Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
  • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
  • G01N21/47—Scattering, i.e. diffuse reflection
  • G01N21/4795—Scattering, i.e. diffuse reflection spatially resolved investigating of object in scattering medium

Definitions

• the present invention relates to the field of structural analysis of objects.
• This type of metrological analysis consists of an optical tomography which makes it possible to determine the surface topology of opaque objects or to reconstruct the volume of transparent objects. It is then likely to be applied in particular in the biological and medical fields (tomography of cells, skin) and materials (tomography of structured materials, reading of invisible 3D structures such as impurities, memories, counterfeits). It relates more particularly to a structural analysis system of an object, comprising means for generating an input light beam arranged to interact with the input beam generated with at least a part of the object, and means for receiving the output light beam resulting from the interaction between the input beam and the object.
• Elie also relates to a method of structural analysis of an object, comprising a step of generating an input light beam capable of interacting with at least a part of the object, and a step of receiving the light beam output from the interaction between the input beam and the object.
• the best-known tomography solution consists of X-ray tomography, as described for example in US patent document 2005/01 17696 A1.
• an X-ray generator is imaged on a two-dimensional X-ray sensor so as to measure the absorbance of these rays through the object to be analyzed.
• the generated X-ray beam illuminates the object and the sensor is arranged so that the object is interposed between the generator and the sensor.
• These are turned relative to the object so have different orientations with respect to the object, the object always remaining interposed between them.
• the pooling of the absorbance measurements according to the different orientation angles then allows a 3D reconstruction of the object.
• the general problem that arises in the field of optical tomography concerns the three-dimensional reconstruction of the complex refractive index of a sample.
• This information which depends both on the geometric parameters (the dimensions) of the object and its optical properties (three-dimensional distribution of the complex refractive index), can not be determined, in the general case, solely from measurements.
• of light intensity of the field diffracted by the sample obtained with a conventional detector. It is necessary to know the complex value of the diffracted field.
• the intensity measured by conventional sensors is only the square of the amplitude of this field. In addition to this intensity measurement, it is necessary to measure the phase of the field diffracted by the sample.
• One solution envisaged for solving this problem consists in producing a diffractive holographic tomography system.
• a laser and a reference path are used to acquire several holograms of an object by interferometry, which allows access at the same time to the phase and the amplitude of the wave diffracted by the object.
• the three-dimensional structure of the object is then recovered by numerical methods such as that proposed by Emil Wotf in 1969, in his article "Three-Dimensional Structure Determination of Semitransparent Objects from Holography Data" (Opt.Commun., 1. IS3- 156, 1969).
• the publication "Tomography Phase Microscopy” discloses a tomography system allowing three-dimensional measurements of the index refraction of cellular or multicellular organisms which requires neither disturbance of the sample nor immersion in a specific medium.
• the system comprises a Mach-Zender heterodyne interferometer, which provides phase images from time-spaced interference patterns, due to the frequency modification of a reference beam with respect to that which passes through the sample.
• a splitter blade divides a laser beam (from a helium-neon laser) into two portions for passing through the sample arm and the reference arm, respectively.
• An orientable mirror mounted with a galvanometer makes it possible to vary the angle of incidence of the illumination.
• two acousto-optic modulators modify the frequency of the reference beam.
• the beams are then recombined to produce an interference pattern in the image plane.
• a camera records multiple images so that the phase shift between sample and reference is equal to ⁇ / 2.
• the phase images are finally calculated by phase shift interferometry.
• phase shift measurements are retro-projected according to the angles of observation, then summed in order to recompose an image.
• the object of the present invention is to overcome this technical problem by allowing a three-dimensional measurement of the refractive index (or permittivity), without requiring a reference channel or an illumination by a maschine source.
• the object of the invention is a system for structural analysis of an object with a view to performing a three-dimensional reconstruction of the structure of the portion of the analyzed object according to at least one three-dimensional reconstruction method, comprising means for generating an input light beam arranged to interact the input beam generated with at least a part of the object, and means for receiving the output light beam resulting from the interaction between the input beam and the object.
• the reception means comprise a wavefront analyzer arranged to measure the electromagnetic field of the wave of the received output beam
• the generation means have a spatial and / or temporal coherence adapted to that receiving means.
• a wavefront analyzer is an apparatus which allows a measurement of the scalar electromagnetic field of a light wave, that is to say which measures both its phase and its intensity. By its nature, it is auto-referenced, the received beam serving as a reference to itself. It therefore has the advantage of not requiring a reference beam and being insensitive to vibrations.
• a wavefront analyzer is a compact detector for imaging electromagnetic fields. Its ability to recover phase and intensity information with large spatial sampling is directly related to its performance for diffractive tomography applications. For these, we will favor high resolution wavefront analyzers.
