The invention concerns a manufacturing process, and the related micromachined capacitive ultra-acoustic transducer, that uses commercial silicon wafer 8 already covered on at least one or, more preferably, on both faces by an upper layer 9 and by a lower layer 9′ of silicon nitride deposited with low pressure chemical vapour deposition technique, or deposition LPCVD deposition. One of the two layers 9 or 9′ of silicon nitride, of optimal quality, covering the wafer 8 is used as emitting membrane of the transducer. As a consequence, the micro-cell array 6 forming the CMUT transducer is grown onto one of the two layers of silicon nitride, i.e. it is grown at the back of the transducer with a sequence of steps that is reversed with respect to the classical technology.