Grown SiO2 Films Research Articles

Spectroscopic ellipsometry studies of very thin thermally grown SiO2 films: Influence of oxidation procedure on oxide quality and stress

The qualitative and structural modifications of very thin SiO2 films with thicknesses between 80 and 500 Å, caused by the oxidation procedure, were studied with spectroscopic ellipsometry (SE). Analysis of the experimental dielectric function obtained by SE provides the structural characteristics and the thickness of the oxide in fairly good agreement with electron microscopy results in cross-sectional geometry. The calculated voids volume fraction was found to drop below 3% (2%) for oxides thicker than 250 (400) Å grown at 900 (1000) °C. The densification of thermal oxides grown at low oxidation temperature predicted by Fourier transform infrared is discussed and compared to the SE results, whereas a relation between the final oxidation time and the viscoelastic relaxation time was found. Furthermore, it is shown that in situ SE can be used to monitor the process of oxide removal with very low-energy Ar+ ions and to control the conditions in order to avoid oxide and Si substrate damage. It is found that Ar+ ions with energy of about 10 eV are required to avoid oxide damage and Si substrate amorphization at a depth below 6 Å. The latter finding is also corroborated by atomic force microscopy images obtained from Si substrates after oxide etching. Finally, the influence of oxide thickness and the oxidation procedure on the E1 structure of c-Si was studied and from the results the stress applied by the oxide on the Si substrate was determined. From this study it was found that the stress depends strongly on the oxidation duration as well as on the oxidation procedure and the type of Si substrate and is minimized at an oxidation temperature 900 °C for oxidation duration longer than 70 min.

Electron-spin-resonance evidence for an impurity-related E′-like hole trapping defect in thermally grown SiO2 on Si

Using electron spin resonance (ESR), a new electrically active point defect in thermally grown SiO2 films on Si has been detected. The defect has a large capture cross section for electrons when it is paramagnetic and holes when it is diamagnetic (ESR inactive). The g-tensor values, symmetry, and microwave power saturation characteristics are all similar to those of the well-known E′ family of amorphous SiO2 defect centers.

Comparison of boranol and silanol reactivities in boron-doped SiO2 chemical vapor deposition from trimethyl borate and tetraethyl orthosilicate

For the first time, the relative rates of consumption of surface boranols and silanols reacting with tetraethyl orthosilicate [TEOS, Si(OCH2CH3)4] have been measured. This comparison has direct bearing on understanding the growth of doped SiO2 films from TEOS and trimethyl borate [TMB, B(OCH3)3] sources since surface boranols and silanols are expected to be present during the thermal chemical vapor deposition process. The measurements were accomplished by first derivatizing a porous silica substrate with boranols and silanols via hydrolysis of the products from an initial trimethyl borate (TMB) chemisorption step. TEOS exposures in the mTorr pressure regime were then carried out in a cold-wall reactor. Reaction products on the surface were identified with Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy in analysis chambers adjoining the reactor. Although TEOS does not react with SiOH at 300 K, it does react with BOH at this temperature. Using the deuterated species (SiOD and BOD) to measure the relative rates of hydroxyl consumption without interference from concurrent hydroxyl formation, the reaction rate constant for boranols with TEOS at 1000 K was determined to be twice that of silanols. At 1000 K, subsequent decomposition of the TEOS chemisorption products regenerates both BOH and SiOH.

Spectroscopic ellipsometry determination of the properties of the thin underlying strained Si layer and the roughness at SiO2/Si interface

The existence of both the strain and microroughness at the interface of thermally grown SiO2 films on Si was ascertained unambiguously for the first time by high accuracy spectroscopic ellipsometry. The dielectric function of the interface was determined by a comprehensive data analysis procedure. By carefully examining the dielectric function obtained by our model, the strain was seen to cause a red shift of 0.042 eV of the interband critical point E1 compared with the bulk silicon value. The thickness of the interface region was found to be 2.2 nm of which a significant part is due to the strain.

Chemical Etching of Thermally-Grown SiO2 Films on SiC Studied by Spectroscopic Ellipsometry

Chemical etching characteristics of thermally grown SiO2 films on 6H-type SiC in aqueous HF solutions have been studied using spectroscopic ellipsometry. Aqueous HF (1.5%) solution can etch thermally grown SiO2 layers on the (0001)Si and (0001̄)C faces of SiC at a rate of ∼90 Å/ min. A linear regression analysis and an effective medium approximation indicate that the SiO2/(0001)Si interface formed by thermal oxidation, followed by aqueous HF etching, is much rougher than the SiO2/(0001̄)C interface produced by the same procedure.

