Lithographic apparatus, metrology apparatus, optical system and method

A method to reduce sensitivity of a level sensor, arranged to measure a height of a substrate, to variations of a property of an optical component in the level sensor includes directing a beam of radiation toward a diffractive element and directing the beam, via an optical system, to a first reflective element at a first angle of incidence. The beam has a first polarization and a second polarization that is perpendicular to the first polarization. The first reflective element reflects the beam toward a second reflective element at a second angle of incidence causing the beam to impinge on the substrate. The first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements.

FIELD

The present disclosure relates to metrology apparatuses and systems, for example, position sensors for lithographic apparatuses and systems.

BACKGROUND

Manufacturing devices, such as semiconductor devices, typically involves processing a substrate (e.g., a semiconductor wafer) using a number of fabrication processes to form various features and often multiple layers of the devices. Such layers and/or features are typically manufactured and processed using, e.g., deposition, lithography, etch, chemical-mechanical polishing, and ion implantation. Multiple devices may be fabricated on a plurality of dies on a substrate and then separated into individual devices. This device manufacturing process may be considered a patterning process. A patterning process involves a pattern transfer step, such as optical and/or nanoimprint lithography using a lithographic apparatus, to provide a pattern on a substrate and typically, but optionally, involves one or more related pattern processing steps, such as resist development by a development apparatus, baking of the substrate using a bake tool, etching the pattern by an etch apparatus, etc. Further, one or more metrology processes are involved in the patterning process.

Metrology processes are used at various steps during a patterning process to monitor and/or control the process. For example, metrology processes are used to measure one or more characteristics of a substrate, such as a relative location (e.g., registration, overlay, alignment, etc.) or dimension (e.g., line width, critical dimension (CD), thickness, etc.) of features formed on the substrate during the patterning process, such that, for example, the performance of the patterning process can be determined from the one or more characteristics. If the one or more characteristics are unacceptable (e.g., out of a predetermined range for the characteristic(s)), one or more variables of the patterning process may be designed or altered, e.g., based on the measurements of the one or more characteristics, such that substrates manufactured by the patterning process have an acceptable characteristic(s).

With the advancement of lithography and other patterning process technologies, the dimensions of functional elements have continually been reduced while the amount of the functional elements, such as transistors, per device has been steadily increased over decades. In the meanwhile, the requirement of accuracy in terms of overlay, critical dimension (CD), etc. has become more and more stringent. Error, such as error in overlay, error in CD, etc., will inevitably be produced in the patterning process. For example, imaging error may be produced from optical aberration, patterning device heating, patterning device error, and/or substrate heating and can be characterized in terms of, e.g., overlay, CD, etc. Additionally or alternatively, error may be introduced in other parts of the patterning process, such as in etch, development, bake, etc. and similarly can be characterized in terms of, e.g., overlay, CD, etc. The error may cause a problem in terms of the functioning of the device, including failure of the device to function or one or more electrical problems of the functioning device. Accordingly, it is desirable to be able to characterize one or more these errors and take steps to design, modify, control, etc. a patterning process to reduce or minimize one or more of these errors.

A metrology system is typically used to characterize errors produced by lithographic processes. A metrology system typically needs reproducible fabrication and arrangements of optical components to accurately measure positions of targets on a substrate. It is desirable for metrology systems that are engineered and manufactured the same should be able to produce measurements that are in agreement, or at least within some predetermined tolerance.

SUMMARY

In some embodiments, a method to reduce sensitivity of a level sensor, arranged to measure a height of a substrate, to variations of a property of an optical component in the level sensor comprises directing a beam of radiation toward a diffractive element and directing the beam, via an optical system, to a first reflective element at a first angle of incidence. The beam has a first polarization and a second polarization that is perpendicular to the first polarization. The first reflective element reflects the beam toward a second reflective element at a second angle of incidence so as to cause the beam to impinge on the substrate. The first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements.

In some embodiments, an optical system for directing a beam of radiation having a first polarization and a second polarization that is perpendicular to the first polarization comprises a diffractive element and first and second reflective elements. The diffractive element is configured to direct the beam in the optical system so as to cause the beam to impinge on the first reflective element at a first angle of incidence. The first reflective element is configured to reflect the beam toward the second reflective element at a second angle of incidence so as to cause the beam to impinge on a substrate. The first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements.

