Isolation Techniques in IC Fabrication

Isolation Techniques

We have studied that on a single silicon wafer, large number of components are fabricated. So it is important to provide isolation between these components electrically. The most commonly used isolation techniques are,

i) p-n junction isolation

ii) Dielectric isolation.

The selection of the isolation technique in fabrication of any IC depends on the application of the IC i.e. for general purpose applications or specialized military applications.

1) p-n junction isolation

Consider p-substrate with n-type epitaxial layer grown over it. To provide isolation, a p-type impurity with high concentration (i.e. p type impurity) is diffused selectively into an epitaxial layer such that it reaches to the p-substrate as shown in the Fig. 1.11.1.

From the Fig. 1.11.1 it is clear that n-epitaxial region forms a region which is surrounded by p-type regions. This region is called island. Two regions are separated by two back to back diodes as shown in the Fig. 1.11.1. These two back to back diodes serve as isolation regions if both are reverse biased. So when very high negative voltage is applied to p-type substrate, both the diodes are reverse biased and thus the electrical isolation is observed between two isolation islands.

The important consideration for the isolation is that the concentration of the acceptor atoms must be higher than the concentration of the p-type substrate. If this condition is not maintained, then the depletion layer of the reverse biased diode penentrates into the p+ region and may connect isolation islands together. 

The main advnatage of p-n junction isolation is that different components can be fabricated within the isolation islands. But the disadvantage of this technique is the presence of undesirable and unavoidable parasitic capacitances at the isolation p-n junctions. These capacitances limit high frequency performance of the circuit. Hence this technique is generally preferred for the general purpose ICs only.

2. Dielectric Isolation Technique

In the dielectric isolation technique, the electrical isolation to each component is provided by surrounding them with a solid dielectric. The most commonly used dielectric are silicon dioxide, ruby etc. It also fulfills the purpose of physical isolation. The dielectric layer thickness is large enough so that the parasitic capacitances are negligible. The dielectric isolation technique is illustrated in the Fig. 1.11.2.

The main advantage of the technique is that within same silicon substrate p-n-p as well as n-p-n transistors can be fabricated.

But the disadvantage is the increase in the cost. As the technique needs additional steps in fabrication to deposit a dielectric layer, this technique is expensive. Hence this technique is used for fabricating ICs for special military applications where performance of the circuit is of prime importance than the cost factor.

Review Questions

1. Explain the need for making isolation islands.

2. Write a note on isolation technique used in ICs.

3. Explain the importance of isolation and discuss the different methods of isolation.

May-14, Marks 16; May-07, Marks 6

4. Compare p-n junction isolation technique with dielectric isolation technique.