System for continuous wave laser processing and SEM characterization of samples

Abstract

The present invention relates to a system (10) for processing and characterizing a sample, the system comprising: a scanning electron microscope (SEM) comprising: a chamber (Ch), a device (COL) configured to generate an electron beam (FE) and 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 scan the continuous wave laser beam (FL), the apparatus includes a moving device configured to move the laser beam on the sample at a defined velocity (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 are configured such that a focused spot on the sample has a defined power (P) and a defined diameter (D).

Description

Technical Field

[0001] The present invention relates to the field of sample processing and characterization, and is applied to materials research and engineering. More specifically, it relates to a continuous wave laser for processing and a scanning electron microscope for characterization. Background Art

[0002] Recently, large-scale developments in industrial processes have involved the development of materials through additive manufacturing using light / matter interactions. These processes result in the formation of microstructures due to the strong thermal gradients experienced by these materials, which are also found in welding. Understanding the results of this unbalanced metallurgy requires more detailed knowledge of what happens during these processes, not only for research purposes but also for the 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 ability to interact optically with matter. For example, very high-precision characterization of microstructures obtained from additive manufacturing requires the part to be removed from the production line and inserted into a scanning electron microscope (SEM). The air in the additive machine is uncontrolled, and the sample can oxidize during the process, making characterization by SEM very difficult if not impossible.

[0004] The publication “A novel combined device for laser modification in SEM”, by K. Wetzig et al. (Scanning, Volume 9, pages 99 - 107 (1987)) describes the association of a pulsed laser with an SEM, where a sample placed in the chamber of the SEM is irradiated with a very high-energy pulsed laser. The aim of this study was to perform an in-situ study of the physical processes occurring at the sub-micron scale during the irradiation of a surface with a pulsed laser at very high energy (3 J per pulse with a duration of 10 - 3 or 10 - 8 s). SEM images were taken before and after irradiation. A double convex lens was used to focus the laser, which reduced the size of the laser spot by a factor of 10, and the focused spot was fixed. The target application was laser ablation or evaporation of matter. This system was limited to studying / characterizing the effects induced by a very high-energy pulsed laser for destructive purposes on a metal sample in a punctual processing area.

[0005] The object of the present invention is to remedy the above-mentioned drawbacks by proposing a general system that is capable of generally processing samples of materials over a millimeter area with a high-energy beam and characterizing large amounts of materials with an electron beam, and that is suitable for a variety of processes, some of which have been developed in recent years. Summary of the Invention

[0006] The present invention relates to a system for processing and characterizing samples, the system comprising:

[0007] Scanning electron microscope, including:

[0008] a chamber containing a gas or under vacuum, the sample being arranged in the chamber, and one wall of the chamber comprising a window,

[0009] a device configured to generate an electron beam and to focus said electron beam onto said sample,

[0010] at least one first detector configured to detect electrons emitted from the sample,

[0011] - a laser source configured to generate a continuous wave laser beam,

[0012] - a two-dimensional scanning device configured to move the laser beam over the sample at a defined speed,

[0013] - a focusing device configured to focus the laser beam onto the sample through the window and to modify the value of the focal length in a controlled manner,

[0014] - The laser source and the focusing device are configured such that the focused spot on the sample has a defined power and a defined diameter.

[0015] According to one embodiment, the focusing device comprises an optical system with a fixed focal length, and a device configured to modify the focal length and arranged upstream of the scanning device.

[0016] According to one embodiment, the system according to the invention comprises a beam splitter arranged upstream of the scanning device, and a display camera arranged in the path of the light beam reflected by the sample and by the beam splitter and configured to display the sample.

[0017] According to one embodiment, the system according to the invention comprises a thermal sensor configured to measure the temperature of a region comprising the laser focus spot on the sample.

[0018] According to one embodiment, the system according to the invention further comprises a device (DIG) for injecting a confinement gas into said chamber.

[0019] According to one embodiment, the first detector is a secondary electron detector.

