Alignment measurement system

Abstract

A substrate alignment measurement system comprising: an illumination source configured to provide a pulsed radiation beam comprising alternating s and p polarized radiation pulses; a first optical module configured to direct the pulsed radiation beam towards an alignment mark on a substrate; a second optical module configured to receive diffracted radiation from the alignment mark, the second optical module comprising a delay loop configured to provide temporally separated s and p polarized sub-pulses from each received pulse of the diffracted radiation; and a detection system configured to receive the s and p polarized sub-pulses from the second optical module and provide an output indicative of the intensity of each sub-pulse.

Description

ALIGNMENT MEASUREMENT SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 737,071 which was filed on 20 December 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to an alignment measurement system. The system may form part of an exposure apparatus such as a lithographic apparatus. The system may form part of a metrology tool.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask also referred to as a reticle) onto a layer of radiationsensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A lithographic apparatus is an example of an exposure apparatus. Other examples are a metrology, or an inspection apparatus, more specifically a mask inspection apparatus and even more specifically an actinic mask inspection apparatus.

[0006] An alignment measurement system may be used to measure a position of an alignment mark on a substrate. In some known alignment measurement systems some measurement information may be not detected, depending on a polarization of measurement radiation that is scattered from the alignment mark.

[0007] It may be desirable to provide a system that obviates or mitigates one or more problems associated with the prior art.SUMMARY

[0008] According to a first aspect of the invention, there is provided a substrate alignment measurement system comprising: an illumination source configured to provide a pulsed radiation beam comprising alternating s and p polarized radiation pulses; a first optical module configured to direct the pulsed radiation beam towards an alignment mark on a substrate; a second optical module configured to receive diffracted radiation from the alignment mark, the second optical module comprising a delay loop configured to provide temporally separated s and p polarized sub-pulses from each received pulse of the diffracted radiation; and a detection system configured to receive the s and p polarized sub-pulses from the second optical module and provide an output indicative of the intensity of each sub-pulse.

[0009] Advantageously, because the substrate alignment measurement system is able to detect s and p polarized sub-pulses, it may provide accurate alignment measurements based on the detected s and p polarized sub-pulses.

[0010] The delay loop may comprise a pair of arms provided on opposite sides of a polarizing beam splitter.

[0011] Each arm may include a quarter- wave plate.

[0012] The delay loop may comprises an optical fiber.

[0013] The delay loop may have an optical path length of at least 30 cm. In some cases the delay loop may have an optical path length of at least 3 cm. The delay loop may have an optical path length of up to 3 m.

[0014] The pulsed radiation beam may have a pulse frequency of up to 100 MHz

[0015] The delay loop may comprise a birefringent material configured to delay sub-pulses with a given polarization relative to sub-pulses with a different polarization.

[0016] The detection system may comprise a self -referencing interferometer.

[0017] The detection system may be configured to provide a comparison of an intensity of positive diffraction and an intensity of negative diffraction.

[0018] The detection system may comprise a reflector located in a pupil which is configured to reflect positive or negative diffraction orders of diffracted radiation towards a detection module.

[0019] The substrate alignment measurement system may further comprise a beamsplitter configured to direct some radiation to the self-referencing interferometer and some radiation to the detection system configured to provide the comparison of the intensity of positive diffraction and the intensity of negative diffraction.

[0020] The detection system may comprise a detection module.

[0021] The detection module may comprise a photodiode connected to a plurality of switched capacitor acquisition circuits which are synchronized with respect to the pulsed radiation beam.

[0022] Each switched capacitor acquisition circuit may comprise a measurement activation switch which connects each switched capacitor acquisition circuit to a detector at a different time.

[0023] Each switched capacitor acquisition circuit may comprise a reset activation switch which is configured to remove charge across a measurement capacitor of the circuit.

[0024] According to a second aspect of the invention there is provided a detection module comprising a photodiode connected to a plurality of switched capacitor acquisition circuits which are synchronized with respect to a pulsed radiation beam.

[0025] Advantageously, the detection module may be able to provide measurements at a faster rate than a conventional detection module.

[0026] Each switched capacitor acquisition circuit may comprise a measurement activation switch which connects each switched capacitor acquisition circuit to a detector at a different time.

[0027] Each switched capacitor acquisition circuit may comprise a reset activation switch which is configured to remove charge across a measurement capacitor of the circuit.

[0028] According to a third aspect of the invention there is provided a lithographic apparatus or lithographic tool comprising the substrate alignment measurement system of the first aspect.

[0029] According to a fourth aspect of the invention there is provided a method of measuring substrate alignment comprising: directing a pulsed radiation beam comprising alternating s and p polarized radiation pulses onto an alignment mark on a substrate; receiving radiation diffracted radiation from the alignment mark and using a delay loop to temporally separate s and p polarized sub-pulses from each received pulse of the diffracted radiation; and using a detection system to measure the s and p polarized sub-pulses from the delay loop and provide an output indicative of the intensity of each sub-pulse.

[0030] Advantageously, because the method detects s and p polarized sub-pulses, it may provide accurate alignment measurements based on the detected s and p polarized sub-pulses.

[0031] The detection system may comprise a self -referencing interferometer.

[0032] The detection system may be configured to provide a comparison of an intensity of positive diffracted radiation and negative diffracted radiation.

