Removal Of Surface Contaminants Research Articles

Contamination-free sounding rocket Langmuir probe

A technique for removing surface contaminants from a sounding rocket spherical Langmuir probe is presented. Contamination layers present on probe surfaces can skew the collected data, resulting in the incorrect determination of plasma parameters. Despite following the usual probe cleaning techniques that are used prior to a launch, the probe surface can become coated with layers of adsorbed neutral gas in less than a second when exposed to atmosphere. The laboratory tests reported here show that by heating the probe from the interior using a small halogen lamp, adsorbed neutral particles can be removed from the probe surface, allowing accurate plasma parameter measurements to be made.

Ion-induced electron emission from Boron-doped diamond: effect of surface hydrogenation and sample temperature

Extremely high ion-induced electron emission yields (γ) from B-doped diamond films induced by impingement of 200-keV protons and 140-keV Ar ions (γ=150 and γ=180, respectively) are reported. The dependence of γ on target temperature, on total accumulated ion-dose and on surface termination (hydrogen- or oxygen-terminated) were studied. Very large γ were found with fresh surfaces at elevated temperatures possibly due to removal of surface contaminants. Progressive ion bombardment severely deteriorated emission properties, presumably due to material modification (graphitization) in the case of Ar, but much less so for hydrogen ion bombardment. Heating the target (up to 500°C) strongly inhibited degradation of γ after some initial degradation, but did not stop it completely. Oxygen-terminated diamond surfaces emitted approximately 30% less efficiently than hydrogen-terminated surfaces. Theoretical and practical implications of these findings are discussed.

Gadolinium Oxide Gate Dielectrics for GaN MOSFETs

ABSTRACTSubstrate preparation of GaN, both in-situ and ex-situ, and the growth of gadolinium oxide, Gd2O3, by Gas source molecular beam epitaxy (GSMBE) have been investigated. Ex-situ cleaning techniques included wet chemical etching and UV-ozone treatments to remove surface contaminants and the native oxide. In-situ cleaning consisted of thermal treatment with and without exposure to an electron cyclotron resonance (ECR) oxygen plasma. A GaN (1x3) streaky RHEED pattern was the final product of this surface treatment study. Various growth initiation techniques were explored to produce Gd2O3 films with different microstructures as evidenced by RHEED, TEM, and XRD. Gd2O3 films planarized the initial GaN surface and stoichiometry was maintained over a range of substrate temperatures (300° to 650°C). Single crystal gadolinium oxide films were grown at substrate temperatures of 600-650°C. These films exhibited a breakdown field strength (EBD) of ∼1MV/cm, and showed high leakage current at high forward bias due to defects within the oxide. Single crystal oxide films were found to be thermally stable at annealing temperatures up to 1000°C. Quasi-amorphous films were grown at a substrate temperature of 100°C. These films exhibited a higher E BD of ∼3MV/cm and an interface state density of 3 × 1011 cm−2eV−1. However, the quasi-amorphous films were not thermally stable at 1000°C, showing evidence of re-crystallization in x-ray diffraction (XRD) scans.

Some experimental results of laser beam interaction with surface layer of brick

Preservation of cultural heritage is gaining increasing attention all over the world. Restoration of ancient artwork and architecture requires even more effective cleaning techniques than are currently available. Laser cleaning appears to be a suitable method for removing surface contaminants from porous materials like masonry. This paper presents some effects of laser beam interaction (ruby laser, free generation regime) with clean and contaminated surface of brick which was subjected to long term exposure to atmospheric conditions. The effects of the laser beam interaction have been examined by the optical microscopy method, scanning electron microscopy and X-ray microanalysis. The present research revealed the potential application of laser irradiation for cleaning of the surface layer from contaminants and possible modification of the chemical composition of cleaned surfaces.

