Maintaining an optical focus of an optical element for improving performance of a metrology system

Abstract

A system includes an optical element, a distance sensor, a controller, and an actuator. The optical element faces a wafer. The optical element is spaced apart from the wafer by a focus distance. The distance sensor can measure a distance to the wafer. The distance sensor can be positioned off axis with respect to an optical axis of the optical element. The controller can be coupled to the actuator and can be configured to generate a control signal to maintain the focus distance. The control signal can be generated based on the measured distance by the distance sensor. The actuator coupled to the optical element and configured to control a position of the optical element based on the control signal.

Description

MAINTAINING AN OPTICAL FOCUS OF AN OPTICAL ELEMENT FOR IMPROVING PERFORMANCE OF A METROLOGY SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 557,046 which was filed on February 23, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to a lithographic apparatus. For example, the present disclosure relates to maintaining an optical focus of an optical element.BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

[0004] During lithographic operations, different processing steps may require different layers to be sequentially formed on the substrate. Accordingly, it can be necessary to position the substrate relative to prior patterns formed thereon with a high degree of accuracy. Generally, alignment marks are placed on the substrate to be aligned and are located with reference to a second object. A lithographic apparatus may use an alignment apparatus for detecting positions of the alignment marks and for aligning the substrate using the alignment marks to ensure accurate exposure from a mask. Misalignment between the alignment marks at two different layers is measured as overlay error.

[0005] To monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on the patterned substrate and critical linewidth of developed photosensitive resist. This measurement can be performed on a product substrate and / or on a dedicated metrology target. There are various techniquesfor making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of a specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. By contrast, angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.

[0006] As discussed above, the alignment apparatus or a metrology system may detect positions of the alignment marks for aligning the substrate. Before performing an alignment measurement, the alignment apparatus can use a through-the-lens focus methodology to check the focus of an objective (optical element) of the alignment apparatus. However, the focus may change during measurements.SUMMARY

[0007] Accordingly, it is desirable to improve the performance of metrology systems. For example, there is a desire to provide inspection systems as discussed in embodiments described herein.

[0008] In some embodiments, a system can include an optical element, a distance sensor, a controller, and an actuator. The optical element can face a substrate (e.g., a wafer). The optical element is spaced apart from the wafer by a focus distance. The distance sensor can measure a distance to the wafer. The distance sensor can be positioned off axis with respect to an optical axis of the optical element. The controller can be coupled to the actuator and can be configured to generate a control signal to maintain the focus distance. The control signal can be generated based on the measured distance by the distance sensor. The actuator coupled to the optical element and configured to control a position of the optical element based on the control signal.

[0009] In some embodiments, a method includes determining a focus distance representative of a relative position between a focus of an optical element and a wafer. The method can output a position signal representative of the focus distance to a controller. The method can then determine a distance to the wafer using a distance sensor, and controlling a position of the optical element based on the position signal and the distance to maintain the optical element at the focus distance. The distance sensor can be positioned off axis with respect to an optical axis of the optical element

[0010] In some embodiments, a system can include an optical element facing a wafer. The system can also include an optical focus module, a distance sensor, an actuator, and a controller. The optical focus module is configured to establish a focus of the optical element and to determine a focus distance. The focus distance can represent a distance between the optical element and the wafer. The distance sensoris configured to measure a distance to the wafer. The actuator can be coupled to the optical element and is configured to maintain the focus distance. The controller is coupled to the actuator and is configured to adjust the actuator to maintain the focus distance based on the measured distance by the distance sensor.

[0011] In some embodiments, a lithographic apparatus can include an illumination system, a projection system, and an inspection system. The illumination system illuminates a pattern of a patterning device. The projection system projects an image of the pattern onto a substrate. The inspection system can include an optical element, a distance sensor, a controller, and an actuator. The optical element can face a wafer. The optical element is spaced apart from the wafer by a focus distance. The distance sensor can measure a distance to the wafer. The distance sensor can be positioned off axis with respect to an optical axis of the optical element. The controller can be coupled to the actuator and can be configured to generate a control signal to maintain the focus distance. The control signal can be generated based on the measured distance by the distance sensor. The actuator can be coupled to the optical element and can be configured to control a position of the optical element based on the control signal.

[0012] Further features of the present disclosure, as well as the structure and operation of various embodiments, are described in detail below with reference to the accompanying drawings. It is noted that the present disclosure is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.BRIEF DESCRIPTION OF THE DRAWINGS / FIGURES

[0013] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present disclosure and, together with the description, further serve to explain the principles of the present disclosure and to enable a person skilled in the relevant art(s) to make and use embodiments described herein.

[0014] FIG. 1A shows a schematic of a reflective lithographic apparatus, according to some embodiments.

[0015] FIG. IB shows a schematic of a transmissive lithographic apparatus, according to some embodiments.

[0016] FIG. 2 shows a more detailed schematic of the reflective lithographic apparatus, according to some embodiments.

[0017] FIG. 3 shows a schematic of a lithographic cell, according to some embodiments.

[0018] FIGS. 4A and 4B show schematics of inspection apparatuses, according to some embodiments.

[0019] FIG. 5A shows a schematic of a side view of an inspection system, according to some embodiments.

[0020] FIG. 5B shows a schematic of a bottom view of the inspection system, according to some embodiments.

[0021] FIG. 6 shows a schematic of an inspection system, according to some embodiments.

[0022] FIG. 7 shows a schematic of an inspection system, according to some embodiments.

[0023] FIG. 8A shows a schematic of a control configuration of the inspection system, according to some embodiments.

[0024] FIG. 8B shows a schematic of a control configuration of the inspection system, according to some embodiments.

[0025] FIG. 9 is a flowchart of a method for controlling an optical element of an inspection system, according to some embodiments.

[0026] FIG. 10 is an example computer system useful for implementing various embodiments.

[0027] The features of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and / or structurally similar elements. Additionally, generally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears. Unless otherwise indicated, the drawings provided throughout the disclosure should not be interpreted as to- scale drawings.DETAILED DESCRIPTION

[0028] This specification discloses one or more embodiments that incorporate the features of the present disclosure. The disclosed embodiment(s) are provided as examples. The scope of the present disclosure is not limited to the disclosed embodiment(s). Claimed features are defined by the claims appended hereto.

[0029] The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

[0030] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

[0031] The term “about” as used herein indicates the value of a given quantity that can vary based on a particular technology. Based on the particular technology, the term “about” can indicate a value of a given quantity that varies within, for example, 10-30% of the value (e.g., ±10%, ±20%, or ±30% of the value).

[0032] Embodiments of the disclosure can be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the disclosure may also be implemented as instructions stored on a machine-readable medium, which can 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 disk 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, and / or instructions can 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.

[0033] Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present disclosure can be implemented.

[0034] Example Lithographic Systems

[0035] FIGS. 1 A and IB show schematic illustrations of a lithographic apparatus 100 and lithographic apparatus 100’, respectively, in which embodiments of the present disclosure may be implemented. Lithographic apparatus 100 and lithographic apparatus 100’ each include the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatus 100 and 100’ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (for example, comprising one or more dies) C of the substrate W. In lithographic apparatus 100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus 100’, the patterning device MA and the projection system PS are transmissive.

[0036] The illumination system IL can include various types of optical components, such as refractive, reflective, catadioptric, magnetic, electromagnetic, electrostatic, or other types of optical components, or any combination thereof, for directing, shaping, or controlling the radiation beam B.

