Metrology Method and Inspection Apparatus, Lithographic System and Device Manufacturing Method

• US 20120123581A1
• Filed: 11/10/2011
• Published: 05/17/2012
• Est. Priority Date: 11/12/2010
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: ASML Netherlands BV (ASML Holding NV)
• Inventors: SMILDE, Hendrik Jan Hidde, BLEEKER, Arno Jan, WARNAAR, Patrick, COENE, Willem Marie Julia Marcel, KUBIS, Michael
• Granted Patent: US 9,140,998 B2
• Time in Patent Office: Days
• US Class Current: 700/105
• CPC Class Codes: G03F 7/70483 Information management; Active and passive control; Testing; Wafer monitoring, e.g. pattern monitoring
• CPC Class Codes: G03F 7/70625 Dimensions, e.g. line width, critical dimension [CD], profile, sidewall angle or edge roughness
• CPC Class Codes: 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
• CPC Class Codes: G03F 7/70641 Focus
• CPC Class Codes: G06N 20/00 Machine learning

Abstract

Methods are disclosed for measuring target structures formed by a lithographic process on a substrate. A grating structure within the target is smaller than an illumination spot and field of view of a measurement optical system. The optical system has a first branch leading to a pupil plane imaging sensor and a second branch leading to a substrate plane imaging sensor. A spatial light modulator is arranged in an intermediate pupil plane of the second branch of the optical system. The SLM imparts a programmable pattern of attenuation that may be used to correct for asymmetries between the first and second modes of illumination or imaging. By use of specific target designs and machine-learning processes, the attenuation patterns may also be programmed to act as filter functions, enhancing sensitivity to specific parameters of interest, such as focus.

Claims

1. A method comprising:

using a lithographic process to form a periodic structure on a substrate;

a first measurement comprising forming and detecting a first image of the periodic structure, while illuminating the structure with a first beam of radiation, the first image being formed using a first selected part of diffracted radiation;

a second measurement comprising forming and detecting a second image of the periodic structure, while illuminating the structure with a second beam of radiation, the second image being formed using a second selected part of the diffracted radiation which is symmetrically opposite to the first part, in a diffraction spectrum of the periodic structure;

using a difference in intensity values derived from the detected first and second images together to determine a property of the periodic structure; and

controlling a spatial light modulator to apply a varying non-binary optical attenuation over the first and second selected parts of the diffracted radiation prior to forming the first and second images respectively.

2. The method of claim 1, wherein:

the first and second measurements are performed without rotating the substrate using different optical paths within a measurement optical system, and

the varying optical attenuation is configured to reduce an influence on the determined property of the difference in optical paths between the first and second measurements.

3. The method of claim 2, wherein a plurality of different patterns of varying attenuation are defined and associated with different available optical paths, the method further comprising:

selecting automatically an attenuation pattern according to the optical path used in each of the measurements and controlling the spatial light modulator to apply the selected attenuation pattern to the diffracted radiation.

4. The method of claim 2, wherein:

the first and second measurements are performed using respectively a first and a second illumination mode of the measurement optical system, such that the first and second beams of radiation are incident on the periodic structure from symmetrically opposed angles relative to an optical axis of the measurement optical system, without rotating the substrate relative to the measurement optical system, and

at least first and second attenuation patterns are defined to compensate for asymmetry between optical paths defining the first and second illumination modes.

5. The method of claim 2, wherein the first and second measurements are performed using respectively a first and a second imaging mode of the measurement optical system, such that the first and second images are formed using portions of radiation diffracted by the periodic structure to diametrically opposed angles relative to an optical axis of the measurement optical system, without rotating the substrate relative to the measurement optical system, the selected part of the diffracted radiation passing through different parts of the spatial light modulator in the first and second imaging modes.

6. The method of any of claim 2, further comprising:

a calibration wherein a pattern for attenuation is at least partially determined using the results of a plurality of calibration measurements performed on a substrate, which is measured at different rotations through each of the optical paths.

7. The method of claim 6, wherein the calibration measurements are performed using an image sensor which is located in a conjugate pupil plane of the optical system.

8. The method of claim 6, wherein the calibration measurements are performed using a calibration target having a periodic structure larger than a field of view of the measurement optical system.

9. The method of claim 1, wherein the varying optical attenuation implements a first and second filter functions for the respective measurement step, the filter functions being calculated to enhance sensitivity of the calculated difference to a property of interest.

10. The method of claim 9, further comprising implementing a machine-learning process to derive the filter functions from measurements of a training set of structures.

