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NBSIR 81-2197

i Semiconductor Technology for the

Non-Technologist

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Robert I. Scace

Electron Devics Division

Center for Electronics

and Electrical Engineering

U S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

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1981

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IS. DEPARTMENT OF COMMERCE

IATIONAL BUREAU OF STANDARDS

NBSIR 81-2197

SEMICONDUCTOR TECHNOLOGY FOR THE

NON-TECHNOLOGIST

National Bureau of Standards

Library, E-01 Admin. Bldg.

MAR 1 1 1981

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Robert I. Scace

Electron Devics Division

Center for Electronics

and Electrical Engineering

U S. Department of Commerce

National Bureau of Standards

Washington, DC 20234

January 1981

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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' . . . ' 3 JAHOI'*.'
Table of Contents

Page

Abstract 1

1 . Introduction 1

2. Basic Materials 1

3. Basic Devices 4

4. Processing Technology 6

4 . 1 Crystal Growth 6

4.2 Units of Measurement 8

4.3 Mechanical Processes 8

4.4 Epitaxy 9

4.5 Oxidation 9

4.6 Lithography 10

4.7 Diffusion 12

4.8 Ion Implantation 12

4.9 Passivation 12

4.10 Metallization 13

4.11 Testing 13

4.12 Dicing 14

4.13 Assembly 14

4.14 Final Testing 15

4.15 Process Control 16

5. Product Development 17

6. Product Types 18

6.1 Diodes 18

6.2 Transistors . 20

6.3 Thyristors 20

6.4 Integrated Circuits 21

6.5 Application Trends and Their Impacts 23

7. Summary ..' 24

iii
Preface

As the semiconductor industry has grown in size, more and more people who are not semiconductor specialists have become involved with the industry. Some

are clerical or other nontechnical employees, some are in other businesses

such as equipment suppliers to the semiconductor industry, and some are in

the communications media. In particular, the recent wave of studies of the

industry has introduced many economists and government people to a highly

technical subject which, at times, seems not to be well understood.

Yet some degree of understanding of the terms and concepts basic to semiconductors is needed by these people, whose specialties are in other fields.

This text, which is intended to meet that need, was originally prepared in

1976 to be part of "A Report on the U.S. Semiconductor Industry" by the Department of Commerce. That report was issued in September 1979. Both before

the issuance of the Commerce report and since, there have been requests for

this material from people who were first getting acquainted with semiconductors. Their responses, and a continuing demand for copies, have suggested

that independent publication would be useful.

The present text has been slightly revised to bring it up to date. The industry moves rapidly, and four years' time has brought changes. It is quite

possible that future re-issues may occur. Suggestions from readers regarding

possible improvements are welcome.

*

IV
SEMICONDUCTOR TECHNOLOGY FOR THE NON-TECHNOLOGIST

Robert I. Scace

Electron Devices Division

National Bureau of Standards

Washington, DC 20234

Abstract

The properties of semiconductor materials, the methods of processing them, and the solid-state products made from them are described

in terms intended to be understandable by the lay person. The

semiconductor industry has grown at a rate of 21 percent per year

compounded for the last twenty years, and its products have de- clined in unit cost by a factor of five in current dollars (a factor of ten in constant dollars) in the same period. This very

satisfactory but anomalous behavior has attracted the interest of

many who are not familiar with the technology of the industry, yet

who need to have some understanding of it. This report is intended

to help meet that need.

Key Words: Integrated circuits; semiconductors; semiconductor devices; semiconductor processes; sem -onductor technology; silicon.

1 . INTRODUCTION

This is a detailed discussion of semiconductor techrology using nontechnical

terms so readers not familiar with the subject can c ^mprehend its complexity

and breadth. While the knowledge underlying semiconductor technology is

primarily in the fields of physics and electrical engineering, many of the

processes used to manufacture semiconductor devices borrow heavily from the

field of chemistry. A manufacturer of semiconductors will have on its professional staff physicists, chemists, metallurgists, ceramists, electrical

engineers, mechanical engineers, industrial engineers, and possibly specialists in optics. Because of the great diversity of technical information

required to make semiconductor devices, no one kind of professional employee

is sufficient to do the entire job. Semiconductor products result from the

team efforts of a variety of professionals.

2. BASIC MATERIALS

Semiconductor materials generally are crystalline substances of relatively

simple physical and chemical composition. Because of this simplicity, good

theoretical understanding of the properties of semiconductors has been devel

- oped. The theory is extensively used to understand the behavior of solidstate devices and to design new ones. While there are many semiconducting

compounds and elements, only a few have commercial significance. Originally

because of the relative simplicity of processing it, germanium was the

material most commonly used. From the middle 1950s to the middle 1960s,

silicon gradually became the primary semiconducting material for several

reasons. First, since silicon is much more abundant than germanium, it is

cheaper. Second, germanium devices generally will not operate above 85°C,

1
whereas silicon devices can operate up to 200°C. Third, and probably most

important, silicon has a tenacious, electrically insulating, protective oxide. The oxide of germanium is not protective. The properties of silicon

dioxide have important consequences in the processing of semiconductor devices, and they have made possible the batch processing necessary to make

solid-state devices at low cost. Therefore, the following discussion will

deal exclusively with silicon, except in special instances which will be

noted.

The crystalline structure of germanium and silicon is basic to semiconducting

action. Pure silicon at room temperature has the very high resistivity of

230,000 ohm-centimeters, which makes it a poor conductor of electricity.

This arises because the atoms in the silicon crystal structure are arranged

so that each atom has four other silicon atoms around it at equal spacings.

This so-called diamond cubic arrangement allows each of the four outer electrons of the silicon atom to be bound to, the four neighboring atoms. In turn

the four neighboring atoms have their electrons similarly bound. Each interatomic bond in the crystal lattice therefore ties up two electrons, one from

each of the atoms connected by any particular bond. Thus, all the available

outer electrons in a perfect silicon crystal are bound together and are not

available to conduct electricity, if the effects of temperature are ignored.

At temperatures other than absolute zero, the atoms in a crystal vibrate

about their nominal positions, and a few bonds may at any instant be disrupted. The electrons released this way do permit a slight amount of electrical conduction, and therefore the resistivity of silicon at room temperature is not infinitely high.

If, however, elements from neighboring columns of the periodic table are introduced into the crystal lattice, this arrangement of electrons is disturbed. For instance, if a phosphorus atom, having five outer electrons, is

substituted for a silicon atom in the crystal, four of the five electrons

will be bound to the crystal lattice, but one will be available to move freely within the crystal. This makes the crystal n-type, meaning that the prin<-

cipal carriers of the electric current are negative charges (electrons).

Conversely, if a boron atom, having three outer electrons, is substituted for

a silicon atom in the crystal, the electrons can complete only three of the

four necessary bonds. The absence of an electron in the one remaining bond

represents a departure from the symmetrical bond arrangement of a perfect

crystal. It is possible for an electron from a neighboring bond to move into

this space and create a vacancy, having a net positive charge, in the bond

left behind. This vacancy, with its positive charge, can propagate through

the crystal in a manner quite similar to that of an electron, with the exception that the sign is positive instead of negative. This charge-carrying

entity is called a hole, and crystals in which this (positive) charge transport mechanism predominates are referred to as p-type.

The elements that cause p-type or n-type conduction are referred to as dopants or impurities. They are impurities only in the sense that they represent a departure from pure silicon crystal. The silicon itself is quite pure

as are the dopant elements. In fact, silicon is the purest raw material ever produced in tonnage quantities. Impurity levels of one part or less per

billion are common in silicon used for semiconductor manufacture.