• the adaptation of the spatial coherence of the generation and reception means aims to have a spatially coherent light at the level of the reception means.
• wavefront analysis technologies use theories based on point light sources, said spatially consistent.
• the analyzer captures a superposition of waves, which has two effects. On the one hand, the measurement is less precise because the marginal points of the source disturb the measurement at the center of its source. On the other hand, each point of the source will be diffracted differently by the object.
• the information useful for the measurement of the complex electromagnetic field is diluted between the different points of the source.
• the invention thus makes it possible to solve the above technical problem, while providing lateral resolution sufficient to benefit from good imaging quality.
• the generating and receiving means are arranged to perform several successive measurements of the output light beam resulting from the interaction between the input beam and the object.
• the generation means have a spatial and / or temporal coherence adapted further to the three-dimensional reconstruction method used.
• the system is provided with means of orientation of the interaction, seen by the receiving means, between the input light beam and the part of the object. Different orientations are applied to each measurement on the object in order to realize this 3D reconstruction. For this, several alternatives can be considered.
• the orientation means can act on the orientation:
• the generating means may be structured so as to generate a plurality of input light beams capable of interacting with at least a portion of the object at different inclinations, which makes it possible to overcome the angular clearance described above.
• Different parts of this structured light source successively illuminate the object in order to achieve this 3D reconstruction, which is equivalent to having made successive measurements at different orientations.
• the system comprises optical conjugation means between the object and the receiving means.
• the system comprises means for spectral selection of the input light beam.
• An alternative for making such chromatism measurements is to provide the system with a plurality of generation means arranged to interact with at least a portion of the object several input light beams of different wavelengths.
• the system comprises means for polarizing the input light beam.
• the generating means may comprise a laser or a temporally incoherent spectral source.
• the system comprises calculation means able to implement an image reconstruction algorithm by digital propagation of the electromagnetic field measured in the plane of the receiving means.
• the wavefront analyzer can be a digital wavefront analyzer (or curvature sensor), a microlitil matrix-type wavefront analyzer (type Shack-Hartmann), a multi-lateral shift interferometry wavefront analyzer (see in this regard patent documents EP 1993/0538126 B1 and EP 2000/1061349 B1), or any other wavefront analyzer based on the study of the effect of the wavefront on the propagation of a structured light wave (Hartmann, ).
• the degree of spatial filtering will be different depending on the technology employed among those above.
• the system comprises a semi-reflective plate disposed between the generating means and the receiving means, and reflection means arranged so as to reflect! a light passing through the object.
• the means of generation and the receiving means are arranged on either side of the object.
• the generating means and the receiving means are disposed on the same side of the object.
• the subject of the invention is also a method of structural analysis of an object with a view to performing a three-dimensional reconstruction of the structure of the portion of the analyzed object according to at least one three-dimensional reconstruction method, comprising a step of generating an input light beam capable of interacting with at least a part of the object, and a step of receiving the output light beam resulting from the interaction between the input beam and the object.
• the phase of its wave is measured by a wavefront analyzer, the spatial and / or temporal coherence of the input light beam being matched to that of the output light beam.
• the steps of generating the light beam for entering and receiving the output light beam are repeated successively, the generation means having spatial and / or temporal coherence further adapted to the three-dimensional reconstruction method used.
• FIG. 1 a diagram of a structural analysis system according to a first embodiment of the invention, with rotating illumination
• FIG. 2 a diagram of a structural analysis system according to a second embodiment, with a rotating sample
• FIG. 3 a diagram of a structural analysis system according to a third embodiment, with an imaging system
• FIG. 4 a diagram of a structural analysis system according to a fourth embodiment, with an imaging system
• FIG. 5 a diagram of a structural analysis system according to a fifth embodiment, by reflection
• FIG. 6 a diagram of a structural analysis system according to a sixth embodiment, with a color filter
• FIG. 7 a diagram of a structural analysis system according to a seventh embodiment, with polarizer, and
• FIG. 8 a diagram of a structural analysis system according to an eighth embodiment, for an opaque object.
• a structural analysis system comprises means 2 for generating an input light beam 3. This light beam is directed on a part of the object 1 to analyze. Reception means 4 make it possible to recover the output light beam 5, resulting from the interaction between the input beam 3 and the object 1.
• Object 1 is the object of tomographic analysis.
• the aim here is to reconstruct this object in a three-dimensional way, by an optical measurement in transmission, according to a three-dimensional tomographic optical microscopy principle.
• This method aims at a dimensional characterization of the object or sample, through a preliminary measurement of its form and its permittivity distribution, with quantitative measurement results.
• the object 1 is here an object having a certain transparency, so that the receiving means 4 can recover a portion of the luminous flux generated by the means 2 after interaction with this object and transmission therethrough.
