Height Measurement Sensor

Abstract

A height measurement sensor comprising a projection and detection unit, the projection unit comprising a radiation source and a projection grating comprising a projection grating spot having a plurality of grating lines, the projection grating being arranged to receive the radiation and output a radiation beam to generate a radiation spot on the surface, the detection unit comprising a detection grating comprising a detection grating spot having a plurality of grating lines and a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot after passing through the detection grating spot, and a controller configured to (i) obtain an output signal of the detector comprising a plurality of periodic components, (ii) take the derivative of two points separated by a period of the periodic components at different positions of the output signal, and (iii) determine the focal plane of the sensor when the derivative changes sign.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to European Patent Application No. 22159153.0, filed February 28, 2022, which is incorporated herein by reference in its entirety.

[0002] [Technical field] The present invention relates to a height measurement sensor, in particular a height measurement sensor for a lithographic apparatus. [Background technology]

[0003] A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may project a pattern (often also referred to as a "design layout" or "design") in a patterning device (e.g., mask) onto a layer of radiation-sensitive material (resist) provided on the substrate (e.g., wafer).

[0004] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. Lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4-20 nm (e.g., 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate than lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

[0005] Low k1 lithography may be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such processes, the resolution equation may be expressed as "CD = k1 x λ / NA", where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optical elements in the lithographic apparatus, CD is the "critical dimension" (typically the smallest feature size to be printed, but in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it is to reproduce on a substrate a pattern resembling the shape and dimensions planned by the circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and / or the design layout. These include, but are not limited to, optical proximity correction (OPC, sometimes denoted as "optical and process correction") in the design layout, such as optimization of the NA, customized illumination schemes, use of phase-shifting patterning devices, various optimizations of the design layout, or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops for controlling the stability of the lithographic apparatus may be used to improve the repeatability of patterns at low k1.

[0006] Generally, the surface of the substrate onto which the pattern is to be projected is not perfectly flat. Furthermore, the substrate may exhibit thickness variations of several microns. This non-flatness and / or thickness variation of the substrate surface may result in an inaccurate projection of the pattern, for example due to focus or imaging errors. In order to correct the non-flatness and / or thickness variation of the substrate, it is known to use a height sensor (often denoted as a level sensor), which is desirably integrated into the lithographic apparatus. Such a height sensor may be used to determine a height map of the substrate before the pattern is transferred (e.g. projected) onto the substrate. This height map may then be used to correct the position of the substrate in order to keep the aerial image of the patterning device in proper focus during the transfer of the pattern onto the substrate. It is understood that "height" in this context refers broadly to a dimension out of plane relative to the substrate and does not imply a particular orientation relative to the earth or gravity.

[0007] A topography measurement system, level sensor or height sensor, is provided to measure the topography of the top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as a height map, may be generated from these measurements, showing the height of the substrate as a function of position on the substrate. This height map may then be used to correct the position of the substrate during transfer of the pattern onto the substrate, in order to provide an aerial image of the patterning device at a suitable focus position on the substrate. "Height" in this context is understood to broadly refer to the dimension out of plane relative to the substrate (also referred to as the Z-axis). Typically, a level or height sensor performs measurements at a fixed position (relative to its own optical system) and relative movement between the substrate and the optical system of the level or height sensor results in height measurements at positions across the substrate.

[0008] Typically, such a height sensor comprises a projection unit, which comprises a projection grating comprising an elongated grating that may be divided into a series of "spots" (herein also referred to as projected grating spots) with grating lines parallel to its axis, and an additional aperture comprising a single grating line arranged to output a capture spot (CS) radiation beam to form a capture spot arranged in front of the array of measurement spots on the substrate surface. Each projection grating spot is arranged to output a separate / discrete measurement spot (MS) radiation beam on the surface of the substrate to form a measurement spot. The height sensor also comprises a detection unit, which typically comprises a first detector for receiving the measurement spot radiation beam after reflection on the substrate and a second detector for receiving the capture spot radiation beam after reflection on the substrate. The processing unit calculates the height level based on the measurement spot radiation beam received by the detector. A change in the surface height results in a shift of the detected image. Summary of the Invention [Problem to be solved by the invention]

[0009] In the known height sensor mentioned above, the (MS) radiation beam is split into two branches at the detector. The light in the two branches is detected by photodiodes. The projected grating image shift is measured by the imbalance in the two branches (AC signal) divided by the total intensity detected in the two branches (DC signal). In a first approximation it can be expressed as: Z MS = gain MS *AC MS / DC MS (Here, gain MS is calibrated) Normalization of the AC signal by the DC signal (Zraw = AC / DC) makes the response of the height sensor independent of the surface reflectance, allowing for accurate calibration. However, normalization is not possible because Zraw MSThis makes the signal periodic as a function of the height of the substrate. The substrate may be a wafer supported by a wafer stage (WS). Accurate calibration and accurate height measurement are only performed at the center / correct period. This is when the complete projected grating image (MS radiation beam) is detected by the detection grating.