Effect of bulk microdefects induced in heat-treated Czochralski silicon on dielectric breakdown of thermal SiO2 films

We examine the effect of bulk microdefects (BMD) intentionally introduced in Czochralski silicon substrates by heat treatment on the dielectric breakdown of thermally grown SiO2 films. Transmission electron microscope observations reveal that the BMD consist of oxygen precipitates, perfect dislocation loops, and faulted dislocation loops. When the BMD are incorporated into the SiO2 film during thermal oxidation, an apparent decrease in the breakdown field is observed. The size of the oxygen precipitates has a clear relationship with the breakdown field: larger oxygen precipitate causes greater degradation. The dislocation loops are unrelated to the breakdown field.

ChemInform Abstract: Growth and Characterization of Room Temperature Anodic SiO2 Films.

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Growth and Characterization of Room Temperature Anodic SiO2 Films

The anodic behavior of Si in dilute solutions has been investigated with the aim of producing thin gate oxides of thickness between 4 and 15 nm. The oxide is formed under potentiostatic conditions and in varying solution concentrations. The thickness is calculated from x‐ray photoelectron spectroscopy (XPS) which has been calibrated with nuclear reaction analysis, using the 16O(d,p) 17O reaction. The anodic growth process yields very clean oxide, contaminated only with adventitious carbon on the surface as well as ≤1 at. percent nitrogen within the bulk (as detected by XPS). The nitrogen impurity is present in various oxidation states depending on the formation conditions. Its concentration can be decreased by annealing the samples, decreasing the concentration of in solution, or by cathodic reduction. The growth mechanism has been investigated using the technique of 18O labeling with analysis by secondary ion mass spectroscopy.

Effect of rf power on remote-plasma deposited SiO2 films

Significant improvement in material and electrical properties of SiO2 films deposited by low-temperature remote plasma-enhanced chemical vapor deposition take place as the rf power to the plasma discharge is increased. The deposition rate increases, and the index of refraction at 632.8 nm, the frequency of the dominant Si—O—Si bond-stretching vibration in the infrared absorption spectrum, the etch rate, and the static dielectric constant of the remote-plasma deposited films all approach those of thermally grown SiO2 films with increasing rf power to the plasma discharge. The total compressive stress in the oxides deposited at high power, ∼300 W, is about 20% higher than that in oxides deposited at lower power, ∼30 W. Comparison of film properties with those of plasma-deposited substoichiometric oxides (SiOx, x≤2) and thermally grown stoichiometric oxides SiO2 leads us to conclude that (i) the films deposited at rf levels from 10 to 300 W are homogeneous stoichiometric oxides SiO2, (ii) correlated variations in the film properties at the higher powers are consistent with a densification of the film, and (iii) systematic changes in the material properties take place along a linear path between two previously identified network morphologies on an n vs ν phase diagram.

Surface Chemistry of Boron-Doped SiO2 CVD: Enhanced Uptake of Tetraethyl Orthosilicate by Hydroxyl Groups Bonded to Boron

AbstractInsight into how dopants can enhance deposition rates has been obtained by comparing the reactivities of tetraethyl orthosilicate (TEOS, Si(OCH2CH3)4) with silanol and boranol groups on SiO2. This comparison has direct relevance for boron-doped SiO2 film growth from TEOS and trimethyl borate (TMB, B(OCH3)3) sources since boranols and silanols are expected to be present on the surface during the thermal chemical vapor deposition (CVD) process. A silica substrate having coadsorbed deuterated silanols (SiOD) and boranols (BOD) was reacted with TEOS in a cold-wall reactor in the mTorr pressure regime at 1000K. The reactions were followed with Fourier transform infrared spectroscopy. The use of deuterated hydroxyls allowed the consumption of hydroxyls by TEOS chemisorption to be distinguished from the concurrent formation of SiOH and BOH that results from TEOS decomposition at this temperature. It was found that TEOS reacts with BOD at twice the rate observed for SiOD, given equivalent concentrations of BOD and SiOD. This demonstrates that hydroxyl groups bonded to boron increase the rate of TEOS chemisorption. In contrast, additional results show that surface ethoxy groups produced by the chemisorption of TEOS decompose at a slower rate in the presence of TMB decomposition products. Possible dependencies on reactor geometries and other deposition conditions may determine which of these two competing effects will control deposition rates. This has significant implications for microelectronics fabrication since the specific dependencies would be expected to affect process reliability. In addition, this may explain (in part) why the rate enhancement effect is not always observed in boron-doped SiO2 CVD processes.

