WO2015069143A9

WO2015069143A9 - Method and device for forming precision holes in optically transparent film using an ultrashort pulse of laser radiation

Info

Publication number: WO2015069143A9
Application number: PCT/RU2014/000747
Authority: WO
Inventors: Сергей Каренович ВАРТАПЕТОВ; Алексей Захарович ОБИДИН; Даниил Валентинович ГАНИН
Original assignee: ОБЩЕСТВО С ОГРАНИЧЕННОЙ ОТВЕТСТВЕННОСТЬЮ "ОПТОСИСТЕМЫ" (ООО "Оптосистемы")
Priority date: 2013-11-07
Filing date: 2014-10-07
Publication date: 2015-09-11
Also published as: WO2015069143A1; CN205927544U; RU2013149554A; RU2551043C1

Abstract

Proposed is a device for forming precision holes in optically transparent film using an ultrashort pulse of laser radiation, in which the laser radiation is focused by a lens with a high numerical aperture through a volume of a medium with a refractive index that is greater than 1.5. The device makes it possible to form a hole with a diameter of up to 5 μm in a film having a thickness of up to 100 μm using a single ultrashort pulse of laser radiation.

Description

Method and device for forming precision holes in an optically transparent film with an ultrashort laser pulse.

Description of the invention.

Technical field

The invention relates to the field of quantum electronics, to laser processing of materials, namely, to the formation of precision holes in an optically transparent film by an ultrashort laser pulse.

State of the art

In recent years, the number of publications on the use of ultrashort pulse lasers for processing materials has increased significantly. The use of lasers with a pulse duration of up to 10 ^" s has an advantage, since an ultrashort laser pulse has a point effect.

Known US patent N ° 7807942, in which a special lens is used to form a cylindrical hole with an ultrashort laser pulse, which increases the focus depth of the system for forming cylindrical holes (cavities) in a transparent material. However, such a lens is an expensive, difficult to align and sensitive optical element.

Known application WO2013138802, which describes a method for processing a transparent material with a laser beam. In this method, the beam is focused into the bulk of the sample at a distance from the surface and is destroyed in the sample due to the self-focusing effect. However, with self-focusing or self-channeling, only about 15% of the beam energy is focused into the channel, which forms fractures in the material. The channel itself is an unstable formation due to the nonlinearity of the self-focusing effect. This method allows you to destroy or weaken the bulk material, but is unsuitable for the formation of precision holes in the film.

The objective of the invention is to develop an installation and method for forming cylindrical holes with a diameter of not more than 5 microns in a film up to 100 microns thick. Disclosure of invention

A method of forming precision holes in an optically transparent film with an ultrashort laser pulse, a film thickness of up to 100 μm, in which an ultrashort laser pulse is focused using a lens with a numerical aperture of at least 0.33, an ultrashort laser pulse is focused through a medium transparent to laser radiation with a refractive index of at least 1.5, into a focal constriction, the length of which is proportional to the thickness of the medium layer to the focus at paraxial approximation, for the focal waist is larger than the film thickness, the optically transparent film is placed in the focal waist so that the focal waist overlaps the part of the film in which the hole is formed, the energy of the ultrashort laser pulse is set such that the laser radiation energy density in the focal waist region exceeds the destruction threshold of the film material . When focusing a laser pulse with a lens in the depth of a material transparent to laser radiation with a refractive index greater than 1, the peripheral rays are focused not in a sphere with a small diameter, but in a cylindrical volume or focal constriction arising due to the refraction of the rays at the air - material interface. The length of the focal constriction, in which conditions for the destruction of the material arise, Δ is determined by the formula

where n is the refractive index of the material;

