Transient ellipsometer or scatterometer and related measurement methods

Abstract

The invention relates to an ellipsometer or scatterometer comprising a light source (1); a polarizer (5); an optical illumination system (2, 4) adapted to direct an incident polarized light beam (11) towards a sample (6); an optical wavefront beam splitter (20) arranged to receive a secondary light beam (12) generated by reflection, transmission or diffraction, the optical wavefront beam splitter (20) being oriented to form three collimated separate light beams; optical polarization modifying means (25) and optical polarization separating means (26) forming six angularly separated light beams; a detection system adapted to detect the six separated light beams; and a processing system adapted to derive ellipsometric or scatterometry measurements therefrom.

Description

Technical Field

[0001] The present invention generally relates to the field of optical instruments for measuring materials and / or thin layers.

[0002] In particular, the present invention relates to a spectroscopic or monochromatic ellipsometer or scatterometer and a spectroscopic or monochromatic ellipsometric or scatterometric measurement method.

[0003] More particularly, the invention relates to spectroscopic or monochromatic ellipsometers or scatterometers having very high measurement rates. Background Art

[0004] A spectroscopic ellipsometer or scatterometer typically comprises an illumination arm and a detection arm. The illumination arm comprises a broadband spectral light source and a polarizer configured to polarize a light beam incident on a sample at a determined angle of incidence (AOI). The detection arm receives the light beam incident on the sample, the light beam formed by reflection or transmission (for an ellipsometer) and the light beam formed by diffraction (for a scatterometer), and comprises a polarization analyzer and a spectrometer to detect the reflected, transmitted or diffracted light beam according to the wavelength.

[0005] Monochromatic ellipsometers or scatterometers comprise the same elements, the light source is usually monochromatic and the detector receives the reflected, transmitted or diffracted beam downstream of the polarisation analyser without the need for a spectrometer.

[0006] In an ellipsometer operating in reflection on the sample, the detection arm is arranged to receive the reflected light beam in an incident plane which forms an angle with respect to the sample normal opposite to the incident angle. In an ellipsometer operating in transmission, the detection arm is arranged to receive the light beam transmitted by the sample in the incident plane.

[0007] In a scatterometer, a detection arm is arranged to receive a beam diffracted by a sample in an incident plane which forms an angle with respect to the sample normal which is usually different from the angle of incidence.

[0008] There are many monochromatic or spectroscopic ellipsometers. Most of these ellipsometers include an optical polarization modulator for temporarily modulating the polarization state of a beam incident on or reflected from a sample in order to obtain polarization components of the beam reflected or transmitted by the sample according to at least two independent polarization states.

[0009] Monochromatic or spectroscopic ellipsometers are known, which are based on a rotating polarizer or a rotating compensator, an optical phase modulator or a birefringent liquid crystal system, which makes it possible to change the polarization of a light beam as a function of time. Regardless of the type of optical polarization modulator, a detection system is used to obtain a series of measurements or spectra modulated as a function of time, i.e. as a function of polarization. The spectrometer usually comprises a photodetector consisting of an array of pixels or a matrix of pixels measuring the intensity of the light beam, each pixel or each pixel column being associated with a wavelength. Under these conditions, the determination of the search parameters of the ellipsometry is performed by analyzing a series of monochromatic measurements or intensity spectra I(λ) as a function of time during at least one modulation period. Many publications have described different ways to derive the elliptical polarization quantity from a series of monochromatic or spectral I(λ) measurements determined for different positions of the components of the modulated polarization. The analysis of the series of monochromatic or spectral I(λ) measurements can be performed in the frequency domain or in the time domain.

[0010] However, this method based on temporal polarization modulation has a certain number of disadvantages. Firstly, the measurement accuracy is affected by the intensity instability of the light source used. Furthermore, the minimum duration of the measurement is fixed by the polarization modulation period and cannot be lower than half of the modulation period. In particular, the use of optical polarization modulators based on rotating components may have disadvantages not only in terms of vibration generation, but also in terms of long-term reliability and robustness. Finally, during the acquisition of a series of spectra within a modulation period, it is generally not possible to adjust the acquisition time of the spectra according to their intensity, since the acquisition duration of the spectra depends on the modulation period. Therefore, it is not possible to use this method to measure each spectrum within the favorable intensity range of the detector dynamics, avoiding both saturation and too low a signal. In that way, it is sometimes difficult to examine with the same monochromator or spectroscopic ellipsometer with very different reflectance values ​​or to use spots with very different sizes.

[0011] On the other hand, a wavelength division polarization spectrometer is known from patent document WO 2014 / 016528 A1, which can divide a source beam into six polarized beams and simultaneously measure the intensities of six spectral images corresponding to the six polarized beams to derive a spectral measurement of the polarization state of the source beam. However, a disadvantage of a wavelength division polarimeter is that the light intensity distribution on different spectral images depends strongly on the experimental conditions. Therefore, the measured value of the light intensity of each image depends not only on the polarization state, but also on the light intensity distribution at the position where the wavelength division is performed. The polarization measurement state with the aid of such a polarimeter may be inaccurate and / or erroneous. Moreover, the document does not describe an ellipsometer or scatterometer that requires a well-defined angle of incidence and reflection, transmission or diffraction angle to analyze the polarization state of a beam reflected, transmitted or diffracted by a sample.

[0012] Generally, in an ellipsometer or scatterometer, in order to avoid introducing systematic measurement errors, it is desirable to avoid inserting any optical system that tends to change the polarization of the light beam between the polarizer and the sample of the illumination arm on the one hand, and between the sample and the polarization analyzer of the detection arm on the other hand. Furthermore, the insertion of an optical system tends to reduce the light intensity of the light beam, restrict the spectral band and / or change the divergence of the light beam.

[0013] Ultimately, it is desirable to obtain monochromatic or spectral ellipsometry or scatterometry measurements at rates easily exceeding 50 Hz.

[0014] In certain applications, it is desirable to obtain time-resolved monochromatic or spectral ellipsometry measurements, for example for in situ monitoring of thin layer growth, deposition or etching processes.

[0015] In these applications, it is desirable to use rotating optical components to avoid mechanical vibrations caused by the rotation.

[0016] On the other hand, it would be desirable to extend the spectral range of ellipsometry or scatterometry measurements while maintaining or improving the quality of these measurements. Summary of the invention

[0017] In order to make up for the above-mentioned shortcomings of the prior art, the present invention proposes an ellipsometer or a scatterometer.

[0018] More specifically, according to the present invention, an ellipsometer is proposed, which includes: a light source suitable for generating a source light beam; a polarizer, suitable for receiving the source light beam and forming a polarized incident light beam; an optical illumination system, suitable for guiding the polarized incident light beam toward a sample along an incident optical axis in an incident plane; an optical wavefront beam splitter, arranged to receive a secondary light beam formed by reflection or transmission of the polarized incident light beam on the sample at a determined incident angle, the secondary light beam propagating along the secondary optical axis in the incident plane, the optical wavefront beam splitter being oriented to form three collimated separated light beams propagating along three different optical axes , the three different optical axes being angularly separated in a plane perpendicular to the plane of incidence; and an optical polarization changing device adapted to receive the three collimated separated light beams and form three light beams polarized according to three different polarization states; an optical polarization beam splitter device arranged and oriented to receive the three light beams polarized according to three different polarization states and form six separated light beams propagating along six optical axes, the six optical axes being angularly separated in a plane perpendicular to the plane of incidence; a detection system adapted to detect the six separated light beams, and a processing system adapted to derive ellipsometry measurements therefrom.

[0019] The ellipsometer can acquire instantaneous ellipsometric measurements without modulating the polarization. The acquisition duration of the ellipsometric measurement can be easily adjusted based on the intensity of each of the six detected beams.

[0020] According to a particular aspect, the ellipsometer is monochromatic.

[0021] According to another specific aspect, the detection system comprises at least one spectrometer adapted to detect six separate light beams.

[0022] According to a specific embodiment, the ellipsometer includes: an optical focusing system, arranged to receive six separated light beams and form six images aligned with each other and spatially separated from each other, and the at least one spectrometer includes an imaging spectrometer, which includes an entrance slit elongated in one direction and arranged to simultaneously receive the six images, the imaging spectrometer is suitable for spectrally dispersing the six images and simultaneously forming six spectral sub-images separated in space on an image detector, the image detector is suitable for acquiring images of the six spectral sub-images, and the processing system is suitable for processing images of the six spectral sub-images and deriving spectroscopic ellipsometric measurements therefrom.

[0023] The acquisition duration of the spectral measurement can be easily adjusted depending on the intensity of the detected spectral sub-image.

[0024] According to a first embodiment, the optical illumination system is suitable for focusing the polarized incident light beam on the sample, and the optical wavefront beam splitter comprises a segmented lens, which comprises three lens segments having the same focal length and optical axes that are parallel to each other and spatially separated from each other, each lens segment having an objective focus, and the three lens segments are assembled so that the three objective focal points are aligned perpendicular to the incident plane on the sample, each lens segment is arranged to receive a different part of the secondary light beam and form a collimated separated light beam, and the three lens segments are assembled together so that the three separated light beams propagate along three different optical axes.

[0025] According to a second embodiment, the optical illumination system is suitable for collimating the polarized incident light beam on the sample, and the optical wavefront beam splitter comprises at least two prisms whose edges are arranged parallel to the incident plane, each prism being arranged to receive a different portion of the secondary light beam and to form a collimated separated light beam propagating along an optical axis angularly offset relative to the secondary optical axis, and the at least two prisms are oriented and assembled so that the three collimated separated light beams propagate along the three different optical axes.

