Classification of Integrated Circuits
An integrated circuit (IC) consists of several interconnected transistors, resistors, capacitors etc., all contained in one small package with external connecting terminals. The circuit may be entirely self-contained, requiring only input and output connections and supply voltage to function. Alternatively, a few external components may have to be connected to make the circuit operative.
On the basis of fabrication techniques used, the ICs can be divided into following three classes.
The word ‘monolithic’ is derived from the Greek monos, meaning ’single’ and lithos, meaning ‘stone’.Thus monolithic circuit is built into a single stone or single crystal i.e. in monolithic ICs, all circuit components, (both active and passive) and their interconnections are formed into or on the top of a single chip of silicon. This type of technology is ideal for manufacturing identical ICs in large quantities and, therefore, provides lowest per unit cost and highest order of reliability. Monolithic ICs are by far the most common type of ICs used in practice, because of mass production , lower cost and higher reliability.
Since their invention, manufacturers have been manufacturing monolithic ICs to carry out all types of functions. Commercially available ICs of this type can be used as ampli¬fiers, voltage regulators, crowbars, AM receivers, TV circuits, and computer circuits. However, the monolithic circuits have the following limitations or drawbacks:
• Low power rating. Since monolithic ICs are of about the size of a discrete small-signal transistor, they typically have a maximum power rating of less than 1 watt. This limits their use to low-power applications.
• Poorer isolation between components.
• No possibility of fabrication of inductors.
• Small range of values of passive components used in the ICs.
• Lack of flexibility in circuit design as for making any variation in the circuit, a new set of masks is required.
Thin and Thick Film ICs.
These devices are larger than monolithic ICs but smaller than discrete circuits. These ICs can be used when power requirement is compara¬tively higher. With a thin-or thick-film IC, the passive components like resistors and capacitors are integrated, but the transistors and diodes are connected as discrete compo¬nents to form a complete circuit. Therefore, commercially available thin- and thick-film circuits are combination of integrated and discrete components. The essential difference between the thin- and thick-film ICs is not their relative thickness but the method of deposition of film. Both have similar appearance, properties and general characteristics.
They are fabricated by depositing films of conducting material on the surface of a glass or ceramic base. By controlling the width and thickness of the films, and by using different materials selected for their resistivity, resistors and conductors are fabricated. Capacitors are produced by sandwiching a film of insulating oxide between two conducting films. Inductors are made by depositing a spiral formation of film. Transistors and diodes can be produced by thin-film technology; but usually tiny discrete components are con¬nected into the circuit.
One method used for producing thin films is vacuum evaporation in which vaporized material is deposited on a substrate contained in a vacuum. In another method, called cathode sputtering, atoms from a cathode made of the desired film material are deposited on a substrate located between a cathode and an anode.
They are sometimes referred to as printed thin-film circuits. In their manu-facturing process silk-screen printing techniques are used to create the desired circuit pattern on a ceramic substrate. The screens are actually made of fine stainless steel wire mesh, and the inks are pastes having conductive, resistive, or dielectric properties. After printing, the circuits are high temperature-fired in a furnace to fuse the films to the substrate. Thick-film passive components are fabricated in the same way as those in thin-film circuits. As with thin-film circuits, active components are added as separate devices. A portion of thick-film circuit is given in figure.
ICs produced by thin-or thick film techniques have the advantages of forming passive components with wider range and better tolerances, better isolation between their com¬ponents, greater flexibility in circuit design and of providing better high-frequency per¬formance than monolithic ICs.
However, such ICs suffer from the drawbacks of larger physical size, comparatively higher cost and incapability of fabrication of active components.