FR3039647A1 - METHOD FOR ANALYZING ORGANIC IMPURITIES PRESENT ON THE SURFACE OF A SUBSTRATE, AND DEVICE FOR COLLECTING ORGANIC IMPURITIES AT THE SURFACE OF A SUBSTRATE - Google Patents

Abstract

The invention relates to a method for analyzing organic impurities (3) on the surface of a substrate, comprising the following steps: • providing a substrate comprising a first main surface, • providing a polymer film (2) having a module of Young less than 3GPa, • applying the polymer film (2) against the first main face of the substrate to recover organic impurities (3) present on the first main face, • analyzing the polymer film by Fourier Transform Infrared Spectroscopy, so as to obtain a Fourier spectrum, • determine the chemical composition of the organic impurities (3) present on the surface of the polymer film (2) from the comparison between the Fourier spectrum and a reference Fourier spectrum. The invention also relates to a device (10) for collecting organic impurities (3) on the surface of a substrate.

Description

Process for the analysis of organic impurities present on the surface of a substrate, and device for collecting organic impurities on the surface of a substrate

TECHNICAL FIELD OF THE INVENTION The invention relates to a method for analyzing organic impurities present on the surface of a substrate, and to a device for collecting organic impurities present on the surface of said substrate. State of the art

The manufacture of silicon wafers for the manufacture of photovoltaic cells is performed by cutting silicon ingots. During this manufacturing step, the platelets are contaminated with elements such as organic residues.

This contamination poses several problems. The first concerns the contamination of photovoltaic cells manufacturing tools that are often arranged in a clean room. The second problem relates to the degradation of the performance of the photovoltaic cell because of parasitic reflections due to organic residues that have not been eliminated.

Also, to avoid this kind of trouble, it is essential to know the nature of the contaminants present on the silicon wafers obtained from the same ingot, so as to perform a suitable cleaning before any subsequent step of manufacturing the photovoltaic cells. .

Techniques for determining the type of organic impurities on the surface of a substrate are already known. They have the advantage of allowing detection of impurities in very small quantities. One of the most common techniques for analyzing the nature of organic residues present on the surface of a substrate is firstly to perform a platelet analysis by thermodesorption, followed by a gas chromatography coupled with an analysis. by mass spectroscopy. This method is still called TD-GC-MS or "ThermoDesorption - Gas Chromatography - Mass Spectroscopy" in English. It makes it possible to determine precisely which volatile contaminants are present on the surface of the substrate, but it has the disadvantage of not detecting the residues present on the surface which can not pass into the gaseous phase. This technique is also complex and expensive to implement, which makes it inaccessible to perform routine analyzes.

An alternative method to TD-GC-MS is Fourier Transform Infrared Spectroscopy, also known as FTIR or Fourier Transform InfraRed Spectroscopy. This can be implemented in three different ways depending on the nature of the sample to be analyzed. The analyzes can be performed grazing incidence, transmission, or attenuated total reflection also called ATR or "Attenuated Total Reflection" in English.

These three techniques make it possible to obtain convincing information for the study of organic molecules, but the ATR method is the most suitable. This consists of attaching a crystal to the surface to be analyzed, and illuminating the surface through the crystal by means of infrared radiation. Part of the energy of the incident beam undergoes multiple reflections in the crystal and then reaches a detector, while another portion of the energy is transmitted to the surface to be analyzed as evanescent waves. The Fourier spectrum of light finally arriving at the detector is characteristic of the analyzed surface. This makes it possible to determine the nature of the organic residues present on the substrate.

This technique can be effective if the roughness of the surface is low, and preferably less than 0.2 μιτι. If the roughness is higher, the contact between the crystal and the surface to be analyzed is not continuous, which causes parasitic reflections and greatly reduces the quality of the Fourier spectrum obtained.

To use this method of analysis, it is therefore necessary to polish the surface of the substrate. This polishing step is not only long and expensive, but it also has the disadvantage of modifying the characteristic parameters of the pollution on the surface of the substrate (nature of the contaminants, concentration, etc.).

It is also necessary to have a reference substrate to subtract the spectrum of the reference substrate from the measured overall spectrum. But it is futile to want to obtain a substrate of identical nature and which is free of any contaminant. Indeed, as the analyzes are not performed, the type of organic residues present on the surface of the substrate is not known precisely, and the surface can not be cleaned appropriately.

