Method and system for improving mass spectrum resolution of three-dimensional atom probe of metal material based on laser polarization and field enhancement effect

Abstract

The invention discloses a method and a system for improving mass spectrum resolution of a three-dimensional atom probe of a metal material based on laser polarization and a field enhancement effect. The method comprises the following steps: preparing a to-be-detected metal material into a nano-scale needle point sample, placing the nano-scale needle point sample on a three-dimensional atom probe sample table, adjusting the polarization direction of the nano-scale needle point sample to be consistent with the axial direction of a sample needle point by taking linear polarization laser as an excitation source, gradually adjusting the distance between the sample needle point and a focusing light spot of the laser through a three-dimensional moving platform, and detecting. In order to realize the method, the invention also provides a related system which comprises a linear polarization laser source, an optical polarization adjustment module, a three-dimensional mobile platform, a three-dimensional atom probe main body and a data acquisition and processing device. According to the method, through laser polarization direction adjustment, near-field positioning and parameter optimization, a laser field enhancement effect is used for replacing a traditional heat effect driving mechanism, the technical problem that the resolution ratio is low due to the heat effect is effectively solved, and the resolution ratio of the three-dimensional atom probe mass spectrum is greatly improved.

Description

Technical Field

[0001] The present invention relates to a three-dimensional atom probe mass spectrometry resolution method, and particularly to a method and system for improving the three-dimensional atom probe mass spectrometry resolution of metal materials based on laser polarization and field enhancement effect, belonging to the field of three-dimensional atom probe analysis. Background Art

[0002] Three-dimensional atom probe (3DAP) is a powerful technique capable of performing compositional analysis and three-dimensional atomic imaging of materials at the atomic scale. In the research of metal materials, 3DAP can provide key information such as the distribution of alloying elements, the composition and size of precipitation phases, which is of great significance for understanding the properties of materials and developing new materials.

[0003] Traditional three-dimensional atom probe technology mainly adopts the electric field evaporation mode, that is, a DC high voltage is applied to the sample and a voltage pulse is superimposed, and the strong electric field at the tip is used to evaporate the atoms on the sample surface one by one. However, this mode has some limitations: the width of the pulse voltage (in the order of kV) is at least about 100 ns, and the too long pulse duration leads to the dispersion of flight time (the uncertainty of the atomic evaporation moment), thus reducing the mass spectrometry resolution. In addition, in order to obtain a high signal-to-noise ratio of the mass spectrum, the pulse voltage is generally set to about 20% of the breakdown voltage, but too high a voltage will cause too many random evaporation events, further reducing the signal-to-noise ratio.

[0004] With the development of technology, a three-dimensional atom probe with laser-assisted atomic evaporation has emerged. Its principle is: a DC high voltage is applied to the tip, and at the same time, a short pulse (pulse width about 10 ps) laser is used to irradiate the sample tip. The heating effect of the laser on the sample is used to reduce the electric field strength required for atomic evaporation at the sample tip, so as to achieve the purpose of evaporating atoms one by one. This method is called the laser mode. The advantage of the laser mode is that the width of the laser pulse is greatly shortened compared with the voltage pulse, the uncertainty of the atomic evaporation moment is reduced, the flight time measurement is more accurate, and the mass spectrometry accuracy is improved. However, due to the laser irradiation, the instantaneous temperature at the sample tip will rise to several hundred or even thousands of K. Although the high temperature duration is very short, as the temperature rises, the atoms on the sample surface will migrate due to thermal motion, the sample will have thermal deformation, local heat damage and other factors, which will not only directly affect the mass spectrometry resolution, but also affect the spatial resolution.

[0005] Currently, the latest three-dimensional atom probe evaporation technology is the voltage and laser collaborative mode, that is, a high DC voltage is applied to the sample, and a lower voltage pulse is superimposed. At this time, it is necessary to ensure that there is no atomic evaporation. Then, within the duration of the pulsed high voltage, the sample tip is irradiated with picosecond laser to further reduce the atomic evaporation barrier and promote atomic evaporation. Although the mass spectrometry resolution has been greatly improved compared with the previous two methods, it still has the disadvantages of the laser mode, such as the deterioration of mass spectrometry resolution caused by laser heating of the sample, the deterioration of spatial resolution caused by sample thermal deformation, and local thermal damage. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the first object of the present invention is to provide a method for improving the mass spectrometry resolution of three-dimensional atom probe of metal materials based on laser polarization and field enhancement effect. This method adjusts the laser polarization direction, localizes the near field and optimizes the parameters, and uses the field enhancement effect of the laser field at the nanoscale tip to replace the traditional thermal effect driving mechanism, effectively solving the technical problem of low resolution caused by the long-existing thermal effect in three-dimensional atom probe technology, providing a new method for atomic-scale material analysis, and having important value for material science research and industrial applications.

