Buried SiO2 Research Articles

Ellipsometric analysis of ultrathin oxide layers on SIMOX wafers

Ellipsometric analysis of surface SiO 2 on Separation by IMplanted OXygen (SIMOX) wafers has been performed. Spectroscopic ellipsometry data in the range of 500–850 nm were fitted based on a structure model composed of surface SiO 2, surface Si, and buried SiO 2 layers as well as transition layers at the SiO 2/Si interfaces. From the fitting results, it is found that there exist transition layers with a thickness of 1.6 and 2.0 Å at the surface SiO 2/surface Si and buried SiO 2/Si interfaces, respectively. Angle-resolved He–Ne laser ellipsometry has been also applied for in-situ monitoring of oxidation processes of SIMOX wafers and it is shown that ultrathin oxide thickness can be determined with an accuracy of 3 Å.

Formation and ordering of self-assembled Si islands by ultrahigh vacuum annealing of ultrathin bonded silicon-on-insulator structure

Thermal agglomeration of Si in an ultrathin (<10 nm) bonded silicon-on-insulator (SOI) wafer was studied by means of X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). It was found that annealing in an ultrahigh vacuum (UHV) causes the formation of square-shaped holes whose sides are parallel to the 〈011〉 directions. Within the hole, single-crystalline Si islands are densely formed on the buried SiO 2 with an ordered alignment in the 〈013〉 directions. The lateral size and the height of a typical island near the center of the hole are about 500 and 70 nm, respectively, and the island is surrounded by facets of {111}, {113} and {100} crystal planes. The initial process of hole opening and island formation is discussed.

Fabrication of Silicon-Based Filiform-Necked Nanometric Oscillators

For the purpose of improving the resolution of force and mass detection of a noncontact-mode atomic force microscope (AFM), we are developing a mechanical oscillator of nanometric size which consists of a head mass supported by an elastic neck. A silicon-on-insulator (SOI) wafer with the laminated structure top Si layer/buried SiO2 layer/base Si is used in the fabrication of the nanometric oscillators. By selective etching of Si and SiO2, nanometric oscillators are successfully obtained. The top Si layer is etched to form a tetrahedral Si dot, which is the mass of the oscillator, and the buried SiO2 layer is etched to form the elastic neck resting on the base Si. The size of the tetrahedral Si dot is determined by the thickness of the top Si layer without depending on the precision of the lithography technique. We found that the cross-sectional shape of the SiO2 neck is a right-angled triangle and that the neck is situated at the center of the tetrahedral Si dot. According to calculations, the oscillators we obtained have spring constants around 1 N/m and a resonance frequency from 3 MHz to 300 MHz according to their dimensions.

AFM and AES studies on the electron‐beam‐irradiation‐induced modifications in superficial and buried SiO2 layers

The interactions between electrons and SiO2 are of interest to modern materials characterization and processing technologies. In this work, electron-beam-irradiation-induced modification has been investigated in a superficial oxide (SiO2/Si) and a buried oxide in Cu/TaN/SiO2/Si using atomic force microscopy (AFM) and Auger electron spectroscopy (AES). In the superficial oxide layer, a hole of 15 nm deep and 2.5 µm wide was observed, after the oxide had been irradiated by a 20 keV 50 nA electron beam rastering over 1.5 × 1.5 µm2 for 30 min. In the buried layer, however, bumps of 40–90 nm high and ∼10 µm wide were generated after irradiation of a 20 keV 150 nA electron beam rastering over 1.5 × 1.5 µm2 for 5–40 min. Surrounding the hole and the bumps were halo-like features. In the bumps, small holes were visible. The observations have been explained by proposing a model whereby, upon irradiation by electron beams, both the swelling and the loss occur in the SiO2 layer. The swelling happened as a result of the generation of voids inside the irradiated volume, whereas the loss was attributed to the desorption of O2 formed by the oxygen atoms originally dwelling at the surface and those diffused to the surface. It is suggested that the TaN barrier layer, which prevented oxygen diffusion from the oxide, played a prominent role in the swelling of the buried SiO2. The damage to SiO2 was demonstrated further with Auger depth profiling analyses, and the corresponding artefact appearing in the analysis was discussed. Copyright © 2000 John Wiley & Sons, Ltd.

