JP5090633B2 - Glass substrate polishing method - Google Patents

Description

The present invention relates to a polishing how the glass substrate, those about the particular method of polishing a glass substrate such as a reflection type mask used for EUV (Extreme Ultra Violet) lithography of the semiconductor manufacturing process.

  Conventionally, in lithography technology, an exposure apparatus for manufacturing an integrated circuit by transferring a fine circuit pattern onto a wafer has been widely used. As integrated circuits become highly integrated, faster, and more functional, miniaturization of integrated circuits advances, and the exposure apparatus is required to image a high-resolution circuit pattern on the wafer surface with a deep focal depth. The wavelength of the exposure light source is being shortened. As an exposure light source, an ArF excimer laser (wavelength 193 nm) has started to be used further from the conventional g-line (wavelength 436 nm), i-line (wavelength 365 nm), and KrF excimer laser (wavelength 248 nm). In order to cope with next-generation integrated circuits in which the circuit line width is 100 nm or less, it is considered promising to use an F2 laser (wavelength 157 nm) as an exposure light source. It seems that it cannot respond.

  Furthermore, in such a technical trend, a lithography technique using EUV light (extreme ultraviolet light) as an exposure light source for the next generation 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 as a lithography light source is being studied. The exposure principle of this EUV lithography (hereinafter abbreviated as “EUVL”) is the same as that of conventional lithography in that the mask pattern is transferred using a projection optical system, but light is transmitted in the EUV light energy region. Since there is no material to be used, the refractive optical system cannot be used, and a reflective optical system is used. (See Patent Document 1)

  A mask used for EUVL basically includes (1) a substrate, (2) a reflective multilayer film formed on the substrate, and (3) an absorber layer formed on the reflective multilayer film. As the reflective multilayer film, one having a structure in which a plurality of materials having different refractive indexes with respect to the wavelength of exposure light are periodically stacked in the order of nm is used, and Mo and Si are known as representative materials. . Also. Ta and Cr have been studied for the absorber layer. As the substrate, a material having a low thermal expansion coefficient is required so that distortion does not occur even under EUV light irradiation, and glass and crystallized glass having a low thermal expansion coefficient are being studied. The substrate is manufactured by polishing and cleaning these glass and crystallized glass materials with high accuracy.

  In general, a method for polishing a magnetic recording medium substrate, a semiconductor substrate, or the like to a highly smooth surface is known. For example, Patent Document 2 discloses polishing for polishing a hard disk of a memory hard disk or polishing a substrate for a semiconductor element that reduces the surface roughness of a polished object after polishing and reduces surface defects such as fine protrusions and polishing scratches. As a method, it describes that a substrate to be polished is polished using a polishing composition containing water, an abrasive and an acid compound, having an acidic pH and an abrasive concentration of less than 10% by weight. . Examples of the abrasive include aluminum oxide, silica, cerium oxide, and zirconium oxide, and examples of acids for making the pH acidic include nitric acid, sulfuric acid, hydrochloric acid, and organic acids.

Further, in Patent Document 3, the surface of a magnetic disk substrate material is softened with a corrosive whose pH is adjusted by adding nitric acid to aluminum nitrate, for example, and then soft colloidal particles such as colloidal silica are used. A method for removing the softened layer is described.
Japanese translation of PCT publication No. 2003-505891 JP 2003-211351 A JP-A-7-240025

  However, in the polishing method of Patent Document 2, when silica particles are used as an abrasive, the particle diameter is in a wide range of 1 to 600 nm in order to improve the polishing rate, and a particularly preferable range is 20 to 200 nm. . From the viewpoint of reducing the fine protrusions and the economical aspect, the concentration of the silica particles is less than 10% by weight, and most preferably 7% by weight or less. In other words, in Patent Document 2, when the concentration of silica particles is increased, microprojections increase, so the concentration is lowered as described above. Instead, the particle diameter of silica particles is set to 1 to 600 nm to obtain a desired polishing rate. It is thought that. As a result, the surface smoothness of the magnetic disk substrate polished with this abrasive is limited in surface roughness (Ra), although microprojections are reduced, and (Ra) in the example is 0.2. It is ˜0.3 nm. In other words, the polishing method of Patent Document 2 can only provide polishing with a surface roughness (Ra) of about 0.2 to 0.3 nm.

