IAN LANG ELECTRONICS
First Diffusion Process.
The first diffusion process is the process that will create a buried n layer for transistors on the IC. An IC transistor looks like the drawing below and from it it may be clear why the buried n layer is referred to as such:
Obviously if we want to bury anything then we first have to dig a hole in which to put it. The way to do this in making monolithic ICs is first to spread a layer over the top of the substrate and then to remove parts of that layer. The following steps illustrate the process but it is important to remember that although we are illustrating only one IC being made the process will be carried on over the whole of the wafer simultaneously; that is dozens of chips will be made at the same time.
In prior steps we have made our substrate out of pure silicon. This substrate is then heated for several hours in an atmosphere of oxygen or steam (which contains oxygen).
The above process causes a layer of silicon oxide to form on the surface of the chip. Silicon oxide is an excellent material for resisting the diffusion process . It is selected parts of this layer that we are going to remove.
Upon the surface of silicon oxide we now coat a layer of photoresistant film. This is going to mark out the sections of silicon oxide that we wish to keep and those we wish to remove.
Now we place a photomask on the wafer. This has been designed previously and will almost certainly have been done by computer. The precision with which this needs to be applied is down to nanometres. We then shine a UV light over the surface.
Wherever the UV light has found a transparent, the photoresistant film has hardened. Where an opaque was encountered, it has remained soft. The soft spots can now be cleaned off with a solvent.
Wherever the photoresist is not present, the silicon oxide layer is etched with hydrofluoric acid. This leaves parts of the silicon substrate open to the surface.
The photoresistant film is removed, and we are left with a layer of silicon dioxide with holes in exposing the silicon substrate underneath. These holes are where we are going to put our buried n layers. After washing thoroughly the wafer is now ready for the first diffusion process.
In the first diffusion the wafer will be put into yet another furnace and heated to 1000 degrees centigrade and exposed to an atmosphere of impurity. The ready doped silicon may have been a P-type, and if we dope the exposed areas with N type (which would be phosphorous) they will slowly begin to change. Let us say the P type doping was boron; then as soon as the concentration of phosphorous atoms exceeds that of boron that part will change to N-type. This is a very slow process as the impurities diffuse into the silicon at about 0.002mm per hour and requires constant monitoring of the atmosphere throughout.
If required the process can be split into two parts. Firstly a highly concentrated atmosphere can be deposited on the surface of the slice, and then it can be moved to another hotter furnace to allow the impurities to diffuse further into the silicon. In this case the first part is called deposition and the second diffusion.
Now that we have completed the first diffusion the next stage is to grow an epitaxial layer.
The term epitaxial comes from the greek for "growth" and simply means a layer of crystal grown on another layer of crystal.