PROJECTION UNIT, METHOD FOR MONITORING HEIGHT, AND LITHOGRAPHIC SYSTEM COMPRISING A PROJECTION UNIT - Patent application

Abstract

The present disclosure provides a projection unit for a level sensor comprising a first light pipe having a first entrance configured to receive radiation from a light source and a first exit, and a second light pipe having a second entrance configured to receive radiation from the first light pipe and a second exit, the unit may comprise a lens arrangement configured to receive radiation from the second exit and output radiation having a predetermined intensity and irradiance distribution.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to European Patent Application No. 21217562.4, filed December 23, 2021, which is incorporated by reference in its entirety.

[0002] The present invention relates to a projection unit, a method for monitoring height and a lithography system with a projection unit, which may be used in a level sensor or applied in other types of metrology assemblies. The level sensor is suitable for example for measuring the height of a surface of a wafer, substrate or reticle in a lithography apparatus. [Background technology]

[0003] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern from a patterning device (e.g. a reticle or mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been scaled down, while the amount of functional elements, such as transistors, per device has been steadily increasing for decades, following a trend commonly referred to as "Moore's Law". To keep up with Moore's Law, the semiconductor industry is pursuing technologies that allow for the creation of ever-smaller features. To project a pattern onto a substrate, a lithography apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that may be patterned on the substrate. Common wavelengths currently in use are 365 nm (i-line), 248 nm (KrF), 193 nm (ArF) and 13.5 nm (EUV). Lithography apparatus using extreme ultraviolet (EUV) radiation, having a wavelength in the range of 4 nm-20 nm, e.g., 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than lithography apparatus using radiation with a wavelength of, e.g., 193 nm.

[0005] At such short wavelengths, accurate positioning of the patterning device and / or substrate within the lithographic apparatus is essential.

[0006] US20070159700 discloses an apparatus for inspecting a mirror surface, comprising a light source, a collector optic for collecting light from the light source, a homogenizing optic for transmitting light from the collector optic, the homogenizing optic having a first microlens array downstream of the collector optic, a second microlens array downstream of the first microlens array, and a Fourier optic for transmitting light from the homogenizing optic onto the mirror surface. The apparatus includes a light source 10 formed by the exit surface of an optical fiber bundle. Other point-like light sources are also contemplated. Light is generated by a flash lamp with a reflector and coupled into the optical fiber bundle.

[0007] US20070052953 discloses an illumination system that can be used with a coherent light source. The system includes a diffuser illuminated by a coherent laser beam. In one such illumination system, the surface of the diffuser is imaged onto the entrance of a homogenizing rod. The Fourier transform plane of the diffuser surface is imaged onto the system pupil, and the exit of the homogenizing rod is imaged onto the system field. In such a system, the light distribution in the system pupil is determined by the scattering distribution from the diffuser. Speckle modulation in the image is minimized by rotating the diffuser during the integration time of the image sensor. The one or more second optical elements may also include one or more optical elements configured to collect the light exiting the homogenizing device.

[0008] US20170219934 discloses providing a compact device for projecting an image onto a substrate. In one embodiment, the image projection device includes a light pipe coupled to a first mounting plate, and a frustrated prism assembly, one or more digital micromirror devices, one or more beam splitters, and one or more projection optics coupled to a second mounting plate. Because the first and second mounting plates are coplanar, the image projection device is compact and can be aligned in a system having multiple image projection devices.

[0009] US20140152991 discloses a level sensor for an exposure apparatus that includes a light source that generates light to be irradiated onto the upper surface of a wafer, a projection unit that is provided on a path of incident light propagating from the light source to the wafer and has a single first slit, a detection unit that is provided on a path of light reflected from the wafer and has a single second slit, and a detector that detects reflected light that is incident on the second slit of the detection unit. Summary of the Invention [Problem to be solved by the invention]

[0010] While the systems and sensors disclosed in the above documents generally work well, there is a constant drive to improve, simplify, streamline, and reduce capital or operational costs. For example, current projection units typically have relatively low light transmission efficiency, many parts, and high associated costs.

[0011] It is an aim of at least one embodiment of at least one aspect of the present invention to obviate or at least mitigate at least one of the above-mentioned disadvantages of the prior art. [Means for solving the problem]

[0012] The present disclosure provides a projection unit for a level sensor, comprising: a first light pipe having a first entrance configured to receive radiation from the light source and a first exit; a second light pipe having a second entrance configured to receive radiation from the first light pipe and a second exit; A projection unit comprising:

[0013] In an embodiment, the projection unit comprises a lens arrangement configured to receive radiation from the second output and to output radiation having a predetermined intensity and irradiance distribution.

[0014] In one embodiment, the lens arrangement has a focal point on one side located at the second exit.

[0015] In one embodiment, the first light pipe has tapered walls and the width and height of the first entrance exceeds the width and height of the first exit.