No technique of analysis of organic residues present on the surface of a substrate is really conclusive to determine with precision which are the impurities, and to set up an adequate protocol of cleaning.

Object of the invention

An object of the invention is a method of analyzing organic impurities on the surface of a substrate that is reliable, easy to implement and cheap.

The method advantageously comprises the following steps: • providing a substrate comprising a first main face, • providing a polymer film having a Young's modulus of less than 3 GPa, • applying the polymer film against the first main face of the substrate to recover impurities organic compounds present on the first main surface, • analyze the polymer film by Fourier Transform Infrared Spectroscopy, so as to obtain a Fourier spectrum, • determine the chemical composition of the organic impurities present on the surface of the polymer film from the comparison between the Fourier spectrum and a reference Fourier spectrum.

According to one aspect of the invention, the polymer film may preferably have a Young's modulus of less than 2 GPa. It can also have a thickness greater than 50 μιτι.

It is generally selected from the family of saturated aliphatic polymers, preferably without OH, CO, NH, C = O bonds, in particular in the family of semicrystalline thermoplastics, for example polyethylene, polypropylene, polymethylpentene or polybutene. -1. It may also be chosen from the family of elastomers such as polyisobutylene, ethylene-propylene copolymers or from paraffin-based films. The analysis of the Fourier Transform Infrared Spectroscopic polymer film may be preferably FTIR-ATR analysis, during which the face of the polymer film having been applied to the substrate is placed against a crystal serving as an internal reflection element. This crystal may for example be a germanium crystal or diamond carbon.

According to a particular embodiment, the polymer film may be applied to the substrate with a pressure of between 104 and 105 Pa. The polymer film may be placed on the substrate and undergo at least one pressure increase phase followed by a phase of pressure decrease.

In addition, the method of analysis may comprise the following preliminary steps: • provide a reference polymer film, • analyze the reference polymer film by Fourier Transform Infrared Spectroscopy, so as to obtain the reference Fourier spectrum.

In this case, the comparison between the Fourier spectrum and the reference Fourier spectrum can be obtained by subtracting the reference Fourier spectrum from the Fourier spectrum. The invention also relates to a device for collecting organic impurities on a main surface of a substrate. This advantageously comprises: • a substrate support, • means for moving a head configured to be in contact with the substrate disposed on the substrate support, the head being covered with a removable polymer film, • a system controller configured to control the moving means and the pressure exerted by the head on the substrate.

According to one aspect of the invention, the head may be a metal head. This can be covered with a damping layer.

According to a particular embodiment of the device, the displacement means can allow both the vertical and horizontal displacement of the head. They may also include means for memorizing the trajectory of the head.

The device may also comprise means for holding the substrate on the support. The invention finally relates to an impurity analysis system comprising a collection device comprising the aforementioned characteristics, as well as a Fourier transform infrared spectrometer configured to perform FTIR-ATR analyzes.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and features will emerge more clearly from the following description of particular embodiments of the invention given as non-restrictive examples and represented in the accompanying drawings, in which: FIGS. 3 schematically illustrate one embodiment of the method for analyzing impurities on the surface of a substrate; FIG. 4 represents Fourier spectra of a reference polymer film and of two polymer films; FIG. 5 shows the residual spectrum of the organic residues; FIG. 6 shows an embodiment of an organic residue collection device; FIG. 7 schematically illustrates, in a top view, a possible trajectory of the device head; collection on the substrate.

detailed description

A monocrystalline ingot obtained by crystal growth is generally cut by means of a diamond wire saw so as to form a batch of platelets. These wafers are themselves subsequently cut in order to produce photovoltaic cells for example.

The oils used when cutting the ingot are partially removed by a rough cleaning, but traces remain and are detrimental during the process of manufacturing photovoltaic cells. Organic residues contaminate the tools used for manufacturing, and degrade the performance of the cells made.

Also, the nature of the contaminants of one of the platelets must be determined accurately to implement a global cleaning protocol adapted to a batch of platelets from the same ingot and / or the same transformation tools.

FTIR analyzes are recommended because they make it possible to precisely determine the chemical composition of a surface. FTIR-ATR analyzes are particularly well suited to the study of organic compounds but to give detailed information, it is necessary that the studied surface is polished optically, that is to say that it has a roughness of less than 0 , 2 μιτι.

