# US 9,753,379 B2 · Inspection apparatus and methods, methods of manufacturing devices

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- Number: US 9,753,379 B2
- Document ID: US9753379B2
- Status: Granted
- Kind: B2
- Application number: 14/794,013
- Filed: 2015-07-08
- Published: 2016-01-14
- Granted: 2017-09-05
- Earliest claimed: 2014-07-09
- Assignees: ASML Netherlands B.V.
- Applicants: ASML Netherlands B.V.
- Inventors: Amandev Singh; Henricus Petrus Maria Pellemans; Patrick Warnaar
- Web page: https://patentgrep.com/patent/US9753379B2

## CPC

- G03F 7/70491: {Information management, e.g. software; Active and passive control, e.g. details of controlling exposure processes or exposure tool monitoring processes}
- G01J 3/2823: {Imaging spectrometer}
- G01J 3/45: Interferometric spectrometry
- G01N 21/4788: {Diffraction (for sizing particles G01N15/0205)}
- G03F 7/70625: {Dimensions, e.g. line width, critical dimension [CD], profile, sidewall angle or edge roughness}
- G03F 7/70633: {Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching}
- G01N 21/956: Inspecting patterns on the surface of objects {(contactless testing of electronic circuits G01R31/308; testing currency G07D; manufacturing processes per se of semiconductor devices implementing a measuring step H10P74/20)}
- G01N 2201/06113: Coherent sources; lasers
- G03F: PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR; (phototypographic composing devices B41B; photosensitive materials or processes for photographic purposes G03C; electrophotography, sensitive layers or processes therefor G03G)
- G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N)
- G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q)

## Abstract

Inspection apparatus (100) is used for measuring parameters of targets on a substrate. Coherent radiation follows an illumination path (solid rays) for illuminating target (T). A collection path (dashed rays) collects diffracted radiation from the target and delivers it to a lock-in image detector (112). A reference beam following a reference path (dotted rays). An acousto-optical modulator (108) shifts the optical frequency of the reference beam so that the intensity of radiation at the lock-in detector includes a time-varying component having a characteristic frequency corresponding to a difference between the frequencies of the diffracted radiation and the reference radiation. The lock-in image detector records two-dimensional image information representing both amplitude and phase of the time-varying component. A second reference beam with a different shift (110) follows a second reference path (dot-dash rays). Interference between the two reference beams can be used for intensity normalization.

## Claims

### Claim 1

An inspection apparatus for measuring properties of a target structure, comprising:
a radiation source;
an image detector; and
an optical system, the optical system defining the following beam paths:
an illumination path for receiving radiation from the radiation source, forming a beam of illuminating radiation having a selected illumination profile and focusing said illuminating radiation onto a target on a substrate;
a collection path for measuring collecting diffracted radiation from said target and delivering a selected portion of the diffracted radiation to the image detector; and
a reference path for receiving radiation from the radiation source and delivering a beam of reference radiation to the image detector so as to interfere with the diffracted radiation,
wherein the image detector comprises an array of pixels for capturing two-dimensional images,
wherein at least one of said illumination path and said reference path includes a device for shifting an optical frequency of the reference radiation so that the intensity of radiation at the image detector includes a time-varying component having a characteristic frequency corresponding to a difference between the frequencies of the diffracted radiation and the reference radiation, and
wherein said image detector comprises a heterodyne lock-in image detector operable with reference to said characteristic frequency to record for each pixel information representing both amplitude and phase of said time-varying component and such that amplitude and phase of multiple angles from the target are measured simultaneously.

### Claim 2 (depends on 1)

The inspection apparatus as claimed in claim 1, further comprising a processor for processing the recorded amplitude and phase information to calculate a measurement of a property of the target.

### Claim 3 (depends on 2)

The inspection apparatus as claimed in claim 2, wherein said optical system is operable to record separately amplitude and phase information from at least two selected portions of a diffraction spectrum of the target, the processor being arranged to calculate a measurement of a property of the target using the recorded amplitude and phase information from at least two selected portions.

### Claim 4 (depends on 3)

The inspection apparatus as claimed in claim 3, wherein said at least two selected portions comprise opposite portions of the diffraction spectrum and said property is asymmetry.

### Claim 5 (depends on 3)

The inspection apparatus as claimed in claim 3, wherein the optical system in said collection path is arranged to deliver the at least two selected portions of the diffraction spectrum to different locations on said image detector, and the processor is arranged to extract amplitude and phase information for each portion from pixels in corresponding regions of the image detector.

