Epson L90P - TM Two-color Thermal Line Printer Manual de usuario descargar pdf (Pagina 86)

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SOLID

STATE

By LOU

GARNER, Semiconductor

Editor

BETTER

QUALITY

semiconductor

de-

vices

at lower

prices may

result from

the use

of

a new type

of

production

test

instrument

developed

at

Bell Telephone

Laboratories.

Dubbed the

"Profilometer,"

the

new instrument

was

invented

by

Bell

scientist

John

A. Copeland (Fig.

1) and is

not only

capable of

much faster

and more

accurate

tests than

earlier machines,

but

costs

only

a fraction as

much to build

and operate.

The majority

of modern

semiconductor de-

vices,

whether transistors,

FET's, or

IC's,

are

manufactured

from thin

wafers of sili-

con, germanium,

or special

alloys.

The elec-

trical properties

of these

wafers are estab-

lished by

adding exact

amounts

of such

impurity

elements as

boron or arsenic.

The

measurement

of wafer impurity

densities

before

processing is

an essential quality

-

control

step

in

the fabrication

of

the final

devices.

The

needed data

was previously

obtained

by using

a costly machine

and a tedious

discrete

measurement

technique.

Test re-

sults

then had to be

processed

by computer

to convert

the raw figures

into

meaningful

information.

These extra

time -consuming

steps help

to

increase

the price of

the final

semiconductor.

In contrast,

the

new Bell

Labs

instrument

can make

the

necessary

measurements and

plot

a "profile"

of

wafer

impurity densities

within seconds,

thus

re-

ducing fabrication cost

and eventually

the

unit cost to the final

purchaser.

In operation, the

test engineer

first de-

posits

tiny

metal dots along

the

surface

of the semiconductor wafer.

A probe

placed

on one of the dots passes

a low

-level

5 -MHz

current through the dot. At

the same

time,

an

increasing

d.c. voltage

forces

mobile

charges

-electrons or

holes -out

of

an in-

creasingly

deep depletion

region

under

the

dot. No measurements

are made

of

the d.c.

voltage since

it

functions

only to vary

the

depth in

which

impurity

densities

are

mea-

sured beneath the

surface of the

wafer.

It

can

do

this

because

no net

charge

exists

within the semiconductor

under

ordinary

conditions.

When an increasing

d.c. voltage

is applied

to the metal

dot, the free

elec-

trons (or

holes) near the

wafer surface

are

forced

farther down,

leaving only

a fixed

94

charge

due

to

impurity

atoms in

the deple-

tion region.

For practical

purposes,

then, each

metal

dot acts

as one

plate of

a capacitor,

while

the depletion

region

acts as

a dielectric.

The

constant

5 -MHz,

r.f. drive

current

causes

the

edge of the

depletion

region

beneath

the dot

(acting as

the other

plate

of the

capacitor)

to oscillate

over a

short

distance,

generating

a small voltage

at the second

harmonic of

the drive

current.

This voltage

is inversely proportional

to the impurity

density

at

the depth

of the depletion

re-

gion in

the

wafer. The

voltage of

the

fundamental frequency

is

proportional

to

the depth

inside the wafer.

Thus, two

signals are obtained

simul-

taneously

-one

proportional

to

depth (be-

neath the

wafer's surface)

and

another

in-

versely proportional

to the impurity

densi-

ty at that

depth. In

practice,

these two

sig-

Fig. 1. Inventor

John A. Copeland,

Bell Telephone

Laboratories,

uses new

Profilometer

to plot

the

densities

of impurities

in

a

semiconductor wafer.

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