Radiation-Induced Interface States of Poly-Si Gate MOS Capacitors Using Low Temperature Gate Oxidation

The effect of gate oxidation temperature on radiation-induced flatband and threshold voltage shifts and interface state buildup for steady-state Co60 irradiation have been studied for poly-Si gate MOS capacitors with pyrogenic and dry gate oxides. The smallest radiation-induced flatband and threshold voltage shifts can be achieved with a pyrogenic oxide grown at 850°C. Total dose effects, applied gate bias during the irradiation and oxide thickness dependence were also evaluated for low temperature pyrogenic oxide MOS capacitors. We obtained a 2/3 power law dependence of radiation-induced interface states on the total dose and the oxide thickness.     

Avalanche Injection of Holes into SiO2

Avalanche injection techniques are used to provide hole currents through MOS capacitors and study the trapping of holes in the oxide layer. Although radiation is not in any way involved in these experiments, the trapped positive charge and surface states resulting from hole injection are similar to those obtained using radiation. The processing and oxide thickness dependence of hole trapping phenomena are also investigated. Prolonged post-oxidation annealing treatments are shown to lead to enhanced hole trapping in "hardened" oxides. Hole trapping cross-sections between 10-13 and 10-14 cm2 and trap densities between 1012 - 1013 cm-2 are measured depending on the processing conditions. The effective charge density is studied over the range of oxide thickness between 200 ? and 600 ? as a function of post-oxidation anneal in these "hardened" oxides. While the effective charge density is only weakly dependent on oxide thickness in unannealed oxides, in annealed oxides it exhibits a strong linear dependence of trapping on oxide thickness. The dependence on post-oxidation anneal time and ambient are also discussed. These results indicate a strong similarity between hole trapping induced by avalanche injection and by radiation.     

Fabrication of Total-Dose-Radiation-Hardened (TDRH) SOI wafer with embedded silicon nanoclusters

Si ion-implantation and post annealing of silicon wafers prior to wafer bonding were used to radiation-harden the thermal oxide layer of Silicon on Insulator structures. After grinding and polishing, Total-Dose-Radiation-Hardened SOI (TDRH-SOI) wafers with several-micron-thick device layers were prepared. Electrical characterization before and after X-ray irradiation showed that the flatband voltage shift induced by irradiation was reduced by this preprocessing. Photoluminescence Spectroscopy (PL), Transmission Electron Microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) results indicated that the improvement of the total dose response of the TDRH-SOI wafer was associated with formation of Si nanoclusters in the implanted oxide layer, suggesting that these were the likely candidates for electron and proton trapping centers that reduce the positive charge buildup effect in the buried oxide.     

Vacuum Ultraviolet Radiation Effects in SiO2

Charging effects observed in MOS structures which have been exposed to sputtering plasmas or electron beam deposition suggest that Vacuum Ultraviolet (VUV) or soft X-radiation is important in producing these effects. Our experiments show that under positive gate bias VUV irradiation produces large positive charging effects for photon energies above 8.8 eV, the threshold for electron-hole pair creation in SiO2. This charging appears to be accompanied by an increase in interface state density. VUV radiation proves to be more useful than higher energy quanta or particles in studying radiation charging. This is true because one can control the depth of radiation absorption into the oxide. Etching experiments show that positive charge is induced near the Si-SiO2 interface even when radiation is absorbed near the gate electrode. This result is strong evidence in support of the hole transport and trapping model. We present evidence that under irradiation with positive bias, positive space charge is formed near both interfaces. We also show how a large positive space charge can be introduced into the oxide without a gate electrode.     

Mechanisms of Charge Buildup in MOS Insulators

The radiation sensitivity of MOS devices has been recognized to be the result of a charge buildup caused by the sweepout of electrons and the trapping of holes following hole-electron pair production by ionizing radiation. Holes have been shown to have a finite, although small, mobility in thermally grown SiO2. Trapping of holes takes place near the Si-SiO2 interface, possibly by oxygen vacancies in the oxide. Normal thermally grown SiO2 possesses only small concentrations of electron traps. Electron traps have been shown to be generated, however, in the oxide by ion implantation, by irradiation with nonpenetrating electrons, and by exposure of the surface of the oxide to negative ions from a corona discharge. Although Na+ and Li+ ions have been shown to be mobile at room temperature in SiO2, contamination can be kept to levels where ionic charge buildup is negligible. The role of contaminants in the formation of hole traps, however, remains to be determined.     

