Defect Generation Rate Research Articles

Physical and predictive models of ultrathin oxide reliability in CMOS devices and circuits

The microelectronics industry owes its considerable success largely to the existence of the thermal oxide of silicon. However, recently there is concern that the reliability of ultra-thin dielectrics will limit further scaling to slightly thinner than 2 mm. This paper will review the physics and statistics of dielectric wearout and breakdown in ultrathin SiO/sub 2/-based gate dielectrics. Electrons or holes tunneling through the gate oxide generate defects until a critical density is reached and the oxide breaks down. The critical defect density is explained by the formation of a percolation path of defects across the oxide. Only <1% of these paths ultimately lead to destructive breakdown, and the microscopic nature of these defects is not known. The rate of defect generation decreases approximately exponentially with supply voltage, below a threshold voltage of about 5 V for hot electron-induced hydrogen release. However, the tunnel current also increases exponentially with decreasing oxide thickness, leading to a decreasing time-to-breakdown and a diminishing margin for reliability as device dimensions are scaled. Estimating the reliability of the dielectric requires an extrapolation from the measurement conditions (e.g., higher voltage) to operation conditions. Because of the diminished reliability margin. it has become imperative to try to reduce the error in this extrapolation, Long-term (>1 year) stress experiments are now being used to measure the wearout and breakdown of ultrathin (<2 nm) dielectric films as close as possible to operating conditions. These measurements have revealed the details of the voltage dependence of the defect generation rate and critical defect density, allowing better modeling of the voltage dependence of the time-to-breakdown, Such measurements are used to guide the technology development prior to the manufacturing stage. We then discuss the nature of the electrical conduction through a breakdown spot and the effect of the oxide breakdown on device and circuit performance. In some cases, an oxide breakdown does not lead to immediate circuit failure, so more research is needed in order to develop a quantitative methodology for predicting the reliability of circuits.

Generation of bulk defects in some semiconductors by laser radiation in the transparency region of the crystal

Laser-stimulated dissociation of regions enriched with metal and redistribution of point defects in the bulk of a semiconductor crystal under the effect of infrared laser radiation (for photon energies lower than the band gap) were studied. It is shown that the defect-generation rate depends on the power density and wavelength of the laser radiation and on the impurity concentration. Two mechanisms of migration for the laser-induced defects were ascertained, and the activation energies for migration during laser irradiation and after irradiation was terminated were estimated.

Self-interstitial migration during ion irradiation of boron delta-doped silicon

Boron delta layers in silicon, grown by molecular beam epitaxy and characterized by the secondary ion mass spectrometry, have been employed to investigate the migration of silicon self-interstitials during irradiation with MeV protons in the 500–850°C temperature range. After growth, the samples were thinned from the backside to a thickness that made them transparent for the proton energies used. As a result, the generation rate of point defects can be considered as essentially uniform throughout the samples. However, the evolution of the boron profiles is almost identical to that observed after injection of self-interstitials caused by thermal oxidation of the samples at elevated temperature. This strongly indicates that the surface acts as a reflective boundary for the migrating self-interstitials or/and an efficient sink for mobile vacancies. Furthermore, higher value of interstitial supersaturation in the near-surface region in proton-irradiated samples is consistent with experimentally detected depth dependence for immobile fraction in boron clusters. Then, activation energy of boron mobilization, (0.9±0.4) eV, was attributed to the dissociation of boron clusters.

Effects of strained layer near SiO2–Si interface on electrical characteristics of ultrathin gate oxides

Ultrathin gate oxides formed by different process technologies are investigated in detail. The following important evidence is found: the discrepant result on the two time-dependent dielectric breakdown (TDDB) lifetime measurements, the constant-current stress, and the constant-voltage stress. The discrepancy is due mainly to the difference in the oxide leakage characteristics. Apparent changes in the activation energy and the defect generation rate during the TDDB testing are also experimentally observed for the two oxides formed by different process technologies. From the analysis based on the x-ray photoelectron spectroscopy by means of the oxide etch by dilute HF and the Fourier-transform infrared attenuated total reflection method, we consider that the above phenomena are induced by the difference in the built-in compressive strain of the Si–O network near the SiO2 and Si interface.

Current-induced instability in Cr–p+ a-Si:H–V thin film devices

Abstract Experimental results on the constant current stressing in hydrogenated amorphous silicon (a-Si:H) Cr–p+–V thin film devices are presented. With increasing injection of charge via either increasing bias or time, the current-voltage characteristics of devices exhibit instability, as shown by a decrease in the reverse current. This is interpreted in terms of the creation of defects in the a-Si:H. The defect generation rate. as measured by the voltage shift ΔV at a constant reverse current in the J-V curve, is found to follow a square-root time dependent law. In addition, a decrease in device conductance after stressing is also observed, which is described by a mechanism of dopant equilibrium during and after stressing.