[0037] The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA with respect to a reference frame, the design of at least one of the lithographic apparatus 100 and 100’, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which can be fixed or movable, as desired. By using sensors, the support structure MT can ensure that the patterning device MA is at a desired position, for example, with respect to the projection system PS.

[0038] The term “patterning device” MA should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B can correspond to a particular functional layer in a device being created in the target portion C to form an integrated circuit.

[0039] The terms “inspection apparatus,” “metrology system,” or the like may be used herein to refer to, e.g., a device or system used for measuring a property of a structure (e.g., overlay error, critical dimension parameters) or used in a lithographic apparatus to inspect an alignment of a wafer (e.g., alignment apparatus).

[0040] The patterning device MA can be transmissive (as in lithographic apparatus 100’ of FIG. IB) or reflective (as in lithographic apparatus 100 of FIG. 1 A). Examples of patterning devices MA include reticles, masks, programmable mirror arrays, or programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase shift, or 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 may be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B, which is reflected by a matrix of small mirrors.

[0041] The term “projection system” PS can encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors, such as the use of an immersion liquid on the substrate W or the use of a vacuum. A vacuum environment may be used for EUV or electron beam radiation since other gases may absorb too much radiation or electrons. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

[0042] Lithographic apparatus 100 and / or lithographic apparatus 100’ can be of a type having two (dual stage) or more substrate tables WT (and / or two or more mask tables). In such “multiple stage” machines, the additional substrate tables WT can be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. In some situations, the additional table may not be a substrate table WT.

[0043] The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0044] Referring to FIGS. 1A and IB, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus 100, 100’ can be separate physical entities, for example, when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatus 100 or 100’, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (in FIG. IB) including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source SO can be an integral part of the lithographic apparatus 100, 100’, for example, when the source SO is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD, if desired, may be referred to as a radiation system.

[0045] The illuminator IL can include an adjuster AD (in FIG. IB) for adjusting the angular intensity distribution of the radiation 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 may be adjusted. In addition, the illuminator IL can comprise various other components (in FIG. IB), such as an integrator IN and a condenser CO. The illuminator IL can be used to condition the radiation beam B to have a desired uniformity and intensity distribution in its cross section.

[0046] Referring to FIG. 1A, the radiation beam B is incident on the patterning device (for example, mask) MA, which is held on the support structure (for example, mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus 100, the radiation beam B is reflected from the patterning device (for example, mask) MA. After being reflected from the patterning device (for example, mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF2 (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor IF1 can be used to accurately position the patterning device (for example, mask) MA with respect to the path of the radiation beam B. Patterning device (for example, mask) MA and substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2.

[0047] Referring to FIG. IB, the radiation beam B is incident on the patterning device (for example, mask MA), which is held on the support structure (for example, mask table MT), and is patterned bythe patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. The projection system has a pupil conjugate PPU to an illumination system pupil IPU. Portions of radiation emanate from the intensity distribution at the illumination system pupil IPU and traverse a mask pattern without being affected by diffraction at the mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

[0048] The projection system PS projects an image of the mask pattern MP, where the image is formed by diffracted beams produced from the mark pattern MP by radiation from the intensity distribution, onto a photoresist layer coated on the substrate W. For example, the mask pattern MP can include an array of lines and spaces. A diffraction of radiation at the array and different from zeroth order diffraction generates diverted diffracted beams with a change of direction in a direction perpendicular to the lines. Undiffracted beams (i.e., so-called zeroth order diffracted beams) traverse the pattern without any change in propagation direction. The zeroth order diffracted beams traverse an upper lens or upper lens group of the projection system PS, upstream of the pupil conjugate PPU of the projection system PS, to reach the pupil conjugate PPU. The portion of the intensity distribution in the plane of the pupil conjugate PPU and associated with the zeroth order diffracted beams is an image of the intensity distribution in the illumination system pupil IPU of the illumination system IL. The aperture device PD, for example, is disposed at or substantially at a plane that includes the pupil conjugate PPU of the projection system PS.

[0049] The projection system PS is arranged to capture, by means of a lens or lens group L, not only the zeroth order diffracted beams, but also first-order or first- and higher-order diffracted beams (not shown). In some embodiments, dipole illumination for imaging line patterns extending in a direction perpendicular to a line may be used to utilize the resolution enhancement effect of dipole illumination. For example, first-order diffracted beams interfere with corresponding zeroth-order diffracted beams at the level of the wafer W to create an image of the line pattern MP at highest possible resolution and process window (i.e., usable depth of focus in combination with tolerable exposure dose deviations). In some embodiments, astigmatism aberration may be reduced by providing radiation poles (not shown) in opposite quadrants of the illumination system pupil IPU. Further, in some embodiments, astigmatism aberration may be reduced by blocking the zeroth order beams in the pupil conjugate PPU of the projection system associated with radiation poles in opposite quadrants. This is described in more detail in US 7,511,799 B2, issued Mar. 31, 2009, which is incorporated by reference herein in its entirety.

[0050] With the aid of the second positioner PW and position sensor IFD (for example, an interferometric device, linear encoder, or capacitive sensor), the substrate table WT can be moved accurately (for example, so as to position different target portions C in the path of the radiation beam B). Similarly, the first positioner PM and another position sensor (not shown in FIG. IB) may be used to accurately position the mask MA with respect to the path of the radiation beam B (for example, after mechanical retrieval from a mask library or during a scan).

[0051] In general, movement of the mask table MT can be realized with the aid of a long-stroke module (coarse positioning) and a short- stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT can be realized using a long-stroke module and a short- stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner), the mask table MT can be connected to a short- stroke actuator only or can be fixed. Mask MA and substrate W can be aligned using mask alignment marks Ml, M2, and substrate alignment marks Pl, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they may be located in spaces between target portions (known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

[0052] Mask table MT and patterning device MA can be in a vacuum chamber V, where an in-vacuum robot IVR can be used to move patterning devices such as a mask in and out of vacuum chamber. Alternatively, when mask table MT and patterning device MA are outside of the vacuum chamber, an out-of- vacuum robot may be used for various transportation operations, similar to the in- vacuum robot IVR. Both the in-vacuum and out-of- vacuum robots need to be calibrated for a smooth transfer of any payload (e.g., mask) to a fixed kinematic mount of a transfer station.

[0053] The lithographic apparatus 100 and 100’ can be used in at least one of the following modes:1. In step mode, the support structure (for example, mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (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.2. In scan mode, the support structure (for example, mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B 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 support structure (for example, mask table) MT can be determined by the (de-)magnification and image reversal characteristics of the projection system PS.3. In another mode, the support structure (for example, mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO can be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation may be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

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

[0055] In a further embodiment, lithographic apparatus 100 includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, theEUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

[0056] FIG. 2 shows the lithographic apparatus 100 in more detail, including the source collector apparatus SO, the illumination system IL, and the projection system PS. The source collector apparatus SO is constructed and arranged such that a vacuum environment may be maintained in an enclosing structure 220 of the source collector apparatus SO. An EUV radiation emitting plasma 210 can be formed by a discharge produced plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor, or Sn vapor in which the very hot plasma 210 is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma 210 is created by, for example, an electrical discharge causing at least a partially ionized plasma. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor, or any other suitable gas or vapor may be required for efficient generation of the radiation. In some embodiments, a plasma of excited tin (Sn) is provided to produce EUV radiation.

[0057] The radiation emitted by the hot plasma 210 is passed from a source chamber 211 into a collector chamber 212 via an optional gas barrier or contaminant trap 230 (in some cases also referred to as contaminant barrier or foil trap), which is positioned in or behind an opening in source chamber 211. The contaminant trap 230 can include a channel structure. Contamination trap 230 can also include a gas barrier or a combination of a gas barrier and a channel structure. The contaminant trap or contaminant barrier 230 further indicated herein at least includes a channel structure.

