Effects of HCl and Cl2 additions on silicon oxidation kinetics

The growth kinetics of SiO₂ films (100-18000Å) on [100], 2Ωcm silicon have been investigated between 900-1100?C with additions of 0-9 vol.% HC1 or 0-2 vol.% Cl₂ to the dry oxygen ambient. The thickness-time data could best be fitted numerically to a mixed linear-parabolic equation that included a correction for fast initial growth. Equivalent amounts of chlorine (e.g., 2 vol.% HC1 or 1 vol.% Cl₂) produced completely different effects on the rate of SiO₂ growth. The quantitative effects of halogen additions were studied in greatest detail at 900?C. At that temperature, the parabolic rate constant increased linearly with the HC1 concentration. At the same time, the linear rate constant remained constant. Both rate constants did change when Cl₂ was used as an additive. The effect of HC1 additions on the parabolic rate constant reaches a maximum around 1000?C. Possible mechanisms for the halogen effects are discussed,← and it is seen that the gas phase reaction 4 HC1 + 0₂ → 2 H₂O + 2 Cl₂ is not reflected in the growth kinetics.     

Electrical characterization of ultrathin oxides of silicon grown by N2O plasma assisted oxidation

The ultrathin (2.0–3.5 nm) oxides of silicon have gained renewed importance in view of ultra large scale integration (ULSI) of the silicon devices. In the present investigation, the ultrathin oxides are grown on (100) oriented p-type single side polished silicon using N₂0 plasma assisted oxidation in a PECVD reactor at 200°C. The oxide growth as a function of oxidation time is studied. The oxidation growth conforms to the reaction limited regime. In order to understand the electrical quality of Si/ultrathin SiO₂ interface, Al-thin SiO₂-Si tunnel capacitors are fabricated and their capacitance-voltage (C-V) and current-voltage (I–V) characteristics are studied. The effect of annealing on these oxides (termed as “post oxidation annealing”) has also been studied. The C-V characteristics of tunnel capacitors with “as grown” oxide showed a frequency dependence, possibly due to the presence of large fast interface state density. These fast interface states are observed to decrease with increasing oxidation time. The tunnel capacitors that the oxides undergone “post oxidation annealing” (POA) at 350°C in N₂ ambient for 20 minutes have shown practically no frequency dependence of the C-V characteristics; this observation along with the data on I-V characteristics confirms that POA reduces the interface state density considerably. The forward and reverse currents of POA capacitors are observed to decrease considerably indicating the reduction in the trap assisted tunneling transport process across the tunnel insulator.     

Low-temperature epitaxy using Si2H6

In the ULSI processes of the 1990s, the emphasis is expected to change from lateral to vertical down sizing in an attempt to achieve better device performance. Low-temperature CVD Si epitaxy will aid in this trend. It also offers the advantage of controllable selective deposition. We proposed three applications of low-pressure, low-temperature Si₂H₆ epitaxy with new designed reactors. One is half-micron epitaxy on buried layer. We successfully fabricated the bipolar transistor of half-micron epitaxy using the batch reactor of Si₂H₆ epitaxy. The other applications are formation of transistors having reduced parasitic capacitance and formation of shallow junction. These advanced applications require flexible conditions and low-temperature prebaking. We examined low temperature prebaking of 874° C using H₂ and of 800° C using SiH₂F₂ with the singlewafer reactor. Also, low-temperature epitaxy of 632° C was observed with the singlewafer reactor.     

Characterization of GaN films etched using reactive ion etching technique by secondary ion mass spectrometry

We investigated GaN films etched by using reactive ion etching (RIE) technique to fabricate the GaN-based devices. The samples were grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD), and Ti/Al contacts were formed on n-GaN surfaces after etching processes. The effects of the kinds of reactive gases were evaluated by secondary ion mass spectrometry (SIMS). The results showed that in the sample etched using BCl₃ gas, the signal from boron contaminations was strongly detected at the interface between the contact metal and n-GaN, and we found that additional etching in Cl₂ plasma after etching with BCl₃ gas was essential to make a good contact.     

Norm constraint regard for stable H2 controllers

For stable output controllers, the standard H₂ problem is modified by additionally including the H₂ controller norm.      

