METHOD FOR MANUFACTURING HIGH HEAT-RESISTANT QUARTZ GLASS
Technical field The present invention relates to a method for manufacturing high heat-resistant quartz glass, which is used for a high temperature poly-Si TFT-LCD (Thin Film Transistor- Liquid Crystal Display) substrate and in a semiconductor process.
Background Art Commercially available glasses are generally classified into 6 basic groups, based on the each composition thereof. Among those 6 groups, 5 groups of glasses other than quartz glass have their own specific composition. Soda-lime glass is the most commonly used glass, which is used to a manufacture of window glass or bottle glass. The soda-lime glass is made of a mixture of silicon oxide, calcium oxide and sodium oxide, and such composition is similar to that of the first glass made by human. Soda-lime glass, which is also often referred as lime glass for short, covers approximately 90% of the total glass production, since it can be manufactured with the lowest production cost and easily formed to various shapes of glasses. It has good durability to chemicals, however has poor resistance to high temperature and dramatic temperature changes. Lead-alkali glass has relatively higher cost than that of soda lime glass. However, since it has an excellent electric insulation property, it is widely used in electric fields and also used for a manufacture of a thermometer tube and artistic products. On the other hand, it has poor resistance to high temperature and dramatic temperature changes. Borosilicate glass has long been used for the longest time as a glass that has a strong resistance to thermal shock or dramatic temperature changes. With the borosilicate
glass, it is possible to work at a high temperature condition compared to soda-lime glass and lead-alkali glass, and it has very good chemical durability, however its production is difficult than those of soda-lime glass and lead-alkali glass. Although its production cost is higher than that of soda-lime glass, it is agreeable in view of its wide range of applications, for example a pipeline, a light, photochromic glass, headlight glass in cars, experimental devices, dishes and cooking utensil, and the like. Aluminosilicate glass has a strong resistance to thermal shock, and can stand the higher temperature compared to the temperature at which borosilicate glass resists. However, its production is far much difficult than that of borosilicate glass. When an electroconductive film is coated with aluminosilicate glass, the film can be served as a resistance inside an electronic circuit. 96% silica glass is referred to a glass manufactured by a proprietary process, and it can stand extreme thermal shock and very high temperature such as 900 °C, thereby being suitably used as various industrial products such as a suspended window of an electric furnace, a supporting die for drying. Additionally, it is further used as an outer window of a spaceship, since it can resist the heat generated during the approach of a spaceship to the atmosphere layer of the earth. Quartz glass is uniquely comprised of only monoxide unlike other 5 groups of glasses, and particularly composed of amorphous, i.e. non-crystalline silica. When other material is added to the quartz glass, it becomes a different type of glass. Quartz glass is the most expensive one among other glasses, and has the strongest resistance to thermal shock, and the possible highest temperature condition where it can be used is 900 °C for a long term use and 1200 °C for a short term use. In the light of many aspects, quartz glass is the best, and used to fields where specific requirements are demanded, such as mirror blank of an astronomical telescope, an optical waveguide, a crucible for crystal
development and the like. However, its production is very difficult, and the shape, which can be formed, is also restricted. In case of a semiconductor wafer, which resists high temperature condition well, the process is relatively independent of the temperature, but in case of an LCD, which uses glass as a substrate, the heat resistance of the glass should be primarily concerned, since LCD is sensitive to temperature. Glass used for TFT-LCD should meet the required conditions such as low density, high heat resistance, high durability, chemical resistance, mechanical properties and the like. Generally, the heat resistance of quartz glass, i.e. the viscosity at high temperature greatly depends on the concentration of OH group contained in the quartz glass in a way that the heat resistance is improved when the concentration of OH group is low. The heat resistance of quartz glass can be evaluated by thermal deformation, thermal shrinkage and the like, which has a close relation with the concentration of OH group.
Disclosure of invention Technical Problem For removing OH groups, conventional processes for high heat resistant quartz glass use an inert gas such as N2, He, Ar and the like, or a halogen gas such as Cl2, SiF4 and the like. When using an inert gas such as N2, the gas should be provided as a dried form such that the dew point of the gas is about -70 °C for efficient use, therefore it involves problems that the devices introduced in the process become complex and the cost is increased (See. Japanese Patent laid-open 1993-139775). When using a halogen gas, hundreds ppm of Cl or F are contained in the glass even after removing OH groups, thereby deteriorating the heat resistance (See. Japanese Patent laid-open 2000-239031). Therefore, there have been needs for a glass manufacturing method to remove OH
groups, which deteriorate the heat resistance of quartz glass, in convenient and simple way. Technical Solution The object of the present invention is to solve the above-mentioned problems in conventional arts and to provide an economic and simple process for removing OH groups contained in porous silica to lower the concentration of OH groups in the quartz glass to lOOppm or less, thereby providing quartz glass with improved heat resistance. In order to achieve the above object, the present inventors have found that the concentration of OH groups in glass can be decreased to lOOppm or less, through a rapid heating process under a vacuum condition to remove OH groups contained in porous silica and a transparent glass forming step, and thus completed the present invention which relates to a method for manufacturing quartz glass with improved heat resistance. The present invention relates to a method for manufacturing high heat resistant quartz glass comprising the steps of: a step of removing OH groups where porous silica is rapidly heated to high temperature under a vacuum condition; and a step of forming transparent glass. Further, the present invention relates to a method for manufacturing high heat resistant quartz glass, characterized in that the step of removing OH groups comprises rapid heating to 1000°C~1300°C, more preferably to 1200°C~1300°C, under a vacuum condition, at the rate of temperature elevation of 150~250 "C/min to remove OH groups. When the rate of the temperature elevation is less than 150°C7min, the effect of removing OH groups is lower, and the rate of the temperature elevation of not less than 250°C/min is hardly achieved in the current technical state of the art. Further, when the temperature is less than 1000°C, the effect of removing OH groups is lower, and when the temperature is more than 1300 °C, the glass forming process is proceeded in the state that OH groups are not removed, thereby generating damages and being impossible to obtain transparent glass.
