Papers

AVS2001 Session DI-TuA: High K Dielectrics III

Tuesday, October 30, 2001 2:00 PM in Room 130

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		DI-TuA-1 Post-deposition Densification of Yttrium Oxide High Dielectric Constant Insulators Deposited by Oxygen Plasma Assisted Chemical Vapor Deposition D. Niu, R.W. Ashcraft, G.N. Parsons (North Carolina State University) Understanding and controlling interface and bulk chemical stability of CVD high-k dielectrics is an important research issue. Several groups report that the equivalent oxide thickness of vapor deposited dielectrics changes during post-deposition processing, but mechanisms that control the changes are not well understood. We examine the effect of post-deposition annealing on yttrium-based high-k dielectrics formed by oxygen plasma assisted CVD at temperatures between 350 and 450°C. Using yttrium-based materials, our group has previously demonstrated dielectrics with equivalent oxide thickness of 10-12Å with k=14. In this work, depositions were compared on Si(100) using two different yttrium diketonate precursors introduced downstream from a remote oxygen plasma source. Physical film thickness ranged from <40Å to >1000Å, and films were characterized using IR, XPS and IV and CV electrical analysis, before and after annealing at temperatures >900°C. For the thin films, XPS indicates mixing of yttrium and oxygen with silicon at the substrate interface during deposition. Over a wide range of thickness, as deposited Y₂O₃ films show evidence for O-H and C-H bonds in the IR spectra. After annealing at 900°C in N₂ (with >10^-5 Torr O₂ pressure), the IR shows almost complete removal of O-H and C-H bonds. The films also show a remarkable decrease in macroscopic thickness (e.g. from 600Å to ~400Å after annealing at 900°C for 10 minutes), which cannot be accounted for simply by film crystallization. After anneal CV analysis shows good behavior, with evidence for positive fixed charge and substrate oxidation. This data suggests that density of as deposited CVD dielectrics is a critical issue, and hydroxyl bonds present in as-deposited films can react at the interface during post-deposition annealing. Approaches to control hydroxide incorporation and improve material stability will be presented and discussed.
2:40 PM		DI-TuA-3 Substitutional Effects of the Dielectric Constant in Ta₂O₅ K. Larsson, J. Westlinder, H.-O. Blom, J. Olsson (Uppsala University, Sweden) Tantalum pentoxide (Ta₂O₅) is considered to be a promising candidate as a dielectric material. The reported dielectric constant (ε? for Ta₂O₅ ranges between 20-40. Depending on annealing temperature, process temperature and process method, the Ta₂O₅ films have crystallised into different phases. Since ε is greatly dependent of phase of Ta₂O₅ as well as dependent of various solutions of additives (e.g., Ti, Zr, Nb), it is of a large interest to study and compare different phases and solutions separately. The purpose with the present study has been to theoretically investigate the substitutional effect of various additives (Ti, Zr, Hf, V, Nb, Sn) in the β-phase of Ta₂O₅ on the dielectric constant. The ionic, as well as the electronic, part of the dielectric function have then been calculated for mono-crystalline materials, using the first principle Density Functional Theory. In addition, the electronic part of ε has also been calculated for a polycrystalline material as well. In the present study, an atomic substitution of 12 % of Ta with Ti, Zr, Hf, V, Nb and Sn, respectively, has been performed with a maintained ideal stoichiometry. For the situation with a Ti solution, the substitutions of 8 and 13 atom% were also used. As a general result, dielectric constants very similar to experimentally obtained values have been theoretically calculated in the present investigation.
