The introduction of deliberate impurities into a material’s molecular structure is one of the key steps in semiconductor manufacturing, this is known as doping. The unique electrical properties of semiconductors are wholly dependent on the amount of electron holes and free electrons available in its atomic bands. The concentrations of these are roughly equivalent when under stable thermal conditions.
The reason dopants are introduced is to increase the conductivity of semiconductors closer to that of a conventional conductor. Introducing an impurity such as gallium (Ga), boron (Bn), or phosphorous (P) into a silicon (Si) semiconducting wafer for example, adds to the amount of free electrons available, and so reducing the material’s electrical resistivity.
Doping semiconductor materials is executed by utilizing diffusion furnaces which can operate under either vacuum or atmospheric environments. This article will explore the use of diffusion furnaces for semiconductor doping in more detail.
What is a Diffusion Furnace?
Diffusion furnaces are made up of cylindrical heating chambers which can be arranged either horizontally or vertically. Each orientation allows operators to maximize thermal efficiency by providing a uniform space between the radiating surface of heating elements and the silicon wafers.
This establishes that the solid-phase semiconductor is heated with a high degree of accuracy and consistency over the entire surface, which is essential for chemical vapor deposition (CVD) processes.
Diffusion of dopants into a heated semiconductor is usually reached by introducing gas phase impurities into the heated atmosphere of a diffusion furnace. Gaseous molecules can permeate silicon substrates and diffuse through the solid at raised temperatures, which alters the chemical composition of the semiconductor.
For example, modulating the electronic behavior of silicon with phosphorous can be achieved through diffusion of phosphine (PH3) at elevated temperatures. To get rid of unwanted impurities from the diffusion furnace atmosphere, the precursor gas may be introduced with an inert carrier such as nitrogen (N).
To decrease the number of unwanted gas-phase reactions further, this process is usually carried out under vacuum or low-pressure conditions. Low-pressure chemical vapor deposition (LPCVD) is presently one of the main methods for producing intermediate semiconductors for utilization in commercial displays, solar cells, thin film structures, and more.
This information has been sourced, reviewed and adapted from materials provided by Thermcraft, Inc.
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