JP3402907B2 - Laser reflector - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser reflecting mirror suitable for laser light in a wavelength range from the ultraviolet region to the infrared region.

[0002]

2. Description of the Related Art Generally, a dielectric multilayer film in which a λ / 4 film (high refractive index film) of a high refractive index material and a λ / 4 film (low refractive index film) of a low refractive index material are alternately laminated The larger the difference in the refractive index between the high-refractive index material and the low-refractive index material, the more effective the reflection mirror becomes, the more the required reflectance can be obtained with a smaller number of layers, and the higher the reflectance band width, and the higher the optical performance. It is known that an excellent and inexpensive reflector can be realized. However, among high refractive index materials, it is known that the higher the refractive index material, the higher the laser resistance (laser damage threshold value: the threshold value of laser energy that causes laser irradiation to cause film destruction).

Therefore, various measures have been taken in order to obtain a reflecting mirror for a laser which requires a small number of layers and is therefore inexpensive and has high laser resistance. For example, by using two kinds of high refractive index materials having different refractive indexes, a high refractive index film on the incident side of laser light is formed by a high refractive index material having a lower refractive index and smaller absorption, A high refractive index material having a high absorption and a high refractive index film formed on the substrate side (see US Pat. No. 3,853,386), and λ /
In the four films, a peak of the electric field intensity in the film occurs at the boundary surface of the films on the laser light incident side. Therefore, the film thickness of each film is λ / 4 (1/4).
Of a high refractive index film that suppresses the electric field strength in the film to a low value by shifting the wavelength optical film thickness (JH ApfeL Ap).
pl. Opt. 16, p1880 (1977), U.S. Pat. No. 4,147,409) and the like have been developed.

Further, in Japanese Patent Laid-Open No. 2-204702 and Japanese Patent Laid-Open No. 4-145677, a film structure using a high refractive index film having a low refractive index on the incident side of laser light, for example, "air / (ZrO ₂ / SiO ₂ ) ^m / (TiO ₂ / SiO
₂ ) ⁿ / substrate ”or“ air / (Al ₂ O ₃ / SiO ₂ ).
^m / (HfO ₂ / SiO ₂ ) ⁿ / substrate ”, the number of pairs m having the higher refractive index film with the higher refractive index is determined based on the required reflectance and the laser resistance of each high refractive index film. Selection is disclosed. Furthermore, if it is difficult to increase the peak reflectivity or widen the high reflectivity bandwidth only with a dielectric multilayer film, a method has been developed to reduce the number of layers required by combining a metal film with the dielectric multilayer film. (See US Pat. No. 4,714,306), and the center wavelength of the dielectric multilayer film is adjusted to the laser wavelength, and the reflectance of the metal film is used for light with low energy such as alignment light and lamp light. A multilayered dielectric film as a reflection enhancing film to compensate for the reflectance of a metal film, or a reflective mirror for an excimer laser or the like, which has a dielectric multilayer film laminated on an aluminum metal film. Have also been developed (Japanese Patent Laid-Open Nos. Sho 63-208801 and Sho 6).
(See 4-76788, U.S. Pat. No. 4,856,019)

[0005]

However, according to the above-mentioned conventional technique, in order to improve the laser resistance of the dielectric multilayer film, the optical characteristics required when only the high refractive index film having a low refractive index is used. To increase the total number of layers to obtain the laser reflection mirror, it is preferable to combine a high refractive index film with a high refractive index and a high refractive index film with a low refractive index to reduce the required number of layers. It becomes difficult to obtain good optical characteristics.
That is, when two kinds of high-refractive index materials having different refractive indexes are used, the laser resistance is improved but the high-reflectance bandwidth is remarkably reduced as compared with a film structure having only a high-refractive index film having a high refractive index. Optical properties tend to be significantly reduced. Therefore, it is extremely difficult to find a film structure that has high laser resistance, is inexpensive, and has excellent optical characteristics.

Further, in the case of a reflecting mirror in which a dielectric multilayer film is combined with a metal film, if the number of layers of the dielectric multilayer film is increased, the optical characteristics and the laser resistance are improved, but the number of layers is increased. Only the manufacturing cost becomes high. As described above, it is extremely difficult to obtain a reflecting mirror having the required laser resistance and optical characteristics with a small number of layers even when a metal film is combined.

The present invention has been made in view of the above-mentioned unsolved problems of the prior art, and has excellent optical characteristics and sufficient laser resistance, and the total number of layers is small, so that the manufacturing cost is low. It is an object of the present invention to provide a low-reflectance mirror for a laser.

[0008]

In order to achieve the above object, a laser reflecting mirror of the present invention has a substrate and a dielectric multilayer film laminated on the surface thereof, and the dielectric multilayer film is on the air side.
And a second portion on the substrate side,
The first part is the first high refractive index film and a double of multiple a high yield strength
A plurality of low refractive index films are alternately laminated,
In the portion, a plurality of second high refractive index films having a higher refractive index than the first high refractive index film and a plurality of low refractive index films are alternately laminated.
Te Do Ri, the first high refractive index film in the film field strength of the maximum value E ₁ and the maximum value E of the film in an electric field intensity of the second high refractive index film
It is characterized in that the following relationship is established between the _two .

[0009]     E₂<0.20 · E₁・ T₂/ T₁   Where T₁: Single layer laser proof stress of the first high refractive index film           T₂: Single layer film laser resistance of the second high refractive index film In addition, the metal film formed on the surface of the substrate and the metal film formed on the metal film
Having a dielectric multilayer filmIs multipleHigh
With refractive index filmpluralLow refractive index filmAnd are stacked alternately
The maximum value E of the in-film electric field strength of the high refractive index film₃And the above
Maximum value E of the electric field strength in the film of the metal film_FourThe following relationship between
It is characterized by being configured to hold
Good.

