21st Century Manufacturing Episode 1 Part 10 doc

Pure semiconductor material ties called "dopants." When fabricating an integrated circuit, the transistors, resistors, and capacitors, as well as their interconnections, are fabricated t

some companies-such as Intel, Compaq, Dell, and Gateway-have understood and exploited this new landscape,whereas other companies-such as Apple-have had more mixed success in recent years

The key to building faster, cheaper, smaller, and more powerful computers is to

ance characteristics: more components in a small area increase the circuit's energy efficiency and processing speed The usual way to measure miniaturization isbythe length,L G,of the polysilicon gate bridging the source and drain region of a tran-sistor.TIlls dimension is shown in later figures

A key component of an integrated circuit (IC) is the transistor.Transistors are the largest member of a family of solid-state devices called "semiconductors." They are built from a special class of materials with electrical properties somewhere between those of conductors and those in insulators Pure semiconductor material

ties called "dopants."

When fabricating an integrated circuit, the transistors, resistors, and capacitors,

as well as their interconnections, are fabricated together-integrated-in a contin-uous substrate of semiconductor material Active circuit elements are formed by

semiconductor substrate material because it has overall cost, performance, and pro-cessing advantages

With each new IC generation, device geometries have become smaller and 10; have become more powerful In 1965, Gordon E Moore, then with Fairchild Corpo-ration but later an Intel cofounder, observed an important trend that was later

ele-transistors that could be integrated on a single die would grow exponentially with time, roughly doubling every 18 to 24 months Moore correctly anticipated today's ICs, which can hold several millions of transistors on a chip, providing far more

func-functionality A log plot of "dollars per function" over time measured in years shows

a linear decrease In simplest form this means that any chip with a given functionality will be about half its original cost in 18 to 24 months

Producing miniaturized devices requires precise and sophisticated design and microfabrication Computer aided design tools have significantly improved the pre-cision and level of complexity achievable in circuit layout planning Automated process technologies, advanced clean room systems, and testing equipment have helped bring chip fabrication to submicron levels The explosion in Ie applications

is also producing a boom in advanced manufacturing equipment It includes advanced lithography equipment, specialized ion-beam machines,

10'

10'

10

Year

F1pre 5.1 Trends in integrated circuit density (from Digitallntegrated Circuits by

Rabaey, © 1996 Reprinted by permission of Prentice-Hall, Inc., Upper Saddle River,NJ)

mechanical polishing equipment to achieve ultraflat surfaces, lasers, and high-vacuum systems

The semiconductor industry is currently focused on producing larger wafers and smaller process geometries Larger wafers reduce raw material costs and increase chip processing outputs Current state-of-the-art semiconductor manufac-turing systems produce 200 millimeter (8 inch) wafers with 0.25 to 0.35 micron line

beginning to use 0.13 to 0.18 micron processes This will further accelerate the trend shown in Figure 5.2

During the time this book goes to press and gets published, some of the first

300 millimeter (12 inch) wafers will be in production By the year 2010, the

Semi-widths on 450 millimeter (18 inch) wafers Quite simply, these larger wafers mean more chips per batch, which means lower processing costs per chip, Actually, this is not entirely new news Henry Ford applied analogous principles to automobile

man-born, Michigan, meant more cars per hour and lower processing costs per batch of

the simple economics are about spreading the fixed costs of the factory, the people, and the manufacturing equipment over a greater number of individual products (see Figure 5.3)

