Techniques of IC fabrication

Most complex aspect of microelectronic devices is the fabrication of monolithic ICs. There are mainly two types of monolithic fabrication techniques.

1) DIFFUSION METHOD
2) EPITAXIAL METHOD

Here Diffusion method is described below:
DIFFUSION METHOD – To start with the DIFFUSION process highly polished silicon wafer is placed in an oven. The oven contains a concentration impurity made up of impurity atoms which yield the desired electrical characteristics. The impurity atoms are diffused into the wafer under the controlled temperature and the time that the silicon wafer will remain in the oven. This process is called DOPING. When the wafer has been uniformly doped, the fabrication of semiconductor devices may begin. Hundreds of circuits can be fabricated at the same time on the wafer. Important steps to fabricate the IC are performed in the following order: (Let’s take an example of NPN transistor)

1. An oxide coating is thermally grown over the n-type silicon starting material.

2. By applying the photolithographic process, a window is opened through the oxide layer. This is done through the use of masks.

3. The base of the transistor is formed by placing the wafer in a diffusion furnace containing a p- type impurity, such as boron. By controlling the temperature of the oven and the length of time that the wafer is in the oven, you can control the amount of boron diffused through the window (the boron will actually spread slightly beyond the window opening). A new oxide layer is then allowed to form over the area exposed by the window.

4. A new window, using a different mask much smaller than the first, is opened through the new oxide layer.

5. An n-type impurity, such as phosphorous, is diffused through the new window to form the emitter portion of the transistor. Again, the diffused material will spread slightly beyond the window opening. Still another oxide layer is then allowed to form over the window.

6. By means of precision-masking techniques, very small windows (about 0.005 inch in diameter) are opened in both the base and emitter regions of the transistor to provide access for electrical currents.

7. Aluminum is then deposited in these windows and alloyed to form the leads of the transistor or the IC.