• This object 1 may optionally be deposited on a substrate, itself disposed along an axis perpendicular to the optical axis A of the system.
• the means for generating the beam 3 can consist, for example, of a temporally coherent light source (for example a laser) or an incoherent light source (such as a white light source).
• a temporally coherent light source for example a laser
• an incoherent light source such as a white light source.
• the present invention finds its particularity in that it makes it possible to use an incoherent light, where the other tomography systems require light sources with a high temporal coherence.
• This tomography system is not limited by the chromatic dispersion of the sample and the sensitivity of the detector constituting the analyzer.
• the receiving means 4 are arranged so that at least a portion of the beam 3 incident on the object 1 is directed towards them.
• the range of wavelengths applicable to the structural analysis system according to the invention only depends on the available matrix detectors, which may range in particular from the Terahertz domain (microbolo-terahertz imager) to ultraviolet and x-ray. More specifically, those skilled in the art will note that it is advisable to work at a wavelength whose order of magnitude corresponds to that of the size of the details to be observed and analyzed.
• These means 4 furthermore comprise a wavefront analyzer making it possible to measure the complex electromagnetic field of the wave of the received output beam, ie the amplitude and the phase of the complex field diffracted by the 1.
• This type of analyzer allows a coupled measurement of the average optical index and the mechanical thickness of the object sample, without the need for a reference path, without beam splitting before the sample, using only light diffracted by this sample.
• the average optical index is deduced from the local complex index, which is a function of both the phase and the intensity of the diffracted wave analyzed.
• an inversion algorithm is used from the measurements of the electromagnetic field, this algorithm being able to be based on the resolution of the Maxwell equations.
• an image reconstruction algorithm by digital propagation of the electromagnetic field generated by the input light beam 3.
• the interference fringes generated by the wavefront analyzer are recorded by the CCD detector of a camera.
• the deformation of this interferogram then makes it possible to deduce the deformation of the wavefront. Indeed, if the received beam has a perfectly flat surface, the image recorded by the acquisition means (a camera) will be a perfect sinusoidal grid. If the received beam contains aberrations, this regular maiilage will be deformed.
• the study of these deformations by spectral analysis methods then makes it possible to find the gradients of the spatial phase, as well as the intensity (square of the modulus of the electromagnetic field). After integrating these gradients, we obtain a phase map with a measurement point per interference fringe.
• Several alternative embodiments of the wavefront analyzer 4 are possible to achieve the measurement of the electromagnetic field.
• a preferred variant consists of a multilateral shift interferometry wavefront analyzer.
• One of the achievements of this technique is 4-wave interferometry.
• a diffraction grating also called a modified two-dimensional Hartmann mask, replicates the beam to be analyzed in four sub-beams perfectly identical to the first and which propagate in slightly inclined directions with respect to the optical axis. Due to their slight inclinations, after a few millimeters of propagation, these beams will be slightly separated. Interference fringes will then appear, with an interference pitch depending on the angle between the directions of propagation.
• Spatial coherence is mainly related to the extent of the light source. In the case of a iaser, this range is given by the size of the Airy spot. For the so-called white light illuminations, for example with halogen sources or for light-emitting diodes, this range is the size of the emitting zone.
• Time coherence deals with the temporal statistical properties of the source. It is mainly related to the spectral extent of the source.
• this range or spectral width varies from a few fractions to a few tens of nanometers, whereas for a so-called white light source, this spectrum covers the entire visible range, ie several hundred nanometers.
• the adaptation of the spatial coherence between the generation means 2 and the reception means 4 is ensured by spatial filtering means 15. These means can for this purpose be placed between the generation means 2 and the object 1.
• the adjustment of the parameters of these means 15 makes it possible to select a part of the output luminous flux 5 in order to filter a part of it.
• the determination of these adjustment parameters for the adaptation of the spatial coherence between the means 2 and 4 is a practice within the reach of those skilled in the art.
• the adaptation of the temporal coherence is carried out using spectral filtering systems at the level of the generation means 2 or at the level of the reception means 4. From the point of view of the measurement of the electromagnetic field, these adaptation means 15 thus make it possible to have a fuminous beam whose spatial coherence is sufficient with respect to the CCD matrix forming the detector plane integrated in the front analyzer 4. It has been shown in the publication of Pierre Bon et al. Opt. Express 17, 2009, pp. 13080-13094, that in the case of 4-wave shift interferometry, the contrast of the interference fringes depends on the spatial coherence of light arriving on the analyzer.
• This contrast is directly related to the ratio between the angular extent of the source and the angle of view of a period of the diffraction grating from the CCD sensor, called critical angle of the analyzer.
