Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2001-06-14
2003-12-16
Crane, Sara (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S324000, C257S335000, C365S184000
Reexamination Certificate
active
06664588
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to FLASH, electrically erasable, programmable read only memory (EEPROM) and nitride, programmable read only memory (NROM) cells in general.
BACKGROUND OF THE INVENTION
Dual bit cells are known in the art although they are not common. Some dual bit cells have multiple threshold voltage levels, where every two threshold voltage levels together store a different bit. Others store one bit on either side of the cell. A dual bit cell of the latter kind, known as nitride, programmable read only memory (NROM) cell, is disclosed in U.S. Pat. No. 6,011,725, assigned to the common assignee of the present invention. The disclosure of the above-identified patent is incorporated herein by reference.
FIGS. 1A
,
1
B and
1
C, to which reference is now made, schematically illustrate the dual bit NROM cell. The cell has a single channel
100
between two bit lines
102
and
104
but two separated and separately chargeable areas
106
and
108
. Each area defines one bit. For the dual bit cell of
FIGS. 1
, the separately chargeable areas
106
and
108
are found within a nitride layer
110
formed in an oxide-nitride-oxide sandwich (layers
109
,
110
and
111
) underneath a polysilicon layer
112
.
To read the left bit, stored in area
106
, right bit line
104
is the drain and left bit line
102
is the source. This is known as the “read through” direction, to indicated by arrow
113
. To read the right bit, stored in area
108
, the cell is read in the opposite direction, indicated by arrow
114
. Thus, left bit line
102
is the drain and right bit line
104
is the source.
FIG. 1B
generally indicates what occurs within the cell during reading of the left bit of area
106
. An analogous operation occurs when reading the right bit of area
108
.
To read the left bit in area
106
, the left bit line
102
receives the source voltage level V
s
, typically of 0V, and the right bit line
104
receives the drain voltage V
d
, typically of 2V. The gate receives a relatively low voltage V
g
, which typically is a low voltage of 3V.
The presence of the gate and drain voltages V
g
and V
d
, respectively, induce a depletion layer
54
and an inversion layer
52
in the center of channel
100
The drain voltage V
d
is large enough to induce a depletion region
55
near drain
104
which extends to the depletion layer
54
of channel
100
. This is known as “barrier lowering” and it causes “punch-through” of electrons from the inversion layer
52
to the drain
104
. The punch-through current is only minimally controlled by the presence of charge in right area
108
and thus, the left bit can be read irrespective of the presence or absence of charge in right area
108
.
Since area
106
is near left bit line
102
which, for this case, acts as the source (i.e. low voltage level), the charge state of area
106
will determine whether or not the inversion layer
52
is extended to the source
102
. If electrons are trapped in left area
106
, then the voltage thereacross will not be sufficient to extend inversion layer
52
to the source
102
and a “0” will be read. The opposite is true if area
106
has no charge.
Like floating gate cells, the cell of
FIGS. 1A and 1B
is erasable and programmable. Thus, the amount of charge stored in areas
106
and
108
can be controlled by the user.
For NROM cells, each bit is programmed in the direction opposite that of its reading direction. Thus, to program left bit in area
106
, left bit line
102
receives the high programming voltage (i.e. is the drain) and right bit line
104
is grounded (i.e. is the source). This is shown in FIG.
1
C. The opposite is true for programming area
108
.
The high programming voltage pulls electrons from the source
104
. As the electrons speed up toward the drain
102
, they eventually achieve enough energy to “jump” into the nitride layer
110
. This is known as “hot electron injection” and it only occurs in the area close to the drain
102
. When the drain voltage is no longer present, the oxide layer
109
prevents the electrons from moving back into the channel
100
.
The bits are erased in the same directions that they are programmed. However, for erasure, a negative erasure voltage is provided to the gate
112
and a positive voltage is provided to the bit line which is to be the drain. Thus, to erase the charge in left area
106
, the erase voltage is provided to left bit line
102
. The highly negative erase voltage creates an electric field in the area near the left bit line
102
which pulls the electrons stored in the area close to the drain. However, the electric field is strong only close to the drain and thus, the charge in right area
108
is not depleted.
Typically, programming and erasure are performed with pulses of voltage on the drain and on the gate. After each pulse, a verify operation occurs in which the threshold voltage level of the cell (i.e. the gate voltage level at which the cell becomes conductive) is measured. During programming, the threshold voltage level Vtp is steadily increased so that the cell will not pass any significant current during a read operation. During erasure, the opposite is true; the threshold voltage level Vte is decreased until a significant current is present in the cell during reading.
Unfortunately, multiple erase and programming cycles change the number of pulses needed to achieve the desired threshold voltage levels. For the pulses, either the voltage level can remain constant and the number of pulses can be increased or the voltage level can be increased until the desired threshold voltage level is achieved.
The cell will no longer be considered functional once the gate voltage required for erasure is too negative and/or the number of programming pulses is reduced to one.
FIGS. 2A
,
2
B and
2
C present experimental results of multiple programming and erase cycles, on log-linear charts. In this experiment, the gate voltage level for erasure was increased, as necessary, and the cell ceased to function after 20,000 cycles.
FIG. 2A
graphs the programming and erase threshold voltage levels for both bits. Curves
60
and
62
illustrate the programming threshold voltage levels for the left and right bits, respectively, where the threshold voltage level for the right bit is measured in the forward (and not the reverse) direction. Curves
64
and
66
illustrate the erase threshold voltage levels for the left and right bits, respectively. It is noted that all curves remain relatively constant until about 2000 cycles at which point the threshold voltage levels increase. It is also noted that the programming threshold voltage level for the left bit, read in the reverse direction, is significantly higher than that for the right bit. However, the erase threshold voltage levels of each bit are smaller than their programming threshold voltage levels.
FIG. 2B
illustrates the read current Ir after programming (curve
70
) and after erasure (curve
72
). Both currents decrease strongly after about 4000 cycles.
FIG. 2C
illustrates the number of programming pulses (curve
74
) and the gate voltage during erasure (curve
76
). The number of programming pulses drops to one and the gate voltage drops from −6V to −9V after about 3000 cycles.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved NROM cell which can endure an increased number of programming and erase cycles.
There is therefore provided, in accordance with a preferred embodiment of the present invention, an NROM cell which has an oxide-nitride-oxide layer over at least a channel and a pocket implant self-aligned to at least one bit line junction. The cell also includes at least one area of hot electron injection within the ONO layer and over the pocket implant and at least one area of hot hole injection generally self-aligned to the area of hot electron injection.
Additionally, in accordance with a preferred embodiment of the present invention, the pocket implant can be a single or a double pocket implant.
There is also
Crane Sara
Eitan, Pearl, Latze & Cohen Zedek, LLP
Saifun Semiconductors Ltd.
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