CN113305106A

CN113305106A - Method for cleaning micro-nano particle pollutants by laser and application

Info

Publication number: CN113305106A
Application number: CN202110618382.4A
Authority: CN
Inventors: 韩敬华; 冯国英; 何长涛; 丁坤艳; 李玮
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2021-06-03
Filing date: 2021-06-03
Publication date: 2021-08-27
Anticipated expiration: 2041-06-03
Also published as: CN113305106B

Abstract

The invention discloses a method for cleaning micro-nano particle pollutants by laser and application thereof. The method is obtained by cleaning a model through laser, wherein the cleaning model is constructed by the following models: the optical field intensity distribution model at the substrate interface including the modulation effect of the pollutant particles, the temperature distribution model at the substrate interface including the temperature distribution nonuniformity phenomenon caused by the modulation effect of the pollutant particles, the thermal stress distribution model at the substrate interface including the thermal stress variation effect caused by the temperature variation, and the optical force distribution model at the substrate interface. According to the invention, an optimal scheme for cleaning the transparent substrate micro-nano particles can be obtained by establishing a three-dimensional cleaning model comprising a scattered light field, a temperature field, a thermal force field and an electromagnetic field.

Description

Method for cleaning micro-nano particle pollutants by laser and application

Technical Field

The invention relates to the technical field of laser cleaning.

Background

In a large-scale high-power laser device, after an optical element is polluted by pollutants, the load capacity of the optical element can be greatly reduced, and the optical element is very easy to damage. The existing research indicates that the nano-scale pollutants are the main cause of damage of laser in ultraviolet band, so the removal of nano-scale particles is the common goal of many technologies. In the conventional cleaning, the substrate is easy to damage, and the conventional cleaning technology is more difficult to apply for nano-scale particles, and in order to solve the problem, part of the prior art provides a laser cleaning means.

The laser cleaning technology is a non-mechanical contact type surface cleaning technology, has the advantages of high cleaning efficiency, small pollution, wide application range and the like, and is widely applied to the industrial field. However, the application of extensive laser cleaning has led to the study of non-transparent substrates, and less difficult to study transparent substrates based on their optical and thermodynamic properties. For example, in 2003, albertobanone proposed a back surface dry laser cleaning of an opaque sample, which can effectively clean the surface of paper without damaging ink marks on the paper, in 2012, Jung-kyupak proposed a novel laser cleaning technique, femtosecond laser plasma shock waves, to remove nanoparticles on the surface of a silicon wafer, and pointed out the removal conditions without damaging the substrate, among these techniques, the strict requirements on laser removal and parameter selection thereof are one of the difficulties that the cleaning technique cannot break through. Meanwhile, the imperfect laser cleaning mechanism is also one of the reasons for limiting the development. For example, in 2006, francesco bloisi et al proposed a one-dimensional model describing a back laser cleaning technique for removing particles from a substrate, but the mechanism is only focused on the one-dimensional model in a single way, and the scattering field of the particles and a three-dimensional thermal expansion model are not described in detail. In 2001, Y.W.ZHEN et al were studying laser pair Sio₂During particle cleaning, the corresponding cleaning threshold is obtained based on the three-dimensional thermal expansion effect mechanism, but the electromagnetic force is not discussed.

Disclosure of Invention

The invention aims to overcome the defects of the prior art, and obtains the best scheme for cleaning transparent substrate micro-nano particles by establishing a three-dimensional cleaning model comprising a scattered light field, a temperature field, a thermal force field and an electromagnetic field.

The invention firstly provides the following technical scheme:

a method for cleaning micro-nano particle pollutants by laser comprises the following steps: the cleaning method comprises the following steps of obtaining a cleaning scheme through a laser cleaning model, and cleaning according to the cleaning scheme, wherein the cleaning model comprises the following model: the optical field intensity distribution model at the substrate interface including the pollutant particle modulation effect, the temperature distribution model at the substrate interface including the temperature distribution nonuniformity phenomenon caused by the pollutant particle modulation effect, the thermal stress distribution model at the substrate interface including the thermal stress variation effect caused by the temperature variation, and the optical force distribution model at the substrate interface.

According to some preferred embodiments of the invention, the light field intensity distribution model is as follows:

wherein, I₀(t) represents a laser smoothing pulse,. phi.represents an incident laser flux,. t represents time,. t represents_l＝0.409t_FWHWRepresenting the pulse time calculation coefficient, where t_FWHMDenotes the duration of the pulse, r₀The radial length of an area of increased field intensity of the optical field caused by modulation of pollutant particles under a polar coordinate system is shown; r represents the radial coordinate of the incident laser pulse point; r is_shIndicating by contaminationThe radial length of the region of reduced optical field intensity caused by the modulation of the object particles; r represents the radius of the pollutant particles; s₀Represents a field enhancement factor which is the optical field intensity I and the incident light field intensity I at the near-field focus generated by incident laser light at the bottom of a contaminant particle (i.e. the interface between the particle and the substrate material supporting the particle) caused by the modulation of the contaminant particle₀Ratio of (i.e. S)₀＝I/I₀， S₁Represents the area of the region of reduced optical field strength caused by modulation of contaminant particles, and

according to some preferred embodiments of the invention, the temperature distribution model is as follows:

wherein r represents the radial coordinate of the contaminant particle in a polar coordinate system,

represents the thermal diffusion coefficient of the base material; t is t₁Representing time units, F (z, t) represents a function as follows:

where erfc represents the complementary error function, c represents the speed of light, and e represents a natural constant.

