JP2001293442A - Optical element cleaning method - Google Patents

Abstract

(57) Abstract: The present invention is not limited to SiO ₂ based optical element, CaF ₂
Of a cleaning method that removes adsorbed organic matter on the surface of an optical element that can be applied to optical elements of a system, and a cleaning method that can be applied even if the optical element has an optical thin film such as an antireflection film. The task is to The present invention provides a method for cleaning an optical element, comprising: (1) a step of cleaning the optical element with an organic solvent;
The present invention proposes a method for cleaning an optical element, which comprises at least (2) a step of irradiating the optical element with ultraviolet light in an atmosphere containing oxygen and a step of (3) heating and cleaning the optical element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method for cleaning an optical element used in an optical system of an apparatus using ArF excimer laser light. In the present invention, an optical element refers to a material obtained by processing a material for an optical element into a predetermined shape.
In this case, the optical element includes an optical thin film (for example, an antireflection film) formed on the surface.

[0002]

2. Description of the Related Art As semiconductor devices become more highly integrated,
Exposure light used in a lithography process in a semiconductor manufacturing process is becoming shorter in wavelength. Recently, as next-generation exposure light,
An ArF excimer laser beam having a wavelength of 93 nm has attracted attention.

[0003] As an optical element used in an optical device for ArF excimer laser light, artificial quartz and fluorite, which are optically isotropic and have sufficient light transmittance and light reflectivity even with short wavelength light, are used. Can be

Conventionally, the mainstream of cleaning these optical elements is a cleaning method that combines cleaning with an aqueous cleaning solution and cleaning with an organic solvent system. In addition, as a relatively new attempt, an optical element is heated and dried only after an organic solvent washing sequence as described in JP-A-9-155309, or after acid cleaning as described in JP-A-10-158435. Finally, attempts have been made to irradiate UV or laser.

However, these cleaning methods have not been able to sufficiently remove contamination by trace amounts of organic substances and water in the atmosphere adsorbed on the optical element surface. These substances are suspended in the atmosphere (also in the clean room atmosphere), and are phthalates, siloxanes, and the like generated from building materials, wall surfaces, and the like.

When these organic substances are adsorbed on the surface of the optical element, the optical characteristics of the optical element obviously deteriorate. For example, Japanese Patent Application Laid-Open No. 10-158035 reports that the transmittance of light having a specific wavelength decreases by about 0.5% when an optical element made of synthetic quartz glass is left in a clean room for 24 hours.

[0007] The contamination of the surface of the optical element by these adsorbed organic substances is slight for each optical element. However, since about 100 optical elements are usually used in an optical device, the optical element is contaminated. Deterioration of the characteristics is synergistically increased, causing a problem that the device is malfunctioned.

[0008] Among the prior art, Japanese Patent Application Laid-Open No. 10-1580
No. 35 points out the presence of an organic substance adsorbed on an optical element, and proposes a cleaning method for removing the organic substance and suppressing its re-adhesion.

[0009] It is necessary to perform HF treatment on the produced optical element, inactivate the Si terminal on the surface of the optical element by fluorination or hydrogenation, and then perform a heat treatment. It comprises a step of performing light cleaning by an excimer laser.

The method disclosed in Japanese Patent Application Laid-Open No. 10-158035 is a very excellent cleaning method that not only removes organic matter contamination but also suppresses re-adhesion. However, HF treatment of optical elements requires very strict control of the reaction temperature, reaction time, and HF concentration, and, if unsuccessful, involves the risk of increasing the surface roughness of the optical element. Met.

Further, the method of JP-A-10-158035 Patent Publication, since a method of fluorination or hydrogenation of the Si of the optical element surface by HF treatment, substantially only an optical element composed mainly of SiO ₂ Could not be applied.

Further, when a thin film made of a different material such as an anti-reflection film is formed on the surface of the optical element, the organic substance adsorbed on the surface of the thin film has a higher optical characteristic than the organic substance adsorbed on the surface of the optical element. However, the method described in Japanese Patent Application Laid-Open No. H10-158035 could not cope with this.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of these problems, and is not limited to SiO ₂ -based optical elements, but is also applicable to CaF ₂ -based optical elements. The problem is to propose a cleaning method for removing organic substances.

Another object of the present invention is to propose a cleaning method applicable even when the optical element has an optical thin film such as an antireflection film.

The optical characteristics of the optical element fluctuate sensitively depending on the surface condition. Normally, when these optical elements are left in the air, organic substances in the air gradually adhere to the element surface and change the optical characteristics. In particular, among the optical characteristics, the transmittance tends to be more pronounced than the reflectance, and the transmittance is reduced due to the attachment of an organic substance. Further, this phenomenon is more remarkable in a wavelength band in an ultraviolet region than in a visible region.

