Papers

AVS1996 Session EM-MoA: Ultrathin Dielectrics

Monday, October 14, 1996 1:30 PM in Room 204A

Monday Afternoon

Start	Invited?	Item
1:30 PM		EM-MoA-1 High Resolution MEIS Characterization of Oxynitride Films on Si(100) E. Gusev, H. Lu, E. Garfunkel, T. Gustafsson (Rutgers University); M. Green (Bell Laboratories); L. Feldman (Vanderbilt University) A growing number of studies suggests that the performance of logic devices with ultrathin oxynitride dielectrics strongly correlates with the concentration and distribution of nitrogen in the film. In this study, medium energy ion scattering (MEIS) has been used to examine the distribution of nitrogen in sub-5 nm oxynitride dielectrics with the depth sensitivity of 0.3 - 0.5 nm, much higher than offered by other techniques. The oxynitridation was performed by thermal (both RTO and furnace) oxidation of Si(100) in N\sub 2\O, NO or O\sub 2\/NO. The nitrogen was found to be located near the dielectric/substrate interface with both distribution and concentration strongly dependent on the oxidation conditions. If compared to N\sub 2\O, oxynitridation in NO results in higher concentration and sharper distribution of nitrogen incorporated into the film. In contrast to recent SIMS experiments, we did not observe nitrogen atoms on the substrate side of the interface after oxidation in NO. We will also discuss mechanistic aspects of the oxynitridation, specifically a retardation of the oxidation reaction at the interface and nitrogen removal from the film, as studied by isotopic (\super 18\O\sub 2\) labeling technique and reoxidation in N\sub 2\O/O\sub 2\, respectively. * Supported by NSF and PRF grants.
1:50 PM		EM-MoA-2 X-ray Photoelectron Study of Nitrogen Incorporation into Silicon Dioxide Thin Films M. Du, R. Opila, C. Liu, Y. Ma (Bell Laboratories) As gate oxides for ULSI CMOS chips become thinner, integrity becomes more important. Incorporating N and controlling its distribution in the gate oxide not only prevents the diffusion of B and alkali metals through the oxide, but also improves the reliability and process control of the oxide. N can be incorporated in two ways--either by N implantation before oxidation or by nitriding the oxide during oxidation. In this work we have used XPS to characterize the chemical state of N, O, and Si in the oxide, and to determine the distribution of N in the oxide layer as a function of oxide growth conditions. The Si 2p peak in the oxide monotonically shifts as a function of oxide thickness for oxide thicknesses from 10 to 40 Angstrom thick, independent of N incorporation. The distribution of N for oxides grown in a nitriding ambient appears to be uniform. On the other hand, oxides grown after nitrogen implant appear to have more N near the oxide/Si interface. Oxides that are grown in a nitriding environment after implanting show the effects of both treatments. The incorporation of N into the oxide layer significantly decreases the rate of growth of the oxide. Reasons for this decrease will be discussed.