According to some preferred embodiments of the present invention, the thermal stress distribution model is obtained by jointly solving the temperature distribution model and the thermal stress model.

According to some preferred embodiments of the invention, the thermal stress model is set as follows:

wherein f is_rDenotes thermal stress, p_pRepresenting the material density of the contaminant particles, f (t) representing the displacement function of the contaminant particles, d representing the differential sign.

According to some preferred embodiments of the invention, the light force distribution model is as follows:

<F_total>＝∫_S<T>·dS (12)

wherein the content of the first and second substances,<F_total>which represents the average total light force,<T>a maxwell stress tensor representing the time domain average, S represents the area,

representing binary multiplication, D electric flux density, E^*Denotes the conjugate of the electric field strength, H denotes the magnetic field strength, B^*Denotes the conjugate of the magnetic flux density, E denotes the incident electric field,

representing the unit tensor of the light field, D^*Represents the conjugate of the electric flux density.

According to some preferred embodiments of the present invention, the laser cleaning model further comprises the following cleaning condition models:

wherein a represents a contact radius of the particle with the substrate; rho_pRepresenting the material density of the contaminant particles, f (t) representing the displacement function of the contaminant particles, d representing the differential sign,

denotes a Lifshitz constant, h denotes a separation distance, and

p represents the loading force, σ₁、σ₂Representing the Poisson coefficient, E, of the particles and of the substrate material, respectively₁、E₂Respectively, the young's modulus of the particle and the substrate material, and R the radius of the contaminant particle.

According to some preferred embodiments of the present invention, the laser cleaning model is simulated by finite element analysis to obtain a numerical solution thereof.

According to some preferred embodiments of the present invention, based on the laser cleaning model, the cleaning threshold is obtained by the following threshold calculation model:

in the forward purge:

when F is present_{Heat generation}＝F_{Glue stick}+F_{Light (es)}Then, a cleaning threshold is obtained, wherein F_{Heat generation}Denotes the resulting thermal stress, F_{Glue stick}The table demonstrates the Delaware force, i.e. the right-hand equation in the inequality (8), F_{Light (es)}Representing the light force obtained by the light force model;

in back-washing:

when F is present_{Heat generation}＝F_{Glue stick}-F_{Light (es)}Then, a cleaning threshold is obtained.

The invention further provides an application of the cleaning method in cleaning optical elements, in particular an application in cleaning optical elements polluted by micro-nano alumina particles.

The invention can optimally remove micro-nano particles attached to the transparent substrate.

Drawings

FIG. 1 shows a test apparatus according to an embodiment.

FIG. 2 is a schematic diagram of forward cleaning (a) and reverse cleaning (b) according to an embodiment.

FIG. 3 is a graph showing the distribution of light intensity of alumina particles on the incident surface side of quartz glass obtained from simulation experiments.

FIG. 4 is a graph showing the light intensity distribution of alumina particles on the transmission surface side of quartz glass obtained from simulation experiment.

FIG. 5 shows the field enhancement factor S obtained from simulation experiment₀Graph relating to particle size.

FIG. 6 shows the bottom intensity distribution I/I of the particles obtained from the simulation experiment₀Schematic representation.

FIG. 7 is a schematic diagram of axial electric vector distribution obtained from simulation experiments.

FIG. 8 is a graph showing the temperature change of the contact surface of the particles and the substrate in the forward cleaning process obtained from the simulation experiment.

FIG. 9 is a graph showing the temperature change of the contact surface of the particles and the substrate in the back washing process, which is obtained by the simulation experiment.

FIG. 10 is a graph showing the forward cleaning stress distribution obtained from the simulation experiment.

FIG. 11 is a graph of the back-side cleaning stress profile from a simulation experiment.

FIG. 12 is a graph showing the variation of the forward cleaning light force with the particle radius obtained from the simulation experiment.

FIG. 13 is a graph of back-washing light force as a function of particle radius from simulation experiments.

Fig. 14 shows the relationship between the acceleration required for cleaning and the particle size obtained by the simulation experiment.

Fig. 15 is a graph showing the change of the stress with the energy density in the front surface laser cleaning according to the simulation experiment, in which (a) the particle radius is 0.2um and (b) the particle radius is 2 um.

Fig. 16 is a graph showing the variation of each stress with energy density in the back laser cleaning process, which is obtained from the simulation experiment, wherein (a) the particle radius is 0.2um, and (b) the particle radius is 2 um.

Fig. 17 is a graph showing a relationship between an optimal removal energy density control range of micro-nano particles and a particle size obtained by a simulation experiment.

FIG. 18 is a graph comparing the effect of contaminants with particle radius of 2um in forward cleaning in test experiments, wherein the laser energy density from left to right is 0.1-0.25mJ/cm²、0.26mJ/cm²And 0.3mJ/cm²。

FIG. 19 is a graph comparing the effect of contaminants with particle radius of 2um in back-side cleaning in test experiments with laser energy densities of 1-17.4mJ/cm from left to right²、17.5mJ/cm²And 19mJ/cm²。

FIG. 20 is a graph showing the comparison of the effect of contaminants with a particle radius of 200nm in a forward cleaning process in a test experiment, wherein the laser energy densities from left to right are 1-1.1mJ/cm²、1.13mJ/cm²And 1.5mJ/cm²。

FIG. 21 is a graph comparing the effect of contaminants with particle radius of 2um in back-side cleaning in test experiments, wherein the laser energy density from left to right is 0.1-0.82mJ/cm²、0.83mJ/cm²And 0.9mJ/cm²。

FIG. 22 is a SEM comparison of cleaned 200nm particles in a test experiment in which (a) the bulk feature is cleaned in a forward direction (b) a partial magnified view (c) the bulk feature is cleaned in a reverse direction (d) a partial magnified view.

