Oxidation Rate Of Silicon Research Articles

Nitride etching with hydrofluorocarbons. I. Selective etching of nitride over silicon and oxide materials by gas discharge optimization and selective deposition of fluorocarbon polymer

Selective etching of silicon nitride over silicon and silicon oxide is one of the most critical processes in the fabrication of complementary metal–oxide–semiconductor devices. In a typical spacer process, the gate is electrically isolated from the source and drain region by the deposition of thin nitride. A plasma etch is employed to remove the nitride from the source/drain regions without or with minimal damage to the exposed surfaces. The authors show that the typical plasma process that enables this process is based on the oxidation rate of silicon and has many limitations when applying this process to devices of 30 nm critical dimension (CD) or lower. The authors show novel gas discharges with which nitride can be etched differently, in particular, because the etch rates are controlled by selective polymer deposition. The novel etch mechanism is explained in detail and advantages and challenges are discussed, in particular. Selected studies of the feedgas chemistry lead to optimized dissociation as evidenced by the respective etch selectivities. The authors demonstrate that the novel etch mechanism is able to reduce nitride thinning and substrate damage significantly, enabling further pitch and CD scaling of spacer etch when employing this novel chemistry C4H9F. The authors also show that because of these advantages, the novel chemistry is a very promising candidate to enlarge the process window for spacer processes of nonplanar devices.

Engineering Silicon to Porous Silicon and Silicon Nanowires by Metal-Assisted Chemical Etching: Role of Ag Size and Electron-Scavenging Rate on Morphology Control and Mechanism.

We demonstrate controlled fabrication of porous Si (PS) and vertically aligned silicon nanowires array starting from bulk silicon wafer by simple chemical etching method, and the underlying mechanism of nanostructure formation is presented. Silicon-oxidation rate and the electron-scavenging rate from metal catalysis play a vital role in determining the morphology of Si nanostructures. The size of Ag catalyst is found to influence the Si oxidation rate. Tunable morphologies from irregular porous to regular nanowire structure could be tailored by controlling the size of Ag nanoparticles and H2O2 concentration. Ag nanoparticles of size around 30 nm resulted in irregular porous structures, whereas discontinuous Ag film yielded nanowire structures. The depth of the porous Si structures and the aspect ratio of Si nanowires depend on H2O2 concentration. For a fixed etching time, the depth of the porous structures increases on increasing the H2O2 concentration. By varying the H2O2 concentration, the surface porosity and aspect ratio of the nanowires were controlled. Controlling the Ag catalyst size critically affects the morphology of the etched Si nanostructures. H2O2 concentration decides the degree of porosity of porous silicon, dimensions and surface porosity of silicon nanowires, and etch depth. The mechanisms of the size- and H2O2-concentration-dependent dissociation of Ag and the formation of porous silicon and silicon nanowire are described in detail.

Correlation Between Oxidant Concentration and Morphological Properties of Silicon Nanowires Obtained by Silver-Assist Electroless Etching

In this work, a correlation between oxidant concentration and the morphological changes of silicon nanowires formed by a two-step silver-assist electroless etching method is established. It reveals that a textured silicon surface appears for samples etched at relatively H2O2 concentration lower than 2%. However, The dynamic and kinetics of silicon nanowires for different H2O2 concentration (5%, 7% and 8%) are studied by scanning electron microscopy. We found that the thickness of etched silicon nanowires as a function of time fallows a linear law. The length of silicon nanowires is not only H2O2 concentration dependent but a critical is necessary to overcome length saturation. We prove also that the oxidation rate of silicon facing Ag particles can limit the dynamic of wire formation, due to the generation of silicon hexafluoride ion (SiF6)2-.

Formation of Mosaic Silicon Oxide Structure during Metal-Assisted Electrochemical Etching of Silicon at High Current Density

We have used constant-current, metal-assisted electrochemical etching of silicon in HF/H2O2/ethanol electrolyte to fabricate porous silicon. We found that, at large enough current density, the sponge-like porous silicon structure is replaced by a mosaic structure, which includes islands of various shapes emerging between trenches that have been etched downward. Energy-dispersive x-ray analysis showed that the surface of the mosaic pieces was covered with silicon oxide, while little silicon oxide developed on the surface of trenches. We suggest that the appearance of the mosaic structure can be explained by the increase in the oxidation rate of silicon when the anodic current density increases, combined with no change in the dissolution rate of silicon oxide into the solution. Consequently, above a certain value of anodic current density, there is sufficient residual silicon oxide on the etched surface to create a continuous thin film. However, if the silicon oxide layer is too thick (e.g., due to too high anodic current density or too long etching time), it will become cracked (formation of mosaic pieces), likely due to differences in thermal expansion coefficient between the amorphous silicon oxide layer and crystalline silicon substrate. The oxide is cracked at locations with many defects, and the cracks reveal the silicon substrate. Therefore, at the locations where cracks occur, etching will go sideways and downward, creating trenches.