[0058] The collector chamber 212 can include a radiation collector CO, which can be a so-called grazing incidence collector. Radiation collector CO has an upstream radiation collector side 251 and a downstream radiation collector side 252. Radiation that traverses collector CO can be reflected off a grating spectral filter 240 to be focused in a virtual source point INTF. The virtual source point INTF is commonly referred to as the intermediate focus, and the source collector apparatus is arranged such that the intermediate focus INTF is located at or near an opening 219 in the enclosing structure 220. The virtual source point INTF is an image of the radiation emitting plasma 210. Grating spectral filter 240 is used in particular for suppressing infra-red (IR) radiation.

[0059] Subsequently the radiation traverses the illumination system IL, which can include a faceted field mirror device 222 and a faceted pupil mirror device 224 arranged to provide a desired angular distribution of the radiation beam 221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the radiation beam 221 at the patterning device MA, held by the support structure MT, a patterned beam 226 is formed and the patterned beam 226 is imaged by the projection system PS via reflective elements 228, 229 onto a substrate W held by the wafer stage or substrate table WT.

[0060] More elements than shown may generally be present in illumination optics unit IL and projection system PS. The grating spectral filter 240 can optionally be present, depending upon the type of lithographic apparatus. Further, there may be more mirrors present than those shown in the FIG. 2,for example there may be one to six additional reflective elements present in the projection system PS than shown in FIG. 2.

[0061] Collector optic CO, as illustrated in FIG. 2, is depicted as a nested collector with grazing incidence reflectors 253, 254, and 255, just as an example of a collector (or collector mirror). The grazing incidence reflectors 253, 254, and 255 are disposed axially symmetric around an optical axis O and a collector optic CO of this type is preferably used in combination with a discharge produced plasma source, often called a DPP source.

[0062] Exemplary Lithographic Cell

[0063] FIG. 3 shows a lithographic cell 300, also sometimes referred to a lithocell or cluster, according to some embodiments. Lithographic apparatus 100 or 100’ can form part of lithographic cell 300. Lithographic cell 300 can also include one or more apparatuses to perform pre- and post-exposure processes on a substrate. In some examples, these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH, and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / Ol, I / O2, moves them between the different process apparatuses and delivers them to the loading bay LB of the lithographic apparatus 100 or 100’. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatuses may be operated to maximize throughput and processing efficiency.

[0064] Exemplary Inspection Apparatus

[0065] In order to control the lithographic process to place device features accurately on the substrate, alignment marks are generally provided on the substrate, and the lithographic apparatus includes one or more inspection apparatuses for accurate positioning of marks on a substrate. These alignment apparatuses are effectively position measuring apparatuses. Different types of marks and different types of alignment apparatuses and / or systems are known from different times and different manufacturers. A type of system widely used in current lithographic apparatus is based on a self-referencing interferometer as described in U.S. Patent No. 6,961,116 (den Boef et al.). Generally marks are measured separately to obtain X- and Y-positions. A combined X- and Y-measurement may be performed using the techniques described in U.S. Publication No. 2009 / 195768 A (Bijnen et al.) and in U.S. Patent No. 11,360,399 B2 (Goorden et al.). The full contents of which is incorporated herein by reference., however. The full contents of both of these disclosures are incorporated herein by reference.

[0066] FIG. 4A shows a schematic of a cross-sectional view of an inspection apparatus 400 that can be implemented as a part of lithographic apparatus 100 or 100’, according to some embodiments. In some embodiments, inspection apparatus 400 can be configured to align a substrate (e.g., substrate W) with respect to a patterning device (e.g., patterning device MA). Inspection apparatus 400 can be further configured to detect positions of alignment marks on the substrate and to align the substrate with respect to the patterning device or other components of lithographic apparatus 100 or 100’ using the detectedpositions of the alignment marks. Such alignment of the substrate may ensure accurate exposure of one or more patterns on the substrate.

[0067] In some embodiments, inspection apparatus 400 can include an illumination system 412, a beam splitter 414, an interferometer 426, a detector 428, a beam analyzer 430, and an overlay calculation processor 432. Illumination system 412 can be configured to provide an electromagnetic narrow band radiation beam 413 having one or more passbands. In an example, the one or more passbands may be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands may be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. Illumination system 412 can be further configured to provide one or more passbands having substantially constant center wavelength (CWL) values over a long period of time (e.g., over a lifetime of illumination system 412). Such configuration of illumination system 412 can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current alignment systems. And, as a result, the use of constant CWL values may improve long-term stability and accuracy of alignment systems (e.g., inspection apparatus 400) compared to the current alignment apparatuses.

[0068] In some embodiments, beam splitter 414 can be configured to receive radiation beam 413 and split radiation beam 413 into at least two radiation sub-beams. For example, radiation beam 413 can be split into radiation sub-beams 415 and 417, as shown in FIG. 4A. Beam splitter 414 can be further configured to direct radiation sub-beam 415 onto a substrate 420 placed on a stage 422. In one example, the stage 422 is movable along direction 424. Radiation sub-beam 415 can be configured to illuminate an alignment mark or a target 418 located on substrate 420. Alignment mark or target 418 can be coated with a radiation sensitive film. In some embodiments, alignment mark or target 418 can have one hundred and eighty degrees (i.e., 180°) symmetry. That is, when alignment mark or target 418 is rotated 180° about an axis of symmetry perpendicular to a plane of alignment mark or target 418, rotated alignment mark or target 418 can be substantially identical to an unrotated alignment mark or target 418. The target 418 on substrate 420 can be (a) a resist layer grating comprising bars that are formed of solid resist lines, or (b) a product layer grating, or (c) a composite grating stack in an overlay target structure comprising a resist grating overlaid or interleaved on a product layer grating. The bars may alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. One inline method used in device manufacturing for measurements of line width, pitch, and critical dimension makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999), which are both incorporated by reference herein in their entireties. In scatterometry, light is reflected by periodic structures in the target, and the resultingreflection spectrum at a given angle is detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled- Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, may be input to the reconstruction process, performed by processing unit PU, from knowledge of the printing step and / or other scatterometry processes.

[0069] In some embodiments, beam splitter 414 can be further configured to receive diffraction radiation beam 419 and split diffraction radiation beam 419 into at least two radiation sub-beams, according to an embodiment. Diffraction radiation beam 419 can be split into diffraction radiation subbeams 429 and 439, as shown in FIG. 4A.

[0070] It should be noted that even though beam splitter 414 is shown to direct radiation sub-beam 415 towards alignment mark or target 418 and to direct diffracted radiation sub-beam 429 towards interferometer 426, the disclosure is not so limiting. It would be apparent to a person skilled in the relevant art that other optical arrangements may be used to obtain the similar result of illuminating alignment mark or target 418 on substrate 420 and detecting an image of alignment mark or target 418.

[0071] As illustrated in FIG. 4A, interferometer 426 can be configured to receive radiation sub-beam 417 and diffracted radiation sub-beam 429 through beam splitter 414. In an example embodiment, diffracted radiation sub-beam 429 can be at least a portion of radiation sub-beam 415 that can be reflected from alignment mark or target 418. In an example of this embodiment, interferometer 426 comprises any appropriate set of optical-elements, for example, a combination of prisms that may be configured to form two images of alignment mark or target 418 based on the received diffracted radiation sub-beam 429. It should be appreciated that a good quality image need not be formed, but that the features of alignment mark 418 should be resolved. Interferometer 426 can be further configured to rotate one of the two images with respect to the other of the two images 180° and recombine the rotated and unrotated images interferometrically.

