10-keV X-ray Research Articles

Analysis of Total Dose-Induced Dark Current in CMOS Image Sensors From Interface State and Trapped Charge Density Measurements

The origin of total ionizing dose induced dark current in CMOS image sensors is investigated by comparing dark current measurements to interface state density and trapped charge density measurements. Two types of photodiode and several thick-oxide-FETs were manufactured using a 0.18-μm CMOS image sensor process and exposed to 10-keV X-ray from 3 krad to 1 Mrad. It is shown that the radiation induced trapped charge extends the space charge region at the oxide interface, leading to an enhancement of interface state SRH generation current. Isochronal annealing tests show that STI interface states anneal out at temperature lower than 100°C whereas about a third of the trapped charge remains after 30 min at 300°C.

Heavy-Ion Induced Charge Yield in MOSFETs

The heavy-ion induced electron/hole charge yield in silicon-oxide versus electric field is presented. The heavy-ion charge yield was determined by comparing the voltage shifts of MOSFET transistors irradiated with 10-keV X-rays and several different heavy ions. The obtained charge yield for the heavy ions is in average nearly an order of magnitude lower than for the X-rays for the entire range of measured electric fields.

Radiation-induced defects in fluorine-doped silica-based optical fibers: Influence of a pre-loading with H2

We investigated the effects of 10-keV X-ray radiation on the transmission properties of F-doped optical fibers in the 200–850 nm range of wavelengths (1.5–6 eV). We also studied the influence of pre-loading this kind of fibers with hydrogen on its radiation sensitivity. Our results showed that, for our experimental conditions (pre-treatment with H2 several months before irradiation with diffusion of all the H2 out the fiber core and cladding before X-ray exposure), this pre-treatment increases the radiation-induced attenuation in the ultraviolet part (200–300 nm) of the spectrum. A previous H2-loading has no influence at greater wavelengths. The nature of the radiation-induced point defects is discussed from the decomposition analysis of the attenuation spectra with Gaussian bands.

Total Ionizing Dose Effects on Strained ${\rm HfO}_{2}$-Based nMOSFETs

Radiation-induced charge trapping and mobility degradation are measured on uniaxially stressed HfO2-based nMOSFETs. Controlled external mechanical stress is applied via a four-point bending jig while the samples are irradiated using 10-keV X-rays. Positive charge trapping is observed for unstressed devices, and for devices irradiated under both compressive and tensile stress. Reduced trapped charge is measured as the uniaxial stress level increases. These results suggest that the increased stress leads to a reconfiguration of defect microstructure, as compared to the unstressed devices. The reconfiguration consists of changes in bond lengths and angles and a corresponding change in trap energy levels, which can (1) reduce the probability that a defect can capture holes, (2) increase the probability for electron and trapped-hole recombination, and/or (3) increase the mobility of the transporting holes in the oxides of the devices that are irradiated under stress. These results show that increased mechanical stress in high-k dielectrics does not degrade their radiation hardness. We also observe that the post-irradiation channel mobility degrades less in uniaxially stressed devices than unstressed devices.

Radiation Effects on the $1/f$ Noise of Field-Oxide Field Effect Transistors

Low-frequency (1/ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> ) noise measurements were performed at room temperature as a function of gate bias on field-oxide field effect transistors (FOXFETs) that were irradiated with 10-keV X-rays and then annealed at room temperature. The resulting oxide-trap and interface-trap charge densities were estimated by the midgap charge separation technique. Some devices also were exposed to 85% relative humidity at 130 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> C for three days after irradiation and annealing. Effective border trap densities were estimated from noise measurements in each case. The noise magnitude increases after irradiation and decreases first during annealing and then even more significantly after moisture exposure. The post-humidity-exposure noise for the irradiated devices is well below the pre-irradiation noise for these devices and experimental conditions. The increased noise due to irradiation is attributed to O vacancies and hydrogen bridge (Si-H-Si) defects in the near-interfacial field oxide. The reduction in noise with room-temperature annealing and humidity exposure is due to a combination of thermal and tunnel annealing processes and defect passivation reactions involving water.

Investigation of Proton and X-Ray Irradiation Effects on Nanocrystal and Floating Gate Memory Cell Arrays

We compared the radiation tolerance of nanocrystal and floating gate memories, fabricated with the same technology. We investigated the effects of 5-MeV proton and 10-keV X-ray irradiations, focusing on the charge loss, the permanent degradation of the electrical characteristics, and the data retention. We also presented a first order model of the charge loss and the permanent threshold voltage shift. The model and the experimental results show that nanocrystal memories feature improved radiation robustness against total ionizing dose. Nanocrystal memories can withstand a radiation dose 3 and 10 times larger than floating gate memories, in terms of charge loss and data retention, respectively.