2
Dopant elements may be added to the silicon at several points in the semiconductor device manufacturing process. They can be added at the time that the

single crystal of silicon is formed, or later by localized melting and re- crystallization processes (alloying), by high temperature diffusion processes, during the growth of additional layers of silicon, by high energy bombardment (implantation), or by nuclear transmutation. As purchased, the

doping of silicon is such that its resistivity usually falls between 0.001

and 1000 ohm-centimeters. Values of resistivity outside this range are achievable but not often used. The effects of dopant elements are quite

significant, permitting silicon to have nearly any desired resistivity and

both p- and n-type conductivity.

The boundary between a p-type region and an n-type region is called a p-n

junction. The active portion of a semiconductor device consists of p- and n- type regions and the junctions between them. The processes for making such

devices provide the capability to introduce dopant atoms in predetermined

locations and amounts with precise control and reproducibility. The ways in

which this is done will be discussed later.

Various external layers can be arranged to provide electrical insulation or connection to parts of the device. Of the insulators, silicon dioxide is

most common partly because it is a byproduct of many process steps and is

there anyway, and partly because of its excellent electrical insulation properties. In some cases, layers of aluminum oxide or silicon nitride are used.

Where insulation thicknesses of more than one micrometer are needed, various

special glass formulations can be deposited on the wafer surface.

The most common metals used for connection .to the silicon are aluminum and

gold, since both are readily available in very high purity,’ can be vacuum- evaporated easily, and form good electrical connections to silicon. For

special purposes, very complex sequences of metal layers can be used, though

at considerable extra cost. Even heavily doped silicon itself is used.

The various metal films serve to connect the often minuscule regions of the

semiconductor device together with each other, and to provide regions to

which external connecting leads can be attached.

In general, all materials used in a semiconductor device—the silicon, its

dopants, the coatings applied, and the metallization—must be extremely pure.

Because elements other than dopants often adversely affect the electrical

properties of the device if they find their way into the silicon, they must

be excluded. The only way this can be done is to use materials of the highest purity. Uncontrolled elements can lead to uncontrolled effects in processing and to uncontrolled device electrical characteristics at the end of

the process. The same stringent demands of purity are made of the various

chemicals used in cleaning, etching, and otherwise processing the silicon

from beginning to end of the process. One of the major technical achievements, not widely known outside the industry, towards making modern semiconductor devices possible was the achievement of suitable purity levels in the

wide range of ancillary materials needed by the industry. Twenty-five years

ago, such high purity materials were known only in the research laboratory.

Today they are available off the shelf in most technically advanced parts of

the world.

3
3. BASIC DEVICES

The p-n junction diode and the transistor, both elementary devices, are useful in themselves and in combination with one another can make up integrated

circuits.

Figure 1 illustrates a p-n junction diode. It is divided into two portions

by the p-n junction. In the upper portion the majority carrier of electrical

current is electrons, while in the lower portion holes carry the current. If

the upper electrode is biased negatively and the lower positively, then the

charge carriers in those regions are repelled towards the p-n junction, and

in crossing that junction find themselves in regions of the device in which

they are not in equilibrium. An electron, for example, flowing downward past

the p-n junction will combine with one of the holes with which it is sur- rounded and disappear. Other electrons will suffer the same fate. Conversely, holes flowing upward into the n-type region will combine with electrons

there and disappear. This flow of electrons and holes across the p-n junction constitutes an electric current through the device and through the ex- ternal circuit. This direction of polarity is called forward bias. If the

upper electrode has a positive potential applied and the lower electrode a negative potential, the charges in the respective upper and lower regions are attracted away from the p-n junction leaving behind a region with no free

electric charge carriers. This region acts as an effective insulator, and

virtually no current exists. This direction of polarity is referred to as reverse bias. This diode or rectifier then acts as an electrical valve that

permits the passage of current in one direction and obstructs current in the

opposite direction.

Figure 2 is a schematic sketch of an npn transistor. (There are other kinds

of transistors, too.) The upper nrregion is biased rather strongly positively, and the upper p-n junction is therefore reverse biased. There is very

little current in the circuit connecting the upper n-region with the lower n- region. If a positive potential is applied to the central p-region, holes

pass from the p-region into the lower n-region and electrons pass from the

lower n-region into the central p-region. A current therefore exists in the

left-hand loop of the sketch. Although the figure does not show it, the pregion is extremely thin. Therefore, most of the electrons passing from the

lower n-region into the central p-region do not have time to recombine with

holes there, but they survive to reach the upper p-n junction. At this point

they are attracted to the much larger positive potential on the other side of

the junction and are swept across it. This constitutes a current in the

right-hand loop of the sketch. If current in the left-hand loop is interrupted by breaking the circuit, the current in the right loop ceases. Similarly, an increase in current in the left-hand loop will lead to an increased

current in the right-hand loop. Thus one current controls another, and this

is the essential property of the transistor. The device illustrated is a junction transistor, so called because it contains two p-n junctions. Other

transistors, such as the junction field-effect transistor and the metaloxide-semiconductor field-effect transistor, accomplish a similar function

although by different means.

The two devices described above depend in their action upon the flow of electrons and holes under the influence of external currents and voltages. While

4
1

CATHODE

n

p-n JUNCTION

6 ANODE

Figure 1. Elementary junction diode.

Figure 2. Elementary npn transistor.
the operation of semiconductor devices will not be discussed in detail, it

should be kept in mind that all devices, from the simplest p-n diode to the

most complex integrated circuit, operate using these elementary concepts.

The art of making semiconductor devices is simply that of arranging the

n-type and p-type regions, the junctions between them, and their interconnections so that useful electrical properties result.

4. PROCESSING TECHNOLOGY

Many techniques are used to manipulate the dopants and other parts of the

semiconductor device structure so as to achieve useful results. The manufacture of a given kind of device may use only some of these processes, but they

will all be briefly described to illustrate the wide range of technical disciplines necessary to make semiconductor devices. Any semiconductor manufacturing plant must be able to perform these processes in a controlled and

reproducible fashion if it is to make devices with any degree of success.

4. 1 Crystal Growth

Most of the metallic and ceramic objects with which we are familiar are com- posed of many tiny crystals of the material in question and are described as polycrystalline. Generally, these small crystals can be seen only under high

power microscopes. In contrast, semiconductor devices are manufactured using

a starting material that is a single crystal. This is, required because the

boundaries between crystallites in ordinary materials have different physical

properties from the body of the crystal. If one attempted to make semiconductor devices from polycrystalline silicon, the diffusion processes and

other techniques used to introduce the dopants into the silicon would result

in dopant concentrations at the crystallite boundaries different from the

concentrations in the interior of the crystallites and the desired degree of

control over the structure would be impossible.

Semiconductor grade silicon is prepared in a classical chemical plant. Most

manufacturers of semiconductor silicon are also substantial manufacturers of

other chemicals. One of the intermediate materials in silicone* manufacture is particularly suited for the preparation of the semiconductor grade

silicon. This is reduced to polycrystalline high purity silicon in the form

of long slender rods. These rods are converted into single crystal silicon

of the appropriate doping and physical dimensions in one of two ways.

In the Czochralski method, a quantity of silicon is melted in a quartz crucible under an atmosphere free from oxygen. A' slender single-crystal bar

(seed) of silicon is introduced into the melt from above and a portion of the

melt begins to freeze onto the seed. If the temperature and other conditions

are correct, the growth onto the seed crystal is a single crystal whose crystal structure is a continuation of that of the seed crystal. In this fashion

the liquid silicon in the melt is converted into a cylindrical ingot of sili- con without any intercrystalline boundaries. The doping of a Czochralski

crystal is performed by adding appropriate dopant elements to the melt prior

*Distinct from silicon. Silicones are a class of chemicals similar to organic chemicals, but with a silicon-oxygen chain as a structural basis rather

than a carbon-carbon chain.