[0033] The detection system may further comprise a detection module which includes switched capacitor acquisition circuits that are synchronised with respect to the pulsed radiation beam.

[0034] The switched capacitor acquisition circuits may be used to time-demultiplex the incident s and p polarized sub-pulses.

[0035] Features of different aspects of the invention may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0037] - Figure 1 schematically depicts a lithographic apparatus which includes an alignment measurement system according to an embodiment of the invention;

[0038] - Figure 2 schematically depicts an alignment measurement system according to an embodiment of the invention;

[0039] - Figure 3 schematically depicts the alignment measurement system of Figure 2 in more detail;

[0040] - Figure 4 is a circuit diagram which depicts a detection module according to an embodiment of the invention; and

[0041] - Figure 5 schematically depicts an alignment measurement system according to an alternative embodiment of the invention.DETAILED DESCRIPTION

[0042] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The lithographic apparatus LA is an example of an exposure apparatus. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a first substrate table WT1 configured to support a first substrate Wl. In addition, the lithographic apparatus comprises a second substrate table WT2 configured to support a second substrate W2. The lithographic apparatus LA further comprises an alignment measurement system 16 according to an embodiment of the invention. The alignment measurement system 16 is connected to a processor 17 which receives an output signal from the alignment measurement system.

[0043] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device Ml and a facetted pupil mirror device M2. The faceted field mirror device Ml and faceted pupil mirror device M2 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device Ml and faceted pupil mirror device M2.

[0044] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the first substrate W 1. For that purpose, the projection system PS may comprise a plurality of mirrors M3,M4 which are configured to project the patterned EUV radiation beam B’ onto the first substrate Wl held by the first substrate table WT1. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors M3,M4 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0045] The second substrate table WT2 and second substrate W2 are located adjacent to the first substrate table WT1 and the first substrate WL The location of the second substrate table WT2 may bereferred to as a measurement location, and the location of the first substrate table WT1 may be referred to as an exposure location. Exposure of the first substrate W1 by the patterned EUV radiation beam B’ takes place at the exposure location. Measurement of the second substrate W2 by the alignment measurement system 16 and by a substrate height measurement system (not depicted) takes place at the measurement location. The alignment measurement system 16 measures the positions of alignment marks on the second substrate W2 relative to alignment marks on the second substrate table WT2. After the alignment measurements and height measurements have been completed, the second substrate table WT2 is moved to the exposure location. Exposure of the second substrate W2 takes into account the alignment measurements.

[0046] During exposure of the second substrate W2, the first (exposed) substrate W1 is removed from the first substrate table WT1. Anew substrate (not depicted) is loaded onto the first substrate table WT1, and is measured by the alignment measurement system 16 and by the height measurement system.

[0047] The above method may be repeated many times in order to expose many substrates W using the lithographic apparatus LA.

[0048] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0049] The lithographic apparatus LA and radiation source SO described herein can be used in method for performing a circuit layout patterning process. A circuit layout patterning method comprises receiving a substrate with a photoresist layer. The method further comprises directing EUV radiation from radiation source to the photoresist layer to form a patterned photoresist layer. The method further comprises developing and etching the patterned photoresist layer to form a circuit layout.

[0050] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1 , which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel (i.e., a target material), such as tin (Sn) which is provided from, e.g., a fuel generator 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel generator 3 may comprise a nozzle configured to direct the fuel, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the fuel at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons with ions of the plasma 7.

[0051] The EUV radiation from the plasma 7 is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A firstone of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0052] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beam expander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0053] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0054] To clarify the disclosure, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axis. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y-axis is referred to as an Ry -rotation. A rotation around about the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the disclosure and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the disclosure. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane. In the figures, the height of the substrate W is indicated as the Z-direction.

[0055] Description of alignment measurements

[0056] Integrated circuits (ICs) are built up layer by layer, and modern ICs can have 30 or more layers. On Product Overlay (OPO) is a measure of a system's ability to print these layers accurately on top of each other. Successive layers or multiple processes on the same layer must be accurately aligned to the previous layer. Otherwise, electrical contact between structures will be poor and the resulting devices will not perform to specification. Good overlay improves device yield and enables smaller product patterns to be printed. The overlay error between successive layers formed in or on the patterned substrate is controlled by various parts of the the lithographic apparatus.

[0057] Process-induced wafer errors are a significant impediment to OPO performance. Process- induced wafer errors are attributable to the complexity of printed patterns as well as an increase of the number of printed layers. This error is of relatively high spatial variation that is different from wafer to wafer, and within a given wafer.

[0058] In order to control the lithographic process to place device features accurately on the substrate, one or more alignment marks are generally provided on, for example, the substrate, and the lithographicapparatus includes one or more alignment measurement systems by which the position of the mark may be measured accurately. The alignment measurement system may be effectively a position measuring apparatus. Different types of marks and different types of alignment measurement systems are known from different times and different manufacturers. Measurement of the relative position of several alignment marks within the field can correct for process-induced wafer errors. Alignment error variation within the field can be used to fit a model to correct for OPO within the field

[0059] Lithographic apparatus are known to use various alignment measurement systems to align the substrate with respect to the lithographic apparatus. The data can for example be obtained with any type of alignment measurement system. A first example is a SMASH (SMart Alignment measurement system Hybrid) sensor, as described in U.S. Pat. No. 6,961,116, issued Nov. 1, 2005 and titled “Lithographic Apparatus, Device Manufacturing Method, and Device Manufactured Thereby,” which is hereby incorporated by reference herein in its entirety, and which employs a self-referencing interferometer with a single detector and four different wavelengths and extracts the alignment signal in software. A second example is an ATHENA (Advanced Technology using High order ENhancement of Alignment) sensor, as described in U.S. Pat. No. 6,297,876, issued Oct. 2, 2001 and titled “Lithographic Projection Apparatus with an Alignment measurement system for Aligning Substrate on Mask,” which is hereby incorporated by reference in its entirety, and which directs each of seven diffraction orders to a dedicated detector. A third example is an ORION sensor, which uses multiple polarizations per available signal (color), and may use multiple colors.

