Papers

AVS2001 Session DI-TuM: High K Dielectrics II

Tuesday, October 30, 2001 8:20 AM in Room 130

Tuesday Morning

Start	Invited?	Item
8:20 AM		DI-TuM-1 Vibrational and Band-Gap Engineering of Transition Metal Oxides for High-k Gate Applications S. Zollner, R. Liu (Motorola SPS) CMOS devices will change around 2005, when the SiO2 thickness reaches 15-20 A and the gate length 0.1 um. Beyond that, the physical thickness of the gate oxide can no longer shrink, since the leakage currents due to tunneling render the devices useless. Instead of reducing the physical thickness, scaling the gate length is accompanied by increasing the low-frequency dielectric constant k of the gate oxide. Apart from a number of requirements related to stability under CMOS processing, new gate oxide materials need to have properties that can be engineered by changing the electronic band structure and the vibrational properties: (1) A large band gap with reasonable band offsets compared to the valence and conduction bands of Si. (2) A large lattice polarizability, which can be associated with a soft phonon driving a transition between two crystalline phases. In a way, engineering of the band gaps and band offsets of high-k gate oxides is not too different from band-engineered semiconductors, such as SiGe:C or InGaP for heterostructure bipolar transistors. Optical spectroscopies (spectroscopic ellipsometry from 300 um to 9 eV, FTIR transmission and reflection, UV Raman scattering) and x-ray structural analysis are ideally suited for a rapid screening of a large class of materials for potential high-k applications, since they do not require processing for electrical measurements. Ab initio band structure calculations can assist in the interpretation of experimental results. This talk will describe recent work on SrTiO3 and various transition metal oxides (TiO2, ZrO2, and HfO2). Specifically, we will show how the band gap of SrTiO3 can be increased by 0.5 eV or more through changing the composition, which reduces the leakage currents of metal-gate CMOS devices. Collaborators: R. Gregory, P. Fejes, A. Demkov, J. Curless, Z. Yu, J. Ramdani, R. Droopad, K. Reid, B.-Y. Nguyen, T.E. Tiwald, J.N. Hilfiker, J.A. Woollam.
9:00 AM		DI-TuM-3 Electrical and Physical Characteristics of Sputtered HfO₂ Films for Alternative Gate Dielectrics S.-W. Nam, J.-H. Yoo, H.-J. Choi, S. Nam, D.-H. Ko (Yonsei University, Korea); J.-H. Ku (R&D Center Semiconductor Samsung Electronics Co., Korea); M.-H. Cho (Yonsei University, Korea); S. Choi (R&D Center Semiconductor Samsung Electronics Co., Korea); C.-W. Yang (Sungkyunkwan University, Korea) SiO₂ has been used as the primary gate dielectric material in MOSFET devices for over 40 years. As the thickness of SiO₂ decreases, a direct tunneling of carriers through the potential barrier occurs, which results in the significant leakage current through the SiO₂ layer. Because of this issue, a conventional SiO₂ process shows limitation in the fabrication of CMOS devices in sub-2.0nm thickness regime. Therefore, the development of alternative dielectric materials is required for the fabrication of sub-0.1µm MOSFET devices. Dielectric materials with a high dielectric constant, large band-gap, low interface state density, and good thermal stability have drawn a lot of attention as alternative gate dielectric materials. Recently, HfO₂ has been considered as promising alternative materials due to high dielectric constant and good thermal stability with Si substrates. We have studied the microstructures and electrical properties of HfO₂ films. The HfO₂ thin films on (100) silicon substrate treated by a HF solution, chemical oxide, and NH₃ nitridation were deposited by reactive dc magnetron sputtering for gate dielectrics applications, followed by Pt sputtering for gate electrode. The resulting films were analyzed by ellipsometry, XRD, HRTEM, RBS, and XPS/AES. The crystalline peaks with orthorhombic phase of HfO₂ were observed in the films annealed at elevated temperatures (>650°C). By HRTEM and XPS/AES analyses, we observed the properties of HfO₂ films and the interfacial layers between the HfO₂ films and the Si substrate upon annealing. The electrical properties were assessed by C-V and I-V measurements for MOS structures.
9:40 AM		DI-TuM-5 Solid State Reactions in Hafnium Silicate Thin Films H.T. Johnson-Steigelman, A.V. Brinck, P.F. Lyman (University of Wisconsin-Milwaukee) Oxides and silicates of hafnium are promising high-dielectric candidates for microelectronics applications because it seems likely that these materials will be stable at their interface with silicon¹ due to the large heat of formation of Hf oxides. If hafnium oxides and silicates are indeed stable against reduction by Si at the dielectric/silicon interface, then, as a corollary, Hf metal should be able to reduce SiO₂. This supposition was tested by forming nm-thick films of hafnium silicates by solid state reaction of Hf metal films deposited on SiO₂ in ultrahigh vacuum. Charge transfer during the reaction was monitored by examining the binding energies of the Hf 4f and Si 2p core levels using x-ray photoelectron spectroscopy (XPS). As predicted, the Hf levels shifted to deeper binding energies upon annealing, while the Si core level corresponding to SiO₂ shifted to shallower binding energy. These shifts are consistent with charge being transferred from the Hf to the Si atoms, as would be expected for reduction of SiO₂ by Hf. Other aspects of the reaction between Hf, O, and Si were explored using thicker (µm) films. It is clear from these investigations that Hf metal can consume SiO₂ layers, with sometimes surprising results. ¹ ¹ G.D. Wilk and R.M. Wallace, Appl. Phys. Lett. V.76, p.112 (2000)
