Abstract:
Device for characterizing a sample includes a measuring instrument for determining a physical characteristic of the sample at one point thereof; a positioning system for positioning the measuring instrument relative to the sample, to obtain a measurement at a point localized on the sample. The positioning system includes: a locating target connected to the sample and defining a reference system linked thereto; elements for acquiring and analyzing images, including lighting elements for illuminating the target; an optical imaging system connected to the measuring instrument for acquiring an image of at least one portion of the target; and image analysis elements for analyzing the image to determine the position and orientation of the optical imaging system relative to the target; calibration elements for determining the position of the measuring instrument relative to the optical imaging system; and processing elements for processing the results of the image analysis and of the calibration.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a characterization device intended to measure a physical characteristic of a sample. 
     The invention more particularly relates to a characterization device including at least a measuring instrument and a positioning system allowing to position in an absolute manner the measuring instrument in relation to the sample. 
     The invention also relates to a characterization device including two measuring instruments, the positioning system allowing to perform co-localized measurements on the sample by means of the two measuring instruments. 
     With the coming of nanotechnologies, the capacity to perform extremely accurate measurements in order to control the manufacturing, the operation and the ageing of different components at the nanometer scale has become essential. A recurrent problem is then the “co-localization” of the measurements performed with different measuring instruments or at different instants of time. 
     By co-localization of the measurements, it is meant the possibility to perform different measurements at the same places of a sample. 
     The co-localization of measurements requires, on the one hand, a high spatial accuracy, i.e. the measuring instrument has to be very accurately positioned in relation to a sample to be characterized during a punctual measurement. 
     On the other hand, it requires a high repeatability of the measurements. It is indeed essential to be able to perform the same measurement at different instants and to find the same result for a stable sample. 
     It is known in the field of metrology that a sample characterization device includes a measuring instrument adapted to determine a physical characteristic of this sample at one point of the latter. 
     When using such a characterization device, it is useful to correctly place the sample in relation to the measuring instrument so as to perform the measurement at one particular point of the sample. 
     DESCRIPTION OF THE RELATED ART 
     A characterization device is known, for example, from the document U.S. Pat. No. 7,630,628, which also includes a positioning system allowing to position the measuring instrument in relation to the sample to be characterized at one localized measurement point of the sample. The positioning system of the document U.S. Pat. No. 7,630,628 comprises in particular a sample carrier plate on which the sample has been placed, with piloting means for displacing this plate in an accurate and repeatable manner. This allows, if the sample is made integral with the sample carrier plate and if the sample is not handled between two measurements, to perform two measurements in two substantially identical measurement points. 
     However, the positioning system of the document U.S. Pat. No. 7,630,628 does not allow to accurately know the position of the measuring instrument in relation to the sample, i.e. to position in an absolute manner the measuring instrument in relation to the sample. 
     Moreover, a characterization device is known from the document U.S. Pat. No. 5,177,110, which includes a positioning system allowing to accurately position the measuring instrument above a determined point of a sample to be characterized. 
     The document U.S. Pat. No. 5,177,110 discloses a characterization device in which the position system, herein formed of an optical microscope, is used in a first time, and the measuring instrument, including a scanning tunneling microscope, in a second time. The positioning and measurement operations are hence not performed simultaneously. 
     One of the objects of the invention is to make so that measurements at the nanometer scale are co-localizable for a great variety of measuring instruments, hence performing a multimodal characterization of a sample, i.e. analysing the latter by different techniques. 
     SUMMARY OF THE INVENTION 
     To remedy the above-mentioned drawback of the prior art, the present invention proposes a device for characterizing a sample, allowing to position in an absolute manner a measuring instrument in relation to the sample. 
     For that purpose, the invention relates to a device for characterizing a sample including:
         a measuring instrument adapted to determine a physical characteristic of said sample at one point of said sample, and   a positioning system adapted to position said measuring instrument in relation to said sample to be characterized, at one localized measurement point of said sample,       

     characterized in that said positioning system comprises:
         a localization target made integral with said sample and defining a referential system linked to said sample,   image acquisition and analysis means including:
           means for illuminating said localization target;   an optical imaging system, integral with said measuring instrument, adapted to acquire an image of at least one portion of said localization target, and   image analysis means adapted to analyse the image of said portion of the localization target to determine the position and orientation of said optical imaging system in relation to said localization target,   
           calibration means adapted to determine the relative position of said measuring instrument in relation to said optical imaging system, and   means for processing the results of the image analysis and of the calibration, adapted to determine the absolute position of said localized measurement point in said referential system linked to said sample, said measuring instrument being positioned for the measurement at said localized measurement point and said physical characteristic of said sample being determined by said measuring instrument at said localized measurement point.       

     The characterization device according to the invention hence allows, thanks to its positioning system, to locate the localized measurement point of said sample in a referential system linked to the sample. 
     Indeed, the acquisition and analysis of the image of a portion at least of the localization target by the image acquisition and analysis means of the positioning system allows to know accurately what are the position and orientation of the optical imaging system in relation to the localization target, and hence in relation to the sample, the localization target being made integral with the sample. 
     By determining moreover, thanks to the calibration means, the relative position of the measuring instrument in relation to the optical imaging system, i.e. the position of the measuring instrument in a referential system linked to the optical imaging system, the positioning system can determine, when the measuring instrument is positioned for the measurement at said localized measurement point, what is the absolute position of the localized measurement point in a referential system linked to the sample. 
     Thanks to the characterization device according to the invention, neither the sample to be characterized, nor the localization target, are moved between the moment where the position of the localized measurement point is determined by the positioning system, and the moment where the measurement of the sample is made by the measuring instrument. The positioning and sample measurement operations are hence performed simultaneously. 
     The positioning system allows to position the measuring instrument at any localized measurement point of the sample by reading the corresponding position on the localization target. The characterization device hence allows to make an accurate cartography of a portion or the totality of the sample to be characterized. 
     Moreover, the characterization device according to the invention allows to perform with the measuring instrument two time-separated measurements on a sample, and that at the same localized measurement point, even if the sample has been displaced in relation to the measuring instrument between the two successive measurements. 
     The characterization device according to the invention is more particularly adapted to the case where it is desired to characterize a same sample by two different measuring instruments, whether these two measuring instruments perform a measurement at a same localized measurement point or at two distinct localized measurement points. 
     Hence, the invention also relates to:
         a characterization device including another measuring instrument adapted to determine another physical characteristic of said sample at one point of said sample, said positioning system being adapted to position said other measuring instrument in relation to said sample at a second localized measurement point of said sample, said optical imaging system being also integral with said other measuring instrument, said calibration means of said positioning system being adapted to determine the relative position of said other measuring instrument in relation to said optical imaging system, and said result processing means being adapted to determine the absolute position of said second localized measurement point in said referential system linked to said sample, said other physical characteristics of said sample being determined by said other measuring instrument at said second measurement point;   a characterization device including another measuring instrument adapted to determine another physical characteristic of said sample at one point of said sample, said positioning system being adapted to position said other measuring instrument in relation to said sample at a second localized measurement point of said sample and comprising another optical imaging system, integral with said other measuring instrument, adapted to acquire another image of at least one portion of said localization target, said image analysis means being adapted to analyse said other image of the portion of said localization target to determine the position and orientation of said other optical image system in relation to said localization target, said calibration means of said positioning system being adapted to determine the relative position of said other measuring instrument in relation to said other optical imaging system, and said result processing means being adapted to determine the absolute position of said second localized measurement point in said referential system linked to said sample, said other physical characteristic of said sample being determined by said other measuring instrument at said second measurement point, and   a characterization device including another measuring instrument adapted to determine another physical characteristic of said sample at one point of said sample, said positioning system being adapted to position said other measuring instrument in relation to said sample at a second localized measurement point of said sample and comprising:
           another optical imaging system, integral with said other measuring instrument, adapted to acquire another image of at least one portion of said localization target, said image analysis means being adapted to analyse said other image of the portion of the localization target to determine the position and orientation of said other optical imaging system in relation to said localization target, and   other calibration means adapted to determine the relative position of said other measuring instrument in relation to said other optical imaging system,
 
