In this paper, an AlN piezoelectric micromachined ultrasonic transducer (PMUT) compatible with pre-processed CMOS substrate is presented. Compared to traditional PMUTs, given that this one is on top of a CMOS pre-processed wafer, it will provide benefits in terms of fabrication without the complexity to use wafer bonding. These benefits will be clearly reflected i.e. in arrays, since the smaller pitch achievable between PMUTs, with no added bond-pads needed, will enable a higher fill factor with the advantages of reduced cost, area and power consumption. Pulse echo measurements of acoustic pressure in liquid provide competitive values compared with the state-of-the-art AlN PMUTs.