Double-pulse system and method for improving depth resolution of laser ionization mass spectrum

Abstract

The invention discloses a double-pulse system and method for improving depth resolution of laser ionization mass spectrometry, and relates to the technical field of mass spectrometers, the system comprises a control system for controlling femtosecond laser parameters generated by a femtosecond laser and nanosecond laser parameters of a nanosecond laser; the laser energy regulation and control module is used for carrying out phase adjustment and detection on the input femtosecond laser and outputting the femtosecond laser meeting set parameters based on the femtosecond laser parameters; the light beam shaping and light beam morphology analysis module is used for shaping femtosecond laser and focusing the femtosecond laser on the surface of the sample for laser denudation; the nanosecond laser performs off-resonance multi-photon ionization on the denudated neutral particles to generate ionized ions for mass spectrometry. According to the invention, the depth resolution of the laser ionization mass spectrum can be improved to nanoscale, the ionization efficiency of ions is improved under the condition of reducing the sampling amount, and meanwhile, the sensitivity of an instrument and the accuracy of quantitative analysis are improved.

Description

Technical Field

[0001] The invention relates to the technical field of mass spectrometers, and in particular to a double-pulse system and method for improving the depth resolution of laser ionization mass spectrometry. Background Art

[0002] Thin films are key components of modern functional materials, usually with a thickness ranging from nanometers to several micrometers, and can achieve properties that are not available in bulk materials. The optical / mechanical / electrical properties of thin film materials largely depend on the chemical composition, material purity, interfaces between different layers and the spatial distribution of elements. As the thickness of thin films continues to decrease, it is increasingly important to characterize thin films at the nanoscale.

[0003] Therefore, many analytical techniques have been used to measure the elemental depth distribution of various thin films. Among them, secondary ion mass spectrometry (SIMS) and glow discharge mass spectrometry (GDMS) are favored for their excellent performance. However, SIMS has a very serious matrix effect when performing quantitative analysis, and the spectrum of GDMS is easily interfered by gas peaks and composite peaks and cannot perform micro-area analysis. In recent years, the use of lasers to directly sample solid samples in micro-areas has attracted much attention. It has the advantages of fast speed, strong controllability, high reproducibility, low sample consumption, and no need for pre-treatment. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and laser ionization mass spectrometry (LIMS) have emerged. Although the emergence of femtosecond lasers has improved the depth resolution of LA-ICPMS to the nanometer level, the three processes of LA-ICPMS, namely laser ablation, sample transmission and ICP ionization, may cause element fractionation, making the element fractionation effect more complicated and affecting the accuracy and precision of the analysis results.

[0004] LIMS, which is also based on laser sampling technology, has been limited by a problem in its early development. To achieve direct atomization and ionization of solid samples, the incident laser must provide a sufficiently high power density (greater than 108W / cm2). However, high-power density lasers often lead to the generation of multivalent ions, which interfere with spectral analysis. At the same time, the ions generated by high-energy laser ionization generally have a large initial kinetic energy dispersion, resulting in a serious reduction in mass resolution. Helium, as an inert gas, is generally difficult to react with other substances. By introducing helium as an auxiliary gas into the sample chamber, high-energy ions collide frequently with the auxiliary gas. In this way, high-energy ions can be cooled and dissociated, and multivalent ions can undergo trimerization to reduce valence, making monovalent ions the main component of ions in the plasma.

[0005] Although the introduction of auxiliary gas improves the quality of the spectrum, the frequent collisions between ions and auxiliary gas reduce the number of ions, and ions will inevitably be lost during the transmission process of multi-stage vacuum, resulting in reduced sensitivity of the instrument. If you want to improve the depth resolution of laser ionization mass spectrometry, you must reduce the sampling volume, and the reduction in sampling volume will reduce the sensitivity of the instrument. Therefore, there is an urgent need for a mass spectrometry system and method that can improve the ionization efficiency of ions while reducing the sampling volume to enhance the sensitivity of the instrument. Summary of the invention

[0006] In response to the above problems, the present invention proposes a dual-pulse system and method for improving the depth resolution of laser ionization mass spectrometry. The system can improve the depth resolution of laser ionization mass spectrometry to the nanometer level, and improve the ionization efficiency of ions while reducing the sampling amount, while improving the sensitivity of the instrument and the accuracy of quantitative analysis. The system and method will further enhance the application potential of laser ionization mass spectrometry in thin-layer analysis.

