16th August, 2013

Why the Czochralski Process needs Spruce Pine

I posted an article from the BBC earlier this week, which explained that Spruce Pine was vital to the Semicon industry. I thought I'd dig a little deeper into this, and try to explain just what about the process of making semiconductors and high-end monocrystalline solar panels makes high purity quartz such a necessity.

The Czochralski process is one way of growing a single crystal silicon ingot, the base for semiconductor chips and high efficiency solar panels. In very basic terms, the Czochralski process uses molten polysilicon and a seed crystal to slowly grow a long ingot of single crystal silicon.

From Wikipedia.

It is this process that necessitates the use of high purity quartz. The most obvious reason for using quartz is that it has a high melting temperature (>2,000°C), and thus can withstand the temperatures needed to melt polysilicon (around 1,000°C). Quartz (SiO2) also shares the same Si element as polysilicon (Si), meaning that the polysilicon is less likely to become contaminated from a quartz crucible, compared to other materials that could be used. Additionally, quartz is notably non-reactive, meaning that the crucible used in this process will not interfere with the chemical composition of the ingot. This is very important, as even the smallest flaw in a wafer can interrupt the flow of electric signals within the wafer.

This is where high purity quartz becomes vital. Quartz, in its raw form, is non-reactive. However, mined quartz contains various metallic impurities, which, when the quartz is heated, can travel from the quartz crucible to the silicon ingot, causing irregularities within the crystal. As such, very high purity quartz is required for the Czochralski process, especially within the semiconductor industry. Spruce Pine quartz is important because it is naturally very pure: Purer than any other mine yet discovered. With processing techniques, the ppm content of metallic impurities can be lowered to <10ppm. This is as pure as natural quartz can be, using current technology. Due to the unfeasible costs of synthetic quartz and the time it takes to fuse synthetic quartz, compared to natural quartz, Spruce Pine sand and the companies that use this material have become vital to the solar and semiconductor industry.