Design of micromachined resonators for fish identification

The ID tag presented here was designed to give a tag of small size that could be produced at a low prize, and that could be read remotely in live fish, even in seawater. The last condition precludes use of electromagnetic waves for interrogation of the tags, and acoustic interrogation is then a clear alternative. The solution presented is a passive tag with a set of acoustic resonances that may be detected acoustically. The tag operates in the 200 to 400 kHz range. The identity of the tag is given by a unique combination of resonances in this frequency range. For the tags presented here there are five resonances per tag. If five or more resonances are chosen from a predetermined set of say 17 resonance frequencies, a total number of at least 3000 to 4000 different tags are available. This is adequate for classification of fish at the batch level in fish farms, or of local wild fish tribes. The resonators on a tag consists of a thin, nominally 500 nm thick silicon nitride membrane suspended over separate evacuated cavities, made by bulk silicon micromachining. The resonators were designed to have Q-factors in the range 27 to 35 with viscous losses in the water neglected. The resonators have been measured in water and in dead or live anesthetized fish from distances up to 30 cm. Sharp resonances in fair accordance with the tag design were achieved. Some alterations of the tag response with change of the angular orientation of the tag relative to the ultrasound beam are seen. This is also theoretically expected.