Doping (Junction Formation)
Dopants are impurity elements added to the semiconductor crystal to form electrical junctions or boundaries between "n" and "p" regions in the crystal. An n-type region is an area containing an excess of electrons for conduction of electricity. A p-type region contains an excess of electron holes or acceptors. The difference in electric potentials between the two regions facilitates the flow of electrons through the circuit. The junctions form the essential element for all semiconductor functions. The most common doping methods include diffusion and ion implantation. Materials used for dopants mainly include compounds of antimony, arsenic, phosphorous, and boron, in gaseous, liquid, and solid physical states. Table 5 identifies various dopants used for both diffusion and ion implantation.
Diffusion occurs when impurity atoms or molecules migrate from an area of high concentration to an area of low concentration. Diffusion usually occurs in two steps: predeposition and drive-in.
During predeposition, the impurity dopant is added to the wafer substrate. Predeposition is done in a furnace at temperatures around 1000-1250ºC. The dopant is introduced into the furnace, and may be in the form of a gas, solid, or liquid. Gaseous dopants are mixed with an inert carrier gas, such as nitrogen or argon, and introduced into the furnace. Solid dopants are often applied in a powder form. The solid is heated and a stream of carrier gas moves the dopant into the furnace. Liquid sources are used by bubbling an inert carrier gas through the liquid dopant, and the gas saturated with the liquid is added to the furnace.
The wafers are then put into a second furnace at higher temperatures (about 1300ºC) to "drive-in" the dopant. The drive-in process usually occurs in an oxidizing atmosphere so that a protective layer of SiO2 is grown over the diffused layer.
During ion implantation, the dopants are ionized (stripped of electrons), accelerated using an electric field, and deposited in the silicon wafer. Upon striking the wafer, the dopant is embedded at various depths, depending on its mass and energy.
Typically, a gaseous dopant is ionized by electric discharge or by heat from a hot filament. The ions are separated using an electromagnetic field that bends the positively-charged particles to a selected band. This ion band is then passed through a high-current accelerator. The high-velocity beam of ions is focused on the wafer, causing the dopant ions to strike the wafer surface and penetrate. Sometimes a mask is used to implant a designated pattern on the wafer. As with diffusion, ion implantation allows the formation of junctions by changing the conductivity characteristics of precise regions in the wafer.
Implantation can damage the surface of the wafer. A high-temperature annealing step (800-1000ºC) is performed to return the wafer to its original condition and to further incorporate the dopant atoms into the silicon crystal lattice. Stack furnaces, high-energy lasers, electron beams, or flash lamps can be used for annealing.
The following are the potential hazards of doping.
- Radiofrequency (RF) and Infrared (IR) Radiation
- Thermal Burns
- Flammable, Explosive, and Pyrophoric Gases
- Toxic, Irritative, and Corrosive Gases and Liquids
- Reaction-Product Residues
- X-ray Radiation
Toxic, Irritative, and Corrosive Gases and Liquids
- Possible employee exposure to toxic, irritative, and corrosive gases and liquids (see Table 5).
- Potential chemical exposures to maintenance personnel working on reaction chambers, pumps, and other associated equipment that may contain reaction-product residues. Substances such as arsenic, arsine, phosphine, etc., may be found in ion implantation equipment.
- Arsenic. OSHA Safety and Health Topics Page.