In the fabrication of a semiconductor integrated circuit (IC), it is desirable to fabricate the IC with materials having a low resistivity (i.e., property of resistance to current flow) in order to optimize its electrical performance by decreasing the resistance of the IC. Lower resistance ICs allow faster processing of information due to a smaller delay time associated with resistance to current flow therethrough.
Individual devices are typically connected within an IC using metal lines (i.e., conductive layers), such as aluminum or copper layers. Resistivity of metal lines plays an increasingly important role in the overall resistance of an IC. As ICs become more dense, wiring length increases. Furthermore, wiring pitch decreases, which effectively decreases the wiring width. As the wiring width decreases, resistivity of the wiring material becomes a dominant factor as compared to parasitic capacitance between wires (i.e., that associated with device resistance). Thus, it is desirable to decrease the resistivity of wiring material within an IC.
It is preferable to use copper within an IC, particularly for interconnect lines and structures (i.e., conductive digit lines and plugs connecting the conductive layers), because copper has a lower resistivity and a higher resistance to electromigration (i.e., the transport of metal atoms in conductors carrying large current densities, resulting in morphological degradation of the conductors) than aluminum. Voids (i.e., regions of macroscopic depletion of atoms) and hillocks (i.e., regions of macroscopic accumulation of atoms) are produced by electromigration. One reason why copper is less susceptible to voiding than aluminum is because the grain boundary diffusion of vacancies in copper has a significantly higher activation energy than the same in aluminum.
Techniques for deposition of copper during fabrication of a semiconductor IC have not been selectively controlled in the past. Such conventional copper deposition techniques include evaporation, sputtering, and chemical vapor deposition (CVD). When using nonselective deposition techniques, excess copper often needs to be removed from surfaces to which it adheres, but on which copper is not desired. This requires an extra processing step. An etchant, such as a dry, chlorine-based plasma etchant is typically used for removing excess copper on a surface.
Deposition of copper on certain materials, such as titanium-containing materials (e.g., titanium and titanium nitride), has also been problematic in the past. Titanium-containing materials are utilized in the fabrication of interconnect structures in which copper is typically formed. For example, titanium-containing diffusion barrier layers are beneficial when formed between copper and silicon because copper has a tendency to diffuse into silicon. The use of diffusion barrier layers prevents the degree of copper migration seen in the absence of a diffusion barrier layer. By using a titanium-containing diffusion barrier layer, the degree of lattice mismatch between copper and silicon is also minimized, as the lattice spacing of titanium is intermediate between that of copper and that of silicon. Previous deposition of copper on titanium-containing materials, however, has repeatedly been plagued with nonuniform thicknesses, poor adhesion, and poor step coverage on complex surfaces, such as contact holes and vias.
One deposition technique involves the chemical reduction of a metal ion from a metal compound contained in solution onto a catalytically active surface. This is known as electroless deposition. Consequently, this technique has the potential to selectively deposit on catalytically active surfaces. Conventional electroless copper deposition on titanium-containing materials, however, has not been perfected. For example, electrodeposition baths (i.e., ionic solutions without any external electrodes) containing copper sulfate (CuSO.sub.4) and sulfuric acid (H.sub.2 SO.sub.4) often experience difficulties with rapid oxidation (i.e., the formation of native oxides) of the titanium-containing material surface during deposition, which prevents adequate copper layer adhesion. Similarly, baths containing copper pyrophosphate (Cu.sub.2 P.sub.2 O.sub.7), potassium pyrophosphate (K.sub.2 H.sub.2 P.sub.2 O.sub.7), ammonium hydroxide (NH.sub.4 OH), and ammonium nitrate (NH.sub.4 NO.sub.3), or baths containing copper fluoroborate (Cu[BF.sub.4 ].sub.2), fluoroboric acid (HBF.sub.4), and boric acid (HBO.sub.3) do not provide adequate copper layer adhesion due to the fact that deposited copper dissolves away in water. Another electrodeposition bath containing tetra-ammonium cuprate (Cu[NH.sub.3 ].sub.4) and ammonium hydroxide (NH.sub.4 OH) does not deposit copper well on titanium-containing diffusion barrier layer materials either.
Conventional electroless copper deposition baths often contain alkali elements, such as lithium, sodium, and potassium, to increase the pH and thereby increase the reaction rate of the electroless deposition process. Alkali components provide a relatively large increase in the pH of an electroless copper deposition bath for a given amount of the alkali components. However, it is undesirable to utilize large amounts of alkali elements in the fabrication of ICs because residual alkali metal ions easily drift under applied electric fields to interfaces, such as silicon/silicon dioxide interfaces within an IC, introducing positive ionic charge in undesired areas that alters device characteristics. This phenomenon potentially causes IC failure, due to such altering of device characteristics.
Another technique for depositing copper on a substrate from an electroless deposition solution includes using a nucleating layer between the substrate and the deposited copper layer in order to initiate copper deposition. In the past, aluminum has been utilized for such a nucleating layer. While the use of such a layer catalyzes the reaction, it does not increase the reaction rate of copper deposition enough to allow for an alkali-free electroless deposition solution to be used in conjunction therewith. Furthermore, when using such a technique, an aluminum layer remains between the copper layer and the substrate. This can potentially cause adhesion problems and increase the resistivity of interconnects when used therein due to the higher resistivity of aluminum as compared to copper. Resistivity of interconnects formed in such a manner is also typically further increased when using such a technique because an oxide layer usually remains on the aluminum layer prior to depositing the copper layer thereon.
Another problem with conventional electroless copper deposition is that implanted nucleation sites need to be formed on the surface on which copper is to be deposited when a nucleation layer is not formed on the underlying surface. Conventionally, electroless deposition of copper onto certain materials, such as titanium-containing materials, will not occur in the absence of implants (e.g., gold, silver, palladium, or platinum) in the underlying surface or activation baths containing similar components. The presence of implants in the underlying surface or the activation bath is needed to provide nucleation sites for the copper deposition reaction. Once copper nucleation is initialized, however, deposited copper acts as its own catalyst via an autocatalytic mechanism (i.e., copper provides more of the catalyzing mechanism--itself--as it is created).
Electroless deposition of copper has been used in printed circuit board (PCB) manufacturing and other applications where critical dimensions are ten microns or greater. Typically, deposition of copper within an IC requires that copper be accurately deposited to much smaller critical dimensions. However, many conventional electroless copper deposition techniques have not perfected accurate deposition at such small critical dimensions.
Thus, there is a need for a method for electroless deposition of copper and other materials that is generally fast and efficient. There is a further need for a method for electroless deposition, such as copper electroless deposition, that does not require nucleation layers, implants, or activation baths for providing nucleation sites on surfaces on which the material is to be deposited. It is further desirable to provide a method for electroless deposition that provides a high conductivity layer of deposited material that adheres well to a substrate.