Science Fair Project Encyclopedia
Semiconductor device fabrication
Semiconductor device fabrication is the process used to create chips, the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconductor material. Silicon is the most commonly used semiconductor material today, although gallium arsenide, germanium, and many other materials are used in special applications.
A wafer is made out of extremely purified silicon that is grown into mono-crystalline cylindrical ingots, using the Czochralski process. These ingots are then sliced into 0.75 mm thick wafers and polished to obtain a very regular and flat surface.
Once the wafers are prepared, many process steps are necessary to produce the desired semiconductor integrated circuit. In general the steps can be grouped into four areas:
- Front end processing
- Back end processing
Front End Processing refers to the most crucial steps in the fabrication. In this stage the actual semiconductor devices or transistors are created. A typical front end process includes the following: 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 aluminium 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.
Wafer test and device test
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 ceramic or plastic packages with pins or other connectors to the outside world.
The packaged chips are then retested to ensure that they were not damaged during packaging and that the die-to-pin interconnect operation was performed correctly.
List of steps
- Wafer fabrication
- Wet cleans
- Ion implantation (in which dopants are embedded in the wafer creating regions of increased ( or decreased ) conductivity)
- Dry Etching
- Wet Etching
- Plasma ashing
- Thermal treatments
- Chemical vapor deposition (CVD)
- Physical vapor deposition(PVD)
- Molecular beam epitaxy (MBE)
- Chemical mechanical polish (CMP)
- Wafer testing (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 smartcard or PCMCIA card.)
- Die preparation
- IC packaging
- IC testing
When feature widths were far greater than about 10 micrometres, purity was not the issue that it is today in device manufacture. But as the devices became more integrated the cleanrooms became even cleaner. Today, the facilities are pressurized with filtered air, to remove even the smallest particles, which could come to rest on the wafers, and contribute to defects.
The workers in a semiconductor fabrication facility now wear bunny suits and step into specially pressurized rooms which force out dust particles from the workers themselves, and into the ordinary environment.
It is important for these workers to minimize their exposure to the toxic materials used in manufacture, such as arsenic; for this reason, the semiconductor fabrication facilities are highly automated.
Memory chips are typically the first devices to use a technology, as they are highly regular, and can be used to test a technology.
In an effort to increase profits, semiconductor device manufacture spread from Texas and California in the 1960s to the rest of the world, such as Malaysia and Japan, and is a global business today. Some Malaysian facilities are now in their fourth decade of operation, for example.
Microprocessor manufacture is still strong in the United States, but the world's market leader is in Taiwan, represented by major companies such as Taiwan Semiconductor Manufacturing Company and United Microelectronics Corporation (see UMC web site). Singapore is the second biggest semiconductor manufacturer in the world, where Chartered Semiconductor Manufacturing dominates.
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