Properties of Ultra-Thin Thermal Silicon Nitride
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Properties of Ultra-Thin Thermal Silicon Nitride Katherine M. Buchheit, Hideki Takeuchi, and Tsu-Jae King EECS Department, University of California, Berkeley CA 94720-1770
ABSTRACT Growth behavior and film properties of ultra-thin Si3N4 layers formed by RTN (rapid thermal nitridation) were characterized. The self-limiting growth characteristics of the RTN process are appealing for precise thickness control in the range of 0.5-2.5nm. From SCA (surface charge analysis), the existence of negative fixed charge and electron traps in nitrides was found, in contrast to positive fixed charge and hole traps in oxides, and improvements in interface properties were seen after high temperature annealing. From annealing ambient dependences, the physical origin of the electron traps is likely N-H bonds in the nitride films. AFM (atomic force microscopy) analysis revealed that an atomically flat nitride surface is obtained by post-nitridation spike annealing, but a longer anneal leads to a rougher surface. Effectiveness of ultra-thin nitride barriers against boron penetration from a p+ poly-Si gate was also confirmed by SRA (spreading resistance analysis). INTRODUCTION For CMOS scaling beyond the 65nm technology node, high-k dielectrics will be needed to suppress excessive gate leakage current [1]. HfO2 seems promising for its thermal stability in contact with Si [2] but is known to form an interfacial layer at the Si interface, resulting in an undesirable increase of EOT (effective oxide thickness) [3]. It has been reported that NH3 pre-treatment prior to HfO2 deposition is effective for suppressing the EOT increase [4]. It has been also shown that HfO2 with a nitrided interface yields superior hot carrier reliability over conventional SiO2 [5], although process optimization to avoid carrier mobility degradation is still necessary [6]. In addition, high-quality Si3N4 film is promising as a tunneling dielectric for scaled nonvolatile memories (NVM), both FG (floating gate)-type [7] and SONOS-type [8]. It has been demonstrated that a tunneling nitride layer formed by RTN provides superior endurance characteristics as compared with tunneling oxide, due to less interface-state generation, as well as faster programming speed [8]. In spite of its promising applications, the detailed properties of Si3N4 formed by RTN are not well understood. In this paper, we investigated the process dependences of interface charge, surface morphology, and boron penetration immunity of ultra-thin Si3N4 films grown by RTN. Rapid Thermal Nitridation Process Growth characteristics of ultra-thin nitrides grown by RTN were investigated. After HFlast pre-cleaning of 4” (100) Si substrates, samples were immediately loaded into a coldwall type rapid thermal processing (RTP) system at room temperature, and an N2 purge was performed to minimize native oxide growth. After growing nitride films by flowing NH3 at atmospheric pressure with various temperatures and nitridation times, film
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