Manufacturing method of synthetic quartz glass ingot and synthetic quartz glass ingot

Abstract

The invention provides a synthetic quartz glass ingot manufacturing method and a synthetic quartz glass ingot, and belongs to the technical field of quartz glass. The preparation method comprises the following steps: mixing synthetic silicon dioxide powder with an alkali metal aqueous solution serving as a crystallization accelerant, controlling the concentration of mixed sodium to be 50-1000ppm, converting the synthetic silicon dioxide into beta-cristobalite in a heating process of a vacuum furnace, heating to 1800 DEG C or above while keeping open pores, and cooling to room temperature to obtain the beta-cristobalite. Therefore, the large synthetic quartz glass ingot with few bubbles and low price can be manufactured and can be applied to many fields such as semiconductors and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of quartz glass, and in particular relates to a method for manufacturing a synthetic quartz glass ingot and the synthetic quartz glass ingot. Background Art

[0002] Synthetic silica powder can be produced by various methods, including the sol-gel method using organosilanes, granulation of fumed silica, and ion exchange with alkaline silicates. This synthetic silica powder itself has no practical application value and can only be used in various applications after sintering. The furnaces used in the sintering process are similar to those used to melt natural quartz powder, such as plasma furnaces, oxyhydrogen furnaces, continuous electric furnaces, and vacuum furnaces.

[0003] Both the oxyhydrogen melting method and the plasma melting method heat a rotating target with an oxyhydrogen burner or plasma flame, causing the raw material powder to fall and deposit on the target, thereby producing cylindrical quartz glass. The vacuum melting method, on the other hand, fills a mold made of graphite or other materials with raw material powder and heats it under reduced pressure to produce quartz glass.

[0004] There are differences in the properties of synthetic silica particles and natural quartz particles. This is because natural quartz is formed through a long process of dissolution and solidification under high temperature and high pressure, while synthetic silica particles are formed from glass under low temperature and normal pressure. For example, the density of crystallized quartz is 2.65g / cm 2 , while synthetic silica is 2.21g / cm 2 , and because of this, there are large differences in thermal conductivity and specific heat capacity.

[0005] Therefore, when synthesizing synthetic silica powder into synthetic quartz glass ingots using an oxyhydrogen burner or plasma melting, the melting temperature of natural quartz powder must be raised, or the raw material must be pulverized finer to reduce the supply volume. This also results in a smaller diameter for the resulting cylindrical synthetic quartz glass ingot.

[0006] Vacuum melting is even more different. Natural quartz powder must be melted at 1400-1600°C to transform β-quartz into β-cristobalite, then heated to above 2000°C. This is because β-cristobalite has a clear melting point of 1723°C. Furthermore, β-cristobalite is non-sticky and difficult to sinter, making it easy to degas, resulting in transparent quartz glass ingots with few bubbles. However, synthetic silica powder is prone to sintering, resulting in poor degassing during the sintering process, which leaves bubbles trapped and creates a translucent area. Therefore, synthetic silica powder cannot be vacuum melted.

[0007] As mentioned above, oxyhydrogen melting and plasma melting methods for converting synthetic silica powder into synthetic quartz glass ingots are costly and can only produce small-diameter cylindrical ingots. Furthermore, when synthetic silica powder is filled into a mold and melted in a vacuum, the synthetic silica sinters, resulting in poor degassing, the formation of bubbles, and opacity. Summary of the Invention

[0008] The object of the present invention is to overcome the defects of the prior art and provide a method for manufacturing a synthetic quartz glass ingot and a synthetic quartz glass ingot.

[0009] The present invention provides the following technical solutions:

[0010] The method for manufacturing a synthetic quartz glass ingot comprises the following steps: mixing synthetic silicon dioxide powder with a crystallization accelerator, and heating the mixture to above 1800° C. under vacuum conditions to obtain the synthetic quartz glass ingot.

[0011] Furthermore, the crystallization accelerator is sodium hydroxide or sodium oxide, and the amount of the crystallization accelerator added is 50 to 1000 wt.ppm relative to the synthetic silica powder, calculated as sodium.

[0012] Furthermore, the heating is performed in a vacuum furnace.

[0013] Furthermore, the particle size of the synthetic silicon dioxide powder is 100-300 μm.

[0014] Furthermore, the synthetic silicon dioxide powder is mixed with a crystallization accelerator and stirred, and then dried and placed into a graphite mold.

[0015] Furthermore, the inner wall of the graphite mold is coated with graphite powder or boron nitride soluble in water.

[0016] Furthermore, the vacuum degree during heating is 10 -3 Torr.

[0017] Furthermore, the heating process is: firstly heating to 1500°C within 2 hours and maintaining for 10 hours; then heating to 2000°C within 2 hours and maintaining for 5 hours.

[0018] The present invention also provides a synthetic quartz glass ingot produced by the above-mentioned method for producing a synthetic quartz glass ingot.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The invention adds alkali metal as a crystallization promoter to synthetic silica powder, converting it into beta-cristobalite during heating, thereby manufacturing quartz glass ingots in the same way as melting natural quartz powder.