said result processing means being adapted to determine the absolute position of said second localized measurement point in said referential system linked to said sample, said other physical characteristic of said sample being determined by said other measuring instrument at said second measurement point.
   
               

     Hence, the characterization device has for advantage to propose a positioning system allowing to make many measuring instruments compliant with each other. 
     Such characterization devices may be used to perform co-localized measurements at the same place or at distinct places on the sample with different measuring instruments, separated in space and/or in time. 
     The characterization device hence allows to couple virtually different techniques of measurement implemented in different measuring instruments. This fulfils a need to perform multimodal studies on samples. 
     Furthermore, the characterization device may in particular include a measuring instrument determining, during a single measurement, said physical characteristic of the sample on an extended area substantially centred about said localized measurement point. 
     Among the measuring instruments that can be used in a characterization device as described above, it may be mentioned, for example, the following measuring instruments:
         wide-field or laser-scanning (confocal microscope), contrasting absorption, reflexion, elastic scattering or Raman, phase, interference, polarization or fluorescence digital optical microscope in the visible, ultraviolet and infrared regions;   local probe microscope (for example, an atomic force microscope);   scanning, transmission or scanning transmission electron microscope, Auger spectrometer, X photoelectron spectrometer;   mechanical profilometer;   surface plasmon resonance imaging system;   mass spectrometer;   X-ray absorption or fluorescence spectrometer;   cathodoluminescence spectrometer.       

     Moreover, other advantageous and non-limitative characteristics of the characterization device are as follows:
         said localization target is engraved on or in the sample through mechanical or photolithographic techniques, or is printed on the sample by inking or serigraphy;   said localization target comprises a flexible or rigid support added on the sample to make the support integral with said sample;   said localization target is formed of an adhesive sheet;   said localization target is made integral with the second face of a substantially planar sample including a first face and a second face, said localized measurement point being located on said first face of said sample;   said localization target extends spatially over a localization area that is greater than a measurement area of said sample intended to be characterized by said measuring instrument;   said localization target extends over the whole second face of said sample;   said localization target is manufactured simultaneously with said sample;   said localization target includes micro- or nanostructured patterns;   said localization target is formed of a plurality of elementary cells forming a regular two-dimensional pavement;   each elementary cell includes a positioning pattern indicating the position of said elementary cell in said referential system linked to said sample, and an orientation pattern indicating the orientation of said elementary cell in said referential system linked to said sample;   each elementary cell includes periodic patterns allowing to improve the accuracy of positioning of said elementary cell in relation to said sample;   each elementary cell includes an identification pattern coding information relating to said sample and/or said localization target;   said identification pattern is identical for each of said elementary cells;   said optical imaging system is arranged in such a manner that said image of the portion of said localization target includes an image of said periodic patterns;   said optical imaging system is arranged in such a manner that said image of the portion of the localization target includes an image of said identification pattern.       

     The use of a localization target having micro- or nanostructured patterns allows to reach an accuracy of positioning of the measuring instrument in relation to the sample lower than 0.1 micrometer (μm). This reveals particularly advantageous, particularly for measuring instruments of the Raman microspectrometer, atomic force microscope or electron microscope type, for example. 
     The invention especially relates to a device for characterizing a substantially planar sample including a first face and a second face, in which:
         said measuring instrument includes an optical microscope that comprises a place intended to receive a condenser when said optical microscope is used in trans-illumination mode;   said localization target is made integral with said second face of said sample, said localized measurement point being located on said first face of said sample, and   said optical imaging system is arranged at the place of said condenser.       

