Static information storage and retrieval – Floating gate – Particular biasing
Reexamination Certificate
1999-11-24
2001-06-19
Nelms, David (Department: 2818)
Static information storage and retrieval
Floating gate
Particular biasing
C365S185040, C365S185320, C365S189011
Reexamination Certificate
active
06249456
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor memories, and, more particularly, to a secured electrically modifiable non-volatile memory.
BACKGROUND OF THE INVENTION
The expression “secured EEPROM memory” is understood to mean any memory to which write access is restricted to protect its contents. The protection may be either a software protection or a hardware protection. This type of EEPROM is protected especially against the electrical erasure of its cells.
One technique that has been devised by certain fraudulent individuals to circumvent protection against electrical erasure includes erasure by ultraviolet (UV) radiation although these memories, in theory, are protected by their packages against such erasure. These fraudulent individuals open the package of the memory to access its memory array and modify the state of the cells therein by exposing them to ultraviolet radiation. This fraudulent erasure does not necessarily stop the performance of the application in which the memory is used, and the fraud is therefore not necessarily detected.
SUMMARY OF THE INVENTION
An object of the invention is to provide a secured EEPROM type memory enabling the detection of the above described fraud, and making it possible to know if the cells of the memory have been subjected to ultraviolet radiation.
A secured electrically modifiable non-volatile memory comprises a matrix of memory cells and means for reading and writing in the memory cells. The memory further comprises at least one additional memory cell, called a reference cell, and associated read means capable of detecting any erasure of the reference cell by ultraviolet radiation. At each access to the memory, the reference cell is read by the associated read means.
The secured EEPROM type memory of the invention has only one reference cell. According to a first embodiment of the invention, the reference cell is initially in an erased electrical state. To detect any erasure of the reference cell by an ultraviolet radiation, the associated reading means has a detection threshold whose value ranges from the threshold value of conduction of an electrically erased cell to the threshold value of conduction of a cell erased by an ultraviolet radiation.
According to a second embodiment of the invention, the reference cell is initially in a programmed electrical state. To detect any erasure of the reference cell by an ultraviolet radiation, the associated read means have a detection threshold whose value ranges from the conduction threshold value of an electrically programmed cell to the conduction threshold value of a cell erased by ultraviolet radiation.
If the state of the reference cell detected by the associated read means is different from its initial electrical state, this indicates that the memory has been subjected to ultraviolet radiation, and the operation of the memory is then neutralized or stopped.
REFERENCES:
patent: 4947378 (1990-08-01), Jinbo et al.
patent: 5198997 (1993-03-01), Arakawa
patent: 0 300 885 A1 (1989-01-01), None
patent: 0 417 008 A1 (1991-03-01), None
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Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Galanthay Theodore E.
Lam David
Nelms David
STMicroelectronics S.A.
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