Thermo-electron emission suppression method based on TTM-MD simulation

Abstract

The invention discloses a hot electron emission suppression method based on TTM-MD simulation, and the method comprises the steps: regulating and controlling the electron temperature through adjusting the energy distribution and delay time of double pulses; and according to the relationship between the heating time and the delay, the maximum delay time introduced under the condition that the processing requirement is met is obtained. According to the method, the maximum delay which can be introduced on the premise that the processing requirement is met is obtained, the electron temperature is reduced, and hot electron emission is restrained.

Description

Technical Field

[0001] The present invention belongs to the field of laser technology, and in particular relates to a method for suppressing thermal electron emission based on TTM-MD simulation. Background Art

[0002] With the rapid development of the manufacturing industry, the demand for machining precision is increasing. Traditional mechanical machining methods can no longer meet the needs of machining complex shapes at the micron and nanometer scales. While currently mature precision manufacturing methods offer extremely high machining resolution, they are limited by the substrate and machining environment, and require lengthy processing steps, making them unsuitable for general machining scenarios and limiting their economic and environmental performance.

[0003] The widespread use of femtosecond lasers has opened a new avenue for precision machining at the micro- and nanoscale. Their short pulse width and high power effectively minimize the area affected by heat, reducing thermal damage to the substrate and enabling cold processing. In ablation, it's desirable to limit the area where material removal occurs; in sintering, it's desirable to interconnect the irradiated areas to avoid material removal.

[0004] Material removal is primarily caused by thermionic emission. Under femtosecond laser irradiation, photon energy is first absorbed by electrons, increasing their energy to form hot electrons. These electrons then transfer energy to the lattice through electron-lattice energy coupling, heating the material. However, if the electrons absorb too much energy in a short period of time—in other words, their "temperature" becomes too high—they can be excited into free electrons, a phenomenon known as thermionic emission. At this point, the material particles exhibit positive electrical repulsion, causing them to be blown away, resulting in material removal.

[0005] Lowering the electron temperature to suppress thermal electron emission can be achieved by controlling the energy, but simply reducing the laser energy may make it impossible to achieve processing requirements; increasing the exposure time to compensate for the reduction in energy may cause heat to be deposited around the substrate and the processing area, thereby expanding the heat-affected zone and failing to fully utilize the advantages of femtosecond lasers. Summary of the Invention

[0006] In response to the above problems, the present invention provides a method for suppressing thermal electron emission based on TTM-MD simulation, aiming to obtain the maximum delay that can be introduced while meeting processing requirements, thereby reducing the electron temperature and suppressing thermal electron emission.

[0007] To achieve the above objectives, the present invention provides a method for suppressing thermal electron emission based on TTM-MD simulation. The method regulates the electron temperature by adjusting the energy distribution and delay time of the double pulses; and based on the relationship between the heating time and the delay, obtains the maximum delay time introduced while meeting the processing requirements.

[0008] A further technical solution of the present invention is to use a beam splitter and a displacement platform to split the laser beam and introduce a delay.

[0009] A further technical solution of the present invention is that the temperature rise of the material under femtosecond laser irradiation is specifically expressed as:

[0010]

[0011] m i 、v i is the mass and velocity of each atom, U is the potential energy of the system, Langevin thermostat to describe electron-lattice energy transfer; C e 、k e 、g p , S represent the heat capacity, thermal conductivity, electroacoustic coupling coefficient and external energy of electrons respectively; T e 、T l represent the temperature of electrons and lattice respectively.

[0012] A further technical solution of the present invention is that the TTM-MD simulation model is two adjacent copper nanoparticles, and the melting point of the copper nanoparticles at the nanoscale is determined to select the simulated laser energy.

[0013] A further technical solution of the present invention is to determine the melting point of copper nanoparticles at the nanoscale, and the specific method includes:

[0014] Simulations were performed under conditions of slow heating at a constant rate and heating with a high-energy femtosecond laser to obtain the relationship between the system's potential energy and temperature.

[0015] A further technical solution of the present invention is to determine the melting point to be 1100K-1200K and the simulated laser energy flux density to be 60J / m 2 ~80J / m 2 .

[0016] A further technical solution of the present invention is to utilize MD simulation based on a microcanonical ensemble to obtain that the introduction of delay has a regulating effect on the electron temperature.

[0017] A further technical solution of the present invention is that the maximum delay time is 16 ps to 20 ps.

[0018] The disclosed embodiments of the present invention provide a method for suppressing thermal electron emission based on TTM-MD simulation. This method studies, at the atomic scale, the regulation of electron temperature by a delayed dual-pulse laser. By adjusting the energy distribution and delay time of the dual pulses, the electron temperature is effectively controlled. Furthermore, considering certain constraints and using the relationship between the heating time and delay as a reference, the maximum delay that can be introduced while meeting processing requirements is determined, thereby reducing the electron temperature and suppressing thermal electron emission.

[0019] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present invention and, together with the description, serve to explain the principles of the present invention.