The Effects of X-Ray and Proton Irradiation on a 200 GHz/90 GHz Complementary $(npn + pnp)$ SiGe:C HBT Technology

We investigate the effects of both X-ray and proton irradiation on a novel 200 GHz/90 GHz (npn/pnp) complementary SiGe:C HBT technology. The DC forward mode total dose tolerance of the pnp HBTs is shown to exceed that of the npn HBTs by a significant margin after being subjected to both 63-MeV proton and 10-keV X-ray sources, while the AC characteristics of both devices exhibit no degradation up to X-ray doses as high as 1.8 Mrad(SiO2). Pre- and post-irradiation results from a current feedback operational amplifier implemented in this technology and irradiated up to a dose of 1.8 Mrad(SiO2) are presented, showing no degradation in performance metrics under two low current density bias configurations.

A broadband laser plasma x-ray source for application in ultrafast chemical structure dynamics

A plasma source free from characteristic emission lines is described, based on laser irradiation of a water jet in a helium atmosphere. Various key aspects of the laser interaction are presented along with practical characterization of the observed isotropic approximately 4-10 keV x-ray emissions, measurements of which indicate subpicosecond duration. Observations are consistent with a vacuum heating plasma mechanism at the helium-water interface and indicate strong potential for in-house ultrafast chemical structure dynamics application when coupled to contemporary detector developments.

Influence of nitrogen element on total-dose radiation response of high-k Hf-based dielectric films

The purpose of this paper is to report some total-dose radiation response experimental results of HfON dielectric films compared with HfO2 dielectric films, then discuss the influence of the added nitrogen element on total-dose radiation tolerance of HfON and HfO2 dielectric films. Results of electrical measurements showed that the equivalent oxide thickness (EOT) of the HfON and HfO2 gate dielectrics was 19.67nm and 8.15nm, respectively. The MIS structures with HfON and HfO2 dielectric films were irradiated with 10keV X-rays. The flatband voltage shifts of HfON MIS structures are much smaller than HfO2 MIS structures under different total-dose from 0 to 3×104Gy(Si).

Effects of Switched-bias Annealing on Charge Trapping in HfO$_{2}$ Gate Dielectrics

Charge trapping characteristics are investigated for MOS capacitors with 6.8 nm HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers and 1.0 nm interfacial silicon oxynitrides; the effective oxide thickness of the high-kappa gate dielectric layers is 2.1 nm. These devices were irradiated with 10-keV X-rays or subjected to constant voltage stress, and then annealed for ten minute intervals of alternating positive and negative gate bias at temperatures between 25 and 150degC. The resulting oxide-trap and interface-trap charge densities exhibit reversible buildup and annealing that depend strongly on bias and temperature. Additional defect-density growth with time was observed as a result of charge injection into the gate stack during the annealing process. This defect-density growth increases with increasing annealing time and temperature. After irradiation, the most of the reversibility in the charge trapping is due to metastable electron traps in the near-interfacial dielectric layers. After constant voltage stress, the motion, reactions, and trapping of protons at or near the Si/oxynitride interface are more important to the observed device response than are metastable electron traps. This is a result of reduced electron-hole pair creation during low-energy constant-voltage stress, as compared to high-energy X-ray irradiation. These results illustrate the importance of both electron traps and protons to the ionizing radiation response and long-term reliability of MOS devices with high-kappa gate dielectrics

Total Dose Radiation Response of Nitrided and Non-nitrided SiO$_{2}$/4H-SiC MOS Capacitors

The total dose radiation response of nitrided and non-nitrided n-type 4H-SiC is reported for metal oxide semiconductor capacitors exposed to 10-keV X-rays under positive bias. The radiation response is affected strongly by differences in the SiC band gap and interface/near interface SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> trap density from typical Si MOS devices. Significantly higher net trapped positive charge densities were observed in nitrided n-SiC MOS capacitors compared to the non-nitrided samples. The mechanisms contributing to the differences in the charge trapping in these devices are discussed. Differences in the interfacial layer between SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiC are responsible for the observed dissimilarities in charge trapping behavior

Characterization and cross calibration of Agfa D4, D7, and D8 and Kodak SR45 x-ray films against direct exposure film at 4.0–5.5keV

