Introduction to Quartz Glass Materials

       The formation of quartz glass is the result of its high melt viscosity at high temperature. It is used in the manufacture of semiconductors, electric light sources, semiconductor communication devices, lasers, optical instruments, laboratory instruments, electrical equipment, medical equipment and high temperature and corrosion resistant chemical instruments, chemical industry, electronics, metallurgy, building materials and national defense industries. very broad. High-purity quartz glass can be used to make optical fibers.

       With the development of semiconductor technology, quartz glass is widely used in various processes of semiconductor production. For example, the Czochralski method converts polycrystalline into monocrystalline silicon; cleaning tanks for cleaning; diffusion tubes and groove boats for diffusion; bell jars for ion implantation, etc.

        Quartz glass is a kind of special glass that contains only a single component of silica. Due to different types, processes and raw materials, foreign countries are often called silicate glass, quartz glass, fused silica, fused silica, synthetic fused silica, and transparent, translucent, and opaque quartz without a clear concept. In my country, it is collectively referred to as quartz glass, which is mostly classified according to the process method, use and appearance, such as fused transparent quartz glass, continuous fused quartz glass, gas-smelted transparent quartz glass, synthetic quartz glass, opaque quartz glass, optical quartz glass, and semiconductor quartz glass. , Quartz glass for electric light source, etc. People are accustomed to using a simple word such as "quartz" to name this material, which is absolutely inappropriate, because "quartz" is a general term for the crystalline state of silica, which has the same physical and chemical properties as vitreous silica. There is a difference above.

        Quartz glass has extremely low thermal expansion coefficient, high temperature resistance, excellent chemical stability, excellent electrical insulation, low and stable ultrasonic delay performance, best UV spectral performance and visible light and near-infrared spectral transmission performance, and has higher mechanical properties than ordinary glass. Therefore, it is one of the indispensable excellent materials in space technology, atomic energy industry, national defense equipment, automation system, and semiconductor, metallurgy, chemical industry, electric light source, communication, light industry, building materials and other industries in modern cutting-edge technology.

        Quartz glass is made of natural crystalline quartz (crystal or pure silica) or synthetic silane by melting at high temperature. The molten product has excellent processing properties, in its high viscosity range, tubes and rods can be thermally processed like ordinary glass fine work, and high-speed machining can be carried out with abrasive tools made of diamond or silicon carbide. Processing, so as to make various complex shapes of instruments and special products. The performance of quartz glass mainly depends on its purity, followed by the process or thermal system. The existence of trace impurities will have a significant impact on the performance of quartz glass; at the same time, due to the slight negligence of the technological process or thermal system, it will bring various defects to the appearance quality, resulting in a large number of waste products.

        Purity is an important indicator of quartz glass, which has a great influence on physical and chemical properties and performance, such as devitrification, high temperature strength, softening point, light conduction, thermal stability, chemical stability, radiation resistance, fluorescence properties, etc.; in addition, Quartz glass used in the semiconductor industry has more stringent requirements for purity, and trace impurities will seriously affect the electrical properties, life and integration of semiconductor materials. Since the purity of semiconductor materials is required to be controlled below the ppb order of magnitude, the quartz glass should be controlled in the PPm order of magnitude to meet the needs of the semiconductor industry. The segregation coefficient of B is close to 1, which is difficult to remove and is one of the most harmful impurities. Cu, Fe, Ti, etc. affect the minority carrier lifetime of semiconductors. K, Na, and Li are harmful impurities that cause micro-defects in single crystal materials.

