Comparative Study on Cu-CVD Nucleation Using β-diketonato and Amidinato Precursors for Sub-10-nm-Thick Continuous Film Growth

Abstract

We demonstrate the growth of sub-10-nm-thick continuous Cu films using chemical vapor deposition (CVD) for next-generation Cu interconnects for ultra-large-scale integration (ULSI). The thickness of such films is equivalent to that of Cu during coalescence, and optimized operating conditions and substrate materials are required to form high-density nucleates. Ru was used as an underlayer, and the time evolution of nucleation and grain growth were studied with systematically varied conditions using two Cu precursors: conventional β-diketonato and newly developed amidinato precursor compounds. The revealed geometry of the initial nano-scale Cu grains prior to coalescence suggests the required nucleate density for 7-nm-thick continuous film growth, and which was 2.4 × 10 11 /cm 2 . The maximum nucleate density was achieved with the lowest deposition temperature and highest precursor concentration for both precursors; i.e., 6.9 × 10 11 /cm 2 for β-diketonato at 100 • C, and 4.6 × 10 11 /cm 2 for amidinato at 150 • C. A 10-nm-thick continuous Cu film was formed using amidinato under the optimized conditions. Furthermore, the framework used in this study to enable a high nucleate density suggests that it is possible to form thinner (4 nm∼) Cu films using amidinato. Because of the inherent good step coverage of CVD, this process is a promising candidate for next-generation ULSI Cu interconnects. Cu interconnects for ultra-large-scale integration (ULSI) are currently fabricated using the so-called 'damascene process,' which consists of: (i) the formation of three-dimensional features (i.e., trenches and via-holes) in the inter-layer dielectric using etching; (ii) sputtering of TaN x onto the inter-layer dielectric to form a Cu diffusion barrier; (iii) sputtering of Ta as an adhesion layer; (iv) sputtering of Cu as a seed layer for subsequent electro-plating; and (v) electro-plating of Cu to fill the remaining gaps. As the dimensions of complementary metal-oxide-semiconductor (CMOS) transistors in ULSI have reduced, the feature size of the Cu interconnects has also reduced. 1After 2016, the Cu line width will become smaller than 22 nm, so the Cu seed layer must be thinner than 7 nm.2 With such small dimensions, conventional physical vapor deposition (PVD) processes (including sputtering) face difficulties in forming conformal Cu films on narrow and high aspect-ratio (AR) features. Hence, it is essential to develop alternative deposition processes that are capable of highly conformal film growth. The current leading candidates are atomic layer deposition (ALD), 3,4 chemical vapor deposition (CVD), 5-9 and supercritical fluid deposition (SCFD).10 ALD-grown films exhibit the required properties for this purpose, but the growth rate of 0.1-1.9 Å/cycle 11 is too slow for mass production. SCFD is an emerging technology, but it requires a high-pressure environment, which is not compatible with current vacuum processes and now under investigation. 12,13 CVD is the most widely studied method, due to its fast growth rate of 2.0-14...

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