[0072] In some embodiments, detector 428 can be configured to receive the recombined image via interferometer signal 427 and detect interference as a result of the recombined image when alignment axis 421 of inspection apparatus 400 passes through a center of symmetry (not shown) of alignment mark or target 418. Such interference may be due to alignment mark or target 418 being 180° symmetrical, and the recombined image interfering constructively or destructively, according to an example embodiment. Based on the detected interference, detector 428 can be further configured to determine a position of the center of symmetry of alignment mark or target 418 and consequently, detect a position of substrate 420. According to an example, alignment axis 421 can be aligned with an optical beam perpendicular to substrate 420 and passing through a center of image rotation interferometer 426. Detector 428 can be further configured to estimate the positions of alignment mark or target 418 by implementing sensor characteristics and interacting with wafer mark process variations.

[0073] In a further embodiment, detector 428 determines the position of the center of symmetry of alignment mark or target 418 by performing one or more of the following measurements:1. measuring position variations for various wavelengths (position shift between colors);2. measuring position variations for various orders (position shift between diffraction orders); and3. measuring position variations for various polarizations (position shift between polarizations).

[0074] This data may, for example, be obtained with any type of alignment sensor, for example a SMASH (SMart Alignment Sensor Hybrid) sensor, as described in U.S. Patent No. 6,961,116 that employs a self-referencing interferometer with a single detector and four different wavelengths, and extracts the alignment signal in software, or Athena (Advanced Technology using High order ENhancement of Alignment), as described in U.S. Patent No. 6,297,876, which directs each of seven diffraction orders to a dedicated detector, which are both incorporated by reference herein in their entireties.

[0075] In some embodiments, beam analyzer 430 can be configured to receive and determine an optical state of diffracted radiation sub-beam 439. The optical state may be a measure of beam wavelength, polarization, or beam profile. Beam analyzer 430 can be further configured to determine a position of stage 422 and correlate the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420 can be accurately known with reference to stage 422. Alternatively, beam analyzer 430 can be configured to determine a position of inspection apparatus 400 or any other reference element such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400 or any other reference element. Beam analyzer 430 can be a point or an imaging polarimeter with some form of wavelength-band selectivity. In some embodiments, beam analyzer 430 can be directly integrated into inspection apparatus 400, or connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other embodiments.

[0076] In some embodiments, beam analyzer 430 can be further configured to determine the overlay data between two patterns on substrate 420. One of these patterns can be a reference pattern on a reference layer. The other pattern may be an exposed pattern on an exposed layer. The reference layer may be an etched layer already present on substrate 420. The reference layer may be generated by a reference pattern exposed on the substrate by lithographic apparatus 100 and / or 100’ . The exposed layer may be a resist layer exposed adjacent to the reference layer. The exposed layer may be generated by an exposure pattern exposed on substrate 420 by lithographic apparatus 100 or 100’. The exposed pattern on substrate 420 can correspond to a movement of substrate 420 by stage 422. In some embodiments, the measured overlay data may also indicate an offset between the reference pattern and the exposure pattern. The measured overlay data may be used as calibration data to calibrate the exposure pattern exposed by lithographic apparatus 100 or 100’ , such that after the calibration, the offset between the exposed layer and the reference layer may be minimized.

[0077] In some embodiments, beam analyzer 430 can be further configured to determine a model of the product stack profile of substrate 420, and can be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement. The product stack profile contains information on the stacked product such as alignment mark, target 418, or substrate 420, and can include mark process variation-induced optical signature metrology that is a function of illumination variation. The product stack profile may also include product grating profile, mark stack profile, and mark asymmetry information. An example of beam analyzer 430 is Yieldstar™, manufactured by ASML, Veldhoven, The Netherlands, as described in U.S. Patent No. 8,706,442, which is incorporated by reference herein in its entirety. Beam analyzer 430 can be further configured to process information related to a particular property of an exposed pattern in that layer. For example, beam analyzer 430 can process an overlay parameter (an indication of the positioning accuracy of the layer with respect to a previous layer on the substrate or the positioning accuracy of the first layer with respective to marks on the substrate), a focus parameter, and / or a critical dimension parameter (e.g., line width and its variations) of the depicted image in the layer. Other parameters are image parameters relating to the quality of the depicted image of the exposed pattern.

[0078] In some embodiments, an array of detectors (not shown) may be connected to beam analyzer 430, and allows the possibility of accurate stack profile detection as discussed below. For example, detector 428 can be an array of detectors. For the detector array, a number of options are possible: a bundle of multimode fibers, discrete pin detectors per channel, or CCD or CMOS (linear) arrays. The use of a bundle of multimode fibers enables any dissipating elements to be remotely located for stability reasons. Discrete PIN detectors offer a large dynamic range but each need separate pre-amps. The number of elements is therefore limited. CCD linear arrays offer many elements that may be read-out at high speed and are especially of interest if phase- stepping detection is used.

[0079] In some embodiments, a second beam analyzer 430’ can be configured to receive and determine an optical state of diffracted radiation sub-beam 429, as shown in FIG. 4B. The optical state may be a measure of beam wavelength, polarization, or beam profile. Second beam analyzer 430’ can be identical to beam analyzer 430. Alternatively, second beam analyzer 430’ can be configured to perform at least all the functions of beam analyzer 430, such as determining a position of stage 422 and correlating the position of stage 422 with the position of the center of symmetry of alignment mark or target 418. As such, the position of alignment mark or target 418 and, consequently, the position of substrate 420, can be accurately known with reference to stage 422. Second beam analyzer 430’ can also be configured to determine a position of inspection apparatus 400, or any other reference element, such that the center of symmetry of alignment mark or target 418 can be known with reference to inspection apparatus 400, or any other reference element. Second beam analyzer 430’ can be further configured to determine the overlay data between two patterns and a model of the product stack profile of substrate 420. Second beam analyzer 430’ can also be configured to measure overlay, critical dimension, and focus of target 418 in a single measurement.

[0080] In some embodiments, second beam analyzer 430’ can be directly integrated into inspection apparatus 400, or it can be connected via fiber optics of several types: polarization preserving single mode, multimode, or imaging, according to other embodiments. Alternatively, second beam analyzer 430’ and beam analyzer 430 can be combined to form a single analyzer (not shown) configured to receive and determine the optical states of both diffracted radiation sub-beams 429 and 439.

[0081] In some embodiments, processor 432 receives information from detector 428 and beam analyzer 430. For example, processor 432 can be an overlay calculation processor. The information may comprise a model of the product stack profile constructed by beam analyzer 430. Alternatively, processor 432 can construct a model of the product mark profile using the received information about the product mark. In either case, processor 432 constructs a model of the stacked product and overlay mark profile using or incorporating a model of the product mark profile. The stack model is then used to determine the overlay offset and minimizes the spectral effect on the overlay offset measurement. Processor 432 can create a basic correction algorithm based on the information received from detector 428 and beam analyzer 430, including but not limited to the optical state of the illumination beam, the alignment signals, associated position estimates, and the optical state in the pupil, image, and additional planes. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines the azimuth angle of the radiation. Processor 432 can utilize the basic correction algorithm to characterize the inspection apparatus 400 with reference to wafer marks and / or alignment marks 418.

