Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
1998-05-14
2001-01-09
Crane, Sara (Department: 2811)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C257S295000, C438S004000
Reexamination Certificate
active
06171871
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ferroelectrics such as oxides having a layered crystal structure that belong to what is called the aurivillius crystallographic group and that tend to have strains in crystal lattices. The invention also relates to a memory device using such a ferroelectric and manufacturing methods of the ferroelectric and the memory device.
2. Description of the Related Art
Ferroelectrics have hysteresis in their electric field vs. polarization characteristic. Based on this fact, proposals for realizing, by utilizing ferroelectrics, nonvolatile memory devices that can hold data without being backed up by a power supply were made in 1960's. However, the attempts to develop such nonvolatile memory devices were stopped because at that time the ferroelectric thin film forming technology has not been established yet and there occurred various problems such as interference between memory cells. However, because of marked improvements in the thin-film technology that occurred thereafter, in recent years attempts to develop nonvolatile memory devices utilizing ferroelectrics have become active again. (For example, refer to C. Araujo, J. Scott, R. Goddfrey, and L. McMillan, Appl. Phys. Lett., 48 (1986) 1439; and W. Kinney, W. Shepherd, W. Miller, J. Evans, and R. Womack, IEDM Tech. Dig., (1987) 850.)
Among ferroelectrics to constitute nonvolatile memory devices, bismuth strontium tantalate (Bi
2
SrTa
2
O
9
; hereinafter referred to as “BiSTa”) particularly attracts attention which belongs to what is called the aurivillius crystallographic group and is superior in fatigue characteristic. (For example, refer to C. A-Paz de Araujo, J. D. Cuchiaro, L. D. McMillan, M. C. Scott, and J. F. Scott, Nature, 374 (1995) 627; K. Amanuma, T. Hase, and Y. Miyasaka, Appl. Phys. Lett., 66 (1995), 221; and S. B. Desu and D. P. Vijay, Master Sci. and Eng., B32 (1995) 75.) The aurivillius crystallographic group includes crystals that are represented by a stoichiometric composition formula [Bi
2
O
2
]
2+1
[Me
m−1
R
m
O
3m+
]
2−
where m is an integer of 2 or more, Me is at least one element selected from the group consisting of sodium (Na), potassium (K), calcium (Ca), barium (Ba), strontium (Sr), lead (Pb), and bismuth (Bi), R is at least one element selected from the group consisting of iron (Fe), niobium (Nb), tantalum (Ta), and tungsten (W).
Recently, there have been made reports of successes in producing a BiSTa thin film by MOCVD (metal organic chemical vapor deposition). (Refer to T. Ami, K. Hironaka, C. Isobe, N. Nagel, M. Sugiyama, Y. Ikeda, K. Watanabe, A. Machida, K. Miura, and M. Tanaka, Mater. Res. Soc. Symp. Proc., 415 (1996) 195; and T. Li, Y. Zhu, S. B. Desu, C-H. Peng, M. Nagata, Appl. Phys. Lett. 68 (1996) 616.)
However, examinations of the characteristics of actually produced BiSTa single crystals revealed that one crystal had portions exhibiting anisotropy and portions not exhibiting anisotropy though these portions did not have any difference in composition. Further, observations with a transmission electron microscope (TEM) showed that portions not exhibiting anisotropy had such large strains that the symmetry of crystal lattices was lost. Similar lattice strain was found in BiSTa polycrystalline thin films by using a TEM. That is, it was found that materials having a complex crystal structure such as BiSTa have a problem that they tend to have such large strains that the symmetry of crystal lattices is lost and hence tend to have portions not exhibiting anisotropy and portions exhibiting anisotropy but not showing superior characteristics.
For the above reasons, when a memory device is formed by using a ferroelectric having a complex crystal structure such as BiSTa, its characteristics deteriorate depending on the volume ratio of portions not exhibiting anisotropy and portions not showing superior characteristics. Further, the device characteristics vary depending on the proportion of portions not exhibiting anisotropy and portions not showing superior characteristics.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems, and an object of the invention is therefore to provide a ferroelectric that exhibits superior characteristics, a memory device using such a ferroelectric, and their manufacturing methods.
A ferroelectric according to the invention is such that 98% or more of the entire body exhibits ferroelectricity.
In a memory device according to the invention, a pair of electrodes are connected to a ferroelectric film, and 98% or more of a section of the ferroelectric film to which a voltage is to be applied via the electrodes is a ferroelectric exhibiting ferroelectricity.
A manufacturing method of a ferroelectric according to the invention comprises a crystal growth step of growing a crystal that is to constitute the ferroelectric; and a voltage application step of applying, after at least part of the crystal has been grown, a voltage to at least part of the crystal to at least partially correct strains of crystal lattices existing in the crystal.
According to the invention, a method for manufacturing a memory device in which a pair of electrodes are connected to a ferroelectric film, comprises a ferroelectric film forming step of forming a ferroelectric film; and a voltage application step of applying, after at least part of the ferroelectric film has been grown, a voltage to at least part of the ferroelectric film to at least partially correct strains of crystal lattices existing in the ferroelectric film.
In the above ferroelectric, 98% or more of the entire body exhibits ferroelectricity, whereby the ferroelectric exhibit superior characteristics.
In the above memory device, polarization occurs in the ferroelectric film when a voltage is applied between the pair of electrodes. The voltage vs. polarization characteristic of the ferroelectric film has hysteresis. Data storage and readout are performed by utilizing the hysteresis. Superior characteristics can be obtained because the ferroelectric film is made of a ferroelectric in which 98% or more of the section of the ferroelectric film to which a voltage is applied via the electrodes exhibits ferroelectricity.
In the above manufacturing method of a ferroelectric, after at least part of a crystal to constitute the ferroelectric is grown in the crystal growth step, a voltage is applied to at least part of the crystal in the voltage application step. As a result, at least part of strains of crystal lattices existing in the crystal are corrected.
In the above manufacturing method of a memory device, after at least part of a ferroelectric film is formed in the ferroelectric film forming step, a voltage is applied to at least part of the ferroelectric film in the voltage application step. As a result, at least part of strains of crystal lattices existing in the crystal are corrected.
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Masayuki Suzuki, et al.: “Preparation of Layered Ferroelectric Bi2SrTa2O9Single-Crystal Platelets” Japanese Journal of Applied Physics, Part 2 (Letters), May 1, 1996, Publication Office, Japanese Journal Appl. Phys, Japan, vol. 35, No. 5A, ISSN 0021-4922, pp. L564-L567, XP000626563.
Ami Takaaki
Machida Akio
Nagasawa Naomi
Suzuki Masayuki
Crane Sara
Sonnenschein Nath & Rosenthal
Sony Corporation
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