[0060] Reference is made in particular to the European application No. EP 1 372040 Al, granted Mar. 5, 2008 and titled “Lithographic Apparatus and Device Manufacturing Method”, which document is hereby incorporated by reference in its entirety. EP 1 372 040 Al describes an alignment measurement system using a self-referencing interferometer that produces two overlapping images of an alignment mark. These two images are rotated over 180° with respect to each other. EP 1 372 040 Al further describes the detection of the intensity variation of the interfering Fourier transforms of these two images in a pupil plane. These intensity variations correspond to a phase difference between different diffraction orders of the two images, and from this phase difference positional information is derived, which is required for the alignment process. Reference is also made to U.S. Pat. No. 8,610,898, “Self- Referencing Interferometer, Alignment measurement system, and Lithographic Apparatus” issued Dec. 17, 2013, the entire contents of which are hereby incorporated by reference in their entirety.

[0061] Such alignment measurement systems typically measure alignment marks in the form of scribe lane marks to determine a position of each mark. A wafer grid is determined from these mark positions. An accurate wafer grid reduces the potential for overlay errors.

[0062] Figure 2 schematically illustrates an alignment measurement system 16 according to an embodiment of the invention. An illumination source 11 emits a spatially coherent beam of radiation 18 which is reflected by an angled reflector 19 and illuminates an alignment mark WM on a substrate (e.g., a wafer). The alignment mark WM scatters the radiation into positive and negative diffraction orders+n and -n. These diffraction orders are collimated by an objective lens 12 and enter a self-referencing interferometer 13. The zero diffraction order may be blocked by the angled reflector 19. The selfreferencing interferometer 13 outputs two images of the input, which have a relative rotation of 180°, and which overlap and which can therefore be made to interfere. In a pupil plane 14, the overlapping Fourier transforms of these images can be seen and be made to interfere (the different diffraction orders being separated in this pupil plane 14). Two detection modules 15 in the pupil plane detect the interfered diffraction orders to provide position information. Based on this position information a substrate can be aligned accurately with respect to a lithographic apparatus. The right-hand part of Figure 2 shows the formation of two overlapping images in the pupil plane 14; for one image +n'and -n'are rotated by +90° relative to the input diffraction orders +n and -n; for the other image +n" and -n"are rotated by -90° relative to the input diffraction orders +n and -n. In the pupil plane the orders of respectively (+n'and - n'), and (+n"and -n') interfere. The self -referencing interferometer 13 and one or more detection modules may be referred to as a detection system.

[0063] The detection modules 15 may comprise single pixel detectors (e.g. photodiodes). Outputs from the detection modules 15 provide a measurement of the intensity variation arising from the interference between the + order and - order. This interference creates two channels, a SUM channel in which the electric fields are added and a DIFF channel in which the electric fields are subtracted. These two channels are 180 degrees out of phase with each other. The alignment position of the mark is computed by the processor 17 by measuring the phase of the signal at the DIFF or SUM or a combination of the two channels. Further details regarding the generation of the two channels are provided in US10558131B2.

[0064] The alignment measurement system 16 further comprises a delay loop 20. The delay loop 20 is illustrated in more detail in Figure 3.

[0065] Figure 3 schematically depicts the alignment measurement system 16, including a schematic depiction of the delay loop 20.

[0066] The incident radiation beam 18 is made up of alternating p-polarized and s-polarized pulses. These are schematically indicated by eight pulses at the bottom left of Figure 3, diagonal line shading indicating p-polarized pulses and horizontal line shading indicating s-polarized pulses. The radiation beam 18 is incident upon an alignment mark WM on a substrate. The alignment mark WM may be a grating. The radiation beam 18 is scattered by the alignment mark (e.g. grating) WM. The scattering of the radiation beam 18, which may be referred to as diffraction, modifies the polarization of the radiation beam. Specifically, a p-polarized or s-polarized radiation pulse is converted on scattering by the alignment mark WM into radiation which has a spread of polarizations. The scattered radiation is no longer p or s polarized. The scattered radiation may have a circular polarization, elliptical polarization, a linear polarization at an angle relative to the p or s polarization directions (e.g. at around 45° relative to the p or s polarization direction), or some other polarization which is not p or s polarization. In general, each pulse of scattered radiation may include a p polarization component and an s polarizationcomponent. In Figure 3, p-polarized radiation is indicated using rectangles and s-polarized radiation is indicated using circles. The radiation scattered from the alignment mark WM includes p-polarized radiation and s-polarized radiation.

[0067] The delay loop comprises a polarizing beam splitter 22. The polarizing beam splitter 22 is configured to reflect s-polarized radiation and to transmit p-polarized radiation. The reflected s- polarized part of a radiation pulse may be referred to as an s-polarized sub-pulse. The transmitted p- polarized part of the radiation pulse may be referred to as p-polarized sub-pulse.

