Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
2002-06-26
2003-06-10
Thomas, Tom (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S296000, C257S298000, C257S301000, C438S003000, C438S238000
Reexamination Certificate
active
06576942
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a diffusion prevention film and fabricating method therefor and relates further to a semiconductor storage device that employs a ferroelectric film or a high dielectric film for the dielectric layer of a capacitor for charge storage use and fabricating method therefor.
The conventional nonvolatile memory devices of EPROM (Erasable Programmable Read Only Memory), EEPROM (Electrically Erasable Programmable Read Only Memory) and flash memory, which have a read time equivalent to that of DRAM (Dynamic Random Access Memory), are however not expected to operate at high speed since the write time thereof is long. In contrast to this, the ferroelectric memory, which is a nonvolatile memory that employs a ferroelectric capacitor as a charge storage section, is a randomly accessible nonvolatile memory that has a read time and a write time equivalent to those of DRAM and is able to have high speed operation expected.
Accordingly, there is the possibility of replacing these memories with sole ferroelectric memory in a system of a combination of the conventional nonvolatile memory of flash memory and DRAM used as a work memory.
Moreover, there can be considered a memory of a form like MFS (metal-ferroelectric-semiconductor) or MFIS (metal-ferroelectric-insulator-semiconductor) in which the gate electrode of a transistor is replaced by a ferroelectric. However, the device structure of the general ferroelectric memory, which starts being widely put into practical use at present, is one memory cell constructed of one ferroelectric capacitor and one select transistor.
As the material of the ferroelectric film used for the ferroelectric capacitor, which is the charge storage section of the ferroelectric memory, attention has been paid to PbZr
x
Ti
1−x
O
3
(PZT: lead zirconate titanate) that has been thoroughly examined up to now, SrBi
2
Ta
2
O
9
(SBT: Strontium Bismuth Tantalate) that has a fatigue resistance characteristic in comparison with PZT and is able to operate on a low voltage, Bi
4
Ti
3
O
12
(BIT) and so on, which are currently being energetically examined.
The forming methods of the above-mentioned ferroelectric film include the MOD (Metal Organic Deposition) method, the sol-gel method, the MOCVD (Metal Organic Chemical Vapor Deposition) method, the sputtering method and so on. According to any of the film forming methods, the ferroelectric film, which is an oxide, is therefore required to be crystallized by heat treatment in an oxidative atmosphere at an elevated temperature of about 600° C. to 800° C. On the other hand, since the electrode material of the ferroelectric capacitor is required to have heat resistance in the high-temperature oxidative atmosphere for crystallizing the ferroelectric, there are widely used platinum that has oxidation resistance, iridium that exhibits electrical conductivity despite its being an oxide, and so on for the upper electrode and the lower electrode.
When the ferroelectric capacitor is formed by using the above-mentioned electrode material and dielectric material, there is the practice of successively depositing a lower electrode layer, a ferroelectric layer and an upper electrode layer and thereafter processing the layers into a tiered form by the dry etching method.
In the above-mentioned ferroelectric memory, the ferroelectric film is the oxide, the ferroelectric film is disadvantageously reduced if the film undergoes a heat treatment process in a reductive atmosphere through a device forming process subsequent to the heat treatment process for crystallizing the ferroelectric film, and this disadvantageously causes bad influence such as the increase in the leak current of the ferroelectric film and the disappearance of ferroelectricity itself. Therefore, according to the structure disclosed in Japanese Patent Laid-Open Publication No. HEI 8-335673, a diffusion prevention film is deposited just on the capacitor so as to cover the entire capacitor. The diffusion prevention film of this structure is used for preventing the structure of the direct contact of the ferroelectric with the layer insulation film. The adoption of this structure can expect the effect of enabling the prevention of diffusion of hydrogen, which becomes a reducing agent, to the ferroelectric capacitor. Therefore, after the deposition of the diffusion prevention film made of alumina (aluminum oxide) for the purpose of providing a hydrogen barrier just on the ferroelectric capacitor, the layer insulation film is deposited.
Although the ferroelectric memory, which employs the ferroelectric film in the charge storage section, has been described hereinabove, a similar situation occurs in the case of a high-integration DRAM that employs a high dielectric film in the charge storage section. That is, the high dielectric film of DRAM is also an oxide similar to the ferroelectric film. If the film undergoes a heat treatment process in a reductive atmosphere in the device forming process after the formation of the high dielectric film, then the high dielectric film is disadvantageously reduced. This causes an increase in leak current of the high dielectric film and a reduction in dielectric constant, disadvantageously causing bad influences such that the sufficient electric charges for making the memory function cannot be maintained. Therefore, for the purpose of protecting the charge storage section from hydrogen that becomes a reducing agent, a diffusion prevention film of alumina is deposited just on the capacitor.
The problem intended to be solved by this invention is the hydrogen barrier property of alumina, which is the diffusion prevention film of the ferroelectric memory and DRAM that employs the high dielectric film. In the ferroelectric memory and the DRAM that employs the high dielectric film, a process using hydrogen is carried out in a device forming process after the formation of a capacitor, and a film that contains hydrogen is employed for the layer insulation film. Therefore, hydrogen diffuses into the capacitor section and unfortunately partially reduces the ferroelectric film and the high dielectric film of the oxide. Otherwise, when a film that contains a large amount of moisture in the layer insulation film is employed, the hydrogen generated through the reaction of the moisture that has desorbed from the layer insulation film with the metallic layer diffuses only by being maintained at an elevated temperature even in the process that does not directly use hydrogen and disadvantageously reduces the dielectric layer of the capacitor, which is the oxide, in a similar way. Consequently, there are caused the degradation and the like of the capacitor characteristics, such as an increase in leak current, a reduction in dielectric constant and the deterioration of the hysteresis loop in the case of the ferroelectric.
Although the amount of permeative hydrogen can be reduced to some degree by devising the hydrogen barrier film producing method and the like, the hydrogen cannot completely be barriered. In particular, alumina, which is the diffusion prevention film, cannot sufficiently maintain the hydrogen barrier property at an elevated temperature of not lower than 400° C. Since the film is required to undergo a process that uses a temperature zone up to 450° C. also at least after the formation of metallic aluminum interconnection, hydrogen is required to be stably barriered also in the temperature zone at this level. However, since hydrogen is practically made to abruptly permeate at and above a temperature of about 300° C. to 400° C., the forming process after the capacitor formation has restrictions of temperature and the high-temperature process time, which is required to be as short as possible. After undergoing a long-time process at an elevated temperature of not lower than 400° C. at the request of another fabricating process, the characteristics of the capacitor in which a ferroelectric film or a high dielectric film is employed for the dielectric layer varies or becomes unstable after the process, a
Ishihara Kazuya
Okutoh Akira
Im Junghwa
Thomas Tom
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