Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
1998-12-30
2001-09-04
Whitehead, Jr., Carl (Department: 2822)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S253000, C438S396000
Reexamination Certificate
active
06284588
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a highly integrated memory device; and, more particularly, to a ferroelectric capacitor memory device capable of preventing volatility of elements contained in a ferroelectic film.
DESCRIPTION OF THE PRIOR ART
In general, a Pt film has been widely used as a lower electrode in highly integrated DRAM cells employing high dielectric materials as well as non-volatile memory device having ferroelectric materials such as BST[Ba(Sr,Ti)O
3
] or PZT. The ferroelectric material, such as the PZT, has a few hundreds to a few thousands of dielectric constant in two stable states in remanent polarization. Based on the remanent polarization, the thin film formed by the ferroelectric material is typically used as a capacitor in a nonvolatile memory device. In case where the ferroelecric film is employed in the nonvolatile memory device, data are input into cell capacitors by controlling the polarization direction in response to the applied electric field and such a digital signal “0” or “1” is stored by the remanent polarization which is caused by the removal of the applied electric field.
FIG. 1
is a cross-sectional view illustrating a conventional ferroelectric memory device having a Pt film as a lower electrode of a capacitor. As shown in
FIG. 1
, a capacitor in the conventional memory device is made up of a polysilicon plug
6
, a diffusion preventing film
7
and a lower electrode
8
such as a Pt film. Since the Pt film, which is commonly used as the lower electrode
8
, doesn't act as a barrier film preventing oxygen atoms from diffusing into its underlayer, the oxygen atoms are diffused into a diffusion preventing film
7
through the Pt film. In
FIG. 1
, unexplained reference numeral
1
denotes a semiconductor substrate,
2
a field oxide film,
3
a gate,
4
a bit line,
5
an interlayer insulating film, and
9
a ferroelectric film.
A TiN/Ti film is widely used as the diffusion preventing film
7
. The barrier metal films, such as TiN and Ti films, and the polysilicon film for plug vigorously react on oxygen atoms from the dielectric film which is formed by the CVD (Chemical Vapor Deposition) method. Accordingly, the electrical interconnection between the lower electrode and an active region of the transistor is broken down. With the increase of the deposition temperature of ferroelectric materials, this problem is getting more serious.
In case of ferroelectric materials, such as BST, which is one of the prevailing materials for the ferroelectric capacitor, the temperature required in the deposition and crystallization is very high. Therefore, in order to fabricate the ferroelectric memory device on the COB (capacitor on bit line) structure, it is most important to electrically connect the Pt lower electrode to the active region of the MOSFET. As a result, other deposition methods capable of improving electric characteristics of the diffusion barrier metal are required.
In particular, it should be noted that the composition of the ferroelectic film can be changed by volatile substance as well as the out-diffusion oxygen atoms. In order to form a high quality of PZT film of the ferroelectric capacitor, the physical deposition such as a radio frequency plasma sputtering method can be used instead of the CVD. However, in any deposition method, it is difficult to control the composition of the PZT film in a subsequent thermal treatment carried out after the PZT deposition because of the volatility of Pb and the vacancy caused by the oxygen out-diffusion. These space charge defects may be moved toward grain boundaries or the domain walls, thereby generating the space charge layer which deteriorates the voluntary polarization.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method for fabricating a ferroelectric capacitor capable of preventing a variation and defects of PZT film in semiconductor devices.
It is another object of the present invention to provide a method for fabricating an improved PZT ferroelectric capacitor by controlling the deposition conditions of a ferroelectric material.
In accordance with an aspect of the present invention, there is provided a method for fabricating a semiconductor capacitor, comprising the steps of: forming a lower electrode of the semiconductor capacitor; forming a ferroelectric film on the lower electrode, wherein the ferroelectric film bears a volatile element; forming a capping oxide film on the ferroelectric film; applying a rapid thermal process to the ferroelectric film and the capping oxide film; and cooling the ferroelectric film, whereby a preferred orientation of atoms in the ferroelectric film and domain boundaries thereof are perpendicular to a semiconductor substrate for which the semiconductor capacitor is provided.
REFERENCES:
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patent: 5471364 (1995-11-01), Summerfelt et al.
patent: 5504330 (1996-04-01), Summerfelt et al.
patent: 5548475 (1996-08-01), Ushikubo et al.
patent: 5572052 (1996-11-01), Kashihara et al.
patent: 5614018 (1997-03-01), Azuma et al.
patent: 5650646 (1997-07-01), Summerfelt
patent: 5719417 (1998-02-01), Roeder et al.
patent: 5757061 (1998-05-01), Satoh et al.
patent: 5780886 (1998-07-01), Yamanobe
Blakely & Sokoloff, Taylor & Zafman
Hyundai Electronics Industries Co,. Ltd.
Jr. Carl Whitehead
Thomas Toniae M.
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