Height measurement sensor

Abstract

A height measurement sensor includes a projection unit and a detection unit. The projection unit comprises 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 radiation and to output a radiation beam onto a surface to produce a radiation spot. The detection unit includes: a detection grating including a detection grating spot having a plurality of grating lines; a detector arranged to receive a reflected radiation beam comprising radiation from the radiation spot after the detection of the grating spot; and a controller configured to (i) obtain a detector output signal, the detector output signal comprising a plurality of periodic components; (ii) deriving two points of different positions of the output signal, where the two points are separated by a period of the periodic component, and (iii) determining a focal plane of the sensor when the derivative changes sign.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European patent application 22159153.0 filed on February 28, 2022, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a height measuring sensor, in particular to a height measuring sensor for a photolithography device. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithographic apparatus may be used in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus may project a pattern (also often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature that can be formed on a substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. A lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate than a lithographic apparatus using radiation with, for example, a wavelength of 193 nm.

[0006] Low-k1 lithography can be used to process features with sizes smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optical element in the lithographic apparatus, CD is the "critical dimension" (usually the minimum feature size printed, but in this case it is the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on the substrate a pattern similar in shape and size to that planned by the circuit designer to achieve specific electrical functions and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. For example, these include but are not limited to optimization of NA, customized illumination schemes, use of phase-shifted patterning equipment, various optimizations of the design layout (such as optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout) or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] In general, the surface of the substrate on which the pattern should be projected is not completely flat. In addition, the substrate can show a thickness variation of several microns. The unevenness and / or thickness variation of the substrate surface can lead to incorrect projection of the pattern, for example due to focusing error or imaging error. In order to correct the unevenness and / or thickness variation of the substrate, it is well known to use a height sensor (sometimes referred to as a level sensor) that is ideally integrated in the lithography apparatus. This height sensor can be used to determine the height map of the substrate before the pattern is transferred (e.g., projected) on the substrate. This height map can then be used to correct the position of the substrate during the pattern transfer on the substrate, so as to keep the aerial image of the patterning device in proper focus. It will be understood that "height" in this context refers to a generalized dimension from a plane to the substrate, and does not mean any specific orientation relative to the earth or gravity.

[0008] A topographic measurement system, level sensor or height sensor is arranged to measure the topography of the top surface of a substrate (or wafer). A topographic map (also called a height map) of the substrate can be generated from these measurements, indicating the height of the substrate according to the position on the substrate. This height map can then be used to correct the position of the substrate during the transfer of the pattern on the substrate so as to provide an aerial image of the patterning device at the correct focus position on the substrate. It will be understood that "height" in this context refers to the generalized dimension from the plane to the substrate (also called the Z axis). Typically, the level sensor or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level sensor or height sensor results in height measurements at a substantially position.

[0009] Typically, such a height sensor comprises a projection unit, including: a projection grating, the projection grating comprising an elongated grating, the elongated grating being divided into a series of "spots" (each spot being referred to herein as a projection grating spot), wherein the grating lines are parallel to its axis, each projection grating spot being arranged to output a separate / discrete measurement spot (MS) radiation beam onto the surface of the substrate to form a measurement spot; and an additional aperture, comprising a single grating line, arranged to output a capture spot (CS) radiation beam to form a capture spot in front of the array of measurement spots on the substrate surface. The height sensor typically also comprises a detection unit, the detection unit comprising 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. Changes in the surface height result in a shift in the detected image. Summary of the invention

[0010] In the known height sensor mentioned above, at the detector, the (MS) radiation beam is split into two branches. The light in the two branches is detected by a photodiode. The projected grating image offset 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 order approximation, we make:

[0011] Z MS =gain MS *AC MS / DC MS , where gain MS is calibrated.

[0012] Normalizing the AC signal by the DC signal (Zraw = AC / DC) makes the height sensor response independent of the surface reflectivity and enables accurate calibration. However, normalization makes Zraw dependent on the height of the substrate. MS The signal has periodicity. The substrate may be a wafer supported by a wafer stage (WS). Accurate calibration and accurate height measurement are only performed in the center / correct period, i.e. when the full projection grating image (MS radiation beam) is detected by the detection grating.

