Wafer manufacturing process
The wafer is the basic material for manufacturing semiconductor chips. The most important raw material for semiconductor integrated circuits is silicon, so the corresponding is a silicon wafer.
Silicon is widely found in rocks and gravel in the form of silicate or silicon dioxide in nature. The manufacture of silicon wafers can be summarized into three basic steps: silicon refining and purification, single crystal silicon growth, and wafer molding.
The first is the purification of silicon. The raw materials of sand and stone are put into an electric arc furnace with a temperature of about 2000°C and a carbon source. At high temperatures, carbon and silica in the sand undergo a chemical reaction (carbon combines with oxygen, The remaining silicon), to obtain pure silicon with a purity of about 98%, also known as metallurgical grade silicon, which is not pure enough for microelectronic devices, because the electrical properties of semiconductor materials are very sensitive to the concentration of impurities, so it is suitable for metallurgical grade silicon. Further purification: the crushed metallurgical grade silicon is chlorinated with gaseous hydrogen chloride to generate liquid silane, and then through distillation and chemical reduction processes, high-purity polysilicon is obtained, with a purity of up to 99.999999999%, which becomes electronic-grade silicon.
Next is the growth of single-crystal silicon, the most commonly used method is called the Czochralski method. As shown in the figure below, high-purity polycrystalline silicon is placed in a quartz crucible and heated continuously with a graphite heater surrounded by the outside. The temperature is maintained at about 1400 ℃. The air in the furnace is usually an inert gas to melt the polycrystalline silicon. Produce unwanted chemical reactions. In order to form single crystal silicon, it is also necessary to control the direction of the crystal: the crucible is rotating with the polycrystalline silicon melt, a seed crystal is immersed in it, and the seed crystal is rotated in the opposite direction by the pulling rod while slowly and vertically. The ground is pulled upward from the silicon melt. The molten polysilicon will stick to the bottom end of the seed crystal and grow continuously in the direction in which the seed crystal lattice is arranged. Therefore, the directionality of the grown crystal is determined by the seed crystal. After it is pulled out and cooled, it grows into a single crystal silicon rod in the same direction as the crystal lattice inside the seed crystal. After being grown by the Czochralski method, the single crystal rod will be cut according to the appropriate size, and then polished to grind away the concave and convex cut marks, and then use a chemical mechanical polishing process to make at least one surface smooth as a mirror, and the wafer manufacturing is completed Up.
The diameter of the single-crystal silicon rod is determined by the speed at which the seed crystal is pulled out and the rotation speed. Generally speaking, the slower the pull-up rate, the larger the diameter of the grown single crystal silicon rod. The thickness of the cut wafer is related to the diameter. Although the preparation of semiconductor devices is only completed within a few microns of the top of the water, the thickness of the wafer generally needs to reach 1mm to ensure sufficient mechanical stress support. The thickness of the wafer will increase with the increase in diameter.
The wafer fabrication plant melts this polycrystalline silicon, and then seed crystals are planted in the molten liquid, and then slowly pulled out to form a cylindrical single crystal silicon ingot, because the silicon ingot is determined by the orientation of a crystal plane The seed crystals are gradually formed in the molten silicon raw material. This process is called "crystal growth". After the silicon ingots are cut into sections, barreled, sliced, chamfered, polished, laser engraved, and packaged, they become the basic raw material of the integrated circuit factory-silicon wafers, which are "wafers".