AVS1997 Session EM+AS-MoA: Ultra Thin Silicon Oxides: Growth and Electrical Characterization

Monday, October 20, 1997 2:00 PM in Room C3/4

Monday Afternoon

Start	Invited?	Item
2:00 PM		EM+AS-MoA-1 Investigation of Charge Spreading and Trapping in SiO₂ Films by Ballistic Electron Emission Microscopy B. Kaczer, H.-J. Im, J.P. Pelz (The Ohio State University) We use the ability of ballistic electron emission microscopy (BEEM) to locally measure density and depth of trapped charge in buried insulating films¹ to investigate the trapped charge distribution in moderately thin (10-25 nm) SiO₂ films after hot electrons were injected at one location. The trapped charge increases the potential barrier in the oxide and suppresses the BEEM current I_c (i.e. the flux of hot BEEM electrons across the oxide). Some of the I_c-t curves taken during charging show step-like behavior, possibly corresponding to individual traps being filled. We use the BEEM current suppression to "image" the lateral distribution of the trapped charge and correlate it with its local density as measured by BEEM I_c-V_T curves. We find the trapped charge to be distributed close (3-4 nm) to the injection interface with lateral spreading of tens of nm. We also find that the size of this spreading depends on the oxide thickness and on the electric field across the oxide during injection. Presently we use BEEM to investigate ultra-thin (2 nm) SiO₂ films. ¹B. Kaczer, Z. Meng, and J. P. Pelz, Phys. Rev. Lett. 77, 91 (1996); B. Kaczer and J. P. Pelz: J. Vac. Sci. Technol. B 14, 2864 (1996).
2:20 PM		EM+AS-MoA-2 Localized Degradation Studies of Ultrathin Gate Oxides. H.J. Wen, R. Ludeke (IBM T.J. Watson Research Center) One of the key challenges of the SIA Roadmap and a potential show stopper is gate oxide reliability. This challenge is based on the continuing reduction in oxide thickness and the increasing oxide fields during device operation. We present here studies on the limits of oxide reliability on a local, microscopic scale, using an STM to inject hot electrons (13 eV) into the conduction band of SiO₂ that is embedded in an MOS structure. Recent studies indicated that oxide breakdowns were difficult to achieve locally.¹ It now appears that these rare breakdowns were preceded by gate metal failure. We have overcome this problem and find that breakdowns are not observed even for injected charge dosages of 5x10⁵ Coul/cm² at equivalent Fowler-Nordheim stress fields of 27 MV/cm. Stressing with electrons exceeding 2 eV produces well known electron traps, whose density exceed 10¹³ cm^-2. Evidence of anode hole injection (another postulated breakdown mechanism) is also observed under high field application (>7 MV/cm). Since breakdown was not achieved even while applying 8 MV/cm fields and injecting 13 eV electrons, we conclude that trap creation and hole injection are not sufficient to cause oxide failure. We conclude, furthermore, that moderate fields do not appear to be relevant to breakdown, and that when failure occurs it is associated with a yet to be identified defect of relatively low density. ¹R. Ludeke, H.J. Wen and E. Cartier, J. Vac. Sci. Technol. B 14, 2855 (1996).