In some embodiments, a metrology system comprises a radiation source, an optical system, and a detector. The radiation source is configured to generate a beam of radiation having a first polarization and a second polarization that is perpendicular to the first polarization. The optical system is configured to direct the beam of radiation toward a substrate. The optical system comprises a diffractive element and first and second reflective elements. The diffractive element is configured to direct the beam in the optical system so as to cause the beam to impinge on the first reflective element at a first angle of incidence. The first reflective element is configured to reflect the beam toward the second reflective element at a second angle of incidence so as to cause the beam to impinge on the substrate. The first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements. The detector is configured to receive radiation scattered by the substrate and generate a signal based on the received radiation. The signal comprises information of a height of the substrate.

The features and advantages of the present invention 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

The term “about” can be used herein to indicate 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).

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.

Example Lithographic Systems

FIGS.1A and1Bshow schematics of a lithographic apparatus100and lithographic apparatus100′, respectively, according to some embodiments. In some embodiments, lithographic apparatus100and lithographic apparatus100′ 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 apparatus100and100′ 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 apparatus100, the patterning device MA and the projection system PS are reflective. In lithographic apparatus100′, the patterning device MA and the projection system PS are transmissive.

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.

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 apparatus100and100′, and other conditions, such as whether or not the patterning device MA is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT can be a frame or a table, for example, which can be fixed or movable, as required. 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.

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.

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 can be used for EUV or electron beam radiation since other gases can absorb too much radiation or electrons. A vacuum environment can therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

Lithographic apparatus100and/or lithographic apparatus100′ 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 can 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.

Referring toFIGS.1A and1B, the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatus100,100′ 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 apparatus100or100′, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD (inFIG.1B) 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 apparatus100,100′—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 required, can be referred to as a radiation system.

The illuminator IL can include an adjuster AD (inFIG.1B) for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as “σ-outer” and “σ-inner,” respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL can comprise various other components (inFIG.1B), 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.

Referring toFIG.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 apparatus100, 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 IF1can 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 M1, M2and substrate alignment marks P1, P2.

Referring toFIG.1B, 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. 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 PPU conjugate 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 a mask pattern and create an image of the intensity distribution at the illumination system pupil IPU.

With the aid of the second positioner PW and position sensor IF (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 inFIG.1B) can 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).

In some embodiments, 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 M1, M2, and substrate alignment marks P1, P2. Although the substrate alignment marks (as illustrated) occupy dedicated target portions, they can 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 can be located between the dies.

Mask table MT and patterning device MA can be in a vacuum chamber, 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 can 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.

The lithographic apparatus100and100′ can be used in at least one of the following modes:

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 can be readily applied to maskless lithography that utilizes a programmable patterning device, such as a programmable mirror array.

Combinations and/or variations on the described modes of use or entirely different modes of use can also be employed.

In some embodiments, lithographic apparatus100includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system, and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source.

Subsequently the radiation traverses the illumination system IL, which can include a facetted field mirror device222and a facetted pupil mirror device224arranged to provide a desired angular distribution of the radiation beam221, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the beam of radiation221at the patterning device MA, held by the support structure MT, a patterned beam226is formed and the patterned beam226is imaged by the projection system PS via reflective elements228,230onto a substrate W held by the wafer stage or substrate table WT.

More elements than shown can generally be present in illumination optics unit IL and projection system PS. The grating spectral filter240can optionally be present, depending upon the type of lithographic apparatus. Further, there can be more mirrors present than those shown in the figures, for example there can be 1-6 additional reflective elements present in the projection system PS than shown inFIG.2.

Collector optic CO, as illustrated inFIG.2, is depicted as a nested collector with grazing incidence reflectors253,254and255, just as an example of a collector (or collector mirror). The grazing incidence reflectors253,254and255are 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.

Exemplary Lithographic Cell

FIG.3shows a schematic of a lithographic cell300, also sometimes referred to a lithocell or cluster. Lithographic apparatus100or100′ can form part of lithographic cell300. Lithographic cell300can also include apparatus to perform pre- and post-exposure processes on a substrate. Conventionally 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/O1, I/O2, moves them between the different process apparatus and delivers then to the loading bay LB of the lithographic apparatus. 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 the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatus can be operated to maximize throughput and processing efficiency.

