JP2002350601A - Method for cleaning lens substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lens substrate cleaning method by which a lens substrate can be cleaned until the surface of the cleaned lens substrate has almost ideal cleanliness. SOLUTION: The surface of the lens substrate consisting of CaF2 single crystal is ground to a prescribed surface shape. After that, the lens substrate is subjected to acid treatment as the pretreatment of a cleaning step to clean the ground surface of the lens substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for cleaning a lens substrate used for applications requiring high performance.

[0002]

Conventionally, optical elements such as lenses made of fluoride crystal material of CaF _2, MgF _2, etc., its optical properties are very over a wide range of wavelength bands with low dispersion with having good transmissivity For this reason, it has been used for high-precision lenses, such as high-grade camera lenses and television camera lenses, which require high functions.

Further, since an optical element made of a fluoride-based crystal material such as CaF ₂ or MgF ₂ has a high transmittance even for short-wavelength light such as an excimer laser, it is studied to use it as an optical element for short wavelength. Is starting to be.

[0004]

In the cleaning of the optical element as described above, conventionally, it has been general that the lens is immersed in a cleaning liquid in a cleaning tank and cleaned by an ultrasonic cleaning method. . When several such washing tanks are provided, and after washing with a surfactant or pure water, and finally isopropyl alcohol washing is performed, the washing tank becomes large, and the washing machine itself becomes expensive and huge. In addition, the amount of solvent used for drying and the like also becomes very large.

Further, from the viewpoint of environmental protection, it is necessary to reduce the use of a solvent or to use a cleaning method that does not use a solvent in the future. As described above, the conventional high-precision lens cleaning is performed by a so-called WET method. However, in order to further remove contaminants on the lens surface of an optical element that handles ultraviolet light having a short wavelength, the WET method is used. Cleaning alone is not sufficient, and DRY cleaning has also become important. That is, in the wavelength region of the excimer laser, the transmittance of the lens decreases due to the contaminants on the lens surface, but it is understood that organic residues as the contaminants and abrasive residues in the polishing process cause the decrease in the transmittance of the lens. Have been.

Conventionally, in cleaning a lens substrate of CaF ₂ single crystal, a cleaning process is set by a combination of such a WET method and a DRY method. In some cases, sufficient optical characteristics cannot be obtained by only the cleaning method.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object thereof is to provide a method of cleaning a lens substrate capable of reducing the surface cleanliness after cleaning the lens substrate to almost an ideal state. To provide.

[0008]

In order to achieve the above object, according to the first aspect of the present invention, a surface of a lens substrate made of CaF ₂ single crystal is polished into a predetermined surface shape, and then the surface of the lens substrate is polished. The lens substrate is subjected to an acid treatment as a pretreatment of a cleaning step of cleaning.

The invention according to claim 2 is characterized in that, in the invention according to claim 1, oxalic acid, tartaric acid or citric acid is used for the acid treatment.

[0010]

Embodiments of the present invention will be described below.

As described above, it has been pointed out that a problem in the cleaning method generally performed conventionally is that the optical characteristics after cleaning are insufficient due to the shortening of the wavelength of the excimer laser. That is, it is expected that the transmittance of the lens after cleaning does not reach the theoretical value, and contaminants remain on the lens substrate surface without being removed.

Factors of the surface that cause deterioration of the transmittance of the lens include: 1) scattering due to surface roughness 2) absorption by organic residues remaining on the surface 3) absorption by inorganic particles such as abrasive remaining on the surface 4) surface Absorption by the affected layer is considered.

As for the surface roughness of 1), it has been found from the results of verification experiments that if the surface roughness is controlled to about 0.5 nm (average roughness RMS) or less, the influence on the transmittance is very small. Regarding 2), 3) and 4), it is expected that the transmittance will remain in the wavelength band of the excimer laser, particularly at the wavelength of 193 nm, when the cleaning is performed by the conventional cleaning method.

Therefore, the present invention employs the following method.

That is, the surface of the lens substrate made of CaF ₂ single crystal is polished into a predetermined surface shape, and then the lens substrate is subjected to an acid treatment as a pretreatment of a cleaning step of cleaning the surface of the lens substrate. I do.

In a normal lens processing step, a rough lens shape is first formed from a lens material, and then a final surface shape and surface roughness are obtained by polishing. For this reason, the contaminants in the lens processing step vary depending on the processing step, but are basically inorganic substances such as abrasives and dust, and organic substances such as oil and fingerprints. These contaminants are generated in each processing step of the lens by covalent bonding, electrostatic force, van der Waals force, and the like.

