Method and apparatus for lithographic imaging

Abstract

A method for compensating for focus deviations on a substrate having multiple layers thereon includes generating a focus prediction map for the substrate. In one approach, the focus prediction map is generated by acquiring key performance indicator (KPI) data on the substrate using an alignment sensor, determining a correlation between the KPI data and focus offset data for locations on the substrate, and using the correlation and the KPI data to generate a focus prediction map for the substrate. In another approach, the prediction map is generated by acquiring a first layer height map for a first layer, measuring a second layer height map for a second layer above the first layer with a level sensor, and subtracting the first height map from the second height map to obtain a delta height map for the substrate.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to provisional patent application US63 / 291,634, filed December 20, 2021, which is incorporated by reference in its entirety.

[0002] [Technical field] The present disclosure relates generally to metrology methods and tools for use in a lithographic apparatus, and more particularly to methods and tools that provide focus correction functionality. [Background technology]

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). In this case, a patterning device, also denoted as a mask or reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern may be transferred onto a target portion (e.g. comprising part of a die, a die, or several dies) on the 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, which are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is illuminated by exposing the entire pattern onto the target portion at once, and so-called scanners, in which each target portion is illuminated by scanning the pattern through a radiation beam in a given direction (the "scanning" direction) while simultaneously scanning the substrate parallel or non-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Summary of the Invention [Problem to be solved by the invention]

[0004] In order to allow many patterned layers to be placed on the substrate, it is desirable to accurately position the substrate relative to the radiation beam and the patterning device. This may be done by accurately positioning the substrate on a substrate table, and accurately positioning the substrate table relative to the radiation beam and the patterning device.

[0005] Alignment of the substrate may be performed. In an alignment system, a number of alignment marks on the substrate are measured to obtain a coordinate system which is compared to a modeled grid to obtain the positions of features on the substrate. Wafer distortions occurring during clamping of the substrate to the substrate table or during non-lithographic processing steps may result in distortions of the substrate which may be monitored by comparing the measurements to the grid. A model may be generated describing the wafer grid which is used when exposing the wafer to compensate for the distortions.

[0006] One characteristic of particular interest is overlay (i.e., the alignment of successive layers formed on a substrate). Measurement of overlay may be performed using a modeled grid as described above. A grid model describing the overlay error across a substrate relative to a previous layer may be generated and used in a control loop to ensure lot-to-lot consistency. [Means for solving the problem]

[0007] The present description relates to a method and apparatus for compensating for focus deviations on a substrate having multiple layers thereon, the method and apparatus including: acquiring Key Performance Indicator (KPI) data for each of multiple alignment marks on the substrate using an alignment sensor of a photolithography apparatus; determining a correlation between the KPI data and focus offset data for positions corresponding to positions of the multiple alignment marks on the substrate; and using the correlation and the KPI data to generate a focus prediction map for the substrate. In one embodiment, the positions on a focus grid model may be used instead of the positions of the alignment marks themselves.

[0008] In one aspect, a method is provided for compensating for focus deviations on a substrate having multiple layers thereon, the method including obtaining a first layer height map for a first layer of the multiple layers, measuring a second layer height map for a second layer above the first layer of the multiple layers using a level sensor, subtracting the first layer height map from the second layer height map to obtain a delta height map for the substrate, and using the delta height map to generate a focus prediction map for the substrate.

[0009] In one aspect, there is provided a method for manufacturing a device in which a device pattern is applied to a series of substrates using a lithographic process, the method comprising exposing the device pattern onto the substrates using the methods described herein and correcting focus for subsequent layers.

[0010] In one aspect, a non-transitory computer program product is provided comprising machine-readable instructions configured to cause a processor to perform the methods described herein. [Brief description of the drawings]

[0011] Embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

[0012] FIG. 1 depicts a schematic diagram of an embodiment of a lithographic apparatus.

[0013] FIG. 2 illustrates a schematic of one embodiment of a lithographic cell or cluster.

[0014] FIG. 3 illustrates diagrammatically an embodiment of a scatterometer for use as a metrology device.

[0015] FIG. 4 depicts diagrammatically another embodiment of a scatterometer for use as a metrology device.

[0016] FIG. 5 shows a composite metrology target formed on a substrate.

[0017] FIG. 6 shows the correlation between the measured wafer quality and μDBO (microdiffraction based overlay) measurement results.

[0018] FIG. 7 illustrates a workflow according to one embodiment.

[0019] FIG. 8 illustrates a workflow according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Before describing the embodiments in detail, an example of an environment in which the embodiments may be implemented is presented for reference.

[0021] 1 illustrates diagrammatically a lithographic apparatus LA. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. DUV or EUV radiation), a support structure (e.g. a mask table) MT configured to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters, a substrate table (e.g. a wafer table) WTa configured to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern formed in the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0022] Illumination systems may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping or controlling radiation.

[0023] The patterning device support structure 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 whether or not the patterning device is held in a vacuum environment. The patterning device support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support structure may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support structure may ensure that the patterning device is at a desired position relative to the projection system or the like. Any use of the terms "reticle" or "mask" herein may be interpreted as synonymous with the more general term "patterning device".