[0007] On the one hand, a dual-pulse system for improving the depth resolution of laser ionization mass spectrometry includes a control system, a laser energy control module, a beam shaping and beam profile analysis module;

[0008] The control system controls the femtosecond laser parameters generated by the femtosecond laser and the nanosecond laser parameters of the nanosecond laser;

[0009] The laser energy control module performs phase adjustment and detection on the input femtosecond laser, and outputs a femtosecond laser that meets the set parameters based on the femtosecond laser parameters;

[0010] The beam shaping and beam profile analysis module transforms the femtosecond laser that meets the set parameters from a Gaussian laser beam into a circular flat-top laser beam, focuses the circular flat-top laser beam and outputs it to the surface of the sample contained in the sample cavity, and performs laser ablation on the sample to generate neutral particles;

[0011] The neutral particles and nanosecond laser are focused so that the focal point is in the area where the neutral particles are located, so that the neutral particles undergo non-resonant multi-photon ionization to generate ionized ions; the ionized ions are used for mass spectrometry analysis; the femtosecond laser and nanosecond laser are emitted in the form of pulses, and the nanosecond laser pulse lags behind the femtosecond laser pulse.

[0012] Preferably, the laser energy regulation module includes a first Glan-Taylor polarizer, a second Glan-Taylor polarizer, a first beam splitter and an energy detector; the femtosecond laser passes through the first Glan-Taylor polarizer and the second Glan-Taylor polarizer in sequence, and is then split into two laser beams by the first beam splitter, one laser beam serves as the output of the laser energy regulation module, and the other laser beam enters the energy detector.

[0013] Preferably, the beam shaping and beam profile analysis module includes an optical diffraction element DOE, a second beam splitter, an attenuation plate, a first focusing lens, a beam analyzer and a second focusing lens; the femtosecond laser that meets the set parameters passes through the optical diffraction element DOE, and is then divided into two laser beams by the second beam splitter, one laser beam passes through the second focusing lens as the output of the beam shaping module, and the other laser beam passes through the attenuation plate and the second focusing lens and enters the beam analyzer.

[0014] Preferably, the setting parameter is femtosecond laser power; the magnitude of the femtosecond laser power is the ablation threshold of the sample plus the power margin, and the femtosecond laser power can perform nanoscale laser ablation on the sample.

[0015] Preferably, the power margin is 1% to 50% of the ablation threshold of the sample.

[0016] Preferably, the delay time of the nanosecond laser pulse relative to the femtosecond laser pulse is 1 ns to 1 s.

[0017] Preferably, the focus of the focused nanosecond laser is located on the femtosecond laser path, and the distance from the focus of the femtosecond laser on the sample surface is 1 nm to 10 cm.

[0018] Preferably, the direction of the nanosecond laser is perpendicular to the direction of the femtosecond laser.

[0019] Preferably, the sample chamber is filled with helium.

