Coating processes – Electrical product produced – Metallic compound coating
Reexamination Certificate
1998-11-20
2001-12-25
Chen, Bret (Department: 1762)
Coating processes
Electrical product produced
Metallic compound coating
C427S255350, C427S255360, C427S255700, C427S079000, C427S100000, C204S192100, C204S192180
Reexamination Certificate
active
06333066
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming a PZT (lead zirconate titanate: Pb(Zr
x
Ti
1 −x
)O
3
) thin film, and more particularly, to a method for forming a PZT thin film using a seed layer.
2. Description of the Related Art
FIG. 1
is a cross-sectional view of a general PZT capacitor formed such that a PZT thin film is deposited on an electrode. The PZT thin film has good pyroelectricity, piezoelectricity and ferroelectricity and is widely employed for use in sensors, piezoelectric elements and memory devices. As shown in
FIG. 1
, a PZT thin film
14
is formed on a Pt electrode
13
. In
FIG. 1
, reference numeral
11
denotes a Si substrate, reference numeral
12
denotes a SiO
2
insulation layer, and reference numeral
15
is an upper Pt electrode. Here, in order for the PZT thin film
14
to exhibit a ferroelectric property, the PZT thin film
14
must have a phase having a special crystal structure, that is, a perovskite crystal structure. Lead (Pb) must be supplied sufficiently in an initial stage of seed formation in order to properly form a perovskite phase on a substrate made of a metal such as platinum (Pt). If Pb is not sufficiently supplied, it is difficult to form an initial thin film having a perfect perovskite phase, the initial thin film formed at an interface between a thin film and a substrate. Instead, the initial thin film may have both a pyrochlore phase and a perovskite phase.
FIG. 2
is a scanning electron microscopy (SEM) photograph illustrating the cross-section of the crystal structure of a PZT thin film
24
in the case when the PZT thin film
24
is formed on a Pt electrode
23
which is most widely utilized as an electrode. Here, the Pt electrode
23
is formed on a Si substrate
21
having a SiO
2
insulation layer
22
coated thereon. As shown, since the PZT thin film
24
has different-sized grains, the surface of the PZT thin film
24
is rough due to protruding large grains. Such different grain sizes are attributed to different growth rates of the respective grains. In more detail, the grains grow at different growth rates according to their crystal orientation. It is known that the growth rate of PZT is highest in a <100> orientation, which is perpendicular to the surface of a substrate, and is lowest in a <111> orientation. Also, of the same orientation crystals, an earlier generated grain starts to grow earlier and thus is larger than a later generated grain, which is prominent in the case of a low nucleation density.
Also, in this case, the grain size is made larger than that in the case of a high nucleation density, so that the height of a protrusion becomes larger.
Also, the surface of the Pt electrode
23
is in good condition before pre-heating, as shown in FIG.
3
A. However, the surface of the Pt electrode
23
becomes defective after pre-heating at 600° C. for about 15 minutes, as shown in
FIG. 3B
, due to a difference between a deposition temperature and a pre-heating temperature. In other words, in the process of pre-heating, the Pt thin film experiences grain growth, thermal expansion/contraction, diffusion, oxidation, etc. It will also have larger grains, more intercrystalline defects, and a more rough surface all of which are impediments to the growth of the PZT thin film.
If the surface of the PZT thin film becomes rough by the above impediments, it is difficult to proceed with subsequent processes for device fabrication. Further, if an interface between the PZT thin film and an upper electrode becomes severely rough, the performance of a capacitor is degraded accordingly.
To date, in order to overcome such problems, there have been attempts to manufacture a seed layer by forming a uniform amorphous thin film at a low temperature and crystallizing the same through subsequent heat treatment, or to deposit a good thin film made of only a perovskite phase by facilitating generation of a perovskite seed by thinly depositing a material such as PbTiO
3
as a seed layer and then depositing a PZT thin film thereon.
However, in the case when the amorphous phase is crystallized to be used as a seed, perfect crystallization of the amorphous phase cannot be achieved due to problems of composition change or recrystallization. Thus, since the amorphous phase remains between the substrate and the PZT thin film, the quality of the capacitor is deteriorated. Also, in the case of using PbTiO
3
, the generation of a seed is facilitated, compared to the case where PZT is directly deposited on a Pt substrate. However, since it is difficult to prepare high quality PbTiO
3
and the surface of the thin film is rough, PbTiO
3
is not suitable as a seed layer.
SUMMARY OF THE INVENTION
To solve the above problems, it is an objective of the present invention to provide a method for forming a PZT thin film using a seed layer, in which a dense PZT thin film made of small grains is formed by growing the PZT thin film using a PbO thin film as a seed layer.
Accordingly, to achieve the above objective, there is provided a method for forming a PZT thin film using a seed layer, comprising the steps of forming an electrode on a substrate, forming a PbO seed layer on the electrode by injecting a Pb precursor into a chamber, and forming a PZT thin film on the PbO seed layer.
In the present invention, the electrode is formed of platinum (Pt), the step of forming the PbO seed layer is performed using a metal organic chemical vapor deposition method or a sputtering method, and the PbO seed layer is deposited at a temperature of 400-500° C. to a thickness of 50-500 nm.
Also, in the step of forming the PbO seed layer, Zr and Ti precursors are simultaneously injected into the chamber in addition to the Pb precursor using the metal organic chemical vapor deposition method, to form a PZT seed layer, where excess Pb is injected into the chamber such that the injection ratio of Pb to Zr and Ti, Pb/(Zr+Ti), for forming the seed layer, is greater than that for forming the PZT thin film.
REFERENCES:
patent: 5500988 (1996-03-01), Moynihan et al.
patent: 5817170 (1998-10-01), Desu et al.
patent: 5820946 (1998-10-01), Kim et al.
patent: 5876503 (1999-03-01), Roeder et al.
patent: 6080593 (2000-06-01), Kim et al.
patent: 6190728 (2001-02-01), Suzuki et al.
patent: 6211035 (2001-04-01), Moise et al.
patent: 6229166 (2001-05-01), Kim et al.
patent: 0222711 (1996-08-01), None
patent: 1012832 (1998-01-01), None
Chen Bret
Rothwell Figg Ernst & Manbeck
Samsung Electronics Co,. Ltd.
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