Abstract: A bulk acoustic wave, BAW, resonator device comprising first and second metal layers (10, 20) and an intervening piezoelectric layer (30), the first metal layer (10) comprising spaced first and second portions (12, 14), wherein the first and second portions (12, 14) are each arranged as a plurality of interconnected fingers (16, 18), and wherein each of the plurality of fingers (16) of the first portion (12) is acoustically coupled to at least one of the fingers (18) of the second portion (14). In one embodiment the fingers of the first portion (12) are interlaced with the fingers (18) of the second portion (14), thereby providing direct coupling. In another embodiment the acoustic coupling between the fingers of the first and second portions is provided indirectly by further portions (15) of the first metal layer (10).