[0007] The second object of the present invention is to provide a system for improving the mass spectrometry resolution of three-dimensional atom probe of metal materials based on laser polarization and field enhancement effect. The system uses a linearly polarized laser source, adjusts the polarization direction of the laser through an optical polarization adjustment module to make it along the axial direction of the sample tip, then adjusts the distance between the sample tip and the laser spot through a three-dimensional moving platform, synchronously monitors the ion extraction efficiency and the heating condition of the sample, so as to determine the optimal position parameters, and further improves the accuracy and reliability of data analysis through a data acquisition and processing device.

[0008] To achieve the above technical solutions, the present invention provides a method for improving the mass spectrometry resolution of three-dimensional atom probe of metal materials based on laser polarization and field enhancement effect, including: preparing the metal material to be tested into a nanoscale tip sample, placing it on the three-dimensional atom probe sample stage, then using a linearly polarized laser as the excitation source, adjusting its polarization direction to be consistent with the axial direction of the sample tip, and then gradually adjusting the distance between the sample tip and the focused laser spot through a three-dimensional moving platform for detection.

[0009] In the prior art, methods such as the laser-assisted mode and the voltage-laser collaborative mode rely on the thermal effect of the laser to reduce the atomic evaporation barrier. However, the instantaneous high temperature will cause thermal migration, deformation, and damage to the sample, which will seriously affect the resolution. In the technical solution provided by the present invention, the polarization direction of the linearly polarized laser is consistent with the axial direction of the tip, and through the local plasma resonance and optical antenna effect of the nano-tip, the significant enhancement of the electric field strength is achieved, and the atoms can be efficiently evaporated without heating the sample, fundamentally avoiding the negative impact of the thermal effect on the mass spectrometry resolution.

[0010] As a preferred solution, the sample tip is located in the near-field region of the laser field, 50-200 nm away from the center of the laser spot.

[0011] The present invention places the sample tip in the near-field region of the laser field rather than the direct irradiation region, which not only utilizes the near-field enhancement effect but also avoids direct laser heating. By optimizing the distance between the sample and the laser spot through experiments and numerical simulations, while ensuring the field enhancement effect, the thermal effect is greatly reduced.

[0012] As a preferred solution, the metal material is a metal element or an alloy.

[0013] As a preferred solution, the pulse width of the laser is 1-20 ps, and the single-pulse energy is 10-100 nJ.

[0014] The present invention optimizes parameters such as the laser wavelength, pulse width, and energy through theoretical calculations and experiments to balance the field enhancement effect and the thermal effect. Among them, within the above range, the pulse width can effectively shorten the evaporation time dispersion, and the pulse energy can ensure that the inherent electric field strength of the laser is high enough.

[0015] As a preferred solution, the calculation process of the electric field strength of the laser is as follows: Equation 1: ; Equation 2: ; In Equation 1 and Equation 2: is the peak light intensity of the laser, is the single-pulse energy, is the laser spot size, is the pulse width, is the vacuum permittivity, is the amplitude of the laser electric field strength, is the polarization axis angle, is the electric field strength when the polarization axis angle is

[0016] ​The present invention can achieve the required electric field strength for evaporation without relying on high laser energy. It mainly meets the actual requirements through the field enhancement of the tip, significantly reducing the power requirement for the laser, while reducing random evaporation events and improving the signal-to-noise ratio.

[0017] As a preferred solution, the size of the laser spot is <5 μm, and the electric field enhancement factor η ≥ 50.

[0018] As a preferred solution, the resolution of the mass spectrometry resolution is 300 - 1000, the lateral spatial resolution is 0.18 - 0.22 nm, and the depth is 0.08 - 0.12 nm.

