JP5842645B2 - Glass substrate cleaning method - Google Patents

Description

The present invention relates to a glass substrate cleaning method. More specifically, the present invention relates to a method for cleaning a silica glass substrate containing TiO ₂ .
A silica glass substrate containing TiO ₂ (hereinafter referred to as SiO ₂ —TiO ₂ glass) is abbreviated as “EUVL” using EUV (Extreme Ultra Violet) light. ) Is preferably used as a base material for reflective masks and reflective mirrors (hereinafter abbreviated as “EUVL optical base material”).

  Conventionally, in a semiconductor manufacturing process, an exposure apparatus for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has been widely used. In recent years, along with the higher integration and higher functionality of semiconductor integrated circuits, the miniaturization of integrated circuits has progressed, and an optical substrate used in a photomask of an exposure apparatus to accurately form a circuit pattern on a wafer surface. The glass substrate for materials is required to have high flatness and smoothness.

  Further, in such a technical trend, a lithography technique using EUV light as a next generation exposure light source (ie, EUVL technique) is considered to be applicable over a plurality of generations of 45 nm and after, and has attracted attention. EUV light refers to light in the wavelength band of the soft X-ray region or the vacuum ultraviolet region, specifically, light having a wavelength of about 0.2 to 100 nm. At present, the use of 13.5 nm wavelength light as a lithography light source is being studied. The EUVL exposure principle is the same as that of conventional lithography in that a mask pattern is transferred using a projection optical system, but a refractive optical system is used because there is no material that transmits light in the EUV light energy region. Therefore, a reflective optical system is used, and a reflective mask or a reflective mirror is used (see Patent Document 1).

A reflective mask used for EUVL includes (1) a base material, (2) a reflective multilayer film formed on the base material (for example, a Mo film as a high refractive index film, and a Si film as a low refractive index film) (Mo / Si reflective multilayer film in which a plurality of layers are alternately laminated) and (3) an absorber layer formed on the reflective multilayer film. In the case of a reflective mirror, it is basically composed of (1) a base material and (2) a reflective multilayer film formed on the base material.
As a base material (EUVL optical base material) used for manufacturing a reflective mask or a reflective mirror, a material having a small coefficient of linear thermal expansion (CTE) so that distortion does not occur even under EUV light irradiation. Is needed. As a material having a small coefficient of linear thermal expansion (CTE), SiO ₂ —TiO ₂ glass is known as an ultra-low thermal expansion material having a coefficient of linear thermal expansion smaller than that of quartz glass. Depending on the TiO ₂ content in the glass, etc. The thermal expansion coefficient can be controlled. Since a zero-expansion glass having a linear thermal expansion coefficient (CTE) close to 0 is obtained, it has been studied to use a SiO ₂ —TiO ₂ glass substrate as an EUVL optical base material.

  A glass substrate used as an EUVL optical base material (hereinafter abbreviated as “glass substrate for EUVL optical base material”) is polished on its main surface before being used for manufacturing a reflective mask or a reflective mirror. It is cleaned to a high cleanliness.

In the cleaning of the glass substrate for EUVL optical substrate, in order to remove the fine particles on the substrate surface, acid and alkali are used as a cleaning liquid, cleaning using a chemical reaction by these cleaning liquids, and cleaning using a physical action. Implemented in combination. Cleaning using physical action includes scrub cleaning, ultrasonic cleaning with a cleaning liquid applied with ultrasonic waves, and two-fluid cleaning in which N ₂ gas and cleaning liquid are mixed and jetted, but the cleaning power is high and recontamination with the cleaning liquid. Therefore, ultrasonic cleaning is preferably used.

  The ultrasonic cleaning apparatus used for such a purpose includes a batch type ultrasonic cleaning apparatus that simultaneously cleans a plurality of glass substrates using a cleaning tank or the like, and a single-wafer type cleaning apparatus that cleans glass substrates one by one. Although there is an ultrasonic cleaning device, the glass substrate for EUVL optical substrate can be cleaned with a new cleaning solution at all times, so there is no risk of recontamination by the cleaning solution, and cleaning using a chemical reaction is used in combination. In this case, since the concentration of the cleaning liquid can be kept constant, it is preferable to use a single wafer type ultrasonic cleaning apparatus.

  When cleaning a glass substrate for EUVL optical base material using a single wafer type ultrasonic cleaning device, for example, as disclosed in Patent Document 1, the glass substrate is rotated while rotating the glass substrate to be cleaned. A cleaning liquid in which ultrasonic waves are applied to the surface to be cleaned is ejected from the nozzle.

