Diffusion

Diffusion and ion implant are the two major processes by which chemical species or dopants are introduced into a semiconductor such as silicon to form the electronic structures that make integrated circuits useful (although ion implant is now much more widely used for this purpose than thermal diffusion).

Diffusion is the movement of a chemical species from an area of high concentration to an area of lower concentration.  The controlled diffusion of dopants into silicon is the foundation of forming a p-n junction and fabrication of devices during wafer fabrication.

Diffusion is used primarily to alter the type and level of conductivity of semiconductor materials.  It is used to form bases, emitters, and resistors in bipolar devices, as well as drains and sources in MOS devices. It is also used to dope polysilicon layers.

Fig. 1. Examples of Diffusion Furnaces

The mathematics of diffusion are based on Fick's laws.  If there is a chemical concentration gradient in a finite volume of silicon (or any matrix substance for that matter), the impurity material will tend to move such that the concentration gradient is decreased.  If the flow is sustained for a sufficient amount of time, the substance will become homogeneous and diffusion will cease.  Fick's First Law is an equation describing the flow of an impurity in a substance, showing that the flux of material across a given plane is proportional to the concentration gradient across the plane.

More frequently, however, Fick's First Law is not an adequate description of the diffusion process since the concentration gradient of an impurity in a finite volume of material decreases with time.  Fick's Second Law describes this phenomenon.  The solution to Fick's Second Law depends greatly on the method by which the impurities are supplied to the substrate.

There are two major ways by which to deposit impurities into a substance by thermal diffusion.  In the first method, known as predeposition, a flux of impurities continuously arrives at the surface of the substrate such that the concentration gradient of the impurity remains constant at the surface of the substrate.  In the second method, known as redistribution or drive-in diffusion, a thin layer of the impurity material  is deposited on the substrate.  In this case, the impurity gradient at the surface of the substrate decreases with time.

Wafer Fab Links:  Incoming Wafers Epitaxy Diffusion Ion Implant Polysilicon

Dielectric Lithography/Etch Thin Films Metallization Glassivation Probe/Trim