[0020] According to one embodiment, the system according to the invention further comprises a second removable detector configured to detect electrons backscattered by said sample.

[0021] According to one embodiment, the system according to the invention further comprises a removable crystallographic camera (CC) configured to detect electrons backscattered by said sample.

[0022] According to one embodiment, the system according to the invention comprises a removable calorimeter for calibrating the laser output power.

[0023] According to one embodiment, the scanning electron microscope is of ambient type.

[0024] 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 focusing device are arranged in a single housing, the system further comprising a coupling portion configured to interface with the housing and the scanning electron microscope.

[0025] According to another aspect, the invention relates to a method for processing and characterizing a sample, the sample being arranged in a chamber of a scanning electron microscope, the chamber comprising a gas or being under vacuum, the walls of the chamber comprising a window, the method comprising:

[0026] - a step 100 of generating a continuous wave laser beam with modifiable power,

[0027] - a processing step 200 comprising focusing the laser beam onto the sample by passing the laser beam through the window, the value of the focal length being modifiable in a controlled manner, and performing the two-dimensional scanning of the laser beam so as to move the laser beam over the sample at a defined speed, the focused spot on the sample having a defined power and a defined diameter,

[0028] A characterization step 300 consisting in producing, with the scanning electron microscope, a first image, called a post-processed image, by irradiating the sample with a focused electron beam and detecting electrons emitted from the sample.

[0029] According to one embodiment, the method according to the invention further comprises, before the processing step 200 , a step 50 consisting in generating, using said scanning electron microscope, a second image, referred to as a pre-processed image.

[0030] The following description presents several exemplary embodiments of the device of the invention: these examples do not limit the scope of the invention. These exemplary embodiments contain not only the features essential to the invention but also additional features associated with the embodiment in question. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will be better understood, and other features, objects and advantages thereof will become apparent, from the following detailed description provided with reference to the accompanying drawings, which are defined by way of non-limiting examples, and in which:

[0032] Figure 1 Shows a system for processing and characterizing a sample according to the present invention.

[0033] Figure 2 Shows an example of an implementation of the system according to the present invention, wherein the SEM operates in "SE" mode and includes a display camera and a thermal camera.

[0034] Figure 3 Shows an example of an implementation of the system according to the present invention, wherein the SEM operates in "BSE" mode and includes a device for injecting a defined gas.

[0035] Figure 4 Shows an example of an implementation of the system according to the present invention, wherein the SEM operates in "EBSD" mode and includes a crystallography camera.

[0036] Figure 4a Shows an example of an implementation of the system according to the present invention, wherein a laser source, an output optical fiber, a two-dimensional scanning device, and a focusing device are arranged in a single housing, and wherein the system includes a coupling part configured to dock the housing and the SEM.

[0037] Figure 5 Shows a first example of the use of a system for characterization in SEM mode according to the present invention.

[0038] Figure 6 Shows a second example of the use of a system for characterization in EBSD mode according to the present invention, where an SEM image A is taken before processing and an SEM image B is taken after processing.

[0039] Figure 7 Shows a third example of the use of a system for characterization in BSE mode according to the present invention. Detailed Description

[0040] The system 10 according to the present invention for processing and characterizing a sample is shown in Figure 1 It includes a scanning electron microscope SEM, which includes a chamber Ch under vacuum or partial gas pressure, wherein the sample to be processed / characterized is arranged in the chamber Ch. One of the walls of the SEM contains a window H. In a known manner, the SEM includes a device COL (commonly called a column), which is configured to generate an electron beam FE and focus the electron beam onto the sample, and at least one detector Det, which is configured to detect electrons emitted from the sample.

[0041] System 10 according to the present invention further includes: a laser source LAS configured to generate a continuous wave laser beam FL; a focusing device DFOC configured to focus the laser beam onto a sample through a window H and modify the value of the focal length in a controlled manner; and a two-dimensional scanning device DS configured to move the laser beam over the sample at a defined speed v.