E ₄ <E ₃ · T ₄ / T ₃ where T _{3 is} the monolayer film laser proof strength of the high refractive index film T _{4 is} the monolayer film laser proof strength of the metal film.

[0011]

[Operation] The first when the dielectric multilayer film is irradiated with laser light
Of the first high refractive index film so that the above-mentioned relationship is established between the maximum value E ₁ of the in-film electric field strength of the high refractive index film and the maximum value E ₂ of the in-film electric field strength of the second high refractive index film. With a film structure in which the number of layers of the film is set, the damage of the film by the laser beam starts within the first high refractive index film, which has a high yield strength. It is possible to secure sufficient durability as a mirror. Therefore, by selecting one having a small number of layers of the first high refractive index film from such a film structure, by using the first high refractive index film having a low refractive index and a high yield strength in general, By suppressing the unavoidable deterioration of optical characteristics and increase in the number of layers of the entire dielectric multilayer film,
It is possible to realize an inexpensive laser reflecting mirror having excellent optical characteristics and high laser resistance.

Further, in the case of a laser reflecting mirror composed of a metal film and a dielectric multilayer film, the maximum value E ₃ of the in-film electric field strength of the high refractive index film and the film of the metal film when the laser reflecting mirror is irradiated with laser light. If the film structure is such that the number of pairs of the dielectric multilayer film is set so that the above relationship is established between the maximum value E ₄ of the internal electric field strength, the damage of the film structure by the laser beam starts in the dielectric multilayer film. Therefore, the overall laser proof strength is high, and sufficient durability as a laser reflecting mirror can be secured.

Therefore, by selecting a dielectric multilayer film having a small number of pairs from such a film structure, it is possible to realize a laser reflecting mirror having high laser resistance and low manufacturing cost.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a laser reflecting mirror M according to the first embodiment.
₁ shows a film structure of _{No. 1} , which has a peak reflectance of 98.
It is a reflecting mirror for the third harmonic (355 nm) of an Nd: YAG laser having a high yield strength of 5% or more.

The reflecting mirror M ₁ for laser is formed by laminating 6 pairs of a SiO ₂ film 2 which is a low refractive index film and a ZrO ₂ film 3 which is a _second high refractive index film on the surface of the substrate 1, and further, a SiO ₂ film 4 is low refractive index film, an Al ₂ O ₃ film 5 which is the first high-refractive index film refractive index than the ZrO ₂ film 3 is a high strength low 9
It has a dielectric multilayer film which is a highly reflective film laminated in pairs, each film is a λ / 4 film, and the overall film structure is “air / (Al ₂ O ₃
/ SiO ₂ ) ⁹ / (ZrO ₂ / SiO ₂ ) ⁶ / substrate ". In this film structure, all high refractive index films have Zr
Laser resistance is higher than that of an O ₂ film, and the number of layers required to make a reflecting mirror with a peak reflectance of 98.5% compared to when all high refractive index films are Al ₂ O ₃ films The present invention realizes an inexpensive reflecting mirror for a laser, which has a high reflectance and a wide bandwidth of a reflectance of 95% or more, has excellent optical characteristics, and has sufficient laser resistance.

FIG. E. Newnam: NBS
Spec. Pub. 19 is a graph showing the results of calculating the in-film electric field intensity of the laser reflection mirror M _{1 by} a known method as shown in 372, p123 (1972). As can be seen from this figure, with the film structure shown in FIG. 1, the maximum in-film electric field strength E ₁ of the Al ₂ O ₃ film 5 of the dielectric multilayer film and the ZrO ₂ film 3 are formed.
The following relationship holds between the maximum in-film electric field strength E ₂ of

E ₂ <0.20 · E ₁ · T ₂ / T ₁ = e (1) where, T ₁ : single layer film laser resistance of each Al ₂ O ₃ film 5 T ₂ : each ZrO Single-layer film of ₂ film 3 If the maximum in-film electric field strength E ₂ of a high-refractive index film such as a ZrO ₂ film having a high laser resistance bending rate but a low laser resistance becomes larger than the value e shown in equation (1), It has been clarified by the experiments described later that the laser resistance of the laser is significantly reduced. This is because if the film structure in which damage due to laser light irradiation starts within a high-refractive-index film having a high refractive index such as an Al ₂ O ₃ film having a low refractive index, the entire dielectric multilayer film has a high laser-resistant strength and a high refractive index. There is a high tendency that becomes a film structure that begins with the ZrO ₂ film high-refractive-index film, such as a dielectric multilayer film overall laser damage threshold is significantly decreased, ZrO ₂
Low yield strength of film, etc. Maximum value of electric field strength E ₂ in film in high refractive index film
It is presumed that when the value e exceeds the value e shown in the formula (1), the damage due to the laser beam starts at the uppermost layer in the low proof stress high refractive index film (ZrO ₂ film or the like).

Therefore, if the number of layers of high yield strength and high refractive index films such as Al ₂ O ₃ film is set so as to satisfy the condition shown in the equation (1), it is sufficient to enhance the laser yield strength of the reflecting mirror. is there. Therefore, in the film constitution satisfying the formula (1), Al ₂
By estimating a film configuration in which the number of layers of a high proof stress and high refractive index film such as an O ₃ film is a minimum value or slightly larger than this, an Al having a high laser proof strength can be used instead of a ZrO ₂ film having a low laser proof strength.
By using a ₂ O ₃ film or the like, it is possible to obtain a laser reflecting mirror having high laser resistance, low cost and excellent optical characteristics, by suppressing the deterioration of optical characteristics and the increase in the number of necessary layers which cannot be avoided. . Next, an experimental example will be described.

