Neutron Depth Profiling Research Articles

Molten fluoride salts incorporation into pristine and ion-modified carbon allotropes and metallic foils

Incorporation of molten fluoride salts into different carbon allotropes (glassy carbon, pyrolytic graphite etc.) and metallic foils (Ni, Ti, etc.), pristine and ion- treated substances, has been studied using non-destructive, depth sensitive nuclear analytical methods—Neutron Depth Profiling (NDP) and Rutherford Backscattering (RBS). Strong interaction between the molten LiF and LiF+KF+NaF salts and the tested materials was found. The results are of great interest for accelerator driven transmutation technology (ADTT) which is a promising way towards effective liquidation of nuclear wastes.

High fluence boron implantation into polyimide

100 keV B + ions are implanted at high fluences into polyimide and the boron depth distributions are measured by the neutron depth profiling technique. Subsequently the implanted samples are annealed isochronally to determine the diffusional, trapping and detrapping behaviour of the boron atoms. The boron depth profiles of as-implanted samples differ significantly from those predicted by TRIM code. Pronounced inward and outward profile tails point at increased mobility and redistribution of boron atoms after implantation. Thermal annealing to the temperatures below 150°C does not change the total boron content in 1 μm thick surface layer and the boron depth profiles as well. For higher annealing temperatures a continuous desorption and significant redistribution of boron atoms is observed.

Study of LiOH etching of polyethyleneterephtalate irradiated with 11.4 MeV/amu Pb ions by neutron depth profiling and alpha particle transmission

Polyethyleneterephtalate (PETP) foils, 23 μm thick, irradiated with 11.4 MeV/amu Pb ions to the fluence of about 1×107 cm−2 were etched in 5M LiOH solution at the temperature of 40°C for 30–570 min and the etching process kinetics was examined by combined alpha particle transmission (APT) and neutron depth profiling (NDP) techniques. The etching process was visualized from very initial stages up to the breakthrough and the appearance of first openings after about 300 min of etching. Several parameters characterizing the etching process were determined and the pore internal profile was determined by comparing the measured APT spectra with those simulated by Monte-Carlo method.

Doping of ion irradiated poly(aryl ether ether ketone) from water solution of LiCl

Poly(aryl ether ether ketone) (PEEK) films were irradiated with 2 MeV O + ions to fluences up to 6×10 14cm −2 and then doped from a 5 molar water solution of LiCl. The depth distribution of the incorporated LiCl was investigated using neutron depth profiling, and electron paramagnetic resonance was used for the determination of the concentration of free radicals. The structural changes of the modified PEEK were examined by IR spectroscopy. Measurement of the sheet resistivity and its temperature dependence gave information on the electrical properties of the modified PEEK. The results show that the ion irradiation enhances the penetration and incorporation of the LiCl dopant in the radiation-damaged PEEK surface layer. The Li atoms are chemically bound to free radicals created by radiation-induced degradation of PEEK macromolecules. The ion implantation leads to an increase in the PEEK conductance, which is further elevated by the LiCl doping. The conductance enhancement is partly a variable-range hopping mechanism and also electron hopping due to bound Li + ions.

Pulse-shape discrimination in neutron depth profiling technique

Pulse-shape discrimination (PSD) is used for the first time for reduction of unwanted background in analyses of solid surfaces by neutron depth profiling method (NDP) based on the detection of charged particles from the (n, p) and (n, α) nuclear reactions induced by thermal neutrons on some light elements. The experimental PSD arrangement is described and its performance is demonstrated on the measurement of real sample. Background reduction by about two orders of magnitude in the energy region below 1 MeV leads to sensitivity improvement by about one order of magnitude and to extension of measurable depth region for some of light elements like N and Cl.

Plasma processing of in a hydrogen/oxygen radio-frequency discharge

Monocrystalline was processed in a radio-frequency (27.12 MHz) discharge in hydrogen and subsequently in oxygen and the composition of the thus produced surface layers evaluated. The hydrogen plasma treatment caused a heavy blackening of the sample due to chemical reduction. At low pressures ( Torr) the niobate structure on the sample surface was completely destroyed; only niobium and its oxides remained on the surface. At higher pressures ( Torr) the niobate structure was preserved. Hydrogenation of the surface layer was found which persisted up to 500C. Subsequent treatment in an oxygen plasma under similar processing conditions brought about the restoration of the transparency of the sample. The following methods were used for the modified surface structure analysis: neutron depth profiling, Rutherford backscattering, x-ray photoelectron spectroscopy and VIS and IR spectrometry.

