Target post film thickness measurement imaging system and film thickness measurement method for laser ion acceleration

Abstract

The application discloses a target post film thickness measurement imaging system for laser ion acceleration, and forms the following optical paths after the target: an incident light path, in which incident light rays are emitted by an illumination light source, enter a microscopic objective lens after passing through a first beam splitting cube, and are focused by the microscopic objective lens to irradiate the target; and a reflected light path, in which reflected light is formed after the target is irradiated by the incident light, enters the first beam splitting cube through the microscopic objective lens, is split into two beams by the first beam splitting cube, one of the two beams enters a spectrometer, and the other enters a second beam splitting cube, is split into two beams again by the second beam splitting cube, one of the two beams enters a first camera, and the other enters a second camera. The target post film thickness measurement imaging system for laser ion acceleration disclosed by the application solves the problems of target imaging illumination and target film thickness detection simultaneously through one system, reduces the complexity of the system, and saves space.

Description

Technical Field

[0001] This invention relates to a target-back film thickness measurement imaging system and film thickness measurement method for laser ion acceleration, belonging to the field of thickness measurement. Background Technology

[0002] Beam-target coupling (coupling the laser beam to a thin film target) is a key issue in laser-ion acceleration. It involves adjusting the target position before using the laser beam to strike the target so that the laser focal length is located at a specified position on the target.

[0003] Existing methods primarily rely on a post-target system. Specifically, using a post-target camera as a reference, the laser focal spot is aligned spatially with the thin-film target to achieve beam-target coupling. However, this method typically requires independent components or optical paths when illuminating the target within the target cavity, and different optical paths are needed for illuminating transparent and metallic targets. This results in complex optical path setups and the need to switch between different units to achieve beam-target coupling, wasting significant costs and time.

[0004] In addition, target film thickness is also an important parameter in laser ion acceleration. Existing post-target systems cannot measure the target film thickness in real time while imaging the target, so additional means are needed to measure the target film thickness.

[0005] Existing film thickness measurement methods and equipment cannot measure thin films within a vacuum chamber. Traditional measurement methods are all offline, and the equipment used for these measurements occupies a large space, making them difficult to place in a vacuum chamber. For these reasons, it is necessary to further study beam-target coupling and back-target film thickness measurement for laser ion acceleration to solve these problems. Summary of the Invention

[0006] To overcome the above problems, the inventors conducted in-depth research and designed a target-back film thickness measurement imaging system for laser ion acceleration. A microscope objective 2, a first beam-splitting cube 31, a second beam-splitting cube 32, a first camera 41, a second camera 42, an illumination source 5, and a spectrometer 6 are arranged behind the target 1, thus forming the following optical path:

[0007] The incident light path is formed by the illumination source 5, which emits incident light rays, which enter the microscope objective 2 after passing through the first beam splitting cube 31, and is focused by the microscope objective 2 to illuminate the target 1.

[0008] The reflected light path is as follows: after the incident light irradiates the target 1, it forms reflected light. The reflected light enters the first beam splitting cube 31 through the microscope objective 2. After passing through the first beam splitting cube 31, it is split into two beams. One beam enters the spectrometer 6, and the other beam enters the second beam splitting cube 32. After passing through the second beam splitting cube 32, it is split into two beams again. One beam enters the first camera 41, and the other beam enters the second camera 42.

[0009] In a preferred embodiment, the spectrometer 6 and the illumination source 5 are disposed outside the vacuum chamber, while the target 1, the first beam splitter cube 31, the second beam splitter cube 32, the first camera 41 and the second camera 42 are disposed inside the vacuum chamber.

[0010] In a preferred embodiment, the incident light emitted by the illumination source 5 enters the optical path through the feedthrough fiber, and the spectrometer 6 acquires the reflected light in the optical path through the feedthrough fiber.

[0011] The feedthrough fiber connecting the illumination source 5 and the feedthrough fiber connecting the spectrometer 6 are bundled together, so that the incident light and the reflected light are coaxial, thereby enabling the spectrometer 6 to measure the incident light and the reflected light.