(First Experimental Example) First, a film material having a small absorption was selected, and the laser resistance (single-layer film laser resistance) and the refractive index of each single-layer film state were measured. Table 1 shows the results of measuring the relationship between the film material and the laser proof strength and refractive index. The pulse width was 0.5 ns, and the unirradiated portion was irradiated with different energy for each shot, and the threshold energy of the laser light when the film was broken was measured.

[0021]

[Table 1] From Table 1, Al ₂ O, which is a high refractive index film with high laser resistance
It is speculated that when a dielectric multilayer film is manufactured by alternating layers of _three films and a SiO ₂ film which is a low refractive index film, a reflecting mirror with high durability is manufactured. However, the refractive index of SiO ₂ is 1.477,
Since the refractive index of Al ₂ O ₃ is 1.623, and the difference in refractive index is smaller than that in the case of combining the ZrO ₂ film and the SiO ₂ film, about 48 layers are required to make the reflectance 98.5% or more. Becomes Further, in the case of an alternate multilayer film of ZrO ₂ film and SiO ₂ film, 18 layers have a content of 98.5% or more, but it is presumed that the laser proof strength becomes low due to the influence of the ZrO ₂ film.

Therefore, in order to obtain a film structure of a highly reflective film having a high laser resistance, a small number of required layers, a peak reflectance of 98.5% or more and a wide bandwidth of a high reflectance, air / (Al ₂ O 3 ₃ / SiO ₂ ) ^m / (ZrO ₂ / Si
O ₂ ) ⁿ / m and n of the film structure of the substrate were changed, and the optical characteristics and laser proof strength were measured. The incident angle of the laser light was 0 degree, and the film thicknesses were all 1/4 wavelength optical film thicknesses.

First, regarding the m and n values, the peak reflectance is 98.
It was determined by simulation so that it would be 5 or more. In Table 2,
According to the simulation, the m and n values of the film structure having the peak reflectance of 98.5% or more and the 95% reflectance bandwidth Δλ (R
= 95%) and the maximum values of the in-film electric field strengths E ₁ and E of the Al ₂ O ₃ film and the ZrO ₂ film in the respective film configurations.
_The result of calculating ₂ is shown.

The calculation of the electric field strength in the film is performed as described above.
B. E. Newnam: NBS Spec. Pub. Three
72, p123 (1972), calculated based on known materials.

FIG. 3 is a graph in which the horizontal axis is based on the results of Table 2 as a ratio of the number of (ZrO ₂ / SiO ₂ ) pairs to the total number of pairs.

[0026]

[Table 2] As shown in the simulation results shown in Table 2 and FIG. 3, when the number of pairs (ZrO ₂ / SiO ₂ ) n increases, the number of required layers decreases, the high reflectance bandwidth expands, and the optical characteristics improve. When the electric field strength in the ZrO ₂ film increases and exceeds the value e shown in the formula (1), the ZrO ₂ film having low laser resistance is damaged, and as a result, the laser resistance of the entire dielectric multilayer film is significantly reduced. .

Therefore, a high reflection film (dielectric multilayer film) having these film structures is actually manufactured, and its high reflectance bandwidth Δλ is obtained.
(R = 95%) and laser resistance were measured. The laser proof strength was measured in the same manner as the single layer laser proof strength shown in Table 1. The third harmonic of the YAG laser (wavelength 355 nm, pulse width 0.5 ns)
Was used to change the energy on a new surface of the film for each irradiation, and the threshold energy at which damage occurred was measured. In addition, the damaged part was observed in detail with a Nomarski microscope.

The measurement results are shown in FIG. From this figure, the film structure "air / (Al ₂ O ₃ / SiO ₂ ) ^m / (ZrO ₂ /
SiO ₂ ) ⁿ / substrate ”(Al ₂ O ₃ / SiO _2
2 ) It can be seen that in the film structure in which the number of pairs m is 24 to 9, the laser proof strength remains almost unchanged and decreases sharply when m becomes smaller than 9. From the observation of the damaged portion with a Nomarski microscope, it was confirmed that the surface of the dielectric multilayer film was damaged when m was 24 to 9, and the uppermost layer of Al ₂ O
_There was damage on the _three membranes. Also, it was confirmed that when m was smaller than 9 and the number of (ZrO ₂ / SiO ₂ ) pairs n was larger than 6, damage was generated inside the film, and damage was first generated in the uppermost ZrO ₂ film. It seems that

In this way, it was clarified that the laser resistance is high in the film structure in which damage is first caused in the Al ₂ O ₃ layer, but the laser resistance is lowered in the film structure in which damage is started in the ZrO ₂ layer. .

Therefore, in the film structure of the high reflection film, which has a high laser resistance, a small number of layers, a wide bandwidth of high reflectance, and a low incidence of defective products due to manufacturing errors, the layer in which damage occurs first is Al. It has been found that a film structure having a slightly larger number of Al ₂ O ₃ layers is more optimal than a film structure at the boundary where the ₂ O ₃ layer moves to the ZrO ₂ layer.

That is, it is a reflecting mirror for the third harmonic (355 nm) of an Nd: YAG laser, has a high laser resistance, has a small number of layers, and has a wide high reflectance bandwidth so that there are few defects due to manufacturing errors. The optimal film structure of the load-bearing mirror is air / (Al ₂ O ₃ / SiO ₂ ) ⁹ / (ZrO ₂ / Si
It has been clarified that the laser resistance is sufficient if the film structure is O ₂ ) ⁶ / substrate, and the film structure having a large number of Al ₂ O ₃ / SiO ₂ pairs is sufficient.