[0024] The term "patterning device" as used herein should be broadly interpreted as referring to any device that can be used to generate a pattern in the cross-section of a radiation beam, for example to generate a pattern in a target portion of a substrate. It should be noted that the pattern generated in 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 generated in the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0025] A patterning device may be of a transmissive or reflective type. 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 that can be individually tilted to reflect an incoming beam in different directions. The tilted mirrors form a pattern in a radiation beam that is reflected by the mirror matrix.

[0026] The term "projection system" as used herein should be interpreted broadly to encompass any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, appropriate for the exposure radiation used, or other factors such as the use of an immersion liquid or the use of a vacuum. Use of the term "projection lens" herein may be interpreted as synonymous with the more general term "projection system".

[0027] As here depicted, the apparatus can be of a transmissive type (e.g. employing a transmissive mask) or it may be of a reflective type (e.g. employing a programmable mirror array of a type as described above, or a reflective mask).

[0028] The lithographic apparatus may be of a type having two (dual stage) or more tables (e.g. two or more substrate tables, two or more patterning device support structures, or a substrate table and a metrology table). In such a "multiple stage" apparatus, the additional tables may be used in parallel, and preparatory steps may be carried out on one or more tables while one or more of the other tables are being used for exposure.

[0029] The lithographic apparatus may be of a type in which at least a part of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill a space between the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example between the mask and the projection system. Immersion techniques are well known techniques for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not imply that a structure such as the substrate has to be submerged in liquid, but only that a liquid is located between the projection system and the substrate during exposure.

[0030] With reference to Figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not to be construed as forming part of the lithographic apparatus, and the radiation beam is passed from the source SO to the illuminator IL by a beam delivery system BD, which may include, for example, suitable directing mirrors and / or beam expanders. In other cases, for example when the source is a mercury lamp, the source may be part of the lithographic apparatus. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0031] 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 "σ-outer" and "σ-inner", respectively) of the intensity distribution in a pupil plane of the illuminator may 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 adjust the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0032] The radiation beam B is incident on a patterning device (e.g. mask) MA, which is held on a patterning device support (e.g. mask table MT), and is patterned by the patterning device. Having passed through the patterning device (e.g. mask) MA, the radiation beam B passes through a projection system PS which focuses the beam on a target portion C of the substrate W. By means of the second positioner PW and a position sensor IF (e.g. an interferometric device, a linear encoder, a two-dimensional encoder or a capacitive sensor), the substrate table WTa can be accurately driven (e.g. to position different target portions C on the path of the radiation beam B). Similarly, the first positioner PM and other position sensors (not explicitly shown in FIG. 1 ) can be used to accurately position the patterning device (e.g. mask) MA with respect to the path of the radiation beam B (e.g. after mechanical retrieval from a mask library or during a scan). In general, movement of the patterning device support (e.g. mask table) MT may be realized by 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 WTa may 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 (but not a scanner), the patterning device support (e.g. mask table) MT may be connected to a short-stroke actuator only, or may be fixed.

[0033] The 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 in the illustrated example occupy dedicated target portions, they may be located in spaces between the target portions (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 be included among the device features within a die. In this case it is desirable for the markers to be as small as possible and not require different imaging or process conditions than nearby features. Alignment systems for detecting alignment markers are described further below.

[0034] The depicted apparatus can be used in at least one of the following modes: In step mode, the patterning device support (e.g. mask table) MT and the substrate table WTa are kept substantially stationary while the entire pattern formed in the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure), and the substrate table WTa is then shifted in the X and / or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure. In scan mode, the patterning device support (e.g. mask table) MT and the substrate table WTa are scanned simultaneously while a pattern formed in the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WTa relative to the patterning device support (e.g. mask table) MT may be determined by the magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, while the length of the scanning motion determines the height (in the scanning direction) of the target portion. In another mode, the patterning device support (e.g. mask table) MT is kept substantially stationary holding a programmable patterning device and the substrate table WTa is driven or scanned while a pattern formed in the radiation beam is projected onto a target portion C. In this mode, a pulsed radiation source is typically employed and the programmable patterning device is updated as required after each movement of the substrate table WTa or in between successive radiation pulses during a scan. This mode of operation can readily be applied to maskless lithography employing a programmable patterning device such as a programmable mirror array of the type described above.

[0035] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0036] The lithographic apparatus LA is of the so-called dual stage type, having two tables WTa, WTb (e.g. two substrate tables) and two stations (i.e. an exposure station and a measurement station between which the tables can be exchanged). For example, while one substrate on one table is being exposed in the exposure station, another substrate is loaded on the other substrate table in the measurement station and various preparation steps can be performed. The preparation steps may include mapping the surface control of the substrate using the level sensor LS and measuring the position of an alignment marker on the substrate using the alignment sensor AS. Both of these sensors are supported by a reference frame RF. If the position sensor IF cannot measure the position of the table while in the measurement station or the exposure station, a second position sensor may be provided to be able to track the position of the table in both stations. As another example, while a substrate on one table is being exposed in the exposure station, the other table without a substrate waits in the measurement station (where measurement activities may optionally take place). This other table may have one or more measurement devices and may optionally have other tools (e.g. cleaning devices). When a substrate has completed exposure, the table without the substrate is moved to, for example, an exposure station to perform a measurement, and the table with the substrate is moved to a location where the substrate is removed and another substrate is loaded (for example, a measurement station). These multiple table configurations can substantially increase the throughput of the apparatus.

