At 07:54 AM 5/7/1998 +1000, Paul B. Webster wrote:

(re erasing OTP parts with x-rays)

*** big snip ***

>  Let me give you a suggestion.  If you want to try it with your
>dentist, use the absolute *lowest* anode voltage to which the machine
>can be set.  Whenever this discussion comes up, people start talking
>about using industrial x-ray machines and the like, set up for
>ridiculously high anode voltages.  It seems not to be intuitive to them
>that "hard" X-rays which simply travel straight through the plastic
>package, silicon die, object holding the chip, the table, the floor and
>just keep on going have *no* effect erasing a chip!
>
>  Cheers,
>        Paul B.
>

Hi Paul,

I disagree with you on your comments above.  It doesn't matter if you use
soft x-rays or hard x-rays (the latter from industrial x-ay machines, for
example) - you will likely deposit some energy in the device (i.e. the
silicon die of the semiconductor).  Hard x-rays do not pass straight
through the chip, table, floor etc, without interacting.  They can interact
(e.g. scatter) with **any** material and deposit energy in that material.
[The amount of interaction depends on such factors as the thickness,
density and Z of the material.]  If you use soft x-rays with sufficiently
low energy, it might be possible that the majority of x-rays would be
absorbed in the plastic or ceramic IC packaging material and very little
would reach the semiconductor die.

BTW, it is relatively easy to generate x-rays with a home made rig.  In a
previous posting, I suggested that this should NOT be attempted by the home
experimenter, as this might lead to the person being needlessly exposed to
harmful levels of ionzing radiation.  [Many high school physics teachers
discontinued the use of Crooke's tubes for demonstrating low pressure
ionization of air, due to the fact that these devices did generate x-rays -
often in an unpredicatable pattern.  Many teachers were totally unaware of
this phenomena and I'll bet that many still are totally ignorant of this!]
Using photographic films to determine if there is leakage or to determine
the distribution patterns of x-rays from a home-made rig is NOT a safe way
to ensure that you are not exposing yourself!

Using x-rays to erase a OTP device is not a practical method in any event.
If it were possible to erase a device without destroying it (from the
accumulated (total) ionizing radiation dose), you would likely introduce
enough electrical changes in the device that the device could not be
expected to perform within the manufacturer's quoted specs.  The long term
reliability of the device might be affected too.

Do the intelligent thing - if you need code revision in a programmed OTP
device, chuck it in the garbage and start over with a fresh
(non-programmed) device.

BTW, I used to perfrom R&D in the field of radiation effects on
electronics.  ;-)

Best regards,
Gary Pepper

Ottawa, Ontario, Canada
e-mail: gpepper@capitalnet.com