Further, from FIG. 3 and Table 2, the damage is first
The resulting layer is Al₂ O₃ Layer to ZrO ₂ Boundary membrane moving to layers
In the structure, ZrO₂ Maximum electric field strength E of the film₂ And A
l₂O₃ , ZrO₂ Single layer film laser resistance T of film₁ , T₂ 
And Al₂ O₃ The maximum electric field strength of the film is E₁ Between the following functions
It turned out to be involved. Therefore, as described above,
Satisfies the relationship of formula (1) by examining the electric field strength in the film of the film.
Like Al₂ O₃ Same as above by setting the number of membrane layers
The optimal membrane configuration can be estimated.

E ₂ = 0.20 · E ₁ · T ₁ / T ₂ = e (Second Experimental Example) High reflectivity band so that laser resistance is high, the number of layers is small, and there are few defective products due to manufacturing errors. The optimum film configuration of the high reflection film for a KrF excimer laser (wavelength 248 nm) having a wide width and a peak reflectance of 95.8% or more was examined.

KrF excimer laser resistance of a single layer film of a film material that can be used at a wavelength is set to KrF with a pulse width of 20 ns.
It was measured using an excimer laser. Table 3 shows the measurement results.

[0035]

[Table 3] From this result, “air / (Al ₂ O ₃ / SiO ₂ ) ^m /
When the m and n values of the film structure of “(HfO ₂ / SiO ₂ ) ⁿ / substrate” were determined so as to satisfy the condition of the formula (1), the layer where damage started to occur by laser light irradiation was an Al ₂ O ₃ layer. The film composition of the boundary from the HfO ₂ layer to the HfO ₂ layer is air / (Al ₂ O ₃ / SiO ₂ ) ¹² / (HfO ₂ / Si
It was estimated to be an O ₂ ) ⁵ substrate.

Therefore, a high-reflection film having the following three types of film constitution was actually manufactured, and the laser resistance thereof was set to K with a pulse width of 20 ns.
It measured using the rF excimer laser. The incident angle was 0 degree, and the film thickness of each layer was ¼ wavelength optical film thickness.

Air / (Al ₂ O ₃ / SiO ₂ ) ²⁴ / Substrate Air / (Al ₂ O ₃ / SiO ₂ ) ¹² / (HfO ₂ /
The measurement result of SiO ₂ ) ⁵ / substrate air / (HfO ₂ / SiO ₂ ) ⁹ / substrate laser proof strength was 5.0 J / cm ² , 4.
8 J / cm ^2, was 2.0 J / cm ^2, the laser strength hardly changes, therefore, be easily manufactured widely reduced and high reflectivity bandwidth the number of layers towards the film structure is a film structure of the Is clear.

Similar results were obtained by using a Sc ₂ O ₃ film having almost the same laser resistance and refractive index instead of the HfO ₂ film.

(Third Experimental Example) Ar having a high laser resistance, a small number of layers required, a wide high reflectance bandwidth so that there are few defective products due to manufacturing errors, and a peak reflectance of 99% or more.
We have developed a high yield strength and high reflection film for F excimer laser (wavelength 193nm).

In the experimental development procedure, first, a film material having a small absorption was selected, and the laser resistance and the refractive index of these single layer films were measured. Table 4 shows the results of measuring the film material, the laser proof stress, and the refractive index.

The pulse width was 20 ns, and the non-irradiated portion of the film surface was irradiated with different energy for each shot, and the threshold energy of the laser beam when the film was broken was measured.

[0042]

[Table 4] From this result, high refractive index material GdF with high laser resistance
_{When a} high reflection film is manufactured by alternate layers of a film selected from ₃ , NdF ₃ and LaF ₃ HoF ₃ and a film selected from low refractive index materials SiO ₂ , MgF ₂ and AlF ₃ , laser light irradiation is performed. It is speculated that a film with high yield strength will be manufactured.

[0043] However, the use of MgF ₂ as a low refractive index film, film stress occurs is large film cracking problems, AlF ₃
If a film is used, GdF ₃ , NdF ₃ , LaF ₃ , HoF ₃
Since the refractive index of the high refractive index film such as 1.60 is 1.60 to 1.66, about 70 layers are required to form a reflecting mirror having a reflectance of 99% or more.

Further, the Al ₂ O ₃ film having a high refractive index and SiO
_{In the case of} an alternating multi-layer film of _two films, the number of 40 layers is 99% or more, but it is presumed that the laser proof strength becomes low due to the influence of the Al ₂ O ₃ film.

Therefore, in order to obtain a film structure of a high reflection film (dielectric multilayer film) having a high laser resistance, a small number of layers, and a wide high reflectance band width, air / (GdF ₃ / SiO ₂ ) ^m / (Al ₂ O ₃ / Si
O ₂ ) ⁿ / m and n of the film structure of the substrate were changed, and the optical characteristics and laser proof strength were measured. The incident angle of the laser light was 0 degree, and the film thicknesses were all 1/4 wavelength optical film thicknesses.

First, for the m and n values, the peak reflectance is 99.
It was calculated by simulation so as to be 5% or more. Table 5
Further, by simulation, the m and n values of the film structure having the peak reflectance of 99% or more and the 95% reflectance bandwidth Δλ (R
= 95%) and Gd of the electric field strength in the film in each film constitution.
The results of calculating the maximum values E ₁ and E ₂ in the F ₃ film and the Al ₂ O ₃ film are shown.

[0047]

[Table 5] As shown in the simulation result of Table 5, (Al ₂ O ₃
/ SiO ₂ ) When the number of pairs increases, the number of required layers decreases and the high reflectance bandwidth increases, which is advantageous in manufacturing, but the maximum electric field strength in the Al ₂ O ₃ film increases, and Al ₂ with low laser resistance O ₃
It is considered that the film is likely to be damaged.