2:40 PM		EM+AS-MoA-3 Properties of Low-Temperature-Grown Ultrathin Oxide with Post-Oxidation Anneal T. Sakoda, M. Matsumura, Y. Nishioka (Texas Instruments Tsukuba Research & Development Center Ltd., Japan) Ultrathin gate oxides are required to be thinner than 3nm, and the low temperature oxidation is effective for the atomically-controlled oxide growth. We found that the post-oxidation anneal (POA) in nitrogen improves the electrical properties of ultrathin oxides grown at very low temperature of 650C. Metal-oxide-semiconductor (MOS) capacitors with 3nm thick oxides on p-type Si substrates were fabricated. The oxidations were performed ranging from 650 to 850C, and POA temperature was 850C. A large leakage current was observed in the oxides grown at 650C without POA. The leakage current at a low gate voltage below 1.5V was abruptly reduced by POA, and this improvement was most significant in the oxides thinner than 3nm. This is correlated to the reduction of the interface traps, and may be explained by the direct tunneling of the electrons from the capacitor electrode to the interface traps ¹. The constant voltage stressing was performed at a room temperature. Longer time to breakdown and fewer hole traps during stressing were observed in the oxides grown at 650C with 850C anneal as opposed to the oxides grown at 850C. These improvements are believed to be due to the MOS interfacial strain releases after the anneal ². To confirm this view, grazing incidence x-ray reflection (GIXR) measurements were performed and obtained the density profiles of oxides. GIXR results indicated that the MOS interface of low-temperature-grown oxides with POA has smaller density gradient than that of the oxides grown at a high temperature. The oxides without POA seemed to have slightly denser regions than the bulk oxides near oxide/Si interface. This strained layer was considered to contain more oxygen vacancies and strained bonds related with the hole trap sites. In conclusion, we found the superior characteristics of low-temperature-grown ultrathin oxides with POA. The control of the MOS interface structures will be more important in future MOS devices. ¹T.P.Ma and R.C.Barker, Solid State Electronics, vol.17, p.913, (1974) ²I.Zekeriya and T.P.Ma, IEEE Trans. on Nuclear Science, vol.NS-31, p.1261, (1984)
3:00 PM		EM+AS-MoA-4 Oxynitridation of Si(100) Using a Remote Ar-N₂O Plasma B.C. Smith, H.H. Lamb (North Carolina State University) The controlled incorporation of nitrogen at the Si-SiO₂ interface has been reported to improve gate dielectric quality and reliability in Si field effect transistors (FETs). Lucovsky and coworkers have shown that Si oxidation by exposure to a remote He-N₂O plasma yields approximately a monolayer of interfacial N. In this work, we have investigated the oxynitridation of Si(100) using a radio-frequency (rf) remote Ar-N₂O plasma. On-line Auger electron spectroscopy (AES), angle-resolved x-ray photoelectron spectroscopy (ARXPS) and surface infrared spectroscopy (SIRS) were employed to determine the concentration, spatial distribution and local chemical bonding of N in the resultant thin dielectric layers. Nitrogen incorporation in the films occurs primarily at the Si-SiO₂ interface irrespective of rf power in the 5-50 W range; however, the interfacial N concentration increases sharply with rf power near the lower end of this range. Real-time optical emission spectroscopy (OES) and quadrupole mass spectrometry (QMS) were used in an effort to ascertain which plasma-generated species are responsible for nitridation. The results indicate that the concentration of NO species is maximized at low rf power (approximately 10 W), whereas the concentrations of N₂ and O₂ species increase monotonically with rf power. Nitrogen molecular ions (N₂⁺) were not detected by OES under any of the conditions investigated. The evidence suggests that an electronically excited molecular nitrogen species (N₂*) is responsible for interface nitridation.
3:20 PM		EM+AS-MoA-5 Development of Ultra-thin Gate Dielectric with Nitrogen Incorporation into Oxide for 0.25 micron Technology Y. Ma, P.K. Roy (Bell Laboratories, Lucent Technologies) With continuous down-scaling of device dimension, gate oxide thickness in metal-oxide-semiconductor devices is in the range of 5 nm. The integrity and reliability of gate dielectric have become the key to the success of development of new generation technologies. Nitrogen incorporation into oxide has shown to improve gate oxide quality. Conventionally, nitrogen was introduced into oxide by annealing oxide in N₂O or NO gases. With gate oxide thickness of 5 nm or thinner, it has become more difficult to control oxide thickness due to fast oxidation rate from run to run. Numerous efforts have been made such as reduction of oxidation temperature and oxidant partial pressure to slow down the oxidation rate. These efforts such as lower oxidation temperature resulted in lower quality oxide. In this study, oxide growth rate was slowed down by oxidizing Si substrate with N₂O first. The nitrogen containing layer functions as an oxygen diffusion barrier thus reducing the oxidation rate. The second step is oxidize the thin layer in O₂ underneath the first lightly nitrided layer. The final step is annealing the stack in N₂O ambient. With these three step synthesis, an "oxynitride-oxide-oxynitride" (NON) pseudo-structure was formed. Nitrogen at the oxynitride/Si interface enhances interface strength against current stress, while nitrogen at the oxynitride/poly-Si interface act as higher barrier for boron penetration. Device quality and manufacturable gate dielectric has been achieved.
