EP2596325A1

EP2596325A1 - Device and method for polarimetric measurement with microscopic resolution, polarimetry accessory for a microscope, ellipsomicroscope and ellipsometric contrast microscope

Info

Publication number: EP2596325A1
Application number: EP11754441.1A
Authority: EP
Inventors: Olivier Acher
Original assignee: Horiba Jobin Yvon SAS
Current assignee: Horiba Jobin Yvon SAS
Priority date: 2010-07-19
Filing date: 2011-07-18
Publication date: 2013-05-29
Also published as: FR2962804B1; US8908180B2; US20130265576A1; WO2012017158A1; FR2962804A1

Abstract

The present invention relates to a polarimetric measurement device with microscopic resolution, in particular comprising polarization conversion means able to modify the polarization of a beam so as to switch from a spatially uniform distribution to a distribution that is cylindrically symmetric about the optical axis, and vice versa, said conversion means being placed on the axis of a focusing objective (7) for focusing said cylindrically symmetric polarized beam onto the surface of a sample to be measured. The invention also relates to a microellipsometer, to an interference contrast microscope, to a polarimetric accessory for a microscope and to a polarimetric measurement method.

Description

The present invention relates to a microscopic resolution ellipsometer and an ellipsometric contrast microscope. Microscopic resolution polarimetric measuring device and method, microscopy polarimetry accessory, ellipso-microscope and ellipsometric contrast microscope.

We first make a state of the art in the technical fields where are combined the techniques of polarimetry and microscopy.

a) Classic ellipsometry

Ellipsometry is a well-known optical metrology technique that provides information on the optical properties of thin materials and deposits, such as the refractive index of materials and / or the thickness of thin layers. The ellipsometry is based on the analysis of the reflection (or transmission) of a beam of obliquely polarized light on a sample to be studied. It is generally accepted that the sensitivity of an ellipsometer is maximum when the angle of incidence Θ is close to the Brewster angle Θ _Β , which on a number of usual materials is between 50 and 65 °.

FIG. 1 schematically represents a conventional ellipsometer comprising a light source 1, an illumination arm 2 which comprises polarizing optical components so as to define the polarization state of the incident beam and an analysis arm 3 which comprises components polarizing optics capable of analyzing the polarization state of the beam reflected or transmitted by a sample 5.

Different types of ellipsometers are known, for example:

the cancellation ellipsometer is based on the adjustment of polarization control components in the illumination and / or analysis arm to obtain the extinction of the signal on the detector 4;

the rotating polarizer or rotary compensating ellipsometer which is based on the rotation of a polarization control component, such as a polarizer or a biregring blade;

the polarization modulation ellipsometer which relies on the use of an active component, such as a photo-elastic modulator, for modulating the state of polarization of light as a function of time;

- the Mueller ellipsometer or Muellermeter which allows the complete measurement of the parameters of the Mueller matrix of a sample (see for example the "MM16" ellipsometer of HORIBA Jobin Yvon which performs a series of measurements for different polarization states obtained by operating several liquid crystal systems).

Some ellipsometers are adapted to the quantitative measurement of the ellipsometric signal at a given point of the sample, corresponding to the point of focus of the incident beam on the sample. The ability to measure this signal Ellipsometric over a large range of wavelength is an asset, it is called a spectroscopic ellipsometer. Other types of ellipsometers, called ellipsometric imagers, make it possible to produce images by collecting the ellipsometric signal on a camera.

One of the goals of recent years is to make an ellipsometer with a very small spot size.

Another goal is to provide high resolution ellipsometric images through a microscope objective.

However, the combination of ellipsometry and microscopy techniques poses a number of optical configuration problems.

b) Ellipse microscope with oblique incidence

According to a first approach, there is also known a microscope operating at oblique incidence and comprising two objectives of relatively limited numerical aperture and polarization control devices placed respectively upstream and downstream of the assembly (illumination objective - sample - reception objective). However, this microscope has the disadvantage of working objectives not at normal incidence but under an inclined axis relative to the sample. However, the size of the objectives imposes a minimum working distance relatively large compared to the sample, which limits the accessible magnification. Moreover, this configuration does not allow to have a focus on the whole field of the sample, because of the shallowness of the objectives and the variations of distance between the plane of the objective and the sample. inclined. (See, for example, "Microscope at Brewster Angle ...", S. Hénon and J. Meunier, Rev. Sci Instrum 62, 936 (1991), or "Description of a single modular optical setup for ellipsometer, plasmid surface , waveguide modes, and their corresponding imaging techniques, Brewster Angle microscopy, M. Harke et al., Rev. Sci., Instrum., 68 3130 (1997)).

(c) Microscopy and polarization in normal incidence

The alliance of microscopy and polarized light has a number of achievements. Thus, the polarized light observation between a polarizer P and an analyzer A is conventional. It is quite common to add phase blades on the optical path.

Observations under a microscope are distinguished in orthoscopy and conoscopy. In orthoscopy, the aperture diaphragm is very closed, so as to select the rays incident on the sample at a near normal incidence. On the contrary, in conoscopy, we are interested in the effects of incidence of light, the aperture diaphragm is open, the field diaphragm being closed so as to analyze only a small portion of the sample. These polarized light microscopy methods are mainly used for the study of birefringent samples. It should be noted here that these microscopic configurations do not make use of the high sensitivity of oblique incidence polarized light reflection, which is the basis for the success of ellipsometry, including non-birefringent samples. This is an illustration of the current limitations of microsopy in polarized light. The fundamental reasons for the lack of exploitation of oblique incidence polarized light microscopy are probably related to the difficulties of analyzing signals after reflection. Indeed, to exploit the sensitivity of the ellipsometry, it is necessary to be interested in the evolutions of the polarization at the reflection not with respect to an arbitrary direction perpendicular to the optical axis of the microscope, but with respect to a plane of impact. However, the objective of the microscope creates a multitude of incidence planes, and thus a multitude of orientations of the polarization with respect to these planes, even though the illumination polarization has been selected by a spatially homogeneous polarizer, or even a combination of a linear polarizer and a birefringent blade.

The effects associated with the multitude of incidence plans can be ignored when all the rays incident on the sample are close to normal. However, since the numerical aperture of the objective becomes large (typically greater than 0.6), these effects become preponderant. However, lenses with a numerical aperture of 0.8 or 0.95 are commonly used.

One way to overcome the effects associated with the multiplicity of incidence plans is to exploit the properties of substrates having particular optical properties, such that for the range of incidence considered, the substrate modifies very little the state of incidence. incident polarization regardless of the orientation of the incidence plan.

Thus, patent FR2841339 describes the use of a substrate having particular optical properties ("Anti-reflective supports and contrast enhancement supports for reflection-polarized light", D. Aussere et al.,) So that the polarization of the reflected light is independent of the azimuth of the plane of incidence. However, this is not true for conventional substrates. However, the above proposed solution of non-depolarizing substrates only works strictly for an angle of incidence. The incidence range considered is then reduced and can in no case extend to large numerical apertures.

d) Radial selection micro-ellipsometer

An ellipsometer works theoretically with a plane-polarized light beam (ie of zero aperture) in a plane of incidence and at a non-zero angle of incidence relative to the normal to the sample. On the other hand, a microscope generally uses a lighting beam whose aperture is high and whose axis is perpendicular to the surface of the sample. Of course, to illuminate a sample at a given angle of incidence through a microscope objective, it is easy to selecting by means of an annular diaphragm on the illumination beam an incident light ring, corresponding to the chosen angle of incidence. However, in this case, a microscope does not then comprise a single incidence plane, but a multitude of incidence planes, each of the incidence planes being defined by an azimuth angle around the axis of the objective . If the incident beam has a spatially uniform polarization distribution, the orientation of the polarization with respect to each incidence plane becomes nonuniform after reflection on the sample and through the lens, thereby blurring the ellipsometric measurement.

e) Micro Ellipsometer resolved in azimuth

To solve the problem related to the multiplicity of incidence planes, a known solution in microscopy consists in retaining only a small part of the illumination beam, by means of a diaphragm placed on the incident beam to select a single azimuth . This solution of course has a major drawback in terms of brightness.

Another known solution consists in spatially resolving the signal on an imaging detector as a function of the azimuth and / or by means of a digital signal processing.

It is also conceivable not to use a diaphragm and to collect all the information on an imager for which each pixel corresponds to an angle of incidence and an azimuth (see for example "Angle-resolved annular data acquisition method for microellipsometry", Blood Hee Ye et al., Optics Express 15 (2007) 18056). The disadvantage of this solution is that complex calculations are necessary to extract the ellipsometric data. This configuration does not lend itself simply to imaging. In addition, the measurement configuration is optimal only for a single azimuth, and not for all of these azimuths. Finally, an additional difficulty concerns the quantitative calibration of this apparatus.