Kodak direct exposure film (DEF) [B. L. Henke et al., J. Opt. Soc. Am. B 3, 1540 (1986)] has been the standard for moderate energy (1–10keV) x-ray diagnostic applications among the high-energy-density and inertial confinement fusion research communities. However, market forces have prompted Kodak to discontinue production of DEF, leaving these specialized communities searching for a replacement. We have conducted cross-calibration experiments and film characterizations on five possible substitutes for Kodak DEF. The film types studied were Kodak’s Biomax MR (BMR) and SR45 along with Agfa’s D8, D7, and D4sc. None of the films tested matched the speed of DEF. BMR and D8 were closest but D8 exhibited lower noise, with superior resolution and dynamic range. Agfa D7, Agfa D4sc, and Kodak SR45 were significantly less sensitive than BMR and D8, however, the improvements they yielded in resolution and dynamic range warrant their use if experimental constraints allow.

The impact of mechanical stress on the total-dose response of linear bipolar transistors with various passivation layers

In this work, we investigate how externally applied mechanical stress impacts the total dose hardness and enhanced low-dose-rate sensitivity of linear bipolar ICs fabricated with different passivation layers. To determine the effects of externally applied stress and/or radiation exposure on the current gain, various compressive or tensile stresses were applied to lateral PNP bipolar transistors at the wafer level, using a cantilever method. The devices were then exposed to 10-keV X-rays or Cs-137 /spl gamma/-rays. The effect of externally applied stress on pre and post irradiation gain degradation is found to be relatively minor compared to the effects of changes in passivation layers on the radiation response of devices.

Defect attributed variations of the photoconductivity and photoluminescence in the HVPE and MOCVD as-grown and irradiated GaN structures

The effect of native and radiation induced defects on the photoconductivity transients and photoluminescence spectra have been examined in GaN epitaxial layers of 2.5 and 12 μm thickness grown on bulk n-GaN/sapphire substrates by metal-organic chemical vapor deposition (MOCVD). For comparison, free-standing GaN as-grown samples of 500 μm thickness, fabricated by hydride vapor phase epitaxy (HVPE), were investigated. Manifestation of defects induced by 10-keV X-ray irradiation with the dose of 600 Mrad and 100-keV neutrons with the fluences of 5 × 1 0 14 and 10 16 cm −2 as well as of 24 GeV/ c protons with fluence 10 16 cm −2 have been revealed through contact photoconductivity and microwave absorption transients. The amplitude of the initial photoconductivity decay is significantly reduced by the native and radiation defects density. Synchronous decrease of the steady-state PL intensity of yellow, blue and ultraviolet bands peaked at 2.18, 2.85, and 3.42 eV, respectively, with density of radiation-induced defects is observed. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime in the photoconductivity transients, which is due to excess-carrier multi-trapping. The decay fits the stretched exponent approximation exp[ - ( t / τ ) α ] with the different factors α in as-grown material ( α ≈ 0.7 ) and irradiated samples ( α ≈ 0.3 ). The fracton dimension d s of disordered structure changes from 4.7 to 0.86 for as-grown and irradiated material, respectively, and it implies the percolative carrier motion on an infinite cluster of dislocations net in the as-grown material and cluster fragmentation into finite fractons after irradiations.

Characterization of as-grown and heavily irradiated GaN epitaxial structures by photoconductivity and photoluminescence

The influence of radiation defects on photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN/sapphire substrates. Defects induced by 10-keV X-ray irradiation with a dose of 600 Mrad and 100-keV neutrons with fluences of 5×10 14 and 10 16 cm −2 have been revealed through contact photoconductivity and microwave absorption transients. The amplitude of the initial photoconductivity decay is significantly reduced by the radiation defect density. A simultaneous decrease with radiation-induced defect density is also observed in the steady-state PL intensity of yellow, blue and ultraviolet bands peaked at 2.18, 2.85, and 3.42 eV, respectively. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess carrier multi-trapping. The decay can be described by the stretched exponential approximation exp[ - ( t / τ ) α ] with different values of α in as-grown material ( α ≈ 0.7 ) and irradiated samples ( α ≈ 0.3 ). The value of the fracton dimension d s of the disordered structure, evaluated as d s = 2 α / ( 1 - α ) , changes from 4.7 to 0.86 for as-grown and irradiated material, respectively, implying percolative carrier motion on an infinite cluster of dislocations net in the as-grown material and cluster fragmentation into finite fractons after irradiation.

