Semiconductor device

Details Semiconductor device Semiconductor device

2002-03-11

2004-06-08

Nelms, David (Department: 2818)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S352000, C257S353000, C257S507000, C438S149000, C438S479000, C438S517000

Reexamination Certificate

active

06747317

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices, and, more particularly, to a semiconductor device that has an oxide film formed by a silicon substrate and an epitaxial film grown on the silicon substrate.
Conventionally, the formation of an oxide film on a silicon substrate has been commonly performed. For most cases, the oxide film is an amorphous film, and is mainly employed as an insulating film or a dielectric film.
In a semiconductor device that utilizes the properties of an oxide film, such as a ferroelectric memory, a crystallized oxide film is employed to realize desired functions. Some oxide crystals exhibit many properties including not only insulation and dielectric properties but also ferroelectricity, piezoelectricity, pyroelectricity, and superconductivity. By forming oxide crystals having these properties as a thin film on a single-crystal silicon substrate, a device having various functions such as a memory, a sensor, and a filter, can be obtained. These functions derive from the properties of the oxide crystals. In an amorphous state, however, the oxide films cannot exhibit the properties or can exhibit only a part of the properties.
A ferroelectric film used in a ferroelectric memory obtains the above properties through crystallization in the existence of oxygen at a temperature of several hundred degrees centigrade. However, a conventional ferroelectric film is a polycrystalline film, in which the orientations of crystals in a direction perpendicular to the substrate, for instance, are aligned, but the orientations of crystals in other directions are generally at random, resulting in defects with the grain boundaries, for instance. For this reason, a semiconductor device including a conventional crystalline oxide film only has a limited ability to exhibit the properties of the oxide crystal.
Meanwhile, it has been very difficult to form an oxide film having an epitaxial orientation in which the crystal orientations are aligned not only in a direction perpendicular to the substrate surface but also in a direction parallel to the substrate surface.
To develop an epitaxial oxide thin film on a single-crystal silicon substrate, it is necessary to utilize the orientations on the surface of the single-crystal silicon substrate. However, a single-crystal silicon substrate has the same chemical properties as metals. If exposed to an oxygen atmosphere at a high temperature, the surface of a single-crystal silicon substrate is quickly oxidized to form a silicon oxide (SiOx) film. Since a silicon oxide film is amorphous and does not have a crystal orientation, an epitaxial oxide thin film cannot grow on a silicon oxide film. It is also essential for epitaxial growth that the reaction and diffusion between a growing thin film and a single-crystal silicon substrate should be minimized. For this reason, not all oxides can be formed through epitaxial growth on a single-crystal silicon substrate. Materials that are known to date as suitable for epitaxial growth on a single-crystal silicon substrate only include rare earth element oxides such as yttrium fully stabilized zirconia (YSZ: see J.Appl.Phys. vol. 67, (1989) pp. 2447), magnesia spinel (MgAl
2
O
4
: ISSCC Digest of Tech. Papers (1981) pp. 210), and cerium oxide (CeO
2
: Appl.Phys.Lett, vol. 56, (1990) pp. 1332), and strontium titanate (StTiO3: Jpn.J.Appl.Phys, 30 (1990) L1415).
The index of crystal quality of an epitaxial oxide thin film formed on a silicon substrate is a half value width that is obtained through X-ray diffraction (Full Width at Half Maximum, FWHM). A half value width is a value determined from a rocking curve obtained by scanning fixed 2 &thgr; axes of an X-ray peak. More specifically, the half value width (FWHM) is determined by the peak width at a half of the strength of the peak top of the rocking curve. This indicates the degree of crystal tilt in the thin film. A smaller value indicates properties similar to those of a single-crystal material, which exhibits better crystallization and orientation. With aligned orientations of crystals in a thin film, the electric properties (such as leak properties with improved hysteresis properties) are improved. It is therefore essential that a thin film having as small half value width (FWHM) as possible should be produced for suitable application to a device.
Materials having perovskite structures, including barium titanate, are ferroelectric materials that are desirable in terms of piezoelectricity, dielectricity, pyroelectricity, semiconductivity, and electric conductivity. However, it has been conventionally difficult to develop a material having a perovskite structure through epitaxial growth directly on a single-crystal silicon substrate. This is because an amorphous-phase SiOx film or a reaction phase such as silicide is formed on a single-crystal silicon substrate.
The only epitaxial perovskite thin film conventionally employed and formed on a single-crystal silicon substrate is strontium titanium (SrTiO
3
). A metal strontium film as an intermediate layer is interposed between a strontium titanium thin film and a single-crystal silicon substrate. Since titanium and silicon are reactive to each other, a strontium titanate film is formed to prevent reaction between titanium and silicon. More specifically, after a metal strontium film is formed on the surface of a silicon substrate, strontium and titanium are supplied in the existence of oxygen so as to produce a strontium titanate film. If the metal strontium film as an intermediate layer is thin enough, the titanium diffuses into the metal strontium film during the formation of a SrTiO
3
film. As a result, a SrTiO
3
film that appeases to have developed through epitaxial growth directly on the single-crystal silicon substrate can be obtained.
To develop a strontium titanate film through epitaxial growth in the above manner, a process control is necessary at the atomic layer level, and, therefore, a molecular beam epitaxy (MBE) technique is employed. Alternatively, Japanese Laid-Open Patent Application No. 10-107216 discloses a method of forming a strontium titanate (SrTiO
3
) film. More specifically, high-vacuum laser ablation is performed on a SrO target in a high vacuum of 10
−8
Torr, so as to form a strontium oxide (SrO) film as an intermediate layer. A strontium titanate (SrTiO
3
) film is then formed on the SrO film. If the SrO intermediate layer is thin enough, the titanium diffuses into the SrO intermediate layer during the formation of the SrTiO
3
film. As a result, it appears that the SrTiO
3
film has developed through epitaxial growth directly on the single-crystal silicon substrate.
Alternatively, another method has been suggested in which an intermediate layer is formed to prevent the reaction between a single-crystal silicon substrate and a perovskite oxide, and the formation of a SiOx phase. Intermediate layers that are known to date include yttria partially stabilized zirconia (YSZ: J.Appl.Phys. 67 (1989) pp. 2447) and magnesia spinel (MgAl
2
O
4
: ISSCC Digest of Tech. Papers (1981) pp. 210). With any of these intermediate layer, a 2-layered structure in which the intermediate layer and a perovskite film are laminated in this order on a single-crystal silicon substrate is obtained.
A yttria partially stabilized zirconia (YSZ) thin film formed through epitaxial growth on a single-crystal silicon substrate is obtained by a pulse laser deposition technique using YSZ ceramics as a target. Where a perovskite film is formed on such a YSZ film on a single-crystal silicon substrate, an epitaxial phenomenon in which the (011)-plane of the perovskite film is orientated in a direction corresponding to the (001)-plane of the YSZ film can be seen. However, the spontaneous polarization direction of a tetragonal perovskite film is the <001>-direction. If the (011)-plane of a perovskite film is orientated, the spontaneous polarization direction is tilted by 45 degrees with respect to the substrate surface. In such a case, the apparent po

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