Response of MNOS Capacitors to Ionizing Radiation at 80°k

MNOS capacitors with oxide thicknesses 85?-600? and silicon nitride thicknesses 200-2000? have been irradiated with 2 MeV electrons at 80°K. Measured flatband shifts are found to depend on both polarity and magnitude of the applied field, oxide thickness, nitride thickness, and variations in device processing. For negative gate bias and effective applied fields 1-2?106 V/Cm, ?VFB is independent of device processing and magnitude of the applied field. For these bias conditions, it is shown that flatband shifts in all MNOS samples may be explained by considering only generation and trapping of holes in the oxide. The holes travel a mean free path of 125± 25? in the oxide before being trapped. For positive gate bias, electrons generated in the oxide are trapped at the oxide-nitride interface and/or in the bulk of the nitride, compensating the effect of the positively charged trapped holes in the oxide, and producing a relatively smaller ?VFB for positive bias. The electron trapping process is considerably processing dependent. For high effective applied fields exceeding ± 2×106 V/cm, a strongly field-dependent mechanism of charge generation in the gate insulator is observed.     

Generation of Oxide Charge and Interface States by Ionizing Radiation and by Tunnel Injection Experiments

Results of irradiation and high field tunnel injection experiments on MOS capacitors are discussed. The midgap voltage shift as a function of dose is caused by hole trapping only. In the case of tunnel injection, the generation of electron-hole pairs by impact ionization requires a much larger electron density and high fields. Thus a model of charge build-up is established which takes into account the hole trapping in neutral oxide states, the subsequent electron trapping in now positively charged states and detrapping of captured electrons. By means of this model, the prediction of the radiation hardness of MOS devices is feasible, provided that the impact ionization coefficient a is known accurately. If this is not the case, the combined techniques of ionizing irradiation and tunnel injection can be utilized to determine ? = ?o exp(-H?/F) as a function of the electrical field F. Electron capture and detrapping crosssections ?n and ?n, resp., can be deduced by fitting the model to the experimental results. An F-3 dependency for ?n and an exp(-H?/F) dependency for ?n are found. Only a weak dependence on different processing parameters is observed. The proposed model is verified by a sequence of irradiation and injection steps. The generation of oxide charge is accompanied by an increase in interface state density Dit with a distribution, which peaks at about 0.15 eV above midgap, in both experiments. The results indicate that the generation of interface states is proportional to the amount of trapped holes.     

Chemical and Structural Aspects of the Irradiation Behavior of SiO2 Films on Silicon

Similarly to vitreous silica, irradiation of Sio2 films on silicon releases bond strain by creating network defects and a small increase in density and a decrease in polarizability. In contrast, the density of quartz crystal decreases and its polarizability increases during irradiation. These effects are due to the basic trend of maximizing ?-bonding and minimizing bond strain in the Si-O network. From the irradiation-generated electron-hole pairs, the holes are trapped in narrow and localized ?-bands at ~0.4 eV above the SiO2 valence band while the electrons move rather freely. This hole trapping is an intrinsic property of the Si-O bond. Hole trapping also occurs at the Si/SiO2 interface where interface states are generated. It is suggested that this process involves breaking surface Si-H bonds. Results obtained with various analytical techniques demonstrate that hydrogen present in various forms in the oxide film plays a crucial and complex role in the irradiation behavior of Si/SiO2 interface structures.     

MOS Hardness Characterization and Its Dependence upon Some Process and Measurement Variables

The effects of oxidation time (or oxide thickness) on the hardness of 1000°C dry oxides on silicon is shown to result in a post-irradiation flatband voltage shift (?VFB) that is proportional to oxide thickness squared (dox2) for high energy irradiation and ?VFB ? dox for vacuum ultraviolet (VUV) radiation and negative corona charging. This is in disagreement with previously reported work in which ?VFB ? dox3 was measured for high energy irradiation. The present experiments strongly suggest that hole traps are located mainly at the Si-SiO2 interface and have a total area density that is independent of oxidation time or oxide thickness at a constant oxidation temperature. Trace amounts of water were added to the oxidation ambient of a modified hard-oxide furnace tube in levels ranging from 16 parts per million (ppm) to 50,000 ppm. The results of these experiments show that trace levels of water do not significantly degrade the radiation hardness of dry oxides.     

Trapping Effects in Irradiated and Avalanche-Injected MOS Capacitors

Avalanche-injection of holes and electrons into nonirradiated and irradiated MOS capacitors, respectively, were used to study hole traps in the SiO2. The trapping parameters for holes, and for electrons in the presence of trapped holes, were obtained in the range 10-14 - 10-13 cm2 for oxide thicknesses in the range 200 - 1000?. A dominant bulk specie is determined to tail off from the Si/SiO2 interface with a characteristic distance of 150-200? for dry oxide and approximately 400? for wet oxide. The electron-injection is shown to be an effective probe of the trapped-hole distribution in the SiO2 after irradiation. The effect of electron compensation of trapped holes during irradiation had been included in the trapping kinetics. C-V shifts and interface state build-up near mid- band after irradiation were found, from irradiation experiments, to follow the same linear dependence on the integrated electron and hole flux crossing the Si/SiO2 interface.     