[0037] As shown in FIG. 2, the lithographic apparatus LA may form part of a lithographic cell LC, sometimes denoted lithocell or lithocluster, which also includes apparatus for performing one or more pre-exposure and post-exposure processes on the substrate. Conventionally, these include one or more spin coaters SC for forming a resist layer, one or more developers DE for developing the exposed resist, one or more cooling plates CH, and one or more bake plates BK. A substrate handler or robot RO picks up the substrates from the input / output ports I / O1, I / O2, moves them between the different processing devices, and delivers them to a loading bay LB of the lithographic apparatus. These devices, often collectively referred to as tracks, are under the control of a track control unit TCU, which may itself be controlled by a supervisory control system SCS. The supervisory control system SCS also controls the lithographic apparatus via a lithography control unit LACU. In this way, the different apparatus may be operated to maximize throughput and processing efficiency.

[0038] In order for substrates exposed by a lithographic apparatus to be consistently and correctly exposed, it is desirable to inspect the exposed substrate to measure one or more characteristics, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. If an error is detected, adjustments may be made to the exposure of one or more subsequent substrates. This may be particularly useful if inspection can be performed quickly, for example while other substrates of the same batch are not yet exposed. Also, substrates that have already been exposed may be removed and reprocessed to improve yield, or may be discarded to avoid performing exposures on substrates that are found to be defective. If only some target portions of a substrate are defective, further exposures may be performed only on good target portions that are free of defects. Another possibility is to adjust the settings of subsequent processing steps to compensate for the error. For example, the time of a trim etch step may be adjusted to compensate for CD variations between substrates due to lithographic processing steps.

[0039] In an embodiment, the patterning device MA may be provided with a functional pattern (i.e. a pattern that forms part of an operational device). Alternatively or additionally, the patterning device may be provided with a measurement pattern that does not form part of the functional pattern. The measurement pattern may for example be arranged to one side of the functional pattern. The measurement pattern may for example be used to measure the alignment of the patterning device relative to the substrate table WT (see FIG. 1) of the lithographic apparatus or may be used to measure some other parameter (e.g. overlay). The techniques described here may be applied to such a measurement pattern.

[0040] According to various embodiments of the present invention, the measured or simulated features of the wafer and attributes of the lithography system may be used to update the design for the reticle to improve performance. In one example, the location of the metrology target (measurement pattern) may be positioned according to the measured and / or simulated features of the wafer such that the effects of the wafer features and attributes of the system are reduced. Alternatively, similar features of the wafer and / or lithography system may be used to update the position and / or orientation for the functional patterns.

[0041] By way of introduction, the operation of an inspection apparatus utilizing a metrology target is described. The inspection apparatus is used to determine one or more properties of a substrate, in particular how one or more properties of different substrates or different layers of the same substrate vary between layers and / or across substrates. The inspection apparatus may be integrated into the lithography apparatus LA or lithocell LC, or may be a stand-alone device. To enable the fastest measurements, it is desirable for the inspection apparatus to measure one or more properties in the exposed resist layer immediately after exposure. However, the latent image in the resist has very low contrast. There is only a very small refractive index difference between the parts of the resist that are exposed to radiation and the parts that are not exposed. Furthermore, not all inspection apparatuses have sufficient sensitivity to make useful measurements of the latent image. For this reason, measurements may be made after a post-exposure bake step (PEB), which is customarily the first step performed on an exposed substrate, enhancing the contrast between exposed and unexposed parts of the resist. At this stage, the image in the resist may be described as semi-latent. It is also possible to make measurements on 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 reworking defective substrates, but may still provide useful information for process control purposes, for example.

[0042] Figure 3 shows an embodiment of a scatterometer SM1. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate 6. The reflected radiation is passed to a spectrometer detector 4 which measures the spectrum 10 of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile on which the detected spectrum is based may be reconstructed by the processing unit PU, for example by Rigorous Coupled Wave Analysis (RCWA) and non-linear regression, or by comparison with a library of simulated spectra as shown in the lower part of figure 3. Typically, for reconstruction, the approximate morphology of the structure is known, some parameters are assumed from knowledge of the process which forms the structure, and only a few structure parameters remain to be determined from the scatterometry data. Such a scatterometer may be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0043] Another embodiment of a scatterometer SM2 is shown in Figure 4. In this device, radiation emitted by a radiation source 2 is collected using a lens system 12 through an interference filter 13 and a polarizer 17, reflected by a partially reflective surface 16 and collected on a substrate W via a microscope objective lens 15 having a high numerical aperture (NA), preferably at least 0.9 or 0.95. An immersion scatterometer may have a lens with a numerical aperture greater than 1. The reflected radiation is then transmitted through the partially reflective surface 16 to a detector 18 for detection of the scatter spectrum. The detector may be located in a back-projected pupil plane 11 at the focal length of the lens 15. However, the pupil plane may alternatively be re-imaged onto the detector 18 by auxiliary optics (not shown). The pupil plane is the plane in which the radial position of the radiation defines the angle of incidence and the angular position defines the azimuthal angle of the radiation. The detector is preferably a two-dimensional detector so that a two-dimensional angular scatter spectrum of the substrate target (i.e. a measurement of the intensity as a function of the scatter angle) can be measured. Detector 18 may be, for example, an array of CCD or CMOS sensors and may have, for example, an integration time of 40 milliseconds per frame.