US Patent 6,698,51, J. R. Léger and Q. Z. Zhan, discloses an imaging ellipsometer based on the use of a single microscope objective and spatial filters. It is noted that in this device, the illumination of the sample is done with light polarized uniformly and linearly. Another example is presented in US Pat. No. 6,275,291 ("Micropolarimeter and Ellipsometer", M. Abraham and M. Eberhardt) which describes a set of micro-ellipsometers treating the beam passing through the objective as a multitude of different beams, respectively analyzed by a multitude of microscopic ellipsometers.

f) Microscope and ellipsometer with radial symmetry

Other configurations have been proposed. For example, Qiwen Zhan and James Léger proposed an ellipsometer having a radial symmetry adapted to the symmetry of an objective around the optical axis of the microscope: "Microellipsometer with Radial symmetry, Qiwen Zhan and James R. Léger", Applied Optics 41, 4630 (2002). The choice displayed in this publication is to illuminate with a spatially uniform circular polarization state over the entire illumination zone and to analyze the reflected beam by means of a radial analyzer. A radial analyzer is an optical component that passes the locally oriented linear polarization component radially from the center of this radial analyzer, whose optical axis is itself aligned with the optical axis of the objective. More recently, this same principle of illumination by means of a uniformly polarized beam in circular polarization and of analysis by means of a non-uniform analyzer with radial symmetry placed in front of a matrix detector, has been applied to realize a micro- ellipsometer ("High spatial resolution nulling microellipsometer using a rotational polarization symmetry", Alain Tschimwang and Qiwen Zhan, Applied Optics 49, 1574 (2010)). US 6,693,71 1 (Ellipsometer using Radial Symmetry, JR Léger and QZ Zhan) discloses other radially symmetrical micro-ellipsometer concepts. All these concepts are based on the use of an illumination whose polarization is radially symmetrical and have the distinction of operating on the reflected signal with radial symmetry polarization, to generate an ellipsometric signal also with radial symmetry. The optical detection is then performed on a signal having a spatial distribution of radial symmetry. However, working on a spatially distributed beam radially symmetrical to generate an ellipsometric signal, also radially symmetrical, is not easy, since most ellipsometric configurations known and controlled rely on the use of a beam of spatially uniform polarization state distribution. Moreover, even if it is possible to use beams having a spatial distribution of non-uniform polarization states, the great majority of polarization-control optical components are components having two plane-specific axes of which orientation and effects are uniform on an optical beam. Thus, the quarter-wave plates, the polarizers, the modulators or the compensators commonly used to make ellipsometers, have two proper axes in a plane transverse to the optical axis of the beam, and are not suitable for the generation of an ellipsometric signal with radial symmetry state distribution.

Thus, even if the principle proposed by Zhan and Léger has the advantage of respecting the radial symmetry of a microscope objective, it is a great disadvantage to have to realize a radially symmetrical ellipsometer, from the illuminator to the detector.

g) Radial polarization converter

To generate an incident beam having a radial symmetry polarization state distribution, or to radially analyze a polarization state distribution, not a conventional linear polarizer but a polarization polarizer is used. Polarization converters are known, used in particular in microscopy to achieve radial polarization illumination, which have some interests: Brewster angle illumination and polarization that is not reflected at this angle (US2006 / 0268265 by Yung-Ho Chuang et al. "Reduced variation in energy reflected from a sample due to thin film interference"); a confocal type microscope using different radial symmetry polarizations for illumination (US2009 / 0284835 patent application, Doron Meshulah et al., "Scanning microscopy using inhomogeneous polarizations"); better focus (US2010 / 0007863 C. Jorsdanoska); and various applications presented in (Advances in Optics and Photonics, Vol.1, pp1-57 (2009), by Qiwen Zhan).

It is true that a polarization polarization converter makes it possible to transform the spatial distribution of a polarization state from a spatially uniform linear polarization state to a linear polarization state having a radial distribution when an axis of the converter is aligned. with the uniform input bias state, or respectively to generate at a linear polarization state having an azimuthal distribution, when an axis of the converter is oriented perpendicular to the input uniform polarization state.

However, in ellipsometry, we try to probe the variation of the ratio r _p / r _s during the reflection or refraction on the surface of a sample. In a conventional ellipsometer, the input polarizer is thus oriented at 45 ° to the axes of the plane of incidence to probe both the polarization state p and the polarization state s.

However, a radial converter does not easily combine two transverse linear polarization states.

Likewise, a polarization polarization analyzer makes it possible to analyze the state of polarization either according to a state of linear polarization (p) or according to the state of linear polarization (s) but does not make it possible to obtain information on the modification of the ratio r _P / r _s .

In radial analyzer ellipsometers, the sample and the radial polarization converter affect the output signal.

A polarization polarizer not only modifies the spatial distribution of a polarization state, but also modifies the polarization state itself in a spatially inhomogeneous manner. Only a linear polarization state is parallel or perpendicular to an axis of a radial converter retains a linear polarization state of radial distribution or azimuth respectively.

Any uniform polarization state at the input of a radial converter that is not a linear polarization state is transformed into a polarization state distribution that does not generally have radial symmetry.

The object of the invention is to overcome the drawbacks of the prior devices and to propose increasing the spatial resolution of an ellipsometer while maintaining a high brightness; an imaging ellipsometer providing ellipsometric measurements of good quality; a microscope compatible with ellipsometric measurements solved at the same time as a function of the angle of incidence and the azimuth angle; a polarimetric measurement method with a high spatial resolution (micrometric or sub-micrometric).

The invention relates to a polarimetric measuring device with microscopic resolution comprising:

an excitation portion comprising:

a light source able to emit an incident light beam over a wavelength range and,

a polarization state generator able to generate an incident optical beam having a spatially uniform polarization state distribution;

first polarization conversion means adapted to receive said spatially uniform incident polarization incident optical beam and to convert it into a beam having a distribution of the polarization vector having a cylindrical or semi-cylindrical symmetry about the optical axis and according to which each polarization vector comprises a linear combination of a non-zero radial component and a non-zero azimuthal component;

optical focusing means having an optical axis for focusing said cylindrical symmetry polarization beam onto the surface of a sample to be measured, said optical axis coinciding with the normal to the sample;

an analysis part comprising:

optical collection means for collecting the light beam reflected or transmitted by the sample, said optical collection means having an optical axis coincident with the optical axis of said reflected or transmitted light beam;

second polarization conversion means arranged to receive said collected light beam, said second polarization conversion means being able to convert a beam having a distribution of the cylindrical symmetry polarization vector according to which a polarization vector comprises a combination linear of a radial component and an azimuthal polarization component into a beam having a spatially uniform distribution of polarization state;

a polarization state analyzer adapted to receive the optical beam converted by the second polarization means and to analyze the state of polarizing said optical beam according to a Cartesian base of polarization states;

detection means capable of receiving said light beam analyzed in polarization by the analyzer.

According to a particular embodiment, the first polarization conversion means and the second polarization conversion means are formed by a single polarization converter used in two opposite propagation directions and in that said optical focusing and collection means are confused.

According to a particular embodiment, said polarization converter is a liquid crystal converter comprising a Cartesian symmetry face and a cylindrical symmetry face.

According to a particular embodiment, said polarization converter is a polarization converter capable of converting the Cartesian polarization state distribution to cylindrical while maintaining the state of polarization of the beam.

According to a particular embodiment, the polarization state generator is a spatially uniform linear polarizer and in that the polarization axis of said linear polarizer is oriented with respect to an own axis of the first polarization conversion means so as to form a non-zero angle.

According to a particular embodiment, the polarization state analyzer is a spatially uniform linear analyzer and the polarization axis of said linear analyzer is oriented with respect to an own axis of the second polarization conversion means so as to form a polarization analyzer. non-zero angle.

According to various aspects of particular embodiments, the polarimetric measuring device further comprises:

optical means for separating the incident beam on the sample and the beam reflected or transmitted by the sample, said optical separation means being arranged on the optical path between the polarization state generator and the polarization converter; a field diaphragm, an aperture diaphragm and / or means for spectroscopic analysis of the detected signals;

a Nomarski prism, the converter being located between the Nomarski prism and said optical focusing means.

According to a particular embodiment, said polarimetric measurements are ellipsometric measurements, Mueller ellipsometry measurements or scalerometry measurements.