  Thus, with surface smoothness having a surface roughness (Ra) of 0.2 to 0.3 nm, a glass substrate for a reflective mask used for EUVL, in particular, an optical system of an exposure apparatus for producing semiconductors of 45 nm or later generations. It is difficult to use it as a glass substrate that requires extremely high surface accuracy and smoothness, such as the reflective mask used in the above.

  Further, the average surface roughness of the disk substrate obtained by the polishing method of Patent Document 3 is 0.158 nm even at the best shown in the examples. Similarly, it is insufficient as a reflective mask used in the optical system of the exposure apparatus.

  The present invention has been made in view of the above problems, and a method of polishing a glass substrate that requires extremely high surface smoothness and surface accuracy, such as a glass substrate for a reflective mask used in EUVL, and An object of the present invention is to provide a glass substrate having a smaller surface roughness than conventional ones.

  In order to solve the above-mentioned problems, the present inventors have intensively studied polishing of a glass substrate for a reflective mask that can be used in an optical system of an exposure apparatus for semiconductor production of generations of 45 nm and subsequent generations. The inventors have found that the surface of the polishing slurry containing smaller colloidal silica and water can be polished to a surface having a small surface roughness by adjusting the pH to be acidic, thereby completing the present invention.

That is, the present invention provides the following polishing how that can polish a glass substrate with a high surface precision.
(1) A glass substrate polishing method for polishing a glass substrate mainly composed of SiO ₂ for a reflective mask used in an optical system of an exposure apparatus for semiconductor manufacturing using EUV light, using a polishing slurry, The polishing slurry contains colloidal silica, water, and acid. The colloidal silica has an average primary particle diameter of 5 nm or more and less than 20 nm, and the content of the colloidal silica in the polishing slurry is 10 to 30% by mass. Further, the pH of the polishing slurry is adjusted to be in the range of 1 to 3, and the surface roughness Rms measured with an atomic force microscope on the surface of the glass substrate using the polishing slurry is 0. Polishing method of glass substrate, wherein polishing is performed to 10 nm or less.
(2) The method for polishing a glass substrate according to (1), wherein the number of concave defects having a width of 60 nm or more is 3 or less within a range of 142 mm × 142 mm.
(3) The glass according to (1) or (2), wherein the glass substrate after being polished with the polishing slurry is washed with a hot solution of sulfuric acid and hydrogen peroxide solution and further washed with a neutral surfactant solution. A method for polishing a substrate.
(4) The method for polishing a glass substrate according to any one of the above (1) to (3), wherein the surface of the glass substrate is preliminarily polished and then finish-polished with the polishing slurry.

  According to the present invention, a glass substrate can be polished to a surface with extremely small surface roughness, smoothness and high surface accuracy, so that it can be used as a reflective mask or the like required for an optical system of a semiconductor manufacturing exposure apparatus of 45 nm or later generation It is possible to obtain a highly accurate glass substrate excellent in smoothness.

  Further, since the concentration of colloidal silica contained in the polishing slurry can be increased, the glass substrate can be efficiently polished at a desired polishing rate.

  In the present invention, the glass to be polished for a glass substrate has a small coefficient of thermal expansion and a variation of the variation in order to obtain a glass substrate that can be used as a reflective mask for EUVL that can cope with high integration and high definition of integrated circuits. Small glass is used. Specifically, low-expansion glass having a thermal expansion coefficient of 0 ± 30 ppb / ° C. at 20 ° C. is preferable, and ultra-low expansion glass having a thermal expansion coefficient of 0 ± 10 ppb / ° C. at 20 ° C. is particularly preferable. If the reflective mask is formed of glass having such a small coefficient of thermal expansion, a high-definition circuit pattern can be satisfactorily transferred in response to temperature changes in the semiconductor manufacturing process.