Fig. 23 is a SEM comparison of washed 2 μm particles in a test experiment, wherein (a) the bulk profile of the forward wash (b) is magnified in section (c) the bulk profile of the reverse wash (d) is magnified in section.

Fig. 24 is an SEM comparison of the interface of laser cleaning in the testing experiment, wherein (a) the substrate topography was cleaned forward (b) the substrate topography was cleaned backward (c) the particles topography was cleaned forward (d) the particles topography was cleaned backward.

Detailed Description

The present invention is described in detail below with reference to the following embodiments and the attached drawings, but it should be understood that the embodiments and the attached drawings are only used for the illustrative description of the present invention and do not limit the protection scope of the present invention in any way. All reasonable variations and combinations that fall within the spirit of the invention are intended to be within the scope of the invention.

According to the technical scheme of the invention, a specific cleaning method comprises the following steps:

s1 construction of cleaning model

More specifically, the cleaning model may further include:

laser pulse model, as follows:

wherein, I₀(t) represents a laser smoothing pulse,. phi.represents an incident laser flux,. t represents time,. t represents_l＝0.409t_FWHMRepresenting the pulse time calculation coefficient, where t_FWHMIndicating the duration of the pulse.

The inventors have surprisingly found that when the particle size of the contaminant is large, the contaminant can modulate the incident laser, and when the particle size is small, the contaminant can obstruct the incident laser. Further, due to the partial transmission effect of the contaminant particles to the laser and the modulation effect under a certain condition, the optical field distribution around the particles will be uneven and locally overheated, thereby causing the actual cleaning threshold to be 1-2 orders of magnitude different from that of the uniform surface heating cleaning.

Based on the above findings, the cleaning model of the present invention further comprises: the model comprises a substrate interface optical field intensity distribution model including particle modulation, a substrate interface temperature distribution model including temperature nonuniformity caused by particle modulation, a substrate interface stress distribution model including thermal stress variation caused by temperature variation, and a substrate interface electromagnetic force distribution model.

Wherein, further, the substrate interface optical field distribution model including the particle modulation effect can be set as follows:

wherein r is₀Representing the radial length of an area of increased field intensity of the optical field caused by the modulation action of pollutant particles under a polar coordinate system; r represents the radial coordinate of the incident laser pulse point; r is_shRepresents the radial length of the region of reduced optical field strength resulting from modulation of the contaminant particles; r represents the radius of the pollutant particles; s₀Represents a field enhancement factor, which is the optical field strength I and the incident light field strength I at the near-field focus generated by the incident laser light at the bottom of the contaminant particle (i.e., the interface between the particle and the substrate material supporting the particle) caused by the modulation of the contaminant particle₀Ratio of (i.e. S)₀＝I/I₀，S₁Represents the area of the region of reduced optical field strength caused by modulation of the contaminant particles, and

further, the specific optical field distribution conditions of the pollutant particles with different particle sizes at the substrate interface can be obtained by a finite element analysis method according to the optical field distribution model.

In the distribution model, the field enhancement part (r) at the bottom of the particle₀Part) of the shaded area in the incident light profile obtained after incidence of the laser light on the substrate material supporting the particles, i.e. the part of the field intensity resulting from modulation of the light field by the particles which is lower than the incident field intensity (r)_shPart) and an approximation of the intensity distribution of the light field at the bottom of the particle is obtained by summing the three gaussian functions.

Based on the model, 1) when r<r₀The time is a field enhancement zone, the enhanced field intensity is S₀I₀(ii) a 2) When r is₀<r<r_shThe field intensity is very small in the field weakening area; 3) when r is>r_shWith R + λ (where R denotes the particle radius and λ denotes the wavelength of the light), the field strength tends to be uniform in one-dimensional intensity.

The modulation condition of the particles to the optical field can be obtained through the simultaneous connection of the formulas (1) - (3), and the changes of the electric field intensity along with the changes of the radial coordinate can be obtained, so that the foundation of further thermal stress analysis is laid.

Further, in one embodiment, the temperature distribution model at the substrate interface, including the temperature non-uniformity caused by particle modulation, can be established based on the following thermal equation:

T|_Z＝∞＝T|_{x，y＝±∞}＝T|t＝0＝0 (5)

wherein, c_sRepresents the specific heat capacity of the substrate material carrying the contaminant particles;

representing the density of the substrate material; k is a radical of_sRepresents the thermal conductivity of the base material; a. the₀Represents the heat absorption rate of the base material; α represents a heat absorption coefficient of the base material; t represents temperature, and gradT represents gradient thereof;

represents the reciprocal of time; i (x, y, t) represents a light field function under different variables, wherein x, y represent abscissa and ordinate, and t represents time; z represents a vertical coordinate in a rectangular coordinate system.