[0010] The measurement spots are positioned in a positive period by performing a capture scan. The height measurement of the capture spots is similar to that of the measurement spots: Z CS = gain CS *AC CS / DC CS During the capture scan, the wafer stage scans a certain range around the expected MS positive period and both MS and CS signals are recorded. The capture signal is not periodic (because the capture spot is constituted by a single slit) and an algorithm is used to detect the capture position. This is called "Zraw" which identifies the MS positive period by design. CS = 0(AC CS = 0). [Means for solving the problem]

[0011] The capture spot zero crossing is, by design, very close to the measurement spot zero crossing in the positive period. The inventors have identified that the signal can be significantly distorted due to local topography on the substrate and interactions with the processing layer stack at the CS and MS locations. In an example where the substrate is a wafer, the processing layer stack comprises a series of layers of the wafer having different transmittances. Local variations in transmittance can occur due to structures (e.g., memory cells, transistors, metal lines) printed on the wafer. Such signal distortion results in a significant shift of the CS and MS zero crossings. This prevents the capture algorithm from identifying the correct MS period.

[0012] In an embodiment of the invention, no capture spots are required since only the MS signal is used to identify positive cycles.

[0013] According to one aspect of the disclosure, there is provided a height measurement sensor for measuring a position of a surface of a substrate, the height measurement sensor comprising a projection unit and a detection unit, the projection unit comprising a radiation source and a projection grating comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to receive radiation emitted by the radiation source and output a radiation beam onto the surface to generate a radiation spot on the surface comprising the plurality of radiation lines, the detection unit comprising a detection grating comprising a detection grating spot comprising a plurality of detection grating lines and a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through the detection grating spot, and a controller configured to (i) obtain an output signal of the detector comprising a plurality of periodic components, (ii) take a derivative of two points separated by a period of the periodic components at different positions of the output signal, and (iii) determine that the substrate is located in a focal plane of the height measurement sensor when the derivative changes sign.

[0014] Thus, embodiments of the present invention ensure that the focal plane of the height measurement sensor can be reliably and accurately detected. As a result of the projection grating not needing a capture spot, the projection grating can be miniaturized. As the detection grating does not need a detection grating spot to pass the capture spot radiation beam reflected from the surface of the substrate, the detection grating can also be miniaturized. Furthermore, the detection unit is simplified in that a separate detector for measuring the capture spot radiation beam is not required.

[0015] The projected grating spot may have a certain total number of projected grating lines and the detected grating spot may have said total number of detected grating lines.

[0016] In some implementations, the projection grating comprises a plurality of projection grating spots, each of which comprises a plurality of projection grating lines, and the projection grating outputs a radiation beam to generate a plurality of radiation spots on the surface, each of which comprises a plurality of radiation lines.

[0017] The detection grating may comprise a number of detection grating spots, each of which comprises a number of detection grating lines.

[0018] The controller may be configured to control the height of the substrate relative to a reference height in a direction perpendicular to a surface of the substrate within a time window during which the output signal is acquired.

[0019] The controller may be able to control the height of the substrate relative to the reference height within a height range, and the controller may be configured to control the height of the substrate relative to the reference height within a sub-portion of said height range within said time window. In one example where the substrate is a wafer supported by a wafer stage (WS) of a lithographic apparatus, for throughput reasons the wafer stage may not scan its entire height range when determining the focal plane, but instead scan a smaller height range to save time. If the entire height range is scanned, the focal plane of the height measurement sensor can be identified when a maximum output signal from the detector is detected. If a smaller height range is scanned, this is not possible. By looking at the derivative of the output signal and identifying the point where the derivative of the output signal is zero, it can be ensured that the true maximum is detected, since the monotonicity of the output signal varies with the true maximum.

[0020] The controller may be configured to move the substrate in a direction perpendicular to the emission lines within the time window in which the output signal is acquired. That is, the substrate may be shifted in the y direction (substrate is scanned diagonally) while scanning in the z direction (substrate is raised) so that the same area on the substrate is illuminated. This increases the robustness of the method for determining the focal plane of the height measurement sensor by compensating for light moving on the substrate by making the method robust to reflectivity variations between the projected grating lines of the projected grating spots.

[0021] In some implementations, all of the multiple radiation lines of the radiation spot generated by the projection grating are detected by a detector at the focal plane of the height measurement sensor.

[0022] The detector may comprise an optical sensor (eg a photodiode) for measuring a reflected radiation beam that passes through the detection grating spot.