The invention also relates to a polarimetric microscope accessory comprising: a polarization state generator adapted to receive an incident optical beam and to generate an optical beam having a spatially uniform polarization state distribution;

a radial or cylindrical polarization converter adapted to convert a beam propagating in a first direction of axial propagation and having a spatially uniform polarization state distribution in a beam having a distribution of the cylindrical symmetry bias vector around the axis optical, and conversely said converter being adapted to convert a beam propagating in a second contra-propagative axial direction to the first direction and having a polarization vector distribution having a cylindrical symmetry about the optical axis in a beam having a distribution spatially uniform polarization state; a polarization state analyzer according to a Cartesian polarization state base;

said converter being disposed on the optical path between the polarization state generator and the microscope objective on the one hand and between the microscope objective and the polarization state analyzer on the other hand. The invention also relates to an imaging microscope-ellipsometer comprising an accessory according to one embodiment of the invention, a digital aperture microscope objective greater than or equal to 0.7 and an imaging detector.

The invention also relates to an ellipsometric contrast microscope comprising an accessory according to one embodiment of the invention, comprising:

a first converter for directing an optical beam towards a sample to be measured,

a second converter for directing the optical beam reflected or transmitted by the sample towards a reference substrate,

said first and second converters having their own axes oriented at 90 degrees relative to one another.

Finally, the invention also relates to a polarimetric measurement method comprising the following steps:

generating an incident light beam having a spatially uniform polarization state distribution;

the polarization vector distribution of said incident beam is converted into a polarization state distribution having a cylindrical or semi-cylindrical symmetry about the optical axis, wherein each polarization vector comprises a linear combination of a radial component non-zero and a non-zero azimuthal component; - Focusing said polarization beam with cylindrical or semi-cylindrical symmetry on the surface of a sample to be measured along an optical axis coincident with the normal to the sample;

the light beam reflected or transmitted by the sample is collected;

the polarization state spatial distribution of the collected beam is converted to convert a cylindrical symmetry polarization state distribution around the optical axis into a spatially uniform polarization state distribution;

the polarization state of the beam collected and converted into polarization is analyzed;

the analyzed beam is detected.

According to a particular embodiment, the polarimetric measurement method further comprises one of the following steps:

the detected beam is spectroscopically analyzed;

an image of the surface of the sample is detected on an imaging detector; and or

an image of an optically conjugated plane of the surface of the sample is detected.

Another way of expressing things is this. The invention relates to an amplitude and phase polarimetric measurement device of the ellipsometric measurement or Mueller ellipsometry measurement type or a scalerometric measurement, said microscopic resolution polarimetric measuring device comprising:

an excitation portion comprising:

a light source able to emit an incident light beam over a wavelength range and,

optical focusing means having an optical axis for focusing said incident light beam on the surface of a sample to be measured; an analysis part comprising:

optical collection means for collecting the light beam reflected or transmitted by the sample, said optical collection means having an optical axis;

a polarization state analyzer adapted to receive the optical beam collected by said optical collection means and to analyze the polarization state of said optical beam according to a Cartesian base of polarization states; and detection means able to receive said light beam analyzed by the analyzer in polarization;

said polarization state generator and / or said polarization state analyzer further comprising bias modulation means.

According to the invention, said optical focusing means are arranged in such a way that said optical axis of said optical focusing means coincides with the normal to the sample; said optical collection means are arranged in such a way that said optical axis of said collection optical means is coincident with the optical axis of said reflected or transmitted light beam; the excitation portion comprises first polarization conversion means disposed between said polarization state generator and said focusing optical means, said first polarization converting means being oriented to receive said spatially uniform polarization incident optical beam and to converting it into a beam having a polarization vector distribution having a cylindrical or semi-cylindrical symmetry about the optical axis and wherein each polarization vector comprises a linear combination of a non-zero radial component and an azimuth component non-zero; and the analyzing portion comprises second polarization converting means disposed between said collection optical means and said polarization state analyzer, said second polarization converting means being oriented to receive said collected light beam, said second polarization converting means being polarization conversion being capable of converting a beam having a cylindrical symmetry bias vector distribution according to which a polarization vector comprises a linear combination of a radial component and an azimuth polarization component into a beam having a spatially uniform distribution polarization state.

According to a particular aspect of the device of the invention, said polarimetric measurements are ellipsometric measurements, Mueller ellipsometry measurements or scalerometry measurements, the matrix of said polarization conversion means expressing the coordinates in the cylindrical coordinate system of the polarization. of an emergent beam of the cylindrical symmetry face, as a function of the coordinates in the cartesian coordinate system of the polarization of the incident beam on the face with Cartesian symmetry, being diagonal of the type:

he ολ

^M - = β)

where the coefficients, β of this matrix are uniform on each half of the component. The invention also relates to an amplitude and phase polarimetric measuring accessory of the ellipsometric measurement or Mueller ellipsometry measurement type or microscale scalerometric measurement comprising:

a polarization state generator adapted to receive an incident optical beam and to generate an optical beam having a spatially uniform polarization state distribution;

a polarization state analyzer according to a Cartesian polarization state base;

polarization modulating means disposed between said polarization state generator and said polarization state analyzer, and

a radial or cylindrical polarization converter arranged on the optical path between the polarization state generator and the microscope objective on the one hand and between the microscope objective and the polarization state analyzer on the other hand ,

said converter being oriented with respect to the polarization state generator so as to convert a beam propagating in a first axial propagation direction and having a spatially uniform polarization state distribution to a beam having a distribution of the polarization vector of cylindrical symmetry around the optical axis according to which each polarization vector comprises a linear combination of a non-zero radial component and a non-zero azimuth component and conversely said converter ensuring the conversion of a beam propagating in a second direction counter-propagative axial axis at the first direction and having a polarization vector distribution having cylindrical symmetry about the optical axis in a beam having a spatially uniform polarization state distribution.

The invention also relates to a polarimetric measurement method in amplitude and in phase of the ellipsometric measurement or Mueller ellipsometry measurement type or a scalerometric measurement comprising the following steps:

the light beam reflected or transmitted by the sample is collected; the polarization state spatial distribution of the collected beam is converted to convert a cylindrical symmetry polarization state distribution around the optical axis into a spatially uniform polarization state distribution;

the analyzed beam is detected.

The invention will find a particularly advantageous application in micro-ellipsometry, ellipsometric contrast microscopy, or ellipsometric imaging micrometric resolution.

The present invention also relates to the features which will emerge in the course of the description which follows and which will have to be considered individually or in all their technically possible combinations.

This description is given by way of nonlimiting example and will better understand how the invention can be made with reference to the accompanying drawings in which:

- Figure 1 shows schematically an ellipsometer;

FIG. 2A diagrammatically shows in an exploded view the two faces of a cartesian-cylindrical polarization converter known in the prior art; FIG. 2B schematically represents a projection in a plane transverse to the optical axis of the cartesian and cylindrical markers respectively associated with the two faces 1 1 and 12 of the converter as well as the orientation of the axes of the liquid crystals to move from one face to the other side of the converter;

FIG. 3 diagrammatically represents a micro-ellipsometer according to one embodiment of the invention;

FIG. 4 schematically represents the propagation of a uniform polarization beam through a microscope objective and after reflection on a sample;

- Figure 5 schematically shows the propagation of a uniform polarization beam through a device according to one embodiment of the invention; FIG. 6 schematically represents the propagation of a polarized beam through a converter and a microscope objective in any plane of incidence; FIG. 7 schematically represents the propagation of a polarized beam through an ellipsometric contrast microscope device according to one embodiment of the invention;

- Figure 8 schematically shows a sectional view of an episcopic illumination microscope and operating in polarized light according to the prior art;

FIG. 9 schematically represents a sectional view of an ellipsometric microscope according to one embodiment of the invention;

- Figure 10 schematically shows a sectional view of the detection part of a microscope according to a particular embodiment of the invention;

FIG. 11 schematically represents a sectional view of an ellipsometric microscope according to one embodiment of the invention using the detection part shown in FIG.

The invention relies on the use of a component called a "Cartesian to Cylindrical" polarization converter and a polarization analyzer.

