Total Ion Dose Research Articles

Focused ion beam lithography for position-controlled nanowire growth

To exploit the promising properties of semiconductor nanowires and ensure the uniformity required to achieve device integration, their position on the growth substrate must be controlled. This work demonstrates the direct patterning of a SiO2/Si substrate using focused ion beam (FIB) patterning to control self-catalyzed GaAsSb nanowire growth in molecular beam epitaxy (MBE). Besides position control, FIB patterning parameters influence nanowire yield, composition and structure. Total ion dose per hole is found to be the most important parameter. Yield of single nanowires ranges from ≈34% to ≈83%, with larger holes dominated by multiple nanowires per hole. Areas exposed to low ion beam doses are selectively etched by routine pre-MBE HF cleaning, enabling patterning and nanowire nucleation with minimal damage to the Si substrate. The optical and electronic properties of nanowires are found to depend on the ion dose used during patterning, indicating the potential for FIB patterning to tune nanowire properties. These findings demonstrate the possibility for a FIB lithography protocol which could provide a rapid and direct patterning process for flexible controlled nanowire growth.

Source shot noise mitigation in focused ion beam microscopy by time-resolved measurement

Focused ion beam microscopy suffers from source shot noise – random variation in the number of incident ions in any fixed dwell time – along with random variation in the number of detected secondary electrons per incident ion. This multiplicity of sources of randomness increases the variance of the measurements and thus worsens the trade-off between incident ion dose and image accuracy. Repeated measurement with low dwell time, without changing the total ion dose, is a way to introduce time resolution to this form of microscopy. Through theoretical analyses and Monte Carlo simulations, we show that three ways to process time-resolved measurements result in mean-squared error (MSE) improvements compared to the conventional method of having no time resolution. In particular, maximum likelihood estimation provides reduction in MSE or reduction in required dose by a multiplicative factor approximately equal to the secondary electron yield. This improvement factor is similar to complete mitigation of source shot noise. Experiments with a helium ion microscope are consistent with the analyses and suggest accuracy improvement for a fixed source dose by a factor of about 4.

Degradation of Radiation-Hardened Vertical Double-Diffused Metal-Oxide-Semiconductor Field-Effect Transistor During Gamma Ray Irradiation Performed After Heavy Ion Striking

The combined impact of single event effect (SEE) and total ion dose (TID) on Si-based radiation-hardened Vertical Double-diffused Metal-Oxide-Semiconductor (VDMOS) Field-Effect Transistor (VDMO-SFET) is investigated in this letter. As previous research of VDMOS on TID effect and SEE carried out separately, in fact, the combined impact could degrade the device performance more seriously, because of the greater numbers of oxide-trapped charges and interface states generated, which is more pronounced during multiple gamma irradiation and post-irradiation annealing. This phenomenon indicates that SEE is not only a transient effect, influencing whether the devices can be driven to failure, but also a potential threat to the long-term reliability of MOSFETs used in aerospace applications.

Radiation-Hardening Techniques for Spin Orbit Torque-MRAM Peripheral Circuitry

Magnetic random access memory (MRAM) is experimentally proved intrinsically immune to radiation effects including heavy-ion irradiation and total ion dose as the data are represented by the spin instead of charges. Hence, it is considered as a promising candidate for aerospace and avionic electronics. However, its CMOS peripheral read/write circuits are much more vulnerable to radiation-induced single-event upset (SEU) and double-node upset (DNU) induced by charge sharing. This paper presents a novel radiation-hardening design of the peripheral circuitry for spin orbit torque (SOT)-MRAM to address abovementioned issues effectively. Transient simulations are performed based on a physics-based SOT-magnetic tunnel junction compact model and a 65 nm CMOS design kit. Simulation results validate that the proposed hardening read/write circuits are strongly robust against SEUs and DNUs. Furthermore, the proposed hardening read circuit shows the improvement of hardware area and reliability compared with the previous works.