Then, the solution of the above linear thermal equation, i.e., the temperature distribution model, can be expressed as:

6)

wherein r represents the radial coordinate of the pollutant particles in a polar coordinate system, and does not need to be converted with the vertical coordinate z in a rectangular coordinate system;

indicating the thermal diffusivity of the substrate material；t₁Representing time units, F (z, t) represents a function as follows:

Further, in different cases of forward or backward cleaning, specific temperature distributions of contaminant particles of different particle sizes at the substrate interface can be obtained by finite element analysis according to the above model.

Further, according to the fact that the adsorption between the particles and the substrate is mainly caused by van der waals force due to the interaction between dipoles, and by Hamaker theory, the attraction force can be expressed by (8) not the right expression, the cleaning model under the contaminant particle removal condition is expressed by the formulas (8) to (11), wherein the left side of the formula (8) represents the cleaning force generated by the laser action, i.e., the thermal stress thereof:

wherein a represents a contact radius of the particle with the substrate; rho_pRepresenting the material density of the contaminant particles, f (t) representing the displacement function of the contaminant particles, wherein d represents the differential sign,

denotes the Lifshitz constant, h denotes the separation distance, h is

P represents the loading force, σ₁、σ₂Representing the Poisson's coefficients of the particles and the substrate material, respectively, E₁、E₂Respectively, the young's modulus of the particle and the substrate material, and R the radius of the contaminant particle.

Further, the specific thermal stress generated by pollutant particles with different particle sizes in different situations of forward or backward cleaning can be calculated according to the calculation formula of the thermal stress

And its combination with the temperature distribution model was obtained by finite element analysis.

Further, due to the uneven distribution of the optical field intensity, the present invention integrates Maxwell Stress Tensor (MST) on the closed surface when the object is irradiated by SC mode, and establishes the following optical force model, by which the force of the total optical force under time average due to the uneven distribution of the optical field intensity can be obtained:

〈F_total〉＝∫_S〈T〉·dS (12)

The specific magnitude of the optical force on the pollutant particles with different particle sizes can be obtained by a finite element analysis method according to the model.

S3 obtaining a specific cleaning scheme according to the model solving result

It may further comprise:

obtaining the field enhancement factor S by finite element analysis of the light field distribution model₀；

Based on the field enhancement factor S₀Carrying out finite element analysis on the temperature distribution model to obtain temperature distribution, and obtaining thermal stress distribution of pollutant particles through coupling analysis of the temperature distribution model and the thermal stress model on the basis of the temperature distribution;

the thermal stress at the time of contaminant particle removal and the thermal stress d obtained by the cleaning model shown in the formulas (8) to (11)²f (t)/dt²The particle acceleration generated by the expressed thermal stress and the magnitude of the light force acceleration under the same energy are obtained;

the obtained magnitude of thermal stress, the adhesion force acceleration and the light force acceleration are input into a comsol model to obtain a particle removal threshold value.

The particle removal threshold is obtained by the following calculation model:

forward cleaning threshold calculation model:

when F is present_{Heat generation}＝F_{Glue stick}+F_{Light (es)}Obtaining a contaminant particle removal threshold, wherein F_{Heat generation}Denotes the resulting thermal stress, F_{Glue stick}Represents van der Waals force, that is, the right-middle equation of the inequality (8), F_{Light (es)}Representing the light force obtained by the light force model;

back-to-back cleaning threshold calculation model:

when F is present_{Heat generation}＝F_{Glue stick}-F_{Light (es)}Then, a contaminant particle removal threshold is obtained.

According to the above embodiments, the present invention has performed the following simulation experiment:

and carrying out finite element analysis on the model through COMSOL, wherein the specific parameter setting comprises the following steps:

TABLE 1 parameter settings in COMSOL

In the finite element analysis, a gaussian function gp1 is set, position: 0, standard deviation: standard; analyzing the function: pg (x, y) ═ P gp1(x) × gp1(y), the upper and lower limits of x and y are R1 and-R1 respectively, wherein the unit of pg is W/m ^2

The piecewise function is set to distinguish dwell times of the laser beams as follows

The boundary conditions were set as shown in table 2 below:

TABLE 2 boundary conditions

Meanwhile, a grid division method is adopted in calculation, and when grid division is carried out, the grid precision of a laser action area is properly increased, and the grid precision of an unaffected material area is reduced. Therefore, the calculation progress is ensured, and the calculation amount is reduced.

Other parameter settings are shown in table 3 below:

TABLE 3 Main parameter settings

Under the above parameters, according to the light field distribution model, the light intensity distribution diagram of the alumina particles on the incident surface side of the quartz glass shown in FIG. 3, the light intensity distribution diagram of the alumina particles on the transmission surface side of the quartz glass shown in FIG. 4, and the field enhancement factor S shown in FIG. 5 can be obtained by simulation experiments₀Graph relating to the particle size of alumina particles, intensity distribution I/I at the bottom of the particles shown in FIG. 6₀Schematic diagram and axial electric vector distribution diagram shown in figure 7.

It can be seen that when the particles are located on the incident surface side of the transparent substrate as shown in fig. 3, the focused hot spot caused by the scattering effect of the particles is located on the interface of the particles and the substrate. For the electric field distribution of the backward laser cleaning, as shown in fig. 4, the focused hot spot is far away from the surface of the substrate, and the substrate is not easily damaged. The simulation results were consistent with the test experiment results described below.