Enhancement MOSFET/

Bipolar transistor

~~~:~[ MESFET

Assuming a 10 mm x to mm chip

and the maximum theoretical yield

FigureS.3 Trends in silicon wafer diameter

5.5 TRANSISTORS

5.5.1 Historical Background

The earliest electronic computers used bulky vacuum tubes resembling short neon

computations and logical functions In the 19408,it took thousands of vacuum tubes

to create the famous computers that occupied several rooms Not surprisingly, this was a rather costly and tedious way to go about building a calculating machine

In 1947, vacuum tube computing was rendered obsolete, ahnost overnight,by thetransistor. Three Bell Labs scientists-William Shockley, Walter Brattain, and John Bardeen-are credited with a series of inventions that introduced, refined, and then commercially launched the transistor.' Their invention was smaller, faster, and cheaper; handled more complex operations; and generated less heat than its

embedded in a solid piece of semiconductor material Transistors were thus called

"solid-state" devices because electric current nows through a solid semiconductor rather than through a vacuum tube

IIbe importance of the vision of M Kelly, Bell Labs' research director at the time, is also usually stressed He understood that vacuum tubes were holding back the electronics industry and fostered an

phones were made from vacuum tubes, the device would be as big as the Washington Monument in the

Transistor technology started the microelectronics revolution by making high-performance inexpensive electronics possible Transistors showed up in a burgeoning array of electronic products-from rockets to portable radios-throughout the 1950s Also, with fewer power, heat, and size constraints, computer designers could build faster, more reliable computers that occupied much less space But properly connecting hundreds of transistors with thousands of other electric circuit compo-nents was an enormous design, manufacturing, and performance problem The problems of interconnecting the discrete devices in computers were over-come with the invention of the integrated circuit in 1958 by Jack Kilby at Texas Instruments This enabled the fabrication of circuit components and their intercon-nections on a single chip

Integrated circuits are classified into analog and digital Analog integrated cir-cuits include a large family of circir-cuits used in power electronics, instrumentation, telecommunications, and optics Digital integrated circuits are usually classified into two types, memory and logic chips:

• Memory chips consist of memory cells and associated circuits for address

selec-tion and amplificaselec-tion Process technologies are extremely well developed for

16 and 64 megabyte dynamic random access memories (DRAMs) DRAMs are inexpensive commodity products differentiated by speed, power con-sumption, configuration, and package type From an integrated design and fab-rication viewpoint, specialty DRAMs and video RAMs are the more emerging technologies of interest

• Logic chips contain the circuits needed to petform arithmetic, logic, and

con-trot functions central to the microprocessor Application specific integrated circuits (ASles) are tailored to a customer's particular requirement, as opposed to one of the standard "Intel-inside" microprocessors Rapid advances in Ie design and process technologies meant that chips could

be made at commercially viable scales by the early 19608.Improvements in minia-turization technology permitted ever-increasing numbers of components to fit on smaller and smaller chips (Table 5.1)

By1971, a single integrated circuit(Ie)was built that combined logic functions, arithmetic functions, memory registers, and the ability to send and receive data This

device was called the microprocessor It was used in many applications and spurred

TABLE5.1 Trends in Ie Integration Levels

Integration scale Abbreviation Devices per chip

Small scele mtegration

Medium scale integration

Large scale integration

Very large scale integration

Ultra large scale integration

Future capabilities

55I MSI

LSI VLSI

21050 50105,000 5,00010100,000 100,000 to 1,000,000

>1,000,000

>1,000,000,000

the factory-floor robotics revolution of the late 19708 (see Figure 1.2) For the robotics industry, the microcorurolter was a cheap and reasonably powerful special-ized control system built around the microprocessor Of course, the microprocessor

also made possible the development of the microcomputer-or the personal

com-puter (PC)

5.5.2 Semiconductors:" Type and ~ Type

A semiconductor is a crystalline material (usually silicon) with electrical properties

and glass Silicon crystallizes in a diamond-shaped lattice, with each atom surrounded

