Silicon is the eighth most common element in the universe by mass, but very rarely occurs as the pure element in the Earth’s crust. More than 90% of the Earth’s crust is composed of silicate minerals, making silicon the second most abundant element in the Earth’s crust (about 28% by mass), after oxygen. The base ingredient of any semiconductor-grade silicon is extracted from desert sand. If someone states that world will run out of silicon, the limiting factor won’t be the amount of available raw material.

Silicon material is found abundantly in the earth’s crust in compounds that consists of around 25% to 50% silicon dioxide. Most elemental silicon produced remains as a ferrosilicon alloy, and only approximately 20% is refined to metallurgical grade purity. About 15% of the world production of metallurgical grade silicon is further refined to semiconductor purity. This typically is the “nine-9” or 99.9999999% purity, nearly defect-free single crystalline material. It’s not easy to find a material with better purity than that of Si.

Semiconductor-grade Silicon is grown as ingots using different techniques. The growth technique has a major contribution to the number of impurities in Si. The vast majority of Si used for semiconductor devices is grown using Czochralski (CZ) method. The Si used by the CMOS foundries is typically CZ Si and it is available in up to 450 mm wafers. In the CZ method, a fused silica crucible in a furnace is filled with electronic-grade polysilicon. The desired dopant atoms are added either in the form of doped Si or in elemental form. A small single crystal seed piece of known crystal orientation is dipped into molten Si and pulled out of the melt. During the crystal pulling process, the grown Si ingot and the crucible with molten Si are rotated in opposite directions. In the Czochralski growth, the fused silica crucible dissolves slowly in the molten liquid and contributes to the number of impurities. The most common impurities are carbon, oxygen, aluminum and boron and they’ll end up in the grown Si ingot in some extent.

The detector grade Si has typically very high purity and is typically grown using Float Zone (FZ) or Magnetic CZochralski (MCZ) methods and the size of the wafers is limited to 200 mm size. Most of the Si used for pixel detectors and power electronics is using FZ silicon, because of its high purity and resistivity. The MCZ silicon differs from the typical CZ, because it has fewer impurities thanks to the magnetic fields that are applied during the crystal growth. The magnetic field reacts with the molten Si and it, and it has an impact on the concentration and amount of the impurities in the grown Si crystal.

The float zone (FZ) Si is very clean and its resistivity can be up to 30k-Ohm-cm. The major advantage is the crucible-less growth that prevents contamination of the silicon from the vessel itself. In FZ growth process an ultra-pure rod of electronic-grade polysilicon rod with a seed crystal at the end will be scanned by the heating coils. The process is carried out in an evacuated process chamber or under inert pure gas. The moving heating element will create a localized molten zone in the Si leading to high-purity single-crystal silicon.

After the growth of the ingot, the ingot will be sliced to wafers with a wire saw. The wafers will be further ground to close to the target thickness and polished. The purpose of the grinding and polishing steps is to have a smooth Si surface. The polishing step will remove damage layer on the Si surface left by the grinding process. After polishing the surface of the wafer is practically defect free and the wafers are mechanically robust. The wafers can be single side or double side polished.

Sensor grade silicon wafer processing requires cleanliness from the process line. The high resistivity Si wafers are sensitive to any organic or inorganic contamination. Any organic or metallic impurities that cannot be cleaned from the Si wafers before the high temperature annealing or oxidation (> 1000C) steps will diffuse to the lattice of Si and ruin the high resistivity (i.e., purity of material) and leads to degradation of the electrical characteristics of the devices. Even small traces of impurities will contaminate the Si wafers during annealing steps because of increased diffusion rate. The impurities can be seen as reduced lifetime of the charge carriers and as increased leakage current in silicon detectors. Metallic impurities like copper or gold ruin the semiconductor properties of Si. The so-called IC clean processes only allow very limited number of metals to be processed on them (I.e., Al, Mo, Ti etc.)