US-12619170-B2 - Projection unit for a level sensor, method of monitoring height of a substrate, and lithographic system comprising the projection unit

US12619170B2US 12619170 B2US12619170 B2US 12619170B2US-12619170-B2

Abstract

A projection unit for a level sensor, the projection unit including: a first light pipe having a first inlet configured to receive radiation from a source and a first outlet; and a second light pipe having a second inlet configured to receive the radiation from the first light pipe and a second outlet. The unit may include a lens device configured to receive radiation from the second outlet and to output radiation having a predetermined distribution of intensity and irradiance.

Inventors

Ahmet Burak CUNBUL
Ferry Zijp
Teunis Willem Tukker
Peter Fernand William Jozef DENDAS
Abraham Franciscus Hubertus VAN GESSEL

Assignees

ASML NETHERLANDS B.V.

Dates

Publication Date: 20260505
Application Date: 20221205
Priority Date: 20211223

Claims (20)

1 . A projection unit for a level sensor, the projection unit comprising: a first light pipe having a first inlet configured to receive radiation from a source and a first outlet; a second light pipe having a second inlet configured to receive the radiation from the first light pipe and a second outlet; a lens system configured to receive radiation from the second outlet and to output radiation having a predetermined distribution of intensity and irradiance; a grating arranged to be illuminated by the radiation exiting the lens system, the grating comprising an array of grating units, each grating unit comprising a number of slits having a predetermined size to filter the radiation; and an image multiplier device arranged to receive radiation exiting the grating, the radiation exiting the grating having an aspect ratio, and the image multiplier device configured to increase the aspect ratio.
2 . The projection unit of claim 1 , wherein the lens system has a focal point on one side located at the second outlet.
3 . The projection unit of claim 1 , wherein the first light pipe has tapering walls, and wherein a width and height of the first inlet exceeds a width and height of the first outlet.
4 . The projection unit of claim 1 , wherein the first light pipe is symmetrical along its longitudinal axis.
5 . The projection unit of claim 1 , wherein the second light pipe has tapering walls, and wherein a width and height of the second outlet exceeds a width and height of the second inlet.
6 . The projection unit of claim 1 , wherein the grating units are rectangular, and the slits are arranged obliquely with respect to sides of the respective grating unit.
7 . The projection unit of claim 1 , wherein the radiation exiting the projection grating has an aspect ratio of width to height of at least 1:1.5, and the image multiplier device is configured to increase the aspect ratio to at least 1:2.
8 . The projection unit of claim 1 , further comprising: a light module comprising a light source to produce the radiation; and a light guide configured to receive the radiation from the light module and transfer the radiation to the first light pipe.
9 . The projection unit of claim 8 , wherein the light guide is a single multimode fiber.
10 . A lithographic system comprising a level sensor for measuring height of a substrate, the level sensor comprising the projection unit according to claim 1 .
11 . A method of monitoring height of a substrate, the method comprising: receiving radiation from a radiation source into a first light pipe of a projection unit of a level sensor, wherein at least some of the radiation reflects off an interior of the first light pipe; receiving radiation exiting the first light pipe into a second light pipe of the projection unit of the level sensor, wherein at least some of the radiation reflects off an interior of the second light pipe; and providing the radiation from the second light pipe toward the substrate for monitoring of the height of the substrate.
12 . The method of claim 11 , further comprising using a lens system to receive radiation exiting the second light pipe, the lens system configured to output radiation having a predetermined distribution of intensity and irradiance.
13 . The method of claim 12 , wherein the lens system has a focal point on one side located at an outlet of the second light pipe.
14 . The method of claim 11 , further comprising illuminating a grating by the radiation exiting the lens system, the grating comprising an array of grating units, each grating unit comprising a number of slits having a predetermined size to filter the radiation.
15 . The method of claim 14 , wherein the radiation exiting the grating has an aspect ratio and further comprising increasing the aspect ratio using an image multiplier device arranged to receive radiation exiting the grating.
16 . The method of claim 15 , wherein the radiation exiting the projection grating has an aspect ratio of width to height of at least 1:1.5, and comprising increasing the aspect ratio to at least 1:2 using the image multiplier device.
17 . The method of claim 14 , wherein the grating units are rectangular, and the slits are arranged obliquely with respect to sides of the respective grating unit.
18 . The method of claim 11 , wherein the first light pipe has tapering walls, and wherein a width and height of an inlet of the first light pipe exceeds a width and height of an outlet of the first light pipe.
19 . The method of claim 11 , wherein the second light pipe has tapering walls, and wherein a width and height of the second outlet exceeds a width and height of the second inlet.
20 . A projection unit for a level sensor, the projection unit comprising: a first light pipe having a first inlet configured to receive radiation from a source and a first outlet; a second light pipe having a second inlet configured to receive the radiation from the first light pipe and a second outlet; a lens system configured to receive radiation from the second outlet and to output radiation having a predetermined distribution of intensity and irradiance; a grating arranged to be illuminated by the radiation exiting the lens system and configured to provide the radiation toward a substrate for measurement by the level sensor.

