Residual Surface Contamination Research Articles

Surface ground contamination and soil vertical distribution of 137Cs around two underground nuclear explosion sites in the Asian Arctic, Russia

Vertical distributions of 137Cs have been determined in vegetation-soil cores obtained from 30 different locations around two underground nuclear explosion sites – “Crystal” (event year – 1974) and “Kraton-3” (event year – 1978) in the Republic of Sakha (Yakutia), Russia. In 2001–2002, background levels of 137Cs surface contamination densities on control forest plots varied from 0.73 to 0.97 kBq m −2 with an average of 0.84 ± 0.10 kBq m −2 and a median of 0.82 kBq m −2. 137Cs ground contamination densities at the “Crystal” site ranged from 1.3 to 64 kBq m −2; the activity gradually decreased with distance from the borehole. For “Kraton-3”, residual surface contamination density of radiocaesium varied drastically from 1.7 to 6900 kBq m −2; maximal 137Cs depositions were found at a “decontaminated” plot. At all forest plots, radiocaesium activity decreased throughout the whole vertical soil profile. Vertical distributions of 137Cs in soil for the majority of the plots sampled ( n = 18) can be described using a simple exponential function. Despite the fact that more than 20 years have passed since the main fallout events, more than 80% of the total deposited activity was found in the first 5 cm of the vegetation-soil cores from most of the forested landscapes. The low annual temperatures, clay-rich soil type with neutral pH, and presence of thick lichen-moss carpet are the factors which may hinder 137Cs transport down the soil profile.

Characteristics of FeSi2quantum dots on silicon

Self-assembled β-FeSi 2 quantum (QD) dots were grown on n-type Si and investigated in this work. Secondary electron images show the shape and distribution of the quantum dots depends on the temperature and thickness of the Fe deposition. Electrical characteristics were measured in MIS devices prepared by covering the quantum dots by an oxide or by a photoresist layer. The cold deposited and subsequently annealed Fe layers were found to generate large concentration of deep level defects, compensating the upper few microns layer of the silicon wafer. Reactive deposition epitaxy (RDE) growth of QDs where the iron is deposited on hot substrate- generated much lower concentration of defect, in some devices with characteristics comparable to the reference wafer. The significant scatter on the surface is attributed to inhomogeneous growth conditions and to residual surface contamination on the surface.

SiC via holes by laser drilling

Through-wafer vias were formed in 400-µm-thick bulk 4H-SiC substrates by CO2 laser drilling (1.06-µm λ) with a o-switched pulse width of ∼30 nsec and a pulse frequency of 8 Hz. The resultant pulse energy delivered to the SiC surface was on the order of 60 mJ/pulse. Laser drilling produces much higher etch rates (229–870 µm/min) than conventional dry etching (0.2–1.3 µm/min) and the via entry can be tapered to facilitate subsequent metallization. Laser drilling combines optical and mass spectroscopic methods to in-situ monitor and control the laser ablation plume and ionized debris, reducing the total residual surface contamination.

Studies of oxide desorption from GaAs substrates via Ga2O3 to Ga2O conversion by exposure to Ga flux

Removal of the native oxide from GaAs wafers in the process of thermal desorption in the presence of As flux causes very significant surface degradation. We examine the merits of an alternative oxide desorption method that consists in a standard thermal oxide removal procedure preceded by a partial oxide removal at lower temperatures via conversion of the stable Ga2O3 surface oxide into a volatile Ga2O oxide by a pulsed supply of Ga in the absence of As flux. We find that a reduction of the substrate roughness by more than one order of magnitude is obtained on most epi-ready GaAs substrates regardless of their age, even if only 70% of the original gallium oxide is removed with Ga pulses, and that nearly atomically smooth surfaces are obtained with 90% oxide removal. In addition, we demonstrate that the Ga-induced oxide removal process is laterally inhomogeneous, making this method vulnerable to the accumulation of Ga droplets on the GaAs surface when more than about 90% of the surface oxide is removed. The Ga2O desorption pattern monitored with quadrupole mass spectrometry indicates that other nonvolatile species initially present on the top of the gallium oxide are also converted to a volatile species by Ga exposure before large areas of Ga2O3 become exposed. We examine the residual surface contamination left at the substrate–epilayer interface and find no significant influence of the age of the substrate, or the oxide removal method, on the measured levels of silicon, oxygen, and carbon.

