High Filament Temperatures

The high filament temperature used causes additional radiative heating of the substrate [530, 531]. Feenstra et al. [531, 548] have developed a heat transport model of their setup (Figure 68). All heat exchange is assumed to occur via ra-... 

Li, D. M., Hemberg, R. and Mantyla, T. (1998), Diamond nucleation under high CH4 concentration and high filament temperature. Diam. Relat. Mater., 7(2-5) 188-192. 

The carrier gas must be flowing into the detector whenever the voltage is applied to the bridge current to avoid excessively high filament temperature. Some gas chromatographs may incorporate a pressure transducer to turn off the filament current if the carrier flow is intermpted. 

In case of excessive carbonization the rate of boiling should be increased or the filament temperature slightly lowered. In general the vapor velocity should be as high as possible without exceeding the capacity of the copper condenser. 

We use commercial Ti02 crystals (Pi-Kent) cut and polished to within 0.3° of the (110) face and we prepare them further with cycles of Ar + bombardment and U H V annealing to approximately 950-1100 K, typically 5-10 min for each cycle. The samples are mounted onto tantalum back-plates via strips of tantalum spot-welded to the back-plate. Annealing is performed by high-energy electron bombardment of the back-plate from a hot filament. Temperatures are measured from optical pyrometers (Minolta) focused on the back-plate. The temperatures are not measured directly from the samples because they are translucent and get darker with each sputter/anneal cycle. 

There are some disadvantages for HFCVD method. Filament is relatively easy to break, and its life time is around 100 h. Second, due to the high working temperature, the filament metal element can evaporate into the gas phase and deposit in the diamond film, which is a kind of contamination (Venter and Neethling 1994). Mehta Menon s group reported that tungsten filament yielded the lowest impurity level (few ppm by mass), whereas rhenium yielded the highest (parts per thousand) (Mehta Menon et al. 1999). Metal contamination in the diamond would affect the electronic application of diamond, even in ppm level, but this is not a big problem for the electrochemical application. 

Specific conductive silicon substrates have to be carefully prepared before use. For the diamond-deposition process, substrates have to be cleaned, seeded with diamond nanocrystalline seeds at high surface density, and then coated with a grown thick diamond film (from less than 1 pm up to several p,m) by hot filament chemical vapor deposition (HF-CVD). At Adamant, deposition processes are performed automatically in programmable controlled process units, which allow growing diamond on scale up to 0.5 m2. The process is performed under low pressure (1 < 0.1 bar) and high temperature (filament temperature 2,500°C and substrate temperature 800-1,000°C) with a gas mixture composed of CH4, H2 (CH4/H2 ratio <1%), and a boron source (typically trimethyl boron). 

Two methods for the evaporation of precursors may be employed - resistance heating and electron beam collision. The first method employs a simple alumina crucible that is heated by a W filament. Temperatures as high as 1,800°C may be reached inside the chamber, which is enough for some metals or metal salts to vaporize. Deposition rates for this method are 1-20 A s . The use of an electron beam to assist in the precursor evaporation results in temperatures on the order of 3,000°C, being more suited for the deposition of refractory metals/alloys and metal oxides such as alumina, titania, and zirconia. Since the temperature of the chamber interior is much higher than the walls, the gas-phase ions/atoms/molecules condense on the sidewalls as well as the substrate this may lead to film contamination as the nonselective coating flakes off the chamber walls. 

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