[0068] The delay loop 20 further comprises a first quarter-wave plate 24 and a first reflector 26. The first reflector 26 may for example be a mirror or a corner-cube. The first quarter-wave plate 24 receives s-polarized radiation reflected by the polarizing beam splitter 22 and converts the s-polarized radiation into circularly polarized radiation. The first reflector 26 reflects the radiation back through the first quarter-wave plate 24, which converts the radiation to p-polarized radiation.

[0069] The p-polarized radiation is incident at the polarizing beam splitter 22 and passes through to an opposite side of the polarizing beam splitter. The radiation is incident at a second quarter-wave plate 26 and then incident at a second reflector 28. The second reflector 28 may for example be a mirror or a corner-cube. The second quarter-wave plate 26 converts the p-polarized radiation to circularly polarized radiation. The second reflector 30 reflects the radiation back through the second quarter- wave plate 28. The second quarter-wave plate 28 converts the circularly polarized radiation to s-polarized radiation. The s-polarized radiation enters the polarising beam splitter 22 and is reflected by the polarizing beam splitter towards the self-referencing interferometer 13.

[0070] The self -referencing interferometer 13 therefore receives p-polarized radiation sub-pulses which have passed directly through the polarizing beam splitter 22 without being delayed by the delay loop 20, and receives s-polarized radiation sub-pulses which have been delayed by the delay loop 20. That is, the s-polarized radiation sub-pulses are delayed relative to the p-polarized radiation sub-pulses when they enter the self-referencing interferometer 13.

[0071] The illumination source 11 comprises a laser and a delay loop. The delay loop is polarization- selective and is configured to provide as an output p-polarized radiation pulses which have not been delayed and s-polarized radiation pulses which have been delayed (or vice versa). The delay loop may have a path length which is equal to half of the separation between incident consecutive pulses, so that the p and s polarized pulses are equally separated in time. A different path length, and thus a different separation in time, may be used. Other methods may be used to generate alternating p and s polarized pulses.

[0072] The radiation beam 18 provided by the illumination source 11 is reflected by an angled reflector 19 and is focussed by a lens 12 onto the alignment mark WM. The alignment mark WM scatters the radiation as described above. The scattered radiation comprises s and p polarized components. The scattered radiation is collimated by the lens 12. Zero-order radiation is blocked by the angled reflector 19. Other diffraction orders are not blocked and pass to the polarizing beam splitter 22.

[0073] The scattered radiation beam as incident at the polarizing beam splitter 22 comprises p- polarization and s-polarization (irrespective of the polarization of the radiation beam pulse that was incident at the alignment mark WM.) This is schematically depicted in Figure 3 which shows the scattered radiation as comprising p-polarized radiation (rectangles) and s-polarized radiation (circles). The p-polarized radiation travels directly through the polarizing beam splitter 22 to the self-referencing interferometer 13. The s-polarized radiation is delayed by the delay loop 20 before it enters the selfreferencing interferometer 13. Radiation output from the self-referencing interferometer 13 is incident at detection modules 15.

[0074] The self -referencing interferometer 13 provides an output signal which consists of pairs of pulses, each pair comprising a p-polarized pulse and an s-polarized pulse. The p-polarized pulse precedes the s-polarized pulse. Four pairs of pulses 30a-d are depicted in Figure 3. The first pair of pulses 30a (which may be referred to as sub-pulses) has horizontal line shading, indicating that it corresponds to an s-polarized pulse of the incident radiation beam 18. The p-polarized pulse of the pulse pair 30a arrives at the detection modules 15 first because it has travelled directly through the polarizing beam splitter 22. The s-polarized pulse of the pair 30a has been delayed by the delay loop 20 and is received at the detection modules 15 after the p-polarized pulse.

[0075] A second pair of pulses 30b (which may be referred to as sub-pulses) has diagonal line shading, indicating that it was generated by a p-polarized pulse of the incident radiation beam 18. The second pair 30b consists of a p-polarized pulse which is detected first, followed by an s-polarized pulse. A third pair of pulses 30c (which may be referred to as sub-pulses) has horizontal line shading, indicating that it was generated by an s-polarized pulse of the incident radiation beam 18. The third pair 30c consists of a p-polarized pulse which is detected first, followed by an s-polarized pulse. A fourth pair of pulses 30d (which may be referred to as sub-pulses) has diagonal line shading, indicating that it was generated by a p-polarized pulse of the incident radiation beam 18. The fourth pair 30b consists of a p-polarized pulse which is detected first, followed by an s-polarized pulse. Other pairs of pulses which are not depicted are incident at the detection modules 15.

[0076] Advantageously, signals output from the detection modules 15 provide all information carried by p and s pulses of scattered radiation for incident p-polarized radiation pulses and also incident s- polarized radiation pulses. Thus, the output from the detection modules 15 provides complete polarization state information (all elements of the Jones matrix are provided). Because complete polarization information is provided, a more accurate measurement of the position of the alignment mark WM may be achieved.

[0077] In an embodiment (not depicted) a birefringent material may be located upstream of the polarizing beam splitter 22. The birefringent material may delay one polarization of the scattered radiation beam relative to an orthogonal polarization. This may provide an additional delay which is added to the delay provided by the delay loop 20.

