Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2001-04-27
2004-03-23
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S701000
Reexamination Certificate
active
06709776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Art Field
The present invention relates to a multilayer thin film including a ferroelectric thin film and an electron device comprising such a multilayer thin film. Typically, the multilayer thin film may be applied to semiconductor memories, thin-film ferroelectric devices such as infrared sensors, recording media for recording information by polarization reversal of ferroelectrics by AFM (atomic force microscope) probes or the like, thin-film vibrators, thin-film VCOs and thin-film filters used for mobile communications equipment, thin-film piezoelectric devices used for liquid injectors or the like.
2. Background Art
Electronic devices comprising dielectric films, ferroelectric films, piezoelectric films or the like formed and packed on Si substrates or semiconductor crystal substrates have been invented and intensively studied. For instance, LSIs having ever higher packing densities and dielectric separated LSIs using SOI technologies have been invented through combinations of semiconductors and dielectric materials, semiconductors storage devices such as nonvolatile memories through combinations of semiconductors and ferroelectric materials, film bulk acoustic resonators or FBARs, thin-film VCOs, thin-film filters, etc. through combinations of semiconductor substrates and piezoelectric films.
To allow such electron devices to have the optimum device performance and its reproducibility, it is desired that single crystals be used as dielectric materials, ferroelectric materials and piezoelectric materials. The same goes for thin-film materials. With polycrystal materials, it is difficult to obtain satisfactory device performance for the reason of physical quantity disturbances due to grain boundaries, and so epitaxial films as close to perfect single crystals as possible are now desired. A requirement for FBAR devices is that they be formed on Si single crystal substrates because the substrates should be processed with high accuracy. In addition, when ferroelectric materials such as PZT are used as FBAR materials, it is believed that the largest output is obtained when the spontaneous polarization of a ferroelectric material lines up in one direction. For this reason, it is ideally desired that a (001) uniaxially oriented ferroelectric thin film be formed on an Si single crystal substrate by epitaxial growth.
Typical ferroelectric thin films include those of PbTiO
3
, PZT, BaTiO
3
, etc. To apply these perovskite oxide thin films to actual devices, it is required to form them on semiconductor substrates. However, it is very difficult to form a uniaxially oriented ferroelectric thin film such as a (001) uniaxially oriented BaTiO
3
film of good crystallographic properties on a semiconductor substrate such as an Si (100) substrate. To overcome such difficulty, the inventors have filed patent applications (JP-A 09-110592, etc.) to come up with a process wherein a ferroelectric epitaxial thin film can be easily formed on an Si single crystal substrate.
Usually, however, a ferroelectric thin film formed on an Si substrate as an example have properties vastly inferior to those derived from ferroelectric's own properties. The properties of a ferroelectric material, e.g., dielectric constant, Curie temperature, coercive electric field and residual polarization change with stresses that the ferroelectric material has. A thin-film form of ferroelectric material is likely to generate stresses in association with film formation, and so stress control is of importance to form a ferroelectric thin film having improved properties. Stresses in particular have a great influence on the deterioration of the properties of a ferroelectric thin film formed on an Si substrate.
For instance, J.A.P. 76(12), 15, 7833 (1994) and A.P.L. 59(20), 11, 2524 (1991) teach that two-dimensional stresses in a film plane have a strong influence on the properties of a ferroelectric material on an MgO single crystal substrate, not an Si single crystal substrate. A leading cause for stress generation is a difference between the underlying substrate and the ferroelectric in physical properties, e.g., the coefficient of thermal expansion and lattice constant. To apply a ferroelectric thin film to a device, therefore, any desired ferroelectric properties cannot be stably obtained without such stress reductions as mentioned above.
Here ferroelectric materials having preferred properties include Pb-base ferroelectric materials such as PbTiO
3
, PLT (PbTiO
3
with La added thereto), PZT (PbZrO
3
—PbTiO
3
solid solution) and PLZT (PbZrO
3
—PbTiO
3
solid solution with La added thereto). The Pb-base ferroelectric materials, for the most part, have their axes of polarization in the [001] direction; they should preferably be uniaxially oriented films in terms of ferroelectric properties. When a Pb-base ferroelectric thin film is formed on an Si single crystal substrate, however, a domain structure is likely to occur, in which structure (001) oriented crystals coexist with (100) oriented crystals.
Set out below is one possible reason why the Pb-base ferroelectric domain structure is easily formed on the Si single crystal substrate. In what follows, PZT is used as an example of the Pb-base ferroelectric material.
Si is much smaller in the coefficient of thermal expansion than PZT. Accordingly, if a PZT thin film is formed at a temperature of 600° C. for instance, the contraction of the PZT thin film is then disturbed by the Si substrate in the process of cooling the thus formed thin film down to room temperature and, as a result, relatively large two-dimensional tensile stresses are generated within the plane of the PZT thin film. To make up for such tensile stresses, PZT must be forcibly formed into a 90° degree domain structure film in which (001) oriented crystals coexist with (100) oriented crystals. As the PZT thin film is cooled down, the tensile stresses remain generated therein even after domain formation and so the ferroelectric properties thereof become low.
The same holds true for the case where the PZT thin film is used as a piezoelectric material. To enhance the piezoelectric properties of the PZT thin film, it is of importance to increase the proportion of the (001) oriented crystals as much as possible, and to reduce the tensile stresses on the PZT thin film as much as possible.
On the other hand, the inventors have proposed a process for obtaining a ferroelectric thin film having a tetragonal (001) orientation while making use of elastic distortion resulting from a difference in lattice constant between both, called a misfit, as set forth in JP-A's 10-223476 and 11-26296, wherein a perovskite oxide thin film is formed on an electrically conductive oxide thin film. With this process, it is possible to form on an Si (100) substrate a (001) uniaxially oriented ferroelectric thin film of several tens of nanometers in thickness.
IEEE ELECTRON DEVICE LETTERS, Vol. 18 (1997), pp. 529-531, Jpn. J. Appl. Phys. Vol. 137 (1988), pp. 5108-5111 and JP-A 11-274419, too, describe that as in the aforesaid process, a perovskite oxide such as BSTO is formed on an electrically conductive oxide such as SrRuO
3
, whereby a dielectric film is elongated in the c-axis direction while making use of elastic distortion due to the misfit. Likewise, it is possible to obtain a ferroelectric film of several tens of nanometers in thickness having the (001) direction.
In this regard, it is noted that the effect of elastic distortion due to the misfit becomes slender with increasing film thickness, because of being absorbed by rearrangement. When a thin film is used as a capacitor or the like, it is unnecessary to increase film thickness except for the purpose of reducing leakage. To use a ferroelectric thin film in the form of a piezoelectric film for thin-film bulk vibrators as an example, it is required to make use of resonance in the thickness direction of the thin film. To be more specific, a thickness of the order of at least several hundred nanometers is needed for obtaining a frequen
Abe Hidenori
Noguchi Takao
Saitou Hisatoshi
Yano Yoshihiko
Jones Deborah
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Sperty Arden
TDK Corporation
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