Fowler-Nordheim Injection Research Articles

Identification and localisation of gate oxide weaknesses in n -MOS transistors through Fowler-Nordheim tunnelling and SPICE simulation

Analysis of the I/V characteristics after Fowler-Nordheim (F-N) injection of n-MOS transistors with weak source-drain overlap regions show the presence of an anomalous ‘hump’. SPICE simulations show that the same hump can arise in localised damage regions along the source-drain periphery, and that this is possibly in the bird's beak region. The results suggest that SPICE simulations and F-N injection can be used to study localised damage in MOS transistors.

Electron tunneling from polysilicon asperities into poly-oxides

We have examined quantum mechanical tunneling of electrons from polysilicon asperities into the poly-oxide as a function of the electric field at the injector. Barrier lowering due to image charge interaction is also calculated for the structure. The local cathode field is determined by the radius of the asperity, and hence, the local barrier height is lowered as this field increases. The consequences of this action causes a local increase in the tunneling current. Previously, workers considered a constant barrier height for all fields and the local tunneling current was determined by the local electric field and Fowler-Nordheim injection. A closed form analytical expression for electron tunneling probability including image force effects has been derived and compared with the commonly utilized Fowler-Nordheim expression. The analysis is relevant to the temperature dependent discharge of floating gate memory structures. The analysis is also applicable for analyzing scanning tunneling microscopy data.

Capture cross section of Si-SiO2 interface states generated during electron injection

The evolution of the capture cross section of the Si-SiO2 interface states is analyzed when metal-oxide-semiconductor capacitors are stressed by Fowler–Nordheim injection of electrons from the n-type substrate. The capture cross sections of the energy distribution of the states are measured by a trap filling method using deep level transient spectroscopy. We found that the capture cross sections of near-midgap states increase by a factor 10, while they remain unchanged for the states localized near the conduction-band edge. The capture cross sections are temperature dependent and their associated activation energies increase with the stress from 30 to 100 meV. The exponential prefactor in the temperature-dependence law of the capture cross section is increased from 10−16 to 10−14 cm2 , and we suggest that the Fowler–Nordheim degradation induces new interface states of donor type in the upper part of the silicon band gap.

Improvement of SiO2/Si Interface Properties Utilising Fluorine Ion Implantation and Drive-in Diffusion

Thermal drive-in diffusion of ion-implanted F atoms has been employed to fluorinate SiO2/Si interfaces and thereby improve their electrical properties. The interface state density can be lowered with little fixed charge generation. Correspondingly, pn-junction surface leakage current decreases. Furthermore, the interfaces can be hardened against hot-electrons due to Fowler-Nordheim current injection and avalanche current at the junction surface. As a result, a fluorinated MOSFET shows higher hot-carrier immunity. It is pointed out that there is an optimal F dose for these improvements to be achieved.

Buried coaxial conductors for high-speed interconnections : Richard C. Landis. IEEE Trans. Compon. Hybrids mfg Technol.CHMT-10, 204 (1987)

We have investigated the generation of interface traps during 60Co irradiation and Fowler-Nordheim (FN) injection in MOS structures with thin plasma nitrided gate oxides. By using deep level transient spectroscopy and capacitance-voltage measurements, we have studied the impact of nitridation time on the creation of fast and slow states in the silicon bandgap. After FN injection, electrical relaxation of the interface has been interpreted as the superposition of two mechanisms: an irreversible physico-chemical time-dependent transformation and a reversible bias-dependent neutralization.

Effects of the atmosphere on the resistance of ITO thin films : Josep Calderer. Vacuum37, 441 (1987)

We have investigated the generation of interface traps during 60Co irradiation and Fowler-Nordheim (FN) injection in MOS structures with thin plasma nitrided gate oxides. By using deep level transient spectroscopy and capacitance-voltage measurements, we have studied the impact of nitridation time on the creation of fast and slow states in the silicon bandgap. After FN injection, electrical relaxation of the interface has been interpreted as the superposition of two mechanisms: an irreversible physico-chemical time-dependent transformation and a reversible bias-dependent neutralization.

Comparison of interface positive charge generated in metal-oxide-silicon devices by high-field electron injection and x-ray irradiation

Interface positive charge induced in thin oxides (582 Å thick) of metal-oxide-silicon devices by Fowler–Nordheim injection and x-ray irradiation was evaluated based on measurements of room-temperature annealing characteristics. Results showed that in the low-dose regime, the dose dependence of the density and distribution of the positive charge was different for the two forms of stress. However, at higher doses, the positive charge density produced by both methods increased monotonically while its spatial extent decreased and became more localized at the interface. These latter results are attributed to a buildup of negative charge in the oxide that neutralizes the positive charge furthest from the interface, whereas the differences at low dose are due either to the different character of the positive charge generated by the two techniques or to the much smaller electron density present during irradiation than injection.

Charge trapping and breakdown in thin SiO 2 polysilicon-gate MOS capacitors

Charge trapping during high-field (Fowler-Nordheim) injection in thin SiO 2 films is investigated. Constant and pulsed current injection as well as C-V measurements are used to determine the evolution of trapped charge and interface state densities. The relation between these charges, which can be influenced by temperature and γ radiation, and dielectric breakdown is studied. We establish the dominant role of negative trapped charge.

