IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 9, NO. 3, MAY2010 335 Variable Interface Dipoles of Metallated Porphyrin Self-Assembled Monolayers for Metal-Gate Work Function Tuning in Advanced CMOS Technologies Mrunal A. Khaderbad, Urmimala Roy, M. Yedukondalu, M. Rajesh, M. Ravikanth, and V. Ramgopal Rao, Senior Member, IEEE Abstract—This paper presents a technique for continuous tun- ing of the metal-gate work function (Φ metal ) using self-assembled monolayer (SAM) of metallated porphyrins. Porphyrin SAM was prepared on SiO 2 followed by Al evaporation to form MOS capac- itors (MOSCAPs). The variation in the dipole moment achieved by changing the central metal ion (Zn, Cu, Ni, and Co) in metal- lated porphyrins has been shown as a way to modify the gate work function. Thermal gravimetric analysis (TGA) on Zn-porphyrin shows that the molecule is stable upto 450 ◦ C. Temperature stabil- ity experiments on MOSCAPs show that the above method can be effectively implemented in advanced CMOS technologies involving the gate-last process. Index Terms—CMOS, interface dipole, self-assembled mono- layer (SAM), work function. I. INTRODUCTION W ITH the aggressive scaling of CMOS technology, the conventional poly-Si gate is known to suffer from short- channel effects such as polydepletion, high gate resistance, and boron penetration. Thus, new gate electrode materials, includ- ing metal gates, are being investigated to replace the traditional poly-Si gate for the 45 nm node and below CMOS technolo- gies [1]. In this context, the importance of metal-gate work function tuning is very important for the advanced CMOS tech- nologies. The work function tuning for metal-gate technologies has been successfully implemented using various techniques such as metal inter-diffusion, dopant implantation, silicidation, interface dipoles, nitridation, and alloying [2]. For bulk devices, the required metal work functions for replacing the conven- tional n- and p-poly-Si gates are about 4 and 5 eV, respectively to control threshold voltage swings. Without doubt, the use of two different metals for PMOS and NMOS devices requires complicated selective deposition and etching processes for their integration into the conventional CMOS technologies. In this work, we present a viable and inexpensive alternate solution by Manuscript received September 23, 2008; revised December 14, 2009. Date of publication February 22, 2010; date of current version May 14, 2010. The review of this paper was arranged by Associate Editor B. Yu. M. A. Khaderbad, U. Roy, M. Yedukondalu, M. Rajesh, and M. Ravikanth are with the Centre for Excellence in Nanoelectronics, Indian Institute of Tech- nology (IIT) Bombay, Mumbai 400076, India. V. R. Rao is with the with the Electrical Engineering Department and the Centre for Excellence in Nanoelectronics, Indian Institute of Technology (IIT) Bombay, Mumbai 400076, India (e-mail: rrao@ee.iitb.ac.in). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2010.2043681 use of self-assembled monolayer (SAM) of organic molecules, sandwiched between a metal and a gate dielectric, to selectively tune the work function of a specific metal [3]. Previous works have reported the use of SAMs to tune the work functions of Au, Ag metal electrodes in organic electronic devices such as polymer LEDs and photovoltaic cells. Use of SAMs of alkanethiols to manipulate the charge injection into organic FETs has been demonstrated [4]. Modification of work function of Ti (Φ Ti ) was achieved by self-assembled mono- layer molecules of aminopropyl triethoxy silane on SiO 2 /p-Si. The change in Φ Ti was attributed to the change in the elec- trical potential at the Ti/SAM interface in the presence of the SAM molecules with dipole moment. It was emphasized that controlling the magnitude of the work function value has an enormous potential for the fabrication of CMOS devices, as well as possibilities to tailor the behavior of opto- and bionan- odevices [3]–[5]. In our previous work, we demonstrated the use of 5-(4- hydroxyphenyl)-10,15,20-tri(p-tolyl)Zn(II) porphyrin (ZnTTP- OH) SAM as a Cu diffusion barrier for ultralarge-scale inte- gration (ULSI) metallization and for microfluidics [6]. In this work, TTP-OH and their metal derivatives (Zn, Cu, Ni, and Co) have been used to tune the metal-gate work function. Besides excellent thickness control, the main advantage of using por- phyrins is the wide range of its derivatives. We can tune the dipole moment not only by changing the central metal ion, but also by changing the groups attached to the porphyrin ring. In the following sections, the surface morphological studies carried out using (AFM) on SiO 2 and on SAM/SiO 2 are pre- sented. The redox chemistry of metalloporphyrins is presented. Water contact-angle measurements were done to check the for- mation of above SAMs on SiO 2 . The results of density func- tional (DFT) simulations and UV-visible absorption spectra of metallo-TTP-OH in toluene are discussed. Thermal gravimet- ric analysis (TGA) is done to determine degradation temper- ature of porphyrin molecule. High-frequency capacitance volt- age (HFCV), variable oxide thickness, and temperature-stability analysis on MOS capacitors (MOSCAPs) with SAMs show that the shift in flat-band voltage is due to the dipole moment asso- ciated with the central metal ion in the porphyrin ring. II. EXPERIMENTATION Fig. 1 shows the chemical structure of TTP-OH with different central metal ions. The silicon dioxide substrate used to prepare porphyrin SAM was prepared by thermally growing SiO 2 on an RCA-cleaned p-type (1 0 0) Si wafer. Then, the SiO 2 sub- strate was dipped in sulphochromic acid to create –OH groups 1536-125X/$26.00 © 2010 IEEE Authorized licensed use limited to: INDIAN INSTITUTE OF TECHNOLOGY BOMBAY. Downloaded on July 30,2010 at 06:03:34 UTC from IEEE Xplore. Restrictions apply.