[0020] On the other hand, the double pulse method for improving the depth resolution of laser ionization mass spectrometry is based on the double pulse system for improving the depth resolution of laser ionization mass spectrometry, and the specific steps are as follows:

[0021] S1, regulating the femtosecond laser according to the laser energy regulation module and the control system so that the femtosecond laser meets the set parameters;

[0022] S2, placing the sample at the position to be tested in the sample chamber;

[0023] S3, the femtosecond laser passes through the laser energy control module, beam shaping and beam profile analysis module in sequence, and is focused on the sample surface to generate neutral particles;

[0024] S4, emits nanosecond laser, focuses the nanosecond laser on the neutral particle area, performs non-resonant multiphoton ionization, and produces ionized ions;

[0025] S5, performing mass spectrometry analysis on the ionized ions to complete the analysis of the elements at the position to be measured;

[0026] S6, moving the position of the sample to change the position to be tested, and repeating steps S3-S6 until the element composition and distribution of all the positions to be tested of the sample are obtained.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The present invention sets a femtosecond laser as the ablation source. Compared with a nanosecond laser, a femtosecond laser has a shorter pulse width and stops working before the electrons transfer heat to the lattice. This allows the sample to be quickly heated and evaporated, effectively reducing the hot melting area, avoiding the matrix effect and element fractionation effect caused by the thermal effect, and realizing stoichiometric sampling.

[0029] (2) The present invention realizes the controllable laser energy density by setting a laser energy control module. According to different samples, a femtosecond laser with an energy density slightly higher than the sample ablation threshold is used for ablation, which can improve the depth resolution of laser ionization mass spectrometry from micrometer level to nanometer level.

[0030] (3) The present invention converts the femtosecond laser from a Gaussian beam into a circular flat-top laser beam with relatively uniform energy distribution by setting a beam shaping and beam profile analysis module, thereby ensuring that the ablation depth at different positions of the spot tends to be consistent under each laser action, further improving the depth resolution. At the same time, the core area of ​​the neutral particles produced by the flat-top beam ablation is larger and more stable, which is conducive to the smooth progress of post-ionization.

[0031] (4) The present invention sets a high-energy-density nanosecond laser as a post-ionization source, and uses the high-energy-density nanosecond laser to perform non-resonant multiphoton post-ionization on the neutral particles generated by the femtosecond laser, thereby achieving ionization of most elements and improving the sensitivity of the instrument. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention is further described in detail below in conjunction with the accompanying drawings;

[0033] Figure 1This is a schematic diagram of the overall structure of a dual-pulse system for improving the depth resolution of laser ionization mass spectrometry according to an embodiment of the present invention;

[0034] Figure 2 A schematic diagram of the timing of a three-dimensional displacement module, a femtosecond laser pulse, and a nanosecond laser pulse of a dual-pulse system for improving the depth resolution of laser ionization mass spectrometry according to an embodiment of the present invention;

[0035] Figure 3 Schematic diagram of focusing nanosecond laser onto neutral particles above a sample stage in a dual-pulse system for improving the depth resolution of laser ionization mass spectrometry according to an embodiment of the present invention.

[0036] Figure numerals: 1. femtosecond laser; 2. first Glan-Taylor polarizer; 3. second Glan-Taylor polarizer; 4. first beam splitter; 5. energy detector; 6. DOE; 7. second beam splitter; 8. attenuation plate; 9. first focusing lens; 10. beam analyzer; 11. second focusing lens; 12. three-dimensional displacement module; 13. sample; 14. sample cavity; 15. third focusing lens; 16. nanosecond laser. DETAILED DESCRIPTION

[0037] The present invention is further described below through specific implementation modes.

[0038] like Figure 1 As shown, a dual-pulse system for improving the depth resolution of laser ionization mass spectrometry includes a laser energy control module, a beam shaping and beam profile analysis module, a sample cavity 14, a three-dimensional displacement module 12, a third focusing lens 15, a femtosecond laser 1, a nanosecond laser 16, and a control system.

[0039] Helium is introduced into the sample chamber 14 as an auxiliary gas, and a three-dimensional displacement module 12 is provided; the three-dimensional displacement module 12, the femtosecond laser and the nanosecond laser are electrically connected to the control system, and the control system controls the movement of the three-dimensional displacement stage and the laser emission parameters and emission pulses of the laser. Specifically, the three-dimensional displacement module 12 can move horizontally along the XY axis and vertically along the Z axis. The lateral resolution of the thin film sample is 20um, and the depth resolution is nanometer level.

