JP2009141239A - Brewster window and laser oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a brewster window improved in laser durability. <P>SOLUTION: The brewster window is disposed inside a laser oscillator such that laser light is oscillated by linear polarization, and includes: a substrate 1 which is constituted of an alkaline earth metal fluoride monocrystal and of which the surface is flattened; and fluoride films 2a, 2a' of a polycrystal or amorphous structure formed on at least one surface of the substrate. The fluoride film 2a, 2a' is constituted of any of gadolinium fluoride, magnesium fluoride, lanthanum fluoride, aluminum fluoride and cryolite. The fluoride film may also be a multilayer film like fluoride films 2b, 2b'. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a laser oscillator such as an excimer laser and a Brewster window used for the laser oscillator. In particular, it relates to a technique for improving the laser resistance of a Brewster window.

  FIG. 5 is a schematic diagram of a laser oscillator 200 that oscillates polarized light such as an excimer laser. The laser oscillator 200 includes a resonator including a laser chamber 201 in which a laser gas as a laser medium is sealed, and reflecting mirrors 202 and output mirrors 203 disposed on both sides of the laser chamber 201.

  When a voltage is applied to the discharge electrode 207, the laser medium in the laser chamber 201 is excited, that is, pumped, and spontaneous emission and amplification by stimulated emission become possible. The light in the resonator is amplified by dielectric emission every time it passes through the laser medium in the laser chamber 201, and is repeatedly reflected between the reflecting mirror 202 and the output mirror 203 to form a standing wave. The output mirror 203 transmits part of the incident light and outputs it as output light to the outside.

The Brewster windows 205 are provided at both ends in the optical axis direction of light transmitted through the laser medium of the laser chamber 201. The Brewster window 205 is an optical substrate installed so as to intersect the optical axis 206 at a Brewster angle. As the optical substrate used for the Brewster window 205, for example, fluorite (CaF ₂ ) having excellent transparency and laser resistance to short wavelength / high energy light is used.

  As shown in FIG. 6, the reflectance of S-polarized light (polarized light perpendicular to the incident surface) increases as the incident angle increases, but the reflectance of P-polarized light (polarized light parallel to the incident surface) is incident. It is known that the angle gradually decreases as the angle increases and becomes 0 at a certain incident angle (in this example, around 56 degrees). The incident angle at which the reflectance of the P-polarized light is 0 is called the Brewster angle. By tilting the optical substrate by the Brewster angle, the transmittance of P-polarized light is ideally 100%, so that the optical substrate does not require an antireflection coating.

  However, when the laser oscillator 200 is a high-power laser device such as an excimer laser, the intensity of light in the laser chamber 201 is increased, and the Brewster window 205 is irradiated with a high-power, short-wavelength laser many times. There is a known problem that the Brewster window 205 deteriorates due to damage.

In order to solve this problem, the uneven SSD surface exposed by etching away the quasi-Birby layer formed by optically polishing the calcium fluoride (CaF ₂ ) forming the optical substrate and flattening the surface. There is provided a technique for improving the laser resistance of the Brewster window 205 by forming an optical thin film of an appropriate material and flattening the surface by optically polishing the optical thin film again (for example, Patent Documents). 1). This technique pays attention to light absorption and heat generation of a quasi-Birby layer generated by optically polishing calcium fluoride (CaF ₂ ) constituting an optical substrate to flatten the surface.
JP 2006-72364 A

  In the technique described in Patent Document 1, there is a problem that it is necessary to perform optical polishing again after the quasi-beer bee layer is removed by etching, which is troublesome.

  An object of the present invention is to provide a Brewster window and a laser oscillator that can be simply configured and have excellent laser resistance.

  The Brewster window according to the present invention is arranged inside the laser resonator so that the laser beam oscillates in a linearly polarized light, and is made of an alkaline earth metal fluoride single crystal and has a flattened surface, and the substrate And a fluoride film having a polycrystalline or amorphous structure formed on at least one surface.