Incident ion dose evolution of damaged layer thickness in Si substrate exposed to Ar and He plasmas

The effect of ion flux on the formation of a damaged layer in a Si substrate was investigated in detail. An inductively coupled plasma source was used to control the flux (Γion) and energy (Eion) of incident ions. The progressive behavior of the total damaged layer thickness (ddam) estimated by spectroscopic ellipsometry primarily depends on the total ion dose (Dion = Γion × tpr, where tpr is the process time) and Eion. The evolution of ddam then saturates and becomes independent of Dion. The characteristic total ion dose (Ddam) is defined, which distinguishes the progressive and saturation phases. Experimental results suggest that Ddam is estimated to be in the range from 1 × 1016 to 3 × 1016 cm−2, regardless of Ar and He plasmas in the present study. This Dion-dependent behavior was also confirmed by transmission electron microscopy and capacitance–voltage measurements. The interface thickness dIL of the damaged layer was found to be independent of Dion. This result confirms that dIL can be used as a measure of the Eion-dependent plasma-induced damage (PID) for in-line monitoring. The present Dion dependence can be implemented in the conventional PID range theory, particularly, in the designs of high-density plasma sources.

Analysis of proton and γ-ray radiation effects on CMOS active pixel sensors**Project supported the National Natural Science Foundation of China (Grant No. 11675259), the West Light Foundation of the Chinese Academy of Sciences (Grant Nos. XBBS201316, 2016-QNXZ-B-2, and 2016-QNXZ-B-8), and Young Talent Training Project of Science and Technology, Xinjiang, China (Grant No. qn2015yx035).

Radiation effects on complementary metal–oxide–semiconductor (CMOS) active pixel sensors (APS) induced by proton and γ-ray are presented. The samples are manufactured with the standards of CMOS technology. Two samples have been irradiated un-biased by 23 MeV protons with fluences of and , respectively, while another sample has been exposed un-biased to 65 krad(Si) 60Co γ-ray. The influences of radiation on the dark current, fixed-pattern noise under illumination, quantum efficiency, and conversion gain of the samples are investigated. The dark current, which increases drastically, is obtained by the theory based on thermal generation and the trap induced upon the irradiation. Both γ-ray and proton irradiation increase the non-uniformity of the signal, but the non-uniformity induced by protons is even worse. The degradation mechanisms of CMOS APS image sensors are analyzed, especially for the interaction induced by proton displacement damage and total ion dose (TID) damage.

Detergent decellularization methods affect the surface molecular functionality of biologic scaffolds

Detergent decellularization methods affect the surface molecular functionality of biologic scaffolds

Radiation defect dynamics in Si at room temperature studied by pulsed ion beams

The evolution of radiation defects after the thermalization of collision cascades often plays the dominant role in the formation of stable radiation disorder in crystalline solids of interest to electronics and nuclear materials applications. Here, we explore a pulsed-ion-beam method to study defect interaction dynamics in Si crystals bombarded at room temperature with 500 keV Ne, Ar, Kr, and Xe ions. The effective time constant of defect interaction is measured directly by studying the dependence of lattice disorder, monitored by ion channeling, on the passive part of the beam duty cycle. The effective defect diffusion length is revealed by the dependence of damage on the active part of the beam duty cycle. Results show that the defect relaxation behavior obeys a second order kinetic process for all the cases studied, with a time constant in the range of ∼4–13 ms and a diffusion length of ∼15–50 nm. Both radiation dynamics parameters (the time constant and diffusion length) are essentially independent of the maximum instantaneous dose rate, total ion dose, and dopant concentration within the ranges studied. However, both the time constant and diffusion length increase with increasing ion mass. This demonstrates that the density of collision cascades influences not only defect production and annealing efficiencies but also the defect interaction dynamics.

Time constant of defect relaxation in ion-irradiated 3C-SiC

Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by the mobility and interaction of ballistically generated point defects. Here, we study the dynamics of radiation defects in 3C-SiC bombarded at 100 °C with 500 keV Ar ions, with the total ion dose split into a train of equal pulses. Damage–depth profiles are measured by ion channeling for a series of samples irradiated under identical conditions except for different durations of the passive part of the beam cycle. Results reveal an effective defect relaxation time constant of ∼3 ms (for second order kinetics) and a dynamic annealing efficiency of ∼40% for defects in both Si and C sublattices. This demonstrates a crucial role of dynamic annealing at elevated temperatures and provides evidence of the strong coupling of defect accumulation processes in the two sublattices of 3C-SiC.