As the low-expansion glass and the ultra-low expansion glass, quartz glass mainly composed of SiO ₂ can be used. Specific examples include low-expansion glass or low-expansion crystal glass such as synthetic quartz glass containing SiO ₂ as a main component and TiO ₂ , ULE (registered trademark: Corning Code 7972), ZERO DUR (registered trademark of German Shot). be able to. The glass substrate is usually polished with a rectangular plate-like body, but the shape is not limited thereto.

The polishing method of the present invention can be carried out using a polishing slurry containing colloidal silica and water and adjusted to have a pH in the range of 1 to 4. That is, the present invention polishes a glass substrate with a polishing slurry containing colloidal silica (silica particles) as an abrasive, an acid for adjusting pH, and water to be slurried. Here, the average primary particle diameter of colloidal silica is 50 nm or less, preferably less than 20 nm, more preferably less than 15 nm. Further, the lower limit of the average primary particle diameter of the colloidal silica is not limited, but is preferably 5 nm or more, more preferably 10 nm or more from the viewpoint of improving the polishing efficiency. When the average primary particle diameter of colloidal silica is more than 50 nm, it becomes difficult to polish the glass substrate to a desired surface roughness, and the glass substrate is suitable for optical system parts of exposure apparatuses for semiconductor manufacturing of generations of 45 nm and later. May not be obtained. Moreover, as colloidal silica, it is desirable to contain as little secondary particles as possible by agglomerating primary particles from the viewpoint of finely managing the particle diameter. Even when secondary particles are included, the average particle size is preferably 70 nm or less. The particle size of the colloidal silica in the present invention, is obtained by measuring the fifteen to one hundred and five × 10 ³ times of the image using the SEM (scanning electron microscope).

Moreover, content of colloidal silica is 10-30 mass% in polishing slurry . Preferably it is 18-25 mass% , and it is especially preferable that it is 18-22 mass%. When the colloidal silica content is less than 10% by mass, the polishing efficiency is lowered, and therefore, the polishing time becomes long. In particular, in the present invention, since colloidal silica having a fine average primary particle diameter is used as an abrasive as described above, if the colloidal silica content is less than 10% by mass, the polishing efficiency is deteriorated and economical polishing is obtained. It may not be possible. On the other hand, if the content of colloidal silica exceeds 30% by mass, the amount of colloidal silica used is increased, which is not preferable from the viewpoints of economy and detergency.

In the present invention, the polishing slurry is adjusted with acid to have a pH of 1 to 4, preferably 1 to 3, more preferably 1.8 to 2.5 as described above. The purpose of adjusting the pH of the polishing slurry in the present invention is substantially the same as that in the conventional acidic polishing. By making the polishing slurry acidic in this way, the surface of the glass substrate is chemically and mechanically polished. It becomes possible to polish it. That is, when mechanical polishing is performed with an acidic polishing slurry, the convex portions on the glass surface are softened by the acid of the polishing slurry, so that the convex portions can be easily removed by mechanical polishing. As a result, the polishing efficiency is improved, and the glass powder or glass dust removed by polishing is softened, so that it is possible to prevent the generation of new scratches due to the glass dust or the like. Therefore, a method for adjusting the pH of the polishing slurry to be acidic is effective as a method for efficiently polishing the glass substrate with good smoothness . When the pH is less than 1, corrosion of the polishing machine is at a level that does not cause a problem, but handling of the polishing slurry is deteriorated. On the other hand, if the pH is higher than 4, the chemical polishing effect on the glass is lowered, which is not preferable.

  In the present invention, the pH adjustment of the polishing slurry can be performed by using an acid selected from an inorganic acid or an organic acid alone or in combination. For convenience, many inorganic acids or organic acids known as pH adjusters for acidic polishing polishing slurries can be appropriately selected and used. For example, as the inorganic acid, nitric acid, sulfuric acid, hydrochloric acid, perchloric acid, phosphoric acid and the like can be mentioned. Among them, nitric acid is preferable in terms of ease of handling. Acids that are highly erodible to glass such as hydrofluoric acid cannot be used because they cause scratches. Examples of the organic acid include oxalic acid and citric acid.

