We investigate the properties of highly confined and isolated surface modes in a phononic crystal based on a stripped membrane combined with disc shape thin films deposited on the micro-strip surface. The structure is made of an Aluminum Nitride (AlN) thin film membrane and Gold (Au) for the disc shape thin film. These materials are chosen owing to the strong contrast in their elastic and density properties and to their compatibility with electroacoustic devices technology. An optimal choice of the geometrical parameters of the membrane and the grooves enables us to obtain wide stop-bands frequency. The introduction of disc shape thin films on the grooved surface enables us to introduce nearly flat modes within the band-gap and consequently paves the way to implement advanced design of electroacoustic filters and high performance cavity resonators.