Patent Application
20050006770
The present invention generally relates to CMOS interconnect structures, and more specifically relates to an interconnect liner for a CMOS interconnect structure.
Since the introduction of 0.25 μm technology, interconnects are becoming the limiting factor for device speed, primarily due to interconnect resistivity as well as the RC delay induced by adjacent interconnects. A solution for reducing the resistance of an interconnect is to use copper rather than aluminum. Copper has been chosen because it is a metal with better conductivity than aluminum, and it has good resistance to electromigration (EM).
Since copper cannot be easily dry etched, the use of damascene or dual damascene structures is becoming a key solution to realize copper integration. In a damascene-type structure, the pattern is etched into a dielectric layer, followed by metal fill and planarization. Dual damascene structures are advantageous in that they incorporate both lines and vias in one deposition step. This reduces the number of process steps and, as a result, is cost-effective. More specifically, two patterning steps are used to create features of two different depths, blanket metal is deposited, and then a single chemical mechanical polishing (CMP) step is used to create the inlaid structure. This is the current process used by many integrated circuit (IC) companies to integrate copper into a circuit.
Implementation of copper and low-K materials as major components of interconnect structures has resulted in some new electromigration (EM)-induced problems that have not been detected in the case of aluminum interconnects. This is especially true for copper metallization characterized by the complicated geometry of dual damascene interconnect segments, and by much lower critical stress for void nucleation than aluminum. The latter fact clarifies an important role of non-uniform stress, current distribution and temperature distribution in failure development. Another important aspect is the presence of a “weak” interface between copper and an etch stop layer such as silicon nitride (SiN) or silicon carbide (SiC). This interface is characterized by fast migration of the copper atoms in comparison with other migration channels, such as grain bulks, grain boundaries and interfaces between copper and diffusion bather liners, for example, Ta/TaN. Experimental studies demonstrate that such copper dual damascene segments result in bimodal failure. As such, some structures fail early, which results in a very short mean-time-to-failure (MTF), which is a factor of 100-1000 shorter than a typical MTF. A lot of effort has been spent to increase a copper-based interconnect's resistance to EM-induced failure. A main direction has been incorporation of an additional process step, which is the deposition of cobalt-tungsten alloy capping. This additional layer removes the copper-SiN(SiC) interface, which is responsible for the reduced MTF of the copper interconnect.
This approach can improve the overall interconnect resistance to EM-induced failure. However, it cannot affect early failure, which is perhaps the most dangerous type of failure in that it renders the chip unuseable. On the basis of available experimental data, it can be concluded that one of the characteristics which contributes to early failure is a void formed at the bottom of a via. If nucleation of such type of voids can be suppressed, then early failure can be eliminated. Of course, this would not mean that the interconnect will be completely EM-failure resistant, as any of the via-containing segments can fail. However, the failure will take longer to occur, and will be caused by a void nucleated at the top corner of the upper line and extended along the line.
In the past, a transient, three-dimensional, fully-linked EM-model has been developed. The developed model targets void nucleation and its growth kinetics as a function of the segment geometry, interconnect architecture and the electrical loading. Coupling of the electromagnetics, heat transfer, structural mechanics and atom migration models, based on a direct solution of the system of partial differential equations in the finite element method (FEM) environment, has allowed for the simulation of stress-induced void nucleation and growth in different interconnect segments. Implementation of this model into a simulation scheme illustrates that there is void nucleation in the case of dual damascene via segment, and that the void leads to long-term failure in the case of highly-conductive liners. Simulation results demonstrate that the character of void evolution strongly depends on the liner conductivity. If this conductivity is very low, then the void nucleates quickly at the bottom of the via. The lower the liner conductivity, the faster void nucleation occurs. Simulations with regard to almost nonconductive liners show that the void at the bottom of the via results in early failure. It should be noted that the nucleation time for the latter void is shorter, by a factor of 100-1000, than for the previous one.
Hence, to be able to suppress void nucleation at the bottom of a via, thereby eliminating the cause of early failure, the conductivity of the via liner must be increased. Liners in the case of copper interconnect are intended to protect the inter-layer dielectric (ILD) from copper diffusion. Typically, refractory metals are used as an interconnect liner and there is a reason. Refractory materials such as Ti/TiN or Ta/TaN are characterized by an extremely low intrinsic diffusivity as well as diffusivity of impurities, which is important property for a diffusion barrier. However, at the same time, these materials are characterized by a low electrical conductivity, which might be responsible for void nucleation at the bottom of a via.
An object of an embodiment of the present invention is to provide a copper dual damascene-based interconnect which is characterized by an enhanced resistance to early failure.
Another object of an embodiment of the present invention is to provide a copper dual damascene-based interconnect which is characterized by a low intrinsic diffusivity, a low diffusivity of impurities, and high electrical conductivity.
Still another object of an embodiment of the present invention is to provide a copper dual damascene-based interconnect liner which adheres well to both copper and silicon oxide.
Still yet another object of an embodiment of the present invention is to provide a copper dual damascene-based interconnect liner which eliminates early failure, increases interconnect MTF, and improves overall interconnect reliability.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a dual damascene-based interconnect structure which includes a liner of aluminum-0.5% copper alloy. The alloy can be implemented by depositing the alloy using a conventional PVD technique. To completely secure against copper atoms possibly penetrating through the aluminum-0.5% copper alloy, one or more Ta/TaN liners can be employed in addition to the aluminum-0.5% copper alloy liner. If Ta/taN is to be used, preferably the Ta/TaN is deposited before the aluminum-0.5% copper alloy is deposited.
The organization and manner of the structure and operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawing, wherein:
While the invention may be susceptible to embodiment in different forms, there are shown in the drawings, and herein will be described in detail, specific embodiments with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that as illustrated and described herein.
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The aluminum-0.5% copper alloy has a conductivity which is much higher than the conductivity of conventional liners. Additionally, the low diffusivity of copper as well as aluminum atoms is reached by copper atom precipitations at the aluminum grain boundaries. Good adhesion of aluminum to the copper and to silicon oxide, as well as to any oxygen containing compounds, provides additional benefits. Implementing aluminum-0.5% copper alloy, such as by depositing by a conventional PVD technique, as a liner for copper-based interconnect eliminates early failure and improves overall interconnect reliability. To be completely secured of the possible copper atoms penetration through the aluminum-0.5% copper alloy-based diffusion barrier into the ILD, one or more Ta/TaN liners is used in addition to aluminum-0.5% copper alloy liner.
While increasing the thickness of the Ta/TaN layer is a possible way to increase liner conductivity (without employing the aluminum-0.5% copper alloy liner), this will either cause a via width increase, which results in an increase in die size (which is not allowed), or a reduction in the inside-via copper cross-section, which results in total resistivity increase negatively effecting chip performance.
While embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications of the present invention without departing from the spirit and scope of the appended claims.