by four other atoms in a tetrahedron The atoms share valence electrons, which give each atom a complete valence shell In its pure state, a semiconductor material exhibits

impurities(dopanrs) to the crystal structure of the semiconductor lowers its resistivity and allows current to flow through the material The atomic structure of the dopant

determines whether the resulting material will be "a-type" or "p-type."

• n-type silicon is typically created by doping silicon with phosphorus which has five electrons in its outer shell In comparison with the four-electron silicon,

response to a voltage Since most of the conduction is carried by negatively charged electrons, the material is called n-type

•p-type silicon is typically created by doping silicon with boron Boron has only

three electrons in its outer shell Since all the silicon atoms were nicely bal-anced with four electrons in their outer shell, the presence of the boron intruder creates additional vacancies, or "holes," in the material These holes

fill this hole and, in doing so, leave behind another hole The holes thus seem

tion occurs by way of the positively charged holes, the material is called p-type.

Modifying the concentration of dopants controls the resulting change in semi-conductor conductivity The process of doping semisemi-conductor materials to selec-tively increase their conductivity is fundamental to the manufacture of advanced semiconductor devices because it makes possible the fabrication of basic circuit substructures

Silicon is the material of choice for microelectronics devices because of its numerous advantages As one of the most abundant elements on the planet, silicon

manium, the next most popular semiconductor resource Silicon also has critical Pre-cessing advantages It easily oxidizes to form silicon dioxide, an excellent insulator among circuit components Silicon dioxide is also extremely useful during the fabri-cation process because it is an effective barrier layer during multiple doping

opera-tiona Gallium arsenide rather than silicon is increasingly used in optoelectronic and high-frequency communication devices

5.5.3The Transistor

The region where p-type and a-type semiconductors meet forms a crucial structure known as apnjunction (Figure 5.4) Apnjunction is basic to the operation of most electronic devices For example, a diode is apnjunction that allows the flow of cur-rent in one direction and blocks it in the opposite direction A bipolar junction tran-sistor(BIT) is made by sandwiching three different semiconductor slices into one solid block, such that the center slice is of one type and the two outer slices are of the opposite type In effect, this creates twopnjunctions Depending on how the

junc-tions are combined, the transistor is either"npn" or "pnp" (see Figure 5.5) In an npn

transistor, electrons can flow from the emitter(n),across the base(P),to the collector

(rl).More significantly, applying a voltage to the base vigorously rips electrons from the emitter and sends them rocketing across the base into thecollector-s-in effect, amplifying the input current to the base The stronger the voltage on the base, the stronger the resulting flow of current through the transistor This amplification is more utilized in analog devices such as an electric guitar For the ICs in computers, the primary function is the ultrafast switching ability for logic

Figure 5.5 shows a simple sandwichlike npn arrangement By contrast, Figure

5.6 shows the horizontal layout of the field effect transistor (FET) The

termi-nology of the npn transistor-emitter, base, and collector-is now changed to

source, gate,anddrain for the FET To activate the transistor, voltage is applied to the polysilicon control gate (center of Figure 5.6) Electrons flow out of the source

region (marked n+)through the channel (part of the p-type substrate) and into the

drain (also markedn+).The amount of flow is precisely controlled by the voltage applied to the gate For the n-type device (NMOS) a positive voltage is applied to the polysilicon gate' The gate and the p-type substrate form the plates of a capac-itor with the gate oxide (Si02) as the dielectric of the capacitor The reader is referred to a text such as Rabaey's (1996) for the relationship between the applied gate voltage and the current flow between the source and the drain

Metallurgical junction

••••• 0.•••

•••bP•

•••••

•••••

• •• -Op.

a-type

JlIpre SA Schematic structure of apnsemiconductor junction in a silicon

This transistor is off

This Iransistor is on

vouage apphed ro base

Elcrtron flow in

flpre S.6 Basic structure of an a-type NMOS Ie (from Dtgital lmegmted

Circuils by Rablley, <0 1996 Reprinted by permissionof Prentice-Hell, Inc., Upper Saddle Rlver.Nj)

5.5.4MOSFETs as the B ••ic Building Block of the Integrated

