Papers

AVS1996 Session EM+SS-MoM: Surface Chemistry

Monday, October 14, 1996 8:20 AM in Room 204A

Monday Morning

Start	Invited?	Item
8:20 AM		EM+SS-MoM-1 Chemical Reactions at Bonded Interfaces: Silicon Wafer Bonding M. Weldon, Y. Chabal, S. Christman, E. Chaban, D. Hamann (Bell Laboratories) A detailed understanding of surface/interface phenomena is becoming more critical to the microelectronics community, as manufacturing moves into the deep sub-micron device regime for ULSI applications. Direct silicon wafer bonding is one such area where the technique is widely used to make pin diodes, silicon-on-insulator (SOI) substrates, and a variety of micromechanical devices; yet the physical forces when two clean wafers are joined, and the chemical reactions taking place at the interface when the wafers are annealed to establish permanent bonding are not well understood. Interface characterization is difficult since most surface science techniques cannot penetrate the substrates. We have used IR absorption spectroscopy along with bond strength and TEM measurements to determine the species and their environment at the interface of both hydrophobic (H-terminated) and hydrophilic (oxide-terminated) Si wafers after joining and as a function of annealing. For atomically flat hydrophobic surfaces, we observe a 18 cm\super -1\ red shift of the Si-H stretching vibration that is due to the physical interaction between the two H-terminated surfaces. Atomically rough surfaces only interact over small density of defect sites. In both cases, the formal interface disappears after H diffuses out at 1000\super o\C. Hydrophilic surfaces are initially held by H interaction among 2 to 4 monolayers of water trapped at the interface. Upon annealing, water dissociates and diffuses away from the interface to form silicon oxide, as evidenced by a loss of H\sub 2\O vibrations and an increase in Si oxide phonons. The formal interface disappears at 1100\super o\C, with the formation of Si-O-Si bridging species.
8:40 AM		EM+SS-MoM-2 Physical Processes for Thin Silicon Film Exfoliation Y. Chabal, V. Marsico, D. Jacobson, S. Christman, A. Mills, J. Sapjeta (Bell Laboratories) A new process for the fabrication of thin crystalline Silicon-on-Insulator is generating excitement in the Microelectronics Industry. The method is based on monoenergetic H ion implantation of one Si wafer with subsequent bonding of that wafer to another oxidized wafer. The remarkable feature is the shearing of the implanted wafer upon annealing at 500\super o\C at the location of highest H concentration (determined by the implantation energy), leaving a uniformly thin (+- 50\Ao\), perfectly crystalline Si layer attached to the oxide on the second wafer. The present study addresses the fundamental mechanisms leading to such exfoliation, using IR absorption, Raman and positron spectroscopies, as well as optical, atomic force and transmission electron microscopies. An important feature is the initial disruption of the Si lattice by H ions in the straggling region, in a well defined buried region. For very high doses (>10\super 18\/cm\super 2\), the damage is so great that spontaneous exfoliation occurs. For useful doses (e.g., 5x10\super 16\), exfoliation only occurs upon annealing. Upon implantation, a wealth of Si-H stretching lines are observed by IR, indicating that most defects observed in TEM are decorated by H. The majority of the defect lines disappear by 400\super o\C, giving rise by 600\super o\C to modes corresponding to H at internal surfaces (mostly 111 platelets). These internal surfaces (cracks) can be traced with positronium lifetime measurements, in which changes correlate well with the behavior of IR bands. TEM shows that these platelets are the initiators of cracks leading to separation. The absence of any detectable H\sub 2\ in Raman and IR spectra confirms that high pressure build up in microbubbles is not the mechanism for shearing as in the case of noble gas implantation of metal and semiconductors. Instead, the mobility and chemical activity of H plays a critical role in crack formation and expansion.
