PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



279 



gases admitted to the coating chamber, on the temperature of this chamber, 

 and on the duration of pyrolysis. The rate of deposition of carbon is independ- 

 ent of this duration except during the first moments of deposition when the 

 initial deposits formed serve to catalyze subsequent reaction, whose steady 

 state is ordinarily quickly achieved. In Fig. 3 the film resistance of carbon 

 films produced in a batch furnace is shown as a function of the duration of 

 deposition. Included in the figure is a scale giving the film thickness which, 

 as discussed in a later section,* is inversely proportional to the film resistance 

 over the range shown. The linearity of the relationship thus bespeaks a con- 

 stant deposition rate. 



400 

 300 



zs|:: z:::: ::z: 



0.2 



10 



20 30 



0.3 0.4 0.6 1.0 2 3 4 5 6 



DURATION OF PYROLYSIS IN HOURS 



Fig. 3 — Dependence of the thickness of carbon films on the duration of pyrolysis at 

 1000 deg C and 30 per cent methane concentration. Film thickness expressed in terms of 

 film resistance. 



The pyrolyzing temperature, more than any other single condition, de- 

 termines the actual rate of deposition of pyrolytic carbon. Figure 4 gives 

 the fihn resistance for a given duration of pyrolysis, inversely proportional 

 to the deposition rate, in a continuous furnace as a function of temperature 

 in dynamic equilibrium and illustrates the need for precise temperature 

 control when films of constant and reproducible film resistance are to be 

 produced. 



Increase in the hydrocarbon concentration in the furnace atmosphere 

 increases the rate of carbon deposition, as is shown by Fig. 5 for the case of 

 methane, which gives the dependence of fihn resistance on concentration for 



* See Section 5.4. 



