Charge Trapping and Degradation of High Permittivity TiO 2 Dielectric Metal-Oxide-Semiconductor Field Effect Transistors
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TiO 2 Dielectric
Hyeon-Saeg Kim*, S.A. Campbell*, D.C. Gilmer"*, D.L. Polla* * Dept. of Electrical Eng. , **Dept. of Chemistry University of Minnesota, Minneapolis, Minnesota 55455, [email protected], [email protected] ABSTRACT Suitable replacement materials for ultrathin SiO 2 in deeply scaled MOSFETs such as lattice polarizable films, which have much higher permittivities than SiO 2, have bandgaps of only 3.0 to 4.0 eV. Due to these small bandgaps, the reliability of these films as a gate insulator is a serious concern. Ramped voltage, time dependent dielectric breakdown, and capacitance-voltage measurements were done on 190A layers of TiO, which were deposited through the metal-organic chemical vapor deposition of titanium tetrakis-isopropoxide. Measurements of the high and low frequency capacitance indicate that virtually no interface states are created during constant current injection stress. The increase in leakage upon electrical stress suggests that uncharged, nearinterface states may be created in the TiO 2 film near the SiO 2 interfacial layer that allow a tunneling current component at low bias. INTRODUCTION The preparation of high permittivity thin films have received considerable interest for the fabrication of charge storage insulators for new generations of dynamic random access memories (DRAMs) [1], [2]. In another application, the use of high permittivity materials is considered as one of the best alternative to conventional ultra-thin silicon dioxide [3], [4]. For MOSFETs scaling into the deep submicron, there is a strong need to reduce the gate oxide thickness. Tunneling currents however, limit the scaling of Si0 2 to approximately 25 A. Therefore, scaling of the gate oxide must end or an alternate material must be used. Suitable replacement materials such as lattice polarizable films which have much higher permittivities than Si0 2 have bandgaps of only 3.0 to 4.0 eV [5], [6]. Due to these small bandgaps, the reliability of these films as a gate electrode is a serious concern. Although wearout information is now being obtained on some storage capacitor materials, very little is known regarding charge trapping and breakdown for high permittivity films when used as a gate dielectric and therefore when deposited directly on silicon. The interface of the gate electrode/gate insulator is also important for the reliability of the MOS system. A poor electrode/gate oxide interface with high interface state density results in high current through the gate film, and a high probability of charge trapping in the gate film, reducing breakdown strength and the charge to breakdown in time dependent dielectric breakdown (TDDB) tests. These states will also lead to low mobilities and poorly controlled threshold voltages when used for FETs. In this paper we report on leakage currents and the effect of electrical stress on TiO 2 and the performance of TiO 2 MOSFETs using one such high permittivity material. EXPERIMENT The TiO 2 MOS capacitors and TiO 2 MOSFETs were fabricated on 9-15 U-cm boron doped substrates
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