IEEE ELECTRON DEVICE LETTERS, VOL. 26, NO. 7, JULY 2005 445 Characteristics and Mechanism of Tunable Work Function Gate Electrodes Using a Bilayer Metal Structure on SiO and HfO Ching-Huang Lu, Gloria M. T. Wong, Michael D. Deal, Wilman Tsai, Prashant Majhi, Chi On Chui, Mark R. Visokay, James J. Chambers, Luigi Colombo, Bruce M. Clemens, and Yoshio Nishi Abstract—In this letter, we investigate a method to adjust the gate work function of an MOS structure by stacking two metals with different work functions. This method can provide work func- tion tunability of approximately 1 eV as the bottom metal layer thickness is increased from 0 to about 10 nm. This behavior is demonstrated with different metal combinations on both SiO and HfO gate dielectrics. We use capacitance–voltage – charac- teristics to investigate the effect of different annealing conditions and different metal/metal bilayer couples on the work function. By comparing the as-deposited and annealed films, and by comparing with metals that are relatively inert with each other, we deduce that the work function tuning behavior likely involves metal/metal in- terdiffusion. Index Terms—HfO , high- dielectrics, metal gate, SiO , work function. I. INTRODUCTION A S CMOS technology continues to scale, metal gate elec- trodes need to be introduced to overcome the deleterious effects of doped polysilicon, namely gate electrode depletion, high resistance, and incompatibility with high- gate dielectrics. In order to optimize low threshold voltage devices, metal gate electrodes will require work functions that can be tuned to a de- sired value. Many approaches including implanted metals [1], fully silicided gates [2], and alloy metals [3] have been studied in an effort to achieve tunable work function metal gates. Recently, the use of metal bilayers has been reported as another method to set the work function [4]–[7]. Here the work function is contin- uously changed from that of one metal to the other by changing the thickness of the bottom metal layer. Several bilayer metal Manuscript received January 26, 2005; revised April 5, 2005. This work was supported by Stanford University’s Center for Integrated Systems (CIS) and the Initiative for Nanoscale and Materials and Process (INMP) and by the National Science Foundation under Grant ECS-9731293. The review of this letter was arranged by Editor K. De Meyer. C.-H. Lu, G. M. T. Wong, and B. M. Clemens are with the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 USA. M. D. Deal is with the Department of Electrical Engineering, Stanford Uni- versity, Stanford, CA 94305 USA. W. Tsai and C. O. Chui are with the Intel Corporation, Santa Clara, CA 95054 USA. P. Majhi is with Sematech Incorporated, Austin, TX 78741 USA. M. R. Visokay, J. J. Chambers, and L. Colombo are with Texas Instruments, Inc., Dallas, TX 75243 USA. Y. Nishi is with the Department of Electrical Engineering and the Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 USA. Digital Object Identifier 10.1109/LED.2005.851232 structures including Al/Ni [4], [5], Al/TaN [6], Ti/Pt and Ti/TiN [5], [7] have been fabricated on SiO to achieve work function tuning. Several mechanisms were proposed to explain how the work function is changed in the bilayer stacks, including carrier redistribution [4], [6] and metal islanding of the thin film [7]. In this letter, we report on the use of a bilayer metal struc- ture to tune the work function over a range of eV with a variety of metals on SiO and HfO dielectrics. From the work function characteristics of as-deposited and annealed samples, a diffusion mechanism is proposed to explain the work function change as a function of bottom metal thickness. II. EXPERIMENT MOS capacitors were fabricated on (100) Si wafers with thermally grown SiO or metal–organic chemical vapor de- position HfO of varying gate dielectric thickness. For the gate electrode, two metals with different work functions were deposited sequentially to form a bilayer structure. The metals were deposited by E-beam evaporation (unless otherwise specified) with the bottom layer thickness varying from approx- imately 1.5 to 10 nm and the top layer thickness being at least 30 nm. Samples with single layers of each of the two metals were also fabricated. After plasma gate etch, the gate stacks were subjected to a forming gas anneal (FGA) at temperatures up to 400 C. The flat-band voltage, , was extracted from the capacitance–voltage ( – ) characteristics based on the NCSU – program [8] and plotted against the effective oxide thickness (EOT) to determine the work function for a range of gate metal thickness [9]. III. RESULTS AND DISCUSSION Fig. 1(a) shows the work function versus bottom layer (Pt) thickness after a 300 C FGA for a Ti/Pt/SiO bilayer electrode structure. It is clearly seen that the work function shifts from that of Ti eV to that of Pt eV as the Pt thick- ness is increased from 0 to nm. We have also investigated Al/Ni/SiO , Ti/W/SiO , and Ta/TaN/SiO structures. In all of these systems, a similar gradual work function transition is ob- served; at least 6 nm of bottom layer metal is required to change the work function from that of one metal to the other. Our re- sults are consistent with reports of other bilayer systems such as Al/TaN [6], Pt/TiN, and Ti/TiN [7] on SiO . The bilayer struc- ture was also investigated on HfO using Ti/Pt and Pt/Ti stacks as the gate electrodes. The work function dependence on the 0741-3106/$20.00 © 2005 IEEE