Damage profile examination on ion irradiated peek by 6Li doping and neutron depth profiling technique

Depth structure of radiation damaged surface layer of poly(aryl–ether–ether ketone) (PEEK) a polymer was studied using doping with 6Li atoms combined with nondestructive neutron depth profiling (NDP) method. The PEEK foils were irradiated with 2 MeV O + ions up to a fluence of 6 × 10 14 ions/cm 2. The damage profiles in the samples were visualized by doping of the samples with 5 M LiCl water solution at room temperature (RT) for 22.5 h. The Li ions are trapped on ion-produced radiation defects and the Li depth profiles are determined by the NDP method. NDP experiments were performed before and after leaching of excess of lithium atoms from the samples in distilled water at RT for 2 h. The leaching leads to dramatic changes in the Li depth distribution which, at low ion fluences, is similar in shape to the electronic energy loss profile of 2 MeV O + ions. For the higher fluences double-peaked profile occurs, which indicates a competition between different degradation processes in ion irradiated polymer.

LiNbO 3 exposed to radio-frequency plasma

Radio-frequency (27.12 MHz) glow discharge in H 2 and O, have been used for surface modification of LiNbO 3 wafers. The properties of plasma modified surface layers were dependent on the gas pressure and radio frequency (RF) input power. At pressures below 1 Torr of H 2 and input powers of 1–10 W/cm 3 , a low-resistance layer was created with a sheet resistivity of 2.6 mΩ cm. At pressures of several Torrs, an Li density drop up to a depth of 1 μm appeared, accompanied by OH − radical formation. The sample processing in oxygen plasma was aimed at restoring light transparency of the samples, which was lost during hydrogen processing. The restoration is attainable in a short time and at much lower sample temperature than under thermal bleaching. Processing under certain plasma parameters decreased the density of point defects in the material. The methods used for characterizing the plasma-modified layers were neutron depth profiling (NDP) and Rutherford backscattering (RBS), X-ray induced photoelectron spectroscopy (XPS), X-ray diffraction, visible and infrared transmission and reflection spectroscopy. The paper summarizes the results of a four year study and suggests possible applications of the findings.

Neutron depth profiling study of lithium niobate optical waveguides

The relation between optical properties and the structure of proton exchanged and annealed proton exchanged optical waveguides in lithium niobate was studied using the mode spectroscopy and neutron depth profiling methods. We have found a close correlation between the lithium depletion and the depth profile of the extraordinary refractive index. The form of the observed dependence between Li depletion and refractive index depends on the fabrication procedure by which the waveguide was prepared but it is highly reproducible for specimens prepared by the same procedure.

RBS measurement of depth profiles of erbium incorporated into lithium niobate for optical amplifier applications

Rutherford Backscattering Spectrometry (RBS) was used for the determination of Er 3+ concentration profiles in locally doped lithium niobate. The doped layers are the basic substrates for the fabrication of optical waveguiding structures which may be utilized as planar optical amplifiers and waveguiding lasers making use of the 4I 13/2 → 4I 15/2 transition in Er 3+, which falls into the third low loss telecommunication window (1.5 μm). We present a new aproach of fabrication of locally doped lithium niobate single crystal wafers. The doping occurs under moderate temperature (∼350°C) from reaction melts containing ca. 10 wt% of erbium nitrate. The erbium content in particular cuts varies dramatically between ca. 3 at.% in the Y- and Z-cut up to 20 at.% in the X-cuts. Erbium ions are localized in a 50 nm thick layer, but they can be diffused deeper into the substrate by subsequent annealing at 350°C. The Er concentrations of the samples doped at moderated temperature are compared with the Er concentrations of the samples doped by a standard high-temperature diffusion (>1000°C) from evaporated metal layers. To utilize the Er doped substrates in integrated optic circuits high quality waveguides must be subsequently fabricated. For that we used the Annealed Proton Exchange (APE) method with adipic acid. For the actual fabrication of the waveguides the following order of operation should be kept: the erbium doping should be done before the APE because the substantially changed structure of APE layers prevents the doping process. The APE process is checked by measurements of lithium depth profiles by Neutron Depth Profiling (NDP).

Prompt nuclear analytical techniques for material research in accelerator driven transmutation technologies: Prospects and quantitative analyses

Accelerator driven transmutation technology (ADTT) is a promissing way toward liquidation of spent nuclear fuel, nuclear wastes and weapon grade Pu. The ADTT facility comprises a high current (proton) accelerator supplying a subcritical reactor assembly with spallation neutrons. The reactor part is supposed to be cooled by molten fluorides or metals which serve, at the same time, as a carrier of nuclear fuel. Assumed high working temperature (400–600°C) and high radiation load in the subcritical reactor and spallation neutron source put forward the problem of optimal choice of ADTT construction materials, especially from the point of their radiation and corrosion resistance when in contact with liquid working media. The use of prompt nuclear analytical techniques in ADTT related material research is considered and examples of preliminary analytical results obtained using neutron depth profiling method are shown for illustration.

