System for the continuous-wave laser processing and sem characterisation of a sample

Abstract

The invention relates to a system (10) for processing and characterising a sample (Ech), the system comprising: - a scanning electron microscope (SEM) comprising: • a chamber (Ch); • a device (COL) configured to generate an electron beam (FE) and to focus the electron beam onto the sample; • at least one first detector (Det); - a laser source (LAS) configured to generate a continuous-wave laser beam (FL); - a two-dimensional scanning device (DS) configured to move the laser beam over the sample at a given speed (v); - a focusing device (DFOC) configured to focus the laser beam onto the sample through the window and to modify a value of the focal length in a controlled manner; - the laser source and the focusing device being configured so that the focusing spot on the sample has a given power (P) and a given diameter (D).

Description

DESCRIPTION TITLE: SYSTEM FOR TREATING A SAMPLE BY CONTINUOUS WAVE LASER AND CHARACTERIZING A SAMPLE BY SEM FIELD OF THE INVENTION

[0001] The present invention relates to the field of sample processing and characterization, applied to materials research and engineering, and more particularly using a continuous wave laser for processing and a scanning electron microscope for characterization. STATE OF THE ART

[0002] Recent large-scale developments in industrial processes involve the development of materials through additive manufacturing using light / matter interaction. These processes lead to the formation of microstructures resulting from the high thermal gradient to which these materials are subjected, also found in welding. Understanding the result of this non-equilibrium metallurgy requires an ever-increasing knowledge of what happens during these processes, both for study purposes and for optimal practical implementation of these processes.

[0003] Today, this need for knowledge is only partially satisfied due to the complexity of the microstructures obtained and the capabilities of the interaction of light with the material. For example, the characterization with very high precision of microstructures resulting from additive manufacturing requires removing the part from the production line to insert it into a scanning electron microscope (SEM). The atmosphere in the additive machine is not controlled and the sample can oxidize during processing, making characterization by SEM very difficult or even impossible.

[0004] The publication "A novel combined device for laser modification in SEM" by K. Wetzig et al (SCANNING Vol 9, pages 99-107 (1987) describes the association of a pulsed laser with a SEM, a sample placed in the SEM chamber being illuminated by the very high energy pulsed laser. The objective of this study is to carry out an in-situ study of the physical processes implemented at a submicron scale during the irradiation of a surface by a very high energy pulsed laser. energy (3J per 10' draw 3 or 10' 8s). A SEM image is taken before and after irradiation. The laser beam is focused with a biconvex lens that reduces the laser spot size by a factor of 10, and the focusing spot is fixed. The intended applications are laser ablation or evaporation of material. This system is limited to a study / characterization of the effects induced on a metal sample by a very high energy pulsed laser on a point treatment area for destructive purposes.

[0005] An aim of the present invention is to remedy the aforementioned drawbacks by proposing a versatile system, capable of treating a sample of material typically over a millimetric area by an energetic light beam, and of characterizing a large number of materials by an electron beam, and adapted to a wide variety of processes, some of which have been developed in recent years. DESCRIPTION OF THE INVENTION

[0006] The present invention relates to a system for processing and characterizing a sample comprising: - a scanning electron microscope comprising: • a chamber containing a gas or under vacuum, in which said sample is placed and one wall of which includes a porthole, • a device configured to generate an electron beam and to focus the electron beam onto the sample, • at least one first detector configured to detect electrons from the sample, - a laser source configured to generate a continuous wave laser beam, - a two-dimensional scanning device configured to move the laser beam over said sample with a determined speed, - a focusing device configured to focus the laser beam on the sample through the window and to modify a focal value in a controlled manner, - the laser source and the focusing device being configured so that the focusing spot on the sample has a determined power and diameter.

[0007] According to one embodiment, the focusing device comprises a fixed focal length optic and a device configured to modify said focal length arranged upstream of the scanning device.

[0008] According to one embodiment, the system according to the invention comprises a beam splitter arranged upstream of the scanning device and a viewing camera arranged on the path of an optical beam reflected by the sample and reflected by the beam splitter, and configured to view the sample.

