A monolithic IC is one that has all components of a circuit and their interconnections made on a single wafer. They are the cheapest and thus more popular than other types of integrated circuits. The basic process of production of monolithic ICs is as follows.
1. P-substrate
The first step in the manufacture of monolithic ICs is called the p-substrate. In this step a wafer or substrate which is the bottom most layer of the circuit is made. A wafer is a polished silicon crystal cut in to required size. A single base or substrate can be cut and used to
2. Epitaxial Growth
In this step, the wafers are placed in a diffusion furnace of about 1,200oC. The furnace has a gas mixture containing silicon and pentavalent atoms which is passed over the wafers. The atoms form a thin layer of p-type semi conductor. All passive and active components of the circuit are fabricated. This layer is called the epitaxial layer.
3. Insulating Layer
The insulating layer is composed of silicon dioxide formed over the surface of the epitaxial layer to prevent the epitaxial layer from contamination. Pure oxygen is passed over the epitaxial layer to form the insulating layer.
4. Photlithographic and Etching Process
The photolithographic process involves removal of some portions of the silicon dioxide. The removal of the insulating layer on selected parts of the wafer allows manufactures to add impurities to the epitaxial layer. The impurities are what make up the
Photo-resist, a layer of photo-sensitive material, is coated on the wafer and then exposed to the sun to mark out the desired regions for etching. After etching and diffusing the impurities, a solvent for example sulphuric acid is used to peel off the photo-resist
5. Component Diffusion
Diffusion is the process of fabricating the circuit components on the wafer. The etched wafer is now sub-divided into portions, ones that cannot take impurities because they still have the SiO2 layer remaining. The second are diffusion acceptor portions that have the SiO2 removed.
Controlled and very high temperatures are used to diffuse p-type impurities into the exposed n-type epitaxial layer. This result in n-type regions called isolation islands; they permit isolation of the various components of the IC, the components will later be formed separately on different islands. The diffusion process is repeated a number of times as various components are fabricated on to the IC. Aluminium is used to create the required interconnections between the components.
6. Scribing and mounting
The ready wafers now need to be put in chips. Hundreds of ICs can be produced from a single wafer. The wafer is then cut and assembled in the required chips. The chips have a ceramic casing and external leads are made after mounting the IC on to the cover, a process called encapsulating. The IC chips are now ready for use.