JP2003098341A - Method for manufacturing optical element for laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture an optical element for laser at low cost which has high reliability by using Zr or Hf as a vapor deposition material. SOLUTION: The optical element for laser composed of an optical multilayer film 4 in which a film 2 of a high refractive index material and a film 3 of a low refractive index material are laminated on a base plate 1 is manufactured. The film 2 of the high refractive index material is composed of ZrO2 or HfO2 . In a first process, metal Zr or Hf is heated and evaporated in a chamber in which the base plate 1 is set. In a second process, gaseous O2 is introduced into the chamber which is filled with the vapor of Zr or Hf. In a third process, dense ZrO2 or HfO2 is deposited on the base plate by accelerating while causing a reaction of Zr or Hf and O in a gaseous phase. In the third process, the dense ZrO2 or HfO2 is deposited by accelerating by an ion plating method or an ion assist method. For example, the film 2 composed of the high refractive index material composed of ZrO2 or HfO2 and the film 3 of the low refractive index material composed of SiO2 are alternately laminated on the base plate 1, thus the optical multilayer film is formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in an apparatus using a high-power laser having an ultraviolet wavelength to a near infrared wavelength,
The present invention relates to a method for manufacturing an optical element for laser. More specifically, in a method for manufacturing an optical element for laser comprising an optical multilayer film in which an optical film made of a high refractive index material and an optical film made of a low refractive index material are laminated on a substrate, particularly, an optical film made of a high refractive index material Regarding film forming technology.

[0002]

2. Description of the Related Art A device incorporating a laser optical system,
Various optical components such as mirrors, lenses and beam splitters are used. These optical components are required to have durability against laser power. When an optical component is irradiated with a laser beam, the portion having the lowest durability is the optical multilayer film laminated on the substrate. In particular, it has been confirmed that the optical film made of a high refractive index material has poor durability. In order to improve durability, it is necessary to select materials having high resistance to laser light irradiation for the substrate material and the film material. Further, measures are taken to reduce defects existing on the surface of the substrate and the optical film as much as possible. Furthermore, attempts have been made to adjust the film thickness and shift the peak of the electric field intensity of laser light from the interface of the optical film of a high-refractive index material having particularly low durability. As described above, the optical multilayer film has a laminated structure in which films of high refractive index material and films of low refractive index material are alternately stacked. When a laser beam is incident on such an optical multilayer film, from the film interface of the high refractive index material, which is the weakest part to the irradiation of the laser beam,
The optical design is performed so that the position where the electric field of the laser beam becomes the maximum is shifted to other parts such as the film of the low refractive index material. Laser light is also an electromagnetic wave, and when considering the influence of light, an optical design considering the strength of the electric field is effective.

Among the various measures described above, it is particularly effective to use a metal oxide having a high durability against laser light as a high refractive index material. In recent years, as a film material exhibiting a high refractive index, HfO ₂ and ZrO ₂ are more durable than TiO ₂ and Ta ₂ O ₅ , which have been widely used in the past, in the wavelength region from ultraviolet to near infrared. It has been confirmed to be excellent, and is being used. Generally, a film with high durability against laser light has no absorption in the laser wavelength range,
There are few film defects (pinholes, pits, internal defects, etc.),
Further, it is considered that the smaller the refractive index is, the more excellent the durability is. Generally, if the band gap is wide, the probability of occurrence of “electron avalanche” caused by laser light irradiation is said to be small, and if the refractive index is small, the band gap tends to be wide. Further, it is considered that the durability against laser light increases as the packing density of the film increases. In this respect, HfO ₂ and ZrO ₂ are TiO ₂ and Ta ₂ O which are usually used as optical film materials having a high refractive index.
_The refractive index is a little smaller than that of _5, and there is a possibility that film absorption and film defects are small.

Various methods for forming a metal oxide film have been developed in the past, for example, Japanese Patent Laid-Open No. 10-10738.
Japanese Patent Publication No. 1 discloses a method in which oxygen plasma is generated from a metal as a raw material to generate a metal oxide. or,
Japanese Patent Application Laid-Open No. 10-233336 discloses a mechanism for generating high-density plasma with respect to the ion plating method. Further, JP-A-11-101903 discloses an optical multi-layer film element for excimer laser. This optical multilayer film element is made of a high refractive index material (Nd
Fluorides such as F ₃ and LaF ₃ and Al ₂ O ₃ ) are prepared by the IAD method.

