Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-05-12
2001-09-04
Wojciechowicz, Edward (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S310000, C257S532000, C361S313000, C361S321500
Reexamination Certificate
active
06285051
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor memory device for storing electrical charges as information in a capacitive insulating film made of a material with a high relative dielectric constant, and also relates to a method for fabricating the same.
As the multimedia applications have been broadening in recent years, it has become more and more necessary to store and process an increasing quantity of digital information using various types of information processors. Under the circumstances such as these, the quantity of information to be processed per unit time, i.e., the capacity of each single semiconductor memory device used for storing that information thereon, would continue to rise by leaps and bounds. However, in a dynamic random access memory (DRAM), for example, as the required capacity increases, the charge storage capacity of each cell should be maintained at an approximately conventional level (i.e., about 30 fF), even though the size of each cell should be reduced to satisfy a downsizing requirement. Accordingly, to ensure a sufficient capacity and reduce the size of a cell at the same time, various materials with a high relative dielectric constant, such as barium strontium titanate (BST), have lately been applied to the capacitive insulating film of a memory cell more and more often. In this specification, a capacitive insulating film of this type, which is made of a material with a high dielectric constant and in which electrical charges can be stored as information, will be called a “charge-storable dielectric film”.
Nevertheless, since a BST film is usually in the form of a polycrystalline thin film, it is important to suppress the leakage current flowing therethrough.
To solve such a problem, a “multi-layered dielectric film” is used as disclosed in Japanese Laid-Open Publication No. 7-161833, for example.
FIG. 8
illustrates a cross-sectional structure of a charge-storable dielectric film disclosed in the publication identified above. As shown in
FIG. 8
, this structure includes: a titanium (Ti) film
101
; a titanium dioxide (TiO
2
) film
102
; a high-dielectric-constant film
103
made of (Ba, Pb)(Zr, Ti)O
3
; a TiO
2
film
104
; and a Ti film
105
, which are stacked in this order on a substrate
100
.
In this conventional DRAM structure, a charge-storable dielectric film is formed by interposing a pair of TiO
2
films
102
and
104
between the high-dielectric-constant film
103
and the Ti films
101
and
105
functioning as respective electrodes, thereby suppressing the leakage current flowing through the high-dielectric-constant film
103
. A smaller amount of current leaks through the TiO
2
films
102
and
104
as compared with the (Ba, Pb) (Zr, Ti)O
3
film
103
, and the TiO
2
films
102
and
104
have a higher dielectric breakdown voltage than the (Ba, Pb)(Zr, Ti)O
3
film
103
. Accordingly, a charge-storable dielectric film can be formed with a leakage current reduced in total.
However, in this conventional DRAM structure, the highdielectric-constant film
103
, which is originally intended to be a capacitive insulating film, is sandwiched between the TiO
2
films
102
and
104
to reduce the leakage current. As a result, the relative dielectric constant of these three insulating films decreases in total. This is because the relative dielectric constant of the TiO
2
film is as low as about 25. Also, the total relative dielectric constant of these three insulating films interposed between the upper and lower Ti films
101
and
105
is calculated the same way as the capacitance of a circuit in which three capacitors, formed by these insulating films, are connected in series to each other. Thus, the DRAM structure shown in FIG.
8
and disclosed in the above-identified document has decreased charge storability er unit area. This is one of the factors interfering with he downsizing of an overall memory structure.
SUMMARY OF THE INVENTION
An object of the present invention is providing a downsized semiconductor memory device and a method for fabricating the same by suppressing an increase in leakage current flowing through a charge-storable dielectric film and a decrease in relative dielectric constant thereof.
A semiconductor memory device according to the present invention includes a plurality of cells. Each of these cells includes: a lower electrode; a charge-storable dielectric film, formed on the lower electrode and made of a dielectric in which information is storable; and an upper electrode formed on the charge-storable dielectric film. The chargestorable dielectric film includes first and second dielectric layers. The first dielectric layer is made of an oxide containing at least two metal elements and a dielectric with a high relative dielectric constant or a ferroelectric. The second dielectric layer is formed over and/or under the first dielectric layer and made of an oxide containing at least two metal elements. A smaller amount of leakage current than that flowing through the first dielectric layer flows through the second dielectric layer.
In this structure, the second dielectric layer is an oxide film containing at least two metal elements, not an oxide film containing a single metal element such as a TiO
2
film conventionally used. Thus, by adjusting the composition of the second dielectric layer, the relative dielectric constant and the amount of leakage current of the second dielectric layer can be appropriately controlled. For example, if the mole fraction of one of these two metal elements, which can contribute to the suppression of the leakage current better than the other, is increased to precipitate an oxide of the metal element into the grain boundary, then the leakage current can be suppressed effectively. And since the crystal grains themselves are those of a binary metal oxide with a higher relative dielectric constant, the total dielectric constant of the charge-storable dielectric film can be kept high. Accordingly, the semiconductor memory device can be further downsized.
In one embodiment of the present invention, the first dielectric layer preferably has an approximately stoichiometric composition, while the second dielectric layer preferably has a composition deviated from the stoichiometric composition thereof.
In such an embodiment, since the second dielectric layer preferably has a non-stoichiometric composition, an oxide of one of at least two metal elements contained in the second dielectric layer precipitates into the grain boundary. Accordingly, a high relative dielectric constant can be easily maintained with the leakage current suppressed.
In another embodiment, the first dielectric layer preferably contains the three elements of Ba, Sr and Ti.
A material containing these three elements of Ba, Sr and Ti is known as a material called “BST” with a high dielectric constant. By forming the first dielectric layer out of this BST layer, the total dielectric constant of the chargestorable dielectric film can be kept high.
In still another embodiment, the first dielectric layer preferably has a composition expressed by Ba
x
Sr
1−x
TiO
3
, where 0<x<1.
In such an embodiment, the first dielectric layer is a BST film containing Ti at 50%. Accordingly, a high relative dielectric constant can be attained.
In still another embodiment, the second dielectric layer preferably contains the three elements of Ba, Sr and Ti.
In such an embodiment, since the relative dielectric constant of the second dielectric layer is also kept high, increase in leakage current flowing through the entire chargestorable dielectric film and decrease in relative dielectric constant thereof can be suppressed with more certainty.
In still another embodiment, the number of Ti atoms preferably accounts for larger than 50% and equal to or smaller than 60% of the total number of Ba, Sr and Ti atoms in the second dielectric layer.
It is already known empirically that the dielectric constant of a BST film is relatively high and the amount of leakage current is relatively small within this range.
Ohtsuka Takashi
Ueda Michihito
Harness & Dickey & Pierce P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Wojciechowicz Edward
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