     The invention finally relates to a method for characterizing a sample, by means of a measuring instrument adapted to determine a physical characteristic at one point of said sample, including steps consisting in: 
     a) determining the relative position of said measuring instrument in relation to an optical imaging system, integral with said measuring instrument, 
     b) placing the sample to be characterized in such conditions to be measured by said measuring instrument at one localized measurement point of said sample, wherein a localization target has been made integral with said sample, said target defining a referential system linked to said sample; 
     c) illuminating said localization target and acquiring, by means of said optical imaging system, an image of at least one portion of said localization target, 
     d) determining, from the analysis of the image of said portion of the localization target, the position and orientation of said optical imaging system in relation to said localization target, and 
     e) deducing from steps a) and d) the absolute position of said localized measurement point in said referential system linked to said sample, when said measuring instrument is positioned for the measurement at said localized measurement point, so as to determine said physical characteristic of said sample by said measuring instrument at said localized measurement point. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Embodiments of the invention will be described in detail with reference to the drawings, in which: 
         FIG. 1  is a schematic view of a characterization device including a measuring instrument and a positioning system; 
         FIG. 2  is a schematic view of a first embodiment of the characterization device including an optical microscope as a measuring instrument; 
         FIG. 3  shows a schematic view of the structure of a localization target added on the sample as a label; 
         FIG. 4  is a schematic view of a sample including on its lower face a localization target having a regular arrangement of elementary cells; 
         FIG. 5  is a detail view of four elementary cells of  FIG. 4  including micro-structured patterns; 
         FIG. 6  is a schematic view of an elementary cell of  FIG. 5 ; 
         FIG. 7  shows a schematic view of the elementary cell of  FIG. 6  indicating how some information is coded in two areas of this elementary cell; 
         FIG. 8  is a detail view of the elementary cell of  FIG. 6 ; 
         FIG. 9  is a schematic view of a target imaging system including an optical imaging system and illumination means; 
         FIG. 10  is a schematic view of an array of detectors and of an associated image-reference system; 
         FIG. 11  is a schematic view of an image of the localization target of  FIG. 4 , a portion of which is acquired by the array of detectors of  FIG. 10 ; 
         FIG. 12  is a detail view of  FIG. 11  showing the array of detectors and the image of a portion of the localization target; 
         FIG. 13  is a detail view of  FIG. 12  showing the image of a few elementary cells and the reference systems of the array of detectors and of the localization target; 
         FIG. 14  shows a schematic view of an example of calibration target that can be used during the step of calibration of a characterization device; 
         FIG. 15  shows a schematic view of the characterization device of  FIG. 1  during the step of calibration; 
         FIG. 16  is a schematic view of a second embodiment of the characterization device including an optical microscope equipped with two lenses; 
         FIG. 17  is a schematic view of a third embodiment of the characterization device including an atomic force microscope and an optical microscope. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a schematic view of a characterization device  1  including a measuring instrument  2  and a positioning system  3 . The characterization device  1  of  FIG. 1  serves to characterize a sample  11  placed in the device. 
     The measuring instrument  2  allows to determine a physical characteristic of the sample  11  at one point of the latter. 
     The positioning system  3  allows to position the measuring instrument  2  in relation to the sample  11  to be characterized, the measuring instrument  2  performing the measurement of the physical characteristic at one localized measurement point of the sample  11 . 
     The characterization device  1  of  FIG. 1  also includes means  13  for processing the results of the image analysis and of the calibration that deduce, based on information received from the measuring instrument  2  and from the positioning system  3 , the absolute position of the localized measurement point in a referential system linked to the sample  11 . The physical characteristic of the sample  11  is hence determined by the measuring instrument  2  at the localized measurement point. 
     In  FIGS. 2, 16 and 17  are shown different embodiments of a characterization device  10 ,  110 ,  210  intended for the characterization of the sample  11 . 
     This sample  11  is consisted of a piece of a silicon wafer having an area of interest  11 C in which are engraved microelectronic circuits. 
     The sample  11  is substantially planar and has a first face  11 A and a second face  11 B. 
     Hereinafter, the first face  11 A will be called the upper face and the second face  11 B the lower face. 
     The upper face  11 A is the face of the sample  11  on which are performed the measurements performed by the different characterization devices  10 ,  110 ,  210 . 
     The sample  11  is square-shaped and has a width of 50 millimeters (mm) and a length of 50 mm. Its thickness is herein equal to 275 micrometers (μm). 
     The microelectronic circuits, which are engraved on the upper face  11 A of the sample  11 , at the area of interest  11 C, have characteristic sizes of the order of a few hundreds of nanometers (1 nm=10 −3  micrometers). 
     The characterization devices  10 ,  110 ,  210  of  FIGS. 2, 16 and 17 , respectively, each include an identical positioning system  30 . 
     The positioning system  30  first includes a localization target  31 , which is also substantially planar. The localization target  31  comprises in particular a flexible support, in the form of an adhesive sheet, added on the sample  11  to make the localization target  31  integral with the latter. By integral, it is meant that the localization target  31  does not move with respect to the sample  11  during the characterization thereof. 
     Advantageously, the localization target  31  has stable dimensional and physical properties at a time scale of the order of several months to a few years. The localization target  31  is preferably resistant to the effects of temperature and humidity variation under laboratory conditions, and withstands high vacuum conditions. 
     As a variant, the positioning system could for example include a substantially planar sample carrier, with an upper face and a lower face. In this case, the sample may be fixed on the upper face of the sample carrier and the localization target may be fixed on the lower face thereof. 
     The localization target  31  is in the form of an adhesive label fixed to the lower face  11 B of the sample  11 . 
     In  FIG. 3  is shown a schematic view of the cross-sectional structure of the localization target  31 , with the different layers included in the adhesive label, before the latter is stuck on the sample  11 . 
     The adhesive label includes five layers  31 A,  31 B,  31 C,  31 D and  31 E, and its total thickness is herein lower than 200 μm. 
     The first layer  31 A is formed of a protective film for the second layer  31 B formed by an adhesive layer. During the sticking of the localization target  31  on the sample  11 , the protective film  31 A is removed so that the adhesive layer  31 B can be applied, by pressure, against the lower face  11 B of the sample  11 . The second layer  31 B may be used to stick the localization target  31  on a great number of supports such as: glass, metal, plastic, crystal, semi-conductor or ceramic. The adhesion of the adhesive layer  31 B is such that the localization target  31  does not unstick from the sample  11  during the whole duration of the characterization of the sample  11  by the characterization device  10 ,  110 ,  210 . 