3:00 PM		DI-TuA-4 Ta d-state Derived Electron Traps in non-Crystalline Al₂O₃-Ta₂O₅ Alloys Prepared by Remote PECVD R.S. Johnson, J.G. Hong, G. Lucovsky (North Carolina State University) The bonding coordination of Al and Ta in Al₂O₃ and Ta₂O₅ is known, respectively, from Al27 nuclear magnetic resonance, NMR,¹ and X-ray diffraction, XRD.² There are 4- and 6-fold coordinated Al-atoms in non-crystalline Al₂O₃ and these are bonded to 3-fold coordinated O-atoms. XRD of high temperature crystalline Ta₂O₆ indicates a mix 6- and 8-fold coordinated Ta, with 2- and 3-fold coordinated O-atom neighbors. FTIR and Raman studies confirm similar bonding arrangements in non-crystalline Ta₂O₅. Non-crystalline Al₂O₃-Ta₂O₅alloys have been prepared by remote PECVD. Robertson³ has shown that the anti-bonding Ta d-state energy levels are significantly below the p-states of Al and Si and cause a reduced conduction band offset with Si of ~0.36 eV. Miyazaki⁴ has measured band offsets by photoemission and has found good agreement with theory. The temperature dependence of C-V and J-V traces for capacitors with Al₂O₃-Ta₂O₅ dielectrics indicate trapping and trap release that is consistent with the trapping states being associated with anti-bond d-states of Ta. In particular, hysteresis in the C-V traces is consistent with electron trapping, and is significantly increased by addition of Ta₂O₅ into Al₂O₃. The temperature dependence of J-V traces, combined with the C-V traces is consistent with different activation energies for electron trapping, and trap release. The energies obtained from analysis of the data are in agreement with conduction band offset energies determined from the photoemission studies of Miyazaki.⁴ The activation for electron injection into Ta trapping states is 0.30±0.05 eV, consistent with the energy of empty Ta-atom d-states relative to the Si conduction band. The activation energy for trap release is 1.5±0.1 eV, in agreement with the energy difference between these Ta d-states and the Al₂O₃ conduction band position as determined from band offset energy measurements.⁴ Supported by the Office of Naval Research and the SEMATECH/SRC Front End Processing Center ¹ D. Muller, W. Gessner, H.J. Behrens, and G. Scheller, Chem. Phys. Lett. 59, 79 (1981). ² N.C. Stephenson and R.S. Roth, J. Solid State Chem. 3, 145 (1971). ³ J. Robertson, J. Vac. Sci. Technol. B 18, 1785 (2000). ⁴ S. Miyazaki, presented at PCSI 28, Lake Buena Vista, FL, 7-11 January 2001, submitted to J. Vac. Sci. Tech. B (2001).
3:20 PM		DI-TuA-5 Angle-Resolved XPS and Auger Analysis of Ultra-Thin Al₂O₃ Films Deposited by Atomic Layer Deposition O. Renault, D. Rouchon, L. Gosset, A. Ermolieff (CEA/Grenoble-LETI, France) Ultra-thin (1-4.5 nm) aluminium oxide (Al₂O₃) films prepared by Atomic Layer Deposition on HF-passivated Si substrates were characterized by ARXPS and AES. Results were analysed in terms of chemical quality of the oxide layer as well as physico-chemical characteristics of the intermediate layer, before and after RTP annealing. For both as-deposited and annealed films, Al2p and Si2p line analysis revealed that only Al-O bonds were present (neither Al-Al bonds¹ nor silicate-type compounds² were observed at the interface with the substrate); the films, as well as 50 nm-thick ones, were found to be stoechiometric with O/Al ratios around 1.5. For as-deposited films, decomposition of the O1s line evidenced the formation of Al-OH groups³ during the deposition process; their concentration was maximum at the surface and decreased as the depth probed increased; this additional contribution of Al-OH bonds to the O1s signal was not observed on annealed films. Results gained from both decomposition of the Si2p line at different analysis angles and AES depth profiling showed that an intermediate layer of oxidized Si grows up upon annealing, its thickness being related to that of Al₂O₃ and equal to 0.6±0.2 nm for 2.5 nm-thick films; sub-oxides appear to be localized at the Si interface whereas fully oxidized Si forms above. Additional results concerning films deposited on thermally grown SiO2-coated Si substrates will be also presented. ¹ Yang et al., Surf. Coat. Technol. 131 (2000) 79-83 ² Klein et al., Appl. Phys. Lett. 75 (25), 4001 (1999). ³ Alexander et al., Surf. Interface Anal. 29, 468 (2000).
