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Materials Used in Semiconductor Manufacturing

Basic Elements Used

Silicon, symbol Si, is the most commonly used basic building block of integrated circuits. Silicon is a semiconductor, which means that its electrical behavior is between that of a conductor and an insulator at room temperature. With the proper addition of dopant elements, p-n junctions can be formed on silicon. Useful electronic components and integrated circuits can be built from p-n junctions. Silicon is obtained by heating silicon dioxide (SiO2), or silica, with a reducing agent in a furnace. Silicon dioxide is the main component of ordinary sand.

Aside from being used as semiconductor substrate, silicon is also widely used as dielectric in integrated circuits, usually in the form of silicon dioxide. Dielectric layers are used to isolate conductive lines and the individual components in the circuit from each other. Polycrystalline silicon, or polysilicon, is also used for making resistors or conductors in integrated circuits. The top glassivation used to mechanically and electrically protect the die is also usually composed of silicon in the form of silicon nitride. Silicon is also widely used in semiconductor packaging, being the main ingredient of plastic encapsulants for integrated circuits. Silicon is also used in die overcoats.

Table 1. Silicon's Basic Properties

Property	Value	Property	Value
Atomic Number	14	Specific Gravity	2.33
Atomic Group	14 or IVA	Hardness	7 for crystalline silicon
Atomic Weight	28.086	Band Gap/Energy Gap	1.11 eV
Melting Point	1410 deg C	Thermal Conductivity	1.57 W/cm deg C
Boiling Temp	2355 deg C	Coeff. of Thermal Expansion	2.63e-6/deg C

Aluminum, symbol Al, is a lightweight metal with silvery appearance. It is the most abundant metallic element on earth. Aluminum is used in many aspects of semiconductor manufacturing. On the integrated circuit, Al metal lines are commonly used as the main conductor between components, mainly because of its low resistivity (2.7 mohm-cm). As a thin film, it also has good adherence to silicon dioxide. Aluminum is also the metallization used for the bonding and probing pads on the die. When used for IC metallization, Al is usually very lightly doped with other elements such as Si and/or Cu to improve its characteristics and reliability. In semiconductor assembly, ceramic packages are composed mainly of alumina. Aluminum is also used for wirebonding integrated circuits in ceramic packages.

Table 2. Aluminum's Basic Properties

Property	Value	Property	Value
Atomic Number	13	Melting Point	660 deg C
Atomic Group	13 or IIIA	Boiling Temp	2467 deg C
Atomic Weight	26.9815	Specific Gravity	2.7
Density	2.7 g/cm³	Resistivity	2.7 mohm-cm

Gold, symbol Au, is a soft metallic element that is bright yellowish in color. A good conductor of heat and electricity, it is also the most malleable and ductile of all metals. Gold is used in many aspects of semiconductor manufacturing, particularly in the assembly or packaging processes. Its most widespread use is in wirebonding. Because of gold's excellent conductivity and ductility, it is extensively used in making wires for the connection of the integrated circuit to the leads of the package. Aside from manufacturability, the ductility of gold wires offers one more advantage when used in plastic-encapsulated devices, i.e., it makes the wires resistant to wire breaking during the encapsulation process. Gold is also used as die attach material for the eutectic die attach process, which is commonly used in old hermetic assembly processes. Gold is also used to cover the die cavity and bonding posts of ceramic packages to protect these from chemical degradation.

Table 3. Gold's Basic Properties

Property	Value	Property	Value
Atomic Number	79	Melting Point	1064 deg C
Atomic Group	Transition Elements	Boiling Temp	2808 deg C
Atomic Weight	196.97	Specific Gravity	19.3

Silver, symbol Ag, is a shiny metallic element used for ornamental and coinage purposes since the ancient times. It is next only to gold in terms of malleability and ductility, and is also a good conductor of heat and electricity. In fact, silver is the best conductor of electricity, better even than copper and gold. Silver, like gold, is used in many facets of semiconductor manufacturing, again more particularly in the assembly or packaging processes. Most epoxy die attach materials contain silver fillers for increased thermal and electrical conductivity. Silver is also used to cover the surfaces of the die pad and bonding fingers of the leadframes of plastic packages to prevent chemical degradation of these areas, which may lead to die attach and bonding problems.

Table 4. Silver's Basic Properties

Property	Value	Property	Value
Atomic Number	47	Melting Point	962 deg C
Atomic Group	Transition Elements	Boiling Temp	2212 deg C
Atomic Weight	107.868	Specific Gravity	10.5

Copper, symbol Cu, is one of the most widely used metals in the history of mankind, mainly because of its many desirable properties. It is the second best conductor of electricity, next only to silver but better even than gold. It also is very malleable and ductile, and is also a good conductor of heat. Copper is also widely used in semiconductor assembly. For instance, most leadframes for plastic packages are composed of copper. The leadframe is the skeletal support of a plastic package. Copper, being an excellent conductor, would've been a very good candidate for use in metal lines in an integrated circuit, but difficulties in the manufacturing of IC's using copper for metallization resulted in Al being the metal of choice for this purpose. Recent technological advancements though have already allowed the use of copper as metal lines in semiconductor devices.

Table 5. Copper's Basic Properties

Property	Value	Property	Value
Atomic Number	29	Melting Point	1083 deg C
Atomic Group	Transition Elements	Boiling Temp	2567 deg C
Atomic Weight	63.546	Specific Gravity	8.9

Chemicals/Gases; Hazardous Chemicals; IC Manufacturing; Semiconductor Eqpt.

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