  Coating the substrate with a fluoride film increases the laser resistance of the Brewster window. Since the fluoride film is provided on the surface of the substrate having a planarized surface, the fluoride film also has a sufficiently flat surface. For this reason, re-polishing after forming the fluoride film is unnecessary, and in that respect, it can be simply configured.

  An embodiment of a Brewster window according to the present invention will be described with reference to FIG. 1A. FIG. 1A is a schematic view illustrating a cross section of a Brewster window according to the present invention.

The Brewster window is composed of a substrate 1 and fluoride films 2a and 2a ′.
The substrate 1 is made of an alkaline earth metal fluoride single crystal such as calcium fluoride, barium fluoride, magnesium fluoride, or strontium fluoride. Here, the case where the alkaline earth metal fluoride single crystal is calcium fluoride (fluorite, CaF ₂ ) will be described as an example. The substrate 1 is planarized by an ordinary CMP (Chemical Mechanical Polish / Planarization) process using an Oscar-type polishing machine until the average roughness becomes about 2 mm or less, for example.

Fluoride films 2 a and 2 a ′ having a polycrystalline or amorphous structure are formed on at least one surface of the substrate 1. For example, as illustrated in FIG. 1A, single-layer fluoride films 2 a and 2 a ′ are provided on both surfaces of the substrate 1. Examples of the fluoride films 2a and 2a ′ include magnesium fluoride (MgF ₂ ), gadolinium fluoride (GdF ₃ ), lanthanum fluoride (LaF ₃ ), aluminum fluoride (AlF ₃ ), and cryolite (Na ₃ AlF). ₆ ) etc. can be used. As the fluoride films 2a and 2a ′, the same fluoride may be used, or different fluorides may be used.

  The thickness of the fluoride film is such that the wavelength of the laser beam is λ, and the optical film thickness in the optical path direction when the laser beam passes through the Brewster window including the fluoride film inside the laser resonator 200 is λ / A thickness that is an integral multiple of 2, that is, the refractive index of the fluoride film is n, and the physical film thickness in the direction perpendicular to the film surface is multiplied by an integer multiple of λ / (2n) by the cosine of the Brewster angle To be. When fluoride films are formed on both surfaces of the substrate 1, the physical film thickness in the direction perpendicular to the film surface of each fluoride film is multiplied by an integer multiple of λ / (2n) by the cosine of the Brewster angle. Try to be large. Thereby, the antireflection performance of the Brewster window of the present invention can be maintained. In the case of an ArF excimer laser, λ = 193 nm.

  The method for forming the fluoride film is arbitrary. For example, an ion beam sputtering method or a vapor deposition method can be used. Since the film is formed on the substrate 1 which has been planarized in advance, the fluoride film also has a sufficiently flat surface. For this reason, it is not necessary to polish the Brewster window again after forming the fluoride film.

Further, as illustrated in FIG. 1B, multilayer fluoride films 2b and 2b ′ may be provided on both surfaces of the substrate 1, respectively. In FIG. 1B, the fluoride films 2b and 2b ′ are two-layer films and are composed of first layers 21 and 21 ′ and second layers 22 and 22 ′, respectively. The first layers 21, 21 ′ and the second layers 22, 22 ′ are respectively magnesium fluoride (MgF ₂ ), gadolinium fluoride (GdF ₃ ), fluoride, similarly to the fluoride films 2a, 2a ′ in FIG. 1A. It consists of lanthanum (LaF ₃ ), aluminum fluoride (AlF ₃ ), cryolite (Na ₃ AlF ₆ ), and the like. Note that the first layer, the second layer,...

  The physical film thickness in the direction perpendicular to the layer surface of each layer constituting the fluoride films 2b and 2b 'is set to a value obtained by multiplying an integral multiple of λ / (2n) by the cosine of the Brewster angle.

