Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2000-03-17
2003-05-27
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S100000, C117S103000, C117S108000, C117S109000, C117S939000
Reexamination Certificate
active
06569240
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for forming an excellently crystalline dielectric film with a high dielectric constant, like a CeO
2
film, or ferroelectric film out of metal Ce and oxygen on an Si substrate.
In recent years, the number of C-MOS devices that can be integrated together on a single Si substrate has increased significantly because those devices have been tremendously downsized. To catch up with this trend, reduction in thickness of a gate insulating film, which is part of a MOSFET, is also in high demand. A thinner gate insulating film is needed because of the following reasons.
First of all, although the operating voltage has been reduced day after day to conserve power as much as possible, the quantity of charge required for the operation of a device remains almost the same and has not been reduced so much. Since the relationship of Q=CV (where Q is the quantity of charge, C is static electricity and V is voltage) should be met, the static electricity C that can be retained in a gate insulating film must be increased to reduce the voltage V with the quantity of charge Q kept substantially constant. The static electricity C is given by C=(∈r·S)/d, where ∈r is relative dielectric constant, S is the area of a capacitor and d is a space between electrodes. Accordingly, the static electricity C can be increased if the thickness d of a gate insulating film, which is currently made of SiO
2
in many cases, is reduced. For that purpose, the gate insulating film has been thinned to a thickness between 10 and 15 nm or less than 10 nm.
However, if a gate insulating film is thinned that much, then various inconveniences might be concerned about lately; the breakdown strength of the gate insulating film might decrease or the leakage current might increase.
In view of these potential disadvantages, alternative gate insulating film materials, which have a relative dielectric constant ∈ r higher than that of SiO
2
and yet exhibit pretty good electrical properties comparable to those of SiO
2
, have been searched for. That is to say, if the relative dielectric constant ∈ r is higher, then the static electricity C can be kept high even when the thickness d is increased to a certain degree. Accordingly, the required charge quantity Q is attainable even with a reduced operating voltage. Taking these points into account, methods for forming, on an Si substrate, an insulating film made of a novel insulating material with high dielectric constant and breakdown strength and low interface level and leakage current have been researched and developed to attain characteristics comparable to those of the SiO
2
gate insulating film currently used.
Efforts have also been made to form an insulating film of non-SiO
2
insulator material on an Si substrate by a different type of demand. For example, an example disclosed in Japan Journal of Applied Physics 35, 4987, (1996) (which will be herein called a “first document” for convenience sake) report-research for implementing a transistor with memory function by providing a thin film with ferroelectric for the gate of field effect transistor. According to the technique disclosed in this document, a thin film of PbZr
1-x
Ti
x
O3 (PZT) with ferroelectric, i.e., a PZT film, is formed as an exemplary thin film of that type. However, since it is difficult to form the PZT film directly on an Si substrate, an insulating film of CeO
2
for example, is interposed as a buffer layer between the PZT film and the Si substrate.
Methods for forming a novel insulator film on an Si substrate to attain those characteristics, including high dielectric constant and breakdown strength and low interface level and leakage current, as in the gate insulating film mentioned above, have also been researched such that a ferroelectric or other dielectric film (e.g., superconductor film) can be formed on the Si substrate.
According to any of these suggested techniques, a CeO
2
film is one of very attractive insulator materials for a buffer layer. This is because the lattice constant of CeO
2
is closer to that of Si than any other known material and a lattice mismatch between CeO
2
and Si is only −0.37% (i.e., a
ceO2
=5.411 Å and a
si
=5.431 Å). In addition, since the crystal structure of CeO
2
is like that of fluorite, CeO
2
can form a continuous crystal lattice with the Si substrate having a diamond structure. The coordination number for all the atoms is four in Si, whereas the coordination number for oxygen atoms is four and that for Ce atoms is eight in CeO
2
. However, since both Si and CeO
2
crystals belong to a cubic system, which is represented as a face-centered cubic lattice as a matter of principle, Si and CeO
2
crystals can be stacked one upon the other by epitaxial growth (because mole ratio of oxygen to Ce is 2:1). Thus, it is possible to form a thin film with excellent crystallinity on the Si substrate, and it is easier to stack a ferroelectric or superconductor film with high crystallinity thereon. Furthermore, since the relative dielectric constant of CeO
2
is as high as around 26, it is very likely that CeO
2
will be used as a novel gate insulating film material in place of SiO
2
.
Various techniques of forming CeO
2
on an Si substrate have been proposed in numerous other documents as well as the first document. Following is typical examples of them.
According to an example disclosed in Japan Journal of Applied Physics, 1765, (1993) (which will be herein called a “second document” for convenience sake), CeO
2
is evaporated from a pellet-like CeO
2
sintered compact by irradiating the compact with an electron beam (EB) in a molecular beam epitaxy (MBE) system including an EB evaporation unit, thereby forming an excellently crystalline CeO
2
thin film on an Si substrate. In this case, decline in crystallinity of the CeO
2
thin film due to the oxygen deficiency is prevented by supplying oxygen gas while CeO
2
is being evaporated. In the first document, the CeO
2
film is also formed by the same method.
An example disclosed in Japan Journal of Applied Physics 270, 1994 (which will be herein called a “third document” for convenience sake) uses a thin film forming technique different from that of the first and second documents. In the third document, a reactive sputterer including a target of metal Ce is used and Ce atoms are sputtered out of the target with oxygen gas supplied thereto and reacted with oxygen on the Si substrate, thereby forming an excellently crystalline CeO
2
thin film on the Si substrate.
An example disclosed in Applied Physics Letters 2027, (1991) (which will be herein called a “fourth document” for convenience sake) forms a CeO
2
film by a different technique from any of the techniques mentioned above. In the fourth document, an MBE system, into which ArF excimer laser radiation can be introduced externally, is used, a pellet-like CeO
2
sintered compact placed inside is irradiated with the laser radiation to evaporate CeO
2
therefrom and oxygen gas is introduced at the same time. In this manner, an excellently crystalline CeO
2
thin film is formed on the Si substrate.
These methods of forming a crystalline CeO
2
thin film as disclosed in the documents cited above, however, have the following shortcomings.
It should be noted that a family of crystallographic planes including (100), (010), (001) and so forth will be collectively referred to as a (001) plane in the following description, although such a family of planes should be labeled {001}. The same statement will be applicable to a (011) or (111) plane. Similarly, (001), (011) or (111) substrate or film will mean a substrate or film with a (001), (011) or (111) plane as its principal surface.
First, in accordance with the example disclosed in the first and second documents, oxygen and Ce are supplied at the same time by evaporating CeO
2
from a pellet-like CeO
2
sintered compact being heated. That is to say, since Ce and oxygen reach the surf
Iijima Kenji
Nishikawa Takashi
Kunemund Robert
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
Studebaker Donald R.
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