[0012] The feedthrough fiber connecting the lighting source 5 and the feedthrough fiber connecting the spectrometer 6 are bundled together through the fiber feedthrough flange.

[0013] In a preferred embodiment, the film thickness of the target 1 is obtained by measuring the incident and reflected light using a spectrometer 6.

[0014] In a preferred embodiment, the magnification of the first camera 41 is less than that of the second camera 42;

[0015] The first camera 41 has a large field of view. By moving the position of the target 1, a light spot is obtained in the first camera 41, and the position of the target 1 is roughly determined.

[0016] The second camera 42 has a high imaging magnification. By adjusting the front and rear positions of the target 1 in the optical path, the light spot in the second camera 42 becomes circular and has the smallest diameter. At this time, the target 1 is located at the focusing position of the optical path, and the angle between the incident light and the target film is 90 degrees, thus completing the beam-target coupling.

[0017] This invention also discloses a method for measuring the thickness of a target film used in laser ion acceleration, which measures the target film thickness through the following steps:

[0018] S1. Irradiate the target, detect the incident light and reflected light, and obtain the target film reflectivity.

[0019] S2. Determine the target film thickness based on the relationship between reflectivity, incident light wavelength, and target film thickness.

[0020] In a preferred embodiment, before S1, there is also S0, where beam-target coupling is performed:

[0021] By moving the target position, a light spot is obtained in the first camera to roughly determine the target position; then the front and back positions of the target in the optical path are adjusted so that the light spot in the second camera is circular and has the smallest diameter, so that the target is located at the focal position of the optical path.

[0022] In a preferred embodiment, in S2, the relationship between reflectivity, incident light wavelength, and target film thickness is as follows:

[0023]

[0024] Where λ represents the wavelength of the incident light, θ represents the incident angle, and h represents the thickness of the target film;

[0025] R(h,λ,θ) represents the reflectivity of the target film to incident light, R0(λ,θ) represents the reflectivity of the target film interface as determined by Fresnel's law of reflection, and δ(h,λ,θ) represents the optical path difference of the target film, which is expressed as:

[0026]

[0027] n(λ) represents the refractive index function of the target film material.

[0028] In a preferred embodiment, at least two wavelengths of monochromatic light are used as incident light, and a target film thickness is obtained independently for each monochromatic light through S1 to S2.

[0029] In a preferred embodiment, the method further includes:

[0030] S3. Obtain the film thickness resolution corresponding to different monochromatic lights, and select the target film thickness obtained corresponding to the monochromatic light with the smaller film thickness resolution as the final measurement result.

[0031] The film thickness resolution is used to indicate the accuracy of film thickness measurement.

[0032] The beneficial effects of this invention include:

[0033] (1) By using a single system, the problems of target imaging illumination and target film thickness detection are solved simultaneously, reducing the complexity of the system and saving space;

[0034] (2) The target film thickness can be obtained in real time and online in a vacuum chamber, and the film thickness measurement accuracy is high. Attached Figure Description

[0035] Figure 1 The diagram illustrates a schematic of a target-back film thickness measurement imaging system for laser ion acceleration according to a preferred embodiment of the present invention.

[0036] Figure 2-4 The variation of reflectivity of thin films with wavelength is shown for different thicknesses;

[0037] Figure 5 This shows the relationship between thickness resolution and film thickness and wavelength when s-light is incident normally on a thin film.

[0038] Figure label:

[0039] 1-Target;

[0040] 2-Microscopic objectives;

[0041] 5-Lighting source;

[0042] 6-Spectrometer;

[0043] 31 - First beam cube;

[0044] 32 - Second beam cube;

[0045] 41-First Camera;

[0046] 42 - Second camera. Detailed Implementation

[0047] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Through these descriptions, the features and advantages of the present invention will become clearer and more apparent.

[0048] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments. Although various aspects of embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated otherwise.

