SIMOX Oxides Research Articles

Total-ionizing-dose effects on isolation oxides in modern CMOS technologies

This paper presents experimental data on the total dose response of deep sub-micron bulk CMOS devices and integrated circuits. Ionizing radiation experiments on shallow trench isolation (STI) field oxide MOS capacitors (FOXCAP) indicate a characteristic build-up of radiation-induced defects in the dielectric. In this paper, capacitors fabricated with STI, thermal, SIMOX and bipolar base oxides of similar thickness are compared and show the STI oxide to be most susceptible to radiation effects. Experimental data on irradiated shift registers and n-channel MOSFETs are also presented. These data indicate that radiation damage to the STI can increase the off-state current of n-channel devices and the standby current of CMOS integrated circuits.

Measurement of buried oxide thermal conductivity for accurate electrothermal simulation of SOI device

Finite element simulations demonstrate that the thermal conductivity of the buried oxide is an important parameter for the modeling of the thermal behavior of silicon-on insulator (SOI) devices. There is uncertainty about the conductivity of different forms of SiO/sub 2/, particularly that of buried oxides. This paper presents a novel approach to measure this conductivity, using structures that are compatible with standard bipolar or CMOS processes. Thermal conductivity values of 0.66 and 0.82 W/mK, respectively, were found for 300-nm BESOI and 420-nm SIMOX oxides at room temperature. The measured variations of thermal conducitivity with temperature agree well with bulk SiO/sub 2/ behavior. Better agreement between measurement and finite element simulation of MOSFET thermal resistance is obtained by using these extracted thermal conductivity values. It is also shown that the role of the silicon substrate in determining the thermal resistance of the device can be calculated using a simple analytical model. This is important when one wishes to calculate accurately individual thermal resistances of transistors in a given circuit.

Radiation induced trap levels in SIMOX oxides: low temperature thermally stimulated luminescence

Thermally Stimulated Luminescence (TSL) properties of SIMOX oxides, induced by x-irradiation, have been studied in the temperature range 100-400 K. For the first time studied in SIMOX, a series of TSL peaks at 110, 150 and 200 K appear clearly. A continuous distribution of trap levels ranging from 0.4 to 1.1 eV has been found to be responsible for the broad and intense peak observed at 200 K; moreover, the wavelength distribution of the emitted light has revealed the presence of an emission centred at 2.7 eV. A comparison with the TSL properties of synthetic silica is presented, and the results are discussed also in the light of previous characterizations of trap levels obtained by electrical techniques. This study gives new insight concerning the electron and hole traps structures in SIMOX compared to bulk silica.

Interaction of Deuterium with the Simox Oxide

ABSTRACTThe interaction of deuterium with silicon-implanted-with-oxygen (SIMOX) was studied for samples with and without silicon top layer using thermal desorption measurements. Deuterium is incorporated in the buried oxide by disruption of the Si-O bridging bonds. The data reveal that the top layer reduces the uptake at 1073K. Furthermore, it retards release; a moderate (≈125K) and a high-temperature (≈1400K) retention were observed. It is proposed that release is accompanied by the reconstruction of the Si-O bonds and that the bare oxide surface constitutes an abundant source for defects thus enhancing the generation and elimination of Si-O bridging bond defects.

Radiation effects at low electric fields in thermal, SIMOX, and bipolar-base oxides

We have performed thermally-stimulated-current (TSC) and capacitance-voltage measurements on 370-1080 nm thermal, SIMOX, and bipolar-base oxides as functions of bias, dose rate, and temperature during irradiation. Base oxides built in a development version of Analog Devices' RF25 process show much more interface-trap buildup than XFCB oxides. Both net-oxide-trap and interface-trap charge densities for RF25 capacitors are enhanced significantly during low-dose-rate or high-temperature irradiation at 0 V over high-rate, 25/spl deg/C exposures. TSC measurements show the increase in net-oxide-trap charge density is due to a decrease in trapped electron density with decreasing dose rate or increasing irradiation temperature (at least to 125/spl deg/C), and not by increased trapped hole density. Similar enhancement of net-oxide-trap and interface-trap charge density with decreasing dose rate is found for soft thermal oxides irradiated at 0 V, but not 5 V. These results strongly suggest that space charge effects associated with holes metastably trapped in the bulk of the oxide can cause the enhanced bipolar gain degradation seen at low dose rates and/or high temperatures in many technologies. No enhanced radiation-induced charge trapping is observed for low-dose-rate or high-temperature, 0 V irradiation of SIMOX capacitors. Implications for hardness assurance tests are discussed.

