In semiconductor processing, oxidation is used to produce a thin layer of oxide onto a wafer’s surface. Since a wafer is often made of silicon, the resulting effect of oxidation is a silicon dioxide layer. Below we will discuss the uses of silicon dioxide layers in semiconductor devices, thermal oxidation growth, and oxidant sources.
Silicon dioxide layers are useful in semiconductor devices by pacifying the silicon surface, acting as a doping barrier, being a surface dielectric, and serving as a dielectric part of the device. Semiconductor devices are extremely sensitive to contamination, and the silicon dioxide layer helps to protect the device from contaminates. Silicon dioxide layers are nonporous and very hard, allowing it to act as a physical barrier between the device and contaminants. Protection is also provided in a chemical nature. As the oxidation process occurs, any contaminants that were on the device end up in the new layer of oxide and away from the active surface. During doping, holes are created in the surface layer to allow for dopants to be introduced into the exposed wafer surface. The silicon dioxide layer blocks the dopant and prevents it from reaching any further than the required depth. As a surface dielectric, silicon dioxide acts as an insulator as well as preventing induction. Finally, the oxide is a dielectric where its thickness is specifically chosen to allow induction of a charge in the gate region of the device.
Thermal oxide growth is a chemical reaction shown by the equation below. While this reaction can occur at room temperature, extremely high temperatures, between 900 and 1200°C are necessary to produce quality oxides for circuits and devices. When a wafer is placed in a heated chamber and exposed to oxygen gas, oxygen atoms combine readily with the silicon atoms forming the oxide layer. This is called the linear stage because oxide grows equally with time (equation below). However, the resulting oxide layer prevents the continued contact of the silicon and oxygen atoms. In thermal growth of silicon dioxide, the oxygen atoms diffuse through the initial oxide layer to the silicon wafer surface. Each new oxide layer that is formed requires that the oxygen atoms to move farther to reach the silicon wafer. This results in a slowing of the oxide growth rate over time, and is called the parabolic stage (equation below). Multiple factors can determine when the change from linear to parabolic occurs. These may include the oxidizing temperature or intentionally included impurities in the oxide, such as hydrogen chloride (HCL).
Reaction of silicon and oxygen to form silicon dioxide
Si (solid) + O2 (gas) ® SiO2 (solid)
Linear oxidation of silicon
X = B/At
Parabolic oxidation of silicon
X = √(Bt)
X = oxide thickness
B = parabolic rate constant
B/A = linear rate constant
t = oxidation time
The oxidant sources necessary to produce silicon dioxide can come from a handful of sources. These could include dry oxygen, water vapor, bubblers, dry oxidation, or chlorine-added oxidation. When oxygen is used as the oxidant, it often comes from a facility source or a tank of compressed oxygen. The gas must be dry. If water vapor is present, when not intended, the oxidation rate would increase, causing the oxide layer to not meet specifications. In some cases water vapor is intentionally used as an oxidant source, but this method depends on the level of thickness and cleanliness control required of the oxide layer. Bubblers were one of the initially methods of creating steam vapor. The major drawbacks of this method were contamination, and the lack of control of the amount of water vapor leaving the bubbler as the water level and temperature change. Tight specifications and heightened cleanliness standards resulted in the dry oxidation, or dryox, system. During this method oxygen and hydrogen gas mix in the oxidation tube under high temperatures to form steam. This method allows for the reassurance of clean and dry gases, as well as the ability to control the amounts of gases going into the tube. The drawback of this method however, is hydrogen’s explosive properties.
Thermal oxidation growth allows for the creation of a protective silicon dioxide layer, allowing for the semiconductor fabrication process to continue.
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Van Zant, P. (2000). Microchip fabrication, a practical guide to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.