Thin Film Targets Research Articles

Measurement of the efficiency of solid state detectors using atomic-field bremsstrahlung

We report on the current status of our efforts to use the atomic-field bremsstrahlung spectrum from the bombardment of thin-film targets by an electron beam to measure the photon energy dependence of the efficiency of HPGe and Si(Li) detectors. Results are presented for an HPGe detector from 15 to 100 keV and for an Si(Li) detector from 2 to 40 keV. Points on the efficiency curve are also obtained using calibrated radioactive sources for comparison and for absolute normalization. Problems associated with the background in bremsstrahlung measurements are also discussed.

Inelastic scattering of 120 keV electrons from carbon, aluminum, vanadium and silver foils

The inelastic scattering of 120 keV electrons from thin-film targets of carbon, aluminum, vanadium, and silver has been measured at 120° and 135°. The number of inelastically scattered electrons at each angle has been normalized to the elastically scattered electrons and the difference of the spectra at the two angles has been computed to correct for backscattering in the Si detector. The results are compared with a double scattering process of ionization followed by elastic scattering in the target. Significantly more backscattering is observed than can be explained by either the single scattering processes of ionization or bremsstrahlung or by the double process considered. Additional double processes need to be considered to resolve the discrepancy.

Thin films for rewritable optical recording by use of photoconductive effect.

A TV camera tube equipped with a photoconductive thin-film target is, in a sense, a two-dimensional rewritable optical memory. To apply the target of a camera tube to memory devices, however, its storage time, that is, time constant, must be sufficiently long to store optical information. This is, in general, possible by optimizing the composition and structure of photoconductive targets.In this article, various kinds of photoconductive materials are reviewed in detail. To realize optical recording media, it is advantageous to combine a photoconductive film with another material. Various combinations of photoconductor and insulator, elastomer, thermoplastics, electro-optical single crystal, and ferroelectric ceramics are also discussed.

A test of two depth profiling techniques using PIXE

A destructive and a nondestructive technique for depth profiling using PIXE is tested on various concentration profiles of Zn depletion in initially homogeneous Ag 3 at.% Zn alloys. The first consists of measuring the yield of X-rays emerging from thin film targets prepared by deposition of Ag and Zn hydroxides originating from slabs of controlled thickness removed from the analysed sample. The second consists of deconvoluting a set of X-ray yield measurements carried out with various energies. Both techniques are cross-referenced with electron microprobe analysis run on a transverse section of the corresponding specimens, but can also be applied to profiles extending over depths too small for analysis on a transverse section. Internal coherence between the different concentration profiles obtained is checked on the basis of Zn diffusivity in Ag-Zn alloys.

Monte Carlo calculations of keV electron and positron slowing down in solids. II

A Monte-Carlo simulation technique based on the screened Rutherford differential cross section for the elastic scattering and Gryzinski's semiempirical expression for the inelastic core and valence electron excitation is used to describe electrons and positrons slowing down in solids. The theoretical results are compared with the experimental backscattering, absorption and transmission results for aluminum, silicon, copper, and gold thin film and semi-infinite targets and good agreement is observed. The simulated stopping profiles are fitted with a simple analytic expression. The profiles are Laplace-transformed to give a useful data base for analyzing phenomena associated with slow positron re-emission from solids.

Sputtering and secondary ion yields of TiAl alloys subjected to oxygen ion bombardment

Total sputtering and secondary ion yields of single phase TiAl alloys containing 0–35 at% Al under oxygen ion bombardment were measured for thin film targets with known thicknesses and compositions. It was found that the total sputtering yield initially decreased with increasing Al content and became constant beyond 20 at% Al. On the other hand, the secondary ion yield of Al roughly increases with increasing Al content, while that of Ti shows a complementary decrease. Their variations, however, show a large flucutation with Al content. Detailed observations revealed that the secondary O 2 ion yield also exhibited the same pattern of fluctuation as that of Al and Ti. The modified degrees of ionization of both Al and Ti in the consideration for the secondary oxygen ion yield indicated an exponential dependence upon the alloy composition. The trends of the modified degrees of ionization revealed that the matrix effect due to the alloy composition affected more strongly on Ti than on Al.

