Play Meter

By William Arkush
Microelectronics Division Manager
Exidy. Incorporate d
Sunnyvale . California
In the· 'chips'
SEMICONDUCTOR DEVICE TECHNOLOGY
First of a three -part article to be continued in UPDATE
With so many devices being in -
corporated into computer-age tech -
nology . today's technicians' daily
exist e nce has become a nightmare of
device numbers . We are not only
required to know the integrated
circuit
identification
numbers
( 7400s . 5400s. 2500s, 2100s, etc .),
we are also required to know the
manufacturers' numbers. For ex-
ample . one semiconductor manufac-
turer's 74181 is made as a 25181 by
another manufacturer .
If that isn 't enough of a problem,
there are the various technologies of
devices which read like a who's who
column. To start with, there are the
MOSs ( CMOS , NMOS , VMOS ,
DMOS , and their brother pmos) .
Then there are the Polars ( bipolar
and isoplanar) , and the logic families
T2L, 12L , ECL , and DTL. Too ,
there are chips , gates , buffers ,
drivers , processors , micros , minis ,
UARTS ,
ROMS ,
RAMS ,
PROMS , EPROMS , EAROMS ,
and EEPROMS .
Oh yes, there are PROMS that
program by blowing their junctions to
create a program , and there are
PROMS where you grow the junc-
tion to create a program within the
device . ( Quite simply , it is the
bipolar PROM versus the AIM
device - Avalanche Injected Migra-
tion).
Clearly , there are many aspects in
understanding the hardware of a
logic system . Regardless of whether it
is a simple digital timer or a complex
computer syster , it is amazing that all
of the technology has a common
base of silicon ( pure sand) about 25
mills ( 0 .025 inch) square. No matter
what function the device has been
designed to perform , the active
ingredient is the actual circuit chip
( i.e ., dice) . There are many proces-
ses known for creating a chip of
silicon, perhaps as many as the
different technologies .
Technologies in this case refers to
the many process methods used in
creating the device itself . The
E-beam evaporator , for example , is a
machine which controls an electron
gun similar to the gun of the CRT to
deposit sub-atomic layers of dopant
material into the wafer . The typical
three -inch silicon wafer is .010 inch
thick and extremely brittle.
This thin little wafer will be
processed in many ways before it is
an array of completed semiconductor
die ( individual chips) . To mention a
few , the pure silicon wafer will be
immersed in the most caustic of
chemicals, exposed to severe radia-
tion exposure of electron beam
particles, or subjected to high
intensity molecular collisions of gas
atoms and metal molecules; it will be
deposited on with photochemicals ,
etched away by acids . Some wafers
are evaporated onto while rotating in
a planetary vacuum evaporator and
yet other wafer proceses sputter
targeted materials onto the wafer
substrate . It may be surprising that an
integrated monolithic circuit exists
when you consider all the process
steps a wafer is subjected to before it
becomes a circuit die (chip) , the
subject of this article . Alter all , the
chip is merely a component itself
within a larger assembly.
The end product , of course , is the
integrated circuit itself.
The wafer
By and large , " wafer" here means
a thin slice of an oblong ingot of pure
silicon . Since the silicon material is a
crystalline atomic structure , it is
predictable . In fact, it is easy to orient
new atoms and adhere them to
adjacent atoms because the atoms
themselves are cubic building blocks.
The two basic types of silicon
orientation used to produce silicon
ingots are : 1-1-1 and 1-0 -0 . This
symbolizes the lattice structure of the
crystal which is comprised of a
nucleus and the electron spacing and
orbital patters which build the
structure of the crystal's lattice .
Silicon ingots are grown in an
oven at about 600 degrees C . and
built from pulling a seed crystal of the
material through an atmosphere of
Si + Si02 to yield 2Si0 in a special
vertical furnace . The atoms then
combine as the spindle with the seed
crystal is pulled while it rotates at
5 -10 RPM .
The end product is an oblong
cylinder about 18 inches long and
about three inches in diameter and
tapered at each end . From this
shape , the cylinder is tooled and then
sliced into .010 inch thick wafers of
uniform size and shape.
After cutting these thin slices , each
one has to be mechanically polished ,
chemically polished , and electro-
chemically processed before the
wafer fabrication process even starts .
When the wafers have been sub-
jected to chemical etchants, the raw
silicon is exposed to air . This is not a
favorable condition for the wafer
because the silicon begins an oxyda-
tion process which is uncontrolled
and eventually will ruin the wafer
surface .
It may not seem significant , but the
surface aggravation of a polished and
processed wafer is less than 20 A
( Angstrom) units . One A equals one
ten -thousandth of a micron. One
micron equals 1/ 25 of a mil/. One
mill is equal to 1/ 1000 of an inch .
Since the spectrum of light is
measured in Angstron units, the
introduction
of
photochemical
oxides - or oxygen - is sufficient to
alter the thickness of the silicon base
material. As you can see , this begins
the controlled process of the wafers ,
so after they have been polished and
treated , the wafers are kept in
nitrogen -purged dry boxes .
The nitrogen purges out the
oxygen
from
the
immediate
atmosphere and the silicon cannot
attract ion exchange between the
nitrogen gas or the non -present
oxygen. From this point on , each
step of the process is carefully
controlled and monitored .
Whatever process is chosen ,
depending on the device to be
fabricated , the process is like a recipe
in that time , temperature, ingredient