Paper EM+TF-WeM6
Electroless Deposition of Co on SiO₂ Surfaces Modified by an Aminosilane Self-Assembled Monolayer

Wednesday, October 31, 2012, 9:40 am, Room 009

As device interconnects continue to shrink in size, the formation of diffusion barriers between dielectric and metal surfaces becomes more difficult. Self-assembled monolayers (SAMs) can be used to chemically activate a variety of surfaces, and their potential for uniform and defect-free monolayer formation makes them attractive alternatives for barrier layers. SAMs can also serve as adhesion layers to promote the electroless deposition (ELD) of metals on some dielectric surfaces. A 3-aminopropyltrimethoxysilane (APTMS) SAM was formed on a well-hydroxylated SiO₂ surface and studied as a function of solvent (methanol, IPA, and toluene), APTMS concentration (5.72 mM and 57.2 mM), and post-deposition rinsing in methanol, IPA or chloroform depending on the solvent used. An aminopropylsilane monolayer with a thickness of 7.8±0.2 Å and roughness of 3±1 Å was formed at an APTMS concentration of 5.72 mM in methanol and IPA, but multilayers were formed in toluene, which were deposited at a rate of 0.4±0.06 Å/min and roughness of 31 ±18 Å. The N 1s XPS peak at a binding energy (BE) of 398.8 eV verified that a primary amine group (-NH₂) was present together with an equal coverage of protonated amine (-NH₃⁺). Based on N 1s XPS peak areas, the molecular density of the aminopropylsilane monolayer was 4.2±0.6 molecules/nm², which is about equal to the accepted value of the OH group density on the Si surface. ELD processes are known to be sensitive to surface termination and require a metal catalyst, such as palladium, to activate the surface. A layer of Pd atoms was deposited by immersing the APTMS SAM surfaces in an 80 mM PdCl₂-HCl solution for 2 min, yielding one Pd atom bonded to two amine groups based on XPS peak areas. These results suggest that Pd atoms are bonded to primary amines rather than protonated amines resulting in a coverage of half of the N sites. Cobalt was plated by immersing the Pd-coated surface in a solution of 0.05 M CoSO₄, 0.2 M dimethylaminoborane (DMAB), and 0.01 M diethylenetriamine (DETA). Cobalt replaced the Pd atoms on the surface depositing a seed layer that auto-catalytically yielded a thick cobalt film on the surface. The thickness of the Co increased with the deposition time as verified by both the Co 2p peak at a BE of 777.2 eV and attenuation of the Si 2p peak with time. These results demonstrate that an aminopropyl silane adhesion layer binds metals such as Co. The next step is to test the effectiveness as a diffusion barrier.