16 Silicon chip Australia’s electronics magazine siliconchip.com.au
- Secondary particles can be gener-
ated by the interaction of primary
particles when they enter electronic
structures, eg, a cosmic ray which
strikes the encapsulation of a device. - Gamma and neutron radiation is pro-
duced in nuclear reactors and can
affect electronics inside a shield-
ed area. - Particle accelerators such as the
Large Hadron Collider produce var-
ious types of radiation that can af-
fect unshielded sensors and control
circuitry. - Nuclear explosions can produce a
powerful electromagnetic pulse and
a large variety of particles that can
affect electronics and power grids. - Trace radioactive elements in elec-
tronic chip packaging and wafer ma-
terials were found to be a problem in
the 1970s. Alpha particles (helium
nuclei) in older packaging materi-
als could discharge the capacitors in
DRAM, but this effect has been mini-
mised today by using purer packag-
ing materials and more sophisticated
error correction.
Origins of damage or effects
to electronic materials
Radiation damage or effects to elec-tronic materials may be either perma-
nent or temporary while the source
of such radiation can be in the form
of neutrons, protons, alpha particles,
ions, x-rays, parts of the UV spectrum
and gamma rays.
In terms of damage to electronics ra-
diation can be divided into two main
types. One type is high energy radia-
tion which is capable of causing dis-
ruption of atoms in a device’s crystal
lattice and permanent damage. The
other type is that comprising of low-
er energy radiation that is not able to
cause disruption in a crystal lattice
but can cause disruption of electron-
ic charge carriers in a crystal lattice.
Permanent damage can be in the
form of “lattice displacement” where-
by atoms are moved from their correct
positions, causing the formation of
new electronic structures such as re-
combination centres, and worsening
the properties of semiconductor junc-
tions due to rearrangement of charge
carriers within the crystal.
Although such lattice displacement
damage is usually permanent, in some
cases limited self-repair is possible
due to “annealing” whereby displaced
atoms can move back or partially back
to their correct locations.Fig.3: a proton or neutron impacting
a semiconductor crystal lattice can
displace an atom from its correct
location and alter its electronic
properties. Meanwhile, it continues
through the crystal (with reduced
energy), where it can potentially
cause additional damage or
electronic disruption.
Fig.4: a radiation
particle, in this case an
ion, passing through a
field effect transistor
(FET) structure. This
can disrupt thousands
of electrons. The flow
of current passing
through the structure is
affected, possibly causing
a malfunction in the
circuit. The damage is
usually temporary.
Image courtesy Windows
to the Universe.Individual instances of lattice dis-
placement won’t necessarily cause no-
ticeable degradation of a device.
However, the effect is cumulative
and multiple instances of lattice dis-
placement cause long term degrada-
tion in the performance of a device.
This could include, for example,
alteration of the switching threshold
voltage of a transistor, causing a tran-
sistor to remain permanently switched
on or off, or reducing the output of a
solar cell on a spacecraft.
Another source of damage in semi-
conductor crystal materials is ionisa-
tion. The energy of particles involved
in ionisation effects is generally too
low to cause permanent damage but
can create “soft errors” such as corrup-
tion of memory contents or alteration
of circuit logic states (Fig.4).
The damage can become permanent
if a condition is generated such as a
Single Event Latchup (SEL), which
can lead to permanent damage under
certain conditions (more on that later).Main types of
radiation-induced effects
Based on the above mechanisms, ra-
diation effects in electronic structures
can be broadly categorised as:When a Soviet pilot flying a MiG-25 de-
fected to the West in 1976, experts were
surprised to find that a majority of its avi-
onics were built with vacuum tubes.
This represented old technology for
the time, but it was concluded that the
Soviet decision to use vacuum tubes was
due to their better tolerance of tempera-
ture extremes than solid state electronics
of the time.
It was also considered that this meant
that the avionics bays would not need environmental controls,
and vacuum tubes were also more resistant to the electromag-Soviet ‘retro’ radiation hardening technology
netic pulse (EMP) from nuclear explosions
than solid-state devices.
Also, the tubes enabled the aircraft radar to
operate at an extremely high power of 600kW.
Having said that, at the time, the more mod-
ern electronics of the West was quite capable
of withstanding adverse environmental con-
ditions and EMP, so the real reason the So-
viets used vacuum tubes was probably that
their electronic industry was less advanced
than that of the West.
But there are still situations today where vacuum tubes are consid-
ered for use in space-based applications, because of their robustness.