Optical alignment system and method

Abstract

The present disclosure provides an optical alignment system comprising: an illumination system configured to condition a radiation beam to form a first off-axis monopole; a marker configured to diffract the first off-axis monopole to form zero-order and first-order diffracted beams; a projection system configured to collect the zero-order and first-order diffracted beams and form an image of the marker; and a sensor device configured to detect an image of the marker.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP Application No. 22196619.5 filed on September 20, 2022, and the entire contents of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present disclosure relates to an optical alignment system and method. The optical alignment system can be applicable to a lithography device. The present disclosure is particularly applicable to an EUV lithography device and an EUV lithography tool. Background Art

[0004] A lithographic apparatus is a machine used to apply a desired pattern to a substrate. For example, a lithographic apparatus can be used to manufacture integrated circuits (ICs). A lithographic apparatus can project a pattern onto a patterning device (e.g., a mask) and project it onto a layer of radiation-sensitive material (photoresist) provided on a substrate.

[0005] 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 that can be formed on the substrate. For example, a lithographic apparatus using extreme ultraviolet (EUV) radiation, whose wavelength is in the range of 4-20 nm (e.g., 6.7 nm or 13.5 nm), can form smaller features on a substrate than a lithographic apparatus using radiation with a wavelength of 193 nm.

[0006] The lithographic apparatus may include an optical alignment system that can be used to determine and improve the alignment between a patterning device and a substrate. The patterning device may include a marker that can be imaged by a projection system of the lithographic apparatus. The marker can impart a mark to the radiation beam that can subsequently be measured to deduce one or more properties of the lithographic apparatus. The optical alignment system may include a sensor device configured to detect an image of the marker to determine the position of the substrate relative to the patterning device.

[0007] It is beneficial to provide an optical alignment system to overcome or alleviate the problems associated with the prior art. The embodiments described in the present disclosure may be used in an EUV lithography apparatus. The embodiments may also be used in a DUV lithography apparatus or other forms of lithography apparatus. Summary of the invention

[0008] According to a first aspect of the present disclosure, there is provided an optical alignment system, comprising an illumination system configured to condition a radiation beam to form a first off-axis monopole. The optical alignment system comprises a marker configured to diffract the first off-axis monopole to form a zero-order and a first-order diffracted beam. The optical alignment system comprises a projection system configured to collect the zero-order and the first-order diffracted beams and form an image of the marker. The optical alignment system comprises a sensor device configured to detect the image of the marker.

[0009] The optical alignment system may be adapted for use in a lithographic apparatus. Radiation reflected from the marker may enter a projection system of the lithographic apparatus. Radiation reflected from the marker may be projected by the projection system onto a sensor located at or near an image plane of the projection system.

[0010] Off-axis can refer to the axis of the optical alignment system. An off-axis monopole can include illuminating only a region of the projection system pupil. The monopole can be located within the pupil such that the monopole is away from the center of the pupil (e.g., half of the pupil plane, or one quadrant of the pupil plane) and thus off-axis relative to the pupil.

[0011] The inventors have discovered that relatively large and isolated features of known markers in known optical alignment systems do not provide accurate alignment measurements when using off-axis monopole illumination. This is because the image formed by the known marker is severely skewed and extremely non-telecentric. This skewness and non-telecentric behavior causes unwanted positional shifts in the image formed by the marker, which are then detected by the sensor device. Focus fluctuations between different off-axis monopole illuminations may cause further unwanted positional shifts in the image formed by the marker.

[0012] The markers of the present invention advantageously diffract off-axis monopoles so as to fill more of the pupil of the optical alignment system, thereby reducing the skewness and increasing the telecentricity of the image formed by the marker compared to known markers. The markers of the present invention can advantageously improve the radiation balance on different regions of the projection system pupil compared to known markers. For example, the zeroth order diffracted beam can at least partially fill the first half of the pupil, and the first order diffracted beam can at least partially fill the second half of the pupil. This is advantageous when measuring radiation reflected from the marker order and radiation output from the projection system to determine the alignment of components of the lithographic apparatus. Typically, the radiation output from the projection system can be measured at or near the substrate level of the lithographic apparatus, which substrate level can correspond to the image plane of the projection system. In such an embodiment, it may be desirable for the radiation to substantially fill the pupil of the projection system.

[0013] Thus, the optical alignment system of the present invention advantageously improves the accuracy of optical alignment measurements performed using off-axis monopole illumination compared to known systems.

[0014] The marker may comprise a plurality of reflective regions configured to preferentially reflect the radiation beam. The marker may comprise a plurality of absorptive regions configured to preferentially absorb the radiation beam. The reflective regions and the absorptive regions may be arranged to form a reflective diffraction grating.

[0015] The reflective region mentioned herein is configured to preferentially reflect radiation of a given wavelength, which should be interpreted as the reflective region being configured so that the reflectivity of the reflective region is higher at the given wavelength than at other wavelengths. The reflective region may also reflect radiation having wavelengths other than the given wavelength.

[0016] For example, the reflective region may include a multilayer structure including layers of two or more materials having different refractive indices. Radiation may be reflected from interfaces between different layers. The layers may be arranged to provide separation between interfaces, resulting in constructive interference between radiation reflected at different interfaces. The separation between interfaces resulting in constructive interference between radiation reflected at different interfaces depends on the wavelength of the radiation. Therefore, the multilayer reflective region may be configured to preferentially reflect radiation of a given wavelength by providing separation between layer interfaces, resulting in constructive interference between radiation of a given wavelength reflected from different interfaces.