[0078] The illumination source 11 may be a laser system. The illumination source 11 may be configured to provide pulses at a repetition rate of for example 5 MHz or more, e.g. up to 10 MHz or up to 100 MHz. The delay loop 20 doubles the number of pulses of the radiation beam. This effectively doubles the repetition rate of pulses to be detected by the detection modules 15 if the delay loop 20 is sufficiently long to add a separation which is half of the period between pulses output from the illumination source. The delay loop may have a length which corresponds with half of the period between pulses output from the illumination source. The delay loop may have a length which is less than half of the period between pulses output from the illumination source. A pair of pulses detected by the detection modules 15 may be closely separated in time (e.g. separated by as little as 1 ns).

[0079] The detection modules 15 may be synchronised with respect to the pulses which are emitted by the illumination source 11. Synchronisation may for example be provided by connecting an output signal from the illumination source 11 to the detection modules 15. Alternatively, synchronisation may be provided using a signal from a detector which is configured to detect pulses of the radiation beam 18 emitted illumination source 11. In another example, synchronisation may be achieved by using the same clock signal to control operation of the illumination source 11 and control operation of the detection modules 15.

[0080] In an embodiment, not depicted, the delay loop comprises an optical fiber. The optical fiber may be a polarization preserving optical fiber. Radiation reflected by the polarizing beamsplitter is coupled into the optical fiber (e.g. using fiber coupling optics). The radiation travels along the optical fiber, which may have a length selected to provide a desired delay of sub-pulses of radiation. Radiation is coupled from an output end of the optical fiber (e.g. e.g. using fiber coupling optics). This output radiation is incident at an opposite face of the polarizing beamsplitter. The delayed radiation sub-pulses are reflected by the polarizing beamsplitter to the self-referencing interferometer.

[0081] Figure 4 is a circuit diagram which depicts a detection module 15 according to an embodiment of the invention. The detection module 15 comprises a photodiode 52 and a series of measurement circuits 54a, b. The detection module 15 comprises further measurement circuits which are not depicted in Figure 4. A first measurement circuit 54a comprises a capacitor 56a which on one side is connected via a switch 58a to the photodiode 52, and which is connected on the opposite side to ground. The capacitor 56a is referred to here as a charge storage capacitor. The switch 58a is referred to here as a measurement activation switch. The measurement activation switch is switchable between a charge accumulation position (depicted by a solid line in Figure 4) and a charge measurement position (depicted by a dashed line). When the measurement activation switch 58a is in the charge accumulation position, radiation incident upon the photodiode 52 generates charge which is stored at the capacitor 56a. The capacitor 56a may be referred to as the charge accumulation capacitor.

[0082] The measurement circuit 54a further comprises an operational amplifier 60a which has an input connected via the switch 58a to the charge accumulation capacitor 56a. An output from the operational amplifier 60a is connected to an analogue digital convertor 62a. A capacitor 64a and a switch 66a areconnected in parallel across the operational amplifier 60a. The capacitor 64a may be referred to as a measurement capacitor 64a. The switch 66a may be referred to as a reset switch. The reset switch 66a is switchable between a reset position (depicted by a solid line in Figure 4) and a charge to voltage conversion position (depicted by a dashed line).

[0083] The measurement circuit 54a (and other measurement circuits) may be referred to as a switched capacitor acquisition circuit because it uses a switch 58a to connect and disconnect a capacitor 56a to the photodiode 52.

[0084] The second measurement circuit 54b has the same configuration as the first measurement circuit 54a. The second measurement circuit thus comprises a charge accumulation capacitor 56b, measurement activation switch 58b, operational amplifier 60b, analogue to digital convertor 62b, measurement capacitor 64b, and reset switch 66b. Further measurement circuits may have the same configuration as the first and second measurement circuits.

[0085] Operation of the measurement activation switches 58a, b, and the reset switches 64a, b is synchronized with the pulses of radiation output from the illumination source 11 (see Figure 3). The switches 58a, b, 66a, b may initially have the configurations depicted in Figure 4. A first radiation pulse is incident at the photodiode 52. The photodiode 52 provides an output current which charge the first charge accumulation capacitor 56a. The amount of charge delivered to the first charge accumulation capacitor 56a depends upon the intensity of the pulse which is incident at the photodiode 52.

[0086] During accumulation of the charge from the photodiode 52 at the first charge accumulation capacitor 56a, the reset switch 66a is moved from the reset position to the charge to voltage conversion position. That is, the switch 66a is moved from the depicted position to the other position. This operation of the switch may occur before accumulation of the charge from the photodiode.

[0087] Once the radiation pulse incident at the photodiode 52 has finished (as determined by the synchronisation system together with knowledge of the radiation pulse duration), the measurement activation switch 58a is switched to connect the charge accumulation capacitor 56a to the operational amplifier 60a. The operational amplifier 60a and measurement capacitor 64a convert the charge from the charge accumulation capacitor 56a into a voltage. The analogue to digital convertor 62a converts the voltage to a digital value. The digital value is output to the processor 17 (see Figure 3).

[0088] The reset switch 66a is then moved to the reset position to allow the charge across the measurement capacitor 64a to be removed. This resetting is desirable to prevent the operational amplifier 560a from reaching its maximum output voltage (approx, equal to its supply voltage).

