JPS63142301A - Optical thin film influenced only slightly by cummulative effect - Google Patents

Abstract

PURPOSE:To reduce cummulative effect by constituting optical thin film of a fluoride dielectrics when a substrate consists of fluorite, and constituting it of an oxide dielectrics when the substrate consists of quartz. CONSTITUTION:In a multilayered or single layered optical thin film wherein incident light intensity of far ultraviolet laser light is >=1.5J/cm<2> when the substrate supporting the optical thin film consists of fluorite, or >=0.8J/cm<2> when the substrate consists of quartz, the thin film is constituted of a fluoride dielectrics when the substrate is constituted of fluorite, and the thin film is constituted of an oxide dielectrics when the substrate is constituted of quartz. If quartz is selected for the substrate, an oxide dielectrics having relatively high refractive index such as Al2O3 or ZrO2 or that having relatively low refractive index such as SiO2, etc. is used selectively. If fluorite is selected for the substrate, a fluoride dielectrics having relatively high refractive index such as NdF3 or LaF3, or that having relatively low refractive index such as MgF2, ThF2, etc. is used selectively. By this constitution, the cummulative effect is reduced even for optical thin film to be used for an optical system using far ultraviolet laser light.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical thin film into which high-power deep ultraviolet laser light is incident, particularly an optical thin film with little cumulative effect.

[Conventional technology]

Based on optical theory, optical thin films such as anti-reflection films, interference filters, and interference mirrors have a wavelength of λ of 4. (λ.

= reference wavelength) is a single layer or multilayer stack of thin films on a substrate with an optical thickness of approximately It consists of alternating layers of thin films.

Examples of constituent materials of the relatively high refractive index thin film include AltOff, Zr0t, Hf0z, and 5CZ.
O3, Th0z.

Examples of constituent materials of the baking soda 11 with relatively low refractive index, such as NdFs, MgO, and Yz03, include 5i02. Mg
Fg, ThFz.

5rFt, LiF, NaF, etc. are used.

On the other hand, since this type of optical thin film is extremely thin and has no self-supporting properties, it is formed on a suitable substrate, generally a glass plate.

In this case, the substrate itself may be a suitable optical element such as a lens, prism, mirror, window material, etc.

By the way, with recent advances in technology, the light used in optical systems is increasingly required to have a higher intensity and shorter wavelength, and it has finally become possible to achieve a light intensity of 500 mJ/c+d or more and a wavelength of I=19.
Deep ultraviolet laser light of 5 to 325 nm is becoming a reality. At present, the only light source for such laser light is an excimer laser.

[Problem that the invention seeks to solve]

According to the research of the present inventors, such light intensity is 500
It has been found that in optical systems that use strong far-ultraviolet laser light of mJ/cm2 or more, the optical thin film is physically or chemically damaged by the laser light. The method for measuring the strength of failure 11 is as follows.

An excimer laser of λ=248 nm is used as a light source, and the optical thin film is irradiated with 20 nsec pulsed light lXl0' times. After that, the surface of the optical thin film is observed using a differential interference microscope, and if there is any damage such as holes, it is considered as "damage".
If there is no damage, it is determined as "Non-damage (Non-damage) J."
This is repeated several times for the same type of optical Fjt film while changing the intensity, and the maximum value of the laser light intensity when it is determined to be r Non-damageJ is determined.

Similarly, the number of pulsed light irradiations, that is, the number of pulses, is 2X10'
, 3X10', 4X10', 5xlQ'----
---Increase to 1.[Non-daa+age J
Find the maximum value of the laser beam when it is determined that

Then, it was observed that there was a difference between the maximum value when the number of irradiations was small and the maximum value when the number of irradiations was large, and the maximum value when the number of irradiations was large tended to become smaller. This tendency is referred to herein as the "A product effect."

Conventionally, such a cumulative effect was not observed when the light intensity was low, but when the optical thin film is composed of an oxide dielectric, the light intensity is 0.8 J/cm2 or more, and when the optical thin film is made of an oxide dielectric, is composed of a fluoride dielectric, the light intensity is 1.5 J/cfl! Problems arise with the optical thin film upon which the far-ultraviolet laser beam enters, and as a result, the optical thin film has problems such as a short life span and low reliability.

An object of the present invention is to provide an optical thin film into which a deep ultraviolet laser beam with high light intensity is incident, and which has little cumulative effect (including cases where no cumulative effect is observed).

Means for Solving Problem c] At present, substrate materials that allow deep ultraviolet laser light are limited to marble or quartz; however, the present inventors have
As a result of extensive research, we found that when fluorite (CaFz) is used as a substrate, optical thin films can be formed using fluoride dielectrics such as MaF.
z, NdF3, etc., and the substrate is quartz (SiO
z) and the optical thin film is composed of an oxide dielectric such as 5iOz or AhQs, the light intensity is 1.5 J/el+! As mentioned above, in the case of a quartz substrate, the light intensity is 1.5 J/co! The inventors have discovered that even in the optical thin film into which the far-ultraviolet laser light is incident, the cumulative effect is small (or not observed at all), leading to the present invention.