Therefore, a high reflection film having such a film structure was actually manufactured and its 95% reflectance bandwidth Δλ (R = 95%)
And the laser proof strength was measured. Laser resistance is measured by ArF
Excimer laser (wavelength 193 nm, pulse width 20 ns)
The threshold energy for damage was measured by changing the energy on a new surface of the film with each irradiation. Also,
The damaged part was observed in detail with a Nomarski microscope. The measurement results are shown in Table 6.

[0049]

[Table 6] From this result, "air (GdF ₃ / SiO ₂ ) ^m / (A
l ₂ O ₃ / SiO ₂ ) ⁿ / substrate ”film structure (GdF ₃ /
It can be seen that when the number of pairs of SiO ₂ ) m is 35 to 23, the laser proof strength hardly changes, and when m becomes smaller than 18, it sharply decreases. In the observation of the damaged portion with a Nomarski microscope, it was confirmed that the m changed from 24 to the internal damage of the reflective film at the boundary of 24, and the film where the damage started to occur was changed from the GdF ₃ film to the Al ₂ O ₃ film. It seems to have changed.

That is, the laser resistance is high in the film structure in which the damage is first generated in the GdF ₃ layer, but the laser resistance is lowered in the film structure in which the damage is first generated in the Al ₂ O ₃ layer. The same applies to the case, and therefore, the film structure of the high-reflection film having a high laser resistance, a small number of layers, and a wide high-reflectance bandwidth so that there are few defects due to manufacturing errors is such that the layer in which damage first occurs is GdF ₃ Layer from Al ₂
It was found that the film structure having a slightly larger number of GdF ₃ layers was more suitable than the film structure at the boundary where the O ₃ layer was transferred.

Thus, the layer in which damage first occurs is Gd.
The film structure of the boundary from the F ₃ layer to the Al ₂ O ₃ layer is as shown below. This is the most (GdF ₃ / SiO ₂ ) pair among the film structures satisfying the condition of the formula (1). There are few.

Air / (GdF ₃ / SiO ₂ ) ²³ / (Al
₂ O ₃ / SiO ₂ ) ⁷ / Substrate (fourth experimental example) Laser reflectivity is high, the number of layers is small, and the high reflectance bandwidth is wide so that there are few defects due to manufacturing errors.
Nd: YAG laser with a peak reflectance of 99.5% or more (1
We have developed a high yield strength and high reflective film for 064 nm).

In the development procedure, first, a film material having a small absorption was selected and the single layer film laser proof strength and the refractive index thereof were measured. Table 7 shows the results of measuring the film material, the laser proof stress, and the refractive index. The pulse width was 1 ns, and the unirradiated portion of the film surface was irradiated with different energy for each shot, and the threshold energy of laser light at which the film was broken was measured.

[0054]

[Table 7] From this result, a high refractive index material Al ₂ O having high laser resistance
₃ , a film selected from ZrO ₂ and a low refractive index material SiO
It is presumed that if a high-reflection film is made of alternating layers of films selected from ₂ and MgF ₂ , a high-reflection film with high resistance to laser light irradiation is produced.

However, when MgF ₂ is used as the low refractive index film, there is a problem that the film stress is large and the problem of film cracking occurs. Therefore, using SiO ₂ as the low refractive index film,
For the high refractive index films of ZrO ₂ and Al ₂ O ₃ having almost the same laser resistance, a ZrO ₂ film having a large refractive index was selected. ZrO ₂
Peak reflectivity of 99.5% in alternate multilayer film of SiO ₂ film and SiO ₂ film
The number of layers as described above was 26 layers.

Further, the TiO ₂ film having a high refractive index and the SiO ₂ film are
In the case of alternating multi-layer films, 16 layers have 99.5% or more, but it is presumed that the laser proof strength is lowered due to the influence of the TiO ₂ film.

Therefore, in order to obtain a film structure of a highly reflective film having a high laser resistance, a small number of layers, and a wide high reflectance bandwidth, air / (ZrO ₂ / SiO ₂ ) ^m / (TiO ₂ / SiO _2
2 ) The optical characteristics and laser proof strength were measured by changing m and n of the film structure of the ⁿ substrate. The incident angle of the laser light was 0 degree, and the film thicknesses were all 1/4 wavelength optical film thicknesses.

First, for the m and n values, the peak reflectance is 99.
It was calculated by simulation so as to be 5% or more. Table 8
Further, by simulation, the m and n values of the film structure having the peak reflectance of 99% or more and the 95% reflectance bandwidth Δλ (R
= 95%) and Zr of the electric field strength in the film in each film constitution
The results of calculating the maximum values E ₁ and E ₂ in the O ₂ film and the TiO ₂ film are shown.

[0059]

[Table 8] As the simulation results in Table 8 show (TiO ₂ /
When the number of pairs of SiO ₂ ) increases, the total number of layers decreases and the high reflectance bandwidth increases, which is advantageous in manufacturing. However, the maximum electric field strength in the TiO ₂ film increases, and a TiO ₂ film with low laser resistance is used. Damage is likely to occur.

Therefore, a high-reflecting film having such a film structure is actually manufactured, and its 95% reflectance bandwidth Δλ (R = 95%).
And the laser proof strength was measured. Laser resistance measurement is Nd:
A YAG laser (wavelength 1064 nm, pulse width 1 ns) was used to change the energy on a new surface of the film for each irradiation, and the threshold energy at which damage occurred was measured. In addition, the damaged part was observed in detail with a Nomarski microscope. The measurement results are shown in Table 9.

[0061]

[Table 9] From this result, "air / (ZrO ₂ / SiO ₂ ) ^m /
(TiO ₂ / SiO ₂ ) ⁿ / substrate ”film constitution of (ZrO ₂
It can be seen that when the number of pairs of / SiO ₂ ) m is 13 to 4, the laser proof strength hardly changes, and when m becomes smaller than 4, it sharply decreases. In the observation of the damaged portion with a Nomarski microscope, it was confirmed that the surface damage of the reflective film was changed to the damage inside the film when m was 4 as a boundary, and the film where the damage began to be changed from the ZrO ₂ film to the TiO ₂ film. Presumed to be.