[0089] Advantageously, the reset switch 66a, by removing the charge across the measurement capacity 64a, causes the entire circuit to behave more-deterministically. That is, for every detected radiation pulse, the circuit will always start from the same state and thus the analogue to digital convertor 62a ADC will be exposed to a circuit response that is consistent with respect to the energy of the detected radiation pulses. This helps with calibration / correction of the circuit response. It is also possible to(selectively) integrate multiple radiation pulses before resetting using the reset switch 66a (e.g. resetting every 4th pulse). However, this does not provide any significant benefit.

[0090] At a time between the end of the first pulse of radiation and the start of the second pulse of radiation, the measurement activation switch 58b is moved to the charge accumulation position. This may be done immediately after the end of the first pulse of radiation. The second measurement circuit 54b is then ready to receive charge from the photodiode 52. The reset switch 66b may have already been used to remove charge across the measurement capacitor 64b. The reset switch 66b is in the voltage conversion position (as depicted).

[0091] Operation of the second measurement circuit 54b is the same as the above described operation of the first measurement circuit 54a. That is, charge is accumulated at the charge accumulation capacitor 56b during the radiation pulse. One the radiation pulse has ended, the measurement activation switch 58b is moved to the charge measurement position. The charge is converted to a voltage by the second operational amplifier 60b and then converted to a digital value by the second analogue to digital convertor 62b.

[0092] Additional measurement circuits (not depicted) are used in series to detect additional pulses of the radiation beam. Although only two measurement circuits are depicted, many more measurement circuits may be provided. For example, ten or more measurement circuits may be provided, 100 or more measurement circuits may be provided. Operation of each measurement circuit may be as described above in connection with the first and second measurement circuits 54a, b. In one example, the laser source 11 may have a repetition rate of 200 MHz, and the delay loop 20 will double this to 400 MHz. If the analogue to digital convertor has a maximum sampling rate of 2 million samples per second, then 200 measurement circuits may be needed. In another example, the laser source 11 may have a repetition rate of 5 MHz, and the delay loop 20 will double this to 10 MHz. If the analogue to digital convertor has a maximum sampling rate of 2 million samples per second, then 5 measurement circuits may be needed. Other numbers of measurement circuits may be used.

[0093] In general, when one charge accumulation capacitor 56a, b is connected to the photodiode 52, the other charge accumulation capacitors are not connected to the photodiode. Thus, at any given time a radiation pulse is only measured by one charge accumulation capacitor 56a, b of the detection module 15.

[0094] Advantageously, the detection module 15 depicted in Figure 4 provides very fast detection of radiation pulses. This allows measurement of the intensity of each pulse incident at the photodiode 52. Advantageously, this is achieved without connecting a resistor in parallel with each measurement capacitor 64a, b. Connecting a resistor in parallel with each measurement capacitor would cause the measurement circuit to operate at a much lower frequency, meaning that it would not be suitable for pulse by pulse measurements that are provided by embodiments of the invention. Advantageously, by providing pulse by pulse measurements, embodiments of the invention avoid phase delay drift that may negatively affect alignment measurements.

[0095] The detection module 15 may be provided as an application- specific integrated circuit (ASIC). Providing the detection module 15 as an ASIC will reduce signal noise compared with building the detection module on a PCB. Multiple detection modules 15 may be provided as a single ASIC. The switches 58a,b, 64a, b of the detection module may for example be MOSFETS.

[0096] The detection module 15 may be capable of operating in the GHz domain.

[0097] Two detection modules 15 may be provided, for example as depicted in Figures 2 and 3. Advantageously, because the detection modules 15 are able to separately detect p-polarized and s- polarized radiation scattered from the alignment mark WM, there is no need to use separate selfreferencing interferometers to separately detect the p-polarized and s-polarized radiation. This provides a substantial reduction of costs.

[0098] Although a particular form of delay loop is described above in connection with Figure 3, any suitable form of delay loop may be used.

[0099] The above described embodiment of the measurement system includes a self-referencing interferometer which measures the position of an alignment mark. In other embodiments a different apparatus may be used. For example, the measurement system may include an apparatus which forms an image of an alignment mark at a detection grating.

[0100] In some situations it may be desirable to only detect a single scattered radiation pulse per incident radiation pulse. For example, it may be desirable to only detect p-polarized scattered radiation for s-polarized incident radiation, and to only detect s-polarized scattered radiation for p-polarized incident radiation. In another example, it may be desirable to only detect p-polarized scattered radiation for p-polarized incident radiation, and to only detect s-polarized scattered radiation for s-polarized incident radiation. Where this is the case, the timing of operation of the measurement circuits 54a, b may be selected accordingly.

[0101] The switched capacitor acquisition circuits are used to time-demultiplex the incident s and p polarized radiation sub-pulses.

[0102] An alternative embodiment of the invention is schematically depicted in Figure 5. The embodiment of Figure 5 corresponds with the embodiment of Figure 3 except that a beamsplitter 70 is provided after the delay loop 20 and before the self-referencing interferometer 13. The beamsplitter 70 is not a polarizing beamsplitter, but instead splits incident radiation irrespective of polarization. A portion of incident radiation is transmitted by the beamsplitter 70 and enters the self-referencing interferometer 13. Operation of the self -referencing interferometer 13 and the detection modules 15 is as described above.

