60-nm이하 선폭의 trench pattern에 적용가능한 무전해도금용 activation process의 연구

Abstract

Currently, the sputtering process is used as formation method of Cu conductive layer (Cu seed layer) for Cu metallization process. However, as critical dimensions are scaled down to below 4x nm, the sputtered seed layer intrinsically suffers from limited coverage of the sidewalls of trenches and contact holes due to the shadow effect. This problem in step-coverage causes Cu to pinch-off during electroplating so that defects such as voids, pores, and inclusions are created inside the Cu interconnect. In this study, to deposit the conformal Cu conductive seed layer on the trench patterns with various line widths, the electroless deposition (ELD) process was introduced because the ELD process shows equal deposition rate over the entire trench pattern. To improve the conformality of Cu seed layer prepared by ELD, we have also developed a novel activation technique by combining a self assembled monolayers (SAM) and Au catalyst. The substrate was modified with an amine group using 3-aminopropyltriethoxysilane, followed by the deposition of uniform, continuous Au nano-catalysts for ELD. Cu film was then electrolessly deposited on the Au-catalyzed substrate. The seed layer formed by this approach showed a good conformality (approximately 95 % step-coverage), small grain size, and excellent adhesion to substrate. The surface roughness was measured to be about 1.4 nm, revealing a very smooth surface. The resistivity of the 20-nm-thick Cu layer is about 4.3 寅cm, which value is suitable for the seed layer of electrodeposition (ED). After formation of Cu conductive seed layer, the superconformal ED of Cu in 60 nm trench was demonstrated using an acid cupric sulfate electrolyte containing chloride ion (Cl-), polyethylene glycol (PEG), and bis (3-sulfopropyl)disulfide (SPS). Examination of I-E deposition characteristics of the electrolytes revealed that the combination of Cl-and PEG inhibited the Cu deposition rate while SPS accelerated the deposition rateat the trench bottom. Also, the degree of acceleration of Cu deposition by SPS increased with increasing the concentration of SPS. The electrical resistivity of an electrolessly deposited very thin Cu seed layer, whose thickness is below ~7-8 nm, is too high to be used to conductive seed layer for ED. To overcome the limitation of the electrolessly deposited Cu seed layer, a new method for depositing a Cu seed layer on trench pattern based on combination of ELD and electron beam (E-beam) evaporation was developed. To enhance the electrical conductivity of the Cu seed layer, an additional Cu layer was deposited on top of the trench by E-beam evaporator. The experimental results show that the ~12 nm thick Cu layer that was additionally coated on the top of trenches by electron beam evaporation led to decrease of the electrical resistivity down to 4.4 μΩcm, which is suitable for the conductive seed layer for ED. After the annealing process to eliminate the chemical impurities in the Cu films, the resistivity is increased by approximately 4-5 %. The increased resistivity with the annealing process is mainly due to the diffusion of Au atoms from the substrate into the Cu surface. The trench pattern with sub 4x nm line width is also filled by ELD process by introducing a novel surface activation. In this method, a coupling agent to anchor Au complex ions instead of metallic catalysts on the substrate was used, which enabled a defect free Cu filling of trenches by minimizing the electrical repulsion among the Au catalysts in the solution. The ELD process, combined with a novel surface activation, showed an excellent gap fill capability with no voids and defects in the sub 4x nm trench pattern. The electrolessly deposited Cu layer has a (111) preferred orientation, which indicates that the electro-migration resistance was improved. Also, the electrical resistivity of 30-nm-thick Cu layer slightly increased from 4.21 μΩcm to 4.58 μΩcm after annealing. Nevertheless, the measured resistivity was very close to the recommended value by the 2007 ITRS.