[0023] The period of the periodic component may depend on the angle of incidence of the radiation beam and the periodicity of the projected grating lines.

[0024] The radiation emitted by the radiation source may have a wavelength in the range of 600 nm to 1050 nm.

[0025] Alternatively, the radiation emitted by the radiation source may have a wavelength in the range of 100 nm to 400 nm.

[0026] According to another aspect of the present disclosure, there is provided a lithographic apparatus comprising a height measurement sensor according to any embodiment described herein.

[0027] According to another aspect of the present disclosure, there is provided an evaluation system including a height measurement sensor according to any embodiment described herein.

[0028] According to another aspect of the present disclosure, there is provided a metrology system comprising a height measurement sensor according to any embodiment described herein.

[0029] According to another aspect of the disclosure there is provided a method for determining a focal plane of a height measurement sensor comprising emitting radiation at a radiation source of the height measurement sensor onto a projection grating of the height measurement sensor comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to output a radiation beam onto the surface to generate a radiation spot on the surface of a substrate comprising a plurality of radiation lines, receiving at a detector of the height measurement sensor a reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through a detection grating spot of the detection grating comprising the plurality of detection grating lines, and a controller of the height measurement sensor configured to (i) obtain an output signal of the detector comprising a plurality of periodic components, (ii) take a derivative of two points separated by a period of the periodic components at different positions of the output signal, and (iii) determine the focal plane of the height measurement sensor when the derivative changes sign. [Brief description of the drawings]

[0030] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: FIG. 1 shows a schematic overview of a lithographic apparatus. FIG. 2 shows a schematic overview of a lithography cell. FIG. 3 shows a schematic representation of Holistic Lithography, which illustrates the collaboration between three key technologies to optimize semiconductor manufacturing. FIG. 4 illustrates a height measurement sensor. FIG. 5a illustrates an example of a projection grating. FIG. 5b illustrates the radiation spots produced on the surface of the substrate. FIG. 5c illustrates an example of a detection grid. Figures 6a-d illustrate how the measured reflected radiation varies as the substrate is moved upwards. FIG. 7a illustrates an example of an output signal of a detector. FIG. 7b illustrates an example of a differential signal. FIG. 8 illustrates diagonal scanning of a substrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (e.g., extreme ultraviolet radiation having a wavelength in the range of about 5-100 nm).

[0032] The terms "reticle," "mask," or "patterning device" as used in this text may be broadly interpreted to refer to a general patterning device that may be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created in a target portion of a substrate. The term "light valve" may also be used in this context. Besides the classic mask (transmissive or reflective; binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0033] 1 illustrates diagrammatically a lithographic apparatus LA, which includes an illumination system (also denoted illuminator) IL configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. mask table) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer table) WT configured to hold a substrate (e.g. resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. 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.

[0034] In operation, the illumination system IL receives a radiation beam from the radiation source SO, for example via the beam delivery system BD. The illumination system IL may include various types of optical components, or any combination thereof, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0035] The term "projection system" PS as used herein should be interpreted broadly to encompass various types of projection systems including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, appropriate for the exposure radiation being used, and / or other factors such as the use of an immersion liquid or vacuum. Use of the term "projection lens" herein may be interpreted as synonymous with the more general term "projection system" PS.

[0036] The lithographic apparatus LA may be of a type (also referred to as immersion lithography) 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 PS and the substrate W. More information about immersion techniques is given in US6952253, which is incorporated herein by reference.

[0037] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also referred to as "dual stage") In such a "multi-stage" apparatus, the substrate supports WT may be used in parallel and / or preparation steps for a subsequent exposure of a substrate W may be performed on a substrate W located on one substrate support WT while another substrate W on the other substrate support WT is being used to expose a pattern onto another substrate W.

[0038] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is provided to hold a sensor and / or a cleaning device. The sensor may be provided to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a number of sensors. The cleaning device may be provided to clean parts of the lithographic apparatus, for example parts of the projection system PS or parts of a system for providing immersion liquid. The measurement stage may move underneath the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0039] In operation, the radiation beam B is incident on a patterning device, such as a mask MA, which is held on a mask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. After the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of a substrate W. By means of the second positioner PW and the position measurement system IF, the substrate support WT may be accurately driven, for example to position different target portions C at focusing and alignment positions in the path of the radiation beam B. Similarly, the first positioner PM and suitable other position sensors (not explicitly shown in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may also be located in spaces between the target portions. The substrate alignment marks P1, P2 located between the target portions C are known as scribe-lane alignment marks.