[0017] The reflective region may be disposed on the absorption layer, and the absorption region may include a region of the absorption layer where the reflective region is not disposed.

[0018] The radiation beam may include extreme ultraviolet radiation.

[0019] The pitch of the reflective diffraction grating may be of the order of the wavelength of the EUV radiation.

[0020] The minimum pitch of a reflective diffraction grating can be determined by the following formula:

[0021]

[0022] Where λ is the wavelength of the radiation beam and NA is the numerical aperture of the projection system.

[0023] The maximum pitch of the reflective diffraction grating may be less than about twice the minimum pitch.

[0024] The pitch of the reflective diffraction grating may be in the range of about 24 nm to about 44 nm, inclusive. The pitch of the reflective diffraction grating may be about 28 nm.

[0025] The reflective diffraction grating may include a terminal reflection and absorption region and a non-terminal reflection and absorption region located between the terminal reflection and absorption regions. The duty ratio of the terminal reflection and absorption region may be different from the duty ratio of the non-terminal reflection and absorption region.

[0026] The duty ratio of the non-terminal reflective and absorptive regions can be about 50% reflective region to about 50% absorptive region (i.e., about 50:50). The duty ratio of the terminal reflective region may not be about 50:50. The range of the terminal reflective region along the pitch of the reflective diffraction grating may be greater or less than the range of the non-terminal reflective region. Introducing a duty ratio difference in the terminal reflective region can advantageously reduce or avoid non-telecentricity associated with the terminal region of the reflective diffraction grating.

[0027] The marker may include a sub-resolution alignment feature.

[0028] Sub-resolution alignment features can be considered to be features that are small enough that after the lithography steps of development and etching, they do not create significant features of their own, while still being large enough to affect the imaging of adjacent features. Thus, the size and form of sub-resolution features can depend at least in part on the process used, and can be specific to the substrate layer. In general, any feature that is less than half the size of a feature to be imaged by the projection system can be considered "sub-resolution".

[0029] According to a second aspect of the present invention, there is provided a marker for use in an optical alignment system according to the first aspect of the present invention.

[0030] According to a third aspect of the present disclosure, a lithographic apparatus is provided, which includes the optical alignment system of the first aspect of the present disclosure. The lithographic apparatus includes a support structure, which is configured to support a pattern forming device, which is capable of imparting a pattern to the off-axis monopole in a cross-section of the off-axis monopole to form a patterned radiation beam. The marker forms a part of the support structure or the pattern forming device. The lithographic apparatus includes a substrate stage configured to accommodate a substrate. The sensor device forms a part of the substrate stage. The projection system is configured to project the patterned radiation beam onto the substrate. The optical alignment system is configured to determine the alignment between the pattern forming device and the substrate.

[0031] The illumination system may be configured to condition the radiation beam to form a second off-axis monopole.The first off-axis monopole and the second off-axis monopole may be located in different regions of a pupil plane of the lithographic apparatus.

[0032] The lithographic apparatus may be configured to perform a first lithographic exposure using a first off-axis monopole to form a first image of the patterning device on the substrate. The lithographic apparatus may be configured to perform a second lithographic exposure using a second off-axis monopole to form a second image of the patterning device on the substrate. The substrate stage may be configured to move between the first lithographic exposure and the second lithographic exposure such that the first and second images of the patterning device substantially overlap on the substrate.

[0033] According to a fourth aspect of the present disclosure, there is provided an optical alignment method, comprising adjusting a radiation beam to form a first off-axis monopole. The optical alignment method comprises diffracting the first off-axis monopole to form zero-order and first-order diffracted beams. The optical alignment method comprises collecting the zero-order and first-order diffracted beams. The optical alignment method comprises forming an image using the zero-order and first-order diffracted beams. The optical alignment method comprises detecting the image.

[0034] According to a fourth aspect of the present disclosure, a lithography exposure method is provided, comprising using the optical alignment method of the fourth aspect of the present disclosure to determine the alignment between a pattern forming device and a substrate. The lithography exposure method comprises using the pattern forming device to impart a pattern to the off-axis monopole on a cross-section of the off-axis monopole to form a patterned radiation beam. The lithography exposure method comprises projecting the patterned radiation beam onto the substrate.

[0035] The lithographic exposure method may include adjusting the radiation beam to form a second off-axis monopole.The first off-axis monopole and the second off-axis monopole may be located in different regions of the pupil plane.

[0036] The lithographic exposure method may include performing a first lithographic exposure using a first off-axis monopole to form a first image of a patterning device on a substrate. The lithographic exposure method may include performing a second lithographic exposure using a second off-axis monopole to form a second image of the patterning device on the substrate. The lithographic exposure method may include moving the substrate between the first lithographic exposure and the second lithographic exposure so that the first and second images of the patterning device substantially overlap on the substrate.

[0037] Features of different aspects of this disclosure may be combined with features of other aspects of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which:

[0039] Figure 1 A lithography system including a lithography apparatus, a radiation source and an optical alignment system according to the present disclosure is schematically shown.

[0040] Figure 2A A lithographic exposure using bipolar illumination mode is schematically shown.