AFM and AES studies on the electron-beam-irradiation-induced modifications in superficial and buried SiO2 layers

The interactions between electrons and SiO 2 are of interest to modern materials characterization and processing technologies. In this work, electron-beam-irradiation-induced modification has been investigated in a superficial oxide (SiO 2 /Si) and a buried oxide in Cu/TaN/SiO 2 /Si using atomic force microscopy (AFM) and Auger electron spectroscopy (AES). In the superficial oxide layer, a hole of 15 nm deep and 2.5 μm wide was observed, after the oxide had been irradiated by a 20 keV 50 nA electron beam rastering over 1.5 x 1.5 μm 2 for 30 min. In the buried layer, however, bumps of 40-90 nm high and ∼10 μm wide were generated after irradiation of a 20 keV 150 nA electron beam rastering over 1.5 x 1.5 μm 2 for 5-40 min. Surrounding the hole and the bumps were halo-like features. In the bumps, small holes were visible. The observations have been explained by proposing a model whereby, upon irradiation by electron beams, both the swelling and the loss occur in the SiO 2 layer. The swelling happened as a result of the generation of voids inside the irradiated volume, whereas the loss was attributed to the desorption of O 2 formed by the oxygen atoms originally dwelling at the surface and those diffused to the surface. It is suggested that the TaN barrier layer, which prevented oxygen diffusion from the oxide, played a prominent role in the swelling of the buried SiO 2 . The damage to SiO 2 was demonstrated further with Auger depth profiling analyses, and the corresponding artefact appearing in the analysis was discussed.

Fabrication of submicrometer structures by PSE/MBE

In this paper we present the first application of the selective area growth by periodic supply molecular beam epitaxy (PSE/MBE) to the fabrication of highly uniform GaAs submicrometer structures grown on GaAs(0 0 1) substrates patterned with a SiO 2 mask. By a combination of selective and lateral growth, the merging of epilayers from adjacent open windows occurs and submicrometer structures of ridge and groove are fabricated above the buried SiO 2 mask. We have studied the morphology dependence of the epilayers on the geometry of the pattern, that is, on the window width and on the mask width. It was found that the geometry has a strong influence both on the morphology and on the possibility of merging of epilayers. The geometry has also a strong correlation with the deposition time and the different morphologies can be regarded as a time evolution of the growth of the epilayers. Besides, the morphology was found to be influenced by the crystallographic orientation of the line-shaped open windows. At optimized growth conditions and PSE parameters and under optimized geometry, we were able to grow uniform GaAs structures of excellent smoothness above the buried oxide mask.

Effect of external stress on creation of buried SiO 2 layer in silicon implanted with oxygen

The effect of external stress exerted by enhanced (up to 1.5 GPa) hydrostatic pressure (HP) of argon ambient during annealing of oxygen-implanted silicon (oxygen dose ≤1×10 17 cm −2) up to 1470 K on oxygen agglomeration has been investigated by secondary ion mass spectrometry, transmission electron microscopy, and X-ray and photoluminescence methods. HP treatment results in oxygen distribution shift and massive creation of oxygen precipitates, whereas creation of dislocations is strongly suppressed.

In Situ Rutherford Backscattering Spectrometry Analysis of Films by Combination with Sputter Etching

We have set up an experimental system consisting of an ion gun and a Rutherford backscattering spectrometry(RBS) analysis chamber. Using this system, in situ 2 MeV 4He+ RBS analysis of films is carried out by combination with sputter etching of low energy Ar+ ions. As an example of the sputtering/RBS method, the analysis of three samples, i.e., Si/(GexSi1-x/Si)/Si(100), WSix/SiO2/Si and CoSix/Si, is presented in this paper. After an appropriate fraction of the thick layer is removed by sputtering, the back edge of the Ge peak is separated from Si RBS spectrum on the interface and the O peak of the buried SiO2 layer can be identified. The change of the doped Ti and W concentrations related to Co on the top surface is observed. The advantages of this analytical method and its possible applications in film are discussed.

Deuterium sintering of silicon-on-insulator structures: D diffusion and replacement reactions at the SiO2/Si interface

We use dynamic secondary ion mass spectrometry (SIMS) to examine the mechanism of H (D) incorporation into and retention within a buried SiO2 film at 625 °C. We find that diffusion of H2 (D2) through the Si/SiO2/Si structure at this temperature is facile and that isotopic exchange occurs at the interfaces upon subsequent forming gas anneals at 625 °C. A detailed examination of the isotopic exchange process indicates that the interfaces do not exhibit equivalent behavior. We also describe the artifacts observed in the SIMS profiles by comparing positive and negative secondary ion profiles.