Further, there is a case where a deteriorated layer is present in a minute region near the surface due to the processing in the polishing step. It is also conceivable that this work-affected layer becomes a short-wavelength absorption layer.

After the final surface shape and surface roughness of the lens are obtained, an antireflection film for improving the transmittance of the lens is formed on the surface. Therefore, final cleaning after the lens processing is completed is very important. In particular, for a lens for an excimer laser, for example, used in the ultraviolet region, a transmittance close to a theoretical value is required. Therefore, final cleaning after lens processing affects the optical performance of the lens.

Further, as described above, it is known that factors that cause such transmittance deterioration in the wavelength region are the residue of an organic substance, the residue of an abrasive, the inorganic particles, the deteriorated layer, and the like. Therefore, it is important to clean the lens surface.

Therefore, in the present invention, it has been proposed to perform an acid treatment as a pre-cleaning treatment in order to sufficiently exert a cleaning effect in final cleaning after lens processing.

The organic residue, the inorganic particles, and the degraded layer remaining on the substrate surface can be removed from the surface by an acid treatment by an etching treatment. Can be eliminated.

However, since CaF ₂ is a very chemically unstable material, the use of a highly reactive inorganic acid, such as nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid, etc., will damage the substrate surface. These acids cannot be used. or,
Even if organic acids with low reactivity are used, there is a problem that the surface will be deteriorated if immersed in these acids for a long time, so measures can be taken if the processing conditions are strictly determined. .

By etching away the absorbing layer adhered to the lens substrate surface or existing on the substrate surface with an acid, an almost ideal surface can be obtained. An extremely high transmittance with an excimer laser (193 nm) can be obtained.

Embodiment An embodiment of the present invention will be described below.

<Example 1> First, as a sample, C
What was cut out from the aF ₂ single crystal substrate was used. still,
The material grade has an internal absorption of 0.2% or less. For sample shape, φ30 × 2tmm
Was used. Using these substrates, the transmittance at 193 nm under each processing condition was evaluated.

The surface roughness was measured with an AFM measuring device and evaluated by the average roughness RMS. Polishing was performed using a diamond abrasive. The acid treatment is performed in a 1% aqueous oxalic acid solution at room temperature for 30 minutes.
After immersion for 2 seconds, the substrate was rinsed in pure water for 1 minute, and then immersed in acetone to finish by hand wiping and drying.

Thereafter, a step of decomposing and removing organic residues by UV / O ₃ treatment was introduced. The processing time is 5 minutes, the distance to the light source is 30 mm, the irradiation intensity is 12 mW / cm ²
Met. The final 19 of the substrate processed under these processing conditions
99.20% was obtained as the transmittance at 3 nm. This value was almost equal to the theoretical transmittance of CaF ₂ , and an ideal cleanliness was obtained. The surface roughness of this sample is 0.5 nm
(RMS).

<Embodiment 2> First, as a sample, C
What was cut out from the aF ₂ single crystal substrate was used. still,
The material grade has an internal absorption of 0.2% or less. For sample shape, φ30 × 2tmm
Was used. Using these substrates, the transmittance at 193 nm under each processing condition was evaluated.

The surface roughness was measured with an AFM measuring device and evaluated by the average roughness RMS. Polishing was performed using a diamond abrasive. The acid treatment was performed by immersing in a 2% tartaric acid aqueous solution at 3 room temperature for 30 seconds, rinsing in pure water for 1 minute, then immersing in acetone, and finishing by hand wiping and drying.

Thereafter, a step of decomposing and removing organic residues by UV / O ₃ treatment was introduced. The processing time is 5 minutes, the distance to the light source is 30 mm, the irradiation intensity is 12 mW / cm ²
Met. The final 19 of the substrate processed under these processing conditions
A transmittance at 3 nm of 92.18% was obtained. This value was almost equal to the theoretical transmittance of CaF ₂ , and an ideal cleanliness was obtained. The surface roughness of this sample is 0.65 nm
(RMS).

[0031]

As is apparent from the above description, according to the present invention, after the surface of a lens substrate made of CaF ₂ single crystal is polished to a predetermined surface shape, a cleaning step of cleaning the surface of the lens substrate is performed. Since the lens substrate is subjected to the acid treatment as the pre-treatment, an effect is obtained that the cleanliness of the surface after cleaning the lens substrate can be made almost ideal.

Claims (2)

[Claims] 1. A lens substrate made of a CaF ₂ single crystal is polished to a predetermined surface shape, and then the lens substrate is subjected to an acid treatment as a pretreatment of a cleaning step of cleaning the surface of the lens substrate. Cleaning method for the lens substrate. 2. The method according to claim 1, wherein the acid treatment uses oxalic acid, tartaric acid or citric acid.