[0041] Figure 2B Shows Figure 2A Figure 3. Intensity of the constituent aerial images and composite aerial images formed by lithography exposure as a function of wafer-level position.

[0042] Figure 3A The first off-axis monopole exposure of a dual monopole lithography exposure is schematically shown.

[0043] Figure 3BThe second off-axis monopole exposure of a dual monopole lithography exposure is schematically shown.

[0044] Figure 4A A pupil plane view of a known marker and an off-axis monopole used to illuminate the known marker is schematically shown.

[0045] Figure 4B A marker configured to diffract off-axis monopoles in accordance with the present disclosure is schematically shown.

[0046] Figure 5 A flow chart of an optical alignment method according to the present disclosure is shown. DETAILED DESCRIPTION

[0047] Figure 1 A lithographic system is shown, comprising a radiation source SO and a lithographic apparatus LA. 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 substrate table WT configured to support a substrate W.

[0048] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. To this end, the illumination system IL may include a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide the EUV radiation beam B with a desired cross-sectional shape and intensity distribution. The illumination system IL may include other mirrors or arrangements in addition to the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0049] After being so conditioned, the EUV radiation beam B interacts with the pattern forming 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 substrate W. To this end, the projection system PS may include a plurality of mirrors 13, 14, configured to project the patterned EUV radiation beam B' onto the substrate W supported by the substrate table WT. The projection system PS may apply a scaling factor to the patterned EUV radiation beam B' so as to form an image of features that are smaller than the corresponding features on the pattern forming device MA. For example, a scaling factor of 4 or 8 may be applied. Although Figure 1 The projection system PS in FIG. 1 is shown with only two mirrors 13 , 14 , but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0050] The substrate W may include a previously formed pattern. If this is the case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B′ with the previously formed pattern on the substrate W.

[0051] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, the illumination system IL and / or the projection system PS.

[0052] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source capable of generating EUV radiation.

[0053] As described above, a lithographic apparatus may be used to expose a portion of a substrate W to form a pattern in the substrate W. In order to improve the accuracy of transferring a desired pattern to the substrate W, one or more properties of the lithographic apparatus LA may be measured. For example, these properties may be measured regularly before and / or after each substrate W is exposed, or more infrequently as part of a calibration process. Examples of properties of the lithographic apparatus LA that may be measured include the relative alignment of components of the lithographic apparatus LA. For example, measurements may be made to determine the relative alignment of a support structure MT (for supporting a pattern forming device MA) and a substrate table WT (for supporting a substrate W). Determining the relative alignment of the support structure MT and the substrate table WT helps to project a patterned radiation beam onto a desired portion of the substrate W. This may be particularly important when projecting patterned radiation onto a substrate W that has already been exposed to radiation, so as to improve the alignment of the patterned radiation with previously exposed areas.

[0054] The above-described measurements can be made by irradiating the reflective marker 17 (e.g. Figure 1 The marker 17 is a reflective feature that, when placed in an optical system (e.g. Figure 1 When the reflective marker is in the field of view of the lithographic apparatus LA in the optical system, it appears in the image produced by the optical system. The reflective markers described herein are suitable for use as reference points and / or as a metric for measuring properties of an image formed by the optical system. For example, radiation reflected from the reflective marker can be used to determine the alignment of the optical system components.

[0055] exist Figure 1In the illustrated embodiment, the reflective marker 17 is part of the patterning device MA. One or more markers 17 may be provided on the patterning device MA for performing lithographic exposures. The marker 17 may be placed outside a pattern area of ​​the patterning device MA and irradiated with radiation during lithographic exposures. In some embodiments, the marker 17 may also be provided on the support structure MT. For example, dedicated hardware, typically referred to as a fiducial, may be provided on the support structure MT, which fiducial may include one or more markers. For the purposes of this description, the fiducial is considered to be an example of a patterning device. In some embodiments, a patterning device MA specifically designed for measuring one or more properties of a lithographic device LA may be placed on the support structure MT to perform a measurement process. The patterning device MA may include one or more markers 17 for irradiation as part of the measurement process.

[0056] exist Figure 1 In the embodiment shown, the lithography apparatus LA is an EUV lithography apparatus and therefore a reflective patterning apparatus MA is used. Therefore, the marker 17 is a reflective marker 17. The configuration of the marker 17 may depend on the nature of the measurement to be performed using the marker 17. Known markers include reflective pinhole features of one or more reflective regions surrounded by an absorptive region, reflective line features, an arrangement of multiple reflective line features, etc.

[0057] In order to measure one or more properties of the lithographic apparatus LA, a sensor arrangement 19 (e.g. Figure 1 ) for measuring the radiation output from the projection system PS. For example, the sensor device 19 may be Figure 1 The support structure MT may be provided on the substrate table WT as shown. In order to perform a measurement process, the support structure MT may be positioned so that the marker 17 on the patterning device MA is illuminated with radiation. The substrate table WT may be positioned so that radiation reflected from the marker is projected by the projection system PS onto the sensor arrangement 19. The sensor arrangement 19 is in communication with the controller CN which may determine one or more properties of the lithographic apparatus LA from the measurements of the sensor arrangement 19. In some embodiments, a plurality of markers 17 and / or sensor arrangements 19 may be provided and the properties of the lithographic apparatus LA may be measured at a plurality of different field points of the projection system PS (i.e. positions in the field or object plane of the projection system PS).