[0049] This invention provides a target-back film thickness measurement imaging system for laser ion acceleration, such as... Figure 1 As shown, a microscope objective 2, a first beam splitter cube 31, a second beam splitter cube 32, a first camera 41, a second camera 42, an illumination source 5, and a spectrometer 6 are arranged behind the target 1, thus forming the following optical path:

[0050] The incident light path is formed by the illumination source 5, which emits incident light rays, which enter the microscope objective 2 after passing through the first beam splitting cube 31, and is focused by the microscope objective 2 to illuminate the target 1.

[0051] The reflected light path is as follows: after the incident light irradiates the target 1, it forms reflected light. The reflected light enters the first beam splitting cube 31 through the microscope objective 2. After passing through the first beam splitting cube 31, it is split into two beams. One beam enters the spectrometer 6, and the other beam enters the second beam splitting cube 32. After passing through the second beam splitting cube 32, it is split into two beams again. One beam enters the first camera 41, and the other beam enters the second camera 42.

[0052] A beam splitter cube is a semi-reflective mirror that can strictly divide the reflected light and transmitted light into a 1:1 ratio. In this invention, the beam splitter cube is a non-polarizing beam splitter cube.

[0053] In a preferred embodiment, the target 1, the first beam splitter cube 31, the second beam splitter cube 32, the first camera 41 and the second camera 42 are disposed in the vacuum cavity, while the spectrometer 6 and the illumination source 5 are disposed outside the vacuum cavity, thereby saving the volume of the vacuum cavity.

[0054] In a preferred embodiment, the incident light emitted by the illumination source 5 enters the optical path through the feedthrough fiber, and the spectrometer 6 acquires the reflected light in the optical path through the feedthrough fiber.

[0055] Furthermore, the feedthrough fiber connecting the illumination source 5 and the feedthrough fiber connecting the spectrometer 6 are bundled together, so that the incident light and the reflected light are coaxial, thereby enabling the spectrometer 6 to measure the incident light and the reflected light.

[0056] In this invention, the incident and reflected rays are coaxial, and the spectrometer 6 measures the incident and reflected rays to obtain the film thickness of the target 1. The position of the target 1 is adjusted by the imaging of the first camera 41 and the second camera 42 to achieve beam-target coupling. The microscope objective in the system can play both imaging and focusing roles, that is, a single system can simultaneously achieve film thickness measurement and target illumination imaging.

[0057] In a preferred embodiment, the feedthrough fiber connecting the lighting source 5 and the feedthrough fiber connecting the spectrometer 6 are bundled together via a fiber optic splitter and a fiber optic feedthrough flange.

[0058] According to the present invention, the magnification of the first camera 41 is less than that of the second camera 42;

[0059] The first camera 41 has a large field of view. By moving the position of the target 1, a light spot is obtained in the first camera 41, and the position of the target 1 is roughly determined.

[0060] The second camera 42 has a high imaging magnification. By adjusting the front and rear positions of the target 1 in the optical path, the light spot in the second camera 42 becomes circular and has the smallest diameter. At this time, the target 1 is located at the focusing position of the optical path, and the angle between the incident light and the target film is 90 degrees, thus completing the beam-target coupling.

[0061] According to the present invention, the target is a transparent thin film target with a target film thickness of less than 150 nm.

[0062] Traditional methods for measuring film thickness generally include reflectance spectroscopy. Figure 2-4The reflectance curves of films with different thicknesses are shown. According to the reflectance spectroscopy method, the reflectance spectrum corresponding to different thicknesses is unique. Therefore, the film thickness can be deduced by measuring the reflectance spectrum of the film. However, the inventors found that this method has significant limitations. When using conventional light sources for measurement, when the film thickness is very small, the oscillation of the reflectance spectrum becomes very slow, and the oscillation frequency is close to the Gaussian envelope of a typical white light source, leading to large errors. Therefore, when the thickness of the film to be measured is less than 150 nm, the measurement accuracy deteriorates significantly, and in some cases, the value cannot be measured at all. Although the measurement accuracy can be further improved by using a broadband ultraviolet light source, the cost is very high. Even with a broadband ultraviolet light source, it is only possible to measure the thickness of films above 60 nm, and it is still not possible to measure the thickness of films below 50 nm. According to the present invention, a method for measuring the thickness of a target-backed film for laser ion acceleration is implemented using the above system, and the thickness of a thick film is measured by reflectance spectroscopy.