     The third layer  31 C is an opaque layer, optically occulting for the light in the visible region. This occulting layer  31 C prevents the light from passing through the localization target  31 , either by absorbing it or by reflecting it. 
     The fourth layer  31 D is a layer including patterns and having an optical contrast allowing to encode some information in the localization target  31 . The arrangement of the patterns in this optically contrasted layer  31 D will be seen hereinafter. 
     The fifth and last layer  31 E is a protective layer of the optically contrasted layer  31 D. It allows to protect the fourth layer  31 D during the application of the localization target  31  against the lower face  11 B of the sample  11 . This fifth layer  31 E is optically transparent in the wavelength range for which the fourth layer  31 D has an optical contrast, which allows the optically contrasted layer  31 D to be visible when observed through the last layer  31 E. 
     The fifth layer  31 E is herein optically transparent for the light in the visible region. 
     As a variant, the localization target could be engraved on or in the sample by means of mechanical or photolithographic techniques. 
     As another variant, the localization target could be printed on the sample by inking or by serigraphy. 
     As a variant, the localization target is a glass microscope slide, on which the patterns are made by photolithography, the sample being applied and/or stuck on the slide. 
     In  FIG. 4  is shown a schematic bottom view of the sample  11 , making appear the lower face  11 B of the sample  11  as well as the localization target  31  that is stuck thereon. The localization target  31  extends over the major portion of the lower face  11 B of the sample  11 . 
     Advantageously, the localization target  31  extends spatially over a localization area that is greater than the area of interest  11 C that constitutes a measurement area of the sample  11  intended to be characterized by the characterization device  10 ,  110 ,  210 . 
     As a variant, the localization target could for example extend over the whole lower face of the sample. 
     In all the embodiments of the invention, the localization target  31  includes a lateral patterning at the micro- or nanometer scale, with micro- or nanostructured patterns. These patterns correspond to the patterns of the above-described optically contrasted layer  31 D. 
     Hence, the localization target  31  is herein formed of a plurality of elementary cells  310  forming a regular two-dimensional pavement, in the plane of the localization target  31 . The elementary cells  310  are hence periodically distributed along the two orthogonal directions  11 X,  11 Y, herein represented in  FIG. 4  by the axes X mire  and Y mire  perpendicular to each other. 
     Moreover, the point  31 R located at the top left corner (see  FIG. 4 ) of the localization target  31  is considered. This point  31 R constitutes a fixed reference point of the localization target  31 , which is integral with the sample  11 . This reference point  31 R is hence also a point linked to the sample  11 . 
     Hence, the reference point  31 R, the axes  11 X and  11 Y form together a referential system  31 R,  11 X,  11 Y linked to the sample  11 , in relation to which it is possible to locate in an absolute manner any point of this sample  11 . 
     In  FIG. 5  is shown a detail view of four elementary cells  3101 ,  3102 ,  3103 ,  3104  of the localization target  31 , these four elementary cells  3101 ,  3102 ,  3103 ,  3104  being those which are indicated in  FIG. 4  by the black square  3100 . Each elementary cell  3101 ,  3102 ,  3103 ,  3104  has herein a squared shape of about 200 μm side. 
     Preferably, each elementary cell  3101 ,  3102 ,  3103 ,  3104  has dimensions along the axes  11 X,  11 Y comprised between 40 μm and 1 mm. 
     Each elementary cell  3101 ,  3102 ,  3103 ,  3104  includes different micro-structured patterns, whose function will be detailed hereinafter. 
     For that purpose, it is considered in  FIG. 6  the elementary cell  3101  which is the cell located on the top left corner of the square  3100  of  FIG. 5 . 
     This elementary cell  3101  may be divided into four distinct sub-cells  3101 A (sub-cell in the top left corner of the elementary cell  3101 ),  3101 B (in the top right corner),  3101 C (in the bottom left corner), and  3101 D (in the bottom right corner). Each sub-cell  3101 A,  3101 B,  3101 C,  3101 D is herein square and of about 100 μm side. 
     Sub-Cell  3101 A ( FIG. 6 ) 
     The sub-cell  3101 A of the elementary cell  3101  is first considered. This sub-cell  3101 A may be itself subdivided into 5×5=25 sub-sub-cells, as shown in  FIG. 7 . 
     The four sub-sub-cells located in the top left corner of the sub-cell  3101 A include an orientation pattern  3101 A 1 . The orientation pattern  3101 A 1  has herein the shape of a right-angle square, each branch of the square having a length equal to the size of a sub-sub-cell of the sub-cell  3101 A. 
     The shape of the orientation pattern  3101 A 1  provides the latter with the geometrical property that it is invariant by no rotation in the plane parallel to the plane of the localization target  31 . 
     The orientation pattern  3101 A 1  is arranged in the first four sub-sub-cells so as to define two orthogonal axes  3101 X and  3101 Y oriented in the direction of each of the branches of the square forming the orientation pattern  3101 A 1 . Hence, the orientation pattern  3101 A 1  defines an orthogonal reference system linked to the elementary cell  3101 . 
     In the case of  FIG. 7 , the orthogonal axes  3101 X and  3101 Y are each parallel to one side of the elementary cell  3101 , so that the two orthogonal axes  3101 X and  3101 Y are parallel to the two orthogonal axes X mire  and Y mire , respectively. 
     Hence, the orientation pattern  3101 A 1  indicates the orientation of the elementary cell  3101  in the referential system  31 A,  11 X,  11 Y linked to the sample  11 . 
     As a variant, the orientation pattern may be arranged in any way inside an elementary cell so that the reference axes defined by the orientation pattern are not parallel to the sides of the elementary cell to which it belongs. 
     It will be observed moreover in  FIGS. 4 and 5  that each elementary cell  310 ,  3101 ,  3102 ,  3103 ,  3104  includes an identical orientation pattern, of same shape and same size, arranged and oriented in the same manner in the elementary cell  310 ,  3101 ,  3102 ,  3103 ,  3104  in which it is located. 
     Hence, the orthogonal reference systems defined by the orientation patterns of the elementary cells  310 ,  3101 ,  3102 ,  3103 ,  3104  are all oriented in the same manner in relation to the localization target  31 . It will be noticed in particular herein that the axes  3101 X and  3101 Y of the elementary cell  3101  are parallel to the axes  11 X and  11 Y, respectively, of the localization target  31  (see  FIG. 4 ). 
     The twenty-one other sub-sub-cells of the sub-cell  3101 A code for a positioning pattern  3101 A 2 , whose coding principle will be described hereinafter. 
     The twenty-one other sub-sub-cells of the sub-cell  3101 A are numbered from  1  to  21 , the numbering being performed from top to bottom, and from left to right (cf.  FIG. 7 ). 
     As can be seen in  FIG. 8 , each sub-sub-cell can be either white (case of the sub-sub-cells # 1  to  3 ,  5  to  8 ,  10 ,  12  to  17 , and  19 ), or black (case of the sub-sub-cells # 4 ,  9 ,  11 ,  18 ,  20 , and  21 ). They hence form the positioning pattern  3101 A 2 . 
     This positioning pattern  3101 A 2  indicates the position of the elementary cell  3101  in the referential system  31 A,  11 X,  11 Y linked to the sample  11 . 
     For that purpose, it is assigned to each of the sub-sub-cells a binary digit (or “bit”), according to the following convention:
         if the sub-sub-cell is black, then the value of the bit associated with this sub-sub-cell is equal to 0;   if the sub-sub-cell is white, then the value of the bit associated with this sub-sub-cell is equal to 1.       