Patent Document 2 describes that when a porous silicon semiconductor substrate is ultrasonically cleaned, a fine and dense porous structure existing on the surface of the substrate is damaged by cavitation impact.
However, in the case of a glass substrate for EUVL optical base material, the main surface to be cleaned is highly polished in the stage before cleaning, so that it is smooth and compared with the porous silicon semiconductor substrate. Since it is a much harder glass material, it was thought that the surface to be cleaned was not damaged by cavitation impact. In fact, ultrasonic cleaning is widely applied to the cleaning of the main surface of quartz glass substrates used for applications such as transmission lithography mask substrates, and sufficient knowledge about suitable conditions for various parameters set during ultrasonic cleaning is obtained. It has been. As long as the ultrasonic cleaning is performed under such suitable conditions, the main surface of the quartz glass that becomes the surface to be cleaned was not damaged.

Japanese Patent No. 2944598 Japanese Patent Laid-Open No. 10-64870

When the main surface of the SiO ₂ —TiO ₂ glass substrate is subjected to ultrasonic cleaning, various parameters may be set based on suitable conditions for ultrasonic cleaning of the main surface of the quartz glass substrate.
However, when various parameters are set on the basis of suitable conditions for ultrasonic cleaning of the main surface of the quartz glass substrate and the main surface of the SiO ₂ —TiO ₂ glass substrate is ultrasonically cleaned, SiO ₂ that becomes the surface to be cleaned is obtained. It became clear that a concave defect may occur on the main surface of the —TiO ₂ glass substrate.
In the inspection of the glass substrate after cleaning, if a concave defect equivalent to 50 to 55 nm or more in terms of SiO ₂ particles is detected, there is a possibility that the required characteristics for the glass substrate for EUVL base material may not be satisfied. It becomes.
Also, when the size of the concave defect is small, it may not be detected by inspection of the glass substrate after cleaning, but when a reflective multilayer film or the like is formed on the main surface of the glass substrate during the production of a reflective mask or reflective mirror Thus, since the reflectivity of the substrate surface is improved and the concave defect is emphasized, a concave defect of 50 nm or more in terms of SiO ₂ particles may be detected. In this case, there is a possibility that the required characteristics for the reflective mask and the reflective mirror may not be satisfied.

In order to solve the above-described problems of the prior art, an object of the present invention is to provide a method for cleaning a SiO ₂ —TiO ₂ glass substrate that exhibits a high degree of cleaning while suppressing the occurrence of concave defects. .

In order to achieve the above object, the present invention supports an object to be cleaned while being rotatable about an axis, and rotates the object to be cleaned with respect to the surface to be cleaned while rotating about the object. A method of cleaning the main surface of the object to be cleaned using a single wafer type ultrasonic cleaning apparatus that ejects a cleaning liquid to which ultrasonic waves have been applied in advance from a cleaning nozzle,
The object to be cleaned is a silica glass substrate containing TiO ₂ ;
The frequency of ultrasonic waves applied to the cleaning liquid, the output of the ultrasonic waves, and the cleaning liquid sprayed from the cleaning nozzle so that the average center radius of cavitation at a position corresponding to the main surface of the object to be cleaned is 10 μm or less. Provided is a cleaning method, wherein at least one of a flow rate and a distance between a tip of the cleaning nozzle and a main surface of the object to be cleaned is adjusted.

  In the cleaning method of the present invention, the average center radius of the cavitation is preferably adjusted to be 0.1 μm or more and 10 μm or less.

  In the cleaning method of the present invention, it is preferable that the distance between the tip of the cleaning nozzle of the single wafer ultrasonic cleaning apparatus and the main surface of the object to be cleaned is 1 to 10 mm.

  In the cleaning method of the present invention, it is preferable that the frequency of the ultrasonic wave applied to the cleaning liquid sprayed from the cleaning nozzle of the single-wafer ultrasonic cleaning apparatus is 500 KHz to 6 MHz.

  In the cleaning method of the present invention, it is preferable that the output of the ultrasonic wave applied to the cleaning liquid sprayed from the cleaning nozzle of the single wafer ultrasonic cleaning apparatus is 5 to 200 W.

  In the cleaning method of the present invention, it is preferable that the flow rate of the cleaning liquid sprayed from the cleaning nozzle of the single-wafer ultrasonic cleaning apparatus is 0.5 to 3.0 L / min.

  In the cleaning method of the present invention, pure water, ammonia water, a mixed liquid of ammonia water and hydrogen peroxide water, hydrogen water, or carbonated water is used as the cleaning liquid sprayed from the cleaning nozzle of the single wafer ultrasonic cleaning apparatus. Is preferred.

In the cleaning method of the present invention, the content of TiO ₂ in the silica glass substrate is preferably 3 to 10% by mass.

  In the cleaning method of the present invention, acid cleaning, scrub cleaning, or UV cleaning of the main surface of the object to be cleaned may be performed before or after the cleaning using the single-wafer ultrasonic cleaning apparatus.

