Semiconductor device fabrication is a multiple step sequence to create chips used in everyday electrical and electronic devices. Silicon is the most common semiconductor used today, although Gallium Arsenide, Germanium and some other materials are also used in special applications.
To create the standard silicon based chips that are used in most computers, raw silicon dioxide, which is the most common part of sand or glass, is heated in the presence of hydrogen to produce pure silicon and water which is extracted. The silicon is then melted and formed into ingots, that are sliced into wafers.
Once the wafers are prepared a large number of process steps are still necessary to produce the desired semiconductor integrated circuits. In general the steps can be grouped into four areas: Front End Processing, Back End Processing, Test and Packaging.
Front End Processing refers to the initial steps in the fabrication. In this stage the actual semiconductor devices or transistors are created. A typical front end process includes: preparation of the wafer surface, patterning and subsequent implantation of dopants to obtain the desired electrical properties, growth or deposition of a gate dielectric, and growth or deposition of insulating materials to isolate neighboring devices.
Once the various semiconductor devices have been created they must be interconnected to form the desired electrical circuits. This "Back End Processing" involves depositing various layers of metal and insulating material in the desired pattern. Typically the metal layers consist of aluminum or more recently copper. The insulating material was traditionally a form of SiO2 or a silicate glass, but recently new low-K materials are being used. The various metal layers are interconnected by etching holes, called "vias" in the insulating material and depositing Tungsten in them.
Once the Back End Processing has been completed, the semiconductor devices are subjected to a variety of electrical tests to determine if they function properly.
Finally, the wafer is cut into individual die, which are then packaged in creamic or plastic packages with pins or other connectors to the outside world.
During the fabrication a number of process steps are used including:
- Wet Cleans
- Ion implantation (where dopants are pushed into the wafer to create regions of varible resistance)
- Plasma Ashing
- Chemical Vapor Deposition(CVD)
- Test (where the electrical performance is verified)
- Wafer Backgrinding (to reduce the thickness of the wafer so the resulting chip can be put into a thin device like a smart card or PCMCIA card.)
- Die Cutting
- Wire Bonding