NA - aperture of the lens;

fd is the depth of focus in the material during paraxial approximation. The diameter of the focal waist is proportional to the wavelength. When the energy density in the focal constriction is greater than the binding energy density of the molecules of the material, conditions arise for the destruction of the material in the entire cylindrical volume. The length of the focal waist is proportional to the thickness of the medium layer to the focus during paraxial approximation. With a lens aperture of at least 0.33 and a refractive index of at least 1.5, the length of the focal waist can be made larger than the thickness of the optically transparent film. When placing an optically transparent film in the area of the focal waist so that the focal the waist overlapped the part of the film in which the hole is formed, and the pulse energy was supplied such that the energy density of the laser radiation in the focal waist region exceeded the destruction threshold of the film material, the hole in the film can be obtained with a single laser pulse. It should be noted that up to 40% of the pulse energy can be focused into the focal constriction, depending on the values of the aperture, refractive index, and focusing depth. The proposed method allows predictably, with efficient use of laser pulse energy, to form precision holes in an optically transparent film with a thickness of up to 100 μm, and each hole can be formed by a single pulse.

A device for forming precision holes in an optically transparent film with an ultrashort laser pulse, comprising a source of ultrashort laser pulses, an optical beam forming and guiding system, a lens with a numerical aperture of at least 0.33 installed in air, a film thickness of up to 100 microns, the device contains a transparent for laser radiation, a medium with a refractive index of at least 1.5, the film is mounted behind the layer of the medium so that the focus of the lens during paraxial approximation is located film focus geometric boundary lens peripheral rays is a film. The film and the material of the medium are transparent to laser radiation. When a laser pulse is focused by a lens through a medium with a refractive index of at least 1.5, the peripheral rays are focused into a cylindrical volume or focal constriction arising due to the refraction of the rays at the air - material interface. The length of the focal constriction, in which conditions of material destruction occur, is proportional to the thickness of the medium layer. The diameter of the focal waist is proportional to the radiation wavelength. With a lens aperture of at least 0.33 and a refractive index of at least 1.5, the length of the focal waist can be made larger than the film thickness so that the focus during the paraxial approximation is located in front of the film and the geometric focus of the boundary rays of the peripheral rays is located behind the film. In this case, the focal constriction will block the entire film. When the energy density in the focal waist is higher than the binding energy density of the molecules of the material, the entire focal waist, which is a cylindrical volume with a high aspect ratio, creates conditions for the destruction of the material, and a precision hole in the film can be obtained by one laser pulse. Approximately 40% of the beam energy can be concentrated in the focal constriction with a medium refractive index of 1, 5 and a numerical aperture of 0.33, depending on the thickness of the medium layer. Thus, the proposed device is an effective tool for forming precision holes in optically transparent films.

The medium is a liquid closed from the lens by a transparent material with a refractive index equal to the refractive index of the liquid, the film is located in the liquid. Waves, splashes may appear on the surface of the liquid. To protect the lens and to eliminate interference caused by surface waves, a transparent material with a refractive index equal to the refractive index of the liquid is installed on the surface of the liquid. When focusing an ultrashort high-energy pulse in a medium, conditions may arise for self-focusing, in addition, the length of the focal waist is determined by formula (1) and is greater than the film thickness. The rarefaction regions arising in the liquid due to the self-focusing effect under the influence of an ultrashort laser pulse disappear, the required hole is obtained only in the film. Liquid can be used for a long time to form many precision holes in the film. Such a device is reliable and can work for a long time without changing the environment.

The device comprises a film moving system in at least one coordinate. For most technological tasks, it is necessary that many holes are made in the film. This requires relative movement of the film and the position of the focal constriction. This can be done by moving the film along at least one coordinate.

The optical system for beam formation and guidance contains a scanning system for at least one coordinate. To form multiple holes in the film, you can use the beam scanning system at least in one coordinate. Moving the film along one coordinate and scanning with a beam is the optimal solution for forming multiple holes in the film.

A liquid is a solution of NaCl in water. One embodiment of the device is the use of a solution of NaCl in water as a liquid. By changing the concentration of the solution, it is possible to choose the refractive index of the solution equal to the refractive index of the plate mounted on the surface of the liquid between the liquid and the lens.

