Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating predominantly nonmetal substrate
Reexamination Certificate
1997-12-10
2001-07-10
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating predominantly nonmetal substrate
C205S164000, C205S184000, C205S104000, C205S170000, C205S241000, C205S263000, C205S271000, C205S266000, C205S283000, C205S252000, C205S291000, C205S300000, C205S305000, C205S158000
Reexamination Certificate
active
06258241
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electroplating metals. In general, metals are difficult to electroplate to a resistive substrate, such as a conductive plastic, a carbon-filled plastic, an insulator with a resistive coating, and a resistive porous electrode. Because of the resistivity of the substrate, metal ions in the electroplating bath are deposited around electrical contact points of the resistive substrate, causing a non-uniform metal layer to develop on the substrate. In particular, a large accumulation of the metal will occur around the electrical contact points, and a negligible amount of metal will accumulate elsewhere.
For example, as shown in the cross-sectional view of
FIG. 1
, to electroplate a copper layer
1
onto a silicon wafer
2
, a barrier film
3
having resistive properties is first placed on the silicon wafer
2
. Conventionally, when copper is electroplated onto the barrier film, copper is deposited around the electrical contact points, which are usually located on the perimeter of the silicon wafer. This resulting copper layer is non-uniform, as illustrated in FIG.
1
. In particular, the copper is deposited in an upwardly extending rim around the perimeter of the barrier film
3
, and negligible amounts of copper are deposited in the middle of the barrier film
3
.
To correct for this non-uniform layer, an initial copper layer is deposited on the barrier film through a means other than electroplating. For example, sputtering, chemical vapor deposition or electroless plating is conventionally used to deposit the initial copper layer on the barrier film. This results in forming a layer with relatively low resistance, and which is conducive to being electroplated with copper. However, this conventional approach has drawbacks. In particular, the sputtering and chemical vapor deposition are expensive. Moreover, electroless plating is slow and is difficult to control. Thus, improved methods for electroplating metals, particularly copper, onto a resistive substrate are desired.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for electroplating a metal layer having substantially uniform thickness onto a resistive substrate. The phrase “substantially uniform thickness” defines a metal layer thickness having a minimum thickness of approximately the same order of magnitude as the maximum thickness for the metal layer, and the phrase “non-uniform thickness” defines a metal layer thickness that is not substantially uniform.
It is another object of the present invention to provide a process for electroplating a metal layer having substantially uniform thickness onto a thin resistive film covering a resistive or insulating substrate.
It is a further object of the present invention to provide a process for electroplating a metal layer having substantially uniform thickness onto a resistive substrate without first depositing a non-resistive layer on the resistive substrate.
It is an additional object of the present invention to electroplate a metal layer having substantially uniform thickness onto a resistive substrate without first depositing a film of the metal using, for example, sputtering, chemical vapor deposition or electroless plating.
The above objects and advantages of the present invention are achieved by a process for electroplating metal on a resistive substrate and by an article of manufacture derived therefrom. The method comprises electroplating a metal layer onto the resistive substrate in an electroplating bath having a polarization parameter &xgr; less than approximately 10 such that the metal layer is substantially uniform. In particular, the polarization parameter &xgr; is defined as &xgr;=i
o
r
1
2
&agr;
c
F/&kgr;hRT, where i
o
is the exchange current density of the electroplating bath; r
i
2
is the square of the maximum distance between a point of the electroplated area and the nearest point of electrical contact with a power supply lead; &agr;
c
is the cathodic transfer coefficient; F is Faraday's constant; &kgr; is the conductivity of the metal; h is the thickness of the metal layer; R is the gas constant; and T is the temperature. Note that the exchange current density, i
o
, is defined based on a kinetic model, for example, the Butler-Volmer model discussed below (see Eqn. (7) and its surrounding discussion).
A polarization parameter &xgr; of less than approximately 10 can be achieved, for example, by a low metal ion concentration of less than approximately 0.1×10
−3
mole/cm
3
(or 0.1 M) in the electroplating bath, by adding an additive to the electroplating bath, or by both providing a low metal ion concentration and adding an additive to the electroplating bath.
A substrate is defined to be resistive if it has a film resistance parameter Rf greater than approximately 10 ohm-cm
2
, where the film resistance parameter Rf is defined as Rf=r
1
2
/&kgr;h.
Before electroplating the metal layer, a thin resistive film can be provided on the resistive substrate, where the thin resistive film comprises a material other than the material of the resistive substrate. Further, before electroplating the metal layer, an initial metal layer can be deposited on the resistive substrate, or on a thin resistive film first provided on the resistive substrate.
To increase the rate at which the metal is electroplated after an initial metal layer is formed using the electroplating bath having a polarization parameter &xgr; less than approximately 10, the metal ion concentration in the electroplating bath can be increased or the resistive substrate with the formed initial layer can be transferred to a conventional electroplating bath.
In addition, to increase the rate at which the metal is electroplated after the initial metal layer is formed, the plating current density i
s
in the electroplating bath can be increased in relation to the increasing thickness of the metal layer while keeping the polarization parameter &xgr; constant.
Further, the electroplating can be accomplished by pulse electroplating.
Moreover, a product can be produced according to the process of the present invention.
The article of manufacture comprises a resistive substrate and a metal layer of substantially uniform thickness electroplated onto the resistive substrate in an electroplating bath having a polarization parameter &xgr; less than approximately 10.
The above objects and advantages of the present invention are illustrative, and not exhaustive, of those which can be achieved by the present invention. Thus, these and other objects and advantages of the present invention will be apparent from the description herein or can be learned from practicing the invention, both as embodied herein and as modified in view of any variations which may be apparent to those skilled in the art.
REFERENCES:
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patent: 4347108 (1982-08-01), Willis
patent: 4374709 (1983-02-01), Combs
patent: 4473448 (1984-09-01), Deeman
patent: 4666567 (1987-05-01), Loch
patent: 4673469 (1987-06-01), Beach et al.
patent: 5275715 (1994-01-01), Tuttle
patent: 5328589 (1994-07-01), Martin
Antisiferov, et al., “Electrodeposition of Metals on Highly Porous Cellular Materials,” Proc. Int. Conf. Surf. Sci. Eng. (1995), Abstract, No Month available.
Gluck Jeffrey W.
Lucent Technologies - Inc.
Sartori Michael A.
Venable
Wong Edna
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