[0044] For example, a reference beam is often used to measure the intensity of incident radiation. To do this, when a radiation beam is incident on the partially reflective surface 16, a part of it is transmitted through the surface as a reference beam directed to the reference mirror 14. The reference beam is then projected onto a different part of the same detector 18.

[0045] One or more interference filters 13 can be used to select the wavelength of interest, for example in the range 405-790 nm, or lower, such as 200-300 nm. The interference filters may be tunable instead of having a set of different filters. Alternatively or in addition to one or more interference filters, a grating may be used.

[0046] The detector 18 may measure the intensity of the scattered radiation at a single wavelength (or a narrow range of wavelengths), may measure the intensity separately at multiple wavelengths, or may measure the intensity integrated over a range of wavelengths. Additionally, the detector may separately measure the intensity of TM (Transverse Magnetic) and TE (Transverse Electric) polarized radiation and / or the phase difference between TM and TE polarized radiation.

[0047] It is also possible to use a broadband radiation source 2 (i.e., one with a wide range of radiation frequencies or wavelengths (hence colors)) that provides a large étendue and allows mixing of multiple wavelengths. Each of the multiple wavelengths in the broadband desirably has a bandwidth of δλ and a spacing of at least 2δλ (i.e., twice the wavelength bandwidth). Several radiation "sources" may be different parts of an extended radiation source, separated, for example, using fiber bundles. In this way, angle-resolved scattering spectra may be measured in parallel at multiple wavelengths. 3D spectra (wavelength and two different angles) may be measured, which contain more information than 2D spectra. This allows more information to be measured and increases the robustness of the measurement process. This is described in more detail in U.S. Patent Application Publication No. US2006-0066855, which is incorporated herein by reference in its entirety.

[0048] One or more properties of the substrate may be determined by comparing one or more properties of the beam before and after it is redirected by the target. This may be done, for example, by comparing the redirected beam with a theoretical redirected beam calculated using a model of the substrate, and searching for the model that gives the best fit between the measured and calculated redirected beam. Typically, a parameterized generic model is used, and the parameters of the model (e.g., width, height and sidewall angle of the pattern) are varied until the best match is obtained.

[0049] Two main types of scatterometers are used: Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered within a specific narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity (or intensity ratio and phase difference in the case of elliptically polarised structures) of the scattered radiation as a function of angle. Alternatively, measurement signals of different wavelengths may be measured separately and combined at the analysis stage. Polarised radiation may be used to generate multiple spectra from the same substrate.

[0050] To determine one or more parameters of the substrate, a best match is typically found between a theoretical spectrum generated from a model of the substrate and a measured spectrum as a function of wavelength (spectroscopic scatterometry) or angle (angular resolved scatterometry) generated by a redirected beam. There are various methods that may be combined to find the best match. For example, a first method is an iterative search method in which a first set of model parameters is used to compute a first spectrum and a comparison is made with the measured spectrum. Then a second set of model parameters is selected, a second spectrum is computed, and the second spectrum is compared with the measured spectrum. These steps are repeated with the goal of finding the set of parameters that gives the best matching spectrum. Typically, information from the comparison is used to guide the selection of subsequent sets of parameters. This process is known as an iterative search technique. The model with the set of parameters that gives the best match is taken to be the best description of the measured substrate.

[0051] The second method is to create a library of spectra, each corresponding to a particular set of model parameters. Typically, the set of model parameters is chosen to cover all or almost all possible variations in the substrate properties. The measured spectrum is compared to the spectra in the library. As with the iterative search method, the model with the set of parameters corresponding to the spectrum that gives the best match is taken to be the best description of the measured substrate. In this library search technique, interpolation techniques may be used to more accurately determine the best set of parameters.

[0052] In either method, enough data points (wavelengths and / or angles) in the calculated spectrum should be used to allow an accurate match, typically between 80 and 800 or more data points for each spectrum. Using an iterative method, each iteration for each parameter value involves calculations on 80 or more data points. This is multiplied by the number of iterations required to obtain the correct profile parameters. Thus, many calculations may be required. In practice, this leads to a compromise between accuracy and speed of processing. With the library approach, there is a similar compromise between accuracy and the time required to set up the library.