Exemplary Metrology System

FIG.4shows a schematic of a metrology system400that can be implemented as a part of lithographic apparatus100or100′, according to some embodiments. In some embodiments, metrology system400can be configured to measure height and height variations on a surface of substrate W. In some embodiments, metrology system400can be 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 lithography apparatus100or100′ using the detected positions of the alignment marks.

In some embodiments, metrology system400can include a radiation source402, a projection grating404, a detection grating412, and a detector414. Radiation source402can be configured to provide an electromagnetic narrow band radiation beam having one or more passbands. In some embodiments, the one or more passbands can be within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, the one or more passbands can be discrete narrow passbands within a spectrum of wavelengths between about 500 nm to about 900 nm. In another example, radiation source402generates light within the ultraviolet (UV) spectrum of wavelengths between about 225 nm and 400 nm. Radiation source402can 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 radiation source402). Such configuration of radiation source402can help to prevent the shift of the actual CWL values from the desired CWL values, as discussed above, in current metrology systems. And, as a result, the use of constant CWL values can improve long-term stability and accuracy of metrology systems (e.g., metrology system400) compared to the current metrology systems.

Projection grating404can be configured to receive the beam (or beams) of radiation generated from radiation source402, and provide a projected image onto a surface of a substrate408. Imaging optics406can be included between projection grating404and substrate408, and can include one or more lenses, mirrors, gratings, etc. In some embodiment, imaging optics406is configured to focus the image projected from projection grating404onto the surface of substrate408.

In some embodiments, projection grating404provides an image on the surface of substrate408at an angle θ relative to the surface normal. The image is reflected by the substrate surface and is re-imaged on detection grating412. Detection grating412can be identical to projection grating404. Imaging optics410can be included between substrate408and substrate detection grating412, and can include one or more lenses, mirrors, gratings, etc. In some embodiment, imaging optics410is configured to focus the image reflected from the surface of substrate408onto detection grating412. Due to the oblique incidence, a height variation (Zw) in the surface of substrate408will shift the image projected by projection grating404when it is received by detection grating412over a distance (s) as given by the following equation (1):
s=2Zwsin(θ)  (1)

In some embodiments, the shifted image of projection grating404is partially transmitted by detection grating412and the transmitted intensity, which is a periodic function of the image shift. This shifted image is received and measured by detector414. Detector414can include a photodiode or photodiode array. Other examples of detector414include a CCD array. In some embodiments, detector414can be designed to measure wafer height variations as low as 1 nm based on the received image.

Exemplary Optical System

As mentioned earlier, a metrology system typically requires reproducible fabrication and arrangement of optical components to accurately make optical measurements. Additionally, identically engineered metrology systems should be able to produce measurements that are in agreement, or at least within some predetermined tolerance. However, tolerance limitations of optical component fabrication (e.g., a mirror needing precise thin film layer depositions) can adversely affect the performance of a metrology system. In the case of metrology systems for lithographic apparatuses, the required tolerances for some optical components may be difficult to achieve. The variations between seemingly identical optical components causes one metrology system to produce measurement results that are different from another metrology system using the same arrangement of optical components. It is desirable to reduce the variation between metrology systems, or machine-to-machine (M2M) variations, in order to improve accuracy and reliability of metrology systems.

FIG.5shows a perspective schematic of optical system500, according to some embodiments. In some embodiments, optical system500comprises a mirror502, a mirror504, a mirror506, a mirror508, and a mirror510. Optical system500can be implemented in a metrology system, e.g., in place of imaging optics406in metrology system400(FIG.4). In this scenario, projection grating404can be absorbed into imaging optics406. Similarly, optical system500can be implemented in place of imaging optics410and optionally absorb detection grating412. Therefore, in some embodiments, optical system500comprises a grating512. In some embodiments, mirror506comprises one or more holes516configured to allow a beam of radiation514to pass through mirror506.

