Charge Deposition Profiles Research Articles

Comparison of Pulsed Electroacoustic Measurements and AF-NUMIT3 Modeling of Polymers Irradiated with Monoenergetic Electrons

Successful spacecraft design and charging mitigation techniques require precise and accurate knowledge of charge deposition profiles. This paper compares models of charge deposition and transport using a venerable deep dielectric charging code, AF-NUMIT3, with direct measurements of charge profiles via pulsed electroacoustic (PEA) measurements. Eight different simulations were performed for comparison to PEA experiments of samples irradiated by 50 or 80 keV monoenergetic electrons in vacuum and at room temperature. Two materials, polyether-ether ketone (PEEK) and polytetrafluoroethylene (PTFE), were chosen for their very low conductivities so that minimal charge migration would occur between irradiation and PEA measurements. PEEK was found to have low acoustic attenuation, while PTFE has high acoustic attenuation through the sample thicknesses of 125 and for each material. The measurements were directly compared to AF-NUMIT3 simulations to validate aspects of the code and to investigate the importance of various simulation options, as well as to characterize the PEA instrumentation, measurement methods, and signal processing used. The measurement and simulation values for magnitude of charge deposition, penetration depth, and charge deposition spatial profiles are largely in agreement, though spatial and temporal distributions in incident electron flux and effects of radiation-induced conductivity (RIC) and delayed RIC during the deposition process complicate the process. This work provides an experimental validation of the AF-NUMIT3 deep dielectric charging code and insight into the accuracy and precision of the PEA method.

New Approach for Pulsed-Laser Testing That Mimics Heavy-Ion Charge Deposition Profiles

A novel approach for two-photon absorption (TPA) pulsed-laser testing produces extended charge deposition profiles that are analogous to those produced by heavy ions. In this approach, which utilizes an axicon rather than a spherical lens, the conventional Gaussian beam is replaced by a quasi-Bessel beam. The key feature of a quasi-Bessel beam, relevant to pulsed-laser single-event effects (PL-SEE) studies, is that its radial size ( $\sim 1~\mu \text{m}$ ) remains constant over the axial length of the beam (several 100 s of $\mu \text{m}$ ) resulting in a charge-deposition profile that more closely mimics that produced by a heavy ion. This elongated charge distribution directly lends itself to a simple and intuitive description in terms of linear-energy transfer (LET), facilitating laser/ion correlation studies. Experimental results comparing a conventional TPA PL-SEE focusing geometry and the axicon focusing geometry are presented for three different test vehicles: a 45-nm silicon-on-insulator (SOI) nFET, a bulk silicon photodiode, and an LM124 operational amplifier. By using the axicon approach, strong laser/ion correlation is observed in the silicon photodiode for single-event transients (SET) across a wide range of LETs. The correlation is achieved by “tuning” the laser to the desired LET rather than relying on preexisting heavy-ion data, and therefore, this approach holds promise as a predictive tool.

Comparison of Single-Event Transients in SiGe HBTs on Bulk and Thick-Film SOI

A comparison of heavy-ion-induced single-event transients (SETs) in silicon–germanium heterojunction bipolar transistors (SiGe HBTs) fabricated on both bulk and silicon-on-insulator (SOI) substrates is presented. Experimental heavy-ion data show a reduction of sensitive volume of $\approx 350\times $ for the SiGe HBTs fabricated on SOI compared to those on a bulk substrate. Furthermore, the results indicate that the sensitive volume of the SOI SiGe HBTs is confined to the intrinsic device. These results have been confirmed using pulsed laser data and TCAD simulations. Limiting the charge collection depth of SiGe HBTs by fabricating them on SOI facilitates correlation of ion and laser results. The improved ease of correlation is a result of suppressing diffusive charge collection mechanisms that are difficult to replicate with the charge deposition profile generated by two-photon absorption using a tightly focused laser beam. However, a potential issue with pulsed laser testing is determining the loss of the pulse energy through the buried oxide, which requires detailed knowledge of the semiconductor platform. Overall, the reduction of sensitive volume for the SOI SiGe HBTs compared to those on a bulk substrate is a clear advantage from a single-event upset vulnerability perspective.

Determination of Energy and Charge Deposition Profiles in Elemental Slabs From an Isotropically Equivalent Electron Source Using Monte Carlo Simulations

Monte Carlo simulations using the MCNP5 transport code have been made for electrons with 10.0 keV to 5.0 MeV kinetic energies incident on thick slabs of elemental materials with atomic numbers in the range 6–79. An electron point source near the surface of the target slab was used to simulate isotropic incidence, allowing for the inclusion of backscatter and secondary-electron emissions in the simulation. Thus, the results should be uniquely useful for the modeling of deep-dielectric charging in spacecraft over a large range of energies and materials. Due to the minimum electron energy cutoff of MCNP5, accuracy of the simulations decreases at the lowest incident energies. A path has been identified for developing an algorithm that can quickly reproduce and interpolate between the Monte Carlo results for both incident energies and target materials in the ranges studied. Such an algorithm should prove useful to modelers of deep-dielectric charging in realistic spacecraft environments.