In the cleaning method of the present invention, it is preferable that the main surface of the object to be cleaned is cleaned a plurality of times using a single wafer ultrasonic cleaning apparatus.
In this case, the ultrasonic cleaning conditions may be changed so that the cavitation size at the time of each cleaning is different.
In this case, acid cleaning, scrub cleaning, or UV cleaning of the main surface of the object to be cleaned may be performed between the cleaning performed a plurality of times.

According to the present invention, it is possible to clean the SiO ₂ —TiO ₂ glass substrate with a high degree of cleaning while suppressing the occurrence of concave defects.
The SiO ₂ —TiO ₂ glass substrate cleaned by the method of the present invention is suitable for use as an EUVL substrate.

FIG. 1 is a schematic diagram showing a configuration example of a branch and leaf type ultrasonic cleaning apparatus used in the cleaning method of the present invention.

The cleaning method of the present invention is a method for cleaning a SiO ₂ —TiO ₂ glass substrate. In particular, it is preferable as a method for cleaning a SiO ₂ —TiO ₂ glass substrate used as an EUVL optical substrate.
Hereinafter, the cleaning method of the present invention will be described with reference to the drawings.
In the cleaning method of the present invention, the main surface of the SiO ₂ —TiO ₂ glass substrate, which is an object to be cleaned, is cleaned using a single-wafer ultrasonic cleaning apparatus. Here, the main surface of the SiO ₂ —TiO ₂ glass substrate refers to a surface that functions as a substrate when in use, and specifically the front and back surfaces excluding the end surfaces.

As described above, the EUVL optical substrate is required to have a low coefficient of linear thermal expansion (CTE) so that distortion does not occur even under EUV light irradiation. Specifically, the linear thermal expansion coefficient at 20 ° C. is required to be 0 ± 30 ppb / ° C., more preferably 0 ± 10 ppb / ° C., and still more preferably 0 ± 5 ppb / ° C.
SiO ₂ —TiO ₂ glass is known as an ultra-low thermal expansion material having a CTE smaller than that of quartz glass. Since CTE can be controlled by the TiO ₂ concentration in the glass, the above-mentioned CTE can be satisfied. Suitable as a material.
The TiO ₂ content in the SiO ₂ —TiO ₂ glass is preferably 3 to 10% by mass in order to satisfy the above CTE.

In addition, the EUVL optical base material has a main surface that is excellent in smoothness and flatness, specifically, a surface roughness of 0.15 nm rms or less and a flatness of 100 nm or less. Although required, the SiO ₂ —TiO ₂ glass substrate can achieve the above surface roughness and flatness by polishing the main surface.
In the substrate cleaning method of the present invention, it is mainly used for the purpose of cleaning the main surface of the SiO ₂ —TiO ₂ glass substrate that has been polished so that the surface roughness and flatness satisfy the above.

In addition, the EUVL optical substrate is required to have excellent resistance to a cleaning liquid used for cleaning a reflective mask blank for EUVL or a reflective mask for EUVL, but a SiO ₂ —TiO ₂ glass substrate satisfies this requirement. Fulfill.

The size and thickness of the EUVL optical base material are appropriately determined depending on the design value of a reflective mask or a reflective mirror produced using the EUVL optical base material. In the examples described later, a SiO ₂ —TiO ₂ glass substrate having an outer shape of 152 mm square and a thickness of 6.35 mm is assumed.

FIG. 1 is a schematic view showing a configuration example of a branch and leaf type ultrasonic cleaning apparatus used in the substrate cleaning method of the present invention.
Foliage ultrasonic cleaning apparatus 10 shown in FIG. 1, the substrate supporting mechanism 11 for supporting the SiO ₂ -TiO ₂ glass substrate 100 to be cleaned, the object to be cleaned that is supported by the substrate support mechanism 11 (SiO ₂ -TiO ₂ glass substrate 100) rotating mechanism 12 for rotating the axial center of the substrate support mechanism object to be cleaned that is supported by the 11 (SiO ₂ -TiO ₂ glass substrate 100) of the cleaning nozzle 13 injects the cleaning liquid to the cleaned surface 110 and, And a nozzle support mechanism 14 for supporting the cleaning nozzle 13.
In the branch and leaf type ultrasonic cleaning apparatus 10 shown in FIG. 1, a nozzle having a truncated cone shape is shown as the cleaning nozzle 13. Hereinafter, the cleaning nozzle having this shape is referred to as a “jet nozzle” in the present specification. The jet nozzle is preferable because the cleaning power is high because the jetting pressure of the cleaning liquid is high. However, since the jet nozzle has a small cleaning area, when the cleaning liquid is sprayed over the entire surface to be cleaned, it is necessary to scan the nozzle on the surface to be cleaned. The nozzle support mechanism 14 also functions as a nozzle scanning mechanism that scans the cleaning nozzle 13 in the direction of the arrow in the figure.
Although not shown, the mechanism for supplying the cleaning liquid to the cleaning nozzle 13 is provided with an ultrasonic generation mechanism for applying ultrasonic waves to the cleaning liquid. A cleaning liquid to which ultrasonic waves are applied in advance is ejected.
Cavitation occurs in the cleaning liquid by application of ultrasonic waves. The cleaning liquid is sprayed from the cleaning nozzle 13 onto the surface to be cleaned 110 of the object to be cleaned (SiO ₂ —TiO ₂ glass substrate 100). Cavitation in the cleaning liquid collapses when it collides with the surface to be cleaned 110 of the object to be cleaned (SiO ₂ —TiO ₂ glass substrate 100). When the cavitation breaks down, it generates a high-speed jet water flow and shock wave to exert its cleaning action.