Negative Transferred Arc Cleaning: A Method for Roughening and Removing Surface Contamination from Beryllium and Other Metallic Surfaces

TA cleaning has been investigated for preparing the surface of beryllium plasma facing components (PFC’s) inside of the International Thermonuclear Experimental Reactor (ITER) prior to depositing beryllium by plasma spraying. Plasma spraying of beryllium was evaluated during the ITER Engineering Design Activity (EDA) for in-situ repair and initial fabrication of the beryllium first wall armor. Results have shown that surface roughening of beryllium, during the TA cleaning process, can result in bond strengths greater than 100 MPa between beryllium surfaces and plasma sprayed beryllium. In addition, the TA cleaning process was shown to be an effective method for removing contaminate layers of carbon and tungsten from the surface of beryllium. Investigations have been performed to characterize the different arc-types that occur during the TA cleaning process (Type I, II and III arcs) and the effectiveness of the TA cleaning process for potentially removing co-deposited layers of carbon and deuterium from the surface of beryllium, stainless steel and tungsten.

Tips and traps in the 14C bio-AMS preparation laboratory

Maintaining a contamination free sample preparation lab for biological 14C AMS requires the same or more diligence as a radiocarbon dating prep lab. Isotope ratios of materials routinely range over 4–8 orders of magnitude in a single experiment, dosing solutions contain thousands of DPM and gels used to separate proteins possess 14C ratios of 1 amol 14C/mg C. Radiocarbon contamination is a legacy of earlier tracer work in most biological laboratories, even if they were never hot labs. Removable surface contamination can be found and monitored using swipes. Contamination can be found on any surface routinely touched: door knobs, light switches, drawer handles, water faucets. In general, all surfaces routinely touched need to be covered with paper, foil or plastic that can be changed frequently. Shared air supplies can also present problems by distributing hot aerosols throughout a building. Aerosols can be monitored for 14C content using graphitized coal or fullerene soot mixed with metal powder as an absorber. The monitors can be set out in work spaces for 1–2 weeks and measured by AMS with regular samples. Frequent air changes help minimize aerosol contamination in many cases. Cross-contamination of samples can be minimized by using disposable plastic or glassware in the prep lab, isolating samples from the air when possible and using positive displacement pipettors.

Test of Opticlean strip coating material for removing surface contamination

The strip coating material, Opticlean, which has been reformulated, has been shown to remove 1-5-microm-diameter particles as well as contamination remaining from previous drag wipe cleaning on a used silicon wafer. In addition, no residue that produced scattering was found on a fresh silicon wafer when Opticlean was applied and then stripped off. The total integrated scattering technique used for the measurements could measure scattering levels of He-Ne laser light as low as a few ppm (parts in 10(6)), corresponding to a surface roughness of <1 A rms.

Surface energy as a function of self-bias voltage in oxygen plasma wafer bonding

A limitation in the use of wafer bonding has been the necessity for high-temperature annealing after contacting the wafers at room temperature. In this paper, we try to find the highest surface energy as a function of self-bias voltage in oxygen plasma-activated wafer bonding, in order to achieve a low-temperature bonding process. The bonding was performed in situ the vacuum chamber. It was found that oxygen plasma has a smoothing effect on the surface roughness, rather independent of the plasma self-bias. However, a moderate self-bias voltage proved to give the highest surface energy for the bonded wafers, both at room-temperature and after annealing at 200°C. We believe that this is due to the fact that a moderate self-bias is the most efficient in removing surface contaminants, like water and hydrocarbons. It was also found that even after annealing at higher temperatures, 480°C and 720°C, the plasma-bonded wafers showed higher surface energy values than wafers bonded in ambient air. This investigation was focused on low-effect plasmas, <200 W, keeping the induced plasma damages at a minimum.

Dose Assessment for Process Water Tunnels at Hanford Site

The RESRAD-BUILD and RESRAD computer codes were used for dose assessment of the 105-C Process Water Tunnels at the Hanford Site. The evaluation assessed three different exposure scenarios: recreational use, tunnel maintenance worker, and residential use. The recreationist and maintenance worker scenarios were evaluated by using RESRAD-BUILD, a computer model for analyzing the radiological doses resulting from remediation and occupancy of structures contaminated with radioactive material. The recreationist was assumed to use the tunnels as an overnight shelter for eight hours per day for one week. The maintenance worker was assumed to spend 20 hours per year working in the the tunnel. Six exposure pathways were considered for both scenarios in dose assessment. The gradual removal of surface contamination over time and ingrowth of decay products were considered in calculating the dose at different times. The maximum dose would occur immediately after the release and was estimated to be 1.9 mrem/yr for the recreationist and 0.9 mrem/yr for the maintenance worker. The residential scenario was evaluated by using the probabilistic RESRAD code. It was assumed that total activity from the tunnels would be brought into the near-surface layer by future human activities. Eight exposure pathways were considered. The maximum yearly dose for this very unlikely scenario would occur immediately after the release and was less than 4 mrem/yr for the maximally exposed individual. The assessment demonstrates that both codes are suitable for nuclear facility decontamination and decommissioning sites, where buildings and structures with residual radioactivity must be evaluated to facilitate property transfer or release.