     Hence, thanks to the twenty sub-sub-cells numbered from  1  to  20 , two binary numbers can be formed as follows:
         a first binary number formed of the bits of the sub-sub-cells numbered from  1  (least significant bit) to  10  (most significant bit), and   a second binary number formed of the bits of the sub-sub-cells numbered from  11  (least significant bit) to  20  (most significant bit).       

     The first binary number codes for the position of the elementary cell  3101  along the axis  11 X and the second binary number codes for the position of the elementary cell  3101  along the axis  11 Y. 
     The bit of the 21 st  sub-sub-cell (numbered  21  in  FIG. 7 ) is used to correct possible decoding errors. It corresponds to the sum of the first twenty bits (sub-sub-cells # 1  to  20 ), i.e. it is equal to “1” if this sum is odd and equal to “0” if this sum is even. 
     As shown in  FIG. 8 , the positioning pattern  3101 A 2  of the elementary cell  3101  is such that:
         the first binary number is 0100001000,   the second binary number is 1010000001, and   the 21 st  bit is equal to 1 (the sum of the first twenty bits being odd, equal to 5).       

     It can be noticed in  FIG. 5  that the positioning pattern  3101 A 2  is different for each of the elementary cells  3101 ,  3102 ,  3103  and  3104 , such that it codes each times for a different position of the elementary cells  3101 ,  3102 ,  3103  and  3104  in the referential system  31 R,  11 X,  11 Y linked to the sample  11 . 
     Sub-Cells  3101 B and  3101 C ( FIG. 6 ) 
     In  FIG. 8 , it is observed that the elementary cell  3101  includes periodic patterns in each of the sub-cells  3101 B and  3101 C. These latter are indeed formed of a draughtboard with white squares  3101 B 1 ,  3101 C 1  and black squares  3101 B 2 ,  3101 C 2 . Each draughtboard comprises ten lines (or ten columns) of ten squares alternately white or black, i.e. a total of 100 squares. Each square of the draughtboard has herein an about 10 μm side. 
     Preferably, the squares of the draughtboard may have dimensions comprised between 2 μm and 50 μm. 
     As can be seen in  FIGS. 4 and 5 , each elementary cell  310 ,  3102 ,  3103 ,  3104  comprises two sub-cells formed similarly to the two sub-cells  3101 B and  3101 C, i.e. they have a draughtboard structure with white squares and black squares. 
     It will be seen hereinafter how these periodic patterns allow to improve the accuracy of positioning of the target imaging system  320  in relation to the sample  11 . 
     Sub-Cell  3101 D ( FIG. 6 ) 
     The sub-cell  3101 D may be subdivided into twenty-five sub-sub-cells, as shown in  FIG. 7 . These sub-sub-cells are numbered from  1  to  25 , the numbering being performed from top to bottom, and from left to right. 
     As for the sub-cell  3101 A, and as can be seen in  FIG. 8 , each sub-sub-cell may be either white (case of the sub-sub-cells # 1  and  2 ,  5  and  6 ,  12  to  20 ,  22 ,  23  and  25 ), or black (case of the sub-sub-cells # 3  and  4 ,  7  to  11 ,  21  and  24 ). 
     The first twenty-four sub-sub-cells of the sub-cell  3101 D hence form an identification pattern  3101 D 1  shown in  FIG. 8 . 
     The identification pattern  3101 D 1  of the elementary cell  3101  codes for information relating to the sample  11  and to the localization target  31 . 
     Indeed, as for the sub-cell  3101 A, one or several binary numbers are formed from the value of the bits associated with each of these sub-sub-cells of the sub-cell  3101 D. It is reminded that the value of the bit is equal to 0 if the sub-sub-cell is black, or equal to 1 if the sub-sub-cell is white. 
     In the embodiments of the invention, the first twenty-first sub-sub-cells numbered from  1  to  24  form two binary numbers as follows:
         a first binary number formed of the twelve bits of the sub-sub-cells numbered from  1  (least significant bit) to  12  (most significant bit), and   a second binary number formed of the twelve bits of the sub-sub-cells numbered from  13  (least significant bit) to  24  (most significant bit).       

     The first binary number herein codes for a reference of the sample  11  and the second binary number codes for the size of a square of the draughtboard of the sub-cells  3101 B and  3101 C. 
     The bit of the 25 th  sub-sub-cell (numbered  25  in  FIG. 7 ) is also used to correct possible decoding errors. It corresponds to the sum of the first twenty-four bits (sub-sub-cells # 1  to  24 ), i.e. it is equal to 1 if this sum is odd and equal to 0 if this sum is even. 
     As shown in  FIG. 8 , the identification pattern  3101 D 1  of the elementary cell  3101  is such that: 
     the first binary number is 100000110011,
         the second binary number is 011011111111, and   the 25 th  bit is equal to 1 (the sum of the first twenty-four bits being odd, equal to 15).       

     Preferably for the three embodiments of the invention, the identification pattern  3101 D 1  is identical for each of the elementary cells  310  of the localization target  31 . It is hence observed in  FIG. 5  that the identification pattern  3101 D 1  is for example the same for each of the elementary cells  3101 ,  3102 ,  3103 , and  3104 , such that it codes each times for the same information relative to the sample  11  and to the localization target  31 , herein the reference of the sample  11  and the size of a square of the draughtboard of the sub-cells B or C. 
     As a variant, the identification pattern could for example code for a reference of the localization target, for a scale of the elementary cell, for a code allowing to interpret correctly the information of localization of the localization target. The first twenty-four sub-sub-cells of the associated sub-cell are then used to form as many binary numbers as required. 
     As another variant, the identification pattern could for example be different for each of the elementary cells of the localization target. 
     The positioning system  30  of the characterization devices  10 ,  110 ,  210  also includes image acquisition and analysis means comprising a target imaging system  320  and image analysis means  33  herein located of the side of the lower face  11 B of the sample  11 . So placed, the target imaging system  320  faces the localization target  31  so as to be able to take an image of a portion of the localization target  31 . 
     In  FIG. 9  are shown the target imaging system  320  used in the three embodiments of the invention. This target imaging system  320  first includes illumination means  321  allowing to illuminate the localization target  31 . These illumination means  321  herein include:
         an electroluminescent diode emitting a visible or near-infrared radiation along an optical axis  326 ,   a splitting cube  325  transmitting the light coming from the illumination means  321  towards the localization target  31 , and   a first group  323  of optical lenses allowing to collimate the light coming from the illumination means  321  to the localization target  31  and to illuminate uniformly the latter.       

     As a variant, when the measuring instrument of the characterization device includes light sources, the illumination means could for example use these same light sources. Hence, advantageously, when the measuring instrument is an optical microscope, the illumination means may comprise a white lamp, or a laser. 
     The illumination means  321 , the splitting cube  325  and the first group of lenses  323  are herein arranged so that the optical axis  326  is perpendicular to the localization target  31 . 
     The incident light (beam parallel to the optical axis  326 ), then reflected, scattered or diffracted by the localization target  31 , may be advantageously used to make an image of a portion of the localization target  31 . 
     For that purpose, the target imaging system  320  also comprises:
         a second group  324  of optical lenses refracting the light reflected by the splitting cube  325 , and   an optical imaging system  322  collecting the light refracted by the second doublet  324 .       

     The optical imaging system  322  herein comprises a digital camera of the CMOS (Complementary Metal Oxide Semiconductor) type, with a planar array  322 A of monochrome detectors. 
       FIG. 10  shows a detail view of the planar array  322 A of detectors. The latter comprises a rectangular array of 640×480 pixels  322 B, with a period of 6 μm in the two directions along the lines and the columns of the array of detectors  322 A. 
     The 640×480 pixels  322 B of the array of detectors  322 A are arranged regularly so that two orthogonal axes X image    322 X and Y image    322 Y can be defined as follow (see  FIG. 10 ):
         the axis X image    322 X is oriented parallel to the lines of pixels of the array of detectors  322 A, and   the axis Y image    322 Y is oriented parallel to the columns of pixels of the array of detectors  322 A.       