2:10 PM		EM-MoA-3 Nitrogen Incorporation at the Top Surface of Ultra-thin Gate Oxides by Low-temperature Plasma-assisted Processing H. Niimi, G. Lucovsky, C. Parker, K. Koh (North Carolina State University) There is considerable interest in incorporating nitrogen (N-) atoms into ultra thin gate oxides. N-atoms serve two functions: i) if incorporated at Si-SiO\sub 2\ interfaces they suppress defect generation; and ii) if incorporated anywhere in the oxide they inhibit transport of boron atoms from p\super +\ doped poly-Si gate electrodes to Si-SiO\sub 2\ interfaces. This paper reports on incorporation of N-atoms at the top surface of ultra thin (1.0-3.0 nm) gate oxides during a low-temperature remote plasma-assisted oxidation/nitridation of crystalline Si(100). The same process also incorporates N-atoms at monolayer concentrations at Si-SiO\sub 2\ interfaces, but not into the bulk of the oxide films. A plasma-excited He/N\sub 2\O gas mixture is used for the oxide growth/nitridation process. The RF power and substrate temperature for this process are 60-100W and 300\super o\C, respectively. On-line Auger Electron Spectroscopy (AES) has been used to i) estimate (\+-\ 0.05 nm) thicknesses of nitrided oxides, ii) track the evolution of film growth, and iii) determine the nitrogen distribution. For low RF powers \<=\ 30W, AES spectra have established a N-atom terminated interface. Further evidence for localization of N-atoms at the interface comes from studies of O- and N-atom terminated interfaces by non-linear optical second harmonic generation. In addition, on-line AES studies show that N-atoms are not incorporated at the top surface of the oxide when the RF power is low, \<=\30W. However, if the RF power is increased from 30W to 60-100W, N-atoms are additionally incorporated at the top surface of the oxide during the growth process. The incorporated nitrogen is stable with respect to high-temperature downstream processing such as rapid thermal annealing at 900\super o\C. The simultaneous incorporation of N-atoms at the Si-SiO\sub 2\ interface and top surface of ultra thin oxides make this an interesting approach for fabricating deep sub-micron devices.
2:30 PM		EM-MoA-4 Atomic Layer Controlled Al\sub 2\O\sub 3\ Films Grown on Si(100) Using Binary Reaction Sequence Chemistry A. Ott, J. Klaus, J. Johnson, S. George (University of Colorado, Boulder) Al\sub 2\O\sub 3\ has a high dielectric constant and may replace SiO\sub 2\ (\epsilon\~3.8) in semiconductor devices. Al\sub 2\O\sub 3\ films with precisely controlled thicknesses and excellent conformality were grown at temperatures from 350 to 650 K. This controlled deposition was achieved by separating a binary reaction for Al\sub 2\O\sub 3\ chemical vapor deposition ( 2Al(CH\sub 3\)\sub 3\ + 3H\sub 2\O -->\Al\sub 2\O\sub 3\ + 6CH\sub 4\) into two half reactions: (A) AlOH* + Al(CH\sub 3\)\sub 3\ --> Al-O-Al(CH\sub 3\)\sub 2\* + CH\sub 4\ (B) AlCH\sub 3\* + H\sub 2\O --> AlOH* + CH\sub 4\ The trimethylaluminum [Al(CH\sub 3\)\sub 3\](TMA) and H\sub 2\O were employed alternatingly in an ABAB... binary reaction sequence to deposit the Al\sub 2\O\sub 3\ film. At the optimal reaction temperature of 450 K, a growth rate of 1.1 \Ao\/AB cycle was measured on Si (100) using ellipsometry. These Al2O3 films had an index of refraction of n=1.65 and a corresponding density of \rho\= 3.50 g/cm\super 3\. Atomic force microscope images revealed that the deposited Al\sub 2\O\sub 3\ films were exceptionally flat with a surface roughness of only 3 \Ao\ (rms) after 500 AB cycles and the deposition of a ~560 \Ao\ thick film. Cross sectional scanning electron microscopy (SEM) demonstrated that conformal films could be grown on trench structures with high aspect ratios. Preliminary electrical measurements have also measured a dielectric constant of \epsilon\~6.8.