Third Row Transition Metals by X-ray Photoelectron Spectroscopy

XPS spectra have been obtained from 11 metals in the third row of transition elements using a Quantum 2000 Scanning ESCA Microprobe. The metals analyzed include La, Hf, Ta, W, Re, Ir, Pt, Au, Tl, Pb, and Bi. Each sample was Ar+ ion etched before XPS analysis to remove surface contamination and/or oxide. The spectra include standard survey scans and high-energy resolution scans of the photoelectron peaks, as well as selected x-ray induced Auger peaks. Each spectrum was collected using a 100 μm diameter monochromatic Al Kα x-ray beam scanned over a 1.5 mm × 0.2 mm area of each sample. Survey scans were collected using a 58.7 eV pass energy, while high energy resolution scans were collected using a 23.5 eV pass energy.

Preparation and operation of hydrogen cleaned GaAs(100) negative electron affinity photocathodes

We report on the preparation and operating conditions of negative electron affinity GaAs, cesium-oxygen activated photocathodes, cleaned using atomic hydrogen generated by a thermal cracking source. Atomic hydrogen is effective in removing surface contaminants and in producing a smooth reconstructed surface free of oxides and carbon compounds at a relatively low GaAs temperature. A clean (2×4) reconstructed GaAs(100) reflection high-energy electron diffraction (RHEED) pattern is obtained after atomic hydrogen cleaning. From the RHEED patterns, the thickness of the cesium layer that causes maximum photoemission is estimated to be ∼0.5 monolayer. High quantum efficiency photocathodes (∼12%) are produced after activation to negative electron affinity. Atomic hydrogen cleaning is effective not only in the initial cleaning but also in reviving the photocathode after its performance is degraded by operation. The GaAs is kept at 500 °C during the cleaning process, a temperature that preserves the arsenic stabilized surface necessary for negative electron affinity activation. By reducing the accelerating voltage of the photoemitted electrons and the photocurrent intensity, and by keeping a low level of continuous cesiation, the photocathode lifetime can be significantly enhanced.

Removal of surface contaminants by cryogenic aerosol jets

Removal of surface contaminants by various cryogenic aerosol jets has been experimentally investigated. Simplified theoretical consideration of their removal mechanism has been also presented based on the impact power of the aerosol jets. Under atmospheric operation, water vapor and carbon dioxide could make their particles independent of their concentrations in the carrier gas while argon and nitrogen could hardly solidify to their own particles. The cryogenic aerosol jets were very effective in removing both submicron particle contaminants and photoresist films on wafers. The rate of the PR film removal strongly depended on the hardness of the film. Molecular organic films could be also removed with the aerosol jets. In general, the removal of the contaminants depends primarily on the physical impact. The removal rate increased with the mass concentration of the aerosol particles, regardless of their nature. The rate also increased with the impact velocity of the jets which was controlled by either the chamber pressure or the distance between the nozzle tip and the contaminant surface. The cryogenic aerosolfree jet was much less effective than the corresponding aerosol jets but had some effectiveness compared to the noncryogenic one. The thermal shock of the film was, therefore, supposed to have a secondary effect on the contaminant removal.

Effects of Grazing Exclusion and Reseeding on a Former Uranium Mill Site in the Great Basin Desert, Arizona