According to one embodiment of the invention, use is made in a particular configuration of a liquid crystal polarization converter as described in particular by Stalder et al. Opt. Lett. 21 p. 1948 (1996). FIG. 2A diagrammatically shows an exploded view of the two faces of such a Cartesian-cylindrical polarization converter with liquid crystals. Such a liquid crystal polarization converter has two distinct faces designated herein: Cartesian face 1 1 and cylindrical face 12 respectively. The Cartesian face 1 1 has two proper axes, oriented at 90 °, these proper axes are identified by u _x and u _y , one of the axes u _x , u _y being an alignment axis of the liquid crystal. The "cylindrical" face has a center O and it is convenient to associate a reference in cylindrical coordinates (centered in O) u ₉ , u _r . FIG. 2B shows a projection in a plane transverse to the optical axis of the cartesian and cylindrical markers respectively associated with the two faces 1 1 and 12 of the converter as well as the orientation of the axes of the liquid crystals to pass from one face to the other side of the converter. The polarization converter receives on its Cartesian face 1 1 an incident beam parallel to its optical axis. According to Stalder et al. it is known that when the incident beam on the cartesian face is polarized linearly and of uniform polarization, the axis of the incident polarization being aligned on one of the axes u _x or respectively u _y , the converter transforms this beam into a beam of radial or azimuth polarization. More generally, when the incident beam on the Cartesian face has a spatially uniform polarization of the type: at _x + bu _y (expressed in the Cartesian base) the converter transforms this incident beam of uniform polarization into a beam emerging from its cylindrical face, from non-uniform polarization, but whose distribution of polarization states is of cylindrical symmetry around the axis optical and which is expressed (in the cylindrical base) by [at ₉ - b.exp (j.%). u _r ] ^* Sign (-y). The factor exp (j.%) Expresses the fact that there may be a phase shift χ between the radial component and the azimuth component of the wave, this phase shift being introduced during the crossing of the polarization converter. The coefficient Sign (-y) is equal to -1 if y is positive (or, which amounts to the same, if φ is between 0 and π) and Sign (-y) is equal to +1 if y is negative ( or, which amounts to the same, if φ is between π and 2π). The behavior of this converter is not defined in the immediate vicinity of 0 nor on the axis where y is equal to 0. Conversely, a beam having a cylindrical polarization polarization incident on the cylindrical face, the distribution of states of polarization is given in the cylindrical coordinate system by ₉ + bu _r , this beam leaves the Cartesian face of the converter in the form of a beam whose distribution of state of polarization is uniform, and given by [at _x -b. exp (j.%). u _y ] ^* Sign (-y).

It will be noted that it is the direction of the emergent polarization which is cylindrically symmetrical, its meaning, it being not the same in the two half-parts of the component, being affected by Sign (-y).

Note that the Stalder publication essentially discloses the use of a polarization converter in a configuration where the direction of propagation of the beam goes from the Cartesian face to the cylindrical face, and where the incident polarization vector is aligned according to one of the axes of the converter to generate a polarization beam either radial or azimuthal. This publication does not disclose the use of such a converter to generate a generalized cylindrical vector beam or generalized CV Beam, comprising radial and azimuth components. This document does not disclose the use of such a converter with a beam propagating from the cylindrical face to the Cartesian face. In particular, the coupling of a generalized cylindrical polarization beam, comprising a linear combination of non-zero radial and azimuth components, to the cylindrical face of such a converter for generating a spatially uniform polarization beam is not described.

In other words, the polarization converter of Stalder and Schadt is described, for the passage of a wave polarized uniformly in a Cartesian coordinate system, by the following transformation matrix:

M_ = .¾ «(-> ^ _ _exp ° ₍ .J where the phase shift χ depends on the wavelength.

For the passage from the cylindrical face to the Cartesian face, the coordinate transformation matrix (expressed in the coordinate system relative to each face) is as follows: Μ_ = .¾ "(-> ·) ^ _ _exp ₍ .J

It may be advantageous to use a perfect polarization converter, that is, for which χ is close to zero. Such a converter retains the ellipticity of the incident polarization, the ellipticity of the outgoing polarization being identical to that of the incident polarization. A suitable converter for implementing the invention is according to a preferred embodiment based on a liquid crystal converter.

Other embodiments of the invention may use other types of polarization converters, based, for example, on the use of segmented half-waves, cylindrical symmetric diffractive or subdiffractive networks,

A polarization converter can be used in the invention since, during its crossing, the matrix expressing the coordinates in the cylindrical coordinate system of the polarization of a beam emerges from the face with cylindrical symmetry, as a function of the coordinates in the Cartesian coordinate system. of the polarization of the incident beam on the face with Cartesian symmetry, is diagonal, that is to say of the type:

where the coefficients, β of this matrix are uniform on each half of the component.

The converter thus makes it possible to transform an optical beam having a distribution of spatially uniform polarization states into a beam having a distribution of cylindrical symmetry polarization states around the optical axis.

A converter is also used to transform a beam having a cylindrical symmetry polarization state distribution into an optical beam having a uniform polarization state distribution in a Cartesian coordinate system.

According to a preferred embodiment of the invention, shown diagrammatically in FIG. 3, the same converter 10 is used in both directions of propagation along the optical axis 8 of a focussing lens 7: a first time in FIG. the Cartesian to cylindrical conversion direction and a second time in the cylindrical to Cartesian conversion direction. The light path passes twice through the converter 10, once before hitting the sample 5, and the second time at the return. On the other hand, the conditioning and analysis components 3 of the polarization placed respectively upstream and downstream of the converter are different for the incident and reflected beam. In addition, we do not use an optical component to filter a specific own polarization (radial or azimuth), since we seek precisely to have information on the two polarizations (unlike devices using a radial polarizer and not a polarization converter ).

I. Embodiments of the invention 1) Ellipsometer with microscopic resolution

In the invention, a micro-ellipsometer is constructed consisting of two subsets:

a conventional ellipsometer (or Mueller-meter), comprising an illumination arm 2 in which the outgoing polarization is uniform in the Cartesian frame u _x and u _y . It can be written in _x + bu _y . The ellipsometer also comprises a detection arm 3, adapted to measure the incoming polarization state in the same frame u _x , u _y . If we note the polarization entering the detection arm r _x .u _x + r _y .u _y , the conventional ellipsometer measures the complex ratio r _y / r _x . In the case of an imaging ellipsometer, a map of r _y / r _x is measured in the plane (x, y);

- The polarization converter 10 and optical parts for intercepting the input to its Cartesian face 1 1 all or part of the illumination arm of the ellipsometer, the proper axes of its Cartesian face January 1 coinciding with the proper axes of the ellipsometer, oriented by the vectors u _x and u _y . At the output, the converter 10 sends all or part of the emergent light to the detection arm 3 of the ellipsometer. The light beam emerges by the cylindrical face 12 of the converter goes towards the objective 7 of the microscope, the optical axis 8 of the objective 7 being aligned with the center O of the cylindrical face 12 of the polarization converter 10.

- A microscope objective 7, placed substantially perpendicular to the sample 5 to be studied.

Regarding the optical parts for sending the beam of the illumination arm 2 to the converter 10, and the beam from the converter 10 to the detection arm 3, we can use separator blades 6. It is preferred to place the plane of incidence of these blades 6 and the different interfaces, either according to u _x or according to u _{y in} order to allow a simple calibration of the changes of amplitude and phase in the reflection which could be translated by r _p / r _s different from 1 .

2) Microscope comprising an ellipsometry accessory

Let us start from a microscope suitable for episcopal polarized light observation, as shown diagrammatically, for example, in Figure 8. Rather than limit the influence of polarization impact effects by working in orthoscopy, or rather than to use particular substrates, our approach is conceptually totally different. We start from a polarization field of the incident light having a spatially uniform distribution of polarization state, which is expressed in a cartesian coordinate system in the form of _x + bu _y , with constant a and b on the object field ( x, y). This polarization state distribution is transformed by means of components having their uniform optical axes so as to generate a cylindrical symmetry polarization field which is expressed as: [au, -b.exp (jx) .u _r ] ^* Sign (-y)

in the cylindrical coordinate system having as axis the optical axis 8 of the objective 7.

The factor exp (j.%) Expresses the fact that there may be a phase shift between the radial component and the azimuth component of the wave, introduced by the polarization converter 10. This transformation is performed by a polarization converter 10 whose principal axes are oriented according to u _x , u _y , and whose center is on the optical axis 8 of the objective 7. In this reference, the radial vector u _r is in the plane of incidence and the orthoradial vector (also called azimuthal) ιι _φ is perpendicular to the plane of incidence. The reflected ray is therefore expressed (if we write r _p (0) and r _s (0) the reflection coefficients under an incidence angle θ, for a polarization respectively parallel and perpendicular to the plane of incidence):

[r _p (0) .to _"p - r _s (0) .b. exp (j.%). u _r ] ^* Sign (-y)

By going back in the opposite direction, in the polarization converter, the beam considered has a polarization which is expressed as follows:

[r _p (0) .au _x + r _s (0) .bu _y ] ^* Sign (-y) ^* Sign (+ y) = - [r _p (0) .au _x + r _s (0) .bu _y ]

The formula above remains valid regardless of the azimuth φ with which the ray passes through the lens. Everything happens as if the system according to the invention "simplified" the microscope objective into a "2D plan" objective having a single and unique incidence plan.