[0103] A portion of incident radiation is reflected by the beamsplitter 70 and travels to a pair of reflectors 72a, b. The pair of reflectors 72a, b are positioned to receive the reflected radiation. The pair of reflectors 72a, b are located in a pupil with respect to the alignment mark WM. A first reflector 72a receives positive (+) diffraction orders (e.g. the +lstdiffraction order as depicted), and directs the positive diffracted radiation to a first detection module 15a. A second reflector 72b receives negative (-) diffraction orders (e.g. the -1stdiffraction order as depicted), and directs the negative diffracted radiation to a second detection module 15b. The detection modules 15a, b may be as described above in connection with Figure 4.

[0104] The radiation that is incident at the first detection module 15a is schematically depicted. The radiation is positive diffraction orders (e.g. the +lstdiffraction order as depicted), and consists of pairs of pulses, each pair comprising a p-polarized pulse and an s-polarized pulse. The p-polarized pulse precedes the s-polarized pulse. Three pairs of pulses 74a-c are depicted in Figure 3. The first pair of pulses 74a (which may be referred to as sub-pulses) has horizontal line shading, indicating that it corresponds to an s-polarized pulse of the incident radiation beam 18. The p-polarized pulse of the pulse pair 74a arrives at the detection module 15a first because it has travelled directly through the polarizing beam splitter 22. The s-polarized pulse of the pair 74a has been delayed by the delay loop 20 and is received at the detection module 15a after the p-polarized pulse. The next pair of pulses 74b has diagonal line shading, indicating that it corresponds to a p-polarized pulse of the incident radiation beam 18. The next pair of pulses 74c has horizontal line shading, indicating that it corresponds to an s-polarized pulse of the incident radiation beam 18.

[0105] The radiation that is incident at the second detection module 15b is not depicted. This radiation corresponds with the depicted radiation except that radiation is negative diffraction orders (e.g. the -1stdiffraction order as depicted).

[0106] The first and second detection modules 15a,b provide measurements of the intensity of the electric field for the positive and negative diffraction orders.

[0107] Signals output from the detection modules 15a, b may be processed by a processor 17. The signals output from the detection modules 15a, b contain information that can be used to correct for phase position offset arising from an asymmetric alignment mark WM. Specifically, when the alignment mark WM is asymmetric it will preferentially scatter radiation into the positive or negative diffraction orders. The embodiment depicted in Figure 5, by separately measuring the positive and negative diffraction orders, is able to provide a measurement of the alignment mark asymmetry.

[0108] In general, separation of the positive and negative diffraction orders may be obtained by locating a reflector in the pupil, the reflector being configured to reflect part of the radiation in the pupil (the positive diffraction orders or the negative diffraction orders). Using a beamsplitter to split off some of the radiation allows these measurements to be made at the same time as measurements are made using the self-referencing interferometer 13.

[0109] In general, separation of the positive and negative diffraction orders may be obtained by locating detection modules 15a,b in the pupil. One detection module may be configured to detect positive diffraction orders. The other detection module may be configured to detect negative diffraction orders.

[0110] An arrangement in which different detection modules are configured to detect positive and negative diffraction orders may be referred to as a detection system.

[0111] Although the embodiment depicted in Figure 5 includes the self-referencing interferometer 13 in addition to the detection of positive and negative diffraction orders, in other embodiments the selfreferencing interferometer may be omitted.

[0112] In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A substrate alignment measurement system comprising: an illumination source configured to provide a pulsed radiation beam comprising alternating s and p polarized radiation pulses; a first optical module configured to direct the pulsed radiation beam towards an alignment mark on a substrate; a second optical module configured to receive diffracted radiation from the alignment mark, the second optical module comprising a delay loop configured to provide temporally separated s and p polarized sub-pulses from each received pulse of the diffracted radiation; and a detection system configured to receive the s and p polarized sub-pulses from the second optical module and provide an output indicative of the intensity of each sub-pulse.2. The substrate alignment measurement system of clause 1, wherein the delay loop comprises a pair of arms provided on opposite sides of a polarizing beam splitter.3. The substrate alignment measurement system of clause 2, wherein each arm includes a quarter-wave plate.4. The substrate alignment measurement system of clause 1, wherein the delay loop comprises an optical fiber.5. The substrate alignment measurement system of clause 2 or clause 3, wherein the delay loop has an optical path length of at least 30 cm.6. The substrate alignment measurement system of any preceding clause wherein the pulsed radiation beam has a pulse frequency of up to 100 MHz7. The substrate alignment system of any preceding clause, wherein the delay loop comprises a birefringent material configured to delay sub-pulses with a given polarization relative to sub-pulses with a different polarization.8. The substrate alignment measurement system of any preceding clause, wherein the detection system comprises a self-referencing interferometer.9. The substrate alignment measurement system of any preceding clause wherein the detection system is configured to provide a comparison of an intensity of positive diffraction and an intensity of negative diffraction.10. The substrate alignment measurement system of clause 9, wherein the detection system comprises a reflector located in a pupil which is configured to reflect positive or negative diffraction orders of diffracted radiation towards a detection module.11. The substrate alignment measurement system of clause 8 and clause 9, further comprising a beamsplitter configured to direct some radiation to the self-referencing interferometer and some radiation to the detection system configured to provide the comparison of the intensity of positive diffraction and the intensity of negative diffraction.12. The substrate alignment measurement system of any preceding clause wherein the detection system comprises a detection module.13. The substrate alignment measurement system of clause 12, wherein the detection module comprises a photodiode connected to a plurality of switched capacitor acquisition circuits which are synchronized with respect to the pulsed radiation beam.14. The substrate alignment measurement system of clause 13, wherein each switched capacitor acquisition circuit comprises a measurement activation switch which connects each switched capacitor acquisition circuit to a detector at a different time.15. The substrate alignment measurement system of clause 13 or clause 14, wherein each switched capacitor acquisition circuit comprises a reset activation switch which is configured to remove charge across a measurement capacitor of the circuit.16. A lithographic apparatus or lithographic tool comprising the substrate alignment measurement system of any of clauses 1 to 15.17. A method of measuring substrate alignment comprising: directing a pulsed radiation beam comprising alternating s and p polarized radiation pulses onto an alignment mark on a substrate; receiving radiation diffracted radiation from the alignment mark and using a delay loop to temporally separate s and p polarized sub-pulses from each received pulse of the diffracted radiation; and using a detection system to measure the s and p polarized sub-pulses from the delay loop and provide an output indicative of the intensity of each sub-pulse.18. The method of clause 17, wherein the detection system comprises a self-referencing interferometer.19. The method of clause 17 or clause 18, wherein the detection system is configured to provide a comparison of an intensity of positive diffracted radiation and negative diffracted radiation.20. The method of any of clauses 17 to 19, wherein the detection system further comprises a detection module which includes switched capacitor acquisition circuits that are synchronised with respect to the pulsed radiation beam.21. The method of clause 20, wherein the switched capacitor acquisition circuits are used to timedemultiplex the incident s and p polarized sub-pulses.