Germinable seed in the soil seed bank and vegetation were characterized at a former uranium mill site in the Great Basin desert , Arizona , 10 years after a remediation program was conducted to remove surface contamination and revegetate the site . The objective of the study was to evaluate the effectiveness of reseeding as routinely practiced to revegetate such sites . Three different conditions at the site were evaluated: (1) an area that had been bladed to remove topsoil then reseeded with exotic and native species and fenced to exclude livestock (ungrazed - bladed - reseeded) (2)a control area inside the fence that had not been bladed or reseeded (ungrazed), and (3) for further comparison , an area outside the fence that was undisturbed by the milling and remediation efforts but has received normal grazing pressure (grazed). Each condition was represented by three plots , from which soil samples and transect data were collected . The diversity of species and total number of viable seeds in the seed bank (top 5 cm of soil) were lowest in the ungrazed - bladed - reseeded plots (P < 0.05). These plots also had lower plant cover (15%) than the ungrazed plots (24%) (P < 0.05), comparable to the cover on grazed plots (11%), even after 10 years of grazing exclusion . We conclude that at this site the results of topsoil removal and replacement were not effectively remediated by reseeding . Although these methods may be effective in moister climates , more intensive efforts to reintroduce vegetation may be required in desert sites such as this .

Comparison of methods for monitoring solid particulate surface contamination in the workplace

This paper reports an assessment of various methods for sampling particulate surface contamination for use in the field. The results from the study will be used to develop guidance for monitoring particulate surface contamination in the workplace.Three types of adhesive tape, two manual and two `semi-automatic' wipe methods, and one Smair method were assessed. A field assessment of selected methods was carried out in two stages. In stage one, the manual wipes, semi-automatic wipes and adhesive tape (Scotch Tape®) were compared. In stage two, the adhesive tape (Scotch Tape®), black forensic tape, clear forensic tape and Smair were compared.Visits were made to ten metal processing facilities and particulate contamination was sampled from six locations at each site. Sampling locations were chosen for their potential as points of worker dermal contact and also to represent a range of surface characteristics. The metal content of the samples were analysed using Atomic Absorption Spectrophotometry (AAS) and X-ray Fluorescence Spectrometry (XRF).It was found that the Smair method proved to be inefficient in removing surface contamination, particularly where the surface was damp or greasy. Tape methods were found to be the most satisfactory and have considerably greater efficiency than wipe sampling. The black forensic tape is the preferred sampling medium as it is highly efficient, reproducible and easy to use.

Removal of Carbonaceous Contamination from SERS−Active Silver by Self-Assembly of Decanethiol

The presence of carbonaceous contamination on common surface-enhanced Raman scattering (SERS) surfaces has been a primary limitation to the analytical utility of SERS; background contamination significantly affects reproducibility and often determines the limit of detection. Due to the strong interaction between thiols and Ag or Au, self-assembly of alkanethiols onto these metals results in the displacement of "adventitious material" from the metal. The cleaning procedure presented here utilized self-assembly of decanethiol on an electrochemically roughened Ag foil for removal of surface contamination. The self-assembled layer was subsequently removed by rapid oxidation with ozone, followed by rinsing in methanol. The clean Ag surface exhibited a tremendously improved background and a 3-fold increase in the enhancement factor for a probe molecule.

Electronic Desorption of Surface Species Using Short-Pulse Lasers

New methods of removing surface contaminants from microelectronic and microelectromechanical systems (MEMS) devices are needed since the decreasing size of their components is reducing the allowable contamination levels. By choosing the pulse duration and fluence to optimize electronic rather than thermal desorption in short-pulse laser processing, surface species can be removed without exceeding maximum temperature constraints. A two-temperature model for short-pulse laser heating of, and subsequent desorption from, metal surfaces is presented. A scaling analysis indicates the material properties and laser parameters on which the ratio of electronic to thermal desorption depends. Regimes of predominantly electronic and thermal desorption are identified, and predicted desorption yields from gold films show that electronic desorption is the primary desorption mechanism in certain short-pulse laser processes.

Chemical alteration of the native oxide layer on LiGaO2(001) by exposure to hyperthermal atomic hydrogen

