Membrane resonator array with backside exposure cavity for gas/chemical sensing

A fluid sensor, having a first side and an opposite second side, for sensing at least one target substance in an analyte that is disposed adjacent to the second side, comprising:

• (a) a substrate;
• (b) a plurality of sensing devices formed on the substrate, each sensing device including:
• (i) a cavity defined through the substrate, the cavity configured to receive the analyte therein;
• (ii) a membrane, having a top side and a bottom side, the bottom side affixed to the substrate so as to isolate the top side of the membrane and the first side of the sensor from the first cavity;
• (iii) an actuator affixed to the top side of the membrane and configured to cause the membrane to vibrate at a first frequency;
• (iv) a functionalizing material applied to the bottom side of the membrane and exposed to the cavity, the functionalizing material having a property such that when the target substance comes in contact with the functionalizing material, the membrane resonates at a second frequency that is different from the first frequency;
• and
• (v) a resonant frequency detector that generates a signal that indicates whether the membrane is resonating at the first frequency or at the second frequency;
• and
• (c) an electronic circuit that is responsive to the signal from the resonant frequency detector and that is configured to generate an output indicative of whether the target substance is present in the analyte based on the signal from the resonant frequency detector, the electronic circuit being disposed on the first side of the sensor so as to be isolated from the analyte.

A method of making a membrane resonator, comprising the steps of:

• (a) depositing a membrane layer onto a top side of a rigid layer, the rigid layer also having a bottom side;
• (b) depositing a first conductive layer onto the membrane layer;
• (c) depositing a piezoelectric layer onto the first conductive layer;
• (d) depositing a second conductive layer onto the piezoelectric layer;
• (e) applying a mask onto the bottom side of the rigid layer, the mask defining a central hole passing therethrough;
• etching the bottom side of the rigid layer through the hole so as to define a cavity through the rigid layer so that the membrane layer is exposed through the cavity;
• (g) applying a functional material to the membrane layer through the cavity;
• and
• (h) coupling an actuating and sensing circuit to the first conductive layer and the second conductive layer.