[0113] In this document, references to scattering by an alignment mark WM may be considered to encompass diffraction by the alignment mark.

[0114] While the methods described herein have been described in relation to a substrate W that has, or will be, exposed to lithographic radiation (i.e. the radiation beam B), as will be clear to the skilled person, the methods (and corresponding apparatus) may beneficially be adapted for use with substrates that are not exposed to lithographic radiation.

[0115] Embodiments of the invention may form part of a lithographic apparatus (e.g. as depicted). Embodiments of the invention may form part of an exposure apparatus, which may for example be a lithographic apparatus or an actinic inspection apparatus. Embodiments of the invention may form part of a lithographic tool. Examples of lithographic tools are mentioned further below.

[0116] Likewise, while the apparatus described herein has been described in relation to an EUV lithographic apparatus, the apparatus may be beneficially used with a DUV lithographic apparatus.

[0117] Although specific reference may be made in this text to the use of a 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, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0118] 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 (e.g. other exposure apparatus). 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 mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0119] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0120] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executingthe firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0121] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A substrate alignment measurement system comprising: an illumination source configured to provide a pulsed radiation beam comprising alternating s and p polarized radiation pulses; a first optical module configured to direct the pulsed radiation beam towards an alignment mark on a substrate; a second optical module configured to receive diffracted radiation from the alignment mark, the second optical module comprising a delay loop configured to provide temporally separated s and p polarized sub-pulses from each received pulse of the diffracted radiation; and a detection system configured to receive the s and p polarized sub-pulses from the second optical module and provide an output indicative of the intensity of each sub-pulse.

2. The substrate alignment measurement system of claim 1, wherein the delay loop comprises a pair of arms provided on opposite sides of a polarizing beam splitter.

3. The substrate alignment measurement system of claim 2, wherein each arm includes a quarterwave plate.

4. The substrate alignment measurement system of claim 1, wherein the delay loop comprises an optical fiber.

5. The substrate alignment measurement system of claim 2 or claim 3, wherein the delay loop has an optical path length of at least 30 cm.

6. The substrate alignment measurement system of any preceding claim, wherein the pulsed radiation beam has a pulse frequency of up to 100 MHz7. The substrate alignment system of any preceding claim, wherein the delay loop comprises a birefringent material configured to delay sub-pulses with a given polarization relative to sub-pulses with a different polarization.

8. The substrate alignment measurement system of any preceding claim, wherein the detection system comprises a self-referencing interferometer.

9. The substrate alignment measurement system of any preceding claim, wherein the detection system is configured to provide a comparison of an intensity of positive diffraction and an intensity of negative diffraction.

10. The substrate alignment measurement system of claim 9, wherein the detection system comprises a reflector located in a pupil which is configured to reflect positive or negative diffraction orders of diffracted radiation towards a detection module.

11. The substrate alignment measurement system of claim 8 and claim 9, further comprising a beamsplitter configured to direct some radiation to the self-referencing interferometer and some radiation to the detection system configured to provide the comparison of the intensity of positive diffraction and the intensity of negative diffraction.

12. The substrate alignment measurement system of any preceding claim wherein the detection system comprises a detection module.

13. The substrate alignment measurement system of claim 12, wherein the detection module comprises a photodiode connected to a plurality of switched capacitor acquisition circuits which are synchronized with respect to the pulsed radiation beam.

14. The substrate alignment measurement system of claim 13, wherein each switched capacitor acquisition circuit comprises a measurement activation switch which connects each switched capacitor acquisition circuit to a detector at a different time.

15. The substrate alignment measurement system of claim 13 or claim 14, wherein each switched capacitor acquisition circuit comprises a reset activation switch which is configured to remove charge across a measurement capacitor of the circuit.

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