JP2000235101A - Uv ray transmitting optical material and uv ray optical device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain desired optical performance with higher internal transmissivity of a UV transmitting optical material using a light source of <=185 nm wavelength by using a material having a specified reduction rate of the internal transmissivity for radiation of light having specified or lower energy density. SOLUTION: The material used has <=0.5% per 10 mm reduction of the internal transmissivity when the material is irradiation with light having <=50 mJ/cm2/pulse energy density. Since moisture or hydrocarbons depositing on the surface of an optical device causes energy loss, the internal loss allowable as an optical material is specified to 0.5%/cm. When lead remains in a calcium fluoride crystal due to lead fluoride added as a scavenger, the crystal shows three absorption bands. Among these bands, the presence of an absorption band at 154 nm is in correlation with durability against a F2 laser, and therefore, the material is produced without using lead fluoride. Lead impurities in calcium fluoride vary depending on the growing method of the crystal, and the steeper the temp. gradient on the solid-liquid interface near the melting point, the lesser the impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical material using vacuum ultraviolet light having a wavelength of 185 nm or less, an optical lithography apparatus using the optical material, a laser processing machine, and the like.

[0002]

BACKGROUND OF THE INVENTION The present invention F ₂ laser (157nm), Xe ₂ (175n
m), Kr ₂ (146 nm), Ar ₂ (126 nm) excimer lasers, solid-state lasers using nonlinear optical effects, and other optical lithography devices that use vacuum ultraviolet light, such as steppers and scanners. It relates to calcium iodide crystals.

In recent years, VLSIs have become more highly integrated and more sophisticated, and require fine processing techniques on wafers.
As a processing method, a method using optical lithography is generally performed. At present, the exposure wavelength becomes shorter and shorter, and ArF excimer laser light (wavelength 193 nm)
It is expected that a stepper with a light source will appear soon. There are very few optical materials that can be used for photolithography at wavelengths below 193 nm, and they are designed with calcium fluoride crystals and quartz glass.

Attempts have been made to further reduce the wavelength of the light source in order to perform fine processing, and the demand for optical lithography technology using F ₂ laser light has been increasing in recent years. However, at that wavelength, quartz glass is no longer expected to be used, and it is considered that only calcium fluoride crystals remain. Calcium fluoride is conventionally used by the Bridgman-Stockberger method (reduction method)
Manufactured in. A fluorinating agent (such as lead fluoride) called a scavenger is added to artificially synthesized calcium fluoride powder with a small amount of impurities, and the temperature is raised in a vacuum,
After melting, the crucible is pulled down at a rate of 0.5 to 5 mm / hour, and crystal growth is performed. After undergoing a heat treatment for removing thermal stress, it is cut and processed into a material for an optical material.

From these materials, prisms, mirrors and various lenses are manufactured for use in optical lithography.
Optical polishing is performed with the required accuracy and the required coating is applied. Japanese Patent Application Laid-Open No. 7-281001 discloses an ultraviolet optical fluorite used in an ultraviolet optical system, which has an energy density of 50 to 500 mJ / cm ² and a laser beam of 50 to 500 Hz when irradiated with 10 ⁴ to 10 ⁷ shots. Transmission is 90.0 per 10mm thickness
% Is disclosed.

[0006]

Meanwhile [0008] In the 185nm UV light below such F ₂ laser, it is used in the following energy density 50 mJ / cm ^2. Further, from the viewpoint of optical design, it is desired that the internal transmittance has a higher numerical value. Therefore, a desired optical performance may not be obtained at 90.0% per 10 mm.

[0007]

The inventors of the present invention have conducted intensive studies, and as a result, have found that the energy density of the material for ultraviolet transmission optical using a light source having a wavelength of 185 nm or less is 50 mJ / cm ² / cm.
The rate of decrease in internal transmittance when irradiating light of less than pulse is 10
By using 0.5% or less material per mm, it was possible to achieve desired optical performance.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As a light source for optical lithography or a laser processing machine, at a wavelength of 185 nm or less, F ₂ laser (157 nm), Xe ₂ (175 nm), Kr ₂ (146 nm), ArKr (134 nm), ArKr (134 nm)
₂ A solid-state laser using an excimer laser (126 nm) or a nonlinear optical effect is considered. Here is a description of examples as an example F ₂ laser, the present invention is the same for a light source of other wavelengths, are not intended to be specified by the F ₂ laser.

