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IAN POOLE REPORTS DISCOVERS THAT “TEST TUBE” TAKES ON A NEW
MEANING FOR MICROELECTRONICS. AND THAT OXYGENATION EXTENDS CHIP
PERFORMANCE.
There are two ideas high-
lighted this month. The first is
associated with minute connec-
tions required within microelec-
tronics circuits. The second new
development relates to the
performance that needs to be
achieved by integrated circuits
today.
nanometers! The investigations
being performed on these
minute tubes could ultimately
have important applications in
microelectronics where ex-
tremely small conductors and
other structures are required.
Electric Light
It has also been found that
the nanotubes themselves act
as conductors. In a phe-
nomenon known as
ballistic
conductance,
these minute
tubes give very low values of
resistance. It has also been
shown that the heating effect
caused by the current flow is
much reduced upon what would
normally be expected.
Enormous current densities
have been seen, with values of
10MA/cm2 being achieved. Had
the current behaved in the
normal manner, then this would
have heated the tubes to tem-
peratures in excess of 10,000K.
Investigations show that the
current is flowing in a different
manner to that normally experi-
enced. In this case, the current
is flowing through the nanotubes
as if it was light passing through
an optical waveguide.
These effects have only
been noted in nanotubes that
are less than 5um in length. At
greater lengths, the ballistic
conductance effect appears to
be destroyed by the scattering
effect of the electrons.
Whilst the maximum allow-
able length of these nanotubes
may limit their usefulness, there
will be many applications within
microelectronics. In addition to
this, further development may
enable their applications to be
extended. Despite the prob-
lems, nanotubes should be
another technique that enables
IC designers to increase the lev-
els of integration.
Test Tube Chips
Whilst the work is still in its
early stages, the researchers
have been able to put materials
into the tubes and manipulate
them. By doing this they have
been able to induce chemical
reactions. This has opened up
new ways of thinking about
the structures and they can be
thought of as extremely small
test tubes.
The first stage in producing
conductors was to try to deposit
a conductor on the inside of the
walls of the nanotube. In order
to achieve this, both ends of the
tube were opened. Silver nitrate
(AgNO
3
) was allowed to fill the
tube by capillary action. Then
the silver nitrate was decom-
posed so that it formed metallic
silver. This was achieved by
heating the tubes with a beam
from an electron microscope.
Initially, the process only
produced tiny silver beads in
the tube, because a small
pocket of gas separated each
bead. The gas was under
extreme pressure, estimated to
be over 1000 atmospheres. The
ultimate idea was to enable
minute conductors to be manu-
factured that could be used in
integrated circuit manufacture.
Even Smaller
Miniaturization in electron-
ics is continuing at an ever-
increasing rate. To achieve this,
one of the main areas for
research is naturally that of
achieving smaller feature sizes
within the integrated circuits.
New developments in lithogra-
phy and the like are all under
investigation.
However, there are many
other new and interesting ideas
that are equally important to the
miniaturization process. In the
years to come, they will make
large contributions to the
increase in the level
of integration.
Some of these ideas may
seem far removed from the
core electronics business, but
this does not mean to say that
they will not affect it. In the past
many ideas that may have
appeared to have no bearing on
one area of technology have
had an enormous impact.
In one area of research
being undertaken at the Georgia
Institute of Technology, nan-
otubes are being investigated.
These tiny tubes are made from
carbon and have inside diame-
ters measuring less than ten
Copyright © 1998 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, December 1998 - www.epemag.com - 115
New technology Updates
Silicon on Insulator
The idea of silicon on insu-
lator (SOI) chips has been
known for many years. How-
ever, they have not been as
widely used as many would like.
The problem has been in en-
abling them to be made in com-
mercial volumes.
Now researchers at IBM
have succeeded in developing a
process that can be easily
applied to a whole range of
CMOS processes. In fact, the
process has been qualified for
their 0⋅22um CMOS process,
and is being developed for their
0⋅15um technology. The new
advance results from a process
called
separation by implanta-
tion of oxygen
(SIMOX).
The advantage of SOI is
that as the device itself sits on
an insulating layer its high fre-
quency performance is greatly
improved. This results from two
main effects. One is known as
the “body effect,” where the size
of the substrate affects the
performance. The other is in the
reduction of stray capacitance
from components in the device
to the substrate.
As a result, figures for SOI
devices typically show a 35 per
cent improvement in perfor-
mance. This is not only useful
for RF applications, but also to
give the increased speeds
required by today's processors
and associated chips.
In addition to the higher
speeds, the SIMOX process as
implemented by IBM offers the
possibility of better low power
devices. The reason for this is
that the performance of SIMOX
devices degrades less with volt-
age than do conventional
CMOS devices. In tests
carried out using SRAMs,
SIMOX devices consumed
less power for an equivalent ac-
cess time.
separated from the substrate by
a thin oxide layer.
Development of the process
has been hailed a success
because once the basic SIMOX
process has been completed
the substrate can be processed
in the normal way. As the gen-
eration of the insulating layer
takes place during the prepara-
tion of the wafer itself, rather
than during the fabrication of
the components in the device,
this can be undertaken off-line.
In this way the fab lines are not
disturbed. A further advantage
is that it can be added to
existing products, by simply
using wafers that have under-
gone the SIMOX process.
In view of the fact that this
process can be incorporated
into existing lines, the cost of its
introduction is much less than
one needing a completely new
line to be set up. This will mean
that the ICs fabricated using the
new process will soon reach the
marketplace.
Also, the cost of the chips
will be less than those requiring
a completely new process with
all the vast sums of investment
to get them up and running.
This is all good news for the end
user who will be able to see im-
proved performance for little
added cost.
Cosmic Success
A final advantage is in the
reliability of memory devices.
On occasions, cosmic rays and
background radioactivity can
cause data errors. It is found
that the level of these errors is
reduced with devices fabricated
using SOI rather than conven-
tional CMOS ICs. In fact, the
process was first developed for
devices to be used in space.
Here the levels of radiation
are higher, and the degrees of
reliability required are much
greater.
The process itself involves
the use of very heavy doses of
oxygen, followed by annealing
at high temperature until a thin
oxide layer is formed. The over-
all result of the process is to
give a thin layer of N material
Copyright © 1998 Wimborne Publishing Ltd and
Maxfield & Montrose Interactive Inc
EPE Online, December 1998 - www.epemag.com - 116

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