2:50 PM		EM-MoA-5 SiO2/Si Interface Structures and Electrical Properties A. Ishitani (Association of Super-Advanced Electronic Technologies, Japan) Although there are many reports on SiO2/Si interface structures, the conclusions are a little bit confused. For instance, XPS studies produce both a compositional transition layer and a structural transition layer. On the other hand, backscattering spectroscopy measurements conclude that an SiO2 film is stoichiometric to approximately one monolayer at the interface. The transition layer thickness is also confused. According to lattice image TEM observations, the transition layer thickness is a few atomic layers. But the thickness obtained by XPS and etching speed studies is reported to be several nanometers which is suitable for explaining electrical properties. The present work shows that a 7-8 nm thick structural transition layer of SiO2 is formed at the SiO2/Si interface by thermal oxidation of Si. The transition layer density is measured using grazing-incidence diffraction of synchrotron radiation, and found to be around 2.4 g/cm3 which is larger than bulk SiO2 density (2.2 g/cm3). When Fowler-Nordheim tunneling current is injected into a thin SiO2 film, the dielectric breakdown mainly occurs in the structural transition layer because of Si-Si bond formations due to hypervalent properties of Si atoms and compressive stress. Introduction of nitrogen atoms into the transition layer improves injected charge-to-breakdown of thin SiO2 films due to stress relaxation. The structural transition layer seems to be formed by volume expansion due to thermal oxidation of Si, because the transition layer is not observed in CVD oxides. Two-layer model for SiO2 film structure has been already proposed by Tiller in 1983. Recently, in addition to the present work, some FT-IR and neutron analysis studies support the two- layer model. Interfacial structures and electrical properties of thin SiO2 films is essential for ULSI applications.
3:30 PM		EM-MoA-7 Ballistic Electron Emission Spectroscopy and Monte Carlo Simulations of Hot Electron Transport through MOS Structures H. Wen, E. Cartier, R. Ludeke (IBM T.J. Watson Research Center) Ballistic Electron Emission Microscopy/Spectroscopy (BEEM/BEES) is a Scanning Tunneling Microscope (STM) based technique used to inject hot electrons (0-9 eV) into the conduction band of SiO\sub 2\. The simultaneous application of an oxide bias can further increase the energy of the electrons emerging from the oxide at the SiO\sub 2\-Si interface. This allows the achievement of energies beyond anything reachable with conventional Fowler-Nordheim injection of electrons into oxides with thickness of <10 nm. The application of an oxide bias gives clear evidence of a downward shift of the threshold in the BEEM spectra. The lowering of the threshold can be accurately described by classical image force effects. Fits to the data yield a zero field Pd-SiO\sub 2\ barrier height of 4.08\+-\0.02 eV, as well as an effective image force dielectric constant of 2.74. The latter value is substantially higher than the so-called optical dielectric constant of 2.15 that was measured years ago by more indirect techniques and was generally assumed to be valid ever since. Argument will be given to account for this difference. The inclusion of image force effects dramatically affects the scattering in the oxide and contributes to the beam spreading as the electrons traverse the oxide. Knowledge of this spreading, calculated by Monte-Carlo methods, is essential to assess the area of potential hot electron damage induced locally in the oxide by the STM-injected electrons. The effective area of the beam spread is needed to estimate the charge-to-breakdown. The BEEM-stressed charge-to-breakdowns are appreciably higher than those obtained on macroscopic samples, which suggests that breakdowns have not yet achieved intrinsic limits, but are still dominated by defects and/or impurities in the SiO\sub 2\.
3:50 PM		EM-MoA-8 Ballistic-Electron Emission Microscopy Characterization of Hot-Electron-Induced Trapped Charge in SiO\sub 2\ B. Kaczer, H. Im, J. Pelz (Ohio State University) We demonstrate that ballistic-electron emission microscopy (BEEM)\super 1,2\ can be used to inject and characterize trapped charge in moderately thick (25 nm) SiO\sub 2\ films at nanometer length scales. Local injection of hot electrons into the oxide conduction band causes build-up of trapped charge in the oxide\super 3\ which is accompanied by local suppression in the BEEM current (i.e., the current transmitted across the oxide) and by local shifts in the apparent Pt/SiO\sub 2\ barrier height as determined from BEEM threshold voltage shifts.\super 4\ We use these threshold shifts (and their dependence on the oxide bias) to determine the local depth and density of the trapped charge in the oxide. We also observe lowering of the Pt/SiO\sub 2\ barrier due to the image force effect, and are able to extract the intrinsic barrier height at the Pt/SiO\sub 2\ interface. The suppression of the BEEM current allows us to laterally "image" trapped charge and to estimate its lateral spread after local injection of hot electrons. We also study the dynamics of charge trapping by monitoring the time dependent decay of the BEEM current. Our measurements show that BEEM can be used to inject, and be sensitive to very small numbers of electrons in the oxide film. We are presently applying BEEM to investigate charge trapping in thinner (3.5 - 10 nm) SiO\sub 2\ films, and plan to test different atomic models for trap creation in SiO\sub 2\ by utilizing the ability of the BEEM technique to control the energy of hot electrons, the oxide field and the microscopic position during injection. \super 1\ L. D. Bell and W. J. Kaiser, Phys. Rev. Lett. 61, 2368 (1988). \super 2\ R. Ludeke, A. Bauer, and E. Cartier, J. Vac. Sci. Technol. B 13, 1830 (1995). \super 3\ D. J. DiMaria, E. Cartier, and D. Arnold, J. Appl. Phys. 73, 3367 (1993). \super 4\ B. Kaczer, J. P. Pelz, accepted to J. Vac. Sci. Technol. B; B. Kaczer, Z. Meng, and J. P. Pelz, accepted to Phys. Rev. Lett.
