After devices have been fabricated in the silicon substrate, connections must be made to link the circuits together. This process is called metallization. Metal layers are deposited on the wafer to form conductive pathways. The most common metals include aluminum, nickel, chromium, gold, germanium, copper, silver, titanium, tungsten, platinum, and tantalum. Selected metal alloys may also be used. Metallization is often accomplished with a vacuum deposition technique. The most common deposition processes include filament evaporation, electron-beam evaporation, flash evaporation, induction evaporation, and sputtering.
Filament evaporation, also called resistive evaporation, is the simplest method. This process is usually carried out in a bell jar, in which a filament is heated by thermal resistance. As the temperature rises, the metal to be deposited is melted and wets the filament. The current through the filament is increased further, until the metal vaporizes. The metal vapor then condenses on the cooler surface of the semiconductor wafers, forming the desired metal layer.
Electron-beam evaporation, frequently called "e-beam," uses a focused beam of electrons to heat the metal for deposition. The metal is kept in a water-cooled crucible and exposed to the electron beam, causing it to vaporize and condense on the wafers.
In flash evaporation, a ceramic bar is heated by thermal resistance. Wire is continuously fed from a spool until it contacts the heated bar. Upon contact, the metal evaporates and is deposited on the substrate.
Induction evaporation uses radiofrequency radiation to evaporate the metal in a crucible. The metal is then deposited as with other methods.
To perform sputtering, ions of an inert carrier gas (such as argon) are introduced into a low-pressure or
partial-vacuum atmosphere. An electric field is used to ionize the atoms and draw them to one place in the chamber called the target. The target is comprised of the metal used for deposition. When the ions strike the target, they dislodge, or sputter, these metal atoms. The dislodged atoms are then deposited in a thin film on the silicon substrate facing the target. Sputtering can be done using both direct current and radiofrequency voltages and can be used to deposit almost any material.
The following are the potential hazards of metal deposition.
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