Papers

AVS1997 Session EM2-ThA: Novel Dielectrics and Materials for Semiconductors

Thursday, October 23, 1997 2:00 PM in Room C1/2

Thursday Afternoon

Start	Invited?	Item
2:00 PM		EM2-ThA-1 Deposition of Ultra Thin CVD Ta₂O₅ Films on Si(100) Using Ta(N(CH₃)₂)₅ and O₂ for Gate Dielectric Applications K.-A. Son, A.Y. Mao, A. Kamath, Y.-M. Sun, E. Pylant, B. Kim, F.I. Liu, J.M. White, D.L. Kwong (University of Texas, Austin); D.A. Roberts, R. Vrtis (Schumacher, A Unit of Air Products and Chemicals, Inc.) Ta₂O₅ CVD films grown with Ta(N(CH₃)₂)₅ precursor have been investigated as a function of deposition temperature (420 to 720°C) and oxygen flow rate (100 to 900 sccm). The chamber pressure during film deposition was 3 to 8 torr depending on O₂ flow rate. No heating of the Ta(N(CH₃)₂)₅ precursor was necessary to grow the film, and the film growth rate was ~20 Å/min. at 520°C. Based on XPS analysis, the estimation about O/Ta ratio in the film is ~2.3, and this ratio does not change significantly with oxygen flow rate or deposition temperature. On the other hand, the amount of carbon in the film decreases with increased oxygen flow rate and with decreased deposition temperature; The C/Ta ratio of the film grown at 420°C with 900 sccm is ~0.15. No nitrogen contaminant has been observed in these films in contrast to the films prepared at ~10^-3 torr, where decomposition of precursor on the surface is significant resulting nitrogen impurity in the film. Improvement of film quality with the increased O₂ flow rate is also evidenced in leakage current measurements. The leakage current of the as-deposited film (t_eff~15 Å) grown at 450°C with 900 sccm of O₂ flow rate is ~10^-5 amp/cm² at ±1V, and the film grown with 100 sccm of O₂ flow rate at 450°C shows more than three order higher leakage current. Ta₂O₅ films grown on SiO_xN_y passivated layers show much improved electrical properties. This is because SiO_xN_y layer prevents SiO₂ formation at the interface during film growth as proved by depth profile SIMS. The effect of post annealing on structural and electrical properties of films have also been investigated.
2:20 PM		EM2-ThA-2 Structure and Growth of Cerium Oxide Films by Molecular Beam Epitaxy Y.J. Kim, S. Thevuthasan, Y. Gao, S.A. Chambers (Pacific Northwest National Laboratory) Metal oxide single crystals are difficult to obtain with reproducible surface composition, structure, and morphology. Epitaxial growth of insulating oxides as thin films on various substrates has many useful electronic applications. Highly-oriented single-crystalline epitaxial films of cerium dioxide (CeO₂), a very stable insulator even at high temperatures, have been grown on various substrates including Si(111), Si(100), and MgO(001) using oxygen-plasma-assisted molecular beam epitaxy. The growth mode and structural parameters of the grown films have been characterized using in situ reflection high-energy electron diffraction, low-energy electron diffraction, and x-ray photoelectron spectroscopy and diffraction, and ex situ atomic force microscopy, Rutherford backscattering and channeling measurements, and x-ray diffraction. The growth parameters and choice of substrate along with the associated effects on film composition and surface morphology will be discussed.
2:40 PM		EM2-ThA-3 A Simple Efficient N Atom Source for Nitride MOCVD based on a Dielectric Barrier Discharge T.-H. Kim, J.M. Lee (Seoul National University, Korea) MOCVD of nitride thin films such as GaN, TiN, BN have attracted much attention in recent years for optical device, barrier layer, and hard coating applications, respectively. In MOCVD of these materials NH₃ and N₂ are commonly used as a N source gas. However, dissociation of these molecules are difficult due to their strong bonds, and consequently a high substrate temperature and high flux of these gases are required to obtain a reasonable growth rate. In order to enhance the dissociation efficiency of these molecules, RF or microwave plasma excitation is often employed, but transport of the activated N-containing radicals onto the substrate surface is difficult because of a relatively low operating pressure of such plasmas. We report here the design and operation of a simple but efficient remote plasma N atom source for nitride MOCVD based on a dielectric barrier discharge. It can be operated at high pressures, i. e. 10 Torr ≤ P ≤ 1 atm, so that N atoms generated in the source can be easily delivered to the substrate surface. When the source was operated with a N₂ (0.1-10 %) - Ar gas mixture at a few hundred Torr, 100 watt, and a few l/min flow rate, the flow rate of N atom measured by photochemical titration with NO at 10 cm downstream of the plasma region amounted to 10¹⁶ - 10¹⁸ atoms/sec. To demonstrate the efficiency of the present N atom source, AlN MOCVD has been performed on silicon(100) substrate using trimetylaluminum (TMA). The growth rate as high as 3 µm/hr was obtained at a substrate temperature of 600 C. The growth rate is found to strongly depend on the N atom flow rate.