Therefore, the present invention provides [a light intensity of 1.5 J/cj or more when the substrate supporting the optical thin film is fluorite, a light intensity of 0.8 J/cI11 or more when the substrate is quartz,
In a multi-layer or single-layer optical thin film on which deep ultraviolet laser light is incident, if the substrate is made of fluorite, the thin film is made of a heptaide dielectric; if the substrate is made of quartz, it is made of an oxide dielectric. To provide an optical thin film with little cumulative effect characterized by the following structure.

[Effect]

In the present invention, the substrate is made of marble or quartz as described above, but the substrate itself may constitute an optical element such as a lens, prism, mirror, or window material.

Single layer or multilayer optical F! of the invention! Films are classified into antireflection films, reflective films, interference filters, ND filters, etc. depending on their purpose.

The layers constituting these optical thin films can be made of oxide dielectrics such as those with a relatively high refractive index: AhOi, ZrO2, l, if quartz is selected as the substrate as described.
1fOz+Sin, 5ctOs+'lx's Something with a relatively low refractive index: If Si (1g, etc.) is selected and used according to optical theory, and if you select travertine, then a fluoride dielectric, such as something with a relatively high refractive index: NdFi
, LaFs, ThFt with relatively low refractive index:
MgF, ThFz, 5rFz, etc. are selected and used according to optical theory.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited thereto.

[Example 1] An Al2O3N film (na
-1,72) is vacuum deposited, followed by 5tox? with an optical thickness of λ/4 as a low refractive index thin film. 3! Membrane (n4=
A two-layer antireflection film (design reference wavelength λ=248 ru++) was prepared by vacuum evaporating 1.44 ).

[Comparative Example 1] The same two-layer antireflection film as in Example 1 (design standard wavelength λ = 2413 n
m) was prepared.

[Example 2] A NdFs thin film (n4
= 1.66), and then a thin Inn4 of MgF having an optical thickness of λ/4 as a low refractive index thin film.
405) by vacuum evaporation to form a 2N antireflection film (
A design reference wavelength λ" (248 nm) was prepared.

The spectral reflectance characteristics when this anti-reflection film was provided on one side of the substrate and the spectral transmittance characteristics when it was provided on both sides were measured, and the results are shown in Figures 3 and 4. And in Figure 4, the center wavelength is slightly shifted from the design wavelength,
This is a manufacturing error.

[Comparative Example 2] The same 2N antireflection film as in Example 2 was created on a quartz substrate.

[Test example]

An excimer laser with λ=248n- is used as a light source, and the optical thin film is irradiated with 20nset pulsed light lXl0' times. After that, the surface of the optical thin film is observed with a differential interference microscope, and if there is damage such as holes [damage (Da
If there is no damage, it is determined as [Non-damage (Non-damage) J].

Then, by changing the intensity and repeating it several times for the same type of optical thin film, the maximum value of the laser light intensity when it is determined that rNon-da+mageJ is determined.

Similarly, the number of pulsed light irradiations, that is, the number of pulses, is set to 2 x lO
' , 3 xlO' , 4 xlO' , 5 xlO'
The maximum value of the laser beam when r is determined to be Non-damageJ is determined by increasing the value of r.

Previous example! The results of testing the optical thin films of Comparative Example 1 and Comparative Example 1 are shown in FIG. 5, and the results of testing the optical thin films of Example 2 and Comparative Example 2 are shown in FIG. 6, respectively.

As is clear from this, when the optical thin film is made of fluoride, no cumulative effect is observed when the substrate is made of fluorite, and when the optical thin film is made of oxide, no cumulative effect is observed when the substrate is made of quartz.

〔Effect of the invention〕

As described above, according to the present invention, even in an optical thin film for an optical system that uses deep ultraviolet laser light with high light intensity, the cumulative effect is reduced.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the layer structure of a two-layer anti-reflection film made of a fluoride dielectric, and FIG. 2 is an explanatory diagram showing the layer structure of a two-layer anti-reflection film made of an oxide dielectric. be. FIG. 3 is a graph obtained by measuring the spectral reflectance characteristics of the antireflection film (single-sided coating) of Example 2. FIG. 4 is a graph showing the spectral transmittance characteristics of the antireflection film (coated on both sides) of Example 2. FIG. 5 is a graph comparing the cumulative effects of the optical thin films of Example 1 and Comparative Example 1. FIG. 6 is a graph comparing the cumulative effects of the optical thin films of Example 2 and Comparative Example 2.

Claims (1)

[Claims] When the substrate supporting the optical thin film is made of fluorite, the light intensity is 1.5 J/cm^2 or more, and when the substrate is quartz, the light intensity is 0.8 J/cm^2. In the multi-layer or single-layer optical thin film described above into which deep ultraviolet laser light is incident, the thin film is composed of a fluoride dielectric when the substrate is fluorite, and an oxide dielectric when the substrate is quartz. An optical thin film with little cumulative effect, characterized by comprising: