The active components like transistors and diodes etc

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ductors can be made by depositing a spiral formation of film. The active components like transistors and diodes etc. are ex- ternally added and inter-connected by wire bonds. Following two methods are used to produce thin films : ( i ) vacuum evaporation In this method, the vaporised material is deposited through a set of masks on the glass or ceramic substrate contained in vacuum. ( ii ) cathode sputtering In this method, atoms from a cathode made of the desired film material are deposited on the substrate which is located between the cathode and the anode. ( b ) Thick-film ICs Such type of integrated circuits are sometimes referred to as printed thin-film circuits. They are so called because silk-screen printing techniques are employed to create the desired circuit pattern on the surface of the substrate. The screens are made of fine stainless steel wire mesh and the ‘inks’ are pastes (of pulverised glass and aluminium) which have conductive, resistive or dielectric properties. After printing, the circuits are high-temperature fired in a furnace to fuse the films to the insulating substrate. As with thin-film ICs , active elements are added externally as discrete components. 3. Hybrid or Multichip ICs As the name implies, such circuits are formed either by inter-connecting a number of individual chips or by a combi- nation of film and monolithic IC techniques. In such ICs , active components are first formed within a silicon wafer (us- ing monolithic technique) which is subsequently covered with an insulating layer such as SiO 2 . Film techniques are then employed to form passive components on the SiO 2 surface. Connections are made from the film to the monolithic struc- ture through ‘windows’ cut in the SiO 2 layer. Resist SIO 2 SIO 2 Positive Resist Mask Positive Resist Etch-back lift-off Negative Resist Thin film ICs Hybrid or Multichip ICs
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2476 Electrical Technology 67.7. Comparison Between Different ICs Each type of IC has its own advantages and disadvantages. Monolithic circuits have the advantage of lowest cost and highest reliability . However they have the following disadvantages : 1. isolation between components is poorer, 2. range of values of passive components used in the circuits is comparatively small, 3. inductors cannot be fabricated, 4. they afford no flexibility in circuit design because for making any changes in the circuit, a new set of masks is required. The film circuits have the advantage of forming passive components with broader range of values and reduced tolerances as compared to monolithic circuits. Isolation between their compo- nents is also better since they are deposited on a substrate that is an insulator. Use of external discrete active components allows greater flexibility in circuit design. These circuits also give better high frequency performance than monolithic circuits.
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