Relationship between the structure and electrical characteristics of diamond-like carbon films

To elucidate the relationship between the structure and the electrical characteristics of diamond-like carbon (DLC) films, DLC films were synthesized in a well-controlled glow discharge with the aid of photoelectrons in an argon/methane atmosphere. The dielectric constant and breakdown strength of the films exhibited opposite behaviors, depending on the total pressure during the synthesis. The product of these two values decreased monotonically as the pressure increased. The Raman spectra were analyzed with a Voigt-type formula. Based on the results, the authors propose the “sp² cluster model” for the DLC structure. This model consists of conductive clusters of sp² carbons surrounded by a dielectric matrix sea of sp² carbon, sp³ carbon, and hydrogen, and indicates that the dielectric constant of the whole DLC film is determined by the balance between the dielectric constant of the matrix and the total size of the clusters, while the breakdown strength is determined by the reciprocal of the cluster size. The model suggests that a high-κ DLC film can be synthesized at a middle pressure and consists of well-grown sp² clusters and a dense matrix. A low-κ DLC film can be synthesized both at low and high pressures. The sp² cluster model explains that a low-κ DLC film synthesized at low pressure consists of a dense matrix and a low density of sp² clusters, and exhibits a high breakdown strength. On the other hand, a low-κ film synthesized at high pressure consists of a coarse matrix and a high density of clusters and exhibits a low breakdown strength.