Ion implantation and optical devices

Ion beams can be used to change many surface properties, such as chemical stability, stress, electrical conductivity, energy gap, defect energy states or refractive index. The depth scale accessible with small ion implanters (i.e. 0–400 kV) is a few microns and therefore modification of thin films or controlled etching is possible. For optical components in integrated optic circuits ion beams can be used to define mask patterns, etch surface features, develop planar lenses and write optical waveguides. Many of the techniques are identical to those applied to semiconductors. However, properties of specifically optical interest are the refractive index, electro-optic coefficient and optical absorption. Localized index changes can be introduced by the addition of specific impurity ions of selected polarizability or by changes in lattice density from energy deposited from the ion beam. In glass systems the relaxation of the network may enhance or reduce the index, depending on the details of the structure and the implanted impurity. For crystalline solids radiation damage dominates and the trend is to form lower index amorphous systems. The optical absorption associated with the radiation damage may be annealed without loss of control of the index. Alternatively the colour centres may be utilized for luminescence or photoconductive properties. Electro-optic properties are generally degraded by the original implantation and optically active elements are best defined by implantation in the regions adjoining the waveguides. Several simple electro-optic components have been formed by ion implantation and these will be discussed. The development of complex devices and the full exploitation of ion beams with thin film targets for optical systems is still in its infancy, but the great potential of these combined techniques is already apparent.

Spatial resolution limits in electron beam nanolithography

Computer simulation of high energy primary electron scattering and subsequent generation of ‘‘fast’’ secondary electrons in thin film targets is demonstrated with Monte Carlo techniques. A hybrid Monte Carlo model is utilized to calculate the generation and subsequent spatial trajectory of each secondary electron in the target. The three-dimensional spatial distribution of energy dissipation by such fast secondary electrons is shown to be the fundamental resolution limit for electron beam lithography with high voltage beams (100 keV) and thin film polymer targets. The dependence of resolution on beam voltage and film thickness is presented, and quantitative comparison is made between these new Monte Carlo calculations and the limited amount of experimental data available in the scientific literature.

The preparation of 235UIr 2 targets for use in g-factor measurements of fission isomers

The cubic, nonparamagnetic alloy, 235UIr 2, was found to be a good host for the preservation of nuclear alignment in the g-factor measurement of the τ = 122(10) ns fission isomer of 237Pu. This paper discusses, in detail, the preparation of the thin-film targets used in this measurement. These techniques were found to apply to other similar actinide alloys such as NpRu 2.

Use of Bremsstrahlung in the Measurement of the Efficiency of Solid State Detectors below 5 keV

Atomic-field bremsstrahlung produced by the bombardment of atoms by an electron beam of 5-10 keV energy provides a continuous x-ray source which is suitable for the measurement of the relative efficiency of solid state detectors in the energy region below 5 keV. The relative efficiency as a function of photon energy can be obtained by normalizing the observed spectrum to the theoretical bremsstrahlung cross section. In this paper we will present an example of this technique using a thin film target and a gas beam target to illustrate some of the problems and to demonstrate the potential of this technique.

Correlation of Spontaneous Magnetic Field with Net Energy Absorbed in Laser-Plasmas Produced from Thin Film Target

Experimental as well as computational studies have been performed on spontaneously generated magnetic-field and net absorbed laser energy bylaser-produced plasmas where one ruby-laser beam with relatively lower power level is focused onto one-side of an aluminum foil target with various thickness. It is found that these two physical quantities are experimentally in good correlation and the computational results based upon a simplified fluid-dynamic model explain this correlation.

Preparation of Nuclear Accelerator Targets by Vacuum Evaporation

In many cases an accelerator target for physical or chemical research work has to be prepared in form of a uniform, as far as possible self supporting thin film of thickness 0.1-10 ?m. Typical target materials are stable or radioactive isotopes for nuclear physics and nuclear chemistry experiments, mixtures of different elements for material analysis with ion beams as well as single crystal samples for special solid state physics applications. A powerful method to prepare thin film nuclear accelerator targets is the procedure of vacuum evaporation-condensation. In the present paper the physical and technological background of this method is discussed and the construction of suitable evaporation systems and related equipment is displayed. Different sublimation techniques including resistance-, induction- and electron beam heating as well as ion beam sputtering are described considering their specific benefits and disadvantages. Particular attention is paid to the collection efficiency of these various techniques in view of the low abundance and high cost of isotopically enriched target material. Since in many cases a target backing severely disturbs the actual measurement, emphasis is also given to procedures yielding unsupported high purity targets. In addition, special techniques for preparation, handling and storage of chemical reactive targets and of radioactive targets are discussed. Finally the preparation of single crystal thin film targets is briefly mentioned as well as target preparation by chemical vapour deposition, which in a physical sense is closely related to the vacuum evaporation-condensation method.

