Patent Application
20250060661
This application claims priority of EP application Ser. No. 21/216,952.8 which was filed on Dec. 22, 2021 and which is incorporated herein in its entirety by reference.
BACKGROUND
The present invention relates to a target arrangement for metrology of a lithographic process and to a method to measure a parameter of a lithographic process.
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, may 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., including 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. In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, a measure of the accuracy of alignment of two layers in a device. Overlay may be described in terms of the degree of misalignment between the two layers, for example reference to a measured overlay of 1 nm may describe a situation where two layers are misaligned by 1 nm.
Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.
The targets used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target as it can be regarded as infinite. However, in order to reduce the size of the targets, e.g., to 10 μm by 10μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically such targets are measured using dark field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed. Examples of dark field metrology can be found in international patent applications WO 2009/078708 and WO 2009/106279 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in patent publications US20110027704A, US20110043791A and US20120242970A. Modifications of the apparatus to improve throughput are described in US2010201963A1 and US2011102753A1. The contents of all these applications are also incorporated herein by reference. Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Targets can comprise multiple gratings which can be measured in one image.
One important parameter of a lithographic process which requires monitoring is focus. There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this, it is necessary to decrease the size of the components and therefore to increase the resolution of the projection system, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. As the critical dimension (CD) in lithography shrinks, consistency of focus, both across a substrate and between substrates, becomes increasingly important. CD is the dimension of a feature or features (such as the gate width of a transistor) for which variations will cause undesirable variation in physical properties of the feature.
Traditionally, optimal settings were determined by “send-ahead wafers” i.e. substrates that are exposed, developed and measured in advance of a production run. In the send-ahead wafers, test structures were exposed in a so-called focus-energy matrix (FEM) and the best focus and energy (exposure dose) settings were determined from examination of those test structures. More recently, focus metrology targets are included in the production designs, to allow continuous monitoring of focus performance. These metrology targets should permit rapid measurements of focus, to allow fast performance measurement in high-volume manufacturing. Ideally, the metrology targets should be small enough that they can be placed among the product features without undue loss of space.
Current test structure designs and focus measuring methods have a number of drawbacks. Known focus metrology targets require sub-resolution features and/or grating structures with large pitches. Such structures may contravene design rules of the users of lithographic apparatuses. Asymmetry in a grating structure can be measured effectively using high-speed inspection apparatus such as a scatterometer, working at visible radiation wavelengths. Known focus measuring techniques exploit the fact that focus-sensitive asymmetry can be introduced into structures printed in a resist layer by special design of the patterns on a patterning device that defines the target structure. For EUV lithography, where printing is performed using radiation of a wavelength less than 20 nm, for example 13.5 nm, the creation of sub-resolution features becomes even more difficult. For EUV lithography, resist thickness, and therefore the thickness of target structures, is smaller. This weakens the diffraction efficiency, and hence the signal strength, available for focus metrology.
For these reasons, there is a need to develop new techniques for the measurement of focus performance in lithographic processes, particularly in EUV lithography, but also for projection-based lithography in general.
It would be desirable to be able to perform metrology of a lithographic process with increased accuracy.
The invention in a first aspect provides a method of measuring a focus parameter from a focus target, the focus target comprising at least one self-referenced pair of sub-targets, the self-referenced pair of sub-targets comprising a first sub-target and a second sub-target, wherein each of said first sub-target and second sub-target comprises at least a periodic main feature; wherein a respective pitch and/or dimensional parameter of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and/or pitch; the method comprising: obtaining a first measurement signal from said first sub-target and a second measurement signal from said second sub-target; determining a difference signal of said first measurement signal and second measurement signal; and determining said focus parameter from said difference signal.
The invention in a second aspect provides a substrate comprising at least one focus target for measuring a focus parameter, the focus target comprising at least one self-referenced pair of sub-targets, the self-referenced pair of sub-targets comprising a first sub-target and a second sub-target, wherein each of said first sub-target and second sub-target comprises at least a periodic main feature; wherein a respective pitch and/or dimensional parameter of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and/or pitch.
The invention in a third aspect provides a substrate comprising at least one focus target or sub-target thereof comprising at least a periodic main feature having a main feature pitch, wherein a center-of-mass and/or main feature pitch is focus dependent; and a periodic reference feature having a reference feature pitch different from said main feature pitch, the main feature and reference feature being arranged such that scattered radiation from the main feature and the reference feature interfere to form a beat signal or Moiré signal.
