Part 2, Oxidation Process

Semiconductors have become an essential part of our lives. Today, we are back at Eight Essential Semiconductor Processes series to walk you through the semiconductor manufacturing process. As the first part of this series, we discovered the manufacturing process of wafers. Continuing onto the next step, we will delve into the oxidation process.

An Oxide Layer (SiO₂) That Protects and Insulates Wafers

Before it can be used as a raw material for the integrated circuit, silicon extracted from sand goes through a series of purification processes to form a silicon rod called an ingot. This ingot is then cut to a uniform thickness, gets polished and eventually becomes a wafer, which is the foundation for semiconductors. The resultant thin disc-shaped wafer is not conductive. Therefore, the wafer requires further processing to become “semi-conductive,” having both conductive and non-conductive properties. To achieve this, various substances are transferred onto the wafer, and then the circuit pattern is etched onto the surface. And these processes are repeated several times.

Oxidation is the groundwork for all of the processes mentioned above. The oxidation process creates an SiO2 layer, which serves as an insulating layer that blocks leakage current between circuits. The oxide layer also protects the silicon wafer during the subsequent ion implantation and etching processes. In other words, the silicon dioxide layer serves as a reliable shield during the semiconductor manufacturing process. The role of the oxide layer is critical because even tiny contaminants have a detrimental effect on electrical properties of integrated circuits, which require extremely high precision. How is this protective oxide layer created?

A silicon wafer forms an oxide layer when it is exposed to oxygen in the air or in other chemicals. This is as if iron (Fe) rusts when it becomes oxidized in the air. There are a variety of oxidation methods, such as thermal oxidation, plasma-enhanced chemical vapor deposition (PECVD), and electrochemical anodic oxidation. Among them, most widely used is the thermal oxidation procedure performed at high temperatures of 800–1200 ℃ to form a thin, uniform silicon dioxide layer. Thermal oxidation can be either wet or dry, depending on the gas used for the oxidation reaction. Dry oxidation uses pure oxygen (O₂), and consequently, the oxide layer grows more slowly, making it ideal for creating a thin layer. Oxides created by the dry method have excellent electronic properties. Wet oxidation uses both oxygen (O₂) and vapor (H₂O). As a result, the oxide layer grows faster and forms a thicker layer. However, oxides created by the wet method are not as dense as those created by dry oxidation. Under identical time and temperature conditions, the oxides formed by the wet method are about five to ten times thicker than those formed by the dry method. This brings us to the end of the second part of the series, where we looked at what oxide layers do and how they are created. Stay tuned for the next part of the series, as we will explain how the circuit pattern is imprinted on the oxide-clad wafer.