FIG. 2 shows the experimental results of evaluating the laser resistance of the Brewster window when a fluoride film is formed by ion beam sputtering. Specifically, experimental results comparing laser damage thresholds using an ArF excimer laser are shown. In this experiment, calcium fluoride (CaF ₂ ) is used as the substrate 1. Further, Brewster angle of about 56 degrees, the maximum output energy of the laser 9 mJ, laser wavelength is 193.4 nm, the average energy density at the sample position 210 mJ / cm ^2, the spot size is 0.0036Cm ^2, sample position The energy density was 650 to 700 mJ / cm ² and the irradiation frequency was 1 kHz. Here, the maximum energy density at the sample position is the maximum energy density at the sample position when the laser is irradiated perpendicularly to the incident surface. The irradiation frequency is the number of laser irradiations per second.

The horizontal axis of FIG. 2 shows the material of the fluoride film. “Substrate alone” is experimental data when a fluoride film is not formed. “GdF ₃ ” is a fluoride made of gadolinium fluoride (GdF ₃ ) on both surfaces of the substrate 1 and having a physical film thickness of λ / (2n) in the laser beam irradiation direction (direction along the Brewster angle). It is an experimental data at the time of forming a film | membrane. “LaF ₃ ” is an experiment in which a fluoride film made of lanthanum fluoride (LaF ₃ ) and having a physical thickness of λ / (2n) in the laser beam irradiation direction is formed on both surfaces of the substrate 1. It is data. “MgF ₂ ” is an experiment in which a fluoride film having a physical film thickness of λ / (2n) is formed on both surfaces of the substrate 1 and is made of magnesium fluoride (MgF ₂ ). It is data. “GdF ₃ + MgF ₂ ” has a thickness of magnesium fluoride (MgF ₂ ) and λ / (2n) in which the first layer and the second layer have a physical film thickness of λ / (2n) in the laser light irradiation direction, respectively. This is experimental data in the case of gadolinium fluoride (GdF ₃ ).

The vertical axis in FIG. 2 indicates the number of laser irradiations. The unit of the vertical axis is 1 × 10 ⁶ shots. “Damaged” indicates that the substrate 1 was damaged by the number of times of irradiation of the corresponding laser. “No damage” means that the substrate 1 was not damaged even when the laser was irradiated as many times as the corresponding laser irradiation. FIG. 3 illustrates a photograph in which damage to a single substrate caused by laser irradiation is magnified 100 times.

From this experimental result, it is understood that the Brewster window in which the fluoride film is provided on the substrate is higher in laser resistance than the Brewster window of the substrate alone made of calcium fluoride (CaF ₂ ). It can also be seen that the laser resistance is high when magnesium fluoride (MgF ₂ ) is used as the fluoride film.

  FIG. 4 shows the experimental results of evaluating the laser resistance of a Brewster window on which a fluoride film was formed by vapor deposition. The conditions other than the film formation by the vapor deposition method are the same as the experimental conditions described above. From this experimental result, it is understood that the laser resistance is improved even in the Brewster window formed by the vapor deposition method.

  The reason why the laser resistance can be increased when a fluoride film is formed on an alkaline earth metal fluoride single crystal substrate can be considered as follows.

  In general, it is known that the surface of the substrate has a weaker atomic bonding state than the inside of the substrate. When high-energy laser is irradiated on the surface of a weakly bonded substrate, the bond between the metal atom on the substrate surface (Ca when the substrate is calcium fluoride) -fluorine atom is broken and the fluorine atom is removed. Release. When an electron is trapped in a fluorine atom deficiency generated by the elimination of a fluorine atom, the crystal structure changes in the ionic crystal. When the crystal structure changes in a single crystal substrate in which atoms are periodically arranged, a weaker bonded portion is generated on the surface, which becomes a starting point for the next defect of fluorine atoms. By repeating this, it is considered that fluorine atoms are successively lost and the substrate is destroyed.