Obtaining such roughness is impossible for saw blades cut with a diamond wire saw. One aspect of the invention therefore consists in collecting on the surface 2a of a polymer film 2 organic impurities 3 initially present on a wafer or more generally on a substrate 1. The analysis of the contaminants is advantageously carried out before the step of polishing to avoid polluting tools used for polishing.

So that the organic residues 3 is moved from a first main face 1a of the substrate 1 to the surface 2a of the polymer film 2, the polymer film 2 is placed at least once against the first main face 1a of the substrate 1. This corresponds to the implementation step illustrated in Figure 1.

When contacting the substrate 1 and the polymer film 2, a certain amount of organic impurities 3 present on the surface of the substrate 1 is transferred to the surface 2a of the polymer film 2 (see FIG. The surface 2a is then analyzed by Fourier Transform Infrared Spectroscopy, in order to obtain a Fourier spectrum characteristic of its chemical composition (see FIG.

It will be noted that FIGS. 1 to 3 are not made to scale, and that the organic impurities 3 have a much smaller thickness (from a few nanometers to a few microns) than the thickness of the polymer film 2 (a few tens of microns). and substrate 1 (a few hundred microns). At this stage, the Fourier spectrum contains both information on the polymer film 2 and on the nature of the organic impurities 3 present on the surface 2a. To determine which are the organic impurities 3, the Fourier spectrum must be compared to a reference Fourier spectrum which is characteristic of the polymer film devoid of any organic residue, or devoid of organic impurities to be removed.

The reference spectrum can for example be obtained from Fourier Transform Infrared Spectroscopy analysis of the polymer film 2 before the latter is placed against the substrate 1. It can also be determined from another polymer film. 2 preferably from the same polymer plate so that the roughness of the reference film and the film on which the impurities are found are comparable. The comparison between the two Fourier spectra is carried out by subtracting the reference Fourier spectrum from the Fourier spectrum of the polymer film 2 which has been contaminated by the organic impurities 3.

The properties of the polymer film 2 must be judiciously chosen so that the Fourier spectrum makes it possible to determine precisely which impurities are present on the surface 2a.

The polymer film 2 preferably has a Young's modulus of less than 3 GPa. It is advantageously less than 2GPa and may be greater than 1 MPa and still advantageously greater than 100 MPa. Such a Young's modulus confers sufficient ductility to the material so that the latter can be deformed without breaking. Using a polymer film having this property therefore makes it possible to exert stresses on the film during the collection of impurities (positioning of the film on the head of the collecting device as will be seen later, and adaptation to the roughness of the substrate), and during FTIR-ATR analyzes (positioning the film against the crystal so as to make intimate contact). This makes it possible in particular to limit the loss of infrared signal during analyzes.

The polymer film 2 must also resist friction during the collection of impurities on the substrate 1. If it is too thin, it may be damaged and therefore it is preferable that the thickness of the polymer film 2 used is greater than at 50 μιτι. If it is too thick, the polymer film 2 may become too rigid for the collection of organic impurities to be effective and subsequent analyzes to be performed accurately. The polymer film therefore preferably has a thickness of less than 500 μιτι.

The chemical nature of the polymer film 2 also plays an important role, since the contribution of the organic residues 3 to the Fourier spectrum must be easily distinguishable from that of the film 2. The polymer chosen for producing the film 2 preferably has a chain comprising as CH bonds, to have a very low absorbance in order to influence the Fourier spectrum as little as possible. It is also advantageously chemically and hydrophobically inert.

It is preferably selected from the family of saturated aliphatic polymers, preferably without O-H, C-O, N-H or C = O bonds, and in particular in the family of semi-crystalline thermoplastics. The polymer film may for example be polyethylene, polypropylene, polymethylpentene (PMP), polybutene-1 (PB-1), be selected from the family of elastomers such as polyisobutylene (PIB), ethylene-propylene copolymers (EPR) for ethylene propylene rubber or EPM) or among paraffin-based films. In order to guarantee the flexibility of the material and therefore the quality of the FTIR measurements, when the film is made of polyethylene, it is preferable that the polyethylene be of low density.

Beyond the properties that have just been stated, the polyethylene has the advantage of being translucent and thus allowing the visualization of organic impurities 3 on the surface 2a of the film 2. This polymer also has the particularity of being inert, and therefore not to react chemically with organic impurities 3 during collection.