[0016] In one embodiment, the first light pipe is symmetrical along its longitudinal axis.

[0017] In one embodiment, the second light pipe has tapered walls and the width and height of the second exit aperture exceeds the width and height of the second entrance aperture.

[0018] In one embodiment, the second light pipe is asymmetric along its longitudinal axis.

[0019] In one embodiment, the second entrance aperture has a width to height ratio in the range of 1:1.5 to 1:14 and / or the second exit aperture has a width to height ratio in the range of 1:1.

[0020] In one embodiment, a grating is provided located on the opposite side of the lens system illuminated by radiation exiting the lens arrangement, the grating comprising an array of grating units, each grating unit having a number of slits of a predetermined size for filtering the radiation.

[0021] In one embodiment, the grating units are rectangular and the slits are disposed diagonally relative to the sides of each grating unit.

[0022] In one embodiment, the system includes an image intensifier arranged to receive radiation exiting the projection grating, the radiation exiting the projection grating having an aspect ratio, the image intensifier configured to increase the aspect ratio.

[0023] In one embodiment, the radiation exiting the projection grating has a width to height aspect ratio of at least 1:1.5, and the image intensifier is configured to increase the aspect ratio to at least 1:2.

[0024] In one embodiment, the projection unit comprises a light module including a light source that generates radiation, and a light guide configured to receive radiation from the light module and transfer the radiation to a first light pipe.

[0025] In one embodiment, the light guide is a single multimode fiber.

[0026] According to another aspect, the present disclosure provides a lithography system including a level sensor for measuring a height of a substrate, the level sensor comprising a projection unit as described above.

[0027] According to yet another aspect, the present disclosure provides a metrology assembly including a projection unit as described above.

[0028] According to another aspect, the present disclosure provides a method for monitoring a height of a substrate using a level sensor having a projection unit, the method including directing radiation to a first light pipe and receiving radiation exiting the first light pipe using a second light pipe.

[0029] In one embodiment, the method includes receiving radiation exiting the second light pipe using a lens system, the lens system configured to output radiation having a predetermined intensity and irradiance distribution. [Brief description of the drawings]

[0030] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: [Figure 1] FIG. 1 depicts an example of a lithography system comprising a lithographic apparatus and a radiation source. [Figure 2A] 2 illustrates a schematic diagram of an exemplary level sensor suitable for the lithographic apparatus of FIG. 1; [Figure 2B] 1 is a schematic diagram of an exemplary level sensor. [Diagram 3] 3 is a schematic diagram of an embodiment of a projection unit according to the present disclosure suitable for the level sensor of FIG. 2; [Figure 4] 1 is a perspective view of an embodiment of a portion of a projection unit according to the present disclosure. [Diagram 5] 1 is a schematic side view of an embodiment of a lens system of a projection unit according to the present disclosure. [Figure 6] 1 is a schematic front view of an embodiment of a projection grating of a projection unit according to the present disclosure; [Figure 7A] 1 is a schematic side view of an embodiment of a projection unit according to the present disclosure; [Figure 7B] 1 is a schematic side view of an embodiment of an image intensifier for a projection unit according to the present disclosure. [Figure 7C] 2 is a schematic side view of another embodiment of an image intensifier device for a projection unit according to the present disclosure. [Figure 8A-8C] 2A to 2C are exemplary diagrams of the irradiance distribution (top) and intensity distribution (bottom) of radiation emitted from each element of the projection unit of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] 1 shows a lithography system comprising a radiation source SO configured to generate a beam of (EUV) radiation B and to provide the EUV radiation beam B to the lithography apparatus LA. The lithography 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.

[0032] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. Together the facetted field mirror device 10 and the facetted pupil mirror device 11 provide an EUV radiation beam B having 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 facetted field mirror device 10 and the facetted pupil mirror device 11.

[0033] After being so conditioned, the EUV radiation beam B interacts with the patterning device MA. This interaction results in a patterned EUV radiation beam B'. The projection system PS is configured to project the patterned EUV radiation beam B' onto a substrate W. To that end, the projection system PS may comprise a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto a substrate W held by a substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV radiation beam B', thereby forming an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although the projection system PS is shown in FIG. 1 as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (for example 6 or 8 mirrors).

[0034] Substrate W may include a previously formed pattern, in which case lithographic apparatus LA aligns the image formed by patterned EUV radiation beam B' with the previously formed pattern on substrate W.

[0035] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure significantly below atmospheric pressure, may be provided within the source SO, illumination system IL and / or projection system PS.

