Patent Application
20070278089
We believe that dopants for copper seed layer can be divided into two main classes, reactive and non-reactive, producing different effects.
Reactive dopants react with the copper and other metals to form a stable compound alloy similar to an intermetallic alloy. The surface of a such a copper alloy bonds well with the underlying barrier layers formed of other refractory metals such as tantalum, titanium, tungsten and their nitrides. That is, the copper alloy layer adheres well to the barrier layer.
Horizontal electrical interconnections are formed in a multi-layer structure. In typical copper metallization, the copper line is deposited over a tantalum-based barrier layer, most usually a Ta/TaN bilayer with the metallic tantalum contacting the copper and the nitride contacting the oxide. The barrier layer in turn is deposited over an oxide dielectric layer, usually based on silicon oxide, perhaps including dopants such as fluorine. During fabrication and operation, the multi-layer structure is subjected to temperature cycling, and stress develops between the layers. Such stress is often relieved by the relatively mobile copper migrating horizontally to regions of lower stress in a process often called stress migration. However, such migration tends to thin out portions of the copper line relative to the thicker portions of the line, thus increasing the local resistivity of the thinned portion of the line. As electrical current is passed through the line, the increased resistivity results in an increased temperature of the thinned portion relative to the thicker portions, thereby further increasing the stress and promoting further stress migration. If the process continues, portions of the line may delaminate from the barrier layer because of excessive stress.
However, the increased adhesion provided by the reactive dopants lessens the stress migration since the copper is more firmly bonded to the barrier layer and further decreases the probability of delamination even in the presence of high stress.
Reactive dopants may be characterized as metals having a low electro negativity with respect to copper such that there a chemical reaction with the copper is more easily activated. The most useful reactive dopants are titanium, magnesium, and aluminum.
On the other hand, non-reactive dopants do not bond strongly with copper and instead tend to diffuse through the copper. However, copper as well as aluminum is almost always formed of small crystallites (grains) with grain boundaries between them. The non-reactive dopants tend to diffuse to the surface of the crystallites and form a non-reactive surface layer of the dopant species between the crystallites.
Electromigration is a well known effect in metal interconnects in which metal atoms tend to migrate along the conduction path of the current carried by the metal line. The electromigration is promoted by elevated temperatures. As a result, any inhomogeneous thinning in a metal line causes a localized region of higher temperature relative to the thicker or defect-free portions of the line. As a result, the metal tends to migrate away from the thinned area to thicker areas. The electromigration thereby increases the resistivity in the thinned portion and the temperature there rises further such that electromigration is further increased. Eventually, the thinned portion is reduced to nothing and the line ruptures at that point.
It is believed that electromigration of metal atoms occurs primarily along the grain boundaries since the metal is only loosely bound at the grain surface and voids may exist within the boundaries.
One of the advantages of copper over aluminum for metallization is its reduced electromigration. Nonetheless, copper electromigration continues to be an issue as the line thickness decreases and the current density increases for advanced interconnects.
It is believed that the non-reactive surface layer passivates the grain surfaces and impedes electromigration along the grain boundaries.
The non-reactive dopants useful with copper seed doping may be characterized as metals having a high electro negativity though not necessarily greater than that of copper. They may also be characterized as being noble or nearly noble. The most useful non-reactive dopants are palladium, tin, indium, iridium, and silver. These metals are clearly less reactive than the reactive metals mentioned above.
The doping concentrations in copper for either the reactive or non-reactive dopants copper may be in the range of 0.1 to 20 wt %. The target otherwise is principally composed of copper, that is, the copper fraction is at least 80 wt %. Beneficial results have been obtained with doping levels below 10 wt % and even below 5 wt %. However, two factors should be observed in selecting the doping levels. First, the doping concentration must be kept low enough that it does not exceed the solid solubility limit above which the dopant precipitates rather than forms an alloy. Secondly, any dopants increase the resistivity relative to pure copper. As a result, excessive doping levels should be avoided to minimize interconnect resistance. That is, only the minimum doping concentrations should be used to achieve the desired result, whether it be reducing stress migration or electromigration to acceptable predetermined levels. However, a substantial fraction of the seed dopants may diffuse into the ECP during the anneal after ECP to improve the electromigration and stress migration of the ECP layer. Accordingly, a high doping concentration in the seed layer may result in a substantially reduced doping concentration in the ECP layer and hence relatively little effect on the overall interconnect resistance.
Despite its high cost, palladium is favored for the non-reactive dopant because of its small increase in resistivity when alloyed with copper. The cost of the noble dopant metal further motivates the minimization of their doping levels.
The separation of dopants into those reducing stress migration and those reducing electromigration allows the two doping concentrations to be separately determined. Stress migration failure levels may be determined by subjecting the circuit to repeated thermal cycles, for example, between −50 and 300° C., with no or low levels of current being applied to the interconnect lines. On the other hand, electromigration failure levels may be determined by subjecting the interconnect lines of a circuit to a long period of high current, typically somewhat in excess of that intended in operation. At the end of the separate migration test period, a large number of interconnects are tested for integrity or excessive resistance.
In
The overall process is illustrated in the flow diagram of
The effects of non-reactive dopants can be determined by using x-ray crystallography or other technique to determine the average grain size of copper crystallites. Generally, the smaller the grain size the better. The grain size of the copper seed layer will affect the grain size of the ECP copper plated on it, thus after its metallurgical behavior and device reliability.
A sputtering target 50, illustrated in the cross-sectional view of
A copper sputter chamber 60, illustrated in the cross-sectional view of
Although the invention was developed for a copper seed layer underlying an ECP copper fill, many proposals have been made to use copper sputtering to completely fill the via hole. In this case, the problems of electro migration and stress migration can still occur. Accordingly, the advantages of the invention can be applied to a copper sputter fill process.
The invention thus allows separate doping optimizations for different effects in advanced copper interconnects. The principal change needed over the conventional copper seed sputtering is a change of the target composition.