The invention in a fourth aspect provides a method of measuring a focus parameter from a focus target on a substrate as off the third aspect, comprising: imaging Moiré fringes resulting from interference between at least one diffraction order from the main feature and at least one corresponding diffraction order from the reference feature to obtain a measurement signal; and determining said focus parameter from said measurement signal.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention 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.
Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.
The illumination optical system may include various types of optical or non-optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of components, or any combination thereof, for directing, shaping, or controlling radiation.
The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.
As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).
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.
Referring to
The illuminator IL may include an adjuster AD 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 can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section.
The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the patterning device support (e.g., mask table MT), and is patterned by the patterning device. Having traversed the patterning device (e.g., mask) MA, the radiation beam B passes through the projection optical system PS, which focuses the beam onto a target portion C of the substrate W, thereby projecting an image of the pattern on the target portion C. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., 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 (which is not explicitly depicted in
Patterning device (e.g., mask) MA and substrate W may 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 may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks may be located between the dies. Small alignment markers may also be included within dies, in amongst the device features, in which case it is desirable that the markers be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment markers is described further below.
Lithographic apparatus LA in this example is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations—an exposure station and a measurement station-between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. The preparatory steps may include mapping the surface control of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. This enables a substantial increase in the throughput of the apparatus.
The depicted apparatus can be used in a variety of modes, including for example a step mode or a scan mode. The construction and operation of lithographic apparatus is well known to those skilled in the art and need not be described further for an understanding of the present invention.
As shown in
In order that the substrates that are exposed by the lithographic apparatus are exposed correctly and consistently, it is desirable to inspect exposed substrates to measure properties such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. Accordingly a manufacturing facility in which lithocell LC is located also includes metrology system MET which receives some or all of the substrates W that have been processed in the lithocell. Metrology results are provided directly or indirectly to the supervisory control system SCS. If errors are detected, adjustments may be made to exposures of subsequent substrates, especially if the inspection can be done soon and fast enough that other substrates of the same batch are still to be exposed. Also, already exposed substrates may be stripped and reworked to improve yield, or discarded, thereby avoiding performing further processing on substrates that are known to be faulty. In a case where only some target portions of a substrate are faulty, further exposures can be performed only on those target portions which are good.
Within metrology system MET, an inspection apparatus is used to determine the properties of the substrates, and in particular, how the properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection apparatus may be integrated into the lithographic apparatus LA or the lithocell LC or may be a stand-alone device. To enable most rapid measurements, it is desirable that the inspection apparatus measure properties in the exposed resist layer immediately after the exposure. However, the latent image in the resist has a very low contrast-there is only a very small difference in refractive index between the parts of the resist which have been exposed to radiation and those which have not—and not all inspection apparatuses have sufficient sensitivity to make useful measurements of the latent image. Therefore measurements may be taken after the post-exposure bake step (PEB) which is customarily the first step carried out on exposed substrates and increases the contrast between exposed and unexposed parts of the resist. At this stage, the image in the resist may be referred to as semi-latent. It is also possible to make measurements of the developed resist image—at which point either the exposed or unexposed parts of the resist have been removed—or after a pattern transfer step such as etching. The latter possibility limits the possibilities for rework of faulty substrates but may still provide useful information.
A metrology apparatus is shown in
As shown in
At least the 0 and +1 orders diffracted by the target T on substrate W are collected by objective lens 16 and directed back through beam splitter 15. Returning to
A second beam splitter 17 divides the diffracted beams into two measurement branches. In a first measurement branch, optical system 18 forms a diffraction spectrum (pupil plane image) of the target on first sensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first order diffractive beams. Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast orders. The pupil plane image captured by sensor 19 can be used for focusing the metrology apparatus and/or normalizing intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.
In the second measurement branch, optical system 20, 22 forms an image of the target Ton sensor 23 (e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane that is conjugate to the pupil-plane. Aperture stop 21 functions to block the zeroth order diffracted beam so that the image of the target formed on sensor 23 is formed only from the −1 or +1 first order beam. The images captured by sensors 19 and 23 are output to processor PU which processes the image, the function of which will depend on the particular type of measurements being performed. Note that the term ‘image’ is used here in a broad sense. An image of the grating lines as such will not be formed, if only one of the −1 and +1 orders is present.