        Devitrification (also called devitrification) is an inherent defect of quartz glass. From a thermodynamic point of view, the internal energy of quartz glass is higher than that of crystalline cristobalite, which is a thermodynamically unstable metastable state. When the temperature is higher than 1000 °C When the SiO2 molecules are vibrated and accelerated, crystals will be formed after a long period of rearrangement and orientation. The devitrification is expressed by the growth rate of the crystal nucleus. The crystallization rates of opaque quartz glass at 1520°C and transparent quartz glass at 1620°C reach the maximum values respectively. Crystallization mainly occurs on the surface, followed by internal defects. The reason is that these places are easy to contaminate, causing local agglomeration of impurity ions, especially alkali ions (such as K, Na, Li, Ca, Mg, etc.) enter the network and cause viscosity decreased, and accelerated devitrification. Since the thermal expansion coefficient and specific gravity of quartz glass are similar to those of the crystallization product β-cristobalite, when used continuously at high temperature, although the crystallization area continues to expand, the volume change is not obvious, and it can still be used satisfactorily. Reduce the plastic deformation of the glass and improve the refractoriness. When the crystallized product was cooled to 800°C, a fine network of cracks appeared. When it continues to cool to 200-275 °C, the structural change of cristobalite from high temperature type to low temperature type (that is, β-cristobalite → a-cristobalite), accompanied by volume fusion, if the devitrification layer is very deep, then The quartz glass also cracked. Since crystallization often occurs in places with impurities, the surface state and surrounding refractory materials and atmosphere before high temperature use are very important.

        Quartz glass is an acid material, except for hydrofluoric acid and hot phosphoric acid, it is inert to any other acid, and it is the best acid-resistant material. The degree of corrosion of quartz glass by alkalis and salts at room temperature is also extremely small, so the use of quartz glass in these reagents is not excluded. Transparent quartz glass has better chemical stability than opaque quartz glass due to the increased surface area of the latter exposed to the etchant due to the presence of air bubbles.

        The structure of quartz glass is very relaxed, and even at high temperatures it allows ions of certain gases to diffuse through the network, with sodium ions diffusing the fastest. This property of quartz glass is particularly important for users. For example, when quartz glass is used in the semiconductor industry as a high-temperature container or a diffusion tube, since the semiconductor material requires high purity, the refractory material that is required to be in contact with the quartz glass as a furnace lining must be prepared in advance. After high temperature and cleaning treatment, alkaline impurities such as potassium and sodium are removed, and then it can be put into quartz glass for use.

        Quartz glass has its unique optical properties. It can transmit far ultraviolet spectrum, which is the best of all ultraviolet transmitting materials, and can transmit visible light and near infrared spectrum. Users can choose any desired varieties from the 185-3500mμ band range according to their needs. Because quartz glass has high temperature resistance, extremely small thermal expansion coefficient, good chemical thermal stability, bubbles, stripes, uniformity, and birefringence comparable to general optical glass, it is a high-stability optical system that works in various harsh occasions. essential optical materials. The structure of quartz glass, impurity content, OH gene, NO, CO and other contents are the main factors affecting the spectral transmittance. Poor oxygen atoms have an absorption peak at 0.24μ, and quartz glass containing OH groups has an absorption peak at 2.7μ. There will be obvious absorption peaks due to molecular vibration, and the low UV transmittance is mainly due to the atomic absorption spectrum caused by many metal impurities. Spectral characteristic curve of quartz glass Fused silica glass is a good infrared-transmitting material, but due to the existence of impurities, the ultraviolet transmittance is low. The quartz glass obtained by melting crystal with oxyhydrogen flame has an absorption peak at 0.24μ due to the defect of oxygen structure, and also contains OH groups, so the infrared transmission is extremely low. High-purity optical quartz glass refined with synthetic raw material gas is the best UV-transmitting material, but has a serious OH absorption peak at 2.7μ. Only the optical quartz glass made of synthetic raw materials through electrofusion or hydrogen-free flame melting can transmit the continuous spectrum from far ultraviolet to near infrared well.

        The thermal expansion coefficient of quartz glass is small, which is 5.5×10-7/℃, which is only 1/12~1/20 of ordinary glass. The standard stipulates that the sample should be burned to 1200℃ and then quickly thrown into cold water. Repeating more than three times is not allowed. burst. Quartz glass can also be made into a zero-expansion coefficient material after adding an appropriate amount of titanium, which is used in laser technology, astronomy and cutting-edge technology.