As shown in fig. 5, since the field intensity under the optical resonance effect particle oscillates with the change of the particle size, the optical oscillation effect can be understood as that the incident light wave and the reflected light wave are overlapped in the bottom of the particle after the light wave undergoes reflection in the particle, and the formation of the traveling wave and the standing wave determines the existence of the oscillation. From the figure, S can be seen₀The ratio of the "hot spot" field strength to the incident field strength increased with increasing particle size, which was consistent with the results of the test experiments.

As shown in FIG. 6, the ordinate represents the field enhancement factor and the abscissa represents the radial coordinate for the temperature distribution at the bottom of the particles. The field enhancement is about 10 times at the bottommost part of the particle (r is 0), and as the radial coordinate r is increased, a region which cannot be irradiated by laser, namely a shadow region, exists, and as r is increased, the laser is uniformly irradiated. Fig. 7 shows the variation of the electric field intensity as the laser passes through the particle.

Under the above parameters, according to the forward temperature distribution model, a simulation experiment can obtain a profile of the model in forward cleaning and a temperature variation curve of the contact surface between the particles and the substrate as shown in fig. 8, wherein (a), (b), (c), (d), (e), and (f) are temperature distribution clouds when t is 1ns, 9ns, 12ns, 21ns, 30ns, and 40ns, respectively.

According to the back temperature distribution model, a cross-sectional view of the back cleaning model and a temperature variation curve of the contact surface between the particles and the substrate as shown in fig. 9 can be obtained, wherein (a), (b) and (c) are temperature distribution clouds with t ═ 1ns, 12ns and 40ns, respectively. (a-1), (b-1) and (c-1) are partial enlarged views of the contact surface of the particles (a), (b) and (c) with the substrate, respectively. As is evident from a comparison of fig. 8 and 9, the greatest difference between the application of laser light to the front surface and the application of laser light to the rear surface is that the "hot spot" is located differently. When the alumina particles are being cleaned in a forward direction, the "hot spot" is located at the interface of the contaminant particles and the substrate, whereas when cleaned in a reverse direction, the "hot spot" is located away from the interface of the substrate and the particles. This is extremely important for laser cleaning, as can be seen from the temperature profile of the interface of the contaminating particles with the substrate over time in fig. 9, the temperature continues to rise from 0 to 12ns, which can be attributed to the absorption of the laser by the alumina particles; the temperature at the position does not greatly increase and decrease in the period of 12-20ns and is basically kept constant, because the laser stops working at the time of 12ns, the particles do not absorb the laser, and in addition, the heat dissipated outwards at the position is equal to the heat transmitted from a particle bottom hot spot to the contact surface of the particles and the substrate; during the period of 20ns-40ns, the temperature continues to rise again because the "hot spot" at the bottom of the particle spreads more heat to the particle-substrate interface than it dissipates.

Under the above parameters, according to the forward cleaning thermal stress distribution model, a simulation experiment can obtain a forward cleaning stress distribution diagram as shown in fig. 10, where (a), (b), (c), (d), (e), and (f) are stress distribution clouds when t is 1ns, 9ns, 12ns, 21ns, 30ns, and 40ns, respectively, and it can be seen that: FIG. (a): after the impurities absorb the laser energy, the temperature of the impurities rises, and when the laser energy is not enough to cause the impurities to generate phase change or ionization, the impurities can generate thermal expansion, and downward acting force can be applied to the substrate, and relatively speaking, the substrate can also apply upward reaction force to the particles, so that the particles are accelerated upwards. For the substrate, the distribution and magnitude of the stress experienced by the substrate is not shown in fig. 10, since the stress experienced by the substrate is orders of magnitude different from the stress experienced by the particles. FIG. (b): at this point the particle thermal stress reaches a maximum, the upward velocity becomes zero and the upward displacement of the particles due to thermal expansion is greatest. FIG. (c): due to the sudden decrease in temperature, the amount of thermal expansion will decrease and the particles will have a downward displacement. Graph (d) shows a slower temperature drop and a slower thermal stress drop than graph (c), so there is a smaller downward velocity relative to graph (c). FIGS. (e) and (f): since the thermal stress variation is small, the downward displacement of the particles tends to be gentle.

According to the back-cleaning thermal stress distribution model, a back-cleaning stress distribution diagram as shown in fig. 11 can be obtained through simulation experiments, and it can be seen that the back-cleaning stress is larger than that of the forward cleaning, and like the forward cleaning, the back-cleaning stress continuously rises due to the action of the laser pulse in the period of 0-9ns, and the thermal stress gradually decreases along with the conduction of the temperature to the low-temperature position in the period of 9-40 ns. There is a downward (particle removal direction) displacement of the particles due to thermal expansion of the particles. Specifically, during the time period from 0ns to 9ns in the figure, a "hot spot" is formed at the intersection point of the particle far from the particle and the substrate, and the formation of the hot spot causes the top of the particle to expand, and the particle is caused to move far from the substrate due to the property of the material which prevents the expansion of the particle, but the expansion also generates a slight pressure on the substrate, and when t is 9ns, fig. 11(b), the thermal stress reaches a maximum value. In contrast to forward cleaning, forward laser cleaning, on the one hand, relies on the reaction of the substrate to the particles to remove, while backward laser cleaning relies on the material itself to resist particle expansion.