[0040] As shown in Fig. 2, the lithographic apparatus LA may form part of a lithographic cell LC, which may also be denoted a lithocell or (litho)cluster and may also include apparatus for performing pre-exposure and post-exposure processing on a substrate W. Conventionally, these include a spin coater SC for forming a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a bake plate BK for adjusting the temperature of the substrate W, e.g. to adjust the solvent in the resist layer. A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between different processing equipment, and delivers the substrates W to a loading bay LB of the lithographic apparatus LA. The devices in the lithocell, which may also be collectively referred to as a track, are typically under the control of a track control unit TCU, which may itself be controlled, e.g., by a supervisory control system SCS, which may control the lithographic apparatus LA via a lithography control unit LACU.

[0041] In order for a substrate W to be exposed by the lithographic apparatus LA to be exposed accurately and consistently, it is desirable to inspect the substrate in order to measure properties of the patterned structures, such as overlay error between successive layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, particularly if inspection is performed before other substrates W of the same batch or lot are exposed or processed, adjustments may be made, for example, to the exposure of subsequent substrates or to other processing steps performed on the substrate W.

[0042] The inspection apparatus, which may also be denoted metrology apparatus or evaluation apparatus, is used to determine the properties of the substrate W, in particular the variation of the properties of different substrates W or the layer-to-layer variation of the properties for different layers of the same substrate W. The inspection apparatus may alternatively be configured to identify defects on the substrate W and may, for example, be part of a lithocell LC, integrated in the lithographic apparatus LA or may be a stand-alone device. The inspection apparatus may measure properties of the latent image (image in the resist layer after exposure), the semi-latent image (image in the resist layer after a post-exposure bake step PEB), the developed resist image (from which exposed or unexposed parts of the resist have been removed) and the etched image (after a pattern transfer step such as etching).

[0043] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, requiring high accuracy of the dimensions and placement of the structures on the substrate W. To achieve this high accuracy, three systems may be combined in a so-called "holistic" control environment, as shown diagrammatically in FIG. 3. One of these systems is the lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and a computer system CL (third system). The essence of such a "holistic" environment is to optimize the cooperation between these three systems to improve the overall process window and provide a tight control loop so that the patterning performed by the lithographic apparatus LA stays within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process leads to a defined result (e.g. a functional semiconductor device). Typically, within that range, variations of the process parameters in the lithographic or patterning process are allowed.

[0044] The computer system CL may use (part of) the design layout to be patterned to predict the resolution enhancement technique to be used and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings will achieve the maximum overall process window of the patterning process (indicated in FIG. 3 by the double-headed arrow at the first scale SC1). Typically, the resolution enhancement technique is provided to match the patterning capabilities of the lithography apparatus LA. The computer system CL may be used to detect (e.g., using input from the metrology tool MT) where in the process window the lithography apparatus LA is currently operating, for example, to predict whether defects are present due to non-optimal processing (indicated in FIG. 3 by the arrow pointing to "0" at the second scale SC2).

[0045] The metrology tool MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA, for example to identify possible drifts in the calibration status of the lithographic apparatus LA (indicated in FIG. 3 by multiple arrows at the third scale SC3).

[0046] 4 illustrates a level sensor LS (herein also referred to as height measurement sensor) according to an embodiment of the present invention. The level sensor LS comprises a projection unit 300 and a detection unit 350.

[0047] The projection unit 300 comprises a radiation source 302 and a projection grating 304. The radiation source 302 may be a narrowband or broadband radiation source, for example a polarized or unpolarized, pulsed or continuous supercontinuum light source, such as a polarized or unpolarized laser beam. The radiation source 302 may include multiple radiation sources, such as multiple LEDs, having different colors or wavelength ranges. The radiation source 302 of the level sensor LS is not limited to visible radiation, but may additionally or alternatively include UV and / or IR radiation, or any range of wavelengths suitable for reflection from the surface of the substrate. In one example, the radiation emitted by the radiation source 302 has a wavelength in the range of 600 nm to 1050 nm. In another example, the radiation emitted by the radiation source 302 has a wavelength in the range of 100 nm to 400 nm.

[0048] A projection grating 304 is provided to receive the radiation emitted by the radiation source 302. The projection grating 304 comprises one or more projection grating spots, each having a number of projection grating lines. The projection grating lines allow the radiation emitted by the radiation source 302 to pass through the projection grating 304, resulting in a beam of radiation having a periodically varying intensity. The beam of radiation having a periodically varying intensity is directed towards a measurement location on the surface of a substrate (e.g. a wafer) W. In contrast to known techniques, the projection grating 304 does not have a capture spot.

[0049] 4 illustrates a projection grating 304 comprising a single projection grating spot having three projection grating lines that pass a respective radiation beam 306a towards the surface of the substrate W. It will be understood that three projection grating lines is merely an example and that a projection grating spot may have more than two projection grating lines. Furthermore, the projection grating 304 may comprise multiple projection grating spots, each having multiple projection grating lines that pass a respective radiation beam towards the surface of the substrate W (thus illuminating different areas on the surface of the substrate).