3:40 PM	Invited	EM+AS-MoA-6 The Growth and Composition of Ultrathin Oxynitride Films on Si(100) E.P. Gusev, H.C. Lu, E. Garfunkel, T. Gustafsson (Rutgers University); M.L. Green (Bell Laboratories, Lucent Technologies) Silicon oxides or oxynitrides will remain the materials of choice for the gate dielectric in MOS structures through the next decade. We have used several methods to develop a better understanding of the key atomic-scale issues concerning ultrathin oxynitride film growth and structure. High resolution medium energy ion scattering (MEIS) is used to accurately determine the depth and elemental concentrations in the films¹^,²^,³ and x-ray photoemission (XPS) yields a better understanding of local chemistry. We have examined oxynitrides thermally (and rapid thermally) grown on Si(100) in N₂O, NO, O₂, and various isotopes. Our results demonstrate that: i) NO reacts similar to O₂, with the predominant growth reaction occurring by NO diffusion to the substrate-dielectric interface; ii) oxynitridation of clean Si(100) in NO results in a relatively homogeneous nitrogen distribution; iii) oxynitridation in N₂O results in a lower concentration of nitrogen incorporated into the film; and iv) for N₂O oxynitridation, or for a thin SiO₂ film subsequently annealed in NO, the nitrogen is concentrated in a region 1-2 nm wide (processing condition dependent) on the oxide side of the interface. We have not observed nitrogen on the substrate side of the interface (i.e., where no oxygen is present). Under certain processing conditions, nitrogen in an oxynitride film can be removed. We favor a model in which atomic oxygen causes nitrogen removal, as O is more stable in the film than N, N₂, O₂, NO and O are produced during the N₂O gas-phase decomposition at high temperatures, and the absolute and relative concentrations of these species strongly influence the film growth chemistry and stoichiometry. We have also reacted Si with various NO, N₂O and O₂ sequences and analyzed the resultant depth profiles. These experiments show that NO and O₂ do not effectively remove N from the bulk of an oxynitride film, whereas N₂O does. Atomic oxygen remains the most likely species responsible for N removal from the film. Finally, we review recent results on oxynitride layer procedures and modeling of gas phase N₂O and NO chemistry. ¹H.C. Lu, E.P. Gusev, T. Gustafsson, E. Garfunkel, M.L. Green, D. Brasen, and L.C. Feldman, Appl. Phys. Lett. 69 (1996) 2713. ²E.P. Gusev, H.C. Lu, T. Gustafsson, and E. Garfunkel, Phys. Rev. B52, 1759 (1995). ³H.C. Lu, T. Gustafsson, E.P. Gusev, and E. Garfunkel, Appl. Phys. Lett. 67, 1742 (1995).
4:20 PM		EM+AS-MoA-8 New Method for Low-Temperature Si Oxidation H. Kobayashi, T. Yuasa, K. Yamanaka, Y. Nakato (Osaka University, Japan); K. Yoneda (Matsushita Electronic Corp., Japan); Y. Todokoro (Matsushita Electric Industrial Co., Ltd., Japan) A new method for the low-temperature formation of silicon oxide layers is developed using the catalytic activity of a platinum (Pt) overlayer. In this method, a 0.5~1nm-thick oxide layer is formed chemically on Si(100) substrates and then a 3nm-thick Pt layer is deposited on it. The heat treatment of the specimens at 300 in oxygen increases the oxide thickness between the Pt layer and the Si substrate to ~6nm. The thin oxide layer is found to prevent the silicide formation effectively, and consequently, the oxide layer grows between the Si substrate and the Pt layer, but not on the Pt surface. Measurements of the oxide thickness vs. the heat treatment time show that the reaction and diffusion are the rate-determining step in the initial and subsequent oxidation stages, respectively. By applying a positive (or negative) bias to the Si substrate with respect to the Pt layer during the heat treatment, the oxidation is enhanced (or retarded), indicating that oxygen ions such as O^- and O^2- as well as oxygen atoms¹ are the moving species through the oxide layer. It is likely that oxidation proceeds at low temperatures with the following mechanism. Oxygen is dissociatively adsorbed on the Pt surface, and dissociated oxygen diffuses to the Pt/oxide interface. Then, oxygen atoms and ions are injected into the oxide layer, diffuse to the Si/oxide interface, and react with Si atoms there. Due to the small size of oxygen atoms and ions, they can diffuse through the oxide layer at low temperatures, and due to the low activation energy, the interfacial reaction occurs easily. ¹H. Kobayashi, T. Yuasa et al. J. Appl. Phys. 80, 4124 (1996).
