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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 The following are the potential hazards of metal deposition. Metals and Solvents Potential Hazard
Possible Solutions
Additional Information
 Similar photolithographic techniques that are used during device fabrication are used to deposit conductive patterns during metallization. In one technique, the metal is deposited then covered with a patterned photoresist, and subsequently etched. In another method the resist is applied first, followed by deposition of the metal. The wafer is then placed in a solvent that causes swelling of the resist. As the resist swells it lifts the overlaid metal away from the wafer surface. The following are the potential hazards of photolithography. Photoresist Chemicals Potential Hazard
Possible Solutions
Additional Information
OSHA Safety and Health Topics Pages:
Flammable Liquids, Fire Potential Hazard
Possible Solutions
Additional Information OSHA Safety and Health Topics Pages: Etching deposited metals requires a more aggressive chemical attack than etching SiO2 during device fabrication. Consequently, the resists need to be tougher. The process of silyation is frequently performed to harden the photoresist before it is subjected to etching. During silyation, silicon atoms are introduced into the surface of the organic resist. This process can be accomplished with either wet or dry procedures. Most commonly, a wet bath using either hexamethyldisilazane (HMDS) or silazone in xylene is used. The following are the potential hazards of silyation.  Reactive ion etching (RIE) is commonly used to etch metal layers. This process uses a combination of physical sputtering and chemically reactive species for etching at low pressures. RIE uses ion bombardment to achieve directional etching and a chemically reactive gas (carbon tetrafluoride, carbon tetrachloride, boron trichloride, and others) to maintain good etched layer selectivity. A wafer is placed into a chamber and given a negative electrical charge. The chamber is heated and brought to a low pressure, and then filled with a positively charged plasma of the reactive gas. The opposing electrical charges cause the rapidly moving plasma molecules to align themselves and strike the wafer surface vertically, thereby reacting with and volatizing the exposed metal layer. After etching, the remaining photoresist is stripped in a similar manner as during Device Fabrication. The following are the potential hazards of metal etching. Reactive Gases Potential Hazard
Possible Solutions
Additional Information OSHA Safety and Health Topics Pages:
After the metallized interconnections have been deposited and etched, a final step of alloying and annealing may be performed. To perform alloying, the metallized substrate is placed in a low-temperature diffusion furnace. Usually aluminum is placed in the furnace to form a low-resistance contact between the aluminum metal and silicon substrate. Finally, either during the alloy step or following it, the wafers are often exposed to a gas mixture containing hydrogen in a diffusion furnace at 400-500ºC. This annealing step is designed to optimize and stabilize the characteristics of the device by combining hydrogen with uncommitted atoms at or near the silicon-silicon dioxide interface. The following are the potential hazards of alloying and annealing. Metals Potential Hazard
Possible Solutions
Additional Information OSHA Safety and Health Topics Pages:
Silicon nitride is often used as a final passivating or protective layer for silicon devices. Passivation is accomplished with a chemical vapor deposition process using silane and ammonia gases. Sometimes a layer known as "p-glass" is deposited with a layer of SiO2 doped with phosphorous. Phosphine gas is used as a source for phosphorous for this type of deposition. The following are the potential hazards of passivation. Toxic Gases Potential Hazard
Possible Solutions
Additional Information
OSHA Safety and Health Topics Pages:
A final processing step called backlapping is sometimes performed. The backside of the wafer may be lapped or ground down using a wet abrasive solution under pressure. Backside metallization with a metal such as gold may be deposited on the back of the wafer with sputtering. This makes attachment of the separated die to the package easier in the final assembly. The following are the potential hazards of backlapping and backside metallization. Metals Potential Hazard
Possible Solutions
Additional Information OSHA Safety and Health Topics Pages:
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