The medium is a plane-parallel plate of material, the focus of the lens during paraxial approximation is located behind the plate, the numerical aperture of the lens is at least 0.5. Another embodiment of the device is the location of the film behind the plate and the focusing of radiation by a lens with a numerical aperture of at least 0.5. The lens focus during paraxial approximation is located behind the plate, therefore, when focusing a laser pulse, the focal constriction arising due to radiation refraction at the material boundary with a refractive index of at least 1.5 is located outside the plate. Therefore, the plate is not damaged by the action of an ultrashort pulse and can be used for a long time. The numerical aperture of the lens is at least 0.5, since with a smaller aperture the length of the focal waist will be insufficient for the formation of a cylindrical cavity in the film with one pulse.

The technical result of the proposed technical solution is to create a method and device that allows one ultra-short pulse of radiation to form a precision cylindrical hole in an optically transparent film with a thickness of up to 100 microns.

Brief Description of the Drawings

In figure 1. The scheme of occurrence of focal constriction is presented.

Figure 2. presents a diagram of a device for forming holes in which the film is located in a liquid. In Fig.Z. a diagram of a hole forming apparatus is shown in which the film is located behind a transparent plane-parallel plate.

In FIG. 4 shows micrographs of a matrix of cylindrical cavities, each of which is formed as a result of a single pulse in the bulk of the material.

5. photographs of holes in an optically transparent film 50 μm thick are presented, each of the holes being formed by a single pulse. Embodiments of the invention

In figure 1. the ray path is shown when laser radiation is focused by a lens 1 with a numerical aperture NA on Wednesday 2 with a refractive index of n. A parallel beam from a source of ultrashort laser pulses (not shown) is focused by a lens 1 mounted in air. In the absence of medium 2, the rays would be focused to the point F0, with a paraxial approximation, the rays are focused to the point F1. The peripheral rays are focused into the constriction F1F2, the length of which Δ is determined by the formula (1), where / d is the distance from the surface of the medium 2 to F1. When the pulse energy density in the waist region is greater than the binding energy density of the molecules of the material, conditions for the destruction of the material arise in the entire waist volume. A cylindrical fracture volume with a length equal to the waist length occurs with each radiation pulse. With insufficient pulse energy, the length of the cylindrical fracture cavity decreases, however, the upper boundary of the cavity is F1. When the energy density is significantly higher than the optical breakdown threshold and NA <0.3, conditions for the self-focusing effect arise in the material, and the destruction region extends toward the lens from the F1 point.

Figure 2. a diagram of a device for forming holes in an optically transparent film is presented, in which the film is located in a liquid. A liquid 4 with a refractive index n is located in the cell 6. The film 3 is held by the system 7 in a liquid 4, on the surface of which a plate 5 with a refractive index n is located. A lens 1 with a numerical aperture NA is located so as to focus ultrashort laser pulses from the source (on drawing not shown) in the depth of the liquid. The film 3 has a thickness less than the length of the focal waist, determined by the formula (1), and

b positioned so that the focal constriction completely overlaps the film 3. The focus of the rays with a paraxial approximation F1 is located in front of the film, the focus of the peripheral peripheral rays F2 is located behind the film. When you turn on the source of ultrashort pulses of laser radiation, the radiation is focused into the focal waist F1F2, each pulse creates an opening in the film 3. Film

3 is held at a selected distance from the lens and moves in fluid

4 using the system 7. The laser system is equipped with a scanning module (not shown in the drawing) installed between the laser radiation source and the lens 1. The scanning module together with the film moving system 7 is allowed to perforate the film according to the developed template. The rarefaction cavities that occur in a liquid when exposed to laser pulses disappear after a short period of time. Such a system can be used for a long time. Since the film is perforated by a focal constriction with a length greater than the film thickness, small deviations of the vertical position of the film do not affect the process.