[0063] The thickness of the target film is measured using the following steps:

[0064] S1. Irradiate the target, detect the incident light and reflected light, and obtain the target film reflectivity;

[0065] S2. Determine the target film thickness based on the relationship between reflectivity, incident light wavelength, and target film thickness.

[0066] Preferably, before S1, there is also S0 for beam-target coupling.

[0067] In S0, by moving the position of target 1, a light spot is obtained in the first camera 41, and the position of target 1 is roughly determined; then the front and rear positions of target 1 in the optical path are adjusted so that the light spot in the second camera 42 is circular and has the smallest diameter, so that target 1 is located at the focal position of the optical path.

[0068] According to the present invention, in S1, the specific formula for calculating reflectance is a conventional technique in the art and will not be elaborated here.

[0069] According to the present invention, in S2, the relationship between reflectivity, incident light wavelength, and target film thickness is as follows:

[0070]

[0071] Wherein, R(h,λ,θ) represents the reflectivity R, which is a parameter related to the target film thickness h, the wavelength λ of the incident light, and the incident angle θ; R0(λ,θ) represents the reflectivity R0 of the target film interface, which is a parameter related to the wavelength λ of the incident light and the incident angle θ; and δ(h,λ,θ) represents the optical path difference δ of the target film, which is a parameter related to the target film thickness h, the wavelength λ of the incident light, and the incident angle θ. In this invention, the incident angle θ is 90 degrees.

[0072] Furthermore, the optical path difference of the target film is expressed as:

[0073]

[0074] n(λ) represents the refractive index function of the target film material. Although it is a parameter associated with the wavelength λ of the incident light, the error it introduces is very small. Therefore, in general, n(λ) is set as a constant. In this invention, preferably, the refractive index of the target film material when the wavelength λ = 600 is taken as the value of n(λ).

[0075] Furthermore, according to the present invention, when the incident light is S-ray, the reflectivity R0(λ,θ) of the target film interface determined by Fresnel's law of reflection is denoted as R s (λ,θ) is represented as:

[0076]

[0077] When the incident light is P-ray, the reflectivity R0(λ,θ) of the target film interface, as determined by Fresnel's law of reflection, is denoted as R. p (λ,θ) is represented as:

[0078]

[0079] S-rays and P-rays are commonly used classification methods for light reflection and refraction. S-rays refer to light whose polarization direction is perpendicular to the plane formed by the incident light and its normal, while P-rays refer to light whose polarization direction lies in the plane formed by the incident light and its normal.

[0080] According to a preferred embodiment of the present invention, at least two wavelengths of monochromatic light are used as incident light, and a target film thickness is obtained independently for each monochromatic light through S1 to S2.

[0081] Furthermore, according to the present invention, step S3 is also included: obtaining the film thickness resolution corresponding to different monochromatic lights, and selecting the target film thickness obtained corresponding to the monochromatic light with the smaller film thickness resolution as the final measurement result.

[0082] The film thickness resolution is used to indicate the accuracy of film thickness measurement.

[0083] The inventors discovered that when the target film thickness is within the range of [0, λ / 4], the reflectivity of the target film for monochromatic light with wavelength λ increases monotonically with the target film thickness, such as... Figure 5 As shown, when the target film thickness is in the range of [0, λ / 4], the target film thickness can be directly calculated by S3 using the reflectivity obtained in S2. However, the target film thickness calculated under different wavelength light sources is different. How to determine which wavelength light source has higher accuracy in calculating the target film thickness is one of the problems to be solved by this invention.

[0084] In this invention, this problem is solved by setting the film thickness resolution. The film thickness resolution indicates the accuracy of film thickness measurement. The smaller the film thickness resolution, the more accurate the measurement result. When the film thickness resolution is greater than or equal to 1, it indicates that the accuracy of the test result is insufficient.