[0026] The following are further non-limiting and advantageous features of the ellipsometer or scatterometer according to the invention, which are derived individually or in all technically possible combinations:

[0027] - the optical wavefront beam splitter further comprises a mask comprising three spatially separated apertures;

[0028] - said optical polarization-changing means comprise at least two wave plates, preferably achromatic, each having a different retardation, one wave plate being arranged on one of said three separate beams and another wave plate being arranged on another of said three separate beams;

[0029] - the optical polarization beam splitter device comprises a Wollaston prism, a Rocon prism, a Sennamonte prism or a diffraction wave plate;

[0030] - the optical wavefront beam splitter is adapted to angularly split the three separated light beams at an angle ALPHA between adjacent separated light beams, and the optical polarization beam splitter device is adapted to angularly split the six separated light beams at an angle BETA between adjacent separated light beams, the angle BETA being between 0.6*ALPHA / 2 and 1.5*ALPHA / 2;

[0031] - the light source comprises a halogen lamp, a xenon flash lamp, a supercontinuum laser source and / or a fiber laser and / or a pulse source;

[0032] - the light source is a supercontinuum laser source and further comprises a cylindrical lens arranged in the optical path of the secondary light beam upstream of the entrance slit of the imaging spectrometer, the cylindrical lens being adapted and oriented to magnify the six images perpendicular to the elongated direction of the entrance slit.

[0033] Advantageously, the ellipsometer and the scatterometer comprise a first source aperture, the optical illumination system being suitable for forming a first image of the first source aperture on the sample, and / or comprise a second source aperture, the optical illumination system being suitable for forming a second image of the second source aperture on the sample, the surface area ratio of the first source aperture and the second source aperture being greater than or equal to 10.

[0034] According to a specific and advantageous embodiment, the light source is suitable for generating a first series of N1 light pulses, wherein N1 is a natural integer, the image detector is suitable for obtaining a first image of the six spectral sub-images of the first series of N1 light pulses when the image detector is not saturated, the light source is suitable for generating a second series of N2 light pulses, wherein N2 is a natural integer, N2 being comprised between N1+1 and 25.N1, the image detector is suitable for obtaining a second image of the second series of N2 light pulses on a first part of the six spectral sub-images when the image detector is saturated, and on a second part of the six spectral sub-images when the image detector is not saturated, and wherein the processing system is suitable for combining a first part of the six spectral sub-images of the first image with a second part of the six spectral sub-images of the second image so as to derive the spectral ellipsometric measurement therefrom.

[0035] In a specific and advantageous manner, at least two spectral sub-images are spatially separated from each other by an intermediate region, and the image processing system is adapted to measure the stray light intensity in the intermediate region and to subtract it from at least a part of the image of the six spectral sub-images.

[0036] According to a specific aspect, the ellipsometer also includes a system for moving a sample holder, the light source is suitable for generating a series of light pulses, the detection system is suitable for acquiring a series of acquisitions of six separate light beams corresponding to the series of light pulses during the displacement of the sample holder, and the processing system is suitable for processing the series of acquisitions and deriving a series of ellipsometry measurements therefrom based on the displacement of the sample holder.

[0037] According to another specific aspect, the ellipsometer or scatterometer comprises an auxiliary light source adapted to illuminate the sample surface and / or an auxiliary camera adapted to acquire at least one image of the sample.

[0038] The present invention also provides an ellipsometry measurement method, comprising the following steps:

[0039] - generating a polarized incident light beam;

[0040] - directing the polarized incident light beam towards the sample along an incident optical axis in an incidence plane;

[0041] - receiving a secondary light beam formed by reflection or transmission of the polarized incident light beam on the sample at a determined angle of incidence, the secondary light beam propagating along a secondary optical axis in the incident plane;

[0042] - splitting the secondary beam by wavelength division optics so as to form three collimated separate beams propagating along three different optical axes, the three different optical axes being angularly separated in a plane perpendicular to the plane of incidence;

[0043] - changing the polarization of at least two of the three collimated separate beams to form three beams polarized according to three different polarization states;

[0044] - polarization-separating the three polarized light beams to form six separate light beams propagating along six optical axes, the six optical axes being angularly separated in a plane perpendicular to the plane of incidence;

[0045] - detecting the six separate beams to derive ellipsometric measurements therefrom.

[0046] According to a specific aspect, the method comprises the following additional steps:

[0047] - simultaneously focusing the six separate beams into six aligned and spatially separated images onto an entrance slit of a spectrometer;

[0048] - spectrally scattering the six images to simultaneously form six spectral sub-images spatially separated on an image detector;

[0049] - acquiring images of the six spectral sub-images; and

[0050] - Processing the images of the six spectral sub-images to deduce spectroscopic ellipsometry or scatterometry therefrom.

[0051] The present invention also relates to a scatterometer, comprising: a light source suitable for generating a source light beam; a polarizer suitable for receiving the source light beam and forming a polarized incident light beam; an optical illumination system suitable for guiding the polarized incident light beam toward a sample along an incident optical axis in an incident plane; an optical wavefront beam splitter arranged to receive a secondary light beam formed by diffracting the polarized incident light beam on the sample at a determined incident angle, the secondary light beam propagating along the secondary optical axis in the incident plane, the optical wavefront beam splitter being oriented to form three collimated separated light beams propagating along three different optical axes, the three The different optical axes are angularly separated in a plane perpendicular to the incident plane; an optical polarization changing device, suitable for receiving three collimated separated light beams and forming three light beams polarized according to three different polarization states; an optical polarization beam splitter device, arranged and oriented to receive three light beams polarized according to three different polarization states and form six separated beams propagating along six optical axes, which are angularly separated in a plane perpendicular to the incident plane; and a detection system, suitable for detecting the six separated light beams, and a processing system, suitable for extracting scattering measurements therefrom.

[0052] According to a specific embodiment, the scatterometer includes: an optical focusing system, arranged to receive the six separated light beams and form six images that are aligned and spatially separated; and an imaging spectrometer, including an entrance slit that is elongated in one direction and arranged to simultaneously receive the six images, the imaging spectrometer is suitable for spectrally scattering the six images and simultaneously forming six spectral sub-images that are spatially separated on an image detector, the image detector is suitable for acquiring images of the six spectral sub-images, and the processing system is suitable for processing images of the six spectral sub-images and deriving spectral scattering measurements therefrom. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The following description related to the accompanying drawings, given by way of non-limiting examples, will allow a good understanding of what the invention consists of and how to implement it.

[0054] In the attached picture:

[0055] - Figure 1 schematically shows a cross-sectional view of an ellipsometer or scatterometer according to a first embodiment configured with an incident beam focused on a sample;

[0056] - Figure 2 Schematically shows the Figure 1 A view of section BB of the detection arm of the ellipsometer or scatterometer shown;

[0057] - Figure 3 schematically shows a front view of a first mask of an optical wavefront beam splitter;

[0058] - Figure 4 Schematically shows Figure 1 and 2 A perspective view of a segmented lens used in a detection arm in FIG.

[0059] - Figure 5 schematically shows a 3D view of a spectroscopic ellipsometer or scatterometer according to a first embodiment;

[0060] - Figure 6 schematically shows a cross-sectional view of an ellipsometer or scatterometer according to a second embodiment configured with a collimated incident beam;

[0061] - Figure 7 Schematically shows the Figure 6 A view of section CC of the detection arm of the ellipsometer or scatterometer shown;

[0062] - Figure 8 schematically shows a spectral polarization image through a source aperture of a spectroscopic ellipsometer or scatterometer according to one of the embodiments, without optical aberrations;

[0063] - Fig. 9 schematically shows a spectral polarization image through a source aperture of a spectroscopic ellipsometer or scatterometer according to one of the embodiments, integrating optical aberrations;

[0064] - Fig.10 A cross-section of an ellipsometer or scatterometer with auxiliary light source and auxiliary visualization is schematically shown;

[0065] - Fig.11 Schematically shows an example of a spectral sub-image used to form a PSI image and an image of a DELTA of a sample;

[0066] - Fig.12 An example of an auxiliary visualization image is shown, where the elliptically polarized spot is overlapped with the sample image. DETAILED DESCRIPTION

[0067] exist Figures 1 to 5 An ellipsometer or scatterometer according to a first embodiment is shown in FIG.

[0068] The ellipsometer or scatterometer comprises an illumination arm 50 and a detection arm 60, which are arranged on either side of a sample 6 to be measured. The illumination arm 50 comprises a light source 1, a first optical system 2, a source aperture 3, a further optical focusing system 4 and a polarizer 5. Preferably, the polarizer 5 is arranged downstream of the optical focusing system 4. Alternatively, the polarizer 5 may be arranged upstream of the optical focusing system 4 or between different components of the optical focusing system 4.

[0069] The light source 1 emits a source light beam 10, which is preferably in a spectrally extended wavelength range, for example 360 ​​nm to 700 nm for a xenon flash lamp. The first optical system 2 forms an image of the source on a source aperture 3. As an alternative, a fiber-shaped light source is used, the end of which forms the source aperture 3. The light source 1 can be a light source that emits light radiation that is continuous in time, or a pulsed light source as described below. The light source 1 comprises, for example, a halogen lamp, a xenon flash lamp, a supercontinuum laser source and / or a fiber laser. As a variant, the light source 1 emits a monochromatic source light beam 10.

[0070] Polarizer 5 receives source beam 10 and forms polarized incident beam 11. Optical focusing system 4 forms an image of source aperture 3 on sample 6, so that polarized incident beam 11 illuminates sample 6 over a spatially limited area. Polarized incident beam 11 propagates along incident optical axis 9 towards the sample.

[0071] In a first embodiment, the polarized incident light beam 11 is a light beam focused on the sample, in other words a converging light beam corresponding to a spherical wavefront.

[0072] The optical axis 9 of the polarized incident light beam 11 and the normal 7 to the sample define an incident plane 8, where the incident plane 8 is aligned with the Figure 1 The polarizer 5 is, for example, a Rocon type polarizer, whose polarization axis makes an angle of 45 degrees with the incident plane.

[0073] The source aperture 3 is arranged in a plane perpendicular to the incident optical axis 9 and is comprised in an aperture inscribed in a rectangle having a dimension perpendicular to the incident plane (denoted as W l，s ) and the dimension parallel to the plane of incidence (denoted as W l，p ). For example, the source aperture 3 includes a size W l，s and W l，p A rectangular aperture that is placed in front of the lamp or on the image of the lamp's emission area.

