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  • Muhammad Zakky Nurrachman posted an update 7 years, 8 months ago

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    Introduction
    The recent intense and focused search for alternatives
    to
    3
    He-based detectors has shown that detectors
    based on the boron isotope
    10
    B are feasible alterna-
    tives [
    1
    ,
    2
    ] both in their performance and economi-
    cally [
    3

    10
    ].
    10
    B has a relatively high neutron
    absorption cross section compared to
    3
    He, and by
    optimizing the design of the detector, a detection
    efficiency comparable to
    3
    He has been presented
    [
    3
    ,
    11
    ]. When a neutron is absorbed by a
    10
    B atom,
    there is a 94 % probability that the nuclear reaction
    10
    B
    ?
    n
    ?
    7
    Li (0.84 MeV)
    ?
    4
    He (1.47 MeV)
    ?
    c
    (0.48 MeV) takes place, otherwise
    10
    B
    ?
    n
    ?
    7
    Li
    (1.02 MeV)
    ?
    4
    He (1.78 MeV).
    For many
    10
    B-based detector applications,
    10
    B has
    to be deposited as ‘‘high-quality’’ thin films onto
    various substrate types. In this case, ‘‘high quality’’ is
    defined by neutron-hard, well-adhering films of
    thicknesses
    [
    1
    l
    m featuring low residual stresses,
    maximum amounts of the neutron absorbing element
    10
    B, and thus a minimum of unfavorable impurities
    like H, O, C, and N. The process must also be scalable
    to several hundred square meters of two-side coated
    substrates at affordable production prices. Natural
    boron contains 20 %
    10
    B, but the isotope separation is
    relatively easy and
    [
    95 %
    10
    B-enriched material is
    commercially available.
    In previous publications [
    3
    ,
    12
    ,
    13
    ], direct current
    magnetron sputtering (DCMS) was shown to provide
    suitable deposition processes for the production of
    10
    B
    4
    C large area neutron detectors.
    10
    B
    4
    C is the pre-
    ferred material, instead of
    10
    B,
    10
    BN, or other
    10
    B-
    containing compounds, due to its relatively high
    boron content in combination with excellent wear
    resistance and thermal and chemical stability [
    14

    17
    ].
    Additionally, the radiation hardness has recently
    been shown to be appropriate for these neutron
    detector applications [
    18
    ].
    Reference [
    12
    ] addresses adhesion issues that often
    arise for micrometer-range-thick B
    4
    C films due to
    high residual film stresses in combination with low
    adhesive forces between the B
    4
    C film and the sub-
    strate. Here, the film adhesion on Al substrates was
    reported to improve significantly as elevated sub-
    strate temperatures between 300 and 400
    °
    C are used.
    However, there is still a need for a well-working
    process at substrate temperatures below 200
    °
    C,
    allowing the deposition of adhering, high-quality
    10
    B
    4
    C coatings on temperature sensitive substrates.
    Additionally, the inherently poor step coverage of
    coatings deposited by DCMS on macrostructured
    (commonly grooved) Al blades [
    4
    ] needs further
    investigation. The poor step coverage arises due to
    the line-of-sight deposition nature of this technique
    [
    19
    ]. As was pointed out by Stefanescu et al.,
    10
    B
    4
    C
    coating thickness non-uniformity on such
    macrostructured blades may lead to detector effi-
    ciency losses of up to 10 % [
    20
    ].
    A possible solution to the above mentioned con-
    cerns, yet still using an industrial-scale magnetron
    sputtering process, may be high-power impulse
    magnetron sputtering (HiPIMS). In HiPIMS pro-
    cesses, the flux of ionized target material usually
    exceeds the flux of ionized working gas [
    21

    25
    ]. This
    implies not only benefits with regard to the film
    morphology and density, but also for the residual
    stress [
    26
    ,
    27
    ] and the step coverage [
    21
    ]. Although
    ion bombardment of the growing film has frequently
    been reported to yield high film stresses [
    28
    ], the
    comparison of films deposited by DCMS and HiPIMS
    showed significantly reduced stresses without sacri-
    ficing film hardness or density in case HiPIMS was
    used [
    29
    ]. Here, mainly the sputter gas and target
    material properties, i.e., mass and ionization poten-
    tials, together with appropriate bias voltage setting
    were found decisive.
    Therefore, this study explores DCMS and HiPIMS
    process parameters for the growth of B
    4
    C coatings on
    temperature-sensitive or macrostructured substrates.
    In order to put the quality of coatings deposited
    using HiPIMS or DCMS at low substrate temperature
    into perspective, their properties are compared to
    high-grade coatings deposited by DCMS at elevated
    substrate temperature. The aim is the deposition of
    uniform, high-quality B
    4
    C films onto Si and various
    Al substrates with a thickness of
    [
    1
    l
    m at low sub-
    strate temperatures without adhesion-enhancing
    interlayers in order to meet the requirements of dif-
    ferent
    10
    B-based neutron detector technologies.
    Experimental details
    B
    4
    C films were deposited in an industrial coating unit
    (CC800/9, CemeCon AG, Germany). A base pressure
    of less than 0.5 mPa was achieved prior to deposition.
    The depositions were carried out in DCMS and
    HiPIMS modes. All coating processes utilized two
    rectangular B
    4
    C compound targets with an area of
    J Mater Sci (2016) 51:10418–10428
    10419
    440 cm
    2
    . The B
    4
    C targets were mounted on two, each
    other facing cathodes and sputtered in Ar
    atmosphere.
    Prior to deposition, the sputter system was evacuated
    at full pumping speed for 2 h and the substrates were
    degassed at the intended deposition temperature. The
    depositionof the B
    4
    C filmswasconducted at75 % of the
    full pumping speed. The influences of the substrate
    temperature and deposition pressure on the B
    4
    Ccoat-
    ing properties grown in DCMS and HiPIMS modes
    were investigated. For our experiments, the deposition
    temperatures of 100 and 400
    °
    C were chosen. The
    working gas pressures were adjusted to 300, 450, 600,
    and 800 mPa by the Ar flow and kept constant
    throughout the deposition. In DCMS mode, a power of
    3500 W was applied to each cathode. In HiPIMS mode,
    the same average target power of 3500 W together with
    a pulse frequency of 700 Hz and a pulse width of 200
    l
    s
    was used. The pulse parameters yielded an energy per
    pulse (E
    pP
    ) of 5 Ws. No additional bias voltage was
    supplied tothe substratetable in bothdeposition modes
    in order to reduce residual coating stresses and to pro-
    vide well-adhering coatings. The floating potential was
    measured to be approximately

