Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-06-28
2004-04-06
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S295000, C257S296000, C257S414000, C365S117000, C365S185040, C365S145000
Reexamination Certificate
active
06717195
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ferroelectric memory and a method of manufacturing the ferroelectric memory, and more particularly to a ferroelectric memory comprising a ferroelectric film which is an inorganic dielectric film having a low dielectric constant.
2. Description of the Related Art
A ferroelectric memory which is currently studied is roughly divided into two parts. One of them is a memory using a method of detecting the inversed electric charge quantity of a ferroelectric capacitor and constituted by the ferroelectric capacitor and a selective transistor.
The other is a memory using a method of detecting a change in the resistance of a semiconductor by the spontaneous polarization of a ferroelectric. The method typically includes an MFSFET. The MFSFET has an MIS structure using a ferroelectric for a gate insulating film. With this structure, it is necessary to directly form a dielectric on the surface of the semiconductor and the interface of the ferroelectric/semiconductor is hard to control. For this reason, it is very hard to manufacture a memory element of high quality. Currently, a memory structure having a buffer layer provided on a ferroelectric/semiconductor interface is a mainstream. However, the inventors have proposed an FET having an MFMIS structure in which a metal layer (M) and an insulating layer (I) are provided as buffer layers on the ferroelectric/semiconductor interface as shown in FIG.
4
. The FET having the MFMIS structure comprises a gate oxide film
5
, a floating gate
6
, a ferroelectric film
7
and a control gate
8
which are sequentially provided on a channel region
4
formed between source and drain regions
2
and
3
of a semiconductor substrate
1
.
With this structure, usually, when the semiconductor substrate
1
is provided and a positive voltage is applied to the control gate
8
, the ferroelectric film
7
causes a polarization inversion. Even if the voltage of the control gate
8
is removed, a negative electric charge is generated in a channel formation region CH by the residual polarization of the ferroelectric film
7
. This state is set to be “1”.
To the contrary, when a negative voltage is applied to the control gate
8
, the ferroelectric film
8
causes a polarization inversion in a reverse direction. Even if the voltage of the control gate
8
is removed, a positive electric charge is generated in the channel formation region CH by the residual polarization of the ferroelectric film
8
. This state is set to be “0”. Thus, information “1” or “0” can be written to the FET.
The written information is read by the application of a reading voltage V
r
to the control gate
8
. The reading voltage V
r
is set to be a value between a threshold voltage V
th1
in the state of “1” and a threshold voltage V
th0
in the state of “0”. By detecting that a drain current flows or not when the reading voltage V
r
is applied to the control gate
8
, it is possible to decide whether the written information is “1” or “0”.
According to the FET having the MFMIS structure, thus, one memory cell can be constituted by one element and nondestructive readout can be carried out well.
However, such an FET having the MFMIS structure has the following problem. At time of writing, the FET has such a configuration that a capacitor C
f
(a capacitance C
f
) of the ferroelectric film
7
and a capacitor C
ox
(a capacitance C
ox
) of the gate oxide film
5
are connected in series (see FIG.
5
). C
D
represents a drain capacitance and is disregarded herein. In the case in which a voltage V is applied between the substrate
1
and the control gate
8
, therefore, the voltage V is divided into V
f
and V
ox
so that the following equation (1) is obtained.
V=V
f
+V
ox
C
f
V
f
=C
ox
V
ox
=q
(
q
: quantity of electric charge generated by capacitor) (1)
Accordingly, a partial pressure V
f
expressed in the following equation is applied to the capacitor C
f
of the ferroelectric film
7
.
V
f
=C
ox
/(
C
f
+C
ox
)·
V
(2)
On the other hand, it is necessary to increase V
f
to some extent in order to carry out a polarization inversion over the ferroelectric film
7
during writing.
Accordingly, it is necessary to reduce the capacitance of the ferroelectric film against the capacitance of the gate insulating film. For example, however, there is a problem in that the relative dielectric constant of PZT is approximately 200 to 1000 and is much higher than a relative dielectric constant of 3.9 of a silicon oxide film constituting the gate insulating film.
For this reason, it is hard to increase the partial pressure V
f
in the equation (1). Accordingly, there is a problem in that it is hard to carry out the polarization inversion over the ferroelectric film
7
during writing.
SUMMARY OF THE INVENTION
In order to solve the problem, it is necessary to reduce the thickness of a film in order to decrease the relative dielectric constant of the ferroelectric film as much as possible. By reducing the thickness of the film, thus, it is possible to increase the partial pressure V
f
. On the other hand, when the thickness of the film is reduced, a leakage current is actually generated between the floating gate and the control gate, causing a deterioration in a memory characteristic.
In order to increase the speed of a ferroelectric memory and to reduce power consumption, thus, it is an important object to reduce a dielectric constant with a decrease in the relative dielectric constant of a ferroelectric film.
The insulating film has variously been devised in order to reduce a dielectric constant. In general, the following methods have conventionally been proposed to reduce the dielectric constant of the insulating film:
(1) to add fluorine to a silica film to be an inorganic insulating film;
(2) to form an organic insulating material having a low dielectric constant as a parent material; and
(3) to intentionally form a porous film.
In the case of the method (1), however, since the heat resistance of the insulating film is deteriorated, the addition is carried out in an element ratio of several % at most. Consequently, there is a problem in that the relative dielectric constant can be reduced by only 10 to 15% of that of a conventional insulating film.
In the case of the method (2), moreover, the organic material is formed. For this reason, there is a problem in that the heat resistance is considerably deteriorated as compared with a conventional silica based insulating film, resulting in a reduction in the reliability of a semiconductor element. Therefore, the organic material cannot be applied to a ferroelectric film at all.
In the case of the (3), furthermore, since a porous structure is random, the mechanical strength of the insulating film is remarkably reduced and the insulating film is apt to be broken in packaging, causing a reduction in the reliability of a semiconductor element.
In many cases, moreover, the porous structure is not closed. If the porous structure is not closed, the moisture resistance of the insulating film is remarkably deteriorated, causing a reduction in the reliability of the semiconductor element.
In the conventional insulating film as well as the ferroelectric film, thus, there is a problem in that the dielectric constant cannot be reduced sufficiently, and furthermore, the mechanical strength is also insufficient.
In consideration of the circumstances, it is an object of the invention to reduce a leakage current and to enhance the data holding characteristic of a ferroelectric memory in order to increase the speed of the ferroelectric memory and to decrease power consumption.
More specifically, it is an object of the invention to provide a ferroelectric film having a low dielectric constant and a high mechanical strength.
The invention is characterized in that a ferroelectric layer is constituted by an inorganic film having a vacancy rate of 50% or more in an FET having an MFMIS structure.
More specifically, the invention
Merchant & Gould P.C.
Rohm & Co., Ltd.
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