Oxide scaling limit for future logic and memory technology

The limit of MOSFET oxide scaling is examined from the viewpoint of reliability. Measurements of the voltage dependence of the defect generation rate and the thickness dependence of the critical defect density, together with the breakdown statistics for ultra-thin oxides, are used to provide a general framework for predicting the lifetime of ultra-thin oxides at operating voltage. It is argued that reliability is the limiting factor for oxide thickness reduction.

Microphotoluminscence of C60 fullerenes doped with ions of rare-earth elements

Optical properties of various fullerene matrices based on C60 were studied by scanning microphotoluminescence in the range of 660–2300 nm. Variations observed in the photoluminescence spectra at room temperature after prolonged exposure to focused He−Ne laser irradiation directly after implantation of ions of rare-earth elements (Yb+ Nd+, or Er+) or after an isochronal thermal annealing of ion-implanted samples in the temperature range of 50–150°C are attributed to changes in the ratio of the generation and recombination rates of native lattice defects. Photoluminescence of fullerene matrices implanted with Yb+ ions has an intracenter origin at liquid-nitrogen temperature. A variation in the photoluminescence of fullerene matrices following dissolution of the samples in toluene and subsequent recrystallization indicates that Yb atoms are involved in the formation of fullerene-based endohedral or more intricate cluster complexes. The results obtained make it possible to identify a number of technological factors that are important for the production of new photoluminescent media, including those for laser operation.

Energy spectrum and the nature of irradiation-induced defects in alloys with inverse band structure

This paper is devoted to the investigation of the effect of electron irradiation (T = 300 K, E = 6 MeV, ) on the galvanomagnetic properties of Se (x = 0.20, 0.25) alloys. Experimental results were explained in the frame of the earlier proposed model of the energy spectrum for electron-irradiated alloys, assuming that electron irradiation leads to appearance of a wide radiation defect band of the acceptor type in the forbidden band of the alloys and to redistribution of electrons between the conduction band and the radiation defect band. By comparing theoretical and experimental dependences of the electron concentration on the radiation flux the main model parameters of the defect formation process (the defect generation rate , the width and the energy position of the band ) were determined in the alloys under investigation. It was shown that agreement between experimental and theoretical data took place only under the assumption that the introduction rate of radiation defects decreases with an increase in the radiation flux. In the present work we put forward the model according to which the major mechanism of the defect formation process in the alloys is the generation of complexes including the primary radiation defects and the intrinsic structure defects typical of initial crystals.

Reliability of nitrided wet silicon dioxide thin films in WSi2 or TaSi2 polycide process : influence of the nitridation temperature

In this paper it is investigated the influence of two polycide processes using WSi2 chemical vapor deposition (CVD) or TaSi2 deposition by sputtering on the integrity of wet and nitrided tunnel oxides (t ox= 6.6 nm) used in Electrical Erasable Programmable Read Only Memory (EEPROM) under negative Fowler Nordheim electron injection (FNEI). It was confirmed that the nitridation reduces the generation rates of defects created by FNEI (interface states, neutral traps, positive and negative charges) and the stress induced leakage current (SILC).This reduction is more pronounced when TaSi2 is used rather than WSi 2. In sputtering TaSi 2 process compared to the CVD WSi2 one, it was observed : ( i) - the generation of positive charges and the SILC are higher while the generation rates of interface states and negative charges are smaller ; ( ii) - the generation rate of neutral traps is higher in wet oxide while it is smaller in nitrided oxides. Whatever the polycide process, the generation of the different types of defects created by FNEI is reduced by increasing the nitridation temperature while the SILC is increased.

Charge centroid and origin of generated and intrinsic bulk defects at 293 and 100 K in insulated gate field effect transistors

Intrinsic and generated bulk defects in the gate insulator of silicon insulated gate field effect transistors were examined using a continuous forward-bias pulsed injection technique to inject up to 1017 e/cm2 at 293 and 100 K, for insulator thicknesses ranging between 5.4 and 50.5 nm. The amount of trapping observed at 100 K was about 30 times greater than that at 293 K. The additional trapping at the reduced temperature was determined to come from two sources. One is trapping by existing shallow bulk defects, and the other is an increase in the density of generated bulk defects. The defect generation process is thought to be related to the neutral hole trap becoming unstable during injection, acting as an electron trap. This instability appears to be enhanced as the temperature is reduced to 100 K by a “freeze out’’ effect, or by higher energy carriers that result from a reduction in the thermal scattering. The defect generation rate follows a power law, much like a chemical rate equation, i.e., the rate of defect generation is dependent on the injection current density, much like a chemical reaction is dependent on pressure of the reactive species. The charge centroid of the generated defects, measured from the substrate/oxide interface, was determined at both temperatures and the centroid of the shallow electron traps was determined at 100 K. These were found to be in the range of 6–8 nm at 100 K and 10–16 nm at 293 K. Also, a defect free, or tunneling, region of 2–4 nm extent was determined to exist at each interface. This implies that when the oxide thickness decreases to about 4–8 nm, no threshold voltage shift should result from carrier injection at room, or low temperature, and in fact this behavior was observed in these devices (at least up to 1017 e/cm2 injected). It was found that the shallow traps can be rapidly depopulated by subjecting the devices to ordinary white light during normal device use, pointing to a possible method to improve device reliability at 100 K.