According to a particular mode of implementation of the method, the Fourier Transform Infrared Spectroscopy analyzes can be carried out using an ATR module.

In this case, the surface 2a of the polymer film 2 having been in contact with the first main face 1a of the substrate 1 is placed against a crystal 4 of the ATR module serving as an internal reflection element. The polymer film 2 can simply be placed against the crystal 4 or pressed against the latter in order to make intimate contact.

The polymer film 2 is held for example by means of a torque screw, so as to exert a pressure of a few N / mm, preferably less than 50 N / mm, and advantageously less than 10 N / mm. The advantage of this type of system is to allow reproducible clamping of the polymer films, and thus to easily perform comparative analyzes of different films.

It is advantageous to have good contact between the film 2 and the crystal 4, so that spurious reflections are few and therefore increase the signal-to-noise ratio. The Fourier spectrum obtained is more precise, and the determination of the nature of the organic residues present on the surface 2a of the film 2 is facilitated.

Different crystals 4 can be used to perform the FTIR-ATR analyzes. These can be for example silicon, germanium or diamond. Germanium is preferred here for its high refractive index to multiply the reflections in the crystal. The light beam used during the FTIR-ATR analyzes goes through the path 5 schematically represented in FIG.

The collection of the impurities present on the first main face 1a can be carried out according to different protocols that can be adjusted for example according to the amount of visible organic residues 3 on the substrate 1. The polymer film 2 can for example be pressed against the first main face 1a of the substrate 1, and possibly be moved to the surface, so as to collect a maximum of impurities.

According to one particular embodiment, the polymer film 2 can be applied to the substrate 1 with a pressure of between 10 4 and 10 5 Pa in order to collect a high percentage of impurities 3 while keeping the film 2 intact.

It is also possible to collect repeatedly organic impurities present on a substrate 1 with the same polymer film 2, so as to increase the amount of residues to be analyzed and to determine more precisely what their chemical characteristics. To carry out the collection repeatedly, the pressure exerted during the application of the polymer film on the substrate may be variable, and comprise at least one pressure increase phase and at least one pressure reduction phase.

FIG. 4 shows several Fourier spectra obtained during FTIR-ATR analyzes carried respectively on a clean polymer film (curve B), a first polymer film (curve A) and on a second polymer film (curve C). In particular, it is observed that the Fourier spectra are very different in the 1000-1300 cm -1, 1550-1700 cm -1 and 3000-3600 cm -1 zones, which indicates the presence of impurities in different concentrations on the first and second polymeric films.

If advantageously an identical pressure has been applied during the collection of impurities, it is possible to compare the two curves A and C. The peaks of the Fourier spectrum of the second polymer film being higher than those of the Fourier spectrum of the first polymer film it is deduced that the concentration of impurities is higher on the second film than on the first. This may be due to the application of the polymer film to a dirtier substrate, or to a longer or repetitive application on the substrate.

To exploit the data obtained in the FTIR-ATR analyzes, a comparison must be made between the spectrum characteristic of the contaminated polymer films, and a reference spectrum. The reference spectrum may for example be a characteristic spectrum of the clean polymer film derived from data in the literature, or be a spectrum obtained by FTIR-ATR analysis on the clean polymer film before it is contaminated by the organic residues 3 of the substrate 1. If the polymer film 2 placed against the substrate 1 corresponds to a portion of a plate of larger size, it is also possible to use the spectrum obtained by FTIR-ATR analysis of a part of the plate different from the part used for the analysis of organic impurities 3. The second and third solutions are preferred to perform a quality comparison, and thus obtain precise information on the chemical nature of the organic impurities 3.

The Fourier spectrum of the polymer film contaminated with the organic residues 3 and the reference Fourier spectrum of a clean polymer film can be compared with each other by subtraction, so that the residual Fourier spectrum corresponds only to the Fourier spectrum of organic impurities 3. The exploitation of the Fourier spectra of the polymer films makes it possible to obtain a residual spectrum such as that represented in FIG. 5. The first peak observed in the 1000-1300 cm -1 zone is characteristic of a simple C-0 bond in an elongation vibration mode. The range of wave number, as well as the high intensity of the peak reflect the presence of molecules comprising alcohol groups on the polymer film 2.

