Diffusion process of IC fabrication

Diffusion

The process of doping i.e. adding impurity to the silicon wafer is called diffusion. Diffusion is used to form bases, emitters and resistors in bipolar device technology and to form source and drain regions in MOS device technology. The dopant can be added into the silicon wafer by using one of the most commonly used methods. 

a) Diffusion from a chemical source in a vapour form at very high temperature.

b) Diffusion from doped-oxide source, and

c) Diffusion from ion-implanted layer.

In general, diffusion process takes place in two steps namely pre-deposition diffusion and drive-in diffusion. In the first step, i.e. pre-deposition diffusion step, very high concentration of dopant atoms are implanted on the surface of silicon. These dopants or impurities are added to the surface in the form of vapour at about 1000 °C. The bonds between the neighbouring atoms of the silicon in the lattice are broken and the silicon atoms move out of the lattice structure. Hence high density of vacancies is created. When dopant is added at 1000 °C, the impurity atoms diffuse into the silicon at the locations of vacancies. In predeposition step, a shallow, heavily doped layer of impurities is formed at the surface. The impurity concentration is maintained constant over the surface of the wafer. Hence it is also called constant source diffusion. In drive-in step, the impurity atoms are redistributed. Without adding new impurity, the present impurity atoms are driven deep into the silicon.

As new impurity atoms are not added during drive-in diffusion, the total amount of impurity remains constant, the main advantage of the drive-in diffusion step is that the surface concentration is reduced is also called constant dose diffusion. The Fig. 1.9.1 illustrates the n impurity substrate in p substrate.

The commonly used dopants are boron for p-type layers and phosphorus, antimony, arsenic for n-type layers. Arsenic may also be used for n+ type source-drain and emitter diffusions. It is observed that practically, instead of above mentioned element, the compounds of these elements are more suitable. Such compounds may be derived from solid, liquid or gaseous sources.

A schematic representation of the diffusion process is as shown in the Fig. 1.9.2. The silicon wafers are stacked vertically on a movable quartz boat inside quartz furnace tube. The temperature of the quartz furnace tube is increased with the help of resistance heaters. The dopant to be introduced is kept in a container. The dopant in the liquid form e.g. POCl₃ may be placed inside the quartz tube in low temperature region or preferably outside the tube with precisely controlled temperature to maintain dopant in the liquid form. Typically nitrogen and oxygen are used as carrier gases. 

When the carrier gases pass over the container, they carry dopant vapour into furnace. In furnace, the gases are deposited on the silicon surface. The layer containing silicon, oxygen, phosphorus is formed on the surface. At high temperature such as 900 °C to 1000 °C, the dopant gets diffused into silicon. For drive-in diffusion, the temperature is further increased to 1100 °C. So that the dopant gets diffused with more depth in silicon. For this step, with a proper control circuitary, the impurity supply is stopped. The proper depth can be achieved by controlling parameters such as time and temperature of the process.

Review Questions

1. Describe in detail about the diffusion process of IC fabrication.

Dec.-06, 11, May-16, Marks 8

2. Explain the diffusion process as applied to IC technology.

3. What is diffusion ? Give different techniques which are commonly used.

4. Distinguish diffusion and ion Implantation process in IC fabrication.