Oxide-substrate Interface Research Articles

Determination of the SiO_2/Si interface roughness by diffuse reflectance measurements

The problem of determining the roughness of the SiO(2)/Si interface is treated. A model is used based on the Fresnel formalism and scalar scattering theory. The resulting formulas express the diffuse reflectance as a function of the optical constants of the two materials, the oxide thickness and the rms roughness of the interfaces. Using the roughness values as adjustable parameters, quantitative information about the interface roughness is obtained from the diffuse reflectance spectra for an SiO(2)/Si double layer. Excellent agreement between calculated and experimental spectra is obtained for an rms roughness of 9.0 nm at the front surface and 2.2 nm at the oxide substrate interface for the case of a low-pressure low-temperature CVD film of SiO(2) on Si.

Determination of the SiO_2/Si interface roughness by diffuse reflectance measurements

The problem of determining the roughness of the SiO(2)/Si interface is treated. A model is used based on the Fresnel formalism and scalar scattering theory. The resulting formulas express the diffuse reflectance as a function of the optical constants of the two materials, the oxide thickness and the rms roughness of the interfaces. Using the roughness values as adjustable parameters, quantitative information about the interface roughness is obtained from the diffuse reflectance spectra for an SiO(2)/Si double layer. Excellent agreement between calculated and experimental spectra is obtained for an rms roughness of 9.0 nm at the front surface and 2.2 nm at the oxide substrate interface for the case of a low-pressure low-temperature CVD film of SiO(2) on Si.

The microstructure of laterally seeded silicon-on-oxide

The production of large scale integrated circuits in thin silicon films on insulating substrates is currently of much interest in the electronics industry. One of the most promising techniques of forming this composite structure is by lateral seeding. We have used optical microscopy and transmission electron microscopy to characterize the microstructure of silicon-on-oxide formed by scanning CW laser induced lateral epitaxy. The primary defects are dislocations. Dislocation rearrangement leads to the formation of both small angle boundaries (stable, regular dislocation arrays) and grain boundaries. The grains were found to be misoriented to the direction perpendicular to the film plane by ≤ 4° and to the directions in the plane of the film by ≤ 2°. Internal reflection twins are a common defect. Microtwinning was found to occur at the vertical step caused by the substrate-oxide interface if the substrate to oxide step height was > 120 nm. The microstructure is continuous across successive scan lines. Microstructural defects are found to initiate at the same topographical location in different oxide pads. We propose that this is due to the meeting of two crystallization growth fronts. The liquid silicon between the fronts causes large stresses in this area because of the 9% volume increase during solidification. The defects observed in the bulk may form by a similar mechanism or by dislocation generation at substrate-oxide interface irregularities. The models predict that slower growth leads to improved material quality. This has been observed experimentally.

Ion beam characterization of the GaAs–GaAs oxide interface for plasma and anodic oxides

Ion beam channeling has been used to measure the effects of oxide growth and annealing on the substrate at the GaAs–GaAs oxide interface. Plasma and anodic oxides have been grown on 〈100〉 GaAs substrates, and the oxides were removed with an etch in hydrofluoric acid. The substrates were then examined with a beam of 1.5 MeV 3He particles channeled down the 〈100〉 axis. The resultant Ga and As surface peaks can be resolved with Rutherford backscattering and are a measure of the Ga and As at the substrate–oxide interface. Anodic oxides grown by the constant-current method (50 μA) show no change from virgin material at the interface; however, annealing of the oxide prior to its removal results in As enhancement at the surface (?2.0 monolayers). Plasma oxides grown by direct immersion in an rf plasma show As enhancement at the interface without annealing. When grown in a fluorinated plasma, the data show an enhanced Ga surface peak consistent with a thicker interface and modified composition of the substrate. The differential measurements were made with a precision of ±0.2 monolayer.