[0040] In this embodiment, a femtosecond laser 1 is arranged as the ablation source. Compared with a nanosecond laser, the femtosecond laser 1 has a shorter pulse width and stops working before the electrons transfer heat to the lattice, so that the sample can be quickly heated and evaporated, the hot melting area can be effectively reduced, the matrix effect and element fractionation effect caused by the thermal effect can be avoided, and stoichiometric sampling can be realized.

[0041] The laser energy control module is used to control the laser energy. The module includes a first Glan-Taylor polarizer 2, a second Glan-Taylor polarizer 3, a first beam splitter 4, and an energy detector 5. After passing through the first Glan-Taylor polarizer 2 and the second Glan-Taylor polarizer 3, the femtosecond laser 1 changes its phase, filters stray light, and improves the quality and transmission efficiency of light. The femtosecond laser 1 is reflected to the energy detector 5 through the first beam splitter 4. The energy detector 5 can control the laser energy according to the ablation threshold of different samples. The laser can bombard the sample 13 with an energy slightly higher than the ablation threshold, reduce the sampling amount of the sample 13, and reduce the ablation depth. Specifically, the angle between the reflection surface of the first beam splitter 4 and the incident direction of the femtosecond laser 1 is less than 90°.

[0042] This embodiment realizes the controllable laser energy density by setting a laser energy control module. According to different samples, a femtosecond laser with an energy density slightly higher than the sample ablation threshold is used for ablation, which can improve the depth resolution of laser ionization mass spectrometry from micrometer level to nanometer level.

[0043] The beam shaping and beam morphology analysis module shapes the femtosecond laser from a Gaussian laser beam to a flat-top circular beam and focuses the beam on the surface of the sample 13. The module includes DOE (Diffractive Optical Elements, optical diffraction elements) 6, a second beam splitter 7, an attenuation plate 8, a first focusing lens 9, a beam analyzer 10, and a second focusing lens 11. After the femtosecond laser 1 passes through DOE 6, the laser beam is shaped from a Gaussian laser beam to a circular flat-top beam with a relatively uniform energy distribution, which is reflected to the attenuation plate 8 through the second beam splitter 7. The laser beam with attenuated light intensity is focused to the beam analyzer 8 through the first focusing lens 9. The light intensity distribution and collective status of the current laser beam can be obtained according to the beam analyzer 10. The circular flat-top laser beam of the femtosecond laser 1 in another direction split by the second beam splitter 7 passes through the surface of the sample 13 focused by the second focusing lens 11, and the sample is stripped into neutral particles. Specifically, the angle between the reflection surface of the second beam splitter 7 and the incident direction of the femtosecond laser 1 is less than 90°.

[0044] This embodiment converts the femtosecond laser 1 from a Gaussian beam into a circular flat-top laser beam with a relatively uniform energy distribution by setting a beam shaping and beam profile analysis module, ensuring that the erosion depth at different positions of the spot tends to be consistent under each laser action, further improving the depth resolution. At the same time, the core area of ​​neutral particles produced by the flat-top beam erosion is larger and more stable, which is conducive to the smooth progress of post-ionization. The traditional Gaussian laser beam has high central energy, resulting in the existence of "local hot spots" that make sample sampling uneven and easily form conical erosion pits, affecting the accuracy of depth analysis. The beam shaping module set in this embodiment solves this problem.

[0045] The above module reduces the sampling amount of the sample 13, reduces the ablation depth, and improves the depth resolution of the laser ionization ablation. However, the reduction in the sampling amount will also cause a decrease in the sensitivity of the instrument. Therefore, in this embodiment, a nanosecond laser 16 is set as a post-ionization source, see Figure 2 As shown, the emission pulse of the nanosecond laser 16 is delayed by t compared with the emission pulse of the femtosecond laser 1. Figure 3 As shown, the nanosecond laser 16 is focused to a distance d from the surface of the sample 13 by the third focusing lens 15. The nanosecond laser 16 is used as a post-ionization source to cause non-resonant multi-photon ionization of the neutral particles generated by the ablation of the femtosecond laser 1, thereby achieving saturated ionization of most elements, improving the ionization efficiency of the ion source, and compensating for the sensitivity loss caused by reducing the sampling amount.