6
to growth. In commercial practice, crystals up to 125 mm (5 inches) in diameter and up to 750 mm (30 inches) long are available. Larger crystals can be

made. While the Czochralski process produces semiconductor silicon appropriate for manufacture of many kinds of devices, the silicon contains an appreciable amount of oxygen dissolved from the quartz crucible in which the melt

was held. In addition, during the growth process some segregation of the

doping element takes place. That is, the solubility of most dopants is

greater in the melt than it is in the crystal. The concentration of dopant

in the melt thus increases continuously during the growing process. The

finished single crystal does not contain a uniform concentration of the doping element from one end to the other and contains oxygen that is troublesome

in the manufacture of some devices, especially those required to operate at

high voltages.

Some of these difficulties can be overcome by growth by the float-zone method. In this technique, a cylindrical rod of polycrystalline silicon is sus- pended in a chamber in an atmosphere free from oxygen.

The bottom of the rod is melted with a high frequency induction coil, forming

a liquid drop hanging from the bottom of the silicon bar. A seed crystal is

introduced into this silicon drop from below, and by controlling the temperature of the molten region and the other thermal parameters of the apparatus,

silicon is made to grow upon the seed as a single crystal. By raising the

induction heating coil, the molten region of the bar is raised slowly, dissolving silicon from the polycrystalline rod above and growing it as single

crystal onto the crystal ingot supported by the seed below. The molten re- gion is held in place by a combination of surface tension and magnetic forces

from the induction heating coil. Doping in this process is either by means

of a small charge of concentrated dopant added to the initial melted drop

before growth on the seed has begun, or by incorporating a gaseous compound

of the dopant into the atmosphere around the growing crystal in a concentration such that it dissolves in the molten region at the same rate at which

dopant atoms are withdrawn from the molten region by the growing single crystal. Float-zone silicon of 100-mm (4-inch) diameter is currently commercially available. This material is quite free from oxygen and can have a much

more uniform distribution of doping species incorporated in it in comparison

with Czochralski silicon.

Another technique for producing exceptionally uniformly doped n- type crystals

of silicon employs neutron irradiation. If an undoped crystal is exposed to

neutrons in a nuclear reactor, some of the silicon atoms will absorb a neu- tron and be transmuted into phosphorus atoms. Since this occurs at random,

the newly created phosphorus (an n-type dopant) is distributed uniformly

throughout the crystal. Such a degree of uniformity of doping is impossible

by the methods described earlier. Local fluctuations in resistivity caused

by nonuniform conventional doping often amount to >20 percent of the average.

Since resistivity is one of the principal factors influencing the maximum

blocking voltage of a p-n junction, these deviations are one cause of variations in the voltage ratings of devices made from conventionally doped silicon. This is a particular problem in large, high-voltage devices used for

power control. Because of the large area of the silicon slices from which

these devices are made, there is a high probability that there will be a lower than average resistivity region somewhere in the slice. This causes

7
the breakdown voltage at that point to be less than at other places, thus

limiting the voltage rating of the device- to a lower value than it otherwise

would be. The uniform resistivity of neutron- irradiated silicon allows this

difficulty to be avoided. Devices made from this material can have a 15- to

20 -percent higher maximum voltage rating without the necessity for making any

changes in design and without affecting any of the design trade-offs that

would otherwise have to be made.

4.2 Units of Measurement

At this point we must digress for a moment to become familiar with the terms

of measurement to be used below. The semiconductor industry, being international in scope, uses metric measures to a great extent. Most Americans are not yet accustomed to thinking in these units, and they especially do not

have an intuitive feeling for very small dimensions expressed in metric

terms. This discussion is meant to relate metric linear measures to familiar

objects.

A nickel is about 2 centimeters in diameter.

1 centimeter (cm) = 0.3937 inch

The diameter of paper clip wire is about 1 millimeter.

1 millimeter (mm) =0.1 cm

A human hair is about 100 micrometers in diameter,

and cigarette paper is about 25 micrometers thick.

1 micrometer ( ym) = 0.001 mm (one-thousandth mm)

The wavelength of green light is about 0.5 micrometers, and this dimension is

close to that of the finest detail that can be seen using a high-powered

light microscope.

There are no familiar objects in the size range from 1 to 25 micrometers

simply for the reason that they are so small that they cannot be seen by the

unaided eye; yet, this is just the region of dimensions of many of the parts

of semiconductor devices as will be seen presently. Now, let us return to our discussion of silicon.

4.3 Mechanical Processes

Crystals obtained by either the Czochralski or float-zone method are usually

ground to a predetermined diameter using diamond grinding equipment. Since

uniform diameter of the crystal is important in subsequent processing operations, most semiconductor manufacturers buy their silicon to a specified

diameter. At this point, the silicon has a market value ranging from 50

cents to $1 a gram ($230 to $460 a pound). Because of this high value, subsequent operations must be conducted so as to minimize the waste of silicon.

The crystal next is sawed into thin slices, using diamond-edged blades. The

difficulty of this operation can be appreciated when one considers that silicon is as brittle as glass, and the slices may be as thin as 0.25 mm. Largediameter thin slices are delicate, and the sawing operation must be conducted

with great care. For many applications the slices need not be so thin, and

8
for reasons of mechanical strength are more often in the thickness range of

0.5 to 0.8 mm. Slices are etched and lapped on both sides to remove the

structural damage to the surface layers of the slice done during the sawing

operation. For most transistor and integrated circuit manufacture, the

slices are then polished to a mirror finish on one side. In this way all

mechanical damage to the crystal on that side is removed, preparatory to

further processing.

It is necessary at this point and at many subsequent points to clean the

slices thoroughly. Mechanical operations are usually relatively dirty. The

dirt includes small particles of silicon as well as other materials. These

must all be removed from the silicon slice in preparation for later work.

Cleaning involves solvent degreasing, heated detergent solutions under ultrasonic agitation, and physical wiping operations. In many cases the final

cleaning step is a chemical etching operation, which removes some of the

silicon from the surface.

4.4 Epitaxy

The cleaned, polished slice is now ready for growth of a layer of silicon by

an epitaxial process. Epitaxy denotes a crystalline growth such that the

atomic arrangement on the surface of the original slice (substrate) is con- tinued into the growing layer. Growth is done at high temperature in an atmosphere containing a silicon compound that decomposes on the hot slice and

leaves silicon there. A dopant compound is also present, which decomposes in

a similar way to provide the desired doping of the growing silicon layer.

Generally, epitaxial layers are from 2 to 20 micrometers thick and are the

silicon layer in which the final devices will be formed. The function of the

substrate is simply as a host surface for the epitaxial layer and as a thick

mechanical support. The use of epitaxy permits the manufacturer to buy sili- con in large quantities to relatively undemanding specifications (thus keeping the cost down) , and to grow a layer tailor-made for the specific kind of

product he makes.

4.5 Oxidation

As a first step to a diffusion operation, the slice is put in a high temperature furnace for growth of an oxide layer over its entire surface. The oxidation takes place at a precisely maintained temperature, usually between 900

to 1200°C, in a controlled atmosphere containing oxygen. Not only are the

composition of the atmosphere and its purity maintained, but the temperature

is also closely controlled. When diffusion became an important semiconductor

process, manufacturers usually built their own furnaces because no suitable

one could be bought. Typical present-day furnaces for semiconductor manufacture can maintain temperatures of 1000°C with a precision of one-half degree over a space exceeding 1 meter in length and 15 cm in diameter. Before the

advent of semiconductor devices, there was no requirement for furnaces with

such precise control. Most of the capital equipment used in semiconductor

processing has similarly evblved from laboratory-made equipment to refined,

commercially available equipment as new processes develop from laboratory

scale to standard manufacturing operations. The extreme demands of cleanliness, precise dimensional control, or uniformity of temperature, unique to

9
the semiconductor industry, have led to the creation of supporting industries

manufacturing highly precise equipment for use in making semiconductor devices.