[0009] F ₂ laser industrial that are currently commercially available, the LAMBDA Physik, repetition frequency 500 Hz, there is an average power 10 W. When this energy is applied to an area with a diameter of 10 mm, 10/500 / ((π / 4) 1 × 1) × 1,000
= 25.5 [mJ / cm ² / pulse]. The energy density is 50mJ if the power of the laser body rises
It may exceed / cm ² / pulse, but at present it will be used below.

At the wavelength of the F ₂ laser, the loss of absorption and scattering at the coated portion of the optical material is much worse than in the wavelength region longer than 185 nm. In addition, moisture and hydrocarbons adhering to the optical element surface cause energy loss, and in order to obtain a desired amount of light with the combined optical system, it is necessary that the internal loss of the optical material be smaller. . In view of the various factors as described above, the present inventors have estimated the internal loss allowable as an optical material to be 0.5% / cm.

On the other hand, in calcium fluoride crystals,
Due to lead fluoride added as a scavenger, if lead remains in the crystal as an impurity, 204 nm, 164 nm, 154 nm
There are three absorption bands (Zhurnal Prikladnoi Spekt)
roskopii, vol.32, 103 (1980)). The present inventors have found that the presence of the 154 nm absorption band among these absorption bands is correlated with the durability to the F ₂ laser. Then, if prepared without lead fluoride, the absorption band no, resistance to F ₂ laser was also found high.

It has been found that the lead impurity in calcium fluoride changes depending on the crystal growth method. As a result of various experiments, it was found that the steeper the temperature gradient of the solid-liquid interface near the melting point, the smaller the amount of lead impurities. as a result,
Absorption of 154nm is reduced, resistance to F ₂ laser is to improve. Hereinafter, as an example of the ultraviolet optical device as a light source an F ₂ laser, an outline of a projection exposure apparatus. The details of the optical system of the projection exposure apparatus as a light source an F ₂ laser, are described in detail in Japanese Patent Application No. 10-370143.

FIG. 3 is a diagram schematically showing the overall configuration of a projection exposure apparatus having a catadioptric optical system. In FIG. 3, the Z axis is parallel to the optical axis AX of the catadioptric optical system 8 constituting the projection optical system, the X axis is parallel to the plane of FIG. 3 in a plane perpendicular to the optical axis AX, and The Y axis is set vertically. The illustrated projection exposure apparatus uses an F2 laser (having an oscillation center wavelength of 15) as a light source for supplying illumination light in the ultraviolet region.
7.6 nm). The light emitted from the light source 1 uniformly illuminates the mask 3 on which a predetermined pattern is formed, via the illumination optical system 2.

In the optical path from the light source 1 to the illumination optical system 2, one or a plurality of bending mirrors for deflecting the optical path are arranged as required. Also, the illumination optical system 2
Is a field stop for defining the size and shape of the illumination area on the mask 3, an optical integrator comprising a fly-eye lens or an internal reflection type integrator to form a surface light source of a predetermined size and shape, this field stop And an optical system such as a field stop imaging optical system for projecting the image on the mask. Further, an optical path between the light source 1 and the illumination optical system 2 is sealed by a casing (not shown).
The space from to the optical member closest to the mask in the illumination optical system 2 is replaced with an inert gas having a low absorption rate of exposure light.

The mask 3 is held on a mask stage 5 via a mask holder 4 in parallel to the XY plane. A pattern to be transferred is formed on the mask 3 and has a rectangular shape (slit shape) having a long side along the Y direction and a short side along the X direction in the entire pattern area.
Are illuminated. The mask stage 5 is two-dimensionally movable along a mask plane (that is, an XY plane), and its position coordinates are measured and controlled by an interferometer 7 using a mask moving mirror 6. ing.