Effect of deuterium substitution for hydrogen in surface functionalisation of hydrophilic nanosilicon particles on their spectral and dynamic properties

Broadband femtosecond spectroscopy has been used to study two types of hydrophilic silicon nanoparticles: (1) photoluminescent, passivated with deuterium and oxidised in fully deuterated dimethyl sulphoxide, and (2) nonluminescent (control samples having a similar crystalline core), passivated with hydrogen and oxidised in dimethyl sulphoxide. We have found significant differences in ultrafast spectral – temporal induced absorption dynamics between the two types of nanoparticles in the energy range corresponding to their calculated band gap. The observed distinction is due to the considerably higher oxidation rate of silicon on the surface of the deuterated samples in comparison with the undeuterated ones and with the associated increase in the number of photoluminescence centres on the surface of the nanoparticles. In the samples containing self-trapped exciton (STE) energy states responsible for the photoluminescence in the red spectral region, carrier capture at these levels and carrier relaxation to the ground state have characteristic times in the femtosecond range. In the samples free of STE states, excited carriers relax to the conduction band bottom in a characteristic time of several picoseconds.

Differential ion beam sputtering of segregated phases in aluminum casting alloys

Differential sputtering of materials is an important phenomenon in materials science with many implications. One of the practical applications of this phenomenon is the modification of the interface between a substrate and coating during sputter coating of materials. Aluminum casting alloys, as common materials in many applications, are suitable candidates to investigate this phenomenon due to their phase separated microstructures.Changes at the sample surface under ion bombardment can be characterized by a range of complimentary techniques. The novel SIMS–SPM instrument used here enables a thorough investigation into the evolution of topography and composition caused by ion beam sputtering. For the alloy examined in this work, the aluminum regions are removed faster than the silicon particles. The faster oxidation rate of silicon compared to aluminum in the exposed surface can also be deduced from this study.

ChemInform Abstract: Thermal Oxidation of Silicon Substrates Through Oxygen Diffusion

A great many studies on the thermal oxidation of silicon substrates have been carried out owing to its industrial importance. In this paper, the conventional studies are overviewed from the point of view of the diffusion of oxygen. First, the mechanism for dry oxidation of silicon has been discussed on the basis of conventional studies. The diffusion mechanism of oxygen in thermal oxide is considered and it is suggested that oxygen which causes the oxidation of silicon dissolves in and diffuses through the interstitial site in SiO2 as oxygen molecules, but the micromechanism for the chemical reaction at the SiSiO2 interface is not fully understood at present. Second, the influences of halogens, dopants, an electric field and the plane orientation on the oxidation rates of silicon is reviewed. The influences have been fully elucidated experimentally but the mechanisms causing such phenomena cannot be successfully explained theoretically. Finally, the future trends in the thermal oxidation of silicon are presented.

Oxidation kinetics of silicon strained by silicon germanium

This paper reports on the studies of oxidation kinetics of silicon strained by silicon germanium layers. Experimental results of natural, chemical and thermal oxide formation are presented. The oxidation rates of silicon strained by SiGe layers have been compared with the rates of pure Si oxidation. The oxidation kinetics was studied using the parallel model proposed by Beck and Majkusiak. This model was fitted with good result to the obtained experimental data and the parameter that is most probably responsible for the strain effect was identified, as well as its dependence on Ge content in the SiGe layer.

Impact of dose and energy of argon ( 40Ar +) and fluorine ( 19F +) ion implantation on uniformity of silicon oxidation

Oxidation rate of silicon was altered by implanting argon ( 40Ar +) and fluorine ( 19F +) ions at doses ranging from 5×10 13 to 1×10 16 ions/cm 2 and energies ranging from 20 to 70 keV. Silicon wafers were oxidized using a rapid thermal oxidation system to retain post-implant conditions of the wafers until onset of oxidation cycle. Dramatic change in the uniformity of grown oxide was observed as a function of implantation dose and energy. Uniformity was found improving with increased implantation dose, whereas, it deteriorated with increased implantation energy. Two phenomenological models are proposed to explain the improvement in uniformity of the grown oxide films.