[0009] According to one embodiment, the system according to the invention comprises a thermal sensor configured to measure the temperature of an area comprising the laser focusing spot on the sample.

[0010] According to one embodiment, the system according to the invention further comprises a device for injecting a specific gas (DIG) into the chamber.

[0011] According to one embodiment, the first detector is a secondary electron detector.

[0012] According to one embodiment, the system according to the invention further comprises a second removable detector configured to detect electrons backscattered by the sample.

[0013] According to one embodiment, the system according to the invention further comprises a removable crystallographic camera (CC) configured to detect electrons back-diffracted by the sample.

[0014] According to one embodiment, the system according to the invention comprises a removable calorimeter allowing the calibration of the output power of the laser.

[0015] According to one embodiment, the scanning electron microscope is of the environmental type.

[0016] According to one embodiment, the laser source is coupled to an optical fiber, and furthermore the laser source, the two-dimensional scanning device and the device focusing are arranged in a single housing, the system further comprising a coupling part configured to interface the housing and the scanning electron microscope.

[0017] According to another aspect the invention relates to a method for treating and characterizing a sample, the sample being placed in a chamber of an electron scanning microscope, the chamber comprising a gas or being under vacuum, a wall of the chamber comprising a porthole, the method comprising: - a step 100 of generating a continuous wave laser beam having a modifiable power, - a processing step 200 comprising focusing the laser beam on the sample by passing through the window, the focal length value being modifiable in a controlled manner, and scanning the laser beam in two dimensions so as to move it over said sample with a determined speed, the focusing spot on the sample having a determined power and diameter, - a characterization step 300 consisting of producing a first so-called post-processing image with the scanning electron microscope, by illuminating the sample with a focused electron beam and detecting electrons from the sample.

[0018] According to one embodiment, the method according to the invention further comprises a step 50 consisting of producing a second so-called pre-processing image with the scanning electron microscope before the processing step 200.

[0019] The following description presents several exemplary embodiments of the device of the invention: these examples are not limiting of the scope of the invention. These exemplary embodiments present both the essential characteristics of the invention as well as additional characteristics linked to the embodiments considered.

[0020] The invention will be better understood and other features, objects and advantages thereof will become apparent during the detailed description which will follow and with regard to the attached drawings given as non-limiting examples and in which:

[0021] Figure 1 illustrates a system for processing and characterizing a sample according to the invention.

[0022] Figure 2 illustrates an example of implementation of the system according to the invention in which the MEB operates in “SE” mode and comprises a viewing camera and a thermal camera.

[0023] Figure 3 illustrates an example of implementation of the system according to the invention in which the MEB operates in “BSE” mode and comprising a device for injecting a specific gas.

[0024] Figure 4 illustrates an example of implementation of the system according to the invention in which the SEM operates in “EBSD” mode and comprises a crystallographic camera.

[0025] Figure 4bis illustrates an exemplary implementation of the system according to the invention in which the laser source, the output optical fiber, the two-dimensional scanning device and the focusing device are arranged in a single housing and in which the system comprises a coupling part configured to interface the housing and the SEM.

[0026] Figure 5 illustrates a first example of use of the system according to the invention with characterization in SEM mode.

[0027] Figure 6 illustrates a second example of use of the system according to the invention with characterization in EBSD mode, the SEM A image being taken before processing and the SEM B image being taken after processing.

[0028] Figure 7 illustrates a third example of use of the system according to the invention with a characterization in BSE mode. DETAILED DESCRIPTION OF THE INVENTION

[0029] The system 10 for treating and characterizing a sample Ech according to the invention is illustrated in figure 1. It comprises a scanning electron microscope SEM comprising a chamber Ch is under vacuum or under partial gas pressure, in which the sample to be treated / characterized is placed. One of the walls of the SEM contains a window H. In a known manner, the SEM comprises a device COL (commonly called a column) configured to generate an electron beam FE and to focus this electron beam on the sample, and at least one detector Det configured to detect electrons coming from the sample.

[0030] The system 10 according to the invention also comprises a laser source LAS configured to generate a continuous wave laser beam FL, a focusing device DFOC configured to focus the laser beam on the sample through the window H and to modify the value of the focal length in a controlled manner, and a two-dimensional scanning device DS configured to move the laser beam on said sample with a determined speed v.