[0019] The present invention also provides a system for improving the mass spectrometry resolution of three-dimensional atom probe based on laser polarization and field enhancement effect, which is used to implement the method described in any one of the above. It includes a linearly polarized laser source, an optical polarization adjustment module, a three-dimensional moving platform, a three-dimensional atom probe main body, and a data acquisition and processing device.

[0020] As a preferred solution, the optical polarization adjustment module includes a half-wave plate and a polarizer, which are used to adjust the polarization direction of the laser.

[0021] As a preferred solution, the three-dimensional moving platform is used to precisely control the relative position of the sample tip and the laser spot.

[0022] The present invention uses a high-precision three-dimensional moving platform to adjust the relative position of the tip and the laser spot to ensure being in the near-field enhancement region. Then, by real-time monitoring of the ion signal and heating situation, the position parameters are optimized to maximize the field enhancement efficiency and minimize the thermal effect, improving the stability and repeatability of data acquisition.

[0023] As a preferred solution, the data acquisition and processing device is used to collect the ion signal generated by the sample, statistically analyze and remove noise, and its processing methods include filtering, noise reduction, and peak fitting.

[0024] The present invention uses algorithms such as filtering, noise reduction, and peak fitting to process the ion signal and remove noise interference, which can further improve the signal-to-noise ratio and peak resolution ability of the mass spectrometry diagram. Especially in trace element analysis, it can more accurately identify low-abundance ion signals.

[0025] Compared with the prior art, the beneficial technical effects of the technical solutions provided by the present invention are:

[0026] 1) The mass spectrometry resolution method provided by the present invention adjusts the laser polarization direction, performs near-field positioning and parameter optimization, and uses the laser field enhancement effect to replace the traditional thermal effect driving mechanism, effectively solving the technical problem of low resolution caused by the long-existing thermal effect in three-dimensional atom probe technology, providing a new method for atomic-scale material analysis, and having important value for materials science research and industrial applications.

[0027] 2) The system provided by the present invention uses a linearly polarized laser source, adjusts the polarization direction of the laser through an optical polarization adjustment module to make it along the axial direction of the sample tip, then adjusts the distance between the sample tip and the laser spot through a three-dimensional moving platform, synchronously monitors the ion extraction efficiency and the heating condition of the sample, thereby determining the optimal position parameters, and further improving the accuracy and reliability of data analysis through a data acquisition and processing device.

[0028] 3) In the technical solution provided by the present invention, the ion extraction efficiency is improved through the field enhancement effect, the evaporation time dispersion is reduced, the full width at half maximum of the mass spectrometry peak is significantly reduced, and the isotope resolution ability is enhanced. This solution effectively avoids sample deformation and atomic migration caused by laser heating, and low laser energy can greatly reduce random evaporation events. Combined with a data processing algorithm, the noise level is significantly reduced, and the detection limit of the three-dimensional atom probe is further improved. Detailed implementation manners

[0029] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below with reference to specific implementation cases. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, these embodiments are provided to make the understanding of the disclosure of the present invention more thorough and comprehensive. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0030] Example 1

[0031] This example provides a method for improving the mass spectrometry resolution of three-dimensional atom probe of metal materials based on laser polarization and field enhancement effect, specifically as follows: 1) Laser system debugging: Select a pulsed laser with a wavelength of 355 nm, a pulse width of 10 ps, a repetition frequency of 500 kHz, and a single pulse energy of 10 nJ. Through optical elements such as an optical focusing lens and a polarizer, adjust the focused laser spot to be less than 5 μm, adjust the polarization direction of the laser to make it along the axial direction of the tip, and precisely measure and adjust the parameters of the laser through devices such as a photodetector and a polarization analyzer; 2) Calculation and analysis of the laser electric field strength: Calculate through Equation 1 and Equation 2: Formula 1: ; Formula 2: ; In Formula 1 and Formula 2: = 10 nJ, = 5μm, = 10 ps, =8.854×10 −12 F / m, =3×10 8 m / s. Substitute the above parameters to calculate the peak optical intensity of the laser : ; Furthermore, can be solved: ; Considering the polarization of the laser, assuming that the polarization direction of the laser is along the axial direction of the sample, the electric field strength at an angle θ with the polarization axis is as shown in Formula 2. It can be seen from this that the electric field strength along the polarization axis of the laser is the strongest. Therefore, adjusting the polarization direction of the laser to be along the axial direction of the sample tip can maximize the utilization of the laser electric field strength.