The particular forms of aperture plate 13 and field stop 21 shown in
In order to make the measurement radiation adaptable to these different types of measurement, the aperture plate 13 may comprise a number of aperture patterns formed around a disc, which rotates to bring a desired pattern into place. Note that aperture plate 13N or 13S can only be used to measure gratings oriented in one direction (X or Y depending on the set-up). For measurement of an orthogonal grating, rotation of the target through 90° and 270° might be implemented. Different aperture plates are shown in
Once the separate images of the overlay targets have been identified, the intensities of those individual images can be measured, e.g., by averaging or summing selected pixel intensity values within the identified areas. Intensities and/or other properties of the images can be compared with one another. These results can be combined to measure different parameters of the lithographic process.
In the following disclosure, techniques will be illustrated for measuring focus performance of a lithographic process that uses oblique illumination on a reflective type of patterning device. These techniques may be applied in particular in EUV lithography, where reflective optics in a near-vacuum environment are required. Metrology targets including certain focus metrology patterns will be printed on the substrate, at the same time as product features are printed. Asymmetry in these printed patterns will be measured using for example diffraction based techniques in the apparatus of
In the context of lithographic apparatuses working in the DUV wavelength range, targets for diffraction-based focus (DBF) measurements have been designed and used successfully. A known type of DBF target is produced by including sub-segmented features in a grating pattern on the reticle. These features have dimensions below the imaging resolution of the lithographic apparatus, alongside more solid features. Consequently, they do not print as individual features in the resist layer on the substrate, but they influence the printing of the solid features, in a manner that is sensitive to focus error. Specifically, the presence of these features creates an asymmetric resist profile for each line in the grating within the DBF metrology target, with the degree of asymmetry being dependent upon focus. Consequently a metrology tool such as the inspection apparatus of
Unfortunately, the known DBF metrology target designs are not suitable for use in all situations. In EUV lithography, resist film thicknesses are significantly lower than those used in DUV immersion lithography, leading to low diffraction efficiency and difficulty extracting accurate asymmetry information from diffracted radiation in the scatterometer. In addition, since the resolution of the imaging system is inherently higher in EUV lithography, features having dimensions below the printing resolution of DUV immersion lithography become “solid” features printable by EUV lithography. To provide analogous sub-resolution features on an EUV reticle is rather impractical, and/or may violate semiconductor manufacturer's “design rules”. Such rules are generally established as a means to restrict the feature designs to ensure the printed features conform to their process requirements. In any case, working outside the design rules makes it difficult to simulate the performance of the process on the DBF targets, so that the optimum target design and the calibration of focus measurements becomes a matter of trial-and-error. The desire to conform to design rules applies to DBF targets in DUV lithography, not only EUV lithography.
As such, methods and target designs for obtaining a stronger and/or more robust signal from a focus target using a scatterometer are desirable. To achieve this a focus target for measuring a focus parameter relating to the focus target is disclosed, comprising at least one self-referenced pair of sub-targets, the self-referenced pair of sub-targets comprising a first sub-target and a second sub-target, wherein each of said first sub-target and second sub-target comprises at least a periodic main feature; wherein a respective pitch and/or dimensional parameter (e.g., critical dimension CD) of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and/or pitch measureable using a scatterometer.
A focus parameter can be determined from measuring the focus target with a scatterometer, the focus parameter relating to a focus setting of a lithographic exposure process on which said target was exposed. Also disclosed is a substrate comprising at least one such target and a reticle or mask comprising target forming features for forming at least one such target.
As such, also disclosed is a method of measuring a focus parameter, from a focus target comprising at least a first sub-target and a second sub-target, each having at least a periodic main feature, wherein a respective pitch and/or dimensional parameter of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and/or pitch; the method comprising: obtaining a first measurement signal from said first sub-target and a second measurement signal from said second sub-target; determining a difference signal of said first measurement signal and second measurement signal; and determining said focus parameter from said difference signal substrate and associated patterning device.
The “center-of mass” of each sub-target or structure may comprise the peak position of the first harmonic of the pattern defined by the printed structure.
To obtain a stronger signal when measuring the center-or-mass of the targets, it is proposed that the focus target may be configured for a self-referenced phase measurement of the center-of-mass of the sub-targets using a scatterometer. This is particularly useful for EUV applications with thin resists, as purely intensity based (or intensity asymmetry based) focus metrology techniques suffer from very low signal strength and unreliable inference (poor repro) when measuring known focus targets in such thin resists. Self-referenced in this context means that the detected fringe position (phase measurement) indicative of center-of-mass or pitch from a first sub-target is compared to the fringe position of the second sub-target.
