Seedless Superfill: Copper Electrodeposition in Trenches with Ruthenium Barriers

The successful application of copper electrodeposition in the synthesis of low resistivity interconnect metallization is due to the ability to achieve void and seam-free filling of submicrometer features. This is accomplished through the superconformal bottom-to-top filling process called "superfill" caused by the impact of area change on local coverage of a rate-enhancing heterogeneous catalyst.^{1 2 3 4} The success of the superconformal portion of the feature filling process makes fabrication of copper seed layers the bottleneck for further increases of aspect ratio and decrease of feature size. Usually, the copper seed layers are deposited on the diffusion barriers, generally TiN or Ta, that prevent reactions between the conductor and the surrounding dielectric. In the optimized conventional process, the surface oxide on the copper seed is removed during immersion into the electrolyte and there is an insignificant barrier to nucleation during subsequent copper electrodeposition. The copper seed layer also improves wafer length scale current distribution during plating.

Sputter deposition of copper seed layers is widespread in the current implementation of damascene processing. However, it provides poor step coverage in high-aspect-ratio features, as is typical of line-of-sight, physical vapor deposition processes. For this reason other approaches have been tried for growing copper seed layers, including chemical vapor,⁵ ionized physical vapor,⁶ and electroless^{7 8} deposition as well as combinations of the above.⁹ Seeds fabricated by these processes have their own drawbacks (or limitations). These include surface roughness and selectivity for electroless processes and poor adhesion for chemical vapor deposition processes. Significant effort is being expended on barrier modification to promote adhesion and wettability¹⁰ as well as the introduction of additional processes for seed layer repair.¹¹ Furthermore, surface control issues do not end with deposition of the copper seed; degradation due to oxidation sharply limits shelf lives and prompts additional studies to understand seed aging induced defects¹² and oxide removal.¹³

One alternative is elimination of the copper seed layer in its entirety. Ruthenium as all, or part, of a barrier layer upon which copper could be electrodeposited directly is a candidate for such an effort. The immiscibility of ruthenium in copper is a particularly favorable attribute for a barrier material. Ruthenium has an electrical conductivity that is approximately twice that of Ta as well as correspondingly higher thermal conductivity.¹⁴ In ultrahigh vacuum (UHV) studies of copper deposition on Ru (0001), the first layer grows pseudomorphically followed by a series of strain-relieved structures which accommodate the 5.5% lattice mismatch between hexagonal close-packed (hcp) Ru(0001) and face-centered cubic (fcc) Cu(111). By the fourth layer the in-plane lattice parameter approaches that for Cu(111).^{15 16 17} Importantly, two studies comparing the behavior of thin copper films grown by either evaporation in vacuum or electrochemical deposition revealed very similar behavior. The first copper monolayer was bound more strongly to ruthenium than to bulk copper as evidenced by underpotential deposition peaks in cyclic voltammetry experiments as well as a distinct monolayer signal in thermal desorption spectroscopy (TDS).^{18 19} The details of monolayer formation were sensitive to the electrolyte composition and deserve further study.²⁰

Recent work has also demonstrated control of the nucleation density, and thus the surface roughness, of copper electrodeposits on planar ruthenium electrodes in sulfuric acid-copper sulfate electrolytes.¹⁴ As is shown in this work, ruthenium barriers are also capable of being a substrate for seedless, superconformal electrodeposition of copper in fine trenches. In this work physical vapor deposition was used to deposit the ruthenium barrier layer. As seen below, this gives rise to less than ideal step coverage as well as substantial roughness. Looking to the future, an atomic layer deposition (ALD) process for ruthenium has been described that offers a path to excellent conformality in high-aspect-ratio features.²¹ In this article, though, attention is focused on the ability to superfill deep submicrometer ruthenium-lined trenches with copper using a single-step electrodeposition process.

Experimental

Patterned substrates were fabricated by International Sematech. Trenches were patterned in 200 nm thick silicon dioxide dielectric films on a silicon nitride etch-stop layer. The patterned wafers possessed neither barrier nor seed layers. Electron-beam evaporation was used to deposit first a titanium or tantalum adhesion layer and then the ruthenium layer. The deposition system used has three independent sources and a base vacuum of Vacuum during metal deposition did not exceed

The metal flux in evaporation-type deposition processes is highly ballistic, giving line-of-sight deposition. For this reason, the water-cooled stage on which the substrates were mounted was systematically tilted during deposition (the trenches parallel to the rotation axis) to try to maximize metal coverage on the sidewalls of the features. As dictated by consideration of shadowing, the tilt angle used was decreased from 15 to 9° during the Ru deposition (±tilt angles were required for deposition on both sidewalls).

The Ru deposits on the sidewalls are visibly porous (Fig. 1a, d, and h). This is due to the oblique angles, relative to the sidewalls, at which the metals were deposited. Protrusions on the bottoms of the larger features arise from the non-normal flux of atoms coupled with shadowing by the sidewalls.

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Such low density structures are frequently seen in thin films fabricated by glancing angle deposition (GLAD).^{22 23} In contrast, copper seeds^{1 2 3} and silver seeds²⁴ fabricated in the same system using similar processing conditions are essentially fully dense. The different behavior is likely a result of much lower surface mobility of the depositing Ru, consistent with its higher melting point (2310°C). These deposits are sufficient for demonstrating seedless, superconformal electrodeposition of copper.

Specimens with smoother ruthenium barriers can be fabricated; observation of superfill in finer features is then possible (Fig. 2). The barriers in these specimens are composed of 10 nm tantalum covered with 20 nm ruthenium (in the field), as compared to 10 nm titanium covered with 70 nm ruthenium for the specimens in Fig. 1. The tilt angle decreased from 15 to 14° during deposition.

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Copper was electrodeposited onto the ruthenium-coated wafer fragments using an electrolyte containing 1.8 mol/L 0.24 mol/L 88 μmol/L polyethylene glycol (PEG, Mw 3400 g/mol), 1 mmol/L NaCl, and 50 μmol/L (SPS). Electrolytes with similar compositions have been previously shown to yield superconformal deposition of copper in fine trenches and vias possessing copper seeds.^{1 2 3 4} Samples were plated potentiostatically at −0.2 V vs. a calomel reference electrode (SCE), which corresponds to a copper deposition overpotential of approximately −0.250 V.

After metal deposition, the specimens were covered in epoxy and a glass coverslip and cross sectioned. Polishing was accomplished using diamond lapping films down to 1 μm grit followed by ion polishing at 13° from the plane of the cross sectioned specimen. Because sputter removal rates of the epoxy and copper were rapid compared to those of the Ru and Si substrate, specimens were shielded so that they only saw the flux across the Si substrate. The cross sectioned specimens were examined in a field-emission scanning electron microscope.

Results

Conclusion