Through comparative analysis of the two, the stress concentration point can be formed at the bottom of the particle by forward laser cleaning, and the stress concentration point of the backward laser cleaning is positioned at the top of the particle; the removal force of forward laser cleaning is due to the reaction of the substrate to the particles caused by particle expansion, and the removal path is opposite to the laser cleaning direction. The backward laser cleaning is from the thermal expansion of the material, and the removal path is the same as the propagation direction of the light beam; the maximum thermal stress of the model when the forward laser cleaning and the backward laser cleaning reach the cleaning threshold is 3.41MPa and 731MPa respectively. It is evident that the front surface cleaning threshold is lower than the back surface cleaning threshold in this case.

Under the above parameters, according to the electromagnetic force distribution model, the variation curve of the forward cleaning light force with the particle radius as shown in fig. 12 and the variation curve of the backward cleaning light force with the particle radius as shown in fig. 13 can be obtained. It can be seen from the figure that the overall variation curve of the optical power gradually increases with increasing particle size. Meanwhile, due to the influence of the small sphere Mie scattering, the light force curve of the object can fluctuate. Meanwhile, comparing the two figures can see that the back laser cleaning has stronger light value. This is because the field enhancement effect of the self-reverse laser cleaning is stronger than that of the forward laser cleaning, i.e. the reverse S₀Greater than positive S₀When the model is simplified into two dimensions, the value (ordinate) is F_{Light (es)}and/2R, multiplying the value by the particle diameter to obtain the required light force. This force corresponds to F in FIG. 2₂The direction points in the direction of the enhancement of the light field. At the same time, the optical force has a corresponding effect on the cleaning threshold of the laser, and the direction of the optical force is the same as the direction of van der Waals force for front surface laser cleaning, so the discussion as resistance force, namely F_{Heat generation}＝F_{Model (A) of}+F_{Light (es)}For back surface laser cleaning, the direction of the optical force is opposite to the van der Waals force, as a dynamic discussion, i.e., F_{Heat generation}+F_{Light (es)}＝F_{Model (A) of}。

Under the above parameters, the simulation experiment simultaneously obtains the relationship between the acceleration required for cleaning and the particle size as shown in fig. 14, the curve of the change of each stress with the energy density of the front surface laser cleaning as shown in fig. 15, wherein (a) the particle radius is 0.2um, (b) the particle radius is 2um, the curve of the change of each stress with the energy density of the back laser cleaning as shown in fig. 16, wherein (a) the particle radius is 0.2um, (b) the particle radius is 2um, and the graph of the relationship between the control range of the optimal removal energy density and the particle size as shown in fig. 17. As can be seen from fig. 14, the acceleration required gradually decreases with increasing particle size, i.e., the larger the particles, the easier it is to clean.

As can be seen from the graph of fig. 15 showing the graphs of the adhesive force, thermal stress, optical force and resultant force (thermal stress dimming force) corresponding to the laser cleaning of the front surface at different energy densities, and the threshold energy density at which the intersection point of the two line segments of the resultant force and the adhesive force corresponds to the particle removal, the optical force in the graph (a) is stronger than the cleaning force, so that the particles cannot be effectively removed within the particle size. And the influence of the light force on the cleaning threshold value in the step (b) is very little, and the cleaning threshold value is 0.2085mJ/cm ^2, which is consistent with the phenomenon of the test experiment described later. Comparing the graphs (a) and (b), it can be seen that the optical force effect at a radius of 200nm is more significant because of the higher laser energy required to remove the particles.

As can be seen from FIG. 16, which is a plot of adhesion (Van der Waals forces) and cleaning power (optical forces + thermal stresses) for laser cleaning of the back surface at different energy densities, and the abscissa of the intersection of the two line segments is the threshold energy density for removal of the particle, it can be seen that (a) the particle size is 0.2 μm and the cleaning threshold is 0.820J/cm 2, (b) the particle size is 2 μm and the cleaning threshold is 17.373mJ/cm 2. Comparing FIGS (a) and (b), it can be seen that the influence of the light intensity on the particle size of 0.2 μm is larger. Comparing fig. 15 and 16, it can be seen that the backward laser cleaning threshold is lower than the forward laser cleaning threshold at a smaller particle size, and the forward laser cleaning threshold is lower than the backward laser cleaning threshold at a larger particle size. In addition, the optical power is stronger as the energy density increases.

The curve of the change of the front and rear surface particle removal threshold values along with the particle size is shown in FIG. 17, and it can be observed from the graph that the front surface removal threshold value is lower due to the aggregation action of the particles when the particle size is larger than or equal to 225 nm; particle size < 225nm, the back-cut threshold is low due to the elimination of "hot spots" and the particles act as barriers to the incident laser light. In addition, the particle cleaning threshold of the front surface increases sharply in this particle size range, and the optical power increases accordingly. The front surface in this size range does not provide effective removal of particles. Also, front and back surface damage threshold curves are plotted, and it can be seen that the back surface damage threshold is higher than the front surface damage threshold because the modulation of the laser by the particles causes the stress of the front surface substrate to be stronger than the stress of the back surface substrate at the same laser energy. In the figure, a shaded portion (1) is a control range of energy density for removing particles by back laser cleaning, and a shaded area 2 is a control range of energy density for removing particles by forward cleaning. Within this range, the cleaning effect is excellent and no damage is caused.