That is, the laser resistance is high in the film structure in which damage is first caused in the ZrO ₂ layer, but the laser resistance is lowered in the film structure in which damage is first caused in the TiO ₂ layer. It has been found that the same applies to the wavelength (1064 nm).

Thus, the layer in which damage first occurs is Zr.
The film structure at the boundary from the O ₂ layer to the TiO ₂ layer is as shown below. This is the film structure with the smallest number of (ZrO ₂ / SiO ₂ ) pairs among the film structures satisfying the formula (1). is there.

Air / (ZrO ₂ / SiO ₂ ) ⁴ / (Ti
O ₂ / SiO ₂ ) ⁶ / Substrate FIG. 5 shows a film structure of the laser reflecting mirror M ₂ according to the second embodiment, which is a KrF having a peak reflectance of 99% or more.
It is a reflecting mirror for an excimer laser (248 nm).

The reflecting mirror M ₂ for laser is provided with a metal film 12 of Al (aluminum) on the surface of a substrate 11, and an Al ₂ O ₃ film 13b which is a high refractive index film and a SiO which is a low refractive index film on the metal film 12. A high-reflection film in which a dielectric multilayer film consisting of _two films 13a16 pairs is laminated and each film is a λ / 4 film, and the overall film configuration is "air / (Al ₂ O ₃ / SiO ₂ ) ¹⁶ / Al / substrate. ”. This film structure has the above-mentioned optical characteristics, has a sufficient laser resistance against the KrF excimer laser, and has a small number of (Al ₂ O ₃ / SiO ₂ ) pairs, and thus has a low manufacturing cost. It has been proved by an experiment to be described later that it is the most suitable mirror for a vehicle. Further, the following relationship between the maximum value E ₃ in the film field strength of the Al ₂ O ₃ film of the maximum value E ₄ and the dielectric multilayer film having an electric field strength of the metal film is established, the first experimental example Similarly, the optimum film structure can be estimated by examining the electric field strength in the film.

E ₄ <E ₃ · T ₄ / T ₃ (2) where, T ₃ : Al ₂ O ₃ film single layer film laser proof strength T ₄ : Metal film single layer film laser proof strength Next, an experimental example will be described.

(Fifth Experimental Example) First, a material of a film having a small absorption and a material of a metal film having a high reflectance were selected, and respective single layer film laser proof strengths and refractive indexes were measured. The only metal film having a high reflectance at a wavelength of 248 nm was Al.

Table 10 shows the results of measuring the film material, the laser proof strength, and the refractive index. For the measurement of the laser resistance, a KrF excimer laser with a pulse width of 20 ns was used and the unirradiated portion of the film surface was irradiated with different energy for each shot, and the threshold energy of the laser light at which the film was broken was measured.

[0069]

[Table 10] From this result, a high refractive index material Al ₂ O having high laser resistance
It can be seen that when a highly reflective film is made of the dielectric multilayer film of ₃ and the low refractive index material SiO ₂ and the metal film of Al, a film having high resistance to laser irradiation is manufactured. Therefore, the substrate _{/ Al / (SiO 2 / Al} 2 O 3) m / dielectric incident angle of 0 degrees with different number of pairs m of the multilayer film of the air film structure produced a highly reflective film, reflectance and laser damage threshold Was measured. In addition, the in-film electric field strength of each film structure was also calculated.

FIG. 6 shows the number of pairs of dielectric multilayer films, m and 248.
When the reflectance in nm is measured, it can be seen that the reflectance is 98.5% or more when m is 8 pairs or more. m = 12, 20
An example of the measurement result of the spectral characteristics of is shown in FIG.

Further, FIG. 8 shows the measurement results of the laser proof strength of the highly reflective film having each film structure. Laser proof is pulse width 2
Using a 0 ns KrF excimer laser, the unirradiated portion of the film surface was irradiated with different energy for each shot, and the threshold energy of the laser light at which the film was broken was measured. When m was 16 pairs or more, the laser proof strength was as high as the laser proof strength of the high reflection film having only the dielectric multilayer film. It is considered that this is because the film destruction due to laser irradiation occurs in the dielectric multilayer film, particularly in the Al ₂ O ₃ film having a high monolayer laser resistance. Also,
When m is 8 pairs or less, the laser resistance is low, and it is considered that the Al metal film having low single layer laser resistance is damaged. When a damaged part in which m is 16 or more and a damaged part in which m is 8 or less are observed with a Nomarski microscope, it is found that the former does not cause the destruction of the Al metal film, but the latter does cause the destruction of the Al film. confirmed.

It is considered that the electric field strength in the film greatly depends on the film breakdown of the high reflection film, and the result of calculating the maximum electric field strength in the metal film of each film constitution is shown in FIG. From this result, the film configuration in which the layer in which film destruction starts to occur by laser irradiation is changed from the metal film to the dielectric multilayer film is m = 16. m = 16
The maximum values of the in-film electric field strengths of the metal film and the Al ₂ O ₃ film in the above film structure are E ₄ , E ₃ , respectively,
It has been found that the following relations are established when the single layer laser proof stress of the ₂ O ₃ film is T ₄ and T ₃ .

E ₄ = E ₃ · T ₄ / T ₃ (Sixth Experimental Example) Al with a high reflection film for KrF excimer laser
We investigated a film structure that uses the above metal film to reduce the number of film layers and has the same laser resistance as a high-reflection film with only a dielectric multilayer film. To increase the high reflectance band, the film materials in Table 10 have lower laser proof strength than Al ₂ O _3, but HfO ₂ with a high refractive index is used, and substrate / Al / (SiO ₂ / HfO ₂ ) ^m / air A high yield strength and high reflection film with the above film structure was manufactured.