Radiation induced thermally stimulated luminescence and conductivity in SIMOX oxides

Thermally Stimulated Luminescence (TSL) and Conductivity (TSC) induced by X-ray irradiation in SIMOX buried oxides have been studied from room temperature up to 400/spl deg/C. The characteristics of an X-ray induced TSL glow peak detected around 62/spl deg/C are presented: specifically, results on the emission wavelength and trap depth are shown. The X-ray induced TSC, observed at approximately 70/spl deg/C, is due to the same trapped species responsible of the TSL emission. The stability after irradiation and dose dependence of both TSL and TSC signals have also been investigated. The results have been compared with similar studies on high temperature annealed thermal SiO/sub 2/ films and bulk materials.

Trapping-detrapping properties of irradiated ultra-thin SIMOX buried oxides

Electron and hole trapping and detrapping have been investigated in new SIMOX buried oxides of different thicknesses (80 to 400 nm) after X-ray irradiation. Isochronal annealing and thermally stimulated current results are presented along with trap densities and cross section values extracted from static measurements on n-channel MOSFETs. The charge trapping properties are shown to be similar in the different SIMOX oxides. Charge detrapping results show the extension of the trapped charge distribution towards higher energy than in thermal SiO/sup 2/.

Negative bias instability at the SIMOX buried oxide-silicon overlayer interface

A study is made of negative bias instability associated with the buried oxide - silicon overlayer interface. Generated positive charge levels are measured from the shift in MOSFET transfer characteristics and interface state generation by charge pumping. The conclusion is that the same mechanism believed to account for NBI in thermal oxides can explain the instability seen in SIMOX oxide. The work has relevance for the long term stability of SOI based circuits which are likely to be particularly prone to NBI due to the device self heating which will thermally activate the instability.

Evidence of negative charge trapping in high temperature annealed thermal oxide

The effect of high temperature processing of Si/SiO/sub 2//Si sandwich structures has been studied on a thermal dry oxide, by considering charge trapping after irradiation. The radiation-induced charge trapping is shown to be very different in annealed and unannealed structures. The latter reveal interfacial trapping behavior, whereas annealed structures exhibit a large trapping of both holes and electrons in the bulk of the oxide, similar to SIMOX oxide behavior.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electron trapping studies in multiple- and single-implant SIMOX oxides

A detailed study of electron traps in SIMOX buried oxides prepared by both single- and multiple-implant acid anneal technologies is reported. The techniques of avalanche electron injection and constant voltage stressing (CVS) have been used to reveal three distinct electron traps in SIMOX. The largest, with a capture cross section of the order 10-14 cm2, is thought to be related to the so-called E' centres associated with excess Si in the oxide matrix. Its concentration is seen consistently to be higher in single-implant material which is known to contain a significantly higher concentration of excess Si than multiple-implant material. SIMOX also contains a smaller trap, with a cross section typically of the order 10-16 cm2, and it is suggested that it is a result of contamination during implantation. The third and smallest trap has a cross section of the order 10-17 cm2 and may be the so-called water-related trap seen in thermal oxides. The experiments also show that useful quantitative information can be obtained from the relatively crude but simple technique of constant voltage stressing. This is mainly because the field necessary to induce significant electron current through the oxide is significantly lower than in conventional thermal oxide as the injection is aided by field intensification arising from interfacial asperities. CVS has therefore been developed into a simple oxide assessment technique which could be readily applied to starting substrates on simple, mesa-defined MOS capacitors.

Annealing-temperature dependence of the thermal conductivity of LPCVD silicon-dioxide layers

The authors point out that the reliability and performance of electronic circuits are influenced by heat conduction in low-pressure chemical-vapor-deposited (LPCVD) silicon dioxide layers. Here, the effective thermal conductivity k/sub eff/ for conduction normal to films of LPCVD silicon dioxide layers as a function of annealing temperature, as well as for films of thermal and SIMOX oxides, is measured. The LPCVD oxide thermal conductivity increases by 23% due to annealing at 1150 degrees C. The conductivities k/sub eff/ of LPCVD layers of thicknesses between 0.03 and 0.7 mu m are higher than those reported previously for CVD layers, and vary between 50% and 90% of the conductivities of bulk fused silicon dioxide. The values of SIMOX and thermal oxide layers are within the experimental error of the values for bulk fused silicon dioxide.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radiation Hardening of Simox Buried Oxide by Nitridation

