1. Field of the Invention
The present invention relates to method for manufacturing ferroelectric thin film, substrate covered with ferroelectric thin film, and capacitor. A substrate covered with a ferroelectric thin film and a capacitor are used a ferroelectric memory device, a pyroelectric sensor, a piezoelectric device or the like.
2. Related Art
Ferroelectrics have been widely used for the development of devices such as a condenser, an oscillator, an optical modulator and an infrared sensor since ferroelectrics have a variety of characteristics such as spontaneous polarization, high dielectric constant, electro-optical effect, piezoelectric effect and pyroelectric effect.
With the advance in the technique of forming thin films, the application field of ferroelectric thin film is expanding. For example, reduction of capacitor area for high integration of devices and improvement of reliability have been achieved by applying the high dielectric characteristics to various kinds of semiconductor devices such as a DRAM. Furthermore, particularly recently, the development of ferroelectric non-volatile memories (FRAMs) having high density and high operation speed has been active by combining them with semiconductor memory devices such as a DRAM. Ferroelectric non-volatile memories eliminate the need for back-up power supply by utilizing the ferroelectric properties (hysteresis effect) of the ferroelectrics. For the development of such devices, it is necessary to use a material having characteristics such as large remanent spontaneous polarization (Pr), small coercive field (Ec), small leakage currents, and large resistance to repetition of polarization inversion. Further, it is desired to realize the above properties with a thin film having a thickness of 200 nm or less so as to reduce the operation voltage and to conform to fine processing of semiconductors.
For the purpose of applying thin films to FRAM or the like, study is under way on the formation of high quality thin film of ferroelectrics comprising a group of bismuth-based oxides having a layer crystal structure and expressed by the following general formula: EQU Bi.sub.2 A.sub.m-1 B.sub.m O.sub.3m+3
where, A is selected from Na.sup.1+, K.sup.1+, Pb.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+, Bi.sup.3+, etc.; B is selected from Fe.sup.3+, Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, W.sup.6+, Mo.sup.6+, etc.; and m represents an integer of 1 or larger. Concerning their crystal structure, the bismuth-based oxides comprise a layered perovskite layer consisting of a sequence of (m-1) units of ABO.sub.3 perovskite lattices interposed between (Bi.sub.2 O.sub.2).sup.2+ layers. Among the group of materials, those comprising a combination of Sr, Ba, or Bi for A-site and Ti, Ta, or Nb for B-site tend to exhibit ferroelectric properties.
In the ferroelectrics expressed by the general formula above, Bi.sub.4 Ti.sub.3 O.sub.12 (bismuth titanate) has a layered perovskite structure (rhombic crystal/lattice constants :a=5.411 .ANG., b=5.448 .ANG., and c=32.83 .ANG.) with strong anisotropy. The ferroelectric property of its single crystal is such that, along the a-axis, the remanent spontaneous polarization is Ps=50 .mu.C/cm.sup.2 and the coercive field is Ec=50 kV/cm; and, along the c-axis, the remanent spontaneous polarization is Ps=4 .mu.C/cm.sup.2 and the coercive field is Ec=4 kV/cm. Thus, among the bismuth-based oxide ferroelectrics, bismuth titanate exhibits the greatest a-axis component of spontaneous polarization and an extremely small c-axis component of the coercive field. It will be possible to apply this ferroelectric to electronic devices such as ferroelectric non-volatile memories if the orientation of thin films can be controlled suitable for utilizing the properties of large spontaneous polarization and small coercive field which Bi.sub.4 Ti.sub.3 O.sub.12 has. However, the cases that have been reported so far utilize only the c-axis orientation along which the spontaneous polarization is small, or the random orientation. Thus, the large spontaneous polarization along the a-axis has not been fully utilized to the present.
On the other hand, formation of a thin film using Bi.sub.4 Ti.sub.3 O.sub.12 has been attempted by employing MOCVD (metal-organic chemical vapor deposition) or sol-gel method. However, a conventional sol-gel method for forming a thin film with favorable ferroelectric properties requires thermal treatment at a temperature of 650.degree. C. or higher, and, moreover, most of the thin films thus obtained exhibit random orientation or c-axis orientation. Furthermore, the surface morphology of the resulting thin film revealed that the thin film consists of crystal particles about 0.5 .mu.m in size. Thus, the film was found difficulties in applying it to highly integrated semiconductor devices which require fine processing.
On the other hand, thin films of Bi.sub.4 Ti.sub.3 O.sub.12 having c-axis orientation is being formed at a temperature of 600.degree. C. or higher on a Pt-electrode layer/SiO.sub.2 /Si-substrate (Pt/SiO.sub.2 /Si substrate) or on a Pt substrate by MOCVD method. However, these substrates cannot be applied to semiconductor devices directly as they are. More specifically, as is the case with the Pt/SiO.sub.2 /Si substrate, a bonding layer such as a Ti film must be formed between the Pt electrode layer and the underlying SiO.sub.2 in order to ensure the bonding strength between the Pt electrode layer and the SiO.sub.2.
However, it has been reported that, if a Bi.sub.4 Ti.sub.3 O.sub.12 thin film is formed by MOCVD on the Pt electrode layer having a bonding layer as above, the film surface tends to exhibit, not only a morphology consisting of coarse crystal particles, but also a tendency of generating pyrochlore phase (Bi.sub.2 Ti.sub.2 O.sub.7) [see Jpn. J. Appl. Phys., 32 (1993), p.4086, and J. Ceramic Soc. Japan, 102 (1994), p.512]. If the film surface morphology consists of coarse crystal particles, the thin film cannot be applied to a highly integrated device which is subjected to fine processing. Moreover, in case films are too thin, they tend to cause pinholes which lead to the generation of current leakage. Furthermore, the incorporation of a non-ferroelectric phase, i.e., pyrochlore phase, deteriorates the ferroelectric properties of the entire thin film. Accordingly, it was difficult to form a ferroelectric thin film having good ferroelectric properties with a thickness of 200 nm or less by a conventional technique.
Jpn. J. Appl. Phys., 33(1994) p.5215-5218 discloses that at the film formation temperature 600.degree. C. or higher, Bi.sub.4 Ti.sub.3 O.sub.12 having an orientation along c-axis can be obtained at the specific flow rate of oxygen gas. However, it discloses that at the flow rate of oxygen gas except the above-mentioned flow rate, Bi.sub.4 Ti.sub.3 O.sub.12 mixed with the one having an orientation along (117) can be obtained. That is, it was difficult to control the orientation of Bi.sub.4 Ti.sub.3 O.sub.12 thin film optionally.
As described above, the conventional techniques for forming thin films involve problems that the large spontaneous polarization of Bi.sub.4 Ti.sub.3 O.sub.12 along the a-axis has not been used to the full extent, and that the density and the flatness of the film surface necessary for fine processing and suppressing current leakage in view of applying the ferroelectric thin film to highly integrated devices has not been obtained. Moreover, because the film surface morphology of a Bi.sub.4 Ti.sub.3 O.sub.12 thin film having c-axis orientation reported to the present consists of coarse crystal particles, a thin film cannot sufficiently prevent current leakage from generating.
To implement the ferroelectric non-volatile memory (FRAM) having high capacity, the large spontaneous polarization of Bi.sub.4 Ti.sub.3 O.sub.12 along the a-axis is effective for the reduction of capacitor area; and, to realize low voltage drive, a small coercive field in along the c-axis is effective. However, conventional methods of film formation had been found unable to fully utilize the aforementioned characteristics of Bi.sub.4 Ti.sub.3 O.sub.12.