FIG. 4 schematically represents a microelectronics device according to the prior art. A source 1 emits a light beam. FIG. 4 shows two radii 31a and 31b parallel to the optical axis 8 of the lens 7 and located at the same distance from the optical axis 8 but at different azimuths. Polarization means (not shown) define a spatially uniform polarization state of the incident beam. The radii 31a (solid line) and 31b (dashed) therefore have the same state of polarization (represented as an elliptical polarization). The beams 31a and 31b are directed towards a lens 7 (or a microscope objective) of high numerical aperture. Consider the propagation of the two radii of identical aperture and azimuth different (respectively in solid line and in dashed lines) through the lens 7 before and after reflection on the sample 5, that is to say in two plans of incidence. In a first plane of incidence, the radius shown in solid lines, the polarization beam 31a through the lens 7 and becomes the beam 32a incident on the sample. The lens 7 does not modify the polarization state of the transmitted beams 32a, 32b. The beam located in the first plane of incidence is elliptically polarized, with an axis of the polarization ellipse 32a located in its plane of incidence. During the reflection on the sample 5, this beam changes and becomes a beam 33a whose major axis is multiplied by the reflection coefficient r _p , relative to the reflection of a ray in the plane of incidence, and of which the small axis by r _s , relative to the reflection of a radius perpendicular to the plane of incidence. In a second plane of incidence, the radius in dashed lines has a polarization ellipse 32b whose axis is no longer in the second plane of incidence after having a first crossing of the lens 7. After reflection on the sample 5, the state of polarization 33b is different from the state of polarization 33a dashed. The state of polarization is therefore not the same in separate incidence planes. Therefore, it is not possible to collect together rays such as dotted and dotted-line to make ellipsometric imaging. The ellipsometric signal thus depends on the plane of incidence (or azimuth) under which they crossed the lens, which makes the analysis complicated. As indicated in the prior art, it is then necessary to make a selection in azimuth or to radially analyze the signal 33a, 33b as a function of the angle of incidence and the azimuth angle.

Figure 5 explains the operation of the invention. In the device of the invention (FIG. 5), a polarization converter 10 has been added, whose cylindrical symmetrical face 12 is disposed on the side of the objective 7 and centered on the optical axis 8 of the objective. The incident beam has a spatially uniform distribution of polarization state. In FIG. 5, the radii 31a (solid line) and 31b (dashed lines) have an initial elliptical polarization whose major axis is oriented parallel to the x axis of the polarization converter 10. At the output of the converter 10 , the polarization ellipses 34a and 34b of the transmitted rays have their major axis in the radial direction and when the rays 34a and 34b thereafter pass through the lens 7, this major axis is in the plane of incidence of each ray. In this case, for each of the rays, the polarization ellipse 35a, respectively 35b experiences the same evolution during the reflection on the surface of the sample 5: the major axis of each ellipse is multiplied by the reflection coefficient R _p , relating to the reflection of a ray in the plane of incidence, and the minor axis by R _s , relative to the reflection of a ray perpendicular to the plane of incidence. It can thus be seen that the cylindrical symmetry of the polarization state distribution is not modified by the reflection on the sample 5 or by the second crossing of the lens 7.

But that's not all, because on return, the polarization converter 10 also plays a decisive role, since it allows, as seen on the radius drawn in dashed lines, to put the polarization ellipse 36b in the Cartesian coordinate system uniform. Thus, all the rays with the same angle of incidence irrespective of their azimuth φ, which have the same state of polarization 35a, 35b in a polar reference before crossing the polarization converter 10, also have the same state of polarization 36a, 36b in a Cartesian coordinate system after having made a round trip in this converter 10 and the objective 7 (or the lens) with high numerical aperture. All these rays 36a and 36b carry the "ellipsometric" information relative to the reflection at oblique incidence on the surface 5 and contribute in the same way to the ellipsometric signal for the same angle of incidence and at the wavelength considered. whatever their azimuth φ. It is then possible to add the contributions of the rays 36a and 36b in order to analyze them in a conventional manner using a linear polarization analyzer, as is commonly the case in ellipsometry and Mueller-metry.

The arrangement of Figure 5 is essential for imaging.

It will be noted that FIG. 5 represents the case where the polarization converter 10 does not induce a phase shift between the two eigenpolarizations (χ = 0) for the sake of simplicity. It is also an advantageous case since it can simplify the calibration, and the fact of obtaining χ = 0 over a large wavelength range facilitates the work in white light, because this component is practically achromatic.

Note also that the fact that each light beam makes a forward path and a return path in the polarization converter 10, two azimuths that differ by 180 °, that we can use a polarization converter 10 which has a different phase shift according to the half-plane along which it passes through (this is the "Sign (- y)" in the formulas above, which corresponds to a phase shift of π when the radius passes through the component at an abscissa y> 0 ). Indeed, a light ray reflecting on the surface of the sample crosses once the converter to a positive abscissa, the other time to a negative abscissa. Consequently, a uniform phase shift is obtained during the round-trip irrespective of the azimuth of the radius.

Figure 6 shows schematically the propagation of an elliptically polarized beam in any plane of incidence through a converter 10 and a microscope objective 7. An incident beam is defined by its spatially uniform polarization state. This polarized beam 21 is incident on the Cartesian face 1 1 of the converter 10. The cylindrical face 12 of center 0 is centered on the optical axis 8 of the objective 7. The emerging beam of the cylindrical face 12 of the converter 10 has a polarization state distribution 22 of cylindrical symmetry about the axis 8. The objective 7 is chosen not to change the polarization state of the beam. The incident beam 23 on the sample 5 is of cylindrical symmetry about the axis 8. After reflection, the elliptically polarized beam sees its polarization state modified by the reflection coefficients Rp and Rs of the sample. The reflected beam has a modified bias state 24 compared to the beam 23. However, the incident beam having a cylindrical symmetry bias distribution, in each plane of incidence, the polarization change is identical. On the return path, the beam 24 crosses the lens 7, to form a polarized beam 25, which is incident on the cylindrical face 12 of the converter 10. The state of polarization of the beams 22, 23, 24 and 25 is uniform in a polar coordinate system. After crossing the converter 10 in the polar-Cartesian direction, the beam 26 thus has a uniform polarization state distribution in a cartesian frame. By the operation of the converter 10, all incidence planes are equivalent to that shown in Figure 6 and have the same contribution to the signal. The use of the polarization converter 1 0 located just upstream of the lens 7 and the return path of the light, the orientation of an analyzer, a polarizer and possibly a compensator located downstream polarizer or upstream of the analyzer, actually allow to define and analyze the state of polarization relative to the plane of incidence on the sample. The positions of the various components are marked in the frame u _x , u _y defined by the Cartesian axes of the polarization converter 1 0.

Furthermore, it should be noted that the sensitivity of the ellipsometric technique is particularly important in the vicinity of the Brewster angle ΘΒ of the substrate used, where the angle ΘΒ is equal to Arctan (N) where N is the index of the substrate. Incident angles of 60 ° or more can be used, which corresponds to a numerical aperture greater than 0.85. A numerical aperture goal of 0.9 or higher is advantageously used, values that can be obtained for commercial purposes. Nevertheless, incidence angles starting from 45 ° can be used, ie numerical apertures greater than 0.7. Note however that it is important that the lens 7 used does not change the radial polarization state distribution. It is important that objective 7 does not exhibit a birefringence that can modify the polarization state spatial distribution in a heterogeneous or random manner. Preferably, an objective 7 compatible with observations in polarized light or a Nomarski type objective compatible with interference contrast observation is used.

Finally, it is quite feasible to perform ellipsometric measurements in immersion, which makes it easier to reach Brewster conditions. In the case of an immersion objective, the Brewster angle ΘΒ is equal to Arctan (N / N _immerS i ₀ n) where N is the index of the substrate and N _immersi is the index of the immersion medium.

FIG. 8 represents a microscope operating in polarized light according to the prior art. The microscope comprises an illuminator 27, generally of the Kohler type. The illuminator 27 comprises a light source 1 placed behind a frosted surface 1a, an aperture diaphragm 16a, a field diaphragm 17a, and several lenses 1b, 18b, 18c. The microscope tube includes a splitter plate 6, an optical axis lens 7, a tube lens 9, a pupil 19, a field diaphragm 17a or 17b, an eyepiece 20, and a detector 4. To operate in accordance with FIG. polarized light, the microscope comprises a polarizer 1 3 in the illumination arm, and / or an analyzer 14 in the detection arm. The orientation of the axes of the polarizer 1 3 and / or the analyzer 14 allows the observation of the sample 5 between crossed polarizers for example.