[0058] As described above, in some embodiments, radiation reflected from a marker can be used to determine the relative alignment of the components of the lithographic apparatus LA. In these embodiments, the marker 17 may include a feature that, when illuminated with radiation, imparts an alignment feature to the radiation. The feature may include one or more reflective patterns in the form of a grating structure.

[0059] This can be accomplished by positioning the substrate W at a level such as, for example, Figure 1The sensor arrangement 19 on the substrate table WT shown in FIG. 1 measures the position of alignment features in the radiation beam B. The sensor arrangement 19 is operable to detect the position of alignment features incident thereon. This allows the alignment of the substrate table WT relative to a marker on the patterning device MA to be determined. Knowing the relative alignment of the patterning device MA and substrate table WT, the patterning device MA and substrate table WT can be moved relative to each other so as to form a pattern at a desired position on the substrate W (using a patterned radiation beam B reflected from the patterning device MA). The position of the substrate W on the substrate table WT can be determined using a separate measurement process.

[0060] For example, the sensor device 19 may be a transmission image sensor (TIS). A TIS is a sensor that can be used to measure the position of a projected aerial image of a marker 17 at the level of a mask (reticle) MA at the level of a substrate W. The TIS is configured to measure the image of the marker 17 using a transmission mode with a radiation sensor located below the transmission mode. The measurement data generated by the sensor device can be used to measure the position of the mask MA relative to the substrate table WT in six degrees of freedom (three translations and three rotations). In addition, the magnification and scaling of the projected marker 17 image can be measured.

[0061] The illumination system IL may be configured to condition the radiation beam B to form different illumination modes. The illumination mode may be defined by the number of regions of a pupil plane of the lithographic apparatus LA illuminated by the radiation beam B. For example, a dipole illumination mode may include two opposing pupil plane portions illuminated with radiation, a quadrupole illumination mode may include four pupil plane regions illuminated with radiation, and so on. The pupil plane may be defined by the numerical aperture (NA) of the lithographic apparatus LA. That is, the pupil plane may be defined by the maximum angular distribution of radiation received by the lithographic apparatus LA. The pupil plane may be a Fourier transform plane of a plane in which the substrate W lies (which may be referred to as an object plane). Therefore, the electric field intensity distribution of the radiation on the pupil plane may be related to the Fourier transform of an object (e.g., a marker 17) on the object plane. In particular, the electric field intensity distribution of the radiation on the pupil plane (i.e., the angular distribution of radiation scattered by an object, such as the marker 17) may be given by: (a) the electric field intensity distribution of the radiation on the illuminated pupil plane (i.e., the angular distribution of radiation illuminating the object, such as the marker 17) and (b) the Fourier transform of the object.

[0062] Some errors may occur when using multipole illumination mode. Figure 2A A lithographic exposure performed using a dipole illumination mode is schematically depicted. The lithographic exposure may be performed using Figure 1 The lithography device LA is used in the Figure 2AIn the example of FIG. 1 , the dipole illumination pattern includes a right pole 201 and a left pole 202. The right pole 201 and the left pole 202 (i.e., the dipoles) are incident on a pattern forming device including a grating mask 210. The dipoles 201, 202 are formed by EUV radiation, and the grating mask 210 may, for example, have a pitch of about 32 nm. In practice, the grating mask 210 is a reflective grating mask configured to reflect the dipoles 201, 202. However, for simplicity and to aid understanding of the schematic diagram, Figure 2A The dipoles 201, 202 in FIG. 2 are shown as being transmitted through the grating mask 210. The dipoles 201, 202 diffract when interacting with the grating mask 210 and form a plurality of diffracted beams 220, 221. The right pole 201 diffracts to produce zero-order and first-order diffracted beams 220, 221, and the left pole diffracts to produce zero-order and first-order diffracted beams 220, 221. Other order diffracted beams (e.g., second order, third order, fourth order, etc.) are not collected by the projection system PS because they propagate at large angles and do not enter the limited numerical aperture of the projection system. Figure 2A In the example of FIG. 2 , the zero-order diffracted beam 220 and the first-order diffracted beam 221 are collected by the projection system PS and focused through the aperture 230 to form an aerial image 240 of the grating mask 210. At EUV wavelengths, the grating mask 210 interacts with the dipoles 201, 202 as a three-dimensional structure rather than a two-dimensional pattern. The three-dimensional effect at EUV wavelengths introduces a phase shift between the zero-order diffracted beam 220 and the first-order diffracted beam 221. The phase shift itself causes a position shift of the constituent aerial images formed by the dipoles of the right pole 201 and the left pole 202. The constituent aerial images combine to form a composite aerial image.

[0063] Figure 2B Shows Figure 2A The intensity of the composite aerial image 251, 252 and the composite aerial image 260 formed by the lithography exposure of the wafer level varies with the position. The position movement direction of the composite aerial image 251 formed by the right pole 201 is opposite to the position movement direction of the composite aerial image 252 formed by the left pole 202. Figure 2B In the example of , the constituent aerial image 251 formed by the right pole 201 moves about -3 nm, and the constituent aerial image 252 formed by the left pole 202 moves about +3 nm. The composite aerial image 260 formed by the right pole and left pole constituent aerial images 251, 252 has a reduced image contrast than the constituent aerial images 251, 252 taken separately. In general, the smaller the pitch of the grating mask 210, the more severe the reduction in image contrast. It is generally desirable to maximize image contrast. On the other hand, it may be desirable to use dipole illumination to maximize the depth from focus.