X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) have been used to examine the near-surface region and outermost atomic layer of air-exposed, solvent-cleaned, LiGaO2(001) substrates, respectively, before and after room-temperature exposures to the flux produced by a novel electron stimulated desorption hyperthermal H-atom source. The native oxide layer on the solvent-cleaned LiGaO2(001) substrate is nonhomogeneous and contains primarily LiO, Ga, and small amounts of C. Li is initially present in the near-surface region as Li2O, LiGaO2, and a small amount of LiOH. Several forms of O are present including adsorbed water, LiGaO2, Li, and Ga-oxides, and hydroxyl groups with Ga2O3 as the predominant species. Upon exposure to the hyperthermal H-atom flux, low-temperature removal of oxygen and carbon contaminants occurs, and the near-surface region approaches the stoichiometry of a clean LiGaO2(001) surface except for an increased O concentration. The usually difficult to observe Li 1s photoelectron peak increases in intensity significantly with H-atom exposure. XPS binding energies for LiGaO2(001) have been assigned for Li 1s, O 1s, Ga 3d, and 2p peaks at 56.0, 530.2, 19.4, and 1116.9 eV, respectively. According to the He+ ISS data, the H-atom flux is very effective in removing surface contamination and enriches the outermost atomic layer in O through diffusion of subsurface O to the surface under a chemically induced driving potential. This O enrichment implies that the diffusion of O to the surface is more rapid than removal of surface O by impinging H atoms.

Plasma cleaning and nitridation of sapphire substrates for AlxGa1−xN epitaxy as studied by x-ray photoelectron diffraction

The influence of plasma and thermal treatments on the structure and composition of sapphire (00.1) surfaces has been studied by hemispherically recorded x-ray photoelectron spectroscopy and diffraction (XPD) to better understand the surface conditioning during the growth initiation procedure for the epitaxy of GaN. A treatment involving an O2 plasma generated by electron cyclotron resonance is shown to efficiently remove surface contamination and to produce well-ordered surfaces. After a thermal treatment in vacuum the sapphire (00.1) surface becomes terminated by Al atoms. AlN films with good short-range order are obtained by a simple high temperature nitridation step in the metal organic chemical vapor deposition reactor. A novel direct crystallographic method termed CHRISDA is employed for the analysis of the XPD data. By analysis of the XPD patterns the crystal structure of sapphire is found to extend into the probed near surface region without significant perturbations. The AlN layers formed by thermal nitridation were found to crystallize in the hexagonal phase and to be terminated by nitrogen atoms.

Elemental Chromium by XPS

X-ray photoelectron spectroscopy (XPS) spectra of elemental chromium (Cr) are presented. The specimen is a polycrystalline solid and was analyzed after argon ion-sputtering to remove surface contamination. The spectra were collected with a Fisons Instruments ESCALab 220i-XL instrument using monochromatized Al Kα x-rays. Spectra include a survey scan and the Cr 2p, 2s, 3p, 3s, and valence band regions. Also included is the Cr L3M3,2M4,5 Auger transition.

Topographic and crystallographic characterization of a grazing-ion-bombarded GaAs(110) surface by time-of-flight ion-scattering spectrometry

We studied the topography and the atomic structure of a clean GaAs(110) surface by time-of-flight ion-scattering spectrometry (TOF-ISS). In a first series of measurements the surface was cleaned by standard cycles of ion bombardment and annealing (500 eV Ar${}^{+}$, 500$\ifmmode^\circ\else\textdegree\fi{}$C). This method was very efficient to remove surface contaminants but not to smooth out the damage produced in TOF-ISS experiments. A cleaning method consisting of grazing bombardment with 20 keV Ar${}^{+}$ combined with annealing at 500$\ifmmode^\circ\else\textdegree\fi{}$C resulted in a clear improvement of the surface flatness. This was confirmed by measurements of electron energy distributions recorded under grazing proton bombardment and by a topographical analysis with an atomic force microscope. The crystallographic structure of the grazing ion bombarded surface was then studied by TOF-ISS. The quasisingle backscattered intensity measured for 5 keV Ne${}^{+}$ presented strong variations with the incident and azimuthal angles which are consistent with the generally accepted relaxed GaAs(110) surface. From the comparison of critical angles measured and focusing regions calculated with a code recently developed we have obtained an As-Ga first interlayer spacing $\ensuremath{\Delta}Z=(0.66\ifmmode\pm\else\textpm\fi{}0.08)$\AA{}, and the spacings between the first and second As layers $\ensuremath{\Delta}{Z}_{1,2}^{\mathrm{As}}=(2.25\ifmmode\pm\else\textpm\fi{}0.08)$ \AA{} and between the first and second Ga layers $\ensuremath{\Delta}{Z}_{1,2}^{\mathrm{Ga}}=(1.57\ifmmode\pm\else\textpm\fi{}0.1)$ \AA{}.