[0050] Figure 5a shows an example of a projection grating 304. In the example shown in Figure 5a, the projection grating 304 comprises three projection grating spots 502. Each of the three projection grating spots comprises five projection grating lines 504 through which radiation from the radiation source 302 can pass. The five projection grating lines are separated by a pitch "p" which defines the periodicity of the projection grating lines.

[0051] As shown in Figure 4, the radiation beam 306a is directed at the surface of the substrate W. The radiation beam 306a may have an angle of incidence, relative to an axis normal to the substrate surface on which it is incident (the Z-axis), greater than 0 degrees and less than 90 degrees. The angle of incidence is typically between 70 degrees (or greater) and 80 degrees (or less). By way of example only, the angle of incidence may be 70 degrees or 78 degrees. In practice, the level sensor LS typically includes further optical elements along the path between the projection grating 304 and the substrate W. These further optical elements have been omitted from Figure 4 for simplicity of illustration.

[0052] When the radiation beam 306a is incident on the surface of the substrate W at a measurement location, a radiation spot (the area on the surface of the substrate W that is illuminated by the radiation source 302) is generated. The radiation spot comprises a plurality of radiation lines which correspond to the plurality of grid lines of the projected grating spot.

[0053] Figure 5b illustrates three radiation spots 512, each with five radiation lines 514 corresponding to the five projected grating lines 504, generated on the surface of the substrate W when the example projection grating 304 is used. For simplicity of illustration, the radiation lines 514 are shown having a rectangular shape corresponding to the projected grating lines 504. It will be understood that the shape of the radiation lines 514 shown in Figure 5b does not correspond to radiation beams 306a having angles of incidence in the typical range between 70 degrees (or greater) and 80 degrees (or less).

[0054] The radiation beam 306a is reflected by the surface of the substrate W at the measurement location and the reflected radiation beam 306b is directed to a detection unit 350. The detection unit 350 comprises a detection grating 354, a detector 356 and a processing unit 360.

[0055] A detection grating 354 is provided to receive the reflected radiation beam 306b. The detection grating is preferably substantially the same as the projection grating 304 and is divided to correspond to the spot array pattern. As such, the detection grating 354 comprises one or more detection grating spots, each having a number of detection grating lines. The detection grating lines allow the reflected radiation beam 306b to pass through the detection grating 354 towards the detector 356.

[0056] 4 illustrates a detection grating 354 comprising a single detection grating spot having three detection grating lines that pass the reflected radiation beam 306b towards the detector 356. It will be understood that three detection grating lines is merely an example and the detection grating spot may have more than two detection grating lines. Furthermore, the detection grating 354 may comprise multiple detection grating spots, each having multiple detection grating lines that pass a respective reflected radiation beam towards the detector 356.

[0057] Figure 5c shows an example of a detection grating 354. In the example shown in Figure 5c, the detection grating 354 comprises three detection grating spots 522. Each of the three detection grating spots comprises five detection grating lines 524. The five detection grating lines are separated by a pitch "p" that defines the periodicity of the detection grating lines.

[0058] In practice, the level sensor LS will typically include further optical elements along the path between the substrate W and the detection grating 354. These further optical elements have been omitted from Figure 4 for simplicity of illustration.

[0059] The detector 356 is configured to output an output signal to the controller 360. The detector 356 may generate a detector output signal indicative of the received light (e.g. indicative of the intensity of the received light) and may comprise a photodetector. Alternatively, the detector 356 may represent the spatial distribution of the received intensity and may comprise a camera. The detector DET may comprise any combination of one or more detector types. For each detection grating spot 522, the detector 356 may comprise a light sensor (or a number of light sensors) for measuring the reflected radiation beam passing through the respective detection grating spot 522. The light sensor may be a photodiode or any other light sensitive sensor. In contrast to known techniques, the detector 356 does not comprise a light detector for measuring the captured spot radiation beam after reflection on the substrate.

[0060] As will be explained in more detail below, the voltage level of the output signal corresponds to the intensity of the radiation measured by the detector 356. According to an embodiment of the invention, the controller 360 is configured to process the output signal received from the detector 356 to determine when the substrate W is located in the focal plane of the level sensor LS. The controller 360 is configured to control the height of the substrate W relative to a reference height in a direction perpendicular to the surface of the substrate W during the time window during which the output signal is acquired. That is, during the time window during which the output signal is acquired, the substrate W is moved upwards (z direction shown in FIG. 4) away from the reference height. The substrate may be a wafer supported by a wafer stage (WS). In these embodiments, the controller 360 is configured to raise (and lower) the wafer stage by communicating with an actuator (e.g., a Lorentz actuator) that controls the movement of the WS.