[0031] The LAS laser source and the DFOC focusing device are configured so that the focusing spot on the sample has a determined power P and diameter D. To vary P, the laser power is adjusted, and to vary D, the focal length value is adjusted. The FL beam enters the Ch chamber via an interface porthole H, preferably laser-quality treated (specific surface treatment). The porthole is transparent to the laser wavelength and also seals the chamber. The porthole seals the chamber and allows the FL laser beam to enter the Ch chamber and access the sample.

[0032] Typically an SEM image of the sample is taken after laser treatment, and optionally also before treatment for comparison.

[0033] The invention thus makes it possible to treat and characterize a sample of material in the same instrument, with a continuous wave laser beam whose parameters (P, D, v) can be modified.

[0034] With the use of a continuous wave laser, the energy input is constant over time, and according to a completely controlled thermal gradient. This allows the study and implementation of physical and chemical processes: - modification of the material, such as change of state, phase, change of microstructure, surface condition, evolution of roughness, assembly, etc. - restoration or repair (solidification or resolidification).

[0035] With the system according to the invention the spot parameters P (power) and D (size) are adjustable, which makes it possible to adapt the surface energy density incident on the sample and the temperature gradient to which it is subjected, which can reach one million degrees / cm. Typically the beam diameter varies, depending on the application, over a range included in the interval [45 - 500 pm],

[0036] Furthermore, the sample treatment is carried out in the controlled atmosphere (vacuum, humidity, neutral or reducing gas, etc.) of the SEM Ch chamber. For example, laser treatment under vacuum protects the sample from damage caused by humidity and oxidation.

[0037] The adjustment of the scanning speed v of the DS device provides a multi-scale character, with an accessible scanning amplitude typically ranging from micron to millimeter, and it is also possible to complexify the scanning shape (scanning strategy).

[0038] This allows us to control the time, temperature and atmosphere factors.

[0039] Thanks to the system according to the invention integrating the control of the aforementioned parameters, it is possible to increase the surface quality and the mechanical properties of the sample material. It is also possible to obtain new properties of the laser-treated material by changing its local microstructure and to characterize the changes produced by the laser treatment.

[0040] The system according to the invention can be implemented with various materials such as ceramics, minerals, metals, polymers, etc.

[0041] The scientific study of the results obtained with the system according to the invention opens up a field of experimental and digital knowledge.

[0042] With the system according to the invention, it is possible to reproduce heat treatment similar to that which takes place in different industrial processes (additive manufacturing, heat treatment, tempering, quenching, welding, etc.), making it possible to understand the nature of the interactions between industrial lasers and materials during these processes, and to propose optimized heat treatments during and / or after the implementation of these processes.

[0043] The system according to the invention also finds its place in industrial centers, for example additive manufacturing or surface structuring. It allows to control the in situ evolution of the material treated by the laser. The system according to the invention is integrated upstream or downstream of a process to verify the consequence(s) of the modification of a manufacturing parameter on the evolution of the microstructure of the material.

[0044] The system for processing and characterizing a sample according to the invention is thus extremely versatile and meets a need for equipment that has not been met to date, as explained above.

[0045] The system according to the invention has different embodiments which can be combined with each other.

[0046] According to an embodiment illustrated in Figure 2, the focusing device comprises an optic with a fixed focal length L and a device 20 configured to modify the value of the focal length and arranged upstream of the scanning device, itself arranged upstream of the optic L. The device 20 is for example a varioScan R which includes a diverging lens that diverges the beam at the laser output, and therefore makes it possible to modify the size of the laser spot impacting the sample.

[0047] The scanning device is for example a goniometric mirror.

[0048] Accessories also illustrated in Figure 2 allow visualization of the beam impact zone on the sample.

[0049] According to one embodiment, the system also comprises a beam splitter BS arranged upstream of the scanning device and a viewing camera CV arranged on the path of the optical beam FLR reflected by the sample Ech and then by the beam splitter BS. The visible camera thus recovers the light reflected by the sample, which makes it possible to view the surface of the sample, the future trajectory of the laser as well as the location where the laser spot has passed.