[0042] The laser source LAS and the focusing device DFOC are configured such that the focused spot on the sample has a defined power P and a defined diameter D. To change P, the power of the adjustable laser can be changed, and to change D, the value of the focal length is changed. The beam FL enters the chamber Ch via the interface window H, which is preferably treated for laser quality (specific surface treatment). The window is transparent to the wavelength of the laser and also ensures the sealing of the chamber. The window seals the chamber and allows the laser beam FL to enter the chamber Ch and approach the sample.

[0043] Typically, the SEM image of the sample is taken after laser processing and optionally also processed before comparison.

[0044] Thus, the present invention makes it possible to process and characterize samples of materials in the same instrument using a continuous wave laser beam whose parameters (P, D, v) can be modified.

[0045] By using a continuous wave laser, the energy input is constant over time and according to a fully controlled thermal gradient. This allows the study and implementation of physical and chemical processes:

[0046] - Modification of materials, such as changes in state or phase, changes in microstructure or surface state, evolution of roughness, assembly, etc.,

[0047] - Recovery or repair (solidification or re-solidification).

[0048] Using the system according to the present 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 it undergoes (which can reach one million degrees per centimeter). Typically, the diameter of the beam varies within the range [45–500 μm] depending on the application.

[0049] Furthermore, the sample is processed in a controlled atmosphere (vacuum, humidity, neutral gas or reducing gas, etc.) in the chamber Ch of the SEM. For example, laser processing under vacuum protects the sample from moisture and oxidation.

[0050] Adjustment of the scanning speed v of the device DS provides multi-scale characterization, where the accessible scanning amplitude is typically in the range from microns to millimeters and can also complicate the shape of the scan (scanning strategy).

[0051] Therefore, the factors of time, temperature, and air are controlled.

[0052] With the aid of a control system integrating the above parameters according to the present invention, it becomes possible to increase the surface quality and mechanical properties of the material of the sample. It is also possible to obtain new properties of the laser-treated material by changing its local microstructure and to characterize the changes generated by the laser treatment.

[0053] The system according to the present invention can be implemented with various materials, such as ceramics, minerals, metals, polymers, etc.

[0054] Scientific research on the results obtained using the system according to the present invention has opened up fields of experimental and numerical knowledge.

[0055] Using the system according to the present invention, it is possible to reproduce heat treatments similar to those occurring in various industrial processes (additive manufacturing, heat treatment, annealing, tempering, welding, etc.), making it possible to understand the nature of the interaction between the industrial laser and the material during these processes and to propose optimized heat treatments during and / or after the implementation of these processes.

[0056] The system according to the present invention also finds applications in industrial centers, such as additive manufacturing or surface structuring. It makes it possible to control the in-situ evolution of the material treated by the laser. The system according to the present invention is integrated upstream or downstream of a process for verifying the results or consequences of modifying manufacturing parameters on the evolution of the microstructure of the material.

[0057] Therefore, the system for treating and characterizing samples according to the present invention is extremely versatile and meets a need for equipment that has not been met so far, as explained above.

[0058] The system according to the present invention has various embodiments that can be combined with each other.

[0059] According to Figure 2 the embodiment shown in, the focusing device includes an optical system L with a fixed focal length, and a device 20, which is configured to modify the value of the focal length and is arranged upstream of the scanning device, which itself is arranged upstream of the optical system L. The device 20 is, for example, a varioScanR, which includes a diverging lens that diverges the beam at the laser output and thus makes it possible to modify the size of the laser spot affecting the sample.

[0060] The scanning device is, for example, a goniometer.

[0061] Also shown in Figure 2 the accessory allows visualization of the area of the sample affected by the beam.

[0062] According to one embodiment, the system further comprises a beam splitter BS arranged upstream of the scanning device, and a visualization camera CV arranged in the path of the light beam FLR reflected by the sample and then by the beam splitter BS. The visualization camera thus recovers the light reflected by the sample, which makes it possible to visualize the surface of the sample, the future path of the laser light, and the positions through which the laser spot has already passed.

