Effects of Seeding Layers on Electroless Copper Deposition
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Mat. Res. Soc. Symp. Proc. Vol. 564 ©1999 Materials Research Society
before electroless deposition of Cu films. The objective of this work is to compare the microstructure of electroless deposited Cu using different seeding layers. The microstructures of as-deposited Cu films are studied using both the Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM), and the crystallinity is analyzed using X-Ray Diffraction (XRD). EXPERIMENT With Seeding Layer In our study, a three-layered metal structure consisting of adhesive metal (Ti), seeding layer (Cu or Pd), and sacrificial layer (Al) is employed. These metal layers are deposited on oxidized silicon substrate via electron-beam evaporation. The first layer of metal, Ti is employed as a adhesion-promoting layer since Ti adheres well to most dielectric substrates. Cu is employed as the seeding material since it is the best homogenous catalyst for electroless Cu deposition. The last metal layer, Al serves as a sacrificial layer to prevent Cu oxidation prior to immersing into the electroless deposition solution. The next seeding material investigated in this work is Pd. Although Pd does not oxidize easily in air, for the sake of comparison, a three layered structure, Al/Pd/Ti is again used. The Ti adhesion promoter layer is still needed for the Pd seeding layer unless Pd silicide can be formed after annealing. The electroless deposition solution used here consists of 3 g/l of CuSO 4 , which serves as an oxidant; 8 g/l of EDTA to function as a complexing agent so as to prevent Cu precipitation; and 4 ml/l of HCHO to act as a reductant. A small amount of Triton X-100 surfactant is used to ensure complete wetting of all surfaces and improve surface morphology. The electroless bath is maintained at 65°C with a pH value of 12.6. In order to avoid H 2 evolution during plating, the electroless Cu bath is purged with N2 . The N 2 gas is uniformly dispersed over the reactive surface so that the H2 gas can be expelled out of the as-deposited Cu surface as it is being formed. With TiN BarrierLaver On TiN barrier film, the PdC12/HCI/HF activation solution is employed to activate the TiN surface so as to form Pd nuclei seeding islands. The HF in the activation solution serves to remove the thin titanium oxide layer on the TiN surface. Electroless Cu is then deposited on the Pd nuclei under the above plating condition. This approach avoids the PVD process required for the deposition of seeding layers and has a good filling capability with high aspect ratios. The surface topography and texture of the Cu and Pd seeding layers, Pd activated TiN surface and the electroless deposited Cu films on the three surfaces have been examined and compared using AFM. RESULTS AND DISCUSSION Seeding Layer Characterization Both PVD Pd and Cu seed layers have been analyzed using XRD and are presented in Fig. I and 2. An AFM micrograph of the PVD Cu seeding layer is given in Fig. 3. The average grain size of PVD Pd and Cu seeding layers is approximately 30 nm to 50 nm and their surface rough
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