Microsystem-Compatible Heterogeneous Micro-Hydrophone for Use at High Static Pressures

Abstract

Underwater acoustic sensing is important for multiple applications, including seismic sensing for the oil and gas industry, SONAR at extended ocean depth, localization of underwater vehicles, implementation in autonomous microsystems, and marine mammal research, to name a few; however, there is a lack of hydrophones that can operate at ambient pressures in the range of 5-50 MPa while still offering a miniature form factor, compatibility with lithographic manufacturing methods, and compatibility with autonomous microsystems. A miniature, high pressure hydrophone should additionally provide high sensitivity and sufficient bandwidth for a given application, while being immune to corrosive brines, debris, and water contamination. Recently, a sapphire-based micromachining process was reported that shows promise in addressing these concerns. The thin-film MEMS process uses surface micromachining technology to create hermetically vacuum sealed, variable gap capacitive sensing elements with touch mode capability that eliminates element rupture due to pressure overloading. An insulating sapphire substrate provides low parasitic capacitance that allows for electrical connectivity in parallel for high sensitivity. The primary focus of this report is to investigate the suitability of this process flow for fabrication of micro hydrophones that satisfy the above criteria, as well as to demonstrate use of the sensor in several applications of interest. The surface micromachined micro-hydrophone utilizes a heterogeneous arrayed architecture to provide high responsivity (up to 3.0 pF/MPa) over a wide range of static pressures (≥50 MPa) while maintaining a miniature form factor (1.4 × 1.6 × 0.5 mm3). The sensor, H106, uses 106 multiply sized sensing elements with diaphragms ranging from ϕ104 μm to ϕ56 μm. Experimental characterization was performed to verify the static pressure response (up to 34.406 pF capacitance change) over a pressure range of 50 MPa, verify repeatability, quantify how responses change over time, and verify sensor bandwidth in air and paraffin oil (EnerpacTM LX101). The first diaphragm resonances in air and oil were experimentally verified to occur at 4.105 MHz and 2.319 MHz, respectively, at atmospheric pressure. Hydrophone operation in an autonomous, battery operated microsystem, intended for application to oil and gas exploration, was demonstrated through sensor-system integration and manipulation of existing microcontroller (MCU) programming. Static pressure sensing capability was demonstrated at up to 50 MPa, with sub-psi resolution throughout the entire static pressure range. Seismic sensing capability was demonstrated using pressure waves as large as 100 psi in ambient pressure levels up to 50 MPa and frequency up to 100 Hz. Minimum detectable pressure (MDP) at frequency was also quantified at several different static pressures and is below 0.1 psi over nearly the entire static pressure range above 2 Hz. The sensor’s potential use as a deep-sea hydrophone was demonstrated through integration with a custom charge amplifier circuit. The circuit provides a 3-dB bandwidth up to 8.9 MHz. All testing was performed in paraffin oil. The sensor was calibrated at atmospheric pressures up to 100 kHz frequency (-221 +/- 1.9 dB re V/μPa, 10V bias). Response up to 2 MHz was also demonstrated using a pulse-echo procedure. Flat band hydrophone sensitivity was experimentally characterized at up to 50 MPa static pressure. Lastly, noise power spectral density and resulting MDP were characterized (24.5 mPa/√Hz, 1 kHz, 1 atm, 40V bias). Bandwidth, form factor, and static pressure rating in particular compare extremely favorably to existing hydrophones.