[0036] The radiation source SO shown in Fig. 1 is, for example, of the type also referred to as a laser-produced plasma (LPP) source. A laser system 1, including, for example, a CO2 laser, is configured to deposit energy via a laser beam 2 in a fuel, for example tin (Sn), provided by a fuel emitter 3. Although tin is mentioned 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 an alloy. The fuel emitter 3 may comprise a nozzle for directing the tin, for example in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident on the tin in the plasma formation region 4. The deposition of the laser energy in the tin forms a plasma 7 in the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of electrons and ions of the plasma.

[0037] EUV radiation from the plasma is collected and focused by a collector 5. The collector 5 may, for example, comprise a near normal incidence radiation collector 5 (sometimes more generally referred to as a normal incidence radiation collector). The collector 5 may comprise a multi-layer mirror structure configured to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may comprise an ellipsoidal structure having two foci. A first one of the foci may be located at the plasma formation region 4 and a second one of the foci may be located at an intermediate focus 6, as described below.

[0038] The laser system 1 may be spatially separated from the radiation source SO. In this case, the laser beam 2 may pass from the laser system 1 towards the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or beam expanders and / or other optics. The laser system 1, the radiation source SO and the beam delivery system together may be considered as a radiation system.

[0039] Radiation reflected by collector 5 forms an EUV radiation beam B. EUV radiation beam B is focused at intermediate focus 6 to form an image at intermediate focus 6 of the plasma present in plasma formation region 4. The image at intermediate focus 6 acts as a virtual source of the illumination system IL. The radiation source SO is configured such that the intermediate focus 6 is located at or near an opening 8 in an enclosure 9 of the radiation source SO.

[0040] Although FIG. 1 illustrates the source SO as a laser-produced plasma (LPP) source, any suitable source may be used to produce EUV radiation, such as a discharge-produced plasma (DPP) source or a free-electron laser (FEL).

[0041] In the following, the level sensor LS is generally described with reference to a wafer W in the lithographic apparatus LA of FIG. 1, however it should be noted that the level sensor LS is equally suitable for measuring the height of the surface of any substrate in any lithographic apparatus, including but not limited to lithographic apparatus based on EUV, DUV (deep ultraviolet), or UV (ultraviolet) radiation.

[0042] FIG. 2A shows a typical example of a level sensor LS for measuring the height of a wafer W. A topography measurement system, level sensor or height sensor, which may be integrated into a lithographic apparatus LA, is typically arranged to measure the topography of a top surface of a substrate (such as a wafer W). A map of the topography of the substrate, also referred to as a height map, may be generated from these measurements, indicating the height of the substrate as a function of position on the substrate. This height map may then be used to correct the position of the substrate during transfer of a pattern to the substrate, to provide an aerial image of the patterning device at a properly focused position on the substrate. In this context, it will be understood that "height" refers generally to the out-of-plane dimension (also referred to as the Z-axis) relative to the substrate. Typically, a level or height sensor performs measurements at a fixed location (relative to its optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at each location across the substrate.

[0043] An example of a level or height sensor LS is shown diagrammatically in FIG. 2A. The figure illustrates only the principle of operation. In this example, the level sensor comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum source, polarized or unpolarized, such as a polarized or unpolarized laser beam, pulsed or continuous. The radiation source LSO may include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0044] The projection grating PGR may comprise a periodic grating with a periodic structure resulting in a radiation beam BE1 with a periodically varying intensity. Radiation passing through the PGR results in a radiation beam BE1 with a periodically varying intensity. See FIG. 2A. In use, the radiation beam BE1 with a periodically varying intensity is directed to a measurement location MLO, for example on the substrate W, at an angle of incidence ANG relative to an axis perpendicular to the incident substrate surface (the Z-axis in FIG. 2A). The angle ANG is between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed to a detection unit LSD.

[0045] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grid DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be identical to the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, e.g. indicative of the intensity of the received light as in a photodetector or representative of the spatial distribution of the received intensity as in a camera. The detector DET may comprise any combination of one or more detector types.

[0046] By means of triangulation techniques the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and on the (oblique) angle of incidence ANG.

[0047] Referring to FIG. 2B, still at a general level, the level sensor LS may include a light guide LG for guiding radiation from the light source LSO to the illumination optics IO. The illumination optics may modify the shape and structure of the radiation, including the irradiance distribution and the intensity distribution. The radiation proceeds towards the projection grating PGR. The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR. As shown diagrammatically in FIG. 2B, the projection side of the level sensor LS comprises the projection optics PO. The detection side of the level sensor LS comprises the detection optics DO. The projection optics and / or the detection optics guide and direct the light, so that the projector LSP and the detector LSD can be located at any position. This position is relatively far from the substrate W, so that interference between the level sensor LS and elements inside or outside the projection system LS or associated with the projection system LS can be minimized.

[0048] In an embodiment of the level sensor, the detection grating DGR may be omitted and the detector DET may be placed at the location of the detection grating DGR, such that an image of the radiation emerging from the projection grating PGR and reflected by the surface of the substrate can be detected more directly.