  In the present invention, pure water or ultrapure water from which foreign substances have been removed can be preferably used as the water used for adjusting the concentration or slurrying of colloidal silica. In other words, the foreign matter (fine particles) to be removed is pure water having a maximum diameter of 0.1 μm or more, which is measured by a light scattering method using a laser beam or the like, regardless of the material or shape, and is substantially 1 / ml or less. Or ultrapure water is preferable. If more than 0.1 µm of foreign matter is mixed in water, the foreign matter acts as a kind of abrasive during polishing and causes surface defects such as scratches and pits on the polished surface of the glass. Therefore, it becomes difficult to obtain a high-quality polished surface with excellent smoothness. In addition, although the foreign material in water can be removed by filtration with a membrane filter or ultrafiltration, for example, the removal method is not limited to this.

  In the present invention, the polishing of the glass substrate can be performed by supplying a polishing slurry having an average primary particle size, concentration and pH of colloidal silica adjusted to a polishing apparatus. Although this polishing apparatus is not shown, for example, the glass substrate is sandwiched with a predetermined load from both sides by a polishing machine to which a polishing tool such as a nonwoven fabric or a polishing cloth is attached, and the polishing disk is glassed while supplying the polishing slurry to the polishing tool. Polishing is possible by rotating the substrate relative to the substrate. In this case, the supply amount of the polishing slurry, the polishing load, the rotation speed of the polishing disk, etc. are appropriately determined in consideration of the polishing speed and the polishing accuracy.

The polishing method of the present invention is particularly suitable as final polishing performed at the end when a glass substrate is polished in a plurality of polishing steps. For this reason, the glass substrate is preferably polished in advance to a predetermined thickness before being polished by the method of the present invention, and preliminarily polished so that the surface roughness thereof is below a certain level. This preliminary polishing can be performed by one or more polishing steps.
Although the polishing method is not limited, for example, a plurality of double-sided lapping machines are connected, and the glass substrate is preliminarily polished to a predetermined thickness and surface roughness by sequentially polishing with the polishing machine while changing the polishing material and polishing conditions. it can. The surface roughness (Rms) of this preliminary polishing is preferably, for example, 3 nm or less, more preferably 1.0 nm or less, and still more preferably 0.5 nm or less.

Furthermore, the glass substrate finally polished by the polishing method of the present invention is cleaned. By this cleaning, it is possible to remove and clean the polishing agent, polishing glass debris and other foreign matters adhering to the surface of the polished glass substrate, and further neutralize the surface of the glass substrate. Therefore, this cleaning is important as a process incidental to polishing. Insufficient cleaning not only causes inconvenience in subsequent inspection, but also prevents the required quality as a glass substrate from being obtained.
One preferred washing method is, for example, a method of first washing with a hot solution of sulfuric acid and hydrogen peroxide, then rinsing with water, and then washing with a neutral surfactant solution. However, the cleaning method is not limited to this, and other methods may be used.

  The glass substrate polished by the polishing method of the present invention has a surface roughness Rms measured by an atomic force microscope (hereinafter referred to as AFM) of 0.15 nm or less, more preferably 0.10 nm or less. By polishing the glass substrate using the polishing slurry containing colloidal silica having an average primary particle diameter of 50 nm or less as described above and adjusted to have a pH in the range of 1 to 4, A high-definition smooth surface with an Rms of 0.15 nm or less can be obtained. Here, as AFM, SPI3800N made by Seiko Instruments Inc. is used. When the surface roughness is greater than 0.15 nm, the optical system components of a 45 nm or higher generation semiconductor manufacturing exposure apparatus, for example, EUVL reflective masks and mirrors, for which higher integration and higher definition are more strongly demanded. It cannot be used as a glass substrate.

  FIG. 1 schematically illustrates a part of the surface of a glass substrate that has been finally polished with a polishing slurry and observed with, for example, a surface inspection machine M1350 (manufactured by Lasertec) in a plan view. It is explanatory drawing which shows an example of the surface roughness in the AA part of FIG. As shown in FIGS. 1 and 2, the surface of the glass substrate 3 after polishing and cleaning generally has a concave defect 1 and a convex defect 2 in addition to the surface roughness Rms. The concave defect 1 is considered to be generated due to the influence or bias of the particle size of the abrasive, and it is recognized that the larger the particle size, the larger the shape and the greater the number of generated particles. Therefore, reducing the particle size of the abrasive (colloidal silica) is also effective in reducing or reducing the concave defects. Since this concave defect 1 is formed in the glass of the glass substrate 3, it becomes a permanent problem defect that cannot be removed even by washing.