FIG. 9 represents an ellipsometric microscope according to one embodiment of the invention. The components of the microscope of FIG. 8 are found again. In addition, the microscope comprises a converter 10 inserted between the splitting plate 6 and the objective 7. To select a given angle of incidence Θ, it is advantageous to use an annular diaphragm 16 inserted at the opening diaphragm 16a of the microscope or in a plane which represents a Fourier plane of the sample 5. By orienting the components 13 (polarizer), 14 (analyzer) and / or 15, 15 (compensator) we can look for the conditions close to the "nulling ellipsometry", able to cancel the emerging light on the detector 4, so as to bring out the smallest variations of properties on the surface 5 to be observed. All state-of-the-art knowledge regarding the observation under nulling ellipsometry conditions can then be reused, but benefiting from the convenience of the fact that the impact is achieved by means of a single objective of which the axis is confused with the normal to the sample.

There are two ways of using this system:

an "imaging" mode, when the image formed on the eyepiece or the camera 4 is that of the sample, for a given angle of incidence Θ. The system is then an "ellipso-microscope" (shown in Figure 9);

- an "ellipsometric signal depending on the angle of incidence" mode (illustrated

Figures 10 and 1 1). If one chooses to be interested in the ellipsometric signal as a function of the angle of incidence 6> on a given zone of the sample, in which case the field diaphragm 17a or 17b is closed, a lens of Bertrand 28, and obtained directly on the display device of the microscope the signal reflected as a function of the incidence Θ and azimuth φ of the incident polarization. Unlike azimuth-resolved ellipsometry devices that do not use a polarization converter, each observation circle corresponding to a given incidence is homogeneous (in the case of an isotropic sample) and can therefore be integrated directly. The ellipsometric signal is relative to the light spot created by the objective 7 of the microscope. This spot can be made very small (ten μιτι or 5 μιτι, or even 1 μιτι) by restricting the field diaphragm to a very small aperture. The system is then a "micro-ellipsometer". The device of the invention therefore does not require the use of a radial analyzer, a complex system of radial imaging, or signal processing.

It can be observed that the device can function as well in episcopia (that is to say, the light illuminates the sample while passing through the lens), or in transmitted light. In transmitted light, two polarization converters are required, a first Cartesian-cylindrical converter 10 between the polarizer and the objective, and a second converter 10 'cylindrical-Cartesian between the objective and the analyzer.

In order to increase the contrast and the signal-to-noise ratio of an ellipsometric contrast microscope, a weak modulation of the two polarization components around the cancellation position and a detection is advantageously used. synchronous image with respect to each of these modulations. For example, a first modulation may be applied by a small rotation of the analyzer around its cancellation position. Another solution is to slightly modulate the delay of a compensator made from a voltage controlled nematic crystal plate, by slightly changing the voltage applied to this voltage-controlled delay blade around the value allowing the cancellation.

3) Ellipsometric contrast microscope

It is considered that the invention opens a new field of applications in microscopy, by providing an ellipsometric contrast microscope self-compensated over a wide range of incidence and wavelength.

This extension of the invention consists in a way of observing (in imaging) with increased contrast objects having one or two very small dimensions (10 nm to less than one tenth of a nm), without being restricted to a length of wave, nor to a range of incidence for illumination, while benefiting from the ellipsometric contrast. Being able to work with all the numerical aperture of the lens is an advantage in terms of lateral resolution.

In order to better explain this extension of the invention, we begin by describing in detail, in Jones matrix formalism, what happens during a path that passes through the polarization converter 10, the objective 7 to large opening, the reflection on the sample 5, then follows a return path, as shown schematically in FIG. 6. The output polarization vector is obtained by making the product of the polarization vector at the output by the matrix M obtained by multiplying each of the Jones matrices of individual components:

It will be noted that cylindrical coordinates are worked in the space after the polarization converter 10 and in Cartesian coordinates in the x, y coordinate for the beams 21 and 26 in FIG. 6. t _p and t _s are the transmission coefficients through the lens or the lens 7. For a high aperture, there is generally a significant phase difference between t _p and t _s .

When performing the calculation, we find:

Y? ² oio ^

M

^■ r _p .t _p exp (2 jz) 0 {t _p / t _s f exp (2j.z) .r _p / r _s

In the case where we are interested in the ellipsometric quantity r _p / r _s , we can

\ t 11 T exo (2 / y)

different measurements and calibration determine ^{K ps / J 'A} ', which depends on both the wavelength and the angle of incidence. We can also place on the optical path, in the "Cartesian" zone, elements that have in the Cartesian frame

\ t I 't Γ exo (2 / y)

(x, y) a Jones matrix which makes it possible to reduce the influence of ^{K ps / J 'A} , and in particular to reduce its variations as a function of the wavelength. The manner of designing a delayed or dichroic blade system for this is known to those skilled in the art. However, this compensation system may work best for a range of incidence and limited wavelength.

Now, let's consider the case where we want to make imaging of objects of extremely small thickness (a few tenths of a few nm to a few nm), and presenting very low contrast optical properties (transparent objects for example), arranged on a plane substrate. The ellipsometric magnitude r _p / r _s of the substrate will be modified in r _p i / r _s i at these objects, and the expected advantage of the ellipsometric contrast imaging is to reveal with a very good contrast these small variations by canceling as much as possible the signal of the bare substrate.

For this, a particular embodiment of the invention, illustrated in FIG. 7, consists in ensuring that each ray contributing to the formation of the image has a path that compensates for the factor related to the instrument and the bare substrate:

(t _p / t _s } cxp (2j.z) .r _p / r _s

For this, use is made of a second polarization converter subsystem 10 '- objective 7' (or high numerical aperture lens), identical to that used to look at the object 5. This second subset comprises a reference substrate 5 _RE F, completely blank of defects and objects to be examined and preferably identical to the substrate used for the examination of objects. The second set is arranged so that the path in the second set polarization converter 1 O'-objective 7'-reference substrate 5 _RE F, so that it is traversed by all the rays used to constitute the image at ellipsometric contrast, under the same angle of incidence conditions, but with a polarization pattern turned 90 °: a linearly polarized ray along the "x" axis of the first set will cross the second parallel to the "y" axis ₂ "of the second set.

Thus, the matrix corresponding to the crossing of the two sets is of the following form: . {tt) ² cxp (2j.z)

r .t 0 / rMr r)

The factor in front of the matrix represents a polarization-independent attenuation and phase shift, whatever the incidence and the wavelength. An observation between crossed polarizers makes it easy to isolate a contrast related to the difference between ta / ^r * ⁱ M ^r _P ^{/ r} _s ) _and _a day. We can add a small modulation of a phase shift to allow the detection of the phase of the quantity ta ^{/ r} ^ M ^{rp / r} J _is note that these measurements can be made over a wide range of angular incidences and over a wide range of wavelengths.

Thus, a new type of microscope is obtained.

We note that the performance of this new microscope can be further improved by providing a device Digital Interference Contrast (also called Nomarski).

FIG. 7 represents an example of a self-compensated ellipsometric contrast microscope over the entire spectral and incidence range.

Two identical semi-transparent polarization objective-mirror objective converter assemblies, one focused on a reference substrate 5 _RE F, the other on a substrate comprising objects to be observed 5, are arranged in such a way that a ray originating from of the light source, arriving on the linearly polarized converter 1 0 parallel to the direction y-ι (converted into a radial polarization by the converter, according to our notation conventions), arrives on the second converter 1 0 'with a polarization according to the x ₂ direction of this second converter 1 0 '(that is to say converted into an azimuthal polarization). Reciprocally, a light beam coming from the source and incident on the converter 1 0 with a linear polarization parallel to x _1; is incident on the second converter 1 0 'with a polarization parallel to y ₂ . Thus a ray 28, in a first plane of incidence, is polarized linearly in a direction p parallel to the plane of incidence. After passing through the converter 1 0, its polarization state is radial and therefore remains a p-type polarization state. After crossing the objective 7, reflection on the sample 5, the state of polarization of the beam remains p to the second set of converter 1 0 '(shown in top view). The axes {x ₂ , y _Σ ) of the second converter 1 0 'being oriented at 90 degrees of the axes {xi, yi) of the first converter 1 0, the beam becomes after passing through the converter 10', a transverse polarization beam or s . This polarized beam s is then incident on the reference sample 5 _RE F-

In the same way a beam 27, in a second plane of incidence oriented at 90 degrees with respect to the first plane of incidence. The beam 27 is initially polarized linearly. After crossing the converter 1 0 the polarization state is radial and in a p-type polarization direction. After crossing the objective 7, reflection on the sample 5, the state of polarization of the beam remains p up to the second set of converter 1 0 '(in the top view). The axes {x ₂ , y _Σ ) of the second converter 1 0 'being oriented at 90 degrees of the axes {xi, yi) of the first converter 10, the beam 27 becomes after passing through the converter 1 0', a transverse polarization beam or s. This polarized beam s is then incident on the reference sample 5 _RE F- The same beam is incident in polarization directions respectively p on the sample 5 to be observed then s on the reference sample 5REF (and vice versa)

Upstream and downstream of this device are the illumination and detection arms 2 of a conventional ellipsometric imager. Preferably, a polarizer 13, a compensator 15, and an analyzer 14 will be used, which corresponds to the configuration of a conventional cancellation ellipsometer. Preferably, the linear polarizer 13 is oriented such that the axis of the vector polarization incident on the first converter 10 is inclined at an angle of between 20 and 70 ° relative to the y axis _1; and even more preferably, inclined at 45 ° +/- 10 °.