3:00 PM		EM2-ThA-4 Atomic Layer Growth Process Characterization of Dimethylethylamine Alane and Ammonia on Si(100) for AlN Thin Film Deposition J.S. Kuo, J.W. Rogers (University of Washington) Numerous recent studies have shown that dimethylethylamine alane (DMEAA) is a viable precursor for depositing aluminum and aluminum nitride thin films. However, for atomic layer growth purposes, the self-limiting behavior of DMEAA on silicon substrate has not been rigorously verified. While some aftergrowth observations support the possibility of self-limiting adsorption, this particular system is often cited to be self-limiting solely on the basis of analogous chemistry with trimethylethylamine alane (TMAA). In this work, adsorption of DMEAA on Si(100) is characterized in a custom UHV chamber coupled with a CVD cell for higher pressure deposition, using X-ray Photoelectron Spectroscopy, Temperature-Programmed Desorption, and Secondary Ion Mass Spectrometry. The surface reaction of DMEAA with ammonia on silicon substrate to form AlN will also be presented.
3:20 PM		EM2-ThA-5 Analysis of AlN Thin Films Grown on Si(100) by Pulsed Supersonic Jet Molecular Beams V.W. Ballarotto, M.E. Little, M.E. Kordesch (Ohio University) Non-skimmed pulsed supersonic free jets of trimethylaluminum (TMA) and ammonia were used to grow thin films of AlN. The use of pulsed jets will increase the intensity of the molecular beams. The translational kinetic energy of the precursors was 130 meV for TMA and 28 meV for ammonia. All films were grown in the average pressure regime of ≤10^-5 Torr. The effects of varying each pulse rate, substrate temperature, deposition time, and relative flux of TMA:ammonia on film composition was studied. A typical ammonia to TMA flux was 1000. Post-deposition analysis included AES,SEM, XRD, X-ray Fluoresence and Photoemission Electron Microscopy (PEEM). Supportedby BMDO/ONR: N00014-96-1-0782 and -1060.
3:40 PM		EM2-ThA-6 Direct Measurement of Ultrathin SiO₂ Thickness by AFM with Self-Assembled-Monolayer Islands as a Self-Patterned-Mask K. Namba, T. Komeda, Y. Nishioka (Texas Instruments, Tsukuba R&D Center, Japan) The shrinkage of ultra-large-scale integration (ULSI) circuit requires an ultrathin gate oxide as thin as 4 nm in 0.1 µm scale metal-oxide-semiconductor (MOS) devices. Since oxide thickness (T_ox) is one of the most important device parameters, the precise control of T_ox is the critical issue. T_ox is most commonly measured by nulling ellipsometry due to its easy sample preparation and short measurement time. However, in the very thin thickness regime, below 5 nm, accurate measurement of ellipsometric parameters, T_ox and the refractive index, becomes very difficult because these parameters couple each other. In this paper, a novel method for measuring ultrathin (2-12 nm) SiO₂ film thickness is discussed.¹ The process consists of: (1) formation of self-assembled-monolayer (SAM) islands on SiO₂ of which thickness to be measured. (2) removal of the SiO₂ layers not covered by the SAM islands. (3) measurement of the height difference between the etched and non-etched areas by atomic-force-microscopy (AFM). Experimentally, we use octadecyltrichlorosilane (OTS, C₁₈H₃₇SiCl₃). The OTS-SAM islands are formed by immersing the sample into a 1.0 mM solution of OTS in the mixture of 8 % CHCl₃+ 12 % CCl₄+ 80 % n-hexadecane (by volume) for 20 sec at room temperature. Subsequently, the SiO₂ of non-masked portion is etched by HF (HF:H₂O=1:20 by volume). It is observed that OTS-SAM forms islands on SiO₂ layer which can be regarded as self-patterned-mask (SPM) for HF etching. No degradation of OTS-SAM mask is observed during the etching of SiO₂ as thick as 12 nm. The unique thickness makes it unnecessary to remove them for the thickness measurement. Practical usefulness is demonstrated not only by the compatibility of the measured values with ellipsometry and TEM observation, but also by the short measurement period resulting from the directness of the method. ¹T. Komeda, K. Namba, and Y. Nishioka, will be appeared on Appl. Phys. Lett. 6/28 issue .