By way of a first such example, to obtain a fringe pattern indicative of center-of-mass or pitch, each sub-target may be configured such that the scatterometer can capture first and second diffraction orders and measure a center-of-mass/pitch and therefore focus dependent phase therefrom. These orders will interfere to provide a fringe pattern on the scatterometer's detector (camera), which will shift in position with focus. By measuring the difference in fringe position, and therefore the difference in center-of-mass, for each sub-target, a monotonic relationship of measurement signal with focus is obtained.
Alternatively, each sub-target may comprise a periodic reference feature having a different pitch to the periodic main feature, such that scattered radiation from the focus dependent main feature and the reference feature interfere to form a beat signal or Moiré signal. This Moiré signal can then be imaged using the scatterometer, with the difference in Moiré fringe position for the two sub-targets measured to determine a difference in center-of-mass for each sub-target indicative of focus. In an embodiment, the pitches of the main feature Pmain and reference feature Pref have
a small difference (e.g., a difference of less than 20% or 10% of the pitch of the main feature). This creates two 1st order diffracted beams with a slightly different direction. Upon recombination in the image a Moiré beat pattern becomes visible with a pitch 1/Pbeat=1/Pref−1/Pmain. As such, the Moiré effect amplifies the phase shift; e.g., a 10% pitch difference provides up to a 10× amplification of the shift.
The main feature of each of the sub-targets may comprise a periodic (e.g., 1D or 2D) array of main feature elements, where each main feature element comprises a larger first sub-element and multiple smaller second sub-elements. The second sub-elements may comprise features having a CD and/or pitch at or close to product dimensions, and as such may be referred to as at-resolution sub-elements. For example, the second sub-elements may comprise a 1D array of such at-resolution second sub-elements, either parallel or perpendicular to the (longest dimension of the) first sub-element. The first sub-element may be a solid element, or else it may be a periodic element (e.g., having a CD and/or pitch at or close to product dimensions).
The main pitch of main feature may be sufficiently large to be compatible with (measurable by) a scatterometer apparatus (e.g., such as illustrated in
To impose the different best focus for each sub-target, it is proposed that the CD and/or pitch of the at-resolution features may differ between the sub-targets. This may be the CD and/or pitch of the second sub-elements and/or, where the first sub-element is a periodic element, the CD and/or pitch of the first sub-element. Best focus may indicate a focus setting for which CD variation is minimal over a small range (e.g., +/−5 nm) around the best focus and/or a focus setting at the peak or trough of a corresponding Bossung curve for that sub-target; i.e., where CD sensitivity for small focus changes is lowest.
The pitch and/or CD of these at-resolution features may be optimized in a computational lithography step (e.g., using source-mask optimization SMO) to obtain a sufficient and/or to maximize the best focus difference between the sub-targets. Also, additional techniques, such as altering the absorber on the mask between the two sub-targets, may be employed to further maximize the best focus offset; e.g., by forming one (or both) targets on the mask using a different mask absorber than the standard Ta-based absorber (e.g., low-n absorbers with refractive index n<0.92 and other EUV absorbers).
In the reference feature embodiments, the reference feature may be comprised in another layer of the device, and/or may be interlaced with the main feature. In a specific example, the reference feature may be interlaced in a direction perpendicular to the periodicity of the main feature. In such an embodiment, the perpendicular pitch of the main feature in this perpendicular direction PPP may be sufficiently small to be unresolved. For example this perpendicular pitch PPP may meet the criterion λ/PPP>1.2, where λ is the radiation wavelength of the measurement radiation.
A main pitch Pmain of the target should be compatible with the scatterometer used to measure the target, so as to generate at least detectable first diffraction orders. Where reference features are provided, it is sufficient that only detectable first orders are generated to enable performance of a phase measurement. If the target does not have reference features then the main pitch Pmain should be configured to generate at least detectable first and second orders to enable performance of a phase measurement.