[0074] The detection arm 60 analyzes the secondary beam 12 formed by the reflection, transmission or diffraction of the polarized incident beam 11 on the sample 6. When the secondary beam 12 is reflected or transmitted by the sample 6, it is said to be a reflection ellipsometer or a transmission ellipsometer. When the secondary beam 12 is diffracted on the sample 6, it is said to be a scatterometer.

[0075] The secondary beam 12 propagates along a secondary optical axis 19 in the plane of incidence 8. In an ellipsometer, the absolute value of the angle between the secondary optical axis 19 and the normal 7 of the sample is equal to the angle of incidence (AOI). In a scatterometer, the angle between the secondary optical axis 19 and the normal 7 of the sample is usually different from the angle of incidence.

[0076] In a first embodiment, the secondary beam 12 is a diverging beam corresponding to a spherical wavefront.

[0077] Figure 2 A detection arm 60 according to a first embodiment is schematically shown in section BB perpendicular to the plane of incidence 8, comprising a secondary optical axis 19. The detection arm 60 comprises an optical wavefront beam splitter 20, an optical polarization changing device 25, an optical polarization beam splitter device 26, an optical focusing system 27 and a detection system. In the case of a spectroscopic ellipsometer or a scatterometer, the detection system comprises at least one spectrometer 30. In a particularly advantageous manner, the spectrometer 30 is of the imaging spectrometer type.

[0078] More precisely, in the first embodiment, the optical wavefront beam splitter 20 comprises a first mask 21 and a segmented lens 22. Optionally, the optical wavefront beam splitter 20 comprises a second mask 24 arranged between the segmented lens 22 and the optical polarization beam splitter device 26.

[0079] Figure 3 The front side of a first mask 21 comprising three apertures 211, 212, 213 is shown, which are spatially separated from each other and surrounded by an opaque strip. The first mask 21 is formed, for example, in a black nickel sheet; the thickness of the sheet is between 12 μm and 80 μm. The three apertures 211, 212, 213 are arranged in a plane perpendicular to the secondary optical axis 19. In the example shown, the central aperture 212 of the mask 21 is arranged in the incident plane 8, and the lateral apertures 211 and 213 are arranged symmetrically with respect to the incident plane. Preferably, the three apertures 211, 212, 213 are rectangular in shape, arranged side by side, with one side parallel to the incident plane and the other side perpendicular to the incident plane. By way of example, the three apertures 211, 212, 213 have a width D of 2 mm perpendicular to the incident plane, a length L of 1.5 mm in the incident plane and a distance G of 0.5 mm between adjacent apertures. The distance G corresponds to the thickness of the opaque strip.

[0080] The apertures 211, 212, 213 of the mask 1 have a finite length L in order to limit the angle of incidence of the secondary beam 12. Alternatively, at least one further aperture stop is arranged on the illumination arm and / or on the detection arm, respectively, to limit the angle of incidence of the incident beam and / or respectively the reflected, transmitted or diffracted beam.

[0081] The segmented lens 22 Figure 4. The segmented lens 22 is formed by an assembly of three lens segments 221, 222, 223 having the same focal length. The three lens segments 221, 222, 223 can be taken from the same spherical lens or three similar spherical lenses. The lens segments 221, 222, 223 are cut and reassembled from isotropic (non-birefringent) material to avoid the formation of birefringent patterns that tend to change the polarization of the light beam. The lens segments 221, 222, 223 are, for example, joined. The lens segments must be cut and reassembled so that the joining surfaces between the side faces are parallel to each other and to the plane of incidence. The segmented lens 22 is a multifocal lens. The trifocal segmented lens used here is not a specific variant of what is generally called a lens, but, on the contrary, a different and specific optical component.

[0082] Let Fa, Fb, Fc be the objective focal points of the lens segments 221, 222, 223 on the optical axes 225, 226, 227, respectively. The segmented lens 22 is arranged so that the three optical axes 225, 226, 227 are parallel to each other, and the three objective focal points Fa, Fb and Fc are aligned in a direction perpendicular to the plane of incidence, and at least one objective focal point Fa, Fb, Fc is on the sample 6. For example, the optical axis of the central lens segment 222 is aligned with the optical axis 19 of the secondary beam 12, and the objective focal point Fb is at the intersection of the secondary beam 12 on the sample 6. In other words, the objective focal point Fb is located at the spot 66 of the beam focused on the sample.

[0083] As a non-limiting example, the three lens segments 221 , 222 , 223 have a focal length of 100 mm, a height H of 3 mm in the plane of incidence, and a thickness E of 2 mm perpendicular to the plane of incidence.

[0084] The first mask 21 is preferably arranged near the front surface of the segmented lens 22 so that the optical axes 225, 226, 227 pass through the apertures 211, 212, 213 of the first mask 21, respectively. The mask 21 in combination with the segmented lens thus splits the wavefront of the secondary beam 12 into three separate beams. The width D of the apertures 211, 212, 213 and the thickness G of the opaque strips of the first mask 21 vary according to the thickness E of the lens segments 221, 222, 223 so that each lens segment of the segmented lens 22 receives one and only one separate beam. The lens segments 221, 222, 223 collimate the separate beams 13, 14, 15, respectively. The lens segment 221 forms the first separate beam 13, which is collimated and propagates along an optical axis that is inclined at an angle ALPHA relative to the optical axis 19 of the secondary beam 12. The lens segment 222 forms a second split beam 14 which is collimated and propagates along the optical axis 19 of the secondary beam 12. The lens segment 223 forms a third split beam 15 which is collimated and propagates along an optical axis which is preferably symmetrically inclined with respect to the optical axis 19 of the secondary beam 12 by an angle -ALPHA.

[0085] Therefore, the three collimated separate beams 13, 14, 15 propagate along three different optical axes 16, 17, 18. The three collimated separate beams 13, 14, 15 each have a plane wavefront. The three different optical axes 16, 17, 18 are perpendicular to the plane of incidence. Figure 2 Any two adjacent beams of the three collimated separate beams 13, 14, 15 are angularly separated at the angle ALPHA at the exit of the optical wavefront beam splitter 20.

[0086] According to a variant of the first embodiment, an optical system comprising a segmented lens and a combination of one or several conventional diverging or converging lenses is used to obtain an angular separation of the secondary beam into at least three beams.

[0087] Optionally, a three-aperture second mask 24 similar to mask 21 is arranged to block stray scattering in segmented lens 22. Second mask 24 has a similar structure to that of first mask 21, with three apertures separated by opaque strips, having the same orientation as first mask 21.

[0088] The optical polarization changing device 25 is placed between the segmented lens 22 and the optical polarization beam splitter device 26. The optical polarization changing device 25 comprises at least two birefringent wave plates 251, 253, each birefringent wave plate 251, 253 having a different retardation. Figure 2 In the example shown, a birefringent wave plate 251 is inserted into the optical path of the first collimated light beam 13, and another birefringent wave plate 253 is inserted into the optical path of the third collimated separated light beam 15. In this example, a plurality of wave plates are arranged on the path of the second collimated separated light beam 14. For example, the wave plate 251 is an achromatic half-wave plate, whose axis is at an angle of about 22.5 degrees to the incident plane, and the other wave plate 253 is an achromatic quarter-wave plate, whose axis points to the incident plane. The wave plates 251, 253 do not change the propagation optical axis of the collimated separated light beams 13, 15, but only change the polarization of the collimated separated light beams 13, 15. In addition, the wave plates 251, 253 introduce a constant delay according to time. Therefore, the wave plates 251, 253 do not introduce polarization modulation according to time.

[0089] Thus, at the exit of the optical polarization changing device 25, three light beams are obtained which are collimated and polarized according to three different polarization states propagating along three optical axes 16, 17, 18 in a plane perpendicular to the plane of incidence.

[0090] The optical polarization beam splitter device here comprises a Wollaston prism 26. The Wollaston prism 26 is placed downstream of the optical polarization changing device 25. The incident surface 261 of the Wollaston prism 26 is perpendicular and centered with respect to the optical axis of the second collimated separated light beam 14. Arranged in this way, the Wollaston prism 26 intercepts the first, second and third collimated separated light beams 13, 14, 15 polarized according to three different polarization states incident on its incident surface 261.

[0091] The Wollaston prism 26 is here formed by two right prisms 264, 265 which are placed side by side on the plane separation interface 262. The Wollaston prism separates each incident light beam 13, 14, 15 according to a separation plane polarization perpendicular to the plane of incidence. Figure 2 The plane corresponds to the separation plane. The separation angle between the two polarized separated light beams at the exit of the Wollaston prism 26 from the same collimated incident light beam is expressed as BETA. The Wollaston prism 26 is selected so that the angle BETA is between 0.6×ALPHA / 2 and 1.5×ALPHA / 2. Preferably, BETA is selected to be equal to ALPHA / 2. For example, for a wavelength of 589nm, the angle BETA is equal to 2.65 degrees. The Wollaston prism can, for example, be composed of two prisms made of quartz, barium borate, calcite crystals, whose separation angle BETA is 2.65 degrees, or made of magnesium fluoride, whose separation angle is 0.75 degrees.