The light from the pattern formed on the mask 3 forms a mask pattern image on a wafer 9 as a photosensitive substrate via a catadioptric projection optical system 8. The wafer 9 is placed on a wafer stage 11 via a wafer holder 10.
Above, it is held parallel to the XY plane. And
In order to optically correspond to a rectangular illumination area on the mask 3, the wafer 9 has a long side along the Y direction and X
A pattern image is formed in a rectangular exposure area having a short side along the direction.

The wafer stage 11 is two-dimensionally movable along the wafer plane (ie, XY plane), and its position coordinates are measured and controlled by an interferometer 13 using a wafer moving mirror 12. Is configured.
In the illustrated projection exposure apparatus, the inside of the projection optical system 8 is configured to maintain an airtight state, and the gas inside the projection optical system 8 is replaced with an inert gas.

Further, a mask 3 and a mask stage 5 are arranged in a narrow optical path between the illumination optical system 2 and the projection optical system 8. (Not shown) is filled with an inert gas. In a narrow optical path between the projection optical system 8 and the wafer 9, the wafer 9 and the wafer stage 11 are disposed, but inside a casing (not shown) that hermetically surrounds the wafer 9 and the wafer stage 11. Is filled with an inert gas such as nitrogen or helium gas.

As described above, an atmosphere in which the exposure light is hardly absorbed is formed over the entire optical path from the light source 1 to the wafer 9. As described above, the viewing area (illumination area) on the mask 3 and the projection area (exposure area) on the wafer 9 defined by the projection optical system 8 are X
It has a rectangular shape with short sides along the direction. Therefore,
Mask 3 using drive system and interferometer (7, 13)
While controlling the position of the wafer 9, the mask stage 5 and the wafer stage 11, that is, the mask 3 and the wafer 9 are synchronously moved along the short side direction of the rectangular exposure region and the illumination region, that is, in the X direction. By performing (scanning), the mask pattern is scanned and exposed on the wafer 9 in a region having a width equal to the long side of the exposure region and having a length corresponding to the scanning amount (movement amount) of the wafer 9. You.

In FIG. 3, all refractive optical members (lens components) constituting the projection optical system 8 are made of an optical material composed of calcium fluoride crystal, and all or a part of the optical material of the present invention is used. Use materials. FIG.
4 is a diagram showing a lens configuration of the catadioptric optical system (projection optical system 8) according to FIG.

The projection optical system composed of the catadioptric optical system shown in FIG. 4 includes a first imaging optical system K1 for forming a primary image (intermediate image) I of the pattern of the mask 3 and a light from the primary image I. A second imaging optical system K2 for forming a secondary image of the mask pattern on the wafer 9 which is a photosensitive substrate based on
It is composed of The first imaging optical system K1 includes, in order from the mask side, a first lens group G1 having a positive refractive power;
It comprises an aperture stop S and a second lens group G2 having a positive refractive power.

The second imaging optical system K2 includes, in order from the mask side, a main mirror M1 having a surface reflection surface R1 with a concave surface facing the wafer side and having an opening at the center, a lens component L2,
And a secondary mirror M2 provided on the wafer-side lens surface and having a reflective surface R2 having an opening at the center. That is, according to another viewpoint, the sub-mirror M2 and the lens component L2 constitute a back surface reflection mirror, and the lens component L2 constitutes a refraction portion of the back surface reflection mirror.

Incidentally, all the optical elements (G1, G2, M1, M2) constituting the projection optical system are arranged along a single optical axis AX. The primary mirror M1 is arranged near the position where the primary image I is formed, and the secondary mirror M2 is arranged close to the wafer 9. Thus, the light from the pattern of the mask 3 forms the primary image (intermediate image) I of the mask pattern via the first imaging optical system K1. The light from the primary image I is
The light is reflected by the secondary mirror M2 via the central opening of the primary mirror M1 and the lens component L2, and the light reflected by the secondary mirror M2 is reflected by the primary mirror M1 via the lens component L2. The light reflected by the primary mirror M1 forms a secondary image of the mask pattern on the surface of the wafer 9 at a reduction magnification through the lens component L2 and the central opening of the secondary mirror M2.