As has been described, each of the sub-targets 600a, 600b have a different best focus setting. This is achieved through Mask3D effects, i.e., the effect resultant from non-normal illumination on a reflective mask comprising 3D patterning features. In particular, and more specifically the pitch and/or CD of the second sub-elements 630a may be different to the second sub-elements 630b to exploit this Mask3D effect. In the specific example shown, the pitch Psse1 of the second sub-elements 630a is different to the pitch Psse2 of the second sub-elements 630b. Furthermore, the CD of the second sub-elements 630a is different to the CD of the second sub-elements 630b. However, only one of pitch and CD need be varied.
As stated, the center-of-mass may be inferred from the difference in position (translation) from each sub-target of imaged fringes of interference of the first and second diffraction orders, or from Moiré fringes where the target comprises reference features. Other methods include inferring the center-of-mass from an intensity difference of the two sub-targets.
The method may comprise an initial calibration stage, e.g., to calibrate the relationship of the center-of-mass difference ΔCoM with focus (the relationship illustrated in
Note that this Figure is not shown as a complete sub-target as it would be printed normally, but is represented here with individual feature elements printed at different focus settings (purely for illustration), with an approximate best focus BF (noting that each sub-target has a different best focus) in the center and moving away to greater defocus in each direction along the arrows. Each feature element may comprise the same width/center-of-mass on the mask, and (for a single focus setting) the actual target as printed would also comprise feature elements having substantially/approximately uniform center-of-mass which is dependent on that focus setting.
This target type provides a focus dependent change in main feature pitch, which may be measured in the same manner as has been described; e.g., via fringes imaged from first and second diffraction orders or Moiré fringes where a reference feature is also provided.
In
The perpendicular pitch of the main feature in the perpendicular direction PPP may be sufficiently small to be unresolved. For example this perpendicular pitch PPP may meet the criterion λ/PPP>1.2, where λ is the radiation wavelength of the measurement radiation.
Moiré focus target are presently unknown, and therefore also disclosed herein is a focus target or sub-target thereof comprising at least a periodic main feature having a main feature pitch and wherein the center-of-mass and/or main feature pitch is focus dependent; and a periodic reference feature having a reference feature pitch different from said main feature pitch, the main feature and reference feature being arranged such that scattered radiation from the main feature and the reference feature interfere to form a beat signal or Moiré signal.
In all the embodiments described, each target as described may be paired with a mirrored or 180 degree rotated version of that target such that overlay content can be removed/separated from the focus inference. For the mirrored pair, any overlay content will cause a fringe shift in the same direction for the pair, while the focus shift will be in opposite directions, enabling their separation.
In association with the physical grating structures of the targets realized on substrates and patterning devices, an embodiment may include a computer program containing one or more sequences of machine-readable instructions describing methods of measuring targets on a substrate and/or analyzing measurements to obtain information about a lithographic process. This computer program may be executed for example within unit PU in the apparatus of
The program may optionally be arranged to control the optical system, substrate support and the like to perform the steps necessary to calculate the overlay error for measurement of asymmetry on a suitable plurality of targets.
Therefore, disclosed is a target arrangement suitable for metrology of a lithographic process comprising at least two targets positioned within the target arrangement such that the target arrangement has a symmetry upon rotation. The at least two targets may be positioned within the target arrangement such that a measured property of the at least two target has a symmetry upon rotation. Also disclosed is a method to measure a parameter of a lithographic process comprising measuring at least two targets of a metrology target arrangement by illuminating the targets with radiation and detecting the radiation scattered by the targets and determining a property in the measurement of the targets wherein the property has a symmetry upon rotation.
Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may 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 may 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.
Further embodiments according to the present invention are described in below numbered clauses:
1. A method of measuring a focus parameter from a focus target, the focus target comprising at least one self-referenced pair of sub-targets, the self-referenced pair of sub-targets comprising a first sub-target and a second sub-target, wherein each of said first sub-target and second sub-target comprises at least a periodic main feature; wherein a respective pitch and/or dimensional parameter of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and/or pitch; the method comprising: obtaining a first measurement signal from said first sub-target and a second measurement signal from said second sub-target; determining a difference signal of said first measurement signal and second measurement signal; and determining said focus parameter from said difference signal.
2. A method according to clause 1, wherein said first measurement signal and second measurement signal are phase signals, and the method comprises using each sub-target to reference the phase of the other sub-target.
3. A method according to clause 1 or 2, wherein a main feature pitch of the main feature and wavelength of measurement radiation is configured to provide at least a detectable first diffraction order from each sub-target.
