Раздел 3 Второе занятие

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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 (непрозрачный) 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 (отжиг), 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 a p-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. Ap-type region is thus cre­ated. The oxidization treatment is now repeated and, in this high-tem- perature 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 /7-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 defme the contact areas. Finally, the contacts are made by the evaporation of aluminium.

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.

10 Бух

Микроэлектроника настоящее и будущее

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High Pressure Oxidation of Silicon

Silicon oxidation has been a fundamental process of silicon de­vice technology fora long time. However, an understanding of oxida­tion methods and the phenomena involved is far from complete. An oxidation method that has received increased attention over the last few years is a high pressure oxidation method. This method is known to offer a practical means for thermally growing silicon oxides at lower temperatures and faster rates than those grown in conventional wet (влажный) oxidation. Presently, efforts to implement\ow temperature processes have become a significant driving force in the evolution of silicon device fabrication technology. The lower temperature aspect of high pressure oxidation has its greatest potential impact in the high density world of submicron VLSI where improvements in process con­trol precision will have a significant effect on performance and yield.

Thin oxide film grown at low temperature by high pressure oxi­dation has excellent dielectric breakdown strength.

Developments in high pressure oxidation will become more im­portant with progress in other low temperature processes such as ion implantation, laser annealing, and plasma enhanced technology dur­ing the next few years.

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Molecular Electronics

Molecular electronics is a new concept of electronic systems. Basically, it seeks to integrate into a solid block of the material the