[0082] In some embodiments, processor 432 can be further configured to determine printed pattern position offset error with respect to the sensor estimate for each mark based on the information received from detector 428 and beam analyzer 430. The information includes but is not limited to the product stack profile, measurements of overlay, critical dimension, and focus of each alignment marks or target 418 on substrate 420. Processor 432 can utilize a clustering algorithm to group the marks into sets of similar constant offset error, and create an alignment error offset correction table based on the information. The clustering algorithm may be based on overlay measurement, the position estimates, and additional optical stack process information associated with each set of offset errors. The overlay is calculated for a number of different marks, for example, overlay targets having a positive and a negative bias around a programmed overlay offset. The target that measures the smallest overlay is taken as reference (as it is measured with the best accuracy). From this measured small overlay, and the known programmed overlay of its corresponding target, the overlay error may be deduced. Table 1 illustrates how this may be performed. The smallest measured overlay in the example shown is -1 nm. However, this is in relation to a target with a programmed overlay of -30 nm. The process may have introduced an overlay error of 29 nm.

[0083] The smallest value may be taken to be the reference point and, relative to this, the offset may be calculated between measured overlay and that expected due to the programmed overlay. This offset determines the overlay error for each mark or the sets of marks with similar offsets. Therefore, in the Table 1 example, the smallest measured overlay was -1 nm, at the target position with programmed overlay of 30 nm. The difference between the expected and measured overlay at the other targets is compared to this reference. A table such as Table 1 may also be obtained from marks and target 418 under different illumination settings, the illumination setting, which results in the smallest overlay error, and its corresponding calibration factor, may be determined and selected. Following this, processor 432 can group marks into sets of similar overlay error. The criteria for grouping marks may be adjusted based on different process controls, for example, different error tolerances for different processes.

[0084] In some embodiments, processor 432 can confirm that all or most members of the group have similar offset errors, and apply an individual offset correction from the clustering algorithm to each mark, based on its additional optical stack metrology. Processor 432 can determine corrections for each mark and feed the corrections back to lithographic apparatus 100 or 100’ for correcting errors in the overlay, for example, by feeding corrections into the inspection apparatus 400.

[0085] In some aspects, metrology tools may control a focus between an objective and a wafer in order to measure metrology targets. In some aspects, it is desired to monitor a focus of an optical element of the inspection apparatus. In some aspects, the inspection apparatus may employ a thru the lens methodology for acquiring metrology marks for analysis of lithography overlay, focus, and exposure. For example, the inspection apparatus may determine a contrast between defocused pinholes. These schemes can view light thru an optical column, which is often also used for analysis of metrology marks. These techniques may have several shortcomings.

[0086] In some aspects, the light source used for focusing and for analysis are physically switched in order to achieve enough illumination for their respective functions. In addition, a wavelength of the light used for focusing may be different from the wavelength used for analysis. Thus, a switch in the illumination paths may also be desired. The light source and / or illumination paths switching may result in loss of focus control during analysis measurements.

[0087] In some aspects, metrology tools can employ focus sensors mechanically in parallel with the optical element (e.g., objective), that allows for continuous focus control while metrology measurements are being acquired.

[0088] Embodiments of the present disclosure, minimize the costs of optics by eliminating some optical elements, improve the range of focus, and minimize the volume used by the focusing modules.In addition, embodiments of the present disclosure provide for continuous servo control of the optical element positioning. In some aspects, off-axis distance sensors such as air gauges have no process sensitivity.

[0089] Example inspection systems

[0090] FIG. 5A shows a side view of an inspection system 500, according to some embodiments. FIG. 5B shows a bottom view of inspection system 500, according to some embodiments. In some embodiments, inspection system 500 can also represent a more detailed view of beam analyzer 430. Note that for the sake of simplicity, FIG. 5 only shows some of the components of inspection system 500.

[0091] Inspection system 500 can include an optical element 502. Optical element 502 is shown inspecting a wafer 506. Optical element 502 can include a plurality of optical elements (e.g., lenses) that form an objective assembly of inspection system 500. In some aspects, optical element 502 can be focused on wafer 506. A focus distance A can represent the distance between optical element 502 and wafer 506 when the optical element is in focus. Inspection system 500 can include a distance sensor 510. In some aspects, distance sensor 510 is positioned off axis with respect to an optical axis of optical element 502. In some aspects, distance sensor 510 can be mounted parallel to the optical axis of optical element 502.

[0092] In some aspects, distance sensor 510 can monitor a gap to wafer 506. In some aspects, distance sensor 510 can measure a distance B to wafer 506. Distance sensor 510 can be coupled to a controller 504. Controller 504 can control a position of optical element 502 to maintain optical element 502 at the focus distance A. In some aspects, optical element 502 can be mounted on a stage 508. Controller 504 can be coupled to an actuator (not shown) of stage 508.

[0093] In some aspects, controller 504 can generate a control signal based on the measured distance B. Controller 504 can output the control signal to the actuator. The actuator controls the position of optical element 502 based on the control signal. For example, controller 504 can compare the measured distance B and the focus distance A. Controller 504 can generate the control signal based on the comparison. In some aspects, control signal may represent a desired position or a desired movement to maintain optical element at the focus distance A. Thus, a feedback loop is established between distance sensor 510 and an actuator of optical element 502 (e.g., actuator of stage 508). This enables continuous focus control during measurements (e.g., alignment measurements, overlay measurements) by continuously providing optical element positioning feedback irrespective of what function inspection system 500 is performing.

[0094] In some aspects, distance sensor 510 can include a plurality of distance sensors. Configurations including a multitude of distance sensors allows for focus control even when a metrology mark is near the edge of wafer 506 as further described in relation to FIGS. 8A and 8B. Distance sensor 510 can include a first distance sensor 510a, a second distance sensor 510b, a third distance sensor 510c, and a fourth distance sensor 510d. In some aspects, first distance sensor 510a, second distance sensor 510b,third distance sensor 510c, and fourth distance sensor 5 lOd can be positioned on stage 508. For example, first distance sensor 510a, second distance sensor 510b, third distance sensor 510c, and fourth distance sensor 510d can be positioned on respective corners of stage 508.

[0095] In some aspects, distance sensor 510 can include a plurality of capacitor probes, a plurality of air gauges, a plurality of optical sensors (e.g., fiber optic based optical sensors), a plurality of mechanical probes, a plurality of ultrasonic probes, or the like.

[0096] In some aspects, each of first distance sensor 510a, second distance sensor 510b, third distance sensor 510c, and fourth distance sensor 510d can be a capacitor probe.

[0097] In some aspects, distance sensor 510 can include a plurality of air gauge sensors. In some aspects, each of first distance sensor 510a, second distance sensor 510b, third distance sensor 510c, and fourth distance sensor 51 Od can be an air gauge sensor. An air gauge can include a pneumatic bridge. A constant mass flow M is divided between two identical branches. When gaps are equal then there is zero differential pressure. A change in the gap between distance sensor 510 and wafer 506 causes a proportional change in the differential pressure that may be more readily measured compared to small changes in an absolute pressure.

[0098] FIG. 6 shows a schematic of an inspection system 600, according to some embodiments. Structures and functions of commonly numbered elements in FIGS. 5 A, 5B, and 6, are described above (e.g., matching elements can have reference numbers that share the two right-most numeric digits). Such elements can include a distance sensor 610, a wafer 606, and an optical element 602. Note that for the sake of simplicity, FIG. 6 only shows some of the components of inspection system 600.