Description

CROSS-REFERENCE TO RELATED APPLICATION The application is the U.S. national phase entry of PCT Patent Application No. PCT/EP2022/084431 which was filed on Dec. 5, 2022, which claims priority of European Patent Application No. 21217562.4 which was filed on Dec. 23, 2021 and which is incorporated herein in its entirety by reference. FIELD The present invention relates to a projection unit, a method of monitoring height, and a lithographic system comprising the projection unit. The projection unit may be used in a level sensor or applied to another kind of metrology assembly. The level sensor may be suitable for measuring a height of, for instance, a surface of a wafer, a substrate or a reticle in a lithographic apparatus. BACKGROUND A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate. As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore's law’. To keep up with Moore's law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that are patterned on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm (KrF), 193 nm (ArF) and 13.5 nm (EUV). A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm. At such short wavelengths, precise positioning of the patterning device and/or substrate within the lithographic apparatus is essential. US20070159700 discloses an apparatus for inspecting a specular surface, comprising: a light source; a collector optics for collecting the light from the light source; a homogenizing optics for transmitting the light from the collector optics having a first micro-lens array downstream of the collector optics; a second micro-lens array downstream of the first micro-lens array; a Fourier optics for transmitting the light from the homogenizing optics onto the specular surface. The apparatus includes a light source 10 formed by the output surface of a fiberoptic bundle. Other point-like light sources are also conceivable. The light is generated by a flash lamp with a reflector and coupled into the fiber-optic bundle. US20070052953 discloses an illumination system that can be used with a coherent light source. The system includes a diffuser that is 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 into the system pupil, and the exit of the homogenizing rod is imaged into the system field. In such a system, the scatter distribution from the diffuser determines the light distribution in the system pupil. Speckle modulation in the image is minimized by rotating the diffuser during the integration time of the imaging sensor. The one or more second optical elements may also include one or more optical elements that are configured to collect light exiting the homogenizer. US20170219934 discloses compact apparatuses for projecting an image onto a substrate are provided. In one embodiment, an image projection apparatus includes a light pipe coupled to a first mounting plate, and a frustrated prism assembly, one or more digital micro-mirror devices, one or more beamsplitters, and one or more projection optics, which are coupled to a second mounting plate. The first and second mounting plates are coplanar, such that the image projection apparatus is compact and may be aligned in a system having a plurality of image projection apparatuses. US20140152991 discloses a level sensor of an exposure apparatus including a light source that generates light that is illuminated onto a top surface of a wafer, a projection part provided in a path of incident light propagating from the light source to the wafer and having a single first slit, a detection part provided in a path of light reflected from the wafer and having a single second slit, and a detector that detects the reflected light that is incident on the second slit of the detection part. Although the systems and sensors disclosed in the documents referenced above ge