[0005]

[Problems to be Solved by the Invention] Conventionally, HfO ₂ and Zr
O ₂ is obtained by processing these oxides into pellets in advance and forming a film on the substrate by electron beam evaporation. At that time, Hf
The O ₂ and ZrO ₂ pellets have the property of melting only the portion irradiated with the electron beam. Therefore, as the film formation progresses,
Since the portion vapor-deposited from the pellets was depressed, the film thickness distribution was likely to change, which was a cause of variation. Further, since only the vapor deposition portion is depressed, the material is consumed quickly and it is difficult to increase the film thickness. Further, the pellet of HfO ₂ used as a high refractive index material is very expensive, and there is a problem that the cost of the produced laser optical element becomes high.

[0006]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, an object of the present invention is to manufacture a reliable and inexpensive optical element for laser using Zr or Hf as a vapor deposition material. The following measures have been taken to achieve this purpose. That is, the present invention relates to a method for manufacturing an optical element for a laser, which comprises an optical multi-layer film in which a film of a high refractive index material and a film of a low refractive index material are laminated on a substrate, and the film of the high refractive index material is ZrO 2. ₂ or HfO ₂ , a first step of heating and evaporating metal Zr or Hf in a chamber in which a substrate is set, and a _{second step} of introducing O ₂ gas into a chamber filled with Zr or Hf vapor. And Zr or H
It is characterized in that a film is formed by a third step of accelerating f and O while reacting with each other in the vapor phase to deposit dense ZrO ₂ or HfO ₂ on the substrate. Preferably, in the third step, a dense ZrO ₂ or HfO ₂ is deposited by acceleration by an ion plating method or an ion assist method. Also, ZrO
A film of a high refractive index material made of ₂ or HfO ₂ and a film of a low refractive index material made of SiO ₂ are alternately laminated on a substrate to form an optical multilayer film.

According to the present invention, a film of a high-refractive index material (hereinafter sometimes referred to as a high-refractive index film) in a method of manufacturing an optical element for a laser used in a laser optical system in the ultraviolet to near infrared region. Zr or H as a raw material when making
Use f. Metal oxides (ZrO ₂ , HfO ₂ ) are created by evaporating these metal raw materials with an electron beam or the like and introducing oxygen gas into the film forming chamber. Furthermore, in order to promote the degree of oxidation and reduce the metal simple substance and lower oxide components in the film that cause the deterioration of durability against laser light and improve the film packing density, an ion assist method, an ion plating method, etc. The method is used to accelerate the deposition material to form a film. Incidentally, the lower oxide means, for example, in the case of an oxide of Zr, ZrO _x , and particularly x <2. Further, the film packing density is defined by (volume occupied by film constituent material / volume of entire film) * 100%. Ideally, the packing density should be 100%. However, when a thin film is formed, a gap is formed in the film and the packing density does not reach 100%. In the atmosphere, water may be adsorbed in the gaps, and the reliability of the optical multilayer film may be impaired.

The present invention uses Zr as a raw material for a high refractive index film.
Or, Hf is used. Zr (zirconium) and H
f (hafnium) is a transition element in the same family as Ti (titanium). The chemical properties of both zirconium and hafnium elements are very similar because the size of the atoms and ions are nearly equal. Zirconium and hafnium almost always have an ionic valence state in the compound, and the oxide is Zr.
It becomes O ₂ and HfO ₂ . Zirconium and hafnium simple substances are metals having high melting points and boiling points. Zirconium has a melting point of 1860 ° C., a boiling point of 4750 ° C., a density of 6.53, and a crystal structure of hexagonal closest packing. Hafnium has a melting point of 2200
C., boiling point is 5100.degree. C., density is 13.1, and crystal structure is hexagonal closest packed. Instead of the oxide of ZrO ₂ and HfO _{2 as in} the prior art, Zr and Hf of simple metals are used as the vapor deposition raw materials. As a result, the raw material placed in the evaporation crucible during film formation is melted uniformly. Even if the film thickness is increased, the liquid level of the metal simple substance melted in the crucible only decreases with time, and there is almost no adverse effect on the film thickness distribution.
Compared to the conventional example that uses pellets of ZrO ₂ and HfO ₂ as the evaporation material, the amount of material stored in the crucible is large, and the whole melts, so the amount usable for film formation per crucible is extremely high. Therefore, it becomes easy to produce an optical element that requires a large number of layers. Conventionally used Zr
Since the oxides of O ₂ and HfO ₂ are melted only in the portion irradiated with the electron beam, many raw material portions are wasted and the vapor deposition film thickness varies.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic view showing an optical element for a YAG laser showing a reflectance of 50% manufactured according to the present invention, (A) is a sectional view, and (B) is a plan view. As shown in (B), the laser optical element has, for example, a rectangular shape, and the effective range is 40 mm in the longitudinal direction. At a wavelength of 1064 nm and an incident angle of 45 degrees, the reflectance R of the random light at the substrate center position is 50 ± 2%, and the variation of the reflectance R with respect to the longitudinal position on the substrate is 0.4% / mm. It has a simple optical design. However, this is an example and does not limit the scope of the present invention. This laser optical element is a design example of a YAG laser reflection mirror, and the laser output at the time of YAG laser irradiation (wavelength 1064 nm, 10 ns pulse) is intended to be 10 J / cm ² or more.

