JP2000128695A - MANAGEMENT IN CLEANING CaF2 SINGLE CRYSTAL SUBSTRATE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for managing the cleanliness of the substrate surface made of CaF2 single crystal and having a preferred plane shape. SOLUTION: When the method for managing the cleanliness of the substrate surface made of CaF2 single crystal comprises (A) polishing the surface of a substrate made of CaF2 single crystal so as to have a preferred plane shape and (B) cleaning the above surface after that, the surface energy of a polished surface of the CaF2 single crystal substrate after being cleaned is measured and the value of its hydrogen bond term is managed so as to become >=20 dyne/cm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the cleaning of a lens used for an application requiring a high function, particularly a high-precision optical element.

[0002]

Conventionally, optical elements such as lenses made of fluoride crystal material of CaF _2, MgF ₂ or the like, with its optical properties with extremely wide good transmittance over the wavelength band of, certain low dispersion From that, luxury camera lens,
It has been used for high-precision lenses, such as television cameras, which require high performance. Optical elements made of fluoride-based crystal materials such as CaF ₂ and MgF ₂ have high transmittance even for short-wavelength light such as excimer lasers. Therefore, use of such optical elements as short-wavelength optical elements has begun to be studied.

In the cleaning of the optical element as described above,
Conventionally, it has been common practice to immerse a lens in a cleaning bath containing a cleaning liquid and clean the lens by an ultrasonic cleaning method. When several such washing tanks are provided and sequentially washed with a surfactant, pure water and the like, and finally isopropyl-alcohol washing is performed, the washing tank becomes large, and the washing machine body becomes expensive and huge, Further, the amount of solvent used for drying and the like also becomes very large. From the viewpoint of environmental protection, it is necessary to reduce the use of solvents or to use a solvent-free cleaning method in the future.

As described above, the conventional high-precision cleaning of the lens has been performed by the so-called WET method. However, in order to remove contaminants on the lens surface of an optical element that handles ultraviolet light having a shorter wavelength. However, DRY cleaning has become important because cleaning by the WET method alone is not sufficient.

That is, in the wavelength region of an excimer laser, the transmittance of a lens is reduced by a contaminant on the lens surface. In particular, it has been found that an organic residue as a contaminant reduces the transmittance of the lens.

As a means for evaluating the cleanliness of the surface after lens cleaning, in the case of these high-precision lenses, the most common method has been to evaluate the optical characteristics of the substrate after cleaning. This method has no problem in terms of reliability, reproducibility, and accuracy, but has a problem in actual production because it takes too much time for measurement.

[0007]

As described above,
A problem in the evaluation method after cleaning that has been generally performed in the past is that the time required for measurement is extremely long, and the production efficiency is reduced. For example, in a vacuum ultraviolet wavelength range used for an ArF excimer stepper, light is absorbed in the atmosphere, so that to evaluate the optical characteristics of the lens, the atmosphere of the lens to be measured must be replaced with nitrogen. In particular, in the case of a lens having a large lens diameter, it takes a long time for the nitrogen replacement, which affects the production tact.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and manages the cleanliness of the surface after cleaning in a short time without requiring a very long measurement time as in the prior art. The aim is to provide a possible method.

[0009]

The present invention for achieving the above object is as follows. That is, the surface of the substrate made of CaF ₂ single crystal is polished to a predetermined surface shape, and then the C
in the process step of cleaning the surface of a substrate made of aF ₂ single crystal, the surface energy of the polishing surface of the CaF ₂ single crystal substrate after cleaning was measured, the value is 20 dyne / c of the hydrogen bond
This is a method for managing cleaning of a substrate made of a CaF ₂ single crystal, characterized in that the substrate is controlled to at least m.

Further, in this control method, the value of the dispersion term in the surface energy of the cleaning machine is set to 30 to 40 dyne / c.
m, the value of the polarity term is controlled in the range of 0 to 5 dyne / cm.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a normal lens processing step,
First, a rough lens shape is formed from the lens material, and then a final surface shape and surface roughness are obtained by polishing.

The contaminants in the lens processing step vary depending on the processing step, but are basically abrasives, inorganic substances such as dust, and organic substances such as oil and fingerprints. These pollutants are
It occurs in each processing step of the lens due to covalent bonding, electrostatic force, van der Waals force, and the like. 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 lens processing is completed is very important. In particular,
Since a very high transmittance is required for a lens used in the ultraviolet region, for example, for an excimer laser, the final cleaning after lens processing affects the optical performance of the lens. Furthermore, since it has been found that organic substances are involved as a factor causing the transmittance deterioration in such a wavelength region, removal of organic substances on the lens surface is important in cleaning.

