Patent Application
20110045171
The present invention relates to wiring structures and more particularly, to techniques for forming a ruthenium (Ru) capping layer over a copper (Cu) wire.
Copper (Cu) wires are generally formed by first patterning a dielectric, e.g., using photolithography, with a layout of the wires. A diffusion barrier layer is deposited in the pattern. The pattern is filled with Cu to form the wires. A capping layer is then deposited over the wires, which serves to protect the wires during subsequent processing. In conventional configurations, the capping layer typically comprises a layer of dielectric over the wires.
From the standpoint of electromigration, however, ruthenium (Ru) as a capping layer for Cu wires provides improved performance over the conventional dielectric capping layer. Unfortunately, one obstacle to the use of Ru as a capping layer is that during deposition of the Ru over the Cu wires, the dielectric can become contaminated with the Ru. It is essential to avoid contaminating the dielectric with Ru as this can lead to electrical leakage through the dielectric and the formation of short circuits. Chemical vapor deposition (CVD) is the best known method for depositing Ru over Cu wires. CVD, however, has only limited selectivity with respect to deposition of the Ru on the Cu wire, as compared to on the dielectric.
Therefore, techniques for depositing a Ru capping layer over a Cu wire, without contaminating the dielectric would be desirable.
The present invention provides techniques for forming a ruthenium (Ru) capping layer on a copper (Cu) wire. In one aspect of the invention, a method of forming a Ru capping layer on at least one exposed surface of a Cu wire embedded in a dielectric structure is provided. The method includes the following steps. A first Ru layer is selectively deposited onto the Cu wire and the dielectric structure by chemical vapor deposition (CVD) for a period of time during which selective nucleation of the Ru occurs on the surface of the Cu wire. Any nucleated Ru present on the dielectric structure is oxidized. The oxidized Ru and an aqueous acid are contacted to remove the oxidized Ru from the dielectric structure based on a selectivity of the aqueous acid in dissolving the oxidized Ru. A second Ru layer is selectively deposited onto the first Ru layer by CVD to produce a thicker Ru layer. The step of oxidizing and the step of contacting the oxidized Ru and an aqueous acid are repeated until a Ru layer having a thickness that is suitable for use as a Ru capping layer on at least one exposed surface of the Cu wire embedded in the dielectric structure is achieved.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.
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According to an exemplary embodiment, the Ru CVD is performed in a vacuum chamber (which can be, but is not required to be, a multi-chamber system) containing a Ru CVD reactor. Prior to the Ru deposition, Cu wire 102 may optionally be subjected to a cleaning treatment to remove oxide from the exposed surfaces thereof. The cleaning treatment can include, but is not limited to, heating the Cu wire and dielectric structure in a vacuum, exposing the structure to a reducing gas (such as molecular hydrogen) and/or sputtering the surface of Cu wire 102 with an inert gas ion beam to physically remove a surface oxide layer. Such cleaning treatments are known to those of skill in the art and thus are not described further herein. The Cu wire and dielectric structure is then inserted into the Ru CVD reactor and brought to a temperature of about 180 degrees Celsius (° C.) (with a deposition pressure of about 10 milliTorr (mtorr)) and exposed to a precursor gas stream comprising, for example, Ru3(CO)12 (a Ru carbonyl precursor), carbon monoxide (CO) and argon (Ar).
Ru CVD exhibits limited selectivity for deposition of Ru on Cu wire 102 over deposition on dielectric structure 104 (a low-k dielectric, see above). This selectivity depends on the difference in the induction times for the Ru to nucleate on the two different materials. On Cu the Ru begins to nucleate immediately, thus the induction time is essentially zero (to within experimental error). As such, Ru layer 108 begins to grow immediately on Cu wire 102 and wets the surface well. By contrast, under typical deposition conditions (i.e., such as those exemplary deposition conditions presented above) it takes about 30 seconds before scattered Ru nuclei 110 begin to form on dielectric structure 104. During this period, it is possible to deposit Ru layer 108 to an arbitrary thickness, e.g., from about two nanometer (nm) to about 2.5 nm on the surface of Cu wire 102. However, for the manufacture of a robust capping layer, a thickness of from about five nm to about 10 nm is desired. Thus, as will be described in detail below, the deposition process can be repeated x number of times until Ru layer 108 achieves a desired thickness. Therefore, the term “arbitrary thickness” makes reference to the fact that since the Ru layers deposited in each iteration of the process are less than the desired final thickness, the thickness of the Ru layer formed in each deposition is generally not important.
It is notable that the thickness of the Ru layer deposited as described above can vary depending, for example, on the exact nature of the reactor and/or exact chemical nature of the dielectric. For example, the lower the dielectric constant (of the dielectric), the more selective the process is for Ru deposition on the Cu wire versus on the dielectric.
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The present teachings are further described way of the following non-limiting example: a SiCOH dielectric sample, with k value approximately 2.2 was placed in a CVD reactor and exposed at 180° C. to Ru3(CO)12 precursor. The sample was then cooled and removed from the reactor, and introduced into an x-ray photoemission spectrometer (XPS) for analysis.
The sample was then removed from the XPS spectrometer and immersed in 100:1 dilute HF for one minute, rinsed in distilled water, air dried, re-immersed in the dilute HF, re-rinsed and re-dried. The sample was reintroduced into the XPS spectrometer and the spectrum from the lower curve of graph 200 was obtained. The results show a level of Ru contamination reduced to below the detection limit of the technique. Thus the incipient Ru nuclei, which give rise to the unwanted deposition the dielectric, have been removed.
Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope of the invention.