[0064] Another illumination mode provided by the illumination system IL may be referred to as an off-axis monopole illumination mode. The off-axis monopole illumination mode includes illuminating only a region of the pupil plane that is offset from the center of the pupil plane (e.g., half or a quadrant of the pupil plane). The off-axis monopole illumination mode has an optimal focus position on the substrate W. Thus, image contrast errors associated with conventional multipole illumination modes associated with optimal focus positions are avoided using the off-axis monopole illumination mode. This avoidance of errors facilitates the lithography apparatus LA to print features that were previously unprintable and / or to use less expensive and / or complex components (e.g., less expensive and / or complex reticle MA) when printing features.

[0065] Another illumination mode provided by the illumination system IL may be referred to as a dual monopole illumination mode. The dual monopole illumination mode involves splitting a conventional dipole illumination mode into a first off-axis monopole exposure (corresponding to either the right pole or the left pole) and a second off-axis monopole exposure (corresponding to the other pole). Each off-axis monopole exposure uses half the illumination radiation dose used by the conventional dipole exposure. Figure 3A The first off-axis monopole exposure of a dual monopole lithography exposure is schematically depicted. Figure 3A In the example, the first off-axis monopole corresponds to Figure 2A The double monopole exposure involves moving the wafer stage to take into account the positional movement of the constituent aerial images 251, 252 formed by the right pole 201 and the left pole 202. Figure 3A In the example of , the wafer stage is moved about -3 nm from the center position to account for the positional movement of the constituent aerial image formed by the right pole. The movement of the wafer stage is represented by the first arrow 271. The right pole 201 is diffracted by the grating mask 210. The zero-order diffracted beam 220 and the first-order diffracted beam 221 are collected by the projection system PS and focused through the aperture 230 to form a first aerial image 281. Given that only half the conventional dipole exposure dose is used, the intensity of the first aerial image 281 formed by the first off-axis monopole exposure is Figure 2A The photolithography exposure forms half of the aerial image 240 .

[0066] Figure 3B The second off-axis monopole exposure of the dual monopole lithography exposure is schematically depicted. Figure 3B In the example, the second off-axis monopole corresponds to Figure 2A The left pole 202 of the dipole. In order to consider the position shift of the spatial image formed by the left pole, Figure 3BIn the example of , the wafer stage is moved about +3 nm from the center position. The movement of the wafer stage is indicated by a second arrow 272. The left pole 202 is diffracted by the grating mask 210. The zero-order diffracted beam 220 and the first-order diffracted beam 221 are collected by the projection system PS and focused through the aperture 230 to form a second aerial image 282. Given that only half the conventional dipole exposure dose is used, the intensity of the second aerial image 282 formed by the second off-axis monopole exposure is Figure 2A The image contrast ratio of the composite aerial image formed by the first aerial image 281 and the second aerial image 282 is better than that of the composite aerial image 240 formed by the photolithography exposure of the wafer stage. Figure 2B That is, dual monopole illumination (e.g., Figure 3A , B) avoids the problems with conventional dipole irradiation (e.g. Figure 2A , B) associated image contrast loss. The image contrast of the composite aerial image formed by the first aerial image 281 and the second aerial image 282 may be equivalent to the image contrast of each of the constituent aerial images 281, 282 formed by the first and second off-axis monopole exposures. It should be understood that dual monopole exposure requires two separate exposures, rather than a single exposure, which requires more time, thereby introducing a throughput penalty compared to conventional dipole exposure. Therefore, dual monopole exposure may be reserved for particularly complex mask patterns where the improvement in image contrast is worth the reduction in throughput.

[0067] Known optical alignment system markers (e.g., known TIS markers) include relatively large and isolated features. That is, the area and / or pitch of the known marker may be significantly larger than the wavelength of the radiation used to illuminate the known marker. This is because the use of relatively large and isolated features may be advantageous for creating an aerial image that can measure various multipole illumination patterns. For example, a known EUV marker has an area of ​​approximately 100-500 μm by 100-500 μm and consists of large, isolated lines with a line width of approximately 100nm to 200nm and a spacing of approximately 1 μm to approximately 10 μm.

[0068] Known markers include large, isolated sets of lines. These features (e.g., lines) are described as isolated because the spacing between the features is so large that individual interference lines are not observed, and the diffraction pattern produced by the features is controlled by the size (e.g., line width) rather than the spacing. Multiple isolated lines can be formed into a set and used as a measurement marker to increase the total measurement signal and improve the signal-to-noise ratio of the measurement. In some cases, the spacing between the isolated lines in the set is constant, forming a grating-like structure. However, as previously described, the spacing between the lines in the grating-like structure is so large that individual interference lines are not observed, and the diffraction pattern produced by the lines is controlled by the line width, rather than the spacing. That is, the grating-like structure does not act as a diffraction grating.

[0069] Different reticle MAs may require the use of different illumination modes for optimal performance. A set of known markers includes relatively large and isolated features that can be placed on different reticle MAs and can provide adequate spatial images of various multipole illumination modes for alignment measurements. However, the relatively large and isolated features of the known markers do not provide accurate alignment measurements when using off-axis monopole exposure.