[0032] 3) Adjustment of the position of the sample and the laser: Install the prepared sample on the sample stage of the three-dimensional atom probe. Through a high-precision three-dimensional moving platform, adjust the relative position between the sample tip and the laser spot. Use equipment such as a laser detector to observe the positional relationship between the sample tip and the laser spot to ensure that the sample tip is within the near-field region of the laser field; during the adjustment process, the distance between the sample tip and the laser spot can be gradually changed, and at the same time, monitor the ion extraction efficiency and the heating condition of the sample, and determine the optimal position parameters through optimization.

[0033] 4) Data acquisition and analysis: Turn on the three-dimensional atom probe device and the data acquisition system. Under laser excitation, collect the ion signals generated by the sample. Conduct statistical analysis on the large number of collected ion signals, remove the noise signals, and obtain high-quality mass spectrometry and three-dimensional atom distribution information. Advanced data processing algorithms, such as filtering, noise reduction, peak fitting and other technologies, can be used to further improve the accuracy and reliability of data analysis.

[0034] It should be noted that although the maximum electric field strength of the selected laser is only 0.2 V / nm, it doesn't seem to be much help for atomic evaporation (usually dozens of V / nm are required. For example, to evaporate Al + requires 19 V / nm, Fe 2+(which requires 33 V / nm). However, the sample tip used in three-dimensional atom probe usually needs to be processed to a scale of dozens of nanoseconds. At the nanoscale, due to reasons such as local plasma resonance and optical antenna effect, the laser field can achieve an enhancement effect of dozens or even hundreds of times in the electric field. Therefore, it is not necessary to make the electric field strength of the laser reach dozens of V / nm, which greatly reduces the requirements for the laser.

[0035] The present invention also provides a system for improving the mass spectrometry resolution of three-dimensional atom probe based on laser polarization and field enhancement effect, which is used to implement the above method, and includes a linearly polarized laser source, an optical polarization adjustment module, a three-dimensional moving platform, a three-dimensional atom probe main body and a data acquisition and processing device; The optical polarization adjustment module includes a half-wave plate and a polarizer, which are used to adjust the polarization direction of the laser; the three-dimensional moving platform is used to accurately control the relative position between the sample tip and the laser spot; The present invention uses a high-precision three-dimensional moving platform to adjust the relative position between the tip and the laser spot, ensuring to be in the near-field enhancement region, and then optimizes the position parameters by real-time monitoring of ion signals and heating conditions to maximize the field enhancement efficiency and minimize the thermal effect, improving the stability and repeatability of data acquisition; The data acquisition and processing device is used to collect ion signals generated by the sample, statistically analyze and remove noise, and its processing methods include filtering, noise reduction and peak fitting.

[0036] The present invention processes ion signals using algorithms such as filtering, noise reduction, and peak fitting to remove noise interference, which can further improve the signal-to-noise ratio and peak resolution ability of the mass spectrometry diagram. Especially in trace element analysis, it can more accurately identify low-abundance ion signals.

[0037] To further illustrate the advantages of the technical solutions provided by the embodiments of the present invention, the above method and system are used to detect aluminum metal. The full width at half maximum mass resolution in the above system is 300-1000, and no energy compensator is set. The calculation process of the mass resolving power (MRP) is as follows: Time-of-flight error of three-dimensional atom probe equipment There are three sources: physical time error , electronics timing error and ion evaporation time error , and satisfy the following relationship: Equation 3: ; Among them, the physical time error is the time error caused by the errors of physical parameters and . According to the law of conservation of energy, the following equation can be obtained: Equation 4: ; In Equation 4, is the initial energy when the ion detaches from the sample, is the charge state of the ion, is the electron charge, is the potential difference between the sample and the detector, is the mass of the ion, is the velocity of the ion, is the flight distance of the ion, is the flight time of the ion; therefore, the flight time of the ion can be calculated from Equation 4 as: Equation 5: ; Furthermore, for Al ions, their , , are all constants, then the flight time error caused by and is: Equation 6: ; Equation 7: ; The electronics timing error is the time uncertainty caused by the electronics and is a definite value for a given experimental setup. In this embodiment, its value is 100 ps.