FIG. 3A is a sectional view taken along line AA of the laser optical element shown in FIG. As shown,
In the optical element for a laser of the present invention, a film 1 of a high refractive index material (high refractive index film) 2 and a film of a low refractive index material (low refractive index film) 3 are provided on a substrate 1.
The optical multilayer film 4 is laminated on the above. The substrate 1 is made of quartz, for example, and its refractive index is 1.45. The high refractive index layer 2 is made of ZrO ₂ and is formed by the following process. That is, in the chamber in which the substrate 1 is set, the metal Zr
The first step of heating and evaporating is carried out. Next, a second process of introducing O ₂ gas into the chamber filled with Zr vapor is performed. Finally, the third step of depositing dense ZrO ₂ on the substrate 1 by accelerating Zr and O while reacting with each other in the gas phase is performed. In the third step, the dense ZrO _{2 is} accelerated by the ion plating method or the ion assist method.
The high refractive index layer 2 consisting of is deposited. In this example, ZrO ₂
An optical multilayer film 4 is formed by alternately laminating a high refractive index film 2 made of SiO ₂ and a low refractive index film 3 made of SiO ₂ on a substrate 1. The refractive index of ZrO ₂ is 2.05. SiO ₂
Has a refractive index of 1.45. In addition, in order to form the high refractive index film 2, Hf may be used instead of Zr described above. In this case, the high refractive index film 2 is made of HfO ₂ .

The illustrated optical multilayer film 4 has a laminated structure in which a high refractive index layer 2 made of ZrO ₂ and a low refractive index layer 3 made of SiO ₂ are alternately laminated in total 10 layers. The optical film thickness of the high refractive index layer 2 is represented by H. When the coefficient of H is 1, it indicates that the optical film thickness of the high refractive index layer 2 is exactly λ0 / 4. It should be noted that λ0 is the wavelength of the incident laser light, which is set to 1064 nm of the YAG laser in this example.
Similarly, the optical film thickness of the low refractive index layer 3 is represented by L.
When the coefficient of L is 1, it indicates that the optical film thickness of the low refractive index layer 3 is exactly λ0 / 4.

FIG. 2 is a graph showing optical characteristics and physical characteristics of the laser optical element shown in FIG. 1 at 1064 nm (wavelength of YAG laser light). The horizontal axis indicates the distance from the center of the substrate, and the vertical axis indicates the reflection characteristic on the left side. The film thickness ratio is shown on the right side of the vertical axis. As indicated by the straight line a, the reflection characteristic of the optical element for the laser is 0.50 (50%) at the center of the substrate, and the reflection characteristic at the left side of the center is the − (minus) direction, and at the right side of the center of the substrate. It is designed so that the reflection characteristics are in the + (plus) direction. In order to realize the inclination of the reflection characteristic along the longitudinal direction of the substrate, the film thickness ratio is designed so that the thickness of the optical multilayer film has an inclination. Specifically, the total thickness of the optical multilayer film 4 is set in the − (minus) direction at a point separated from the center of the substrate to the left, as indicated by the straight line B, compared to the center. On the contrary, at the point away from the center of the substrate to the right, the film thickness ratio is +
It is set in the (plus) direction.

