SYSTEM AND METHOD FOR SUPER-RESOLUTION FULL-FIELD OPTICAL METROLOGY ON THE FAR-FIELD NANOMETRE SCALE

Abstract

A system of super-resolution full-field optical metrology for delivering information on the surface topography of a sample or object on the far-field nanometre scale, including a light source, an interferometer (1a, 1b, 1c, 1d) including a reference arm incorporating a micro bead and a mirror, an object arm including a micro bead similar to the micro bead and arranged in immediate proximity to the surface of the object, receiving structure for capturing the interference figures, and a processor for processing these interference figures in such a way as to produce surface topography information. The light source is temporally coherent or partially coherent. The interferometer and the processor for processing interference figures are designed to reconstruct the surface of the object by phase shifting interferometry.

Claims

1. A super-resolution full-field optical metrology system for delivering information on the surface topography of a sample or object on the far-field nanometre scale, comprising: a coherent or partially coherent light source; an interferometer comprising an object arm incorporating a transparent microsphere and placed in immediate proximity to the surface of the object; a reference arm incorporating a mirror; receiving means for capturing interference figures and means for processing said interference figures so as to produce said surface topography information; said interferometer and said means for processing interference figures being arranged in order to reconstruct the topography of the object by phase-shifting interferometry.

2. The system according to claim 1, characterized in that the light source is temporally coherent or quasi-coherent with a wavelength in the visible spectrum.

3. The system according to claim 1, characterized in that the light source is temporally coherent or quasi-coherent with a wavelength in the infrared spectrum.

4. The system according to claim 1, characterized in that the light source is temporally coherent or quasi-coherent with a wavelength in the ultraviolet spectrum.

5. The system according to claim 1, characterized in that the interferometer is arranged in order to achieve measurements in a reflective configuration.

6. The system according to claim 5, characterized in that the interferometer is of a type selected from the group of Michelson, Twyman-Green, Mirau and Mach-Zehnder interferometers.

7. The system according to claim 1, characterized in that the interferometer is arranged in order to achieve measurements in transmissive configuration.

8. The system according to claim 7, characterized in that the interferometer is of the Mach-Zehnder type.

9. The system according to claim 1, characterized in that the reference arm also comprises a microsphere similar to the microsphere of the object arm, said microsphere of the reference arm being placed in order to compensate for the dispersion, and placed in immediate proximity to the surface of the mirror.

10. The system according to claim 1, characterized in that it comprises, in the object arm and in the reference arm, a plurality of microspheres arranged in the form of a translatable matrix of microspheres.

11. The system according to claim 1, characterized in that the microspheres(s) microsphere or microspheres are spherical, elliptical, hemispherical or convex in shape.

12. The system according to claim 1, characterized in that the microsphere or microspheres are placed in contact with the surface of the object or the surface of the reference mirror.

13. The system according to claim 1, characterized in that the microsphere or microspheres are held away from contact with the surface of the object or with the surface of the reference mirror.

14. The system according to claim 13, characterized in that the microsphere or microspheres are placed in a transparent layer placed on the surface of the object and having a refractive index less than that of said microsphere or microspheres.

15. The system according to claim 13, characterized in that the microsphere or microspheres are held above the surface of the object by a micromanipulator arm equipped with means for keeping said microsphere or microspheres.

16. The system according to claim 13, characterized in that the microspheres are held above the object by an optical tweezer.

17. The system according to claim 13, characterized in that the microspheres are held above the object by a piezoelectric system.

18. The system according to claim 13, characterized in that the microsphere or microspheres are placed in a micro-grid placed above the surface of the object and comprising holes of diameter substantially less than that of said microsphere or microspheres.

19. A super-resolution full-field optical metrology method for delivering information on the surface topography of an object on the far-field nanometre scale, implemented in an optical metrology system according to claim 1, said system incorporating an interferometer comprising an object arm equipped with a microsphere placed in immediate proximity to the surface of the object and being arranged in order to achieve interference figures, this method comprising: illuminating the surface via said microsphere, by means of a temporally coherent or partially coherent light source; and processing the interference figures in order to reconstruct the surface of the object by phase-shifting interferometry.