Characterization of pre-β- and α-migrating HDL subfractions in Russian men with high and low HDL cholesterol level

Vertical distributions of 137Cs have been determined in vegetation-soil cores obtained from 30 different locations around two underground nuclear explosion sites – “Crystal” (event year – 1974) and “Kraton-3” (event year – 1978) in the Republic of Sakha (Yakutia), Russia. In 2001–2002, background levels of 137Cs surface contamination densities on control forest plots varied from 0.73 to 0.97 kBq m−2 with an average of 0.84 ± 0.10 kBq m−2 and a median of 0.82 kBq m−2. 137Cs ground contamination densities at the “Crystal” site ranged from 1.3 to 64 kBq m−2; the activity gradually decreased with distance from the borehole. For “Kraton-3”, residual surface contamination density of radiocaesium varied drastically from 1.7 to 6900 kBq m−2; maximal 137Cs depositions were found at a “decontaminated” plot. At all forest plots, radiocaesium activity decreased throughout the whole vertical soil profile. Vertical distributions of 137Cs in soil for the majority of the plots sampled (n = 18) can be described using a simple exponential function. Despite the fact that more than 20 years have passed since the main fallout events, more than 80% of the total deposited activity was found in the first 5 cm of the vegetation-soil cores from most of the forested landscapes. The low annual temperatures, clay-rich soil type with neutral pH, and presence of thick lichen-moss carpet are the factors which may hinder 137Cs transport down the soil profile.

Tritium Behavior Intentionally Released in the Radiological Controlled Room under the US-Japan Collaboration at TSTA/LANL

A series of planned tritium release experiments into the Tritium Systems Test Assembly (TSTA) tritium processing rooms (3000 m3), were performed under the US-Japan collaboration program at Los Alamos National Laboratory (LANL). These experiments were carried out to acquire data on the behavior of tritium in an actual tritium facility. These experiments were performed safely with no impact on the environment or worker safety using 37 GBq (1 Ci). The results showed that the migration of tritium within the facility was dominated by the residual airflow in the room and reached a uniform value in 30 ~ 40 minutes. After restoring the normal once through ventilation (5 air changes per hour), room tritium levels decreased to background in less than 1 hour. Residual surface contamination was detected (max. 1 Bq/cm2 level). Residual contamination levels were found to be in order of decreasing contamination: linoleum > epoxy coating > acrylic resin > butyl rubber > stainless steel, from soaking results. The surface contamination increase as a result of the tritium release, was reduced to background with the normal ventilation within a few days.

Facts and artifacts of TEM specimen preparation for YBa 2Cu 3O 7− x superconductors

In this study, transmission electron microscopy (TEM) specimens of polycrystalline YBa 2Cu 3O 7− x (YBCO) superconductors were prepared by a variety of common techniques: crushing, ion beam thinning, and electrochemical-jet-polishing. Experimental techniques employed in the characterization of these specimens were SQUID measurements, transmission electron energy loss spectroscopy (EELS) analysis, and secondary ion mapping. In addition, a Monte Carlo computer simulation, known as transport of ions in matter (TRIM), was implemented to model the damage introduced during ion milling. Not surprisingly, the results suggest that oxygen stoichiometry of YBCO depends heavily upon the TEM specimen-preparation technique utilized. Crushing is the most optimal technique. The next most favorable technique is ion beam thinning at liquid-nitrogen temperature with low ion energy. Electrochemical-jet-polishing is promising, but care must be taken to remove residual surface contaminants deposited by the electrolyte solution. These results suggest the need for post-TEM specimen-preparation treatments, such as ozone annealing, to facilitate reoxygenation of the as-prepared TEM specimens.

GaAs surface plasma treatments for Schottky contacts

The properties of different rectifying metallizations (Al, Ti/Pt, WN x ) on GaAs have been investigated for various surface preparation procedures. In particular, in situ hydrogen plasma treatments were used to remove residual surface contamination (mainly O and C) and a nitrogen plasma to grow a thin mixed nitride layer. Al and Ti/Pt Schottky diodes with an ideality factor very close to 1, but with reduced barrier height, were found after the H 2 plasma as a consequence of H diffusion into GaAs. The Schottky barrier height was further reduced if a H 2 + N 2 plasma was performed. The N content in the sputtering environment during the WN x deposition affects the diode properties of plasma-treated WN x contacts. By increasing the N 2 partial pressure, the diode barrier height is reduced, probably due to nitridization of the GaAs surface. Such differences disappear after annealing the diodes in arsine overpressure at 800°C. WN x contacts deposited under different conditions on H 2 plasma treated substrates also show a similar Schottky barrier height after such annealing.

GaAs(100) substrate cleaning by thermal annealing in hydrogen

The removal of residual surface contaminants from chemically polished GaAs wafers has been examined with x‐ray photoelectron spectroscopy. Al Kα‐excited As 2p3/2 and Ga 2p3/2 core level spectra with a shallow probing depth show complete desorption of As3+ surface oxide after annealing in a stream of pure hydrogen at atmospheric pressure and 400 °C, whereas a temperature of 600 °C is required for entire removal of the Ga3+ surface oxide. Carbonaceous surface impurities are also shown to be efficiently reduced by this hydrogen processing. The recorded core level peak intensities indicate formation of a fully stoichiometric substrate surface, with no depletion of arsenic. The chemical structure of the hydrogen processed surface compares favorably with that found for substrates annealed in an ultrahigh vacuum environment. In contrast, annealing at 600 °C in 1 atm of high‐purity inert gas (argon) is shown to promote formation of a thick Ga–oxide on the substrate surface, thus emphasizing the reactive nature of...

Dynamic Control of Water Concentration to Prolong The Life Time of Encapsulated Electronic Devices in High Humidity Environments

AbstractSilicone resins and gels have been used extensively for IC protection. Reduction of interfacial moisture and ionic contamination is considered crucial to device reliability.The present research has revealed an interesting phenomenon. Prolonged lifetime was obtained for comb pattern samples using a non-conventional encapsulation method. In order to increase specimen sensitivity to interfacial moisture, samples were contaminated prior to encapsulation. Three groups of contaminated samples encapsulated with polysiloxane were produced. Group I was encapsulated with a polymer/CaCl2/polymer/substrate structure. The structure of group II consisted of only the polymer on the contaminated substrates. Group III was encapsulated with a CaCl2 embedded polymer/polymer /substrate structure. Following exposure at 85°C, 100−10 amperes after 2,000 hrs, with no visible corrosion. Group II and III samples showed visible corrosipn after 390 to 470 hrs with leakage current of the order of 10−5 amperes. Direct measurement of RH at the polymer/substrate interface and water sorption experiments were done to explain the mechanisms of these packaging structures.The experimental results suggest that a desiccating multilayer coating can maintain low interfacial water concentrations. During exposure to high external RH, water from the surroundings diffuses through the top layer, producing an interlayer consisting of a solid CaCl2 and a CaCl2-saturated solution. Until all of the salt dissolves, the water chemical potential of the saturated solution defines the water content at the polymer/substrate interface. Hence the RH at the polymer/substrate interface was maintained below that required to dissolve the residual surface contamination for a longer time than by conventional encapsulation. This technique could be useful to reduce the dependence of IC reliability upon pre-encapsulation cleaning.

In-situ light scattering studies of substrate cleaning and layer nucleation in silicon MBE

Light scattering has been used to detect changes in surface topography during removal of substrate surface oxide and initial deposition of silicon films. The scattered intensity is interpreted by reference to XPS, TMS, SIMS and TEM measurements. The scattering signal observed during oxide removal depends on the chemical nature of the oxide, while that observed during initial film growth is sensitive to substrate temperature and to the nature of residual surface contamination. An intense peak occurs during initial growth above 700–750°C on (100) Si and above 800–840°C on (111) Si, and is accounted for by a temperature-dependent interaction of residual surface contaminants with the lateral flow of growth steps.

Influence of residual gases and surface contaminants on the aging behavior of island silver films

The aging of island silver films deposited on glass at room temperature and at a pressure of 2×10−5 Torr was studied by monitoring the dc electrical resistance of the films. Films in the resistance range 1–11 MΩ/⧠ were studied under different conditions to ascertain the role of the residual gases and surface contaminants on the agglomeration rate. It was found that residual gases do not significantly affect the agglomeration rate, but a film of water vapor adsorbed on the glass surface retards the agglomeration to a great extent. An expression is arrived at for the functional dependence of the interisland spacing with time for the different conditions of study, assuming that the conduction in these island films is by quantum mechanical tunneling.

Growth, nucleation, and electrical properties of molecular beam epitaxially grown, As-doped Ge on Si substrates

Epitaxial Ge is grown on (100)Si substrates by molecular beam epitaxy (MBE). The effect of various MBE growth conditions on both the nucleation and morphology of Ge grown on Si is studied by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), and reflection high-energy electron diffraction (RHEED). These studies indicate that, at the substrate temperatures examined (300–600 °C), heteroepitaxy of Ge on Si favors three-dimensional growth, which is enhanced by both higher growth temperatures and substrate preparation techniques that leave residual surface contamination. Heavily doped n+ Ge layers are obtained using an elemental As source. The electrical properties of these films are evaluated by Hall–van der Pauw measurements. Growth temperatures of 250 °C and optimum As:Ge flux ratios yield electron concentrations as high as 2.5×1020 cm−3. Secondary ion mass spectroscopy (SIMS) and Hall effect data show that for As concentrations which exceed this optimum level, a decrease in both the electron concentration and drift mobility is observed, indicating the presence of electrically inactive As.