Characterization of Si homoepitaxial films grown with and without surface photoactivation by ArF excimer laser-induced photodissociation of Si2H6

Silicon homoepitaxial films have been grown by photodissociation of Si₂H₆ by the 193 nm line of an ArF excimer laser in an ultra-high vacuum system. Silicon epitaxy has been achieved in two ways: one, in which the laser shines into the chamber parallel to the substrate and another, in which the laser is directly incident on the substrate at grazing angles (87° with respect to the substrate normal). Controllable growth rates of 0.5–4Å/min have been achieved for crystalline films by the first method using substrate temperatures as low as 250° C, Si₂H₆ partial pressures of 20 mTorr and photon flux densities of 10¹⁶ photons/pulse.cm². In the second method, where the laser beam is directly incident on the wafer at grazing angles, very high growth rates of up to 80Å/min have been achieved at 300° C, 20 mTorr Si₂H₆ partial pressure and a photon flux density of 2 × 10¹⁵ photons/pulse.cm². A comparison of the microstructure of the films grown by the two methods is presented on the basis ofin situ reflection high energy electron diffraction (RHEED) analysis and selected area transmission electron microscopy (TEM) studies. In both cases, the growth rates are found to be linearly dependent on the photon flux density for the process parameter ranges studied.     

Low temperature cleaning of Si by a H2/AsH3 plasma prior to heteroepitaxial growth of GaAs by metalorganic chemical vapor deposition (MOCVD)

A method using a H₂/AsH₃ plasma to clean the Si surface before GaAs heteroepitaxy was investigated and the dependence of the effectiveness of this treatment on arsine partial pressure was studied. Thin GaAs-on-Si films deposited on the plasma-cleaned Si were analyzed using plan-view TEM, HRXTEM and SIMS. Although not optimized, this method of Si cleaning makes heteroepitaxial deposition of GaAs possible. Some roughening of the Si surface was observed and a possible explanation is offered. Using the results of this study, thick (2.5–3.0μm) epitaxial GaAs films were then deposited and their quality was evaluated using RBS, XTEM and optical Nomarski observation. All Si surface cleaning and GaAs deposition were carried out at temperatures at or below 650°.     

Low temperature Si2H6 Si epitaxy in-situ doped with AsH3/SiH4

The use of disilane (Si₂H₆) as a silicon source for epitaxial deposition was investigated for both very low pressure chemical vapor deposition (thermal CVD) and plasma enhanced chemical vapor deposition (PECVD) from 600 to 800° C. The growth rates observed for temperatures at or below 750° C were at least an order of magnitude higher than those observed for silane (SiH₄) using similar deposition conditions. An argon plasma was used to sputter clean the silicon surface, in-situ, immediately before the deposition. It was found that a low dc bias on the substrate during the argon sputter cleaning process helped remove carbon and oxide from the surface of the silicon substrate. A 16 min Ar sputter clean at 650° C, 2.5 W rf power, and •50 V dc bias resulted in a carbon and oxygen concentration at the epilayer-substrate of less than 4 × 10¹⁸/cm³ and 2 × 10¹⁸/cm³, respectively. In situ arsenic doping during disilane epitaxial growth was carried out by thermal CVD and PECVD using arsine (AsH₃) diluted in silane (SiH₄) at 800° C. The results were compared to similar experiments using only SiH₄ as the silicon source. Up to 500 ppm of arsine was diluted in the reactant gas and it was found that the Si₂H₆ growth rates were insensitive to the arsine concentraton in the gas phase.     

Low temperature dopant activation of BF2 implanted silicon

Boron activation and carrier mobility were measured after low temperature furnace heat treatments, in silicon layers implanted with BF ₂ ⁺ ions at 60 keV and at fluence in the 1 − 5 × 10¹⁵ ions cm⁻² range. These quantities were correlated with boron and fluorine chemical depth profiles obtained with secondary ion mass spectrometry (SIMS), and with the lattice defects revealed by transmission electron microscopy (TEM). High dopant activation, well above the extrapolated boron solid solubility, was found for all the fluences investigated after a thermal treatment of 20 min at 600‡ C. In the high fluence implanted samples, the solid phase epitaxial regrowth of the amorphous layer induces a severe fluorine redistribution which causes the formation of a defective band at the sample surface containing microtwins and small precipitates; a decrease in both the activated dopant concentration and carrier mobility was found in this region. The comparison with dopant activation data obtained in samples diffused at higher temperature (from 900 to 1000‡ C) shows that twins are electrically active only when they are decorated by isolated impurities and/or in presence of very small precipitates.     

Preparation and characterization of thin films of MgO,Al2O3 and MgAl2O4 by atomic layer deposition

MgO, Al₂O₃ and MgAl₂O₄ thin films were deposited on silicon substrates at various temperatures by the atomic layer deposition (ALD) method using bis(cyclopentadienyl)magnesium, triethylaluminum, and H₂O and were characterized systematically. High-quality polycrystalline MgO films were deposited for a substrate temperature above 500°C, and amorphous thin films were deposited around 400°C. The deposited Al₂O₃ and MgAl₂O₄ thin films were characterized as amorphous in structure. Applicability of ALD to complex oxides is discussed.     