[0070] Figure 4A A pupil plane view 410 of a known marker 400 and an off-axis monopole 420 used to illuminate the known marker 400 is schematically depicted. The known marker 400 includes a reflective region 440 located between two absorptive regions 442, 444. Light reflected from the reflective region 440 is collected by a projection system (not shown) to form an aerial image 430 of the reflected portion 440. The intensity of the aerial image 430 is displayed along the axis of the pupil plane 410. Figure 4A In the example of FIG. 4 , the off-axis monopole 420 is located only to the left of the pupil plane 410. Likewise, the aerial image 430 of the reflected portion is also located only to the left of the pupil plane 410. The aerial image 430 formed by the known mark 400 is severely distorted and extremely non-telecentric due to the relatively finite and asymmetric angular distribution of the off-axis monopole 420. This non-telecentric behavior results in an unwanted position shift of the aerial image 430 at the sensor arrangement. This unwanted position shift may result in overlay errors when used as part of a lithographic apparatus.

[0071] Figure 4B Schematically depicted is a marker 450 configured to diffract an off-axis monopole 420 according to an embodiment of the present disclosure. The marker 450 is shown next to a pupil plane view 410 of the off-axis monopole 420 used to illuminate the marker 450. The marker is configured to diffract the off-axis monopole 420 to form zero-order and first-order diffracted beams 460, 461. Figure 4BIn the example of FIG. 4 , the zero-order diffracted beam 460 is located on the left side of the pupil plane 410, and the first-order diffracted beam 461 is located on the right side of the pupil plane 410. Therefore, the distortion of the aerial images 460 and 461 is reduced and the confocality is greater than Figure 4A Aerial image 430 (located only to the left of pupil plane 410). Projection system (not shown), e.g. Figure 1 The projection system PS is configured to collect the zeroth and first order diffracted beams 460, 461 and form an image of the marker 450. A sensor device (not shown), such as Figure 1 The sensor device 19 is configured to detect the image of the marker 450.

[0072] The marker 450 includes a plurality of reflective regions 470 configured to preferentially reflect off-axis monopoles 420, and a plurality of absorptive regions 472 configured to preferentially absorb off-axis monopoles 420. The reflective regions 470 and the absorptive regions 472 are arranged as a reflective diffraction grating that diffracts the off-axis monopoles 420. The reflective regions 470 may, for example, include a multilayer structure consisting of two or more layers of materials having different refractive indices. Radiation may be reflected from interfaces between different layers. The arrangement of the layers may provide a separation between the interfaces, thereby causing coherent interference between radiation reflected from different interfaces. The separation between the interfaces that causes the coherent interference depends on the wavelength of the radiation. Therefore, the multilayer reflective regions 470 may be configured to preferentially reflect radiation of a given wavelength (e.g., EUV radiation) by providing a separation between the interfaces that causes coherent interference between radiation reflected from different interfaces. The reflective regions 470 may be disposed on an absorptive layer, and the absorptive regions 472 may include regions of the absorptive layer where the reflective regions 470 are not disposed.

[0073] exist Figure 4B In the example of FIG. 4 , the off-axis monopole 420 is formed by extreme ultraviolet radiation. The pitch of the reflective diffraction grating formed by the reflective region 470 and the absorptive region 472 is comparable to the wavelength of the extreme ultraviolet radiation.

[0074] The minimum pitch of the reflective diffraction grating may be large enough so that the zero-order and first-order diffracted beams 460, 461 only fit into the pupil 410 (i.e., the zero-order and first-order diffracted beams are only collected by the projection system). The minimum pitch of the reflective diffraction grating may be determined by the following equation:

[0075]

[0076] Where λ is the wavelength of the radiation beam and NA is the numerical aperture of the projection system. For example, for EUV radiation with a wavelength of about 13.5 nm and a projection system with a numerical aperture of about 0.33, the minimum pitch of the reflective diffraction grating may be about 24 nm. The maximum pitch of the reflective diffraction grating may only be to prevent the second-order diffracted beam from also fitting into the pupil 410 (i.e., only prevent the second-order diffracted beam from entering the projection system). The maximum pitch of the reflective diffraction grating may be less than twice the minimum pitch. For example, for EUV radiation with a wavelength of about 13.5 nm and a projection system with a numerical aperture of about 0.33, the minimum pitch of the reflective diffraction grating may be about 44 nm. It should be understood that the maximum pitch is not a strict limitation, but more of a practical consideration. In view of the above, for example, the pitch of the reflective diffraction grating of the EUV lithography device may be about 28 nm.

[0077] To improve performance, the pitch of the markers may be matched to the illumination pattern of the illumination system to reduce or minimize the non-telecentricity and / or skewness of the aerial image of the markers. That is, the markers according to the present invention act to improve the radiation distribution or balance of the pupil. The position of the zero-order diffraction beam 460 in the pupil 410 may depend at least in part on the incident radiation beam or the illumination pattern, which in turn may depend at least in part on the spacing of the structure to be imaged by the projection system (i.e., to create improved or optimal telecentricity for the structure to be imaged by the projection system). Once the position of the zero-order diffraction beam 460 in the pupil 410 is determined, a first-order diffraction beam 461 can be formed by selecting an appropriate marker spacing to substantially mirror the position of the zero-order diffraction beam 460 in the pupil 410.