[0050] According to another embodiment, the system 10 according to the invention comprises a CT thermal sensor for measuring the temperature of an area comprising the laser focusing spot on the sample. Preferably, a 2D thermal camera is used which makes it possible to carry out infrared thermography, such as visualizing the distribution of the temperature field induced by laser treatment.

[0051] In Figure 2 the electron detector shown is an electron detector commonly called secondary DES, classically used in scanning electron microscopy operating in secondary electron mode or "Secondary Electron Mode" SEM in English. Classically this mode is installed by default in commercial SEMs, and makes it possible to obtain a topographic contrast image of the observed surface.

[0052] Figure 2 also shows a removable BP protective shield, which protects the MEB column during laser firing.

[0053] Figure 3 illustrates an embodiment of the system according to the invention in which the SEM operates in so-called "BSE" mode for BackScattered Electron in English. This mode produces an atomic number contrast image allowing information to be obtained on the chemical nature of the elements observed. In this mode the system comprises a second electron detector which is a backscattered electron detector DER. This detector is preferably removable and is only positioned in the chamber when the SEM must operate in this mode.

[0054] According to an embodiment also illustrated in Figure 3, the system 10 comprises a device for injecting a specific DIG gas into the chamber Ch, which allows control of the environment during laser treatment.

[0055] Figure 4 illustrates an embodiment of the system according to the invention in which the SEM operates in so-called "EBSD" mode for "Electron Back Scattered Diffraction" in English. In this mode the system comprises a removable DC crystallographic camera configured to detect electrons backscattered / diffracting by the sample. This mode makes it possible to view the result of the laser treatment in order to characterize the microstructure from a crystallographic point of view (specific imaging).

[0056] According to one embodiment, the scanning microscope is of the environmental type, or MEBE, which allows the control of the gaseous environment of the part to be treated. In addition, the use of a MEBE in the system according to The invention allows the implementation of all related physical and chemical techniques, using particle physics (X-ray spectrometry, electron diffraction, IR thermography).

[0057] According to one embodiment, the system comprises a removable calorimeter allowing calibration of the laser output power.

[0058] . According to a non-limiting example, the LAS laser is a class 4 fiber laser having an adjustable fiber output power of 20 to 200 W (collimated beam) and emitting at a wavelength of 1070 + / - 2 nm.

[0059] According to one embodiment, the laser is a laser coupled to an output optical fiber. The laser source, the output optical fiber, the two-dimensional scanning device and the focusing device are arranged in a single housing BT. According to one embodiment, the system also comprises a coupling part PA configured to interface the housing and the SEM, as illustrated in Figure 4bis. This part makes it possible to confine the focusing lens of the housing.

[0060] Figure 5 illustrates a first example of use of the system according to the invention. This secondary electron image (SEM) makes it possible to identify the location and to evaluate the width of the area affected by the passage of the laser. In this experiment, five laser shots were carried out with a power of 24 W, with a spot size of 60 pm. Tracks 1 to 5 correspond respectively to a laser feed speed of 50, 100, 250, 500 and 1000 mm / s.

[0061] Figure 6 illustrates a second example of use of the system according to the invention, image A is the image before laser treatment and image B after. The backscattered electron diffraction type analysis obtained with the EBSD camera highlights the crystallographic nature of the microstructure of the material and the modification of the latter by the laser. Comparison of images A (before laser treatment) and B (after laser treatment) shows that the grains constituting the microstructure and the surface state are modified by the passage of the laser (zone 60).

[0062] Figure 7 illustrates a third example of use of the system according to the invention. The backscattered electron type imaging (BSE mode) obtained with The electron microscope highlights the contrast in chemical composition constituting the microstructure of the observed material and the modification of the latter by the laser. In the lower part (raw area) we observe the network (lighter in the image due to an enrichment in chromium and molybdenum) and the cells (interconnected honeycomb network, darker in the image). In the upper treated part we see that the network changes, its topography is greatly reduced and the cells are smaller. This change in structure induces modifications in the mechanical properties of the material.