  Further, the convex defect 2 is an abrasive remaining after cleaning, impurities (foreign matter) contained in water, or the like. Unlike the concave defect 1, this convex defect can be reduced by changing the cleaning method or by using cleaning water from which foreign substances are sufficiently removed.

In the glass substrate in a preferred embodiment of the present invention, the number of concave defects 1 having a width of 60 nm or more is 3 or less, preferably 1 or less within a range of 142 mm × 142 mm. Moreover, it is preferable that the convex defect 2 having a width of 60 nm or more does not substantially exist. The width of the concave defect 1 here refers to the maximum diameter w of the concave defect 1 as shown in FIGS.
The width of the convex defect 2 is the same as this. If there are more than three concave defects 1 having a width w of 60 nm or more in the range of 142 mm × 142 mm, not only the flatness of the polished surface is deteriorated, but also when a reflective multilayer film is formed on the surface, This is not preferable because of the uneven defect.

A synthetic quartz glass ingot containing 7% by mass of TiO ₂ produced by a flame hydrolysis method is cut into a plate shape of 153.0 mm in length, 153.0 mm in width, and 6.75 mm in thickness using an inner peripheral blade slicer. 60 plate samples of synthetic quartz glass (hereinafter referred to as “sample base material”) were prepared. These were then chamfered using a commercially available NC chamfering machine using a # 120 diamond grindstone so that the vertical and horizontal outer dimensions were 152 mm and the chamfering width was 0.2 to 0.4 mm.

  This sample substrate was pre-polished by the following method. That is, first, using a 20B double-sided lapping machine manufactured by Speed Fem Co., as a sample base material, GC # 400 (manufactured by Fujimi Incorporated) substantially consisting of SiC as an abrasive is suspended in filtered water by 18 to 20% by mass. Using the slurry, the main surface was polished until the thickness reached 6.63 mm.

Furthermore, using another 20B double-sided lapping machine, the sample was prepared by using a slurry in which 18 to 20% by mass of FO # 1000 (produced by Fujimi Incorporated) whose main component is Al ₂ O ₃ as an abrasive was suspended. The substrate was polished until the thickness was 6.51 mm. Then, the outer periphery of the sample substrate was polished by 30 μm using a slurry and buff mainly composed of cerium oxide, and the end surface was mirror-finished to a surface roughness (Ra) of 0.05 μm.

  Next, these sample base materials are used as a primary polish, and a SpeedBam 20B double-side polish machine is used, LP66 (trade name made by Rhodes) as an abrasive cloth, and Mille 801A (trade name made by Mitsui Kinzoku Co., Ltd.) as an abrasive. The slurry was suspended at 10 to 12% by mass and polished on both sides by 50 μm.

  Further, a 20B double-side polish machine is used as the secondary polish, Seagull 7355 (trade name, manufactured by Toray Cortex Co., Ltd.) is used as the polishing cloth, and the abrasive is polished 10 μm on both sides using the above-mentioned Mille 801A, and then simple cleaning is performed. Carried out. The (Rms) of this pre-polished sample substrate was about 0.8 nm.

Next, 60 samples of the pre-polished sample base material were divided into 3 groups of 20 sheets, and final polishing was performed. That is, the first group used a conventional polishing slurry containing colloidal silica having an average primary particle size and water. The second group used a polishing slurry in which nitric acid was added to a polishing slurry containing the same colloidal silica and water as in the first group to adjust the pH to a predetermined value. The third group used a polishing slurry in which nitric acid was added to a polishing slurry containing colloidal silica having an average primary particle size and water according to the present invention to adjust the pH to the same as that of the second group. Table 1 shows the method for preparing the polishing slurry of each group. Further, the finish polishing conditions other than the method for preparing the polishing slurry of each group are the same and are as follows.
(Polishing conditions)
Polishing tester: Double-sided 24B polishing machine polishing pad manufactured by Hamai Sangyo Co., Ltd .: Bellatrix K7512 manufactured by Kanebo
Polishing platen rotation speed: 35rpm
Polishing time: 50 minutes Polishing load: 80 g / cm ²
Dilution water: Pure water (specific resistance value 4.2 MΩ · cm, foreign matter filtration of 0.2 μm or more)
Slurry flow rate: 10 l / min