[0013] By performing a capture scan, the measurement spot is positioned in the correct period. The capture spot height measurement is similar to the measurement spot:

[0014] Z CS =gain CS *AC CS / DC CS

[0015] During the capture scan, the wafer stage scans a certain range around the expected MS correct period, and both the MS signal and the CS signal are recorded. The capture signal is not periodic (since the capture spot consists of a single slit), and an algorithm is used to detect the capture position. This is Zraw CS =0(AC CS =0), which identifies the correct period of the MS by design.

[0016] In the exact cycle, the capture spot zero crossing point is designed to be very close to the measurement spot zero crossing point. The inventors have confirmed that the signal can be significantly distorted due to the interaction with the local topography on the substrate and the process layer stack at the CS position and the MS position. In the example where the substrate is a wafer, the process layer stack includes a series of wafer layers with different transparency. Due to the structures printed on the wafer (such as memory cells, transistors, metal lines, etc.), the transparency can vary locally. Due to this signal distortion, the CS and MS zero crossing points are significantly offset. This causes the capture algorithm to be unable to identify the correct MS cycle.

[0017] In an embodiment of the present invention, only the MS signal is used to identify the correct period, so there is no need to capture the light spot.

[0018] According to one aspect of the present disclosure, there is provided a height measurement sensor for measuring the position of a surface of a substrate, comprising:

[0019] A projection unit, wherein the projection unit comprises:

[0020] Radiation sources;

[0021] 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 a radiation source and output a radiation beam onto a surface to produce a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines; and

[0022] A detection unit, wherein the detection unit comprises:

[0023] A detection grating, wherein the detection grating comprises a detection grating light spot, and the detection grating light spot comprises a plurality of detection grating lines;

[0024] 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

[0025] A controller is configured to (i) obtain an output signal of the detector, wherein the output signal includes a plurality of periodic components; (ii) differentiate the output signal at two points at different locations, wherein the two points are separated by a period of the periodic components; and (iii) determine when the substrate is positioned on a focal plane of the height measurement sensor when the derivative changes sign.

[0026] Thus, embodiments of the present invention ensure that the focal plane of the height measurement sensor can be reliably and accurately detected. Since the projection grating does not require a capture spot, the projection grating can be made smaller. The detection grating does not require a detection grating spot to pass the capture spot radiation beam reflected from the surface of the substrate, so the detection grating can also be made smaller. In addition, the detection unit is simplified because a separate detector for sensing the capture spot radiation beam is not required.

[0027] The projection grating spot may have a total number of projection grating lines and the detection grating spot may have said total number of detection grating lines.

[0028] In some embodiments, the projection grating includes a plurality of projection grating spots, each of the plurality of projection grating spots includes a plurality of projection grating lines, and the projection grating outputs a radiation beam to generate a plurality of radiation spots on a surface, each of the plurality of radiation spots includes a plurality of radiation lines.

[0029] The detection grating may include a plurality of detection grating spots, each of which includes a plurality of detection grating lines.

[0030] 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 during a time window in which the output signal is obtained.

[0031] The controller may be capable of controlling the height of the substrate relative to a reference height within a height range, and during the time window, the controller may be configured to control the height of the substrate relative to the reference height within a sub-portion of the height range. Taking the example of a substrate being a wafer supported by a wafer stage (WS) of a lithography apparatus, the wafer stage may not scan its entire height range when determining the focal plane due to throughput reasons, but may 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 the maximum output signal from the detector is detected. This is not possible when scanning a smaller height range. 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 guaranteed that the true maximum is detected because the monotonicity of the output signal changes at the true maximum.

[0032] The controller can be configured to move the substrate in a direction perpendicular to the plurality of radiation lines during the time window in which the output signal is obtained. That is, the substrate can be moved in the y direction while being scanned in the z direction (the substrate is lifted) so that the same area on the substrate is illuminated (the substrate is scanned diagonally). This improves the robustness of the method to compensate for light moving on the substrate by making the method robust to reflectivity variations between the projected grating lines of the projected grating spot, thereby determining the focal plane of the height measurement sensor.

[0033] In certain embodiments, at the focal plane of the height measurement sensor, all of the plurality of radiation lines of the radiation spot generated by the projection grating are detected by the detector.

[0034] The detector may comprise an optical sensor (eg a photodiode) for sensing the reflected radiation beam passing through the detection grating spot.

[0035] 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.

[0036] The radiation emitted by the radiation source may have a wavelength in the range of 600 nm to 1050 nm.

