The CN

x

films were grown on Si(001) substrates using r.f. magnetron sputtering pyrolytic graphite in pure N

2

. During the deposition

substrate temperature is kept a constant of 350℃ (T

s

). The r.f. power and PN

2

were fixed at 300W and 0.5Pa

respectively. Only substrate bias (

V

b

) is varied from 0V to -150V.The effect of substrate bias on adhesion and roughness of CN

x

film is discussed. The optimized condition for growing CN

x

films with the smooth and good adhesion on Si (001) is desired. Atomic force microscopy (AFM) is used to characterize the surface morphology and roughness of the CN

x

films deposited on the Si (001) substrate. The scratch test was utilized for measurement of coating adhesion of the CN

x

films on Si (001). The AFM and scratch test results show that the adhesion and roughness of CN

x

films on Si (001) are highly dependent on substrate bias. It can be found that increased substrate bias causes a rougher surface of CN

x

films

and correspondingly the minimum value of rms roughness (0.8nm) was obtained while substrate bias increase up to -100V.But when substrate bias increases continuously from -100V to -150V

the rms roughness of the films is enhanced. It is similar to the change of adhesion of the films with substrate bias. The films grown at

V

b

=-100V obtain better adhesion to Si substrate than that grown at other substrate bias value. The change trend of roughness and adhesion of films with substrate bias is related to the ion energy reaching to the film surface during deposition. It can be concluded that the adhesion and smoothness of CN

x

films are highly dependent on the energy of the ions reaching on the surface of the films

whereas substrate bias can control this energy

consequently determine the final quality of the deposited films. Thus an appropriate substrate bias is favorable to deposit good quality films by controlling properly the ion energy reaching on the surface of the films. For the deposition of CN

x

films at T

s

=350℃ using r.f. magnetron sputtering

V

b

=-100V is optimal condition for obtaining the quite smooth CN

x

thin films with good adhesion on Si (001) substrate.