4.6 Lithography

To form a doped pattern in the slice of silicon by diffusion, openings must

first be made in the layer of oxide created in the preceding step. A master

pattern, prepared photographically on a glass plate, contains an image of the

desired array of openings. A layer of photosensitive material (resist) is

spread on the surface of the silicon wafer and dried. It is exposed to

ultraviolet light through the master pattern to expose certain areas of the

resist. The pattern is developed chemically, leaving areas of the slice

covered with resist while other areas are clear. The slice is then etched to remove the silicon dioxide in the clear areas, following which the remaining

resist is stripped away. The regions now free of oxide can be accessed by a diffusion process to add impurities to these areas.

The photographic plate containing the pattern that was used in the preceding

step is generated itself by a complex sequence of operations. This may

start with a precise master drawing of the pattern at 100 to 200 times its

final size. This drawing is frequently generated by computer-controlled

drafting equipment. Using specially designed cameras, the drawing is reduced

in size, generally in two steps, to an image on a glass plate at the final

size of the microcircuit. The single final image is used in a step-andrepeat camera to generate photographically a master mask containing many

copies of the same pattern.

Most integrated circuit processes require a set of four to ten such patterns

for the sequential, steps necessary to fabricate the device. Each of the

patterns in the set is reproduced in multiple. The resulting master set

of plates, containing many images each, is used to reproduce working copies

that are either used directly in manufacture or are the working masters to

create the plates used in manufacture.

The foregoing procedure with its many optical steps is difficult to control

to the degree of precision needed for making today's complex integrated

circuits, and two newer methods are more widely used. The pattern generator

technique employs a computer-controlled machine which produces tiny, intense

pulses of light. The machine scans the photographic plate in a series of

parallel lines, in much the same way as that in which a television picture is

painted on the screen. The light pulses are produced as needed to expose the

plate and build up, point by point, the desired image. Integrated circuit

designs are produced with the aid of computers, and the data for controlling

the pattern generator are produced by the computer design system on a magnetic tape. The image formed by the pattern generator can be the same

size as the final circuit, or it can be few times larger. In the latter

case, the final size image is produced on the silicon slice by optical

reduction at the time the silicon is being processed.

The smallest images are produced with electron beams instead of by light.

Because light has a wavelength of dimensions comparable to the dimensions of

many parts of an integrated circuit, it is not possible to produce sharply

10
defined images at the desired scale with light. Electrons which have been

accelerated by a potential of 10 kilovolts or so have a much smaller wavelength than light, and are used in a machine which operates in much the same way as a pattern generator to draw tiny patterns on a photographic plate.

For the most demanding imaging, at dimensions less than one micrometer in

size, electron-beam equipment can be used to write the pattern directly on each slice. Special electron-sensitive resists are used. The process at

present is much slower than the conventional optical one, and requires the use of a machine which costs well over a million dollars. The cost has so far limited the use of direct electron-beam writing to those circuits which

must be made in this way if they are to be made at all.

In repeating photomasking operations, it is necessary to align the new pattern precisely with that already on the slice. This is done with the aid of

a sophisticated mechanical apparatus called a photomask aligner. In making

complex integrated circuits, it is necessary to align patterns to within

typically 1 ym across the entire area of a 100 mm slice of silicon.

Difficulties in this process will be appreciated when it is realized that

linewidths of 3 ym are commonly used in making integrated circuits. A com- plex integrated circuit may contain thousands of lines of these dimensions.

The patterns in sequential masks in a set thus must align with one another to

within substantially less than that dimension, such that all the patterns in

the sequence will be correctly aligned everywhere on a large slice of silicon. Further, any defects in the images will be reproduced on the silicon

slice as defects in the pattern of resist. This leads to diffusion where it

is not wanted or a lack of diffusion where it is wanted, and frequently to

failure of that particular segment of the slice to produce a good final device. Scratches, dirt, and misalignment all contribute to losses of what

would otherwise be good devices.

Such defects in pattern definition are a major cause of yield loss in device

manufacture. In large, complex integrated circuits the yield may be as low

as 5 percent. That is, only one good circuit is produced for every 20 that

are started at the beginning of processing of the silicon slice. In some

cases, duplicate photomasking operations are done, using two different photomasks having the same image patterns. Flaws in the pattern from random

causes will not generally be in identical locations on both photomasks, and

thus some of the defects produced by one mask will be corrected by the second. Of course, this doubles the labor expense of the operation.

The most advanced optical photolithographic printers use a projection method,

rather than the simpler contact printing procedure. Masks have a short life

in contact printing since they are exposed to the hazards of contamination or scratching. Projection printing allows the mask to be better protected. To

reduce the incidence of defects in the patterns on the silicon, masks for

projection printing are rigorously inspected and defects are repaired before

the masks are used. This is a costly procedure, but the long life of masks

in a projection printer and the reduction of yield loss makes it worthwhile.

There are serious optical problems related to the photomasking process. The

lenses used for reduction of original drawings or for projection printing are specially designed for this purpose and are very expensive. Such lenses must

be capable of forming an image without distortion and with excellent edge

11
definition. In the final images of the photomask, the dimensions are so small that accurate measurement is difficult. Lines on these masks are com- monly 3 vim wide (only 6 wavelengths of green light), and sometimes only 1 viti. They do not appear well defined in measuring microscopes because of their

size is so close to the dimensions of the light waves used to observe them.

The problem of verifying that each of the thousands of lines and spaces on a photomask is of the proper size and in the right place is staggering.

4.7 Diffusion

Following the creation of open areas in the oxide coating, and the removal of

the photoresist, slices can be diffused with a dopant. This is a batch process, in which a number of slices are inserted in a furnace at a precise

temperature, usually between 900 and 1200°C, in a closely controlled atmosphere. The atmosphere contains a quantity of the dopant to be diffused into

the silicon surface to dope the regions that are free from oxide. The con- centration of the dopant in the silicon is related directly to its concentration in the atmosphere, and the depth to which it penetrates is related to

the square root of the time that the slices are exposed to the atmosphere in

the furnace. Diffusion times may range from a few seconds to many hours

depending on the depth of diffused layer to be produced. High purity sources

of diffusing atoms may be either gaseous, liquid, or solid.

4.8 Ion Implantation

Ion implantation permits the placement of dopant atoms into the slice at room

temperature, rather than at the high temperatures required in diffusion. In

ion implantation, a high-energy stream of dopant atoms is directed toward the

silicon slice in a high vacuum. They penetrate into the surface of the target slice to a depth depending upon their kinetic energy. The number of

atoms implanted in a given volume of the target slice can be very precisely

controlled, as can the depth of penetration. Furthermore, distributions of

dopant atoms not possible to achieve by other means can be produced by ion

implantation. This technique has become a production process only during the

last decade, but it is now routinely used in the manufacture of high perfor- mance integrated circuits. For example, the circuits for pocket calculators

require the use of ion implantation to permit their operation at low power

levels and thus prolong battery life.

For most products it is necessary to repeat the preceding steps of oxidation,

photomasking, and diffusion or implantation a number of times to create the

desired structure. Simple transistors require two or more diffusion steps

plus a masking operation to expose the surface regions where electrical con- tact is required. After the metal connecting layer is deposited as described

below, a further photomasking and etching operation permits delineation of

the desired pattern of metal.