[0021] Specifically, by mixing synthetic silica powder with an aqueous alkali metal solution as a crystallization promoter, controlling the sodium concentration of the mixture to 50 to 1000 ppm, converting the synthetic silica into β-cristobalite during heating in a vacuum furnace, and heating to above 1800°C while maintaining open pores, large, low-cost synthetic quartz glass ingots with minimal bubbles can be produced, which can be used in many fields such as semiconductors. DETAILED DESCRIPTION

[0022] The present invention is described in further detail below with reference to the embodiments.

[0023] Those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Where specific techniques or conditions are not specified in the examples, the methods were performed according to the techniques or conditions described in the literature in the art or according to the product specifications. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased.

[0024] The synthetic silica powder used in the present invention can be produced by known methods, but from a cost perspective, it is more ideal to use synthetic silica powder made from inexpensive alkali silicate. This synthetic silica powder can be used even without heat treatment, but heat treatment is preferred because a higher specific gravity results in fewer bubbles. Furthermore, for similar reasons, the particle size of the synthetic silica powder is preferably between 100 and 300 μm, but is not limited to this.

[0025] Crystallization promoters include hydroxides of sodium, potassium, lithium, and calcium. Ideally, synthetic silica is converted to β-cristobalite at 1200-1500°C and volatilized at 1500-1700°C under vacuum. Because sodium oxide boils at 1950°C, potassium oxide at 1370°C, lithium oxide at 2600°C, and calcium oxide at 2850°C, lithium and calcium easily remain in the synthetic quartz glass. Potassium oxide has a lower boiling point, so crystallization of synthetic silica must occur at around 1200°C, requiring a larger addition amount. The optimal crystallization agent is sodium hydroxide. Its boiling point is 1950°C, but it begins to evaporate at 1500°C in a vacuum, posing no problem. The addition amount, calculated as sodium, should be 50-1000 wt.ppm relative to the synthetic silica powder. If the addition is less than 50 wt.ppm, the conversion to β-cristobalite at 1200-1500°C will be slow and inefficient. However, due to the high purity of synthetic silica, its transformation to β-cristobalite is more than twice as fast as that of natural quartz. If the sodium concentration exceeds 1000 wt.ppm, sodium may remain in the synthetic quartz glass ingot, which is undesirable. Regarding the mixing method, known methods can be used. For example, a 5N aqueous solution of electronic-grade sodium hydroxide at a specified concentration and synthetic silica powder are placed in a resin rotary flask and mixed. After mixing, the mixture is dried using a spray dryer, hot air dryer, microwave dryer, or other drying method.

[0026] When heating this sodium-containing synthetic silica powder in a vacuum furnace, molds made of graphite, molybdenum, or tungsten can be used. Reusable graphite molds are preferred for cost reasons, but to prevent sticking, it's advisable to coat the inner surface of the mold with water-soluble graphite powder or boron nitride. Heaters can be top-mounted or side-mounted, but side-mounted designs present challenges when manufacturing large products. Therefore, it's best to use multiple graphite molds or a graphite core for porous ingots.

[0027] The present invention has no special requirements for vacuum levels; it can be maintained within the range achievable by oil-type rotary pumps and mechanical boosters, eliminating the need for vacuum control. When the temperature of synthetic silica particles placed in a mold exceeds 1200°C, they begin to sinter due to viscous flow. Adding sodium reduces viscosity, allowing sintering at lower temperatures. If sintering continues, the gaps between particles close, preventing degassing from these gaps, thus forming bubbles. To prevent sintering, at least the surface of the synthetic silica particles must be converted to β-cristobalite, which is difficult to sinter. The rate of conversion from synthetic quartz to β-cristobalite is related to the amount of sodium added as a crystallizing agent and the temperature. Sodium forms crystal nuclei at temperatures slightly above the glass transition temperature. Crystal nuclei grow at higher temperatures. Therefore, the sodium content is related to the number of crystal nuclei, while the crystal growth rate is related to temperature. For example, when the sodium content is 500 wt.ppm, the glass transition temperature is 850°C, and crystal nuclei form at around 1000°C.

[0028] The transformation to β-cristobalite and sintering occur in a similar temperature range. However, even if sintering occurs, as long as the gaps between the particles remain open, it will not hinder degassing. In view of this, taking advantage of the fact that the degree of sintering changes exponentially with time, after sintering and β-cristobalite nucleation are completed, in order to promote the growth of β-cristobalite, the temperature should be raised to a temperature at which β-cristobalite grows faster as soon as possible. Specifically, the temperature is raised from 1400°C to 1500°C within 2-3 hours and maintained at this temperature for several hours to promote the transformation to β-cristobalite.

[0029] β-cristobalite is crystalline and does not undergo sintering. Its melting point is 1723°C. The porous sintered body after β-cristobalite transformation contains numerous open pores, allowing sodium to volatilize through degassing. This process can be carried out at temperatures between 1400 and 1500°C. This is because, in a vacuum environment, even sodium oxide, with a boiling point of 1950°C, will volatilize due to vapor pressure. If the sodium content is below 1000 wt.ppm, no sodium residue will remain in the resulting synthetic quartz glass ingot.