The glass manufactured according to the present invention is characterized by having the OH concentration of 400ppm or less, and preferably lOOppm or less.
In the present invention, the rapid heating process is advantageous in an economic aspect of making the process time shorter to improve productivity, and a technical aspect of increasing the temperature at which porous silica is shrank, thereby removmg OH groups more effectively. If OH groups are not removed before the shrinkage of the porous silica, air bubbles are likely to be remained in the final glass products resulted from the subsequent transparent glass forming process. When using rapid heating for removing OH groups, the temperature is higher than the temperature elevated by a general heating process, and thus the more OH groups can be removed. In the step of removing OH groups, when the heating temperature is excessively high, interparticle agglomeration of porous silica is preceded, and thus gases in the atmosphere become locked in the glass. Those gases remained in the glass make impossible to obtain transparent glass. On the other hand, when the heating temperature is excessively low, the sintering rate of the porous silica becomes lowered, resulting significant decrease in productivity. Therefore, taking both of productivity and efficiency into consideration, the heating temperature in the step of removing OH groups is suitably in the range oflOOO °C~1300 °C , and preferably in the order of 1200 °C~1300°C . It is effective that the heating process for removing OH groups is practiced in vacuum condition. In an inert gas atmosphere such as Ar and the like or in the air, OH groups are not sufficiently removed, and further damages may be occurred after the transparent glass forming process, thereby hardly obtaining transparent glass. Under a vacuum condition of approximately 10"2 ton-, transparent glass could be obtained.
Best Mode Hereinafter, the constitution of the present invention is further described in detail, through the following examples. However, the scope of the present invention is by no means restricted or limited by the examples, which have only illustrative purpose.
Glass thickness (t), transmittance at 2.6μm (Ta), transmittance at the wavelength of 2.73μιn (Tb) were measured. With those measured values, the concentration of the residual OH groups in the glass was estimated by using the following equation. [OH]=(l/t)logl0(Ta/Tb)x910 The concentration of the residual OH groups was determined by an infrared spectrometer (JASCO, FT/IR 430).
<Example 1> Porous silica was heated in a graphite furnace at the rate of 200 °C/min, maintained at 1000°C for 10 hours to remove OH groups, heated again at the rate of 10°C/min, maintained at 1450 °C for 1 hour for glass forming and cooled. In the process, the atmosphere was maintained under vacuum of 10"2torr. The surface of the resulted glass was polished. By using an infrared spectrometer, the concentration of OH groups was detennined to be 339ppm.
<Example 2> The same process as in Example 1 was carried out, except that the heated porous silica was maintained for 20 hours in the step of removing OH groups. The resulted concentration of OH groups was 329ppm.
<Example 3> The same process as in Example 1 was carried out, except that the heated porous silica was maintained for 40 hours in the step of removing OH groups. The resulted concentration of OH groups was 292ppm.
<Example 4> The same process as in Example 2 was carried out, except that the heating temperature was 1100°C in the step of removing OH groups. The resulted concentration of OH groups was 270ppm.
<Example 5> The same process as in Example 2 was carried out, except that the heating temperature was 1200 °C in the step of removing OH groups. The resulted concentration of OH groups was 59ppm.
AExample 6> The same process as in Example 2 was carried out, except that the heating temperature was 1300 °C in the step of removing OH groups. The resulted concentration of OH groups was 25ppm.
<Comparative Example 1> Porous silica was heated in a graphite furnace at the rate of 10°C/min, maintained at 1000 °C for 10 hours to remove OH groups, heated again at the rate of 10°C/min, maintained at 1450 °C for 1 hour for glass forming and cooled. In the process, the atmosphere was maintained in Ar atmosphere. It was failed to obtain transparent glass,
thereby being impossible to determine the concentration of OH groups.
< Comparative Example 2> The same process as in Comparative Example 1 was carried out, except that the atmosphere during heating was changed to the air. It was failed to obtain transparent glass, thereby being impossible to determine the concentration of OH groups.
< Comparative Example 3> The same process as in Comparative Example 2 was carried out, except that the heating temperature was changed to 1100°C in the step of removing OH groups. It was failed to obtain transparent glass, thereby being impossible to determine the concentration of OH groups.
< Comparative Example 4> The same process as in Comparative Example 2 was carried out, except that the heating temperature was changed to 1200°C in the step of removing OH groups. It was failed to obtain transparent glass, thereby being impossible to determine the concentration of OH groups.
< Comparative Example 5> The same process as in Comparative Example 2 was carried out, except that the heating temperature was changed to 1300 °C in the step of removing OH groups. It was failed to obtain transparent glass, thereby being impossible to determine the concentration of OH groups.
Test conditions such as atmosphere during heat treatment, conditions for removing OH groups, conditions for forming transparent glass and the like, and the resulted concentration of OH groups were summarized in Table 1 below.
[Table 1]
*N.D. - Not determined
Industrial Applicability According to the present invention, it is possible to produce quartz glass, which is used for a high temperature Poly-Si TFT-LCD substrate or in a semiconductor process, through a relatively simple device and process, which comprises rapidly heating porous silica under vacuum atmosphere to effectively remove OH groups, and caπying out a transparent glass forming process. By the method of the present invention, the process time is reduced so that the quartz glass can be mass-produced in economic way. Further, the present invention provides high heat-resistant quartz glass being free of the contamination of halogen atoms.