QUESTION:
I know that they radiation hardened the chips for Pathfinder and Sojourner.
What does radiation hardening consist of and how is it done?

ANSWER from Mark Adler on August 6, 1997:
It depends a lot on what the part is and what it's used for.  Parts can be
designed to be radiation-hard, but they are less common nowadays.  For those
cases, special properties of oxide insulators and annealing properties of
the semiconductors are adjusted to improve their tolerance to the total
radiation dose.  More common nowadays is to test commercial parts for
radiation susceptibility, which can actually vary with small changes in the
processes used to make the parts.  Then parts from a lot that passed are
qualified for use in space at a certain dosage level.  Then any hardening
that is done is simply shielding the parts from radiation and additional
spot- shielding of the more susceptible parts.

Aside from the total dose effects, there are also upset effects, where the
device isn't damaged, but instead the radiation causes an occasional, random
mistake.  In cases where the mistake can be corrected, such as in memory,
radiation-soft parts are commonly used. So here the hardening is adding more
memory chips to allow crosschecks of the data and corrections of errors.
Also if a mistake would have little impact, such as contributing noise to an
analog-to-digital conversion for an image, the level of errors of a soft
part may be acceptable.

There are certain design misfeatures of integrated circuits in which a
radiation upset can result in an electrical short and subsequent burnout of
the chip.  This "latchup" potential of the design can be analyzed and
tested, and such parts are simply avoided.  If a latchup disables a part but
does not destroy it, then the part may be acceptable if the latchup can be
cleared by a power on/off or reset, and does not occur too often.  So part
of the radiation hardening may be in how the device is used, e.g. frequent
resets.

mark