Room temperature oxidation of Cu/Si 0.76Ge 0.24 annealed at 200 to 300°C

The Cu 3Si-catalyzed oxidation behavior of Cu/Si 0.76Ge 0.24 after annealing at a temperature of 200–300°C was studied using transmission electron microscopy (TEM). For the Cu/Si 0.76Ge 0.24 samples annealed at 200°C and followed by exposure in air for 1–4 weeks an SiO 2 layer embedded with precipitates containing Cu, Ge, Si, and O was formed on the surface of the Cu 3(Si 1− x Ge x ) film. During exposure the Cu atoms released from Cu 3(Si 1− x Ge x ) by oxidation diffused down to the residual Si 0.76Ge 0.24 film and subsequently the Si substrate to form new Cu 3(Si 1− x Ge x ) and Cu 3Si, respectively. After exposure for 5–6 weeks not only the oxidation of the surface layer became severe but also the growth of the buried SiO 2 layer was initiated at the Cu 3(Si 1− x )/Cu 3Si interface. Concurrently, the Cu 3Si-catalyzed oxidation of Si by inward movement of the SiO 2/Si interface was also observed. As compared with the annealed Cu/Si samples the presence of Ge significantly lowered the oxidation rate of the annealed Cu/Si 0.76Ge 0.24 samples. Higher temperature annealing promoted the oxidation rate because of Ge segregation out of the Cu 3(Si 1− x Ge x ) layer and the formation of a larger fraction of the Cu 3(Si 1− x Ge x )/Cu 3Si interface where the buried SiO 2 layer was initially formed.

Silicon-based resonant-cavity-enchanced photodiode with a buried SiO2 reflector

We report on a silicon-based resonant cavity photodiode with a buried silicon dioxide layer as the bottom reflector. The buried oxide is created by using a separation by implantation of oxygen technique. The device shows large Fabry–Pérot oscillations. Resonant peaks and antiresonant troughs are observed as a function of the wavelength, with a peak responsivity of about 50 mA/W at 650 and 709 nm. The leakage current density is 85 pA/mm2 at −5 V, and the average zero-bias capacitance is 12 pF/mm2. We also demonstrate that the buried oxide prevents carriers generated deep within the substrate from reaching the top contacts, thus removing any slow carrier diffusion tail from the impulse response.

Mechanism for the reduction of interstitial supersaturations in MeV-implanted silicon

We demonstrate that the excess vacancies induced by a 1 MeV Si implant reduce the excess interstitials generated by a 40 keV Si implant during thermal annealing when these two implants are superimposed in silicon. It is shown that this previously observed reduction is dominated by vacancy annihilation and not by gettering to deeper interstitial-type extended defects. Interstitial supersaturations were measured using B doping superlattices (DSL) grown on a silicon-on-insulator (SOI) substrate. Implanting MeV and keV Si ions into the B DSL/SOI structure eliminated the B transient enhanced diffusion normally associated with the keV implant. The buried SiO2 layer in the SOI substrate isolates the deep interstitials-type extended defects of the MeV implant, thereby eliminating the possibility that these defects getter the interstitial excess induced by the keV Si implant.

Fabrication of a Separation by Implantation of Oxygen Silicon Nano-Gap Using Thin Film Stress

We have fabricated and controlled a novel nanometer scale silicon gap in a separation by implantation of oxygen (SIMOX) structure using the thin film stress generated during high temperature annealing and cooling, and then elucidated the formation mechanism and crystal orientation dependency of the gaps. The technique is very simple and controllable in shaping a wedge type nano-structure with a several hundred-nanometer gap. The fabricated structure has a 190 and 250 nm narrow gap with 2 µm and 11 µm side etching of the buried SiO2, respectively.

History of SIMOX Material

Watanabe and Tooi first reported on the formation of silicon oxide by oxygenion implantation into silicon in 1966. Further investigations of such oxygen-implanted oxide layers have been carried out by several workers, with the result that the oxide has equivalent isolation characteristics to those for thermally grown silicon oxide. Practical applications of the oxygen-implanted oxide to semiconductor devices have been reported by only a few workers, with a suggestion of their usefulness. Unfortunately Watanabe and Tooi's work has not been followed by additional silicon-on-insulator (SOI) studies.In 1978 Izumi, Doken, and Ariyoshi succeeded in fabricating a complementary-metal-oxide-semiconductor (CMOS) ring oscillator using a buried SiO2 layer formed by oxygenion (16O+) implantation into silicon. They named the new SOI technology “SIMOX,” which is short for separation by implanted oxygen. Since then Izumi and his research group have continued their study of SIMOX technology.

A modification of formulas used in SOI capacitors analysis and its application in total dose radiation

The capacitance-voltage (C-V) technique used to characterize the irradiation response of separation by implantation of oxygen (SIMOX) material was improved and then used in the analysis of the total-dose radiation effects of the silicon on insulator (SOI) capacitors. The capacitor samples were made up by using the etch-back technique to pull off several layers of fluorine ions (F+) implanted and non-implanted SIMOX materials. According to the results of deconvolution of C-V data of those samples, after irradiation, it indicated that the tolerane of SIMOX materials to total dose radiation was improved by F+ ions implanted into the buried SiO2 layer.