4. A method according to clause 3, wherein a ratio of wavelength of the measurement radiation to said main feature pitch is between 0.7 and 1.3.
5. A method according to any preceding clause, comprising capturing at least one first diffraction order and at least one second diffraction order from measurement radiation scattered by each of said first sub-target and said second sub target;
6. A method according to clause 5, wherein a main feature pitch of the main feature and wavelength of measurement radiation is configured to provide at least a detectable first diffraction order and second diffraction order from each sub-target.
7. A method according to any of clauses 1 to 4, wherein each sub-target comprises a periodic reference feature having a reference feature pitch which is different from a main feature pitch of the main feature, such that scattered radiation from the main feature and the reference feature interfere to form a Moiré signal.
8. A method according to clause 7, wherein said main feature pitch and reference feature pitch differ by less than 20% of the main feature pitch.
9. A method according to clause 7 or 8, comprising imaging Moiré fringes resulting from interference between at least one diffraction order from the main feature of said first sub-target and at least one corresponding diffraction order from the reference feature of said first sub-target to obtain said first measurement signal; and imaging Moiré fringes resulting from interference between at least one diffraction order from the main feature of said first sub-target and at least one corresponding diffraction order from the reference feature of said second sub-target to obtain said second measurement signal.
10. A method according to clause 7, 8 or 9, wherein said reference feature of each sub-target is comprised in a layer below a layer of interest comprising said main feature of each sub-target.
11. A method according to clause 7, 8 or 9, wherein said reference feature of each sub-target and said main feature of each sub-target are both comprised within the same layer.
12. A method according to clause 11, wherein, for each sub-target, the main feature and reference feature are interlaced in a direction of periodicity of the main feature and reference feature, such that main feature elements of the main feature alternate with reference feature elements of the reference feature in said direction of periodicity.
13. A method according to clause 11, wherein, for each sub-target, the main feature and reference feature are interlaced in a direction perpendicular to the periodicity of the main feature and reference feature, such that main feature elements of the main feature alternate with reference feature elements of the reference feature in said direction perpendicular to the periodicity of the main feature and reference feature.
14. A method according to clause 13, wherein a perpendicular pitch of the main feature in said perpendicular direction below the resolution limit of the apparatus used to perform said measurement signals.
15. A method according to clause 13 or 14, wherein a ratio of wavelength of the measurement radiation to said perpendicular pitch is greater than 1.2.
16. A method according to any of clauses 5 to 15, wherein said difference signal comprises the difference in position of corresponding said fringes in said first measurement signal and second measurement signal.
17. A method according to any preceding clause, wherein each said main feature is formed with a focus dependent center-of-mass, said center-of-mass of each sub-target describing the peak position of a first harmonic of a pattern defined by the sub-target when printed.
18. A method according to any of clauses 1 to 16, wherein each said main feature is formed with a focus dependent pitch, said main feature being substantially symmetrical around an axis of symmetry parallel to the direction of periodicity of the main feature.
19. A method according to any preceding clause, wherein said main feature of each of the sub-target comprises a periodic array of main feature elements.
20. A method according to clause 19, wherein some or all of said main feature elements each comprise a larger first sub-element and multiple smaller second sub-elements.
21. A method according to clause 20, wherein said second sub-elements comprises features having a CD and/or pitch at or close to product dimensions.
22. A method according to clause 20 or 21, wherein said second sub-elements forms a periodic structure with periodicity in a direction parallel to the longest dimension of the first sub-element.
23. A method according to clause 20 or 21, wherein said second sub-elements forms a periodic structure with periodicity in a direction perpendicular to the longest dimension of the first sub-element.
24. A method according to any of clauses 20 to 23, wherein said at least some sub-elements of the main feature which are configured such that said first sub-target and second sub-target have a respective different best focus value comprise said second sub-elements.
25. A method according to any of clauses 20 to 24, wherein said first sub-elements each comprise a solid or single filled structure.
26. A method according to any of clauses 20 to 24, wherein said first sub-elements each comprise a periodic structure.
27. A method according to clause 26, wherein said at least some sub-elements of the main feature which are configured such that said first sub-target and second sub-target have a respective different best focus value comprise said first sub-elements.
28. A method according to any preceding clause, comprising obtaining a calibrated relationship between said difference signal and said focus parameter to determine said focus parameter from said difference signal.
