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
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
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.
Flash Evaporation
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
Induction evaporation uses radiofrequency radiation to evaporate the metal in a crucible. The metal is then
deposited as with other methods.
Sputtering
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.
Potential Hazards
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Metals and Solvents |
Potential Hazard
- Possible employee exposure to metals and solvents during evaporator cleaning
and maintenance operations. Typical metal exposures include silver. Methanol is a popular
cleaning solvent.
Possible Solutions
Additional Information
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