US-12624940-B2 - Illumination source and associated metrology apparatus

US12624940B2US 12624940 B2US12624940 B2US 12624940B2US-12624940-B2

Abstract

Disclosed is an illumination source comprising a gas delivery system being configured to provide a gas target for generating an emitted radiation at an interaction region of the gas target, and an interferometer for illuminating at least part of the gas target with an interferometer radiation to measure a property of the gas target.

Inventors

Wenjie Jin
Petrus Wilhelmus SMORENBURG
Nan Lin
Christina Lynn PORTER
David O'DWYER
Cord Louis ARNOLD
Sjoerd Nicolaas Lambertus Donders

Assignees

ASML NETHERLANDS B.V.

Dates

Publication Date: 20260512
Application Date: 20210614
Priority Date: 20200721

Claims (18)

1 . A method comprising: providing a gas target using a gas delivery system; illuminating at least part of the gas target with a pump radiation at an interaction region to generate a plasma and an emitted radiation; illuminating at least part of the gas target with a first branch of a first radiation of an interferometer while a second branch of the first radiation does not pass through the gas target; and receiving the first and second branches of the first radiation at a detector of the interferometer to measure a property of the gas target comprising a property of the plasma; wherein the first radiation has a first wavelength, the pump radiation has a pump wavelength, and the first wavelength and the pump wavelength are different.
2 . The method of claim 1 , wherein the first radiation is a second, third or higher harmonic of a second radiation generated by a harmonic generation process.
3 . The method of claim 2 , wherein the second radiation and the pump radiation are split from one radiation source.
4 . The method of claim 1 , wherein the illuminating with the first radiation comprises illuminating with pulses.
5 . The method of claim 4 , wherein the pump radiation comprises pulses, and the pulses of the first radiation and the pump radiation are at least partly synchronized.
6 . The method of claim 5 , further comprising using a time delay unit to adjust a time delay between the pulses of the first radiation and the pump radiation.
7 . The method of claim 5 , further comprising measuring a buildup of the plasma during the illuminating with the pump radiation by scanning a time delay of the pulses of the first radiation such that a time of arrival of each of the pulses of the first radiation at the gas target varies during the illuminating with the pump radiation.
8 . The method of claim 5 , further comprising measuring a decay of the plasma after the illuminating with the pump radiation by scanning a time delay of the pulses of the first radiation such that a time of arrival of each of the pulses of the first radiation at the gas target varies between a period defined by an end of the illuminating with the pump radiation and an end of the decay.
9 . The method of claim 1 , wherein the gas delivery system comprises a gas nozzle and the first branch of the first radiation at least partly illuminates at least part of the gas nozzle.
10 . The method of claim 1 , wherein the illuminating at least part of the gas target with the pump radiation comprises generating the emitted radiation as one or more harmonics of the pump wavelength.
11 . The method of claim 1 , wherein the interaction region is disposed within a capillary structure.
12 . The method of claim 1 , wherein the emitted radiation has a wavelength in a range from 0.01 nm to 100 nm.
13 . The method of claim 1 , wherein the receiving the first and second branches of the first radiation at the detector comprises measuring a phase shift introduced by the gas target and/or the plasma based on interference fringes generated by interference of the first and second branches of the first radiation.
14 . The method of claim 1 , wherein the receiving the first and second branches of the first radiation at the detector comprises measuring a position of the plasma as an indicator of an alignment of the pump radiation relative to the gas target.
15 . The method of claim 1 , further comprising measuring a plasma evolution over time based on the property of gas target and/or the property of the plasma measured by the detector of the interferometer.
16 . The method of claim 15 , further comprising measuring, with the detector, a state of the gas target before, during, and after the illuminating with the pump radiation to measure a state of the plasma.
17 . An illumination source comprising: a gas delivery system configured to provide a gas target at an interaction region; a pump radiation source configured to illuminate at least part of the gas target with a pump radiation at the interaction region to generate a plasma and an emitted radiation; and an interferometer comprising a first branch of a first radiation configured to pass through at least part of the gas target and a second branch of the first radiation configured to not pass through the gas target; wherein the first branch radiation and the second branch radiation interfere to measure a property of the gas target comprising a property of the plasma; and wherein the first radiation has a first wavelength, the pump radiation has a pump wavelength, and the first wavelength and the pump wavelength are different.
18 . A metrology apparatus comprising: an illumination source comprising: a gas delivery system configured to provide a gas target at an interaction region; a pump radiation source configured to illuminate at least part of the gas target with a pump radiation at the interaction region to generate a plasma and an emitted radiation; and an interferometer comprising a first branch of a first radiation configured to pass through at least part of the gas target and a second branch of the first radiation configured to not pass through the gas target; wherein the first branch radiation and the second branch radiation interfere to measure a property of the gas target comprising a property of the plasma; and wherein the first radiation has a first wavelength, the pump radiation has a pump wavelength, and the first wavelength and the pump wavelength are different; an illumination system configured to focus the emitted radiation toward a target disposed on a substrate support; a detector configured to measure the emitted radiation reflecting from the target; and a processor configured to calculate a property of the target based on the measured radiation.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application claims priority of EP application 20187019.3 which was filed on 2020 Jul. 21 and EP application 20192841.3 which was filed on 2020 Aug. 26 and EP application 21169180.3 which was filed on 2021 Apr. 19 and whom are incorporated herein in their entirety by reference. FIELD The present invention relates to illumination source, metrology apparatus and a method of inspecting a gas target of an illumination source. 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 (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer). To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 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. Low-k1 lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD=k1×λ/NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k1 is an empirical resolution factor. In general, the smaller k1 the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and/or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k1. In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. Examples of known scatterometers often rely on provision of dedicated metrology targets. For example, a method may require a target in the form of a simple grating that is large enough that a measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target. In addition to measurement of feature shapes by reconstruction, diffraction-based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Examples of dark field imaging metrology can be found in numerous published patent applications, such as for example US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image, using a composite grating target. The known scatterometers tend to use light in the visible or near-infrared (IR) wave range, which requires the pitch of the grating to be much coarser than the actual product structures whose properties are actually of interest. Such product features may be defined using