In Situ Cleaning of Probe Surfaces by Pulsed Laser Heating

Inconel 600, Inconel 625 and austenitic steel (AISI 304LN) surfaces were cleaned in UHV by laser pulses of 1J total energy. Residual surface contamination layers were dissociated and desorbed. The surface cleanness degree reached was equivalent to that obtained by conventional cleaning techniques like bulk heating and sputtering by ion bombardment. A comparison between these three techniques is presented. The laser cleaning efficiency was found to be strongly dependent on the initial surface contamination degree and on the residual gas composition. In particular the effect of laser shots on the activation of the surface oxidation process at ambient pressures of about 10-9 mbar of CO was studied. The possibilities of using the laser heating technique as a tool in plasma edge diagnostic (in situ cleaning of probes,analysis of trapped particles, redeposition measurements,…) in fusion devices is discussed.

Origin and Elimination of Crescent‐Shaped Growth Defects in LPE Layers of InGaAs / InP Alloys

Crescent‐shaped growth defects have been occasionally observed in liquid epitaxy layers grown on substrates. The source of these defects is traced to residual surface contamination involving S, adsorbed oxygen, and oxides of P and In from pregrowth etching of the substrate in solutions. Auger study of the cleaning procedure suggests that a thorough rinsing in DI water following the etch is essential to remove the S and reduce the O contamination to eliminate these crescent‐shaped defects.

An Ionic Contamination Detection System with Improved Performance for Quantizing Residual Ionic Species

Mildly activated rosin flux is commonly used to clean mating metallic surfaces during soldering operations. This practice is generally recognized as necessary and usually promotes a good soldered connection. However, the active constituents in fluxes are often ionic species, which, if not adequately removed, might lead to electrical surface leakage and unacceptably low circuit reliability. Therefore, both an effective flux removal procedure and some technique for quantizing residual ionic surface contamination as a function of the cleaning procedure are needed to insure acceptable surface cleanliness. Some innovations employed in designing and building a sensitive ionic contamination detection system (ICDS) with improved performance are described. The completed system is capable of automatically detecting ionic contamination at a level as low as the calibrated equivalent of 0.35 &microg NaCl with a reproducibility within 2 percent. Manual override can be used to lower the detectable limit to the calibrated equivalent of ~0.10-t&microg NaCl. The basic system includes a recirculating alcohol-water loop, a conductivity cell, an ion exchange column, a sample reservoir with provision for an argon blanket over the fluid, and an analog-to-digital converter to interface with an HP 2114B minicomputer for accurate rapid data reduction. This system has recently been used to detect ionic residues on printed wiring board circuits. It has also been used to determine the effectiveness of several cleaning procedures for flux removal on both thin- and thick-film circuits that have been subjected to external terminal attachment by soldering.

Ellipsometric study of 400ev ion damage in silicon

Lattice damage in silicon induced by low-energy argon ions is characterized ellipsometrically in two ways. In one the optical model consists of residual surface contamination and the partially-damaged substrate. The characterizing parameter in this case is an apparent substrate index N ' s which is a composite of the refractive indices of the damaged and undamaged portions of the substrate. Alternatively the damage can be analyzed using a three-layer model consisting of residual surface contaminant, the damaged layer and the underlying undamaged substrate. Using the latter model the spatial extent of the damage and the optical constants of the damage layer can be determined. The bombardment causes an increase in the absorption in the infrared, a decrease in the reflectivity in the ultra-violet, and broadens the absorption edge. The degree of damage increases with ion energy and can be removed by annealing at 800°C.

Surface Effects of Radiation on Transistors*

Observation of surface effects of ionizing radiation on several types of transistors indicates that in reverse-biased devices these effects occur at much lower radiation dosage than in unbiased devices or bulk semiconductor material. Further, the total radiation dose rather than dose rate seems often to be the more important factor in the effect. The type of particle used in irradiation is unimportant; the significant factor is the ionization it produces. The effects seem to arise from ionization of gases within a transistor encapsulation and interaction between the ionized gas and residual semiconductor surface contaminants. This results in inversion layers at the device surface and thus in alteration of junction characteristics. The changes in device properties are not permanent, but the recovery after removal from radiation is complex and proceeds with characteristic times between seconds and days. Different types of devices may respond quite differently to exposure, and the response is different even between different batches and individuals, indicating a dependence upon device processing.

Growth mechanism and defect structures in epitaxial silicon

Abstract Thin layers of silicon have been grown epitaxially on silicon substrates by hydrogen reduction of trichlorosilane. The initial mode of growth of these films has been studied using electron microscopic methods to examine substrates and surfaces exposed to slow growth rate conditions for short periods. These observations have been supplemented by glancing angle electron diffraction studies of the same surfaces, and have shown that growth commences by a three-dimensional nucleation process, probably as a result of slight residual surface contamination. Chemical etching of grown surfaces has revealed that defect structures are present in the epitaxial layers. The nature and origin of these defects have been investigated by transmission electron microscopy and electron diffraction.