 1, wherein the map of points is obtained by scanning a surface of the patterning device with an illumination beam, and each point in the map where contaminant is suspected to be present is a point on the patterning device where a reflection of the illumination beam indicative of the presence of contaminant is detected, further comprising: generating a second map by subtracting the subset of points from the first map; determining that the patterning device is contaminated if the second map contains at least one point; and determining that the patterning device is not contaminated if the second map is empty.

3. The method of claim 1, wherein the detecting the geometric pattern comprises identifying unique groups of three or more points in the first map that are substantially collinear, further comprising: for each of the unique groups of three or more points that are substantially collinear, incrementing a score associated with each of the points forming the group, wherein the detecting the geometric pattern further comprises determining whether the points in each unique group of three or more points that are substantially collinear are also substantially equidistant.

4. The method of claim 3, further comprising: for each of the unique groups of three or more points that are substantially collinear and equidistant, incrementing the score associated with each of the points forming the group, wherein the detecting the geometric pattern further comprises determining whether the points in each unique group of three or more points that are substantially collinear and substantially equidistant are also within a predetermined particle size range; and for each of the unique groups of three or more points that are substantially collinear and equidistant and within the predetermined particle size range, incrementing the score associated with each of the points forming the group.

5. The method of claim 1, wherein the detecting the geometric pattern comprises calculating an angular distribution by: for each unique pair of points from the points in the first map, determining the angle of a straight line joining the pair of points and associating the angle therewith; and classifying the unique pairs of points into bins of angular ranges according to the respective associated angles, further comprising: determining whether the number of unique pairs of points classified into each of the bins of angular ranges exceeds a predetermined threshold; and for each bin in which the predetermined threshold is exceeded, incrementing the score associated with each of the points forming each of the unique pairs of points classified into the bin.

6. The method of claim 1, wherein the detecting the geometric pattern comprises: receiving patterning features of the patterning device; identifying points in the first map that coincide with the patterning features; and incrementing the score associated with each of the points that coincides with the patterning features, further comprising: determining whether the score associated with each of the points in the first map exceeds a second predetermined threshold; and for each score that exceeds the second predetermined threshold, adding the point associated therewith to the subset of points at which the patterning device is determined to be free from contaminant.

7. The method of claim 1, wherein: the steps of detecting the geometric pattern and selecting the subset of the points are performed using a trained machine learning model; and the trained machine learning model takes one or more of the following as input: the unique groups of three or more points in the first map that are identified to be substantially collinear; the unique groups of three or more points in the first map that are identified to be substantially collinear and equidistant; the angular distribution of the unique pairs of points in the first map; the patterning features of the patterning device; an image of the intensity of reflected light indicative of the presence of contaminantobtained by scanning a surface of the patterning device with an illumination beam; and a particle size distribution extracted from the image.

8. The method of claim 1, wherein the patterning device is of the transmissive type or is of the reflective type.

9. A method of inspecting a patterning device comprising: scanning a surface of the patterning device with an illumination beam; generating a map of points on a patterning device where contaminant is suspected to be present, each point of the map being a point where a reflection of the illumination beam indicative of the presence of contaminant is detected; and the method of any one of the preceding claims.

10. The method of claim 9, further comprising rejecting the patterning device if it is determined to be contaminated.

11. A method of manufacturing devices comprising the method of claim 1.

12. A computer program product comprising instructions which, when executed by a processor, cause the processor to perform the method of claim 1.

13. A patterning device handler assembly configured to perform the method of claim 1.

14. A lithographic apparatus comprising the patterning device handler assembly of claim 13.

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