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Each individual
cell in an eprom is programmed by injection of high energy
electrons and due to the absence of any electrical connections
each gate acts as a charged capacitor. Programming converts
each addressed cell to a "0" state. Again, as there are no
connections, erasure is achieved by non electrical means.
The standard method is to expose the whole cell array to high
intensity ultra violet light at 253.7nm for a defined period
of time. The UV radiation causes an ionising action resulting
in the charge leaking into the silicon substrate. When sufficient
charge has been drawn from the cell it is no longer programmed
in the "0" state.
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Neither programming
nor erasure occur instantaneously. Programming may require
100 or more pulses to achieve the required change of cell
charge and a considerable exposure time to UV light is required
to remove the programme. Eprom manufacturers specify the desired
exposure level in watt-seconds/sq cm and since the light output
of a new UV tube is determined by the lamp manufacturer the
end user only has control of the exposure period. One of the
most common causes of failure in equipment containing MOS
eproms is improper or incomplete erasure before programming.
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Correct erasure
requires the use of a high quality eraser and the application
of the correct UV exposure for an adequate period. The duration
of the period varies with the device to be erased, previous
programing-erasing history, power of the UV source, cleanliness
of both the eprom window and UV lamp surface.
Each manufacturer
specifies minimum erase dosage usually in the range 6 - 15
watt-sec/sq cm but in practice these should only be used as
a rough guide.
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As the UV lamp
ages, its intensity gradually diminishes increasing the required
exposure time. Similarly, reduced AC voltage supplies, increased
distance from the lamp and dirt or finger marks on the lamp
and eprom window surface also extend the erase time.
For these reasons
it is recommended to check the correct erase time periodically
as shown below.
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