Circuit

Metal-oxide semiconductors (MOSFETs) are one type offield effect transistor They are the essential building blocks of integrated circuits MOSFETs can be made either

p- or n-type The a-type devices (NMOS) are faster than p-type devices (PMOS) In

practice, the most common type is complementary MOS-type (CMOS) circuits In

this case, a single circuit simultaneously controls pairs of n-type and p-type

transis

-Small or no etcctrou nowIf1

riddoxide (SiO:) Dra!n

Gate Gilteo~id"

p'ficldimpl:Jllt

power-consumption efficencies (see review on P: 4 of Rabaey, 1996) The precise high-speed switching of MOSFET devices allows transistors to carry out the rapid binary data processing that lies at the foundation of modem computing 5.5.5The NMOS Transistor

Key terminology includes:

• Substrates, which are p-type fur NMOS

•Active transistor areas, which are n+ in NMOS

• Polysilicon layers for the gate electrode

• Select regions, or field implant regions, which are p+in NMOS

• Field oxide regions of silicon dioxide (SiOz)

• Interconnect layers, usually of aluminum

• Contact layers for interconnections between different layers

• Wells, which are a-type within a p-type substrate for CMOS transistors

The basic structure of anIedepends on the specific transistor technology used

In MOS-based chips, source and drain regions are formed by selectively "doping"

por-device is made up of n+ source/drain these n+ areas arise from the selective doping of

desired regions in ap-type substrate The conductive gate is made with a thin film of polycrystalline silicon (usually referred to as polysilicon) Comparatively thick layers of

p+ in NMOS) insulate neighboring n+areas Aluminum layers provide the intercon-nections among circuits Copper will increasingly be used for this purpose 5.5.6The CMOS Circuit

The complementary MOS process is preferred over basic NMOS because it leads to the creation of more circuits on a chip This is shown in Figure 5.7.The process starts with a p-substrate, which will eventually be doped in certain areas for n+type transistors (on the left) A mask is used early on in the process to define many

addi-tional n-weIJs-shown on the right-which will then contein p" transistors

e-well

5.8 DESIGN

The design of integrated circuits-say, for the embedded systems in cell phones, PDAs, and cameras-is outside the scopeof this book However, typical design levels for such devices are shown in Figures 5.8 and 5.9 In Figure 5.8,8 hierarchy is shown that breaks down a simple IC's description into several levels of abstraction These include:

• The defined global function of the device

• Subfunctions, which must coordinate withthe global goal Therefore iterative high-level simulations are needed These iterations are indicated by the feed-back loop at the top of the diagram

• The assembly of these subfunctions into cells or functional blocks

• The creation of specific transistor and circuit layouts that deliver the performance

of the desired functional blocks while stillbeing manufacturable in a standard "feb," Figure 5.9 is similar but for a more complex device such as a wireless net-worked computer or a wireless PDA Such a device needs three main divisions (shown in three columns) of the design abstraction for (a) analog data processing, (b) digital data processing, and (c) protocols and control (seehttp://bwrc.eecs

berkeley.edu) Some common development tools from Figure 5.9 are listed in Table 5.2 For one of these complex devices, with more than a million transistors, today'sIedesigners target the gate level netlist description in the fifth row of the

Figure 5.8 Design flow,typical of the early 199Os,for a simple device Define function

Partition design

High-level simulation

Assemblellayout

functional blocks

Layout rules

Low-level simulation

of new blocks

Device performance files

Check for layout

rule violation

GeneratePG

(mask) file

Mask fabrication

The Destgu Euvironment

TABlE5.2 Design Flow, Typical for Today's More Complex Devleas Shown in Figure 5,9 The Table Was Prepared with the Help of Ahatt Davis,

Level of design Company (tool in parentheses)

Functional specifications

Register transfer level (RTL) coding and

bebavioralsimulation

Logicsyntbesis

Floor planning

Placement and route

Matbworks (Matlab, Simulink); Cadence (SPW,VCC) Synopsys (VSS); Cadence (Verilog-XL); Mentor Gmphics (VHDL simulator) Synopsys (Design Complier; Module Compiler; Behavior Compiler)

Synopsys Synopsys (VSS); Cadence (Verilog-XL); Mentor Graphics (VHDL)

Cadence (Design Planner, Pillar);Avanl! (Apollo) Cadence (Silicon Ensemble, IC Craftsman)

Test Insertion and automated test pattern generation

Gate level netlist simulation

- -Ana4ogd.~

pr~

l';lptocolsllM _hol