[0038] Furthermore, from the relationship between the time error and the mass resolving power (MRP) can be deduced: Equation 8: ; For a pure aluminum sample, calculated according to Al + ions, the values are as follows: , C, kg, mm, mm, kV, V, ps, ps, eV, after calculation, its .

[0039] Comparative Example 1

[0040] In this comparative example, the three-dimensional atom probe test of metallic aluminum was carried out by the laser heating evaporation method. For the pure aluminum sample in this comparative example, the values calculated according to Al + ions are exactly the same as those in Example 1. The difference is that: ps, eV.

[0041] For laser heating evaporation, the sample locally absorbs the laser energy, the temperature rises sharply, and it takes a relatively long time to cool down. During the period when the temperature rises, there is a certain probability that atoms will be evaporated, which leads to an error in the ion evaporation time increase. In addition, the smaller the laser beam spot, the smaller the heated area, and the shorter the required cooling time.

[0042] Furthermore, the maximum temperature of the laser-heated metal sample is about 300 K is positively correlated with the temperature. At 300 K, its value is 0.025 eV. Therefore, for this comparative example, its .

[0043] In addition, for the method provided in Embodiment 1 of the present invention, only depends on the duration of the laser field (on the order of ~10 ps), so the value is greatly reduced, and this method is carried out at low temperature with basically no thermal effect. Therefore, can be ignored and calculated as 0. Therefore, compared with the laser heating scheme, the mass resolution of the method of the embodiment of the present invention is greatly improved.

[0044] As can be seen from the above Embodiment 1 and Comparative Example 1, by reasonably selecting laser parameters and adjusting the polarization direction of the laser, the present invention can, without heating the sample, utilize the near-field enhancement effect of the laser field to significantly improve the resolution and signal-to-noise ratio of the mass spectrometry, thereby realizing more accurate atomic-scale analysis of metal materials.

Claims

1. A method for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects, characterized in that Comprising: Preparing the metal material to be measured into a nanoscale tip sample, placing it on a three-dimensional atom probe sample stage, then using a linearly polarized laser as the excitation source, adjusting its polarization direction to be consistent with the axial direction of the sample tip, and then gradually adjusting the distance between the sample tip and the focused laser spot through a three-dimensional moving platform for detection, thus obtaining the result.

2. A method for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects, characterized in that: The sample tip is located in the near-field region of the laser field, 50 - 200 nm away from the center of the laser spot; the metal material is a metal element or an alloy.

3. A method for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects, characterized in that: The pulse width of the laser is 1 - 20 ps, and the single-pulse energy is 10 - 100 nJ.

4. A method for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effect, characterized in that: The calculation process of the electric field strength of the laser is as follows: Formula 1: ; Formula 2: ; In Formula 1 and Formula 2: is the peak optical intensity of the laser, is the single-pulse energy, is the laser spot size, is the pulse width, is the permittivity of vacuum, is the amplitude of the laser electric field strength, is the polarization axis angle, is the electric field strength when the polarization axis angle is ​ 5. A method for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effect, characterized in that: The size of the laser spot < 5 μm, and the electric field enhancement factor η ≥ 50.

6. A method for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects according to any one of claims 1 to 5, characterized in that: The resolution of the mass spectrometry resolution is 300 - 1000, and the spatial resolution is 0.18 - 0.22 nm in the lateral direction and 0.08 - 0.12 nm in the depth direction.

7. A system for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects, characterized in that: Used to implement the method according to any one of claims 1 - 6, which includes a linearly polarized laser source, an optical polarization adjustment module, a three-dimensional moving platform, a three-dimensional atom probe main body, and a data acquisition and processing device.

8. A system for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects, characterized in that: The optical polarization adjustment module includes a half-wave plate and a polarizer, and is used to adjust the polarization direction of the laser.

9. A system for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effect, characterized in that: The three-dimensional moving platform is used to precisely control the relative position between the sample tip and the laser spot.

10. A system for improving the resolution of three-dimensional atom probe mass spectrometry of metal materials based on laser polarization and field enhancement effects, characterized in that: The data acquisition and processing device is used to collect the ion signals generated by the sample, perform statistical analysis and remove noise, and its processing methods include filtering, noise reduction, and peak fitting.

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