Step 5
Metallization
It is used to create contact with silicon and to make interconnections on chip. A thin layer of aluminum is deposited over the whole wafer. Aluminium is selected because it is a good conductor, has good mechanical bond with silicon, forms low resistance contact and it can be applied and patterned with single deposition and etching process.
Making successive layers: - The process such as masking, etching, doping will be repeated for each successive layers until all integrated chips are completed. Between the components, silicon dioxide is used as insulator. This process is called chemical vapor deposition. To make contact pads, aluminum is deposited. The fabrication includes more than three layers separated by dielectric layers. For electrical and physical isolation a layer of solid dielectric is surrounded in each component which provides isolation. It is possible to fabricate PNP and NPN transistor in the same silicon substrate. To avoid damage and contamination of circuit, final dielectric layer (passivation) is deposited. After that, the individual IC will be tested for electrical function. Check the functionality of each chip on wafer. Those chips are not passed in the test will be rejected.
Assembly and packaging
Each of the wafers contains hundreds of chips. These chips are separated and packaged by a method called scribing and cleaving. The wafer is similar to a piece of glass. A diamond saw cut the wafer into single chips. The diamond tipped tool is used to cut the lines through the rectangular grid which separates the individual chips. The chips that are failed in electrical test are discarded. Before packaging, remaining chips are observed under microscope. The good chip is then mounted into a package. Thin wire is connected using ultrasonic bonding. It is then encapsulated for protection. Before delivered to customer, the chip is tested again. There are three configurations available for packaging. They are metal can package, ceramic flat package and dual in line package. For military applications, the chip is assembled in ceramic packages. The complete integrated circuits are sealed in anti static plastic bags.