[0099] In some aspects, optical element 602 and distance sensor 610 can be positioned on a stage 608. Distance sensor 610 can provide off-axis measurements to a wafer 606 with respect to optical element 602. In some aspects, wafer 606 can include a target (e.g., a mark) 612. During measurements corresponding to target 612, distance sensor 610 (or different probes of distance sensor 610) can be measuring distances to different parts of wafer 606. For example, a distance B can be between optical element 602 and target 612. Distance sensor 610 can measure a distance A to a top surface of wafer 606. An offset distance C can be between distance B and distance A. Offset distance C may represent an offset in the x-direction and y-direction from optical element 602 to distance sensor 610. The offset may be different for each target 612 on wafer 606.

[0100] In some aspects, distance sensor 610 can measure at different planes in the z-direction from target 612 (e.g., due to scribe lanes). In some aspects, distance sensor 610 can measure a distance to a top surface of wafer 606. However, target 612 can be at a bottom of the scribe lanes or on top of a circuit stack. In addition, some distance sensors such as capacitance probes may suffer from inherent process sensitivity, resulting in measurements that differ from the top surface based on wafer stack materials and layouts. Accordingly, in some aspects, it is desirable to improve the performance of off-axis distance sensors.

[0101] FIG. 7 shows a schematic of an inspection system 700, according to some embodiments. Structures and functions of commonly numbered elements in FIGS. 5A, 5B, 6, and 7, are described above (e.g., matching elements can have reference numbers that share the two right-most numeric digits). Such elements can include a distance sensor 710, a wafer 706, a stage 708, and an optical element 702. Note that for the sake of simplicity, FIG. 7 only shows some of the components of inspection system 700.

[0102] Inspection system 700 can include optical element 702, distance sensor 710, a controller 704, and an optical sensing system 714. Optical element 702 is shown inspecting wafer 706. Optical element 702 can include a plurality of optical elements (e.g., lens) that form an objective assembly of inspection system 700. In some aspects, optical element 702 can be focused on wafer 706. A focus distance A can represent the distance between optical element 702 and wafer 706 when the optical element is in focus. In some aspects, distance sensor 710 is positioned off axis with respect to an optical axis of optical element 702. In some aspects, distance sensor 710 can be mounted parallel to the optical axis of optical element 702.

[0103] In some aspects, distance sensor 710 can monitor a gap between distance sensor 710 and wafer 706. In some aspects, distance sensor 710 can measure a distance B to wafer 706.

[0104] To deal with the X, Y and Z offsets described above in relation to FIG. 6, a latch mode can be used by controller 704. An optical focus function can be provided by optical sensing system 714. Optical sensing system 714 can include a light source 716 and a detector 718. The optical focus function of optical sensing system 714 can be used to establish an initial positioning of optical element 702. The initial positioning may be performed upon arrival at a target (e.g., target 612 of FIG. 6) to be measured. In some aspects, once the initial positioning is performed by optical sensing system 714, the optical train can pivot to measuring the target without losing focus control.

[0105] In some aspects, controller 704 can maintain the distance between the wafer and optical element 702 at the initial positioning based on measurements from distance sensor 710. Thus, controller 704 using a feedback loop can maintain optical element 702 and wafer 706 in a desired position to maintain focus control. During operation (e.g., during alignment measurements after the initial positioning), distance sensor 710 can maintain optical element 702 at the focus distance. Distance sensor 710 can continuously monitor the distance B and output the measured distance to controller 704. Controller 704 can generate a control signal to an actuator of stage 708 based on the measured distance (e.g., by comparing to the initial distance measured during the initial positioning)

[0106] In some aspects, distance sensor 710 can include one or more air gauges, capacitor probes, or optical distance sensors.

[0107] In some aspects, distance sensor 710 can have a linear range greater than a linear range of optical sensing system 714. Linear range can refer to the range of the sensor in which the output of the sensor is proportional to an input (i.e., the sensitivity of the sensor is constant).

[0108] In some aspects, distance sensor 710 can include a plurality of capacitor probes. The capacitor probes may have a linear range of about 1200 pm. The linear range of optical sensing system 714 can be about 6 pm.

[0109] In some aspects, distance sensor 710 can include a plurality of air gauges. The air gauges may have a linear range of about 500 pm.

[0110] In some aspects, the distance sensor can be used to determine a distance to wafer 706 before initial positioning by optical sensing system 714 because of the greater linear range of distance sensor 710. Due to variations in a thickness of structure and / or wafer 706, distance sensor 710 can be used to determine an initial distance to avoid an undesired contact between optical element 702 and wafer 706.

[0111] In some aspects, references between optical sensing system 714 and distance sensor 710 can be established during recipe setup. The references can be fed forward to subsequent wafer measurements. This provide better throughput since distance sensor 710 can identify a proper setpoint prior to arrival at the target. During measurements, optical sensing system 714 may not provide the focus distance and the focus distance is maintained based on the references and measurements from distance sensor 710.

[0112] FIG. 8A is a schematic that shows a control configuration of an inspection system 800, according to some embodiments. FIG. 8B is a schematic that shows inspection system 800 performing measurement on a target 812 located on an edge of a wafer 806. Structures and functions of commonly numbered elements in FIGS. 5A, 5B, 6, and 7, are described above (e.g., matching elements can have reference numbers that share the two right-most numeric digits). Such elements can include a distance sensor 810, a wafer 806, stage 808, and an optical element 802. Note that for the sake of simplicity, FIGS. 8A and 8B only show some of the components of inspection system 800.

[0113] In some aspects, distance sensor 810 can include a plurality of distance sensors. For example, distance sensor 810 can include a first sensor 810a and a second sensor 810b. First sensor 810a can measure a distance A to wafer 806. Second sensor 810b can measure a distance B to wafer 806. The controller (not shown in FIG. 8A) can determine the distance based on the measurements from one or more sensors of the plurality of distance sensors (e.g., from first sensor 810a and / or second sensor 810b). In some aspects, the controller may determine an average of distance A and distance B. In some aspects, the controller may add distance A and distance B.

[0114] In some aspects, one or more distance sensors may not be able to provide reliable measurement because they may be outside of the area of wafer 806. In some aspects, targets can be located near an edge of wafer 806 (e.g., target 812 shown in FIG. 8B). This may result in some distance sensors being off wafer 806 or outside a region of interest of wafer 806 (e.g., second sensor 810b as shown in FIG. 8B). For example, an exclusion region may be used around an outer edge of wafer 806 topography to prevent substandard structures from being printed.

[0115] In some aspects, the controller may identify one or more sensors from the plurality of sensors based on a horizontal position of the sensor and use measurements from the identified sensors to control a position of optical element 802. The horizontal position may indicate the position of the sensor in afirst direction and a second direction in a plane perpendicular to the optical axis of optical element 802 (e.g., x-direction and y-direction). For example, the controller may identify first sensor 810a when performing measurements for target 812. The controller can determine the distance to the wafer based on measurements from first sensor 810a. In some aspects, the controller may identify a plurality of sensors that can provide reliable measurements (e.g., over wafer 806) and may determine the distance using an average of the measurements.

[0116] In some aspects, the controller may identify a sensor that may not provide reliable measurements (off wafer) (e.g., second sensor 810b). The controller may use a previously stored value representing a measurement before the identified become off-wafer in determining the distance to the wafer. For example, controller may use a stored value form second sensor 810b (e.g., distance B) when second sensor 810b is off wafer 806.

[0117] FIG. 9 shows method steps (e.g., using one or more processors) for performing a method 900 including functions described herein, according to some embodiments. The method 900 of FIG. 9 can be performed in any conceivable order and it is not required that all steps be performed. Moreover, the method steps of FIG. 9 described above merely reflect an example of steps and are not limiting.