20. The method according to claim 19, characterized in that it achieves interferometric measurements in reflective configuration.

21. The method according to claim 19, characterized in that it achieves interferometric measurements in transmissive configuration.

22. The method according to claim 19, characterized in that it achieves interferometric measurements in matrix configuration.

23. The method according to claim 19, characterized in that the processing of the interference figures comprises: based on a phase measurement in images of interference figures, producing a raw signal of the phase measured modulo 2; cutting off said raw phase signal in an area of interest of the object so as to limit the boundary effects; two-dimensional unwrapping of the phase image thus obtained, surface-fitting said thus-unwrapped phase image so as to remove the effects of aberrations; converting said thus-unwrapped, then surface-fitted, phase image into a height distribution; and processing said height distribution in order to plot surface profiles of said object.

24. The method according to claim 19, characterized in that the processing of the interference figures comprises an optimization algorithm used in order to seek to bring the measurements closer to the results of a simulation describing the ball-object interaction.

Description

DESCRIPTION OF THE FIGURES

[0044] Other advantages and characteristics of the invention will become apparent on reading the detailed description of implementations and embodiments that are in no way limitative, and from the following attached figures:

[0045] FIG. 1 diagrammatically illustrates four optical configurations used in reflection and an optical configuration used in transmission, for a metrology system according to the invention,

[0046] FIG. 2 diagrammatically illustrates a device for positioning a microsphere with respect to the surface of a sample,

[0047] FIG. 3 diagrammatically illustrates a matrix arrangement of microspheres (in this case, of the hemispherical type),

[0048] FIG. 4 diagrammatically illustrates a succession of steps implemented in the optical metrology method according to the invention, and

[0049] FIG. 5 diagrammatically illustrates a variant of the device for positioning a microsphere with respect to the surface of a sample, utilizing a piezoelectric actuator.

DETAILED EMBODIMENTS

[0050] As these embodiments are in no way limitative, variants of the invention can be considered in particular comprising only a selection of the characteristics described or illustrated hereinafter, in isolation from the other characteristics described or illustrated (even if this selection is isolated within a phrase containing these other characteristics), if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, and/or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

[0051] With reference to FIG. 1, three variants of an optical metrology system operating in reflective configuration based respectively on a Michelson interferometer 1a, a Twyman-Green interferometer 1b and a Mirau interferometer 1c, and a variant operating in transmissive configuration based on a Mach-Zehnder interferometer 1d are described according to the invention.

[0052] The components common to these four embodiment variants are mentioned below with identical references.

[0053] The configuration of the Michelson type 1a requires an illumination part comprising a source 2, temporally coherent or partially coherent, a collimator and a beam splitter 3, and an imaging part comprising the Michelson interferometer, a tube lens 4, a detector 5 and a device 8 for processing these interference figures in order to generate surface profiles of an object or sample 6.

[0054] An assembly (not shown in FIG. 1) of lenses and diaphragms makes it possible to obtain an illumination of the object that is homogeneous in intensity. In the Michelson interferometer, the reference and object arms are perpendicular to one another. The beam incident on a converging lens or an assembly of lenses 11 is split into beam fractions by a beam splitter 12 and oriented in the reference arm and the object arm. The reference arm comprises a microsphere or a matrix of microspheres 9 (spherical, elliptical, hemispherical, convex in shape) and a mirror 10. The microsphere is or is not in contact with the mirror. The object arm comprises a microsphere or a matrix of microspheres 7 similar to said microsphere of the reference arm, and the object or sample 6 to be characterized in reflection mode.

[0055] The detector 5 captures interference figures produced by the interference of an object beam originating from the object arm and a reference beam originating from the reference arm, and a device 8 processes these interference figures in order to generate surface profiles of the sample 6.