[0049] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0050] Various height sensors of the general type are disclosed, for example, in U.S. Patent Nos. 7,265,364 and 7,646,471, which are incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a compact height sensor that uses a multi-element detector to detect and recognize the position of a grid image without the need for a detection grid.

[0051] 3 and 4A-4C, an embodiment of the projector LSP comprises a first light pipe 50 having a first entrance 52 arranged to receive radiation from the light guide LG, and a first exit 54 for outputting the radiation. The projector comprises a second light pipe 56 having a second entrance 58. The second entrance is arranged to receive radiation from the first light pipe 50. The second light pipe comprises a second exit 60 for outputting the radiation.

[0052] The projection unit LSP may comprise an optical module LSO including a light source for generating radiation. A light guide LG may be provided for receiving radiation from the optical module and forwarding the radiation to the first light pipe 50. In a practical embodiment, the light guide LG may be a single multimode fiber. The single fiber may have a substantially circular cross section. Alternatively, the single fiber may have a cross section of substantially any other suitable shape, e.g. square, semicircular, rectangular, etc. For additional details, see Figures 8A-8C and the associated description below.

[0053] The projection unit LSP may comprise a lens or lens arrangement 70. The lens arrangement may be configured to receive the radiation output from the second output 60 and to output radiation having a predetermined intensity and irradiance distribution.

[0054] In one embodiment, the lens arrangement 70 has a first focal point on a first side of the lens arrangement, the first focal point being located substantially at the second outlet 60. By at the second outlet, it is meant that the first focal point is located substantially at the plane of the second outlet 60 of the second light pipe. Here, the lens 70 may be referred to as a Fourier lens.

[0055] In one embodiment, the projector LSP may comprise an image intensifier 72. The image intensifier 72 may be arranged to receive the radiation exiting the projection grating 70. The radiation exiting the projection grating 70 may have a particular aspect ratio, meaning the ratio between the width and the height of the radiation beam. The image intensifier may be configured to increase said ratio.

[0056] 3 and 4A, the first light pipe 50 may have tapered walls, where the width and height of the first entrance 52 is greater than the width and height of the first exit 54. The first entrance 52 and / or the first exit 54 may be substantially rectangular. In an actual embodiment, the first entrance and / or the first exit are substantially square.

[0057] A light pipe is a rod that collects, guides, and redistributes light with high efficiency from a source to a target. The redistributed light has uniform irradiance. Light pipes are used in many optical systems, such as backlight units in liquid crystal displays, lighting, and automobile dashboards. Light pipes can be made of metal and hollow, utilizing the reflectivity of the metal to reflect light and move the beam inside the pipe. Light pipes can also be made of dielectrics and can use total internal reflection (TIR) ​​to reflect light off the walls of the pipe. Whether the pipe is hollow or glass filled, the action of a light pipe is the same. The difference is the loss of light. In a light pipe made of a dielectric, total internal reflection is lossless. On the other hand, a metal light pipe loses light at every reflection, and if R is the reflection coefficient of the metal, the transmitted flux F is:

number

[0058] Due to the reduced number of reflections in the dielectric light pipe, the transmission flux through the dielectric light pipe is higher than that of a metallic light pipe. Additionally, mirrors require adhesives and expensive packaging solutions to withstand UV light, which is not necessary for light pipes made from a single piece of glass.

[0059] When the input beam is sent through the fiber LG, the light intensity incident on the first pipe is uniform. The light pipe then homogenizes the irradiance distribution. The radiation exiting the first and / or second light pipe has a substantially uniform irradiance distribution.

[0060] FIG. 4B shows details of the optical system IO and illustrates an example of the connection of the first light pipe 50 and the second light pipe 56. The distance between the first and second light pipes, i.e. the distance between the first outlet and the second inlet, varies for example in the range of 0-50 cm. The second light pipe may have tapered walls. In this case, the width and height of the second outlet exceed the width and height of the second inlet. In other words, the second light pipe tapers from a smaller cross-sectional area of ​​the second inlet to a wider cross-sectional area of ​​the second outlet.

[0061] In one embodiment, the second light pipe 56 may be asymmetric along its longitudinal axis. See FIG. 4B. For example, the second entrance 58 may be longitudinal. The second light pipe may be oriented such that the second entrance 58 is upright (long side facing up) or flat (long side oriented horizontally, as shown in FIG. 4B).

[0062] In practical embodiments, the sides of the second entrance 58 may have a ratio in the range of 1:1.5 to 1:14, for example about 1:3 to 1:6. Said ratio may relate to width to height or height to width. In practical embodiments, the second entrance is arranged such that its long side is substantially vertical. The second exit may have a width to height ratio in the range of 1:0.8 to 1:1.2. In a preferred embodiment, the second exit is square and its width to height is 1:1. The taper of the second light pipe is determined by the ratio of entrance height to exit height (i.e. the height of the second entrance to the height of the second exit), which may be in the range of about 1:2 to 1:7. In this regard, see FIG. 4C.