  The branch-and-leave ultrasonic cleaning apparatus used in the substrate cleaning method of the present invention supports an object to be cleaned while being rotatable about an axis, and cleans the object to be cleaned while rotating the object to be cleaned about the axis. Any structure may be used as long as the cleaning liquid to which ultrasonic waves are applied in advance is sprayed from the cleaning nozzle on the surface, and is not limited to the illustrated apparatus.

In the substrate cleaning method of the present invention, various parameters set at the time of ultrasonic cleaning, such that the average center radius of cavitation at a position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 is 10 μm or less, Includes the frequency of the ultrasonic wave applied to the cleaning liquid, the output of the ultrasonic wave, the flow rate of the cleaning liquid sprayed from the cleaning nozzle 13, and the tip of the cleaning nozzle 13 and the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100. At least one of the distances L is adjusted.

Are we cavitation at the location corresponding to the main surface 110 of the SiO ₂ -TiO ₂ glass substrate 100, determines cavitation when striking the cleaned surface 110 of the cleaning object (SiO ₂ -TiO ₂ glass substrate 100) This is to make it an indicator.
As described above, in ultrasonic cleaning, cavitation collapses upon the surface to be cleaned 110 of the object to be cleaned (SiO ₂ —TiO ₂ glass substrate 100) upon collision, thereby generating a high-speed jet water flow and a shock wave to generate a cleaning action. Demonstrate. Here, it is considered that the larger the cavitation, the faster the jet water flow and the greater the shock wave, and thus the greater the cleaning action.
However, it is considered that the faster jet water flow and the generation of a larger shock wave cause a concave defect in the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 that is the surface to be cleaned. Therefore, the occurrence of concave defects on the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 is adjusted by adjusting the cavitation size when colliding with the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 that is the surface to be cleaned. Can be suppressed.

However, since the cavitation size at the time of collision with the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 that is the surface to be cleaned cannot be measured, the substrate support mechanism 11 of the branch-and-leaf ultrasonic cleaning apparatus 10 shown in FIG. which is a cleaning object SiO ₂ -TiO ₂ glass substrate 100 in a state that does not support, the cavitation size at a position corresponding to the main surface 110 of the SiO ₂ -TiO ₂ glass substrate 100 is measured, and this decision metrics.
In addition, the average center radius of cavitation is used as a judgment index as the cavitation size because it is considered that the cavitation particle size distribution can be accurately measured when measured by the laser scattering method described later. The average center radius of cavitation most closely reflects the measurement result of the diameter distribution.
The measurement of the particle size distribution of the cavitation at the position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 and the average center radius are performed by the following procedure.
In the state where the substrate support mechanism 11 of the branch-and-leave type ultrasonic cleaning apparatus 10 shown in FIG. 1 does not support the SiO ₂ —TiO ₂ glass substrate 100 that is the object to be cleaned, a cleaning liquid to which ultrasonic waves have been applied in advance from the cleaning nozzle 13 is ejected. To do. Laser light is irradiated from within a surface parallel to the surface to be cleaned 110 of the object to be cleaned (SiO ₂ —TiO ₂ glass substrate 100), and corresponds to the tip of the cleaning nozzle 13 and the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100. The particle size distribution of the cavitation at the position to be moved (position corresponding to the distance L between the tip of the cleaning nozzle 13 and the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100) is measured by a laser scattering method, and SiO ₂ —TiO ₂ glass is measured. An average center radius of cavitation at a position corresponding to the main surface 110 of the substrate 100 is obtained.

By setting the average center radius of cavitation at a position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 to 10 μm or less, the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 that is the surface to be cleaned is formed. The occurrence of concave defects can be suppressed.
In the substrate cleaning method of the present invention, the average center radius of cavitation at a position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 is more preferably 7 μm or less, and preferably 3.5 μm or less.
On the other hand, as apparent from the above points, the cleaning action decreases as the cavitation size at the time of collision with the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 that is the surface to be cleaned decreases. Therefore, the average center radius of cavitation at a position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 is preferably 0.1 μm or more, more preferably 0.5 μm or more, and 1 μm or more. Most preferred.

