Furnace annealing is a heating and cooling method frequently used on metal, glass, ceramics, and silicon wafers. It generally improves qualities in the various materials — such as durability in glass and ceramics, ductility and strength in metals, and electrical conductivity in silicon wafers. This helps develop the machinability of the different materials. As its name implies, this process typically takes place inside an oven, known as an annealing furnace. This is generally done so that the temperature can be controlled during heating and cooling.
One of the products that furnace annealing is typically used in making is metal wire. This practice changes the internal structure of many metals, like steel, copper, and silver, among others, so that their ductility, or ability to stretch, is enhanced. This happens because the different metals are heated to just beyond the temperature at which the metals will recrystallize, meaning that the atoms in the metals move to a state of equilibrium. When metals do this, they are usually healed of any stresses that existed in them, which would cause them to break, as opposed to being stretched.
Sometimes, when metal is being put thorough this process, a bright annealing furnace is used. It typically uses either a special mix of gases, which reduce the presence of oxygen, carbon, or nitrogen that the metal is exposed to, or it can anneal metals in a vacuum. The furnace is then referred to, more specifically, as a vacuum annealing furnace. With either method, the end product maintains its luster.
The temperature for furnace annealing is dependent on the kind of material being treated. Each metal has a different critical temperature at which it will recrystallize. Similarly, glass and ceramics have differing temperatures at which their atoms move to their most stable configuration. In addition, each substance has a different time and temperature to which it should be cooled for annealing to be complete. For instance, the high temperature just above silver recrystallization is 1,778°F (970°C). It should then be cooled to 932°F (500°C) for several hours before it is fully annealed.
When furnace annealing is used to process silicon wafers for manufacture of semiconductors, a batch annealing furnace is often utilized. In this manner many silicon wafers can be annealed at once. Since this can take several hours, using a furnace capable of handling many pieces at once can be economical and time-saving.