SiO2 + C → Si + CO2Silicon prepared in this manner is called “metallurgical grade” since most of the world’s production actually goes into steel-making. It is about 98% pure. MG-Si is not pure enough for direct use in electronics manufacturing. A small fraction (5% – 10%) of the worldwide production of MG-Si gets further purified for use in electronics manufacturing. The purification of MG-Si to semiconductor (electronic) grade silicon is a multi-step process, shown schematically in Figure 2. In this process, MG-Si is first ground in a ball-mill to produce very fine (75% < 40 µM) particles which are then fed to a Fluidized Bed Reactor (FBR). There the MG-Si reacts with anhydrous hydrochloric acid gas (HCl), at 575 K (approx. 300ºC) according to the reaction:
Si + 3HCl → SiHCl3 + H2
The hydrochlorination reaction in the FBR makes a gaseous product that is about 90% trichlorosilane (SiHCl3). The remaining 10% of the gas produced in this step is mostly tetrachlorosilane, SiCl4, with some dichlorosilane, SiH2Cl2. This gas mixture is put through a series of fractional distillations that purify the trichlorosilane and collect and re-use the tetrachlorosilane and dichlorosilane by-products. This purification process produces extremely pure trichlorosilane with major impurities in the low parts per billion range. Purified, solid polycrystalline silicon is produced from high purity trichlorosilane using a method known as “The Siemens Process.” In this process, the trichlorosilane is diluted with hydrogen and fed to a chemical vapor deposition reactor. There, the reaction conditions are adjusted so that polycrystalline silicon is deposited on electrically-heated silicon rods according to the reverse of the trichlorosilane formation reaction:
SiHCl3 + H2 → Si + 3HC
By-products from the deposition reaction (H2, HCl, SiHCl3, SiCl4 and SiH2Cl2) are captured and recycled through the trichlorosilane production and purification process as shown in Figure 2. The chemistry of the production, purification and silicon deposition processes associated with semiconductor grade silicon is more complex than this simple description. There are also a number of alternative chemistries that can be, and are, used for polysilicon production.
Higher purity silicon can be produced by a method known as Float Zone (FZ) refining. In this method, a polycrystalline silicon ingot is mounted vertically in the growth chamber, either under vacuum or inert atmosphere. The ingot is not in contact with any of the chamber components except for the ambient gas and a seed crystal of known orientation at its base (Figure 4). The ingot is heated using non-contact radio-frequency (RF) coils that establish a zone of melted material in the ingot, typically about 2 cm thick. In the FZ process, the rod moves vertically downward, allowing the molten zone to move up the length of the ingot, pushing impurities ahead of the melt and leaving behind highly purified single crystal silicon. FZ silicon wafers have resistivities as high as 10,000 ohm-cm.
The final stage in silicon wafer manufacture involves chemically etching away any surface layers that may have accumulated crystal damage and contamination during sawing, grinding and lapping; followed by chemical mechanical polishing (CMP) to produce a highly reflective, scratch and damage free surface on one side of the wafer. The chemical etch is accomplished using an etchant solution of hydrofluoric acid (HF) mixed with nitric and acetic acids that can dissolve silicon. In CMP, silicon slices are mounted onto a carrier and placed in a CMP machine where they undergo combined chemical and mechanical polishing. Typically, CMP employs a hard polyurethane polishing pad combined with a slurry of finely dispersed alumina or silica abrasive particles in an alkaline solution. The finished product of the CMP process is the silicon wafer that we, as users, are familiar with. It has a highly reflective, scratch and damage free surface on one side on which semiconductor devices can be fabricated.
Table 1 provides a list of the elemental and binary (two element) compound semiconductors along with the nature of their band gap and its magnitude. In addition to the binary compound semiconductors, ternary (three element) compound semiconductors are also known and used in device fabrication. Ternary compound semiconductors include materials such as aluminum gallium arsenide, AlGaAs, indium gallium arsenide, InGaAs and indium aluminum arsenide, InAlAs. Quarternary (four element) compound semiconductors are also known and used in modern microelectronics.
The unique light-emitting ability of compound semiconductors is due to the fact that they are direct band gap semiconductors. Table 1 denotes which semiconductors possess this property. The wavelength of the light emitted by devices built from direct band gap semiconductors depends on the band gap energy. By skillfully engineering the band gap structure of composite devices built from different compound semiconductors with direct band gaps, engineers have been able to produce solid state light emitting devices that range from the lasers used in fiber optic communications to high efficiency LED light bulbs. A detailed discussion of the implications of direct versus indirect band gaps in semiconductor materials is beyond the scope of this work.
Simple, binary compound semiconductors can be prepared in bulk, and single crystal wafers are produced by processes similar to those used in silicon wafer manufacturing. GaAs, InP and other compound semiconductor ingots can be grown using either the Czochralski or Bridgman-Stockbarger method with wafers prepared in a manner similar to silicon wafer production. Surface conditioning of compound semiconductor wafers, (i.e., making them reflective and flat) is complicated by the fact that at least two elements are present and these elements can react with etchants and abrasives in different fashions.
|Material System||Name||Formula||Energy Gap (eV)||Band Type(I = indirect; D = direct)|
Table 1. The elemental semiconductors and the binary compound semiconductors.