Semiconductor device manufacturing: process – Making passive device – Stacked capacitor
Reexamination Certificate
2003-06-25
2004-11-16
Le, Dung A. (Department: 2818)
Semiconductor device manufacturing: process
Making passive device
Stacked capacitor
C438S253000, C257S295000
Reexamination Certificate
active
06818523
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor storage device and, more particularly, a semiconductor storage device having a capacitor using a high dielectric constant or ferroelectric film.
BACKGROUND ART
A conventional capacitor using a dielectric having a dielectric constant higher than that of a silicon oxide film or a ferroelectric having a dielectric constant higher than that of a silicon nitride film has a large capacitance per unit area. Applications require large capacitance with small area, particularly applications such as large-scaled DRAMs have been being examined. In such structures, the dielectric having a high dielectric constant is defined as a material having a dielectric constant higher than that of the silicon oxide film. Ferroelectric materials have a spontaneous polarization which can be inverted by an electric field. In particular, as ferroelectrics, complex-metal oxides such as (Ba, Sr)TiO
3
(hereinbelow, BST) and Pb(Zr, Ti)0
3
(hereinbelow, PZT) are being examined. In order to suppress degradation of the oxides upon film formation, a noble metal such as platinum having oxidation resistance is typically used as a lower electrode. Meanwhile, since an upper electrode is generally formed after film formation, in order to avoid reaction with the dielectric having a high dielectric constant in a heat treatment process after formation of a capacitor, the upper electrode is generally made of platinum.
For example, according to U.S. Pat. No. 5,005,102, a lower electrode has a structure of platinum/titanium nitride/titanium and an upper electrode has a structure of aluminum/titanium/platinum. In particular, with respect to the upper electrode, it is described that aluminum serves as an electrical contact layer, titanium serves as a diffusion barrier layer, and platinum serves as a plate layer.
In the case of fabricating a memory using those elements, after forming a capacitor, a wiring layer for electrically connecting to the capacitor and a wiring layer related to a peripheral circuit part for performing electrical conversion between a memory cell and the outside of the memory chip are formed. In order to obtain electrical insulation between the wiring layers and between the wiring layers and the capacitor, it is necessary to form interlayer insulating films. This process is performed in a reducing or weak acidic atmosphere in order to prevent degradation of the wiring layers. Since a through hole for electrically connecting the peripheral circuit and the wiring layers generally has a shape with a high aspect ratio, which is deep as compared with the size of the opening, tungsten or the like is deposited by a CVD process. The atmosphere at this time is a reducing one. It is known that the capacitor is seriously damaged by being subjected to the treatment in the reducing atmosphere. For example, according to “Material Research Society Symposium Proceedings”, Vol. 310, pp. 151 to 156, 1993, it is reported that by forming an SiO
2
film by a CVD process, PZT as a ferroelectric loses its ferroelectricity characteristics and a leakage current increases.
Further, although the characteristics of a semiconductor active device degrade due to a heat treatment in a capacitor fabricating process, a plasma process in a wiring process, and the like, by additionally performing a heat treatment in hydrogen at approximately 400 degrees after completion of the wiring process, the degradation can subsequently be repaired. Consequently, a hydrogen treatment is generally performed after completion of the wiring process. It is known, however., that the hydrogen treatment exerts an influence on the characteristics of the capacitor in a manner similar to the interlayer insulating film process. For instance, according to “8th International Symposium on Integrated Ferroelectrics, 11c”, 1996, it is reported that, in the case where SrBi
2
Ta
2
O
9
(hereinbelow, SBT) is used as a ferroelectric, the capacitor is peeled off or, when the capacitor is not peeled off, a leakage current characteristic largely deteriorates.
The dielectric having a high dielectric constant and the ferroelectric will be generically called a high dielectric constant or ferroelectric film hereinbelow.
It is an object of the invention to obtain a very reliable semiconductor storage device in which the high dielectric constant or ferroelectric film is prevented from degrading.
SUMMARY OF THE INVENTION
The object is achieved by providing a capacitor electrode with a film which reduces an amount of hydrogen molecules reaching the capacitor electrode to 10
13
/cm
2
or smaller. It is preferable to use a film by which the hydrogen molecules become 10
12
/cm
2
or smaller.
As a result of examination of the cause of degradation in a treatment using hydrogen, we have found that using platinum as an electrode is related to a degradation process. Specifically, the following mechanism was uncovered. When platinum is used as an electrode, hydrogen molecules are decomposed by the platinum, active hydrogen such as hydrogen atoms and hydrogen radicals are generated, and the active hydrogen is promptly diffused in platinum, thereby degrading the high dielectric constant or ferroelectric film.
It was also found out that, because of the existence of the mechanism, the capacitor characteristics degrade or an electrode is peeled off even at a low temperature such as 300° C. At such a temperature, it has not previously been expected that the high dielectric constant or ferroelectric is reduced and degraded.
By providing a film which prevents hydrogen molecules from reaching the electrode as much as possible, the high dielectric constant or ferroelectric film can be prevented from degrading.
To be specific, it is sufficient to provide a film whose adsorption of the hydrogen molecules is 10
13
/cm
2
or smaller, preferably, 10
12
/cm
2
or smaller. By providing such a film which substantially does not adsorb hydrogen (hereinbelow, called an “adsorption inhibiting layer”), the amount of hydrogen molecules reaching a platinum film as a part of the capacitor electrode is decreased and, as a result, the amount of active hydrogen can be reduced. Thus, degradation and peeling in the wiring forming process of the high dielectric constant or ferroelectric capacitor are suppressed and improvement in the long-term reliability is realized. As a material of the film, silver, aluminum, silicon, lead, bismuth, gold, zinc, cadmium, indium, germanium, and tin are effective. Since the surface of each of these materials has an atomic arrangement which prevents the adsorption of hydrogen, it is effective in preventing the adsorption. As described above, by providing a layer which does not adsorb hydrogen much as compared with platinum, an effect on suppression of generation of the active hydrogen is produced. In the case where only an aluminum film is used, since the adsorption of hydrogen is relatively good, it is preferable to further provide a diffusion preventing layer which will be described hereinbelow. When the films are formed so as to be in contact with the platinum electrode, mutual diffusion occurs. It is therefore preferable to provide a reaction preventing layer such as a titanium nitride film or a tungsten nitride film therebetween.
A film in which the diffusion of hydrogen molecules is 10
13
/cm
2
or smaller, preferably, 10
12
/cm
2
or smaller may be provided. By providing a film which substantially prevents the diffusion of the hydrogen molecules (hereinbelow, called a “hydrogen diffusion preventing layer”), the diffusion amount of the hydrogen molecules becomes extremely small, the amount of hydrogen molecules reaching the capacitor electrode is decreased, and the amount of active hydrogen generated by the action of the capacitor electrode can be reduced. As a hydrogen diffusion preventing layer, specifically, besides tungsten, a conductive oxide such as ruthenium oxide, iridium oxide, palladium oxide, osmium oxide, or platinum oxide, ruthenium, iridium, palladium, osmium, or an oxide of an alloy of any of these mate
Fujisaki Yoshihisa
Kushida Keiko
Miki Hiroshi
Nakai Hiromi
Shimamoto Yasuhiro
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Le Dung A.
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