[0021] Figure 1 is a potential energy-temperature curve diagram in an embodiment of the present invention;

[0022] Figure 2 is an electron / lattice temperature-time curve diagram in an embodiment of the present invention;

[0023] Figure 3 is a graph showing the maximum electron temperature-delay curve in an embodiment of the present invention;

[0024] Figure 4 (a) is a heating time-delay curve diagram in an embodiment of the present invention, Figure 4 (b) Yes Figure 4 (a) Graph after the curve is differentiated. DETAILED DESCRIPTION

[0025] The present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are intended only to illustrate the present invention and are not intended to limit the present invention. It should also be noted that, for ease of description, the accompanying drawings only illustrate portions relevant to the present invention, not all structures.

[0026] Before discussing the exemplary embodiments in more detail, it should be mentioned that some exemplary embodiments are described as processes or methods depicted as flow charts. Although the flow charts describe the steps as sequential processes, many of the steps can be implemented in parallel, concurrently, or simultaneously. In addition, the order of the steps can be rearranged. The process can be terminated when its operation is completed, but can also have additional steps not included in the accompanying drawings. The process can correspond to a method, function, procedure, subroutine, subprogram, etc.

[0027] Lowering the electron temperature to suppress thermal electron emission can be achieved by controlling the energy. A feasible solution is to split the laser pulse while keeping the total energy flux unchanged, introduce a picosecond delay, allow the first laser beam to enter the material first, and allow the second laser beam to arrive before the electron-lattice energy coupling is completely completed. The peak temperature of the thermal electrons is lowered while reaching the target temperature, thereby effectively suppressing the additional material removal caused by thermal electron emission.

[0028] A method for suppressing thermal electron emission based on two-temperature equation-molecular dynamics (TTM-MD) simulation is proposed. The method regulates the electron temperature by adjusting the energy distribution and delay time of the double pulses. Based on the relationship between the heating time and the delay, the maximum delay time introduced while meeting the processing requirements is obtained.

[0029] Specifically, the use of beam splitters and displacement platforms can split the laser beam and introduce a slight delay. The microscopic behavior of materials under femtosecond laser irradiation needs to be simulated by coupling the two-temperature equation (TTM) with molecular dynamics (MD). In MD, matter is modeled as atomic clusters with a certain speed (kinetic energy) and bound to each other by a certain force (potential energy), and there is a one-to-one correspondence between the temperature of the lattice and the kinetic energy of the atoms. Therefore, the temperature rise of the material under femtosecond laser irradiation can be described by the following two equations:

[0030]

[0031] where m i 、v i is the mass and velocity of each atom, U is the potential energy of the system, Langevin thermostat to describe electron-lattice energy transfer; C e 、k e 、g p , S represents the heat capacity, thermal conductivity, electroacoustic coupling coefficient and external energy of the electron; T e 、T l Represents the temperature of electrons and lattice.

[0032] The simulation model is two adjacent copper nanoparticles, and the coordinates and velocity of each atom are calculated. First, the melting point at the nanoscale is determined, and the laser energy used in the subsequent simulation is selected based on this. Simulations are performed under conditions of slow heating at a constant rate and heating with a high-energy femtosecond laser to examine the relationship between the system's potential energy and temperature. Figure 1As shown. The curve shows that under the condition of constant rate heating, the potential energy of the system changes dramatically at a temperature of 1100K-1250K, and its melting point should be in the range of 1175K±75K; under the action of femtosecond laser, the dramatic change of potential energy occurs at 1200K-1350K, and its melting point is in the range of 1275K±75K. The difference in the melting point range under the two conditions can be explained as follows: under the condition of slow heating, the temperature (kinetic energy) and potential energy of the system have enough time to reach equilibrium, and under the action of femtosecond laser, the system heats up rapidly, and at the same temperature, the kinetic energy has not yet reached equilibrium with the potential energy, and the system has already risen to a higher temperature. Therefore, the melting point range under both conditions is acceptable. Here, 1150K, which is slightly higher than the critical temperature, is taken as the melting point, and the energy flux density is selected as 60J / m 2 ~80J / m 2 , preferably 70 J / m 2 Femtosecond laser simulation.

[0033] At 70 J / m 2 The total energy flux density is simulated under the conditions of single pulse and double pulse (35 / 35) with the same energy and a delay of 2ps-12ps. The curve of electron / lattice temperature changing with time is as follows: Figure 2 As shown. In the MD simulation, the microcanonical ensemble (NVE) was selected, that is, the volume of the simulation box of the system remains unchanged, and the total energy change of the system except for the heating source / cold source is not considered. Since the energy coupling between the electron and the lattice occurs within tens of ps, this time window is almost negligible compared with the heat dissipation time between the copper film and the room temperature environment under the macroscopic state. Therefore, in the simulation, the external heat dissipation of the system can be ignored. When a delay is introduced, the copper nanoparticles under the action of the femtosecond laser can eventually reach the same temperature after a slightly extended coupling time, that is, it is considered that there is a delayed double pulse, which can achieve the same processing effect as a single pulse; at the same time, it can be seen from the graph that the maximum temperature of the electrons shows a downward trend with the introduction of the delay. The introduction of the delay has a certain regulatory effect on the electron temperature, which can effectively suppress the damage of the processed surface caused by thermal electron emission.

