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SUMMARY: Not-so-volatile volatile memory

About a month ago, I asked for comments about recovering data from
semiconductor memory after power had been removed.  After much procrastinating,
I've finally finished the summary of what people sent me.  Many thanks to
everyone who responded, in particular to Bob Hale for answering many questions
about the possibility of recovering data from DRAM's.
If anyone has any further comments to add to this (I'm particularly interested
in actual figures for data retention in DRAM cells, although I've been told
this is burn-before-reading proprietary information), you can send it to me at
the above address.

-- Summary: Data retention in semiconductor memory --
Contrary to conventional wisdom, "volatile" semiconductor memory does not
entirely lose its contents when power is removed.  Both static (SRAM) and
dynamic (DRAM) memory retain some information on the data stored in it while
power was still applied.  SRAM is particularly susceptible to this problem, as
storing the same data in it over a long period of time has the effect of
altering the preferred power-up state to the state which was stored when power
was removed.  Older SRAM chips could often "remember" the previously held state
for several days.  In fact, it is possible to manufacture SRAM's which always
have a certain state on power-up, but which can be overwritten later on - a
kind of "writeable ROM".
DRAM can also "remember" the last stored state, but in a slightly different
way.  It isn't so much that the charge (in the sense of a voltage appearing
across a capacitance) is retained by the RAM cells, but that the thin oxide
which forms the storage capacitor dielectric is highly stressed by the applied
field, or is not stressed by the field, so that the properties of the oxide
change slightly depending on the state of the data.  One thing that can cause a
threshold shift in the RAM cells is ionic contamination of the cell(s) of
interest, although such contamination is rarer now than it used to be, because
robotic handling of the materials and the purity of chemicals is greatly
improved.  However, even a perfect oxide is subject to having its properties
changed by an applied field.  When it comes to contaminants, sodium is the most
common offender - it is found virtually everywhere, and is a fairly small (and
therefore mobile) atom with a positive charge.  In the presence of an electric
field, it migrates towards the negative pole with a velocity which depends on
temperature, concentration of the sodium, the oxide quality, and the other
impurities in the oxide such as dopants from the processing.  If the electric
field is zero and, given enough time, the sodium contamination tends to spread
itself around evenly.
Other factors which affect the rate of change are temperature, the field
strength of the stored charge, the quality of the oxide, and the concentration
of dopants and other impurities which have already been mentioned above.  The
stress on the cell a cumulative effect, much like charging an RC circuit.  If
the data is applied for only a few milliseconds then there is very little
"learning" of the cell, but if it is applied for hours then the cell will
acquire a strong (relatively speaking) change in its threshold.
The effects of the stress on the RAM cells can be measured using the built-in
self test capabilities of the cells, which provide the the ability to impress a
weak voltage on the storage cell in order to measure its margin.  Cells will
show different margins depending on how much oxide stress has been present.
Many DRAM's have undocumented test modes which allow some normal I/O pin to
become the power supply for the RAM core when the special mode is active.
One way to activate the special test mode might be to underdrive a pin and turn
on its protection diodes(s), which will be recognized internally and will
change a multiplexer so that the core is powered by some pin which is normally
a digital I/O pin.  Another way, if the DRAM has suitable clocks, is to
recognise an invalid combination of clocks (such as CAS before RAS, if the DRAM
doesn't use that mode for higher speed operation) to enable the test mode.
Great care must be taken to ensure that the test mode isn't inadvertently
entered so that the memory system appears to be malfunctioning (for example in
the first case if the system has substantial undershoot at the wrong time, the
test mode could be activated).  This problem can be avoided by designing the
test mode signals such that a certain set of states which would not occur in a
normally-functioning system has to be traversed to activate the mode.
Manufacturers won't admit to such capabilities in their products because they
don't want their customers using them and potentially rejecting devices which
comply with their spec sheets, but have little margin beyond that.
One way to speed up the annihilation of stored bits in semiconductor memory is
to heat it.  Both DRAM's and SRAM's will lose their contents a lot more quickly
at Tjunction = 140C than they will at room temperature.  Several hours at that
temperature with no power applied will clear their contents sufficiently to
make recovery difficult.  Conversely, to extend the life of stored bits with
the power removed, drop the temperature below -60C (some people even claim that
you can permanently "imprint" an SRAM with its stored bits by rapidly cooling
it below liquid nitrogen's boiling point).  In any case it should lead to
weeks, instead of hours or days, of data "retention".
Simply repeatedly overwriting the data held in DRAM with new data isn't nearly
as effective as it is for magnetic media.  The new data will begin stressing or
relaxing the oxide as soon as it is written, and the oxide will immediately
begin to take a "set" which will either reinforce the previous "set" or will
weaken it.  The greater the amount of time that new data has existed in the
cell, the more the old stress is "diluted", and the less reliable the
information extraction will be.  Generally, the rates of change due to stress
and relaxation are in the same order of magnitude.  Thus, a few microseconds of
storing the opposite data to the currently stored value will have little effect
on the oxide.  Ideally, the oxide should be exposed to as much stress at the
highest feasible temperature and for as long as possible to get the greatest
"erasure" of the data.  Unfortunately if carried too far this has a rather
detrimental effect on the life expectancy of the RAM.
Therefore the goal to aim for when sanitising memory is to store the data for
as long as possible rather than trying to change it as often as possible.
Conversely, storing the data for as short a time as possible will reduce the
chances of it being "remembered" by the cell.  Based on tests on DRAM cells, a
storage time of one second causes such a small change in threshold that it
probably isn't detectable.  On the other hand, one minute probably is
detectable, and 10 minutes is certainly detectable.
The most practical solution to the problem of DRAM data retention is therefore
to constantly flip the bits in memory to ensure that a memory cell never holds
a charge long enough for it to be "remembered".  While not practical for
general use, it is possible to do this for small amounts of data such as
encryption keys.