The second peak located in the 1550-1700 cm -1 zone reveals the presence of compounds comprising a C = O double bond in an elongation vibration mode. The position and intensity of the peak in the Fourier spectrum are characteristic of molecules having C = O bonds such as amide, ketone, aldehyde or aesthetic groups.

Finally, the peak located in the third zone 3000-3600 cm-1 may be characteristic of both a single NH bond present in an amide group in an elongation vibration mode, a single OH bond of a group alcohol in a bound elongation vibration mode, or a single OH bond of an alcohol group in a free elongation vibration mode. From these chemical characteristics, the chemical functions present in the organic residues 3 can be determined, which can make it possible to find the nature of the residues, for example by coupling these results to chromatographic analyzes.

The organic impurities 3 present on the surface of the substrate 1 can advantageously be collected by means of a collection device 10 such as that represented in FIG.

The device 10 comprises a support 11 of substrate 1, such as a plate. This support 11 may for example be designed to move horizontally and vertically to adapt to the constraints of the user. It is possible that the substrate 1 is more or less thick, and more or less large, and that it must be moved vertically or horizontally to achieve the collection of organic impurities 3.

To prevent inadvertent movement of the substrate 1 relative to the support 11, the substrate 1 can be fixed on the support 11 by means of holding means (not shown). These may for example be clips fixed on the support 11 and configured to clamp the edges of the substrate 1. They may also comprise pivoting blades for blocking the substrate 1 when they are placed on the edges. These holding means are however not optimal when the substrate 1 is thin and the main face 1a has a very low surface roughness. The holding means could indeed weaken the substrate 1, or even break, or scratch the surface irreversibly.

Alternatively, it is possible to maintain the substrate 1 on the support 11 and prevent it from moving by exerting a depression between the substrate 1 and the support 11. The suction can for example be performed with a vacuum pump. This kind of system has the advantage of not damaging the substrate 1 and in particular the first main face 1a which is here the face opposite the support 11.

To pick up the impurities present on the substrate 1, the device 10 comprises a head 12 advantageously covered with the polymer film 2. As mentioned in connection with the organic impurity analysis method 3, the polymer film 2 may advantageously be produced in a polymer selected from polyolefins.

The polymer film 2 is advantageously removably mounted on the head 12 to be removed at the end of the collection process, and applied against the crystal 4 to perform the FTIR-ATR analysis.

The head 12 covered with the polymer film 2 can be moved vertically in order to be put in contact with the substrate 1 and to exert a pressure both sufficient to allow the collection of organic impurities 3, but also low enough not to breaking the substrate 1. For example, for a substrate 1 such as a silicon wafer having a minimum thickness of 50 μιτι and for example between 80 μη) and 2000 μη), the pressure exerted by the head 12 on the substrate 1 must be between 104 and 105 Pa.

In order for the pressure exerted by the head 12 on the substrate 1 to be sufficient, the material of the head 12 must be chosen so that its Young's modulus is not too weak and is not deformed when it is applied on the substrate 1. The material of the head 12 may for example be selected from metals and metal alloys.

The head 12 may advantageously be covered with a damping layer 13 intended to prevent the deterioration of the substrate 1 and the polymer layer 2 during collection. The damping layer 13 preferably has a Young's modulus of less than 500 MPa, and advantageously less than 100 MPa. It may for example be made of silicone, foam or rubber.

The head 12 is displaced by displacement means 14 comprising a spring 14a mounted on an articulated arm 14b. This mechanism allows a displacement of the head both horizontal and vertical. The displacement means 14 are directed by a control system 15 also controlling the pressure exerted by the head on the substrate 1. The control system 15 may also comprise storage means capable of recording the three-dimensional coordinates of the head 12, and thus to memorize its trajectory or pre-record a course to be made.

During collection, it is conceivable to perform a coarse spatial adjustment of the device 10 by means of the head 12 and the support 11 of the substrate 1, so as to center the head horizontally relative to the substrate 1, and to maintain the latter at a minimum. short distance from the substrate 1. Then, the collection of organic impurities 3 can begin by laterally displacing the support 11 of substrate 1, and by vertically moving the head 12. cycles of increase and decrease of pressure can for example be set in order to collect a quantity of doping impurities sufficient to carry out the FTIR-ATR analyzes.