Ionizing radiation effects on floating gates

Floating gate (FG) memories, and in particular Flash, are the dominant among modern nonvolatile memory technologies. Their performance under ionizing radiation was traditionally studied for the use in space, but has become of general interest in recent years. We are showing results on the charge loss from programmed FG arrays after 10keV x-rays exposure. Exposure to ionizing radiation results in progressive discharge of the FG. More advanced devices, featuring smaller FG, are less sensitive to ionizing radiation that older ones. The reason is identified in the photoemission of electrons from FG, since at high doses it dominates over charge loss deriving from electron/hole pairs generation in the oxides.

Submicron resolution of secondary radiation in LIGA polymethyl-methacrylate resist exposure

Secondary radiation during LIGA polymethyl-methacrylate (PMMA) resist exposure adversely affects feature definition, sidewall taper, and overall sidewall offset. Additionally, it can degrade the resist adjacent to the substrate, leading to the loss of free-standing features through undercutting during resist development or through mechanical failure of the degraded material. The source of this radiation includes photoelectrons, Auger electrons, fluorescence photons, etc. Sandia's Integrated Tiger Series (ITS), a coupled electron/photon Monte Carlo transport code, is used to compute dose profiles within 1 to 2 µm of the absorber edge and near the interface of the resist with a metallized substrate. The difficulty of submicron resolution requirement was overcome by solving a few local problems, having carefully designed micron-scale geometries. The results for a 10-keV x-ray photons source indicate a 2-µm dose transition region near the absorber edge, resulting from PMMA photoelectrons. This region leads to sidewall offset and to tapered sidewalls following resist development. The results also show a dose boundary layer of around 1 µm near the substrate interface due to electrons emitted from the substrate metallization layer. The maximum dose at the resist bottom under the absorber can be very high and can lead to feature loss during development. This model is also used to investigate resist doses resulting from multilayer substrate.

Radiation-defect-dependent photoconductivity transients and photoluminescence in semi-insulating GaN

Effect of radiation defects on the photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN∕sapphire substrates. Manifestation of defects induced by 10keV x-ray irradiation with the dose of 600Mrad and 100keV neutrons with the fluence of 5×1014cm−2 have been revealed through steady-state and pulsed PL as well as through contact photoconductivity and microwave absorption transients. Synchronous decrease of the PL intensity of yellow, blue, and ultraviolet bands peaked at 2.19, 2.85, and 3.42eV, respectively, with density of radiation-induced defects is observed. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess-carrier multi-trapping. The decay fits the stretched exponent approximation exp[−(t∕τ)α] with the different factors α in as-grown material (α≈0.7) and irradiated samples (α≈0.3).

The use of the Micromegas technology for a new imaging system

The Micromegas (Micromesh Gaseous) detector technology was developed by I. Giomataris and G. Charpak, in the mid 90s, for applications in the field of experimental Particle Physics. The most recent development is a novel Micromegas detector designed to detect photons of energies 1–10keV (X-ray range), for a discovery experiment of the hypothetical particles called axions, installed and currently taking data at CERN (the European Laboratory for Particle Research in Geneva). This detector has an X–Y readout capability of resolution less than 100μm, an energy resolution down to 14%, for this energy range, and an overall efficiency of 70%. With planned modifications, similar performances can be achieved for operation in the energy regime of the technetium gammas. This could lead to a novel γ-ray imaging device with spatial resolution in the submillimeter range. Initial results are presented obtained using the current detector with a parallel hole collimator to image thin capillary phantoms filled with a 99mTc water solution.

Total-dose radiation response of hafnium-silicate capacitors

Hafnium-silicate capacitors with 4.5-nm equivalent oxide thickness gate insulators were irradiated with 10-keV X-rays. The midgap and flatband voltage shifts in these devices increase linearly with dose and are significantly larger than the shifts seen in high quality, thermal SiO/sub 2/ gate oxides of similar electrical thickness. The standard trapping efficiency equation is adapted for calculating effective trapping efficiencies in alternative dielectrics and used to compare the radiation response of hafnium silicate to SiO/sub 2/ from several manufacturers. The effects of common reliability screens such as "burn-in" and bias stress tests are also discussed. It is shown that baking these devices can degrade their capacitance-voltage characteristics, and large applied voltages inject excess charge into the dielectric, which can lead to a misinterpretation of the radiation results. However, the radiation responses of these devices, coupled with the demonstrated resistance of these films to heavy-ion induced gate rupture in previous studies, suggest that alternative dielectrics to SiO/sub 2/ potentially could be integrated into future electronics technologies for many low-power space applications.