IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 54, NO. 4, APRIL 2007 807 Comparison of the Work Function of Pt–Ru Binary Metal Alloys Extracted From MOS Capacitor and Schottky-Barrier-Diode Measurements Ravi M. Todi, Student Member, IEEE, Matthew S. Erickson, Student Member, IEEE, Kalpathy B. Sundaram, Senior Member, IEEE, Katayun Barmak, Member, IEEE, and Kevin R. Coffey, Senior Member, IEEE Abstract—This paper describes a systematic comparison of work function for the Pt–Ru binary-alloy metals, extracted from capacitance–voltage (C –V ) characteristics of MOS capaci- tors and the current–voltage (I –V ) and C –V characteristics of Schottky-barrier diodes. Our results indicate that the work func- tion of the Pt–Ru binary-alloy system can be tuned over the wide range of 4.8–5.2 eV. Furthermore, the results indicate that the change of film properties, i.e., resistivity, work function, and crys- tal structure, with composition is consistent with the equilibrium- phase diagram and that the work function in the face-centered cubic and the hexagonal-close-pack single-phase regions is only weakly dependent on composition while a strong dependence is observed in the intermediate compositional range. It is also ob- served that work-function values obtained from the Schottky I –V analysis are significantly lower than those extracted from the MOS C –V data. Index Terms—Gate electrode, metal alloy, Pt–Ru, Schottky, work function. I. INTRODUCTION A S THE silicon dioxide (SiO 2 ) gate dielectric in CMOS technology is thinned below 20 Å for higher density and performance; limitations associated with the polysilicon gate become increasingly important. These limitations include increasing polydepletion effects, high gate resistance, boron penetration, and compatibility issues with high-permittivity gate dielectric films [1], [2]. Metal gates can potentially address these limitations and are, therefore, attracting research interest [3], [4]. Replacement of the poly-Si gate with a metal gate eliminates the depletion and reduces the electrical thickness by the (SiO 2 ) equivalent of 0.3–0.5 nm, without a substantial increase in leakage. Metal gates lower gate resistivity from values of 1000–3000 μΩcm, typical for the doped poly-Si [4] to the range of a few micro-ohms centimeter to tens of micro-ohms centimeter typical of metals. Furthermore, without Manuscript received June 28, 2006; revised October 19, 2006. The review of this paper was arranged by Editor V. R. Rao. R. M. Todi, M. S. Erickson, K. B. Sundaram, and K. R. Coffey are with the University of Central Florida, Orlando, FL 32816 USA (e-mail: rtodi@ mail.ucf.edu). K. Barmak is with the Carnegie Mellon University, Pittsburgh, PA 15213 USA. 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/TED.2007.892352 boron-doped polysilicon gates for p-type field-effect transistors (p-FETs), boron penetration will no longer be a concern. Polysilicon can react with some high-κ insulators and form undesirable silicides or silicates [5]–[7]. Metal gate electrodes that are thermally stable adjacent to the high-κ insulators will eliminate these reactions. Several approaches including implanted metals [8], fully silicided gates [9], and metal alloys [10] have been investigated in an effort to achieve tunable work- function metal gates. Recently, the use of metal bilayers has been reported as another method to achieve the desired work function [11], [12], where the work function is continuously changed from that of one metal to the other by changing thickness of a bottom metal layer. A key parameter for the choice of the metal-gate material is its work function. For p-channel MOS (pMOS), Pt and Ru are both possible choices due to their higher work-function value. Alloys of these metals can also be considered to achieve an op- timized or “tunable” work function. Replacing the polysilicon in the gate typically requires a dual metal approach: a metal with a pMOS work function and a second metal with nMOS work function. The work function (Φ m ) of the metal must be near the conduction-band edge for nMOS and near valence- band edge of silicon for pMOS in order to satisfy the low-power and low-supply-voltage demands. Ideally, metal gate electrodes will have work functions that are within 0.2 eV of the silicon conduction and valence-band edges, namely work functions of 5.0–5.2 eV for pMOS and 4.1–4.3 eV for nMOS [13]. The element Ru has attractive combination of characteristics that makes it a strong candidate for pMOS devices. Some of these characteristics are that Ru can be patterned by dry etching [14], is thermally stable for anneals up to 1000 ◦ C, has high diffusivity for hydrogen and low diffusivity for oxygen, has low resistivity (7.3 μΩcm, bulk Ru), and forms strongly 〈0001〉 fiber texture films [15]. The ability to tune gate metal work function for Pt–Ru alloys as a possible pMOS metal-gate- electrode material has been previously reported [15], and in this paper, we describe a systematic comparison of the work function of the Pt–Ru binary-alloy metals extracted from three types of measurements: capacitance–voltage (C–V ) charac- teristics of the MOS capacitors and current–voltage (I –V ) and C–V characteristics of Schottky-barrier diodes. We also provide a detailed discussion on the crystal structure and its influence on the alloy work function in this paper. 0018-9383/$25.00 © 2007 IEEE