[0118] In some aspects, method 900 illustrates a method for controlling an optical element of an inspection system.

[0119] In some aspects, in 902 a focus distance representative of a relative position between a focus of an optical element and a wafer is determined.

[0120] In 904, a position signal representative of the focus distance may be output to a controller.

[0121] In 906, a distance to the wafer is determined using a distance sensor. The distance sensor is positioned off axis with respect to an optical axis of the optical element.

[0122] In 908, a position of the optical element is controlled based on the position signal and the distance to maintain the optical element at the focus distance.

[0123] In some aspects, the distance sensor can include a plurality of sensors. A first distance can be determined using a first sensor from the plurality of sensors. A second distance can be determined using a second sensor from the plurality of sensors. The distance to the wafer can be determined based on the first distance and the second distance. In some aspects, the distance can be an average of the first distance and the second distance.

[0124] In some aspects, the position of the distance sensor can be determined in a x-direction and in a y-direction. In some aspects, the distance sensor comprises a plurality of sensors. A sensor can be identified from the plurality of sensors based on the position. In some aspects, the distance to the wafer is determined using at least a measurement from the identified sensor. In some aspects, the distance is determined using the at least a measurement from the identified sensor and a stored measurement obtained from another sensor from the plurality of sensors.

[0125] In some aspects, the focus distance can be based on an on-axis measurement.

[0126] Various embodiments may be implemented, for example, using one or more well-known computer systems, such as computer system 1000 shown in FIG. 10. One or more computer systems 1000 can be used, for example, to implement any aspect of the disclosure discussed herein, as well as combinations and sub-combinations thereof.

[0127] Computer system 1000 can include one or more processors (also called central processing units, or CPUs), such as a processor 1004. Processor 1004 can be connected to a communication infrastructure or bus 1006.

[0128] Computer system 1000 can also include customer input / output device(s) 1003, such as monitors, keyboards, pointing devices, etc., which may communicate with communication infrastructure 1006 through customer input / output interface(s) 1002.

[0129] One or more of processors 1004 can be a graphics processing unit (GPU). In an embodiment, a GPU may be a processor that is a specialized electronic circuit designed to process mathematically intensive applications. The GPU may have a parallel structure that is efficient for parallel processing of large blocks of data, such as mathematically intensive data common to computer graphics applications, images, videos, etc.

[0130] Computer system 1000 can also include a main or primary memory 1008, such as random access memory (RAM). Main memory 1008 can include one or more levels of cache. Main memory 1008 can have stored therein control logic (i.e., computer software) and / or data.

[0131] Computer system 1000 can also include one or more secondary storage devices or memory 1010. Secondary memory 1010 can include, for example, a hard disk drive 1012 and / or a removable storage device or drive 1014. Removable storage drive 1014 can be a floppy disk drive, a magnetic tape drive, a compact disk drive, an optical storage device, tape backup device, and / or any other storage device / drive.

[0132] Removable storage drive 1014 can interact with a removable storage unit 1018. Removable storage unit 1018 can include a computer usable or readable storage device having stored thereon computer software (control logic) and / or data. Removable storage unit 1018 can be a floppy disk, magnetic tape, compact disk, DVD, optical storage disk, and / any other computer data storage device. Removable storage drive 1014 can read from and / or write to removable storage unit 1018.

[0133] Secondary memory 1010 can include other means, devices, components, instrumentalities or other approaches for allowing computer programs and / or other instructions and / or data to be accessed by computer system 1000. Such means, devices, components, instrumentalities or other approaches may include, for example, a removable storage unit 1022 and an interface 1020. Examples of the removable storage unit 1022 and the interface 1020 can include a program cartridge and cartridge interface (such as that found in video game devices), a removable memory chip (such as an EPROM or PROM) and associated socket, a memory stick and USB port, a memory card and associated memory card slot, and / or any other removable storage unit and associated interface.

[0134] Computer system 1000 can further include a communication or network interface 1024. Communication interface 1024 can enable computer system 1000 to communicate and interact with any combination of external devices, external networks, external entities, etc. (individually and collectively referenced by reference number 1028). For example, communication interface 1024 can allow computer system 1000 to communicate with external or remote devices 1028 over communications path 1026, which may be wired and / or wireless (or a combination thereof), and which may include any combination of LANs, WANs, the Internet, etc. Control logic and / or data may be transmitted to and from computer system 1000 via communication path 1026.

[0135] Computer system 1000 can also be any of a personal digital assistant (PDA), desktop workstation, laptop or notebook computer, netbook, tablet, smart phone, smart watch or other wearable, appliance, part of the Internet-of-Things, and / or embedded system, to name a few non-limiting examples, or any combination thereof.

[0136] Computer system 1000 can be a client or server, accessing or hosting any applications and / or data through any delivery paradigm, including but not limited to remote or distributed cloud computing solutions; local or on-premises software (“on-premise” cloud-based solutions); “as a service” models (e.g., content as a service (CaaS), digital content as a service (DCaaS), software as a service (SaaS), managed software as a service (MSaaS), platform as a service (PaaS), desktop as a service (DaaS), framework as a service (FaaS), backend as a service (BaaS), mobile backend as a service (MBaaS), infrastructure as a service (laaS), etc.); and / or a hybrid model including any combination of the foregoing examples or other services or delivery paradigms.

[0137] Any applicable data structures, file formats, and schemas in computer system 1000 can be derived from standards including but not limited to JavaScript Object Notation (JSON), Extensible Markup Language (XML), Yet Another Markup Language (YAML), Extensible Hypertext Markup Language (XHTML), Wireless Markup Language (WML), MessagePack, XML User Interface Language (XUL), or any other functionally similar representations alone or in combination. Alternatively, proprietary data structures, formats or schemas may be used, either exclusively or in combination with known or open standards.

[0138] In some embodiments, a tangible, non-transitory apparatus or article of manufacture comprising a tangible, non-transitory computer useable or readable medium having control logic (software) stored thereon may also be referred to herein as a computer program product or program storage device. This includes, but is not limited to, computer system 1000, main memory 1008, secondary memory 1010, and removable storage units 1018 and 1022, as well as tangible articles of manufacture embodying any combination of the foregoing. Such control logic, when executed by one or more data processing devices (such as computer system 1000), may cause such data processing devices to operate as described herein.

[0139] Based on the teachings contained in this disclosure, it will be apparent to persons skilled in the relevant art(s) how to make and use embodiments of this disclosure using data processing devices,computer systems and / or computer architectures other than that shown in FIG. 10. In particular, embodiments can operate with software, hardware, and / or operating system implementations other than those described herein.