As described above, it is necessary to evaluate whether the cleaning effect is sufficient in the final cleaning after the lens processing in order to manage the process. As a result of various studies, CaF ₂
The surface energy of the polished surface of the single crystal substrate was measured, and it was found that the value of the hydrogen bond term was particularly important.

That is, the surface energy of the cleaved surface of the CaF ₂ single crystal substrate was measured and the hydrogen bond term was calculated. As a result, immediately after cleavage, the value of the hydrogen bond term was 24 dyne / cm.
Although high, the dispersion term and the polar term hardly change when left to stand in the air, whereas the value of the hydrogen bond term decreases with time. This is considered that the surface of the CaF ₂ substrate is contaminated by hydrocarbons or the like existing in the atmosphere, and as a result, the surface energy is also reduced with time. From this, it can be determined that the normality of the cleaned surface is very high if the value of the hydrogen bond term immediately after cleavage of the cleavage surface is equal to the value of the hydrogen bond term after cleaning.

The cleaved surface is in an ideal state as a clean surface, and the values of the dispersion term and the polarity term of the surface energy after cleaning are considered to be in the same range as the value of the cleaved surface. Can be adopted as a value.

By adopting such a method, it is possible to evaluate the cleanliness in a shorter time as compared with the conventional method, so that a lens of good quality can be stably manufactured without lowering the productivity of lens processing. Will be possible.

[0017]

Embodiments of the present invention will be described below.

Embodiment 1 First, a method for evaluating the surface energy will be described. The evaluation of the surface energy was performed by a conventionally well-known method. That is, the contact angle on the substrate surface is measured using a liquid whose surface energy is known in advance, and the dispersion term, the polarity term,
Find each hydrogen bond term. In the present invention, three kinds of liquids for evaluation were alpha bromonaphthalene, diiodomethane, and pure water.

Using such a technique, Ca
The surface energy of the surface of the material cleaved from the F ₂ single crystal substrate was determined. Next, the surface energy before and after the final cleaning of the lens polished surface was determined by the same method. Table 1 shows the results. It should be noted that the lens after cleaning was confirmed to be sufficiently cleaned by the evaluation of the optical characteristics by the conventional method.

[0020]

[Table 1] In this way, comparing the values of the dispersion term, the polarity term, and the hydrogen bond term from the surface energy measurement results, the dispersion term and the polarity term hardly changed before and after cleaning, but the hydrogen bond term clearly increased. are doing.

In comparison with the cleaved surface, it can be seen that the surface energy after cleaning shows a very close value. From these results, it was determined that the cleaning effect could be evaluated because the surface energy after cleaning was almost equal to the surface energy of the cleaved surface.

From the above, it is considered that the cleaning effect is sufficient if the value of the hydrogen bond term after cleaning is 20 dyne / cm or more.

It can be inferred that this result has the following mechanism.

For a polymer which is an organic material, this surface energy has already been measured, and the results are shown in Table 2 (literature values).

[0025]

[Table 2] Looking at these values, the hydrogen bonding terms of the organic material as the polymer material are all zero. By the way, the value of the hydrogen bond term of the surface energy before the CaF ₂ cleaning shows zero as can be seen from the results of Table 1. This means that before cleaning, CaF ₂
It is conceivable that an organic material such as a polymer was present on the surface, and this was probably due to the fact that what adhered in the polishing step remained. After the cleaning, the surface energy obtained by the same method shows a value substantially equal to that of the cleaved surface due to an increase in the hydrogen bond term. This is by washing
It can be assumed that the organic material on the surface of CaF ₂ was removed and a surface with extremely high cleanliness was obtained. According to this evaluation method, organic matter as described is addition to effective removal management of contamination adhering to the membrane, a very thin can detect also the organic film excellent in CaF ₂ surface cleanliness far Think of it as a management method.

[0026]

As described above, the CaF according to the present invention is used.
_{(2) If a} single-crystal substrate cleaning management method is adopted, the degree of cleanliness can be evaluated more easily than in the past, so that high-quality lenses can be stably manufactured without lowering the productivity of lens processing. Will be possible. As a result, an improvement in product yield can be expected, and cost can be reduced.

Claims (2)

[Claims] In a processing step of polishing a surface of a substrate made of a CaF ₂ single crystal into a predetermined surface shape and then cleaning the surface of the substrate made of a CaF ₂ single crystal, the cleaned CaF ₂ single crystal substrate A method for controlling the cleaning of a substrate made of a single crystal of CaF ₂ , wherein the surface energy of the polished surface is measured and the value of the hydrogen bond term is controlled to be 20 dyne / cm or more. 2. In the surface energy of the polished surface after cleaning,
The value of the dispersion term is 30-40 dyne / cm, and the value of the polarity term is 0-5.
cleaning management method of a substrate made of CaF ₂ single crystal according to claim 1, characterized in that managing to be in the range of dyne / cm.