[0063] According to another embodiment, the system 10 according to the invention comprises a thermal sensor CT for measuring the temperature of the area comprising the laser focus spot on the sample. Preferably, a 2D thermal camera is used, which makes it possible to perform infrared thermography, for example to visualize the distribution of the temperature field caused by the laser treatment.

[0064] exist Figure 2 In FIG, the electron detector shown is an electron detector commonly known as a secondary electron detector DES, which is conventionally used in scanning electron microscopes operating in secondary electron mode. This mode is conventionally installed by default in commercial SEMs and makes it possible to obtain topographic contrast images of the observed surface.

[0065] Figure 2 Also shown is a removable protective cover BP, which protects the SEM column during laser firing.

[0066] Figure 3 An embodiment of a system according to the present invention is shown in which the SEM is operated in backscattered electron (BSE) mode. This mode produces atomic number contrast images, allowing information to be obtained on the chemical properties of the observed elements. In this mode, the system includes a second electron detector, a backscattered electron detector (DER). This detector is preferably removable and is only placed in the chamber when the SEM must be operated in this mode.

[0067] According to the still Figure 3 In the embodiment shown in FIG. 1 , the system 10 comprises a device DIG for injecting a confining gas into the chamber Ch, which allows controlling the environment during laser processing.

[0068] Figure 4 An embodiment of a system according to the present invention is shown in which the SEM is operated in electron backscatter diffraction (EBSD) mode. In this mode, the system includes a removable crystallographic camera CC configured to detect electrons backscattered / diffracted by the sample. This mode enables viewing the results of laser processing to characterize microstructures from a crystallographic perspective (specifically imaging).

[0069] According to one embodiment, the scanning microscope is of the environmental type or ESEM, which allows monitoring of the gas environment of the part to be processed. Additionally, the use of an ESEM in the system according to the present invention allows implementation of all relevant physical and chemical techniques using particle physics (X-ray spectroscopy, electron diffraction, IR thermography).

[0070] According to one embodiment, the system includes a removable calorimeter for calibrating the laser output power.

[0071] By way of non-limiting example, the laser LAS is a 4-level fiber laser with an adjustable fiber output power of 20 to 200 W (collimated beam) and emitting at a wavelength of 1070 + / - 2 nm.

[0072] According to one embodiment, the laser is a laser coupled to an output fiber. The laser source, output fiber, two-dimensional scanning device, and focusing device are arranged in a single housing BT. According to one embodiment, the system further includes a coupling member PA configured to engage the housing and the SEM, as Figure 4a shown. This part makes it possible to limit the focusing lens of the housing.

[0073] Figure 5 A first example of the use of the system according to the present invention is shown. This secondary electron image (SEM) makes it possible to identify the position and evaluate the width of the area affected by the laser channel. In this experiment, five laser emissions were made with a power of 24 W and a spot size of 60 μm. Racks 1 to 5 correspond to laser advance speeds of 50, 100, 250, 500, and 1000 mm / s respectively.

[0074] Figure 6 A second example of the use of the system according to the present invention is shown; Image A is the image before laser treatment, while Image B is the image after laser treatment. Analysis of the type of backscattered electron diffraction obtained using an EBSD camera reveals the crystalline characteristics of the material microstructure and the modification of the latter by the laser. Comparison of Image A (before laser treatment) and B (after laser treatment) shows that the grains constituting the microstructure and surface state are modified by the laser channel (area 60).

[0075] Figure 7 A third example of the use of the system according to the present invention is shown. Backscattered electron imaging (BSE mode) obtained using an electron microscope reveals the contrast between the chemical composition of the microstructure constituting the observed material and the modification of the latter by the laser. The lower part (untreated area) shows a network (lighter in the image due to chromium and molybdenum enrichment) and cells (interconnected honeycomb network darker in the image). The upper part of the treatment shows that the network is modified; its topography is greatly reduced and the cells are smaller. This structural change causes a modification of the mechanical properties of the material.