In some embodiments, grating512is configured to direct beam of radiation514toward mirror502. Grating512is further configured to diffract beam of radiation514such that beam of radiation514is decomposed into one or more diffraction orders and/or one or more wavelengths. Mirror502is configured to direct beam of radiation514from grating512and toward mirror504. Mirror504is configured to direct beam of radiation514from mirror502and toward mirror506. Mirror506is configured to receive beam of radiation514from mirror504and direct beam of radiation514back to mirror504. Mirror504is further configured to receive beam of radiation514from mirror506and direct beam of radiation514toward mirror508.

In some embodiments, when mirror502directs beam of radiation514from grating512toward mirror504, mirror502is configured to do so by directing beam of radiation through one of the one or more holes516of mirror506. In some embodiments, when mirror504directs beam of radiation514from mirror506toward mirror508, mirror504is configured to do so by directing beam of radiation through one of the one or more holes516of mirror506.

In some embodiments, mirror508is configured to receive beam of radiation514, from mirror504, at a first angle of incidence and direct beam of radiation514toward mirror510. Mirror510is configured to receive, at a second angle of incidence, the beam reflected by mirror508and output beam of radiation514, e.g., toward a substrate518.

In some embodiments, the optical functions described above can be reversed. For example, if optical system500is used in a detection arrangement, radiation scattered from substrate518is the radiation that is input into optical system500(e.g., by impinging on mirror510). An example detection arrangement will be discussed later in reference toFIG.8.

In some embodiments, the first and second angles of incidence do not exceed approximately 80 degrees. In some embodiments, the first angle of incidence is between approximately 60 to 80 degrees. In some embodiments, the first angle of incidence is between approximately 45 to 60 degrees. In some embodiments, the second angle of incidence is between approximately 60 to 80 degrees. In some embodiments, the second angle of incidence is between approximately 45 to 60 degrees. In some embodiments, the first and second angles of incidence are the same or substantially similar.

In some embodiments, optical system500can be implemented in a metrology system, for example, to measure a height of a substrate (e.g., a level sensor). In this scenario, a detector in the metrology system is configured to receive radiation scattered from a substrate and generate a signal based on the received radiation. The signal can comprise information of a height of the substrate. The height can be defined as the position along an axis perpendicular to the substrate surface. A critical metric of M2M variations of level sensors is a so-called height process dependency (HPD). HPD refers to a level sensor's response characteristics (e.g., sensitivity) with respect to height from a substrate. The ratio of transmission, through an optical system (e.g., a lens, a lens objective, imaging optics, and the like), is a parameter that affects HPD, particularly the ratio of transmission for vertical and horizontal polarization—referred to as Rs/Rp when considered relative to the plane of substrate. More specifically, an optical system's Rs/Rp in turn is proportional to the product of Rs/Rp of, for example, two or more fold mirrors (e.g., mirror508and mirror510). Rs/Rp of each mirror is a function of the angle of incidence of a beam impinging on each mirror. An important parameter for HPD is variation of Rs/Rp due to, for example, manufacturing errors. A primary contributor to HPD M2M variation of the optical system are the coating thickness errors of the mirrors, particularly at high angles of incidence. Due to the nature of reflection phenomena, Rs/Rp is dependent on angle of incidence and, for example, thickness and composition of a coating(s) on reflective surfaces—composition can affect optical properties, such as transmission and dielectric properties (e.g., permittivity, refractive index, and the like). Embodiments of the present invention provide structures and methods that allow a reduction of HPD M2M variation and increasing the accuracy and reliability of metrology systems.

FIG.6shows a perspective schematic of optical system600, according to some embodiments. In some embodiments, optical system600comprises a mirror602, a mirror604, a mirror606, a mirror608, and a mirror610. Optical system600can be implemented in a metrology system, e.g., in place of imaging optics406in metrology system400(FIG.4). In this scenario, projection grating404can be absorbed into imaging optics406. Similarly, optical system600can be implemented in place of imaging optics410and optionally absorb detection grating412. Therefore, in some embodiments, optical system600comprises a grating612. In some embodiments, mirror606comprises one or more holes616configured to allow a beam of radiation614to pass through mirror606.

In some embodiments, grating612is configured to direct beam of radiation614toward mirror602. Grating612is further configured to diffract beam of radiation614such that beam of radiation614is decomposed into one or more diffraction orders and/or one or more wavelengths. Mirror602is configured to direct beam of radiation614from grating612and toward mirror604. Mirror604is configured to direct beam of radiation614from mirror602and toward mirror606. Mirror606is configured to receive beam of radiation614from mirror604and direct beam of radiation614back to mirror604. Mirror604is further configured to receive beam of radiation614from mirror606and direct beam of radiation614toward mirror608.