[0037] Alternatively, the radiation emitted by the radiation source may have a wavelength in the range of 100 nm to 400 nm.

[0038] According to another aspect of the present disclosure, there is provided a lithographic apparatus comprising a height measurement sensor according to any one of the embodiments described herein.

[0039] According to another aspect of the present disclosure, an evaluation system is provided, comprising a height measurement sensor according to any one of the embodiments described herein.

[0040] According to another aspect of the present disclosure, a measurement system is provided, comprising a height measurement sensor according to any one of the embodiments described herein.

[0041] According to another aspect of the present disclosure, there is provided a method for determining a focal plane of a height measurement sensor, the method comprising:

[0042] emitting radiation onto a projection grating of the height measurement sensor using a radiation source of the height measurement sensor, the projection grating comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to output a radiation beam onto a surface of the substrate to generate a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines;

[0043] receiving, with a detector of the height measurement sensor, a reflected radiation beam comprising radiation from a radiation spot reflected from the surface after passing through a detection grating spot of a detection grating, the detection grating spot comprising a plurality of detection grating lines; and

[0044] A controller of a height measurement sensor (i) obtains an output signal of a detector, wherein the output signal includes a plurality of periodic components, (ii) derives the output signal at two points at different positions, wherein the two points are separated by a period of the periodic components, and (iii) determines a focal plane of the height measurement sensor when the derivative changes sign. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0046] - Figure 1 A schematic overview of the photolithography apparatus is depicted;

[0047] - Figure 2 A schematic overview of the lithography unit is depicted;

[0048] - Figure 3 Depicted is a schematic representation of the overall lithography technology, showing the collaboration between the three key technologies to optimize semiconductor manufacturing;

[0049] - Figure 4 A height measurement sensor is illustrated;

[0050] - Figure 5a An example projected grating is illustrated;

[0051] - Figure 5b The figure shows the radiation spot generated on the surface of the substrate;

[0052] - Figure 5c An example detection grating is illustrated;

[0053] - Figures 6a to 6d illustrates how the sensed reflected radiation changes as the substrate moves upward;

[0054] - Figure 7a An example output signal of the detector is illustrated;

[0055] - Figure 7b illustrates an example derivative signal; and

[0056] - Figure 8 A diagonal scan of the substrate is illustrated. DETAILED DESCRIPTION

[0057] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365nm, 248nm, 193nm, 157nm or 126nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5nm to about 100nm).

[0058] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds to the pattern produced in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmed mirror arrays and programmable LCD arrays.

[0059] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask stage) 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 MA according to certain parameters, a substrate support (e.g. a wafer stage) WT configured to accommodate a substrate (e.g. a 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. a refractive projection lens system) PS configured to project a pattern delivered to 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.

[0060] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0061] The term "projection system" as used herein should be interpreted broadly as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate to the exposure radiation used and / or other factors, such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the broader term "projection system" PS.

[0062] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0063] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or steps in preparation for subsequent exposure of the substrate W may be performed on a substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0064] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measuring table. The measuring table is arranged to accommodate sensors and / or cleaning equipment. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measuring table may accommodate a plurality of sensors. The cleaning equipment may be arranged to clean parts of the lithographic apparatus, for example parts of the projection system PS or parts of a system for providing an immersion liquid. The measuring table may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0065] In operation, a radiation beam B is incident on a patterning device, such as a mask MA, which is fixed on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the radiation beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly a further position sensor (which is not in the position sensor) are arranged in a manner such that the radiation beam B is patterned by a pattern (design layout) present on the patterning device MA. Figure 1 The patterning device MA (depicted explicitly in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as shown occupy dedicated target portions, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe lane alignment marks.

[0066] like Figure 2 As shown, the lithography apparatus LA may form part of a lithocell LC, sometimes also referred to as a lithocell or a (lithography) cluster, which typically also includes devices for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these processes include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a baking plate BK, for example for regulating the temperature of the substrate W, for example for regulating a 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 the different process devices and transfers the substrates W to a loading and unloading area LB of the lithography apparatus LA. The equipment in the lithocell (also often collectively referred to as a track) is typically under the control of a track control unit TCU, which itself may be controlled by a monitoring control system SCS, which may also control the lithography apparatus LA, for example via a litho control unit LACU.