[0063] The first light pipe 50 may have four side walls 80, 82, 84 (FIG. 4A). The first light pipe 50 may be symmetrical along its longitudinal axis. The second light pipe 56 may have four side walls 90, 92, 94, 96 (FIG. 4C). The second light pipe 56 may be symmetrical or asymmetrical along its longitudinal axis.

[0064] In practical embodiments, the first light pipe and / or the second light pipe may be made of a transparent material such as glass, or the first light pipe and / or the second light pipe may have reflective sidewalls, where the entrance and exit of each light pipe and the interior volume between the walls are substantially open.

[0065] For tapered light pipes, i.e. light pipes with non-parallel side walls, θ out =θ in / τ, where θ out is the angle of the incident radiation (the radiation entering the entrance of each light pipe), θ inis the angle of radiation leaving each light pipe, and τ = D / d is the ratio of the exit surface area D to the entrance surface area d of each light pipe. As a result, a change in the area from entrance to exit of each light pipe results in a change in the numerical aperture NA. The change in NA is proportional to the change in surface area. Thus, the first light pipe reduces the NA and the second light pipe increases the NA.

[0066] The added taper allows a higher input NA to be used, improving irradiance uniformity, while the output NA is low enough to limit strain on the lens and meet the target requirements.

[0067] In principle, the same result can be achieved with a single light pipe. However, combining a first light pipe (reduced NA) with a second light pipe (increased NA) can limit the overall length of the light pipe assembly. A single light pipe would in practice result in a relatively long light pipe, e.g., greater than several meters in length. The assembly of the first light pipe 50 and the second light pipe 56 may in practice not exceed a total length of one meter.

[0068] The size of the illumination spot on the target y' is pre-determined. The outgoing NA from the second light pipe (NA out ) determines the object field of the Fourier lens 70. In a practical embodiment, the object field angle of the Fourier lens 70 is limited to less than 10°. Thus, the lens remains in the paraxial regime, making the lens easier to design and resulting in an error due to the paraxial approximation of less than 0.5%. This is because the NA out With these two parameters fixed, the focal length of lens 70 is:

number

[0069] The exit face of the second light pipe is at the pupil of the lens 70, and the NA of the target (on the grating PGR) (which is given by NA target Knowing the focal length calculated above (approximately 0.03), the front width w of the second light pipe is:

number

[0070] This means that the irradiance distribution at the output of the second light pipe is symmetrical, since the irradiance distribution of the light pipe is transformed by the system of Fourier lens 70 and the symmetrical intensity distribution on the target. Therefore, the output side of the pipe is square, since in a practical embodiment this is what determines the irradiance shape at the exit of the pipe. Therefore, w x =w y It is.

[0071] The entrance NA of the first and second light pipes (NA in ) can be derived as follows:

number

[0072] For the first light pipe, NA in Since d exceeds the source NA, in the embodiment, a coupling optical system is added between the fiber LG and the first light pipe. Next, the input width of the first light pipe is set to d x It can also be derived as =D / τ.

[0073] The height of the input side of the first light pipe shapes the intensity distribution leaving the light pipe and therefore the irradiance distribution at the target. Since the target, i.e. the grating PGR, can be asymmetric, it is possible that at least the input side of the second light pipe will also be asymmetric if the source distribution is symmetric and has the same height to width ratio as the illumination spot size. In that case:

number

[0074] The length of the light pipe is selected to optimize the homogenization of the irradiance distribution at the output of the light pipe, taking into account other geometric parameters.

[0075] Referring to FIG. 5, the lens arrangement 70 can include one or more lenses 100, 102, 104. In one embodiment, the lens arrangement can include a single lens or a limited number of lenses, such as three lenses. On the first side 106, the lens arrangement 70 has a first focal point, which in a preferred embodiment is located substantially at or in the plane of the second exit 60 of the second light pipe. On the opposite side 108, the lens arrangement outputs radiation having a substantially parallel radiation. The output of the lens arrangement 70 can be directed to a grating PGR.

[0076] Referring to FIG. 6 , the grating PGR may be disposed on one side 108 of the lens arrangement 70. The grating PGR is positioned to be illuminated by radiation exiting the lens arrangement 70. The grating PGR may include an array 110 of grating units 112. Each grating unit may be provided with a plurality (multiple) slits 114. The slits may have a predetermined size to filter the radiation. The grating units 112 may be rectangular. The slits may be disposed obliquely with respect to a side 116 of each grating unit 112. In a practical embodiment, the slits 114 are oriented at an angle q with respect to the side 116 of each grating unit 112. s The angle may be between 0 and 90 degrees, for example about 10 or 20 degrees.

