Eight Major Steps to Semiconductor Fabrication, Part 2: The Oxidation Process
In the previous part of this series, we discussed the manufacturing process of the wafer, an indispensable part of a semiconductor integrated circuit. Continuing onto the next step of the disc production stage, we will delve into the oxidation process that produces a thin layer of silicon dioxide (SiO2).
A reliable oxide layer that shields the wafer’s surface
Before it can be used as a raw material for the integrated circuit, silicon extracted from sand goes through a purification process and is shaped into an ingot. This conic object is then cut to a uniform diameter, polished and eventually becomes a wafer.
The polished wafers start out pure in a non-conductive state. To make them semi-conductive, various substances are transferred onto the wafer, and then the circuit pattern is etched onto the surface.
Oxidation, the groundwork for the sequential procedures mentioned above, is a process in which a thin layer composed of various materials is deposited. The technique forces oxygen, or vapor, to diffuse into the wafer surface at high temperatures between 800 and 1200°C so that a thin, smooth layer of silicon dioxide can be created.
This layer protects the surface from chemical impurities and pollutants that permeate during the processes. Even tiny contaminants invisible to the naked eye can alter resistivity or conductivity and consequently damage the circuit’s electrical properties. Therefore, shielding the surface from these substances with a protective layer is crucial.
The silicon dioxide layer doubles as a trustworthy guardian against unintended adulteration during the ion implementation stage, and as an insulator that separates each part of the electrical circuit on the wafer to prevent a short circuit.
So what kind of chemical reaction creates this dependable oxide layer?
When exposed to oxygen in the atmosphere or within chemicals, an oxide layer begins to build on the wafer’s surface, just as iron (Fe) rusts when it becomes oxidized in the air.
There are a variety of oxidation methods, such as thermal oxidation, electrochemical anodic oxidation and plasma-enhanced chemical vapour deposition (PECVD). Among them, the thermal oxidation procedure performed at a high temperature is most widely used.
Thermal oxidation can be either wet or dry. Dry oxidation only uses oxygen to forge a thinner layer, whereas wet oxidation uses both oxygen and vapour to fashion a thicker layer.
Although oxides created by the dry method have excellent electronic properties, they grow much slower when compared to the wet method. Under identical time and temperature conditions, the wet method can present an outcome that is five or ten times thicker than that of the dry method.
This brings us to the end of the second part of the series. Stay tuned for Part 3, as Samsung Tomorrow will explain next week how the circuit pattern is imprinted on the oxide-clad wafer.
In Korean, http://samsungsemiconstory.com/110.
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