[0061] The functionality of the controller 360 may be implemented by a code (software) stored in a memory comprising one or more storage media and arranged for execution on a processor comprising one or more processing units. The code is configured to perform operations according to the embodiments discussed below when read from the memory and executed on the processor. Alternatively, it is not excluded that some or all of the functionality of the controller 360 is implemented by dedicated hardware circuits or configurable hardware circuits such as FPGAs.

[0062] Here, the substrate W is said to be in the focal plane of the level sensor LS when all of the radiation lines 514 are detected by the detector 356 (all of the radiation that generated the radiation lines 514 is reflected by the surface of the substrate W and passes through the detection grid lines 524 of the detection grid spots 522). It is understood that when substrates such as wafers are manufactured, their thickness may vary. Thus, when a wafer W is placed on a wafer stage WS in the lithographic apparatus LA, due to thickness variations, the surface of the wafer will not necessarily be at the same height above the reference height as a wafer previously placed on the wafer stage WS. An embodiment of the present invention allows each substrate to be accurately placed in the focal plane of the level sensor LS.

[0063] Figure 6a illustrates a scenario 600 in which no reflected radiation is detected by detector 356 through detection grid lines 524. In this scenario, the voltage level of the output signal output by detector 356 is zero. An example of an output signal 700 is shown in Figure 7a.

[0064] 6b illustrates a scenario 620 in which the substrate W has been moved upwards (positive z-direction) relative to scenario 600, and some of the emission lines 514 overlap with the detection grid lines 524. In this scenario, the voltage level of the output signal 700 output by the detector 356 increases, but is not at a maximum. FIG. 7a illustrates the voltage output in scenario 620 by dotted line 720. Because not all of the emission lines 514 overlap with the detection grid lines 524, the voltage level of the output signal 700 output by the detector 356 is not at a maximum. The circle 602 in FIG. 6b indicates that two of the emission lines 514 do not overlap with the detection grid lines 524.

[0065] Figure 6c illustrates a scenario 640 in which the substrate W has been moved further upwards (in the positive z direction) relative to scenario 620, such that all of the emission lines 514 completely overlap all of the detection grid lines 524. In this scenario, the voltage level of the output signal 700 output by the detector 356 has increased to a maximum. Figure 7a illustrates the voltage output in scenario 640 by the dotted line 740. The substrate W is now in the correct MS period as previously described.

[0066] FIG. 6d illustrates a scenario 660 in which the substrate W has been moved further upward (in the positive z direction) relative to scenario 640, such that some of the emission lines 514 no longer overlap with the detection grid lines 524. In this scenario, the voltage level of the output signal 700 output by the detector 356 is reduced but not zero (some of the emission lines 514 still overlap with the detection grid lines 524). FIG. 7a illustrates the voltage output in scenario 660 by the dotted line 760. Since not all of the emission lines 514 overlap with the detection grid lines 524, the voltage level of the output signal 700 output by the detector 356 is not at a maximum. The circle 604 indicates that two of the emission lines 514 do not overlap with the detection grid lines 524.

[0067] As the substrate W is moved upwards in the positive z-direction, the output signal 700 is provided to the controller 360 for processing. As shown in Figure 7a, the output signal 700 comprises multiple periodic components with increasing voltage steps corresponding to more radiation lines being detected by the detector 356 as the substrate moves upwards towards the focal plane and decreasing voltage steps corresponding to fewer radiation lines being detected by the detector 356 as the substrate moves upwards away from the focal plane. The period of the periodic components depends on the angle of incidence of the radiation beam 306a and the pitch "p" of the projected grating lines.

[0068] The controller 360 is configured to calculate the derivative of two points at different positions of the output signal 700 (where the two points are separated by a period of the periodic component) to obtain the derivative signal 702 shown in FIG. 7b. The two points separated by one period share a robust condition: one additional emission line is detected (or not detected) regardless of its intensity. Thus, when calculating the derivative of two points separated by one period, the sign of the derivative changes (from positive to negative) when all of the emission lines 514 completely overlap all of the detection grid lines 524 and the substrate is located in the focal plane. FIG. 7b illustrates the derivative changing sign at the focal plane position (defined as the substrate at Z=0). The controller 360 is configured to determine the focal plane of the level sensor when the derivative changes sign.

[0069] The process may be performed dynamically in that the controller 360 may take the differential signal 702 and detect when the differential signal changes sign as the substrate W is moved upwards in the positive z-direction. Having detected when the differential changes sign, the controller 360 may prevent further upward movement of the substrate W such that the substrate W is kept in the focal plane of the level sensor.