29. A method according to any preceding clause, comprising an initial step of exposing said focus target on a substrate; and
30. A method according to clause 29, comprising using a scatterometer to measure said focus target in said measuring step.
31. A method according to any preceding clause, wherein said focus target is paired with a mirrored or 180 degree rotated version of said focus target; and the method comprises removing overlay content from the determined focus parameter from measurement signals obtained from the paired focus targets.
32. A method according to any preceding clause, wherein said dimensional parameter is critical dimension.
33. A method according to any preceding clause, wherein said focus parameter describes a focus setting of a lithographic exposure process in which said focus target was exposed.
34. A substrate comprising at least one focus target for measuring a focus parameter, the focus target comprising at least one self-referenced pair of sub-targets, the self-referenced pair of sub-targets comprising a first sub-target and a second sub-target, wherein each of said first sub-target and second sub-target comprises at least a periodic main feature; wherein a respective pitch and/or dimensional parameter of at least some sub-elements of the main feature are configured such that said first sub-target and second sub-target have a respective different best focus value; and wherein each said main feature is formed with a focus dependent center-of-mass and/or pitch.
35. A substrate according to clause 34, wherein each sub-target comprises a periodic reference feature having a reference feature pitch which is different from a main feature pitch of the main feature, such that scattered radiation from the main feature and the reference feature interfere to form a Moiré signal.
36. A substrate according to clause 35, wherein said main feature pitch and reference feature pitch differ by less than 20% of the main feature pitch.
37. A substrate according to clause 34, 35 or 36, wherein said reference feature of each sub-target is comprised in a layer below a layer of interest comprising said main feature of each sub-target.
38. A substrate according to clause 34, 35 or 36, wherein said reference feature of each sub-target and said main feature of each sub-target are both comprised within the same layer.
39. A substrate according to clause 38, wherein, for each sub-target, the main feature and reference feature are interlaced in a direction of periodicity of the main feature and reference feature, such that main feature elements of the main feature alternate with reference feature elements of the reference feature in said direction of periodicity.
40. A substrate according to clause 38, wherein, for each sub-target, the main feature and reference feature are interlaced in a direction perpendicular to the periodicity of the main feature and reference feature, such that main feature elements of the main feature alternate with reference feature elements of the reference feature in said direction perpendicular to the periodicity of the main feature and reference feature.
41. A substrate according to any of clauses 34 to 40, wherein each said main feature has been formed with a focus dependent center-of-mass, said center-of-mass of each sub-target describing the peak position of a first harmonic of a pattern defined by the sub-target when printed.
42. A substrate according to any of clauses 34 to 40, wherein each said main feature is formed with a focus dependent pitch, said main feature being substantially symmetrical around an axis of symmetry parallel to the direction of periodicity of the main feature.
43. A substrate according to any of clauses 34 to 42, wherein said main feature of each of the sub-target comprises a periodic array of main feature elements.
44. A substrate according to clause 43, wherein some or all of said main feature elements each comprise a larger first sub-element and multiple smaller second sub-elements.
45. A substrate according to clause 44, wherein said second sub-elements comprises features having a CD and/or pitch at or close to product dimensions.
46. A substrate according to clause 44 or 45, wherein said second sub-elements forms a periodic structure with periodicity in a direction parallel to the longest dimension of the first sub-element.
47. A substrate according to clause 44 or 45, wherein said second sub-elements forms a periodic structure with periodicity in a direction perpendicular to the longest dimension of the first sub-element.
48. A substrate according to any of clauses 44 to 47, wherein said at least some sub-elements of the main feature which are configured such that said first sub-target and second sub-target have a respective different best focus value comprise said second sub-elements.
49. A substrate according to any of clauses 44 to 48, wherein said first sub-elements each comprise a solid or single filled structure.
50. A substrate according to any of clauses 44 to 48, wherein said first sub-elements each comprise a periodic structure.
51. A substrate according to clause 50, wherein said at least some sub-elements of the main feature which are configured such that said first sub-target and second sub-target have a respective different best focus value comprise said first sub-elements.
52. A substrate according to any of clauses 34 to 51, wherein said focus target is paired on said substrate with a mirrored or 180degree rotated version of said focus target.
53. A substrate according to any of clauses 34 to 52, wherein said dimensional parameter is critical dimension.
54. A substrate according to any of clauses 34 to 53, wherein said focus parameter describes a focus setting of a lithographic exposure process in which said focus target was exposed.