FIG. 7 presents as such an example of a trajectory that the head 12 can make while being placed against the substrate 1. The trajectory of the head 12 can also be carried out randomly by a control algorithm, or on the contrary be controlled by a user who wishes to analyze a specific area of the substrate 1, especially if it is heterogeneously soiled. At the end of the collection, the polymer film 2 is detached from the head 12 of the device 10, and the FTIR-ATR analyzes are performed. A reference spectrum of the polymer film 2 alone can for example be obtained by first analyzing a non-contaminated area of the film, and a contaminated area thereafter. Even if the polymer film 2 has been stretched to be attached to the head 12 during the collection, this does not change the chemical nature of the film, and therefore does not disturb the analyzes performed.

Claims (18)

claims 1. Method for analyzing organic impurities (3) on the surface of a substrate (1), the analysis method comprising the following steps: • providing a substrate (1) comprising a first main face (1 a) • providing a polymer film (2) having a Young's modulus less than 3GPa, • applying the polymer film (2) against the first main face (1a) of the substrate (1) to recover organic impurities (3) present on the first main face (1a), • analyzing the polymer film (2) by Fourier Transform Infrared Spectroscopy, so as to obtain a Fourier spectrum, • determine the chemical composition of the organic impurities (3) present on the surface of the film polymer (2) from the comparison between the Fourier spectrum and a reference Fourier spectrum. 2. Analysis method according to claim 1, wherein the polymer film (2) has a Young's modulus of less than 2 GPa. 3. Analysis method according to any one of claims 1 or 2, wherein the polymer film (2) has a thickness greater than 50 μιτι. 4. Analysis method according to any one of claims 1 to 3, wherein the polymer film (2) is made of a material selected from the family of saturated aliphatic polymers, preferably without OH, CO, NH, C bond = 0, and preferably in the family of semi-crystalline thermoplastics or elastomers. An assay method according to claim 4, wherein the polymeric film (2) is made of a material selected from polyethylene, polypropylene, polymethylpentene, polybutene-1, polyisobutylene, ethylene-propylene copolymers, or in a paraffin-based material. The method of analysis of any one of claims 1 to 5, wherein the analysis of the polymer film (2) by Fourier Transform Infrared Spectroscopy is an FTIR-ATR analysis. 7. Analysis method according to claim 6, wherein the face of the polymer film (2) having been applied to the substrate (1) is placed against a crystal (4) serving as an internal reflection element during the analysis. FTIR-ATR. 8. Analysis method according to claim 7, wherein the crystal (4) is a germanium crystal or diamond. 9. Analysis method according to any one of claims 1 to 8, wherein the polymer film (2) is applied to the substrate (1) with a pressure of between 104 and 105 Pa. 10. Method of analysis according to any one of claims 1 to 9, wherein the polymer film (2) is placed on the substrate (1), and undergoes at least one pressure increase phase followed by a phase pressure decrease. 11. Analysis method according to any one of claims 1 to 10, comprising the following preliminary steps: • provide a reference polymer film, • analyze the reference polymer film by Fourier Transform Infrared Spectroscopy, so as to obtain the reference Fourier spectrum. 12. Analysis method according to any one of claims 1 to 11, wherein the comparison between the Fourier spectrum and the reference Fourier spectrum is obtained by subtraction of the reference Fourier spectrum to the Fourier spectrum. 13. An impurity analysis system comprising: a device (10) for collecting organic impurities (3) on a main (1a) of a substrate (1) comprising: o a substrate support (11) ( 1), o moving means (13) of a head (12) configured to be in contact with the substrate (1) disposed on the support (11) of substrate (1), the head (12) being covered with a removable polymeric film (2), a control system (14) configured to control the displacement means (13) and the pressure exerted by the head (12) on the substrate (1), and • an infrared spectroscope to Fourier transform configured to perform FTIR-ATR analyzes. The impurity analysis system of claim 13, wherein the head (12) of the collection device (10) is a metal head. 15. A system for analyzing impurities according to any one of claims 13 or 14, wherein the head (12) of the collection device (10) is covered with a damping layer. 16. A system for analyzing impurities according to any one of claims 13 to 15, wherein the moving means (13) of the collection device (10) allow the vertical and horizontal movement of the head. 17. A system for analyzing impurities according to any one of claims 13 to 16, wherein the collection device (10) comprises means for storing the trajectory of the head. 18. A system for analyzing impurities according to any one of claims 13 to 17, wherein the collection device (10) comprises means for holding the substrate (1) on the support (11).