[0070] Alternatively, the controller 360 may process the output signal 700 after a scan has been performed in which the substrate is moved above the reference height (in the z direction shown in FIG. 4), and the controller 360 can subsequently control the substrate W to be in the determined focal plane.

[0071] As illustrated in Figure 8, within the time window in which output signal 700 is acquired, substrate W may be shifted in the y direction while scanning is performed in the z direction, such that the same area on substrate W is illuminated (substrate is scanned diagonally). In particular, Figure 8 illustrates components of radiation beam 306a incident on an area 802 of the surface of substrate W when substrate W is in a first position. After substrate W is moved upwards (z direction) and shifted to a second position (y direction), the same area 802 of the surface of substrate W is illuminated.

[0072] Although specific reference may be made in this text to a substrate being a wafer, embodiments also encompass measuring the position of a surface of a substrate other than a wafer.

[0073] 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. Possible 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, etc. In the context of such alternative applications, those skilled in the art will appreciate that the use of the terms "substrate" or "die" herein may be interpreted 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 (stack), and the terms "substrate" and "substrate surface" as used herein may refer to a substrate already including multiple processed layers and the surfaces of each of these layers.

[0074] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum or atmospheric (non-vacuum) conditions.

[0075] 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 understood that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0076] Although specific embodiments of the invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to one skilled in the art that changes may be made to the invention as described without departing from the scope of the claims set out below. Other aspects of the invention are set out in the following numbered paragraphs: Item 1: 1. A height measurement sensor for measuring a position of a surface of a substrate, comprising: A projection unit; A detection unit; Equipped with The projection unit includes: A radiation source; a projection grating comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to receive radiation emitted by the radiation source and to output a radiation beam onto the surface to generate a radiation spot on the surface comprising a plurality of radiation lines; Equipped with The detection unit includes: a detection grating comprising a detection grating spot comprising a plurality of detection grating lines; a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through the detection grating spot; a controller configured to: (i) obtain an output signal of the detector having a plurality of periodic components; (ii) take the derivative of two points at different positions of the output signal that are separated by a period of the periodic components; and (iii) determine a focal plane of the height measurement sensor when the derivative changes sign; Equipped with Height measurement sensor. Item 2: the projected grating spot has a total number of projected grating lines; the detection grating spot has the total number of detection grating lines; 2. The height measuring sensor according to item 1. Item 3: the projection grating comprises a plurality of projection grating spots, each of which comprises a plurality of projection grating lines; the projection grating outputs the radiation beam to generate a plurality of radiation spots on the surface, each radiation spot comprising a plurality of radiation lines. 3. A height measuring sensor according to item 1 or 2. Item 4: 4. A height measurement sensor according to any one of items 1 to 3, wherein the detection grating comprises a plurality of detection grating spots, each of which comprises a plurality of detection grating lines. Item 5: 5. A height measurement sensor as described in any of items 1 to 4, wherein the controller is configured to control the height of the substrate relative to a reference height in a direction perpendicular to the surface of the substrate within a time window in which the output signal is acquired. Item 6: the controller is operable to control the height of the substrate relative to the reference height within a range of heights; the controller is configured to control, within the time window, the height of the substrate relative to the reference height within a sub-portion of the height range. 6. A height measuring sensor according to any one of items 1 to 5. Item 7: 7. The height measurement sensor of claim 5 or 6, wherein the controller is configured to move the substrate in a direction perpendicular to the plurality of radial lines within the time window during which the output signal is acquired. Item 8: 8. A height measuring sensor according to any one of items 1 to 7, wherein in the focal plane of the height measuring sensor, all of the multiple radiation lines of the radiation spot generated by the projection grating are detected by the detector. Item 9: 9. A height measurement sensor according to any of the preceding claims, wherein the detector comprises an optical sensor for measuring the reflected radiation beam passing through the detection grating spot. Item 10: 10. The height measurement sensor according to item 9, wherein the optical sensor is a photodiode. Item 11: 11. A height measurement sensor according to any of items 1 to 10, wherein the period of the periodic component depends on the angle of incidence of the radiation beam and the periodicity of the projected grating lines. Item 12: 12. A height measuring sensor according to any one of items 1 to 11, wherein the radiation emitted by the radiation source has a wavelength in the range of 600 nm to 1050 nm. Item 13: 12. A height measuring sensor according to any one of items 1 to 11, wherein the radiation emitted by the radiation source has a wavelength in the range of 100 nm to 400 nm. Item 14: Item 14. A lithographic apparatus comprising a height measurement sensor according to any of items 1 to 13. Item 15: 14. An evaluation system comprising a height measurement sensor according to any one of items 1 to 13. Item 16: 14. A measurement system comprising a height measuring sensor according to any one of items 1 to 13. Item 17: 1. A method for determining a focal plane of a height measurement sensor, comprising: - emitting radiation at a radiation source of the height measurement sensor onto a projection grating of the height measurement sensor comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to output a radiation beam onto the surface of the substrate to generate a radiation spot on the surface comprising a plurality of radiation lines; receiving a reflected radiation beam at a detector of the height measurement sensor, the reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through a detection grating spot of a detection grating comprising a plurality of detection grating lines; a controller of the height measurement sensor that (i) acquires an output signal of the detector having a plurality of periodic components, (ii) takes the derivative of two points at different positions of the output signal that are separated by a period of the periodic components, and (iii) determines the focal plane of the height measurement sensor when the derivative changes sign; A method for providing the above.