[0034] Further study the effect of the delay between pulses on the maximum electron temperature, and consider adjusting the energy distribution of the double pulses to make the double pulse energies different (20 / 50, 50 / 20), such as Figure 3As shown. Within a short delay time, the maximum electron temperature increases with the delay, showing an exponential decay pattern. As the delay gradually increases to 10ps, the maximum electron temperature decreases by hundreds of K. Changing the energy distribution of the pulse will also affect the regulation of the electron temperature. When using a large pulse / small pulse combination, the effect of the delay time on the maximum temperature is more obvious within a shorter delay time (about 10ps); however, when using a small pulse / large pulse combination, the rate of electron temperature decrease is slow and always higher than the other two cases. This phenomenon is easy to explain: the energy coupling between electrons and the lattice is large at high electron temperatures. After using a large pulse, the electrons are first excited to a higher temperature and transfer energy to the lattice at a faster rate. Therefore, compared with the small pulse / large pulse combination, at the same delay time, the energy remaining in the electron system after adding the second pulse is less than the latter, that is, the regulation effect on the maximum electron temperature is better.

[0035] It should be noted that, as mentioned above, the simulation here is performed without considering external heat dissipation. Therefore, after introducing the delay, it must be ensured that the entire coupling time is still much shorter than the external heat dissipation time in actual processing, so that the simulation is still applicable to the premise of ignoring external heat dissipation. In other words, the second pulse must be introduced before the coupling under the action of the first pulse is completed. Otherwise, it can no longer be regarded as a double pulse, but rather a "single pulse with two beams that cannot complete the processing." This puts a limit on the delay that can be introduced.

[0036] from Figure 2 It is easy to see that the lattice temperature rise curve with time is a convex function, and the slope eventually approaches 0. Therefore, when two temperature rise processes are superimposed, if the previous process has almost been completed, then increasing the delay will only increase the temperature rise time by the same amount. Taking the melting point of 1150K as an example, plot the melting time versus delay curve, as shown in Figure 4 (a) is shown. After 30 ps, ​​the slope is close to 1, which is consistent with the situation mentioned that "the previous process is almost completed". That is, after 30 ps, ​​the two pulses are completely unrelated. At this time, it is very likely that the processing purpose cannot be achieved. It is no longer meaningful to study the control of the maximum electron temperature by delay. At the same time, it can be seen that the slope changes significantly before 20 ps. Therefore, after fitting the curve, the differential is obtained, as shown in Figure 4 (b) The first-order differential of the fitted curve is an upward-convex function. Around 16 ps, its slope is almost half of its initial value. At this point, the connection between the two pulses is already very weak, so the maximum delay that can be introduced can be achieved around this time. This delay allows the dual pulses to achieve the processing goal while minimizing the electron temperature.

[0037] Based on the above examples, a method for suppressing thermal electron emission based on TTM-MD simulation was developed to study, at the atomic scale, the effect of delayed dual-pulse lasers on electron temperature. By adjusting the energy distribution and delay of the dual pulses, the electron temperature was effectively controlled. Furthermore, considering certain constraints and using the relationship between the heating time and delay as a reference, the maximum delay that can be introduced while meeting processing requirements was determined.

[0038] In this document, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a step or method that comprises a series of elements includes not only those elements, but also includes other elements not expressly listed, or also includes elements inherent to such step or method.

[0039] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art of the present invention, without departing from the concept of the present invention, several simple deductions or substitutions can be made, which should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for suppressing thermal electron emission based on TTM-MD simulation, characterized in that: The method regulates the electron temperature by adjusting the energy distribution and delay time of the double pulses, and obtains the maximum delay time introduced while meeting the processing requirements according to the relationship between the heating time and the delay.

2. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 1, characterized in that: A beam splitter and a translation stage are used to split the laser beam and introduce a delay.

3. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 1, characterized in that: The temperature rise of the material under femtosecond laser irradiation is specifically expressed as: m i 、v i is the mass and velocity of each atom, U is the potential energy of the system, Langevin thermostat to describe electron-lattice energy transfer; C e , k e , g p , S represent the heat capacity, thermal conductivity, electroacoustic coupling coefficient and external energy of electrons respectively; T e , T l Represent the temperature of electrons and lattice respectively.

4. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 1, characterized in that: The TTM-MD simulation model is two adjacent copper nanoparticles. The melting point of the copper nanoparticles at the nanoscale is determined to select the simulation laser energy.

5. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 4, characterized in that: Determine the melting point of copper nanoparticles at the nanoscale. The specific methods include: The relationship between the system's potential energy and temperature was obtained by simulating the system under conditions of slow heating at a constant rate and heating with a high-energy femtosecond laser.

6. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 4, characterized in that: The melting point is determined to be 1100K~1200K, and the simulated laser energy flux density is 60J / m 2 ~80J / m 2 .

7. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 6, characterized in that: Using MD simulation based on microcanonical ensemble, it is found that introducing delay has a regulatory effect on the electron temperature.

8. The method for suppressing thermal electron emission based on TTM-MD simulation according to claim 6, characterized in that: The maximum delay time is 16ps~20ps.

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