17-08-2012, 04:24 PM
Dopant Diffusion
[attachment=31910]
As indicated previously the main front-end processing in building a device or integrated circuit is to selectively introduction of dopant atoms into silicon wafer. Dopant introduction by high temperature diffusion is one of the two major processes for achieving this. Diffusion is carried out at high temperature ( 800 to 1000 °C) in a dopant-rich gaseous ambient.
Diffusion also covers the redistribution in the wafer of dopant atoms introduced into the wafer by other methods such as ion implantation.
For simplicity the diffusion of dopant in silicon is modelled by the simple diffusion theory described by Fick’s law in which the flux of dopant atoms at any point is assumed to be proportional to the gradient of the dopant concentration at that point. The proportionality constant is the diffusion coefficient, D . This is assumed to be a constant for a given dopant species at a given temperature. Actually D is not constant by is dependent on dopant concentration.
This section covers the diffusion techniques used and the simple 1-D analysis describing the dopant distribution into the wafer.
Diffusion Mechanisms
Vacancy: atoms jump from one lattice site to the next.
Interstitial: atoms jump from one interstitial site to the next.
Diffusion System
The most common practice of introducing dopant into wafers by diffusion is the horizontal quartz tube furnaces similar to that used for thermal oxidation of silicon wafers. A separate furnace, wafer holder etc are reserved for a particular dopant to avoid contamination by other dopants.
In general the dopant atoms are introduced in two separate diffusion steps, each in its own tube furnace. The first step is referred to as pre-deposition diffusion, it is aimed to introduce a controlled amount of dopant atoms into the silicon surface (dopant atoms per unit surface area). The second step known as the drive-in diffusion which is aimed (at least theoretically) to redistribute the dopant atoms introduced during predeposition further into the silicon and reducing the dopant concentration near the surface.
Predeposition Diffusion
In pre-deposition diffusion the wafer is exposed to excess amount of dopant atoms in a gaseous ambient to ensure that the wafer surface has the maximum concentration of dopant atoms determined by the diffusion temperature (referred to as the solid solubility limit). Not all introduced dopant atoms are electrically active.
Drive-in Diffusion The drive-in diffusion is typically carried out in an oxidising ambient but with no dopant atoms. This is carried out after in a furnace very similar to an oxidation furnace usually at a temperature higher than the predeposition temperature. Prior to the drive-in diffusion, a very thin layer of dopant rich silicon and/or silicon dioxide formed during predeposition is etched away (hydrofluoric acid “dip”).
[attachment=31910]
As indicated previously the main front-end processing in building a device or integrated circuit is to selectively introduction of dopant atoms into silicon wafer. Dopant introduction by high temperature diffusion is one of the two major processes for achieving this. Diffusion is carried out at high temperature ( 800 to 1000 °C) in a dopant-rich gaseous ambient.
Diffusion also covers the redistribution in the wafer of dopant atoms introduced into the wafer by other methods such as ion implantation.
For simplicity the diffusion of dopant in silicon is modelled by the simple diffusion theory described by Fick’s law in which the flux of dopant atoms at any point is assumed to be proportional to the gradient of the dopant concentration at that point. The proportionality constant is the diffusion coefficient, D . This is assumed to be a constant for a given dopant species at a given temperature. Actually D is not constant by is dependent on dopant concentration.
This section covers the diffusion techniques used and the simple 1-D analysis describing the dopant distribution into the wafer.
Diffusion Mechanisms
Vacancy: atoms jump from one lattice site to the next.
Interstitial: atoms jump from one interstitial site to the next.
Diffusion System
The most common practice of introducing dopant into wafers by diffusion is the horizontal quartz tube furnaces similar to that used for thermal oxidation of silicon wafers. A separate furnace, wafer holder etc are reserved for a particular dopant to avoid contamination by other dopants.
In general the dopant atoms are introduced in two separate diffusion steps, each in its own tube furnace. The first step is referred to as pre-deposition diffusion, it is aimed to introduce a controlled amount of dopant atoms into the silicon surface (dopant atoms per unit surface area). The second step known as the drive-in diffusion which is aimed (at least theoretically) to redistribute the dopant atoms introduced during predeposition further into the silicon and reducing the dopant concentration near the surface.
Predeposition Diffusion
In pre-deposition diffusion the wafer is exposed to excess amount of dopant atoms in a gaseous ambient to ensure that the wafer surface has the maximum concentration of dopant atoms determined by the diffusion temperature (referred to as the solid solubility limit). Not all introduced dopant atoms are electrically active.
Drive-in Diffusion The drive-in diffusion is typically carried out in an oxidising ambient but with no dopant atoms. This is carried out after in a furnace very similar to an oxidation furnace usually at a temperature higher than the predeposition temperature. Prior to the drive-in diffusion, a very thin layer of dopant rich silicon and/or silicon dioxide formed during predeposition is etched away (hydrofluoric acid “dip”).