Chemical influence of inert gas on the thin film stress in plasma-enhanced chemical vapor deposited alpha-SiN:H films

The growth of amorphous hydrogenated silicon nitride (a-SiN:H) films by plasma enhanced chemical vapor deposition (PECVD) of SiH4-NH3-N-2 reactive gas mixtures has been studied. Films were deposited at low temperature (T < 250 degrees C) in a commercial PECVD system commonly used to grow a-SiN:H for semiconductor integrated circuit passivation. It has been observed that the stress of the a-SiN:H film can be controlled through dilution of the film precursors with an inert gas. Experiments indicate that the influence of the inert gas on the process extends from growth kinetics and plasma chemistry to hydrogen bonding, elemental composition, and biaxial elastic modulus. The stress in films deposited without dilution is tensile. When argon is added to the plasma, Si-H, plasma chemistry and film hydrogen bond density change producing a reduction in the amount of tensile stress. Dilution with helium can be used to shift the film stress from tensile to compressive with minimum change in growth rate. The observed helium/film stress relationship is associated with helium-based Penning ionization processes, which create metastable reactive gas species. In turn, the metastables influence nitrogen and hydrogen incorporation into the film. Nitrogen incorporation produces volume expansion of the film, increasing the compressive character of the film stress. This effect is similar to that observed when the RF power is varied or when low or multifrequency plasma excitation is used during PECVD growth of a-SiN:H.