[0056] The tube lens 4 is placed at the exit of the beam splitter 3 in order to converge the two interference beams, measurement and reference, towards the detector 5, while the second lens 11 is placed between the first splitter device 3 and the second splitter device 12 in order to converge the illumination beam towards the object 6 to be measured.

[0057] The numerical aperture of the lens 11 is in practice limited by its working distance and thus is generally less than 0.3. With a microsphere of diameter greater than 30 m, this therefore makes it possible to obtain a wide field of view.

[0058] The Twyman-Green configuration 1b shown in FIG. 1 is a variant of the Linnik configuration which is itself an improvement of the Michelson configuration insofar as it achieves an improved lateral resolution. This architecture requires an illumination part comprising a source 2, temporally coherent or partially coherent, equipped with a collimator, and an imaging part comprising a Twyman-Green interferometer, a tube lens 4, a detector 5 connected to a signal processing unit 8 in order to generate the topography of an object or sample 6. An assembly (not shown in FIG. 1) of lenses and diaphragms makes it possible to obtain an illumination of the object that is homogeneous in intensity.

[0059] In the Twyman-Green interferometer, the reference and object arms are perpendicular to one another and coupled by a beam splitter 12. The beam fractions are incident on two convergent lenses or two assemblies of lenses (one in each arm) 13 and 14. A portion of the beam is transmitted in the reference arm. The beam is then focused by the lens 14 and the microsphere or a matrix of microspheres 9 (spherical, elliptical, hemispherical, convex in shape) on the reference mirror 10 and reflected by the latter. The microsphere 9 is or is not in contact with the mirror 10. The reflected wave is collected by the microsphere 9 then the lens 14. The second part of the beam is reflected by the splitter 12 then directed into the object arm of the interferometer. The second lens 13 focuses the beam on the surface of the object 6 to be characterized in reflection mode, via a microsphere 7 similar to the microsphere 9 of the reference arm and placed in immediate proximity to this object. The wave is thus reflected or diffused by the surface of the object 6 then collected via the microsphere 7 by the lens 13. Like the reference wave, the object wave is transmitted by the tube lens 4 then imaged on the detector 5.

[0060] The detector 5 captures interference figures produced by the interference of an object beam originating from the object arm and a reference beam originating from the reference arm, and a device 8 processes these interference figures in order to generate surface profiles of the sample 6.

[0061] The tube lens 4 is placed at the exit of the splitter 12 in order to converge the two interference beams, measurement and reference, towards the detector 5.

[0062] The numerical aperture of the two identical lenses 13 and 14 is in practice not limited by its working distance and thus makes it possible to carry out the acquisition with a high lateral resolution. The Twyman-Green architecture provides the benefit of a compromise between lateral resolution and field of view.

[0063] The Mirau configuration 1c shown in FIG. 1 has an advantage with respect to the other architectures; that of a reduction in bulk. In fact, the reference arm is superimposed on the object arm and the optical axes of the reference and object arms are then merged. This architecture requires an illumination part comprising a source 2, temporally coherent or partially, coherent, equipped with a collimator and a beam splitter 3, and an imaging part comprising the Mirau interferometer, a tube lens 4, a detector 5 and a device 8 for processing these interference figures. An assembly (not shown in FIG. 1) of lenses and diaphragms makes it possible to obtain an illumination of the object that is homogeneous in intensity.

[0064] In the Mirau interferometer, the reference and object arms are parallel to one another. The beam incident on a convergent lens or an assembly of lenses 11 is split into fractions by a beam splitter 12 and oriented in the reference arm and the object arm. The reference arm comprises a microsphere or a matrix of microspheres 9 (spherical, elliptical, hemispherical, convex in shape) and a mirror 10. The microsphere is or is not in contact with the mirror. The object arm comprises a microsphere or a matrix of microspheres 7 similar to the microsphere of the reference arm, and the object 6 to be characterized in reflection mode.

[0065] The detector 5 captures interference figures produced by the interference of an object beam originating from the object arm and a reference beam originating from the reference arm, and a device 8 processes these interference figures in order to generate surface profiles of the sample 6.