Field- and Time-Dependent Radiation Effects at the SiO2/Si Interface of Hardened MOS Capacitors

The field and temperature dependence of the interface-state density as a function of time following pulsed e-beam irradiation, and the dose dependence of the interface-state density following steady state Co60 irradiation were examined in MOS capacitors with both hardened dry and wet (pyrogenic) gate oxides. From the results of the pulsed e-beam experiment, we show that in the wet oxide the electric field affects the time scale for the buildup of interface states as well as the final or saturation value of interface states at late times (~105 s), but that in the dry oxide there is no marked field dependence. For the wet oxide, we observed that temperature affects only the time scale for the buildup of interface states. From total-dose Co60 measurements, we report a power law dependence on dose, D0.65, for both wet and dry oxide capacitors. The buildup of interface states in the wet-oxide capacitors is considerably larger than in the dry.     

Role Transport and Charge Relaxation in Irradiated SiO2 MOS Capacitors

The transient response of SiO2 gate-insulator MOS capacitors to pulsed electron beam irradiation was studied. The radiation-induced flatband voltage shift (?VFB) in SiO2 MOS capacitors was measured with a fast C-V technique from 70 ?sec to 1000 sec after a 60 krad radiation pulse for temperatures from 80 to 293 K. In complementary experiments, the post-irradiation charge displacement in the MOS capacitors was measured with an integrating picoammeter. By correlating the relaxation of the flatband voltage with integrated current measurements, it was established that the measured response is dominated by hole transport and trapping in the SiO2 film. The temporal and temperature dependences of the hole transport are well described by a stochastic hopping model based on a continuous time random walk (CTRW). The essential feature of the CTRW is that the transport occurs via a carrier hopping (phonon assisted) process between localized sites randomly distributed in the amorphous SiO2. Since the carriers do not require excitation to the band edges in order to be mobilized, the activation energy for conduction (tunneling) is independent of the optical excitation energy.     

Displacement Damage in MOS Transistors

The changes in MOS device characteristics produced by neutron irradiation from the Northrop TRIGA reactor have been observed. Three damage mechanisms have been identified: an increase in net surface state density, a decrease in substrate resistivity, and a decrease in carrier mobility in the channel. The surface effect is usually dominant, although the bulk resistivity effect becomes increasingly important as the resistivity of the substrate is decreased. An exact closed form expression for the turn-on voltage has been derived by obtaining a solution to Poisson's equation in the gate region. The open circuit gate to substrate capacitance as afunction of gate to substrate voltage has been obtained by numerical integration techniques in terms of the charge density and dielectric constants present in the SiO2 layer and Si substrate. For both enhancement and depletion devices, an increase in positive charge density was apparent in the oxide region. It is postulated that the observedincrease in positive charge density is due to creation of mobile positive ions, and that the decreasing net accumulation rate with increasing flux is due to a diffusion or recombination process competing with the creation process. On this basis, a rate effect is expected.     

Experimental Observations of the Chemistry of the SiO2/Si Interface

Changes in silicon surface preparation prior to thermal oxidation are shown to leave a signature by altering the final SiO2/Si interface structure. Surface analytical techniques, including XPS, static SIMS, ion milling, and newly developed wet-chemical profiling procedures are used to obtain detailed information on the chemical structure of the interface. The oxides are shown to be essentially SiO2 down to a narrow transitional interface layer (3-7 ?). A number of discrete chemical species are observed in this interface layer, including different silicon bonds (e.g., C-, OH-, H-) and a range of oxidation states of silicon (0 ? +4). The effect of surface preparation and the observed chemical species are correlated with oxide growth rate, surface-state density, and flatband shifts after irradiation.     

Radiation-Induced Charge Transport and Charge Buildup in SiO2 Films at Low Temperatures

Studies of the temporal, temperature, and electricfield dependences of radiation-induced charge transport have been performed for radiation-hardened SiO2 films. At room temperature for high applied fields, nearly all electrons and holes generated in the oxide by a pulse of ionizing radiation (5-keV electrons) drift to the interfaces, whereas at low temperatures only electrons contribute to observed transport for relatively low fields. Below ~130°K at high fields, field-induced emission of trapped holes occurs, giving rise to collection within seconds of a significant fraction of the total number of holes generated. The present hole transport data are accounted for quite well in terms of a multiple-trapping model with a spread in trap levels ranging from ~0.3 to ~0.5 eV from the valence band. Comparison with the stochastic hopping transport model is made and that model is found to be less satisfactory in explaining these data. Charge buildup was examined in a Co60 environment and it is demonstrated that oxides exhibiting radiation tolerance at room temperature display severe radiation-induced changes at 77°K. It is also demonstrated that low-temperature charge buildup problems can be alleviated either by employing an ion-implanted oxide or by applying a relatively high field to the oxide during irradiation.     

