Metallization refers to the metal layers that electrically interconnect  the various device structures fabricated on the silicon substrate.  Thin-film aluminum is the most widely used material for metallization, and is said to be the third major ingredient for IC fabrication, with the other two being silicon and SiO2. 


Al is very suitable for this purpose with its very low resistivity and its adhesion compatibility with SiO2.  A disadvantage of Al as the metallization material is its low melting temperature (660 deg C) and the low Al-Si eutectic temperature (577 deg C).  These restrict the maximum processing temperature once the Al layer has been deposited.



Fig. 1. Aluminum Metal Lines as seen under a SEM (left), a low-power

microscope (middle), and a high-power microscope (right)


Actually, aluminum alloys (lightly doped Al) such as Al-Cu are preferred to pure aluminum for metallization because these inhibit problems like electromigration and junction spiking.     


Al metal layers are usually deposited  through Physical Vapor Deposition (PVD) by sputtering.  Sputtering may be described as a series of four steps: 1) high-energy ions are generated and are used to bombard a target (the source of material for deposition); 2) the ions sputter (eject) atoms from the target; 3) the sputtered atoms reach the substrate; and 4) the sputtered atoms condense and form a thin film over the substrate.



Fig. 2. Metal lines after deposition


Upon exposure to oxygen, aluminum readily forms a native thin oxide on its surface (Al2O3), even at ambient temperature.  The presence of such an oxide layer can increase the contact resistance of the Al layer.  It can also inhibit the sputtering of an Al target or etching of an Al thin film, resulting in processing difficulties.


Al can also easily suffer from corrosion.  All it takes is the presence of a corrosive contaminant and moisture.  For instance, if phosphorus-doped silicon dioxide is deposited over Al lines, phosphoric acid can result if moisture ingress through the glass occurs.  The acid will lead to corrosion of the Al lines.


Fig. 3. Example of a Sputter Deposition System that can be used for depositing Al metal lines


Wafer Fab Links:  Incoming Wafers Epitaxy Diffusion Ion Implant Polysilicon

Dielectric Lithography/Etch Thin Films Metallization Glassivation Probe/Trim


See Also:  Properties of Various Metallizations;  IC ManufacturingWafer Fab Equipment




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