Nickel aluminide thin film fabrication via ion beam sputtering of compound targets

We report recent results from our use of Kaufman-type ion beam sputtering to fabricate boron-doped nickel aluminide thin films on substrates of Si, NaCl, and polycarbonate resin plastic. Compound targets of boron-doped Ni 3Al were bombarded with ≈1 keV Ar ions to form films having thicknesses up to 2000 Å and areas of approximately 2.5 cm × 2.5 cm. Films with Ni/Al ratios similar to the target have been produced on NaCl and Si as determined by instrumental neutron activation analysis. Grazing incidence X-ray diffraction scans show the (111) plane of Ni 3Al for films grown on NaCl or Si substrates. Monte Carlo simulations of the compound target sputtering process, taking into account primary ion energy, target surface binding energy, beam-target angle and anisotropy of sputtered species, are compared with film composition as determined by INAA. Results from these measurements comprise complementary data for investigating boron distributions in the films via neutron depth profiling.

Depth distribution of boron determined by slow neutron induced lithium ion emission

Neutron Depth Profiling (NDP) has been established as a non-destructive technique to determine the near surface distribution of light elements, particularly boron. By analyzing the residual energy spectrum of the emitted particles of known initial energy as a result of nuclear capture within the target material, information about the site and amount of the reactions can be deduced. In the event of 10B neutron capture, an alpha particle (1473 keV) and an excited 7Li ion (840 keV) are emitted, both conveying the same information. However, because the Li ion has a greater charge, the stopping power in a given matrix is higher than that for the alpha particle. Consequently, for boron near the surface, the location of the origin of the emission can be determined with better depth resolution. At the NIST NDP facility, routine analysis using the alpha particle has been established earlier. This paper reports the progress of using the 7Li ion stopping power to determine the boron depth distribution in the near surface of several matrices. This study has been performed on semiconductor device-related systems – boron in silicon glass, and carbon matrices. Various factors affecting the depth resolution are assessed when comparing the analysis of the alpha particle with that of the 7Li ion.

Development of a Titanium Nitride Thin Film Standard

ABSTRACTTitanium nitride films are used in a variety of applications in the semiconductor industry, but the stoichiometry and homogeneity of the films can be difficult to control and characterize. The National Institute of Standards and Technology (NIST) is developing a TiN thin film on silicon to be used as a physical and chemical standard for the semiconductor industry. A prototype 100 nm thick film has been manufactured by dual ion beam deposition. NIST is characterizing the film by x-ray interferometry for thickness and density, neutron depth profiling for nitrogen content and homogeneity as a function of depth, neutron activation analysis for titanium content, and x-ray fluorescence spectromety, electron probe microanalysis, grazing incidence x-ray photoelectron spectroscopy, and transmission electron microscopy for homogeneity and composition. The intent of this research is to create a TiN film of known thickness, density, stoichiometry, and homogeneity that can be used by the semiconductor industry as a primary, traceable standard for characterization of industry thin films. The intent of the standard is to promote more accurate characterization and improved TiN manufacturing capabilities as well as to aid manufactures in more easily employing ISO 9000 related traceability and quality assurance practices.

High fluence boron implantation into polymers

100 keV B+ ions are implanted at high fluence into three polymers of technological importance and into a polymeric mixture, respectively. The boron depth distributions are measured by the neutron depth profiling technique. It is shown that the boron atoms redistribute after their implantation according to the nuclear (collisional) energy transfer distribution. This contrasts to low fluence implantation, where the boron atoms redistribute according to their electronic energy transfer distributions. Subsequently, the samples are annealed isochronally. The change of the boron depth profiles with annealing temperature is then evaluated to determine the diffusional, trapping and detrapping behavior of the boron atoms. At, or slightly above room temperature, intrinsic boron impurities of the examined polymer foils become mobile and getter in the ion-implanted region. At higher temperatures, the thermal desorption spectra show a nearly continuous desorption of both the implanted and gettered boron, with no pronounced desorption peaks. Due to the high polymeric destruction yield, the different polymers show little difference in their desorption behavior.

