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

Abstract

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 silicon, which is vibrating in a selected balanced torsion mode. The resonance frequency changes with the density of the fluid in the tube due to the change of the inertial mass of the vibrating system. The sensor is fabricated and encapsulated at wafer level using silicon micromachining techniques. The encapsulation is performed by anodically bonding the silicon densitometer in vacuum between two glass lids with metal electrodes for electrostatic excitation and capacitive detection. The sample volume is only 0.035 ml and the size of the encapsulated device is 14 mm × 23 mm × 1.85 mm. The measurements were performed using a novel excitation and detection technique based on discontinuous, `burst' excitation. This principle enabled us to eliminate the electrical crosstalk between excitation and detection. The electrodes could be placed on top of the glass lids without using electrical feedthroughs, and a cavity gap of 100 µm could be formed between the recessed glass lid surface and the silicon tube to reduce squeeze-film damping. The closed-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 Q-factor of 3400 for air in the tube and low temperature sensitivities of -29 ppm °C-1 in the range 20-100 °C.