[0067] In order for the substrate W exposed by the lithography apparatus LA to be correctly and consistently exposed, the substrate needs to be inspected to measure properties of the patterned structure, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. To this end, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, particularly if the inspection is performed before other substrates W of the same batch or lot are still exposed or processed.

[0068] The inspection device (which may also be referred to as a metrology device or an evaluation device) is used to determine properties of the substrate W, and in particular, to determine how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection device may alternatively be configured to identify defects on the substrate W and may, for example, be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection device may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB) or a developed resist image (where exposed or unexposed parts have been removed) or even an etched image (after a pattern transfer step such as etching).

[0069] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in dimensioning and placement of structures on the substrate W. To ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as Figure 3 Schematically shown. One of these systems is a lithography apparatus LA which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device) - typically within this range, variations in process parameters in the lithography process or patterning process are allowed.

[0070] The computer system CL can use (portions of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layouts and lithography apparatus settings achieve the maximum overall process window for the patterning process (e.g., Figure 3Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect the current operating position of the lithographic apparatus LA in the process window (e.g. using input from the metrology tool MT) to predict the position of the lithographic apparatus LA in the process window due to, for example, suboptimal processing (e.g. Figure 3 The arrow in the second scale SC2 points to "0" to determine whether there may be a defect.

[0071] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drift, for example in the calibration state of the lithographic apparatus LA (e.g. Figure 3 (depicted by multiple arrows in the third scale SC3).

[0072] Figure 4 A level sensor LS (also referred to herein as a height measurement sensor) according to an embodiment of the present invention is illustrated. The level sensor LS includes a projection unit 300 and a detection unit 350 .

[0073] The projection unit 300 includes a radiation source 302 and a projection grating 304. For example, the radiation source 302 can be a narrowband or broadband radiation source, such as a supercontinuum light source, a polarized or non-polarized light source, a pulsed or continuous light source, such as a polarized or non-polarized laser beam. The radiation source 302 may include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source 302 of the level sensor LS is not limited to visible radiation, but may additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate. In one example, the radiation emitted by the radiation source 302 may have a wavelength in the range of 600nm to 1050nm. In another example, the radiation emitted by the radiation source 302 may have a wavelength in the range of 100nm to 400nm.

[0074] The projection grating 304 is arranged to receive radiation emitted by the radiation source 302. The projection grating 304 includes one or more projection grating spots, each of which has a plurality of projection grating lines. The projection grating lines allow the radiation emitted by the radiation source 302 to pass through the projection grating 304, thereby generating a radiation beam with a periodically varying intensity. The radiation beam with a periodically varying intensity is directed to a measurement position on the surface of a substrate (e.g., a wafer) W. Compared to known techniques, the projection grating 304 does not have any capture spots.

[0075] Figure 4The projection grating 304 is illustrated, comprising a single projection grating spot having three projection grating lines that deliver a radiation beam 306a toward the surface of the substrate W. It will be appreciated that the three projection grating lines are merely examples, and that the projection grating spot may have two or more projection grating lines. Furthermore, the projection grating 304 may comprise a plurality of projection grating spots, each having a plurality of projection grating lines that deliver respective radiation beams toward the surface of the substrate W (thereby illuminating different areas on the substrate surface).

[0076] Figure 5a An example projected grating 304 is illustrated. Figure 5a In the example shown, the projected grating 304 includes three projected grating spots 502. Each of the three projected grating spots includes five projected grating lines 504 through which radiation from the radiation source 302 can pass. The five projected grating lines 504 are separated by a pitch "p" that defines the periodicity of the projected grating lines.

[0077] like Figure 4 As shown, the radiation beam 306a is directed to the surface of the substrate W. The radiation beam 306a may have an incident angle relative to an axis (Z axis) perpendicular to the incident substrate surface, which is greater than 0 degrees and less than 90 degrees. Typically, the incident angle is between 70 degrees and 80 degrees (inclusive). For example, the incident angle may be 70 degrees or 78 degrees. In practice, the level sensor LS typically includes additional optical elements along the path between the projection grating 304 and the substrate W. For ease of illustration, these additional optical elements have been removed from the image sensor LS. Figure 4 Omitted in .

[0078] When the radiation beam 306a is incident on the surface of the substrate W at the measurement position, a radiation spot is generated, which is the area on the surface of the substrate W illuminated by the radiation source 302. The radiation spot comprises a plurality of radiation lines corresponding to the plurality of grating lines of the projected grating spot.