     A image-reference system of the array of detectors  322 A can then be defined by considering the first pixel  322 D, located at the 1 st  line and the 1 st  column of the array of detectors  322 A, as the origin of this image-reference system, the two orthogonal axes) X image    322 X and Y image    322 Y forming an orthogonal base of this image-reference system. It will be seen hereinafter that the orientation of the optical imaging system  322  in relation to the localization target  31  can be defined by means of the orthogonal axes X image    322 X and Y image    322 Y. 
     The array of detectors  322 A finally comprises an image-centre  322 C located at the centre thereof (see  FIG. 10 ). It will be seen hereinafter that the position of the optical imaging system  322  in relation to the localization target  31  can be defined thanks to this image-centre  322 C. 
     The optical imaging system  322  of the target imaging system  320  acquires an image  31 P of at least one portion of the localization target  31 , this image  310  being formed in the plane of the array of detectors  322 A. 
     As the object field of the optical imaging system  322  does not cover herein all the localization target  31 , the portion of the localization target  31  that is imaged by the optical imaging system  322  is (see  FIG. 11 ) the portion delimited by the black rectangle showing the array of detectors  322 A. 
     It can be seen in  FIG. 11  that, on the one hand, the image-centre  322 C of the array of detectors  322 A is not located at the centre of the localization target  31  and that, on the other hand, neither of the orthogonal axes X image    322 X and Y image    322 Y is oriented in such a manner that they are parallel to one of the orthogonal axes X mire    11 X or Y mire    11 Y of the localization target  31 . 
     Indeed, without any particular precaution about the arrangement of the target imaging system  320  in relation to the localization target  31  other than to suitably illuminate the latter, the optical imaging system  322  has any position and orientation in relation to the localization target  31 . 
     To determine the position and orientation of the optical imaging system  322  in relation to the localization target  31 , the positioning system  30  further includes image analysis means  33  that analyse the image  31 P of the portion of the localization target  31  acquired by the optical imaging system  322 . 
     It will be described hereinafter how the image analysis means  33  exploit the acquired image  31 P as well as the particular information coded in the elementary cells  310  of the localization target  31  to perform this determination. 
     Determination of the Position and Orientation of the Optical Imaging System in Relation to the Localization Target 
     In  FIG. 12  is shown the image  31 P of the portion of the localization target  31  acquired by the optical imaging system  322 . This image  31 P of the portion of the localization target  31  comprises the images  310 P of several elementary cells  310  of the localization target  31 . 
     In particular, the optical imaging system  322  is herein advantageously arranged so that the image  31 P of the portion of the localization target  31  includes:
         an image of a positioning pattern and an orientation pattern,   an image of the periodic patterns forming sub-cells, and   an image of an identification pattern.       

     The image  31 P of the portion of the localization target  31  is analysed by the image analysing means  33  of the positioning system  30 . By conventional shape recognition techniques, the image analysis means  33  identify all the orientation patterns present in the image  31 P and determine the common orientation of each of them in relation to the two axes X image    322 X and Y image    322 Y. 
     This may be understood thanks to  FIG. 13 , in which is represented an enlarged view of the image  31 P of the portion of the localization target  31  in the area defined by the black circle in  FIG. 12 . In  FIG. 13  is also shown the two orthogonal axes X image    322 X and Y image    322 Y attached to the array of detectors  322 A of the optical imaging system  322 . 
     Based on the identification of the orientation patterns, the image analysis means  33  determine that the localization target  31  is oriented according to the two other orthogonal axes X mire    11 X and Y mire    11 Y, as shown in  FIG. 13 . 
     The image analysis means  33  hence determine that:
         the optical imaging system  322  is oriented according to the two orthogonal axes X image    322 X and Y image    322 Y, and   the localization target  31  is oriented according to the two orthogonal axes X mire    11 X and Y mire    11 Y.       

     Hence, by comparison, the image analysis means  33  determine the orientation of the optical imaging system  322  in relation to the localization target  31 . 
     In the case shown in  FIG. 13 , this relative orientation may be, for example, quantified in simple manner by the measurement of the angle oriented between the axis X image    322 X and the axis X mire    11 X. 
     Likewise, the image analysis means  33  determine the position of the optical imaging system  322  in relation to the localization target  31 . For that purpose, the image analysis means  33  determine the position of the image-centre  322 C thanks to the analysis of the image  31 P of the portion of the localization target  31 . 
     The image analysis means  33  identify in particular the central elementary cell  310 C containing the image centre  322 C. The image analysis means  33  then decode the positioning pattern of the central elementary cell  310 C so as to determine a first positioning of the image-centre  322  in relation to the localization target  31 . 
     The image analysis means  33  also identify by shape recognition the positioning pattern of the central elementary cell  310 C and deduce therefrom the values of the first binary number that codes for the position of the central elementary cell  310 C according to the axis  11 X and of the second binary number that codes for the position of the central elementary cell  310 C according to the axis  11 Y. 
     The image analysis means  33  then determine a second positioning of the image-centre  322  in relation to the localization target  31 . This second, more accurate, positioning is made by means of the sub-cells including the periodic draughtboard patterns of the central elementary cell  310 C. 
     Indeed, by conventional image processing techniques, the image analysis means  33  allow a sub-pixel positioning of these sub-cells, i.e., for example, the position of each of these sub-cells is determined with an accuracy better than 3/100 th  of a pixel. For that purpose, the magnification of the target imaging system  320  is chosen so that each square of the draughtboards covers a surface equivalent to about six to twelve pixels. 
     Hence, the periodic patterns of the sub-cells allow to improve the accuracy of the positioning of the image-centre  322 C in relation to the localization target  31 . 
     That way, these periodic patterns allow to improve the accuracy of the positioning of the optical imaging system  322  in relation to the sample  11 . 
     To sum-up, the image analysis means  33  determine from the image  31 P of a portion of the localization target  31 :
         the orientation of the optical imaging system  322  in relation to the localization target  31  thanks to the identification of the orientation patterns of the different elementary cells  310  present in the image  31 P, and   the position of the optical imaging system  322  in relation to the localization target  31  thanks to the reading of the positioning pattern of the central elementary cell  310 C and to the sub-pixel positioning of the sub-cells of the central elementary cell  310 C that comprise periodic patterns.       

     The different embodiments of the characterization device described hereinafter all include a positioning system that comprises a localization target  31  and image acquisition and analysis means such that those described hereinabove. 
     1 st  Embodiment 
     In the first embodiment shown in  FIG. 2 , the characterization device  10  includes a measuring instrument that is a digital optical microscope  20 . 
     According to the invention, the measuring instrument  20  and the optical imaging system  322  are integral with each other. It is meant by this that there exists a mechanical coupling between them, i.e. any displacement of the measuring instrument  20  in a plane substantially parallel to the sample  11  causes an identical displacement of the optical imaging system  322  in relation to the sample  11 . 
     This is shown in  FIG. 2  by the full line  12  between the optical microscope  20  and the target imaging system  320  of the positioning system  30 . 
     Advantageously, the optical imaging system  322  is herein arranged at the place of the condenser of the optical microscope  20 . 
     The digital optical microscope  20  moreover comprises:
         a ×10 magnification lens  21  allowing to image a portion of the area of interest  11 C of the sample  11 , and   a digital camera  22  allowing to acquire an image of the portion of the area of interest  11 C.       