Kinetics of Rapid Thermal Oxidation of Silicon

We have proposed, for the first time, a practical model for ultrathin (<10 nm) SiO2 film growth in rapid thermal oxidation (RTO) kinetics. The results showed that the overall RTO growth can be well described by the linear-parabolic model proposed by Deal and Grove [J. Appl. Phys. 36 (1965) 3770]. Moreover, the model proposed indicates that in order to fit the experimental data, the oxide growth in a ramp-up process must be included in the linear-parabolic scheme. As a result, we have succeeded in explaining the RTO growth kinetics in the RTO temperature range from 950 to 1200°C and in a wide thickness range from 15 to 250 Å without making any special assumptions. The resultant activation energies for the linear rate constant (B/A) and for the parabolic rate constant (B) are 2.0 eV and 1.74 eV, respectively.

Current—Voltage Measurements of Thermally Grown SiO2 Films on Etched Silicon Surfaces

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Properties of Buried SiO2 Films in Simox Structures

ABSTRACTImplantation of 1.8×1018O+/cm2 into silicon results in a buried oxide (BOX) layer, nominally 400 nm thick. The as-implanted BOX has ∼1020 cm−3 dangling bonds, ∼1020cm−3 reactive sites for interaction with H (D); the dielectric properties are ill-defined, and, similarly to densified or ion-bombarded silica glass, the SiO2 network contains an increased proportion of high energy Si-O bonds, i.e. it is “strained”. After annealing above 1300°C the properties approach those of thermally grown SiO2, the dangling Si bonds are eliminated, the D-uptake decreases, the Si/SiO2 interfaces become sharper, and the dispersed silicon agglomerates into clusters of various kind. However, the properties of BOX are still significantly different from those of thermally grown SiO2 films as shown by pronounced electron trapping, large concentration of different hole traps, decreased etch rate in HF, increased D-uptake and E' center generation rate, as well as by increased bulk conduction and the presence of localized conducting defects. These phenomena are likely to be related to some oxygen deficiency, e.g. O3SiO3 groups, as oxygen treatments reduce, but not completely eliminate, the difference between BOX and thermal oxide.

Synchrotron-Radiation-Induced Modification of Silicon Dioxide Film at Room Temperature

Thermally grown SiO2 films were irradiated by synchrotron radiation at room temperature and changes in film properties were analyzed to clarify the irradiation effects and explain the mechanism of the photo-stimulated desorption (PSD) of SiO2. The irradiated SiO2 was chemically weakened throughout the film of 89-nm thickness by Si-O bond breaking following core electron excitation. The broken bonds were recombined by postannealing at 850°C. The irradiated surface was deoxidized by oxygen desorption and the remaining Si-rich layer was evaporated by postannealing over 700°C. Results indicate that irradiation effects are bond breaking and the desorption of oxygen and that thermal effects are the recombination of broken bonds and the evaporation of the deoxidized layer. It is supposed that the PSD of SiO2 at elevated temperature consists of photo-desorption of oxygen and thermal evaporation of SiO.

In situ spectroscopic ellipsometry study of the electron cyclotron resonance plasma oxidation of silicon and interfacial damage

The growth of SiO2 films on Si and the evolution of interfacial damage resulting from electron cyclotron resonance plasma oxidation was studied using in situ during process spectroscopic ellipsometry. Accelerated growth under positive substrate bias indicates that negative atomic species dominate the growth above an oxide thickness of 4 nm. Below this thickness bias appears less important. The interfacial damage is different in both nature and extent from that caused by ions with higher energies. It appears that the damage layer is composed of SiO2 with a-Si and is due to the oxidation reaction rather than the ions from the plasma.

