Thermal oxidation is the result of exposing a silicon wafer to a combination of oxidizing agents and heat to make a layer of silicon dioxide (SiO2). This layer is most commonly made with hydrogen and/or oxygen gas, although any halogen gas can be used. Silicon dioxide growth takes place on wafers in ambient air to about 20Å (angstroms) thick; however, for most specifications thermal oxide growth uses a heat source in order to catalyze this reaction and create oxide layers up to 25,000Å thick.
There are numerous applications for thermal oxide wafers, but their primary use is as dielectric material and in MEMS (micro-electromechanical systems) devices.
There are two primary ways to perform thermal oxidation on silicon wafers, and both require growth of oxygen on the surface of the wafer, as shown in Figure 1. This differs from CVD applications, where the oxide layer is deposited on top of the wafer.
The first method is rapid thermal processing (RTP) which is usually used on thin dry thermal oxide, or implant annealing applications. In RTP, wafers experience a rapid heating to over 1,000°C for a very short period of time. After a few seconds of exposure, the wafers slowly cool down to room temperature to prevent cracking. The result is a thin silicon dioxide layer, capable for use in a variety of applications.
The second and more common method is diffusion. Thermal oxide diffusion takes place between ~800° and 1,100°C, and there are two ways to grow this film: ‘wet’ or ‘dry’.
Wet Thermal Oxide
Wet thermal oxide films are generally for applications where a thicker silicon dioxide layer is necessary. To prevent impurities, these films usually grow in quartz tubes using a combination of heat and pure steam. Most major manufacturers use external heaters housed in a quartz tube separate from the furnace, shown in Figure 2. Although uncommon, some systems use an internal flame to heat the wafers instead. During the oxidation process, the temperature in the external heater is raised to more than 800°C. This causes the gases to spontaneously ignite and produce a blue flame without an ignition source. The flame creates pure steam, hence the name wet thermal oxide. The pure steam moves through the tube that houses the flame into the furnace where the wafers are. Once the steam enters the quartz chamber, it expands and evenly distributes throughout the furnace.
The wafers, which either stand horizontally (as shown in Figure 2) or stack vertically, are in the furnace for several hours, as time can vary depending on the target film thickness. The growth of SiO2 is not linear, so when a wafer is in a 1,000°C furnace for 5 hours, an oxide layer of approximately 10,000Å will form; and if that same wafer remains in the furnace for 24 hours, a layer of approximately 25,000Å will form. The second wafer was in the furnace for nearly five times longer, but the oxide layer is only about twice as thick. This is because as the oxide layer grows, it becomes more difficult for oxygen to penetrate the device layer and interact with the silicon substrate to create SiO2.
Dry Thermal Oxide
Dry thermal oxide results in a much thinner silicon dioxide layer than wet oxide and the process takes much longer. Due to these limitations, dry silicon dioxide layers are no thicker than 1,000Å.
Dry thermal oxide growth is a very similar process to wet thermal oxide. The only difference is that this process uses molecular oxygen instead of pure steam to create the device layer. This produces a high uniformity, high dielectric strength, and high-density silicon dioxide wafer.
Thick Thermal Oxide
Thick oxide is a silicon dioxide layer that is greater than 2μm (20,000Å).
When thicker oxide layers are needed, it is typically grown using wet thermal oxide. This technique has a much faster growth rate than dry thermal oxide and can result in much thicker silicon dioxide films.
Thick Oxide Applications:
Wet thermal oxide is generally used as a barrier on thick-film silicon on insulator wafers between the handle and carrier wafer. It is a consumable in chemical mechanical planarization (CMP) for breaking in tools and pads.
Particle-sensitive oxide is the process of growing thermal oxide on a substrate while adding the fewest number of particles during the growth process. In order to protect the wafers from particles, all processing takes place in a cleanroom. The starting wafers must also have low particle count and be in original factory packaging to minimize particle contamination.
To grow this coating, wafers undergo wet or dry thermal oxide inside a quartz furnace. It is important to use a clean quartz furnace to protect the wafers from particles in the surrounding environment.
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