Optical and electrical properties of Al2O3 films containing silicon nanocrystals

Optical, electrical, and structural properties of Al₂O₃ films subjected to silicon-ion implantation and annealing were investigated by means of photoluminescence measurements, current-voltage measurements and transmission electron microscopy. Transmission electron microscopy revealed that silicon nanocrystals were epitaxially formed in ϑ-Al₂O₃. Visible photolum inescence was observed, for the first time, from Al₂O₃ films containing silicon nanocrystals. Observed visible photoluminescence seems to be related to quantum size effects in silicon nanocrystals as well as localized radiative recombination centers located at the interface between silicon nanocrystals and matrix, similar to porous Si and other Si nanostructures. The conduction mechanism in the samples was studied by using dc current-voltage measurements. The conduction properties depend on temperature and applied electric fields. The conduction behavior in low electric fields consists of thermally activated region dominated by the Schottky conduction and nonthermally activated region in which carrier transport is controlled by space-charge-limited currents. The conduction behavior under relatively high electric fields is almost independent of temperature and well fitted by space-charge-limited conduction.     

Epitaxial Ge layers on Si via GexSi1-xO2 reduction: The roles of the hydrogen partial pressure and the Ge content

The epitaxial growth of an epi-Ge layer via Ge_xSi_1-xO₂ reduction in hydrogen annealing is reported. Ge_xSi+_1-x alloys with x = 0.52 and 0.82 were first grown epitaxially on Si substrates. They were then oxidized in a wet ambient and subsequently annealed in 5% or 100% H₂. The reduction of Ge from its oxide state is observed in both samples with both ambients. However, an epitaxial Ge growth is only observed in the sample with x = 0.82 after the 5% H₂ annealing. The other three cases result in the formation of polycrystalline Ge. The roles of the hydrogen partial pressure and the Ge content are discussed and conditions under which this novel mode of solid-phase epitaxy can occur are explained.     

A comparative investigation of the oxidation of silicon using H2/O2, TCA/O2 and HCl/O2 mixtures

The oxidation of single-crystal silicon wafers has been investigated using an industrial thermal oxidation system. The growth characteristics and electrical properties of the oxides resulting from pure hydrogen/oxygen (H₂/O₂), trichloroethane/oxygen (TCA/O₂) and hydrogen chlorid/oxygen (HCl/O₂) mixtures have been investigated and compared. The addition of both HCl and TCA to oxygen produces higher growth rates and improved electrical characteristics. It is shown that the oxidation rate for TCA/O₂ is approximately 30%–40% higher than for HCl/O₂ and that comparable electrical properties can be readily obtained. A TCA/O₂ ratio of 1 mol% gives the optimum process for VLSI applications, though 3 mol% HCl/O₂ gives comparable results. It is suggested that the overall mechanisms governing the processes are similar. However, the TCA process is a safer and cleaner alternative because it generates HCl in situ in the oxidation chamber.     

Characterization of the CH4/H2/Ar high density plasma etching process for HgCdTe

High density plasma etching of mercury cadmium telluride using CH₄/H₂/Ar plasma chemistries is investigated. Mass spectrometry is used to identify and monitor etch products evolving from the surface during plasma etching. The identifiable primary etch products are elemental Hg, TeH₂, and Cd(CH₃)₂. Their relative concentrations are monitored as ion and neutral fluxes (both in intensity and composition), ion energy and substrate temperature are varied. General insights are made into surface chemistry mechanisms of the etch process. These insights are evaluated by examining etch anisotropy and damage to the remaining semiconductor material. Regions of process parameter space best suited to moderate rate, anisotropic, low damage etching of HgCdTe are identified.     

The oxidation of titanium silicide

This paper investigates the morphology changes that occur with the oxidation of a ti-tanium silicide—polysilicon system. These changes were studied as a function of poly-silicon doping and silicide formation parameters. Emphasis was placed on transmission electron microscopy studies of the samples by planar and cross sectional techniques. Various surface analysis methods have also been used to characterize the films. This study helps to define the possible use and shortcomings of a self aligned titanium silicide insulator. The results show that varying quality insulators result, dependent largely on the initial conditions of the titanium silicide. After oxidation the Auger and TEM anal-ysis show that in all cases some form of silicon dioxide was created, but typically a considerable amount of titanium oxide was also present. For instance, it was apparent that more titanium oxide formed on the samples RTA’ed for 1 min at 700° C than the 5 min at 800° C and considerably more on the arsenic doped sample than the boron doped. The silicide also had morphology changes as the result of the oxidation. There was a phase change from the C49 to C54 phase for the 1 min at 700° C samples as would be expected at the time and temperature of the oxidation. There also was a sig-nificant amount of agglomeration and epitaxial growth observed. Further work is re-quired to completely characterize these phenomena.     