Geant 4-based calculation of electric field in deep dielectric charging

Based on Geant4 particle transfer simulation, the internal current density, dose rate and charge deposition profiles of Teflon are acquired, and then the electric field under high energy and low density e-beam radiation is calculated with radiation-induced conductivity model of charging dynamics, which includes the continuity equation, Poisson equation and deep trap equation. According to Geant4 package, the charging model of dielectric is simplified as a compound structure consisting of shielding aluminum and multiple-layer plates radiated by 1.0MeV energy and 0.1 pA/cm2 flux density surface source. With statistical method, the distributions of internal current density, dose rate and charge deposition are obtained and normalized with the above flux density. Substituting these results into RIC model, the distributions of electric field in Teflon are calculated under various grounding conditions. It shows that the charge density, electric field and potential for the back side grounding are higher than these for the front side grounding, which tends to induce internal discharging. The electric field calculated for the critical internal discharging conditions in space is also consistent with the threshold field of dielectric Teflon(1e7V/m).

Charge-collection efficiency of GaAs field effect transistors fabricated with a low-temperature grown buffer layer: Dependence on charge deposition profile

The dependence of the charge-collection processes of LT GaAs field-effect transistors on the depth profile of the deposited carriers is examined using computer simulation and laser-induced charge-collection measurements. The charge-collection simulations reveal a surprising dependence of the charge-collection efficiency on the location of the deposited charge, such that the charge-collection efficiency is largest for charge deposition below the LT GaAs buffer layer. These results implicate the significant role of charge-enhancement phenomena in the charge-collection processes of LT GaAs FETs. Experimental measurements performed as a function of the optical penetration depth support the conclusions of the simulation study.

Effects related to dose deposition profiles in integrated optics structures

Results from exposures of lithium tantalate and lithium niobate integrated optic structures to pulses of high energy X-rays and fast electrons are related to dose and charge deposition profiles. Anomalous effects in the tantalate are ascribed to induced electric fields.

The transport equation theory applied to the prediction of the spatial distribution of an electron beam in a target

The author makes a contribution to the understanding of the phenomenon of the electron distribution under the circumstances of a beam-induced charge build-up in a target. Such phenomena are encountered in electron-beam-assisted microanalysis or in Auger electron analysis. The theoretical problem of the motion of a beam of charged particles in a solid is resolved by interpreting the transport equation in terms of an integro-differential equation. A treatment of the Boltzmann equation is developed for the energy dependence in order to predict the electron penetration in targets bombarded under the simultaneous influence of scattered and decelerated incident particles. The exact values of the specific moments of the electron distribution in a semi-infinite medium are calculated. The author suggests a procedure applied to the calculation of an analytic expression characterizing the asymptotic trend of the electron spatial distribution at very small residual energies. The perturbing effects of insulator irradiation are treated and charge deposition profiles near the surface are calculated for mixed alkali silica glasses. Results are compared with previous data; agreement is in most cases fairly good.

Fokker-Planck Calculations Using Standard Discrete Ordinates Transport Codes

A method is developed for using standard discrete ordinates neutron transport codes to perform Fokker-Planck calculations in one-dimensional slab and spherical geometries. No modification of the codes is necessary and time-dependent, steady-state, forward, or adjoint calculations can be performed. It is shown that energy-angle integrated quantities such as energy and charge deposition profiles can be accurately and efficiently calculated for electrons. However, in certain types of problems, the number of groups required to converge the differential energy spectra can be prohibitively large.

Charge Deposition Profiles near Co60 Irradiated Material Interfaces

Charge deposition profiles in thin planar layers of aluminum, tin and tantalum next to various combinations of thick layers of aluminum, copper, tin and lead have been measured under cobalt 60 irradiation. The gamma ray beam is collimated and its energy spectrum is measured. The gamma intensity is accurately determined so that the absolute charge deposition can be related to the gamma beam intensity and spectrum. The geometry is one dimensional slab. Data is tabulated for 44 combinations of the various materials allowing for extensive comparison with computer calculations.

Charge Deposition Profiles near Irradiated Material Interfaces

Charge deposition profiles in thin planar layers of aluminum, tin and tantalum next to various combinations of thick layers of aluminum, copper, tin and lead have been measured under cobalt 60 irradiation. The gamma ray beam is collimated and its energy spectrum is measured. The gamma intensity is accurately determined so that the absolute charge deposition can be related to the gamma beam intensity and spectrum. The geometry is one dimensional slab. Data is tabulated for 44 combinations of the various materials allowing for extensive comparison with computer calculations.

Numerical Solution of the Spencer-Lewis Equation for Kilovolt Electron Transport in Plane Slabs

We have developed a computer program BOLT for the numerical solution of the Spencer-Lewis equation for kilovolt electron transport in finite slabs. The method of solution used is a discrete ordinates method utilizing Gauss-quadrature angles equivalent to a 20th order Legendre polynomical expansion of the electron flux distribution function. The ultimate goal of the program is to compute x-ray photoelectron emission from thin targets and energy and charge deposition profiles near solid-vacuum boundaries and at interfaces between materials of differing atomic number. In this paper results are presented for a uniformly distributed monoenergetic electron source function, anisotropic in angle with form (1 + ß cos θ). The values for the charge and energy flow and deposition profiles agree within a few percent with analytic solutions characteristic of an infinite medium and with Monte Carlo transport calculations using simulated electron trajectories. The equilibrium value for the net flow or "Compton current" for 30 kilovolt electrons is found to be about 7 percent higher than the analytic value while all other quantities are believed to be accurate to within 2 percent. The principal source of error appears to be in the evaluation of the collision integral involving the exceedingly narrow deflections characteristic of kilovolt electron scattering. The discretization error due to finite grid sizes in the discrete ordinates scheme used was found to be very small.