Boron carbide and boron carbonitride thin films as protective coatings in ultra-high density hard disk drives

Boron carbide (B₄C) and boron carbon nitride (B_x C_yN_z) thin films were synthesized by pulsed DC magnetron sputtering. Effects of target power, target pulse frequency, substrate bias and pulse frequency on surface roughness were studied by AFM over sampling areas of 20 μm × 20 μm. For B₄ C, the combination of target power of 75 W/50 kHz and substrate bias of -100 V/2 kHz results in the smallest surface roughness. Compressive stress in these films is around 2.5 GPa. These B₄C films have a hardness of 30 ± 5 GPa and reduced modulus of 250 ± 50 GPa. Nitrogen incorporation into B₄C films, which gives B_xC_yN_z thin films, has a beneficial effect. When deposited under similar conditions with substrate bias pulsed at -100 V/20 kHz, the root-mean-square surface roughness decreases to 0.19 nm, compared with 0.28 nm for B₄C. The hardness of B_xC_yN_z is 20 ± 3 GPa, and reduced modulus is 210 ± 30 GPa. Auger electron spectroscopy was used to characterize the film composition. High-resolution cross-sectioned TEM images and diffraction show that both films are amorphous. Corrosion studies indicate that B_x C_yN_z is a better protective coating for hard disk substrates than B₄C and CN_x films. This is attributed mainly to the smoother morphology of B_x C_yN_z films.