The device may furthermore comprise optical separation means (semi-transparent mirror, separating plate 6 or the like), for separating the incident beam from the beam reflected by the sample. These beam splitting means are preferably arranged between the polarizer 13 and the converter 10 on the one hand and between the converter 10 and the analyzer 14 on the other hand.

4) Ellipsometer and microscope combination

In order to have superior performance in terms of the quality of the ellipsometric measurements, it is known that it is preferable to prefer the ellipsometers with modulation to the ellipsometers with cancellation. In order to benefit from the accuracy of a modulation ellipsometer, it is possible to modify a microscope by inserting an ellipsometric illumination arm 2 downstream of the light source, an ellipsometric analysis arm 3 upstream of the detector 4 and a polarization converter. 10 immediately upstream of the objective 7. One of the ellipsometric arms preferably comprises a dynamic polarization modulator. The components of the illumination and analysis arm are oriented with respect to the u _x , u _{y mark} of the converter, in the same way as they are with respect to the directions p and s. The ellipsometric quantities are calculated in a conventional manner.

It is also noted that in all the areas of the microscope where the light rays have a small inclination with respect to the normal, it is possible without difficulty to introduce polarization components having Cartesian symmetry, since the cylindrical symmetry polarization effects related to the incidence on the lenses, are low. This is the case of the zones before and after the separator of an episcopic microscope, respectively on the side of illumination and observation (illumination conduit and "tube" of a confocal microscope). The polarization control and modulation components are therefore preferably arranged in these zones.

In a number of ellipsometers (with photoelastic modulator or rotating compensator), one of the arms (illumination or detection) comprises a single polarizer whose linear polarization axis is oriented at 45 ° degrees of the direction P. It is advantageous to use the polarizer present in a microscope and oriented so that the linear polarization incident on the polarization converter is located at 45 ° (within +/- 15 degrees) of the axis u _x of the converter and we add a polarization-modulated ellipsometric detection arm at or after the microscope tube, which is the part of the optical path close to the eyepiece and / or the viewing camera. Alternatively, it may be preferable to add the modulation on the illumination arm after the source and to use the analyzer as an ellipsometric analysis arm, oriented so that the linear polarization incident on the polarization converter is located at 45 °. (within +/- 15 degrees) of the axis u _x of the converter.

Because of this orientation at about 45 °, it is noted that the state of polarization of the light incident on the sample is not purely radial of purely azimuth, but comprises a combination of the two components of radial and azimuthal polarization . The device of the invention thus makes it possible to generate an incident beam having an incident radial and azimuth combined polarization state, which is different from the state of polarization generated in prior devices where a polarization converter is used essentially as a polarizer. either radial or azimuthal. 5) Combination Mueller-meter and microscope

Similarly, a device that measures the Muller matrix and a microscope can be combined. For example, the measurement device of the Mueller matrix marketed by Jobin Yvon under the name of ellipsometer MM16, comprising an illumination arm and a detection arm can be arranged with downstream a polarization converter and a large lens. aperture (NA> 0.5 preferably NA> 0.8) or a microscope objective (also of great aperture, the best being NA> = 0.9 but one can start working from NA> 0.5).

According to a variant of this embodiment, a microscope and a Mueller imager are combined, as described in "Mueller Polarimetric Imaging System with Liquid Crystals" B. Laude-Boulesteix, A. De Martino, B. Drévillon, and L. Schwartz, Applied Optics, Vol. 43, Issue 14, pp. 2824-2832 (2004).

II. Variants of the invention We will now see some particular modes of operation that apply to all of the above devices

1) Spectroscopic operation According to a particular embodiment, as is conventional in ellipsometry, one works at a fixed wavelength, corresponding to the emission wavelength of the source (laser or diode for example), or to a length of spectrally filtered wave by means of a filter or a monochromator.

According to another particular embodiment, a spectrometer is advantageously used at the detection level to obtain spectroscopic measurements.

2) Operation in multi-wavelength imaging

When ellipsometric contrast imaging is performed with an extinction ellipsometer, the extinction setting generally operates over a limited range of wavelengths because the ellipsometric quantities are wavelength-dependent (and also because of the chromaticism of certain optical components, such as a delay plate). In this case, the wavelength range is adjusted so as to obtain good extinction conditions.

However, when transparent and very thin objects are observed on transparent substrates, such as glass, whose index varies slightly with the wavelength, the quenching conditions can be stable over a wide range of times. wave length. An achromatic compensator is then preferably used to benefit from a high ellipsometric contrast in white light.

To be able to obtain ellipsometric information resolved spectroscopically on the objects placed on the transparent substrate, a conventional color camera is advantageously used, or several cameras equipped with wavelength filters.

3) Calibration procedures

When crossing the microscope objective with a high aperture, the light rays generally undergo a phase shift that depends on the polarization of the wave, the eigenpolarizations corresponding to a linear polarization in the radial plane of the lens (marked by u _r ), or its azimuthal plane (indicated by ιι _φ ). Let us call in general t _r and t ₉ the transmission coefficients for these two eigenpolarizations over the entire "cylindrical symmetry" domain delimited by the polarization converter, corresponding to the round trip in the objective for a system in reflection or crossing the condenser and lens for a transmission system). The coefficients t _r and t ₉ depend on the distance r to the axis 8 of the objective 7 (which can be connected to the angle of incidence Θ when the field diaphragm is closed enough).

The ellipsometric measurement actually gives no r _p / r _s but (r _p .t _r ) / (r _s .t ₉ ). We measure t _r / t ₉ by performing an ellipsometric measurement on a reference substrate, for different angles of incidence Θ. We can then correct the ellipsometric measurement of the effect of the objective.

4) Multi-angle of incidence

In the case where very thin samples are observed on a substrate, it is desired to increase the luminous flux intended for observation. According to a particular embodiment, there is a delay blade (commonly called quarter wave plate) in the plane of the opening diaphragm. It is preferable to use a blade whose axes are uniform in a Cartesian coordinate system, but whose thickness varies as a function of the distance to the optical axis in order to introduce a delay adapted to the angle of incidence. This blade makes it possible to preserve good cancellation conditions of the ellipsometric bottom.

According to another particular embodiment, in the plane of the aperture diaphragm, a color filter comprising three color rings as well as diaphragm rings is also available to allow three different incidence ranges to be associated with three different colors. Direct observation of the image obtained by means of a three-color camera (RGB for example) makes it possible to extract images corresponding respectively to the different angles of incidence.

5) Scalerometer mode operation

According to another embodiment of the invention, rather than analyzing the specularly reflected radiation, it is also possible to analyze the light scattered angularly. For this purpose, an annular aperture diaphragm is used and the polarization is analyzed by means of a Bertrand lens type lens placed in the tube of the microscope, according to an ellipsometric signal analysis configuration at normal incidence.

6) Joint use of an ellipso-microscope or an ellipsometric contrast microscope with a device with interferential contrast (also called "Nomarsky")

According to yet another embodiment of the invention, the benefits of ellipsometric and Nomarski contrast imaging are combined by modifying a microscope equipped with a Nomarski contrast device (also called Differential Interference Contrast) to add to it:

^* A polarization converter between the Nomarski prism and the objective, the converter being centered on the axis of the lens, its "Cartesian" side being the side of the Nomarski, its face "cylindrically symmetric" being on the side of the objective, and its proper directions u _x , u _y , being substantially parallel to the natural directions of the Nomarski prism, and at 45 degrees of the axes of the polarizer; ^* An annular aperture stop, for selecting a range of incidence;

^* an orientable delay blade located before the analyzer.