FIG. 3 is a graph showing the spectral reflection characteristics in the central portion of the substrate of the laser optical element shown in FIG. The spectral reflection characteristic is measured at the central portion of the substrate, the vertical axis of the graph represents the reflection characteristic (reflectance), and the horizontal axis represents the incident wavelength. In any case of random polarization (curve B), S wave (curve B), and P wave (curve C), the peak of the reflection characteristic is designed to come near the wavelength of 950 nm.

FIG. 4 is a schematic block diagram showing a reactive ion plating apparatus used for manufacturing the laser optical apparatus shown in FIG. As shown, this device
The chamber 11 includes a substrate holder 12, a substrate holder 12, an evaporation source 13, an electron gun 14, an ion gun 15, and the like. The chamber 11 can be evacuated by an exhaust unit 16 including a vacuum pump. or,
A gas pipe is connected to the chamber 11, for example, O ₂
Gas can be introduced during film formation. A substrate made of quartz is mounted on the substrate holder 12. The evaporation source 13 includes
The vapor deposition material stored in the crucible is stored. When forming the high refractive index layer 2 of the optical multilayer film 4, a metal Z is used as an evaporation source.
r is used. In some cases, Hf having similar chemical properties and physical properties may be used instead of Zr.

In this reactive ion plating apparatus, the electron beam emitted from the electron gun 14 melts / dissolves the material.
At the same time as the vaporization, O ₂ gas is introduced into the chamber 11, and large electron particles are released from the ion gun 15 into the chamber 11 to generate oxygen plasma. Evaporation source 13
The metal particles M + evaporated from are ionized and accelerated toward the substrate holder 12 side by self bias, and
It combines with oxygen and is deposited on the substrate as an oxide.

The crucible for containing the evaporation material in the evaporation source 13 is made of a material that can withstand the high evaporation temperature of Zr or Hf, and is made of tungsten that does not react with the film forming raw material. A carbon crucible may be used as long as the reaction with the metal material does not proceed. The chamber 11 was brought to 2.7 × 10 ^-2 by introducing oxygen gas.
While maintaining the pressure of Pa, an electron beam was emitted from the electron gun 14 toward the evaporation source 13 to dissolve the metal Zr and form a high refractive index layer made of ZrO ₂ on the substrate. Incidentally, Z
The evaporation temperature of r is 2900 ° C. or higher. The Hf evaporation temperature is 3200 ° C. or higher. Both are values measured in the atmosphere. By the way, the melting point of Mo is 2620 ° C., Ta
Has a melting point of 2996 ° C., C has a melting point of 3727 ° C., and W has a melting point of 3380 ° C. Since the crucible is water-cooled when the evaporation material is melted during film formation, the evaporation material V. P. (boiling point)> M. P. Even if it is (melting point), it may be usable.

The formed ZrO ₂ film has a physical film thickness of about 0.
At 35 nm, the refractive index was 1.974 and the light absorption amount was 0.4% (value at incident wavelength 930 nm).

The above data is the result when no acceleration means is employed. On the other hand, the feature of the present invention is to form a dense film by using an acceleration means such as an ion plating method or an ion assist method. By forming a dense film, the degree of oxidation of the film can be improved. Operate the ion gun 15 with the device of FIG.
When the film was formed by the reactive ion plating method, the high refractive index film had a refractive index of 2.223 (measurement wavelength: 470 nm), and almost no absorption was observed. For example, YAG
Assuming that the dispersion at the laser output wavelength of 1064 nm is almost the same as that of the ordinary film formation method, the refractive index when the film is formed by the reactive ion plating method is 2.05 to 2 at 1064 nm. It is estimated to be between 10 and 10. The low refractive index film can also be formed by the same reactive ion plating method to form the optical multilayer film 4 including the alternating layers of the high refractive index layer 2 and the low refractive index layer 3. As the material of the low refractive index film 3, SiO ₂ is used. This makes it possible to form a laser optical element having excellent durability. Note that the vapor deposition material used this time is a metal simple substance, unlike the conventional oxide, and exhibits conductivity when melted, so there is no adverse effect due to charge-up by electron beam irradiation or irradiation of the ion gun used in the reactive ion plating method. Can be avoided. In the case of oxides (dielectrics), as the electron beam is irradiated, negative charges are accumulated on the surface of the material, causing cracks in the material such as when using pellets, which makes the deposition rate unstable. Also, splashes and lumps are likely to appear. On the other hand, when the reactive ion plating method is used, an electron beam is emitted from an electron gun and, at the same time, large electron particles are emitted from the ion gun toward the evaporation material. As a result, plasma is generated by the arc discharge. If the evaporation material is not electrically conductive at this point, the arc discharge cannot be stably maintained. In this respect, since the present invention uses a simple metal as the evaporation material, it is possible to stably maintain the arc discharge.