In order to design the number of pairs m of the optimum film configuration, H
fO ₂ film and Al metal film each monolayer laser damage threshold T ₃ of, and T ₄ and the HfO ₂ film up film in an electric field strength E ₃ of the metal film of Al a maximum electric field strength E ₄ calculated by the following equation .

E ₄ = E ₃ · T ₄ / T ₃ As a result, E ₄ = 0.128, and the optimum film configuration is m =
It was calculated to be 4.

Therefore, (A) a film structure "substrate / Al / (Si
O ₂ / HfO ₂ ) ⁴ / air ”, and (B). High-reflectivity film consisting of only dielectric multilayer film
The reflectance and the laser proof stress of “(SiO ₂ / HfO ₂ ) ⁸ / air” were compared.

The reflectivity of both (A) and (B) is 99% or more, and the laser resistance is (A) 2.3 J / cm ² , (B).
Was 2.4 J / cm ² which was almost the same value.
From this, it has been found that a film reflector having a number of (SiO ₂ / HfO ₂ ) pairs of 4 can realize an inexpensive laser reflecting mirror having sufficient laser resistance.

(Seventh Experimental Example) With a film structure in which a metal film and a dielectric multilayer film are combined, the number of layers is smaller than that of a high reflection film having only a dielectric multilayer film, and a high reflection film having only a dielectric multilayer film is provided. We have developed a high-strength, high-reflection film for ArF excimer lasers (193 nm) with a laser resistance and a peak reflectance of 99% or more.

The development procedure was as follows. First, a film material having a small absorption at a wavelength of 193 nm was selected, and a single layer film laser proof stress and a refractive index were measured. As the metal film, Al showing high reflection at a wavelength of 193 nm was selected, and the laser resistance of the single layer film was measured. Table 11
The results of measurement of the film material, laser proof strength, and refractive index are shown in Fig.

For the measurement of the laser resistance, an ArF excimer laser with a pulse width of 20 ns was used, and the unirradiated portion of the film surface was irradiated with different energy for each shot, and the threshold energy of the laser light at which the film was broken was measured.

[0081]

[Table 11] From this result, (A) “Substrate / Al / (SiO ₂ / Al
₂ O ₃ ) ^m / air ”, (B)“ Substrate / Al / (MgF ₂
/ Al ₂ O ₃₎ ^m / air ", (C)" substrate / Al / (A
1F ₃ / GdF ₃ ) ^m / air ”, the number of layers is smaller than that of the high reflection film having only the dielectric multilayer film, and the laser resistance is about the same as the high reflection film of only the dielectric multilayer film. , ArF excimer laser with peak reflectance of 99% or more (193
The film design of the high yield strength and high reflection film of (nm) was performed in the same manner as in the sixth experimental example, and the value of the number of pairs m of the dielectric multilayer film was calculated.

As a result, the following was obtained.

(A) Optimal film configuration. Substrate / Al / (Si
Optimal film composition of O ₂ / Al ₂ O ₃ ) ¹¹ / air (B). Substrate / Al / (MgF ₂ / Al ₂
Optimal film composition of O ₃ ) ⁶ / air (C). Substrate / Al / (AlF ₃ / GdF
₃ ) ⁸ / air Then, the reflecting mirror having the optimum film structure of (A), (B), and (C) and the following film structures (E), (F), (only for the dielectric multilayer film for comparison. The reflector of G) was manufactured, and the reflectance and the laser proof strength were measured.

(E). Substrate _{/ (SiO 2 / Al 2 O} 3) 21 / air (F). Substrate _{/ (MgF 2 / Al 2 O} 3) 12 / air (G). Substrate / (AlF ₃ / GdF ₃ ) ¹⁶ / air Table 12 shows the measurement results.

[0085]

[Table 12] As described above, when the film structure is designed based on the electric field strength in the film based on the formula (2), the film structure is a combination of the Al metal film and the dielectric multilayer film, and the number of layers is higher than that of the high reflection film having only the dielectric multilayer film. A, which has a peak reflectivity of 99% or more and has a laser proof strength comparable to that of a high reflection film having only a dielectric multilayer film.
A high yield strength and high reflection film for rF excimer laser (193 nm) can be manufactured.

(Eighth Experimental Example) A high yield strength and high reflection film for Nd: YAG laser (1064 nm) having a peak reflectance of 99% or more is a combination of a metal film and a dielectric multilayer film as in the case of KrF and ArF excimer lasers. It is possible to fabricate a high-reflection film with a different film structure, which has a smaller number of layers than a high-reflection film having only a dielectric multilayer film, and which has a laser resistance equivalent to that of a high-reflection film having only a dielectric multilayer film. It confirmed using the membrane material of Table 13. Table 13 shows the measurement results of the single-layer film laser proof stress of the film material and the metal film that can be used for the Nd: YAG laser reflecting mirror. The procedure for developing the optimum film configuration is the same as in the fifth to seventh experimental examples.

[0087]

[Table 13]

[0088]

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

It is possible to realize a laser reflecting mirror having excellent optical characteristics and sufficient laser resistance, and having a small number of layers as a whole and a low manufacturing cost.

[Brief description of drawings]

FIG. 1 is a diagram showing a film structure of a laser reflecting mirror according to a first embodiment.

FIG. 2 is a graph showing a distribution of in-film electric field strength of the first example.

FIG. 3 shows (ZrO ₂ / SiO ₂ ) in the first experimental example.
It is a graph which shows the result of having investigated the relationship of the ratio of the number of pairs, the electric field strength in a film, and the high reflectance bandwidth.