In some embodiments, when mirror602directs beam of radiation614from grating612toward mirror604, mirror602is configured to do so by directing beam of radiation through one of the one or more holes616of mirror606. In some embodiments, when mirror604directs beam of radiation614from mirror606toward mirror608, mirror604is configured to do so by directing beam of radiation through one of the one or more holes616of mirror606.

In some embodiments, mirror608is configured to receive beam of radiation614, from mirror604, at a first angle of incidence and direct beam of radiation614toward mirror610. Mirror610is configured to receive, at a second angle of incidence, the beam reflected by mirror608and output beam of radiation614, e.g., toward a substrate618.

In some embodiments, the optical functions described above can be reversed. For example, if optical system600is used in a detection arrangement, radiation scattered from substrate618is the radiation that is input into optical system600(e.g., by impinging on mirror610). An example detection arrangement will be discussed later, in reference toFIG.8.

In some embodiments, optical system600can be implemented in a metrology system, for example, to measure a height of a substrate (e.g., a level sensor). In this scenario, a detector in the metrology system is configured to receive radiation scattered from a substrate and generate a signal based on the received radiation. The signal can comprise information of a height of the substrate.

It was mentioned earlier that the nature of reflection phenomena causes Rs/Rp to respond to angles of incidence, with large variations occurring at high angles of incidence. Therefore, in some embodiments, the first and second angles of incidence do not exceed approximately 45 degrees. In some embodiments, the first angle of incidence is between approximately 30 to 45 degrees. In some embodiments, the first angle of incidence is between approximately 10 to 30 degrees. In some embodiments, the second angle of incidence is between approximately 30 to 45 degrees. In some embodiments, the second angle of incidence is between approximately 10 to 30 degrees. In some embodiments, the first and second angles of incidence are the same or substantially similar. In some embodiments, mirror608and mirror610are arranged such that beam of radiation614received by mirror608crosses beam of radiation614reflected by mirror610. In this scenario, the sum of the first and second angles of incidence is less than 90 degrees. In an alternative description, mirror608and mirror610are arranged such that beam of radiation614received by mirror608is folded onto beam of radiation614reflected by mirror610. Embodiments using reduced angles of incidence reduce the sensitivity of optical systems, which in turn can counteract the sensitivities arising from errors in fabrication of the mirrors. A significant advantage of the structures and methods described herein is that Rs/Rp variations can be reduced without tightening coating tolerances of the mirrors, which in turn provides significant time and cost advantages in the production, implementation, and maintenance of optical systems.

A skilled artisan will appreciate that mirrors in optical system500or optical system600may be removed, added, or rearranged while still achieving the results and advantages described herein. For example, the arrangements of mirror608and mirror610can be maintained as described above while the arrangement of the other mirrors that deliver beam of radiation614to mirror608is altered.

FIG.7shows method steps for reducing sensitivity of a level sensor, arranged to measure a height of a substrate, to variations of a property of an optical component in the level sensor, according to some embodiments. In step702, a beam of radiation is directed toward a diffractive element. The beam comprises a first polarization and a second polarization that is perpendicular to the first polarization. In step704, the beam is directed, via an optical system, to a first reflective element at a first angle of incidence. In step706, the beam is reflected, using the first reflective element, toward a second reflective element at a second angle of incidence. The first and second angles of incidence are selected (e.g., via arrangement of the first and second reflective elements) to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements. In step708, the beam is output from the optical system so as to impinge on a substrate.

In some embodiments, the first and second angles of incidence do not exceed approximately 45 degrees. In some embodiments, the first angle of incidence is between approximately 30 to 45 degrees. In some embodiments, the first angle of incidence is between approximately 10 to 30 degrees. In some embodiments, the second angle of incidence is between approximately 30 to 45 degrees. In some embodiments, the second angle of incidence is between approximately 10 to 30 degrees. In some embodiments, the first and second angles of incidence are the same or substantially similar. In some embodiments, the first and second reflective elements are arranged such that the beam received by the first reflective element crosses the beam reflected by the second reflective element. In this scenario, the sum of the first and second angles of incidence is less than 90 degrees. In an alternative description, the first and second reflective elements are arranged such that the beam received by the first reflective element is folded onto the beam reflected by the second reflective element.

