Integrated Circuits

VLSI Technology (Integrated Circuits)

1 | P r e p a r e d b y V i m a l K a n t P a n d e y

1. Integrated circuits

With the small and effective transistor at their hands, electrical engineers of the 50s saw the

possibilities of constructing far more advanced circuits than before. However, as the complexity

of the circuits grew, problems started arising. When building a circuit, it is very important that all

connections are intact. If not, the electrical current will be stopped on its way through the circuit,

making the circuit fail. Before the integrated circuit, assembly workers had to construct circuits

by hand, soldering each component in place and connecting them with metal wires. Engineers

soon realized that manually assembling the vast number of tiny components needed in, for

example, a computer would be impossible, especially without generating a single faulty

connection. Another problem was the size of the circuits. A complex circuit, like a computer,

was dependent on speed. If the components of the computer were too large or the wires

interconnecting them too long, the electric signals couldn't travel fast enough through the circuit,

thus making the computer too slow to be effective.

In the summer of 1958 Jack Kilby at Texas Instruments found a solution to this

problem. Because he was newly employed, Kilby had no vacation like the rest of the staff.

Working alone in the lab, he saw an opportunity to find a solution of his own to the

miniaturization problem. Kilby's idea was to make all the components and the chip out of the

same block (monolith) of semiconductor material. When the rest of the workers returned from

vacation, Kilby presented his new idea to his superiors. He was allowed to build a test version of

his circuit. In September 1958, he had his first integrated circuit ready. It was tested and it

worked perfectly! Although the first integrated circuit was pretty crude and had some problems,

the idea was groundbreaking. Jack Kilby is probably most famous for his invention of the

integrated circuit, for which he received the Nobel Prize in Physics in the year 2000.

Another scientist Robert Noyce of Fairchild Semiconductors came up with his own idea for the

integrated circuit. He did it half a year later than Jack Kilby. Noyce's circuit solved several

practical problems that Kilby's circuit had, mainly the problem of interconnecting all the

components on the chip. This was done by adding the metal as a final layer and then removing

some of it so that the wires needed to connect the components were formed. This made the

integrated circuit more suitable for mass production.

An integrated circuit is also known as IC, microcircuit, microchip and silicon chip. It is a

miniature electronic circuit that consists of several interconnected transistors, resistors,

capacitors etc., all contained in one small package with external connecting terminals that

has been manufactured in the surface of a thin substrate of semiconductor material. 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.



The main advantages of Integrated circuits over discrete circuits are cost and performance. Costs

are generally low as the chips (along with their components), are printed as a unit by

photolithography, so are not constructed one transistor at a time. The performance is high

because the components switch extremely quickly and efficiently as they consume little power,

as they are small and very close together. Now we see up to 1 million transistors per mm.

1.1 Classification of Integrated Circuits

Classifications of IC’s are done by the following ways:

a) Based on fabrication

b) Based on complexity

c) Based on application

1.1.1 Based on fabrication

Based on fabrication IC’s are classified into following three classes:

a) Monolithic IC’s

b) Thin and Thick film IC’s

c) Hybrid IC’s

Monolithic IC’s: The word „monolithic‟ is derived from the two Greek words “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. The below figure shows monolithic IC‟s in different types of

packaging.

http://www.circuitstoday.com/wp-content/uploads/2009/09/Monolithic-IC-Plastic-Package.jpg



Limitations:

1) Low power rating. Since monolithic ICs are about the size of discrete small signal

transistors, they have a maximum power rating of less than 1 watt. This limits their use to

low power application.

2) Poorer isolation between the components.

3) No possibility of fabrication of inductors.

4) Small range of values of passive components used in the ICs

5) Lack of flexibility in circuit design as for making any variation in the circuit, a new set of

mask 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 comparatively 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 components 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.