ABSTRACTRadiation hardening of SIMOX buried oxides by nitridation is demonstrated in this paper. Bare SIMOX oxide substrates, obtained by selective reactive ion etching of the top silicon film, were nitrided by annealing in ammonia at a high temperature. Al-Si thin film capacitors were fabricated on the SIMOX oxide and also on concurrently processed dry thermal oxides of similar thickness. Radiation hardness was tested by C-V measurements before and after the exposure to x-rays in an ARACOR x-ray source. SIMOX oxides were found to be inherently more resistant to radiation damage than the thermal oxides. Nitridation improved the radiation hardness of SIMOX oxides similarly to that of thermal oxides. Reoxidation nearly reversed the radiation hardening effect of nitridation in these thick (&gt; 300 nm) oxides. For example, the following shifts in the flat band voltage were obtained when the capacitors were biased with ±5 V on the Al-Si dots and exposed to 100 krad (SiO2): -24 V for SIMOX oxide, -14.2 V for nitrided SIMOX oxide, and -22.8 V for reoxidized nitrided SIMOX oxide.

Thermal Annealing of Radiation-Induced Trapped Charge in Simox and Thermal Oxide Thin Films

ABSTRACTThe results of charge trapping and thermal annealing in X-ray irradiated SiO2 are presented. A comparative study is made between thermal oxide and SIMOX buried oxide behavior. At low dose (10 krad(SiO2)), X-ray induced charge trapping is found to be very different in thermal and SIMOX oxides. Results are interpreted in terms of a relatively large bulk trapping in SIMOX whereas thermal oxide behavior is dominatedby interface trapping. However, at high dose, both oxides behave similarly and are dominated by interface trapping. Etch-back experiments performed on SIMOX irradiated at 10krad (SiO2) and 1 Mrad (SiO2) reveal these two trapping regimes. Isochronal annealings have been performed up to 300°C on irradiated samples. Recovery data are interpreted assuming an energy distribution of trapped charge densities. Detrapping energies located at about 1.05 eV and 1.35 eV have been obtained in SIMOX whereas the thermal oxide exhibits a unique peak at 1.35 eV. Based on these data, SIMOX and thermal oxides are demonstrated to differ significantly.

A pressure contact current-voltage technique for rapid characterization of thin dielectric films

A technique and precision instrumentation developed for rapid characterization of dielectric layers is described. It involves the use of a kinematically stable, flattened probe placed directly on the dielectric layer formed on top of a conducting substrate. This technique does not involve device fabrication and hence avoids any fabrication-induced changes in the dielectric layer. Electrical characterization involves static I-V measurements to breakdown, self-healing behaviour, charge to breakdown and time to breakdown distributions in constant current and constant voltage modes. The technique was used to characterize thin (38 nm) silicon dioxide films and also relatively thick (320 nm) buried oxide layers in two types of SIMOX SOI (separation by implanted oxygen - silicon on insulator) substrates. The I-V study clearly showed that SIMOX oxides formed by the multiple oxygen implantation process are superior to those formed by a single oxygen implantation step.

Radiation-Induced Oxide Charge Distributions in Simox Buried Oxides

AbstractIn ionizing-radiation environments, oxide charge buildup in the buried oxide of SIMOX SOI material causes an increase in the back-channel leakage current, resulting in an instability in electronic circuits formed on SIMOX wafers. We report the measured distributions of radiation-induced oxide charge density in buried oxides formed by oxygen implantation. These are obtained by etch-back experiments including C-V measurements using a mercury probe and ellipsometer measurements. The results show a region of higher charge density about 1500A from the top interface with a lower density region within a few hundred angstroms of the top interface. The results also show that the oxide charge density decreases with annealing temperature. Some unusual optical properties of SIMOX oxide observed in ellipsometer measurements are discussed comparatively with thermal oxide.

Interaction of Deuterium with Buried Oxides in Silicon

AbstractSilicon with buried oxides formed by ion implantation (SIMOX) or zone-melt recrystallization (ZMR) was exposed to deuterium gas at temperatures from 773 K to 1273 K, and the depth profile of the D was then determined by nuclear-reaction analysis. The D was localized within the buried oxide, with no measurable quantity in the Si phase. Uptake was controlled by permeation through the Si overlayer, and the permeability of D in Si was determined at 873 K. The sample dependence of D uptake indicated substantially fewer defect-trap sites in SIMOX oxide annealed at 1678 K as opposed to 1548 K, with still smaller defect densities in the ZMR oxide. Hydrogen exposure at 1273 K substantially disrupted the SIMOX structures.