Carnegie Mellon Logo

High Sensitivity Integrated Capacitive Chemical Sensing

Nathan Lazarus, Gary Fedder

Left: Electric panel for a residential building with current transformers installed to collect current waveforms. Right: Start-up transient for a refrigerator in the building.

Integrating chemical sensors on the same silicon die as that on which electronics is tested has the potential to reduce fabrication costs because it shares process steps. Eliminating the need to take signals off chip to external circuitry also reduces noise in the system, improving the lower limit of detection. This project is focused on using MEMS fabrication techniques to make high sensitivity integrated capacitive chemical sensors. Since capacitive sensing is commonly used in humidity sensors, an improved sensor would have significant commercial potential.

Past integrated capacitive chemical sensors have consisted of interdigitated electrodes coated with polymer. This type of sensor simplifies fabrication, requiring only the addition of polymer, but the capacitance through the oxide under the electrodes results in a degradation of sensitivity. This work has investigated several methods for improving the sensitivity of these sensors by using MEMS micromachining. The sensor design shown above creates a vertical parallel plate sensor by etching away a layer in the CMOS metal stack and filling the resulting cavity with polyimide. All of the electric field lines in this design pass through the polyimide layer, resulting in a much higher sensitivity for the device.

These techniques have been successfully demonstrated for humidity sensing. Future efforts will be focused on investigating other capacitive materials and demonstrating that these sensors can be used for a wide variety of chemicals, rather than simply for water vapor.

This work is supported primarily by the National Institute for Occupatonal Health and Safety/Centers for Disease Control and Prevention (NIOSH/CDC).