[0079] Figure 5b Three radiation spots 512 are illustrated, each of which includes five radiation lines 514, corresponding to the five projected grating lines 504 that will be produced on the surface of the substrate W when the example projection grating 304 is used. For ease of illustration, the radiation lines 514 have been shown to have a rectangular shape corresponding to the projected grating lines 504. It will be understood that Figure 5b The shape of the radiation line 514 shown does not correspond to the radiation beam 306a, which has an angle of incidence in the typical range of 70 degrees to 80 degrees, inclusive.

[0080] The radiation beam 306a is reflected by the surface of the substrate W at the measurement position and the reflected radiation beam 306b is directed to the detection unit 350. The detection unit 350 comprises a detection grating 354, a detector 356 and a processing unit 360.

[0081] The detection grating 354 is arranged to receive the reflected radiation beam 306b. The detection grating is preferably substantially the same as the projection grating 304 and is subdivided to correspond to the dot pattern. Therefore, the detection grating 354 includes one or more detection grating spots, each of which has a plurality of detection grating lines. The detection grating lines allow the reflected radiation beam 306b to pass through the detection grating 354 to the detector 356.

[0082] Figure 4 The detection grating 354 is illustrated, including a single detection grating spot having three detection grating lines that pass the reflected radiation beam 306b toward the detector 356. It will be understood that the three detection grating lines are merely examples, and the detection grating spot may have two or more detection grating lines. In addition, the detection grating 354 may include a plurality of detection grating spots, each having a plurality of detection grating lines that pass their respective reflected radiation beams toward the detector 356.

[0083] Figure 5c An example detection grating 354 is shown. Figure 5c In the example shown, the detection grating 354 comprises three detection grating spots 522. Each of the three detection grating spots 522 comprises five detection grating lines 524. The five detection grating lines are separated by a pitch "p" which defines the periodicity of the detection grating lines.

[0084] In practice, the level sensor LS typically includes additional optical elements along the path between the substrate W and the detection grating 354. For ease of illustration, these additional optical elements have been removed from Figure 4 Omitted in .

[0085] The detector 356 is configured to output an output signal to the controller 360. The detector 356 may generate a detector output signal indicating the received light, for example indicating the intensity of the received light, and may therefore include a photodetector. The detector 356 may alternatively represent the spatial distribution of the received intensity, and may therefore include a camera. The detector DET may include any combination of one or more detector types. For each detection grating spot 522, the detector 356 may include an optical sensor (or multiple optical sensors) for sensing the reflected radiation beam passing through the respective detection grating spot 522. The optical sensor may be a photodiode or any other photosensitive sensor. In contrast to known techniques, the detector 356 does not include an optical detector for sensing the capture spot radiation beam after reflection on the substrate.

[0086] As described in more detail below, the voltage level of the output signal corresponds to the intensity of the radiation sensed by the detector 356. According to an embodiment of the present invention, the controller 360 is configured to process the output signal it receives 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 the reference height in a direction perpendicular to the surface of the substrate W during the time window in which the output signal is obtained. That is, during the time window in which the output signal is obtained, the substrate W is moved upward away from the reference height (in a direction perpendicular to the surface of the substrate W). Figure 4 The substrate may be a wafer supported by a wafer stage (WS). In these embodiments, the controller 360 is configured to raise (and / or lower) the wafer stage by communicating with an actuator (e.g., a Lorentz actuator) that controls the movement of the WS.

[0087] The functions of the controller 360 may be implemented in a code (software) stored on a memory including one or more storage media, and arranged to be executed on a processor including one or more processing units. The code is configured to perform operations consistent with the following embodiments when it is retrieved from the memory and executed on the processor. Alternatively, it is not excluded that some or all of the functions of the controller 360 are implemented in a dedicated hardware circuit device or a configurable hardware circuit device (such as an FPGA).

[0088] When all radiation lines 514 are detected by the detector 356 (all radiation generating radiation lines 514 is reflected by the surface of the substrate W and passes through the detection grating lines 524 of the detection grating spots 522), we refer here to the substrate W being in the focal plane of the horizontal sensor LS. It will be appreciated that when substrates (such as wafers) are produced, their thickness may vary. Therefore, when the wafer W is placed onto the wafer stage WS in the lithography apparatus LA, due to the thickness variation, there is no guarantee that the surface of the wafer is at the same height above the reference height as the previous wafer placed on the wafer stage WS. Embodiments of the present invention enable each substrate to be accurately positioned in the focal plane of the horizontal sensor LS.

