Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2000-05-17
2003-05-13
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S325000
Reexamination Certificate
active
06563163
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nonvolatile memory using a semiconductor device.
2. Related Art Statement
Among memory devices using semiconductor devices, a memory device in which stored data is maintained without performing any special maintaining operation is called a nonvolatile memory.
A nonvolatile memory using a Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) has become much more important in accordance with a progress of the information-oriented society.
FIG. 1
is a cross sectional view showing a known floating gate type nonvolatile memory which has been used widely at present. A tunnel oxide film
106
is provided on a semiconductor substrate
101
in a surface of which are formed junction regions
102
and
103
for source electrode
104
and drain electrode
105
, respectively, and a floating gate
107
made of a conductive polysilicon is formed on the tunnel oxide film
106
. On the floating gate
107
, an oxide film
108
and a control gate
109
are formed successively.
A substrate electrode
111
is provided to be connected to a rear surface of the semiconductor substrate
101
, and a gate electrode
110
is formed to be connected to the control gate
109
.
A voltage is applied across the substrate electrode
111
and the gate electrode
110
or across the substrate electrode
111
and the drain electrode
105
to inject carriers into the floating gate
107
through the tunnel oxide film
106
and the thus injected carriers are contained by the oxide film
108
between the control gate
109
and the floating gate
107
.
When carriers are contained in the floating gate
107
, a threshold voltage for a drain current assumes a first value, and when no carrier is contained, the threshold voltage assumed a second value which is different from said first value. In this manner, the memory device has two conditions in its characteristics. Then, information data can be stored memorized as these two conditions.
In the floating gate type nonvolatile memory explained above, it is important that carriers could be injected into the floating gate efficiently and the once injected carriers could be retained in the floating gate. By reducing a thickness of the tunnel oxide film
106
, it is possible to raise the carrier injection efficiency, and the device property can be improved. However, this causes the deterioration of the tunnel oxide film, and the proper operation of the memory circuit might be lost. Therefore, a thickness of the tunnel oxide film could not be reduced beyond a certain limit in order to attain a reliability of the tunnel oxide film, and as a result, the operating voltage must be high for injecting a sufficient amount of carriers.
In order to retain the injected carriers, it is required that the oxide film
108
between the control gate
109
and the floating gate
107
has a certain thickness. It is apparent that the thick oxide film
108
might reduce the efficiency of the carrier injection into the floating gate
107
from the tunneling oxide film
106
. In order to increase the carrier injection efficiency even increasing a thickness of the oxide film
108
, it has been proposed to make a difference in a surface area between the control gate
109
and the floating gate
107
. However, this solution make a structure of the memory device and a miniaturization of the memory device could not be attained.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide a nonvolatile memory having a novel structure in which an operation voltage can be lowered, and miniaturization of the memory device can be realized easily.
According to the invention, a nonvolatile memory comprises a semiconductor substrate having at least one step formed in a first surface thereof; source and drain junction regions formed in the first surface of the semiconductor substrate on respective sides of said step; source and drain electrodes formed on said source and drain junction regions, respectively; an oxide film formed on the first of the semiconductor substrate to have a corner structure corresponding to said step of the semiconductor substrate; and a gate provided on said oxide film such that information is stored in the memory in accordance with a change in a threshold voltage due to carriers electrically injected into said gate film and captured therein.
As mentioned in H. J. Mattausch et al., Appl. Phys. Lett., vol. 71, p. 3391 (1997), it has been known that a stable carrier capturing is performed at the corner structure in comparison with the plane part. Such a phenomenon is called a deep level capture. This deep level capture is very stable, and even if a heating treatment at a higher temperature is performed, the capture of carriers at the corner structure is stable.
In the nonvolatile memory according to the invention, carriers are injected into the oxide film by an application of an electric field across the semiconductor substrate and the gate or across the gate and the drain. Though a part of the injected carriers flows into the gate electrode, a remaining part of the injected carriers is captured in the oxide film stably to change a threshold voltage for the drain current.
In the known floating gate type nonvolatile memory, carriers are injected into the floating gate through the tunnel oxide film and are contained therein. Contrary to this, in the memory according to the invention, carriers are captured in the oxide film. Therefore, the memory can be operated at a lower voltage than the conventional floating gate type nonvolatile memory.
Furthermore, according to the invention, it is no more necessary to provide the oxide film for containing carriers, and thus the memory has a very simple structure. Therefore, the manufacturing cost of the nonvolatile memory according to the invention can be reduced greatly.
An optimum operating voltage of the nonvolatile memory according to this invention is determined by a thickness of the oxide film. In other words, a thickness of the oxide film can be determined by a desired operating voltage. It should be noted that when the oxide film is thinner than 10 nm, the deep level capture of carriers is decreased abruptly. Therefore, it is preferable that a thickness of the oxide film should not be less than 10 nm.
In an embodiment of the nonvolatile memory according to the invention, a thickness of the oxide film is not less than 10 nm.
As mentioned above, in the conventional floating gate type nonvolatile memory, the tunnel oxide file must be made thin in order to improve the device property. In the nonvolatile memory according to the invention, since carriers are captured in the oxide film, if the oxide film is too thin, carriers could not be effectively captured by the oxide film and the memory device could not work satisfactorily. Therefore, the nonvolatile memory according to the invention can be manufactured easily, precisely and reliably by means of the well developed manufacturing technology, and the manufacturing cost can be reduced.
In the memory according to the invention, the drain junction region is formed to extend under the oxide film. In this case, a distance from the corner to an inner edge of the drain junction region is determined in accordance with a channel length and a desired property of MOSFET. Particularly, it is preferred to extend the drain junction region under the oxide film such that the inner edge of this region is apart from the corner not less than 0.1 &mgr;m.
REFERENCES:
patent: 4672577 (1987-06-01), Hirose et al.
patent: 5274588 (1993-12-01), Manzur et al.
patent: 5278440 (1994-01-01), Shimoji
patent: 5519653 (1996-05-01), Thomas
patent: 5621683 (1997-04-01), Young
patent: 5753951 (1998-05-01), Geissler
patent: 47-26937 (1972-10-01), None
patent: 53070682 (1978-06-01), None
patent: 55-34432 (1980-03-01), None
patent: 61003450 (1986-01-01), None
patent: 363248176 (1988-10-01), None
patent: 1-18268 (1989-01-01), None
patent: 3100991 (1991-04-01), None
patent: 07115142 (1995-05-01), None
patent: WO 92/02885 (1992-02
Mattausch Hans Jürgen
Miura Michiko
Ono Tsuyoshi
Alston & Bird LLP
Fahmy Wael
Hiroshima University
Pham Hoai
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