Question

State any one process for wafer fabrication with diagram

Answer 1

Wafer Processing: The basis raw material used is a disk of silicon, which is between 75 mm to 150 mm in diameter and is less than 1mm thick used in semi-conductor plants. Wafers are cut from ignots of silicon crystal that have been pulled from a crucible melt of pure molten polysilicon silicon. This method is known as Czochralski [C-Z] method. {An ingot is a piece of material, usually metal, that is cast into a shape suitable for further processing}

For getting crystals with required electrical properties controlled amounts of impurities are added to the melt. To initiate single-crystal growth, the crystal orientation is determined by seed crystal is dipped into the melt and slowly pulled out. As the seed crystal is pulled out of the melt, it brings with it a solidified mass of silicon with the same crystalline structure as that of the seed. During the crystal pulling process, the seed crystal and the crucible are rotated in opposite direction in order to produce ingots of circular cross- section. The diameter of the ingot is controlled by pulling rate and melt temperature. Ingot diameter is about 10 to 15 cm and length of order 100 cm. The ignot is also ground flat slightly to get reference plane. The ignot is then sliced using a stainless steel blade with industrial diamonds embedded into inner diameter cutting edge. This produces circular wafers of slices. The silicon wafers obtained have very rough surface due to slicing operation. These wafers undergo a number of polishing steps to produce a flat surface. Then one side of wafer is given a fine mirror smooth highly polished finish, where as the other side is simply lapped on an abrasive lapping machine to obtain acceptable degree of flatness. Finally the wafers are thoroughly rinsed and dried. A raw of thickness about 0.6 to 1mm produces wafer of about 0.15 to 0.8 mm thickness after all polishing steps.

OR

It consists of Quartz crucible, which is surrounded by a graphite radiator. The graphite is heated by radio frequency induction heating and temperature maintained a few degrees above the melting point of silicon (approx. 1425⁰C), the atmosphere just above the polysilicon melt is typically helium or orgon for freezing. A polycrystalline Si is melted in the crucible and controlled amount of impurities (p type or n type) are added to the melt to provide the crystal with required electrical properties. After the seed (single crystal silicon piece) is dipped into the melt, the seed is gradually withdrawn vertically from the melt while simultaneously being rotated. The molten polycrystalline silicon melts the tip of the seed and it is withdrawn, refreezing occurs. As the melt freezes, it assumes the single crystal form of the seed. This process is continued until the melt is consumed. The diameter of the ingot (rod of silicon) is determined by the seed withdrawn rate and seed rotation rate. The produced crystalline silicon rod is then slicing into wafers using cutting tools like diamond blades. Following slicing at least one face of the wafer is polished to flat scratch free mirror finish surface.