[0077] With reference to Figure 7A, the system of the present disclosure may include an image multiplier 72. The image multiplier 72 may be configured to intensify an image 120 provided by the second light pipe, grating PGR and / or lens 70 to provide an intensified image 122. Figure 7A illustrates how an image 118 having a particular shape, intensity and irradiance (see Figure 8B) enters the second light pipe 56 and exits as image 120.

[0078] The image intensifier 72 may comprise a beam splitter 130 with one or more reflecting devices, such as, for example, prisms 132, 134. See FIG. 3. FIG. 7B shows an embodiment of the image intensifier 72 with an integrated beam splitter 130 and multiple prisms 132. The image 120 may be intensified multiple times along its length. For example, the image 120 is intensified by a factor of three or four. The beam splitter 130 includes a partially reflective inner surface 136. The surface 136 partially transmits the image 120 and partially reflects the image towards the respective prism.

[0079] The split ratio of each beam splitter is P n =P0×1 / N, where P0 is the radiant power of the incident light, P n is the power of the nth split image and N is the total number of image copies. Said power is preferably the same for each copy. In other words, Φ0 is the initial total flux to be split, N is the number of splits we want to achieve, Φ1, Φ2, . . . , Φ N If is the luminous flux of each spot in the resulting intensified image, then:

number

[0080] In the example where the image is intensified by a factor of N, the projection unit contains N-1 beam splitters. The transmission T of the kth beam splitter is given by

number

[0081] As an example, for N=3 (see FIG. 3 and FIG. 7A), the first beam splitter has a transmittance of about 33.3% and the second beam splitter has a transmittance of 50%.

[0082] In another embodiment (see FIG. 7C), the image intensifier comprises a Bowen-Walraven type image slicer, which for example comprises a first mirror 140 and a second mirror 142 arranged parallel to the first mirror. Radiation is reflected between said mirrors and a part of the reflected radiation is directed towards a target, i.e. a grating PGR.

[0083] In practical embodiments, the image 120 of the radiation emerging from the second light pipe 56 or grating PGR has an aspect ratio of width to height of at least 1:1.5, or at least 1:2, for example in the range of about 1:3 to 1:6. The image intensifier 72 may be configured to increase said ratio. The image intensifier may result in an aspect ratio of at least 1:2. The image intensifier may copy the image 120 at least twice to at least double the aspect ratio. In practice, the resulting aspect ratio may be greater than 1:8, for example about 1:9 to 1:15. The image intensifier 72 allows a shorter main light pipe 56 to be used.

[0084] In a practical embodiment, when the projection unit includes a beam splitter 72, the main light pipe 56 may have a length of about 40 cm or less, for example about 20-35 cm. Here, the length refers to the distance between the second entrance 58 and the second exit 60. The first light pipe 50 may be shorter than the main light pipe 56. For example, the first light pipe 50 may have a length of about 20 cm or less, for example about 10-15 cm. Here, the length refers to the distance between the first entrance 52 and the first exit 54.

[0085] Depending on the length of the first and second light pipes, the radiation may be reflected one or more times within said pipes. In practical embodiments, some of the radiation is reflected multiple times, while another part of the radiation may go straight through without reflection or may be reflected only once. This results in a change in the distribution of irradiance and intensity. Irradiance (or flux density) is a radiometric term and is defined as the radiative flux received by a surface per unit area. In the SI system, it is expressed as W / m 2 It is measured in units of watts per square meter. The term intensity, or light intensity, has the same units as irradiance, although they are not the same quantity. Intensity is defined as the amount of energy passing through an area perpendicular to the beam, while irradiance refers to the amount of energy reaching a particular surface in a particular direction.

[0086] 8A-8C show examples of both irradiance (top) and intensity (bottom) distributions of a practical embodiment of the disclosed device and method, where initially the irradiance and intensity distributions are substantially circular (as generated by the light source LSO and transmitted by the (optional) light guide LG). See diagram in FIG. 8A. Note that the light guide of the disclosed system can be a simple single circular radiation-transmitting fiber. This system eliminates the need for complex multi-fiber light guides to shape the beam into a predefined shape.

[0087] The light pipes transform the shape and distribution of the radiation. Figure 8B shows a schematic example of the output of the first and second light pipes. The first light pipe changes the irradiance distribution to a substantially square while the intensity distribution is still substantially circular. The second light pipe, due to its asymmetric shape, keeps the irradiance distribution square while changing the intensity distribution to a substantially elliptical. As shown in Figure 8C, lens 72 then changes the irradiance distribution to a substantially elliptical while the intensity distribution is circular.

