JPH06235806A - Reflection mirror for laser - Google Patents

Abstract

PURPOSE:To obtain a reflection mirror having high durability against laser beams of high average power. CONSTITUTION:A thin film of diamond or the like is formed by vapor deposition to 50nm to 1mm thickness on a substrate, and then a multilayer film of dielectric material is formed by vapor deposition thereon. Since diamond has the highest thermal conductivity among all of materials, it effectively diffuses the thermal energy absorbed by the multilayer film and prevents temp. increase of the multilayer film. Thereby, even when the mirror is irradiated with laser beams of high output with a high frequency of repetition, the multilayer film does not cause thermal deformation or peeling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention particularly relates to a laser resonator for generating a laser beam having a high output and a high repetition frequency, and thus a high average power, or a laser suitable for an optical system using such a laser beam. It relates to a reflector.

[0002]

2. Description of the Related Art A laser reflecting mirror for reflecting laser light having a wavelength from the ultraviolet region to the infrared region is a substrate made of optical glass or quartz, on which a multilayer film made of a dielectric material having low absorption in the wavelength region is formed. It is generally formed, especially for excimer laser light used as exposure light or alignment light for exposure equipment, which has a high output and a high repetition frequency, and is therefore a reflection for a laser used for high average power laser light. Mirrors have been devised in various ways to improve their durability. For example, by performing undercoating or overcoating which is an integral multiple of the 1/2 wavelength optical film thickness, the laser damage threshold value of the multilayer film may be increased (Japanese Patent Laid-Open No. 2-97901).
(See Japanese Laid-Open Patent Publication No. 2004-242242), a metal having a high thermal conductivity is used as a material of the substrate, or a silicon compound having a high thermal conductivity is vapor-deposited on the surface of the substrate to form a multilayer film to dissipate heat from the multilayer film. A method for promoting it (see JP-A-2-248093 and JP-A-60-66202) has been proposed.

[0003]

However, according to the above conventional technique, when the multilayer film has a weak absorption for the laser light, even if the absorption is minute, the average power of the laser light is high. The thermal energy accumulated in the multilayer film also increases, and even if a metal is used as the material of the substrate or a silicon compound is vapor-deposited on the substrate as the first layer as described above, the amount of heat released by these is still insufficient. Therefore, the temperature rise of the multilayer film cannot be avoided. As a result, problems such as thermal deformation of the multilayer film and deterioration of adhesion occur.

In addition, since the absorption coefficient of metals and silicon compounds is larger than that of dielectric materials from the visible region to the near-infrared region, the incident angle of laser light on the multilayer film deviates from the design value, or the multilayer film. If the reflectance of is less than 100%,
Part of the laser light may reach the substrate, which may damage the substrate, and the visible range (around 560 nm).
Since there is an absorption edge at, transmission alignment by a visible light laser is difficult.

The present invention has been made in view of the above-mentioned unsolved problems of the prior art, and an object of the present invention is to provide a reflecting mirror for a laser having excellent durability against a laser beam having a high average power. Is.

[0006]

In order to achieve the above-mentioned object, a laser reflecting mirror of the present invention is a laser reflecting mirror comprising a substrate and a multilayer film of a dielectric material. It is characterized in that a thin film made of a diamond type material is provided between the films.

Further, the thickness of the thin film made of a diamond type material is preferably 50 nm to 1 mm.

Further, it is preferable that the surface roughness of the thin film made of a diamond type material is 100 nm rms or less.

[0009]

[Function] Since diamond materials have the highest thermal conductivity in all substances, by disposing such a diamond thin film between the substrate and the multilayer film, the heat energy absorbed by the multilayer film is radiated. Can be greatly promoted and the temperature rise of the multilayer film can be prevented. In addition, since diamonds have an absorptance near zero for light in the visible to near-infrared wavelength range, the incident angle of the laser light incident on the laser reflecting mirror may deviate from the design value, or the reflection of the multilayer film may occur. Rate is 100
Even if it is less than%, there is no possibility that the laser light reaches the substrate and damages it. In addition, since there is an absorption edge near 240 nm, transmission alignment with a visible light laser becomes easy.

The thin film made of diamond has a film thickness of 50.
The thickness is preferably 1 nm to 1 mm, and when the surface roughness of the thin film is 100 nm rms or less, the adhesion of the multilayer film can be improved.

[0011]

EXAMPLES Examples of the present invention will be described.