[0053] In any of the scatterometers described above, the target on the substrate W may be a grating printed such that after development, bars of solid resist lines are formed. The bars may alternatively be etched into the substrate. The target pattern is chosen to be sensitive to parameters of interest such as focus, dose, overlay, chromatic aberrations in the lithographic projection apparatus, etc., such that variations in the parameters of interest are manifested as variations in the printed target. For example, the target pattern may be sensitive to chromatic aberrations and illumination symmetry in the lithographic projection apparatus (particularly the projection system PL), the presence of such aberrations being manifested as variations in the printed target pattern. In this way, scatterometry data of the printed target pattern is used to reconstruct the target pattern. From knowledge of the printing steps and / or other scatterometry processes, parameters of the target pattern such as line width and shape may be input to the reconstruction process performed by the processing unit PU. The lines in the target may be constructed by subunits including near or sub-resolution features that together define the lines of the grating, as described in US Pat. No. 7,466,413.

[0054] Although a scatterometer embodiment has been described herein, in an embodiment other types of metrology devices may be used, for example a dark field metrology device such as that described in U.S. Patent No. 8,797,554, the entirety of which is incorporated herein by reference. Furthermore, these other types of metrology devices may use entirely different technologies than scatterometry.

[0055] Targets as described herein may be, for example, overlay targets designed for use in the "Yieldstar" standalone or integrated metrology tools and / or alignment targets as typically used with "TwinScan" lithography systems, both of which are available from ASML, Veldhoven, The Netherlands.

[0056] In general, metrology targets for use with such systems should be printed on the wafer with dimensions that satisfy the design specifications for the particular microelectronic device to be imaged on that wafer. As processes at advanced process nodes approach the imaging resolution limits of lithographic devices, design rules and process compatibility requirements put stress on the selection of an appropriate target. As the targets themselves advance, often requiring the use of resolution enhancement techniques such as phase shift masks and optical proximity correction, the printability of the target within the process design rules becomes uncertain. As a result, proposed marks may be subject to testing to ensure their validity from both a printability and detectability standpoint. In a commercial environment, good overlay mark detectability may be interpreted as a combination of low total measurement uncertainty and short move-acquire-move times (since slow acquisition negatively impacts the total throughput for a production line). Modern micro-diffraction based overlay targets (μDBOs) are typically 40x160μm in size as used in the context of wafer monitoring. 2The thickness of the target may be on the order of 10 μm on one side, providing an inherently lower detection signal compared to a target of 10 μm on the other side.

[0057] In addition, once a mark that satisfies the above criteria is selected, its detectability may vary with respect to process variations such as film thickness variations, various etching biases, and geometric asymmetries resulting from the etching and / or polishing process. For this reason, it may be useful to select a target that has low detectability variation and low overlay / alignment variation with respect to various process variations. Similarly, the fingerprint of the particular apparatus used to produce the microelectronic device to be imaged (e.g., printing characteristics including lens aberrations) generally affects the imaging and production of the target mark. Thus, it may be useful to ensure that the mark is resistant to the effects of the fingerprint, since some patterns are more or less susceptible to a particular lithographic fingerprint.

[0058] FIG. 5 shows a composite metrology target formed on a conventional substrate. The composite target comprises four gratings 32, 33, 34, 35 closely positioned within a measurement spot 31 formed by an illumination beam of a metrology device. Thus, all four targets are simultaneously illuminated and imaged onto the sensor 4, 18. In one example for overlay measurement, the gratings 32, 33, 34, 35 are themselves composite gratings formed by overlapping gratings patterned in different layers of a semiconductor device formed on the substrate W. The gratings 32, 33, 34, 35 may have different biased overlay offsets to facilitate measurement of overlay between the layers on which different parts of the composite grating are formed. The gratings 32, 33, 34, 35 may differ in their orientation as shown to diffract incoming radiation in the X and Y directions. In one example, the gratings 32 and 34 are X-direction gratings with biases of +d and -d, respectively. This means that the grating 32 has components arranged so that if both were printed exactly at their nominal positions, one of the components would be offset relative to the other by a distance d. The grating 34 has components arranged so that if printed perfectly, there would be an offset of d in the opposite direction from the first grating, etc. The gratings 33 and 35 may be Y-direction gratings with offsets of +d, -d, respectively. Although four gratings are illustrated, other embodiments may include larger matrices to obtain the desired accuracy. For example, a 3x3 array of nine composite gratings may have biases of -4d, -3d, -2d, -d, 0, +d, +2d, +3d, +4d. Separate images of these gratings may be identified in the images captured by the sensors 4, 18.

[0059] An example of an alignment sensor is a transmission image sensor used to determine the relative positions of a mask and a substrate. The mask comprises a mask mark. An image of the mask mark is formed by a projection system on a transmission image sensor (TIS) arranged on the substrate table using a radiation beam. The transmission image sensor (TIS) comprises a grating structure having transmissive and reflective (or absorptive) elements (e.g. a transmissive pattern in a layer of chrome). The transmissive elements correspond to the image when the image is focused and aligned with the grating structure. A detector (such as a photodiode) arranged to measure the intensity of the radiation behind the grating structure is arranged behind the grating structure.

[0060] When the image is focused and aligned on the structure, all of the radiation passes through the structure resulting in maximum intensity at the detector. If the image is not focused or aligned on the grating structure, some of the radiation will hit the reflective (or absorptive) elements resulting in a lower intensity measured by the detector behind the structure.