Fast room-temperature growth of SiO2 films by molecular-layer dosing

A molecular-layer dosing technique for room-temperature growth of α-SiO2 thin films has been developed. The process, based on the reaction of H2O and SiCl4 adsorbates, is catalyzed by the hydrated SiO2 growth surface and requires a specific surface phase of hydrogen-bonded water. Careful adjustment of the coverage of this last phase is used to moderate continuous or pulsed growth. Thicknesses can be controlled to molecular-layer precision; alternatively, fast conformal growth to rates exceeding 100 nm/min can be achieved by slight depression of the substrate temperature below room temperature. Potential applications are trench filling for integrated circuits and hermetic ultrathin layers, for multilayer photoresists. Excimer-laser-induced surface modification has been used to achieve projection-patterned selective-area growth on silicon. This last result relies on the conversion from hydrogen to hydroxyl termination of the initial growth surface.

Evidence for anomalous structural relaxation in SiO2 films

Evidence from both thermal oxide films and bulk samples is discussed for whether amorphous SiO2 exhibits the anomalous property that the annealing of volume or refractive index may become more rapid at high densities solely due to structural effects. Unlike typical glasses, volume changes in SiO2 are not governed by free volume but by reorientations of SiO4 tetrahedra. This may be reflected in the larger initial volume changes found in the annealing of bulk SiO2 corresponding to higher initial densities at the same temperature. It is also suggested by an apparent reverse asymmetry between volume expansion and volume contraction which appears to be unique to SiO2 but not previously noted. These features are incorporated into a self-consistent annealing model for an analysis of the Landsberger–Tiller annealing data for thermally grown SiO2 films which is based on structural relaxation and which is compatible with high temperature data.

Photostimulated evaporation of SiO2 films by synchrotron radiation

Irradiation by synchrotron radiation on SiO2 films induces continuous removal of this material at elevated temperatures. The photostimulated evaporation rate for a thermally grown SiO2 film increases steeply with temperature giving an activation energy of 0.7 eV. The experimental results indicate that photon-induced bond breaking assists decomposition and thermal desorption of the film. Applications to microfabrication of a line-and-space pattern and low-temperature cleaning of Si(100) surface are demonstrated.

Stress in silicon dioxide films deposited using chemical vapor deposition techniques and the effect of annealing on these stresses

Using an in situ stress measurement technique that measures stress as a function of annealing temperature, instabilities in mechanical stress induced by heat treatment in a variety of doped/undoped SiO2 films deposited by atmospheric-pressure chemical vapor deposition (APCVD), low-pressure chemical vapor deposition (LPCVD), and plasma-enhanced chemical vapor deposition (PECVD) techniques have been investigated. A large hysteresis in mechanical stress, caused by first heat treatment to which the as-deposited films are subjected, has been observed in films deposited by APCVD/LPCVD techniques. No such hysteresis is observed in films deposited by PECVD technique. Hysteresis in APCVD/LPCVD films is found to vanish once the films are heat treated at or above 800 °C. Stress behavior of heat treated (≥800 °C) films is found to resemble that of thermally grown SiO2 films and can be described by simple elastic theory. During the first annealing cycle, APCVD/LPCVD films have been found to develop stress levels as large as an order of magnitude higher than their as-deposited stress value. Maximum stress developed in the film during heat treatment and the temperature TR at which maximum stress occurs have been found to be dependent on phosphorous content in the film. For heat treatment below this temperature TR, the stress on cooling becomes significantly larger than the as-deposited stress. The results are discussed in terms of oxide densification, the presence of hydrogenous species, and phosphorous.

Growth and properties of thin SiO2 films by inductively coupled low-temperature plasma anodisation

The increasing trend towards smaller MOS devices for VLSI has led to the need the thinner gate dielectrics grown at lower temperatures. The electrical quality of thin ( approximately 20 nm) films of SiO2 prepared by inductively coupled plasma anodisation at temperatures down to room temperature is investigated. Using MOS capacitors with plasma-grown dielectric, the oxide quality is measured in terms of dielectric breakdown strength, Eb, interface state density, Dit, and the net oxide fixed charge density, Nf, and is shown to be a function of plasma conditions. It is also shown that the DC biasing voltage, rather than the DC biasing current, is a critical parameter on the quality of the SiO2. Thin plasma oxides comparable with high-quality thermally grown oxides are demonstrated having average dielectric breakdown strengths Eba>10 MV cm-1, interface state density at mid-gap Ditm<6*1010 cm-2 eV-1 and Nf<1.5*1011 cm-2. Small-geometry MOS transitions fabricated using this technique are also demonstrated with threshold voltages of 0.4 V and n-channel mobilities in excess of 650 cm2 V-1 s-1.