Text 6. An integrated circuit

An integrated circuit is comprised of a single silicon chip containing transistors, diodes, resistors and capacitors, suitably connected to form a complete circuit. The first successful attempt to produce an integrated circuit, in 1959, made use of mesa construction, but this method is known to be quickly replaced by the use of planar techniques.

The important feature of the planar process is the deposition of a silicon dioxide layer on the top surface of the epitaxial wafer which acts as a mask against diffusion. The process involves exposing the wafer to an oxygen atmosphere at high temperature.

After the oxidation process it is necessary to etch holes in the oxide, through which diffusion can take place. The process used is similar to that employed in the manufacture of printed circuit boards. Initially the oxidized surface is coated with a thin film of photo-sensitive emulsion (photoresist). A mask is manufactured, the pattern of which defines the area to be etched, it being opaque where etching is to be performed and transparent where the oxide is to be retained. The mask is brought into contact with the wafer and exposed to ultraviolet light. The photoresist under the transparent area of the mask being subjected to the light becomes polymerized and is not affected by the trichlorethylene developer which is subsequently used to dissolve the unexposed resist. When fixed, by baking, the remaining photoresist protects the oxide from the window where diffusion is required and, after the surface has been cleaned, the chip is ready for the first diffusion process.

For а р-type diffusion the most generally used dopant proves to be boron. This is deposited on the wafer at high temperature, and diffuses through the window into the silicon. А р-type region is thus created. The oxidization treatment is now repeated and, in this high-temperature process, the open window is sealed with an oxide layer and the base dopant is driven deeper into the silicon. A new mask is used in a second photoresist and etching stage, which opens a window for the diffusion of the emitter region.

For n-type diffusion the most generally used dopants are phosphorus and arsenic. The cycle is supposed to be repeated yet a third time. The emitter window is sealed by oxidization, the emitter dopant is driven in, and new windows are etched in the oxide layer to define the contact areas. Finally the contacts are made by the evaporation of aluminum.

In practice many devices are manufactured at the same time on a single sheet of silicon. These are separated by scribing with a diamond stylus and breaking into individual chips. They are then mounted in suitable packages which allow electrical connections to be readily made and power, dissipated as heat, to escape.

It is necessary to be able to electrically isolate individual devices from each other. This is done by surrounding each component with material of opposite polarity and reverse biasing the semiconductor junction so formed.

Date: 2015-12-13; view: 578; Нарушение авторских прав