Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Of specified material other than unalloyed aluminum
Reexamination Certificate
1999-05-17
2001-08-07
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Of specified material other than unalloyed aluminum
C257S762000, C257S771000
Reexamination Certificate
active
06271591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated circuit fabrication techniques. In particular, the present invention relates to copper-aluminum metallization.
2. Discussion of the Related Art
In the past, pure aluminum metallization had been used in integrated circuits because of the low cost, good ohmic contact, and high conductivity of aluminum. More recently, aluminum alloys that provide advantages over pure aluminum have also been developed. For example, aluminum is alloyed with copper to provide an aluminum alloy with improved electromigration resistance. However, increasing circuit densities and faster operating speeds of integrated circuit technology require the use of metals having higher conductivities than that of aluminum.
One example of a metal with a conductivity higher than that of aluminum is copper. In the fabrication of an integrated circuit device utilizing pure copper metallization, copper is deposited into vias, trenches, or other recesses to interconnect semiconductor devices or conductive layers formed on a semiconductor substrate. For example,
FIG. 1
illustrates a cross-section of a recess
18
in a portion of a prior art copper metallization structure
10
. Referring to
FIG. 1
, in copper metallization structure
10
, a copper wire
16
is used to interconnect two or more conductive regions formed in a substrate. Copper wire
16
is formed in a recess
18
defined in a dielectric layer
12
. Dielectric layer
12
is typically formed out of silicon dioxide. A diffusion barrier
14
which is formed out of a metal such as tantalum (Ta), Tungsten (W), Chromium (Cr) or a metal composite such as titanium-nitride, titanium-tungsten, tungsten-nitride, or tantalum nitride, is provided as both a diffusion barrier
14
and as an adhesive layer which improves the adhesion of copper wire
16
to dielectric layer
12
.
A standard technique such as physical vapor deposition or chemical vapor deposition can be used to form diffusion barrier
14
and copper wire
16
in recess
18
. However, physical vapor deposition of copper has poor step-coverage, resulting in void or seam formation. Further, a chemical vapor deposition technique requires careful control of selectivity, processing temperatures, and incurs high capital costs. As a result, electroless copper deposition has been suggested as a superior alternative technique for fabricating copper metallization. This is because electroless deposition techniques incur less capital costs, provides high quality deposited films, is inherently selective and is a conformal deposition process.
One example of electroless copper deposition is described in copending patent application Ser. No. 08/887,264 filed Jan. 16, 1996. Electroless copper deposition, which proceeds at relatively low temperatures, can be used to fill vias, trenches, or other recesses in dielectrics, and to fabricate in-laid copper metallization. Moreover, electroless copper deposition also offers advantages of low cost, high throughput, high quality electroless copper films, and superior recess filling capability.
However, pure copper metallization also has undesirable qualities that present integrated circuit reliability problems. For example, without proper measures, copper is easily oxidized and diffuses into silicon and silicon oxide, causing device failure. Also, pure copper does not adhere well to dielectrics such as silicon dioxide, and presents corrosion problems. As a result, aluminum has been suggested as a doping material to be introduced into the copper metallization to improve oxidation resistance and to act as a diffusion barrier. (See P. J. Ding, W. A. Lanford, S. Hymes, and S. P. Murarka, “Effects of the addition of small amounts of Al to copper: Corrosion, resistivity, adhesion, morphology, and diffusion,” Journal of Applied Physics, Volume 75 Number 7).
Fabricating copper-aluminum metallization can be accomplished using a variety of well known techniques. For example, a copper/aluminum bilayer can be deposited by sputtering. Alternatively, a copper-aluminum alloy film can be obtained by annealing an aluminum film provided above or below a copper film. In a conventional process, an annealing step and a wet etch step are required after the copper/aluminum bilayer or the copper-aluminum alloy is formed. Thus, a process for fabricating copper-aluminum metallization based on electroless copper deposition is desired.
Unfortunately, an electroless copper deposition process for fabricating copper-aluminum metallization has proved to be extremely difficult to develop. One difficulty is that co-deposition of aluminum with copper cannot be achieved by electroless plating, because the electrochemical potential of aluminum reduction is highly negative. Another difficulty is that electroless copper deposition on aluminum films cannot be achieved because aluminum films dissolve in a basic electroless copper solution (pH>11).
Accordingly, it would be desirable to provide an improved method for fabricating copper-aluminum metallization using electroless copper deposition.
SUMMARY OF THE INVENTION
The present invention provides an improved method for fabricating copper-aluminum metallization. The present invention also provides improved copper-aluminum metallization structures formed using the fabrication method.
The first step in the method of the present invention is to form a copper-aluminum alloy film in a recess defined in a dielectric such as silicon dioxide. Alternatively, a copper/aluminum bilayer film is deposited on the surfaces of the recess. In either process, a diffusion barrier layer can be formed on the surfaces of the recess before depositing the copper-aluminum alloy film or copper/aluminum bilayer. The diffusion barrier layer includes a metal composite selected from the group of tantalum (Ta), tantalum nitride (TaN), tungsten (W), tungsten nitride (WN), titanium tungsten (TiW), titanium nitride (TiN), tantalum silicon nitride (TaSiN), titanium silicon nitride (TiSiN), or tungsten silicon nitride (WSiN).
One embodiment of the present invention deposits electroless copper into a recess in the substrate. In particular, this embodiment uses an electroless copper deposition to form a copper seed layer on the copper-aluminum alloy film, followed by a copper electroplating step to fill the recess. In another embodiment of the present invention, the recess is filled by depositing electroless copper on a copper-aluminum alloy film.
In the method of the present invention, chemical-mechanical polishing can be applied to the plated copper.
In one embodiment of the present invention low-temperature annealing of in-laid electroless copper deposited on the copper-aluminum alloy film can be used to form a self-encapsulated copper-aluminum metallization structure. In particular, the annealing of the in-laid electroless copper deposited on the copper-aluminum alloy film forms an in-laid copper-aluminum alloy. The annealing results in a metallization structure that is self-encapsulated because (i) a layer of aluminum oxide (Al
2
O
3
) is formed on the exposed surfaces of the in-laid copper-aluminum alloy, and (ii) aluminum oxide and copper-aluminum alloy (Al
2
O
3
+CuAl) is formed on surfaces of the in-laid copper-aluminum alloy that are in contact with the diffusion barrier layer.
Thus, the present invention provides a method for fabricating copper-aluminum metallization using electroless copper deposition. Hence, the method of the present invention represents a significant improvement over prior art fabrication methods.
The self-encapsulated copper-aluminum metallization structures formed in a fabrication method of the present invention provide the following advantages over prior art copper metallization structures: (i) improved adhesion, (ii) a self-aligned diffusion barrier (i.e., aluminum oxide (Al
2
O
3
) on exposed surfaces of the in-laid copper-aluminum alloy, and aluminum oxide and copper-aluminum alloy (Al
2
O
3
+CuAl) on surfaces of the in-laid copper-aluminum alloy that are in contac
Dubin Valery
Ting Chiu
Advanced Micro Devices , Inc.
Kwok, Esq. Edward C.
Potter Roy
Skjerven Morrill & MacPherson LLP
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