Fundamentals of Monolithic IC Technology

Fundamentals of Monolithic IC Technology

We have already studied that in monolithic ICs all the active as well as passive components along with interconnections are integrated on a single crystal. The word 'monolithic' is originated from Greek word and means 'single stone' or 'one stone'. Thus this word is appropriate as the components are integrated on a single piece of silicon crystal. The fabrication of different discrete devices such as diodes, transistors and integrated circuits is carried out by the same technology. The various processes involved  in the fabrication of different devices are carried out in a single plane. Hence this is also referred as a planar technology.

One might think that building complicated circuits with many interconnected components on a single silicon substrate would be cost effective. But using modern technologies, it is possible to fabricate an IC reliably and relatively inexpensively. The main reason behind this is that many identical circuits are fabricated on a single silicon chip simultaneously. This process is called batch fabrication or batch production. The illustration of a single silicon wafer and an integrated chip with complex circuit is as shown in the Fig. 1.4.1.

As shown in the Fig. 1.4.1 a single IC chip which may vary in area 1 mm² to 100 mm² , contains upto several million devices. Eventhough the fabrication processes are complex and expensive, the resulting large number of the integrated circuits make ultimate cost of the individual IC fairly low.

Another important consideration is the percentage of usuable devices, i.e. fault free devices. Generally the faulty devices occur due to some defects in Si or defects in the fabrication steps. The percentage of the usable devices per chip area is called yield. Now let us consider a silicon wafer of 150 mm diameter consisting approximately 12,000 chips of rectangular shape of sides 1 mm. Each IC may contain the components varying in number from few tens to millions. And in one batch, 15 such wafers are fabricated. Hence, in one batch, simultaneously we can get 1,80,000 ICs. However assuming that the percentage of the usable devices (i.e. fault free devices) per chip area is 25 % only. Then also we can have as many of 45,000 perfect fault free IC chips in one batch.

1. Basic Planar Processes in IC Fabrication

The basic planar processes involved during the fabrication of ICs are as listed below.

1. Crystal growth and wafer preparation.

2. Epitaxial growth.

3. Oxidation.

4. Lithography.

5. Reactive plasma etching.

6. Diffusion.

7. Ion implantation.

8. Metallization.

9. Assembly techniques and packaging.

Let us study each process in detail one by one.

Review Questions

1. Explain meaning of monolithic with reference to integrated circuits.

2. List various processes involved in IC fabrication.