[0092] According to a first type of Wollaston prism, the polarizations of the separated light beams are respectively parallel and perpendicular to the separation plane. Another type of Wollaston is preferably used here, in which the polarizations of the two light beams resulting from the polarization separation are at an angle of 45 degrees to the separation plane. The reason for this proposal to polarization-separate the light beams at +45 degrees or -45 degrees to the separation plane is as follows: It is well known that the efficiency of a diffraction grating spectrometer depends on the polarization. In the case of a Wollaston prism according to the first type, the polarization of the light beam separated by the polarization table is parallel or perpendicular to the lines of the grating, depending on the light beam considered. The two light beams of equal intensity and different polarizations would then generate different signals on the image sensor of the imaging spectrometer, due to the different efficiency of the polarization. In contrast, by separating the polarization of the light beams by +45 degrees or -45 degrees to the separation plane, the diffraction grating diffracts the light beams of different polarizations with the same efficiency, independently of the polarization I of the light beams. i,+ or I i,- where index i = 1, 2, 3, represents one of the beams 13, 14, 15 separated by the wavefront beam splitter, index + represents the +45 degree component, and index - represents the -45 degree component, separated by the Wollaston prism. This property is important for obtaining the form δ i =(I i，+ -I i，- ) / (I i，++I i，- ) are useful for ellipsometric or scatterometry measurements that are not affected by errors related to differences in the efficiency of the grating network as a function of polarization. If the responses of the different paths can be corrected by appropriate calibration, then this recommended configuration may not perform such calibration, or, if it is performed, the calibration coefficients may be less sensitive to sources of error, such as their dependence on wavelength.

[0093] exist Figure 2 In the example shown, according to the first type of Wollaston prism, the optical axis of the first right prism 264 is parallel to the incident plane 8 and perpendicular to the three optical axes 16, 17, 18 of the three separate light beams 13, 14, 15, and the optical axis of the second right prism 265 is perpendicular to the incident plane 8.

[0094] So oriented, the Wollaston prism 26 generates two polarized light beams 131 , 132 from the first collimated split light beam 13 incident on the splitting interface 262 , which emerge through the exit surface 263 of the Wollaston prism 26 .

[0095] The two polarized light beams 131, 132 are polarized according to two polarization states orthogonal to each other, preferably at ±45 degrees. The two polarized light beams 131, 132 propagate along two propagation directions angularly separated by the separation angle BETA of the Wollaston prism (here 2.65 degrees). The two polarized light beams 131, 132 are collimated.

[0096] In the same manner, the Wollaston prism 26 generates two polarized beams 133 , 134 and two polarized beams 135 , 136 from each of the second separated beam 14 and the third separated beam 15 incident on the separation interface 262 , respectively, which are presented through the exit surface 265 of the Wollaston prism 26 .

[0097] The two polarized light beams 133 , 134 (respectively the two polarized light beams 135 , 136 ) are polarized according to two polarizations that are perpendicular to each other and propagate along two different propagation directions separated by the separation angle BETA of the Wollaston prism 26 .

[0098] Therefore, the Wollaston prism 26 generates a total of six polarized light beams 131 , 132 , 133 , 134 , 135 , 136 .

[0099] The separation angle of the Wollaston prism 26 is such that the six polarized light beams 131, 132, 133, 134, 135, 136 are separated at an angle from each other and propagate in a plane perpendicular to the plane of incidence 8 (see Figure 5Advantageously, when BETA=ALPHA / 2, any two adjacent beams among the six polarized beams 131, 132, 133, 134, 135, 136 are angularly separated at the exit of the Wollaston prism 26 by an angle BETA.

[0100] As a variant, instead of a Wollaston prism, the polarization separation means 26 may comprise, for example, a Rocon prism, a Sénarmonte prism or a diffractive wave plate.

[0101] Advantageously, an optical focusing system 27 is placed at the exit of the Wollaston prism 26. The optical focusing system 27 comprises, for example, a lens having a focal length of 10 mm. The optical focusing system 27 receives the six collimated polarized light beams 131, 132, 133, 134, 135, 136 and focuses them on the detection system into six images 141, 142, 143, 144, 145, 146. In one example, the detection system comprises several independent detectors to obtain six intensity measurements corresponding to the six polarized light beams 131, 132, 133, 134, 135, 136. In another example, an imaging detector receives the six images 141, 142, 143, 144, 145, 146 and obtains six intensity measurements. In the specific example of a spectroscopic ellipsometer or scatterometer, six images 141, 142, 143, 144, 145, 146 are focused on the entrance slit 31 of the imaging spectrometer 30. The six images are spatially separated from each other while being aligned in a direction perpendicular to the plane of incidence 8.

[0102] According to a particular aspect of the present disclosure, the imaging spectrometer 30 is placed so that its entrance slit 31 is in the plane of the six images generated by the optical wavefront beam splitter 20, the Wollaston prism 26 and the optical focusing system 27. Preferably, the entrance slit 31 has a rectangular shape extending along an axis parallel to the alignment direction of the six images 141, 142, 143, 144, 145, 146, i.e. perpendicular to the incident plane 8.

[0103] The imaging spectrometer 30 is a spectrometer of a known type having an entrance slit 31, scattering and focusing means and an image sensor 33. The image obtained represents a set of spectra corresponding to light incident at different horizontal coordinates along the entrance slit 31. An example of an imaging spectrometer is the spectrometer CP140 from the company HORIBA or the CP20 from the same company. These spectrometers are based on the use of a concave diffraction grating corrected from chromatic aberrations, which ensures spectral scattering and focusing of the spectrum on the image sensor. There are other imaging spectrometers, such as the Offner imaging spectrometer, which are also suitable for this purpose. The imaging spectrometer is arranged so that the spectral scattering direction is orthogonal to the axis of the entrance slit. In the case of a diffraction grating imaging spectrometer, the lines of the diffraction grating 21 are preferably parallel to the entrance slit 31.

[0104] according to Figure 8 In one aspect of the present disclosure shown, the size of the source aperture 3 and more precisely the dimension W perpendicular to the plane of incidence is l，s The size of is small enough, taking into account the magnification of the optical systems 2, 4 of the illumination arm and the optical systems 22 and 27 of the detection arm, so that the six images 141, 142, 143, 144, 145, 146 of the source aperture 3 are spatially separated from each other on the detection system by a distance greater than the height W. f，s Specifically, in the case of a spectroscopic ellipsometer or scatterometer including an imaging spectrometer, the six images 141, 142, 143, 144, 145, 146 of the source aperture 3 are spatially separated in the plane of the entrance slit 31 of the imaging spectrometer by a distance greater than the height W f，s In other words, the six images 141, 142, 143, 144, 145, 146 do not overlap one another in the plane of the entrance slit 31 of the imaging spectrometer 30 or of the detection system. In an optional but recommended manner, another dimension W of the source aperture 3 is l，p Small enough so that another dimension W of each of the 6 images 141, 142, 143, 144, 145, 146 parallel to the plane of incidence is f，p is smaller than the width of the entrance slit 31 of the imaging spectrometer 30. Then, according to the present disclosure, the diffraction grating 32 of the imaging spectrometer 30 generates an image including six spectral sub-images (or more) of the secondary beam 12 on the image sensor 33. These six spectral sub-images, respectively indicated as 41, 42, 43, 44, 45, 46, do not intersect spatially on the surface of the image sensor 33. Each spectral sub-image 4j corresponds to the spectrally extended image 14j.

[0105] The image sensor 33 has a detection surface suitable for recording one image of 6 spectral sub-images 41, 42, 43, 44, 45, 46. The image sensor 33 comprises, for example, a grayscale CMOS camera comprising n rows and m columns of pixels. For example, a black and white CMOS Sony camera comprising 1200×1920 pixels is used, and for fast reading a surface area of ​​about 400×1800 pixels is used. The surface area of ​​each pixel is 5.86×5.68 μm. Each spectral sub-image 4j (where j=1, ..., 6) extends over at least one row of pixels. The CMOS camera is selected to have a high absorption density, preferably an absorption density higher than or equal to 800 electrons / μm 2 The image sensor 33 operates at an image acquisition rate of 1 Hz, 50 Hz, 100 Hz or even higher than or equal to 200 Hz.

[0106] The ellipsometer or scatterometer also includes an image processing system adapted to perform the following steps:

[0107] - add up all pixels of the same column of the spectral sub-image corresponding to the same wavelength to obtain 6 (or more) intensity spectra;

[0108] - Calculate the ratio δ for each wavelength and each index i i =(I i，+ -I i，- ) / (I i，+ +I i，- ), where i=1, 2, 3, where index 1,+ corresponds to spectral sub-image 41, index 1,− corresponds to spectral sub-image 42, index 2,+ corresponds to spectral sub-image 43, index 2,− corresponds to spectral sub-image 44, index 3,+ corresponds to spectral sub-image 45, and index 3,− corresponds to spectral sub-image 46;

[0109] - Calculate the reduced Stokes vector by multiplying the vector (δ1, δ2, δ3) by a suitable matrix, ie the Stokes vector divided by its first component. This matrix will be determined by calibration with the aid of measurements on known polarization states.

[0110] Similarly, the monochromatic ellipsometer or scatterometer also includes a processing system adapted to perform the following steps:

[0111] - receiving 6 intensity acquisitions corresponding to 6 polarized light beams 131, 132, 133, 134, 135, 136;

[0112] - Calculate the intensity ratio δ for each index i i =(I i，+ -I i，- ) / (I i，+ +I i，- ), where i=1, 2, 3, where index 1, + corresponds to polarized beam 131, index 1, − corresponds to polarized beam 132, index 2, + corresponds to polarized beam 133, index 2, − corresponds to polarized beam 134, index 3, + corresponds to polarized beam 135, and index 3, − corresponds to polarized beam 136;

[0113] - Calculate the reduced Stokes vector by multiplying the vector (δ1, δ2, δ3) by a suitable matrix, ie the Stokes vector divided by its first component. This matrix will be determined by calibration with the aid of measurements on known polarization states.

[0114] The skilled person then knows how to determine the values ​​Psi and Delta commonly used in ellipsometry from the measured Stockes vector and knowledge of the polarization state of the incident light. The depolarization coefficient is also determined simultaneously as well as the diffraction coefficient used in scatterometry.

[0115] Thus, instantaneous monochromatic or spectral ellipsometric or scatterometric measurements are obtained, i.e. based on a single acquisition on the detection system, for example based on the acquisition of a single image on an image detector, without polarization modulation and without vibrations induced by rotation of the optical components. The acquisition rate is limited by the readout rate of the image sensor. Thus, monochromatic or spectral ellipsometric or scatterometric measurements can be performed at rates of one to several tens of Hertz or even above 100 Hz or 200 Hz over a broadband spectrum.

