- Integrated Circuits-Merits and Demerits
- Classification of Integrated Circuits
- Monolithic IC
- Monolithic IC Manufacturing Process
- Monolithic IC-Component Fabrication
- IC Packages
Integrated Circuits (IC’s)
The concept of IC was first introduced in the year 1958. Since then this concept has reached great technological heights than any other concepts and has helped in the miniaturization of a lot of components like mobiles, computers, laptops, and many more devices in the digital world.
The digital era started with the invention of vacuum tubes. Vacuum based computers were rare and expensive. This was then replaced by transistors, which were faster in use and smaller in size, cost effective, less power consuming and reliable. Then came the invention of integrated circuits which just revolutionized the use of computers. Due to its small dimension, low cost, and very high reliability even the common man is familiar with its applications like smart phones and laptops.
The IC’s also found its way in military applications, state of the art communication systems, and industrial applications due to its high reliability and compact size. Nowadays, an IC that has the size of a fingernail consists of more than a million transistors and other discrete components embedded into it. Thus an integrated circuit can also be called a microchip and is basically a collection of some discrete circuits on a small chip that is made of a semiconductor material like silicon.
The use of discrete circuits was replaced by IC’s due to two factors. One is space consumption. A discrete circuitry consists of transistors, resistors, diodes, capacitors, and many other discrete devices. Each of them is soldered on to printed circuit boards (PCB) according to the need of circuitry. In the end PCB will occupy a large space. Another drawback is that the soldered components will show less reliability due to the use of many components. Both these factors urged engineers to invent microcircuits that have more reliability and consume less space.
The idea of an integrated circuit was first proposed by Geoffrey W. A. Dummer in the year 1952. But the attempt to build it led to failure. Another idea was proposed by Jack Kilby. He came up with an idea to create small ceramic wafers where each wafer carried a small miniature discrete component. All these wafers could then be wired to form a compact circuit. But this concept, though developed for the U.S army failed to find momentum and was discarded.
Shortly soon, the very same Jack Kilby came up with the original idea of making an IC while he was working for Texas Instruments. He started building his first IC and finally completed it on 12th September 1958. He made his IC using germanium as the semiconductor chip. This invention won him the Nobel prize for physics in the year 2000. Soon Robert Noyce developed his own prototype of an IC, using silicon as a semiconductor material. This invention helped in resolving many practical problems that Jack Kilby’s IC had.
All IC’s consist of both active and passive components and the connections between them are so small that it may be impossible to see them even though a microscope. All the components (active and passive) are interconnected through fabrication process.
In a circuit diagram, there is no common symbol for representing an IC. They are mostly available as dual in-line packages, metal cans and also ceramic flat packs. They may be 8-pin, 10-pin, or 14-pin depending on the specification of the manufacturer.
Advantages of Integrated Circuits
1. Miniature in size. As fabrication process is used for the integration of active and passive components on to a silicon chip, the IC becomes a lot smaller. When compared to a discrete circuit, it may be at least a thousand times smaller.
2. Due to small size, the weight of the IC also reduces, when compared to the discrete circuit.
3. To produce hundreds of discrete circuits on a PCB for the same logic takes more time and increase the cost factor. But for the production of hundreds of IC’s the cost of production will be very low and less time consuming.
4. The PCB consisting soldered joints will be less reliable. This problem is omitted in IC’s because of no soldered joints, with fewer interconnections, and thus highly reliable.
5. The small size of IC’s causes lesser power consumption and lesser power loss.
6. In a discrete circuitry, if a single transistor becomes faulty, the whole circuit may fail to work. This transistor has to be desoldered and replaced. It is difficult to find out which component has failed. This problem can be omitted in an IC by replacing an entire IC as it is low in cost.
7. Increased operating speed because of absence of parasitic capacitance effect.
8. As the IC’s are produced in bulk the temperature coefficients and other parameters will be closely matching.
9. Improved functional performance as more complex circuits can be fabricated for achieving better characteristics.
10. All IC’s are tested for operating ranges in very low and very high temperatures.
11. As all the components are fabricated very close to each other in an IC, they are highly suitable for small signal operation, as there won’t be any stray electrical pickup.
12. As all the components are fabricated inside the chip, there will not be any external projections.
Disadvantages of Integrated Circuits
1. Some complex IC’s maybe costly. If such integrated circuits are used roughly and become faulty, they have to be replaced by a new one. They cannot be repaired as the individual components inside the IC are too small.
2. The power rating for most of the IC’s does not exceed more than 10 watts. Thus it is not possible to manufacture high power IC’s.
3. Some components like transformers and inductors cannot be integrated into an IC. They have to be connected externally to the semiconductor pins.
4. High grade P-N-P assembly is not possible.
5. The IC will not work properly if wrongly handled or exposed to excessive heat.
6. It is difficult to achieve low temperature coefficient.
7. It is difficult to fabricate an IC with low noise.
8. It is not possible to fabricate capacitors that exceed a value of 30pF. Thus, high value capacitors are to be connected externally to the IC.
