What Are Eleven Nines Semiconductors?
In the world of semiconductors, ‘Eleven Nines’ refers to a semiconductor with near-perfect levels of purity, specifically 99.999999999%.
This level of purity is crucial for the production of semiconductor devices like integrated circuits (ICs). Semiconductors are often made from silicon, and at Eleven Nines purity, the silicon becomes extremely non-conductive. However, the presence of even a tiny amount of impurity can dramatically alter the electrical properties of the material, affecting how it conducts electricity, leading to potential malfunctions in the final device.
By using this highly pure silicon, manufacturers can create highly reliable and predictable semiconductors that form the building blocks of modern electronics.
Why Is Silicon Needed At Such High Purity For Semiconductors?
Silicon needs such high purity for semiconductors because even tiny traces of impurities can significantly disrupt the delicate electrical behaviour required for these devices. There are two main reasons for this:
- Precise Control Of Conductivity: Semiconductors are neither great conductors like metals nor perfect insulators. However, their conductivity can be precisely controlled. In an Eleven Nines pure silicon wafer, the electrical behaviour is dominated by the intrinsic properties of silicon itself. This allows for the introduction of specific impurities, called dopants, in very controlled amounts. These dopants create regions within the silicon with either an excess of negative charges (n-type) or positive charges (p-type).
The interaction between these n- and p-type regions forms the basis for electronic components like transistors. Any unwanted impurities can throw off this delicate balance and lead to unpredictable or malfunctioning devices.
- Minimising Recombination: For semiconductors to function efficiently, the flow of electrons and their partner holes (absence of an electron) is crucial. Impurities in the metal act as recombination centres. When an electron encounters an impurity, it can recombine with a hole, essentially cancelling out its contribution to the current flow. This reduces the overall efficiency of the device, especially in applications like solar cells where maximising electron movement is key.
How Is Eleven Nines Purity Achieved?
Achieving Eleven Nines purity for silicon in semiconductors is a multi-step process that involves meticulous refining techniques. The following is a breakdown of some common methods, though the actual process can vary significantly, depending on the desired level of purity, the manufacturer’s preferences, and the use cases:
- Starting With High-Purity Silicon Source: The journey begins with a source material that’s already quite pure, often metallurgical-grade silicon obtained from mining quartzite or sand. This silicon typically has a purity of around 98%.
- Chemical Purification: Next, several chemical processes are employed to remove impurities. These might involve techniques like:
- The Siemens Process: This method uses high-temperature chlorination to convert silicon to volatile silicon tetrachloride (SiCl₄). The SiCl₄ is then purified through distillation and subsequently decomposed back into silicon with high purity.
- Fluidised Bed Reactor (FBR): Here, silicon is reacted with hydrogen to form volatile silicon hydrides (SiH₄). These hydrides are then purified through distillation and decomposed to get ultra-pure silicon.
- Zone Refining: This technique takes advantage of the differing melting points of silicon and its impurities. A molten zone is created along a silicon rod. As this zone moves, it purifies the silicon by leaving behind impurities that solidify first at the cooler end of the rod. This process can be repeated multiple times for further purification.
- Crystal Growth: The final stage involves growing a single crystal of high-purity silicon. This is often done using a technique called the Czochralski process (CZ process). In this method, a seed crystal of high-purity silicon is dipped into molten silicon. As the seed is slowly pulled upwards and rotated, it extracts pure silicon from the melt, forming a large, single crystal with minimal impurities.
