An improved method of plasma chemical-vapor deposition (CVD) of a thin film of boron carbide has been devised. Boron carbide is useful because it is hard, is electrically insulating, withstands high temperature, and resists chemical attack. Plasma CVD of boron carbide involves the thermal dissociation of feed gases BCl3, CH4, and H2. Prior to the development of the improved method, it had been found that in plasma CVD, the rate of growth is enhanced when the deposition substrate is biased with a positive dc voltage, which gives rise to a secondary discharge. However, the applied dc voltage must be limited (1) so that the current drawn by the secondary discharge does not exceed the capacity of the bias power supply, and/or (2) the current drawn is not so large as to adversely affect the boron carbide deposit.
In the improved method, the bias potential is pulsed instead of steady. Taking advantage of the ability of a pulse circuit to sustain high voltage and the associated high current for a short time, pulses of bias potential higher than the limiting dc bias potential can be applied to the deposition substrate. The pulse-repetition frequency and duty cycle can be chosen so that for a given high potential, the time-averaged secondary-discharge current does not exceed the capability of the power supply or is not so high as to harm the deposit. For a given pulse-repetition frequency, one can trade applied potential versus duty cycle at the limiting time-averaged current; in other words, one can choose a potential as high as desired, as long as one makes the duty cycle short enough.
The beneficial effect of increasing the applied potential is attributed to a consequent increase in electron temperature. The higher the electron temperature, the greater the rate of dissociation of feed-gas molecules by impact of electrons.
During CVD of boron carbide and some other hard materials, part of the momentum of impinging ions becomes converted to internal stresses, which accumulate as the deposits grow. The buildup of internal stresses limits the maximum desirable thickness of a deposit in the sense that if one attempts to make the deposit thicker, the internal stresses are released through the formation of cracks. In the improved method, the peak potential, pulse-repetition frequency, and duty cycle can be regarded as process parameters that can be adjusted to prevent the formation of cracks: The magnitude of the bias defines the momentum and kinetic energy of the impinging ions, whereas the duty cycle and the pulse-repetition frequency define the time during which the deposited material can release some of its internal stresses.
This work was done by Joachim V. R. Heberlein and Olivier B. Postel of the University of Minnesota for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tspunder the Materials category.
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Refer to LEW-16716.