4:10 PM		EM-MoA-9 Effect of Surface Roughness on 5 Nm Oxide Charge Trapping and Fowler-Nordheim Tunneling Behavior -- Experiment and 3D Simulation H. Lin (Stanford University); T. Yamanaka (Hitachi, Ltd., Japan); S. Fang, C. Helms (Stanford University) Ultra thin (< 5 nm) gate oxides are becoming more important for ULSI technology while gate insulator surface and Si/SiO2 roughness becomes potentially more important. In this paper, poly gate MOS transistors and capacitors, fabricated by a simplified LOCOS process are investigated as a function of surface and interface roughness.Initial surfaces with roughness from 0.2 to 4 nm RMS were prepared by a combination of wet chemical processing and sacrificial oxidations so that roughness can be varied independently. AFM measurements showed different roughness at the oxide surfaces and at the Si/SiO2 interface.IV, CV, QBD and charge pumping measurement techniques were used to investigate charge trapping effects and oxide quality for these different roughened capacitances and transistors. The IV curves for different roughness capacitors were similar before constant current stressing. However, after stressing, voltage shifts were higher for rougher interface MOS capacitors, indicating that the rougher surface is more vulnerable to charge trapping.A 3 dimensional Possion simulator was developed to investigate the potential and electric field distribution inside oxides with different surface and interface roughness. This combined with a quasi one dimensional Fowler-Nordheim calculation was applied to simulate the I-V characteristic of MOS capacitors. The field distribution and tunneling behavior is sensitive to a combination of the wavelength of the roughness, its aspect ratio, and the oxide thickness. These effects will be discussed and compared to the experimental data, including the electrical measurements and AFM measurements of both oxide surface and Si/SiO2 interface.
4:30 PM		EM-MoA-10 Evolution of Si/SiO\sub 2\ Interfacial Roughness during ECR Plasma Oxidation C. Zhao, Y. Hu, E. Irene (University of North Carolina, Chapel Hill) This is a study of the evolution of Si/SiO\sub 2\ interfacial roughness during the oxidation process under the influence of an electric field. Purposely roughened single crystalline Si wafers were oxidized in a microwave electron cyclotron resonance (ECR) plasma system for different lengths of time, at different temperatures and with different DC bias voltages applied to the Si substrate. The resulting Si/SiO\sub 2\ interface was analyzed by spectroscopic ellipsometry and spectroscopic immersion ellipsometry, and the morphology of the Si surface before and after oxidation was examined by atomic force microscopy together with Fractal analysis after the grown SiO\sub 2\ film was removed by brief HF etching. Both Si/SiO\sub 2\ interfacial region thickness and Si surface roughness were found to decrease with oxidation, most probably due to the local electric field enhancement effect around Si protrusions at the interface. Oxidation temperature and DC bias voltage were also found to affect the interfacial roughness. The new results for Si oxidation in an ECR plasma are compared with previous ones for thermally oxidized rough Si surfaces.