RU2263567C1 - Method for wet cleaning of rigid surfaces with use of laser - Google Patents

Abstract

FIELD: systems for cleaning rigid surfaces.

SUBSTANCE: in order to improve quality of cleaning, surface is irradiated by means of space-modulated beam of laser irradiation.

EFFECT: improved quality of cleaning surfaces.

1 cl, 2 dwg

Description

The invention relates to the field of laser processing of materials and can be used in instrumentation, cleaning of optical and electrical elements.

A known method of ultrasonic cleaning of hard surfaces (IPMaragopoulos, CJMartin and JSM Hutchison. Measurement of field distribution in ultrasonic cleaming baths: implications for cleaming efficiency. 15 August 1995, p. 1897-19080), which consists in removing particles from a solid surface under the action ultrasound. However, the lower size limit of the removed particles is about 0.5 μm. There is no protection against additional contamination and damage to the work surface.

Known methods for laser cleaning of hard surfaces, for example, a method of dry laser cleaning (M.N. Hong, T. C. Chong, Laser removal of particles from solid surfaces. Riken Review, January 2001, p. 64). It is associated with pulsed irradiation of the treated surface with laser radiation, this entails the rapid thermal expansion of the substrate and / or polluting particles, which leads to the removal of particles from the solid surface. However, with dry laser cleaning on a cleaned surface in places where particles were removed, surface damage is observed: the formation of depressions, which is explained as a result of local ablation of the substrate.

A known method of wet laser cleaning of hard surfaces (M. Mosbacher, V. Dobler, P. Leiderer. Universal threhold for the steam laser cleaning of submicron spherical particles from silicon. Appl. Phys., A 70, 2000, p.669-672) , including applying a layer of liquid to the surface and irradiating it with pulsed laser radiation, causing the liquid to boil and remove it from the surface along with polluting particles.

This method for the combination of features is the closest to the proposed invention and is selected by the authors for the prototype.

The mentioned prototype method does not allow to increase the surface area to be cleaned.

The problem to which the invention is directed, is to create a method that allows to increase the surface area to be cleaned while reducing the power density of the laser radiation.

This problem is solved in the implementation of the invention by achieving the boiling point of the liquid only at the maxima of the distribution of the intensity of the laser radiation.

где τ - длительность импульса излучения, а - наибольшее значение из двух величин: температуропроводности облучаемого материала и температуропроводности жидкости.The specified technical result in the implementation of the invention is achieved by the fact that in the method of wet laser cleaning of the surface, including applying a layer of liquid to the surface and irradiating it with pulsed laser radiation, causing the liquid to boil and remove it from the surface together with contaminating particles, the surface is irradiated with a spatially modulated beam laser radiation. In this case, the distance between the maxima and minima of the intensity of the spatially modulated beam should be

where τ is the duration of the radiation pulse, and is the largest of the two values: thermal diffusivity of the irradiated material and thermal diffusivity of the liquid.

При этом условии распределение температуры в слое жидкости на поверхности материала не сглаживается вследствие теплопроводности, а соответствует распределению плотности мощности излучения. Энергия лазерного излучения поглощается в поверхностном слое твердого тела, и слой жидкости нагревается за счет теплопроводности, на поверхности подложки образуются и растут паровые пузырьки. Удаление частиц происходит под действием акустической волны, возникающей при расширении и последующем сжатии и исчезновении пузырьков. При облучении поверхности с нанесенным слоем жидкости пространственно-модулированным пучком излучения температура жидкости повышается не во всей области облучения, а только вблизи максимумов интенсивности излучения. Вблизи минимумов плотность мощности излучения близка к нулю, и значительного повышения температуры жидкости не происходит. Поскольку расстояние между максимумами и минимумами распределения интенсивности излучения превышает величину

то температурное распределение в слое жидкости не сглаживается за счет теплопроводности. Для инициирования пузырькового кипения, вызывающего очистку поверхности, достаточно достижения температуры, соответствующей кипению, только в максимумах распределения интенсивности. Поэтому размер зоны обработки при использовании предлагаемого способа и, соответственно, производительность очистки увеличивается по сравнению с использованием способа-прототипа.The above set of essential features of the invention allows to achieve the desired technical result and solve the problem. When using the proposed method, the liquid is applied immediately before irradiation. The surface is irradiated with a spatially modulated radiation beam. In addition, the distance between the maxima and minima of the intensity of the spatially modulated beam should be

Under this condition, the temperature distribution in the liquid layer on the surface of the material is not smoothed due to thermal conductivity, but corresponds to the distribution of the radiation power density. The energy of laser radiation is absorbed in the surface layer of a solid, and the liquid layer is heated due to thermal conductivity, vapor bubbles form and grow on the surface of the substrate. Particles are removed under the influence of an acoustic wave arising from the expansion and subsequent compression and disappearance of bubbles. When a surface with a liquid layer deposited by a spatially modulated radiation beam is irradiated, the temperature of the liquid does not increase in the entire irradiation region, but only near the radiation intensity maxima. Near the minima, the radiation power density is close to zero, and a significant increase in the temperature of the liquid does not occur. Since the distance between the maxima and minima of the radiation intensity distribution exceeds the value

then the temperature distribution in the liquid layer is not smoothed due to thermal conductivity. To initiate bubble boiling, which causes surface cleaning, it is sufficient to achieve a temperature corresponding to boiling, only at the maxima of the intensity distribution. Therefore, the size of the treatment zone when using the proposed method and, accordingly, the cleaning performance is increased compared with the use of the prototype method.

