Requirements for Quartz Sand in Quartz Crucible Manufacturing

Quartz crucibles are indispensable containers widely applied for silicon crystal growth in advanced semiconductors and photovoltaic industries. This high-performance components are manufactured from high-purity quartz sands via electric arc fusion processes. As high-performance fused silica products, quartz crucibles feature prominent advantages including high purity, excellent temperature resistance, large available dimensions with high precision, favorable thermal insulation, energy conservation and stable quality.

As the critical feedstock for crucible production, high-purity quartz sand must meet the following requirements：

1. Purity and Impurity Content Requirements

Purity is the most essential quality metric for quartz sand, which will directly determine quartz crucibles’ purity level and impurity content. Semiconductor-grade quartz crucibles impose extremely high purity requirements on quartz sand. The purity of their inner layer are typically required to reach the 4N8 level (SiO₂ ≥ 99.998%) or higher, with an impurity content of less than 10 ppm. In comparison, the purity requirements for the outer layer are relatively lower, 4N5 grade (SiO₂ ≥ 99.995%) is sufficient.

Impurity content is also a vital metric for evaluating quartz sand quality, which will impact the service performance and lifespan of quartz crucibles. For example:

A. Alkali metal impurities

Alkali metals including Lithium (Li), Sodium (Na) and Potassium (K) severely degrade the thermal properties of quartz crucibles, lowering the temperature resistance and melting point of quartz components and deteriorating high-temperature performance. These elements also trigger localized viscosity reduction on the inner and outer surfaces of crucibles, accelerating devitrification and crystallization.

B. Transition metal impurities

Transition metals such as Iron (Fe), Copper (Cu), Nickel (Ni) and Chromium (Cr) alter the electrical conductivity of silicon melt, reducing the minority carrier lifetime of monocrystalline silicon ingots and impairing power generation efficiency of solar cells. N-type silicon ingots are especially sensitive to such impurities.

C. Aluminum element

Aluminum (Al) exists as lattice impurities within quartz. Despite its generally low concentration, aluminum is extremely difficult to remove, making it one of the key elements restricting the final quality of high-purity quartz sand. Notably, aluminum content is not required to be as low as possible. An excessively high ratio of alkali metal content to aluminum content reduces quartz viscosity and lowers the melting point of crucibles.

D. Boron element

Quartz crucibles are significantly affected by Boron (B), so low boron concentration is required for quaetz sand raw materials.

2. Particle Size Distribution Requirements

The particle size distribution of quartz sand exerts a direct impact on the forming quality and performance of crucibles. More uniform particle distribution is demanded for quartz sand used in crucible inner layers: particle sizes are generally controlled within 0.1–0.3 mm (100–300 µm), with cumulative mass fraction of particles within this range ≥90%. Less stringent particle size requirements apply to sand for outer layers.

In conventional manufacturing processes, uneven quartz sand particle sizes (particle size difference up to 11 times, surface area difference up to 130 times) lead to varied specific surface areas, adsorption capacity and crystallization rates among particles. This substantially influences the crystallization behavior of quartz crucibles. Therefore, precise control of particle size distribution is an important prerequisite to guarantee consistent and stable crucible quality.

3. Bubble Inclusion Content

Bubble inclusions are a key quality indicator for quartz sand. Bubble inclusions (micro-bubbles) mainly consist of crystal water and gases, including CO₂, H₂O, H₂O₂, N₂, CH₄, CO and other components. The strictest limits for bubble inclusion content apply to quartz sand for crucible inner layers.

This is because excessive bubble inclusions bring severe risks: On one hand, micro-bubbles in the inner layer expand under high crystal-pulling temperatures, rupturing the inner layer. Detached particles fall into silicon melt, causing edge cracking of silicon ingots and reducing crystal yield. On the other hand, gases released into silicon melt form void defects in silicon wafers in subsequent processes, leading to product scrapping.

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