The invention further provides a cleaning effect test experiment carried out by the test device shown in the attached figure 1, which comprises the following components:

1: a laser transmitter; 2: a beam splitter; 3: a focusing lens; 4: a three-dimensional mobile platform; 5: a glass slide; 6: alumina particles; 7: a detector; 8: an energy meter; 9: and (4) a computer. Wherein the ratio of transmitted light to reflected light energy after beam splitting of the beam splitter is 8:2, the reflected laser is connected with an energy meter for energy detection, the transmitted laser is focused on the Al coating through a lens with the focal length of 200mm₂o₃The rear side of the silica glass sample of the particles, in which the off-focus amount was 30mm, was held by a three-dimensional moving platform so that the laser irradiated the sample point by point. The figure shows back irradiation, where the particles are on the transmission surface of laser irradiation and the incident surface of converting the particles into laser is forward irradiation.

The laser was generated by a solid-state pulse laser (model SGR-10) manufactured by radium-Bao corporation, the pulse width was 12ns, the laser wavelength was 1064nm, and the mode of the laser was TEM₀₀The pulse frequency was 1 Hz.

Al with different grain sizes₂O₃A sample obtained by loading particles as contaminants on a glass slide, and performing a cleaning effect simulation, wherein the sample is performed by the following processes:

the slide glass is cut into a rectangle with the size of 25mm multiplied by 50mm from the position of 75mm multiplied by 25mm multiplied by 1 mm;

placing the cut glass slide in deionized water for ultrasonic cleaning for 30 min;

0.037g of Al₂O₃Placing the particles in a beaker containing 100ml of ethanol, placing the beaker in an ultrasonic cleaning machine, ultrasonically oscillating for 30min so as to disperse the particles, and then stirring the solution for 3h at a stirring speed of 350r/min by using a magnetic stirrer;

and (3) dripping the stirred solution on a cleaned glass slide by using a dropper, and then placing the glass slide in a dry and well-ventilated environment until the ethanol solution is completely volatilized to obtain the sample.

The prepared sample was subjected to a single point scanning ablation experiment, with one pulse applied at each point and 10 points applied at each energy. Wherein the moving speed of the three-dimensional displacement platform is 5mm/s, and the laser energy is gradually increased until the particles are removed. Firstly, determining a basic energy range for removing particles, secondly, gradually increasing the laser energy according to the increment of 0.1mJ, and finally, comparing an experimental picture to obtain a cleaning threshold value.

The experimental results are shown in FIGS. 18 to 24, in which FIG. 18 is a graph comparing the effect of contaminants having a particle radius of 2um in forward cleaning, and the laser energy densities thereof from left to right are respectively 0.1 to 0.25mJ/cm²、 0.26mJ/cm²And 0.3mJ/cm²(ii) a FIG. 19 is a graph comparing the effect of contaminants with particle radius of 2um in back-to-back cleaning with laser energy densities of 1-17.4mJ/cm from left to right²、17.5mJ/cm²And 19mJ/cm²(ii) a FIG. 20 is a graph showing the comparison of the effect of contaminants having a particle radius of 200nm in forward cleaning with laser energy densities of 1-1.1mJ/cm from left to right²、1.13mJ/cm²And 1.5mJ/cm²(ii) a FIG. 21 is a graph comparing the effect of contaminants with particle radius of 2um in back-to-back cleaning with laser energy densities of 0.1-0.82mJ/cm from left to right²、0.83mJ/cm²And 0.9mJ/cm²(ii) a FIG. 22 is a SEM contrast view of cleaned 200nm particles, wherein (a) the forward cleaning topography (b) is a partial magnified view of (c) the reverse cleaning topography (d) is a partial magnified view; FIG. 23 is an SEM comparison of washed 2 μm particles from a test experiment in which (a) a forward wash topography (b) is magnified in section (c) a reverse wash topography (d) is magnified in section.

Wherein, FIG. 18-21 shows the cleaning threshold values of the front and rear surfaces at two particle sizes (2um, 0.2um), wherein the cleaning effect is not achieved at the energy density of the leftmost image, the removal effect is achieved at the energy density of the middle image, the energy density is the cleaning threshold value, and the laser energy density of the rightmost image is greater than the cleaning threshold value, which is better than that of the middle image. In addition, it can be obtained that for particle contamination with a radius of 2um, the front surface cleaning threshold is much lower than the back surface laser cleaning threshold, and since particle removal depends on the temperature of the particle-substrate interface, the front surface has a higher temperature at the particle contamination-substrate interface at the same energy density, which is due to Al₂O₃The modulation of the laser by the particles. The cleaning threshold for the front surface is greater than the cleaning threshold for the back surface for contaminants with a radius of 200nm, suggesting that back laser cleaning results in a higher temperature rise at the particle-substrate interface at the same energy, consistent with modeling calculations.

In a further enlargement of the experimental picture by SEM, fig. 22(a), (c) are respectively the global topography of the forward and reverse cleaning up to the cleaning threshold and (b), (d) are respectively the partial enlargement (SEM) of (a), (c), the particle size is 200nm, it can be seen that the reverse laser cleaning is more efficient for the small particles (200nm), from (b) it can be seen that the particles melt due to the action of the laser and form a contamination layer on the substrate. Although the particles are partially removed, secondary pollution is caused to the substrate, the particle removing effect is very obvious from the step (d), and although the melting phenomenon of individual particles exists, a large-area pollution layer is not formed, thereby conforming to the model result.