[0088] As an example, the length D calculated above as the entrance pupil of the Fourier lens is the height of the exit face of the second light pipe. Since the exit face of the pipe is symmetric, D fixes the height and width of the exit face. In an exemplary embodiment, it is determined to overfill in the x direction so that the irradiance is uniform across the entire grating PGR. The height to width ratio is determined by the Fourier plane of the lens 70, e.g., in the y direction. p It can be fixed at ' / x'=4. Therefore, the result is d y =3d x Therefore, the output NA of the second light pipe can be fixed; that is, if the taper is fixed, the NA of the light coupled into the pipe according to the above formula is also fixed. If the taper is too high compared to the length of the pipe, the mixing will be impaired and a kaleidoscope-like image will start to appear in the irradiance distribution. In a practical embodiment, the lowest taper that provides a uniform irradiance distribution is selected. Said taper is about 3 times.

[0089] In a practical embodiment, the parameters of the second light pipe are: pipe length approximately 40 cm, entrance face width approximately 0.30-0.40 mm, height approximately 1.2-2 mm, exit face width and height approximately 3-6 mm, entrance NA approximately 0.3-0.5, exit NA approximately 0.1-0.3.

[0090] Using a relatively small number of simple components, the disclosed system and method allows both irradiance and intensity distributions to be shaped according to a predetermined shape, form, and level. The limited number of components minimizes absorption of radiation. The system allows the use of a single commercially available multimode fiber as a light guide. Thus, the system of the present invention eliminates the need for complex beam-shaping multi-fiber structures.

[0091] The present invention provides a redesign of the illumination optics (IO) and simplifies the light guide (LG) accordingly. In the disclosed system, homogenization of the irradiance distribution by MO is not necessary. As a result, the number of components can be reduced (e.g. from 12 components to 5). The LG fiber bundle can be replaced by a single LG fiber. The number of components and the associated costs are reduced and the transmission efficiency is increased. The coupling pipe (first light pipe) enlarges the numerical aperture (NA) between the LG and the main light pipe (second light pipe), which homogenizes the irradiance distribution and shapes the light intensity distribution to provide uniform illumination with the correct aspect ratio (height to width ratio). The Fourier lens functions to match the NA required at the target, transforms the intensity distribution of the incident beam into an irradiance distribution on the target and transforms the incident irradiance distribution into an intensity distribution on the target surface. The projection grating is placed in the target surface.

[0092] The projection unit of the present disclosure achieves uniformity of irradiance: light is delivered to the IO module from a simplified light guide by a simple multimode fiber that ensures uniformity of the intensity distribution of the light delivered to the IO module.

[0093] The projection unit and method of the present disclosure achieves uniform illumination from the light source to the projection grating PGR, both in spatial and angular space, using fewer and simpler elements than conventional systems. As a result, the unit is relatively affordable. The projection unit can achieve uniform and symmetrical intensity distribution and uniform and asymmetrical irradiance distribution according to the shape of the projection grating.

[0094] Those skilled in the art will appreciate that in these other applications, the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, by a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology unit, and / or an inspection unit. Where applicable, the disclosure herein may also be applied to this or other substrate processing tools. A substrate may also be processed multiple times, for example to produce a multi-layer IC, in which case the term "substrate" herein refers to a substrate including multiple processing layers that have already been processed.

[0095] Although specific reference may be made in this specification to the use of lithographic apparatus in the manufacture of ICs, it will be appreciated that the lithographic apparatus described herein may have other applications, such as in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0096] Although specific reference may be made herein to embodiments of the invention in relation to lithographic apparatus, embodiments of the invention may also be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0097] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that, where the context permits, the invention is not limited to optical lithography and may also be used in other applications, such as, for example, imprint lithography.

[0098] 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 above description is intended to be illustrative and not limiting. Thus, those skilled in the art will appreciate that modifications may be made to the invention as described without departing from the scope of the claims set forth below. Other aspects of the invention are described in the following numbered clauses.

