CT On A Chip: Enabling High-Resolution Polar In-Situ Data Collection

The effects of anthropogenic climate change are being felt globally, but there is still much unknown about the long-term impacts of these changes. Global and regional-scale climate modeling can help us better understand these complex interactions, especially over the long term as the oceans help to buffer much of the response by taking up excess carbon dioxide and heat. Most of this heat is taken up by the Southern Ocean, and then is distributed around the globe through the thermohaline circulatory system via the Antarctic Bottom and Intermediate Waters. Melting plays a major role in generating this cold, fresh water, but melt rates are difficult to measure under hundreds of meters of ice, and different parameterizations of this critical metric lead to large variations in model estimates, making in situ measurements critical to model and parameterization improvements. Salinity, which can be used to determine melt rates in these difficult-to-access locales, is calculated using the conductivity, temperature, and pressure measurements taken by a CTD instrument. These devices, however, tend towards large and expensive tools that require boats and cranes to deploy. Hand-held devices are generally expensive and delicate, as well. Microelectromechanical systems offer one alternative to these bulky sensors by taking advantage of microfabrication techniques used for fabricating integrated circuits to shrink measurement volumes for improved accuracy and resolution. However, while work has been done to develop these devices, little has been done to take advantage of their improved abilities. This work looks at addressing that unknown by examining how changes in the geometry of cell affect the overall response. I developed a set of finite element models to better understand the physics of the system, using COMSOL electro-physical simulations and an algorithm proposed previously in the literature to calculate cell constants for a large number of simulated chips and MATLAB to build a number of variations of linear regression models to help determine which parameters were important. I then fabricated over a hundred chips of various geometries on silicon using standard microfabrication techniques, with a 3μm oxide layer for insulation and 110nm chrome/gold electrodes. Testing and characterization of these devices was done with a Keysight impedance measurement system (LCR E4980A) and demonstrated that the response of the cell was largely dictated by the width of the driving electrode and the interelectrode spacings, with wider electrodes and spacings leading to weakening responses. Finally, I developed an instrument in a 1000m-rated soda-can-sized housing with a commercial pressure sensor and thermistor to test these chips in the field. Deployments in Antarctica during the 2021/22 austral summer were successful, and demonstrated the potential of the system as a whole, with some post-field debugging and diagnostics discussed with solutions implemented. Future opportunities for continuing this work are provided at the end.

Date

2024-04-27

Resource Type

Text

Resource Subtype

Dissertation

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