Module 1 part B4

Production of Silicon Materials

Silicon is the second most abundant element in the Earth's crust, primarily found in various forms such as silicon carbide and silicon oxide within rocks and sands. The extraction process involves placing natural silicon-rich materials in a furnace and heating them to approximately 1600-1800 degrees Celsius, which enables the separation of silicon from other compounds. The result is a mixture of silicon in liquid form along with silicon monoxide and carbon monoxide gases. After this initial phase, the silicon produced is not highly pure, typically containing around 98% silicon, which classifies it as metallurgical grade silicon; however, this grade is unsuitable for electronic device manufacturing due to its impurities.

Purification Techniques

To attain higher purity levels required for electronic applications, various purification techniques are utilized. One common method involves solidifying the metallurgical grade silicon and treating it with hydrochloric acid gas at around 300 degrees Celsius, which produces trichlorosilane gas. This is followed by the chemical vapor deposition (CVD) process, where hydrogen gas is introduced, effectively removing impurities. The end result is a solid silicon material typically reaching purities of approximately 99.9999%, referred to as electronic grade silicon.

Crystal Growth

Once electronic grade silicon is obtained, it is transformed into a molten state by heating it in a crucible at temperatures between 1400 and 1600 degrees Celsius. Seed crystals are placed on top of the molten silicon, and by carefully controlling temperature gradients and pulling the seeds upward slowly, a single crystal, known as an ingot, begins to form. This crystallization process is critical as it determines the structural integrity and quality of the silicon ingot, which can range from 1 to 10 meters in length and come in various diameters (2 to 6 inches or more) based on application requirements.

Characteristics of Silicon Ingots

Post-crystallization, the silicon ingot undergoes several steps to ensure purity and quality. The top and bottom are trimmed, and any defects or impurities are removed. The orientation of the silicon crystal during growth is essential, as the physical properties differ depending on the crystallographic direction, with the <100> and <111> directions being particularly significant in device fabrication. For instance, the macroscopic properties of silicon along different crystallographic planes require careful consideration during the device design phase.

Identification of Silicon Type

When silicon wafers are produced from the ingots, they exhibit specific cuts which allow for the identification of their type. Visual inspections of these cuts provide immediate information regarding the crystallographic orientation of the wafers. For instance, in the case of <100> wafers, p-type silicon features a primary flat cut and an additional cut at right angles, whereas n-type silicon displays an additional cut at a 45-degree angle. In both <111> and <100> orientations, cuts indicate whether the material is p-type or n-type, essential for determining device characteristics.