IAN LANG ELECTRONICS
The Epitaxial Layer.
The stage described before has given us our buried n layers and they take the form of the transistor part of the diagram below:
In this diagram the silicon oxide mask is still on, represented by the diagonal stripes. the substrate is the large grey area and the buried n layer the black and grey stripe in the transistor section. As the next step the
silicon oxide layer is to be removed chemically. After doing that, the slice is ready to be put into a reaction chamber, where it will be heated to 1200 degrees centigrade and an atmosphere of silicon tetrachloride and hydrogen will be introduced. In such heat the silicon tetrachloride breaks down and forms a further layer of silicon on the surface. The addition of an impurity such as phosphorous means the doping turns this layer into a n type.
An interesting phenomenon occurs here in that the epataxial layer follows exactly the crystal formation of the substrate, and so no further purifying actions are necessary. The stage happens quite quickly, growing 0.001 mm per minute. At the end of it we have a construction similar to the drawing below:
In this diagram the epitaxial layer is represented by the spotty area. You will note that the silicon oxide layer we used to diffuse the buried layer has gone, and with the addition of the epitaxial layer the buried layer is now in fact buried, hence its name.
We are going to use the epitaxial layer for the following: the transistor collector, the diode cathode, a junction for the capicitor and an extra piece of isolation for the resistor. Therefore we need to create another mask of silicon oxide to protect the parts we want to keep, and diffuse again using a p-type impurity to change the doping on the parts of the epitaxial layer we want to be isolators. When we've done, it should look like this:
The transistor in particular is no good like this as it does not yet have a base or emitter. We will need to do some more diffusing.
Next we will add p type bases.