On the other hand, fluoride films such as magnesium fluoride (MgF ₂ ) and gadolinium fluoride (GdF ₃ ) formed on the substrate have a polycrystalline or amorphous structure, and are random compared to a single crystal substrate. The array is stabilized. For this reason, when a laser having high energy is irradiated on the film, even if fluorine atom defects occur as in the case of the substrate, it is difficult to generate a portion where the bonding state of atoms is weak on the surface. As a result, it is considered that the loss of fluorine atoms is suppressed and laser resistance is improved as compared with the substrate alone.

  In order to configure the laser oscillator using the Brewster window according to the present invention, the Brewster window according to the present invention is used instead of the Brewster window 205 of the laser oscillator 200 described in the background art with reference to FIG. Good.

[Modifications, etc.]
When the fluoride film is composed of three layers, for example, the first layer of the fluoride film is magnesium fluoride (MgF ₂ ), the second layer is gadolinium fluoride (GdF ₃ ), and the third layer is magnesium fluoride. (MgF ₂ ) is preferred.

  A fluoride film may be formed only on one surface of the substrate 1.

  When forming fluoride films on both surfaces of the substrate 1 respectively, the number of fluoride film layers formed on one surface of the substrate 1 and the number of fluoride films formed on the other surface of the substrate 1 The number of layers may be different.

  It is not necessary to cover the entire surface of the substrate 1 with a fluoride film. That is, it is only necessary to form the fluoride film only on the portion of the surface of the substrate 1 through which the laser passes.

The schematic diagram which illustrates the cross section of one Example of the Brewster window by this invention. FIG. 4A is a schematic view illustrating a cross section of an example of a Brewster window in which a single-layer fluoride film is formed on both surfaces of a substrate. B is a schematic view illustrating a cross section of an example of a Brewster window in which two layers of fluoride films are formed on both sides of a substrate. The figure which shows the experimental result which evaluated the laser tolerance of the Brewster window which formed the fluoride film into a film by ion beam sputtering method. The figure showing the example of damage of the board | substrate which arose by laser irradiation. The figure which shows the experimental result of the Brewster window which formed the fluoride film | membrane by the vapor deposition method. The schematic diagram of the laser oscillator 200. FIG. The graph showing the reflectance in each incident angle of S polarized light and P polarized light.

Claims (8)

A Brewster window arranged inside the laser resonator so that the laser beam oscillates with linearly polarized light,
A substrate made of an alkaline earth metal fluoride single crystal and having a flat surface,
A polycrystalline or amorphous fluoride film formed on at least one surface of the substrate;
Brewster window characterized by comprising. Brewster window according to claim 1,
The fluoride film is made of any of gadolinium fluoride, magnesium fluoride, lanthanum fluoride, aluminum fluoride, and cryolite.
Brewster window characterized by that. Brewster window according to claim 1,
The fluoride film is a multilayer film, and each layer of the multilayer film is made of any one of gadolinium fluoride, magnesium fluoride, lanthanum fluoride, aluminum fluoride, and cryolite.
Brewster window characterized by that. Brewster window according to claim 3,
The first layer of the multilayer film is made of magnesium fluoride.
Brewster window characterized by that. In the Brewster window according to claim 1 or 2,
The wavelength of the laser beam that passes through the Brewster window is λ, and the optical film thickness of the fluoride film in the direction in which the laser beam is transmitted is an integral multiple of λ / 2.
Brewster window characterized by that. In the Brewster window according to claim 3 or 4,
The wavelength of the laser beam that passes through the Brewster window is λ, and the optical film thickness of each layer of the multilayer film in the direction in which the laser beam is transmitted is an integral multiple of λ / 2.
Brewster window characterized by that. In the Brewster window according to any one of claims 1 to 6,
The alkaline earth metal fluoride single crystal is calcium fluoride,
Brewster window characterized by that.   A laser oscillator comprising the Brewster window according to any one of claims 1 to 7.