     It will be considered herein that the measuring instrument  20  determines the optical contrast of the sample  11  on the area of interest  11 C. During a single measurement, the optical contrast of the sample  11  is measured over an extended area substantially centred about a localized measurement point. This localized measurement point is herein the image-centre  322 C of the image  31 P acquired by the digital optical microscope  20 . 
     The measuring instrument  20  also comprises image processing means  23  allowing to determine the optical contrast on the whole digital image of a portion of the area of interest  11 C of the sample  11  that is acquired by the digital camera  22 . The image processing means  23  determine in particular the optical contrast at the localized measurement point of the sample  11 . 
     Hence, the characterization device  10  includes:
         a measuring instrument  20  allowing to determine the optical contrast of the sample  11  at a localized measurement point of the sample  11 , and   a positioning system  30  allowing to determine the position and orientation of the optical imaging system  322  in relation to the localization target  31 , and hence in relation to the sample  11  that is integral therewith.       

     Furthermore, to determine the relative position of the measuring instrument  20  in relation to the optical imaging system  322 , the positioning system  30  of the characterization device  10  also includes calibration means. 
     These calibration means first comprise a thin glass slide  34  such as those conventionally used in optical microscopy. This glass slide  34  comprises a calibration target  34 A engraved on the upper face of the glass slide  34  by conventional techniques of photolithography allowing to reach sub-micron engraving resolutions. 
     The calibration target  34 A has advantageously a structure similar to the localization target  31  fixed on the sample  11 . Preferably, the calibration target  34 A is semi-transparent: for example, patterns appearing black are opaque on a background at least partially transparent. 
     As a variant, the calibration target could for example comprise a multi-scale multimodal tag as shown in  FIG. 14 . It is a self-similar structure, having no symmetry of rotation and observable by several instrumental techniques. Such a calibration target may be made by metal deposition on a glass slide, providing it with, on the one hand, a sufficient contrast to be observed by optical microscopy with different magnifications and by electron microscopy, and on the other hand, a topographic structure observable by means of an atomic force microscope. The position of this tag may be accurately known either because it belongs to the location pattern and has been manufactured simultaneously with the latter, in this case, it may for example replace the identification pattern of a known elementary cell; or because it is deposited lately on a support including a calibration target and its position is measured by means of an already-calibrated measuring instrument, for example a low-magnification digital optical microscope. 
     During a step of calibration, the glass slide  34  is arranged in the characterization device  10  in the same way as the sample  11 . This situation is shown in  FIG. 15 . 
     The glass slide  34  hence deposited on the characterization device  10 , the calibration target  34 A can be observed simultaneously by both sides and imaged from above by the digital optical microscope  20  and from below by the target imaging system  320 . 
     On one side, the image analysis means  33  of the positioning system  30  determine, in the same way as the localization target  31 , the position and orientation of the optical imaging system  322  in relation to the calibration target  34 A, i.e. in a referential system linked to the calibration target  34 A. 
     On the other side, the image processing means  23  of the measuring instrument  20  determine, also in the same way, the position and orientation of the measuring instrument  20  in relation to the calibration target  34 A, i.e. in the same referential system linked to the calibration target  34 A. 
     As a variant, if a multi-scale multimodal tag is used, it is advisable to observe it by means of the measuring instrument. The conventional image processing techniques then allow to determine its exact position and orientation. 
     The calibration means also comprise data processing means  35  to which are transmitted:
         by the image analysis means  33 : the position and orientation of the optical imaging system  322  in the referential system linked to the calibration target  34 A, and   by the image processing means  23 : the position and orientation of the measuring instrument  20  in the referential system linked to the calibration target  34 A.       

     The data processing means  35  then determine the relative position of the measuring instrument  20  in relation to the optical imaging system  322 . This relative position corresponds to the vector shift in the referential system linked to the optical imaging system  322  between the localized measurement point of the calibration target  34 A observed by the measuring instrument  20  and the image-centre  322 C of the optical imaging system  322 . 
     The data processing means  35  comprise data storage means allowing to record this relative position so that the latter can be lately exploited by the characterization device  10 . 
     To better understand the operation of the first embodiment of the characterization device  10 , the characterization method according to the invention allowing to characterize the silicon wafer  11  constituting the sample will now be described. 
     Characterization Method 
     a) Calibration 
     An operator places the calibration target  34  in the characterization device  10 , so that it can observed it on its both faces, on one side by the optical microscope  20  and on the other side by the optical imaging system  322 . 
     The optical microscope  20  acquires a first image of a portion of the calibration target  34  that is processed by the image processing means  23  to determine the position and orientation of the optical microscope  20  in relation to the calibration target  34 A. 
     The optical imaging system  322  acquires a second image of a portion of the calibration target  34  that is analysed by the image analysis means  33  to determine the position and orientation of the optical imaging system  322  in relation to the calibration target  34 A. 
     The data processing means  35  then determine the relative position of the optical microscope  20  in relation to the optical imaging system  322 . 
     b) Setting of the Sample 
     The operator gets the silicon wafer  11  to be characterized and fixes the localization target  31  by adhesive on the lower face  11 B of the silicon wafer  11 . The localization target  31  is hence made integral with the sample  11 . The localization target  31  is of the type of that shown in  FIG. 4 . By these patterns, this localization target  31  defines a referential system linked to the sample  11 . 
     Then, the operator places the silicon wafer  11  in the characterization device  10  for the measurement. The optical microscope  20  acquires an image of a portion of the area of interest  11 C of the silicon wafer, the image being centred at the localized measurement point. This image of the portion of the area of interest  11 C is processed by the image processing means  23 , which then determine the value of the optical contrast of the sample  11  at the localized measurement point. 
     c) Acquisition of an Image of the Localization Target 
     The target imaging system  320  illuminates the localization target  31  thanks to the illumination means  321  and the optical imaging system  322  acquires an image of at least one portion of the localization target  31 . 
     d) Analysis of the Acquired Image 
     The image previously acquired by the optical imaging system  322  is analysed by the image analysis means  33 , which then determine the position and orientation of the optical imaging system  322  in relation to the localization target  31 . 
     e) Determination of the Position of the Measurement Point 
     The image analysis means  33  transmit to the data processing means  35  the position and orientation of the optical imaging system  322  in relation to the localization target  31 , i.e. in the referential system linked to the silicon wafer  11 . 
     The characterization device  10  further includes means  13  for processing the results of the image analysis and of the calibration to which are transmitted, on the one hand, the result of the measurement of the optical contrast at the localized measurement point by the image processing means  23 , and on the other hand, the relative position of the optical microscope  20  in relation to the optical imaging system  322  by the data processing means  35 . 
     The means  13  for processing the results of the image analysis and of the calibration then deduce therefrom the absolute position of the localized measurement point in the referential system linked to the sample  11 . 
     Hence, the operator of the characterization device  10  knows at the end of the characterization procedure:
         the value of the optical contrast of the sample  11  at the localized measurement point, and   the absolute position of the localized measurement point in a referential system linked to the sample  11 .       

