Electrical connections to dielectric materials

Details Electrical connections to dielectric materials Electrical connections to dielectric materials

2001-02-07

2001-08-14

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S313000, C257S306000, C257S310000

Reexamination Certificate

active

06275370

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to improving electrical connections to materials with high-dielectric-constants, such as in the construction of capacitors.
BACKGROUND OF THE INVENTION
Without limiting the scope of the invention, its background is described in connection with current methods of forming electrical connections to high-dielectric-constant materials, as an example.
The increasing density of integrated circuits (e.g. DRAMs) is increasing the need for materials with high-dielectric-constants to be used in electrical devices such as capacitors. The current method generally utilized to achieve higher capacitance per unit area is to increase the surface area/unit area by increasing the topography, such as in trench and stack capacitors using SiO
2
or SiO
2
/Si
3
N
4
as the dielectric. This approach becomes very difficult in terms of manufacturability for devices such as the 256 Mbit and 1 Gbit DRAMs.
An alternative approach is to use a high permittivity dielectric material. Tc, be useful in electronic devices, however, reliable electrical connections should. generally be constructed which do not diminish the beneficial properties of these high-dielectric-constant materials.
Heretofore, in this field, single and multiple metal layers are generally used to form electrical contacts to high-dielectric-constant materials. For example, to provide an electrical connection to a high-dielectric-constant material which makes up a capacitor on the surface of a semiconductor substrate, the following techniques are among those now employed: (a) dielectric/platinum/substrate, (b) dielectric/platinum/tantalum/substrate, and (c) dielectric/platinum/titanium/substrate. The layering sequence in these examples is from the top down to the substrate (e.g. silicon). A similar metallization scheme may be used for the top of the dielectric film to complete the capacitor structure.
SUMMARY OF THE INVENTION
As used herein the term high-dielectric-constant (hereafter abbreviated HDC) means a dielectric constant greater than about 20. HDC materials are useful for the fabrication of many electrical devices, such as capacitors. However, HDC materials are generally not chemically stable when deposited directly on a semiconductor substrate, so one or more additional layers are required to provide the electrical connection between the HDC material and the substrate. The additional layer or layers should generally be chemically stable when in contact with the substrate and also when in contact with the high-dielectric-constant material.
Current methods provide for using platinum as the noble layer to contact the high-dielectric-constant material, along with tantalum or titanium as the sticking layer to contact the substrate. However, HDC materials (e.g. ferroelectrics) are generally deposited at elevated temperatures (greater than about 500° C.) in an O
2
atmosphere. It has been discovered that, in this environment, oxygen diffuses through the platinum and forms a resistive layer of Ta
2
O
5
or TiO
2
when it comes in contact with the Ta or Ti, causing high contact resistance. Further, the substrate (e.g. silicon) itself can become oxidized during the deposition of the HDC material. As used herein, the term oxidizable layer refers to the underlying sticking layer, or substrate, which becomes more insulating when oxidized.
The disclosed structures generally provide electrical connection to HDC materials without the disadvantages of the current structures. One embodiment of this invention comprises an oxidizable layer, an oxygen gettering layer overlaying the oxidizable layer, a noble metal layer overlaying the oxygen gettering layer, and a high-dielectric-constant material layer overlaying the noble metal layer. A method of forming an embodiment of this invention comprises the steps of forming a oxygen gettering layer on an oxidizable layer, forming a noble metal layer on the oxygen gettering layer, and forming a high-dielectric-constant material layer on the noble metal layer. Examples of processes for depositing the lower electrode materials are sputtering, evaporation, and chemical vapor deposition. Examples of processes for depositing the high-dielectric-constant material are ion-beam sputtering, chemical vapor deposition, and pulsed laser deposition.
These are apparently the first structures wherein an electrical connection to high-dielectric-constant materials comprises an oxygen gettering layer. The oxygen gettering layer controls oxygen diffusion, minimizing the formation of a resistive layer either in the oxidizable layer or at the oxidizable layer/oxygen gettering layer interface. The oxygen gettering layer acts as a gettering site for oxygen, wherein the oxygen oxidizes the reactive metal portion of the layer, leaving the noble metal portion of the layer intact. While the oxides or suboxides that are formed are resistive, they are dispersed within the noble metal matrix, leaving a conductive path from the top of the layer to the bottom. The oxygen gettering layer should generally contain enough reactive metal to successfully combine with most or all of the diffused oxygen, but not so much that there is not a conductive path remaining via the noble metal component of the layer. Generally, the required thickness and composition of the oxygen gettering layer depend on the specific deposition parameters (temperature, O
2
pressure, etc.) of the high-dielectric-constant material. If enough oxygen reaches the oxidizable layer (e.g. the sticking layer or the substrate), a resistive layer will be formed, significantly increasing the contact resistance. The noble metal layer between the high-dielectric-constant material layer and the oxygen gettering layer is desirable, as it both minimizes undesirable reduction of the high-dielectric-constant material layer and lowers the amount of oxygen which enters the oxygen gettering layer. This invention generally provides a stable electrode for HDC materials while using standard integrated circuit materials to facilitate and economize the manufacturing process.


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