In the substrate cleaning method of the present invention, when the average center radius of cavitation at a position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 is 10 μm or less, the frequency of ultrasonic waves applied to the cleaning liquid, The adjustment of at least one of the sound wave output, the flow rate of the cleaning liquid sprayed from the cleaning nozzle 13, and the distance L between the tip of the cleaning nozzle 13 and the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 is as follows: This is because the cavitation size changes depending on these parameters.
In the substrate cleaning method of the present invention, any one of these parameters may be adjusted, or two or more may be adjusted. The preferable range of each parameter is shown below.

The frequency of the ultrasonic wave applied to the cleaning liquid ejected from the cleaning nozzle 13 is preferably 200 kHz to 6 MHz. If it is less than 200 kHz, the generation of cavitation with an average center radius exceeding 10 μm at the position corresponding to the main surface 110 of the SiO ₂ —TiO ₂ glass substrate 100 cannot be suppressed. Due to the increase and the directivity of the ultrasonic wave, there may be a problem such as unevenness in the cleaning action.
The frequency of the ultrasonic wave applied to the cleaning liquid ejected from the cleaning nozzle 13 is more preferably 500 kHz to 3 MHz, and further preferably 1 MHz to 3 MHz.

The output of the ultrasonic wave applied to the cleaning liquid ejected from the cleaning nozzle 13 is preferably 5 to 200W. If it is less than 5 W, it is difficult to stably generate cavitation because the ultrasonic intensity is low, and there is a possibility that a problem arises in making the cavitation size uniform. If it exceeds 200 W, unevenness occurs when converting electrical energy into mechanical vibration energy, which may cause a problem in making the cavitation size uniform.
The output of the ultrasonic wave applied to the cleaning liquid ejected from the cleaning nozzle 13 is more preferably 10 W to 80 W, and further preferably 15 W to 70 W.

The flow rate of the cleaning liquid ejected from the cleaning nozzle 13 is preferably 0.5 to 3.0 L / min. If it is less than 0.5 L / min, the area of the surface to be cleaned from which the cleaning liquid is ejected from the cleaning nozzle 13 is small, which may cause problems such as a decrease in cleaning power and an increase in cleaning time. If it exceeds 3.0 L / min, an increase in cost due to an increase in the amount of cleaning liquid used may be a problem.
The flow rate of the cleaning liquid ejected from the cleaning nozzle 13 is more preferably 1.0 L / min to 2.5 L / min.

  In the substrate cleaning method of the present invention, the cleaning liquid sprayed from the cleaning nozzle 13 is not particularly limited as long as the cleaning liquid contains dissolved gas. For example, pure water, ammonia water, ammonia and hydrogen peroxide A mixed solution with water, sulfuric acid, a mixed solution of sulfuric acid and hydrogen peroxide solution, dilute HF water, ozone-containing water, hydrogen water, or carbonated water can be used. Among them, pure water, ammonia water, a mixed solution of ammonia water and hydrogen peroxide water, hydrogen water, and carbonated water damage the members (cleaning nozzle 13, diaphragm (not shown), etc.) that apply ultrasonic waves to the cleaning liquid. It is preferable because the cleaning can be performed without bringing particles into the cleaning liquid.

In the cleaning of the main surface of the EUVL substrate after the polishing treatment, cleaning is usually performed a plurality of times. Also in the substrate cleaning method of the present invention, cleaning of the main surface of the SiO ₂ —TiO ₂ glass substrate after the polishing treatment may be performed a plurality of times using a single wafer ultrasonic cleaning apparatus.
By performing the cleaning of the main surface of the SiO ₂ —TiO ₂ glass substrate using a single wafer ultrasonic cleaning apparatus a plurality of times, a higher degree of cleaning can be achieved.
Cleaning of the main surface of the SiO ₂ —TiO ₂ glass substrate using a single wafer ultrasonic cleaning apparatus is preferably performed twice or more, more preferably four times or more, and six times. It is more preferable to carry out the above.
When the main surface of the SiO ₂ —TiO ₂ glass substrate is cleaned a plurality of times using a single-wafer ultrasonic cleaning apparatus, the ultrasonic cleaning conditions may be changed so that the cavitation size at each cleaning is different. . Specifically, for example, among the plurality of implementations, in the first half, the cavitation size is increased to increase the degree of cleaning, and in the second half, the cavitation size is used to suppress the occurrence of concave defects on the surface to be cleaned. It is conceivable to reduce.