[0140] The embodiments may further be described using the following clauses:1. A system, comprising: an optical element facing a wafer, wherein the optical element is spaced apart from the wafer by a focus distance; a distance sensor configured to measure a distance to the wafer, wherein the distance sensor is positioned off axis with respect to an optical axis of the optical element; a controller coupled to an actuator and configured to generate a control signal to maintain the focus distance, wherein the control signal is generated based on the measured distance by the distance sensor; and the actuator coupled to the optical element and configured to control a position of the optical element based on the control signal.2. The system of clause 1, further comprising: an optical sensing system, associated with the optical element, configured to measure an on-axis distance between the wafer and the optical element.3. The system of clause 2, wherein the optical sensing system is further configured to determine the focus distance and to output a position signal representative of the focus distance to the controller.4. The system of clause 2, wherein a linear range of the distance sensor is greater than the linear range of the optical sensing system.5. The system of clause 1, wherein the distance sensor is positioned parallel to the optical axis of the optical element.6. The system of clause 1, wherein the distance sensor comprises a plurality of capacitor probes.7. The system of clause 1, wherein the distance sensor comprises a plurality of air gauges.8. The system of clause 1, wherein the distance sensor comprises a plurality of optical distance sensors.9. The system of clause 1, wherein the distance sensor comprises a first sensor configured to generate a first measurement and a second sensor configured to generate a second measurement; and wherein the controller is configured to generate the control signal based on the first measurement and the second measurement.10. The system of clause 1, wherein the distance sensor comprises a plurality of sensors; wherein the controller is configured to generate the control signal based on a measurement from a sensor of the plurality of sensors; and wherein the sensor is identified from the plurality of sensors based on a horizontal position of the sensor with respect to the wafer, the horizontal position representing the position of the sensor in a first direction and in a second direction.11. A method comprising : determining a focus distance representative of a relative position between a focus of an optical element and a wafer; outputting a position signal representative of the focus distance to a controller; determining a distance to the wafer using a distance sensor, wherein the distance sensor is positioned off axis with respect to an optical axis of the optical element; and controlling a position of the optical element based on the position signal and the distance to maintain the optical element at the focus distance.12. The method of clause 11, wherein the distance sensor comprises a plurality of sensors, and wherein the method further comprises: determining a position of the distance sensor in a x-direction and in a y-direction; identifying a sensor from the plurality of sensors based on the position; and determining the distance using at least a measurement from the identified sensor.13. The method of clause 12, further comprising: determining the distance using the at least a measurement from the identified sensor and a stored measurement obtained from another sensor from the plurality of sensors.14. The method of clause 11, wherein the distance sensor comprises a plurality of sensors, and wherein the method further comprises: determining a first distance using a first sensor from the plurality of sensors; determining a second distance using a second sensor from the plurality of sensors; and determining the distance to the wafer based on the first distance and the second distance.15. The method of clause 14, wherein determining the distance further comprises: determining an average of the first distance and the second distance.16. The method of clause 11, wherein determining the focus distance is based on an on-axis measurement.17. A system comprising: an optical element facing a wafer; an optical focus module configured to establish a focus of the optical element and to determine a focus distance, wherein the focus distance represents a distance between the optical element and the wafer; a distance sensor configured to measure a distance to the wafer; an actuator coupled to the optical element and configured to maintain the focus distance; and a controller coupled to the actuator and configured to adjust the actuator to maintain the focus distance based on the measured distance by the distance sensor.18. The system of clause 17, wherein the distance sensor is mounted in parallel to an optical axis of the optical element.19. The system of clause 17, wherein the distance sensor comprises a plurality of capacitor probes.20. The system of clause 19, wherein the controller is further configured to: acquire a first measurement from a first capacitor probe from the plurality of capacitor probes; acquire a second measurement from a second capacitor probe from the plurality of capacitor probes; and generate the control signal based on the first measurement and the second measurement.

[0141] Although specific reference can 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 patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film 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 can be considered as specific examples of the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track unit (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) and / or a metrology unit. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example in order 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.

[0142] Although specific reference may have been made above to the use of embodiments of the present disclosure in the context of optical lithography, it will be appreciated that the present disclosure can be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0143] It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present disclosure is to be interpreted by those skilled in relevant art(s) in light of the teachings herein.

[0144] The terms “radiation,” “beam of radiation” or the like as used herein can encompass all types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength I of 365, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV or soft X-ray) radiation (for example, having a wavelength in the range of 5-20 nm such as, for example, 13.5 nm), or hard X-ray working at less than 5 nm, as well as matter beams, such as ion beams or electron beams. The terms “light,” “illumination,” or the like can refer to non-matter radiation (e.g., photons, UV, X-ray, or the like). Generally, radiation having wavelengths between about 400 to about 700 nm is considered visible radiation; radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, the term “UV” also applies to the wavelengths that can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and / or, I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by gas), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in some embodiments, an excimer laser can generate DUV radiation used within a lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a desired wavelength band, of which at least part is in the range of 5-20 nm.

[0145] It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the present disclosure and the appended claims in any way.

[0146] The present disclosure has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed.

[0147] While specific embodiments of the disclosure have been described above, it will be appreciated that embodiments of the present disclosure may be practiced otherwise than as described. The descriptions 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 disclosure as described without departing from the scope of the claims set out below.

[0148] The foregoing description of the specific embodiments will so fully reveal the general nature of the present disclosure that others can, by applying knowledge within the skill of the art, readily modify and / or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present disclosure. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein.

[0149] The breadth and scope of the protected subject matter should not be limited by any of the abovedescribed exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

CLAIMS1. A system, comprising: an optical element facing a wafer, wherein the optical element is spaced apart from the wafer by a focus distance; a distance sensor configured to measure a distance to the wafer, wherein the distance sensor is positioned off axis with respect to an optical axis of the optical element; a controller coupled to an actuator and configured to generate a control signal to maintain the focus distance, wherein the control signal is generated based on the measured distance by the distance sensor; and the actuator coupled to the optical element and configured to control a position of the optical element based on the control signal.

2. The system of claim 1, further comprising: an optical sensing system, associated with the optical element, configured to measure an on- axis distance between the wafer and the optical element.

3. The system of claim 2, wherein the optical sensing system is further configured to determine the focus distance and to output a position signal representative of the focus distance to the controller.

4. The system of claim 2, wherein a linear range of the distance sensor is greater than the linear range of the optical sensing system.

5. The system of claim 1, wherein the distance sensor is positioned parallel to the optical axis of the optical element.

6. The system of claim 1, wherein the distance sensor comprises a plurality of capacitor probes.

7. The system of claim 1, wherein the distance sensor comprises a plurality of air gauges.

8. The system of claim 1, wherein the distance sensor comprises a plurality of optical distance sensors.

9. The system of claim 1, wherein the distance sensor comprises a first sensor configured to generate a first measurement and a second sensor configured to generate a second measurement; and wherein the controller is configured to generate the control signal based on the first measurement and the second measurement.

10. The system of claim 1, wherein the distance sensor comprises a plurality of sensors; wherein the controller is configured to generate the control signal based on a measurement from a sensor of the plurality of sensors; and wherein the sensor is identified from the plurality of sensors based on a horizontal position of the sensor with respect to the wafer, the horizontal position representing the position of the sensor in a first direction and in a second direction.

11. A method comprising : determining a focus distance representative of a relative position between a focus of an optical element and a wafer; outputting a position signal representative of the focus distance to a controller; determining a distance to the wafer using a distance sensor, wherein the distance sensor is positioned off axis with respect to an optical axis of the optical element; and controlling a position of the optical element based on the position signal and the distance to maintain the optical element at the focus distance.

12. The method of claim 11, wherein the distance sensor comprises a plurality of sensors, and wherein the method further comprises: determining a position of the distance sensor in a x-direction and in a y-direction; identifying a sensor from the plurality of sensors based on the position; and determining the distance using at least a measurement from the identified sensor.

13. The method of claim 12, further comprising: determining the distance using the at least a measurement from the identified sensor and a stored measurement obtained from another sensor from the plurality of sensors.

14. The method of claim 11, wherein the distance sensor comprises a plurality of sensors, and wherein the method further comprises: determining a first distance using a first sensor from the plurality of sensors; determining a second distance using a second sensor from the plurality of sensors; and determining the distance to the wafer based on the first distance and the second distance.

15. The method of claim 14, wherein determining the distance further comprises: determining an average of the first distance and the second distance.

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