4:00 PM		EM2-ThA-7 Studies on Ultrathin Silicon Oxides Grown by Low Temperature and Low Pressure Wet Oxidation. V.K. Bhat, A. Subrahmanyam, K.N. Bhat (Indian Institute of Technology, India) The interest in thin oxides on silicon has been evoked with the scaling down of device dimensions. The ultrathin oxides are also important in the case of highly efficient(>18%) Metal Insulator Semiconductor Inversion Layer (MISIL) solar cells and scaled Oxide -Nitride-Oxide (ONO) multi layer dielectrics. The frequently occurring problems in the growth of ultra thin oxides on silicon are: the presence of pinholes in the oxide layer and the reproducibility of the oxide layer thickness. Since the growth of thinner oxide requires high control over the growth parameters, standard oxidation techniques at atmospheric (oxygen) pressure may not be suitable. Thus other technique such as low-pressure, low temperature oxidation are necessiated to achieve the required control. In the present investigation, ultrathin oxide layers, 2-5 nm thick, have been grown on FZ grown (100) p-Silicon (lap polished), of resistivity 0.5-1.0 ohm cm. by wet oxidation technique. The wet oxidation is carried out in a quartz furnace maintained at 600^oC by varying the time of oxidation(15-120 minutes) and the partial pressure of water vapour passing through the quartz chamber (0.04 to 1.00 atmospheres). The thickness of the grown oxides are measured using an ellipsometer. Al-SiO₂-Si tunnel diodes are fabricated using metal masks on the oxide grown (with an oxide layer thickness between 2nm and 4nm). The current(I)-voltage(V) and capacitance(C)-voltage(V) characteristics of these diodes are being studied. Diode ideality factor (n), reverse saturation current density (J_s), series resistance (R_s) and the barrier heights (φ_b) have been evaluated for these diodes to assess the quality of the thin oxide layer. Results indicate that reproducible ultrathin silicon oxides with control can be grown by low temperature, low pressure wet oxidation technique. Experiments are also being carriedout to investigate the growth kinetics of these thin oxides on the pre-oxidation cleaning of the silicon wafer.
4:20 PM		EM2-ThA-8 Transition Metal Nitride Formed at 40K by Simultaneous Physisorption and Thermal-Evaporation ; TiN/Si(100) S. Ahn, J.Y. Han, J.M. Seo (Jeonbuk National University, South Korea) It has been confirmed by in-situ XPS analysis that Ti nitride can be athermally formed without substrate disruption by simultaneous NH₃ exposure and Ti evaporation to a clean Si(100)-2x1 held at 40K under UHV. One of the critical points in the present nitride formation at 40K is solving the kinetic constraint problem by simultaneous adsorption of consisting elements. Compared to TiN formed by other techniques, this athermally formed Ti nitride does not contain O contamination but bear TiN precursors, TiN_xH_y. Post-heat treatment partially converts TiN_xH_y to Ti nitride and induces the excessive metallic Ti to be mixed with the previously formed Ti nitride. When the Ti nitride is exposed to the air, Ti in the Ti nitride is preferentially oxidized and forms the Ti oxide on the top side. By mild-annealing it up to 550C under UHV, O is transferred from Ti oxide to the Si substrate and the original Ti nitride is gradually recovered. Even for annealing it up to the temperature high enough to desorb the oxidized Si, the Ti nitride hardly desorbs.
4:40 PM		EM2-ThA-9 Scanning Tunneling Microscopy and Ballistic Electron Emission Microscopy Studies of Epitaxial Pt/CaF₂/Si(111) V.P. LaBella (Rensselaer Polytechnic Institute); C.A. Ventrice, Jr. (University of New Orleans); Y. Shusterman, L.J. Schowalter (Rensselaer Polytechnic Institute) The interest in understanding electron transport properties in ultra-thin insulating layers has prompted the study of the Pt/CaF₂/Si(111) metal-insulator-semiconductor structure. Several groups have demonstrated growth of high quality, single crystal layers of CaF₂ as thin as 2 monolayers (ML). The electronic and morphologic properties of these Pt/CaF₂/Si(111) structures have been studied with our in-situ scanning tunneling microscope (STM) which also has the ability to perform ballistic electron emission microscopy (BEEM) measurements. BEEM has been proven to be an excellent probe for looking at local electrical transport properties. Thin (5-10Å), relatively defect free CaF₂ layers were grown on both well-oriented and off-axis Si(111) wafers by molecular beam epitaxy. The STM images of these structures show flat terraces and steps of CaF₂ as well as island formation and step bunching (for vicinal substrates). Platinum was deposited in-situ on top of the CaF₂/Si(111) structures with thicknesses ranging from sub-monolayer to 55Å. The STM images of the Pt/CaF₂/Si(111) structures show the atomic steps of the underlying CaF₂ morphology, as well as the formation of Pt nodules. For the well oriented substrates, STM images for sub monolayer Pt coverages show that these nodules nucleate on defect sites and step edges of the CaF₂ layer. The BEEM spectra for thicker Pt coverages show an onset near the conduction band minimum of the CaF₂. For thinner Pt coverages, an additional BEEM peak appears at 2eV, which has not been previously observed in other metal/CaF₂/Si(111) studies. This low energy peak is consistent with resonant tunneling through the metal quantum well demonstrating coherent electron scattering at room temperature. In addition, the BEEM current at high tip biases (>6 V) can be used to estimate the relative efficiency of Auger scattering to produce lower energy electrons which can still cross the CaF₂ barrier.