10:00 AM		DI-TuM-6 Kinetic Study on ZrO₂-CVD using Zirconium-Tetra-tertiary-Butoxide T. Kawamoto, Y. Shimogaki (University of Tokyo, Japan) ZrO₂ films have lately attracted much attention as high-k dielectrics for high performance ULSI gate applications. We carried out kinetic study on ZrO₂-CVD by using hot-wall tubular reactor to understand the basic reaction mechanisms. Zirconium-Tetra-tertiary-Butoxide (ZTB) was used as the Zr source and injected into the reactor by bubbling with He. We investigated the fractured surface of the sample by FE-SEM to obtain the growth-rate and step coverage of deposited ZrO₂ films on Si substrate with microtrenches. The deposition profile at 663K showed the increase in the up-stream part, and then the decrease was observed in the down-stream part. This implies that this reactive process has the special path, precursors decomposed into intermediate species by the gas-phase reaction, and the intermediate species deposits on growing surface. Furthermore, we examined depositions into the trench substrates with different aspect ratio at several positions in the reactor, and could found out that the step coverage became poorer, as samples were laid further from inlet. As a result of carrying out fitting these experimentally observed step coverage profiles with a single species model, the predicted composition profiles did not match well with the all observed profiles. This result indicates that some growth species contribute to film growth. Then, by fitting with two species model, we determined reactive sticking coefficients of two growth species; η₁=0.0095 and η₂=0.20, respectively. And we found out that the less active growth species (η₁=0.0095) existed dominantly in the up-stream part, on the other hand, the high active growth species (η₂=0.20) did the same role in the down-stream part. The dependence of the step coverage quality on the distance from inlet could be explained by the relative ratio of these species. The film properties may have different characteristics by this precursor ratio.
10:40 AM		DI-TuM-8 Correlations Between Local Bonding and Electronic Structure, and Gate Dielectric Performance of Zirconium Silicate Alloys G. Rayner, Jr., D. Kang, M. Schultz, G. Appel, G. Lucovsky, H. Ade, D.E. Aspnes, D.E. Sayers (North Carolina State University) Zirconium silicate alloys have been prepared by remote plasma enhanced metal organic chemical vapor deposition, RPE-MOCVD, using down-stream injected SiH₄ as the Si-atom source, Zr(IV) t-butoxide as the Zr-atom source, and plasma excited O₂/He mixtures to produce active oxygen species to drive the deposition reactions. Bulk film properties have been studied for as-deposited films, and films subjected to post-deposition annealing at temperatures to 1000C in an Ar ambient. This paper extends previously reported studies of thermal stability (chemical phase separation and crystallization) by infrared, IR, spectroscopy and x-ray diffraction, XRD.¹ Specific spectroscopic studies include i) x-ray photoelectron spectroscopy, XPS, ii) x-ray absorption spectroscopy, XAS, iii) extended x-ray absorption fine structure, EXAFS, and iv) spectroscopic ellipsometry, SE. The spectroscopic studies have confirmed that the lowest lying anti-bonding states are derived from d-states of the Zr-atoms. Results demonstrate that the energy of these states relative to vacuum, and the Si conduction band, do not change as a function of alloying, or as a function of long or short range order. This result is consistent with a molecular orbital description of the electronic structure derived from the Zr d-states. This aspect of the electronic structure has important implications for the interpretation of electrical studies, e.g., current-voltage and capacitance-voltage measurements that are also reported in this talk. For example, changes in the coordination and bonding symmetry of the bonding and anti-bonding orbitals derived from Zr d-states provide insights into the microscopic mechanism responsible for enhancements in the dielectric constant at low ZrO₂ concentrations.² Supported by ONR, SRC and the Sematech SRC Center for Front End Processes ¹ G. Rayner, R. Therrien and G. Lucovsky, MRS Symp Proc 611, c1 3.1 (2001) ² G. Lucovsky and G. Rayner, Appl. Phys. Lett. 77, 2912 (2000).
11:20 AM		DI-TuM-10 Interface Reactions of High-K Y₂O₃ and Gd₂O₃ Gate Oxides with Si B.W. Busch (Agere Systems); W.H. Schulte (Rutgers University); R. Kwo, M. Hong, J.P. Mannaerts, B.J. Sapjeta (Agere Systems); T. Gustafsson, E. Garfunkel (Rutgers University) Ultrathin Y2O3 and Gd2O3 films were e-beam evaporated onto Si(100) and investigated by high-resolution medium energy ion scattering with and without in-situ Si capping layers. Si-capped metal oxide films were stoichiometric (M2O3), and their interface with the Si substrate was sharp. Uncapped films that were exposed to air, however, contained excess oxygen and showed a 6-8 Å thick interfacial layer. Other than limited intermixing between the capping Si and metal oxide layers during deposition, the oxides did not react with the initially-amorphous Si overlayer until ~800 °C. Uncapped films showed additional Si uptake from the substrate at ~700 °C, while the capped films did not react with the substrate until ~900 °C. Results are discussed within the framework of the relevant solid-solid and solid-gas thermodynamics and kinetics. This work demonstrates the critical importance of gas ambient during growth and processing of high-K gate oxides.