Below is our eighth article intended as an overview for those who are not technical specialists in the semiconductor processing field. We will describe the basics of chemical vapor deposition in the semiconductor processing industry.
It is believed that the German chemist, Robert Bunsen, documented the first account of the manufacturing technique known as chemical vapor deposition (CVD) in 1852. He did so while observing that iron oxide condenses as crystals from hot volcanic gases containing hydrogen chloride. While at the time Bunsen was more concerned with the phenomenon than in the materials synthesis, a few years later St. Claire de Ville became the first scientists to put CVD to use when he formed oxides of magnesium, titanium, and tin.
[i] CVD has gone on to become a widely used industrial tool that produces a vast range of materials, especially in the semiconductor processing industry.
While process engineers of the 1960s only had the option of atmospheric chemical vapor deposition, today’s engineers have many more options. In the semiconductor industry, the term deposition refers to any process where a material is physically deposited on the wafer surface. CVD is primarily used to deposit films on the wafer. During the process, a chemical (C) containing the atoms or molecules necessary for the final film are mixed in a deposition chamber to form a vapor (V). The atoms or molecules are then deposited (D) on the wafer surface to form a film. Usually the addition of energy to the system is necessary for the process to take place. This can come in the form of heating the chamber or the wafer itself. The deposited film growth occurs in several specific stages, the first being nucleation. Nucleation occurs as the first few atoms or molecules deposit on the wafer surface. These initial atoms or molecules form islands that grow larger. Next, these islands spread until they form a continuous film. This is the transition stage. During this stage the film has different chemical and physical properties then the final, thicker film. After the transition film is formed, the growth of the final film begins.
Basic CVD System Design
In general a CVD system has the same parts as a tube furnace: source cabinet, reaction chamber, energy source, wafer holder, and loading and unloading mechanisms. With the chemical housed in the source cabinet, vapors are generated from pressurized gas cylinders or liquid source bubblers. Pressure regulators, mass flow meters, and timers control the gas flow. The deposition itself occurs in the reaction chamber. The energy sources used are either heat, induction RF, radiant, plasma, or ultraviolet. The chamber configuration and heat sources used determine the style and material of the wafer holder.
Various CVD System Types
There are primarily two types of CVD systems: atmospheric pressure (AP) or low pressure (LP), however the most advanced device films are deposited in low-pressure systems. Another variation of systems is either cold wall or hot wall. In a cold-wall system the wafer holder or wafers are directly heated with induction or radiant heating, while the walls of the chamber remain cold. This allows the reaction to occur only at the heated wafer holder. Hot-wall systems heat the wafer, wafer holders, and the chamber walls, causing the reaction to occur throughout the chamber. This leaves reaction products on the inside of the chamber walls, and thus requiring frequent cleaning to prevent wafer contamination. CVD systems use primarily two energy sources: thermal and plasma. Thermal sources include tube furnaces, hot plates, and RF induction. Plasma, combined with lower pressure, offers the benefit of lowered temperatures and good film composition and coverage. Additional benefits of low-pressure CVD include less dependence on gas flow dynamics, less time for gas phase reaction particles to form, and the process can be performed in a standard tube furnace.
We hope that you found this review of CVD helpful. Please feel free to comment below and let the bloggers at Glew Engineering know if there is a specific topic you’d like us to blog about in the future.
Van Zant, P. (2000). Microchip fabrication, a practical guid to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.