Active solid-state devices (e.g. – transistors – solid-state diode – Integrated circuit structure with electrically isolated... – Passive components in ics
Reexamination Certificate
2000-07-06
2003-01-07
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Integrated circuit structure with electrically isolated...
Passive components in ics
C257S295000, C257S296000, C257S310000, C438S381000
Reexamination Certificate
active
06504228
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device comprising a ferro-electric capacitor or ferro-electric memory composed of highly dielectric materials.
2. Description of the Related Art
Year after year the integration of a semiconductor device has been sophisticated and miniaturization of the circuit used therein has been promoted. Caused by such demand it is required that capacitors maintain their volume while the effective area be smaller and therefore, it is required that films of the capacitors be thinner or highly dielectrics materials be used for the capacitors. With respect to the thickness of the films, the films have reached their lowest possible thickness and electric field strength, which are next to the dielectric breakdown, and it is impossible to make the film thinner. Accordingly, use of dielectrics is indispensible.
Highly dielectric materials mean materials having a higher relative dielectric constant than that of SiO
2
and Si
3
N
4
which have been used for the prior capacitor. Such materials are generally dielectric oxides and when development of such materials was started, monometallic oxide such as Ta
3
O
5
was examined. In recent years, perovskite dielectrics oxide such as SrTiO
3
, Ba
x
Sr
1-x
TiO
3
(BST), PbSr
x
Ti
1-x
O
3
(PZT), Pb
1-y
La
y
Zr
x
Ti
1-x
O
3
(PLZT), and SrBi
2
Ta
2
O
9
have been studied and if such dielectrics are realized, a capacity of 500 times or more that of the prior capacitor, can be ensured.
In connection with formation of ferro-electric memory, particularly, use of highly dielectric materials such as PZT can contribute to producing ferro-electric nonvolatile memory (FeRAM) that cannot delete information though external voltage is cut off, and so it has been attracting attention. Ferro-electric materials have spontaneous polarization and an electric field can reverse its orientation. Since ferro-electric materials generally have an ABO
3
type perovskite constitution (here, A represents one or more elements chosen from Ba, Sr, Pb, La, or divalent metal and B represents one or more elements chosen from Ti, Zr, or tetravalent metal), electric field having the reverse orientation needs to be impressed in order to transfer atoms at B sites to the other stable sites. Therefore, ferro-electric materials show a hysteresis characteristic and even when the electric field is 0, they maintain residual polarization and so are suitable and can be used as a memory. However, when the above mentioned dielectric oxide was employed for forming capacitors and memory, the following problems are produced.
Forming films of the dielectric oxide in an oxidizing atmosphere causes the first problem. Dielectric oxide is formed into films by a sol-gel method or a CVD (chemical vapor deposition) method. With respect to the sol-gel method, gels of metallic compound composing dielectric oxide are applied on the substrate by spinning and the substrate is dried, and then a high temperature, heat treatment is performed for the purpose of crystallizing dielectric oxide, and the heat treatment is carried out in an oxidizing atmosphere to prevent oxygen from being lost. The sputtering method is conducted in plasma containing oxygen and so, this method employs a so-called reactive sputtering style. The CVD method is performed by utilizing such energy as heat, plasma and light, and all these processes are carried out in an oxidation atmosphere to prevent oxygen from being lost.
Since an electrode film of ferro-electric capacitors is composed of the inoxidizable platinum family including Pt, or metals, that are conductive even when they are oxidized, such as Ir, Ru, or Os, forming a film of dielectric oxide in an oxidation atmosphere does not cause any problems. What causes problems is that oxygen infiltrates the electrode film and diffuses while it is being formed, and whereby a contact plug made of polycrystalline Si connected to the electrode film and a barrier layer made of Tin are oxidized. When the contact plug made of polycrystal Si and a barrier layer are oxidized, problems have been brought about in that resistance at the electrode is increased and adhesion is lowered, and accordingly, a ferro-electric capacitor thus produced cannot satisfy required performance and its yielding percentage is poor. For instance, K. Kusida-Abdelghafar et al. reported that when a barrier layer made of TiN was laminated on the Si substrate and thereon a Pt crystal film having a columnar structure was formed as a lower electrode film, while a PZT thin film was being formed, oxygen diffused through the grain boundary of Pt crystals, having a columnar structure and constituting the lower electrode film, over the surface of a TiN film and TiO
2
was formed in the midst of Pt in Mater. Res., 1998, Vol.13, p.3265.
The cause of oxygen infiltrating the lower electrode film easily is that the electrode film manufactured by a well-known prior method is composed of crystal grain layers having a columnar structure. Since the crystal grain layers in a columnar structure have large crystal grain diameters and crystal grain boundaries exit along the direction of electric conduction, conductivity is favorable, but oxygen permeability is high as well and so, the oxygen barrier performance is low.
As a means to enhance the oxygen barrier performance of the electrode film Matsui et al. disclosed a method for forming an electrode film, using Pt crystal having a granular structure, which had been difficult in the past and made a report that the oxygen barrier performance was improved compared with that of an electrode film having a columnar structure in the proceedings of the lecture meeting of the 44
th
Applied Physics Society Association, 1997, Vol. 2, p.437. However, since the granular structure is composed of micro-crystal grains, compared with the columnar structure, its oxygen barrier performance was improved, but resistivity was also higher and resulted in conduction failure. Further, crystallinity of the dielectric oxide thin film is affected by that of the touching electrode film and accordingly, when Pt in a granular structure, which is inferior in terms of crystallinity to a columnar structure, was employed as an electrode film, crystallinity of dielectric oxide is also worsened and resulted in lower specific inductive capacity and decreased residual polarization.
The second problem is conduction failure and adhesion failure occurring at the connected surface between respective films, caused by inter diffusion of constituent substances including oxygen occurred through the electrode films between the dielectric oxide thin film and contact plugs or barrier film. This problem causes not only difficulty while the films of dielectric oxide are being formed but also economical difficulty. For instance, the life of dielectric capacitors is shortened and reliability is lowered. More specifically, after a ferro-electric capacity is manufactured, inter diffusion of constituent substances including oxygen gradually occurs through the electrode films and results in conduction failure and peeling at the connected surfaces between respective layers. When the above-mentioned Pt crystals having a columnar structure are used as the electrode films, sufficient conductivity of currents is secured, but substance transfer through grain boundaries of the columnar structure occurs relatively easily. On the other hand, when the electrode films are composed of crystals having a granular structure, barrier performance is high, but conductivity of current is low and the films were not good for practical use.
The third problem is that, when ferro-electric memory is formed of dielectric oxide, rewritability is insufficient. Dielectric oxide has so-called fatigue characteristics, namely a repetition of reverse polarization results in a decrease in residual polarization. The chief causes of the fatigue characteristics are diffusion of the metals constituting the electrode films into the dielectric oxide thin films, leakage currents through crystal gra
Hayes & Soloway P.C.
NEC Corporation
Nelms David
Tran Mai-Huong
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