Semiconductor device manufacturing: process – Making passive device – Stacked capacitor
Reexamination Certificate
2000-05-12
2002-08-20
Christianson, Keith (Department: 2813)
Semiconductor device manufacturing: process
Making passive device
Stacked capacitor
C438S003000, C438S238000, C438S381000
Reexamination Certificate
active
06436786
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to a semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a semiconductor device, including a capacitor, which can suppress leakage current sufficiently, and to a method for fabricating the device.
As a semiconductor device has been downsized, it has become more and more necessary to improve its performance by taking full advantage of the properties of materials for the device. For example, if the number of devices integrated on a single chip should be increased for a dynamic or ferroelectric random access memory (i.e., DRAM or FeRAM), which stores information thereon using transistors and capacitors in combination, then not only the transistors but also capacitors should be miniaturized.
As for a capacitor, its capacitance should be at least equal to a predetermined value, and preferably more, even if its area on the chip has been reduced as a result of downsizing. This is because noise and soft error might seriously affect the performance of an overly downsized capacitor. A capacitor has often been made of a multi-layered dielectric film, e.g., an ONO film in which silicon dioxide and silicon nitride layers are stacked one upon the other. However, since a higher capacitance should be now attained from a capacitor of a reduced size, alternative materials with dielectric constants higher than that of ONO have been looked for. A film made of any of those high-dielectric-constant materials will be herein called a “functional material film”.
Examples of proposed functional material films with high dielectric constants include a perovskite oxide dielectric thin film of tantalum pentoxide (Ta
2
O
5
), strontium titanate (SrTiO
3
, which will be abbreviated as “STO”), barium titanate (BaTiO
3
, which will be abbreviated as “BTO”) or barium strontium titanate (Ba
x
Sr
1−x
TiO
3
, which will be abbreviated as “BST”).
However, if one of these functional materials is used for a capacitive insulating film of a capacitor, then the electrode of the capacitor cannot be made of polysilicon anymore. This is because those functional material films are often formed within an oxidizing ambient. That is to say, if polysilicon is exposed to the oxidizing ambient, then its surface is oxidized to form a silicon dioxide film with a relatively low dielectric constant. In such a situation, even if the capacitive insulating film is made of a material with a high dielectric constant, the effective quantity of storable charge decreases due to the existence of the silicon dioxide film with a low dielectric constant.
Thus, when one of those functional materials is used, the electrode is made of a noble metal such as platinum (Pt), ruthenium (Ru) or iridium (Ir).
An exemplary structure with a capacitor made of Ta
2
O
5
, is disclosed in Japanese Journal of Applied Physics, 37, (1998), pp. 1336-1339, while an exemplary structure with a capacitor made of BST is disclosed in Technical Digest of International Electron Device and Materials (1998), pp. 253-256.
Among these noble metal materials, Ru is particularly promising as an alternative electrode material. This is because ruthenium dioxide (RuO
2
), one of its oxides, is a conductor and because ruthenium tetroxide (RuO
4
), another oxide thereof, has a high vapor pressure at a low temperature and can be shaped by dry etching.
However, the higher the dielectric constant of such a functional material film, the higher the density of leakage current flowing between electrodes when a voltage is applied thereto. The increase in density of leakage current adversely decreases the quantity of stored charge with time. As a result, the charge is storable by a DRAM for a shorter period of time.
Next, a technique disclosed in Articles for the 54
th
Symposium on Semiconductor and Integrated Circuit Technology, pp. 12 to 17, will be briefly described. According to the technique disclosed in this article, a lower electrode of ruthenium is formed to a thickness of 100 nm on a thermal oxide film that has been formed on a silicon substrate, and then annealed for 30 seconds within a nitrogen ambient at 700° C.
Thereafter, a Ta
2
O
5
, film is formed to a thickness of 24 nm on the lower electrode by a chemical vapor deposition (CVD) process. In such a state, the composition of the Ta
2
O
5
film deviates from that defined by stoichiometry and the Ta
2
O
5
film contains Ta excessively. A very large amount of leakage current will flow as it is. To avoid such an unfavorable situation, the Ta
2
O
5
film is annealed for an hour within an oxygen ambient at 550° C. As a result of this annealing, the density of leakage current will be 1×10
−8
A/cm
2
for a field intensity of 1 MV/cm, and the dielectric constant will be 30.
According to another technique disclosed in the above-identified document, oxygen may also be supplemented by performing oxygen plasma annealing at 300° C. for 10 minutes. As a result of this oxygen plasma annealing, the dielectric constant will also be about 30 and the density of leakage current will be 1×10
−8
A/cm
2
for a field intensity of 1.5 MV/cm.
The prior art capacitor has a low leakage current value after having been annealed for an hour within an oxygen ambient at 550° C. or for 10 minutes within oxygen plasma at 300° C. However, at this point in time, the Ta
2
O
5
film has not been crystallized yet and has a dielectric constant as low as about 30, which is not so much greater compared to a conventional capacitor using an oxynitride film and a polysilicon electrode with a roughened surface. Thus, it is not advantageous to use the ruthenium electrode and the Ta
2
O
5
film.
To increase the dielectric constant, the Ta
2
O
5
film should be crystallized by being annealed at about 700° C. According to the technique disclosed in the above-identified article, the Ta
2
O
5
film is crystallized by being annealed for 60 seconds within a nitrogen ambient at 750° C . After the Ta
2
O
5
film has been crystallized, the electrical characteristics of the capacitor improve to a certain degree. Specifically, a sample that has been annealed for an hour within an oxygen ambient at 550° C. has a dielectric constant of about 60 and a leakage current density of 1×10
−5
A/cm
2
for a field intensity of 1 MV/cm.
A sample that has been annealed with oxygen plasma to supplement oxygen thereto has a dielectric constant of about 60 and a leakage current density of 1×10
−8
A/cm
2
for a field intensity of 1 MV/cm or 1×10
−6
A/cm
2
for a field intensity of −1 MV/cm. As can be seen, in supplementing oxygen, the oxygen plasma annealing attains a lower leakage current density compared to the annealing within the oxygen ambient. However, even the lower leakage current density is far from being sufficiently low.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to reduce the leakage current sufficiently when an electrode for a capacitor is made of a noble metal.
A more specific object of the present invention is to reduce the leakage current sufficiently for a capacitor including a functional material film and a noble metal electrode.
To achieve these objects, when a conductor film for an electrode, e.g., a ruthenium film, is crystallized, the crystal grains of ruthenium are grown to have stepped surfaces according to the present invention.
Specifically, a first inventive semiconductor device includes an electrode, which is formed over a substrate and contains ruthenium. Crystal grains of ruthenium contained in the electrode have stepped surfaces.
In the first semiconductor device, the crystal grains of ruthenium, which is a material for the electrode, have stepped surfaces. That is to say, since the surface area of each crystal grain is greater, the apparent dielectric constant increases compared to normal crystal grains with no stepped surfaces. In addition, a plane linking adjacent crystal grains together forms an obtuse angle with the surface of each adj
Mori Yoshihiro
Okuno Yasutoshi
Tsuzumitani Akihiko
Christianson Keith
Huynh Yennhu B.
Matsushita Electronics Corporation
Nixon & Peabody LLP
Studebaker Donald R.
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