3:40 PM		DI-TuA-6 In-situ, Real Time Studies of Interface Formation of BST Thin Films on Si Substrates A.H. Mueller, N.A. Suvorova, E.A. Irene (University of North Carolina, Chapel Hill); O. Auciello (Argonne National Laboratory); J.A. Schultz (Ionwerks, Inc.) The decrease in feature size of electronic devices and the commensurate electronic properties scaling has resulted in a search for new materials to achieve the electronic properties required for such miniature technologies. Ba_{0.5√sub 0.5}TiO₃ (BST) and other oxide films have become the front-runners in a search for materials to replace existing dielectrics in future technologies. Precluding the integration of BST as a gate dielectric into silicon devices are issues regarding the interface quality between the dielectric and semiconductor as well as the metal contact, as these affect critical electrical properties of the film such as the magnitude of the dielectric constant and the leakage current. In-situ studies of the oxygen incorporation into these films and the interface formation between BST films and Si substrates using time of flight ion scattering and recoil spectrometry (ToF-ISARS) as well as spectroscopic ellipsometry (SE) are presently being employed to ascertain conditions which minimize interface intermixing and maximize oxygen incorporation during reactive ion beam sputter deposition, and these studies comprise the focus of this presentation. Ex-situ material and electronic characterizations have also been used to characterize devices (Ir/ BST/ Si) prepared completely in vacuo and have shown the film's overall dielectric constant to be reduced by an intermixed layer of BST and Si substrate possessing an intermediate dielectric constant, while leakage characteristics of the film indicate a dependence upon the degree of oxygen deficiency as well as the BST/ metal contact interface quality. The electronic characterization studies will be presented separately.¹ ¹ N.A. Suvorova, A.H. Mueller, E.A. Irene, O. Auciello, and J.A. Schultz. Electrical properties of BST thin films on Si substrates. Present Conference Proceedings.
4:00 PM		DI-TuA-7 Direct Observation of Atomic Disordering at the SrTiO₃/Si Interface Due to Oxygen Diffusion V. Shutthanandan, S. Thevuthasan, Y. Liang (Pacific Northwest National Laboratory); Z. Yu, R. Droopad (Motorola Labs) Since CMOS devices based on the conventional dielectric material, SiO₂, will soon reach their dimensional limits in device technology, alternative high dielectric materials received much attention in recent years. Several oxides are being considered as alternative dielectric materials and strontium titanate is one of the most attractive choices of such materials. Recently, single crystal SrTiO₃(100) films have been successfully grown on Si(100) substrates at Motorola.¹ The stability of these films that were grown at Motorola was studied as a function of temperature under various control environments including vacuum, hydrogen and oxygen using Rutherford backscattering spectrometry (RBS) along with channeling techniques, nuclear reaction analysis (NRA) and x-ray photoelectron spectroscopy (XPS) at Pacific Northwest National Laboratory (PNNL).² Results from channeling measurements are consistent with the reported thin silicate/silica layer at the interface of an as grown sample. Annealing experiments in the vacuum and hydrogen indicate more disordering at the interface and in the bulk of the film due to oxygen movement from the film to the interface for the growth of silica. On the other hand, annealing in oxygen show improvements in crystalline quality of both film and interface. Experiments with isotopic labeled oxygen (¹⁸O) were carried out to understand the oxygen diffusion through the film to the interface using ¹⁶O and ¹⁸O nuclear reactions. Film collapses in the temperature range of 1070-1120 K regardless of the heating environment. ¹ Z. Yu, J. Ramdani, J.A. Curless, C.D. Overgaard, J.M. Finder, R. Droopad, K.W. Eisenbeiser, J.A. Hallmakrk, and W.J. Ooms, J. Vac. Sci. Technol. B 18, 2139 (2000). ² Work conducted at PNNL was supported by the DOE Laboratory Technology Research (LTR) Program and by Office of Biological and Environmental Research (OBER).