[0085] In S3, the film thickness resolution Δ is expressed as:

[0086]

[0087] λ represents the wavelength of the incident light, θ represents the incident angle, h represents the target film thickness, and n represents the refractive index of the target film material. The target film thickness is the film thickness measured for the monochromatic light. Preferably, the refractive index of the target film material at a wavelength of λ = 600 nm is taken as the value of the refractive index n of the target film material in this invention. In another preferred embodiment, when the wavelength of the monochromatic light is 405 nm, n = 1.505; when the wavelength of the monochromatic light is 780 nm, n = 1.485.

[0088] In a preferred embodiment, of the two wavelengths of monochromatic light, one is monochromatic light with a wavelength greater than 600 nm, and the other is monochromatic light with a wavelength less than 600 nm.

[0089] More preferably, 780nm monochromatic light and 405nm monochromatic light were selected as the incident light. After extensive experiments, the combination of these two monochromatic lights resulted in film thickness measurement results with extremely high accuracy.

[0090] Example

[0091] Example 1

[0092] use Figure 1 The device shown measures a target film with a thickness of 50 nm. The incident light path is formed by light emitted from the illumination source 5, passing through the first beam splitter cube 31 and entering the microscope objective 2, where it is focused onto the target 1. The reflected light path is formed by the incident light illuminating the target 1, passing through the microscope objective 2 and entering the first beam splitter cube 31. The reflected light is split into two beams by the first beam splitter cube 31; one beam enters the spectrometer 6, and the other enters the second beam splitter cube 32. The second beam splitter cube 32 then splits the light into two beams again; one beam enters the first camera 41, and the other enters the second camera 42.

[0093] The film thickness measurement steps are as follows:

[0094] S0. By moving the target 1, a light spot is obtained in the first camera 41, and the position of the target 1 is roughly determined; then, the front and rear positions of the target 1 in the optical path are adjusted so that the light spot in the second camera 42 is circular and has the smallest diameter, so that the target 1 is located at the focal point of the optical path.

[0095] S1. Irradiate the target, detect the incident light and reflected light, and obtain the target film reflectivity.

[0096] S2. Determine the target film thickness based on the relationship between reflectivity, incident light wavelength, and target film thickness.

[0097] S3. Obtain the film thickness resolution corresponding to different monochromatic lights, and select the target film thickness obtained corresponding to the monochromatic light with the smaller film thickness resolution as the final measurement result.

[0098] In S2, the relationship between reflectivity, incident light wavelength, and target film thickness is as follows:

[0099]

[0100]

[0101] The incident light is S-ray, and the reflectivity R0(λ,θ) of the target film interface, as determined by Fresnel's law of reflection, is denoted as R. s (λ,θ) is represented as:

[0102]

[0103] Specifically, when the incident light is a 405nm light source, the refractive index function n of the target film of this material is a constant of 1.505; when the incident light is a 780nm light source, the refractive index function n of the target film of this material is a constant of 1.485.

[0104] 780nm monochromatic light and 405nm monochromatic light were used as incident light. Each monochromatic light was used to obtain a target film thickness through S1 to S2. The target film thickness obtained by the 780nm monochromatic light was 51.11nm, and the target film thickness obtained by the 405nm monochromatic light was 50.86nm.

[0105] In S3, the film thickness resolution Δ is expressed as:

[0106]

[0107] The resolution corresponding to 780nm monochromatic light is 0.3435, and the resolution corresponding to 405nm monochromatic light is 0.2526. Therefore, the final measured film thickness is 50.86nm, which deviates from the actual film thickness by 1.72%.

[0108] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front," and "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship in the working state of this invention, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first," "second," "third," and "fourth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0109] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0110] The present invention has been described above with reference to preferred embodiments; however, these embodiments are merely exemplary and illustrative. Various substitutions and modifications can be made to the present invention based on these embodiments, all of which fall within the scope of protection of the present invention.