Fig. 23(a) and (c) are the overall topography of the forward and reverse cleaning reaching the cleaning threshold, respectively, (b) and (d) are the partial enlarged views (SEM) of (a) and (c), respectively, and the particle size is 2 μm, so that the cleaning effect of both can be observed to be very effective, which is in line with the model result.

Meanwhile, in the test experiment, it is observed that for large particles (2um), pit formation is accompanied by backward laser cleaning, and local overheating occurs to particles in both cleaning modes, in the front surface laser cleaning effect under the action of laser energy density of 1mJ/cm ^2 as shown in FIG. 24, the graphs (a) and (b) are the appearance graphs of the substrate and particles cleaned forward by laser, and (c) and (d) are the appearance graphs of the substrate and particles cleaned backward by laser, from (a), pits appear on the substrate after forward laser cleaning can be seen, and from (b), the interface (bottom) between the particles and the substrate in forward laser cleaning is firstly melted, which can further illustrate that the modulation of the particles leads to the highest temperature of the bottom of the particles, and no pits appear in graph (c) and some traces of particle removal are left, the melting region is clearly seen in the graph (d) away from the particle-substrate interface (top), indicating that the alumina particles modulate the laser such that the temperature at the bottom of the particle is higher than the temperature at the top of the particle, which has an effect on the laser cleaning threshold.

Based on the above test experiment results, it can be found that the forward laser cleaning has a lower threshold than the backward laser cleaning for alumina particles with larger particle size because of its modulating effect on the laser. In the range of small particle size, the back laser cleaning threshold is lower because the particles can block the incident laser. In addition, on the premise of reaching the cleaning threshold, the back laser cleaning effect is more remarkable, and the forward laser cleaning can cause the existence of a pollution layer. Consistent with the modeling results of the present invention.

The above examples are merely preferred embodiments of the present invention, and the scope of the present invention is not limited to the above examples. All technical schemes belonging to the idea of the invention belong to the protection scope of the invention. It should be noted that modifications and embellishments within the scope of the invention may be made by those skilled in the art without departing from the principle of the invention, and such modifications and embellishments should also be considered as within the scope of the invention.

Claims

1. A method for cleaning micro-nano particle pollutants by laser is characterized by comprising the following steps: it includes: obtaining a cleaning scheme through a laser cleaning model, and cleaning according to the cleaning scheme, wherein the cleaning model is constructed by the following models: the optical field intensity distribution model at the substrate interface including the modulation effect of the pollutant particles, the temperature distribution model at the substrate interface including the temperature distribution nonuniformity phenomenon caused by the modulation effect of the pollutant particles, the thermal stress distribution model at the substrate interface including the thermal stress variation effect caused by the temperature variation, and the optical force distribution model at the substrate interface.

2. The method of claim 1, wherein: the light field intensity distribution model is as follows:

wherein, I₀(t) represents a laser smoothing pulse,. phi.represents an incident laser flux,. t represents time,. t represents_l＝0.409t_FWHMRepresenting the pulse time calculation coefficient, where t_FWHMDenotes the duration of the pulse, r₀Representing the radial length of an area of increased field intensity of the optical field caused by the modulation action of pollutant particles under a polar coordinate system; r represents the radial coordinate of the incident laser pulse point; r is_shRepresents the radial length of the region of reduced optical field strength resulting from modulation of the contaminant particles; r represents the radius of the pollutant particles; s₀Representing field enhancement factors which are the optical field intensity I and the incident light field intensity I at the near field focus position generated by incident laser at the bottom of the pollutant particle caused by the modulation effect of the pollutant particle₀Ratio of (i.e. S)₀＝I/I₀，S₁Represents the area of the region of reduced optical field strength caused by modulation of contaminant particles, and

3. the laser cleaning method according to claim 2, characterized in that: the temperature distribution model is as follows:

represents the thermal diffusivity of the substrate material; t is t₁Representing time units, F (z, t) represents a function as follows:

4. The laser cleaning method according to claim 1, characterized in that: the thermal stress distribution model is obtained by jointly solving the temperature distribution model and the thermal stress model.

5. The laser cleaning method according to claim 4, characterized in that: the thermal stress model is set as follows:

6. The laser cleaning method according to claim 1, characterized in that: the light force distribution model is as follows:

<F_total>＝∫_S<T>·dS (12)

representing the unit tensor of the light field, D^*Representing the conjugate of the electric flux density.

7. The laser cleaning method according to claim 1, characterized in that: the laser cleaning model further comprises the following cleaning condition models:

denotes a Lifshitz constant, h denotes a separation distance, and

8. The laser cleaning method according to claim 1, characterized in that: and simulating the laser cleaning model by a finite element analysis method to obtain a numerical solution of the laser cleaning model.

9. The laser cleaning method according to claim 1, characterized in that: based on the laser cleaning model, obtaining a cleaning threshold value through the following threshold value calculation model:

in the forward purge:

when F is present_{Heat generation}＝F_{Glue stick}+F_{Light (es)}Then, a cleaning threshold is obtained, wherein F_{Heat generation}Denotes the resulting thermal stress, F_{Glue stick}Denotes Van der Waals force, i.e., the right-middle equation of the inequality (8), F_{Light (es)}Representing the light force obtained by the light force model;

in back-washing:

10. Use of the laser cleaning method according to claims 1-9 for cleaning optical components.