TaSiN Could Replace TaN and Ta as Copper Barrier

-- Semiconductor International, 7/1/1999

A recent study by engineers at National Semiconductor (Santa Clara, Calif.) and SEMATECH (Austin, Texas) showed that copper-filled damascene lines with tantalum silicon nitride (TaSiN) barriers offer lower film stress, lower surface roughness and better interface properties with copper than TaN or Ta films. They reported that film texture, which correlates with interface bonding strength, is strongest with TaSiN, medium with TaN and lowest with Ta, in spite of the fact that TaSiN is amorphous.

Following an anneal, copper seed layers exhibited significant agglomeration on Ta and TaN films on trench sidewalls, whereas a relatively smooth copper seed was present before and after the anneal on TaSiN. The findings by Qing-Tang Jiang and Hieu Lam of National, and Rick Faust and Jay Mucha of SEMATECH, were presented at IEEE's International Interconnect Technology Conference (IITC) in San Francisco, May 24-26.

The microstructure of electroplated films is significantly different on TaSiN films, affecting device leakage and reliability. In this study, 250 Å barrier layers were sputtered, followed by PVD of 1000 Å of copper. The TaSiN films, with stoichiometry of N:Si:Ta = 0.28:0.53:1, were deposited using a TaSi_0.6 target. The Cu seed layers were electroplated with 1.6 µm of copper using a sulfate plating bath and small amounts of proprietary additives at 25°C. Blanket wafer measurements (Table 1) indicated TaSiN film stress levels one-quarter those of TaN and Ta. Ta exhibited strong Cu (111) crystallinity, TaN weak and TaSiN was amorphous. After annealing at 350°C for 30 minutes, TEM analysis revealed a rough interface between Ta and Cu, a 25 Å transition layer between TaN and Cu and a distinct interface between TaSiN and Cu.

After observing characteristics of blanket films, an array of 0.5 µm damascene trenches with copper seed layers only were annealed for 30-minutes to promote agglomeration. Prior to anneal, all barriers were relatively smooth and continuous. After anneal, islands of copper formed on the Ta and TaN barriers, while the TaSiN promoted a smooth, continuous surface. A lower degree of agglomeration of Cu on Ta compared to TaN suggests that a Ta/TaN/Ta graded barrier may be necessary to promote interfacial bonding with Cu.

Differences in surface roughness (shown in Table 1) indicated stronger interface binding between TaSiN and Cu films. X-ray fiber texture plots of blanket films showing Cu (111) intensity disclosed strongest texture on the TaSiN barrier, medium on Ta and weakest on TaN. The same measurements taken perpendicular to and parallel to the trench direction showed strongest Cu (111) intensity for TaSiN again, but medium for TaN and lowest for Ta. The dramatic difference of Cu line texture indicates differences in sidewall interaction of copper and the surrounding barrier/ dielectrics.

Stress measurements using X-ray diffraction (Table 2) showed severe anisotropy for TaSiN, TaN and Ta, with higher tensile stress along the trench direction than the other two directions. TaSiN stress values were lowest in all directions. D-stress is based on lattice constants measured by X-ray and defined as in-trench stress less the isotropic stress of a blanket copper film.

Each finding in this study -- greater Cu (111) intensity, lower film stress, minimal agglomeration of the Cu seed layer and strong bonding at the TaSiN/Cu interface -- indicates better electromigration resistance and higher reliability using a TaSiN barrier material for copper damascene structures.