4:20 PM		DI-TuA-8 Core-level Photoemission of High-K Dielectrics on Si Substrates J.E. Rowe (Army Research Office); M.D. Ulrich, R.S. Johnson (North Carolina State University); T.E. Madey (Rutgers University); G. Lucovsky (North Carolina State University) High resolution soft X-ray photoelectron spectroscopy (SXPS) with synchrotron radiation is used to study the interface of device quality ultrathin gate oxides of Al, Ta and Al-Ta alloys. Our present studies were performed on thin oxides grown plasma-enhanced CVD. After growth samples were further processed by rapid thermal annealing for 30 sec at temperatures near ~ 900ÂºC. Our photoemission measurements were performed with synchrotron radiation at the National Synchrotron Light Source (NSLS) located at Brookhaven National Laboratory using beamline U4A which has a total instrumental resolution of better than 0.1 eV. Some data were collected at lower resolution (~0.2 eV) in order to increase data collecting efficiency for better sampling depth at higher photon energies (near 200 eV). We find that our data can be well described by an ultrathin ~5 Ã… interface layer of SiO2 with a graded transition to Al and/or Ta oxides. We have studied a number of samples prepared as described above and find that the interface peak energies relative to the Si-2p peak are 0.81 eV, 1.75 eV, 2.47 eV, and 3.60 eV for the Si+1, Si+2, Si+3, and Si+4 peaks respectively. No evidence of sub-oxides were found for either Al or Ta species. The total concentration of Si-suboxide derived from SXPS data is dependent on the uniformity of the SiO2 interfacial layer as well as data modeling, i.e., fitting of the data; both will be discussed.
4:40 PM		DI-TuA-9 Quadrupole Mass Spectrometer Studies of a High Temperature Etch Process St. Schneider, H. Kohlstedt, R. Waser (Forschungszentrum Juelich, Germany) High temperature etch processes are necessary to pattern Platinum successfully. In contrast to conventional, low temperature sputter driven etch processes, a chemical etch component is observed. >From the process perspective, they yield in redeposition free features (no fences) and in good control over sidewall sloping (minimization of CD loss). Furthermore those process regimes are characterized by a good process stability in terms of wafer counts and particle contamination, which is due to the dominating volatile etch products. To systematically investigate possible reactive etch process regions, we used a reactive ion etching (RIBE) tool with a filament free ICP source, that gives us control over the beam energy and the current density, and allows to use reactive gases. In this study we present the results for a chlorine chemistry with oxygen and carbonmonoxide additives. An energy dispersive quadrupole mass spectrometer is used for in-situ process monitoring. In one configuration, we are able to place the probe in the wafer position, opposite to the ion beam source. In this way it is possible to investigate the beam properties which are normally impinging on the wafer surface. In a second position we are able to lock on the wafer from the side, to monitor the etch process. Though the main focus of the study is on Platinum we also present results from selectivity studies to different hard mask materials and substrate materials.
5:00 PM		DI-TuA-10 Spectroscopic Ellipsometry Characterization of High-k Dielectric Thin Films N.V. Nguyen, Y.J. Cho, R.A. Richter, J.R. Ehrstein (National Institute of Standards and Technology) Spectroscopic ellipsometry (SE) characterization of high-k gate dielectric thin films will be presented in this paper. The materials investigated include HfO2, ZrO2, TiO2, and Ta2O5 films on silicon substrates. These films are candidates to replace traditional SiO2 gate dielectrics. Unlike SiO2, the dielectric functions for these high-k films are not yet established and are strongly dependent on deposition process and conditions. Therefore, the objective of this study is to assess and validate optical models that can reasonably represent the dielectric functions of these materials. The films used in this study were fabricated by different techniques including chemical vapor deposition, jet vapor deposition, and sputtering, and were post-deposition annealed at various high temperatures. To model the SE experimental data, single and multiple Tauc-Lorentz (TL) dispersion function(s) were employed for the dielectric functions of the films. It is found that a simple single TL can generally reproduce the dielectric function below and near the absorption edge. Above the edge, a summation of two or more TL dispersions is needed to improve data fitting. For samples annealed at high temperatures, additional new optical features due to structural changes in the films were seen in their dielectric functions, which require even more than two TL dispersions to fit the data. In addition, the effect of a possible oxide layer at the interface between the substrate and the films was examined in the relation with their determined dielectric functions. We also describe and evaluate the use of other optical dispersions such as harmonic oscillator approximations for high-k thin films and compare the results with those of TL dispersion. A brief discussion for using extended spectral range into vacuum ultraviolet region to characterize these and other high-k dielectric thin films will be presented.