CVD Diamond Wires as X-Ray Detectors
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B WIRE
HOT
FILAMENTS MIRE HOHOT
8mm
HOT FIMSNT
2mm
T'r &ASGA
+ -GAS
FLOW
HOT R1AWENI SUJPPLY
Fig. 1 - HFCVD apparatus used for diamond wire fabrication (A) and the particular arrangement of the two hot filaments in order to obtain a uniform deposition (B)
F1ig. 2 - SEM micrograph of a cross section of a )on a W wire with a dii0mond deposition (20 jtm) on 1(100jim diameter at the origin
Two tantalum wires ( 0.25 mm diameter) are wound into helical shapes with about 2 mm turn diameter. The filaments were heated up to 2300°C as measured by an optical pyrometer (Minolta-Land Cyclops 152). The filaments were located 8 mm apart and 6 mm distant from the substrate which consists of a molybdenum wire (50 mm long and 0.3 mm diameter). In order to increase initial diamond nucleation rates, they were manually abraded with 0.25 gim diamond paste and then washed "inacetone. The temperature of the substrate wire, heated by radiation and conduction from the tantalum filaments, was not measured due to the difficulty in placing a thermocouple through the filament and was estimated to be about 950 0C. The tantalum filaments dissociated the gases allowing carbon to
deposit on to the surface of the wires in the form of a polycrystalline diamond film whose thickness, as evaluated by scanning electron microscopy, was about 10 gtm after 8 hours deposition. After deposition, and before contacts were applied, the diamond coated wire was annealed at 600TC in an argon environment and rinsed in acetone.
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A metallic thermal evaporation technique was used to coat 4 mm of CH 4 flow rate 2.5 sccm the diamond surface with successive metals: Ti (40 nm) and Au (100 nm). H2 flow rate 500 sccm The thickness of the metallic layers Pressure 15 mbar was measured during evaporation with Filament temperature 23000 C an in-situ thickness monitor. Titanium was used since it easily forms a carbide Substrate temperature (estimated) 950 0 C structure; gold was used to prevent Deposition Time 8 hours oxidation of the Ti layer. The sample was then rinsed in acetone and annealed at 600'C in a N2 environment Table I Growth parameter used for the deposition to allow the Ti to form a carbide with of diamond film diamond. The electronic properties of the Au/Ti-diamond-Mo structure were investigated by measuring the current-voltage characteristic by means of a mod.617 Keithley electrometer. Parameter
RESULTS AND DISCUSSION The CVD diamond films onto metal wires grow at a reasonable deposition rate (1-2 Itm/h) and, with the particular hot filaments arrangement adopted, quite uniformly. The best films, from the point of view of adhesion and thickness uniformity, are those deposited onto tungsten wires: an example is shown in Fig. 2. Tungsten, however, is relatively fragile, due to the strong carburization; and, moreover, the cleaning procedure before deposition is difficult to apply. This is to some extent disappointing, since Raman spectra indicate the best diamond quality for W deposition. Tantalum giving
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