[0024]

[Example] A trace amount of lead fluoride (0.5 to 1.5 [mol%]) was added as a fluorinating agent to a powdered material of artificially synthesized calcium fluoride by minimizing impurities. In a powder state, the mixture was sufficiently mixed using a stirrer, and heated and melted in a vacuum of 10 ⁻³ to 10 ⁻⁴ Pa to produce a polycrystalline body. 20 kg of this polycrystal
After filling into 200 carbon crucibles, crystal growth was carried out according to the usual Bridgman method. The temperature near the heater is set at 1370-1450 ° C so that the polycrystalline body in the crucible is melted, and the lowering speed is 0.5-3mm
/ Time lowered. With a pulling distance of 350 mm, a calcium fluoride crystal having a total length of about 300 mm of the ingot was grown.

Calcium fluoride crystals A, B, C grown by changing the temperature gradient during crystal growth to 15, 10, 7 [° C./cm], respectively.
A test piece sample having a diameter of 30 mm and a thickness of 30 mm was sampled from the sample, two planes were optically polished with diamond abrasive grains, and irradiated with F ₂ laser. The spectral transmittances before and after irradiation at this time are shown in FIGS. Laser power is 4W, repetition frequency is
200 Hz. The energy density per pulse (fluence) is 33 mJ / cm ² / pulse because the irradiation area is approximately 0.5 × 1.2 [cm ² ]. The decrease in transmittance (wavelength 157 nm) after irradiation with 10,000 pulses was measured with a vacuum ultraviolet spectrophotometer VUV200 manufactured by JASCO Corporation. As shown in Table 1, the decrease was 1.2%, 6.0%, and 30.0%. became. The transmittance reduction per cm can be calculated from this numerical value using Lambert's law.

[0026]

[Table 1]

The calcium fluoride of the present invention did not absorb at 154 nm, and the transmittance reduction (wavelength 157 nm) after irradiating 10,000 pulses of the F ₂ laser was a very small value of 0.4% per 10 mm. . In the calcium fluoride of Comparative Examples B and C,
The reduction amounts were 2.0 and 11.2 [% / cm], exceeding the allowable value in the optical design.

[0028]

According to the present invention, the F ₂ laser,
As an optical material for optical lithography equipment and laser processing machines that use other vacuum ultraviolet light as a light source, it is possible to manufacture calcium fluoride crystals with sufficient durability and process line widths of 100 nm or less. Optical lithography has become possible.

[Brief description of the drawings]

FIG. 1 Spectral transmittance of the present invention (solid line before irradiation of F2 laser, broken line after irradiation, thickness 30 mm)

FIG. 2 shows the spectral transmittance of a comparative example (sample C) (solid line indicates F)
(Before laser irradiation, after dashed line irradiation, thickness 30mm)

FIG. 3 is a diagram schematically showing an overall configuration of a projection exposure apparatus according to the present invention.

4 is a diagram showing a lens configuration of a catadioptric optical system (projection optical system) of the projection exposure apparatus of FIG.

[Explanation of symbols]

 Reference Signs List 1 laser light source 2 illumination optical system 3 mask 4 mask holder 5 mask stage 6, 12 moving mirror 7, 13 interferometer 8 projection optical system 9 wafer 10 wafer holder 11 wafer stage AX optical axis K1 first imaging optical system K2 second imaging Image optical system G1 First lens group G2 Second lens group S Aperture stop M1 Primary mirror M2 Secondary mirror I Intermediate image Li Each lens component

Claims (4)

[Claims] An ultraviolet light transmitting optical material using a light source having a wavelength of 185 nm or less has a decrease rate of an internal transmittance of 0.5% or less per 10 mm when irradiated with light having an energy density of 50 mJ / cm ² / pulse or less. A material for ultraviolet transmission optics, characterized in that: 2. A light source according to claim 1, UV-transparent optical material, which is a F ₂ laser. 3. An ultraviolet transmitting optical material according to claim 1, wherein said material is a calcium fluoride crystal. 4. An ultraviolet optical device having a light source having a wavelength of 185 nm or less and an optical system including an optical material that transmits light from the light source, wherein the optical material has an energy density of 50 mJ / cm ² / pulse or less. An ultraviolet optical device characterized by using a material having a rate of decrease in internal transmittance of 0.5% or less per 10 mm when irradiating light.