9:00 AM		EM+SS-MoM-3 Initial Oxide Formation on SiGe Studied by Ultrahigh Resolution XPS S. Kilpatrick, R. Jaccodine (Lehigh University); P. Thompson (Naval Research Laboratory) Ultrathin oxides have been grown on films of Si\sub 1-x\Ge\sub x\ on (100) Si and analyzed in situ by Angle-Resolved X-Ray Photoelectron Spectroscopy (ARXPS) using the ultrahigh resolution Scienta ESCA-300 spectrometer. Samples with Ge contents up to x = 0.22 were oxidized at 550-750 C and 1.5-3.0 x 10\super -6\ Torr of O\sub 2\ using an electron beam heater, or at 270-370 C and 1.9 x 10\super -5\-760 Torr of O\sub 2\ with a Nichrome heater. These conditions created oxide thicknesses generally in the range of 0.3-3 nm. Thermodynamics strongly favors pure SiO\sub 2\ formation with a segregated layer of Ge at the alloy/oxide interface, although kinetic factors can lead to the growth of mixed oxides\super 1\. In this study of the initial phase of oxide building, varying mole fractions of Si and Ge oxides were produced, even under conditions which would likely produce pure SiO\sub 2\ in thicker oxides\super 2\. Quantitative analysis of the peak intensities was performed to determine the relative fractions of the oxidation states Si\super +1\ to Si\super +4\ and Ge\super +1\ to Ge\super +4\. Correlations have been sought between the fractional quantities of the various suboxides and factors such as alloy content, temperature, and oxidation rate. Successive spectra have been acquired during the evolution of these oxides to find evidence for "conversion of structure", in which Ge which has initially bonded to oxygen is later supplanted by Si to form the thermodynamically favorable Si-O bond. This work is supported under ONR Contract No. N0001492J1308. \super 1\W.S. Liu et al., MRS Symp. Proc. 220, 259 (1991). \super 2\S.J. Kilpatrick et al., submitted to J. Appl. Phys.
9:20 AM		EM+SS-MoM-4 The Surface Reaction during Silicon Oxidation Studied by MEIS H. Lu, E. Gusev, E. Garfunkel, T. Gustafsson (Rutgers University) From isotopic labeling studies [e.g. E.P. Gusev et. el. Phys. Rev. B 52, 1759 (1995)] of the oxidation of silicon, it has been shown that oxygen reacts both at the oxide surface (an exchange reaction) and the oxide/silicon interface (a growth reaction). Though much work has been done on the interface reaction, little effort in the past was spent on understanding the surface reaction. However, this reaction will be important for sub-5nm oxide films used in gate oxides for next-generation devices. We have studied the surface reaction during silicon oxidation for oxides less than 6 nm thick, with isotopic labeling and high-resolution MEIS (Medium Energy Ion Scattering) depth profiling, under different processing (p,T,t) and preparation (including the effect of water and hydrogen). We found the surface reaction has both a lower apparent activation energy (0.4-0.7 eV) and a lower exponent (0.3-0.5) in pressure dependence when compared with the interface reaction (1.3-1.6 eV and p\super 0.6-0.7\, repectively). We also observed a strong dependence of the surface reaction on different preparation conditions. Finally, we will discuss the possible mechanisms for this reaction.
9:40 AM		EM+SS-MoM-5 Selective Oxidation and Low pH Chemical Etching Route to Smooth Si(100) H-terminated Surfaces B. Hinds, H. Niimi, D. Schmidt, D. Aspnes, G. Lucovsky (North Carolina State University) In an effort to form atomically flat H-passivated Si(100) surfaces, wet chemical removal of thermally grown sacrificial SiO\sub 2\ layers on Si(100) substrates has been studied. In particular the pH of etching solutions is reduced to eliminate Si surface oxidation by OH\super -\ and control the SiO\sub 2\ etching species to be primarily HF and H\sub 2\F\sub 2\. The pH is adjusted in the range of -0.32 to 1.6 by adding controlled amounts of H\sub 2\SO\sub 4\ to a 1:30 HF:H\sub 2\O mixture. Of equal importance is the control of oxidizing species in solution which preserve a planar Si interface presumably by selective oxidation of Si surface defects with subsequent removal by HF. Among several examined oxidizing species, SO\sub 4\\super =3D\ and HSO\sub 4\\super -\ anions appear to have favorable oxidative properties and result in relatively smooth Si(100) surfaces as characterized by spectroellipsometry (SE). The smoothest surfaces were obtained at a 1:0.25:30 HF(49wt%):H\sub 2\SO\sub 4\(98wt%):H\sub 2\O etch with a pH of approximately 0.5. Electrical characterization of metal-oxide-semiconductor (MOS) capacitors fabricated on these surfaces with oxide layers prepared by 300=B0C remote plasma assisted oxidation, followed by remote plasma enhanced chemical vapor deposition showed i) the lowest density of interface traps, D\sub it\, ii) the lowest tunneling currents, J\sub o\, and iii) the highest breakdown fields, E\sub bd\, (4x10\super 10\\/cm\super 2\-eV, 11 MV/cm, and 19 \mu\A/cm\super 2\ respectively), occurred with the same etchant solution that produced the smoothest surfaces by SE. In contrast, MOS capacitors fabricated with high-temperature thermally grown oxides were not significantly effected as can be expected for the subsequent planar growth of a thermal oxide-Si(100) interface.
