Series on Semiconductor Processing and Integrated Circuits, Part 7: Ion Implantation
This is our seventh article in a series intended as an overview for those who are not technical specialists in the semiconductor processing field. We will briefly describe ion implantation in the semiconductor processing industry.
Diffusion doping techniques were originally used in the semiconductor processing industry. However, high-density circuits requiring smaller feature sizes, has moved the industry to use ion implantation as the primary dopant introduction technique. Issues with thermal diffusion that placed limits on the production of advanced circuits included: ultra thin junctions, poor doping control, surface contamination interference, lateral diffusion, and dislocation generation. Ion implantation overcomes these limits of diffusion. During ion implantation there is no side diffusion, the process can take place at room temperature, dopant atoms are placed below the surface, and a wide range of doping concentrations are possible. There is also greater control of the location and quantity of dopants put into the wafer.
The same dopant elements that are used in the diffusion process are used in ion implantation. However, while liquid, gas, or solid dopant sources are used in diffusion, only gas and solid sources are used for ion implantation. Gases are used more often because they offer a higher level of control. The gases are mostly fluorine based, such as phosphorus pentafluorid (PF5), arsenic pentafluoride (AsF5), or Phosphorus trifluoride (PF3). In certain applications, solid sources are used, such as phosphorus pentoxide (P2O5).
Ions implanted are ionized atoms of the dopants. Source vapors are fed into a low-pressure chamber where the ionization occurs. Inside the chamber, a filament is heated to where electrons are created on the filament surface. The negatively charged electrons are attracted to an oppositely charged anode in the chamber. As the electrons move from the filament to the anode they collide with the dopant source molecules, thus creating positively charged ions. A cold-cathode technique is another ionization method. In this method, a high-voltage electric field is created between a cathode and anode, which created the electrons.
The next step is ion selection, and occurs in a mass analyzer. This subsystem was developed during the Manhattan Project and used while developing the atomic bomb. The analyzer creates a magnetic field and the species leave the ionization subsystem traveling at a high rate of speed. Each of the positively charged species is bent in an arc, with a radius determined by the mass of the particular species, its speed, and the strength of the magnetic field. A slit at the end of the analyzer allows only one species to exit, the desired ion.
Once the analyzing subsystem in complete, the ion moves into an acceleration tube. This subsystem is designed to accelerate the ion to gain sufficient momentum to penetrate the wafer surface. The necessary momentum is obtained by utilizing the fact that negative and positive charges attract each other. The acceleration tube is linear, with annular anodes along its axis. Each anode is negative in charge, with the charge amount increasing down the tube. Once the positive charged ion enters the tube it accelerates long the tube. The voltage value is determined based on the mass of the ion and the momentum required at the wafer end of the implanter. A higher voltage leads to higher momentum and thus allowing the dopant to be implanted faster and deeper.
Success ion implantation relies on the implantation of only the desired dopant atom. Dangers include the possibility of residue molecules in the system ending up on the wafer surface as well. While ion implantation is still faced with numerous challenges, it allows for the production of advanced circuits in the semiconductor processing industry.
We hope that you found this review of ion implantation 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 guide to semiconductor processing. (4th ed.). New York, NY: McGraw-Hill.