A low-pressure encapsulated resonant fluid density sensor with feedback control electronics

In this paper we present a fully low-pressure encapsulated and closed-loop operated resonant fluid density sensor. The device consists of a tube in si licon, which is vibrating in a selected balanced torsion mode. The resonanc e frequency changes with the density of the fluid in the tube due to the ch ange of the inertial mass of the vibrating system. The sensor is fabricated and encapsulated at wafer level using silicon micromachining techniques. T he encapsulation is performed by anodically bonding the silicon densitomete r in vacuum between two glass lids with metal electrodes for electrostatic excitation and capacitive detection. The sample volume is only 0.035 mi and the size of the encapsulated device is 14 mm x 23 mm x 1.85 mm. The measur ements were performed using a novel excitation and detection technique base d on discontinuous, 'burst' excitation. This principle enabled us to elimin ate the electrical crosstalk between excitation and detection. The electrod es could be placed on top of the glass lids without using electrical feedth roughs, and a cavity gap of 100 mu m could be formed between the recessed g lass lid surface and the silicon tube to reduce squeeze-film damping. The c losed-loop 'burst' technology enabled us to make continuous measurements of fluid densities. The sensor showed high density sensitivities of the order of -200 ppm (kg m(-3))(-1), a high mechanical e-factor of 3400 for air in the tube and low temperature sensitivities of -29 ppm degrees C-1 in the ra nge 20-100 degrees C.