Impact-parameter dependence of L and M vacancies induced in Th by H and O ions

Scattered beam, X-ray coincidence studies have been made for L and M X-rays induced by 2 MeV protons and 60 MeV 16O ions in thin-film targets of thorium. Coincidences were established by the output of a time-to-amplitude converter started and stopped respectively by X-ray and particle pulses, and for each event the three pulse amplitudes were recorded. The L X-ray spectrum was separable into L 1, L α, L β and L γ groups. The data were compared to predictions based on BEA and SCA models. Satisfactory agreement with SCA is found, except for M X-rays at small impact parameters (<0.2 pm), for which the yield becomes anomalously large. This effect is similar to that observed elsewhere in K-vacancy production.

A simple ion source for target preparation via ion beam sputtering

A simple glow discharge ion source has been developed and used to prepare thin-film nuclear targets of W, Pt, Mo, Zn, Ti and Si via ion beam sputtering with good efficiency for separated isotopes.

Polycrystalline film target texture and nuclear backscattering analysis

The influence of target texture on the backscattering yield of 2 MeV 4He particles has been investigated for evaported thin-film targets of Nb, Ag, Au and Bi. The results have been correlated with those from X-ray diffraction. The implications of the texture found for nuclear-backscattering analysis are discussed, and it is suggested that backscattering may constitute a fast, quantitative method for investigations of texture in thin foils.

Stopping cross section measurements with thin supported films

Elemental stopping cross sections, ϵ, can be determined by Rutherford backscattering either with thin-film or bulk targets. Both methods rely on approximations, which are met well for heavy elements. We present a modification of thin-film method which eliminates the need for approximations, but at the cost of precision. Results obtained for both light and heavy elements agree with those obtained by traditional methods. For Al, ϵ values are obtained which deviate from most published results, but agree with previously unexplained AlSi and AlAl 2O 3 yield ratios.

Absolute electron impactK-ionization cross sections of titanium and nickel (?50 keV)

The absoluteK-ionization cross sections of Ti and Ni by electron impact (impact energy ≦50 keV) was measured detecting the X-rays emitted by thin solid film targets of known mass thickness with a flow proportional counter. The experimental method, especially the correction procedures and the measurements are described, the results compared with calculations in different theoretical approaches. For impact energies aboveE0/EK>1.5 (EK=K-shell ionization energy) a systematic deviation of about +20% occurs in comparison with the best agreeing calculations of M.R.H. Rudge and S.B. Schwartz. A fit to Drawin's empirical formula reveals that the measurements are approximated better than ±10 % within the range of comparison.

Depth-profiling of Cu-Ni sandwich samples by secondary ion mass spectrometry

SIMS depth profiles of copper-nickel thin film targets were measured with argon and nitrogen primary ions. While pronounced cone formation is observed in case of argon irradiation the erosion is much more uniform with nitrogen projectiles, probably due to formation of nitride surface layers.

Characteristic X-ray excitation in thin films by 4–50 keV electron bombardment

X-ray excitation by electron bombardment has been used to analyse thin films, the main objective being non-destructive thickness determination. In a specially designed, but otherwise conventional, electron probe the thin film targets were irradiated with 4–50 keV electrons. Al, Mg, Cu and Zn films evaporated and/or sputtered onto aluminium and copper substrates were measured and the influence of the excitation energy on the sensitivity was studied in the thickness range from 20 to 5000 Å. Typical sigmoid-shaped calibration curves are found, provided X-ray excitation by the primary electrons is the dominating process. Good agreement can then be expected with calculations based on the depth distribution of characteristic X-ray generation. This is not true, however, of film-substrate systems in which X-ray excitation by electrons backscattered from the substrate becomes comparable with direct excitation ( e.g. Al on Cu). Artificial enhancement of the ionization density by thin surface layers is demonstrated and applications of this effect are discussed with particular reference to energy-dispersive measurements.

Determination of surface energy of solids and liquids by means of an ultrasonic method : Wawra, H.H. 14 (December 1972) 413

The energy loss of iodine in gold was investigated at energies below the Bragg peak. With the present work, the range of the available experimental data is extended to significantly lower energies, while the nuclear stopping power contribution is taken into account. Experiments were performed on thin film targets in reflection geometry. Electronic stopping cross sections were extracted by different approaches from experimental spectra. The obtained results have been compared to tabulated values from SRIM and previously published experimental data, where available. For all energies the obtained values are consistently higher than predicted by SRIM. Monte Carlo simulations (TRIM) have been performed to study path length distributions and the influence of nuclear stopping on the total energy loss. The results from the calculations indicate that the experimental geometry might be an important factor affecting the observed energy loss due to a selection of specific trajectories.