[0063] Thus, the system according to the invention allows an in situ comparison before / after treatment without moving the sample between laser treatment and characterization. This comparison of images taken in situ represents a significant time saving and the use of the system according to the invention leads to more reliable measurements because they are carried out in a controlled atmosphere. According to another aspect, the invention relates to a method for processing and characterizing a sample Ech. The sample is placed in the chamber Ch of a SEM), the chamber comprising a gas or being under vacuum. A wall of the chamber comprises a porthole H. The method comprises a step 100 of generating a continuous wave laser beam FL having a modifiable power P. Then, in a processing step 200, the laser beam is focused on the sample by passing through the porthole and the laser beam is scanned in two dimensions so as to move it over the sample with a determined speed v. The value of the focal length can be modified in a controlled manner, the focusing spot on the sample having a determined power P and a diameter D.Finally, in a characterization step 300, a first so-called post-processing image is produced with the SEM, by illuminating the sample with a focused FE electron beam and detecting electrons from the sample.

[0064] According to one embodiment, the method according to the invention further comprises a step 50 consisting of producing a second so-called pre-processing image with the SEM before the processing step 200.

Claims

CLAIMS System (10) for processing and characterizing a sample (Ech) comprising: - a scanning electron microscope (SEM) comprising: • a chamber (Ch) containing a gas or under vacuum, in which said sample is placed and one wall of which includes a porthole (H), • a device (COL) configured to generate an electron beam (FE) and to focus the electron beam on the sample, • at least one first detector (Det) configured to detect electrons from the sample, - a laser source (LAS) configured to generate a continuous wave laser beam (FL), - a two-dimensional scanning device (DS) configured to move the laser beam over said sample with a determined speed (v), - a focusing device (DFOC) configured to focus the laser beam on the sample through the window and to modify a focal value in a controlled manner, - the laser source and the focusing device being configured so that the focusing spot on the sample has a determined power (P) and diameter (D). System according to one of the preceding claims wherein the focusing device comprises an optic with a fixed focal length (L) and a device (20) configured to modify said focal length arranged upstream of the scanning device. System according to one of the preceding claims comprising a beam splitter (BS) arranged upstream of the scanning device and a viewing camera (CV) arranged on the path of an optical beam (FLR) reflected by the sample and reflected by the beam splitter, and configured to view the sample. . System according to one of the preceding claims comprising a thermal sensor (CT) configured to measure the temperature of an area comprising the laser focusing spot on the sample. . System according to one of the preceding claims further comprising a device for injecting a determined gas (DIG) into the chamber. . System according to one of the preceding claims in which the first detector is a secondary electron detector (DES). . System according to one of the preceding claims further comprising a second removable detector (DER) configured to detect electrons backscattered by the sample. . System according to one of the preceding claims further comprising a removable crystallographic camera (CC) configured to detect electrons backdiffracted by the sample. . System according to one of the preceding claims comprising a removable calorimeter allowing the calibration of the output power of the laser. 0.System according to one of the preceding claims in which the scanning electron microscope is of the environmental type (MEBE).

1. System according to one of the preceding claims in which the laser source is coupled to an optical fiber and in which the laser source, the two-dimensional scanning device and the focusing device are arranged in one. single housing, the system further comprising a coupling part configured to interface the housing and the scanning electron microscope. Method for processing and characterizing a sample (Ech), the sample being placed in a chamber (Ch) of a scanning electron microscope (SEM), the chamber comprising a gas or being under vacuum, a wall of said chamber comprising a porthole (H), the method comprising: - a step 100 of generating a continuous wave laser beam (FL) having a modifiable power (P), - a processing step 200 comprising focusing the laser beam on the sample by passing through the window, the focal length value being modifiable in a controlled manner, and scanning the laser beam in two dimensions so as to move it over said sample with a determined speed (v), the focusing spot on the sample having a determined power (P) and diameter (D), - a characterization step 300 consisting of producing a first so-called post-processing image with the scanning electron microscope, by illuminating the sample with a focused electron beam (FE) and by detecting electrons from the sample. Method according to the preceding claim further comprising a step 50 consisting of producing a second so-called pre-processing image with the scanning electron microscope before the processing step 200.

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