In some embodiments, the optical system comprises third, fourth, and fifth mirrors. The diffractive element is configured to direct the beam toward the third reflective element. The third reflective element is configured to reflect the beam from the diffractive element toward the fourth reflective element. The fourth reflective element is configured to reflect the beam from the third reflective element toward the fifth reflective element, receive the beam reflected by the fifth reflective element, and reflect the beam from the fifth reflective element toward the first reflective element at the first angle of incidence.

In some embodiments, the property of the layer comprises a thickness of the layer and/or at least one dielectric property of the layer (e.g., permittivity, refractive index, atomic composition, and the like).

FIG.8shows a perspective schematic of optical system800and optical system800′ respectively configured for illumination sourcing and detection, according to some embodiments. The element of optical system800are functionally and structurally similar to those inFIGS.5and6with like reference numbers, with the left-most digits of a reference number identifying the drawing in which the reference number appears. In some embodiments, optical system800′ comprises elements corresponding to similarly numbered elements of optical system800. The arrangement shown inFIG.8illustrates optical system800′ as a copy of optical system800and rotated 180 degrees. In some embodiments, optical system800′ is arranged as a mirror copy of optical system800.

In some embodiments, a beam of radiation814′ that has been reflected by a substrate818impinges on mirror810′ as beam of radiation814′ is input into optical system800′. Mirror810and mirror810′ need not be the optical elements most proximate to substrate818. Beam of radiation814′ then travels through optical system800′ in a direction that corresponds to the reverse of a beam of radiation814through optical system800.

In some embodiments, an optical element820(e.g., polarization rotator, grating, waveplates, multiple optical elements, and the like) can be disposed between mirror810and substrate818. An optical element820′ can be disposed between mirror810′ and substrate818. Though the arrangements of mirror808, mirror810, mirror808′, and mirror810′ are shown being similar to mirror508and mirror510(FIG.5), a skilled artisan will appreciate that mirror808, mirror810, mirror808′, and mirror810′ can also be arranged similar to mirror608and mirror610(FIG.6).

1. A method to reduce sensitivity of a level sensor, arranged to measure a height of a substrate, to variations of a property of an optical component in the level sensor, the method comprising:

directing a beam of radiation toward a diffractive element, the beam having a first polarization and a second polarization that is perpendicular to the first polarization; and

directing the beam, via an optical system, to a first reflective element at a first angle of incidence,

wherein the first reflective element reflects the beam toward a second reflective element at a second angle of incidence so as to cause the beam to impinge on the substrate, and

wherein the first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements.

2. The method of clause 1, wherein the first and second angles of incidence do not exceed approximately 45 degrees.

3. The method of clause 1, wherein the first and second angles of incidence are between approximately 10 degrees to 30 degrees.

4. The method of clause 1, wherein the first and second reflective elements are configured such that the beam received by the first reflective element crosses the beam reflected by the second reflective element.

5. The method of clause 1, wherein:

the optical system comprises third, fourth, and fifth mirrors;

the diffractive element is configured to direct the beam toward the third reflective element;

the third reflective element is configured to reflect the beam from the diffractive element toward the fourth reflective element; and

the fourth reflective element is configured to reflect the beam from the third reflective element toward the fifth reflective element, receive the beam reflected by the fifth reflective element, and reflect the beam from the fifth reflective element toward the first reflective element at the first angle of incidence.

6. The method of clause 1, wherein the property of the layer comprises a thickness of the layer.

7. The method of clause 1, wherein the property of the layer comprises a dielectric property comprising permittivity, refractive index, and/or atomic composition of the layer.

8. An optical system for directing a beam of radiation having a first polarization and a second polarization that is perpendicular to the first polarization, the optical system comprising:

a diffractive element; and

first and second reflective elements;

wherein the diffractive element is configured to direct the beam in the optical system so as to cause the beam to impinge on the first reflective element at a first angle of incidence,

wherein the first reflective element is configured to reflect the beam toward the second reflective element at a second angle of incidence so as to cause the beam to impinge on a substrate, and

wherein the first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements.

