Bongmook Lee

Ultra-low power sensing platform for personal health and personal environmental monitoring

The vision of the NSF Center on Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) is to develop nano-enabled technologies to achieve a paradigm shift towards long-term health and wellness management. To achieve this, the center is building self-powered, wearable and multimodal sensing systems for correlation of environmental exposures to physiological parameters. This paper presents the latest advances in environmental and personal health sensors that have ultra-low power consumption and are highly selective and sensitive to enable real time, continuous, and wearable platforms.

On the Issue of Work Function Tuning of Nickel Silicide Gates

In the past few years, many research groups have focused on achieving work function (Φm) tuning of nickel silicide gates on various dielectrics. On SiO2, the dopants (P,B,As etc) in NixSiy gates affected a Φm tuning of >600meV. However on hafnium based dielectrics, NixSiy gates were shown to undergo Fermi level pinning due to Hf+Si bonding. The focus soon shifted to composition based phase controlled work function tuning of the NixSiy gates. Using the aforesaid concept, Φm tuning of ~300meV was achieved on HfSiON dielectrics. The objective of our research has been not only to achieve work function tuning but also to study the fundamental mechanism responsible for work function tuning in silicided metal gates. Furthermore, the role of the underlying dielectric was also evaluated and it was found that the work function tuning mechanism strongly depends upon the concentration of SiO2 in the dielectric. To this end, ternary silicides NixTa1-xSi and NixPt1-xSi gates were investigated on SiO2, HfSiOx (with varying concentrations of SiO2) and HfO2 for NMOS and PMOS applications respectively. Full silicidation for NixTa1-xSi was achieved at 700C whereas that for NixPt1-xSi was achieved at 500C. Φm of 4.27eV and 5.1eV was achieved for the cases of Ta rich and Pt rich silicides on SiO2. XRD confirmed the presence of ternary phases that may have been responsible for the observed Φm tuning. However the window of Φm range reduced as hafnium was added to the dielectric such that for the case of HfO2 dielectric, the range reduced to 200meV (Fig. 1). We attribute this to Fermi level pinning due to increase in Hf+Si bonds. Currently the role of dielectric-metal gate interface is being critically investigated. Results of varying the Ni and Si composition on SiO2, HfO2 and HfSiOx dielectrics and resulting variation of EOT and Φm will be presented. It is observed that as the Si in NixSiy gate is increased, the EOT variation is more severe on HfSiOx dielectrics than on HfO2 dielectrics as shown in Fig. 2 and Fig. 3. The corelation of EOT variation with work function of the metal gate will be discussed. 0 20 40 60 80 100 120 140 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0 5.1 5.2 5.3 5.4 Ni(20%)Ta(80%) (Ni(40%)Pt(60%)) (Ni(50%)Ta(50%)) (Ni(20%)Pt(80%)) (Ni(80%)Ta(20%)) PtSix