• critical angle of the analyzer it is possible to set a threshold on the contrast of the fringes to obtain a reliable reconstruction of the electromagnetic field. From this threshold, we go back to the extent of the maximum allowed source on the analyzer. For example, if this contrast threshold is set at 50%, the angular extent should be less than about half of the critical angle.
• the critical angle is given by the ratio between the spacing of the microlenses and their common focal length.
• this coherence volume is given transversely by the wavelength divided by the angular extent of the source where ⁇ is the central wavelength of the source and ⁇ its angular extent) and axially by the source temporal coherence length, directly related to its spectral width ⁇ ⁇ ⁇ ⁇ ⁇ , where ⁇ is the spectral width of the source .
• a condition of validity of reconstruction algorithms is that this volume of coherence must be larger than the object to be reconstructed.
• the angular extent of the source must be lower than 55 mrad and its spectral width lower than 30 nm.
• spatial filtering means 15 are conceivable. Among these, there may be mentioned a Kôhier type assembly or more simply a simple aperture diaphragm. These spatial filtering means can be mounted on all the embodiments described below, although, for the sake of greater clarity of the figures, they are not always represented on them.
• These means of orientation of the interaction 6 consist of a means of rotation of the source 2 with respect to the optical axis A. These means make it possible to tilt the beam 3 generated by the source 2 with respect to the optical axis A. Since the interaction surface of the sample is perpendicular to the optical axis A, an inclination of the incident beam 3 with respect to the sample surface as seen by analyzer 4.
• Another possibility is to use a light source 2 having a structuring of its lighting rather than angular sweeping.
• the generation means 2 of the incident beam 3 are fixed so that this beam 3 is parallel to the optical axis.
• the orientation means of the interaction 7 are now the means of rotation of the sample with respect to the optical axis A. Since the source 2 is arranged so that the incident beam 2 is perpendicular to the optical axis , it follows an inclination of the incident beam 3 relative to the surface of the sample, as seen by the analyzer 4.
• the orientation means of the interaction 6 are identical to those of the first embodiment.
• Optical conjugation means 8 are arranged between the object 1 and the receiving means 4. These means 8 comprise a set of lenses, for the purpose of combining the interaction surface of the object 1 with the integrated detector plane. to the wavefront analyzer 4, which significantly improves the measurement results.
• the interaction orientation means 7 of the second embodiment are used.
• Optical conjugation means 8 are also arranged in order to conjugate the interaction surface of the object with the plane of the detector integrated in the wavefront analyzer.
• the preceding embodiments worked in transmission on the object 1.
• the fifth embodiment (FIG. 5), for its part, always works in transmission on the object, but to operate on it. a retro-reflection.
• the incident beam 3 coming from generation means 2 travels through a separator cube 10 (which may also be a splitter plate), to be reflected towards optical conjugation means 8, then object 1.
• the diffracted beam is then reflected at a mirror 11.
• This beam The reflector 5 crosses again the object 1 and the optical conjugation means 8, the latter always combining the interaction surface of the object 1 with the plane of the detector integrated in the wavefront analyzer 4.
• the transmitted part 5 "of this beam 5' is then directed to the reception means integrating the wavefront analyzer 4.
• the beam is injected before the imaging system 8.
• the imaging system 8 Those skilled in the art will note however that it is possible to inject the beam before or after the imaging system 8 .
• the system may comprise spectral selection means 13 for the input light beam 3.
• This spectral filter 13 makes it possible to change the central wavelength of the illumination and consequently, by successive changes of the spectral filter 13, to measure the chromaticism of the object 1.
• the system may comprise polarization means 14 of the input beam 3.
• polarization means 14 of the input beam 3 Such a polarizer makes it possible to choose the polarization of the light incident on the object, and by that even to go back to the anisotropic properties of the material constituting the object 1.
• the polarization of the input beam 3 is carried out here, by placing the polarizer 14 between the source 2 and the object 1, it should be noted that it is also possible to polarize the beam 5 resulting from the interaction, by placing a polarizer between the object 1 and the detector 4.
• the eighth embodiment presents the case where the object is entirely reflective and therefore has no transparency. In this case, it itself constitutes the reflective element and it is not necessary to use a mirror to operate in retro-reflection (as described above with reference to Figure 5).
• the measurement performed is then a surface topology. More precisely, the source 2 is arranged to direct the incident beam 3 that it generates towards the reflective object 1. Orientation means 6 of this source 2 make it possible to vary the angle of incidence of the beam 3 on the surface of the object 1. Thereafter, the diffracted and reflected beam 5, resulting from the interaction between the beam 3 and the object 1, is received by the receiving means integrating the wavefront analyzer 4. For this, the source 2 and the detector 4 must be on the same side vis-à-vis the object 1.

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• 2009
  • 2009-10-08 FR FR0904806A patent/FR2951269A1/fr not_active Withdrawn
• 2010
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