The role of yttrium on the oxide adherence of Fe-24Cr base alloys

A study on isothermal oxidation of Fe-24Cr base alloys containing 0.063 and 0.87 wt pct yttrium has been made to examine the role of yttrium on the improvement of oxide adherence. The isothermal oxidation resistance of the 0.063Y alloy was nearly equal to that of the 0.87Y alloy at 1000 °C and 1100 °C. The oxide scales of the former spalled partially during cooling to room temperature, while the adherence of the oxide on the latter was extremely strong. A morphological observation of the substrate-oxide interface by a scanning electron microscope revealed that cavities developed at the substrate-oxide interface of the 0.063Y alloy caused the partial spallation. No cavities were observed at the substrate-oxide interface of the 0.87Y alloy and voids centered on Y2O3 particles were observed in the substrate of the alloy. Those voids indicated that internal oxide particles had acted as preferential sinks for excess vacancies which would otherwise have condensed to form voids at the substrate-oxide interface. A model is proposed for a diffusional process based on excess vacancy sinks at the boundaries of internal oxide particles dispersed in the matrix of the Fe-24Cr-O.87Y alloy, and the effect of yttrium on the surface oxide adherence of the alloy.

Electron and acoustic emission accompanying oxide coating fracture

The behavior of an oxide coating during the tensile deformation of its substrate depends on the physical properties of the oxide and the oxide-substrate interface. These properties strongly influence the rate of appearance and extension of cracks in the oxide layer. We investigate the relationship between the growth of cracks in anodized Al 1350 and electron and acoustic emission. We present evidence that both types of emission are strongly influenced by the energy released during crack propagation.

Surface roughness and the optical properties of a semi-infinite material; the effect of a dielectric overlayer

We derive expressions for the rate at which radiation is scattered and absorbed because of surface roughness on a semi-infinite material, in the presence of a dielectric overlayer. We confine our attention to the case of normal incidence. A formalism developed in an earlier paper by the present authors is utilized in the discussion. We also present a series of numerical calculations which explore the roughness-induced scattering and absorption of electromagnetic radiation for aluminum overcoated by aluminum oxide, in the ultraviolet region of the spectrum. The position of the reflectivity dip produced by roughness-induced coupling to the surface plasmon is found to shift toward the visible as the thickness of the oxide layer increases. The size of the dip is controlled strongly by the degree of correlation between the roughness on the vacuum-oxide interface, and that on the oxide-substrate interface. Under conditions discussed in the text of the paper, the presence of the oxide layer can greatly enhance the coupling between the incident radiation and surface plasmons.

Mechanism of oxide adherence on Fe-25Cr-4Al (Y or Sc) alloys

Oxidized alloys of Fe-25Cr-4Al, some containing small additions of elements (Yttrium or Scandium) which strongly promote oxide adherence, have been subjected to extensive structural studies by scanning, replication, and transmission electron microscopy as well as other techniques. The morphological details of the Al2O3 scales, which develop on these alloys during high temperature oxidation, and the structural changes at the oxide-substrate interfaces are discussed with respect to oxidation kinetics and oxide adherence. The oxide scales grow into the alloys at rates consistent with the diffusion of oxygen down grain boundaries of the fine-grained scales. Yttrium and expecially scandium additions tend to increase oxidation rates by providing rapid diffusion paths in the form of yttria and scandia stringers. More importantly, the morphological data led to the conclusion that the spallation of nonadherent scale is caused by distributions of small voids at the oxide-substrate interface. Consistent with this conclusion, it is found that the oxide scale is tenaciously adherent when voids are absent, as is the case when yttrium or scandium is added to the alloy. It is maintained that these alloy additions promote oxide adherence by forming vacancy complexes with excess vacancies or by providing internal oxide boundaries for the condensation of excess vacancies, which would have otherwise condensed at the oxide-substrate interface and formed the voids responsible for oxide spallation. The role of vacancy sinks is further confirmed by showing that oxide adherence is promoted in Fe-25Cr-4Al, in the absence of yttrium of scandium, by a distribution of Al2O3 particles. This result indicates that particles, which have been used in engineering alloys for strengthening purposes, can also promote oxide adherence. The morphological data are also discussed with respect to other mechanisms that had been proposed for oxide adherence.