Method for forming Co-W-P-Au films

Details Method for forming Co-W-P-Au films Method for forming Co-W-P-Au films

2001-09-27

2003-11-11

Fahmy, Wael (Department: 2814)

Active solid-state devices (e.g., transistors, solid-state diode

Combined with electrical contact or lead

Of specified material other than unalloyed aluminum

C257S764000

Reexamination Certificate

active

06646345

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a method for forming quaternary composite films of Co—W—P—Au and devices formed containing such films and more particularly, relates to a method for forming quaternary films of Co—W—P—Au by first forming a Co—W—P alloy film and then electrolessly depositing a Au film on top of the alloy film such that Au diffuses into the alloy film forming the quaternary composite film and devices formed containing such film.
BACKGROUND OF THE INVENTION
The technology of making interconnections for providing vias, lines and other recesses in semiconductor chip structures, flat panel displays, and package applications has been developed for many years. For instance, in developing interconnection technology for very-large-scale-integrated (VLSI) structures, aluminum has been utilized as the primary metal source for contacts and interconnects in semiconductor regions or devices located on a single substrate. Aluminum has been the material of choice because of its low cost, good ohmic contact and high conductivity. However, pure aluminum thin-film conductors have undesirable properties such as a low melting point which limits its use to low temperature processing, possible Si diffusion into Al during annealing which leads to contact and junction failure, and poor electromigration resistance. The electro migration phenomenon occurs when the superposition of an electronic field onto random thermal diffusion in a metallic solid causes a net drift of ions. Consequently, a number of aluminum alloys have been developed which provided advantages over pure aluminum. For instance, U.S. Pat. No. 4,566,177 discloses a conductive layer of an alloy of aluminum containing up to 3% by weight of silicon, copper, nickel, chromium and manganese to improve electromigration resistance. U.S. Pat. No. 3,631,304 discloses aluminum alloys with aluminum oxide which were also used to improve electromigration resistance.
More recently VLSI and ULSI technology has placed more stringent demands on the wiring requirements due to the extremely high circuit densities and faster operating speeds required of such devices. This leads to higher current densities in increasingly smaller conductor lines. As a result, higher conductance wiring is desired which requires either larger cross-section wires for aluminum alloy conductors or a different wiring material that has a higher conductance. The obvious choice in the industry is to develop the latter using copper based on its desirable high conductivity.
In the formation of VLSI and ULSI interconnection structures such as vias and lines, copper is deposited into a line, via or other recesses to interconnect semiconductor regions or devices located on the same substrate. Copper is known to have problems at semiconductor device junctions due to its fast reaction rate with Si. Any diffusion of copper ions into the silicon substrate can cause device failure. In addition, pure copper does not adhere well to oxygen containing dielectrics such as silicon dioxide and polyimide.
It is therefore an object of the present invention to provide a diffusion barrier layer between a copper interconnect and other semiconductor materials that does not have the drawbacks or shortcomings of the conventional diffusion barriers.
It is another object of the present invention to provide a diffusion barrier layer between a copper interconnect and a silicon substrate in a semiconductor structure.
It is a further object of the present invention to provide a diffusion barrier between a copper interconnect and a dielectric material layer in which the interconnect is formed.
It is another further object of the present invention to provide a diffusion barrier layer for a copper interconnect in a semiconductor structure wherein the barrier layer is a quaternary composite film of Co—W—P—Au.
It is still another object of the present invention to provide a diffusion barrier layer for a copper interconnect in a semiconductor structure wherein the barrier layer consists of a Au film coated on a Co—W—P alloy film.
It is yet another object of the present invention to provide a diffusion barrier layer for a copper interconnect in a semiconductor structure which is formed by two sequential electroless plating processes.
It is still another further object of the present invention to provide a method for forming a quaternary Co—W—P—Au composite film by first electroless plating a Co—W—P film on the surface of a copper conductive region in a plating bath containing cobalt ions, tungstate ions, citrate ions and a reducing agent, and then immersing the substrate in a Au electroless plating solution for depositing a Au layer on top.
It is yet another further object of the present invention to provide an electronic structure which includes a Co—W—P—Au composite film coating in a via opening that is filled with copper for forming a copper interconnect.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for forming a diffusion barrier layer for a copper interconnect in a semiconductor structure and the structure formed are disclosed.
In a preferred embodiment, a method for forming a Co—W—P—Au composite film on a substrate can be carried out by the operating steps of cleaning a substrate which has a copper conductive region in 0.2 M H
2
SO
4
for 10~20 sec., then rinse in H
2
O for 60 sec., the substrate is then immersed in a solution that contains palladium ions for a length of time sufficient for palladium metal to deposit by a redox reaction on surfaces of the copper conductive regions, rinsing the substrate in H
2
O, immersing the substrate in a solution containing at least 15 gr/l sodium citrate or EDTA (Ethylene diaminetetra acetic acid) complexing agents for removing excess palladium ions from the surfaces of the copper conductive regions, rinsing the substrate with distilled water, electroless plating a Co—W—P film on the surfaces of copper conductive regions in a plating solution containing cobalt ions, tungstate ions, citrate ions and a reducing agent, rinsing the substrate with distilled water, and immersing the substrate in a Au electroless plating solution for depositing a Au layer on top of the Co—W—P film alloy.
In the method for forming a Co—W—P—Au film on a substrate, the first and second immersing steps are carried out at ambient temperature. The method may further include the step of mixing a first plating solution of cobalt ions/tungstate ions at a ratio of between about 1 and about 10, stabilizing the plating solution with citrate ions at a citrate ions/cobalt ions ratio of not less than 3, adjusting the pH of the plating solution in the range between 7 and 9 by using a NaOH and boric acid buffer and adding a hypophosphite reducing agent. The method may further include the step of maintaining the first plating solution at a temperature between about 65° C. and about 85° C. The method may further include the step of mixing the first plating solution of cobalt ions/tungstate ions preferably at a ratio of between about 2 and about 4. The method may further include the step of stabilizing the first plating solution with citrate ions at a citrate ions/cobalt ions ratio preferably not less than 5. The method may further include the step of maintaining the first plating solution at a temperature preferably between about 70° C. and about 80° C.
In the method for forming a Co—W—P—Au film on a substrate, the process for forming by adding boric acid at a concentration of at least three times that of the cobalt ions, and adding a surface active agent. The method for forming the first plating solution may further include the step of adding a reducing agent containing hypophosphite at a concentration of at least 1.2 times that of the cobalt ions. The method may further include the step of immersing the substrate in a solution containing preferably at least 30 gr/l of sodium citrate or ethylenediamine tetra acetic acid, sodium salt (EDTA) for removing excess palladium ions.
In the step of electroless plating a Co—W—P film on the substrate

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