### Claim 6 (depends on 5)

The inspection apparatus as claimed in claim 5, wherein said optical system in the collection path includes imaging optics arranged to deliver diffracted radiation from different locations on the target to different locations on the image detector, and further includes beam diverting optics arranged so that for each location on the target the selected portions of the diffracted radiation are directed to different locations on the target, and the processor is arranged to extract amplitude and phase information for each location on the target and each selected portion of the diffracted radiation from pixels in corresponding regions of the image detector.

### Claim 7 (depends on 2)

The inspection apparatus as claimed in claim 2, wherein the optical system in said collection path is arranged to form an image of a pupil plane of the collection path on the detector, such that the image detector records a diffraction pattern representing one or more selected portions of the diffraction spectrum of the target, and the processor is arranged to compare the recorded amplitude and phase information for one or more portions of the diffraction spectrum with modeled amplitude and phase information corresponding to a candidate target structure.

### Claim 8 (depends on 2)

The inspection apparatus as claimed in claim 2, wherein the optical system in said collection path is arranged to deliver at least two selected portions of the diffraction spectrum to the image detector in separate image capture operations, and the processor is arranged to combine information from at least two image capture operations to calculate a property of the target.

### Claim 9 (depends on 1)

The inspection apparatus as claimed in claim 1, wherein different diffraction orders are measured simultaneously.

### Claim 10 (depends on 1)

The inspection apparatus as claimed in claim 1, wherein the optical system is a scanning optical system.

### Claim 11

A method for measuring properties of a target structure, the method comprising the steps:
(a) illuminating a target on a substrate with an illuminating radiation emitted from a radiation source and having a selected illumination profile;
(b) collecting diffracted radiation from said target and delivering a selected portion of the diffracted radiation to an image detector comprising an array of pixels for capturing two-dimensional images;
(c) delivering a beam of reference radiation emitted from the radiation source to the image detector so as to interfere with the diffracted radiation,
wherein steps (a) and (c) include introducing an optical frequency shift between the reference radiation and the illuminating radiation so that the intensity of radiation at the image detector includes a time-varying component having a characteristic frequency corresponding to the frequency shift; and
wherein step (b) includes operating the image detector as a heterodyne lock-in image detector at the characteristic frequency, to record two-dimensional images representing both amplitude and phase of the time-varying component, and such that amplitude and phase of multiple angles from the target are measured simultaneously.

### Claim 12 (depends on 11)

The method of claim 11, wherein the diffracted radiation is measured in a pupil plane.

### Claim 13 (depends on 11)

The method of claim 11, wherein the diffracted radiation is measured in an image plane imaged by imaging optics.

### Claim 14 (depends on 11)

The method of claim 11, further comprising the step:
(d) processing the recorded amplitude and phase information to calculate a measurement of a property of the target.

### Claim 15 (depends on 14)

The method of claim 14, wherein step (d) comprises processing separately amplitude and phase information from at least two selected portions of a diffraction spectrum of the target to calculate a measurement of a property of the target using the recorded amplitude and phase information from at least two selected portions.

### Claim 16 (depends on 15)

The method of claim 15, wherein the at least two selected portions comprise opposite portions of the diffraction spectrum and said property is asymmetry.

### Claim 17 (depends on 16)

The method of claim 16, wherein in step (d) said asymmetry is calculated from amplitude information of the selected portions of the diffraction spectrum, while said phase information is used to calculate and apply a correction before and/or after calculating asymmetry.

### Claim 18 (depends on 16)

The method of claim 16, wherein steps (a) to (d) are repeated to obtain measurements of asymmetry for multiple targets, the method further comprising the step:
(e) using said measurements and known characteristics of the targets to calculate a measurement of a performance parameter of a lithographic process used to form the targets.

### Claim 19 (depends on 18)

A method as claimed in claim 18, wherein said performance parameter of the lithographic process is one of overlay, focus and dose.

### Claim 20 (depends on 18)

The method of claim 18, wherein said phase information is used in the calculation of said performance parameter in step (e).

### Claim 21 (depends on 14)

A method of manufacturing devices wherein device features and metrology targets are formed on a series of substrates by a lithographic process, wherein properties of the metrology targets on one or more processed substrates are measured by a method as claimed in claim 14, and wherein the measured properties are used to adjust parameters of the lithographic process for the processing of further substrates.

### Claim 22 (depends on 14)

A computer program product comprising non-transitory computer readable medium having one or more sequences of instructions for implementing the step (d) of a method of claim 14.

### Claim 23 (depends on 11)

The method of claim 11, further comprising simultaneously measuring different diffraction orders.

### Claim 24 (depends on 11)

The method of claim 11, wherein illuminating the target with the illuminating radiation further comprises scanning the target.