FIG. 4 shows (ZrO ₂ / SiO ₂ ) in the first experimental example.
It is a graph which shows the result of having investigated the relationship between the ratio of the number of pairs and laser proof strength.

FIG. 5 is a diagram showing a film configuration of a laser reflecting mirror according to a second embodiment.

FIG. 6 is a graph showing the results of examining the relationship between the number of pairs of dielectric multilayer films and the reflectance in the fifth experimental example.

FIG. 7 is a graph showing the results of examining the frequency characteristics of reflectance in the case where the number of pairs of the dielectric multilayer film is 12 and 20 in the fifth experimental example.

FIG. 8 is a graph showing the results of examining the relationship between the number of pairs of dielectric multilayer films and laser proof stress in a fifth experimental example.

FIG. 9 is a graph showing the relationship between the number of pairs of dielectric multilayer films and the maximum value of the in-film electric field strength of a metal film in a fifth experimental example.

[Explanation of symbols]

1, 11 Substrate _{2, 4,} 13a SiO ₂ film 3 ZrO ₂ film 5, 13b Al ₂ O ₃ film 12 Metal film

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02B 5/08 G02B 5/28

Claims (11)

(57) [Claims] 1. A substrate and a dielectric multilayer film laminated on the surface thereof, the dielectric multilayer film including a first portion on the air side and a substrate.
It is composed from a second portion of the plate side, the first portion and a plurality of low refractive index film the first high refractive index film of the multiple a high yield strength
Are alternately laminated, and the second portion is the first
A plurality of second high refractive index films having a higher refractive index than the high refractive index film
Ri name and a plurality of low refractive index film are laminated alternately, the first
The following relationship is established between the maximum value E ₁ of the in-film electric field strength of the high refractive index film and the maximum E ₂ of the in-film electric field strength of the second high refractive index film. A laser reflecting mirror. E ₂ <0.20 · E ₁ · T ₂ / T ₁ where T _{1 is} the single layer film laser resistance of the first high refractive index film T _{2 is} the single layer film of the second high refractive index film Laser resistance 2. A reflecting mirror for laser light having a wavelength range of 190 to 1200 nm, which has a reflectance of 90% or more, and is used as a material for a high refractive index film of TiO ₂ , HfO ₂ , ZrO ₂ ,
Al ₂ O ₃ , Ta ₂ O ₅ , Sc ₂ O ₃ , NdF ₃ , La
F _3, GdF _3, or HoF _3, the laser reflecting mirror of claim 1, wherein SiO _2, was MgF ₂ or, characterized in that the use of AlF ₃ as a material for the low refractive index film. 3. A reflection mirror for the third harmonic of an Nd: YAG laser, wherein Al ₂ is used as the material of the first high refractive index film.
_3. The laser reflecting mirror according to claim 1, wherein O ₃ and ZrO ₂ are used as a material of the second high refractive index film. 4. A reflection mirror for a KrF excimer laser, wherein Al ₂ O _{3 is} used as the material of the first high refractive index film and HfO ₂ or Sc ₂ O ₃ is used as the material of the second high refractive index film. The laser reflecting mirror according to claim 1 or 2, wherein. 5. A reflection mirror for an ArF excimer laser, wherein NdF ₃ , La is used as a material of the first high refractive index film.
F ₃ , GdF ₃ or HoF ₃ , and Al ₂ O ₃ is used as a material for the second high refractive index film.
Alternatively, the reflecting mirror for laser according to the item 2. 6. Nd: A reflecting mirror for YAG laser, as a material of the first high refractive index film, Al ₂ O ₃ or ZrO _2, the TiO ₂ used as the material of the second high refractive index film The laser reflecting mirror according to claim 1 or 2, wherein. 7. A metal is deposited on the surface of the substrate film has a dielectric multi-layer film provided thereon, the dielectric multilayer film
Multiple high refractive index films and multiple low refractive index films are alternately stacked.
Ri Na is the maximum value E ₃ in the film field strength of the high refractive index film
And a maximum value E ₄ of the in-film electric field strength of the metal film, the following relationship is established. E ₄ <E ₃ · T ₄ / T ₃ where T _{3 is} the single layer film laser resistance of the high refractive index film T _{4 is} the single layer film laser resistance of the metal film 8. A reflecting mirror for laser light in a wavelength region of 190 to 1200 nm, which has a reflectance of 90% or more and is used as a material for a high refractive index film of TiO ₂ , HfO ₂ , ZrO ₂ ,
Al ₂ O ₃ , Ta ₂ O ₅ , Sc ₂ O ₃ , NdF ₃ , La
F _3, GdF _3, or HoF _3, claim 7, characterized by using Al, Ag, Au, Cu or Pt as the material of the low refractive index film SiO _2, MgF ₂ or AlF _3, as the material of gold Shokumaku The laser reflecting mirror described. 9. A reflection mirror for a KrF excimer laser, characterized in that Al ₂ O ₃ , HfO ₂ or Sc ₂ O ₃ is used as a material for the high refractive index film, and Al is used as a material for the metal film. Item 7. A laser reflecting mirror according to item 7 or 8. 10. A reflecting mirror for an ArF excimer laser, wherein NdF ₃ , LaF ₃ and G are used as a material for a high refractive index film.
9. The laser reflecting mirror according to claim 7, wherein dF ₃ , HoF ₃ or Al ₂ O ₃ and Al are used as the material of the metal film. 11. A reflector for an Nd: YAG laser, wherein Al ₂ O ₃ , ZrO ₂ or TiO ₂ is used as the material of the high refractive index film, and Al, Ag, Cu or Au is used as the material of the metal film. 9. The laser reflecting mirror according to claim 7 or 8, wherein: