V - Index of

----------------------------Programmlng I Erasing of EPROMs
1.2 Erasing
Data strored in the EPROM is erased by releasing
the electric charge from the floating gate through
the exposure of the memory chip to ultraviolet
light. light has an energy that is inversely proportional
to its wavelength. Receiving the energy of
the ultraviolet light, the electrons in the floating
gate are again turned into hot electrons, which jump
across the oxide film into the control gate or substrate.
As a result of this process, the stored
information is, erased. Accordingly, the stored
information can not be erased by such lighu whose
wavelengths are too long to give adequate energy to
jump over the barrier of the oxide film. For
successful erasing, the wavelength and minimum
exposure rate of ultraviolet light are specified as
2,537 A and 15W sec/cm 1 respectively. This condition
is attained by exposing a device to an ultraviolet
lamp of 12,Ooo",W/cm 1 1.2 - 3cm away for
approximately 20 minutes. The ultraviolet light
transmission rate of the transparent lid is about 70
percent. Any contamination or foreign material
at the surface of the capsule lowers the transmission
rate, prolonging the erasing time. So such contamination
should be recovered by use of alcohol or
other solvent that does not damage the package.
Fig. 5 shows typical erasure characteristics for
EPROM.
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Flg.5 Typical Erasing Characteristics
1.3 Data retention characteristic of EPROM
As a result of writing in, approximately 0.5 to 2.0
x 10- 13 coulomb of electrons are accumulated at
the floating gate. With the elapse of time, however,
these electrons decrease, as a result of which the
inversion of stored information can happen. The
mechanism of electron dissipation is generally
explained as follow:
It) Data dissipation by heat
The accumulation of electrons at the floating
gate is an unbalanced state, so the dissipation of
thermally excited electron is unavoidable.
Therefore, the data retention time has a close
relationship with temperature. Fig. 6 shows
typical data retention characteristics. The data
retention time is proportional to the reciprocal
of absolute temperature.
(2) Data dissipation by ultraviolet light
Ultraviolet rays at a wavelength of not greater
than 3,000 - 4,oooA is capable of releasing the
electric charge stored in the memory of the
EPROM with varying efficiencies. Fluorescent
light and sunlight contain some ultraviolet rays,
so prolonged exposure to these lights can cause
data corruption as a result of electric charge
dissipation. Fig. 7 shows examples of the data
retention time under an ultraviolet eraser,
sunlight and fluorescent lighting. But it should
be noted that the data for fluorescent light and
sunlight are not definite because of their'varying
ultraviolet ray contents. The ultraviolet ray
content in sunlight, for example, varies greatly
with seasons, weather and the composition of
the atmosphere.
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Slon,e Te-.erature ltl
Flg.6 Typical Data Retention Characteristics
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Stored Chu,. (RelatIVe)
Flg.7 EPROM'. data retention time
• HITACHI
Hitachi America Ltd. • 2210 O'Toole Avenue • San Jose, CA 95131 • (408) 435-8300 31