Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1996-05-20
2003-02-04
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S411000, C438S003000
Reexamination Certificate
active
06515322
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to a nonvolatile semiconductor memory, and more specifically to a nonvolatile semiconductor memory configured to store information by utilizing polarization of a ferroelectric material.
2. Description of Related Art
A nonvolatile semiconductor memory utilizing a ferroelectric material as a medium for storing information, has an advantage that if deterioration of a ferroelectric material film does not occur, information can be stored for a long term of time, and further, is expected that it can make it possible to reduce a memory cell size and therefore to realize a memory having a large storage capacity.
Reza Moazzami et al, “A Ferroelectric DRAM Cell for High-Density NVRAM's”, IEEE ELECTRON DEVICE LETTERS, Vol. 11, No.10, October 1990, Pages 454-456, (the disclosure of which is incorporated by reference in its entirety into this application) proposed one example of the above mentioned conventional nonvolatile semiconductor memory, in which a capacitor dielectric of a DRAM memory cell capacitor is formed of lead zirconate titanate (PbZr
1−x
Ti
x
O
3
)
Referring to
FIG. 1
, there is shown a diagrammatic sectional view of the nonvolatile semiconductor memory proposed by Reza Moazzami et al. On a principal surface of a P-type silicon substrate
101
, a device isolation oxide film (field oxide)
102
is formed by a selective oxidation such as a LOCOS (local oxidation of silicon) process, and within an active region confined by the device isolation oxide film, a gate electrode
103
is formed through a gate insulator film on the surface of the substrate. A source region
104
and a drain region
105
are formed in a surface region of the substrate at opposite sides of the gate electrode
103
, so as to locate the gate electrode between the source region and the drain region. A first interlayer insulator film
106
is formed to cover a whole surface of the substrate, and a Pt film
107
is formed on the first interlayer insulator film
106
, above a position of the gate electrode
103
. Furthermore, a PZT (PbZr
1−x
Ti
x
O
3
) film
108
is formed to cover the Pt film
107
. A second interlayer insulator film
109
is formed to cover a whole surface of the substrate including the PZT film
108
. In addition, contact holes are formed to reach the drain region
105
and the PZT film
108
, respectively, and an aluminum wiring conductor
110
is formed on the second interlayer insulator film
109
to contact with the drain region
105
and the PZT film
108
through the contact holes.
Referring to
FIG. 2
, there is shown a diagrammatic section view illustrating another example of a conventional nonvolatile semiconductor memory utilizing a ferroelectric material film, in which a gate insulator film of a transistor is formed of a ferroelectric material film.
As shown in
FIG. 2
, a device isolation oxide film
2
is formed on a principal surface of a P-type silicon substrate
1
, and a ferroelectric material film
4
C is formed on the principal surface of the substrate
1
to constitute a gate insulator film. A gate electrode
5
A is formed on the ferroelectric material film
4
C, and a source region
7
and a drain region
8
are formed in a surface region of the substrate at opposite sides of the gate electrode
5
A, so as to locate the gate electrode between the source region and the drain region.
This structure is very effective in reducing the cell size, since the transistor itself has a memory part. The ferroelectric material of the gate insulator film, which is now under consideration, is BaMgF
4
and PbZr
1−x
Ti
x
O
3
.
A construction and an operation principle of this type memory cell is discussed in, for example, “Nonvolatile Memory FET Utilizing A Ferroelectric Material Thin Film”, Report of (Japanese) Society of Electronic Communication, CPM-78-46: 1, 1978, the disclosure of which is incorporated by reference in its entirety into this application.
In the conventional memory cell shown in
FIG. 1
, since the electrode underlying the ferroelectric material film has to be formed of a material such as Pt, which is hard to etch or pattern, a fine patterning is difficult. In addition, since each memory cell consists of a transistor part and a memory part, the structure is complicated. This is inconvenient to microminiaturization.
On the other hand, in the second conventional example shown in
FIG. 2
, since a material, such as PbZr
1−x
Ti
x
O
3
, having a high dielectric constant, is used as the ferroelectric material, it is difficult to form a highly reliable device. In addition, it is difficult to realize a low voltage driving, which is recently strongly demanded by users. The reason for these disadvantages will be described in the following.
In the case that a PbZr
1−x
Ti
x
O
3
film is used as the gate insulating film, when the PbZr
1−x
Ti
x
O
3
film is deposited directly on a silicon substrate, a natural oxide (or native oxide) layer having a thickness of about 2 nm is inevitably formed at a boundary of the silicon substrate.
A coercive electric field (applied electric field when polarization reversal starts) of the PbZr
1−x
Ti
x
O
3
film is on the order of 80 kV/cm, and a dielectric constant of the PbZr
1−x
Ti
x
O
3
film is on the order of 1000. On the other hand, a dielectric constant of a silicon oxide film is on the order of 4. Therefore, when the coercive electric field is applied across the PbZr
1−x
Ti
x
O
3
film, an electric field as high as 20 MV/cm {=80 kV/cm×(1000/4)} is applied across the natural oxide film. However, since the natural oxide film is not an intentionally formed film, the natural oxide film is not so good in film quality, so that there is high possibility that if a high electric field as mentioned above is applied, the natural oxide film is broken down.
Here, assuming that the PbZr
1−x
Ti
x
O
3
film is formed to have a thickness of 100 nm, it is necessary to apply a voltage of 0.8 V across the PbZr
1−x
Ti
x
O
3
film in order to apply a necessary coercive electric field. Incidentally, in order to cause a complete polarization reversal, it is necessary to apply a voltage which is higher than 0.8 V by several ten percents. At this time, on the other hand, a voltage of 4 V (=20 MV/cm×2 nm) is applied across the natural oxide film. Therefore, it is necessary to apply a voltage of 5 V or more to the gate electrode in order to cause the polarization reversal. This means that it is difficult to operate an actual device with a low voltage.
On the other hand, if the gate insulator film is formed of BaMgF
4
, no natural oxide film is formed since BaMgF
4
does not include an oxidizing specie. However, polarizability of this material is relatively low. In addition, if the BaMgF
4
film contains a crystal defect, the polarizability further lowers. Therefore, in order to constitute a satisfactory memory, it is necessary to form a BaMgF
4
film having an excellent film quality, namely, less crystal defect. However, this is not so easy because of difference in lattice constant between BaMgF
4
and a silicon substrate and because of other causes.
Since there exist ferroelectric materials other than oxides having a low dielectric constant, it is possible to prevent formation of the natural oxide by using the ferroelectric materials other than oxides. However, these ferroelectric materials are small in polarizability and poor in heat resistive property, and therefore, it is difficult to use these ferroelectric materials as a material used for manufacturing a semiconductor device.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a nonvolatile semiconductor memory which has overcome the above mentioned defects of the conventional ones.
Another object of the present invention is to provide a nonvolatile semiconductor memory configured to store information by utilizing polarization of a ferroelectric material, the nonvolatile semiconductor
Chaudhuri Olik
Weiss Howard
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