[0116] Figure 6 and Figure 7 FIG. 2 shows an ellipsometer or scatterometer according to a second embodiment. Figures 1 to 5 Similar components or systems.

[0117] Similar to the first embodiment, the illumination arm 50 includes a source aperture 3, an optical system 4 and a polarizer 5. However, unlike the first embodiment, the optical system 4 of the illumination arm 50 is arranged to illuminate the sample 6 with a polarized incident light beam 11 that is collimated and not focused on the sample 6. In the second embodiment, the polarized incident light beam 11 is a light beam that is collimated on the sample corresponding to a plane wavefront. Moreover, in the second embodiment, the secondary light beam 12 formed by reflection, transmission or diffraction of the polarized incident light beam 11 on the sample 6 is also a collimated light beam corresponding to a plane wavefront.

[0118] Figure 7 A detection arm 60 according to a second embodiment is schematically shown in a section CC perpendicular to the plane of incidence 8 , which includes a secondary optical axis 19 .

[0119] In the second embodiment, the optical wavefront beam splitter 20 includes a first mask 21 and at least two prisms 231, 233. The first mask 21 is similar to the combination of Figure 3 The first mask 21 splits the wavefront of the secondary beam 12 into three separate beams which are collimated here. Optionally, the optical wavefront beam splitter 20 comprises a second mask 24 arranged between the prisms 231 , 233 and the optical polarization beam splitter device 26 .

[0120] By way of example, the optical wavefront beam splitter 20 comprises a first prism 231 arranged opposite the aperture 211 of the mask 21 and a second prism 233 arranged opposite the aperture 213 of the mask 21. Thus, each prism 231, 233 receives one and only one separated light beam 13, 15, respectively. Advantageously, no prism is arranged opposite the aperture 212, so that one of the three separated light beams, in the example shown the separated light beam 14, passes without deviation. The prisms 231, 233 are preferably identical and oriented so that their edges each extend in a direction parallel to the plane of incidence and perpendicular to the secondary optical axis 19. For example, prisms of triangular or quadrilateral cross section made of silicon dioxide are used, which have an angle between the entrance face and the exit face of between 1° and 8°. Preferably, the prisms 231, 233 are slightly spectrally dispersed. The prisms 231, 233 are preferably oriented with their respective bottom surfaces facing each other or with their respective edges facing each other and are separated by a distance equal to the distance between the apertures 211 and 213. Thus, the first prism 231 receives the separated light beam 13 and deviates the separated light beam 13 by an angle ALPHA in a plane perpendicular to the plane of incidence relative to the optical axis 19 of the secondary light beam 12. Symmetrically, the second prism 233 receives the separated light beam 15 and deviates the separated light beam 15 by an angle -ALPHA in a plane perpendicular to the plane of incidence relative to the optical axis 19 of the secondary light beam 12.

[0121] Therefore, the three collimated separate beams 13, 14, 15 propagate along three different optical axes 16, 17, 18. In the absence of a prism in the optical path of the separate beam 14, the beam is not deflected and the optical axis 17 coincides with the secondary optical axis 19. The three collimated separate beams 13, 14, 15 each have a plane wavefront. The three different optical axes 16, 17, 18 are perpendicular to the plane of incidence. Figure 7 Advantageously, any two adjacent beams of the three collimated separate beams 13, 14, 15 are angularly separated at the output of the optical wavefront beam splitter 20 by an angle ALPHA.

[0122] According to a variant of the second embodiment, an optical system is used which comprises a combination of a prism and one or more conventional converging lenses in order to obtain an angular separation of the secondary beams into at least three beams.

[0123] Optical polarization changing means 25 are arranged to change the polarization of at least two of the three separate beams to form three collimated beams polarized according to three different polarization states propagating along the three optical axes 16, 17, 18 in a plane perpendicular to the plane of incidence.

[0124] The polarization separator 26 of the second embodiment is similar to the polarization separator of the first embodiment and operates similarly to form six polarized light beams 131, 132, 133, 134, 135, 136 that are angularly separated from each other. The optical focusing system 27 and the imaging spectrometer 30 are similar to those in combination with the first embodiment. Figure 1 , 2 and those described in 7.

[0125] In the two embodiments described above, six spatially separated polarized light beams are obtained on the detection system. Advantageously, in the case of a spectroscopic ellipsometer or scatterometer comprising an imaging spectrometer, the six polarized light beams form six images on the entrance slit of the imaging spectrometer 30 .

[0126] Figure 8 Schematically shows the spectral polarization images of the source aperture through a spectroscopic ellipsometer or scatterometer and a reflective or transparent sample according to the selected configuration. In the plane perpendicular to the incident light axis 9, the source aperture 3 (at Figure 8 (front view) is inscribed in the l，s and W l，p The optical system 4 of the illumination arm forms a rectangular aperture of size W in the sample plane on the sample. e，s and W e，p The image 66 of the source aperture 3. On the one hand, the size W e，s and W l，s The ratio between the two, on the other hand, the dimension W e，p and W l，p The ratio between them depends on the optical magnification of the optical system 4 and the angle of incidence on the sample 6. In the plane of the entrance slit 31 of the imaging spectrometer 30, six images 141, 142, 143, 144, 145 and 146 are obtained, each image having a rectangular shape and having the same size W in the absence of optical aberrations. f，s and W f，p In the plane of the two-dimensional image sensor 33 of the imaging spectrometer 30, six spectral sub-images 41, 42, 43, 44, 45 and 46 are obtained, each of which has a rectangular shape and has the same size W in the absence of optical aberrations. i，s , W i，p. The six spectral sub-images 41, 42, 43, 44, 45 and 46 are spatially separated from each other by one or several rows of pixels. More precisely, the six spectral sub-images 41, 42, 43, 44, 45 and 46 are separated in the plane of the image sensor 33 by at least five intermediate areas, denoted 51, 52, 53, 54 and 55, respectively, in which the detected intensity is zero (unll) or very low. "Zero" or "low" means that once the background signal is subtracted, for each wavelength of interest, the value of the signal in the separated area is at least 10 times lower than the maximum value of the signal observed at the pixel corresponding to the same wavelength in any of the six spectral sub-images 41, 42, 43, 44, 45 and 46.

[0127] In order to separate the six images 141, 142, 143, 144, 145 and 146 in the plane of the entrance slit 31 of the imaging spectrometer, the dimension W is φ relative to the interval between two adjacent images (eg, 131 and 132). f，s must be small enough so that the images do not overlap one another. To this end, the angular spacing between the six paths performed by the optical wavefront beam splitter 20 on the one hand and by the optical polarization beam splitter device 26 on the other hand is large enough so that the six images 141, 142, 143, 144, 145 and 146 are spatially separated in the plane of the entrance slit 31. In the absence of aberrations in the different optical focusing and imaging systems 2, 4, 22, 27, a person skilled in the art will have no particular difficulty in adjusting the parameters of the optical wavefront beam splitter 20 and the optical polarization beam splitter device 26 according to the size of the source aperture 3 and the focal length of the optical system of the illumination arm and the detection arm, so that the six images of the source surface in the plane of the entrance slit 31 of the imaging spectrometer do not overlap one another.

[0128] However, optical systems often exhibit geometric and / or chromatic optical aberrations. Fig. 9 As shown, the width W of each spectral sub-image 41, 42, 43, 44, 45 and 46 observed on the sensor of the imaging spectrometer is i，s It is affected by aberrations, defects and stray light from the optical wavefront beam splitter 20, the optical polarization beam splitter device 26 and / or the optical focusing system 4 and / or 27 of the imaging spectrometer 30.

[0129] According to a specific aspect of the present disclosure, intermediate regions 51, 52, 53, 54 and 55, in which the detected intensity is zero or very low, are advantageously used to ensure that there is no crosstalk between the 6 spectral sub-images, which are associated with 6 complementary polarization information. On the other hand, even in the absence of crosstalk, there is still a certain level of stray light. This stray light corresponds to the fact that very bright areas of the image generate a stray illumination that extends beyond the only very bright area. It is difficult to reduce the level of stray light below 0.5% in a compact spectrometer. Now, in ellipsometers, measurement accuracies better than 0.1% are usually required. It is then advantageous to be able to determine and then subtract the stray light. To this end, use is made of the fact that the stray light is usually almost out of focus, i.e., the level of the stray light changes slowly with changes in the spatial coordinates. The intermediate regions 51, 52, 53, 54 and 55 theoretically receive no signal. According to a particular aspect of the disclosure, the signal is measured in one or several intermediate regions 51, 52, 53, 54 and / or 55 to estimate the level of stray light on adjacent pixels. Adjacent pixels in this context refer to pixels located in a rectangle of about ten to several dozen pixels on the side. The measured values ​​are corrected in the spectral sub-images 41, 42, 43, 44, 45 and 46 by subtracting from the pixels corresponding to the path of the search signal an estimate of the stray light obtained, for example, by the average value of a certain number of adjacent pixels of the closest intermediate regions 51, 52, 53, 54 and 55, in other words, in a rectangle of about ten to several dozen pixels on the side, i.e. for adjacent wavelengths.

[0130] As described above, the reduced Stokes vector is obtained by measuring six spectral sub-images 41, 42, 43, 44, 45 and 46 or six polarized beams separated spatially and polarization-separated on the image detector. The measurement can be acquired at the acquisition rate of the detection system, for example 50 Hz or 100 Hz. It is emphasized here that no temporal polarization modulation is required.