Saturation of the metastable defect density in amorphous silicon semiconductor

We report on a study of the metastable defect creation under pulsed light illumination in undoped hydrogenated amorphous silicon. Defect generation rate, saturation value of defect density, N sat, and characteristic time, τ c , required to reach N sat have been investigated. Results show approximately a t 1/2 time-dependence of the defect generation, different from a t 1/3 time-dependence under continuous wave light illumination. It is observed that the saturation value of the defect density is about one order of magnitude higher than by cw illumination in device-quality films. It is found that the characteristic time decreases with hydrogen content and microstructure fraction, but increases with substrate temperature. Results are discussed within the framework of existing models for the light-induced defect generation.

Quantitative estimation of generation rates of Si/SiO/sub 2/ interface defects by MeV He single ion irradiation

Generation rates of the interface defects, namely, the oxide trapped holes and the interface states for both n-ch MOSFETs (nMOS) and p-ch MOSFETs (pMOS) by MeV He single ions at zero bias conditions have been investigated quantitatively. The number of oxide trapped holes induced in nMOS and pMOS by 2 MeV He single ion have been estimated to be about 17.5 and 8.5, respectively. The number of interface states induced in nMOS and pMOS have been estimated to be about 6.6 and 2.4, respectively. The difference between nMOS and pMOS in the number of oxide trapped holes has been attributed to the difference in the surviving rate of holes during ion irradiation. By investigating the field-dependence of single ion induced oxide trapped holes, it has been found that the surviving rate of holes generated in oxide layer by ion irradiation for 2 MeV He ion varies as a power law dependence on electric field with an exponent of about 0.7 in the range between 0.0 to 2.0 MV/cm at room temperature.

Evidence for diffusion-limited kinetics of ion-beam-induced epitaxial crystallization in silicon.

The theoretical predictions of point defect related models for ion-beam-induced epitaxial crystallization (IBIEC) with reaction- or diffusion-limited kinetics are compared to each other and with experimental results. It is shown that the diffusion model provides the correct nuclear energy deposition and dose rate dependence of the IBIEC rate, whereas striking differences are observed for the reaction model. A compilation of several experimental data sets indicates a uniform diffusion regime for IBIEC in a wide range of defect generation rates. In this range the IBIEC rate can be estimated by a simple formula. The expected layer thickness dependence of the IBIEC rate is verified by in situ time-resolved reflectivity measurements. From these results it can be concluded that point defects originating from both the amorphous and the crystalline side contribute to IBIEC. It is speculated that divacancies are the defects which diffuse and stimulate the recrystallization at the amorphous/crystalline interface.

Numerical Simulation of Point Defect Distributions in a Growing Czochralski Silicon Crystal in Response to an Abrupt Change in the Growth Conditions

AbstractNumerical simulation of point defect distributions in a growing Czochralski silicon crystal with an abrupt change in the crystal growth rate from 1.0 to 0.4 mm/min was performed. The result was fitted to the experimental data for the flow pattern defects obtained from a crystal grown under simulated conditions. From the simulation result, it was observed that the axial temperature distribution shifts slightly upwards as a result of the growth rate reduction. Based upon the argument that the flow pattern defects are of vacancy-type, it is proposed that the generation rate of the flow pattern defects during crystal growth can be described by the classical nucleation rate theory proposed by Becker [Proc.Phys.Soc., 52, 71(1940)]. In addition, it is suggested that the vacancy concentration in the flow pattern defects depends upon the reaction time between the silicon interstitials and the flow pattern defects and thus the crystal growth rate.