Interior surface plasma immersion ion implantation of a small oval tube

We use a two-dimensional particle-in-cell model to investigate ions dynamics implanted into the inner sidewall of an oval tube target during a high voltage pulse in plasma immersion ion implantation. Our study shows that the potential structure in the plasma sheath adjacent to the surface of the oval tube acts as two cylindrical divergent potential lenses with different focal lengths which principal axes along the major and minor axes on the elliptical cross-section respectively. The focusing properties of the two potential lenses results in non-uniform total incident ion dose distribution along the inner elliptical arc surface of the oval tube, an ion dose peak between the vertices of the major and minor axes of the elliptical arc surface and nearer the vertex of the major is observed. It is notable that non-uniform ion dose distribution is obtained at any time during the whole implantation process from the beginning of the implantation until all ions exhausted. The calculation of the cross-correlation of the ion dose distributions at different time shows that the ion dose distribution along the surface remains unchanged.

GaN‐based high‐frequency devices and circuits: A Fraunhofer perspective

AbstractWe present the current status of our technology for GaN‐based HEMTs and MMICs as well as results ranging from the L‐band up to the W‐band. Epitaxial growth is carried out on 4H‐SiC(0001) substrates by both MOCVD and MBE. Processing is done using standard III–V equipment including both frontside and backside processing. For L‐band power bars we arrive at output powers, efficiencies and gains beyond 100 W, 60% and 17 dB, all measured under cw conditions at 50 V drain bias. The X‐band MMICs are characterized by a high efficiency above 40% for two‐stage amplifiers. Towards mm‐wave applications we have fabricated HEMTs with transit frequencies above 100 GHz and W‐band MMICs delivering 0.5 W/mm at 94 GHz with 7% PAE. First quaternary InAlGaN barriers show promising results for this new materials system. Reliability tests return a very good long‐term stability of our devices even at an elevated channel temperature of 200 °C with an extrapolated lifetime of 5 × 105. Initial space capability tests including total ion dose radiation insensitivity, radiation displacement damage, hydrogen poisoning and single event effect are successfully passed.

Fabrication of discrete gallium nanoislands on the surface of a Si(001) substrate using afocused ion beam

A gallium focused ion beam (FIB) has been used to implant Ga at specificsites on the surface of undoped Si(001) substrates. Upon annealing at600 °C, discrete nanoscale surface islands form within the FIB patternedregions when the total Ga ion dose, or fluence, is greater than1.0 × 1016 ions cm − 2. The number of islands depends on the size of the irradiated regionand a single island can be achieved for a FIB milled region that is100 nm × 100 nm.The average sizes of the islands were found to range from 24.5 nm when exposed to a total ion dose of1.2 × 1016 ions cm − 2 to 45 nm fora dose of 3.0 × 1016 ions cm − 2. We have confirmed that these surface islands are metallic Ga by performing aselective chemical etch that removes the islands and by transmission electronmicroscopy characterization. These patterned Ga surface templates could serve asnucleation sites for the lateral arrangement of discrete quantum dot structures.

Two-dimensional particle-in-cell plasma source ion implantation of a prolate spheroid target

A two-dimensional particle-in-cell simulation is used to study the time-dependent evolution of the sheath surrounding a prolate spheroid target during a high voltage pulse in plasma source ion implantation. Our study shows that the potential contour lines pack more closely in the plasma sheath near the vertex of the major axis, i.e. where a thinner sheath is formed, and a non-uniform total ion dose distribution is incident along the surface of the prolate spheroid target due to the focusing of ions by the potential structure. Ion focusing takes place not only at the vertex of the major axis, where dense potential contour lines exist, but also at the vertex of the minor axis, where sparse contour lines exist. This results in two peaks of the received ion dose, locating at the vertices of the major and minor axes of the prolate spheroid target, and an ion dose valley, staying always between the vertices, rather than at the vertex of the minor axis.

Attempts for molecular depth profiling directly on a rat brain tissue section using fullerene and bismuth cluster ion beams

The capabilities of time of flight secondary ion mass spectrometry (TOF-SIMS) have been recently greatly improved with the arrival in this field of polyatomic ion sources. This technique is now able to map at the micron scale intact organic molecules in a range of a thousand Daltons or more, at the surface of tissue samples. Nevertheless, this remains a surface analysis technique, and three-dimensional information on the molecular composition of the sample could not be obtained due to the damage undergone by the organic molecules during their irradiation. The situation changed slightly with the low damage and low penetration depth of the C 60 fullerene ion beams. Recent promising studies have shown the possibility of organic molecular depth profiling using this kind of beams onto model samples. This possibility has been tried out directly onto a rat brain tissue section, which is the most commonly used biological tissue model in TOF-SIMS imaging method developments. The tissue surface has been sputtered with a 10 keV energy fullerene ion beam, and surface analyses were done with a 25 keV Bi 3 + ion beam at regular time intervals. The total depth which was analysed was more than two microns, with total primary ion doses of more than 10 16 ions cm −2. Although not in contradiction with results previously published but with much lower doses, it is found that the molecular damage remains too large, thus making molecular imaging very difficult. In addition, most of the lipids, which are usually the main observable molecules in TOF-SIMS, are concentrated close to the sample surface in the first hundreds of nanometers.