[0061] At several relative positions between the mask and the substrate stage, the intensity of the radiation passing through the mask mark and the grating is measured by the detector in order to find the position where the measured intensity is maximum. This relative position corresponds to the mask mark being focused and aligned to a structure of a transmission image sensor (TIS). Alternative embodiments are also possible. For example, a mark may be provided on the mask table or on a reference part on the mask table in order to align the mask table to the substrate table. Also, several marks may be present on the mask or on the mask table and several reference parts may be present on the mask table in order to qualify the projection system. This may be done, for example, by determining the magnification of the projection system by measuring the positions of the images of several mask marks on the mask one by one using a grating structure and a detector and comparing the distance between the measured positions with the distance of several marks on the mask. Also, several grating structures may be present on the substrate table in order to determine the rotation of the mask relative to the substrate table.

[0062] Another type of alignment sensor that may be used is a diffractive alignment sensor, which measures the position of an alignment mark. During alignment, the alignment mark is illuminated with an alignment beam of radiation. The alignment beam of radiation is diffracted by the alignment mark into several diffraction orders, such as +1, -1, +2 and -2. Each set of corresponding diffraction orders (e.g. +1 and -1) is used to form an image of the alignment mark on a reference plate using optical elements. The reference plate comprises a reference grating for each set of corresponding diffraction orders to be measured. Behind each reference grating, a separate detector is provided to measure the intensity of the radiation in the image passing through the reference grating. By moving the alignment mark relative to the reference plate, a position with the highest intensity for one or more images (giving an aligned position) is found.

[0063] Radiation in the "zeroth diffraction order" that reaches the detector (e.g., radiation scattered from the edge of a spot mirror, radiation scattered from surface roughness (random scattering), radiation scattered from target edges, etc., that does not contain signal information related to the parameter being measured) limits the dynamic range of the sensor. Wafer quality (WQ) varies as a result of the presence of these structures and can be significantly reduced as a result. Wafer quality is a measure (ratio) of the actual alignment signal strength to the signal generated by the fiducial marks.

[0064] To improve performance, the intensity of several images is measured and the alignment beam of radiation consists of multiple colors.

[0065] Another type of alignment sensor is a self-referencing sensor, for example as described in detail in European Patent EP1372040, which is incorporated herein by reference in its entirety.

[0066] As the number of layers on a wafer increases, material thickness variations across the wafer surface can accumulate; that is, some portions or locations of the wafer may have one or more layers that are thicker than other locations. This may be due to a single layer having a different thickness across the die, or several layers stacked together, each with a thickness that varies across the die. The variation in thickness may be large enough that the system's ability to focus on the surface is affected. The thicker portions may be defocused in the negative z direction, while the thinner portions may be defocused in the positive z direction.

[0067] Generally, wafer quality is reduced by the presence of multiple layers of structures on the wafer. Alignment marks are generally on the base layer of the wafer and are therefore partially or completely covered by subsequent layers that are deposited. Thus, interference can occur due to the interaction of alignment light passing through the layers and alignment light that is partially reflected at the interfaces, such as due to changes in refractive index between the layers. Similarly, surface roughness, material type, and other considerations can affect the strength of the signal acquired by the alignment system.

[0068] In some applications, the associated alignment marks may be on a layer above the base layer, and the top set of alignment marks may be located on any of several layers on the wafer.

[0069] The inventors have found that monitoring wafer quality can provide information about film thickness on the surface of the wafer. In particular, a correlation can be determined between signal strength and / or diffraction efficiency and thickness of structures on the wafer, which correlation can be used to correct the focus for imaging of subsequent layers. Wafer quality or signal strength constitutes one KPI (Key Performance Indicator) that can be used to characterize the response of the alignment system measurements, although in principle other KPIs may be used.

[0070] In one embodiment of the method, raw focus data is collected for a wafer with multiple layers stacked, for example using μDBO measurements over multiple points per wafer. In a typical application, dozens (e.g., 40) of points may be measured per wafer, each point may be measured using one or more alignment wavelengths and one or more alignment polarizations. For a given alignment wavelength and polarization, it is possible to fit a function that correlates μDBO measurements and wafer quality, as shown in FIG. 6. The function may be a 2D first order polynomial function as illustrated, although higher order functions may be used as appropriate.

[0071] In an embodiment, multiple wavelengths and / or polarizations may be used in combination, for example, the edge of the wafer may be measured at a different alignment system wavelength than the center of the wafer.

[0072] The μDBO data can be mapped to locations on the surface of the wafer, and using the previously determined correlation, the wafer quality data can be used to correct focus to achieve improved uniformity of performance.

[0073] When generating wafer quality data, it may be useful to include a step in which intra-field height data is taken into account. On the surface of a wafer, several substantially identical dies are typically produced during the manufacturing process. Each die may have some height variation within it, but this variation should be uniformly repeatable between dies. So, for each imaging field, there is a surface height variation, but this variation is consistent across the field.

[0074] As illustrated in Fig. 7, raw focus data 102 is collected using a µDBO measurement using system 100. Intrafield data 103 is then removed from the raw data to generate smoothed inter-field data 104. The smoothed inter-field data is then used to modify a focus prediction map for use in imaging subsequent layers. That is, the normal leveling and wafer quality data 106 used to generate a focus recipe for the subsequent layer is modified by the addition of information acquired with the smoothed inter-field data to generate a delta focus prediction 108.

