(Invited) Dependency of Calcination Temperature on Silanol and Bubble Concentration in High−Purity Synthetic Quartz Powder
Authors: Choi Jiho , Lee Woo-Guk , Shim Tae-Hun , Park Jinsub , Park Jea-Gun 
Published: December 2024

Recently, high-purity synthetic quartz powder has emerged as a critical component in the semiconductor industry, extensively utilized to manufacture various quartz wares due to its high heat resistance (up to 1600 °C), high optical transmittance for ultraviolet and visible light wavelengths, and low thermal expanding coefficient (0.5 × 10−6/°C). In particular, quartz powder of 6N purity or higher is used in the semiconductor industry to produce blank masks for the ArF immersion process or crucibles for Si ingot growth. In this case, not only the impurities of the quartz powder but also the concentration of internal silanol groups are important factors. Silanol groups in quartz powder can induce the formation of bubbles and defects during the production of quartz glass, leading to a deterioration in both its optical transmittance and thermal expansion properties. When utilized in specific applications such as blank masks or crucibles, the presence of residual bubbles within the quartz can significantly impair the product's functionality. These bubbles strongly affect the overall quality, and in the case of blank masks, they contribute to light scattering, which can drastically reduce the precision and effectiveness of the mask in practical applications. However, It has not been reported what chemical properties of pure synthetic quartz powder produce bubbles.

In this study, we investigated the dependency of calcination temperature on silanol and bubble concentration in high-purity synthetic quartz powder. The synthetic quartz powder, which was synthesized using the sol-gel process via ion exchange process [1], the bubble and OH contents were analyzed as a function of calcination temperatures of 900°C, 1000°C, 1100°C, and 1200°C. We observed a decreasing tendency in OH content from 196.38 ppm to 18.5 ppm as the calcination temperature increased, measured using FTIR analysis via the KBr pellet method. Furthermore, through thermogravimetric analysis [Fig 1(a)], we identified the removal temperature and mechanism of various types of silanols [Fig. 1(b)] within the quartz powder. To effectively remove each silanol group, we applied a 3-step temperature calcination process at 300°C, 700°C, and 1200°C, thereby reducing the OH content within the quartz powder to 0.0 ppm[Fig. 1(c)]. Finally, quartz glass substrates were fabricated via vacuum fusion using quartz powders calcined under these conditions, and after undergoing cutting and polishing processes, their characteristics were evaluated. The bubble density and OH concentration within the glass substrates, depending on the calcination process conditions, will be presented in detail, along with an elucidation of the reduction mechanism.

Acknowledgement

This work was supported by BrainKorea21 Four and Technology Innovation Program(00266205, HFC Substitute material development having low GWP for Semiconductor process) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea)

References

[1] Choi, J.-H et al. Fumed Silica-Based Ultra-High-Purity Synthetic Quartz Powder via Sol-Gel Process for Advanced Semiconductor Process beyond Design Rule of 3 nm. Nanomaterials
13, 390 (2023)

Figure 1