[0099] (Section) 1. A projection unit for a level sensor, comprising: a first light pipe having a first entrance configured to receive radiation from the light source and a first exit; a second light pipe having a second entrance configured to receive radiation from the first light pipe and a second exit; A projection unit comprising: 2. The projection unit described in item 1, comprising a lens arrangement configured to receive radiation from the second output and output radiation having a predetermined intensity and irradiance distribution. 3. The projection unit described in item 2, wherein the lens device has a focal point on one side located at the second exit. 4. A projection unit described in any one of items 1 to 3, wherein the first light pipe has a tapered wall and the width and height of the first entrance aperture exceed the width and height of the first exit aperture. 5. A projection unit according to any one of claims 1 to 4, wherein the first light pipe is symmetrical along its longitudinal axis. 6. A projection unit described in any one of items 1 to 5, wherein the second light pipe has a tapered wall and the width and height of the second exit aperture exceed the width and height of the second entrance aperture. 7. A projection unit according to any one of claims 1 to 6, wherein the second light pipe is asymmetric along its longitudinal axis. 8. The projection unit described in item 7, wherein the width-to-height ratio of the second entrance aperture is in the range of 1:1.5 to 1:14, and / or the width-to-height ratio of the second exit aperture is in the range of 1:1. 9. A projection unit as described in any one of clauses 1 to 8, comprising a grating arranged on the opposite side of the lens system illuminated by radiation exiting the lens device, the grating comprising an array of grating units, each grating unit being provided with a number of slits of a predetermined size for filtering the radiation. 10. A projection unit as described in item 9, wherein the grating units are rectangular and the slits are arranged obliquely with respect to the side surfaces of each grating unit. 11. A projection unit as described in clause 9 or 10, comprising an image intensifier arranged to receive radiation exiting a projection grating, the radiation exiting the projection grating having an aspect ratio, the image intensifier configured to increase the aspect ratio. 12. The projection unit of claim 11, wherein radiation exiting the projection grating has a width to height aspect ratio of at least 1:1.5, and the image intensifier is configured to increase the aspect ratio to at least 1:2. 13. An optical module including a light source for generating radiation; a light guide configured to receive radiation from the light module and transfer radiation to the first light pipe; Item 13. The projection unit according to any one of items 1 to 12, comprising: 14. The projection unit according to claim 13, wherein the light guide is a single multimode fiber. 15. A lithography system having a level sensor for measuring the height of a substrate, comprising: Item 1. A lithography system comprising a projection unit, the level sensor comprising: 16. A method for monitoring the height of a substrate using a level sensor having a projection unit, comprising: directing radiation into a first light pipe; using a second light pipe to receive radiation exiting the first light pipe; The method includes: 17. The method of claim 16, comprising receiving radiation exiting the second light pipe using a lens system, the lens system configured to output radiation having a predetermined intensity and irradiance distribution. 18. A metrology assembly comprising a projection unit according to any one of clauses 1 to 14.

Claims

1. A projection unit for a level sensor, comprising: a first light pipe having a first entrance configured to receive radiation from the light source and a first exit; a second light pipe having a second entrance configured to receive radiation from the first light pipe and a second exit; A projection unit comprising:

2. 10. A projection unit as claimed in claim 1, comprising a lens arrangement configured to receive radiation from the second output and to output radiation having a predetermined intensity and irradiance distribution.

3. 3. The projection unit of claim 2, wherein the lens arrangement has a focal point on one side located at the second exit.

4. 4. A projection unit as claimed in claim 1, wherein the first light pipe has tapered walls, and a width and height of the first entrance aperture exceeds a width and height of the first exit aperture.

5. 5. A projection unit according to claim 1, wherein the first light pipe is symmetrical along its longitudinal axis.

6. 6. A projection unit as claimed in claim 1, wherein the second light pipe has tapered walls and the width and height of the second exit aperture exceeds the width and height of the second entrance aperture.

7. 7. A projection unit according to claim 1, wherein the second light pipe is asymmetric along its longitudinal axis.

8. 8. A projection unit as claimed in claim 7, wherein a width to height ratio of the second entrance aperture is in the range of 1:1.5 to 1:14 and / or a width to height ratio of the second exit aperture is in the range of 1:

1.

9. 9. A projection unit as described in any one of claims 1 to 8, comprising a grating arranged on an opposite side of the lens system illuminated by radiation exiting the lens arrangement, the grating comprising an array of grating units, each grating unit provided with a number of slits of predetermined size for filtering the radiation.

10. 10. The projection unit of claim 9, wherein the grating units are rectangular, and the slits are disposed obliquely with respect to a side surface of each grating unit.

11. 11. A projection unit as claimed in claim 9 or 10, comprising an image intensifier arranged to receive radiation exiting a projection grating, the radiation exiting the projection grating having an aspect ratio, the image intensifier being configured to increase the aspect ratio.

12. 12. The projection unit of claim 11, wherein radiation exiting the projection grating has a width to height aspect ratio of at least 1:1.5 and the image intensifier is configured to increase the aspect ratio to at least 1:

2.

13. an optical module including a light source for generating radiation; a light guide configured to receive radiation from the light module and transfer radiation to the first light pipe; 13. A projection unit according to claim 1, comprising:

14. 14. A projection unit as claimed in claim 13, wherein the light guide is a single multimode fibre.

15. 1. A lithography system comprising a level sensor for measuring a height of a substrate, the level sensor comprising: A lithography system, wherein the level sensor comprises a projection unit according to claim 1 .

16. 1. A method for monitoring a height of a substrate using a level sensor having a projection unit, comprising: directing radiation into a first light pipe; using a second light pipe to receive radiation exiting the first light pipe; The method includes:

17. 17. The method of claim 16, comprising receiving radiation exiting the second light pipe using a lens system, the lens system configured to output radiation having a predetermined intensity and irradiance distribution.

18. A metrology assembly comprising a projection unit according to any preceding claim.

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