Thin film ICs:

Fabricated by depositing films of conducting material on the surface of a glass or ceramic

base.

http://www.circuitstoday.com/wp-content/uploads/2009/09/Monolithic-IC-Can-Type-Enclosure.jpg



By controlling the width and thickness of the films, and by using different materials

selected for their resistivity, resistors and conductors are fabricated.

Methods used for producing thin films are 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.

Thick film ICs:

They are sometimes referred to as printed thin-film circuits.

In their manufacturing 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 components,

greater flexibility in circuit design and of providing better high-frequency performance than

monolithic ICs.

Drawbacks:

Larger physical size.

Comparatively higher cost

Incapability of fabrication of active components.

Hybrid or Multichip ICs:

The circuit is fabricated by interconnecting a number of individual chips.

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. Wiring or a metalized pattern provides connections between chips.

Hybrids ICs are widely used for high power audio amplifier applications from 5 W to

more than 50 W.

The structure of a hybrid or multi-chip IC is shown in figure.



Like thin- and thick-film ICs, hybrid ICs usually has better performance than

monolithic ICs.

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.

1.1.2 Based on complexity

Based on complexity ICs can be classified as follows:

a) SSI (1 to 10 components per chip)

b) MSI(10 to 100 components per chip)

c) LSI(100 to 10000 components per chip)

d) VLSI(more than 10000 components per chip)

e) ULSI(highly dense chips and have more than 3 millions of transistors per

chip) f) WSI (wafer scale integration): Wafer-scale integration (WSI) is a system

of building extremely large integrated circuits that uses a whole silicon

wafer to produce a single "super-chip". Through a combination of large

size and reduced packaging, WSI could lead to dramatically reduced costs

for some systems, notably in massively parallel supercomputers. The

name is taken from the term Very-Large-Scale Integration, the current

state of the art during the development time of WSI.

g) SOC (system on chip): It is an integrated circuit where all the components

needed for a computer (or other system), are included on a single chip.

The design of this device can be costly and extremely complex, and also

building disparate components on a single piece of silicon, could

compromise the efficiency of some of its elements. Nevertheless these

drawbacks are offset by low manufacturing and assembly costs, and by a

http://www.circuitstoday.com/wp-content/uploads/2009/09/Hybrid-Multichip-IC.jpg



vastly reduced power budget (as the signals among the components are

kept on-die, much less power is required).

h) 3D-IC: Three Dimensional Integrated Circuit (3D-IC) has two or more

layers of active electronic components; these are integrated both

horizontally and vertically into a single circuit. Communication between

the layers relies on on-die signaling, so the power consumption is lower

than that of equivalent separate circuits. Sensible use of short vertical

wires can substantially reduce the total wire length, for faster operation

and efficiency.

Features of IC:

1. An IC performs complex functions whereas elementary devices (i.e. transistors) can

perform same function only in combination with other components. e.g. single transistor

cannot amplify signal whereas IC can.

2. Reliability and cost are not affected by an increase in functional complexity of IC.

3. The sizes of most of SSI & MSI are comparable with discrete transistor but the function

performed by an IC is much more complex.

4. In an IC, the adjacent components are spaced mostly 50-100um apart. At such small

distance the components are not very likely to show variations in electrical, chemical and

physical properties. Whereas the parameters are changed in different discrete elements

e.g. change in temperature coefficient due the change in the temperature variation.

5. In an IC, interconnection of individual elements is made through metallization (i.e.

without soldering or welding) hence it is more reliable in comparison to discrete

components in which large number of soldering and welding is used.

Advantages:

6. Increased reliability due to lesser number of connections.

7. Extremely small size due to fabrication of various circuit elements in a single chip.

8. Lesser weight and space requirements.

9. Low power requirement.

10. Low cost etc.

Disadvantages:

11. If any component in an IC goes out of order then the whole IC has to replace by new one.

12. For high values of capacitances discrete components exterior to chip are connected.

13. It is not possible to fabricate inductors and transformers.

14. It is not possible to produce high power IC’s greater than 10 watt.

15. There is lack of flexibility in an IC.

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Integrated Circuits