[0078] The reflective diffraction grating 450 includes terminal reflective regions 480, 482. The terminal reflective regions 480, 482 are adjacent to an absorptive region that does not continue the pattern of the reflective diffraction grating 450. Therefore, there is an asymmetry in the terminal reflective regions 480, 482, which may be seen as a different spacing compared to the spacing of the non-terminal regions 470, 472 of the diffraction grating 450. The terminal reflective regions 480, 482 may introduce skewness and / or non-telecentricity to the aerial image 460, 461 of the mark 450. In order to reduce this effect, the duty cycle of the terminal reflective regions 480, 482 may be different from the duty cycle of the non-terminal reflective and absorptive regions 470, 472 located between the terminal reflective regions 480, 482. For example, the duty cycle of the non-terminal reflective and absorptive regions 470, 472 may be about 50% reflective region 470 to 50% absorptive region 472 (i.e., 50:50). However, the duty cycle of the terminal reflection regions 480, 482 may not be 50:50. That is, the extension of the terminal reflection regions 480, 482 along the pitch of the reflective diffraction grating 450 may be greater or less than the extension of the non-terminal reflection regions. Adjusting the duty cycle of the terminal reflection regions may be beneficial to reduce or avoid non-telecentricity associated with the ends of the reflective diffraction grating.

[0079] The marker 450 may include one or more sub-resolution alignment features. A sub-resolution alignment feature may be considered small enough to not result in a noticeable feature after the lithography development and etching steps, while still being large enough to affect adjacent features. Thus, the size and shape of the sub-resolution features may depend at least in part on the process used, and may be specific to the substrate layer. In general, any feature that is less than half the size of a feature to be imaged by the projection system may be considered "sub-resolution".

[0080] Reference again Figure 1 , Figure 4B The marker 450 in may correspond to Figure 1 The marker 17 in the patterning device MA. The marker 17 may be part of the support structure MT or the patterning device MA. Figure 1 The illumination system IS, the marker 17, the projection system PS and the sensor device 19 in the embodiment can together serve as an optical alignment system. The lithography device LA can be used to perform a dual monopolar exposure, such as Figure 3A and 3B The optical alignment system can be configured to determine and improve the alignment between the patterning device MA and the substrate W before the dual lithography exposure. For example, the illumination system IL can adjust the radiation beam B to form a first off-axis monopole, such as Figure 3A The right off-axis monopole 201 is shown. The optical alignment system can use the first off-axis monopole 201 to perform measurements of the marker 17, similar to Figure 4B The measurements shown are taken to determine the alignment between the patterning device MA and the substrate W. The lithography device LA can then perform a first lithography exposure using the first off-axis monopole 201 to form a first image 281 of the patterning device MA on the substrate, as shown in FIG. Figure 3A The first monopole exposure is shown. Alignment information determined by the optical alignment system can be used to ensure that the substrate table WT is moved 271 from the center position to account for the position shift of the constituent aerial image 281 formed by the first off-axis monopole 201. The illumination system IS can be configured to adjust the radiation beam B to form a second off-axis monopole, such as Figure 3B The left monopole 202 is shown. The first off-axis monopole 201 and the second off-axis monopole 202 are located in different regions (e.g., left and right) of the pupil plane of the lithographic apparatus LA. The lithographic apparatus LA can then perform a second lithographic exposure using the second off-axis monopole 202 to form a second image 282 of the patterning device MA on the substrate, as shown in FIG. Figure 3BThe second monopole exposure is shown. Alignment information determined by the optical alignment system may be used to ensure that the substrate table WT is moved 272 from the center position to account for a position shift of the constituent aerial image 282 formed by the second off-axis monopole 202. That is, the substrate table WT is configured to move between the first lithographic exposure and the second lithographic exposure such that the first and second images 281, 282 of the patterning device MA substantially overlap on the substrate.

[0081] Figure 5 A flow chart of an optical alignment method according to the present disclosure is shown. A first step 500 of the method includes adjusting a radiation beam to form a first off-axis monopole. A second step 501 of the method includes diffracting the first off-axis monopole to form zero-order and first-order diffracted beams. A third step 502 of the method includes collecting the zero-order and first-order diffracted beams and forming an image using the zero-order and first-order diffracted beams. A fourth step 503 of the method includes detecting the image.

[0082] The first step of the photolithography exposure method may include using Figure 5 The optical alignment method of the lithography exposure method may include determining the alignment between the pattern forming device MA and the substrate W. The second step of the lithography exposure method may include using the pattern forming device MA to impart a pattern to the first off-axis monopole in a cross-section of the first off-axis monopole to form a patterned radiation beam B'. The third step of the lithography exposure method may include projecting the patterned radiation beam B' onto the substrate, thereby performing a first lithography exposure using the first off-axis monopole to form a first image of the pattern forming device MA on the substrate. The fourth step of the lithography exposure method may include adjusting the radiation beam B to form a second off-axis monopole. The first off-axis monopole and the second off-axis monopole may be located in different areas of the pupil plane of the lithography device LA. The fifth step of the lithography exposure method may include performing a second lithography exposure using the second off-axis monopole to form a second image of the pattern forming device MA on the substrate. The lithography exposure method may include moving the substrate W using the alignment determined by the optical alignment method so that the first image and the second image of the pattern forming device MA substantially overlap on the substrate.