    40 V.
    Films with thicknesses between 1.4 and 1.9
    l
    m
    were grown onto Si(001) wafer pieces, on flat Al
    blades (alloy EN AW-5754) [
    30
    ], and on macrostruc-
    tured Al blades [
    20
    ]. The chosen substrates allow for
    various material analysis techniques and correspond
    to frequently used substrates in
    10
    B-based neutron
    detectors. All Al blades were mounted on a sample
    carousel with a 2-axis planetary rotation for 2-sided
    deposition. The Si wafer pieces were attached with
    stainless steel wires to the flat Al blades and mounted
    in a similar position as the flat Al substrates without
    Si. The macrostructured Al blades were mounted
    inside the deposition chamber so that the grooves
    were vertical and rotated around their primary axis
    using twofold rotation.
    Cross-sectional scanning electron microscopy
    (SEM, LEO 1550 Gemini, Zeiss, Germany) was car-
    ried out in order to determine the B
    4
    C thickness and
    hence the deposition rates of the sputter processes.
    The instrument, equipped with an in-lens detector,
    was operated at an acceleration voltage of 5 kV at a
    working distance of
    *
    3 mm.
    In order to study the thickness uniformity of B
    4
    C
    coatings on grooved Al blades, cross-sectional SEM
    was conducted. For sample preparation, the grooved
    Al blades were cut perpendicular to the grooves,
    embedded into Bakelite resin (Polyfast, Struers), and
    subsequently mirror polished. The above mentioned
    instrument settings were applied for SEM imaging.
    The composition and bonding states of the B
    4
    C
    films were examined by X-ray photoelectron spec-
    troscopy (Axis UltraDLD, Kratos Analytical, Manch-
    ester, UK) using monochromatic Al(K
    a
    ) X-ray
    radiation (h
    m
    =
    1486.6 eV). The base pressure in the
    analysis chamber during acquisition was less than
    1
    9
    10

    7
    Pa. The XPS survey spectrum and core-level
    spectra of the B 1s, Ar 2p, C 1s, and O 1s regions were
    recorded on the as-received samples and after Ar
    ?
    etching with a 4 keV Ar
    ?
    ion beam. In order to
    remove the surface oxide layer that is generated upon
    exposure to air, the Ar
    ?
    beam was rastered over an
    area of 3
    9
    3mm
    2
    at an incidence angle of 20
    °
    .
    Automatic charge compensation was applied
    throughout the acquisition. After subtraction of a
    Shirley-type background, the compositions were
    extracted from the core-level spectra obtained from
    sputter cleaned samples applying elemental cross
    sections provided by Kratos Analytical.
    Isotope-specific compositional analysis was per-
    formed with time-of-flight elastic recoil detection
    analysis (ToF-ERDA), using a 36 MeV
    127
    I
    9
    ?
    beam at
    66
    °
    incidence and 45
    °
    recoil scattering angle. The
    recoil energy of each element was converted to rela-
    tive elemental depth profiles using the CONTES code
    [
    31
    ].
    The residual stresses in the films were determined
    by the wafer curvature method assessed by X-ray
    diffraction (XRD, PANalytical Empyrean) [
    32
    ]. The
    diffractometer, equipped with a Cu K
    a
    1 source, was
    operated at 45 kV and 40 mA. The Stoney formula for
    anisotropic single crystal Si(001) was used to extract
    residual coating stress from the measured substrate
    curvature. Here, uniform plane stress in the film was
    assumed [
    33
    ]. The same instrument was chosen to
    study the film density by X-ray reflectivity (XRR).
    The density was evaluated using the PANalytical
    X’Pert reflectivity software. Here, a 3-layer model,
    resembling the substrate, the B
    4
    C films, and a surface
    oxide layer, was applied.