4.9 Passivation

Frequently, it is necessary to provide an inorganic coating on the semiconductor slice to protect it from exterior influences. Such layers are called

passivation layers, and they range from a 1-pm thick layer of silicon dioxide

to relatively thick deposits of special glasses. The objective is to render

12
the semiconductor chip inert to attack by external chemical species. In

former times, this protection was provided by a hermetic package. Today,

however, large numbers of devices are packaged in less expensive nonhermetic

assemblies. It is necessary to arrange for the protection of the chip in

spite of the nonhermeticity of the package. One can regard passivation as a technique for shrinking the hermetic package down directly onto the surface

of the semiconductor chip. Particularly in power devices, passivation is a necessary step to provide the low surface leakage necessary for high-voltage

junctions. Many power device processes leave the intersection of the p-n

junction and the surface of the device unprotected by any material. Considering the fact that the dielectric breakdown strength within a p-n junction

may be as high as 200,000 volts per centimeter, and that most gases, including air, break down at around 2000 volts per centimeter, the need for passivation of the edge of the p-n junction becomes obvious. Regardless of the

kind of device, a p-n junction exposed to the outside world will not have

stable reverse-biased electrical characteristics. In fact, many p-n junctions that are protected by a thin silicon dioxide layer are still not stable over long periods of time due to diffusion of mobile ions through the silicon

dioxide overcoat. Passivation of the device requires the isolation of the

junction from these influences by the addition of inert surface layers. For

low-power devices, silicon nitride and aluminum oxide have been used with

considerable success in protecting oxide-coated junctions from diffusion of

external species. Higher voltage devices generally employ thicker coatings

of glass applied in a variety of ways.

4.10 Metallization

Silicon is not a metal. It is not possible to make electrical connection to

silicon in the ways commonly used to make connections to metals. Yet external connections are required, and are usually provided by the evaporation in

vacuum of thin metal films onto selected areas of the surface of the semiconductor chip. In discrete devices, these provide metallic surfaces to which

further metal members may be attached. In integrated circuits, these evaporated layers also provide the interconnections between regions of the surface

that need to be connected together electrically. In many cases these interconnection runs never are connected to the outside world at all. Various

metals and multimetal systems are used for these purposes. Nearly every

metal in the periodic table has been used in a production semiconductor device for making electrical connection to the silicon. However, the most common are aluminum, gold, and multilayer systems involving platinum, titanium, chromium, nickel, silver, or copper.

4.11 Testing

Once the processing of the slice has reached a point at which metal contact

regions are available, the devices can be electrically tested to some extent.

It is not possible to carry out the full range of electrical tests, because

the device is not yet mounted in a package that permits removal of substantial amounts of heat. However, low-power characteristics can be measured on discrete devices and the functioning of most integrated circuits can be

checked. Testing at this stage is one of the most valuable tools in controlling cost, since defective parts can be weeded out and discarded without

incurring further cost in their processing. At this point, a slice of sili13
con will typically contain a very large number of apparently identical semiconductor devices in it. The testing apparatus has an array of precisely

positioned, needle-like probes, which are arranged to descend on the contact

pads that make electrical connections to each individual device in turn. In

most cases in the interests of speed and accuracy the testing regime is con- ducted by computer-controlled equipment. When a defective device is detected, the usual practice is to mark it with a dot of identifying ink. This

permits that particular device to be discarded after the slice has been separated into individual parts.

4.12 Dicing

The separation of individual dice* from the slice may be accomplished by

either scribing the surface of the wafer with a diamond- pointed tool, or by

engraving the surface in the same pattern by the use of a high-powered laser

beam, and subsequently cracking the slice along these marks. Both operations

are referred to as scribing, the diamond-point-scribing process being the

older. Laser scribing provides improved control of the breaking process that

follows. Alternately, the dice may be cut apart with a thin diamond-edged

saw blade. In many cases, this scribing operation is performed at the plant

location in which previous processing has been carried out, but the wafers

are not broken until they are delivered to the plant where the dice are to be

assembled into completely packaged devices, which may be halfway around the

world. In this way the operations requiring the highest degrees of skill and

equipment investment are confined to the processing plant, while operations

requiring lesser skills are at the assembly location.

4.13 Assembly

The regularity of the pattern of dice in each slice is valuable to the assem- bler of devices. If the slice is broken and the dice become displaced from

their initial positions, this regularity is lost. At assembly, each dice

must be identified, oriented and put in the proper position for further assembly. This costs money. If the position and orientation of each die can

be maintained, cost can be saved. There is a variety of techniques in use

for mounting a scribed wafer on a thin plastic membrane and then stretching

the membrane uniformly in two directions. The individual dice follow the

stretched membrane and retain their relative position and orientation. During subsequent assembly operations, the devices can be picked from the mem- brane and positioned with a minimum of additional effort and cost.

The assembly of transistors and integrated circuits into finished devices

generally involves mounting each individual die into a package. Often a metallic bond is created, using a layer of gold between the silicon chip and

the metal surface of the package. The package and chip are raised in temperature until the gold fuses with the silicon and forms a strong bond. A more recently developed technique uses electrically conducting epoxy cements.

Dice, chips, and pellets are the same. These terms refer to a segment of

the wafer which contains usually one device or integrated circuit and which

will be assembled into a package.

14
In devices that must dissipate a significant amount of power, other techniques are employed. Generally, the part to which the device is to be

attached is made of copper whose high thermal conductivity is important in

extracting heat from the semiconductor chip. Bonding of this type of semiconductor device is done using a variety of metallurgical methods, but rarely

an epoxy or other material with poor thermal conductivity.

In very large devices, the silicon may be bonded between two pieces of a metal such as tungsten. Tungsten has a thermal expansion coefficient similar

to that of silicon and has good thermal and electrical conductivity. Its

high elastic modulus provides a rigid backup for the silicon, protecting the

silicon from external mechanical forces. In hockey-puck shaped packages,

this tungsten-silicon-tungsten sandwich is assembled between copper end

plates, but not bonded to them. The flexible diaphragm- like end plates of

the package then permit the copper surfaces to be clamped by external pressure to the silicon-tungsten sandwich to provide good electrical and thermal

paths to the device. The absence of a solid metallic bond permits differential expansion to take place between the tungsten and the copper, which have

differing thermal expansion coefficients, without the risk of deforming a metallic joint and consequent fatigue failure after many heating and cooling

cycles.

In low-power devices, once the die is mounted in the package, it is necessary

to connect the individual metal contact patterns to the external connections

of the package. The usual approach employs fine gold wire and the thermocompression bond technique. In this procedure, the end of the gold wire is

melted into a spherical droplet, which is then solidified before being

pressed against a metallic area on the silicon chip. A combination of temperature not as high as the melting point and high physical pressure serves

to weld the wire ball to the metallization on the surface of the silicon.

The wire is then led from this point to the external package connection where

it is welded in a similar fashion. These wires range in diameter from 50 to

150 pm.

Once the electrical connections are complete, it is necessary to enclose the

device so as to protect it mechanically from the shocks of the external environment. Silicon is brittle and will not tolerate physical abuse. Therefore, the package in which it is contained must protect it. In many cases

the packages are molded of plastic materials, while in other cases the package is a metal and ceramic assembly. Semiconductor dice are also sold without any packaging at all. These are used by sophisticated customers who

mount and connect these devices to other circuits of their own manufacture.

With reasonable care it is possible to test and ship unpackaged dice to an- other location in large quantities without damage.