[0046] Specifically, the delay time t of the nanosecond laser pulse is 1ns to 1s compared to the femtosecond laser pulse. The nanosecond laser 16 is focused to the neutral particles at a distance of 1nm to -10cm parallel to the sample surface through the third focusing lens 15 in a direction perpendicular to the femtosecond laser 1. The focal points of the lasers emitted by the two lasers differ only by the distance from the sample, and are projected at one position along the direction of the sample stage. The nanosecond laser 16 is generated by a nanosecond laser, which is a 266nm laser (SGR-S800, Rabo Optoelectronics) with a maximum output energy of 80mJ.

[0047] In this embodiment, a high-energy-density nanosecond laser 16 is provided as a post-ionization source, and the high-energy-density nanosecond laser 16 performs non-resonant multiphoton post-ionization on the neutral particles generated by the femtosecond laser 1, thereby achieving ionization of most elements and improving the sensitivity of the instrument.

[0048] A double pulse method for improving the depth resolution of laser ionization mass spectrometry, the process of using a double pulse system for improving the depth resolution of laser ionization mass spectrometry for analysis is as follows:

[0049] The femtosecond laser 1 adjusts the phase through the first Glan-Taylor polarizer 2 and the second Glan-Taylor polarizer 3, thereby improving the light quality and transmission efficiency. The femtosecond laser 1 reflects a laser beam to the energy detector 5 through the first beam splitter 4. The energy detector 5 can adjust the femtosecond laser energy to be slightly greater than the ablation threshold of the sample 13, thereby reducing the ablation depth. The femtosecond laser energy of this embodiment is greater than 1% to 50% of the ablation threshold of the sample. After the other beam of the first beam splitter 4 passes through the DOE 6 in the direction of the original femtosecond laser 1, it is shaped from a Gaussian beam to a circular flat-top beam, and then reflected by the second beam splitter 7 to the attenuation plate 8. The laser light with attenuated light intensity passes through the first focusing lens 9 to the beam analyzer 10, and the current beam morphology is obtained through the beam analyzer 10. After obtaining the required laser intensity and beam morphology, the matrix crater experiment is started.

[0050] After the sample 13 is moved to the experimental position using the three-dimensional displacement module 12, the femtosecond laser 1 is emitted and focused on the surface of the sample 13 through the second focusing lens 11. A certain amount of neutral particles are stripped from the sample 13. The emission pulse of the nanosecond laser 16 is delayed by a time t compared to the femtosecond laser. Figure 2 As shown; after a delay of t, the nanosecond laser 16 is focused by the third focusing lens 15 to an area at a distance d from the sample surface, as shown Figure 3 As shown; the neutral particles here are fully post-ionized through non-resonant multiphoton ionization, and the ionized ions pass through the ion funnel into the ion transmission system, and then from the ion transmission system to the time-of-flight mass spectrometer for elemental analysis.

[0051] The above operation is repeated continuously to perform matrix ablation on the sample 13 in a certain area, and the elemental composition of the components at each ablation point is obtained through an analyzer.

[0052] In summary, the dual-pulse system and method for improving the depth resolution of laser pulse ionization of this embodiment achieves an extremely low sample ablation rate by using a femtosecond laser with an energy density slightly higher than the sample ablation threshold, and improves the ionization efficiency by performing non-resonant multi-photon ionization on the ablated neutral particles through a high-power post-ionization laser, thereby achieving nanometer-level depth resolution.