[0089] Figure 6a A scenario 600 is illustrated in which no reflected radiation is detected by the detector 356 via the detection grating lines 524. In this scenario, the voltage level of the output signal output by the detector 356 is zero. Figure 7a An example output signal 700 is shown.

[0090] Figure 6b Scenario 620 is illustrated in which substrate W has moved upwards (in the positive z direction) relative to scene 600, causing portions of radiation lines 514 to overlap detection grating lines 524. In this scenario, the voltage level of output signal 700 output by detector 356 has increased, but is not a maximum. Figure 7a The voltage output in the scene 620 is illustrated by the dashed line 720. The voltage level of the output signal 700 output by the detector 356 is not a maximum, because not all radiation lines 514 overlap with the detection grating lines 524. Figure 6b The circle 602 in FIG. 5 shows that there are two radiation lines 514 that do not overlap with the detection grating lines 524 .

[0091] Figure 6c Scenario 640 is illustrated in which substrate W has moved further upward (in the positive z direction) relative to scene 620, causing all radiation lines 514 to completely overlap detection grating lines 524. In this scenario, the voltage level of output signal 700 output by detector 356 has increased to a maximum value. Figure 7a The voltage output in scenario 640 is illustrated by dashed line 740. Here, substrate W is in the correct MS cycle mentioned above.

[0092] Figure 6d A scene 660 is illustrated in which the substrate W has moved further upward (in the positive z-direction) relative to the scene 640, which causes portions of the radiation lines 514 to no longer overlap with the detection grating lines 524. In this scene, the voltage level of the output signal 700 output by the detector 356 has increased, but is not zero (portions of the radiation lines 514 still overlap with the detection grating lines 524). Figure 7a The voltage output in scene 660 is illustrated by dashed line 760. The voltage level of output signal 700 output by detector 356 is not a maximum because not all radiation lines 514 overlap with detection grating lines 524. Circle 604 shows that there are two radiation lines 514 that do not overlap with detection grating lines 524.

[0093] When the substrate W moves upward in the positive z-direction, the output signal 700 is provided to the controller 360 for processing. Figure 7a As shown, the output signal 700 includes a plurality of periodic components, wherein increasing voltage steps correspond to more radiation lines detected by the detector 356 as the substrate moves upward and closer to the focal plane, and decreasing voltage steps correspond to fewer radiation lines detected by the detector 356 as the substrate moves upward and away from the focal plane. The period of the periodic component depends on the angle of incidence of the radiation beam 306a and the pitch 'p' of the projected grating lines.

[0094] The controller 360 is configured to calculate the derivative of two points at different locations of the output signal 700 (where the two points are separated by the period of the periodic component) to obtain Figure 7b Derivative signal 702 is shown. Two points separated by one period share the robust condition that an additional radiation line is detected (or not detected), regardless of its intensity. Therefore, when calculating the derivative of two points separated by one period, the derivative changes sign (from positive to negative) when all radiation lines 514 completely overlap with all detection grating lines 524 and the substrate is in the focal plane. Figure 7b The derivative changes sign at the focal plane position (defined as the substrate at Z = 0) as shown. The controller 360 is configured to determine the focal plane of the level sensor when the derivative changes sign.

[0095] The process may be performed dynamically in the controller 360, which may obtain the derivative signal 702 and detect when the derivative signal changes sign when the substrate W moves upward in the positive z-direction. Upon detecting when the derivative changes sign, the controller 360 may prevent any further upward movement of the substrate so that the substrate W remains in the angular plane of the horizontal sensor.

[0096] Alternatively, after performing a scan, wherein the substrate is upward (in Figure 4 The controller 360 may process the output signal 700. The controller 360 may then control the substrate W to be located on the determined focal plane.

[0097] During the time window in which the output signal 700 is obtained, the substrate W may be moved in the y direction while scanning is performed in the z direction so that the same area on the substrate W is illuminated (the substrate is scanned diagonally), as shown in FIG. Figure 8 shown. Specifically, Figure 8 It is illustrated that a component of the radiation beam 306a is incident on an area 802 of the surface of the substrate W when the substrate W is in a first position, and illuminates the same area 802 of the surface of the substrate W after the substrate W moves upward (in the z-direction) and laterally (in the y-direction) to a second position.