In addition, for cleaning the main surface of the EUVL substrate after the polishing treatment, organic substances present on the surface to be cleaned are decomposed and removed by acid cleaning or scrub cleaning using an acid as a cleaning liquid, or UV (ultraviolet) irradiation. UV cleaning may be used. Here, scrub cleaning refers to physical cleaning in which cleaning is performed by pressing a cleaning tool such as a rotating brush against the surface to be cleaned. UV cleaning also has the effect of removing foreign matter present on the surface to be cleaned by improving the wettability of the surface to be cleaned.
In the substrate cleaning method of the present invention, acid cleaning, scrub cleaning, or UV cleaning is performed on the main surface of the SiO ₂ —TiO ₂ glass substrate before or after the cleaning using the single-wafer ultrasonic cleaning apparatus. May be. Further, when the cleaning using the single-wafer ultrasonic cleaning apparatus is performed a plurality of times, acid cleaning, scrub cleaning, or UV cleaning may be performed between the cleanings that are performed a plurality of times.
Here, the acid cleaning is preferably performed before the cleaning using the single-wafer ultrasonic cleaning device, during the cleaning using the single-wafer ultrasonic cleaning device that is performed a plurality of times, or both.
The scrub cleaning is preferably performed after the cleaning using the single-wafer ultrasonic cleaning apparatus, between the cleanings performed a plurality of times, or both.
The UV cleaning is preferably performed before and after the cleaning using the single-wafer ultrasonic cleaning apparatus, between cleanings performed a plurality of times, or both.
Further, when performing acid cleaning, scrub cleaning or UV cleaning, these cleanings may be performed a plurality of times.

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples.
An ingot of a synthetic quartz glass (SiO ₂ —TiO ₂ glass) containing 7% by mass of TiO ₂ produced by a flame hydrolysis method is formed into a plate shape having a length of 153 mm, a width of 153 mm, and a thickness of 6.75 mm. 60 pieces of synthetic quartz glass (SiO ₂ —TiO ₂ glass) plate material samples (hereinafter referred to as “sample substrates”) were produced. Further, these sample substrates were chamfered using a commercially available diamond grindstone so that the vertical and horizontal outer dimensions were 152 mm and the chamfer width was 0.2 to 0.4 mm.
Next, after polishing the main surface of the sample substrate using a double-sided lapping machine (manufactured by Speedfam) until the thickness becomes 6.51 mm, a double-sided polishing machine (manufactured by Speedfam) is used. Pre-polishing was performed so that the surface roughness (rms) was about 0.8 nm. The outer periphery of the sample substrate was also polished, and the end surface was mirror-finished to a surface roughness (Ra) of 0.05 μm.
Next, the pre-polished sample substrate was finish-polished. The used polishing slurry was used by adding nitric acid to a polishing slurry containing 20 mass% of colloidal silica having an average primary particle diameter of 10 to 20 nm in pure water and adjusting the pH to 2. The polishing pad used was a suede pad and was dressed with an electrodeposition diamond prior to the final polishing of the sample substrate.
After the sample substrate is finish-polished under the above conditions, acid cleaning and scrub cleaning are performed using a single wafer cleaning device without drying the sample substrate to remove polishing slurry and particles adhering to the surface of the sample substrate. Removed.

  After irradiating the sample substrate from which the polishing slurry and particles have been removed with an excimer UV lamp having a wavelength of 172 nm for 60 seconds, the sample substrate is divided into four quadrants in the first to fourth quadrants. Each of the four quadrants is subjected to ultrasonic cleaning for 10 minutes each using a cleaning liquid (ultra pure water) to which ultrasonic waves are applied under the following conditions using the single wafer ultrasonic cleaning apparatus shown in FIG. Rinse for a minute and spin dry. Here, ultrasonic condition 1 is a comparative example, and ultrasonic conditions 2 and 3 are examples. Further, the first quadrant where no cleaning was performed was used as a reference (reference example) for the concave defect caused by polishing.

(Ultrasonic application condition 1) (second quadrant)
Frequency: 400KHz
Output: 70W
Cleaning liquid flow rate: 2.5 L / min
Distance between the tip of the cleaning nozzle and the surface to be cleaned: 1.5 cm

(Ultrasonic application condition 2) (3rd quadrant)
Frequency: 1.5MHz
Output: 100W
Cleaning liquid flow rate: 1.5 L / min
Distance between the tip of the cleaning nozzle and the surface to be cleaned: 1.5 cm

(Ultrasonic application condition 3) (4th quadrant)
Frequency: 1.5MHz
Output: 60W
Cleaning liquid flow rate: 1.5 L / min
Distance between the tip of the cleaning nozzle and the surface to be cleaned: 4.5 cm