Claims

1. A post-target film thickness measurement imaging system for laser ion acceleration, characterized in that, A microscope objective (2), a first beam splitting cube (31), a second beam splitting cube (32), a first camera (41), a second camera (42), an illumination light source (5) and a spectrometer (6) are arranged behind the target (1), thereby forming the following optical paths: An incident light path, the incident light emitted by the illumination light source (5) enters the microscope objective (2) after passing through the first beam splitting cube (31), and is focused by the microscope objective (2) to irradiate the target (1); A reflected light path, after the incident light irradiates the target (1), the reflected light enters the first beam splitting cube (31) through the microscope objective (2), and is split into two beams by the first beam splitting cube (31), one of which enters the spectrometer (6), and the other enters the second beam splitting cube (32), and is split into two beams again by the second beam splitting cube (32), one of which enters the first camera (41), and the other enters the second camera (42).

2. The target post film thickness measurement imaging system for laser ion acceleration according to claim 1, wherein the spectrometer (6) and the illumination light source (5) are arranged outside the vacuum chamber, and the target (1), the first beam splitting cube (31), the second beam splitting cube (32), the first camera (41) and the second camera (42) are arranged in the vacuum chamber.

3. The target post film thickness measurement imaging system for laser ion acceleration according to claim 1, wherein the incident light emitted by the illumination light source (5) enters the optical path through a feedthrough optical fiber, and the spectrometer (6) acquires the reflected light in the optical path through a feedthrough optical fiber; The feedthrough optical fiber connected to the illumination light source (5) and the feedthrough optical fiber connected to the spectrometer (6) are combined to make the incident light and the reflected light coaxial, so that the spectrometer (6) can measure the incident light and the reflected light; The feedthrough optical fiber connected to the illumination light source (5) and the feedthrough optical fiber connected to the spectrometer (6) are combined through an optical fiber feedthrough flange.

4. The target post film thickness measurement imaging system for laser ion acceleration according to claim 1, wherein the film thickness of the target (1) is obtained by measuring the incident light and the reflected light through the spectrometer (6).

5. The target post film thickness measurement imaging system for laser ion acceleration according to claim 1, wherein the imaging magnification of the first camera (41) is smaller than that of the second camera (42); The first camera (41) has a large field of view, and the position of the target (1) is adjusted to obtain a light spot in the first camera (41) and roughly determine the position of the target (1); The second camera (42) has a higher imaging magnification, and the front and back positions of the target (1) in the optical path are adjusted so that the light spot in the second camera (42) is circular and has the smallest diameter, at this time the target (1) is located on the focusing position of the optical path, the angle between the incident light and the target film is 90 degrees, and the beam-target coupling is completed. The target film thickness is measured by the target post film thickness measurement imaging system for laser ion acceleration according to claim 1 through the following steps: S1, irradiating the target, detecting the incident light and the reflected light, and obtaining the reflectivity of the target film, S2, determining the target film thickness according to the relationship among the reflectivity, the incident light wavelength and the target film thickness. ​ 6. A method for measuring the thickness of a post-target film for laser ion acceleration, characterized in that, ​ ​ ​ 7. The method of claim 6, wherein, Before S1, there is also S0, which is beam-target coupling: By moving the target position, the light spot is obtained in the first camera, and the target position is roughly determined; then the front and back positions of the target in the light path are adjusted so that the light spot in the second camera is circular and has the smallest diameter, so that the target is located on the focusing position of the light path.

8. The method of claim 6, wherein, In S2, the relationship between reflectivity, incident light wavelength and target film thickness is: wherein, represents the wavelength of the incident light, represents the angle of incidence, represents the target film thickness; Rtarget represents the reflectivity of the target film to the incident light, Rinterface represents the interface reflectivity of the target film as determined by the Fresnel reflection law, OPDtarget represents the optical path difference of the target film, the optical path difference of the target film being expressed as: denotes the refractive index function of the target film material.

9. The method of claim 6, wherein, At least two wavelengths of monochromatic light are used as incident light, and for each monochromatic light, a target film thickness is obtained by S1-S2 independently.

10. The method for post-target film thickness measurement for laser ion acceleration of claim 6, wherein, The method further comprises: S3, obtaining the film thickness resolution corresponding to different monochromatic light, and selecting the target film thickness corresponding to the monochromatic light with smaller film thickness resolution as the final measurement result, Wherein, the film thickness resolution is used to indicate the film thickness measurement accuracy.

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