[0030] After the sodium evaporates, the temperature is raised to above the melting point of β-cristobalite. Typically, the temperature inside the vacuum furnace is raised and maintained at approximately 2000°C. Since no sintering occurs, degassing is performed efficiently, resulting in a synthetic quartz glass ingot with minimal bubbles.

[0031] Next, examples will be described, but the present invention is not limited to the following examples.

[0032] The synthetic silica powder used in the following examples was made of sodium silicate and purchased from Ningbo Puhang Semiconductor Materials Co., Ltd. under the trade name PS200 (particle size 100-300 μm, Al: 0.05 ppm, Fe: 0.03 ppm, Na: 0.03 ppm, K: 0.05 ppm, Ca: 0.05 ppm).

[0033] The purity of the electronic industry grade sodium hydroxide used in the following examples is 99.999%, i.e., the impurity content is ≤10ppm.

[0034] Example 1

[0035] 100 kg of synthetic silica powder was mixed with 0.87 L of 5N (5 mol / L) electronics-grade sodium hydroxide (1000 ppm sodium content relative to the synthetic silica powder) in a 300 L nylon kettle. The mixture was stirred at 10 rpm for 1 hour. The mixture was then dried in a clean dryer at 200°C for 2 hours to obtain Material I.

[0036] Subsequently, 40 kg of the above-mentioned material I was filled into a graphite mold with an inner diameter of 300 mm, an outer diameter of 350 mm, and a height of 500 mm, and then placed in a vacuum furnace for heating. The pressure was first reduced to 10 -3 Torr, then began heating. The temperature was raised to 1500°C over 2 hours and held for 10 hours. Subsequently, the temperature was raised to 2000°C over 2 hours and held for 5 hours. Finally, the power was turned off, and the ingot was removed after 20 hours. The ingot had a diameter of 300 mm and a height of 250 mm. Samples were taken from the top, center, and bottom of the ingot and analyzed for bubbles and impurities. The results are shown in Table 1.

[0037] Example 2

[0038] An ingot was produced under the same conditions, except that 0.435 L of 5N electronics-grade sodium hydroxide was added (converted to a sodium content of 500 ppm relative to the synthetic silica powder). The ingot had a diameter of 300 mm and a height of 250 mm. Evaluation was performed in the same manner as in Example 1, and the results are listed in Table 1.

[0039] Example 3

[0040] An ingot was produced under the same conditions, except that 0.087 L of 5N electronics-grade sodium hydroxide was added (converted to a sodium content of 100 ppm relative to the synthetic silica powder). The ingot had a diameter of 300 mm and a height of 240 mm. Evaluation was performed in the same manner as in Example 1, and the results are shown in Table 1.

[0041] Example 4

[0042] An ingot was produced under the same conditions, except that 0.0434 L of 5N electronics-grade sodium hydroxide was added (converted to a sodium content of 50 ppm relative to the synthetic silica powder). The ingot had a diameter of 300 mm and a height of 280 mm. Evaluation was performed in the same manner as in Example 1, with the results shown in Table 1.

[0043] Table 1 shows the results of Examples 1 to 4.

[0044] Table 1 Evaluation results

[0045]

[0046] As can be seen from Table 1, as the amount of Na added gradually increases, the number of bubbles in the produced synthetic quartz glass ingot decreases.

[0047] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for producing a synthetic quartz glass ingot, characterized in that: The following steps are involved: The synthetic silica powder is mixed with a crystallization accelerator and heated to above 1800°C under vacuum conditions to produce a synthetic quartz glass ingot.

2. The method for producing a synthetic quartz glass ingot according to claim 1, wherein: The crystallization accelerator is sodium hydroxide or sodium oxide. The amount of the crystallization accelerator added is calculated based on the sodium content in the synthetic silica powder, and reaches 50 to 1000 ppm after addition.

3. The method for producing a synthetic quartz glass ingot according to claim 1, wherein: The heating is performed in a vacuum furnace.

4. The method for producing a synthetic quartz glass ingot according to claim 1, wherein: The particle size of the synthetic silicon dioxide powder is 100-300 μm.

5. The method for producing a synthetic quartz glass ingot according to claim 1, wherein: The following steps are also included: The synthetic silicon dioxide powder is mixed with a crystallization accelerator and stirred, and then dried and placed in a graphite mold.

6. The method for producing a synthetic quartz glass ingot according to claim 5, wherein: The inner wall of the graphite mold is coated with graphite powder or boron nitride soluble in water.

7. The method for producing a synthetic quartz glass ingot according to claim 1, wherein: The vacuum degree during heating is 10 -3 Torr.

8. The method for producing a synthetic quartz glass ingot according to claim 1, wherein: The heating is specifically as follows: firstly, the temperature is raised to 1500° C. within 2 hours and maintained for 10 hours; then, the temperature is raised to 2000° C. within 2 hours and maintained for 5 hours.

9. A synthetic quartz glass ingot produced by the method for producing a synthetic quartz glass ingot according to any one of claims 1 to 8.

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