表から明らかなように、キャビテーションサイズと凹欠点数には相関が明確に観察され、キャビテーションサイズを小さくすることで凹欠点の発生を抑制することができる。さらに、被洗浄面に相当する位置におけるキャビテーションの平均中心半径を３．５μｍ以下とすることで、基板検査時に５０〜５５ｎｍ相当の凹欠点が検出されないレベルまで凹欠点の発生を抑制することができる。 The sample substrate after spin drying washed by the above procedure is inspected with a photomask surface defect inspection machine (MAGICS manufactured by Lasertec), the number of concave defects within 142 mm × 142 mm is measured, and the defects in the first to fourth quadrants are measured. The number was specified. Thereafter, a Mo / Si multilayer film (2.3 nm + 4.5 nm) × 40 cycles) was formed on the surface of the sample substrate, and again inspected with a surface defect inspection machine for photomasks to compare the number of concave defects. By forming a Mo / Si multilayer on the sample substrate surface, the reflectivity of the substrate surface is improved, and the concave defects are emphasized. Can be detected. The detection size of the concave defect by the inspection before forming the Mo / Si multilayer film is equivalent to 50 to 55 nm in terms of SiO ₂ particles, and the detection size of the concave defect by the inspection after forming the Mo / Si multilayer film. Is equivalent to 50 nm in terms of SiO ₂ particles.
For Examples 1 and 2 and Comparative Example 1, the cavitation size (average center radius) at a position corresponding to the distance between the nozzle tip in the ultrapure water to which ultrasonic waves were applied and the surface to be cleaned was determined by the laser scattering method. It was measured. The results are shown in the table below.

As is apparent from the table, a correlation is clearly observed between the cavitation size and the number of concave defects, and the generation of concave defects can be suppressed by reducing the cavitation size. Furthermore, by setting the average center radius of cavitation at a position corresponding to the surface to be cleaned to 3.5 μm or less, it is possible to suppress the occurrence of concave defects to a level at which a concave defect corresponding to 50 to 55 nm is not detected during substrate inspection. .

表から明らかなように、ＴｉＯ₂を含まない合成石英ガラスを用いて作製した試料基板を洗浄した場合には、キャビテーションサイズに対する凹欠点数がはるかに小さく、キャビテーションの平均中心半径を７．０μｍ以下とすることでＭｏ／Ｓｉ多層膜成膜後に５０ｎｍ相当の凹欠点を発生させることなく洗浄することが可能である。
上記の結果は、ＴｉＯ₂を含まない石英ガラス基板に比べて、ＳｉＯ₂−ＴｉＯ₂ガラス基板は、超音波洗浄時に凹欠点が発生しやすく、ＴｉＯ₂を含まない石英ガラス基板の超音波洗浄時の好適条件に基づいて、各種パラメータを設定して、ＳｉＯ₂−ＴｉＯ₂ガラス基板を超音波洗浄すると、ＳｉＯ₂−ＴｉＯ₂ガラス基板の被洗浄面に凹欠点が発生することを示している。 (Reference example)
A sample substrate manufactured using a synthetic quartz glass not containing TiO ₂ was subjected to ultrasonic cleaning in the same procedure as described above, and a substrate inspection and a post-film formation inspection were performed. The results are shown in the table below.

As is clear from the table, when a sample substrate prepared using synthetic quartz glass not containing TiO ₂ was washed, the number of concave defects relative to the cavitation size was much smaller, and the average center radius of cavitation was 7.0 μm or less. By doing so, it is possible to perform the cleaning without generating a concave defect corresponding to 50 nm after the Mo / Si multilayer film is formed.
The above results show that, compared with a quartz glass substrate not containing TiO ₂ , the SiO ₂ —TiO ₂ glass substrate is more likely to have a concave defect during ultrasonic cleaning, and during the ultrasonic cleaning of a quartz glass substrate not containing TiO _2. It is shown that when various parameters are set based on the preferred conditions and the SiO ₂ —TiO ₂ glass substrate is ultrasonically cleaned, a concave defect is generated on the surface to be cleaned of the SiO ₂ —TiO ₂ glass substrate.