11. The method of claim 10, wherein the machine-learning process comprises principal component analysis.

12. The method of claim 1, wherein the lithographic process is performed so that a certain asymmetry of profile of the periodic structure is made sensitive to a particular performance parameter of the lithographic process.

13. The method of claim 1, wherein the lithographic process is an optical lithography process and is performed so that asymmetry between side wall angles in the periodic structure is sensitive to focus variation in the lithographic process.

14. The method of claim 9, wherein the filter functions are calculated such that the calculated difference is related directly to the performance parameter of the lithographic process, rather than a feature of the periodic structure itself.

15. The method of claim 1, wherein the periodic structure occupies less than half the area of the field of view such that first or second images of a plurality of periodic structures can be formed and detected imaged simultaneously.

16. The method of claim 1, wherein a correction is applied to the detected first and second images and/or to the measured asymmetry, based at least in part on the position of the periodic structure within a field of view of the measurement optical system.

17. The method of claim 1, wherein the spatial light modulator is used also in the selection of the first and second parts of the diffracted radiation.

18. An inspection apparatus comprising:

an illumination arrangement operable to deliver conditioned beams of radiation to the substrate for use in measurements;

a detection arrangement operable during such measurements to form and detect respective images of the substrate using radiation diffracted from the substrate the illumination arrangement and the detection arrangement forming a measurement optical system; and

a stop arrangement within the detection arrangement,

wherein the illumination arrangement and stop arrangement together are operable to select which part of a diffraction spectrum of the diffracted radiation contributes to each image, and

wherein the detection arrangement further comprises a spatial light modulator operable to apply a varying optical attenuation over the selected part of the diffracted radiation prior to forming the first and second images respectively.

19. The inspection apparatus of claim 18, further comprising:

a controller for controlling the apparatus to make first and second images of a periodic structure on a substrate, the first image being formed in a first measurement using a first selected part of diffracted radiation, the second image being formed in a second measurement using a second selected part of the diffracted radiation which is symmetrically opposite to the first part in the a diffraction spectrum of the periodic structure without rotating the substrate; and

a computational arrangement arranged to use a difference in intensity values derived from the detected first and second images together to determine an asymmetry-related parameter.

20. The inspection apparatus of claim 19, wherein the first and second measurements are performed without rotating the substrate using different optical paths within the measurement optical system, and the controller is further arranged to control the spatial light modulator to reduce an influence on the determined asymmetry-related parameter of the difference in optical paths between the first and second measurement steps.

21. The inspection apparatus of claim 20, wherein the computational arrangement is arranged to change an illumination mode of the illumination arrangement between the first and second measurement steps, and to select the attenuation pattern applied by the spatial light modulator in accordance with the selected illumination mode.

22. A lithographic system comprising:

a lithographic apparatus comprising;

an illumination optical system arranged to illuminate a pattern;

a projection optical system arranged to project an image of the pattern onto a substrate; and

an inspection apparatus comprising,

an illumination arrangement operable to deliver conditioned beams of radiation to the substrate for use in measurements;

a detection arrangement operable during such measurements to form and detect respective images of the substrate using radiation diffracted from the substrate the illumination arrangement and the detection arrangement forming a measurement optical system; and

a stop arrangement within the detection arrangement,

wherein the illumination arrangement and stop arrangement together are operable to select which part of a diffraction spectrum of the diffracted radiation contributes to each image, and

wherein the detection arrangement further comprises a spatial light modulator operable to apply a varying optical attenuation over the selected part of the diffracted radiation prior to forming the first and second images respectively,

wherein the lithographic apparatus is arranged to use the measurement results from the inspection apparatus in applying the pattern to further substrates.

23. A method of manufacturing devices comprising:

a device pattern is applied to a series of substrates using a lithographic process;

inspecting at least one periodic structure formed as part of or beside the device pattern on at least one of the substrates using an inspection method comprising,

using a lithographic process to form a periodic structure on a substrate;

a first measurement comprising forming and detecting a first image of the periodic structure, while illuminating the structure with a first beam of radiation, the first image being formed using a first selected part of diffracted radiation;

a second measurement comprising forming and detecting a second image of the periodic structure, while illuminating the structure with a second beam of radiation, the second image being formed using a second selected part of the diffracted radiation which is symmetrically opposite to the first part, in a diffraction spectrum of the periodic structure;

using a difference in intensity values derived from the detected first and second images together to determine a property of the periodic structure; and

controlling a spatial light modulator to apply a varying non-binary optical attenuation over the first and second selected parts of the diffracted radiation prior to forming the first and second images respectively; and

controlling the lithographic process for later substrates in accordance with the result of the inspection method.