Electronic-grade silicon

Electronic Grade Silicon (EGS). As the first step in the production of electronic grade silicon (EGS), an impure grade of silicon is pulverized and reacted with anhydrous hydrochloric acid, to yield primarily tricholorosilane, HSiClg. This reaction is carried out in afluidizedbed at approximately 300°C in the presence of a catalyst. At the same time, the impurities in the starter impure silicon reactto form their respective chlorides. These chlorides are liquid at room temperature with the exception of vanadium dichloride and iron dichloride, which are soluble in HSiCl3 at the low concentration prevailing. Purification is accomplished by fractional distillation. 

Different techniques are currently used for the production of large scale electronic grade silicon. Metallurgical grade silicon powder at 300°C in the presence of a catalyst is reacted with anhydrous HC1 ... 

The diffusion length of electronic grade silicon wafers is about 0.5 mm and therefore in the order of the wafer thickness. Illumination of the backside of a silicon electrode may, as a result, influence the electrochemistry at the front side, as discussed in Section 10.3. 

These chemical processes result in electronic-grade silicon (EG-Si), with a purity of 99.999999999% that is, only one out of every billion atoms in the solid is something other than silicon To put this into perspective, imagine stacking yellow tennis balls from the earth s surface to the moon replacing only one of these with a blue ball would represent the level of impurities in EG-Si. Every year, between 15,000 and 20,000 tones of EG-Si is manufactured throughout the world for an ever-increasing number of applications. 

Since the resistivity of coals decreases sharply with increasing pyrolysis temperature (39), conductivity effects on ESR measurements should be anticipated. To investigate this possibility, the sample was ground under nitrogen to pass 200 mesh and diluted with electronic grade silicon... 

Fig. 5 Schematic process flow for making electronic-grade silicon.

Concerning eosts, silicon electronics is and will remain an expensive teehno-logy [5]. Production of electronic-grade silicon is an expensive process, as are the subsequent vacuum evaporation and lithography steps needed to make ehips out of the material. Due to the high costs, application of silicon in large-scale low-end eleetronics is not likely. 

Before we leave this section there is one additional example of the importance of CVD. All of the electronic grade silicon (EGS) used in the fabrication of silicon wafers is made by CVD. The highest purity EGS is made by decomposition of silane (SiHt), which itself can be prepared in extremely high purity. The substrate is an electrically heated U-shaped rod made of single crystal silicon. The silane decomposes to form silicon, which deposits on the U-shaped rod in the form of... 

The largest applications for semiconductors use extrinsic material. The entire electronic materials industry is built around doped silicon. However, there are applications that require intrinsic semiconductors. One such application is X-ray detectors used on transmission electron microscopes (TEMs) and scanning electron microscopes (SEMs) for chemical analysis. Unfortunately it is essentially impossible to produce pure silicon. Even electronic grade silicon contains small amounts of boron (a p-type dopant). To create intrinsic material a dopant is added that produces an excess of electrons that combine with the holes formed by the residual boron. The process involves diffusing lithium atoms into the semiconductor. Ionization of the lithium produces electrons that recombine with the holes. It is possible to produce germanium crystals with much higher purity, and intrinsic Ge detectors are used on some TEMs. 

Jach cubic unit cell (edge length a — 543 pm) contains eight Si atoms. If there are 1.0 X 10 boron atoms per cubic centimeter in a sample of pure silicon, how many Si atoms are there for every B atom in the sample Does this sample satisfy the 10 purity requirement for the electronic grade silicon ... 

Silicon is the most commonly used substrate for semiconductor wafers. It is heated with hydrogen chloride gas to form trichlorosilane. This is purified by reaction with hydrogen at high temperatures to form electronic grade silicon. Crystal growth requires a starter seed crystal placed at the end of a rod. The rod... 

We will now create a flowsheet for a process to produce elemental silicon used in the manufacture of integrated circuits and other electronic devices. This seems easy - silicon is a nontoxic element, abundant in the Earth s crust, and it is stable at reasonable temperatures and pressures. However, the lack of a means to produce electronic-grade silicon once inhibited the mass production of solid-state electronic devices, such as transistors and diodes. The complication is the extraordinary purity needed. 

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