[0053] The above is only a specific implementation of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial changes to the present invention using this concept shall be deemed as an infringement of the protection scope of the present invention.

Claims

1. A dual pulse system for improving the depth resolution of laser ionization mass spectrometry, characterized in that: It includes a control system, a laser energy control module, and a beam shaping and beam profile analysis module; The control system controls the femtosecond laser parameters generated by the femtosecond laser and the nanosecond laser parameters of the nanosecond laser; The laser energy control module performs phase adjustment and detection on the input femtosecond laser, and outputs a femtosecond laser that meets the set parameters based on the femtosecond laser parameters; The beam shaping and beam profile analysis module transforms the femtosecond laser that meets the set parameters from a Gaussian laser beam into a circular flat-top laser beam, focuses the circular flat-top laser beam and outputs it to the surface of the sample contained in the sample cavity, and performs laser ablation on the sample to generate neutral particles; The neutral particles and nanosecond laser are focused so that the focal point is in the area where the neutral particles are located, so that the neutral particles undergo non-resonant multi-photon ionization to generate ionized ions; the ionized ions are used for mass spectrometry analysis; the femtosecond laser and nanosecond laser are emitted in the form of pulses, and the nanosecond laser pulse lags behind the femtosecond laser pulse.

2. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The laser energy regulation module includes a first Glan-Taylor polarizer, a second Glan-Taylor polarizer, a first beam splitter and an energy detector; the femtosecond laser passes through the first Glan-Taylor polarizer and the second Glan-Taylor polarizer in sequence, and is then split into two laser beams by the first beam splitter, one laser beam is used as the output of the laser energy regulation module, and the other laser beam enters the energy detector.

3. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The beam shaping and beam profile analysis module includes an optical diffraction element DOE, a second beam splitter, an attenuation plate, a first focusing lens, a beam analyzer and a second focusing lens; the femtosecond laser that meets the set parameters passes through the optical diffraction element DOE, and is then divided into two laser beams by the second beam splitter, one laser beam passes through the second focusing lens as the output of the beam shaping and beam profile analysis module, and the other laser beam passes through the attenuation plate and the second focusing lens and enters the beam analyzer.

4. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The setting parameter is the femtosecond laser power; the magnitude of the femtosecond laser power is the ablation threshold of the sample plus the power margin, and the femtosecond laser power can perform nano-level laser ablation on the sample.

5. The double pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 4, characterized in that: The power margin is 1% to 50% of the ablation threshold of the sample.

6. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The delay time of the nanosecond laser pulse relative to the femtosecond laser pulse is 1ns-1s.

7. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The focus of the focused nanosecond laser is located on the femtosecond laser path, and the distance from the focus of the femtosecond laser on the sample surface is 1 nm to 10 cm.

8. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The direction of the nanosecond laser is perpendicular to the direction of the femtosecond laser.

9. The dual pulse system for improving the depth resolution of laser ionization mass spectrometry according to claim 1, characterized in that: The sample chamber is filled with helium.

10. A double pulse method for improving the depth resolution of laser ionization mass spectrometry, characterized in that: Based on the double-pulse system for improving the depth resolution of laser ionization mass spectrometry as described in any one of claims 1 to 9, the specific steps are as follows: S1, regulating the femtosecond laser according to the laser energy regulation module and the control system so that the femtosecond laser meets the set parameters; S2, placing the sample at the position to be tested in the sample chamber; S3, the femtosecond laser passes through the laser energy control module, beam shaping and beam profile analysis module in sequence, and is focused on the sample surface to generate neutral particles; S4, emits nanosecond laser, focuses the nanosecond laser on the neutral particle area, performs non-resonant multiphoton ionization, and produces ionized ions; S5, performing mass spectrometry analysis on the ionized ions to complete the analysis of the elements at the position to be measured; S6, moving the position of the sample to change the position to be tested, and repeating steps S3-S6 until the element composition and distribution of all the positions to be tested of the sample are obtained.

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