55. A substrate comprising at least one focus target or sub-target thereof comprising at least a periodic main feature having a main feature pitch, wherein a center-of-mass and/or main feature pitch is focus dependent; and a periodic reference feature having a reference feature pitch different from said main feature pitch, the main feature and reference feature being arranged such that scattered radiation from the main feature and the reference feature interfere to form a beat signal or Moiré signal.
56. A substrate according to clause 55, wherein said main feature pitch and reference feature pitch differ by less than 20% of the main feature pitch.
57. A substrate according to clause 55 or 56, wherein said reference feature is comprised in a layer below a layer of interest comprising said main feature of each sub-target.
58. A substrate according to clause 55 or 56, wherein said reference feature and said main feature are both comprised within the same layer.
59. A substrate according to clause 58, wherein the main feature and reference feature are interlaced in a direction of periodicity of the main feature and reference feature, such that main feature elements of the main feature alternate with reference feature elements of the reference feature in said direction of periodicity.
60. A substrate according to clause 58, wherein, for each sub-target, the main feature and reference feature are interlaced in a direction perpendicular to the periodicity of the main feature and reference feature, such that main feature elements of the main feature alternate with reference feature elements of the reference feature in said direction perpendicular to the periodicity of the main feature and reference feature.
61. A substrate according to any of clauses 55 to 60, wherein said main feature has been formed with a focus dependent center-of-mass, said center-of-mass of each sub-target describing the peak position of a first harmonic of a pattern defined focus target when printed.
62. A substrate according to any of clauses 55 to 60, wherein each said main feature is formed with a focus dependent pitch, said main feature being substantially symmetrical around an axis of symmetry parallel to the direction of periodicity of the main feature.
63. A substrate according to any of clauses 55 to 62, wherein said main feature comprises a periodic array of main feature elements.
64. A substrate according to clause 63, wherein some or all of said main feature elements each comprise a larger first sub-element and multiple smaller second sub-elements.
65. A substrate according to clause 64, wherein said second sub-elements comprises features having a CD and/or pitch at or close to product dimensions.
66. A substrate according to clause 64 or 65, wherein said second sub-elements forms a periodic structure with periodicity in a direction parallel to the longest dimension of the first sub-element.
67. A substrate according to clause 64 or 65, wherein said second sub-elements forms a periodic structure with periodicity in a direction perpendicular to the longest dimension of the first sub-element.
68. A substrate according to any of clauses 64 to 67, wherein said first sub-elements each comprise a solid or single filled structure.
69. A substrate according to any of clauses 64 to 67, wherein said first sub-elements each comprise a periodic structure.
70. A substrate according to any of clauses 55 to 69, wherein said focus target is paired on said substrate with a mirrored or 180 degree rotated version of said focus target.
71. A substrate according to any of clauses 55 to 70, wherein said focus parameter describes a focus setting of a lithographic exposure process in which said focus target was exposed.
72. A patterning device comprising target patterning features for forming said focus target on said substrate to obtain the substrate of any of clauses 34 to 71
73. A method of measuring a focus parameter from a focus target on a substrate according to any of clauses 55 to 72, comprising: imaging Moiré fringes resulting from interference between at least one diffraction order from the main feature and at least one corresponding diffraction order from the reference feature to obtain a measurement signal; and determining said focus parameter from said measurement signal.
74. A computer program comprising program instructions operable to cause a metrology apparatus to perform the method of any of clauses 1 to 33, when run on a suitable apparatus.
75. A non-transient computer program carrier comprising the computer program of clause 74.
76. A processing arrangement comprising: the non-transient computer program carrier of clause 74; and a processor operable to run said computer program.
77. A metrology apparatus comprising the processing arrangement of clause 76.
78. A lithographic apparatus comprising: a patterning device support for supporting a patterning device; a substrate support for supporting a substrate; wherein the lithographic apparatus is operable to perform use the determined focus parameter value in correcting a further lithographic exposure.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), A well A particle beams, such A ion beams or electron beams.
The term “lens”, where the context allows, may refer to any one or combination of various types of components, including refractive, reflective, magnetic, electromagnetic and electrostatic components.
The foregoing description of the specific embodiments will so fully reveal the general nature of the invention 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 invention. 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. It is to be understood that the phraseology or terminology herein is for the purpose of description by example, and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.
The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21216952.8 | Dec 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/083486 | 11/28/2022 | WO |
