Preparation and Characterization of Ohmic Contacts
The fabrication of ohmic contacts is a much-studied part of materials engineering that nonetheless remains something of an art. The reproducible, reliable fabrication of contacts relies on extreme cleanliness of the semiconductor surface. Since a native oxide rapidly forms on the surface of silicon, for example, the performance of a contact can depend sensitively on the details of preparation.
The fundamental steps in contact fabrication are semiconductor surface cleaning, contact metal deposition, patterning and annealing. Surface cleaning may be performed by sputter-etching, chemical etching, reactive gas etching or ion milling. For example, the native oxide of silicon may be removed with an HF dip, while GaAs is more typically cleaned by a bromine-methanol dip. After cleaning, metals are deposited via sputter deposition, evaporation or chemical vapor deposition (CVD). Sputtering is a faster and more convenient method of metal deposition than evaporation but the ion bombardment from the plasma may induce surface states or even invert the charge carrier type at the surface. For this reason the gentler but still rapid CVD is increasingly preferred. Patterning of contacts is accomplished with standard photolithographic methods such as lift-off, where contact metal is deposited through holes in a photoresist layer that is later dissolved away. Post-deposition annealing of contacts is useful for relieving stress as well as for inducing any desirable reactions between the metal and the semiconductor.
The measurement of contact resistance is most simply performed using a four-point probe although for more accurate determination, use of the transmission line method is typical.
Read more about this topic: Ohmic Contact
Famous quotes containing the words preparation and, preparation and/or contacts:
“It’s sad but true that if you focus your attention on housework and meal preparation and diapers, raising children does start to look like drudgery pretty quickly. On the other hand, if you see yourself as nothing less than your child’s nurturer, role model, teacher, spiritual guide, and mentor, your days take on a very different cast.”
—Joyce Maynard (20th century)
“It’s sad but true that if you focus your attention on housework and meal preparation and diapers, raising children does start to look like drudgery pretty quickly. On the other hand, if you see yourself as nothing less than your child’s nurturer, role model, teacher, spiritual guide, and mentor, your days take on a very different cast.”
—Joyce Maynard (20th century)
“The modern nose, like the modern eye, has developed a sort of microscopic, intercellular intensity which makes our human contacts painful and revolting.”
—Marshall McLuhan (1911–1980)