9. There is a large value of saturation resistance of transistors.
All the IC’s have interconnected discreet devices inside the chip and the corresponding external connecting terminals outside. Each pin may have each function and may vary according to the manufacturer’s design. In order to make the circuit fully operative, the pins in the IC must be used for supply voltage, input and output connections, and also some external components according to the needs of the manufacturer.
ICs can be classified on the basis of their chip size as given below:
Small scale integration (SSI)—3 to 30 gates/chip.
Medium scale integration (MSI)—30 to 300 gates/chip.
Large scale integration (LSI)—300 to 3,000 gates/chip.
Very large scale integration (VLSI)—more than 3,000 gates/chip.
On the basis of applications ICs are of two types namely: Linear Integrated Circuits and Digital Integrated Circuits.
Linear IC’s are used in cases when the relationship between the input and output of a circuit is linear. An important application of linear IC is the operational amplifier commonly referred to as op-amp.
When the circuit is either in on-state or off-state and not in between the two, the circuit is called a digital circuit. IC’s used in such circuits are called digital IC’s. They find wide applications in computers and logic circuits.
Here are some further classification of integrated circuits based on the fabrication techniques used.
1. Monolithic Integrated Circuits
The word ‘monolithic’ comes from the Greek words ‘monos’ and ‘lithos’ which means ‘single’ and ’stone’. As the name suggests, monolithic IC’s refer to a single stone or a single crystal. The single crystal refers to a single chip of silicon as the semiconductor material, on top of which all the active and passive components needed are interconnected. This is the best mode of manufacturing IC’ as they can be made identical, and produces high reliability. The cost factor is also low and can be manufactured in bulk in very less time. They have been found applicable for C’s used for AM receivers, TV circuits, computer circuits, voltage regulators, amplifiers and so on.
A detailed article explaining the concept and fabrication process of different components and monolithic IC production process is explained here – Monolithic Integrated Circuit.
Being as it is, monolithic IC’s have some limitations as well.
1. Monolithic IC’s have low power rating. They cannot be used for low power applications as they cannot have a power rating of more than 1 watt.
2. The isolation between the components inside the IC is poor.
3. Components like inductor cannot be fabricated to the IC.
4. The passive components that are fabricated inside the IC will be if small value. For higher values they have to be connected externally to the IC pins.
5. It is difficult to make a circuit flexible for any kind of variation; a new set of masks is required.
2. Thin and Thick Film Integrated Circuit
Thick and thin film IC’s are comparatively larger than monolithic IC’s and smaller than discrete circuits. They find their use in high power applications. Though it is a little large in size, these IC’s cannot be integrated with transistors and diodes. Such devices have to be externally connected on to its corresponding pins. Passive components like resisters and capacitors can be integrated.
Both thick and thin film IC’s are explained in detailed below. Though both the IC’s have similar appearance, properties, and general characteristics, the main difference between the two of them is the manner in which the film is deposited on to the IC.
Thin Film Integrated Circuits
This IC is fabricated by depositing films of conducting material on the surface of a glass or ceramic base. The resistors are fabricated by controlling the width and thickness of the films and by using different materials selected for their resistivity. For capacitors, a film of insulating oxide is sandwiched between two conducting films. A spiral form of film is deposited onto the IC to create an inductor.
Mainly two methods are used for producing thin films. One method, called vacuum evaporation is used in which vaporized material is deposited on a substrate contained in a vacuum. The other method is called cathode sputtering in which atoms from a cathode made of the desired film material are deposited on a substrate located between a cathode and an anode.
Thick Film Integrated Circuits
They are also commonly called as printed thin film circuits. The desired circuit pattern is obtained on a ceramic substance by using a manufacturing process called silk-screen printing technique.
The inks used for printing are usually materials that have resistive, conductive, or dielectric properties. They are selected accordingly by the manufacturer. The screens are actually made of fine stainless steel wire mesh. The films are fused to the substrate after printing by placing them in hot high temperature furnaces.
The fabrication techniques used for thin film passive components are adopted for thick films as well. As with thin-film circuits, active components are added as separate devices. A portion of thick-film circuit is given in the figure below.
When compared to monolithic IC’s, thick and thin film IC’s do have some advantages. They have the advantage of better tolerance, better isolation between components, and greater flexibility in circuit design that further helps in providing high frequency performance. But, these are the only factors that must be considered for the application of such IC’s as they are costly in making, and has higher dimensions than monolithic IC’s. They also cannot be used to fabricate active components which further increase the size.
3. Hybrid or Multi-chip Integrated Circuits
As the name suggests, the circuit is fabricated by interconnecting a number of individual chips. Hybrids ICs are mostly used for high power audio amplifier applications from 5 Watts to more than 50 Watts. The active components are diffused transistors or diodes. The passive components may be group of diffused resistors or capacitors on a single chip, or they may be thin-film components. Interconnection between the individual chips is made by wiring process or a metallized pattern.
The diagram of a hybrid or multi-chip IC is shown in the figure above. Hybrid IC’s are also known to provide a better performance than monolithic IC’s. Although the process is too expensive for mass production, multi-chip techniques are quite economical for small quantity production and are more often used as prototypes for monolithic ICs.
Based upon the active devices employed the ICs can be classified as bipolar ICs using bipolar active devices (BJT) and unipolar IC’s using unipolar active devices like FET.