[0076] Thus, the system according to the present invention allows in-situ comparison before / after processing without moving the sample between laser processing and characterization. This comparison of in-situ captured images represents a significant time saving, and the use of the system according to the present invention results in more reliable measurements as they are carried out under controlled air.

[0077] According to another aspect, the present invention relates to a method for processing and characterizing a sample. The sample is disposed in a chamber Ch of an SEM, the chamber being either gaseous or under vacuum. One wall of the chamber includes a window H. The method includes 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 onto the sample by passing through the window, and the laser beam is scanned in two dimensions so as to move the laser beam across the sample at a defined speed v. The value of the focal length can be modified in a controlled manner, and the focused spot on the sample has a defined power P and a defined diameter D. Finally, in a characterization step 300, a first image called a post-processing image is generated using the SEM by irradiating the sample with a focused electron beam FE and detecting the electrons emitted from the sample.

[0078] According to one embodiment, the method according to the present invention further includes a step 50 of generating, using the SEM, a second image called a pre-processing image before the processing step 200.

Claims

1. A system (10) for processing and characterizing a sample, the system comprising: A scanning electron microscope (SEM), comprising: A chamber (Ch) that contains gas or is under vacuum, wherein the sample is disposed in the chamber, and one wall of the chamber includes a window (H), 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) configured to detect electrons emitted 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 the sample at a defined speed (v), A focusing device (DFOC) configured to focus the laser beam through the window onto the sample and to modify the value of the focal length in a controlled manner, The laser source and the focusing device are configured such that the focused spot on the sample has a defined power (P) and a defined diameter (D).

2. The system according to any one of the preceding claims, wherein, The focusing device includes an optical system (L) with a fixed focal length, and a device (20) configured to modify the focal length and disposed upstream of the scanning device.

3. The system according to any one of the preceding claims, comprising: A beam splitter (BS) disposed upstream of the scanning device; And a display camera (CV) disposed in the path of the light beam (FLR) reflected by the sample and reflected by the beam splitter, and configured to display the sample.

4. The system according to any one of the preceding claims, comprising: A thermal sensor (CT) configured to measure the temperature of a region including the laser focused spot on the sample.

5. The system according to any one of the preceding claims, further comprising: A device (DIG) for injecting a defined gas into the chamber.

6. The system according to any one of the preceding claims, wherein, The first detector is a secondary electron detector (DES).

7. The system according to any one of the preceding claims, further comprising: A second removable detector (DER) configured to detect electrons backscattered by the sample.

8. The system according to any one of the preceding claims, further comprising: A removable crystallography camera (CC) configured to detect electrons backscattered by the sample.

9. The system according to any one of the preceding claims, comprising: A removable calorimeter for calibrating the laser output power.

10. The system according to any one of the preceding claims, wherein, The scanning electron microscope has an environmental type (ESEM).

11. The system according to any one of the preceding claims, wherein, The laser source is coupled to an optical fiber, and wherein the laser source, the two-dimensional scanning device, and the focusing device are disposed in a single housing, and the system further includes a coupling portion configured to dock the housing and the scanning electron microscope.

12. A method for processing and characterizing a sample, the sample being disposed in a chamber (Ch) of a scanning electron microscope (SEM), the chamber including gas or being under vacuum, a wall of the chamber including a window (H), the method comprising: Step 100, which generates a continuous wave laser beam (FL) having a modifiable power (P), A processing step 200, which includes focusing the laser beam onto the sample by passing the laser beam through the window, the value of the focal length being modifiable in a controlled manner, and two-dimensionally scanning the laser beam in order to move the laser beam over the sample at a defined speed (v), the focused spot on the sample having a defined power (P) and a defined diameter (D), Characterization step 300, which consists in generating a first image called a post-processed image by irradiating the sample with a focused electron beam (FE) using the scanning electron microscope and detecting electrons emitted from the sample.

13. The method according to claim 12, further comprising: Step 50 of generating a second image called a pre-processed image using a scanning electron microscope before the processing step 200.

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