Comparison of metal gate electrodes on MOCVD HfO2

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Abstract

Metal gate electrodes of sputtered aluminum (Al), titanium nitride (TiN) and nickel aluminum nitride (NiAlN) are investigated in this work. They are compared with respect to their compatibility with metal organic chemical vapor deposited (MOCVD) hafnium dioxide (HfO2) gate dielectrics. TiN, with a midgap work function of 4.65 eV on SiO2, exhibits promising characteristics as metal gate on HfO2. In addition, encouraging results are presented for the ternary metal NiAlN, whereas classic Al electrodes are found unstable in conjunction with HfO2.

Introduction

The International Technology Roadmap for Semiconductors demands a vast amount of material innovations to keep up the transistor scaling trend of the last decades [1]. Gate leakage currents, polysilicon gate depletion, dopant penetration and high gate resistivity pose a limit to future use of SiO2/polysilicon gate stacks in use today [2]. One of the most urgent issues is therefore the integration of novel gate stacks with a combination of high-k materials and metal gates into CMOS technology. The integration of these materials, however, faces many challenges such as carrier mobility, thermal- and chemical stability, interface passivation, plasma damage anneal, work function control and gate leakage currents [3].

Hafnium dioxide (HfO2) is currently considered one of the prime high-k candidates to replace SiO2 as gate dielectric for the next two CMOS technology generations. It is not clear, however, which metal will be used in conjunction with HfO2, since the underlying dielectric influences the thermal, chemical and electrical characteristics of the gate electrode [4]. A variety of materials must therefore be investigated concerning work function, thermal and chemical stability and the ability to anneal oxide defects.

Metal-organic chemical vapor deposition (MOCVD) is one of several high-k deposition processes contending to be used for future novel high-k dielectric materials. This is due to its low temperature budget, conformal step coverage, adaptability to large scale processing, promising dielectric qualities and low operational cost compared to other deposition techniques such as molecular beam epitaxy (MBE) or atomic layer deposition (ALD).

In this work, various metals are experimentally compared as gate electrodes for MOCVD HfO2 using MOS capacitors as basic devices. Aluminum (Al) is chosen because it is the standard gate electrode for the evaluation of SiO2 gate dielectrics. Titanium nitride (TiN) is used for its good thermal stability and its well known properties as diffusion barrier for copper metallization. Nickel aluminum nitride (NiAlN) is selected for its high oxidation resistance and thermal stability. In addition, NiAlN has shown promising characteristics in a low temperature process [5].

Section snippets

Experimental

MOS-capacitors have been fabricated using both HfO2 and SiO2 as gate dielectrics together with sputtered Al, TiN and NiAlN gate electrodes to evaluate the compatibility of the high-k/metal gate combinations.

One batch of silicon substrates has been dry oxidized in a Centrotherm diffusion furnace at 1000 °C to form SiO2 gate dielectrics of tox = 15.7 nm. A second batch has been used to grow HfO2 layers of tHfO2 = 8.2 nm, 11.4 nm and 17.5 nm by MOCVD on a thin, wet chemically grown SiO2 buffer layer. The

Aluminum gate electrodes

A standard combination of SiO2 dielectric and aluminum gate electrode has been chosen to investigate the influence of the sputtering process and subsequent annealing step on dielectric integrity. An HFCV measurement with forward and reverse sweep of such a capacitance is shown in Fig. 1. A hysteresis is not discernible for this gate stack, ruling out the presence of oxide trapped charge. The measured flat band voltage of VFB = −0.95 V relates to the expected p-type work function for Al and a low

Conclusion

Electrical characteristics of various metal gate electrodes on MOCVD grown HfO2 high-k dielectrics have been compared in this work. Aluminum yields excellent results from capacitance voltage measurements on SiO2, but appears to be incompatible with HfO2.

TiN and NiAlN exhibit excellent characteristics on SiO2. On HfO2, however, a high oxide trapped charge density is observed after a standard forming gas annealing process. A modified annealing step is therefore needed for these materials. TiN is

Acknowledgments

The authors wish to thank T. Bobek of the Institute for Semiconductor Electronics, Aachen University for careful SNMS analysis. This work has been supported by the German Bundesministerium für Bildung und Forschung (bmb + f) under contract number 01M3142A (“KrisMOS”) and by the European Commission under the frame of the Network of Excellence “SINANO” (Silicon-based Nanodevices, IST-506844).

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