[0131] According to a first exemplary embodiment, the lighting arm 50 comprises a xenon flash lamp 1 with a power of 5 W, and the first optical system 2 comprises a lens which focuses the image of the arc of the xenon flash lamp 1 onto a device carrying a plurality of source apertures, the size of which is W. l，s =540μm×W l，p = 250 μm first source aperture, size W l，s =50μm×W l，p =25μm second source aperture. The optical focusing system includes a lens 4 with a focal length of 50mm, which forms an image of the source aperture with a magnification of 1 on a sample 6 placed 100mm from the lens 4. The Rocon polarizer 5 is placed on the output side of the lens 4, with its polarization axis at an angle of 45 degrees to the incident plane 8. The receiving arm 60 includes a segmented lens 22 with a focal length of 100mm. Figure 4 As shown, the segmented lens 22 is manufactured from a biconvex lens with a focal length of 100 mm by cutting three segments 221, 222, 223 with a width of E≈1 mm and a height of H≈8 mm, and the three segments 221, 222, 223 are assembled by joining. The segmented lens 22 is placed on the optical path of the reflected secondary beam 12 100 mm away from the sample to form three collimated beams 13, 14, 15, which are separated by an angle of about 1.8 degrees. The separation plane is a plane perpendicular to the incident plane 19 containing the optical axis of the secondary beam 12. A mask 21 with three apertures 211, 212, 213 is placed in front of the segmented lens 22 so as to effectively separate the three collimated beams 13, 14, 15 while avoiding a portion of the secondary beam incident on the joining area between two adjacent segments from entering through one lens segment and exiting through another lens segment. After the segmented lens 22, a wave plate 251 is placed in front of the lens segment 221, and another wave plate 253 is placed in front of the other lens segment 223, so as to accurately intercept the collimated beams 13, 15, each passing through the two lens segments 221, 223, respectively. The wave plate 251 is, for example, an achromatic half-wave plate, whose axis is at an angle of about 22.5 degrees to the incident plane. The wave plate 253 is, for example, an achromatic quarter-wave plate, whose axis points to the incident plane. The Wollaston prism 26 is placed in the path of all three collimated beams 13, 14, 15, and the non-polarized beam is separated at an angle BETA of 0.9 degrees into two polarized lights at +45 degrees and -45 degrees to the separation plane, and the separation plane is perpendicular to the incident plane. A lens 27 with a focal length of 10 mm is placed at the output of the Wollaston prism 26 to focus the six separated light beams 131, ..., 136 onto the entrance slit 31 of the spectrometer CP20 of HORIBA Corporation. The entrance slit with a large size of 2 mm is located in the scattering plane of the six separated light beams 131, ..., 136. The small size of the entrance slit is 25 μm. The image detector 33 equipped with the spectrometer CP20 is a Sony monochrome sensor with a pixel of 5.86 μm side.

[0132] Figure 8 The image obtained by the sensor is schematically shown, together with the different regions 41, ..., 46 defined above and associated with the signal processing. The intensity values ​​acquired on each of these regions 41, ..., 46 make it possible to deduce therefrom the spectral values ​​of Psi and Delta of the sample.

[0133] The spectroscopic ellipsometer or scatterometer disclosed herein makes it possible to adjust the duration of the measurement according to the size of the source aperture 3 and the properties of the sample to be measured. In practice, the reflection ellipsometer can be used on highly reflective materials such as aluminum, or on materials with very poor reflectivity such as glass. Now, it is known that the signal-to-noise ratio obtained with a matrix image sensor is generally better when the exposure time is chosen so that the maximum signal is close to saturation without reaching saturation, so it is best to use the dynamic variation of the image sensor 33. The system according to the disclosure makes it possible to adjust the acquisition duration of the image sensor 33 so as to obtain a maximum signal over the wavelength band of interest, i.e. a signal close to saturation without reaching such saturation. According to the disclosure, the acquisition duration of the image sensor 33 can be adapted independently of the experimental conditions in a wide range, for example from 4 ms to 1 s, the acquisition rate from one Hz to tens of Hz to 100 Hz or more than 200 Hz, each spectral measurement extending over a spectral range of hundreds of nanometers, with a spectral resolution of a few nanometers.

[0134] In contrast, in a temporal polarization modulation ellipsometer, e.g. based on a rotating optical assembly, the acquisition duration of a complete measurement is fixed by the modulation frequency, which requires acquiring a series of spectral measurements within one modulation period, e.g. a rotation period of the rotating optical assembly.

[0135] According to another specific aspect, a plurality of source apertures 3 with different sizes and / or shapes are arranged. For example, at least two source apertures 3 are arranged with an area ratio of at least 10 to each other. A switch makes it possible to select a source aperture 3 with a size adjusted according to the desired measurement value. On the other hand, a large source aperture is used to form a larger spot, for example 540 μm x 250 μm, for fast measurements of samples without patterns or with heterogeneities smaller than the spot surface. On the other hand, a small source aperture is used to form a small-sized spot, for example 50 μm x 25 μm, on samples with small patterns, or to acquire ellipsometric images with good spatial resolution. In this case, the maximum light flow reaching the image sensor is a hundred times lower than with a large source aperture: interestingly, the acquisition duration is then increased by a factor of 100 in order to make maximum use of the dynamics of the image sensor according to the spot size used to probe the sample.

[0136] An important advantage of the ellipsometer or scatterometer according to the present disclosure is that it allows the use of light sources whose intensity is not constant over time. The light source 1 can be a conventional lamp that emits a light flux that is unstable over time, for example fluctuating by a few percent, or a pulsed source, such as a flash lamp or a pulsed laser. In particular, a light source of the supercontinuum type, also called a white light laser, is interesting here because it emits very high brightness, typically light in a wide spectral range from 390 nm to 1.2 μm, at a high repetition rate, such as 20 kHz, although it is known that this type of pulsed source is hardly repeatable from one pulse to another in terms of intensity and / or spectral shape. Advantageously, when a pulsed source is used, the pulsed source and the detection system are synchronized so that the number of pulses collected during it can be controlled. The ellipsometer according to the present invention can be easily and advantageously used with sources and / or pulsed sources that are not very stable as a function of time. In contrast, conventional ellipsometers using temporal polarization modulation work very poorly in the case of unstable or pulsed sources. In fact, in this case, the evolution of the signal over time cannot be attributed solely to the polarization modulation, and the signal processing performed is wrong.

[0137] According to a specific aspect, a pulsed light source, such as a xenon flash lamp or a supercontinuum source, is combined with an adjustment of the acquisition duration so that the number N of light pulses produces an image on the image sensor 33 of the imaging spectrometer 30 whose intensity is just below saturation in the target wavelength range. In other words, the duration of the acquisition is adjusted so that for N pulses, the image is not saturated, while for N+1 pulses, the image is saturated.

[0138] According to another specific aspect, it has been observed that the maximum signal received by an image sensor varies by decades according to wavelength. This variation depends on the variation of the wavelength according to the source intensity, the optical properties of the sample, the diffraction grating efficiency, and / or the sensor efficiency. When the acquisition duration is determined as described above to avoid saturation at wavelengths where the signal is stronger, it can be observed that the signal values are several orders of magnitude lower than the saturation values at wavelengths where the signal intensity is less strong. For low-signal wavelengths, this results in sources of error or uncertainty, which are related, for example, to the quantization noise of the image sensor. One way to address this drawback is to acquire two consecutive images corresponding to two different values N1 and N2 of the number of pulses of a pulsed light source. N1 is chosen such that the signal does not saturate at any wavelength of interest. Then N2 is chosen in the range: 4N1 < N2 < 25N1, for example N2 = 10N1. Then the two images acquired with N1 and N2 pulses respectively are processed as described above to obtain the ellipsometric values of Psi and Delta. Only the values of Psi and Delt obtained from the N2-pulse image at certain wavelengths are retained, at which wavelengths the image is not saturated. The Psi and Delta spectra at other wavelengths are obtained by processing the image obtained with N1 pulses. Thus, due to the use of a larger dynamic range of the image sensor, the two image captures can obtain the Psi and Delta values under significantly improved signal conditions.

[0139] According to an exemplary embodiment, the illumination arm 50 includes a supercontinuum source 1 that delivers a source beam 10 having a spectral range extending from 400 nm to several microns at the output of a single-mode fiber. One or more concave mirrors focus the source beam 10 into a spot 66 on the sample 6. Similar to that described in the first example above, the reflected secondary beam 12 is captured by an illumination arm similar to that described in the first embodiment above. During the acquisition by the image sensor, the supercontinuum source 1 emits a sequence of 10 pulses. A first image 33a is read by a computer, and it is confirmed that the maximum signal of the signal does not reach the saturation of the image detector 30 but reaches or exceeds 75% of the saturation. Then, the supercontinuum source 1 emits a sequence of 100 pulses, and a second image 33b acquired during this sequence of 100 pulses is read on the computer. To obtain the ellipsometric signal over the entire spectral range, the first image 33a and the second image 33b are processed according to the steps detailed above. Then, the retained ellipsometric values are those from the second image 33b that have a higher signal, except at wavelengths where saturation is observed, for which wavelengths the retained ellipsometric values are those from the first image 33a.

[0140] According to another specific aspect, the estimator is further extracted so that the measurement noise level can be quantified. Ellipsometry is often used to adjust the parameters of digital simulation models of materials and / or thin layers. The search for optical model parameters is usually based on minimizing the difference between the measured ellipsometry angles Psi and Delta and the ellipsometry angles from the model. These differences are weighted by parameters that reflect the accuracy of the experimental measurements. The main source of experimental uncertainty is the shot noise of the detector. In an ellipsometer, the noise on the ellipsometry angles Psi and Delta is approximately the inverse of the square root of the number of photons measured in the spectral box under consideration. In this article, this means that "spectral box" refers to the pixels of the six spectral sub-images 41 to 46 assigned to the same wavelength. In an ellipsometer according to the present disclosure, the Stokes vector not only provides Psi, Delta and depolarization values, but the image sensor also provides the total number of photons measured. This total number of photons is easily derived from the sum of the pixel values ​​at the wavelength under consideration multiplied by the number of photons per bit, the last quantity being a known characteristic of the image sensor 30. Quantities associated with the latter can be calculated, for example, such as the inverse of the square root of the number of photons detected in the spectral box divided by 2, which directly provides an estimate of the quantization noise of the ellipsoidal polarization angles Psi and Delta if they are expressed in radians. Other quantities representative of the signal-to-noise ratio can be calculated from the sum of the pixel values ​​of a sub-area of ​​the image sensor 30.