Negative ions in argon–oxygen discharges

The temporal behavior of negative oxygen ions in argon/oxygen mixtures is investigated in a pulsed inductively excited modified GEC reactor. The negative ion densities are measured by laser photodetachment of negative ions in combination with a Langmuir probe as well as by absorption of photons due to photodetachment with cavity ring-down spectroscopy. An unexpected high density of negative oxygen ions was found at high plasma densities. The high density of negative oxygen ions may account for the high oxidation rates of silicon in microwave discharges. The main loss mechanism for negative oxygen ions at plasma densities above 5·1010 cm−3 is mutual neutralization with positive ions. Measurements of atomic oxygen densities show that collisions of the atoms with negative ions are a minor loss process for the negative ions. According to predictions of common models it is unlikely that the density of the negative ions is high at such plasma densities. It is, therefore, necessary to invoke an additional production channel for the negative ions in order to explain the observed high negative ion density. It is likely that negative ions are generated to a considerable amount by dissociative attachment to excited oxygen molecules too. Two possible reactants for this generation can be considered, namely Rydberg oxygen molecules or metastable oxygen molecules O2M(A3Σu+, C3Δu, c1Σu−).

Recent Progress in Theoretical Study of Formation of Semiconductor Surfaces and Interfaces Based on Microscopic Processes. Simulation of Thermal Silicon Oxidation Based on a New Physical Model.

We have simulated dry and wet oxidation of silicon taking into account the emission of a large number of silicon species from the interface that governs the silicon-oxidation rate. The silicon emission model enables the simulation to be done by use of the oxidant self-diffusivity in the oxide with a single activation energy. Using a unified set of physically reasonable parameters, the whole range of oxide thickness for both dry and wet oxidation is fitted in a wide range of oxidation temperatures (800−1200oC) and oxygen pressures (1−20 atm) for both (100) and (111) silicon substrates without any empirical modifications.

Uniaxial and biaxial strain field dependence of the thermal oxidation rate of silicon

We have investigated the effect of homogenous uniaxial and biaxial strains on the thermal oxidation processes of Si with newly developed techniques. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements have revealed that the biaxial compressive strain suppresses the oxidation rate at temperatures lower than 800 °C, while the uniaxial strain does not affect the oxidation rate. It has been also found that stress during the thermal oxidation does not cause any change in the electronic states of the oxide which are characterized by XPS and x-ray absorption near edge structure.

Self-aligned silicon-on-insulator nano flash memory device

This paper reports on the fabrication of a silicon-on-insulator nano flash memory device based on the differential oxidation rate of silicon resulting from gradients in the arsenic doping concentration. The key processes involved are the formation of the desired arsenic doping profile, electron beam lithography and wet oxidation. The resulting device is a triangular-channel MOSFET with a nanocrystal floating gate embedded in the gate oxide. The length, width and height of the nanocrystal are 10, 10 and 20 nm, respectively. As long as the control gate voltage does not exceed ±2V, the device behaves like a thin and narrow P-channel MOSFET. When a voltage of −5 or +5 V is applied to the control gate at room temperature, holes are injected into the floating gate or removed from it, respectively. This effect induces a persistent shift of the threshold voltage of the device, which acts as a miniature EEPROM.

Enhancement of silicon oxidation rate due to tensile mechanical stress

Oxidation of silicon wafers under external mechanical stress was studied in this work. From the oxide thickness profile measured by an automatic ellipsometer, it was found that the oxidation kinetics of silicon was affected by the mechanical stress. The tensile stress strongly enhances the oxidation rate of silicon. A concept was proposed to explain this phenomenon by using a well-known physical Si–SiO2 lattice model. The tensile stress in the silicon will enlarge the atom spacing of silicon and make the oxidation to be easier and faster. A simulated deformation of silicon substrate under tensile stress was also given to support this concept. This work is a direct evidence of the effect of mechanical stress on silicon oxidation.

Field Oxide Thinning Behavior in Local Oxidation of Silicon Process under Enhanced Oxidation Conditions

The field oxide thinning behavior in a modified local oxidation of silicon isolation process has been studied. The oxidation rates of silicon were deliberately modified by implantation or by high pressure oxidation (HiPOX). Although oxidation of heavily doped silicon with arsenic (As) or germanium (Ge) greatly enhanced the oxidation rate on the wide‐field region severe field oxide thinning was observed at the small isolation spacings. The dry HiPOX process also showed aggravated thinning compared to the atmospheric dry oxidation process. These results can be attributed to the increased oxide stresses during field oxidation due to low temperature and to fast oxidation rate, respectively. These results suggest that, in order to mitigate the thinning, the stress should be relieved by the slower oxide growth at the higher oxidation temperatures. Much reduced thinning was observed, as was expected, when the stress was relieved by introducing an interanneal during field oxidation.

Determining the Wafer Temperature in Atmospheric Thermal Silicon Oxidation Reactors

It has been shown that, in ultrathin gate oxidation processes, temperature ramp‐up in atmospheric thermal silicon oxidation reactors subjects the wafers to severe temperature nonuniformities. The extent of this nonuniformity has been determined via a method based on the principle that for a given type of silicon substrate, in the absence of nonuniform oxygen partial pressure distributions, the oxidation rate of silicon depends only on substrate temperature. By experimentally determining within‐wafer and wafer‐to‐wafer growth rate variabilities, and by using an growth rate computer simulation model, substrate temperature has been determined.