The formation of silicon nanocrystals in SiO2 layers by the implantation of Si ions with intermediate heat treatments

The effect of heat treatments at 1100°C on an ion-beam synthesis of Si nanocrystals in SiO2 layers is studied. The ion-implanted samples are subjected either to a single heat treatment after the total ion dose (1017 cm−2 has been implanted, two heat treatments (a heat treatment after the ion implantation of each half of the total dose), or three heat treatments (a heat treatment after each third of the dose). The total duration of the heat treatments is maintained at 2 h. It is found that the intermediate heat treatments lead to a shift of the Raman spectrum of the nanocrystals to longer wavelengths and to a shift of the photoluminescence spectrum to shorter wavelengths. Study using electron microscopy shows that the size of the nanoprecipitates decreases, which is accompanied by the disappearance of the characteristic features of crystallinity; however, the features of photoluminescence remain characteristic of the nanocrystals. The experimental data obtained are accounted for by a preferential drain of Si atoms to newly formed clusters, which is consistent with the results of a corresponding numerical simulation. It is believed that small nanocrystals make the main contribution to photoluminescence, whereas the Raman scattering and electron microscopy are more sensitive to larger nanocrystals.

Polyatomic primary ion bombardment of organic materials: experiences in routine analysis

In order to investigate the differences between monoatomic (Au 1) and polyatomic (Au 3) primary ion bombardment with respect to routine TOF-SIMS analysis, systematic studies were performed by taking ion spectra and images for identical sample positions with total primary ion dose densities below the static SIMS limit. Samples were mainly of organic origin but also inorganic materials were included. The data were evaluated qualitatively (peak patterns) as well as quantitatively (secondary ion yields). It could be shown that with few exceptions the overall peak patterns are very similar and secondary ion yields for Au 3 bombardment are significantly higher than those for Au 1. Differences occur, however, with respect to the order of this enhancement. Highest differences (more than a factor of 100) are observed for intact molecular ions from thick layers or substrates of low density. Additionally, selective enhancement effects could be observed for nitrogen containing species, sulphates and phosphates. A selective yield reduction was found for monolayer coverages of fatty acid residues on metal substrates.

Modification of Electronic Structure of Mesoscopic Perchlorate-Doped Polypyrrole Films by Ion Irradiation

The effects of low-energy (1−3 keV) argon ion irradiation on a 100 nm thick polypyrrole film deposited potentiostatically in an aqueous perchlorate solution have been studied by X-ray photoelectron spectroscopy (XPS). Dramatic spectral changes in the valence band structure and Cl 2p region with increasing ion impact energy and total ion dose have been observed for the perchlorate-doped conductive polymer surface. Higher impact energy (3 keV) is found to be more effective in modifying the polymer backbone, particularly in the breakage and formation of terminal bonding units, while higher ion flux (for impact energy above 1 keV) appears to produce complete dissociation of the perchlorate counterions, leaving behind only Cl atoms. These observations are also consistent with the spectral changes in the C 1s, N 1s, and O 1s regions observed under different ion irradiation conditions.

Atomic mixing and chemical bond formation in MoS x/Fe thin-film system deposited from a laser plume in a high-intensity electrostatic field