10:00 AM		EM+SS-MoM-6 Fundamental Chemistry of Molecular Precursors on Compound Semiconductor Surfaces J. Creighton (Sandia National Laboratories) Organometallic and hydride compounds are widely used as precursors for the epitaxial growth of GaAs and other compound semiconductors. We have investigated the surface chemical properties of these precursors on GaAs (also on AlAs to a lesser extent) using a variety of surface science diagnostics. Results of these experiments have shed light on the mechanisms of precursor decomposition which lead to film growth and carbon doping. By comparing results on surfaces of varying stoichiometry and crystallographic orientation, the role of specific surface sites and surface structure can be probed. For instance, the polar surfaces of GaAs (e.g. (100) and (111)A) irreversibly chemisorb trimethylgallium (TMGa) and evolve methyl radicals between 700-750 K during TPD. In contrast, the nonpolar (110) surface reversibly chemisorbs TMGa. In this case the atom vacancies that are intrinsic to the reconstructions of the polar surfaces serve as sites for irreversible decomposition. There is also evidence that on the polar surfaces chemisorption may create a new class of adsorbate-induced reconstructions. The best example of this is the (1 X 2) reconstruction formed by adsorbed methyl groups on a Ga-terminated GaAs(100) surface. We have studied this reconstruction with a wide variety of tools including polarized surface infrared spectroscopy. Our recent efforts have focused on using in-situ diagnostics to examine the state of GaAs(100) surfaces during MOCVD. The most successful technique to date is reflectance-difference spectroscopy (RDS), as pioneered by Aspnes. We have extended the RDS database by measuring the spectra of key adsorbate covered surfaces. Originally, RDS was "benchmarked" in an MBE system only for the arsenic and gallium terminated surfaces, hence the effect of adsorbates such as methyl groups or hydrogen was not determined.
10:40 AM		EM+SS-MoM-8 Photo Chemistry of Dimethylcadmium on Compound Semiconductor Surfaces D. Slater, P. Lu, P. Lasky, R. Osgood (Columbia University) Metal alkyls are important precursor molecules in a wide variety of chemical vapour deposition techniques, both for the epitaxial growth of compound semiconductors and for metal films. Frequent claims for photo-enhanced deposition have been made. In this paper we present studies of both the photon induced and thermal surface reactions of such a metal alkly, Cd(CH\sub 3\)\sub 2\, on two different semiconductor surfaces, GaAs(110) and CdTe(110), using time-of flight and thermal desorption mass spectroscopy techniques. The thermal chemistry observed is found to be strongly substrate dependent with simple, non-dissociative adsorption/desorption occurring on CdTe whereas on the GaAs surface dissociative chemisorption results in complex desorptive behaviour attributed to the formation of a number of surface bound intermediate species. Laser induced photochemistry on the CdTe surface is found to be dominated by direct photon absorption by the precursor molecule. Fragment kinetic energies measured in this case are in good agreement with those observed for the gas phase molecule, the primary effect of the surface in this case is to grossly reduce the photofragmentation yield by providing efficient relaxation channels to the photoexcited intermediate. No evidence for electron induced chemistry is observed. Radically different photochemistry is observed following adsorption on the GaAs surface an observation consistent with the presence of surface species chemically distinct from the precursor. In this case multiple features observed in the TOF spectrum may be correlated with the presence or absence of features in the thermal desorption spectrum and thus with specific surface bound intermediates.
