Method for driving semiconductor memory

Details Method for driving semiconductor memory Method for driving semiconductor memory

2001-07-09

2002-07-16

Elms, Richard (Department: 2824)

Static information storage and retrieval

Systems using particular element

Ferroelectric

C365S149000

Reexamination Certificate

active

06421268

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a method for driving a semiconductor memory including a ferroelectric capacitor.
A known semiconductor memory including a ferroelectric capacitor is composed of, as shown in
FIG. 6
, a field effect transistor (hereinafter referred to as the FET)
1
having a drain region
1
a
, a source region
1
b
and a gate electrode
1
c
, and a ferroelectric capacitor
2
having an upper electrode
2
a
, a lower electrode
2
b
and a ferroelectric film
2
c
. This semiconductor memory employs the non-destructive read-out system in which the lower electrode
2
b
of the ferroelectric capacitor
2
is connected to the gate electrode
1
c
of the FET
1
, so as to use the ferroelectric capacitor
2
for controlling the gate potential of the FET
1
. In
FIG. 6
, a reference numeral
3
denotes a substrate.
In writing a data in this semiconductor memory, a writing voltage is applied between the upper electrode
2
a
of the ferroelectric capacitor
2
, which works as a control electrode, and the substrate
3
.
For example, when a data is written by applying a voltage (control voltage) positive with respect to the substrate
3
to the upper electrode
2
a
, downward polarization is caused in the ferroelectric film
2
c
of the ferroelectric capacitor
2
. Thereafter, even when the upper electrode
2
a
is grounded, positive charge remains in the gate electrode
1
c
of the FET
1
, and hence, the gate electrode
1
c
has positive potential.
When the potential of the gate electrode
1
c
exceeds the threshold voltage of the FET
1
, the FET
1
is in an on-state. Therefore, when a potential difference is induced between the drain region
1
a
and the source region
1
b
, a current flows between the drain region
1
a
and the source region
1
b
. Such a logical state of the ferroelectric memory is defined, for example, as “1”.
On the other hand, when a voltage negative with respect to the substrate
3
is applied to the upper electrode
2
a
of the ferroelectric capacitor
2
, upward polarization is caused in the ferroelectric film
2
c
of the ferroelectric capacitor
2
. Thereafter, even when the upper electrode
2
a
is grounded, negative charge remains in the gate electrode
1
c
of the FET
1
, and hence, the gate electrode
1
c
has negative potential. In this case, the potential of the gate electrode
1
c
is always lower than the threshold voltage of the FET
1
, the FET
1
is in an off-state. Therefore, even when a potential difference is induced between the drain region
1
a
and the source region
1
b
, no current flows between the drain region
1
a
and the source region
1
b
. Such a logical state of the ferroelectric memory is defined, for example, as “0”.
Even when the power supply to the ferroelectric capacitor
2
is shut off, namely, even when the voltage application to the upper electrode
2
a
of the ferroelectric capacitor
2
is stopped, the aforementioned logical states are retained, and thus, a nonvolatile memory is realized. Specifically, when power is supplied again to apply a voltage between the drain region
1
a
and the source region
1
c
after shutting off the power supply for a given period of time, a current flows between the drain region
1
a
and the source region
1
b
if the logical state is “1”, so that the data “1” can be read, and no current flows between the drain region
1
a
and the source region
1
b
if the logical state is “0”, so that the data “0” can be read.
In order to correctly retain a data while the power is being shut off (which characteristic for retaining a data is designated as retention), it is necessary to always keep the potential of the gate electrode
1
c
of the FET
1
to be higher than the threshold voltage of the FET
1
when the data is “1” and to always keep the potential of the gate electrode
1
c
of the FET
1
at a negative voltage when the data is “0”.
While the power is being shut off, the upper electrode
2
a
of the ferroelectric capacitor
2
and the substrate
3
have ground potential, and hence, the potential of the gate electrode
1
c
is isolated. Therefore, ideally, as shown in
FIG. 7
, a first intersection c between a hysteresis loop
4
obtained in writing a data in the ferroelectric capacitor
2
and a gate capacitance load line
7
of the FET
1
obtained when a bias voltage is 0 V corresponds to the potential of the gate electrode
1
c
obtained in storing a data “1”, and a second intersection d between the hysteresis loop
4
and the gate capacitance load line
7
corresponds to the potential of the gate electrode
1
c
obtained in storing a data “0”. In
FIG. 7
, the ordinate indicates charge Q appearing in the upper electrode
2
a
(or the gate electrode
1
c
) and the abscissa indicates voltage V.
Actually, however, the ferroelectric capacitor
2
is not an ideal insulator but has a resistance component, and hence, the potential of the gate electrode
1
c
drops through the resistance component. This potential drop is exponential and has a time constant obtained by multiplying parallel combined capacitance of the gate capacitance of the FET
1
and the capacitance of the ferroelectric capacitor
2
by the resistance component of the ferroelectric capacitor
2
. The time constant is approximately 10
4
seconds at most. Accordingly, the potential of the gate electrode
1
c
is halved within several hours.
Since the potential of the gate electrode
1
c
is approximately 1 V at the first intersection c as shown in
FIG. 7
, when the potential is halved, the potential of the gate electrode
1
c
becomes approximately 0.5 V, which is lower than the threshold voltage of the FET
1
(generally of approximately 0.7 V). As a result, the FET
1
that should be in an on-state is turned off in a short period of time.
In this manner, although the ferroelectric memory using the ferroelectric capacitor for controlling the gate potential of the FET has an advantage that a rewrite operation is not necessary after a data read operation, it has the following problem: The gate electrode
1
a
of the FET
1
obtains potential after writing a data, and the ability for keeping the gate potential determines the retention characteristic. Since the time constant until discharge of the ferroelectric capacitor
2
is short due to the resistance component of the ferroelectric capacitor
2
, the data retaining ability is short, namely, the retention characteristic is not good.
SUMMARY OF THE INVENTION
In consideration of the aforementioned conventional problem, an object of the invention is improving the retention characteristic of a semiconductor memory including a ferroelectric capacitor for storing a multi-valued data in accordance with displacement of polarization of a ferroelectric film thereof.
In order to achieve the object, the first method of this invention for driving a semiconductor memory including a ferroelectric capacitor for storing a multi-valued data in accordance with displacement of polarization of a ferroelectric film thereof and a reading field effect transistor that is formed on a substrate and has a gate electrode connected to a first electrode corresponding to one of an upper electrode and a lower electrode of the ferroelectric capacitor for detecting the displacement of the polarization of the ferroelectric film, comprises a first step of writing a multi-valued data in the ferroelectric capacitor by applying a relatively high first writing voltage or a relatively low second writing voltage between the first electrode and a second electrode corresponding to the other of the upper electrode and the lower electrode of the ferroelectric capacitor; a second step of removing a potential difference induced between the first electrode and the second electrode; and a third step of reading the multi-valued data by detecting the displacement of the polarization of the ferroelectric film by applying a reading voltage between the second electrode and the substrate, and the reading voltage has the same polarity as the first writing voltage and is set to such magn

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