Neutron Depth Profiling with the New NIST Cold Neutron Source

Neutron depth profiling (NDP) is a method of near-surface analysis for isotopes that undergo neutron-induced positive Q-value charged particle reactions, e.g. (n, α) and (n, p). The method is based on measuring the energy loss of the charged particles as they exit the specimen. Depending on the material under study, depths of up to 10 μm can be profiled and depth resolutions of the order of 15 nm can be obtained. The most studied analytes via NDP at the National Institute of Standards and Technology (NIST) are boron, lithium and nitrogen. These analytes have been determined in a variety of matrices. The NIST research reactor has recently undergone a major upgrade, including the addition of a new liquid-hydrogen cold source, which reaches a lower effective temperature and more fully illuminates the neutron guides. The cold neutron depth profiling (CNDP) facility has been placed on a curved super-mirror guide. Because of the guide curvature, the NDP chamber does not directly view the reactor and its associated fast neutrons and gamma rays, and therefore a 13.5 cm thick sapphire filter used previously in front of the beam port is no longer necessary. The increase in the neutron fluence rate at the end of the guide leads to an improvement in detection limit of more than a factor of four over the previous depth profiling instrument. The reduction in the gamma-ray background in the chamber will improve our ability to measure elements such as nitrogen. © 1997 by John Wiley & Sons, Ltd.

Neutron Depth Profiling with the New NIST Cold Neutron Source

Neutron depth profiling (NDP) is a method of near-surface analysis for isotopes that undergo neutron-induced positive Q-value charged particle reactions, e.g. (n, α) and (n, p). The method is based on measuring the energy loss of the charged particles as they exit the specimen. Depending on the material under study, depths of up to 10 μm can be profiled and depth resolutions of the order of 15 nm can be obtained. The most studied analytes via NDP at the National Institute of Standards and Technology (NIST) are boron, lithium and nitrogen. These analytes have been determined in a variety of matrices. The NIST research reactor has recently undergone a major upgrade, including the addition of a new liquid-hydrogen cold source, which reaches a lower effective temperature and more fully illuminates the neutron guides. The cold neutron depth profiling (CNDP) facility has been placed on a curved super-mirror guide. Because of the guide curvature, the NDP chamber does not directly view the reactor and its associated fast neutrons and gamma rays, and therefore a 13.5 cm thick sapphire filter used previously in front of the beam port is no longer necessary. The increase in the neutron fluence rate at the end of the guide leads to an improvement in detection limit of more than a factor of four over the previous depth profiling instrument. The reduction in the gamma-ray background in the chamber will improve our ability to measure elements such as nitrogen. © 1997 by John Wiley & Sons, Ltd.

Neutron depth profiling applications at The University of Texas research reactor

The neutron depth profiling (NDP) technique has become an increasingly important method to nondestructively measure the absolute concentration versus depth of various elements in substrates. A permanent NDP facility is operational at a tangential beam port of the 1-MW TRIGA Mark II research reactor at The University of Texas at Austin (UT). This facility was developed to perform materials research, specifically measurements of interest to the microelectronics industry. Applications of the UT-NDP facility include measurements of boron-10 profiles in borophosphosilicate glass samples and helium-3 depth profiles of implanted helium-3 in metals, alloys and amorphous materials. A study is underway to determine radiation damage and microstructural changes in stainless steel samples by helium irradiation using NDP and Transmission Electron Microscopy. Another study, currently planned, is to measure surface wear by measuring the depth profiles of implanted beryllium-7 and sodium-22 in various materials.

Doping of Ion Irradiated Polyethylenterephtalate from Water Solution of LiCl

Polyethylenterephtalate foils (10 μm thick with the density of ρ = 1.3 g cm−3) were irradiated with 150 keV Ar+ ions to fluences from 5×1011 to 1×1015 cm−2 and one year after the irradiation they were exposed to a 5 M water solution of LiCl at the boiling point for times ranging from 15 s up to 8 h. The depth profiles of incorporated Li atoms as a function of the ion fluence and the doping time were determined using the neutron depth profiling technique based on the 6Li(nth, α)3H nuclear reaction. The Li content in the 600 nm thick surface layer achieves saturation very rapidly, already after 15 s doping time, and it exhibits a local, pronounced maximum at 2 or 4 h doping times for the specimens irradiated to fluences below and above 5×1014 cm−2, respectively. The concentration depth profiles of incorporated Li atoms consist of a pronounced surface component, obviously connected with radiation damages created by the ion irradiation and a long inward tail which is due to regular diffusion in pristine polymer. As a function of ion fluence, the Li content increases up to the fluence of 5×1013 cm−2 and then declines in most cases. The surface component of the Li depth profiles changes dramatically with increasing ion fluence from bell-shaped ones for fluences below 5×1014 cm×2 to those characterized by a depleted surface layer and a rather sharp concentration maximum at depths significantly exceeding the calculated ion projected range.

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a58Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the γ-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).