[0066] The tube lens 4 is placed at the exit of the beam splitter 3 in order to converge the two interference beams, measurement and reference, towards the detector 5, while the second lens 11 is placed between the first splitter device 3 and the second splitter device 12 in order to converge the illumination beam towards the object 6 to be measured and the reference mirror 10.

[0067] The numerical aperture of the lens 11 is in practice limited by its working distance and thus is generally less than 0.5. With a microsphere of diameter greater than 30 m, this thus makes it possible to obtain a wide field of view.

[0068] A configuration 1d of an optical metrology system, utilizing an interferometer of the Mach-Zehnder type, suitable for transmission measurements, will now be described with reference to FIG. 1. Transmission measurements are mainly used in biology, because the samples are often transparent at the wavelength.

[0069] A transmission measurement makes it possible to find the optical path difference induced by the object passed through. By knowing the refractive index of the object, the geometric height of the object is found, and vice versa.

[0070] The light beam originating from a source, coherent or partially coherent 2, is divided in two by a beam splitter 12. The beam transmitted by the splitter 12, called object beam, passes through an object or sample 6, after being optionally focused by an optional lens 15 which is provided in order to concentrate the light energy on the desired field of view and thus subsequently collect more light.

[0071] A microsphere 7 then a lens 11 collect the beam diffused by the object 6. A mirror 16 directs this object beam onto a detector 5, passing through a beam splitter 18 and a tube lens or relay lens 4.

[0072] The beam reflected by the beam splitter 12, called reference beam, is directed by a mirror 17 towards the beam splitter 18 where it is again directed onto the detector 5 via the tube lens 4.

[0073] The device 8 for processing the interference figures then makes it possible to find the lateral distribution (i.e. along X and Y) of the optical path of the object, in particular items of refractive index and geometric height information.

[0074] In the four configurations 1a, 1b, 1c and 1d which have just been described, the reference mirror 10 can be fixed to a piezoelectric device (not shown) which is controlled in order to achieve a lateral displacement of this mirror 10 around an equilibrium position, in order to obtain the phase shift. The microspheres 7 can be held at a short distance from the object 6 by another piezoelectric device (not shown).

[0075] It is important to note that, according to the invention, in the optical metrology systems that have just been described, it is possible to modify the polarization via polarizers and retardation plates, the uniformity of the lighting of the object via a lighting system, and the angles of the rays incident on the sample.

[0076] The microspheres 7, 9, which are utilized in the optical metrology systems 1a, 1b, 1c and 1d according to the invention described above with reference to FIG. 1, can be placed in air or immersed in a transparent material of the gaseous, liquid or solid type (for example, a polymer such as polydimethylsiloxane or PDMS).

[0077] In the different cases that have just been described, the measurable quantity in an optical metrology system according to the invention is an image or a series of 2D intensity images which is more usually called an interference figure. The items of information found are thus the surface topography of the object via phase-shifting interferometry. This phase-shift method is quicker than the known method of detecting the coherence function peak as it requires fewer acquisitions, and provides a better axial resolution. Four images are sufficient to reconstruct the surface topography of the object. The calculated phase shift between the reference wave and the object wave (interpreted as a retardation of the wave) makes it possible to find surface reliefs, i.e. the topography, via a conventional formula taking account of the dispersion of the microsphere.

[0078] For the different embodiments that have been described, the light source 2 must supply a high coherence. In addition, digital simulations as well as experimental measurements have shown that the use of a light source with a short wavelength provides a greater lateral resolution. For example, a blue light source close to UV provides a greater lateral resolution.

[0079] The light source 2 can be: [0080] coherent, for example a laser source with a coherence length on a scale of metres, [0081] quasi-coherent, for example a laser diode with a coherence length on a scale of centimetres, [0082] partially coherent, for example a superluminescent diode with a coherence length on a scale of hundreds of micrometres, [0083] partially incoherent, for example a light-emitting diode with a coherence length on a scale of tens of micrometres, [0084] narrow filtering by wavelength, for example a supercontinuum and a filter, [0085] in all cases, preferably with a short wavelength or with a spectrum centred on a short wavelength in the green, the blue, or even the ultraviolet, for example a blue LED at 450 nm.