[0098] Although specific reference may be made herein to the substrate being a wafer, embodiments extend to measuring the position of a surface of a substrate other than a wafer.

[0099] Although particular reference may be made herein 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 modes for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like. It will be understood by those skilled in the art that any use of the term "substrate" or "die" herein in the context of such alternative applications may be considered synonymous with the more general term "substrate" or "target portion", respectively. The substrate referred to herein may be processed, 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, before or after exposure. Where applicable, the present disclosure may be applied to such substrate processing tools and other substrate processing tools. In addition, a substrate may be processed more than once, for example to produce a multi-layer IC (stack), so that the terms substrate and substrate surface used herein may also refer to a substrate that already contains multiple processed layers and the surfaces of each of these layers.

[0100] Although embodiments of the invention may be specifically mentioned herein in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0101] Although particular reference may be made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0102] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may not be practiced in the manner described. The above description is intended to be illustrative, not limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the following claims. Other aspects of the present invention are described in the following numbered clauses:

[0103] 1. A height measurement sensor for measuring the position of a surface of a substrate, comprising:

[0104] A projection unit, wherein the projection unit comprises:

[0105] Radiation sources;

[0106] 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 a radiation source and output a radiation beam onto a surface to produce a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines; and

[0107] A detection unit, wherein the detection unit comprises:

[0108] A detection grating, wherein the detection grating comprises a detection grating light spot, and the detection grating light spot comprises a plurality of detection grating lines;

[0109] 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

[0110] A controller is configured to (i) obtain an output signal of the detector, wherein the output signal includes multiple periodic components; (ii) differentiate the output signal at two points at different positions, wherein the two points are separated by the period of the periodic components; and (iii) determine the focal plane of the height measurement sensor when the derivative changes sign.

[0111] 2. The height measuring sensor according to clause 1, wherein the projection grating spot has a total number of projection grating lines and the detection grating spot has said total number of detection grating lines.

[0112] 3. A height measurement sensor according to claim 1 or 2, wherein the projection grating includes a plurality of projection grating spots, each of the plurality of projection grating spots includes a plurality of projection grating lines, and the projection grating outputs a radiation beam to generate a plurality of radiation spots on a surface, each of the plurality of radiation spots includes a plurality of radiation lines.

[0113] 4. The height measuring sensor according to any one of the preceding clauses, wherein the detection grating comprises a plurality of detection grating spots, each of the plurality of detection grating spots comprising a plurality of detection grating lines.

[0114] 5. A height measurement sensor according to any of the preceding clauses, 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 during a time window in which the output signal is obtained.

[0115] 6. A height measurement sensor according to any of the preceding clauses, wherein the controller is capable of controlling the height of the substrate relative to a reference height within a height range, and during the time window, the controller is configured to control the height of the substrate relative to the reference height within a sub-portion of the height range.

[0116] 7. A height measurement sensor according to clause 5 or 6, wherein the controller is configured to move the substrate in a direction perpendicular to the plurality of radiation lines during a time window in which the output signal is obtained.

[0117] 8. A height measuring sensor according to any of the preceding clauses, wherein on a focal surface of the height measuring sensor, a plurality of radiation lines of the radiation spot generated by the projection grating are all detected by the detector.

[0118] 9. A height measurement sensor according to any of the preceding clauses, wherein the detector comprises an optical sensor for sensing a reflected radiation beam passing through a detection grating spot.

[0119] 10. The height measurement sensor according to clause 9, wherein the optical sensor is a photodiode.

[0120] 11. A height measurement sensor according to any of the preceding clauses, 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.

[0121] 12. A height measuring sensor according to any of the preceding clauses, wherein the radiation emitted by the radiation source has a wavelength in the range of 600 nm to 1050 nm.

[0122] 13. A height measuring sensor according to any of clauses 1 to 11, wherein the radiation emitted by the radiation source has a wavelength in the range of 100 nm to 400 nm.

[0123] 14. A lithographic apparatus comprising a height measurement sensor according to any of the preceding clauses.

[0124] 15. An evaluation system comprising a height measurement sensor according to any of clauses 1 to 13.

[0125] 16. A measurement system comprising a height measurement sensor according to any one of clauses 1 to 13.

