Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1998-05-27
2001-06-12
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S450000, C428S701000, C428S702000, C427S255320, C427S585000, C427S596000, C423S263000, C423S275000, C423S593100, C423S599000, C423S605000, C365S117000, C365S145000, C204S192100
Reexamination Certificate
active
06245451
ABSTRACT:
TECHNICAL FIELD
This invention relates to a ferroelectric material and method of manufacturing the same, which is possible to constitute a non-volatile memory, a thin-film capacitor, an electrooptic device, and so on, and a semiconductor memory device using the same material. More particularly, the invention relates to a ferroelectric material having a basic structure of REMNO
3
(RE refers hereinafter to a lanthanoid group element) and method for forming a thin-film thereof, and to a semiconductor memory device using the thin-film and method of manufacturing the same.
BACKGROUND ART
Conventionally, there has been an FET having a structure formed by a metal film, a ferroelectric film and a semiconductor layer (hereinafter referred to as an MFS structure) as a typical memory (semiconductor memory device) that is adapted to detect a resistance change in the semiconductor layer due to residual polarization in the ferroelectric film. This FET uses a ferroelectric material for a gate dielectric film. As shown in FIGS.
4
(
a
)-(
b
) with residual polarization, an inversion layer is created in the channel region by ferroelectric residual polarization, enabling write-in. The memory of this type can perform reading out in a non-destructive manner, being advantageous in increasing the rewritable cycle life. In FIGS.
4
(
a
)-(
b
),
21
is a semiconductor substrate, for example, of a p-type.
22
,
23
are respectively source and drain regions formed by introducing an n
+
impurity.
26
is a channel region sandwiched between the source region
22
and the drain region
23
. The channel region
26
is formed thereon with a ferroelectric film
27
and a gate electrode
28
. FIG.
4
(
a
) illustrates an ON state that the gate electrode
28
is applied with a positive potential, while FIG.
4
(
b
) shows an OFF state that the gate electrode
28
is applied with a negative potential. This ferroelectric film
27
conventionally has an oxide perovskite structure such as of BaTiO
3
, PZT (Pb(Zr
1−x
Ti
x
)O
3
), PLZT (Pb
1−y
La
y
(Zr
1−a
Ti
a
)
1−y/4
O
3
).
In the MFS structure, however, when a ferroelectric film
27
is formed on an Si semiconductor substrate
21
, an unwanted film such as of SiO
2
is formed at an interface of between them. This causes not only increase in operating voltage but also injection of electric charges by appearance of trap into the ferroelectric film
27
. This in turns causes that electric charges are canceled by t he residual polarity. In order to avoid this problem, considerations are being given to a MFMIS structure having overlying layers of, from the above, a control electrode, a ferroelectric film, a floating gate, a gate oxide (SiO
2
) , and a Si substrate. This structure enables the ferroelectric material to be film-formed on an electrode metal material so that a ferroelectric film can be formed with proper matching onto the electrode by selecting a metal material.
If an oxide of the oxide-perovskite structure, other than REMNO
3
, is used as a ferroelectric material as the conventionally done, the Si substrate on which a dielectric film is to be directly formed will have an oxide film on the surface due to oxidation. This oxide film is low in dielectric constant and consumes a voltage much more than the ferroelectric film having a greater dielectric constant, raising a problem of requiring high write-in voltage. Further, there is a possibility that oxygen deficiency occurs in the conventionally-used oxide perovskite structured dielectric, resulting in change of valence number and hence increase of space charges. This raises a problem of lower in ferroelectric characteristics.
Meanwhile, the present inventors has proposed a use of REMNO
3
material for nonvolatile memories as disclosed in “Proposal of REMNO
3
Thin Film to Nonvolatile Memories” in 56th Applied Physics Academy Study Lecture Preliminary Paper, page 440 (published on Aug. 26, 1995), wherein the same material is an oxide of lanthanoid group elements RE including Y and Mn , and has an advantage of possessing a dielectric characteristic and a small dielectric constant. However, REMNO
3
is difficult to determine a film-forming condition for forming a complete crystalline structure. Therefore, it is poor in dielectric characteristic such as of leak current and hence not placed in practical applications.
The present invention has been made in view of such circumstances, and it is an object to provide a ferroelectric material that has a basic REMNO
3
structure having improved dielectric characteristics and excellent crystallinity, wherein where used for a semiconductor memory or the like the characteristics thereof can be improved.
It is another object of the present invention to provide a concrete film-forming method using a ferroelectric film by which it is possible to form a ferroelectric film having REMNO
3
thus improved in ferroelectric characteristics on a semiconductor substrate or the like.
It is further object of the present invention to provide a semiconductor memory device utilizing the inventive ferroelectric material and a method of manufacturing the same.
DISCLOSURE OF THE INVENTION
The present inventors has eagerly studied in order to obtain a ferroelectric film having a basic structure of REMNO
3
excellent in ferroelectric characteristics on an Si substrate or the like. As a result, it was found that if the ferroelectric material comprises RE and Mn one of which is contained in excess of the other to a limit of 20 at. % instead of the ratio thereof 1:1, it is possible to make the composition uniform and reduce leak current thereby improving ferroelectric characteristics.
Here, RE means a lanthanoid group element including Y, Er, Ho, Tm, Yb, Lu, etc. at. % means atomic %. For example, RE being in excess by 20 at. % means that RE and Mn are 1.2:1 in atomic % .
The present inventors has further eagerly studied in order to obtain a ferroelectric film having a basic structure of REMNO
3
to improve ferroelectric characteristics. As a result, it was found that REMNO
3
is small in bandgap and has a tendency to increase leak current and turn into a p-type due to a presence of somewhat carriers. It was found that the addition of a 4-valence element provides a fine and homogeneous texture and reduce leak current.
Here, the 4-valence element means an element that becomes ions with 4 valences when ionized.
The present inventors has further eagerly studied in order to put a ferroelectric film having a basic structure of REMNO
3
into practical application for a semiconductor memory, a thin-film capacitor, etc. on a cause of degradation in crystallinity and increase in leak current if formed on a semiconductor substrate or the like. As a result, found was the followings. Since RE and Mn tend to oxidize, if the oxygen partial pressure is high during vacuum deposition, laser abrasion, sputtering, or the like or if REMNO
3
is used as a target, RE and Mn in a material source state oxidize, or their oxides are created during scatter before reaching the surface of the substrate to be film-formed. The formation of RE-rich RE
2
O
3
, Mn-rich Mn
3
O
4
, REMn
2
O
5
, etc. are promoted. The target surface is changed of composition. If a film is formed in a form of these oxides on the substrate surface, the crystallinity deteriorates. If the oxygen partial pressure is lowered than the usual within the film-forming reactor to use a non-oxide target of a RE-Mn alloy and blowing an oxidizing source only to the vicinity of the substrate surface to be formed with a ferroelectric film, a nice crystalline structure of REMNO
3
is obtained without creating individual oxides of RE and Mn. Thus a ferroelectric film excellent in ferroelectric characteristics, e.g. reduction in leak current, can be formed. This can also prevent the composition from changing in the evaporation source or during growing a target.
Here, the oxidizing source means a gas or ion, such as oxygen, ozone, N
2
O, radical ion sources, that can cause an element present therewith to oxidize. Also, the oxygen parti
Fujimura Norifumi
Kamisawa Akira
Arent Fox Kintner & Plotkin & Kahn, PLLC
Jones Deborah
Rohm & Co., Ltd.
Stein Stephen
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