Static information storage and retrieval – Floating gate – Particular biasing
Reexamination Certificate
1999-07-15
2001-01-23
Elms, Richard (Department: 2824)
Static information storage and retrieval
Floating gate
Particular biasing
C365S185290, C365S185330
Reexamination Certificate
active
06178119
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of erasing a non-volatile semiconductor memory cell having a floating gate electrode by radiating the floating gate electrode with short-wave radiation, more particularly by radiating with radiation in the ultraviolet range of the spectrum. In most embodiments, the memory cell together with a large number of other, identical cells forms a non-volatile programmable memory, mostly referenced as EPROM in the literature. The cell is then formed by an n-channel MOS transistor in which the conductance of the channel area is determined by the charging state of the floating gate electrode provided over the channel area and electrically insulated therefrom. Starting from the supposition that (by way of example) the uncharged state of the floating gate electrode corresponds to a “0”, the cell may be programmed by storing electrons on the floating gate as a result of which the threshold voltage V
th
of the transistor is increased (logic “1”) compared to the uncharged state. By applying to the control gate of the transistor a voltage between these two threshold voltage values and determining the conductance by the transistor, the stored information may be read out. As is generally known, the information may be erased by means of UV radiation, so that the electric charge of the floating gate electrode is removed and the cell is rendered suitable for re-use.
In practice it has appeared that the lowest value for the threshold voltage that may be obtained by erasing with UV radiation is rather high. This value depends to a certain extent on the production process and on a number of other factors such as wavelength of the UV radiation. For a specific process and a standard-wavelength UV radiation, the threshold voltage of the erased cell was about 1.9 volts, and 4 volts for the programmed cell. This means that a gate voltage of at least about 3 volts is to be applied for the reading operation. This value is therefore considered to be high, because in electronics there has been a tendency the past few years towards lower supply voltages and lower powers.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide, inter alia, a method of erasing by means of UV radiation the information stored on the floating gate, so that a lower threshold voltage may be obtained than with the known method. A method according to the invention is therefore characterized in that the cell is heated during radiation. Experiments have shown that due to the higher temperature during erasure a considerable reduction of the threshold voltage may be obtained. For example, a reduction of 0.2-0.3 volt was advantageously obtained at a temperature of 100° C. A preferred embodiment of a method according to the invention is therefore characterized in that the cell is heated up during radiation to a temperature in the region from 200° C. to 300° C. With a temperature in this region, a reduction of about 0.5 volt (relative to 1.9 volts) of the threshold voltage may be obtained. As a result, a gate voltage of, for example, no more than 1.8 volts can be used for reading.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
REFERENCES:
patent: 5386388 (1995-01-01), Atwood et al.
patent: 6028787 (2000-02-01), Sansbury et al.
Druijf Klaas G.
Lindeman Jan
Biren Steven R.
Elms Richard
Nguyen Vanthu
U.S. Philips Corporation
LandOfFree
Method of erasing a non-volatile memory does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of erasing a non-volatile memory, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of erasing a non-volatile memory will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2494335