Concentration-dependent redistribution of arsenic in silicon during thermal oxidation

The redistribution phenomena (such as pile-up, push-back) of arsenic impurities in silicon during thermal oxidation are dependent upon the oxidation rate, the diffusivities of arsenic in silicon and SiO2, and the segregation rate of arsenic impurities at the interface between the oxide and silicon. The diffusivity of arsenic in SiO2 is known to be negligible compared with the diffusivity of arsenic in silicon and the oxidation rate of silicon. The diffusivity of arsenic in silicon is also dependent on the arsenic concentration. The pile-up at the Si-SiO2 interface as a result of the concentration dependence of arsenic, has not reported so far. For silicon samples implanted with low fluences (1 × 1015 or 3 × 1015cm−2) of arsenic at 100 keV, a pile-up of arsenic was observed during thermal oxidation at 1050 °C, using Rutherford backscattering spectroscopy. For silicon samples implanted with fluences greater than 3 × 1016cm−2, push-back phenomena were observed. These phenomena can be explained only by the diffusivity of arsenic, dependent upon the concentration of arsenic in the silicon.

The effects of trichloroethane HCl and ion-implantation on the oxidation rate of silicon

The thermal oxidation of silicon was studied using a large-scale industrial oxidation system. The characteristics of the oxides resulting from pure hydrogen/oxygen (H2/O2), trichloroethane/oxygen (TCA/O2) and hydrogen chloride/oxygen (HCl/O2) mixtures are compared. Both HCl and TCA addition to oxygen produced an enhanced oxidation rate. The oxidation rate for TCA/O2 was approximately 30–40% higher than for HCl/O2 mixtures. A molar ratio of TCA/O2 of 1% gives an optimum process for very-large-scale industrial (VLSI) applications. However, 3% HCl/O2 gives comparable results to 1% TCA. In addition, boron and phosphorus implantation are observed to increase the oxidation rate. Phosphorus doping of the silicon yields a higher rate than boron-doped wafers. This behaviour is explained in terms of surface damage and chemistry. It appears that the overall mechanisms governing all these processes are similar.

O 1s investigation of SiO 2/Si interface formation using an alkali metal promoter

We have investigated the role of pre-adsorbed potassium in the promotion of silicon surface oxidation by core level and valence band photoemission experiments using AlKα (1,486.6 eV) and ZrMζ (151.4 eV) X-ray lines. We dose between 10 +15 and 10 +18 oxygen molecules onto a Si(100)2 × 1 surface modified by a K overlayer. Under these conditions the oxidation rate of silicon is enhanced by about 3 to 4 orders of magnitude with formation of a silicon oxide layer having high oxidation states (Si 3+) at room temperature. The alkali metal catalyst is removed by thermal annealing (at about 600°C), leading to the formation of a SiO 2/Si interface at lower temperatures than by direct thermal oxidation. The O 1s core level line clearly displays two components which suggests that the oxygen atoms have different bond configurations including binding to the substrate and the adsorbate, as also indicated by the corresponding valence band results. Interestingly, the respective intensities of the O 1s components behave very differently upon increasing oxygen exposure. The Si related component grows proportionally to the logarithm of the number of dosed oxygen molecules, whereas the K related one is already saturated at the smallest dose and yields a constant value for the whole exposure range. This feature demonstrates that the potassium atoms do not “store” oxygen, but only a small fraction of oxygen atoms is bound weakly to the catalyst. The micromechanisms of promotion will be described and compared to the latest state-of-the-art ab initio theoretical calculations using the local density functional approach.

Thermal oxidation of silicon substrates through oxygen diffusion

A great many studies on the thermal oxidation of silicon substrates have been carried out owing to its industrial importance. In this paper, the conventional studies are overviewed from the point of view of the diffusion of oxygen. First, the mechanism for dry oxidation of silicon has been discussed on the basis of conventional studies. The diffusion mechanism of oxygen in thermal oxide is considered and it is suggested that oxygen which causes the oxidation of silicon dissolves in and diffuses through the interstitial site in SiO 2 as oxygen molecules, but the micromechanism for the chemical reaction at the SiSiO 2 interface is not fully understood at present. Second, the influences of halogens, dopants, an electric field and the plane orientation on the oxidation rates of silicon is reviewed. The influences have been fully elucidated experimentally but the mechanisms causing such phenomena cannot be successfully explained theoretically. Finally, the future trends in the thermal oxidation of silicon are presented.