Degradation of metal/oxide/semiconductor structures by Fowler-Nordheim tunnelling injection

The behaviour of thin metal/oxide/semiconductor (MOS) structures with polycrystalline silicon (poly-Si) or silicon aluminium gates subjected to stresses caused by Fowler-Nordheim tunnelling injection was analysed. Native electron capture centres in the bulk SiO 2 were characterized by their capture cross-section, their density and their centroid localization in the oxide layer by means of capacitance and conductance measurements. The trapping was independent of the gate bias polarity during injection. The other aging mechanism is interface (SiO 2-Si and gate-SiO 2) degradation owing to the generation of new centres in the oxide. The new electrically active sites acting as stable electron traps were analysed in correlation with the technological process parameters such as oxidation ambient environment (dry or wet, with or without HCl) and gate type (aluminium or poly-Si). Our results seem to be consistent with the hypothesis that aging of MOS structures under electrical injection is essentially controlled by interfacial phenomena involving the formation of intrinsic defects at the Si-SiO 2 interface. It appears that poly-Si gate structures are the least sensitive to intense Fowler-Nordheim injection stresses.

Electron spin resonance study of high field stressing in metal-oxide-silicon device oxides

We find that two paramagnetic ‘‘trivalent silicon’’ centers appear to be responsible for damage resulting from Fowler–Nordheim injection of electrons into thermal oxides on silicon.

A model for the electrical conduction in polysilicon oxide

A model is presented for the current injection in oxide layers on polycrystalline silicon. The model is based on the representation of the interface by a collection of asperities with different heights. The electric field at the asperities, the resulting Fowler-Nordheim injection, and the trapping of charge carriers are calculated numerically. A uniform distribution of traps is assumed in the oxide. The presence of electron traps has been demonstrated by thermal detrapping experiments. From a comparison with experiments it turns out that the magnitude of the electric field at the interface exhibits an exponential distribution. Using this distribution, a simple equation is derived for the current-voltage characteristics of polyoxides.

Hot-electron induced interface traps in metal/SiO2/Si capacitors: The effect of gate-induced strain

It is shown that the generation of interface traps by Fowler–Nordheim injection, like those generated by ionizing radiation, is a function of the mechanical strain at the silicon-silicon dioxide interface. However, because of the current enhancement at the edge of a metal-oxide-semiconductor gate when the gate serves as the hot-electron injector, this phenomenon may only be evident when the device is stressed with the gate biased positively. It is also shown that the capture rate of electrons in the silicon dioxide depends on the strain in the film.

Effect of high field stresses on interface states of n-MOS capacitors

High electrical stresses have been applied to n-MOS capacitors prepared following two thermal oxidation methods. The increase of the interface state density was investigated by means of a conductance method and incidentally by a quasi-static method. A threshold effect, possibly related to a Fowler-Nordheim injection, has been found in the increase of this density versus field stress. At high field stresses, a second conductance peak has been observed, which can be explained in terms of new interface states, rather than lateral nonuniformities.

Observation of positively charged state generation near the Si/SiO2 interface during Fowler–Nordheim tunneling

In previous work, we have established that thin oxide (40–50 Å) MOS structures exhibit, prior to large levels of Fowler–Nordheim injection, the ideal tunneling behavior of a uniform trapezoidal barrier with thick oxide properties. With increasing levels of electron injection towards the Si/SiO2 interface (up to 6×1018 e/cm2), a build‐up of excess current results. This behavior is shown to be due to the generation of positive states in the SiO2 about 14 Å from the Si/SiO2 interface, giving excellent agreement with our analysis of charge‐assisted tunneling. The approach is seen to offer a sensitive probe of the oxide interfacial region, capable of locating and detecting as few as 1010 cm−2 positive states. Comparisons are made among three oxidation processes, revealing wet oxides to be more susceptible than dry oxides to the state generation process. The results are consistent with the mechanism of breaking strained Si–O–Si bonds near the interface, observed in x‐ray photoelectron spectroscopy experiments.

High-field dark currents in thin CVD silicon nitride

Thin films of Si3N4 have been prepared by chemical vapor deposition (CVD) onto n+-Si substrates at 875 °C using an ammonia/silane ratio of 200 : 1. In the thickness range 93⩽τ⩽1021 Å, ellipsometric measurements have revealed that the refractive index decreases with film thickness, indicating a graded composition of silicon oxynitride with increasing oxygen content as the Si-Si3N4 interface is approached. Measurements of steady-state dark currents, injected into Si3N4 from negatively biased Al contacts at a constant average field Eav=Vτ−1=6 MV/cm, have been fitted to an empirical relation J=J0 exp(τ0/τ), where τ is the film thickness and τ0=1039 Å. The apparent increase in film conductivity with decreasing thickness τ is attributed to diminished electron trapping as τ approaches the centroid of trapped charge δ∼100 Å. Below approximately 200 Å, CVD Si3N4 films cannot sustain the continuous average field of 6 MV/cm needed for efficient Fowler-Nordheim injection from an Al contact. Thus, the maximum dissipative energy density of CVD Si3N4 has been estimated to be of the order of 10 kW/cm3. Measurements as a function of temperature in the range 276⩽T⩽470 °K indicate that the traps participating in electric field modification of Fowler-Nordheim injection lie in a narrow range 0.16⩽φT⩽0.18 eV below the conduction band of Si3N4.

The influence of nitride thickness variations on the switching speed of MNOS memory transistors

The influence of nitride thickness variations on the switching speed of MNOS memory transistors is examined. The switching time constant is calculated as a function of the nitride thickness using a model of modified Fowler-Nordheim injection. The calculated characteristics compare well with measured characteristics and show a strong dependence on the nitride thickness.