Monolithic IC-Component Fabrication
Fabrication of Components on Monolithic IC
The outstanding feature of an IC is that it comprises a number of circuit elements inseparably associated in a single small package to perform a complete electronic circuit. We shall now see how various circuit elements (such as transistors, diodes, resistors etc.) can be fabricated in an IC form.
1. Transistors and Diodes.
Transistors and diodes are usually formed using the epitaxial planar diffusion process described in previous blog post.Collector, base, and emitter regions are diffused into a P-type silicon substrate,as shown in figure, and surface terminals are provided for connection. In discrete transistors the substrate is normally employed as collector. If this were done with transistors in monolithic IC, all transistors fabricated on one substrate would have their collectors connected together. This is the reason that separate collector regions are diffused into the substrate. Even though separate collector regions are formed, they are not completely isolated from the substrate. For proper functioning of the circuit it is necessary that the P-type substrate is always kept negative with respect to the transistor collector. This is achieved by connecting the substrate to the most negative terminal of the circuit supply.The unwanted or parasitic junctions, even when reverse-biased, can still affect the circuit performance adversely. The junction reverse leakage current can cause a serious problem in circuits operating at very low current levels. The capacitance of the reverse-biased junction may affect the circuit high-frequency performance, and the junction break down voltage imposes limits on the usable level of supply voltage. All these adverse effects can be reduced to the minimum if highly resistive material is employed for the substrate. If the substrate is very lightly doped, it will behave almost as an insulator.
Integrated circuit diodes
They are usually fabricated by diffusion exactly as transistors. Only two of the regions are used to form one P-N junction. In figure, collector-base junction of the transistor is used as a diode. Anode of the diode is formed during the base diffusion of the transistor and the collector region of the transistor becomes the cathode of the diode. For high speed switching emitter base junction is used as a diode.
2. Diffused Resistors.
In IC resistors, the resistance value can be controlled by varying the concentration of doping impurity and depth of diffusion. The range of resistor values that may be produced by the diffusion process varies from ohms to hundreds of kilohms. The typical tolerance, however, may be no better than ± 5%, and may even be as high as ± 20%. On the other hand, if all the resistors are diffused at the same time, then the tolerance ratio may be good. Most resistors are formed during the base diffusion of the integrated transistor, as shown in figure. This is because it is the highest resistivity region. For low resistance values, emitter region is used as it has much lower resistivity.Alternatively, resistors for ICs can be produced by using thin-film technique. In this process a metal film is deposited on a glass or Si02 surface. The resistance value can be controlled by varying thickness, width and length of the film. Since diffused resistors can be processed while diffusing transistors, the diffusion technique is the cheapest and, therefore, the most widely used.
All P-N junctions have capacitance so capacitors may be produced by fabricating suitable junctions. As shown in figure, P- and N-regions form the capacitor plates and depletion region between them is the dielectric. The depletion region width and, therefore, the junction capacitance also varies with change in reverse bias. The value is limited to 100 p F. IC capacitors may also be fabricated by utilizing the Si02 surface layer as a dielectric. A heavily-doped N-region is diffused to form one plate of the capacitor. The other plate is formed by depositing a film of aluminium on the Si02 formed, on the wafer surface. The breakdown voltage of such a capacitor is much larger than for diffused capacitor and voltage of any polarity can be used.
There is no feasible process for fabrication of inductors as part of monolithic structure. It is added externally as a discrete component, when required.