Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2001-08-29
2003-06-03
Jones, Deborah (Department: 1775)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C428S699000, C428S701000, C428S702000
Reexamination Certificate
active
06573211
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having capacitor elements. In particular, the present invention relates to a high-dielectric-constant film and a ferroelectric film which is made by using an organometal material gas, and is used for a capacitor or a gate of a semiconductor integrated circuit.
2. Description of the Related Art
In recent years, nonvolatile ferroelectric memories, which utilizes a ferroelectric capacitor, and dynamic random access memories (DRAMs), which utilizes a high-dielectric capacitor, have been actively investigated and developed. These ferroelectric memories and DRAMs comprise selective transistors and data-storing capacitors, and in a memory cell a capacitor connected to one diffusion layer of the selective transistor. A ferroelectric capacitor uses a ferroelectric film made of Pb(Zr,Ti)O
3
(hereinafter, referred to as “PZT”) etc., as a capacitor insulating film, and thereby storing non-volatile information by polarizing a ferroelectric.
On the other hand, a high-dielectric capacitor uses a high-dielectric thin film made of (Ba,Sr)TiO
3
(hereinafter, referred to as “BST”) etc., as a capacitor insulating film. Therefore, the capacitance of the capacitor can be increased to miniaturize a device. When such a ceramic material is used for a semiconductor device, it is very important to electrically isolate such a ceramic material of each memory cell as a minute capacitor. A ceramic material film is deposited on a crystallize-assisting conductive film that is to be a lower electrode.
As a method for depositing ceramic thin film, a conventional sol-gel process, sputtering, and chemical vapor deposition (CVD) have been reported.
The sol-gel process is a method in which organometallic materials dissolved in an organic solvent are applied to a wafer having a lower electrode formed thereon by a spin-coating, and crystallized by annealing in the presence of oxygen. This process causes crystallization in a solid phase, so that a very high temperature is required for crystallization. In the case where a metal oxide dielectric film is PZT, a crystallization temperature that allows sufficient ferroelectric characteristics to be exhibited is 600° C. In the case where the metal oxide dielectric film is BST, a crystallization temperature that allows sufficient high-dielectric characteristics to be exhibited is 650° C. A crystal formed by the foregoing method has many defects, and uneven crystal orientation. Furthermore, according to this process, it is difficult to handle a large-diameter wafer, and step-coverage is poor. Thus, the sol-gel process is not suitable for fabrication of highly integrated devices.
Next, the sputtering method is a method in which a crystal is formed on a wafer having lower electrode film, by reactive sputtering using Ar+O
2
plasma and a ceramic sintered-body target, and then subjected to crystallization by anealing in oxygen. According to sputtering, film uniformity could be obtained by enlarging the diameter of a target, and a sufficient deposition rate is obtained by increasing a plasma injection power. However, sputtering also has a drawback that a high temperature is required for crystallization. In the case where a metal oxide dielectric film is PZT, a crystallization temperature that allows sufficient ferroelectric characteristics to be exhibited is 600° C. In the case where the metal oxide dielectric film is BST, a crystallization temperature that allows sufficient high-dielectric characteristics to be exhibited is 650° C. Furthermore, according to sputtering, a composition is almost determined by that of a target; therefore, a target is required to be exchanged in order to vary the composition of a film, which is inconvenient in terms of production steps.
Next, according to CVD, source gases are introduced in a gas phase into a deposition chamber wherein a heated substrate is set, then a film is formed on the substrate. CVD has excellent deposition characteristics in uniformity of a large-diameter wafer and a step-coverage, so that it is considered to be promising method as a mass-production technique when applied to ULSI. Organometals are often employed for CVD source because there are few appropriate hydrides and chlorides for constituent metal element of ceramics including Ba, Sr, Bi, Pb, Ti, Zr, Ta, and La, and so on. However, organometals have rather low vapor pressure, and in most cases, present as a solid or a liquid at room temperature, so that a transportation method utilizing a carrier gas is used.
However, in the case of using the above-mentioned method, it is difficult to quantify the flow rate of an organometal gas in a carrier gas and to control the flow rate thereof exactly. More specifically, the carrier gas contains an organometal source gas with a saturation vapor pressure (determined by the temperature of a source vessel) or higher, and this flow rate depends upon not only the flow rate of a carrier gas but also the surface area of a solid material, the temperature of a thermostat, and the like. Furthermore, according to the description on film formation of PTO (lead titanate: PbTiO
3
) using the above-mentioned film formation method, in Jpn, J. Appl. Phys. Vol. 32 (1993) p.4175), a film formation temperature of PTO is also very high (570° C.), and crystal orientation is not uniform.
Hitherto, for formation of a ferroelectric memory and a DRAM, the above-mentioned film formation method has been used. However, heating at a high temperature of about 600° C. or higher in an oxygen atmosphere is necessary, and it is also difficult to control crystal orientation.
According to a structural aspect of a semiconductor device, in order to make a ferroelectric capacitor and a high-dielectric capacitor well functioning, it is required to electrically connect either one of capacitor electrodes to diffusion layers of a selective transistor. Conventionaly, in a DRAM, polysilicon layer connected to one diffusion layer of a selective transistor is used as lower electrode of a capacitor, and a SiO
2
film, a Si
3
N
4
film, or the like is formed as an insulating film of the capacitor on the surface of the polysilicon layer to construct the capacitor. However, since a ceramic thin film is made of an oxide, if the ceramic thin film is formed directly on polysilicon, the polysilicon is oxidized. Thus, a structure in which a ceramic film directry formed on a polysilicon electrode cannot be obtained. Therefore, as a one alternate structure, a cell structure in which an upper electrode of a capacitor and a diffusion layer are connected to each other by local wiring of metal made of Al or the like is described in 1995 Symposium on VLSI Technology Digest of Technical Papers, p. 123. Furthermore, according to International Electron Devices Meeting Technical Digest, 1994, p. 843, a technique for forming a PZT capacitor by using TiN barrier metal on polysilicon is described. Regarding a DRAM, for example, according to International Electron Devices Meeting Technical Digest, 1994, p. 831, a technique is described for forming a STO (strontium titanate: SrTiO
3
) thin film on a RuO
2
/TiN lower electrode formed on a polysilicon plug, thereby forming a capacitor.
On the other hand, Japanese Patent Application Laid-open No. 11-317500 discloses a memory cell structure in which a capacitor is connected to a diffusion layer by alternately stacked plugs and metal pads formed simultaneously with a multi-layered metal line, unlike a conventional memory cell structure in which a capacitor is connected to a diffusion layer via local wiring and a polysilicon plug.
Incidentally, perovskite crystal has two a-axes (which may be referred to as an a-axis, b-axis) and a c-axis, as shown in
FIG. 14. A
polarization direction in which ferroelectricity is exhibited is a c-axis direction, and polarization does not occur in an a-axis direction. In case of a thin film, Ferroelectricity is exhibited by a component of the c-axis in a direction vertical to the film surface. Thus, the ratio o
Jones Deborah
NEC Corporation
Sperty Arden B.
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