On the segregation of metals during low-energy oxygen bombardment of silicon

Segregation of metal impurities in Si under oxygen ion bombardment has been studied using secondary ion mass spectrometry. A strong evidence has been found for the thermodynamic driving force for segregation. This includes segregation of metal atoms at both interfaces of a buried SiO2 layer and the `antisegregation' behaviour of metal atoms having lower heats of oxide formation than Si. Segregation is enhanced at elevated temperatures when the metal diffusivity in amorphous Si is higher.

Spectroellipsometric characterization of SIMOX with nanometre-thick top Si layers

The size dependence in optical properties of the top Si layer of SIMOX (separation by implanted oxygen) structures has been studied by spectroscopic ellipsometry. The ellipsometric parameters Ψ and Δ were measured in the spectral range from 230–800 nm in air. The sample structure analyzed is [top SiO 2]/[top Si]/[buried SiO 2]/[Si substrate]. By using the bulk dielectric functions, the thickness of the top SiO 2, top Si, and buried SiO 2 layers have been determined by the SIMPLEX method. The best fit to the data was realized by applying a fluctuating thickness model for the buried SiO 2 layer. The fluctuating thickness model is found to yield a much better fit than a model including an EMA interface roughness layer. Furthermore the MDF (model dielectric function) developed by Adachi was applied for the nanometre (nm) thick top Si layers in order to improve the fitting. As a result, a size dependence in the dielectric functions of the nm thick top Si layers was observed.

A Nonvolatile MOSFET Memory Device Based on Mobile Protons in the Gate Dielectric

AbstractEver since the introduction of the metal-oxide-silicon field-effect-transistor (MOSFET), the nature of mobile and trapped charge in the oxide layer has been studied in great detail. For example, contamination with alkali ions such as sodium, causing instability of the flat-band voltage, was a major concern in the early days of MOS fabrication. Another SiO2 impurity of particular interest is hydrogen, because of its beneficial property of passivating charge traps. In this work we show that annealing of Si/SiO2/Si structures in forming gas (Ar:H2; 95:5) above 400 °C can introduce mobile H+ ions into the SiO2 layer. These mobile protons are confined within the oxide layer, and their space-charge distribution is well controllable and easily rearrangeable by applying a gate bias, making them potentially useful for application in a reliable nonvolatile MOSFET memory device. We present speed, retention, endurance, and radiation tolerance data showing that this non-volatile memory technology can be competitive with existing Si-based non-volatile memory technologies such as Flash.The chemical kinetics of mobile-proton reactions in the SiO2 film are also analyzed in greater detail. Our data show that the initial buildup of mobile protons during hydrogen annealing is limited by the rate of lateral hydrogen diffusion into the buried SiO2 films. The final density of mobile protons is determined by the cooling rate which terminates the annealing process and, in the case of subsequent anneals, by the temperature of the final anneal. To explain the observations, we propose a dynamical equilibrium model. Based on these insights, the incorporation of the proton generation process into standard semiconductor process flows is discussed.

Mechanism for Si island retention in buried SiO2 layers formed by oxygen ion implantation

The density of silicon islands trapped in buried SiO2 layers produced by the implantation of oxygen into (001)Si substrates is monitored by atomic force microscopy for the oxygen dose range just above that required for continuous oxide formation. In addition to an exponential increase in the Si island density with oxygen dose, the regularly shaped remnants of an SiO2 phase with a reduced HF etch rate were found. The formation of this additional oxide phase as a result of enhanced internal pressure inside the Si crystal is proposed to account for the retention of Si islands in the buried oxide.

Structural inhomogeneity and silicon enrichment of buried SiO2 layers formed by oxygen ion implantation in silicon

The microstructure and electrical properties of buried SiO2 layers produced in silicon by the implantation of oxygen ions are analyzed in terms of implantation parameters and supplemental incorporation of oxygen. The buried oxides show inhomogeneous etching in aqueous HF, revealing the presence of a crystalline oxide phase and Si-enriched regions. Silicon enrichment in SiO2 is found in the form of Si inclusions and oxygen deficient network defects. The former are found to be sensitive to the oxygen implantation profile, and may arise as a result of a blockage of Si outdiffusion by crystalline oxide inclusions. The network defects, in turn, are predominantly generated during high temperature postimplantation annealing, caused possibly by some mechanism of silicon transport from the interfaces into the bulk of oxide. The electron trapping and electrical conduction characteristics of buried oxides are found to correlate with the density and size of the inhomogeneities. By contrast, hole trapping and the generation of positive charge at the Si/oxide interfaces by exposure to hydrogen at elevated temperature are controlled by the network defects in the bulk of the oxide and in the near interfacial layers, respectively.