(First embodiment) Diameter 40 mm, thickness 3 mm
After depositing a thin film of a diamond-like material with a film thickness of 2 μm on the synthetic quartz substrate which is the substrate of No. 2 by the microwave CVD method, the fundamental wave of the Nd: YAG laser (106
A multilayer film for 4 nm) and a multilayer film for the third harmonic (355 nm) were manufactured. The multilayer film for the fundamental wave includes a first group in which five thin films of SiO ₂ and TiO _{2 having} a thickness of ¼ wavelength are alternately laminated, and SiO ₂ and Zr having a thickness of ¼ wavelength are also used.
The multilayer film for triple harmonics is composed of a second group in which thin films of O ₂ are alternately laminated by 6 layers each, and the multilayer film for triple harmonics is made of SiO having a quarter wavelength film thickness.
₂ and ZrO ₂ are alternately laminated in groups of 5 layers, and SiO ₂ and Al ₂ O ₃ having the same quarter wavelength thickness are alternately arranged to form 12 layers.
It is composed of a second group in which each layer is laminated, and the surface of the final layer is provided with an overcoat of ½ wavelength film of SiO ₂ , and the reflectance is 99% or more at the laser wavelength.

Next, in order to compare the durability of the laser reflecting mirror of this embodiment with respect to the laser beam to the conventional example, a 2 μm-thickness SiC thin film is vapor-deposited on a similar synthetic quartz substrate and the same as above. Sample A is a laser reflecting mirror having a multilayer film formed thereon, and sample is a laser reflecting mirror having the same multilayer film formed as described above after vapor-depositing a hard carbon thin film having a thickness of 2 μm on a similar synthetic quartz substrate. B, the laser reflecting mirror of the present embodiment is referred to as a sample C, and the laser reflecting mirror in which a multilayer film is directly formed on a similar synthetic quartz substrate is referred to as a sample D. The 1-shot laser proof strength and the laser proof strength of 20 Hz continuous irradiation of each sample. Was measured. The results are shown in Table 1. The 1-shot laser proof stress is obtained by irradiating the fundamental wave and the triple harmonic wave of the Nd: YAG laser at different irradiation positions and energies for each shot, and observing whether the laser reflecting mirror is damaged or not. The laser resistance of 20Hz continuous irradiation is
The same irradiation position was irradiated with laser light of the same energy for 5 minutes at a frequency of 20 Hz, and the presence or absence of damage to the laser reflecting mirror was observed.

[0014]

[Table 1] From Table 1, almost the same tendency was confirmed for the fundamental wave and the third harmonic, and the 1-shot laser proof stress was the best for Sample D and Sample C in which the multilayer film was directly formed on the synthetic quartz substrate, and then for Sample A. , Sample B, but in the laser proof strength of 20 Hz continuous irradiation, sample C,
Sample D, sample A, and sample B are in this order. Particularly, in the case of a fundamental wave, sample C, that is, a thin film of the diamond-like material of this embodiment is deposited on a synthetic quartz substrate and then a multilayer film is formed. It was found that the laser yield strength of the sample was significantly higher than that of other samples.

(Second embodiment) Diameter 40 mm, thickness 3 mm
On the synthetic quartz substrate which is the substrate of No. 3, a thin film made of a diamond material having a film thickness of 5 μm is vapor-deposited by a high frequency sputtering method (RF sputtering method), and the surface of the thin film has a surface roughness of 10 n.
After polishing to less than mrms, KrF (248n
m) A multilayer film for an excimer laser was manufactured. The multilayer film is formed by alternately stacking 28 layers of Al ₂ O ₃ and SiO ₂ thin films.

Next, in order to compare the durability of the laser reflecting mirror of this embodiment with respect to the laser beam to the conventional example, a 5 μm-thickness SiC thin film was vapor-deposited on a similar synthetic quartz substrate, and the surface thereof was treated as described above. A laser reflecting mirror having the same surface roughness as above and having a multilayer film similar to the above formed thereon was designated as sample A, and the laser reflecting mirror of this example was designated as sample B, and a synthetic quartz substrate similar to the above was used. A durability test was conducted on each sample, with the sample C having a multilayer film directly formed thereon as described above. The durability test was performed with a pulse width of 15 ns K
rF laser, frequency 200Hz, spot diameter 5mm
After irradiating 5 × 10 ⁷ times with the energy kept constant, the surface of each sample was observed with a Nomarski microscope. 3 energy per shot
Table 2 shows the results of the durability test performed by changing the stages.

[0017]

[Table 2] ◯: No change Δ: Slight change, no film peeling ×: Film peeling From Table 2, when the energy of the laser light to be irradiated is high, the durability of Sample B, that is, the laser reflecting mirror of the present example. It turns out that the sex far exceeds the other two samples.

[0018]

Since the present invention is configured as described above, it has the following effects.

The durability of the laser reflecting mirror for a laser beam having a high average power can be greatly improved.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Koji Sawamura 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (3)

[Claims] 1. A laser reflection mirror comprising a substrate and a multilayer film of a dielectric material, wherein a thin film made of a diamond-like material is provided between the substrate and the multilayer film. Laser reflector. 2. The laser reflecting mirror according to claim 1, wherein the thin film made of a diamond material has a thickness of 50 nm to 1 mm. 3. The laser reflecting mirror according to claim 1, wherein the thin film made of a diamond material has a surface roughness of 100 nmrms or less.