9. The optical system of clause 8, wherein the first and second angles of incidence do not exceed approximately 45 degrees.

10. The optical system of clause 8, wherein the first and second angles of incidence are between approximately 10 degrees to 30 degrees.

11. The optical system of clause 8, wherein the first and second reflective elements are configured such that the beam received by the first reflective element crosses the beam reflected by the second reflective element.

12. The optical system of clause 8, further comprising third, fourth, and fifth mirrors, wherein:

the diffractive element is configured to direct the beam toward the third reflective element;

the third reflective element is configured to reflect the beam from the diffractive element toward the fourth reflective element; and

the fourth reflective element is configured to reflect the beam from the third reflective element toward the fifth reflective element, receive the beam reflected by the fifth reflective element, and reflect the beam from the fifth reflective element toward the first reflective element at the first angle of incidence.

13. The optical system of clause 8, wherein the property of the layer comprises a thickness of the layer.

14. The optical system of clause 8, wherein the property of the layer comprises a dielectric property comprising permittivity, refractive index, and/or atomic composition of the layer.

15. A metrology system comprising:

a radiation source configured to generate a beam of radiation having a first polarization and a second polarization that is perpendicular to the first polarization;

an optical system configured to direct the beam of radiation toward a substrate, the optical system comprising:a diffractive element;first and second reflective elements;wherein the diffractive element is configured to direct the beam in the optical system so as to cause the beam to impinge on the first reflective element at a first angle of incidence,wherein the first reflective element is configured to reflect the beam toward the second reflective element at a second angle of incidence so as to cause the beam to impinge on the substrate, andwherein the first and second angles of incidence are selected to reduce variations of a ratio of intensities of the first polarization to the second polarization of the beam imparted by a property of a layer of at least one of the first and second reflective elements; and

a detector configured to receive radiation scattered by the substrate and generate a signal based on the received radiation, wherein the signal comprises information of a height of the substrate.

16. The metrology system of clause 15, wherein the first and second angles of incidence do not exceed approximately 45 degrees.

17. The metrology system of clause 15, wherein the first and second angles of incidence are between approximately 10 degrees to 30 degrees.

18. The metrology system of clause 15, wherein the first and second reflective elements are configured such that the beam received by the first reflective element crosses the beam reflected by the second reflective element.

19. The metrology system of clause 15, further comprising third, fourth, and fifth mirrors, wherein:

the diffractive element is configured to direct the beam toward the third reflective element;

the third reflective element is configured to reflect the beam from the diffractive element toward the fourth reflective element; and

the fourth reflective element is configured to reflect the beam from the third reflective element toward the fifth reflective element, receive the beam reflected by the fifth reflective element, and reflect the beam from the fifth reflective element toward the first reflective element at the first angle of incidence.

20. The metrology system of clause 15, wherein the property of the layer comprises a thickness of the layer, permittivity, refractive index, and/or atomic composition of the layer.

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 synonymous with 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), a metrology unit and/or an inspection 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.

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.

In the embodiments described herein, the terms “lens” and “lens element,” where the context allows, can refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic, and electrostatic optical components. The term “mirror” as used herein can refer to any one or combination of various types of reflective elements or optical components that direct/redirect radiation via reflection. The term “grating” as used herein can refer to any one or combination of various types of diffractive elements or optical components that direct/redirect radiation via diffraction.

Further, the terms “radiation,” “beam,” and “light” used herein may encompass all types of electromagnetic radiation, for example, ultraviolet (UV) radiation (for example, having a wavelength λ 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 particle beams, such as ion beams or electron beams. 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 certain wavelength band, of which at least part is in the range of 5-20 nm.

The term “substrate” as used herein may describe a material onto which material layers are added. In some embodiments, the substrate itself can be patterned and materials added on top of it may also be patterned, or may remain without patterning.

Although specific reference can be made in this text to the use of the apparatus and/or system according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus and/or system has many other possible applications. For example, it can be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, LCD panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle,” “wafer,” or “die” in this text should be considered as being replaced by the more general terms “mask,” “substrate,” and “target portion,” respectively.

While specific embodiments of the invention have been described above, it will be appreciated that the invention can be practiced otherwise than as described. The description is not intended to limit the invention.