The potential of pulsed laser deposition in an applied uniform electrostatic field was investigated. A flat, positively charged, fine-celled-grid counter electrode was used to provide bias voltage of up to +50 kV with respect to the substrate. This enabled control of the atomic mixing and made it possible to initiate chemical bond formation at the interfaces of the films formed by deposition from the laser-induced plume. As an example, the results of multilayer 56Fe/MoS x / 57Fe film deposition are presented. At first, a bilayer MoS x / 57Fe film was grown in the absence of the electric field. This was followed by 56Fe film deposition in an applied field. A relatively sharp interface between the MoS x and 57Fe films was observed. In contrast, after 56Fe deposition, effective atom mixing was observed and new chemical bonds between Fe, S and Mo were detected. By penetrating through the interface, accelerated 56Fe ions gave rise to the growth of an amorphous layer of up to 50 nm in thickness. It consisted of rather evenly distributed Fe, S and Mo atoms (at total ion dose of 2.5×10 16 cm −2). The ion flux destroyed MoS chemical bonds, and the S atoms released preferably bound Fe atoms, thus forming a FeS 2-type phase. The Mo atoms, as a lower-oxidation-state species (apparently together with S atoms), were localized in the vicinity of Fe atoms and affected the hyperfine magnetic fields. The technique developed has made it possible to study the ion-induced processes occurring at the interfaces of multilayer films. It can also be applied to improve the tribological functionality of thin films.

Evolution of tungsten film deposition induced by focused ion beam

Direct write metallization is an important approach for circuit modification and prototyping. We investigate the evolution of the chemical vapor deposition of tungsten induced by a 50 keV focused Ga+ ion beam. Time resolved imaging in combination with atomic force microscopy reveals that chemical vapor deposition of tungsten by focused ion beam proceeds via two clearly distinguishable regimes of layer growth. Deposition starts with the nucleation of nanoscale tungsten deposits scattered over the substrate surface. Despite local impacts of the ion beam within the irradiated area of the substrate the localization of the nucleation spots is not correlated to the scan path of the ion beam. The nanoscale tungsten particles preserve their position and typical shape during further deposition. Only after merging of the particles into a contiguous tungsten layer, does the second regime of growth characterized by deposition of tungsten on a tungsten surface set in. In this regime the deposition process is determined by the total ion dose and the average current density the sample was subjected to. Deposition yields up to 3.5 atoms per incident gallium ion are achieved. The layer quality is determined by Auger electron analysis, which shows fractions of Ga, C, Si and O in the W layer. Depth profiling by secondary ion mass spectroscopy showed the depth profiles of these constituents and confirmed the existence of a 50–100 nm thick transition zone between the tungsten layer and the substrate. Electrical resistivity of metal layers of 250 μ Ω cm and current densities up to 3.5×106 A/cm2 are measured by means of van der Pauw test structures. In order to give a concise description of the experimental findings the data were interpreted utilizing an analytic model that mainly incorporates the precursor gas coverage, precursor gas transformation cross section and ion induced sputtering. The critical ion current density, where ion sputtering exceeds the deposition, was identified by the model. Because the model shows excellent agreement with the measurement it should be suitable for a further survey of focused ion beam process development.

Time Resolved Studies of Focused Ion Beam Induced Tungsten Deposition

ABSTRACTIn this study we investigate the nucleation and growth mechanisms of tungsten films processed by focused ion beam (FIB) induced chemical vapor deposition. For our investigation we use a 50 keV Ga+ ion beam focused on the substrate target and tungsten hexacarbonyl (W(CO)6) as precursor gas. Mediated by the substrate the energy of the impinging ions leads to the decomposition of the tungsten hexacarbonyl molecules adsorbed on the substrate into volatile parts and nonvolatile residues forming a metal deposit. Time resolved FIB secondary electron microscope imaging in combination with atomic force microscopy reveal first the formation of isolated nuclei and further their coalescence finally resulting in the formation of a contiguous metal layer. Despite the local impacts of the ion beam within the irradiated area of the substrate the localization of the nucleation spots is neither correlated to the spot centers nor to the scan path of the ion beam. After formation, the nanoscale tungsten nuclei preserve their positions and typical shapes during further deposition. Only after merging the nuclei to a contiguous tungsten layer, a further regime of growth sets on which is characterized by deposition of tungsten on a tungsten surface. In this regime the deposition process is determined by the total ion dose and the average current density the samples are subjected to. In this regime, deposition yields up to 3.5 atoms per incident gallium ion are achieved. The contiguous layer quality is determined by Auger electron analysis. The measured growth data were interpreted by adopting the analytic Ruedenauer Steiger approach mainly incorporating ion current density, precursor gas transformation rate, and ion induced sputtering. As a result, the critical ion current density, where ion sputtering exceeds deposition, was identified by the model. Because the model shows excellent agreement with the measurement it should be suitable for further survey concerning focused ion beam process development.