How to Grow Silicon Dioxide (SiO2) on Wafers?

The oxidation process is a crucial step in CMOS integrated circuit manufacturing, used to grow silicon dioxide (SiO2) on the surface of silicon wafers (Si wafers). The process of growing SiO2 can be likened to "dressing" the wafer in a protective layer that serves functions such as insulation, protection, and isolation.

1. Prepare the Substrate (Silicon Wafer)  

First, a high-purity silicon wafer must be prepared. This is akin to painting on a blank canvas; the wafer serves as the foundation for the CMOS circuit.

2. Choosing the Oxidation Furnace  

The oxidation furnace is the core equipment used to grow SiO2. Typically, the temperature range of the oxidation furnace is between 900℃ and 1200℃. You can think of this furnace as a high-temperature "steamer," where the temperature is high and stable. High temperature is a necessary condition for the chemical reaction between silicon (Si) and oxygen.

3. Choosing the Growth Medium: O2 or H2O  

The oxidation process can be performed in two ways: dry oxidation and wet oxidation. The difference between these two methods lies in the medium used to provide oxygen:  

   - Dry oxidation uses O2 gas, similar to "baking" the wafer in a pure oxygen environment.  

   - Wet oxidation utilizes an environment containing water vapor, which can be likened to "steaming" the wafer in a high-temperature atmosphere with moisture.  

Both methods allow the wafer to react with oxygen to form SiO2, but wet oxidation is faster because the oxygen atoms provided by water molecules can diffuse more easily to the wafer's surface.

4. Chemical Reaction Process  

In the oxidation furnace, when the silicon wafer comes into contact with oxygen (O2) or water vapor (H2O), silicon atoms react with oxygen atoms according to the following chemical reactions:  

   - Dry oxidation reaction equation:  

     Si + O2  →  SiO2

   - Wet oxidation reaction equation:  

     Si + 2H2O  →  SiO2 + 2H2

In both reactions, silicon atoms on the wafer's surface combine with oxygen atoms to generate a layer of silicon dioxide. This layer of SiO2 covers the surface of the wafer, forming a very uniform and dense oxide layer.

5. Control of Growth Thickness  

The temperature and time of the oxidation process directly influence the thickness of the SiO2 layer. You can think of this process like boiling an egg: the higher the temperature and the longer the time, the thicker the egg white solidifies. Similarly, the higher the oxidation temperature and the longer the time, the thicker the SiO2 produced. In practical operations, these two parameters must be precisely controlled according to the requirements of the circuit design.

6. Uses and Functions  

The grown SiO2 layer has various applications in CMOS processes, such as:  

   - Serving as a gate oxide layer, providing isolation and capacitive effects in transistors;  

   - Acting as a field oxide layer to isolate devices and reduce leakage current;  

   - Functioning as a mask layer to protect the wafer surface during ion implantation and other processes.  

In summary, the entire process of "growing SiO2" involves placing the silicon wafer into a high-temperature "CVD Furnace" and using either dry or wet oxidation methods to generate a uniform SiO2 layer on the wafer's surface. The primary control parameters for this process are temperature, time, and oxidation atmosphere. The grown SiO2 layer plays a critical role in providing protection and insulation in CMOS processes.