4.14 Final Testing

Final electrical testing of semiconductor devices is done mostly by computercontrolled equipment.. The number and complexity of tests are large, and the

precision with which they must be conducted is high. Furthermore, given the

large amount of data obtained during a test sequence on a single integrated

circuit and the complex logic that aoverns decisions regarding the fate of

15
the device under test, the manufacturer quickly decides to use a computer for

control of the test sequence.

In most cases it is not possible for a semiconductor process line to deliver

a part that is characterized by a single type number. The number of serial

steps in the process and the inevitable variability of the product mean that

the output distribution from a product line will be wide enough that it must

be subclassified into different parts. This sorting may take place on the

basis of electrical characteristics, response of the device to temperature

extremes, or other factors. Devices with a high degree of uniformity and

that are operable over wide temperature ranges command premium prices. Other

devices, perhaps made on the identical line at the same time, which have less

desirable characteristics but which still function adequately, are sold for

lesser prices. Additional sampling tests are done to explore the performance

of the device in high stress environments or over long periods of time, to assure stability and long life. The testing costs incurred in qualifying

devices for very high reliability service, such as in cardiac pacemakers or

in satellites, may exceed the cost of manufacturing the device many times

over.

4.15 Process Control

The subject of process control deserves separate discussion. The reader can appreciate from the foregoing description that semiconductor device manufacture is a long and delicate chain of events with more opportunity for failure

than for success. Disaster in the form of catastrophically low yield is

always lurking nearby, and the process engineer is the person who must con- trol the many variables and keep the factory running.

Each of the processes described above produces a change in the silicon wafer,

be it a pattern in photoresist, a diffused layer, or a deposit of contact

metal. The majority of such changes are irreversible. If the process has

been wrongly done, expensive scrap has been created.

The key to control is measurement. Too often the variables one needs to measure are in such tiny areas that they are not accessible to measurement,

or the measurement is destructive. On the other hand, if the measurement is

not done and an error in processing has been made, the money spent in further

work on the material is wasted.

One way to deal with this problem is to provide on each silicon wafer a small area devoted to special test patterns for process control purposes. These

patterns range from simple rectangular regions for measurement of sheet re- sistance of diffused layers, or dot patterns for checking alignment of photoresist processes, to transistor structures typical of those buried deep within an integrated circuit. Measurements routinely made on these structures

can tell the process engineer much about the day-to-day reproducibility of

processes and can help him pinpoint the troublesome process when disaster

strikes. This information more than adequately compensates for the small area of each wafer used to carry the test structures and thus not available

for salable devices.

16
Semiconductor manufacturing processes are by no means thoroughly understood,

largely because of the lack of adequate measurement methods. Most process

steps involve high purity materials, thin layers, small dimensions, low con- centrations, or other extremes of conditions which make measurements difficult at best. What is needed are simple, reproducible, nondestructive mea- surement techniques that can be done routinely in a factory environment by

technically unsophisticated people. There is a large gap between the need

and what is possible at present. The cost of this unsatisfactory measurement

and control situation is yield loss, which in total has been estimated at

about $2 billion annually for the U.S. semiconductor industry. Not all of

this loss can be avoided solely by improved measurement—people will always

make mistakes—but a significant amount certainly could be.

5 . PRODUCT DEVELOPMENT

Successful development of a new semiconductor product requires a confluence

of several factors. First, there must be a need established for the product.

If there is no need, there is no market, and the development is an exercise.

While this seems obvious, it has not always been recognized. The need implies a set of performance specifications for the new product if it is to be

successfully applied.

Given a set of characteristics, a device structure can be designed to have at

least a first approximation of these characteristics. While designs for

discrete devices can frequently be done without recourse to computer systems,

the complexity of today's integrated circuits and the very large number of

individual devices that go together to make up a single integrated circuit

require the use of computer assistance. Modern integrated circuits may con- tain more than 100,000 individual transistors in a single device; the number

of devices overtaxes human abilities to position and interconnect them cor- rectly. Since most of these devices are exact repetitions of other devices

within the same integrated circuit, differing only in location and perhaps

orientation, computer aid is natural. In addition, since the final set of

masks to produce the product requires registration of many levels of extreme

complexity, the ability of the computer to remember what exists in the other

levels aids immeasurably in creating the proper form for the one being worked

on at the moment. Thus registration errors between masks can be reduced.

In addition, the particular characteristics of the processes in use by each

manufacturer are different. These properties lead to requirements, called

design rules, which apply in the preparation of the photomasks, and which are different for different manufacturers—even for different process sequences

within the same manufacturer's house. In general, processes in use by different firms, even to produce devices with essentially identical properties,

are sufficiently different that the masks used by manufacturer A will not

work in manufacturer B's environment.

When the initial design is complete, and the masks are available, processing

of silicon can begin. Initial attempts to build the product are usually done

in a development laboratory. The products of this effort are tested and the

results compared with the initial specifications for the product. Any design

corrections are made, and the fabrication process is repeated. Once the

design to produce the new product is established, and the process is consid17
ered well enough controlled to give satisfactory yields, manufacture of the

product in the factory can begin. The transfer from laboratory to factory is

fraught with difficulties, because what was previously done by highly skilled

people must now be done by relatively less skilled factory workers who do not

in general understand the details of what they are doing. If the new product

uses the same sequence of processes as other products already being manufactured, less difficulty is usually encountered. If, however, an entirely new process sequence is required, the road to successful manufacture of a new product is studded with obstacles.

Initial yields of complex integrated circuits are frequently below five percent. This is obviously not satisfactory. Through continuous refinement

these yields improve, leading to a final product cost that decreases sharply

with time. The semiconductor industry's record of product cost reduction is

probably unmatched.

Semiconductor manufacturers have always provided substantial technical assistance to their customers in the use of their products. In the beginning,

when semiconductors were replacing vacuum tubes in old applications, the

sharply different characteristics of semiconductors created considerable

difficulty in application. As a part of marketing operations, engineering

assistance was extended to customers to help them to apply semiconductors

correctly. With the proliferation of types of semiconductor products, this

application assistance has continued to be required.

6 . PRODUCT TYPES

In the following paragraphs each of the major kinds of semiconductor devices

will be described briefly.

6.1 Diodes

Signal diodes are elementary one- junction small rectifiers, by definition

limited to forward average currents of 100 millamperes or less. Their re- verse voltage rating is generally 200 volts or less. Typically, these devices are packaged in small glass envelopes with axial leads. They are ma- ture products; that is, their design and manufacturing processes are stable

and their consumption is steady or decreasing with time. Many of the circuit

functions formerly done by signal diodes are now performed by integrated

circuits.

An important class of signal diodes is made using a slightly different junction structure than has been described up until now. If a layer of a properly selected metal is evaporated onto a silicon slice, it is found that the

metal-silicon interface possesses rectifying characteristics. This is termed

a Schottky barrier. Devices utilizing this effect are called Schottky diodes; they have a very fast recovery time from the forward-conducting to the

reverse-blocking mode, a feature that is important in some applications.

This type of signal diode will probably increase its utility with time over

the next few years, because of the value of the fast recovery characteristic

in microwave applications. Schottky barrier devices are made in larger sizes

as well, and a small part of the rectifier market described below is supplied

this way. Because of the limited reverse-blocking voltage of Schottky de18
vices, however, the impact of this technique in the rectifier business is

substantially less than it is for signal diodes.

Rectifiers are defined as simple p-n junction devices having two leads and

carrying currents greater than 100 millamperes. They can be very large indeed; rectifiers rated in the thousands of amperes are made and are packaged

in enclosures 8 cm (3 inches) or more in diameter. Devices for these applications may also have very high reverse blocking voltages, ranging up to

3,500 volts.