[0075] 8 illustrates a production process integrating focus correction according to one embodiment. A photolithography system 110 acquires leveling and wafer quality information similar to that in a conventional imaging process. Focus mapping as illustrated above is applied to generate a delta focus prediction 108. The delta focus prediction 108 may then be used as a correction to the imaging process for subsequent layers, and the system generates a focus corrected wafer 112. In one embodiment, the delta focus prediction may be made based solely on wafer quality data as described above, or in combination with additionally measured level sensor data.

[0076] In one embodiment, a delta focus map may be generated more directly. In this approach, a leveling map is generated or obtained for a first lower layer (which may be, for example, the bottom layer). In principle, this layer may be any layer on which layers are stacked, and does not have to be the exact bottom layer. Generally, the top layer in the stack, which also includes alignment marks, is the one used to generate a baseline height map. In a typical application, once the lower levels are covered, it may be difficult to obtain measurements of that layer, but if a height map has been generated in an earlier step, it may be stored or made available for use in a later stage. This baseline measurement may be made, for example, using a level sensor, but may be generated using any height mapping sensor, including an air gauge. A leveling map for a second higher layer (which may be, for example, the top layer) is generated in a similar manner. Generally, the second layer may be any layer that is directly or indirectly above the first layer. The top layer is generally the layer that is most easily measured, but in principle other layers may be used as the second layer. The difference between the maps is a delta focus map that can be used in a feedforward manner similar to that produced by the methods described above.

[0077] Contrast as discussed herein includes, for the aerial image, image log slope (ILS) and / or normalized image log slope (NILS), and for the resist, dose sensitivity and / or exposure latitude.

[0078] As used herein, the terms "optimize," "optimizing," and "optimization" refer to adjusting lithographic process parameters such that the lithographic result and / or process has more desirable characteristics (such as greater accuracy in the projection of the design layout onto the substrate, a larger process window, etc.).

[0079] An embodiment of the present invention may take the form of a computer program comprising one or more sequences of machine-readable instructions describing the method as disclosed herein, or a data storage medium having such a computer program stored thereon (e.g., a semiconductor memory, a magnetic or optical disk). Further, the machine-readable instructions may be embodied in two or more computer programs. The two or more computer programs may be stored on one or more different memories and / or data storage media.

[0080] This computer program may for example be contained within or outside the imaging device of Figure 1 and / or the control unit LACU of Figure 2. For example, where an existing device of the type shown in Figures 1 and 2 is already in production and / or in use, an embodiment may be implemented by the provision of an updated computer program product to cause a processor of the device to carry out the methods as described herein.

[0081] Any controllers described herein may operate, alone or in combination, when one or more computer programs are read by one or more computer processors located in at least one component of the lithographic apparatus. The controllers, alone or in combination, may have any suitable configuration for receiving, processing, and transmitting signals. The one or more processors are configured to communicate with at least one controller. For example, each controller may include one or more processors for executing computer programs including machine-readable instructions for the methods described above. The controllers may include data storage media for storing such computer programs and / or hardware for receiving such media. In this manner, the controllers may operate according to machine-readable instructions of one or more computer programs.

[0082] Although specific reference may have been made to the use of an embodiment in the context of lithography using radiation, it will be understood that an embodiment of the present invention may be used in other applications such as imprint lithography and is not limited to lithography using radiation where the context permits. 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 and the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is then removed from the resist leaving a pattern behind after the resist has been cured.

[0083] Furthermore, although specific reference may have been 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. Other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like. In the context of such alternative applications, those skilled in the art will appreciate that the use of the terms "wafer" or "die" herein may be construed as synonymous with the more general terms "substrate" or "target portion", respectively. Substrates referred to herein may be processed, before or after exposure, for example, in a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein may be applied to such other substrate processing tools. Furthermore, a substrate may be processed multiple times, for example to form a multi-layer IC, and the term "substrate" as used herein may refer to a substrate already including multiple processed layers.

[0084] The patterning devices described herein may be referred to as lithographic patterning devices. Thus, the term "lithographic patterning device" may be interpreted to mean a patterning device suitable for use in a lithographic apparatus.

[0085] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of about 365, 355, 248, 193, 157 or 126 nm), extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), and particle beams such as ion beams or electron beams.

[0086] The term "lens", where the context allows, may refer to each or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0087] References to the described embodiments, and to "embodiments," "examples," and the like in the specification, indicate that the described embodiments may include a particular feature, structure, or characteristic, but that not all embodiments necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Moreover, when a particular feature, structure, or characteristic is described with respect to one embodiment, it is understood that it is within the knowledge of one of ordinary skill in the art to enable such feature, structure, or characteristic with respect to other embodiments, whether or not expressly stated.

