Static information storage and retrieval – Floating gate – Particular biasing
Reexamination Certificate
2000-12-05
2002-08-13
Ho, Hoai V. (Department: 2818)
Static information storage and retrieval
Floating gate
Particular biasing
C365S185030, C365S185170, C257S324000
Reexamination Certificate
active
06434053
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device which has a planarly dispersed charge storing means (for example, in a MONOS type or a MNOS type, charge traps in a nitride film, charge traps near the interface between a top insulating film and the nitride film, small particle conductors, etc.) in a gate insulating film between a channel forming region and a gate electrode in a memory transistor and is operated to electrically inject a charge into the charge storing means to store the same therein and to extract the same therefrom and a method of operating the device.
2. Description of the Related Art
Nonvolatile semiconductor memories offer promise as large capacity, small size data-storage media. Along with the recent spread of broadband information networks, however, write speeds equivalent to the transmission rates of the networks (for example, a carrier frequency of 100 MHZ) are being demanded. Therefore, nonvolatile memories are being required to have good scaling and be improved in write speed to one or more orders of magnitude higher than the conventional write speed of 100 &mgr;s/cell.
As nonvolatile semiconductor memories, in addition to the floating gate (FG) types wherein the charge storing means (floating gate) that hold the charge is planarly formed, there are known MONOS (metal-oxide-nitride-oxide semiconductor) types wherein the charge storing means are planarly dispersed.
In an MONOS type nonvolatile semiconductor memory, since the carrier traps in the nitride film [Si
x
N
y
(0<x<1, 0<y<1)] or on the interface between the top oxide film and the nitride film, which are the main charge-retaining bodies, are spatially (that is, in the planar direction and thickness direction) dispersed, the charge retention characteristic depends on not only the thickness of a tunnel insulating film (bottom insulating film), but also on the energy and spatial distribution of the charges captured by the carrier traps in the Si
x
N
y
film.
When a leakage current path is locally generated in the tunnel insulating film, in an FG type, a large amount of charges easily leak out through the leakage path and the charge retention characteristic declines. On the other hand, in an MONOS type, since the charge storing means are spatially dispersed, only the charges near the leakage path will locally leak from it, therefore the charge retention characteristic of the entire memory device will not decline much.
As a result, in a MONOS type, the disadvantage of the degradation of the charge retention characteristic due to the reduction in thickness of the tunnel insulating film is not so serious as in an FG type. Accordingly, a MONOS type is superior to an FG type in scaling of a tunnel insulating film in a miniaturized memory transistor with an extremely small gate length.
Moreover, when a charge is locally injected into the plane of distribution of the planarly dispersed charge traps, the charge is held without diffusing in the plane and in the thickness direction like in an FG type memory.
To realize a miniaturized memory cell in a MONOS type nonvolatile semiconductor memory, it is important to improve the disturbance characteristic. Therefore, it is necessary to set the tunnel insulating film thicker than the normal thickness of 1.6 nm to 2.0 nm. When the tunnel insulating film is formed relatively thick, the write speed is in the range of 0.1 to 10 mo, which is still not sufficient.
In other words, in a conventional MONOS type nonvolatile semiconductor memory etc., to fully satisfy the requirements of reliability (for example, data retention, read disturbance, data rewrite, etc.), the write speed is limited to 100 &mgr;s.
A high speed is possible if the write speed alone is considered, but sufficiently high reliability and low voltage cannot be achieved. For example, a source-side injection type MONOS transistor has been reported wherein the channel hot electrons (CHE) are injected from the source side (
IEEE Electron Device Letter,
19, 1999, p. 153). In this source-side injection type MONOS transistor, in addition to the high operation voltages of 12V for write operations and 14V for erasure operations, the road disturbance, data rewrite, and other facets of reliability are not sufficient.
On the other hand, taking note of the fact that it is possible to inject a charge into part of dispersed charge traps area by the conventional CHE injection method, it has been reported that by independently writing binary data into the source and drain side of a charge storing means, it is possible to record 2 bits of data in one memory cell. For example,
Extented Abstract of the
1999
International Conference on Solid State Devices and Materials,
Tokyo, 1999, pp. 522-523, considers that by changing the direction of the voltage applied between the source and drain to write 2 bits of data by injecting CHE and, when reading data, applying a specified voltage with a direction reversed to that for writing, i.e., the so-called “reverse read” method, correct reading of the 2 bits of data is possible even if the write time is short and the amount of the stored charge is small. Erasure is achieved by injecting hot holes.
By using this technique, it becomes possible to increase the write speed and largely reduce the cost per bit.
Furthermore, a split gate type MONOS nonvolatile memory able to record 2 bits in one cell was recently proposed (“Twin MONOS Cell with Dual Control Gates”, 2000
Symposium on VLSI Technology Digest of Technical Papers,
pp. 122-123).
In this MONOS type nonvolatile memory, a split gate structure is employed to provide a control gate electrode in addition to the gate electrode so as to try to increase the write speed. The principle of this write method is basically channel hot electron injection. Since the impurity concentration around the drain is made relatively high comparing with that at the center of the channel, the injection efficiency of hot electrons is greatly improved.
However, in a conventional CHE injection type MONOS type or 2 bit/cell recordable MONOS type nonvolatile semiconductor memory, since electrons are accelerated in the channel to produce high energy electrons (hot electrons), it is necessary to apply a voltage larger than the 3.2 eV energy barrier of the oxide film, in practice a voltage of about 4.5V, between the source and drain. It is difficult to decrease this source-drain voltage. As a result, in a write operation, the punch-through effect becomes a restriction and good scaling of the gate length is difficult.
In addition, with the CHE injection method, since the efficiency of charge injection into the charge storing means is as low as 1×10
−6
to 1×10
−5
, a write current of a few hundred &mgr;A is needed. As a result, there is another problem that it is impossible to write in parallel a large number of memory cells simultaneously. To solve this problem, the write current has been reduced to 10 &mgr;A per cell in the recently reported split gate type cells, but it is still difficult to write memory cells of more than 1 k bits in parallel because of the current restriction of the peripheral charge pump circuitry.
Moreover, with these three types of cells using the CHE injection method, because the write operation is performed with a current flowing in the channel of a memory transistor, it is impossible to simultaneously write at the source side and the drain side for the purpose of the aforesaid 2-bit data storage.
Furthermore, in the aforesaid 2-bit data recordable memory cells and split gate type memory cells, due to the necessity of local erasure, the method of erasure of injecting hot holes from the source or drain side utilizing FN tunneling or a band-to-band tunneling current has been employed. However, with this method, since passage of hot holes may cause deterioration of the oxide film, a decline in the reliability, in particular, the data rewrite, cannot be avoided.
Therefore, in a conventional MONOS type nonvolatile s
Ho Hoai V.
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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