[0126] 17. A method for determining a focal plane of a height measurement sensor, the method comprising:

[0127] emitting radiation onto a projection grating of the height measurement sensor using a radiation source of the height measurement sensor, the projection grating comprising a projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to output a radiation beam onto a surface of the substrate to generate a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines;

[0128] receiving, with a detector of the height measurement sensor, a reflected radiation beam comprising radiation from a radiation spot reflected from the surface after passing through a detection grating spot of a detection grating, the detection grating spot comprising a plurality of detection grating lines; and

[0129] A controller of a height measurement sensor (i) obtains an output signal of a detector, wherein the output signal includes a plurality of periodic components, (ii) derives the output signal at two points at different positions, wherein the two points are separated by a period of the periodic components, and (iii) determines a focal plane of the height measurement sensor when the derivative changes sign.

Claims

1. A height measurement sensor for measuring the position of a surface of a substrate, comprising: A projection unit, wherein the projection unit comprises: Radiation sources; a projection grating, comprising a projection grating spot, the 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 produce a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines; and A detection unit, wherein the detection unit comprises: A detection grating, wherein the detection grating comprises a detection grating light spot, and the detection grating light spot comprises a plurality of detection grating lines; a detector arranged to receive a reflected radiation beam comprising radiation from said radiation spot reflected from said surface after passing through said detection grating spot; and A controller is configured to (i) obtain an output signal of the detector, wherein the output signal includes multiple periodic components; (ii) derive the output signal at two points at different positions, wherein the two points are separated by the period of the periodic components; and (iii) determine the focal plane of the height measurement sensor when the derivative changes sign. 2 . The height measurement sensor according to claim 1 , wherein the projection grating spot has a total number of projection grating lines and the detection grating spot has the total number of detection grating lines.

3. A height measurement sensor according to claim 1 or 2, wherein the projection grating includes a plurality of projection grating spots, each of the plurality of projection grating spots includes a plurality of projection grating lines, and the projection grating outputs the radiation beam to generate a plurality of radiation spots on the surface, each of the plurality of radiation spots includes a plurality of radiation lines. 4 . The height measuring sensor according to claim 1 , wherein the detection grating comprises a plurality of detection grating spots, each of the plurality of detection grating spots comprising a plurality of detection grating lines.

5. The height measurement sensor according to any one of the preceding claims, 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 during a time window in which the output signal is obtained.

6. A height measurement sensor according to any of the preceding claims, wherein the controller is capable of controlling the height of the substrate relative to the reference height within a height range, and during the time window, the controller is configured to control the height of the substrate relative to the reference height within a sub-portion of the height range. 7 . The height measurement sensor according to claim 5 , wherein the controller is configured to move the substrate in a direction perpendicular to the plurality of radiation lines during the time window in which the output signal is obtained. 8 . The height measuring sensor according to claim 1 , wherein on the focal surface of the height measuring sensor, all of the plurality of radiation lines of the radiation spot generated by the projection grating are detected by the detector.

9. A height measurement sensor according to any one of the preceding claims, wherein the detector comprises an optical sensor for sensing the reflected radiation beam passing through the detection grating spot. 10 . The height measurement sensor according to claim 9 , wherein the optical sensor is a photodiode.

11. A height measurement sensor according to any of the preceding claims, 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 . The height measuring sensor according to claim 1 , wherein the radiation emitted by the radiation source has a wavelength in the range of 600 nm to 1050 nm. 13 . The height measuring sensor according to claim 1 , 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 measuring sensor according to any one of claims 1 to 13.

16. A measurement system comprising the height measurement sensor according to any one of claims 1 to 13.

17. A method for determining a focal plane of a height measurement sensor, the method comprising: emitting radiation onto a projection grating of the height measurement sensor using a radiation source of the height measurement sensor, the projection grating comprising a projection grating spot, the projection grating spot comprising a plurality of projection grating lines, the projection grating being arranged to output a radiation beam onto a surface of a substrate to generate a radiation spot on the surface, the radiation spot comprising a plurality of radiation lines; receiving, with 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 a detection grating, the detection grating spot comprising a plurality of detection grating lines; and The controller of the height measurement sensor (i) obtains an output signal of the detector, wherein the output signal includes a plurality of periodic components, (ii) takes a derivative of two points at different positions of the output signal, wherein the two points 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.

Images