The analysis of the prior art, including a search by patents and scientific and technical sources of information, allowed us to establish that the authors did not find a method characterized by features identical to all the essential features of the claimed invention, and with respect to the technical result, a combination of essential distinguishing features was revealed. Therefore, the claimed method meets the requirement of novelty.

To verify the conformity of the proposed method to the requirements of the inventive step, an additional search was carried out for known solutions to identify features that match the distinctive features of the prototype of the claimed invention.

The results of this search show that the inventive method does not follow explicitly from the prior art for a specialist. The invention is illustrated in figure 1, which shows an example of a design of a device that allows you to implement the proposed method.

The scheme of the device that allows the proposed method to be implemented includes YAG: Nd - laser 1, mirror 2, deflecting the laser beam, telescopic system 3, increasing the beam diameter in accordance with the required size, functional unit 4, spatially modulating the laser beam on the surface of the part 5 located on the coordinate table 6 with a stepper motor (figure 1).

The operation of the device is as follows. Before irradiation, a layer of liquid is sprayed onto the surface to be cleaned. The beam of laser radiation 1 hits the mirror 2. Reflecting it, it hits the telescopic system 3. The radiation transmitted through the telescopic system 3 and the functional unit 4, which forms the spatially modulated radiation, falls on the part 5 with a layer of liquid deposited on it. When radiation is absorbed, a rapid increase in the temperature of the surface layers of the part occurs. If the intensity of laser radiation is such that a liquid is heated at the interface with the substrate, which is sufficient for it to boil at normal pressure, then vapor bubbles form and grow on the surface of the part. Particles are removed under the influence of an acoustic wave arising from the expansion and subsequent compression and disappearance of bubbles.

As an example, consider the removal of contaminants from the surface of a silicon wafer. Immediately before the radiation, we apply a layer of liquid to the plate. In our example, this is water. A YAG was chosen as the emitter: an Nd laser with a wavelength of λ = 1.06 μm, a pulse duration of τ = 15 ns, a pulse repetition rate of f = 2 Hz and a power of P _imp = 1.3 · 10 ⁶ W, the diameter of the radiation beam at the laser output is d ₀ = 1.5 mm.

The radiation power density q necessary for heating the liquid at the interface with the substrate by ΔТ = Т _m -Т _n :

A_S - поглощательная способность кремния, а - температуропроводность

T_нач - начальная температура [°С], q - плотность мощности лазерного излучения, индекс "l" - относится к жидкости, "s" - к кремниевой пластине.where T _m is the temperature to which the water layer must be heated to initiate bubble boiling under the influence of a radiation pulse of τ = 15 ns, is 153 ° C [Yavas O., Schilling A., Bischof J. et al. Study of nucleation processes during laser cleaning of surfaces. Laser Physics, v. 7. No 2, 1997, p.343-348], K - thermal conductivity

A _S - absorption capacity of silicon, and - thermal diffusivity

T _beg is the initial temperature [° C], q is the power density of the laser radiation, the index “l” refers to a liquid, and “s” refers to a silicon wafer.

As a result, the power density required for wet laser cleaning is:

Those. when using a uniform distribution of intensity, the size of the simultaneously processed area (prototype method) is:

where P _imp is the pulsed power of the laser radiation.

Cleaning performance in this case:

where f is the pulse repetition rate.

When using a spatially modulated radiation beam, the initiation of bubble boiling occurs when the temperature T _m is reached only at the maxima of the radiation intensity distribution, i.e.

Under conditions of coherent radiation, the average value of the radiation power density over the irradiated region:

As a result, when using a spatially modulated radiation beam (the proposed method), the size of the simultaneously processed area is:

The increase in the telescopic system, providing such a magnitude of the irradiated area on the surface of the part:

Cleaning Performance:

В рассматриваемом случае наибольшее значение

The distance between the maxima and minima of the intensity of the spatially modulated beam should be:

In this case, the highest value

Consider the example as a functional node 4, which will ensure the fulfillment of the set conditions, a beam splitting cube 7 with a mirror 8, the rotation angle of which can be changed (figure 2). The distance l between the maximum and minimum intensity of the spatially modulated beam is determined as follows:

число максимумов интенсивности на диаметр пятна.Where

the number of intensity maxima per spot diameter.

The surface to be machined is positioned so that rays II and III fall on it at equal angles β. Then:

x ₁ = AD = AB · sinβ; x ₂ = CB = AB

here AB is the linear dimension characterizing the interaction region of the interfering beams II and III, AD is the path difference of the extreme rays a and b of beam III, BC is the path difference of the extreme rays a 'and b' of beam II, λ is the laser radiation wavelength, α - angle of incidence of ray II on the surface of the reflecting mirror 8, β is the angle of incidence of rays III and II on surface 5, β = π / 4-α.

At α = 15 °, the value of l in our example will be l = 1.06 · 10 ^-6 (m).

Based on the foregoing, the claimed invention will increase the area of the simultaneously processed surface.

Claims (1)

A method of wet laser cleaning of solid surfaces, including applying a liquid layer to the surface, irradiating the surface with pulsed laser radiation, characterized in that the surface is irradiated with a spatially modulated laser beam, the distance between the intensity maxima of the spatially modulated beam being

where τ is the duration by the minima of laser radiation, α is the largest value of thermal diffusivity of two quantities: thermal diffusivity of the irradiated material and thermal diffusivity of the liquid.

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