Although most of the organic substances can be removed by the conventional cleaning method, optical characteristics are slightly deteriorated because a small amount of organic substances and organic solvents remain as residues.

Even if the optical characteristics of individual optical elements are slightly deteriorated, there is a problem that desired performance cannot be obtained in an optical device having a large number of optical elements built therein because the fluctuations of the optical characteristics are synergistic. Was.

[0018]

According to the present invention, there is provided a method for cleaning an optical element, comprising: (1) a step of cleaning the optical element with an organic solvent; and (2) a step of cleaning the optical element with ultraviolet light in an atmosphere containing oxygen. And (3) a step of heating and cleaning the optical element to provide an optical element cleaning method.

According to the present invention, after cleaning the optical element with an organic solvent,
This is a cleaning method of performing irradiation cleaning with ultraviolet light, and further heating and drying.

In particular, it is suitable for cleaning an optical element used in an optical device that handles an ArF excimer laser.

Most of organic substances adsorbed on the surface of the optical element when left in the atmosphere are removed by first wiping with an organic solvent. However, this alone cannot be completely removed, and a small amount of organic matter remains on the element surface.

In order to remove it, the surface of the optical element is irradiated with ultraviolet light in an atmosphere containing oxygen. As a result, active oxygen generated on the element surface reacts with the residual organic matter to become a gas (H ₂ O, CO _2, etc.) and is desorbed from the element surface. In this way, a trace amount of organic substances on the element surface can be removed.

On the other hand, although the organic solvent is highly volatile,
Some of them do not evaporate easily and remain on the sample surface even after irradiation with ultraviolet light. Therefore, heating and drying are performed to remove this.

Here, the reason why the heating is performed after the irradiation with the ultraviolet light is that if the heating is performed in a state where the organic substance remains, the organic substance carbonizes and adheres to the surface of the optical element and cannot be removed. .

In the present invention, through these steps, the surface of the optical element can be cleaned more than the conventional method.

The cleaning method of the present invention can be widely applied regardless of the material of the optical element, but is particularly effective when the optical element is made of SiO ₂ or CaF ₂ . Further, the cleaning method of the present invention is applicable even if the optical element has an optical thin film such as an antireflection film on the surface.

In the present invention, it is preferable to use a quick-drying substance as the organic solvent used for washing. Specifically, a mixture of methyl alcohol and ether at a mass ratio of 1: 9 is preferable as the organic solvent.

Here, the washing with the organic solvent in the step (1) is optional as long as it can remove organic contaminants adsorbed on the surface of the optical element sufficiently and does not damage the surface of the optical element. However, sufficient cleanliness can be ensured by, for example, manual cleaning using a cleaning cloth or cleaning paper.

However, when cleaning is performed manually, it is necessary to pay close attention to reattachment of contamination.

Immediately after the completion of the washing with the organic solvent, it is desirable to carry out washing by irradiation with ultraviolet rays. This is because the longer the standing time after the washing, the larger the amount of organic substances attached from the atmosphere.

When the oxygen concentration in the ultraviolet irradiation atmosphere is 5%
It is desirable that this is the case. Furthermore, oxygen partial pressure is 0.1Pa
It is desirable that this is the case. If the oxygen partial pressure is within this range, a sufficient amount of active oxygen is generated near the optical element surface,
Organic substances remaining on the element surface are effectively gasified. It is desirable to use ultraviolet light having a wavelength of 300 nm or less as the ultraviolet light. Particularly preferably 185, 254n
m ultraviolet rays. This is because ultraviolet light of this wavelength has a particularly high active oxygen generation rate. The ultraviolet irradiation step in the oxygen atmosphere in the step (2) is desirably performed in a dedicated container. The container has a light source that emits ultraviolet light of at least a predetermined wavelength. Further, if necessary, the container may have an oxygen source for supplying oxygen. Here, the oxygen atmosphere is necessarily oxygen 10
The atmosphere may not be 0%, and may be any atmosphere that can generate a sufficient amount of active oxygen by ultraviolet irradiation. For example, ultraviolet irradiation can be performed by introducing air into the container. However, since the atmosphere contains organic matter and moisture, it is desirable to introduce purified air into the container through a filter or the like.

Further, it is desirable that the heating in the step (3) is performed at a pressure of 1 Pa or less.