[0088] The above description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims presented below. For example, one or more aspects of one or more embodiments may be combined with or substituted for one or more aspects of one or more other embodiments, where appropriate. Such adaptations and modifications are therefore intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is understood that the expressions or terms herein are intended to be descriptive by way of example and are not limiting of the invention, and that the terms or expressions herein will be interpreted by those skilled in the art under the teaching and guidance. The breadth and scope of the invention should not be limited by any of the exemplary embodiments described above, but should be determined solely in accordance with the following claims and their equivalents. Other aspects of the invention are presented as in the following numbered items. Item 1: 1. A method for compensating focus deviations on a substrate having a plurality of layers present thereon, comprising the steps of: acquiring KPI (Key Performance Indicator) data for each of a plurality of alignment marks on the substrate using an alignment sensor of a photolithography apparatus; determining a correlation between the KPI data and focus offset data for a position on the substrate; and generating a focus prediction map for the substrate using the correlation and the KPI data; and A method for providing the above. Item 2: 2. The method of claim 1, wherein the positions on the substrate correspond to positions of the plurality of alignment marks. Item 3: 2. The method of claim 1, wherein the positions on the substrate correspond to positions defined by a focus model for the substrate. Item 4: 2. The method of claim 1, wherein the KPI data is signal strength data. Item 5: 2. The method of claim 1, wherein the correlation is process and layer dependent. Item 6: 2. The method of claim 1, wherein the correlation is expressed as a mathematical function. Item 7: 7. The method of claim 6, wherein the correlation is expressed as a first order 2D polynomial function. Item 8: and removing at least a portion of intra-field focus data from the KPI data to generate smooth inter-field focus data prior to determining the correlation. using the correlation and the KPI data comprises using the smooth inter-field focus data. The method according to item 1. Item 9: 2. The method of claim 1, wherein the KPI data is obtained for each of the plurality of alignment marks using multiple wavelengths of alignment sensor radiation. Item 10: Item 10. The method of item 1, wherein the generating further comprises using level sensor data measured from the substrate. Item 11: Item 10. The method of item 1, further comprising using the focus prediction map to adjust focus for an imaging process of a subsequent layer of the substrate. Item 12: 1. A method for compensating focus deviations on a substrate having a plurality of layers present thereon, comprising the steps of: obtaining a first layer height map for a first layer of the plurality of layers; measuring a second layer height map for a second layer above the first layer of the plurality of layers using a level sensor; subtracting the first layer height map from the second layer height map to obtain a delta height map for the substrate; using the delta height map to generate a focus prediction map for the substrate; A method for providing the above. Item 13: Item 13. The method of item 12, wherein obtaining the first layer height map comprises reading stored height map data for the first layer. Item 14: Item 13. The method of item 12, wherein the first layer height map is obtained using a level sensor. Item 15: the first layer being a top layer containing alignment or overlay metrology marks; the second layer being the layer that is exposed in a subsequent exposure operation; Item 13. The method according to item 12. Item 16: Item 13. The method of item 12, further comprising using the focus prediction map to adjust focus for an imaging process of a subsequent layer of the substrate.

Claims

1. 1. A method for compensating focus deviations on a substrate having a plurality of layers present thereon, comprising the steps of: acquiring KPI (Key Performance Indicator) data for each of a plurality of alignment marks on the substrate using an alignment sensor of a photolithography apparatus; determining a correlation between the KPI data and focus offset data for a position on the substrate; and generating a focus prediction map for the substrate using the correlation and the KPI data; and A method for providing the above.

2. The method of claim 1 , wherein the locations on the substrate correspond to locations of the plurality of alignment marks.

3. The method of claim 1 , wherein the positions on the substrate correspond to positions defined by a focus model for the substrate.

4. The method of claim 1 , wherein the KPI data is signal strength data.

5. The method of claim 1 , wherein the correlation is process and layer dependent.

6. The method of claim 1 , wherein the correlation is expressed as a mathematical function.

7. The method of claim 6 , wherein the correlation is expressed as a first order 2D polynomial function.

8. and removing at least a portion of intra-field focus data from the KPI data to generate smooth inter-field focus data prior to determining the correlation. using the correlation and the KPI data comprises using the smooth inter-field focus data. The method of claim 1.

9. The method of claim 1 , wherein the KPI data is obtained for each of the plurality of alignment marks using multiple wavelengths of alignment sensor radiation.

10. The method of claim 1 , wherein the generating further comprises using level sensor data measured from the substrate.

11. The method of claim 1 , further comprising using the focus prediction map to adjust focus for an imaging process of a subsequent layer of the substrate.

12. 1. A method for compensating focus deviations on a substrate having a plurality of layers present thereon, comprising the steps of: obtaining a first layer height map for a first layer of the plurality of layers; measuring a second layer height map for a second layer above the first layer of the plurality of layers using a level sensor; subtracting the first layer height map from the second layer height map to obtain a delta height map for the substrate; using the delta height map to generate a focus prediction map for the substrate; A method for providing the above.

13. The method of claim 12 , wherein obtaining the first layer height map comprises reading stored height map data for the first layer.

14. The method of claim 12 , wherein the first layer height map is obtained using a level sensor.

15. the first layer being a top layer containing alignment or overlay metrology marks; the second layer being the layer that is exposed in a subsequent exposure operation; The method of claim 12.

16. The method of claim 12 , further comprising using the focus prediction map to adjust focus for an imaging process of a subsequent layer of the substrate.

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