     2 nd  Embodiment 
     In  FIG. 16  is shown a second embodiment of the characterization device  110  according to the invention. 
     In this second embodiment, the characterization device  110  first includes a first measuring instrument  120 A (see dashes in  FIG. 16 ) comprising a rotary plate  124 , a first ×10 magnification lens  121 A fixed on the rotary plate  124 , a digital camera  122 , and image processing means  123 . 
     The characterization device  110  also includes a second measuring instrument  120 B (see dashes in  FIG. 16 ). This second measuring instrument  120 B shares with the first measuring instrument  120 A the rotary plate  124 , the digital camera  122  and the image processing means  123 . 
     The second measuring instrument  120 B moreover includes a second ×50 magnification lens  121 B fixed on the rotary plate  124 . 
     The rotary plate  124  is able to rotate in its plane, which allows the operator using the characterization device  110  to select which object to use during a measurement. 
     In this second embodiment, the characterization device  110  includes a positioning system  30  identical to that of the first embodiment and operating in the same manner to determine the position of the optical imaging system  322  in relation to the calibration target  31  integral with the sample  11 . 
     An operator wishing to study the sample  11  by means of the first and second measuring instruments  120 A,  120 B equipped with the two lenses  121 A,  121 B, respectively, for example to perform a measurement located at the same localized measurement point of the sample  11 , will implement twice the characterization method described above for the first embodiment. 
     Advantageously, the operator will be able to carry out the steps of calibration of the two lenses  121 A and  121 B one after the other, before performing the successive measurements by means of these two lenses  121 A,  121 B. 
     In the case where the optical axes between the two lenses  121 A,  121 B are shifted by a few tens of micrometers, the same calibration procedure could no longer be performed, the field of view of the digital camera  122  of the measuring instruments  120 A,  120 B being then too reduced. 
     The calibration may be performed with the same calibration sample (the glass slide of  FIG. 15 , with its calibration target), by taking care of having in the field of view of the digital camera an image of the calibration target where the orientation pattern is visible. By acquiring and recording an image coming from the digital camera of the optical microscope, the orientation pattern is searched (manually or automatically) and its relative position in relation to the centre of the image, as well as its orientation, are determined. By making the hypothesis of being still in the same area of the calibration target, whose absolute position has been determined during the first calibration, it is possible to go back to the absolute position and to the orientation of the field of view of the high-magnification optical microscope. 
     As a variant, the multi-scale multimodal tag described hereinabove (see  FIG. 14 ) may be used. In this case, it is no longer necessary to suppose a small shift between the different lenses of the optical microscope. 
     It may hence be advantageous to arrange the target imaging system in such a manner that its centre of observation, i.e. the image-centre of the optical imaging system coincides with or is close enough to the measurement point of the measuring instrument or of the centre of its field of observation. 
     3 rd  Embodiment 
     In  FIG. 17  is shown a third embodiment of the characterization device  210  according to the invention. 
     In this third embodiment, the characterization device  210  includes two measuring instruments: a digital optical microscope  20  and an atomic force microscope  220  (called hereinafter AFM). 
     The digital optical microscope  20  is identical to that used in the first embodiment (see  FIG. 2 ): it includes a lens  21 , a digital camera  22  and image processing means  23  to process the images acquired by the digital camera  22 . 
     The AFM  220  comprises a tip  221 , an amplifier  222  and signal processing means  223  processing the signal exiting from the amplifier  222 . The AFM also comprises a visualisation device  224  allowing to acquire an image of the area explored by the tip  221  of the AFM  220 . The visualisation device  224  herein comprises a video camera providing an image of the sample  11  under a low magnification. 
     In this third embodiment, the positioning system  230  of the characterization device  210  comprises:
         a first target imaging system  320 , comprising a first optical imaging system (not shown) and associated with first image analysis means  33 ;   a second target imaging system  2320 , comprising a second optical imaging system (not shown) and associated with the second image analysis means  233 .       

     Although it is not schematized in  FIG. 17 , in this third embodiment, on the one hand, the first optical imaging system is integral with the digital optical microscope  20  and, on the other hand, the second optical imaging system is integral with the AFM  220 . 
     The first and second target imaging systems  320 ,  2320 , associated with the first and second image analysis means  33 ,  233 , respectively, operate in the same way as for the two preceding embodiments. 
     In particular, during a step of calibration or a step of measurement, they determine and transmit to the data processing means  35  the position and orientation of the first and second optical imaging systems in relation to the target they observe (a calibration target during a calibration and a localization target during a measurement). 
     In this third embodiment, the procedure of calibration of the positioning system  30  is performed separately with each of the two target imaging systems  320 ,  2330 , so as to determine, on the one hand, the relative position of the first measuring instrument  20  in relation to the first optical imaging system, and on the other hand, the relative position of the second measuring instrument  220  in relation to the second optical imaging system. 
     In particular, the calibration of the tip  221  is performed in the same manner as that of a high-magnification optical lens (see the 2 nd  embodiment). An AFM topographic measurement of the surface of a calibration sample, such as the glass slide provided with its calibration target, is performed, so as to find the position and orientation of the orientation pattern. 
     In the case of an atomic force microscope, by proceeding in the same manner as in the case of a digital optical microscope, it may hence be proceeded to the calibration of the visualisation device  224 . 
     In an alternative embodiment, the calibration means for the atomic force microscope and for the optical microscope may be different. 
     Once the calibration of each of the measuring instruments  20 ,  220  terminated, the characterization device  210  may proceed to the measurements on the sample  11 . 
     Thanks to this third embodiment, it is possible to study the silicon wafer  11  by means of two very different measuring instruments. It is in particular possible to characterize this sample  11  at a same localized measurement point by two different techniques. 
     To sum up, the characterization devices are all equipped with an optical imaging system that observes permanently a localization target integral with the studied sample. By interpreting the image of the localization target, the positioning system allows to deduce the absolute position of the observation location in the reference system of the sample it-self. The positioning system allows to reproduce the observation point when the sample is transferred from one measuring instrument to one another or when successive observations are performed with the same measuring instrument, but separated in time. 
     An advantage of the invention is to make so that measurements at the nanometer scale are co-localizable for a great variety of measuring instruments.