Next, after finishing and polishing 40 sample substrates (SiO ₂ —TiO ₂ glass substrate) under the same conditions as described above, the sample substrate is not dried, and the sample substrate is dried using the single wafer ultrasonic cleaning apparatus shown in FIG. Sonic cleaning was performed 6 times to remove polishing slurry and particles adhering to the surface of the sample substrate. Of the 6 ultrasonic cleanings, 5 times used pure water as the cleaning liquid, and 1 time used a mixed solution of ammonia and hydrogen peroxide water in a mass ratio of 20/1 as the cleaning liquid. The washing time per time was 1 to 2.5 minutes. In the process of performing ultrasonic cleaning six times, acid cleaning (using a mixed solution of sulfuric acid and hydrogen peroxide solution at a mass ratio of 4/1) and scrub cleaning were each performed once. In the process of performing ultrasonic cleaning six times, rinsing with ultrapure water or hydrogen peroxide water is performed as appropriate, and after all cleaning is completed, rinsing with ultrapure water is performed for 2 minutes, followed by spin drying. It was.
Of the 40 sample substrates, 10 were cleaned without applying ultrasonic waves (Comparative Example 2). The other 10 sheets were subjected to ultrasonic cleaning under the ultrasonic wave application condition 2 (Example 3). Another 10 sheets were subjected to ultrasonic cleaning under the ultrasonic application condition 3 (Example 4). Another 10 sheets were subjected to ultrasonic cleaning under the following ultrasonic wave application condition 4 (Example 5).
(Ultrasonic application condition 4)
Frequency: 1.5MHz
Output: 30W
Cleaning liquid flow rate: 1.5 L / min

表から明らかなように、キャビテーションサイズと凸欠点数には相関が明確に観察され、キャビテーションサイズを大きくすることで凸欠点の発生を抑制することができる。また、超音波を印加しない洗浄では凸欠点除去性能が大きく低下する。 The sample substrate after spin drying was inspected with a photomask surface defect inspection machine (MAGICS, manufactured by Lasertec Corporation), and the number of convex defects within 142 mm × 142 mm was measured. The number of convex defects shown in the following table is an average value of inspection results on 10 sample substrates.

As is apparent from the table, a correlation is clearly observed between the cavitation size and the number of convex defects, and the occurrence of convex defects can be suppressed by increasing the cavitation size. In addition, the convex defect removing performance is greatly reduced by cleaning without applying ultrasonic waves.

10: Branch and leaf type ultrasonic cleaning device 11: Substrate support part 12: Rotating mechanism 13: Cleaning nozzle 14: Nozzle support mechanism (nozzle scanning mechanism)
100: SiO ₂ —TiO ₂ glass substrate 110: main surface (surface to be cleaned)

Claims (11)

The cleaning object is sprayed from the cleaning nozzle by applying ultrasonic waves to the surface to be cleaned while rotating the object to be cleaned while supporting the object to be cleaned while being rotatable about the axis. A method for cleaning the main surface of an object to be cleaned using a single wafer ultrasonic cleaning apparatus,
The object to be cleaned is a silica glass substrate containing TiO ₂ ;
The frequency of the ultrasonic wave applied to the cleaning liquid, the output of the ultrasonic wave, and the cleaning nozzle so that the average center radius of cavitation at a position corresponding to the main surface of the object to be cleaned is 0.1 μm or more and 10 μm or less. A cleaning method comprising adjusting at least one of a flow rate of cleaning liquid to be sprayed and a distance between a tip of the cleaning nozzle and a main surface of the object to be cleaned. The cleaning method according to claim 1, wherein the distance between the tip of the cleaning nozzle of the single wafer ultrasonic cleaning apparatus and the main surface of the object to be cleaned is 1 to 10 mm . The cleaning method according to claim 1 or 2, wherein a frequency of an ultrasonic wave applied to a cleaning liquid sprayed from a cleaning nozzle of the single wafer ultrasonic cleaning apparatus is set to 500 KHz to 6 MHz . The washing | cleaning method in any one of Claims 1-3 which makes the output of the ultrasonic wave applied to the washing | cleaning liquid injected from the washing | cleaning nozzle of the said single wafer type ultrasonic washing apparatus 5 to 200W . The cleaning method according to any one of claims 1 to 4, wherein a flow rate of a cleaning liquid sprayed from a cleaning nozzle of the single wafer ultrasonic cleaning apparatus is set to 0.5 to 3.0 L / min . The pure water, ammonia water, a mixed solution of ammonia water and hydrogen peroxide water, hydrogen water, or carbonated water is used as the cleaning liquid ejected from the cleaning nozzle of the single wafer ultrasonic cleaning apparatus. A cleaning method according to claim 1 . The TiO ₂ content in the silica glass substrate is 3 to 10 wt%, The method of cleaning according to claim 1. The acid cleaning, scrub cleaning or UV cleaning of the main surface of the object to be cleaned is performed at least before or after the cleaning using the single-wafer ultrasonic cleaning apparatus. Cleaning method . The cleaning method according to claim 1, wherein the main surface of the object to be cleaned is cleaned a plurality of times using a single-wafer ultrasonic cleaning apparatus . The cleaning method according to claim 9, wherein the object to be cleaned is cleaned under different ultrasonic cleaning conditions so that the cavitation sizes at the time of each cleaning are different . The cleaning method according to claim 9 or 10, wherein acid cleaning, scrub cleaning, or UV cleaning of the main surface of the cleaning object is performed between cleanings that are performed a plurality of times .

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