[0086] The performance of a super-resolution profilometer according to the invention depends on several parameters such as the combination of the lens or the assembly of collection lenses and the microsphere, and the wavelength. It has been shown that an interferometer of the Twyman-Green configuration 1b described above with reference to FIG. 1 and comprising a close, short-wavelength light source and a glass microsphere having a diameter between 10 m and 30 m, makes it possible to resolve patterns of 100 nm in size. The microscope objective 13 placed in an immersion medium must have a numerical aperture of 0.9.

[0087] In the embodiment example illustrated in FIG. 2, a microsphere 7 intended to be placed in a measurement beam 23 within one of the optical metrology systems shown in FIG. 1, is included in an immersion layer 21. The refractive index of the medium constituting the layer 21 is less than that of the microsphere 7. This immersion layer 21 is placed on the surface 22 of the object 6 to be measured, for example a substrate, itself placed on a support 24. The microsphere 7 then collects the beam 25 reflected or diffused by the surface 22 of the object 6.

[0088] In an embodiment variant of the device in FIG. 2, illustrated in FIG. 5 in which components common to the two embodiments have common references, a coverslip 27 transparent at the wavelength of the light source is placed on the microsphere 7.

[0089] This coverslip 27 can be made from glass or any other transparent material and may have or may not have one and the same refractive index as the microsphere 7, which can be bonded, fused or held by a force, to the coverslip 27. The coverslip 27 is fastened to a piezoelectric actuator 28 which can control a vertical and/or horizontal displacement.

[0090] The refractive index contrast to be taken into account for the evaluation of the imaging performances is that between the microsphere 7 and the layer 21. For example, the microsphere 7 can be made from barium titanate and included in a layer 21 made from PDMS. It is also possible to provide for the microsphere to be placed in a perforated micro-grid of a diameter slightly less than the size of the microspheres in order to support them, or held by a micromanipulator arm with a tweezer or another adhesion system, or even held by an optical tweezer.

[0091] In addition, a matrix configuration of microspheres can be envisaged as illustrated in FIG. 3, where, in this example, a matrix of microspheres of the hemisphere type 26 is shown. The matrix of hemispheres provided to be placed in a measurement beam 23 within one of the optical metrology systems shown in FIG. 1, is included in an immersion medium 21. The refractive index of the medium 21 is less than that of the microsphere 26. This immersion layer 21 is placed on the surface 22 of the object 6 to be measured, for example a substrate, itself placed on a support 24. The microsphere 7 then collects the beam 25 reflected or diffused by the surface 22 of the object 6.

[0092] This matrix arrangement of microspheres is particularly suitable with the use of a matrix of Mirau interferometers due to the reduced bulk, and it makes it possible to increase the field of view while maintaining a similar acquisition rate.

[0093] A practical example of a processing of the interference figures obtained with the optical metrology method according to the invention will now be described with reference to FIG. 4. Processing the signal in phase-shifting interferometry requires the unwrapping of the phase.

[0094] The raw signal 30a of the phase measured modulo 2 is cut off at an area of interest 30b of the object in order to limit the boundary effects. The phase image is then unwrapped 30c in two dimensions then surface-fitted in order to remove the effects of aberrations.

[0095] This thus-processed image is then converted to a height distribution 30e. A dedicated programme then makes it possible, from this height distribution, to plot surface profiles 30f.

[0096] Of course, the invention is not limited to the examples that have just been described, and numerous modifications may be made to these examples without exceeding the scope of the invention. Of course, the various characteristics, forms, variants and embodiments of the invention can be combined together in various combinations inasmuch as they are not incompatible or mutually exclusive. In particular, all the variants and embodiments described above can be combined together.