4:40 PM		EM+AS-MoA-9 Atomic Layer Controlled SiO₂ Growth at Room Temperature Using Catalyzed Self-limiting Surface Chemistry J.W. Klaus, A.W. Ott, S.M. George (University of Colorado, Boulder) SiO₂ thin films were deposited with atomic layer control using self-limiting surface chemistry. The SiO₂ growth was accomplished by separating the binary reaction SiCl₄ + 2H₂O --> SiO₂ + 4HCl into two half-reactions. Successive application of the half-reactions in an ABAB… binary reaction sequence produced SiO₂ deposition. Without the catalyst, SiO₂ films could be grown at temperatures of 600-800 K with reactant pressures of 1-10 Torr. Using pyridine as a catalyst, SiO₂ films were deposited for the first time at room temperature. The pyridine catalyst lowered the required SiO₂ deposition temperature from > 600 K to 300 K. The pyridine also lowered the reactant flux required for complete reactions from 10⁹ L to 10⁴ L. The self-limited growth of the SiO₂ thin films was monitored with an in-situ spectroscopic ellipsometer. SiO₂ growth rates were measured versus temperature and reactant exposure time. The uncatalyzed SiO₂ deposition rates per AB cycle were shown to drop from 1.1 Å per AB cycle at 600 K to 0.74 Å per AB cycle at 800 K. This drop off correlates with the loss of reactive Si-OH* surface species due to surface dehydration. The surface topography of all the SiO2 films measured using atomic force microscopy were extremely smooth with a roughness nearly identical to the initial substrate.
5:00 PM		EM+AS-MoA-10 Electrical Properties of the Ultrathin (3nm) Silicon Oxide Film Grown by Low Density Oxygen Plasma at Room Temperature M. Kitajima, I. Kamioka (National Research Institute for Metals, Japan); M. Matsumura, T. Sakoda, Y. Hirose, Y. Nishioka (Texas Instruments Tsukuba Research and Development Center Ltd., Japan) We report on the electrical properties of the ultrathin (3nm) silicon oxide film grown on Si(100) at room temperature (RT) using oxygen plasma generated by radio frequency (RF) discharge. The low density (2-4x10⁷ cm^-3) plasma in our system results in the very slow and bias dependent growth rate which has been advantageously applied to the control of such ultrathin oxide growth in the previous work ¹. For realizing metal oxide semiconductor (MOS) devices, however, the electrical properties of these oxide films should yet be evaluated. For this purpose, we characterize the MOS capacitors with the oxides grown by this technique. The surface of Si(100) sample cleaned by the standard RCA procedure and by HF is oxidized by the oxygen plasma in an ultrahigh vacuum chamber with the base pressure of 5x10^-7 Pa. The plasma frequency and the power are 13.56MHz and 300W. The oxide films are grown with the different sample bias conditions, i.e. from -60V to +60V. The final thickness of the oxide films is controlled to be 27.6±1.1Å with the assist of the real time ellipsometer. The Al electrode is then formed by vacuum evaporation. The high frequency (1MHz) C-V measurement shows the oxide fixed charge of low 10¹²cm^-2 level and the AC conductance method extracts the interface state density of low 10¹³ cm^-2eV^-1 range. We stress the fact that these numbers are obtained from the samples receiving no post heat treatment. In addition, in contrast to the previous report on the thicker film growth ², even with the negative bias condition, the electrical properties comparable to those with the positive biases are obtained reproducibly. These results are apparently the first data indicating the possibility of the RT growth of the ultrathin oxide film with the device grade electrical properties by this class of technique. The ongoing investigations such as the carrier transport through the film and the temperature effect will also be discussed. ¹M. Kitajima et al., Phys. Rev. B, Vol. 53, p.3993 (1996) ²D. A. Carl et al., J. Appl. Phys., Vol 70, p.3301 (1991)