[0141] The advantages of using a supercontinuum source with very high brightness over a wide spectral range, e.g. from 390 nm to 1.2 μm, have already been mentioned. The radiation emitted by the supercontinuum source emerges from an optical fiber with a core diameter typically less than 5 μm. The incident beam can be focused onto a very small spot on the sample: for example, W e，s =10μm, W e，p = 20 μm. The surface area of ​​the source is small, and its six images in the plane of the imaging spectrometer's entrance slit 31 are also small and very spatially separated: for example, W f，s and W f，p is about 5 μm, separating the different spots by 450 μm along the length of the slit 31. This spatial separation of the six images on the entrance slit allows very good spectral resolution to be obtained, since the spectral resolution is better for imaging spectrometers than for conventional spectrometers, since the slit (real or virtual) width W at the spectrometer input is f，p Six tracks are then observed on the image sensor 33 of the image spectrometer 30, each track corresponding to a spectral expansion for each polarization, with a width W compared to the spacing between two adjacent areas, for example 41 and 42. i，s Very small (see Figure 8). According to a variant, in combination with a small-scale source, such as a supercontinuum source, a converging or diverging cylindrical lens is added in the detection arm 60 between the sample 6 and the entrance slit 31 of the imaging spectrometer, preferably in the space where the light beam is collimated. The cylindrical lens is oriented to scatter the 6 images spatially in the direction of the entrance slit 31 while avoiding that the 6 images overlap each other. Thus, the intensity from the supercontinuum source is distributed over more pixels of the image sensor 33 without compromising the spectral resolution. This variant makes it possible to sense more light during each acquisition of the image sensor without reaching saturation of the image sensor.

[0142] According to another specific aspect, the ellipsometer or scatterometer also includes a stage for moving the sample holder. The mobile stage generally includes two translation axes perpendicular to the sample normal. Such a mobile stage allows the acquisition of elliptical polarization spectra at different points of the sample. Thus, a map of the sample, such as a semiconductor plate, is obtained. In a polarization modulation ellipsometer, the displacement must be stopped to acquire complete measurements because the duration of the measurement is at least half of the modulation period. However, due to the repeated stopping and moving of the mobile stage, point-by-point acquisition is time-consuming. It is desirable to obtain the sample map faster. The instantaneous ellipsometer or scatterometer according to the present disclosure makes it possible to combine the use of a short-pulse pulsed light source with a continuously moving mobile stage. In this article, "short pulse" refers to a pulse that is short enough so that the displacement is negligible during the pulse duration. In this way, since the instantaneous elliptical polarization measurement is acquired during a single pulse (or several pulses), it is not necessary to stop the displacement of the sample holder at each measurement. This acquisition mode makes it possible to instantly acquire a series of measurement results on each source pulse without having to stop the mobile stage to measure on time. The acquisition frequency of the measurement is determined by the repetition frequency and / or the reading frequency of the source of the image detector 33 of the imaging spectrometer 30 to be synchronized. Using a CMOS camera, a reading frequency of 50 Hz or even higher than 100 Hz can be achieved. A xenon flash lamp can produce a large amount of light at a rate of the same order of magnitude or higher on a pulse of a few nanoseconds. The repetition frequency of a supercontinuum source may reach 20 kHz. In that way, the spectroscopic ellipsometer according to the present disclosure makes it possible to obtain a map including hundreds of points in a few seconds. In contrast, a conventional ellipsometer based on temporal polarization modulation needs to be collected while stopping the moving stage within at least one modulation cycle. Now, the modulation frequency of the rotating optical assembly is usually a maximum of a few Hz to about 20 Hz, which is much lower than the maximum acquisition frequency of the ellipsometer working under a short pulse pulse source according to the present disclosure. In addition, it is necessary to stop the workbench and then restart the workbench to limit the displacement frequency to a few Hz. What is thus to follow is that the time polarization modulation ellipsometer enables images to be obtained with a maximum frequency of the order of ten Hertz, i.e. at least one order of magnitude lower than the maximum rate of an ellipsometer according to the present disclosure operating with a short pulsed source combined with continuous movement of a mobile stage.

[0143] According to Fig.10 In the variant shown, the ellipsometer or scatterometer also includes an auxiliary imaging system. The auxiliary imaging system includes an auxiliary light source 70, which emits a beam 75 with a narrow spectral width, for example a beam of LED type, which is associated with an auxiliary imaging mask 71, the image of which is projected onto the sample 6, on a surface area of ​​the order of 1.5 mm×4 mm, and an auxiliary camera 78 with a lens 77, arranged in the optical path after the optical polarization beam splitter device 26. Fig.10 As shown, a reflective optical component 73 is arranged between the source aperture 3 and the optical focusing system 4 so that an auxiliary beam 75 on the incident optical axis 9 can be directed toward the sample. Another reflective optical component 76 makes it possible to reflect an auxiliary beam 79 reflected on the secondary optical axis 19 toward an auxiliary camera 78. The auxiliary camera 78 is optically conjugate with the six images from the auxiliary imaging mask 71, wherein the six images do not overlap or at most partially overlap. The field of view that can be imaged on the sample can range from a few square millimeters to tens of square millimeters.

[0144] Due to the segmented lens and Wollaston prism as described above, the auxiliary camera 78 receives six images 341, ..., 346 of the area illuminated by the auxiliary illuminator, for example Fig.11 As shown. When the illuminated area on the sample includes patterns with different elliptical polarization properties at the wavelength of the auxiliary light source 70, the optical response of these patterns appears differently on each of the 6 images due to the fact that these 6 images correspond to different polarizations. In order to extract the images of ellipticity parameters Psi and Delta from them, it is only necessary to process the received intensity as shown in p14, line 20 and thereafter, for each group of points 361, 362, 363, 364, 365, 366 with the same coordinates in each image, so as to determine the value of Psi and the value of Delta at the same point. Therefore, at the wavelength of the auxiliary light source 70 on the illuminated area of ​​the sample 6, two images are obtained, image 340 representing PSI and another image 349 representing DELTA. Therefore, a method for elliptical polarization imaging at one wavelength has been shown. Using an auxiliary light source of the RGB diode type, elliptical polarization images of the sample at three different wavelengths can be continuously acquired.

[0145] Another use of the auxiliary imaging system is to visualize the spot position of the spectroscopic ellipsometer on the sample. In contrast to the above example, the polarized incident beam 11 from the main light source 1 has been turned off or blocked, in which case the auxiliary camera 78 receives images of the main light source 1 and the auxiliary light source 70 at the same time. The much higher intensity of the spot of the main ellipsometer 371 makes it possible to determine its position without ambiguity. Therefore, Fig.12An example is shown which enables visualization when a spot of an elliptically polarized light beam 371 is superimposed on an image 341 of a larger area of ​​the sample 6. The image 341 of the larger area of ​​the sample 6 can be one of the six images 341, ..., 346, or the image 340 in PSI and / or the image 349 in DELTA.

[0146] Visualizing a portion of a sample and the location of a spot on the sample on an ellipsometer and how to do it on a modulation ellipsometer is well known, but not how to do it in a transient ellipsometer, nor how to obtain it.

[0147] According to a particular aspect of this variant, the plane of the sensor of the auxiliary camera is tilted so as to satisfy the Scheimpflug condition. More precisely, the tilt angle i3 of the plane of the sensor of the auxiliary camera relative to the optical axis is related to the angle of incidence i on the sample at the focal length F1 of the segmented lens 22 and at the focal length F3 of the lens 77 of the auxiliary camera 78 by:

[0148]

[0149] The auxiliary imaging mask 71 is also tilted, and forms an angle i0 with the normal of the incident light beam, which is a function of the previous parameters and the magnification G of the optical illumination system 4, and the relationship is:

[0150]

[0151] In a particularly advantageous manner, at each point of the sample present in the six images 341...346 formed simultaneously on the auxiliary camera 78, the processing for obtaining the Psi and Delta for each spectral polarization path is applied to the wavelength of the auxiliary illumination to obtain a graph of the Psi and Delta of the sample observed without displacement.

[0152] In an exemplary embodiment, an auxiliary light source 70 comprising a diode emitting a beam of 640 nm over a spectral width of about 10 nm is added to the ellipsometer described in the first example. Furthermore, it is assumed that the ellipsometer operates at an incident angle of i=70°. The auxiliary beam emitted by the diode is imaged onto a light source of size W. a，s =0.75 and W a，p =2 mm, which is itself reimaged on the sample 6 by the magnifying optical system 2. The normal of this auxiliary mask 71 is inclined at an angle i0 of 53 degrees relative to the optical axis of the auxiliary beam 75. The semi-reflective sheet 73 allows the beams from the two light sources 1 and 70 to be combined, and the distance between the two light sources and the focusing lens 4 of the illumination arm 50 is 100 mm.

[0153] The detection arm 60 is similar to the detection arm of Example 1 and also comprises a partition plate 76 located after the Wollaston prism 26. The lens 77 of the auxiliary camera 78 with a focal length F3=35 mm focuses the acquired light beam on the sensor of the auxiliary camera 78. Advantageously, as described above, according to the Scheimpflug condition, the normal to the plane of the sensor of the auxiliary camera 78 makes an angle i3 of 43 degrees with the optical axis.