Still another two-terminal single- junction device is the Zener, or voltage

reference, diode. These devices are designed and made to be used in the

reverse-biased direction, with the voltage breakdown characteristic of the

blocking junction carefully controlled. By passing a small current through

the device in the reverse direction, beyond the normal reverse voltage rating

of an ordinary device, a very stable reference voltage drop occurs. These

devices are used widely to establish fixed values of voltage in power supplies and other specialized applications.

Another specialized rectifier is the variable capacitance or varactor diode.

When a p-n junction is biased in the reverse direction, the free charge car- riers are swept away from a region in the neighborhood of the p-n junction.

This produces a layer free of any mobile charge, and in every respect this

layer functions as a dielectric. A dielectric bounded on either side by a conducting medium forms a capacitor. Capacitors made in this fashion, in

silicon, have the valuable feature that their capacitance can be controlled

electrically. By changing the magnitude of the applied reverse voltage, the

width of the blocking layer is changed and the capacitance also changes. By

controlling the distribution of the dopant atoms on either side of the p-n

junction, a variety of voltage-capacitance characteristics can be designed.

These devices have increasing applications in electronically tuned radio and

television receiving equipment.

Yet another specialized class of p-n junction diode is used for detection of

light. If light is allowed to be absorbed in silicon, pairs of holes and

electrons are created. If this occurs in the neighborhood of a reverse biased p-n junction, these holes and electrons form a current much larger than

the small current that would pass through that junction in the dark. In

connection with yet another specialized p-n junction device, the lightemitting diode, devices can be made for sensing holes in computer cards and

for communicating signals between circuits at different potentials, for

example.

Light-emitting diodes are not made of silicon, but of gallium-arsenic com- pounds. These compounds have the property that when holes and electrons

recombine in a forward-biased junction, the energy that they represent appears in the form of light with a modest but useful efficiency. The wavelength of this light from pure gallium arsenide is in the near infrared and

therefore invisible to the human eye. If part of the arsenic is replaced by

phosphorus, however, diodes made of gallium arsenide-phosphide emit light

which is bright red, the familiar color of the digits on many pocket calculators and electronic watches. This same red light is readily absorbed by

silicon. If, therefore, a light-emitting diode and a photodiode are arrayed

19
in proximity, current passing through the light-emitting diode in the forward

direction will be detected by the photodiode as a reverse current increase.

Such devices are called opto-electronic couplers. Not all opto-electronic

couplers use photodiodes. Many instead use a photosensitive transistor that

provides greater sensitivity to the light signals.

Other specialized diodes are used as oscillators at ultrahigh frequencies.

This class of very high frequency signal sources includes more than one type

of p-n junction diode; tunnel diodes, IMPATT diodes, and Gunn effect diodes

are all in this category. They all work in different ways, but the end re- sult is similar. All generate ac signals of extremely high frequency that

are useful in communications equipment in the microwave regions.

6.2 Transistors

All the devices heretofore discussed have two external leads. We will now

consider three-leaded devices—transistors, of which there are many specialized types. Signal transistors, in which the power dissipation is 1 watt or less, are used in great variety but in decreasing numbers. The signal transistor is being displaced slowly by integrated circuits.

Larger transistors, capable of dissipating more than 1 watt of power, are classified as power transistors. „They range to over 100 amperes in currentcarrying capability, or up to nearly 2000 volts in voltage breakdown rating.

All transistors share the common characteristic that the output current is a predictable and generally proportional function of the input current.

Included in signal and power transistor categories are two specialized devices used at frequencies above 300 megahertz. They are specialized in the sense that they are made with extremely small elements. For example, a mi- crowave signal transistor to operate at 3 gigahertz has lines in its metallization pattern of 1 micrometer wide, spaced 1 micrometer apart. This very

fine geometry puts extreme requirements on the photoresist processing techniques used to make these devices. Similarly, microwave power transistors

also use a very large number of tiny elements in their construction to permit

them to operate efficiently at very high frequencies. These devices comprise

a rapidly developing segment of the semiconductor business; they are being

used in high-frequency radio transmission and in cable television systems.

6.3 Thyristors

A device that has extremely important applications in industrial and power

control equipment is the thyristor. It is sometimes referred to as the

silicon-controlled rectifier, or SCR, and is used as a switching device. Its

useful properties stem from the fact that when it is in its nonconducting

state only a very small current exists, and when it is in the conducting

state large currents can be passed with a small voltage drop. Thus a relatively small device can switch quite large amounts of power. Switching is

initiated by means of current into a control terminal, called the gate. The

thyristor is normally in the off-condition, and when a certain amount of

current is conducted into the gate, it switches into the on-condition. The

gate signal needed is only a small impulse, because once switching has oc- curred the gate loses control. The thyristor then remains in the on-state

20
until the current through it is reduced to nearly zero by the external circuit. At this time the blocking state is recovered, and the device remains

in the blocking state until another gai:e signal is received. Thyristors are available with current ratings as high as 2500 amperes and with voltage ratings as high as 5000 volts.

6.4 Integrated Circuits

An integrated circuit is nothing more than a collection of individual transistors, diodes, and resistors, all made extremely small, arrayed in a single

piece of silicon and interconnected by a pattern of metallization on the

surface of the die. The arrangement is made to perform a specific circuit

function that may be quite simple, as in the case of a gate circuit for a computer, or that may be extremely complex and embody a large fraction of the

circuitry for an entire television receiver. From the smallest to the largest integrated circuit, however, the building blocks are still the same.

The earliest types of integrated circuits were used in computers. Integrated

circuits are particularly well suited to computer applications because com- puters are made up of large numbers of a few kinds of circuits; there may be

tens of thousands of gate circuits in a computer. The earliest application

of integrated circuits was simply to replace a gate circuit made of discrete

parts with a gate circuit made in a single piece of silicon. Immediate econ- omies of scale existed in this application, and integrated circuits have

invaded the computer domain to the greatest degree.

Since computer circuits are generally arranged to operate in either an on- condition or an off-condition (binary circuitry), ICs used in this manner

are referred to as digital circuits. Their behavior is defined only in the on or off state. Digital ICs are divided into families having certain characteristics in common. First, the input and output voltage swings of all

devices of a family fall within the same ranges. Second, the power supply

requirements are similar. Third, the relative speeds of all devices in the

family are similar. These characteristics are achieved by using similar

circuitry for all members of the family. Families are named according to the

type of basic circuit element that is used. There are, among the obsolete

families of digital circuits, such types as diode-transistor logic ( DTL )

,

resistor-transistor logic (RTL), and so on. These families have been re- placed by families that are either faster, use less power, or are less expensive. Now the important bipolar digital families of ICs are transistortransistor logic ( TTL ) , emitter-coupled logic (ECL), and integrated-injection

logic (I 2 L). Within families there are varieties that differ in speed or power consumption or both. But, for example, all TTL circuits can work together, sharing common power supply voltages and common input-output voltage

levels. Interconnection between families, however, is often more difficult,

although it is done when necessary. The present bipolar digital families of

interest mentioned above are expected to remain the significant members of

this class of device for several years to come.

Digital circuits are further subdivided into bipolar integrated circuits,

using transistors of the types dismissed previously, or MOS (metal-oxidesilicon) circuits. MOS transistors are a special class of transistor in

which an electric field outside the silicon controls the current parallel to

21
and beneath the surface of a semiconductor chip. The electric field is cre- ated between a metal layer and the semiconductor, with a thin silicon dioxide

dielectric interposed between (the metal-oxide-silicon structure). MOS digital circuits are distinguished by the fact that their power requirements are significantly lower than bipolar digital circuits, although in general their

speed is also less. In many applications this is an acceptable compromise

and has led to development of a variety of MOS device families. There are pchannel MOS (pMOS), n-channel MOS (rzMOS), complementary MOS (CMOS), siliconon-sapphire (SOS), and charge-coupled devices (CCD).