The present inventors investigated the relationship between the pressure during heating and the recovery rate of the optical characteristics by changing the pressure during heating (FIG. 4). As a result, the transmittance of the optical element was recovered. Required time was shorter under reduced pressure.
As described above, by performing heating under reduced pressure, the time required for cleaning the optical element can be reduced.

In the step (3), the heating temperature is desirably 70 ° C. or more and 300 ° C. or less.

A suitable range for the heating temperature of the optical element is selected mainly depending on the material of the optical element. The optical element made of SiO ₂ can be heated without deformation at 500 ° C. or lower.

In the case of fluorite, if the temperature is 300 ° C. or lower, heating can be performed without generating cracks.

The lower limit of the heating temperature may be at least the temperature at which the attached organic substance can be desorbed, and according to the experiments conducted by the inventors, it is preferable that the heating temperature be at least 70 ° C.

In the step (3), the heating time is desirably 40 minutes or more. The present inventors have investigated the relationship between the heating time and the optical characteristics (transmittance), and found that the transmittance increases with the heating time, reaches an upper limit determined by the material, and becomes constant. The heating temperature is
It is related to the heating time up to this upper limit. The higher the heating temperature, the shorter the time to reach the upper limit. Even when heating is performed at atmospheric pressure, if heating is performed for 40 minutes, the upper limit of the transmittance is reached.
Desirably, it is at least minutes.

[0039]

(Example 1) The transmittance and reflectance of a polished synthetic quartz substrate having a thickness of 2 mm having two parallel surfaces were measured without cleaning. Thereafter, the mixture was mixed at a ratio of methyl alcohol: ether = 1: 9 (mass ratio) to prepare an organic solvent for washing.

An appropriate amount of this organic solvent was impregnated into a cleaning cloth, and the cloth was wiped and cleaned by hand. In order to prevent the dirt taken into the cleaning cloth from re-adhering, the wiping with one cleaning cloth was performed only in one direction, and the back wiping was not performed. In addition, each time of wiping, wiping was performed using a new cleaning cloth. Then, wiping and cleaning was performed on at least three different directions (for example, vertical, horizontal, and slug) on one cleaning surface.

In the present embodiment, the wiping and cleaning is performed manually. However, the main cleaning operation is not limited to the wiping, and a known method such as an ultrasonic cleaning method can be used.

The wiped and cleaned optical element was immediately irradiated with ultraviolet light.

The ultraviolet irradiation was performed at room temperature in an atmosphere containing 80% oxygen. Ultraviolet irradiation was performed for 10 minutes using a low-pressure mercury lamp as the ultraviolet irradiation lamp. In addition to the light source used for UV irradiation,
Although a deuterium lamp and a xenon lamp are mentioned, a low-pressure mercury lamp is most preferable in consideration of the life of the lamp.

According to the results of the ultraviolet irradiation experiment, the oxygen concentration at the time of ultraviolet irradiation is preferably 5% or more. If the oxygen concentration is equal to or higher than the lower limit, active oxygen is generated at a practically acceptable level, which is effective in removing trace organic substances remaining on the cleaning surface.

The wavelength of the ultraviolet light used is preferably 300 nm or less. In particular, ultraviolet rays of 185 nm and 254 nm, which can generate active oxygen efficiently by ultraviolet irradiation, are most preferable.

The irradiation time of the ultraviolet ray depends on the degree of pre-cleaning, the incident angle of the ultraviolet ray on the cleaning surface, and the wavelength of the ultraviolet ray to be used. Irradiation makes it possible to remove residual organic matter on the surface. More desirably, irradiation is performed for 10 minutes or more under the vertical incidence condition.

When the optical element has a plurality of surfaces as in this embodiment, it is desirable to irradiate each surface with ultraviolet rays.

After irradiating with ultraviolet light for 10 minutes,
The optical element was heated in an air atmosphere at 150 ° C. for 30 minutes in a heating furnace to complete the cleaning.

At each stage of the washing, the transmittance and the reflectance of the optical element were measured. FIG. 1 shows the data of the internal transmittance of the obtained results. From this result, it can be seen that the transmittance increases in the order of cleaning before cleaning, (organic solvent) cleaning, (organic solvent + ultraviolet light irradiation) cleaning, and (organic solvent + ultraviolet light irradiation + heating) cleaning. Finally, the transmittance of this quartz substrate was about 90.7% at a wavelength of 193 nm. This almost coincides with the ideal transmittance of the synthetic quartz substrate having a thickness of 2 mm, which suggests that the cleaning method of the present invention has sufficiently cleaned the substrate.

Example 2 Next, a polished fluorite substrate having two parallel surfaces and a thickness of 2 mm was washed in the same manner as in Example 1.

