Effect of magnetron sputtered silicon nitride on the water-vapor-permeation-rate of polyimide thin film

A series of PIs were first derived from 4,4′-oxydianiline (ODA) and various dianhydrides, including pyromellitic dianhydride (PMDA), 4,4′-oxydiphthalic dianhydride (ODPA), and 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) (IDPA), to produce PI matrix with various degrees of rigidity. In order to reduce the water-vapor-transmission-rate (WVTR) of PI, a moisture barrier was deposited on each PI substrate by an RF magnetron sputtering system and the WVTR was investigated as a function of rigidity of PI matrix. The deposited thin film (SiNxOy) with the thickness of 100 nm on PI is an excellent moisture barrier that the WVTR of all PI thin films is significantly reduced. The relatively rigid PI matrix leads to a denser packing of barrier layer, and thus exhibits lower WVTR than other two PI systems. The WVTR of ODA–PMDA/SiNxOy declines to 5 g/m2-day compared with 123 g/m2-day for parent PI film. On the other hand, the moisture barrier on a flexible PI matrix (ODA–IDPA) shows a higher reliability on the WVTR after bending treatment to 15,000 cycles. In addition to the moisture barrier feature, the SiNxOy-deposited PI thin films maintain favorable thermal stability and mechanical strength for practical applications.