11:00 AM		EM+SS-MoM-9 Novel Method for Growing CdS on CdTe Surfaces for Passivation and Heterojunction Formation A. Nelson (Colorado School of Mines); D. Levi (National Renewable Energy Laboratory) Large-grain polycrystalline CdTe was subjected to in-situ H\sub 2\S plasma processing. Plasma processing was performed for 30 min. at normal incidence and at ambient, 100C and 200C. High resolution X-ray photoemission spectroscopy (XPS) of the Cd 4d, Te 4d, S 2p core-levels and valence band was used to determine the presence and chemical environment of S at the surface following plasma processing. XPS compositional depth profiling was also used to determine the distribution of S in the near surface region. The XPS results clearly show a CdS/CdS\sub 1-x\Te\sub x\/CdTe graded heterojunction device structure as a result of the H\sub 2\S plasma processing. Furthermore, time-resolved photoluminescence was used to measure carrier lifetimes and, thus, determine the degree of passivation of CdTe surface states. Results of these measurements prove that the thin layer of CdS reduces the surface recombination velocity through passivation of surface states. This is a new and novel method of fabricating CdS/CdTe heterojunction devices.
11:20 AM		EM+SS-MoM-10 Metal Dependent Fermi-level Movement in the Metal/Sulfur-passivated InGaP Contact Y. Kim, S. Kim (KAIST, Korea); S. Ahn, J. Seo (Jeonbuk National University, Korea) Through the high resolution core level photoemission studies on S-passivated n-InGaP(100) using Pohang Light Source, it has been identified that S-related chemical shifts of -0.54, -0.81, and -3.86eVs relative to the bulk In, Ga, and P are due to In-, Ga- monosulfides and P-polysulfides, respectively. The band bending(BB) of this surface is 0.7eV smaller than the sputtered InGaP with P-vacancy related defects(V\sub P\). The Al deposition preferentially reduces monosulfides, Ga-S and In-S, and increases BB by 0.22eV. The reduced elemental In segregates towards the top and Ga remains near the interface, Post-annealing at 673 K induces the remaining P-polysulfides reduction, P-deposition, Ga-clustering, Al partial-diffusion into the bulk and forms the interfacial Al sulfide, which flattens BB by 0.22eV. On the contrary, Au deposition initially forms InAu alloy, leaves elemental S near the interface, but BB doesn't change. Post-annealing at 673 K causes the interfacial Ga-S reduction, AuGa mixing, Au partial-diffusion into the bulk and increases BB by 0.48eV. Such different behaviors of BB in the metal contact with S/InGaP tells us that, in order to supress BB, it must be considered how to keep S at the interface and remove V\sub P\ with stable S compounds not having gap states.
11:40 AM		EM+SS-MoM-11 Site-Specific Carbon Doping by MOVPE R. Hicks, H. Qi (University of California, Los Angeles) For efficient carbon doping, without simultaneous etching of the semiconductor surface, carbon precursors for p-type doping of III-V compounds must not contain halogen atoms and must have weak carbon-ligand bonds. In this study, we have examined the adsorption and decomposition of a new precursor, trisdimethylaminomethane (HC(NMe\sub 2\)\sub 3\, TDMAM), as well as dimethylamine (DMA) on GaAs(100) surfaces in ultrahigh vacuum. Dimethylamine adsorbs non-dissociatively on GaAs(001) at 300 K, producing an N-H stretching vibration at 3165 cm\super -1\, and C-H stretching and bending vibrations at 2970, 1460, 1395 and 1115 cm\super -1\. Polarized infrared spectroscopy reveals that the DMA molecule adsorbs with surface N-Ga bonds oriented along the [110] direction and C-N bond along [-110] direction. Upon heating the crystal to 353 K, all the DMA desorbs off the surface. By contrast, TDMAM adsorbs dissociatively on GaAs(100) at 300 K, as evidenced by the appearance of C-H stretching vibrations for CH\sub x\ groups bonded to Ga and As atoms. During heating, these species remain on the surface after all the amine groups have desorbed. Hydrogen titration of the GaAs(100) surface after TDMAM adsorption at 413K reveals that carbon has been deposited on the Ga sites. At the meeting, IR, TPD and XPS results will be presented which show the mechanism for TDMAM decomposition and carbon incorporation into GaAs.