Degradation of silicon dioxide during selective silicon epitaxy in a dichlorosilane environment

The electrical degradation of dry thermal SiO₂ upon exposure to selective silicon epitaxy using dichlorosilane has been investigated. Capacitors were fabricated with thermal gate oxides (120 to 440A thick) grown on p-type silicon (100) substrates. Prior to the gate electrode formation, the oxides were exposed to hydrogen and dichlorosilane + hydrogen anneals. Leakage current and electric field breakdowns were measured to evaluate the effects of these anneals on the SiO₂ degradation. The SiO₂ degradation occurring because of dichlorosilane exposure was studied as a function of the temperature and time. While dichlorosilane exposure at temperatures above 850°C was found to cause high leakage current and breakdowns at low electric fields for silicon dioxide films thinner than 440Å, little effect was observed as a result of hydrogen and chlorine exposures. The degradation mechanism was attributed to pinhole etching via volatile SiO formation along defects present in the as-grown SiO₂.     

Plasma enhanced chemical vapor deposition of SiO2 films at low temperatures using SiCl4 and O2

Silicon dioxide films have been deposited by Plasma-Enhanced Chemical Vapor Deposition (PECVD) technique using SiCl₄ and O₂ as reactive materials. Infra-red transmittance, Auger electron spectroscopy analysis, ellipsometry, electrical, and chemical etch measurements have been used to characterize these films. It is possible to obtain good quality oxides at a substrate temperature of 200° C using a low flow of reactant gases. High flow of reactant gases results in highly non-homogeneous porous films. The best oxide films obtained show destructive breakdown at electrical fields above 4 MV/cm and a fixed charge density of the order of 2.6 × 10¹¹ charges/cm².     

MIS characterization and modeling of the electrical properties of the epitaxial Caf2/Si(111) interface

CaF₂ layers have grown by molecular beam epitaxy on bothn-type andp- type Si(111). Capacitance-voltage (C-V) and current-voltage (I-V) measurements have been made on metal-insulator-semiconductor (MIS) capacitors to characterize these structures. For devices onp-type Si, C-V characteristics show only the insulator value of the capacitance over a wide range of applied voltages, indicating an accumulated surface. At room temperature, C-V characteristics ofn-type devices show the minimum value of the capacitance (corresponding to inversion) for small voltages, but modulation of the capacitance at larger voltages. At 200 K, this modulation is no longer present in the C-V curves. I-V curves show a rapid increase in the leakage current at relatively low fields at room temperature, and this leakage decreases dramatically with temperature. These results are largely independent of cleaning procedure, growth temperature, and the degree of misalignment of the substrate. The characteristics are modeled by assuming the Fermi level to be pinned at the valence-band edge, and the modulation in the C-V characteristics ofn-type samples to be driven by leakage currents. Work done at GE while a graduate student in the School of Electrical Engineering, Cornell University. Present address: Jet Propulsion Laboratory, 4800 Oak Grove Dr., Pasadena, CA 91109.     

Effects of annealing conditions on the properties of tantalum oxide films on silicon substrates

The formation of a SiO₂ layer at the Ta₂O₅/Si interface is observed by annealing in dry O₂ or N₂ and the thickness of this layer increases with an increase in annealing temperature. Leakage current of thin (less than 40 nm thick) Ta₂O₅ films decreases as the annealing temperature increases when annealed in dry O₂ or N₂. The dielectric constant vs annealing temperature curve shows a maximum peak at 750 or 800° C resulting from the crystallization of Ta₂O₅. The effect is larger in thicker Ta₂O₅ films. But the dielectric constant decreases when annealed at higher temperature due to the formation and growth of a SiO₂ layer at the interface. The flat band voltage and gate voltage instability as a function of annealing temperature can be explained in terms of the growth of interfacial SiO₂. The electrical properties of Ta₂O₅ as a function of annealing conditions do not depend on the fabrication method of Ta₂O₅ but strongly depend on the thickness of Ta₂O₅ layer.     

Dry etching characteristics of In1-x-y Ga x Al y as alloys in CCl2F2:Ar and CH4:H2:Ar discharges

The etching characteristics of InGaAlAs alloys lattice-matched to InP were investigated using low pressure (1 mTorr) electron cyclotron resonance CH₄:H₂:Ar or CCl₂F₂:Ar discharges with additional radiofrequency biasing of the samples. Using CCl₂F₂:Ar discharges with ≥250V negative bias it is possible to obtain equi-rate etching of the material for all compositions between In_0.53Ga_0.47As and In_0.52Al_0.48As. At lower bias values, formation of A1F₃ on the surface leads to an inhibition of the etch rates. By making use of the differential etch rates of InGaAlAs layers of different compositions in CH₄:H₂:Ar mixtures, it is possible to choose dc bias values that allow one to stop the etching at a pre-selected depth in a multi-layer structure. For example, for -150 V bias, one can etch through In_0.53Ga_0.47As, In_0.53Ga_0.40Al_0.07As and Ino.53Ga_0.30Al_0.17As layers, and stop at an underlying layer with composition In_0.53Ga_0.20Al_0.27As.