In the same manner as in Example 1, before the cleaning, (organic solvent) cleaning, (organic solvent + ultraviolet light irradiation) cleaning, and (organic solvent + ultraviolet light irradiation + heating) cleaning, the transmittance and The reflectance was evaluated.

FIG. 2 shows the results obtained for the transmittance. According to this, it can be seen that the transmittance increases in this order. Also, as the results show, in fluorite,
Since the difference between the cleanings is remarkably observed in the ultraviolet region having a wavelength of 170 nm or less, it can be said that the cleaning method of the present invention is particularly effective in the ultraviolet region.

EXAMPLE 3 A 2 mm thick synthetic quartz substrate having two polished parallel surfaces was prepared, and an antireflection film having a laminated structure of SiO ₂ and Al ₂ O ₃ was formed on the two polished surfaces. Coated by vacuum evaporation and left in the normal atmosphere for 3 months.

After the standing, cleaning was performed in the same manner as in Example 1. The result is shown in FIG.

FIG. 3 shows that (organic solvent) cleaning and (organic solvent +
(Ultraviolet light irradiation) Cleaning is almost the same, but (organic solvent +
It can be seen that the transmittance was significantly increased by washing with (ultraviolet light irradiation + heating) washing.

In Examples 1 and 2, since the effect of heating is not so remarkable as in this example, it can be said that an optical element coated with a thin film has a larger effect of heating than an element not coated. This is because the organic solvent penetrates into the coating film and is less likely to evaporate as compared with the element without the coating, which is considered to be easily removed by heating.

On the other hand, the reflectance was measured simultaneously with the measurement of the transmittance. However, since the reflectance hardly fluctuated irrespective of each washing step, it was judged that the present invention particularly contributed to the improvement of the transmittance. You.

(Example 4) A 2 mm thick synthetic quartz substrate having two parallel polished surfaces was washed with an organic solvent in which methyl alcohol was mixed with ether at a ratio of 9 to 1 and the surface was washed.
Then, a low-pressure mercury lamp was used in an atmosphere containing oxygen for 10 minutes.
The sample was irradiated with ultraviolet light for one minute, and the transmittance was measured. This was divided into halves, and one was heated at 150 ° C. in the air and the other was similarly heated at 150 ° C. in a vacuum, and the change in transmittance at a measurement wavelength of 193 nm with respect to time was observed. The results are shown in Graph 4. From this graph, it can be seen that the recovery in transmittance is faster when heating in vacuum.

[0059]

According to the cleaning method of the present invention, it is possible to remove not only the SiO ₂ optical element but also the adsorbed organic substances on the surface of the CaF ₂ optical element. Further, the cleaning method of the present invention can be applied to an optical element having an optical thin film such as an antireflection film.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in transmittance due to each cleaning of a quartz substrate.

FIG. 2 is a diagram showing a change in transmittance due to each cleaning of a fluorite substrate.

FIG. 3 is a diagram showing a change in transmittance due to each cleaning of a quartz substrate provided with an antireflection film.

FIG. 4 is a diagram showing a change in transmittance due to a difference in a heating atmosphere.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Minoru Otani 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Yasuyuki Suzuki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon (72) Inventor Hidehiro Kanazawa 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term in Canon Inc. (reference) 3B201 AA01 BA22 BB82 BB95 BC01 CB15 CC21 4G059 AA11 AB01 AB09 AB11 AC30 5F046 AA28 CA04 CB01 5F071 AA06 JJ05

Claims (6)

[Claims] 1. A method for cleaning an optical element, comprising: (1) a step of cleaning the optical element with an organic solvent; and (2) a step of irradiating the optical element with ultraviolet light in an atmosphere containing oxygen. (3) a method for cleaning an optical element, comprising at least a step of heating and cleaning the optical element. 2. The method for cleaning an optical element according to claim 1, wherein the optical element is made of SiO ₂ or CaF ₂ . 3. The method for cleaning an optical element according to claim 1, wherein in the step (2), the oxygen concentration in the atmosphere is 5% or more. 4. The method according to claim 1, wherein in the step (2), ultraviolet light having a wavelength of 300 nm or less is used as the ultraviolet light.
4. The method for cleaning an optical element according to claim 3. 5. The method for cleaning an optical element according to claim 1, wherein the heating in the step (3) is performed at a pressure of 1 Pa or less. 6. In the step (3), the heating temperature is set to 7
The method for producing an optical element according to claim 1, wherein the temperature is 0 ° C. or more and 300 ° C. or less.