Claims

1. An ellipsometer (100), comprising: - a light source (1) suitable for generating a source light beam (10); - a polarizer (5) adapted to receive the source light beam (10) and to form a polarized incident light beam (11); - an optical illumination system (2, 4) adapted to direct said polarized incident light beam (11) towards the sample (6) along an incident light axis (9) in an incident plane (8); - an optical wavefront beam splitter (20) arranged to receive a secondary beam (12) formed by reflection or transmission of the polarized incident light beam (11) on the sample at a determined angle of incidence, the secondary beam (12) propagating along a secondary optical axis (19) in the plane of incidence (8), the optical wavefront beam splitter (20) being oriented to form three collimated separate beams (13, 14, 15) propagating along three different optical axes (16, 17, 18) angularly separated in a plane perpendicular to the plane of incidence (8); as well as - optical polarization changing means (25) adapted to receive the three collimated separate light beams (13, 14, 15) and to form three light beams polarized according to three different polarization states; - an optical polarization beam splitter device (26) comprising a Wollaston prism arranged and oriented to receive the three light beams polarized according to three different polarization states and to form six separated light beams (131, 132, 133, 134, 135, 136) propagating along six optical axes, the six optical axes being angularly separated in a plane perpendicular to the plane of incidence, the Wollaston prism splitting each of the three light beams into two light beams, the Wollaston prism being a Wollaston prism of the type wherein the respective polarizations of the two light beams from the Wollaston prism are at +45 degrees and -45 degrees to the separation plane; - a detection system comprising at least one diffraction grating spectrometer adapted to detect said six separate beams (131, 132, 133, 134, 135, 136), and a processing system adapted to derive ellipsometric measurements therefrom.

2. The ellipsometer according to claim 1, comprising: - an optical focusing system (27) arranged to receive the six separate light beams (131, 132, 133, 134, 135, 136) and to form six images (141, 142, 143, 144, 145, 146) aligned with each other and spatially separated from each other; - and among them, The at least one diffraction grating spectrometer comprises an imaging spectrometer (30), the imaging spectrometer (30) comprising an entrance slit (31) elongated in one direction and arranged to simultaneously receive the six images (141, 142, 143, 144, 145, 146), the imaging spectrometer (30) being adapted to spectrally disperse the six images (141, 142, 143, 144, 145, 146) and simultaneously form six spectral sub-images (41, 42, 43, 44, 45, 46) spatially separated on an image detector (33); - the image detector (33) is suitable for acquiring images of the six spectral sub-images (41, 42, 43, 44, 45, 46); and - said processing system is adapted to process images of said six spectral sub-images and to derive spectroscopic ellipsometry measurements therefrom.

3. The ellipsometer according to claim 1, wherein: The optical illumination system (2, 4) is adapted to focus the polarized incident light beam (11) onto the sample (6), wherein the optical wavefront beam splitter comprises a segmented lens (22), the segmented lens (22) comprising three lens segments (221, 222, 223) having the same focal length and optical axes (225, 226, 227) that are parallel to each other and spatially separated from each other, each lens segment (221, 222, 223) having an objective focal point (Fa, Fb, Fc), the three lens segments ...2, 222, 223) having an objective focal point (Fa, Fb, Fc), the three lens segments (222, 222, 223) having an objective focal point (Fa, Fb, Fc), the three lens segments ( The segments (221, 222, 223) are assembled so that the three objective lens focal points (Fa, Fb, Fc) are aligned perpendicular to the incident plane on the sample (6), each lens segment (221, 222, 223) is arranged to receive a different part of the secondary beam (12) and form a collimated separated beam (13, 14, 15), and the three lens segments (221, 222, 223) are assembled so that the three collimated separated beams (13, 14, 15) propagate along three different optical axes.

4. The ellipsometer (100) according to claim 1, wherein: The optical illumination system (2, 4) is adapted to collimate a polarized incident light beam (11) on the sample (6), and wherein the optical wavefront beam splitter comprises at least two prisms (231, 232, 233) whose edges are arranged parallel to the incident plane (8), each prism (231, 232, 233) being arranged to receive a different portion of the secondary light beam (12) and to form a collimated separated light beam (13, 14, 15) propagating along an optical axis angularly offset relative to the secondary optical axis (19), the at least two prisms (231, 232) being oriented and assembled such that the three collimated separated light beams (13, 14, 15) propagate along the three different optical axes.

5. The ellipsometer (100) according to claim 1, wherein: The optical polarization changing device (25) comprises at least two wave plates, each wave plate having a different delay, one wave plate being arranged on one of the three collimated separate beams (13, 14, 15) and the other wave plate being arranged on the other of the three collimated separate beams (13, 14, 15).

6. The ellipsometer (100) according to claim 1, wherein: The light source (1) comprises a halogen lamp, a xenon flash lamp, a supercontinuum laser source and / or a fiber laser and / or a pulse source.

7. The ellipsometer (100) according to claim 2, wherein: The light source (1) comprises a supercontinuum laser source and further comprises a cylindrical lens arranged in the optical path of the secondary light beam upstream of an entrance slit (31) of the imaging spectrometer (30), the cylindrical lens being adapted and oriented to magnify the six images perpendicular to the elongated direction of the entrance slit (31).

8. The ellipsometer (100) according to claim 1 comprises a first source aperture (3), the optical illumination system (2, 4) being suitable for forming a first image of the first source aperture (3) on the sample (6), and / or comprises a second source aperture, the optical illumination system (2, 4) being suitable for forming a second image of the second source aperture on the sample (6), the surface area ratio of the first source aperture (3) and the second source aperture being greater than or equal to 10.

9. The ellipsometer (100) according to claim 2, wherein: The light source (1) is adapted to generate a first series of N1 light pulses, wherein N1 is a natural integer, the image detector (33) is adapted to obtain a first image of six spectral sub-images of the first series of N1 light pulses when the image detector (33) is not saturated, the light source (1) is adapted to generate a second series of N2 light pulses, wherein N2 is a natural integer and N2 is included between N1+1 and 25.N1, the image detector (33) is adapted to obtain a second image of the second series of N2 light pulses on a first part of the six spectral sub-images when the image detector (33) is saturated and on a second part of the six spectral sub-images when the image detector (33) is not saturated, and wherein the processing system is adapted to combine a first part of the six spectral sub-images of the first image with a second part of the six spectral sub-images of the second image in order to derive ellipsometry measurements therefrom.

10. The ellipsometer (100) according to claim 2 or 9, wherein at least two spectral sub-images (41, 42, 43, 44, 45, 46) are spatially separated from each other by an intermediate region (51, 52, 53, 54, 55), wherein: The image processing system is adapted to measure the stray light intensity in the intermediate region (51, 52, 53, 54, 55) and to subtract the stray light intensity from at least a part of the images of the six spectral sub-images.

11. An ellipsometry measurement method comprising the following steps: - generating a polarized incident light beam (11); - directing the polarized incident light beam (11) towards the sample (6) along an incident light axis (9) in an incident plane (8); - receiving a secondary light beam (12) formed by reflection or transmission of the polarized incident light beam (11) on the sample (6) at a determined angle of incidence, the secondary light beam (12) propagating along a secondary optical axis (19) in the incident plane (8); - optically separating the secondary beam by wavefront splitting so as to form three collimated separate beams (13, 14, 15) propagating along three different optical axes, the three different optical axes being angularly separated in a plane perpendicular to the plane of incidence; - changing the polarization of at least two of the three collimated separate beams (13, 14, 15) to form three beams polarized according to three different polarization states; - polarization separation of the three polarized light beams using a Wollaston prism to form six separated light beams (131, 132, 133, 134, 135, 136) propagating along six optical axes, the six optical axes being angularly separated in a plane perpendicular to the incident plane (8), the Wollaston prism splitting each of the three polarized light beams into two light beams, the Wollaston prism being a Wollaston prism of the type wherein the respective polarizations of the two light beams from the Wollaston prism are at +45 degrees and -45 degrees to the separation plane; - detecting the six separate beams (131, 132, 133, 134, 135, 136) to derive ellipsometric measurements therefrom.

12. A scatterometer (100), comprising: - a light source (1) suitable for generating a source light beam (10); - a polarizer (5) adapted to receive the source light beam (10) and to form a polarized incident light beam (11); - an optical illumination system (2, 4) adapted to direct said polarized incident light beam (11) towards the sample (6) along an incident light axis (9) in an incident plane (8); - an optical wavefront beam splitter (20) arranged to receive a secondary beam (12) formed by diffraction of the polarized incident light beam (11) on the sample at a determined angle of incidence, the secondary beam (12) propagating along a secondary optical axis (19) in the plane of incidence (8), the optical wavefront beam splitter (20) being oriented to form three collimated separate beams (13, 14, 15) propagating along three different optical axes (16, 17, 18) angularly separated in a plane perpendicular to the plane of incidence (8); as well as - optical polarization changing means (25) adapted to receive the three collimated separate light beams (13, 14, 15) and to form three light beams polarized according to three different polarization states; - an optical polarization beam splitter device (26) comprising a Wollaston prism arranged and oriented to receive three light beams polarized according to three different polarization states and to form six separated light beams (131, 132, 133, 134, 135, 136) propagating along six optical axes, the six optical axes being angularly separated in a plane perpendicular to the plane of incidence, the Wollaston prism splitting each of the three light beams into two light beams, the Wollaston prism being a Wollaston prism of the type wherein the respective polarizations of the two light beams from the Wollaston prism are at +45 degrees and -45 degrees to the separation plane; and - a detection system comprising at least one diffraction grating spectrometer, the detection system being adapted to detect the six separate beams (131, 132, 133, 134, 135, 136), and a processing system being adapted to extract scatterometry measurements therefrom.

13. The scatterometer (100) according to claim 12, comprising: - an optical focusing system (27) arranged to receive the six separated light beams (131, 132, 133, 134, 135, 136) and to form six aligned and spatially separated images (141, 142, 143, 144, 145, 146); - the diffraction grating spectrometer comprises an imaging spectrometer (30), the imaging spectrometer comprises an entrance slit (31) elongated in one direction and arranged to simultaneously receive the six images (141, 142, 143, 144, 145, 146), the imaging spectrometer (30) being adapted to spectrally scatter the six images (141, 142, 143, 144, 145, 146) and simultaneously form six spectral sub-images (41, 42, 43, 44, 45, 46) spatially separated on an image detector (33); - the image detector (33) is suitable for acquiring images of the six spectral sub-images (41, 42, 43, 44, 45, 46); and - said processing system is adapted to process images of said six spectral sub-images and to derive spectral scatterometry measures therefrom.

Images