In this embodiment, the ion plating method is used. As mentioned above, the ion plating method
This is a method in which energy is externally applied to an evaporated substance, introduced gas, or the like to generate plasma, and a bias is applied to apply energy to particles attached to a substrate. In the reactive ion plating method, a compound is formed on the substrate by converting the introduced gas into plasma and reacting it with the evaporation material. By this ion plating method,
A dense high refractive index film can be formed. The present invention is not limited to the ion plating method, and a dense film can be formed by, for example, the ion assist method.
The ion assist method is a method in which ions such as argon are irradiated from an ion gun to a substrate during electron beam evaporation. By this ion irradiation, the surface of the deposited film is physically hit and the film is densified.

As an index of the denseness of the film, there is a method of measuring the shift amount of the spectrum when the laser optical element (filter) is exposed to a high temperature state. For example, transmittance 5
When a long-wavelength pass filter whose 0% position is near 550 nm is created by a normal vacuum vapor deposition method, it shifts to a short wavelength side by about 10 nm. On the other hand, when the film is formed by the reactive ion plate method, the shift amount of the spectrum is 0.1 nm.
It is the following.

[0021]

As described above, according to the present invention, a method of using a single metal of Zr or Hf as a film forming material, introducing oxygen during film formation and forming a dense film (ion assist method, By using the ion plating method or the like), it is possible to obtain an optical element for laser that does not absorb and exhibits good durability against laser shock. In addition, since a single metal is used as the vapor deposition source, the film thickness distribution does not fluctuate, stable optical characteristics are obtained, and the amount usable for film formation per crucible is large, so that it is easy to stack in multiple layers. .
In addition, by forming a dense film, it has become possible to inexpensively produce an optical element for laser that exhibits good durability against laser shock.

[Brief description of drawings]

FIG. 1 is a schematic view showing an optical element for a YAG laser showing a reflectance of 50% manufactured according to the present invention.

2 is a laser optical element 1064n shown in FIG.
It is a graph which shows the reflection characteristic in m (wavelength of YAG laser light).

FIG. 3 is a graph showing spectral reflection characteristics in the central portion of the substrate of the laser optical element shown in FIG.

FIG. 4 is a schematic block diagram showing a reactive ion plating apparatus used for making the optical element for laser shown in FIG.

[Explanation of symbols]

1 ... Substrate, 2 ... High refractive index layer, 3 ... Low refractive index layer, 4 ... Optical multilayer film, 11 ... Chamber, 12 ...
..Substrate holder, 13 ... evaporation source, 14 ... electron gun, 15 ... ion gun, 16 ... exhaust unit

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 14/06 C23C 14/06 PF term (reference) 2H048 GA04 GA12 GA33 GA60 GA61 4F100 AA17B AA20C AT00A BA03 BA07 BA10A BA10C EH66B EH66C GB41 JL02 JN18B JN18C 4G059 AA11 AB11 AB19 AC09 EA01 EA05 GA02 GA04 GA11 4K029 AA08 BA43 BA46 BB02 BC08 BD00 CA04 DD03

Claims (3)

[Claims] 1. A method of manufacturing an optical element for laser comprising an optical multi-layered film comprising a high refractive index material film and a low refractive index material film laminated on a substrate, wherein the high refractive index material film is ZrO ₂ Alternatively, a _{first step} of heating and evaporating the metal Zr or Hf in a chamber in which a substrate is set, and a _{second step} of introducing O ₂ gas into a chamber filled with vapor of Zr or Hf. A third step of accelerating Zr or Hf and O while reacting with each other in a vapor phase to deposit dense ZrO ₂ or HfO ₂ on a substrate. 2. The third step is accelerating by an ion plating method or an ion assist method to form a dense ZrO ₂ film.
Alternatively, HfO ₂ is deposited, and the method for manufacturing an optical element for laser according to claim 1. 3. An optical multilayer film is formed by alternately laminating a film of a high refractive index material made of ZrO ₂ or HfO ₂ and a film of a low refractive index material made of SiO ₂ on a substrate. The method for manufacturing an optical element for laser according to claim 1.