Claims

1. 1. A height measurement sensor for measuring a position of a surface of a substrate, comprising: A projection unit; A detection unit; Equipped with The projection unit includes: A radiation source; a projection grating comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to receive radiation emitted by the radiation source and to output a radiation beam onto the surface to generate a radiation spot on the surface comprising a plurality of radiation lines; Equipped with The detection unit includes: a detection grating comprising a detection grating spot comprising a plurality of detection grating lines; a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through the detection grating spot; a controller configured to: (i) obtain an output signal of the detector having a plurality of periodic components; (ii) take the derivative of two points at different positions of the output signal that are spaced apart by a period of the periodic components; and (iii) determine a focal plane of the height measurement sensor when the derivative changes sign; Equipped with Height measurement sensor.

2. the projected grating spot has a total number of projected grating lines; the detection grating spot has the total number of detection grating lines; The height measurement sensor according to claim 1 .

3. the projection grating comprises a plurality of projection grating spots, each of which comprises a plurality of projection grating lines; the projection grating outputs the radiation beam to generate a plurality of radiation spots on the surface, each radiation spot comprising a plurality of radiation lines.

3. A height measuring sensor according to claim 1 or 2.

4. 4. A height measurement sensor according to claim 1, wherein the detection grating comprises a plurality of detection grating spots, each of which comprises a plurality of detection grating lines.

5. 5. A height measurement sensor according to claim 1, wherein the controller is configured to control a height of the substrate relative to a reference height in a direction perpendicular to the surface of the substrate within a time window during which the output signal is acquired.

6. the controller is operable to control the height of the substrate relative to a reference height within a range of heights; the controller is configured to control, within a time window, the height of the substrate relative to the reference height within a sub-portion of the height range. A height measuring sensor according to any one of claims 1 to 5.

7. A height measurement sensor according to claim 5 or 6, wherein the controller is configured to move the substrate in a direction perpendicular to the plurality of radial lines within the time window during which the output signal is acquired.

8. A height measuring sensor according to claim 1 , wherein in the focal plane of the height measuring sensor, all of the plurality of radiation lines of the radiation spot produced by the projection grating are detected by the detector.

9. A height measurement sensor according to any preceding claim, wherein the detector comprises an optical sensor for measuring the reflected radiation beam passing through the detection grating spot.

10. The height measurement sensor of claim 9 , wherein the optical sensor is a photodiode.

11. A height measurement sensor according to claim 1 , wherein the period of the periodic component depends on the angle of incidence of the radiation beam and the periodicity of the projected grating lines.

12. A height measurement sensor according to any preceding claim, wherein the radiation emitted by the radiation source has a wavelength in the range of 600 nm to 1050 nm.

13. A height measurement sensor according to any preceding claim, wherein the radiation emitted by the radiation source has a wavelength in the range of 100 nm to 400 nm.

14. A lithographic apparatus comprising a height measurement sensor according to any one of the preceding claims.

15. An evaluation system comprising a height measurement sensor according to any one of claims 1 to 13.

16. A measuring system comprising a height measuring sensor according to any one of claims 1 to 13.

17. 1. A method for determining a focal plane of a height measurement sensor, comprising: - emitting radiation at a radiation source of the height measurement sensor onto a projection grating of the height measurement sensor comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to output a radiation beam onto the surface of the substrate to generate a radiation spot on the surface comprising a plurality of radiation lines; receiving a reflected radiation beam at a detector of the height measurement sensor, the reflected radiation beam comprising radiation from the radiation spot reflected from the surface after passing through a detection grating spot of a detection grating comprising a plurality of detection grating lines; a controller of the height measurement sensor that (i) acquires an output signal of the detector having a plurality of periodic components, (ii) takes the derivative of two points at different positions of the output signal that are separated by a period of the periodic components, and (iii) determines the focal plane of the height measurement sensor when the derivative changes sign; A method for providing the above.

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