Interface defect formation in MOSFETs by atomic hydrogen exposure

The formation of interface defects that occurs when atomic hydrogen, H/spl deg/, is introduced into the gate oxide of MOSFETs at room temperature has been studied. MOSFETs with the edges of the gate oxide at the source and drain exposed to the ambient were placed downstream of a hydrogen plasma. Baffles prevent UV light from the plasma from reaching the device. The effective channel lengths ranged from 0.9 to 5 /spl mu/m. For comparison, bare thermal oxides were exposed at the same time. The interface state density in the MOSFETs was measured by charge pumping and in the bare oxide by high-low capacitance-voltage measured with a Hg probe. We observe that the induced damage has the same energy distribution of interface traps in both types of samples. This distribution is very similar to that produced by irradiation or hot electron stressing. It is characterized by a broad peak at 0.7 eV above the valence band. A charge pumping analysis has been developed to measure interface states as a function of position above the MOSFET source and drain regions. The degradation decreases exponentially from the point of H/spl deg/ entry at the gate edges near the source and drain. This demonstrates that the hydrogen enters the gate oxide of the MOSFET at the channel edges. Furthermore, the decay length is constant within the experimental uncertainty over three orders of magnitude of H/spl deg/ dose and two orders of magnitude of interface trap density. The exponential decrease of defect generation with increasing lateral diffusion distance is in contrast to the weaker oxide thickness dependence of the defect generation rate observed by Cartier et al. (J. Non-Cryst. Sol., 1994). These results indicate that defects such as Si-H bonds at the Si-SiO/sub 2/ interfaces act as catalysts for H/sub 2/ formation by reactions such as Si- + H/spl deg//spl rarr/SiH, SiH + H/spl deg//spl rarr/Si- + H/sub 2/. Furthermore, the number of these sites is not changed by the H/sub 2/ exposure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Influence of germanium content on the degradation of strained Si1−xGex epitaxial diodes by electron irradiation

The degradation of strained n+-Si/p+-Si1−xGex epitaxial diodes, which are irradiated at room temperature with 1 MeV electrons in a high-voltage transmission electron microscope, is investigated. Special attention is given to the influence of the germanium content on the degradation of electrical characteristics and on the introduction of lattice defects into the epitaxial layer. The diode degradation and the deep level density for the x = 0.12 diodes are larger than for the x = 0.16 diodes. This can be explained by the fact that germanium atoms act as recombination centres for vacancies and interstitials thus decreasing the generation rate of stable radiation defects and by the fact that the energy absorbed in the epitaxial layer during irradiation decreases with increasing germanium content due to the smaller stopping power of germanium atoms.

Damage Production and Annealing During MeV He Ion RBS/Channeling Analysis of GaP

The creation of defects in GaP bombarded with MeV 4He ions has been studied under ultra-high-vacuum condition by Rutherford backscattering/channeling (RBS/C) method. Our experimental results suggest that the defect generation rate is strongly related to the surface condition. When the GaP is covered with a thin amorphous layer, a significant ion beam annealing effect will be observed. The present results show that for GaP the damage is produced mainly by nuclear collision.

Radiation-induced damage in GaP by MeV Helium ions

The creation of defects in GaP bombarded with MeV 4He ions has been studied under ultra-high-vacuum condition by Rutherford backscattering/channeling (RBS/C) method, and the absolute damage cross section σd for GaP has been determined under bombardment of 1.0 MeV 4He ions, with the measured value 8.0×10-18 cm2. Our experimental results suggest that the defect generation rate is strongly related to the surface condition. When the GaP is covered with a thin amorphous layer, a significant ion beam annealing effect will be observed. The present results show that for GaP the damage is produced mainly by nuclear collision.

The influence of radiation-induced segregation on precipitate stability in austenitic steels

The influence of radiation-induced segregation of major components and silicon in austenitic steels on the stability of M 23C 6, γ' and G phases under irradiation is theoretically studied. The low temperature boundary T L of M 23C 6 carbide stability and the high temperature boundary T H of γ' and G silcides stability are estimated by comparing the steady-state chromium and silicon concentrations at point defect sinks with corresponding solubility limits for these precipitates. It is shown that both T L and T H are decreased with increasing sink density and decreasing point defect generation rate. The value of T L drops with increasing carbon content while the value of T H does with decreasing silicon concentration.

Damage-to-dose ratio after low energy silicon ion implantation into crystalline silicon

In this work, we develop a model describing the diffusion of vacancies and self-interstitials and their recombination during ion implantation. The model includes the effect of the moving surface due to regrowth and the defect generation rate as a function of depth based on Monte Carlo simulations. The results are compared to experimental measurements of the damage-to-dose ratio (DDR) after low energy, 40 eV, silicon ion implantation into silicon at 300 and 685 K. We have derived an analytic approximation which agrees with the results of the computational model, implemented on a CM-2 parallel computer. We find that the calculated effective diffusivity, the main adjustable parameter in the simulations, is much lower than predicted based on extrapolation from experiments at higher temperatures. We attribute this difference to the aggregation of self-interstitials. We also find that the effect of interstitial-vacancy recombination on DDR is negligible under the experimental conditions considered; however, the crystal surface motion has a significant impact on the results.