  After final polishing of the sample substrate under the above conditions, the first tank is a 90 ° C. solution of sulfuric acid and hydrogen peroxide, the second tank is warm pure water for rinsing, and the third tank is a neutral surfactant solution. The washing tank was washed with a multistage automatic washing machine composed of a rinse tank with ultrapure water and a drying tank with IPA. The cleaned sample substrate was inspected with a surface defect inspection machine for photomasks manufactured by Lasertec, and the number of defects having a width of 60 nm or more within 142 mm × 142 mm was determined and the unevenness of the defects was determined. Each defect was counted by dividing it into a defect of 60 to 150 nm and a defect of more than 150 nm.

  Further, the surface roughness of the substrate was measured with an atomic force microscope SP13800N manufactured by Seiko Instruments Inc. The measurement of the surface roughness by this atomic force microscope was performed by measuring an arbitrary one place for each sample substrate at a range of 10 μm × 10 μm. These measurement results are shown in Table 2.

  As is apparent from Table 2, the sample base material of the first group polished with a polishing slurry having an average primary particle diameter of colloidal silica of 62 to 80 nm and without adjusting the pH has a concave defect and a convex defect of 60 nm or more. The surface roughness Rms has an average value of 0.1296 nm. On the other hand, in the second group of sample substrates polished by adjusting the pH of the first group polishing slurry to 2, the decrease in the concave defects of 60 nm or more was not so large, but the convex defects were remarkable. is decreasing. However, the average value of the surface roughness Rms was substantially the same as that of the first group because the average primary particle diameter of the colloidal silica was the same.

  On the other hand, the third group of sample substrates polished by the polishing method of the present invention uses a polishing slurry having an average primary particle diameter of colloidal silica of less than 10 to 20 nm and a pH adjusted to 2. Concave defects that could not be realized with the two groups of sample bases are significantly reduced, and the average value of the surface roughness Rms is also significantly reduced.

  Since the present invention can polish a glass substrate to a high-quality surface with extremely small surface roughness, polishing of a glass substrate such as a reflective mask or mirror used as an optical component of an exposure apparatus for semiconductor production of generations of 45 nm and later. Suitable for.

The enlarged partial top view of the grind | polished glass substrate. Explanatory drawing which shows the surface roughness in the AA part of FIG.

Explanation of symbols

1: Concave defect 2: Convex defect 3: Glass substrate

Claims (4)

A glass substrate polishing method for polishing a glass substrate mainly composed of SiO ₂ for a reflective mask used in an optical system of a semiconductor manufacturing exposure apparatus using EUV light using a polishing slurry, the polishing slurry Contains colloidal silica, water and acid, the colloidal silica has an average primary particle diameter of 5 nm or more and less than 20 nm, and the content of the colloidal silica in the polishing slurry is 10 to 30% by mass, Further, the pH of the polishing slurry is adjusted to be in the range of 1 to 3, and the surface roughness Rms of the surface of the glass substrate measured with an atomic force microscope using the polishing slurry is 0.10 nm. A method for polishing a glass substrate, comprising polishing so as to be as follows.   The method for polishing a glass substrate according to claim 1, wherein the number of concave defects having a width of 60 nm or more is 3 or less within a range of 142 mm × 142 mm.   The glass substrate polishing method according to claim 1 or 2, wherein the glass substrate after being polished with the polishing slurry is washed with a hot solution of sulfuric acid and hydrogen peroxide, and further washed with a neutral surfactant solution.   The method for polishing a glass substrate according to any one of claims 1 to 3, wherein the surface of the glass substrate is preliminarily polished, and then finish polishing is performed with the polishing slurry.

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