PMOS剂量计的长期室温退火

为掌握ＰＭＯＳ剂量计的应用方式并提高其应用精度，研究了ＰＭＯＳ剂量计的辐照剂量记录－阈电压在室温下的长期退火表现。结果表明：氧化物电荷的隧道退火与界面态的后生长效应是造成退火的原因，ＰＭＯＳ剂量计辐照及贮藏偏置是决定其退火方向和程度大小的重要因素。负偏置条件能较好地保持其辐照记录，在正偏置贮存下的退火较大。     

Surface Effects of Space Radiation on Silicon Devices

Experimental study indicates that a significant contribution to the radiation-sensitivity of certain contemporary silicon planar transistors is caused by space-charge modifications in the oxide surface region. Certain silicon planar transistors, which are inherently passivated and protected by a silicon dioxide surface layer, have been found to realize appreciable degradation after exposure to space-type ionizing radiation. The model of radiation surface-effects proposed in the literature claims the radiation-induced ionization of the ambient gas of the transistor to be the major cause of transistor radiation-sensitivity. Contrary to this model, silicon planar transistors irradiated in an ultra-high vacuum environment realize comparable changes in electrical charactw. istics to those silicon planar transistors irradiated in gaseous ambients. Since the degradation occurs at radiation doses as low as 104 rads and is dependent upon the polarity and magnitude of electrical bias, it has been determined to be a surfa. ce effect. In order to study the effects of radiation on the silicon-silicon dioxide interface, a metal-oxide-semiconductor (MOS) structure was used. From the capacitance versus voltage characteristics of irradiated MOS transistors it has been confirmed that a major contribution to the radiation-surface degradation in silicon planar devices is the buildup of positive space charge in the silicon dioxide films covering the silicon-device surfaces. This space charge is mobile and able to drift by motion in electric fields. This positive charge can be accumulated at certain areas of the surface causing the silicon surf ace to have a tendency to go n type.     

栅氧化方式对注FMOSFET电离辐射效应的影响

对氢氧合成和干氧栅氧化后注Ｆ的Ｐ沟ＭＯＳＦＥＥＴ和Ｎ沟ＭＯＳＦＥＴ进行了γ射线辐照试验，比较了两种栅介质注Ｆ的电离辐射响应特性。研究表明，氢氧合成栅氧化后注Ｆ的ＭＯＳＦＥＴ具有较强的抑制辐射感生氧化物电荷和界面态生长的能力。用一个新的模型对实验结果进行了讨论，该新模型中用Ｓｉ－Ｆ结键替代其它在辐射场中易成为电荷陷阱的应力键，并考虑到不同氧化方式导致栅介质本身具有的电子陷阱数、空穴迁移率和氧化时所引     

A Simple Model for Predicting Radiation Effects in MOS Devices

A mathematical model for the electrical consequence of charge buildup in metal-oxide-semiconductor devices has been investigated and found to be useful. It can be used (a) for comparing experimental data from different sources and (b) for making predictions of the lifetime of MOS circuits exposed to radiation in space or laboratory environments. A wide range of experimental data has been collected together and is presented in the form of a normalised oxide radiation-sensitivity parameter A, representing the probability of hole capture. The results show that the simple model, based upon a thin sheet of hole traps, must be modified when the rate of interface-state creation is commensurate with the trap filling rate but that this does not destroy its usefulness for a wide range of applications.     

Physical Mechanisms of Radiation Hardening of MOS Devices by Ion Implantation

This paper reports experimental and analytical studies performed on Al+ ion-implanted MOS capacitors. Electrons and holes were excited in SiO2 by a low-energy electron beam, the resultant current measured, and the results then compared to findings for unimplanted specimens. Comparisons were also made of flatband voltage shifts for implanted and unimplanted devices. Analytical expressions, based on a physical model, are developed which explain the experimental results. In this model, decreases in collected current and changes in flatband voltage shift are attributed to both hole and electron trapping in the implanted region. Thus, the model contains three critical parameters: the mobility-lifetime products for both holes and electrons in the implanted region and the depth of this region. While a range of these variables satisfy either the current vs voltage measurements or the flatband-voltage-shift results, considered jointly these experiments provide specific values for all three parameters. The depth of the implanted region so determined agrees well with experimental observation, and the mobility-lifetime products calculated for different experimental conditions are consistent. The product for holes (~3.5 × 10-13 cm2/V) is substantially smaller than that for electrons (~2 × 10-12 cm2/V), indicating that holes are more easily trapped in the implanted zone than electrons. The fact that more electrons than holes are actually trapped in the case of gamma irradiation under positive bias is explained by the model. Bias dependence is also explained, as well as the fact that there is an optimum implantation energy for minimizing flatband shifts.