Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-12-14
2002-04-30
Jackson, Jr., Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S310000, C257S308000, C257S309000
Reexamination Certificate
active
06380573
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor memory device including a film formed of a ferroelectric material and a method for producing the same, and specifically to a non-volatile memory device including a film formed of a ferroelectric material and a method for producing the same.
2. Description of the Related Art
Non-volatile memory devices including a film formed of a ferroelectric material are roughly classified into two types. One type is referred to as a capacitor type device. and the other is referred to as an MFS (metal-ferroelectric-semiconductor) FBT device.
A capacitor type device has a capacitor structure including a ferroelectric thin film interposed between two electrodes. Information is read or written by detecting whether or not there is an inverted current which flows when the spontaneous polarization of the ferroelectric film is inverted. The capacitor type device mainly has the following two problems (1) since the stored information is destroyed when the information is read, rewrite of the information is required; and (2) each time the information is read, the spontaneous polarization is inverted, which causes fatigue.
The capacitor type device also has an advantage in that the ferroelectric film, which is provided on a Pt electrode or the like, tends to have relatively high quality. Due to this advantage, the capacitor type device has been developed for practical use. In order to secure a sufficient amount of signals required to determine the contents of the memory, oxide ferroelectric materials having a relatively high value of spontaneous polarization are the main targets of study. Such oxide ferroelectric materials are, for example, PZT (lead zirconate titanate) represented by Pb (Zr
x
Ti
1−x
)O
3
(0≦x≦1), and Bi-based layered oxides including BrBi
2
Ta
2
O
9
and Bi
4
Ti
3
O
12
.
MFSFET devices allow non-destructive information read. An MFSFET device includes an Si (silicon) substrate having, in a surface area thereof, a source region and a drain region each formed of an impurity diffusive region and a channel region sandwiched between the source region and the drain region; a gate insulating layer formed of a ferroelectric thin film provided on the channel region, and a gate electrode provided on the ferroelectric thin film. The ferroelectric thin film is included in place of a gate oxide film included in a usual MOS (metal-oxide-semiconductor)FET. An MFSFET utilizes the phenomenon, in which charges are exalted on a surface of the semiconductor substrate by spontaneous polarization of the ferroelectric thin film, to control the conductivity in the channel region.
Information is written into the MFSFET device by applying a positive or negative voltage between the gate electrode and the Si substrate to fix the polarization direction of the ferroelectric thin film. Information is read from the MFSFET device in a non-destructive manner by detecting the conductivity state of the channel region. The conductivity state of the channel region changes in accordance with the direction of the polarization direction of the ferroelectric thin film.
The ferroelectric thin film is required to have a polarization charge amount which to only sufficient to change the potential on the surface of the Si substrate Thus, the value of the spontaneous polarization required for the MFSFET device is smaller than that required for the capacitor type device.
As can be appreciated from the above description, the MFSFET device realizes non-destructive information read and accordingly does not require rewrite of the information as is required in the capacitor type device. Thus, the MFSFET device does not have the problem of the fatigue by the polarization which is unavoidable in the case of the capacitor type device. The MFSFET device also needs only a smaller memory cell area than the capacitor type device, and thus is highly suitable for higher integration.
However, the MFSFET device has the following problems in terms of stability.
Formation of a ferroelectric thin film of an oxide ferroelectric material of, for example, PZT, SrBi
2
Ta
2
O
9
or Bi
4
Ti
3
O
12
directly on the Si substrate usually includes a process of heat treatment performed at a temperature as high as 500° C. to 800° C. Accordingly, an element of the ferroelectric material, e.g., Pb or Bi, is diffused in the Si substrate to form an interface reaction layer; or an Si oxide film is formed at a surface of the Si substrate by oxygen. This undesirably results in, for example, deterioration in the crystallinity of the ferroelectric thin film or an increase in the interface state density (problem 1). When an Si oxide film or the like having a relatively low specific dielectric constant is formed, the effective voltage applied on the ferroelectric portion of the film is significantly decreased due to the relatively high specific dielectric constant of the oxide ferroelectric material (200 to 1,000). This results in an increase in the operating voltage (problem 2).
The oxide ferroelectric material is easily reduced by hydrogen gas sintering, etching using hydrogen (H), or metal or insulating film formation which are included in a usual Si-MOSFET process. As a result, the ferroelectric material is changed from an insulating material into a conductive material and thus changes the characteristics thereof. As appreciated from this, the oxide ferroelectric material is not properly usable in the usual Si-MOSFET process (problem 3).
It has been proposed to use a fluoride ferroelectric material such as, for example, BaMgF
4
having a specific dielectric constant as low as about 10 in place of the oxide ferroelectric material (see, for example, S. Sinharoy et al., J. Vac. Sci. Technol., A9(3), page 409, 1991). It is difficult to form a BaMgF
4
layer directly on the Si substrate in consideration of the coefficient of thermal expansion and the lattice constant of Si and BaMgF
4
. Therefore, it has been proposed to form a buffer layer formed of a fluoride between the Si substrate and the BaMgF
4
layer (Japanese Laid-Open Publication No. 8-55919).
Fluorine (F), which is an element of a fluoride, has problems in terms of stability that fluorine is easily diffused in the Si substrate to form an Si—F bond, thus increasing an interface level (problem 4) and that fluorine reacts with water (problem 5).
It has also been proposed to use an SrTiO
3
layer having a relatively high specific dielectric constant as the buffer layer. Like in the case of the oxide ferroelectric material, it is difficult to form an SrTiO
3
layer directly on the Si substrate without causing any interface reaction (problem 6). In order to avoid this, it is proposed to form an SrTiO
3
/SiO
2
two-layer buffer layer as disclosed in Japanese Laid-Open Publication No. 8-335580. However, as disclosed in Japanese Laid-Open Publication No. 7-38061, in the case where the SrTiO
3
layer is formed by sputtering, the fluorine contained in an SrTiO
3
target in a large amount is diffused in the SiO
2
layer during the layer formation or heat treatment performed after the layer formation. As a result, the Si—O bond is cut to form an Si—F bond. At this point, the oxygen (O) released from the Si—O bond it diffused in the Si/SiO
2
interface to increase the thickness of the SiO
2
layer, thus deteriorating the device characteristics (problem 7).
SUMMARY OF THE INVENTION
According to one aspect of the invention, a semiconductor memory device includes a semiconductor substrate having a channel therein; a gate insulating layer formed of a ferroelectric material provided on the semiconductor substrate; and a gate electrode provided on the gate insulating layer. The ferroelectric material includes a nitrogen (N) and at least one element selected from the group consisting of Mg, Sr, Ba and Ca.
In one embodiment of the invention, the ferroelectric material is represented by formula (1):
A
2
BN
3
(1)
where A is one element selected from the group consisting of Mg, Sr, Ba and Ca, and B is one element
Fenty Jesse A.
Jackson, Jr. Jerome
Matsushita Electronics Corporation
Renner Otto Boisselle & Sklar
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