The MOS digital circuits were developed in the order given above, pMOS being

oldest and CCD being newest. With the exception of CCDs, the speed also has

increased with time through the various families from pMOS to SOS. The speed

increase has not always been at the price of increased power consumption,

however; CMOS has extremely low power consumption. As an additional feature,

CCD devices are also photosensitive and can be used to make useful imageforming devices for use in TV cameras.

The low power consumption of MOS ICs has made possible the pocket calculator

and the electronic watch, both of which must be sparing of power to give

satisfactory battery life. More subtly, low power is also important in many

other applications. Since the use of ICs permits great circuit complexity to

be put in a very small volume, even the modest power dissipation of ICs be- comes significant when many such devices are put together into one equipment

enclosure.

In contrast with digital ICs in which the transistors and circuits are designed to operate in either one or the other of two states with rapid transitions between, linear integrated circuits are designed to have a predictable

functional relationship between input and output. For instance, in an IC

amplifier, the output is a magnified version of the input. The total gain

through such a circuit may be 1 million or more. Design and manufacture are very much more difficult for linear integrated circuits than for digital

circuits, a fact which has somewhat delayed their growth. Furthermore, there

is a much larger variety of linear circuits than there is of digital circuits, and each variety of circuit requires a separate design. The cost of

designing a complex integrated circuit may be as much as $500,000 so the task

is not to be undertaken lightly. A substantial number of applications must

exist beforehand. It should be noted that this cost covers only the design;

if a new process sequence, not already in use in the factory, must be developed, the cost can easily be five times this amount. If the circuit is a microprocessor or microcomputer, the cost of developing the associated soft- ware will also run into the millions of dollars.

Six to eight linear integrated circuits can perform all of the functions

required inside a television set with few exceptions, and such circuits now exist. Linear integrated circuits are also available to make AM radios, FM

radios, low-power stereo amplifiers, etc. These applications share the com- mon factors of a large market and relatively standardized circuitry.

In addition, one of the oldest types of linear integrated circuits, the operational amplifier (op amp), was developed for use in analog computers. Uses

of op amps in other applications have now far overshadowed the applications

22
in analog computers, a tribute to the ingenuity of users who, once a microelectronic device is available to do a particular job, are amazingly clever

in finding other uses for the function. Many of these uses could not have

been met at reasonable cost before.

6.5 Application Trends and Their Impacts

Cost and complexity trends in the industry are interesting to consider. In

ICs, the average cost per component in 1961 was $1. By 1972, this figure was

$0.01, and in 1975 it was $0,001. The trend for discrete devices is by no means as steep, but it is strongly downward nonetheless. This rapid decrease

in cost with the passage of time has made today's minicomputers and pocket

calculators economically possible.

On the other hand, complexity has grown. Where in 1959 one device per chip

was the norm, by 1973 over 10,000 components per chip were possible. This

increase rate corresponds closely to doubling each year. It continued at

this rate for 16 years (a 65,000-fold increase by August 1975) but has slowed

somewhat since that time. In 1980, development chips having about a half

million components have been described. This growth has been made possible

by reduction in size per, component and increased chip size, together accounting for a factor of about 2500, and by ingenuity of device and circuit design

that has contributed the remaining factor of 200.

The net effect of these trends has been to reduce substantially the cost of

performing electronic functions. This change appears in products in a variety of ways:

1. Transistor radios, pocket calculators, and electronic watches

have each, on their own time scale, been available at rapidly

falling prices limited ultimately by the material and labor

costs of the whole product. The electronics part of this total

cost is relatively modest.

2. Television sets, major appliances, and automobiles tend to

incorporate more and more sophistication at a relatively con- stant price. Communication and industrial control equipment

also shares this trend. The electronics content of these tends

to increase with time, to provide the added sophistication.

3. The size of the largest practical electronics system keeps

increasing. This is not only for the reason that decreasing

costs per component permit more components in an affordable

system, but also because the failure rate per component in an

IC has dropped at a rate almost identical to the rate of in- crease in the number of components per IC. The failure rate

per IC package has not significantly changed. Practical system

size tends to be limited more by the tolerable failure rate of

the system.

In power semiconductors, the trends are different in kind but similarly

broadening in their impact on our daily life. Applications of power devices

are most apparent in variable lighting controls, speed controls on power

23
tools and appliances, and in other major household appliance control functions. Less visible to the consumer but of more economic importance are industrial controls that are at the heart of many machine tools and that

operate many industrial processes. The economic effect stems from the improved productivity per man-hour that is provided by these sophisticated

kinds of plants and machinery.

The growing concern for our energy sources has caused the development of many

electric power-generating schemes, such as solar cells and windmills, whose

output is either direct current or perhaps alternating current of varying

frequency. The output from these sources requires modification to be compatible with the existing constant-frequency ac power distribution system, and

solid-state power-converting equipment will be necessary.

A final example is the use of power semiconductor devices as the "musclepower" to operate mechanical systems under the primary control of electronic

logic circuits. Such systems are already in use in antiskid controls on large vehicles and are being considered for a variety of other vehicular

subsystems. The need for power solid-state devices to do the work, under

control of logic circuits (computers) that make decisions, is not unique to

the automotive field, of course, but the example illustrates the concept.

7. SUMMARY

We have explored briefly the wide range of technologies used in the semiconductor industry to transform simple, pure crystals of a few kinds into a constantly growing array of solid-state devices and circuits. These have

also been described. The manufacturing process is materials- and processoriented, requiring a wide variety of technical professionals for its conception, development, and implementation. It is an industry of extremes: highest purity, smallest linewidths, most complex products, simple concepts, and

bewildering processes. In less than 35 years since the transistor was invented, a nearly $15 billion annual flood of semiconductors worldwide has

radically changed the electronics industry and our daily lives. Semiconductor devices are at the heart of nearly every electronic product and are the

indispensible components in almost $80 billion worth of goods in 1980 in the

United States alone.

Semiconductor manufacturing operations are limited at any particular time by

materials, processing, and measurement problems. As today's problems are solved and new progress is made, new problems arise and impose new limits.

The past performance of the industry in controlling processes, improving

yields, and raising reliability levels has resulted in dramatic reductions in

cost. In turn, lowered cost has allowed penetration of electronic products

into new areas. Fundamental physical limits will ultimately determine how

complex an integrated circuit can become, for example, but there is still a long way to go.

24
IBS-114A (rev. 2-ec) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NBSIR 81-2197 January 1981

4. TITLE AND SUBTITLE

Semiconductor Technology for the Non- Technologist

5. AUTHOR(S)

Robert I. Scace

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No. N/A

8 . Type of Report & Period Covered

N/A

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City . State. ZIP)

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

The properties of semiconductor materials, the methods of processing them, and the

solid-state products made from them are described in terms intended to be

understandable by the lay person. The semiconductor industry has grown at a rate of

21 percent per year compounded for the last twenty years, and its products have

declined in unit cost by a factor of five in current dollars (a factor of ten in

constant dollars) in the same period. This very satisfactory but anomalous behavior

has attracted the interest of many who are not familiar with the technology of the

industry, yet who need to have some understanding of it. This report is intended to

help meet that need.

12.

KEY WORDS (Six to twelve entries ; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Integrated circuits; semiconductors; semiconductor devices; semiconductor processes;

semiconductor technology; silicon.

13. AVAILABILITY

fyl Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

ri Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

(^] Order From National Technical Information Service (NTIS), Springfield. VA. 22161

14. NO. OF

PRINTED PAGES

28

15. Price

$6.50

U SC OMM-DC 8043-P80
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