In Fig.Z. shows a setup in which ultrashort laser pulses focus the lens 1 through a plate of optically transparent material 8. The plate 8 is located so that the geometric focus of the lens 1 is located on the surface of the plate 8 facing the film 3. The film 3 is held by the system 7 at a distance from the lens greater than the focal length of the system with the paraxial approximation of F1. This arrangement of the plate 8 allows you to minimize the risk of destruction of the plate from focused laser radiation. Each ultrashort laser pulse forms a hole in the film 3. System 7 allows you to hold and move the film in one coordinate. The scanning module (not shown in the drawing) installed between the radiation source and the lens 1 allows you to move the position of the focal waist at least by one coordinate, which together makes it possible to perforate the film 3 according to the selected pattern.

Figure 4 presents microphotographs of a matrix of cylindrical cavities, each of which was obtained using a single pulse in a bulk material. Material - polycarbonate. Laser radiation: wavelength 1.06 μm, pulse duration 350 fsec, pulse energy 7 μJ. The transmitted energy was 3.5 μJ. The objective was an aspherical lens with a numerical aperture NA = 0.545. The refractive index of the material is 1.56. Depth focusing 1, 1 mm. Photo 4 (a) is a top view in natural light, 4 (b) is a side view in natural light, 4 (c) is a view from the side surface of a separate cylindrical cavity. The length of each cavity is 200 μm, the diameter of each cavity is 2 μm.

5. a micrograph of a top view of a matrix of exit openings in a film of a material 50 μm thick in natural light is presented. Lens 54-18-23-1064nm, NA = 0.39, Special Optics. The material of the film is polypropylene. Depth of focus 1, 1 mm. Laser radiation with a wavelength of 1, 06 μm, a pulse duration of 350 fsec, a pulse energy of 5 μJ. The pulse repetition rate of 100 Hz, the scanning speed of 100 μm / sec. The diameter of each of the holes is 5 μm.

Industrial applicability

The proposed technical solution can be used to form micron-sized holes in optically transparent films.

Claims

Claim.

1. The method of forming precision holes in an optically transparent film with an ultrashort laser pulse, the film thickness is up to 100 μm, in which the ultrashort laser pulse is focused by a lens with a numerical aperture of at least 0.33, characterized in that the ultrashort laser pulse is focused through a medium transparent to laser radiation with a refractive index of at least 1, 5 into the focal waist, the length of which is proportional to the thickness of the medium layer to the focus with paraxia noy approximation, the focal length of the constriction over the film thickness; an optically transparent film is placed in the focal waist so that the focal waist overlaps the portion of the film in which the hole is formed; set the energy of the ultrashort laser pulse such that the energy density of the laser radiation in the region of the focal waist exceeds the threshold for the destruction of the film material.

2. A device for forming precision holes in an optically transparent film with an ultrashort laser pulse, comprising an ultrashort laser pulse source, an optical beam forming and guiding system, a lens with a numerical aperture of at least 0.33 installed in air, film thickness up to 100 microns, different in that the device contains a medium transparent to laser radiation with a refractive index of at least 1.5, the film is mounted behind the layer of the medium so that the focus of the lens during paraxial approximation Imitation is located in front of the film, the focus of the boundary peripheral rays of the lens is located behind the film.

3. The device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claim 2, characterized in that the medium is a liquid closed from the lens by a transparent material with a refractive index equal to the refractive index of the liquid, the film is located in the liquid.

4. A device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claim 3, characterized in that it contains a film moving system in at least one coordinate.

5. A device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claims 3 or 4, characterized in the fact that the optical system for forming and guiding the beam contains a scanning system for at least one coordinate.

6. A device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claim 3, wherein the liquid is a solution of NaCl in water.

7. The device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claim 2, characterized in that the medium is a plane-parallel plate of material, the focus of the lens during paraxial approximation is located behind the plate, the numerical aperture of the lens is at least 0.5.

8. The device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claim 7, characterized in that it contains a system for moving the film in at least one coordinate.

9. A device for forming precision holes in an optically transparent film with an ultrashort laser pulse according to claims 7 or 8, characterized in that the optical system for forming and guiding the beam contains a scanning system for at least one coordinate.

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