[0083] Although specific embodiments of using a lithography apparatus in a lithography apparatus for manufacturing ICs are specifically mentioned herein, it should be understood that the lithography apparatus described herein may have other applications. Other possible applications include manufacturing integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0084] Although embodiments of the invention are specifically mentioned herein in the context of a lithographic apparatus, embodiments of the invention may be used in other devices. Embodiments of the invention may be 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 pattern forming device). These apparatuses may be collectively referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0085] If the context permits, embodiments of the invention may be implemented in hardware, firmware, software, or any combination. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium that can 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, acoustic or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. In addition, firmware, software, routines, instructions may be described herein as performing certain operations. However, it should be understood that such descriptions are for convenience only, and that these operations are actually performed by a computing device, processor, controller, or other device executing firmware, software, routines, instructions, etc., and may cause actuators or other devices to interact with the physical world during execution.

[0086] Although specific embodiments of the invention have been described, it will be appreciated that the invention may be implemented in ways other than those described. The foregoing description is intended to be illustrative, not restrictive. 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 claims set forth below.

Claims

1. An optical alignment system, comprising: an illumination system configured to condition the radiation beam to form a first off-axis monopole; a marker configured to diffract the first off-axis monopole to form a zero-order diffracted beam and a first-order diffracted beam, wherein the marker comprises: a plurality of reflective regions configured to preferentially reflect the radiation beam; and, a plurality of absorption regions configured to preferentially absorb the radiation beam, wherein the reflective region and the absorption region are arranged to form a reflective diffraction grating, wherein the reflective diffraction grating includes a terminal reflective and absorption region and a non-terminal reflective and absorption region located between the terminal reflective and absorption regions, wherein a duty ratio of the terminal reflective and absorption region is different from a duty ratio of the non-terminal reflective and absorption region; a projection system configured to collect the zero-order diffraction beam and the first-order diffraction beam and form an image of the marker; and A sensor device is configured to detect the image of the marker.

2. The optical alignment system of claim 1, wherein the radiation beam comprises extreme ultraviolet radiation.

3. The optical alignment system according to claim 1 or 2, wherein the minimum pitch of the reflective diffraction grating is determined by the following formula: Where λ is the wavelength of the radiation beam and NA is the numerical aperture of the projection system.

4. An optical alignment system according to claim 2 or 3, wherein the pitch of the reflective diffraction grating is in the range of about 24 nm to about 44 nm, including the end values ​​of the range.

5. An optical alignment system according to any one of claims 1 to 4, wherein the marker comprises a sub-resolution alignment feature.

6. A marker for use in the optical alignment system according to any one of claims 1 to 5.

7. A lithography apparatus comprising: The optical alignment system according to any one of claims 1 to 6; a support structure configured to support a patterning device capable of imparting a pattern to the off-axis monopole in a cross-section thereof to form a patterned radiation beam, wherein the marker forms part of the support structure or the patterning device; as well as, a substrate table configured to hold a substrate, wherein the sensor arrangement forms part of the substrate table, wherein the projection system is configured to project the patterned radiation beam onto the substrate, And wherein the optical alignment system is configured to determine alignment between the patterning device and the substrate.

8. The lithographic apparatus of claim 7, wherein the illumination system is configured to condition the radiation beam to form a second off-axis monopole, wherein the first off-axis monopole and the second off-axis monopole are located in different regions of a pupil plane of the lithographic apparatus.

9. The lithographic apparatus according to claim 8, wherein the lithographic apparatus is configured to: performing a first lithographic exposure using the first off-axis monopole to form a first image of the patterning device on the substrate; and, performing a second lithographic exposure using the second off-axis monopole to form a second image of the patterning device on the substrate, wherein the substrate stage is configured to move between the first lithographic exposure and the second lithographic exposure such that the first image and the second image of the patterning device substantially overlap on the substrate.

10. The lithographic apparatus of claim 9, wherein the alignment determined by the optical alignment system is used to ensure that the substrate table is moved.

11. An optical alignment method, comprising: adjusting the radiation beam to form a first off-axis monopole; diffracting the first off-axis monopole to form a zero-order diffracted beam and a first-order diffracted beam; collecting the zero-order diffraction light beam and the first-order diffraction light beam; forming an image using the zeroth-order diffracted light beam and the first-order diffracted light beam; as well as The image is detected.

12. A photolithography exposure method, comprising: Determining alignment between a patterning device and a substrate using the optical alignment method of claim 11; imparting a pattern to the off-axis monopole in a cross-section of the off-axis monopole using the patterning device to form a patterned radiation beam; and, The patterned radiation beam is projected onto the substrate.

13. The lithographic exposure method of claim 12, comprising adjusting the radiation beam to form a second off-axis monopole, wherein the first off-axis monopole and the second off-axis monopole are located in different regions of a pupil plane.

14. The photolithography exposure method according to claim 13, comprising: performing a first lithographic exposure using the first off-axis monopole to form a first image of the patterning device on the substrate; performing a second lithographic exposure using the second off-axis monopole to form a second image of the patterning device on the substrate; and, The substrate is moved between the first and second lithographic exposures such that the first and second images of the patterning device substantially overlap on the substrate.

Images