This system is used by looking for the cancellation conditions of the ellipsometric contrast and then inserting the Nomarski prism.

According to a variant of this embodiment, the role of the analyzer and the polarizer can be exchanged, provided that the delay plate is placed after the polarizer.

7) Use as a wide area, high incidence imager

By using a high numerical aperture objective and low magnification, a large field imaging ellipsometer is obtained.

As explained above, the principle of the invention allows new and very promising applications.

The double passage of the light through the converter to make a "Cartesian to Cylindrical" conversion on the one side and a "Cylindrical to Cartesian" conversion on the other hand avoids the use of a radial analyzer and a system for processing data. complex signal dedicated to a radial geometry. On the contrary, the device of the invention makes it possible to use a linear analyzer and standard detection means.

Unlike prior polarimetric polarization converting devices, the invention uses at the sample level a generalized cylindrical type polarization distribution beam, comprising any polarization state, which is neither purely radial, nor ie p, nor purely circumferential, ie s. This is fundamental in ellipsometry, since we are interested in the ratio r _p on r _s . However, in the previous polarimetric devices which exploit only one or the other of polarization states p (in radial) or s (in azimuth), it is not possible to extract information on the report. r _p on r _s .

In the field of ellipsometry, where the invention makes it possible to obtain ellipsometric measurements with a high optical magnification while maintaining an excellent brightness, which makes it possible to obtain measurements with a micrometric resolution on the sample.

In the field of microscopy, it becomes possible to modify a standard microscope to transform it into a micro-ellipsometer by inserting a polarization converter accessory. Finally, in the field of imaging microscopes, the invention makes it possible to improve the contrast of the images by an efficient ellipsometric contrast device.

Claims

CLAIMS. A polarimetric amplitude and phase measurement device of the ellipsometric measurement or Mueller ellipsometry measurement type or a scalerometric measurement, said polarimetric measurement device having a microscopic resolution and comprising:

an excitation portion comprising:

a light source (1) capable of emitting an incident light beam over a range of wavelengths and

a polarization state generator (2, 13) capable of generating an incident optical beam having a spatially uniform polarization state distribution;

optical focusing means (7) having an optical axis (8) for focusing said incident light beam onto the surface of a sample (5) to be measured;

an analysis part comprising:

optical collecting means (7) for collecting the light beam reflected or transmitted by the sample (5), said collection optical means (7) having an optical axis (8);

a polarization state analyzer (3, 14, 15 ') adapted to receive the optical beam collected by said collection optical means (7) and to analyze the polarization state of said optical beam according to a Cartesian base of states polarization; and

detection means (4) able to receive said light beam analyzed by the analyzer in polarization;

said polarization state generator (2, 13) and / or said polarization state analyzer (3, 14) further comprising bias modulation means (15, 15 '),

characterized in that

said optical focusing means (7) are arranged in such a way that said optical axis (8) of said optical focusing means (7) coincides with the normal to the sample (5);

said collection optical means are arranged in such a way that said optical axis (8) of said collection optical means is coincident with the optical axis of said reflected or transmitted light beam;

the excitation part comprises first polarization conversion means (10) arranged between said polarization state generator (2, 13) and said optical focusing means (7), said first polarization converting means (10) being oriented to receive said spatially uniform incident polarization incident optical beam and converting it into a beam having a polarization vector distribution having cylindrical or semi-cylindrical symmetry about the optical axis and wherein each polarization vector comprises a linear combination of a non-zero radial component and a non-zero azimuthal component; and

the analysis part comprises second polarization conversion means (10, 10 ') arranged between said collection optical means (7) and said polarization state analyzer (3, 14, 15'), said second conversion means biasing means (10, 10 ') being oriented to receive said collected light beam, said second polarization converting means being adapted to convert a beam having a cylindrical symmetry polarization vector distribution according to which a polarization vector comprises a linear combination of a radial component and an azimuthal polarization component into a beam having a spatially uniform polarization state distribution.

Polarimetric measuring device according to claim 1, characterized in that the first polarization converting means and the second polarization converting means are formed by a single polarization converter (10) used in two opposite propagation directions and in that that said optical focusing means (7) and collection (7) are combined.

3. polarimetric measuring device according to claim 2 characterized in that said polarization converter (10) is a liquid crystal converter comprising a face with Cartesian symmetry and a cylindrical symmetry face.

4. polarimetric measuring device according to claim 2 characterized in that said polarization converter is a polarization converter capable of converting the Cartesian polarization state distribution to cylindrical while maintaining the state of polarization of the beam.

Polarimetric measuring device according to one of claims 1 to 4, characterized in that the polarization state generator (13, 15) is a spatially uniform linear polarizer and in that the polarization axis of said linear polarizer is oriented with respect to an axis own first polarization converting means (10) so as to form a non-zero angle.

6. polarimetric measuring device according to one of claims 1 to 5 characterized in that the polarization state analyzer is a spatially uniform linear analyzer and in that the polarization axis of said linear analyzer is oriented relative to an own axis of the second polarization converting means so as to form a non-zero angle.

7. polarimetric measuring device according to one of claims 2 to 6 characterized in that it comprises optical means of separation (6) of the beam incident on the sample and the beam reflected or transmitted by the sample, said means separation optics (6) being arranged in the optical path between the polarization state generator (13, 15) and the polarization converter (10).

8. Measuring device according to one of claims 1 to 7 characterized in that it further comprises a field diaphragm (17a), an opening diaphragm (16, 16a) and / or spectroscopic analysis means detected signals.

9. Device according to one of claims 3 or 4 characterized in that said polarimetric measurements are ellipsometric measurements, Mueller ellipsometry measurements or scalerometry measurements, the matrix of said polarization conversion means (10, 10 '). ) expressing the coordinates in the cylindrical coordinate system of the polarization of an emergent beam of the cylindrical symmetry face, as a function of the coordinates in the cartesian coordinate system of the polarization of the incident beam on the Cartesian symmetry face, being diagonal of the type:

10. Device according to one of claims 1 to 8 characterized in that it comprises a Nomarski prism, the converter (1 0) being located between the Nomarski prism and said optical focusing means (7).

1 1. Amplitude and phase polarimetric measurement accessory of the ellipsometric measurement or Mueller ellipsometry measurement type or microscale scalerometric measurement comprising: a polarization state generator adapted to receive an incident optical beam and to generate an optical beam having a spatially uniform polarization state distribution;

a polarization state analyzer according to a Cartesian polarization state base;

a radial or cylindrical polarization converter arranged on the optical path between the polarization state generator (13, 15) and the microscope objective (7) on the one hand and between the microscope objective (7) and the polarization state analyzer (14, 15 ') on the other hand,

said converter (10) being oriented with respect to the polarization state generator so as to convert a beam propagating in a first axial propagation direction and having a spatially uniform polarization state distribution into a beam having a vector distribution of polarization of cylindrical symmetry around the optical axis according to which each polarization vector comprises a linear combination of a non-zero radial component and a non-zero azimuth component and vice versa said converter ensuring the conversion of a beam propagating in a second contra-propagative axial direction at the first direction and having a polarization vector distribution having cylindrical symmetry about the optical axis in a beam having a spatially uniform polarization state distribution.

An imaging microscope-ellipsometer comprising an accessory according to claim 11, a microscope objective (7) having a numerical aperture greater than or equal to 0.7 and an imaging detector.

An ellipsometric contrast microscope comprising an accessory according to claim 11 comprising:

a first converter (10) for directing an optical beam towards a sample (5) to be measured,

a second converter (10 ') for directing the reflected or transmitted optical beam from the sample (5) to a reference substrate (5 _RE F),

said first and second converters (10, 10 ') having their own axes oriented at 90 degrees relative to one another.

14. A polarimetric measurement method in amplitude and in phase of the ellipsometric measurement or Mueller ellipsometry measurement type or a scalerometric measurement comprising the following steps:

the polarization vector distribution of said incident beam is converted into a polarization state distribution having a cylindrical or semi-cylindrical symmetry about the optical axis, wherein each polarization vector comprises a linear combination of a radial component non-zero and a non-zero azimuthal component;

- Focusing said polarization beam with cylindrical or semi-cylindrical symmetry on the surface of a sample (5) to be measured along an optical axis (8) coincident with the normal to the sample (5);

the light beam reflected or transmitted by the sample (5) is collected;

the analyzed beam is detected.

15. Polarimetric measurement method according to claim 14, characterized in that it further comprises one of the following steps:

the detected beam is spectroscopically analyzed;