Nucleation and Growth Dependence of ALD WNC on Substrate Surface Condition
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Nucleation and Growth Dependence of ALD WNC on Substrate Surface Condition Thomas Abell1, Jörg Schuhmacher2, Youssef Travaly2, Karen Maex2,3 Intel affiliate at IMEC, Kapeldreef 75, 3001 Leuven, Belgium ([email protected]) 2 IMEC, Kapeldreef 75, 3001 Leuven, Belgium 3 K.U. Leuven, Belgium
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ABSTRACT ALD WNC nucleation and growth was observed strongly affected by different substrate materials and surface chemistries. Nucleation was inhibited on most pristine low k surfaces, which is attributed to low concentrations of chemisorption sites (Si-OH). Plasma treatments were used to alter the surface chemistry to improve nucleation. Surface closure and surface roughness of the WNC layer were found to strongly correlate with starting surface condition. Resistivities of the resulting films were also found dependent on starting surface treatment. But the relationship between W content of the films, surface treatment and resistivity was not fully comprehended.
INTRODUCTION Atomic layer deposition (ALD) is an attractive method to deposit ultrathin Cu diffusion barrier films for semiconductor interconnect applications due to the high conformality of deposition. Reduction of barrier thickness while maintaining adequate barrier properties is becoming increasingly important as feature sizes are reduced as documented in the ITRS roadmap [1]. The ALD process relies upon chemisorption of precursor molecules onto the surface of the substrate for nucleation [2]. Thus, ALD processes are inherently sensitive to the starting surface chemistry of the substrate. Surface chemistries can be significantly altered by several methods including wet chemistry (e.g. HF) and irradiation (e.g. electron beam, UV, or plasma). The nucleation and growth of ALD films are not completely understood with the island growth model as one of several [3]. The island growth model supposes nucleation of the film at chemisorption sites with subsequent growth on the nucleated island. A continuous film is formed when islands grow together and merge. Integration of ALD with porous low k dielectric materials can be problematic for several reasons. The first is due to the heterogenous nature of the surface chemistry at a molecular level. Most Si-based low k dielectrics incorporate Si-CH3 bonds that have low polarity, induce microporosity (< 2 nm pore diameters) and result in a hydrophobic surface. These Si-CH3 terminating bonds are present along with other surface bonds such as Si-O-Si, Si-OH, Si-NH2 and other contaminant species. It is possible that the number of reactive chemisorption sites for ALD precursors may be the limiting factor determining nucleation density. This would occur in situations where the reaction site concentration is lower than the steric hindrance limit of the precursor molecules on the surface (e.g. maximum packing density of adsorbed molecules). Another difficulty in integrating ALD with porous low k dielectrics is the ability of the gaseous ALD precursor molecules to penetrate into the porous structure that can result in unwante
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